Surface-treated steel sheet

Abstract

To provide a surface treated steel sheet excellent in corrosion resistance without including an electrodeposition film.SOLUTION: A surface treated steel sheet includes: 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. The surface treatment layer includes at least a coating film as an outer layer; the coating film having an average film thickness of 3 μm or more and 10 μm or less includes a binder resin, an antirust and a conductive agent; the antirust includes at least one magnesium compound of magnesium hydroxide and magnesium oxide; the conductive agent includes zinc powder as first electric conductive pigment; and the coating film includes the magnesium compound of 5 volume% or more and 15 volume% or less and the zinc powder of 5 volume% or more and 30 volume% or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Surface-treated steel sheets are used as components of automobiles and the like. The surface-treated steel sheets 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.

[0007] Furthermore, Patent Document 5 discloses a technology for ensuring excellent corrosion resistance by coating a plated steel sheet with a coating composition containing a binder resin, a Zn-based metal powder, and condensed aluminum phosphate containing Mg. [Prior art documents] [Patent documents]

[0008] [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 [Patent Document 5] Japanese Patent Application Publication No. 2017-122186 Summary of the Invention [Problem to be solved by the invention]

[0009] Surface-treated steel sheets used as components for automobiles and the like can be subjected to electrodeposition coating before use. However, depending on the application, the surface-treated steel sheets may be used without electrodeposition coating, with the coating film exposed. In this case, corrosion resistance is required even without the electrodeposition coating film. In particular, at welded joints, the coating film and plating layer are easily lost over a wide area due to the heat effect during welding, which makes them prone to becoming the starting point for corrosion. In conventional surface-treated steel sheets, sufficient research has not been conducted on improving the corrosion resistance without the electrodeposition coating film, particularly the corrosion resistance of welded joints, and there is room for improvement. [Means for solving the problem]

[0010] As one of the means for solving the above problems, the present application provides: 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 3 μm or more and 10 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, the rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, the conductive agent contains zinc powder as a first conductive pigment, the coating film contains the magnesium compound in an amount of 5% by volume or more and 15% by volume or less, The coating film contains 5% by volume or more and 30% by volume or less of the zinc powder. Disclose.

[0011] In the surface-treated steel sheet of the present disclosure, The conductive agent may include at least one second conductive pigment selected from the group consisting of doped oxide particles, Si alloys containing 50% by mass or more of Si, Si compounds containing 50% by mass or more of Si, and composites thereof; The coating film may contain the second conductive pigment in an amount of 5% by volume or more and 20% by volume or less.

[0012] In the surface-treated steel sheet of the present disclosure, the doped oxide particles may be doped zinc oxide particles.

[0013] In the surface-treated steel sheet of the present disclosure, the Si alloy or the Si compound may be ferrosilicon containing 70 mass % or more of Si.

[0014] In the surface-treated steel sheet of the present disclosure, 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, The coating may have an average thickness of 0.1 μm or more and 1.0 μm or less. [Effects of the Invention]

[0015] The surface-treated steel sheet of the present disclosure has excellent corrosion resistance and weldability. In addition, it has excellent corrosion resistance even in the absence of an electrodeposition coating film, and also has excellent corrosion resistance at welds. DETAILED DESCRIPTION OF THE INVENTION

[0016] 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.

[0017] A surface-treated steel sheet according to an embodiment includes 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. The surface treatment layer includes at least a coating film as an outer layer. The coating film has an average film thickness of 3 μm or more and 10 μm or less. The coating film includes a binder resin, a rust inhibitor, and a conductive agent. The binder resin includes an epoxy resin. The rust inhibitor includes at least one magnesium compound of magnesium hydroxide and magnesium oxide. The conductive agent includes zinc powder as a first conductive pigment. The coating film includes 5% by volume or more and 15% by volume or less of the magnesium compound. The coating film includes 5% by volume or more and 30% by volume or less of the zinc powder.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] (binder resin) There are no particular limitations on the type of binder, and any binder resin known to constitute the coating film of surface-treated steel sheets can be used. The binder resin may be one or more resins selected from various thermosetting resins and thermoplastic resins. Specific examples of binder resins include at least one resin selected from epoxy resins, polyester resins, urethane resins, acrylic resins, nylon resins, and olefin resins.

[0025] 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 various curing agents. For example, melamine resin, isocyanate resin, etc. may be used.

[0026] 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.

[0027] (rust inhibitor) The rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide. In this embodiment, the coating film contains a magnesium compound together with zinc powder, as described below, and the following effects can be expected. First, magnesium compounds are easily ionized and have the function of increasing conductivity. This facilitates an increase in the sacrificial protection distance provided by the zinc powder. That is, even in areas where the coating film or Zn-containing plating layer has been lost over a wide area, such as at spot welds, the zinc powder contained in the coating film adjacent to the area can provide corrosion protection over a wide area of ​​the area. Furthermore, the magnesium compound has the effect of depositing a corrosion protection product resulting from the sacrificial protection of the zinc powder. Typically, when zinc powder becomes a corrosion protection product (e.g., Zn hydroxide) through sacrificial protection, the corrosion protection product is easily soluble, and therefore the corrosion protection effect is easily lost due to leaching. In contrast, by combining the corrosion protection product with magnesium, the leaching of the corrosion protection product is suppressed, making it easier for the corrosion protection product to deposit in areas where the coating film or plating layer has been lost, such as at spot welds, thereby facilitating the development of excellent corrosion resistance.

[0028] The content of the magnesium compound in the coating film is 5% by volume or more and 15% by volume or less. If the content of the magnesium compound is too low, it is difficult to ensure the effects of increasing the sacrificial protection distance and depositing the corrosion protection product. On the other hand, if the content of the magnesium compound is too high, voids are likely to form in the coating film due to elution of the magnesium compound, and the corrosion resistance originally ensured by the coating film may be reduced in the portion of the surface-treated steel sheet where the coating film is present. 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 may be 14% by volume or less, 13% by volume or less, or 12% by volume or less.

[0029] The shape of the magnesium compound is not particularly limited. It may be, for example, particulate (powder-like). The average particle size of the magnesium compound is preferably 2.0 times or less the average film thickness of the coating film. If the average particle size of the magnesium compound is too large, the magnesium compound will protrude from the coating film and easily fall off. The average particle size of the magnesium compound may be 1.5 times or less or 1.0 times or less the average film thickness of the coating film. The lower limit of the average particle size of the magnesium compound is not particularly limited. For example, it may be 1 / 10 or more or 1 / 5 or more of the thickness of the coating film. In this application, the "average particle size" of the magnesium compound refers to the average primary particle size when the particles present in the coating film exist as primary particles, and refers to the average secondary particle size when the particles exist as aggregates. The "average particle size" of the magnesium compound contained in the coating film is determined as follows. Specifically, 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 particles of the magnesium compound present in the field of view of the observation image are randomly selected, the equivalent circle diameter of each particle is determined, and the average value is taken as the average particle diameter. Whether or not the particles in the observation image are magnesium compounds can be easily determined by elemental analysis, etc.

[0030] In addition to the magnesium compound, the coating film may contain other types of rust inhibitors (other rust inhibitors). The other rust inhibitors may be inorganic or organic rust inhibitors. The other rust inhibitors may contain at least one of P and V, which are elements that exhibit rust prevention functions. Examples of rust inhibitors containing P include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, and tetraphosphoric acid; ammonium salts such as triammonium phosphate and diammonium hydrogen phosphate; metal phosphates with Na, Mg, Al, K, Ca, Mn, Ni, Zn, and Fe; phosphonic acids such as aminotri(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic acid) and salts thereof; and organic phosphoric acids such as phytic acid and salts thereof. 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. Other rust inhibitors may also be guanidino group-containing compounds, pyruguanidino group-containing compounds, thiocarbonyl group-containing compounds, etc. The form of the other rust inhibitors may be, for example, particulate. The particle size details are the same as those for the magnesium compound described above. The other rust inhibitors may be water-soluble or water-insoluble. When the other rust inhibitors are 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. The content of the other rust inhibitor in the coating film is not particularly limited. The content of the other rust inhibitor may be greater than, less than, or the same as the content of the magnesium compound. For example, the content of the other rust inhibitor in the coating film may be 0% by volume or more, 0.5% by volume or more, 1.0% by volume or more, or 5.0% by volume or more, and may be 15% by volume or less, or 10% by volume or less.

[0031] (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. 3 A conductive agent can be one having a resistivity of Ω / cm or less. As described above, in this embodiment, the conductive agent contains zinc powder as the first conductive pigment. As described above, zinc powder has sacrificial corrosion protection ability, and therefore can function as both a conductive agent and a rust inhibitor, but in this application it is counted as a conductive agent.

[0032] When a surface-treated steel sheet is welded, the heat generated during welding causes the paint film and Zn-containing plating layer to disappear over a wide area of ​​the steel sheet surface. Therefore, it is difficult to ensure corrosion resistance at the welded portion. While corrosion resistance can be ensured by applying electrodeposition coating to the welded portion, electrodeposition coating is not always performed. Therefore, it is important to consider how to improve the corrosion resistance of the welded portion in the absence of an electrodeposition coating film. In the surface-treated steel sheet according to this embodiment, zinc powder is added to the paint film to supply sacrificial corrosion protection components from the paint film, thereby improving corrosion resistance. However, zinc powder alone has a short sacrificial corrosion distance, making it difficult to provide sufficient corrosion resistance to areas where the paint film or plating layer has disappeared over a wide area, such as welded portions. Therefore, in the surface-treated steel sheet according to this embodiment, the above-mentioned magnesium compound is added to the paint film together with zinc powder. As described above, this increases the sacrificial corrosion distance and facilitates the deposition of corrosion protection components, thereby improving the corrosion protection effect. As a result, excellent corrosion resistance can be imparted to areas where the coating or plating layer has been lost over a wide area, such as welded joints.

[0033] The content of the zinc powder in the coating film is 5% by volume or more and 30% by volume or less. If the zinc powder content is too low, it is difficult to ensure the above-mentioned corrosion prevention effect. On the other hand, if the zinc powder content is too high, voids are likely to form in the coating film due to elution of the zinc powder, and the corrosion resistance originally ensured by the coating film may be reduced in the areas of the surface-treated steel sheet where the coating film is present. The content of zinc powder in the coating film may be 7% by volume or more, 10% by volume or more, 12% by volume or more, or 15% by volume or more, or may be 27% by volume or less, 25% by volume or less, 22% by volume or less, or 20% by volume or less.

[0034] The zinc powder may be in a powder form sufficient to be incorporated into the coating film. The average particle size of the zinc powder is preferably 2.0 times or less the average film thickness of the coating film. If the average particle size of the zinc powder is too large, the zinc powder will protrude from the coating film and easily fall off. The average particle size of the zinc powder may be 1.5 times or less or 1.0 times or less the average film thickness of the coating film. The lower limit of the average particle size of the zinc powder is not particularly limited. For example, it may be 1 / 10 or more or 1 / 5 or more of the thickness of the coating film. In this application, the "average particle size" of the zinc powder contained in the coating film refers to the average primary particle size when the particles present in the coating film exist as primary particles, and refers to the average secondary particle size when they exist as aggregates. The "average particle size" of the zinc powder can be determined in the same manner as the particle size of the magnesium compound. 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 under a scanning electron microscope to obtain an observation image. Several zinc particles present in the field of view of the observation image are randomly selected, the circle equivalent diameter of each particle is calculated, and the average value is taken as the average particle diameter. Whether or not the particles in the observation image are zinc powder can be easily determined by elemental analysis, etc.

[0035] In addition to the zinc powder, the coating film may contain other types of conductive agents (other conductive agents), such as various metals and metal compounds. Specifically, the metals may include metals such as magnesium, aluminum, silicon, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, germanium, strontium, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, indium, and tin; alloys such as those of 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 zinc or the above-mentioned metal elements. Among these, in addition to the above-mentioned zinc, magnesium, aluminum, silicon, chromium, iron, nickel, 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 other conductive agents in the coating film is not particularly limited, and may be determined as appropriate taking into consideration the desired weldability and corrosion resistance.

[0036] In particular, when the conductive agent contains at least one second conductive pigment selected from the group consisting of doped oxide particles, Si alloys containing 50% by mass or more of Si, Si compounds containing 50% by mass or more of Si, and composites thereof, it is easy to improve the conductivity (weldability), adhesion of the coating film, etc. In this case, the content of the second conductive pigment in the coating film may be 5% by volume or more and 30% by volume or less.

[0037] 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.

[0038] 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 further 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.

[0039] The other conductive agent may be, for example, particulate. When the other conductive agent is particulate, its average particle size is not particularly limited, and an appropriate size may be selected taking into account factors such as the thickness of the coating film. 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, and may be 2.0 times or less, 1.5 times or less, or 1.0 times or less. The average particle size of the conductive agent may be, for example, 0.1 μm or more, 0.3 μm or more, 0.5 μm or more, or 1.0 μm or more, or 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 the particles exist as aggregates. The average particle size can be determined in the same manner as the average particle size of the magnesium compound described above. That is, a surface-treated steel sheet on which a coating film has been formed is cut, the cross section is exposed and polished, and the polished cross section thus obtained is observed with a scanning electron microscope to obtain an observation image. Several conductive particles present in the field of view of the observation image are randomly selected, and the circle-equivalent diameters of each particle are determined, and the average of these is taken as the average particle diameter. Whether the particles in the observation image are other conductive agents can be easily determined by elemental analysis or the like.

[0040] (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, pigments other than the above-mentioned anti-rust pigments and conductive pigments (such as bright pigments for improving design), lubricants, antifoaming agents, thickeners, etc. The content of other components in the coating film is not particularly limited.

[0041] (average film thickness) In this embodiment, the coating film has an average thickness of 3 μm to 10 μm. If the coating film is too thin, sufficient corrosion resistance may not be obtained. 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 4 μm or more or 5 μm or more, or 9 μm or less or 7 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 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 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 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 may be 1 μm to 10 μm.

[0042] (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 steel sheet is cut to a predetermined size and then the initial weight is measured, and the coating film is removed using a solvent or a special agent capable of dissolving the binder resin, or by a blast treatment using resin beads or alumina beads. The coating weight can be calculated by measuring the weight of the steel sheet from which the coating film has been removed and determining the difference between the weights.

[0043] 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 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.

[0044] (Constituents) 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.

[0045] (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.

[0046] (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.

[0047] 3.Effects As described above, in the surface-treated steel sheet according to this embodiment, the coating contains a predetermined amount of zinc powder and a predetermined amount of magnesium compound. The combined effect of the zinc powder and the magnesium compound increases the sacrificial protection distance and facilitates deposition of anticorrosive components. This allows for excellent corrosion resistance, even in areas where the coating or plating layer has been lost over a wide area, such as welds. Furthermore, the coating contains a conductive agent and has an average coating thickness of at most a certain level, ensuring excellent weldability.

[0048] 4. 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 Zn-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:

[0049] Alternatively, the method for producing a surface-treated steel sheet comprises: Obtaining a plated steel sheet having a Zn-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:

[0050] 4.1 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.

[0051] 4.2 Chemical conversion treatment 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.

[0052] 4.3 Coating film formation In the manufacturing method of the present disclosure, a coating film as an outer layer may be formed by applying a paint containing a binder resin, a rust inhibitor, and a conductive agent to the surface of the plated steel sheet obtained as described above or to the surface of the chemical conversion coating 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 and content of the rust inhibitor contained in the coating film, the thickness of the coating film, etc. [Example]

[0053] 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 the object is achieved without departing from the gist of the present invention.

[0054] 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.

[0055] GA: Galvannealed steel sheet (thickness 0.8 mm, 10% Fe by mass, coating weight 45 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)

[0056] 1.2 Formation of inner layer (chemical conversion coating) Next, the following chemical conversion treatment solution S was prepared and applied to the above-mentioned base metal sheet while changing the bar coating size so as to obtain the coating amount shown in Tables 3 and 4. The solution was 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.

[0057] S: A chemical conversion treatment solution with Nv10% consisting of Zr compounds, silane coupling agents, silica particles, and polyester resin.

[0058] 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 obtain the same solid content concentration as in Table 1, to prepare a coating composition for coating formation. This composition was applied to a base metal sheet or a chemical conversion coating using a bar coater, while changing the bar coat size and dilution ratio to obtain the coating amount shown in Table 2, and then dried in an oven at a maximum temperature of 200°C to form a coating film as an outer layer. The average coating film thickness (μm) was as shown in Table 2. The components contained in the coating composition are shown below.

[0059] (Anti-rust pigment) MgO: Magnesium oxide (average particle size 3 μm) Mg(OH)2: Magnesium hydroxide (average particle size 3 μm) Si1: Silica (oil absorption 50ml / 100g, average particle size 3μm) Si2: Silica (oil absorption 300ml / 100g, average particle size 3μm) PA: Aluminum tripolyphosphate (oil absorption 10 ml / 100 g, average particle size 2 μm) PM: Magnesium phosphate (oil absorption 30ml / 100g, average particle size 2μm)

[0060] (Conductive pigment) Zn: Zinc powder (average particle size 3 μm) FeSi: Ferrosilicon particles (average particle size 3 μm, containing 70% or more by mass of Si) SUS: SUS particles (average particle size 5μm) ZnO: Doped zinc oxide particles (23-Kt, manufactured by Hakusui Tech Co., Ltd., average particle size 0.5 μm)

[0061] (binder resin) B1: Epoxy resin (Mitsubishi Chemical Epoxy Resin 1001B80) B2: Polyester resin (Toyobo polyester resin Vylon 200) B3: Melamine resin: (Allnex melamine resin CYMEL325)

[0062] 2. Performance evaluation test The following performance evaluation tests were carried out on each of the surface-treated steel sheets.

[0063] 2.1 Corrosion resistance test The end faces of the surface-treated steel sheets were sealed with sealing tape, and a cyclic corrosion test was carried out for 240 cycles under the following cycle conditions.

[0064] (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).

[0065] The corrosion state of the flat surface was then observed and the following ratings were given. In the corrosion resistance test, a rating of "3," "4," or "5" was determined to be excellent in corrosion resistance of the flat surface other than the spot welds. The results are shown in Tables 3 and 4. 1: The area ratio of white rust occurring on the evaluation surface (flat surface not subjected to spot welding, same below) is 50% or more, or red rust occurring on the evaluation surface is confirmed. 2: The area ratio of white rust on the evaluation surface is 20% or more but less than 50% 3: The area ratio of white rust on the evaluation surface is 10% or more but less than 20% 4: The area ratio of white rust on the evaluation surface is 5% or more but less than 10% 5: The area ratio of white rust on the evaluated surface is less than 5%

[0066] 2.2 Spot weld corrosion resistance test The surface-treated steel sheets were spot-welded using a CF-type Cr-Cu electrode with a tip diameter of 5 mm and R40 at a pressure of 1.96 kN, a welding current of 8 kA, and a welding time of 12 cycles / 50 Hz. After that, the end faces were sealed with sealing tape, and a cyclic corrosion test was carried out for 60 cycles under the following cycle conditions.

[0067] (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).

[0068] The corrosion state of the spot welds was then observed and the following ratings were given. In the corrosion resistance test, a rating of "3," "4," or "5" was determined to indicate excellent corrosion resistance of the spot welds. The results are shown in Tables 3 and 4. 1: Red rust was found around the spot weld (the area including the spot weld and its surrounding area, the same below). 2: The area ratio of white rust occurring around the spot weld is 20% or more but less than 50% 3: The area ratio of white rust occurring around the spot weld is 10% or more but less than 20% 4: The area ratio of white rust occurring around the spot weld is 5% or more but less than 10% 5: The area ratio of white rust occurring around the spot weld is less than 5%.

[0069] 2.3 Spot weldability A continuous spot welding test was performed on the surface-treated steel sheet 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 the weldability test was considered to be excellent. The results are shown in Tables 3 and 4. 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

[0070] [Table 1]

[0071] [Table 2]

[0072] 3. Results and Discussion The results shown in Tables 1 and 2 reveal the following:

[0073] For Nos. 1 and 2, the magnesium compound content in the coating was too low, less than 5% by volume, making it difficult to ensure the effect of increasing the sacrificial corrosion protection distance of the zinc powder or the effect of depositing the corrosion protection product. As a result, the corrosion resistance of the welded joint after spot welding for both Nos. 1 and 2 was significantly poor. In addition, the corrosion resistance of the flat surface of No. 1 was also poor.

[0074] In the case of No. 6, the content of magnesium compounds in the coating was too high at over 15% by volume, so when the magnesium compounds were eluted from the coating due to salt spray, many voids were formed in the coating, resulting in a decrease in the corrosion resistance of the flat surfaces.

[0075] For Nos. 7 and 8, the zinc powder content in the coating was too low, less than 5% by volume, so the sacrificial corrosion protection effect of the zinc powder was not obtained. As a result, corrosion resistance against salt spray was reduced, and the corrosion resistance of the welded joint after spot welding was also significantly inferior. Furthermore, because the amount of conductive pigment was too low, spot weldability was also reduced.

[0076] In the case of No. 13, the zinc powder content in the coating was too high, exceeding 30% by volume, resulting in reduced spot weldability. The detailed reason is unknown, but as a result of the high ratio of zinc powder on the coating surface, zinc and coating components tended to accumulate on the electrode surface when the electrode and coating came into contact during spot welding, which may have caused electrode wear and reduced continuous welding performance.

[0077] For Nos. 15 to 18, instead of magnesium compounds, only other rust inhibitors were added to the coating, so the effects of magnesium compounds (such as increasing the sacrificial corrosion protection distance of zinc powder and depositing corrosion protection products) were not obtained, resulting in poor corrosion resistance of the welded parts after spot welding.

[0078] In No. 19, like No. 1 and No. 2, the magnesium compound content in the coating was too low at less than 5% by volume, making it difficult to ensure the effect of increasing the sacrificial protection distance of zinc powder and the effect of depositing anticorrosion products. As a result, the corrosion resistance of the welded joint after spot welding was significantly inferior.

[0079] For Nos. 26 to 28, instead of zinc powder, only other conductive agents were added to the coating, so the sacrificial corrosion protection effect of zinc powder was not obtained. As a result, the corrosion resistance of the welded parts after spot welding was poor. In addition, for No. 27, the corrosion resistance of the flat parts was also poor.

[0080] No. 49 is an example using steel sheet without a Zn-containing coating. In No. 49, corrosion resistance was ensured on flat surfaces other than the spot welds due to the zinc powder in the coating, but the amount of zinc as a sacrificial protection component was insufficient in the spot welds, resulting in poor corrosion resistance.

[0081] In Nos. 50 to 52, the average coating thickness was less than 3 μm, which was too thin, and therefore sufficient corrosion resistance was not obtained in either the welded parts or flat parts other than the welded parts.

[0082] For No. 56, the average thickness of the coating was over 10 μm, which was too thick, resulting in a decrease in weldability.

[0083] In contrast, for Nos. 3-5, 9-12, 14, 20-25, 29-48, and 53-55, (1) the coating film had an average thickness of 3 μm to 10 μm, (2) the coating film contained a binder resin, a rust inhibitor, and a conductive agent, (3) the rust inhibitor contained at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, (4) the conductive agent contained zinc powder as a first conductive pigment, (5) the coating film contained 5% to 15% by volume of the magnesium compound, and (6) the coating film contained 5% to 30% by volume of the zinc powder. The combined effect of the magnesium compound and the zinc powder increased the sacrificial protection distance and allowed the deposition of corrosion-preventing components. This provided excellent corrosion resistance even to welds where the coating film or plating layer had been extensively lost. Furthermore, the inclusion of the conductive agent in the coating film and its average thickness below a certain level ensured excellent weldability.

[0084] From the above results, it can be said that surface-treated steel sheets that satisfy the following requirements have excellent weldability and also ensure excellent corrosion resistance in both welded and non-welded areas, even in the absence of electrodeposition coating.

[0085] 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 3 μm or more and 10 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, the rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, the conductive agent contains zinc powder as a first conductive pigment, the coating film contains the magnesium compound in an amount of 5% by volume or more and 15% by volume or less, The coating film contains 5% by volume or more and 30% by volume or less of the zinc powder. 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 3 μm or more and 10 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, the rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, the conductive agent includes zinc powder as a first conductive pigment, the coating film contains the magnesium compound in an amount of 5% by volume or more and 15% by volume or less, The coating film contains the zinc powder in an amount of 5% by volume or more and 27% by volume or less. Surface-treated steel sheet.

2. 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 3 μm or more and 10 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, the rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, the conductive agent includes zinc powder as a first conductive pigment, the coating film contains the magnesium compound in an amount of 5% by volume or more and 15% by volume or less, The coating film contains the zinc powder in an amount of 5% by volume or more and 30% by volume or less, The conductive agent is doped oxide particles, a composite of doped oxide particles and a Si alloy containing 50% by mass or more of Si; A composite of doped oxide particles and a Si compound containing 50% by mass or more of Si, and Composite of doped oxide particles, Si alloy containing 50% by mass or more of Si, and Si compound containing 50% by mass or more of Si At least one second conductive pigment selected from the group consisting of: the coating film contains the second conductive pigment in an amount of 5% by volume or more and 20% by volume or less; Surface-treated steel sheet.

3. 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 3 μm or more and 10 μm or less, the coating film contains a binder resin, a rust inhibitor, and a conductive agent, the rust inhibitor contains at least one magnesium compound selected from magnesium hydroxide and magnesium oxide, the conductive agent includes zinc powder as a first conductive pigment, the coating film contains the magnesium compound in an amount of 5% by volume or more and 15% by volume or less, The coating film contains the zinc powder in an amount of 5% by volume or more and 30% by volume or less, The conductive agent is Ferrosilicon containing 70% by mass or more of Si, and a composite of ferrosilicon containing 70% by mass or more of Si and doped oxide particles; At least one second conductive pigment selected from the group consisting of: the coating film contains the second conductive pigment in an amount of 5% by volume or more and 20% by volume or less; Surface-treated steel sheet.

4. the doped oxide particles are doped zinc oxide particles; The surface-treated steel sheet according to claim 2 or 3.

5. 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 claims 1 to 4.

Images