ORIGINAL Description Title of Invention: ROLLED STEEL BAR OR WIRE FOR HOT FORGING Technical Field [ 0 0 0 11 The present invention relates to a rolled steel bar or a wire rod for hot forging for use as starting material of a component such as a gear and a pulley. More specifically, the present invention relates to a rolled steel bar or a wire rod for hot forging roughly formed through hot forging, wh,ich are excellent in machinability before carburizing or carbonitriding', and also excellent in bending fatigue strength and surface fatigue strength of a carburized or carbonitrided component. Background Art [0002] Conventional steel components such as gears and pulleys of automobiles or industrial machinery are made by using, as starting materials, rolled steel bars or wire rods of alloy steel for mechanical structures such as SCr420, SCM 420, and SNCM 420 specified by JIS standard, which are roughly' formed through hot forging or cold forging. After normalized if necessary, the, roughly formed rolledsteel bars or the wire rods are machined, - 2 - and then carburizing-quenched or carbonitriding-quenched, and thereafter are tempered at a temperature of not more than 200°C. The rolled steel bars or the wire rods are further subjected to shot peening processing if necessary for production, thereby securing a property required for each component such as contact fatigue strength, bending fatigue strength, and wear resistance. [0003] Due to recent progress in weight reduction and size reduction of components in order to achieve improvement of fuel efficiency of automobiles and high output performance of engines, load applied to the components tends to be increased. Meanwhile, in the light of cost reduction, there are also strong needs to omit additional surface processing such as shot peening after carburizing-quenching. There are also strong needs to enhance machinability because the percentage of machining cost in the total processing cost of components is great. [0004] It is common to add more alloying elements in order to enhance fatigue strength of components, but this often deteriorates machinability. Hence, it has been desired to cope with both b,ending/contact fatigue strength and machinability of the components at a high level. [0005] The aforementioned "contact fatigue" includes "surface fatigue", "linear fatigue", and "point fatigue", but there barely occur a "linear" contact or a "point" contact in reality; thus the "surface fatigue strength" is handled as the contact fatigue strength. [0006] Pitching is one of fracture morphologies of the surface fatigue, and the damage morphology of the surface fatigue caused on a surface tooth of a gear or'a pulley, etc., is chiefly pitching. Hence, enhancement of the , pitching strength corresponds to enhancement of the ' . aforementioned surface fatigue strength, and-thus the "pitching" will be described as the "surface fatigue", and the "pitching strength" is referred to as the "surface fatigue strength", hereinafter. [0007] JP60-21359A, JP7-242994A, JP7-126803A suggest improvement of steel for gears. Specifically, JP60- 21359A discloses steel for gears specified to contain Si: not more than 0.1% and P: not more .than 0.01% so as to provide gears excellent in strength and stiffness, and having high reliability. JP7-242994A discloses steel for gears, gears, and a method of producing the gears specified to contain Cr: 1.50 to 5.0%, and 7.5%>2.2xSi ( % ) +2.5xMn ( % ) +Cr ( % ) +5.7xMo ( % ) if necessary, or Si:0.40 to 1.0% so as to be excellent in tooth surface strength. JP7-126803A discloses carburized steel for gears specified to contain Si: 0.35 to not more than 3.0%, and V: 0.05 to 0.5% so as to be preferable to provide gears excellent in wear resistance and surface fatigue strength as well as bending fatigue strength. Disclosure of the Invention Since no account is taken for the surface fatigue strength in JP60-21359A, the surface fatigue strength is insufficient. Since no account is taken for the bending fatigue strength in JP7-242994A, the bending fatigue strength is insufficient. The machinability is also . ' insufficient. Since sufficient account is not taken for the bending fatigue strength in JP7-126803A, the bending fatigue strength is insufficient. Since V-addition significantly increases hardness of steel after hot forging, the machinability is also insufficient. [0009] As described in JP60-21359A, JP7-242994A, and JP7- 126803A, it has been well known that adjustment of contents of Si and Cr produces steel material excellent in bending/surface fatigue strength after carburizing or carbonitriding. In general, it is, however, difficult to cope with both the banding/surface fatigue strength and the machinability at a high level, which conflict with each other. [OOlO] An object of the present invention is to provide a rolled steel bar or a wire.rod for hot forging to be roughly formed through hot forging, which is capable of coping with both machinability and bending/surface fatigue strength of a carburizing-quenched or carbonitriding-quenched component at a high level. Solution to Problem and Advantageous Effects of Invention The rolled steel bar or the wire rod for hot forging according to the present invention includes: a composition containing, in mass%, C: 0.1 to 0.25%, Si: 0.01 to 0.10%, Mn: 0.4 to 1.0%, S: 0.003 to 0.05%, Cr: 1.60 to 2.00%, Mo: 0.10% or less (including O%), Al: 0.025 to 0.05%, and N: 0.010 to 0.025%, in which contents of Cr and Mo satisfies 1.82rough I L ( an image analysis'in accordpnce with a common method'i : I 1 i The microstructure of every1 specimen included no , , , I I I martensite structure, and whs constituted by any one1 of I I the ferrite-pearlite structure, the ferrite-pearlite- I bainite structure, or the f,errite-bainite structure.;, , 1 [0052] I I i j Each rolled steel bar 'for hot forging having a 1 I ( 1 diameter of 50 mm, which was produced by using each3steel I of Table 1 underithe condition shown in Table 2, was, I k ' heated at a temperature of 1200°C for 30 minutes, and was f ' roller pitching test shownI in Figure 1, and each sp1e cimen 1 having a notched portion ;or the Ono-type rotating / I then hot-forged at a finishing temperature of 950°C ;br , I : bending fatigue test a shape shown in ~i~urd'j2 I:/ more, so as to be' produced 1 ' into a round bar having I I diameter of 35 mm. Each small roller specimen for a' I i.1 Condition: cutting speed was 200 m/min., feed rate was 0.30 mm/rev., depth of cut was 1.5 mm, and watersoluble cutting fluid was Gsed. Measurement item: amount of flank wear at major cutting edge after ten minutes of cutting time. [0060] The test result of each test above was shown in Table 4. The target value in the cutting test was specified to have amount of flank wear .at the major cutting edge of 20% or more than 20% smaller than that in Test No. 2 specified to be 100, where Test No. 2 was produced by carburizing the steel B that is a conventional high strength material satisfying the specification of SCM822H of JIS standard. Test -No. 1 2 -3 -4 -5 6 7 -8 9 -10 Classification I Steel Comparative Ex. 'A Comparative Ex. *B Comparative EX. C Inventive Ex. C Comparative Ex. C Comparative Ex. C Comparative Ex. C Comparative Ex. C Inventive Ex. C Comparative Ex. C Table 21 Average Ferrite 2 F+P 1.5 (2) F+P+B 1.7 (1) F+P+B '2.3 (2) F+P+B 1.7 (3) F+P+B '2.2 (4) F+P+B '2.8 - (5) F+P+B *2.4 (6) F+ B '3.2 (7) F+P+B 1.3 (8) F+P+B *2.. 1 Mediumcycle1Bending Fatigue Strength (Specified) Standard (100) #I12 #I12 120 #114 #I02 #I10 #I 04 125 #I14 HighcyclelBending Fatigue Strength (Specified) Standard (1 00) #I12 118 120 118 #I12 116 #I14 120 118 I *) represents deviation from the scope of the present invention. I Surface Fatigue Strength (Specified) Standard (100) 120 120 130 120 #I15 120 #I10 i 30 125 I #) represents that the target of the present invention is not achieved. Cutting TesVWear Amount at Main Cutting Edge (Specified) 70 Standard (1 00) 75 70 75 75 75 75 70 70 As show in Table 4, in each test for Test No, that .deviated from the condition specified by the present invention, at least one of the target bending fatigue strength, surface fatigue strength, and machinability was not achieved. [0062] In each test for Test No. that satisfied the condition specified by the present invention, the target bending fatigue strength, surface fatigue strength, and machinability were all achieved. Example 2 [0063] Quality governing was applied to steels D to T having the chemical compositions shown in Table 5 in the 70-ton converter, and rectangular ca'st pieces of 400 mm x 300 mm were produced through continuous casting, and were cooled down to not more than 600°C. [Table 51 [0064] Rolling reduction was applied to each steel while being solidified in the continuous casting. Each cast piece was heated.under the condition shown in Table 2, - 33 - and thereafter the cast piece was formed into a square billet of 180 mm x 180 mm through the blooming, and then was cooled down to the room temperature. Subsequently, the billets were heated under the condition shown in Table 2, and thereafter, were hot-rolled into steel bars having a diameter of 50 mm and 70 rnrn under the condition shown in Table 2. The investigation item and the investigation method were the same as those described in Example 1. [0065] The test result of each test was shown in Table 6. As show in Table 6, in each test for Test No. that deviated from the condition specified by the present invention, at least one of the target bending fatigue strength, surface fatigue strength, and machinability was not achieved. [00671 In each test for Test No. that satisfied.the condition specified by the present invention, the target bending fatigue strength, surface fatigue strength, and machinability were all achieved. The tests for Test No. 31 and Test No. 33 containing Nb had results significantly exceeding the target value. The tests for Test No. 39 to Test No. 41 containing at least one of Cu and Ni had results significantly exceeding the target value. [0068] The embodiment of the present invention has been explained as described above, but the aforementioned , embodiment was nothing but an example for carrying out the present invention. Accordingly, the present invention is not limited to the aforementioned embodiment, and various modifications and variations of the aforementioned embodiment may be carried out without departing from the scope of the invention. We claim: [Claim 11 A rolled steel bar or a wire rod for hot forging comprising: a composition containing, in mass%, C: 0.1 to 0.25%, Si: 0.01 to 0.10%, Mn: 0.4 to 1.0%, S: 0.003 to 0.05%, Cr: 1.60 to 2.00%, . Mo: 0.10% or less (including O % ) , Al: 0.025. to 0.05%, and N: 0.010 to 0.025%, contents of Cr and Mo satisfying 1.821fn112.10 if a value of fnl expressed by a following formula (1) is given : fnl=Cr+2xMo (1) I where an symbol of each element in the formula (1). represents a content of the element in mass%; a balance being Fe and impurities having a composition containing P: 0.025% or less, Ti: 0.003% or less, and 0 (oxygen) : 0.002% or less; a structure including any one of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, and a ferrite-bainite structure; and a cross section in which a maximum value/a minimum value of an average ferrite grain diameter is 2.0 or less - 37 - when measurement by observation is .randomly carried out in 15 visual fields with an area per visual field set to be 62500 pn2. [Claim 23 The rolled steel bar or the wire rod for hot forging according to claim 1, wherein the rolled steel bar or the wire rod contains Nb: 0.08% or less in mass% instead of part of Fe. [Claim 31 The rolled steel bar or the wire rod for ,hot forging according to claim 1 or 2, wherein the rolled steel bar or the w.ire rod contains at least one of Cu: 0.4% or less and Ni: 0.8% or less in mass% instead of part of Fe.