Coated plated steel sheet or coated plated steel strip

Abstract

A coated steel sheet or coated steel strip comprises: a steel sheet or steel strip; a coating disposed on one or both sides of the steel sheet or steel strip and containing zinc; a chemically converted coating disposed on the coating disposed on one side of the steel sheet or steel strip, or disposed on at least one of the coatings disposed on both sides of the steel sheet or steel strip; and a single-layer or multi-layer coating disposed on the chemically converted coating. For the single-layer coating or the multi-layer coating in contact with the chemically converted coating, the solid component of the single-layer coating or the multi-layer coating in contact with the chemically converted coating contains, at a concentration of 0.01 to 10.0% by mass, a cerium compound that is soluble in 100g of water or more at room temperature.

Description

Technical Field

[0001] This disclosure relates to coated or plated steel sheets or strips. Background Technology

[0002] In home appliances, building materials, and other applications, pre-coated steel sheets with colored organic films are increasingly being used, replacing the previous method of painting steel sheets after forming and processing. Pre-coated steel sheets are steel sheets that have undergone rust prevention treatment or are plated with colored organic films. They have the following characteristics: an attractive appearance, good processability, and good corrosion resistance.

[0003] Pre-coated steel sheets (hereinafter also referred to as coated steel sheets) are steel sheets shipped with a coating pre-formed to meet the user's requirements for the product. This eliminates the need for painting or related work, and also eliminates the need for equipment used in such work, thus expanding their application in various fields. Initially, chromate-treated steel sheets, which underwent rust prevention treatment using chromates, were used as the base steel sheets for coating. Later, due to concerns about the toxicity of hexavalent chromium that could leach from the chromate-treated film, chromate-free coated steel sheets, which used base steel sheets treated with hexavalent chromium-free rust prevention, gained attention as a replacement for chromate-treated coated steel sheets, and their use has increased particularly in the home appliance industry in recent years. On the other hand, chromate-coated steel sheets are the mainstream choice in the building materials industry, where long-term outdoor corrosion resistance is required.

[0004] With the aim of expanding the application of chromate-free coated steel sheets in the building materials field, and in order to achieve the high corrosion resistance of chromate chemical conversion treatment through chromate-free treatment, much research has been conducted to date. However, a chromate-free treatment that fully meets the required performance has not yet been provided.

[0005] On the other hand, recently, the high corrosion resistance of coated steel sheets, which have become the base for chromate-free treatment, has also been developed. From the previously mainstream galvanized steel sheets, zinc alloy coated steel sheets with added aluminum, magnesium, silicon, etc. are also being used. As a result, chromate-free treatment with high corrosion resistance is required for these zinc alloy coated steel sheets.

[0006] To date, there have been research examples on chromate-free post-treatment of coated steel sheets with zinc-containing coatings.

[0007] For example, Patent Document 1 discloses "a surface-treated metal plate and a metal surface treatment liquid for the same, characterized in that it is coated with a film mainly composed of rare earth metal element compounds and a resin matrix, wherein the resin matrix physically holds the rare earth metal element compounds on the surface of the metal plate and has a tight bond with the metal plate."

[0008] Moreover, according to Patent Document 1, it describes "a surface-treated metal plate and a metal surface treatment liquid that can provide excellent corrosion resistance and significantly reduce environmental impact by completely eliminating the use of hexavalent chromium".

[0009] Furthermore, Patent Document 2 discloses "a surface treatment film for zinc-plated steel sheets, which suppresses the formation of white rust by setting the main components of the film to oxides and / or hydroxides of Ce and oxides and / or hydroxides of Si." Patent Document 2 states that "the ratio of oxides and / or hydroxides of Ce in the film to all oxides is 0.10 to 0.60; the ratio of oxides and / or hydroxides of Si in the film to all oxides is in the range of 0.2 to 0.8, and the ratio of SiO2 in the oxides and / or hydroxides of Si is preferably 0.15 to 0.90."

[0010] Furthermore, according to Patent Document 2, it is described that "a novel chromate-free treated steel sheet can be provided, which can suppress the formation of zinc-induced white rust even when treated with a treatment bath that does not contain harmful ions".

[0011] Patent document 3 discloses "a surface treatment agent for aluminum or its alloys, characterized in that, relative to 10-1000 parts by weight of cerium ions, it contains 10-500 parts by weight of zirconium ions, 10-500 parts by weight of phosphate ions, and contains effective fluoride ions in a ratio of 1-50 parts by weight. 2) A surface treatment bath for aluminum or its alloys, characterized in that, relative to 10-1000 ppm of cerium ions, it contains 10-500 ppm of zirconium ions, 10-500 ppm of phosphate ions, and contains effective fluoride ions in a ratio of 1-50 ppm."

[0012] Patent document 4 discloses "a highly corrosion-resistant surface-treated steel sheet, characterized in that a surface-treated film with a thickness of 0.01 to 1.0 μm is formed on the surface of a zinc-plated steel sheet by coating a surface-treated composition containing the following components (a) to (c) and then drying it."

[0013] (a) Aqueous epoxy resin dispersion

[0014] (b) Silane coupling agents

[0015] (c) Phosphoric acid and / or hexafluorometallic acid

[0016] The upper layer has an upper film with a thickness of 0.3 to 2.0 μm, formed by coating an upper film coating composition containing the following components (D) to (G) and drying it.

[0017] (D) A resin solution obtained by reacting a compound having two or more carboxyl groups in one molecule (d2) and a hydrazine derivative having active hydrogen (d3) with bisphenol A type epoxy resin (d1).

[0018] (E) Curing agents having functional groups that react with hydroxyl groups

[0019] (F) Non-chromium rust inhibitors

[0020] (G) Complex solid lubricant.

[0021] Patent document 5 discloses "a surface-treated metal plate, characterized in that it has an intermediate layer with oxides, hydroxides, oxyacid compounds and / or oxyacid hydrogen compounds of group IVA elements as the main components, and a corrosion-resistant coating layer with rare earth and / or oxyacid compounds or oxyacid hydrogen compounds of group IVA elements or mixtures thereof as the main components (however, the intermediate layer and the corrosion-resistant coating layer are not of the same composition)".

[0022] In addition, research examples regarding pigments were also presented.

[0023] For example, Patent Document 6 discloses "a coloring metallic pigment comprising at least a metallic pigment, an amorphous silicon oxide film layer formed on the surface of the metallic pigment, a metal layer formed on the surface of the amorphous silicon oxide film layer, and metal particles formed on the surface of the metal layer, wherein the coloring metallic pigment is formed in such a manner that a portion of the metal layer is directly coated by the metal particles".

[0024] According to Patent Document 6, it is described that the weather resistance is improved by forming a layer containing the aforementioned coloring metallic pigment in a coating on a steel plate that has undergone zinc phosphate chemical conversion treatment and intermediate coating, and then drying it.

[0025] Furthermore, Patent Document 7 discloses "a rust-preventive pigment composition, characterized in that it is in the form of the following general formula: Me (0.5m+1.5n) (X) m ·(PO4) n [In the formula, Me represents Ca, Mg, or Ba, X represents OH, F, CO3, NO3, or NO2, and m and n represent coefficients.] The mixture of apatite and other insoluble metal phosphates represented by the formula is used as the active ingredient.

[0026] According to Patent Document 7, the weather resistance of a steel plate is improved by coating it with the aforementioned anti-rust pigment composition and then drying it.

[0027] In addition, there were research examples on cationic electrodeposition coating compositions that do not contain chromium compounds.

[0028] For example, Patent Document 8 discloses "a cationic electrodeposition coating composition, wherein it contains an amino-containing modified epoxy resin (A), a capped polyisocyanate curing agent (B), a phenolic resin (C), a metal compound (D), and nitrogen oxide ions (E)".

[0029] According to Patent Document 8, it is described that by using the above-mentioned cationic electrodeposition coating composition, it is possible to obtain a coated article with excellent corrosion resistance on a steel plate.

[0030] In addition, a case study on chromate-free post-treatment of aluminum materials was conducted.

[0031] For example, Patent Document 9 discloses "a surface treatment agent containing a water-soluble or water-dispersible organic polymer (A) having carbonyl and / or hydroxyl groups in a unit structure and an organic compound having phosphine groups (B)".

[0032] According to Patent Document 9, "a chromate-free surface treatment agent can be provided, a method for manufacturing a surface treatment film using the surface treatment agent, and an aluminum or aluminum alloy having the surface treatment film. The chromate-free surface treatment agent does not use one or more iron group compounds selected from oxides of iron, nickel, or cobalt, their hydroxides, or their hydroxyl oxides, and can form a surface treatment film between the aluminum or the like and the upper film that has excellent adhesion to the aluminum or the like and imparts excellent corrosion resistance to the aluminum or the like."

[0033] Patent Document 1: Japanese Patent Application Publication No. 10-337530

[0034] Patent document 2 Japanese Patent Application Publication No. 2001-158973

[0035] Patent document 3 Japanese Patent Application Publication No. 2-25579

[0036] Patent document 4 Japanese Patent Application Publication No. 2010-201353

[0037] Patent document 5 Japanese Patent Application Publication No. 2000-309879

[0038] Patent document 6 International Publication No. 2007 / 094253

[0039] Patent document 7 Japanese Patent Application Publication No. 5-43212

[0040] Patent document 8 Japanese Patent Application Publication No. 2011-84729

[0041] Patent Document 9 International Publication 2017 / 138464 Summary of the Invention

[0042] The problem that the invention aims to solve

[0043] In recent years, in order to make coated steel sheets with zinc-containing coatings suitable for applications in household appliances and building materials, there has been a demand for further improvements in corrosion resistance. Furthermore, to address cracks that occur in the coating or film due to processing of zinc-plated steel sheets containing aluminum, magnesium, etc. (processing parts such as bending and pressing), there is a demand for further improvements in corrosion resistance, particularly for further suppression of white rust in the processed parts.

[0044] In particular, the technology in Patent Document 9 is a chromate-free post-treatment technology for aluminum materials, not a chromate-free post-treatment technology for coated steel sheets with zinc-containing coatings.

[0045] Therefore, it is desirable to have coated steel sheets or strips with zinc-containing coatings that meet these performance requirements.

[0046] Therefore, the purpose of this disclosure is to provide a chromate-free coated steel sheet or coated steel strip that can suppress the formation of white rust even when cracks occur in the plating or coating due to bending, pressing, etc., without utilizing hexavalent chromium, and also has high corrosion resistance.

[0047] Methods for solving problems

[0048] The means used to solve the problem include the following solutions.

[0049] <1> A coated steel sheet or coated steel strip, comprising:

[0050] Steel plate or steel strip;

[0051] A coating, which is provided on one or both sides of the aforementioned steel plate or strip, and contains zinc;

[0052] A chemically converted coating is disposed on one side of the coating on the steel plate or strip, or on at least one of the coatings disposed on both sides of the steel plate or strip.

[0053] A single-layer or multi-layer coating is applied to the aforementioned chemically converted film.

[0054] For the single-layer coating or the multi-layer coating that is in contact with the chemically converted film, the coating that is in contact with the chemically converted film contains, relative to the solid component of the single-layer coating or the multi-layer coating that is in contact with the chemically converted film, 0.01 to 10.0% by mass of a cerium compound that is soluble in 100g of water or more at room temperature.

[0055] <2> According to the above <1> The coated steel sheet or coated steel strip mentioned above, wherein the cerium compound includes cerium(III) compound.

[0056] <3> According to the above <1> The coated steel sheet or coated steel strip mentioned above, wherein the cerium compound comprises a cerium(IV) compound.

[0057] <4> According to the above <3> The coated steel sheet or coated steel strip mentioned above, wherein the cerium(IV) compound comprises diammonium cerium(IV) nitrate.

[0058] <5> According to the above <1> ~ <4> In any one of the coated steel sheets or coated steel strips, the content of cerium chloride in the single-layer coating or the multi-layer coating that is in contact with the chemically converted coating is 0.01% by mass or less relative to the solid content of the single-layer coating or the multi-layer coating that is in contact with the chemically converted coating.

[0059] <6> According to the above <1> ~ <5> The coated steel sheet or coated steel strip according to any one of the following methods, wherein the chemical composition of the coating, in mass % is:

[0060] Al: 0-60.0%

[0061] Mg: 0–15.0%

[0062] Si: 0-2.0%

[0063] Ni: 0-1.0%

[0064] Cr: 0–1.0%

[0065] Ti: 0-1.0%, and

[0066] Remaining components: Zn and impurities.

[0067] <7> According to the above <1> ~ <5> The coated steel sheet or coated steel strip according to any one of the following methods, wherein the chemical composition of the coating, in mass % is:

[0068] Al: 0.5–60.0%

[0069] Mg: 0.5–15.0%

[0070] Si: 0–2.0%, and

[0071] Ni: 0-1.0%

[0072] Cr: 0–1.0%

[0073] Ti: 0-1.0%

[0074] Remaining components: Zn and impurities.

[0075] <8> According to the above <1> ~ <7> In any one of the coated steel sheets or coated steel strips, the thickness of the coating in contact with the chemically converted coating film in the single-layer coating or the multi-layer coating film is more than 5 μm.

[0076] <9> According to the above <1> ~ <8> The coated steel sheet or coated steel strip as described in any one of the above-mentioned methods, wherein the coating in contact with the chemical conversion treated film in the single layer or the multi-layer coating comprises any one or more of polyester resin, epoxy resin, acrylic resin and polyurethane resin.

[0077] <10> According to the above <1> ~ <9> In any one of the coated steel sheets or coated steel strips, the coating in contact with the chemically converted coating film, whether in a single layer or a multilayer, comprises one or more of vanadate, tungstate, silicate, and phosphate.

[0078] Invention Effects

[0079] According to this disclosure, a chromate-free coated steel sheet or coated steel strip can be provided that can suppress the formation of white rust even if cracks occur in the plating or coating due to bending, pressing, etc., without utilizing harmful hexavalent chromium, and also has high corrosion resistance. Detailed Implementation

[0080] The following describes an example of the coated steel sheet and coated steel strip disclosed herein.

[0081] It should be noted that, in this specification, the numerical range represented by "~" refers to the range that includes these values ​​as lower and upper limits, unless the values ​​before and after "~" are marked with "exceeding" or "below". Conversely, the numerical range that does not include these values ​​as lower or upper limits refers to the range that does not include these values.

[0082] Within the numerical range described in this specification, the upper limit of a certain numerical range of a particular stage can be replaced by the upper limit of other numerical ranges described in different stages, or by the values ​​shown in the embodiments. Similarly, the lower limit of a certain numerical range of a particular stage can be replaced by the lower limit of other numerical ranges described in different stages, or by the values ​​shown in the embodiments.

[0083] In addition, regarding concentration or content, "%" refers to "mass %".

[0084] As a concentration or content (%), "0~" means that the ingredient is optional and may not be present.

[0085] In this specification, "a coating containing only zinc as a coating component" is also referred to as "zinc coating", "a coating containing aluminum, magnesium, etc. as coating components in addition to zinc" is also referred to as "zinc alloy coating", and "zinc coating", "zinc alloy coating" and "coating containing zinc" are collectively referred to as "zinc-based coating".

[0086] In addition, "steel sheet with zinc coating" is also called "galvanized steel sheet", "steel sheet with zinc alloy coating" is also called "zinc alloy coated steel sheet", and "steel sheet with zinc-based coating" is also called "zinc-based coated steel sheet".

[0087] The coated steel sheet or coated steel strip disclosed herein has the following characteristics:

[0088] Steel plate or steel strip;

[0089] A coating, which is provided on one or both sides of the aforementioned steel plate or strip, and contains zinc;

[0090] A chemically converted coating is disposed on one side of the coating on the steel plate or strip, or on at least one of the coatings disposed on both sides of the steel plate or strip; and

[0091] A single-layer or multi-layer coating is applied to the aforementioned chemically converted film.

[0092] Furthermore, in the coated steel sheet disclosed herein, the coating that is in contact with the chemically converted coating film (hereinafter, "the coating that is in contact with the chemically converted coating film" is also referred to as "the coating that is in contact with the chemically converted coating film") contains, at a concentration of 0.01 to 10.0% by mass of a cerium compound that is soluble in 100g of water at room temperature, at a concentration of 0.01 to 10.0% by mass.

[0093] The coated steel sheet or coated steel strip disclosed herein, through the above-described configuration, becomes a chromate-free coated steel sheet or coated steel strip that, without utilizing harmful hexavalent chromium, suppresses the formation of white rust even when cracks occur in the coating or film due to bending, pressing, or other processes, and also possesses high corrosion resistance.

[0094] The coated or plated steel sheet or strip disclosed herein was discovered through the following observations.

[0095] Here, in chromate-free coated steel sheets, the interface where corrosion occurs is either the interface between the coating and the chemical conversion treated film, or the interface between the coating and the coating film in contact with the chemical conversion treated film. However, the chemical conversion treated film is only tens to hundreds of nanometers smaller than the coating film in contact with it, making it impossible to clearly determine the interface using cross-sectional observation with a scanning electron microscope. Furthermore, the thickness of the corrosion products generated is relatively thick compared to the thickness of the chemical conversion treated film, making it impossible to determine at which interface the corrosion products form. Therefore, unless otherwise specified, the site of corrosion is defined as the area between the coating and the coating film in contact with the chemical conversion treated film. That is, the corrosion products formed at each interface are referred to as corrosion products formed between the coating and the coating film in contact with the chemical conversion treated film.

[0096] It should be noted that a chemical conversion film is formed between the coating and the chemically converted film in contact. However, the chemically converted film is usually nanometer thick. In cases of uneven film thickness, the coating and the film are sometimes in direct contact. Therefore, corrosion products are sometimes formed at the interface between the coating and the chemically converted film.

[0097] First, in order to understand why white rust is easily generated during bending, pressing, and other processing, the inventors conducted the following research.

[0098] For coated steel sheets that have undergone general chromate-free chemical conversion treatment and coating, and coated steel sheets that have undergone chromate chemical conversion treatment and coating, a corrosion-promoting test was conducted on the coated steel sheets that had undergone bending through a composite cycle test, and cross-sections were observed after the corrosion test. As a result, the inventors obtained the following insights.

[0099] Under the general chromate-free chemical conversion treatment, the corrosion behavior of the processed parts differs between coated steel sheets (hereinafter "coated galvanized steel sheets") made from galvanized steel sheets with zinc coatings and coated steel sheets (hereinafter "coated zinc alloy coated steel sheets") made from zinc alloy coated steel sheets containing aluminum, magnesium, etc.

[0100] In the case of coated galvanized steel sheets, corrosion occurs throughout the zinc coating, starting from cracks formed during bending. In contrast, in coated zinc alloy coated steel sheets, corrosion also occurs between the coating and the chemically converted coating, originating from cracks in the coating caused by bending. This is presumably because: in the case of zinc alloy coatings, the leaching or formation of protective corrosion products by coating components, including aluminum and magnesium (which have higher oxidizing power than zinc), is more potent than zinc coatings; and oxidation caused by the leaching of coating components from cracks into the coating or the formation of corrosion products occurs through the presence of a specific, highly oxidizing coating between the coating and the chemically converted coating, resulting in white rust.

[0101] On the other hand, regarding coated steel sheets that have undergone chromate-containing chemical conversion treatment and coating, regardless of whether they have a zinc coating or a zinc alloy coating containing aluminum, magnesium, etc., corrosion is less likely to occur between the coating and the chemically converted coating film, even if the crack in the coating or film caused by bending processing is the starting point. The formation of white rust is also minimal. The reasons are as follows: Regarding chromate chemical conversion treatment, the high oxidizing power of the chemically converted coating film and the high affinity between the metal constituting the coating or its oxide and the chemically converted coating film allow the chemically converted coating film to form a rapid and strong bond with the surface of the coating, which is effective for the corrosion resistance of the processed parts.

[0102] It is believed that chromates, due to their multiple valences, have an oxidizing effect on steel plates and can self-repair the film, which is the reason for their high corrosion resistance in the processed parts.

[0103] Therefore, Ce, which has multiple valences similar to chromates and thus has a high affinity for the components of zinc-plated steel sheets, was selected. Specifically, cerium compounds (hereinafter referred to as "specific cerium compounds") that can dissolve more than 0.10 g in 100 g of water at room temperature were chosen. This is because specific cerium compounds possess the following characteristic: if contained in a coating that comes into contact with a chemically converted coating, cerium ions in the coating dissolve from the coating into the plating or steel sheet under corrosive conditions, thus inhibiting the corrosion reaction.

[0104] The addition of specific cerium compounds to the coating in contact with the chemically converted coating resulted in the suppression of white rust formation on the bent areas after a combined cyclic corrosion test (CCT). Therefore, specific cerium compounds were found to be effective inhibitors of zinc-based coatings.

[0105] One reason for the reduced corrosion resistance of the processed part is believed to be the over-dissolution of the plating components from the cracks in the coating. It is presumed that the cerium ions contained in certain cerium compounds inhibit the dissolution of the plating components.

[0106] Furthermore, by adding a specific cerium compound to the coating film in contact with the chemical conversion treated film, a coated steel sheet with high corrosion resistance is produced, which can suppress the formation of white rust even when cracks occur in the plating or coating film due to bending, pressing, or other processes, compared to the case where a specific cerium compound is added to the chemical conversion treated film. The reasons for this are speculated as follows.

[0107] By adding a specific cerium compound to the coating that comes into contact with the chemically converted film, cerium ions dissolve from the coating in a corrosive environment, oxidizing the coating or steel surface. This improves the corrosion resistance of the coating and inhibits over-dissolution, thereby protecting the areas where the basic reactions that cause corrosion of the coating or steel, namely anodic or cathodic reactions, are located.

[0108] It should be noted that the above content also applies to coated and plated steel strips.

[0109] Based on the above observations, it has been found that the coated steel sheet or coated steel strip of this disclosure, through the above-described configuration, becomes a chromate-free coated steel sheet or coated steel strip that inhibits the formation of white rust even when cracks occur in the coating or film due to bending, pressing, or other processes without utilizing harmful hexavalent chromium, and also has high corrosion resistance.

[0110] The coated steel sheet disclosed herein will be described in detail below.

[0111] <steel plate>

[0112] Steel sheet is the sheet of steel to which a coating is formed. There are no particular limitations on the steel sheet. For example, steel sheets of any type can be used, such as very low C type (ferrite-based structure), Al-k type (structure containing pearlite in ferrite), two-phase structure type (e.g., structure containing martensite in ferrite, structure containing bainite in ferrite), work-induced phase transformation type (structure containing retained austenite in ferrite), and fine-grained type (ferrite-based structure).

[0113] The tensile strength of the steel plate is not particularly limited, but it is preferably 270-780 MPa, more preferably 270-590 MPa, and even more preferably 270-440 MPa.

[0114] Tensile strength was measured using test piece No. 5 according to JIS Z 2241:2011. The sampling position of the tensile test piece was set to 1 / 4 of the distance from the end in the width direction of the plate, and the direction perpendicular to the rolling direction was set as the length direction.

[0115] The thickness of the steel plate is not particularly limited, but it is preferably 0.20 to 2.0 mm, more preferably 0.25 to 1.2 mm, and even more preferably 0.30 to 1.0 mm.

[0116] Zinc-based coatings

[0117] The zinc content in a zinc-based coating (a coating containing zinc) is preferably 25.0 to 100.0% by mass relative to the overall chemical composition of the zinc-based coating. If necessary, the lower limit of the zinc content can also be set to 30.0%, 35.0%, 45.0%, 55.0%, or 65.0% by mass.

[0118] Specifically, zinc-based coatings include zinc coatings, zinc-aluminum-magnesium coatings, zinc-aluminum-magnesium-silicon coatings, zinc-aluminum coatings, and zinc-aluminum-silicon coatings.

[0119] Zinc-based coatings can also include coatings containing small amounts of cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc., as dissimilar metal elements or impurities.

[0120] From a corrosion resistance perspective, zinc-based coatings are preferably those that contain aluminum in addition to zinc, or those that contain both aluminum and magnesium. In other words, using zinc alloy-coated steel sheets as the base material provides superior corrosion resistance compared to galvanized steel sheets, and is therefore preferred.

[0121] More specifically, the preferred chemical composition of the zinc-based coating, expressed in % by mass, is:

[0122] Al: 0-60.0%

[0123] Mg: 0–15.0%

[0124] Si: 0-2.0%

[0125] Ni: 0-1.0%

[0126] Cr: 0–1.0%

[0127] Ti: 0-1.0%, and

[0128] Remaining components: Zn and impurities.

[0129] If cracks occur in the zinc-based coating due to bending, pressing, or other processes, resulting in the aforementioned composition, white rust and corrosion may sometimes occur. However, by applying a chemical conversion coating to the coating, or by applying a single-layer or multi-layer coating to the chemical conversion coating containing a specific cerium compound at the aforementioned concentration, the formation of white rust can be suppressed, and corrosion resistance can be improved.

[0130] Specifically, the zinc-based coating is preferably a coating containing 0.5 to 60.0% by mass of aluminum, with the remainder consisting of zinc and impurities. More preferably, it is a coating containing 0.5 to 60.0% by mass of aluminum, and further 0.5 to 15.0% by mass or less of magnesium, with the remainder consisting of zinc and impurities. In this case, the zinc-based coating may also contain 1.0% by mass or less of Si, Ni, Cr, or Ti. The content of Al, Mg, Si, Ni, Cr, or Ti is not essential, and the lower limit is 0%.

[0131] The lower limit of Al content is preferably 1.0% by mass, 1.5% by mass, or 2.0% by mass.

[0132] The upper limit of Al content is preferably 55.0% by mass, 50.0% by mass, or 45.0% by mass.

[0133] The lower limit of Mg content is preferably 1.0% by mass, 1.5% by mass, or 2.0% by mass.

[0134] The upper limit of Mg content is preferably 12.5% ​​by mass, 10.0% by mass, or 7.5% by mass.

[0135] The lower limit of Si content is preferably 0.2% by mass, 0.4% by mass, or 0.6% by mass.

[0136] The upper limit of Si content is preferably 1.8% by mass, 1.6% by mass, or 1.4% by mass.

[0137] The lower limits for Ni, Cr, and Ti content are preferably 0.1% by mass, 0.2% by mass, or 0.3% by mass, respectively.

[0138] The upper limits for Ni, Cr, and Ti content are preferably 0.8% by mass, 0.6% by mass, or 0.4% by mass, respectively.

[0139] Examples of zinc alloy coatings containing any one of zinc, aluminum, and magnesium include zinc-aluminum-magnesium coatings and zinc-aluminum-magnesium-silicon coatings. Depending on the proportions of the components, there are various Zn-6%Al-3%Mg coatings, Zn-11%Al-3%Mg-0.2%Si coatings, Zn-55%Al-2%Mg-1.6%Si coatings, and coatings containing trace amounts of Ni, Cr, Ti, etc.

[0140] Specifically, for zinc-based coatings, the chemical composition of the above coatings can also be expressed in mass percent as follows:

[0141] Al: 0.5–60.0%

[0142] Mg: 0.5–15.0%

[0143] Si: 0-2.0%

[0144] Ni: 0-1.0%

[0145] Cr: 0–1.0%

[0146] Ti: 0-1.0%, and

[0147] The remainder consists of Zn and impurities. That is, zinc-based coatings can also be zinc alloy coatings.

[0148] There are no particular limitations on the method for forming zinc-based coatings; it can be any of the well-known methods such as electroplating, hot-dip plating, vapor deposition, dispersion plating, and vacuum plating.

[0149] There is no particular limitation on the amount of zinc-coated steel sheet applied to each side, but it is preferably 15 g·m. -2 Above and 140g·m -2 The following is preferred. More preferably, it is 30 g·m -2 Above and 90g·m -2 the following.

[0150] If the zinc coating adhesion amount is 15 g·m -2 The above can suppress the formation of uncoated areas and improve the corrosion resistance of the coating. Furthermore, if the zinc coating adhesion is 140 g·m... -2 The following methods improve corrosion resistance and reduce the likelihood of the coating turning black and discolored.

[0151] <Chemical conversion treatment of the coating (hereinafter also referred to as "coating")>

[0152] The chemical conversion coating is formed by removing oil and other impurities and surface oxides adhering to the surface of the coated steel sheet through degreasing and washing processes, followed by chemical conversion treatment.

[0153] The chemical conversion treatment film may also contain one or more of the following: resin, silane coupling agent, zirconium compound, silicon dioxide, phosphate and its salts, fluoride, and vanadium compound. The inclusion of these substances improves the film-forming properties after the chemical conversion treatment solution is applied, the film's barrier properties (density) against corrosive agents such as moisture or corrosive ions, and the film's adhesion to the plated surface, thus contributing to an improved level of corrosion resistance.

[0154] In particular, if the chemically converted film contains one or more of silane coupling agents and zirconium compounds, a cross-linked structure is formed in the film, which strengthens the bond with the coated surface, thereby improving the film's adhesion and barrier properties.

[0155] Furthermore, if the chemically converted coating contains any one or more of silicon dioxide, phosphate and its salts, fluorides, and vanadium compounds, it acts as an inhibitor, thereby improving corrosion resistance by forming a precipitated coating or a passive coating on the coating or steel surface.

[0156] Furthermore, chemical conversion treatment membranes may also contain specific cerium compounds.

[0157] [Resin]

[0158] The resin is not particularly limited; for example, known organic resins such as polyester resin, polyurethane resin, epoxy resin, phenolic resin, acrylic resin, and polyolefin resin can be used. To further improve adhesion to the coated steel sheet, it is preferable to use at least one of the resins (polyester resin, polyurethane resin, epoxy resin, acrylic resin, etc.) that have forced sites or polar functional groups in their molecular chains. The resin can be used alone or in combination of two or more.

[0159] The resin content (dry film concentration = mass % relative to the solid content of the chemically converted film) is preferably 0% by mass or more and 85% by mass or less relative to the solid content of the film. More preferably, it is 0% by mass or more and 60% by mass or less, and even more preferably, it is 1% by mass or more and 40% by mass or less. If the resin content exceeds 85% by mass, the proportion of other film components may decrease, and the performance required for the film other than corrosion resistance may be reduced.

[0160] [Silane coupling agent]

[0161] As a silane coupling agent, various silane compounds other than the carboxylic acid derivatives with silanol groups mentioned above can be used.

[0162] Examples of silane coupling agents include γ-(2-aminoethyl)aminopropyltrimethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-(2-aminoethyl)aminopropyltriethoxysilane, γ-(2-aminoethyl)aminopropylmethyldiethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, γ-methacryloyloxypropylmethyldimethoxysilane, γ-methacryloyloxypropyltriethoxysilane, γ-methacryloyloxypropylmethyldiethoxysilane, N-β-(N N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropylmethyldimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltriethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane γ-Methyldimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-mercaptopropylmethyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, vinyltriacetoxysilane, γ-chloropropyltrimethoxysilane, γ-chloropropylmethyldimethoxysilane, γ-chloropropyltriethoxysilane, γ-chloropropylmethyldiethoxysilane, hexamethyldisilazane, γ-anilinopropyltrimethoxysilane, γ-anilinopropylmethyldimethoxysilane, γ-anilinopropyltriethoxysilane, γ-anilinopropylmethyldiethoxysilane Vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldimethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(triethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldiethoxysilyl)propyl]ammonium chloride, γ-chloropropylmethyldimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, etc.

[0163] Among these, silane coupling agents with glycidyl ether groups (such as γ-glycidylpropoxypropyltrimethoxysilane and γ-glycidylpropoxypropyltriethoxysilane with glycidyl ether groups) exhibit particularly improved processing adhesion to the coating (undercoat) in contact with the chemically converted film. Furthermore, the use of triethoxy-type silane coupling agents enhances the storage stability of the substrate treatment agent. This is believed to be because triethoxysilanes are relatively stable in aqueous solutions and exhibit slow polymerization rates.

[0164] One type of silane coupling agent can be used, or two or more can be used together.

[0165] [Zirconium compounds]

[0166] As zirconium compounds, there are no particular limitations, but examples include zirconium n-propoxide, zirconium n-butoxide, zirconium tetraacetylacetonate, zirconium monoacetylacetonate, zirconium diacetylacetonate, zirconium monoethylacetoacetate, zirconium di(ethylacetoacetate)acetylacetonate, zirconium acetate, zirconium monostearate, zirconium carbonate, ammonium zirconium carbonate, potassium zirconium carbonate, sodium zirconium carbonate, etc.

[0167] Zirconium compounds can be used alone or in combination of two or more.

[0168] It should be noted that the zirconium carbonate compound undergoes a cross-linking reaction with the resin to form a film with a cross-linked structure of zirconium and resin. Furthermore, during the coating and drying process, the carbonate ions volatilize, and the remaining zirconium bonds with each other via oxygen, resulting in a higher molecular weight. In this process, the -Zr-OH groups form Zr-OM bonds with the surface of the coating (M: the metal element in the coating).

[0169] [Total content of silane coupling agent and zirconium compound]

[0170] The content of silane coupling agent and zirconium salt (dry film concentration = mass % relative to the solid content of the chemically converted film) is preferably 5% by mass and 80% by mass in the film. More preferably, it is 20% by mass and 70% by mass. If the content is less than 5% by mass, the improvement in adhesion to the substrate and corrosion resistance may not be obtained. If it exceeds 80% by mass, the processability may be reduced.

[0171] Silicon dioxide

[0172] Silica refers to silica particles that, due to their fine particle size, can stably maintain a water-dispersible state when dispersed in water. Silica is effective in improving the corrosion resistance of coated steel sheets and enhancing the adhesion of coatings (undercoatings) in contact with chemically converted coatings.

[0173] There are no particular limitations on the silica used, but colloidal silica or fumed silica microparticles with a primary particle size of 5–50 nm are preferred. Commercially available silica products such as "SNOWTEX N," "SNOWTEX C," "SNOWTEX UP," and "SNOWTEX PS" (all manufactured by Nissan Chemical Industries), "ADELITE AT-20Q" (manufactured by Asahi Denka Kogyo), or powdered silica such as AEROSIL#300 (manufactured by Nippon Aerosil) can be used. The silica should be selected appropriately based on the desired properties.

[0174] Silicon dioxide can be used in combination with one type or two or more types.

[0175] The silica content (dry film concentration = mass % relative to the solid content of the chemically converted film) is preferably 0% by mass or more and 30% by mass or less relative to the solid content of the film. More preferably, it is 1% by mass or more and 20% by mass or less. If the silica content exceeds 30% by mass, the film may become brittle, reducing the processability during the forming and processing of the coated steel sheet.

[0176] Phosphate and its salts

[0177] Examples of phosphoric acid salts include orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, and their salts; ammonium salts such as triammonium phosphate and diammonium hydrogen phosphate; phosphonic acids such as aminotris(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic acid); and organic phosphoric acids such as phytic acid and their salts. It should be noted that salts of phosphoric acid, other than ammonium salts, include metal salts containing Na, Mg, Al, K, Ca, Mn, Ni, Zn, and Fe.

[0178] Phosphate and its salts can be used alone or in combination of two or more.

[0179] The content of phosphate and its salts (dry film concentration = mass % relative to the solid content of the chemically converted film) is preferably 0% or more and 20% or less relative to the solid content of the film. More preferably, it is 1% or more and 10% or less.

[0180] If the content of phosphate and its salts exceeds 20% by mass, the coating may become brittle, reducing the processability of the coating during the forming process of the coated steel sheet.

[0181] [Fluorides]

[0182] Examples of fluorides include ammonium fluorozirconate, ammonium fluorosilicate, ammonium fluorotitanate, sodium fluoride, potassium fluoride, calcium fluoride, lithium fluoride, fluorotitanic acid, and fluorozirconic acid.

[0183] Fluorides can be used alone or in combination of two or more.

[0184] The fluoride content (dry film concentration = mass % relative to the solid content of the chemically converted film) is preferably 0% by mass or more and 20% by mass or less relative to the solid content of the film. More preferably, it is 1% by mass or more and 10% by mass or less. If the fluoride content exceeds 20% by mass, the film may become brittle, reducing the processability of the film during the forming process of the coated steel sheet.

[0185] [vanadium compounds]

[0186] Examples of vanadium compounds include those obtained by reducing pentavalent vanadium compounds such as vanadium pentoxide, metavanadate, ammonium metavanadate, sodium metavanadate, and vanadium trichloride to 2-4 valences using a reducing agent; and vanadium compounds with oxidation states of 4-2 such as vanadium trioxide, vanadium dioxide, vanadium oxysulfate, vanadium oxyoxalate, vanadium oxyacetylacetonate, vanadium acetylacetonate, vanadium trichloride, vanadium phosphomolybdic acid, vanadium sulfate, vanadium dichloride, and vanadium oxide.

[0187] Vanadium compounds can be used alone or in combination of two or more.

[0188] The content of vanadium compounds (dry film concentration = mass % relative to the solid content of the chemically converted film) is preferably 0% by mass or more and 20% by mass or less relative to the solid content of the film. More preferably, it is 1% by mass or more and 10% by mass or less. If the content of vanadium compounds exceeds 20% by mass, the film may become brittle, reducing the processability of the film during the forming process of the coated steel sheet.

[0189] Certain cerium compounds may also be contained in chemically converted coatings.

[0190] As a specific cerium compound contained in the coating through chemical conversion treatment, the same compounds as those contained in the coatings described later can be listed. That is, cerium compounds that can dissolve more than 0.10g of cerium relative to 100g of water at room temperature can be listed.

[0191] Furthermore, the lower limit of the content of the specific cerium compound contained in the chemically converted coating is preferably 0.01% by mass, more preferably 0.5% by mass, and even more preferably 1.0% by mass, relative to the solid content of the coating in contact with the chemically converted coating.

[0192] The upper limit of the content of a specific cerium compound relative to the solid composition of the coating in contact with the chemically converted film is preferably 10.0% by mass, more preferably 8.0% by mass, and even more preferably 5.0% by mass.

[0193] [Specific examples of the composition of chemically converted films]

[0194] Specific examples illustrating the composition of chemically converted films are provided.

[0195] As an example of the composition of a chemically converted coating, it preferably includes a resin, as well as any one of a silane coupling agent and a zirconium compound.

[0196] In this case, the resin content relative to the solid content of the film is preferably 40-85% by mass, more preferably 45-80% by mass, and even more preferably 50-75% by mass.

[0197] Furthermore, the content of the silane coupling agent or zirconium compound relative to the solid content of the film is preferably 15 to 60% by mass, more preferably 20 to 55% by mass, and even more preferably 25 to 50% by mass.

[0198] As another example of the composition of the chemically converted coating, a composition comprising the components described below is preferred.

[0199] Resin

[0200] • Any of the silane coupling agents and zirconium compounds

[0201] • Any one or more of silicon dioxide, phosphate and its salts, fluorides, and vanadium compounds

[0202] In this case, the resin content relative to the film solids content is preferably 10-75% by mass, more preferably 15-70% by mass, and even more preferably 20-65% by mass.

[0203] Furthermore, the content of silane coupling agent or zirconium compound relative to the solid content of the film is preferably 5 to 59% by mass, more preferably 12 to 53% by mass, and even more preferably 18 to 47% by mass.

[0204] Furthermore, the total content of silicon dioxide, phosphate and its salts, fluoride and vanadium compound is preferably 1 to 40% by mass relative to the solid content of the film, more preferably 2 to 30% by mass, and even more preferably 3 to 20% by mass.

[0205] [Methods for forming chemical conversion treatment agents]

[0206] There are no particular limitations on the manufacturing method of chemical conversion treatment agents, but examples include mixing the various film-forming components and stirring, dissolving, or dispersing them using a disperser. To improve the solubility or dispersibility of the individual film-forming components, known hydrophilic solvents may be added as needed. In chemical conversion treatment agents, acids, bases, etc., may also be added for pH adjustment, within a range that does not impair their performance.

[0207] To form a chemical conversion coating, a chemical conversion agent is applied to the plated steel sheet, and the coating is then heated and dried.

[0208] There are no particular limitations on the coating method of chemical conversion treatment agents. It can be any commonly known coating method, such as roller coating, air spraying, airless spraying, or dipping.

[0209] The optimal heating and drying temperature is 50–250°C. Below 50°C, the evaporation rate of moisture is slow, resulting in insufficient film formation and sometimes inadequate rust prevention. Above 250°C, the alkyl portion of the silane coupling agent used in organic compounds may be modified due to thermal decomposition, leading to reduced adhesion and corrosion resistance. A more preferred heating temperature is 70–160°C.

[0210] There are no particular limitations on the heating and drying method; methods such as hot air, induction heating, near-infrared radiation, and direct flame can be listed individually or in combination. For example, when using hot air drying, the preferred heating and drying time is 1 second to 5 minutes.

[0211] [Amount of chemical conversion coating adhered to each side of the galvanized steel sheet]

[0212] The amount of chemical conversion coating on each side of the coated steel sheet, based on solid content, is preferably 10–1000 mg / m². 2 Below 10 mg / m³ 2 Sufficient processing seal and corrosion resistance cannot be guaranteed at times; if it exceeds 1000 mg / m³, 2 Sometimes the tightness of the process is reduced.

[0213] The lower limit of the chemical conversion coating adhesion amount on each side of the coated steel sheet is preferably 20 mg / m². 2 Or 30mg / m 2 More preferably 40 mg / m 2 Or 50mg / m 2 The upper limit of the chemical conversion coating adhesion amount on each side of the coated steel sheet is preferably 800 mg / m². 2 Or 600mg / m 2 More preferably 400 mg / m 2 300mg / m 2 Or 200mg / m2 The thickness of the chemical conversion treatment film on each side of the coated steel sheet is preferably less than 2.0 μm, more preferably less than 1.0 μm, less than 0.8 μm, or less than 0.6 μm. It is not necessary to specifically specify a lower limit for the thickness of the chemical conversion treatment film on each side of the coated steel sheet, but it can also be set to 0.01 μm, 0.03 μm, or 0.05 μm.

[0214] Coating

[0215] A coating is a single-layer or multi-layer film formed on a chemically converted coating. Most coatings consist of 1 to 3 layers on each side of the coated steel sheet. In cases where the coating has two or more layers, the layer in contact with the chemically converted coating is sometimes specifically referred to as the primer, primarily to ensure adhesion and corrosion resistance between the two layers. On the other hand, the upper coatings are mostly used to ensure decorative patterns, barrier properties, or other surface functionalities. It should be noted that, unless otherwise specified, the coatings shown here refer to those in contact with the chemically converted coating.

[0216] The coating contains a specific cerium compound.

[0217] Furthermore, the coating film may contain, for example, a resin. The coating film preferably contains pigments. In addition to these components, the coating film may also contain additives such as leveling agents, defoamers, colorants, viscosity modifiers, and UV absorbers. It should be noted that the coating liquid used to form the coating film is preferably obtained by dispersing or dissolving the above-mentioned components in a solvent.

[0218] [Specific cerium compounds]

[0219] A specific cerium compound is one that can dissolve more than 0.10g of cerium relative to 100g of water at room temperature.

[0220] Here, "normal temperature" refers to the temperature range of 15 to 25°C.

[0221] Whether a cerium compound is soluble in more than 0.10g of the solvent is determined by the following steps.

[0222] First, weigh 100g of water. Next, weigh 0.10g of the cerium compound to be tested. Add the weighed solvent and the cerium compound to the beaker, keeping the solvent at room temperature and stirring. Then, visually determine whether the cerium compound has dissolved.

[0223] From the viewpoint of further suppressing the formation of white rust and further improving corrosion resistance, the specific cerium compound is preferably dissolved in 0.1g or more of 100g of water at room temperature, more preferably dissolved in 1.0g or more of the specific cerium compound, and even more preferably dissolved in 10g or more of the specific cerium compound.

[0224] In addition, there are no particular limitations, but a specific cerium compound can also be a specific cerium compound that dissolves in less than 500g of water at room temperature relative to 100g.

[0225] Examples of specific cerium compounds include cerium(III) compounds and cerium(IV) compounds. That is, in the coating that comes into contact with the chemically converted film, the specific cerium compound preferably includes at least one of cerium(III) compounds and cerium(IV) compounds.

[0226] In particular, as a specific cerium compound, it is preferable to include at least a cerium(IV) compound. This is because cerium(IV) compounds have a higher leaching inhibition effect on the plating components compared to cerium(III) compounds.

[0227] Examples of cerium(III) compounds include cerium(III) nitrate hexahydrate (cerium nitrate hexahydrate), cerium(III) sulfate octahydrate (cerium sulfate octahydrate), cerium(III) sulfate tetrahydrate, bis(trifluoromethanesulfonyl)imide cerium(III), ammonium nitrate cerium(III) tetrahydrate, diammonium nitrate cerium(III) tetrahydrate, cerium(III) sulfate n hydrate, cerium(III) bromide, cerium(III) chloride heptahydrate, cerium(III) chloride n hydrate, cerium(III) acetate hydrate, cerium(III) trifluoromethanesulfonate, cerium(III) chloride, and cerium(III) iodide.

[0228] Among them, as cerium(III) compounds, cerium(III) nitrate hexahydrate (cerium nitrate hexahydrate), cerium(III) sulfate octahydrate (cerium sulfate octahydrate), ammonium cerium(III) nitrate tetrahydrate, diammonium cerium(III) nitrate tetrahydrate, cerium(III) sulfate n hydrate, and cerium(III) acetate hydrate are preferred.

[0229] Examples of cerium(IV) compounds include diammonium cerium(IV) nitrate, cerium(IV) n-hydrate, tetraammonium cerium(IV) dihydrate, anhydrous cerium(IV) sulfate, tetraammonium cerium(IV) tetrahydrate, and methoxyethanol cerium(IV).

[0230] Among them, as cerium(IV) compounds, diammonium cerium(IV) nitrate, cerium(IV) n hydrate, tetraammonium cerium(IV) dihydrate, anhydrous cerium(IV) sulfate, and tetraammonium cerium(IV) tetrahydrate are preferred, and diammonium cerium(IV) nitrate is more preferred.

[0231] Examples of cerium compounds that do not conform to a specific cerium compound include cerium phosphates and cerium oxides.

[0232] The solubility of the aforementioned cerium phosphates and cerium oxides at room temperature relative to 100g of water is less than 0.10g.

[0233] The content of a specific cerium compound is 0.01 to 10.0% by mass relative to the solid content of the coating in contact with the chemically converted film.

[0234] If the content of cerium compound is less than 0.01% by mass, the dissolution inhibition effect of the plating components cannot be obtained, the formation of white rust cannot be suppressed, and the corrosion resistance is reduced. Therefore, the content of cerium compound is set to be 0.01% by mass or more.

[0235] If the cerium compound content exceeds 10.0% by mass, the concentration of cerium compounds in the coating that comes into contact with the chemically converted film may be too high, leading to a decrease in the processability of the coating and making it difficult to fully obtain properties other than corrosion resistance. Therefore, the cerium compound content is set to be below 10.0% by mass.

[0236] The lower limit of the content of a specific cerium compound relative to the solid content of the coating in contact with the chemically converted film is preferably 0.50% by mass, more preferably 1.0% by mass, and even more preferably 1.5% by mass.

[0237] The upper limit of the content of a specific cerium compound relative to the solid composition of the coating in contact with the chemically converted film is preferably 8.0% by mass, more preferably 5.0% by mass, and even more preferably 4.0% by mass.

[0238] Cerium chloride tends to have a limited effect on improving the corrosion resistance of the processed parts. The reason for this is unclear, but it is presumed to be due to the fact that cerium chloride forms complex salts with metals such as iron, thereby increasing its solubility. The lower the content of cerium chloride in the coating that comes into contact with the chemically converted film, the better.

[0239] Therefore, in the coating that comes into contact with the chemically converted film, the content of cerium chloride relative to the solid content of the coating that comes into contact with the chemically converted film is preferably 0.01% by mass or less, more preferably 0.005% by mass or less, and even more preferably 0.001% by mass or less.

[0240] In the coating that comes into contact with the chemically converted film, the content of cerium chloride is preferably 0% by mass relative to the solid content of the coating that comes into contact with the chemically converted film (i.e., the coating that comes into contact with the chemically converted film preferably does not contain cerium chloride), but it may contain more than 0.001% by mass.

[0241] Examples of cerium chloride include cerium(III) chloride, cerium(III) chloride heptahydrate, and cerium(III) chloride n hydrate.

[0242] [Resin]

[0243] There are no specific limitations on the resin. Examples include polyester resin, acrylic resin, epoxy resin, polyurethane resin, and fluoropolymer resin.

[0244] Other examples of resins include those obtained by crosslinking these resins with a crosslinking agent component of butylated melamine resin, methylated melamine resin, butylmethyl mixed melamine resin, urea resin, isocyanate resin, or a mixture thereof.

[0245] Other examples of resins include electron beam curable resins and ultraviolet curable resins.

[0246] Among them, the preferred adhesive resins are any one or more of polyester resin, epoxy resin, acrylic resin and polyurethane resin.

[0247] These adhesive resins can be used alone or in combination of two or more.

[0248] The resin content is preferably 20-90% by mass relative to the solid content of the coating film. If the resin content is less than 20% by mass, there is less component that forms the matrix, which can easily lead to cracking during processing and sometimes affects corrosion resistance. Furthermore, if the resin content exceeds 90% by mass, the content of anti-rust pigments is insufficient, which may compromise the corrosion resistance and adhesion of the coating film. From the viewpoints of processability and corrosion resistance, a resin content of 30-80% by mass is more preferable.

[0249] [pigment]

[0250] As pigments, they can be broadly classified into rust-preventing pigments, coloring pigments, and extender pigments.

[0251] Examples of rust-preventive pigments include vanadates, tungstates, silicates, and phosphates.

[0252] As coloring pigments, well-known inorganic and organic coloring pigments can be listed. Examples of inorganic coloring pigments include titanium dioxide, zinc oxide, zirconium oxide, calcium carbonate, barium sulfate, aluminum oxide, kaolin clay, carbon black, and iron oxide. Examples of organic coloring pigments include Hansa Yellow, pyrazolone orange, phthalocyanine, and azo pigments.

[0253] Examples of pigments that can be used as body pigments include talc, clay, silica, mica, aluminum oxide, calcium carbonate, and barium sulfate.

[0254] From the viewpoint of improving corrosion resistance, rust-preventive pigments are preferred as pigments. That is, the coating film (undercoat film) in contact with the chemically converted film preferably contains one or more of vanadates, tungstates, silicates, and phosphates.

[0255] Here, examples of vanadates include calcium vanadate, magnesium vanadate, ammonium metavanadate, potassium vanadate, sodium vanadate, ammonium vanadate, phosphorus vanadate, and vanadium oxide.

[0256] Examples of tungstates include sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, and magnesium tungstate.

[0257] Examples of silicates include sodium silicate, potassium silicate, lithium silicate, and calcium ion-exchanged silicon dioxide.

[0258] Examples of phosphates include sodium dihydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, aluminum tripolyphosphate, magnesium tripolyphosphate, sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate, and calcium hypophosphite.

[0259] The pigment content is preferably 5-70% by mass relative to the solid content of the coating film. If the pigment content is less than 5% by mass, the coating film's rigidity and cohesion may decrease, making it easier for the coating surface to peel off (coating adhesion) during the pressing process of coated steel sheets when rubbing against the mold. Furthermore, if the pigment content exceeds 70% by mass, processability may decrease. From the viewpoint of balancing corrosion resistance, chemical resistance, and processability, the pigment content is more preferably 15-70% by mass, and even more preferably 20-50% by mass.

[0260] [Coating thickness]

[0261] The coating thickness is preferably 4 μm or more (e.g., 4–50 μm). When the coating is multilayered, the film thickness refers to the total thickness of the multilayers.

[0262] However, when the coating is multilayered, the thickness of the coating in contact with the chemically converted coating (the lower coating layer) is preferably 4 to 15 μm. If the film thickness is 4 μm or more, sufficient corrosion resistance and chemical resistance are easily obtained. On the other hand, if the film thickness is 15 μm or less, processability is easily improved. From the viewpoint of achieving a good balance between corrosion resistance, chemical resistance, and processability, the film thickness of the coating in contact with the chemically converted coating (the lower coating layer) is more preferably in the range of 4 to 10 μm.

[0263] Furthermore, when the coating is multilayered, the thickness of the coating layer that is on top of the coating layer in contact with the chemically converted film is preferably 5 to 30 μm. If the film thickness is 5 μm or more, chemical resistance, corrosion resistance, and color concealment are easily improved, and pattern design becomes easier. Furthermore, if the film thickness is 30 μm or less, processability is easily improved. The thickness of the upper coating layer is more preferably 10 to 25 μm.

[0264] It should be noted that when the upper coating is a multilayer coating, the film thickness of the upper coating refers to the film thickness of each individual coating layer in the upper layer.

[0265] In the case of a single layer or multiple layers of coating, from the viewpoint of further suppressing the formation of white rust and further improving corrosion resistance, the thickness of the coating in contact with the chemically converted film is preferably more than 5 μm, more preferably more than 6 μm, and even more preferably more than 7 μm.

[0266] In the case of a single layer or multiple layers of coating, from the viewpoint of improving processability, the thickness of the coating in contact with the chemically converted film is preferably 25 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less.

[0267] [Methods for forming coatings]

[0268] A coating film is formed by applying a coating liquid and then drying and curing the coating film.

[0269] There are no particular limitations on the coating method, and examples include dip coating, curtain flow coating, roller coating, bar coating, electrostatic coating, brush coating, T-die coating, lamination, and spray coating. In addition, wet-on-wet coating and multilayer simultaneous coating methods can also be listed.

[0270] Heating and curing can be achieved using any method, such as hot air, near-infrared rays, far-infrared rays, high-frequency induction heating, or a combination of these methods.

[0271] [Specific forms of coated steel sheets]

[0272] The coated steel sheet disclosed herein is a pre-coated steel sheet.

[0273] A pre-coated steel sheet is a flat steel sheet obtained by forming a coating on a steel sheet before forming. Here, the coating formed on the steel sheet before forming is the coating in the coated steel sheet of this disclosure.

[0274] On the other hand, the coated steel sheet of this disclosure does not have an electrodeposited coating film. That is, the coated steel sheet of this disclosure is a steel sheet with a coating film as its outermost surface.

[0275] <Painted and plated steel strip>

[0276] The coated steel strip in this embodiment is a strip of steel.

[0277] As an example of the shape of the coated steel strip in this embodiment, a coated steel strip wound into a roll shape can be cited.

[0278] The coated steel strip of this embodiment has the same structure as the coated steel sheet described above, except that it is strip-shaped.

[0279] Coated steel strips are obtained, for example, by rolling the aforementioned coated steel sheet into a coil.

[0280] There are no particular limitations on the method of rolling the above-mentioned coated steel sheet into a coil shape. For example, a method using a known winding machine can be listed.

[0281] Example

[0282] The following examples illustrate this disclosure in detail. However, these examples do not limit the scope of this disclosure.

[0283] (1) Zinc-coated steel sheet

[0284] Prepare zinc-coated steel sheets with zinc coatings on both sides as shown in Table 1. The zinc-coated steel sheets used are mild steel sheets with a thickness of 0.5 mm. The zinc-coated steel sheets are used after surface degreasing with alkali, washing, and drying.

[0285] (2) Formation of the film by chemical conversion treatment

[0286] The chemical conversion agent used to form the chemical conversion treated film is prepared by combining the Ce compound shown in Table 2, the resin shown in Table 3, the silane coupling agent shown in Table 4, the zirconium compound shown in Table 5, the silica shown in Table 6, the phosphate and its salts shown in Table 7, the fluoride shown in Table 8, and the vanadium compound shown in Table 9 in the proportions shown in Table 12 (dry film concentration = mass % relative to the solids content of the chemical conversion treated film) and stirring with a disperser.

[0287] Next, the zinc-coated steel sheet prepared in (1) above is plated on both sides at a density of 100 mg / m² on each side. 2 The chemical conversion treatment agent is applied by a roller coater and dried on a steel plate at 100°C to form a chemical conversion treatment film.

[0288] It should be noted that the content (dry film concentration) of each component in the chemically converted coating in Table 12 is relative to the mass percentage of the solid content of the coating. In Table 12, except for Ce compounds, the amounts of resin, silane coupling agent, zirconium compound, silica, phosphate and its salts, fluoride, and vanadium compound in the chemically converted coating are recorded in 1% units. Therefore, the total amount of all compounds including Ce compounds may not necessarily be 100%.

[0289] (3) Coating

[0290] The coating solution used to form the coating film is prepared by mixing the Ce compound shown in Table 2, the resin shown in Table 10, and the pigment shown in Table 11 in the proportions shown in Table 12 (dry film concentration = mass % of solids relative to the coating film) and stirring with a disperser.

[0291] Next, the zinc-plated steel sheet with a chemically converted coating prepared in (2) above is coated with a coating liquid on both sides in a manner that achieves a specified film thickness using a roller coater, and then dried at the steel sheet reaching a temperature of 220°C to form a coating film.

[0292] It should be noted that, in the case of a two-layer coating, a Nippon Paint Coatings FLC100 coating is applied to the coating (lower coating) that is in contact with the chemically converted film using a roller coater at a thickness of 15 μm, and then dried at a substrate temperature of 230°C to form a coating film.

[0293] It should be noted that the content (concentration of dry film) of each component in the coating film in Table 12 is relative to the mass of the solid content of the coating film.

[0294] (4) Evaluation methods and evaluation criteria

[0295] Test plates were collected from the coated steel plates obtained in (3) above, and the test plates were evaluated using the evaluation methods and evaluation criteria shown below.

[0296] (width of white rust when cut crosswise)

[0297] The end faces of the test plate (50×100mm) were sealed with tape. A cross-cut was made into the steel plate using a cutter, creating scratches until reaching the steel substrate. A composite cyclic corrosion test according to CCT-JASO M609 was then conducted for 60 cycles. The maximum width of the white rust coating from the cross-cut was measured after the test, and the results were evaluated according to the following criteria.

[0298] Rating 5: White rust width less than 2mm

[0299] Rating 4: White rust width is 2mm or more but less than 4mm

[0300] Rating 3: White rust width is 4mm or more but less than 6mm

[0301] Rating 2: White rust width is 6mm or more but less than 8mm

[0302] Rating 1: White rust width is 8mm or more

[0303] (White rust from Erichsen's machining department)

[0304] After sealing the end faces of the test plate (50×100mm size) with tape, a 6mm Erickson extrusion was performed on the center of the test plate, and a composite cyclic corrosion test based on CCT-JASO M609 was conducted for 60 cycles. The proportion of white rust plating on the circular portion of the test plate after the test, which was extruded through Erickson processing, was measured and evaluated according to the following evaluation criteria.

[0305] Rating 5: White rust covers less than 20% of the area.

[0306] Rating 4: The area of ​​white rust is more than 20% but less than 30%.

[0307] Rating 3: The area of ​​white rust is more than 30% but less than 40%.

[0308] Rating 2: The area of ​​white rust is between 40% and 50%.

[0309] Rating 1: The area of ​​white rust exceeds 50%.

[0310] In Table 2, “Solubility in water (0.10% or more)” will be marked as “Y” if the cerium compound dissolves in 0.10g or more of 100g of water at room temperature. On the other hand, the dissolution rate of less than 0.10g will be marked as “N”.

[0311] Table 1

[0312] No. Coated steel sheet (% = mass%) a1 Hot-dip galvanized steel sheet a2 Zn-11%Al-3%Mg-0.2%Si hot-dip galvanized alloy steel sheet a3 Zn-6%Al-3%Mg hot-dip galvanized alloy steel sheet a4 Zn-55%Al-2%Mg-1.6%Si hot-dip galvanized alloy steel sheet a5 Zn-55%Al-1.6%Si hot-dip galvanized alloy steel sheet

[0313] Table 2

[0314]

[0315] Table 3

[0316] No. Resin for chemical conversion treatment of film f1 Polyester resin f2 polyurethane resin f3 Epoxy resin f4 Phenolic resin f5 acrylic resin

[0317] Table 4

[0318] No. Silane coupling agents for chemical conversion treatment of membranes g1 3-Glycidoxypropyltrimethoxysilane g2 3-Aminopropyltriethoxysilane

[0319] Table 5

[0320] No. Zirconium compounds chemically converted from coating h1 ammonium zirconium carbonate

[0321] Table 6

[0322] No. Chemical conversion treatment of silica in film i1 Colloidal silica

[0323] Table 7

[0324] No. Chemical conversion treatment of phosphate and its salts in film j1 Phosphoric acid j2 ammonium phosphate

[0325] Table 8

[0326] No. Chemical conversion treatment of fluoride in film k1 Fluorotitanic acid k2 ammonium fluoride

[0327] Table 9

[0328] No. Vanadium compounds from chemically converted coatings l1 Vanadyl acetylacetonate

[0329] Table 10

[0330] No. The resin of the lower coating layer (layer 1) in contact with the chemically converted film m1 Polyester resin m2 acrylic resin m3 Epoxy resin m4 polyurethane resin m5 Butylmethyl mixed melamine resin m6 Isocyanate resin

[0331] Table 11

[0332] No. Pigment in the lower coating layer (layer 1) that is in contact with the chemically converted film n1 Calcium vanadate-magnesium n2 Sodium tungstate n3 Sodium silicate n4 Calcium ion-exchanged silica (manufactured by Grace: SHIELDEX C303) n5 Aluminum phosphate (manufactured by TAYCA: K-WHITE#82)

[0333]

[0334]

[0335]

[0336]

[0337]

[0338]

[0339]

[0340]

[0341] It was found that the width of the white rust in the cross section and the proportion of white rust area in the Erickson machining section were small in coated steel sheets (Tests No. 11-26, 29-34, 36-71, 75-79, 83) containing specific cerium compounds in the coating that came into contact with the chemically converted coating.

[0342] On the other hand, it was found that the cross-sectional white rust width and the proportion of white rust area in the Erickson machining section of coated steel sheets (Tests No. 1-10, 27, 28, 35, 72-74, 80-82, 84-87) without specific cerium compounds in the coating that came into contact with the chemically converted coating were larger than those of coated steel sheets containing specific cerium compounds.

[0343] It should be noted that, although the cross-sectional white rust width and the white rust score of the Erickson machining section of the coated steel sheets containing specific cerium compounds in the chemical conversion treatment film (Tests No. 84-87) are the same as those of the coated steel sheets in Test No. 2, the results show that the cross-sectional white rust width and the proportion of white rust area in the Erickson machining section of the coated steel sheets containing specific cerium compounds in the chemical conversion treatment film (Tests No. 84-87) are smaller.

[0344] The results above show that, in this embodiment, compared with the comparative example, even if cracks occur in the plating or coating due to bending or pressing processes, the formation of white rust is suppressed and high corrosion resistance is also achieved without utilizing harmful hexavalent chromium.

Claims

1. A coated steel sheet or coated steel strip, comprising: Steel plate or steel strip; A coating, which is provided on one or both sides of the steel plate or strip, and contains zinc; A chemically converted coating is disposed on a coating on one side of the steel plate or strip, or on at least one of the coatings disposed on both sides of the steel plate or strip; and A single-layer or multi-layer coating is applied to the chemically converted film. For the single-layer coating or the multi-layer coating that is in contact with the chemically converted film, the solid component of the single-layer coating or the multi-layer coating that is in contact with the chemically converted film contains 0.01 to 10.0% by mass of a cerium compound that is soluble in more than 0.10g of water at room temperature. The cerium compound includes cerium IV compound.

2. The coated plated steel sheet or strip according to claim 1, wherein, The cerium compound includes cerium III compounds.

3. The coated steel sheet or coated steel strip according to claim 1, wherein, The cerium IV compound comprises diammonium cerium IV nitrate.

4. The coated steel sheet or coated steel strip according to any one of claims 1 to 3, wherein, In the single-layer coating or the multi-layer coating that is in contact with the chemically converted film, the content of cerium chloride in the coating that is in contact with the chemically converted film is less than 0.01% by mass relative to the solid content of the coating that is in contact with the chemically converted film.

5. The coated steel sheet or coated steel strip according to any one of claims 1 to 3, wherein, The chemical composition of the coating, expressed as a percentage by mass, is: Al：0～60.0%、 Mg: 0–15.0% Si: 0~2.0%, Ni: 0~1.0%, Cr：0～1.0%、 Ti: 0–1.0%, and Remaining components: Zn and impurities.

6. The coated steel sheet or coated steel strip according to any one of claims 1 to 3, wherein, The chemical composition of the coating, expressed as a percentage by mass, is: Al：0.5～60.0%、 Mg: 0.5–15.0% Si: 0–2.0%, and Ni: 0~1.0%, Cr：0～1.0%、 Ti: 0~1.0%, Remaining components: Zn and impurities.

7. The coated steel sheet or coated steel strip according to any one of claims 1 to 3, wherein, The thickness of the single-layer coating or the multi-layer coating in contact with the chemically converted film exceeds 5 μm.

8. The coated steel sheet or coated steel strip according to any one of claims 1 to 3, wherein, The single-layer coating or the multi-layer coating that is in contact with the chemically converted film includes one or more of polyester resin, epoxy resin, acrylic resin and polyurethane resin.

9. The coated steel sheet or coated steel strip according to any one of claims 1 to 3, wherein, The single-layer coating or the multi-layer coating that is in contact with the chemically converted film contains one or more of vanadate, tungstate, silicate, and phosphate.

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