Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/08—Iron or steel
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/08—Iron or steel
  • C23G1/081—Iron or steel solutions containing H2SO4
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/38—Chromatising
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D21/00—Processes for servicing or operating cells for electrolytic coating
  • C25D21/12—Process control or regulation
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium
  • C25D3/06—Electroplating: Baths therefor from solutions of chromium from solutions of trivalent chromium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/06—Wires; Strips; Foils
  • C25D7/0614—Strips or foils
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials
  • C25D9/08—Electrolytic coating other than with metals with inorganic materials by cathodic processes
  • C25D9/10—Electrolytic coating other than with metals with inorganic materials by cathodic processes on iron or steel

Definitions

• the present disclosure 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 coated part.
• the surface-treated steel sheet of the present disclosure can be suitably used in containers such as cans.
• the present disclosure also relates to a method of producing the surface-treated steel sheet.
• tinplate Sn-plated steel sheet
• TFS tin-free steel sheet
• Tinplate and TFS are used with an organic resin coating such as epoxy-based paint or PET film to accommodate a variety of contents.
• an organic resin coating such as epoxy-based paint or PET film
• the steel sheet is subjected to electrolysis or immersion treatment in an aqueous solution containing hexavalent Cr to form a chromium oxide layer on the outermost surface.
• the chromium oxide layer exhibits excellent adhesion to an organic resin coating layer. Therefore, deformation of the organic resin coating layer also follows the steel sheet deformation that accompanies can manufacturing, thereby ensuring corrosion resistance to the various contents even after can manufacturing.
• Examples of known methods of forming surface-treated steel sheets without using hexavalent chromium are the methods proposed in PTL 3 to 6.
• a surface treatment layer is formed by performing electrolysis in an electrolytic solution containing a trivalent chromium compound such as basic chromium sulfate.
• a surface treatment layer can be formed without using hexavalent chromium. Also according to PTL 3 to 6, the above method can obtain a surface-treated steel sheet with excellent adhesion to epoxy-based paint. According to PTL 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 obtained.
• the surface-treated steel sheet according to an embodiment of the present disclosure is a surface-treated steel sheet having a chromium-containing layer on at least one surface of the steel sheet.
• any steel sheet can be used as the above steel sheet without any particular limitation, but a steel sheet for cans is preferred.
• a steel sheet for cans is preferred.
• an ultra low carbon steel sheet or low carbon steel sheet can be used as the steel sheet.
• the method of producing the steel sheet is not limited, and a steel sheet produced by any method may be used, but it typically suffices to use a cold-rolled steel sheet.
• the cold-rolled steel sheet can be produced by general production processes, for example, hot rolling, pickling, cold rolling, annealing, and temper rolling.
• the chemical composition of the steel sheet is not limited, but C, Mn, P, S, Si, Cu, Ni, Mo, Al, and inevitable impurities may be contained to the extent that the effects of the scope of the present disclosure are not impaired.
• a steel sheet with the chemical composition specified in ASTM A623M-09, for example, can be suitably used as the steel sheet.
• the thickness of the steel sheet is not particularly limited but is preferably 0.60 mm or less. On the other hand, no lower limit is placed on the thickness, but the thickness is preferably 0.10 mm or more.
• the term "steel sheet” as used here includes a "steel strip”.
• the steel sheet has a chromium-containing layer on at least one surface thereof.
• the components forming 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.
• the impurities include metal elements such as Ni, Cu, Sn, and Zn that are mixed as impurities in the electrolytic solution, which will be described later.
• the metal elements are typically thought to be present in the chromium-containing layer in a metallic state but may also be present as compounds.
• the chromium-containing layer preferably has a total content of elements forming the metallic chromium and the chromium compound of 90 atomic% or more.
• the total content is the ratio, expressed as a percentage, of the total number of atoms of elements constituting the metallic chromium and chromium compound to the total number of atoms of all elements other than Fe.
• the total content can be determined by measuring the contents (atomic%) of the metallic chromium and the elements constituting the chromium compound contained in the chromium-containing layer by X-ray photoelectron spectroscopy (XPS) and adding up the contents.
• XPS X-ray photoelectron spectroscopy
• 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 the element.
• the content of chromium carbide can be determined from the integrated intensity of the peak, appearing near 281.0 eV, of the C 1s carbide.
• the C content atomic ratio to the total of all elements other than Fe
• the Cr 2 O 3 content can be determined from the integrated intensity of the peak, appearing near 576.7 eV, of the Cr 2p oxide.
• the CrO 3 content can also be determined from the integrated intensity of the peak, appearing near 579.2 eV, of the Cr 2p oxide portion.
• the contents of other chromium compounds can be determined using, for example, the integrated intensities of the peaks listed below.
• the content of metallic chromium is determined by calculating the Cr content from the integrated intensity of the Cr 2p peak appearing near 573.8 eV and subtracting the content of Cr atoms contained as chromium compounds from the chromium content.
• 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 the metallic chromium and the elements constituting the chromium compound.
• the total content refers to the value at the intermediate position in the thickness of the chromium-containing layer.
• the intermediate position can be determined by the following procedure. First, the chromium-containing layer is sputtered from the outermost surface, while the Fe content and the total content of the metallic chromium and the elements constituting the chromium compound are measured by the above-described method. The position that is intermediate (1/2) between the position (depth) where the measured total content of the metallic chromium and elements constituting the chromium compound is equal to the Fe content and the outermost surface of the chromium-containing layer is determined as the intermediate position.
• a scanning X-ray photoelectron spectroscopic analyzer PHI X-tool produced by ULVAC-PHI, Inc. can be used.
• the X-ray source is a monochrome AlK ⁇ ray
• the voltage is 15 kV
• the beam diameter is 100 ⁇ m
• the take-off angle is 45°
• the sputtering conditions are Ar ions with an accelerating voltage of 1 kV and a sputtering rate of 1.50 nm/min in terms of SiO 2 .
• the spatial structure of the components constituting the chromium-containing layer is not particularly limited.
• the components may be, for example, separated as separate layers in the chromium-containing layer or may be mixed throughout the chromium-containing layer. That is, the spatial structure of the components that form the chromium-containing layer can contain either or both of separate and mixed layers.
• the coating weight of chromium in the chromium-containing layer is not particularly limited. However, if the coating weight of chromium in the chromium-containing layer is excessive, cohesive failure may occur in the chromium-containing layer during processing of the surface-treated steel sheet. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free coated part, the chromium coating weight of the chromium-containing layer is preferably 500.0 mg/m 2 or less per side. The chromium coating weight is preferably 450.0 mg/m 2 or less per side.
• the chromium coating weight of the chromium-containing layer is preferably 40.0 mg/m 2 or more per side.
• the chromium coating weight is more preferably 50.0 mg/m 2 or more per side.
• the "chromium coating weight” refers to the total coating weight of chromium present in various forms.
• the chromium coating weight can be measured by X-ray fluorescence analysis. More specifically, the chromium coating weight is measured by the following procedure. First, the Cr content (total Cr content) in the surface-treated steel sheet is measured using an X-ray fluorescence device. Next, the Cr content in the steel sheet before the formation of the chromium-containing layer or in the steel sheet after the chromium-containing layer has been removed (original sheet Cr content) is measured using the X-ray fluorescence device. The chromium coating weight of the chromium-containing layer is determined by subtracting the chromium content of the original sheet from the total chromium content. The chromium-containing layer can be removed using, for example, a commercially available hydrochloric acid-based chromium plating remover.
• 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) included in a scanning electron microscope (SEM) or a transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).
• EDS energy dispersive X-ray spectroscopy
• WDS wavelength dispersive X-ray spectroscopy
• SEM scanning electron microscope
• TEM transmission electron microscope
• 3DAP three-dimensional composition analysis using a three-dimensional atom probe
• the coating weight of chromium oxide in the chromium-containing layer is not particularly limited. However, if the coating weight of chromium oxide in the chromium-containing layer is excessive, cohesive failure may occur with the chromium oxide in the chromium-containing layer as an initiation point when the surface-treated steel sheet is processed, and the corrosion resistance of the BPA-free coated part may deteriorate. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free coated part, the chromium oxide coating weight of the chromium-containing layer is preferably 40.0 mg/m 2 or less per side. The chromium oxide coating weight is preferably 35.0 mg/m 2 or less per side.
• the chromium-containing layer may be completely free of chromium oxide. Therefore, no lower limit is placed on the coating weight of chromium oxide in the chromium-containing layer, and the coating weight may be 0.0 mg/m 2 per side.
• the chromium oxide coating weight can be measured by X-ray fluorescence analysis. More specifically, the chromium oxide coating weight is measured by the following procedure. First, the Cr content (total Cr content) of the surface-treated steel sheet is measured. Next, the surface-treated steel sheet is subjected to an alkali treatment by immersion in 7.5N-NaOH at 90 °C for 10 minutes to remove the chromium oxide. The surface-treated steel sheet after the alkali treatment is thoroughly washed with water, and the Cr content (Cr content after alkali treatment) is measured again using an X-ray fluorescence device. The chromium oxide coating weight in the chromium-containing layer is determined by subtracting the chromium content after the alkali treatment from the total chromium content.
• the chromium-containing layer may be amorphous or crystalline. That is, the chromium-containing layer can 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.
• the mechanism by which the chromium-containing layer is formed is not clear, but it is believed that partial crystallization occurs when the amorphous phase is formed, resulting in a chromium-containing layer that contains both amorphous and crystalline phases.
• the area ratio of crystalline regions is not particularly limited, but the area ratio 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 %.
• the crystalline regions in the chromium-containing layer can be confirmed by removing the base steel sheet portion from the surface-treated steel sheet to prepare a single-layer sample of the chromium-containing layer and then observing the single-layer sample of the chromium-containing layer from the surface side using a TEM or STEM.
• the method of preparing the single-layer sample of the chromium-containing layer is not particularly limited, but for example, the sample can be prepared by irradiating an ion beam of Ar or the like from the base steel sheet side to subject the steel sheet to ion milling.
• the ion beam is irradiated at an accelerating voltage of 5 kV or less onto the base steel sheet at an incidence angle in the range of 1 to 5 degrees, thereby ensuring a field of view of a single chromium layer region of several ⁇ m 2 or more.
• the bottom surface of the chromium-containing layer is also milled to some extent, and the thickness of the chromium-containing layer may be reduced, but this does not affect the measurement results of the area ratio of the crystalline region.
• the area ratio of the crystalline regions in the chromium-containing layer can be measured by a TEM. Specifically, first, a diffraction pattern of the chromium-containing layer is obtained by selected area diffraction of a TEM. Next, dark-field images are obtained for all the diffraction spots in the diffraction pattern, and the regions that appear with high brightness in the dark-field images are determined to be crystalline regions. The area of the obtained crystalline regions is calculated by image processing and is divided by the area of the chromium-containing layer within a selected area aperture to calculate the area ratio of the crystalline regions. The area ratio can be calculated using image interpretation software such as image-J.
• the chromium-containing layer may contain C. No upper limit is placed on the C content in the chromium-containing layer, but the C content is preferably 50 % or less in terms of atomic ratio to Cr. The C content is more preferably 45 % or less in terms of atomic ratio to Cr.
• the chromium-containing layer need not contain C, and therefore no lower limit is placed on the atomic ratio of C contained in the chromium-containing layer to Cr, and this atomic ratio may be 0 %.
• 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 determined by sputtering to a depth of 0.2 nm or more in terms of SiO 2 from the outermost layer, quantifying the integrated intensities of the narrow spectra of Cr2p and C1s as atomic ratios by the relative sensitivity coefficient method, and calculating the C atomic ratio/Cr atomic ratio.
• XPS for example, a scanning X-ray photoelectron spectroscopic analyzer PHI X-tool produced by ULVAC-PHI, Inc. can be used.
• the X-ray source is a monochrome AlK ⁇ ray, the voltage is 15 kV, the beam diameter is 100 ⁇ m, the take-off angle is 45°, and the sputtering conditions are Ar ions with an accelerating voltage of 1 kV and a sputtering rate of 1.50 nm/min in terms of SiO 2 .
• 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) included in a scanning electron microscope (SEM) or a transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).
• EDS energy dispersive X-ray spectroscopy
• WDS wavelength dispersive X-ray spectroscopy
• SEM scanning electron microscope
• TEM transmission electron microscope
• 3DAP three-dimensional composition analysis using a three-dimensional atom probe
• the chromium-containing layer may contain metal impurities such as K, Na, Mg, and Ca contained in the water; Sn, Ni, Cu, Zn, and the like contained in the aqueous solution; and S, N, Cl, Br, and the like.
• the presence of these elements may reduce the corrosion resistance in the BPA-free coated part. Therefore, the total atomic ratio of elements other than Cr, O, Fe, and C to Cr is preferably 3 % or less. Such elements are more preferably not contained at all (0 %).
• the contents of the above elements are not particularly limited but can be measured, for example, by XPS in the same manner as the C content.
• the coverage of the oxygen-enriched region in the region in the vicinity of the interface is preferably 50 % or more, and more preferably 70 % or more. No upper limit is placed on the coverage of the oxygen-enriched region in the region in the vicinity of the interface, and the coverage may be 100 %.
• the oxygen-enriched region refers only to the region present in the region in the vicinity of the interface.
• a typical chromium-containing layer formed from a hexavalent chromium bath or a trivalent chromium bath is composed of metallic chromium or chromium oxide.
• Such a surface-treated steel sheet having a chromium-containing layer is generally processed into a can or the like after an organic resin coating is formed on the surface.
• metallic chromium has poor workability, and therefore the chromium-containing layer cannot completely follow the steel sheet deformation that accompanies processing. This results in damage to the organic resin coating present on the chromium-containing layer. As a result, the corrosion resistance after processing decreases.
• chromium oxide is provided on the uppermost layer to ensure corrosion resistance after processing.
• 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 production.
• an oxygen-enriched region is provided in the region in the vicinity of the interface, thereby realizing excellent corrosion resistance in the BPA-free coated part.
• the strain introduced into the chromium-containing layer during processing of the surface-treated steel sheet is dispersed, enabling the chromium-containing layer to follow the deformation of the steel sheet, and because the oxygen-enriched region itself exhibits excellent corrosion resistance.
• the present disclosure is based on a technical concept that is completely different from conventional technology, namely, improving the deformability of the chromium-containing layer itself and improving the corrosion resistance in the vicinity of the interface, rather than the adhesion to the paint.
• the region in which O is present at an atomic concentration of 10 % or more is defined as an oxygen-enriched region.
• 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 region in the vicinity of the interface and the oxygen-enriched 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 region in the vicinity of the interface includes the interface between the chromium-containing layer and the steel sheet.
• the coverage is defined as follows. First, for each ion map, the percentage is calculated using the formula O/I ⁇ 100, where I is the number of voxels in the region in the vicinity of the interface and O is the number of voxels in the oxygen-enriched region. The average value of the percentages for each ion map is defined as the coverage of the oxygen-enriched region in the region in the vicinity of the interface. In this manner, the coverage of the oxygen-enriched region is measured by three-dimensional composition analysis using a three-dimensional atom probe.
• the oxygen-enriched region may contain C. No upper limit is placed on the C content in the oxygen-enriched region, but the C content is preferably 20 atomic% or less.
• the oxygen-enriched region need not contain C, and therefore no lower limit is placed on the C content in the oxygen-enriched region, and the C content may be 0 atomic%.
• 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 a large amount, the corrosion resistance in the BPA-free coated part may decrease. From this viewpoint, the total content of O, Fe, Cr and C in the oxygen-enriched region is preferably 70 atomic% or more. From a similar viewpoint, the oxygen-enriched region preferably contains absolutely no oxides of Si and Mn (0 %). The content of the oxide can be measured by, for example, XPS.
• the atomic concentration of each element in the oxygen-enriched region can be measured by three-dimensional composition analysis using 3DAP, in the same manner as when determining the region in the vicinity of the interface 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 the measurement regions are subjected to three-dimensional composition analysis by 3DAP to obtain five ion maps. The ion map is then divided into voxels of 2 nm ⁇ 2 nm ⁇ 2 nm, and the composition of each voxel is calculated in terms of atomic concentration.
• 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.
• the average compositions for each measurement region are further averaged to calculate the atomic concentration of each element in the oxygen-enriched region.
• 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, the oxygen-enriched region is preferably amorphous. Whether an oxygen-enriched region is amorphous 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 an oxygen-enriched region is acquired, the oxygen-enriched region is amorphous if no diffraction spots appear in the diffraction pattern.
• a surface-treated steel sheet with the aforementioned characteristics can be produced by the method described below.
• a method of producing a surface-treated steel sheet according to one embodiment of the present disclosure is a method of producing a surface-treated steel sheet having a chromium-containing layer on at least one surface of the steel sheet, and the method includes a steel sheet surface conditioning process and a cathodic electrolysis process. Each process is described below.
• Amount of aqueous solution over 30.0 g/m 2 and 60.0 g/m 2 or less Holding time: 0.10 seconds to 20.0 seconds
• the mechanism by which the oxygen-enriched region is formed in the region in the vicinity of the interface by the steel sheet surface conditioning process is not clear but is thought to be as follows.
• 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 the pH of the steel sheet surface to rise.
• the aqueous solution on the steel sheet becomes extremely thin, and therefore the amount of dissolved oxygen in the aqueous solution increases. As a result, the above reaction is further promoted.
• the state in which the aqueous solution is present on the steel sheet is not particularly limited, but from the viewpoint of making the reaction uniform, the aqueous solution is preferably in the form of a liquid film.
• the Fe ions are oxidized to Fe oxide, which accumulates on the steel sheet surface in very small amounts.
• the minute amount of deposited Fe oxide is reduced and a chromium-containing layer is formed.
• the minute amount of Fe oxide deposited by the above method can form a chromium-containing layer on the surface even if the Fe oxide 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 in the vicinity of the interface.
• the amount of the aqueous solution is preferably 32.0 g/m 2 or more.
• the amount of the aqueous solution is more preferably 35.0 g/m 2 or more.
• the amount of the aqueous solution is preferably 58.0 g/m 2 or less.
• the amount of the aqueous solution is more preferably 55.0 g/m 2 or less.
• the holding time is preferably 0.2 seconds or more.
• the holding time is more preferably 0.3 seconds or more.
• the holding time is preferably 18.0 seconds or less.
• the holding time is more preferably 15.0 seconds or less.
• 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 at the steel sheet surface is measured by a moisture meter using a filter-type infrared absorption method, and the amount of aqueous solution is determined from the absorbance using a previously determined calibration curve.
• the calibration curve can be prepared by the following procedures. 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 determined from the weight of the steel sheet before and after the aqueous solution is dropped.
• the resulting weight of the aqueous solution is divided by the area of the steel sheet to determine the amount of aqueous solution per unit area.
• the absorbance on the surface of the steel sheet is measured using a moisture meter based on a filter-type infrared absorption method. The above measurements are performed multiple times while varying the amount of aqueous solution, and a calibration curve representing the correlation between the amount of aqueous solution and absorbance is created. A linear approximation of the correlation between the amount of aqueous solution and absorbance can be used as the calibration curve.
• the method of adjusting the amount of aqueous solution present on the steel sheet surface is not limited, and any method can be used. Examples of methods that can be used include squeezing the liquid with a wringer roll or wiping.
• the composition of the aqueous solution is not particularly limited but is preferably an aqueous sulfuric acid solution, such as dilute sulfuric acid.
• the aqueous sulfuric acid solution refers to an aqueous solution of sulfuric acid and comprises the case in which components other than sulfuric acid are included.
• the pickling solution can also be used as the aqueous solution in the steel sheet surface conditioning process.
• pickling solutions generally contain pickling inhibitors and pickling accelerators, but these components do not particularly hinder the formation of the oxygen-enriched region. Therefore, even if a pickling inhibitor 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 process.
• concentration is preferably 3 g/L or higher.
• concentration is more preferably 5 g/L or higher.
• concentration is preferably 200 g/L or lower.
• concentration is more preferably 150 g/L or lower.
• No lower limit is placed on the temperature of the aqueous solution, but the temperature is preferably 10 °C or higher, more preferably 15 °C or higher.
• No upper limit is placed on the temperature of the aqueous solution, but the temperature is preferably 70 °C or lower. The temperature is more preferably 60 °C or lower.
• the steel sheet is preferably washed with water to remove the aqueous solution adhering to the steel sheet.
• 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 can be used as the trivalent chromium ion source as long as the compound can supply trivalent chromium ions.
• 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.
• the temperature of the electrolytic solution during the cathodic electrolysis is not particularly limited but is preferably 40 °C or more in order to efficiently form the chromium-containing layer.
• the temperature of the electrolytic solution is preferably 70 °C or less. From the viewpoint of stably producing the above-described surface-treated steel sheet, it is preferable to monitor the temperature of the electrolytic solution in the cathodic electrolysis process and maintain the temperature of the electrolytic solution in the temperature range of 40 °C to 70 °C.
• the current density in the cathodic electrolysis 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 electrolysis device becomes excessive. Therefore, the current density is preferably 200.0 A/dm 2 or less. The current density is more preferably 100.0 A/dm 2 or less. No lower limit is placed on the current density, but if the current density is excessively low, hexavalent Cr may be produced in the electrolytic solution, causing the bath to lose stability. Therefore, the current density is preferably 5.0 A/dm 2 or more. The current density is more preferably 10.0 A/dm 2 or more.
• the number of times that the steel sheet is subjected to the cathodic electrolysis is not particularly limited and may be any number of times.
• cathodic electrolysis can be performed using an electrolysis treatment device having one pass or any number of passes greater than one.
• 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 reduced productivity. Therefore, the electrolysis time per pass is preferably 5 seconds or less. The electrolysis time is more preferably 3 seconds or less. There is no particular lower limit to the electrolysis time per pass, but if the electrolysis time is excessively 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. The electrolysis time is more preferably 0.01 seconds or more.
• the coating weight of chromium in the chromium-containing layer formed by cathodic electrolysis can be controlled by the total electrical charge density, which is expressed as the product of the current density, the electrolysis time, and the number of passes.
• the total electrical charge density which is expressed as the product of the current density, the electrolysis time, and the number of passes.
• the type of anode used when carrying out cathodic electrolysis is not particularly limited, and any anode can be used.
• An insoluble anode is preferably used as the anode.
• 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, examples of the insoluble anode include an anode in which the surface of a Ti substrate is coated with platinum, iridium oxide, or ruthenium oxide.
• the concentration of the electrolytic solution is constantly changing due to the influence of the formation of the chromium-containing layer on the steel sheet, the introduction and removal of the solution, the evaporation of water, and the like. Since the change in the concentration of the electrolytic solution in the cathodic electrolysis process varies depending on the configuration of the equipment and the manufacturing 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 electrolytic solution in the cathodic electrolysis process and maintain the concentrations within the ranges described below.
• the steel sheet is preferably washed with water at least once. Water washing removes any residual electrolytic solution from the surface of the steel sheet.
• the water washing is not limited and may be performed by any method.
• a water washing tank can be provided downstream of an immersion tank for carrying out the immersion treatment, so that the steel sheet after immersion can be continuously immersed in water.
• Water washing may also be performed by spraying water on the steel sheet after the immersion.
• 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 is preferably 100 ⁇ S/m or less.
• the electrical conductivity is more preferably 50 ⁇ S/m or less.
• the electrical conductivity is even more preferably 30 ⁇ S/m or less.
• the temperature of water used for the water washing is not limited and may be any temperature. However, since excessively high temperatures place an excessive burden on the water washing equipment, the temperature of the water used for water washing is preferably 95 °C or lower. No lower limit is placed on the temperature of water used for water washing either, but the temperature is preferably 0 °C or higher. The temperature of the water used in the water washing may be room temperature.
• Drying may optionally be performed after the water washing.
• the drying method is not limited, and ordinary dryers or electric furnace drying methods, for example, can be applied.
• the temperature during the drying treatment is preferably 100 °C or less from the viewpoint of preventing deterioration of the surface-coating layer. Although no lower limit is placed on the temperature, the lower limit is typically around room temperature.
• the steel sheet Prior to the steel sheet surface conditioning process, the steel sheet may be subjected to any pretreatment.
• the pretreatment preferably includes at least one of degreasing, pickling, and water washing.
• Degreasing removes rolling oil, anti-rust oil, and the like from the steel sheet.
• the degreasing is not limited and can be performed by any method. After degreasing, water washing is preferably performed to remove any degreasing treatment solution adhering to the steel sheet surface.
• the natural oxide film present on the surface of the steel sheet can be removed, so that the surface can be effectively conditioned in the subsequent steel sheet surface conditioning process.
• the pickling is not limited and can be performed by any method. After pickling, water washing is preferably performed to remove any 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 subject the steel sheet to the steel sheet surface conditioning process as is.
• the method of preparing the electrolytic solution used in the cathodic electrolysis process is not particularly limited, but by preparing the electrolytic solution as described below, it is possible to provide the electrolytic solution for the cathodic electrolysis process stably for a longer period of time.
• a trivalent chromium ion source When preparing the aforementioned electrolytic solution, it is preferable to first mix a trivalent chromium ion source, a carboxylic acid compound, and water to prepare an aqueous solution.
• any compound can be used as the trivalent chromium ion source as long as the compound can supply trivalent chromium ions.
• 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.
• the content of the trivalent chromium ion source in the aqueous solution needs to be 0.05 mol/L or more in terms of trivalent chromium ions.
• the content of the trivalent chromium ion source is preferably 0.08 mol/L or more.
• the content of the trivalent chromium ion source is more preferably 0.10 mol/L or more. No upper limit is placed on the content of the trivalent chromium ion source, but the content is preferably 1.50 mol/L or less in terms of trivalent chromium ions.
• the content of the trivalent chromium ion source is more preferably 1.30 mol/L or less.
• BluCr ® BluCr is a registered trademark in Japan, other countries, or both
• TFS A from Atotech Corporation can be used.
• 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 number of carbon atoms in the aliphatic carboxylic acid is preferably 1 to 10.
• the number of carbon atoms is more preferably 1 to 5.
• the number of carbon atoms in the aliphatic carboxylate is preferably 1 to 10.
• the number of carbon atoms is 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.
• the content of the carboxylic acid compound is more preferably 0.15 mol/L or more.
• the content of the carboxylic acid compound is preferably 5.5 mol/L or less.
• the content of the carboxylic acid compound is more preferably 5.3 mol/L or less.
• BluCr ® TFS B from Atotech Corporation can be used.
• Water can be used as a solvent for preparing the aqueous solution. It is preferable to use at least one of ion-exchanged water and distilled water as the water.
• the aqueous solution further contains at least one type of halide ion.
• the content of the halide ions is not particularly limited but is preferably 0.05 mol/L or more.
• the content of the halide ions is more preferably 0.10 mol/L or more.
• the content of the halide ions is preferably 3.0 mol/L or less.
• the content of the halide ions is more preferably 2.5 mol/L or less.
• BluCr ® TFS C1 and BluCr ® TFS C2 by Atotech can be used.
• hexavalent chromium is added to the above-described aqueous solution. It has been confirmed that, in principle, hexavalent chromium is not formed in the cathodic electrolysis process. 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 concentration of hexavalent chromium in the electrolytic solution does not increase.
• the aforementioned metal ions are not limited, but examples thereof include Cu ions, Zn ions, Fe ions, Sn ions, and Ni ions.
• the content of each of these is preferably 0 mg/L or more and 40 mg/L or less.
• the content is more preferably 0 mg/L or more and 20 mg/L or less.
• the content is most preferably 0 mg/L or more and 10 mg/L or less.
• Fe ions may dissolve in the above-described electrolytic solution during the cathodic electrolysis process and the immersion process and be co-deposited in the coating, but this does not affect the corrosion resistance in the BPA-free coated part.
• the Fe ion concentration is preferably in the aforementioned range when the bath is prepared, but it is also preferable to maintain the Fe ion concentration in the electrolytic solution within the aforementioned range during the cathodic electrolysis process and the immersion process. If the Fe ions are controlled within the aforementioned range, the formation of the chromium-containing layer is not inhibited, and a necessary amount of the chromium-containing layer can be formed.
• the pH of the aqueous solution it is preferable to adjust the pH of the aqueous solution to 4.0 to 7.0 and to adjust the temperature of the aqueous solution to 40 °C to 70 °C, thereby preparing the electrolytic solution.
• the pH of the aqueous solution after mixing it is preferable to adjust the pH of the aqueous solution after mixing to 4.0 to 7.0.
• the pH is more preferably 4.5 or higher.
• the pH is more preferably 6.5 or lower.
• Any reagent can be used to adjust the pH.
• the temperature of the aqueous solution after mixing it is preferable to adjust the temperature of the aqueous solution after mixing to 40 °C to 70 °C.
• the holding time in the temperature range of 40 °C to 70 °C is not particularly limited.
• the electrolytic solution obtained by the above procedure can be stably used in the cathodic electrolysis process for a long period of time.
• the electrolytic solution prepared by the above procedure can be stored at room temperature.
• the surface-treated steel sheet of the present disclosure is particularly suitable as a surface-treated steel sheet for containers used in the production of various types of containers, such as food cans, beverage cans, pails, and 18-liter cans, for example.
• surface-treated steel sheets were produced by the following procedures, and their properties were evaluated.
• electrolytic solutions having compositions A to G listed in Table 1 were prepared under the conditions listed in Table 1. That is, each component listed in Table 1 was mixed with water to prepare an aqueous solution, and then the aqueous solution was adjusted to the pH and temperature listed in Table 1.
• the electrolytic solution G corresponds to the electrolytic solution used in the examples of PTL 4.
• ammonia water was used in all cases.
• sulfuric acid was used in electrolytic solutions A, B, and G, hydrochloric acid in electrolytic solutions C and D, and nitric acid in electrolytic solutions E and F.
• the steel sheet used was a cold-rolled 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. The pickling was performed by immersing the steel sheet in an aqueous sulfuric acid solution having the sulfate ion concentration listed in Table 2. The steel sheet after the pickling was subjected to the subsequent steel sheet surface conditioning process without being washed with water.
• the steel sheet after the pickling was subjected to surface conditioning. Specifically, the pickling solution remaining on the surface of the steel sheet was squeezed out with a wringer roll to adjust the coating weight of the pickling solution to the amount listed in Table 2 as "amount of aqueous solution.” Thereafter, while maintaining the coating weight, the steel sheet was held for the holding time listed in Table 2 and then washed with water to remove the pickling solution.
• the steel sheet was subjected to cathodic electrolysis under the conditions listed in Table 2.
• the electrolytic solution during the cathodic electrolysis was maintained at the pH and temperature listed in Table 1.
• the current density during the cathodic electrolysis was 40 A/dm 2 , and the electrolysis time and number of passes were appropriately changed.
• As the anode during the cathodic electrolysis an insoluble anode having a Ti substrate coated with iridium oxide was used. After the cathodic electrolysis, the substrate was washed with water having an electric conductivity of 100 ⁇ S/m or less and then dried at room temperature using a blower.
• the chromium coating weight per side of the steel sheet and the chromium oxide coating weight per side of the steel sheet in the chromium-containing layer were measured by the method described above.
• 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.
• the presence or absence of an oxygen-enriched region was judged by determining, in the vicinity, an oxygen-enriched region containing 10 % or more of O in atomic concentration by the method described above.
• the coverage of the oxygen-enriched region was measured by the method described above. The measurement results are listed in Table 3.
• the chromium-containing layer resulting from the cathodic electrolysis contained chromium compounds, such as chromium oxide and chromium carbide, in addition to metallic chromium.
• the total content of the metallic chromium and the elements constituting the 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.
• the surface of the surface-treated steel sheet was coated with BPA-free paint to prepare a BPA-free prepainted steel sheet.
• BPA-free paint a polyester-based paint for the inner surface of a can (BPA-free paint) was used.
• the BPA-free paint was applied to the surface of the surface-treated steel sheet, and the sheet was then baked at 180 °C for 10 minutes.
• the coating weight of the paint was 60 mg/dm 2 .
• the obtained BPA-free prepainted steel sheet was provided with cross cuts penetrating through to the base steel sheet, and an Erichsen tester was then used to yield a stretch formation 4 mm in height, centered on the intersection of the cross cuts, to prepare a test piece.
• test piece was immersed in a Teflon ® (Teflon is a registered trademark in Japan, other countries, or both) container containing a test liquid, and the container was covered with a lid. In this state, the container was subjected to a retort treatment at a temperature of 121 °C for 1 hour. Subsequently, the test piece was removed from the container, washed with water to remove the test liquid, and then dried with a blower.
• Teflon ® Teflon is a registered trademark in Japan, other countries, or both
• test pieces After drying, the test pieces were subjected to tape peeling twice, and the surfaces of the test pieces were then observed using a microscope or the like.
• the areas of peeled coating and areas of discoloration such as rust were visually evaluated and rated on a 5-point scale. 1 represented the poorest performance, and 5 represented the best performance.
• the same evaluation was performed on two samples per level, and the arithmetic mean of the ratings was calculated and used as an index of the corrosion resistance in the BPA-free coated part.
• the rating is equal to or higher than that of the conventional TFS, the sample can be evaluated as having excellent corrosion resistance in the BPA-free coated part, but it is more preferably for the rating to be equal to or greater than that of the conventional TFS and to be 3.0 or more.

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