Surface-treated steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide a surface-treated steel sheet having excellent film corrosion resistance and post-paint corrosion resistance and exhibiting good appearance even when produced without using hexavalent chromium, and a method for producing the surface-treated steel sheet.　To solve the above problem, the surface-treated steel sheet of the present invention comprises a base steel sheet and a chromium-containing layer formed on at least one surface of the base steel sheet. The chromium-containing layer comprises a metallic chromium layer and a chromium oxide layer formed on the surface of the metallic chromium layer. The chromium oxide layer is characterized in that it has a chromium-converted coating weight of 1.0 to 100.0 mg / m2 per side and has a thick-film portion that protrudes in an island-like manner and has a specific area.

Description

Surface-treated steel sheet and method for manufacturing the same

[0001] The present invention relates to a surface-treated steel sheet, and more particularly to a surface-treated steel sheet that is excellent in film adhesion, paint corrosion resistance, and appearance. The surface-treated steel sheet of the present invention is suitable for use in containers such as cans.

[0002] Sn-plated steel sheets (tinplate) have excellent corrosion resistance, weldability, and workability, and are easy to manufacture, so they have been used for over 200 years as a material for various metal cans such as beverage cans, food cans, pails, and 18-liter cans. However, because tin is a relatively expensive material, surface-treated steel sheets that do not use tin, so-called tin-free steel sheets (TFS), have been developed. Tin-free steel sheets are typically surface-treated steel sheets in which a metallic chromium layer and a chromium oxide layer are sequentially formed on the surface of the steel sheet, and are manufactured by electrolytically treating the steel sheet in an electrolyte containing hexavalent Cr (Patent Document 1). Because such tin-free steel sheets have excellent corrosion resistance and paint adhesion, they are now commonly used as a substitute for tinplate in containers.

[0003] Furthermore, surface-treated steel sheets used for containers are required to have excellent adhesion to paints or films. For this reason, chromate treatment is commonly applied to Sn-plated steel sheets. Chromate treatment is a type of surface treatment that uses a treatment solution containing chromium compounds such as chromic acid or chromate salts. For example, as described in Patent Document 1, it is a technique that forms a metallic chromium layer and a chromium oxide layer on the surface of a steel sheet by performing cathodic electrolysis in an electrolyte solution containing a hexavalent chromium compound.

[0004] On the other hand, in recent years, due to growing environmental awareness, there has been a global trend toward restricting the use of hexavalent chromium. Therefore, in the field of surface-treated steel sheets used in containers and the like, there is a need to establish manufacturing methods that do not use hexavalent chromium. Methods for forming surface-treated steel sheets without using hexavalent chromium are known, for example, as proposed in Patent Documents 2 and 3. In these methods, a surface treatment layer is formed by performing an electrolytic treatment in an electrolyte containing a trivalent chromium compound such as basic chromium sulfate. According to the method proposed in Patent Document 2, a surface treatment layer can be formed without using hexavalent chromium, and a surface-treated steel sheet with good adhesion to resin films in humid environments (hereinafter referred to as "film wet adhesion") and adhesion to paints in humid environments (hereinafter referred to as "paint secondary adhesion") can be obtained. Furthermore, Patent Document 3 proposes a method for depositing a chromium oxide layer without using hexavalent chromium, and a surface-treated steel sheet with improved adhesion due to the increased chromium oxide layer can be obtained.

[0005] JP-A-58-110695 Publication Special Publication No. 2016-505708 Publication Special Publication No. 2022-521962

[0006] However, when considering the use of the surface-treated steel sheet proposed in Patent Document 2 as a steel sheet for containers, its adhesion to the film and paint is insufficient, and there is a risk that subsequent corrosion resistance (film corrosion resistance, paint corrosion resistance) may deteriorate, so further improvement was desired. In addition, although surface-treated steel sheets obtained by conventional methods such as those proposed in Patent Document 3 have good wet adhesion to the film and secondary adhesion to the paint, their appearance was sometimes inferior to that of surface-treated steel sheets formed using hexavalent chromium.

[0007] In view of these circumstances, the present invention aims to provide a surface-treated steel sheet and a method for manufacturing the same that exhibits excellent film corrosion resistance and paint corrosion resistance, as well as a good appearance, even when manufactured without the use of hexavalent chromium.

[0008] The present inventors have investigated a surface-treated steel sheet comprising a base steel sheet and a chromium-containing layer formed on at least one surface of the base steel sheet in order to solve the above problems. As a result, they have found that by setting the amount of chromium oxide layer constituting the chromium-containing layer to a specific range, and by having thick film portions that protrude in an island-like manner, and by setting the thickness of the thick film portion, the area of ​​each thick film portion when observed on the surface, and the area ratio of the thick film portion to the chromium oxide layer to a specific range, it is possible to improve the film corrosion resistance and paint corrosion resistance with the chromium oxide layer, even when manufactured without using hexavalent chromium, while simultaneously suppressing interference colors on the surface and improving the appearance.

[0009] The present invention is based on the above findings, and its gist is as follows: 1. A surface-treated steel sheet comprising a base steel sheet and a chromium-containing layer formed on at least one surface of the base steel sheet, wherein the chromium-containing layer consists of a metallic chromium layer and a chromium oxide layer formed on the surface of the metallic chromium layer, and the amount of adhesion of the chromium oxide layer per side is 1.0 to 100.0 mg / m² in terms of chromium ２ Furthermore, it has thick film portions that protrude in an island-like manner, the thickness of the thick film portions is 200 nm or less, and the area of ​​each thick film portion when the surface of the chromium oxide layer is observed is 0.010 mm². ２ A surface-treated steel sheet characterized by the following, and wherein the area ratio of the thick film portion to the chromium oxide layer is 3 to 80%. 2. The amount of the chromium-containing layer deposited on one side is 50 to 600 mg / m² in terms of chromium. ２ The surface-treated steel sheet according to item 1, characterized in that the atomic concentration of solid-solution carbon present in the chromium-containing layer is 0.2 to 50.0%. 3. The surface-treated steel sheet further comprising a sn-based plating layer between the base steel sheet and the chromium-containing layer, wherein the amount of sn-based plating layer deposited per side is 0.1 to 20.0 g / m². ２ A surface-treated steel sheet according to 1 or 2 above, characterized in that... 4. A Ni-based plating layer is further provided between the base steel sheet and the Sn-based plating layer, wherein the amount of Ni-based plating layer deposited on one side is 2 to 2000 mg / m² ２The surface-treated steel sheet according to item 3 above, characterized in that it is a base steel sheet and a chromium-containing layer formed on at least one surface of the base steel sheet, comprising: an electrolyte preparation step of preparing an aqueous solution by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, and then adjusting the pH of the aqueous solution to 4.0 to 7.0 and the temperature to 40 to 70°C to obtain an electrolyte containing trivalent chromium ions; and a film formation step of performing a cathode electrolytic treatment C1 on the base steel sheet using the electrolyte containing trivalent chromium ions, and then performing a cathode electrolytic treatment C2 using a second electrolyte containing a trivalent chromium ion source and a buffer.

[0010] According to the present invention, even when manufactured without using hexavalent chromium, it is possible to provide a surface-treated steel sheet and a method for manufacturing the same that exhibits excellent film corrosion resistance and paint corrosion resistance, as well as a good appearance.

[0011] This figure schematically shows a cross-section of one embodiment of the surface-treated steel sheet according to the present invention. This figure schematically shows a cross-section of another embodiment of the surface-treated steel sheet according to the present invention, where (a) is an embodiment further comprising a Sn-based plating layer, and (b) is an embodiment further comprising a Sn-based plating layer and a Ni-based plating layer. This is an image obtained when the surface of the chromium oxide layer was photographed with a scanning electron microscope (SEM), and the captured backscattered electron image was binarized and image analysis was performed, where (a) is the image before binarization and (b) is the image after binarization. This is a curve showing the relationship between the distance from the surface of the chromium-containing layer and the intensity of O and Cr, obtained by line analysis of the thick film portion of the chromium oxide layer using energy-dispersive X-ray spectroscopy (EDX).

[0012] The following describes embodiments of the surface-treated steel sheet of the present invention. However, the present invention is not limited to these embodiments. The thickness and shape of each layer constituting the surface-treated steel sheet of the present invention are shown schematically in the drawings for convenience of explanation and may differ from the actual embodiment.

[0013] <Surface-treated steel sheet> The surface-treated steel sheet of the present invention comprises a base steel sheet and a chromium-containing layer formed on at least one surface of the base steel sheet. The applications of the surface-treated steel sheet of the present invention are not particularly limited, but it is particularly suitable as a surface-treated steel sheet for containers used in the manufacture of various containers such as food cans, beverage cans, pails, and 18-liter cans.

[0014] (Base Steel Sheet) The base steel sheet that forms the base of the surface-treated steel sheet of the present invention is not particularly limited, and any steel sheet can be used depending on the required performance. For example, the base steel sheet is preferably a steel sheet for cans. Also, as the base steel sheet, for example, an ultra-low carbon steel sheet or a low carbon steel sheet can be used. Furthermore, the manufacturing method of the base steel sheet is not particularly limited, and a steel sheet manufactured by any method can be used. Usually, a cold-rolled steel sheet can be used as the steel sheet. The cold-rolled steel sheet can be manufactured by a general manufacturing process, for example, hot rolling, pickling, cold rolling, annealing, and temper rolling.

[0015] (Chromium-containing layer) As shown in Figure 1, the surface-treated steel sheet of the present invention includes a chromium-containing layer 20 on the base steel sheet 10. The chromium-containing layer 20 consists of a metallic chromium layer 21 and a chromium oxide layer 22 and a thick film portion 23 of the chromium oxide layer formed on the surface of the metallic chromium layer 21. In other words, the chromium-containing layer 20 is formed sequentially from the base steel sheet 10 side, consisting of the metallic chromium layer 21, the chromium oxide layer 22 and the thick film portion 23 of the chromium oxide layer. Note that the chromium-containing layer 20 only needs to be formed on at least one surface of the base steel sheet 10, and may be formed on both surfaces.

[0016] Furthermore, if the amount of chromium-containing layer attached is small, the number of adhesion points with the film or paint may decrease, resulting in inferior film corrosion resistance and paint corrosion resistance. Therefore, the amount of chromium-containing layer attached to one side should be 50 mg / m² in terms of chromium equivalent. ２ Preferably, it is 80 mg / m² or more. ２The above is more preferred. On the other hand, regarding the adhesion amount per side of the chromium-containing layer, if the adhesion amount of the chromium-containing layer is excessive, cracks may occur in the chromium-containing layer, resulting in poor film corrosion resistance and paint corrosion resistance, so 600 mg / m ２ or less is preferable, and 500 mg / m ２ or less is more preferred.

[0017] The chromium-converted adhesion amount of the chromium-containing layer is measured by the following procedure using an X-ray fluorescence device. First, the amount of Cr (total Cr amount) in the surface-treated steel sheet is measured using an X-ray fluorescence device. Next, the amount of Cr (base sheet Cr amount) in the steel sheet before forming the chromium-containing layer or the steel sheet after removing the chromium-containing layer is measured using an X-ray fluorescence device. The value obtained by subtracting the base sheet Cr amount from the total Cr amount is taken as the Cr adhesion amount of the chromium-containing layer. For removing the chromium-containing layer, for example, a commercially available chromium plating remover such as a hydrochloric acid-based one can be used.

[0018] The metallic chromium layer constituting the chromium-containing layer is a layer made of unoxidized metallic Cr. The metallic Cr constituting the metallic chromium layer may be amorphous Cr or crystalline Cr. That is, the metallic chromium layer can contain one or both of amorphous Cr and crystalline Cr. A metallic chromium layer produced by the method described later generally contains amorphous Cr, and may further contain crystalline Cr in some cases. Although the formation mechanism of the metallic chromium layer is not clear, it is considered that partial crystallization progresses when amorphous Cr is formed, resulting in a metallic chromium layer containing both an amorphous phase and a crystalline phase.

[0019] The proportion of crystalline Cr to the total amount of amorphous Cr and crystalline Cr contained in the metallic chromium layer is preferably 80% or less, and more preferably 50% or less. Here, the proportion of crystalline Cr can be measured by observing a cross-section along the stacking direction of the metallic chromium layer with a scanning transmission electron microscope (STEM). Specifically, a STEM image is acquired at a magnification of about 2 million to 10 million times with a beam diameter that can obtain a resolution of 1 nm or less, and in the obtained STEM image, the region in which lattice fringes can be confirmed is considered the crystalline phase, and the region in which maze patterns can be confirmed is considered amorphous, and the areas of both are calculated. From the calculated area results, the ratio of the area of ​​crystalline Cr to the total area of ​​amorphous Cr and crystalline Cr is calculated.

[0020] Furthermore, while the thickness of the metallic chromium layer is not particularly limited, from the viewpoint of further improving corrosion resistance, it is preferable to have a thickness of 3 nm or more, more preferably 4 nm or more, and even more preferably 5 nm or more. On the other hand, while the upper limit of the thickness of the metallic chromium layer is not particularly limited, if the metallic chromium layer is excessively thick, cracks may occur in the metallic chromium layer, impairing the corrosion resistance of the film and the coating. Therefore, from the viewpoint of ensuring more stable corrosion resistance, it is preferable to have a metallic chromium layer thickness of 100 nm or less, more preferably 90 nm or less, and even more preferably 80 nm or less. The thickness of the metallic chromium layer can be measured using X-ray photoelectron spectroscopy (XPS) by the method described in the examples.

[0021] The chromium oxide layer constituting the chromium-containing layer is a layer made of Cr oxide formed on the metallic chromium layer. When this Cr oxide bonds with the film or paint, adhesion is improved, and the corrosion resistance of the film and the paint can be improved.

[0022] Here, the amount of chromium oxide layer deposited on each side is set to 1.0 mg / m² in chromium equivalent, from the viewpoint of improving film corrosion resistance and paint corrosion resistance. ２ The above is 4.0 mg / m². ２ Preferably, it is 8.0 mg / m² or more. ２is more preferably at or above. On the other hand, if the adhesion amount of the chromium oxide layer is excessive, the chromium oxide layer causes cohesive failure, which may reduce the film corrosion resistance and coating corrosion resistance. Therefore, the adhesion amount of the chromium oxide layer, in terms of chromium conversion, is 100.0 mg / m ２ or less, and is 80.0 mg / m ２ or less, preferably 60.0 mg / m ２ or less, and more preferably at or below this value.

[0023] The chromium-converted adhesion amount of the chromium oxide layer is measured by the following procedure using an X-ray fluorescence spectrometer. First, the Cr content (total Cr content) of the surface-treated steel sheet is measured. Thereafter, the surface-treated steel sheet is subjected to an alkali treatment of immersion in 7.5 N NaOH at 90°C for 10 minutes to remove the chromium oxide layer. After sufficiently washing the alkali-treated surface-treated steel sheet with water, the Cr content (Cr content after alkali treatment) is measured again using an X-ray fluorescence spectrometer, and the value obtained by subtracting the Cr content after alkali treatment from the total Cr content is taken as the adhesion amount of the chromium oxide layer.

[0024] As shown in FIG. 1, the chromium oxide layer 22 has thick film portions 23 protruding in an island shape. Generally, as the adhesion amount of the chromium oxide layer 22 increases, the surface of the surface-treated steel sheet exhibits interference color and tends to be inferior in appearance. Therefore, in the surface-treated steel sheet of the present invention, by forming the thick film portions 23 of the chromium oxide layer 22 in an island shape, adhesion can be ensured with sufficient chromium oxide in the thick film portions 23, and interference color can also be suppressed. If the film thickness of the thick film portions 23 of the chromium oxide layer 22 is excessively large, there is a risk of local interference color occurrence. Therefore, the maximum film thickness of the thick film portions 23 is 200 nm, preferably 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less.

[0025] Furthermore, the maximum film thickness of the thick film portion is not limited to the island-like protruding portions, but is the thickness of the chromium oxide layer in the portion where the thick film portion is formed. Specifically, the thick film portion can be identified and then its thickness measured and calculated as follows. First, a cross-sectional sample of the surface-treated steel sheet is prepared by focused ion beam (FIB) spectroscopy. The cross-sectional sample is observed at 5,000x magnification using a scanning transmission electron microscope (TEM), and the film thickness of the chromium oxide layer is measured at the thickest and thinnest parts of the chromium-containing layer within the 5,000x observation field. The magnification is adjusted as appropriate when measuring the film thickness of the chromium oxide layer. Then, line analysis is performed using energy-dispersive X-ray spectroscopy (EDX), and as shown in Figure 4, the interface between the metallic chromium layer and the chromium oxide layer is derived by taking the intersection of the O intensity curve and the Cr intensity curve in the O and Cr intensity curves (horizontal axis: distance, vertical axis: intensity) as the interface between the metallic chromium layer and the chromium oxide layer. The distance from the derived interface to the outermost layer of the chromium-containing layer is defined as the thickness of the chromium oxide layer. The thicknesses of the thickest and thinnest chromium oxide layers are measured in the field of view observed at 5,000x magnification. Then, by setting the magnification appropriately and taking the average of the thicknesses of the thickest (H1) and thinnest (H2) chromium oxide layers, the average thickness of the chromium oxide layer (H3) is calculated. Areas in the chromium oxide layer where the thickness is thicker than this average H3 are defined as thick-film areas. For the thick-film areas in the chromium oxide layer, the thickness of the chromium oxide layer at the locations where the thick-film areas are formed is measured using the method described above (a method for measuring the thickness of the chromium oxide layer from the interface derived from the intensity curves of O and Cr), and this is defined as the thickness of the thick-film area. The thickness is measured at 10 thick-film areas randomly selected from the observation field of view, and the value with the thickest thickness among them is defined as the maximum thickness.

[0026] Furthermore, in the treated steel sheet of the present invention, when the surface of the chromium oxide layer is observed, the area of ​​each thick film portion is 0.010 mm². ２ The following conditions apply, and the area ratio of the thick film portion to the chromium oxide layer is 3 to 80%. By setting the area and area ratio of the thick film portion on the surface of the chromium oxide layer within the above range, it is possible to achieve both film corrosion resistance, paint corrosion resistance, and appearance.

[0027] When observing the surface of the chromium oxide layer, if the area of ​​each thick film portion is too large, interference colors may appear locally. Therefore, the area of ​​each thick film portion should be 0.010 mm². ２ The following is the value: 0.008 mm ２ Preferably, the following: 0.005 mm ２ The following is more preferable: Here, "the area per thick film when the surface of the chromium oxide layer is observed is 0.010 mm²." ２ The following means that the area of ​​each individual thick film in the observation field is 0.010 mm². ２ The following conditions must be met: the maximum area of ​​each thick film in the observation field is 0.010 mm². ２ This means the following. Furthermore, the amount of chromium oxide layer attached is 1.0 mg / m². ２ As a result, adhesion between the film and the coating can be ensured, and therefore, there are no particular limitations on the maximum thickness of the thick film portion or the lower limit of the area per thick film portion.

[0028] Furthermore, if the ratio of the thick film portion to the total area of ​​the chromium oxide layer is too small, the effect of suppressing interference colors due to the presence of the thick film portion cannot be obtained. For this reason, when observing the surface of the chromium oxide layer, the area ratio of the thick film portion to the chromium oxide layer is 3% or more, preferably 5% or more, and more preferably 10% or more. On the other hand, if the area ratio of the thick film portion to the chromium oxide layer is too high, the effect of suppressing interference colors cannot be obtained, so the area ratio is 80% or less, preferably 70% or less, and more preferably 60% or less.

[0029] The maximum area per thick film portion and the area ratio of the thick film portions are measured and calculated as follows. First, carbon deposition is applied to the surface of the surface-treated steel sheet to create an observation sample. Then, as shown in Figure 3, a backscattered electron image is taken at 1,000x magnification with a scanning electron microscope (SEM) (Figure 3(a)), and the captured backscattered electron image is binarized using software (product name: ImageJ) to perform image analysis (Figure 3(b)) to obtain the area per thick film portion and the area ratio. On the surface of the chromium oxide layer, the oxygen concentration is low in the parts other than the thick film portions, and these appear bright in the backscattered electron image, while the thick film portions appear dark (Figure 3(b)). By binarizing the dark areas of the backscattered electron image, it becomes possible to measure the thick film portions. If the contrast is unclear, oxygen-enriched areas can also be extracted and analyzed using surface analysis by energy-dispersive X-ray spectroscopy (EDX). At this time, the binarization threshold is appropriately set so that only the thick film portion defined during the measurement of the maximum film thickness can be extracted. The area of ​​each thick film portion is the maximum area among all observed fields when five fields of view are captured at 1,000x magnification, and the area ratio is the average value of the five fields of view captured at 1,000x magnification.

[0030] Furthermore, the chromium-containing layer inevitably contains carbon (C). This is thought to be because, in the process of forming the chromium-containing layer on the base steel plate, if organic compounds such as carboxylic acid compounds are included in the electrolyte, the organic compounds decompose and are incorporated into the film. The atomic ratio of C to Cr in the chromium-containing layer is 0.2% or more and 50.0% or less. The atomic ratio of C inevitably included in the film during the chromium-containing layer formation process is 0.2% or more. The atomic ratio of C changes depending on the electrolysis conditions, but if it is 50.0% or less, it does not affect the film performance. Therefore, the atomic ratio of C is set to 50.0% or less. If the atomic ratio of C is excessive, the film may become brittle and the film corrosion resistance and paint corrosion resistance may decrease.

[0031] Furthermore, the atomic ratio of C to Cr in the chromium-containing layer can be measured using XPS by the following procedure. First, from the outermost surface of the surface-treated steel sheet, SiO ２Sputtering is performed to a depth of 0.2 nm or more (converted to a specific value), and the integrated intensities of the narrow spectra of Cr2p and C1s are determined. From the obtained integrated intensities, the atomic ratio is quantified using the relative sensitivity coefficient method, and the C atomic ratio / Cr atomic ratio is calculated. Here, for the XPS measurement, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-FI can be used. The X-ray source is monochromatic AlKα rays, the voltage is 15 kV, the beam diameter is 100 μmφ, and the extraction angle is 45°. The sputtering conditions are Ar ions accelerated at a voltage of 1 kV, and the sputtering rate is SiO ２ This can be converted to 1.50 nm / min.

[0032] Furthermore, the chromium-containing layer may also contain Fe. The upper limit of the Fe content in the chromium-containing layer is not particularly limited, but it is preferably 100% or less as an atomic ratio to Cr. Since the chromium-containing layer does not need to contain Fe, the lower limit of the atomic ratio to Cr is not particularly limited and may be 0%. The mechanism by which Fe is contained in the chromium-containing layer is not clear, but it is thought that in the process of forming the chromium-containing layer on the base steel sheet, a small amount of Fe contained in the steel sheet dissolves in the electrolyte and the Fe is incorporated into the film. The Fe content in the chromium-containing layer can be measured by XPS, similar to the C content described above, and the narrow spectra of Cr2p and Fe2p can be used to calculate the atomic ratio.

[0033] Furthermore, the chromium-containing layer may contain, in addition to Cr, O, Fe, and C, metallic impurities such as K, Na, Mg, Ca contained in water, and Sn, Ni, Cu, and Zn contained in aqueous solutions, as well as elemental impurities such as S, N, Cl, and Br. However, since the presence of these elements may reduce the corrosion resistance of the film and the coating, the total amount of impurities other than Cr, O, Fe, and C is preferably 30% or less, more preferably 3% or less, and particularly preferably none at all (0%), as an atomic ratio to Cr. The method for measuring the content of the impurities is not particularly limited, but for example, XPS can be used, similar to the method for measuring the content of C.

[0034] (Sn-based plating layer) Furthermore, as shown in Figures 2(a) and (b), the surface-treated steel sheet of the present invention may further include a Sn-based plating layer 30 between the base steel sheet 10 and the chromium-containing layer 20. By further including the Sn-based plating layer 30, the corrosion resistance of the surface-treated steel sheet can be further improved.

[0035] Here, the Sn-based plating layer may be formed on only one side of the base steel sheet, or on both sides. Furthermore, the Sn-based plating layer may cover only a portion of the base steel sheet, or it may cover the entire surface. Moreover, the Sn-based plating layer may be a continuous layer or a discontinuous layer. An example of such a discontinuous layer is an Sn-based plating layer having an island-like structure.

[0036] Furthermore, the Sn-based plating layer also includes cases where a portion of the Sn-based plating layer is alloyed. For example, cases where a portion of the Sn-based plating layer has become an Sn alloy layer due to a heat melting treatment after Sn plating are also included in the Sn-based plating layer. Examples of the Sn alloy layer include Fe-Sn alloy layers and Fe-Sn-Ni alloy layers.

[0037] One method for obtaining the aforementioned Sn alloy layer is to heat and melt the Sn by means of electric heating after Sn plating, thereby making a portion of the steel sheet side of the Sn-based plating layer an Fe-Sn alloy layer. Alternatively, for a steel sheet having a Ni plating layer on its surface (described later), after applying Sn plating, the Sn can be further heated and melted by means of electric heating, thereby making a portion of the steel sheet side of the Sn-based plating layer an Fe-Sn-Ni alloy layer or an Fe-Sn alloy layer.

[0038] The amount of Sn deposited in the Sn-based plating layer is not particularly limited and can be any amount. However, from the viewpoint of improving the corrosion resistance of the surface-treated steel sheet, the amount of Sn deposited should be 0.1 g / m² per side of the steel sheet. ２ Preferably, it is 0.3 g / m ２ More preferably, the above is true. From a similar viewpoint, the amount of Sn attached is 20.0 g / m² per side of the steel plate. ２ Preferably, the following is present: 15.0 g / m２ The following is more preferable. The amount of Sn deposited in the Sn-based plating layer shall be the value measured by, for example, the electrolytic method or the X-ray fluorescence method described in JIS G 3303.

[0039] Here, the formation of the Sn-based plating layer is not particularly limited and can be carried out by any method, such as electroplating or hot-dip plating. For example, when forming the Sn-based plating layer by electroplating, any plating bath can be used. Examples of usable plating baths include phenol sulfonic acid Sn plating baths, methanesulfonic acid Sn plating baths, or halogen-based Sn plating baths.

[0040] After forming the aforementioned Sn-based plating layer, a reflow process can also be performed. When a reflow process is performed, the initially formed Sn-based plating layer is heated to a temperature above the melting point of Sn (231.9°C) to form an alloy layer, such as an Fe-Sn alloy layer, beneath the Sn-only plating layer (on the steel sheet side). If the reflow process is omitted, a Sn-plated steel sheet having a Sn-only plating layer is obtained.

[0041] (Ni-based plating layer) Furthermore, as shown in Figure 2(b), the surface-treated steel sheet of the present invention may further include a Ni-based plating layer 40 between the base steel sheet 10 and the Sn-based plating layer. By further including the Ni-based plating layer 40, the corrosion resistance of the surface-treated steel sheet can be further improved.

[0042] Here, the Ni-based plating layer may be formed on only one side of the base steel sheet, or on both sides. For example, the surface-treated steel sheet may have a Ni-based plating layer, a Sn-based plating layer, and a chromium-containing layer sequentially formed on at least one side of the base steel sheet.

[0043] As the Ni-based plating layer, any plating layer containing nickel can be used, for example, one or both of a Ni layer and a Ni alloy layer can be used. As the Ni alloy layer, for example, a Ni-Fe alloy layer can be used. Furthermore, by forming the Sn plating layer on the Ni-based plating layer and then performing a reflow treatment, an Fe-Sn-Ni alloy layer or an Fe-Sn alloy layer can be formed below the Sn-only plating layer (on the steel sheet side).

[0044] The method for forming the Ni-based plating layer is not particularly limited. For example, the Ni-based plating layer can be formed using any method, such as electroplating. When forming a Ni-Fe alloy layer as the Ni-based plating layer, for example, the Ni-Fe alloy layer can be formed by forming a Ni layer on the surface of the steel sheet using a method such as electroplating, and then annealing.

[0045] Furthermore, while the amount of Ni deposited in the Ni-based plating layer is not particularly limited, from the viewpoint of further improving corrosion resistance, the amount of Ni deposited per side of the steel sheet should be 2 mg / m². ２ It is preferable to use a dose of 4 mg / m² or more. ２ It is more preferable to have the above. Furthermore, from the viewpoint of manufacturing costs, the amount of Ni attached per side of the steel sheet should be 2000 mg / m². ２ The following is preferable.

[0046] <Method for Manufacturing Surface-Treated Steel Sheets> The method for manufacturing surface-treated steel sheets in one embodiment of the present invention is not particularly limited, but can be carried out as follows, for example. The method for manufacturing surface-treated steel sheets is a method for manufacturing surface-treated steel sheets in which a chromium-containing layer is disposed on at least one surface of the steel sheet, and includes the following steps (1) and (2): (1) an electrolyte preparation step of preparing an electrolyte containing trivalent chromium ions (2) a film formation step of forming a chromium-containing layer

[0047] (1) Electrolyte preparation step) The electrolyte preparation step is a step of mixing a trivalent chromium ion source, a carboxylic acid compound, and water to make an aqueous solution.

[0048] Any compound capable of supplying trivalent chromium ions can be used as the trivalent chromium ion source. For example, at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate can be used as the trivalent chromium ion source. The content of the trivalent chromium ion source in the aqueous solution is not particularly limited, but is preferably 3 g / L or more and 50 g / L or less in terms of trivalent chromium ions, and more preferably 5 g / L or more and 40 g / L or less. Atotech's "BluCr® TFS A" can be used as the trivalent chromium ion source.

[0049] The carboxylic acid compound is not particularly limited, and any carboxylic acid compound can be used. The carboxylic acid compound may be at least one of a carboxylic acid and a carboxylic acid salt, and it is preferable that it is at least one of an aliphatic carboxylic acid and a salt of an aliphatic carboxylic acid. The number of carbon atoms in the aliphatic carboxylic acid is preferably 1 to 10, and more preferably 1 to 5. The number of carbon atoms in the aliphatic carboxylic acid salt is preferably 1 to 10, and more preferably 1 to 5. The content of the carboxylic acid compound is not particularly limited, but it is preferably 0.1 mol / L or more and 5.5 mol / L or less, and more preferably 0.15 mol / L or more and 5.3 mol / L or less. As the carboxylic acid compound, Atotech's "BluCr® TFS B" can be used.

[0050] The water mentioned above is used as a solvent for preparing the electrolyte. It is preferable to use ion-exchanged water from which cations have been removed in advance using an ion exchange resin, or highly purified water such as distilled water. Furthermore, from the viewpoint of reducing the amount of K, Na, Mg, and Ca contained in the electrolyte, it is preferable to use water with an electrical conductivity of 30 μS / m or less.

[0051] Furthermore, in the electrolyte preparation step, it is preferable to further include at least one type of halide ion in the aqueous solution in order to effectively suppress the generation of hexavalent chromium at the electrode in the film formation step described later and to improve the stability of the electrolyte. The content of the halide ion is not particularly limited, but it is preferably 0.05 mol / L or more and 3.0 mol / L or less, and more preferably 0.10 mol / L or more and 2.5 mol / L or less. To include the halide ion, Atotech's BluCr® TFS C1 and BluCr® TFS C2 can be used.

[0052] Furthermore, it is preferable not to add hexavalent chromium to the aqueous solution. As will be described later, the trace amount of hexavalent chromium formed on the electrode or steel plate surface during the film formation process is reduced to trivalent chromium, so the concentration of hexavalent chromium in the electrolyte does not increase.

[0053] Furthermore, it is preferable that the aqueous solution does not intentionally contain metal ions other than trivalent chromium ions. The metal ions are not limited to Cu ions, but examples include Cu ions, Zn ions, Ni ions, Fe ions, Sn ions, etc., and are preferably 0 mg / L to 40 mg / L, more preferably 0 mg / L to 20 mg / L, and most preferably 0 mg / L to 10 mg / L.

[0054] In the electrolyte preparation step, the pH of the aqueous solution is then adjusted to 4.0 to 7.0, and the temperature of the aqueous solution is adjusted to 40 to 70°C to prepare the electrolyte. To manufacture the surface-treated steel sheet described above, it is insufficient to simply dissolve a trivalent chromium ion source and a carboxylic acid compound in water; as described above, it is important to properly control the pH and temperature.

[0055] The reason for adjusting the pH of the aqueous solution to 4.0 to 7.0 is that if the pH is less than 4.0 or greater than 7.0, the stability of the electrolyte decreases, and precipitation occurs, making it impossible to form a chromium-containing layer during the film formation process. In addition, the hexavalent chromium concentration of the electrolyte increases during the electrolytic treatment. From a similar viewpoint, it is preferable that the pH of the aqueous solution be 4.5 to 6.5.

[0056] Furthermore, in the electrolyte preparation step, the temperature of the aqueous solution after mixing is adjusted to 40 to 70°C. If the temperature is below 40°C or above 70°C, the stability of the electrolyte decreases, causing precipitation and preventing the formation of a chromium-containing layer in the film formation step. Also, the hexavalent chromium concentration in the electrolyte increases during the electrolytic treatment. The holding time in the temperature range of 40 to 70°C is not particularly limited.

[0057] By following the above procedure, the electrolyte to be used in the next film formation step can be obtained. The electrolyte produced by the above procedure can be stored at room temperature.

[0058] (Film Formation Process) In the film formation process, the steel sheet or Sn-plated steel sheet is subjected to a cathode electrolytic treatment C1 using the electrolyte prepared in the electrolyte preparation process described above, and then subjected to a cathode electrolytic treatment C2 using an electrolyte (second electrolyte) containing a trivalent chromium source and a buffer. This makes it possible to form the chromium-containing layer described above. In the cathode electrolytic treatment C1, some chromium oxide is also deposited, but mainly a metallic chromium layer is formed.

[0059] In one embodiment of the method for manufacturing surface-treated steel sheets, the surface-treated steel sheet may further comprise a Ni-based plating layer disposed beneath the Sn-based plating layer. Therefore, when manufacturing a surface-treated steel sheet comprising a Ni-based plating layer, a steel sheet having the Ni-based plating layer and the Sn-based plating layer formed on at least one surface may be used for the film formation process.

[0060] First, a base steel sheet or a steel sheet with a plating layer (hereinafter sometimes simply referred to as "steel sheet") is subjected to a cathode electrolytic treatment C1 using the electrolyte. By performing the cathode electrolytic treatment C1, a chromium-containing layer can be formed on the steel sheet.

[0061] The electrical density in the cathode electrolytic treatment C1 is not particularly limited. However, the amount of chromium deposited in the chromium-containing layer can be controlled by the electrical density of the cathode electrolytic treatment C1. Therefore, the electrical density is 5.0 C / dm ２ Preferably, it is 10.0 C / dm ２ It is more preferable that the above values ​​are met. Also, for the same reason, the electrical density is 200.0 C / dm². ２ Preferably, it is 180.0 C / dm ２ The following is more preferable:

[0062] Furthermore, the current density and energizing time of the cathode electrolytic treatment C1 are not particularly limited and can be set as appropriate to achieve the desired electrical density. Note that the electrical density is the current density of the electrolytic treatment (unit: A / dm). ２ It is expressed as the product of the voltage and the energizing time (unit: sec.).

[0063] The temperature of the electrolyte during the cathode electrolytic treatment C1 is not particularly limited, but from the viewpoint of efficiently forming the chromium-containing layer, it is preferable to set the temperature in the range of 40°C to 70°C. From the viewpoint of stably manufacturing the surface-treated steel sheet described above, it is preferable to monitor the temperature of the electrolyte during the cathode electrolytic treatment C1 and maintain it within the above temperature range.

[0064] Furthermore, the pH of the electrolyte when performing the cathode electrolytic treatment C1 is not particularly limited, but it is preferably 4.0 or higher, and more preferably 4.5 or higher. Also, the pH of the electrolyte is preferably 7.0 or lower, and more preferably 6.5 or lower. From the viewpoint of stably manufacturing the surface-treated steel sheet described above, it is preferable to monitor the pH of the electrolyte during the cathode electrolytic treatment C1 and maintain it within the above pH range.

[0065] The type of electrode used when performing the cathode electrolytic treatment C1 is not particularly limited, and any electrode can be used. Preferably, an insoluble electrode is used. Preferably, at least one selected from the group consisting of an electrode coated with one or both of a platinum group metal and / or an oxide of a platinum group metal, and a graphite electrode, is used as the insoluble electrode. More specifically, an example of the insoluble electrode is an electrode in which platinum, iridium oxide, or ruthenium oxide is coated on the surface of a Ti substrate.

[0066] In the cathode electrolytic treatment C1, the concentration of the electrolyte constantly changes due to factors such as the formation of a chromium-containing layer on the steel sheet, the removal and addition of liquid, and the evaporation of water. Since the change in the concentration of the electrolyte in the cathode electrolytic treatment C1 varies depending on the configuration of the apparatus and the manufacturing conditions, it is preferable to monitor the concentration of the components contained in the electrolyte in the cathode electrolytic treatment C1 and maintain it within the above-mentioned concentration range, from the viewpoint of more stably manufacturing surface-treated steel sheets.

[0067] Then, the steel sheet that has undergone the above-described cathodic electrolytic treatment C1 is subjected to cathodic electrolytic treatment C2 using a different electrolyte (second electrolyte) that contains a trivalent chromium ion source and a buffer, but excludes agents for adjusting the pH and does not contain other compounds. By performing the cathodic electrolytic treatment C2, a chromium oxide layer having island-like thick film portions can be formed.

[0068] In the electrolyte containing the aforementioned trivalent chromium ions, the trivalent chromium ions and water molecules normally form a stable complex, so even when cathode electrolysis is performed, the reduction deposition reaction of chromium does not proceed on the steel plate, and the hydrogen evolution reaction proceeds instead. At this time, as the pH of the steel plate surface rises, the oxation reaction of trivalent chromium ions in the second electrolyte proceeds, and the chromium oxide layer is formed by the deposition of Cr oxide formed by the oxation reaction onto the steel plate surface. At this time, by adding a small amount of buffer to the electrolyte, the rise in pH on the steel plate surface is made non-uniform, and island-like thick film portions can be formed.

[0069] The trivalent chromium ion source for the second electrolyte used in the cathode electrolytic treatment C2 can be any compound that can supply trivalent chromium ions. As the trivalent chromium ion source, for example, at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate can be used, and from the viewpoint of efficiently precipitating Cr oxide, it is preferable that the chromium ion concentration is 3 g / L or more and 50 g / L or less, and more preferably 5 g / L or more and 40 g / L or less.

[0070] Furthermore, there are no particular limitations on the type of buffering agent contained in the second electrolyte. For example, commonly used buffering agents such as citrate, phosphate, and boric acid can be used as appropriate. Since the purpose of the buffering agent is to mitigate the uneven rise in pH on the steel plate surface due to hydrogen generation during the cathode electrolytic treatment C2, there are no restrictions on the buffering pH range of the buffering agent used.

[0071] Furthermore, if the concentration of the buffering agent is too low, the uneven rise in pH on the steel plate surface will not occur, leading to a decrease in appearance. For this reason, the concentration of the buffering agent is 0.01 mol / L or higher, preferably 0.05 mol / L or higher, and more preferably 0.10 mol / L or higher. On the other hand, if the concentration of the buffering agent is too high, the uneven rise in pH on the steel plate surface will not occur, leading to a decrease in appearance. For this reason, the concentration of the buffering agent is 5.0 mol / L or lower, preferably 3.0 mol / L or lower, and more preferably 1.5 mol / L or lower.

[0072] Furthermore, the second electrolyte may further contain at least one selected from polymer compounds and polyhydric alcohols. When forming the chromium oxide layer, adding at least one selected from polymer compounds and polyhydric alcohols to the electrolyte as a thickener makes it easier for hydrogen gas to accumulate on the steel sheet surface. Film formation progresses in areas where hydrogen gas does not accumulate, while film formation is locally inhibited in areas where hydrogen gas accumulates. As a result, areas of relatively thick and thin chromium oxide layers are distributed, promoting the formation of island-like thick film portions.

[0073] The types of polymer compounds and polyhydric alcohols contained in the second electrolyte are not particularly limited, and various compounds can be used as long as they can adjust the viscosity of the electrolyte to a predetermined range. For example, examples of polymer compounds include polyethylene glycol, polyvinyl alcohol, carboxymethylcellulose, etc. Examples of polyhydric alcohols include glycerin, ethylene glycol, etc.

[0074] If the viscosity of the second electrolyte is too high, the uniformity of the film will deteriorate significantly, leading to a decrease in appearance. Therefore, the viscosity of the second electrolyte is 50 mPa·s or less, preferably 40 mPa·s or less, and more preferably 30 mPa·s or less. The viscosity of the second electrolyte is based on the dynamic viscosity value (unit: mPa·s) measured at a measurement temperature of 20°C using a vibrating viscometer (e.g., manufactured by Sofraser).

[0075] Furthermore, the electrical density of the cathode electrolytic treatment C2 is 1.0 C / dm ２ If the electrical density is less than 1.0 C / dm², the chromium oxide layer will not be sufficiently formed, resulting in reduced film corrosion resistance and coating corrosion resistance. Therefore, the electrical density should be 1.0 C / dm². ２ The above is 3.0 C / dm ２ Preferably, it is 5.0 C / dm ２ It is more preferable that the above conditions are met. On the other hand, the electrical density is 150.0 C / dm ２ If the electrical density is too high, an excessive chromium oxide layer will form, resulting in a poor appearance. Therefore, the electrical density should be 150.0 C / dm ２ The following is true: 120.0 C / dm ２ Preferably, it is 100.0 C / dm ２ The following is more preferable:

[0076] Furthermore, if the current density of the cathode electrolytic treatment C2 is low, the pH increase on the steel plate surface due to hydrogen generation is gradual, causing non-uniformity to progress only in a narrow area, and the maximum area per thick film portion becomes too large. Therefore, the current density of the cathode electrolytic treatment C2 is 0.10 A / dm². ２ The above is true, 0.50 A / dm ２Preferably, it is 1.00 A / dm ２ The above is more preferable. On the other hand, if the current density of the cathode electrolytic treatment C2 is too high, the pH rise and pH non-uniformity on the steel plate surface will progress rapidly, and the maximum film thickness of the thick film portion will become too large. For this reason, the current density of the cathode electrolytic treatment C2 should be 120.00 A / dm² or less, and 100.00 A / dm² or less. ２ Preferably, it is 80.00 A / dm ２ The following is more preferable:

[0077] The energizing time for the cathode electrolytic treatment C2 is not particularly limited and can be set as appropriate to achieve the desired electrical density.

[0078] Furthermore, the temperature of the second electrolyte when performing the cathode electrolytic treatment C2 is not particularly limited, but it is preferable to set it to a temperature range of 20°C to 70°C in order to efficiently form the chromium oxide layer. From the viewpoint of stably manufacturing the surface-treated steel sheet described above, it is preferable to monitor the temperature of the second electrolyte during the cathode electrolytic treatment C2 and maintain it within the above temperature range.

[0079] Furthermore, from the viewpoint of bath stability, the pH of the second electrolyte when performing the cathode electrolytic treatment C2 is preferably 4.0 or less, and more preferably 3.5 or less. If the pH of the second electrolyte is too high, chromium oxide will precipitate in the second electrolyte, making the electrolyte unstable. The lower limit of the pH of the second electrolyte is not particularly limited, but from the viewpoint of efficiently forming the chromium oxide layer, it is preferably 1.0 or higher, and more preferably 1.5 or higher. From the viewpoint of stably manufacturing the surface-treated steel sheet described above, it is preferable to monitor the pH of the second electrolyte during the cathode electrolytic treatment C2 and maintain it within the above pH range.

[0080] Furthermore, the type of electrode used when performing the cathode electrolytic treatment C2 is not particularly limited, and any electrode can be used. Preferably, an insoluble electrode is used. Preferably, at least one selected from the group consisting of an electrode coated with one or both of a platinum group metal and / or an oxide of a platinum group metal, and a graphite electrode, is used as the insoluble electrode. More specifically, examples of the insoluble electrode include an electrode in which platinum, iridium oxide, or ruthenium oxide is coated on the surface of a Ti substrate.

[0081] In the cathode electrolytic treatment C2 described above, the concentration of the second electrolyte constantly changes due to the formation of a chromium oxide layer on the steel sheet, the removal and addition of liquid, and the evaporation of water. Since the change in the concentration of the second electrolyte in the cathode electrolytic treatment C2 varies depending on the configuration of the apparatus and the manufacturing conditions, from the viewpoint of more stably manufacturing surface-treated steel sheets, it is preferable to monitor the concentration of the components contained in the second electrolyte in the cathode electrolytic treatment C2 and maintain it within the concentration range described above.

[0082] (Water washing) In the method for manufacturing a surface-treated steel sheet according to one embodiment of the present invention, it is preferable to wash the surface-treated steel sheet after the cathode electrolytic treatment C1 and the surface-treated steel sheet after the cathode electrolytic treatment C2 with water at least once each. By performing the water washing, the electrolyte remaining on the surface of the steel sheet can be removed.

[0083] The aforementioned washing can be carried out by any method without particular limitation. For example, a washing tank can be provided downstream of the immersion tank for the immersion treatment, allowing the immersed steel plates to be continuously immersed in water. Alternatively, the immersed steel plates may be washed by spraying them with water.

[0084] The water used for the washing is not particularly limited, but it is preferable to use at least one of reverse osmosis water (RO water), ion-exchanged water, and distilled water. The electrical conductivity of the water used for washing is not particularly limited, but it is preferably 100 μS / m or less, more preferably 50 μS / m or less, and even more preferably 30 μS / m or less.

[0085] Furthermore, the temperature of the water used for washing is not particularly limited and may be any temperature. However, since excessively high temperatures place an excessive burden on the washing equipment, it is preferable that the temperature of the water used for washing be 95°C or lower. On the other hand, the lower limit of the temperature of the water used for washing is not particularly limited, but it is preferable that it be 0°C or higher. The temperature of the water used for washing may be room temperature.

[0086] Furthermore, drying may be performed after the aforementioned washing. The drying method is not particularly limited, and for example, a conventional dryer or electric furnace drying method can be applied. From the viewpoint of suppressing deterioration of the surface treatment film, the temperature during the drying process is preferably 100°C or lower. The lower limit is not particularly limited, but it is usually around room temperature.

[0087] (Pretreatment) In addition, in the method for manufacturing a surface-treated steel sheet according to one embodiment of the present invention, the steel sheet may be optionally pretreated prior to the film formation step. Preferably, the pretreatment involves at least one of degreasing, pickling, and washing.

[0088] By performing the aforementioned degreasing, rolling oil, rust-preventive oil, etc., adhering to the steel plate can be removed. The degreasing is not particularly limited and can be performed by any method. After degreasing, it is preferable to wash the steel plate with water to remove the degreasing solution adhering to the surface.

[0089] By performing the aforementioned pickling, the native oxide film present on the surface of the steel sheet can be removed, thereby enabling the effective formation of a chromium-containing layer in the subsequent coating process. The pickling is not particularly limited and can be carried out by any method. After the pickling, it is preferable to rinse the steel sheet surface with water to remove the pickling solution adhering to it.

[0090] The present invention will be described below with reference to examples. However, the present invention is not limited to the following examples.

[0091] <Preparation of Surface-Treated Steel Sheet Samples> To confirm the effects of the present invention, samples of surface-treated steel sheets were prepared according to the following procedure. (Electrolyte Preparation Process) First, electrolytes having compositions A to G shown in Table 1 were prepared under the conditions shown in Table 1. That is, each component shown in Table 1 was mixed with water to make an aqueous solution, and then the aqueous solution was adjusted to the pH and temperature shown in Table 1. Ammonia water was used to increase the pH in all cases, and sulfuric acid was used for electrolytes A and B, hydrochloric acid for electrolytes C and D, and nitric acid for electrolytes E, F, and G to decrease the pH. (Pretreatment of Base Steel Sheet) Cold-rolled steel sheet was used as the base steel sheet. More specifically, a can steel sheet (T4 base sheet) with a thickness of 0.17 mm was used. The base steel sheet was subjected to electrolytic degreasing, water washing, pickling by immersion in dilute sulfuric acid, and water washing in sequence as pretreatment. (Coating Formation Process) Subsequently, the base steel sheet was subjected to cathodic electrolytic treatment C1 and cathodic electrolytic treatment C2 in that order under the conditions shown in Table 2(1) and Table 2(2). The electrolyte (second electrolyte) during each electrolytic treatment was maintained at the same pH and temperature as shown in Table 1. For each electrolytic treatment, an insoluble electrode was used, which consisted of Ti as the substrate coated with iridium oxide. After performing cathodic electrolytic treatment C2, the sample of surface-treated steel sheet was obtained by washing with water having an electrical conductivity of 100 μS / m or less and drying at room temperature using a blower. For the obtained sample of surface-treated steel sheet, the amount of chromium deposited and the atomic concentration of solid-solution carbon per side of the chromium-containing layer, the amount of chromium oxide layer deposited per side, the maximum film thickness of the thick film portion, the maximum area and area ratio were measured and calculated using the method described above. The results of each measurement and calculation are shown in Table 3(1) and (2).

[0092] <Evaluation> The obtained surface-treated steel sheet samples were evaluated for film corrosion resistance, paint corrosion resistance, and appearance using the following methods. The evaluation results are shown in Tables 3(1) and (2).

[0093] (1) Preparation of evaluation samples Laminated steel sheets were prepared by laminating both sides of the obtained surface-treated steel sheet samples with an isophthalic acid copolymer polyethylene terephthalate film having a stretch ratio of 3.1 × 3.1, a thickness of 25 μm, a copolymerization ratio of 12 mol%, and a melting point of 224°C. The lamination was carried out under conditions such that the crystallinity of the resin film was 10% or less, specifically, a steel sheet feed speed of 40 m / min, a rubber roll nip length of 17 mm, and a time from pressing to water cooling of 1 sec. The crystallinity of the resin film was determined by the density gradient pipe method in accordance with JIS K7112. The nip length is the length in the transport direction of the part where the rubber roll and the steel sheet are in contact. Painted steel sheets, which were used as samples for evaluating the paint corrosion resistance, were prepared by applying epoxy phenol-based paint to the surface of the obtained surface-treated steel sheet and baking it at 210°C for 10 minutes. The amount of paint adhering to the surface is 50 mg / dm ２ That's what I decided.

[0094] (2) Evaluation of film corrosion resistance and paint corrosion resistance Cross cuts were made using a cutter to a depth that reached the base metal (steel plate) on the film surface of the prepared laminated steel plate and the painted surface of the painted steel plate. The laminated steel plate and painted steel plate with cross cuts were immersed for 168 hours in a test solution at 55°C consisting of a mixed aqueous solution containing 1.5% by mass citric acid and 1.5% by mass sodium chloride. After immersion in the test solution, washing and drying were performed, and cellophane adhesive tape was applied to the film surface of the laminated steel plate and the painted surface of the painted steel plate, respectively, and the tape was peeled off. After the tape peeling was performed, (i) for film corrosion resistance, the film peel width (total width of the left and right sides spreading from the cut) was measured at four arbitrary locations on the cross cut area of ​​the laminated steel plate, and the average value of the four locations was calculated and considered as the corrosion width. (ii) For paint corrosion resistance, the paint peeling width (total width extending from the cut to the left and right) was measured at four arbitrary locations on the cross-cut portion of the painted steel sheet, and the average value of the four locations was calculated and considered as the corrosion width. The obtained film corrosion resistance and paint corrosion resistance were evaluated on the following four levels. In practical terms, a rating of 1 to 3 indicates excellent corrosion resistance. 1: Corrosion width less than 0.3 mm 2: Corrosion width 0.3 mm or more and less than 0.5 mm 3: Corrosion width 0.5 mm or more and less than 1.0 mm 4: Corrosion width 1.0 mm or more

[0095] (3) Evaluation of appearance: For the surface-treated steel sheet that was prepared, 1 dm ２ The number of plating irregularities on the surface was visually inspected and measured, and evaluated on the following four levels. In practical terms, an evaluation of 1 to 4 indicates excellent appearance. 1: 0 plating irregularities 2: 1-3 plating irregularities 3: 4-6 plating irregularities 4: 7-9 plating irregularities 5: 10 or more plating irregularities

[0096]

[0097]

[0098]

[0099] The results in Tables 3(1) and (2) show that the surface-treated steel sheet samples according to the invention example all possess excellent film corrosion resistance, paint corrosion resistance, and appearance, even when manufactured without the use of hexavalent chromium.

[0100] According to the present invention, even without chromate treatment, it is possible to provide a surface-treated steel sheet with excellent corrosion resistance and workability, as well as a method for manufacturing the same.

[0101] 10 Base steel plate 20 Chromium-containing layer 21 Metallic chromium layer 22 Chromium oxide layer 23 Thick film section 30 Sn-based plating layer 40 Ni-based plating layer

Claims

1. A surface-treated steel sheet comprising a base steel sheet and a chromium-containing layer formed on at least one surface of the base steel sheet, wherein the chromium-containing layer consists of a metallic chromium layer and a chromium oxide layer formed on the surface of the metallic chromium layer, and the amount of chromium deposited on one side of the chromium oxide layer is 1.0 to 100.0 mg / m² in terms of chromium. ２ Furthermore, it has thick film portions that protrude in an island-like manner, the thickness of the thick film portions is 200 nm or less, and the area of ​​each thick film portion when the surface of the chromium oxide layer is observed is 0.010 mm². ２ A surface-treated steel sheet characterized by the following, and wherein the area ratio of the thick film portion to the chromium oxide layer is 3 to 80%.

2. The amount of chromium-containing layer deposited on one side is 50 to 600 mg / m² in terms of chromium equivalent. ２ The surface-treated steel sheet according to claim 1, characterized in that the atomic concentration of solid-solution carbon present in the chromium-containing layer is 0.2 to 50.0%.

3. A sn-based plating layer is further provided between the base steel plate and the chromium-containing layer, wherein the amount of the sn-based plating layer deposited on one side is 0.1 to 20.0 g / m². ２ A surface-treated steel sheet according to claim 1 or 2, characterized in that it is the same as the above.

4. A Ni-based plating layer is further provided between the base steel plate and the Sn-based plating layer, wherein the amount of Ni-based plating layer deposited on one side is 2 to 2000 mg / m². ２ The surface-treated steel sheet according to claim 3, characterized in that it is the same as the above.

5. A method for manufacturing a surface-treated steel sheet, comprising a base steel sheet and a chromium-containing layer formed on at least one surface of the base steel sheet, comprising: an electrolyte preparation step of preparing an aqueous solution by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, and then adjusting the pH of the aqueous solution to 4.0 to 7.0 and the temperature to 40 to 70°C to obtain an electrolyte containing trivalent chromium ions; and a film formation step of performing a cathode electrolytic treatment C1 on the base steel sheet using the electrolyte containing trivalent chromium ions, and then performing a cathode electrolytic treatment C2 using a second electrolyte containing a trivalent chromium ion source and a buffer.

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