Surface-treated steel sheet

Abstract

[Problem] To further improve adhesion and corrosion resistance of a coating film after forming processing. [Solution] The surface-treated steel sheet of the present invention has: a plated layer on at least one surface of a steel sheet, and containing zinc and nickel; a first composite layer between the steel sheet and the plated layer, and containing Fe, Zn, and Ni; a chromium-containing composite layer on the plated layer, and containing at least Cr; and a coating film on the chromium-containing composite layer, and containing an epoxy resin, an isocyanate resin, and silica particles, the plated layer having an attached amount of 2 to 50 g / m 2 2 per one side, the chromium-containing composite layer including: a second composite layer on the plated layer side, and containing Cr, Zn, and Ni; and a third composite layer on the coating film side, and containing Si, N, and Cr, the chromium-containing composite layer having an attached amount of 10 to 200 mg / m 2 2 in terms of metal Cr, the coating film having a thickness of 0.5 to 2.0 µm.

Description

Technical Field

[0001] This invention relates to surface-treated steel sheets. Background Technology

[0002] Various types of galvanized steel sheets are used as raw materials for manufacturing fuel tanks for motorized two-wheeled vehicles. During fuel tank manufacturing, the galvanized steel sheets used as raw materials must be machined into the desired shape. Furthermore, the surface of the galvanized steel sheet exposed to fuels such as gasoline requires corrosion resistance to prevent fuel-induced corrosion. Therefore, the galvanized steel sheets used as raw materials for fuel tanks must balance workability and corrosion resistance.

[0003] To achieve such a balance between formability and corrosion resistance, for example, Patent Document 1 discloses an organic composite coated steel sheet with a chromate coating and a coating composed of a coating composition formed on the surface of a zinc, aluminum, or zinc-based alloy coated steel sheet.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Publication No. 4-28539 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] Here, the inventors conducted further research on the organic composite coated steel sheet disclosed in the aforementioned patent document 1, and found that there is room for further improvement in terms of balancing the adhesion and corrosion resistance of the coating after forming and processing.

[0009] Therefore, the present invention was made in view of the above-mentioned points, and the object of the present invention is to provide a surface-treated steel sheet that can further improve the adhesion and corrosion resistance of the coating after forming.

[0010] Solution for solving the problem

[0011] In order to solve the above problems, the inventors conducted in-depth research and found that by appropriately forming a specific composite layer in the laminated structure realized by surface-treated steel plate, it is possible to further improve the adhesion and corrosion resistance of the coating after forming and processing.

[0012] The present invention, based on this insight, is as follows.

[0013] (1) A surface-treated steel sheet comprising: a coating on at least one surface of the steel sheet and containing zinc and nickel; a first composite layer between the steel sheet and the coating and containing Fe, Zn, and Ni; a chromium-containing composite layer on top of the coating and containing at least Cr; and a coating film on top of the chromium-containing composite layer and containing epoxy resin, isocyanate resin, and silica particles, wherein the coating has an adhesion amount of 2 to 50 g / m² per side. 2 The chromium-containing composite layer comprises: a second composite layer located on the plating side and containing Cr, Zn, and Ni; and a third composite layer located on the coating side and containing Si, N, and Cr. The adhesion amount of the chromium-containing composite layer, calculated in terms of metallic Cr, is 10–200 mg / m³. 2 The thickness of the coating is 0.5 to 2.0 μm.

[0014] (2) The surface-treated steel plate according to (1), wherein the ratio (d2 / d1) of the average thickness d2 of the second composite layer to the average thickness d1 of the first composite layer is 0.2 to 150.0, and the ratio (d3 / d1) of the average thickness d3 of the third composite layer to the average thickness d1 of the first composite layer is 0.2 to 150.0.

[0015] (3) The surface-treated steel sheet according to (1) or (2), wherein the coating contains polyethylene wax in a proportion of 0.1 to 10.0% by mass relative to the coating solids, the epoxy resin content is 30.0% by mass or more relative to the coating solids, the isocyanate resin content is 0.10 to 2.00 by mass relative to the epoxy resin, the silica particles content is 5.0 to 50.0% by mass relative to the coating solids, and the coating contains the epoxy resin, the isocyanate resin, the silica particles and the polyethylene wax in a manner that totals less than 100% by mass.

[0016] (4) The surface-treated steel sheet according to (1) or (2), wherein the epoxy resin is a bisphenol A type epoxy resin with a number average molecular weight of 300 to 100,000.

[0017] (5) The surface-treated steel plate according to (1) or (2), wherein the silica particles are at least one of fumed silica or colloidal silica, the average particle size of the fumed silica is 5 to 40 nm, and the average particle size of the colloidal silica is 5 to 200 nm.

[0018] (6) The surface-treated steel sheet according to (1) or (2) is used as a blank for a fuel tank.

[0019] The effects of the invention

[0020] As explained above, according to the present invention, the adhesion and corrosion resistance of the coating after forming of the surface-treated steel sheet can be further improved. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the structure of a surface-treated steel sheet according to an embodiment of the present invention.

[0022] Figure 2 This is an explanatory diagram used to illustrate the first composite layer in a surface-treated steel sheet of the same embodiment.

[0023] Figure 3 This is an explanatory diagram used to illustrate the first composite layer in a surface-treated steel sheet of the same embodiment.

[0024] Figure 4 This is an explanatory diagram used to illustrate the chromium-containing composite layer in a surface-treated steel sheet of the same embodiment.

[0025] Figure 5 This is an explanatory diagram used to illustrate the chromium-containing composite layer in a surface-treated steel sheet of the same embodiment.

[0026] Figure 6 This is an explanatory diagram used to illustrate the chromium-containing composite layer in a surface-treated steel sheet of the same embodiment.

[0027] Figure 7 This is a flowchart illustrating an example of a method for manufacturing a surface-treated steel sheet according to the same embodiment. Detailed Implementation

[0028] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that in this specification and the accompanying drawings, constituent elements having substantially the same functional structure are omitted from repeated description by using the same reference numerals.

[0029] <About the overall composition of surface-treated steel plates>

[0030] First, refer to Figure 1 The overall structure of the surface-treated steel plate in this embodiment will be described. Figure 1 This is an explanatory diagram schematically showing the structure of the surface-treated steel sheet according to this embodiment.

[0031] like Figure 1 As shown, the surface-treated steel sheet 1 of this embodiment has a steel sheet 10, a plating layer 20, a chromium-containing composite layer 30, and a coating 40. This surface-treated steel sheet 1 is suitable for use as a blank for fuel tanks of various automobiles (especially motorized two-wheeled vehicles).

[0032] It should be noted that, Figure 1 The illustration shows a case where a plating layer 20, a chromium-containing composite layer 30, and a coating 40 are provided on one side of the steel plate 10. However, these plating layers 20, chromium-containing composite layers 30, and coatings 40 can also be provided on both sides of the steel plate 10.

[0033] [Regarding steel plate 10]

[0034] The steel plate 10 serves as the base material for the surface-treated steel plate 1 in this embodiment, and various steel plates can be used depending on the required strength (e.g., tensile strength) of the surface-treated steel plate 1. Examples of such steel plates include ordinary steel, low-carbon steel, and high-strength steel.

[0035] However, when manufacturing a fuel tank, which is one of the important applications of the surface-treated steel sheet 1 in this embodiment, a high degree of machinability is required for the steel sheet used as raw material. Therefore, as the steel sheet 10 in this embodiment, IF steel with excellent machinability is preferably used, and furthermore, in order to ensure airtightness after welding, secondary machinability, etc., a steel sheet containing a few ppm or more of boron (B) is more preferably used.

[0036] In addition, the thickness of the steel plate 10 can be appropriately set according to the required strength of the surface-treated steel plate 1 and the allowable mass of the article manufactured by processing the surface-treated steel plate 1.

[0037] [Regarding coating 20]

[0038] The coating 20 in the surface-treated steel sheet 1 is a layer provided to ensure the corrosion resistance of the steel sheet 10, which serves as the base material. In this embodiment, the coating 20 is located on at least one surface of the steel sheet 10 and is a coating containing zinc (Zn) and nickel (Ni).

[0039] By providing a zinc and nickel plating layer 20, this plating layer 20 exhibits excellent resistance weldability, processability, and corrosion resistance (especially gasoline resistance). In this plating layer 20, the Ni content is preferably in the range of 5 to 15% by mass, with the balance being Zn and impurities.

[0040] By setting the Ni content to 5% by mass or more, resistance weldability and processability are not reduced, and corrosion resistance can be further improved. A Ni content of 8% by mass or more is more preferable. On the other hand, by setting the Ni content to 15% by mass or less, large cracks in the coating 20 can be further suppressed, and the adhesion of the coating 20 can be further ensured. A Ni content of 13% by mass or less is more preferable.

[0041] The coating 20 has an adhesion amount of 2-50 g / m² per single side. 2 Within the range. By setting the adhesion amount of coating 20 to 2 g / m 2The above fully demonstrates the resistance welding properties, processability, corrosion resistance, and other characteristics of the coating 20. The preferred adhesion amount of the coating 20 is 5 g / m². 2 That's all. On the other hand, by making the adhesion amount of coating 20 50g / m 2 The following optimal balance is achieved between resistance weldability, processability, and corrosion resistance. The preferred adhesion amount of coating 20 is 40 g / m². 2 the following.

[0042] It should be noted that the amount of coating 20 can be calculated from the mass difference before and after removal of a known area of ​​coating 20 from the steel plate 10 by physical or chemical methods. There are no particular limitations on the method used to remove coating 20 from the steel plate 10; for example, sandblasting can be used as a physical method, and acid dissolution can be used as a chemical method. Furthermore, the Ni content of coating 20 can be determined by measuring the coating 20 removed from the steel plate 10 as described above using a fluorescence X-ray analysis device.

[0043] [Regarding chromium-containing composite layer 30]

[0044] In this embodiment, the chromium-containing composite layer 30 is located on the plating layer 20 and is a composite layer containing at least chromium (Cr). This chromium-containing composite layer 30 helps to improve the adhesion between the plating layer 20 as described above and the coating film 40 described later, and also helps to improve the corrosion resistance of the surface-treated steel plate 1.

[0045] The adhesion amount of the chromium-containing composite layer 30, calculated in terms of metallic Cr, is 10–200 mg / m³. 2 The adhesion amount of the chromium-containing composite layer 30 is less than 10 mg / m². 2 In this case, the amount of chromium-containing composite layer 30 attached becomes too small, making it difficult to exhibit the improved adhesion effect described above. By setting the amount of chromium-containing composite layer 30 attached to 10 mg / m³, 2 The above measures improve the adhesion between the plating layer 20 and the coating film 40 described later. The preferred adhesion amount of the chromium-containing composite layer 30 is 20 mg / m³. 2 The above, more preferably 30 mg / m² 2 above.

[0046] On the other hand, the adhesion amount of the chromium-containing composite layer 30 exceeds 200 mg / m². 2 In some cases, the amount of chromium-containing composite layer 30 adhering becomes excessive, and sometimes the coating 40 described later is easily peeled off. By reducing the amount of chromium-containing composite layer 30 to 200 mg / m³, [the problem can be mitigated]. 2 The following methods can stably improve the adhesion between the plating layer 20 and the coating film 40 described later. The preferred adhesion amount of the chromium-containing composite layer 30 is 150 mg / m³. 2 The following is more preferably 100 mg / m²2 the following.

[0047] Here, in the surface-treated steel sheet 1 of this embodiment, a layer that meets the following conditions is designated as the chromium-containing composite layer 30. For example, when obtaining the depth distribution related to elemental distribution from the surface of the coating 40 (described later) using high-frequency glow discharge optical emission spectrometry (RF-GD-OES), the chromium-containing composite layer 30 can be identified based on the depth distribution of chromium. That is, when the position where the intensity related to elemental chromium reaches its maximum in the chromium depth distribution located between the coating 40 and the plating layer 20 is taken as the peak intensity position, the position where the peak intensity of chromium on the coating 40 side is halved can be defined as the position of the interface of the chromium-containing composite layer 30 on the coating 40 side, and the position where the peak intensity of chromium on the plating layer 20 side is halved can be defined as the position of the interface of the chromium-containing composite layer 30 on the plating layer 20 side.

[0048] It should be noted that the above determination using RF-GD-OES can be performed using a glow discharge luminescent surface analyzer (such as the Markus-type high-frequency glow discharge luminescent surface analyzer GD-Profiler2 manufactured by HORIBA Jobin Yvon), under the conditions of 30 seconds of gas replacement time, 30 seconds of pre-sputtering time, 600 Pa pressure, and 35 W output power, until the ferrography originating from the substrate steel is detected in terms of intensity.

[0049] Furthermore, the amount of chromium-containing composite layer 30 can be calculated, for example, by quantitative analysis using fluorescence X-ray analysis, from a standard curve showing the relationship between the fluorescence X-ray intensity of chromium and the amount of chromium. Alternatively, the amount of chromium-containing composite layer 30 can also be calculated by analyzing an aqueous solution containing the chromium-containing composite layer 30 using inductively coupled plasma (ICP) emission spectroscopy.

[0050] Here, when determining the adhesion amount of the aforementioned chromium-containing composite layer 30 from surface-treated steel sheets that have already been processed into various components such as fuel tanks, test pieces are cut from the components of interest at locations that have never undergone various joining processes such as welding, deep drawing, bending, etc., and used as the analysis objects. The size of the cut test piece is the same as that of a flat plate, for example, approximately 50×50 mm. Using a portion of the obtained test piece, RF-GD-OES measurements are performed as described above to determine the locations in the test piece corresponding to the chromium-containing composite layer 30. Based on this, fluorescence X-ray analysis or ICP emission spectroscopy analysis is performed on the locations in the obtained test piece corresponding to the chromium-containing composite layer 30.

[0051] It should be noted that the detailed composition of the chromium-containing composite layer 30 will be explained again below.

[0052] [Regarding coating 40]

[0053] In this embodiment, the coating 40 is located on the chromium-containing composite layer 30 and is a resin layer containing epoxy resin, isocyanate resin, and silica particles. Furthermore, this coating 40 preferably contains polyethylene wax in addition to the aforementioned components. By providing this coating 40, the surface-treated steel sheet 1 of this embodiment exhibits excellent corrosion resistance, processability, and electrodeposition coating properties.

[0054] Epoxy Resins

[0055] The epoxy resin contained in the coating film 40 of this embodiment functions as an adhesive resin. By containing epoxy resin in the coating film 40, not only are the water resistance and alkali resistance of the coating film improved, but the adhesion between the coating film and other layers is also improved. In the coating film 40 of this embodiment, a bisphenol A type epoxy resin is preferably used, for example.

[0056] Furthermore, the number average molecular weight of the epoxy resin contained in the coating 40 of this embodiment is preferably in the range of 300 to 100,000.

[0057] By making the number average molecular weight of the epoxy resin 300 or more, the resin can be fully polymerized through a crosslinking reaction, further improving the corrosion resistance of the coating film. More preferably, the number average molecular weight of the epoxy resin is 1000 or more, and even more preferably 2000 or more. On the other hand, by making the number average molecular weight of the epoxy resin 100,000 or less, the resin can be fully polymerized through a crosslinking reaction, further improving the corrosion resistance of the coating film. More preferably, the number average molecular weight of the epoxy resin is 20,000 or less, and even more preferably 10,000 or less.

[0058] Furthermore, among epoxy resins having a number average molecular weight within the aforementioned range, epoxy resins with an epoxy equivalent in the range of 1000 to 3500 are even more preferred. By having an epoxy equivalent of 1000 or more, the epoxy resin can be cross-linked more appropriately, further improving the corrosion resistance of the coating film. An epoxy equivalent of 1500 or more is more preferred, and 2000 or more is even more preferred. On the other hand, by having an epoxy equivalent of 3500 or less, excessive polymerization of the epoxy resin can be suppressed, maintaining the processability of the coating film 40, and further improving the corrosion resistance of the coating film. An epoxy equivalent of 3000 or less is more preferred, and 2500 or less is even more preferred.

[0059] Isocyanate resins

[0060] The isocyanate resin contained in the coating 40 of this embodiment functions as a curing agent, enabling the coating 40 to cure appropriately and exhibiting excellent corrosion resistance, processability, and adhesion. Here, the isocyanate resin of this embodiment refers to a resin having isocyanate groups or a low-molecular-weight compound having isocyanate groups. Examples of such isocyanate resins include polyisocyanate compounds and end-capped polyisocyanate compounds.

[0061] Examples of polyisocyanate compounds include aliphatic or alicyclic diisocyanate compounds, aromatic diisocyanate compounds, and triisocyanate compounds.

[0062] Here, examples of aliphatic or alicyclic diisocyanate compounds include hexamethylene diisocyanate, isophorone diisocyanate, and hydrogenated diphenylmethane diisocyanate. Examples of aromatic diisocyanate compounds include toluene diisocyanate and diphenylmethane-4,4'-diisocyanate. Examples of triisocyanate compounds include adducts of 1 mole of trimethylolpropane and 3 moles of the above-mentioned diisocyanates, trimers of diisocyanates such as hexamethylene diisocyanate and toluene diisocyanate.

[0063] Furthermore, the capped polyisocyanate compound is formed by capping the polyisocyanate compound exemplified above with a capping agent. As such a capping agent, it is preferable to have an adduct formed by addition to the isocyanate group that is stable at room temperature and dissociates to reform free isocyanate groups when a coating is formed by baking.

[0064] Examples of such capping agents include lactam-based capping agents, oxime-based capping agents, alcohol-based capping agents, phenol-based capping agents, and ester-based capping agents.

[0065] Here, examples of lactam-based capping agents include ε-caprolactam and γ-butyrolactam. Examples of oxime-based capping agents include methyl ethyl ketone oxime and cyclohexanone oxime. Examples of alcohol-based capping agents include methanol, ethanol, and isobutanol. Examples of phenol-based capping agents include phenol, p-tert-butylphenol, and cresol. Examples of ester-based capping agents include ethyl acetoacetate and methyl acetoacetate. It should be noted that, among the capping agents described above, methyl ethyl ketone oxime and ethyl acetoacetate are particularly preferred from the viewpoint of dissociation at low temperatures and stability in the condition of the coating used for film formation.

[0066] In the coating 40 of this embodiment, polyisocyanate compounds, such as those described above, or end-capped polyisocyanate compounds, or combinations of two or more, can be used alone. Furthermore, the compounds illustrated above are merely examples of isocyanate resins; other isocyanate resins may also be used.

[0067] Silica particles

[0068] The silica particles contained in the coating 40 of this embodiment are components included to prevent the components in the coating 40 from dissolving out of the coating and to further improve the corrosion resistance of the coating 40. Examples of such silica particles include fumed silica and colloidal silica. In the coating 40 of this embodiment, at least one of such silica particles is used.

[0069] Here, the average particle size of the fumed silica is preferably in the range of 5 to 40 nm. By making the average particle size of the fumed silica 5 nm or more, the corrosion resistance and adhesion of the coating film 40 can be further improved. The average particle size of the fumed silica is more preferably 7 nm or more. On the other hand, by making the average particle size of the fumed silica 40 nm or less, the corrosion resistance can be further improved while maintaining the smoothness of the coating film 40. The average particle size of the fumed silica is more preferably 16 nm or less.

[0070] Furthermore, the average particle size of the colloidal silica is preferably in the range of 5 to 200 nm. By making the average particle size of the colloidal silica 5 nm or more, the corrosion resistance and adhesion of the coating film 40 can be further improved. The average particle size of the colloidal silica is more preferably 9 nm or more, and even more preferably 12 nm or more. On the other hand, by making the average particle size of the colloidal silica 200 nm or less, the corrosion resistance can be further improved while maintaining the smoothness of the coating film 40. The average particle size of the colloidal silica is more preferably 60 nm or less, and even more preferably 22 nm or less.

[0071] Polyethylene Wax

[0072] The polyethylene wax contained in the coating 40 of this embodiment functions as a lubricant, further improving the processability of the surface-treated steel sheet 1 of this embodiment. Specifically, it is preferable to use a polyethylene wax with a density of 0.94 or higher, a molecular weight in the range of 1000 to 10000, and an acid value of 15 KOH mg / g or less. By using a polyethylene wax having the density, molecular weight, and acid value described above, the processability of the surface-treated steel sheet 1 can be further improved.

[0073] In addition, the coating 40 in this embodiment may contain various lubricants such as polyolefin lubricants, carboxylic acid ester lubricants, carboxylic acid metal salts, polyalkylene glycol lubricants, molybdenum disulfide, organosilicon compounds, and fluorine compounds, in addition to the polyethylene wax mentioned above.

[0074] The content of the above-mentioned components

[0075] In the coating 40 of this embodiment, the contents of the epoxy resin, isocyanate resin, silica particles and polyethylene wax preferably satisfy the following conditions.

[0076] That is, the content of epoxy resin is 30.0% by mass or more relative to the solid components of the coating (i.e., relative to the total mass of the coating 40), the content of isocyanate resin is 0.10 to 2.00 by mass relative to the epoxy resin, the content of silica particles is 5.0 to 50.0% by mass relative to the solid components of the coating (i.e., relative to the total mass of the coating 40), and the content of polyethylene wax is 0.1 to 10.0% by mass relative to the solid components of the coating (i.e., relative to the total mass of the coating 40). Preferably, the epoxy resin, isocyanate resin, silica particles, and polyethylene wax are contained in a manner that totals 100% by mass or less.

[0077] By ensuring that the epoxy resin content is 30.0% by mass or more relative to the total mass of the coating solids (i.e., coating 40), embrittlement of the coating 40 can be suppressed, and the processing adhesion of the coating 40 can be further improved. More preferably, the epoxy resin content is 45.0% by mass or more relative to the coating solids, and even more preferably, 50.0% by mass or more.

[0078] By making the content of isocyanate resin at 0.10 or more by mass ratio to the epoxy resin, a preferred cross-linking state between the epoxy resin and the isocyanate resin can be achieved, resulting in a coating film with excellent corrosion resistance. The content of isocyanate resin is more preferably 0.15 or more by mass ratio to the epoxy resin, and even more preferably 0.20 or more. On the other hand, by making the content of isocyanate resin at 2.00 or less by mass ratio to the epoxy resin, the adhesion of the coating film 40 can be further improved while maintaining its water resistance and alkali resistance. The content of isocyanate resin is more preferably 0.90 or less by mass ratio to the epoxy resin, and even more preferably 0.50 or less.

[0079] By ensuring that the content of silica particles is 5.0% by mass or more relative to the total mass of the coating solids (i.e., coating 40), the corrosion resistance of the coating 40 can be further improved. More preferably, the content of silica particles is 18.0% by mass or more relative to the coating solids, and even more preferably, 23.0% by mass or more. On the other hand, by ensuring that the content of silica particles is 50.0% by mass or less relative to the total mass of the coating solids, a decrease in the processability of the coating 40 can be prevented. More preferably, the content of silica particles is 38.0% by mass or less relative to the coating solids, and even more preferably, 33.0% by mass or less.

[0080] By making the content of polyethylene wax 0.1% by mass or more relative to the total mass of the coating solids (i.e., coating 40), the surface friction resistance of the coating 40 can be reduced, preventing coating peeling during processing. The content of polyethylene wax relative to the coating solids is more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more. On the other hand, by making the content of polyethylene wax 10.0% by mass or less relative to the total mass of the coating solids, the generation of uneven shrinkage patterns of the polyethylene wax can be prevented, resulting in a coating 40 with excellent appearance. The content of polyethylene wax relative to the coating solids is more preferably 3.0% by mass or less, and even more preferably 2.5% by mass or less.

[0081] Regarding other ingredients

[0082] In addition to the components described above, the coating 40 of this embodiment may also contain a methyl phenolic resin. By including a methyl phenolic resin in the coating 40, the coating film formation reaction can be promoted at lower temperatures (e.g., temperatures reaching a maximum of approximately 100-130°C). In this case, the content of the methyl phenolic resin relative to the content of the isocyanate resin is preferably set in the range of 0.1 to 10.0 by mass. This suppresses the decrease in the alkali resistance of the coating 40 and promotes the coating film formation reaction at lower temperatures.

[0083] In addition to the components described above, the coating 40 of this embodiment may also contain rust-preventive pigments, extender pigments, coloring pigments, rust inhibitors, dispersants, and stabilizers.

[0084] On Thickness

[0085] In the coating 40 of this embodiment, the thickness of the coating 40 is in the range of 0.5 to 2.0 μm. If the thickness of the coating 40 is less than 0.5 μm, the coating 40 is too thin, and the surface-treated steel sheet 1 cannot exhibit sufficient corrosion resistance. By setting the thickness of the coating 40 to 0.5 μm or more, the surface-treated steel sheet 1 can exhibit excellent corrosion resistance even when exposed to various fuels, such as gasoline. The thickness of the coating 40 is preferably 0.6 μm or more, and more preferably 0.7 μm or more.

[0086] On the other hand, when the thickness of the coating 40 exceeds 2.0 μm, the weldability and appearance of the surface-treated steel plate 1 may decrease, so it is not preferred. By setting the thickness of the coating 40 to 2.0 μm or less, the decrease in weldability and appearance can be suppressed, and excellent corrosion resistance can be exhibited. The thickness of the coating 40 is preferably 1.5 μm or less.

[0087] Here, the thickness d1 of coating 40 can be determined by direct observation from a cross-section. Specifically, the surface-treated steel plate of interest is embedded in room-temperature-drying epoxy resin in a cross-section where the thickness direction can be observed. The embedded surface is mechanically ground and then refined to a mirror finish, followed by observation using a scanning electron microscope (SEM). The thickness is measured at multiple arbitrary locations (e.g., 10 locations), and the average of the obtained thicknesses is taken as the thickness of coating 40. Alternatively, the coating 40 can be determined by combining elemental mapping based on energy dispersive X-ray spectrometry (EDS) and electron probe microanalysis (EPMA).

[0088] In addition, the film thickness can be determined by high-frequency glow discharge emission spectroscopy (RF-GD-OES) to measure the depth distribution from the surface side to the steel plate side of the surface-treated steel plate. The sputtering time corresponding to the depth distribution of the portion of the coating 40 is compared with the thickness of the coating 40 obtained by SEM observation from the above cross section, and thus converted into the thickness of the coating 40 per unit sputtering time.

[0089] It should be noted that the position of the interface between the coating 40 and the chromium-containing composite layer 30 side can also be determined using the method described above in the depth distribution obtained by high-frequency glow discharge emission spectroscopy. That is, in the chromium depth distribution, focusing on the intensity peak within the range corresponding to the chromium-containing composite layer 30, the position where the intensity of chromium from the peak towards the coating 40 side is halved can be taken as the position of the interface between the coating 40 and the chromium-containing composite layer 30.

[0090] Here, given the detailed characteristics of the coating 40 determined from surface-treated steel sheets already processed into various components such as fuel tanks, test pieces are cut from areas of the component of interest that have never undergone joining processes such as welding, deep drawing, or bending, and these sections are used as the objects of analysis. The size of the cut test pieces is similar to that of a flat sheet, for example, approximately 50 × 50 mm. Various measurements are then performed using these test pieces.

[0091] For example, to determine the amount of the aforementioned components present in the portion of the test piece corresponding to coating 40, Fourier transform infrared spectroscopy (FT-IR) can be used to analyze the portion corresponding to coating 40. The determination is based on whether peaks from epoxy groups or isocyanate groups are detected in the obtained infrared absorption spectrum. If a peak from epoxy groups is detected in the obtained infrared absorption spectrum, it can be determined that coating 40 of interest contains epoxy resin. Similarly, if a peak from isocyanate groups is detected in the obtained infrared absorption spectrum, it can be determined that coating 40 of interest contains isocyanate resin.

[0092] Furthermore, regarding the content of epoxy resin and isocyanate resin, an epoxy resin / isocyanate resin mixed coating film with a known mixing ratio of epoxy resin and isocyanate resin is prepared in advance. Using FT-IR, standard curves are pre-prepared for the peak intensities of the epoxy group peak and the isocyanate group peak of this mixed coating film. Based on this, in the FT-IR analysis obtained as described above, the content of epoxy resin and isocyanate resin can be determined from the peak intensities of each obtained peak.

[0093] In addition, the number-average molecular weight of epoxy resin can be determined as follows: a sample is collected from the portion of the test piece corresponding to coating 40. For this sample, the molecular structure of the resin is analyzed by field desorption-mass spectrometry (FD-MS) to determine its molecular weight distribution.

[0094] Furthermore, the presence or absence of polyethylene wax can be determined as follows: The test piece is immersed in a heated solvent (more specifically, 2-butanone is heated to approximately 80°C) to dissolve the polyethylene wax into the solvent, and the extract is analyzed using FT-IR. The presence or absence of a peak originating from the methylene group in the resulting infrared absorption spectrum is used for determination. If a peak originating from the methylene group is detected in the resulting infrared absorption spectrum, it can be determined that the coating of interest 40 contains polyethylene wax. Additionally, the percentage of polyethylene wax present can be determined based on the mass difference before and after the extraction of polyethylene wax, as described above.

[0095] Furthermore, the presence or absence of silica particles can be determined, for example, as follows: After confirming the presence of Si and O elements in the portion of the test piece corresponding to coating 30 using elemental analysis methods such as fluorescence X-ray diffraction, and then performing FT-IR analysis on the portion corresponding to coating 30, the presence or absence of peaks originating from Si-O bonds in the resulting infrared absorption spectrum is used to determine whether these peaks are detected. If the presence of Si and O elements is confirmed by elemental analysis, and peaks originating from Si-O bonds are detected in the infrared absorption spectrum, it can be determined that the coating 30 of interest contains silica particles.

[0096] Above, refer to Figure 1 The overall structure of the surface-treated steel plate 1 in this embodiment has been described.

[0097] <Regarding the first composite layer 50>

[0098] Next, refer to Figure 2 and Figure 3 The first composite layer of the surface-treated steel plate 1 in this embodiment will be described in detail. Figure 2 and Figure 3 This is an explanatory diagram used to illustrate the first composite layer in the surface-treated steel sheet of this embodiment.

[0099] In the surface-treated steel sheet 1 of this embodiment, the first composite layer is located between the steel sheet 10 and the coating layer 20. Here, Figure 1 The layer structure of the surface-treated steel plate 1 in the region R enclosed by the dashed line (in other words, the layer structure near the interface between the steel plate 10 and the coating 20) is shown enlarged and schematically. Figure 2 exhibit.

[0100] exist Figure 2 In this process, the layer located between the steel plate 10 and the coating 20 is the first composite layer 50. This first composite layer 50 is a composite layer containing iron (Fe) from the steel plate 10 and Zn and Ni from the coating 20. The first composite layer 50 is a layer formed by the interdiffusion and composite of Fe from the steel plate 10 and Zn and Ni from the coating 20 through a manufacturing method described later.

[0101] The presence of the first composite layer 50 improves the adhesion between the steel plate 10 and the coating 20. Combined with the detailed configuration of the chromium-containing composite layer 30 described below, the surface-treated steel plate 1 of this embodiment can further improve the adhesion and corrosion resistance of the coating after forming.

[0102] Here, the average thickness of the first composite layer 50 ( Figure 2The thickness d1 of the first composite layer 50 is preferably in the range of 0.01 to 0.50 μm. By setting the average thickness d1 of the first composite layer 50 to 0.01 μm or more, the adhesion between the steel plate 10 and the coating 20 can be further improved. The average thickness d1 of the first composite layer 50 is more preferably 0.03 μm or more, and even more preferably 0.05 μm or more. On the other hand, by setting the average thickness d1 of the first composite layer 50 to 0.50 μm or less, sufficient formability can be obtained. The average thickness d1 of the first composite layer 50 is more preferably 0.40 μm or less, and even more preferably 0.30 μm or less.

[0103] Here, refer to Figure 3 The method for identifying the first composite layer 50 and the method for measuring its average thickness are described in detail.

[0104] In this embodiment, the method for determining the first composite layer 50 and the method for measuring its average thickness focus on the depth distribution of iron, zinc, and nickel obtained by high-frequency glow discharge emission spectroscopy (RF-GD-OES) from near the interface of the coating layer 20 to near the interface of the steel plate 10.

[0105] That is, in the depth distribution of iron, zinc, and nickel obtained by RF-GD-OES from near the interface of coating 20 to near the interface of steel plate 10, the position where the peak of zinc or nickel, as an element from coating 20, is halved is defined as the position of the interface on the coating 20 side of the first composite layer 50. At this time, the position closer to the steel plate 10 than the position defined by zinc is taken as the position of the interface on the coating 20 side of the first composite layer 50. For example, in Figure 3 In the example shown, when the peak intensity of Zn or Ni is Ia on the side closer to the steel plate 10, the position corresponding to the sputtering time t1 with an intensity of 0.5 × Ia for the element of interest is the position of the interface on the coating 20 side of the first composite layer 50.

[0106] Furthermore, in this embodiment, the position where the peak of iron, an element from steel plate 10, is halved is defined as the position of the interface on the steel plate 10 side of the first composite layer 50. For example, in Figure 3 In the example shown, when the peak intensity of Fe is Ib, the position corresponding to the sputtering time t2 when the intensity of Fe is 0.5×Ib becomes the position of the interface on the steel plate 10 side of the first composite layer 50.

[0107] Based on the above definition, the area between the two specified interfaces constitutes the range of the first composite layer 50. Figure 3In the example shown, the positions corresponding to sputtering times t1 to t2 are designated as the positions of the first composite layer 50, and the thickness corresponding to sputtering times t1 to t2 is designated as the average thickness of the first composite layer 50. Here, based on the time required to sputter a layer with a known average thickness, insights related to the thickness per unit sputtering time can be obtained, thus enabling the conversion of sputtering time into thickness.

[0108] It should be noted that when the first composite layer 50 is determined by a surface-treated steel sheet that has already been processed into various components such as fuel tanks, test pieces can be cut from the parts of the component of interest that have never undergone various welding or joining processes, deep drawing, bending, or other processing, and used as the object of analysis. In this case, the size of the cut test piece is the same as the size when the test piece is made into a flat plate, for example, about 50×50mm.

[0109] <Detailed structure of chromium-containing composite layer 30>

[0110] Next, refer to Figure 4 and Figure 5 The detailed structure of the chromium-containing composite layer 30 in this embodiment will be described. Figure 4 and Figure 5 This is an explanatory diagram used to illustrate the chromium-containing composite layer in the surface-treated steel sheet of this embodiment.

[0111] The chromium-containing composite layer 30 of this embodiment, as detailed in the manufacturing method described below, is a layer formed by combining a portion of a plating layer, a chromate coating formed on the plating layer, and a portion of a coating film formed on the chromate coating layer through a manufacturing process.

[0112] like Figure 4 As schematically shown, the chromium-containing composite layer 30 has a second composite layer 60 located on the side of the plating layer 20 and a third composite layer 70 located on the side of the coating film 40. The second composite layer 60 is a composite layer containing chromium (Cr), Zn and Ni, and the third composite layer 70 is a composite layer containing silicon (Si), nitrogen (N) and Cr.

[0113] Based on the assumption that the composite layer contains Cr, Zn and Ni, the second composite layer 60 is a composite layer of Cr present in the chromate coating before composite formation and Zn and Ni present in the coating before composite formation.

[0114] Furthermore, based on the assumption that the composite layer contains Si, N, and Cr, the third composite layer 70 is a composite layer consisting of Cr present in the chromate coating before composite formation, Si present in the silica particles in the coating before composite formation, and N present in the isocyanate resin.

[0115] In addition, such as Figure 4As illustrated, sometimes there is a layer 80 between the second composite layer 60 and the third composite layer 70 that is different from these two composite layers.

[0116] Layer 80, which can exist between the second composite layer 60 and the third composite layer 70, can be, for example, a layer from an uncomposite chromate coating, or a composite layer containing Cr, Zn, Ni, Si, and N (in other words, a layer formed by further composited second composite layer 60 and third composite layer 70). Additionally, the chromium-containing composite layer 30 may sometimes be a layer consisting of the second composite layer 60 and the third composite layer 70 without the layer 80 described above. Furthermore, the second composite layer 60 and the third composite layer 70 may overlap.

[0117] By including the second composite layer 60 and the third composite layer 70 as described above, the adhesion between the chromium-containing composite layer 30 and the plating layer 20, as well as the adhesion between the chromium-containing composite layer 30 and the coating film 40, are improved. In conjunction with the first composite layer 50 as described above, the surface-treated steel sheet 1 of this embodiment can further improve the adhesion and corrosion resistance of the coating after forming.

[0118] Here, the average thickness of the second composite layer 60 ( Figure 4 The thickness d2 of the second composite layer 60 is preferably in the range of 0.1 to 1.5 μm. By setting the average thickness d2 of the second composite layer 60 to 0.1 μm or more, the adhesion between the plating layer 20 and the chromium-containing composite layer 30 can be further improved. The average thickness d2 of the second composite layer 60 is more preferably 0.2 μm or more, and even more preferably 0.5 μm or more. On the other hand, by setting the average thickness d2 of the second composite layer 60 to 1.5 μm or less, the stress applied to the steel sheet during forming can be appropriately dispersed. The average thickness d2 of the second composite layer 60 is more preferably 1.2 μm or less, and even more preferably 1.0 μm or less.

[0119] In addition, the average thickness of the third composite layer 70 ( Figure 4 The thickness d3 of the third composite layer 70 is preferably in the range of 0.1 to 1.5 μm. By setting the average thickness d3 of the third composite layer 70 to 0.1 μm or more, the adhesion between the chromium-containing composite layer 30 and the coating film 40 can be further improved. The average thickness d3 of the third composite layer 70 is more preferably 0.2 μm or more, and even more preferably 0.5 μm or more. On the other hand, by setting the average thickness d3 of the third composite layer 70 to 1.5 μm or less, the stress applied to the steel plate during forming can be appropriately dispersed. The average thickness d3 of the third composite layer 70 is more preferably 1.2 μm or less, and even more preferably 1.0 μm or less.

[0120] Furthermore, the ratio (d2 / d1) of the average thickness d2 of the second composite layer to the average thickness d1 of the first composite layer is preferably 0.2 to 150.0. Additionally, the ratio (d3 / d1) of the average thickness d3 of the third composite layer to the average thickness d1 of the first composite layer is preferably 0.2 to 150.0.

[0121] By making the ratio (d2 / d1) 0.2 or higher, the adhesion between the plating layer 20 and the chromium-containing composite layer 30 can be further improved. The ratio (d2 / d1) is more preferably 0.5 or higher, and even more preferably 1.7 or higher. On the other hand, by making the ratio (d2 / d1) 150.0 or lower, the stress applied to the steel sheet during forming can be appropriately dispersed. The ratio (d2 / d1) is more preferably 40.0 or lower, and even more preferably 20.0 or lower.

[0122] By making the ratio (d3 / d1) 0.2 or higher, the adhesion between the chromium-containing composite layer 30 and the coating film 40 can be further improved. The ratio (d3 / d1) is more preferably 0.5 or higher, and even more preferably 1.7 or higher. On the other hand, by making the ratio (d3 / d1) 150.0 or lower, the stress applied to the steel sheet during forming can be appropriately dispersed. The ratio (d3 / d1) is more preferably 40.0 or lower, and even more preferably 20.0 or lower.

[0123] Here, refer to Figure 5 and Figure 6 The identification methods for the second composite layer 60 and the third composite layer 70, as well as the methods for measuring the average thicknesses d2 and d3 of each composite layer, are described in detail.

[0124] In this embodiment, the method for determining the second composite layer 60 and the method for measuring its average thickness focus on the depth distribution of chromium, zinc, and nickel obtained by high-frequency glow discharge emission spectroscopy (RF-GD-OES) from near the interface of the coating 40 to near the interface of the plating layer 20.

[0125] That is, in the depth distribution of chromium, zinc, and nickel obtained by RF-GD-OES from near the interface of coating 40 to near the interface of plating 20, the position where the peak of the element from the chromium-containing composite layer 30 is halved is defined as the position of the interface on the plating 20 side of the second composite layer 60. For example, in Figure 5 In the example shown, when the peak intensity of Cr is Ic, the position corresponding to the sputtering time t3 when the intensity of Cr is 0.5×Ic is the position of the interface on the coating 20 side of the second composite layer 60.

[0126] Furthermore, in this embodiment, the position where the peak of zinc or nickel, which are elements from the plating layer 20, is halved is defined as the position of the interface on the coating layer 40 side of the second composite layer 60. At this time, the position closer to the coating layer 40 side of the position defined by zinc and the position defined by nickel is defined as the position of the interface on the coating layer 40 side of the second composite layer 60. For example, in Figure 5 In the example shown, when the peak intensity of Zn or Ni is Id on the side closer to the coating 40, the position corresponding to the sputtering time t4 with an intensity of 0.5×Id for the element of interest is the position of the interface on the coating layer 40 side of the second composite layer 60.

[0127] Based on the above definition, the area between the two specified interfaces constitutes the range of the second composite layer 60. Figure 5 In the example shown, the positions corresponding to sputtering times t3 to t4 are the positions of the second composite layer 60, and the thickness corresponding to sputtering times t3 to t4 is the average thickness of the second composite layer 60. Here, based on the time required to sputter and form a layer of known thickness, insights into the thickness per unit sputtering time can be obtained, thus allowing the sputtering time to be converted into thickness.

[0128] Furthermore, in this embodiment, the method for determining the third composite layer 70 and the method for measuring its average thickness focus on the depth distribution of silicon and chromium obtained by high-frequency glow discharge emission spectroscopy (RF-GD-OES) from near the interface of the coating 40 to near the interface of the plating layer 20.

[0129] That is, in the depth distribution of silicon and chromium obtained by RF-GD-OES from near the interface of coating 40 to near the interface of plating 20, the position where the peak of silicon, as an element from coating 40, is halved is defined as the position of the interface on the plating 20 side of the third composite layer 70. For example, in Figure 6 In the example shown, when the peak intensity of Si is Ie, the position corresponding to the sputtering time t5 when the Si intensity is 0.5×Ie is the position of the interface on the coating 20 side of the third composite layer 70.

[0130] Furthermore, in this embodiment, the position where the peak of chromium from the chromium-containing composite layer 30 is halved is defined as the position of the interface on the coating 40 side of the third composite layer 70. For example, in Figure 6 In the example shown, when the peak intensity of Cr is If, the position corresponding to the sputtering time t6 when the intensity of Cr is 0.5 × If is the position of the interface on the coating 40 side of the third composite layer 70.

[0131] Based on the above definition, the area between the two specified interfaces constitutes the range of the third composite layer 70. Figure 6In the example shown, the positions corresponding to sputtering times t5 to t6 are the positions of the third composite layer 70, and the thickness corresponding to sputtering times t5 to t6 is the average thickness of the third composite layer 70. Here, based on the time required to sputter and form a layer of known thickness, insights into the thickness per unit sputtering time can be obtained, thus allowing the sputtering time to be converted into thickness.

[0132] It should be noted that, when determining the detailed structure of the aforementioned chromium-containing composite layer 30 based on the surface-treated steel sheet that has already been processed into various components such as fuel tanks, test pieces can be cut from the parts of the component of interest that have never undergone various joining processes such as welding, deep drawing, bending, etc., and used as the analysis objects. In this case, the size of the cut test piece is the same as the size when the test piece is made into a flat plate, for example, approximately 50×50 mm. Based on this, the measurements described above can be performed using the obtained test piece.

[0133] Above, refer to Figures 1-6 The surface-treated steel sheet of this embodiment has been described in detail.

[0134] (Regarding the manufacturing method of surface-treated steel sheets)

[0135] Next, refer to Figure 7 The manufacturing method of the surface-treated steel sheet of this embodiment described above will be explained. Figure 7 This is a flowchart illustrating an example of the process for manufacturing a surface-treated steel sheet according to this embodiment.

[0136] like Figure 7 As shown, the manufacturing method of the surface-treated steel sheet of this embodiment includes a pretreatment process (step S11), an electroplating process (step S13), a cleaning process (step S15), an electrolytic chromate treatment process (step S17), a cleaning process (step S19), and a coating formation process (step S21).

[0137] The pretreatment process (step S11) is a process used to perform various pretreatments on the steel plate used as the base material, such as alkali degreasing, water washing, and pickling, to produce a clean steel plate surface. There are no special provisions for this pretreatment process; the usual treatment methods can be followed.

[0138] The electroplating process (step S13) is a process of forming a coating on the surface of a steel sheet, which serves as the base material, by electroplating. In this electroplating process, a plating bath (containing Ni and other elements as needed, with the balance being Zn) with a composition capable of achieving the desired coating composition is prepared. Using this plating bath and the steel sheet, electroplating is performed with a specified current density and energizing time.

[0139] Here, the bath temperature for plating is preferably set to 50–70°C. Furthermore, the current density is preferably set to 5–100 A / dm³. 2 Furthermore, the power-on time is preferably set to 10 to 100 seconds, and more preferably 12 to 50 seconds.

[0140] As described above, in the electroplating process (step S13) of this embodiment, a longer energizing time is used compared to the usual zinc-nickel electroplating. As a result, the immersion time of the steel sheet in the plating bath is longer, and consequently, the diffusion of Fe in the steel sheet is also increased. Therefore, composite formation is performed in a larger area than usual, forming the first composite layer 50 of this embodiment with an appropriate thickness.

[0141] It should be noted that, regarding the coating adhesion amount in this electroplating process, in order to achieve the coating adhesion amount of coating 20 as described above, it is preferably set to 2-50 g / m². 2 Within the range.

[0142] The cleaning process (step S15) is a process of cleaning the steel plate after the electroplating process. There are no special provisions for this cleaning process; the usual processing methods can be followed.

[0143] The electrolytic chromate treatment process (step S17) is a process of performing electrolytic chromate treatment on the steel sheet (more specifically, the steel sheet with a coating) after the cleaning process.

[0144] As an electrolytic bath for electrolytic chromate treatment, it is preferable to use a Cr-based electrolyte. 6+ An electrolytic chromate bath is used as the main component, and also contains sulfuric acid or halide ions, or an electrolytic bath that further contains inorganic colloids such as SiO2 and / or TiO2, and metal ions such as Co and / or Zn, relative to the above components. Using this electrolytic chromate bath, Cr ions are electrodeposited in a trivalent state to form a Cr-based chromate bath. 3+ A poorly soluble chromate coating is formed. By forming a poorly soluble chromate coating, chromium can be prevented from leaching into the treatment solutions used for subsequent chemical conversion treatments and electrodeposition coatings on the surface-treated steel sheet.

[0145] Here, the bath temperature for the electrolytic chromate bath is preferably set to 50–70°C. Furthermore, the current density is preferably set to 5–100 A / dm³. 2 Furthermore, the power-on time is preferably set to 1 to 10 seconds, and more preferably 2 to 8 seconds.

[0146] In the manufacturing method of the surface-treated steel sheet of this embodiment, as a chromate treatment, an electrolytic chromate treatment based on a specific current density and energizing time, as described above, is performed. As a result, the dissolution of components from the pre-formed coating also occurs, and in conjunction with the coating formation process described below, a chromium-containing composite layer 30 of this embodiment is formed.

[0147] Furthermore, regarding the amount of chromate coating adhered during the electrolytic chromate treatment process, in order to achieve the Cr adhesion amount described above, it is preferable to have an amount of 10 to 200 mg / m³, calculated in terms of metallic Cr. 2 Within the range.

[0148] The cleaning process (step S19) is a process of cleaning the steel plate after the electrolytic chromate treatment process. There are no special provisions for this cleaning process; the usual treatment method can be followed.

[0149] The coating formation process (step S21) is a process of treating a steel sheet that has undergone a cleaning process (more specifically, a steel sheet with a plating and chromate coating) to form a coating.

[0150] In this process, a coating for forming a film is prepared in a manner that achieves the component content of the coating film 40 as described above. In this coating, each component of the coating film 40 as described above is dissolved or dispersed in an organic solvent.

[0151] In this embodiment, ketone-based organic solvents are preferably used as the organic solvent in the coating film forming process. By containing ketone-based organic solvents at a rate of 40% or more by mass of the coating material, and adjusting the concentration of the coating solids to 10-50% by mass, a uniform coating film can be easily formed.

[0152] Examples of ketone-based organic solvents include methyl isobutyl ketone, acetone cyclohexanone, and isophorone. Other ketone-based organic solvents may also be used. However, it is important to avoid solvents that can react with isocyanate resins when selecting a solvent.

[0153] The prepared coating is applied to the surface of the steel plate after the cleaning process using various methods such as roller coating and curtain coating. The thickness of the coating is preferably adjusted to take into account the formation of the third composite layer 70, and the thickness of the dried coating film 40 is preferably within the range of 0.5 to 2.0 μm.

[0154] Next, the coated steel plate is held at a maximum temperature of 100–200°C for 10–60 seconds, thereby causing the solvent in the coating to evaporate and the solid components of the coating to dry and solidify, forming a coating film 40. Furthermore, during this heat treatment process, the components contained in the coating, the components contained in the chromate coating (and, depending on the situation, the components contained in the first composite layer, the plating layer, etc.) undergo thermal diffusion, ultimately forming the previously described chromium-containing composite layer 30.

[0155] After the above-described processes, the surface-treated steel sheet 1 of this embodiment described above is manufactured.

[0156] Above, refer to Figure 7 The manufacturing method of the surface-treated steel sheet according to this embodiment is described in detail.

[0157] Example

[0158] Hereinafter, the surface-treated steel sheet of the present invention will be specifically described with reference to embodiments and comparative examples. It should be noted that the embodiments shown below are merely examples of the surface-treated steel sheet of the present invention, and the surface-treated steel sheet of the present invention is not limited to the examples described below.

[0159] (1) Galvanized-nickel steel sheet

[0160] The galvanized steel sheet is a zinc-nickel coating formed by electroplating on cold-rolled steel sheet (manufactured by Nippon Steel Corporation) with a specified nickel content. It should be noted that the coating adhesion amount refers to the coating adhesion amount on each side. The zinc-nickel steel sheet coating is degreased by a 5-second spray using a commercially available sodium silicate-based alkaline cleaning solution, followed by rinsing and drying.

[0161] (2) Chemical conversion treatment and coating treatment

[0162] Prepare chromate chemical conversion treatment solution and coating.

[0163] In the chemical conversion treatment of chromates, electrolytic chromate treatment is carried out at a liquid temperature of 50–70℃ and a current density of 5–100 A / dm³. 2 Electrolysis was performed within the specified time range as shown in Tables 1-1 to 1-3 below. Then, each coated steel sheet was washed and dried.

[0164] In addition, a coating is prepared by mixing the epoxy resin, isocyanate resin, silica particles, and polyethylene wax shown below in a specified proportion. It should be noted that the contents shown in Tables 1-1 to 1-3 represent the contents in the coating film (solid components).

[0165] [Epoxy Resin]

[0166] Epoxy Resin A: Manufactured by Mitsubishi Chemical Corporation, No. 1007, Molecular Weight: 2900, Epoxy Equivalent: 1750-2200

[0167] Epoxy Resin B: Manufactured by Mitsubishi Chemical Corporation, No. 827, Molecular Weight: 360, Epoxy Equivalent: 180-190

[0168] Epoxy Resin C: Manufactured by Mitsubishi Chemical Corporation, No. 1001, Molecular Weight: 900, Epoxy Equivalent: 450-500

[0169] Epoxy Resin D: H-360 manufactured by DIC Corporation, molecular weight: 25000

[0170] Epoxy Resin E: Manufactured by Mitsubishi Chemical Corporation, No. 4250, Molecular Weight: 60000, Epoxy Equivalent: 7500-8900

[0171] [Isocyanate resin]

[0172] Isocyanate A: Hexamethylene diisocyanate manufactured by Tosoh Corporation

[0173] Isocyanate B: Ethyl acetoacetate end-capped

[0174] [Silica particles]

[0175] Fumed silica

[0176] Fumed silica A: AEROSIL 300 manufactured by AEROSIL Corporation of Japan (average particle size: 7nm)

[0177] Fumed silica B: AEROSIL 200 manufactured by AEROSIL Corporation of Japan (average particle size: 12nm)

[0178] Fumed silica (C): AEROSIL R972 CF manufactured by AEROSIL Corporation of Japan (average particle size: 16 nm)

[0179] Fumed silica D: AEROSIL MOX80 manufactured by AEROSIL Corporation of Japan (average particle size: 30nm)

[0180] Fumed silica E: AEROSIL OX50 manufactured by AEROSIL Corporation of Japan (average particle size: 40nm)

[0181] Colloidal silica

[0182] Colloidal silica A: SNOWTEX N manufactured by Nissan Chemical Co., Ltd. (average particle size: 12nm)

[0183] Colloidal silica B: SNOWTEX ST-NXS manufactured by Nissan Chemical Co., Ltd. (average particle size: 5nm)

[0184] Colloidal silica C: SNOWTEX ST-NS manufactured by Nissan Chemical Co., Ltd. (average particle size: 9nm)

[0185] Colloidal silica D: SNOWTEX ST-N-40 manufactured by Nissan Chemical Co., Ltd. (average particle size: 22nm)

[0186] Colloidal silica E: SNOWTEX ST-YL manufactured by Nissan Chemical Co., Ltd. (average particle size: 60nm)

[0187] Colloidal silica F: SNOWTEX MP-2040 manufactured by Nissan Chemical Co., Ltd. (average particle size: 200nm)

[0188] Colloidal silica G: SNOWTEX MP-454M manufactured by Nissan Chemical Co., Ltd. (average particle size: 450nm)

[0189] [Polyethylene wax]

[0190] CERIDUST 3620 manufactured by Clariant Chemicals Co., Ltd.

[0191] The coating adjusted as described above is applied using a roller coater and held at the maximum reach temperature (PMT) of 165–185°C for 10–60 seconds to allow it to dry and cure. The thickness of the dried and cured coating is shown in Tables 1-1 to 1-3.

[0192] For the surface-treated steel sheets obtained above, the depth distribution of elements Fe, Zn, Ni, Cr, Si, and N was measured using the method described above based on RF-GD-OES. This confirmed whether the specified elements were present in each composite layer. The results are shown in Tables 1-1 to 1-3.

[0193] [Table 1-1]

[0194]

[0195] [Table 1-2]

[0196]

[0197] [Table 1-3]

[0198]

[0199] In addition, the surface treatment properties of the obtained steel plates were evaluated, including the tightness of the machined area, the corrosion resistance of the machined area, the corrosion resistance of the surface, and the corrosion resistance after degreasing. The evaluation methods and criteria are described below. The results are summarized in Tables 2-1 to 2-3 below.

[0200] <Post-processing sealing>

[0201] Test pieces were cut from various surface-treated steel plates, with a punch diameter of: Punch shoulder radius (R): 5mm, blank diameter: The cylindrical cup deep-drawing process was carried out under the conditions of a drawing ratio of 2.0 and a pressing pressure of 1t. For the formed product obtained by the above-described cylindrical cup deep-drawing process, after cross-scribing the main body of the cylindrical cup, it was immersed in boiling water for 1 hour, and the post-processing airtightness was evaluated. It should be noted that the cross-scribing was performed at two opposite points on the main body of the deep-drawn cylindrical cup, located at the midpoint between the rolling direction (L direction) and the orthogonal direction (C direction) of the steel plate (at a 45° angle to both directions). The cross-scribing was performed as follows: starting from a position 5mm above the deep-drawn end face of the main body, towards the bottom surface of the cylindrical cup, one scribing line was 40mm long, and the two scribing lines were orthogonal at 90°. At the two cross-scribing points on the main body of the deep-drawn cylindrical cup, the peel width of the coating from the scribing point was measured, and the largest peel width was scored. A score of 2 or higher was considered acceptable.

[0202] [Evaluation Criteria]

[0203] Rating 5: Below 0.1mm

[0204] 4: 0.1mm or more but less than 0.2mm

[0205] 3: ≥0.2mm but <0.3mm

[0206] 2: ≥0.3mm and ≤0.4mm

[0207] 1: Exceeding 0.4mm

[0208] <Corrosion resistance after processing>

[0209] Test pieces were cut from various surface-treated steel plates. After protruding the ball head to a height of 7 mm using an Ericsson testing machine, a 500-hour salt spray test (JIS Z 2371:2015) was conducted. The evaluation was based on the area ratio of white rust formation at the ball head protrusion. The end face and back face were sealed for evaluation. A score of 2 or higher was considered acceptable.

[0210] [Evaluation Criteria]

[0211] Rating 5: Below 5%

[0212] 4: More than 5% and less than or equal to 7%

[0213] 3: More than 7% and less than or equal to 10%

[0214] 2: More than 10% and less than or equal to 12%

[0215] 1: More than 12%

[0216] <Flat surface corrosion resistance>

[0217] Cut test pieces from each surface-treated steel plate, conduct a 500-hour salt spray test (JIS Z 2371: 2015), and evaluate based on the area ratio of white rust generation. Seal the end faces and back faces for evaluation. A score of 2 or above is considered qualified.

[0218] [Evaluation criteria]

[0219] Score 5: 1% or less

[0220] 4: More than 1% and less than or equal to 3%

[0221] 3: More than 3% and less than or equal to 5%

[0222] 2: More than 5% and less than or equal to 10%

[0223] 1: More than 10%

[0224] <Corrosion resistance after degreasing>

[0225] Cut test pieces from each surface-treated steel plate, use a commercially available sodium metasilicate-based alkaline cleaning solution, spray and degrease for 2 minutes at 60°C, then conduct a 500-hour salt spray test (JIS Z 2371: 2015), and evaluate based on the area ratio of white rust generation on the flat surface. Seal the end faces and back faces for evaluation. A score of 2 or above is considered qualified.

[0226] [Evaluation criteria]

[0227] Score 5: 5% or less

[0228] 4: More than 5% and less than or equal to 7%

[0229] 3: More than 7% and less than or equal to 10%

[0230] 2: More than 10% and less than or equal to 12%

[0231] 1: More than 12% [Table 2-1]

[0232]

[0233] [Table 2-2]

[0234]

[0235] [Table 2-3]

[0236]

[0237] As can be seen from Tables 2-1 to 2-3 above, the surface-treated steel sheet corresponding to the embodiments of the present invention shows that the processing part sealing, processing part corrosion resistance, planar corrosion resistance and degreasing corrosion resistance are all qualified. On the other hand, the surface-treated steel sheet corresponding to the comparative examples of the present invention has at least one unqualified result in processing part sealing, processing part corrosion resistance, planar corrosion resistance or degreasing corrosion resistance.

[0238] For example, in Comparative Examples No. 1 and No. 8, the amount of Cr adhering to the chromium-containing composite layer was outside the scope of this invention due to the energizing time during the electrolytic chromate treatment. Therefore, the adhesion of the processed area, the corrosion resistance of the processed area, the corrosion resistance of the plane, and the corrosion resistance after degreasing were all unqualified. Similarly, in Comparative Examples No. 9 and No. 16, the amount of Cr adhering to the chromium-containing composite layer was outside the scope of this invention due to the chromium concentration in the electrolytic bath used. Therefore, the adhesion of the processed area, the corrosion resistance of the processed area, the corrosion resistance of the plane, and the corrosion resistance after degreasing were all unqualified. Furthermore, in Comparative Examples No. 17 and No. 24, the film thickness of the coating was outside the scope of this invention due to inappropriate conditions of the roller coater during coating application. Therefore, the adhesion of the processed area, the corrosion resistance of the processed area, the corrosion resistance of the plane, and the corrosion resistance after degreasing were all unqualified.

[0239] Comparative Example No. 98 failed in its processing part sealing, processing part corrosion resistance, surface corrosion resistance, and post-degreasing corrosion resistance because the amount of Zn-Ni coating adhered was outside the scope of this invention. Similarly, Comparative Example No. 99 failed in its processing part sealing because the amount of Zn-Ni coating adhered was outside the scope of this invention.

[0240] Comparative Example No. 100 failed to meet the requirements for corrosion resistance of the machined part, surface corrosion resistance, and corrosion resistance after degreasing because the chromium concentration in the electrolytic bath used was outside the scope of this invention. Similarly, Comparative Example No. 101 failed to meet the requirements for sealing of the machined part because the chromium concentration in the electrolytic bath used was outside the scope of this invention. Furthermore, Comparative Example No. 102 failed to meet the requirements for corrosion resistance of the machined part, surface corrosion resistance, and corrosion resistance after degreasing because the temperature of the electrolytic bath used was set to 40°C, resulting in a chromium content in the chromium-containing composite layer outside the scope of this invention.

[0241] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to these examples. Various modifications and alterations will be readily apparent to anyone skilled in the art to which this invention pertains, within the scope of the technical concept set forth in the claims, and these are, of course, also understood to fall within the technical scope of this invention.

[0242] The embodiments disclosed herein are illustrative and not restrictive in all respects. The above embodiments can be omitted, substituted, or modified in various ways without departing from the appended claims, the structure and spirit of the invention as described below, which fall within the technical scope of the invention. For example, the constituent elements of the above embodiments can be arbitrarily combined without impairing their effects. Furthermore, based on such arbitrary combinations, the functions and effects of each constituent element involved in the combination can naturally be obtained, and according to the description herein, other functions and effects that can be understood by those skilled in the art can be obtained.

[0243] Furthermore, the effects described in this specification are merely illustrative or exemplary and not limiting. That is, the technology involved in this invention can achieve other effects, in addition to those described above, or in place of those described above, that are obvious to those skilled in the art based on the description herein.

[0244] It should be noted that the following configurations also fall within the technical scope of this invention. (1)

[0246] A surface-treated steel sheet, comprising:

[0247] A coating, which is located on at least one surface of a steel plate and contains zinc and nickel;

[0248] The first composite layer is located between the steel plate and the coating, and contains Fe, Zn and Ni;

[0249] A chromium-containing composite layer, situated above the plating layer, and containing at least Cr; and

[0250] A coating film, situated on top of the chromium-containing composite layer, contains epoxy resin, isocyanate resin, and silica particles.

[0251] The coating thickness is 2-50 g / m² per single side. 2 ,

[0252] The chromium-containing composite layer comprises:

[0253] A second composite layer, located on the side of the coating, contains Cr, Zn, and Ni; and

[0254] The third composite layer, located on the coating side, contains Si, N, and Cr.

[0255] The adhesion amount of the chromium-containing composite layer, calculated based on metallic Cr, is 10–200 mg / m³. 2 ,

[0256] The thickness of the coating is 0.5 to 2.0 μm. (2)

[0258] According to the surface-treated steel plate described in (1), wherein,

[0259] The ratio (d2 / d1) of the average thickness d2 of the second composite layer to the average thickness d1 of the first composite layer is 0.2 to 150.0.

[0260] The ratio (d3 / d1) of the average thickness d3 of the third composite layer to the average thickness d1 of the first composite layer is 0.2 to 150.0. (3)

[0262] According to the surface-treated steel sheet described in (1) or (2), wherein,

[0263] The coating contains polyethylene wax in a proportion of 0.1 to 10.0% by mass relative to the solid content of the coating.

[0264] The epoxy resin content is 30.0% by mass or more relative to the solid content of the coating film.

[0265] The content of the isocyanate resin relative to the epoxy resin is 0.10 to 2.00 by mass.

[0266] The content of the silica particles is 5.0% to 50.0% by mass relative to the solid content of the coating film.

[0267] The coating contains the epoxy resin, the isocyanate resin, the silica particles, and the polyethylene wax in a total of less than 100% by mass. (4)

[0269] The surface-treated steel sheet according to any one of (1) to (3), wherein the epoxy resin is a bisphenol A type epoxy resin with a number average molecular weight of 300 to 100,000. (5)

[0271] The surface-treated steel sheet according to any one of (1) to (4), wherein,

[0272] The silica particles are at least one of fumed silica or colloidal silica.

[0273] The average particle size of the fumed silica is 5–40 nm.

[0274] The average particle size of the colloidal silica is 5–200 nm. (6)

[0276] The surface-treated steel sheet according to any one of (1) to (5) is used as a blank for a fuel tank.

[0277] Explanation of reference numerals in the attached figures

[0278] 1. Surface-treated steel plate

[0279] 10 steel plates

[0280] 20 coatings

[0281] 30 Chromium-containing composite layer

[0282] 40 Coating

[0283] 50 First composite layer

[0284] 60 Second composite layer

[0285] 70 Third Composite Layer

Claims

1. A surface-treated steel plate, comprising: A coating, which is located on at least one surface of a steel plate and contains zinc and nickel; The first composite layer is located between the steel plate and the coating, and contains Fe, Zn and Ni; A chromium-containing composite layer, situated above the plating layer, and containing at least Cr; and A coating film is located on the chromium-containing composite layer and contains epoxy resin, isocyanate resin and silica particles. The coating thickness is 2-50 g / m² per single side. 2 , The chromium-containing composite layer comprises: A second composite layer, located on the side of the coating, contains Cr, Zn, and Ni; and The third composite layer is located on the coating side and contains Si, N and Cr; The adhesion amount of the chromium-containing composite layer, calculated based on metallic Cr, is 10–200 mg / m³. 2 , The thickness of the coating is 0.5 to 2.0 μm.

2. The surface-treated steel plate according to claim 1, wherein, The ratio d2 / d1 of the average thickness d2 of the second composite layer to the average thickness d1 of the first composite layer is 0.2 to 150.

0. The ratio d3 / d1 of the average thickness d3 of the third composite layer to the average thickness d1 of the first composite layer is 0.2 to 150.

0.

3. The surface-treated steel plate according to claim 1 or 2, wherein, The coating contains polyethylene wax in a proportion of 0.1 to 10.0% by mass relative to the solid content of the coating. The epoxy resin content is 30.0% by mass or more relative to the solid content of the coating film. The content of the isocyanate resin relative to the epoxy resin is 0.10 to 2.00 by mass. The content of the silica particles is 5.0% to 50.0% by mass relative to the solid content of the coating film. The coating contains the epoxy resin, the isocyanate resin, the silica particles, and the polyethylene wax in a total of less than 100% by mass.

4. The surface-treated steel plate according to claim 1 or 2, wherein, The epoxy resin is a bisphenol A type epoxy resin with a number average molecular weight of 300 to 100,000.

5. The surface-treated steel plate according to claim 1 or 2, wherein, The silica particles are at least one of fumed silica or colloidal silica. The average particle size of the fumed silica is 5–40 nm. The average particle size of the colloidal silica is 5–200 nm.

6. The surface-treated steel sheet according to claim 1 or 2, which is used as a blank for a fuel tank.

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