STEEL SHEET FOR SOFT-NITRIDlNG TREATMENT, MIiTHOD OF 5 MANUFACTURING SAME, AND SOFT-NITRIDED STEEL Technical Field [OOOl] The present invention relates to a steel sheet for soft-nitriding treatment and 10 a method of manufacturing the same, and in particular, a steel sheet for soft-nitriding treatment which is to be subjected to soft-nitriding treatment after being subjected to press working and a method of manufacturing the same. Further, the present invention relates to a soft-nitrided steel, and in particular, a soft-nitrided steel having excellent press-moldability before nitriding treatment and excellent fatigue 15 characteristics after the nitriding treatment. Background Art [0002] Surface hardening treatment is treatment for generating residual stress on a 20 surface of steel to improve abrasion resistance and fatigue resistance simultaneously with hardening the surface of the steel. Examples of method for typical surface hardening treatment that are currently in practical use include carburizing treatment and nitriding treatment. [0003] 25 The carburizing treatment is treatment involving increasing the temperature of the steel to a y region and diffusing and permeating carbon over the surface of the steel. After the carburizing, quenching is performed to attempt surface hardening. Since the temperature of the steel is increased to the high temperature region in the carburizing treatment, deep hardening can be achieved. However, since it is 30 necessary to perform quenching and tempering after the carburizing, strain is likely to be generated. Therefore, the steel subjected to the carburizing treatment cannot be used for the parts that are used ibr components accompanying rotation such as a transmission of an automobile. Although the strain can be removed by carrying out special treatment such as press-tempering treatment after the quenching, loss in time and cost accompanied by the special treatment cannot be avoided. 5 [0004] On the other hand, nitriding treatment is treatment involving diffusing and permeating nitrogen at temperature lower than or equal to an A1 point. Since the heating temperature in the nitriding treatment is 500 to 550°C, which is low, so that phase transformation does not occur due to heating, thus, no strain is generated in the 10 steel while it is the case with the carburizing treatment. However, thc time taken for the treatment is 50 to 100 hours, which is remarkably long, and it is also necessary to remove a brittle compound layer that has been generated on the surface after the treatment. Also in this case, loss in time and cost cannot be avoided. [0005] 15 Accordingly, there has been developed a method called soft-nitriding treatment. In the sof-nitriding treatment, the steel sheet is heated to temperature lower than or equal to the A, transformation temperature, and nitrogen is diffused and permeated from the surface of the steel sheet. In this event, by using a carburizing atmosphere, carbon is also additionally diffused and permeated. Since 20 no quenching is necessary as in the case with the carburizing treatment, no strain is generated due to the phase transformation. Further, since the treatment is carried out at relatively low temperature, thermal strain is small. Accordingly, the surface layer of the steel sheet can be hardened without deteriorating precision of a shape of a part. In addition, the time taken for the treatment is approximately half the time 25 taken for the nitriding treatment. Therefore, the soft-nitriding treatment has rapidly been spread widely recently as a method of the surface hardening treatment for parts used in a mechanical structure. [0006] Moreover, the soft-nitriding treatment is often carried out after performing 30 press working to obtain a desired shape of the part. In particular, a part used in a mechanical structure such as a transmission part of an automobile is subjected to the press working from the viewpoint of productivity. Accordingly, a dcnland is increasing, for a steel shcct for soft-nitriding treatment having excellent moldability which is suitable for a inaterial of a part used in a mechanical structure such as a transmission part of an automobile, and various techniques have been proposed so far. 5 [0007] For example, Patent Literature 1 discloses a method of manufacturing a steel nitride member having excellent cold forgeability and fatigue characteristics, and Patent Literature 2 discloses a method of manufacturing a steel nitride member having small strain caused by heat treatment. Further, Patent Literatures 3 and 4 10 each disclose a steel sheet for nitriding having excellent moldability. [0008] Patent Literature 5 discloses a steel for soft-nitriding treatment whose cost is low and which has satisfactory press workability. Further, Patent Literature 6 discloses a thin steel sheet for nitriding treatment which can obtain, after the nitriding 15 treatment, high surface hardness and sufficient hardening depth. Still further, Patent Literature 7 discloses a steel sheet for soft-nitriding treatment having both processability and fatigue characteristics, and Patent Literature 8 discloses a steel sheet for soft-nitriding treatment having excellent moldability and strength stability after the soft-nitriding treatment. 20 Citation List Patent Literature [0009] Patent Literature 1 : JP 87-286257A Patent Literature 2: JP H8-49059A Patent Literature 3: JP H9-25543A Patent Literature 4: JP H9-25544A Patent Literature 5: JP 2003-105489A Patent Literature 6: JP 2003-277887A Patent Literature 7: JP 2009-68057A Patent Literature 8: JP 2012-177176A Summary of Invention Technical Problem (00 101 5 The steel nitride members disclosed in Patent Literatures 1 and 2 each have the C content of more than or equal to 0.10%, which is high, and also have high Cr and V contents, and therefore have poor processability including elongation. The C content in Patent Literature 3 is 0.01 to less than 0.08%, and the C content in Patent Literature 4 is less than or equal to 0.01%, which are extremely low. However, 10 since the steel sheets disclosed in Patent Literatures 3 and 4 contain large amounts of expensive elements such as Cr and V, there is a problem that manufacturing cost increases. [0011] Further, although Patent Literature 5 evaluates surface hardness, hardening 15 depth, and adhesion bendability after the soft-nitriding treatment, and makes it clear that excellent results are obtained, no examination is carried out for the fatigue characteristics of actual parts, and hence, there leaves room for improvement. The technology described in Patent Literature 6 aims to improve durability, however, evaluation is only carried out on surface hardness and hardening depth, and fatigue 20 characteristics are not considered sufficiently. [0012] Still further, in each of Patent Literatures 7 and 8, a nitrided layer is hardened by containing Cr as an element for forming a nitride, and strength of a base material is simultaneously adjusted by adding an extremely minute amount of Nb, 25 thereby improving the fatigue characteristics. However, the plane bending fatigue strengths of the steel sheets described in Patent Literatures 7 and 8 are approximately 300 to 420MPa, and there is a problem in that the steel sheets cannot be applied to the parts used in a mechanical structure which are used under a state in which large stress is applied. 30 [0013] 'The present invention attempts to inlprovc the Fdtiguc charactcristics which are not sufficiently improved using the conventional technology, and aims to provide a steel sheet for soft-nitriding treatment having both processability and fatigue characteristics after the soft-nitriding treatment, and a method of manufacturing the 5 same. Further, the present invention attempts to improve the fatigue characteristics which are not sufficiently improved using the conventional technology without reducing productivity and cost, and aims to provide a soft-nitrided steel having excellent processability before the soft-nitriding treatment and also having high fatigue characteristics by being subjected to the soft-nitriding treatment. 10 Solution to Problem [0014] The inventors of the present invention have conducted intensive studies on technology for obtaining a soft-nitrided steel having both processability before the 15 soft-nitriding treatment and fatigue characteristics afler the soft-nitriding treatment. As a result, the inventors have achieved the following findings. [OO 151 (a) In order to achieve both the processability before the soft-nitriding treatment and the fatigue characteristics after the soft-nitriding treatment, it is 20 necessary to adjust the alloy composition and the metal structure of the steel sheet such that desired surface hardness, hardening depth, and hardness of a base material can be obtained by the soft-nitriding treatment, without deteriorating the moldahility before the soft-nitriding treatment. [0016] 25 (b) In order to make the processability of the steel sheet before the softnitriding treatment satisfactory, it is necessary to have a metal structure that mainly contains ferrite. The ferrite area ratio can be set to more than or equal to a predetermined amount by causing an appropriate amount of Mn and A1 to be contained as composition components of the steel sheet, and appropriately selecting 30 manufacturing conditions in accordance with the composition components. [0017] (c) Precipitation or (Mn, Al) nitrides occurs in thc soft-nitriding treatment and sufficient surface hardness can be obtained by adjusting Mn and A1 contents within an appropriate range. [OOl S] 5 (d) It is important to adjust ferrite dislocation density on the surface of the steel sheet in controlling the precipitation of nitride in the soft-nitriding treatment. This is because the precipitation of nitride can be promoted by increasing the ferrite dislocation density on the surface of the steel sheet. [00 191 10 (e) Further, the crystal composition of the nitride that precipitates in this case is M3N2 (M represents an alloying element) mainly containing Mn. The amount of nitrogen necessary for forming nitride in the case of M3Nz is smaller than the amount of nitrogen necessary for forming nitride in the case of MlNl having another crystal composition. Therefore, nitrogen is diffused deeper in the steel 15 sheet, and thereby making it possible to obtain a large hardening depth. [0020] (f) In addition, by allowing carbides to precipitate inside the steel sheet during the soft-nitriding treatment, the hardness of the base material can he increased owing to precipitation strengthening. Accordingly, it is necessary that Ti, Nb, Mo, 20 V, and Cr, which are elements for forming carbides, he dissolved as a solid solution at more than or equal to a certain amount in the steel sheet before the soft-nitriding treatment. [0021] (g) In order to improve the fatigue characteristics after the soft-nitriding 25 treatment, it is important to form a hardened layer having hardness in Vickers hardness at the depth of 50 pm fiom the outermost surface of the steel of more than or equal to 600 HV, and a hardening depth of more than or equal to 0.35 mm. [0022] (h) In order lo obtain desired surface hardness and hardening dcpth, it is 30 particularly necessary to rcgulate the content of nitride-forming elements in the sleel. COO231 (i) Additionally, as a result of analyzing surface layer parts of various pieces of soft-nitrided steel using a transmission electron ~nicroscope (TEM), it has been found that it is necessary to control a precipitation form, a composition, and a number density at a depth position of 50 bm from the outermost surface of the steel, 5 among nitrides formed by the sofl-nitriding treatment. [0024] The present invention has been achieved on the basis of the above findings, and the gist of the present invention is to provide the following steel material, method of manufacturing the same, and soft-nitrided steel. 10 [0025] [11 A steel sheet for soft-nitriding treatment which has a chemical composition consisting of, in mass%, C: more than or equal to 0.02% and less than 0.07%, Si: less than or equal to 0.10%, Mn: 1.1 to 1.8%, P: less than or equal to 0.05%, S: less than or equal to 0.01 %, Al: 0.10 to 0.45%, N: less than or equal to 0.01%, Ti: 0.01 to 0.10%, Nb: 0 to 0.1%, Mo: 0 to 0.1%, V: 0 to 0.1%, Cr: 0 to 0.2%, and the balance: Fe and impurities, satisfies the following formula (i), and has a total content of Ti, Nb, Mo, V, and Cr present as precipitates in the steel sheet of less than 0.03% in mass%, 30 wherein the steel sheet for soft-nitriding treatment has a metal structure in which a ferrite area ratio is more than or equal to SO%, and a ferrite dislocation dcnsity at a position of 50 pm from a surface of the steel shect is 1x10't~o 1x 10'"n- 2 Mn+A1>1.5 6) where each chemical symbol included in the formula represents a content 5 (mass%) of each element contained in the steel sheet. [0026] 121 The steel sheet for soft-nitriding treatment according to [I], wherein the chemical composition includes, in mass%, one or more selected 10 from Nb: 0.005 to 0.1%, Mo: 0.005 to 0.1%, V: 0.005 to 0.1%, and Cr: 0.005 to 0.2%. 15 [0027] PI A method of manufacturing a steel sheet for soft-nitriding treatment, the method including: starting rolling of a steel raw material having a chemical composition 20 recited in [I] or [2] after the steel raw material is heated to higher than or equal to 115O0C, and ending the rolling at finishing temperature of higher than or equal to 900°C; performing coiling, after cooling, in a temperature region of 470 to 530°C to cause ferrite area ratio to be more than or equal to 80%; 25 thereafter subjecting the steel raw material to pickling; and subjecting the steel raw material to skin pass rolling after the pickling in conditions in which a rolling reduction ratio is 0.5 to 5.0%, and FIT (mm), is more than or equal to 8000, said FlT(mm) being a ratio of a line load F (kglmm) determined by dividing a rolling mill load by a width of the steel ,sheet to a load T 30 (kglmm2) per unit ares applied in a longitudinal direction of the steel sheet. [0028] M A sofi-nitrided steel having a chemical composition consisting of, in mass%, C: more than or equal to 0.02% and less than 0.07%, Si: less than or equal to 0.10%, Mn: 1.1 to 1.8%, P: less than or equal to 0.05%, S: less than or equal to 0.01%, Al: 0.10 to 0.45%, Ti: 0.01 to 0.10%, Nb: 0 to 0.1%, Mo: 0 to 0.1%, V: 0 to 0.1%, Cr: 0 to 0.2%, and the balance: Fe and impurities, 15 wherein, at a depth position of 50 pm fiom an outermost surface, nitrides are precipitated on a {001 ) plane in a ferrite crystal, an average value of maximum lengths of the respective nitrides is 5 to 10 nm, and a number density of nitrides is more than or equal to 1x 102 4 m -3 . 20 [0029] [51 The soft-nitrided steel according to [4], wherein the chemical composition contains, in mass%, one or more selected fiom 25 Nh: 0.01 to 0.1%, Mo: 0.01 to 0.1%, V: 0.01 to 0.1%, and Cr: 0.01 to 0.2%. [0030] 30 [61 The soft-nitridcd steel according to 141, whercin a Mn concentration in metal elements included in the nitrides is more than or equal to 80 at%. [003 I ] Note that thc "steel sheet for soft-nitriding treatment" according to ihe 5 present invention includes "steel strip" which is steel in a belt shape. Further, although there is a case where an iron nitride layer having a thickness of approximately several tens of pm is formed on the surface of the steel after the softnitriding treatment depending on a surface treatment condition, the "outermost surface of the steel" according to the present invention indicates the surface of the 10 steel that includes the above iron nitride layer. Advantageous Effects of Invention COO321 According to the present invention, there can be provided the steel sheet for 15 soft-nitriding treatment having excellent press-moldability such as stretch flangeability and hole expandability before the soft-nitriding treatment without deteriorating productivity and economic efficiency. Further, there can be provided the soft-nitrided steel in which a hardened layer having a sufficient thickness from the surface is formed after the soft-nitriding treatment, and which is excellent in 20 fatigue characteristics. The steel sheet for soft-nitriding treatment according to ihe present invention having such characteristics is suitable for being subjected to the soft-nitriding treatment after being processed into a predetermined part shape, and being used as a part for a general structure such as a part for an automobile. Further, the soft-nitrided steel according to the present invention is suitable for being used as 25 a part for a general structure such as a part for an automobile. COO331 Here, the "press working" refers to a processing method that collectively rcpresents deep drawing, bending, ironing, blanking, and the like, and "excellent in prcss workability" refers to the case in which the pres's working is capable without 30 applying large strength to the steel material and no cracks or the like occurs which may becomc substantial defects in the press-molded body in the event oT being subjected to press working. Brief Description of Drawings 5 [0034] [FIG. I] FIG. 1 is a diagram showing an image of nitrides present in ferrite observed using a transmission electron microscope (TEM). [FIG. 21 FIG. 2 is a diagram showing spectra of energy dispersive X-ray spectrometry (TEM-EDS) obtained from nitrides and a parent phase. 10 Description of Embodiments [0035] Hereinafter, respective matters of the present invention will be described in detail. 15 [0036] 1. Chemical composition The reasons for limiting the respective elements are as follows. Note that T,Oh ,, used for a content in the following description represents "mass%". [0037] 20 C: more than or equal to 0.02% and less than 0.07% C is an element for improving the strength by being combined with a carbide-forming element and precipitating a carbide, and contributes to press workability of the steel and base material hardness after the soft-nitriding treatment. With decrease in the C content, precipitation density of cementite decreases and the 25 press workability improves, but on the other hand, the amount of precipitation of carbides during the soft-nitriding treatment decreases, and sufficient hardness of the base material in the steel sheet cannot be obtained after the soft-nitriding treatment. Accordingly, the C content is more than or equal to 0.02%. On the other hand, in the case where the C content in the steel is more than or equal to 0.07%, the press 30 workability of the steel deteriorates, and hence, the C content is less than 0.07%. The C content is preferably more than or equal to 0.03%, and preferably less than or equal to 0.06%. [0038] Si: less than or equal to 0.10% 5 Although Si is a useful element as a deoxidizer at a stage of steelmaking process, Si does not contribute to improvement of the surface hardness in the nitriding treatment and decreases the hardening depth. Accordingly, the Si content is less than or equal to 0.10%. The Si content is preferably less than or equal to 0.05%. Note that, in attempting to obtain an effect as the dcoxidizer, the Si content I0 is preferably more than or equal to 0.01%. [0039] Mn: 1.1 to 1.8% Mn has an effect of enhancing the surface hardness by forming a nitride through the soft-nitriding treatment, and is an exceedingly important element in the 15 present invention. When the Mn content is less than 1.1%, the effect of enhancing the surface hardness owing to the nitride formation is not sufficient, the desired hardness distribution cannot be obtained after the soft-nitriding treatment, and hence, it is difficult to obtain satisfactory abrasion resistance and fatigue characteristics. On the other hand, when the Mn content exceeds 1.8%, an influence of center 20 segregation becomes notable, and the processability of the steel sheet is deteriorated. Accordingly, the Mn content is 1.1 to 1.8%. The Mn content is preferably more than or equal to 1.2%, and preferably less than or equal to 1.7%. [0040] P: less than or equal to 0.05% 25 P is an impurity contained in molten iron, segregates at a grain boundary, and is an element that decreases the toughness with increase in the content. Accordingly, the P content is preferably as low as possible. The P content exceeding 0.05% has an adverse effect on the processability, and hence is limited to less than or equal to 0.05%. In particular, taking into account the hole 30 expandability and the weldability, the P content is desirably less than or equal to 0.02%. Note that, since it is difficult to make the P content 0% in terms of operation, 0% is not included. [004 1 ] S: less than or equal to 0.01% 5 S is an impurity contained in molten iron, and when the content is too much, S not only decreases the toughness and causes a crack in a hot-rolling process, but also deteriorates hole expandability. Accordingly, the S content should be decreased to the utmost. Since the S content is in an acceptable range when it is less than or equal to 0.01%, the S content is limited to less than or equal to 0.01%. 10 Note that, since it is difficult to make the S content 0% in terms of operation, 0% is not included. [0042] Al: 0.10 to 0.45% A1 has an effect of enhancing the surface hardness by forming a nitride 15 through the soft-nitriding treatment, and is an exceedingly important element in the present invention. Accordingly, it is necessary that the A1 content be more than or equal to 0.10%. On the other hand, when the A1 content exceeds 0.45%, the hardening depth becomes small, and the fatigue characteristics are poor. Accordingly, the A1 content is 0.10 to 0.45%. The Al content is preferahly more 20 than or equal to 0.15%, and preferahly less than or equal to 0.40%. [0043] N: less than or equal to 0.01% When the N content exceeds 0.01% before the soft-nitriding treatment, N combines with A1 or Ti in the steel sheet to form a nitride, to thereby deteriorate the 25 processability of the steel sheet. Further, since Ti dissolved as a solid solution in the steel sheet decreases, sufficient hardness of the base material cannot be obtained after the soft-nitriding treatment. Accordingly, the N content is less than or equal to 0.01%. The N content is preferahly less than or equal to 0.008%. Note that, after the soft-nitriding treatment, a concentration gradient is generated in the sheet 30 thickness direction owing to the diffusion of N during the treatment. N after the sohitriding treatment is dissolved as a solid solution in Fe, and also forms nitride precipitate whose precipitate dcnsity depcnds on thc N concentration. Furthcr, the fatigue characteristics do not depend on N dissolved as a solid solution, and can be secured when the precipitation density and the size are satisfied. The soft-nitrided steel includes N dissolved as a solid solution in Fe and N forming the nitride 5 precipitate. However, defined in Claims is not the amount of N but only the number density of nitrides. Further, Table 3, which will be described below, describes the amount of N dissolved as a solid solution (EPMA) at a depth position of 50 pm from the surface layer, and makes it understandable that there is no dependence on the amount of N. 10 [0044] Ti: 0.01 to 0.1% Ti has an effect of enhancing the hardness of the base material by being precipitated as a carbide in the base material during the soft-nitriding treatment, and is an exceedingly important component in the present invention. When the Ti 15 content is less than 0.01%, the above effect is not sufficiently obtained. On the other hand, when the Ti content exceeds 0.1%, heating temperature for solution treatment of titanium carbon nitride in hot-rolling is high and heating temperature increases, which raises manufacturing costs. Therefore, the Ti content is 0.01 to 0.1%. The Ti content is preferably more than or equal to 0.02% and preferably less 20 than or equal to 0.09%. [0045] Nb: 0 to 0.1% Mo: 0 to 0.1% v: 0 to 0.1% Cr: 0 to 0.2% Nh, Mo, V, and Cr are elements each having an effect of enhancing the hardness of the base material by forming a carbide in the base material during the soft-nitriding treatment. Accordingly, one or more selected from the above elements may be contained. However, when the Nb content, the Mo content, and 30 the V content each exceed 0.1%, and the Cr content exceeds 0.2%, heating temperature for solution treatment ol: carbon nitride in hot-rolling is high and heating temperature increases, which raiscs manufacturing costs. Accordingly, it is necessary that the content of each element be lcss than or equal to 0.1%. In attempting to obtain the above effect, it is preferred that the content of one or more selected from those elements be more than or equal to 0.005%. Note that, in the 5 case where two or more selected from the above elements are contained in a mixed manner, the total content is preferably 0.005 lo 0.1%. [0046] MnlA121 .S (9 where each chemical symbol included in the formula represents a content 10 (mass%) of each element contained in the steel sheet. In order to obtain sufficient surface hardness by the soft-nitriding treatment, it is not sufficient that the contents of the respective elements be in the above-defined ranges, respectively, and it is necessary that the above formula (i) be satisfied. It is because the surface hardness cannot be enhanced sufficiently if the amount of 15 precipitation of (Mn, Al) nitride formed in the soft-nitriding treatment is small. [0047] The steel material according to the present invention has a chemical composition comprising the above-mentioned elements from C to Cr, and the balance of Fe and impurities. 20 [0048] The "impurities" represent components that are mixed due to various factors of manufacturing processes and of raw materials such as ores and scraps in industrially manufacturing the steel sheet, and indicate those which are allowed to be contained in a range that do not adversely affect the present invention. 25 COO491 Total content of Ti, Nb, Mo, V, and Cr present as precipitates in steel sheet for soft-nitriding treatment before soft-nitriding treatment: less than 0.03% In the present invention, the total content of Ti, Nb, Mo, V, and Cr present as precipitates in the steel is an important index from the viewpoint of improving the 30 fatigue characteristics of Lhe steel sheet after the soft-nitriding treatment. In order to make the fatigue characteristics satisfactory, not only the hardness of the surface of the steel sheet (surface hardness), but also the hardness of the inside oT the steel sheet (hardness of the base material) should be high. By causing carbides to precipitate inside the steel sheet during the soft-nitriding treatment, it is possible to malce the hardness of the base material high owing to precipitation stren9hening. 5 Accordingly, it is necessary that Ti, Nb, Mo, V, and Cr, which are elements for forming carbides, be dissolved as a solid solution at more than or equal to a certain amount in the steel sheet for soft-nitriding treatment. [OOSO] When the total content of Ti, Nh, Mo, V, and Cr present as precipitates is, 10 in mass%, more than or equal to 0.03%, the solid solution concentration decreases, sufficient precipitation strengthening cannot be obtained, the hardness of the base material decreases, and the fatigue characteristics also deteriorates. Therefore, in the present invention, the total content of Ti, Nb, Mo, V, and Cr included in the precipitates present in the steel sheet is, in mass%, less than 0.03%. 15 [0051] Note that the content of each of Ti, Nh, Mo, V, and Cr, which are present as precipitates, is determined using the following extraction residue analysis. A test piece is collected from the steel sheet for soft-nitriding treatment, is immersed in an electrolytic solution (10% of acetilacetone, 1% of tetramethylammonium chloride, 20 and the balance of methanol), is subjected to constant-current electrolysis, and is then caused to filter through a filter having a filtration diameter of 0.2 pm to obtain an extraction residue (carbide). After dissolving the extraction residue to obtain a solution, the solution is analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES), and the concentrations of Ti, Nb, Mo, V, and Cr in the 25 solution are each measured. Then, the measured concentrations are each divided by the mass of the electrolyzed test piece to thereby calculate the content of each of Ti, Nb, Mo, V, and Cr, which are present as precipitates in the steel sheet. LO0521 2. Metal structure of steel sheet for soft-nitriding treatment before soft- 30 nitriding treatment In addition to thc above composition component, the steel sheet according to the present invention has a metal structure in which a ferrite area ratio is more than or equal to 80%, and a ferrite dislocation density at a position of SO pnl from a surface of the steel sheet is 1x10'~t o 1x10 16 m -2. 5 [0053] Ferrite area ratio: more than or equal to 80% In the present invention, the ferrite area ratio is an important indcx for making the processability of the steel sheet satisfactory. If metal structures other than the ferrite is contained and the ferrite area ratio becomes less than 80%, it 10 becomes difficult to achieve both the elongation and the hole expandability of the steel sheet. The other metal structures include austenite, pearlite, bainite, and martensite. Further, in order to achieve the strength, the elongation, and the hole expandability in proper balance, the ferrite grain size is desirably less than 20 pm. [0054] 15 Ferrite dislocation density at position of 50 pm from surface of steel sheet: 16 -2 1x10'~ to 1x10 m The ferrite dislocation density at a position of 50 pm from the surface of the steel sheet is an exceedingly important index for controlling precipitation of a nitride in the soft-nitriding treatment. When the dislocation density of the steel sheet 20 surface layer is increased, nitride is preferentially produced on the dislocation, the precipitation can be promoted, and the surface hardness can be increased. However, when the dislocation density of the steel sheet surface layer is too high, the processability deteriorates and it becomes difficult to be formed into a shape of a part. Accordingly, in the present invention, the dislocation density at the position of 50 pm 25 from the surface of the steel sheet is more than or equal to 1x10'~ m-2 in order to obtain sufficient surface hardness in the soft-nitriding treatment, and is less than or equal to 1 x 1016 m-2 in order to secure the processability of the steel sheet. [OOSS] In the event of increasing the ferrite dislocation density of the surface of ihe 30 steel sheet, if the dislocation is introduced up to the center in the shect thickness direction, the processability may deteriorate. Accordingly, it is desirable that the dislocation density at the center in the sheet thickness direction be not increased [0056] The dislocation density can bc determined as follows. After subjecting 5 the steel sheet to mechanical polishing, the steel sheet is further polished up to a predetermined position in the sheet thickness direction through electrolytic polishing. Lattice strain E is calculated using Williamson-Hall plot from peak integrated intensity of (1101, (2111, {220) obtained by an X-ray diffraction method, and after that, dislocation density p is calculated on the basis of the following formula. Here, 10 b represents a Burgers vector. p=(14.4xc2)/b2 Williamson-Hall plot is disclosed in a known document "Tetsu-to-Hagane, Vol. 100 (2014) No. 10 k a k a et al.", for example. [0057] 15 3. Nitride of soft-nitrided steel after soft-nitriding treatment As described above, in order to improve the fatigue characteristics of the soft-nitrided steel, it is important to form a hardened layer having hardness in Vickers hardness in the case of setting test force to 0.3 kgf at the depth position of 50 pm from the outermost surface of the steel of more than or equal to 600 HV, and a 20 hardening depth or more than or equal to 0.35 mm. For forming such a hardened layer, it is necessary that in the soft-nitrided steel of the present invention, plate-like nitrides be precipitated on a (001) plane in a ferrite crystal at least at a depth position of 50 pm from an outermost surface. In addition, it is necessary that the precipitation form, the composition, and the number density of the above nitrides be 25 defined as shown below. [0058l Since the nitrides precipitated on the {001} plane in the ferrite crystal each have a plate-like shape, the nitrides generate large coherency strain in ferrite crystal lattice, and effectively act on hardness increasing. In order to exhibit this action 30 effectively, it is necessary that the maximum length of a nitride be 5 to 10 nm. When the maximum length is less than 5 nm, sufficiently large coberency strain cannot bc generated in the ferrite crystal lattice. On the other hand, when the rnaximum length exceeds 10 nm, the incoherency increases, and hence, the hardness lowers. [0059] 5 Further, the nitrides that precipitates by the nitriding treatment in the present invention contain Mn, Al, and N as main components, and each show a crystal composition of (Mn, Al),N,. In the case where a nitride present in the soft-nitrided steel precipitates as (Mn, Al)3Nz having a crystal structure of q-Mn3N2 type, the Mn concentration in the metal elements including Mn and A1 forming the nitride is more 10 than or equal to Soat%. This nitride uses, compared to (Mn, A1)]Nl having a crystal structure of NaCl type, small amount of precipitated N which has entered from the surface of the steel and dissolved as a solid solution. Therefore, N enters up to a deeper position during the same time period of soft-nitriding treatment, and the hardening depth increases. Accordingly, the Mn concentration in metal elements 15 included in the nitride present at a depth position of 50 pm fiom the outermost surface is more than or equal to 80 at%. [0060] Conventionally, it has been considered that Mn only has a weak action as an element for forming nitrides. However, by being contained by a predetermined 20 amount in the steel with A1 having a strong action on forming nitrides, the formation of nitrides mainly containing Mn and A1 is promoted. Those nitrides do not show much action that inhibits the difhsion of nitrogen inside afler forming only on the surface of a pole. Accordingly, it becomes possible to effectively form nitrides up to sufficiently deep region fiom the surface of the steel, and thereby making it 25 possible to obtain a large hardening depth. [00611 Additionally, in order to obtain a predetermined hardness at the depth position of 50 pm from the outennost surface of the steel, it is necessary that the nitrides each having the above-mentioned precipitation form be dispersed in high 30 density in the surface layer. Accordingly, the number density of nitridcs is more than or equal to 1x10'~ m-3. Further, in order to increase the improvement of the fatigue characteristics owing to the hardening of the surSace layer, the number density of nitride is preferably more than or equal to 2x 102 4 m- 3 . [0062] Note that, the maximum length of a nitride and the number density of 5 nitrides at the depth position of 50 pm from the outermost surface of the steel can be determined by, for example, observing and analyzing precipitates in the hardened layer of the surface layer using a TEM. The TEM observation is desirably carried out in the condition that a [001] direction of ferrite is parallel to an incident direction of an electron beam. Further, the maximum length is desirably evaluated using an 10 average value of nitrides included in an observed visual field. Note that it is preferred that, regarding nitrides, five visual fields be imaged, each visual field having an area of 50 nmx5O nm, at least 50 nitrides in total be extracted, and the average value be determined. [0063] 15 In determining the number density of nitrides, the nitrides precipitated on the {001} plane in a ferrite crystal can be determined by counting the number of nitrides on a (001) plane, the number of nitrides on a (100) plane, and the number of nitrides on a (010) plane, and totalizing the numbers. However, if it is difficult to observe the nitrides precipitated on the (001) plane, the determination can be 20 performed by counting the number of nitrides on the (100) plane and the number of nitrides on the (010) plane, and multiplying the total number by 1.5. Further, the thickness of a TEM sample of the observed region can be measured by using a logratio method of electron energy loss spectroscopy (EELS). The number density can be determined by dividing the observed number of nitrides by a volume, the volume 25 being determined by multiplying the area of the observed visual field by the thickness. In calculating the number density, it is preferred that at least five visual fields be imaged from different crystal grains at 1000000 to 2000000-fold magnification, the number densities be determined in the respective visual fields, and an average value of the number densities determined in the respective visual fields be 30 employed. [0064] Further, in the present invcntion, rcgarding the Mn concentration in metal elements included in the nitrides, the value determined by an element analysis using TEM energy dipersion x-ray spectroscopy (TEM-EDS) is employed. 100651 5 Note that a sample to be served for the TEM observation may be prepared by a general TEM sample preparation method such as electrolytic polishing, FIB liftout, and Ar-ion polishing. [0066] 4. Manufacturing method 10 The method of manufacturing the steel sheet for soft-nitriding treatment according to the present invention is not particularly limited, and, for example, the steel sheet for soft-nitriding treatment according to the present invention can be manufactured by subjecting the steel raw material having the above chemical composition to the following treatment. 15 100671 The steel raw material is heated to higher than or equal to 1 150°C, and after that, rolling is started. The rolling is ended at finishing temperature of higher than or equal to 900°C. By heating the slab in a heating furnace to have the beforerolling heating temperature of higher than or equal to 11 50°C, precipitation elements 20 contained in the steel can be sufficiently subjected to solution treatment. Note that since the austenite grain size becomes coarse when the heating temperature exceeds 1300°C, the heating temperature is preferably lower than or equal to 1300°C. Further, when the rolling finishing temperature is lower than 90OoC, the deformation resistance becomes high and a load on the rolling mill increases. 25 100681 After the rolling, cooling is performed, and then coiling is performed in a temperature region of 470 to 530°C. Note that, during the period from after the rolling to the coiling, in the time period within 4.0 seconds from the start of the cooling, the cooling is preferably performed in the condition that a cooling rate CR 1 30 ("Cis) satisfies the following fonnula (iii), the formula (iii) having a relatioilship with a value of CeqIIW defined in the following formula (ii), CeqIIW=C+Mn/6+(Cr+Mo+V)l5 (ii) 80-190xCeqIIW~CR