Steel sheet for a can and method of manufacturing the same

Abstract

This invention provides a tank steel plate with excellent adhesion, weldability, and corrosion resistance. The tank steel plate has, from the steel plate side, a metallic chromium layer and a chromium hydrate oxide layer, respectively. The chromium hydrate oxide layer contains chromium (Cr) and at least one element M selected from the group consisting of sodium, magnesium, potassium, and calcium. The molar ratio of element M to chromium (Cr) is M / Cr of 0.010 or more and 0.100 or less. The chromium hydrate oxide layer contains chromium hydroxide (X), and the molar ratio of chromium hydroxide (X) to chromium (Cr) is X / Cr of 0.400 or more and 0.800 or less.

Description

Technical Field

[0001] This invention relates to steel plates for tanks and methods for manufacturing the same. Background Technology

[0002] Previously, it was known that can steel plates had a "metallic chromium layer" and a "chromium hydrate oxide layer" on the surface of the steel plate (Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2005-298864 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] In addition to good corrosion resistance, steel plates used for tanks also require good weldability.

[0008] Furthermore, the steel plates used for tanks sometimes require excellent adhesion to coatings and films (hereinafter also referred to as "adhesion").

[0009] Therefore, the object of the present invention is to provide a can steel plate with excellent adhesion, weldability and corrosion resistance, and a method for manufacturing the same.

[0010] Methods for solving problems

[0011] Through in-depth research, the inventors discovered that the above-mentioned objectives can be achieved by adopting the following configuration, thus completing the present invention.

[0012] That is, the present invention provides the following [1] to [5].

[0013] [1] A steel plate for tanks, wherein a metallic chromium layer and a chromium hydrate oxide layer are sequentially formed on the surface of the steel plate from the side of the steel plate. The chromium hydrate oxide layer contains chromium (Cr) and at least one element M selected from the group consisting of sodium, magnesium, potassium, and calcium, wherein the molar ratio of element M to chromium (Cr) is 0.010 or more and 0.100 or less. The chromium hydrate oxide layer contains chromium hydroxide X, and the molar ratio of chromium hydroxide X to chromium element Cr, X / Cr, is greater than 0.400 and less than 0.800.

[0014] [2] According to the steel plate for tanks described in [1] above, wherein the amount of the chromium layer is 50 mg / m³. 2 Above and 200mg / m 2 the following.

[0015] [3] According to the steel plate for tanks described in [1] or [2] above, wherein the chromium content of the chromium hydrate oxide layer is 3 mg / m³. 2 Above and 20mg / m 2 the following.

[0016] [4] A method for manufacturing a can steel plate, comprising the method for manufacturing a can steel plate as described in any one of [1] to [3] above, wherein, for the steel plate, a cathodic electrolysis treatment C1 is performed using an aqueous solution 1 containing a hexavalent chromium compound, a fluorine compound and sulfuric acid, and then an aqueous solution 2 containing the element M at a concentration of 0.10 to 1.00 mmol / L is used at a concentration of 0.30 to 3.00 C / dm. 2 The charge density was subjected to cathode electrolysis treatment C2.

[0017] [5] According to the method for manufacturing the steel plate for tanks described above [4], the pH of the aqueous solution 2 is above 9.0 and below 11.5.

[0018] Invention Effects

[0019] According to the present invention, a tank steel plate with excellent adhesion, weldability and corrosion resistance, and a method for manufacturing the same, are available. Detailed Implementation

[0020] [Steel plate for tanks]

[0021] The can steel plate of this embodiment has a metallic chromium layer and a chromium hydrate oxide layer sequentially on the surface of the steel plate from the side of the steel plate.

[0022] In this embodiment, the chromium hydrate 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.

[0023] Furthermore, in this embodiment, the chromium hydrate oxide layer 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.

[0024] The steel plate used for the tank in this embodiment has excellent adhesion, weldability, and corrosion resistance.

[0025] The following is a more detailed description of the components of the tank steel plate of this embodiment.

[0026] <Steel Plate>

[0027] There are no particular limitations on the type of steel plate. Generally, steel plates used as container materials (e.g., low-carbon steel plates, ultra-low-carbon steel plates) can be used. There are also no particular limitations on the manufacturing method of steel plates. They can be manufactured using standard steel sheet manufacturing processes such as hot rolling, pickling, cold rolling, annealing, and tempering rolling. As for the composition of steel plates, examples include those specified in ASTM standards, but are not limited to these. There are also no particular limitations on the thickness of steel plates, for example, it can be 0.10 mm or more and 0.60 mm or less. Furthermore, "steel plate" includes the concept of "steel strip."

[0028] <Metallic Chromium Layer>

[0029] The chromium layer suppresses the surface exposure of the steel plate, thereby improving corrosion resistance.

[0030] Adhesion Amount

[0031] From the perspective of achieving good corrosion resistance, the preferred adhesion amount of the metallic chromium layer is 50 mg / m³. 2 The above, more preferably 70 mg / m 2 The above is further preferred to be 80 mg / m 2 The above refers to the amount of adhesion per single steel plate (the same applies below).

[0032] On the other hand, excessive chromium coating may reduce weld strength and cause excessive spatter during welding, damaging the chromium layer and impairing corrosion resistance. Therefore, the preferred chromium coating thickness is 200 mg / m³. 2 The following is more preferably 150 mg / m 2 The following is a further preferred value: 130 mg / m² 2 the following.

[0033] Methods for determining adhesion amount

[0034] The amount of chromium-coated metallic chromium layer and the amount of chromium-converted chromium-coated chromium-coated chromium-coated chromium oxide layer (described later) were determined as follows.

[0035] First, for can steel plates with both a metallic chromium layer and a hydrated chromium oxide layer, the chromium content (total chromium content) was determined using a fluorescence X-ray apparatus. Next, an alkaline treatment was performed, immersing the can steel plate in a 6.5N sodium hydroxide aqueous solution (liquid temperature: 90°C) for 10 minutes, followed by another determination of the chromium content (chromium content after alkaline treatment) using a fluorescence X-ray apparatus. The chromium content after alkaline treatment was taken as the amount of metallic chromium layer adhesion.

[0036] Then, calculate (alkali-soluble chromium content) = (total chromium content) - (chromium content after alkali treatment), and use the alkali-soluble chromium content as the chromium content of the chromium hydrate oxide layer.

[0037] <Chromium Hydrated Oxide Layer>

[0038] Hydrated chromium oxides include oxygen-containing chromium compounds such as chromium oxides and chromium hydroxides.

[0039] By depositing a chromium hydrate oxide layer on a metallic chromium layer, adhesion to coatings and the like can be improved. The chromium hydroxide in the chromium hydrate oxide layer has hydroxyl groups and is polar. The chromium hydroxide exerts its adhesion through hydrogen bonds with coatings and the like.

[0040] Adhesion Amount

[0041] From the perspective of achieving good adhesion, the preferred chromium content of the chromium hydrate oxide layer is 3 mg / m³. 2 The above, more preferably 4 mg / m 2 above.

[0042] On the other hand, if there is too much chromium hydrated oxide layer, it may aggregate and break down at the adhesion interface with coatings, etc., resulting in reduced adhesion. Therefore, the chromium equivalent of the chromium content of the chromium hydrated oxide layer is preferably 20 mg / m³. 2 The following is more preferably 15 mg / m² 2 The following is a further preferred value: 10 mg / m 2 the following.

[0043] Molar ratio (M / Cr)

[0044] The chromium hydroxide in the chromium hydrate oxide layer may dehydrate due to heating during welding, leading to increased contact resistance and reduced weldability. This is believed to be due to dehydration condensation between chromium hydroxides containing hydroxyl groups (-OH), forming Cr-O-Cr bonds and transforming into compounds with low conductivity (such as chromium oxides).

[0045] In this embodiment, in order to suppress this dehydration, the chromium hydrate oxide layer contains trace amounts of element M.

[0046] That is, in addition to chromium (Cr), the chromium hydrate oxide layer also contains at least one element M selected from the group consisting of sodium (Na), magnesium (Mg), potassium (K) and calcium (Ca).

[0047] Therefore, it is believed that the hydrogen atom at the hydroxyl terminus is replaced by the cation of element M, which is difficult to achieve even after dehydration through heating.

[0048] At this point, when the molar ratio (M / Cr) of element M to chromium Cr in the chromium hydrate oxide layer is too low, this effect is difficult to achieve due to the scarcity of element M.

[0049] 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.

[0050] It should be noted that the molar ratio (M / Cr) of the chromium hydrate oxide layer can exceed 0.050.

[0051] At this point, 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.

[0052] On the other hand, when the chromium hydrate oxide layer contains an excessive amount of element M, it may impair the continuity of the chromium hydrate oxide layer, weaken the bonding between the coating and hydroxyl groups, and lead to reduced adhesion. In addition, the chromium hydrate oxide layer ensures corrosion resistance by covering the pinholes (parts of the steel plate that are not completely covered) of the metallic chromium layer, but if the continuity of the chromium hydrate oxide layer is damaged, this coverage becomes insufficient, which may lead to a decrease in corrosion resistance.

[0053] 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.

[0054] Molar ratio (X / Cr)

[0055] As mentioned above, the chromium hydroxide in the chromium hydrate oxide layer contributes to adhesion.

[0056] At this point, if the molar ratio (X / Cr) of chromium hydroxide X to chromium Cr in the chromium hydrate oxide layer is too low, sufficient adhesion may not be achieved due to the scarcity of chromium hydroxide X.

[0057] 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.

[0058] On the other hand, compared with chromium oxides, chromium hydroxides have lower strength, so when there is too much of them, their corrosion resistance may be insufficient.

[0059] 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.

[0060] Methods for Determining Molar Ratio

[0061] The above molar ratios (M / Cr) and (X / Cr) are calculated as follows.

[0062] First, a can steel plate with a metallic chromium layer and a chromium hydrate oxide layer (a can steel plate in its so-called manufacturing state that has not been heated except for drying caused by indoor storage, etc.) is placed in an ultra-high vacuum, and under the following conditions, the Cr2p spectrum of the outermost surface of the chromium hydrate oxide layer is obtained by X-ray photoelectron spectroscopy.

[0063] After background correction, the obtained Cr2p spectrum was separated into peaks representing metallic chromium at 574.4 ± 0.1 eV, chromium hydroxide at 577.4 ± 0.4 eV, and chromium oxide at 578.1 ± 1.7 eV. Peak separation was performed using a curve fitting method based on the nonlinear least squares approach employing the Gaussian-Lorentz composite function. The area of ​​each separated peak was then calculated.

[0064] Thus, for the outermost surface of the chromium hydrate oxide layer, the ratio (X / Cr) of the peak area X of chromium hydroxide to the total peak area Cr of the total chromium peaks (peaks of metallic chromium, chromium hydroxide and chromium oxide) is calculated and used as the molar ratio (X / Cr) mentioned above.

[0065] Furthermore, for the outermost surface of the chromium hydrate oxide layer, the spectra (narrow spectrum) of Na1s, Mg1s, K2p, and Ca2p were obtained. Based on the integrated intensity of the obtained spectra, the relative sensitivity coefficient method was used to quantify the elements Na, Mg, K, and Ca, and the total molar amount M was determined.

[0066] Similarly, the molar amount of chromium (Cr) can be determined from the integrated intensity of the Cr2p spectrum.

[0067] The above molar ratio (M / Cr) is obtained from the molar amounts M and Cr.

[0068] (X-ray photoelectron spectroscopy conditions)

[0069] • Measuring apparatus: X-tool manufactured by Ulvac-PHI

[0070] • Excitation source: Monochromatic Al Kα 25W×15kV

[0071] • Analysis size: 100 μmΦ

[0072] • Angle of departure: 45°

[0073] ·Tongneng

[0074] Wide scan: 280.0 eV

[0075] Narrow scan: 112.0 eV

[0076] [Manufacturing method of steel plates for tanks]

[0077] Next, the method for manufacturing the tank steel plate of this embodiment will be described.

[0078] In this embodiment, generally speaking, the steel plate is first subjected to cathodic electrolysis treatment C1 in an aqueous solution 1 containing a hexavalent chromium compound. This causes a reduction reaction on the surface of the steel plate, precipitating metallic chromium, and subsequently, chromium hydrate oxide, as an intermediate product for the deposition of 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 treatment C1. Thus, a layer of metallic chromium and a layer of chromium hydrate oxide are formed on the surface of the steel plate.

[0079] Furthermore, in this embodiment, after the cathodic electrolysis treatment C1, the cathodic electrolysis treatment C2 is performed using an aqueous solution 2 containing element M. This introduces element M into the formed chromium hydrate oxide layer.

[0080] The following provides a detailed description of aqueous solutions 1-2 and cathodic electrolysis treatments C1-C2.

[0081] <Aqueous Solution 1>

[0082] The aqueous solution 1 used for cathodic electrolysis of C1 contains at least a hexavalent chromium compound, a fluorine compound, and sulfuric acid.

[0083] Examples of hexavalent chromium compounds include chromium trioxide (CrO3), potassium dichromate (K2Cr2O7) and other dichromates, and potassium chromate (K2CrO4) and other chromates.

[0084] Based on the premise that metallic chromium can be deposited stably and efficiently over a long period of time, the content of hexavalent chromium compounds in aqueous solution 1 is preferably 0.50 mol / L or more, more preferably 0.80 mol / L or more, based on the amount of Cr.

[0085] On the other hand, the amount of Cr in aqueous solution 1 is preferably 5.00 mol / L or less, more preferably 3.00 mol / L or less.

[0086] Examples of fluorine-containing compounds include hydrofluoric acid (HF), potassium fluoride (KF), sodium fluoride (NaF), fluorosilicic acid (H₂SiF₆), and fluorosilicates. Examples of fluorosilicates include sodium fluorosilicate (Na₂SiF₆), potassium fluorosilicate (K₂SiF₆), and ammonium fluorosilicate ((NH₄)₂SiF₆).

[0087] The content of fluorine-containing compounds in aqueous solution 1, expressed in terms of F, is preferably more than 0.100 mol / L, more preferably more than 0.110 mol / L, even more preferably more than 0.150 mol / L, and particularly preferably more than 0.200 mol / L.

[0088] On the other hand, the amount of F in 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.

[0089] The adhesion efficiency of metallic chromium can be improved by using sulfuric acid in combination with fluorine-containing compounds.

[0090] Some or all of sulfuric acid can be sulfates such as sodium sulfate, calcium sulfate, and ammonium sulfate.

[0091] The sulfuric acid content in aqueous solution 1 is expressed as SO4. 2- The concentration of the liquid is preferably 0.0001 mol / L or higher, more preferably 0.0003 mol / L or higher, and even more preferably 0.0010 mol / L or higher.

[0092] On the other hand, SO4 in aqueous solution 1 2- The amount is preferably below 0.1000 mol / L, and more preferably below 0.0500 mol / L.

[0093] 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.

[0094] On the other hand, the temperature of the aqueous solution 1 is preferably below 80°C, and more preferably below 60°C.

[0095] In the cathodic electrolysis treatment C1, it is preferable to use only one type of aqueous solution 1.

[0096] <Cathode Electrolysis Treatment C1>

[0097] As described above, during the cathodic electrolysis treatment C2, metallic chromium and chromium hydrate oxide are deposited. This forms a layer of metallic chromium and a layer of chromium hydrate oxide on the surface of the steel plate.

[0098] The preferred current density for cathode electrolysis of C1 is 5 A / dm³. 2 The above is preferred, with 10A / dm.2 The above is further optimized to 20A / dm. 2 above.

[0099] On the other hand, the preferred current density for cathode electrolysis of C1 is 60 A / dm³. 2 The following is more preferably 50A / dm 2 The following is a further preferred value: 40A / dm 2 the following.

[0100] The preferred charge density of C1 in the cathode electrolysis treatment is 70.0 C / dm³. 2 The following is more preferably 60.0C / dm 2 The following is further preferred: 50.0C / dm 2 the following.

[0101] On the other hand, the charge density of C1 in the cathode electrolysis treatment is preferably 10.0 C / dm³. 2 The above, more preferably 20.0C / dm 2 The above is further preferred to be 30.0C / dm. 2 above.

[0102] The energizing time (in seconds) of the cathode electrolysis treatment C1 can be appropriately set according to the current density and charge density.

[0103] Cathode electrolysis C1 may not be a continuous electrolysis process.

[0104] That is, the cathode electrolysis treatment C1 can be an intermittent electrolysis treatment in which electrolysis is carried out on multiple electrodes in industrial production, inevitably resulting in non-electrostatic immersion time. In the case of intermittent electrolysis treatment, the total charge density is preferably within a preferred range.

[0105] The same applies to the cathode electrolysis treatment C2, which will be described later.

[0106] It should be noted that, for purposes such as controlling and improving the adhesion of the chromium hydrate oxide layer, after the cathodic electrolytic treatment of C1, the steel plate can also be non-electrolytically impregnated in an aqueous solution containing hexavalent chromium compounds.

[0107] <Aqueous Solution 2>

[0108] Aqueous solution 2 contains element M (selected from at least one of the group consisting of Na, Mg, K and Ca).

[0109] Aqueous solution 2 is prepared, for example, by adding a compound containing element M (also called "M compound") to water as a solvent.

[0110] As for compound M, sulfates, nitrates, chlorides and fluorides should be avoided, and hydroxides and carbonates are preferred.

[0111] Examples of hydroxides of element M include sodium hydroxide (NaOH), magnesium hydroxide (Mg(OH)2), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2).

[0112] Examples of carbonates that are elements M include sodium carbonate (Na₂CO₃), magnesium carbonate (MgCO₃), potassium carbonate (K₂CO₃), and calcium carbonate (CaCO₃).

[0113] From the viewpoint of introducing sufficient element M into the chromium hydrate 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.

[0114] On the other hand, in order to suppress the introduction of excessive element M into the chromium hydrate 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.

[0115] The total content of element M in aqueous solution 2 was determined by ICP (inductively coupled plasma) emission spectroscopy or ICP mass spectrometry.

[0116] For reasons described later, aqueous solution 2 is preferably weakly alkaline.

[0117] Specifically, 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.

[0118] On the other hand, 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.

[0119] The temperature of aqueous solution 2 is, for example, above 40°C.

[0120] However, from the viewpoint of suppressing the excessive introduction of element M into the chromium hydrate oxide layer, it is preferable to be below 75°C, more preferably below 65°C, and even more preferably below 55°C.

[0121] <Cathode Electrolysis Treatment C2>

[0122] Following cathodic electrolysis treatment C1, the steel sheet with the formed metallic chromium layer and chromium hydrate oxide layer is appropriately washed with water, and then subjected to cathodic electrolysis treatment C2 using an aqueous solution 2 containing element M. Thus, as described above, element M is introduced into the chromium hydrate oxide layer formed by cathodic electrolysis treatment C1. It is believed that the hydrogen atoms at the hydroxyl terminus of the chromium hydroxide in the chromium hydrate oxide layer are replaced by cations of element M.

[0123] Furthermore, by performing cathodic electrolysis on C2, hydroxyl groups are also introduced into the chromium hydrate oxide layer. That is, the amount of chromium hydroxide increases.

[0124] On the surface of the chromium hydrate oxide layer after cathode electrolysis of C1, sulfate ions (SO4) are adsorbed or embedded. 2- Fluorine-containing compounds, etc. In this state, by using a weakly alkaline aqueous solution 2 to perform cathodic electrolysis C2, sulfate ions, etc. are removed and replaced by hydroxyl groups introduced into the chromium hydrate oxide layer.

[0125] When the charge density of C2 during cathodic electrolysis is too low, hydroxyl groups are difficult to introduce into the chromium hydrate oxide layer. From the perspective of promoting the introduction of hydroxyl groups and thus increasing chromium hydroxide, the charge density of C2 during cathodic electrolysis is 0.30 C / dm³. 2 The preferred value is 0.35 C / dm. 2 The above is preferred, with 0.40 C / dm. 2 The above is further preferred to be 0.45 C / dm. 2 The above is particularly preferred, with 0.50 C / dm. 2 above.

[0126] On the other hand, in order to suppress the excessive increase of chromium hydroxide, the charge density of C2 in the cathodic electrolysis treatment is 3.00 C / dm³. 2 The preferred value is 2.50 C / dm. 2 Below, 2.00 C / dm is preferred. 2 The following is further preferred to be 1.50 C / dm. 2 the following.

[0127] Regarding the current density for cathode electrolysis of C2, there are no particular limitations as long as the charge density is within the above range, but 0.5 A / dm is preferred. 2 The above is preferred to be 1.0 A / dm. 2 The above further optimizes the ratio to over 1.5 A / dm. 2 .

[0128] On the other hand, the current density for cathode electrolysis of C2 is preferably less than 10.0 A / dm³. 2 More preferably 9.0 A / dm 2The following is a further preferred value of 8.0 A / dm. 2 the following.

[0129] The energizing time (in seconds) of the cathode electrolysis treatment C2 can be appropriately set according to the current density and charge density, preferably 0.10 seconds or more, and more preferably 0.20 seconds or more.

[0130] There is no particular upper limit, but from the viewpoint of efficiency in continuous manufacturing, the energizing time of the cathode 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.

[0131] <Water Wash>

[0132] After the C2 cathodic electrolysis treatment, the resulting can steel plate can be further washed with ordinary industrial water. This washing removes any electrolyte residue remaining on the surface of the can steel plate. There is no particular time limit for the washing process.

[0133] However, in order to prevent the hydroxyl groups in the chromium hydrate oxide layer that have been replaced by element M from reverting to their original state, the temperature of the cleaning water is preferably below 40°C, and more preferably below 30°C.

[0134] There are no particular limitations on the washing method; any previously known method can be used. For example, a washing tank can be installed downstream of the electrolytic cell used for cathode electrolysis treatments C1 to C2, and the steel plate of the tank after cathode electrolysis treatment C2 can be continuously immersed in the washing water. Alternatively, a sprayer can be used to spray washing water onto the steel plate of the tank after cathode electrolysis treatment C2.

[0135] There is no specific limit to the number of washes. Each wash can be performed using the same method or different methods.

[0136] Example

[0137] The present invention will now be described in detail through embodiments. However, the present invention is not limited to the embodiments described below.

[0138] <Fabrication of Steel Plates for Tanks>

[0139] For steel plates manufactured with a thickness of 0.22 mm (quenching and tempering: T4CA), conventional degreasing and pickling are performed.

[0140] For this steel plate, cathodic electrolysis treatment C1 was performed using aqueous solution 1, and then cathodic electrolysis treatment C2 was performed using aqueous solution 2.

[0141] 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 treatment C1 and cathodic electrolysis treatment C2 are shown in Table 3 below. The pH of the aqueous solution 2 used is in the range of 9.0 or higher and 11.5 or lower. If cathodic electrolysis treatment C2 is not performed, "-" is recorded in the corresponding column of Table 3 below.

[0142] In the cathodic electrolysis treatments C1 to C2, aqueous solutions 1 to 2 are circulated in a flow cell by a pump at a rate equivalent to 100 mpm, using lead electrodes.

[0143] The steel plates for the tanks were thus manufactured. After being manufactured, the steel plates were washed by immersion in industrial water (liquid temperature: 25°C) and then dried at room temperature using a blower.

[0144] <Adhesion amount, etc.>

[0145] For the manufactured can steel plates, the amount of metallic chromium attached and the amount of chromium-converted chromium attached to the chromium hydrate oxide layer were measured (abbreviated as "attachment amount" in Table 3 below).

[0146] Furthermore, the molar ratio (M / Cr) and molar ratio (X / Cr) of the chromium hydrate oxide layer were determined for the manufactured can steel plate.

[0147] The determination methods were all as described above. The results are shown in Table 3 below.

[0148] <Evaluation>

[0149] For the manufactured steel plates used in cans, the adhesion, corrosion resistance, and weldability were evaluated through the following tests. The results are shown in Table 3 below.

[0150] "Tightness"

[0151] Two test pieces were cut from the manufactured can steel plate, and their surfaces were coated. Specifically, a coating of 50 mg / dm³ was applied to the surface of the test pieces. 2 Apply epoxy phenolic coating at a certain coating amount and bake at 210°C for 10 minutes.

[0152] A nylon film was sandwiched between the coated surfaces of two test pieces, preheated at 190°C for 1 minute, and then subjected to a temperature of 3 kgf / cm². 2 The pressure is applied for 30 seconds to create the test specimen.

[0153] The test specimen was then cut to a width of 5 mm and immersed in a test solution (a mixed aqueous solution of 1.5% citric acid and 1.5% sodium chloride) at 55°C for 336 hours.

[0154] After immersion, the test specimen was removed from the test solution, washed with water, and dried. Then, a tensile testing machine was used to stretch two test pieces of the specimen at a tensile speed of 3.33 mm / s. The tensile strength at the point of peeling was taken as the peel strength (unit: kgf / cm²). 2 ) Calculate. In practice, if the peel strength is "A" or "B" as described below, it can be evaluated as having excellent adhesion.

[0155] A: 2.5 kgf / cm 2 above

[0156] B: 2.0 kgf / cm 2 Above and less than 2.5 kgf / cm 2

[0157] C: 1.5 kgf / cm 2 Above and less than 2.0 kgf / cm 2

[0158] D: Less than 1.5 kgf / cm 2

[0159] Weldability

[0160] Two test pieces were cut from the manufactured can steel plate and heated in a batch furnace. Specifically, two heating processes were performed, each holding the plate at 210°C for 10 minutes. The two heated test pieces were then overlapped.

[0161] Then, using a DR-type 1% Cr-Cu electrode (with a tip diameter of 2.3 mm and a radius of curvature of 40 mm), the two overlapping test pieces were clamped together at 40 kgf / cm². 2 The pressure is applied, and a rectangular wave is applied using a DC power supply for 1 ms. The current value at which the test pieces are joined together is determined as the lower limit current (unit: kA), and the current value at which the test piece surface overheats and spatters is determined as the upper limit current (unit: kA). The current range (= upper limit current - lower limit current) is then calculated from these two values. In practical terms, if the current range falls within the range of "A" or "B" below, the weldability is considered excellent.

[0162] A: 1.0 kA or more

[0163] B: Less than 1.0 kA and more than 0.6 kA

[0164] C: Less than 0.6 kA and more than 0.2 kA

[0165] D: Less than 0.2 kA

[0166] Corrosion Resistance

[0167] Two test pieces were cut from the steel plate used for the can. The side with the coating layer and the chromium-containing layer was used as the evaluation surface. The two test pieces were overlapped with the evaluation surfaces (the sides with the metallic chromium layer and the chromium hydrate oxide layer) facing each other, and a surface pressure of 40 MPa was applied between metal rollers.

[0168] Subsequently, epoxy phenolic resin was applied to the evaluation surface of one of the two test pieces, and the piece was subjected to two heating treatments at 210°C for 10 minutes each to form a coating film.

[0169] Next, after making cross-shaped cuts to the depth of the steel plate on the coating, the test piece was immersed in a test solution (a mixed aqueous solution of 1.5% by mass citric acid and 1.5% by mass sodium chloride) at 45°C for 168 hours. After immersion, the test piece was removed from the test solution, washed with water, and dried. Then, a test was conducted to peel off the coating using adhesive tape.

[0170] Measure the peel width (the total width extending left and right from the intersection) at four points within 10 mm of the cross-shaped cut. Calculate the average of the four peel widths as the corrosion width. In practice, if the corrosion width is "A" or "B", it can be evaluated as having excellent corrosion resistance.

[0171] A: Below 1.0 mm

[0172] B: More than 1.0 mm and less than 2.0 mm

[0173] C: More than 2.0 mm but less than 3.0 mm

[0174] D: Over 3.0 mm

[0175] [Table 1]

[0176] [Table 2]

[0177] [Table 3]

[0178] <Summary of Evaluation Results>

[0179] As shown in Table 3 above, the steel plates for tanks in Examples 1 to 20 all exhibit good adhesion, weldability, and corrosion resistance.

[0180] In contrast, the steel plates for tanks in Comparative Examples 1 to 5 are inadequate in at least one of the following aspects: adhesion, weldability, and corrosion resistance.

Claims

1. A steel plate for tanks, wherein, The surface of the steel plate has a metallic chromium layer and a chromium hydrate oxide layer in sequence, starting from the side of the steel plate. The chromium hydrate oxide layer contains chromium (Cr) and at least one element M selected from the group consisting of sodium, magnesium, potassium, and calcium, wherein the molar ratio of element M to chromium (Cr) is 0.010 or more and 0.100 or less. The chromium hydrate oxide layer contains chromium hydroxide X, and the molar ratio of chromium hydroxide X to chromium element Cr, X / Cr, is greater than 0.400 and less than 0.

800.

2. The steel plate for tanks according to claim 1, wherein, The amount of the chromium layer attached is 50 mg / m³. 2 Above and 200mg / m 2 the following.

3. The steel plate for tanks according to claim 1 or 2, wherein, The chromium content of the chromium hydrate oxide layer is 3 mg / m³. 2 Above and 20mg / m 2 the following.

4. A method for manufacturing a steel plate for tanks, comprising the method for manufacturing the steel plate for tanks according to any one of claims 1 to 3, wherein in the method for manufacturing the steel plate for tanks, For the steel plate, cathodic electrolysis C1 is performed using an aqueous solution 1 containing hexavalent chromium compounds, fluorine compounds, and sulfuric acid, followed by an aqueous solution 2 containing element M at a concentration of 0.10–1.00 mmol / L at a concentration of 0.30–3.00 C / dm. 2 The charge density was subjected to cathode electrolysis treatment C2.

5. The method for manufacturing tank steel plates according to claim 4, wherein, The pH of the aqueous solution 2 is above 9.0 and below 11.5.

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