Surface-treated steel material

Abstract

To provide a surface-treated steel material that is excellent in corrosion resistance (especially, white rust resistance) in a water-flowing environment.SOLUTION: A surface-treated steel material includes: a steel material; a plating layer formed on the surface of the steel material; and a chemical conversion coating film formed on the surface of the plating layer. The plating layer is a Zn alloy plating layer including Al of 2.0 mass% or more and less than 20.0 mass% and Mg of 1.0 mass% or more and less than 6.0 mass%. The chemical conversion coating film includes an urethane resin, P, and a silicon oxide. The chemical conversion coating film has a storage elastic modulus of 1.5-4.0 GPa which is measured at a measured temperature of 25°C. In the chemical conversion coating film, an average grain size of the silicon oxide is 5-20 nm and concentration of the silicon oxide is 20-30 mass%. Within a range of 500-600 nm on a side of the chemical conversion coating film from a boundary surface between the plating layer and the chemical conversion coating film, a maximum value P1 of concentration of P is 5.0-10.0 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a surface-treated steel material. [Background technology]

[0002] Conventionally, zinc-plated steel sheets (zinc (Zn)-plated steel sheets), in which a zinc-based plating layer is formed on the surface of steel sheets, have been used in a wide range of applications, including automobiles, building materials, and home appliances. Furthermore, for the purpose of imparting further corrosion resistance, paint adhesion, etc. to the surface of such zinc-plated steel sheet, methods such as chromate treatment using a treatment solution containing chromic acid, dichromic acid, or a salt thereof as a main component, treatment using a metal surface treatment agent that does not contain chromium, phosphate treatment, treatment with a silane coupling agent alone, and organic resin coating treatment are generally known and in practical use.

[0003] As a method for performing an organic resin coating treatment, Patent Documents 1 to 4 disclose a method in which a treatment liquid containing an emulsion of a urethane resin is applied to the surface of a substrate and then dried. Patent Document 1 describes a method for applying a treatment liquid containing a lubricant such as a water-dispersible polyurethane resin, silica particles, or polyolefin wax to a steel sheet, which can reduce the storage modulus E1 to 1.0 × 10 9 dyn / cm 2 As described above, a technique for forming an organic resin film with excellent processability has been disclosed. Furthermore, Patent Document 2 discloses a technology for forming an organic resin coating having excellent heat resistance by adding a resin other than polyurethane resin selected from vinyl chloride-vinyl acetate copolymers, cellulose resins, and (meth)acrylic resins to a polyurethane resin coating to increase the storage modulus of the coating. Furthermore, Patent Documents 3 and 4 disclose a technique for improving the corrosion resistance, tape peeling resistance, solvent resistance, alkali resistance, and abrasion resistance of processed areas by incorporating silicon oxide particles and organic titanium compounds into an organic resin coating primarily composed of polyurethane resin particles and ethylene-unsaturated carboxylic acid copolymer resin particles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-183944 [Patent Document 2] Japanese Patent Application Publication No. 2022-95299 [Patent Document 3] International Publication No. 2007 / 144951 [Patent Document 4] Japanese Patent Application Laid-Open No. 2005-281863 Summary of the Invention [Problem to be solved by the invention]

[0005] The techniques disclosed in Patent Documents 1 to 4 are excellent techniques that have been put to practical use as surface-treated steel sheets that are made mainly of urethane resin and that have undergone chromate-free surface treatment to impart the various functions described above. However, in recent years, customer needs have become more sophisticated, requiring corrosion resistance in more severe environments. For example, corrosion resistance is required in an environment where water droplets (e.g., rainwater) fall and hit the surface of the coated steel sheet, and the water droplets also flow over the surface of the coated steel sheet (hereinafter referred to as a flowing water environment). In such a flowing water environment, water penetrates into the coating and destroys the coating, making rust more likely to occur. As a result of investigations by the present inventors, it has been found that with the techniques of Patent Documents 1 to 4, zinc rust (white rust) may develop early on from the surface-treated steel sheet in such a flowing water environment. Therefore, the present invention aims to provide a surface-treated steel material that also has excellent corrosion resistance (particularly white rust resistance) in a flowing water environment, based on the premise that the surface of the surface-treated steel material has a chemical conversion coating formed on the surface of a zinc-plated steel material that has excellent corrosion resistance. [Means for solving the problem]

[0006] The present inventors have conducted research into improving corrosion resistance (white rust resistance) in a flowing water environment, and have made the following findings. (i) White rust resistance in a flowing water environment (hereinafter sometimes referred to as flowing water corrosion resistance) is affected by the crosslink density of the organic resin contained in the chemical conversion coating, and the higher the crosslink density, the better the flowing water corrosion resistance. (ii) Although it is difficult to measure the crosslink density directly, it correlates with the storage modulus, and the value of the storage modulus can be used as an index. (iii) By dispersing an appropriate amount of silicon oxide fine particles of an appropriate particle size in the chemical conversion coating, corrosion resistance in flowing water is improved. (iv) By adding appropriate amounts of Al and Mg to the zinc plating layer, corrosion resistance in flowing water is improved. (v) When P is concentrated near the interface between the chemical conversion coating and the plating layer, the corrosion resistance in flowing water is further improved.

[0007] The present invention has been made in light of the above findings. [1] A steel material, a plating layer formed on a surface of the steel material, and a chemical conversion coating formed on the surface of the plating layer, wherein the plating layer is a Zn alloy plating layer containing 2.0 mass% or more and less than 20.0 mass% Al and 1.0 mass% or more and less than 6.0 mass% Mg, the chemical conversion coating contains a urethane resin, P, and silicon oxide, the storage modulus of the chemical conversion coating measured at a measurement temperature of 25°C is 1.5 to 4.0 GPa, and the average particle size of the silicon oxide in the chemical conversion coating is 5 to 20 nm. m, the silicon oxide concentration is 20 to 30 mass %, and when the concentrations of C, O, Mg, Al, P, and Zn are continuously measured by linear analysis from the plating layer toward the surface of the chemical conversion coating in the thickness direction of the chemical conversion coating, and the position at which the Zn concentration first becomes 35.0 mass % is defined as the interface between the plating layer and the chemical conversion coating, the maximum P concentration P1 is 5.0 to 10.0 mass % within a range of 500 to 600 nm from the interface toward the chemical conversion coating. [2] The surface-treated steel material according to [1], wherein the boundary region is defined as a region including the interface, between a range of 10 nm from the interface toward the plating layer in the thickness direction and a range of 15 nm toward the chemical conversion coating, and the maximum P concentration in the boundary region is defined as P2, and the ratio of P2 to P1, P2 / P1, is 1.5 or more. [Effects of the Invention]

[0008] According to the above-described aspect of the present invention, it is possible to provide a surface-treated steel material that is also excellent in corrosion resistance (particularly white rust resistance) in a flowing water environment. DETAILED DESCRIPTION OF THE INVENTION

[0009] A surface-treated steel material according to one embodiment of the present invention (surface-treated steel material according to this embodiment) will be described.

[0010] The surface-treated steel material according to this embodiment includes a steel material, a plating layer formed on the surface of the steel material, and a chemical conversion coating formed on the surface of the plating layer.

[0011] [Steel] The surface-treated steel material according to this embodiment has a significant feature in the chemical conversion coating. Therefore, the steel material is not particularly limited. The steel material may be determined based on the product to which it is applied and the required strength and thickness. For example, hot-rolled steel sheets (hot-rolled steel sheets) described in JIS G 3131:2018, JIS G 3113:2018, etc., or cold-rolled steel sheets (cold-rolled steel sheets) described in JIS G 3141:2021, JIS G 3135:2018, etc., can be used. As described above, the steel material can be steel materials other than steel sheets, such as steel pipes, steel wires, and various components made of steel. The following mainly describes the case where the steel material is a steel plate (that is, the case where the surface-treated steel material is a surface-treated steel plate), but the following also applies to the case where the steel material is not in a plate shape (steel plate).

[0012] [Plating layer] The surface-treated steel material according to this embodiment has a plating layer formed on the surface of the steel material. This plating layer is a Zn (zinc) alloy plating layer containing 2.0 mass% or more and less than 20.0 mass% Al and 1.0 mass% or more and less than 6.0 mass% Mg. In this embodiment, the Zn alloy plating layer is a plating layer containing more than 50.0 mass% Zn. That is, the plating layer has a chemical composition containing, in mass %, Al: 2.0% or more and less than 20.0%, Mg: 1.0% or more and less than 6.0%, and Zn: more than 50.0%.

[0013] In the plating layer, the concentrations (contents) of elements other than Zn, Al, and Mg are not limited, but may further contain one or more elements selected from the group consisting of Sn, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb, and B, as necessary. That is, the chemical composition of the plating layer is, in mass %, Al: 2.0% or more and less than 20.0%, Mg: 1.0% or more and less than 6.0%, Sn: 0% or more and less than 20.0%, Bi: 0% or more and less than 5.0%, In: 0% or more and less than 2.0%, Ca: 0% or more and less than 3.0%, Y: 0% or more and less than 0.5%, La: 0% or more and less than 0.5%, Ce: 0% or more and less than 0.5%, Si: 0% or more and less than 2.5%, Cr: 0% or more and less than 0.25%, Ti: 0% or more and less than 0.2 The content may be less than 5%, Ni: 0% or more and less than 0.25%, Co: 0% or more and less than 0.25%, V: 0% or more and less than 0.25%, Nb: 0% or more and less than 0.25%, Cu: 0% or more and less than 0.25%, Mn: 0% or more and less than 0.25%, Fe: 0% or more and less than 5.0%, Sr: 0% or more and less than 0.5%, Sb: 0% or more and less than 0.5%, Pb: 0% or more and less than 0.5%, B: 0% or more and less than 0.5%, with the balance being more than 50.0% Zn and impurities. The reasons for the preferred chemical composition of the plating layer will be explained below. Unless otherwise specified, % for the concentration (content) of each element in the chemical composition of the plating layer is mass %.

[0014] Al: 2.0% or more, less than 20.0% Mg: 1.0% or more, less than 6.0% Al and Mg are elements effective in improving the corrosion resistance of zinc alloy plating layers. When the plating layer (Zn alloy plating layer) contains appropriate amounts of Al and Mg, dense corrosion products are formed when moisture that has permeated the chemical conversion coating reaches the plating layer, improving corrosion resistance in flowing water. In order to obtain the above effects, the Al concentration is set to 2.0% or more and the Mg concentration is set to 1.0% or more. On the other hand, if the Al concentration and Mg concentration of the coating layer are too high, cracks in the coating layer will become large and the corrosion resistance of the processed part will decrease. Therefore, the Al concentration is set to less than 20.0% and the Mg concentration is set to less than 6.0%.

[0015] [Sn: 0% or more, 20.0% or less] [Bi: 0% or more, less than 5.0%] [In: 0% or more, less than 2.0%] These elements contribute to improving corrosion resistance and sacrificial corrosion protection. Therefore, one or more of them may be contained. To obtain the above effects, the concentration of each element is preferably 0.05% or more, and more preferably 0.1% or more. Of these, Sn is preferred because it is a low melting point metal and can be easily incorporated into the plating bath without impairing the properties of the plating bath. On the other hand, if the Sn concentration exceeds 20.0%, the Bi concentration is 5.0% or more, or the In concentration is 2.0% or more, the corrosion resistance decreases. Therefore, it is preferable that the Sn concentration is 20.0% or less, the Bi concentration is less than 5.0%, and the In concentration is less than 2.0%, respectively.

[0016] [Ca: 0% or more, 3.0% or less] Ca is an element that reduces the amount of dross that is easily formed during operation and contributes to improving coating manufacturability. Therefore, Ca may be added. To obtain this effect, it is preferable that the Ca concentration be 0.1% or more. On the other hand, if the Ca concentration is too high, the corrosion resistance of the flat surface of the coating layer itself tends to deteriorate, and the corrosion resistance around the weld may also deteriorate. Therefore, the Ca concentration is preferably 3.0% or less.

[0017] [Y: 0% or more, 0.5% or less] [La: 0% or more, less than 0.5%] [Ce: 0% or more, less than 0.5%] Y, La, and Ce are elements that contribute to improving corrosion resistance. To obtain this effect, it is preferable to contain at least one of these elements in an amount of at least 0.05%, more preferably at least 0.1%. On the other hand, excessive concentrations of these elements increase the viscosity of the coating bath, making it difficult to prepare the coating bath itself, and there is a concern that steel products with good coating properties may not be produced. Therefore, it is preferable that the Y concentration be 0.5% or less, the La concentration be less than 0.5%, and the Ce concentration be less than 0.5%.

[0018] [Si: 0% or more, less than 2.5%] Si is an element that contributes to improving corrosion resistance. Furthermore, when forming a plating layer on a steel sheet, Si prevents the alloy layer formed between the steel sheet surface and the plating layer from becoming excessively thick, thereby enhancing the adhesion between the steel sheet and the plating layer. To achieve these effects, the Si concentration is preferably 0.1% or more. The Si concentration is more preferably 0.2% or more. On the other hand, if the Si concentration is 2.5% or more, excessive Si precipitates in the plating layer, which not only reduces the corrosion resistance but also reduces the workability of the plating layer. Therefore, it is preferable that the Si concentration be less than 2.5%. The Si concentration is more preferably 1.5% or less.

[0019] [Cr: 0% or more, less than 0.25%] [Ti: 0% or more, less than 0.25%] [Ni: 0% or more, less than 0.25%] [Co: 0% or more, less than 0.25%] [V: 0% or more, less than 0.25%] [Nb: 0% or more, less than 0.25%] [Cu: 0% or more, less than 0.25%] [Mn: 0% or more, less than 0.25%] These elements contribute to improving corrosion resistance. To obtain this effect, it is preferable to set the concentration of one or more of these elements to 0.05% or more. On the other hand, if the concentrations of these elements are excessive, the viscosity of the coating bath increases, making it difficult to prepare the coating bath itself, and there is a concern that steel products with good coating properties cannot be produced. Therefore, it is preferable that the concentrations of each element be less than 0.25%.

[0020] [Fe: 0% or more, 5.0% or less] Fe is mixed into the coating layer during production. It may be contained up to about 5.0%, but within this range, the adverse effect on the effects of the coated steel sheet according to this embodiment is small. Therefore, it is preferable to set the Fe concentration to 5.0% or less.

[0021] [Sr: 0% or more, less than 0.5%] [Sb: 0% or more, less than 0.5%] [Pb: 0% or more, less than 0.5%] When Sr, Sb, or Pb is contained in the plating layer, the appearance of the plating layer changes, spangles are formed, and an improvement in metallic luster is confirmed. To obtain this effect, the concentration of one or more of Sr, Sb, and Pb is preferably 0.05% or more, and more preferably 0.1% or more. On the other hand, if the concentrations of these elements are excessive, the viscosity of the coating bath increases, making it difficult to prepare the coating bath itself, and there is a concern that steel products with good coating properties cannot be produced. Therefore, it is preferable that the concentrations of each element be less than 0.5%.

[0022] [B: 0% or more, less than 0.5%] When B is included in a coating layer, it combines with Zn, Al, Mg, etc. to form various intermetallic compounds. These intermetallic compounds have the effect of improving LME. To achieve this effect, the B concentration should preferably be 0.05% or more, and more preferably 0.1% or more. On the other hand, if the B concentration is excessive, the melting point of the coating will rise significantly, which may lead to deterioration in coating operability and failure to obtain a coated steel sheet with good coating properties. Therefore, it is preferable that the B concentration be less than 0.5%.

[0023] [balance: Zn and impurities] The chemical composition of the plating layer may contain only Zn and impurities other than the above-mentioned elements. The Zn concentration in the plating layer is more than 50.0%, preferably 70.0% or more, and more preferably 85.0% or more. Impurities are elements that are mixed in during the manufacturing process. The total concentration of impurities is usually 0.5% or less, but preferably 0.1% or less.

[0024] There is no limit to the amount of plating applied, but to improve corrosion resistance, it is recommended that the coating weight be 10 g / m per side. 2 On the other hand, the coating weight is preferably 500 g / m per side. 2 If the coating weight exceeds 500g / m, the corrosion resistance will be saturated and it will be economically disadvantageous. 2 It is preferable that:

[0025] The chemical composition of the plating layer can be measured by the following method. First, the plating layer is stripped and dissolved using an acid containing an inhibitor that suppresses corrosion of the base steel (steel sheet) (for example, an acid containing 1 mass% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) added to 10 mass% hydrochloric acid) to obtain an acid solution. Next, the obtained acid solution is measured by ICP analysis to determine the chemical composition of the plating layer.

[0026] The coating weight of the plating layer can be measured by the following method. A 30 mm x 30 mm sample is taken from the surface-treated steel sheet, and the plating layer is stripped and dissolved from this sample using an acid containing an inhibitor that suppresses corrosion of the base steel (steel material) (for example, an acid consisting of 10 mass% hydrochloric acid to which 1 mass% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) has been added). The change in weight of the plated steel sheet after stripping and dissolution is measured, and the adhesion weight is calculated from the results.

[0027] [Chemical conversion coating] The surface-treated steel material according to this embodiment has a chemical conversion coating formed on the surface of the plating layer. This chemical conversion coating contains a urethane resin, P (phosphorus), and silicon oxide. The thickness of the chemical conversion coating is not limited, but is, for example, 0.1 to 3.0 μm.

[0028] (storage modulus) The running water corrosion resistance of a chemical conversion coating is affected by the crosslink density of the organic resin that makes up the coating. Specifically, the higher the crosslink density, the fewer locations in the chemical conversion coating where water molecules can penetrate, extending the time it takes for water molecules to reach the plating surface from the coating surface. This also makes the coating harder, reducing damage to the coating caused by dripping water. As a result, running water corrosion resistance improves. For this reason, the crosslink density of the organic resin is increased in the chemical conversion coating according to this embodiment. However, it is difficult to directly measure the crosslink density in a chemical conversion coating. Since crosslink density correlates with the storage modulus and the value of the storage modulus can be used as an index of crosslink density, the value of the storage modulus is used as an index of crosslink density in the chemical conversion coating according to this embodiment. Specifically, the chemical conversion coating according to this embodiment has a storage modulus of 1.5 to 4.0 GPa, measured at a temperature of 25°C. If the storage modulus is less than 1.5 GPa, the crosslink density is low and the effect of improving corrosion resistance in running water is not obtained. On the other hand, if the storage modulus exceeds 4.0 GPa, the crosslink density becomes too high, reducing the elongation of the coating during processing, which may result in the coating being destroyed by processing and reducing the corrosion resistance of the processed area. The measurement temperature is 25° C. because the environment in which the surface treatment material is used is room temperature (25° C.). The measurement frequency is not particularly limited and may be performed under the recommended conditions of the device used.

[0029] (urethane resin) The chemical conversion coating provided on the surface-treated steel material according to this embodiment (sometimes referred to as the chemical conversion coating according to this embodiment) contains a urethane resin as the organic resin that constitutes the matrix of the chemical conversion coating. By using a urethane resin as the organic resin, the effect of improving corrosion resistance (white rust resistance) is obtained. As described below, the presence or absence of the urethane resin in the chemical conversion coating according to this embodiment can be determined by Fourier transform infrared spectroscopy (FT-IR). There are no limitations on the urethane resin as long as it satisfies the above crosslink density. The content of the urethane resin in the chemical conversion coating is preferably 75 to 95% by mass.

[0030] (silicon oxide) By dispersing an appropriate amount of silicon oxide with an appropriate particle size in the chemical conversion coating, the penetration of water molecules is suppressed, improving corrosion resistance in running water. Therefore, in the chemical conversion coating of the surface-treated steel material according to this embodiment, the average particle size of the silicon oxide in the chemical conversion coating is 5 to 20 nm, and the concentration (content) of silicon oxide is 20 to 30 mass %. In the chemical conversion coating according to this embodiment, the silicon oxide in the chemical conversion coating is derived from the colloidal silica in the chemical conversion treatment solution and is mainly in the form of particles. If the average particle size of silicon oxide is less than 5 nm or the concentration of silicon oxide is less than 20 mass %, sufficient effects cannot be obtained. On the other hand, if the average particle size of silicon oxide exceeds 20 nm or the content exceeds 30 mass %, the elongation of the chemical conversion coating during processing decreases, the coating is destroyed by processing, and the corrosion resistance of the processed part decreases.

[0031] (Rin (P)) In the chemical conversion coating, P improves the adhesion between the urethane resin and silicon oxide, thereby inhibiting the penetration of water molecules and improving corrosion resistance in running water. Therefore, in the chemical conversion coating according to this embodiment, the maximum P concentration, P1, is 5.0 to 10.0 mass % within a range of 500 to 600 nm from the interface between the plating layer and the chemical conversion coating toward the chemical conversion coating. If the maximum concentration of P (P1) is less than 5.0 mass%, a sufficient effect cannot be obtained. On the other hand, if P1 exceeds 10.0 mass%, the chemical conversion coating becomes brittle and easily peels off during processing, reducing corrosion resistance during processing. In the chemical conversion coating according to this embodiment, when the concentrations of C, O, Mg, Al, P, and Zn are continuously measured by linear analysis in the thickness direction of the chemical conversion coating from the plating layer toward the surface of the chemical conversion coating, the position where the Zn concentration first becomes 35.0 mass % or less is defined as the interface between the plating layer and the chemical conversion coating. The maximum P concentration is specified in the range of 500 to 600 nm from the interface toward the chemical conversion coating because this position allows the average P concentration in the chemical conversion coating to be obtained. The state of P is not limited, but it is often present as a zinc phosphate compound.

[0032] [Boundary area] In the chemical conversion coating according to this embodiment, when the boundary region is defined as a region extending from the interface between the plating layer and the chemical conversion coating in the thickness direction of the chemical conversion coating, extending 10 nm toward the plating layer and 15 nm toward the chemical conversion coating (i.e., a 25 nm range including the interface), it is preferable that P2 / P1, which is the ratio of P2, the maximum P concentration in the boundary region, to P1, be 1.5 or greater. When P2 / P1 is large, meaning that the P concentration in the boundary region is higher than the P concentration in other locations (in other words, P is concentrated near the interface between the plating layer and the chemical conversion coating), the penetration of water molecules is prevented and zinc phosphate compounds are formed near the interface, improving the adhesion between the plating layer and the chemical conversion coating, resulting in further improved corrosion resistance in flowing water. There is no upper limit to P2 / P1, but in terms of the manufacturing process, it is practically 3.0 or less.

[0033] (Method for measuring storage modulus) The storage modulus of the chemical conversion coating is determined by the following method. A diamond square pyramidal indenter is pressed against the surface of the chemical conversion coating of surface-treated steel at 25°C with a test load of 90 μN, and the load-indentation depth curve is obtained when the device is vibrated at the recommended frequency. The storage modulus is calculated from this load-indentation depth curve. The storage modulus is determined by performing the above measurement at 10 random locations and using the average value of the 10 locations.

[0034] (How to check urethane resin) The surface of the chemical conversion coating of the surface-treated steel is subjected to Fourier transform infrared spectroscopy (FT-IR) to obtain an absorbance spectrum under the following conditions: Measurement method: Microscopic reflection method ·Resolution: 4cm -1 Number of times accumulated: 128 In the obtained absorbance spectrum, the wave number of 1000 cm -1 ~1200cm -1 The first peak is at the position of 1500 cm -1 ~1650cm -1 The absorbances of the first and second peaks are A1 and A2, respectively, and the wavenumber is 2000 cm. -1 ~2500cm -1 When the average absorbance of the above is taken as A0, if A1 / A0 > 1000, A2 / A0 > 100, or A1 / A2 > 1.5, it is determined that urethane resin is present in the chemical conversion coating.

[0035] The concentration of the urethane resin in the chemical conversion coating is measured by the following thermogravimetric method. Peel off 1 mg of the chemical conversion coating from the surface-treated steel material and perform thermogravimetry (TG) under the following conditions to determine the change in weight when heated from 25°C to 800°C. The conditions for this are as follows: Heating atmosphere: He Analysis temperature range: Gas replacement: 30 min → Heat to 40°C, hold for 30 min → Heat to 800°C at an average heating rate of 20°C / min

[0036] (Average particle size and concentration of silicon oxide) The average particle size and concentration of silicon oxide in the chemical conversion coating are determined by the following method. A test piece measuring 10 μm×10 μm and 100 nm thick is cut out from the surface-treated steel material using the cryo-FIB (Focused Ion Beam) method. A bright-field image of the cross section of the chemical conversion coating of the cut-out test piece in the thickness direction was observed using a transmission electron microscope (TEM) at a magnification (100,000 to 1,000,000 times) such that an 800 nm x 300 nm area of ​​the coating was observed within the observation field. TEM-EDS (Energy Dispersive X-ray Spectroscopy) was used to determine that the area in the observation field where Si and O were primarily detected was silicon oxide. Twenty particles were randomly selected from those determined to be silicon oxide, and their minor and major diameters were measured. The average value was calculated to determine the average particle size. The accelerating voltage during observation and EDS analysis is 200 kV. Furthermore, the concentration (content) of silicon oxide in the chemical conversion coating is determined from the area ratio of silicon oxide in the observed field of view.

[0037] (P concentration in the area 500-600 nm from the interface to the chemical conversion coating side and in the boundary region) The P concentration in the region 500 to 600 nm from the interface toward the chemical conversion coating and in the boundary region is measured using TEM-EDS (Energy Dispersive X-ray Spectroscopy). During the measurement, the O, Mg, Al, P, and Zn concentrations are measured continuously in the thickness direction from the plating layer toward the surface of the chemical conversion coating, which is also the surface of the surface-treated steel, at a magnification (100,000 to 1,000,000 times) that allows the entire chemical conversion coating and part of the plating layer to be visible. Based on the measurement results, the maximum P concentration in the range of 500 to 600 nm from the interface toward the chemical conversion coating is designated as P1, and the maximum P concentration in the boundary region is designated as P2.

[0038] [Manufacturing method] The surface-treated steel material according to this embodiment can be produced by a method including the following steps, although the effects can be obtained regardless of the production method as long as it has the above-mentioned characteristics. (I) a plating process for forming a plating layer on the surface of the steel material; (II) A chemical conversion coating forming step of applying a chemical conversion solution to the surface of the steel material after the plating step and drying the solution to form a chemical conversion coating. Each step will be described below.

[0039] (Plating process) In the plating process, a steel material such as a steel sheet is immersed in a plating bath containing Al, Mg, and Zn, or electroplated to form a plating layer on the surface. The conditions for forming the plating layer are not particularly limited. A conventional method may be used to obtain sufficient plating adhesion. The composition of the plating bath may be adjusted depending on the chemical composition of the plating layer to be obtained. After the steel material is removed from the plating bath, the coating weight of the plating layer can be adjusted by wiping, if necessary. The steel material to be subjected to the plating step and its manufacturing method are not limited. In the case of a surface-treated steel sheet, the steel sheet to be immersed in the plating bath may be, for example, a hot-rolled steel sheet (hot-rolled steel sheet) described in JIS G 3131:2018, JIS G 3113:2018, or the like, or a cold-rolled steel sheet (cold-rolled steel sheet) described in JIS G 3141:2021, JIS G 3135:2018, or the like. Furthermore, steel materials other than steel plates, such as steel pipes, steel wires, and various members made of steel, can also be used.

[0040] (Chemical conversion film formation process) In the chemical conversion coating forming step, a chemical conversion solution is applied to the surface of the steel material after the plating step, and then dried to form a chemical conversion coating. The treatment liquid to be applied contains 30 to 50 g / L of urethane resin, 0.5 to 2.5 g / L of crosslinking agent, 50 to 80 g / L of colloidal silica with an average particle size of 5 to 20 μm, and 0.8 to 4.0 g / L of phosphate. If the urethane resin is less than 30 g / L, the ratio of urethane resin in the chemical conversion coating decreases, resulting in reduced corrosion resistance. If it exceeds 50 g / L, the amount of silicon oxide and P contained in the coating decreases, making it impossible to obtain corrosion resistance. If the amount of crosslinking agent is less than 0.5 g / L, the crosslink density of the chemical conversion coating will be insufficient, and if it exceeds 2.5 g / L, the crosslink density will be excessively high. If the amount of colloidal silica is outside the above range, the amount of silicon oxide in the chemical conversion coating will not fall within the specified range. If the amount of phosphate is less than 0.8 g / L or more than 4.0 g / L, the P1 of the chemical conversion coating will not fall within the specified range.

[0041] As the urethane resin, a water-soluble or water-dispersible urethane resin obtained by reacting an organic polyisocyanate compound with a polyol compound can be used, and a self-emulsifying urethane resin is preferred. The crosslinking agent is not particularly limited, but epoxy-based crosslinking agents, oxazoline-based crosslinking agents, carbodiimide-based crosslinking agents, and blocked isocyanate-based crosslinking agents can be used. The type of phosphate is not limited as long as it dissolves in water, and metal phosphates such as orthophosphoric acid, polyphosphoric acid, phosphorous acid, hypophosphorous acid, sodium phosphate, magnesium phosphate, potassium phosphate, manganese phosphate, and zinc phosphate, as well as ammonium phosphate, etc. Among these, it is preferable to use diammonium hydrogen phosphate, which undergoes little change in pH when dissolved in water.

[0042] Next, the chemical conversion treatment solution is dried on the surface-treated steel material to which the chemical conversion treatment solution has been applied, thereby baking the chemical conversion coating. During drying, the heating temperature (maximum temperature reached) PMT is preferably 80 to 200° C., and the time in the oven is preferably 5.0 to 25.0 seconds. The average heating rate up to the PMT during heating is preferably 5 to 20° C. / second. When P is concentrated at the interface between the plating layer and the chemical conversion coating (P2 / P1 is increased), it is preferable that the PMT is 160°C or higher and the average heating rate is 10°C / sec or less. Furthermore, when heating, it is preferable to spray the paint onto the steel plate through a punched metal (a steel plate having a plurality of through holes). There is no limitation on the cooling after reaching the PMT. [Example]

[0043] A hot-rolled steel sheet with a thickness of 0.8 mm that conformed to JIS G 3131:2018 was prepared as the steel sheet. This steel sheet was subjected to hot-dip galvanizing to form a Zn alloy plating layer having the chemical composition shown in Tables 1-1 and 1-2. The impurity content was 0.2% or less in both cases. The plated steel sheet was coated with a chemical conversion treatment solution having the composition shown in Tables 1-1 and 1-2, using a roll coater. Within 5 seconds after applying the chemical conversion treatment solution, hot air was blown onto the steel sheet through a punched metal (a steel sheet with multiple through holes) to heat the steel sheet to the heating temperature (PMT) shown in Table 1-3 for the furnace time shown in Table 1-3. The steel sheet was then cooled to 20°C by air-cooling by blowing air through the punched metal or by water-cooling. The film thickness of the chemical conversion coating was 1.5 μm. In this way, surface-treated steel sheets Nos. 1 to 31 were produced.

[0044] The obtained surface-treated steel sheets were measured in the manner described above for the presence of urethane resin in the chemical conversion coating, the storage modulus of the chemical conversion coating, the average particle size and concentration of silicon oxide in the chemical conversion coating, the maximum P concentration P1 in the range 500 to 600 nm from the interface between the plating layer and the chemical conversion coating toward the chemical conversion coating, and the maximum P concentration P2 in the boundary region. The results are shown in Table 2.

[0045] The surface-treated steel sheets thus obtained were evaluated for white rust resistance (flowing water corrosion resistance) under the water droplet and for white rust resistance in the processed area by the following methods. The results are shown in Table 3.

[0046] <White rust resistance under the water droplets> A 70 mm × 150 mm sample was cut out from the surface-treated steel plate, and with this sample tilted at 15° from the horizontal, a 5 ppm aqueous sodium chloride solution was dripped onto the surface of the surface-treated steel plate at a rate of 4 ml / min (20 drops / min) from a position 100 mm above the surface. The number of test days until white rust with a minor diameter of 10 mm or more appeared below the water droplet was defined as the number of days until white rust appeared. Depending on the number of days for which white rust appeared, the specimens were rated as SS to B according to the following criteria. SS, S, and A were judged to have excellent white rust resistance under the water droplets, while B was judged to have insufficient white rust resistance under the water droplets. (Evaluation criteria) SS: 30 days or more S: 20 days or more, less than 30 days A: 10 days or more, but less than 20 days B: Less than 10 days

[0047] <White rust resistance in processed areas> A sample of 70 mm x 150 mm was taken from the surface-treated steel plate, and a protrusion of 20 mm in diameter and 7 mm in height was formed in the center of this sample. Thereafter, a salt spray test was conducted on the processed area in accordance with JIS Z 2371:2015 (salt spray test method), and the area ratio of white rust on the processed area after 72 hours of testing was observed. According to the white rust occurrence area rate, the sample was evaluated as S to B according to the following criteria. If the sample was rated as S or A, the processed part had excellent white rust resistance, and if the sample was rated as B, the processed part had insufficient white rust resistance. (Evaluation criteria) S: White rust occurrence area rate is 10% or less A: White rust area ratio is over 10% and 20% or less B: White rust occurrence area rate is over 20%

[0048] [Table 1-1]

[0049] [Table 1-2]

[0050] [Table 1-3]

[0051] [Table 2]

[0052] [Table 3]

[0053] As can be seen from Tables 1-1 to 3, in Test Nos. 1 to 19, which are inventive examples, the plating layer and chemical conversion coating were in a favorable state, and were excellent in corrosion resistance in running water and white rust resistance in processed areas. In contrast, in the comparative examples, Test Nos. 20 to 31, the chemical conversion coating did not contain urethane resin, the chemical conversion coating had a low storage modulus, the average particle size or concentration of silicon oxide in the chemical conversion coating was outside the range of the present invention, or P1 was outside the range of the present invention. As a result, in these examples, either or both of the running water corrosion resistance and the white rust resistance in the processed area were insufficient.

Claims

1. Steel and a plating layer formed on the surface of the steel material; a chemical conversion coating formed on the surface of the plating layer; and the plating layer is a Zn alloy plating layer containing 2.0 mass% or more and less than 20.0 mass% Al and 1.0 mass% or more and less than 6.0 mass% Mg, the chemical conversion coating contains a urethane resin, P, and silicon oxide; the storage modulus of the chemical conversion coating film measured at a measurement temperature of 25°C is 1.5 to 4.0 GPa; the silicon oxide in the chemical conversion coating has an average particle size of 5 to 20 nm and a concentration of the silicon oxide of 20 to 30 mass %, When the concentrations of C, O, Mg, Al, P, and Zn are continuously measured by linear analysis in the thickness direction of the chemical conversion coating from the plating layer toward the surface of the chemical conversion coating, the position where the concentration of Zn first becomes 35.0 mass% or less is defined as the interface between the plating layer and the chemical conversion coating, the maximum concentration of P, P1, is 5.0 to 10.0 mass% in a range of 500 to 600 nm from the interface toward the chemical conversion coating; A surface-treated steel material characterized by:

2. a boundary region including the interface, extending from the interface in the thickness direction between a range of 10 nm on the plating layer side and a range of 15 nm on the chemical conversion coating side, the maximum value of the P concentration in the boundary region being P2, and P2 / P1, which is the ratio of P2 to P1, being 1.5 or more; The surface-treated steel material according to claim 1 .

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