Decorative film-coated automobile outer panel, and method for manufacturing decorative film-coated automobile outer panel

Abstract

A decorative film-coated automobile outer panel according to the present embodiment comprises: a steel material; Zn-Al-Mg-based plating layers provided on both surfaces of the steel material; a chemical conversion treatment layer provided on the surface of at least one of the Zn-Al-Mg-based plating layers; and a decorative film layer provided on at least one surface side of the steel material and on the surface of the chemical conversion treatment layer.

Description

Automotive exterior panel coated with decorative film and method for manufacturing an automotive exterior panel coated with decorative film

[0001] This disclosure relates to an automotive exterior panel coated with a decorative film and a method for manufacturing an automotive exterior panel coated with a decorative film. This application claims priority under Japanese Patent Application No. 2025-057172, filed in Japan on March 28, 2025, the contents of which are incorporated herein by reference.

[0002] With the recent advancements in electric vehicle development, the elimination of manufacturing processes during the vehicle body production has attracted attention. If the painting process can be eliminated during the vehicle body production, costs can be significantly reduced.

[0003] As a technology that can eliminate the painting process, Patent Document 1 discloses a decorative molded article for use on the exterior of a vehicle body, having an adherend, an adhesion layer, a design layer, and a surface protection layer in that order, wherein the adherend and the adhesion layer are in direct adhesion, the 180° peel strength at the interface between the adherend and the adhesion layer at 23°C and 50% RH as specified in JIS Z0237:2009 is 30 N / 25 mm or more, the adherend is a resin layer, the adhesion layer is a pressure-sensitive adhesive layer, and the glass transition temperature Tg of the pressure-sensitive adhesive layer is 0 to 30°C.

[0004] Furthermore, as a technology that can omit the painting process, Patent Document 2 discloses a decorative laminate comprising, in order adjacent to each other, a cover film, a clear layer, a design layer, a base film, and an adhesive layer, wherein the cover film is heat-laminated to the clear layer, the design layer is a coating film of a coating paint for the design layer, the clear layer is a coating film of a coating paint for the clear layer, the coating paint for the clear layer contains an acrylic resin, and the glass transition temperature (Tg) of the acrylic resin is 20 to 100°C.

[0005] Patent No. 6926602, Patent No. 7052119

[0006] However, the decorative molded body described in Patent Document 1 lacked sufficient strength because the adherend was made of resin. Furthermore, when the decorative laminate described in Patent Document 2 was used on a conventional highly corrosion-resistant plated steel sheet having a Zn-Al-Mg plating layer, although it could improve the strength and corrosion resistance of the automotive exterior panel, there were problems such as corrosion and blistering of the film due to the intrusion of moisture into the boundary between the Zn-Al-Mg plating layer and the decorative laminate (decorative film), and rust generation from the edge of the plated steel material.

[0007] This disclosure is an invention made in view of the above circumstances, and aims to provide an automotive exterior panel coated with a decorative film that eliminates the painting process and has excellent strength, corrosion resistance, and design, as well as a method for manufacturing such an automotive exterior panel coated with a decorative film.

[0008] To solve the aforementioned problems, the present disclosure proposes the following means: (1) A decorative film-coated automobile exterior panel according to Embodiment 1 of the present disclosure comprises: a steel material; a Zn-Al-Mg plating layer provided on both surfaces of the steel material; a chemical conversion treatment layer provided on the surface of at least one of the Zn-Al-Mg plating layers; and a decorative film layer provided on at least one side of the steel material and on the surface of the chemical conversion treatment layer, wherein the chemical conversion treatment layer contains a resin, the area coverage of the resin in the chemical conversion treatment layer is 0.5 to 1.0, the resin is present on the outermost surface of the chemical conversion treatment layer, and in a cross-section in the thickness direction, the relationship between the total length Lr of the resin present on the outermost surface of the chemical conversion treatment layer and the decorative film layer in any interface length Lc = 600 nm obtained from an electron microscope image obtained by observing the interface between the chemical conversion treatment layer and the decorative film layer with a transmission electron microscope is Lr / Lc ≥ 0.1. (2) Aspect 2 of the present disclosure is an automotive exterior panel coated with a decorative film according to aspect 1, wherein the chemical composition of the Zn-Al-Mg plating layer is, by mass%, Al: 1.00 to 30.00%, Mg: 1.00 to 10.00%, Si: 0% to 2.00%, and the remainder: Zn and impurities. (3) Aspect 3 of the present disclosure is an automotive exterior panel coated with a decorative film according to Aspect 1, wherein the chemical composition of the Zn-Al-Mg plating layer is, in mass%, Al: 5.00 to 30.00%, Mg: 2.00 to 10.00%, Si: 0% to 2.00%, Ca: 0% to 2.00%, Sb: 0% to 0.5000%, Pb: 0% to 0.50%, Sr: 0% to 0.50%, Cu: 0% to 1.00%, Ti: 0% to 1.00%, Cr: 0% to 1.00%, Nb: 0% to 1.00%, Ni: 0% to 1.00%, Mn: 0% to 1.00%, Mo: 0% to 1.00%, Co: 0% to 1.0000%, V: 0% to 1.0000%, Sn: 0% to 1.00%, In: 0% to 1.0000%, Bi: 0% to 1.0000%, Zr: 0% to 1.00%, Ag: 0% to 1.00%, Li: 0% to 1.00%, La: 0% to 0.50%, Ce: 0% to 0.50%, Y: 0% to 0.50%, B: 0% to 0.50%, Fe: 0% to 5.00%, and the remainder: containing Zn and impurities.(4) Aspect 4 of the present disclosure is a decorative film-coated automobile exterior panel of Aspect 2, wherein the chemical composition further contains one or more selected from the group consisting of Si: greater than 0% and 2.00% or less, and Ca: greater than 0% and 2.00% or less. (5) Aspect 5 of the present disclosure is a decorative film-coated automobile exterior panel of Aspect 2 or Aspect 4, wherein the chemical composition further contains one or more selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.50% or less, and Sr: greater than 0% and 0.50% or less. (6) Embodiment 6 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of embodiments 2, 4, or 5, wherein the chemical composition further contains one or more selected from the group consisting of Cu: greater than 0% and 1.00% or less, Ti: greater than 0% and 1.00% or less, Cr: greater than 0% and 1.00% or less, Nb: greater than 0% and 1.00% or less, Ni: greater than 0% and 1.00% or less, Mn: greater than 0% and 1.00% or less, Mo: greater than 0% and 1.00% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less. (7) Embodiment 7 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of Embodiments 2, 4 to 6, wherein the chemical composition further contains one or more selected from the group consisting of Sn: greater than 0% and 1.00% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less. (8) Embodiment 8 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of Embodiments 2, 4 to 7, wherein the chemical composition further contains one or more selected from the group consisting of Zr: greater than 0% and 1.00% or less, Ag: greater than 0% and 1.00% or less, and Li: greater than 0% and 1.00% or less. (9) Embodiment 9 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of embodiments 2, 4 to 8, wherein the chemical composition further contains one or more selected from the group consisting of La: greater than 0% and 0.50% or less, Ce: greater than 0% and 0.50% or less, and Y: greater than 0% and 0.50% or less. (10) Embodiment 10 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of embodiments 2, 4 to 9, wherein the chemical composition further contains B: greater than 0% and 0.50% or less.(11) Embodiment 11 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of embodiments 2, 4 to 10, wherein the chemical composition further contains Fe: 0.01% or more and 5.00% or less. (12) Embodiment 12 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of embodiments 2 to 11, wherein an adhesive layer exists between the decorative film layer and the chemical conversion treatment layer. (13) Embodiment 13 of the present disclosure is a decorative film-coated automobile exterior panel according to any one of embodiments 1 to 12, wherein the chemical conversion treatment layer is a film layer having at least one bond of Zr-O bond, Ti-O bond, or Si-O bond. (14) Embodiment 14 of the present disclosure is a decorative film-coated automobile exterior panel according to embodiment 13, wherein the resin is at least one of acrylic resin, urethane resin, polyester resin, and epoxy resin. (15) In an aspect 15 of the present disclosure, in a decorative film-coated automotive exterior panel of any one of aspects 13 to 14, the chemical conversion treatment layer comprises at least one of phosphorus, vanadium, cobalt, titanium, and chromium. (16) In an aspect 16 of the present disclosure, in a decorative film-coated automotive exterior panel of any one of aspects 1 to 15, the decorative film layer is provided on only one surface of the chemical conversion treatment layer, and the chemical conversion treatment layer that does not have the decorative film layer is covered with a resin film. (17) An aspect 17 of the present disclosure is a decorative film-coated automotive exterior panel of any one of aspects 1 to 16, in which at least a portion of the end face of the steel material is covered with the decorative film layer. (18) An automotive component of an aspect 18 of the present disclosure comprises a decorative film-coated automotive exterior panel of any one of aspects 1 to 17. (19) A method for manufacturing an automotive exterior panel covered with a decorative film according to aspect 19 of the present disclosure involves pressing a chemically treated steel sheet, which comprises a steel sheet, a Zn-Al-Mg plating layer provided on both sides of the steel sheet, and a chemical conversion treatment layer provided on the surface of the Zn-Al-Mg plating layer, to form a part, and then attaching a decorative film to the surface of the chemical conversion treatment layer.(20) A method for manufacturing an automotive exterior panel covered with a decorative film according to aspect 20 of the present disclosure comprises a steel sheet, a Zn-Al-Mg plating layer provided on both sides of the steel sheet, and a chemical conversion treatment layer provided on the surface of the Zn-Al-Mg plating layer. A decorative film is attached to the surface of the chemical conversion treatment layer of the chemical conversion treatment steel sheet, and then press-formed.

[0009] According to each of the above embodiments of this disclosure, it is possible to provide an automotive exterior panel coated with a decorative film that eliminates the painting process and has excellent strength, corrosion resistance, and design, as well as a method for manufacturing such an automotive exterior panel coated with a decorative film.

[0010] This is a schematic cross-sectional view of a decorative film-coated automobile exterior panel according to one embodiment of the present disclosure. This is a schematic diagram of the area near the boundary between the chemical treatment layer and the decorative film layer. This is a schematic cross-sectional view of the decorative film layer. This is a flowchart of the manufacturing method of the decorative film-coated automobile exterior panel according to the first embodiment. This is a flowchart of the manufacturing method of the decorative film-coated automobile exterior panel according to the second embodiment. This is a schematic cross-sectional view of a chemical treatment steel sheet. This is a schematic diagram of a vacuum forming apparatus.

[0011] Hereinafter, with reference to the drawings, a decorative film-coated automobile exterior panel and a method for manufacturing the decorative film-coated automobile exterior panel according to one embodiment of the present disclosure will be described.

[0012] (Decorative Film-Coated Automotive Exterior Panel) Figure 1 is a schematic cross-sectional view of a decorative film-coated automotive exterior panel according to one embodiment of the present disclosure. As shown in Figure 1, the decorative film-coated automotive exterior panel 10 comprises a steel material 1, a Zn-Al-Mg-based plating layer 2 provided on both surfaces of the steel material 1, a chemical conversion treatment layer 3 provided on the surface of at least one of the Zn-Al-Mg-based plating layers 2, and a decorative film layer 4 provided on at least one side of the steel material 1 and on the surface of the chemical conversion treatment layer 3. Here, a structure comprising the steel material 1, the Zn-Al-Mg-based plating layer 2 provided on both surfaces of the steel material 1, and the chemical conversion treatment layer 3 provided on the surface of the Zn-Al-Mg-based plating layer 2 (i.e., a structure excluding the decorative film layer 4) is defined as an automotive exterior panel 8.

[0013] (Steel Material 1) The dimensions, composition, structure, and mechanical properties of the steel material 1 as the base material are not particularly limited. For example, various types of steel materials can be used depending on the mechanical strength (e.g., tensile strength) required for the decorative film-coated automobile exterior panel 10. Examples of such steel materials 1 include iron and steel materials standardized by Japanese Industrial Standards (JIS), such as carbon steel, alloy steel, and high-tensile steel used for general structural and mechanical structural purposes. By using steel material 1, the decorative film-coated automobile exterior panel 10 can obtain high strength.

[0014] Specific examples of such steel materials include cold-rolled steel, hot-rolled steel, hot-rolled steel sheets for automotive structures, hot-rolled high-tensile steel sheets for automotive processing, cold-rolled steel sheets for automotive structures, cold-rolled high-tensile steel sheets for automotive processing, and high-tensile steel materials generally called hot-stamped materials that have been quenched during hot working. The chemical composition of the steel material is not particularly limited, but in addition to Fe and C, it may contain one or more of the following: Si, Mn, S, P, Al, N, Cr, Mo, Ni, Cu, Ca, Mg, Ce, Hf, La, Zr, and Sb. One or more of these optional additive elements can be appropriately selected to obtain the desired material strength and formability, and their content can also be appropriately adjusted.

[0015] Furthermore, the thickness of the steel material 1 is not particularly limited and can be set appropriately according to the mechanical strength and other requirements of the decorative film-coated automobile exterior panel 10.

[0016] (Zn-Al-Mg Plating Layer 2) The Zn-Al-Mg plating layer 2 is provided on both surfaces of the steel material 1. In this embodiment, the Zn-Al-Mg plating layer 2 is a ternary plating. The chemical composition of such a Zn-Al-Mg plating layer 2 is preferably, in mass%, Al: 1.00 to 30.00%, Mg: 1.00 to 10.00%, Si: 0% to 2.00%, and the remainder: Zn and impurities. The chemical composition of the Zn-Al-Mg plating layer 2 is as follows: Al: 3.00-30.00%, Mg: 1.00-10.00%, Si: 0%-2.00%, Ca: 0%-2.00%, Sb: 0%-0.5000%, Pb: 0%-0.50%, Sr: 0%-0.50%, Cu: 0%-1.00%, Ti: 0%-1.00%, Cr: 0%-1.00%, Nb: 0%-1.00%, Ni: 0%-1.00%, Mn: 0%-1.00%, Mo: 0%-1. 0% of the material consists of Zn and impurities, with the following more preferable composition: Co: 0% to 1.0000%, V: 0% to 1.0000%, Sn: 0% to 1.00%, In: 0% to 1.0000%, Bi: 0% to 1.0000%, Zr: 0% to 1.00%, Ag: 0% to 1.00%, Li: 0% to 1.00%, La: 0% to 0.50%, Ce: 0% to 0.50%, Y: 0% to 0.50%, B: 0% to 0.50%, Fe: 0% to 5.00%, and the remainder being Zn and impurities. The chemical composition of the Zn-Al-Mg plating layer 2 will be described in detail below.

[0017] [Al: 1.00% by mass to 30.00% by mass] Al is included in a certain amount or more to ensure corrosion resistance as a decorative film-coated automobile exterior panel 10. If the Al content in the Zn-Al-Mg plating layer 2 is less than 1.00% by mass, it may not be possible to guarantee corrosion resistance as a decorative film-coated automobile exterior panel 10. For this reason, in the Zn-Al-Mg plating layer 2 according to this embodiment, the Al content is 1.00% by mass or more. The Al content is preferably 3.00% by mass or more, and more preferably 5.00% by mass or more. By having an Al content within the above range, it is possible to guarantee corrosion resistance as a decorative film-coated automobile exterior panel 10.

[0018] On the other hand, if the Al content in the Zn-Al-Mg plating layer 2 exceeds 30.00% by mass, the corrosion resistance of the decorative film-coated automobile exterior panel 10 cannot be guaranteed. Therefore, the Al content is 30.00% by mass or less. Preferably, the Al content is 25.00% by mass or less, and more preferably 20.00% by mass or less.

[0019] [Mg: 1.00% by mass to 10.00% by mass] Mg is included in a certain amount to ensure corrosion resistance as a decorative film-coated automobile exterior panel 10. Therefore, in the Zn-Al-Mg plating layer 2 according to this embodiment, the Mg content is 1.00% by mass or more. Preferably, the Mg content is 2.00% by mass or more, and more preferably 3.00% by mass or more. By having the Mg content within the above range, it is possible to ensure corrosion resistance as a decorative film-coated automobile exterior panel 10. If the Mg content exceeds 10.00% by mass, the processability may deteriorate. Therefore, the upper limit of the Mg content is 10.00% by mass.

[0020] On the other hand, if the Mg content in the Zn-Al-Mg plating layer 2 is less than 1.00% by mass, it becomes difficult to ensure corrosion resistance as a decorative film-coated automobile exterior panel 10. Therefore, the Mg content is 1.00% by mass or more. By keeping the Mg content within the above range, it becomes possible to ensure corrosion resistance as a decorative film-coated automobile exterior panel 10.

[0021] Next, the group of elements A that the Zn-Al-Mg plating layer 2 may contain will be described. Preferably, the Zn-Al-Mg plating layer 2 contains one or two elements selected from the group consisting of Si: greater than 0 mass% and 2.00 mass% or less, and Ca: greater than 0 mass% and 2.00 mass% or less (group of elements A). At least one of the elements of the above-mentioned group of elements A is an element that can be contained in the Zn-Al-Mg plating layer 2 in place of a portion of the remaining Zn.

[0022] [Si: 0% to 2.00% by mass] In this embodiment, the Zn-Al-Mg plating layer 2 may not contain Si, so the lower limit of the Si content is 0% by mass. On the other hand, Si is an element that can further improve the adhesion between the Zn-Al-Mg plating layer 2 and the steel material 1. When Si is included in the Zn-Al-Mg plating layer 2, the Si content is preferably 0.05% by mass or more, and more preferably 0.10% by mass or more.

[0023] On the other hand, if the Si content exceeds 2.00% by mass, it may inhibit the formation of Al-Mg oxide, which exhibits a Zn evaporation suppression effect when welding the area where the Zn-Al-Mg plating layer 2 is present. Therefore, it is preferable that the Si content in the Zn-Al-Mg plating layer 2 is 2.00% by mass or less. Furthermore, if the Si content in the plating bath for producing the Zn-Al-Mg plating layer 2 is too high, the viscosity of the plating bath may increase more than necessary, potentially reducing the plating operability. Therefore, by adjusting the Si content in the plating bath from the viewpoint of plating operability, the Si content in the Zn-Al-Mg plating layer 2 is preferably 1.00% by mass or less, and more preferably 0.50% by mass or less.

[0024] [Ca: 0% by mass to 2.00% by mass] In this embodiment, the Zn-Al-Mg plating layer 2 may not contain Ca, so the lower limit of the Ca content is 0% by mass. On the other hand, when Ca is included in the Zn-Al-Mg plating layer 2, it is possible to suppress liquid metal embrittlement cracking (LME) during welding. When Ca is included in the Zn-Al-Mg plating layer 2, this effect of suppressing LME during welding is achieved by setting the Ca content to 0.01% by mass or more. The Ca content in the Zn-Al-Mg plating layer 2 is more preferably 0.05% by mass or more.

[0025] On the other hand, if the Ca content in the Zn-Al-Mg plating layer 2 exceeds 2.00% by mass, the corrosion resistance of the decorative film-coated automobile exterior panel 10 may decrease. From this viewpoint, the Ca content in the Zn-Al-Mg plating layer 2 is 2.00% by mass or less. Preferably, the Ca content in the Zn-Al-Mg plating layer 2 is 1.50% by mass or less, and more preferably 1.00% by mass or less.

[0026] Next, the element group B that the Zn-Al-Mg plating layer 2 may contain will be described. Preferably, the Zn-Al-Mg plating layer 2 contains one or more elements selected from the group (element group B) consisting of Sb: greater than 0% by mass and 0.5000% by mass or less, Pb: greater than 0% by mass and 0.50% by mass or less, and Sr: greater than 0% by mass and 0.50% by mass or less. At least one of the elements of the above-mentioned element group B is an element that can be contained in the Zn-Al-Mg plating layer 2 in place of a portion of the remaining Zn.

[0027] [Sb: 0% by mass to 0.5000% by mass] In this embodiment, it is possible that the Zn-Al-Mg plating layer 2 may not contain Sb, so the lower limit of the content of these elements is 0% by mass. On the other hand, when Sb is contained in the Zn-Al-Mg plating layer 2, spangles are formed on the surface of the Zn-Al-Mg plating layer 2, making it possible to improve the metallic luster. For this reason, from the viewpoint of improving the design in combination with the decorative film layer 4, it is preferable that Sb is contained in the Zn-Al-Mg plating layer 2. This design improvement effect is manifested when the content of at least one of the Sb is 0.0500% by mass or more. For this reason, when Sb is contained in the Zn-Al-Mg plating layer 2, it is preferable that the Sb content be 0.0500% by mass or more.

[0028] On the other hand, when forming a Zn-Al-Mg-based plating layer 2 with an Sb content exceeding 0.5000% by mass, the amount of dross generated in the plating bath used to form the Zn-Al-Mg-based plating layer 2 increases, and it may not be possible to produce a plated steel sheet with good plating properties. For this reason, the Sb content in the Zn-Al-Mg-based plating layer 2 is 0.5000% by mass or less. Preferably, the Sb content is 0.2000% by mass or less.

[0029] [Pb: 0 to 0.50% by mass] In this embodiment, it is possible that the Zn-Al-Mg plating layer 2 may not contain Pb, so the lower limit of the content of these elements is 0% by mass. On the other hand, when Pb is contained in the Zn-Al-Mg plating layer 2, spangles are formed on the surface of the Zn-Al-Mg plating layer 2, making it possible to improve the metallic luster. For this reason, from the viewpoint of improving the design in combination with the decorative film layer 4, it is preferable that Pb is contained in the Zn-Al-Mg plating layer 2. This design improvement effect is manifested when the content of at least one of the Pb is 0.0500% by mass or more. For this reason, when Pb is contained in the Zn-Al-Mg plating layer 2, it is preferable that the Pb content be 0.05% by mass or more.

[0030] On the other hand, when forming a Zn-Al-Mg-based plating layer 2 with a Pb content exceeding 0.50% by mass, the amount of dross generated in the plating bath used to form the Zn-Al-Mg-based plating layer 2 increases, and it may not be possible to produce a plated steel sheet with good plating properties. For this reason, the Pb content in the Zn-Al-Mg-based plating layer 2 is 0.50% by mass or less. Preferably, the Pb content is 0.20% by mass or less.

[0031] [Sr: 0% to 0.50% by mass] In this embodiment, it is possible that the Zn-Al-Mg plating layer 2 may not contain Sr, so the lower limit of the content of these elements is 0% by mass. On the other hand, when Sr is contained in the Zn-Al-Mg plating layer 2, spangles are formed on the surface of the Zn-Al-Mg plating layer 2, making it possible to improve the metallic luster. For this reason, from the viewpoint of improving the design in combination with the decorative film layer 4, it is preferable that Sr be contained in the Zn-Al-Mg plating layer 2. This design improvement effect is manifested when the content of at least one of the Sr is 0.0500% by mass or more. For this reason, when Sr is included in the Zn-Al-Mg plating layer 2, it is preferable that the Sr content be 0.05% by mass or more.

[0032] On the other hand, when forming a Zn-Al-Mg-based plating layer 2 with a Sr content exceeding 0.50% by mass, the amount of dross generated in the plating bath used to form the Zn-Al-Mg-based plating layer 2 increases, and it may not be possible to produce a plated steel sheet with good plating properties. For this reason, the Sr content in the Zn-Al-Mg-based plating layer 2 is 0.50% by mass or less. Preferably, the Sr content is 0.20% by mass or less.

[0033] Next, we will explain the element group C that the Zn-Al-Mg plating layer 2 may contain. The Zn-Al-Mg plating layer 2 preferably contains one or more elements selected from the group (element group C) consisting of Cu: greater than 0 mass% and 1.00 mass% or less, Ti: greater than 0 mass% and 1.00 mass% or less, Cr: greater than 0 mass% and 1.00 mass% or less, Nb: greater than 0 mass% and 1.00 mass% or less, Ni: greater than 0 mass% and 1.00 mass% or less, Mn: greater than 0 mass% and 1.00 mass% or less, Mo: greater than 0 mass% and 1.00 mass% or less, Co: greater than 0 mass% and 1.0000 mass% or less, and V: greater than 0 mass% and 1.0000 mass% or less. At least one of the elements of the above-mentioned element group C is an element that can be contained in the Zn-Al-Mg plating layer 2 in place of a portion of the remaining Zn.

[0034] [Cu: 0 mass% to 1.00 mass%] [Ti: 0 mass% to 1.00 mass%] [Cr: 0 mass% to 1.00 mass%] [Nb: 0 mass% to 1.00 mass%] [Ni: 0 mass% to 1.00 mass%] [Mn: 0 mass% to 1.00 mass%] In this embodiment, the Zn-Al-Mg plating layer 2 may not contain Cu, Ti, Cr, Nb, Ni, or Mn, so the lower limit of the content of these elements is 0 mass%. On the other hand, if at least one of Cu, Ti, Cr, Nb, Ni, or Mn is contained in the Zn-Al-Mg plating layer 2, these elements are incorporated into the Fe-Al intermetallic compound generated during welding, making it possible to improve the corrosion resistance of the formed weld. This effect of improving the corrosion resistance of the welded joint is achieved when the content of at least one of Cu, Ti, Cr, Nb, Ni, and Mn in the Zn-Al-Mg plating layer 2 is 0.01% by mass or more. Therefore, when at least one of Cu, Ti, Cr, Nb, Ni, and Mn is included in the Zn-Al-Mg plating layer 2, it is preferable that the content of each of these elements be independently 0.01% by mass or more.

[0035] On the other hand, when forming a Zn-Al-Mg-based plating layer 2 in which the content of any of Cu, Ti, Cr, Nb, Ni, or Mn exceeds 1.00% by mass, it may not be possible to manufacture an automotive exterior panel 10 with good plating properties that is coated with a decorative film. Therefore, the content of Cu, Ti, Cr, Nb, Ni, and Mn in the Zn-Al-Mg-based plating layer 2 should each be independently 1.00% by mass or less. Preferably, the content of Cu, Ti, Cr, Nb, Ni, and Mn should each be independently 0.20% by mass or less.

[0036] [Mo: 0% by mass to 1.00% by mass] In this embodiment, the Zn-Al-Mg plating layer 2 may not contain Mo, so the lower limit of the Mo content is 0% by mass. On the other hand, when Mo is included in the Zn-Al-Mg plating layer 2, it is possible to further improve the corrosion resistance of the decorative film-coated automobile exterior panel 10. This improvement in corrosion resistance is achieved when the Mo content is 0.01% by mass or more. Therefore, when Mo is included, it is preferable that the Mo content be 0.01% by mass or more.

[0037] On the other hand, when the Zn-Al-Mg based plating layer 2 having a Mo content exceeding 1.00 mass% is formed, it causes a large amount of dross to be generated in the plating bath used, which is not preferable. Therefore, the content of Mo is 1.00 mass% or less. The content of Mo is preferably 0.05 mass% or less.

[0038] [Co: 0 mass% to 1.0000 mass%] [V: 0 mass% to 1.0000 mass%] Since the Zn-Al-Mg based plating layer 2 according to the present embodiment may not contain Co or V, the lower limit of the content of these elements is 0 mass%. On the other hand, when at least one of Co and V is contained in the Zn-Al-Mg based plating layer 2, these elements are incorporated into the Fe-Al based intermetallic compound generated during welding, which makes it possible to improve the corrosion resistance of the formed welded portion. Such an effect of improving the corrosion resistance of the welded portion is exhibited when the content of at least one of Co and V in the Zn-Al-Mg based plating layer 2 is 0.0050 mass% or more. Therefore, when at least one of Co and V is contained in the Zn-Al-Mg based plating layer 2, the content of each of these elements is preferably independently 0.0050 mass% or more.

[0039] On the other hand, when forming the Zn-Al-Mg based plating layer 2 in which any of the content of Co and V exceeds 1.0000 mass%, it may be impossible to produce a decorative film-coated automotive outer panel 10 having good plating properties. Therefore, the contents of Co and V in the Zn-Al-Mg based plating layer 2 are each independently 1.0000 mass% or less. The contents of Co and V are each independently preferably 0.2000 mass% or less.

[0040] Next, the element group D that can be contained in the Zn-Al-Mg based plating layer 2 will be described. The Zn-Al-Mg based plating layer 2 preferably contains one or more elements selected from the group (element group D) consisting of Sn: more than 0% by mass and 1.00% by mass or less, In: more than 0% by mass and 1.0000% by mass or less, and Bi: more than 0% by mass and 1.0000% by mass or less. The elements in the above-mentioned element group D are elements that can be contained in the Zn-Al-Mg based plating layer 2 in place of a part of the remaining Zn.

[0041] [Sn: 0% by mass to 1.00% by mass] Since the Zn-Al-Mg based plating layer 2 according to the present embodiment may not contain Sn in some cases, the lower limit of the Sn content is 0% by mass. On the other hand, when the Zn-Al-Mg based plating layer 2 containing Sn is placed in a corrosive environment, Sn is an element that increases the Mg elution rate. When the Mg elution rate increases, Mg ions are supplied to the exposed portion of the steel material 1, thereby improving corrosion resistance. From this viewpoint, when Sn is contained, the Sn content is preferably 0.01% by mass or more. On the other hand, excessive addition of Sn excessively promotes the Mg elution rate, which may reduce the corrosion resistance of the decorative film-coated automobile outer panel 10. Such an increase in the Mg elution rate becomes remarkable when the Sn content exceeds 1.00% by mass, so the Sn content is 1.00% by mass or less. The Sn content is preferably 0.20% by mass or less.

[0042] [In: 0% by mass to 1.0000% or less] [Bi: 0% by mass to 1.0000% or less] In and Bi may not be present in the Zn-Al-Mg plating layer 2 according to this embodiment, so the lower limit of the In and Bi content is 0% by mass. On the other hand, In and Bi are elements that increase the Mg elution rate when the Zn-Al-Mg plating layer 2 containing In and Bi is placed in a corrosive environment. When the Mg elution rate increases, Mg ions are supplied to the exposed parts of the steel material 1, improving corrosion resistance. From this viewpoint, when In and Bi are included, it is preferable that the In and Bi content be 0.0050% by mass or more, independently of each other. On the other hand, excessive addition of In and Bi may excessively accelerate the Mg elution rate, potentially reducing the corrosion resistance of the decorative film-coated automobile exterior panel 10. Since the increase in the Mg elution rate becomes significant when either the In or Bi content exceeds 1.0000% by mass, the In and Bi content are each independently 1.0000% by mass or less. Preferably, the In and Bi content are each independently 0.2000% by mass or less.

[0043] Next, the element group E that the Zn-Al-Mg plating layer 2 may contain will be described. Preferably, the Zn-Al-Mg plating layer 2 contains one or more elements selected from the group (element group E) consisting of Zr: greater than 0 mass% and 1.00 mass% or less, Ag: greater than 0 mass% and 1.00 mass% or less, and Li: greater than 0 mass% and 1.00 mass% or less. At least one of the elements of element group E shown below may be included in the Zn-Al-Mg plating layer 2 in place of a portion of the remaining Zn.

[0044] [Zr: 0% to 1.00% by mass] [Ag: 0% to 1.00% by mass] [Li: 0% to 1.00% by mass] In this embodiment, the Zn-Al-Mg plating layer 2 may not contain Zr, Ag, or Li, so the lower limit of the content of these elements is 0% by mass. On the other hand, if at least one of Zr, Ag, or Li is included in the Zn-Al-Mg plating layer 2, it is possible to improve the plating operability. This effect of improving plating operability is exhibited when the content of at least one of Zr, Ag, or Li in the Zn-Al-Mg plating layer 2 is 0.01% by mass or more. Therefore, when at least one of Zr, Ag, or Li is included, it is preferable that the content of each of these elements be independently 0.01% by mass or more.

[0045] On the other hand, when forming a Zn-Al-Mg plating layer 2 in which the content of any one of Zr, Ag, or Li exceeds 1.00% by mass, a large amount of dross tends to be generated in the plating bath used to form the Zn-Al-Mg plating layer 2. Therefore, the content of at least one of Zr, Ag, or Li is independently 1.00% by mass or less. Preferably, the content of at least one of Zr, Ag, or Li is independently 0.10% by mass or less.

[0046] Next, the group of elements F that the Zn-Al-Mg plating layer 2 may contain will be described. Preferably, the Zn-Al-Mg plating layer 2 contains one or more elements selected from the group (element group F) consisting of La: greater than 0% by mass and 0.50% by mass or less, Ce: greater than 0% by mass and 0.50% by mass or less, and Y: greater than 0% by mass and 0.50% by mass or less. At least one of the elements of the above-mentioned element group F is an element that can be contained in the Zn-Al-Mg plating layer 2 in place of a portion of the remaining Zn.

[0047] [La: 0% to 0.50% by mass] [Ce: 0% to 0.50% by mass] [Y: 0% to 0.50% by mass] In the Zn-Al-Mg plating layer 2 according to this embodiment, La, Ce, and Y may not be present, so the lower limit of the content of these elements is 0% by mass. When these elements are included in the Zn-Al-Mg plating layer 2, it is possible to suppress liquid metal embrittlement cracking (LME) during welding.

[0048] These effects are achieved by setting the content of each of these elements to 0.01% by mass or more, independently. Therefore, when at least one of La, Ce, and Y is included, it is preferable that the content of each of these elements be 0.01% by mass or more, independently. The content of La, Ce, and Y in the Zn-Al-Mg plating layer 2 is more preferably 0.05% by mass or more, independently.

[0049] On the other hand, if the La, Ce, and Y content in the plating bath for forming the Zn-Al-Mg plating layer 2 is too high, the viscosity of the plating bath may increase excessively, potentially reducing the plating operability. Therefore, from the viewpoint of plating operability, the La, Ce, and Y content in the plating bath is adjusted so that the content of La, Ce, and Y is independently 0.50% by mass or less. Preferably, the content of La, Ce, and Y is independently 0.10% by mass or less.

[0050] Next, we will explain the elements B that the Zn-Al-Mg plating layer 2 may contain. It is preferable that the Zn-Al-Mg plating layer 2 contains B: greater than 0% by mass and less than or equal to 0.50% by mass. The above-mentioned B is an element that may be contained in the Zn-Al-Mg plating layer 2 in place of a portion of the remaining Zn.

[0051] [B: 0% by mass to 0.50% by mass] In this embodiment, the Zn-Al-Mg plating layer 2 may not contain B, so the lower limit of its content is 0% by mass. On the other hand, when B is included in the Zn-Al-Mg plating layer 2, it has the effect of suppressing LME during welding. This improvement effect is achieved by including 0.05% by mass or more of B. Therefore, when B is included, the content of B is preferably 0.05% by mass or more.

[0052] On the other hand, if an excess of B is included in the plating bath in order to include B in the Zn-Al-Mg plating layer 2, it causes a rapid increase in the plating melting point, reducing the plating operability and making it impossible to manufacture an automotive exterior panel 10 with a decorative film coating that has excellent plating properties. This reduction in plating operability becomes significant when the B content exceeds 0.50% by mass, so the B content is 0.50% by mass or less. Preferably, the B content is 0.10% by mass or less.

[0053] [Fe: 0% to 5.00% by mass] Elements constituting the base material steel 1 may be mixed into the Zn-Al-Mg plating layer 2. In particular, in the hot-dip galvanizing method, elements constituting the steel 1 are easily mixed into the Zn-Al-Mg plating layer 2 due to interdiffusion of elements caused by solid-liquid reactions between the steel 1 and the Zn-Al-Mg plating layer 2. Due to such elemental mixing, a certain amount of Fe may be contained in the Zn-Al-Mg plating layer 2. Fe does not need to be contained, so the lower limit is 0% by mass. If Fe is contained in the Zn-Al-Mg plating layer 2, the Fe content may be 0.01% by mass or more. If the above interdiffusion is promoted, the adhesion between the steel 1 and the Zn-Al-Mg plating layer 2 will improve. From the viewpoint of improving the adhesion between the steel material 1 and the Zn-Al-Mg plating layer 2, it is preferable that the Fe content in the Zn-Al-Mg plating layer 2 be 0.20% by mass or more.

[0054] Furthermore, within the limits that do not impair the effects of this disclosure, Fe may be intentionally added to the plating bath used when manufacturing the Zn-Al-Mg plating layer 2. However, if the Fe content in the Zn-Al-Mg plating layer 2 exceeds 5.00% by mass, it may significantly degrade the appearance quality, which is undesirable. From this viewpoint, the Fe content in the plating bath is adjusted so that the Fe content in the Zn-Al-Mg plating layer 2 is 5.00% by mass or less. More preferably, the Fe content in the Zn-Al-Mg plating layer 2 is 4.00% by mass or less.

[0055] [Remainder: Zn and impurities] In the Zn-Al-Mg plating layer 2, the remainder consists of Zn and impurities. Zn is an important element for improving the corrosion resistance of the plated steel. The impurities are elements contained in the raw materials or elements introduced during the manufacturing process, and may be present in amounts that do not impair corrosion resistance.

[0056] [Method for measuring chemical composition] The chemical composition of the above Zn-Al-Mg plating layer 2 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometer) or ICP-MS (Inductively Coupled Plasma Mass Spectrometer). When analyzing chemical composition down to 0.1 mass%, ICP-AES should be used, and when analyzing trace amounts of chemical composition less than 0.1 mass%, ICP-MS should be used. A sample taken from the decorative film-coated automobile exterior panel 10 of interest is immersed in a 10% HCl aqueous solution with an inhibitor added for about 1 minute to peel and dissolve the Zn-Al-Mg plating layer 2, and a solution containing the dissolved Zn-Al-Mg plating layer 2 is prepared. The obtained solution is analyzed by ICP-AES or ICP-MS to obtain the chemical composition as the overall average of the plating layer.

[0057] Here, when taking a sample of the Zn-Al-Mg plating layer 2 from the decorative film-coated automobile exterior panel 10 of interest, the following procedure should be followed. First, the decorative film layer 4 is peeled off from the decorative film-coated automobile exterior panel 10. The method for peeling off the decorative film layer 4 is not specified, and any well-known method may be used. After that, the chemical conversion treatment layer 3 remaining on the steel material 1 is removed using a paint remover. Specifically, for example, the chemical conversion treatment layer 3 is removed from a sample cut from the automobile body and peeled off, in accordance with JIS K3151:1996. Specifically, the chemical conversion treatment layer 3 can be removed by immersing the sample in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes, and then rinsing with water and drying. In this way, a sample for analyzing the chemical components can be obtained. When measuring the chemical components of the Zn-Al-Mg plating layer 2, the sample should be taken from the ends of the steel material 1 and, if there are welded areas, the welded areas should be avoided.

[0058] There are no specific requirements for the amount of Zn-Al-Mg plating layer 2 to be applied, but for example, 15 to 250 g / m² per side of the steel member. ２ It is preferable that the amount of Zn-Al-Mg plating layer 2 adheres to within the above range, thereby enabling the decorative film-coated automobile exterior panel 10 according to this embodiment to exhibit sufficient corrosion resistance.

[0059] The amount of the Zn-Al-Mg plating layer 2 attached is measured as follows. First, a sample measuring 30 mm x 30 mm in plan view is cut from the decorative film-coated automobile exterior panel 10 of interest, in the same manner as described above. When cutting the sample, the entire thickness is cut out, and the chemical conversion treatment layer 3 and decorative film layer 4 are removed using a coating remover. The mass of the sample taken in this manner is measured in advance. A tape seal is applied to one side of the sample to prevent the plating layer on that side from dissolving in the next step. Then, the sample is immersed in a 10% HCl aqueous solution with an inhibitor added to pickle and remove the Zn-Al-Mg plating layer 2, and the mass of the sample after pickling is measured. From the change in mass of the sample before and after pickling, it is possible to determine the amount of the Zn-Al-Mg plating layer 2 attached to each side.

[0060] (Chemical treatment layer 3) The chemical treatment layer 3 is provided on at least one of the Zn-Al-Mg-based plating layers 2 provided on both surfaces of the steel material 1. By providing the chemical treatment layer 3 on the surface of the Zn-Al-Mg-based plating layer 2, the corrosion resistance of the decorative film-coated automobile exterior panel 10 can be improved, and the adhesion between the Zn-Al-Mg-based plating layer 2 and the decorative film layer 4 can be improved. The chemical treatment layer may be provided only on the surface to which the decorative film layer is attached. It may be provided on both sides. It can be selected as needed.

[0061] The chemical conversion treatment layer is preferably a film layer having at least one bond of Zr-O, Ti-O, or Si-O. Furthermore, the chemical conversion treatment layer 3 preferably contains a resin. In addition, in this embodiment, it is preferable that a portion of the resin in the chemical conversion treatment layer 3 is in contact with the decorative film layer 4 or the adhesive layer for attaching the decorative film layer at the boundary with the decorative film layer 4. The components of the chemical conversion treatment layer 3 will be described below.

[0062] The Zr in the chemical conversion treatment layer 3 forms a cross-linked structure (Zr-O bond) with the resin in the chemical conversion treatment layer 3. As a result, the chemical conversion treatment layer 3 has excellent barrier properties. Furthermore, it is presumed that the Zr in the chemical conversion treatment layer 3 forms a Zr-O-M bond (M: metal element in the plating layer) with the surface of the Zn-Al-Mg plating layer 2. As a result, the chemical conversion treatment layer 3 has excellent adhesion to the Zn-Al-Mg plating layer 2. The chemical conversion treatment layer 3 having a Zr-O bond can be obtained by applying an aqueous solution containing Zr (aqueous surface treatment agent) and drying it. Examples of Zr sources to be added to the aqueous solution (aqueous surface treatment agent) include zirconium carbonate compounds such as zirconium carbonate, ammonium zirconium carbonate, potassium zirconium carbonate, and sodium zirconium carbonate, and one or more of these can be used. Applying and drying an aqueous surface treatment agent containing a zirconium carbonate compound is preferable because it crosslinks with the resin in the chemical conversion treatment layer 3, forming a chemical conversion treatment layer 3 having a crosslinked structure between zirconium and the resin. Furthermore, when the aqueous surface treatment agent containing the zirconium carbonate is applied and dried, the carbonate ions volatilize, and the remaining zirconium particles bond with each other via oxygen, resulting in a higher molecular weight. In this process, the -Zr-OH group forms a Zr-O-M bond (M: metal element in the plating layer) with the surface of the Zn-Al-Mg-based plating layer 2, making it even more preferable. Among these, at least one of zirconium carbonate and zirconium ammonium carbonate is more preferable because it readily forms a crosslinked structure and has excellent corrosion resistance.

[0063] The presence of Si-O bonds in the chemical conversion treatment layer 3 forms a chemical bond called an Si-O-Me bond between it and the elements constituting the Zn-Al-Mg plating layer 2. Here, Me represents the main component metal element of the Zn-Al-Mg plating layer 2 (in other words, a metal element whose content is 50% by mass or more of the total mass of the Zn-Al-Mg plating layer 2). In this embodiment, the formation of such primary bonds in the Zn-Al-Mg plating makes the bonding state between the Zn-Al-Mg plating layer 2 and the chemical conversion treatment layer 3 stronger. As a result, the adhesion between the Zn-Al-Mg plating layer 2 and the chemical conversion treatment layer 3 is further improved. Consequently, a state is achieved in which water is less likely to penetrate the interface between the Zn-Al-Mg plating layer 2 and the chemical conversion treatment layer 3 from the outside. This improves the corrosion resistance of the decorative film-coated automobile exterior panel 10 according to this embodiment. A chemical conversion treatment layer 3 having Si-O bonds can be obtained by applying an aqueous solution containing a silane compound (aqueous surface treatment agent) and drying it. A silane coupling agent is preferable as the silane compound. A silane coupling agent is preferable because, during the drying (baking) process of the coating film formed by applying the aqueous surface treatment agent containing it, it undergoes hydrolysis to become silanol, forming a siloxane-type chemical conversion treatment layer 3 that is three-dimensionally crosslinked by siloxane bonds, thus exhibiting excellent corrosion resistance and adhesion. As the silane coupling agent, it is preferable to use an alkoxysilane having two or more, preferably three or more alkoxy groups. A partially hydrolyzed product of the above alkoxysilane may also be used as the silane coupling agent. The alkoxy groups of the silane coupling agent undergo hydrolysis in the aqueous surface treatment agent to become silanol (-Si-OH). If the pH of the aqueous surface treatment agent is 6.5 or lower, the dispersion stability of silanol in the aqueous surface treatment agent is good.

[0064] Examples of silane coupling agents include N-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 2-(3,4 epoxycyclohexyl)ethyltrimethoxysilane, vinyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, and 3-methacryloxypropyltrimethoxysilane.

[0065] The presence of Ti-O bonds in the chemical conversion treatment layer 3 forms a Ti-O-Me chemical bond between it and the elements constituting the Zn-Al-Mg plating layer 2. The formation of such a primary bond strengthens the bond between the Zn-Al-Mg plating layer 2 and the chemical conversion treatment layer 3, further improving adhesion. As a result, water is less likely to penetrate the interface between the Zn-Al-Mg plating layer 2 and the chemical conversion treatment layer 3 from the outside. This improves corrosion resistance in the decorative film-coated automobile exterior panel 10 according to this embodiment. The chemical conversion treatment layer 3 having Ti-O bonds can be obtained by applying an aqueous solution containing Ti (aqueous surface treatment agent) and drying it. Examples of Ti sources to be added to the aqueous solution include titanium fluoride compounds such as titanium fluoride, titanium calcium fluoride, titanium ammonium fluoride, and titanium strontium fluoride, and one or more of these can be used.

[0066] (Resin) The chemical treatment layer 3 preferably contains a resin. The inclusion of a resin in the chemical treatment layer 3 improves the adhesion between the decorative film layer 4 and the chemical treatment layer 3. The resin may be either a water-dispersible aqueous resin that disperses in water or a solvent-based resin that disperses in an organic solvent, but an aqueous emulsion resin is preferred from the viewpoint of ease of mixing with chemicals, manufacturing cost, and environmental suitability. There is no particular requirement for the type of resin in the chemical treatment layer 3, but it is more preferable that the resin in the chemical treatment layer 3 is at least one of acrylic resin, urethane resin, polyester resin, and epoxy resin.

[0067] The acrylic resin used in this embodiment is preferably a resin containing a polymer of alkyl (meth)acrylate, and may be a polymer obtained by polymerizing only alkyl (meth)acrylate, or a copolymer obtained by polymerizing alkyl (meth)acrylate and other monomers. In this specification, "(meth)acrylic" means "acrylic" or "methacrylic". The acrylic resin contributes to improving the adhesion between the chemical conversion treatment layer 3 provided on the Zn-Al-Mg plating layer and the decorative film layer 4, and also contributes to improving the corrosion resistance of the decorative film-coated automobile exterior panel 10. In particular, if the decorative film or this adhesive layer contains acrylic resin, it exhibits a greater adhesion improvement effect.

[0068] As the acrylic resin, it is preferable to use a copolymer of (meth)acrylate alkyl ester and other monomers. As the copolymer, it is preferable to use a copolymer of styrene (b1), (meth)acrylic acid (b2), (meth)acrylate alkyl ester (b3), and acrylonitrile (b4). In particular, it is preferable to use a copolymer of 15 to 25% by mass of styrene (b1), 1 to 10% by mass of (meth)acrylic acid (b2), 40 to 58% by mass of (meth)acrylate alkyl ester (b3), and 20 to 38% by mass of acrylonitrile (b4) as the acrylic resin. By using such a copolymer as the acrylic resin, the adhesion between the Zn-Al-Mg plating layer 2 and the decorative film layer 4 can be further improved, and a corrosion-resistant chemical conversion treatment layer 3 can be obtained.

[0069] Furthermore, by including urethane resin, epoxy resin, and ester resin in the chemical treatment layer 3, the adhesion at the interface between the chemical treatment layer 3 and the decorative film layer 4 is improved. As a result, water is less likely to penetrate the interface between the chemical treatment layer 3 and the decorative film layer 4 from the outside. This improves the corrosion resistance between the chemical treatment layer 3 and the decorative film layer 4 in the decorative film-coated automobile exterior panel 10. The chemical treatment layer 3 containing urethane resin, ester resin, and epoxy resin is more effective against films containing urethane resin, ester resin, or epoxy resin in the decorative film layer 4 or its bonding layer (adhesive layer) 41. Hereinafter, the adhesive layer will be referred to as the bonding layer.

[0070] These resins are preferable if they are aqueous resins, as they mix easily with chemicals for forming Si-O bonds, Zr-O bonds, and Ti-O bonds (for example, the aqueous surface treatment agents mentioned above). Furthermore, aqueous emulsion resins are even more preferable because they tend to exist as particles in the chemical treatment layer 3. Commercially available aqueous emulsion resins can be used. In the case of acrylic, Arakawa Chemical Industries' acrylic aqueous polymer "Tamanori" G-36, DIC's "Boncoat series," and Dainippon Paint Co., Ltd.'s aqueous acrylic resin "Hydran HW-171" can be used. In the case of urethane, Arakawa Chemical Industries' polyurethane "Uriano U series," DIC's "Hydran series," and Daiichi Kogyo Seiyaku's polyurethane aqueous dispersion "Superflex" can be used. In the case of polyester, Arakawa Chemical Industries' polyester resins "Arapole series" and "Arakid" series, and Toyobo MC's Byronal® series can be used.

[0071] The resin content in the chemical conversion treatment layer 3 is preferably 25% to 45% by mass of the total mass of the chemical conversion treatment layer 3. If the resin content in the chemical conversion treatment layer 3 is 25% by mass or more, a sufficient contact area between the decorative film layer 4 (described later) and the resin can be provided, and the adhesion between the decorative film layer 4 and the chemical conversion treatment layer 3 can be maintained at a high level. Furthermore, if the resin content in the chemical conversion treatment layer 3 is 45% by mass or less, the chemical conversion treatment layer 3 contains sufficient Zr-O bonds, Ti-O bonds, or Si-O bonds, and the adhesion between the chemical conversion treatment layer 3 and the Zn-Al-Mg plating layer 2 can be maintained at a high level.

[0072] The resin contained in the chemical conversion treatment layer 3 can be dispersed in various shapes within the chemical conversion treatment layer 3. The shape is not particularly limited, but it is preferable that it is dispersed in a granular form. The granular dispersion of the resin in the chemical conversion treatment layer 3, and the presence of at least one of Zr-O bonds, Ti-O bonds, or Si-O bonds in the chemical conversion treatment layer 3, can improve the adhesion between the Zn-Al-Mg plating layer 2 and the decorative film layer 4.

[0073] Whether or not the resin is dispersed in granular form can be determined by observing a cross-section of the decorative film-coated automobile exterior panel 10 with a transmission electron microscope (TEM). For convenience of explanation, the method for observing granular resin (resin particles) by cross-sectional observation and the method for measuring particle size will be described later. In this embodiment, the resin is defined as granular if the average equivalent circle diameter of the resin particles in the cross-sectional shape observation is 200 nm or less and the aspect ratio is 4 or less. In this embodiment, for granular resin particles, the particle size is preferably 20 to 200 nm. The aspect ratio is the ratio of the major axis to the minor axis of the object to be evaluated (resin particles). The equivalent circle diameter refers to the diameter of a circle having an area equal to the area of ​​the object to be evaluated.

[0074] The resin contained in the chemical conversion treatment layer 3 is not limited to a granular shape. The resin may be cloud-like, marble-like, or layered. During the film formation process, the dispersed resin particles fuse together, resulting in the resin taking on these shapes within the chemical conversion treatment layer 3. In this embodiment, it is preferable that the area envelope (particle cross-sectional area / area within the envelope) of the resin dispersed in the chemical conversion treatment layer 3 is 0.5 to 1.0. Examples of shapes with an area envelope of 0.5 to 1.0 include granular and layered aggregates (aggregates are aggregates of resin particles), cloud-like aggregates, and marble-like aggregates. Here, area envelope (solidity) is a particle contour parameter used for image analysis by simplifying complex particle shapes. For example, let Sb be the area of ​​the region enclosed by the virtual envelope line Lb (dashed line in Figure 2) surrounding the two-dimensional contour of the aggregate as shown in Figure 2, and let Sa be the area of ​​the region enclosed by the actual contour line La (solid line in Figure 2) of the two-dimensional contour of the aggregate. In this case, the area envelope degree is expressed as area Sa / area Sb. The area envelope degree approaches 1 as the surface of the particles is smoother with fewer irregularities, and decreases as the object being evaluated (aggregates, etc.) has more irregularities. A lower area envelope degree makes it easier to form interfacial pathways connected to the interface with the inorganic layer 32, which can become diffusion paths for moisture and oxygen, thus leading to localized variations in corrosion resistance. Furthermore, a lower area envelope degree indicates that the resin is in a state of aggregation, resulting in unevenness in resin density within the chemical conversion treatment layer 3, and a loss of uniformity in the chemical conversion treatment layer 3, thus leading to localized variations in performance such as corrosion resistance.

[0075] The method for calculating the area envelope in this embodiment is as follows.

[0076] A carbon film is deposited as a protective film on the surface of the decorative film-coated automobile exterior panel 10, and then a carbon film of several micrometers is formed using a FIB (Focused Ion Beam Processing apparatus). After that, microsampling is performed using the FIB at an acceleration voltage of 30 kV (finishing process: 5 kV), and this is thinned to obtain a cross-sectional sample of the coating (chemical conversion treatment layer). The cross-sectional sample of the coating (chemical conversion treatment layer) is observed (1 μm × 1 μm) using a TEM with an EDS (Energy Dispersive X-ray Spectrometer), and EDS analysis (elemental mapping) is performed at three locations to obtain a detected element map. The obtained detected element map is divided into 100 squares (10 × 10), and the region where C and O components overlap and the other elements are binarized using the contrast of the image. In the binarized image, the region where C and O components overlap is determined to be the resin region. For regions identified as resin, the area Sa / area Sb is calculated using image analysis software such as ImageJ to determine the area envelope of that region. Similarly, the area envelope of each resin is calculated for five observed fields, and the average value is taken as the area envelope. When calculating the area envelope, regions with an area of ​​5 square nm or more are selected. That is, regions where both area Sa and area Sb are 5 square nm or more are selected, and the area envelope is calculated.

[0077] Examples of water-based resins include water-dispersible resins (emulsions) such as acrylic resins, urethane resins, epoxy resins, ester resins, and mixed resins of two or more of these resins. The emulsion is preferably one with an average particle size of 20 to 200 nm (preferably 20 to 60 nm). If the emulsion particle size is excessively small, when it is formed as a chemical conversion treatment layer, the particle size and aggregate structure of the particles and aggregates contained in the chemical conversion treatment layer will be small, which may reduce adhesion to the chemical conversion treatment layer 3. It may also increase costs. On the other hand, if the emulsion particle size is excessively large, the gaps between the emulsions will be large when it is formed as a film, which may reduce the barrier properties of the chemical conversion treatment layer 3.

[0078] Here, the resin in the chemical conversion treatment layer 3 may be a crosslinked resin having a crosslinked structure, or a non-crosslinked resin not having a crosslinked structure. Preferred crosslinking agents (curing agents) for imparting a crosslinked structure to the resin include melamine, isocyanates, silane compounds, zirconium compounds, and titanium compounds. The amount of crosslinking agent added is preferably 5 to 30 parts by mass per 100 parts by mass of resin solids. If the amount of crosslinking agent added is less than 5 parts by mass, the crosslinking reaction with the resin decreases, and the performance of the coating film may become insufficient. On the other hand, if the amount of crosslinking agent added is more than 30 parts by mass, the crosslinking reaction proceeds too much, causing the chemical conversion treatment layer 3 to become excessively hard and reducing its processability. Furthermore, when silane compounds, zirconium compounds, or titanium compounds are used as crosslinking agents, if the amount of crosslinking agent added exceeds 30 parts by mass, the paint stability may further decrease, which is undesirable.

[0079] Here, whether or not the chemically treated layer 3 according to this embodiment contains a resin, and moreover, whether or not it contains at least one of urethane resin, epoxy resin, ester resin, or acrylic resin, can be determined by the following method. Similarly, whether or not it contains at least one of Zr-O bond, Ti-O bond, or Si-O bond can also be determined by the following method.

[0080] First, the portion of the decorative film-coated automobile exterior panel 10 where the chemical conversion treatment layer 3 is located is cut by oblique cutting to expose its cross-section, and this cross-section is further polished to obtain a cross-sectional sample in the thickness direction of the chemical conversion treatment layer 3. Next, the portion of the cross-sectional sample containing the chemical conversion treatment layer 3 is analyzed using a micro-Fourier transform infrared (FT-IR) spectrometer, and a determination is made based on whether or not vibrational peaks originating from urethane groups, epoxy groups, ester groups, Zr-O bonds, Ti-O bonds, and Si-O bonds are observed in the obtained infrared absorption spectrum of the chemical conversion treatment layer 3. Specifically, in the obtained infrared absorption spectrum, at 910 cm⁻¹ －１ If a peak is observed in the vicinity, it is determined to contain epoxy resin, and the measurement is 1550 cm. －１ Nearby and 1740 cm －１ If a peak is observed in the vicinity, it is determined to contain urethane resin, and the measurement range is 1720-1740 cm. －１When a peak is observed in the vicinity, it is determined that an ester resin is contained, 2950 cm －１ -1 vicinity, and 1725 cm －１ -1 vicinity, 1435 cm －１ -1 vicinity, 1145 cm －１ -1 vicinity, when a peak is observed, it is determined that an acrylic resin is contained. 1000 to 1100 cm －１ -1 vicinity, when a peak is observed, it is determined that a Si-O bond is contained, 450 to 700 cm －１ -1 vicinity, when a peak is observed, it is determined that a Zr-O bond is contained, 400 to 900 cm －１ -1 vicinity, when a peak is observed, it is determined that a Ti-O bond is contained. Regarding IR spectroscopic analysis of Si-O bonds, Zr-O bonds and Ti-O bonds, elemental analysis may be performed by an analysis method combined with other than a microscopic Fourier transform infrared (FT-IR) spectrometer, for example, an electron probe microanalyzer (EPMA), a scanning electron microscope, a transmission electron microscope, a scanning transmission electron microscope combined with energy dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS). When Si and O are detected from the same location, it can be determined that there is a Si-O bond; when Ti and O are detected from the same location, it can be determined that there is a Ti-O bond; and when Zr and O are detected from the same location, it can be determined that there is a Zr-O bond.

[0081] The chemical conversion treatment layer 3 preferably contains at least one or more selected from phosphorus, vanadium, cobalt, titanium and chromium.

[0082] Vanadium in the chemical conversion treatment layer 3 preferentially dissolves into the Zn-Al-Mg plating layer 2 under corrosive conditions, suppressing the pH increase due to the dissolution of the Zn-Al-Mg plating layer 2 and improving the corrosion resistance of the decorative film-coated automobile exterior panel 10. Vanadium can be incorporated into the chemical conversion treatment layer 3 by adding a vanadium compound to the coating solution (e.g., an aqueous surface treatment agent) used to form the chemical conversion treatment layer 3. Examples of vanadium compounds include vanadium pentoxide, metavanadic acid and its salts (e.g., ammonium metavanadate), vanadium trioxide, vanadium dioxide, vanadium oxyacetylacetonate, vanadium acetylacetonate, and vanadium acetate. Considering the effect of improving corrosion resistance, it is particularly preferable to use vanadium acetylacetone complexes such as vanadium acetylacetonate and vanadium oxyacetylacetonate among the above vanadium compounds. The phosphorus in the chemical conversion treatment layer 3 forms a passivation film consisting of a sparingly soluble metal salt, such as zinc phosphate, on the surface of the Zn-Al-Mg plating layer 2, improving the corrosion resistance of the decorative film-coated automobile exterior panel 10. The sparingly soluble metal salt is produced by the reaction of phosphorus with metal ions generated when a portion of the Zn-Al-Mg plating layer 2 dissolves. The sparingly soluble metal salt is formed when a portion of the Zn-Al-Mg plating layer 2 dissolves due to the application of a phosphorus-containing aqueous treatment agent used to form the chemical conversion treatment layer 3 to the Zn-Al-Mg plating layer 2, and / or when the Zn-Al-Mg plating layer 2 is subjected to a corrosive environment after the formation of the chemical conversion treatment layer 3. Phosphorus can be incorporated into the chemical conversion treatment layer 3 by adding a phosphorus component to the coating solution (e.g., an aqueous surface treatment agent) used to form the chemical conversion treatment layer 3. Examples of phosphate components include inorganic phosphate compounds such as phosphate, ammonium phosphate, potassium phosphate, sodium phosphate, and monosodium dihydrogen phosphate.

[0083] The cobalt in the chemical conversion treatment layer 3 improves the resistance to blackening and corrosion of the chemical conversion treated steel sheet 7. In this embodiment, the aluminum and magnesium in the Zn-Al-Mg plating layer 2 exhibit a sacrificial corrosion protection effect in a corrosive environment. Therefore, blackening may occur when the zinc in the Zn-Al-Mg plating layer 2 oxidizes in an oxygen-deficient state. Blackening is more likely to occur in easily soluble parts of the Zn-Al-Mg plating layer 2. The cobalt in the chemical conversion treatment layer 3 reduces the oxidation (corrosion) rate of zinc in the Zn-Al-Mg plating layer 2, thereby preventing blackening. Cobalt can be incorporated into the chemical conversion treatment layer 3 by adding a cobalt compound to the coating solution (e.g., an aqueous surface treatment agent) used to form the chemical conversion treatment layer 3. Examples of cobalt compounds include cobalt chloride, cobalt nitrate, cobalt sulfate, cobalt carbonate, and cobalt acetate.

[0084] The titanium in the chemical conversion treatment layer 3 is deposited on the surface of the Zn-Al-Mg plating layer 2, exhibiting a barrier effect and improving the corrosion resistance of the decorative film-coated automobile exterior panel 10. In particular, titanium organic complexes, such as titanium diisopropoxybisacetylacetonate and titanium tetrakisacetylacetonate, are effective in improving corrosion resistance. Titanium can be incorporated into the chemical conversion treatment layer 3 by adding a titanium organic complex to the coating solution (for example, an aqueous surface treatment agent) used to form the chemical conversion treatment layer 3.

[0085] The chromium in the chemical conversion treatment layer 3 is deposited on the surface of the Zn-Al-Mg plating layer 2, exhibiting a barrier effect and improving the corrosion resistance of the decorative film-coated automobile exterior panel 10. Chromium can be incorporated into the chemical conversion treatment layer 3 by adding a chromium compound to the coating solution used to form the chemical conversion treatment layer 3. Examples of chromium compounds include hexavalent chromium compounds and trivalent chromium compounds, but trivalent chromium compounds are preferred because hexavalent chromium compounds are hazardous substances. Examples of trivalent chromium compounds include chromium acetate, chromium hydroxide, chromium sulfate, basic chromium sulfate, potassium chromium sulfate, and chromium nitrate.

[0086] The average thickness of the chemical conversion treatment layer 3 is not particularly limited, but is preferably, for example, 0.1 μm or more and 2.0 μm or less. More preferably, the average thickness of the chemical conversion treatment layer 3 is 0.3 μm or more and 1.5 μm or less. An average thickness of 0.1 μm or more of the chemical conversion treatment layer 3 makes it easier to ensure bonding strength between the Zn-Al-Mg-based plating layer 2 and the decorative film layer 4 via the chemical conversion treatment layer 3. On the other hand, an average thickness of 2.0 μm or less of the chemical conversion treatment layer 3 suppresses cohesive failure of the chemical conversion treatment layer 3 and improves adhesion between the chemical conversion treatment layer 3 and the decorative film layer 4.

[0087] (Decorative film layer 4) The decorative film layer 4 is provided on at least one side of the steel material 1 and on the surface of the chemical conversion treatment layer 3. The decorative film layer 4 is a film having an adhesive layer on one side. The decorative film layer 4 improves the design of the decorative film-coated automobile exterior panel 10. A known decorative film can be used for the decorative film layer 4. Figure 3 is an example of a schematic cross-sectional view of the decorative film layer 4. In the illustrated example, the decorative film layer 4 comprises an adhesive layer 41 and a resin layer 42 in that order. The adhesive layer 41 may be provided on the surface of the chemical conversion treatment layer 3 by itself, and then a decorative film without an adhesive layer 41 may be attached. If the resin layer 42 has adhesive properties, the adhesive layer 41 may be omitted.

[0088] The bonding layer 41 comes into contact with the chemical treatment layer 3 of the automobile exterior panel 8 when the decorative film layer 4 is bonded to the automobile exterior panel 8 by the bonding layer 41.

[0089] The bonding layer 41 preferably contains an adhesive or tack. The adhesive contained in the bonding layer 41 may contain one or more of the following: acrylic adhesive, epoxy adhesive, polyester adhesive, rubber adhesive, urethane adhesive, and silicone adhesive. It is preferable that the adhesive contained in the bonding layer 41 contains epoxy adhesive, polyester adhesive, or urethane adhesive. This improves the bonding between the binder resin contained in the chemical treatment layer 3 and the bonding layer 41.

[0090] The adhesive contained in the bonding layer 41 may contain one or more of the following: acrylic adhesive, epoxy adhesive, polyester adhesive, rubber adhesive, urethane adhesive, and silicone adhesive. Preferably, the adhesive contained in the bonding layer 41 contains epoxy adhesive, polyester adhesive, or urethane adhesive. This can improve the bonding between the binder resin used in the chemical treatment layer 3 and the bonding layer 41.

[0091] The bonding layer 41 is preferably in the form of a sheet. The thickness of the bonding layer 41 may be, for example, 10 μm or more, 20 μm or more, or 30 μm or more. The thickness of the bonding layer 41 may be 100 μm or less, 80 μm or less, or 60 μm or less.

[0092] The resin layer 42 is provided on the bonding layer 41. The resin layer 42 is preferably in the form of a sheet. The decorative film layer 4 may further include a design layer (not shown) on the resin layer 42, and a protective layer covering the design layer.

[0093] The resin constituting the resin layer 42 is not particularly limited. Examples of resins used in the resin layer 42 include vinyl resins such as polyvinyl chloride (PVC), polyvinyl alcohol (PVA), and ethylene vinyl acetate copolymer (EVA); acrylic resins such as poly(meth)methyl acrylate (PMMA); ionomer resins; polypropylene (PP), polyethylene (PE), polyacetal (POM), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyamide (PA), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polystyrene (PS), thermoplastic polyurethane (TPU), acrylonitrile butadiene styrene copolymer (ABS), and polycarbonate.

[0094] As the material for the resin layer 42, one type of resin may be used alone from the above-mentioned resins, or two or more types of resins may be used. When two or more types of resins are used as the material for the resin layer 42, the resin layer 42 may be a single layer of a mixture of the two or more resins, or it may be composed of multiple layers.

[0095] The resin layer 42 may contain a coloring agent. By using a coloring agent, the resin layer 42 is colored to a desired color. The decorative film-coated automobile exterior panel 10 containing the colored resin layer 42 has excellent design. Known coloring agents can be used. The resin layer 42 may contain one or more known pigments and dyes.

[0096] The thickness of the resin layer 42 is not particularly limited and may be 50 μm or more, 70 μm or more, or 100 μm or more. The thickness of the resin layer 42 may be 500 μm or less, 400 μm or less, 300 μm or less, or 200 μm or less.

[0097] The design layer is provided, for example, on the resin layer 42. The design layer may also be a pattern layer that displays a design. The pattern layer may display figures, patterns, pictures, photographs, marks, letters, numbers, etc. The design layer is not particularly limited as long as it can display a design. For example, the design layer may be formed by a known printing method.

[0098] The protective layer may be provided on the surface of the design layer. The protective layer provides resistance to discoloration, deterioration, etc., of the design layer and resin layer 42 due to outdoor use. The protective layer may be formed from multiple layers, each with its own function.

[0099] (Interface between chemical treatment layer 3 and decorative film layer 4) Figure 2 is a schematic diagram showing an example of the vicinity of the interface between the chemical treatment layer 3 and the decorative film layer 4. In this example, the chemical treatment layer 3 has a resin 31 and an inorganic phase 32. The chemical treatment layer 3 is in contact with the bonding layer 41 of the decorative film layer 4, which will be described later. The resin 31 is the resin of the chemical treatment layer 3 described above. In the example of Figure 2, the resins 31 are fused together and have a marbled appearance. The inorganic phase 32 is a phase consisting of an inorganic compound containing at least one of a Zr-O bond, a Ti-O bond, or a Si-O bond.

[0100] In a cross-section perpendicular to the surface of the Zn-Al-Mg plating layer 2 (cross-section in the thickness direction), at the boundary between the chemical conversion treatment layer 3 and the decorative film layer 4, in the example shown in Figure 2, at least a portion of the resin 31 is in contact with the bonding layer 41 of the decorative film layer 4. The total length L of the region where this resin 31 is in contact with the decorative film layer 4 is defined as the resin contact region length Lr, and the interface length Lc between the chemical conversion treatment layer 3 and the decorative film layer 4 is defined as Lc. The method for measuring Lr is to observe the interface between the chemical conversion treatment layer and the decorative film layer or the adhesive layer using a transmission electron microscope, and in the range of Lc = 600 nm in three arbitrary fields of view obtained from the electron microscope image, the relationship between the total length of resin present on the outermost surface of the chemical conversion treatment layer 3 at the interface (resin contact region length) Lr is defined as Lr / Lc, and the average of the Lr / Lc measured in each of the three arbitrary fields is defined as the average Lr / Lc. The conditions for measuring Lr and Lc were as follows: using a JEOL JEM-2100F field emission transmission electron microscope, acceleration voltage: 200 kV, magnification: 150,000x. Image analysis was performed using image analysis software such as ImageJ.

[0101] When the resin is dispersed in granular form, it is preferable that the average Lr / Lc is in the range of 0.1 or higher. By having an Lr / Lc in the range of 0.1 or higher, it is possible to maintain high adhesion between the chemical conversion treatment layer 3 and the Zn-Al-Mg-based plating layer 2, while also maintaining sufficient adhesion between the chemical conversion treatment layer 3 and the decorative film layer 4.

[0102] Even when the resin is dispersed in a marbled or layered manner, it is preferable that the Lr / Lc ratio be in the range of 0.1 or greater. This allows the chemical conversion treatment layer 3 and the decorative film layer 4 to be integrated over a wide area, thereby achieving high adhesion. If the resin is layered and the boundary between the chemical conversion treatment layer 3 and the decorative film layer 4 is not clear under electron microscopy, it can be determined by analyzing the elements contained in the decorative film layer 4, the adhesive layer, the chemical conversion treatment layer 3, and the resin particles contained therein. For example, a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM) can be used in combination with energy-dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS) to analyze the observed cross-section and analyze the atomic ratios of C, O, N, S, etc., contained in each resin. Based on the results obtained, the boundary can be determined from the atomic ratios. Alternatively, if there are regions where a specific element is detected and regions where it is not detected, the boundary between them can be determined as the interface. Elemental analysis can be performed by point analysis of each area to be measured based on transmission electron microscope images or scanning transmission electron microscope images, or by line analysis from the decorative film layer 4 and the adhesive to the chemical conversion treatment layer 3. Alternatively, a method (elemental mapping) can be used to visualize the distribution of elements in a specified measurement area by image processing the intensity distribution of characteristic X-rays or the concentration distribution of elements of a specified element in a specified area and displaying it two-dimensionally. Furthermore, if the resin in the chemical conversion treatment layer 3 is the same type as the resin in the decorative film layer 4 or the bonding layer, the decorative film layer 4 or the bonding layer will contain some additive other than resin, so the elements contained in this additive other than resin can be used as markers to distinguish it from the resin contained in the chemical conversion treatment layer 3.

[0103] The decorative film-coated automobile exterior panel 10 according to this embodiment has been described above. The decorative film-coated automobile exterior panel 10 according to this embodiment comprises a steel material 1, a Zn-Al-Mg-based plating layer 2 provided on both surfaces of the steel material 1, a chemical conversion treatment layer 3 provided on the surface of the Zn-Al-Mg-based plating layer 2, and a decorative film layer 4 provided on at least one surface of the chemical conversion treatment layer 3. Since the Zn-Al-Mg-based plating layer 2 has the above-described chemical composition, it has excellent corrosion resistance. Because the decorative film-coated automobile exterior panel 10 of this disclosure has excellent corrosion resistance, it can be suitably used as an automobile component. Examples of automobile components include automobile doors, roofs, fenders, hoods, various pillars, side sills, and bumpers. In the case of various pillars, it is one of A-pillar, B-pillar, C-pillar, and D-pillar.

[0104] Furthermore, the decorative film-coated automobile exterior panel 10 only needs to have the decorative film layer 4 on at least one surface. For example, the decorative film layer 4 may be provided so as to cover only one of the two surfaces, and the other surface may not have the decorative film layer 4. In this case, the other surface without the decorative film layer 4 may have only the chemical conversion layer 3, or the surface of the chemical conversion layer 3 may be directly covered with a resin film such as paint. Directly covering the chemical conversion layer 3 with a resin film further improves the corrosion resistance of the decorative film-coated automobile exterior panel 10.

[0105] Furthermore, it is preferable that at least a portion of the end face of the steel material 1 is covered with the decorative film layer 4. It is even more preferable that the entire end face of the steel material 1 is covered with the decorative film layer 4. By covering at least a portion of the end face of the steel material 1 with the decorative film layer 4, corrosion resistance at the end face can be maintained. This further improves the corrosion resistance of the decorative film-coated automobile exterior panel 10. To determine whether the end face of the steel material 1 is covered with the decorative film layer 4, for example, the end face of the decorative film-coated automobile exterior panel 10 obtained by disassembling the automobile body can be observed under a microscope.

[0106] Next, a method for manufacturing the decorative film-coated automobile exterior panel 10 will be described. <First Embodiment> Figure 4 is a flowchart showing a method for manufacturing the decorative film-coated automobile exterior panel 10. In the first embodiment, the method for manufacturing the decorative film-coated automobile exterior panel 10 involves pressing a chemically treated steel sheet 7 to form a part, and then attaching a decorative film to the surface of the chemically treated layer. Specifically, the method for manufacturing the decorative film-coated automobile exterior panel according to the first embodiment includes a plating layer formation step S1 in which a Zn-Al-Mg-based plating layer 2 is formed on a steel sheet 1A, a chemically treated layer formation step S2 in which a chemically treated layer 3 is formed on the surface of the Zn-Al-Mg-based plating layer 2 to obtain the chemically treated steel sheet 7 shown in Figure 6, a part forming step S3 in which the chemically treated steel sheet 7 is pressed to form a part, and an attachment step S4 in which a decorative film layer 4 is attached to the surface of the chemically treated layer 3 of the automobile exterior panel 8 after part forming.

[0107] (Plating layer formation process S1) In plating layer formation process S1, a Zn-Al-Mg-based plating layer 2 having the above-described chemical composition is formed on both sides of the steel sheet 1A. The method for forming the Zn-Al-Mg-based plating layer 2 is not particularly limited as long as a Zn-Al-Mg-based plating layer 2 having the above-described chemical composition can be formed, and methods such as hot-dip galvanizing, thermal spraying, cold spraying, sputtering, vapor deposition, and electroplating can be applied. However, for forming a plating layer of a thickness commonly used in automobiles and the like, the hot-dip galvanizing method is the most preferable in terms of cost. In the case of hot-dip galvanizing, it is preferable to immerse the steel sheet 1A in a molten plating solution made by molten an ingot having the same composition as the Zn-Al-Mg-based plating layer 2, withdraw it, control the amount of plating adhesion by a gas wiping method in which air or nitrogen gas is blown on it during withdrawal, and then cool and plate it. It is preferable to manufacture using a continuous hot-dip galvanizing line called the generally known Zenzimir method, as this increases productivity. Steel plate 1A is not particularly limited as long as it is a steel plate for automotive use, but it is preferable to use an extremely low carbon steel with excellent deep drawability, generally known as IF steel, because it has excellent formability. Furthermore, using high-strength steel with a tensile strength of 590 MPa or higher is preferable because it enhances the collision safety of the automobile.

[0108] (Chemical Conversion Layer Formation Step S2) Next, in the chemical conversion layer formation step S2, a chemical conversion layer 3 is formed on the surface of the Zn-Al-Mg plating layer 2 to obtain a chemical conversion treated steel sheet 7. The chemical conversion layer 3 is formed by coating the surface of the Zn-Al-Mg plating layer 2 with a coating solution containing the material of the chemical conversion layer 3 described above, and then drying and baking it.

[0109] Here, the method for producing the coating solution is not particularly limited, and a solvent (such as water) appropriate to the resin used may be used, and resin particles (which may also be a resin emulsion) and a compound capable of forming at least one of a Zr-O bond and a Ti-O bond or a Si-O bond in the chemical treatment layer 3 may be mixed and stirred in the solvent by various known methods.

[0110] The method for applying the coating solution to form the chemical conversion treatment layer 3 is not particularly limited, and various known methods can be used as appropriate. For example, if the coating solution is a viscous liquid, it can be applied using known methods such as coating by discharge from a slit nozzle or a circular nozzle, brush coating, plate coating, or spatula coating. Furthermore, if the coating solution is in which the above components are dissolved in a predetermined solvent, various known coating methods can be used, such as brush coating, spray coating, bar coating, discharge coating from nozzles of various shapes, die coating, curtain coating, roll coating, or inkjet coating. In addition, various known methods such as screen printing and powder coating can be employed. Among these coating methods, continuous coating using a roll coater or die coater is preferable because it increases productivity and allows for the acquisition of a uniform film thickness.

[0111] When applying the coating solution to the surface of the Zn-Al-Mg plating layer 2, it is preferable to set the temperature of the Zn-Al-Mg plating layer 2, including the steel plate 1A, within the range of 5 to 80°C. Setting the temperature of the Zn-Al-Mg plating layer 2 to 5°C or higher makes it possible to sufficiently increase the migration speed of the organic resin, allowing the resin to move the desired distance within the time from the formation of the film to the start of drying, as described later, making it easier to achieve the desired distribution state. The temperature of the Zn-Al-Mg plating layer 2 when applying the coating solution is more preferably 10°C or higher, and even more preferably 15°C or higher.

[0112] On the other hand, by keeping the temperature of the Zn-Al-Mg plating layer 2 at 80°C or lower when applying the coating solution, it is possible to suppress the occurrence of small, bubble-like blisters or holes (the so-called bubbling phenomenon) that result from excessively rapid evaporation of water in the aqueous treatment solution. The temperature of the Zn-Al-Mg plating layer 2 at the time of coating solution application is more preferably 60°C or lower, and even more preferably 40°C or lower.

[0113] Furthermore, it is preferable that the temperature of the processing solution when applying it to the Zn-Al-Mg plating layer 2 be within the range of 5 to 60°C. By keeping the temperature of the coating solution within the above range, it becomes possible to use a coating method with excellent productivity, such as a roll coater, and further improvements in productivity can be made. The temperature of the coating solution when applying it is more preferably 10°C or higher, and even more preferably 15°C or higher. Also, the temperature of the coating solution when applying it is more preferably 50°C or lower, and even more preferably 40°C or lower.

[0114] Furthermore, it is preferable to hold the steel plate coated with the above-mentioned film coating solution on the Zn-Al-Mg plating layer 2 for 0.5 to 8.0 seconds before starting the drying and baking process described later, without drying or baking. By ensuring such a holding time before drying and baking, it becomes possible to achieve the desired distribution state of the organic resin. The holding time before drying and baking is more preferably in the range of 0.5 to 4.0 seconds.

[0115] After ensuring the holding time described above, the chemical conversion treatment layer 3 is formed by drying and baking at a temperature in the range of 60 to 200°C for 2 to 30 seconds. At this time, the heating rate should be in the range of 3 to 90°C / second. After the above heat treatment, the dried and baked steel plate is cooled to room temperature at a temperature in the range of 10 to 150°C / second.

[0116] Here, if the drying and baking temperature is below 60°C, the baking and curing may be insufficient, potentially reducing the corrosion resistance and sliding properties of the coating. If the drying and baking temperature exceeds 200°C, even if the drying and baking time is shortened as much as possible, thermal degradation of the resin components may occur, potentially reducing processability. It is more preferable that the drying and baking temperature of the coating solution be within the range of 70 to 160°C.

[0117] Furthermore, it is preferable to adjust the drying and baking temperature according to the physical properties of the resin contained in the chemical conversion treatment layer. Specifically, it is preferable that the temperature does not reach the thermal melting temperature of the resin, and for example, it is more preferable that the drying and baking temperature is less than the thermal melting temperature of the resin. If the drying and baking temperature exceeds the thermal melting temperature, the resin particles will melt together, making it difficult to maintain the desired area coverage, and the properties of the coating may deteriorate.

[0118] Furthermore, if the drying and baking time is less than 2 seconds, the formation of chemical bonds between the plating layer and the chemical conversion treatment layer is inhibited, resulting in reduced adhesion, which is undesirable. If the drying and baking time exceeds 30 seconds, productivity is reduced, which is also undesirable. The drying and baking time of the coating solution is more preferably within the range of 5 to 20 seconds.

[0119] If the heating rate is less than 5°C / s, it takes time for the solvent to evaporate, which can cause aggregation of the dispersed resin particles, making it difficult to maintain the desired area coverage and potentially degrading the properties of the coating. This is undesirable. If the heating rate exceeds 90°C / second, it becomes difficult to form a chemical conversion layer with a uniform thickness, which is also undesirable. The heating rate during drying and baking is more preferably in the range of 6 to 34°C / second.

[0120] Furthermore, if the cooling rate after drying and baking is less than 10°C / second, the equipment length will increase, leading to higher construction costs, which is undesirable. If the cooling rate exceeds 150°C / second, uniform cooling will not be possible, and cracks due to differences in shrinkage rates are more likely to occur in the chemical conversion treated layer, which is also undesirable. It is more preferable that the cooling rate after drying and baking be within the range of 30 to 100°C / second.

[0121] The coating solution described above can be applied using generally known coating methods, such as roll coating, curtain flow coating, spraying, dipping, bar coating, etc.

[0122] (Part Forming Process S3) In part forming process S3, the chemically treated steel sheet 7 is pressed or roll-formed to form the part. A method called hydroforming, which involves applying hydraulic pressure to press the sheet against a mold, can also be used. This allows for the production of an automobile exterior panel 8. The pressing method is not particularly limited, and various pressing processes such as shearing, bending, drawing, and forging can be employed.

[0123] (Bonding process S4) In bonding process S4, the decorative film layer 4 is bonded to the chemical treatment layer 3 of the automobile exterior panel 8. The decorative film layer 4 may be bonded by hand, or it may be bonded using the vacuum forming apparatus 100 shown in Figure 7. Other known film bonding methods may also be used. The vacuum forming apparatus 100 includes, for example, a cylinder 51, an upper forming chamber 52, a heater 53, a table 54, a lower forming chamber 55, a cylinder 56, a bolster 57, a fixing jig 58, a vacuum tank 59, and a pressure tank 60. This vacuum forming apparatus 100 includes an upper forming chamber 52 and a lower forming chamber 55, and vacuum forming of the decorative film 4A is performed between the two forming chambers, the upper forming chamber 52 and the lower forming chamber 55. A vacuum circuit and an air circuit are piped to the upper forming chamber 52 and the lower forming chamber 55, respectively. The following describes a method for attaching the decorative film 4A using the vacuum forming apparatus 100.

[0124] The upper molding chamber 52 and the lower molding chamber 55 can be joined and separated, and the upper molding chamber 52 is equipped with a cylinder 51 that allows it to move up and down. A table 54 is also disposed inside the lower molding chamber 55, and the table 54 is configured to move up and down by a cylinder 56. Cylinders 51 and 56 can be, for example, air cylinders, hydraulic cylinders, servo motors, etc. An automobile exterior panel 8 to which a decorative film 4A is attached is placed on the table 54. Specifically, a bolster 57 is placed on the table 54, and the automobile exterior panel 8 is fixed by a fixing jig 58 placed on the bolster 57.

[0125] A heater 53 is incorporated inside the upper molding chamber 52, and the decorative film 4A is heated by the upper molding chamber 52 between the two joined molding chambers. The heater 53 is, for example, a near-infrared heater.

[0126] As shown in Figure 7, the automobile exterior panel 8 is placed on the fixing jig 58 inside the lower molding chamber 55. The decorative film 4A is fixed to the upper surface of the lower molding chamber 55 with clamps or the like. At this time, both the upper molding chamber 52 and the lower molding chamber 55 are under atmospheric pressure.

[0127] Next, the upper molding chamber 52 is lowered, and the upper molding chamber 52 and the lower molding chamber 55 are joined together to close the molding chambers. The upper molding chamber 52 and the lower molding chamber 55 are both reduced from atmospheric pressure to a vacuum state by the vacuum tank 59.

[0128] Both the upper molding chamber 52 and the heater 53 are under vacuum, and the decorative film 4A is heated by the heater 53. When heated, the decorative film 4A tends to sag due to its own weight. At this time, the decorative film 4A is kept in a nearly horizontal position by adjusting the pressure difference in the vacuum between the upper and lower molding chambers.

[0129] Subsequently, the table 54 in the lower molding chamber 55 is raised. At this time, the upper molding chamber 52 and the lower molding chamber 55 are in a near-vacuum state.

[0130] Subsequently, by releasing the vacuum in the upper molding chamber 52 and introducing atmospheric pressure, the decorative film 4A is pressed and adhered to the automobile exterior panel 8. Specifically, the bonding layer 41 of the decorative film 4A is adhered to the chemical treatment layer 3 of the automobile exterior panel 8. Furthermore, by supplying compressed air into the upper molding chamber 52 via the pressure tank 60, the decorative film 4A and the automobile exterior panel 8 can be adhered to each other with even greater force.

[0131] After the application is complete, the heater 53 is stopped, the lower molding chamber 55 is returned to atmospheric pressure, and the upper molding chamber 52 is raised to obtain the decorative film-coated automobile exterior panel 10.

[0132] (Second Embodiment) Figure 5 is another flowchart showing a method for manufacturing a decorative film-coated automobile exterior panel 10. In the method for manufacturing a decorative film-coated automobile exterior panel 10 according to the second embodiment, a decorative film 4A is attached to the surface of the chemical conversion treatment layer 3 of a chemical conversion treated steel sheet 7, and then press-formed. Specifically, the method for manufacturing a decorative film-coated automobile exterior panel according to the second embodiment includes a plating layer formation step T1 in which a Zn-Al-Mg-based plating layer 2 is formed on a steel sheet 1A, a chemical conversion treatment layer formation step T2 in which a chemical conversion treatment layer 3 is formed on the surface of the Zn-Al-Mg-based plating layer 2 to obtain the chemical conversion treated steel sheet 7 shown in Figure 6, an attachment step T3 in which a decorative film layer 4 is attached to the chemical conversion treatment layer 3 of the chemical conversion treated steel sheet 7, and a part forming step T4 in which the chemical conversion treated steel sheet 7 with the decorative film 4A attached is pressed to form a part.

[0133] (Plating layer formation process T1) In the plating layer formation process T1, the product can be manufactured using the same process as the plating layer formation process S1 of the first embodiment.

[0134] (Chemical treatment layer formation step T2) In the chemical treatment layer formation step T2, the manufacturing process can be the same as in the chemical treatment layer formation step S2 of the first embodiment.

[0135] (Bonding process T3) In bonding process T3, a decorative film 4A, which will become the decorative film layer 4, is bonded to the surface of the chemical treatment layer 3 of the chemical treatment steel sheet 7. If the decorative film 4A has a bonding layer 41, it can be bonded by pressing the decorative film 4A directly. Roll bonding is preferred as the bonding method. The pressure and temperature during bonding can be appropriately selected and determined according to the characteristics of the decorative film 4A. In the case of a decorative film 4A that does not have a bonding layer 41, it is preferable to apply an adhesive to the chemical treatment steel sheet 7 and then bond it using the roll bonding method. Known methods such as roll coater, die coater, brush coating, bar coater, and blade coater can be used for applying the adhesive, but a roll coater or die coater is preferable as it increases productivity.

[0136] (Part molding process T4) In part molding process T4, the same process as in part molding process S3 of the first embodiment can be used for manufacturing. This makes it possible to obtain the decorative film-coated automobile exterior panel 10.

[0137] The technical scope of this disclosure is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of this disclosure. In addition, it is possible to replace the components in the embodiments with well-known components as appropriate, without departing from the spirit of this disclosure, and the modifications described above can be combined as appropriate.

[0138] The effects of one aspect of this disclosure will be further illustrated by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of this disclosure. This disclosure is not limited to these examples of conditions. This disclosure may adopt various conditions as long as they do not depart from the gist of this disclosure and achieve the objectives of this disclosure.

[0139] (Steel Sheet) A cold-rolled steel sheet with a thickness of 0.6 mm was obtained by hot-rolling, hot-rolled pickling, and cold-rolling of steel having the chemical composition shown in Table 1. The remainder of the chemical composition in Table 1 consists of Fe and impurities.

[0140] (Plating layer) The prepared cold-rolled steel sheet was annealed in a heating furnace of a continuous hot-dip galvanizing apparatus under conditions that the maximum plate temperature reached was 820°C. Then, it was immersed in a molten plating solution having the chemical compositions shown in Tables 2A, 2B, and 2C, and then removed and cooled to coat the cold-rolled steel sheet with a Zn-Al-Mg-based plating layer 2. The gas atmosphere inside the heating furnace during annealing was H ２ N containing 1 volume% ２ The dew point was set to -40°C based on the atmosphere. Also, the amount of plating was determined by the amount of N when the cold-rolled steel sheet was immersed in the molten plating solution and pulled out, through a slit-shaped nozzle. ２ The process was controlled by a method commonly known as gas wiping, which involves spraying gas onto the surface. The amount of plating deposited was 60 g / m² on each side for all samples. ２ That's what I decided.

[0141] (Chemical Conversion Treatment Layer) For forming the chemical conversion treatment layer, aqueous surface treatment agents with each component shown in Table 3 were prepared as coating solutions (solvent: water). The solid content of the aqueous surface treatment agents was adjusted to 11% by mass.

[0142] (Components of the aqueous surface treatment agent (agent A)) The aqueous surface treatment agent for the chemical conversion treatment layer was prepared by adding and mixing agents A) to F) shown below into pure water. The details of the prepared aqueous surface treatment agent are shown in Table 3.

[0143] A) Agents for forming Zr-O, Ti-O, and Si-O bonds The following reagents were used as agents for forming Zr-O, Ti-O, and Si-O bonds: • Component for forming Zr-O bonds: Zirconium ammonium carbonate (referred to as the Zr-O component in Table 3) • Component for forming Ti-O bonds: Ammonium titanate fluoride (referred to as the Ti-O component in Table 3) • Component for forming Si-O bonds: 3-aminopropyltriethoxysilane (referred to as the Si-O component in Table 3)

[0144] (Resin B) The following resins (Resin B) were used as the resin dispersed in the chemical conversion treatment layer. a) Aqueous emulsion type urethane resin From commercially available aqueous emulsion type urethane resins, the particle size of the emulsion was measured in advance, and those with average particle sizes of 10 nm, 20 nm, 60 nm, 200 nm, and 300 nm were selected and used. The particle size of the emulsion was measured using a light scattering photometer. In Table 3, these are referred to as Urethane 10 (average particle size 10 nm), Urethane 20 (average particle size 20 nm), Urethane 60 (average particle size 60 nm), and Urethane 200 (average particle size 200 nm), respectively. b) Aqueous emulsion type acrylic resin From commercially available aqueous emulsion type acrylic resins, the particle size of the emulsion was measured in advance, and those with an average particle size of 60 nm were selected and used. In Table 3, this is referred to as Acrylic 60. c) Aqueous emulsion polyester resin: From commercially available aqueous emulsion polyester resins, the particle size of the emulsion was measured in advance, and a resin with an average particle size of 60 nm was selected and used. In Table 3, this is referred to as polyester 60. d) Aqueous emulsion epoxy resin: From commercially available aqueous emulsion polyester resins, the particle size of the emulsion was measured in advance, and a resin with an average particle size of 60 nm was selected and used. In Table 3, this is referred to as epoxy 60.

[0145] (Chemicals used to impart phosphorus, vanadium, cobalt, titanium, and chromium to the chemical treatment layer (Chemicals C)) The following chemicals were used to impart phosphorus, vanadium, cobalt, titanium, and chromium to the chemical treatment layer: ・Phosphorus: Phosphoric acid (referred to as P in Table 3) ・Vanadium: Vanadium acetylacetonate (referred to as V in Table 3) ・Cobalt: Cobalt sulfate (referred to as Co in Table 3) ・Titanium: Titanium diisopropoxybisacetylacetonate (referred to as Ti in Table 3) ・Chromium: Chromium nitrate (referred to as Cr in Table 3)

[0146] Next, the prepared coating solution was applied to the prepared plated steel sheet using a roll coater, and then dried in an induction heating furnace to form a chemical conversion treatment layer on the surface of the plated layer. The temperature of the treatment agent and the plated steel sheet during application of the coating solution were set to 25°C, and the holding time from application of the coating solution to drying and baking was set to 2.5 seconds. The thickness of the chemical conversion treatment layer was obtained by adjusting the concentration of the treatment solution with pure water and further controlling the roll peripheral speed and roll pressure of the roll coater. When drying the coating solution, the drying conditions shown in Tables 4A, 4B, 4C, 4D, 4E, 4F, 5A, 5B, 5C, 5D, 5E, and 5F were used, and the cooling rate was 50°C / second. The prepared surface-treated steel sheet samples are shown in Tables 4A, 4B, 4C, 4D, 4E, 4F, 5A, 5B, 5C, 5D, 5E, and 5F.

[0147] (Decorative film layer) The following film was used for the decorative film layer: ・Decorative film: An unoriented copolymer polyester film (a copolymer polyester consisting of terephthalic acid / isophthalic acid (weight ratio 88 / 12) and ethylene glycol (melting point 228°C)) was used.

[0148] (Decorative film-coated automobile exterior panel) <Manufacturing method of the first embodiment> A press die simulating an automobile door was manufactured and inserted into a 2000t press testing machine. Using this, a chemically treated steel sheet was press-formed into the shape of an automobile door. Furthermore, the blank portion generated during pressing was cut by laser welding to produce a simulated automobile door part.

[0149] Next, a decorative film was applied to the created automobile door simulation part using the vacuum forming apparatus 100 shown in Figure 7. The decorative film was applied only to the outer surface of the door, while the inner surface of the door was left with only a chemical conversion treatment layer, without any decorative film or paint. By following these steps, an automobile exterior panel with a decorative film, manufactured using the manufacturing method of the first embodiment, was obtained.

[0150] <Manufacturing Method of the Second Embodiment> The decorative film described above was attached to only one side of the prepared chemically treated steel sheet. The attachment method was a roll-pressure type laminating line, with the laminating roll temperature set to 150°C.

[0151] A press die simulating an automobile door was fabricated and inserted into a 2000-ton press test machine. Using this machine, a steel sheet with a decorative film attached to a chemically treated steel sheet was press-formed into the shape of an automobile door. Furthermore, the blank portion generated during pressing was cut by laser welding to create a simulated automobile door part. During the press-forming process, the side with the decorative film attached was processed to face the outside of the door. The inside surface of the processed door was left with only the chemically treated layer, without any decorative film or paint. By following these steps, an automobile exterior panel with a decorative film, manufactured using the manufacturing method of the second embodiment, was obtained.

[0152] (Evaluation of decorative film-coated automobile exterior panels) (Observation of resin dispersion state in the cross-section of the chemical conversion layer and measurement of Lr / Lc and area envelopment) Materials were cut from three arbitrary locations on the fabricated automobile door simulation part, and samples were prepared for observation of the cross-section in the thickness direction using a focused ion beam processing device (FIB / SII Nanotechnology SMI3050SE). The prepared samples were observed using a JEOL JEM-2100F field emission transmission electron microscope at an acceleration voltage of 200kV. The distribution state of the resin was observed from the transmission electron microscope (TEM) image, and if the particles were dispersed in the chemical conversion layer, it was evaluated as "granular," and if it was determined that the distribution was cloudy or marbled based on the above judgment criteria, it was evaluated as "cloudy / marbled." The evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the first embodiment are shown in Tables 4A, 4B, 4C, 4D, 4E, and 4F, and the evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the second embodiment are shown in Tables 5A, 5B, 5C, 5D, 5E, and 5F.

[0153] Furthermore, the total length of the region where the resin 31 in the chemical conversion treatment layer 3 is in contact with the decorative film layer 4 was determined by calculating Lr / Lc, which is the relationship between the resin contact region length Lr and the interface length Lc between the chemical conversion treatment layer 3 and the decorative film layer 4. Lr was determined by measuring Lr included in an arbitrary Lc = 600 nm range in the TEM image of the cut sample cross-section. The average of the Lr / Lc obtained from samples processed from materials cut from any three locations on the simulated automobile door part was taken as the Lr / Lc for each simulated automobile door part. Furthermore, if the average Lr / Lc of the samples taken from the three locations was 0.3 or higher, it was evaluated as "Rank A", if it was 0.1 or higher but less than 0.3, it was evaluated as "Rank B", and if it was less than 0.1, it was evaluated as "Rank C". Ranks A and B were considered acceptable. The evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the first embodiment are shown in Tables 4A, 4B, 4C, 4D, 4E, and 4F, and the evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the second embodiment are shown in Tables 5A, 5B, 5C, 5D, 5E, and 5F.

[0154] Furthermore, EDS analysis (elemental mapping) was performed on the cross-sectional sample of the film to obtain a detected element map. From the obtained detected element map, the region of resin 31 in the chemical conversion treatment layer 3 was determined, and the area Sa / area Sb was calculated to determine the area envelope of resin 31 in the cross-sectional view of the film. The area envelope was calculated as the average value of the area envelope obtained for each of the five observation fields. The evaluation results of the decorative film-coated automobile exterior panel manufactured by the manufacturing method of the first embodiment are shown in Tables 4A, 4B, 4C, 4D, 4E, and 4F, and the evaluation results of the decorative film-coated automobile exterior panel manufactured by the manufacturing method of the second embodiment are shown in Tables 5A, 5B, 5C, 5D, 5E, and 5F.

[0155] (Observation of delamination of decorative film) The surface of the automobile door simulation member manufactured by the manufacturing method of the second embodiment was observed, and the state of delamination at the processed parts and cut ends was visually inspected. A score of "3" was given if there was no delamination at all, a score of "2" was given if no delamination was observed visually but a very slight delamination was observed with a 10x magnifying glass, and a score of "1" was given if delamination was observed visually. A score of 2 or higher was considered a pass. The evaluation results of the decorative film-coated automobile exterior panel manufactured by the manufacturing method of the first embodiment are shown in Tables 4A, 4B, 4C, 4D, 4E, and 4F, and the evaluation results of the decorative film-coated automobile exterior panel manufactured by the manufacturing method of the second embodiment are shown in Tables 5A, 5B, 5C, 5D, 5E, and 5F.

[0156] (Corrosion Test) Test specimens measuring approximately 200 mm x 200 mm were cut from the prepared decorative film-covered automobile exterior panels. Cuts reaching the bare steel plate were made on the decorative film-covered surface using an NT cutter, and the cut edges were further coated with commercially available rust-preventive paint to prevent corrosion from progressing from the cut edges. When cutting the samples, the processed areas were also included. The prepared test specimens were subjected to a combined cycle corrosion test described in the automotive industry standard JASO. M609-1991 for 120 cycles.

[0157] For test pieces after 120 cycles of combined cycle corrosion testing, the width of film swelling from the cut defect (the width of swelling as corrosion factors penetrate from the defect and corrosion progresses beneath the film) was measured (door exterior corrosion test). A rating of "3" was given if the swelling width was 5 mm or less, "2" if it was between 3 mm and 10 mm, and "1" if it was greater than 10 mm, with a score of 3 being considered a pass. The evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the first embodiment are shown in Tables 4A, 4B, 4C, 4D, 4E, and 4F, and the evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the second embodiment are shown in Tables 5A, 5B, 5C, 5D, 5E, and 5F. Furthermore, the inner surface where the decorative film was not applied was visually inspected and evaluated as follows: "4" if white rust was present but no red rust was present; "3" if the processed area had red rust to the extent of the ceiling; "2" if red rust was present over a relatively wide area in the processed area; and "1" if red rust was present across the entire inner surface of the test piece (inner door corrosion test). The evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the first embodiment are shown in Tables 4A, 4B, 4C, 4D, 4E, and 4F, and the evaluation results of the decorative film-coated automobile exterior panels manufactured by the manufacturing method of the second embodiment are shown in Tables 5A, 5B, 5C, 5D, 5E, and 5F.

[0158] In the examples of the present invention, which included a plating layer, a chemical conversion treatment layer, and a decorative film, the adhesion of the decorative film was excellent, and the corrosion resistance was also excellent. In the comparative examples, which lacked a chemical conversion treatment layer, red rust occurred in all cases.

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[0176] 1. Steel material, 2. Plating layer, 3. Chemical conversion treatment layer, 4. Decorative film layer

Claims

1. An automotive exterior panel with a decorative film coating, comprising: a steel material; a Zn-Al-Mg plating layer provided on both surfaces of the steel material; a chemical conversion treatment layer provided on the surface of at least one of the Zn-Al-Mg plating layers; and a decorative film layer provided on at least one side of the steel material and on the surface of the chemical conversion treatment layer, wherein the chemical conversion treatment layer contains a resin, the area coverage of the resin in the chemical conversion treatment layer is 0.5 to 1.0, the resin is present on the outermost surface of the chemical conversion treatment layer, and in a cross-section in the thickness direction of the panel, the relationship between the total length Lr of the resin present on the outermost surface of the chemical conversion treatment layer and the decorative film layer obtained from an electron microscope image of the interface between the chemical conversion treatment layer and the decorative film layer using a transmission electron microscope is Lr / Lc ≥ 0.1 within an arbitrary interface length Lc = 600 nm between the chemical conversion treatment layer and the decorative film layer.

2. The decorative film-coated automobile exterior panel according to claim 1, characterized in that the chemical composition of the Zn-Al-Mg plating layer is, by mass%, Al: 1.00 to 30.00%, Mg: 1.00 to 10.00%, Si: 0% to 2.00%, and the remainder: Zn and impurities.

3. The chemical composition of the Zn-Al-Mg plating layer is, in mass%, Al: 5.00 to 30.00%, Mg: 2.00 to 10.00%, Si: 0% to 2.00%, Ca: 0% to 2.00%, Sb: 0% to 0.5000%, Pb: 0% to 0.50%, Sr: 0% to 0.50%, Cu: 0% to 1.00%, Ti: 0% to 1.00%, Cr: 0% to 1.00%, Nb: 0% to 1.00%, Ni: 0% to 1.00%, Mn: 0% to 1.00%, Mo: 0% to 1.00%, Co: 0% to 1.0000%, V: 0% to 1.0000%. The decorative film-coated automobile exterior panel according to claim 1, characterized in that it comprises Sn: 0% to 1.00%, In: 0% to 1.0000%, Bi: 0% to 1.0000%, Zr: 0% to 1.00%, Ag: 0% to 1.00%, Li: 0% to 1.00%, La: 0% to 0.50%, Ce: 0% to 0.50%, Y: 0% to 0.50%, B: 0% to 0.50%, Fe: 0% to 5.00%, and the remainder: Zn and impurities.

4. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains one or more selected from the group consisting of Si: greater than 0% and 2.00% or less, and Ca: greater than 0% and 2.00% or less.

5. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains one or more selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.50% or less, and Sr: greater than 0% and 0.50% or less.

6. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains one or more selected from the group consisting of Cu: greater than 0% and 1.00% or less, Ti: greater than 0% and 1.00% or less, Cr: greater than 0% and 1.00% or less, Nb: greater than 0% and 1.00% or less, Ni: greater than 0% and 1.00% or less, Mn: greater than 0% and 1.00% or less, Mo: greater than 0% and 1.00% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less.

7. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains one or more selected from the group consisting of Sn: greater than 0% and 1.00% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less.

8. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains one or more selected from the group consisting of Zr: greater than 0% and 1.00% or less, Ag: greater than 0% and 1.00% or less, and Li: greater than 0% and 1.00% or less.

9. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains one or more selected from the group consisting of La: greater than 0% and 0.50% or less, Ce: greater than 0% and 0.50% or less, and Y: greater than 0% and 0.50% or less.

10. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains B: more than 0% and 0.50% or less.

11. The decorative film-coated automobile exterior panel according to claim 2, characterized in that the chemical composition further contains Fe: 0.01% or more and 5.00% or less.

12. The decorative film-coated automobile exterior panel according to claim 2, characterized in that an adhesive layer exists between the decorative film layer and the chemical conversion treatment layer.

13. The decorative film-coated automobile exterior panel according to claim 1, characterized in that the chemical conversion treatment layer is a film layer having at least one bond of Zr-O bond, Ti-O bond, or Si-O bond.

14. The decorative film-coated automobile exterior panel according to claim 13, characterized in that the resin is at least one of acrylic resin, urethane resin, polyester resin, and epoxy resin.

15. The decorative film-coated automobile exterior panel according to claim 13, characterized in that the chemical conversion treatment layer contains at least one of phosphorus, vanadium, cobalt, titanium, and chromium.

16. The decorative film layer is provided on only one surface of the chemical conversion treatment layer, and the chemical conversion treatment layer that does not have the decorative film layer is covered with a resin film, as described in claim 1.

17. The decorative film-coated automobile exterior panel according to claim 1, characterized in that at least a portion of the end face of the steel material is covered with the decorative film layer.

18. An automotive component having a decorative film-coated automotive exterior panel as described in claim 1.

19. A method for manufacturing an automotive exterior panel with a decorative film coating, characterized by pressing a chemically treated steel sheet comprising a steel sheet, a Zn-Al-Mg-based plating layer provided on both sides of the steel sheet, and a chemical conversion treatment layer provided on the surface of the Zn-Al-Mg-based plating layer, forming a part, and then attaching a decorative film to the surface of the chemical conversion treatment layer.

20. A method for manufacturing an automobile exterior panel with a decorative film, comprising a steel sheet, a Zn-Al-Mg-based plating layer provided on both sides of the steel sheet, and a chemical conversion treatment layer provided on the surface of the Zn-Al-Mg-based plating layer, characterized by attaching a decorative film to the surface of the chemical conversion treatment layer of the chemical conversion treatment steel sheet and then press-forming it.

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