Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/38—Chromatising
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials
  • C25D9/08—Electrolytic coating other than with metals with inorganic materials by cathodic processes
  • C25D9/10—Electrolytic coating other than with metals with inorganic materials by cathodic processes on iron or steel

Definitions

• the present invention relates to steel sheets for cans and a manufacturing method thereof.
• Patent Document 1 steel sheets for cans that have a "metallic chromium layer” and a “hydrated chromium oxide layer” on the surface of the steel sheet have been known.
• an object of the present invention is to provide a steel sheet for cans which is excellent in all of adhesion, weldability and corrosion resistance, and a method for manufacturing the same.
• a steel sheet for cans comprising a surface of a steel sheet having, in this order from the steel sheet side, a metallic chromium layer and a chromium hydrated oxide layer, the chromium hydrated oxide layer containing chromium element Cr and at least one element M selected from the group consisting of sodium, magnesium, potassium and calcium, a molar ratio M/Cr of the element M to the chromium element Cr being 0.010 or more and 0.100 or less, and the chromium hydrated oxide layer containing chromium hydroxide X, a molar ratio X/Cr of the chromium hydroxide X to the chromium element Cr being 0.400 or more and 0.800 or less.
• the present invention provides a steel sheet for cans that has excellent adhesion, weldability, and corrosion resistance, and a manufacturing method thereof.
• the steel sheet for cans of this embodiment has, on the surface of the steel sheet, a metallic chromium layer and a chromium hydrate oxide layer in this order from the steel sheet side.
• the chromium hydrated oxide layer contains chromium element Cr and at least one element M selected from the group consisting of sodium, magnesium, potassium and calcium, and the molar ratio of element M to chromium element Cr (M/Cr) is 0.010 or more and 0.100 or less.
• the chromium hydroxide layer contains chromium hydroxide X, and the molar ratio of chromium hydroxide X to elemental chromium Cr (X/Cr) is 0.400 or more and 0.800 or less.
• the steel sheet for cans of this embodiment is excellent in all of adhesion, weldability, and corrosion resistance.
• each configuration of the steel sheet for cans of the present embodiment will be described in more detail.
• the type of steel plate is not particularly limited. Steel plates that are usually used as container materials (e.g., low carbon steel plate, ultra-low carbon steel plate) can be used.
• the manufacturing method of the steel plate is also not particularly limited. The steel plate is manufactured through a normal steel billet manufacturing process, followed by hot rolling, pickling, cold rolling, annealing, temper rolling, and other processes.
• the composition of the steel plate includes, but is not limited to, compositions defined by the ASTM standard.
• the thickness of the steel plate is also not particularly limited, and is, for example, 0.10 mm or more and 0.60 mm or less.
• the term "steel plate” is a concept that includes "steel strip”.
• the metallic chromium layer reduces the surface exposure of the steel sheet and improves corrosion resistance.
• the coating weight of the metal chromium layer is preferably 50 mg/m 2 or more, more preferably 70 mg/m 2 or more, and even more preferably 80 mg/m 2 or more.
• the coating weight is the coating weight per one side of the steel sheet (hereinafter the same).
• the deposition amount of the metal chromium layer is preferably 200 mg/ m2 or less, more preferably 150 mg/ m2 or less, and even more preferably 130 mg/ m2 or less.
• the coating weight of the metallic chromium layer and the coating weight in terms of chromium of the hydrated chromium oxide layer described below are measured as follows. First, the chromium amount (total chromium amount) of a steel sheet for cans having a metallic chromium layer and a chromium hydrate oxide layer is measured using an X-ray fluorescence device.
• the steel sheet for cans is subjected to an alkali treatment by immersing it in a 6.5N aqueous sodium hydroxide solution (liquid temperature: 90°C) for 10 minutes, and then the chromium amount (amount of chromium after alkali treatment) is measured again using an X-ray fluorescence device.
• the amount of chromium after alkali treatment is defined as the amount of adhesion of the metallic chromium layer.
• (amount of alkali-soluble chromium) (total amount of chromium)-(amount of chromium after alkali treatment) is calculated, and the amount of alkali-soluble chromium is regarded as the amount of deposition of the chromium hydrous oxide layer in terms of chromium.
• Chromium hydrate oxides include oxygen-containing chromium compounds such as chromium oxide and chromium hydroxide.
• oxygen-containing chromium compounds such as chromium oxide and chromium hydroxide.
• the coating amount of the chromium hydrate oxide layer is preferably 3 mg/m 2 or more, and more preferably 4 mg/m 2 or more, in terms of chromium.
• the chromium-equivalent coating amount of the chromium hydrated oxide layer is preferably 20 mg/ m2 or less, more preferably 15 mg/ m2 or less, and even more preferably 10 mg/m2 or less .
• the chromium hydroxide in the chromium hydrated oxide layer may dehydrate due to heating during welding, increasing the contact resistance and decreasing the weldability. This is thought to be because chromium hydroxides having hydroxyl groups (-OH) undergo dehydration condensation with each other to form Cr-O-Cr bonds, which change into compounds with low electrical conductivity (such as chromium oxide).
• a trace amount of element M is contained in the chromium hydrate oxide layer.
• the chromium oxide layer contains, in addition to chromium element (Cr), at least one element M selected from the group consisting of sodium (Na), magnesium (Mg), potassium (K) and calcium (Ca). It is believed that this causes the hydrogen atom at the end of the hydroxy group to be replaced with a cation of element M, making dehydration less likely to proceed even when heated, etc.
• Cr chromium element
• element M selected from the group consisting of sodium (Na), magnesium (Mg), potassium (K) and calcium (Ca). It is believed that this causes the hydrogen atom at the end of the hydroxy group to be replaced with a cation of element M, making dehydration less likely to proceed even when heated, etc.
• the molar ratio (M/Cr) of the chromium hydrate oxide layer is 0.010 or more, preferably 0.015 or more, more preferably 0.020 or more, even more preferably 0.025 or more, and particularly preferably 0.030 or more.
• the molar ratio (M/Cr) of the chromium hydrous oxide layer may be greater than 0.050.
• the molar ratio (M/Cr) of the chromium hydrate oxide layer is preferably 0.055 or more, more preferably 0.060 or more, even more preferably 0.065 or more, particularly preferably 0.070 or more, and most preferably 0.075 or more.
• the chromium hydrated oxide layer ensures corrosion resistance by covering pinholes (parts where the steel sheet is not completely covered) in the metal chromium layer, but if the continuity of the chromium hydrated oxide layer is impaired, this coverage may become insufficient, resulting in reduced corrosion resistance.
• the molar ratio (M/Cr) of the chromium hydrate oxide layer is 0.100 or less, preferably 0.090 or less, more preferably 0.080 or less, even more preferably 0.070 or less, and particularly preferably 0.065 or less.
• the chromium hydroxide in the chromium hydrous oxide layer contributes to adhesion.
• the molar ratio (X/Cr) of chromium hydroxide X to elemental chromium Cr in the chromium hydrate oxide layer is too low, the amount of chromium hydroxide X is too small, and sufficient adhesion may not be obtained. Therefore, from the viewpoint of obtaining good adhesion, the molar ratio (X/Cr) of the chromium hydrate oxide layer is 0.400 or more, preferably 0.410 or more, more preferably 0.415 or more, even more preferably 0.420 or more, and particularly preferably 0.425 or more.
• the molar ratio (X/Cr) of the chromium hydrate oxide layer is 0.800 or less, more preferably 0.750 or less, further preferably 0.650 or less, and particularly preferably 0.550 or less.
• molar ratio (M/Cr) and molar ratio (X/Cr) are determined as follows. First, a steel sheet for cans having a metallic chromium layer and a chromium hydrate oxide layer (a so-called as-manufactured steel sheet for cans which has not been subjected to heating or the like except for drying by indoor storage) is placed in an ultra-high vacuum, and a Cr2p spectrum of the outermost surface of the chromium hydrate oxide layer is obtained by X-ray photoelectron spectroscopy under the following conditions.
• the obtained Cr2p spectrum is background-corrected and separated into a metallic chromium peak appearing at 574.4 ⁇ 0.1 eV, a chromium hydroxide peak appearing at 577.4 ⁇ 0.4 eV, and a chromium oxide peak appearing at 578.1 ⁇ 1.7 eV.
• the peaks are separated by a curve fitting method using a nonlinear least-squares method with a Gauss-Lorentz composite function. The area of each separated peak is then calculated.
• the ratio (X/Cr) of the chromium hydroxide peak area X to the total peak area Cr of all chromium peaks (peaks of metallic chromium, chromium hydroxide, and chromium oxide) is calculated, and this is determined as the molar ratio (X/Cr) described above.
• the spectra (narrow spectra) of Na1s, Mg1s, K2p and Ca2p are obtained for the outermost surface of the chromium hydrate oxide layer. From the integrated intensities of the obtained spectra, the elements Na, Mg, K and Ca are quantified by the relative sensitivity coefficient method to obtain the total molar amount M. Similarly, the molar amount of chromium element, Cr, is determined from the integrated intensity of the Cr2p spectrum. The above-mentioned molar ratio (M/Cr) is calculated from the molar amount M and the molar amount Cr.
• cathodic electrolysis C1 is performed on a steel sheet in an aqueous solution 1 containing a hexavalent chromium compound.
• a reduction reaction occurs on the surface of the steel sheet, causing metallic chromium to precipitate, and further, chromium hydrate oxide, which is an intermediate product of the metallic chromium, is precipitated on the surface of the metallic chromium.
• the amount of precipitation can be arbitrarily controlled, for example, by the conditions of the cathodic electrolysis C1.
• a metallic chromium layer and a chromium hydrate oxide layer are formed on the surface of the steel sheet. Furthermore, in this embodiment, after the cathodic electrolytic treatment C1, a cathodic electrolytic treatment C2 is carried out using an aqueous solution 2 containing the element M. As a result, the element M is introduced into the formed chromium hydrous oxide layer.
• the aqueous solutions 1 and 2 and the cathodic electrolytic treatments C1 and C2 will be described in detail below.
• the aqueous solution 1 used in the cathodic electrolytic treatment C1 contains at least a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid.
• hexavalent chromium compounds include chromium trioxide (CrO 3 ); dichromates such as potassium dichromate (K 2 Cr 2 O 7 ); and chromates such as potassium chromate (K 2 CrO 4 ).
• the content of the hexavalent chromium compound in the aqueous solution 1 is preferably 0.50 mol/L or more, and more preferably 0.80 mol/L or more, in terms of the amount of Cr, because this allows metallic chromium to be precipitated stably for a long period of time with high efficiency.
• the amount of Cr in the aqueous solution 1 is preferably 5.00 mol/L or less, and more preferably 3.00 mol/L or less.
• fluorine-containing compounds examples include hydrofluoric acid (HF), potassium fluoride (KF), sodium fluoride (NaF), hydrosilicic acid (H 2 SiF 6 ), and salts of hydrosilicic acid.
• salts of hydrosilicic acid include sodium silicic acid (Na 2 SiF 6 ), potassium silicic acid (K 2 SiF 6 ), and ammonium silicic acid ((NH 4 ) 2 SiF 6 ).
• the content of the fluorine-containing compound in the aqueous solution 1, in terms of the F amount, is preferably more than 0.100 mol/L, more preferably 0.110 mol/L or more, even more preferably 0.150 mol/L or more, and particularly preferably 0.200 mol/L or more.
• the amount of F in the aqueous solution 1 is preferably 4.000 mol/L or less, more preferably 3.000 mol/L or less, even more preferably 2.000 mol/L or less, and particularly preferably 1.000 mol/L or less.
• the use of sulfuric acid in combination with a fluorine-containing compound improves the deposition efficiency of metallic chromium.
• the sulfuric acid may be partially or entirely in the form of a sulfate salt such as sodium sulfate, calcium sulfate, or ammonium sulfate.
• the content of sulfuric acid in the aqueous solution 1, in terms of the amount of SO 4 2- is preferably 0.0001 mol/L or more, more preferably 0.0003 mol/L or more, and even more preferably 0.0010 mol/L or more.
• the amount of SO 4 2- in the aqueous solution 1 is preferably 0.1000 mol/L or less, and more preferably 0.0500 mol/L or less.
• the liquid temperature of the aqueous solution 1 is preferably 20° C. or higher, more preferably 30° C. or higher, and even more preferably 40° C. or higher. On the other hand, the liquid temperature of the aqueous solution 1 is preferably 80° C. or lower, and more preferably 60° C. or lower. In the cathodic electrolysis C1, it is preferable to use only one type of aqueous solution 1.
• the current density of the cathodic electrolysis C1 is preferably 5 A/dm2 or more , more preferably 10 A/dm2 or more , and further preferably 20 A/dm2 or more .
• the current density of the cathodic electrolysis treatment C1 is preferably 60 A/dm2 or less, more preferably 50 A/dm2 or less , and further preferably 40 A/dm2 or less .
• the electrical charge density of the cathodic electrolysis treatment C1 is preferably 70.0 C/dm2 or less, more preferably 60.0 C/dm2 or less , and even more preferably 50.0 C/dm2 or less .
• the electrical charge density of the cathodic electrolysis C1 is preferably 10.0 C/dm 2 or more, more preferably 20.0 C/dm 2 or more, and even more preferably 30.0 C/dm 2 or more.
• the energization time (unit: s) of the cathodic electrolysis treatment C1 is appropriately set based on the current density and the charge density.
• the cathodic electrolysis C1 does not have to be a continuous electrolysis. That is, the cathodic electrolysis C1 may be an intermittent electrolysis in which electrolysis is performed separately for a plurality of electrodes in industrial production, and therefore a non-current immersion period is unavoidably present. In the case of the intermittent electrolysis, it is preferable that the total electric charge density is within a suitable range. The same applies to the cathodic electrolysis C2 described below.
• the steel sheet may be electrolessly immersed in an aqueous solution containing a hexavalent chromium compound for the purposes of controlling the amount of adhesion of the chromium hydrate oxide layer and modifying it.
• the aqueous solution 2 contains an element M (at least one element selected from the group consisting of Na, Mg, K, and Ca).
• the aqueous solution 2 is prepared, for example, by adding a compound containing element M (also referred to as "M compound") to water, which is a solvent.
• M compounds sulfates, nitrates, chlorides and fluorides are avoided, and hydroxides and carbonates are preferred.
• hydroxides of element M include sodium hydroxide (NaOH), magnesium hydroxide (Mg(OH) 2 ), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH) 2 ).
• Examples of carbonates of element M include sodium carbonate (Na 2 CO 3 ), magnesium carbonate (MgCO 3 ), potassium carbonate (K 2 CO 3 ), and calcium carbonate (CaCO 3 ).
• the total content of element M in aqueous solution 2 is 0.10 mmol/L or more, preferably 0.20 mmol/L or more, more preferably 0.30 mmol/L or more, and even more preferably 0.40 mmol/L or more, from the viewpoint of introducing a sufficient amount of element M into the chromium hydrous oxide layer.
• the total content of the element M in the aqueous solution 2 is 1.00 mmol/L or less, preferably 0.90 mmol/L or less, more preferably 0.80 mmol/L or less, even more preferably 0.70 mmol/L or less, and particularly preferably 0.60 mmol/L or less.
• the total content of element M in aqueous solution 2 is measured by ICP (Inductively Coupled Plasma) emission spectrometry or ICP mass spectrometry.
• ICP Inductively Coupled Plasma
• the aqueous solution 2 is preferably weakly alkaline for reasons described below.
• the pH of the aqueous solution 2 is preferably 9.0 or higher, more preferably 9.5 or higher, and even more preferably 10.0 or higher.
• the pH of the aqueous solution 2 is preferably 11.5 or less, more preferably 11.0 or less, and even more preferably 10.5 or less.
• the liquid temperature of the aqueous solution 2 is, for example, 40° C. or higher. However, from the viewpoint of preventing excessive incorporation of element M into the chromium hydrous oxide layer, the temperature is preferably 75° C. or less, more preferably 65° C. or less, and even more preferably 55° C. or less.
• hydroxyl groups are also introduced into the chromium hydrous oxide layer, i.e., the amount of chromium hydroxide increases.
• Sulfate radicals (SO 4 2- ), fluorine-containing compounds, etc. are adsorbed or incorporated on the surface of the chromium hydrous oxide layer after the cathodic electrolysis treatment C1.
• the cathodic electrolysis treatment C2 using a weakly alkaline aqueous solution 2
• the sulfate radicals, etc. are removed and instead, hydroxyl groups are introduced into the chromium hydrous oxide layer.
• the electrical charge density of the cathodic electrolysis C2 is 0.30 C/dm2 or more , preferably 0.35 C/dm2 or more , more preferably 0.40 C/dm2 or more , even more preferably 0.45 C/dm2 or more , and particularly preferably 0.50 C/dm2 or more .
• the electrical charge density of the cathodic electrolysis treatment C2 is 3.00 C/dm 2 or less, preferably 2.50 C/dm 2 or less, more preferably 2.00 C/dm 2 or less, and even more preferably 1.50 C/dm 2 or less.
• the current density of the cathodic electrolysis treatment C2 is not particularly limited as long as the electric charge density is within the above range, but is preferably 0.5 A/dm2 or more , more preferably 1.0 A/dm2 or more , and even more preferably more than 1.5 A/dm2.
• the current density of the cathodic electrolysis treatment C2 is preferably less than 10.0 A/ dm2 , more preferably 9.0 A/ dm2 or less, and even more preferably 8.0 A/dm2 or less .
• the energization time (unit: s) of the cathodic electrolysis treatment C2 is appropriately set based on the current density and the charge density, but is preferably 0.10 s or more, and more preferably 0.20 s or more. Although there is no particular upper limit, from the viewpoint of efficiency in continuous production, the current application time of the cathodic electrolytic treatment C2 is preferably 5.00 s or less, more preferably 3.50 s or less, and even more preferably 2.00 s or less.
• the obtained steel sheet for cans may be further washed with water using general industrial water as washing water.
• the washing time is not particularly limited.
• the liquid temperature of the cleaning water is preferably 40° C. or lower, and more preferably 30° C. or lower.
• the method of washing with water is not particularly limited, and a conventionally known method can be adopted.
• a washing tank may be provided downstream of the electrolytic cells used in the cathodic electrolysis treatments C1 to C2, and the steel sheet for cans after the cathodic electrolysis treatment C2 may be continuously immersed in the washing water.
• the washing water may be sprayed onto the steel sheet for cans after the cathodic electrolysis treatment C2 using a spray.
• the number of water washes is not particularly limited. Each water wash may be performed by the same method or by different methods.
• cathodic electrolysis treatment C2 When cathodic electrolysis treatment C2 was not performed, "-" is entered in the corresponding column in Table 3 below.
• the aqueous solutions 1-2 were circulated in a flow cell by a pump at the equivalent of 100 mpm, and lead electrodes were used.
• the steel sheet for cans after preparation was washed by immersing it in industrial water (liquid temperature: 25° C.) and dried at room temperature using a blower.
• Adhesion Two test pieces were cut out from the prepared steel plate for cans, and the surfaces of the test pieces were painted. Specifically, an epoxy phenol-based paint was applied to the surface of the test piece in an amount of 50 mg/ dm2 , and the test piece was baked at 210°C for 10 minutes. A nylon film was placed between the coated surfaces of two test pieces, preheated at 190° C. for 1 minute, and then pressed together at a pressure of 3 kgf/cm 2 for 30 seconds to prepare a test specimen. Thereafter, the test specimen was sheared to a width of 5 mm, and then immersed in a test liquid (a mixed aqueous solution of 1.5 mass % citric acid and 1.5 mass % sodium chloride) at 55° C. for 336 hours.
• a test liquid a mixed aqueous solution of 1.5 mass % citric acid and 1.5 mass % sodium chloride
• the test specimen was removed from the test solution, washed with water, and then dried. The two test pieces were then pulled together at a tensile speed of 3.33 mm/s using a tensile tester, and the tensile strength at the time of peeling was determined as the peel strength (unit: kgf/cm 2 ).
• the peel strength is "A" or "B” below, it can be evaluated as having excellent adhesion.
• B 2.0 kgf/ cm2 or more, less than 2.5 kgf/ cm2 C: 1.5 kgf/ cm2 or more, less than 2.0 kgf/ cm2 D: less than 1.5 kgf/ cm2
• the current range is "A" or "B” below, it can be evaluated as having excellent weldability.
• C 0.2 kA or more but less than 0.6 kA
• D Less than 0.2 kA
• Corrosion resistance Two test pieces were cut out from the prepared steel sheet for cans. The surfaces on which the coating layer and the chromium-containing layer were formed were used as evaluation surfaces. The two test pieces were superimposed with the evaluation surfaces (the surfaces on which the metallic chromium layer and the chromium hydrated oxide layer were formed) facing each other, and passed between metal rolls to apply a surface pressure of 40 MPa. Thereafter, an epoxy phenol resin was applied to the evaluation surface of one of the two test pieces, and a treatment of heating at 210° C. for 10 minutes was carried out twice to form a coating film.
• a cross cut was made in the coating film to a depth reaching the steel plate, and the test piece was then immersed in a test liquid (a mixed aqueous solution of 1.5 mass% citric acid and 1.5 mass% sodium chloride) at 45°C for 168 hours. After immersion, the test piece was removed from the test liquid, washed with water, and dried. Then, a test was performed to peel off the coating film using tape. The peel width (the total width extending from the intersection to the left and right) at four points within 10 mm from the intersection of the crosscut was measured, and the average of the peel widths at the four points was calculated as the corrosion width. In practice, if the corrosion width is "A" or "B", it can be evaluated as excellent corrosion resistance.

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