Surface-treated steel sheet and manufacturing method for parts

Abstract

Disclosed is a surface-treated steel plate having excellent weldability and corrosion resistance after electropainting. A surface-treated steel plate according to the present disclosure is characterized by including a plated steel plate including a Zn-containing plating layer and a surface-treated layer provided on at least one main surface of the plated steel plate, wherein the surface-treated layer includes at least a coating film serving as an outer layer, the coating film has an average film thickness of 0.5-5.0 µm, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, and the binder resin contains a phenol resin with a crosslinking degree of 40-80%.

Description

[Technical Field]

[0001] The present application discloses a surface-treated steel sheet and a method for manufacturing a part. [Background technology]

[0002] Surface-treated steel sheets are used as components of automobiles and the like. The surface-treated steel sheets include, for example, a plated steel sheet having a Zn-containing plating layer and a surface treatment layer provided on at least one main surface of the plated steel sheet. In conventional technology, a coating film is used as the surface treatment layer, and the types and contents of components constituting 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 weldability and corrosion resistance of a surface-treated steel sheet having a coating film on at least one side of the plated steel sheet by adding predetermined amounts of a binder resin, non-oxide ceramic particles containing V, and doped zinc oxide particles to the coating film.

[0004] Furthermore, Patent Document 2 discloses a technique for improving the corrosion resistance of the end face of a coated steel sheet obtained by forming two or more coating layers on at least one side of a zinc-containing plated steel sheet, by providing an outermost layer of the coating film with a predetermined thickness, including a predetermined non-chromium compound, and devising a configuration of the coating film so that when the coated steel sheet is immersed in ion-exchanged water under predetermined conditions, the electrical conductivity of the immersion water is 30 μS / cm or more.

[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 incorporating a specified resin having a urethane bond and specified conductive particles into the organic coating.

[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 incorporating a predetermined amount of conductive particles of a predetermined particle size into the coating layer. [Prior art documents] [Patent documents]

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

[0008] Surface-treated steel sheets used as components for automobiles and the like are required to have not only the above-mentioned weldability and corrosion resistance, but also paint adhesion after electrodeposition coating. For example, a warm saltwater immersion test (SDT) is a test for evaluating paint adhesion after electrodeposition coating. According to the inventor's new findings, when conventional surface-treated steel sheets are subjected to SDT after electrodeposition coating, blisters and paint peeling may occur on the surface after SDT. If blisters or paint peeling occur on the surface-treated steel sheet, this can cause red rust, making it difficult to ensure sufficient corrosion resistance. For conventional surface-treated steel sheets, sufficient research has not been conducted to suppress the occurrence of blisters and paint peeling during SDT while also improving corrosion resistance after SDT. As described above, a new technology is needed that can achieve both weldability, corrosion resistance, and paint adhesion after electrodeposition coating in surface-treated steel sheets. [Means for solving the problem]

[0009] The present application discloses the following aspects as one of means for solving the above problems. <Aspect 1> A surface-treated steel sheet, a plated steel sheet having a Zn-containing plating layer; a surface treatment layer provided on at least one main surface of the plated steel sheet, the surface treatment layer has at least a coating film as an outer layer, The coating film has an average film thickness of 0.5 μm or more and 5.0 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, The binder resin contains a phenol resin having a crosslinking degree of 40% or more and 80% or less. Surface-treated steel sheet. <Aspect 2> It is for electrocoating, The surface-treated steel sheet of embodiment 1. <Aspect 3> the conductive agent comprises doped oxide particles; The coating film contains 5% by volume or more and 30% by volume or less of the doped oxide particles. The surface-treated steel sheet according to embodiment 1 or 2. <Aspect 4> the doped oxide particles are doped zinc oxide particles; The surface-treated steel sheet of embodiment 3. <Aspect 5> the rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, The coating film contains the magnesium compound in an amount of 5% by volume or more and 15% by volume or less. The surface-treated steel sheet according to any one of embodiments 1 to 4. <Aspect 6> the surface treatment layer has an inorganic or organic-inorganic composite coating as an inner layer between the coating film and the plated steel sheet, The coating has an average thickness of 0.1 μm or more and 1.0 μm or less. The surface-treated steel sheet according to any one of embodiments 1 to 5. <Aspect 7> A method for manufacturing a part, comprising: Preparing a surface-treated steel sheet according to any one of aspects 1 to 6; and laminating an outermost layer on the surface of the coating film of the surface-treated steel sheet, and then heating the laminate to crosslink the phenolic resin and bond the coating film and the outermost layer together. Manufacturing method. <Aspect 8> an electrodeposition coating film is laminated on the surface of the coating film as the outermost layer; The manufacturing method of embodiment 7. [Effects of the Invention]

[0010] The surface-treated steel sheet of the present disclosure has excellent weldability, corrosion resistance, and paint adhesion after electrodeposition coating. For example, the occurrence of blisters and paint peeling during SDT is suppressed, and the steel sheet has excellent corrosion resistance even after SDT. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described. Note that these descriptions are intended to be merely examples of embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0012] 1. Surface-treated steel sheets The surface-treated steel sheet according to the embodiment comprises a plated steel sheet having a Zn-containing plating layer and a surface treatment layer provided on at least one main surface of the steel sheet. The surface treatment layer has at least a coating film as an outer layer. The coating film has an average film thickness of 0.5 μm or more and 5.0 μm or less. The coating film contains a binder resin, a rust inhibitor, and a conductive agent. The binder resin contains a phenolic resin having a cross-linking degree of 40% or more and 80% or less.

[0013] 1.1 Galvanized steel sheet The plated steel sheet has, for example, a base steel sheet and a Zn-containing plating layer provided on at least one main surface of the base steel sheet. The "main surface" referred to in this application refers to the surface corresponding to the front or back side of the sheet. The Zn-containing plating layer may be provided on only one main surface of the base steel sheet, or on both main surfaces. Furthermore, the Zn-containing plating layer may be provided on the entire main surface of the base steel sheet, or on a part of the main surface.

[0014] The base steel sheet may have various chemical compositions and metallographic structures. The base steel sheet may be an ordinary steel sheet or a steel sheet containing additive elements such as chromium, and the chemical composition and metallographic structure of the base steel sheet may be adjusted taking into consideration 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.

[0015] The Zn-containing plating layer may have a chemical composition known to those skilled in the art. For example, the Zn-containing plating layer may contain additional elements such as Al in addition to Zn, and may also contain Fe if alloyed. For example, the Zn-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 these elements, in mass %, may be Al: 0-60%, Mg: 0-10%, Si: 0-2%, Mn: 0-1%, Ni: 0-1%, Sb: 0-1%, and Fe: 0-20%. The Zn-containing plating layer may be a galvannealed layer, a hot-dip galvanized layer, or an electrogalvanized layer. The coating weight of the zinc-containing plating layer relative to the base steel sheet is not particularly limited and may be a general coating weight. For example, the thickness of the Zn-based alloy plating layer may be 1-30 μm.

[0016] 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 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 Zn-containing plating layer among the surfaces of the plated steel sheet.

[0017] The surface treatment layer has at least a coating film as an outer layer. The surface treatment layer may consist of only a coating film, or may have a two-layer structure consisting of a coating film as an outer layer and a chemical conversion coating 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.

[0018] 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.

[0019] (binder resin) The binder resin contains a phenolic resin with a crosslinking degree of 40% or more and 80% or less. When the coating film contains a semi-crosslinked phenolic resin, for example, heating during electrodeposition coating can promote crosslinking of the semi-crosslinked phenolic resin, improving adhesion between the coating film and the electrodeposition coating film. Alternatively, the surface-treated steel sheet according to this embodiment can also improve adhesion between the coating film and an outermost layer other than the electrodeposition coating film. Any thermosetting phenolic resin (e.g., a resol-type phenolic resin) can be used. There are no particular restrictions on the types of phenols, aldehydes, or catalysts that constitute the semi-crosslinked phenolic resin. In any case, in this embodiment, the crosslinking degree of the phenolic resin is sufficient as long as it is 40% or more and 80% or less. The crosslinking degree of the phenolic resin is measured as follows: The surface-treated steel sheet is cut into two pieces of a predetermined size using a shearing machine. One piece is designated as Sample A, and the other piece is left standing in an oven set to 200°C for one hour, then cooled and designated as Sample B. Using two samples, FT-IR was used to measure the 3200-3400 cm -1 Transmittance (%) of hydroxyl groups A , I B After that, the coating film of each of samples A and B was removed with resin shot or solvent, and then the surface of the steel sheet was measured at 3200 to 3400 cm -1 Transmittance (%) of hydroxyl groups A0, I B0 ((I A0 -I A ) / (I B0 -I B ) × 100) can be calculated as the "degree of crosslinking of the phenolic resin (%)."

[0020] According to the findings of the present inventors, phenolic resins can form films that are excellent in hardness and adhesion, and also in weldability, even in the semi-crosslinked state described above. It is difficult to achieve these effects when a resin other than phenolic resin is used as the semi-crosslinked resin. For example, when polyester resin is used as the semi-crosslinked resin, the coating film tends to peel off when the surface-treated steel sheet is subjected to press processing or the like. Furthermore, when polyester resin is used as the semi-crosslinked resin, the coating film tends to become sticky, which may make pressing and welding, for example, difficult.

[0021] The binder resin may contain other resins in addition to the phenolic resin. Examples of other resins include various thermosetting resins and thermoplastic resins. For example, at least one resin selected from epoxy resins, polyester resins, urethane resins, acrylic resins, nylon resins, and olefin resins may be used. The epoxy resin may be an aromatic epoxy resin, an aliphatic epoxy resin, or an amine epoxy resin. Specific examples of epoxy resins include bisphenol A epoxy resins, bisphenol F epoxy resins, hydrogenated bisphenol A epoxy resins, brominated bisphenol A epoxy resins, bisphenol S epoxy resins, phenol novolac epoxy resins, cresol novolac epoxy resins, bisphenol A novolac epoxy resins, biphenyl epoxy resins, naphthalene epoxy resins, dicyclopentadiene epoxy resins, and triphenylmethane epoxy resins. The epoxy resin may be combined with a curing agent. Various epoxy curing agents, such as phenolic resins, may be used as the curing agent. 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 a number average molecular weight of 3,000 to 30,000. When a urethane resin is used as the binder resin, the urethane resin may have a Tg of 0 to 50°C and a number average molecular weight of 5,000 to 25,000. When an acrylic resin is used as the binder resin, the acrylic resin may have a Tg of 0 to 50°C and a number average molecular weight of 3,000 to 25,000. The binder resin may contain a curing agent other than an epoxy curing agent. For example, a melamine resin or an isocyanate resin may be used.

[0022] The content of the binder resin in the coating film is not particularly limited, and may be, for example, 50% by volume or more, or 60% by volume or more, or 90% by volume or less, 80% by volume or less, or 70% by volume or less. Furthermore, the proportion of the phenolic resin in the binder resin is not particularly limited, and, assuming the total binder resin to be 100% by volume, the proportion of the phenolic resin in the binder resin may be 50% by volume or more, 60% by volume or more, 70% by volume or more, 80% by volume or more, or 90% by volume or more.

[0023] (rust inhibitor) The rust inhibitor may be an inorganic rust inhibitor or an organic rust inhibitor. The rust inhibitor may contain, for example, at least one magnesium compound of magnesium hydroxide and magnesium oxide. In this embodiment, the coating film contains a magnesium compound as a rust inhibitor, which further improves corrosion resistance. In this case, the content of the magnesium compound in the coating film is not particularly limited, but better corrosion resistance is likely to be achieved when the coating film contains 5% by volume or more and 15% by volume or less of the magnesium compound. The content of the magnesium compound in the coating film may be 6% by volume or more, 7% by volume or more, or 8% by volume or more, or 14% by volume or less, 13% by volume or less, or 12% by volume or less.

[0024] The rust inhibitor may contain, in addition to or instead of the magnesium compound, at least one of P and V, which are elements that exhibit rust prevention function. 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. Examples of V-containing rust inhibitors include vanadium pentoxide, metavanadate HVO3, ammonium metavanadate, vanadium oxytrichloride VOCl3, vanadium trioxide VO2O3, vanadium dioxide, vanadium oxysulfate VOSO4, vanadium oxyacetylacetonate VO(OC(=CH2)CH2COCH3)3, vanadium acetylacetonate V(OC(=CH2)CH2COCH3)3, and vanadium trichloride VCl3. The rust inhibitor may also contain a guanidino group-containing compound, a pyruguanidino group-containing compound, a thiocarbonyl group-containing compound, or the like. The rust inhibitor may be water-soluble or water-insoluble. When the rust inhibitor is water-soluble, for example, when the coating film is exposed to a humid environment, the rust inhibitor dissolves and elutes in water, thereby exhibiting its rust-preventing function of suppressing corrosion of the plating layer.

[0025] The rust inhibitor may be in the form of particles, for example. When the rust inhibitor is in the form of particles, the average particle size is not particularly limited, and an appropriate size may be selected taking into account the thickness of the coating film, etc. If the particle size of the rust inhibitor is too large compared to the thickness of the coating film, the rust inhibitor will easily fall off from the coating film. The average particle size of the rust inhibitor 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 or the same as or less. The average particle size of the rust inhibitor 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 "average particle size" of the rust inhibitor 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. The polished cross section thus obtained is observed with a scanning electron microscope to obtain an observation image. Several rust inhibitor 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 is taken as the average particle size. Whether or not the particles in the observation image are rust inhibitor can be easily determined by elemental analysis, etc.

[0026] The total content of the rust inhibitor in the coating film is not particularly limited. For example, the total content of the rust inhibitor in the coating film may be 5.0 vol% or more, 6 vol% or more, 7 vol% or more, or 8 vol% or more, or 15 vol% or less, 14 vol% or less, 13 vol% or less, or 12 vol% or less.

[0027] (Conductive agent) The conductive agent has the function of improving the conductivity of the coating film and improving the weldability of the surface-treated steel sheet. 3The conductive agent may be one having a resistivity of Ω / cm or less. Examples of the conductive agent include metals and metal compounds. Specific examples include 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, rhodium, 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. The content of the conductive agent in the coating film is not particularly limited, and may be determined as appropriate taking into consideration the desired weldability and corrosion resistance.

[0028] In particular, when the conductive agent contains doped oxide particles, it is easy to improve both conductivity and corrosion resistance. In this case, the coating film may contain 5 to 30 volume % of the doped oxide particles. Alternatively, the conductive agent may be a Si alloy containing 50 mass % or more of Si, a Si compound containing 50 mass % or more of Si, or a composite of these.

[0029] When the conductive agent contains doped oxide particles, a specific example of the doped oxide particles is doped zinc oxide particles. Examples of doped zinc oxide particles include zinc oxide particles whose conductivity has been improved by doping them with at least one doping element selected from the group consisting of elements from Group 13 of the Periodic Table, such as B, Al, Ga, and In, and elements from Group 15 of the Periodic Table, such as P and As. When the doping element is Al or Ga, the conductivity is more likely to be 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, relative to the undoped zinc oxide particles.

[0030] When the conductive agent contains a Si alloy or a Si compound, a specific example of the Si alloy or Si compound is ferrosilicon containing 70% by mass or more of Si. By including ferrosilicon as a conductive agent in the coating film, it is easy to improve the conductivity and corrosion resistance of the coating film. In particular, ferrosilicon containing 70% by mass or more of Si is excellent in corrosion resistance and formability.

[0031] 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 taking into consideration the thickness of the coating film, etc. If the particle size of the conductive agent is too small relative to the thickness of the coating film, the conductivity is likely to decrease. On the other hand, if the particle size of the conductive agent is too large relative to the thickness of the coating film, the conductive agent is likely to fall off from the coating film. In this regard, the average 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 or the same as the thickness. 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 "average particle size" of the conductive agent 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 they 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 then polished. The polished cross section thus obtained is observed with a scanning electron microscope to obtain an observation image. Several conductive 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 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, etc.

[0032] The total content of the rust inhibitor in the coating film is not particularly limited. For example, the total content of the rust inhibitor in the coating film may be 5% by volume or more, 6% by volume or more, 7% by volume or more, or 8% by volume or more, or may be 30% by volume or less, 25% by volume or less, 20% by volume or less, or 15% by volume or less.

[0033] (Other ingredients) The coating film may contain other components in addition to the components described above. Examples of other components include various additives. For example, bright pigments for improving design, lubricants, antifoaming agents, thickeners, etc. The content of other components in the coating film is not particularly limited.

[0034] (average film thickness) In this embodiment, the coating film has an average thickness of 0.5 μm to 5.0 μm. If the coating film is too thin, sufficient corrosion resistance may not be obtained. Furthermore, the barrier properties against water vapor may be reduced, and water may accumulate at the interface between the coating film and the plating due to water vapor pressure, which may result in peeling. On the other hand, if the coating film is too thick, spot weldability may be reduced. The average thickness of the coating film may be 1.0 μm or more or 2.0 μm or more, or 4.5 μm or less or 4.0 μm or less. The average thickness of the coating film 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 then polished. The polished cross section thus obtained is observed with a scanning electron microscope to obtain an observation image. The thickness of the coating film present in the field of view of the observation image is measured at 10 or more points at 1 μm intervals in the surface direction of the plated steel sheet, and the average value is taken as the average thickness. Alternatively, the average thickness of the coating film may be determined by calculating the density of the coating film from the components contained in the coating film and then measuring the weight of the coating film. In this embodiment, the average thickness characterized by either method is sufficient as long as it is 0.5 μm or more and 5.0 μm or less.

[0035] (adhesion amount) The coating weight of the coating film is not particularly limited. For example, the coating weight of the coating film is 2.0 g / m 2 More than 3.5g / m 2 or more than 5.0g / m 2 It may be 20 g / m or more. 2 Below 15g / m 2 Less than or equal to 10g / m 2 The coating weight of the coating film on the surface-treated steel sheet can be measured by a gravimetric method or by cross-sectional observation. When measuring the coating weight by a 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 or a special agent capable of dissolving the binder resin, or by a blast treatment using resin beads, and the coating weight of the steel sheet from which the coating film has been removed is measured, and the coating weight can be calculated by finding the difference between the weights.

[0036] 1.2.2 Chemical conversion coating In the surface-treated steel sheet according to this embodiment, the surface treatment layer may have an inorganic or organic-inorganic composite coating as an inner layer between the coating film and the plated steel sheet, and the coating may have an average film thickness of 0.1 μm or more and 1.0 μm or less. This coating may also be called a chemical conversion coating. In other words, the surface treatment layer may have a two-layer structure consisting of a coating film as an outer layer and a chemical conversion coating as an inner layer.

[0037] (Components) By providing a chemical conversion coating as an inner layer on the surface of a plated steel sheet and then providing the above-described coating film on the surface of the chemical conversion coating, the adhesion of the coating film to the plated steel sheet is improved. The chemical conversion coating may be a layer that is substantially free of chromium (a chromate-free layer). Examples of chromate-free treatment solutions used in chemical conversion treatment include silica-based treatment solutions containing silicon compounds such as liquid-phase silica, gas-phase silica, and silicates as the main component, zircon-based treatment solutions containing zircon compounds as the main component, and mixtures thereof. The chemical conversion coating may contain a binder resin. For example, the chemical conversion coating may contain at least one of the binder resins exemplified above as components that can constitute the coating film. There are no particular limitations on the content of the binder resin in the chemical conversion coating or the content of components other than the binder resin. For example, the content of binder resin in the chemical conversion coating may be 0% by volume or more and 50% by volume or less, and the content of components other than the binder resin may be 50% by volume or more and 100% by volume or less. The chemical conversion coating serving as the inner layer may be an inorganic coating containing an inorganic component as a binder, or may be an organic-inorganic composite coating. The chemical conversion coating may contain various additives, such as bright pigments, lubricants, antifoaming agents, and thickeners, to improve the design. There are no particular limitations on the content of other components in the chemical conversion coating.

[0038] (average film thickness) The average thickness of the chemical conversion coating is not particularly limited. From the viewpoint of further improving adhesion between the plated steel sheet and the coating, further improving corrosion resistance and weldability, etc., the average thickness of the chemical conversion coating is preferably 0.1 μm or more and 1.0 μm or less. The average thickness of the chemical conversion coating can be measured in the same manner as the average thickness of the coating. That is, a surface-treated steel sheet on which a chemical conversion coating has been formed is cut, the cross section is exposed, and polished. The polished cross section thus obtained is observed with a scanning electron microscope to obtain an observation image. The thickness of the chemical conversion coating present in the field of view of the observation image is measured at 10 or more points at intervals of 1 μm in the in-plane direction of the plated steel sheet, and the average value is taken as the average thickness. Alternatively, the average thickness of the chemical conversion coating may be determined by determining the density of the chemical conversion coating from the components contained in the chemical conversion coating and then measuring the weight of the chemical conversion coating.

[0039] (adhesion amount) In the surface-treated steel sheet, the coating weight of the chemical conversion coating is not particularly limited. For example, 2 More than 2000mg / m 2 If the value is below 100%, the corrosion resistance of the surface-treated steel sheet is more likely to be improved. The deposition weight of the chemical conversion coating 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 with fluorescent X-rays, and the deposition weight of the prepared chemical conversion treated sheet is calculated from the X-ray intensities of the contained elements and the X-ray intensity of the calibration curve plate.

[0040] 1.3 Effects As described above, in the surface-treated steel sheet according to this embodiment, the coating contains a semi-crosslinked phenolic resin, which allows the phenolic resin to be crosslinked by heating or the like during subsequent electrodeposition coating. That is, when an outermost layer (e.g., an electrodeposition coating) is provided on the coating, the adhesion between the coating and the outermost layer can be improved. As a result, blisters and coating peeling during SDT can be suppressed, and excellent corrosion resistance can be ensured even after SDT. Furthermore, by keeping the average coating thickness below a certain level, excellent weldability can also be ensured. Thus, the surface-treated steel sheet according to this embodiment has excellent weldability, corrosion resistance, and coating adhesion after electrodeposition coating, and exhibits excellent effects when used in electrodeposition coating, for example.

[0041] Furthermore, as described above, in the surface-treated steel sheet according to this embodiment, a phenolic resin is used as the semi-crosslinked resin, which makes the coating hard and provides excellent adhesion. As a result, for example, a coating film with excellent scratch resistance can be formed on the surface-treated steel sheet according to this embodiment. When the scratch resistance of the coating film of the surface-treated steel sheet according to this embodiment is evaluated using, for example, a pencil with a hardness of H according to JIS K5600, no coating defects occur at scratched areas, and the coating film does not peel off. That is, the coating film of the surface-treated steel sheet according to this embodiment may have a scratch hardness of H or higher according to the pencil method specified in JIS K5600-5-4.

[0042] 2. Manufacturing method of surface-treated steel sheet The above-mentioned surface-treated steel sheet can be produced, for example, by the following method. 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:

[0043] Alternatively, the method for producing a surface-treated steel sheet comprises: Obtaining a plated steel sheet having a zinc-containing plating layer; forming a chemical conversion coating 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 the surface of the chemical conversion coating to form a coating film; may include:

[0044] 2.1 Preparation of plated steel sheets A plated steel sheet having a Zn-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 skin-passing, etc. The continuous casting conditions, hot rolling conditions, coiling conditions, cold rolling conditions, annealing conditions, and plating conditions may be conventionally known general conditions.

[0045] 2.2 Chemical conversion coating In the manufacturing method of the present disclosure, a chemical conversion coating may be formed as an inner layer by applying a chemical conversion treatment to at least one main surface of the plated steel sheet obtained as described above. The chemical conversion treatment can be carried out by applying one of the various treatment solutions described above to the surface of the steel sheet and drying it.

[0046] 2.3 Coating film formation In the manufacturing method of the present disclosure, a coating film may be formed as an outer layer 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 formed as described above, and then drying the paint. Here, the surface-treated steel sheet according to the above embodiment can be obtained by adjusting the type of rust inhibitor contained in the coating film, the content of the rust inhibitor, the thickness of the coating film, and the like. However, in this embodiment, it is important that the phenolic resin contained in the coating film is in a semi-crosslinked state, as described above. To obtain such a semi-crosslinked phenolic resin, the manufacturing method according to this embodiment preferably involves adjusting, for example, the heating temperature and heating time.

[0047] 3. Parts manufacturing method The technology of the present disclosure also has an aspect as a method for manufacturing a part. Preparing the surface-treated steel sheet of the present disclosure; and and laminating an outermost layer on the surface of the coating film of the surface-treated steel sheet, and then heating the laminate to crosslink the phenolic resin and bond the coating film and the outermost layer together.

[0048] Various types of outermost layer can be used as long as they can be bonded to the coating film by heating. When laminating the outermost layer on the coating film, heating not only crosslinks the semi-crosslinked phenolic resins in the coating film but also induces a bonding reaction between the semi-crosslinked phenolic resin and the resin terminal groups in the outermost layer, improving adhesion between the coating film and the outermost layer. As described above, particularly, excellent effects are easily obtained when an electrodeposition coating film is laminated on the surface of the coating film as the outermost layer. In this case, general electrodeposition coating conditions (e.g., type of electrodeposition paint, voltage) may be employed. In the manufacturing method of the present disclosure, for example, after electrodeposition coating on a surface-treated steel sheet, the electrodeposition coating film is baked, thereby crosslinking the semi-crosslinked phenolic resin contained in the coating film on the surface-treated steel sheet and inducing a bonding reaction with the resin in the electrodeposition coating film. This improves adhesion between the coating film and the electrodeposition coating film. The baking temperature is preferably, for example, 150°C or higher and 190°C or lower, and the baking time is preferably, for example, 10 minutes or higher and 40 minutes or lower.

[0049] The type of part is not particularly limited, and may be, for example, an electrodeposition coated part as described above, or other parts. [Example]

[0050] 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 allows various conditions to be adopted as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0051] 1. Manufacturing of surface-treated steel sheets 1.1 Preparation of galvannealed 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 immersion in an aqueous solution (2.5 mass%, 40°C) of an aqueous alkaline degreasing agent (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for 2 minutes, followed by rinsing with water and drying to prepare base metal sheets for surface treatment.

[0052] GA: Galvannealed steel sheet (thickness 0.8 mm, 10% Fe by mass, coating weight 45 g / m2 ) ZA1: Zn-Al-Mg ternary hot-dip galvanized steel sheet (Zn-11%Al-3%Mg-0.2%Si) (sheet thickness 0.8 mm, 10% Fe by mass, coating weight 60 g / m 2 ) ZA2: Zn-Al-Mg ternary hot-dip galvanized steel sheet (Zn-6%Al-3%Mg) (sheet thickness 0.8 mm, 10% Fe by mass, coating weight 60 g / m 2 ) ZL: Electrolytic Zn-10% Ni alloy plated 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 sheet (thickness 0.8 mm, unplated)

[0053] 1.2 Formation of inner layer (chemical conversion coating) Next, the following chemical conversion treatment solution S was prepared, and the treatment solution was applied to the above-mentioned base metal sheet while changing the bar coating size, and then dried in a hot air oven so that the temperature reached on the metal sheet surface was 70°C, and then air-dried to form a chemical conversion treatment film on the surface of the metal sheet. The average film thickness of the chemical conversion treatment film was 0.2 μm.

[0054] S: A chemical conversion treatment solution with Nv10% consisting of Zr compounds, silane coupling agents, silica particles, and polyester resin.

[0055] 1.3 Formation of outer layer (paint film) Next, to form a coating film having the composition ratio (volume %) shown in Table 1, the components were mixed to a solids concentration similar to that shown in Table 1 to prepare a coating composition for coating film formation. The coating composition was applied to a base metal sheet or a chemical conversion coating using a bar coater, while varying the bar coat size and dilution ratio, and then dried in an oven at maximum temperatures (PMT) of 140°C, 160°C, 180°C, or 200°C to form a coating film as an outer layer. The degree of crosslinking of the phenolic resin in the coating film was adjusted by the maximum temperature achieved during drying. The average coating film thickness (μm) was as shown in Table 2. The components contained in the coating composition are listed below.

[0056] (Anti-rust pigment) MgO: Magnesium oxide (average particle size 3 μm) Mg(OH)2: Magnesium hydroxide (average particle size 3 μm) PA: Aluminum tripolyphosphate (oil absorption 10 ml / 100 g, particle size 2 μm)

[0057] (Conductive pigment) ZnO: Doped zinc oxide particles (23-Kt, manufactured by Hakusui Tech Co., Ltd., average particle size 0.5 μm) FeSi: Ferrosilicon particles (average particle size 3 μm, containing 70% or more by mass of Si) SUS: SUS particles (average particle size 5μm)

[0058] (binder resin) B1: Phenolic resin (DIC water-soluble resol phenolic resin GG-1490) B2: Phenolic resin (DIC resol type phenolic resin 5010) B3: Urethane resin (Superflex 150 manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) B4: Polyester resin (Vylonal MD1480 manufactured by Byron) B5: Melamine resin (Allnex Cymel 325)

[0059] 2. Performance evaluation test The following performance evaluation tests were carried out on each of the surface-treated steel sheets.

[0060] 2.1 Measurement of the degree of cross-linking When a phenolic resin was used as the binder in the coating film, the degree of crosslinking of the phenolic resin contained in the coating film was measured as follows. The results are shown in Table 2.

[0061] (Method for measuring the degree of crosslinking of phenolic resin) The surface-treated steel sheet prepared as described above was cut into two pieces of a predetermined size using a shearing machine, one of which was designated as Sample A, and the other was left standing in an oven set at 200°C for 1 hour, then cooled and designated as Sample B. Using the two samples, the 3200 to 3400 cm of the coating film was measured by FT-IR. -1 Transmittance (%) of hydroxyl groups A , I B After that, the coating films of samples A and B were removed with resin shot or solvent, and the 3200 to 3400 cm -1 Transmittance (%) of hydroxyl groups A0 , I B0 ((I A0 -I A ) / (I B0 -I B ) × 100) was measured as the "degree of crosslinking of the phenolic resin (%)."

[0062] 2.2 Hot saltwater immersion test (SDT) (Advance preparation) Each surface-treated steel sheet was subjected to surface conditioning at room temperature for 20 seconds using a surface conditioning treatment agent, Preparen X (trade name), manufactured by Nihon Parkerizing Co., Ltd. Furthermore, a chemical conversion treatment (phosphate treatment) was performed using a chemical conversion treatment solution (zinc phosphate treatment solution), Palbond 3020 (trade name), manufactured by Nihon Parkerizing Co., Ltd. The temperature of the chemical conversion treatment solution was set to 43°C, and the hot press-molded material was immersed in the chemical conversion treatment solution for 120 seconds, then rinsed with water and dried. After the chemical conversion treatment (phosphate treatment), the material was electrocoated with a cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd. using a ramp current of 160V, followed by baking at a baking temperature of 170°C for 20 minutes. The average thickness of the electrocoated film after electrocoating was 10 μm for all samples.

[0063] (Corrosion resistance during SDT (blister area ratio)) After the electrodeposition coating, the edge of the surface-treated steel sheet was sealed with sealing tape and immersed in a 3% NaCl aqueous solution at 50°C for 500 hours. After the immersion test, the sample was removed and dried, and the area ratio of blisters present on the electrodeposition coating surface was measured visually. In this corrosion resistance test, a score of "3" was determined to have a certain degree of corrosion resistance, and a score of "4" or "5" was determined 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 but less than 50% 3: Blister area ratio from the evaluation surface is 1% or more and less than 5% 4: From the evaluation point of view, blisters occur in less than 1% of cases. 5: No blistering

[0064] 2.3 Corrosion resistance test after SDT After the warm saltwater immersion test, the surface-treated steel sheets that had been subjected to the electrodeposition coating were subjected to a cyclic corrosion test under the following cycle conditions for 120 cycles.

[0065] (Cycle conditions) One cycle consisted 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).

[0066] Thereafter, the state of corrosion 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 determined 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 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 evaluated surface is less than 1%

[0067] 2.4 Spot weldability Each surface-treated steel sheet 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 welding pressure of 1.96 kN, a welding current of 8 kA, and a current flow time of 12 cycles / 50 Hz. The number of spots just before the nugget diameter fell below 3√t (t is the sheet thickness) was determined. The spot weldability was evaluated using the following evaluation points. A score of "4," "5," or "6" in this weldability test was considered to be excellent. The results are shown in Table 2. 1: No nuggets are 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 or more RBIs but less than 200 RBIs 4: RBIs are between 200 and 1000 5: RBIs are between 1000 and 2000 6: 2000 or more RBIs

[0068] 2.5 Scratch resistance A scratch test was carried out on the coating film of each surface-treated steel sheet in accordance with JIS K5600 using a pencil with hardness H manufactured by Mitsubishi Uni. The condition of the coating film after scratching was observed, and scratch resistance was evaluated using the following evaluation points. A rating of "2" or "3" in this scratch resistance test was judged to have excellent scratch resistance. The results are shown in Table 2. 1: Paint peeling occurs in part or all of the scratched area 2: There are paint defects in the scratched areas, but the paint has not peeled off 3: No paint damage in scratched area

[0069] [Table 1]

[0070] [Table 2]

[0071] 3. Results and Discussion The results shown in Tables 1 and 2 reveal the following:

[0072] No. 13 did not contain any anti-rust pigment in the coating, and had poor corrosion resistance after SDT.

[0073] No. 14 did not contain conductive pigment in the coating, and was therefore inferior in spot weldability.

[0074] In the case of No. 19, the binder that makes up the coating film is not phenolic resin, so blisters occurred during SDT and the corrosion resistance after SDT was also poor.

[0075] In the case of No. 23, the coating thickness was too thin at 0.2 μm, and the corrosion resistance after SDT was poor.

[0076] No. 28 had poor spot weldability because the coating thickness was 10 μm, which was too thick.

[0077] No. 34 was made of a steel sheet without a Zn-containing coating layer, and therefore had poor corrosion resistance after SDT.

[0078] For Nos. 41 to 44, phenolic resin was used as the binder for the coating film, but the degree of cross-linking of the phenolic resin was too high, so the cross-linking of the phenolic resin could not proceed during electrodeposition coating, resulting in poor adhesion between the coating film and the electrodeposition coating. As a result, blisters occurred during SDT and the corrosion resistance after SDT was also poor.

[0079] In No. 45, the degree of cross-linking of the phenolic resin in the coating was too low at 20%, so the coating did not have sufficient hardness or adhesion.

[0080] For No. 46, the use of uncrosslinked polyester resin resulted in insufficient hardness and adhesion of the coating. Furthermore, the electrode stuck to the coating during welding, preventing proper welding. Furthermore, peeling of the coating occurred during the scratch test.

[0081] For No. 47, polyester resin was used as the semi-crosslinked resin in the coating, and melamine resin was used as the crosslinking agent, which resulted in insufficient adhesion of the coating. Furthermore, the electrode stuck to the coating during welding, preventing proper welding. Furthermore, peeling of the coating occurred during the scratch test.

[0082] In contrast, Nos. 1-12, 15-18, 20-22, 24-27, 29-33, and 35-40 showed almost no blisters during SDT and exhibited excellent corrosion resistance after SDT, as well as excellent spot weldability and scratch resistance. Nos. 1-12, 15-18, 20-22, 24-27, 29-33, and 35-40 have the following surface-treated steel sheets: (1) a plated steel sheet having a Zn-containing plating layer; (2) a coating film as a surface treatment layer having an average film thickness of 0.5 μm or more and 5.0 μm or less; (3) the coating film contains a binder resin, an anti-rust pigment, and a conductive pigment; and (4) the binder resin contains a phenolic resin with a crosslinking degree of 40% or more and 80% or less. In other words, the inclusion of the anti-rust pigment and conductive pigment in the coating film is thought to have improved corrosion resistance and spot weldability. Furthermore, by using a semi-crosslinked phenolic resin as the binder resin, the crosslinking of the phenolic resin can be promoted by heating during electrodeposition coating, which is thought to have improved the adhesion between the coating film and the electrodeposition coating film.Furthermore, by using a phenolic resin as the semi-crosslinked resin in the coating film, the coating film becomes hard and has excellent adhesion, which is thought to have improved scratch resistance.

[0083] From the above results, it can be said that a surface-treated steel sheet that satisfies the following requirements has excellent weldability and corrosion resistance.

[0084] A surface-treated steel sheet, a plated steel sheet having a Zn-containing plating layer; a surface treatment layer provided on at least one main surface of the plated steel sheet, the surface treatment layer has at least a coating film as an outer layer, The coating film has an average film thickness of 0.5 μm or more and 5.0 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, The binder resin contains a phenol resin having a crosslinking degree of 40% or more and 80% or less. Surface-treated steel sheet.

Claims

1. A surface-treated steel sheet, a plated steel sheet having a Zn-containing plating layer; a surface treatment layer provided on at least one main surface of the plated steel sheet, the surface treatment layer has at least a coating film as an outer layer, the coating film has an average film thickness of 0.5 μm or more and 5.0 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, The binder resin contains a phenol resin having a crosslinking degree of 40% or more and 80% or less. Surface-treated steel sheet.

2. It is for electrocoating, The surface-treated steel sheet according to claim 1.

3. the conductive agent comprises doped oxide particles; the coating film contains 5% by volume or more and 30% by volume or less of the doped oxide particles; The surface-treated steel sheet according to claim 1.

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

5. the rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, the coating film contains the magnesium compound in an amount of 5% by volume or more and 15% by volume or less; The surface-treated steel sheet according to claim 1.

6. the surface treatment layer has an inorganic or organic-inorganic composite coating as an inner layer between the coating film and the plated steel sheet, The coating has an average thickness of 0.1 μm or more and 1.0 μm or less. The surface-treated steel sheet according to claim 1.

7. A method for manufacturing a part, comprising: Providing a surface-treated steel sheet according to any one of claims 1 to 6; and laminating an outermost layer on the surface of the coating film of the surface-treated steel sheet, and then heating the laminate to crosslink the phenolic resin and bond the coating film and the outermost layer together. Manufacturing method.

8. an electrodeposition coating film is laminated on the surface of the coating film as the outermost layer; The method of claim 7.

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