Surface-treated steel sheet and method for producing same

Abstract

Provided is a surface-treated steel sheet which can be produced without using hexavalent chromium and has excellent film corrosion resistance, coating corrosion resistance, coating secondary adhesiveness, and weldability. The surface-treated steel sheet comprises a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, wherein: the N / L determined by a prescribed method is 0.023 nm-1 to 0.085 nm-1; and the chromium-containing layer has an atomic ratio of C to Cr of 0.2% to 50.0%.

Description

Surface-treated steel sheet and its manufacturing method

[0001] The present invention relates to a surface-treated steel sheet, particularly to a surface-treated steel sheet excellent in film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability. The surface-treated steel sheet of the present invention can be suitably used for containers such as cans. The present invention also relates to a method for manufacturing the surface-treated steel sheet.

[0002] Sn-plated steel sheet (tinplate) has excellent corrosion resistance, weldability, and workability, and is easy to manufacture, so it has been used for over 200 years as a material for various metal cans such as beverage cans, food cans, pail cans, and 18-liter cans.

[0003] However, since Sn is an expensive material, tin-free steel sheets (TFS), which are surface-treated steel sheets that do not use Sn, have been developed. Tin-free steel sheets are surface-treated steel sheets in which a metallic chromium layer and a chromium oxide layer are formed on the surface of the steel sheet, and are usually produced by electrolyzing the steel sheet in an electrolyte containing hexavalent chromium (Patent Documents 1 to 3). Tin-free steel sheets have excellent corrosion resistance and are currently very commonly used as steel sheets for containers to replace tinplate. However, general tin-free steel sheets have poor weldability because they have a chromium oxide layer, which is an insulating coating, on the surface.

[0004] Therefore, as a tin-free steel sheet with excellent weldability, a steel sheet for cans is known in which granular protrusions are formed on the surface of the steel sheet by performing anodic electrolysis between multiple cathodic electrolysis treatments in electrochromic chromium plating (Patent Documents 4 and 5).

[0005] However, in recent years, due to growing environmental awareness, there has been a trend toward restricting the use of hexavalent chromium worldwide, and therefore, in the field of surface-treated steel sheets used for containers, etc., there is a demand for the establishment of a manufacturing method that does not use hexavalent chromium.

[0006] Known methods for forming a surface-treated steel sheet without using hexavalent chromium include those proposed in, for example, Patent Documents 6, 7, and 8. In these methods, a surface treatment layer is formed by electrolysis in an electrolytic solution containing a trivalent chromium compound such as basic chromium sulfate.

[0007] JP-A-58-110695 JP-A-55-134197 JP-A-57-035699 JP-A-61-213399 JP-A-63-186894 Publication Special Publication No. 2016-505708 Publication Special Publication No. 2015-520794 Publication Patent No. 6593574

[0008] According to the methods proposed in Patent Documents 6, 7, and 8, a surface treatment layer can be formed without using hexavalent chromium. According to Patent Documents 6, 7, and 8, these methods make it possible to obtain a surface-treated steel sheet that has excellent adhesion to paint in a humid environment (hereinafter referred to as "secondary paint adhesion"), film corrosion resistance, and paint corrosion resistance.

[0009] However, although the surface-treated steel sheets obtained by the conventional methods proposed in Patent Documents 6, 7, and 8 are excellent in secondary paint adhesion, film corrosion resistance, and paint corrosion resistance, they are inferior in weldability. Therefore, their performance is insufficient to be used as a substitute for surface-treated steel sheets produced by methods using hexavalent chromium.

[0010] Therefore, there is a demand for a surface-treated steel sheet that can be produced without using hexavalent chromium and that combines film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a surface-treated steel sheet that can be produced without using hexavalent chromium and that is excellent in film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability.

[0012] As a result of extensive research into achieving the above object, the inventors of the present invention have discovered the following (1) and (2).

[0013] (1) In a surface-treated steel sheet having a chromium-containing layer disposed on at least one surface of the steel sheet, parameters relating to the surface properties of the chromium-containing layer determined by a predetermined method and the atomic ratio of C to Cr in the chromium-containing layer are controlled within specific ranges, respectively, thereby providing a surface-treated steel sheet having excellent film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability.

[0014] (2) The surface-treated steel sheet can be produced by carrying out cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using an electrolytic solution prepared by a predetermined method, while controlling the electrical density of anodic electrolysis A1 and cathodic electrolysis C2 within a specific range. Furthermore, by preparing the electrolytic solution by a predetermined method, it is possible to prevent an increase in hexavalent chromium in the electrolytic solution during film formation.

[0015] The present invention has been completed based on the above findings. The gist of the present invention is as follows.

[0016] 1. A surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, wherein a cutoff value λ is applied to a cross-sectional curve extracted from a cross-sectional image of the chromium-containing layer. ｓ = 5 nm low-pass filter and cutoff value λ ｃ In the roughness curve of the chromium-containing layer obtained by applying a high-pass filter with a filter width of 65 nm, the number N of peaks whose height from the mean line of the roughness curve at the apex exceeds 1.2 nm and the evaluation length L of the roughness curve is N / L 0.023 nm. －１ 0.085nm or more －１ wherein the chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less.

[0017] 2. The chromium coating amount of the chromium-containing layer is 40.0 to 500.0 mg / m per side. ２ 2. The surface-treated steel sheet according to 1 above,

[0018] 3. The chromium oxide deposition amount of the chromium-containing layer is 40.0 mg / m per side. ２ 3. The surface-treated steel sheet according to 1 or 2 above, wherein:

[0019] 4. A method for producing a surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, the method comprising: an electrolytic solution preparation step of preparing an electrolytic solution containing trivalent chromium ions; and a film formation step of forming the chromium-containing layer, wherein the electrolytic solution preparation step comprises mixing a trivalent chromium ion source, a carboxylic acid compound, and water, and adjusting the pH to 4.0 to 7.0 and the temperature to 40 to 70°C to prepare the electrolytic solution; and wherein the film formation step comprises subjecting the steel sheet to cathodic electrolysis treatment C1, anodic electrolysis treatment A1, and cathodic electrolysis treatment C2 in this order using the electrolytic solution, and wherein the electrical charge density of the anodic electrolysis treatment A1 is 0.50 C / dm ２ 20.00C / dm or more ２ The electric charge density of the cathodic electrolysis treatment C2 is 50.0 C / dm or less. ２ 150.0C / dm or more ２ The method for producing a surface-treated steel sheet is as follows.

[0020] According to the present invention, it is possible to provide a surface-treated steel sheet that is excellent in film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability without using hexavalent chromium. The surface-treated steel sheet of the present invention can be suitably used as a material for containers, etc.

[0021] FIG. 1 is a diagram showing a roughness curve.

[0022] A method for carrying out the present invention will be specifically described below. Note that the following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

[0023] A surface-treated steel sheet according to one embodiment of the present invention is a surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet. －１ 0.085nm or more －１ It is important that the atomic ratio of C to Cr in the chromium-containing layer is 0.2% or more and 50.0% or less. Each of the constituent requirements of the surface-treated steel sheet will now be described.

[0024] [Steel Sheet] The steel sheet is not particularly limited, and any steel sheet can be used. The steel sheet is preferably a steel sheet for cans. The steel sheet can be, for example, an ultra-low carbon steel sheet or a low carbon steel sheet. The method for manufacturing the steel sheet is also 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 that includes, for example, hot rolling, pickling, cold rolling, annealing, and temper rolling.

[0025] The composition of the steel sheet is not particularly limited, but for example, a steel sheet having a composition specified in ASTM A623M-09 can be suitably used.

[0026] In one embodiment of the present invention, the alloy contains, by mass%, C: 0.0001 to 0.13%, Si: 0 to 0.020%, P: 0 to 0.020%, S: 0 to 0.030%, Al: 0 to 0.20%, and N: 0 to 0.040%, and optionally further contains, by mass%, Mn: 0.01 to 0.60%, Cu: 0 to 0.20%, Ni: 0 to 0.15%, Cr: 0 to 0.10%, Mo: 0 to 0.05%, Ti: 0 to 0.020%, Nb: 0 to 0.020%, B: 0 to 0.020%, Ca: 0 to 0.020%, Sn: 0 to 0.020%, and It is preferable to use a steel sheet having a chemical composition containing at least one element selected from the group consisting of Sb: 0 to 0.020%, and the balance consisting of Fe and unavoidable impurities. Of the chemical compositions, the lower the contents of Si, P, S, Al, and N, the more preferable they are, and Mn, Cu, Ni, Cr, Mo, Ti, Nb, B, Ca, Sn, and Sb are components that can be added as desired.

[0027] The thickness of the steel plate is not particularly limited, but is preferably 0.60 mm or less. Here, "steel plate" is defined to include "steel strip." On the other hand, the lower limit of the thickness is not particularly limited, but is preferably 0.10 mm or more.

[0028] [Chromium-Containing Layer] A chromium-containing layer is present on at least one surface of the steel sheet. The components constituting the chromium-containing layer are not particularly limited, but may include metallic chromium and a chromium compound. The chromium compound is not particularly limited and may include any chromium compound. The chromium compound may include, for example, at least one selected from the group consisting of chromium oxide, chromium carbide, chromium sulfide, chromium nitride, chromium chloride, chromium bromide, and chromium boride. In addition to metallic chromium and the chromium compound, the chromium-containing layer may also contain impurities. Examples of such impurities include metal elements such as Ni, Cu, Sn, and Zn that are mixed as impurities in the electrolytic solution described below. The metal elements are typically considered to exist in the chromium-containing layer in a metallic state, but may also exist as compounds.

[0029] In one embodiment of the present invention, the chromium-containing layer preferably has a total content of elements constituting metallic chromium and chromium compounds of 90 atomic % or more, where the total content is the ratio, expressed as a percentage, of the total number of atoms of elements constituting metallic chromium and chromium compounds to the total number of atoms of all elements other than Fe.

[0030] The total content can be determined by measuring the content (atomic %) of each element constituting the metallic chromium and the chromium compound contained in the chromium-containing layer by X-ray photoelectron spectroscopy (XPS) and adding them up. In measuring the content by XPS, the content (atomic ratio) of each element can be calculated by the relative sensitivity factor method from the integrated intensity of the peak corresponding to each element.

[0031] For example, chromium carbide (Cr ２ C ３ The content of Cr) can be determined from the integrated intensity of the C 1s carbide peak appearing near 281.0 eV. For example, if the C content (atomic ratio to the total of all elements other than Fe) calculated from the integrated intensity of the peak is 6 atomic %, then Cr ２ C ３ The content of is 6×(2+3) / 3=10 atomic %.

[0032] Regarding chromium oxide, from the integrated intensity of the 2p oxide peak of Cr that appears around 576.7 eV, Cr ２ O ３ The content of CrO can be calculated from the integrated intensity of the peak of the 2p oxide part of Cr that appears around 579.2 eV. ３ The content can be calculated.

[0033] Similarly, the content of other chromium compounds can be determined using the integrated intensities of the following peaks: Chromium sulfide (Cr ２ S ３ Chromium nitride (CrN): S 2p sulfide peak appearing around 162.3 eV Chromium chloride (CrCl): N 1S peak appearing around 397.3 eV ３ Chromium bromide (CrBr): Cl 2p peak appearing around 199.8 eV ３ Chromium boride (CrB): Br 1s peak appearing around 188.2 eV

[0034] On the other hand, the content of metallic chromium can be determined by calculating the Cr content from the integrated intensity of the 2p peak of Cr appearing near 573.8 eV and subtracting the content of Cr atoms contained as chromium compounds from the chromium content.

[0035] The total content of metallic chromium and the elements constituting the chromium compound can be determined by adding together the content of metallic chromium obtained by the above method and the content of each element constituting the chromium compound.

[0036] The total content refers to the value at the half-thickness position of the chromium-containing layer. The half-thickness position can be determined by the following procedure. First, the chromium-containing layer is sputtered from its outermost surface, while the total content of elements constituting metallic chromium and chromium compounds and the Fe content are measured by the above-mentioned method. The position (depth) at which the measured total content of elements constituting metallic chromium and chromium compounds and the Fe content become equal is determined as the interface between the chromium-containing layer and the steel sheet. The thickness from the outermost surface of the chromium-containing layer to the interface is defined as the thickness of the chromium-containing layer, and the half-thickness position is determined.

[0037] For the measurement by XPS, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI, Inc. The X-ray source is a monochromatic AlKα ray, the voltage is 15 kV, the beam diameter is 100 μmφ, the take-off angle is 45°, and the sputtering conditions are Ar ions with an acceleration voltage of 1 kV and a sputtering rate of SiO ２ This can be converted to 1.50 nm / min.

[0038] The spatial structure of the components constituting the chromium-containing layer is not particularly limited, and may be, for example, separated as separate layers within the chromium-containing layer or may be mixed throughout the chromium-containing layer. That is, the spatial structure of the components constituting the chromium-containing layer may include one or both of separate layers and mixed layers.

[0039] The chromium deposition amount of the chromium-containing layer is not particularly limited. However, if the chromium deposition amount of the chromium-containing layer is excessive, it may hinder weldability and may cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and secondary paint adhesion, it is preferable that the chromium deposition amount of the chromium-containing layer is 500.0 mg / m per side. ２ It is preferable that the concentration is 450.0 mg / m or less. ２ On the other hand, from the viewpoint of further improving the coating corrosion resistance and the film corrosion resistance, it is more preferable that the chromium deposition amount of the chromium-containing layer is 40.0 mg / m or less per side. ２ It is preferable that the concentration is 50.0 mg / m or more. ２More preferably, it is set to be equal to or greater than this.

[0040] The chromium coating weight is measured using an X-ray fluorescence analyzer according to the following procedure. First, the Cr amount (total Cr amount) in the surface-treated steel sheet is measured using the X-ray fluorescence analyzer. Next, the Cr amount (original sheet Cr amount) in the steel sheet before the chromium-containing layer is formed or in the steel sheet after the chromium-containing layer has been stripped is measured using the X-ray fluorescence analyzer. The value obtained by subtracting the original sheet Cr amount from the total Cr amount is taken as the Cr coating weight of the chromium-containing layer. To strip the chromium-containing layer, for example, a commercially available chromium plating stripper such as a hydrochloric acid-based stripper can be used.

[0041] [Chromium Oxide Deposition Amount] Chromium oxide may be present in the chromium-containing layer. The location of the chromium oxide is not particularly limited. The location of O can be confirmed by, for example, composition analysis using energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) attached to a scanning electron microscope (SEM) or a transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).

[0042] The amount of chromium oxide deposited in the chromium-containing layer is not particularly limited. However, if the amount of chromium oxide deposited in the chromium-containing layer is excessive, it may impair weldability and cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and secondary paint adhesion, the amount of chromium oxide deposited in the chromium-containing layer is set to 40.0 mg / m per side. ２ Preferably, the concentration is 35.0 mg / m or less. ２ On the other hand, the chromium-containing layer may not contain any chromium oxide. Therefore, the lower limit of the chromium oxide coating amount of the chromium-containing layer is not particularly limited, and is preferably 0.0 mg / m or less per side. ２ It may be.

[0043] The chromium oxide deposition amount is measured using an X-ray fluorescence analyzer according to the following procedure. First, the Cr amount (total Cr amount) of the surface-treated steel sheet is measured. Next, the surface-treated steel sheet is subjected to an alkali treatment by immersing it in 7.5N-NaOH at 90°C for 10 minutes to remove chromium oxide. After the alkali treatment, the surface-treated steel sheet is thoroughly rinsed with water, and the Cr amount (post-alkali-treatment Cr amount) is measured again using the X-ray fluorescence analyzer. The value obtained by subtracting the post-alkali-treatment Cr amount from the total Cr amount is defined as the chromium oxide deposition amount of the chromium-containing layer.

[0044] The chromium-containing layer may be amorphous or crystalline. That is, the chromium-containing layer may contain one or both of an amorphous phase and a crystalline phase. The chromium-containing layer produced by the method described below generally contains an amorphous phase and may also contain a crystalline phase. Although the mechanism of formation of the chromium-containing layer is unclear, it is believed that partial crystallization occurs during the formation of the amorphous phase, resulting in a chromium-containing layer containing both an amorphous phase and a crystalline phase. The area ratio of the crystalline region is not particularly limited, but is preferably 30% or less when the chromium-containing layer is observed from the surface direction. The lower limit of the area ratio of the crystalline region is not particularly limited and may be 0%.

[0045] The crystalline region in the chromium-containing layer can be confirmed by preparing a chromium-containing single layer sample by etching the substrate steel sheet and observing the sample from the surface side using a TEM or scanning transmission electron microscope (STEM). The method for preparing the chromium-containing single layer sample is not particularly limited, but for example, the sample can be prepared by irradiating an ion beam such as Ar from the substrate steel sheet side and ion milling the steel sheet. When preparing a chromium-containing single layer region using an ion beam, the ion beam is irradiated at an acceleration voltage of 5 kV or less and at an incident angle in the range of 1 to 5 degrees relative to the substrate steel sheet, thereby forming a crystalline region of several μm. ２ The field of view of the chromium single layer region can be secured. At this time, the bottom surface of the chromium-containing layer is also milled to some extent, and the film thickness of the chromium-containing layer may become thinner, but this does not affect the measurement results of the area ratio of the crystalline region.

[0046] The area ratio of the crystalline region in the chromium-containing layer can be measured using a TEM. Specifically, a diffraction pattern of the chromium-containing layer is obtained by selected-area diffraction using a TEM, and dark-field images are obtained at all diffraction spots in the pattern. The regions that appear brightest in the dark-field image are defined as crystalline regions. The area of ​​the obtained crystalline region is calculated by image processing, and the area ratio of the crystalline region is calculated by dividing the area by the area of ​​the chromium-containing layer within the selected-area aperture. Image analysis software such as Image-J can be used to calculate the area ratio.

[0047] [Atomic Ratio of C to Cr] In the present invention, it is important that the chromium-containing layer contains C, and that the atomic ratio of C to Cr in the chromium-containing layer is 0.2% or more and 50.0% or less. When the atomic ratio of C is 0.2% or more and 50.0% or less, the chromium-containing layer is destroyed by volume change during welding pressure or initial heat input, making it easier to conduct current, thereby lowering the minimum welding current, i.e., improving weldability. If the atomic ratio of C is too low, the above-mentioned effect of improving weldability cannot be obtained. Therefore, the atomic ratio of C is set to 0.2% or more. From the viewpoint of ensuring more stable weldability, the atomic ratio of C is preferably set to 0.3% or more. On the other hand, if the atomic ratio of C is excessive, the weld heat-affected zone will excessively harden, causing weld cracking. Therefore, the atomic ratio of C is set to 50.0% or less. From the viewpoint of ensuring more stable weldability, the atomic ratio of C is preferably set to 40.0% or less.

[0048] The atomic ratio of C to Cr in the chromium-containing layer is measured using XPS in the following procedure. ２ The sample is sputtered to a depth of 0.2 nm or more in terms of atomic conversion, and the integrated intensity of the narrow spectrum of Cr2p and C1s is determined. From the integrated intensity, the atomic ratio is quantified using the relative sensitivity factor method, and the C atomic ratio / Cr atomic ratio is calculated. For the XPS measurement, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI, Inc., can be used. The X-ray source is a monochromatic AlKα ray, the voltage is 15 kV, the beam diameter is 100 μmφ, and the take-off angle is 45°. The sputtering conditions are Ar ions with an acceleration voltage of 1 kV, a sputtering rate of SiO２ This can be converted to 1.50 nm / min.

[0049] The mechanism by which C is incorporated into the chromium-containing layer is not clear; however, it is thought that if a carboxylic acid compound is contained in the electrolytic solution during the process of forming the chromium-containing layer on the steel sheet, the carboxylic acid compound decomposes and is incorporated into the coating.

[0050] The chromium-containing layer may 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 in terms of atomic ratio to Cr. The chromium-containing layer may not contain Fe, and therefore the lower limit of the Fe atomic ratio to Cr is not particularly limited and may be 0%. The Fe content in the chromium-containing layer can be measured by XPS, similar to the C content. The atomic ratio can be calculated using narrow spectra of Cr2p and Fe2p.

[0051] The mechanism by which Fe is incorporated into the chromium-containing layer is not clear, but it is thought that during the process of forming the chromium-containing layer on the steel sheet, a small amount of Fe contained in the steel sheet dissolves in the electrolyte, and the Fe is incorporated into the coating.

[0052] In addition to Cr, O, Fe, and C, the chromium-containing layer may contain metal impurities such as K, Na, Mg, and Ca contained in the water, Sn, Ni, Cu, and Zn contained in the aqueous solution, as well as S, N, Cl, Br, etc. However, the presence of these elements may reduce the corrosion resistance of the coating. Therefore, the total atomic ratio of elements other than Cr, O, Fe, and C to Cr is preferably 3% or less, and more preferably no elements (0%) are contained. The content of these elements is not particularly limited, but can be measured, for example, by XPS, in the same way as the C content.

[0053] [N / L] In the present invention, N / L determined by a predetermined method is 0.023 nm. －１ 0.085nm or more －１It is important that N / L is the number N of peaks in the roughness curve of the chromium-containing layer, the height of which from the mean line of the roughness curve at the apex exceeds 1.2 nm, divided by the evaluation length L of the roughness curve. Here, the roughness curve is obtained by applying a cutoff value λ to a cross-sectional curve extracted from a cross-sectional image of the chromium-containing layer. ｓ = 5 nm low-pass filter and cutoff value λ ｃ = 65 nm high-pass filter was applied.

[0054] First, the method for determining N / L will be described in detail.

[0055] First, a cross-sectional image of the chromium-containing layer is obtained. Specifically, a dark-field image of a cross section perpendicular to the surface of the coated steel sheet is taken by STEM, and the image is used as the cross-sectional image. STEM has a high spatial resolution sufficient for observing the chromium-containing layer, and dark-field observation allows the chromium-containing layer region to be clearly distinguished from the background.

[0056] In order to obtain accurate cross-sectional curves and roughness curves, the resolution of the cross-sectional images is set to 0.5 nm or less per pixel. In addition, in order to obtain average information on the chromium-containing layer, the images are taken at a magnification at which a chromium-containing layer having a length of 150 nm or more can be confirmed, and cross-sectional images of five or more randomly selected fields are used.

[0057] The cross-sectional image is set so that the left-right direction coincides or nearly coincides with the longitudinal direction of the chromium-containing layer, and the up-down direction coincides or nearly coincides with the thickness direction of the chromium-containing layer. If the line connecting the end points of the chromium-containing layer is inclined by 5 degrees or more from the left-right direction, the captured cross-sectional image is rotated by image processing. However, when performing the rotation process, the resolution of the image before rotation is set to 0.25 nm or less per pixel.

[0058] To avoid a decrease in the accuracy of the extraction of the cross-sectional curve due to unnecessary information, the cross-sectional image is trimmed so that it does not include areas more than 100 nm above or below the chromium-containing layer. To reduce the variability of the results due to noise in the cross-sectional image, a median filter with a kernel of 3 × 3 or larger is applied to the observed dark-field image to remove noise. However, to avoid underestimating the surface roughness, the length of one side of the kernel is set to 2 nm or less.

[0059] Next, a profile curve is extracted from the cross-sectional image after noise removal. The profile curve is a curve along the surface of the chromium-containing layer on the surface side of the surface-treated steel sheet. First, the cross-sectional image is segmented into regions of the chromium-containing layer and the base steel sheet and other regions (e.g., background, and layers such as paint and film that may be applied to the surface). For segmentation, any of a method of identifying using a brightness threshold, a method of identifying by hand painting, and a method of identifying by image analysis using machine learning may be used. Then, a profile curve is extracted by connecting the highest points in the thickness direction of the chromium-containing layer in the obtained regions of the chromium-containing layer and the base steel sheet.

[0060] Next, a cutoff value λ is applied to the profile curve p(x) extracted by the above procedure. ｓ = 5 nm low-pass filter and cutoff value λ ｃ The roughness curve r(x) is obtained by applying a high-pass filter of r = 65 nm.

[0061] Specifically, a Gaussian filter, which is a phase compensation filter, is used as the filter applied to the cross-sectional curve p(x). The weighting function s of the Gaussian filter is λ (x) is defined by the following formula: where x is the coordinate, λ is the cutoff value, and a is a constant (a=0.4697). Using the profile curve p(x) and the above weighting function, the roughness curve r(x) is calculated by the following equation.

[0062] FIG. 1 shows a schematic diagram of the roughness curve r(x).

[0063] N is the number of peaks whose height from the mean line of the roughness curve exceeds 1.2 nm at their apexes. The mean line conforms to JIS B0601:2013 and coincides with the x-axis in FIG. 1. L is the evaluation length of the roughness curve (length in the x-axis direction). For example, the roughness curve r(x) in FIG. 1 has three peaks whose height exceeds 1.2 nm. Therefore, N / L = 3 / L. N can be expressed by the following equation: where # is the number of elements in the set, and c = 1.2 nm. ｍａｘ (x, d) indicates the maximum value of r(x) within a range of d / 2 before and after, and is expressed by the following equation. Here, d = 10 nm. That is, the peaks are spaced at least 10 nm apart. N / L is calculated from cross-sectional images of five or more randomly selected fields of view using the above-described method, and the average value is used as N / L.

[0064] The cutoff value λ ｓ and λ ｃ is a value smaller than the cutoff value commonly used to derive roughness curves, and by using this cutoff value, parameters suitable for expressing fine surface shapes can be obtained. Furthermore, by setting 1.2 nm as the reference height, the state of granular protrusions present on the surface of the chromium-containing layer can be expressed. In the present invention, N / L obtained using the cutoff value and reference height accurately reflects factors in the surface shape of the chromium-containing layer that affect weldability and secondary paint adhesion.

[0065] Next, N / L calculated by the above method was set to 0.023 nm. －１ 0.085nm or more －１ The reasons for this will be explained in detail below.

[0066] N / L corresponds to the number density of grain protrusions having a given height present on the surface of the chromium-containing layer. －１ If the N / L is less than 0.023 nm, there are few granular protrusions of sufficient size on the surface of the chromium-containing layer, which reduces the surface area and therefore reduces the secondary paint adhesion. －１ or more, preferably 0.028 nm －１More preferably, 0.032 nm －１ On the other hand, N / L is 0.085 nm or more. －１ When N / L is greater than 0.085 nm, the number density of the granular protrusions is excessively high and dense, so that stress concentration on the granular protrusions when pressurized is alleviated and destruction of the metal oxide on the surface is unlikely to occur. －１ or less, preferably 0.070 nm －１ or less, more preferably 0.055 nm －１ The following applies.

[0067] [Manufacturing Method] In a manufacturing method of a surface-treated steel sheet according to one embodiment of the present invention, a surface-treated steel sheet having the above-described properties can be manufactured by the method described below.

[0068] A method for producing a surface-treated steel sheet according to one embodiment of the present invention is a method for producing a surface-treated steel sheet having a chromium-containing layer disposed on at least one surface of the steel sheet, and includes the following steps (1) and (2). Each step will be described below: (1) An electrolytic solution preparation step for preparing an electrolytic solution containing trivalent chromium ions; (2) A film formation step for forming a chromium-containing layer.

[0069] [Electrolyte Solution Preparation Step] (i) Mixing In the electrolyte solution preparation step, first, a trivalent chromium ion source, a carboxylic acid compound, and water are mixed to prepare an aqueous solution.

[0070] The trivalent chromium ion source can be any compound capable of supplying trivalent chromium ions, such as at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate.

[0071] The content of the trivalent chromium ion source in the aqueous solution is not particularly limited, but is preferably 3 g / L or more, more preferably 5 g / L or more, calculated as trivalent chromium ions. The content of the trivalent chromium ion source is preferably 50 g / L or less, more preferably 40 g / L or less. BluCr (registered trademark) TFS A from Atotech can be used as the trivalent chromium ion source.

[0072] Carboxylic acid stabilizes trivalent chromium ions in the electrolyte. Therefore, by adding a carboxylic acid compound to the aqueous solution, it is possible to suppress an increase in the hexavalent chromium concentration in the film formation process, particularly the anodic electrolysis process A1, described below. While carboxylic acid compounds are not typically used in electrolysis processes using hexavalent chromium, in the present invention, it is necessary to add a carboxylic acid compound to the aqueous solution. 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 is preferably at least one of an aliphatic carboxylic acid and an aliphatic carboxylic acid salt. The aliphatic carboxylic acid preferably has 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms. The aliphatic carboxylic acid salt preferably has 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms. The content of the carboxylic acid compound is not particularly limited, but is preferably 0.1 mol / L or more, more preferably 0.15 mol / L or more. The content of the carboxylic acid compound is preferably 5.5 mol / L or less, more preferably 5.3 mol / L or less. As the carboxylic acid compound, BluCr (registered trademark) TFS B manufactured by Atotech can be used.

[0073] In the present invention, water is used as a solvent for preparing the electrolyte solution. It is preferable to use highly pure water, such as ion-exchanged water from which cations have been removed in advance using an ion exchange resin or distilled water, as the water. Furthermore, from the viewpoint of reducing the amounts of K, Na, Mg, and Ca contained in the electrolyte solution, it is preferable to use water having an electrical conductivity of 30 μS / m or less. The lower limit of the electrical conductivity is not limited and may be 0 μS / m.

[0074] In order to effectively suppress the generation of hexavalent chromium at the electrode during the film formation process and improve the stability of the above-mentioned electrolyte, it is preferable that the aqueous solution further contain at least one type of halide ion. The content of the halide ion is not particularly limited, but is preferably 0.05 mol / L or more, and more preferably 0.10 mol / L or more. Furthermore, the content of the halide ion is preferably 3.0 mol / L or less, and more preferably 2.5 mol / L or less. To incorporate the halide ion, BluCr (registered trademark) TFS C1 and BluCr (registered trademark) TFS C2 from Atotech can be used.

[0075] It is preferable not to add hexavalent chromium to the above aqueous solution. As will be described later, the trace amount of hexavalent chromium formed on the surface of the electrode or steel sheet in the film formation step is reduced to trivalent chromium, so the hexavalent chromium concentration in the electrolytic solution does not increase.

[0076] It is preferable that no metal ions other than trivalent chromium ions are intentionally added to the aqueous solution. The metal ions are not limited to, but include Cu ions, Zn ions, Ni ions, Fe ions, Sn ions, etc., and the concentration of each of these ions is preferably 0 mg / L or more and 40 mg / L or less, more preferably 0 mg / L or more and 20 mg / L or less, and most preferably 0 mg / L or more and 10 mg / L or less.

[0077] (ii) Adjustment of pH and Temperature Next, the electrolytic solution is prepared by adjusting the pH of the aqueous solution to 4.0 to 7.0 and adjusting the temperature of the aqueous solution to 40 to 70° C. In order to produce the above-mentioned surface-treated steel sheet, it is not sufficient to simply dissolve a trivalent chromium ion source and a carboxylic acid compound in water; it is important to appropriately control the pH and temperature as described above.

[0078] pH: 4.0 to 7.0 In the electrolyte solution preparation step, the pH of the aqueous solution after mixing is adjusted to 4.0 to 7.0. If the pH is less than 4.0 or more than 7.0, the stability of the electrolyte solution decreases, causing precipitation and preventing the formation of a chromium-containing layer in the film formation step. Furthermore, the hexavalent chromium concentration in the electrolyte solution increases during electrolysis. The pH is preferably 4.5 or higher. The pH is preferably 6.5 or lower.

[0079] Temperature: 40 to 70°C In the electrolyte preparation step, the temperature of the aqueous solution after mixing is adjusted to 40 to 70°C. If the temperature is lower than 40°C or higher than 70°C, the stability of the electrolyte decreases, causing precipitation and preventing the formation of a chromium-containing layer in the film formation step. In addition, 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.

[0080] By the above procedure, an electrolyte solution to be used in the next film formation step can be obtained. The electrolyte solution produced by the above procedure can be stored at room temperature.

[0081] [Film Formation Step] In the film formation step, the steel sheet is subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using the electrolytic solution prepared in the electrolytic solution preparation step. Specifically, the steel sheet is immersed in the electrolytic solution and subjected to each of the electrolytic treatments. This allows the chromium-containing layer to be formed.

[0082] <<Cathode Electrolysis Treatment C1>> First, the steel sheet is subjected to the cathode electrolysis treatment C1 using the electrolytic solution. By performing the cathode electrolysis treatment C1, a chromium-containing layer can be formed on the steel sheet.

[0083] The electrical charge density of the cathodic electrolysis treatment C1 is not particularly limited. However, the amount of chromium deposited in the chromium-containing layer can be controlled by adjusting the electrical charge density of the cathodic electrolysis treatment C1. Therefore, the electrical charge density is set to 5.0 C / dm ２ More than 10.0 C / dm ２ For the same reason, the charge density is preferably 200.0 C / dm ２Preferably, 180.0 C / dm or less ２ The following is more preferred:

[0084] The current density and current application time of the cathodic electrolysis treatment C1 are not particularly limited, and can be appropriately set to achieve the desired value of the electricity density. The electricity density is the current density (unit: A / dm ２ ) and the energization time (unit: sec.).

[0085] The temperature of the electrolytic solution when performing cathodic electrolysis C1 is not particularly limited, but in order to efficiently form a chromium-containing layer, it is preferably in the temperature range of 40° C. or more and 70° C. or less. From the viewpoint of stably producing the above-mentioned surface-treated steel sheet, it is preferable to monitor the temperature of the electrolytic solution in cathodic electrolysis C1 and maintain it in the above temperature range.

[0086] The pH of the electrolytic solution used in the cathodic electrolytic treatment C1 is not particularly limited, but is preferably 4.0 or higher, more preferably 4.5 or higher. Furthermore, the pH is preferably 7.0 or lower, more preferably 6.5 or lower. From the viewpoint of stably producing the above-described surface-treated steel sheet, it is preferable to monitor the pH of the electrolytic solution during the cathodic electrolytic treatment C1 and maintain it within the above pH range.

[0087] The type of electrode used in performing the cathodic electrolysis treatment C1 is not particularly limited, and any electrode can be used. It is preferable to use an insoluble electrode as the electrode. It is preferable to use at least one selected from the group consisting of an electrode in which Ti is coated with one or both of a platinum group metal and an oxide of a platinum group metal, and a graphite 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 Ti as a substrate.

[0088] In cathodic electrolysis C1, the concentration of the electrolytic solution constantly changes due to the influence of the formation of a chromium-containing layer on the steel sheet, the introduction and removal of the solution, evaporation of water, etc. The change in the concentration of the electrolytic solution in cathodic electrolysis C1 varies depending on the configuration of the apparatus and the production conditions. Therefore, from the viewpoint of more stable production of surface-treated steel sheets, it is preferable to monitor the concentrations of the components contained in the electrolytic solution in cathodic electrolysis C1 and maintain them within the above-mentioned concentration ranges.

[0089] <Anodic Electrolysis Treatment A1> Next, the steel sheet after cathodic electrolysis treatment C1 is subjected to anodic electrolysis treatment A1 using the electrolytic solution. By performing anodic electrolysis treatment A1, the chromium-containing layer formed by cathodic electrolysis treatment C1 is dissolved, forming a generation site for granular precipitates of metallic chromium and chromium compounds in cathodic electrolysis treatment C2. In the following description, the granular precipitates of metallic chromium and chromium compounds may be simply referred to as granular chromium. Furthermore, when anodic electrolysis treatment A1 is performed, trivalent chromium is oxidized to hexavalent chromium on the steel sheet surface. However, by using the above-mentioned electrolytic solution, the hexavalent chromium is instantly reduced to trivalent chromium, so that in practice, no hexavalent chromium is present in the electrolytic solution. Therefore, it is important to use the electrolytic solution prepared in the above-mentioned electrolytic solution preparation step, particularly to make anodic electrolysis treatment A1 a treatment that does not use hexavalent chromium.

[0090] The electrical charge density of the anodic electrolysis treatment A1 is 0.50 C / dm ２ If the N / L ratio is less than 0.023 nm, the chromium-containing layer is not sufficiently dissolved, so that no generation sites for granular chromium are formed, and granular chromium is not sufficiently precipitated in the subsequent cathodic electrolytic treatment C2. As a result, the N / L ratio of the finally obtained surface-treated steel sheet is 0.023 nm. －１ Therefore, the charge density is less than 0.50 C / dm ２ or more, and 0.60 C / dm ２ More than 0.70 C / dm ２ On the other hand, the charge density is preferably 20.00 C / dm or more. ２ If the electrical charge density is more than 20.00 C / dm, the oxidation reaction of trivalent chromium will proceed locally on the steel sheet surface, increasing the hexavalent chromium concentration and making the electrolyte unstable. In addition, the amount of chromium oxide deposited may become excessive. Therefore, the electrical charge density should be set to 20.00 C / dm２ or less, 18.00 C / dm ２ Preferably, 16.00 C / dm or less ２ The following is more preferred:

[0091] The current density and current application time of the anodic electrolysis treatment A1 are not particularly limited, and can be set appropriately so as to achieve the desired value for the electrical charge density.

[0092] The temperature of the electrolytic solution when performing anodic electrolysis A1 is not particularly limited, and a suitable embodiment is the same as that of cathodic electrolysis C1. From the viewpoint of stably producing the above-mentioned surface-treated steel sheet and more reliably suppressing an increase in the hexavalent chromium concentration, it is preferable to monitor the temperature of the electrolytic solution in anodic electrolysis A1 and maintain it within the above temperature range.

[0093] The pH of the electrolytic solution when performing anodic electrolysis A1 is not particularly limited, and a suitable embodiment is the same as that of cathodic electrolysis C1. From the viewpoint of stably producing the above-mentioned surface-treated steel sheet and more reliably suppressing an increase in the hexavalent chromium concentration, it is preferable to monitor the pH of the electrolytic solution in anodic electrolysis A1 and maintain it within the above pH range.

[0094] The type of electrode used in the anodic electrolysis treatment A1 is not particularly limited, and the preferred embodiment is the same as that in the cathodic electrolysis treatment C1.

[0095] From the same viewpoint as in the cathodic electrolysis treatment C1, it is preferable to monitor the concentrations of the components contained in the electrolytic solution in the anodic electrolysis treatment A1 and maintain them within the above-mentioned concentration ranges.

[0096] Cathodic Electrolysis Treatment C2 Next, the steel sheet that has been subjected to anodic electrolysis treatment A1 is subjected to cathodic electrolysis treatment C2 using the electrolytic solution described above. By performing cathodic electrolysis treatment C2, a chromium-containing layer can be formed on the steel sheet, and particulate chromium can be precipitated starting from the generation sites described above.

[0097] The electrical density of the cathodic electrolysis treatment C2 is 50.0 C / dm ２ If the N / L ratio is less than 0.023 nm, the granular chromium will not precipitate sufficiently, and as a result, the final surface-treated steel sheet will have an N / L ratio of 0.023 nm or less. －１Therefore, the charge density is less than 50.0 C / dm ２ or more, and 55.0 C / dm ２ More than 60.0 C / dm ２ On the other hand, the charge density is preferably 150.0 C / dm or more. ２ If it exceeds 0.085 nm, granular chromium will be excessively precipitated, resulting in a dense surface. As a result, the above-mentioned N / L in the finally obtained surface-treated steel sheet will be 0.085 nm or less. －１ Therefore, the charge density is 150.0 C / dm ２ or less, 120.0 C / dm ２ Preferably, 100.0 C / dm or less ２ The following is more preferred:

[0098] The current density and current application time of the cathodic electrolysis treatment C2 are not particularly limited, and can be set appropriately so as to achieve the desired value of the electrical charge density.

[0099] The temperature of the electrolytic solution when performing cathodic electrolysis treatment C2 is not particularly limited, and a suitable embodiment is the same as that of cathodic electrolysis treatment C1. From the same viewpoint as that of cathodic electrolysis treatment C1, it is preferable to monitor the temperature of the electrolytic solution in cathodic electrolysis treatment C2 and maintain it within the above temperature range.

[0100] The pH of the electrolytic solution when performing cathodic electrolysis treatment C2 is not particularly limited, and a suitable embodiment is the same as that of cathodic electrolysis treatment C1. From the same viewpoint as that of cathodic electrolysis treatment C1, it is preferable to monitor the pH of the electrolytic solution in cathodic electrolysis treatment C2 and maintain it within the above pH range.

[0101] The type of electrode used in the cathodic electrolysis treatment C2 is not particularly limited, and the preferred embodiment is the same as that in the cathodic electrolysis treatment C1.

[0102] From the same viewpoint as in the cathodic electrolysis treatment C1, it is preferable to monitor the concentrations of the components contained in the electrolytic solution in the cathodic electrolysis treatment C2 and maintain them within the above-mentioned concentration ranges.

[0103] [Water washing] After the film forming step, the surface-treated steel sheet is preferably washed with water at least once, which makes it possible to remove the electrolytic solution remaining on the surface of the steel sheet.

[0104] The water rinsing can be carried out by any method without any particular limitation. For example, a water rinsing tank can be provided downstream of an immersion tank for carrying out the immersion treatment, and the steel sheet after immersion can be continuously immersed in water. Alternatively, the steel sheet after immersion can be rinsed by spraying water onto it.

[0105] The water used for 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.

[0106] The temperature of the water used for washing is not particularly limited and may be any temperature. However, an excessively high temperature places an excessive burden on the washing equipment, so the temperature of the water used for washing is preferably 95°C or less. On the other hand, the lower limit of the temperature of the water used for washing is not particularly limited, but is preferably 0°C or higher. The temperature of the water used for washing may be room temperature.

[0107] After the water washing, drying may be carried out as desired. The drying method is not particularly limited, and for example, a conventional dryer or electric oven drying method can be applied. The temperature during the drying treatment is preferably 100°C or less from the viewpoint of suppressing deterioration of the surface treatment film. The lower limit is not particularly limited, but is usually around room temperature.

[0108] [Pretreatment] Prior to the film forming step, the steel sheet may be subjected to any pretreatment, which is preferably at least one of degreasing, pickling, and water washing.

[0109] By degreasing, rolling oil, rust-preventive oil, etc. adhering to the steel sheet can be removed. The degreasing can be carried out by any method without any particular limitation. After degreasing, it is preferable to wash the steel sheet with water to remove the degreasing treatment solution adhering to the surface of the steel sheet.

[0110] By performing pickling, the natural oxide film present on the surface of the steel sheet can be removed, and therefore a chromium-containing layer can be effectively formed in the subsequent film formation process. The pickling can be performed by any method without any particular limitation. After the pickling, the steel sheet is preferably washed with water to remove the pickling solution adhering to the surface of the steel sheet.

[0111] The use of the surface-treated steel sheet of the present invention is 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, pail cans, and 18-liter cans.

[0112] In order to confirm the effect of the present invention, surface-treated steel sheets were produced according to the procedure described below, and their properties were evaluated.

[0113] (Electrolyte Solution Preparation Step) First, electrolyte solutions 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 prepare an aqueous solution, and then the aqueous solution was adjusted to the pH and temperature shown in Table 1. Note that electrolyte solution G corresponds to the electrolyte solution used in the examples of Patent Document 6. Ammonia water was used to increase the pH in all cases, and sulfuric acid was used for electrolyte solutions A, B, and G, hydrochloric acid for electrolyte solutions C and D, and nitric acid for electrolyte solutions E and F to decrease the pH.

[0114] (Pretreatment of Steel Sheet) A cold-rolled steel sheet was used as the steel sheet. More specifically, a steel sheet for cans (T4 base sheet) having a thickness of 0.17 mm was used. As pretreatment, the steel sheet was subjected to electrolytic degreasing, water washing, pickling by immersion in dilute sulfuric acid, and water washing in that order.

[0115] (Film Formation Step) Next, the steel sheet was subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order under the conditions shown in Table 2. The electrolytic solution during each electrolysis was maintained at the pH and temperature shown in Table 1. An insoluble electrode in which iridium oxide was coated on a Ti substrate was used as the electrode for each electrolysis treatment. After performing cathodic electrolysis C2, the steel sheet was washed with water having an electrical conductivity of 100 μS / m or less and dried at room temperature using a blower. When the above electrolysis treatment was performed using electrolytic solution G, the stability of the electrolytic solution decreased, and a significant chromium-containing layer could not be formed due to the generation of precipitation and hexavalent chromium, so subsequent measurements and evaluations were not performed.

[0116] For each of the obtained surface-treated steel sheets, the chromium deposition amount per side of the chromium-containing layer and the chromium oxide deposition amount per side of the steel sheet were measured by the methods described above. Furthermore, for each of the obtained surface-treated steel sheets, the N / L ratio and the atomic ratio of C to Cr in the chromium-containing layer were measured by the methods described above. The measurement results are shown in Table 3.

[0117] Furthermore, the obtained surface-treated steel sheets were evaluated for film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability by the following methods. The evaluation results are also shown in Table 3.

[0118] (Preparation of Samples) Laminated steel sheets were prepared as samples to be used for evaluating the corrosion resistance of films by the following procedure.

[0119] Both sides of the obtained surface-treated steel sheet were laminated with an isophthalic acid copolymerized 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 to produce a laminated steel sheet. The lamination was performed under conditions that resulted in a resin film with a crystallinity of 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 pressure bonding to water cooling of 1 sec. The crystallinity of the resin film was determined by a density gradient tube method in accordance with JIS K7112. The nip length refers to the length in the conveying direction of the portion where the rubber roll and the steel sheet contact each other.

[0120] In addition, coated steel sheets were prepared as samples to be used for evaluating the coating corrosion resistance and secondary coating adhesion by the following procedure.

[0121] An epoxy phenol-based paint was applied to the surface of the obtained surface-treated steel sheet, and the surface was baked at 210°C for 10 minutes to prepare a painted steel sheet. The coating weight was 50 mg / dm ２ It was decided.

[0122] (Film Corrosion Resistance, Paint Corrosion Resistance) Crosscuts were made on the film surface of the prepared laminated steel sheet and the painted surface of the painted steel sheet using a cutter, reaching the steel substrate (steel sheet). The crosscut laminated steel sheet and painted steel sheet were immersed for 96 hours in a test solution at 55°C consisting of a mixed aqueous solution containing 1.5% by mass of citric acid and 1.5% by mass of salt. After immersion, washing, and drying, cellophane adhesive tape was applied to the film surface of the laminated steel sheet and the painted surface of the painted steel sheet, and then peeled off. For film corrosion resistance, the film peel width (total width extending from the cut) was measured at four arbitrary locations on the crosscut portion of the laminated steel sheet, and the average of the four values ​​was calculated and considered to be the corrosion width. For paint corrosion resistance, the paint peel width (total width extending from the cut) was measured at four arbitrary locations on the crosscut portion of the painted steel sheet, and the average of the four values ​​was calculated and considered to be the corrosion width. The corrosion resistance of the film and the corrosion resistance of the coating were evaluated according to the following four levels. In practice, a rating of 1 to 3 can be said to be excellent in corrosion resistance: 1: Corrosion width less than 0.3 mm 2: Corrosion width 0.3 mm or more but less than 0.5 mm 3: Corrosion width 0.5 mm or more but less than 1.0 mm 4: Corrosion width 1.0 mm or more

[0123] (Paint Secondary Adhesion) Two coated steel plates prepared under the same conditions were laminated with the painted surfaces facing each other, sandwiching a nylon adhesive film between them, and then bonded together under pressure of 2.94 × 105 Pa, a temperature of 190°C, and a compression time of 30 seconds. This was then divided into 5 mm wide test pieces. The divided test pieces were immersed for 168 hours in a test solution at 55°C consisting of a mixed aqueous solution containing 1.5 mass% citric acid and 1.5 mass% table salt. After immersion, washing, and drying, the two steel plates of the divided test pieces were peeled apart using a tensile tester, and the tensile strength at the time of peeling was measured. The average value of three test pieces was evaluated using the following four levels. In practical terms, a rating of 1 to 3 indicates excellent paint secondary adhesion. 1: 2.5 kgf or more; 2: 2.0 kgf or more but less than 2.5 kgf; 3: 1.5 kgf or more but less than 2.0 kgf; 4: Less than 1.5 kgf.

[0124] (Weldability) The obtained surface-treated steel sheets were subjected to a heat treatment at 210°C for 10 minutes, simulating a paint baking process, and then two samples were sandwiched between DR-type 1 mass% Cr-Cu electrodes (electrodes processed to have a tip diameter of 2.3 mm and a curvature radius of 40 mm), and a current was applied under the following conditions: Amada Miyachi transistor power supply: MDA-8000A, welding head: AH-200, pressure: 45 kgf, current application time: 1.6 msec (slope 0.2 msec), waveform: square wave

[0125] The appropriate current range (= upper limit current - lower limit current) was determined from the lower limit current at which sufficient strength can be obtained and the upper limit current at which no expulsion occurs, and was evaluated using the following four levels. In practice, a rating of 1 to 3 can be said to indicate excellent weldability: 1: 0.6 kA or more 2: 0.4 kA or more, less than 0.6 kA 3: 0.2 kA or more, less than 0.4 kA 4: Less than 0.2 kA

[0126] As is clear from the results shown in Table 3, all of the surface-treated steel sheets satisfying the conditions of the present invention were produced without using hexavalent chromium, but they all had excellent film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability.

[0127]

[0128]

[0129]

Claims

1. A surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, wherein a cutoff value λ is applied to a cross-sectional curve extracted from a cross-sectional image of the chromium-containing layer. ｓ = 5 nm low-pass filter and cutoff value λ ｃ In the roughness curve of the chromium-containing layer obtained by applying a high-pass filter with a filter width of 65 nm, the number N of peaks whose height from the mean line of the roughness curve at the apex exceeds 1.2 nm and the evaluation length L of the roughness curve is N / L 0.023 nm. －１ 0.085nm or more －１ or less, wherein the chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less.

2. The chromium coating amount of the chromium-containing layer is 40.0 to 500.0 mg / m per side. ２ The surface-treated steel sheet according to claim 1 , 3. The chromium oxide deposition amount of the chromium-containing layer is 40.0 mg / m per side. ２ The surface-treated steel sheet according to claim 1 or 2, wherein:

4. A method for producing a surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, the method comprising: an electrolyte solution preparation step of preparing an electrolyte solution containing trivalent chromium ions; and a film formation step of forming the chromium-containing layer, wherein the electrolyte solution preparation step comprises mixing a trivalent chromium ion source, a carboxylic acid compound, and water, and adjusting the pH to 4.0 to 7.0 and the temperature to 40 to 70°C to prepare the electrolyte solution; and the film formation step comprises subjecting the steel sheet to cathodic electrolysis treatment C1, anodic electrolysis treatment A1, and cathodic electrolysis treatment C2 in this order using the electrolyte solution, and the electrical charge density of the anodic electrolysis treatment A1 is 0.50 C / dm ２ 20.00C / dm or more ２ The electric charge density of the cathodic electrolysis treatment C2 is 50.0 C / dm or less. ２ 150.0C / dm or more ２ The method for producing a surface-treated steel sheet is as follows.

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