Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/10—Electroplating with more than one layer of the same or of different metals
  • C25D5/12—Electroplating with more than one layer of the same or of different metals at least one layer being of nickel or chromium
  • C25D5/14—Electroplating with more than one layer of the same or of different metals at least one layer being of nickel or chromium two or more layers being of nickel or chromium, e.g. duplex or triplex layers
• • 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
  • C25D5/16—Electroplating with layers of varying thickness
• • 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.
• 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.
• 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.
• 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, and weldability.
• the inventors of the present invention have discovered that in a surface-treated steel sheet having a chromium-containing layer disposed on at least one surface of the steel sheet, by controlling the outermost surface structure of the chromium-containing layer and the atomic ratio of C to Cr in the chromium-containing layer, it is possible to obtain a surface-treated steel sheet with excellent film corrosion resistance, paint corrosion resistance, and weldability.
• the present invention was completed based on the above findings.
• the gist of the present invention is as follows:
• a steel plate A surface-treated steel sheet comprising a chromium-containing layer disposed on at least one surface of the steel sheet, granular protrusions are present on the outermost surface of the chromium-containing layer, and the average area of Voronoi polygons obtained by performing Voronoi tessellation on the distribution of the centroids of the granular protrusions is 10.00 nm2 or more and 1000.00 nm2 or less,
• the chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less.
• the present invention it is possible to provide a surface-treated steel sheet that has excellent film corrosion resistance, paint corrosion resistance, 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 schematic diagram of the distribution of the centers of gravity of the granular protrusions.
• FIG. 2 is a Voronoi diagram obtained from the distribution of the centroids of the granular protrusions shown in FIG.
• a surface-treated steel sheet comprises a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet. It is important that granular protrusions are present on the outermost surface of the chromium-containing layer, that the average area of Voronoi polygons obtained by performing Voronoi tessellation on the distribution of the centroids of the granular protrusions is 10.00 nm2 or more and 1000.00 nm2 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 features 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: 0.01-0.60%, 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 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 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 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.
• granular protrusions are present on the outermost surface of the chromium-containing layer. It is important that the average area of Voronoi polygons obtained by performing Voronoi tessellation on the distribution of the centroids of the granular protrusions is 10.00 nm2 or more and 1000.00 nm2 or less.
• the granular protrusions can be confirmed by observing the surface of the surface-treated steel sheet using a scanning electron microscope (SEM).
• SEM scanning electron microscope
• the magnification of the SEM should be such that 500 or more granular protrusions can be confirmed within one field of view.
• the surface-treated steel sheet is painted or laminated with a film
• the surface-treated steel sheet is immersed in the agent until the paint or film peels off, then rinsed with running water and dried before observation.
• the agent is selected to match the paint or film; for example, sulfuric acid or hydrogen peroxide can be used.
• the immersion temperature is adjusted appropriately depending on the type of agent, etc.
• Voronoi division is performed on the obtained distribution. Specifically, image processing is performed on the obtained SEM image to extract the granular protrusions and their centroids (center of gravity positions). Then, using the extracted centroids as the generating points, Voronoi division is performed to create a Voronoi diagram.
• Voronoi tessellation is a method of dividing a plane containing multiple points (generator points) by drawing perpendicular bisectors connecting adjacent generator points and dividing the plane into areas (Voronoi polygons) enclosed by these perpendicular bisectors.
• a diagram created by Voronoi tessellation (a diagram composed of Voronoi polygons) is also called a Voronoi diagram.
• Figure 1 is a diagram showing a schematic representation of the distribution of centroids of granular protrusions.
• Figure 2 is a Voronoi diagram created by performing Voronoi division on the distribution of centroids of the granular protrusions shown in Figure 1.
• the area of each Voronoi polygon that makes up the Voronoi diagram is calculated, and the average area of all Voronoi polygons in five randomly selected fields of view is calculated as the average area of the Voronoi polygons.
• the presence of the granular protrusions on the outermost surface of the chromium-containing layer promotes the destruction of the metal oxide on the surface of the chromium-containing layer when pressure is applied during welding, making it easier to obtain conductivity that serves as the starting point for welding. Furthermore, the density of the granular protrusions increases as the average area of the Voronoi polygons decreases. If the average area of the Voronoi polygons exceeds 1000.00 nm2 , the density of the granular protrusions decreases, and the destruction of the metal oxide on the surface of the chromium-containing layer is not promoted, resulting in poor weldability, particularly in welding with high pressure.
• the average area of the Voronoi polygons is set to 1000.00 nm2 or less, preferably 950.00 nm2 or less, and more preferably 900.00 nm2 or less.
• the average area of the Voronoi polygons is set to 10.00 nm 2 or more, preferably 20.00 nm 2 or more, and more preferably 50.00 nm 2 or more.
• the variance ⁇ 2 calculated from the areas of all Voronoi polygons in the five fields of view obtained by the above-mentioned method is preferably small. When the variance is small, heat is generated uniformly during welding, resulting in better weldability.
• a surface-treated steel sheet having the above-described properties can be produced by the method described below.
• 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 electricity density of the cathodic electrolytic treatment C2 is 1.0 C/ dm2 or more and 150.0 C/dm2 or less .
• the production method includes the following steps (1) and (2). Each step will be described below. (1) An electrolyte preparation step for preparing an electrolyte containing trivalent chromium ions; (2) A film formation step for forming a chromium-containing layer.
• 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.
• 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.
• 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, by adding a carboxylic acid compound to the aqueous solution, it is possible to 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, in a preferred embodiment of the present invention, a carboxylic acid compound is added to the aqueous solution.
• the carboxylic acid compound is not particularly limited, and any carboxylic acid compound can be used.
• 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 each of the electrolytic treatments. This allows the chromium-containing layer to be formed.
• 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 pH of the electrolyte used in cathodic electrolysis C1 is not particularly limited, but is preferably 4.0 or higher, and more preferably 4.5 or higher. Furthermore, the pH is preferably 7.0 or lower, and more preferably 6.5 or lower. From the perspective of stably producing the above-mentioned surface-treated steel sheet, it is preferable to monitor the pH of the electrolyte during cathodic electrolysis C1 and maintain it within the above pH range.
• an insoluble electrode as the electrode. It is preferable to use at least one selected from the group consisting of an electrode in which Ti is coated with one or both of a platinum group metal and an oxide of a platinum group metal, and a graphite electrode as the insoluble electrode. More specifically, examples of the insoluble electrode include an electrode in which platinum, iridium oxide, or ruthenium oxide is coated on the surface of Ti as a substrate.
• the charge density of the anodic electrolysis treatment A1 is less than 0.50 C/ dm2 , the chromium-containing layer is not sufficiently dissolved, and therefore, no generation sites for granular chromium are formed. As a result, granular chromium is not sufficiently precipitated in the subsequent cathodic electrolysis treatment C2. As a result, the average area of the Voronoi polygons described above may be excessively large in the finally obtained surface-treated steel sheet. Therefore, the charge density is 0.50 C/dm2 or more , preferably 0.60 C/dm2 or more , and more preferably 0.70 C/dm2 or more .
• the charge density exceeds 20.00 C/ dm2 , the oxidation reaction of trivalent chromium may proceed locally on the steel sheet surface, increasing the hexavalent chromium concentration, destabilizing the electrolyte, and possibly resulting in excessive deposition of chromium oxide. Therefore, 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 pH of the electrolyte when performing anodic electrolysis A1 is not particularly limited, and the preferred embodiment is the same as that for cathodic electrolysis 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 pH of the electrolyte during anodic electrolysis A1 and maintain it within the above pH 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 1.0 C/ dm2 , granular chromium may not be sufficiently precipitated, resulting in an excessively large average area of the Voronoi polygons in the finally obtained surface-treated steel sheet. Therefore, the electricity density is 1.0 C/dm2 or more , preferably 3.0 C/dm2 or more , and more preferably 5.0 C/dm2 or more .
• the electricity density exceeds 150.0 C/ dm2 , excessive granular chromium may be precipitated, resulting in an excessively small average area of the Voronoi polygons in the finally obtained surface-treated steel sheet. Therefore, the electricity density is 150.0 C/dm2 or less, preferably 120.0 C/dm2 or less , and more preferably 100.0 C/dm2 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.
• cathodic electrolysis treatment C1 From the same perspective as in cathodic electrolysis treatment C1, it is preferable to monitor the concentrations of the components contained in the electrolyte in cathodic electrolysis treatment C2 and maintain them within the concentration ranges described above.
• 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.
• 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 methods described above. Furthermore, for each of the obtained surface-treated steel sheets, the average area of the Voronoi polygons 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
• 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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