Classifications

• • 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
  • 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
  • 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
  • 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
  • C25D5/627—Electroplating characterised by the visual appearance of the layers, e.g. colour, brightness or mat appearance
• • 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 in particular to a surface-treated steel sheet having excellent corrosion resistance at bisphenol A (BPA)-free painted worked part.
• the surface-treated steel sheet according to the present disclosure is suitable for use for containers such as cans.
• the present disclosure also relates to a production method for the surface-treated steel sheet.
• Sn coated steel sheets (tinplate) and tin-free steel sheets (TFS) have been widely used as materials for various metal cans such as beverage cans, food cans, pails, and 18-liter cans.
• Tinplate and TFS are used with organic resin coatings such as epoxy-based paint and PET films in order to accommodate various contents.
• organic resin coatings such as epoxy-based paint and PET films
• a Cr (chromium) oxide layer formed on the outermost surface of the steel sheet by subjecting the steel sheet to electrolysis treatment or immersion treatment in an aqueous solution containing hexavalent Cr exhibits excellent adhesion to the organic resin coating layer.
• the deformation of the organic resin coating layer follows the deformation of the steel sheet during can production, as a result of which corrosion resistance to various contents is ensured after can production.
• Examples of known methods of forming a surface-treated steel sheet without using hexavalent chromium include the methods proposed in PTL 3 to PTL 6. These methods form a surface-treatment layer by performing electrolysis treatment 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.
• a surface-treated steel sheet with excellent adhesion to epoxy-based paint can be obtained by the methods.
• a surface-treated steel sheet that exhibits excellent corrosion resistance even after being painted (coated) with epoxy-based paint and deformed can be obtained.
• the surface-treated steel sheet is suitable for use as a material for containers and the like.
• a surface-treated steel sheet in one embodiment of the present disclosure is a surface-treated steel sheet comprising: a steel sheet; and a chromium-containing layer disposed on a surface of the steel sheet on at least one side.
• the steel sheet is not limited and any steel sheet may be used.
• the steel sheet is preferably a steel sheet for cans.
• As the steel sheet for example, an ultra low carbon steel sheet or a low carbon steel sheet may be used.
• the production method for the steel sheet is not limited, and a steel sheet produced by any method may be used.
• a cold-rolled steel sheet may be used as the steel sheet.
• the cold-rolled steel sheet can be produced, for example, by a typical production process that includes hot rolling, pickling, cold rolling, annealing, and temper rolling.
• the chemical composition of the steel sheet is not limited, and may contain C, Mn, P, S, Si, Cu, Ni, Mo, Al, and inevitable impurities within such ranges that do not undermine the effects according to the present disclosure.
• a steel sheet having a chemical composition specified in ASTM A623M-09 can be suitably used as the steel sheet.
• the sheet thickness of the steel sheet is not limited, but is preferably 0.60 mm or less. No lower limit is placed on the sheet thickness, but the sheet thickness is preferably 0.10 mm or more.
• the term “steel sheet” is defined to include “steel strip”.
• the chromium-containing layer is present on at least one side of the steel sheet.
• the components constituting the chromium-containing layer are not limited, but may include metallic chromium and one or more chromium compounds.
• the one or more chromium compounds are not limited, and any chromium compound(s) may be contained.
• the chromium-containing layer may also contain impurities in addition to metallic chromium and chromium compounds.
• impurities include metallic elements such as Ni, Cu, Sn, and Zn that are mixed in the below-described electrolytic solution as impurities.
• the metallic elements are considered to typically exist in the chromium-containing layer in a metallic state, but may exist as compounds.
• the total content of metallic chromium and elements constituting chromium compounds is preferably 90 at % (atomic %) or more.
• the total content is the ratio of the total atomic number of metallic chromium and elements constituting chromium compounds to the total atomic number of all elements other than Fe, expressed as a percentage.
• the total content can be determined by measuring the content (at %) of each of the metallic chromium and elements constituting chromium compounds contained in the chromium-containing layer by X-ray photoelectron spectroscopy (XPS) and adding them up.
• XPS X-ray photoelectron spectroscopy
• the content (atomic ratio) of the element can be calculated from the integrated intensity of the peak corresponding to the element by the relative sensibility coefficient method.
• the Cr 2 O 3 content can be determined from the integrated intensity of the Cr 2p oxide peak that appears around 576.7 eV.
• the CrO 3 content can be determined from the integrated intensity of the Cr 2p oxide peak that appears around 579.2 eV.
• the contents of other chromium compounds can be determined using the integrated intensities of the following peaks, for example.
• the content of metallic chromium is determined by calculating the Cr content from the integrated intensity of the Cr 2p peak that appears around 573.8 eV and subtracting, from the chromium content, the content of Cr atoms contained as chromium compounds.
• Adding the content of metallic chromium and the contents of elements constituting chromium compounds obtained by this method can yield the total content of metallic chromium and elements constituting chromium compounds.
• the total content refers to the value at the 1/2 position (i.e. position of 1/2) of the thickness of the chromium-containing layer.
• the 1/2 position can be determined by the following procedure. First, while sputtering the chromium-containing layer from its outermost surface, the total content of metallic chromium and elements constituting chromium compounds and the Fe content are measured by the foregoing method. The position (depth) at which the measured total content of metallic chromium and elements constituting chromium compounds and Fe content are equal is taken to be the interface between the chromium-containing layer and the steel sheet. The thickness from the outermost surface of the chromium-containing layer to the interface is taken to be the thickness of the chromium-containing layer, and the 1/2 position of the thickness is determined.
• a scanning X-ray photoelectron spectrometer PHI X-tool produced by ULVAC-PHI, Inc. can be used for the measurement by XPS, for example.
• the X-ray source is a monochromatic AlK ⁇ ray
• the voltage is 15 kV
• the beam diameter is 100 ⁇ m ⁇
• the extraction angle is 45°
• the sputtering conditions are Ar ions with an acceleration 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 limited.
• the components may be separated as separate layers within the chromium-containing layer, or may be mixed throughout the chromium-containing layer.
• the spatial structure of the components constituting the chromium-containing layer may contain one or both of separate layers and a mixed layer.
• the chromium coating weight of the chromium-containing layer is not limited. If the chromium coating weight of the chromium-containing layer is excessively high, however, cohesive fracture may occur in the chromium-containing layer when working the surface-treated steel sheet. Therefore, from the viewpoint of more stably ensuring corrosion resistance at BPA-free painted worked part, the chromium coating weight of the chromium-containing layer per one side is preferably 500.0 mg/m 2 or less and more preferably 450.0 mg/m 2 or less. From the viewpoint of further improving corrosion resistance at BPA-free painted worked part, the chromium coating weight of the chromium-containing layer per one side is preferably 40.0 mg/m 2 or more and more preferably 50.0 mg/m 2 or more.
• the term “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 amount (total Cr amount) in the surface-treated steel sheet is measured using an X-ray fluorescence instrument. Next, the Cr amount (blank sheet Cr amount) in the steel sheet on which the chromium-containing layer has not been formed yet or the steel sheet from which the chromium-containing layer has been peeled off is measured using the X-ray fluorescence instrument. The value obtained by subtracting the blank sheet Cr amount from the total Cr amount is taken to be the chromium coating weight of the chromium-containing layer. For example, a commercially available chromium coating separating agent such as a hydrochloric acid-based agent may be used to peel off the chromium-containing layer.
• a commercially available chromium coating separating agent such as a hydrochloric acid-based agent may be used to peel off the chromium-containing layer
• Chromium oxide may be present in the chromium-containing layer.
• the location of chromium oxide is not limited, and chromium oxide may be present in the form of O concentrated in the below-described linear regions.
• the location of O can be determined, for example, by composition analysis using energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) attached to a scanning electron microscope (SEM) or 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 oxide coating weight of the chromium-containing layer is not limited. If the chromium oxide coating weight of the chromium-containing layer is excessively high, however, cohesive fracture may occur starting from Cr oxide in the chromium-containing layer when working the surface-treated steel sheet, causing degradation in corrosion resistance at BPA-free painted worked part. Therefore, from the viewpoint of more stably ensuring corrosion resistance at BPA-free painted worked part, the chromium oxide coating weight of the chromium-containing layer per one side is preferably 40.0 mg/m 2 or less and more preferably 35.0 mg/m 2 or less.
• the chromium-containing layer may contain no chromium oxide. Thus, no lower limit is placed on the chromium oxide coating weight of the chromium-containing layer, and the chromium oxide coating weight of the chromium-containing layer per one side may be 0.0 mg/m 2 .
• 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 amount (total Cr amount) in the surface-treated steel sheet is measured. Next, the surface-treated steel sheet is subjected to alkali treatment of immersing in 7.5N—NaOH at 90° C. for 10 minutes to remove chromium oxide. The surface-treated steel sheet after the alkali treatment is thoroughly washed with water, and then the Cr amount (the Cr amount after alkali treatment) is measured again using an X-ray fluorescence instrument. The value obtained by subtracting the Cr amount after alkali treatment from the total Cr amount is taken to be the chromium oxide coating weight of the chromium-containing layer.
• the chromium-containing layer may be amorphous or crystalline.
• the chromium-containing layer may contain one or both of amorphous and crystalline phases.
• the chromium-containing layer produced by the below-described method usually contains amorphous phase, and may also contain crystalline phase.
• the mechanism by which the chromium-containing layer is formed is not clear, but it is presumed that, during formation of amorphous phase, the amorphous phase is partially crystallized and as a result a chromium-containing layer containing both amorphous and crystalline phases is formed.
• the area ratio of the crystalline region is not limited, but is preferably 30% or less when the chromium-containing layer is observed from the surface direction. Since the crystalline region may not be present, the lower limit of the area ratio of the crystalline region may be 0%.
• the crystalline region in the chromium-containing layer can be determined by removing the base steel sheet from the surface-treated steel sheet to prepare a single-layer sample of the chromium-containing layer and 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 limited.
• the single-layer sample of the chromium-containing layer can be prepared by applying an ion beam of Ar or the like from the base steel sheet side and ion milling the steel sheet.
• the ion beam is applied with an acceleration voltage of 5 kV or less and an incidence angle of 1 degree to 5 degrees relative to the base steel sheet, thereby ensuring an observation field of a single chromium layer region of several ⁇ m 2 or more.
• the bottom of the chromium-containing layer is also milled to some extent and as a result the film thickness of the chromium-containing layer decreases in some cases. This, however, does not affect the measurement result of the crystalline region.
• the area ratio of the crystalline region in the chromium-containing layer can be measured using a TEM. Specifically, first, a diffraction pattern of the chromium-containing layer is obtained by selected area diffraction of the TEM. Next, a dark field image is obtained for all diffraction spots in the diffraction pattern, and a region that appears with high brightness in the dark field image is taken to be the crystalline region. The area of the obtained crystalline region is calculated by image processing, and the calculated area is divided by the area of the chromium-containing layer in the selected area aperture to calculate the area ratio of the crystalline region. For example, image analysis software such as Image-J may be used to calculate the area ratio.
• image analysis software such as Image-J may be used to calculate the area ratio.
• the chromium-containing layer may contain C. No upper limit is placed on the C content in the chromium-containing layer, but the atomic ratio of C to Cr is preferably 50% or less and more preferably 45% or less.
• the chromium-containing layer may not contain C. Thus, no lower limit is placed on the atomic ratio of C to Cr in the chromium-containing layer, and the lower limit may be 0%.
• the C content in the chromium-containing layer can be measured by XPS.
• XPS X-ray photoelectron spectrometer
• the X-ray source is a monochromatic AlK ⁇ ray
• the voltage is 15 kV
• the beam diameter is 100 ⁇ m ⁇
• the extraction angle is 45°
• the sputtering conditions are Ar ions with an acceleration 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 limited, and C may be present in the form of being concentrated in the below-described linear regions.
• the location of C can be determined, for example, by composition analysis using energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) attached to a scanning electron microscope (SEM) or 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 Fe. No upper limit is placed on the Fe content in the chromium-containing layer, but the atomic ratio of Fe to Cr is preferably 100% or less.
• the chromium-containing layer may not contain Fe. Thus, no lower limit is placed on the atomic ratio of Fe to Cr, and the lower limit may be 0%.
• the Fe content in the chromium-containing layer can be measured by XPS, as with the C content. The atomic ratio can be calculated using the narrow spectra of Cr2p and Fe2p.
• Fe contained in the chromium-containing layer dissolves in a small amount into the electrolytic solution and is incorporated into the layer.
• the chromium-containing layer may contain metal impurities such as K, Na, Mg, and Ca contained in water and Sn, Ni, Cu, and Zn contained in the aqueous solution, and S, N, Cl, Br, etc., besides Cr, O, Fe, and C.
• metal impurities such as K, Na, Mg, and Ca contained in water and Sn, Ni, Cu, and Zn contained in the aqueous solution, and S, N, Cl, Br, etc.
• the total content of elements other than Cr, O, Fe, and C is preferably 3% or less and more preferably 0% (i.e. the other elements are not contained at all) in terms of atomic ratio to Cr.
• the content of these elements can be measured by XPS as with the C content, without being limited thereto.
• the number of the linear regions is 5.0 or more per 100 nm.
• the number of the linear regions is preferably 7.0 or more per 100 nm and more preferably 10.0 or more per 100 nm. No upper limit is placed on the number of the linear regions, and the number of the linear regions may be, for example, 50.0 or less per 100 nm, 45.0 or less per 100 nm, or 40.0 or less per 100 nm.
• a typical chromium-containing layer formed from a hexavalent Cr bath or trivalent Cr bath is composed of metallic chromium and chromium oxide.
• a surface-treated steel sheet including such a chromium-containing layer is usually worked into cans or the like after an organic resin coating is formed on its surface.
• metallic chromium has poor workability, the chromium-containing layer cannot fully follow the deformation of the steel sheet during working, and consequently the organic resin coating on the chromium-containing layer is damaged. This causes a decrease in post-working corrosion resistance.
• chromium oxide is provided in the top layer to ensure post-working corrosion resistance. Since chromium oxide has excellent adhesion to epoxy-based paint, even though metallic chromium cannot follow the deformation of the base steel sheet, the chromium-containing layer and the epoxy-based paint firmly adhere to each other and the coating properties of the epoxy-based paint can be maintained even after can production.
• the surface-treated steel sheet according to the present disclosure achieves excellent corrosion resistance at BPA-free painted worked part by including 5.0 or more linear regions per 100 nm in its chromium-containing layer as described above.
• the present disclosure is thus based on a technical idea completely different from the conventional ones, i.e. improving the deformability of the chromium-containing layer itself rather than the adhesion to paint.
• a region in which an element smaller in atomic number than chromium is detected in a larger amount than the average composition of the chromium-containing layer by 20 at % or more in an EDS quantitative map obtained by STEM/EDS analysis of the chromium-containing layer is defined as a “linear region in which an element smaller in atomic number than chromium is concentrated”.
• the STEM/EDS analysis is performed using a single-layer sample of the chromium-containing layer.
• the single-layer sample of the chromium-containing layer can be prepared by the foregoing method.
• the number of linear regions can be measured, for example, from an EDS quantitative map of the chromium-containing layer. Any ten 100 nm lines are drawn on the map image, the intersections of each line with linear regions are counted, and the arithmetic mean value is taken to be the number of linear regions.
• the element concentrated in the linear region is not limited, and may be any element smaller in atomic number than chromium.
• the element may include at least one selected from the group consisting of O, C, N, and S.
• the linear regions may be isolated from each other, may intersect with each other, or may be connected in a mesh-like manner.
• the linear regions preferably have a mesh-like connected structure.
• the size, number, and shape of the mesh are not limited, but the standard deviation of the equivalent circular diameter of the mesh is preferably 30 nm or less and more preferably 20 nm or less from the viewpoint of further improving corrosion resistance at BPA-free painted worked part. No lower limit is placed on the standard deviation, and the standard deviation may be, for example, 0.5 nm or more or 1.0 nm or more.
• the equivalent circular diameter of the mesh can be calculated by image analysis of an STEM/EDS map. Specifically, from an STEM/EDS map observed at a magnification of 450,000 times, the number of pixels in the region surrounded by the mesh is calculated using image analysis software such as Image-J, and the calculated number is multiplied by the area per pixel to obtain the area of the mesh. Then, the equivalent circular diameter is calculated from the obtained area. The standard deviation of the equivalent circular diameter is calculated from a total of 100 pieces of equivalent circular diameter data.
• the shape of the mesh is not limited, but is desirably close to a perfect circle. Specifically, the average roundness of the mesh is preferably 0.5 or more.
• the roundness is 1.
• the average roundness may be 1.0 or less.
• the roundness of the mesh can be calculated by image analysis of an STEM/EDS map, as with the equivalent circular diameter. Specifically, an STEM/EDS map observed at a magnification of 450,000 times is analyzed using image analysis software such as Image-J, and a circle inscribed in the mesh and a circle circumscribed around the mesh are drawn. The diameter of the inscribed circle is divided by the diameter of the circumscribed circle to find the roundness. The roundness is calculated for a total of 100 inscribed circles, and the average value is taken to be the roundness of the mesh.
• a surface-treated steel sheet having the above-described properties can be produced by the method described below.
• the production method for a surface-treated steel sheet in one embodiment of the present disclosure is a production method for a surface-treated steel sheet that includes: a steel sheet; and a chromium-containing layer disposed on a surface of the steel sheet on at least one side, and comprises a steel sheet surface adjustment process and a cathodic electrolysis process.
• Amount of Aqueous Solution 1.0 g/m 2 to 30.0 g/m 2
• the mechanism by which the linear regions in which an element smaller in atomic number than chromium is concentrated are formed as a result of the steel sheet surface adjustment process is not clear, but it is presumed as follows.
• an aqueous solution containing sulfate ions 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 surface of the steel sheet increases.
• the amount of the aqueous solution is within the foregoing range, the thickness of the aqueous solution on the steel sheet is very thin, so that dissolved oxygen in the aqueous solution increases. This further promotes the foregoing reactions.
• the state in which the aqueous solution is present on the steel sheet is not limited, but the aqueous solution is preferably in the form of a liquid film from the viewpoint of making the reactions uniform.
• the Fe ions are oxidized to Fe oxide, which accumulates on the surface of the steel sheet in a very small amount.
• the accumulated small amount of Fe oxide is reduced and a chromium-containing layer is formed.
• the surface potential is microscopically different between the parts where the accumulated small amount of Fe oxide exists and the parts where the accumulated small amount of Fe oxide does not exist. As a result, linear regions in which an element smaller in atomic number than chromium is concentrated are formed.
• the amount of the aqueous solution is preferably 2.0 g/m 2 or more and more preferably 3.0 g/m 2 or more. From the same viewpoint, the amount of the aqueous solution is preferably 28.0 g/m 2 or less and more preferably 25.0 g/m 2 or less.
• the holding time is preferably 0.2 seconds or more and more preferably 0.3 seconds or more. From the same viewpoint, the holding time is preferably 18.0 seconds or less and more preferably 15.0 seconds or less.
• the amount of the aqueous solution present on the surface of the steel sheet can be measured with a moisture meter using a filter type infrared absorption method. Specifically, the absorbance on the surface of the steel sheet is measured with the moisture meter using the filter type infrared absorption method, and the amount of the aqueous solution is calculated from the absorbance using a calibration curve obtained in advance.
• the calibration curve can be created by the following procedure. First, the steel sheet is placed on an electronic balance. The aqueous solution is dropped onto the steel sheet with a pipette to form a liquid film over the entire surface of the steel sheet.
• the weight of the aqueous solution present on the steel sheet is calculated from the weight of the steel sheet before the aqueous solution is dropped and the weight of the steel sheet after the aqueous solution is dropped.
• the calculated weight of the aqueous solution is divided by the area of the steel sheet to determine the amount of the aqueous solution per unit area.
• the absorbance on the surface of the steel sheet is measured with the moisture meter using the filter type infrared absorption method.
• the above measurements are carried out a plurality of times while changing the amount of the aqueous solution to create a calibration curve that represents the correlation between the amount of the aqueous solution and the absorbance.
• As the calibration curve a linear approximation of the correlation between the amount of the aqueous solution and the absorbance can be used.
• the method of adjusting the amount of the aqueous solution present on the surface of the steel sheet is not limited, and any method may be used. For example, squeezing the solution with a wringer roll, wiping, or the like may be used.
• composition of the aqueous solution is not limited, but a sulfuric acid aqueous solution such as dilute sulfuric acid is preferable.
• sulfuric acid aqueous solution means an aqueous solution of sulfuric acid, and includes cases where components other than sulfuric acid are contained.
• the pickling liquid may also be used as the aqueous solution in the steel sheet surface adjustment process.
• pickling liquids typically contain pickling inhibitors, pickling accelerators, etc., these components do not particularly hinder the formation of linear regions. Thus, even when the pickling liquid contains pickling inhibitors, pickling accelerators, etc., the pickling liquid can be used as the aqueous solution in the steel sheet surface adjustment process.
• concentration of sulfate ions contained in the aqueous solution is preferably 3 g/L or more and more preferably 5 g/L or more.
• concentration of sulfate ions contained in the aqueous solution is preferably 200 g/L or less and more preferably 150 g/L or less.
• the temperature of the aqueous solution is preferably 10° C. or more and more preferably 15° C. or more.
• No upper limit is placed on the temperature of the aqueous solution, but the temperature of the aqueous solution is preferably 70° C. or less and more preferably 60° C. or less.
• the steel sheet is subjected to cathodic electrolysis treatment in an electrolytic solution containing 0.05 mol/L or more of trivalent chromium ions.
• a chromium-containing layer can be formed on the steel sheet.
• the trivalent chromium ion source any compound that can supply trivalent chromium ions may be used.
• the trivalent chromium ion source for example, at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate may be used.
• the temperature of the electrolytic solution during the cathodic electrolysis treatment is not 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 and maintain the temperature within a temperature range of 40° C. to 70° C. in the cathodic electrolysis process.
• the pH of the electrolytic solution during the cathodic electrolysis treatment is not limited, but is preferably 4.0 or more and more preferably 4.5 or more.
• the pH is preferably 7.0 or less and more preferably 6.5 or less. From the viewpoint of stably producing the above-described surface-treated steel sheet, it is preferable to monitor the pH of the electrolytic solution and maintain the pH within the foregoing pH range in the cathodic electrolysis process.
• the current density in the cathodic electrolysis treatment is not limited and may be adjusted appropriately so that the desired surface-treatment layer will be formed. If the current density is excessively high, an excessive load is put on the cathodic electrolytic device.
• the current density is therefore preferably 200.0 A/dm 2 or less and more preferably 100 A/dm 2 or less. Although no lower limit is placed on the current density, if the current density is excessively low, hexavalent Cr may form in the electrolytic solution, causing the stability of the bath to be lost.
• the current density is therefore preferably 5.0 A/dm 2 or more and more preferably 10.0 A/dm 2 or more.
• the number of times the steel sheet is subjected to cathodic electrolysis treatment is not limited and may be any number.
• cathodic electrolysis treatment can be performed using an electrolytic device having any number of passes where the number is one or more.
• the electrolysis time per pass is not limited. If the electrolysis time per pass is excessively long, however, the conveyance speed (line speed) of the steel sheet decreases and productivity decreases.
• the electrolysis time per pass is therefore preferably 5 seconds or less and more preferably 3 seconds or less. Although no lower limit is placed on the electrolysis time per pass, if the electrolysis time is excessively short, the line speed needs to be increased accordingly, which makes control difficult.
• the electrolysis time per pass is therefore preferably 0.005 seconds or more and more preferably 0.01 seconds or more.
• the Cr coating weight of the chromium-containing layer formed by cathodic electrolysis treatment can be controlled by the total electric charge density, which is expressed as the product of the current density, the electrolysis time, and the number of passes.
• An excessively small Cr coating weight may impair corrosion resistance at BPA-free painted worked part, and an excessively large Cr coating weight may cause cohesive fracture in the chromium-containing layer during working, as mentioned above. From the viewpoint of more stably ensuring corrosion resistance at BPA-free painted worked part, it is preferable to control the total electric charge density so that the Cr coating weight of the chromium-containing layer per one side of the steel sheet will be within an appropriate range.
• the type of the anode used when performing cathodic electrolysis treatment is not limited and any anode may be used.
• an insoluble anode is preferably used.
• the insoluble anode 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 is preferably used.
• a more specific example of the insoluble anode is an anode in which the surface of Ti as a substrate is coated with platinum, iridium oxide, or ruthenium oxide.
• the concentration of the electrolytic solution changes constantly due to the formation of the chromium-containing layer on the steel sheet, the introduction and removal of liquid, the evaporation of water, etc. Since the change in the concentration of the electrolytic solution in the cathodic electrolysis process varies depending on the structure of the device and the production conditions, it is preferable to monitor the concentration of each component in the electrolytic solution and maintain it within the below-described concentration range in the cathodic electrolysis process from the viewpoint of more stably producing the surface-treated steel sheet.
• the steel sheet after the cathodic electrolysis process is preferably washed with water at least once. As a result of water washing, the electrolytic solution remaining on the surface of the steel sheet can be removed.
• the water washing is not limited and may be performed by any method.
• a water washing tank may be provided downstream of the immersion tank for performing the immersion process so that the steel sheet after immersion can be continuously immersed in water.
• Water washing may be performed by spraying water on the steel sheet after immersion.
• the water used for the water washing is not 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 water washing is not limited, but is preferably 100 ⁇ S/m or less, more preferably 50 ⁇ S/m or less, and further preferably 30 ⁇ S/m or less.
• the temperature of the water used for the water washing is not limited and may be any temperature. Since an excessively high temperature puts an excessive load on the water washing equipment, the temperature of the water used in the water washing is preferably 95° C. or less. No lower limit is placed on the temperature of the water used in the water washing, but the temperature is preferably 0° C. or more. The temperature of the water used in the water washing may be room temperature.
• drying may be optionally performed.
• the drying method is not limited, and a typical dryer or electric furnace drying method may be used.
• the temperature during the drying treatment is preferably 100° C. or less, from the viewpoint of suppressing the deterioration of the surface-coating layer. Although no lower limit is placed on the temperature during the drying treatment, the temperature is typically about room temperature.
• the steel sheet may be optionally subjected to pretreatment before the steel sheet surface adjustment process. It is preferable to perform at least one of degreasing, pickling, and water washing as the pretreatment.
• Degreasing can remove rolling oil, rust preventive oil, etc. adhering to the steel sheet.
• the degreasing is not limited and may be performed by any method. After the degreasing, it is preferable to perform water washing to remove the degreasing liquid adhering to the surface of the steel sheet.
• Pickling can remove a natural oxide layer present at the surface of the steel sheet, with it being possible to effectively adjust the surface in the subsequent steel sheet surface adjustment process.
• the pickling is not limited and may be performed by any method. After the pickling, it is preferable to perform water washing to remove the pickling liquid adhering to the surface of the steel sheet. If an aqueous solution containing sulfate ions is used as the pickling liquid, it is preferable to use the aqueous solution containing sulfate ions in the steel sheet surface adjustment process.
• the method of preparing the electrolytic solution used in the cathodic electrolysis process is not limited, but the below-described electrolytic solution preparation process enables the electrolytic solution to be provided in the cathodic electrolysis process stably for a long period of time.
• a trivalent chromium ion source a trivalent chromium ion source, a carboxylic acid compound, and water are mixed to prepare an aqueous solution.
• trivalent chromium ion source any compound that can supply trivalent chromium ions may be used.
• the trivalent chromium ion source for example, at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate may be used.
• the content of the trivalent chromium ion source in the aqueous solution needs to be 0.05 mol/L or more and is preferably 0.08 mol/L or more and more preferably 0.10 mol/L or more in terms of trivalent chromium ions. No upper limit is placed on the content of the trivalent chromium ion source, but the content of the trivalent chromium ion source is preferably 1.50 mol/L or less and more preferably 1.30 mol/L or less in terms of trivalent chromium ions.
• BluCr® (BluCr is a registered trademark in Japan, other countries, or both) TFS A produced by Atotech can be used.
• the carboxylic acid compound is not limited and any carboxylic acid compound may 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 an aliphatic carboxylate.
• 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 limited.
• the content of the carboxylic acid compound is preferably 0.1 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 0.15 mol/L or more.
• the content of the carboxylic acid compound is more preferably 5.3 mol/L or less.
• BluCr® TFS B produced by Atotech can be used.
• Water may be used as a solvent for preparing the aqueous solution.
• As the water it is preferable to use at least one of ion-exchanged water and distilled water.
• the aqueous solution further contains at least one type of halide ion.
• the content of the halide ion is not limited.
• the content of the halide ion is preferably 0.05 mol/L or more.
• the content of the halide ion is preferably 3.0 mol/L or less.
• the content of the halide ion is more preferably 0.10 mol/L or more.
• the content of the halide ion is more preferably 2.5 mol/L or less.
• BluCr® TFS Cl and BluCr® TFS C2 produced by Atotech can be used to contain the halide ion.
• hexavalent chromium it is preferable not to add hexavalent chromium to the 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 that the concentration of hexavalent chromium in the electrolytic solution does not increase.
• metal ions are not limited, and examples thereof include Cu ions, Zn ions, Fe ions, Sn ions, and Ni ions.
• the content of each of these ions is preferably 0 mg/L or more and 40 mg/L or less, more preferably 0 mg/L or more and 20 mg/L or less, and most preferably 0 mg/L or more and 10 mg/L or less.
• Fe ions may dissolve in the electrolytic solution in the cathodic electrolysis process and the immersion process and co-deposit in the layer (coating), but does not affect corrosion resistance at BPA-free painted worked part.
• the concentration of Fe ions is preferably within the foregoing range at initial make-up of electrolytic bath. It is also preferable to maintain the concentration of Fe ions in the electrolytic solution within the foregoing range in the cathodic electrolysis process and the immersion process. If Fe ions are controlled within the foregoing range, the formation of the chromium-containing layer is not hindered and the necessary amount of the chromium-containing layer can be formed.
• the pH of the aqueous solution is adjusted to 4.0 to 7.0 and the temperature of the aqueous solution is adjusted to 40° C. to 70° C. to prepare the electrolytic solution.
• the pH of the aqueous solution after mixing is adjusted to 4.0 to 7.0.
• the pH is preferably 4.5 or more.
• the pH is preferably 6.5 or less.
• Any reagent may be used to adjust the pH.
• the temperature of the aqueous solution after mixing is adjusted to 40° C. to 70° C.
• the holding time in the temperature range of 40° C. to 70° C. is not limited.
• the electrolytic solution obtained by this procedure can be provided in the cathodic electrolysis process stably for a long period of time.
• the electrolytic solution produced by this procedure can be stored at room temperature.
• the surface-treated steel sheet according to the present disclosure is not limited.
• the surface-treated steel sheet is suitable as a surface-treated steel sheet for containers used in the production of various containers such as food cans, beverage cans, pails, and 18-liter cans.
• surface-treated steel sheets were produced by the procedure described below and their properties were evaluated.
• electrolytic solutions having compositions A to G shown in Table 1 were prepared under the conditions shown in Table 1.
• the components shown in Table 1 were mixed with water to prepare an aqueous solution, and the aqueous solution was then adjusted to the pH and temperature shown in Table 1.
• Electrolytic solution G corresponds to the electrolytic solution used in the examples in PTL 6.
• Ammonia water was used to increase the pH for each electrolytic solution.
• Sulfuric acid was used to decrease the pH for electrolytic solutions A, B, and G.
• Hydrochloric acid was used to decrease the pH for electrolytic solutions C and D.
• Nitric acid was used to decrease the pH for electrolytic solutions E and F.
• steel sheets cold-rolled steel sheets were used. More specifically, steel sheets for cans (T4 blank sheets) with a sheet thickness of 0.17 mm were used. As pretreatment, each steel sheet was subjected to electrolytic degreasing, water washing, and pickling in sequence. For the pickling, a sulfuric acid aqueous solution with the 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 next steel sheet surface adjustment process without water washing.
• the steel sheet after the pickling was subjected to surface adjustment. Specifically, the pickling liquid remaining on the surface of the steel sheet was squeezed with a wringer roll to adjust the amount of the pickling liquid attached to the surface to the amount shown in Table 2 as “amount of aqueous solution”. After this, the steel sheet was held for the holding time shown in Table 2 while maintaining the amount of the pickling liquid attached. The steel sheet was then washed with water to remove the pickling liquid.
• the steel sheet was then subjected to cathodic electrolysis treatment under the conditions shown in Table 2.
• the electrolytic solution during the cathodic electrolysis treatment was maintained at the pH and temperature shown in Table 1.
• the current density was 40 A/dm 2 , and the electrolysis time and the number of passes were varied as appropriate.
• anode in the cathodic electrolysis treatment an insoluble anode composed of a Ti substrate coated with iridium oxide was used.
• the steel sheet was washed with water with an electrical conductivity of 100 ⁇ S/m or less, and dried at room temperature using a blower.
• the chromium coating weight of the chromium-containing layer per one side of the steel sheet and the chromium oxide coating weight of the chromium-containing layer per one side of the steel sheet were measured by the above-described methods.
• the number of linear regions in which an element smaller in atomic number than chromium is concentrated, whether a mesh structure was present, the standard deviation of the mesh, and the roundness of the mesh were measured by the above-described methods. The measurement results are shown in Table 3.
• the chromium-containing layer obtained by cathodic electrolysis treatment contained chromium compounds such as chromium oxide and chromium carbide in addition to metallic chromium.
• the total content of metallic chromium and elements constituting chromium compounds in the chromium-containing layer was 90 mass % or more.
• at least one selected from the group consisting of O, C, N, and S was concentrated. In particular, O was observed to be concentrated in the linear regions in all examples.
• BPA-free paint was applied to the surface of the surface-treated steel sheet to prepare a BPA-free painted steel sheet.
• the BPA-free paint 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 then baked at 80° C. for 10 minutes.
• the coating weight of the paint was 60 mg/dm 2 .
• the obtained BPA-free painted steel sheet was cross-cut through to the base steel sheet, and then a 4 mm-high overhang was formed around the intersection of the cross-cut using an Erichsen tester 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 covered with a lid. In this state, retort treatment was performed 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
• the dried test piece was subjected to tape peeling twice. After this, the surface of the test piece was observed with a microscope or the like, and the area of paint peeling and the area of discoloration such as rust were visually evaluated and scored on a 5-point scale. 1 is the poorest performance and 5 is the most excellent performance.
• the same evaluation was performed on two samples for each example, and the arithmetic mean value of the scores was calculated and taken to be an index of corrosion resistance at BPA-free painted worked part. Practically, if the score is higher than or equal to that of conventional TFS, the corrosion resistance at BPA-free painted worked part can be evaluated as excellent. It is more preferable if the score is higher than or equal to that of conventional TFS and is 3.0 or more.

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