The present invention relates to a coated steel sheet having excellent corrosion resistance and contamination resistance. Particularly, the present invention relates to a coated steel sheet having excellent corrosion resistance and raindrop contamination resistance in a case where defects had occurred in a coating film. Priority is claimed on Japanese Patent Application No. 2014-145821, filed on July 16,2014, the content of which is incorporated herein by reference. [Related Att] [0002] External sheets of metallic apparatuses such as home appliances used indoors and outdoors are required to have excellent corrosion resistance. In addition, contaminants often adhere to the external sheet of a metallic apparatus for outdoor use, and thus the external sheet is required to have excellent contamination resistance. In particular, streaky or spot-like contaminants are likely to adhere to the external sheet of a metallic apparatus used in a rainy envirmm1ent due to rainwater containing dust such as soil particles, and thus the external sheet is required to have excellent raindi·op contamination resistance. For these reasons, a coated steel sheet in which a coating film for improving corrosion resistance and contamination resistance is formed on a substrate steel sheet is generally used as the external sheet of a metallic apparatus. [0003] Patent Document 1 discloses a resin coated metal sheet, which has excellent raindrop contamination resistance and contamination resistance and also has a high - I - degree of formability. According to Patent Document I, it is said that a high degree of workability can be obtained without cracks in aT-bend test. In addition, Patent Document 2 discloses a chromium-free coated steel sheet, which exhibits excellent corrosion resistance without containing a chromium-based anti-rust pigment having a large environmental burden. According to Patent Document 2, it is said that by overlaying an undercoat coating film mixed with a chromium-free anti-rust pigment with a hydrophilic overcoat coating film, rainwater is prevented from remaining on the coating fihn surface as a thick water film, and thus a coated steel sheet which exhibits sufficient corrosion resistance can be obtained without chromium-based anti-rust pigment mixed therein. [Prior Art Document] [Patent Document] [0004] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-288963 [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2007-260953 [Disclosure of the Invention] [Problems to be Solved by the Invention] [0005] There may be cases where defects that reach the substrate steel sheet occur on the coating film surface in the external sheet (coated steel sheet) of the metallic apparatus used outdoors. In tllis case, there is concern that corrosion may progress from the defects as starting points in the substrate steel sheet. However, in Patent Documents 1 and 2, corrosion resistance in a case where defects had occurred in the -2- coating film is not considered. As long as the progress of corrosion in the substrate steel sheet can be suppressed even in a case where the substrate steel sheet is exposed, it is possible to significantly extend the life of the external sheet (coated steel sheet) of the metallic apparatus. [0006] An object of the present invention is to provide a coated steel sheet which has excellent contamination resistance (raindrop contamination resistance) and also has excellent corrosion resistance in a case where defects had occurred in a coating film. [Means for Solving the Problem] [0007] The gist of the present invention is as follows. [0008] (I) According to an aspect of the present invention, a coated steel sheet includes: a substrate steel sheet; an inner coating film disposed on the substrate steel sheet; and an outer coating film which is disposed on the inner coating film and acts as an outermost film, in which the outer coating film contains a silicate and a melamine, a region from an outer surface of the outer coating film to a depth of, but not including, 10 nm toward the inner coating film is referred to as an outermost layer of the outer coating film, a region from the outer surface to a depth of, but not including, 0.1 fll11 toward the illller coating film is referred to as a surface layer of the outer coating film, a region excluding the surface layer in the outer coating film is referred to as a body layer of the outer coating film, a Si content of the outermost layer is referred to as Shs in terms of unit mass%, a Si content of the surface layer is referred to as Sis in terms of unit mass%, a Si content of the body layer is referred to as Sin in terms of unit mass%, Sirs, Sis, and Sin simultaneously satisfY Sin< Sis< Sirs, 7.0 :S Sis :S 50, and 7.0 :S - 3 - II SiTs/SiB :<:50, aN content of the outermost layer is referred to as NTs in terms of unit mass%, aN content of the surface layer is referred to as Ns in terms of unit mass%, a N content of the body layer is referred to as N8 in terms of unit mass%, NTs, Ns, and NB simultaneously satisfy Ns < NTs, 30 :<: NTs :<:50, and 1.0 < NTs/NB :<:3.0, a depth profile curve of aN content of the outer coating film has an absolute maximum value in the region from a depth of 0 to a depth of, but not including, I 0 nm, and has a local maximum value itr a region from a depth of I 0 run to a depth of, but not including, 0.1 run, and a value obtained by dividing the local maximum value by the absolute maximum value is equal to or higher than 0.8 and lower than 1.0. (2) In the coated steel sheet described in (1 ), the outer coating film may contain, as the silicate, one or more of a tetraalkoxysilane and a partial hydrolysis condensates thereof. (3) In the coated steel sheet described in (1) or (2), the outer coating film may fmiher contain a polyester resin. ( 4) In the coated steel sheet described in any one of (1) to (3), the substrate steel sheet may be a galvanized steel sheet. [Effects of the Invention] [0009] According to the aspect of the present invention, it is possible to provide a coated steel sheet which is excellent in contamination resistance (raindrop contamination resistance) and also excellent in .corrosion resistance in a case where a defect had occurred in a coating film. Therefore, the coated steel sheet according to the aspect of the present invention can be suitably used as the external sheet of a metallic apparatus used outdoors. [Brief Description ofthe Drawings] -4- II [0010] FIG. I is a schematic sectional view of a coated steel sheet according to an embodiment of the present invention. FIG. 2 is an enlarged schematic sectional view illustrating main parts of the coated steel sheet according to the embodiment and is an enlarged schematic sectional view of a part corresponding to A in FIG. I. FIG. 3 shows a depth profile curve regarding theN content of an outer coating film of the coated steel sheet according to the embodiment. [Embodiment of the Invention] [00 II] Hereinafter, a preferred embodiment of the present invention will be described in detail. However, the present invention is not limited only to configurations disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention. [0012] The inventors intensively studied a coated steel sheet which is a coated steel sheet having good contamination resistance, specifically excellent raindrop contamination resistance and can suppress the progress of corrosion even in a case where defects that reach a substrate steel sheet are generated on the coating film surface, and as a result, obtained the following knowledge. FIGS. I and 2 show sectional schematic views ofthe coated steel sheet according to this embodiment as a reference. [0013] That is, in a case of a coated steel sheet I includes: a substrate steel sheet 2; an i1mer coating film 3 disposed on the substrate steel sheet 2; and an outer coating film 4 - 5 - which is disposed on the inner coating film 3 and acts as an outermost film, in which the outer coating film 4 contains silicate and melamine, a region from an outer surface 43 (coating film surface) of the outer coating film 4 to a depth of, but not including, 10 nm toward the i1mer coating film 3 is referred to as an outermost layer 42a of the outer coating film 4, a region from the outer surface 43 to a depth of, but not including, 0.1 pm toward the iimer coating filmJ is referred to as a surface layer 42 of the outer coating film 4, a region excluding the surface layer 42 in the outer coating film 4 is referred to as a body layer 41 of the outer coating film4, the average Si content of the outermost layer 42a is referred to as Shs in terms of unit mass%, the average Si content of the surface layer 42 is referred to as Sis in terms of unit mass%, and the average Si content of the body layer 41 is referred to as Sin in terms of unit mass%, Shs, Sis, and Sin simultaneously satisfY the following expressions 1, 2, and 3, the average N content of the outermost layer 42a is referred to as NTs in terms of unit mass%, the average N content of the surface layer 42 is referred to as Ns in terms of unit mass%, and the average N content ofthe body layer 41 is referred to as N8 in terms of unit mass%, NTs, Ns, and N8 simultaneously satisfy the following expressions · 4, 5, and 6, a depth profile curve 5 of theN content of the outer coating film4 has an absolute maximum value 51 in the region from a depth of 0 to a depth of, but not including, 10 nm, and has a local maximum value 52 in a region from a depth of I 0 nm to a depth of, but not including, 0.1 pm, and a value obtained by dividing the local maximum value 52 by the absolute maximum value 51 is equal to or higher than 0.8 and lower than 1.0, raindrop contamination resistance and corrosion resistance when defects are generated on the coating film surface (hereinafter, referred to as defect corrosion resistance) can be simultaneously improved. Sin< Sis < Shs ... (Expression I) - 6 - 7.0 <::Sis<:: 50 ... (Expression 2) 7.0 <:: Shs/SiB '<:50 ... (Expression 3) ... (Expression 4) 30 <:: NTs <::50 ... (Expression 5) ... (Expression 6) [0014] In general, in order to improve the contamination resistance of the coated steel sheet, particularly the raindrop contamination resistance, it is necessary to enhance the hydrophilicity (wettability) and the barrier properties of the outer coating fihn of the coated steel sheet. That is, by enhancing the hydrophilicity, contaminants on the coating fihn surface are easily washed, and by enhancing the barrier properties, contaminants are less likely to penetrate into the coating film. When the outer coating film of the coated steel sheet contains silicate and melamine, Si (silicon) derived from the silicate has an effect of enhancing the hydrophilicity of the outer coating film, and N (nitrogen) derived fi·om the melamine has an effect of enhancing the barrier properties of the outer coating film. [0015] In addition to the above-described knowledge, the inventors studied the relationship between the Si content of the outer coating film and the defect corrosion resistance. As a result, it became clear that when the Si content of the outer coating film satisfies the above conditions, qualitatively speaking, when the Si content increases toward the coating film surface, not only the hydrophilicity but also the defect corrosion resistance are improved together. Although a detailed mechanism for improving defect corrosion resistance is not clear yet, the following is presumed. [0016] -7- For example, in a case where a galvanized steel sheet is used as the substrate steel sheet and the substrate steel sheet is exposed due to scratches on the coating film surface, cations such as Zn2+ are eluted from the plating layer of the substrate steel sheet under a corrosive environment. These cations react with silicate ions such as (SiOJ/- derived from the silicate in the outer coating film to produce insoluble salts, which adhere to the defect pmiions. It is thought that these insoluble salts suppress the progress of corrosion of the substrate steel sheet. That is, it is thought that the silicate ions derived from the silicate act an inhibitor and improve the defect corrosion resistance of the coated steel sheet. In addition, as the Si content increases toward the coating film surface in this range, silicate ions are more likely to be eluted under a cmTOsive environment. Timt is, it is thought that when the Si content of the outer coating film satisfies the above expressions 1 to 3, silicate ions are suitably eluted from the outer coating film under the corrosive environment to act as an inhibitor, and as a result, the defect corrosion resistance of the coated steel sheet is preferably improved. [0017] More specifically, as the Si content increases toward the coating film surface in this range, the hydrophilicity of the outer coating film increases, and the wettability of the coating film surface increases. That is, it is thought that when the Si content of the outer coating film satisfies the above expressions I to 3, the contact area between the coating film surface and raindrops increases, the silicate ions are suitably eluted from the outer coating film, and as a result, the defect corrosion resistance of the coated steel sheet is preferably improved. J [0018] As described above, as the Si content increases toward the coating film surface in this range, specifically, when the Si content of the outer coating film - 8- satisfies the above expressions 1 to 3, the defect corrosion resistance of the coated steel sheet is improved, and hydrophilicity necessary to improve raindrop contamination resistance is also increased. However, in order to improve the raindrop contamination resistance of the coated steel sheet, it is necessary to enhance the barrier properties in addition to the hydrophilicity. [00 19] The barrier prope11ies of the outer coating film are affected by theN content of the outer coating film. As a result of examination by the inventors, it became clear that when the N content of the outer coating film satisfies the above conditions, qualitatively speaking, when theN content reaches the highest value at the coating film surface, in addition to the hydrophilicity, the barrier prope11ies are also improved. [0020] Since the cross-link density of the coating film surface increases as theN content on the coating film surface increases in the above range, the barrier prope1ties of the outer coating film are enhanced. Specifically, when theN content of the outer coating film satisfies the above expressions 4 to 6, the barrier prope1iies of the outer coating film are suitably increased, and contaminants are less likely to penetrate into the coating film. As a result, the raindrop contamination resistance of the coated steel sheet is preferably improved. [0021] That is, in order to preferably achieve both the raindrop contamination resistance and the defect corrosion resistance of the coated steel sheet, it is necessary for the Si content and theN content of the outer coating film to simultaneously satisfY the above expressions I to 6. [0022] - 9- However, since surface enrichment of Si and surface enrichment ofN are competitive reactions, theN content on the coating film surface tends to decrease as the Si content increases toward the coating film surface in the above range. As described above, generally, it is not easy to simultaneously increase the Si content and theN content toward the coating film surface. That is, it is difficult to simultaneously control the Si content and theN content of the outer coating film to satisfy the above expressions I to 6. [0023] For example, during baking of the outer coating film by a hot-air fbrnace, the outer coating film is baked and cured by heat transfer from the atmosphere, and thus the coating film surface excessively rises in temperature. Accordingly, Si is likely to volatilize from the coating film smface. Therefore, the Si content of the coating film surface decreases, and theN content of the coating film surface becomes excessively high. On the other hand, during baking of the onter coating film by an II-I furnace, the outer coating film is baked and cured by heat transfer from the induction-heated substrate steel sheet, and thus an excessive rise in the temperature of the coating film surface is suppressed. Accordingly, volatilization of Si from the coating film surface is suppressed. Therefore, the N content of the coating film surface decreases while the Si content increases toward the coating film surface. [0024] In order to solve these problems, in the coated steel sheet according to this embodiment the outer coating film is optimally controlled. Specifically, in the coated steel sheet according to this embodiment, the outer :coating film is controlled so that a depth profile curve of theN content of the outer coating film has an absolute maximum value in a region from a depth of 0 to a depth of, but not including, I 0 nm, and has a - I 0- II local maximum value in a region from a depth of 10 nm to a depth of, but not including, 0.1 pm, and a value obtained by dividing the local maximum value by the absolute maximum value is equal to or higher than 0.8 and lower than 1.0. When the depth profile curve of theN content of the outer coating film satisfies the above conditions, the Si content and theN content of the outer coating film are preferably controlled to satisfy the above expressions 1 to 6. [0025] FIG. 3 shows the depth profile curve of theN content of the outer coating film of the coated steel sheet according to this embodiment. The depth profile curve shown in FIG. 3 can be achieved only by optimally controlling the baking conditions of the outer coating film. Specifically, at the time of baking the outer coating film, each of induction heating (IH) for heat transfer from the substrate steel sheet toward the coating film surface, and hot air blowing for heat transfer from the coating film surface toward the substrate steel sheet is optimally controlled. [0026] It is thought that the depth profile curve 5 in FIG. 3 is formed by superimposing a first waveform 53 on a second waveform 54 as shown in FIG.'3. It is thought that the first waveform 53 of the depth profile curve 5 is derived from selfc'ondensates of layered melamine present in the outermost layer. On the other hand, it is thought that the second waveform 54 of the depth profile curve 5 is derived from self-condensates of granular melamine and melamine cross-linked with polyester. [0027] Induction heating for heat transfer from the substrate steel sheet to the coating film surface and hot air blowing for heat transfer from the coating film surface toward the substrate steel sheet are heat transfer processes that take place in opposite - 11 - directions. Therefore, only by separately and optimally controlling the conflicting control conditions for the purpose of intentionally producing the depth profile curve of the N content, a depth profile curve that satisfies the above conditions of this embodiment can be obtained. [0028] The depth profile curve of theN content of the outer coating film is complexly affected by various baking conditions of the outer coating film. Therefore, under simple baking conditions uniquely defined, the depth profile curve of theN conte!ltwhich.satisfies.th~.above conditions of this embodiment cannot be obtained. It is necessary to separately, intentionally, and optimally control the induction heating for heat transfer from the substrate steel sheet toward the coating film smface and the hot air blowing for heat transfer from the coating film surface toward the substrate steel sheet. In a case where only one heating method of the above-mentioned two heating methods is dominant, the depth profile curve that satisfies the above conditions cannot be obtained. As a result, it becomes difficult to simultaneously increase the Si content and the N content toward the coating film surface. [0029] In the coated steel sheet according to this embodiment, the depth profile curve of theN content tha:rsatisfies the above conditions is intentionally produced by optimally controlling each of the conflicting control conditions, although detailed control conditions will be described later. As a result, it is possible to control the Si content and theN content of the outer coating film to preferably satisfY the above expressions I to 6. [0030] - 12- fl In addition, when the depth profile curve of theN content of the outer coating film satisfies the above conditions, the workability of the outer coating film is preferably improved. [0031] It is thought that in a case of hot air heating, since heat is transferred from the coating film surface toward the substrate steel sheet, self-condensates of melamine present in the form of layers are likely to be preferentially formed in the outermost layer of the outer coating film. It is thought that the self-condensates oflayered melamine enhance barrier properties (raindrop contarnination.resistance) against. · .. · . contaminants. However, since the self-condensates oflayered melamine is full hard, the workability of the outer coating film is likely to deteriorate by the self-condensates oflayered melamine. On the other hand, in a case of induction heating, since heat is transferred from the substrate steel sheet toward the coating film surface, in the region of the outer coating film fi'om a depth 10 1llll to a depth of, but not including, 0.1 Jlm, self-condensates of melamine present in a granular form and melamine cross-linked with polyester are likely to be preferentially formed. It is thought that the selfcondensates of granular melamine preferably improve the workability of the outer coating film due to the shapes thereof. However, the self-condensates of granular melamine are likely to deteriorate the barrier propei'ties (raindrop contamination resistance) against contaminants. [0032] In a case where the outer coating film is baked only by the hot air heating, it is difficult to obtain the depth profile curve of theN content having the above-mentioned local maximum value in the region of the outer coating film from a depth of 10 run to a depth of, but not including, 0.1 fllll. Therefore, theN content of the outermost layer - 13 - ll becomes excessively high, while the Si content is likely to decrease. On the other hand, in a case where the outer coating film is baked only by the induction heating, it is difficult to obtain the depth profile curve of theN content having the above-mentioned absolute maximum value in the region of the outer coating film from a depth of 0 to a depth of, but not including, I 0 IIlli. Therefore, theN content is likely to decrease while the Si content of the outermost layer increases. [0033] In the coated steel sheet according to this embodiment, as described above, the baking conditions during the hot air heating and induction heating arc optimally . controlled. As a result, the depth profile curve of the N content of the outer coating film satisfies the above conditions. Since the self-condensates of layered melamine have a high cross-link density and are thus full hard, the self-condensates of layered melamine preferably enhance the barrier properties when present in the outermost layer. However, when the self-condensates oflayered melamine arc present in a large amount over the entire region in the thickness direction of the outer coating film, the outer coating film is likely to excessively harden. In the coated steel sheet according to tllis embodiment, it is thought that since the self-condensates of granular melamine and the like are preferably present in the region of the outer coating film from a depth of 10 nm to a depth of, but not including, 0.1 f.!m, the workability of the outer coating film is preferably improved. [0034] That is, in the outer coating film of the coated steel sheet according to this embodiment, the defect corrosion resistance is improved and the hydrophilicity necessary to improve the raindrop contamination resistance is enhanced by the Si controlled in the above range, the barrier prope1iies necessary to improve the raindrop - 14- contamination resistance are enhanced by theN controlled in the above range, and furthermore, the workability is also improved by the depth profile curve of theN content controlled to satisfy the above conditions. [0035] Hereinafter, the coated steel sheet according to this embodiment will be described in detail. [0036] I. Outer coating film .The outer coating film formed in the coated steel sheet according to this embodiment contains silicate. Si derived from this silicate affects the hydrophilicity (wettability) and defect corrosion resistance of the coated steel sheet. [0037] SiTs, which is the Si content of the outermost layer of the outer coating film, Sis, which is the Si content of the surface layer of the outer coating film, and Sin, which is the Si content of the body layer of the outer coating film need to satisfy Si8 < Sis < Shs, as shown in the above expression 1. When Sin ~Gh of Examples Nos. 1 to 15 of the present invention was a coated steel sheet which satisfied the ranges of the present invention and had excellent defect corrosion resistance and excellent raindrop contamination resistance. Although not shown in the tables, the examples of the present invention were also excellent in workability evaluated by a 1 T bending test. [0093] On the other hand, as shown in Tables 4 and 5, each of Comparative Examples Nos. I to I 7 was a coated steel sheet in which an outer coating film was formed only by induction heating, a coated steel sheet in which an outer coating film was formed only by hot air heating, or a coated steel sheet in which an onter coating fiim was formed by using both induction heating and hot air heating but the baking conditions were not preferable. [0094] Comparative Example No. 1 was a coated steel sheet that did not contain silicate in the outer coating film, and Sis, Sio < Sis < SiTS, Ns· < NTs, and the depth profile of theN content of the outer coating film did not satisfY the conditions. -34- Therefore, the defect corrosion resistance and raindrop contamination resistance became insufficient. [0095] Comparative Example No. 2 was a coated steel sheet which did not contain melamine in the outer coating film, and the cross-linking reaction of the outer coating film was insufficient. Therefore, the Si content, N content, defect corrosion resistance, and raindrop contamination resistance of the outer coating film could not be evaluated. [0096] ,. Comparative Example No. 3 was a coated steel sheet in which the average wind speed of hot air was 1.1 m/second although the outer coating film was baked by using both induction heating and hot air heating, and NTs, Shs/SiB, and the depth profile of theN content of the outer coating film did not satisfy the conditions. Therefore, the defect corrosion resistance and raindrop contamination resistance became insufficient. [0097] Comparative Example No. 4 was a coated steel sheet in which the average wind speed of hot air was 0.05 mlsecond although the outer coating film was baked by using both induction heating and hot air heating, arid NTs, Ns:::: 7 B 10 Contained Contained 35 31 22 28 3 27 ~ 4-< 0 8 B 10 Contained Contained 28 33 16 30 3 27 1""' 9 B 10 Contained Contained 25 35 15 32 2 27 10 B 10 Contained Contained 46 34 28 32 3 27 ~""' 11 B 10 Contained Contained 26 42 7 3.7 2 27 12 B 10 Contained Contained 46 32 18 28 2 26 13 B 10 Contained Contained 29 40 12 36 2 26 14 B 10 Contained Contained 38 36 31 32 2 27 15 B 10 Contained Contained 23 32 14 31 3 27 *The underlined numerical values, indicate values out of the ranges of the present inventiOn. -40- .s= ::1 9 B 10 Contained Contained 35 28 28 27 3 27 ·.p "g' 10 B 10 Contained Contained 20 53 17 43 3 27 E II B 10 Contained Contained 35 28 28 27 3 27 0 u 12 B 10 Contained Contained 16 54 14 41 3 27 13 B 10 Contained Contained 31 28 29 29 3 27 14 B 10 Contained Contained 4 52 1 40 3 27 15 B 10 Contained Contained 18 54 16 44 3 27 16 B 10 Contained Contained 55 li 53 33 3 27 17 B 10 Contained Contained 3 55 l 44 3 27 ~The underlmed numencal values mdtcate values out of the ranges of the present mventton. -42- "' 0".. § &l ·;">:: "~' "Q'. . § u - [0115] [Table 5] No. Paint I A 2 D 3 B 4 B 5 B 6 B 7 B 8 B 9 B 10 B II B 12 B 13 B 14 B 15 B 16 B 17 B Production results Outer coating film GDS analysis results Sis