Surface-treated steel sheet and manufacturing method therefor

Abstract

Provided is a surface-treated steel sheet which can be manufactured without using hexavalent chromium and has excellent corrosion resistance in a BPA-free coated part. This surface-treated steel sheet comprises a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, wherein: an oxygen-enriched region containing at least 10% of O in terms of atomic concentration is present in the vicinity of a boundary surface between the steel sheet and the chromium-containing layer; and the vicinity is a region in which the atomic ratio of Fe to Cr is in the range of 0.7-1.3.

Description

Surface-treated steel sheet and its manufacturing method

[0001] The present invention relates to a surface-treated steel sheet, and more particularly to a surface-treated steel sheet having excellent corrosion resistance in a BPA (bisphenol A)-free painted area. 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 sheets (tinplate) and tin-free steel sheets (TFS) have been widely used as materials for various metal cans such as beverage cans, food cans, pail cans, and 18-liter cans.

[0003] Tinplate and TFS are used with organic resin coatings such as epoxy paints and PET films to accommodate a variety of contents. When applying an organic resin coating to tinplate or TFS, the steel sheet is electrolytically treated or immersed in an aqueous solution containing hexavalent chromium to form a chromium oxide layer on the outermost surface, which exhibits excellent adhesion to the organic resin coating. Therefore, the deformation of the organic resin coating follows the deformation of the steel sheet during can manufacturing, ensuring corrosion resistance to a variety of contents even after can manufacturing.

[0004] On the other hand, it has been suggested that BPA contained in epoxy-based paints may have harmful effects on humans. Therefore, development of BPA-free paints using polyester-based resins that do not contain BPA is underway (Patent Documents 1 and 2), and there is a demand for replacing epoxy-based paints with BPA-free paints. However, conventional tinplate and TFS have poorer adhesion to BPA-free paints than to epoxy-based paints, so the BPA-free paints cannot keep up with the deformation of the steel plate that occurs during can manufacturing, and sufficient corrosion resistance against various contents after can manufacturing has not been ensured. Therefore, the application of BPA-free paints to various metal cans has not progressed at present.

[0005] Furthermore, 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 various metal cans, 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 3 to 6. 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] Japanese Patent Application Laid-Open No. 2013-144753 Japanese Patent Application Laid-Open No. 2008-50486 Japanese Patent Application Laid-Open No. 2016-501985 Japanese Patent Application Laid-Open No. 2016-505708 Japanese Patent Application Laid-Open No. 2020-172700 Japanese Patent Application Laid-Open No. 2020-172701

[0008] According to the methods proposed in Patent Documents 3 to 6, a surface treatment layer can be formed without using hexavalent chromium. According to Patent Documents 3 to 6, a surface-treated steel sheet having excellent adhesion to an epoxy-based paint can be obtained by the methods. Furthermore, according to Patent Documents 3 and 4, a surface-treated steel sheet that exhibits excellent corrosion resistance even after being coated with an epoxy-based paint and subjected to deformation can be obtained.

[0009] However, although the surface-treated steel sheets obtained by the conventional methods proposed in Patent Documents 3 to 6 have excellent adhesion to epoxy-based paints and excellent corrosion resistance in the epoxy-based painted areas, the corrosion resistance in the BPA-free painted areas is insufficient. Therefore, it has not been possible to replace conventional epoxy-based paints with BPA-free paints while maintaining corrosion resistance to various contents.

[0010] Therefore, there is a demand for a surface-treated steel sheet that can be produced without using hexavalent chromium and has excellent corrosion resistance in the BPA-free painted area.

[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 has excellent corrosion resistance in a BPA-free painted area.

[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 on at least one side, by providing an oxygen-enriched region between the steel sheet and the chromium-containing layer, it is possible to obtain a surface-treated steel sheet having excellent corrosion resistance in the BPA-free painted area.

[0014] (2) The surface-treated steel sheet is prepared by contacting a steel sheet with an aqueous solution containing sulfate ions, and applying the aqueous solution to the surface of the steel sheet at a concentration of 30.0 g / m ２ Super 60.0g / m ２ The steel sheet is then subjected to cathodic electrolysis in an electrolytic solution containing 0.05 mol / L or more of trivalent chromium ions, after which the steel sheet is maintained in the presence of 0.10 to 20.0 seconds.

[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 an oxygen-enriched region containing 10% or more O in atomic concentration exists near the interface between the steel sheet and the chromium-containing layer, and the vicinity is a region where the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3.

[0017] 2. The surface-treated steel sheet according to 1 above, wherein the coverage of the oxygen-enriched region in the vicinity is 50% or more.

[0018] 3. The chromium coating amount of the chromium-containing layer is 40.0 to 500.0 mg / m per side. ２ 3. The surface-treated steel sheet according to 1 or 2 above.

[0019] 4. The chromium oxide deposition amount of the chromium-containing layer is 40.0 mg / m per side. ２ 4. The surface-treated steel sheet according to any one of 1 to 3, which is as follows:

[0020] 5. 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 contacting the steel sheet with an aqueous solution containing sulfate ions, and applying the aqueous solution to the surface of the steel sheet at a concentration of 30.0 g / m ２ Super 60.0g / m ２and a cathodic electrolysis treatment step of cathodic electrolysis of the steel sheet in an electrolytic solution containing 0.05 mol / L or more of trivalent chromium ions.

[0021] 6. The method for producing a surface-treated steel sheet according to 5 above, wherein the electrolytic solution is prepared by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, adjusting the pH to 4.0 to 7.0, and adjusting the temperature to 40 to 70°C.

[0022] According to the present invention, it is possible to provide a surface-treated steel sheet having excellent corrosion resistance in a BPA-free painted area without using hexavalent chromium. The surface-treated steel sheet of the present invention can be suitably used as a material for containers, etc.

[0023] 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.

[0024] A surface-treated steel sheet according to one embodiment of the present invention is a steel sheet having a chromium-containing layer on at least one surface of the steel sheet. It is important in the present invention that an oxygen-enriched region exists between the chromium-containing layer and the steel sheet. Hereinafter, each of the constituent elements of the surface-treated steel sheet will be described.

[0025] [Steel Sheet] The steel sheet is not particularly limited and any steel sheet can be used, but it is preferable to use a steel sheet for cans. For example, an ultra-low carbon steel sheet or a low carbon steel sheet can be used as the steel sheet. The manufacturing method of the steel sheet is also not particularly limited and a steel sheet manufactured by any method can be used, but typically a cold-rolled steel sheet can be used. 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.

[0026] The composition of the steel sheet is not particularly limited, but may contain C, Mn, P, S, Si, Cu, Ni, Mo, Al, and unavoidable impurities within a range that does not impair the effects of the present invention. In this case, for example, a steel sheet having a composition specified in ASTM A623M-09 can be suitably used as the steel sheet.

[0027] In one embodiment of the present invention, the composition is, in mass%, C: 0.0001 to 0.13%, Si: 0 to 0.020%, Mn: 0.01 to 0.70%, P: 0 to 0.15%, S: 0 to 0.050%, Al: 0 to 0.20%, N: 0 to 0.040%, 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%, Sb: 0 to 0.020%, It is preferable to use a steel sheet having a composition consisting of Cr, P, S, Al, and N, and the balance being Fe and unavoidable impurities. Of the above composition, the lower the content of Si, P, S, Al, and N, the more preferable it is. Cu, Ni, Cr, Mo, Ti, Nb, B, Ca, Sn, and Sb are components that can be added as desired.

[0028] The thickness of the steel plate is not particularly limited, but is preferably 0.60 mm or less. On the other hand, the lower limit of the thickness is also not particularly limited, but is preferably 0.10 mm or more. Here, "steel plate" is defined to include "steel strip."

[0029] [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.

[0030] 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.

[0031] 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.

[0032] 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 %.

[0033] 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.

[0034] 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

[0035] 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.

[0036] 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.

[0037] The total content refers to a value at a midpoint in the thickness of the chromium-containing layer. The midpoint can be determined by the following procedure. First, the chromium-containing layer is sputtered from its outermost surface, and the total content of elements constituting the metallic chromium and the chromium compound and the Fe content are measured by the method described above. Then, the midpoint (half) between the position (depth) where the measured total content of elements constituting the metallic chromium and the chromium compound and the Fe content become equal is determined as the midpoint between the outermost surface of the chromium-containing layer and the depth.

[0038] 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.

[0039] 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.

[0040] The chromium coating amount of the chromium-containing layer is not particularly limited. However, if the chromium coating amount of the chromium-containing layer is excessive, cohesive failure may occur in the chromium-containing layer when the surface-treated steel sheet is processed. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free painted processed portion, the chromium coating amount of the chromium-containing layer is set to 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 corrosion resistance of the BPA-free painted portion, 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. ２ Here, the "chromium deposition amount" refers to the total deposition amount of chromium present in various forms.

[0041] The chromium deposition weight can be measured by X-ray fluorescence analysis. More specifically, the chromium deposition weight is measured by the following procedure. First, the Cr amount (total Cr amount) in the surface-treated steel sheet is measured using an X-ray fluorescence analyzer. Next, the Cr amount (original sheet Cr amount) in the steel sheet before the chromium-containing layer is formed or after the chromium-containing layer has been stripped is measured using the X-ray fluorescence analyzer. The chromium deposition weight of the chromium-containing layer is determined by subtracting the original sheet Cr amount from the total Cr amount. For example, a commercially available hydrochloric acid-based chromium plating stripper can be used to strip the chromium-containing layer.

[0042] [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).

[0043] 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, cohesive failure may occur from the chromium oxide in the chromium-containing layer when the surface-treated steel sheet is processed, and the corrosion resistance of the BPA-free painted area may deteriorate. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free painted area, 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.

[0044] The chromium oxide deposition weight can be measured by X-ray fluorescence analysis. More specifically, the chromium oxide deposition weight is measured by 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 an 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 weight of the chromium-containing layer.

[0045] The chromium-containing layer may be amorphous or crystalline. That is, the chromium-containing layer may contain one or both of an amorphous 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 chromium-containing layer formation 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 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. On the other hand, since the crystalline region does not necessarily have to exist, the lower limit of the area ratio of the crystalline region may be 0%.

[0046] The crystalline region in the chromium-containing layer can be confirmed by preparing a chromium-containing layer single layer sample by removing the substrate steel sheet portion from the surface-treated steel sheet, and observing the chromium-containing layer single layer sample from the surface side using a TEM or STEM. The method for preparing the chromium-containing layer single layer sample is not particularly limited, but it can be prepared, for example, 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 layer 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.

[0047] 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 first obtained by selected area diffraction using a TEM. Next, dark-field images are obtained for all diffraction spots in the diffraction pattern, and the regions that appear brighter in the dark-field image are identified 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.

[0048] The chromium-containing layer may contain C. Although the upper limit of the C content in the chromium-containing layer is not particularly limited, it is preferably 50% or less, more preferably 45% or less, in terms of atomic ratio to Cr. The chromium-containing layer may not contain C, and therefore the lower limit of the atomic ratio of C to Cr contained in the chromium-containing layer is not particularly limited and may be 0%.

[0049] The C content in the chromium-containing layer can be measured by XPS. ２ The XPS measurement can be performed using, 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, and the take-off angle is 45°. The sputtering conditions are Ar ions with an acceleration voltage of 1 kV, a sputtering rate of SiO ２ This can be converted to 1.50 nm / min.

[0050] 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.

[0051] The location of C in the chromium-containing layer is not particularly limited, and 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).

[0052] 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.

[0053] The mechanism by which Fe is incorporated into the chromium-containing layer is not clear, but it is thought that in 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.

[0054] 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 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 BPA-free painted area. Therefore, the total atomic ratio of elements other than Cr, O, Fe, and C to Cr is preferably 3% or less, and more preferably 0%. 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.

[0055] [Oxygen-enriched Region] The surface-treated steel sheet of the present invention has an oxygen-enriched region near the interface between the steel sheet and the chromium-containing layer (hereinafter, this vicinity may also be referred to as the near-interface region), which contains O at an atomic concentration of 10% or more. Here, the near-interface region refers to a region where the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3. The presence of this oxygen-enriched region in the near-interface region thus enables the realization of excellent corrosion resistance of the BPA-free painted area. From the viewpoint of further improving the corrosion resistance of the BPA-free painted area, the coverage of the oxygen-enriched region in the near-interface region is preferably 50% or more, more preferably 70% or more. The upper limit of the coverage of the oxygen-enriched region in the near-interface region is not particularly limited and may be 100%. In the following description, the oxygen-enriched region refers only to the region present in the near-interface region.

[0056] The reason why the corrosion resistance of the BPA-free painted processed portion is improved by providing an oxygen-enriched region as described above will be explained below.

[0057] First, a typical chromium-containing layer formed from a hexavalent chromium bath or a trivalent chromium bath is composed of metallic chromium or chromium oxide. Surface-treated steel sheets with such a chromium-containing layer are typically processed into cans and other products after an organic resin coating is formed on the surface. However, because metallic chromium has poor workability, the chromium-containing layer cannot completely adapt to the deformation of the steel sheet during processing, resulting in damage to the organic resin coating on the chromium-containing layer. This, in turn, reduces corrosion resistance after processing.

[0058] Therefore, in conventional surface-treated steel sheets, corrosion resistance after processing has been ensured by providing a chromium oxide top layer. That is, because chromium oxide has excellent adhesion to epoxy-based paints, even if metallic chromium cannot follow the deformation of the base steel sheet, the chromium-containing layer and the epoxy-based paint adhere firmly to each other, and the coating properties of the epoxy-based paint can be maintained even after can making.

[0059] However, such conventional surface-treated steel sheets have poor adhesion to BPA-free paints, and therefore have poor corrosion resistance in processed areas when coated with BPA-free paint.

[0060] In contrast, the surface-treated steel sheet of the present invention, as described above, provides an oxygen-enriched region in the region near the interface, thereby achieving excellent corrosion resistance in the BPA-free painted area. This is thought to be because the strain introduced into the chromium-containing layer during processing of the surface-treated steel sheet is dispersed, allowing the chromium-containing layer to follow the deformation of the steel sheet, and the oxygen-enriched region itself exhibits excellent corrosion resistance. Thus, the present invention is based on a completely different technical concept from conventional ones, which is to improve the deformability of the chromium-containing layer itself and the corrosion resistance near the interface, rather than the adhesion to the paint.

[0061] In the present invention, the near-interface region and the oxygen-enriched region are determined as follows. First, five measurement regions are selected to include both the chromium-containing layer and the steel sheet, and five ion maps are obtained by performing three-dimensional composition analysis on the measurement regions using 3DAP. Next, the ion map is divided into 2 nm x 2 nm x 2 nm voxels, and the composition of each voxel is calculated in terms of atomic concentration. The near-interface region is defined as a region consisting of voxels in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3. Furthermore, the oxygen-enriched region is defined as a region consisting of voxels in the near-interface region in which O is present at an atomic concentration of 10% or more. In this way, the near-interface region and the oxygen-enriched region are determined by measuring the atomic ratio of Fe to Cr and the atomic concentration of O by three-dimensional composition analysis using a three-dimensional atom probe. In defining the near-interface region, the position where the atomic ratio of Fe to Cr is 1.0 is considered to be the interface between the chromium-containing layer and the steel sheet. That is, the near-interface region includes the interface between the chromium-containing layer and the steel sheet.

[0062] In the present invention, the coverage is defined as follows: First, for each ion map, the number of voxels in the near-interface region is defined as I, and the number of voxels in the oxygen-enriched region is defined as O, and the percentage is calculated using the formula O / I x 100. Then, the average value of the percentages for each ion map is defined as the coverage of the oxygen-enriched region in the near-interface region. In this way, the coverage of the oxygen-enriched region is measured by three-dimensional composition analysis using a three-dimensional atom probe.

[0063] The oxygen-enriched region may contain C. The upper limit of the C content in the oxygen-enriched region is not particularly limited, but is preferably 20 atomic % or less. The oxygen-enriched region may not contain C, and therefore the lower limit of the C content in the oxygen-enriched region is not particularly limited and may be 0 atomic %.

[0064] In addition to O, Fe, Cr, and C, the oxygen-enriched region may contain metals such as Si and Mn as oxides. However, if elements other than O, Fe, Cr, and C are present in large amounts, the corrosion resistance of the BPA-free painted portion may be reduced. From this perspective, it is preferable that the total content of O, Fe, Cr, and C in the oxygen-enriched region is 70 atomic % or more. From the same perspective, it is preferable that the oxygen-enriched region contains no Si or Mn oxides (0%). The content of the oxides can be measured, for example, by XPS.

[0065] The atomic concentration of each element in the oxygen-enriched region can be measured by three-dimensional composition analysis using 3DAP, as in determining the near-interface region and the oxygen-enriched region. More specifically, five measurement regions are first selected to include both the chromium-containing layer and the steel sheet, and five ion maps are obtained by performing three-dimensional composition analysis using 3DAP on the measurement regions. Next, the ion maps are divided into 2 nm × 2 nm × 2 nm voxels, and the composition of each voxel is calculated in terms of atomic concentration. Then, the compositions of the voxels within the oxygen-enriched region for each ion map are averaged to calculate the average composition for each ion map, i.e., the average composition for each measurement region. Finally, the average compositions for each measurement region are further averaged to calculate the atomic concentration of each element in the oxygen-enriched region.

[0066] The oxygen-enriched region may be amorphous or crystalline. That is, the oxygen-enriched region may contain one or both of an amorphous and a crystalline phase. However, from the viewpoint of improving corrosion resistance, it is preferable that the oxygen-enriched region be amorphous. Whether the oxygen-enriched region is amorphous or not can be determined, for example, by preparing an observation sample including the oxygen-enriched region and observing the observation sample with a TEM or the like. That is, when a diffraction pattern of the oxygen-enriched region is obtained, if no diffraction spots appear in the diffraction pattern, the oxygen-enriched region is amorphous.

[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 on at least one surface of the steel sheet, and includes a steel sheet surface conditioning step and a cathodic electrolytic treatment step. Each step will be described below.

[0069] [Steel Sheet Surface Conditioning Step] In the present invention, it is important to carry out a steel sheet surface conditioning step, prior to the cathodic electrolytic treatment described below, in which the steel sheet is brought into contact with an aqueous solution containing sulfate ions and maintained for a predetermined time in a state in which a predetermined amount of the aqueous solution is present on the surface of the steel sheet.

[0070] Amount of aqueous solution: 30.0 g / m ２ Super 60.0g / m ２ In order to provide an oxygen-enriched region in the region near the interface in the finally obtained surface-treated steel sheet, the steel sheet is brought into contact with an aqueous solution containing sulfate ions in a steel sheet surface conditioning step, and the aqueous solution is applied to the surface of the steel sheet at a concentration of 30.0 g / m ２ Super 60.0g / m ２ It is necessary to maintain the state in which the current exists for 0.10 seconds or more and 20.0 seconds or less.

[0071] The mechanism by which an oxygen-enriched region is formed in the region near the interface by the steel sheet surface conditioning step is not clear, but is thought to be as follows: When a steel sheet is brought into contact with an aqueous solution containing sulfate ions, a dissolution reaction of Fe and a decomposition reaction of dissolved oxygen occur on the surface of the steel sheet, causing an increase in the pH of the steel sheet surface. In this case, if the amount of the aqueous solution is within the above range, the thickness of the aqueous solution on the steel sheet becomes very thin, and the amount of dissolved oxygen in the aqueous solution increases. As a result, the above reaction is further accelerated. Note that the state of the aqueous solution on the steel sheet is not particularly limited, but from the viewpoint of ensuring a uniform reaction, it is preferable that the aqueous solution be in the form of a liquid film.

[0072] If dissolved Fe ions are present on the steel sheet surface where the pH has increased, the Fe ions are oxidized to Fe oxide, which accumulates on the steel sheet surface in very small amounts. In the subsequent cathodic electrolysis treatment step, the accumulated Fe oxide is reduced and a chromium-containing layer is formed. Although the mechanism is unclear, the Fe oxide accumulated in small amounts by the above method can form a chromium-containing layer on top of the Fe oxide even if it is not completely reduced in the subsequent cathodic electrolysis treatment step. As a result, it is presumed that an oxygen-enriched region is formed in the region near the interface.

[0073] From the viewpoint of forming more oxygen-enriched regions and further improving the corrosion resistance of the BPA-free painted processed portion, the amount of the aqueous solution is 32.0 g / m ２ It is preferable that the density is 35.0 g / m or more. ２ From the same viewpoint, the amount of the aqueous solution is more preferably 58.0 g / m or more. ２ It is preferable that the density is 55.0 g / m or less. ２ It is more preferable to set the following:

[0074] In order to form more oxygen-enriched regions and further improve the corrosion resistance of the BPA-free coated part, the holding time is preferably 0.2 seconds or more, more preferably 0.3 seconds or more, and from the same viewpoint, the holding time is preferably 18.0 seconds or less, more preferably 15.0 seconds or less.

[0075] The amount of aqueous solution present on the surface of the steel sheet can be measured using a moisture meter based on a filter-type infrared absorption method. Specifically, the absorbance on the steel sheet surface is measured using a moisture meter based on a filter-type infrared absorption method, and the amount of aqueous solution is determined from the absorbance using a previously obtained calibration curve. The calibration curve can be created by the following procedure. First, a steel sheet is placed on an electronic balance. The aqueous solution is dropped onto the steel sheet using a pipette to form a liquid film over the entire surface of the steel sheet. The weight of the aqueous solution present on the steel sheet is calculated from the weight of the steel sheet before and after the dropping of the aqueous solution. The amount of aqueous solution per unit area is calculated by dividing the resulting weight of the aqueous solution by the area of ​​the steel sheet. Simultaneously, the absorbance on the steel sheet surface is measured using a moisture meter based on a filter-type infrared absorption method. The above measurements are performed multiple times while changing the amount of aqueous solution, and a calibration curve showing the correlation between the amount of aqueous solution and absorbance is created. The calibration curve can be a linear approximation of the correlation between the amount of aqueous solution and absorbance.

[0076] The method for adjusting the amount of aqueous solution present on the steel sheet surface is not particularly limited, and any method can be used, such as squeezing the solution with a wringer roll or wiping.

[0077] The composition of the aqueous solution is not particularly limited, but it is preferably an aqueous sulfuric acid solution such as dilute sulfuric acid. Here, the aqueous sulfuric acid solution means an aqueous solution of sulfuric acid, and includes cases where components other than sulfuric acid are contained.

[0078] When a sulfuric acid aqueous solution is used as the pickling solution in the pretreatment step described below, the pickling solution can also be used as the aqueous solution in the steel sheet surface conditioning step. Although pickling solutions generally contain pickling inhibitors, pickling accelerators, and the like, these components do not particularly hinder the formation of oxygen-enriched regions. Therefore, even if a pickling inhibitor, pickling accelerator, or the like is added to the pickling solution, the pickling solution can be used as the aqueous solution in the steel sheet surface conditioning step.

[0079] The lower limit of the concentration of sulfate ions contained in the aqueous solution is not particularly limited, but is preferably 3 g / L or more, more preferably 5 g / L or more.The upper limit of the concentration of sulfate ions contained in the aqueous solution is not particularly limited, but is preferably 200 g / L or less, more preferably 150 g / L or less.

[0080] The lower limit of the temperature of the aqueous solution is not particularly limited, but is preferably 10° C. or higher, and more preferably 15° C. or higher. The upper limit of the temperature of the aqueous solution is not particularly limited, but is preferably 70° C. or lower, and more preferably 60° C. or lower.

[0081] After the steel sheet surface conditioning step, the steel sheet is preferably washed with water to remove the aqueous solution adhering to the steel sheet.

[0082] [Cathodic Electrolysis Treatment Step] Next, the steel sheet is subjected to cathodic electrolysis in an electrolytic solution containing 0.05 mol / L or more of trivalent chromium ions. This cathodic electrolysis treatment allows a chromium-containing layer to be formed on the steel sheet. 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.

[0083] The temperature of the electrolytic solution during cathodic electrolysis is not particularly limited, but is preferably 40°C or higher in order to efficiently form a chromium-containing layer. For the same reason, the temperature of the electrolytic solution is preferably 70°C or lower. 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 the cathodic electrolytic treatment step and maintain the temperature of the electrolytic solution in a temperature range of 40 to 70°C.

[0084] The pH of the electrolytic solution used in the cathodic electrolysis treatment 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-mentioned surface-treated steel sheet, it is preferable to monitor the pH of the electrolytic solution in the cathodic electrolysis treatment step and maintain it within the above pH range.

[0085] The current density in the cathodic electrolysis treatment is not particularly limited and may be adjusted appropriately so that a desired surface treatment layer is formed. However, if the current density is excessively high, the burden on the cathodic electrolysis treatment device will be excessive. Therefore, the current density is set to 200.0 A / dm ２ It is preferable that the current is 100.0 A / dm or less. ２ The lower limit of the current density is not particularly limited, but if the current density is too low, hexavalent Cr may be generated in the electrolyte, which may cause the stability of the bath to be lost. Therefore, the current density should be 5.0 A / dm or less. ２ It is preferable that the current is 10.0 A / dm or more. ２ More preferably, it is set to be equal to or greater than this.

[0086] The number of times that the steel sheet is subjected to cathodic electrolysis is not particularly limited and can be any number of times. In other words, cathodic electrolysis can be performed using an electrolytic treatment apparatus having one or any number of passes (two or more). For example, it is also preferable to perform cathodic electrolysis continuously by passing the steel sheet (steel strip) through multiple passes while conveying it. Note that, if the number of cathodic electrolysis treatments (i.e., the number of passes) is increased, a corresponding number of electrolytic cells will be required, so the number of cathodic electrolysis treatments (number of passes) is preferably 20 or less.

[0087] The electrolysis time per pass is not particularly limited. However, if the electrolysis time per pass is too long, the conveying speed (line speed) of the steel sheet decreases, resulting in a decrease in productivity. Therefore, the electrolysis time per pass is preferably 5 seconds or less, and more preferably 3 seconds or less. There is no particular limit to the lower limit of the electrolysis time per pass, but if the electrolysis time is too short, it becomes necessary to increase the line speed accordingly, making control difficult. Therefore, the electrolysis time per pass is preferably 0.005 seconds or more, and more preferably 0.01 seconds or more.

[0088] The chromium deposition amount of the chromium-containing layer formed by cathodic electrolysis can be controlled by the total electrical density, which is expressed as the product of the current density, the electrolysis time, and the number of passes. As described above, an excessively low chromium deposition amount impairs the corrosion resistance of the BPA-free coated area, while an excessively high chromium deposition amount may cause cohesive failure within the chromium-containing layer during processing. Therefore, from the perspective of more stably ensuring the corrosion resistance of the BPA-free coated area, it is preferable to control the total electrical density so that the chromium deposition amount per side of the steel sheet in the chromium-containing layer is within an appropriate range. However, since the relationship between the chromium deposition amount per side of the steel sheet in the chromium-containing layer and the total electrical density varies depending on the configuration of the equipment used in the cathodic electrolysis process, the actual electrolysis conditions can be adjusted to suit the equipment.

[0089] The type of anode used when performing cathodic electrolysis is not particularly limited, and any anode can be used. It is preferable to use an insoluble anode as the anode. It is preferable to use at least one selected from the group consisting of an anode 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 anode. More specifically, examples of the insoluble anode include an anode in which platinum, iridium oxide, or ruthenium oxide is coated on the surface of Ti as a substrate.

[0090] In the cathodic treatment step, 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 take-out and take-in of the solution, the evaporation of water, etc. The change in the concentration of the electrolytic solution in the cathodic electrolytic treatment step 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 the cathodic electrolytic treatment step and maintain them within the concentration ranges described below.

[0091] After the cathodic electrolytic treatment step, the 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] Prior to the steel sheet surface conditioning step, the steel sheet may be optionally pretreated, and the pretreatment preferably includes at least one of degreasing, pickling, and water washing.

[0097] 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.

[0098] By performing pickling, the natural oxide film present on the surface of the steel sheet can be removed, allowing the surface to be effectively conditioned in the subsequent steel sheet surface conditioning step. 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 treatment solution adhering to the surface of the steel sheet. When an aqueous solution containing sulfate ions is used as the pickling treatment solution, it is preferably subjected to the steel sheet surface conditioning step as is.

[0099] The method for preparing the electrolyte used in the cathodic electrolysis treatment step is not particularly limited, but by preparing the electrolyte as described below, it is possible to provide the electrolyte for the cathodic electrolysis treatment step stably for a longer period of time.

[0100] [Preparation of Electrolyte Solution] (i) Mixing When preparing the above-mentioned electrolyte solution, it is preferable to first mix a trivalent chromium ion source, a carboxylic acid compound, and water to prepare an aqueous solution.

[0101] 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.

[0102] The content of the trivalent chromium ion source in the aqueous solution must be 0.05 mol / L or more, preferably 0.08 mol / L or more, and more preferably 0.10 mol / L or more, calculated as trivalent chromium ions. The upper limit of the content of the trivalent chromium ion source is not particularly limited, but is preferably 1.50 mol / L or less, more preferably 1.30 mol / L or less, calculated as trivalent chromium ions. BluCr (registered trademark) TFS A from Atotech can be used as the trivalent chromium ion source.

[0103] 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 a salt of an aliphatic carboxylic acid. 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 from Atotech can be used.

[0104] The solvent for preparing the aqueous solution can be water, and it is preferable to use at least one of ion-exchanged water and distilled water.

[0105] In order to effectively suppress the generation of hexavalent chromium at the anode in the cathodic electrolytic treatment step and improve the stability of the above-mentioned electrolyte, it is preferable that the aqueous solution further contains 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 contain the halide ion, BluCr (registered trademark) TFS C1 and BluCr (registered trademark) TFS C2 from Atotech can be used.

[0106] It is preferable not to add hexavalent chromium to the above-mentioned aqueous solution. It has been confirmed that hexavalent chromium is not formed in principle even in the cathodic electrolysis process, but even if a small amount of hexavalent chromium is formed at the anode or the like, it is immediately reduced to trivalent chromium, so the hexavalent chromium concentration in the electrolyte does not increase.

[0107] 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, but include Cu ions, Zn ions, Fe ions, Sn ions, Ni ions, etc., each of which preferably has a concentration of 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. Among the metal ions, Fe ions may dissolve in the electrolyte during the cathodic electrolysis and immersion steps and co-deposit into the coating, but this does not affect the corrosion resistance of the BPA-free coated part. While the Fe ion concentration is preferably within the above range during bath preparation, it is also preferable to maintain the Fe ion concentration in the electrolyte within the above range during the cathodic electrolysis and immersion steps. Controlling the Fe ion concentration within the above range ensures that the required amount of chromium-containing layer is formed without inhibiting the formation of the chromium-containing layer.

[0108] (ii) Adjustment of pH and Temperature Next, it is preferable to prepare the electrolytic solution 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. As described above, in order to stably provide the aqueous solution for the cathodic electrolytic treatment step for a long period of time, it is preferable not only to simply dissolve the trivalent chromium ion source and the carboxylic acid compound in water but also to appropriately control the pH and temperature as described above.

[0109] In preparing the electrolytic solution, the pH of the aqueous solution after mixing is preferably adjusted to 4.0 to 7.0. The pH is more preferably 4.5 or higher. The pH is more preferably 6.5 or lower.

[0110] Any reagent can be used to adjust the pH. For example, it is preferable to use hydrochloric acid, sulfuric acid, nitric acid, etc. to lower the pH, and to use ammonia water, etc. to raise the pH.

[0111] Temperature: 40 to 70° C. In preparing the electrolytic solution, it is preferable to adjust the temperature of the aqueous solution after mixing to 40 to 70° C. The time for which the solution is kept at a temperature range of 40 to 70° C. is not particularly limited.

[0112] The electrolyte solution obtained by the above procedure can be stably used in the cathodic electrolysis treatment step for a long period of time. The electrolyte solution produced by the above procedure can be stored at room temperature.

[0113] 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.

[0114] 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.

[0115] (Preparation of Electrolyte Solutions) 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 form 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 4. 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.

[0116] (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, and pickling in this order. For the pickling, an aqueous sulfuric acid solution having a sulfate ion concentration shown in Table 2 was used, and the steel sheet was immersed in the aqueous solution to perform pickling. After the pickling, the steel sheet was subjected to the subsequent steel sheet surface conditioning step without rinsing with water.

[0117] (Steel Sheet Surface Conditioning Step) Next, the steel sheet after pickling was subjected to surface conditioning. Specifically, the pickling solution remaining on the surface of the steel sheet was squeezed with a wringer roll to adjust the amount of the pickling solution adhered to the surface to the amount shown as "amount of aqueous solution" in Table 2. Thereafter, the steel sheet was held for the holding time shown in Table 2 while maintaining the adhered amount, and then the steel sheet was washed with water to remove the pickling solution.

[0118] (Cathodic electrolysis treatment step) Next, the steel sheet was subjected to cathodic electrolysis treatment under the conditions shown in Table 2. The electrolytic solution during the cathodic electrolysis treatment was maintained at the pH and temperature shown in Table 1. The current density during the cathodic electrolysis treatment was 40 A / dm ２ The electrolysis time and number of passes were changed as appropriate. An insoluble anode made of a Ti substrate coated with iridium oxide was used as the anode during the cathodic electrolysis treatment. After the cathodic electrolysis treatment, the sample was washed with water having an electrical conductivity of 100 μS / m or less and dried at room temperature using a blower.

[0119] 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 using the method described above. Furthermore, for each of the obtained surface-treated steel sheets, the region in which the atomic ratio of Fe to Cr was in the range of 0.7 to 1.3 was determined as the vicinity of the interface between the steel sheet and the chromium-containing layer using the method described above. The presence or absence of the oxygen-enriched region was determined by determining, using the method described above, an oxygen-enriched region containing 10% or more O in atomic concentration in the vicinity. Furthermore, the coverage of the oxygen-enriched region was measured using the method described above. The measurement results are shown in Table 3.

[0120] In all examples, the chromium-containing layer obtained by cathodic electrolysis contained chromium compounds such as chromium oxide and chromium carbide in addition to metallic chromium. The total content of elements constituting the metallic chromium and chromium compounds in the chromium-containing layer was 90 mass% or more. It was also confirmed that the oxygen-enriched region did not contain oxides of Si and Mn. It was also confirmed that the oxygen-enriched region was composed of an amorphous material.

[0121] (Corrosion Resistance of BPA-Free Painted Section) Next, the corrosion resistance of the BPA-free painted section of each of the obtained surface-treated steel sheets was evaluated according to the procedure described below.

[0122] First, a BPA-free paint was applied to the surface of a surface-treated steel sheet to prepare a BPA-free painted steel sheet. A polyester-based paint for the inner surface of a can (BPA-free paint) was used as the BPA-free paint. For the coating, the BPA-free paint was applied to the surface of the surface-treated steel sheet, and then the sheet was baked at 180°C for 10 minutes. The coating weight was 60 mg / dm². ２ It was decided.

[0123] The obtained BPA-free coated steel sheet was provided with a cross-cut that penetrated all the way to the base steel sheet, and then an Erichsen tester was used to form a 4 mm high overhang centered on the intersection of the cross-cut to prepare a test specimen.

[0124] Next, a corrosion resistance test was performed using the test specimens according to the following procedure. First, the test specimens were immersed in a Teflon (registered trademark) container containing a test liquid and covered with a lid. In this state, they were subjected to a retort treatment at a temperature of 121°C for 1 hour. Thereafter, the test specimens were removed from the container, washed with water to remove the test liquid, and then dried with a blower.

[0125] After drying, the test specimens were subjected to tape peeling twice, and the surfaces of the test specimens were observed using a microscope or the like. The areas of peeled paint and discolored areas such as rust were visually evaluated and rated on a 5-point scale, with 1 being the poorest and 5 being the best. The same evaluation was performed on two samples per level, and the arithmetic mean of the scores was calculated and used as an index of the corrosion resistance of the BPA-free painted processed part. In practice, a score equal to or higher than that of conventional TFS can be evaluated as excellent in corrosion resistance of the BPA-free painted processed part, but a score equal to or higher than that of conventional TFS and a score of 3.0 or higher is more preferable.

[0126] In order to simulate the difference in the corrosive environment depending on the contents when the surface-treated steel sheet is used in a can, the corrosion resistance test was carried out using four test solutions having the following compositions (1) to (4). The evaluation results are shown in Table 4.

[0127] (1) Cysteine ​​Sodium dihydrogen phosphate: 3.56 g / L Disodium hydrogen phosphate dodecahydrate: 14.52 g / L L-cysteine ​​hydrochloride monohydrate: 0.5 g / L

[0128] (2) Lactic acid ・Lactic acid: 22.5g / L

[0129] (3) Citric acid Citric acid: 19.2 g / L L(+)-ascorbic acid: 3.92 g / L

[0130] (4) Salt + acetic acid Salt: 18.7 g / L Acetic acid: 30 g / L

[0131] As is clear from the results shown in Table 4, all of the surface-treated steel sheets satisfying the conditions of the present invention can be produced without using hexavalent chromium, and they also have excellent corrosion resistance in the BPA-free painted area, equivalent to or better than that of conventional TFS. However, (4) retort treatment in the test solution of salt and acetic acid is an extremely severe corrosive environment, and therefore, like the conventional TFS, the surface-treated steel sheets satisfying the conditions of the present invention received a rating of less than 3.0. Therefore, when applying the surface-treated steel sheets of the present invention to contents containing acetic acid, in particular, it is necessary to take the same precautions as when using conventional TFS, such as double-coating the BPA-free paint and optimizing the retort treatment conditions.

[0132]

[0133]

[0134]

[0135]

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 an oxygen-enriched region containing 10% or more O in atomic concentration exists near the interface between the steel sheet and the chromium-containing layer, and the atomic ratio of Fe to Cr in the vicinity of the interface is in the range of 0.7 to 1.

3.

2. The surface-treated steel sheet according to claim 1, wherein the coverage of the oxygen-enriched region in the vicinity is 50% or more.

3. 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 or 2, 4. The amount of chromium oxide deposited in the chromium-containing layer is 40.0 mg / m per side. ２ The surface-treated steel sheet according to any one of claims 1 to 3, wherein:

5. 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, comprising contacting the steel sheet with an aqueous solution containing sulfate ions, and applying the aqueous solution to the surface of the steel sheet at a concentration of 30.0 g / m ２ Super 60.0g / m ２ and a cathodic electrolysis treatment step of cathodic electrolysis of the steel sheet in an electrolytic solution containing 0.05 mol / L or more of trivalent chromium ions.

6. The method for producing a surface-treated steel sheet according to claim 5, wherein the electrolytic solution is prepared by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, adjusting the pH to 4.0 to 7.0, and adjusting the temperature to 40 to 70°C.

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