Surface-treated steel sheet and automobile component therewith

Abstract

To provide a surface-treated steel sheet excellent in slidability, weldability, and corrosion resistance.SOLUTION: A surface-treated steel sheet comprising a base material, an alloyed galvanized layer formed on the surface of the base material, and a coating film formed on at least a part of the surface of the alloyed galvanized layer, where the alloyed galvanized layer has, in order from the base material side, a Γ phase, a δ phase, and a ζ phase; the mass ratio ζ / δ of the ζ phase to the δ phase in the alloyed galvanized layer is 0.100 to 0.400; the mass ratio Γ / δ of the Γ phase to the δ phase in the alloyed molten zinc coating layer is 0.010 to 0.100; and the average thickness of the coating film is 0.5 to 3.0 μm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a surface-treated steel sheet and an automotive member using the same. [Background technology]

[0002] Surface-treated steel sheets are steel sheets with a plating and / or coating formed on the surface, and are used in a variety of applications, including automotive components, machinery components, home appliance components, and building materials. Surface-treated steel sheets used as automotive components in particular require many properties due to the complexity of vehicle body manufacturing.

[0003] Automotive components are manufactured by pressing surface-treated steel sheets. Therefore, surface-treated steel sheets are required to have excellent sliding properties that minimize frictional resistance between the steel sheet and the mold to facilitate pressing. Furthermore, since pressed automotive components are assembled to the vehicle body by spot welding, excellent weldability is required. Furthermore, since the finished automobile is exposed to wind and rain, excellent corrosion resistance is also required.

[0004] Patent Document 1 discloses a galvannealed steel sheet having excellent low-temperature chipping resistance and workability, and a method for producing the same. Patent Document 2 discloses a surface-treated steel sheet having excellent electromagnetic wave shielding properties, corrosion resistance, and heat dissipation properties. Patent Document 3 discloses a surface-treated steel sheet and a coated member having excellent corrosion resistance and weldability. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-257456 [Patent Document 2] Japanese Patent Application Publication No. 11-200000 [Patent Document 3] International Publication No. 2018 / 092244 Summary of the Invention [Problem to be solved by the invention]

[0006] Surface-treated steel sheets used as materials for automotive components may have a coating film formed on a substrate made of alloyed zinc-plated steel sheet. The coating film formed on the surface-treated steel sheet protects the steel sheet from the outside and contributes to improving corrosion resistance, and also serves as a lubricant during steel sheet processing, contributing to improving sliding properties. However, because the coating film is insulating, there is a problem in that spot welding performed after press processing cannot be performed properly. Therefore, a conductive pigment is usually added to the coating film to allow current to flow between the steel sheet and the welding electrode, making spot welding easier.

[0007] However, while conductive pigments enable electrical conduction, in surface-treated steel sheets using some pigments, the resin components disappear when electrical conduction occurs, and the pigment components adhere to the welding electrode. The pigment components oxidize due to welding heat and adhere to the welding electrode, hindering electrical conduction, which can lead to the problem of not being able to perform continuous spot welding. In this case, the welding electrode needs to be cleaned, and frequent cleaning can reduce the production efficiency of automotive parts. From the perspective of improving weldability while maintaining the sliding properties and corrosion resistance of surface-treated steel sheets, the inventions disclosed in Patent Documents 1 to 3 leave room for improvement.

[0008] An object of the present invention is to solve the above problems and to provide a surface-treated steel sheet that is excellent in sliding properties, weldability, and corrosion resistance. [Means for solving the problem]

[0009] The present invention has been made to solve the above-mentioned problems, and is summarized as the following surface-treated steel sheet.

[0010] (1) A surface-treated steel sheet comprising a base material, a galvannealed hot-dip galvanized layer formed on a surface of the base material, and a coating film formed on at least a part of the surface of the galvannealed hot-dip galvanized layer, the galvannealed layer has, in order from the base metal side, a Γ phase, a δ phase, and a ζ phase, the mass ratio ζ / δ of the ζ phase to the δ phase in the galvannealed layer is 0.100 to 0.400; the mass ratio Γ / δ of the Γ phase to the δ phase in the galvannealed layer is 0.010 to 0.100; The average thickness of the coating film is 0.5 to 3.0 μm. Surface-treated steel sheet.

[0011] (2) A chemical conversion coating layer is provided between the galvannealed layer and the coating film. The surface-treated steel sheet according to (1) above.

[0012] (3) The coating film contains a binder resin, 0 to 30.0 mass% of a conductive pigment, 0 to 20.0 mass% of a rust inhibitor, and 0 to 2.0 mass% of a lubricant. The surface-treated steel sheet according to (1) or (2) above.

[0013] (4) The conductive pigment is a doped oxide particle, The doped oxide particles have an average particle size of 0.1 to 2.0 μm. The surface-treated steel sheet according to (3) above.

[0014] (5) An automotive component using the surface-treated steel sheet according to (1) or (2) above. [Effects of the Invention]

[0015] According to the present invention, a surface-treated steel sheet having excellent sliding properties, weldability, and corrosion resistance can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0016] In order to solve the above-mentioned problems, the present inventors have conducted detailed investigations into the sliding properties, weldability, and corrosion resistance of surface-treated steel sheets, and have come to the following findings. In the description of the present invention, the side of a galvannealed (hereinafter also referred to as "GA") layer or the like that is closer to the base material is also referred to as the "base material side." In a GA layer or the like, the side opposite to the base material side, in other words, the direction toward the surface of the surface-treated steel sheet, is also referred to as the "surface side."

[0017] The GA layer consists of three phases: Γ, δ, and ζ. The δ phase is the thickest layer and is more noble than the Γ and more noble than the ζ phase. This improves the mutual corrosion resistance of these three phases. Furthermore, the δ phase has excellent ductility, making it less susceptible to plating peeling during press forming. Therefore, it is ideal for the GA layer to be essentially a single δ phase. However, while a high amount of Γ phase can cause the GA layer to peel during press forming, it also contributes to improved corrosion resistance after painting, specifically the blister corrosion resistance of the coating after electrocoating. Therefore, controlling the amount of Γ phase relative to the amount of δ phase within an appropriate range can improve corrosion resistance.

[0018] Next, we conducted an investigation from the perspective of ensuring weldability. Even if a coating film is formed on a GA steel sheet, spot welding is possible as long as current can be passed between the welding electrode and the GA layer. Therefore, the inventors focused on the ζ phase, which is formed on the surface of the GA layer. The ζ phase has an acicular structure. If the acicular structure of the ζ phase is grown and the surface roughness (hereinafter also referred to as "roughness") of the GA steel sheet is increased, when a coating film is formed on the GA layer, the thickness of the coating film will vary. In other words, there will be locations where the distance between the surface of the coating film and the GA layer is small. As a result, current can be passed between the welding electrode and the GA layer at these locations, ensuring weldability.

[0019] Furthermore, if the amount of ζ phase is controlled to a level that does not significantly affect the surface friction resistance of the coated steel sheet, it is possible to ensure not only corrosion resistance and weldability, but also sliding properties. Therefore, by controlling the amount of ζ phase within an appropriate range relative to the amount of δ phase, weldability and sliding properties can be ensured. Furthermore, forming a coating film on a GA layer with a high degree of roughness has the advantage of improving adhesion between the GA layer and the coating film.

[0020] However, when a coating film is formed on the GA layer, it becomes difficult to measure the morphology based on the acicular structure of the ζ phase, in other words, the roughness of the ζ phase. Since the roughness tends to increase with the amount of ζ phase, and the roughness can be determined from the amount (mass) of the ζ phase, in this invention, the mass of each phase is measured, and the morphologies of the Γ, δ, and ζ phases are identified from their mass ratios.

[0021] The present invention was made based on the above findings. Each of the requirements of the present invention will be described in detail below.

[0022] (A) GA layer The surface-treated steel sheet of the present invention has a GA layer formed on the surface of a base material. The configuration of the GA layer will be described in detail below.

[0023] Mass ratio of ζ phase to δ phase ζ / δ: 0.100 to 0.400 If the mass ratio ζ / δ is less than 0.100, the roughness is small, and when a coating film is formed on the ζ phase, the coating film thickness remains constant. As a result, there are no areas where the coating film is thin, which prevents electrical conductivity during spot welding and results in poor weldability. If the mass ratio ζ / δ is greater than 0.400, the roughness is large, and even when a coating film is formed on the ζ phase, the roughness of the ζ phase is reflected in the coating film, resulting in a large roughness of the coating film. As a result, the sliding properties deteriorate. Furthermore, since the ζ phase has a lower Fe content than the δ phase, Γ phase, and base material, if the mass ratio ζ / δ is greater than 0.400, the coating film may blister and have poor corrosion resistance after electrocoating. Therefore, the mass ratio ζ / δ is limited to 0.100 to 0.400. The mass ratio ζ / δ is preferably 0.120 or greater, more preferably 0.150 or greater. The mass ratio ζ / δ is preferably equal to or less than 0.390, and more preferably equal to or less than 0.350.

[0024] Mass ratio of Γ phase to δ phase Γ / δ: 0.010 to 0.100 If the mass ratio Γ / δ is less than 0.010, the Fe concentration in the GA layer will be low, resulting in reduced corrosion resistance. If the mass ratio Γ / δ is more than 0.100, the Γ phase will be abundant, making interfacial fracture more likely to occur at the interface between the GA layer and the base material. As a result, the adhesion of the GA layer to the base material will be reduced, making the GA layer more likely to peel off during press working and deteriorating the sliding properties. Therefore, the mass ratio Γ / δ is set to 0.010 to 0.100. The mass ratio Γ / δ is preferably 0.020 or more, more preferably 0.035 or more. Furthermore, the mass ratio Γ / δ is preferably 0.090 or less, more preferably 0.080 or less.

[0025] The mass ratios ζ / δ and Γ / δ are determined by the following procedure. First, a sample of a predetermined size is cut out from the manufactured surface-treated steel sheet, and the coating is removed using a coating remover (Neoliver) manufactured by Sansai Kako Co., Ltd. to obtain a plated steel sheet consisting of a GA layer and base material. As described below, if the surface-treated steel sheet has a coating and a chemical conversion coating layer described below, the coating and chemical conversion coating layer are removed.

[0026] Next, the ζ and δ phases of this plated steel sheet were electrolyzed using a saturated calomel electrode to determine the masses of the ζ and δ phases. Specifically, the ζ phase was electrolyzed by constant-potential electrolysis at -1030 mV versus the saturated calomel electrode. After confirming the disappearance of the ζ phase using X-ray diffraction, the mass of the ζ phase was determined from the mass difference between the sample before and after the constant-potential electrolysis. Next, the δ phase was electrolytically eliminated by constant-potential electrolysis at -940 mV versus the saturated calomel electrode, and the mass of the δ phase was determined in the same manner as the ζ phase. Furthermore, the Γ phase remaining in the plated steel sheet was dissolved in a 5% hydrochloric acid solution containing an inhibitor, and the mass of the Γ phase was determined from the mass difference between the plated steel sheet before and after dissolution. The mass ratios ζ / δ and Γ / δ were then calculated from the masses of the ζ, δ, and Γ phases obtained.

[0027] (B) Paint film By adjusting the mass ratios ζ / δ and Γ / δ as described above and then forming a coating film of an appropriate thickness on the GA layer, the corrosion resistance of the surface-treated steel sheet can be improved. The coating film is formed on at least a portion of the surface of the GA layer. The coating film may be formed on only one side of the base material having the GA layer, or on both sides. Note that when a chemical conversion coating layer (described later) is formed in addition to the GA layer, the chemical conversion coating layer is formed between the GA layer and the coating film.

[0028] If the average thickness of the coating film is less than 0.5 μm, the corrosion resistance of the coated steel sheet cannot be improved. Furthermore, the sliding properties of the coated steel sheet may be deteriorated. On the other hand, if the average thickness of the coating film exceeds 3.0 μm, it becomes difficult to form a portion where the distance between the surface of the coated steel sheet and the GA layer is small, and electrical conductivity cannot be ensured during spot welding, making it impossible to improve weldability. Therefore, the average thickness of the coating film is set to 0.5 to 3.0 μm.

[0029] The thickness of the coating film can be determined by calculating the area of ​​the coating film from an electronic image of a cross section of the surface-treated steel sheet taken in the thickness direction using a scanning electron microscope (SEM), and dividing this area by the length in the image field perpendicular to the coating film thickness direction. In this case, to reduce measurement variability, the coating film thickness is measured in five fields, and the average value is taken as the average coating film thickness.

[0030] The composition of the coating film is not particularly limited. However, the coating film preferably contains a binder resin, 0 to 30.0 mass% of a conductive pigment, 0 to 20.0 mass% of a rust inhibitor, and 0 to 2.0 mass% of a lubricant. Note that the binder resin, conductive pigment, rust inhibitor, and lubricant are not necessarily essential for the surface-treated steel sheet of the present invention, and therefore the lower limit of their content is 0 mass%.

[0031] <Binder resin> The binder resin may be either a water-soluble or water-dispersible aqueous resin that dissolves or disperses in water, or a solvent-based resin that dissolves or disperses in an organic solvent, but from the standpoints of production cost and environmental friendliness, a water-based resin is preferred.

[0032] Examples of the water-based resin include water-soluble or water-dispersible resins such as polyester resin, urethane resin, polyolefin resin, acrylic resin, epoxy resin, phenol resin, and mixed resins of two or more of these resins.

[0033] When a polyester resin is used, the molecular weight is preferably 10,000 to 30,000. A molecular weight of 10,000 or more ensures sufficient processability. On the other hand, a molecular weight of 30,000 or less increases the number of bonding sites of the resin itself, ensuring excellent adhesion to the electrodeposition coating film. Furthermore, when crosslinking is performed using a curing agent such as melamine, the crosslinking reaction is carried out sufficiently, ensuring the performance of the coating film.

[0034] When a urethane resin is used, the urethane resin is preferably in the form of an emulsion with an emulsion particle size of 10 to 100 nm (preferably 20 to 60 nm). When the emulsion particle size is 10 nm or more, costs can be kept low. On the other hand, when the emulsion particle size is 100 nm or less, the gaps between the emulsion particles are small when the coating film is formed, ensuring the barrier properties of the coating film. Examples of urethane resin types include ether-based, polycarbonate-based, ester-based, and acrylic graphite-based types. These may be used alone or in combination.

[0035] Examples of solvent-based resins include polyester resins, urethane resins, epoxy resins, acrylic resins, and mixed resins of two or more of these resins.

[0036] Here, the binder resin may be a crosslinked resin having a crosslinked structure or a non-crosslinked resin not having a crosslinked structure, but a non-crosslinked resin is preferable from the viewpoint of low-temperature film formation of the coating film. As a crosslinking agent (curing agent) that imparts a crosslinked structure to the binder resin, a water-soluble crosslinking agent is preferable. Specific examples of the crosslinking agent include melamine, isocyanate, silane compounds, zirconium compounds, and titanium compounds.

[0037] The amount of crosslinking agent added is preferably 5 to 30 parts by mass per 100 parts by mass of resin solids. If the amount of crosslinking agent added is 5 parts by mass or more, the crosslinking reaction with the binder resin is ensured, resulting in sufficient performance as a coating film. On the other hand, if the amount of crosslinking agent added is 30 parts by mass or less, the crosslinking reaction proceeds moderately, preventing the coating film from becoming excessively hard. This ensures processability. In addition, the paint stability of the crosslinking agents, silane compounds, zirconium compounds, and titanium compounds, can be ensured.

[0038] <Conductive pigment> Conductive pigments may be added as needed to improve the electrical conductivity of the coating film and improve weldability. However, if the conductive pigment content exceeds 30.0 mass%, the pigment adheres to the electrode during spot welding in large quantities, reducing productivity. Therefore, the conductive pigment content is preferably 0 to 30.0 mass%.

[0039] Examples of conductive pigments include oxide particles (doped zinc oxide particles, doped tin oxide particles, tin oxide-titanium oxide composite particles, tin oxide-barium sulfate composite particles, nickel oxide-aluminum composite particles, etc.), non-oxide ceramic particles (boride ceramics, carbide ceramics, nitride ceramics, silicide ceramics, etc.), iron alloy particles (ferrosilicon particles, etc.), stainless steel particles, particles other than iron alloys (particles of metals such as zinc, nickel, aluminum, cobalt, manganese, copper, tin, etc., or alloys thereof, etc.), carbon particles (conductive carbon, graphite powder, etc.), and carbides (titanium carbide, silicon carbide, etc.).

[0040] The conductive pigment is preferably a doped oxide particle. The use of the doped oxide particle can improve the electrical conductivity of the coating film. Examples of the doped oxide particle include doped zinc oxide particles, doped tin oxide particles, tin oxide-titanium oxide composite particles, tin oxide-barium sulfate composite particles, and nickel oxide-aluminum composite particles.

[0041] It is preferable to use doped zinc oxide particles as the doped oxide particles. When doped zinc oxide particles are used, they act in the same way as when spot welding steel sheets having only an uncoated GA layer, and therefore electrode wear can be made less severe than with other doped oxide particles.

[0042] The average particle size of the conductive pigment is not particularly limited. However, by setting the average particle size of the conductive pigment to 0.1 μm or more, electrical conductivity can be ensured. Furthermore, by setting the average particle size of the conductive pigment to 2.0 μm or less, the binder resin can cover the conductive pigment, thereby preventing corrosion factors such as chloride ions from penetrating the interface between the binder resin and the conductive pigment. As a result, corrosion resistance can be improved. Therefore, the average particle size of the conductive pigment is preferably set to 0.1 to 2.0 μm. Note that, from the viewpoint of reliably ensuring electrical conductivity, the average particle size of the conductive pigment may be larger than the average film thickness of the coating film.

[0043] The average particle size of the conductive pigment in the coating is determined by the average particle size of the conductive pigment used when mixing the paint, but it can also be calculated from cross-sectional images of the surface-treated steel sheet taken with an SEM. That is, the particle size of the conductive pigment can be determined by calculating the circle-equivalent particle size from the area of ​​each conductive pigment in the image field and averaging these. In this case, to reduce measurement variability, the particle size of the conductive pigment is calculated in five fields of view, and the average is used as the average particle size of the conductive pigment.

[0044] <Rust inhibitor> A rust inhibitor may be added as needed to improve corrosion resistance. However, if the rust inhibitor content exceeds 20.0 mass%, excessive adhesion of the rust inhibitor in the coating to the electrode occurs during spot welding, resulting in reduced weldability. Therefore, the rust inhibitor content is preferably 0 to 20.0 mass%.

[0045] As the rust inhibitor, for example, organic rust inhibitors such as organic acids, organic acid salts, amine salts, and esters, and rust inhibitor pigments are used to impart high weldability. When an organic rust inhibitor is used, it burns and carbonizes due to the heat during welding, making it difficult for it to adhere to the welding electrode. Among these, organic acids are preferred because they can impart high corrosion resistance.

[0046] Examples of organic acids include carboxylic acids (tartaric acid, tannic acid, oleic acid, dimer acid, naphthalene acid, etc.). Examples of organic acid salts include carboxylic acid metal soaps (lanolin calcium, naphthenate zinc, oxidized wax calcium, oxidized wax barium salt, etc.) and sulfonates (sodium sulfonate, calcium sulfonate, barium sulfonate). Examples of esters include glycerin esters of higher fatty acids, sorbitan monoisostearate, sorbitan nooleate, etc.

[0047] Examples of anti-rust pigments include silica particles and metal phosphates (e.g., aluminum tripolyphosphate, etc.). When silica is used as the anti-rust agent, some elements of the silica may be exchanged with other elements such as calcium by ion exchange.

[0048] <Lubricant> A lubricant may be added as needed to improve sliding properties. However, if the lubricant content exceeds 2.0% by mass, the proportion of the lubricant component on the coating surface increases, resulting in a decrease in the coatability of the electrodeposition coating when electrodeposition coating is performed. This increases the likelihood of water, water vapor, and corrosion factors accumulating at the interface between the electrodeposition coating and the surface-treated steel sheet. As a result, corrosion resistance decreases. Therefore, the lubricant content is preferably 0 to 2.0% by mass.

[0049] Lubricants can impart excellent lubricity to the coating film and also improve the powdering resistance of the coating film. Examples of lubricants include the organic lubricants (1) and (2) below. In particular, when an organic lubricant is used, it burns and carbonizes due to the heat during welding, making it difficult for the organic lubricant to adhere to the welding electrode. Among these, it is preferable to use polyolefin wax as the organic lubricant, as it can impart high corrosion resistance. (1) Polyolefin wax, paraffin wax: for example, polyethylene wax, synthetic paraffin, natural paraffin, microcrystalline wax, chlorinated hydrocarbons, etc. (2) Fluorine resin wax: for example, polyfluoroethylene resin (polytetrafluoroethylene resin, etc.), polyvinyl fluoride resin, polyvinylidene fluoride resin, etc.

[0050] When polyethylene wax is used as the lubricant, the average particle size is preferably 0.5 to 10 μm. If the average particle size of the polyethylene wax is 0.5 μm or more, surface thickening of the polyethylene wax is suppressed, and a decrease in adhesion between the coating film and the electrodeposition coating film can be suppressed. On the other hand, if the average particle size of the solid lubricant is 10 μm or less, peeling of the polyethylene wax from the coating film is suppressed, and lubricity and corrosion resistance can be ensured. The average particle size of the solid lubricant is more preferably 1.0 to 5.0 μm in order to obtain excellent adhesion between the coating film and the electrodeposition coating film, corrosion resistance, lubricity, and powdering resistance.

[0051] (C) Chemical conversion coating layer The surface-treated steel sheet according to the present invention preferably has a chemical conversion coating layer between the GA layer and the coating film. By having the chemical conversion coating layer, corrosion resistance can be further improved. In addition, by having the chemical conversion coating layer, adhesion at the interface between the GA layer and the coating film can be improved.

[0052] By making the thickness of the chemical conversion coating layer 0.05 μm or more, corrosion resistance can be improved. Furthermore, adhesion between the GA layer and the paint film can be improved. Furthermore, by making the thickness of the chemical conversion coating layer 0.20 μm or less, the roughness of the GA layer can be reflected on the surface of the surface-treated steel sheet. Furthermore, cohesive failure of the chemical conversion coating layer can be suppressed, and adhesion between the GA layer and the paint film can be maintained. Therefore, the thickness of the chemical conversion coating layer is preferably 0.05 to 0.20 μm.

[0053] The thickness of the chemical conversion coating layer on a surface-treated steel sheet can be measured using a transmission electron microscope (TEM). Specifically, a thin film sample is first prepared and imaged using a TEM. The area of ​​the chemical conversion coating layer is calculated from the obtained electron image, and the thickness of the chemical conversion coating layer can be determined by dividing this area by the length in the image field in the direction perpendicular to the thickness direction. In this case, to reduce measurement variability, the thickness of the chemical conversion coating layer is measured in five fields of view, and the average value is taken as the average thickness of the chemical conversion coating layer.

[0054] The thickness of the chemical conversion coating layer can also be measured using an X-ray fluorescence analyzer. In this case, a calibration curve is first prepared from multiple steel sheets that have been subjected to chemical conversion treatment, and then the surface-treated steel sheet to be measured is measured using X-ray fluorescence. The X-ray intensity of Si is then applied to the calibration curve to determine the amount of adhesion of the chemical conversion coating layer, and the thickness of the chemical conversion coating layer can be calculated. This measurement method can be used easily.

[0055] The chemical conversion coating layer may be, for example, a layer that does not substantially contain 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, vapor-phase silica, and silicates as the main component, zircon-based treatment solutions containing zircon compounds as the main component, and mixtures thereof.

[0056] The chemical conversion coating layer may contain a binder resin. For example, the chemical conversion coating layer may contain at least one of the binder resins exemplified above as possible components of the coating film, or may contain a polyester resin. The content of the binder resin and the content of components other than the binder resin (such as the silicon compounds described above) in the chemical conversion coating layer are not particularly limited. For example, the content of the binder resin in the chemical conversion coating layer may be 0% by mass or more and 50% by mass or less, and the content of components other than the binder resin may be 50% by mass or more and 100% by mass or less.

[0057] (D)Application The surface-treated steel sheet according to the present invention has excellent sliding properties, weldability, and corrosion resistance, and can therefore be used as an automotive component. The surface-treated steel sheet according to the present invention can be used as an automotive component such as a side sill, which is a component having a closed cross section that has been pressed and spot-welded to form an overlap weld. The surface-treated steel sheet according to the present invention may also be electrocoated and used as an automotive component.

[0058] (E) Manufacturing method There are no particular limitations on the method for producing the surface-treated steel sheet of this embodiment. For example, the method may include a step of producing a base material, a step of forming a GA layer on the surface of the base material, and a step of forming a coating film. If necessary, the method may also include a step of forming a chemical conversion coating layer. Each step will be described in detail below.

[0059] <Base material manufacturing process> In the base material production process, the base material for the surface-treated steel sheet is produced. For example, molten steel having a predetermined chemical composition is produced, and this molten steel is used to produce a slab by a casting method or an ingot by an ingot-making method. The slab or ingot is then hot-rolled to obtain the base material (hot-rolled sheet).

[0060] The base material may be a cold-rolled sheet obtained by subjecting the hot-rolled sheet to pickling and then cold-rolling the hot-rolled sheet after the pickling treatment. Furthermore, the base material may be a hot-rolled annealed sheet or a cold-rolled annealed sheet obtained by subjecting the hot-rolled sheet or the cold-rolled sheet to annealing.

[0061] <Plating process> In the plating process, a zinc plating layer is formed on the surface of the base material to produce a plated steel sheet. The zinc plating layer can be formed, for example, by hot dip plating.

[0062] For example, an example of forming a galvanized layer by hot-dip galvanizing is as follows. First, a cold-rolled sheet is degreased with an alkali, then washed with water, and dried. If the base material is a hot-rolled sheet, a hot-rolled annealed sheet, or a cold-rolled annealed sheet, it may be pickled, washed with water, and dried. Then, the cold-rolled sheet or the like is annealed (1 to 20% H2) and immersed in a hot-dip galvanizing bath containing Zn, Al, and impurities to deposit a galvanized layer on the surface of the base material. The chemical composition of the hot-dip galvanizing bath is such that the Zn content is 90% by mass or more. Al, Mg, Pb, Si, etc. may be contained, but the total content of these elements is preferably 10% by mass or less.

[0063] Next, the base material with the zinc plating layer attached thereto is pulled out of the plating bath. The speed at which the steel sheet is pulled out of the plating bath is preferably 10 to 200 mm / s. The flow rate of the wiping gas is preferably 10 m / s or more. The subsequent cooling rate is preferably 1 to 100°C / s. The amount of zinc plating thus produced is 25 to 100 g / m 2 It is preferable to set the following.

[0064] Thereafter, an alloying heat treatment is carried out to form a galvannealed layer. There are no particular restrictions on the conditions for the alloying heat treatment, but the heating temperature is preferably 440 to 600°C, and the heating time is preferably 1 to 30 seconds.

[0065] <Coating film formation process> In the coating film formation process, a coating film is formed on the surface of the GA layer to produce a surface-treated steel sheet. The binder resin, conductive pigment, rust inhibitor, and lubricant are mixed in a solvent primarily composed of water or a known organic solvent to prepare a coating composition. This coating composition is applied to the surface of the GA layer or chemical conversion coating using a bar coater. The coating is then dried in an oven to form a coating film.

[0066] <Chemical conversion coating process> After the plating process, a chemical conversion coating layer may be formed on the surface side of the GA layer, and then a coating film formation process may be carried out to produce a surface-treated steel sheet. Forming a chemical conversion coating layer does not affect the coating film on top of it. To form a chemical conversion coating, the resin and compound described above are mixed in the above-mentioned mass percentages in a solvent primarily composed of water or a known organic solvent to prepare a treatment solution. This treatment solution is applied to the surface side of the GA layer using a bar coater. The applied treatment solution is then heated and dried in a hot air oven so that the temperature reaches 30 to 150°C, followed by air drying.

[0067] After producing the surface-treated steel sheet as described above, an automobile component may be produced by press working. After press working, electrodeposition coating may be performed by a known method.

[0068] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0069] 1. Manufacturing of surface-treated steel sheets 1.1 Preparation of base material A 0.8 mm thick galvannealed steel sheet was prepared and immersed in an aqueous solution (2.5 mass %, 40°C) of a water-based alkaline degreasing agent (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for 2 minutes to degrease the surface, followed by rinsing with water and drying to prepare a substrate for surface treatment.

[0070] The coating weight was controlled by adjusting the coating pull-up speed and wiping amount during coating preparation to achieve the coating weights shown in Tables 1 and 2. Regarding the abundance ratio of each phase, increasing the alloying temperature resulted in less ζ phase and more Γ phase, while decreasing the alloying temperature resulted in more ζ phase and less Γ phase. The abundance ratio of each phase was controlled by adjusting the alloying time and, primarily, the alloying temperature, to produce a GA layer with the abundance ratios shown in Tables 1 and 2.

[0071] [Table 1]

[0072] [Table 2]

[0073] 1.3 Formation of chemical conversion coating The following chemical conversion treatment solutions S1 and S2 were prepared, and treatment solution S was applied to some of the substrates while changing the bar coating size to achieve the deposition amount shown in Tables 1 and 2. The substrates were then heated and dried in a hot air oven so that the surface of treatment solution S1 or S2 reached a temperature of 70°C, and then air-dried to obtain surface-treated steel sheets. A chemical conversion treatment film with an average film thickness of 0.03 to 0.40 μm was formed on the surface of the substrate. S1: A chemical conversion treatment solution consisting of a silane coupling agent, silica particles, urethane resin, phosphoric acid compound, and fluorine compound with a solids concentration of 10%, and the remaining solvent (water). S2: A chemical conversion treatment solution consisting of a Zr compound, a silane coupling agent, a phosphate compound, and a vanadium compound with a solids concentration of 10%, the remainder being solvent (water).

[0074] 1.4 Coating film formation To form a coating film having the composition ratio (mass %) shown in Tables 1 and 2, the components shown below were mixed in the proportions shown in Table 3 to prepare a coating composition for coating formation. This coating composition was applied to the substrate or chemical conversion coating with a bar coater, varying the bar coat size and dilution ratio, and then dried in an oven at 350°C to form a coating film, resulting in a surface-treated steel sheet. Tables 1 and 2 also show the average film thickness (μm) of each coating film.

[0075] (binder resin) J1: Epoxy resin (ADEKA ADEKA RESIN EM-0461N) J2: Polyester resin (Vylonal MD1480 manufactured by Byron) J3: Urethane resin (Superflex 150 manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) J4: Epoxy resin (ADEKA ADEKA RESIN EP-4100) J5: Polyester resin (Byron 200 manufactured by Byron) J6: Melamine resin (Allnex Cymel 325) J7: Silane coupling agent (KBE403 manufactured by Shin-Etsu Chemical Co., Ltd.)

[0076] (Conductive pigment) Z1: Doped zinc oxide particles (23-Kt, manufactured by Hakusui Tech Co., Ltd., average particle size 0.5 μm) Z2: Doped zinc oxide particles (23-K manufactured by Hakusui Tech Co., Ltd., average particle size 0.2 μm) Z3: Doped zinc oxide particles (Pazet CK, manufactured by Hakusui Tech Co., Ltd., average particle size 0.03 μm) Z4: Zinc oxide particles (Kanto Chemical Co., Ltd., average particle size 0.5 μm) Z5: Titanium nitride particles (Kanto Chemical Co., Ltd., average particle size 2.0 μm)

[0077] (rust inhibitor) B1: Tannic acid (Kanto Chemical Co., Ltd.) + phosphoric acid (Kanto Chemical Co., Ltd.) mixed in a mass ratio of 1:1 B2: Tartaric acid (Kanto Chemical Co., Ltd.) + phosphoric acid (Kanto Chemical Co., Ltd.) mixed in a mass ratio of 1:1 B3: Tannic acid (Kanto Chemical Co., Ltd.) + trizinc phosphate (Kanto Chemical Co., Ltd.) mixed in a mass ratio of 1:1 B4: Tartaric acid (Kanto Chemical Co., Ltd.) + trizinc phosphate (Kanto Chemical Co., Ltd.) mixed in a mass ratio of 1:1 B5: Colloidal silica (Nissan Chemicals, particle size 20 nm) B6: Aluminum dihydrogen tripolyphosphate (manufactured by Teika, particle size 2 μm) B7: Calcium ion-exchanged silica (Grace, Ca exchange rate 9%) (particle size 2 μm)

[0078] (lubricant) W1: Polyethylene wax (Sanyo Chemical) W2: Microcrystalline wax (manufactured by Nippon Seiro) W3: PTFE particles (BYK)

[0079] [Table 3]

[0080] 3. Tissue Observation 3.1 Mass ratio ζ / δ and mass ratio Γ / δ The mass ratios ζ / δ and Γ / δ were determined by the following procedure. First, the coating was removed from a sample cut to a predetermined size from the manufactured surface-treated steel sheet using a coating remover (Neoliver) manufactured by Sansai Kako Co., Ltd., to obtain a plated steel sheet consisting of a GA layer and base material. When the surface-treated steel sheet had a coating and a chemical conversion coating layer described below, the coating and chemical conversion coating layer were removed.

[0081] Next, the ζ and δ phases were electrolyzed using a saturated calomel electrode to determine the masses of the ζ and δ phases. Specifically, the ζ phase was electrolyzed at −1030 mV vs. the saturated calomel electrode. After confirming the disappearance of the ζ phase using X-ray diffraction, the mass of the ζ phase was determined from the mass difference between the sample before and after the electrolysis. Next, the δ phase was electrolyzed at −940 mV vs. the saturated calomel electrode, and the mass of the δ phase was determined similarly to the ζ phase. Furthermore, the Γ phase remaining in the plated steel sheet was dissolved in a 5% hydrochloric acid solution containing an inhibitor. The mass of the Γ phase was determined from the mass difference between the plated steel sheet before and after dissolution. The mass ratios ζ / δ and Γ / δ were then calculated from the masses of the ζ, δ, and Γ phases.

[0082] 3.2 Thickness of chemical conversion coating layer The thickness of the chemical conversion coating layer was measured using a TEM. First, a thin film sample was prepared and photographed using a TEM. The area of ​​the chemical conversion coating layer was calculated from the resulting electronic image, and the thickness of the chemical conversion coating layer was determined by dividing this area by the length in the image field perpendicular to the thickness direction. Taking into account measurement variability, the average value measured over five fields of view was taken as the average thickness of the chemical conversion coating layer.

[0083] 3.3 Coating thickness The thickness of the coating film was determined by photographing the cross section of the surface-treated steel sheet in the thickness direction using an SEM and dividing the area of ​​the coating film by the length in the image field perpendicular to the coating film thickness direction. The thickness of the coating film was measured in five fields of view, and the average value was taken as the average coating film thickness.

[0084] 4.Evaluation Test The following evaluation tests were carried out on each of the surface-treated steel sheets.

[0085] 4.1 Spot weldability Each surface-treated steel sheet was spot-welded at 500 points using a CF-type Cr-Cu electrode with a tip diameter of 5 mm and R40, with a pressure of 1.96 kN and a welding time of 12 cycles / 50 Hz. The welding current was adjusted to produce a nugget diameter of 4√t (t = thickness of the surface-treated steel sheet). After spot welding, the nugget diameter and electrode condition were observed at the end of the 500 welding points. Spot welds where no nuggets were formed were judged to have no current and were assigned the following rating. A rating of "3," "4," or "5" was considered to indicate excellent weldability. The results are shown in Tables 4 and 5. 1:500 RBI not possible 2: 500 dots can be performed, but there may be no current flowing in 10 or fewer dots (including no current flowing), and at the end of 500 dots, the diameter of the electrode contact surface using pressure-sensitive paper must be 1.7 times or more the diameter of the electrode contact surface at the first dot. 3: 500 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 500 dots is 1.4 times or more but less than 1.7 times the diameter of the electrode contact surface at the first dot. 4: 500 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 500 dots is 1.1 times or more but less than 1.4 times the diameter of the electrode contact surface at the first dot. 5: 500 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 500 dots is less than 1.1 times the diameter of the electrode contact surface at the first dot

[0086] 4.2 Sliding properties (press workability) Each surface-treated steel sheet was punched to a diameter of 110 mm to prepare test specimens. First, the mass of the test specimen was measured, and cylindrical drawing was performed using a press with a die that achieved a drawing ratio of 2.2. Next, 24 mm-wide tape (Nichiban Co., Ltd., Model No. CT-24) was applied to the entire front and back walls of the test specimen after cylindrical drawing. The tape was then peeled off to remove the coating and plating layer that had peeled off during cylindrical drawing, and the mass of the test specimen was measured again. The amount of plating peeled off was calculated from the difference in mass before and after cylindrical drawing, and the sliding properties were evaluated. The evaluation was based on the following criteria: A rating of "3," "4," or "5" was considered to indicate excellent sliding properties (press workability). The results are shown in Tables 4 and 5. 1: Check for cracks during pressing 2: Large plating peeling (over 50 mg), no cracks when pressed 3: Plating peeling (30 mg or more but less than 50 mg), no cracks when pressed 4: Minor plating peeling (10 mg to less than 30 mg), no cracks when pressed 5: Very little plating peeling (less than 10 mg), no cracks when pressed

[0087] 4.3 Corrosion resistance <Corrosion resistance test 1: Cross-cut corrosion resistance test> Each surface-treated steel sheet was immersed in a surface conditioning treatment agent, "Preparen X (trade name)" manufactured by Nihon Parkerizing Co., Ltd., for 20 seconds at room temperature to condition the steel sheet surface. Subsequently, the steel sheet was immersed in a chemical conversion treatment solution (zinc phosphate treatment solution), "Palbond 3020 (trade name)" manufactured by Nihon Parkerizing Co., Ltd., for 120 seconds at 43°C, followed by rinsing with water and drying, for chemical conversion treatment (phosphate treatment). The steel sheet surface was then 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 thickness of the electrocoated film after electrocoating was approximately 10 μm for all steel sheets.

[0088] A corrosion cycle test was conducted on the electrodeposition-coated steel sheets, with cross-cuts made with a cutter knife down to the substrate. A corrosion cycle consisted of 2 hours of salt spray (SST, 5% NaCl, 35°C atmosphere), 2 hours of dry (60°C), and 4 hours of wet (50°C, 98% RH) exposure, for a total of 360 cycles. Evaluation was based on the following criteria, with a rating of "3," "4," or "5" indicating excellent corrosion resistance. The results are shown in Tables 4 and 5. 1: Red rust appears on the flat surface 2: Paint blister or white rust from flat area, or paint blister width from cross cut area exceeds 4mm 3: No rust on flat surfaces, paint blister width from cross-cut area is over 2mm and within 4mm 4: No rust on flat surfaces, paint blister width from cross-cut area is over 1mm and less than 2mm 5: No rust on flat surfaces, paint blister width within 1mm from cross-cut area

[0089] <Corrosion resistance test 2: Corrosion resistance test of spot welds> Two surface-treated steel plates cut to 70 mm x 150 mm and 30 mm x 100 mm were spot-welded together (two locations) to prepare a laminated test piece. A spacer was inserted between the steel plates to adjust the clearance to approximately 200 μm during spot welding. The test piece was then electrocoated in the same manner as in Corrosion Resistance Test 1.

[0090] The electrodeposition-coated test specimens were subjected to 360 cycles of salt spray (SST, 0.5% NaCl, 35°C atmosphere) for 2 hours, dry (60°C) for 2 hours, and wet (50°C, 98% RH) for 4 hours. After 360 cycles, the spot welds were removed, the corrosion products were removed, and the corrosion depth was measured. Evaluation was based on the following criteria: A rating of "3," "4," or "5" was considered to indicate excellent corrosion resistance. The results are shown in Tables 4 and 5. 1: Maximum corrosion depth: Over 0.4 mm, holes appear 2: Maximum corrosion depth: Over 0.2 mm and up to 0.4 mm 3: Maximum corrosion depth: Over 0.1mm and up to 0.2mm 4: Maximum corrosion depth: Over 0mm and within 0.1mm 5: Maximum corrosion depth: 0mm

[0091] [Table 4]

[0092] [Table 5]

[0093] As can be seen from Tables 4 and 5, the test pieces corresponding to the surface-treated steel sheets satisfying the present invention were good in all of weldability, sliding property, and corrosion resistance. On the other hand, the test pieces not satisfying the present invention did not achieve the desired properties in any of weldability, sliding property, and corrosion resistance.

[0094] In other words, the test piece of Steel No. 33 had a thin coating and could not ensure corrosion resistance. The sliding properties also deteriorated. The test piece of Steel No. 34 had a thicker coating than the test piece of Steel No. 33, and although corrosion resistance was ensured in the corrosion resistance test of the spot welded joint, corrosion resistance was not ensured in the cross-cut corrosion resistance test. The test piece of Steel No. 39 had a thick coating and did not allow for proper current flow during spot welding, so weldability was not ensured.

[0095] The test piece of Steel No. 40 had a small mass ratio ζ / δ, and weldability could not be ensured. This is thought to be because the roughness of the galvannealed layer could not be increased, and therefore current could not be passed properly during spot welding. The test piece of Steel No. 41 also had a small mass ratio ζ / δ, and weldability could not be ensured, just like the test piece of Steel No. 40. In addition, the test piece of Steel No. 41 also had a large mass ratio Γ / δ, and sliding properties could not be ensured. This is thought to be because the Γ layer in the galvannealed layer increased, reducing adhesion and resulting in frequent peeling of the galvannealed layer.

[0096] The test specimens of Steel No. 43, Steel No. 45, Steel No. 46, and Steel No. 49 also had a large mass ratio Γ / δ, and were unable to ensure sliding properties. The test specimen of Steel No. 47 had a small mass ratio Γ / δ, and was able to ensure corrosion resistance in the spot welds, but was unable to ensure corrosion resistance in the cross-cut corrosion resistance test. The test specimens of Steel No. 50 and Steel No. 51 also had a large mass ratio ζ / δ, which increased the roughness of the galvannealed coating layer, which is thought to have resulted in increased roughness of the coating film and a deterioration in sliding properties. Corrosion resistance was also unable to be ensured. [Industrial Applicability]

[0097] According to the present invention, in the galvannealed steel sheet, the coating layer and the coating film formed thereon have a consistent structure, and therefore not only are the coating layer and the coating film formed thereon excellent in corrosion resistance and sliding properties, but also, even when spot welding is performed, the pigment component is less likely to adhere to the welding electrode, and continuous spot welding is possible, so that productivity is not reduced. Therefore, the surface-treated steel sheet of the present invention is particularly suitable for use as a material for automotive parts, and can be used as automotive parts such as side sills and other parts having a closed cross section that is pressed and spot-welded to form a spot-welded lap weld.

Claims

1. A surface-treated steel sheet comprising a base material, a galvannealed layer formed on a surface of the base material, and a coating film formed on at least a part of a surface of the galvannealed layer, the galvannealed layer has, in order from the base metal side, a Γ phase, a δ phase, and a ζ phase, a mass ratio ζ / δ of the ζ phase to the δ phase in the galvannealed layer is 0.100 to 0.400; a mass ratio Γ / δ of the Γ phase to the δ phase in the galvannealed layer is 0.010 to 0.100; The average film thickness of the coating film is 0.5 to 3.0 μm. Surface-treated steel sheet.

2. A chemical conversion coating layer is provided between the galvannealed layer and the coating film. The surface-treated steel sheet according to claim 1.

3. the coating film contains a binder resin, 0 to 30.0 mass % of a conductive pigment, 0 to 20.0 mass % of a rust inhibitor, and 0 to 2.0 mass % of a lubricant; The surface-treated steel sheet according to claim 1 or 2.

4. the conductive pigment is a doped oxide particle, The doped oxide particles have an average particle size of 0.1 to 2.0 μm; The surface-treated steel sheet according to claim 3.

5. An automotive member using the surface-treated steel sheet according to claim 1 or 2.

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