Steel sheets for cans

Abstract

The present invention provides a steel sheet for cans 1 having excellent paint adhesion and surface appearance. The steel sheet for cans 1 has, on the surface of a steel sheet 2, a metallic chromium layer 3 and a chromium hydrate oxide layer 4, in this order from the steel sheet 2 side. The coating weight of the metallic chromium layer 3 is 50 to 200 mg / m 2 The chromium-equivalent coating amount of the chromium hydrate oxide layer 4 is 3 to 50 mg / m 2 The metal chromium layer 3 includes a flat portion 3a and protruding portions 3b provided on the flat portion 3a. The minimum value of the birth size of the protruding portions 3b is -500 nm or more. The number of 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 protrusions with a death size of 750 nm or more is 0.02 / μm 2 The birth size and death size of the protrusion are calculated by creating a zero-order persistence diagram for the protrusion.

Description

[Technical Field]

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

[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 having "granular protrusions." [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 098991 Summary of the Invention [Problem to be solved by the invention]

[0004] Steel sheets for cans having granular protrusions are expected to exhibit good paint adhesion (adhesion to paint applied to the surface of the steel sheet for cans) because the anchoring effect of the granular protrusions improves paint adhesion. However, when steel sheets for cans have granular protrusions, a good surface appearance may not be obtained in some cases, 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. [Means for solving the problem]

[0005] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention. That is, the present invention provides the following [1]. [1] A steel sheet has a surface having 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 2and 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 That's it for can steel sheets. However, the birth size and death size of the protrusion are determined by creating a zero-order persistence diagram for the protrusion. [Effects of the Invention]

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

[0007] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a steel sheet for cans. [Figure 2] This is an SEM image of steel sheet for cans. [Figure 3] 1 is a zero-order persistence diagram of a protrusion on a steel sheet for cans. [Figure 4] This is a histogram of death size. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Steel sheets for cans] FIG. 1 is a cross-sectional view schematically showing a steel sheet for cans 1. As shown in FIG. As shown in Fig. 1, steel sheet for cans 1 has a steel sheet 2. Steel sheet for cans 1 further has, on the surface of the steel sheet 2, a metal chromium layer 3 and a chromium hydrate oxide layer 4 in this order from the steel sheet 2 side. The metal chromium layer 3 includes a flat plate portion 3a that covers the steel plate 2 and protruding portions 3b provided on the flat plate portion 3a. The chromium hydrate oxide layer 4 is disposed on the metal chromium layer 3 so as to follow the shape of the protruding portions 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 sheet is not particularly limited. Steel sheets that are normally used as container materials (for example, low-carbon steel sheets, ultra-low-carbon steel sheets) can be used. The manufacturing method and material of the steel sheet 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 chrome layer> A metal chromium layer is disposed on the surface of the steel sheet, which prevents the surface of the steel sheet from being exposed, thereby improving corrosion resistance.

[0012] Adhesion amount The coating weight of the metal chromium layer is 50 mg / m because it provides excellent corrosion resistance to steel sheets for cans. 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, making it 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 Less than 160 mg / m 2 The following is more preferred:

[0013] (Method for measuring adhesion amount) The deposition weight of the metallic chromium layer and the deposition weight in chromium equivalent of the hydrated chromium oxide layer described below are measured as follows. First, the chromium content (total chromium content) of a steel sheet for cans on which a metallic chromium layer and a chromium hydrate 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 sodium hydroxide solution (liquid temperature: 90°C) for 10 minutes, and then the chromium content (chromium content after alkali treatment) is measured again using the X-ray fluorescence analyzer. The chromium content after alkali treatment is defined as the deposition amount of the metallic chromium layer. Next, (amount of alkali-soluble chromium) = (total amount of chromium) - (amount of chromium after alkali treatment) is calculated, and the amount of alkali-soluble chromium is taken as the amount of deposition of the chromium hydrous oxide layer in terms of chromium.

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

[0015] 《Flat plate part》 The flat portion of the metal chromium layer mainly covers the surface of the steel plate and improves corrosion resistance. The flat portion of the metal 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 portion and expose the steel sheet. Because the corrosion resistance of steel sheets for cans is excellent, the coating weight of the metal chromium layer on the flat plate is 10 mg / m 2 More than 30 mg / m is preferable. 2 More preferably, 40 mg / m or more 2 The above is more preferable.

[0016] 《Protrusion》 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 existence of the protrusions can be evaluated by creating a zero-order persistent 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 for confirmation of 500 or more protrusions within one field of view. Figure 2 is an SEM image of a steel sheet for cans, and it can be seen that there are both areas with sparse protrusions and areas with dense protrusions.

[0018] The obtained SEM image is subjected to image processing to extract protrusions, which may be extracted by hand painting. Next, for the extracted protrusions, a calculation for creating a zero-order persistent diagram is performed using a known persistent homology method, for example, as follows. First, using an SEM image in which protrusions have been extracted, the boundary between protrusions and non-protrusions is set to 0 nm, and the distance is calculated so that the distance increases in the negative direction for protrusions and in the positive direction for non-protrusions as the distance increases from the boundary. This results in a distance distribution in which the boundary between protrusions and non-protrusions is set to 0 nm. The space of this distance distribution is then filled starting from the maximum negative value 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. That is, the absolute value of the birth size corresponds to the radius of the protrusion, and a small birth size means that the diameter of the protrusion is large. On the other hand, the Death size increases as the spacing between adjacent protrusions increases. Therefore, a large Death size means a low density of protrusions. 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] Figure 3 is a zero-order persistence diagram of a protrusion on a steel sheet for cans. FIG. 4 is a histogram of Death size, which is a histogram created for the Deaths (Death sizes) shown in FIG. In Figure 4, the numerical values ​​at the top indicate the frequency. Figure 4 shows the frequency 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 become coarse, and in this case, the effect of improving paint adhesion cannot be sufficiently obtained. For this reason, in this embodiment, the minimum birth size of the protrusions (ie, all birth sizes) is -500 nm or more, and is preferably -450 nm or more, more preferably -400 nm or more, because this provides better paint adhesion. The upper limit is not particularly limited, and may be, for example, −1 nm or less, −3 nm or less, or −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 with dense protrusions increases, improving paint adhesion. 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 2.0 particles / 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 of protrusions with a Death size of 0 nm or more and less than 750 nm (number density) is 10.0 / μm2 9.0 particles / μm or less, which is why the paint adhesion is better. 2 Preferably less than 8.0 particles / μm 2 The following is even more preferred:

[0023] 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. This is because the protrusions absorb light on the short wavelength side (blue), attenuating the reflected light and causing a reddish-brown color; the protrusions scatter light, reducing the overall reflectance and causing a darker color; etc. In contrast, by having a mixture of areas with many protrusions and areas with few protrusions, light scattering is suppressed and good color tone can 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 the surface appearance is good. Therefore, the number of protrusions with a death size of 750 nm or more (number density) is 0.02 / μm 2 or more, 0.04 particles / μm 2 The above is preferable. The upper limit is not particularly limited. The number (number density) of protrusions having a death size of 750 nm or more is, for example, 0.40 / μm 2 less than 0.30 particles / μm 2 It may be 0.20 pieces / μm or less. 2 It may be the following:

[0025] <Chromium hydrate oxide layer> The chromium hydrate oxide is precipitated on the surface of the steel sheet simultaneously with metallic chromium, thereby improving corrosion resistance. Examples of the chromium hydrate oxide include chromium oxide and chromium hydroxide.

[0026] Adhesion amount In order to ensure the corrosion resistance of steel sheets for cans, the chromium equivalent coating weight of the chromium hydrate oxide layer 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, which may lead to the generation of dust and splashes and various welding defects such as blowholes due to over-fusion welding, and may result in poor weldability of the steel sheet for cans. The coating weight of the chromium hydrated oxide layer is set at 50 mg / m2 in terms of chromium because it provides excellent weldability for can steel sheets. 2 ≤ 40 mg / m 2 Less than 30 mg / m 2 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 sheets for cans] Next, a method for producing the steel sheet for cans of the present embodiment will be described. In summary, the steel sheet is subjected to cathodic electrolysis C1 using aqueous solution 1 containing a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid, then subjected to immersion treatment using aqueous solution 2 containing a low concentration of sulfuric acid, and then subjected to cathodic electrolysis C2 using aqueous solution 1 again.

[0030] In the cathodic electrolysis treatment C1, flat plate-like metallic chromium is deposited on the surface of the steel sheet. Thereafter, an immersion treatment is carried out (i.e., the steel sheet on which the flat metallic chromium has been formed is immersed in aqueous solution 2 containing sulfuric acid), thereby partially activating the flat metallic chromium. At this time, since the sulfuric acid concentration of aqueous solution 2 is low, differences in the activation state occur depending on the crystal orientation of the steel sheet. Next, when the cathodic electrolytic 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 becomes uneven.

[0031] <Aqueous solution 1> The aqueous solution 1 used in the cathodic electrolysis treatment contains a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid. In aqueous solution 1, the fluorine-containing compounds and sulfuric acid exist in dissociated states (i.e., in the form of fluoride ions, sulfate ions, and hydrogen sulfate ions), which act as catalysts involved in the reduction reaction of hexavalent chromium ions present in aqueous solution 1, which proceeds during cathodic electrolysis. The aqueous solution 1 used in the cathodic electrolysis contains a fluorine-containing compound and sulfuric acid, which reduces the chromium-equivalent deposition amount of the chromium hydrous oxide layer on the resulting steel sheet for cans. The reason for this is not clear, but it is thought that the increase in the amount of anions during the electrolysis reduces the amount of chromium hydrous oxide produced.

[0032] 《Hexavalent chromium compounds》 Examples of hexavalent chromium compounds include chromium trioxide (CrO3); dichromates such as potassium dichromate (K2Cr2O7); chromates such as potassium chromate (K2CrO4); and the like. The content of the hexavalent chromium compound in the aqueous solution 1 is preferably 0.14 mol / L or more, and 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, and more preferably 2.50 mol / L or less, in terms of the amount of Cr.

[0033] Fluorine-containing compounds Examples of fluorine-containing compounds include hydrofluoric acid (HF), potassium fluoride (KF), sodium fluoride (NaF), hydrofluosilicic acid (H2SiF6), and salts of hydrofluosilicic acid. Examples of the salt of hydrosilicofluoric acid include sodium silicofluoride (Na2SiF6), potassium silicofluoride (K2SiF6), and ammonium silicofluoride ((NH4)2SiF6). 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》 The sulfuric acid (H2SO4) may be partially or entirely in the form of a sulfate such as sodium sulfate, calcium sulfate, or ammonium sulfate. The sulfuric acid content in aqueous solution 1 is SO4 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. On the other hand, the content of sulfuric acid in aqueous solution 1 is SO4 2- The amount is preferably 0.1000 mol / L or less, 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] <Cathodic electrolysis treatment C1> The cathodic electrolysis C1 deposits plate-like metallic chromium and hydrated chromium oxide. If the current density of the cathodic 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. Therefore, the current density of the cathodic electrolysis treatment C1 is 10.0 A / dm 2 More than 15.0A / dm is preferable. 2 The above is more preferable. On the other hand, there is no particular upper limit, and the current density of the cathodic electrolysis treatment C1 is, for example, 200.0 A / dm 2 less than 150.0A / dm 2 The following is preferred: The current application time and 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 (H2SO4). If the concentration of sulfuric acid in the 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, which may result in the subsequent cathodic electrolytic treatment C2 resulting in the deposition of protrusions uniformly and entirely in all crystal orientations, resulting in poor surface appearance. Therefore, the concentration of sulfuric acid in the aqueous solution 2 is less than 10% by volume, and preferably less than 8%. On the other hand, from the viewpoint of causing the necessary activation, the concentration of sulfuric acid in the 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 the aqueous solution 2 contains substantially only sulfuric acid in water. In other words, it is preferable that when preparing the aqueous solution 2, no components other than sulfuric acid are added to the water. If the aqueous solution 2 contains components other than sulfuric acid (for example, the above-mentioned hexavalent chromium compounds and fluorine-containing compounds), the hexavalent chromium compounds 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 the 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 amount of Cr. Similarly, the content of the fluorine-containing compound in the 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 amount of F.

[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 that has been subjected to the cathodic electrolysis 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, making it difficult to form protrusions. 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] <Cathodic electrolysis treatment C2> In the cathodic electrolysis treatment C2, protrusions of the metal chromium layer are formed. If the current density of the cathodic electrolysis treatment C2 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 cathodic electrolysis treatment C2 is set to 10.0 A / dm 2 More than 15.0A / dm is preferable. 2 The above is more preferable. 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 less than 150.0A / dm 2 The following is preferred: Furthermore, 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 set to 30.0 C / dm 2 is less than 25.0C / dm 2 Less than 20.0C / dm is preferable. 2 The lower limit is not particularly limited, and for example, 1.0 C / dm 2 and 3.0C / dm 2 is preferred. The energization time of the cathodic electrolysis treatment C2 is appropriately set based on the current density and the charge density. [Example]

[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] <Production of steel sheets for cans> Steel sheets (temper: T4CA) manufactured to a thickness of 0.22 mm were subjected to normal 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 thereafter 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 treatment C1, immersion treatment, and cathodic electrolysis treatment C2, are shown in Table 1 below.

[0045] However, in Comparative Example 1, only the cathodic electrolysis treatment C1 was carried out using Aqueous Solution 1. For this reason, "-" is entered in the columns for Aqueous Solution 2 and Cathodic Electrolysis Treatment C2. In Comparative Example 7, the immersion treatment was carried out using Aqueous Solution 1 instead of Aqueous Solution 2.

[0046] Aqueous solution 1 was prepared by adding chromium trioxide (CrO3), sodium silicofluoride (Na2SiF6), and sulfuric acid (H2SO4) to water. Aqueous solution 2 was prepared by adding only sulfuric acid (H2SO4) to water.

[0047] The aqueous solution was circulated through a flow cell by a pump at a rate equivalent to 100 mpm. Lead electrodes were used for cathodic electrolysis. After the manufacturing process, the steel sheet for cans was washed with water and dried at room temperature using a blower.

[0048] (Amount of adhesion, etc.) For the produced steel sheets for cans, the deposition weight of the metallic chromium layer and the deposition weight of the chromium hydrate oxide layer in terms of chromium (simply referred to as "deposition weight" in Table 1 below) were measured. In addition, a zero-order persistence diagram was created for the protrusions on the metal chromium layer of the produced steel sheet for cans, and 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 were measured. The measurement methods were all as described above, and 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, slashes were made in the paint at 1 mm intervals in a checkerboard pattern, reaching down to the steel sheet, and the paint was peeled off using tape. The peeling condition was observed. The peeling area rate was evaluated according to the following criteria. A rating of "A," "B," or "C" was considered to indicate excellent paint adhesion. A: Peeling area rate 0% B: Peeling area rate more than 0%, less than 2% C: Peeling area rate more than 2%, less than 5% D: Peeling area rate more than 5%, less than 30% E: Peeling area rate over 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 1mm intervals, reaching down to the steel sheet, and the sheet was then retorted at 125°C for 30 minutes. After drying, the paint was peeled off with tape, and the peeling condition was observed. The peeled area ratio was evaluated according to the following criteria. A rating of "A," "B," or "C" was considered to indicate excellent paint adhesion. A: Peeling area rate 0% B: Peeling area rate more than 0%, less than 2% C: Peel area ratio exceeds 2% and is 5% or less D: Peel area ratio exceeds 5% and is 30% or less E: Peel area ratio exceeds 30%

[0052] 《Surface Appearance》 Regarding the produced steel sheet for cans, based on the Hunter color difference measurement defined in the former JIS Z 8730 (1980), the b value was measured, and the surface appearance was evaluated according to the following criteria. If it was “A” or “B”, it was evaluated that the surface appearance was excellent. A: b value ≤ 0 B: 0 < b value ≤ 2.0 C: 2.0 < b value ≤ 3.0 D: 3.0 < b value

[0053]

Table 1

[0054] 〈Summary of Evaluation Results〉 As shown in Table 1 above, the steel sheets for cans of Invention Examples 1 to 23 had both good paint adhesion and surface appearance. The steel sheet for cans of Comparative Example 1 without protrusions (without dipping treatment and without cathodic electrolytic treatment C2) had insufficient paint adhesion. The steel sheet for cans of Comparative Example 2 where the minimum value of the Birth size of the protrusions was -525 nm (the quantity density of cathodic electrolytic treatment C2 was 30.6 C / dm 2 ) had insufficient paint adhesion. The number of protrusions with a Death size of 0 nm or more and less than 750 nm was 0.5 pieces / μm 2 The steel sheet for cans of Comparative Example 3 with an immersion time of 0.05 seconds for the dipping treatment had insufficient paint adhesion. The number of protrusions with a Death size of 0 nm or more and less than 750 nm was 11.0 pieces / μm 2 The steel sheet for cans of Comparative Example 4 with an immersion time of 2.10 seconds for the dipping treatment had insufficient paint adhesion. The number of protrusions with a Death size of 750 nm or more was 0.01 pieces / μm2 The steel sheet for cans of Comparative Example 5 (the concentration of sulfuric acid in the aqueous solution 2 was 11% by volume) had an insufficient surface appearance. The number of protrusions with a death size of 750 nm or more is 0.01 / μm 2 Comparative Example 6 (where the current density of cathodic electrolysis C2 is 220.0 A / dm 2 ) steel sheet for cans had insufficient surface appearance. The number of protrusions with a death size of 750 nm or more is 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. [Explanation of symbols]

[0055] 1: Steel sheets for cans 2: Steel plate 3: Metallic chrome layer 3a: Flat plate part 3b:Protrusion 4: Chromium hydrate oxide layer

Claims

[Claim 1] The surface of the steel sheet has, in order from the steel sheet side, a metallic chromium layer and a chromium hydrate oxide layer, The amount of the metal chromium layer is 50 to 200 mg / m ２ and The chromium-equivalent coating amount of the chromium hydrate oxide layer is 3 to 50 mg / m ２ and The metal chromium layer includes a flat portion and a protrusion portion provided on the flat portion, The protrusion has a minimum birth size of −500 nm or more, The number of the protrusions having a death size of 0 nm or more and less than 750 nm is 1.0 to 10.0 pieces / μm ２ and The number of the protrusions having a death size of 750 nm or more is 0.02 / μm ２ That's all for the steel sheet for cans. However, the birth size and death size of the protrusion are obtained by creating a zero-order persistence diagram for the protrusion.