Surface-treated steel sheet

Abstract

To disclose a surface-treated steel plate with improved corrosion resistance by restraining blister on a coating film in a warm saline water immersion test.SOLUTION: A surface-treated steel plate has a plating steel plate having a zinc-containing plating layer, and a surface treatment layer provided on at least one main face of the plating steel layer. The surface treatment layer has a coating film, and the coating film contains binder resin, an anti-rust agent, and a conductive agent. An adhesive amount of the coating film is 2 g / m2 or more and 30 g / m2 or less, and electric conductivity of immersion water obtained when a predetermined ion exchange water immersion test is performed to the surface-treated steel plate is 10 μS / cm or more and 25 μS / cm or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present application discloses a surface-treated steel sheet.

[0002] Surface-treated steel sheets are used as components of automobiles and the like. The surface-treated steel sheets have, for example, a plated steel sheet and a surface treatment layer provided on at least one main surface of the plated steel sheet. In the prior art, a coating film is used as the surface treatment layer, and the types and contents of components constituting the coating film and the thickness of the coating film are adjusted to improve the weldability and corrosion resistance of the surface-treated steel sheet.

[0003] For example, Patent Document 1 discloses a technique for improving the corrosion resistance of an end face of a coated steel sheet obtained by forming two or more coating layers on at least one side of a Zn-containing plated steel sheet, by providing an outermost layer of the coating film with a predetermined thickness, by containing a predetermined non-chromium compound in the outermost layer of the coating film, and by devising a configuration of the coating film so that when the coated steel sheet is immersed in ion-exchanged water under predetermined conditions, the immersion water has an electrical conductivity of 30 μS / cm or more.

[0004] Furthermore, Patent Document 2 discloses a technique for improving the weldability and corrosion resistance of a surface-treated steel sheet having a coating film on at least one side of a plated steel sheet by making the coating film contain predetermined amounts of a binder resin, non-oxide ceramic particles containing V, and doped zinc oxide particles.

[0005] Furthermore, Patent Document 3 discloses a technique for improving the weldability and corrosion resistance of a coated metal material having an organic coating on the surface of the metal material by making the organic coating contain a specified resin having a urethane bond and specified conductive particles.

[0006] Furthermore, Patent Document 4 discloses a technology for improving the weldability and corrosion resistance of a coated metal plate having a coating layer on the surface of the metal plate by making the coating layer contain a predetermined amount of conductive particles having a predetermined particle size. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2012-136025 A [Patent Document 2] International Publication No. 2018 / 092244 [Patent Document 3] JP 2004-042622 A [Patent Document 4] JP 2004-183080 A Summary of the Invention [Problem to be solved by the invention]

[0008] Surface-treated steel sheets as components for automobiles and the like may be required to have paint adhesion after electrodeposition coating in addition to the above-mentioned weldability and corrosion resistance. A test for evaluating paint adhesion after electrodeposition coating is the hot salt water immersion test (SDT). According to the inventor's new findings, in conventional surface-treated steel sheets, blisters may occur on the paint surface during SDT. When blisters occur on the paint surface of the surface-treated steel sheet, red rust is likely to occur from the blistered area. In conventional surface-treated steel sheets, sufficient studies have not been conducted on suppressing the occurrence of blisters during SDT to improve corrosion resistance, and there is room for improvement. [Means for solving the problem]

[0009] As one of the means for solving the above problems, the present application provides: A plated steel sheet having a zinc-containing plating layer; a surface treatment layer provided on at least one main surface of the plated steel sheet; A surface-treated steel sheet having the following structure: the surface treatment layer has a coating film, The coating film contains a binder resin, a rust inhibitor, and a conductive agent, The coating weight is 2 g / m 2 More than 30g / m 2 is as follows: When the surface-treated steel sheet is subjected to the following ion-exchange water immersion test, the electrical conductivity of the immersion water obtained is 10 μS / cm or more and 25 μS / cm or less. Surface-treated steel sheet Disclose.

[0010] Ion-exchanged water immersion test: The surface-treated steel sheet is cut to a main surface size of 0.5 cm x 4.5 cm to prepare 100 samples, and all of the 100 samples prepared are immersed in 200 ml of ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less for 30 minutes while applying ultrasonic vibrations of 40 kHz to obtain the immersion water.

[0011] In the surface-treated steel sheet of the present disclosure, The rust inhibitor may have the following properties:

[0012] Properties: When the rust inhibitor is dissolved at a concentration of 0.1% by mass in ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less to obtain a solution, the electrical conductivity of the solution is 100 μS / cm or less.

[0013] In the surface-treated steel sheet of the present disclosure, The conductive agent may be a doped oxide particle, a Si alloy containing 50% by mass or more of Si, a Si compound containing 50% by mass or more of Si, or a composite thereof; The content of the conductive agent in the coating film may be 5 vol % or more and 30 vol % or less.

[0014] In the surface-treated steel sheet of the present disclosure, The doped oxide particles may be doped zinc oxide particles.

[0015] In the surface-treated steel sheet of the present disclosure, The Si alloy or the Si compound may be ferrosilicon containing 70 mass % or more of Si.

[0016] In the surface-treated steel sheet of the present disclosure, The rust inhibitor may be amorphous silica having a particle size of 0.5 μm or more and 10 μm or less, The content of the amorphous silica in the coating film may be 5 vol % or more and 30 vol % or less. Effect of the Invention

[0017] The surface-treated steel sheet of the present disclosure suppresses the occurrence of blisters during SDT and has excellent corrosion resistance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, embodiments of the present invention will be described. Note that these descriptions are intended to be merely examples of the embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0019] 1. Surface-treated steel sheets The surface-treated steel sheet according to this embodiment includes a plated steel sheet having a zinc-containing plating layer and a surface treatment layer provided on at least one main surface of the plated steel sheet. The surface treatment layer includes a coating film. The coating film includes a binder resin, a rust inhibitor, and a conductive agent. The coating film has a coating weight of 2 g / m. 2 More than 30g / m 2 When the surface-treated steel sheet according to this embodiment is subjected to the following ion-exchanged water immersion test, the resulting immersion water has an electrical conductivity of 10 μS / cm or more and 25 μS / cm or less.

[0020] Ion-exchanged water immersion test: The surface-treated steel sheet is cut to a main surface size of 0.5 cm x 4.5 cm to prepare 100 samples, and all of the 100 samples prepared are immersed in 200 ml of ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less for 30 minutes while applying ultrasonic vibrations of 40 kHz to obtain the immersion water.

[0021] 1.1 Galvanized steel sheet The plated steel sheet may have, for example, a base steel sheet and a zinc-containing plating layer provided on at least one main surface of the base steel sheet. The "main surface" in this application refers to a surface corresponding to the front side or back side of the sheet. The zinc-containing plating layer may be provided on only one main surface of the base steel sheet, or on both main surfaces. In addition, the zinc-containing plating layer may be provided on the entire main surface of the base steel sheet, or on a part of the main surface.

[0022] As the base steel sheet, those having various chemical compositions and metal structures can be used. The base steel sheet may be an ordinary steel sheet or a steel sheet containing an additive element such as chromium, and the chemical composition and metal structure of the base steel sheet may be adjusted in consideration of the desired mechanical properties, formability, etc. The thickness of the base steel sheet is also not particularly limited, and may be, for example, 0.2 mm or more and 6.0 mm or less.

[0023] The zinc-containing plating layer may be a plating layer having a chemical composition known to those skilled in the art. For example, the zinc-containing plating layer may contain an additive element such as Al other than Zn, and may contain Fe or the like when alloying treatment is performed. As an example, the zinc-containing plating layer may be a Zn-Al-Mg alloy plating layer containing at least Al and Mg, or a Zn-Al-Mg-Si alloy plating layer further containing Si. The contents (concentrations) of each of these may be, in mass%, Al: 0.01 to 60%, Mg: 0.001 to 10%, Si: 0 to 2%, and the balance may be Zn and impurities. The zinc-containing plating layer may be an alloyed hot-dip galvanized layer, a hot-dip galvanized layer, or an electrolytic galvanized layer. The adhesion amount of the zinc-containing plating layer to the base steel sheet is not particularly limited, and may be a general adhesion amount.

[0024] 1.2 Surface treatment layer The surface treatment layer is provided on at least one of the main surfaces of the plated steel sheet. The surface treatment layer may be provided on only one of the main surfaces of the plated steel sheet, or on both main surfaces. The surface treatment layer may be provided on the entire main surface of the plated steel sheet, or on a part of the main surface. The surface treatment layer may be laminated on the surface of the zinc-containing plating layer among the surfaces of the plated steel sheet.

[0025] The surface treatment layer has a coating film. The surface treatment layer may be composed of only a coating film, or may have a two-layer structure of a coating film as an outer layer and a chemical conversion coating layer as an inner layer. When the surface treatment layer has such a two-layer structure, it can exhibit better corrosion resistance, etc. On the other hand, when the surface treatment layer does not have a chemical conversion coating as an inner layer, it can exhibit better spot weldability.

[0026] 1.2.1 Coating In the surface-treated steel sheet according to this embodiment, the coating film contains a binder resin, a rust inhibitor, and a conductive agent.

[0027] (binder resin) The binder resin contained in the coating film may be, for example, at least one resin selected from polyester resin, urethane resin, and acrylic resin. When a polyester resin is used as the binder resin, the polyester resin may have a glass transition temperature Tg of -20 to 70°C, and may have a number average molecular weight of 3000 to 30000. When a urethane resin is used as the binder resin, the urethane resin may have a Tg of 0 to 50°C, and may have a number average molecular weight of 5000 to 25000. When an acrylic resin is used as the binder resin, the acrylic resin may have a Tg of 0 to 50°C, and may have a number average molecular weight of 3000 to 25000. The binder resin may be cured by a curing agent. As the curing agent, for example, a melamine resin, an isocyanate resin, or an epoxy resin may be used. The content of the binder resin in the coating film is not particularly limited, and may be, for example, 50 vol% or more, or 60 vol% or more, and may be 90 vol% or less, 80 vol% or less, or 70 vol% or less.

[0028] (rust inhibitor) The rust inhibitor contained in the coating film may be an inorganic rust inhibitor or an organic rust inhibitor. The form of the rust inhibitor may be, for example, particulate. The rust inhibitor may be water-soluble or water-insoluble. In particular, in the surface-treated steel sheet according to this embodiment, it is preferable to use a rust inhibitor that has low solubility in water as the rust inhibitor contained in the coating film. Specifically, the rust inhibitor may have the following properties.

[0029] Properties: When the rust inhibitor is dissolved at a concentration of 0.1% by mass in ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less to obtain a solution, the electrical conductivity of the solution is 100 μS / cm or less.

[0030] Regarding the above-mentioned properties of the rust inhibitor, if the electrical conductivity of the solution is too high, the rust inhibitor contained in the coating film is likely to be excessively dissolved, and as described below, blisters are likely to occur in the coating film in a hot salt water immersion test (SDT). In this regard, if the electrical conductivity of the solution is 100 μS / cm or less, the problem of blister generation is easily avoided. The electrical conductivity of the solution may be 90 μS / cm or less, 80 μS / cm or less, 70 μS / cm or less, 60 μS / cm or less, 50 μS / cm or less, 40 μS / cm or less, 30 μS / cm or less, or 20 μS / cm or less. The lower limit of the electrical conductivity of the solution is not particularly limited, and may be, for example, 1 μS / cm or more, 5 μS / cm or more, or 10 μS / cm or more.

[0031] In the case where the rust inhibitor of the surface-treated steel sheet according to the present embodiment has the above-mentioned properties, the electrical conductivity of the immersion water obtained when the surface-treated steel sheet is subjected to the above-mentioned ion-exchanged water immersion test is easily controlled to a range of 10 μS / cm to 25 μS / cm. Various rust inhibitors having such properties can be mentioned.

[0032] For example, the rust inhibitor may contain at least one of P and V, which are elements that exhibit rust-preventing functions. Examples of rust inhibitors containing P include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, and tetraphosphoric acid, ammonium salts such as triammonium phosphate and diammonium hydrogen phosphate, metal phosphates with Na, Mg, Al, K, Ca, Mn, Ni, Zn, and Fe, phosphonic acids such as aminotri(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic acid) and salts thereof, and organic phosphoric acids such as phytic acid and salts thereof. In the surface-treated steel sheet according to the present embodiment, it is preferable to select a rust inhibitor that satisfies the above-mentioned properties from among these. Examples of rust inhibitors containing V include vanadium pentoxide, metavanadate HVO3, ammonium metavanadate, vanadium oxytrichloride VOCl3, vanadium trioxide V2O3, vanadium dioxide, vanadium oxysulfate VOSO4, vanadium oxyacetylacetonate VO(OC(=CH2)CH2COCH3)3, vanadium acetylacetonate V(OC(=CH2)CH2COCH3)3, vanadium trichloride VCl3, and the like. For the surface-treated steel sheet according to this embodiment, it is preferable to select from these rust inhibitors that satisfy the above-mentioned properties.

[0033] The rust inhibitor may contain a guanidino group-containing compound, a pyruguanidino group-containing compound, a thiocarbonyl group-containing compound, or the like.

[0034] Anti-rust pigments can also be used as the rust inhibitor. Examples of the anti-rust pigment include silica particles and metal phosphates (e.g., aluminum tripolyphosphate, etc.). When an anti-rust pigment is used as the rust inhibitor, its particle size and content are not particularly limited. In particular, when the rust inhibitor is amorphous silica, particularly when the amorphous silica has a particle size of 0.5 μm or more and 10 μm or less, the solubility of the amorphous silica in water is low, and the occurrence of blisters in the hot salt water immersion test (SDT) is more likely to be suppressed. In this case, the content of amorphous silica in the coating film may be, for example, 5 vol% or more and 30 vol% or less. When silica is used as the rust inhibitor, the silica may be one in which some elements have been exchanged with other elements such as calcium by ion exchange. The solubility of silica in water may change depending on the magnitude of the ion exchange rate with other elements such as calcium. When ion-exchanging other elements with silica, it is advisable to control the ion exchange rate so that the solubility in water does not become too large.

[0035] The content of the rust inhibitor in the coating film can be adjusted depending on the desired rust prevention effect. In the surface-treated steel sheet according to this embodiment, the content of the rust inhibitor is limited so that the electrical conductivity of the immersion water obtained when the surface-treated steel sheet is subjected to the above-mentioned ion-exchange water immersion test is 10 μS / cm or more and 25 μS / cm or less. The content of the rust inhibitor in the coating film may be, for example, 0.5 vol% or more, 1 vol% or more, or 5 vol% or more, or 40 vol% or less, 30 vol% or less, or 20 vol% or less, depending on the type of rust inhibitor.

[0036] (Conductive agent) The conductive agent contained in the coating film has a function of improving the conductivity of the coating film and improving the weldability of the surface-treated steel sheet. 3The conductive agent may have a volume resistivity of Ω / cm or less. Examples of the conductive agent include metals and metal compounds. Specifically, the conductive agent may be metals such as magnesium, aluminum, silicon, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, strontium, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, indium, and tin; alloys such as magnesium, aluminum, silicon, phosphorus, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, arsenic, strontium, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, palladium, silver, cadmium, indium, tin, antimony, and tellurium; or compounds such as oxides of the above-mentioned metal elements. Among these, magnesium, aluminum, silicon, chromium, iron, nickel, zinc, tin, zinc-aluminum alloy, zinc-aluminum-magnesium alloy, zinc-aluminum-magnesium-silicon alloy, zinc-iron alloy, zinc-chromium alloy, zinc-nickel alloy, iron-nickel alloy, iron-chromium alloy, stainless steel, ferrosilicon, ferromanganese, ferrophosphorus, zinc oxide, etc. are easily available. The content of the conductive agent in the coating film is not particularly limited, and may be appropriately determined in consideration of the intended weldability and corrosion resistance.

[0037] In particular, when the conductive agent is a doped oxide particle, a Si alloy containing 50% or more by mass of Si, a Si compound containing 50% or more by mass of Si, or a composite of these, it is easy to improve the adhesion of the electrodeposition coating film to the coating film as well as the conductivity (weldability). In this case, the content of the conductive agent in the coating film may be 5 vol% or more or 10 vol% or more, and may be 30 vol% or less or 25 vol% or less.

[0038] When the conductive agent is a doped oxide particle, a specific example of the doped oxide particle is a doped zinc oxide particle. Examples of the doped zinc oxide particle include zinc oxide particles whose conductivity is improved by doping at least one doping element selected from the group consisting of elements of Group 13 of the periodic table, such as B, Al, Ga, and In, and elements of Group 15 of the periodic table, such as P and As. When the doping element is Al or Ga, the conductivity is more easily improved. The content of the doping element may be, for example, 0.05 atom% or more or 0.1 atom% or more, and 5 atom% or less, based on the undoped zinc oxide particle.

[0039] When the conductive agent is a Si alloy or a Si compound, a specific example of the Si alloy or Si compound is ferrosilicon containing 70 mass% or more of Si. By including ferrosilicon as a conductive agent in the coating film, it is easy to improve the corrosion resistance as well as the conductivity. In particular, ferrosilicon containing 70 mass% or more of Si is excellent in corrosion resistance and moldability.

[0040] The conductive agent may be, for example, particulate. When the conductive agent is particulate, its average particle size is not particularly limited, and an appropriate size may be selected in consideration of the thickness of the coating film. If the particle size of the conductive agent is too small compared to the coating film, the conductivity is likely to decrease. On the other hand, if the particle size of the conductive agent is too large compared to the thickness of the coating film, the conductive agent is likely to be removed from the coating film. In this regard, the particle size of the conductive agent may be 1 / 10 or more or 1 / 5 or more of the thickness of the coating film, or may be 2 times or less. The average particle size of the conductive agent may be, for example, 0.1 μm or more, 0.3 μm or more, 0.5 μm or more, or 1.0 μm or more, or may be 20 μm or less, 10 μm or less, 8.0 μm or less, 6.0 μm or less, 5.0 μm or less, 4.0 μm or less, or 2.5 μm or less. The term "average particle size" refers to the average primary particle size when the particles present in the coating film exist as primary particles, and to the average secondary particle size when the particles exist as aggregates. The average particle size is measured as follows. That is, a surface-treated steel sheet on which a coating film has been formed is cut, the cross section is exposed and polished, and the polished cross section thus obtained is observed with a scanning electron microscope to obtain an observation image. Several particles present in the field of view of the observation image are randomly selected, and the circle equivalent diameter of each particle is determined, and the average value of these is taken as the average particle size. Whether or not the particles in the observation image are conductive agents can be easily determined by elemental analysis or the like.

[0041] (Amount of adhesion) In the surface-treated steel sheet according to this embodiment, the coating weight of the coating film is 2 g / m 2 More than 30g / m 2 If the coating weight is too low, the corrosion resistance is likely to decrease. If the coating weight is too high, the weldability is likely to decrease. In the surface-treated steel sheet according to this embodiment, the coating weight is 3 g / m or less. 2 or more than 4 mg / m 2 or more, and 25 g / m 2 Below 20g / m 2 Less than or equal to 15g / m 2or less. The adhesion amount of the coating film on the surface-treated steel sheet can be measured by a gravimetric method or by cross-sectional observation. In measuring the adhesion amount by the gravimetric method, the initial weight of the steel sheet cut to a predetermined size is measured, and then the coating film is removed by using a solvent capable of dissolving the binder resin or a dedicated agent, or by a blasting treatment using resin beads or alumina beads, and the weight of the steel sheet from which the coating film has been removed is measured, and the adhesion amount can be calculated by finding the difference between the weights.

[0042] 1.2.2 Chemical conversion layer In the surface-treated steel sheet according to the present embodiment, the surface treatment layer may include a chemical conversion treatment layer in addition to the above-mentioned coating film. That is, the surface treatment layer may have a two-layer structure including a coating film as an outer layer and a chemical conversion treatment layer as an inner layer.

[0043] By providing a chemical conversion layer as an inner layer on the surface of a plated steel sheet and further providing the above-mentioned coating film on the surface of the chemical conversion layer, the adhesion of the coating film to the steel sheet is improved. The chemical conversion layer may be a layer that does not substantially contain chromium (chromate-free layer). Examples of chromate-free treatment liquids used in the chemical conversion treatment include silica-based treatment liquids mainly composed of silicon compounds such as liquid-phase silica, gas-phase silica, and silicates, zircon-based treatment liquids mainly composed of zircon-based compounds, and mixtures thereof. The chemical conversion layer may contain a binder resin. For example, the chemical conversion layer may contain at least one of the binder resins exemplified as those that can constitute the above-mentioned coating film, and may contain a polyester resin. The content of the binder resin in the chemical conversion layer and the content of components other than the binder resin (such as the above-mentioned silicon compounds) are not particularly limited. For example, the content of the binder resin in the chemical conversion treatment layer may be 0 vol% or more and 50 vol% or less, and the content of components other than the binder resin may be 50 vol% or more and 100 vol% or less. The chemical conversion treatment layer as the inner layer may be an inorganic coating containing an inorganic component as a binder.

[0044] In the surface-treated steel sheet, the coating weight of the chemical conversion coating layer is not particularly limited. For example, 2 More than 2000mg / m 2 When the content is below 100%, the corrosion resistance of the surface-treated steel sheet is further improved. The adhesion weight of the chemical conversion layer on the surface-treated steel sheet can be measured by fluorescent X-rays and cross-sectional analysis. Specifically, a calibration curve plate is prepared for each chemical conversion treatment. The chemical conversion treated sheet and the calibration curve plate are measured by fluorescent X-rays, and the adhesion weight of the prepared chemical conversion treated sheet is calculated from the X-ray intensity of the contained elements and the X-ray intensity of the calibration curve plate.

[0045] 1.2.3 Other The coating film or the chemical conversion treatment layer may contain other components in addition to the above-mentioned components. Examples of the other components include various additives. For example, pigments other than the above-mentioned rust-preventive pigments (such as bright pigments for improving design), lubricants, defoamers, thickeners, etc. The content of the other components in the surface treatment layer is not particularly limited.

[0046] 1.3 Electrical conductivity of immersion water in ion-exchange water immersion test In the surface-treated steel sheet according to this embodiment, the electrical conductivity of the immersion water obtained when the surface-treated steel sheet is subjected to the above-mentioned ion-exchanged water immersion test is 10 μS / cm or more and 25 μS / cm or less. The electrical conductivity of the immersion water may be 11 μS / cm or more, 12 μS / cm or more, or 13 μS / cm or more, or may be 24 μS / cm or less, 23 μS / cm or less, or 22 μS / cm or less.

[0047] Generally, the coating film provided on the surface-treated steel sheet is unavoidably permeable to moisture such as water vapor. When moisture penetrates the coating film, rust inhibitors and the like are dissolved, and the rust-preventing function and adhesion function of the coating film can be exhibited. When the electrical conductivity of the immersion water obtained by the above-mentioned ion-exchange water immersion test is high, it means that the coating film of the surface-treated steel sheet contains rust inhibitors and the like that are highly soluble in water and easily dissolved as ions. According to the findings of the present inventors, when the electrical conductivity of the immersion water obtained by the above-mentioned ion-exchange water immersion test is too high (i.e., when the coating film contains rust inhibitors and the like that are easily dissolved excessively as ions), improvement in corrosion resistance and the like is expected due to the elution of the rust inhibitor when moisture penetrates the coating film, but a region with a high ion concentration may occur locally in the coating film. Since the ion concentration of the region is higher than that of the warm saltwater in the warm saltwater immersion test (SDT), moisture penetrates into the region due to the osmotic pressure during the SDT, and local blisters are likely to occur in the coating film. Blisters are likely to occur at the coating / chemical conversion layer interface or the coating / plated steel sheet interface, which have higher surface free energy. In particular, when an electrodeposition coating is applied on the coating, water discharge from the coating to the outside of the coating is further suppressed, so that the occurrence of blisters becomes more pronounced. The location where blisters occur in the coating becomes hollow after drying, and when a corrosion test is performed, corrosion factors remain in the hollow, which is likely to cause red rust and the like to occur from this as a starting point. In contrast, the surface-treated steel sheet according to this embodiment has such a property that when the above-mentioned ion-exchange water immersion test is performed, the electrical conductivity of the immersion water is 25 μS / cm or less. Therefore, even if moisture penetrates into the coating, a region with a locally high ion concentration is unlikely to occur in the coating, and as a result, blisters are unlikely to occur even after SDT.

[0048] On the other hand, if the electrical conductivity of the immersion water obtained by the above-mentioned ion-exchanged water immersion test is too low, even if moisture penetrates into the coating film, the rust inhibitor and the like will not be eluted from the coating film, and the rust prevention function and adhesion improvement function are difficult to exhibit. In contrast, the surface-treated steel sheet according to this embodiment has such a property that the electrical conductivity of the immersion water is 10 μS / cm or more when the above-mentioned ion-exchanged water immersion test is performed, so that when moisture penetrates into the coating film, the rust inhibitor and the like can be eluted to a certain extent, and sufficient rust prevention function and adhesion improvement function are exhibited.

[0049] As described above, the electrical conductivity of the immersion water in an ion-exchanged water immersion test for a surface-treated steel sheet can be controlled by the properties of the rust inhibitor contained in the coating film, the content of the rust inhibitor contained in the coating film, the thickness (adhesion amount) of the coating film, etc.

[0050] 2. Manufacturing method of surface-treated steel sheet The above-mentioned surface-treated steel sheet can be produced, for example, by the following method. That is, the method for producing the surface-treated steel sheet is as follows: Obtaining a plated steel sheet having a zinc-containing plating layer; and forming a coating film by applying a coating material containing a binder resin, a rust inhibitor, and a conductive agent to at least one main surface of the plated steel sheet; may also include

[0051] Alternatively, a method for producing a surface-treated steel sheet includes the steps of: Obtaining a plated steel sheet having a zinc-containing plating layer; forming a chemical conversion treatment layer on at least one main surface of the plated steel sheet; and applying a coating material containing a binder resin, a rust inhibitor, and a conductive agent to a surface of the chemical conversion treatment layer to form a coating film; may include:

[0052] 2.1 Preparation of plated steel sheet A plated steel sheet having a zinc-containing plating layer can be obtained, for example, by obtaining a slab by continuous casting, hot rolling the slab to obtain a hot-rolled sheet, coiling the hot-rolled sheet, cold rolling the hot-rolled sheet to obtain a cold-rolled sheet, annealing the cold-rolled sheet, plating the annealed sheet, and optionally performing a skin pass, etc. The continuous casting conditions, hot rolling conditions, coiling conditions, cold rolling conditions, annealing conditions, and plating conditions may be general conditions known in the art.

[0053] 2.2 Chemical treatment In the manufacturing method of the present disclosure, a chemical conversion treatment layer may be formed as an inner layer by subjecting at least one main surface of the plated steel sheet obtained as described above to a chemical conversion treatment. The chemical conversion treatment can be performed by applying various treatment solutions as described above to the surface of the steel sheet and drying it.

[0054] 2.3 Coating film formation In the manufacturing method of the present disclosure, a coating film as an outer layer may be formed by applying a paint containing a binder resin, a rust inhibitor, and a conductive agent to the surface of the plated steel sheet obtained as described above or to the surface of the chemical conversion coating layer formed as described above, and then drying the paint. Here, by adjusting the type of rust inhibitor contained in the coating film, the content of the rust inhibitor, the thickness of the coating film, etc., it is possible to obtain a surface-treated steel sheet in which the electrical conductivity of the immersion water falls within a predetermined range in the above-mentioned ion-exchange water immersion test. EXAMPLES

[0055] The present invention will be further described below with reference to examples, but the present invention is not limited to the following examples. The present invention can adopt various conditions as long as it does not deviate from the gist of the invention and achieves its object.

[0056] 1. Manufacturing of surface-treated steel sheets 1.1 Preparation of plated steel sheet The following five types of zinc-based plated steel sheets and cold-rolled steel sheets were prepared, and the surfaces were degreased by immersing them in an aqueous solution (concentration 2.5 mass%, 40°C) of an aqueous alkaline degreaser (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for 2 minutes, and then rinsed with water and dried to prepare metal sheets for surface treatment.

[0057] GA: Galvannealed steel sheet (thickness 0.8 mm, 10% Fe by mass, coating weight 45 g / m 2 ) ZL: Zn-10% Ni alloy electroplated steel sheet (thickness 0.8 mm, coating weight 40 g / m 2 ) GI: Hot-dip galvanized steel sheet (sheet thickness 0.8 mm, coating weight 60 g / m 2 ) EG: Electrogalvanized steel sheet (sheet thickness 0.8 mm, coating weight 40 g / m 2 ) CR: Cold rolled steel plate (thickness 0.8mm)

[0058] 1.2 Formation of chemical conversion coating layer Next, a chemical conversion coating layer was formed on some of the metal sheets. Specifically, the following chemical conversion coating solution S was prepared, and the composition was applied to the metal sheets so that the adhesion amount after drying was 500 mg / m 2 The bar coat size was changed so that the coating was applied to a metal plate, which was then dried in a hot air oven at a metal surface temperature of 70°C and air-dried to form a chemical conversion coating layer on the surface of the metal plate.

[0059] S: Zr compound, silane coupling agent, silica particles, polyester resin, Nv10% treatment solution for forming undercoat

[0060] 1.3 Coating film formation Next, in order to form a coating film having the composition (vol%) shown in Table 1, each component was mixed to obtain a solid content concentration similar to that in Table 1, to prepare a coating composition for forming a coating film. This composition was applied to a steel plate or a chemical conversion coating layer with a bar coater while changing the number of the bar coat and the dilution ratio so as to obtain the adhesion amount shown in Table 2, and dried in an oven under conditions of a maximum temperature of 200°C, to obtain a surface-treated steel plate having a coating film as a surface treatment layer. The components contained in the coating composition are shown below.

[0061] (Anti-rust pigment) PA: Aluminum tripolyphosphate (average particle size 1-2 μm) PM: Magnesium phosphate (average particle size 1-2 μm) SC1: Calcium ion-exchanged silica (Ca exchange rate 6%) (average particle size 1-2 μm) SC2: Calcium ion-exchanged silica (Ca exchange rate 9%) (average particle size 1-2 μm) Si: Silica (oil absorption 100~1000ml / 100g, specific surface area 200~1000m 2 / g, amorphous silica with an average particle size of 1 to 30 μm) (Fuji Silysia Silomask 02) HP: Hydrocalumite-treated zinc phosphate (TOHO Pigment EXPERT NP-530 N5) (average particle size 1-2 μm) PN: Pentasodium triphosphate (general reagent) (average particle size 3-5 μm) PK: Potassium dihydrogen phosphate (general reagent) (average particle size 3-5 μm) CS: Strontium chromate (average particle size 2-3 μm)

[0062] In addition, for each of the above anti-rust pigments, when a solution was obtained by dissolving each of the pigments at a concentration of 0.1% by mass in ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less, the measurement results of the electrical conductivity of the solution are shown in Table 3.

[0063] (Conductive pigment) FeSi: Ferrosilicon particles (average particle size 3-7μm) SUS: SUS particles (average particle size 3~7μm) ZnO: Doped zinc oxide particles Conductive zinc oxide particles (Hakusui Tech Co., Ltd. 23-Kt (average particle size = 0.5 μm))

[0064] (binder resin) B1: A resin was used which was a blend of polyester resin (Vylon 200 manufactured by Toyobo Co., Ltd.) and melamine (Cymel 325 manufactured by Mitsui Cytec Co., Ltd.) in a solid content ratio of 70:30. B2: Water-based epoxy resin (ADEKA 0434AN)

[0065] 2.Performance evaluation test 2.1 Ion-exchange water immersion test (electrical conductivity of immersion water) (1) For each of the surface-treated steel sheets, 100 test pieces with a main surface size of 0.5 cm x 4.5 cm (the sum of the end face lengths was 10 cm) were cut out by shearing (the total end face length for each sample was 10 cm x 100 = 10 m). (2) These 100 test pieces were immersed together in 200 ml of ion-exchanged water at 50° C. in a beaker placed on an ultrasonic vibration device. (3) While maintaining the temperature at 50° C., ultrasonic vibration of 40 kHz was applied to the beaker for 30 minutes. US CLEANER manufactured by AS ONE Corporation was used as the ultrasonic vibration device. (4) After the application of ultrasonic vibration was completed, the test piece was immediately removed, and the electrical conductivity of the resulting aqueous solution (immersion water) was measured with an electrical conductivity meter (D-54SE manufactured by Horiba, Ltd.).

[0066] 2.2 Warm salt water immersion test (Advance preparation) For each surface-treated steel sheet, surface conditioning was performed at room temperature for 20 seconds using a surface conditioning treatment agent Preparen X (trade name) manufactured by Nihon Parkerizing Co., Ltd. Furthermore, chemical conversion treatment (phosphate treatment) was performed using a chemical conversion treatment liquid (zinc phosphate treatment liquid) "Palbond 3020 (trade name)" manufactured by Nihon Parkerizing Co., Ltd. The temperature of the chemical conversion treatment liquid was set to 43°C, and the surface-treated steel sheet was immersed in the chemical conversion treatment liquid for 120 seconds, followed by washing with water and drying. After the above-mentioned chemical conversion treatment (phosphate treatment) was performed, a cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd. was electrocoated with a slope current of 160V, and further baked at a baking temperature of 170°C for 20 minutes. The average thickness of the electrodeposition coating film after electrocoating was 10 μm for all samples.

[0067] (Hot Salt Water Immersion Test (SDT)) After the above-mentioned electrodeposition coating, the end faces of the surface-treated steel sheets were sealed with sealing tape and immersed in a 3% NaCl aqueous solution at a temperature of 50°C for 500 hours. After the immersion test, the samples were taken out and dried, and the area ratio of blisters present on the surface of the electrodeposition coating was measured visually. In the corrosion resistance test, samples with a score of "3", "4" or "5" were judged to have excellent corrosion resistance. The results are shown in Table 2. 1: Blister area ratio from the evaluation surface is 50% or more 2: Blister area ratio from the evaluation surface is 5% or more and less than 50% 3: Blister area ratio from the evaluation surface is 1% or more and less than 5% 4: Blisters are generated on the evaluation surface, but the area ratio is less than 1% 5: No blistering

[0068] 2.3 Corrosion resistance test after warm salt water test The surface-treated steel sheets that had been subjected to the electrodeposition coating after the warm salt water immersion test were subjected to a cyclic corrosion test for 120 cycles under the following cycle conditions.

[0069] (Cycle conditions) The test was performed in a cycle consisting of 2 hours of salt spray (SST, 5% NaCl, 35°C atmosphere), 2 hours of dryness (60°C), and 4 hours of wetness (50°C, 98% RH).

[0070] Thereafter, the corrosion state was observed from the flat surface and the following rating was given. In the corrosion resistance test, a rating of "3", "4" or "5" was judged to be excellent in corrosion resistance. The results are shown in Table 2. 1: The area ratio of white rust occurring from the evaluation surface is 50% or more, or red rust occurring from the evaluation surface is confirmed 2: The area ratio of white rust occurring on the evaluation surface is 10% or more but less than 50%. 3: The area ratio of white rust on the evaluation surface is 5% or more but less than 10%. 4: The area ratio of white rust on the evaluation surface is 1% or more but less than 5%. 5: The area ratio of white rust on the evaluation surface is less than 1%

[0071] 2.4 Spot weldability The surface treatment thus produced was subjected to a continuous spot welding test using a CF type Cr-Cu electrode with a tip diameter of 5 mm and R40 at a pressure of 1.96 kN, a welding current of 8 kA, and a current flow time of 12 cycles / 50 Hz, and the number of spots just before the nugget diameter fell below 3√t (t is the plate thickness) was determined. The spot weldability was evaluated using the following evaluation points. In the weldability test, a score of "4", "5", or "6" was determined to be excellent in weldability. The results are shown in Table 2. 1: No nugget is formed and no welding points are possible, or the number of welding points is less than 10 2: 10 or more RBIs but less than 50 RBIs 3: 50 to less than 200 RBIs 4: RBIs between 200 and 1,000 5: RBIs between 1000 and 2000 6: 2000 or more RBIs

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] The results shown in Tables 1 to 3 reveal the following:

[0076] For Nos. 3 to 9 in Table 2, the electrical conductivity of the immersion water obtained by the ion-exchange water immersion test was too high, so blisters occurred in the hot water immersion test, and the corrosion resistance after the hot water immersion test decreased. This is presumed to be due to the following mechanism. That is, when the electrical conductivity of the immersion water obtained by the ion-exchange water immersion test is high, it means that the rust inhibitor contained in the coating film is an rust inhibitor that is easily eluted as ions (PM, SC1, SC2, HP, PN, PK, CS). In this case, when moisture penetrates into the coating film, an area with a high ion concentration may occur locally in the coating film due to excessive elution of the rust inhibitor. Since the ion concentration of the area is higher than that of the hot saltwater in the hot saltwater immersion test, moisture penetrates into the area during the test due to the osmotic pressure, and local blisters occur in the coating film. In particular, when an electrodeposition coating is applied on the coating film as in the above example, the discharge of water from the coating film to the outside of the coating film is further suppressed, so the occurrence of blisters becomes even more noticeable. When blisters occur in the coating film in this way, the blistered area becomes hollow, and this becomes the starting point for the development of red rust and the like.

[0077] As for No. 25, since the coating did not contain conductive pigments, the necessary conductivity of the coating could not be secured, and the weldability decreased.

[0078] In the case of No. 31, the amount of coating film adhered was too large, making it difficult to obtain a conductive path during spot welding, and the weldability of the surface-treated steel sheet was reduced.

[0079] In the case of No. 33, the amount of coating film adhered was too small, so the corrosion resistance of the surface-treated steel sheet after the hot water immersion test decreased.

[0080] As for No. 38, since a steel sheet not having a zinc-containing plating layer was used as the base material, the corrosion resistance after the hot water immersion test was reduced.

[0081] In contrast, for Nos. 1, 2, 10 to 24, 26 to 30, 32, and 34 to 37, steel sheets having a zinc-containing plating layer were used as the substrate, the coating weight was within the specified range, and the electrical conductivity of the immersion water obtained by the ion-exchanged water immersion test was within the specified range. Therefore, the occurrence of blisters in the hot water immersion test was suppressed, sufficient corrosion resistance was secured even after the hot water immersion test, and further, the weldability was excellent.

[0082] From the above results, it can be said that the surface-treated steel sheet described below suppresses the occurrence of blisters in the hot salt water immersion test, has excellent corrosion resistance, and further has excellent weldability.

[0083] A plated steel sheet having a zinc-containing plating layer; a surface treatment layer provided on at least one main surface of the plated steel sheet; A surface-treated steel sheet having the following structure: the surface treatment layer has a coating film, The coating film contains a binder resin, a rust inhibitor, and a conductive agent, The coating weight is 2 g / m 2 More than 30g / m 2 is as follows: When the surface-treated steel sheet is subjected to the following ion-exchange water immersion test, the electrical conductivity of the immersion water obtained is 10 μS / cm or more and 25 μS / cm or less. Surface treated steel sheet. Ion-exchanged water immersion test: The surface-treated steel sheet is cut to a main surface size of 0.5 cm x 4.5 cm to prepare 100 samples, and all of the 100 samples prepared are immersed in 200 ml of ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less for 30 minutes while applying ultrasonic vibrations of 40 kHz to obtain the immersion water.

[0084] Furthermore, from the results shown in No. 13, it can be seen that even if the solubility of the rust inhibitor contained in the coating film in water is somewhat high (the electrical conductivity of the solution is around 100 μS / cm), by reducing the content, the electrical conductivity of the immersion water obtained in the ion-exchanged water immersion test falls within a specified range, the occurrence of blisters in the hot water immersion test is suppressed, and corrosion resistance and weldability are ensured even after the hot water immersion test. From the results of No. 13 etc., it can be said that it is preferable for the rust inhibitor contained in the coating film in surface-treated steel sheets to have the following properties.

[0085] Properties: When the rust inhibitor is dissolved at a concentration of 0.1% by mass in ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less to obtain a solution, the electrical conductivity of the solution is 100 μS / cm or less.

Claims

1. A plated steel sheet having a zinc-containing plating layer; a surface treatment layer provided on at least one main surface of the plated steel sheet; A surface-treated steel sheet having the following structure: the surface treatment layer has a coating film, The coating film contains a binder resin, a rust inhibitor, and a conductive agent, The coating weight of the coating film is 2 g / m ２ 30g / m or more ２ is as follows: When the surface-treated steel sheet is subjected to the following ion-exchanged water immersion test, the resulting immersion water has an electrical conductivity of 10 μS / cm or more and 25 μS / cm or less. Surface treated steel sheet. Ion-exchanged water immersion test: The surface-treated steel sheet was cut to a main surface size of 0.5 cm × 4.5 cm to prepare 100 samples, and all of the prepared 100 samples were immersed in 200 ml of ion-exchanged water having a temperature of 50°C and an electrical conductivity of 4 μS / cm or less while applying ultrasonic vibration of 40 kHz for 30 minutes to obtain the immersion water.

2. The rust inhibitor has the following properties: The surface-treated steel sheet according to claim 1. Properties: When the rust inhibitor is dissolved at a concentration of 0.1% by mass in ion-exchanged water having a temperature of 50° C. and an electrical conductivity of 4 μS / cm or less to obtain a solution, the electrical conductivity of the solution is 100 μS / cm or less.

3. the conductive agent is a doped oxide particle, a Si alloy containing 50% by mass or more of Si, a Si compound containing 50% by mass or more of Si, or a composite thereof; The content of the conductive agent in the coating film is 5 vol% or more and 30 vol% or less. The surface-treated steel sheet according to claim 1 or 2.

4. The conductive agent comprises the doped oxide particles, the doped oxide particles are doped zinc oxide particles; The surface-treated steel sheet according to claim 3.

5. The conductive agent contains the Si alloy or the Si compound, The Si alloy or the Si compound is ferrosilicon containing 70 mass% or more of Si. The surface-treated steel sheet according to claim 3 or 4.

6. The rust inhibitor is amorphous silica having a particle size of 0.5 μm or more and 10 μm or less, The content of the amorphous silica in the coating film is 5 vol% or more and 30 vol% or less. The surface-treated steel sheet according to any one of claims 1 to 5.

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