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 chrome layer 3 includes a flat plate portion 3a and a protruding portion 3b provided on the flat plate portion 3a. The maximum equivalent circle diameter of the protruding portion 3b is 1000 nm or less. The average area of ​​the Voronoi polygon obtained by performing Voronoi division on the center of gravity of the protruding portion 3b is 5,000 to 200,000 nm. 2 The simple average of the circle-equivalent diameters of the Voronoi polygons is D AVE , the 50% diameter of the cumulative distribution is D 50 , the 90% diameter of the cumulative distribution is D 90 Then, D 50 / D AVE is less than 1.00, and D 90 / D AVE is less than 3.00.

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 "on the surface of the steel sheet, in this order from the steel sheet side, a metallic chromium layer and a chromium hydrate oxide layer," and further, the metallic chromium layer has "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 show good paint adhesion (adhesion to paint applied to the surface of the steel sheet for cans) because the anchor effect of the granular protrusions improves adhesion to paint. However, when a steel sheet for cans has 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 which is excellent in paint adhesion and surface appearance. [Means for solving the problem]

[0005] As a result of extensive investigations, 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 having 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 chrome layer includes a flat plate portion and a protrusion portion provided on the flat plate 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 D AVE , the 50% diameter of the cumulative distribution is D 50 , the 90% diameter of the cumulative distribution is D 90 Then, D 50 / D AVE is less than 1.00, and D 90 / D AVE is less than 3.00. Effect of the Invention

[0006] According to the present invention, it is possible to provide a steel sheet for cans which is excellent in paint adhesion and surface appearance. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a cross-sectional view that typically shows a steel sheet for cans. [Diagram 2] This is an SEM image of a steel sheet for cans. [Diagram 3] FIG. 2 is a Voronoi diagram of a steel sheet for cans. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Steel sheets for cans] FIG. 1 is a cross-sectional view that typically shows a steel sheet for cans 1. As shown in FIG. 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 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 hydrated 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 the present embodiment will be described in more detail.

[0010] <Steel plate> The type of steel plate is not particularly limited. Steel plates that are usually used as container materials (e.g., low carbon steel plates, ultra-low carbon steel plates) can be used. The manufacturing method and material of the steel plate are not particularly limited. The steel plate is manufactured through a normal steel slab manufacturing process, followed by hot rolling, pickling, cold rolling, annealing, temper rolling, and other processes.

[0011] <Metal Chromium Layer> A metal chromium layer is disposed on the surface of the above-mentioned steel sheet. The metal chromium layer reduces the surface exposure of the steel sheet and improves corrosion resistance.

[0012] 《Amount of adhesion》 The coating weight of the metal chromium layer is set to 50mg / m because the corrosion resistance of the steel sheet for cans is excellent. 2 or more, 70 mg / m 2 More than 80 mg / m is preferable. 2 The above is more preferable. The adhesion amount is the adhesion amount per one side of the steel sheet (the same applies below). On the other hand, even if the coating weight of the metal chromium layer is excessively increased, the corrosion resistance effect is saturated, so it is uneconomical. For this reason, the coating weight of the metal chromium layer is set to 200 mg / m 2 180 mg / m or less 2 Less than 160 mg / m 2 The following is more preferred:

[0013] (Method of measuring adhesion amount) The coating weight of the metallic chromium layer and the coating weight in terms of chromium of the hydrated chromium oxide layer described below are measured as follows. First, the chromium content (total chromium content) of the steel sheet for cans on which the metallic chromium layer and the chromium hydrate oxide layer are formed is measured using an X-ray fluorescence device. Next, the steel sheet for cans is subjected to an alkali treatment by immersing it in a 6.5N 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 an X-ray fluorescence device. The chromium content after alkali treatment is regarded as the amount of adhesion 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 regarded 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 a protruding portion 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 the corrosion resistance. It is preferable that the flat portion of the metal chromium layer 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 to expose the steel sheet. The coating weight of the metal chromium layer on the flat plate is set to 10 mg / m because the corrosion resistance of the steel sheet for cans is excellent. 2 More than 30 mg / m is preferable. 2 More preferably, 40 mg / m 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 believed to be because the anchor effect of the protrusions improves the adhesion to the paint applied to the surface of the steel sheet for cans.

[0017] (Maximum circle equivalent 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 from the viewpoint of superior paint adhesion, it is preferably 900 nm or less, and more preferably 800 nm or less. The lower limit is not particularly limited, and is, for example, 10 nm.

[0018] The maximum equivalent circle diameter of the protrusion is determined as follows. First, the protrusions are observed at a magnification of 50,000 times using a scanning electron microscope (SEM) 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 number of pixels thus determined is multiplied by the size (area) of each pixel to calculate the area A of each protrusion. Furthermore, π(d / 2) 2 Calculate the equivalent circular diameter d using the relational expression: = A. The maximum value of the equivalent circular diameters d of the protrusions in any five fields of view is determined as the maximum equivalent circular diameter of the protrusions (unit: nm).

[0019] (Voronoi polygons) The presence of protrusions can be evaluated by performing Voronoi division. In Voronoi tessellation, a plane on which multiple points (kernel points) are arranged is divided by drawing perpendicular bisectors to the straight lines connecting adjacent kernel points, and dividing the plane into areas (Voronoi polygons) enclosed by these perpendicular bisectors. A diagram created by Voronoi tessellation (a diagram that includes Voronoi polygons) is also called a Voronoi diagram (see Figure 3 below). When Voronoi tessellation is performed using the center of gravity of the protrusion as the generating point, it can be said that the smaller the area of ​​the obtained Voronoi polygon, the higher the density of the protrusion.

[0020] The Voronoi division is performed as follows. First, a scanning electron microscope (SEM) is used to observe the protrusions and obtain an SEM image (a planar image of the steel sheet for cans). The SEM observation is performed at a magnification that allows more than 500 protrusions to be identified within one field of view. Figure 2 is an SEM image of a steel sheet for cans, which shows that there is a mixture of areas with sparse protrusions and areas with dense protrusions. The obtained SEM image is subjected to image processing, whereby the protrusions and their centers of gravity (center of gravity positions) are extracted, and Voronoi division is performed using the extracted centers of gravity as generating points to create a Voronoi diagram including Voronoi polygons. FIG. 3 is a Voronoi diagram of a steel sheet for cans, and more specifically, is a Voronoi diagram created by subjecting the SEM image of FIG. 2 to Voronoi division. For each Voronoi polygon included in the Voronoi diagram, calculate the area A, and then multiply it by π(d / 2). 2 Calculate the circle equivalent diameter d using the relational expression =A. The average of the areas A of each Voronoi polygon in any five fields of view is defined as the average area of ​​the Voronoi polygon (unit: nm 2 ) is calculated. In addition, the circle equivalent diameter of each Voronoi polygon in any five fields of view is calculated by taking the simple average D AVE , 50% diameter of cumulative distribution D 50 , and the 90% diameter of the cumulative distribution D 90 Request.

[0021] ((average area)) The smaller the area of ​​the Voronoi polygon, the better the paint adhesion, because the smaller the area of ​​the Voronoi polygon, the higher the density of the protrusions, resulting in a stronger anchoring effect. Therefore, the average area of ​​a Voronoi polygon is 200,000 nm 2 150,000 nm or less, and because paint adhesion is superior 2 Preferably below 100,000 nm 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. Therefore, the average area of ​​a Voronoi polygon is 5,000 nm 2 Because of the above, and because paint adhesion is superior, 7,000 nm 2 More 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 protrusions absorbing light on the short wavelength side (blue-based) and attenuating the reflected light, resulting in a reddish-brown color; the protrusions scatter light, reducing the overall reflectance and making the color darker; etc. In contrast, by having a mixture of areas with many protrusions and areas with few protrusions, light scattering is suppressed and good color tones 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 is (the distribution is a mixture of sparse and dense protrusions), and in this case, as described above, the surface appearance is excellent. Therefore, the circle equivalent diameter of the Voronoi polygon is D 50 / D AVE is less than 1.00, and more preferably less than 0.95 because this results in a better surface appearance. Similarly, D 90 / D AVE is less than 3.00, and more preferably less than 2.50 because of better surface appearance.

[0025] D 50 / D AVE and D. 90 / D AVE The lower limit is not particularly limited. D 50 / D AVE is, for example, 0.15 or more, may be 0.35 or more, or may be 0.55 or more. D 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, D AVE is not particularly limited. The circle equivalent diameter of the Voronoi polygon, D AVEis, for example, 50 nm or more, may be 100 nm or more, or may be 150 nm or more. In addition, the circle equivalent diameter of the Voronoi polygon, D AVE is, 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, D 50 is not particularly limited. The circle equivalent diameter of the Voronoi polygon, D 50 is, for example, 30 nm or more, may be 70 nm or more, or may be 100 nm or more. In addition, the circle equivalent diameter of the Voronoi polygon, D 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, D 90 is not particularly limited. The circle equivalent diameter of the Voronoi polygon, D 90 is, for example, 50 nm or more, may be 150 nm or more, or may be 200 nm or more. In addition, the circle equivalent diameter of the Voronoi polygon, D 90 is, for example, 550 nm or less, may be 500 nm or less, or may be 450 nm or less.

[0029] <Chromium hydrate oxide layer> The chromium hydrate oxide is precipitated on the surface of the steel sheet simultaneously with metallic chromium, and improves the corrosion resistance. The chromium hydrate oxide includes, for example, chromium oxide and chromium hydroxide.

[0030] 《Amount of adhesion》 In order to ensure the corrosion resistance of steel sheets for cans, the chromium equivalent coating weight of the chromium hydrate oxide layer is 3mg / m 2 4 mg / m 2 The above is preferred.

[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, which may lead to the generation of dust and splashes and various welding defects such as blowholes due to overfusion 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 50mg / m2 in terms of chromium because of the excellent weldability of the steel sheet for cans. 2 40 mg / m or less 2 Less than 30 mg / m is preferred. 2 The following is more preferred:

[0032] The method for measuring the amount of deposition of the chromium-equivalent hydrated chromium oxide layer is as described above.

[0033] [Manufacturing method of steel sheets for cans] Next, a method for producing the steel sheet for cans according to the above-mentioned embodiment will be described. In general, 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 an immersion treatment using an aqueous solution 2 containing a low concentration of sulfuric acid, and thereafter subjected to cathodic electrolysis C2 again using the aqueous solution 1.

[0034] In the cathodic electrolytic 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 is formed is immersed in an aqueous solution 2 containing sulfuric acid), thereby partially activating the flat metallic chromium. At this time, since the concentration of sulfuric acid in the aqueous solution 2 is low, differences in the activation state occur for each crystal orientation of the steel sheet. Next, when performing the cathodic electrolytic treatment C2, protrusions are formed starting from the activated portions of the flat metal chromium. However, since the activation state differs depending on the crystal orientation of the steel sheet, the distribution of the protrusions becomes non-uniform.

[0035] <Aqueous solution 1> The aqueous solution 1 used in the cathodic electrolysis contains a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid. In the aqueous solution 1, the fluorine-containing compound and sulfuric acid exist in a dissociated state (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 the aqueous solution 1, which proceeds in the cathodic electrolysis treatment. The aqueous solution 1 used in the cathodic electrolysis contains a fluorine-containing compound and sulfuric acid, so that the amount of chromium-equivalent deposition of the chromium hydrous oxide layer on the resulting steel sheet for cans can be reduced. Although the reason for this is not clear, it is believed that the amount of chromium hydrous oxide produced decreases as the amount of anions increases during the electrolysis.

[0036] 《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 Cr amount. 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 Cr amount.

[0037] Fluorine-containing compounds Examples of fluorine-containing compounds include hydrofluoric acid (HF), potassium fluoride (KF), sodium fluoride (NaF), hydrofluorosilicic acid (H2SiF6), and salts of hydrofluorosilicic 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, and 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, in terms of the amount of F, is preferably 0.48 mol / L or less, and more preferably 0.40 mol / L or less.

[0038] 《Sulfuric acid》 The sulfuric acid (H2SO4) may be partially or entirely a sulfate salt such as sodium sulfate, calcium sulfate, or ammonium sulfate. The sulfuric acid content in 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 solution 1 is SO4 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, and more preferably 40° C. or higher. On the other hand, the liquid temperature of the aqueous solution 1 is preferably 80° C. or lower, and more preferably 60° C. or lower.

[0040] <Cathodic electrolysis C1> The cathodic electrolysis C1 deposits plate-like metallic chromium and hydrated chromium oxide. If the current density of the cathodic electrolysis C1 is too low, the deposition efficiency of metallic chromium decreases, and the proportion of the chromium hydrous oxide layer tends to increase. Therefore, the current density of the cathodic electrolysis C1 is 10.0 A / dm 2 More than 15.0A / dm is preferable. 2 The above is more preferable. On the other hand, the upper limit is not particularly limited, and the current density of the cathodic electrolysis C1 is, for example, 200.0 A / dm 2 Less than or equal to 150.0A / dm 2 The following is preferred: The current flow time and the electrical charge density (the product of the current density and the current flow time) of the cathodic electrolysis treatment C1 are appropriately set in order to obtain a 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 (H2SO4). If the concentration of sulfuric acid in the aqueous solution 2 is too high, the activation of the flat metallic chromium described above tends to proceed excessively in all crystal orientations of the steel sheet, which may result in the subsequent cathodic electrolytic treatment C2 causing protrusions to precipitate uniformly and entirely in all crystal orientations, resulting in poor surface appearance. For this reason, 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 necessary activation, the concentration of sulfuric acid in the aqueous solution 2 is preferably 1 vol % or more, more preferably 2 vol % or more, further preferably 3 vol % or more, and particularly preferably 4 vol % or more.

[0042] It is preferable that the aqueous solution 2 substantially contains only sulfuric acid in water. In other words, it is preferable that no components other than sulfuric acid are added to the water when preparing the aqueous solution 2. 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, so that in the subsequent cathodic electrolytic treatment C2, protrusions are precipitated uniformly and over the entire surface, which tends to deteriorate the surface appearance.

[0043] 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 Cr amount. Similarly, the content of the fluorine-containing compound in the aqueous solution 2, in terms of the amount of F, 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.

[0044] The liquid temperature of the aqueous solution 2 is preferably 20° C. or higher, and more preferably 40° C. or higher. On the other hand, the liquid temperature of the aqueous solution 2 is preferably 80° C. or lower, and more preferably 60° C. or lower.

[0045] <Immersion treatment> In the immersion treatment, the steel sheet that has been 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, as described above, the flat metallic chromium is partially activated. 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, the activation proceeds excessively, resulting in a structure in which the protrusions are locally dense, 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] <Cathodic electrolysis C2> In the cathodic electrolysis C2, protrusions of the metal chromium layer are formed. If the current density of the cathodic electrolysis 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 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 electrolytic treatment C2 is too high, the protrusions may precipitate suddenly, damaging the surface appearance. For this reason, the current density of the cathodic electrolytic treatment C2 is set to 200.0 A / dm 2 Less than or equal to 150.0A / dm 2 The following is preferred: In addition, if the electric charge density of the cathodic electrolytic treatment C2 is large, the diameter of the protrusion may become too large. Therefore, the electric charge density of the cathodic electrolytic treatment C2 is set to 30.0 C / dm 2 Less than or equal to 25.0C / dm 2 Less than 20.0C / dm is preferable. 2 The lower limit is not particularly limited, and is, 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. EXAMPLES

[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] <Production of steel sheets for cans> Steel plates (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 of the cathodic electrolysis treatment C1, the immersion treatment, and the cathodic electrolysis treatment C2 performed are shown in Table 1 below.

[0049] However, in Comparative Example 1, only the cathodic electrolysis treatment C1 was carried out using the aqueous solution 1. For this reason, "-" is entered in the columns for the aqueous solution 2 and the cathodic electrolysis treatment C2. In Comparative Example 7, the immersion treatment was carried out using Aqueous Solution 1 instead of Aqueous Solution 2.

[0050] 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.

[0051] The aqueous solution was circulated through a flow cell by a pump at the equivalent of 100 mpm. Lead electrodes were used for the cathodic electrolysis. Thus, a steel sheet for cans was produced. After production, the steel sheet for cans was washed with water and dried at room temperature using a blower.

[0052] (Amount of adhesion, etc.) For the produced steel sheets for cans, the adhesion weight of the metallic chromium layer and the adhesion weight in terms of chromium of the hydrated chromium oxide layer (simply referred to as "adhesion weight" in Table 1 below) were measured. In addition, the maximum equivalent circle diameter of the protrusions in the metal chromium layer of the produced steel sheets for cans was measured. Furthermore, Voronoi division is performed to find the average area, D AVE , D 50 , D 90 , D 50 / D AVE , and D 90 / D AVE was measured. The measurement methods were all as described above, and the results are shown in Table 1 below.

[0053] <evaluation> The prepared 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} Paint (epoxy-phenolic resin) was applied to the surface of the prepared steel plate for cans, and then it was heated twice at 210°C for 10 minutes. After that, the paint was scored in a checkerboard pattern at 1 mm intervals deep enough to reach the steel plate, the paint was peeled off using tape, and the peeling condition was observed. The peeled area rate was evaluated according to the following criteria. If it was rated as "A", "B", or "C", it was evaluated as having 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%

[0055] {Secondary paint adhesion} Paint (epoxy-phenolic resin) was applied to the surface of the prepared steel plate for cans, and then the plate was heated twice at 210°C for 10 minutes. After that, the paint was scored in a checkerboard pattern at 1mm intervals, reaching the steel plate, and then retort treatment was performed at 125°C for 30 minutes. After drying, the paint was peeled off with tape, and the peeling condition was observed. The peeled area rate was evaluated according to the following criteria. If it was rated as "A", "B", or "C", the paint adhesion was evaluated as excellent. A: Peel area ratio 0% B: Peel area ratio exceeds 0% and is 2% or less 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%

[0056] 《Surface Appearance》 For the produced steel sheet for cans, the b value was measured based on the Hunter color difference measurement defined in the old JIS Z 8730 (1980), 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

[0057]

Table 1

[0058] 〈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 good surface appearance. The steel sheet for cans of Comparative Example 1 without protrusions (without immersion treatment, without cathodic electrolysis treatment C2) had insufficient paint adhesion. The steel sheet for cans of Comparative Example 2 (the electric quantity density of cathodic electrolysis treatment C2 is 30.6 C / dm 2 ) with the equivalent diameter of the maximum circle of the protrusions being 1050 nm had insufficient paint adhesion. The steel sheet for cans of Comparative Example 3 (the immersion time of the immersion treatment is 0.05 seconds) with the average area of the Voronoi polygon of the protrusions being 201,000 nm 2 had insufficient paint adhesion. The steel sheet for cans of Comparative Example 4 (the immersion time of the immersion treatment is 2.10 seconds) with the average area of the Voronoi polygon of the protrusions being 4,900 nm 2 had insufficient paint adhesion. The D of the Voronoi polygon of the protrusions50 / 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 difference was 1.05, had insufficient surface appearance. D of the Voronoi polygon of the protrusion 90 / D AVE Comparative Example 6 (wherein the current density of cathodic electrolysis C2 is 220.0 A / dm 2 ) steel sheet for cans had insufficient surface appearance. D of the Voronoi polygon of the protrusion 50 / D AVE The steel sheet for cans of Comparative Example 7 (using Aqueous Solution 1 for the immersion treatment) having a surface appearance of 1.07 was insufficient. [Explanation of symbols]

[0059] 1: Steel sheets for cans 2: Steel plate 3: Metal chrome layer 3a: Flat plate part 3b:Protrusion 4: Chromium hydrate oxide layer

Claims

[Claim 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, 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 maximum equivalent circle diameter of the protrusions is 1000 nm or less, The average area of ​​the Voronoi polygon obtained by performing Voronoi division on the center of gravity of the protrusion is 5,000 to 200,000 nm ２ and The circle equivalent diameter of the Voronoi polygon is calculated by taking the simple average 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 The steel sheet for cans, wherein the modulus of elasticity is less than 3.00.