Surface-treated steel sheet

Abstract

This surface-treated steel sheet comprises: a base material steel sheet; a plating layer that is formed on the base material steel sheet and that contains 50 mass% or more of Zn and 0.3 mass% or more of Mg; and a chemical conversion coating formed on the plating layer. The chemical conversion coating contains a silicon compound, P and F, and Mg. The average Si concentration in the chemical conversion coating is 10 mass% or more. The chemical conversion coating has, in a region in contact with an interface between the chemical conversion coating and the plating layer, an F-Mg concentrated layer having a Mg concentration of 1.50-40.00 mass% and a F concentration of 0.50-5.00 mass%. The thickness of the F-Mg concentrated layer is 1.0 nm or more. In a region excluding the F-Mg concentrated layer in the chemical conversion coating, the average Mg concentration is less than 0.50 mass% and the average F concentration is less than 0.50 mass%.

Description

Technical Field

[0001] The present invention relates to a surface-treated steel sheet. This application claims priority based on Japanese Patent Application No. 2022-032606 filed in Japan on March 3, 2022, and incorporates its content herein by reference.

Background Art

[0002] Conventionally, plated steel sheets (zinc-based plated steel sheets) with a plating layer mainly composed of zinc formed on the surface of steel sheets have been used in a wide range of applications such as automobiles, building materials, and household electrical appliances. Among them, in particular, Mg-containing zinc-based plated steel sheets containing 0.5% by mass or more of Mg have high corrosion resistance due to the effect of Mg, and thus have been used in applications such as building materials where particularly strict corrosion resistance is required. Also, in such applications, for the purpose of improving white rust resistance, a chromium-free chemical conversion treatment, for example, a chemical conversion treatment mainly composed of an organosilicon compound having a cyclic siloxane bond, has been performed on the surface of the zinc-based plated steel sheet.

[0003] For example, Patent Document 1 discloses that (1) on the surface of a steel material, (2) a silane coupling agent (A) containing one amino group in the molecule and a silane coupling agent (B) containing one glycidyl group in the molecule are blended at a solid content mass ratio [(A) / (B)] of 0.5 to 1.7, and the resulting product has a formula -SiR in the molecule 1 R 2 R 3 (wherein R 1 、R 2 and R 3(wherein each independently represents an alkoxy group or a hydroxyl group, and at least one represents an alkoxy group), two or more functional groups (a), and at least one hydrophilic functional group (b) selected from a hydroxyl group (separate from those that may be included in the functional group (a)) and an amino group, and an organosilicon compound (W) having an average molecular weight of 1000 to 10000, (3) at least one fluorine compound (X) selected from titanium hydrofluoric acid or zirconium hydrofluoric acid, (4) phosphoric acid (Y), and (5) a vanadium compound (Z) are applied and dried to form a composite film containing each component, and in each component of the composite film, (6) the solid content mass ratio [(X) / (W)] of the organosilicon compound (W) and the fluorine compound (X) is 0.02 to 0.07, (7) the solid content mass ratio [(Y) / (W)] of the organosilicon compound (W) and phosphoric acid (Y) is 0.03 to 0.12, (8) the solid content mass ratio [(Z) / (W)] of the organosilicon compound (W) and the vanadium compound (Z) is 0.05 to 0.17, and (9) the solid content mass ratio [(Z) / (X)] of the fluorine compound (X) and the vanadium compound (Z) is 1.3 to 6.0. A surface-treated steel sheet is disclosed. According to Patent Document 1, it is disclosed that this surface-treated steel sheet satisfies all of corrosion resistance, heat resistance, fingerprint resistance, conductivity, paintability, and black residue resistance during processing.

[0004] Also, Patent Document 2 discloses a hot-dip zinc alloy plated steel sheet excellent in corrosion resistance, in which a chemical conversion film mainly composed of one or more of a hydroxide, oxide, oxyacid, oxygen acid salt, and fluoride of valve metal is formed via an interfacial reaction layer containing one or more selected from magnesium fluoride, magnesium phosphate, and a composite compound of magnesium and valve metal oxyacid salt on a Mg-containing zinc alloy plating layer.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Even when a chemical conversion coating described in Patent Document 1 or Patent Document 2 is formed on the surface of a zinc-based plating layer, a certain effect of improving corrosion resistance can be obtained. However, as a result of the studies by the present inventors, it has been found that in such a chemical conversion treatment, for example, in an environment where steel materials come into contact with running water or in an environment where condensation occurs, such as in civil engineering and construction applications, white rust may occur at an early stage.

[0007] That is, an object of the present invention is to provide a surface-treated steel sheet that can suppress the occurrence of white rust in any environment where it comes into contact with running water and in an environment where condensation occurs, on the premise of not deteriorating general properties such as blackening resistance.

Means for Solving the Problems

[0008] The present inventors examined a method for suppressing the occurrence of white rust in an environment where it comes into contact with running water and in an environment where condensation occurs, on the premise of a Mg-containing zinc-based plated steel sheet subjected to a chemical conversion treatment mainly composed of an organosilicon compound. As a result, it was found that by forming a layer in which F and Mg are concentrated in the region of the chemical conversion coating in contact with the interface between the plating layer and the chemical conversion coating, the corrosion resistance to white rust can be improved particularly in an environment in contact with running water (running water environment). Furthermore, as a result of further examination, after forming a layer in which F and Mg are concentrated in the vicinity of the interface, by reducing the concentration of F in the region other than the layer in which F and Mg are concentrated, it was found that the corrosion resistance to white rust is improved even in an environment where condensation occurs (condensation environment).

[0009] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] The surface-treated steel sheet according to one aspect of the present invention includes a base steel sheet, a plating layer formed on the base steel sheet and containing 50% by mass or more of Zn and 0.3% by mass or more of Mg, and a chemical conversion coating formed on the plating layer. The chemical conversion coating contains a silicon compound, P and F, and Mg. The average Si concentration of the chemical conversion coating is 10% by mass or more. In the region of the chemical conversion coating in contact with the interface between the chemical conversion coating and the plating layer, the Mg concentration is 1.50% by mass or more and 40.00% by mass or less, and the F concentration is 0.50% by mass or more and 5.00% by mass or less, and it has an F-Mg enriched layer. The thickness of the F-Mg enriched layer is 1.0 nm or more. In the region of the chemical conversion coating excluding the F-Mg enriched layer, the average Mg concentration is less than 0.50% by mass, and the average F concentration is less than 0.50% by mass. Further, in the formation treatment film, the thickness of the F-Mg enrichment layer is 5.0 nm or more and 100.0 nm or less, and the chemical composition of the plating layer is, in mass%, Al: 4.0% or more and less than 25.0%, Mg: 0.3% or more and less than 12.5%, Sn: 0% or more and 20% or less, Bi: 0% or more and less than 5.0%, In: 0% or more and less than 2.0%, Ca: 0% or more and 3.0% or less, Y: 0% or more and 0.5% or less, La: 0% or more and less than 0.5%, Ce: 0% or more and less than 0.5%, Si: 0% or more and 2.5% or less, Cr: 0% or more and less than 0.25%, Ti: 0% or more and less than 0.25%, 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 5.0% or less, 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%, and the balance: Zn and impurities, and the adhesion amount of the plating layer is 10 to 200 g / m 2 and the average P concentration of the formation treatment film is 0.01 mass% or more and 10.00 mass% or less, the average F concentration is 0.01 mass% or more and 1.10 mass% or less, the average Mg concentration is 0.01 mass% or more and 1.00 mass% or less, the average Zr concentration is 0 mass% or more and 3.00 mass% or less, the average V concentration is 0 mass% or more and 3.00 mass% or less, and the thickness of the formation treatment film is 0.02 to 2.0 μm.

Advantages of the Invention

[0010] According to the above aspect of the present invention, it is possible to provide a surface-treated steel sheet capable of suppressing the occurrence of white rust in any environment where it comes into contact with running water and any environment where dew condensation occurs.

Brief Description of the Drawings

[0011]

Figure 1

Modes for Carrying Out the Invention

[0012] Hereinafter, a surface-treated steel sheet according to an embodiment of the present invention (the surface-treated steel sheet according to this embodiment) will be described. As shown in FIG. 1, the surface-treated steel sheet 1 according to this embodiment has a base steel sheet 11, a plating layer 12 formed on the base steel sheet 11, and a chemical conversion coating 13 formed on the plating layer 12. Further, the chemical conversion coating 13 has an F-Mg enriched layer 14 in the region in contact with the interface between the chemical conversion coating 13 and the plating layer 12. In FIG. 1, the plating layer 12 and the chemical conversion coating (sometimes simply referred to as the coating) 13 are formed only on one side of the base steel plate 11, but they may also be formed on both sides.

[0013] Hereinafter, the base steel plate 11, the plating layer 12, and the chemical conversion coating 13 will be described respectively.

[0014] <Base steel plate> The surface-treated steel plate 1 according to the present embodiment can obtain excellent corrosion resistance by the plating layer 12 and the chemical conversion coating 13. The base steel plate 11 is a steel material without the plating layer 12 or the chemical conversion coating 13 on its surface, and its material (such as strength) and plate thickness are not particularly limited. The base steel plate 11 may be determined according to the applied product, required strength, plate thickness, etc. For example, hot-rolled mild steel plates or hot-rolled steel plates described in JIS G3131:2018 or JIS G3113:2018, or cold-rolled steel plates described in JIS G3141:2017 can be used.

[0015] <Plating layer> The plating layer 12 provided in the surface-treated steel plate 1 according to the present embodiment is a plating layer (zinc-based plating layer) formed on the surface of the base steel plate 11, with Zn (zinc) as the main component and containing 0.3 mass% or more of Mg. Here, taking Zn as the main component means that the Zn concentration (content) is 50 mass% or more. The Zn concentration (content) may be 55 mass% or more, 60 mass% or more, 65 mass% or more, 70 mass% or more, 75 mass% or more, or 80 mass% or more. The Zn concentration (content) is 99.7 mass% or less, but may also be 95.7 mass% or less, 95 mass% or less, 92 mass% or less, 90 mass% or less, or 86 mass% or less. Mg is an element necessary to form an F-Mg enrichment layer in the chemical conversion coating after chemical conversion treatment. When the Mg concentration (content) is less than 0.3 mass%, the F-Mg enrichment layer is not formed. Therefore, the Mg concentration is set to 0.3 mass% or more.

[0016] In the plating layer 12, the concentrations (contents) of elements other than those described above are not limited. However, the chemical composition of the plating layer is, in mass%, Al: 4.0% or more and less than 25.0%, Mg: 0.3% or more and less than 12.5%, Sn: 0% or more and 20% or less, Bi: 0% or more and less than 5.0%, In: 0% or more and less than 2.0%, Ca: 0% or more and 3.0% or less, Y: 0% or more and 0.5% or less, La: 0% or more and less than 0.5%, Ce: 0% or more and less than 0.5%, Si: 0% or more and 2.5% or less, Cr: 0% or more and less than 0.25%, Ti: 0% or more and less than 0.25%, 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 5.0% or less, 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%, and the balance: Zn and impurities. By this, excellent corrosion resistance can be obtained as a surface-treated steel sheet, which is preferable.

[0017] The reason for the preferable chemical composition of the plating layer 12 will be described. Unless otherwise specified, the % regarding the concentration (content) of each element in the chemical composition of the plating layer is mass%.

[0018] [Al: 4.0% or more and less than 25.0%] Al is an element effective for improving corrosion resistance in the zinc-based plating layer. When sufficiently obtaining the above effects, it is preferable that the Al concentration is 4.0% or more. The Al concentration may be 6.0% or more, 8.0% or more, 10.0% or more, or 13.0% or more. On the other hand, when the Al concentration is 25.0% or more, the corrosion resistance of the cut end face of the plating layer decreases. Therefore, it is preferable that the Al concentration is less than 25.0%. The Al concentration may be 23.0% or less, 20.0% or less, 18.0% or less, or 15.0% or less.

[0019] [Mg: 0.3% or more and less than 12.5%] As described above, for the formation of the F-Mg enrichment layer, the Mg concentration is 0.3% or more. Mg is also an element that has the effect of enhancing the corrosion resistance of the plating layer. When obtaining the effect of improving corrosion resistance, it is preferable to set the Mg concentration to 0.5% or more. The Mg concentration is more preferably 1.0% or more, and even more preferably 2.0% or more or 3.0% or more. The Mg concentration may be 4.0% or more, 5.0% or more, 6.0% or more, or 8.0% or more. On the other hand, when the Mg concentration is 12.5% or more, the effect of improving corrosion resistance saturates, and the workability of the plating layer may decrease. In addition, manufacturing problems such as an increase in the amount of dross generated in the plating bath occur. Therefore, it is preferable to set the Mg concentration to less than 12.5%. The Al concentration may be 12.0% or less, 11.0% or less, 10.0% or less, or 9.0% or less.

[0020] The plating layer 12 may further contain the following elements as its chemical composition. The inclusion of the following elements is not essential, and the lower limit of these elements is 0%.

[0021] [Sn: 0% or more and 20% or less] [Bi: 0% or more and less than 5.0%] [In: 0% or more and less than 2.0%] These elements are elements that contribute to the improvement of corrosion resistance and sacrificial anticorrosion properties. Therefore, any one or more of them may be contained. When obtaining the above effects, it is preferable to set the concentration to 0.05% or more respectively. Among these, Sn is preferable because it is a low melting point metal and can be easily contained without impairing the properties of the plating bath. On the other hand, when the Sn concentration exceeds 20%, 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 to set the Sn concentration to 20% or less, the Bi concentration to less than 5.0%, and the In concentration to less than 2.0% respectively. The Sn concentration may be 15.0% or less, 10.0% or less, 5.0% or less, or 3.0% or less. The Bi concentration may be 4.0% or less, 3.0% or less, 2.0% or less, or 1.0% or less. The In concentration may be 1.5% or less, 1.0% or less, or 0.5% or less.

[0022] [Ca: 0% or more and 3.0% or less] Ca is an element that reduces the amount of dross that is likely to be formed during operation and contributes to the improvement of plating productivity. Therefore, Ca may be contained. When obtaining this effect, it is preferable that the Ca concentration be 0.1% or more. On the other hand, when the Ca concentration is high, the corrosion resistance of the flat part of the plating layer itself tends to deteriorate, and the corrosion resistance around the welded part may also deteriorate. Therefore, it is preferable that the Ca concentration be 3.0% or less. The Bi concentration may be 2.0% or less, 1.0% or less, or 0.5% or less.

[0023] [Y: 0% or more and 0.5% or less] [La: 0% or more and less than 0.5%] [Ce: 0% or more and less than 0.5%] Y, La, and Ce are elements that contribute to the improvement of corrosion resistance. When obtaining this effect, it is preferable that one or more of these be contained at 0.05% or more, respectively. On the other hand, when the concentrations of these elements become excessive, the viscosity of the plating bath increases, and it often becomes difficult to prepare the plating bath itself, and there is a concern that steel materials with good plating properties cannot be manufactured. 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%. The concentrations of these elements may be 0.3% or less, 0.2% or less, or 0.1% or less.

[0024] [Si: 0% or more and less than 2.5%] Si is an element that contributes to the improvement of corrosion resistance. Also, when forming a plating layer on a steel sheet, Si is an element that has the effect of suppressing the formation of an excessively thick alloy layer formed between the steel sheet surface and the plating layer, and enhancing the adhesion between the steel sheet and the plating layer. When obtaining these effects, it is preferable that the Si concentration be 0.1% or more. The Si concentration is more preferably 0.2% or more. On the other hand, when the Si concentration becomes 2.5% or more, excessive Si precipitates in the plating layer, not only reducing the corrosion resistance but also deteriorating the workability of the plating layer. Therefore, it is preferable that the Si concentration is less than 2.5%. More preferably, the Si concentration is 1.5% or less. The Si concentration may be 1.2% or less, 1.0% or less, 0.6% or less, or 0.3% or less.

[0025] [Cr: 0% or more and less than 0.25%] [Ti: 0% or more and less than 0.25%] [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%] These elements are elements that contribute to the improvement of corrosion resistance. When obtaining this effect, it is preferable that the concentration of one or more of these elements is 0.05% or more. On the other hand, when the concentration of these elements becomes excessive, the viscosity of the plating bath increases, and it often becomes difficult to prepare the plating bath itself, and there is a concern that steel materials with good plating properties cannot be manufactured. Therefore, it is preferable that the concentration of each element is less than 0.25%. The concentration of these elements may be 0.20% or less, 0.10% or less, or 0.05% or less.

[0026] [Fe: 0% or more and 5.0% or less] Fe is mixed into the plating layer as an impurity when manufacturing the plating layer. It may be contained up to about 5.0%, but within this range, the adverse effect on the effect of the surface-treated steel sheet according to this embodiment is small. Therefore, it is preferable that the Fe concentration is 5.0% or less. The Fe concentration may be 3.0% or less, 2.0% or less, 1.0% or less, or 0.5% or less.

[0027] [Sr: 0% or more and less than 0.5%] [Sb: 0% or more and less than 0.5%] [Pb: 0% or more, less than 0.5%] When Sr, Sb, and Pb are contained in the plating layer, the appearance of the plating layer changes, spangles are formed, and an improvement in metallic luster is confirmed. When obtaining this effect, it is preferable that the concentration of one or more of Sr, Sb, and Pb is 0.05% or more. On the other hand, when the concentration of these elements becomes excessive, the viscosity of the plating bath increases, and it often becomes difficult to prepare the plating bath itself, and there is a concern that a steel material with good plating properties cannot be produced. Therefore, it is preferable that the concentration of each element is less than 0.5%. The concentration of these elements may be 0.4% or less, 0.2% or less, or 0.1% or less.

[0028] [B: 0% or more, less than 0.5%] B is an element that combines with Zn, Al, Mg, etc. when contained in the plating layer to form various intermetallic compounds. This intermetallic compound has the effect of improving the resistance to LME cracking. When obtaining this effect, it is preferable that the B concentration is 0.05% or more. On the other hand, when the B concentration becomes excessive, the melting point of the plating significantly increases, and there is a concern that the plating operability deteriorates and a surface-treated steel sheet with good plating properties cannot be obtained. Therefore, it is preferable that the B concentration is less than 0.5%. The B concentration may be 0.4% or less, 0.2% or less, or 0.1% or less.

[0029] The deposition amount of the plating layer 12 is not limited, but it is preferably 10 g / m or more per side for improving corrosion resistance. The deposition amount may be 20 g / m or more, 35 g / m or more, 50 g / m or more, or 70 g / m or more per side. On the other hand, when the deposition amount exceeds 200 g / m per side, the corrosion resistance saturates and it becomes economically disadvantageous. Therefore, it is preferable that the deposition amount per side is 200 g / m or less. The deposition amount may be 175 g / m or less, 150 g / m or less, 125 g / m or less per side. 2 or more per side. The deposition amount may be 20 g / m 2 or more, 35 g / m 2 or more, 50 g / m 2 or more, or 70 g / m 2 or more per side. On the other hand, when the deposition amount exceeds 200 g / m 2 per side, the corrosion resistance saturates and it becomes economically disadvantageous. Therefore, it is preferable that the deposition amount per side is 200 g / m 2 or less. The deposition amount may be 175 g / m 2 or less, 150 g / m 2 or less, 125 g / m 2The following or 110 g / m 2 may also be the following.

[0030] <Conversion coating film> [Containing a silicon compound, P and F, and Mg, and having an average Si concentration in the conversion coating film of 10% by mass or more] The conversion coating film 13 included in the surface-treated steel sheet 1 according to the present embodiment is obtained by applying a treatment liquid containing a silane coupling agent, a fluoride, and a P compound such as a phosphate on a plating layer containing zinc under predetermined conditions and drying it. Therefore, the conversion coating film 13 included in the surface-treated steel sheet 1 according to the present embodiment contains a silicon compound containing Si, C, and O derived from the silane coupling agent as a film-forming component, and contains P derived from the P compound and F derived from the fluoride as inhibitor components. Further, the conversion coating film 13 contains Mg derived from an Mg compound or the like. When the silicon compound is a film-forming component, the average Si concentration of the conversion coating film is 10% by mass or more. The average Si concentration may be 11% by mass or more, 12% by mass or more, 14% by mass or more, or 16% by mass or more. The upper limit of the average Si concentration is not limited, but the average Si concentration may be 35% by mass or less. The average Si concentration may be 30% by mass or less, 27% by mass or less, 24% by mass or less, 22% by mass or less, or 20% by mass or less. The maximum value of the P concentration by the measurement method described later is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, 0.05% by mass or more, or 0.10% by mass or more. Although it is not particularly necessary to define the average P concentration, the average P concentration may be 0.01% or more, 0.05% by mass or more, 0.10% by mass or more, 0.20% by mass or more, 0.50% by mass or more, 0.80% by mass or more, or 1.20% by mass or more. The average P concentration may be 10.00% by mass or less, 7.00% by mass or less, 5.00% by mass or less, or 3.00% by mass or less. The maximum value of the F concentration by the measurement method described below is preferably 0.01% by mass or more, 0.05% by mass or more, more preferably 0.10% by mass or more. Although it is not necessary to particularly define the average F concentration, the average F concentration may be 0.01% by mass or more, 0.05% by mass or more, 0.10% by mass or more, 0.15% by mass or more, or 0.20% by mass or more. The average F concentration may be 1.10% by mass or less, 1.00% by mass or less, 0.70% by mass or less, 0.50% by mass or less, 0.40% by mass or less, or 0.35% by mass or less. The maximum value of the Mg concentration by the measurement method described below is preferably 0.05% by mass or more, more preferably 0.10% by mass or more. Although it is not necessary to particularly define the average Mg concentration, the average Mg concentration may be 0.01% by mass or more, 0.05% by mass or more, 0.10% by mass or more, 0.15% by mass or more, or 0.20% by mass or more. The average Mg concentration may be 1.00% by mass or less, 0.70% by mass or less, 0.50% by mass or less, 0.40% by mass or less, or 0.35% by mass or less. Further, if necessary, the chemical conversion coating 13 may contain Zr or V derived from a Zr compound or a V compound. The inclusion of Zr or V derived from a Zr compound or a V compound is optional, and the lower limit of the average Zr concentration and the average V concentration is 0%. The average Zr concentration and the average V concentration may each be 3.00% by mass or less, 2.00% by mass or less, 1.00% by mass or less, 0.70% by mass or less, or 0.50% by mass or less.

[0031] Whether the chemical conversion coating contains P, F, Mg, Zr, and V or not, the average Si concentration in the chemical conversion coating is determined by the following method. A sample of a size that can be inserted into a cryo-FIB processing apparatus is cut out from the surface-treated steel material with a formation treatment film, and a test piece with a thickness of 80 to 200 nm is cut out from the sample by the cryo-FIB (Focused Ion Beam) method. The cross-sectional structure of the cut-out test piece is observed with a transmission electron microscope (TEM: Transmission Electron Microscope) at a magnification at which the entire formation treatment film enters the observation field. To identify the constituent elements of each layer, TEM-EDS (Energy Dispersive X-ray Spectroscopy) is used to perform quantitative analysis of Si, P, F, Mg, Zr, and V at five or more points in the film. The average value of each point of the Si concentration is adopted as the average Si concentration of the formation treatment film. On the other hand, for P, F, Mg, Zr, and V, if even one point is detected among the points (when a value exceeding the detection limit (for example, 0.001 mass% or more or 0.005 mass% or more as the concentration) is obtained), it is determined that they are contained in the film. However, it is assumed that a device with a detection limit value of at least P, F, Mg, Zr, and V of 0.01 mass% or less is used. That is, if there is even one measurement point where the content is 0.01 mass% or more, it is always determined that the element is contained.

[0032] Whether the formation treatment film contains a silicon compound (whether Si exists as a silicon compound) can be confirmed using FT-IR. Specifically, using a general FT-IR device, when a peak in the absorbance at 1030 to 1200 cm indicating a siloxane bond is observed, it is determined that a silicon compound is contained. As the FT-IR device, for example, the model number: Frontier IR manufactured by PERKIN ELMER can be used. -1 In FT-IR, the measurement conditions are as follows, for example. Measurement method: Diffuse reflection method Resolution: 4 cm Number of integrations: 128 times -1 Measurement atmosphere: Air

[0033] ​[In a region in contact with the interface between the coating film and the plating layer, there is an F-Mg enriched layer in which the Mg concentration is 1.50 mass% or more and 40.00 mass% or less, and the F concentration is 0.50 mass% or more and 5.00 mass% or less.] On the premise of a Mg-containing zinc-based plated steel sheet subjected to a chemical conversion treatment mainly composed of an organosilicon compound, the present inventors examined a method for suppressing the generation of white rust in an environment where it comes into contact with running water and an environment where dew condensation occurs. As a result, in a region of the chemical conversion coating film in contact with the interface between the plating layer and the chemical conversion coating film, a layer (F-Mg enriched layer) in which the Mg concentration is 1.50 mass% or more and 40.00 mass% or less, and the F concentration is 0.50 mass% or more and 5.00 mass% or less is formed, and it was found that the white rust resistance in an environment in contact with running water (running water environment) can be improved. Regarding the mechanism of improving the white rust resistance by the F-Mg enriched layer, it is not clear, but the F-Mg enriched layer in which F and Mg are enriched is considered to be an amorphous layer containing an Mg-F composite salt, and it is considered that the white rust resistance is improved because this amorphous layer has a high barrier property. Conventionally, it has been shown that Zn-F composite salts and Al-F composite salts are formed near the interface. However, as a result of the examination by the present inventors, when a test piece after a corrosion test in a running water environment was observed with a transmission electron microscope (TEM), the disappearance of the Zn-F composite salt and the Al-F composite salt was confirmed. On the other hand, the Mg-F composite salt was confirmed to remain even after the corrosion test in the running water environment. That is, the Mg-F composite salt maintains its layer for a longer period than the Zn-F composite salt and the Al-F composite salt even in a running water environment, that is, the barrier effect is maintained. Therefore, when the F-Mg enriched layer is not formed, it is considered that the improvement of the white rust resistance in a running water environment is not sufficient. In a layer where the Mg concentration is less than 1.50 mass% or the Mg concentration is less than 0.50 mass%, the above effects cannot be obtained. Also, even if F and Mg are enriched, in a layer where the Mg concentration exceeds 40.0 mass% or the F concentration exceeds 5.00 mass%, the blackening resistance decreases. Therefore, in the present embodiment, a layer with an Mg concentration of 1.50 mass% or more and 40.00 mass% or less, and an F concentration of 0.50 mass% or more and 5.00 mass% or less is defined as the F-Mg enriched layer. In the present embodiment, having the F-Mg enriched layer means that when measuring the thicknesses of 10 F-Mg enriched layers by the measurement method described later, the average thickness is 1.0 nm or more.

[0034] The thickness of the F-Mg enriched layer (the thickness from the interface between the plating layer and the chemical conversion coating) is preferably 5.0 nm or more and 100.0 nm or less on average. When the thickness of the F-Mg enriched layer is 5.0 nm or more, the improvement in white rust resistance becomes remarkable. Therefore, the thickness of the F-Mg enriched layer is preferably 1.5 nm or more, 2.0 nm or more, 3.0 nm or more, or 5.0 nm or more, and more preferably 10.0 nm or more, 20.0 nm or more, 40.0 nm or more, or 60.0 nm or more. On the other hand, the F-Mg enriched layer is hard. When the thickness of the F-Mg enriched layer is large, when processing the surface-treated steel sheet, the amorphous layer may serve as a starting point and the chemical conversion coating may peel off. In this case, the corrosion resistance of the processed part may decrease. Therefore, from the viewpoint of suppressing the peeling of the coating in the processed part, it is preferable that the thickness of the F-Mg enriched layer is 200.0 nm or less, 150.0 nm or less, or 120.0 nm or less. When obtaining more excellent corrosion resistance of the processed part, it is preferable that the thickness of the F-Mg enriched layer is 100.0 nm or less.

[0035] [In the region excluding the F-Mg enriched layer, the average Mg concentration is less than 0.50 mass%, and the average F concentration is less than 0.50 mass%] In the surface-treated steel sheet 1 according to the present embodiment, when the average Mg concentration in the region excluding the F-Mg enriched layer is 0.50 mass% or more, the blackening resistance decreases. Therefore, in order to ensure sufficient (equal to or better than the conventional) blackening resistance, the Mg concentration in the region excluding the F-Mg enriched layer is set to less than 0.50 mass%. If necessary, the Mg concentration in the region excluding the F-Mg enriched layer may be 0.45 mass% or less, 0.40 mass% or less, or 0.35 mass% or less. Further, as a result of investigations by the present inventors, it has been found that in the surface-treated steel sheet 1 according to the present embodiment, when the average F concentration is 0.50% by mass or more in the region excluding the F-Mg enriched layer, the white rust resistance in an environment where dew condensation occurs deteriorates. Therefore, in the surface-treated steel sheet 1 according to the present embodiment, the average F concentration is set to less than 0.50% by mass in the region excluding the F-Mg enriched layer. If necessary, the F concentration in the region excluding the F-Mg enriched layer may be 0.45% by mass or less, 0.40% by mass or less, or 0.35% by mass or less.

[0036] The thickness of the F-Mg enriched layer (the thickness from the interface between the plating layer and the chemical conversion coating) is determined by the following method. A sample of a size that can be inserted into a cryo-FIB processing apparatus is cut out from the surface-treated steel material on which the chemical conversion coating is formed, and a test piece having a thickness of 80 to 200 nm is cut out from the sample by the cryo-FIB (Focused Ion Beam) method. The cross-sectional structure of the cut-out test piece is observed with a transmission electron microscope (TEM: Transmission Electron Microscope) at a magnification at which the entire chemical conversion coating enters the observation field of view. Based on the observation image, the interface between the plating layer and the chemical conversion coating (chemical conversion layer) is visually determined, and line analysis is performed parallel to the thickness direction of the plating layer to measure the concentrations of F and Mg. At this time, the starting point of the analysis is set at a position 100 nm from the interface between the plating layer and the chemical conversion coating toward the steel sheet side, and the end point is set at the surface of the chemical conversion coating. Also, the measurement pitch of the line analysis is set to 1.0 nm. As a result of the measurement, a range in which the Mg concentration is 1.50% by mass or more and 40.00% by mass or less, and the F concentration is 0.50% by mass or more and 5.00% by mass or less is determined as the F-Mg enriched layer, and this thickness is defined as the thickness of the F-Mg enriched layer. However, the measurement is performed at 10 locations at 100 nm intervals in the direction orthogonal to the thickness direction from an arbitrary point, and the average is defined as the F-Mg enriched layer thickness.

[0037] The average Mg concentration and the average F concentration in the region excluding the F-Mg enriched layer are determined by the following method. When measuring the thickness of the above-mentioned F-Mg enriched layer, starting from the point in the F-Mg enriched layer that is farthest from the interface between the plating layer and the chemical conversion coating (the F-Mg enriched layer is a part of the chemical conversion coating and is formed in the part of the chemical conversion coating adjacent to the plating layer. Therefore, the point in the F-Mg enriched layer that is farthest from the interface between the plating layer and the chemical conversion coating is the point closest to the surface of the chemical conversion coating in the F-Mg enriched layer), and performing line analysis at a pitch of 1.0 nm up to the surface of the chemical conversion coating to measure the Mg concentration and the F concentration, and taking the average value as the average Mg concentration and the average F concentration, respectively.

[0038] The thickness of the chemical conversion coating 13 including the F-Mg enriched layer is preferably 0.02 to 2.0 μm, and more preferably 0.2 to 2.0 μm. During the above-mentioned TEM observation, the thickness of the chemical conversion coating can be easily identified from the difference in contrast between the plating layer and the chemical conversion coating at the boundary. Therefore, the distance from the boundary to the surface of the chemical conversion coating is measured and taken as the thickness. When measuring, it is performed at 10 locations at intervals of 100 nm in a direction orthogonal to the thickness direction from an arbitrary point, and the average of each measurement result is taken as the thickness of the chemical conversion coating.

[0039] <Manufacturing method> Next, a preferred manufacturing method of the surface-treated steel sheet according to the present embodiment will be described. The surface-treated steel sheet according to the present embodiment can obtain the above effects as long as it has the above characteristics regardless of the manufacturing method. However, the following manufacturing method is preferred because it can be stably manufactured.

[0040] That is, the surface-treated steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps. (I) A plating step of immersing a steel sheet in a plating bath containing Zn and Mg, pulling it up, and water-cooling it to form a plating layer on the surface; (II) A coating step of applying a chemical conversion treatment liquid containing a silane coupling agent, a fluoride, acetylacetone (acetylacetonate), a P compound, and Mg to the steel sheet having the plating layer; (III) A heating step of heating the steel sheet coated with the chemical conversion treatment liquid to form a coating film (chemical conversion treatment coating film) containing a silicon compound, P, F, and Mg. Hereinafter, the preferable conditions of each step will be described.

[0041] [Plating step] In the plating step, the steel sheet is immersed in a plating bath containing Zn and Mg, pulled out, and water-cooled to form a plating layer on the surface. Conventionally, as the Mg-containing zinc-based plating layer, those with an Mg concentration on the plating surface of less than 10% by mass have been used. In contrast, in this embodiment, the Mg concentration on the plating surface at the stage of being subjected to the chemical conversion treatment is set to 20% by mass or more. By setting the Mg concentration on the plating surface to 20% by mass or more, the supply of Mg to the interface is promoted. In this case, as will be described later, by applying and heating a predetermined chemical conversion treatment liquid, an F-Mg concentration-enriched layer can be formed in the chemical conversion treatment coating film. On the other hand, when the Mg concentration on the plating surface exceeds 60% by mass, the Mg concentration of the layer formed at the interface becomes excessive. Therefore, the Mg concentration on the plating surface is set to 60% by mass or less.

[0042] The Mg concentration on the plating surface after the plating step (before the chemical conversion treatment) can be controlled by the water-cooling conditions after pulling the steel sheet out of the plating bath. Specifically, during water-cooling, by adjusting the pH of the cooling water to 9.5 or more and controlling the temperature of the steel sheet immediately before contacting the cooling water to 170°C or less, the Mg concentration on the plating surface can be set to 20% by mass or more and 60% by mass or less. The reason why the Mg concentration on the plating surface can be adjusted by controlling the water-cooling conditions will be explained. In the Mg-containing zinc-based plated steel sheet, immediately after the solidification of the plating layer, Mg, which has a high affinity for oxygen, is concentrated in the surface layer of the plating layer with a thickness of about several nm. However, this Mg is extremely unstable and easily dissolves in water during the water-cooling after plating, and the Mg concentration on the surface becomes equal to the Mg concentration in the plating layer. On the other hand, by controlling the water-cooling within the above range, the elution of Mg is suppressed, and the Mg concentration on the surface of the plating layer can be set to 20 - 60% by mass. Although the mechanism by which the elution of Mg is suppressed is not clear, it is considered that by adjusting the pH to 9.5 or higher, Mg approaches the passive region, and the reaction between Mg and water is suppressed due to the low temperature of the steel sheet. When the pH is less than 9.5, the Mg concentration on the plating surface is less than 20% by mass. Also, when the temperature of the steel sheet immediately before contacting the cooling water exceeds 170°C, the Mg concentration on the plating surface is less than 20% by mass. On the other hand, when the pH exceeds 11.0, the appearance of the plating layer deteriorates. In this case, since the appearance after the formation of the chemical conversion coating also deteriorates, the pH is preferably 11.0 or less. After the plating process and before the chemical conversion treatment, the thickness of the Mg-enriched layer with an Mg concentration of 20% by mass or more and 60% by mass or less is preferably 3.0 to 100 nm. By setting the thickness of the Mg-enriched layer to 3.0 to 100 nm, it is advantageous to set the thickness of the F-Mg-enriched layer after the chemical conversion treatment to 5.0 to 100.0 nm. When the thickness of the Mg-enriched layer is 3.0 to 100 nm, the temperature of the steel sheet immediately before contacting the cooling water is preferably 120°C or higher and 150°C or lower.

[0043] The thickness of the Mg-enriched layer with an Mg concentration of 20% by mass or more and 60% by mass or less can be determined by the following method. Cut out a sample of a size that can be inserted into a cryo-FIB processing apparatus from the plated steel sheet before the chemical conversion treatment, and cut out a test piece with a thickness of 80 to 200 nm from the sample by the cryo-FIB (Focused Ion Beam) method. Observe the cross-sectional structure of the cut-out test piece with a transmission electron microscope (TEM: Transmission Electron Microscope) at a magnification at which the entire thickness direction of the plating layer is included in the observation field of view. Based on the observation image, determine the interface between the plating layer and the base metal steel sheet, and perform line analysis in parallel with the thickness direction of the plating layer to measure the Mg concentration. At that time, the starting point of the analysis is set at a position 100 nm from the interface between the plating layer and the steel sheet toward the steel sheet side, and the end point is the surface of the plating layer. Also, the measurement pitch of the line analysis is set to 1 nm. As a result of the measurement, a range where the Mg concentration is 20 mass% or more and 60 mass% or less is determined as the Mg-enriched layer, and this thickness is defined as the thickness of the Mg-enriched layer. However, the measurement is carried out at 10 locations at 100 nm intervals in the thickness direction and the direction orthogonal to the thickness direction from an arbitrary point, and the average is taken as the thickness of the Mg-enriched layer. When measuring, when the thickness of the enriched layer identified by TEM is 5 nm or less, it is preferable to use a TEM having a spherical aberration correction function from the viewpoint of spatial resolution.

[0044] The steel sheet to be subjected to the plating process and its manufacturing method are not limited. As the steel sheet to be immersed in the plating bath, for example, hot-rolled mild steel sheets or hot-rolled steel sheets described in JIS G3131:2018 or JIS G3113:2018, or cold-rolled steel sheets described in JIS G3141:2017 can be used. The composition of the plating bath may be adjusted according to the chemical composition of the plating layer to be obtained. After pulling the steel sheet out of the plating bath, the adhesion amount of the plating layer can be adjusted by wiping. For pH adjustment of the cooling water, various known pH adjusters may be used.

[0045] [Coating process] In the coating process, a chemical conversion treatment liquid is applied to the steel sheet (plated steel sheet) on which the plating layer is formed. As the chemical conversion treatment liquid, a treatment liquid containing a silane coupling agent, a fluoride, acetylacetone (acetylacetonate), a P compound, and an Mg compound may be used. The chemical conversion treatment liquid may contain a Zr compound and a V compound. In the coating process, the coating method of the surface treatment metal agent is not limited. For example, it can be applied using a roll coater, a bar coater, a spray, etc.

[0046] The silane coupling agent is included as a film-forming component. As the silane coupling agent, for example, a Si compound obtained by blending a silane coupling agent (A) containing one amino group in the molecule and a silane coupling agent (B) containing one glycidyl group in the molecule at a solid content concentration ratio (A) / (B) of 0.5 to 1.7 may be used.

[0047] The P (phosphorus) compound contained in the chemical conversion treatment liquid remains as P as an inhibitor component in the chemical conversion treatment film. The corrosion resistance of the chemical conversion treatment film is improved by this P as the inhibitor component. Regarding the blending amount of the P compound (T), it is preferable that the solid content mass ratio [(Ts) / (Ss)] of Si derived from the organosilicon compound (S) and P derived from the phosphorus compound (T) is 0.15 to 0.31. When the solid content mass ratio [(Ts) / (Ss)] of Si derived from the organosilicon compound (S) and P derived from the P compound (T) is less than 0.15, the effect of the P compound (T) as an elution inhibitor cannot be obtained, which is not preferable. On the other hand, when [(Ts) / (Ss)] exceeds 0.31, the film becomes significantly water-soluble, which is not preferable. In the present embodiment, the P compound contained in the chemical conversion treatment liquid is not particularly limited, and examples thereof include phosphoric acid, ammonium phosphate salt, potassium phosphate salt, sodium phosphate salt, etc. Among these, phosphoric acid is more preferable. When phosphoric acid is used, more excellent corrosion resistance can be obtained.

[0048] The fluoride in the chemical conversion treatment liquid reacts with Mg in the plating layer to form an F-Mg enriched layer. Therefore, when obtaining the surface-treated steel sheet according to the present embodiment, the chemical conversion treatment liquid contains fluoride (fluorine compound). Regarding the blending amount of the fluoride (U), it is preferable that the blending amount of the fluoride contained in the chemical conversion treatment liquid is such that the mass ratio [(Us) / (Xs)] of the solid content (X) contained in the chemical conversion treatment liquid and F derived from the fluoride is 0.02 to 0.70. When [(Us) / (Xs)] is less than 0.02, there is a concern that the F concentration in the vicinity of the interface will be less than 0.5% by mass and a predetermined F-Mg layer will not be formed. On the other hand, when [(Us) / (Xs)] exceeds 0.70, there is a concern that the F concentration will exceed 0.50% by mass in parts other than the F-Mg enriched layer. Examples of fluorides contained in the chemical conversion treatment liquid include compounds such as hydrofluoric acid HF, tetrafluoroboric acid BF4H, hexafluorosilicic acid H2SiF6, hexafluorozirconic acid H2ZrF6, hexafluorotitanic acid H2TiF6, ammonium titanium fluoride (NH4)2TiF6, and ammonium zirconium fluoride (NH4)2ZrF6. The compounds may be one type or a combination of two or more types. Among these, hydrofluoric acid is more preferable. When hydrofluoric acid is used, better corrosion resistance and paintability can be obtained.

[0049] Mg contained in the chemical conversion treatment liquid contributes to the formation of the F-Mg enrichment layer. The reason for this is not clear, but it is presumed that it may be the starting point for the formation of the F-Mg enrichment layer near the interface with the plating layer. When Mg is not contained in the chemical conversion treatment liquid, even if Mg is contained in the plating layer, the F-Mg enrichment layer is not sufficiently formed at the interface, and a sufficient effect of improving white rust resistance cannot be obtained. Examples of Mg compounds contained in the chemical conversion treatment liquid include magnesium fluoride, magnesium nitrate, magnesium sulfate, magnesium chloride, and magnesium acetate. When Mg is contained in the chemical conversion treatment liquid in the form of a Mg compound, the blending amount of the Mg compound contained in the chemical conversion treatment liquid is preferably such that the mass ratio [(Vs) / (Xs)] of Mg in the Mg compound to the solid content (X) contained in the chemical conversion treatment liquid is 0.05 to 0.60. When [(Vs) / (Xs)] is less than 0.05, the F concentration near the interface becomes less than 0.5% by mass, and there is a concern that a predetermined F-Mg enrichment layer may not be formed. On the other hand, when [(Vs) / (Xs)] exceeds 0.60, there is a concern that the Mg concentration becomes more than 0.5% by mass in parts other than the F-Mg enrichment layer.

[0050] Acetylacetone (acetylacetonate) contained in the chemical conversion treatment liquid contributes to the stabilization of the Mg compound and suppresses the reaction of the Mg compound with the components in the treatment liquid during storage of the treatment liquid. When acetylacetone is not contained in the chemical conversion treatment liquid, a sufficient F-Mg enrichment layer is not formed. Regarding the blending amount of acetylacetone (W), the molar ratio of acetylacetone (W) to the Mg compound (V) [(Wmol) / (Vmol)] is preferably 1.0 to 10.0. When the molar ratio of acetylacetone (W) to the Mg compound (V) [(Wmol) / (Vmol)] is less than 1.0, the F concentration near the interface becomes less than 0.5% by mass, and there is a concern that a predetermined F-Mg enrichment layer may not be formed. On the other hand, when [(Wmol) / (Vmol)] exceeds 10.0, the stabilizing effect of the Mg compound saturates, resulting in poor economy.

[0051] When the chemical conversion treatment liquid contains a Zr compound, examples thereof include ammonium zirconium carbonate, zirconium hexafluoride hydrofluoric acid, ammonium hexafluorozirconate, and the like. In addition, when the V compound is included, examples thereof include vanadium pentoxide V2O5, metavanadic acid HVO3, ammonium metavanadate, sodium metavanadate, vanadium oxytrichloride VOCl3, vanadium trioxide V2O3, vanadium dioxide VO2, vanadium oxysulfate VOSO4, vanadium oxyacetylacetonate VO(OC(=CH2)CH2COCH3))2, vanadium acetylacetonate V(OC(=CH2)CH2COCH3))3, vanadium trichloride VCl3, phosphovanadomolybdic acid, and the like. Further, a compound obtained by reducing a pentavalent vanadium compound to a tetravalent to divalent state with an organic compound having at least one functional group selected from the group consisting of a hydroxyl group, a carbonyl group, a carboxyl group, a primary to tertiary amino group, an amide group, a phosphate group, and a phosphonic acid group can also be used.

[0052] [Heating step] In the heating step, the steel sheet coated with the chemical conversion treatment liquid is heated, dried, and baked. Thereby, a chemical conversion coating film is formed on the surface of the plating layer. Regarding the heating temperature (drying temperature), if the maximum reaching temperature is less than 60°C, the solvent of the surface treatment metal agent will not completely volatilize, which is not preferable. On the other hand, if the maximum reaching temperature exceeds 200°C, the solvent drying effect by heating will saturate, which is not economical and thus not preferable. Therefore, the maximum reaching temperature is preferably 60 - 200°C, and more preferably 80 - 150°C. In the heating process, the heating method is not limited. For example, it can be heated and dried using IH, a hot air furnace, etc.

Examples

[0053] A cold-rolled steel sheet (plating base plate) with a thickness of 0.8 mm that satisfies JIS G3141:2017 was immersed in a plating bath having the composition shown in Table 1, pulled out, and then adjusted to the adhesion amount shown in Table 8 by wiping with N2 gas. Thereafter, using cooling water with the pH adjusted by adding the pH adjuster shown in Table 2, it was water-cooled under the conditions of Table 8 to obtain plated steel sheets (O1 - O31). In Table 1, for example, Zn-6.0%Al-3.0%Mg indicates a composition containing 6.0 mass% of Al, 3.0 mass% of Mg, and the balance being Zn and impurities.

[0054] Regarding the obtained plated steel sheets, the appearance was visually evaluated. Specifically, when it was locally or entirely whitened, it was judged as "F (Fair)" (it can be applied to parts where appearance is not required or used after maintenance, but it is difficult to directly use for parts where appearance is required and not preferable). On the other hand, when no whitening was observed, it was judged as "G (Good)" (excellent in appearance). Also, the thickness of the region where the Mg concentration from the surface layer of the plating layer is 20 - 60 mass% was measured.

[0055] For the obtained plated steel sheets, aqueous surface treatment metal agents ST1 - ST21 were prepared by mixing the silicon compounds (silane coupling agents), P compounds, fluorides, Mg compounds, and acetylacetone shown in Tables 3 - 7 at the ratios shown in Table 9.

[0056] On the plated steel sheets O1 to O31, the surface treatment metal agents of ST1 to ST21 were applied by a roll coater and dried to form a film. At that time, the coating amount of the film and the combination of the plated steel sheet and the surface treatment metal agent were as shown in Tables 10-1 to 10-4. Drying was carried out by heating to the drying plate temperature in Tables 10-1 to 10-4 (the steel sheet temperature reached) and holding for 2 seconds to form a film. Thereby, surface-treated steel sheets No. 1 to 120 were manufactured.

[0057] For the obtained surface-treated steel sheets, the thickness, Si concentration, P concentration, F concentration, Mg concentration, Zr concentration, and V concentration of the chemical conversion coating film were measured in the above-mentioned manner. The results are shown in Tables 11-1 to 11-4. In the table, the "-" in the columns of Zr concentration and V concentration indicates that a concentration of 0.001 mass% or more was not detected in any measurement. Although not shown in the table, in all cases, as a result of FT-IR measurement, Si existed as a silicon compound. Also, in the above-mentioned manner, the thickness of the F-Mg enriched layer of the chemical conversion coating film was measured. The results are shown in Tables 11-1 to 11-4. At that time, the average of the F concentration and Mg concentration at the 1.0 nm position was as shown in Tables 11-1 to 11-4. Also, in the above-mentioned manner, the F concentration and Mg concentration at the site excluding the F-Mg layer were measured.

[0058] Also, for the obtained surface-treated steel sheets, the corrosion resistance (SST), white rust resistance in an environment in contact with running water, corrosion resistance in a dew condensation environment, corrosion resistance of the Erichsen processed part, blackening resistance, and appearance were evaluated in the following manner. The results are shown in Tables 12-1 to 12-4.

[0059] "Corrosion resistance (SST)" Flat test pieces (100 mm × 100 mm) were prepared, and for each test piece, a salt spray test conforming to JIS Z 2371:2015 was carried out, and the occurrence status of white rust on the surface after 120 hours (the ratio of the area where white rust occurred to the area of the test piece) was evaluated. <Evaluation criteria> EX (Excellent): Rust generation is less than 5% of the total area G (Good): Rust generation is 5% or more but less than 10% of the total area P (Poor): Rust generation is 10% or more of the total area

[0060] "Rust resistance in an environment in contact with running water" Flat test pieces (100 mm × 100 mm) were prepared from the obtained surface-treated steel sheet, and these test pieces were fixed at an angle of 45 degrees with respect to the vertical line of the test surface. Thereafter, for each test piece, salt water with a salt concentration of 50 g / L and a pH of 6.5 to 7.2 was dropped. The salt water was dropped from a tube with an inner diameter of 3 mm. The tip of the tube was aimed at a position 20 mm shifted downward from the center of the upper end of the test piece, and the distance between the test piece and the tip of the tube was 20 mm. The dropping speed was 10 ml / s. The dropping test was performed in the above manner, and the occurrence status of white rust on the surface after 120 hours was evaluated. The part where the salt water was directly dropped from the tube (a region with a diameter of 20 mm centered on the above-mentioned aiming position) is called the dropping part, and the flow path of the salt water flowing from the dropping part is called the running water part. Evaluation was performed according to the following evaluation criteria, and if it was Ex or G, it was judged that the rust resistance was excellent. <Evaluation Criteria> Ex (Excellent): No white rust generation G (Good): White rust generation in the dropping part, no white rust generation in the running water part P (Poor): White rust generation in both the dropping part and the running water part

[0061] "Corrosion resistance in a dew condensation environment" Flat test pieces (100 mm × 100 mm) were prepared from the obtained surface-treated steel sheet, and 5 ml of the salt water used in the neutral salt spray described in JIS Z 2371:2015 was dropped at the center of the test piece. The test piece after the salt water was dropped was stored at 50°C - 98% RH for 240 hours, and the occurrence status of white rust was evaluated. If it was G, it was judged that the corrosion resistance in the dew condensation environment was excellent. <Evaluation Criteria> G (Good): No white rust generation P (Poor): White rust generation

[0062] "Corrosion resistance of Erichsen processed part" Flat test pieces (50 mm × 50 mm) were prepared from the obtained surface-treated steel sheets. After performing Erichsen tests (7 mm extrusion), salt spray tests conforming to JIS Z 2371:2015 were conducted for 120 hours, and the white rust occurrence status was observed. If it was Ex or G, it was judged that the corrosion resistance of the Erichsen processed part was excellent. <Evaluation Criteria> Ex (Excellent): Rust generation is less than 10% of the processed part area G (Good): Rust generation is 10% or more and less than 30% of the processed part area P (Poor): Rust generation is 30% or more of the processed part area

[0063] "Blackening resistance" Test plates (50 mm × 50 mm) were prepared from the obtained surface-treated steel sheets. After holding the test plates in a humid box at a temperature of 70°C and a relative humidity of 80% for 6 days, they were taken out and the blackening status of the test plates was visually judged. The evaluation criteria were as follows. If it was G, it was judged as qualified. If it was Ex, it was judged that the blackening resistance was particularly excellent. Ex (Excellent): The area ratio of the blackened part is less than 1% G (Good): The area ratio of the blackened part is 1% or more and less than 25% P (Poor): The area ratio of the blackened part is 25% or more

[0065]

Table 1

[0066]

Table 2

[0067]

Table 3

[0068]

Table 4

[0069]

Table 5

[0070]

Table 6

[0071]

Table 7

[0072]

Table 8

[0073]

Table 9

[0074]

Table 10-1

[0075]

Table 10-2

[0076]

Table 10-3

[0077]

Table 10-4

[0078]

Table 11-1

[0079]

Table 11-2

[0080]

Table 11-3

[0081]

Table 11-4

[0082]

Table 12-1

[0083]

Table 12-2

[0084]

Table 12-3

[0085]

Table 12-4

[0086] As can be seen from Tables 1 to 12-4, on the steel material, there is a predetermined plating layer and a chemical conversion coating, and in the region where the chemical conversion coating is in contact with the interface between the chemical conversion coating and the plating layer, the Mg concentration is 1.50% by mass or more and 40.00% by mass or less, and the F concentration is 0.50% by mass or more and 5.00% by mass or less, having an F-Mg enrichment layer. In the region of the chemical conversion coating excluding the F-Mg enrichment layer, in the examples (Examples Nos. 1 to 30, Nos. 47 to 54, Nos. 97 to 104) where the average Mg concentration is less than 0.50% by mass and the average F concentration is less than 0.50% by mass, the blackening resistance is good, and the generation of white rust is suppressed in any environment where it comes into contact with running water and in an environment where dew condensation occurs. However, among these, although Nos. 1 to 30 had excellent appearance, in Nos. 47 to 54 and Nos. 97 to 104, the appearance of the plating layer of the plated steel sheet was inferior, so the appearance of the surface-treated steel sheet was inferior. On the other hand, in Comparative Examples Nos. 31 to 46, Nos. 55 to 86, and Nos. 95 to 120, a predetermined F-Mg enrichment layer could not be obtained, and they were inferior in appearance and blackening resistance, and / or white rust occurred in one or both of the environments where they came into contact with running water and in an environment where dew condensation occurred.

Industrial Applicability

[0087] According to the present invention, it is possible to provide a surface-treated steel sheet capable of suppressing the generation of white rust in any environment where it comes into contact with running water and in an environment where dew condensation occurs. This surface-treated steel sheet is applicable to steel sheets for civil engineering and construction uses that are used in an environment where the steel material comes into contact with running water or in an environment where dew condensation occurs, and has high industrial applicability.

Explanation of Reference Numerals

[0088] 1 Surface-treated steel sheet 11 Base metal steel sheet 12 Plating layer 13 Chemical conversion coating 14 F-Mg enrichment layer

Claims

【Claim 1】 a base steel plate, a plating layer formed on the base steel plate and containing 50% by mass or more of Zn and 0.3% by mass or more of Mg, a chemical conversion coating formed on the plating layer, characterized by comprising: the chemical conversion coating contains a silicon compound, P and F, and Mg, the average Si concentration of the chemical conversion coating is 10% by mass or more, the chemical conversion coating has an F-Mg enriched layer in a region in contact with the interface between the chemical conversion coating and the plating layer, wherein the Mg concentration is 1.50% by mass or more and 40.00% by mass or less, and the F concentration is 0.50% by mass or more and 5.00% by mass or less, the thickness of the F-Mg enriched layer is 1.0 nm or more, in a region of the chemical conversion coating excluding the F-Mg enriched layer, the average Mg concentration is less than 0.50% by mass and the average F concentration is less than 0.50% by mass, in the chemical conversion coating, the thickness of the F-Mg enriched layer is 5.0 nm or more and 100.0 nm or less, the chemical composition of the plating layer is, in mass%, Al: 4.0% or more and less than 25.0%, Mg: 0.3% or more and less than 12.5%, Sn: 0% or more and 20% or less, Bi: 0% or more and less than 5.0%, In: 0% or more and less than 2.0%, Ca: 0% or more and 3.0% or less, Y: 0% or more and 0.5% or less, 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 0.25%, 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 5.0% or less, 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%, and the balance: Zn and impurities, the adhesion amount of the plating layer is 10 to 200 g / m², the average P concentration of the chemical conversion coating is 0.01% by mass or more and 10.00% by mass or less, the average F concentration is 0.01% by mass or more and 1.10% by mass or less, the average Mg concentration is 0.01% by mass or more and 1.00% by mass or less, the average Zr concentration is 0% by mass or more and 3.00% by mass or less, and the average V concentration is 0% by mass or more and 3.00% by mass or less, the thickness of the chemical conversion coating is 0.02 to 2.0 µm, a surface-treated steel plate.

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