Surface-treated steel sheet

Abstract

To disclose a surface-treated steel plate with excellent weldability and corrosion resistance.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 chemical conversion treatment layer and a coating film, and the coating film contains binder resin, an anti-rust agent, and a conductive agent. The chemical conversion treatment layer is in contact with the plating steel plate, and a ratio Tmax / Tmin of Tmax and Tmin measured under a predetermined condition using EPMA is 2.0 or more and 8.0 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

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

[0002] Surface-treated steel sheets are used as structural materials for automobiles and the like. The surface-treated steel sheet has, 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 chemical conversion treatment layer and a coating film are adopted as the surface treatment layer, and the weldability and corrosion resistance of the surface-treated steel sheet are improved by adjusting the types and contents of the components constituting the chemical conversion treatment layer and the coating film.

[0003] For example, Patent Document 1 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 including a predetermined amount of a binder resin, non-oxide ceramic particles containing V, and doped zinc oxide particles in the coating film.

[0004] Further, Patent Document 2 discloses a technique for improving the weldability and corrosion resistance of a coated metal material having an organic film on the surface of a metal material by including a predetermined resin having a urethane bond and predetermined conductive particles in the organic film.

[0005] Further, Patent Documents 3 and 4 disclose a technique for improving the weldability and corrosion resistance of a coated metal plate having a coating layer on the surface of a metal plate by including a predetermined amount of conductive particles having a predetermined particle size in the coating layer.

[0006] Further, Patent Document 5 discloses a technique for improving the weldability and corrosion resistance of a surface-treated steel sheet having an undercoat layer on at least one surface of a zinc-based plated steel sheet and an organic resin film thereon by including a predetermined amount of ferrosilicon having a predetermined particle size in the organic resin film and setting the thickness of the organic resin film within a predetermined range.

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] International Publication No. 2018 / 092244 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-042622 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-183080 [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2004-183082 [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2002-172363 [Summary of the Invention] [Problems to be Solved by the Invention]

[0008] In the prior art, in improving the weldability and corrosion resistance of surface-treated steel sheets, although the types and contents of the components constituting the surface treatment layer have been studied, there has been insufficient study on improving the weldability and corrosion resistance of surface-treated steel sheets by devising other factors. In this regard, there is room for improvement in improving the weldability and corrosion resistance of surface-treated steel sheets. [Means for Solving the Problems]

[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, and a surface-treated steel sheet having the above, wherein the surface treatment layer has a chemical conversion treatment layer and a coating film, the coating film contains a binder resin, a rust preventive agent, and a conductive agent, the chemical conversion treatment layer is in contact with the plated steel sheet, and the ratio Tmax / Tmin of Tmax to Tmin measured under the following conditions is 2.0 or more and 8.0 or less, a surface-treated steel sheet is disclosed.

[0010] Condition: With the coating film removed from the surface-treated steel sheet, for the chemical conversion treatment layer present on the main surface of the plated steel sheet, using an electron probe microanalyzer (EPMA), a 10 mm square area is divided into a plurality of 260×260 areas for elemental mapping, the intensity of the chemical conversion treatment components in each area is specified, the intensity that is the top 10% in terms of area% is specified as Tmax, and the intensity that is the bottom 10% in terms of area% is specified as Tmin.

[0011] In the surface-treated steel sheet of the present disclosure, the coating film may contain a polyester resin as the binder resin.

[0012] In the surface-treated steel sheet of the present disclosure, the coating film may contain ferrosilicon as the conductive agent.

[0013] In the surface-treated steel sheet of the present disclosure, the coating film may contain at least one of a phosphorus compound and a vanadium compound as the rust preventive agent.

[0014] In the surface-treated steel sheet of the present disclosure, the coating film may contain silica as the rust preventive agent.

[0015] In the surface-treated steel sheet of the present disclosure, the coating film deposition amount may be 2 g / m 2 or more and 30 g / m 2 or less.

[0016] In the surface-treated steel sheet of the present disclosure, the chemical conversion treatment layer may contain a polyester resin, a silane coupling agent, silica, and tannic acid.

Advantages of the Invention

[0017] In the surface-treated steel sheet of the present disclosure, there is a predetermined unevenness in the amount of the chemical conversion treatment layer on the main surface of the plated steel sheet. In the surface-treated steel sheet of the present disclosure, while high corrosion resistance is ensured by the thick portion of the chemical conversion treatment layer, high conductivity is ensured by the thin portion of the chemical conversion treatment layer, and the balance between weldability and corrosion resistance of the entire surface-treated steel sheet is good.

Brief Description of the Drawings

[0018]

Figure 1

Embodiments for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described. Note that these descriptions are intended merely as examples of the embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0020] 1. Surface-Treated Steel Sheet The surface-treated steel sheet according to the present embodiment has 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 has a chemical conversion treatment layer and a coating film. The coating film contains a binder resin, a rust preventive agent, and a conductive agent. The chemical conversion treatment layer is in contact with the plated steel sheet. In the surface-treated steel sheet according to the present embodiment, the ratio Tmax / Tmin of Tmax to Tmin measured under the following conditions is 2.0 or more and 8.0 or less.

[0021] Condition: With the coating film removed from the surface-treated steel sheet, regarding the chemical conversion treatment layer present on the main surface of the plated steel sheet, using an electron probe microanalyzer (EPMA), a 10 mm square area is divided into a plurality of areas of 260 × 260, elemental mapping is performed, the intensity of the chemical conversion treatment component in each area is specified, the intensity that is the top 10% in terms of area% is specified as Tmax, and the intensity that is the bottom 10% in terms of area% is specified as Tmin.

[0022] 1.1 Plated 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" referred to in the present application is a surface corresponding to the front or back side of the plate. The zinc-containing plating layer may be provided on only one main surface of the base steel sheet, or may be provided on both main surfaces. Further, the zinc-containing plating layer may be provided over the entire main surface of the base steel sheet, or may be provided on a part of the main surface.

[0023] As the base steel sheet, those having various chemical compositions and metal structures can be adopted. The base steel sheet may be a plain steel sheet or a steel sheet containing additive elements such as chromium, and the chemical composition and metal structure of the base steel sheet may be adjusted in consideration of the intended mechanical properties and formability. Further, the thickness of the base steel sheet is not particularly limited, and may be, for example, 0.2 mm or more and 6.0 mm or less.

[0024] 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 additive elements such as Al in addition to Zn, and may contain Fe, etc. 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 may be a Zn-Al-Mg-Si alloy plating layer further containing Si. The respective contents (concentrations) 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 zinc plating layer, a hot-dip zinc plating layer, or an electro-galvanized plating layer. The adhesion amount of the zinc-containing plating layer to the base steel plate is not particularly limited and may be a general adhesion amount.

[0025] 1.2 Surface treatment layer The surface treatment layer is provided on at least one main surface of the plated steel sheet. The surface treatment layer may be provided on only one main surface of the plated steel sheet, or may be provided on both main surfaces. Also, the surface treatment layer may be provided on the entire main surface of the plated steel sheet, or may be provided on a part of the main surface. The surface treatment layer can be laminated on the surface of the zinc-containing plating layer among the surfaces of the above-mentioned plated steel sheet.

[0026] The surface treatment layer has a chemical conversion treatment layer and a coating film. The chemical conversion treatment layer is in contact with the plated steel sheet. Specifically, 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.

[0027] 1.2.1 Coating film In the surface-treated steel sheet according to the present embodiment, the coating film contains a binder resin, a rust preventive agent, and a conductive agent.

[0028] (Binder resin) The binder resin contained in the coating film may be, for example, at least one resin selected from polyester resins, urethane resins, and acrylic resins. In particular, when the coating film contains a polyester resin as the binder resin, high performance is likely to be exhibited. When a polyester resin is employed 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 3,000 to 30,000. When a urethane resin is employed 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 5,000 to 25,000. When an acrylic resin is employed 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 3,000 to 25,000. The binder resin may be cured by a curing agent. As the curing agent, for example, a melamine resin, an isocyanate resin, an epoxy resin, or the like may be employed. The content of the binder resin in the coating film is not particularly limited and may be, for example, 50% by mass or more or 60% by mass or more, and may be 90% by mass or less, 80% by mass or less, or 70% by mass or less.

[0029] (Rust preventive agent) The rust preventive agent contained in the coating film may be an inorganic rust preventive agent or an organic rust preventive agent. The form of the rust preventive agent may be, for example, particulate. The rust preventive agent may be water-soluble or water-insoluble. When the rust preventive agent is water-soluble, for example, when the coating film is exposed to a wet environment, the rust preventive agent in the coating film dissolves in water and elutes, and a rust preventive function of suppressing corrosion of the plating layer or the like can be exhibited.

[0030] The content of the rust preventive agent in the coating film may be appropriately adjusted according to the intended rust preventive effect. For example, the content of the rust preventive agent in the coating film may be 1 vol% or more, 5 vol% or more, or 10 vol% or more, and may be 40 vol% or less, 30 vol% or less, or 20 vol% or less.

[0031] In the surface-treated steel sheet according to the present embodiment, the coating film may contain at least one of a phosphorus compound and a vanadium compound as a rust preventive agent. Examples of the phosphorus compound that can exhibit the function as a rust preventive agent include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, and tetrapolyphosphoric acid; ammonium salts such as ammonium triphosphate and diammonium hydrogen phosphate; metal phosphates with Na, Mg, Al, K, Ca, Mn, Ni, Zn, Fe, etc.; phosphonic acids such as aminotri(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic acid) and their salts; organic phosphoric acids such as phytic acid and their salts, etc. Examples of the vanadium compound that can exhibit the function as a rust preventive agent include vanadium pentoxide, metavanadic acid HVO3, ammonium metavanadate, vanadyl trichloride VOCl3, vanadium trioxide V2O3, vanadium dioxide, vanadyl sulfate VOSO4, vanadyl acetylacetonate VO(OC(=CH2)CH2COCH3)3, vanadium acetylacetonate V(OC(=CH2)CH2COCH3)3, vanadium trichloride VCl3, etc.

[0032] The rust preventive agent may contain a guanidino group-containing compound, a biguanidino group-containing compound, a thiocarbonyl group-containing compound, etc.

[0033] A rust preventive pigment can also be used as the rust preventive agent. Examples of the rust preventive pigment include silica and metal phosphates (e.g., aluminum tripolyphosphate, etc.). In particular, when the coating film contains silica as a rust preventive agent, higher performance is likely to be exhibited. When a rust preventive pigment is used as the rust preventive agent, its particle size and content are not particularly limited.

[0034] (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. In the present application, for example, 1.0×10 3Those having a volume resistivity of 1 Ω / cm or less can be used as the conductive agent. Examples of the conductive agent include metals and metal compounds. Specifically, 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, 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, rhodium, palladium, silver, cadmium, indium, tin, antimony, tellurium; or compounds such as oxides of the above-mentioned metal elements may be used. Among them, 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 desired weldability and corrosion resistance.

[0035] When the conductive agent is doped oxide particles, an Si alloy containing 50% by mass or more of Si, an Si compound containing 50% by mass or more of Si, or a composite thereof, it is easy to improve the conductivity (weldability) and the adhesion of the electrodeposition coating film to the coating film. In this case, the content of the conductive agent in the coating film may be 5% by mass or more and 30% by mass or less.

[0036] When the conductive agent is doped oxide particles, specific examples of the doped oxide particles include doped zinc oxide particles. Examples of the doped zinc oxide particles include those obtained by doping at least one doping element selected from the group consisting of Group 13 elements of the periodic table such as B, Al, Ga, and In, and Group 15 elements of the periodic table such as P and As into zinc oxide particles to improve conductivity. When the doping element is Al or Ga, it is easier to further improve the conductivity. The content of the doping element may be, for example, 0.05 atom% or more or 0.1 atom% or more, and may be 5 atom% or less, based on the undoped zinc oxide particles.

[0037] When the conductive agent is an Si alloy or an Si compound, specific examples of the Si alloy or the Si compound include ferrosilicon, and more specifically, ferrosilicon containing 70% by mass or more of Si. When the coating film contains ferrosilicon as the conductive agent, the corrosion resistance is likely to be improved together with the conductivity. In particular, when the coating film contains ferrosilicon containing 70% by mass or more of Si as the conductive agent, it is excellent in corrosion resistance and moldability.

[0038] The conductive agent may be, for example, in the form of particles. When the conductive agent is in the form of particles, its average particle diameter is not particularly limited, and an appropriate size may be selected in consideration of the thickness of the coating film and the like. If the particle diameter of the conductive agent is too small with respect to the coating film, the conductivity is likely to decrease. On the other hand, if the particle diameter of the conductive agent is too large with respect to the thickness of the coating film, the conductive agent is likely to drop out from the coating film. In this regard, the particle diameter of the conductive agent may be 1 / 10 or more or 1 / 5 or more of the thickness of the coating film, and may also be 2 times or less. The average particle diameter 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, and may also 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. Note that the "average particle diameter" refers to the average primary particle diameter when the particles present in the coating film exist as primary particles, and refers to the average secondary particle diameter when they exist in an aggregated state. The average particle diameter is measured as follows. That is, the surface-treated steel sheet on which the coating film is formed is cut, its cross section is exposed and then 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 arbitrarily selected, the equivalent circle diameter of each particle is determined, and the average value thereof is taken as the average particle diameter. Whether the particles in the observation image are the conductive agent or not can be easily determined by performing elemental analysis or the like.

[0039] (Adhesion amount of coating film) In the surface-treated steel sheet according to the present embodiment, the adhesion amount of the coating film is not particularly limited. For example, the adhesion amount of the coating film may be 2 g / m 2 or more and 30 g / m 2 or less. If the adhesion amount of the coating film is too small, the corrosion resistance of the surface-treated steel sheet is likely to decrease. If the adhesion amount of the coating film is too large, the weldability of the surface-treated steel sheet is likely to decrease. In the surface-treated steel sheet according to the present embodiment, the adhesion amount of the coating film may be 3 g / m 2 or more or 4 g / m 2 or more, and may also be 25 g / m 2 or less, 20 g / m 2 or less, or 15 g / m 2The following may be applicable. In addition, the coating film adhesion amount on the surface-treated steel sheet can be measured by the gravimetric method or cross-sectional observation. As the measurement of the coating film adhesion amount by the gravimetric method, after measuring the initial weight of the steel sheet cut into a predetermined size, a method of removing the coating film using a solvent capable of dissolving the binder resin or a dedicated chemical, or a method of removing the coating film by blasting using resin beads or alumina beads, is used to measure the weight of the steel sheet after removing the coating film, and it is possible to calculate by obtaining these differences.

[0040] 1.2.2 Conversion coating layer In the surface-treated steel sheet according to this embodiment, the surface treatment layer has a conversion coating layer in addition to the above coating film. The conversion coating layer is in contact with the plated steel sheet. That is, in the surface treatment layer, the conversion coating layer is provided on the inner side and the coating film is provided on the outer side.

[0041] By providing a chemical conversion layer as an inner layer on the surface of the 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, and the corrosion resistance and the like of the surface-treated steel sheet are further improved. The chemical conversion layer may be a layer that substantially does not contain chromium (chromate-free layer). Examples of the chromate-free treatment liquid used for 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 those exemplified as the binder resins that can constitute the above-mentioned coating film, and may contain a polyester resin. The content of the binder resin and the content of components other than the binder resin (such as the above-mentioned silicon compounds) in the chemical conversion layer are not particularly limited. For example, the content of the binder resin in the chemical conversion layer may be 0% by mass or more and 80% by mass or less, and the content of components other than the binder resin may be 20% by mass or more and 100% by mass or less. The chemical conversion layer as the inner layer may be an inorganic film containing an inorganic component as a binder. As an example, the chemical conversion layer may contain a polyester resin, a silane coupling agent, silica, and tannic acid.

[0042] In the surface-treated steel sheet according to the present embodiment, the adhesion amount of the chemical conversion layer is not particularly limited as long as the requirements related to elemental mapping by EPMA described later are satisfied. For example, when the adhesion amount of the chemical conversion layer is 50 mg / m 2 or more and 2000 mg / m 2 or less, it is easier to further improve the corrosion resistance and weldability of the surface-treated steel sheet. The adhesion amount of the chemical conversion layer in the surface-treated steel sheet can be obtained as an average value in the range of 30 mm Φ. That is, the amount of a specific element can be obtained by fluorescent X-ray quantitative analysis from the ratio of the specific element contained in the solid content. When a resin is contained, it can also be obtained from an infrared film thickness meter.

[0043] 1.2.3 Others The coating film and the chemical conversion treatment layer may contain other components in addition to the above-described components. Examples of other components include various additives. For example, carbon black, iron oxide, diazo yellow, lubricants, antifoaming agents, thickeners, and the like. The content of other components in the surface treatment layer is not particularly limited.

[0044] 1.3 Tmax / Tmin For the surface-treated steel sheet according to the present embodiment, the ratio Tmax / Tmin of Tmax to Tmin measured under the above conditions is 2.0 or more and 8.0 or less.

[0045] As described above, when measuring Tmax and Tmin, the coating film is removed from the surface-treated steel sheet according to the present embodiment. The method for removing the coating film is not particularly limited as long as it can remove the coating film while leaving the chemical conversion treatment layer. For example, only the coating film can be removed from the surface-treated steel sheet by rubbing it with a sponge containing a solvent (suitable one selected from xylene, toluene, naphtha, dichloroethane, etc.). Alternatively, at the time of manufacturing the surface-treated steel sheet, after forming the chemical conversion treatment layer on the main surface of the plated steel sheet and before forming the coating film, the above-described analysis by EPMA may be performed.

[0046] As described above, after removing the coating film from the surface-treated steel sheet, elemental mapping by EPMA is performed to confirm the distribution state of the chemical conversion treatment layer provided on the main surface of the plated steel sheet. Specifically, a 10 mm square range on the main surface is divided into 260×260 areas, and the intensity of the chemical conversion treatment component is specified for each area. Based on the intensity in each area, the intensity that is in the top 10% in terms of area% is specified as Tmax, and the intensity that is in the bottom 10% in terms of area% is specified as Tmin.

[0047] The "chemical conversion treatment component" that is the analysis target by EPMA may be any component that can confirm the amount of the chemical conversion treatment layer on the main surface of the plated steel sheet. For example, C or Si. Note that the plated steel sheet itself may also contain the same components as those contained in the chemical conversion treatment layer, but the influence of the components contained in the plated steel sheet is within the error range.

[0048] "The intensity that becomes the top 10% of the area%" means that when counting the above 260 × 260 (= 67,600) total areas in descending order of the intensity of the chemical conversion treatment component, it refers to the intensity at the time when the area becomes 10% of the total area. In other words, by comparing the intensities of the chemical conversion treatment components for all areas (67,600), the 6,760th largest intensity can be regarded as Tmax.

[0049] "The intensity that becomes the bottom 10% of the area%" means that when counting the above 260 × 260 (= 67,600) total areas in ascending order of the intensity of the chemical conversion treatment component, it refers to the intensity at the time when the area becomes 10% of the total area. In other words, by comparing the intensities of the chemical conversion treatment components for all areas (67,600), the 6,760th smallest intensity can be regarded as Tmin.

[0050] As shown in Fig. 1(A), the smaller Tmax / Tmin is, the larger the ratio of the portion having an average thickness in the chemical conversion treatment layer on the main surface of the plated steel sheet becomes. As shown in Fig. 1(B), the larger Tmax / Tmin is, the larger the ratio of the portions with a thin or thick chemical conversion treatment layer on the main surface of the plated steel sheet becomes. Note that the intensity distributions shown in Figs. 1(A) and (B) are merely examples, and the intensity distribution in the surface-treated steel sheet according to the present embodiment is not limited to such a form.

[0051] In the surface-treated steel sheet according to the present embodiment, since Tmax / Tmin is as large as 2.0 or more, on the main surface of the plated steel sheet, the abundance of the thin portion of the chemical conversion treatment layer increases, making it easy to ensure excellent weldability, and at the same time, the abundance of the thick portion of the chemical conversion treatment layer also increases, making it easy to ensure excellent corrosion resistance. Tmax / Tmin may be 2.5 or more or 3.0 or more.

[0052] On the other hand, if Tmax / Tmin is too large, the unevenness of the chemical conversion treatment layer becomes too large, and conversely, the weldability and corrosion resistance may decrease. In this regard, in the surface-treated steel sheet according to the present embodiment, when Tmax / Tmin is 8.0 or less, it is easy to achieve a good balance between weldability and corrosion resistance. Tmax / Tmin may be 7.0 or less or 6.0 or less.

[0053] In addition, in the present embodiment, the range (measurement field of view) of elemental mapping by EPMA is as fine as 10 mm square (10 mm × 10 mm), and in such a fine range, thin portions and thick portions of the chemical conversion treatment layer are mixed. That is, in the surface-treated steel sheet according to the present embodiment, it can be said that the thin portions and thick portions of the chemical conversion treatment layer are finely dispersed and present over the entire surface treatment layer. By having a predetermined amount or more of the thin portions and thick portions of the chemical conversion treatment layer in the range of 10 mm square, high corrosion resistance is ensured in any part of the surface treatment layer, and a conductive path during welding is easily obtained, ensuring high weldability.

[0054] 2. Manufacturing method of the surface-treated steel sheet The above-described surface-treated steel sheet can be manufactured, for example, by the following method. That is, the manufacturing method of the surface-treated steel sheet obtaining a plated steel sheet having a zinc-containing plating layer, forming a chemical conversion treatment layer by performing a chemical conversion treatment on at least one main surface of the plated steel sheet, and forming a coating film by applying a paint containing a binder resin, a rust preventive agent, and a conductive agent to the surface of the chemical conversion treatment layer. may include.

[0055] 2.1 Production of the plated steel sheet The plated steel sheet having a zinc-containing plating layer can be obtained, for example, by obtaining a slab by continuous casting, subjecting the slab to hot rolling to obtain a hot-rolled sheet, winding up the hot-rolled sheet, subjecting the hot-rolled sheet to cold rolling to obtain a cold-rolled sheet, annealing the cold-rolled sheet, subjecting the annealed sheet to a plating treatment, and optionally performing a skin pass, and the like. The continuous casting conditions, hot rolling conditions, winding-up conditions, cold rolling conditions, annealing conditions, and plating conditions may be conventional well-known general conditions.

[0056] 2.2 Chemical conversion treatment In the manufacturing method of the present disclosure, a chemical conversion treatment layer as an inner layer can be formed 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 liquids as described above to the steel sheet surface and drying. Here, in the manufacturing method of the present disclosure, it is necessary to devise the application method of the chemical conversion treatment liquid so that the above-mentioned Tmax / Tmin is satisfied with respect to the abundance of the chemical conversion treatment layer on the main surface of the plated steel sheet. In the knowledge of the present inventor, when the following conditions (1) to (3) are satisfied, a chemical conversion treatment layer satisfying the above-mentioned Tmax / Tmin is formed. (1) As a treatment liquid for forming the chemical conversion treatment layer, a treatment liquid having a cup viscosity at the time of application of 1 second or more and 10 seconds or less and a surface tension of 15 mN / m or more and 55 mN / m or less is used. (2) The treatment liquid is applied by a roll coater. (3) When applying the treatment liquid by a roll coater, the rotation direction of the applicator roll and the passing direction of the plated steel sheet to be coated are set in the same direction.

[0057] When the cup viscosity and surface tension of the above treatment liquid are within a predetermined range, the unevenness (concavities and convexities) formed on the surface of the chemical conversion treatment layer immediately after coating is likely to be maintained. Also, in this technical field, when applying the treatment liquid by a roll coater, it is common to form a uniform chemical conversion treatment layer by setting the rotation direction of the applicator roll opposite to the passing direction of the plated steel sheet to be coated. In contrast, in the manufacturing method of the present disclosure, when applying the treatment liquid by a roll coater, by setting the rotation direction of the applicator roll and the passing direction of the plated steel sheet to be coated in the same direction, the thickness of the chemical conversion treatment layer in the passing direction becomes non-uniform (specifically, in the passing direction, thin portions and thick portions of the chemical conversion treatment layer are alternately repeated), and unevenness occurs in the chemical conversion treatment layer in the passing direction. Here, by adjusting the rotation speed of the applicator roll, the passing speed of the plated steel sheet, the coating amount of the treatment liquid by the applicator roll, etc., the degree and interval of the unevenness of the chemical conversion treatment layer formed on the steel sheet surface can be controlled. For example, when the ratio A / B of the peripheral speed A of the applicator roll to the passing speed B of the steel sheet is 0.8 to 3.0, it is easy to form predetermined unevenness in the chemical conversion treatment layer. If the ratio A / B is too small, the chemical conversion treatment layer is likely to become uniform, and if the ratio A / B is too large, the unevenness of the chemical conversion treatment layer is likely to become excessively large. Thus, according to the manufacturing method of the present disclosure, without requiring new equipment, by simply controlling the coating conditions of the chemical conversion treatment liquid, the surface-treated steel sheet according to the above-described embodiment can be manufactured.

[0058] 2.3 Formation of coating film In the manufacturing method of the present disclosure, a coating film as an outer layer can be formed by applying and drying a paint containing a binder resin, a rust inhibitor, and a conductive agent on the surface of the chemical conversion treatment layer formed as described above. The formation conditions of the paint may be the same as the conventional conditions.

Examples

[0059] Hereinafter, the present invention will be further described while showing 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 and achieves its purpose.

[0060] 1. Manufacture of Surface-Treated Steel Sheets 1.1 Preparation of Plated Steel Sheets The following three types of zinc-based plated steel sheets were prepared, immersed in an aqueous solution (concentration 2.5% by mass, 40 °C) of an aqueous alkaline degreasing agent (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for 2 minutes to degrease the surface, and then washed with water and dried to obtain plated steel sheets for surface treatment.

[0061] GA: Galvannealed Steel Sheet (thickness 0.8 mm, 10% by mass Fe, plating adhesion amount 45 g / m 2 ) ZL: Electroplated Zn-10% by mass Ni Alloy Steel Sheet (thickness 0.8 mm, plating adhesion amount 40 g / m 2 ) SD: Zn-11% by mass Al-3% by mass Mg-0.2% by mass Si Alloy Plated Steel Sheet (thickness 0.8 mm, plating adhesion amount 180 g / m 2 )

[0062] 1.2 Formation of Conversion Coating Layer Next, the following treatment liquids for conversion coating were prepared and applied to both sides of the plated steel sheet with a roll coater. After application, it was dried in a hot air furnace at a metal surface reaching temperature of 70 °C and air-dried to form a conversion coating layer on the surface of the plated steel sheet. The roll coater used was a two-roll coater of an applicator roll and a pickup roll. The rotation direction of the applicator roll (AR) was the same direction (N) or the reverse direction (R) with respect to the plate, and the pickup roll (PR) was rotated so that AR and PR were in the same direction. The average adhesion amount of the conversion coating layer was adjusted to 50, 100, 1000, or 2000 mg / m 2 . The average adhesion amount of the conversion coating layer was determined as the average in a range of 30 mm Φ by fluorescent X-ray.

[0063] Conversion Coating Liquid 1: Treatment liquid for forming a conversion coating film with Nv10% composed of tannic acid, silane coupling agent, silica fine particles, and polyester resin Conversion Coating Liquid 2: Treatment liquid for forming a conversion coating film with Nv10% composed of ammonium zirconium carbonate

[0064] 1.3 Formation of Coating Film Next, in order to form a coating film having the composition shown in Table 1, each component was mixed so as to have the same solid content concentration as in Table 1, and a coating composition for forming a coating film was prepared. This composition was applied onto the plated steel sheet or the chemical conversion treatment layer with a bar coater so as to have the coating amount described in Table 1, and dried using an oven under the condition that the maximum temperature reached 200 °C, thereby obtaining a surface-treated steel sheet having a chemical conversion treatment layer and a coating film as a surface treatment layer. The average coating amount of the coating film was adjusted to 7 g / m 2 . Note that the average coating amount of the coating film was measured by the gravimetric method. The components contained in the coating composition are shown below.

[0065] (Rust preventive pigment) Silica: Silomask 02 (manufactured by Fuji Silysia Chemical Ltd.) V compound: Vanadium pentoxide (manufactured by Kishida Chemical Co., Ltd.) P compound: Calcium phosphate (manufactured by Fujifilm Wako Pure Chemical Corporation)

[0066] (Conductive pigment) FeSi: Ferrosilicon particles SUS: Stainless steel particles

[0067] (Binder resin) Polyester resin: Baylon 200 (manufactured by Toyobo Co., Ltd.) Acrylic resin: Acradic A-405 (manufactured by DIC Corporation)

[0068] 2. Performance evaluation test 2.1 Spot weldability Two pieces of the produced surface-treated steel sheets were stacked, and using a CF type Cr-Cu electrode with a tip diameter of 5 mm and R40, after sandwiching them under the condition of a pressing force of 1.96 kN, the resistance values of the surfaces in contact with the upper electrode and the lower electrode when a welding current of 1 kA was passed were measured at five arbitrary points, and the average thereof was taken as the contact resistance value. The superiority or inferiority of the spot weldability was evaluated using the following evaluation points. In such a welding test, a value of "2" or more was judged to have excellent weldability. The evaluation results are shown in Table 1 below. 4: Resistance value is 0 mΩ or more and 1500 mΩ or less 3: Resistance value exceeds 1500 mΩ and is 2500 mΩ or less 2: The resistance value is more than 2500 mΩ and equal to or less than 4000 mΩ 1: The resistance value is more than 4000 mΩ

[0069] 2.2 Rust resistance of the damaged part The end face of the produced surface-treated steel sheet was sealed with tape, and after applying a cut flaw (in the shape of an ×) reaching the steel sheet under the plating, a cycle corrosion test under the following cycle conditions was carried out for 30 cycles.

[0070] (Cycle conditions) Salt spray (SST, 5% NaCl, 35°C atmosphere) for 2 hours, drying (60°C) for 2 hours, and wetting (50°C, 98% RH) for 4 hours were taken as one cycle and carried out.

[0071] The superiority or inferiority of the rust resistance of the damaged part was evaluated using the following evaluation points. In the following evaluation points, a value of "2" or more was judged to be excellent in the rust resistance of the damaged part. The evaluation results are shown in Table 1 below. 5: No rust generation 4: The maximum width of rust from the damaged part is 3 mm or less 3: The maximum width of rust from the damaged part is more than 3 mm and equal to or less than 5 mm 2: The maximum width of rust from the damaged part is more than 5 mm and equal to or less than 7 mm 1: The rust from the damaged part is more than 7 mm

[0072] 2.3 Workability adhesion After bending the produced surface-treated steel sheet, for the test piece cut to a width of 5 cm, 2T bending was carried out in an atmosphere of 20°C according to the test method specified in JIS G3312. For the processed part, a tape peeling test (tape used: tape manufactured by Nichiban Co., Ltd.) was carried out, and the occurrence status of coating film peeling was observed. In the following evaluation points, a value of "2" or more was judged to be excellent in workability adhesion. The evaluation results are shown in Table 1 below. 5: No peeling 4: The peeled coating film is less than 5% of the area where the tape was attached. 3: The peeled coating film is 5% or more and less than 20% of the area where the tape was attached. 2: The peeled coating film is 20% or more and less than 50% of the area where the tape was attached. 1: The area where the peeled coating film is attached to the tape is 50% or more and less than 70%.

[0073] 2.4 EPMA Analysis After removing the coating film from the surface-treated steel sheet, for the main surface of the plated steel sheet with the chemical conversion layer remaining, using EPMA (Electron Probe Micro Analyzer, JXA8500F manufactured by JEOL Ltd.), a 10 mm square range was divided into an area of 260 points × 260 points, and elemental mapping was performed for the components derived from the chemical conversion layer. The intensity of the components derived from the chemical conversion layer in each area was measured. The intensity that became the top 10% in area% was measured as Tmax, and the intensity that became the bottom 10% in area% was measured as Tmin, and the ratio Tmax / Tmin was calculated. The removal of the coating film from the surface-treated steel sheet was carried out by impregnating a sponge with xylene and rubbing it with a weight of 2 kgf / cm 2 until the coating film disappeared. The disappearance of the coating film was determined by previously performing infrared spectroscopic analysis and fluorescent X-ray analysis on the resin and pigment of the coating film from the surface and cross-section, and the fact that they were not detected by infrared spectroscopic analysis and fluorescent X-ray analysis. In the EPMA mapping, as a representative of the components derived from the chemical conversion layer, the abundance of elemental C was analyzed. The results are shown in Table 1 below.

[0074]

Table 1

[0075] From the results shown in Table 1, the following can be understood.

[0076] Regarding Comparative Example 1, by applying the chemical conversion treatment liquid using a bar coater, the chemical conversion layer became too uniform. Also, regarding Comparative Example 2, by applying the chemical conversion treatment liquid using a roll coater and setting the rotation direction of the applicator roll to the direction opposite to the through-plate direction (R), the chemical conversion layer became too uniform. As a result, in both Comparative Examples 1 and 2, it was difficult to achieve both high weldability and corrosion resistance in the surface-treated steel sheet.

[0077] For Comparative Example 3, although a chemical conversion treatment liquid was applied using a roll coater and the rotation direction of the applicator roll was the same direction (N) as the sheet passing direction, unevenness in the chemical conversion treatment layer became excessive because other coating conditions were not appropriate. As a result, it was difficult to achieve both high-level weldability and corrosion resistance in the surface-treated steel sheet.

[0078] For Comparative Example 4, since no chemical conversion treatment layer was provided on the surface-treated steel sheet, corrosion resistance and processing adhesion decreased.

[0079] On the other hand, for Examples 1 to 14, a chemical conversion treatment liquid was applied using a roll coater, the rotation direction of the applicator roll was the same direction (N) as the sheet passing direction, and furthermore, as a result of adjusting other coating conditions, the strength ratio Tmax / Tmin of the chemical conversion treatment layer could be within a predetermined range. As a result, high-level weldability was ensured by the thin portion of the chemical conversion treatment layer, high-level corrosion resistance was ensured by the thick portion of the chemical conversion treatment layer, and the surface-treated steel sheet as a whole was excellent in the balance between weldability and corrosion resistance. Also, the processing adhesion of the coating film was good.

[0080] From the above results, it can be said that the following surface-treated steel sheets can achieve both high-level weldability and corrosion resistance.

[0081] 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 wherein the surface treatment layer has a chemical conversion treatment layer and a coating film, the coating film contains a binder resin, a rust preventive agent, and a conductive agent, the chemical conversion treatment layer is in contact with the plated steel sheet, and the ratio Tmax / Tmin of Tmax and Tmin measured under the following conditions is 2.0 or more and 8.0 or less, A surface-treated steel sheet. Condition: With the coating film removed from the surface-treated steel sheet, for the chemical conversion treatment layer present on the main surface of the plated steel sheet, using an electron probe microanalyzer (EPMA), a 10 mm square range is divided into a plurality of areas of 260 × 260, elemental mapping is performed, the intensity of the chemical conversion treatment component in each area is specified, the intensity that is the top 10% in terms of area% is specified as Tmax, and the intensity that is the bottom 10% in terms of area% is specified as Tmin.

Claims

1. A plated steel sheet having a zinc-containing plating layer, and a surface treatment layer provided on the surface of the zinc-containing plating layer on at least one main surface of the plated steel sheet, wherein the surface treatment layer has a chemical conversion treatment layer that is a chromium-free layer and a coating film, the coating film contains a binder resin, a rust preventive agent, and a conductive agent, the chemical conversion treatment layer is in contact with the plated steel sheet, and the ratio Tmax / Tmin of Tmax to Tmin measured under the following conditions is 2.0 or more and 8.0 or less. Condition: With the coating film removed from the surface treatment steel sheet, for the chemical conversion treatment layer present on the main surface of the plated steel sheet, using an electron probe microanalyzer (EPMA), a 10 mm square area is divided into a plurality of 260×260 areas for elemental mapping, the intensity of the chemical conversion treatment components in each area is specified, the intensity that is the top 10% in terms of area% is specified as Tmax, and the intensity that is the bottom 10% in terms of area% is specified as Tmin. A surface treatment steel sheet.

2. The coating film contains a polyester resin as the binder resin, The surface treatment steel sheet according to Claim 1.

3. The coating film contains ferrosilicon as the conductive agent, The surface treatment steel sheet according to Claim 1 or 2.

4. The coating film contains at least one of a phosphorus compound and a vanadium compound as the rust preventive agent, The surface treatment steel sheet according to any one of Claims 1 to 3.

5. The coating film contains silica as the rust preventive agent, The surface treatment steel sheet according to any one of Claims 1 to 4.

6. The surface treatment steel sheet according to any one of Claims 1 to 5. The adhesion amount of the coating film is 2 g / m ２ or more and 30 g / m ２ or less,

7. The chemical conversion treatment layer contains a polyester resin, a silane coupling agent, silica, and tannic acid, The surface treatment steel sheet according to any one of Claims 1 to 6. ​

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