Surface-treated steel

Abstract

The objective is to provide a surface-treated steel material, such as a surface-treated steel sheet, that has a Zn-containing plating layer (Zn-based plating layer) and a film formed on the surface of the plating layer, and that suppresses the formation of red rust in the unplated areas, especially in the early stages of corrosion (having excellent red rust resistance). [Solution] A surface-treated steel material comprising: a steel material; a Zn-containing plating layer formed on at least a portion of the surface of the steel material; and a film containing P, Si, Al, and Mg formed on the surface of the plating layer, wherein when the cross-section of the film is analyzed by TEM-EDS for P content, Al content, Mg content, and Si content in mass%, the average value of the P / Al ratio (P content to Al content) is 0.010 to 10.00, the average value of the Mg / Si ratio (Mg content to Si content) is 0.025 to 1.00, and the area ratio of cracks present on the surface of the film is 0.1 to 10.0%.

Description

[Technical Field]

[0001] This invention relates to surface-treated steel materials. [Background technology]

[0002] Conventionally, plated steel sheets (zinc-plated steel sheets), in which a zinc-based plating layer is formed on the surface of a steel sheet, have been used in a wide range of applications such as automobiles, building materials, and home appliances. For example, in the building materials sector, research has long been conducted to improve the corrosion resistance of zinc-plated steel sheets in response to the need for longer lifespan of building materials. In this context, the inclusion of Al and Mg in the zinc-plated layer has been investigated as a way to improve corrosion resistance. For example, Patent Documents 1 to 3 disclose plated steel materials that achieve high corrosion resistance by containing a certain amount of Al and Mg in addition to Zn in the plating layer.

[0003] Furthermore, to impart corrosion resistance and paint adhesion to the surface of such zinc-plated steel sheets, methods such as chromate treatment using a treatment solution mainly containing chromic acid, dichromate, or their salts, treatment using a chromium-free metal surface treatment agent, phosphate treatment, treatment with a silane coupling agent alone, and organic resin coating treatment are generally known and in practical use. These coatings can be given various functions by changing their film composition.

[0004] For example, Patent Document 4 describes a chromium-free surface-treated steel sheet that has excellent flatness and corrosion resistance after processing, as well as excellent adhesive bonding properties, even when alkaline-degreased by the user or exposed to a corrosive environment for a long period of time. The surface treatment composition (X) is applied to the surface of a zinc-plated steel sheet or an aluminum-plated steel sheet, and contains 1 to 400 parts by mass of an organic phosphoric acid compound (B) and 50 to 150 parts by mass of an inorganic phosphoric acid compound (C) per 100 parts by mass of solids of a titanium-containing aqueous solution (A) obtained by mixing at least one titanium compound selected from hydrolyzable titanium compounds, low condensates of hydrolyzable titanium compounds, titanium hydroxide, and low condensates of titanium hydroxide with hydrogen peroxide, and then dried to form a surface treatment film with a thickness of 0.01 to 1.0 μm, wherein a solvent-based organic resin (E), a non-chromium rust-preventive additive (F), and [M 2+ 1-x M 3+ x(OH)2][A n- A highly corrosion-resistant surface-treated steel sheet is disclosed, characterized by having an upper layer film with a film thickness of 0.1 to 3.0 μm formed by applying and drying a paint composition (Y) containing a solid lubricant (G) of ]x / n·zH2O.

[0005] Furthermore, Patent Document 5 discloses a chemically treated steel sheet having a chemical treatment coating containing an organic resin and using a molten Zn-Al-Mg alloy plated steel sheet as a base material, which is excellent in all aspects of weather resistance, water resistance, blackening resistance, coating adhesion, and anti-glare properties. Patent Document 5 discloses that by forming a phosphate coating on the surface of a molten Zn-Al-Mg alloy plated steel sheet, and then forming a chemical treatment coating on the phosphate coating, which is made by crosslinking an aqueous fluorine-containing resin prepared without the use of an emulsifier with an organic crosslinking agent and a group 4A metal compound, the weather resistance, water resistance, blackening resistance, coating adhesion, and anti-glare properties of the chemically treated steel sheet using a molten Zn-Al-Mg alloy plated steel sheet as a base material are all improved. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2006-193791 [Patent Document 2] International Publication No. 2011 / 001662 [Patent Document 3] Japanese Patent Publication No. 2021-172878 [Patent Document 4] Japanese Patent Publication No. 2012-77368 [Patent Document 5] Japanese Patent Publication No. 2012-77322 [Overview of the project] [Problems that the invention aims to solve]

[0007] When plated steel is used, it may be cut to a specified size. In this case, the cut surface (cut end surface) will not have a plated layer. Furthermore, even on plated surfaces, there may be areas where the plated layer is not formed (exposed steel material) due to unplated areas, peeling of the plated layer due to scratches, or cracking of the plated layer due to cutting, punching, bending, drawing, etc. In the plated steel materials described in Patent Documents 1 to 3, the plated portion exhibits excellent corrosion resistance, and in particular, when the plating layer contains Mg and Zn, after a certain period of time has elapsed since the start of exposure, the dissolution of the plating layer progresses to some extent, and a compound film containing the constituent elements of the plated portion is formed on the unplated portion, thereby suppressing the progression of corrosion even in the unplated portion. However, as a result of the inventors' investigations, it has been found that even with these plated steel materials, in the early stages of corrosion when the dissolution of the plating layer has not progressed much, red rust may occur on the cut end surface, unplated portion, or portion of the steel sheet that has been exposed due to scratches or processing after the plating layer has been formed (collectively referred to as the unplated portion).

[0008] Furthermore, even in the case of surface-treated steel materials having a coating, such as those described in Patent Documents 4 and 5, the coating is often not formed on unplated areas where the steel material is exposed due to cut surfaces (cut ends), scratches, punching, bending, drawing, etc. In such cases, there is a problem in that red rust is generated (rusting) in the early stages of corrosion. Therefore, there is a need for the development of technologies that can suppress the formation of red rust on unplated areas.

[0009] Considering the above background, the present invention aims to provide a surface-treated steel material, such as a surface-treated steel sheet, which has a Zn-containing plating layer (Zn-based plating layer) and a film formed on the surface of the plating layer, that suppresses the formation of red rust in the unplated areas, especially in the early stages of corrosion (has excellent red rust resistance). [Means for solving the problem]

[0010] The inventors investigated a method for suppressing the formation of red rust in the unplated areas of surface-treated steel materials. As a result, they found that it is possible to suppress the formation of red rust (rusting) in the unplated areas by incorporating a highly eluting agent into the coating and controlling the elemental composition ratio in the coating so that the eluted components form a precipitated film in the unplated areas. Furthermore, we found that creating a certain percentage of cracks on the surface of the coating is effective in increasing the drug dissolution rate, and that lowering the glass transition temperature of the coating is also effective.

[0011] This invention was made in view of the above-mentioned problems. The gist of this invention is as follows. [1] A surface-treated steel material comprising: a steel material; a Zn-containing plating layer formed on at least a portion of the surface of the steel material; and a film formed on the surface of the plating layer containing P, Si, Al, and Mg, wherein when the cross-section of the film is analyzed by TEM-EDS for P content, Al content, Mg content, and Si content in mass%, the average value of the P / Al ratio, which is the ratio of the P content to the Al content, is 0.010 to 10.000, the average value of the Mg / Si ratio, which is the ratio of the Mg content to the Si content, is 0.025 to 1.000, and the area ratio of cracks present on the surface of the film is 0.10 to 10.00%. [2] The average value of (P + Al) / (Mg + Si), which is the ratio of the sum of the P content and the Al content to the sum of the Mg content and the Si content in mass%, is 0.025 to 5.000. The surface-treated steel material according to [1], characterized in that. [3] The glass transition temperature of the film is 40°C or lower. The surface-treated steel material according to [1] or [2], characterized in that. [4] When thin-film X-ray diffraction is performed on the surface, the ratio intensity I1, which is the ratio of the maximum value to the minimum value of the detected intensity at 2θ of 9.25 to 10°, and the ratio intensity I2, which is the ratio of the maximum value to the minimum value of the detected intensity at 2θ of 10 to 12°, one or both of them are 1.50 or more. The thin-film X-ray diffraction uses Cu-Kα rays, which are characteristic X-rays, as the X-rays, the output of the X-rays is 45 kV and 40 mA, and the measurement is performed under the condition that the incident angle of the X-rays is 0.5°. The surface-treated steel material according to any one of [1] to [3], characterized in that. [5] When the area of the exposed surface is 50 cm 2 and it is naturally immersed in 150 mL of a test solution consisting of pure water at 40°C in a sealed state for 24 hours, the P concentration of the test solution after immersion is 0.100 mg / L -1 or more, and the Si concentration of the test solution is 0.100 mg / L -1 or more. The surface-treated steel material according to any one of [1] to [4], characterized in that. [6] When the area of the exposed surface is 50 cm 2 and it is naturally immersed in 150 mL of a test solution consisting of pure water at 40°C in a sealed state for 24 hours, the Zn concentration of the test solution after immersion is 0.100 mg / L -1 or more. The surface-treated steel material according to any one of [1] to [5], characterized in that. [7] The average chemical composition of the plating layer contains 4.00 mass% or more and 70.00 mass% or less of Al and 0.3 mass% or more and 12.5 mass% or less of Mg. The surface-treated steel material according to any one of [1] to [6], characterized in that. [8] The average thickness of the film is 1.00 to 10.00 μm. The surface-treated steel material according to any one of [1] to [7], characterized in that.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide a surface-treated steel material in which the generation of red rust in non-plated portions is suppressed.

Brief Description of the Drawings

[0013] [Figure 1A] This is an example of a SEM photograph of the surface of the film. [Figure 1B] This is an example of an image obtained by binarization to determine the area ratio of cracks on the surface of the film from the SEM photograph of FIG. 1A.

Modes for Carrying Out the Invention

[0014] A surface-treated steel material according to an embodiment of the present invention (the surface-treated steel material according to this embodiment) will be described.

[0015] The surface-treated steel material according to this embodiment has a steel material, a plating layer containing Zn formed on at least a part of the surface of the steel material, and a film formed on the surface of the plating layer and containing P, Si, Al, and Mg.

[0016] <Steel material> The surface-treated steel material according to this embodiment has great features in the plating layer and the film. Therefore, the steel material is not particularly limited. The steel material may be determined according to the applied product, required strength, plate thickness, etc. For example, hot-rolled steel sheets (hot-rolled steel plates) described in JIS G 3131:2018, JIS G 3113:2018, etc. or cold-rolled steel sheets (cold-rolled steel plates) described in JIS G 3141:2021, JIS G 3135:2018, etc. can be used. Also, as described above, the steel material can be a steel material such as a steel pipe, steel wire, or various members made of steel other than steel plates. Hereinafter, an example in which the surface-treated steel material according to this embodiment is a surface-treated steel plate (when the steel material is a steel plate) will be mainly described.

[0017] <Plating layer> The surface-treated steel material according to this embodiment has a plating layer containing Zn formed on at least a portion of the surface of the steel material. From the viewpoint of corrosion resistance, the plating layer preferably contains 20% by mass or more of Zn in its average chemical composition, more preferably 30% by mass or more, and even more preferably 50% by mass or more. The average chemical composition of the plating layer may consist of Zn and impurities, but may also contain 4.00% to 70.00% by mass of Al and 0.3% to 12.5% ​​by mass of Mg. Furthermore, if necessary, it may also contain one or more of the following: Sn, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb, and B. For example, the average chemical composition of the plating layer, in mass%, is: Al: 4.00% or more, 70.00% or less; Mg: 0.3% or more, 12.5% ​​or less; Sn: 0% or more, 20.0% or less; Bi: 0% or more, less than 5.0%; In: 0% or more, less than 2.0%; Ca: 0% or more, 3.0% or less; Y: 0% or more, 0.5% or less; La: 0% or more, less than 0.5%; Ce: 0% or more, less than 0.5%; Si: 0% or more, less than 2.5%; Cr: 0% or more, less than 0.25%; Ti: 0% The above includes 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%, Fe: 0% or more, 5.0% or less, Sr: 0% or more, less than 0.5%, Sb: 0% or more, less than 0.5%, Pb: 0% or more, less than 0.5%, B: 0% or more, less than 0.5%, with the remainder being Zn and impurities. In this case, it is preferable because a more superior corrosion resistance can be obtained as a surface-treated steel sheet, including the portion where the plating layer is formed.

[0018] The reasons for the preferred chemical composition of the plating layer are explained below. Unless otherwise specified, the percentages for the concentration (content) of each element in the chemical composition of the plating layer are given in mass percentages.

[0019] [Al:4.00% or more, 70.00% or less] Al is an effective element for improving corrosion resistance in zinc-based plating layers. Therefore, it may be included. To obtain the above effect sufficiently, it is preferable to have an Al concentration of 4.00% or higher. On the other hand, if the Al concentration exceeds 70.00%, the sacrificial corrosion protection effect of the plating layer decreases. Therefore, it is preferable that the Al concentration be 70.00% or less. More preferably, the Al concentration is 50.00% or less.

[0020] [Mg: 0.3% or more, 12.5% ​​or less] Mg is an element that enhances the corrosion resistance of the plating layer. To obtain the effect of improved corrosion resistance, it is preferable to have an Mg concentration of 0.3% or higher. It is more preferable to have an Mg concentration of 1.0% or higher, and even more preferable to have an Mg concentration of 3.0% or higher. On the other hand, if the Mg concentration exceeds 12.5%, the effect of improving corrosion resistance saturates, and the processability of the plating layer may decrease. In addition, manufacturing problems may arise, such as an increase in the amount of dross generated in the plating bath. For this reason, it is preferable to keep the Mg concentration at 12.5% ​​or less.

[0021] [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 improved corrosion resistance and sacrificial corrosion protection. Therefore, one or more of these elements may be included. To obtain the above effects, it is preferable that the concentration of each element be 0.1% or higher, and more preferably 0.2% or higher. Of these, Sn is preferred because it is a low-melting-point metal that can be easily incorporated 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 higher, or the In concentration is 2.0% or higher, the corrosion resistance decreases. Therefore, it is preferable to keep the Sn concentration at 20.0% or lower, the Bi concentration at less than 5.0%, and the In concentration at less than 2.0%, respectively.

[0022] [Ca: 0% or more, 3.0% or less] Ca is an element that reduces the amount of dross that tends to form during operation, thereby contributing to improved plating manufacturability. Therefore, it may be included. To obtain this effect, it is preferable to have a Ca concentration of 0.1% or higher. On the other hand, if the Ca concentration is high, the corrosion resistance of the flat parts of the plating layer tends to deteriorate, and the corrosion resistance around the weld may also deteriorate. For this reason, a Ca concentration of 3.0% or less is preferable.

[0023] [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 improved corrosion resistance. To achieve this effect, it is preferable to include at least 0.1% of each of these elements, and more preferably 0.2% or more. On the other hand, if the concentrations of these elements become excessive, the viscosity of the plating bath increases, often making it difficult to prepare the plating bath itself, and there are concerns that steel materials with good plating properties cannot be manufactured. For this reason, it is preferable to keep the Y concentration at 0.5% or less, the La concentration at less than 0.5%, and the Ce concentration at less than 0.5%.

[0024] [Si: 0% or more, less than 2.5%] Si is an element that contributes to improved corrosion resistance. Furthermore, when forming a plating layer on a steel sheet, Si has the effect of suppressing the formation of an excessively thick alloy layer between the steel sheet surface and the plating layer, thereby improving the adhesion between the steel sheet and the plating layer. To obtain these effects, it is preferable to have a Si concentration of 0.1% or higher. More preferably, the Si concentration is 0.2% or higher. On the other hand, if the Si concentration exceeds 2.5%, excess Si precipitates in the plating layer, reducing not only corrosion resistance but also the processability of the plating layer. Therefore, it is preferable to keep the Si concentration below 2.5%. More preferably, the Si concentration is 1.5% or less.

[0025] [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 greater, less than 0.25%] These elements contribute to improved corrosion resistance. To achieve this effect, it is preferable to have a concentration of 0.05% or more of one or more of these elements. On the other hand, if the concentrations of these elements become excessive, the viscosity of the plating bath increases, often making it difficult to prepare the plating bath itself, which raises concerns that steel materials with good plating properties cannot be manufactured. For this reason, it is preferable to keep the concentration of each element below 0.25%.

[0026] [Fe: 0% or more, 5.0% or less] Fe is incorporated into the plating layer during its manufacture. While it may be present in concentrations up to approximately 5.0%, within this range, the adverse effect on the surface-treated steel sheet according to this embodiment is small. Therefore, it is preferable to keep the Fe concentration below 5.0%.

[0027] [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, and Pb are included in the plating layer, the appearance of the plating layer changes, spangles are formed, and an improvement in metallic luster is observed. To obtain this effect, it is preferable that the concentration of one or more of Sr, Sb, and Pb be 0.05% or higher, more preferably 0.1% or higher, and even more preferably 0.2% or higher. On the other hand, if the concentrations of these elements become excessive, the viscosity of the plating bath increases, often making it difficult to prepare the plating bath itself, which raises concerns that steel materials with good plating properties cannot be manufactured. For this reason, it is preferable to keep the concentration of each element below 0.5%.

[0028] [B: 0% or more, less than 0.5%] B is an element that, when included in the plating layer, combines with Zn, Al, Mg, etc., to form various intermetallic compounds. These intermetallic compounds have the effect of improving LME (Laser Metal Efficiency). To obtain this effect, it is preferable to have a B concentration of 0.1% or higher, and more preferably 0.2% or higher. On the other hand, if the concentration of B is excessive, the melting point of the plating will rise significantly, leading to concerns that the plating operation will deteriorate and surface-treated steel sheets with good plating properties cannot be obtained. For this reason, it is preferable to keep the concentration of B below 0.5%.

[0029] [Remainder: Zn and purines] In the chemical composition of the plating layer, elements other than those mentioned above may be Zn and impurities. Impurities are elements that are introduced during the manufacturing process. While the total concentration of impurities is usually 0.5% or less, a total concentration of 0.1% or less is preferable.

[0030] The chemical composition of the plating layer can be measured by the following method. First, an acid solution is obtained by stripping and dissolving the plating layer with an acid containing an inhibitor that suppresses corrosion of the base metal (steel plate) (for example, an acid made by adding 1% by mass of Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) to 10% by mass of hydrochloric acid). Next, the chemical composition (average chemical composition) of the plating layer can be obtained by measuring the obtained acid solution with ICP analysis.

[0031] The amount of plating layer to be applied is not limited, but to improve corrosion resistance, 10 g / m² per side is recommended. 2 It is preferable that the amount of adhesion is 450 g / m² per side. 2 Beyond this limit, corrosion resistance saturates, and it becomes economically disadvantageous. Therefore, the amount of adhesive applied per side is 450 g / m². 2 The following is preferable:

[0032] The amount of plating layer can be measured by the following method. A 30mm x 30mm sample is taken from a surface-treated steel sheet. The plating layer is then stripped and dissolved from this sample using an acid containing an inhibitor that suppresses corrosion of the base metal (for example, an acid made by adding 1% by mass of Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) to 10% by mass of hydrochloric acid). The weight change of the plated steel sheet after stripping and dissolving is measured, and the amount of adhesion is calculated from the results.

[0033] <coating> The surface-treated steel material according to this embodiment has a film formed on the surface of the plating layer. The inventors investigated a method to suppress the formation of red rust on unplated areas by incorporating a highly elutable agent into the coating, eluting the agent component in a corrosive environment, and forming a precipitated film on the unplated areas with the eluted component. As a result, we found that by adding P compounds, Al compounds, Mg compounds, and Si compounds as additives to a resin matrix coating, the components leached from the coating form a precipitated film on the unplated areas. This precipitated film then passivates the steel, dramatically improving the resistance to red rust in the unplated areas of surface-treated steel materials. The coating on the surface-treated steel material according to this embodiment is constructed based on the above findings. Specifically, the coating on the surface-treated steel material according to this embodiment is obtained by adding a P compound, an Al compound, an Mg compound, and a Si compound as additives to a resin-based coating. As a result, this coating contains P derived from the P compound, Al derived from the Al compound, Mg derived from the Mg compound, and Si derived from the Si compound. As the matrix resin, known organic resins such as polyester resin, polyurethane resin, epoxy resin, phenolic resin, acrylic resin, and polyolefin resin can be used. To further improve adhesion to the metal plate, it is preferable to use at least one resin (polyester resin, urethane resin, epoxy resin, acrylic resin, etc.) that has forced sites or polar functional groups in its molecular chain. The resin may be used alone or in combination of two or more types. The resin content is preferably more than 0% by mass, 1% or more by mass, and more preferably 5% or more by mass, relative to the film solids. This improves corrosion resistance. Alternatively, the resin content is preferably 85% by mass or less, more preferably 60% by mass or less, and even more preferably 40% by mass or less, relative to the film solids. By setting the resin content to 85% by mass or less, the corrosion resistance of the film can be improved while ensuring other performance requirements for the film.

[0034] However, in order to obtain a sufficient improvement in red rust resistance, it is necessary to control the composition of the coating so that the P content, Si content, Al content, and Mg content in the coating satisfy a predetermined relationship. Specifically, the average value of P / Al, which is the ratio of P content to Al content by mass%, in the coating is set to 0.010 to 10.000, and the average value of Mg / Si, which is the ratio of Mg content to Si content, is set to 0.025 to 1.000. If the average P / Al value is less than 0.010, the amount of additive chemicals leached from the coating will be small, resulting in insufficient improvement in the rust resistance of the unplated areas. On the other hand, if the average P / Al value exceeds 10.000, the amount of additive chemicals leached from the coating will be too large, degrading the corrosion resistance of the flat surfaces. Furthermore, if the average Mg / Si value is less than 0.025, the additive chemicals leached from the coating will not easily form a compound film on the unplated areas, resulting in insufficient improvement in the rust resistance of the unplated areas. On the other hand, if the average Mg / Si value exceeds 1.000, the amount of additive chemicals leached from the coating becomes too large, degrading the corrosion resistance of the flat surfaces.

[0035] Furthermore, it is preferable that the average value of (P+Al) / (Mg+Si), which is the ratio of the sum of the P content and Al content to the sum of the Mg content and Si content in the film, is between 0.025 and 5.000 by mass. By keeping it within this range, the Mg and Al dissolved from the film are formed as a composite compound in the unplated areas, further improving the resistance to red rust in the unplated areas.

[0036] In the coating, the ranges for P content, Al content, Mg content, and Si content are not limited, but it is preferable that the P content is 0.100 to 10.000 mass%, the Al content is 0.100 to 10.000 mass%, the Mg content is 0.100 to 10.000 mass%, and the Si content is 0.100 to 10.000 mass% when calculated as the concentration in the coating.

[0037] [Measurement method] The P, Si, Al, and Mg content of the coating can be measured by quantitative analysis using an energy dispersive X-ray spectroscopy (EDS) (TEM-EDS) attached to a transmission electron microscope (TEM). Specifically, test specimens are cut from surface-treated steel with a coating using the cryo-FIB (Focused Ion Beam) method, and the cross-sectional structure of the cut specimens is observed using a TEM. For example, a transmission electron microscope such as the JEM-2100F (manufactured by JEOL Ltd.) can be used, an EDS measuring device such as the JED-2300T (manufactured by JEOL Ltd.) can be used, and a FIB device such as the NB5000 (manufactured by Hitachi High-Tech Corporation) can be used. In the FIB method, the processing position is determined by SEM observation, but before SEM observation, Au is deposited as an electron irradiation protective film to form a protective film composed of carbon. After SEM observation, processing is performed using the FIB method under acceleration voltage conditions of 40-5kV. A Cu mesh is used for sample holding. To identify the components of the coating, quantitative analysis of P, Al, Mg, and Si is performed using TEM-EDS at five or more points in the center of the coating's thickness direction (any point between 3 / 8 and 5 / 8 of the thickness from the surface is acceptable). The arithmetic mean of the mass percentage content at each point is adopted as the P, Si, Al, and Mg content of the coating. Additionally, the arithmetic mean of the P / Al ratio (mass ratio) of the five points is taken as the average P / Al value, and the arithmetic mean of the Mg / Si ratio (mass ratio) is taken as the average Mg / Si value. Similarly, the average (P+Al) / (Mg+Si) can also be calculated. Observation and EDS analysis are performed under conditions of an acceleration voltage of 200kV and a probe diameter of 1nmΦ.

[0038] In the coating of the surface-treated steel material according to this embodiment, thin-film X-ray diffraction (thin-film XRD) is performed on the surface under conditions of a shallow incident angle of X-rays, and the ratio of the maximum value to the minimum value of the detected intensity (cps) when 2θ is 9.25 to 10° (maximum value / minimum value) is defined as specific intensity I1, and the ratio of the maximum value to the minimum value of the detected intensity (cps) when 2θ is 10 to 12° (maximum value / minimum value) is defined as specific intensity I2. It is preferable that one or both of I1 and I2 are 1.50 or higher. A value of I1 of 1.50 or higher means that the P compound contains a certain amount or more of aluminum phosphate (AlPO4), and a value of I2 of 1.50 or higher means that the P compound contains aluminum dihydrogen triphosphate dihydrate (AlH2P3O 10 This means that it contains a certain amount of (2H2O). These compounds have the effect of suppressing the dissolution of steel, so when either or both of I1 and I2 are 1.50 or higher, the formation of red rust in the unplated areas is suppressed (red rust resistance is improved).

[0039] [Measurement method] The maximum and minimum values ​​of the detection intensity (cps) at a predetermined diffraction angle, used to determine the specific intensities I1 and I2, are measured by thin-film X-ray diffraction. Specifically, a surface-treated steel sample is cut into approximately 2 cm squares. Thin-film XRD is performed on the surface of this sample using characteristic X-rays, Cu-Kα rays, with an X-ray output of 45 kV and 40 mA, and an incident X-ray angle of 0.5°, to determine the minimum and maximum X-ray intensities within a predetermined diffraction angle range. However, for measurement, the measurement range is 2θ = 5 to 60°, the step size is 0.02°, and the time per step is 1 second.

[0040] (Percentage of crack area) Furthermore, in the coating of the surface-treated steel material according to this embodiment, the area ratio of cracks present on the surface of the coating is 0.10 to 10.00%. Cracking is initiated by the added chemicals. Specifically, when minute cracks exist on the surface of the coating, the surface area of ​​the chemical conversion coating increases, and the area ratio of the added chemicals on the crack surface increases, leading to a higher elution rate of the added chemicals. This higher elution rate promotes passivation of the exposed steel material in the unplated areas, improving resistance to red rust. If the crack area ratio on the surface of the coating is less than 0.10%, the effect of increasing the elution rate of the additive is not sufficiently obtained. On the other hand, if the crack area ratio is greater than 10.00%, the corrosion resistance of flat areas other than the unplated areas decreases. Surface cracking can be controlled by adjusting the content of the added chemicals and the conditions for film formation from the treatment solution. In this embodiment, a crack is a fissure-like structure that can be observed in a secondary electron image taken at 1000x magnification using a scanning electron microscope (SEM). Examples of cracks on the surface of the coating are shown in Figures 1A and 1B. In these examples, the area ratio of cracks is 3.70%.

[0041] [Measurement method] The area ratio of cracks on the surface of the coating is determined by the following method. A 20mm square sample is cut from a surface-treated steel material with a coating, and gold approximately 100nm thick is deposited onto the surface of the coating. This surface was examined using a scanning electron microscope (SEM) at a magnification of 1000x, covering an area of ​​8000 μm. 2The above area is observed using a secondary electron image. Since cracks appear darker relative to the film, the secondary electron image is binarized to determine the area of ​​the crack-like portion with lower brightness relative to the film. The ratio of this area to the observation field of view is defined as the crack area ratio. Here, the crack area ratio is measured using the image processing software "ImageJ". Specifically, the secondary electron image is imported into ImageJ, and the image is binarized using the "Threshold" setting in "Adjust" under "Image" so that cracks are displayed in white and areas other than cracks are displayed in black. After binarization, the "Measure" function under "Analyze" is used, and the "Area fraction" of the white area in "Results" is read to determine the crack area ratio. Perform the above measurements at three locations at least 5 mm apart from each other, and use the average of these measurements as the crack area ratio.

[0042] (Glass transition temperature) In the surface-treated steel material according to this embodiment, it is preferable that the glass transition temperature of the coating is 40°C or lower. The lower the glass transition temperature of the resin that forms the base (matrix) of the coating, the higher the elution rate of the additive. A higher elution rate improves resistance to red rust in the unplated areas. The reason why the elution rate of the additive increases with lower glass transition temperatures is not clear, but it is presumed that a lower glass transition temperature makes the resin softer and allows the additive to move more easily at low temperatures, resulting in a larger amount of eluted additive. When the glass transition temperature is greater than 40°C, the effect of increasing the elution rate of the added agent is small. On the other hand, there is no lower limit to the glass transition temperature, but for resins commonly used in coatings, the glass transition temperature is at least around -100°C, and often above -50°C.

[0043] [Measurement method] The glass transition temperature of the coating is determined using a differential scanning calorimetry (DSC). For the measurement, the measurement temperature is set to 250 to -50°C, and the temperature at which the baseline shifts is defined as the glass transition temperature. If the coating contains two or more types of resin, the baseline will shift at two different positions, and the highest temperature among these shifts will be considered the glass transition temperature of the coating.

[0044] (elution rate) As described above, the surface-treated steel material according to this embodiment has excellent resistance to red rust, as the additives contained in the coating are easily leached out. Specifically, when measuring the dissolution rate, it is preferable that it falls within the following range.

[0045] In other words, the exposed surface area is 50 cm². 2 When the test solution, consisting of 150 mL of pure water at 40°C, was immersed in a sealed container for 24 hours, the phosphorus concentration in the test solution after immersion was 0.100 mgL. -1 The above is the result, and the Si concentration of the test solution is 0.100 mgL. -1 The above, and / or the area of ​​the exposed surface is 50 cm². 2 When the sample was immersed in 150 mL of a test solution consisting of pure water at 40°C in a sealed state for 24 hours, the Zn concentration of the test solution after immersion was 0.100 mg / L. -1 The above is preferable. The elution of P and Si is from the additive agent, and the elution of Zn is from the plating layer. In addition to the elution of P and Si from the additive agent, the elution of Zn from the plating layer can further improve the resistance to red rust in the unplated areas. If the above conditions are met, even in environments with high temperature and humidity, where water exposure is prolonged and dissolved chemical components are likely to leak out of the system, a compound film derived from the added chemical can be formed on the non-plated parts, providing resistance to red rust.

[0046] [Measurement method] The elution rate is determined by the following method. A sample containing the coating was cut from the surface-treated steel material, and the exposed surface area of ​​the coated portion of the sample was 50 cm². 2 Seal all surfaces except the exposed ones with tape. The container is immersed in 150 mL of a test solution consisting of pure water at a temperature of 40°C, ensuring that the exposed surface does not come into contact with the container. The container is then sealed with a lid and left in this sealed state for 24 hours. After 24 hours, the concentrations (contents) of P, Si, and Zn in the test solution are measured by ICP analysis.

[0047] (thickness) The average thickness of the coating on the surface-treated steel material according to this embodiment is preferably 1.00 to 10.00 μm. If the average thickness of the coating is less than 1.00 μm, there is a concern that the amount of additive chemicals leached out will be insufficient. On the other hand, if it exceeds 10.00 μm, the effect of improving the resistance to red rust in the unplated area will saturate, while manufacturing costs will increase. Furthermore, there is a concern that it will become difficult to ensure a sufficient amount of zinc leached from the plating layer.

[0048] [Measurement method] The average thickness of the coating is determined by the following method. Prepare a test specimen in the same manner as for measuring the P, Si, Al, and Mg content of the coating, and measure the coating thickness at three points using TEM-EDS, ensuring that the entire coating is visible in the thickness direction. Furthermore, this measurement is performed in a total of three fields of view, and the average of the nine points (3 points x 3 fields of view) is taken as the average thickness of the coating. To measure the thickness, the phosphorus (P) content is measured by EDS analysis at 10 nm intervals from the plating layer toward the surface of the surface-treated steel. The point where the P content first exceeds 1 mass% is determined to be the interface between the film and the plating layer, and the distance from this interface to the surface of the film (surface of the surface-treated steel) is defined as the film thickness. Observation and EDS analysis are performed under conditions of an acceleration voltage of 200 kV and a probe diameter of 1 nmΦ, and the EDS analysis is performed under conditions of analyzing elements Zn, Al, Mg, O, C, P, and Si.

[0049] [Manufacturing method] The surface-treated steel sheet according to this embodiment can achieve the above-described effects regardless of the manufacturing method, as long as it possesses the above-described characteristics, but it can be manufactured by a manufacturing method that includes the following steps. (I) A plating process to obtain a plated steel sheet by forming a plating layer containing Zn on the surface of a steel material (base steel material), (II) Preparation of the treatment solution, which will form a film after application and drying, (III) A coating formation step in which a coating is formed by applying the treatment solution prepared in the treatment solution preparation step to the steel material after the plating process (plated steel material) and drying it, thereby obtaining surface-treated steel material. The preferred conditions for each process will be explained.

[0050] (Plating process) In the plating process, a plating layer is formed on the surface of a steel material, such as a steel plate, by immersing it in a plating bath containing zinc or by electroplating. The conditions for forming the plating layer are not particularly limited. It is acceptable to use a conventional method to ensure sufficient plating adhesion. Furthermore, there are no limitations on the steel materials used in the plating process or their manufacturing methods. In the case of surface-treated steel sheets, for example, hot-rolled steel sheets (hot-rolled steel sheets) as described in JIS G 3131:2018, JIS G 3113:2018, etc., or cold-rolled steel sheets (cold-rolled steel sheets) as described in JIS G 3141:2021, JIS G 3135:2018, etc., can be used as the steel sheets immersed in the plating bath. In addition, steel materials other than steel sheets, such as steel pipes, steel wires, and various components made of steel, can also be used. The composition of the plating bath should be adjusted according to the chemical composition of the plating layer to be obtained. After removing the steel material from the plating bath, the amount of plating layer can be adjusted by wiping as needed.

[0051] After the plating process and before the treatment solution preparation process, the surface of the surface-treated steel material may be degreased with a degreasing agent.

[0052] (Preparation of processing solution) In the treatment solution preparation step, P compounds, Al compounds, Si compounds, and Mg compounds are dispersed or dissolved in pure water to obtain a compound-containing solution. This solution is mixed with a water-based paint containing a resin that will form the film matrix, and stirred at 25-40°C for 24-48 hours to obtain the treatment solution. If P compounds, Al compounds, Si compounds, and Mg compounds are not first dispersed or dissolved in pure water, but are directly mixed into a water-based paint containing resin, or if the stirring time is short, the P compounds, Al compounds, Si compounds, and Mg compounds, which are the starting points for cracking, will not be sufficiently dispersed in the film, resulting in a smaller crack area ratio. On the other hand, if the stirring time is too long, the degree of dispersion of the drug will saturate, and there is a concern that the matrix base resin and the added drug will react, changing the chemical state of the added drug.

[0053] The P compound is a compound containing P, and may contain one or more selected from, for example, inorganic phosphoric acid, organic phosphoric acid, and salts thereof. The inorganic phosphoric acid, organic phosphoric acid, and salts thereof are not particularly limited and any compound can be used. For example, as the inorganic phosphoric acid, it is preferable to use one or more selected from phosphoric acid, monophosphate, dicaphosphate, tertiary phosphate, pyrophosphate, pyrophosphate, tripolyphosphate, tripolyphosphate, phosphorous acid, phosphite, hypophosphorous acid, and hypophosphite. As the organic phosphoric acid, it is preferable to use phosphonic acid (phosphonic acid compound). If the P compound is a salt, the salt is preferably a salt of an element from Group 1 to Group 13 of the periodic table, more preferably a metal salt, and preferably one or more selected from alkali metal salts and alkaline earth metal salts. The added Al compound is not limited, but it is preferably one or more selected from aluminum sulfate, aluminum carbonate, aluminum chloride, aluminum oxide, aluminum hydroxide, condensed aluminum phosphate, and aluminum metaphosphate. The added Mg compound is not limited, but it is preferably one or more selected from magnesium fluoride, magnesium nitrate, magnesium sulfate, magnesium chloride, magnesium acetate, magnesium hydroxide, magnesium silicate, basic magnesium carbonate, and magnesium phosphate. The added Si compound is not limited, but it is preferably one or more selected from silica, sodium silicate, potassium silicate, magnesium silicate, and silane coupling.

[0054] (Film formation process) In the film formation process, the treatment solution prepared in the treatment solution preparation process is applied to the plated steel sheet, dried, and cooled to form a film, thereby obtaining a surface-treated steel material. The application method is not limited. For example, it can be applied using a roll coater, bar coater, spray, etc. After applying the treatment solution, the surface-treated steel material is heated to a heating temperature (maximum temperature reached: PMT) of 120-180°C, and the holding time at the maximum temperature is set to 1.0-10.0 seconds. When heating, it is preferable to spray the material onto the steel plate through perforated metal (a steel plate with multiple through-holes). If the heating temperature is below 120°C, surface cracking is less likely to occur. On the other hand, if the heating temperature exceeds 180°C, the width of each crack increases and the area ratio of the cracks decreases, which reduces the elution rate of the added chemicals. If the holding time is less than 1.0 second, surface cracking is less likely to occur. On the other hand, if the holding time exceeds 10.0 seconds, the area ratio of surface cracks becomes excessively large, the rate of consumption of additives increases, and the end face corrosion resistance becomes inferior. After heating the surface-treated steel material to which the treatment solution has been applied, a two-stage cooling process is performed. The first cooling stage involves cooling from the highest temperature reached to 80°C, and the second cooling stage involves cooling from 80°C to 50°C. The surface-treated steel material is cooled so that the average cooling rate in the first cooling stage is 5.0 to 10.0°C / s, and the average cooling rate in the second cooling stage is 1.0 to 5.0°C / s. If the cooling rate in the first cooling stage is less than 5.0°C / s, the volume contraction of the film due to the temperature drop is slow, which may result in insufficient cracking of the film. On the other hand, if it exceeds 10.0°C / s, the volume contraction of the film is too rapid, which may result in excessive cracking. If the cooling rate in the second cooling stage is less than 1.0°C / s, the cracks formed will become larger, the consumption rate of the additive chemicals will increase, and the end face corrosion resistance will be inferior. On the other hand, if it is greater than 5.0°C / s, the cracks formed will not propagate easily, and the crack area ratio may become smaller, resulting in inferior end face corrosion resistance. The cooling method may be by blowing nitrogen gas or air, mist cooling, or immersion in water. Mist cooling is preferred, and mist cooling with water added to nitrogen is preferred. The cooling rate can be adjusted in two stages by appropriately adjusting the water content and gas flow rate. This forms a film on the surface of the plated steel material. (A surface-treated steel material according to this embodiment is obtained.) [Examples]

[0055] For the steel material, we prepared hot-rolled steel sheets with a thickness of 4.5 mm that satisfy JIS G 3131:2018. A hot-dip galvanizing process was performed on this steel sheet to form a plating layer. The composition of the plating layer is shown in Table 1. In Table 1, for example, Zn-0.18%Al indicates that it contains 0.18% Al, with the remainder being Zn and impurities, and Zn-55.00%Al-1.6%Si indicates that it contains 55.00% Al and 1.6% Si, with the remainder being Zn and impurities. The same applies to the others. The concentration (content) of impurities in the plating layers C1 to C8 was 0.3% or less in all cases. The amount of plating layer attached was 135 g / m² on both the front and back surfaces of the plated surface. 2 That's what I decided. The obtained plated steel sheet (surface-treated steel sheet) was cut with an electric shear to form an end face having a portion with a plated layer and a portion without a plated layer (where the steel sheet is exposed). No unplated areas were formed on the plated surface. After plating, the surface was degreased using the degreasing agent Surf Cleaner 4336LA1+LB.

[0056] [Table 1]

[0057] Furthermore, a treatment solution was prepared to form a protective film. First, the P compound, Al compound, Mg compound, and Si compound listed in Tables 2 to 5 were used to prepare the resin that would serve as the film matrix, as shown in Table 6. Five different compounds were prepared as P compounds. All of the P compounds used were commercially available. Their chemical formulas are also listed in Table 2. Five different compounds were prepared as Al compounds. All of the Al-containing compounds used were commercially available. Their chemical formulas are also listed in Table 3. Five different Mg compounds were prepared. All of the Mg-containing compounds used were commercially available. Their chemical formulas are also listed in Table 4. Four types of Si compounds were prepared. All of the Si-containing compounds used were commercially available. Their chemical formulas are also listed in Table 5. Six different types of resin were prepared. All of the resins used were commercially available.

[0058] [Table 2]

[0059] [Table 3]

[0060] [Table 4]

[0061] [Table 5]

[0062] [Table 6]

[0063] The P, Al, Mg, and Si compounds listed in Tables 2 to 5 were dispersed or dissolved in pure water to obtain a compound-containing solution. The combinations and amounts of these compounds and resins were adjusted so that their concentrations in the dried film were as shown in Tables 7-1, 7-2, and 8. This solution was mixed with a water-based paint containing the resin shown in Table 6, which formed the film matrix. The mixture was stirred using a stirring bar at a temperature of 25-40°C for the times indicated in Tables 7-1, 7-2, and 8 to obtain a treatment solution. Deionized water was added to adjust the solid content of the treatment solution to 20% by mass. For example, P4 and A4, where the same reagent is listed under the specified conditions, this means that the reagent at the indicated concentration was added twice. These treatment solutions were applied to the degreased surface-treated steel sheets obtained above, so as to achieve the dry film thickness shown in Tables 11-1, 11-2, and 12. The sheets were then dried in an induction heating furnace so that the maximum temperature reached and the holding time were as shown in Tables 7-1, 7-2, and 8. Finally, they were cooled with nitrogen gas containing moisture so that the average cooling rates from the maximum temperature to 80°C and from 80°C to 50°C were as shown in Tables 7-1, 7-2, and 8, respectively.

[0064] (Film-forming component) The obtained surface-treated steel sheets were quantitatively analyzed using an energy-dispersive X-ray spectroscopy (EDS) (TEM-EDS) attached to a transmission electron microscope (TEM) to determine the P / Al (mass ratio), Mg / Si (mass ratio), and (P+Al) / (Mg+Si) ratios of the coating. Specifically, test specimens were cut from surface-treated steel materials with a coating using the cryo-FIB (Focused Ion Beam) method, and the cross-sectional structure of the cut specimens was observed using a TEM. A JEM-2100F transmission electron microscope (manufactured by JEOL Ltd.), a JED-2300T EDS measurement device (manufactured by JEOL Ltd.), and an NB5000 FIB device (manufactured by Hitachi High-Tech Corporation) were used. For FIB cutting, the processing position was determined by SEM observation, but before SEM observation, a protective film of Au (gold) was deposited as an electron irradiation protective film, forming a protective film composed of carbon. After SEM observation, processing was performed using the FIB method under acceleration voltage conditions of 40-5kV. A Cu mesh was used for sample holding. To identify the components of the film, quantitative analysis of P, Al, Mg, and Si was performed using TEM-EDS at five points each, centered in the thickness direction of the film (3 / 8 to 5 / 8 of the thickness from the surface). The arithmetic mean of the mass percent content at each point was adopted as the P, Si, Al, and Mg content of the film. Furthermore, the arithmetic mean of P / Al and Mg / Si at the five points were taken as the average P / Al and Mg / Si values, respectively. Additionally, the arithmetic mean of (P+Al) / (Mg+Si) at the five points was taken as the average (P+Al) / (Mg+Si) value. Observation and EDS analysis were performed under conditions of an acceleration voltage of 200kV and a probe diameter of 1nmΦ. The results are shown in Tables 9-1, 9-2, and 10.

[0065] (Specific intensity of X-ray diffraction peaks) To determine the specific intensities I1 and I2, the maximum and minimum intensities (cps) at predetermined diffraction angles were measured using X-ray diffraction. Specifically, surface-treated steel material was cut into approximately 2 cm squares, and X-ray diffraction (thin-film XRD) was performed on this sample using Cu-Kα rays, which are characteristic X-rays, with an X-ray output of 45 kV and 40 mA, and an X-ray incidence angle of 0.5°. The minimum and maximum values ​​of the X-ray intensity within a predetermined diffraction angle range were then determined. However, for the measurement, the measurement range was set to 2θ = 5 to 60°, the step size to 0.02°, and the time per step to 1 second. The results are shown in Tables 9-1, 9-2, and 10.

[0066] (Percentage of cracks on the surface of the coating) The area ratio of cracks on the surface of the coating was determined by the following method. A 20mm square sample was cut from a surface-treated steel material with a coating, and gold approximately 100nm thick was deposited onto the surface of the coating. This surface was examined using a scanning electron microscope (SEM) at a magnification of 1000x, covering an area of ​​8000 μm. 2 The above areas were observed using secondary electron imaging. Since cracks appear darker relative to the film, the secondary electron imaging was binarized to determine the area of ​​the crack-like portion with lower brightness relative to the film. The ratio of this area to the observation field of view was defined as the crack area ratio. Here, the crack area ratio was measured using the image processing software "ImageJ". Specifically, the secondary electron imaging was imported into ImageJ, and the image was binarized using the "Threshold" setting in "Adjust" under "Image" so that cracks were displayed in white and areas other than cracks in black. After binarization, the "Measure" function under "Analyze" was used, and the "Area fraction" of the white area in "Results" was read to determine the crack area ratio. The above measurements were performed at three locations at least 5 mm apart from each other, and the average of these measurements was used as the crack area ratio. The results are shown in Tables 9-1, 9-2, and 10.

[0067] (Glass transition temperature) The glass transition temperature of the coating was determined using differential scanning calorimetry (DSC). For the measurements, the measurement temperature was set to 250 to -50°C, and the temperature at which the baseline shifted was defined as the glass transition temperature. When the coating contains two or more types of resin, the baseline shifts at two different positions, and the highest temperature among these shifts was defined as the glass transition temperature of the coating. The results are shown in Tables 9-1, 9-2, and 10.

[0068] (elution rate) The elution rate was determined using the following method. A sample containing the coating was cut from the surface-treated steel material, and the exposed surface area of ​​the coated portion of the sample was 50 cm². 2 To achieve this, the areas other than the exposed surface were sealed with tape. The container was immersed naturally in 150 mL of a test solution consisting of pure water at a temperature of 40°C, with the lid closed to prevent contact between the exposed surface and the container, and left in a sealed state for 24 hours. After 24 hours, the concentrations (contents) of P, Si, and Zn in the test solution were measured by ICP analysis. The results are shown in Tables 11-1, 11-2, and 12.

[0069] (thickness) The average thickness of the coating was determined by the following method. Test specimens were prepared in the same manner as for measuring the P, Si, Al, and Mg content of the coating. Using TEM-EDS, the coating thickness was measured at three points in a field of view that included the entire coating in the thickness direction. Furthermore, this measurement was performed in a total of three fields of view, and the average of the nine points (3 points x 3 fields of view) was taken as the average thickness of the coating. To measure the thickness, the phosphorus (P) content was measured by EDS analysis at 10 nm intervals from the plating layer toward the surface of the surface-treated steel. The point where the P content first exceeded 1 mass% was determined to be the interface between the film and the plating layer, and the distance from this interface to the surface of the film (surface of the surface-treated steel) was defined as the film thickness. Observation and EDS analysis were performed under conditions of an acceleration voltage of 200 kV and a probe diameter of 1 nmΦ. The EDS analysis was performed using Zn, Al, Mg, O, C, P, and Si as the analytical elements. The results are shown in Tables 11-1, 11-2, and 12.

[0070] <Performance evaluation details> (Corrosion resistance of the end face) Exposure tests were conducted on plated steel materials, and the area percentage of red rust on the end face after 100 days was determined. The exposure conditions were as follows: A sample of plated steel was tilted 30° from the horizontal so that the treated cut end face was facing downwards, and placed facing south for an atmospheric exposure test. After exposure, the sample was evaluated as follows based on the ratio of the area where red rust was formed to the area where the plating layer was not formed. A rating of SS, S, AA, or A indicated excellent corrosion resistance at the end face. A rating of B indicated that the area where red rust was formed exceeded 100%, meaning that red rust was formed not only in the areas where the plating layer was not formed, but also in the surrounding areas. SS: 50% or less S: More than 60%, less than 70% AA: More than 70%, less than 80% A: More than 80%, less than 100% B: More than 100%, less than 115% The results are shown in Tables 11-1, 11-2, and 12.

[0071] [Table 7-1]

[0072] [Table 7-2]

[0073] [Table 8]

[0074] [Table 9-1]

[0075] [Table 9-2]

[0076] [Table 10]

[0077] [Table 11-1]

[0078] [Table 11-2]

[0079] [Table 12]

[0080] As can be seen from Tables 1 to 12, surface-treated steel sheets having a predetermined coating on the plating layer exhibited excellent resistance to red rust on the edge surfaces. On the other hand, when the required film was not formed on the plating layer, the red rust resistance of the end face was poor.

Claims

1. Steel materials, A plating layer containing Zn is formed on at least a portion of the surface of the steel material, A film containing P, Si, Al, and Mg is formed on the surface of the aforementioned plating layer, It has, When the cross-section of the aforementioned film was analyzed using TEM-EDS to determine the P content, Al content, Mg content, and Si content in mass%, The average value of P / Al, which is the ratio of the P content to the Al content, is between 0.010 and 10.

000. The average value of Mg / Si, which is the ratio of Mg content to Si content, is between 0.025 and 1.

000. The area ratio of cracks present on the surface of the aforementioned coating is 0.10 to 10.00%. A surface-treated steel material characterized by the following features.

2. The average value of (P + Al) / (Mg + Si), which is the ratio of the sum of the P content and Al content to the sum of the Mg content and Si content in mass%, is between 0.025 and 5.

000. The surface-treated steel material according to claim 1, characterized in that...

3. The glass transition temperature of the aforementioned film is 40°C or lower. A surface-treated steel material according to claim 1 or 2, characterized in that...

4. When thin-film X-ray diffraction is performed on the surface, 1.50 or greater if one or both of the specific intensity I1, which is the ratio of the maximum value to the minimum value of the detection intensity when 2θ is 9.25 to 10°, and the specific intensity I2, which is the ratio of the maximum value to the minimum value of the detection intensity when 2θ is 10 to 12°, The thin-film X-ray diffraction described above is performed using Cu-Kα rays, which are characteristic X-rays, as the X-rays, with the X-ray output being 45 kV and 40 mA, and the incident angle of the X-rays being 0.5°. The surface-treated steel material according to claim 1, characterized in that...

5. The exposed surface area is 50 cm². ２ When immersed in a sealed state for 24 hours in 150 mL of a test solution consisting of pure water at 40°C, The P concentration of the test solution after immersion is 0.100 mg / L. －１ The above is true, and the Si concentration of the test solution is 0.100 mg / L. －１ That's all. The surface-treated steel material according to claim 1, characterized in that...

6. The exposed surface area is 50 cm². ２ When immersed in a sealed state for 24 hours in 150 mL of a test solution consisting of pure water at 40°C, The Zn concentration of the test solution after immersion is 0.100 mg / L. －１ That's all. The surface-treated steel material according to claim 1, characterized in that...

7. The average chemical composition of the aforementioned plating layer is 4.00% by mass or more and 70.00% by mass or less of Al, Mg in an amount of 0.3% by mass or more and 12.5% ​​by mass or less, including, The surface-treated steel material according to claim 1, characterized in that...

8. The average thickness of the aforementioned film is 1.00 to 10.00 μm. The surface-treated steel material according to claim 1, characterized in that...

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