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, 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.
• 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 with excellent film corrosion resistance and paint corrosion resistance.
• FIG. 1 is a diagram showing a roughness curve.
• 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 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 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 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 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. Furthermore, from the viewpoint of improving weldability, it is even more preferable that it be 7.0 mg/ m2 or less.
• 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.
• 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 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.
• arithmetic mean roughness In the present invention, it is important that the arithmetic mean roughness determined by the method described below is 1.30 nm or more and 15.0 nm or less.
• 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 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.
• the arithmetic mean roughness is calculated from the roughness curve r(x) using the following formula: where Ra is the arithmetic mean roughness and L is the evaluation length of the roughness curve.
• the arithmetic mean roughness is calculated from cross-sectional images of five or more randomly selected fields of view using the above-mentioned method, and the average value is used as the arithmetic mean roughness.
• 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.
• 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 steel sheet is subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using the electrolytic solution prepared in the electrolytic solution preparation step. Specifically, the steel sheet is immersed in the electrolytic solution and subjected to the electrolytic treatments. This allows the chromium-containing layer to be formed. Furthermore, when higher weldability is required, cathodic electrolysis C1, anodic electrolysis A1, cathodic electrolysis C2, and anodic electrolysis A2 may be performed in this order.
• the charge density of the cathodic electrolysis treatment C1 is not particularly limited. However, the amount of chromium deposited in the chromium-containing layer can be controlled by the charge density of the cathodic electrolysis treatment C1. Therefore, the charge density is preferably 5.0 C/dm2 or more , more preferably 10.0 C/dm2 or more . For the same reason, the charge density is preferably 200.0 C/dm2 or less , more preferably 180.0 C/dm2 or less .
• the current density and current application time of the cathodic electrolysis treatment C1 are not particularly limited and can be appropriately set to achieve the desired value of the electricity density, which is expressed as the product of the current density (unit: A/dm 2 ) and the current application time (unit: sec.) of the electrolysis treatment.
• the temperature of the electrolyte when performing cathodic electrolysis C1 is not particularly limited, but in order to efficiently form a chromium-containing layer, it is preferable to set the temperature in the range of 40°C or higher and 70°C or lower. From the perspective of stably producing the above-mentioned surface-treated steel sheet, it is preferable to monitor the temperature of the electrolyte during cathodic electrolysis C1 and maintain it in the above temperature range.
• 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 methods described above. Furthermore, for each of the obtained surface-treated steel sheets, the arithmetic mean roughness 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.
• 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
• a rating of 0 to 3 can be said to indicate excellent weldability. 0: 0.8 kA or more 1: 0.6 kA or more, less than 0.8 kA 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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