Steel sheet for cans

Abstract

The present invention provides a steel sheet 1 for cans, the steel sheet being excellent in terms of coating adhesion and surface appearance. The steel sheet 1 for cans comprises, on the surface of a steel sheet 2, a chromium metal layer 3 and a hydrated chromium oxide layer 4 sequentially from the steel sheet 2 side. The adhesion amount of the chromium metal layer 3 is 50 to 200 mg / m2, and the adhesion amount of the hydrated chromium oxide layer 4 in terms of chromium is 3 to 50 mg / m2. The chromium metal layer 3 comprises a flat plate part 3a and protrusions 3b that are provided on the flat plate part 3a. The minimum value of the birth size of the protrusions 3b is -500 nm or more. The number of protrusions having a death size of not less than 0 nm but less than 750 nm is 1.0-10.0 protrusions / μm2, and the number of protrusions having a death size of 750 nm or more is 0.02 protrusion / μm2 or more. The birth size and the death size of the protrusions are determined by creating a zero-order persistence diagram with respect to the protrusions.

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 protrusions provided on the flat portion, and the protrusions have a minimum birth size of −500 nm or more and a death size of 0 nm or more but less than 750 nm, and the number of the protrusions is 1.0 to 10.0 / μm ２ and the number of the protrusions having a death size of 750 nm or more is 0.02 / μm ２ The above is the steel sheet for cans. However, the birth size and death size of the protrusions are determined by creating a zero-order persistence diagram for the protrusions.

[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; 3 is a zero-order persistence diagram of a protrusion of the steel sheet for cans; and 4 is a histogram of death size.

[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 ２The 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] (Persistence Diagram) The state of the presence of protrusions can be evaluated by creating a zero-order persistence diagram using the persistent homology method as follows. First, the protrusions are observed using a scanning electron microscope (SEM) to obtain an SEM image (a planar image of the steel sheet for cans). The SEM observation is carried out at a magnification that allows 500 or more protrusions to be confirmed within one field of view. Figure 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 together.

[0018] Image processing is performed on the obtained SEM image to extract protrusions. The protrusions may be extracted by hand painting. Next, a calculation to create a zeroth-order persistent diagram for the extracted protrusions is performed using a known persistent homology method, for example, as follows. First, using the SEM image from which the protrusions have been extracted, the boundary between the protrusions and non-protrusions is set to 0 nm, and the distance is calculated so that the further away from the boundary the protrusions become in the negative direction and the non-protrusions become in the positive direction. This obtains a distance distribution in which the boundary between the protrusions and non-protrusions is set to 0 nm. The space of this distance distribution is filled starting from the maximum value in the negative direction of the distance distribution, and the distance when a new point is born in the space is recorded as the birth size, and the distance when the points are connected is recorded as the death size. In other words, the absolute value of the birth size corresponds to the radius of the protrusion, and a smaller birth size means that the diameter of the protrusion is larger. On the other hand, as the distance between adjacent protrusions increases, the Death size also increases. Therefore, a large Death size means that the density of protrusions is low. For example, if the Birth size is -300 nm and the Death size is 500 nm, this means that a different protrusion exists at a position about 1000 nm away from a protrusion with a diameter of 600 nm.

[0019] Fig. 3 is a zero-order persistence diagram of protrusions on a steel sheet for cans. Fig. 4 is a histogram of death size, which is a histogram created for the deaths shown in Fig. 3. In Fig. 4, the numerical values ​​at the top indicate frequencies. Fig. 4 shows the frequencies per unit area for death sizes of 0 nm or more and less than 750 nm, and for death sizes of 750 nm or more.

[0020] ((Birth size)) If the birth size is too small, the protrusions will become coarse, and in that case, the effect of improving paint adhesion will not be sufficiently obtained. For this reason, in this embodiment, the minimum birth size of the protrusions (i.e., all birth sizes) is -500 nm or more, and since this results in better paint adhesion, it is preferably -450 nm or more, and more preferably -400 nm or more. There is no particular upper limit. For example, it is -1 nm or less, or it may be -3 nm or less, or it may be -5 nm or less.

[0021] ((Death size)) When there are many protrusions with a Death size of 0 nm or more and less than 750 nm, the area where the protrusions are dense increases, and the paint adhesion improves. Therefore, the number of protrusions with a Death size of 0 nm or more and less than 750 nm (number density) is 1.0 / μm ２ and the coating adhesion is better, so the number of particles is 2.0 / μm ２ More than 3.0 particles / μm is preferable. ２ The above is more preferable.

[0022] On the other hand, if there are too many protrusions with a Death size of 0 nm or more and less than 750 nm, the protrusions will overlap, and the paint adhesion will decrease. Therefore, the number (number density) of protrusions with a Death size of 0 nm or more and less than 750 nm should be 10.0 pieces / μm ２ The particle size is 9.0 particles / μm or less, and the paint adhesion is better. ２ Preferably, 8.0 particles / μm or less ２ The following is even more preferred:

[0023] When protrusions are uniformly and densely present over the entire surface of a steel sheet, the surface appearance of the steel sheet for cans is impaired. The reasons for this include the protrusions absorbing light on the short wavelength side (blue) and attenuating the reflected light, resulting in a reddish-brown color; the protrusions scattering light, reducing the overall reflectance and darkening the color tone; etc. In contrast, the presence of a mixture of areas with many protrusions and areas with few protrusions suppresses light scattering and allows for a good color tone to be obtained.

[0024] When there are many protrusions with a death size of 750 nm or more, there are many regions where the protrusions are sparse, and a good surface appearance is exhibited. Therefore, the number (number density) of protrusions with a death size of 750 nm or more is 0.02 pieces / μm ２ or more, and 0.04 particles / μm ２ The number (number density) of protrusions having a death size of 750 nm or more is preferably 0.40 / μm. ２ or less, 0.30 particles / μm ２ It may be 0.20 pieces / μm or less. ２ It may be the following:

[0025] <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.

[0026] <<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.

[0027] 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 less２ The following is more preferred:

[0028] The method for measuring the chromium-equivalent coating weight of the chromium hydrate oxide layer is as described above.

[0029] [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.

[0030] 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.

[0031] <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.

[0032] Hexavalent chromium compounds include, for example, chromium trioxide (CrO ３); potassium dichromate (K ２ Cr ２ O ７ ) and other dichromates; potassium chromate (K ２ CrO ４ The content of the hexavalent chromium compound in the aqueous solution 1 is preferably 0.14 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.

[0033] <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.

[0034] 《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.

[0035] 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.

[0036] <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.

[0037] <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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] <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.

[0042] <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.

[0043] The present invention will be specifically described below with reference to examples, although the present invention is not limited to the following examples.

[0044] <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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] <Adhesion Mass, etc.> For the produced steel sheets for cans, 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. Furthermore, for the protrusions on the metallic chromium layer of the produced steel sheets for cans, a zero-order persistence diagram was created to measure the minimum birth size, the number density of protrusions having a death size of 0 nm or more and less than 750 nm, and the number density of protrusions having a death size of 750 nm or more. The measurement methods were all as described above. The results are shown in Table 1 below.

[0049] <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.

[0050] <<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%

[0051] <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%

[0052] <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

[0053]

[0054] <Summary of Evaluation Results> As shown in Table 1 above, the steel sheets for cans of 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 cathodic electrolytic treatment C2 electricity density of 30.6 C / dm 2) in which the minimum birth size of the protrusions was -525 nm had insufficient paint adhesion. ２ The steel sheet for cans of Example 1 had insufficient paint adhesion. The number of protrusions having a death size of 0 nm or more and less than 750 nm was 0.5 / μm. 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. ２ The steel sheet for cans of Comparative Example 5 (the concentration of sulfuric acid in the aqueous solution 2 was 11% by volume) had insufficient surface appearance. ２ Comparative Example 6 (where the current density of cathodic electrolysis treatment C2 is 220.0 A / dm ２ The steel sheet for cans of Example 1 had an insufficient surface appearance. The number of protrusions having a death size of 750 nm or more was 0.01 / μm. ２ The steel sheet for cans of Comparative Example 7 (Aqueous Solution 1 was used for the immersion treatment) had an insufficient surface appearance.

[0055] 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 chromium layer includes a flat portion and a protruding portion provided on the flat portion, the protruding portion has a minimum birth size of −500 nm or more, and the number of the protruding portions having a death size of 0 nm or more and less than 750 nm is 1.0 to 10.0 pieces / μm ２ The number of the protrusions having a death size of 750 nm or more is 0.02 / μm ２ The above is a steel sheet for cans, where the birth size and death size of the protrusion are determined by creating a zero-order persistence diagram for the protrusion.

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