Classifications

• • 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
  • C25D5/18—Electroplating using modulated, pulsed or reversing current
• • 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

Definitions

• the present invention relates to a surface-treated steel sheet, and in particular to a surface-treated steel sheet that has excellent film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability.
• the surface-treated steel sheet of the present invention can be suitably used for containers such as cans.
• the present invention also relates to a method for manufacturing the surface-treated steel sheet.
• Sn-plated steel sheet has excellent corrosion resistance, weldability, and workability, and is easy to manufacture, so it has been used for over 200 years as a material for various metal cans such as beverage cans, food cans, pail cans, and 18-liter cans.
• Tin-free steel sheet is a surface-treated steel sheet in which a metallic chromium layer and a chromium oxide layer are formed on the surface of the steel sheet, and is typically produced by electrolyzing the steel sheet in an electrolyte containing hexavalent chromium (Patent Documents 1 to 3). Due to its excellent corrosion resistance, tin-free steel sheet is now extremely commonly used as a steel sheet for containers, replacing tinplate. However, because typical tin-free steel sheet has an insulating chromium oxide layer on the surface, it has poor weldability.
• Patent Documents 4 and 5 a type of tin-free steel sheet with excellent weldability is known, in which granular protrusions are formed on the surface of the steel sheet by performing anodic electrolysis between multiple cathodic electrolysis treatments during electrochromic chromium plating.
• a surface treatment layer is formed by electrolysis in an electrolyte containing a trivalent chromium compound such as basic chromium sulfate.
• the present invention was made in consideration of the above-mentioned circumstances, and its purpose is to provide a surface-treated steel sheet that can be produced without using hexavalent chromium and that has excellent film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability.
• a surface-treated steel sheet having a chromium-containing layer disposed on at least one surface of the steel sheet parameters relating to the surface properties of the chromium-containing layer determined by a predetermined method and the atomic ratio of C to Cr in the chromium-containing layer are each controlled within specific ranges. This makes it possible to obtain a surface-treated steel sheet with excellent film corrosion resistance, paint corrosion resistance, paint secondary adhesion, and weldability.
• the above-mentioned surface-treated steel sheet can be produced by carrying out cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using an electrolytic solution prepared by a specified method, while controlling the electrical density of anodic electrolysis A1 and cathodic electrolysis C2 within a specific range. Furthermore, by preparing the electrolytic solution by a specified method, it is possible to prevent the hexavalent chromium in the electrolytic solution from increasing during film formation.
• the present invention was completed based on the above findings.
• the gist of the present invention is as follows:
• the chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less.
• a method for producing a surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, the method comprising: an electrolyte solution preparation step of preparing an electrolyte solution containing trivalent chromium ions; a coating formation step of forming the chromium-containing layer,
• a trivalent chromium ion source, a carboxylic acid compound, and water are mixed;
• the electrolyte solution is prepared by adjusting the pH to 4.0 to 7.0 and the temperature to 40 to 70°C.
• the steel sheet is subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using the electrolytic solution,
• the electricity density of the anodic electrolysis treatment A1 is 0.50 C/dm 2 or more and 20.00 C/dm 2 or less
• the method for producing a surface-treated steel sheet, wherein the electricity density of the cathodic electrolysis treatment C2 is 50.0 C/dm 2 or more and 150.0 C/dm 2 or less.
• the present invention it is possible to provide a surface-treated steel sheet that has excellent film corrosion resistance, paint corrosion resistance, secondary paint adhesion, and weldability without using hexavalent chromium.
• the surface-treated steel sheet of the present invention can be suitably used as a material for containers, etc.
• a surface-treated steel sheet according to one embodiment of the present invention comprises a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet. It is important in the present invention that the N/L ratio (described below) is 0.023 nm -1 or more and 0.085 nm -1 or less, and that the chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less. Each of the constituent elements of the surface-treated steel sheet is described below.
• the steel sheet is not particularly limited, and any steel sheet can be used.
• the steel sheet is preferably a steel sheet for cans.
• the steel sheet can be, for example, an ultra-low carbon steel sheet or a low carbon steel sheet.
• the method for manufacturing the steel sheet is also not particularly limited, and a steel sheet manufactured by any method can be used.
• a cold-rolled steel sheet can be used as the steel sheet.
• the cold-rolled steel sheet can be manufactured by a general manufacturing process that includes, for example, hot rolling, pickling, cold rolling, annealing, and temper rolling.
• the chemical composition of the steel plate is not particularly limited, but for example, steel plate having a chemical composition specified in ASTM A623M-09 can be suitably used.
• mass % C: 0.0001 to 0.13%, Si: 0 to 0.020%, P: 0 to 0.020%, S: 0 to 0.030%, Al: 0 to 0.20%, and N: 0 to 0.040%, and optionally further containing, in mass%, Mn: 0.01 to 0.60%, Cu: 0 to 0.20%, Ni: 0 to 0.15%, Cr: 0 to 0.10%, Mo: 0 to 0.05%, Ti: 0 to 0.020%, Nb: 0 to 0.020%, B: 0 to 0.020%, Ca: 0-0.020%, Contains at least one selected from the group consisting of Sn: 0 to 0.020% and Sb: 0 to 0.020%, It is preferable to use a steel sheet having a composition with the balance consisting of Fe and unavoidable impurities. Of the above composition, the lower the content of Si, P, S, Al, and N, the more preferable it is. Mn,
• the thickness of the steel plate is not particularly limited, but is preferably 0.60 mm or less.
• “steel plate” is defined here to include “steel strip.”
• the lower limit of the thickness is not particularly limited, but is preferably 0.10 mm or more.
• a chromium-containing layer is present on at least one surface of the steel sheet.
• the components constituting the chromium-containing layer are not particularly limited, but may include metallic chromium and a chromium compound.
• the chromium compound is not particularly limited, and may include any chromium compound.
• the chromium compound may include, for example, at least one selected from the group consisting of chromium oxide, chromium carbide, chromium sulfide, chromium nitride, chromium chloride, chromium bromide, and chromium boride.
• the chromium-containing layer may also contain impurities.
• impurities include metal elements such as Ni, Cu, Sn, and Zn that are mixed as impurities in the electrolytic solution described below.
• the metal elements are typically considered to exist in the chromium-containing layer in a metallic state, but may also exist as compounds.
• the chromium-containing layer preferably has a total content of elements constituting metallic chromium and chromium compounds of 90 atomic % or more.
• the total content is the ratio, expressed as a percentage, of the total atomic number of elements constituting metallic chromium and chromium compounds to the total atomic number of all elements other than Fe.
• the total content can be determined by measuring the content (atomic %) of each element constituting the metallic chromium and chromium compound contained in the chromium-containing layer using X-ray photoelectron spectroscopy (XPS) and adding them up.
• XPS X-ray photoelectron spectroscopy
• the content (atomic ratio) of each element can be calculated using the relative sensitivity factor method from the integrated intensity of the peak corresponding to that element.
• the content of chromium carbide can be determined from the integrated intensity of the peak of C 1s carbide appearing around 281.0 eV .
• the C content atomic ratio to the total of all elements other than Fe
• the Cr2O3 content can be determined from the integrated intensity of the Cr2p oxide peak appearing near 576.7 eV . Also, the CrO3 content can be determined from the integrated intensity of the Cr2p oxide peak appearing near 579.2 eV.
• Chromium sulfide (Cr 2 S 3 ): S 2p sulfide peak appearing around 162.3 eV
• Chromium nitride (CrN): N 1S peak appearing around 397.3 eV
• Chromium chloride (CrCl 3 ): Cl 2p peak appearing around 199.8 eV
• Chromium bromide (CrBr 3 ): Br 3d peak appearing around 69.1 eV
• Chromium boride (CrB): Br 1s peak appearing around 188.2 eV
• the metallic chromium content can be determined by calculating the Cr content from the integrated intensity of the Cr 2p peak that appears around 573.8 eV, and then subtracting the content of Cr atoms contained as chromium compounds from the chromium content.
• the total content of metallic chromium and the elements that make up the chromium compound can be calculated.
• the total content refers to the value at the half-thickness position of the chromium-containing layer.
• the half-thickness position can be determined by the following procedure. First, the chromium-containing layer is sputtered from its outermost surface, while the total content of elements constituting metallic chromium and chromium compounds and the Fe content are measured using the method described above. The position (depth) where the measured total content of elements constituting metallic chromium and chromium compounds and the Fe content are equal is determined as the interface between the chromium-containing layer and the steel sheet. The thickness from the outermost surface of the chromium-containing layer to this interface is defined as the thickness of the chromium-containing layer, and its half-thickness position is determined.
• the XPS measurement can be performed using, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI, Inc.
• the X-ray source is a 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 acceleration voltage of 1 kV and a sputtering rate of 1.50 nm/min in terms of SiO2 .
• the spatial structure of the components that make up the chromium-containing layer is not particularly limited; for example, they may be separated into separate layers within the chromium-containing layer, or they may be mixed throughout the chromium-containing layer.
• the spatial structure of the components that make up the chromium-containing layer can include either or both separate layers and mixed layers.
• the chromium deposition amount of the chromium-containing layer is not particularly limited. However, an excessive chromium deposition amount of the chromium-containing layer may impair weldability and may cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and paint secondary adhesion, the chromium deposition amount of the chromium-containing layer is preferably 500.0 mg/ m2 or less per side, and more preferably 450.0 mg/ m2 or less.
• the chromium deposition amount of the chromium-containing layer is preferably 40.0 mg/ m2 or more per side, and more preferably 50.0 mg/ m2 or more.
• the chromium deposition weight is measured using an X-ray fluorescence spectrometer according to the following procedure. First, the Cr amount (total Cr amount) in the surface-treated steel sheet is measured using the X-ray fluorescence spectrometer. Next, the Cr amount (original sheet Cr amount) is measured using the X-ray fluorescence spectrometer on the steel sheet before the chromium-containing layer is formed or on the steel sheet after the chromium-containing layer has been stripped off. The value obtained by subtracting the original sheet Cr amount from the total Cr amount is the Cr deposition weight of the chromium-containing layer. To strip the chromium-containing layer, for example, a commercially available hydrochloric acid-based chromium plating stripper can be used.
• Chromium oxide deposition amount Chromium oxide may be present in the chromium-containing layer.
• the location of the chromium oxide is not particularly limited. The location of O can be confirmed by, for example, composition analysis using energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) attached to a scanning electron microscope (SEM) or a transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).
• 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 deposition amount of the chromium-containing layer is not particularly limited. However, if the chromium oxide deposition amount of the chromium-containing layer is excessive, it may impair weldability and may cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and secondary paint adhesion, the chromium oxide deposition amount of the chromium-containing layer is preferably 40.0 mg/m2 or less per side, and more preferably 35.0 mg/ m2 or less . On the other hand, the chromium-containing layer may not contain any chromium oxide at all. Therefore, the lower limit of the chromium oxide deposition amount of the chromium-containing layer is not particularly limited, and may be 0.0 mg/ m2 per side.
• the chromium oxide deposition amount is measured using an X-ray fluorescence analyzer according to the following procedure. First, the Cr amount (total Cr amount) of the surface-treated steel sheet is measured. Next, the surface-treated steel sheet is subjected to an alkali treatment by immersing it in 7.5N NaOH at 90°C for 10 minutes to remove the chromium oxide. After the alkali treatment, the surface-treated steel sheet is thoroughly rinsed with water, and the Cr amount (post-alkali-treatment Cr amount) is measured again using the X-ray fluorescence analyzer. The value obtained by subtracting the post-alkali-treatment Cr amount from the total Cr amount is the chromium oxide deposition amount of the chromium-containing layer.
• 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. Chromium-containing layers manufactured by the method described below generally contain an amorphous phase, and may also contain a crystalline phase. The mechanism by which the chromium-containing layer is formed is unclear, but it is thought that partial crystallization occurs when the amorphous phase is formed, resulting in a chromium-containing layer containing both an amorphous and a crystalline phase.
• the area ratio of the crystalline region is not particularly limited, but it is preferably 30% or less when the chromium-containing layer is observed from the surface direction. The lower limit of the area ratio of the crystalline region is not particularly limited, and may be 0%.
• the crystalline region in the chromium-containing layer can be confirmed by preparing a chromium-containing single-layer sample by etching the substrate steel sheet and observing the sample from the surface side using a TEM or scanning transmission electron microscope (STEM).
• the method for preparing the chromium-containing single-layer sample is not particularly limited, but it can be prepared, for example, by irradiating the steel sheet with an ion beam such as Ar from the substrate steel sheet side and ion milling the steel sheet.
• a field of view of a chromium single-layer region of several ⁇ m2 or more can be ensured by irradiating the ion beam with an acceleration voltage of 5 kV or less and an incident angle relative to the substrate steel sheet in the range of 1 to 5 degrees.
• the bottom surface of the chromium-containing layer may also be milled to some extent, which may result in a thinner film thickness of the chromium-containing layer, but this does not affect the measurement results of the area ratio of the crystalline region.
• the area ratio of crystalline regions in a chromium-containing layer can be measured using a TEM. Specifically, a diffraction pattern of the chromium-containing layer is obtained using selected-area diffraction with a TEM, and dark-field images are obtained at all diffraction spots in the pattern. The areas that appear brightest in the dark-field image are determined to be crystalline regions. The area of the obtained crystalline regions is calculated using image processing, and the area ratio of the crystalline regions is calculated by dividing the area by the area of the chromium-containing layer within the selected-area aperture. Image analysis software such as Image-J can be used to calculate the area ratio.
• Image analysis software such as Image-J can be used to calculate the area ratio.
• the chromium-containing layer contains C, and that the atomic ratio of C to Cr in the chromium-containing layer is 0.2% or more and 50.0% or less.
• the atomic ratio of C is 0.2% or more and 50.0% or less, the chromium-containing layer is destroyed by volume change during welding pressure application or initial heat input, making it easier to conduct current, thereby lowering the minimum welding current limit, i.e., improving weldability. If the atomic ratio of C is too low, the above-mentioned effect of improving weldability cannot be obtained. Therefore, the atomic ratio of C is set to 0.2% or more.
• the atomic ratio of C is preferably set to 0.3% or more.
• the atomic ratio of C is set to 50.0% or less.
• the atomic ratio of C is preferably set to 40.0% or less.
• the atomic ratio of C to Cr in the chromium-containing layer is measured using XPS according to the following procedure. First, sputtering is performed from the outermost layer to a depth of at least 0.2 nm in SiO2 equivalent, and the integrated intensity of the narrow spectrum of Cr2p and C1s is determined. From the integrated intensity, the atomic ratio is quantified using the relative sensitivity factor method, and the C atomic ratio/Cr atomic ratio is calculated.
• XPS measurement for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI, Inc.
• the chromium-containing layer may contain Fe. There is no particular upper limit to the Fe content in the chromium-containing layer, but it is preferable that the atomic ratio relative to Cr is 100% or less.
• the chromium-containing layer may not contain Fe, and therefore the lower limit of the atomic ratio relative to Cr is not particularly limited and 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 narrow spectra of Cr2p and Fe2p.
• the chromium-containing layer may contain metal impurities such as K, Na, Mg, and Ca contained in the water, Sn, Ni, Cu, and Zn contained in the aqueous solution, as well as S, N, Cl, and Br.
• metal impurities such as K, Na, Mg, and Ca contained in the water, Sn, Ni, Cu, and Zn contained in the aqueous solution, as well as S, N, Cl, and Br.
• the total atomic ratio of elements other than Cr, O, Fe, and C to Cr is preferably 3% or less, and it is even more preferable that they are completely absent (0%).
• the content of the above elements is not particularly limited, but can be measured, for example, by XPS, in the same way as the C content.
• N/L is the number N of peaks in the roughness curve of the chromium-containing layer, the height of which from the mean line of the roughness curve at the apex exceeds 1.2 nm, divided by the evaluation length L of the roughness curve.
• a cross-sectional image of the chromium-containing layer is obtained. Specifically, a dark-field image is taken using an STEM of a cross section perpendicular to the surface of the coated steel sheet, and this is used as the cross-sectional image. STEM has a high enough spatial resolution for observing the chromium-containing layer, and dark-field observation allows the chromium-containing layer region to be clearly distinguished from the background.
• the resolution of the cross-sectional image shall be 0.5 nm or less per pixel. Furthermore, in order to obtain average information about the chromium-containing layer, images shall be taken at a magnification that allows a chromium-containing layer with a length of 150 nm or more to be confirmed, and cross-sectional images shall be taken from at least five randomly selected fields of view.
• the cross-sectional image is set so that the left-right direction coincides with or nearly coincides with the longitudinal direction of the chromium-containing layer, and the up-down direction coincides with or nearly coincides with the thickness direction of the chromium-containing layer. If the line connecting the endpoints of the chromium-containing layer is tilted by 5 degrees or more from the left-right direction, the captured cross-sectional image is rotated using image processing. However, if rotation processing is performed, the resolution of the image before rotation must be 0.25 nm or less per pixel.
• the cross-sectional image is trimmed so that it does not include areas more than 100 nm above or below the chromium-containing layer. Furthermore, to reduce the variability in results due to noise in the cross-sectional image, a median filter with a kernel of 3 x 3 or larger is applied to the observed dark-field image to remove noise. However, to avoid underestimating the surface roughness, the length of one side of the kernel is set to 2 nm or less.
• N/L is calculated from cross-sectional images of five or more randomly selected fields of view using the above-described method, and the average value is used as N/L.
• N/L determined by the above-mentioned method is set to 0.023 nm ⁇ 1 or more and 0.085 nm ⁇ 1 will be described in detail.
• N/L corresponds to the number density of granular protrusions having a given height present on the surface of the chromium-containing layer. If N/L is less than 0.023 nm ⁇ 1 , there will be few granular protrusions of sufficient size on the surface of the chromium-containing layer, resulting in a reduced surface area and poor paint adhesion. Therefore, N/L should be 0.023 nm ⁇ 1 or greater, preferably 0.028 nm ⁇ 1 or greater, and more preferably 0.032 nm ⁇ 1 or greater.
• N/L should be 0.085 nm ⁇ 1 or less, preferably 0.070 nm ⁇ 1 or less, and more preferably 0.055 nm ⁇ 1 or less.
• a method for producing a surface-treated steel sheet according to one embodiment of the present invention is a method for producing a surface-treated steel sheet having a chromium-containing layer disposed on at least one surface of the steel sheet, and includes the following steps (1) and (2). Each step will be described below.
• a film formation step for forming a chromium-containing layer is a method for producing a surface-treated steel sheet having a chromium-containing layer disposed on at least one surface of the steel sheet.
• any compound capable of supplying trivalent chromium ions can be used as the trivalent chromium ion source.
• at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate can be used as the trivalent chromium ion source.
• water is used as the solvent for preparing the electrolyte solution. It is preferable to use highly pure water, such as ion-exchanged water from which cations have been removed in advance using an ion exchange resin or the like, or distilled water. Furthermore, from the perspective of reducing the amounts of K, Na, Mg, and Ca contained in the electrolyte solution, it is preferable to use water with an electrical conductivity of 30 ⁇ S/m or less. The lower limit of electrical conductivity is not limited and may be 0 ⁇ S/m.
• the electrolytic solution is prepared by adjusting the pH of the aqueous solution to 4.0 to 7.0 and adjusting the temperature of the aqueous solution to 40 to 70° C.
• it is not sufficient to simply dissolve a trivalent chromium ion source and a carboxylic acid compound in water; it is important to appropriately control the pH and temperature as described above.
• the above procedure allows the electrolyte solution to be obtained and used in the next film formation process.
• the electrolyte solution produced using the above procedure can be stored at room temperature.
• Anodic electrolysis treatment A1 Next, the steel sheet subjected to cathodic electrolysis C1 is subjected to anodic electrolysis A1 using the electrolytic solution.
• the chromium-containing layer formed by cathodic electrolysis C1 is dissolved, forming a generation site for granular precipitates of metallic chromium and chromium compounds in cathodic electrolysis C2.
• the granular precipitates of metallic chromium and chromium compounds may be simply referred to as granular chromium.
• trivalent chromium is oxidized to hexavalent chromium on the steel sheet surface.
• the hexavalent chromium is instantly reduced to trivalent chromium, so that in practice, no hexavalent chromium is present in the electrolytic solution. Therefore, it is important to use the electrolytic solution prepared in the above-mentioned electrolytic solution preparation step, particularly to make anodic electrolysis A1 a process that does not use hexavalent chromium.
• the charge density is 20.00 C/dm2 or less, preferably 18.00 C/dm2 or less , and more preferably 16.00 C/ dm2 or less.
• the current density and current application time for the anodic electrolysis treatment A1 are not particularly limited and can be set appropriately to achieve the desired electrical charge density.
• the temperature of the electrolyte when performing anodic electrolysis treatment A1 is not particularly limited, and the preferred embodiment is the same as that for cathodic electrolysis treatment C1. From the perspective of stably producing the above-mentioned surface-treated steel sheet and more reliably suppressing an increase in the hexavalent chromium concentration, it is preferable to monitor the temperature of the electrolyte during anodic electrolysis treatment A1 and maintain it within the above temperature range.
• the steel sheet that has been subjected to the anodic electrolysis treatment A1 is subjected to cathodic electrolysis treatment C2 using the electrolytic solution described above.
• cathodic electrolysis treatment C2 a chromium-containing layer can be formed on the steel sheet, and particulate chromium can be precipitated starting from the above-mentioned generation sites.
• the electricity density of the cathodic electrolytic treatment C2 is less than 50.0 C/dm 2 , granular chromium is not sufficiently precipitated, resulting in an N/L ratio of less than 0.023 nm 1 in the finally obtained surface-treated steel sheet. Therefore, the electricity density is 50.0 C/dm 2 or more, preferably 55.0 C/dm 2 or more, and more preferably 60.0 C/dm 2 or more.
• the electricity density exceeds 150.0 C/dm 2 , excessive granular chromium is precipitated and becomes dense, resulting in an N/L ratio of more than 0.085 nm 1 in the finally obtained surface-treated steel sheet. Therefore, the electricity density is 150.0 C/dm 2 or less, preferably 120.0 C/dm 2 or less, and more preferably 100.0 C/dm 2 or less.
• the current density and current application time for the cathodic electrolysis treatment C2 are not particularly limited and can be set appropriately to achieve the desired electrical charge density.
• the temperature of the electrolyte when performing cathodic electrolysis treatment C2 is not particularly limited, and the preferred embodiment is the same as that of cathodic electrolysis treatment C1. From the same perspective as for cathodic electrolysis treatment C1, it is preferable to monitor the temperature of the electrolyte during cathodic electrolysis treatment C2 and maintain it within the above temperature range.
• the pH of the electrolyte when performing cathodic electrolysis treatment C2 is not particularly limited, and the preferred embodiment is the same as that of cathodic electrolysis treatment C1. From the same perspective as for cathodic electrolysis treatment C1, it is preferable to monitor the pH of the electrolyte in cathodic electrolysis treatment C2 and maintain it within the above pH range.
• cathodic electrolysis treatment C2 there are no particular restrictions on the type of electrode used when performing cathodic electrolysis treatment C2, and the preferred embodiment is the same as for cathodic electrolysis treatment C1.
• the surface-treated steel sheet is preferably washed with water at least once, which makes it possible to remove the electrolytic solution remaining on the surface of the steel sheet.
• the water washing can be carried out by any method without particular limitations.
• a water washing tank can be provided downstream of the immersion tank used for the immersion treatment, and the steel sheet can be continuously immersed in water after immersion.
• the steel sheet can be washed by spraying water onto it after immersion.
• the water used for the rinsing 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 rinsing is not particularly limited, but it is preferably 100 ⁇ S/m or less, more preferably 50 ⁇ S/m or less, and even more preferably 30 ⁇ S/m or less.
• the temperature of the water used for the washing is not particularly limited and may be any temperature. However, an excessively high temperature places an excessive burden on the washing equipment, so the temperature of the water used for washing is preferably 95°C or less. On the other hand, the lower limit of the temperature of the water used for washing is not particularly limited, but it is preferably 0°C or higher. The temperature of the water used for the washing may be room temperature.
• drying may be carried out as desired.
• a conventional dryer or electric oven drying method can be used.
• the temperature during the drying process be 100°C or less.
• the temperature during the drying process is no particular lower limit, but it is usually around room temperature.
• the steel sheet Prior to the film forming step, the steel sheet may be optionally subjected to a pretreatment, which is preferably at least one of degreasing, pickling, and water washing.
• a pretreatment which is preferably at least one of degreasing, pickling, and water washing.
• Degreasing allows the removal of rolling oil, rust-preventive oil, and other substances adhering to the steel sheet. There are no particular restrictions on the degreasing method, and it can be carried out by any method. After degreasing, it is preferable to rinse the steel sheet with water to remove the degreasing treatment liquid adhering to the surface.
• the natural oxide film present on the surface of the steel sheet can be removed, allowing for the effective formation of a chromium-containing layer in the subsequent film formation process.
• the pickling can be performed by any method without any particular restrictions. After the pickling, it is preferable to rinse the steel sheet with water to remove any pickling solution adhering to the surface.
• the uses of the surface-treated steel sheet of the present invention are not particularly limited, but it is particularly suitable as a surface-treated steel sheet for containers used in the manufacture of various containers such as food cans, beverage cans, pail cans, and 18-liter cans.
• electrolyte preparation process First, electrolyte solutions having compositions A to G shown in Table 1 were prepared under the conditions shown in Table 1. That is, each component shown in Table 1 was mixed with water to prepare an aqueous solution, and then the aqueous solution was adjusted to the pH and temperature shown in Table 1. Note that electrolyte solution G corresponds to the electrolyte solution used in the examples of Patent Document 6. Ammonia water was used to increase the pH in all cases, and sulfuric acid was used for electrolyte solutions A, B, and G, hydrochloric acid for electrolyte solutions C and D, and nitric acid for electrolyte solutions E and F to decrease the pH.
• 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.
• the steel sheet was pretreated by electrolytic degreasing, water washing, pickling by immersion in dilute sulfuric acid, and water washing, in that order.
• the amount of chromium deposited per side of the chromium-containing layer and the amount of chromium oxide deposited per side of the steel sheet were measured using the method described above. Furthermore, for each of the obtained surface-treated steel sheets, the N/L ratio and the atomic ratio of C to Cr in the chromium-containing layer were measured using the method described above. The measurement results are shown in Table 3.
• Laminated steel sheets were produced by laminating an isophthalic acid copolymerized polyethylene terephthalate film with a stretch ratio of 3.1 x 3.1, a thickness of 25 ⁇ m, a copolymerization ratio of 12 mol%, and a melting point of 224°C on both sides of the resulting surface-treated steel sheet.
• the lamination was carried out under conditions that resulted in a crystallinity of the resin film of 10% or less, specifically, a steel sheet feed speed of 40 m/min, a rubber roll nip length of 17 mm, and a time from pressing to water cooling of 1 sec.
• the crystallinity of the resin film was determined using the density gradient tube method in accordance with JIS K7112.
• the nip length refers to the length in the conveying direction of the area where the rubber roll and steel sheet come into contact.
• painted steel plates were prepared as samples to be used to evaluate paint corrosion resistance and secondary paint adhesion using the following procedure.
• An epoxy phenol-based paint was applied to the surface of the obtained surface-treated steel sheet, and baked at 210°C for 10 minutes to prepare a painted steel sheet.
• the coating weight of the paint was 50 mg/ dm2 .
• Film corrosion resistance was evaluated using the following four criteria: film peel width (total width extending from the cut) was measured at four random locations on the crosscut of the laminated steel sheet, and the average of the four values was calculated and considered to be the corrosion width. Paint corrosion resistance was evaluated using the following four criteria: film peel width (total width extending from the cut) was measured at four random locations on the crosscut of the coated steel sheet, and the average of the four values was calculated and considered to be the corrosion width. Film corrosion resistance and paint corrosion resistance were evaluated using the following four criteria: In practice, a rating of 1 to 3 can be said to be excellent in corrosion resistance.

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