TWI841025B - Steel sheet for cans and method of producing same - Google Patents

Abstract

The present invention provides a steel plate (1) for cans having excellent weldability. The steel plate (1) for cans has a metal chromium layer (3) and a chromium hydrate oxide layer (4) on the surface of a steel plate (2) in order from the side of the steel plate (2). The adhesion amount of the metal chromium layer (3) is 50 to 200 mg/ ^m2 , and the adhesion amount of the chromium hydrate oxide layer (4) is 3 to 50 mg/ ^m2 in terms of chromium. The metal chromium layer (3) includes a flat plate portion (3a) and a protrusion portion (3b) provided on the flat plate portion (3a). The height (H) of the protrusion (3b) is greater than 25nm and less than 1000nm, the bottom diameter (D) of the protrusion (3b) is greater than 50nm and less than 1000nm, the ratio H/D of the height (H) to the bottom diameter (D) is greater than 0.50, and the area ratio of the protrusion (3b) is greater than 5%.

Description

Steel plate for tank and manufacturing method thereof

The present invention relates to a steel plate for a can and a method for manufacturing the same.

Patent document 1 discloses a steel plate for cans, which has "a metal chromium layer and a chromium hydrate oxide layer in order from the side of the steel plate on the surface of the steel plate", and the metal chromium layer has "granular protrusions". [Prior art document] [Patent document]

[Patent Document 1] International Publication No. 2017/098991

[The problem that the invention is trying to solve]

Steel sheets for cans having granular protrusions are expected to show good weldability. This is because when the steel sheets for cans are welded together, the chromium hydrate oxide layer is destroyed by the granular protrusions, and the contact resistance is reduced. Therefore, the object of the present invention is to provide a steel sheet for cans having excellent weldability and a method for manufacturing the same. [Means for Solving the Problem]

The inventors of the present invention have made intensive studies and have found that the above-mentioned object can be achieved by adopting the following structure, thereby completing the present invention.

That is, the present invention provides the following [1] to [6]. [1] A steel plate for cans, comprising a metal chromium layer and a chromium hydrate oxide layer on the surface of the steel plate in order from the side of the steel plate, the metal chromium layer having an adhesion amount of 50 to 200 mg/m ² , the chromium-converted adhesion amount of the chromium hydrate oxide layer having an adhesion amount of 3 to 50 mg/m ² , the metal chromium layer comprising a flat plate portion and a protrusion provided on the flat plate portion, the protrusion having a height H of not less than 25 nm and not more than 1000 nm, the protrusion having a bottom diameter D of not less than 50 nm and not more than 1000 nm, the ratio H/D of the height H to the bottom diameter D of not less than 0.50, and the protrusion having an area ratio of not less than 5%. [2] A method for producing a steel sheet for a can, which is a method for producing a steel sheet for a can as described in [1] above, comprising: subjecting the steel sheet to an immersion treatment using an aqueous solution 1, and thereafter subjecting the steel sheet to a cathodic electrolysis treatment using an aqueous solution 2; the aqueous solution 1 contains a water-soluble organic compound, and the aqueous solution 2 contains a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid. [3] The method for producing a steel sheet for a can as described in [2] above, wherein the content of the water-soluble organic compound in the aqueous solution 1 is not less than 10 g/L and not more than 100 g/L. [4] The method for producing a steel sheet for a can as described in [2] or [3] above, wherein the water-soluble organic compound is a water-soluble polymer having a weight average molecular weight of 300 to 100,000. [5] The method for producing a steel sheet for a can as described in [4] above, wherein the water-soluble polymer is polyacrylic acid. [6] The method for producing a steel sheet for a can as described in any one of [2] to [5] above, wherein the current density of the cathodic electrolysis treatment is 10 to 200 A/dm ² . [Effects of the Invention]

According to the present invention, a can steel plate having excellent weldability and a method for manufacturing the same can be provided.

[Form of implementing the invention] [Steel plate for tanks]

FIG. 1 schematically shows a cross-sectional view of a steel plate 1 for a can according to the present embodiment. As shown in FIG. 1 , the steel plate 1 for a can has a steel plate 2. The steel plate 1 for a can further has a metal chromium layer 3 and a chromium hydrated oxide layer 4 on the surface of the steel plate 2 in order from the side of the steel plate 2. The metal chromium layer 3 includes a flat plate portion 3a covering the steel plate 2 and a protrusion 3b provided on the flat plate portion 3a. The chromium hydrated oxide layer 4 is arranged on the metal chromium layer 3 in a manner following the shape of the protrusion 3b. The height H and the bottom diameter D of the protrusion 3b are described below.

Hereinafter, each structure of the steel plate for a can according to this embodiment will be described in more detail.

〈Steel Plate〉 There is no particular limitation on the type of steel plate. Usually, steel plates used as container materials (such as low carbon steel plates and ultra-low carbon steel plates) can be used. There is no particular limitation on the manufacturing method and material of the steel plate. It can be manufactured from the usual steel sheet manufacturing steps, through hot rolling, pickling, cold rolling, annealing, and tempering rolling.

<Metallic chromium layer> A metallic chromium layer is disposed on the surface of the above-mentioned steel plate. The metallic chromium layer suppresses the surface exposure of the steel plate and improves corrosion resistance.

《Adhesion Amount》 Based on the excellent corrosion resistance of can steel sheets, the adhesion amount of the metal chromium layer is 50 mg/ ^m2 or more, preferably 70 mg/ ^m2 or more, and more preferably 80 mg/ ^m2 or more. The adhesion amount refers to the adhesion amount on each side of the steel sheet (the same applies below).

On the other hand, if the amount of the metal chromium layer is too much, the high melting point metal chromium covers the entire steel plate, resulting in a decrease in weld strength or significant generation of dust during welding, and the weldability may be insufficient. Based on the reason that the weldability of the steel plate for cans is excellent, the amount of the metal chromium layer is 200 mg/ ^m2 or less, preferably 180 mg/ ^m2 or less, and more preferably 160 mg/ ^m2 or less.

(Method for measuring the amount of adhesion) The amount of adhesion of the metallic chromium layer and the amount of adhesion of the chromium hydrated oxide layer described later in terms of chromium are measured as follows. First, the amount of chromium (total chromium amount) is measured using a fluorescent X-ray device for a steel plate for a can formed with a metallic chromium layer and a chromium hydrated oxide layer. Next, the steel plate for a can is immersed in a 6.5N sodium hydroxide aqueous solution (liquid temperature: 90°C) for 10 minutes for alkali treatment, and then the amount of chromium (chromium amount after alkali treatment) is measured again using a fluorescent X-ray device. The amount of chromium after alkali treatment is set as the amount of adhesion of metallic chromium. Next, calculate (alkali-soluble chromium amount) = (total chromium amount) – (chromium amount after alkali treatment), and set the alkali-soluble chromium amount as the chromium-converted adhesion amount of the chromium hydrous oxide layer.

Such a metal chromium layer includes a flat plate portion and a protrusion portion provided on the flat plate portion. Next, each of the portions included in the metal chromium layer is described in detail.

"Flat Plate" The flat plate of the metal chromium layer mainly covers the surface of the steel plate to improve corrosion resistance. When the steel plates for cans inevitably come into contact with each other during handling, it is preferred to ensure sufficient thickness in a manner that does not cause the protrusions provided on the surface to damage the flat plate and expose the steel plate. Based on the excellent corrosion resistance of the steel plate for cans, the adhesion amount of the flat plate of the metal chromium layer is preferably 10 mg/ ^m2 or more, more preferably 30 mg/ ^m2 or more, and even more preferably 40 mg/ ^m2 or more.

《Protrusion》 The protrusion of the metal chromium layer is formed on the surface of the above-mentioned flat plate portion, which reduces the contact resistance between the can steel plates and improves the weldability. The estimated mechanism of contact resistance reduction is described below. The chromium hydrate oxide layer coated on the metal chromium layer is a non-conductive film, so its resistance is greater than that of metal chromium, which becomes an obstacle to welding. If a protrusion is formed on the surface of the flat plate portion of the metal chromium layer, the protrusion destroys the chromium hydrate oxide layer due to the surface pressure when the can steel plates are in contact during welding, and becomes the conduction point of the welding current, and the contact resistance is greatly reduced.

(Height H) As the height H of the protrusion increases, the surface pressure when the can steel plates are in contact with each other increases, and the weldability becomes better. Therefore, the height H of the protrusion is 25 nm or more, preferably 200 nm or more, and more preferably 500 nm or more.

On the other hand, if the height H of the protrusion is too large, the flat plate parts of the steel plate for the can are blocked from joining each other during welding, and the weldability may be deteriorated. Therefore, based on the reason that the weldability of the steel plate for the can is excellent, the height H of the protrusion is 1000 nm or less, preferably 900 nm or less, and more preferably 800 nm or less.

(Bottom diameter D) As the bottom diameter D of the protrusion increases, the surface pressure when the can steel plates are in contact with each other increases, and the weldability becomes better. Therefore, the bottom diameter D of the protrusion is 50 nm or more, preferably 200 nm or more, and more preferably 500 nm or more.

On the other hand, if the bottom diameter D of the protrusion is too large, the flat plate parts of the can steel plate are blocked from joining each other during welding, and the weldability may be deteriorated. Therefore, based on the reason that the weldability of the can steel plate is excellent, the bottom diameter D of the protrusion is 1000nm or less, preferably 900nm or less, and more preferably 800nm or less.

(H/D) When the height H is significantly smaller than the bottom diameter D, the weldability may deteriorate. Therefore, the ratio (H/D) of the height H of the protrusion to the bottom diameter D is 0.50 or more, preferably 0.60 or more, and more preferably 0.70 or more. On the other hand, the upper limit of the ratio (H/D) is not particularly limited as long as the height H and the bottom diameter D are within the above range.

((Method for measuring height H and bottom diameter D)) The height H and bottom diameter D of the protrusion are obtained as follows. First, a cross-section sample of a can steel plate formed with a metal chromium layer and a chromium hydrate oxide layer is prepared using a focused ion beam (FIB), and observed at a magnification of 20,000 times using a transmission electron microscope (TEM). For all protrusions within the observation field of view, the vertical distance from the flat plate of the metal chromium layer to the top of the protrusion is measured. This measurement is carried out in 5 fields of view for each can steel plate, and the maximum value in the measurement results is set as the height H of the protrusion of the can steel plate (see Figure 1).

Similarly, the cross-section sample of the can steel plate is observed by TEM, and the distance between the protrusion of the metal chromium layer and the portion (bottom) where the protrusion and the flat plate are in contact is measured for all the protrusions in the observation field. This measurement is performed in 5 fields for each can steel plate, and the maximum value in the measurement results is set as the bottom diameter D of the protrusion of the can steel plate (see Figure 1).

(Area ratio) When the area ratio of the protrusion is high, the welding property is excellent because the current point increases. Therefore, the area ratio of the protrusion is 5% or more, preferably 10% or more, and more preferably 20% or more. There is no particular upper limit on the area ratio of the protrusion, and it may be 100%.

((Measurement method of area ratio)) The area ratio of the protrusion is determined as follows. First, carbon vapor deposition is applied to the surface of a can steel plate formed with a metallic chromium layer and a chromium hydrate oxide layer, and the sample is used for observation. Next, the sample for observation is observed using a scanning electron microscope (SEM), and an SEM image is obtained at a magnification of 20,000 times. For the obtained SEM image, the protrusion is binarized using software (trade name: ImageJ), and the image is analyzed to determine the area ratio of the protrusion (unit: %). The area ratio is set as the average of 5 fields of view.

〈Chromium Hydrous Oxide Layer〉 Chromium hydrous oxide is precipitated on the surface of the steel plate simultaneously with metallic chromium to improve corrosion resistance. Chromium hydrous oxide includes, for example, chromium oxide and chromium hydroxide.

<<Adhesion Amount>> In order to ensure the corrosion resistance of the steel sheet for cans, the adhesion amount of the chromium hydrated oxide layer calculated as chromium is 3 mg/ ^m2 or more, preferably 4 mg/ ^m2 or more.

On the other hand, the conductivity of chromium hydrate oxide is lower than that of metallic chromium. If the amount is too much, it will become too large a resistance during welding, causing the occurrence of dust or spatter and various welding defects such as blowholes accompanying welding, and the welding property of the steel plate for cans may be deteriorated. Based on the reason for the excellent welding property of the steel plate for cans, the adhesion amount of chromium converted to chromium in the chromium hydrate oxide layer is 50 mg/ ^m2 or less, preferably 40 mg/ ^m2 or less, and more preferably 30 mg/ ^m2 or less.

The method for determining the chromium-equivalent adhesion amount of the chromium hydrous oxide layer is as described above.

[Manufacturing method of steel plate for cans] Next, the method for manufacturing the steel plate for cans of the present embodiment described above (hereinafter also referred to as "the present manufacturing method") is described. In general, the present manufacturing method is to perform an immersion treatment on the steel plate using an aqueous solution 1 containing a water-soluble organic compound, and then perform a cathodic electrolytic treatment using an aqueous solution 2 containing a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid. Thereby, a metallic chromium layer and a chromium hydrate oxide layer are generated on the surface of the steel plate, and further, a protrusion is formed on the metallic chromium layer.

The reason is speculated as follows. In the immersion treatment, the steel plate is immersed in the aqueous solution 1. At this time, the water-soluble organic compound is randomly adsorbed on the surface of the steel plate. Thereby, in the subsequent cathodic electrolytic treatment, the precipitation (electrodeposition) of metallic chromium is hindered. Moreover, in the part where the water-soluble organic compound is not adsorbed, metallic chromium is preferentially precipitated, and a protrusion is formed on the metallic chromium layer.

In this way, a steel plate for a can having a protrusion can be manufactured by a simple method of only performing an immersion treatment and a cathodic electrolytic treatment, without adopting a method of performing an anodic electrolytic treatment during a cathodic electrolytic treatment (see Patent Document 1).

<Aqueous solution 1> The aqueous solution 1 used for the immersion treatment contains a water-soluble organic compound.

《Water-soluble organic compounds》 Examples of water-soluble organic compounds include saccharin, 2-butyne-1,4-diol, oxalic acid, imidazole, sodium dodecyl sulfate, thiouric acid, and methanesulfonic acid.

In addition, as a water-soluble organic compound, a water-soluble polymer can be appropriately cited because it has many adsorption points and a large volume on the steel plate, and effectively blocks the precipitation of metallic chromium. As a water-soluble polymer, polyethylene glycol, poly(meth)acrylic acid, polyvinyl alcohol, polyethyleneimine, polyvinylpyrrolidone, etc. can be cited. Among them, poly(meth)acrylic acid is preferred, and polyacrylic acid is more preferred because of its excellent stability in aqueous solution 1 and its high adsorption capacity on the steel plate due to its carboxyl group.

(Content) If the water-soluble organic compound is too little, the amount of adsorption on the steel plate becomes insufficient, and it may be difficult to form the protrusion. Therefore, the content of the water-soluble organic compound in the aqueous solution 1 is preferably 10 g/L or more, more preferably 20 g/L or more, and even more preferably 30 g/L or more.

On the other hand, if the water-soluble organic compound is too much, the water-soluble organic compound is excessively adsorbed on the entire surface of the steel plate, and it is still difficult to form a protrusion. Therefore, the content of the water-soluble organic compound in the aqueous solution 1 is preferably 100 g/L or less, more preferably 90 g/L or less, and even more preferably 80 g/L or less.

(Weight average molecular weight of water-soluble polymer) When the water-soluble organic compound is a water-soluble polymer, if the weight average molecular weight (Mw) of the water-soluble polymer is too small, the number of adsorption starting points per molecule of the water-soluble polymer decreases. Therefore, adsorption to the steel plate is not sufficient, and it is difficult to form protrusions. Therefore, the Mw of the water-soluble polymer is preferably 300 or more, more preferably 500 or more, and even more preferably 1,000 or more.

On the other hand, if the Mw of the water-soluble polymer is too large, the water-soluble polymer itself will be entangled, and the starting point for adsorption on the steel plate will decrease. In this case, it will be difficult to form a protrusion because the water-soluble polymer is not sufficiently adsorbed on the steel plate. Therefore, the Mw of the water-soluble polymer is preferably 100,000 or less, more preferably 50,000 or less, and even more preferably 20,000 or less.

The weight average molecular weight (Mw) is the weight average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC) under the following conditions. ・Apparatus: HLC8020 (manufactured by Tosoh Corporation) ・Column: 3 TSKgelGMHXL connected in series ・Media: tetrahydrofuran ・Flow rate: 1 mL/min ・Concentration: 4 mg/10 mL ・Injection volume: 0.1 mL ・Column temperature: 40°C

The liquid temperature of the aqueous solution 1 is preferably 20°C or higher, more preferably 40°C or higher. On the other hand, the liquid temperature of the aqueous solution 1 is preferably 80°C or lower, more preferably 60°C or lower.

<Immersion treatment> In the immersion treatment, the steel plate is immersed in the aqueous solution 1. Thereby, the water-soluble organic compound is adsorbed on the surface of the steel plate. If the time (immersion time) for immersing the steel plate in the aqueous solution 1 is too short, the water-soluble organic compound is not sufficiently adsorbed on the surface of the steel plate, and it is difficult to form a protrusion. Therefore, the immersion time is preferably 1 second or more, more preferably 5 seconds or more, and particularly preferably 10 seconds or more. There is no particular upper limit on the immersion time. However, if the immersion time is too long, the surface state of the steel plate may deteriorate. Therefore, the immersion time is preferably 300 seconds or less, more preferably 180 seconds or less, and particularly preferably 60 seconds or less.

<Aqueous solution 2> The aqueous solution 2 used for cathodic electrolysis contains a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid. In the aqueous solution 2, the fluorine-containing compound and sulfuric acid exist in a dissociated state (i.e., in the state of fluoride ions, sulfuric acid ions, and hydrogen sulfate ions). These act as catalysts that participate in the reduction reaction of the hexavalent chromium ions present in the aqueous solution 2 during the cathodic electrolysis. The aqueous solution 2 used for cathodic electrolysis can reduce the chromium-converted adhesion amount of the chromium hydrous oxide layer of the obtained can steel plate by containing a fluorine-containing compound and sulfuric acid. Although the reason is unclear, it is believed that the amount of anions in the electrolytic treatment increases, and the amount of chromium hydrous oxide generated decreases.

<< Hexavalent chromium compound>> Examples of the hexavalent chromium compound include chromium trioxide (CrO ₃ ); dichromates such as potassium dichromate (K ₂ Cr ₂ O ₇ ); chromates such as potassium chromate (K ₂ CrO ₄ ). The content of the hexavalent chromium compound in the aqueous solution 2 is preferably 0.14 mol/L or more, more preferably 0.30 mol/L or more, in terms of Cr. On the other hand, the content of the hexavalent chromium compound in the aqueous solution 2 is preferably 3.00 mol/L or less, more preferably 2.50 mol/L or less, in terms of Cr.

《Fluorine-containing compound》 Examples of the fluorine-containing compound include hydrofluoric acid (HF), potassium fluoride (KF), sodium fluoride (NaF), fluorosilicic acid (H ₂ SiF ₆ ), and salts of fluorosilicic acid. Examples of salts of fluorosilicic acid include sodium silicofluoride (Na ₂ SiF ₆ ), potassium silicofluoride (K ₂ SiF ₆ ), and ammonium silicofluoride ((NH ₄ ) ₂ SiF ₆ ). The content of the fluorine-containing compound in the aqueous solution 2 is preferably 0.02 mol/L or more, more preferably 0.08 mol/L or more, in terms of F. On the other hand, the content of the fluorine-containing compound in the aqueous solution 2 is preferably 0.48 mol/L or less, more preferably 0.40 mol/L or less, in terms of F.

《Sulfuric acid》 Sulfuric acid (H ₂ SO ₄ ) is a sulfate salt, part or all of which may be sodium sulfate, calcium sulfate, ammonium sulfate, etc. The content of _sulfuric acid in aqueous solution 2 is ^preferably 0.0001 mol/L or more, more preferably 0.0003 mol/L or more, and particularly preferably 0.0010 mol/L or more, in terms of SO 4 2-. On the other hand, the content of sulfuric acid in aqueous solution 2 is preferably 0.1000 mol/L or less, and more preferably 0.0500 mol/L or less, in terms of SO ₄ ^2- .

The liquid temperature of the aqueous solution 2 is preferably 20°C or higher, more preferably 40°C or higher. On the other hand, the liquid temperature of the aqueous solution 2 is preferably 80°C or lower, more preferably 60°C or lower.

<Cathode electrolysis> Cathodic electrolysis is to precipitate metallic chromium and chromium hydrated oxide. If the current density of the cathodic electrolysis is too low, the precipitation efficiency of metallic chromium is reduced and the proportion of the chromium hydrated oxide layer tends to increase. Therefore, the current density of the cathodic electrolysis is preferably 10A/ ^dm2 or more, more preferably 15A/ ^dm2 or more, and even more preferably 20A/ ^dm2 or more.

On the other hand, if the current density of the cathodic electrolysis treatment is too high, the protrusions are rapidly precipitated, and there is a case where the height H and/or the bottom diameter D of the protrusions become excessively large. Therefore, the current density of the cathodic electrolysis treatment is preferably 200A/ ^dm2 or less, and more preferably 150A/ ^dm2 or less.

The power-on time and charge density (the product of current density and power-on time) of the cathodic electrolysis treatment are appropriately set to obtain the desired adhesion amount. [Example]

The present invention is specifically described below with reference to the following embodiments. However, the present invention is not limited to the following embodiments.

<Production of steel plates for cans> A steel plate (tempering degree: T4CA) produced with a plate thickness of 0.22 mm was subjected to normal degreasing and pickling. The steel plate was subjected to immersion treatment using aqueous solution 1, and then to cathodic electrolysis treatment using aqueous solution 2. The following Table 1 shows the details of aqueous solution 1 and aqueous solution 2 used, as well as the conditions of the immersion treatment and cathodic electrolysis treatment. However, in Comparative Example 1, since the immersion treatment was not performed, "-" is recorded in the columns of "aqueous solution 1" and "immersion treatment" in the following Table 1.

Aqueous solution 1 contains water-soluble organic compounds shown in Table 1. When the water-soluble organic compound is not a water-soluble polymer, "-" is written in the "Mw" column in Table 1. Aqueous solution 2 contains chromium trioxide (CrO ₃ ), sodium fluoride (Na ₂ SiF ₆ ) and sulfuric acid (H ₂ SO ₄ ).

The aqueous solution is circulated in a flow tank by a pump at a speed equivalent to 100 mpm. A lead electrode is used in the cathodic electrolysis. In this way, a steel plate for a tank is manufactured. The manufactured steel plate for a tank is washed with water and dried at room temperature using a blower.

<Adhesion amount, etc.> For the produced steel sheets for cans, the adhesion amount of the metal chromium layer and the adhesion amount of the chromium-converted chromium hydrate oxide layer were measured (in Table 1 below, only "adhesion amount" is recorded). Furthermore, for the protrusions of the metal chromium layer of the produced steel sheets for cans, the height H, bottom diameter D, ratio (H/D) and area ratio were measured. The measurement methods are the same as above. The results are shown in Table 1 below.

<Evaluation> The following tests were carried out on the produced steel plates for cans to evaluate weldability (weldability 1 and weldability 2). The results are shown in Table 1 below.

《Weldability 1: Heat treatment once》 The produced steel plate for cans was subjected to a heat treatment of 210℃×10 minutes (maintained at a plate temperature of 210℃ for 10 minutes), and then the contact resistance was measured. More specifically, two samples were first collected from the steel plate for cans, heat treated in a batch furnace, and then stacked after the heat treatment. Next, a DR-type 1 mass% Cr-Cu electrode (processed to have a tip diameter of 6mm and a curvature of R40mm) was used to sandwich the two stacked samples, and the pressure was set to 1kgf/ ^cm2 and maintained for 15 seconds. Then, a current of 10A was passed, and the resistance between the two samples was measured (unit: μΩ). The 10-point measurement was performed, and the average value was set as the contact resistance value. The weldability was evaluated according to the following criteria. If it was "◎◎", "◎" or "○", the weldability was evaluated as excellent. ◎◎: Contact resistance value 100μΩ or less ◎: Contact resistance value more than 100μΩ and less than 300μΩ ○: Contact resistance value more than 300μΩ and less than 500μΩ △: Contact resistance value more than 500μΩ and less than 1000μΩ ×: Contact resistance value more than 1000μΩ

《Weldability 2: Heat treatment twice》 The produced steel plate for cans was heat treated twice at 210℃ for 10 minutes (maintained at a plate temperature of 210℃ for 10 minutes), and then the contact resistance was measured. Other detailed measurement conditions and evaluation criteria are the same as those of "Weldability 1" above.

<Summary of evaluation results> As shown in Table 1 above, Invention Examples 1 to 26 are excellent in Weldability 1 and Weldability 2. In contrast, Comparative Examples 1 to 7 are at least insufficient in Weldability 2. More specifically, as follows.

《Comparative Example 1》 Comparative Example 1, which was not subjected to immersion treatment, did not form a protrusion of a metal chromium layer, and weldability 1 and weldability 2 were insufficient.

《Comparative Example 2 and Invention Examples 1 to 3》 Comparative Example 2 and Invention Examples 1 to 3 differ only in the content of the water-soluble organic compound (polyacrylic acid) in the aqueous solution 1. In Comparative Example 2, where the content of the water-soluble organic compound is 8 g/L, the ratio (H/D) is not above 0.50, and the weldability 2 is insufficient. If compared with Invention Examples 1 to 3, Invention Examples 2 to 3, where the bottom diameter D of the protrusion is 900 nm or less, have better weldability 2 than Invention Example 1, which does not meet the requirement.

《Comparative Example 3, Invention Examples 4 to 10 and Comparative Example 4》 Comparative Example 3, Invention Examples 4 to 10 and Comparative Example 4 each differ only in the weight average molecular weight (Mw) of the water-soluble organic compound (polyacrylic acid) in the aqueous solution 1. In Comparative Example 3 with Mw of 200, the height H of the protrusion is not more than 25nm, and the ratio (H/D) is not more than 0.50, so the weldability 2 is insufficient. In Comparative Example 4 with Mw of 120,000, the area ratio of the protrusion is not more than 5%, and the weldability 2 is insufficient.

Compared with Invention Examples 4 to 10, Invention Examples 5 to 9, in which the height H of the protrusion is greater than 200 nm, the ratio (H/D) is greater than 0.60, and the area ratio of the protrusion is greater than 10%, have better weldability 2 than Invention Examples 4 and 10 which do not meet these requirements.

Furthermore, among Invention Examples 5 to 9, the height H of the protrusion is greater than 500nm and less than 800nm, the bottom diameter D of the protrusion is greater than 500nm and less than 800nm, the ratio (H/D) is greater than 0.70, and the area ratio of the protrusion is greater than 20%. Invention Example 7 has better weldability 2 than Invention Examples 5 to 6 and 8 to 9 which do not satisfy at least any of the above requirements.

《Invention Examples 11-12 and Comparative Example 5》 Invention Examples 11-12 and Comparative Example 5 are the same as the above-mentioned Comparative Examples 2 and Invention Examples 1-3, except that the content of the water-soluble organic compound in the aqueous solution 1 is different. In Comparative Example 5, in which the content of the water-soluble organic compound is 120 g/L, the bottom diameter D of the protrusion is not more than 50 nm, and the weldability 2 is insufficient.

Comparing Invention Examples 11 and 12, Invention Example 11, in which the bottom diameter D of the protrusion is 200 nm or more, has better weldability 2 than Invention Example 12, which does not meet this requirement.

《Comparative Example 6, Invention Examples 13 to 17 and Comparative Example 7》 Comparative Example 6, Invention Examples 13 to 17 and Comparative Example 7 differ only in the conditions of the cathodic electrolytic treatment (current density, etc.). In Comparative Example 6 with a current density of 5 A/dm ² , the chromium-converted adhesion amount of the chromium hydrate oxide layer is not less than 50 mg/m ² , and the weldability 2 is insufficient. In Comparative Example 7 with a current density of 220 A/dm ² , the height H of the protrusion is not less than 1000 nm, and the weldability 2 is insufficient.

In comparison with Invention Examples 13 to 17, Invention Examples 14 to 16, in which the height H of the protrusion is 900 nm or less and the chromium-converted adhesion amount of the chromium hydrated oxide layer is 40 mg/m ² or less, have better weldability 2 than Invention Examples 13 and 17, which do not satisfy at least any of the above conditions. Furthermore, among Invention Examples 14 to 16, Invention Example 15, in which the height H of the protrusion is 800 nm or less and the chromium-converted adhesion amount of the chromium hydrated oxide layer is 30 mg/m ² or less, has better weldability 2 than Invention Examples 14 and 16, which do not satisfy at least any of the above conditions.

《Invention Examples 18 to 20》 Invention Examples 18 to 20 use polyethylene glycol as a water-soluble organic compound, and each differs only in its content. Invention Example 19, in which the height H of the protrusion is 500 nm or more, the ratio (H/D) is 0.70 or more, and the area ratio of the protrusion is 20% or more, has better weldability 2 than Invention Examples 18 and 20 which do not satisfy at least any of the above conditions.

《Invention Examples 21 to 26》 Invention Examples 21 to 26 use other water-soluble organic compounds. Invention Example 21, in which the height H of the protrusion is 500 nm or more and the ratio (H/D) is 0.70 or more, has better weldability 2 than Invention Examples 22 to 26 that do not satisfy at least any of the above conditions.

1: Steel plate for tank 2: Steel plate 3: Metal chromium layer 3a: Flat plate 3b: Protrusion 4: Chromium hydrate oxide layer D: Bottom diameter of protrusion H: Height of protrusion

[Fig. 1] is a schematic cross-sectional view of a steel plate for a tank according to the present embodiment.

1: Steel plate for tanks

2:Steel plate

3:Metallic chromium layer

3a: Flat plate

3b: protrusion

4: Chromium hydrate oxide layer

D: Bottom diameter of the protrusion

H: Height of the protrusion

Claims (6)

A steel plate for cans has a metal chromium layer and a chromium hydrated oxide layer on the surface of the steel plate in order from the side of the steel plate, the metal chromium layer has an adhesion amount of 50-200 mg/ ^m2 , the chromium-converted adhesion amount of the chromium hydrated oxide layer is 3-50 mg/ ^m2 , the metal chromium layer includes a flat plate portion and a protrusion provided on the flat plate portion, the protrusion has a height H of 25 nm to 1000 nm, the protrusion has a bottom diameter D of 50 nm to 1000 nm, the ratio H/D of the height H to the bottom diameter D is 0.50 or more, and the protrusion has an area ratio of 5% or more. A method for manufacturing a steel plate for a can, which is a method for manufacturing a steel plate for a can as claimed in claim 1, comprising: applying an immersion treatment to the steel plate using an aqueous solution 1, and then applying a cathodic electrolysis treatment using an aqueous solution 2; the aqueous solution 1 contains a water-soluble organic compound, and the aqueous solution 2 contains a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid. The method for manufacturing steel plates for tanks as in claim 2, wherein the content of the water-soluble organic compound in the aqueous solution 1 is greater than 10 g/L and less than 100 g/L. The method for manufacturing steel plates for cans as in claim 2, wherein the aforementioned water-soluble organic compound is a water-soluble polymer with a weight average molecular weight of 300 to 100,000. The method for manufacturing steel plates for cans as in claim 4, wherein the aforementioned water-soluble polymer is polyacrylic acid. The method for manufacturing a steel plate for a can as recited in any one of claims 2 to 5, wherein the current density of the cathodic electrolysis treatment is 10 to 200 A/dm ² .

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