Surface-treated steel sheet and its manufacturing method

Abstract

The present invention provides a surface-treated steel sheet that can be produced without using hexavalent chromium and has excellent corrosion resistance in a BPA-free painted area. 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 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.

Description

[Technical field]

[0001] The present invention relates to a surface-treated steel sheet, and in particular 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 producing the surface-treated steel sheet. [Background technology]

[0002] Sn-plated steel sheet (tinplate) and tin-free steel sheet (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 Cr to form a Cr oxide layer on the outermost surface, which provides excellent adhesion to the organic resin coating layer. Therefore, the deformation of the organic resin coating layer 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, the 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 to replace epoxy-based paints with BPA-free paints. However, compared to their adhesion to epoxy-based paints, conventional tinplate and TFS have poor adhesion to BPA-free paints, so the deformation of the BPA-free paint cannot follow the deformation of the steel plate associated with can manufacturing, and corrosion resistance against various contents after can manufacturing cannot be sufficiently ensured. Therefore, the current situation is that the application of BPA-free paints to various metal cans has not progressed.

[0005] Furthermore, in recent years, due to growing environmental awareness, there has been a trend toward restricting the use of hexavalent chromium worldwide, and as a result, there is a demand for the establishment of manufacturing methods that do not use hexavalent chromium, even in the field of surface-treated steel sheets used in various metal cans.

[0006] Known methods for forming a surface-treated steel sheet without using hexavalent chromium include, for example, the methods proposed in Patent Documents 3 to 6. In these methods, a surface treatment layer is formed by performing an electrolytic treatment in an electrolytic solution containing a trivalent chromium compound such as basic chromium sulfate. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2013-144753 A [Patent Document 2] JP 2008-50486 A [Patent Document 3] Special Publication No. 2016-501985 [Patent Document 4] Special Publication No. 2016-505708 [Patent Document 5] JP 2020-172700 A [Patent Document 6] JP 2020-172701 A Summary of the Invention [Problem to be solved by the invention]

[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, the above methods can provide a surface-treated steel sheet having excellent adhesion to an epoxy-based paint. 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 deformed can be provided.

[0009] However, 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 paint-processed areas, but the corrosion resistance in the BPA-free paint-processed areas is insufficient. Therefore, it has not been possible to replace the conventional epoxy-based paints with BPA-free paints while maintaining corrosion resistance to various contents.

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

[0011] The present invention has been made in consideration of the above-mentioned 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 the BPA-free painted area. [Means for solving the problem]

[0012] As a result of extensive investigations conducted by the inventors of the present invention in order to achieve the above object, the inventors 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 above-mentioned surface-treated steel sheet is prepared by contacting a steel sheet with an aqueous solution containing sulfate ions, and applying 30.0 g / m of the aqueous solution to the surface of the steel sheet. 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 held 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 steel plate; A surface-treated steel sheet comprising a chromium-containing layer disposed on at least one surface of the steel sheet, an oxygen-enriched region containing O at an atomic concentration of 10% or more is present in the vicinity of the interface between the steel sheet and the chromium-containing layer, The vicinity is a region in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3 in this surface-treated steel sheet.

[0017] 2. The surface-treated steel sheet according to 1, 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 amount of chromium oxide deposited in 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 above, 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, comprising the steps of: The steel sheet is brought into contact with an aqueous solution containing sulfate ions, 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 A steel sheet surface conditioning process in which the steel sheet is maintained in the presence of the following for 0.10 to 20.0 seconds; and a cathodic electrolysis 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, and adjusting the pH to 4.0 to 7.0 and the temperature to 40 to 70°C. Effect of the Invention

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

[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] The surface-treated steel sheet according to one embodiment of the present invention is a surface-treated steel sheet having a chromium-containing layer on at least one side of the steel sheet. In the present invention, it is important 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 plate] 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. The steel sheet can be, for example, an ultra-low carbon steel sheet or a low carbon steel sheet. The manufacturing method of the steel sheet is also not particularly limited and any steel sheet manufactured by any method can be used, but usually 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, in mass %, C: 0.0001 to 0.13%, Si: 0 to 0.020%, Mn: 0.01 to 0.70%, P: 0-0.15%, S: 0~0.050%, Al: 0-0.20%, N: 0 to 0.040%, Cu: 0-0.20%, Ni: 0-0.15%, Cr: 0~0.10%, Mo: 0 to 0.05%, Ti: 0 to 0.020%, Nb: 0 to 0.020%, B: 0~0.020%, Ca: 0-0.020%, Sn: 0~0.020%, Sb: 0 to 0.020%, and the balance being Fe and unavoidable impurities. Of the above-mentioned composition, the lower the content of Si, P, S, Al, and N, the more preferable, and Cu, Ni, Cr, Mo, Ti, Nb, B, Ca, Sn, and Sb, are components that may 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 not particularly limited, but is preferably 0.10 mm or more. Here, the term "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. The chromium-containing layer may contain impurities in addition to the metallic chromium and the chromium compound. Examples of the 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 expressed as a percentage of the total atomic number of elements constituting metallic chromium and chromium compounds relative to the total atomic number 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 coefficient method from the integrated intensity of the peak corresponding to each element.

[0032] For example, the content of chromium carbide (Cr2C3) 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 %, the Cr2C3 content is 6×(2+3) / 3=10 atomic %.

[0033] For chromium oxide, the Cr2O3 content can be calculated from the integrated intensity of the Cr 2p oxide peak that appears around 576.7 eV, and the CrO3 content can be calculated from the integrated intensity of the Cr 2p oxide peak that appears around 579.2 eV.

[0034] Similarly, the contents of other chromium compounds can be determined using, for example, the integrated intensities of the peaks listed below. Chromium sulfide (Cr2S3): S 2p sulfide peak appears around 162.3 eV Chromium nitride (CrN): N 1s peak appears around 397.3 eV Chromium chloride (CrCl3): Cl 2p peak appears around 199.8 eV Chromium bromide (CrBr3): Br 3d peak appears around 69.1 eV Chromium boride (CrB): Br 1s peak appears around 188.2 eV

[0035] On the other hand, the content of metallic chromium is determined by calculating the Cr content from the integrated intensity of the 2p peak of Cr appearing around 573.8 eV and subtracting the content of Cr atoms contained as chromium compounds from the chromium content.

[0036] The content of metallic chromium obtained by the above method and the content of each element constituting the chromium compound are added together to obtain the total content of metallic chromium and the elements constituting the chromium compound.

[0037] The total content refers to a value at a middle position in the thickness of the chromium-containing layer. The middle position 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 above-mentioned method. Then, the middle position (1 / 2) between the position (depth) where the measured total content of elements constituting the metallic chromium and the chromium compound and the Fe content are equal and the outermost surface of the chromium-containing layer is determined as the middle position.

[0038] The above-mentioned 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 monochrome 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 ion, the acceleration voltage is 1 kV, and the sputtering rate is 1.50 nm / min in terms of SiO2.

[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 in the chromium-containing layer, or may be mixed throughout the chromium-containing layer, i.e., 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 securing the corrosion resistance of the BPA-free painted part, 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 coating amount of the chromium-containing layer is 40.0 mg / m or less per side. 2 More preferably, it 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 amount can be measured by X-ray fluorescence analysis. More specifically, the chromium deposition amount 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 device. 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 is peeled off is measured using an X-ray fluorescence device. The value obtained by subtracting the original sheet Cr amount from the total Cr amount is the chromium deposition amount of the chromium-containing layer. 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 attached to the chromium-containing layer is not particularly limited. However, if the amount of chromium oxide attached to the chromium-containing layer is excessive, the Cr oxide in the chromium-containing layer may cause cohesive failure during processing of the surface-treated steel sheet, which may deteriorate the corrosion resistance of the BPA-free painted part. Therefore, from the viewpoint of more stably securing the corrosion resistance of the BPA-free painted part, the amount of chromium oxide attached to 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 It is more preferable that the chromium-containing layer does not contain any chromium oxide. Therefore, the lower limit of the amount of chromium oxide deposited on the chromium-containing layer is not particularly limited, and is preferably 0.0 mg / m per side. 2 It may be.

[0044] The chromium oxide adhesion amount can be measured by X-ray fluorescence analysis. More specifically, the chromium oxide adhesion amount 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 in which it is immersed 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 washed with water, and the Cr amount (Cr amount after alkali treatment) is measured again using an X-ray fluorescence device. The value obtained by subtracting the Cr amount after alkali treatment from the total Cr amount is defined as the chromium oxide adhesion amount 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 phase and a crystalline phase. The chromium-containing layer manufactured 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 not clear, it is considered that the chromium-containing layer contains both an amorphous phase and a crystalline phase as a result of partial crystallization progressing when the amorphous phase is formed. 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 may not 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 single-layered chromium-containing layer sample by removing the base steel sheet portion from the surface-treated steel sheet, and observing the single-layered chromium-containing layer sample from the surface side with a TEM or STEM. The method for preparing the single-layered chromium-containing layer sample is not particularly limited, but it can be prepared, for example, by irradiating an ion beam of Ar or the like from the base steel sheet side and ion milling the steel sheet. When preparing a single-layered chromium-containing layer region with an ion beam, the ion beam is irradiated at an acceleration voltage of 5 kV or less and at an incidence angle of 1 to 5 degrees relative to the base steel sheet, thereby obtaining a crystal region of several μm. 2 The field of view of the above 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 result of the area ratio of the crystalline region.

[0047] The area ratio of the crystalline region in the chromium-containing layer can be measured by TEM. Specifically, first, a diffraction pattern of the chromium-containing layer is obtained by selected area diffraction of the TEM. Then, a dark-field image is obtained for all the diffraction spots in the diffraction pattern, and the region that is displayed with high brightness in the dark-field image is determined as the crystalline region. 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 of ​​the chromium-containing layer within the selected area aperture. For example, image analysis software such as Image-J can be used to calculate the area ratio.

[0048] The chromium-containing layer may contain C. The upper limit of the C content in the chromium-containing layer is not particularly limited, but 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 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. That is, the C content in the chromium-containing layer can be measured by sputtering from the outermost layer to a depth of 0.2 nm or more in terms of SiO2, quantifying the integrated intensity of the narrow spectrum of Cr2p and C1s as an atomic ratio by the relative sensitivity coefficient method, and calculating the C atomic ratio / Cr atomic ratio. For the measurement of the XPS, 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, the take-off angle is 45°, and the sputtering conditions are Ar ion with an acceleration voltage of 1 kV and a sputtering rate of 1.50 nm / min in terms of SiO2.

[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 electrolyte 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. The location of C 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 preferable that the Fe content is 100% or less as an atomic ratio to Cr. The chromium-containing layer may not contain Fe, and therefore the lower limit of the 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 by using narrow spectra of Cr2p and Fe2p.

[0053] The mechanism by which Fe is incorporated into the chromium-containing layer is not clear; however, it is thought that during the process of forming the chromium-containing layer on the steel sheet, traces of Fe contained in the steel sheet dissolve in the electrolyte, and the Fe is incorporated into the coating.

[0054] The chromium-containing layer may contain, in addition to Cr, O, Fe, and C, metal impurities such as K, Na, Mg, and Ca contained in water, Sn, Ni, Cu, and Zn contained in the aqueous solution, and S, N, Cl, Br, etc. However, the presence of these elements may reduce the corrosion resistance of the BPA-free painted part. Therefore, the total of elements other than Cr, O, Fe, and C is preferably 3% or less in atomic ratio to Cr, and more preferably no elements are contained (0%). The content of the above elements is not particularly limited, but can be measured by XPS, for example, in the same manner as the content of C.

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

[0056] The reason why the corrosion resistance of the BPA-free painted part 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 generally processed into cans and the like after forming an organic resin coating on the surface. However, because metallic chromium has poor workability, the chromium-containing layer cannot completely follow the deformation of the steel sheet during processing, resulting in damage to the organic resin coating on the chromium-containing layer. As a result, the corrosion resistance after processing is reduced.

[0058] Therefore, in conventional surface-treated steel sheets, chromium oxide is provided as the top layer to ensure corrosion resistance after processing. That is, since 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, and the coverage 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 BPA-free paint is applied.

[0060] In contrast, the surface-treated steel sheet of the present invention realizes excellent corrosion resistance of the BPA-free paint-processed part by providing an oxygen-enriched region in the region near the interface, as described above. This is believed 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 technical idea that is completely different from the conventional one, 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 so as to include both the chromium-containing layer and the steel sheet, and five ion maps are obtained by performing a three-dimensional composition analysis by 3DAP on the measurement regions. Next, the ion map is divided into voxels of 2 nm×2 nm×2 nm, and the composition of each voxel is calculated in terms of atomic concentration. The region consisting of voxels in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3 is determined as the near-interface region. In addition, the region consisting of voxels in the near-interface region in which O is 10% or more in atomic concentration is determined as the oxygen-enriched region. In this way, the atomic ratio of Fe to Cr and the atomic concentration of O are measured by three-dimensional composition analysis using a three-dimensional atom probe, thereby determining the near-interface region and the oxygen-enriched region. In addition, in defining the near-interface region, the position where the atomic ratio of Fe to Cr is 1.0 is the interface between the chromium-containing layer and the steel sheet. In other words, 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 I, and the number of voxels in the oxygen-enriched region is O, and the percentage is calculated from the formula O / I×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] The oxygen-enriched region may contain metals such as Si and Mn as oxides in addition to O, Fe, Cr, and C. However, if elements other than O, Fe, Cr, and C are present in large amounts, the corrosion resistance of the BPA-free painted part may decrease. 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 does not contain any oxides of Si and Mn (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 by 3DAP, similar to the determination of the interface vicinity region and the oxygen-enriched region. More specifically, first, five measurement regions are selected so as to include both the chromium-containing layer and the steel sheet, and five ion maps are obtained by performing three-dimensional composition analysis by 3DAP on the measurement regions. Next, the ion map is divided into voxels of 2 nm x 2 nm x 2 nm, and the composition of each voxel is calculated in terms of atomic concentration. Then, for each ion map, the compositions of the voxels in the oxygen-enriched region are averaged to calculate the average composition for each ion map, that is, the average composition for each measurement region. Finally, the average composition for each measurement region is 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 phase and a crystalline phase. However, from the viewpoint of improving corrosion resistance, it is preferable that the oxygen-enriched region is 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 spot appears in the diffraction pattern, the oxygen-enriched region is amorphous.

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

[0068] The 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 plate surface conditioning process] 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 solution: 30.0g / m 2 Super 60.0g / m 2 below Holding time: 0.10~20.0 seconds In order to provide an oxygen-enriched region in the interface vicinity region in the finally obtained surface-treated steel sheet, in the steel sheet surface conditioning step, the steel sheet is brought into contact with an aqueous solution containing sulfate ions, 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 The following must be present for 0.10 seconds or more and 20.0 seconds or less:

[0071] The mechanism by which the 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 the 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, and the pH of the steel sheet surface increases. 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, so that the amount of dissolved oxygen in the aqueous solution increases. As a result, the above reaction is further promoted. The state of the aqueous solution on the steel sheet is not particularly limited, but from the viewpoint of making the reaction uniform, it is preferable that the aqueous solution is 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 process, 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 it even if it is not completely reduced in the subsequent cathodic electrolysis process. 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 parts, the amount of the aqueous solution is 32.0 g / m 2 More preferably, it 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 thickness is 55.0 g / m or less. 2 It is more preferable to set it as follows:

[0074] From the viewpoint of forming more oxygen-enriched regions and further improving the corrosion resistance of the BPA-free painted part, the holding time is preferably 0.2 seconds or more, more preferably 0.3 seconds or more. 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 the aqueous solution present on the surface of the steel sheet can be measured by a moisture meter using a filter type infrared absorption method. Specifically, the absorbance on the surface of the steel sheet is measured by a moisture meter using a filter type infrared absorption method, and the amount of the aqueous solution is calculated from the absorbance using a calibration curve that has been obtained in advance. 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 on 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 the aqueous solution is dropped and the weight of the steel sheet after the aqueous solution is dropped. The amount of the aqueous solution per unit area is calculated by dividing the weight of the aqueous solution obtained by the area of ​​the steel sheet. At the same time, the absorbance on the surface of the steel sheet is measured by a moisture meter using a filter type infrared absorption method. The above measurements are performed multiple times while changing the amount of the aqueous solution, and a calibration curve showing the correlation between the amount of the aqueous solution and the absorbance is created. As the calibration curve, a linear approximation of the correlation between the amount of the aqueous solution and the absorbance can be used.

[0076] The method for adjusting the amount of the 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 and pickling accelerators, these components do not particularly hinder the formation of oxygen-enriched regions. Therefore, even if a pickling inhibitor and / or a pickling accelerator 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, and 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, and 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 process] Next, the steel sheet is subjected to cathodic electrolysis in an electrolytic solution containing 0.05 mol / L or more of trivalent chromium ions. The cathodic electrolysis can form a chromium-containing layer on the steel sheet. Any compound that can supply 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 the cathodic electrolytic treatment 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 during the cathodic electrolytic treatment is not particularly limited, but is preferably 4.0 or more, more preferably 4.5 or more. The pH is preferably 7.0 or less, more preferably 6.5 or less. 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 electrolytic treatment step and maintain it within the above pH range.

[0085] The current density in the above-mentioned cathodic electrolytic treatment is not particularly limited, and may be appropriately adjusted so that the desired surface treatment layer is formed. However, if the current density is excessively high, the burden on the cathodic electrolytic treatment device becomes excessive. Therefore, the current density is set to 200.0 A / dm 2 It is preferable to set it to 100.0A / dm or less. 2 It is more preferable that the current density is 5.0 A / dm or less. Although there is no particular limit to the lower limit of the current density, if the current density is too low, hexavalent Cr is generated in the electrolyte, which may cause the stability of the bath to be lost. Therefore, the current density is preferably 5.0 A / dm 2 More than 10.0A / dm is preferable. 2 More preferably, it is 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 may be any number of times. In other words, cathodic electrolysis may be performed using an electrolysis treatment device 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, and the productivity decreases. 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, which makes 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 amount of chromium deposited in the chromium-containing layer formed by cathodic electrolysis can be controlled by the total electric charge density, which is the product of the current density, the electrolysis time, and the number of passes. As described above, if the amount of chromium deposited is too small, the corrosion resistance of the BPA-free coating part is impaired, and if the amount of chromium deposited is too large, cohesive failure may occur in the chromium-containing layer during processing. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free coating part, it is preferable to control the total electric charge density so that the amount of chromium deposited per one side of the steel sheet in the chromium-containing layer is within an appropriate range. However, the relationship between the amount of chromium deposited per one side of the steel sheet in the chromium-containing layer and the total electric charge density varies depending on the configuration of the device used in the cathodic electrolysis process, so the actual electrolysis conditions can be adjusted according to the device.

[0089] The type of anode used when performing cathodic electrolysis is not particularly limited, and any anode can be used. As the anode, it is preferable to use an insoluble anode. As the insoluble 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, as the insoluble anode, an anode in which platinum, iridium oxide, or ruthenium oxide is coated on the surface of Ti as a substrate can be exemplified.

[0090] In the above-mentioned cathodic treatment step, the concentration of the electrolyte constantly changes due to the effects of the formation of a chromium-containing layer on the steel sheet, the take-out and take-in of the electrolyte, the evaporation of water, etc. Since the change in the concentration of the electrolyte in the cathodic electrolytic treatment step varies depending on the configuration of the apparatus and the production conditions, 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 electrolyte in the cathodic electrolytic treatment step and maintain them within the concentration ranges described below.

[0091] The steel sheet after the cathodic electrolytic treatment step 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 washing can be carried out by any method without any particular limitation. For example, a water washing tank can be provided downstream of an immersion tank for carrying out an immersion treatment, and the steel sheet after immersion can be continuously immersed in water. Alternatively, the steel sheet after immersion can be washed by spraying water on it with a spray.

[0093] The water used for the washing is not particularly limited, but it is preferable to use at least one of reverse osmosis water (RO water), ion-exchanged water, and distilled water. The electrical conductivity of the water used for the 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 the 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 it is preferably 0° C. or more. The temperature of the water used for the washing may be room temperature.

[0095] After the above water washing, drying may be carried out as desired. The drying method is not particularly limited, and for example, a normal dryer or electric oven drying method can be applied. The temperature during the drying process 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 about 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 carrying out degreasing, rolling oil, rust-preventive oil, and the like 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 in order to remove the degreasing treatment liquid adhering to the surface of the steel sheet.

[0098] By carrying out pickling, the natural oxide film present on the surface of the steel sheet can be removed, and the surface can be effectively conditioned in the subsequent steel sheet surface conditioning step. The pickling can be carried out by any method without any particular limitation. After the pickling, it is preferable to wash the steel sheet with water in order to remove the pickling solution adhering to the steel sheet surface. When an aqueous solution containing sulfate ions is used as the pickling solution, it is preferable to directly supply the steel sheet to the steel sheet surface conditioning step.

[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 long period of time.

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

[0101] As the trivalent chromium ion source, any compound that can supply trivalent chromium ions can be used. 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.

[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, and 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 carboxylate, and is preferably at least one of an aliphatic carboxylic acid and a salt of an aliphatic carboxylic acid. The carbon number of the aliphatic carboxylic acid is preferably 1 to 10, and more preferably 1 to 5. The carbon number of the aliphatic carboxylate is preferably 1 to 10, and more preferably 1 to 5. The content of the carboxylic acid compound is not particularly limited, but is preferably 0.1 mol / L or more, and more preferably 0.15 mol / L or more. The content of the carboxylic acid compound is preferably 5.5 mol / L or less, and more preferably 5.3 mol / L or less. As the carboxylic acid compound, BluCr (registered trademark) TFS B from Atotech can be used.

[0104] As a solvent for preparing the aqueous solution, water can be used, and as the water, 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 to improve the stability of the above-mentioned electrolyte, it is preferable that the aqueous solution further contains at least one kind 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. The content of the halide ion is preferably 3.0 mol / L or less, and more preferably 2.5 mol / L or less. In order 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 in the cathodic electrolytic treatment process, but even if a small amount of hexavalent chromium is formed at the anode, it is immediately reduced to trivalent chromium, so the concentration of hexavalent chromium 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., and each is preferably 0 mg / L to 40 mg / L, more preferably 0 mg / L to 20 mg / L, and most preferably 0 mg / L to 10 mg / L. Among the above metal ions, Fe ions may dissolve in the above-mentioned electrolyte in the cathodic electrolytic treatment step and the immersion step, and may be co-deposited in the film, but this does not affect the corrosion resistance of the BPA-free painted part. It is preferable that the Fe ion concentration is within the above range when the electrolyte is made up, but it is also preferable to maintain the Fe ion concentration in the electrolyte within the above range in the cathodic electrolytic treatment step and the immersion step. If the Fe ions are controlled within the above range, the formation of the chromium-containing layer is not inhibited, and a required amount of the chromium-containing layer can be formed.

[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] pH: 4.0~7.0 In preparing the electrolytic solution, the pH of the aqueous solution after mixing is preferably adjusted to 4.0 to 7.0, more preferably 4.5 or higher, and 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: 40~70℃ 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 in the temperature range of 40 to 70° C. is not particularly limited.

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

[0113] The uses of the surface-treated steel sheet of the present invention are not particularly limited, but it is particularly suitable as a surface-treated steel sheet for containers used in the manufacture of various containers such as food cans, beverage cans, pail cans, and 18-liter cans. EXAMPLES

[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) 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 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 for steel plates) A cold-rolled steel sheet was used as the steel sheet. More specifically, a can steel sheet (T4 base sheet) having a thickness of 0.17 mm was used. The steel sheet was subjected to electrolytic degreasing, water washing, and pickling in this order as pretreatment. 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. The steel sheet after the pickling was subjected to the subsequent steel sheet surface conditioning step without being washed with water.

[0117] (Steel plate surface conditioning process) 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 amount of adhesion, and then the steel sheet was washed with water to remove the pickling solution.

[0118] (Cathodic electrolysis process) Next, the steel sheet was subjected to cathodic electrolysis under the conditions shown in Table 2. The electrolytic solution during the cathodic electrolysis was maintained at the pH and temperature shown in Table 1. The current density during the cathodic electrolysis 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 cathodic electrolysis. After cathodic electrolysis, the sample was washed with water with 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 one side of the steel sheet in the chromium-containing layer and the chromium oxide deposition amount per one side of the steel sheet were measured by 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 by the method described above. Then, the presence or absence of the oxygen-enriched region was determined by the method described above to determine an oxygen-enriched region in the vicinity, in which O was contained at an atomic concentration of 10% or more. Furthermore, the coverage of the oxygen-enriched region was measured by the method described above. The measurement results are shown in Table 3.

[0120] In all of the examples, the chromium-containing layer obtained by the cathodic electrolysis contained chromium compounds such as chromium oxide and chromium carbide in addition to metallic chromium. The total content of elements constituting 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] (BPA-free painted parts have corrosion resistance) Next, the corrosion resistance of the BPA-free painted area 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. In the painting, 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 / dm 2 It was decided.

[0123] The obtained BPA-free painted steel sheet was provided with cross cuts penetrating through to the base steel sheet, and then an Erichsen tester was used to form an overhang 4 mm in height from the intersection of the cross cut to prepare a test specimen.

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

[0125] After drying, the test pieces were tape-peeled twice, and the surface of the test piece was observed with a microscope, etc., and the area of ​​peeled paint and discoloration such as rust were visually evaluated and scored on a 5-point scale, with 1 being the poorest and 5 being the best performance. 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 parts. In practice, a score equal to or higher than that of the conventional TFS can be evaluated as excellent in corrosion resistance of the BPA-free painted parts, but a score equal to or higher than that of the 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 above 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: 3.56g / L Disodium hydrogen phosphate dodecahydrate: 14.52g / L · L-cysteine ​​hydrochloride monohydrate: 0.5g / L

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

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

[0130] (4) Salt + Acetic Acid Salt: 18.7g / L Acetic acid: 30g / 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 manufactured without using hexavalent chromium, and have excellent corrosion resistance of the BPA-free painted parts equal to or greater than that of the conventional TFS. However, (4) retort treatment in a test solution of salt + acetic acid is an extremely severe corrosive environment, so the surface-treated steel sheets satisfying the conditions of the present invention were scored below 3.0, as was the case with the conventional TFS. Therefore, when applying the surface-treated steel sheets of the present invention to contents that contain acetic acid in particular, it is necessary to pay attention to the same precautions as when using the conventional TFS, such as double-coating the paint with BPA-free paint and optimizing the retort treatment conditions.

[0132] [Table 1]

[0133]

Table 2

[0134]

Table 3

[0135]

Table 4

Claims

1. A steel plate, A surface-treated steel sheet comprising a chromium-containing layer disposed on at least one surface of the steel sheet, an oxygen-enriched region containing O at an atomic concentration of 10% or more is present in the vicinity of the interface between the steel sheet and the chromium-containing layer, The vicinity is a region in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.

3.

2. The surface-treated steel sheet according to claim 1 , wherein a coverage rate 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 .

4. The surface-treated steel sheet according to claim 2, wherein the chromium-containing layer has a chromium deposition amount of 40.0 to 500.0 mg / m 2 per side.

5. 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 any one of claims 1 to 4, wherein:

6. 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: The steel sheet is brought into contact with an aqueous solution containing sulfate ions, 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 A steel sheet surface conditioning step of maintaining the steel sheet surface in the state of being present for 0.10 to 20.0 seconds; and a cathodic electrolysis step of cathodic electrolysis of the steel sheet in an electrolytic solution containing 0.05 mol / L or more of trivalent chromium ions.

7. The method for producing a surface-treated steel sheet according to claim 6, 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.