JP6680412B1 - Surface treated steel plate - Google Patents

Abstract

The present invention has a steel sheet, a Zn-based alloy plating layer formed on at least one surface of the steel sheet, and a coating film containing a rust preventive agent and a binder resin formed on the Zn-based alloy plating layer. The concentration of the rust preventive agent in the coat film at a position 10 nm away from the interface between the layer and the coat film is 1.5 to 5.0 times the average concentration of the rust preventive agent in the coat film. The present invention relates to a surface-treated steel sheet.

Description

  The present invention relates to a surface-treated steel sheet having excellent corrosion resistance.

  Various plated steel sheets having excellent corrosion resistance used for home appliances, building materials, automobiles, etc. are known. For example, a galvanized steel sheet is known in which a galvanized layer is formed on the steel sheet by hot dip galvanizing or the like. When the galvanized layer is provided on the steel sheet in this manner, for example, even when the galvanized steel sheet is damaged and the steel sheet is exposed, zinc that is more corrosive than iron forming the steel sheet corrodes first to form a protective film, and The protective film can prevent corrosion of the steel sheet. Therefore, the galvanized steel sheet has been developed for various applications requiring corrosion resistance.

  However, the surfaces of various galvanized steel sheets such as galvanized steel sheets may deteriorate depending on the surrounding environment. For example, there is a problem that the plating layer is oxidized by an electrolyte such as salt contained in the atmosphere, oxygen and water existing in a high temperature and high humidity environment, and white rust is generated. Since the formation of white rust may impair the uniformity of appearance, galvanized steel sheets are required to have higher corrosion resistance.

  As a technique for further improving the corrosion resistance of a galvanized steel sheet, a Zn-based alloy plated steel sheet subjected to Zn-Al-Mg-based alloy plating or the like is known.

  However, even in such a Zn-based alloy plated steel sheet, further improvement in corrosion resistance is required, and in particular, excellent corrosion resistance is ensured by preventing corrosion factors such as oxygen from reaching the alloy plating layer. Such technology is required. Further, it is required that excellent corrosion resistance can be maintained even when such an alloy-plated steel sheet is processed.

  In Patent Document 1, a galvanized steel sheet having excellent corrosion resistance, including a steel sheet, a Zn-Al-Mg-based alloy plating layer formed on the surface of the steel sheet, and a film containing aluminum formed on the alloy plating layer is provided. It is disclosed.

  Further, in Patent Document 2, in a surface-treated metal plate having at least one coating layer on a metal plate or the like, the coating layer formed on the outermost surface is an organic resin having an anionic functional group, Li, etc. Disclosed is a surface-treated metal plate containing at least one cationic metal element selected from the following, wherein the cationic metal element is concentrated in a region near the outer surface of the coating layer, It is taught that such a surface-treated steel sheet can improve alkali resistance and solvent resistance without lowering corrosion resistance.

  Further, Patent Document 3 discloses a ground treatment composition for a coated steel sheet containing a specific organosilicon compound, hexafluorometal acid, a urethane resin having a specific cationic group, a vanadium compound, and an aqueous medium. However, it is taught that by using such a composition, it is possible to form an undercoating layer having corrosion resistance under the eaves on a steel sheet.

  Patent Documents 4 to 6 disclose coated steel plates having a resin film containing, for example, a vanadium rust preventive pigment on a zinc-based plated steel plate.

International Publication No. 2015/075792 JP, 2009-248460, A JP, 2014-214315, A JP, 2005-015834, A JP, 2013-194145, A JP 2001-003181 A

  In the galvanized steel sheet described in Patent Document 1, a Zn-Al-Mg-Si alloy plating layer is provided on the steel sheet, and the corrosion resistance of the galvanized steel sheet is mainly secured by this alloy plating layer. Further, Patent Document 1 teaches that a rust preventive agent can be further added to the coating film on the alloy plating layer, but the concentration distribution of the rust preventive agent in the coating film and its control method are not always sufficient. It has not been examined. Therefore, the galvanized steel sheet described in Patent Document 1 still has room for improvement in the improvement of corrosion resistance.

  Further, the invention described in Patent Document 2 relates to a surface-treated metal plate having a coating film mainly having improved alkali resistance and solvent resistance without lowering corrosion resistance. And, the degree of concentration of the cationic metal element in the coating layer has not necessarily been sufficiently studied, and therefore, even in the surface-treated metal plate described in Patent Document 2, improvement in corrosion resistance is still improved. There is room.

  Further, in the composition described in Patent Document 3, a vanadium compound is used in order to improve the corrosion resistance, but the vanadium compound concentration distribution in the undercoat layer obtained using this composition is not always limited. It has not been sufficiently examined, and there is still room for improvement in corrosion resistance. Similarly, in the inventions described in Patent Documents 4 to 6, the concentration distribution of the rust preventive pigment such as the vanadium compound in the film has not necessarily been sufficiently examined, and there is still room for improvement in improving the corrosion resistance.

  Therefore, in view of the above problems, an object of the present invention is to provide a Zn-based alloy plated steel sheet, which is a surface-treated steel sheet having excellent corrosion resistance.

  In order to obtain a surface-treated steel sheet having excellent corrosion resistance, the present inventors include a rust preventive agent in the coating film formed on the Zn-based alloy plating layer, and the Zn-based alloy plating layer and the coating film. It is important to make the concentration of the rust preventive agent in the coating film at a position 10 nm away from the interface with and 1.5 times to 5.0 times the average concentration of the rust preventive agent in the coating film. I found it. That is, according to the present invention, the rust-preventive agent is concentrated and present in the region near the interface between the coating film and the Zn-based alloy plating layer in the coating film as compared with other regions. Therefore, the concentrated region of the rust preventive agent can suppress corrosion factors such as oxygen from passing through the coating film and corroding the Zn-based alloy plating layer. That is, this concentrated region of the rust preventive agent can serve as a barrier region for the underlying Zn-based alloy plating layer in the coating film. Further, such a barrier region can sufficiently play its role even after processing the surface-treated steel sheet according to the present invention. Therefore, the surface-treated steel sheet according to the present invention having such a coating film can provide extremely excellent corrosion resistance.

The present invention is based on the above findings, and its gist is as follows.
(1)
A steel sheet, a Zn-based alloy plating layer formed on at least one surface of the steel sheet, and a coating film containing a rust preventive agent and a binder resin formed on the Zn-based alloy plating layer,
The chemical composition of the Zn-based alloy plating layer is% by mass,
Al: 0.01-60%,
Mg: 0.001 to 10%, and Si: 0 to 2%,
The concentration of the rust-preventive agent in the coating film at a position 10 nm away from the interface between the Zn-based alloy plating layer and the coating film is 1.5 to 5 of the average concentration of the rust-preventing agent in the coating film. A surface-treated steel sheet characterized by being 0.0 times.
(2)
The surface-treated steel sheet according to (1), wherein the rust preventive agent contains at least one of P, V and Mg.
(3)
The surface-treated steel sheet according to (1) or (2), wherein the average concentration of the rust preventive agent in the coating film is 3 to 15% by mass.
(4)
The surface-treated steel sheet according to any one of (1) to (3), wherein the coating film further contains a bright pigment, and the bright pigment contains at least one of aluminum and an oxide.
(5)
The surface-treated steel sheet according to (4), wherein the oxide is alumina, silica, mica, zirconia, titania, glass, or zinc oxide.
(6)
The surface-treated steel sheet according to (4) or (5), wherein the bright pigment further contains at least one of Rh, Cr, Ti, Ag, and Cu.
(7)
The surface-treated steel sheet according to any one of (4) to (6), characterized in that an average concentration of the bright pigment in the coating film is 5 to 15% by mass.

  According to the present invention, the coating film formed on the Zn-based alloy plating layer contains a rust preventive agent, and the rust preventive agent is at a position 10 nm away from the interface between the Zn-based alloy plating layer and the coating film. Is not less than 1.5 times and not more than 5.0 times the average concentration of the rust preventive agent in the coating film. That is, in the coating film, in the region near the interface between the coating film and the Zn-based alloy plating layer, the rust preventive agent is concentrated and present as compared with other portions. Therefore, the concentrated region of the rust preventive agent serves as a barrier region for the Zn-based alloy plating layer against corrosion factors such as oxygen, and as a result, a surface-treated steel sheet having excellent corrosion resistance can be provided. Further, according to the present invention, it becomes possible to maintain excellent corrosion resistance even when the surface-treated steel sheet according to the present invention is processed.

  Further, according to the present invention, a bright pigment may be contained in the coating film on the Zn-based alloy plating layer. In such a case, due to the metallic appearance of the bright pigment, the brightness of the surface-treated steel sheet according to the present invention is improved, and a surface-treated steel sheet excellent in designability can be provided. Furthermore, when the bright pigment is contained in the coating film, it is contained in the coating film even if the Zn-based alloy plating layer is discolored black (hereinafter, referred to as black discoloration) due to, for example, zinc oxidation of the Zn-based alloy plating layer. It is possible to make the black discoloration invisible by the bright pigment, that is, suppress the change in the appearance of the coating film, and to provide a surface-treated steel sheet excellent in designability.

  Further, according to the present invention, since the acidic coating material having a pH of 3.0 to 5.0 is used when forming the coating film, the oxide film on the surface of the Zn-based alloy plating layer is appropriately removed, and the Zn-based alloy plating is performed. By chemically bonding the layer and the coating film, it becomes possible to have excellent adhesion during processing. Further, according to the present invention, it is possible to prepare a coating material in which a rust preventive agent is stably dissolved by setting the coating material to the above-mentioned pH, and it has excellent storage stability as compared with an alkaline coating material. Is possible.

[Surface-treated steel sheet]
The surface-treated steel sheet of the present invention includes a steel sheet, a Zn-based alloy plating layer formed on at least one surface of the steel sheet, and a coating film containing a rust inhibitor and a binder resin formed on the Zn-based alloy plating layer, The chemical composition of the Zn-based alloy plating layer is mass%, Al: 0.01 to 60%, Mg: 0.001 to 10%, and Si: 0 to 2%. The concentration of the rust preventive agent in the coating film at a position 10 nm away from the interface with the film is 1.5 to 5.0 times the average concentration of the rust preventive agent in the coating film. The constituent features of the surface-treated steel sheet according to the present invention will be described below.

<Steel plate>
The steel plate (plating original plate) in the present invention is not particularly limited, and a general steel plate such as a hot rolled steel plate or a cold rolled steel plate can be used. The steel type is not particularly limited, and it is possible to use, for example, Al-killed steel, ultra-low carbon steel containing Ti, Nb, etc., and high-strength steel containing P, Si, Mn, and other elements. is there. The plate thickness of the steel plate in the present invention is not particularly limited, but may be, for example, 0.25 to 3.5 mm.

<Zn-based alloy plating layer>
The Zn-based alloy plating layer in the present invention is formed on the steel plate. This Zn-based alloy plating layer may be formed on one side or both sides of the steel sheet. The Zn-based alloy plating layer may be a Zn-Al-Mg alloy plating layer containing at least Al and Mg, or may be a Zn-Al-Mg-Si alloy plating layer containing Si. The content (concentration) of each of these is, in mass%, Al: 0.01 to 60%, Mg: 0.001 to 10%, Si: 0 to 2%, and the balance is Zn and impurities. Hereinafter, when the chemical composition of the Zn-based alloy plating layer is simply described as “%”, it means “mass%”.

  When the Al content of the Zn-based alloy plating layer is less than 0.01%, the effect of improving corrosion resistance of the plated steel sheet due to the inclusion of Al is not sufficiently exhibited, and when it exceeds 60%, the effect of improving corrosion resistance is saturated. Therefore, the Al content may be 0.01% or more, for example, 0.1% or more, 0.5% or more, 1% or more, 3% or more, or 5% or more, and 60% or less, for example, , 55% or less, 50% or less, 40% or less, or 30% or less. The Al content is preferably 1 to 60%, more preferably 5 to 60%.

  If the Mg content of the Zn-based alloy plating layer is less than 0.001%, the effect of improving the corrosion resistance of the plated steel sheet due to the inclusion of Mg may not be sufficiently exhibited. On the other hand, if it exceeds 10%, Mg cannot be completely dissolved in the plating bath and floats as an oxide (generally called dross), and when zinc plating is performed in this plating bath, the oxide adheres to the plating surface layer to cause poor appearance, or However, there is a possibility that a non-plated portion (generally called non-plating) may occur. Therefore, the Mg content may be 0.001% or more, for example, 0.01% or more, 0.1% or more, 0.5% or more, 1% or more, or 2% or more, and 10% or less. , For example 8% or less, 6% or less, 5% or less or 4% or less. The Mg content is preferably 1 to 5%, more preferably 1 to 4%.

  The lower limit of the Si content of the Zn-based alloy plating layer may be 0%, but it may be 0.001% to 2% in order to further improve the corrosion resistance of the Zn-based alloy plating layer. The Si content may be, for example, 0.005% or more, 0.01% or more, 0.05% or more, 0.1% or more, or 0.5% or more, and 1.8% or less, It may be 1.5% or less or 1.2% or less. The Si content is preferably 0.1 to 2%, more preferably 0.5 to 1.5%.

  The Zn-based alloy plating layer in the present invention can be formed by a known plating method such as hot dipping or vapor deposition plating. For example, the Zn-based alloy plating layer may have a thickness of 1 to 30 μm.

<Coating film>
The coating film in the present invention is formed on the Zn-based alloy plating layer. The coating film contains a rust preventive agent and a binder resin. In order to improve the brightness of the surface-treated steel sheet, it is preferable that the coating film further contains a bright pigment. In the coating film on the surface-treated steel sheet according to the present invention, the rust preventive agent exists as a fine compound (for example, P compound or V compound). Thus, the rust inhibitor is present as a fine compound in the coating film, and, as described above, in order to form a concentrated region of the rust inhibitor in the interface region between the coating film and the Zn-based alloy plating layer, As the paint for forming the coating film in the present invention, it is effective to use an acidic paint having a pH of 3.0 to 5.0, for example. Since the rust preventive agent is microscopically dispersed in the coating film, in the usual analysis method, the fine rust preventive agent and the binder resin forming the coating film are clearly identified in the coating film. It is difficult to do so, and it is observed that the rust preventive agent and the binder resin are distributed in the same region in the coating film. Therefore, in the present invention, "containing a rust preventive agent" in the coating film means that the coating film contains an element exhibiting a rust preventive function constituting the fine compound, such as P, V, or Mg. Means Therefore, the “concentration” of the rust preventive agent described later means, for example, the total concentration (content) of elements of P, V, and Mg, and the unit thereof is mass%.

  By thus making the coating material for forming the coating film in the present invention acidic, for example, pH 3.0 to 5.0, it becomes possible for the components of the rust preventive agent to exist in a dissolved state in the coating material. . That is, the components of the rust preventive agent according to the present invention are not contained in the paint as a compound state (that is, a solid component), but are contained in the paint as an ionic state (that is, a dissolved component). Therefore, when such a coating material is applied to the surface of the Zn-based alloy plating layer and cured, the rust preventive agent can be made to exist substantially uniformly as a fine compound in the formed coating film.

  Moreover, when an acidic paint having a pH of 3.0 to 5.0 is applied to the surface of the Zn-based alloy plating layer, the acidic paint removes the oxide film on the surface of the Zn-based alloy plating layer, The components of the rust preventive agent in the ionic state react with the components in the Zn-based alloy plating layer near the surface of the. As a result, after the coating material is cured, a region where the reaction product is concentrated can be formed near the interface between the Zn-based alloy plating layer and the coating film. Therefore, in the area where such a reaction product is present in the coating film, not only the fine compound which is almost uniformly present in the coating film as the rust preventive agent but also the reaction product formed as described above Also, since the rust preventive agent (for example, P, V, Mg) is thicker than other regions, the thickened region serves as a barrier region for preventing invasion of corrosion factors in the coating film. To work. Therefore, the surface-treated steel sheet according to the present invention produced by using an acidic paint having a pH of 3.0 to 5.0 has a concentrated region of the rust preventive agent near the interface between the Zn-based alloy plating layer and the coating film. However, it is possible to provide extremely high corrosion resistance.

  The average thickness of the coating film is not particularly limited, but can be, for example, 3 to 15 μm. With the average thickness of the coating film in such a range, the coating film serves as a barrier that sufficiently suppresses corrosion of the underlying Zn-based alloy plating layer, and is sufficient for the surface-treated steel sheet according to the present invention. Can provide corrosion resistance. Further, if the average thickness of the coating film is in the above range, cracks and the like do not occur in the coating film even if the surface-treated steel sheet according to the present invention having such a coating film is processed, and the workability is excellent. It is possible to provide a coating film that is excellent.

  When the average thickness of the coating film is less than 3 μm, the thickness may be insufficient to sufficiently suppress the progress of corrosion of the underlying Zn-based alloy plating layer, and therefore the surface treatment according to the present invention. The corrosion resistance of the steel sheet may become insufficient. On the other hand, if the average thickness of the coating film is more than 15 μm, the effect of increasing the corrosion resistance by increasing the thickness of the coating film becomes small, and it takes time to cure, which may be disadvantageous in terms of cost. is there. Further, if the coating film is too thick, the coating film may crack when the steel sheet having the coating film is subjected to processing such as bending, and the workability of the surface-treated steel sheet according to the present invention may deteriorate. The average thickness of the coating film may be, for example, 3 μm or more, 4 μm or more, or 5 μm or more, and may be 12 μm or less or 10 μm or less. Therefore, the average thickness of the coating film is preferably 3 μm or more and 12 μm or less, and more preferably 5 μm or more and 10 μm or less.

  The "average thickness" of the coating film according to the present invention can be determined by any method known to those skilled in the art. For example, the cross section of a steel sheet having a coating film is observed, and the shortest distances from the five arbitrary positions on the interface between the Zn-based alloy plating layer and the coating film to the surface of each coating film are measured (that is, It can be determined by measuring the distance in the direction perpendicular to the interface) and averaging the measured values.

(Binder resin)
The binder resin used as a component of the coating film of the present invention is not particularly limited as long as it is a resin that can be used in an acidic solvent, but may be, for example, a polyester resin, a urethane resin, or an acrylic resin. The curing agent for the binder resin is not particularly limited as long as it can be used in an acidic solvent and can cure the binder resin, and examples thereof include a melamine resin, an isocyanate resin, or an epoxy resin. Can be used. Preferably, the binder resin in the present invention is a polyester resin and the curing agent is a melamine resin. The polyester resin preferably has a glass transition temperature Tg of −20 to 70 ° C. and an average molecular weight of 3,000 to 30,000. When the binder resin is a urethane resin, it preferably has a Tg of 0 to 50 ° C. and a number average molecular weight of 5,000 to 25,000. When the binder resin is an acrylic resin, it preferably has a Tg of 0 to 50 ° C. and a number average molecular weight of 3,000 to 25,000.

(anti-rust)
In order to improve the corrosion resistance of the surface-treated steel sheet according to the present invention, a rust preventive agent (typically P and / or V) is contained in the coating film. As described above, the rust preventive agent in the present invention exists as a substantially uniformly fine compound in the coating film, but in the present invention, the "rust preventive agent" means the rust preventive agent constituting the rust preventive agent. It means an element that exhibits a function, for example, P element, V element, and Mg element. Since the rust preventive agent present as a fine compound in the coating film is soluble in water, the rust preventive agent in the coating film dissolves in water when the coating film is exposed to, for example, a humid environment. As a result, the components of the rust preventive agent are eluted and the rust preventive function of suppressing the corrosion of the Zn-based alloy plating layer can be exhibited. Further, as described above, in the concentrated region near the interface between the Zn-based alloy plating layer and the coating film, the reaction formation of the components of the rust preventive agent (for example, P, V, etc.) and the components in the Zn-based alloy plating layer. The reaction product is present in the region acting as a barrier region for corrosion factors. Therefore, in the surface-treated steel sheet according to the present invention, the rust preventive agent is present as a fine compound in the coating film, and has a concentrated area of the rust preventive agent in the interface region between the Zn-based alloy plating layer and the coating film. Therefore, it has excellent corrosion resistance.

  As a compound that can be added to a coating material for forming a coating film containing the rust preventive agent according to the present invention (hereinafter referred to as a rust preventive agent source), any compound that can be dissolved in an acidic coating material Can be used. The rust preventive dissolved in such an acidic paint is sometimes called a cation inhibitor.

  Examples of the rust preventive agent source in the present invention include P (phosphorus) compounds, V (vanadium) compounds, and Mg (magnesium) compounds. Preferably, P and V are contained alone or in combination in the coating film of the present invention. More preferably, P alone or a combination of P and V is contained in the coating film.

  When P is contained in the coating film as a rust preventive, the corrosion resistance of the processed part can be particularly improved. The processed part corrosion resistance means the corrosion resistance in the processed part when the steel sheet having a coating film is processed (for example, bent). The reason why the corrosion resistance of the processed portion is improved by including P in the coating film is that P reacts with the surface of the Zn-based alloy plating layer to form a phosphate layer to passivate the processed portion. The effect is that P itself forms a poorly soluble coating film and exhibits a barrier property against corrosion factors, and that P complements the metal ions eluted from the underlying metal plate and forms a sparingly soluble compound together with the metal ion to form a barrier. It is considered that this is because it has the effect of exerting the property. The source of the rust preventive agent containing P in the present invention is not particularly limited, but examples thereof include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, triammonium phosphate and dihydrogen phosphate. Ammonium salts such as ammonium, metal salts with Na, Mg, Al, K, Ca, Mn, Ni, Zn, Fe, etc., aminotri (methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra Examples thereof include phosphonic acids such as (methylenephosphonic acid) and diethylenetriaminepenta (methylenephosphonic acid) and salts thereof, and organic phosphoric acids such as phytic acid and salts thereof. These rust preventive agent sources can be added alone or in combination in the coating material for forming the coating film in the present invention.

Further, when V is contained as a rust preventive in the coating film, the end face corrosion resistance can be particularly improved. The end face corrosion resistance means, for example, the corrosion resistance at the end face when a steel sheet having a coating film is processed (for example, cut). The reason why the corrosion resistance is improved by the inclusion of V in the coating film is that the V eluted from the coating film reacts with Zn or Al eluted from the Zn-based alloy plating layer at the end surface to form corrosion. This is because it is possible to suppress the progress of corrosion by forming a substance to passivate the surface layer of the Zn-based alloy plating layer. Examples of the rust preventive agent source containing V in the present invention include vanadium pentoxide, HVO ₃ metavanadate, ammonium metavanadate, vanadium oxytrichloride VOCl ₃ , vanadium trioxide V ₂ O ₃ , vanadium dioxide, and vanadium oxysulfate VOSO ₄ , Vanadium oxyacetylacetonate VO (OC (= CH ₂ ) CH ₂ COCH ₃ ) ₃ , vanadium acetylacetonate V (OC (= CH ₂ ) CH ₂ COCH ₃ ) ₃ , vanadium trichloride VCl _{3 and the} like. These rust preventive agent sources can be added alone or in combination in the coating material for forming the coating film in the present invention.

Examples of the rust preventive agent source containing Mg in the present invention include magnesium nitrate Mg (NO ₃ ) ₂ , magnesium sulfate MgSO ₄ , magnesium acetate Mg (CH ₃ COO) _{2, and the} like. These rust preventive agent sources can be added alone or in combination in the coating material for forming the coating film in the present invention. Mg can improve the corrosion resistance of the end face portion as in the case of V described above. The reason why the corrosion resistance of the end face portion is improved is considered to be the same as V.

  The average concentration of the rust preventive agent in the coating film can be 3% to 15% by mass. As described above, the “average concentration of the rust preventive agent” is based on the total concentration (mass%) of elements such as P, V and Mg in the coating film. With the average concentration of the rust preventive agent in the coating film in such a range, since there is sufficient rust preventive agent in the entire coating film, it is possible to provide the surface-treated steel sheet according to the present invention with sufficient corrosion resistance. It will be possible. Further, even if the rust preventive agent is concentrated near the interface between the coating film and the Zn-based alloy plating layer as described above, the concentration of the rust preventive agent is not insufficient in other regions, and the entire coating film, that is, the present invention In the surface-treated steel sheet according to the above, sufficient corrosion resistance can be provided.

  When the average concentration of the rust preventive agent in the coating film is less than 3% by mass%, the concentration of the rust preventive agent in the entire coating film is insufficient, and the corrosion resistance improvement due to the effect of the rust preventive agent is limited. Therefore, there is a possibility that sufficient corrosion resistance cannot be obtained. On the other hand, if the average concentration of the rust preventive agent in the coating film exceeds 15%, the effect of improving the corrosion resistance due to the addition of the rust preventive agent is saturated, which is not preferable in terms of cost. The average concentration of the rust preventive agent in the coating film may be 5% or more, 7% or more, or 10% or more by mass%, and therefore, preferably 5% or more and 15% or less, more preferably 7%. It is not less than 15% and more preferably not less than 10% and not more than 15%.

  As used herein, "average concentration of rust inhibitor in coating" is determined by the following method. First, the cross section of the steel sheet having the coating film is observed by TEM, and from the randomly selected position on the surface of the coating film, it is directed to the Zn-based alloy plating layer in the direction (thickness direction) perpendicular to the surface of the coating film. Draw a straight line. Then, the thickness of the coating film is divided into 11 equal parts on the straight line and divided into 11 regions. Then, the concentration of the rust preventive agent, that is, the total concentration of the elements of P, V, and Mg, for example, is measured in 10 regions of the coating film excluding the region closest to the Zn-based alloy plating layer from that region. Then, the measured values are averaged and determined. The concentration of the rust preventive agent at each position can be measured by elemental analysis using an energy dispersive X-ray spectrometer (EDS) attached to the SEM or TEM.

  In the present invention, the concentration of the rust preventive agent in the coat film at a position 10 nm away from the interface between the Zn-based alloy plating layer and the coat film is 1.5 times or more the average concentration of the rust preventive agent in the coat film 5 0.0 times or less. That is, in the coating film, the rust inhibitor is concentrated in the region near the interface between the coating film and the Zn-based alloy plating layer. Thus, when the rust preventive agent is concentrated in the area near the interface between the coating film and the Zn-based alloy plating layer as compared with other areas, the concentrated area of the rust preventive agent becomes Zn against corrosion factors such as oxygen. It becomes possible to act as a barrier region for the system alloy plating layer. Therefore, it is possible to suppress corrosion of the Zn-based alloy plating layer by a corrosion factor to a minimum, and the surface-treated steel sheet can have extremely excellent corrosion resistance. Further, due to the concentrated region of the rust preventive agent as described above, it becomes possible to sufficiently maintain the corrosion resistance even after processing the surface-treated steel sheet.

  If this value is less than 1.5 times, in the coating film, in the vicinity of the interface between the coating film and the Zn-based alloy plating layer side, a corrosion factor passes and corrodes the Zn-based alloy plating layer. The effect as a barrier region to suppress may be weakened, the corrosion factor may reach the Zn-based alloy plating layer, and the coating film may not provide sufficient corrosion resistance. On the other hand, if this value is more than 5.0 times, the degree of thickening in the concentrated area of the rust preventive agent is too high, and therefore, when the surface-treated steel sheet is processed, the coating film is applied in the concentrated area of the rust preventive agent. May cause cohesive failure. If so, the work adhesion decreases, and as a result, the corrosion resistance in the processed portion may not be maintained, and the corrosion resistance may become insufficient. The concentration of the rust inhibitor in the coating film at a position 10 nm away from the interface between the Zn-based alloy plating layer and the coating film is 1.7 times or more and 2.0 times or more the average concentration of the rust agent in the coating film. , Or 2.2 times or more, and 4.8 times or less, 4.5 times or less, 4.2 times or less, 4.0 times or less or 3.5 times or less, preferably It is 2.0 times or more and 4.5 times or less, more preferably 2.0 times or more and 4.0 times or less, and further preferably 2.5 times or more and 4.0 times or less.

  The “concentration of the rust preventive agent in the coating film at a position 10 nm away from the interface between the Zn-based alloy plating layer and the coating film” is determined from the cross section of the steel sheet having the coating film using TEM-EDS. Specifically, from the TEM image of the observed cross section, five positions 10 nm apart from the interface between the randomly selected Zn-based alloy plating layer and the coating film in the direction perpendicular to the surface of the coating film were located. Then, the concentration of the rust preventive agent (that is, the total concentration of P, V, and Mg elements, for example) is measured by TEM-EDS, and the measured values are averaged to determine.

  As described above, the acidic coating material having a pH of 3.0 to 5.0 removes the oxide film on the surface of the Zn-based alloy plating layer, so that the component (for example, P) of the rust preventive agent contained in the coating film in the present invention. And a component (for example, Zn) contained in the Zn-based alloy plating layer reacts near the interface between the coating film and the Zn-based alloy plating layer, and a reaction product (for example, Zn and P) is generated in a region near the interface. Reaction product containing) is formed. In the area where this reaction product is present, both the components of the rust preventive agent that are uniformly dispersed in the coating film and the components of the rust preventive agent that make up the reaction product are present, as in the other areas. are doing. Therefore, in the surface-treated steel sheet according to the present invention, the rust preventive agent (for example, P) is concentrated in the coating film in the region near the interface between the coating film and the Zn-based alloy plating layer as compared with other regions. Exists.

  The area where such a reaction product is present can be measured using an elemental analysis method known to those skilled in the art. Specifically, for example, when P is contained as a rust preventive, when elemental analysis is performed from the surface of the coating film toward the Zn-based alloy plating layer in a direction perpendicular to the surface of the coating film, that is, in the thickness direction, A region where P as a component of the rust preventive is concentrated can be measured near the interface between the coating film and the Zn-based alloy plating layer. Further, the P concentration region measured in this manner is analyzed by a method known to those skilled in the art for measuring the bond energy between atoms, whereby P of the rust preventive component and the component of the Zn-based alloy plating layer are analyzed. The reaction product with Zn or Al can be measured.

(Bright pigment)
In the surface-treated steel sheet according to the present invention, it is preferable that, in addition to the above-mentioned rust preventive agent, a bright pigment is contained in the coating film in order to improve designability. As used herein, "bright pigment" means a pigment that reflects light on its surface. As the bright pigment, one that does not dissolve in the acidic coating material for producing the coating film and is contained in the coating film as it is added to the coating material is used. Therefore, in the present invention, "containing a bright pigment" in the coating film means that the coating contains a metal simple substance, an oxide, or an alloy described below, and in the coating film, the bright pigment is included. And the binder resin forming the coating film can be clearly distinguished and specified. Therefore, the “concentration” of the bright pigment described below means the total concentration of a simple metal, an oxide, an alloy, or the like described below.

  One of the reasons for improving the design is that Zn-based alloy-plated steel sheets used for building materials and outdoor home appliances are generally used in places where users can see them. This is because the base alloy-plated steel plate preferably has good visual quality (appearance). Particularly, in the case of a design in which the bright pigment is close to the Zn-based alloy plating layer, the unevenness of the coating film thickness is less noticeable or the flaw is less noticeable. Therefore, the coating film thickness can be reduced, which is economically preferable.

  Therefore, by using the bright pigment as described above in the coating film, it is possible to improve the brightness of the surface-treated steel sheet due to its metallic appearance (for example, silver color), and a surface having excellent appearance and high designability. It is possible to provide a treated steel sheet. Furthermore, when the bright pigment has the same or similar color tone as that of the Zn-based alloy plating layer, it is possible to make the change in appearance due to scratches less noticeable when the coating film is scratched, thus improving scratch resistance. Therefore, the excellent appearance of the surface-treated steel sheet according to the present invention can be maintained for a long period of time.

  When the surface-treated steel sheet according to the present invention is observed from the direction perpendicular to the surface of the coating film by containing the bright pigment in the coating film, the bright pigment makes the underlying Zn-based alloy plating layer invisible. be able to. In this case, for example, when Zn contained in the Zn-based alloy plating layer is oxidized under the influence of oxygen in the air to form oxygen-deficient Zn oxide and the Zn-based alloy plating layer turns black. However, the blackening can be made invisible by the bright pigment, and the design of the surface-treated steel sheet according to the present invention can be maintained.

  The bright pigment in the present invention is not particularly limited as long as it can be used in an acidic coating material having a pH of 3.0 to 5.0 used in the present invention, that is, it does not dissolve in this pH range, and examples thereof include aluminum and oxides. Can be used. Examples of oxides include, but are not limited to, alumina, silica, mica, zirconia, titania, glass, zinc oxide and the like. These pigments are coated with a metal oxide such as silica and have a metallic appearance (also called metallic appearance). These can be used alone or in combination in the coating film.

  As the bright pigment in the present invention, in addition to the above-mentioned aluminum or oxide, a metal capable of providing high brightness can be further added to the coating film. Examples of such a metal are metals having high brightness and are not particularly limited as long as they can be used in an acidic paint, and examples thereof include Rh (rhodium), Cr (chrome), Ti (titanium), Examples include simple metals such as Ag (silver) and Cu (copper), alloys such as Zn-Cu (brass), and the like. These metals can be used alone or in combination in the coating film. By including a metal capable of providing such a high brightness in the coating film, it becomes possible to further enhance the metallic appearance of the coating film, and therefore, the brightness of the surface-treated steel sheet according to the present invention can be further improved. The designability of can be further improved.

  The average particle diameter of the bright pigment in the present invention is not particularly limited, but may be, for example, in the range of 1 μm or more and 30 μm or less. When the average particle diameter of the bright pigment is in the range of 1 μm or more and 30 μm or less, it is possible to provide sufficient designability while maintaining the corrosion resistance without causing uneven brightness. When the average particle diameter of the bright pigment is less than 1 μm, it becomes difficult to disperse the paint uniformly in the coating material for forming the coating film of the present invention, and the formed coating film has unevenness in color tone and is sufficiently designed. There is a case that the sex cannot be guaranteed. On the other hand, when the average particle diameter of the bright pigment is more than 30 μm, the bright pigment may project from the surface of the coating film, and a corrosion factor may enter from the projected portion, resulting in deterioration of corrosion resistance. Furthermore, if such a protruding portion is present, it becomes difficult to have a uniform appearance, and the designability may be insufficient. The average particle diameter of the bright pigment may be 2 μm or more or 3 μm or more, and may be 25 μm or more, 20 μm or less or 15 μm or less, preferably 3 μm or more and 25 μm or less, more preferably 3 μm or more and 20 μm or less, It is preferably 3 μm or more and 15 μm or less.

As used herein, the "average particle size" for the bright pigments according to the invention can be determined by the following method, as an example. A mapping image of the elements constituting the bright pigment is obtained from a direction perpendicular to the surface of the coating film by a field emission-electron probe micro analyzer (FE-EPMA). The area of the measurement range of the mapping image is 20 mm × 20 mm or more. The contour of the bright pigment existing in the measurement range is specified from the obtained mapping image, and the total area S surrounded by the contour is determined. Further, the number N of bright pigments existing in the measurement range is determined. Then, assuming that the obtained area S is composed of N bright pigments each having a circular cross section with a diameter (particle diameter) D, the average particle diameter of the bright pigment is [D = 2 × (S / (ΠN)) ^0.5 ].

  The shape of the bright pigment in the present invention may be any shape, and may be, for example, a spherical shape, an elliptical shape, a needle shape, a flat shape, a thin plate shape, a scale shape, or the like. Preferably, the bright pigment may have a scaly shape. When the shape of the bright pigment in the present invention is scale-like, the bright pigment can effectively make the underlying Zn-based alloy plating layer invisible, that is, the product is effectively blackened by the Zn-based alloy plating layer. It is possible to provide a surface-treated steel sheet that can suppress changes in appearance and that is extremely excellent in design.

  The average concentration of the bright pigment in the coating film can be, for example, 5 to 15% by mass. By the average concentration of the bright pigment in the coating film in such a range, it is possible to provide a uniform metallic appearance to the surface-treated steel sheet according to the present invention without impairing the processability of the coating film, It is possible to provide a surface-treated steel sheet excellent in designability. If the average concentration of the bright pigment in the coating film is less than 5%, the amount of the bright pigment in the coating film is insufficient, a sufficient metallic appearance cannot be provided, the brightness becomes insufficient, and sufficient designability can be provided. It may disappear. On the other hand, if the average concentration of the bright pigment in the coating film exceeds 15%, the improvement in brightness due to the addition of the bright pigment is saturated, which is not preferable in terms of cost. In addition, the presence of a large amount of bright pigment in the coating film may relatively reduce the proportion of the binder resin constituting the coating film, resulting in a decrease in processability such as cracking of the coating film when processed. . The average concentration of the bright pigment in the coating film is preferably 5% or more and 12% or less, more preferably 6% or more and 10% or less.

  As used herein, the "average concentration of the bright pigment in the coating film" can be determined by a known method. For example, it can be measured by using a glow discharge optical emission spectroscopy (GD-OES). Specifically, when the kind of the bright pigment, that is, the specific compound of the bright pigment is known, first, the coating film is sputtered from the surface toward the Zn-based alloy plating layer to form the main pigment constituting the bright pigment. For the element, the concentration profile in the depth direction is measured every 1.0 μm. Then, the average value of the measured concentrations of the main elements is calculated, the measured concentration is converted based on the molecular weight of the compound of the known coloring pigment, and the average concentration of the bright pigment in the coating film is calculated. Further, the coating film is peeled mechanically or chemically, and the total mass of the coating film is measured. Then, the concentration of the bright pigment contained in the peeled coating film is measured by analysis. As a method for analyzing the concentration of the bright pigment in the peeled coating film, for example, induction plasma emission analysis (ICP) or fluorescent X-ray analysis can be used. When the type of the bright pigment, that is, the specific compound of the bright pigment is unknown, the elements constituting the bright pigment are analyzed by FE-EPMA with respect to the cross section of the coating film (the surface perpendicular to the surface of the coating film). After specifying the type of the bright pigment by doing so, the "average concentration of the bright pigment in the coating film" can be measured as described above. When the bright pigment is brass, which is an alloy, the total content (concentration) of Cu and Zn is the average concentration of the bright pigment in the coating film.

  In the coating film of the present invention, a pigment other than the rust preventive agent and bright pigment of the present invention, an aggregate and the like can be added, if necessary. In addition, wax such as polyethylene wax or PTFE wax, resin beads such as acrylic resin beads or urethane resin beads, and dyes such as phthalocyanine blue, phthalocyanine green, methyl orange, methyl violet, or alizarin may be used in the coating film. It can be added. Addition of these is more preferable because the strength of the coating film can be increased and a desired color can be imparted to the coating film. The addition amount of these may be appropriately determined so as not to be disadvantageous for the coating film of the present invention.

  In particular, a dye can be used as a colorant for imparting a desired color to the coating film of the present invention, and thus the surface-treated steel sheet of the present invention. The dyes may be used alone, or a plurality of dyes may be used in combination. Further, the dye may be used in combination with the color pigment. The type of dye that can be used in the coating film in the present invention is not particularly limited, but known dyes can be used, for example, phthalocyanine blue, phthalocyanine green, methyl orange, methyl violet, or alizarin is used. be able to.

[Method of manufacturing surface-treated steel sheet]
The method for manufacturing the surface-treated steel sheet according to the present invention will be described below. The surface-treated steel sheet according to the present invention, for example, on a Zn-based alloy plating layer formed on the steel sheet, apply an acidic paint of pH 3.0 to 5.0 containing at least a rust inhibitor and a binder resin, It can be manufactured by heating to cure the paint.

<Formation of Zn-based alloy plating layer>
As the steel plate, one having any plate thickness and chemical composition can be used. For example, a cold rolled steel plate having a plate thickness of 0.25 to 3.5 mm can be used. Further, the Zn-based alloy plating layer can be formed to a thickness of 5 to 30 μm using, for example, a Zn-Al-Mg hot-dip bath or a Zn-Al-Mg-Si hot-dip bath at 400 to 550 ° C. .

<Preparation of paint>
The paint is obtained by, for example, mixing a binder resin dispersed in a solvent and a curing agent, and then dispersing a predetermined amount of a rust inhibitor source and optionally a bright pigment in the mixture. You can The order of mixing may be different. The binder resin is not particularly limited, but a polyester resin, a urethane resin, an acrylic resin, or the like can be used, and a melamine resin or the like can be used as a curing agent. Further, an acidic solvent can be used as the solvent, and a rust preventive agent source that can be dissolved in the acidic solvent, for example, a P compound, a V compound, a Mg compound or two or more thereof can be used. On the other hand, the bright pigment can be appropriately selected from pigments that do not dissolve in an acidic solvent. The ratio of the binder resin to the curing agent can be appropriately determined, but can be, for example, in the range of 1: 1 to 9: 1.

  It is important that the pH of the coating material used to obtain the coating film in the present invention is 3.0 or more and 5.0 or less. By setting the pH of the coating material in such a range, not only the rust inhibitor source can be dissolved in the coating material, but also when such coating material is applied to the Zn-based alloy plating layer, the Zn-based alloy plating layer The oxide film on the surface of the can be removed properly. Then, in the vicinity of the surface of the Zn-based alloy plating layer, the components of the rust preventive agent in the ionic state react with the components in the Zn-based alloy plating layer, and as a result, after the coating material is cured, the Zn-based alloy plating layer It becomes possible to form a region where the reaction product is concentrated near the interface between the coating film and the coating film. If the pH of the paint is less than 3.0, the degree of thickening in the concentrated area of the rust preventive agent becomes too high, and when the surface-treated steel sheet is processed, the coating film cohesively fails in the concentrated area of the rust preventive agent. There is a case. If so, the work adhesion decreases, and as a result, the corrosion resistance in the processed portion may not be maintained, and the corrosion resistance may become insufficient. Further, Zn may be eluted in the paint to reduce the storage stability of the paint. On the other hand, if the pH of the coating exceeds 5.0, the oxide film on the surface of the Zn-based alloy plating layer cannot be sufficiently removed, and the rust preventive agent is formed in the region near the interface between the coating film and the Zn-based alloy plating layer. May not be fully concentrated. Further, if the pH is alkaline, that is, more than 7.0, the coating material solidifies (gels) during the preparation of the coating material, resulting in lack of storage stability as the coating material and a problem in use. The pH of the paint may be 3.2 or higher or 3.5 or higher, and may be 4.8 or lower or 4.5 or lower. The pH of the coating material is preferably 3.2 to 4.8, more preferably 3.5 to 4.5. Note that the pH cannot be measured after the coating material is cured to form a coating film.

  The pH of the paint may change depending on the production lot of the raw material solvent and the like. Therefore, it is necessary to adjust the pH using an acid or alkaline aqueous solution. More specifically, after measuring the pH of the paint after preparation, nitric acid, hydrochloric acid or sulfuric acid may be used to lower the pH value depending on the target pH, and an aqueous sodium hydroxide solution may be used to raise the pH value. Etc. can be used. These acid or alkali aqueous solutions are preferably diluted before use for pH adjustment.

<Formation of coating film>
Next, the obtained coating material is applied onto the Zn-based alloy plating layer so that the coating film has a predetermined thickness, baked and cured. The method for applying the coating material is not particularly limited, and any coating method known to those skilled in the art may be used. For example, a roll coater or the like may be used. Baking can be performed under arbitrary heating conditions for curing the paint, for example, heating is performed at a heating rate of 5 to 70 ° C./sec to a steel plate temperature of 180 to 230 ° C.

  As described above, in the surface-treated steel sheet according to the present invention, the rust preventive agent containing, for example, P, V or Mg exists as a fine compound in the coating film. In order to have such a structure, in the method for producing a surface-treated steel sheet according to the present invention, in order to allow the rust preventive agent to exist in the paint in an ionic state, the rust preventive agent source (for example, P compound, V compound or Mg compound) is dissolved to prepare a coating material for forming a coating film in the present invention. The present inventors have found that the use of such a manufacturing method is advantageous in the following points.

  For example, unlike the present invention, when the rust preventive pigment is contained as a solid component (for example, powder) in the coating film, the coating film is formed in order to uniformly distribute the rust preventive pigment in the formed coating film. It is considered necessary to uniformly disperse the rust preventive pigment in the paint for forming the. Moreover, in such a manufacturing method, if a large amount of rust preventive pigment is added to the paint, it becomes difficult to uniformly disperse the rust preventive pigment in the paint, and further, the resin as the main component of the formed coating film. It is thought that there is an upper limit to the amount of the rust preventive pigment added to the coating film, since the ratio may decrease and the coating film may become brittle. In addition, such a coating composition has a dispersion state that is deteriorated during storage of the coating composition until it is used after the coating composition is prepared by dispersing the anticorrosion pigment. There are problems such as not being able to obtain a film.

  Further, for example, unlike the present invention, when a compound that dissolves in an alkaline solvent is used as a source of a rust preventive agent and an alkaline coating material for a coating film is prepared, the addition amount of the compound is increased. In some cases, the source of the rust preventive agent is not sufficiently dissolved and solid matter may be generated in the paint. In addition, the paint may harden (gelate) during storage of the paint, which poses a problem of storage stability of the paint during storage. Further, it is considered that even if the alkaline coating is applied on the Zn-based alloy plating layer, the oxide film on the Zn-based alloy plating layer cannot be sufficiently removed.

  On the other hand, in the present invention, an acidic coating material and a compound that dissolves in the coating material as a rust preventive source are used, and the compound is dissolved in the acidic coating material. Therefore, there is no limitation on uniformly dispersing the components of the rust preventive agent in the coating material as in the case of using a powdered rust preventive pigment. Therefore, in such a manufacturing method, as compared with a paint containing a rust preventive pigment such as powder, many rust preventive agents can be added to the paint in a state in which the rust preventive agent is uniformly dispersed. Further, the acidic paint having a pH of 3.0 to 5.0 for forming the coating film in the present invention is a paint which is hard to be solidified as compared with an alkaline paint even when a large amount of a rust preventive source is added to the paint. It has excellent storage stability. As described above, the coating material for forming the coating film in the present invention can have many rust preventive agent sources added while having the storage stability of the coating material. It is possible to form a coating film containing a rust preventive agent. Therefore, by forming a coating film using such a coating material, it becomes possible to form a surface-treated steel sheet having extremely excellent corrosion resistance.

  Further, as described above, when the present inventors applied such an acidic coating material having a pH of 3.0 to 5.0 on the Zn-based alloy plating layer, it was formed on the surface of the Zn-based alloy plating layer. The oxide film is removed by the coating material, and the components of the rust preventive agent and the components in the Zn-based alloy plating layer react with each other, and as a result, the rust preventive agent is formed in the region near the interface between the coating film and the Zn-based alloy plating layer. It has been found that a reaction product with a metal in the Zn-based alloy plating layer (for example, a reaction product of P and Zn) is formed. This removal of the oxide film is due to the fact that the coating material used in the present invention applied on the Zn-based alloy plating layer is acidic. Then, the removal of the oxide film exposes the active metal under the oxide film of the Zn-based alloy plating layer, and the active metal reacts with the components of the rust preventive agent in the coating film to form the reaction product. It In the region where the reaction product thus generated is present, the rust preventive agent is more concentrated than in other regions. Therefore, the concentrated region acts as a barrier region that prevents a corrosion factor from entering the Zn-based alloy plating layer, and thus the surface-treated steel sheet according to the present invention can have extremely high corrosion resistance.

  The surface-treated steel sheet according to the present invention, that is, the concentration of the rust inhibitor in the coating film at a position 10 nm away from the interface between the Zn-based alloy plating layer and the coating film is 1 times the average concentration of the rust inhibitor in the coating film. The surface-treated steel sheet that is 5 times or more and 5.0 times or less uses an acidic paint having a pH of 3.0 to 5.0, and further has various parameters during production, such as the type of rust preventive agent in the paint, Manufacture by appropriately adjusting the addition amount of rust preventive agent, coating temperature, heating temperature and heating time for curing coating, ratio of binder resin and curing agent, pretreatment of alloy plating layer, etc. You can That is, by using an acidic paint having a pH of 3.0 to 5.0 containing a predetermined amount of a rust preventive component and optionally a bright pigment, and appropriately adjusting such parameters, the anti-corrosion effect in the coating film can be improved. It is possible to adjust the degree of concentration of the rust agent, and thus it is possible to manufacture the surface-treated steel sheet according to the present invention.

  Further, the oxide film of the Zn-based alloy plating layer is removed, and the active metal of the Zn-based alloy plating layer reacts with the components in the coating material, resulting in a strong chemical reaction between the Zn-based alloy plating layer and the coating film. As a result, a surface-treated steel sheet having excellent adhesion between the Zn-based alloy plating layer and the coating film can be obtained. More specifically, without being bound to a particular theory, the components of the rust inhibitor in the paint react to form hydroxides, and the functional groups of the hydroxide react with the resin to cause irreversible As a result, the adhesiveness is improved between the Zn-based alloy plating layer and the coating film. Such adhesion cannot be achieved, for example, when a neutral or alkaline paint is used for forming the coating film, and therefore an acidic paint having a pH of 3.0 to 5.0 is used for forming the coating film. In this case, the adhesion is improved as compared with the case where a neutral or alkaline paint is used.

  The surface-treated steel sheet according to the present invention can be manufactured by using the manufacturing method as described above. That is, a steel sheet, a Zn-based alloy plating layer formed on at least one surface of the steel sheet, and a coating film containing a rust inhibitor and a binder resin formed on the Zn-based alloy plating layer, and a Zn-based alloy plating layer Production of a surface-treated steel sheet in which the concentration of the rust preventive agent in the coating film at a position 10 nm away from the interface with the coating film is 1.5 times or more and 5.0 times or less the average concentration of the rust preventing agent in the coating film. can do.

  In this example, the average concentration and concentration distribution of the rust preventive agent in the coating film, the average concentration of the bright pigment, the type of the rust preventive agent and the bright pigment, the type of the binder resin, and the chemical composition of the Zn-based alloy plating layer were varied. About the surface-treated steel plate manufactured by changing, the corrosion resistance, brightness, work adhesion, and storage stability were evaluated. The surface-treated steel sheet according to the present invention will be described in more detail below with some examples. However, the particular examples described below are not intended to limit the scope of the invention as claimed.

<Preparation of surface treated steel sheet sample>
(Formation of Zn-based alloy plating layer)
A cold-rolled steel sheet having a thickness of 1 mm is immersed in a hot-dip bath having a chemical composition of Al: about 11%, Mg: about 3%, and Zn: about 86% at about 450 ° C. for 3 to 5 seconds, and then placed on the cold-rolled steel sheet. A Zn-11% Al-3% Mg alloy plating layer having a thickness of about 10 μm was formed on the substrate. Further, the composition of the hot dip plating bath was changed, and a Zn-1% Al-1% Mg alloy plating layer and a Zn-40% Al-8% Mg alloy plating having a thickness of about 10 μm were formed on the cold-rolled steel sheet by the same procedure. Layers were formed. Alternatively, a cold-rolled steel sheet having a thickness of 1 mm is placed in a hot dip bath having a chemical composition of Al: about 11%, Mg: about 3%, Si: about 1%, and Zn: about 85% at about 450 ° C. for 3 to 5 ° C. It was immersed for 2 seconds, and a Zn-11% Al-3% Mg-1% Si alloy plating layer having a thickness of about 10 μm was formed on the cold rolled steel sheet. In addition, the composition of the hot dip plating bath was changed, and a Zn-11% Al-3% Mg-0.4% Si alloy plating layer and Zn-11% Al having a thickness of about 10 μm were formed on the cold rolled steel sheet by the same procedure. A -3% Mg-1.5% Si alloy plating layer was formed.

(Preparation of paint)
A polyester resin (molecular weight: 16,000; glass transition point: 10 ° C.) and a polyurethane resin (molecular weight: 10000; glass transition point: 20 ° C.) were dispersed as an emulsion in an acidic solvent as an emulsion. Regarding the paints used in 3 to 21 and 25 to 36, nitric acid or sodium hydroxide was used to adjust the pH to 3.0 to 5.0. An imino group type melamine resin was mixed therein. The ratio of the concentrations of polyester resin and melamine resin was 100: 20. Then, a rust preventive agent source and a bright pigment were added to the mixture to prepare a coating material. Sample No. The paints used in Nos. 1, 2 and 24 were adjusted so that the pH was higher than 5.0, and the sample No. The paints used in Nos. 22 and 23 were adjusted to have a pH of less than 3.0. The pH of the paint used for each sample is shown in Table 1. And No. For No. 25, no bright pigment was added. Orthophosphoric acid, vanadium pentoxide and magnesium sulfate were used as the rust preventive agent sources for the samples containing P, V and Mg as the rust preventive agent, respectively. The bright pigments listed in Table 1 were used.

  The amount of the rust preventive agent added to the paint is determined by TEM-EDS based on the cross section of the obtained paint film, and the average concentration of the rust preventive agent in the desired paint film (3%, 5% %, 10%, 13% or 15%). Further, the concentration of the bright pigment was appropriately adjusted so that the average concentration was 10% or 5% when measured by GD-OES.

(Formation of coating film)
The coating material prepared as described above was applied onto the Zn-based alloy plating layer so that the formed coating film had an average thickness of 5 μm, and was baked to cure. The baking was performed at a heating rate of about 20 ° C./sec and a steel plate temperature of about 200 ° C. until the paint was completely cured.

  The ratio of the concentration of the rust inhibitor in the coating film at the position 10 nm away from the interface between the Zn-based alloy plating layer and the coating film with respect to the average concentration of the rust inhibitor in the coating film should be changed appropriately. I adjusted it.

  From the obtained coating film, the average concentration (mass%) of the rust preventive agent in the coating film was analyzed by elemental analysis using TEM-EDS; and from the interface between the Zn-based alloy plating layer and the coating film with respect to the average concentration. The ratio of the concentration of the rust preventive agent in the coating film at the position 10 nm apart was determined. The values thus determined are shown in Table 1. Table 1 shows the types of rust preventive agents and bright pigments contained in the coating film. When two kinds of rust preventive agents are contained in the coating film, the sum of the average concentrations of the two rust preventive agents corresponds to the average concentration shown in the table, and each rust preventive agent is contained in the coating film. Present in quantity. The same applies to bright pigments.

<Evaluation of sample of surface-treated steel sheet>
Samples of the surface-treated steel sheet were prepared as described above, and each sample as shown in Table 1 was evaluated for corrosion resistance, brightness, work adhesion and storage stability as follows.

(Corrosion resistance evaluation test)
For each sample, a 0.6 mm test material for testing was obtained by processing (7 mm extrusion) according to the Erichsen test (JIS Z2247: 2006), which is a simulation of actual use, and corrosion resistance was obtained for the test material. A salt spray test (based on the JASO M609-91 method) was performed as an evaluation test of. In this salt spray test, (1) salt spray 2 hours (5% NaCl, 35 ° C); (2) drying 4 hours (60 ° C); and (3) wet 2 hours (50 ° C, humidity 95% or more). A total of 120 cycles (total 960 hours) were performed as one cycle. In order to prevent corrosion from the end face, the end face of each sample was sealed with tape and tested.

The corrosion resistance was evaluated by observing the surface (planar portion) of the sample after 960 hours of the salt spray test with an optical microscope and determining the rust generation area ratio Z. Specifically, first, the surface of the sample was read by a scanner. Then, the area in which rust was generated was selected using image editing software, and the rust generation area ratio was calculated. This procedure was performed on five samples, and the "rust generation area ratio Z" was determined as the average of the rust generation area ratios. Based on the "rust generation area ratio Z" thus determined for each sample, the rating of each sample was determined in the following eight stages. A score of 4 or more was taken as a passing score for corrosion resistance.
Rating 8: Z = 0%
Rating 7: 0% <Z ≦ 5%
Score 6: 5% <Z ≤ 10%
Rating 5: 10% <Z ≦ 20%
Rating 4: 20% <Z ≤ 30%
Score 3: 30% <Z ≤ 40%
Rating 2: 40% <Z ≤ 50%
Rating 1: 50% <Z

(Brightness evaluation test)
For each sample, 10 randomly selected testers visually observed the surface of the sample and evaluated the “brightness level” from 1 to 5 as follows.
1 point: Metal appearance is not confirmed at all or slight metal appearance is observed 2 points: Metal appearance is confirmed, but uneven appearance is easily confirmed by observing from the front 3 points: Metal appearance is confirmed However, slight unevenness in appearance is confirmed when observed from the front. 4 points: Metal appearance is confirmed overall, but slight unevenness is observed when observed obliquely 5 points: Metal appearance is entirely observed It is confirmed

Regarding the brightness, according to the total score of the “brightness level” of the above-mentioned 10 testers, the rating of each sample was determined in the following 8 stages. A rating of 4 or more was taken as a pass point for luminance.
Score 8:40 <total score Score 7:35 <total score ≤ 40
Rating 6:30 <total score ≤ 35
Rating score 5:25 <total score ≤ 30
Score 4:20 <Total score <25
Rating 3:15 <total score ≤ 20
Rating 2:10 <total score ≤ 15
Score 1: Total score = 10

(Processing adhesion evaluation test)
As described above, a 0.6 mm test material for testing was obtained by processing (7 mm extrusion) according to the Erichsen test (JIS Z2247: 2006), which is a simulation of actual use. A cellophane adhesive tape having a width of 24 mm (Cellotape manufactured by Nichiban Co., Ltd.) was adhered to the test material on the coating film, and then rapidly peeled off at an angle of 45 degrees. The peeled area ratio Z ′ was obtained from the peeled coating film area and evaluated according to the following criteria.
Rating 5: 0% (no peeling) <Z '≦ 5%
Rating 4: 5% <Z '≤ 10%
Rating 3: 10% <Z '≤ 30%
Rating 2: 30% <Z '≦ 50%
Score 1: 50% <Z '

(Evaluation test for storage stability)
100 g of the coating material prepared at the pH shown in Table 1 was maintained at 25 ° C, and a Zn-11% Al-3% Mg alloy plated steel sheet was immersed therein. The paint after 60 minutes of immersion was visually observed, and the storage stability score of each sample was determined as follows according to the state of the paint before immersion (during paint preparation) and after immersion. A score of 3 or more was regarded as a passing score for storage stability.
Rating 5: No change in paint before or after immersion in steel plate Rating 4: Discoloration or increased viscosity in paint before or after immersion in steel plate Score 3: Change in paint or increase in viscosity before or after immersion in steel plate Both are recognized. Rating 2: The coating solidifies (gels) after the steel plate is dipped.
Rating 1: Solidification (gelation) before immersion (during paint preparation)

  The samples of the surface-treated steel sheet were evaluated for corrosion resistance, brightness, work adhesion and storage stability as described above, and their respective scores were determined. Table 1 shows the obtained results.

  Sample No. In Nos. 1 and 2, the pH of the coating was high, and the ratio of the concentration of the rust preventive agent at the position 10 nm away from the interface between the Zn-based alloy plating layer and the coat film was 1 with respect to the average concentration of the rust preventive agent in the coat film. Since it was less than 0.5, the concentration of the rust preventive agent was insufficient, the concentrated region did not function sufficiently as a barrier layer for protecting the Zn-based alloy plating layer, and the corrosion resistance was insufficient. In addition, the sample No. In Nos. 22 and 23, the pH of the coating was low, and the ratio of the concentration of the rust preventive agent at the position 10 nm away from the interface between the Zn-based alloy plating layer and the coat was 5 with respect to the average concentration of the rust preventive agent in the coat. Since it was more than 0.0, the corrosion resistance was insufficient. This is because, when processed to obtain the test material, the coating film cohesively fails in the concentrated region of the rust preventive agent, the processing adhesion is lowered, and as a result, the corrosion resistance in the processed portion is deteriorated. it is conceivable that. Sample No. In No. 24, the pH of the paint was alkaline, and the paint solidified during preparation of the paint, and a coating film could not be formed, and therefore corrosion resistance, brightness, and processing adhesion could not be evaluated.

  On the other hand, sample No. 3-21, No. In Nos. 25 to 36, the ratio of the concentration of the rust preventive agent at the position 10 nm away from the interface between the Zn-based alloy plating layer and the coat film was 1.5 or more to 5.0 with respect to the average concentration of the rust preventive agent in the coat film. Since it was below, it had excellent corrosion resistance. In particular, the sample containing one or both of P and V as the rust preventive had better corrosion resistance.

Then, the sample No. All of the samples except 25 had sufficient brightness because the coating film contained the bright pigment. Further, in the sample in which the bright pigment contains one or both of aluminum (Al) and oxide (SiO ₂ , alumina, mica), the brightness was more excellent. In particular, the sample further containing, in addition to Al or SiO ₂ , metal Rh, Ti or Ag having high brightness in the coating film had extremely high brightness.

  Sample No. 14 to 17 and No. No. 35 is a sample in which the average concentration of the rust preventive agent in the coating film was changed. All samples had sufficient corrosion resistance.

  According to the present invention, a surface-treated steel sheet having high corrosion resistance can be provided because it has a concentrated region of the rust inhibitor near the interface between the Zn-based alloy plating layer and the coating film. As a result, it becomes possible to provide sufficient corrosion resistance and designability as a steel sheet used for building materials and home appliances products, and therefore the present invention can be said to have extremely high industrial value.

Claims (7)

A steel sheet, a Zn-based alloy plating layer formed on at least one surface of the steel sheet, and a coating film containing a rust preventive agent and a binder resin formed on the Zn-based alloy plating layer,
The chemical composition of the Zn-based alloy plating layer is% by mass,
Al: 0.01-60%,
Mg: 0.001 to 10%, and Si: 0 to 2%,
The rust inhibitor is at least one of P, V and Mg,
The total average concentration of P, V and Mg in the coating film is 3 to 15% by mass,
The concentration of the rust-preventive agent in the coating film at a position 10 nm away from the interface between the Zn-based alloy plating layer and the coating film is 1.5 to 5 of the average concentration of the rust-preventing agent in the coating film. A surface-treated steel sheet characterized by being 0.0 times. The surface-treated steel sheet according to claim 1, wherein the total average concentration of P, V and Mg in the coating film is 5 to 15% by mass. The surface-treated steel sheet according to claim 1 or 2 , wherein the coating film further contains a bright pigment, and the bright pigment contains at least one of aluminum and an oxide. The surface treated steel sheet according to claim 3 , wherein the oxide is alumina, silica, mica, zirconia, titania, glass, or zinc oxide. The surface-treated steel sheet according to claim 3 or 4 , wherein the bright pigment further contains at least one of Rh, Cr, Ti, Ag, and Cu. The average concentration of the bright pigment in the coating film, by mass%, characterized in that 5 to 15%, the surface treated steel sheet according to any one of claims 3-5. The surface-treated steel sheet according to any one of claims 1 to 6, wherein the binder resin is a polyester resin.