Steel sheet for can

Abstract

Provided is a steel sheet for a can 1 excellent in coating adhesion and surface appearance. The steel sheet for a can 1 comprises, in order from a steel sheet 2 side, a metal chromium layer 3 and a hydrated chromium oxide layer 4 which are disposed on a surface of the steel sheet 2. The amount of the metal chromium layer 3 attached is 50-200 mg / m2, and the amount of the hydrated chromium oxide layer 4 attached in terms of chromium is 3-50 mg / m2. The metal chromium layer 3 includes a flat plate part 3a and protrusions 3b provided on the flat plate part 3a. The maximum circle equivalent diameter of the protrusions 3b is 1000 nm or less. The average area of the polygonal shape obtained by performing Voronoi division on the center of gravity of the protrusions 3b is 5,000-200,000 nm2. For the circle equivalent diameter of the Voronoi polygons, D50 / DAVE is less than 1.00 and D90 / DAVE is less than 3.00, when the simple average is taken as DAVE, the 50% diameter of the cumulative distribution is taken as D50, and the 90% diameter of the cumulative distribution is taken as D90.

Description

Steel sheets for cans

[0001] The present invention relates to a steel sheet for cans.

[0002] Patent Document 1 discloses a steel sheet for cans having "a metallic chromium layer and a chromium hydrate oxide layer on the surface of the steel sheet, in this order from the steel sheet side," and further the metallic chromium layer has "granular protrusions."

[0003] International Publication No. 2017 / 098991

[0004] A steel sheet for cans having granular protrusions is expected to exhibit good paint adhesion (adhesion to paint applied to the surface of the steel sheet for cans). This is because the anchoring effect of the granular protrusions improves paint adhesion. However, when a steel sheet for cans has granular protrusions, a good surface appearance may not be obtained. This is presumably because the granular protrusions diffusely reflect visible light. Therefore, an object of the present invention is to provide a steel sheet for cans that is excellent in paint adhesion and surface appearance.

[0005] As a result of extensive research, the present inventors have found that the above object can be achieved by adopting the following configuration, and have thus completed the present invention. That is, the present invention provides the following [1]: [1] A steel sheet having, on the surface thereof, a metallic chromium layer and a chromium hydrate oxide layer, in this order from the steel sheet side, and the coating amount of the metallic chromium layer is 50 to 200 mg / m ２ and the coating amount of the chromium hydrate oxide layer is 3 to 50 mg / m ２ The metal chromium layer includes a flat portion and a protrusion portion provided on the flat portion, the protrusion portion having a maximum circle equivalent diameter of 1000 nm or less, and a Voronoi polygon obtained by performing Voronoi division on the center of gravity of the protrusion portion having an average area of ​​5,000 to 200,000 nm ２ The circle-equivalent diameter of the Voronoi polygon is D ＡＶＥ , the 50% diameter of the cumulative distribution is D ５０ , 90% diameter of cumulative distribution is D ９０ When D ５０ / D ＡＶＥ is less than 1.00, and D ９０ / D ＡＶＥ A steel sheet for cans, wherein the modulus of elasticity is less than 3.00.

[0006] According to the present invention, a steel sheet for cans having excellent paint adhesion and surface appearance can be provided.

[0007] 1 is a cross-sectional view schematically showing a steel sheet for cans, 2 is an SEM image of the steel sheet for cans, and 3 is a Voronoi diagram of the steel sheet for cans.

[0008] [Steel Sheet for Cans] Fig. 1 is a cross-sectional view that schematically shows a steel sheet for cans 1. As shown in Fig. 1, the steel sheet for cans 1 has a steel sheet 2. The steel sheet for cans 1 further has, on the surface of the steel sheet 2, a metal chromium layer 3 and a chromium hydrated oxide layer 4, in this order from the steel sheet 2 side. The metal chromium layer 3 includes a flat portion 3a in a flat plate shape that covers the steel sheet 2, and protrusions 3b provided on the flat portion 3a. The chromium hydrated oxide layer 4 is disposed on the metal chromium layer 3 so as to follow the shape of the protrusions 3b.

[0009] Hereinafter, each configuration of the steel sheet for cans of this embodiment will be described in more detail.

[0010] <Steel Plate> The type of steel plate is not particularly limited. Steel plates that are normally used as container materials (for example, low-carbon steel plates and ultra-low-carbon steel plates) can be used. The manufacturing method and material of the steel plate are also not particularly limited. They are manufactured through a normal steel billet manufacturing process followed by processes such as hot rolling, pickling, cold rolling, annealing, and temper rolling.

[0011] <Metallic Chromium Layer> A metallic chromium layer is disposed on the surface of the above-described steel sheet. The metallic chromium layer prevents the surface of the steel sheet from being exposed, thereby improving corrosion resistance.

[0012] <<Coating Mass>> The coating mass of the metal chromium layer is set to 50 mg / m because the corrosion resistance of the steel sheet for cans is excellent. ２ or more, and ２ More than 80 mg / m 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, even if the coating weight of the metal chromium layer is excessively increased, the corrosion resistance effect saturates, and it is therefore uneconomical. For this reason, the coating weight of the metal chromium layer is set to 200 mg / m ２ or less than 180 mg / m ２ Preferably, 160 mg / m or less ２ The following is more preferred:

[0013] (Method for Measuring Coating Weight) The coating weight of the metallic chromium layer and the coating weight of the chromium hydrated oxide layer described below in terms of chromium are measured as follows. First, the chromium amount (total chromium amount) of a steel sheet for cans on which a metallic chromium layer and a chromium hydrated oxide layer have been formed 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 solution of sodium hydroxide (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 terms of chromium.

[0014] Such a metal chromium layer includes a flat portion and protruding portions provided on the flat portion. Next, each of these portions included in the metal chromium layer will be described in detail.

[0015] <<Flat Plate Portion>> The flat plate portion of the metallic chromium layer mainly covers the surface of the steel sheet and improves corrosion resistance. The flat plate portion of the metallic chromium layer preferably has a sufficient thickness so that when the steel sheets for cans inevitably come into contact with each other during handling, the protrusions provided on the surface layer do not destroy the flat plate portion and expose the steel sheet. For the reason that the corrosion resistance of the steel sheet for cans is excellent, the coating amount of the flat plate portion of the metallic chromium layer is preferably 10 mg / m ２ More than 30 mg / m ２ More preferably, 40 mg / m or more ２ The above is more preferable.

[0016] <<Protrusions>> The protrusions of the metal chromium layer are formed on the surface of the above-mentioned flat plate portion and improve paint adhesion. This is thought to be because the anchor effect of the protrusions improves the adhesion of paint applied to the surface of the steel sheet for cans.

[0017] (Maximum equivalent circle diameter) If the protrusions are too large, the effect of improving paint adhesion cannot be sufficiently obtained. For this reason, the maximum equivalent circle diameter of the protrusions is 1000 nm or less, and is preferably 900 nm or less, more preferably 800 nm or less, for the reason that paint adhesion is better. The lower limit is not particularly limited, and is, for example, 10 nm.

[0018] The maximum equivalent circle diameter of the protrusions is determined as follows. First, a scanning electron microscope (SEM) is used to observe the protrusions at a magnification of 50,000 times to obtain an SEM image (a planar image of the steel sheet for cans). The obtained SEM image is subjected to image processing to determine the number of pixels of each protrusion. The total value of the determined number of pixels is multiplied by the size (area) of each pixel to calculate the area A of each protrusion. Furthermore, π(d / 2) ２ = A. The maximum value of the equivalent circular diameter d of each protrusion in any five fields of view is determined as the maximum equivalent circular diameter (unit: nm) of the protrusion.

[0019] (Voronoi polygons) The presence of protrusions can be evaluated by performing Voronoi tessellation. Voronoi tessellation is a method of dividing a plane on which a plurality of points (generator points) are arranged by drawing perpendicular bisectors to lines connecting adjacent generator points and dividing the plane by areas (Voronoi polygons) enclosed by these perpendicular bisectors. A diagram created by Voronoi tessellation (a diagram including Voronoi polygons) is also called a Voronoi diagram (see Figure 3 described below). When Voronoi tessellation is performed using the center of gravity of the protrusions as the generator point, it can be said that the smaller the area of ​​the resulting Voronoi polygon, the higher the density of the protrusions.

[0020] Voronoi tessellation is performed as follows. First, a scanning electron microscope (SEM) is used to observe the protrusions, and an SEM image (a planar image of the steel sheet for cans) is obtained. The SEM observation is performed at a magnification that allows 500 or more protrusions to be identified within one field of view. FIG. 2 is an SEM image of the steel sheet for cans, and it can be seen that regions with sparse protrusions and regions with dense protrusions are mixed. The obtained SEM image is subjected to image processing. As a result, the protrusions and their centroids (centers of gravity) are extracted, and Voronoi tessellation is performed using the extracted centroids as generating points to create a Voronoi diagram including Voronoi polygons. FIG. 3 is a Voronoi diagram of the steel sheet for cans, and more specifically, it is a Voronoi diagram created by Voronoi tessellating the SEM image of FIG. 2. For each Voronoi polygon included in the Voronoi diagram, the area A is calculated, and further, π(d / 2) ２ The circle-equivalent diameter d is calculated using the relational expression ∑ = A. The average of the areas A of the Voronoi polygons in any five fields of view is taken as the average area of ​​the Voronoi polygons (unit: nm ２ ) is calculated as follows. In addition, the circle equivalent diameter d of each Voronoi polygon in any five fields of view is calculated as a simple average D ＡＶＥ , 50% diameter of cumulative distribution D ５０ , and the 90% diameter of the cumulative distribution D ９０ Ask for.

[0021] ((Average Area)) The smaller the area of ​​the Voronoi polygon, the better the paint adhesion. This is because the smaller the area of ​​the Voronoi polygon, the higher the density of the protrusions and the stronger the anchor effect. For this reason, the average area of ​​the Voronoi polygon is 200,000 nm ２ 150,000 nm or less, and because paint adhesion is better ２ Preferably, 100,000 nm or less ２ The following is more preferred:

[0022] On the other hand, if the area of ​​the Voronoi polygon is too small, the protrusions will overlap, resulting in insufficient paint adhesion. ２ 7,000 nm is preferred because it has better paint adhesion. ２More than 10,000 nm is preferable. ２ The above is more preferable.

[0023] ((Circle equivalent diameter)) When protrusions are uniformly and densely present over the entire surface of a steel sheet, they impair the surface appearance of the steel sheet for cans. The reasons for this include the fact that the protrusions absorb light on the short wavelength side (blue) and the reflected light is attenuated, resulting in a reddish-brown color; the protrusions scatter light, reducing the overall reflectance and darkening the color tone; etc. In contrast, when there are both areas with many protrusions and areas with few protrusions, light scattering is suppressed and a good color tone can be obtained.

[0024] The more the distribution of the equivalent circle diameters of the Voronoi polygons deviates from the normal distribution, the more non-uniform the distribution of the protrusions (the distribution is a mixture of sparse and dense protrusions), and in this case, as described above, the surface appearance is excellent. ５０ / D ＡＶＥ is less than 1.00, and is more preferably less than 0.95 because the surface appearance is better. ９０ / D ＡＶＥ is less than 3.00, and more preferably less than 2.50 because this results in a better surface appearance.

[0025] D ５０ / D ＡＶＥ and D ９０ / D ＡＶＥ The lower limit of is not particularly limited. ５０ / D ＡＶＥ is, for example, 0.15 or more, may be 0.35 or more, or may be 0.55 or more. ９０ / D ＡＶＥ is, for example, 0.90 or more, may be 1.20 or more, or may be 1.50 or more.

[0026] The circle equivalent diameter of the Voronoi polygon is D ＡＶＥ is not particularly limited. The circle equivalent diameter D of the Voronoi polygon ＡＶＥ is, for example, 50 nm or more, may be 100 nm or more, or may be 150 nm or more. ＡＶＥis, for example, 350 nm or less, may be 300 nm or less, or may be 250 nm or less.

[0027] The circle equivalent diameter of the Voronoi polygon is D ５０ is not particularly limited. The circle equivalent diameter D of the Voronoi polygon ５０ is, for example, 30 nm or more, may be 70 nm or more, or may be 100 nm or more. ５０ is, for example, 300 nm or less, may be 250 nm or less, or may be 200 nm or less.

[0028] The circle equivalent diameter of the Voronoi polygon is D ９０ is not particularly limited. The circle equivalent diameter D of the Voronoi polygon ９０ is, for example, 50 nm or more, may be 150 nm or more, or may be 200 nm or more. ９０ is, for example, 550 nm or less, may be 500 nm or less, or may be 450 nm or less.

[0029] <Hydrated Chromium Oxide Layer> Hydrated chromium oxide is precipitated on the surface of the steel sheet simultaneously with metallic chromium to improve corrosion resistance. Examples of hydrated chromium oxide include chromium oxide and chromium hydroxide.

[0030] <<Coating Amount>> In order to ensure the corrosion resistance of the steel sheet for cans, the coating amount of the chromium hydrate oxide layer in terms of chromium is 3 mg / m ２ or more, and ２ The above is preferable.

[0031] On the other hand, chromium hydrate oxide has a lower electrical conductivity than metallic chromium, and if the amount is too large, it will cause excessive resistance during welding, leading to the generation of dust and splashes and various welding defects such as blowholes due to over-fusion welding, which may result in poor weldability of the steel sheet for cans. ２ or less than 40 mg / m ２ Preferably, 30 mg / m or less ２ The following is more preferred:

[0032] The method for measuring the chromium-equivalent coating weight of the chromium hydrate oxide layer is as described above.

[0033] [Manufacturing Method of Steel Sheet for Cans] Next, a method for manufacturing the steel sheet for cans of the present embodiment will be described. Briefly, a steel sheet is subjected to cathodic electrolysis C1 using an aqueous solution 1 containing a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid, then subjected to immersion treatment in an aqueous solution 2 containing sulfuric acid at a low concentration, and thereafter subjected to cathodic electrolysis C2 again using the aqueous solution 1.

[0034] In the cathodic electrolysis treatment C1, flat metallic chromium is deposited on the surface of the steel sheet. Then, an immersion treatment is performed (i.e., the steel sheet on which the flat metallic chromium has been formed is immersed in an aqueous solution 2 containing sulfuric acid), thereby partially activating the flat metallic chromium. Since the sulfuric acid concentration of the aqueous solution 2 is low, the activation state differs depending on the crystal orientation of the steel sheet. Next, when the cathodic electrolysis treatment C2 is performed, protrusions are formed starting from the activated portions of the flat metallic chromium, but since the activation state differs depending on the crystal orientation of the steel sheet, the distribution of the protrusions is non-uniform.

[0035] <Aqueous Solution 1> Aqueous solution 1 used in cathodic electrolysis contains a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid. In aqueous solution 1, the fluorine-containing compound and sulfuric acid exist in dissociated states (i.e., in the form of fluoride ions, sulfate ions, and hydrogen sulfate ions). These act as catalysts involved in the reduction reaction of hexavalent chromium ions present in aqueous solution 1, which proceeds during cathodic electrolysis. By including a fluorine-containing compound and sulfuric acid in aqueous solution 1 used in cathodic electrolysis, the chromium-equivalent deposition weight of the chromium hydrous oxide layer on the resulting steel sheet for cans can be reduced. Although the reason for this is unclear, it is thought that the increase in the amount of anions during electrolysis reduces the amount of chromium hydrous oxide produced.

[0036] Hexavalent chromium compounds include, for example, chromium trioxide (CrO ３ ); potassium dichromate (K ２ Cr ２ O７ ) and other dichromates; potassium chromate (K ２ CrO ４ The content of the hexavalent chromium compound in the aqueous solution 1 is preferably 0.14 mol / L or more, more preferably 0.30 mol / L or more, in terms of the amount of Cr. On the other hand, the content of the hexavalent chromium compound in the aqueous solution 1 is preferably 3.00 mol / L or less, more preferably 2.50 mol / L or less, in terms of the amount of Cr.

[0037] <Fluorine-containing compound> Examples of the fluorine-containing compound 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 SiF ６ ), 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 0.02 mol / L or more, more preferably 0.08 mol / L or more, in terms of the amount of F. On the other hand, the content of the fluorine-containing compound in the aqueous solution 1 is preferably 0.48 mol / L or less, more preferably 0.40 mol / L or less, in terms of the amount of F.

[0038] 《Sulfuric acid》 Sulfuric acid (H ２ SO ４ ) may be partly or entirely sulfates such as sodium sulfate, calcium sulfate, ammonium sulfate, etc. The content of sulfuric acid in aqueous solution 1 is 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, and more preferably 0.0500 mol / L or less.

[0039] 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.

[0040] <Cathode electrolysis treatment C1> The cathode electrolysis treatment C1 deposits flat metallic chromium and chromium hydrate oxide. If the current density of the cathode electrolysis treatment C1 is too low, the deposition efficiency of metallic chromium decreases, and the proportion of the chromium hydrate oxide layer tends to increase. For this reason, the current density of the cathode electrolysis treatment C1 is set to 10.0 A / dm ２ More than 15.0 A / dm ２ On the other hand, the upper limit is not particularly limited, and the current density of the cathodic electrolysis treatment C1 is, for example, 200.0 A / dm ２ or less, 150.0 A / dm ２ The following is preferred: The current application time and the electricity density (product of current density and current application time) of the cathodic electrolytic treatment C1 are appropriately set to obtain the target deposition amount.

[0041] <Aqueous Solution 2> The aqueous solution 2 used in the immersion treatment is, for example, a mixture of water and sulfuric acid (H ２ SO ４ ) is a mixture of sulfuric acid and chromium. If the concentration of sulfuric acid in aqueous solution 2 is too high, the activation of the above-mentioned flat metallic chromium tends to proceed excessively in all crystal orientations of the steel sheet. As a result, protrusions may be precipitated uniformly and entirely in all crystal orientations by the subsequent cathodic electrolysis treatment C2, resulting in poor surface appearance. For this reason, the concentration of sulfuric acid in aqueous solution 2 is less than 10% by volume, and preferably less than 8% by volume. On the other hand, from the viewpoint of achieving the necessary activation, the concentration of sulfuric acid in aqueous solution 2 is preferably 1% by volume or more, more preferably 2% by volume or more, even more preferably 3% by volume or more, and particularly preferably 4% by volume or more.

[0042] It is preferable that aqueous solution 2 contains substantially only sulfuric acid in water. In other words, it is preferable that no components other than sulfuric acid are added to water when preparing aqueous solution 2. If aqueous solution 2 also contains components other than sulfuric acid (for example, the above-mentioned hexavalent chromium compound and fluorine-containing compound), the hexavalent chromium compound and F also activate the steel sheet, and therefore, in the subsequent cathodic electrolytic treatment C2, protrusions are likely to precipitate uniformly and over the entire surface, resulting in poor surface appearance.

[0043] Specifically, for example, the content of the hexavalent chromium compound in aqueous solution 2 is preferably 0.010 mol / L or less, more preferably 0.005 mol / L or less, and even more preferably 0.001 mol / L or less, in terms of the Cr amount. Similarly, the content of the fluorine-containing compound in aqueous solution 2 is preferably 0.010 mol / L or less, more preferably 0.005 mol / L or less, and even more preferably 0.001 mol / L or less, in terms of the F amount.

[0044] 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.

[0045] <Immersion Treatment> In the immersion treatment, the steel sheet subjected to the cathodic electrolytic treatment C1 (the steel sheet on which the flat metallic chromium and chromium hydrate oxide are formed) is immersed in the aqueous solution 2. As a result, the flat metallic chromium is partially activated, as described above. If the time for immersing the steel sheet in the aqueous solution 2 (immersion time) is too short, activation may not proceed sufficiently, and protrusions may not be easily formed. Therefore, the immersion time is 0.10 seconds or more, preferably 0.20 seconds or more, and more preferably 0.30 seconds or more. On the other hand, if the immersion time is too long, activation may proceed excessively, resulting in a structure in which the protrusions are locally densely packed, which may result in poor paint adhesion. Therefore, the immersion time is 2.00 seconds or less, preferably 1.50 seconds or less, and more preferably 1.00 seconds or less.

[0046] <Cathode Electrolysis Treatment C2> In the cathode electrolysis treatment C2, protrusions of the metallic chromium layer are formed. If the current density of the cathode electrolysis treatment C2 is too low, the deposition efficiency of the metallic chromium decreases, and the proportion of the chromium hydrate oxide layer tends to increase. For this reason, the current density of the cathode electrolysis treatment C2 is set to 10.0 A / dm ２ More than 15.0 A / dm ２ On the other hand, if the current density of the cathodic electrolysis treatment C2 is too high, protrusions may precipitate suddenly, damaging the surface appearance. For this reason, the current density of the cathodic electrolysis treatment C2 is set to 200.0 A / dm ２ or less, 150.0 A / dm ２ If the electrical charge density of the cathodic electrolysis treatment C2 is too high, the diameter of the protrusions may become too large. Therefore, the electrical charge density of the cathodic electrolysis treatment C2 is preferably 30.0 C / dm or less. ２ or less, and ２ Preferably, 20.0 C / dm or less ２ The lower limit is not particularly limited, and for example, 1.0 C / dm ２ and 3.0 C / dm ２ The energization time of the cathodic electrolysis treatment C2 is appropriately set based on the current density and the charge density.

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

[0048] <Preparation of Steel Sheet for Can> A steel sheet (temper: T4CA) manufactured to 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, then to immersion treatment using aqueous solution 2, and then to cathodic electrolysis C2 using aqueous solution 1. Details of the aqueous solutions 1 and 2 used, as well as the conditions for the cathodic electrolysis C1, immersion treatment, and cathodic electrolysis C2, are shown in Table 1 below.

[0049] However, in Comparative Example 1, only cathodic electrolysis treatment C1 using aqueous solution 1 was performed. Therefore, "-" is entered in the columns for aqueous solution 2 and cathodic electrolysis treatment C2. In Comparative Example 7, immersion treatment was performed using aqueous solution 1 instead of aqueous solution 2.

[0050] Aqueous solution 1 is a solution of chromium trioxide (CrO ３ ), sodium silicofluoride (Na ２ SiF ６ ) and sulfuric acid (H ２ SO ４ Aqueous solution 2 was prepared by adding sulfuric acid (H ２ SO ４ ) was added only.

[0051] The aqueous solution was circulated in a flow cell at a speed equivalent to 100 mpm using a pump. A lead electrode was used for the cathodic electrolysis. Thus, a steel sheet for a can was produced. After production, the steel sheet for a can was washed with water and dried at room temperature using a blower.

[0052] <Adhesion mass, etc.> The adhesion mass of the metallic chromium layer and the adhesion mass of the chromium hydrate oxide layer in terms of chromium (simply referred to as "adhesion mass" in Table 1 below) were measured for the prepared steel sheets for cans. Furthermore, the maximum equivalent circle diameter of the protrusions on the metallic chromium layer of the prepared steel sheets for cans was measured. Furthermore, Voronoi tessellation was performed to calculate the average area, D ＡＶＥ , D ５０ , D ９０ , D ５０ / D ＡＶＥ , and D ９０ / D ＡＶＥ The measurement methods were as described above. The results are shown in Table 1 below.

[0053] <Evaluation> The produced steel sheets for cans were subjected to the following tests to evaluate paint adhesion (primary paint adhesion and primary paint adhesion) and surface appearance. The results are shown in Table 1 below.

[0054] <<Primary Paint Adhesion>> A paint (epoxy-phenolic resin) was applied to the surface of the prepared steel sheet for cans, and then heated twice at 210°C for 10 minutes. Subsequently, cross-cut cuts were made in the paint at 1 mm intervals, deep enough to reach the steel sheet, and the paint was peeled off using tape, and the peeling state was observed. The peeled area rate was evaluated according to the following criteria. "A," "B," or "C" was evaluated as excellent paint adhesion. A: Peeled area rate 0% B: Peeled area rate more than 0% but 2% or less C: Peeled area rate more than 2% but 5% or less D: Peeled area rate more than 5% but 30% or less E: Peeled area rate more than 30%

[0055] <Secondary Paint Adhesion> A paint (epoxy-phenolic resin) was applied to the surface of the prepared steel sheet for cans, and then the sheet was heated twice at 210°C for 10 minutes. Subsequently, slits were made in the paint in a checkerboard pattern at 1 mm intervals, reaching down to the steel sheet, and then the sheet was retorted at 125°C for 30 minutes. After drying, the paint was peeled off using tape, and the peeling state was observed. The peeled area rate was evaluated according to the following criteria. A rating of "A," "B," or "C" was considered to indicate excellent paint adhesion. A: Peeled area rate 0% B: Peeled area rate greater than 0% and 2% or less C: Peeled area rate greater than 2% and 5% or less D: Peeled area rate greater than 5% and 30% or less E: Peeled area rate greater than 30%

[0056] <Surface Appearance> The b value of the produced steel sheets for cans was measured based on the Hunter color difference measurement method specified in the old JIS Z 8730 (1980), and the surface appearance was evaluated according to the following criteria. If the surface appearance was rated as "A" or "B," it was evaluated as excellent. A: b value ≦ 0 B: 0 < b value ≦ 2.0 C: 2.0 < b value ≦ 3.0 D: 3.0 < b value

[0057]

[0058] <Summary of Evaluation Results> As shown in Table 1 above, the steel sheets for cans of Inventive Examples 1 to 23 were good in both paint adhesion and surface appearance. The steel sheet for cans of Comparative Example 1 (without immersion treatment, without cathodic electrolytic treatment C2) which had no protrusions had insufficient paint adhesion. The steel sheet for cans of Comparative Example 2 (without immersion treatment, without cathodic electrolytic treatment C2) in which the maximum circle equivalent diameter of the protrusions was 1050 nm (with an electricity density of 30.6 C / dm２ The steel sheet for cans of Example 1 had insufficient paint adhesion. The average area of ​​the Voronoi polygons of the protrusions was 201,000 nm ２ The steel sheet for cans of Comparative Example 3 (the immersion time of the immersion treatment was 0.05 seconds) had insufficient paint adhesion. ２ The steel sheet for cans of Comparative Example 4 (the immersion time of the immersion treatment was 2.10 seconds) had insufficient paint adhesion. ５０ / D ＡＶＥ The steel sheet for cans of Comparative Example 5 (the concentration of sulfuric acid in the aqueous solution 2 was 11% by volume), in which the D of the Voronoi polygons of the protrusions was 1.05, had insufficient surface appearance. ９０ / D ＡＶＥ Comparative Example 6 (where the current density of cathodic electrolysis treatment C2 is 220.0 A / dm ２ The surface appearance of the steel sheet for cans of the above test piece was insufficient. ５０ / D ＡＶＥ The steel sheet for cans of Comparative Example 7 (using Aqueous Solution 1 for the immersion treatment) in which the value of the surface roughness was 1.07 was insufficient in appearance.

[0059] 1: Steel plate for cans 2: Steel plate 3: Metallic chromium layer 3a: Flat plate portion 3b: Projection portion 4: Chromium hydrated oxide layer

Claims

1. A steel sheet has a surface having, in order from the steel sheet side, a metallic chromium layer and a chromium hydrate oxide layer, and the coating amount of the metallic chromium layer is 50 to 200 mg / m ２ The chromium-equivalent coating amount of the chromium hydrate oxide layer is 3 to 50 mg / m ２ the metal chrome layer includes a flat portion and a protrusion portion provided on the flat portion, the protrusion portion has a maximum circle equivalent diameter of 1000 nm or less, and the average area of ​​a Voronoi polygon obtained by performing Voronoi division on the center of gravity of the protrusion portion is 5,000 to 200,000 nm ２ The circle equivalent diameter of the Voronoi polygon is expressed as a simple average D ＡＶＥ , the 50% diameter of the cumulative distribution is D ５０ , 90% diameter of cumulative distribution is D ９０ Then, D ５０ / D ＡＶＥ is less than 1.00, and D ９０ / D ＡＶＥ A steel sheet for cans, wherein the modulus of elasticity is less than 3.00.

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