Title of the Invention: PROCESS FOR PRODUCING ENVIRONMENTALLY-FRIENDLY STEEL 5 SHEET FOR CONTAINER MATERIAL, ENVIRONMENTALLY-FRIENDLY STEEL SHEET FOR CONTAINER MATERIAL, AND LAMINATED AND PRE-COATED STEEL SHEET FOR CONTAINER MATERIAL USING THE SAME 10 Technical Field [0001.] The present invention relates to a surface-treated metal material and a method of surface-treating the metal material. More specifically, the present invention 15 relates to an environmentally-friendly steel sheet for a container, wherein a steel sheet can be primer-treated without using a treatment solution containing chrome, fluorine, or nitrate nitrogen, and a process for producing the same. 20 BACKGROUND ART [0002] As the treatment for improving the adhesion between an organic film and a metal material such as steel sheet, 25 zinc-based plated steel sheet, zinc-alloy sheeI, tinbased plated steel sheet and aluminum alloy sheet, there have heretofore been known chromate treatment, phosphate treatment, silane coupling treatment, etc. Among these, the chromate treatment has been broadly utilized in the 30 fields of home electrical appliances, building materials, vehicles, metal containers, etc., due to its superior corrosion resistance and adhesion. However, there has been pointed out the possibility of the toxic substance of hexavalent chrome contaminating the soil, etc., by the 35 leaching thereof into the soil at the time of the disposal of the chromate-treated products. Accordingly, the industries mainly in Europe, are ready to eliminate 2 the chromate treatment at the present stage. [00031 In the field of metal materials to be used for containers, a certain type of chromate treatment method 5 is being utilized, such that a tin-plated steel sheet is treated by cathodic electrolysis in an aqueous solution of sodium bichromate, or a steel sheet is treated by cathodic electrolysis in an aqueous solution of fluorinecontaining anhydrous chromic acid, so as not to leave 10 hexavalent chrome in the resultant film. However, even in the case of the chromate treatment of a type where the treated layer does not include hexavalent chrome, the treatment solution to be used therefor contains the hexavalent chrome, and accordingly, the hexavalent chrome 15 has to be rendered harmless for the treatment or disposal of the wastewater and gas emissions. For this reason, from the viewpoint of the environmental load, a surface treatment is desirable such that the treatment solution does not include hexavalent chrome either. 20 From this viewpoint, in order to make a treatment solution hexavalent chromium--free., attempts to eliminate chromium have come to attract attention, and such chromium-free attempts include investigation on the removal of chromium from a coating film or plating per 25 se, a coat-Lug film or alternative plating whicli is alternative to chromium or chromium plating. Further, with respect to fluorine, boron, nitrate nitrogen, etc,, are also not preferable from the viewpoint of the environmental load. In the future, the 30 industries will be encountered with toughened emission standards therefor. Therefore, the treatment solutions for metal materials to be used for containers may preferably be those which do not contain the substances as described above, 35 [0004.] Therefore, as one measure for reducing the environmental load, there is elimination of the use of 3 chrome. Patent Document 1 discloses an example of the method of surface-treating a tin-plated steel can superior in corrosion resistance and coating adhesion, wherein a container material is chrome-free surface- 5 treated by providing, on a tin-plated surface of a tinplated steel sheet, an organic-inorganic composite coating comprising an organic compound main comprising carbon and an inorganic phosphorus compound. Patent Document 2 discloses, as a surface treatment solution for 10 an aluminum can or tin-plated DI (drawing and ironing) can prior to the coating and printing thereof, an example of the surface treatment solution for DI can, which contains at least one kind of phosphoric acid ions and a zirconium compound and titanium compound, and contains an 15 oxidizing agent and at least one kind of fluoric acid and a fluoride. [0005] Conventionally, the metal containers to be used for beverage can and food can applications have generally 20 been treated so as to bake the coatings at the inside and outside surfaces of the cans, after the manufacturing of the cans. In recent years, as the metal materials to be used for beverage cans or food cans, there have been increasingly used a steel sheet with a film which has 25 been hot-laminated on the steel sheet in advari e, and a pre-coated steel sheet comprising a steel sheet which has been subjected to a coating treatment including printing and baking, in advance. (00061 30 However, in the can manufacturing using DI or DRD (drawing and redrawing), an ironing force acts on the can wall, so in a case where a can is manufactured by using a laminated steel sheet or coating pre-coated steel sheet type of metal material for container, if the adhesive 35 strength between the resin coating and the steel sheet is not sufficient, there is caused a problem such that the resin coating will easily be peeled off. Further, in the 4 sterilization (i.e., retort treatment) which is to be performed after filling of the can with a content, water in the content sometimes permeates the resin coating under the high temperature and high pressure conditions, 5 and the adhesion is liable to be decreased. Accordingly, even in the development of the chromium-free type steel sheet for container material, it is necessary to attain an excellent adhesion between the resin coating and the steel sheet. 10 [0007] With respect to such a requirement for the container materials, as disclosed in Patent Document 3, the present inventors have developed a chromium-free steel sheet for a container material having an excellent adhesion in 15 which a zirconium compound-containing coating film is formed on a tin-plated steel sheet, and have developed a steel sheet for container material having an adhesion which is equal to or greater than that of the conventional chromate treatment. However, the invention 20 disclosed in Patent Document 3 had a drawback such that, in the electrolytic treatment therefor, it is necessary to finely regulate the electrolytic conditions during the treatment in order to maintain the coating amount in an appropriate range. 25 Prior Art Documents Patent Documents °[0008] [Patent Document 1] JP-A (Japanese Unexamined 30 Patent Publication; Kokai) No. 11-264075 [Patent Document 2] JP-A No. 7-48677 [Patent Document 3] JP-A No. 2009-68108 [Summary of the Invention] [Problem to Be Solved by the Invention] 35 [0009] An object of the present invention is to provide a chromium-free steel sheet for a container material having 5 excellent characteristics (for example, adhesion with an organic resin coating such as laminate film or coating material; and resistance to iron dissolution after dent impact), which are equal or comparable to those of the 5 conventional steel sheets for container material which have been subjected to chromium plating or chromate coating treatment. Another object of the present invention is to provide a process for producing a chromium-free steel 10 sheet for a container material having excellent characteristics as mentioned above, which also enables easy and stable production of the chromium-free steel sheet. 15 Means for Solving the Problem (00101 As a result of earnest study on chromium-free processes which do not use chromium in the plating or in the coating film to be disposed thereon for the purpose 20 of solving the above problem, the present inventors have found a process which is environmentally friendly, and can easily and stably produce a chromium-free steel sheet for a container material having an excellent adhesion, as describe hereinbelow. 25 More specifically, the present invention relates to a process for producing an environmentally friendly steel sheet for a container material, comprising a step of subjecting a tin-plated steel sheet to a cathodic electrolytic coating treatment in a treatment solution 30 not containing a chromium compound, fluorine, or a nitrate nitrogen, wherein: a tin oxide layer present on a tin-plated steel sheet before the cathodic electrolytic coating treatment is removed by a cathodic electrolytic treatment in an 35 aqueous solution containing sodium carbonate or sodium hydrogen carbonate, or by an immersion treatment in an aqueous sulfuric acid solution, so as to provide a 6 thickness of 0 mC/cm2 or more and 3.5 mC/cm2 or less as measured by electrolytic stripping method, and then; a coating film having a film coating amount converted to zirconium of 0.1 mg/m2 or more and 20 mg/m2 5 or less is formed by a cathodic electrolytic coating treatment in an aqueous solution of an alkali metal sulfate containing a zirconium compound with an electric conductivity of 0.2 S/m or more and 6.0 S/m or less and a pH of 1.5 or more and 2.5 or less. 10 The present invention also provides an environmentally friendly steel sheet for a container material, comprising a tin-plated steel sheet and a zirconium compound-containing coating film disposed thereon, wherein a tin oxide layer present on the tin- 15 plated steel sheet is 0 mC/cm2 or more and 3.5 mC/cm2 or less, and the zirconium compound-containing coating film has a film coating amount converted to zirconium of 0.1 mg/m2 or more and 20 mg/m2 or less. According to the discovery and investigation of the 20 present inventors, it is presumed that, in the invention disclosed in Patent Document 3, the electrolytic condition in the electrolytic treatment in this document is required to be finely regulated so as to maintain the coating amount in an appropriate range, because the 25 coating amount of the coating film tends to be increased abruptly, with respect to an increase in the current density (see Fig. 2 and Fig. 3 appearing hereinafter). it is also presumed that such a change in the coating amount of the coating film is caused by a pH change 30 (i.e., an increase in the pH) due to the hydrogen gas release in the vicinity of the electrode for electrolysis, to thereby cause a change in the coating amount of the coating film (i.e., an increase in the coating amount of the coating film). In addition, it is 35 presumed that the progress of the film coating process (i.e., consumption of zirconium) per se causes an increase in the pH, and this pH increase accelerates the 7 above pH change. As a result of such phenomena, in the prior art, it presumed to be indispensable, to finely regulate the electrolytic conditions in response to the variation in 5 the process conditions (such as sheet width, line speed, and liquid temperature) so as to maintain the coating amount in an appropriate range, by suitably controlling the above tendency to cause the "abrupt increase in the coating amount of coating film." 10 In contrast thereto, the present inventors have found that a large amount of alkali metal ions such as Na+ and K+ in the electrolytic solution neutralize OH- ions in the vicinity of the cathode, so as to provide a tendency of relieving (or reducing) the local pH variation in the 15 vicinity of the cathode, and on the basis of the tendency, the zirconium oxide ions (ZrO2+) are stabilized. Based on such a discovery, the present inventors have completed the present invention. According to the present invention, a curve showing 20 "changes in the film coating amount converted to zirconium" corresponding to the pH change in the vicinity of the electrode for electrolysis can be smoothened (as shown in the graphs of Fig. 2 and Fig. 3 appearing hereinafter). Therefore, it is presumed that, according 25 to the present invention, "the film coating amount converted to zirconium" can stably be controlled, so as to enable the stable film deposition treatment. In other words, the present invention has a characteristic such that ZrO2 to be deposited on a plated 30 surface is added (actually, zirconium sulfate is added) by using an easily electrolyzable "aqueous solution of an alkali metal sulfate" as a main component. The present invention may include the following 35 embodiments. [1] A process for producing an environmentally friendly steel sheet for a container material, comprising 8 a step of subjecting a tin-plated steel sheet to a cathodic electrolytic coating treatment in a treatment solution not containing a chromium compound, fluorine, or a nitrate nitrogen, wherein: 5 a tin oxide layer present on a tin-plated steel sheet before the cathodic electrolytic coating treatment is removed by a cathodic electrolytic treatment in an aqueous solution containing sodium carbonate or sodium hydrogen carbonate, or by an immersion treatment in an 10 aqueous sulfuric acid solution, so as to provide a thickness of 0 mC/cm2 or more and 3.5 mC/cm2 or less as measured by electrolytic stripping method., and then; a coating film having a film coating amount converted to zirconium of 0.1 mg/m2 or more and 20 mg/m2 15 or less is formed by a cathodic electrolytic coating treatment in an aqueous solution of an alkali metal sulfate containing a zirconium compound with an electric conductivity of 0.2 S/m or more and 6.0 S/m or less and a pH of 1.5 or more and 2.5 or less. 20 [2] The process for producing an environmentally friendly steel sheet for a container material according to [1], wherein the concentration of zirconium contained in the aqueous solution of an alkali metal sulfate is 10 mg/L or more and 2000 mg/L or less. 25 [3] The process for producing an environmentally friendly steel sheet for a container material according to [1] or [2], wherein the alkali metal sulfate is sodium sulfate. [4] The process for producing an environmentally 30 friendly steel sheet for a container material according to [1] or [2], wherein the alkali metal sulfate is potassium sulfate. [5] The process for producing an environmentally friendly steel sheet for a container material according 35 to [1.] or [2], wherein the concentration of the alkali metal sulfate contained in the aqueous solution of the alkali metal. sulfate is 0.1 mass % or more and 8.0 mass % 9 or less, [6] An environmentally friendly steel sheet for a container material, comprising a tin-plated steel sheet and a zirconium compound-containing coating film disposed 5 thereon, wherein a tin oxide layer present on the tin-plated steel sheet is 0 mC/cm2 or more and 3.5 mC/cm2 or less, and the zirconium compound-containing coating film has a film coating amount converted to zirconium of 0.1 mg/m2 or 10 more and 20 mg/m2 or less. [7] An environmentally friendly laminated steel sheet for a container material, comprising the steel sheet for a container material according to [6]. [8] An environmentally friendly precoated steel 15 sheet for a container material, comprising the steel sheet for a container material according to [6]. Effect of the Invention [0011] 20 The steel sheet for a container material having a light environmental load which has been produced by the production process according to the present invention has an adhesion with an organic resin coating film such as a laminated film or a coating material, and also has an 25 excellent performance as a chromium-free steel sheet for a container material such as resistance to iron dissolution after dent impact, which is equal or comparable to that of the conventional chromium-treated steel sheet for a container material. In addition, such 30 a steel sheet for a container material can be produced easily and stably, and therefore the industrial value thereof is very high. Brief Description of the Drawings 35 [0012] [Fig. 1] Fig. 1 is a graph showing a relationship between the - 10 - amount (amount of removal due to electrolytic stripping) of tin oxide on a tin-plated surface, and coating material adhesion (T-peel strength) of a tin-plated steel sheet which has been coated with a film of zirconium 5 compound. [Fig. 2] Fig. 2 is a graph showing a relationship between the current density during an electrolytic treatment and the film coating amount converted to zirconium of a primer 10 coating film in the case of a conventional zirconium sulfate treatment solution, and in the case of a sodium sulfate treatment solution containing a zirconium compound according to the present invention. [Fig. 3] 15 Fig. 3 is a graph showing a relationship between the pH of a treatment solution and the film coating amount converted to zirconium of a primer coating film disposed on a tin-plated steel sheet after electrolytic treatment, in the case of a conventional zirconium sulfate treatment 20 solution, and in the case of a sodium sulfate treatment solution containing a zirconium compound according to the present invention, [Fig. 4] Fig. 4 is a graph showing a relationship between the 25 film coating amount converted to zirconium of primer coating film, and the coating material adhesion (T-peel strength) of a tin-plated steel sheet, which has been -subjected to an electrolytic treatment with a sodium sulfate treatment solution containing a zirconium 30 compound according to the present invention. [Fig. 5] Fig. 5 is a graph showing a relationship between the zirconium concentration of an aqueous sodium sulfate solution containing a zirconium compound according to the 35 present invention, and the film coating amount converted to zirconium of a coating film containing a zirconium compound. 11 - [Fig. 6] Fig. 6 is a graph showing a relationship between the zirconium concentration, and the storage stability of a treatment solution according to the present invention. 5 [Fig. 7] Fig. 7 is a graph showing a relationship between the electric conductivity of a treatment solution, and the rectifier voltage during electrolysis, when sodium sulfate treatment solution containing a zirconium 10 compound having different electric conductivities according to the present invention are electrolyzed while changing the current density. [Fig. 8] Fig. 8 is a graph showing a relationship between the 15 electric conductivity of a treatment solution, and the film coating amount converted to zirconium of a primer coating film, when each of a sodium sulfate treatment solution containing a zirconium compound, or a potassium sulfate treatment solution containing a zirconium 20 compound according to the present invention having different electric conductivities is electrolyzed. [Fig. 9] Fig. 9 is a graph showing a relationship between the pH of a treatment solution, and the film coating amount 25 converted to zirconium of a primer coating film, when sodium sulfate treatment solutions containing a zirconium compound according to the present invention having different pH are electrolyzed. [Fig. 10] 30 Fig. 10 is a graph which shows the storage stability of sodium sulfate treatment solutions containing a zirconium compound according to the present invention having different pH which have been allowed to stand at 40°C for 2 weeks, and shows a relationship between the pH 35 and the results of storage stability evaluation of the solutions. [Fig. 11] - 12 - Fig. 11 is a graph showing a relationship between the sodium sulfate concentration (mass %) and the electric conductivity of a solution, wherein zirconium sulfate has been added to an aqueous sodium sulfate 5 solution so as to provide a zirconium concentration of 10 mg/L, and sulfuric acid is added thereto so that pH of the solution has been adjusted to 1,5 or 2.5. [Fig. 12] Fig. 12 is a graph showing a relationship between a 10 sodium sulfate concentration (mass %), and the electric conductivity of a treatment solution, wherein zirconium sulfate has been added to an aqueous sodium sulfate solution so as to provide a zirconium concentration of 2000 mg/L, and sulfuric acid is added thereto so that pH 15 of the solution has been adjusted to 1.5 or 2.5. [Figs 13] Fig. 13 is a graph showing a relationship between the current density during electrolytic treatment, and the film coating amount converted to zirconium of a 20 primer coating film in the case of a conventional zirconium sulfate treatment solution, and in the case of a sodium sulfate treatment solution containing a zirconium compound according to the present invention. The graph shows that the zirconium deposition amount is 25 stable, even if the zirconium concentration is changed. Modes for Carrying Out the Invention 10013] The present invention relates to a process for 30 producing a steel sheet for a container material, wherein a tin-plated steel sheet is subjected to a cathodic electrolytic coating treatment in a treatment solution which does not contain a chromium compound, fluorine, or nitrate nitrogen. The steel sheet for a container 35 material having a small environmental load which has been obtained by the present invention is a steel sheet wherein a cathodic electrolytic coating treatment layer - 13 - comprising a zirconium compound has been formed on the surface of a tin-plated steel sheet. The present invention specifically relates to a process for producing a steel sheet for a container 5 material having a small environmental load, wherein a cathodic electrolytic coating treatment film can be obtained by cathodic electrolytic coating treatment in an aqueous alkali metal sulfate solution comprising a zirconium compound which does not contain a chromium 10 compound, fluorine or nitrate nitrogen. The best mode for carrying out the invention will be explained below, [0014] 15 The type of the steel sheet to be used in the present invention is not particularly limited. It is possible to use a steel sheet which is the same as the steel sheet which has been used for materials for containers. 20 [0015] The type of the steel sheet to be used in the cathodic electrolytic coating treatment according to the present invention is not particularly limited. However, 25 a tin-plated steel sheet may be most suitable s the environmentally friendly steel sheet for a container material according to the present invention, in view of the reasons such as good record of use in canning applications, freedom from problems in food safety and 30 sanitation, superiority in corrosion resistance, superiority in formability, and comparatively low cost as compared with that of other plating. [0016] The tin-plated steel sheet to be used in the present 35 invention may be a conventional electroplated tin plate, and may be treated by iron-tin alloying (reflow treatment) after the tin plating, as desired. The amount - 14 - of tin plating may preferably be in the range of 0.5 to 12.0 g/m2 from the viewpoint of suppression of iron dissolution from dented parts of the film laminate or coating. If the amount of tin plating is less than 0.5 5 g/m2, the amount of iron dissolution after denting becomes greater and the corrosion resistance falls, so this may not be preferred. On the other hand, even if the amount of tin plating exceeds 12.0 g/m2, the functions are not particularly obstructed, but in the production process, 10 the tin easily sticks to and builds up on the rolls etc., and causes dents or the plating costs swells more than necessary. Thus, this may not be preferred. [001°71 15 The environmentally friendly steel sheet for a container material according to the present invention may not necessarily be a plated steel sheet. However, in order to secure a sufficient corrosion resistance with respect to contents to be contained in a container 20 material, the surface of the side of the container material to be in contact with the contents after the can manufacturing may preferably be plated with tin or an iron-tin alloy. When the tin oxide layer present on the surface of a tin-plated steel sheet is too thick, even in 25 the case of the formation of a zirconium compoundcontaining coating film on the tin oxide layer, the tin oxide layer so fragile that the coating may be peeled off together with the tin oxide layer, to thereby deteriorate the coating adhesion. Accordingly, it is preferred to 30 remove the tin oxide layer, immediately before the cathodic electrolytic coating treatment. [0018] Fig. 1 is a graph showing the results of evaluating the coating adhesion in terms of T-peel strength 35 appearing hereinafter by using a zirconium compoundcontaining coating film having an amount converted to zirconium of 2to 4 mg/m2 which has been formed by using a - 15 zirconium sulfate electrolytic treatment on a tin-plated steel sheet (tin coating amount on one side: 2.8 g/m2) which has been subjected to a tin oxide removal treatment while changing the immersion time in sulfuric acid. 5 [0019] As can be seen from Fig. 1, when the amount of tin oxide on the tin plating is in the range of 0 mC/cm2 to 3.5 mC/cm2 measured by the electrolytic stripping method, the coating adhesion is stable at a T-peel strength of 60 10 or more. On the other hand, when the amount of tin oxide exceeds 3.5 mC/cm2, the coating adhesion is abruptly decreased. It is presumed that an increase in the amount of tin oxide reduces the wettability of the surface, and accordingly the zirconium compound-containing coating 15 film is not deposited uniformly during the electrolytic coating treatment of zirconium sulfate, to thereby decrease the strength of coating adhesion. When the amount of tin oxide on the tin plating exceeds about 3.5 mC/cm2, the tin plating comes to be entirely covered with 20 the tin oxide layer, and accordingly the tin oxide may easily be peeled off from the fragile tin oxide layer, at the time of the forming thereof or under the application of an impact. It is presumed that such a phenomenon causes a decrease in the coating adhesion. 25 [0020] For the above reasons, in order to stabilize the attachment of a zirconium compound on the tin-plated layer or the iron-tin alloy layer, it is preferred to remove the tin oxide layer of the tin-plated steel sheet 30 so as to provide a level thereof of 3.5 mC/cm2 or less measured by the electrolytic stripping method. [0021] From the viewpoint of improvement in the adhesion of the film or coating, it is preferred that no tin oxide 35 layer is present at all. However, even if the tin oxide layer is completely removed, the tin will be oxidized at the uppermost surface, provided that there is even a - 16 - little oxygen present. Therefore, the film lamination or coating of the tin plated surface in a state where no tin oxide is present at all, is difficult by ordinary facilities. Even if this could be realized, the 5 manufacturing costs would swell, so this may not be preferred. [0022] If the tin oxide layer on the tin plating is removed to 0.01 mC/cm2, an equivalent adhesion may be obtained as 10 the state thereof with substantially no tin oxide layer, so the thickness of the tin oxide layer may preferably be in the range from 0.01 mC/cm2 to 3.5 mC/cm2. If the manufacturing costs is not considered, the most preferable lower limit of the thickness of the tin oxide 15 layer is 0 (mC/cm2). The more preferable upper limit of the thickness of the tin oxide layer may be 3.0 (mC/cm2) (mC/cm2) [0023] Herein, the electric stripping method refers to a 20 method of applying the principle of constant current coulometry for constant current electrolysis of a test piece, wherein the change in potential of the test piece accompanying electric stripping is recorded by using a pen recorder, and the amount of electricity, (that is, 25 the amount of deposition of surface tin and the oxide film) is measured from the electrolysis time-potential curve. 10024] As the method of removing the tin oxide layer which 30 has been formed on a tin-plating layer or iron-tin alloy layer, it is most desirable to use a treatment by using cathodic electrolysis in a sodium carbonate or sodium hydrogen carbonate solution, since the tin oxide layer may reliably be removed in a short time and almost no tin 35 dissolving-out is observed. [0025] When a tin-plated steel sheet is subjected to cathodic electrolysis in an aqueous solution of sodium carbonate or sodium hydrogen carbonate, the concentration range of sodium carbonate or sodium hydrogen carbonate may preferably be 1 mass % to 5 mass When the 5 concentration of the aqueous solution of sodium carbonate or sodium hydrogen carbonate is less than 1 mass %, the tin oxide layer may sometimes remain and accordingly this may not be preferred. When the concentration of the aqueous solution of sodium carbonate or sodium hydrogen 10 carbonate exceeds 5 mass %, sufficient washing with water may be required after the treatment, otherwise the sodium carbonate or sodium hydrogen carbonate may sometimes remain, and accordingly this may not be preferred. When the solution temperature during the electrolytic coating 15 treatment is low, the solubility of sodium carbonate or sodium hydrogen carbonate becomes lower, and accordingly the solution temperature may preferably be 5°C or more. The upper limit of the solution temperature is not particularly limited, and any temperature can be used as 20 long as it does not make the handling thereof dangerous. [00261 When the current density during the cathodic electrolysis is too low, the removal of the tin oxide layer may become uneven, and accordingly the treatment 25 with 1 A/dm2 or more of the current density may be preferred. The upper limit of the current density is not be particularly limited, but when the current density is too high, the removal efficiency of tin oxide does not considerably be changed, despite the presence of vigorous 30 generation of hydrogen, and accordingly about 10 A/dm2 or less may be preferred. (00271 Further, it is also preferred to use a method of removing the tin oxide layer which has been formed on a 35 tin-plated layer or the iron-tin alloy layer by the immersion thereof in an aqueous sulfuric acid solution. The concentration of the aqueous sulfuric acid solution 18 - may preferably be 0.5 mass % or more and 5 mass %.or less. When the concentration of the aqueous sulfuric acid solution is less than 0.5 mass %, the tin oxide layer cannot be fully removed and accordingly this may 5 not be preferred. The higher the concentration of the aqueous sulfuric acid solution, the easier the tin oxide is removed. However, a higher concentration thereof may cause rough skin or the residual sulfuric acid so as to reduce the coating adhesion, and accordingly the upper 10 limit of the concentration of the aqueous sulfuric acid solution may preferably be 5 mass % or less. The temperature of the aqueous sulfuric acid solution may preferably be in the range of 10°C or more and 80°C or less. When the liquid temperature of the aqueous 15 sulfuric acid solution is less than 10°C, the rate of removing tin oxide becomes very low, and tin oxide may sometimes remain, and accordingly, this may not be preferred. On the other hand, when the temperature of the aqueous sulfuric acid solution exceeds 80°C, the rate 20 of removing tin oxide becomes significantly high, and the tin-plated surface may excessively be etched, so as to provide uneven gloss, and accordingly, this may not be preferred. [0028] 25 In the cathodic electrolytic coating treatment according to the present invention, a tin-plated steel sheet or an iron-tin alloy plated steel sheet is subjected to a cathodic electrolytic coating treatment in 30 an aqueous solution of an alkali metal sulfate which does not contain a chromium compound, fluorine or nitrate nitrogen, but contains a zirconium compound, wherein the zirconium concentration in the cathodic electrolytic coating treatment solution is 10 mg/L or more and 2000 35 mg/L or less, the electric conductivity of the treatment solution is 0.2 S/m or more and 6.0 S/m or less, and the - 19 - pH of the treatment solution is 1.5 or more and 2.5 or less. [0029] The purpose of using a zirconium compound as a 5 primer agent is to coat the surface of a steel sheet with a zirconium oxide hydrate, so as to form hydrogen bonding between the zirconium oxide hydrate and hydroxy groups contained in the resin coating layer, similarly as in the case of the chromate treatment, to thereby enhance the 10 adhesion with the resin coating film. [0030] For the purpose of obtaining an effect similar to that of a zirconium compound, the present inventors have examined various metal-based oxides for suitability as 15 the cathodic electrolytic coating treatment agent. As a result, the present inventors have found that the cathodic electrolytic coating treatment with a zirconium compound provided the best adhesion with a resin coating film (particular, in view of the secondary adhesion after 20 retort treatment). As a metal salt not containing a chromium compound, fluorine or nitrate nitrogen to be used in the process of depositing:a zirconium compound by using a cathodic electrolytic coating treatment, it is possible to use a carbonate, a sulfate, a halogenated 25 salts Among these, zirconium sulfate may be m°st preferred, since its aqueous solution is stable, and industrially easily available. 10031] As the process of forming a zirconium compound into 30 a cathodic electrolytic coating treatment layer, it is general to use a cathodic electrolytic coating treatment in an aqueous solution of zirconium fluoride. Since a fluoride-containing bath has a heavy load to the waste treatment to be used therefor, Patent Document 3 as 35 mentioned above proposes the use of zirconium sulfate in stead of a zirconium fluoride compound, in the cathodic electrolytic coating treatment. - 20 - However, the method of forming a coating film by the cathodic electrolytic coating treatment of a sulfate compound has a characteristic that the deposition of a zirconium oxide hydrate is markedly changed depending on 5 the current density, and accordingly it is difficult to keep the coating amount of a zirconium oxide hydrate in an appropriate range. When the coating amount of the coating film of a zirconium oxide hydrate is changed, it causes unevenness in the coating adhesion and film 10 adhesion, and accordingly this may not be preferred. Further, the aqueous zirconium sulfate solution has a problem in storage stability, that is, when a high concentration zirconium solution is stored at a high temperature (40°C or higher) for a long time, precipitates 15 of a zirconium oxide hydrate is liable to be formed. [0032] In view of these problems, in the present invention, a zirconium compound is added to an aqueous solution of an alkali metal sulfate, so that the deposition behavior 20 of the zirconium oxide hydrate is stabilized with respect to the current density during the cathodic electrolytic coating treatment, as well as the storage stability of the solution is enhanced. As a result, the unevenness in the coating amount of a zirconium oxide hydrate can be 25 reduced or obviated, even when the operation condition slightly is changed to a certain extent, and accordingly a drastic enhancement in the stability of the solution is attained during a long-time use. [0033] 30 First, a mechanism of forming a zirconium oxide hydrate coating by cathodic electrolytic coating treatment of a tin-plated steel sheet in an aqueous solution of alkali metal sulfate containing a zirconium compound will be explained (hereinafter, there will be 35 described an embodiment using "an aqueous solution of zirconium sulfate to which sodium sulfate has been added" as an example). - 21 - It is presumed that zirconium is be present as ZrO2 in an aqueous sodium sulfate solution, It is also presumed that ZrO2+ is stable at a low pH region, but the stability of ZrO2 becomes lower as pH becomes higher, so 5 that it is liable to be deposited as a hydrated oxide. When sodium sulfate is subjected to a cathodic electrolytic coating treatment, hydrogen gas is generated at the interface with the solution at the tin-plated steel sheet side of the cathode, and as a result, the 10 hydroxide ion concentration in the vicinity of the interface may become higher (i.e., pH id increased). It is presumed that, when the pH of the interface becomes higher, ZrO2 begins to be deposited as a hydrated oxide, and a coating film of zirconium oxide hydrate is formed 15 on the tin-plated steel sheet. [00341 Then, the effect of adding a zirconium compound to an aqueous sodium sulfate solution will be explained. As described above, when a tin-plated steel sheet is 20 subjected to a cathodic electrolytic treatment in an aqueous zirconium sulfate solution, the interface pH is increased so as to form a zirconium hydroxide coating film. Since the diffusion rate of ions in an aqueous solution is slow, it is presumed that a significantly 25 thick high--pH layer is formed in the vicinity o the interface, and when the interface pH reaches a condition for the deposition of zirconium hydroxide, a zirconium oxide hydrate coating film is formed abruptly. Accordingly, in a cathodic electrolysis solution 30 comprising a zirconium sulfate alone, it is expected that the coating amount of the zirconium hydroxide coating film may be changed markedly depending on variation in current density or pH. As a first effect of using an aqueous solution of an 35 alkali metal sulfate as a base solution, the aqueous solution of the alkali metal sulfate acts as an electrolyte, and it reduces the electric resistance of - 22 -- the solution. This has an effect of reducing the burden or load to the rectifier. As a second effect, the alkali metal ion neutralizes the hydroxide ions which have been formed at the 5 interface of the tin-plated steel sheet and the cathodic electrolytic solution by the cathodic electrolytic treatment, so that a high pH layer having an appropriate thickness can be formed on the interface, to thereby provide an effect of suppressing the variation in the 10 coating amount of the zirconium hydroxide coating film due to the variation in the current density variation or in the pH (specifically, in a case where ions such as Na+ and K+ are present in large quantities near the electrode). 15 [0035] Then, there is described the effect on the coating amount in the cathodic electrolytic treatment and the current density during the cathodic electrolytic treatment, and on the pH of the solution, in a case where 20 an aqueous zirconium sulfate solution alone is subjected to cathodic electrolytic treatment of the prior art, and in a case where an aqueous solution of an alkali metal sulfate containing a zirconium compound according to the present invention is subjected to cathodic electrolytic 25 treatment. [0036] Fig. 2 is a graph showing a relationship between the current density during the cathodic electrolytic coating treatment and the zirconium compound coating amount which 30 has been deposited to the steel sheet, when a tin-plated steel sheet (amount of tin-plating: 2.8 g/m2) which has been subjected to a tin oxide removal treatment, is subjected to a cathodic electrolytic coating treatment, by using an aqueous solution, wherein zirconium sulfate 35 has been added to a 402 mass %-aqueous sodium sulfate solution so as to provide a concentration converted to zirconium of 400 mg/L, and the pH thereof has been - 23 - regulated to 1.9 by the addition of sulfuric thereto. As can be seen from Fig. 2, in a case where the cathodic electrolytic coating treatment is performed in a treatment solution comprising zirconium sulfate alone, 5 the increasing rate of the coating amount of the zirconium compound-containing coating film is small in the low current density region, but the increasing rate of the coating amount of the zirconium compoundcontaining coating film tends to be increased abruptly at 10 a specific current density. In contrast thereto, in a case where a sodium sulfate treatment solution to which zirconium compound has been added is used, the variation in the coating amount of the zirconium compound with respect to an increase in the current density is small 15 (i.e., the degree of increase in the coating amount of the zirconium compound-containing coating film with respect to the increase in the current density is moderate), and accordingly, this operational stability is high and preferable. 20 The alkali metal sulfate may appropriately be selected from sodium sulfate and potassium sulfate, since both of them give a similar effect. [00371 As describe above, in a chromium-free treatment 25 process according to the present invention wherein a zirconium compound is added to an aqueous solution of an alkali metal sulfate such as sodium sulfate and potassium sulfate, even if the current density condition may be changed to a certain extent, the variation in the coating 30 amount of a zirconium compound-containing coating film is small, and a stable operation can be realized. [00301 Fig, 3 is a graph showing a relationship between the pH of a solution and the amount of the zirconium 35 compound-containing coating film on a tin-plated steel sheet, when the tin-plated steel sheet is subjected to a cathodic electrolytic coating treatment at a current - 24 - density of 5 A/dm2 for 5 seconds, by using an aqueous zirconium sulfate solution wherein the pH has been lowered by adding sulfuric acid to an aqueous zirconium sulfate solution of pH 1.9, and an aqueous sodium sulfate 5 solution containing a zirconium compound wherein pH is raised by mixing sodium sulfate with an aqueous zirconium sulfate solution of pH 1.6. As can be seen from Fig. 3, in a case where the cathodic electrolytic coating treatment is performed in a 10 treatment solution containing zirconium sulfate alone, a change in pH leads to extreme variation in the coating amount of the zirconium compound-containing coating film. On the other hand, in the case of an aqueous sodium sulfate solution containing a zirconium compound, even 15 when pH is changed, the variation in the coating amount of the zirconium compound-containing coating film is small. Accordingly, in this case, even if pH is lowered by a continuous cathodic electrolytic coating treatment, the coating amount of the zirconium compound-containing 20 coating film does not show an abrupt decrease. That is, the coating amount is stable. (Ob397 As described above, as compared to an aqueous zirconium sulfate solution, in the case of a solution 25 wherein a zirconium compound is added to an aqueous solution of an alkali metal sulfate such as sodium sulfate and potassium sulfate, the variation in the coating amount of the zirconium compound-containing coating film is small with respect to a change in the 30 electrolytic condition, and accordingly it is easy to keep the coating amount of the zirconium compoundcontaining coating film in an appropriate range, and stable production can be attained. [00401 35 With regard to the concentration of the alkali metal sulfate of an aqueous solution of the alkali metal sulfate containing a zirconium compound, the alkali metal 25 - sulfate may be deposited in an environment of 5°C or less, and accordingly the upper limit of the concentration of the alkali metal sulfate may preferably be 8.0 mass % or less. 5 [0041] With regard to the lower limit of concentration of the alkali metal sulfate in an aqueous solution of the alkali metal sulfate containing a zirconium compound, the alkali metal sulfate may not be required, as long as the 10 optimum range of the electric conductivity and the optimum pH of the solution appearing hereinafter are to be satisfied. However, with the aqueous zirconium sulfate solution alone, as described hereinabove, not only the coating amount of the zirconium compound- 15 containing coating film may become unstable with respect to variation in the electrolysis condition, but also the presence of the alkali metal ion in an aqueous solution can enhance the stability of the solution, and accordingly, the alkali metal sulfate is essential. 20 Incidentally, when the lower limit of the zirconium concentration in an aqueous zirconium sulfate solution is 10mg/L and the upper limit of pH is 2.5, 0.1 mass % of the alkali metal sulfate may be required, and accordingly the lower limit range of the concentration of the alkali 25 metal sulfate may be 0.1 mass %. [0042] Then, an appropriate range of the coating amount of She zirconium compound-containing coating film will be explained. 30 Since the coating material adhesion of a tin-plated steel sheet to which a zirconium compound-containing coating film has been applied by a cathodic electrolytic coating treatment may be changed depending on the coating amount of the zirconium compound-containing coating film, 35 it is important to clarify the appropriate range of the coating amount of the zirconium compound-containing coating film. 26 -- Fig. 4 is a graph showing a relationship between the film coating amount converted to zirconium and the coating material adhesion after coating of a tin-plated steel sheet which has been subjected to a cathodic 5 electrolytic coating treatment in an aqueous zirconium sulfate solution. Herein, the coating material adhesion is evaluated by using the T-peel strength appearing hereinafter. As can be seen from Fig. 4, the T-peel strength is 10 stable at 60 N/10 mm or more in the range of a film coating amount converted to zirconium of Oel mg/mz to 20 mg/mz e However, in the zirconium compound-containing coating film amount outside of this range, the T-peel strength is not stable and a sufficient forming adhesion 15 after the coating cannot be obtained. [00431 Then, the concentration of zirconium to be contained in the cathodic electrolytic coating treatment solution according to the present invention will be explained. 20 As shown in Fig. 5, in a case where the concentration of zirconium contained in the cathodic electrolytic coating treatment solution according to the present invention is less than 10 mg/L, for example at a low current density such as 2 A/dmz, the coating amount of 25 the zirconium compound-containing coating film after the cathodic electrolytic coating treatment may become lower than the lower limit as describe above of Oel mg/mz of the film coating amount converted to zirconium, and accordingly this may not be preferred. 30 Therefore, the zirconium concentration in an aqueous solution of an alkali metal sulfate containing a zirconium compound may preferably be 10 mg/L or more, [00441 On the other hand, when the concentration of 35 zirconium contained in the cathodic electrolytic coating treatment solution exceeds 2000 mg/L, the storage stability of the solution may be reduced, and after a - 27 - long-time storage, the sludge of zirconium oxide hydrate may be deposited as shown in Fig. 6, and this may not be preferred. Further, when the concentration of zirconium 5 contained in the cathodic electrolytic coating treatment solution exceeds 2000 mg/L, the zirconium compoundcontaining coating film on the steel sheet surface tends to be uneven, and sludge tends to be formed during the electrolytic treatment, and accordingly this may not be 10 preferred. When the concentration of an aqueous zirconium sulfate solution is high, the amount of the solution to be taken out during a continuous threading operation becomes large, and this is not economical. For the above reasons, the concentration of 15 zirconium contained in the cathodic electrolytic coating treatment solution according to the present invention may preferably be 10 mg/L or more and 2000 mg/L or less. [0045] The electric conductivity of the cathodic 20 electrolytic coating treatment solution according to the present invention may be changed depending on the concentration of an aqueous solution of an alkali metal sulfate, the amount of a zirconium compound and pH, but an appropriate range of the electric conductivity is 0.2 25 S/m or more and 6.0 S/m or less. Hereinbelow, the reasons therefor will be explained in Fig. 7 and Fig. 8. [00461 - Fig. 7 is a graph showing a relationship between the electric conductivity of a solution and the rectifier 30 voltage, when the cathodic electrolytic coating treatment of a tin-plated steel sheet is performed by the changing current density from 1 A/dm2 to 10 A/dm2 by using a solution having a zirconium concentration of 10 mg/L and pH of 1.9, wherein the electric conductivity is changed 35 by changing the concentration of an aqueous solution of sodium sulfate. As can be seen from Fig. 7, when the electric conductivity of the solution becomes lower than - 28 - 0.2 S/m, the voltage of the rectifier comes to exceed 25 V, even if the current density is 1 A/dmZ, to thereby increase the load on the rectifier. In view of the use of the present chromium plating 5 equipment as it is without changing the electrode length or the electrolytic treatment path number, the voltage during the operation should be about 25 V at the highest, since the upper limit of voltage of the actual rectifier is generally about 25 V. 10 On the other hand, if the predetermined value of the current density is lowered, the voltage can be lowered. However, an excessively low current density can make the depositing property of a zirconium compound unstable, and this may not be preferred, but about 1 A/dmZ at the lowest 15 may be preferred. Thus, as can be suggested from Fig. 7, the lower limit of the electric conductivity of an electrolytic solution may preferably be 0.2 S/m or more. [00471 An optimum current density when a tin-plated steel 20 sheet or an iron-tin alloy-plated steel sheet is subjected to a cathodic electrolytic coating treatment by using the cathodic electrolytic coating treatment solution according to the present invention may appropriately be selected based on the coating amount of 25 a zirconium compound-containing coating film which is to be deposited on the tin-plated steel sheet or the irontin alloy-plated steel sheet. However, if the current density is too high, the hydrogen generation from the steel sheet side as a cathode side becomes vigorous, and 30 accordingly the deposited zirconium compound may be peel off by the thus generated hydrogen gas, whereby uneven coating is liable to be caused. Accordingly, the electrolytic treatment at about 30 A/dmZ or less may be preferred. 35 [0048] Then, the upper limit of the electric conductivity of the cathodic electrolytic coating treatment solution - 29 - according to the present invention will be explained. When the concentration of an aqueous solution of an alkali metal sulfate in the cathodic electrolytic coating treatment solution according to the present invention is 5 increasingly raised, the load to the rectifier becomes smaller, and accordingly the current density can be raised. However, if the electric conductivity becomes too high, the coating amount of zirconium compound-- containing coating film tends to be decreased, to thereby 10 cause an uneven outer appearance, which may not be preferred. [0049] Fig. 8 is a graph showing the electric conductivity of the solution, and the coating amount converted to 15 zirconium of a zirconium compound, when a tin--plated steel sheet or a iron-tin alloy-plated steel sheet is subjected to a cathodic electrolytic coating treatment at a current density of 15 A/dm2 using a solution with a zirconium concentration of 50 mg/L and pH of 1.7, wherein 20 the electric conductivity is changed by changing the concentration of an alkali metal sulfate in an aqueous sodium sulfate solution containing a zirconium compound, or an aqueous potassium sulfate solution containing a zirconium compound. 25 [0050] As can be seen from Fig. 8, the coating amount of the zirconium compound-containing coating film tends to be decreased, as the electric conductivity of the solution comes to exceed about 6.0 S/m. 30 In the reaction at the cathode side where no alkali metal ions are present, hydrogen ions first receive electrons to become hydrogen gas to be released, and the hydroxide ion concentration at the interface is increased (i.e., pH is increased), and as a result, zirconium oxide 35 ions (ZrO2+) are deposited as zirconium oxide hydrate. In contrast thereto, it is presumed, when alkali metal ions are present, Na ions also participate in the transfer of - 30 - electrons at the cathode interface (while the deposited metal Na is immediately dissolved and dissociated), and accordingly as compared to a case where no alkali metals are added, the concentration of hydroxide ions which have 5 been formed at the interface becomes lower, whereby the deposition of zirconium oxide hydrates is inhibited. Accordingly, if the electric conductivity is excessively increased by adding the alkali metal ions, the interface pH at the cathode side is less liable to be 10 increased, and the deposition of zirconium hydroxides becomes more difficult. Accordingly, the electric conductivity of the solution may preferably be 6.0 S/m or less. [0051) 15 Next, the optimum pH range of the cathodic electrolytic coating treatment solution according to the present invention will be explained. With regard to the lower limit of pH of the cathodic electrolytic coating treatment solution according to the 20 present invention, when the pH becomes lower, as shown in Fig. 9, the coating amount of the zirconium compoundcontaining coating film tends to be decreased, and at less than pH 1.5, the film coating amount converted to zirconium does not reach the lower limit target of 0,1 25 mg/m2, and accordingly this may not be preferred. It is presumed that the mechanism of the deposition of the zirconium compound-containing coating film is the deposition of zirconium oxide hydrate due to an increase in the concentration of hydroxide ions (i.e., an increase 30 in pH) at the interface caused by the hydrogen gas generation during the cathodic electrolytic treatment, and when the pH of the cathodic electrolytic coating treatment solution is low, the hydroxide ion concentration at the cathode cannot be increased, and as 35 a result, the formation of the coating film of the zirconium oxide hydrate becomes difficult. When the amount of the zirconium oxide hydrate - 31 - deposited is small, the lower limit (a film coating amount converted to zirconium of 0.1 mg/m2 or more) of the amount of the zirconium compound-containing coating film capable of giving the favorable adhesion of the coating 5 material cannot be obtained, and this may not be preferred. Accordingly, the lower limit of pH of the cathodic electrolytic solution according to the present invention may preferably be 1.5 or more. 10 [0052] Then, the upper limit of pH of the cathodic electrolytic coating treatment solution according to the present invention will be explained. Fig. 10 is graph showing the storage stability 15 (which has been evaluated from the presence or absence of precipitate generation in a solution which has been allowed to stand at 40°C for 2 weeks) of an aqueous solution of sodium sulfate containing a zirconium compound, and an aqueous solution comprising zirconium 20 sulfate alone. As can be seen from Fig. 10, in the aqueous solution comprising zirconium sulfate alone, the storage stability of the solution is decreased when pH exceeds 2.1. In an aqueous zirconium sulfate solution, zirconium 25 is present in the form of ZrO2,+Accordingly, i1t is considered that as the pH becomes higher, ZrO2 tends to be deposited in the form of a hydrated oxide, and when an aqueous zirconium sulfate solution having a high pH is stored for a long time or at a high temperature, ZrO2 30 ions which have been dissolved in the solution are deposited as a zirconium oxide hydrate, and the resultant deposition becomes a white precipitate. [0053] On the other hand, in the case of an aqueous sodium 35 sulfate solution containing a zirconium compound as shown in Fig. 10, it is found that the upper limit of the stable pH region of the solution is extended to pH 2.5. _, 32 - This is probably because, in the aqueous solution of an alkali metal sulfate, hydroxide ions are coordinated with the dissociated alkali metal. ions, and as a result, the number of hydroxide ions to be coordinated with ZrO2 5 become smaller, to thereby enhance the stability of ZrO2,+ In the case of an aqueous solution of sodium sulfate containing a zirconium compound, a white precipitate may be formed at a pH of 2.5 as the upper limit, and accordingly the pH may preferably be 2.5 or less. 10 Further, when a continuous electrolytic treatment operation is performed by using a high-pH solution, sludge is liable to be formed in a large amount, and accordingly also in view of the operability and product quality, the pH may preferably be 2.5 or less. 15 [0054] As as alkali metal sulfate to be used in the cathodic electrolytic coating treatment solution according to the present invention, sodium sulfate and potassium sulfate may be preferred in view of easy 20 availability and easy handling. [0055] Fig. 11 is a graph showing a relationship between the sodium sulfate concentration (mass % ) and the electric conductivity of an aqueous solutions of sodium 25 sulfate containing a zirconium compound (the zirconium concentration: 10 mg/L). In this case, the pH of the solution has been regulated to 1.5 and 2.5 by adding sulfuric acid. Fig. 12 is a graph showing a relationship between 30 the sodium sulfate concentration (mass % ) and the electric conductivity of an aqueous solutions of sodium sulfate containing a zirconium compound (the zirconium concentration: 2000 mg/L). In this case, the pH of the solution has been regulated to 1.5 and 2.5 by adding 35 sulfuric acid. [0056] As can be seen from Fig. 11 and Fig. 12, the 33 - electric conductivity of the cathodic electrolytic coating treatment solution according to the present invention may be changed depending on the concentration of the zirconium compound, the concentration of the 5 alkali metal sulfate, and pH. Accordingly, after the determination of the concentration of a zirconium compound, it may be preferred to regulate the pH and the electric conductivity to be in an appropriate range by adding, as appropriate, a suitable amount of the alkali 10 metal sulfate and concentrated sulfuric acid. [0057] With regard to the temperature of a treatment solution during the cathodic electrolytic coating treatment according to the present invention, a range of 15 5°C to 50°C provides a high deposition efficiency of a zirconium compound and a small variation in the concentration due to evaporation, and accordingly this range may be preferred. When the temperature of the solution becomes high, 20 the rate of supplying hydrogen ions to the cathode interface is increased, and the zirconium compound is less liable to be deposited. Accordingly, in order to obtain an appropriate zirconium coating amount, the current density should be enhanced, and as a result, the 25 load to the rectifier becomes excessive. Accordingly, the temperature of the solution may preferably be 50°C or less. Further, if the temperature of the solution is high, the stability of the solution is decreased, and zirconium 30 oxide hydrate is liable to be deposited. For this reason as well, the upper limit of the temperature of the solution may preferably be 50°C or less. [0058] With regard to the lower limit of the temperature of 35 the solution during the cathodic electrolytic coating treatment, when the concentration of the alkali metal 34 - sulfate is high, the alkali metal sulfate may be deposited, if the temperature is below 5°C. Therefore, the lower limit of the temperature of the solution may preferably be 5°C or more. 5 [0059] In the cathodic electrolytic coating treatment according to the present invention, after the treatment, it is preferred to conduct washing with water or washing with warm water. When the cathodic electrolytic coating 10 treatment solution according to the present invention is subjected to an electrolytic treatment, sulfate ions (SO42-) may remain in the zirconium compound-containing coating film, and the excess sulfate ions remaining in the coating film may cause a color change so that a stain 15 on the surface may be caused and the adhesion after coating may be decreased. Accordingly, such residual ions may not be preferred. [0060] After the cathodic electrolytic coating treatment 20 with the cathodic electrolytic coating treatment solution according to the present invention, it is sufficient to conduct washing with water or washing with warm water, in an extent such that the washing operation may not be a heavy burden. The amount of sulfate ions (S042-) 25 remaining in the zirconium compound-containing coating film may preferably be controlled to be within a range (0.2 mg/m2 or more and 7 mg/m2 or less) which is almost equal to that of the remaining sulfate ions after the chromate treatment. 30 [0061] After the cathodic electrolytic coating treatment, it is preferred to conduct drying so as to evaporate the moisture. The drying operation may conducted by natural drying or hot-air drying. When the coating amount of the 35 zirconium compound is large, much moisture may remain in the coating film. Accordingly, in this case, hot-air drying may be more preferred. - 35 - The steel sheet for a container material according to the present invention as described above may preferably be used for the production of a laminated 5 steel sheet for a container material. The structure of such a laminated steel sheet for a container material using a steel sheet for a container material according to the present invention is not particularly limited. For example, such a laminated steel sheet for a container 10 material may preferably comprise, at least, a steel sheet for a container material as describe hereinabove, and a laminate film disposed thereon.