Steel sheet for cans and method for producing same

Abstract

The present invention provides a steel sheet for cans, which excels in all of adhesion, weldability and corrosion resistance. The steel sheet for cans sequentially has, on the surface of a steel sheet, a chromium metal layer and a hydrated chromium oxide layer in this order from the steel sheet side. The hydrated chromium oxide layer contains an elemental chromium Cr and at least one element M that is selected from the group consisting of sodium, magnesium, potassium and calcium. The molar ratio M / Cr of the element M to the elemental chromium Cr is 0.010 to 0.100 inclusive. The hydrated chromium oxide layer contains a chromium hydroxide X. The molar ratio X / Cr of the chromium hydroxide X to the elemental chromium Cr is 0.400 to 0.800 inclusive.

Description

Steel sheet for cans and its manufacturing method

[0001] The present invention relates to a steel sheet for cans and a method for producing the same.

[0002] Conventionally, a steel sheet for cans having a "metallic chromium layer" and a "hydrated chromium oxide layer" on the surface of the steel sheet has been known (Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2005-298864

[0004] Steel sheets for cans are required to have not only good corrosion resistance but also good weldability. Furthermore, steel sheets for cans may also be required to have excellent adhesion to paints and films (hereinafter simply referred to as "adhesion"). Therefore, an object of the present invention is to provide a steel sheet for cans that is excellent in all of adhesion, weldability, and corrosion resistance, and a method for manufacturing the same.

[0005] As a result of extensive investigations, the present inventors have found that the above object can be achieved by employing the following configuration, and have thus completed the present invention. That is, the present invention provides the following [1] to [5]. [1] A steel sheet for cans, which has, on the surface of the steel sheet, a metallic chromium layer and a chromium hydrated oxide layer, in this order from the steel sheet side, the chromium hydrated oxide layer contains elemental chromium (Cr) and at least one element M selected from the group consisting of sodium, magnesium, potassium, and calcium, wherein the molar ratio M / Cr of the element M to the elemental chromium (Cr) is 0.010 or more and 0.100 or less, and the chromium hydrated oxide layer contains chromium hydroxide (X), wherein the molar ratio X / Cr of the chromium hydroxide (X) to the elemental chromium (Cr) is 0.400 or more and 0.800 or less. [2] The coating weight of the metallic chromium layer is 50 mg / m ２ 200mg / m or more ２ [3] The steel sheet for cans according to the above [1], wherein the coating amount of the chromium hydrated oxide layer is 3 mg / m or less. ２ 20mg / m or more ２[4] A method for producing a steel sheet for cans according to any one of the above [1] to [3], comprising the steps of: subjecting the steel sheet to cathodic electrolysis C1 using an aqueous solution 1 containing a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid; ２ [5] The method for producing a steel sheet for cans according to the above [4], wherein the aqueous solution 2 has a pH of 9.0 or more and 11.5 or less.

[0006] According to the present invention, it is possible to provide a steel sheet for cans which is excellent in all of adhesion, weldability and corrosion resistance, and a method for producing the same.

[0007] [Steel Sheet for Cans] The steel sheet for cans of this embodiment has, on the surface of the steel sheet, a metallic chromium layer and a chromium hydrated oxide layer, in this order from the steel sheet side. In this embodiment, 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. In this embodiment, the chromium hydrated oxide layer also contains chromium hydroxide X, and the molar ratio of chromium hydroxide X to chromium element Cr (X / Cr) is 0.400 or more and 0.800 or less. The steel sheet for cans of this embodiment has excellent adhesion, weldability, and corrosion resistance. Hereinafter, each component of the steel sheet for cans of this embodiment will be described in more detail.

[0008] <Steel Plate> The type of steel plate is not particularly limited. Steel plates that are normally used as container materials (e.g., low-carbon steel plates, ultra-low-carbon steel plates) 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 ASTM standards. 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. Note that the term "steel plate" is a concept that encompasses "steel strip."

[0009] <Metallic Chromium Layer> The metallic chromium layer reduces the surface exposure of the steel sheet, thereby improving corrosion resistance.

[0010] <<Coating Mass>> In order to obtain good corrosion resistance, the coating mass of the metal chromium layer is 50 mg / m ２ More than 70 mg / m ２ More preferably, 80 mg / m or more ２ The above is more preferable. The coating weight is the coating weight per one side of the steel sheet (the same applies hereinafter). On the other hand, if the coating weight of the metallic chromium layer is excessively high, the welding strength may decrease, and a large amount of dust may be generated during welding, which may damage the metallic chromium layer and impair corrosion resistance. For this reason, the coating weight of the metallic chromium layer is set to 200 mg / m ２ Preferably, 150 mg / m or less ２ More preferably, 130 mg / m or less ２ The following is even more preferred:

[0011] <<Method for Measuring Coating Weight>> The coating weight of the metallic chromium layer and the coating weight of the chromium hydrated oxide layer (described later) in chromium equivalent 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 hydrated oxide layer is measured using an X-ray fluorescence analyzer. Next, the steel sheet for cans is subjected to an alkali treatment by immersing it in a 6.5 N 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 analyzer. The amount of chromium after alkali treatment is taken as the coating weight of the metallic chromium layer. Next, (amount of alkali-soluble chromium) = (total chromium amount) - (amount of chromium after alkali treatment) is calculated, and the amount of alkali-soluble chromium is taken as the coating weight of the chromium hydrated oxide layer in chromium equivalent.

[0012] <Chromium hydrated oxide layer> The chromium hydrated oxide layer includes oxygen-containing chromium compounds such as chromium oxide and chromium hydroxide. By providing a chromium hydrated oxide layer on the metal chromium layer, adhesion to paints and the like is improved. The chromium hydroxide in the chromium hydrated oxide layer has a hydroxyl group and is polar. The chromium hydroxide exerts adhesion by hydrogen bonding with paints and the like.

[0013] <<Deposition Mass>> In order to obtain good adhesion, the deposition mass of the chromium hydrate oxide layer is 3 mg / m2 in terms of chromium. ２ More than 4 mg / m ２ On the other hand, if the amount of the hydrated chromium oxide layer is too large, cohesive failure may occur at the adhesion interface with the paint, etc., and the adhesion may decrease. Therefore, the amount of the hydrated chromium oxide layer in terms of chromium should be 20 mg / m ２ Preferably, 15 mg / m or less ２ More preferably, 10 mg / m or less ２ The following is even more preferred:

[0014] Molar Ratio (M / Cr) The chromium hydroxide in the chromium hydrated oxide layer may undergo dehydration due to heating during welding, increasing contact resistance and reducing weldability. This is thought to be because chromium hydroxides having hydroxyl groups (—OH) undergo dehydration condensation to form Cr—O—Cr bonds, converting them into compounds with low electrical conductivity (such as chromium oxide). In this embodiment, to suppress the progression of such dehydration, a trace amount of element M is contained in the chromium hydrated oxide layer. That is, the chromium hydrated 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). This is thought to replace the hydrogen atoms at the terminals of the hydroxyl groups with cations of element M, making dehydration less likely to proceed even when heated, etc.

[0015] In this case, if the molar ratio of element M to element chromium Cr (M / Cr) in the chromium hydrate oxide layer is too low, this effect is difficult to achieve due to the small amount of element M. Therefore, from the viewpoint of obtaining good weldability, 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.

[0016] The molar ratio (M / Cr) of the chromium hydrate oxide layer may be greater than 0.050. In this case, the molar ratio (M / Cr) of the chromium hydrate oxide layer is preferably 0.055 or greater, more preferably 0.060 or greater, even more preferably 0.065 or greater, particularly preferably 0.070 or greater, and most preferably 0.075 or greater.

[0017] On the other hand, if the element M is contained in an excessive amount in the chromium hydrate oxide layer, the continuity of the chromium hydrate oxide layer may be impaired, weakening the bond between the paint or the like and the hydroxyl group, which may result in reduced adhesion. Furthermore, the chromium hydrate oxide layer ensures corrosion resistance by covering pinholes (portions of the steel sheet that are not completely covered) in the metal chromium layer. However, if the continuity of the chromium hydrate oxide layer is impaired, this coverage may become insufficient, resulting in reduced corrosion resistance. Therefore, from the viewpoint of obtaining good adhesion and 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.

[0018] <<Molar Ratio (X / Cr)>> As described above, chromium hydroxide in the chromium hydrate oxide layer contributes to adhesion. In this case, if the molar ratio (X / Cr) of chromium hydroxide X to chromium element Cr in the chromium hydrate oxide layer is too low, the amount of chromium hydroxide X will be 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.

[0019] On the other hand, since chromium hydroxide has lower strength than chromium oxide, if the amount of chromium hydroxide is too much, corrosion resistance may be insufficient. Therefore, from the viewpoint of obtaining good corrosion resistance, the molar ratio (X / Cr) of the chromium hydrate oxide layer is 0.800 or less, more preferably 0.750 or less, even more preferably 0.650 or less, and particularly preferably 0.550 or less.

[0020] <<Method for Measuring Molar Ratios>> The molar ratios (M / Cr) and (X / Cr) are determined as follows. First, a steel sheet for cans having a metallic chromium layer and a chromium hydrate oxide layer (i.e., an as-manufactured steel sheet for cans that 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 then 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 curve fitting using a nonlinear least-squares method using a Gauss-Lorentz composite function. Next, the area of ​​each separated peak is determined.

[0021] In this manner, for the outermost surface of the chromium hydrous oxide layer, the ratio (X / Cr) of the peak area X of chromium hydroxide 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.

[0022] Furthermore, spectra (narrow spectra) of Na1s, Mg1s, K2p, and Ca2p are obtained for the outermost surface of the chromium hydrate oxide layer. The elements Na, Mg, K, and Ca are quantified from the integrated intensities of the obtained spectra using the relative sensitivity coefficient method to determine the total molar amount M. Similarly, the molar amount Cr of chromium element is determined from the integrated intensity of the Cr2p spectrum. The molar amount M and the molar amount Cr are used to determine the molar ratio (M / Cr) described above.

[0023] (Conditions for X-ray photoelectron spectroscopy) Measurement device: X-tool manufactured by Ulvac-PHI Excitation source: monoAl Kα 25W×15 kV Analysis size: 100 μmφ Take-off angle: 45° Pass Energy Survey scan: 280.0 eV Narrow scan: 112.0 eV

[0024] [Manufacturing Method of Steel Sheet for Cans] Next, a method for manufacturing a steel sheet for cans according to this embodiment will be described. In this embodiment, roughly speaking, first, cathodic electrolysis C1 is performed on a steel sheet in an aqueous solution 1 containing a hexavalent chromium compound. As a result, a reduction reaction occurs on the surface of the steel sheet, causing metallic chromium to precipitate. Furthermore, chromium hydrate oxide, an intermediate product of the metallic chromium, precipitates on the surface of the metallic chromium. The amount of precipitation can be freely controlled, for example, by the conditions of cathodic electrolysis C1. In this way, a metallic chromium layer and a chromium hydrate oxide layer are formed on the surface of the steel sheet. Furthermore, in this embodiment, after cathodic electrolysis C1, cathodic electrolysis C2 is performed using an aqueous solution 2 containing element M. As a result, element M is introduced into the formed chromium hydrate oxide layer. Below, aqueous solutions 1 and 2 and cathodic electrolysis C1 and C2 will be described in detail.

[0025] <Aqueous Solution 1> 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.

[0026] Examples of hexavalent chromium compounds include chromium trioxide (CrO ３ ); potassium dichromate (K ２ Cr ２ O ７ ) and other dichromates; 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, more preferably 0.80 mol / L or more, in terms of the Cr amount, because this allows metallic chromium to be precipitated stably for a long period of time with high efficiency. On the other hand, the Cr amount in the aqueous solution 1 is preferably 5.00 mol / L or less, more preferably 3.00 mol / L or less.

[0027] Examples of fluorine-containing compounds include hydrofluoric acid (HF), potassium fluoride (KF), sodium fluoride (NaF), and hydrosilicofluoric acid (H ２ SiF ６ ), salts of hydrosilicofluoric acid, etc. Examples of the salts of hydrosilicofluoric acid include sodium silicofluoride (Na ２ SiF６ ), potassium silicofluoride (K ２ SiF ６ ), ammonium silicofluoride ((NH ４ ) ２ SiF ６ The content of the fluorine-containing compound in the aqueous solution 1 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, in terms of the F amount. On the other hand, the F amount 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.

[0028] 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 a sulfate such as sodium sulfate, calcium sulfate, or ammonium sulfate. The content of sulfuric acid in aqueous solution 1 is 0.05% by weight. ４ ２－ The amount 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. ４ ２－ The amount is preferably 0.1000 mol / L or less, more preferably 0.0500 mol / L or less.

[0029] The 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 temperature of the aqueous solution 1 is preferably 80° C. or lower, and more preferably 60° C. or lower. In the cathodic electrolysis treatment C1, it is preferable to use only one type of aqueous solution 1.

[0030] <Cathodic Electrolysis Treatment C1> As described above, in cathodic electrolysis treatment C2, metallic chromium and chromium hydrate oxide are precipitated, thereby forming a metallic chromium layer and a chromium hydrate oxide layer on the surface of the steel sheet.

[0031] The current density of the cathodic electrolysis treatment C1 is 5 A / dm ２ More than 10 A / dm ２More preferably, 20 A / dm ２ On the other hand, the current density of the cathodic electrolysis treatment C1 is 60 A / dm ２ Preferably, 50 A / dm or less ２ More preferably, 40 A / dm ２ The following is even more preferred:

[0032] The electrical charge density of the cathodic electrolysis treatment C1 is 70.0 C / dm ２ Preferably, 60.0 C / dm or less ２ More preferably, 50.0 C / dm or less ２ On the other hand, the electricity density of the cathodic electrolysis treatment C1 is 10.0 C / dm ２ More than 20.0 C / dm ２ More preferably, 30.0 C / dm or more ２ The energization time (unit: s) of the cathodic electrolysis treatment C1 is appropriately set based on the current density and the electrical charge density.

[0033] The cathodic electrolysis treatment C1 does not have to be a continuous electrolysis treatment. That is, the cathodic electrolysis treatment C1 may be an intermittent electrolysis treatment in which electrolysis is performed using multiple electrodes in industrial production, and therefore, a non-energized immersion period is unavoidably present. In the case of an intermittent electrolysis treatment, it is preferable that the total electrical charge density is within a suitable range. This also applies to the cathodic electrolysis treatment C2 described below.

[0034] After the cathodic electrolysis C1, the steel sheet may be electrolessly immersed in an aqueous solution containing a hexavalent chromium compound for the purposes of controlling the deposition amount of the chromium hydrous oxide layer and modifying the layer.

[0035] <Aqueous Solution 2> Aqueous solution 2 contains element M (at least one element selected from the group consisting of Na, Mg, K, and Ca). 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. As the M compound, sulfates, nitrates, chlorides, and fluorides are avoided, and hydroxides and carbonates are preferred. Examples of hydroxides of element M include sodium hydroxide (NaOH), magnesium hydroxide (Mg(OH) ２), potassium hydroxide (KOH), calcium hydroxide (Ca(OH) ２ ) As the carbonate of element M, for example, sodium carbonate (Na ２ CO ３ ), magnesium carbonate (MgCO ３ ), potassium carbonate (K ２ CO ３ ), calcium carbonate (CaCO ３ ) are listed.

[0036] From the viewpoint of introducing a sufficient amount of element M into the chromium hydrated oxide layer, 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. On the other hand, in order to prevent excessive introduction of element M into the chromium hydrated oxide layer, the total content of element M in 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.

[0037] The total content of element M in the aqueous solution 2 is measured by ICP (Inductively Coupled Plasma) emission spectrometry or ICP mass spectrometry.

[0038] For reasons described below, aqueous solution 2 is preferably weakly alkaline. Specifically, the pH of aqueous solution 2 is preferably 9.0 or higher, more preferably 9.5 or higher, and even more preferably 10.0 or higher. On the other hand, the pH of aqueous solution 2 is preferably 11.5 or lower, more preferably 11.0 or lower, and even more preferably 10.5 or lower.

[0039] The 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 hydrate oxide layer, the temperature is preferably 75° C. or lower, more preferably 65° C. or lower, and even more preferably 55° C. or lower.

[0040] <Cathode Electrolysis Treatment C2> After the cathodic electrolysis treatment C1, the steel sheet on which the metallic chromium layer and the chromium hydrous oxide layer have been formed is appropriately rinsed with water and then subjected to the cathodic electrolysis treatment C2 using an aqueous solution 2 containing element M. As a result, as described above, element M is introduced into the chromium hydrous oxide layer formed by the cathodic electrolysis treatment C1. As a result, it is believed that the hydrogen atoms at the terminals of the hydroxy groups of the chromium hydroxides in the chromium hydrous oxide layer are substituted with cations of element M.

[0041] Furthermore, by carrying out the cathodic electrolysis C2, hydroxyl groups are also introduced into the chromium hydrous oxide layer. That is, the amount of chromium hydroxide increases. After the cathodic electrolysis C1, sulfate radicals (SO ４ ２－ In this state, cathodic electrolysis C2 is carried out using a weakly alkaline aqueous solution 2, whereby sulfate radicals and the like are removed and instead hydroxyl groups are introduced into the chromium hydrous oxide layer.

[0042] If the electrical charge density of the cathodic electrolysis treatment C2 is too low, it is difficult to introduce hydroxyl groups into the chromium hydrous oxide layer. From the viewpoint of promoting the introduction of hydroxyl groups and increasing the amount of chromium hydroxide, the electrical charge density of the cathodic electrolysis treatment C2 is set to 0.30 C / dm ２ or more, and 0.35 C / dm ２ More than 0.40 C / dm ２ More preferably, 0.45 C / dm or more ２ More preferably, 0.50 C / dm or more ２ The above is particularly preferred.

[0043] On the other hand, in order to prevent excessive increase of chromium hydroxide, the electrical charge density of the cathodic electrolysis treatment C2 is 3.00 C / dm ２ or less, and ２ Preferably, 2.00 C / dm or less ２ More preferably, 1.50 C / dm or less ２ The following is even more preferred:

[0044] The current density of the cathodic electrolysis treatment C2 is not particularly limited as long as the charge density is within the above range, but is preferably 0.5 A / dm ２ More than 1.0 A / dm is preferable. ２ More preferably, 1.5 A / dm or more ２ On the other hand, the current density of the cathodic electrolysis treatment C2 is more preferably 10.0 A / dm ２ Preferably less than 9.0 A / dm ２ More preferably, 8.0 A / dm or less ２ The following is even more preferred:

[0045] The energization time (unit: s) of the cathodic electrolysis treatment C2 is appropriately set based on the current density and the electricity density, but is preferably 0.10 s or more, more preferably 0.20 s or more. Although there is no particular upper limit, from the viewpoint of efficiency in continuous production, the energization time of the cathodic electrolysis 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.

[0046] <Water Washing> After the cathodic electrolysis treatment C2, the obtained steel sheet for cans may be further washed with general industrial water as washing water. By washing with water, the electrolytic solution remaining on the surface of the steel sheet for cans can be removed. The washing time is not particularly limited. However, in order to prevent the hydroxyl groups in the chromium hydrate oxide layer, whose terminal hydrogen atoms have been substituted with element M, from returning to their original hydroxyl groups, the temperature of the washing water is preferably 40°C or less, more preferably 30°C or less. The method of washing with water is not particularly limited, and a conventionally known method can be used. For example, 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. Alternatively, the steel sheet for cans after the cathodic electrolysis treatment C2 may be sprayed with washing water using a spray. The number of times of washing with water is not particularly limited. Each washing may be performed by the same method or by different methods.

[0047] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0048] <Preparation of Steel Sheet for Can> A steel sheet (temper: T4CA) manufactured with a thickness of 0.22 mm was subjected to conventional degreasing and pickling. This steel sheet was subjected to cathodic electrolysis C1 using Aqueous Solution 1, and then to cathodic electrolysis C2 using Aqueous Solution 2. The composition of Aqueous Solution 1 is shown in Table 1 below, the composition of Aqueous Solution 2 is shown in Table 2 below, and the conditions for Cathodic Electrolysis C1 and Cathodic Electrolysis C2 are shown in Table 3 below. The pH of the Aqueous Solution 2 used was in the range of 9.0 to 11.5. When Cathodic Electrolysis C2 was not performed, a "-" is entered in the corresponding column in Table 3 below. In the cathodic electrolysis treatments C1 to C2, Aqueous Solutions 1 and 2 were circulated in a flow cell at a speed equivalent to 100 mpm using a pump, and a lead electrode was used. In this manner, steel sheets for cans were prepared. The produced steel sheet for cans was washed by immersing it in industrial water (liquid temperature: 25° C.) and then dried at room temperature using a blower.

[0049] <Adhesion Amount, etc.> For the prepared steel sheets for cans, the adhesion amount of metallic chromium and the adhesion amount of the chromium hydrate oxide layer in terms of chromium (simply referred to as "adhesion amount" in Table 3 below) were measured. Furthermore, for the prepared steel sheets for cans, the molar ratio (M / Cr) and molar ratio (X / Cr) of the chromium hydrate oxide layer were measured. The measurement methods were as described above. The results are shown in Table 3 below.

[0050] <Evaluation> The produced steel sheets for cans were subjected to the following tests to evaluate adhesion, corrosion resistance, and weldability. The results are shown in Table 3 below.

[0051] Two test pieces were cut out from the prepared steel sheet for cans, and the surfaces of the test pieces were painted. Specifically, an epoxy phenolic paint was applied to the surface of the test piece at a concentration of 50 mg / dm ２ A nylon film was placed between the coated surfaces of two test pieces, preheated at 190°C for 1 minute, and then baked at 3 kgf / cm for 30 seconds. ２The specimens were then sheared to a width of 5 mm and immersed in a test solution (a mixed aqueous solution of 1.5% by mass of citric acid and 1.5% by mass of sodium chloride) at 55°C for 336 hours. After immersion, the specimens were removed from the test solution, washed with water, and then dried. Then, the two specimens of the specimen were pulled together at a pulling rate of 3.33 mm / s using a tensile tester, and the tensile strength at the time of peeling was measured as the peel strength (unit: kgf / cm ２ In practice, if the peel strength is "A" or "B" below, it can be evaluated as having excellent adhesion. A: 2.5 kgf / cm ２ or more B:2.0kgf / cm ２ Above, 2.5kgf / cm ２ Less than C: 1.5 kgf / cm ２ Above 2.0 kgf / cm ２ Less than D: 1.5 kgf / cm ２ less than

[0052] <<Weldability>> Two test pieces were cut out from the prepared steel sheet for cans and heated in a batch furnace. Specifically, heating was performed twice, with the sheet temperature reaching 210°C and held for 10 minutes. The two heated test pieces were overlapped. Next, the two overlapped test pieces were sandwiched using a DR-type 1 mass% Cr-Cu electrode (an electrode processed to have a tip diameter of 2.3 mm and a curvature radius of 40 mm), and a pressure of 40 kgf / cm was applied. ２ A square wave current was applied for 1 ms (millisecond) using a DC power source at a pressure of 1.0 kA. The current value at which the test pieces were joined together was determined as the lower limit current (unit: kA), and the current value at which the surface of the test piece overheated and spatter occurred was determined as the upper limit current (unit: kA). Furthermore, the current range (= upper limit current - lower limit current) was determined from these two. In practice, if the current range is "A" or "B" below, it can be evaluated as having excellent weldability. A: 1.0 kA or more B: 0.6 kA or more but less than 1.0 kA C: 0.2 kA or more but less than 0.6 kA D: Less than 0.2 kA

[0053] <<Corrosion Resistance>> Two test specimens were cut from the prepared steel sheet for cans. The surfaces on which the coating layer and chromium-containing layer were formed were used as the evaluation surfaces. The two test specimens were stacked with the evaluation surfaces (the surfaces on which the metallic chromium layer and chromium hydrate oxide layer were formed) facing each other and passed through metal rolls under a surface pressure of 40 MPa. An epoxy phenolic resin was then applied to the evaluation surface of one of the two test specimens, and the specimen was heated twice at 210°C for 10 minutes to form a coating film. Next, a cross-cut was made in the coating film deep enough to reach the steel sheet, and the specimen was immersed in a test solution (a mixed aqueous solution of 1.5% by mass of citric acid and 1.5% by mass of sodium chloride) at 45°C for 168 hours. After immersion, the specimen was removed from the test solution, washed with water, and dried. A coating film peeling test was then performed using tape. The peel width (total width extending from the intersection to the left and right) was measured at four locations within 10 mm of the crosscut intersection, and the average of the peel widths at the four locations was calculated as the corrosion width. In practice, if the corrosion width is "A" or "B," it can be evaluated as having excellent corrosion resistance. A: 1.0 mm or less B: More than 1.0 mm and less than 2.0 mm C: More than 2.0 mm and less than 3.0 mm D: More than 3.0 mm

[0054]

[0055]

[0056]

[0057] <Summary of Evaluation Results> As shown in the above Table 3, the steel sheets for cans of Examples 1 to 20 were good in all of adhesion, weldability and corrosion resistance. In contrast, the steel sheets for cans of Comparative Examples 1 to 5 were insufficient in at least one of adhesion, weldability and corrosion resistance.

Claims

1. A steel sheet for cans, comprising a surface 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, the 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, the molar ratio X / Cr of the chromium hydroxide X to the chromium element Cr being 0.400 or more and 0.800 or less.

2. The coating weight of the metal chromium layer is 50 mg / m ２ 200mg / m or more ２ 2. The steel sheet for cans according to claim 1, wherein:

3. The amount of the chromium-equivalent coating of the chromium hydrate oxide layer is 3 mg / m ２ 20mg / m or more ２ The steel sheet for cans according to claim 1 or 2, wherein:

4. A method for producing a steel sheet for cans according to any one of claims 1 to 3, comprising the steps of: subjecting the steel sheet to cathodic electrolysis C1 using an aqueous solution 1 containing a hexavalent chromium compound, a fluorine-containing compound and sulfuric acid; and then subjecting the steel sheet to cathodic electrolysis C2 at 0.30 to 3.00 C / dm using an aqueous solution 2 having a content of the element M of 0.10 to 1.00 mmol / L. ２ The method for manufacturing a steel sheet for cans comprises carrying out cathodic electrolysis C2 at a charge density of 5. The method for producing steel sheet for cans according to claim 4, wherein the pH of said aqueous solution 2 is 9.0 or more and 11.5 or less.

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