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, film wet 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.
• a surface treatment layer is formed by electrolysis in an electrolyte containing a trivalent chromium compound such as basic chromium sulfate.
• Patent Documents 6, 7, and 8 make it possible to form a surface treatment layer without using hexavalent chromium. Furthermore, Patent Documents 6, 7, and 8 also show that these methods can produce surface-treated steel sheets that exhibit excellent adhesion to resin films in humid environments (hereinafter referred to as "film wet adhesion"), film corrosion resistance, and paint corrosion resistance.
• film wet adhesion surface-treated steel sheets that exhibit excellent adhesion to resin films in humid environments
• 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, film wet 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 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 1.0 C/dm 2 or more and less than 50.0 C/dm 2 .
• the present invention it is possible to provide a surface-treated steel sheet that has excellent film corrosion resistance, paint corrosion resistance, film wet 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.
• FIG. 1 is a diagram showing a roughness curve.
• the surface-treated steel sheet comprises a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet.
• the S/L ratio (described below) is 0.10 or more and 0.70 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.
• 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: 0.01-0.60%, 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 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
• 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 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 coating weight of the chromium-containing layer is not particularly limited. However, an excessive chromium coating weight of the chromium-containing layer may impair weldability and cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and film wet adhesion, the chromium coating weight 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 coating weight 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 cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and film wet 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-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 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 X-ray source may be a monochromatic AlK ⁇ ray, with a voltage of 15 kV, a beam diameter of 100 ⁇ m ⁇ , and a take-off angle of 45°.
• the sputtering conditions are Ar ion with an acceleration voltage of 1 kV and a sputtering rate of 1.50 nm/min in SiO2 equivalent.
• 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.
• S/L is the ratio of the total length S in the x-axis direction of the region of the roughness curve whose height from the mean line exceeds 1.9 nm to the evaluation length L of the roughness curve.
• 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 curve is a curve that follows the surface of the chromium-containing layer on the surface side of the surface-treated steel sheet.
• the cross-sectional image is segmented into the chromium-containing layer and base steel sheet regions and other regions (for example, the background, and layers such as paint and film that may be applied to the surface). Segmentation can be performed using any of a method that uses a brightness threshold, a method that uses manual painting, or a method that uses image analysis by machine learning.
• a cross-sectional curve is extracted by connecting the highest points in the thickness direction of the chromium-containing layer.
• a Gaussian filter which is a phase compensation filter, is used as the filter applied to the profile curve p(x).
• the roughness curve r(x) is calculated by the following equation.
• Figure 1 shows a schematic diagram of the roughness curve r(x).
• the mean line conforms to JIS B0601:2013 and coincides with the x-axis in Figure 1.
• L is the evaluation length (length in the x-axis direction) of the roughness curve.
• the roughness curve r(x) in Figure 1 has five regions with a height of 1.9 nm or more.
• the cutoff values ⁇ s and ⁇ c are smaller than those commonly used to derive roughness curves, and by using these cutoff values, parameters suitable for expressing fine surface morphology can be obtained. Furthermore, by setting 1.9 nm as the reference height, the state of granular protrusions present on the surface of the chromium-containing layer can be expressed. In the present invention, the S/L obtained using the cutoff value and reference height accurately reflects factors of the surface morphology of the chromium-containing layer that affect weldability and film wet adhesion.
• S/L corresponds to the area ratio of granular protrusions of a specified height present on the surface of the chromium-containing layer. If S/L is small, there will be few granular protrusions of sufficient size on the surface of the chromium-containing layer, making it difficult for the metal oxide on the surface to break down when pressure is applied, and no electrical continuity can be obtained as a starting point for welding. Therefore, if S/L is less than 0.10, weldability will deteriorate, especially in welding with high pressure. Therefore, S/L should be 0.10 or greater, preferably 0.15 or greater, and more preferably 0.20 or greater.
• 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.
• Electrolytic solution preparation process (i) Mixing In the electrolytic solution preparation step, first, a trivalent chromium ion source, a carboxylic acid compound, and water are mixed to prepare an aqueous solution.
• the content of the trivalent chromium ion source in the aqueous solution is not particularly limited, but is preferably 3 g/L or more, and more preferably 5 g/L or more, calculated as trivalent chromium ions.
• the content of the trivalent chromium ion source is preferably 50 g/L or less, and more preferably 40 g/L or less.
• BluCr (registered trademark) TFS A from Atotech can be used as the trivalent chromium ion source.
• Carboxylic acid stabilizes trivalent chromium ions in the electrolyte. Therefore, adding a carboxylic acid compound to the aqueous solution can suppress an increase in the hexavalent chromium concentration during the film formation process, particularly the anodic electrolysis process A1, described below. While carboxylic acid compounds are not typically used in electrolysis processes using hexavalent chromium, the present invention requires the addition of a carboxylic acid compound to the aqueous solution.
• 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 an aliphatic carboxylic acid salt.
• the aliphatic carboxylic acid preferably has 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms.
• the aliphatic carboxylic acid salt preferably has 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms.
• the content of the carboxylic acid compound is not particularly limited, but is preferably 0.1 mol/L or more, more preferably 0.15 mol/L or more.
• the content of the carboxylic acid compound is preferably 5.5 mol/L or less, more preferably 5.3 mol/L or less.
• Atotech's BluCr (registered trademark) TFS B can be used as the carboxylic acid compound.
• 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 aqueous solution to further contain at least one type of halide ion.
• the amount of halide ion is not particularly limited, but it is preferably 0.05 mol/L or more, and more preferably 0.10 mol/L or more.
• the amount of halide ion is preferably 3.0 mol/L or less, and more preferably 2.5 mol/L or less.
• Atotech's BluCr(R) TFS C1 and BluCr(R) TFS C2 can be used to incorporate the halide ions.
• hexavalent chromium it is preferable not to add hexavalent chromium to the above-mentioned aqueous solution. As will be described later, the trace amounts of hexavalent chromium that form on the electrode or steel sheet surface during the film formation process are reduced to trivalent chromium, so the hexavalent chromium concentration in the electrolyte does not increase.
• the metal ions are not limited, but examples include Cu ions, Zn ions, Ni ions, Fe ions, and Sn ions, and each concentration is preferably 0 mg/L to 40 mg/L, more preferably 0 mg/L to 20 mg/L, and most preferably 0 mg/L to 10 mg/L.
• 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 pH of the aqueous solution after mixing is adjusted to 4.0 to 7.0. If the pH is less than 4.0 or more than 7.0, the stability of the electrolyte decreases, causing precipitation and preventing the formation of a chromium-containing layer in the film formation step. Furthermore, the hexavalent chromium concentration in the electrolyte increases during electrolysis.
• the pH is preferably 4.5 or higher.
• the pH is preferably 6.5 or lower.
• the concentration of the electrolyte constantly changes due to factors such as the formation of a chromium-containing layer on the steel sheet, the introduction and removal of the solution, and the evaporation of water.
• the change in concentration of the electrolyte in cathodic electrolysis C1 varies depending on the equipment configuration and manufacturing conditions. Therefore, from the perspective of more stable production of surface-treated steel sheets, it is preferable to monitor the concentrations of the components contained in the electrolyte in cathodic electrolysis C1 and maintain them within the concentration ranges described above.
• 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 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 methods described above. Furthermore, for each of the obtained surface-treated steel sheets, the S/L ratio and the atomic ratio of C to Cr in the chromium-containing layer were measured using the methods 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 the paint corrosion resistance 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.
• Corrosion width less than 0.3 mm 2 Corrosion width 0.3 mm or more and less than 0.5 mm 3: Corrosion width 0.5 mm or more and less than 1.0 mm 4: Corrosion width 1.0 mm or more
• test pieces were cut from each of the above laminated steel plates: three with the front surface as the target surface and three with the back surface as the target surface. Each test piece measured 30 mm wide and 100 mm long.
• the film on the target surface was left intact and the steel plate was cut away from the film on the side opposite the target surface 15 mm from the top along the length of each test piece.
• the test piece was fixed in place from the bottom 15 mm along the length of the test piece so that the steel plate was perpendicular to the ground, and a 30 mm wide and 15 mm long section above the cut position was left hanging down, connected by the film on the target surface. A 100 g weight was then attached to the hanging section, 30 mm wide and 15 mm long.
• the test specimens in this state were left in a retort atmosphere at a temperature of 130°C and a relative humidity of 100% for 30 minutes, and then exposed to the atmosphere.
• the length of the film on the target surface peeled from the surface-treated steel sheet was taken as the film peel length, and the average film peel length for six test specimens was calculated for each laminated steel sheet.
• the film wet adhesion was evaluated using the following four levels. In practice, a rating of 1 to 3 can be said to indicate excellent film wet adhesion. 1: Peeling length less than 20 mm 2: Peeling length 20 mm or more and less than 40 mm 3: Peeling length 40 mm or more and less than 60 mm 4: Peeling length 60 mm or more
• a rating of 1 to 3 can be said to indicate excellent weldability. 1: 0.6 kA or more 2: 0.4 kA or more, less than 0.6 kA 3: 0.2 kA or more, less than 0.4 kA 4: Less than 0.2 kA

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