NON-HEAT TREATED STEEL FOR NITROCARBURIZING AND NITROCARBURIZED COMPONENT TECHNICAL FIELD [0001] The present invention relates to a non-heat treated steel for nitrocarburizing and a nitrocarburized component. More particularly, the present invention relates to a component that is used after carrying out a nitrocarburizing, such as a crankshaft, which is an engine component for automobiles, construction machines and so on (hereinafter, referred to as a "nitrocarburized component"), and also relates to a steel for nitrocarburizing that is used suitably as a starting material for the said component without a heat treatment such as "quenching-tempering", "normalizing^', "annealing" and so on after being rolled (hereinafter, referred to as a "non-heat treated steel for nitrocarburizing"). StiU more particularly, the present invention relates to a nitrocarburized component having a high bending fatigue strength of 600 MPa or more and an excellent straightenabiUty, and also relates to a non-heat treated steel for nitrocarburizing that is used suitably as a starting material for a nitrocarburized component being required straightening operations, and is capable of providing the aforementioned characteristics to the nitrocarburized component under various nitrocarburizing conditions, in particular even when water cooling is carried out in the cooling step after nitrocarburizing. [0002] The "straightenabiUty" described above means a property that when the straightening operations are carried out in the finishing step after the nitrocarburizing, no crack occurs in a nitrocarburized layer on the surface of the nitrocarburized component until the bending displacement becomes large. BACKGROUND ART .^. z [0003] A component being required a high bending fatigue strength and wear resistance, including a crankshaft for automobiles, construction machines and so on, is often manufactured by subjecting the component to surface hardening treatments such as induction hardening, nitrocarburizing and so on in a non-heat treated state after being formed into a predetermined component rough shape by hot forging and machining. [0004] Among the surface hardening treatments mentioned above, the nitrocarburizing has a major feature such that the strain occurring at the time of surface hardening treatment is smaller than the strain at the time of the induction hardening. [0005] Therefore, in particular, a component such as a crankshaft is often subjected to the nitrocarbiu-izing; however, even in the case of the said nitrocarburizing, the strain cannot be entirely prevented from occurring. [0006] Therefore, with regard to the nitrocarbiu'ized component in which strain has occurred on account of the nitrocarburizing, the strain has been removed by straightening operations in the finishing step after the nitrocarburizing. [0007] However, if the nitrocarburized component the outer layer of which has been hardened excessively is subjected to straightening operations, cracks sometimes occur in the nitrocarburized layer on the surface. If cracks occur in the nitrocarburized layer, the bending fatigue strength that the nitrocarburized component had inherently before the straightening operations decreases significantly. In particular, in the case where treatments in which the cooHng rate is high, such as water cooling, is carried out in the cooling step after the 2) nitrocarburizing, the outer layer hardness of the nitrocarburized component becomes high, so that the reduction in straightenabiHty is unavoidable. Therefore, the nitrocarburized component is required to also have an excellent straightenabiHty. [0008] On the other hand, in some cases, only the water-cooling treatment can be carried out in the cooling step after the nitrocarburizing from the viewpoint of safety and restriction of equipment. [0009] Therefore, there have been demanded a nitrocarburized component that steadily has a high bending fatigue strength and an excellent straightenability even in the case of not only being oil cooled but also being water cooled in the cooling step after the nitrocarburizing, and a non-heat treated steel for nitrocarburizing that is suitable as a starting material for the nitrocarburized component. [0010] Conventionally, both of the high bending fatigue strength and the excellent straightenability have been attained in the nitrocarburized component even in a non-heat treated state by containing an expensive alloying element such as Mo. On the other hand, from the industrial world, there has arisen an increasing demand for providing a high bending fatigue strength and an excellent straightenability to the nitrocarburized component without containing an expensive alloying element as far as possible to save the material cost. [0011] Accordingly, in order to meet the demand mentioned above, a "non-heat treated steel for nitrocarburizing" is disclosed in the Patent Literature 1, and in 4- addition a "nitrocarburized non-heat treated steel member" is disclosed in the Patent Literature 2. [0012] To be concrete, the Patent Literature 1 discloses a "non-heat treated steel for nitrocarburizing" characterized in that the steel consists of, by mass percent, C: 0.2 to 0.6%, Si: 0.05 to 1.0%, Mn: 0.25 to 1.0%, S'- 0.03 to 0.2%, Cr: 0.2% or less, s-Al: 0.045% or less, Ti: 0.002 to 0.010%, N: 0.005 to 0.025%, and 0: 0.001 to 0.005%, and further according to need one kind or two or more kinds of Pb: 0.01 to 0.40%, Ca: 0.0005 to 0.0050%, and Bi: 0.005 to 0.40%, and satisfying the conditions of [0.12 x Ti < O < 2.5 x Ti] and [0.04 x N < O < 0.7 x N], and the balance of Fe and unavoidable impurities, wherein the microstructure after hot forging is a composite microstructure of ferrite and pearhte. [0013] The Patent Literature 2 discloses a "nitrocarburized non-heat treated steel member" characterized in that the steel member consists of a non-heat treated steel having a nitrocarburized layer on the surface thereof and also having a ferrite plus pearlite microstructure in a steel cross-sectional structure excluding the nitrocarburized layer, the composition of the said steel containing, in addition to Fe contained as a main component, by mass percent, C: 0.30 to 0.50%, Si: 0.05 to 0.30%, Mn: 0.50 to 1.00%, S: 0.03 to 0.20%, Cu: 0.05 to 0.60%, Ni: 0.02 to 1.00%, and Cr: 0.05 to 0.30%, and further according to need one or both of (1) Ti: 0.0020 to 0.0120%, N: 0.0050 to 0.0250%, and O: 0.0005 to 0.008% and (2) Ca: 0.0005 to 0.0050%, wherein, when the contents of Cu, Ni and Cr are taken as WCu, WNi, and WCr, respectively, and when composition parameters Fl and F2 are taken as Fl = 185WCr + 50Wcu and F2 = 8 + 4WNi + 1.5WCu - 44WCr, respectively, [Fl > 20] and [F2 > O] are satisfied. LIST OF PRIOR ART DOCUMENT PATENT LITERATURES -^- s [0014] Patent Literature l: JP 2002-226939 A Patent Literature % JP 2007-197812 A DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0015] In the technique disclosed in the aforementioned Patent Literature 1, the bending fatigue strength does not reach 600 MPa. Moreover, in the Patent Literature 1, the cooling method in the coohng step after the nitrocarburizing is not described at aU. [0016] As in the Patent Literatua-e 1, in the technique disclosed in the Patent Literature 2 as well, the bending fatigue strength does not reach 600 MPa, and moreover, the cooling method in the cooling step after the nitrocarburizing is not described. [0017] Accordingly, the objectives of the present invention are to provide a nitrocarburized component steadily having a high bending fatigue strength of 600 MPa or more and an excellent straightenability under the nitrocarbvirizing, in particular even when water cooling is carried out in the cooling step after nitrocarburizing, and a non-heat treated steel for nitrocarburizing that is suitable as a starting material for the nitrocarburized component. MEANS FOR SOLVING THE PROBLEMS [0018] In order to solve the above problems, the present inventors conducted various studies. As a result, the items of (a) to (c) described below were clarified. [0019] (o (a) Mn (manganese) is an element that can give a high bending fatigue strength to the nitrocarburized component at a low cost without adding an expensive alloying element such as Mo and V. [0020] (b) However, if Mn is contained in the steel, much nitrogen intrudes into the outer layer at the time of nitrocarburizing, so that the outer layer of the nitrocarburized component hardens excessively, and thus the straightenability is liable to degrade. [0021] (c) Cr (chromium) is contained in the steel as an impurity, and the content thereof has a great influence on the bending fatigue strength and straightenabiHty of the nitrocarburized component. [0022] Accordingly, the present inventors conducted further studies. As a result, the following findings (d) to (i) were obtained. [0023] (d) By controlling the content of Mn so as to be low, the outer layer of nitrocarburized component can be restrained fi-om hardening excessively, and the straightenability can be prevented fi*om degrading. In this case, however, the bending fatigue strength decreases. [0024] (e) Even if the content of Mn is high, if a steel in which the content of C is controlled so as to be in a specific range is used, a high bending fatigue strength and an excellent straightenability can be given to the nitrocarburized component. [0025] (£) By controlling the content of Cr, which is contained as an impurity, so as to be low, the outer layer of nitrocarburized component can be restrained ^ - from hardening excessively, and the straightenability can be prevented from degrading. [0026] (g) If a specific amount of Al is contained, the diffusion layer depth can be increased while restraining the outer layer of nitrocarburized component from hardening excessively. Thereby, a high bending fatigue strength and an excellent straightenability can be given to the nitrocarbiu-ized component. [0027] (h) By controlling the contents of C, Mn, Cr and Al so as to be in a very proper range, under various nitrocarbiirizing conditions, in particular even when water cooling is carried out in the cooling step after nitrocarburizing, the diffusion layer depth can be increased while the outer layer of nitrocarburized component is restrained from hardening excessively. [0028] (i) Mn not only strengthens the outer layer by increasing the amount of dissolved nitrogen but also strengthens the outer layer by forming fine Mn nitrides. To be concrete, in the case where nitrocarburizing is carried out by using a non-heat treated steel having a raised Mn content, plate-shaped fine ri-Mn3N2 having a thickness of 5 nm or less and a width of 200 nm or less precipitates, while keeping a coherent state, in the ferrite of a composite microstructure of ferrite and pearlite (hereinafter, referred to as a "ferrite-pearlite structure") which forms the diffusion layer. A fine plate-shaped precipitate coherently precipitating in the ferrite, which is a matrix, as described above improves the strength of nitrocarburized component, and contributes to the improvement in the bending fatigue strength. The precipitation of this fine plate-shaped precipitate almost finishes during the time of being held at high temperatures at the time of nitrocarburizing, so that the influence of the cooling rate in the cooling step after nitrocarburizing is small. Therefore, even if water cooling treatment is performed in the cooling step after nitrocarburizing, a high bending fatigue strength and an excellent straightenabihty can be given steadily to the nitrocarburized component. [0029] The present invention has been accompUshed on the basis of the aforementioned findings. The main points of the present invention are the non-heat treated steel for nitrocarburizing shown in the following (l) and the nitrocarburized component shown in the following (2). [0030] (1) A non-heat treated steel for nitrocarburizing characterized in that the steel has a chemical composition consisting of, by mass percent, C: 0.25% or more to less than 0.35%, Si: 0.15 to 0.35%, Mn: 0.85 to 1.20%, S: 0.10% or less, Al: more than 0.010% to 0.030% or less, Ti: 0.003 to 0.020%, and N: 0.010 to 0.024%, and the balance of Fe and impurities, wherein P and Cr among the impurities are P: 0.08% or less and Cr^ 0.10% or less respectively, and further the PI and the P2 expressed by the formulas (l) and (2) are PI > 0.96 and P2 > 0.90 respectively: PI = 0.02C + 0.22Mn + 0.87Cr + 0.85A1 + 0.72 ... (1), P2 = 2.40C - 0.54Mn - 9.26Cr - O.OIAI + 1.59 ... (2), wherein each symbol C, Mn, Cr and Al in the above formulas (l) and (2) represents the content by mass percent of the element concerned. [0031] (2) A nitrocarburized component characterized in that the component in which the chemical composition of the matrix thereof consisting of, by mass percent, C: 0.25% or more to less than 0.35%, Si: 0.15 to 0.35%, Mn: 0.85 to 1.20%, S: 0.10% or less, Al: more than 0.010% to 0.030% or less, Ti: 0.003 to 0.020%, and N: 0.010 to 0.024%, and the balance of Fe and impurities, wherein P and Cr among the impiirities are P: 0.08% or less and Cr: 0.10% or less ^ - respectively, and further the PI and the P2 expressed by the formulas (l) and (2) are PI > 0.96 and P2 > 0.90 respectively, wherein, further the diffusion layer thereof consists of a ferrite-pearlite structure, and the number of plate-shaped precipitates having a thickness of 5 nm or less and a width of 200 nm or less, which have precipitated in the ferrite, is 130 to 250/|j.m2: PI = 0.02C + 0.22Mn + 0.87Cr + 0.85A1 + 0.72 ... (l), P2 = 2.40C - 0.54Mn - 9.26Cr - O.OIAI + 1.59 ... (2), wherein each symbol C, Mn, Cr and Al in the above formulas (l) and (2) represents the content by mass percent of the element concerned. [0032] The term "impvirities" so referred to in the phrase "the balance of Fe and impurities" indicates those elements which come from the raw materials such as ore and scrap, and/or the production environment when the steel is produced on an industrial scale. ADVANTAGEOUS EFFECTS OF THE INVENTION [0033] The nitrocarburized component of the present invention steadily has a high bending fatigue strength of 600 MPa or more and an excellent straightenabiUty under various nitrocarburizing conditions, in particular even when water cooling is carried out in the cooling step after nitrocarburizing," and thus the said nitrocarburized component is suitable as a crankshaft for automobiles and so on. If the non-heat treated steel of the present invention is used as a starting material, the said nitrocarburized component can be manufactured easily. BRIEF DESCRIPTION OF THE DRAWINGS [0034] 10 [Fig. l] Figure 1 is a view showing the shape^ of an Ono type rotating bending fatigue test piece. In this figure, the unit of each numerical value is "mm". [Fig. 2] Figure 2 is a graph summarizing the relationship between the bending fatigue strength and the PI (= 0.02C + 0.22Mn + 0.87Cr + 0.85A1 + 0.72) for the steels AA to AF. [Fig. 3] Figure 3 is a three-side view showing the shape of a four-point bending test piece for measxuring straightenabUity. In this figure, the unit of each numerical value is "mm". [Fig. 4] Figure 4 is a graph summarizing the relationship between the amount of straightenable strain, which is an index of the straightenabUity, and P2 (= 2.40C - 0.54Mn - 9.26Cr - O.OIAI + 1.59) for the steels BA to BF. [Fig. 5] Figure 5 is a schematic view for explaining a method for cutting off a test piece for the observation by using a transmission electron microscope from the four-point bending test piece having been subjected to the nitrocarburizing and having the shape shown in Figure 3. [Fig. 6] Figure 6 is a microphotograph showing one example of a brightfield image in the case where the ferrite in the ferrite-pearlite structure of a diffusion layer after nitrocarburizing is observed by using a transmission electron microscope; in which the portions indicated by white arrow marks are T]-Mn3N2. In the case of this figure, observation is made by exciting systematic reflection of g = (020)a-Fe to reduce the influence of strain existing, for example, around a precipitate and to obtain a clearer observation result in the state in which the incident direction of electron beam is made [OOlla-Fe. The black arrow mark in the figure indicates the direction of g vector of excited systematic reflection in a reciprocal space. [Fig. 7] Figure 7 is a microphotograph showing electron beam diffiraction patterns of the same field as that in Figure 6 in the case where observation is W carried out by using a transmission electron microscope. In this figure, spot-like portions are the difiraction patterns of a-Fe, and portions extending in a streak form longitudinally and transversely are the diffraction patterns of Tj-MnsNa. The fact that the di&action patterns extending in a streak form are obtained as described above is characteristic of a thin plate-shaped precipitate. MODE FOR CARRYING OUT THE INVENTION [0035] In the following, aU of the requirements of the present invention are described in detail. In the following description, the symbol "%" for the content of each element means "% by mass". [0036] (A) Chemical composition of the non-heat treated steel for nitrocarburizing and matrix of nitrocarburized component: C- 0.25% or more to less than 0.35% C (carbon) is an effective element for ensuring the bending fatigue strength after nitrocarburizing. In order to ensure the strength of matrix which is necessary to obtain a high bending fatigue strength of 600 MPa or more, 0.25% or more of C must be contained. However, if the content of C becomes excessive, the outer layer hardness increases excessively. Moreover, the area fraction of ferrite decreases, and the ferrite-pearlite structure does coarsen. Therefore, a siifficient straightenability cannot be obtained. Consequently, the content of C is set to 0.25% or more to less than 0.35%. [0037] Si: 0.15 to 0.35% Si (sihcon) is an element necessary for deoxidizing the steel at the time of melting. In order to achieve this effect, at least 0.15% of Si must be contained. However, since the containing of much Si leads to an excessive deterioration in straightenability, the content of Si is set to 0.15 to 0.35%. The content of Si is preferably set to 0.15% or more, and 0.30% or less. [0038] Mn: 0.85 to 1.20% Like Si mentioned above, Mn (manganese) is an effective element for deoxidizing the steel. At the time of nitrocarburizing, Mn increases the content of dissolved nitrogen in the nitrocarburized layer, and further forms fine plate-shaped Mn nitrides together with intruded nitrogen. The said nitrides coherently precipitate in the matrix; and thus the bending fatigue strength is improved. In order to obtain the aforementioned effects, 0.85% or more of Mn must be contained. On the other hand, if the content of Mn exceeds 1.20%, the content of dissolved nitrogen and the precipitation amount of Mn nitrides increase excessively, and also the outer layer hardness increases excessively, so that the straightenabihty degrades. Therefore, the content of Mn is set to 0.85 to 1.20%. The content of Mn is preferably set to 0.90% or more. [0039] S: 0.10% or less S (sulfur) is contained in the steel as an impvu-ity. Incidentally, if S is added, S has an effect of improving the machinabiUty. However, as the result of addition, if the content of S increases and exceeds 0.10%, the bending fatigue strength and the straightenabihty degrade remarkably. Therefore, the content of S is set to 0.10% or less. The content of S is preferably set to 0.08% or less. In order to obtain the advantageous effect of improving machinabihty, the content of S is preferably set to 0.04% or more. [0040] Al: more than 0.010% to 0.030% or less 13 Al (aluminum) increases the diffusion layer depth at the time of nitrocarburizing; and thus Al is an effective element for improving the bending fatigue strength. In order to achieve this effect, more than 0.010% of Al must be contained. However, if the content of Al becomes excessive, the outer layer hardness increases excessively, so that the straightenabihty degrades. Therefore, the content of Al is set to more than 0.010% to 0,030% or less. [0041] Ti: 0.003 to 0.020% Ti (titanium) is an element that restrains grain coarsening; and thus Ti makes grains fine, and also improves the bending fatigue strength. In order to achieve these effects, 0.003% or more of Ti must be contained. However, if the content of Ti exceeds 0.020%, the straightenabihty degrades. Therefore, the content of Ti is set to 0.003 to 0.020%. The content of Ti is preferably set to 0.005% or more, and 0.015% or less. [0042] N: 0.010 to 0.024% N (nitrogen) is an element for improving the bending fatigue strength and the straightenability. In order to achieve these effects, 0.010% or more of N must be contained. On the other hand, even if 0.024% or more of N is contained, the aforementioned effects are saturated. Therefore, the content of N is set to 0.010 to 0.024%. The content of N is preferably set to 0.012% or more, and 0.022% or less. [0043] With regard to the non-heat treated steel for nitrocarburizing and the matrix of the nitrocarburized component in accordance with the present invention, the contents of P and Cr among the impurities must be restricted to being in the ranges described below. [0044] 14- As already described, the term "impurities" so referred to in the phrase "the balance of Fe and impurities" indicates those elements which come from the raw materials such as ore and scrap, and/or the production environment when the steel is produced on an industrial scale. [0045] P: 0.08% or less P (phosphorus) is contained in the steel as an impurity. P is an unfavorable element for the bending fatigue strength, that is to say, P degrades the bending fatigue strength. In particular, if the content of P exceeds 0.08%, the bending fatigue strength degrades remarkably. Therefore, the content of P among the impurities is set to 0.08% or less. The content of P among the impurities is preferably set to 0.03% or less. [0046] Cr: 0.10% or less Cr (chromium) is contained in the steel as an impurity, and the content thereof has a great influence on the bending fatigue strength and the straightenability. In particular, if the content of Cr exceeds 0.10%, the straightenabihty degrades remarkably. Therefore, the content of Cr among the impurities is set to 0.10% or less. [0047] Pi: 0.96 or more In the non-heat treated steel for nitrocarburizing and the matrix of the nitrocarburized component in accordance with the present invention, the Pi expressed by [PI = 0.02C + 0.22Mn + 0.87Cr + 0.85A1 + 0.72 ... (l)] must satisfy [Pl > 0.96], wherein each symbol C, Mn, Cr and Al in the above formula (l) represents the content by mass percent of the element concerned. [0048] If the chemical composition is in the aforementioned range, the bending fatigue strength can be put in order by the said PI. When the PI is 0.96 or more, a bending fatigue strength of 600 MPa or more is obtained. In the following, this relationship is explained. [0049] The steels AA to AF having the chemical composition shown in Table 1 were melted by use of a 70-tons converter, and then were bloomed into billets having a cross-sectional size of 180 mm x 180 mm. [0050] y [Table 1] / — I I ... - tJ . .. ^^ ^ ^ • = s r ^ - ^ = = = = - , be a 0} t--t«u-l bo Qj • i-i H O O Ifl W W to ^ v - ' C D < a 3 < D < O l O > 0 bfi d PQ , 00 (M -^ 05 lO '^ r* 03 O O O OS OJ A [ , « • « • , O T-l 1-4 t-< O O m , 2 O i-l CO 05 (N »0 +j o o a> CO t - y r H T - i r H r — I C O r - l i —I •^ J5 d d d d d d E-* cS M «5 t~ 00 O CO (M ra <6 CD O d O d m O >0 r-< t- W -^ ^ m o o o o o o jv, ^ d d d d d d «> -s; d + Cl • < S t U 5 ' 5 i < i - < ( M . - i r5 2 « T - ( r H r H i H < M ( N^ '43 PH O O O O O O ^ CO O O O O O O _• n •• •• + g tt; en o o r-i 00 00 t- _ ' ^ O r H i - H i - l O O • 13 ? a o c n o o o o o c o '3 „ l"^ (N --I N CM 1-1 r-4 >2< g TO . . . . . . (M r£3 O O O O O O (M o J + ID 00 (M 1* in lo o O (M o __i d ^ 0.90], wherein each symbol C, Mn, Cr and AI in the above formulas (2) represents the content by mass percent of the element concerned. [0058] If the chemical composition is in the aforementioned range, the straightenabiHty can be put in order by the said P2. When the P2 is 0.90 or more, an excellent straightenabiHty is obtained. In the following, this relationship is explained. [0059] The steels BA to BF having the chemical composition shown in Table 2 were melted by use of a 70"tons converter, and then were bloomed into billets having a cross-sectional size of 180 mm x 180 mm. [0060] ^^ [Table 2] ^^ -X_ _ __ti _ to M o a 3 t > J3 c o < ^ ^ ( N ( N I H r ^ \ w \. ew 3 \ I „ t - CO . 2 * • u o O t - C l O J O O t - • r J * 7 l - l t - l r - H l - l l - l i - l 3 * ^ 0 0 0 0 0 0 a d o c J d o d a •l-l S E ^ O O O O O O i J ^ , d d d d d d 'rt f^ ^ "^ Ti» (M 10 TH eq •^ g < ! o o o o o o ^ o <6 <6 d <6 d cs W w d d d d d d C8 ^ O T O O O O O O O J h - • | 3 ( i < q o o q o o o ' r o d d d o o o c i o J a l^ g ^ o c < i ^ q . w r w g ' ^ r - H r - i i - l . - i i - H « - 4 . " a J j ^ l j i r H r - l r H I N r - t l N l S U^ \JJ r • > • * t ^ ^ C J O O O O O O i f t I Q CO CO (M| d "^^ ^ -jr- 20 [0061] The above-described each billet was forged into a steel bar having a diameter of 90 mm, and further forged into a steel bar having a diameter of 50 mm under the conditions of a heating temperature of 1200°C, and a finishing temperature of 1000 to 1050°C. After forging, the said steel bar was stood to cool in the atmosphere, and thus cooled to room temperature. [0062] From the R/2 portion of each steel bar, having the diameter of 50 mm obtained as described above, the four-point bending test piece for measuring straightenabnity having the shape shown in Figure 3 was cut off. The said test piece was subjected to the nitrocarburizing under the condition of a soaking temperature of 600°C and soaking time of 150 minutes in an atmosphere of NH3 gas : RX gas = 1: i; and thereafter it was water cooled. All of the units of dimensions of the four-point bending test piece shown in Figvire 3 are "mm". [0063] To the notch bottom of the four-point bending test piece obtained as described above, a 2-mm strain gage was bonded, and a bending strain was imparted until the strain gage was broken. In the case where a crack occurs in the nitrocarburized layer, the strain gage bonded to the outer layer is broken. Therefore, the straightenability was evaluated by the strain at the time when the strain gage was broken, that is to say, by the amount of straightenable strain. For aU of the four-point bending test pieces on which the strain gage was broken, the test piece was embedded in a resin so that the R3 notch bottom longitudinally sectioned portion of the four-point bending test piece did be the test plane. Thereafter, the aforementioned test plane was mirror-like polished, and by using an optical microscope, it was checked that a crack was present in the nitrocarburized layer. - ^ - 21 [0064] The target of the straightenability was made such that the amount of straightenable strain was 20,000 |j£ or larger. [0065] The amount of straightenable strain, which is an index of the straightenability, was also shown in Table 2. In addition, the relationship between the P2 and the amount of straightenable strain was shown in Figure 4. [0066] As is apparent from Figure 4, in the case where the P2 is 0.90 or more, the amount of straightenable strain of 20,000 p,8 or larger can be obtained. [0067] The P2 can be a value close to 1.97 in the case where, as the contents of elements defined in the formula (2), C is a value close to 0.35%, Mn is 0.85%, Cr is a value close to 0%, and Al is a value close to 0.010%. [0068] If the non-heat treated steel for nitrocarbvirizing of the present invention, having the above-described chemical composition, is forged under the ordinary hot forging condition, for example, the conditions of a heating temperature of 1200 to 1300°C, and a finishing temperatures of 900 to 1100°C, and thereafter is stood to cool in the atmosphere, and thus cooled to room temperature, a non-heat treated steel material having a ferrite-pearhte structvire, in which the area fraction of ferrite is 30 to 80%, and the balance is pearHte, can be obtained. [0069] (B) Diffusion layer of the nitrocarburized component: The nitrocarburized component of the present invention can be obtained by the following process^ the aforementioned non-heat treated steel material is formed by machining so as to have a component shape, and thereafter is subjected to the nitrocarburizing under the conditions of a soaking temperature 2'Zof 450 to 650°C, and a soaking time of 30 minutes or longer. The said soaking temperature is far lower than the A3 transformation point of the steel; and thus in the matrix and diffusion layer of the said non-heat treated steel material, the phase transformation during the nitrocarbiu'izing does not occur, and the microstructure of the nitrocarburized component becomes the same ferrite-pearUte structure as that of the non-heat treated steel material having not been subjected to the nitrocarburizing. Therefore, the diffusion layer of the nitrocarburized component of the present invention consists of a ferrite-pearlite structure. [0070] Next, the nitrocarburized component of the present invention is such that in the ferrite in the ferrite-pearhte structure of the diffusion layer, 130 to 250/(j,m2 of plate-shaped precipitates each having a thickness of 5 nm or less and a width of 200 nm or less are present. If the aforementioned plate-shaped precipitates are present in the ferrite, the nitrocarburized component can be caused to have both of a high bending fatigue strength of 600 MPa or more and a target straightenability that the amount of straightenable strain is 20,000 us or larger [0071] A compound being fine and precipitating coherently in the matrix improves the strength of matrix as a precipitation strengthening factor, and contributes to precipitation strengthening more greatly as the precipitation amount increases and as the size decreases. Inversely speaking, a precipitate having a large size such that at least either of thickness and width exceeds the above-described value scarcely contributes to the strengthening of the nitrocarburized component of the present invention. In the case where 130/(j.m2 or more of plate-shaped precipitates each having a thickness of 5 nm or less and a width of 200 nm or less are present in the ferrite, a high bending ;>2r- 2 ^ I fatigue strength of 600 MPa or more can be attained steadily. On the other hand, if more than 250/\un.^ of plate-shaped precipitates having the above-described size are present in the ferrite, the outer layer is strengthened excessively, so that the amount of straightenable strain, which is an index of the straightenabihty, does not reach 20,000 |X8. [0072] The thickness and width of the said plate-shaped precipitate that is present in the ferrite are preferably 3 nm or less and 100 nm or less, respectively. Depending on the performance of the observation equipment such as a transmission electron microscope (hereinafter, referred to as a "TEM"), in the case where the TEM observation is made at a magnification of 200,000 times, the precipitate having a thickness of 1 nm and a width of 10 nm becomes the observation Umit. [0073] The diffusion layer of the nitrocarburized component described in this item (B) can be obtained by carrying out the nitrocarburizing under the conditions of a soaking temperature of 450 to 650°C, and a soaking time of 30 minutes or longer by using the steel having the chemical composition described in the above item (A). In addition, in the case where the steel having the chemical composition described in the above item (A) is used, the influence of cooling rate after the nitrocarburizing on the properties of component is not excessively great, and the target is met even when any cooling method is employed. Therefore, the coohng after nitrocarburizing can be carried out by an appropriate method. [0074] In the following, the present invention is explained in detail by referring to examples. EXAMPLES 14 [0075] __.^____ The steels A to N having the chemical composition shown in Table 3 were melted by use of a 70-tons converter, and then were bloomed into billets having a cross-sectional size of 180 mm x 180 mm. [0076] The steels A to E in Table 3 are steels having the chemical composition being within the range regulated by the present invention. On the other hand, the steels F to N are steels having the chemical composition being out of the condition regulated by the present invention. [0077] / [Table 3] / -JWg]! TO O q p CO Ol r-j C£j TP 0> 0 ) C35 00 O d i H r H r H r - H r H r H r - i r H T H O O O Q ' i H ,9 , O Ti* UO T-t C- l-- C^ CO W 05 CO r-i 00 00 > jQ I j 9 P *-? R '^ ^ ^ '^. ^'^ *^ P '"! 9 9 S < D T H T H j H r 4 o O O G d i H T H i - - I r - i t - i "^ • rt 4s +3 . S ^ •• — • — fl 3 o o irt o >* o w o ^ 00 o lo -^ o % p, rH Ti< t - I - t - t - i n 00 t O O t>> a ) - i - ( » - < O r - < r - { r — t r H t - ( O r - < ' - < l - - < O i — ( r - l J3 fefHooqqoooooooooo <:>Sd>SQS<6<6<6<6<6<6<6d '^ o +^ Rj -^ l > Ol iH t O O C O C O I > C O C O C n r - « C » 0 0 '-H g / ^ q o q r s o o o o o o ^ o o r - i .-^^ JJ B c>^ o • ^ C 3 C O O ' ^ O O Q l O C - O O r H Q O < 3 > O O l O -. ^,_':o c o o o o u s c o o o o o i f l O » o o o < M < J < t ;^ O - ^ r - < T H « - ( ( M r H r H r H i - H i - I C M ( M » H » H i H ^ 5 ^3 J^PH q q q o o o o o o q q q o o cpo o g d d o d d d d d o 6 d d c D o d o hfi r-J s - ! O C 0 0 5 t > C O l O O r H I > l O O a > 0 0 » O p ^ ' ^ ^ f c 5 0 5 0 r - 4 Q 0 0 5 C D O O C n O O < M C J 5 t - l > - f r H t ^ S g «2 • J Z j ' ^ O r H r H O O O O O O t H O r - l t H r - l - ' ^^ S -!< •!< ? ? S fi n n o r i ^ 0 5 t - c o c o o o o o o t o i o o > ' H i o i c ' 5 ; * 5 ! d ^ ^ »-j CNl rH iH Cc>(6c>SS (MIO*^ d> a-Fe, and the precipitate having the above-described shape as a plate-shape precipitate, the number of plate-shaped precipitates each having a thickness of 5 nm or less and a width of 200 nm or less was examined. That is to say, the ferrite in the ferrite-pearhte structure of the diffusion layer was observed by using a TEM at a magnification of 200,000 times, and the number of plate-shaped precipitates having the above-described size included in a field of 250 nm x 350 nm was determined. This process was repeated for optional 5 fields, and the number of plate-shaped precipitates each having a thickness of 5 nm or less and a width of 200 nm or less, which were included per unit area, was calculated by dividing the total number for 5 fields of the determined numbers by the total area of observed 5 fields. [0092] Furthermore, the bending fatigue strength and the straightenability were examined by using the aforementioned Ono type rotating bending fatigue test piece and foxir-point bending test piece, respectively, which were water cooled after being subjected to the nitrocarburizing. [0093] That is to say, the fatigue test was carried out under the alternating condition of 3,000 rpm (revolutions per minutes) at room temperature and in an air atmosphere by using the above-described Ono type rotating bending fatigue test pieces that were water cooled after being subjected to the nitrocarburizing, and thereby the bending fatigue strength was examined. The target value of the said bending fatigue strength is set to 600 MPa or more. [0094] A 2-mm strain gage was bonded to the notch bottom of the four-point bending test piece that was water cooled after being subjected to the '^1 nitrocarburizing, and a bending strain was imparted until the strain gage was broken. As described above, in the case where the strain gage was broken, a crack occurred in the nitrocarburized layer. Therefore, the straightenability was evaluated by the strain at the time when the strain gage was broken, that is to say, by the amount of straightenable strain. The target of the amount of straightenable strain, which is an index of the straightenability, is set to 20,000 us or larger. [0095] The above-described test resvdts are summarized in Table 4. [0096] [Table 4] / -5 o i o o u s * a BS^ -g S S ? c 3 C O N < M ( N < N i N M « O i N C O C 0 ' - < ' - f ' - l ' - < r H « g « „ , g J j =» 'ft » » » I .| ^.g ^ rg o i o i o o i o o o o » f t O i f l © o o o o ! 2 ® !rta ^ h n m M ' i N C O ' ^ l O O J C O M O O O O O S O J C O C O t ^ O O S ' " ' r£3 ?>£S f^ 2 ' g g ' t s o « > « o c o c o « > < o w ' o i o i o « > t o « > « > 2 j , . +^ b-S r^i « tS * -3 tj O ?* O fd fl b tn fe S f^ ti « " ^ ''^ .a d irt i ^ 3 o g -ti 43 « I ^ ' S . B ' 0 ' - t ' - < < M I M > - < r - l ^ ) - 4 . - l . - < . - l < M H i N < M < S J Q "SMS* ,^g s3 * ^. -; 'Sj^'S 3 + + + 4- + + + + + + + + + + + + H sfi Q)a™.<'t: I J ^ g "3 ..a « ' S ' r t O o i o o o i o i o o o m o i n i c i o i Q O b n ^ &«Ma}" ^ • S f o f l j S o ' o d d d d d d d d o ' o d d d d a 2 ^a^rt'S g o 2 i^ TS "a «w d 3 1 = tA & .S ^ § -^ "^ a , H , f j - f H i O O O l f t l O r - i i - « M O O O O C O I N O < - t O < M _ _ a ; p , S S ( i - S *^ a s + + + + + + + + + 4- + + 4- + + 4 - . - " 4 3 n w g o .a •S d _,u_jwji_.i_ o o 0 5 0 n ( M « ) r } < i o « 3 r . H ' 2 : r t r , " " o § o ^ _-__—_-_—____«. rtS'g^fiW'S -a • It It ^|1i^-t§i|S ao> r^® iilSe-t3iJH?HBe-idf-' ^ _ ^ ^ ^ ^ ^ ^ i-i <-> s w ft -5 [0097] _,_^_ __=_=======.============ From Table 4, it is apparent that test Nos. 1 to 7 in accordance with the "inventive examples", in which the conditions regulated by the present invention were satisfied, were on targets, that is to say, each bending fatigue strength was 600 MPa or more and each amount of straightenable strain was 20,000 H8 or larger, and had an excellent straightenabiUty in addition to a high bending fatigue strength. [0098] In contrast, in the case of test No. 8, the content of C of the steel F was as low as 0.20%, and deviated from the condition regulated by the present invention. Therefore, although the number of plate-shaped precipitates having a thickness of 5 nm or less and a width of 200 nm or less, which precipitated in the ferrite of the diffusion layer, was 176/p,m2, satisfying the regidation of the present invention, the strength of matrix was insufficient, and the bending fatigue strength as a nitrocarbiu*ized component did not meet the target. [0099] In the case of test No. 9, the content of Mn of the steel G was as low as 0.80%, and deviated from the condition regulated by the present invention. In addition, the number of plate-shaped precipitates having a thickness of 5 nm or less and a width of 200 nm or less, which precipitated in the ferrite of the diffusion layer, was as small as 112/|j,m2, which deviated from the condition regulated by the present invention. Therefore, the bending fatigue strength as a nitrocarburized component did not meet the target. [0100] In the case of test No. 10, the content of Al of the steel H was as low as 0.003%, and deviated from the condition regulated by the present invention. Therefore, the diffusion layer depth at the time of nitrocarburizing did not increase, and the bending fatigue strength as a mtrocarbitrizedcQiBEQnent did not meet the target. [0101] In the case of test No. 11, the parameter PI of the steel I was 0.95, and was lower than the range regulated by the present invention. Therefore, the bending fatigue strength as a nitrocarburized component did not meet the target. [0102] In the case of test No. 12, the content of Mn of the steel J was as high as 1.25%, and deviated from the condition defined in the present invention. In addition, the number of plate-shaped precipitates having a thickness of 5 nm or less and a width of 200 nm or less, which precipitated in the ferrite of the diEfusion layer, was as large as 257/nm2, which deviated from the condition regulated by the present invention. Therefore, the amount of straightenable strain as a nitrocarbiu-ized component did not meet the target, and the straightenability was poor. [0103] In the case of test No. 13, the content of Cr of the steel K was as high as 0.11%, and deviated from the condition regulated by the present invention. Therefore, the amovint of straightenable strain as a nitrocarburized component did not meet the target, and the straightenability was poor. [0104] In the case of test No. 14, the content of Al of the steel L was as high as 0.060%, and deviated from the condition regulated by the present invention. Therefore, the amount of straightenable strain as a nitrocarburized component did not meet the target, and the straightenability was poor. [0105] In the case of test No. 15, the parameter P2 of the steel M was 0.89, and was lower than the range regulated by the present invention. Therefore, the amount of straightenable strain as a nitrocarburized component did not meet the target, and the straightenabiHty was poor. [0106] In the case of test No. 16, the content of C of the steel N was as high as 0.42%, and deviated from the condition defined in the present invention. Therefore, the amount of straightenable strain as a nitrocarburized component did not meet the target, and the straightenability was poor. INDUSTRIAL APPLICABILITY [0107] The nitrocarburized component of the present invention steadily has a high bending fatigue strength of 600 MPa or more and an excellent straightenabiHty under various nitrocarbiwizing conditions, in particular even when water cooling is carried out in the cooUng step after nitrocarburizing; and thus the said nitrocarburized component is suitable as a crankshaft for automobiles and so on. If the non-heat treated steel of the present invention is used as a starting material, the said nitrocarburized component can be manufactured easily. . We claim: 1. A non-heat treated steel for nitrocarburizing characterized in that the steel has a chemical composition consisting of, by mass percent, C: 0.25% or more to less than 0.35%, Si: 0.15 to 0.35%, Mn: 0.85 to 1.20%, S: 0.10% or less, Al: more than 0.010% to 0.030% or less, Ti: 0.003 to 0.020%, and N: 0.010 to 0.024%, and the balance of Fe and impurities, wherein P and Cr among the impurities are P: 0.08% or less and Cr: 0.10% or less respectively, and further the PI and the P2 expressed by the formulas (l) and (2) are PI > 0.96 and P2 > 0.90 respectively: PI = 0.02C + 0.22Mn + 0.87Cr + 0.85A1 + 0.72 ... (l), P2 = 2.40C-0.54Mn-9.26Cr-0.01Al+1.59 ...(2), wherein each symbol C, Mn, Cr and Al in the above formulas (l) and (2) represents the contents by mass percent of the element concerned. 2. A nitrocarburized component characeterized in that the component in which the chemical composition of the matrix thereof consisting of, by mass percent, C: 0.25% or more to less than 0.35%, Si: 0.15 to 0.35%, Mn: 0.85 to 1.20%, S: 0.10% or less, Al: more than 0.010% to 0.030% or less, Ti: 0.003 to 0.020%, and N: 0.010 to 0.024%, and the balance of Fe and impurities, wherein P and Cr among the impurities are P: 0.08% or less and Cr: 0.10% or less respectively, and further the PI and the P2 expressed by the forumulas (l) and (2) are PI > 0.96 and P2 > 0.90 respectively, wherein, further the diffusion layer thereof consists of a ferrite-pearlite structure, and the number of plate-shaped precipitates having a thickness of 5 nm or less and a width of 200 nm or less, which have precipitated in the ferrite, is 130 to 250/pm2: PI = 0.02C + 0.22Mn + 0.87Cr + 0.85A1 + 0.72 ... (l), P2 = 2.40C - 0.54Mn - 9.26Cr - O.OIAI + 1.59 ... (2), wherein each symbol C, Mn, Cr and Al in the above formulas (l) and (2) represents the contents by mass percent of the element concerned.