FIELD The present invention relates to a carburized shafi part. BACKGROUND A shaft part used in a11 automobile or industrial machine (for example, a transmission shaft) is sometimes heated to harden its surface by cahurizing and quenching or induction hardening. As the method of producing a quenched shaft pa& there is for exa~npleth e following method- First, a workpiece of a shape close to the final product is produced- Ned, an oil hole or other hole is f o r d by drilling, etc., to produce a semifil~ished part further closer to the final produ~tF. urther, finally, the semifinished part is quenched (hduction hardened or carburlzed 20 md quenched) to obiah the shaft part. [@004] Usually, a shafi part is formd with various holes includhg an oil hole. The area su~roundinga hole is the pofiion which is the weakest in strength. Therefore, to chance the strength of a shaft part having a hole, it is necessary to focus on strengthenhg the hole and its 25 sunrour~dingsA. technique for enhancing the torsional fatigue strength of a shaft part is disclosed 111 PTE I a d $TI, 2. ~ooos~ PTL 1 discloses a method of produci~~a gs haft part with a high torsional fatigue strengh optimizing the components of the steel material and the carburiziilg t h e . 30 [0006] PTL 2 discloses a shaft excellent in fatigue resistance characterized in that a residual stress s f compression at a surface layer of an oil hole is 50% to 90% of the tensile strength ofthe steel material and a method of lnngrovl~~thga t fatigue strength. 35 [CITATION LIST] [PATENT LITERATURE] [0007] [PTL 11 Japanese Unexamined Patent Publication No. 2005-256082 [PTL 21 Japanese Unexamined Patent Publication No. 2006-1 1 1962 5 SUMMARY [TECHNICAL PROBLEM] [OOOS] In this regard, in recent autoinobiles ai~din dustrial machines, for improving the he1 efficiency, smaller size and lighter weight are being drongly demanded. I11 the midst of all of 10 this, hrlher improvement of the torsional fatigue strength and excellent static torsional strength are being dema~lded& om shaft parts- However, in the shaft part obtained by the technique disclosed in PTL 2, hole making and ilnprovement of strength have not been sufficiently studied and &&her the structure of the surface layer of the hole has not been sufficiently studied, so sometiines achieving both static torsional strength and torsional fatigue strength at high levels is 15 difficult. 100091 In the technique disclosed in PTL 2, an ultrasonic vibration terminal is used to strike the inside of an oil hole to k~troducer esidual stress of compressio~at~ t he surface layer of the oil hole and thereby stmngthen the part of the oil hole forming the sta&ig pokt of fatigue fiacture of the 20 shaft. However, with strking by an ultrasonic vibration ter~ninal,i t is difficult to perfor~ne ven treatmnt over the entire oil hole and there is the possibility that the target strength cannot always be obtained. Furthermore, due to the insufficient study of the constituents of the steel material and structure of the surface layer, sometimes achieving both static torsional strength and torsional fatigue strength at high levels is difficult. 25 loo r ol As a technique for strengthening an oil hole, in addition to striking the hole by an ultrasonic vibration teminal disclosed in PTL 2, treatment to improve the surface by shot peening may also be considered. However, both of these processes requlre different facilities and apparatuses than ~lormapl rocesses, and therefore are economically disadvantageous due to an 3 0 increase in cost. loo1 I] The present invention was made in consideration of the above situation and has as its object the provision of a carburized shaft part excellent in static torsional strength and torsional fatigue strength. 3 5 [SOLUTION TO PROBLEM] [OOlZ] The inventors engaged in intensive studies on a carburized shaft part able to achieve both excellent static torsional strength and torsional fatigue strength. As a result, the inventors discovered that by machining a hole aRer carburizing and quenching, at the time of machining, 5 the rehined austenite at the surface layer part of the Iaole tm~sformtso hard deformation-induced martensite and can make the hard~~ensesa r the hole rise. Furthermore, the inventors found that by inaking the hardness near the hole rise, the formation and progression of cracks fiom a portlo11 near the hole are suppressed, so the static torsional sh-ength and torsional fatigue strength of carburized shaft part can be improved and that by making a larger amui?L of retained 10 austenite transform to deformnation-induced madensite at the time of machining, the static torsional strength auld torsional fatigue strenglh of the carburized shaft part can be improved more. [@@I31 Usually, to control the behavior in transforination to deformation-induced martensite at 15 the time of machining, optimking the ~nachiningc onditions is effective. For this reason, the imve~~toersq erimented with optimlzatlon ofthe mach'ming conditions so as to increase as much as possible the amount oftrmsfor~nationto mwens&e. However, wkh optimizhg just the ~nachiningc osrditions, while the st&ic torsional strengh and brsional fatigue stren@h of the carburlzed shaft part are indeed improved, this does not lead to the targeted values being 20 reached. [0014j Therefore, the hventors took ~~otlocfe t he chemical constituents of the steel material (carburized shaft part) and heat treatment condhions as well to try to k ~ h eirm prove the static torsional strength and torsional fatigue strengh. As a result, they found that by employing 25 specific steel material constituents and heat treatment conditbns, defamation-induced martensite tra~~sformatiomno re easily occurs at the time of n~achininga nd the static torsional strenglh and torsional fatigue streng"rh of the carburlzed shaR part are remarkably in~proved. [0015j In the past, it has been general practice to employ specik chemical constituents of the 30 steel material and heat treatment conditions to control the amount of retained austenite. However, optimizing the chemical constituents of the steel material and heat treat~nent conditions so as to c013tro111ot only the amount of retained austenite but also the behavior of deformation-iraduced martensite transformation at the time of machining is a novel technical idea not found up to now. 3 5 [0016] Due to the above, the inve~~toorbst ained the finding that to dramatically lrnpro.de the static torsional strength and torsional fatigue strength of a carburized sl~afpi art, rather than individually optimizing the chemical constituents of the steel material, heat treatment conditions, and machining conditions, it is desirable to optimize these conditions linked with each other organically. 5 loo r 71 Fuaher, the invento~xo btained the fmdii~gth at by organically optimizing the chemical constituents, heat tre&ment conditions, and rnachiiling conditions of the steel material, the structure after carburking and quenching and the structure afier machining are suitably controlled and in tun1 a carburized shaR part improved in static torsional strenglh and torsional 10 fatigue strenglh with a good balance is obtained. Based on the above finding, the inventors completed the illvention. The gist is as follows. fools] [I] A carburrzed shaft part comprising, at a 3 mrn deptl~fi -om an outer ckcumferential surface or inside deeper than that, by mass %, 15 C: 0.10 to 00.30%, Si: 0.01 to O.30%, Mn: 0-4 to 2.0%, P: 0.050% or less, S: 0.005 to 0.020%, 20 Cr: 0.4 to 3.5%, Al: 0.010 to 0.050%, N: 0.005 to 0-025% 0: 0.003% or less, aid a balai~ceo f Fe and impurities, 25 optlollally fuaher comprising, by mass%, Pb: 0.5% or less, one or more elerne~ltss elected 16rom the group consisting of V, Nb, and Ti: 0.1% or less in total content, one or more elements selected fronl the group consisting of Mo: 3.0% or less and Ni: 30 2.5% or Iess, Cu: 0 to 0.50%, and B: O to 0.020%, satisfying formula ( I ) and formula (21, having a C content o'fa surface layer part (Gs) by mass% of 0.60 to B.OO%, 3 5 having at leas! one hole at the outer circumferential surface, Baaviaag a total voBuane ratio (as?') of martensite and retained ausrien~iteo f9796 or more at a structure at a position of a 1 mm depth fioin the outer circumferential surface in an axial direction of the hole and at a position of a 20 pin depth from the surface of the hole, having a maximum retained austenite volume 1-atio (RI) of 10.0 to 30.0% at a position of a 1 mm depth from the outer circumfel-ential surface in an axial direction of the hole and in a 5 range of up to a 200 pm depth from the surface of the hole, and having a retained austenite reduction rate (Ay) of 20% or more found by a formula (A) fi-om the Rl and a voluine ratio (R2) of retained austenite at a position of a 1 mm depth from the outer circumferential surface in an axial direction of the hole and at a position of a 20 prn depth from the surface of the hole: 10 1~54xC+0.81xSi+l.59xNln+l.65xCr+l~77xMo+0.63xNi>2.(3I5) 11.31-0.1xSi+15~2xMn+7.OxCr+6.7xNI0+6.2xNi~33(2.)8 where, the contents (mass%) of the elements are entered in the notations of the elements in formula (I) and formula (2) and 0 is entered in the case where the elements are not included. Ay(R1 -R2)/Rl x 100 (A) 15 [2] The carburized shaft part according to [I], wherein the R2 is 20% or less. [3] The carburized shaft part according to [I] or 121, wherein the carburized shaft part has a plastic flow layer at the surface of the hole. [4] The carburlzed shaft part according to 131, wherein the plastic flow layer has a thick~lesso f 0.5 to I5 pin. 20 [ADVANTAGEOUS EFFECTS OF NVENTION] E@lol9] Accordi~igto the present invention, it is possible to obtain a carburized shaft part excellent in static torsional strength and torsional fatigue strength. 25 BRIEF DESCRIPTION OF D M [0020] FIG. I(a) is a schematic view of a quenched material and a carburized shaft part, while FIG. I (b) is a view showing a position of a I mm depth &om the outer ckcumference of the 30 quenched material and carburlzed shaft part in an axial direction of the hole and at a crosssection A-A' vertical to an axial center of the hole. FIG. 2 is a view showing a reference position in measurement of a volume ratio of retained austenite of the carbktrked shak part-t. FIG. 3 is a scan electron micrograph of a surface layer of a hole at a position of a 1 mrn 35 depth %om an outer circumference of the carburized shaft part ia-8 an axial direction of the hole and at a cross-section A-A' vertical to the hole. FIG. 4 is a side view of a test piece used for a torsion test. FIG. 5 is a top view of the surrounding parts of a hole in the carburized shaft part according to the present invention. 5 DESCRIPTION OF EMBODIMENTS [OOZl] Below, refewing to the drawings, a carburized shaft part according to an embodiineilt of the present invention will be eqlained in detail. Note that in the figures, the same or corresponding members will be assigned the same notations and explanations will not be 10 repeated. (00221 A carburized shafi part according to an embodiment ofthe present invention is a carburized shaft part comprising, at a 3 mrn depth from an outer circumferential surface or inside 15 deeper than that, by inass %, 6: 0.10 to 0.30%, S1: 0.01 to 0.30%, Mn: 0.4 to 2.0%, P: 0.050% or less, s: 0.003 to 0.020%, Cr: 0.4 to 3.5%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, 0: 0.003% or less, and a balance of Fe and impurities, optionally furl-her comprising, by ~?aass%, Pb: 0.5% or less, one or ~noree lements selected &om the group consisting of V, Nb, and Ti: 0,1% or less in total contellt, 30 one or more elements selected from the group consisting of Mo: 3.0% or less and Ni: 2.5% or less, CU: 0 to 0.50%, and B: 0 to 0.020%, satisfying formula (1) and fo'oramuBa j2), having a G content of a surface layer part (Cs) by mass% sf 0.60 to 1.00%, having at least one hole at the outer circumferential senrface, having a total voluine ratio (a'?) of cnartensite and retained austenite of 97% or Inore at a structure at a position of a 1 mm depth fiom the outer circuinferential surface in an axial direction ofthe hole and at a position of a 20 pm depth fiom the surface ofthe hole, having a ~naximumre tained austenite volu~nera tio (Rl) of 10.0 to 30.0% at a position of 5 a I mm depth &om the outer circumferential surface in an axial direction of the hole and in a range of up to a 200 pm depth from the surface of the hole, and having a retained austenite reduction rate (Ay) of 20% or more found by a forlnula (A) from the R1 and a volume ratio (W) of retained austenite at a position of a I mm depth fiom the outer circumferential surfax in an axial direction of the hole and at a position of a 20 pm depth 10 &om the surface of the hole: 1.54xC-t0.8IxSi+1-59xMn+l.65xCr+l.77xMo+0.63xN22.35( I) 1 1.31-0. I xSi+15.2xMn+7.0xCr+6.7xMo+6.2xNi<33.8 (2) where, the contents (mass%) of the elements are entered in the notalions of the elements in for~nula(1 ) and forlnula (2) and 0 is eiltered in the case where the elements are not included. 15 Ay=(R1 -R2)/R1 x 100 (A) [0@23] The carburked shaA part accordillg to an embodiment of the present invention includes any shaft parts havhg at least one oil hole or other hole at the outer ckcumfere~~tisaulr face and treakd by carbunzdion- It is not padicularly limited but for example includes shaft parts used 20 for automobiles and hdustrial machines, for example, transmission shafks, Further, the carburlzed shaft part accordkg to an ennbodi~nent of the present invention includes any shapes of shaft pa&s. mile not pa&icularly limited, it nay be a hollow or solid tubular shaped or rod shaped shaft part with a diamter of about 150 mm or less, about 100 mm or less, or about 30 mm or less and a length of 5 n~rno r more. 25 [00241 [Chemical Composition of Garburized "Oafi Part (Essential Constitue~~ts)] The cahurizd shaft part has the foilowing chemical composition. Note that the ratios (Oh)o f the elements shown below all lneall mass%. At the carburlzed shaft part, carbon is introduced into the surface layer pad due to t11e carburization, so drictly speaking the surface 30 layer part aiid inside part of the carburlzed shaft part diEer in chemical composition. Therefore, the chemical compositio~sl~~ ownb elow (including the essential constituents, impurities, and optional constituents) refers to the chemical composition at a region not affected by the carburizing, i.e., the 3 mm depth kom the outer ckcumferentlal surface ofthe carburlzed shaft part or the inside part deeper than that6$so, as to match with the chemical composition of the steel 35 material before carburizing. [0025) C: 0.10 to 0.30% Carbon (C) enhances the strength of the carburized shaft part (in particular the strength of the core part). C furthermore produces retained austenite for enhancing the static torsional strength and torsional fatigue strength. If the C content is too low, these effects cannot be 5 obtairted. On the other hand, if the C content is too high, the strength of the steel material for being worked to a carburized shaft part becomes too high. For this reason, the machiileability of the steel material falls. Thei-efore, the C content is 0.10% to 0.30%. The preferable lower limit of the C conte~lits 0.15% or n~oreT- he preferable upper limit ofthe C content is less than 0.25%. 10026) 10 Si: 0.01 to 0.30% Silicon (Si) has the action of enhancing the hadening ability, but at the time of carburizing, ends up incl-easing the carburized abnormal layer. In pa&icular, if the content exceeds 0,30%, the carburized abnormal layer greatly increases, so a soft structure called an "incompletely quenched structure" is formed and the torsional fatigue strength of the carburized 15 shaR part falls. To prevent the fortnation of the carburized abnormal layer, the content of Si is preferably made 0.25% or less, more preferably is made 0.20% or less. Howven; in mass production, it is difficult to make the content of Si less than 0.01%- Therefore, the content of Si was made 0.01 to 0.30%. Note that ifconsidering the producing costs in mass production, in the actually produced parts of the present invention, the Si conte~w~itl l probably often be 0.05% or 20 more. 100271 Mn: 0.4 to 2.0% Mailganese (Mn) enhances the quenching ability of steel and makes the amount of the retallled austenite in the steel increase. Austenite containing Mn is more readily transfor~nedt o 25 deformation-induced marlensite colnpared with austenite not containihlg Mn at the time of machining after carburlziilg and que~lching. A s a result, the static torsional strenglh and tors ional fatigue drength of the carburized shafi part rise. If the Mn content Is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the amount of the retained austenite becomes excessively high after carburizing and quenching and tempering. For this 30 reason, sufficiellt deformation-induced madelzsite transformation will not occur at the time of machining, the amount of the retamed austenite will become excessive even after machining, and in t ~sunffic~ien t deformation-induced nlal-tensite transformatio~w~i ll not occur at the time of machining and the amount of the retained austenite will be hard to reduce even aNer machining. As a result, the static torsional strength and torsiem~alf atigue strength of the carburized shaft part 3 5 aRer machining will fall. Therefore, the Mn content is 0.4 to 2.0%. The preferable Bower Biw~it of the Mn content is 0.8% The preferable upper limit of the Mn content is Pi .8% [0028] P: 0.050% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and lowers the grain boundary strength. As a result, the static torsional strength and torsional fatigue strength of the 5 carburized shaft part fall. Therefore, the P a ~ ~ t eisr 0~.0t5 0% or less. The prefemble upper Iimit of the P antent is 0.030%. The P content should be as low as possible- The preferable lower limit of the P coiltent is 0.0002%. 10029j S: 0.005 to 0.020% 10 Sulhr (S) bonds with Mn to form MnS and ei~hancesth e machineability- If the S content is too low, this eff'ect cannot be obtained. On the other hand, if the S content is too high, coarse MnS are formed and the hot workability and cold workability of the steel and the torsior?al fatigue strength of the carburized shaft part fall. Therefore, the S content is 0-005 to 0.020%. The preferable lower limit ofthe S content is 0.008%. The preferable upper limit of the S col~tenits 15 0-015%. [0030] Cr: 0.4 to 3.5% Gkorne (Cr) enhnces the quenching ability of steel a d makes the amount of the retained austenik increase. If lowring the Cr col~tencth ese effects cmnot be obtaied. On the 20 other hand, if the Cr content is too high, the amount of the rebined austenite after carburlzing and quenching and tempering beconles excessive- In this case, suficient deformation-kduced marlensite tmnsformation does not occur at the time of lnachlning in the hole machining step. The amount of retailled austenite is difficult to reduce even after machinhg. As a result, the static torsional strength and torsional fatigue strength of the carburized shaft part fall. Therefore, 25 the Cr mnLent is 0.4 to 3.5%. The preferable lower limit of the Cr content is 0.5%. The preferable upper Iimit of the Cr content is 3.1 %. 1003 11 Al: 0.010 to 0.050% Aluminum (Al) deoxidizes steel. A1 furthermore bonds with N to fom AIN and refines 30 the crystal grams, As a result, the static torsional strength and torsional fatigue strength of the carburized shaft part rise. If the Al content is too low, these eEects cannot be obtained. On the other hand, ifthe Al content is too high, hard, coarse A1203 is formed, the machineability of the steel falls, and furthermore the torsional fatigue strength also falls. Therefore, the Al content is 0.010 to 0.050%. The preferable lower limit of the Al content is 0.020%. The preferable upper 3 5 limit of the AB conteaat is 0.040%. [0032] N: 0.005 to 0.025% Nitrogen (N) fornis nitrides to refine the crystal grains and enhance the static torsional strength and torsional fatigue strength of the carburized shaft part. Ifthe N content is too low, these effects cannot be obtained. On the other hand, ifthe N co11tent is too high, coarse nitrides 5 are formed and touglmess of the steel falls. Therefore, the N conteilt is 0.005 to 0.025%. The prefemble lower limit ofthe N content is 0.010%. The preferable upper limit of the N content is 0.020%. 10033j 0: 0.003% or less 10 Oxygel~( 0)i s an impurity. O bonds with Al to form hard oxide-based inclusions. The oxide-based inclusions cause the machineability of the steel to fall and also cause the torsional fatigue strength of the carburlzed shaN part to fall. Therefore, the 0 content is 0.003% or less. As bvv as possible a11 0 content the beEer. The preferable lower limit of the 0 content is 0.0001%. [0034] 15 The balance of the che~nlcacl omposition of the carburized shaft part consists of iron (Fe) and impurities* ""Impurities" mean constituents entering from the ore or scraps utilized as raw materials for the steel material or kern the environment of the production process and the lilce and not constituents ktentionally included in the carburized shaft part. Even if impurities enter the carburlzed shaft part, if they are trace ainounts and the prope&ks of the steel material are not 28 detracted &om, the object of the prexnt invention can be achieved- As a specific example, the carburized shaR part according to the present inveliition can achieve the object ofthe present hvelllioion even if containing the ele~nentss hown bebw within the respectively stipulated r-anges: Rare earth metals (REM): 0.0005% or kss, Calcium (Ca): 0.0005% or less, 25 Magnesium (Mg): 0.0005% or less, Tungsten (W): 0.001% or less, Antimony (Sb): 0.001% or less, Bismuth (Bi): 0.001 % or less, Cobalt (Co): 0.001% or less, 3 0 Tantalum (Ta): 0.001% or less. looas] [Chemical Composition of Carburlzed Shafi Part (Optional Constitue~lls)] The carburized shaft part -kmq also contain Pb 111 place of part of the Fe. LO0361 3 5 Pb: 0.5% or less Lead (Pb) is aaa optional eleaaae~at and may be included or not included If it is included, reduction in the tool wear and improveinent in the scrap disposability are achieved. However, if the Pb content is too high, the strength and toughness of the steel fall and the static torsional strength and torsioilal fatigue strength of the carburized shaft past fall. Therefore, the Pb content is preferably made 0.5% or less. The more preferable upper limit of the Pb content is 0.4%. Note that to obtain the above effects, the Pb content is preferably made 0.03% or more. LO0371 The carburized shaft part may also contain one or more elements selected fiom the group consisting of V, Nb, and Ti in place of part of the Fe. f00381 V, Nb, and TL 0.1% or less in total content Vanadiuln (V), niobium Wb), and tiknium (Ti) are optional elements and may be included or may not be included. These elements bond with C and N to form precipitates. The precipitates of these elements assist the refii~emenot fthe crystal grains at the quenched pads formed by AIN. The precipiLates of these elements enhance the static torsional strength and torsional fatigue strength of the carburized shaft past. However, if the total content of these elements exceeds 0.1%, the precipitates coarsen and the torsional fatigue strength falls. Therefore, the total content ofV, Nb, and Ti is preferably 0.1% or less. If one or m r e ofany of V, Nb, and Ti are included as optional elements, the above effect is obtained. The nnore preferable upper limit of the total content of V, Nb, and Ti is 0.08%. To obtain the above eEect by V, Nb, and Ti, inclusion of 0.01 96 or more is prefemble. 180391 The carburized shaft part nay fu~hermoreal so contain one or more elements selected &om the group consisting of Mo and Ni instead of past of the Fe. These ele~nentsa ll enhance the quenching ability of the steel and increase the amount of the retained austenite. f004Oj Mo: 3.0% or less Molybdenum (Mo) is an optional elennent and need not be included. If included, Mo enhances the quenching ability of the steel and makes the amount of the retailled austenite increase. Mo furthermore enhances the residance to ternper softening and enhances the static torsional strength and torsional fatigue strengLh of the carburized shaft part. However, if the Mo content is too high, the amount of the retained austenite after cal-burizing and quenching becomes excessive. In this case, sufficient defomatio~a-inducedm at%ensitet ransforn~ationd oes not arise at the time sf maclii~~inAg.s a result, the static torsional strengfli and torsional fatigue streng"c of the carburked shaft part fall. Therefore, the Mo content i s preferably 3.0% or less. Tlae more preferable upper limit of the MO content is 2.0%. To obtain the above effect by Mo, inclusion of 0.1 % or more is preferable. [0041] Ni: 2.5% or less Nickel (Ni) is an optional element and need not be included. If included, Ni enl~ancesth e quenching ability of the steel and increases the amount of the retained austenite. Ni hrther 5 enhances the toughness of the steel. I-Iowever, if the Ni content is too high, the amount of the retained austenite after carburizing and quenching becomes excessive. In this case, sufficient defomation-induced ma~ensitetr a~~sformatiodno es not occur at the time of machining after tempering. As a result, the static torsional strength and torsioilal fatigue strength of the carburized sl~afpt art fall. Therefore, the Ni content is preferably 2.5% or less. The more 10 preferable upper limit of the Ni content is 2.0%. To obtaii~th e above effect by Ni, inclusion of 0- 1 % or more is preferable- [0@42] CU: 0 to 0.50% Cu dissolves into a solid solution in madensite to enhance the stren@h of the steel 15 material. For this reason, the fatigue strength of the steel inaterial rises. However, if the Cu content is too higl~t,h e element segregates d the grain bou~ldarieso f the steel at the ti~neo f hot forging and induces hot cracking. Therefore, the Cu content is 0.50% or less Note that the Cu content is preferably 0.40% or less, more preferably 0.25% or less. To obtain the above effect by Cu, inclusion of0.l0% or more is preferable. 20 [0043j B: 0 to 0.020% B has the effect of suppressing the grain boundary segregat'lon of P and eilhallcing the toughness. However, if addb~go ver 0.020%, abnomal g r a i ~gr~ov vl-h occurs at the time of carburizing and the torsional fatigue strength falls. Therefore, the B content is 0.020% or less. 25 Note that the B content is preferably 0.015%, more preferably 0.010% or less. To obtain the above effect by B, inclusion of 0.0005% or more is preferable. [0044j (Relationship of Coidents of Elements) T11e relationship of contents of the elements forlning the carburized shaft part satisfies the 30 formula (1) and formula (2) shown below: 1.54~6+0.81xSi+1.59xMn+1.65xCi-cl.77xMo+0.63xNi22.(31g) 11.31-0.I~Si+~5.2xMrat-7.0xCr+6.7xMo+6~2x(N2i)~ 33~8 where, the contents (mass%) of the elements are entered in the notations sf the comespondi~age lements in formula (1) and formula (2) and 0 is entered when elements are not 3 5 included. [0045p$ Regarding Formula (1) Fl=1.54xC+0.81 xSi+l.59xMn+l.65xCr+1.77xMO.63xNi is defined. Fl is a parameter showing the quenching ability of steel. If Fl is too low, the quenching ability ofthe steel becomes lower. In this case, low strength ferrite and pearlite are fonned and the static torsional strength and torsio~~faalt igue strength of the carburized shaft part fall. Therefore, Fl is 2.35 or more- The preferable lower limit of Fl 'Is 3.0- For securing the toughness of the carburized shaft part, the preferable upper limit of Fl is 8.0. 100461 Regardi~lgF orlnula (2) F2=-0- l xSi+l5-2xMn+7.0xCr+6.7xMo+6.2xNi is defined- F2 is a parameter showing the stability of austenite- If F2 is too low, the ratio of the retained austenite obtained after carburiziilg and quenching becomes lower. As a result, the hardening action of the surroundings of the hole due to deformation-induced ma&ensite transformation is not obtained while the static torsional strength and torsional fatigue strength ofthe carburlzed shaft part become lower. On the other hand, if F2 is too high the mount of retained austenite after carburizing and quenching and ternpering becomes excessive and the static torsional strength and torsional fatigue stren@h fall. Furthermore, t k retahed austenhe is stable, so the ratio of deformion-induced martensite transfonnation~o btai~leda t the time of machining also bmomes smaller. From this viewpoint as well the swlc torsional strength and torsional fatigue strengh of the carburized shaft part fall. Therefore, a F2 of 1 1.3 b 33.8 is demanded. The preferable lower Limit of F2 is 12.0. The preferable upper limit of F2 is 33.0. [0047] [At Least 011e Hole at Outer Circumferential Surface of Carburlzed ShaR] The carburized shaft part according to an embodiment of the present invention has one or mnore through hoks or non-through holes vertical to or having a certain angle with respect to the longitudinal (axial) direction of the carburized shaft part, which are opened &om the oLller ckumferential surface of the carburized shafi part. The hole diameter is not particularly limited, but may be for example 0.2 mm to 10 mm. [0048] [C Content of Surface Layer Part (Cs): 0.60 to 1.00%] The G cor~taineda t the surface layer part of the carburized shaft part enhances the static torsional strength and torsioml fatigue strellgdh of the carburlzed shafi part. In the present invention?, the C content of the surface layer par4 of the carburlzed shaR part is measured by the following technique. [0049] The part of the carburized shaft part at a 1 mrn depth &om the outer circu~nferenatid surface in the axial direction of the hole and at 50 ym fiom the hole surface layer was cut out by a machining operation. The C content of the machining powder was quantitatively measured by emission spectroscopy. The value was defined as the C content of the surface layer part. Further, the C concentratioll of the surface layer part of the carburized shafi part can also be 5 quantitatively analyzed by EPMA (electron beam microanalyzer). [OOSO] If the C content contained at the surface layer part (Cs) is low, the carburized layer becomes lower in l~ardnessA. s a result, the carburized shaft part falls in static torsional strengh. On the other hand, if (Cs) is high, hard pro-eutectoid cemelltite is fornled at the surface layer of 10 the carburized shaft part. If Cs is excessively high and the pro-eutectoid cementite is over 3%, the cennentite becomes the starting point of cracks and the static torsional strength and torsional fatigue strength fall. Furlhermore, the tool wear at the time of ~nachiningin creases and the machineability falls.. Therefore, the C contel~ot f the surface layer part (Cs) is 0.60 to 1.00%. The preferable lower limit of Cs is 0.65%. The preferable upper lilnit of Cs is 0.90%. 15 [OOSl] [Total Volume Ratio (a'?) of Martensite and Retained Austenite at Structure at Poshion of Depth of I mm From Outer Clrcuinferential Surface of Carburized Shaft Part in Axial Directlor? of Hole and Depth of20 pm om Surfam of Hole] If ferrite, pearlite, and o"cher low stre~lgthp hases are present in the structure at the 20 position of a I rnrn depth from the outer ckcurnferential surface of the carburized shaft part in the axial direction ofthe hole and at a position sf a 20 pm depth from the surface of the hole, cracks easily form sta~ingk om these phases and the static torsional Grength and torsional fatigue strength of the carburlzed shafi part becoine bwer. Fu~ther,i f pro-eutectoid cementite is presellt-, the tool wear at the time of machining in the producing process of the carburized shaft 25 pa^ increases and, hdher, the cenlentite becolnes the sming points of fatigue hcture, so the torsional fatigue strength falls. Therefore, the total volulne ratio (a1+) of the martensite and retained austenite in the structure at this position is limited to 97% or more. Note that the preferable range of the total volume ratio is 99% or more, [OOSZ] 30 In the present invention, the total volume ratio (a'+y)o f the martensite and retained austenite is lneasured by the following method by observing the structure at the reference position 21 corresponding to the position of a I mm depth &om the outer ckcurnferentB1 surface of the carburked shaft part in an axial direction sfthe hole and a positlo11 sf a 20 pm depth from the surface of the hole (see FIG. 2). A test piece is taken so as to include a hole surface layer part 35 at a pssikiola ofa 1 mrn depth from the outer circumference of the carburized sB-saiA part the axial direction of the hole and at 68: cross-section vertical to the hole axial center and so that the surface vertical to the axial direction of the hole (horizoi~tacl ross-section) becomes the observed surface (FIG. 1A-A'). The inirror polished test piece is etched by a 5% Nital solution. Tl~eet ched surface is observed at three fields by a 1 000X power optical microscope. At this time, the reference position 21 is made the center of the field (FIG. 1-1 1). In the plane of a range of 20 5 pmx 100 pm of 10 pin in the surface direction of the quencl~ed material &om the center of the field, 10 p1n in the direction opposite to the surface ofthe quenched material from the center of the field, and 50 pm each in the two directions vertical to the surface dkection of the quenched material from the center of the field, the area ratios of the phases are found by the usual image analysis method. The average values of the area ratios of the phases obtained for three fields are 10 defined as the volume ratios of the phases. I00531 [Maximum Retained Austen&e Volume Ratio (RI) in Range at Position of Depth of 1 mm From Outer Ckcumferential Surface of Carburlzed Shaft Part 113 Axial Direction of Hole alld Up to Depth of 200 pm From Surface of Hole] 15 The retained austenite introduced by carburjzlng and quenching transforms to defonnatbn-induced marl-ensite at the t l m of machining a bole in the cahurlzed shaft part- Spifically, at the time ofhole ~n&~ngt,h e frictbnal force beWwn the cuaing tool and base material cauxs pm ofthe retained austenite near the surface layer of the hole to transform to deformation-induced ma&ensite. On the other hand, tfae occumence of deformation-induced 20 martensite transformation due to this action becomes stronger the closer to the surfxe of the hole and becomes weaker the fU~$her om the surface of the hole* I00541 As a result of the defamation-induced martensite transformation accompanying holemaking, the carburked shaR part rises in strength and the static torsional sh.ength and torsional 25 fatigue strength rise- To obtain such an effect, the maximum retained austenite volume ratio (RI) at a position of a 1 min depth kom the outer ckcumferential surface of the carburized shaft part in the axial direction of the hole and up to a 200 pm depth &om the surface of the hole has to be 10.00/0 or more. [OOSS] 30 On the other hand, retained austenite is soft, so if the maximum retalned austenite volume ratio (RI) exceeds 30.0%, conversely the static "crsional strength and torsional fatigue strengh of the carburized shaR part fall. [8056] In the present invention, the maxianurn retained austeaaite volume ratio (Rl) is measured 35 by the foliowing method. The carburized shaft part is cut in the axial di1rec8ioa-a of the hole so as to pass through the center and bisect the hole (FIG. 2B-B"). The surface ofthe hole is masked leaving open a 11ole of 91 mm centered about a position of a 1 mm depth from the outer circumferential surface and electrolytically polished. The duration ofthe electrolytic polishing is changed to adjust the amount of polishing and dig a hole of a 30 pm depth. The electrolytic polishing is performed by a voltage of 20V using an electrolytic solutioi~c ontaining 11.6% of 5 amnloniuin chloride, 35.1 % of glycerin, and 53.3% of w&er- The electrolytically polished surface is analyzed by X-ray diffraction to fitld the volume ratio of retailled austellite at a position 30 prn from the surface. This process is repeated to make the hole deeper by 10 pm at a time. Each time, the volume ratio of the retained austen'rte is measured- This is repeated until the hole depth becomes 200 pm. The maximum retailled austenite volume ratio obtained during that 10 ismade(R1)- I00571 At the surface of the electrolFically polished test piece, an X-ray is fred centered on a reference position for analysis by the X-ray diffraction method- For the X-ray diffraction, a product name RINT-2500mIPC made by Rigaku is used. For the light source, a Cr tube is used. 15 The tube voltage is 40kV, the tube currei~ti s 40 mA, and the collimator diameter is 0.5 mm- A V-filter is used to remove the KP rays. Just the Ka rays are used- For d&a analysis, the AutoMATE software (made by K~gaku)is used. The Rach'mger method is used to remove the Ka2 component and use the profile ofthe Kal cotnponent to calculate the volu~nera tio (RI) of retained austenite based on the ratio of integl-ated intensities of the difkactiol~p eaks of the (21 I) 20 f m of the bcc structure and the (220) face of the fcc structure. Note that, the spot size of the irradiated X-ray is made q0.5 rn~no r less fooseg [Retained Austenite Volu~neR atio (R2) at Position of Depth of 1 rnrn from Outer Girculnferentlal Surface of Carburized Shafi Part in Axial Direction of Hole and at Position of 25 Depth of 20 pm fiorn Surface of Hole] The retailled austenite volume ratio (R2) at a position of a I m111 depth from the outer circumferential surface of the carburized shaft part in the axial direction of the hole and at a position of a 20 pm depth from the surface of the hole is preferably 20% or less. If the volume ratio of the retained austenite after machining is too high, hard ma&ensite canr~obt e obtained and 30 the static torsional strensh and torsional fatigue stren@h fall. [0059] In the presel~ti l~ventiont, he retained austenite volume ratio (W)is measured by the following method. The carburized shaft part is cut in the axial direct is^^ of the hole so as to pass through the center and bisect the hole (FIG. 2B-B'). The surface of the hole is masked leaving 35 open a hole of cpl mm centered about a position of a I m1-n depth &o~tnh e outer ckcumferential starface and electrolyti&i63allgicy polished. The duration ofthe electrolytic polisl~ingi s changed to adjust the amount of polishing and form a hole of a 20 pm depth. An X-ray of a spot size of (p0.5 mm is fired at the center of the hole and the retained austenite volume ratio (R2) is measured i11 the same way as the retained austenite volurne ratio (Rl). [0060] 5 [Retained Austenite Reduction Ratio Ay Found From R1 and R2 by Formula (A): Ay=(R1- R2)R 1 x 1 001 The retailled austenite reduction ratio (Ay) found from Rl and R2 by the above formula (A) is 20% or more. [@061] 10 The retained austenite reduction ratio (Ay) shows the exlent of deformation-induced martensite transformation at the time of machining. If Ay is high, it means that a larger amount of defor~l~diion-induce~dn artensitetr ansformation occurs at the time of machinbg. The static torsionaal strengh and torsional fatigue strengh are improved. To obtain such an effect, Ay has to be 20% or more. Note that the preferable value of Ay is 25% or more. 15 100621 plastic Flow Layer of Hole Surface] The carburized shaft part according to an embodiment of the present invention may also have a plastic flow layer at the surface of the hole, This plastic Row layer is a layer fomed by occurrence of large deformation at the surface layer pad of the hole at the time of machining the 20 hole. This plastic Row layer is hard. If the thickness is 0.5 pm or more, the static torsional strength and torsional fatigue strength of the carburked shaft part can be improved, However, the plastic flow layer is briQle, so if it B thin in thickness, a cedaii~e aent of defamation is possible, but if the thickness is over 15 pm, cracks occur and form sarting points of eactures, so sometimes the torsional fatigue strenglh conversely falls. Furthermore, if the plastic flow layer 25 exceeds 15 prn in thickness, sometimes the machineability falls and the load on the tool at the tiine of machining iil~reasess o that tool life remarkably falls. Due to the above, the thickstess of the plastic flow layer of the surface layer of the carburized shaft part is preferably 0.5 to 15 pm. Note that to hdher improve the static torsional strength and torsional fatigue strellgth of the carburized shaft part, the thickness of the plastic flow layer ofthe surface layer of the carburlzed 30 shaft part is preferably made I pm or more, more preferably 3 p1n or more. Further, the preferable upper limit is is3 pm, more preferably 10 pm. [0063] The thicknmess of the plastlc flow layer at the surface of the hole is measured by the fillowing anethod. A test piece is taken so as to include a hole surface layer part at a position of a 35 1 rnm depth 6om the outer circumference of the carbaarized shaft part in the axial direction of the bole and at a cross-section vertical to tila; hole and so that the surface ve~ticaHto the axial direction of the hole (horizontal cross-section) becomes the observed surface (FIG. IA-A'). 'The inin-or polished test piece is etched by a 5% Nital solution. The etched surface is observed by a 5000X power scan electron microscope (SEM). One example of the obtained SEM image is shown in FIG. 3. In the figure, the plastic flow layer 3 1 is the part where the structure is curved 5 along the surface of the hole with respect to the base material 32 (fkom the left direction to the right direction ofthe paper surface at FIG. 3). The distance fiom the surface of the hole to the end of the curved structure is defined as the thickness of the plastic flow layer 3 1. l0064) [Hardened Layer of Mole Surface Layer Past] 10 The carburlzed shaft past according to an elnbodiinent of the present invention has a layer, inclrading the above plastic flow layer, hardened fi-om the hole surface over a certain depth. This hardened layer includes a layer for~nedb y the retained austenite of the hole surface layer part transforming to deformation-induced martellsite at the time of machining a hole (defor~nation-inducedm astensite layer) and for example has a thickiless of about 200 to 300 pm. 15 The carburizexl shaft part according to the presei~ti nvention, as shown in FIG. 5, realizes excellent stalic torsional stret~gtha nd torsional fatigue strengh overall by providing a Ilardened layer includillg a plastic flow layer 3 1 and deforiaLion- i~lduced ma&ensite layer 5 I, in particular a hard defamation-il~duced madensite layer 5 1, arouild a I~ole4 3 able to beco~lnea cause of lowering the static torsional strength and torsional fatigue strength. 20 [0065j/ The carburized shaft past accordirlg to an embodiment of the present invention can be produced by machining a hole after caa'burking and quenching. For example, it can be produced by the methods shown in following Modes I and 2. 25 100661 (Mode I) The method of producing a carburlzed shaR part coqrises workkg a steel lnaterlal to obtain a workpiece (workpiece producing step), carburizing and quenching the workpiece to obtain a carburlzed material (carburlzed material producing step), and machining a hole in the 30 quenched material to obtain a carburized shaft past (hole machining step). More specifically, the method of producing the carburlzed shaft part coinprises obtaking a workpiece by workkg a steel mnaterial comprising, by mass%, C: 0.10 to 0.309'6, Si: 0.01 to 0.30%, Mn: 0.4 "8 22.0%, P: 0.050% or less, S: 0.005 to 0.020%, Cr: 0.4 to 3.5%, AI: 0.010 to 0.050%, N: 0.005 to 0.025%, 0: 0.003% or less, and a balance of Fe and impurities, optionally furl-her comprising, by mass%, Pb: 0.5% or less, one or Inore elements selected from the group consist'mg of V, M, and Ti: 0.1% 10 or less in total content, , one or more elements selected goin the group consisting of Mo: 3.8% or less and N1: 2.5% or kss, Cu: 0 to 0-50O/o, and B: 0 to 0.020% satisfying formula (1) and formula (2) (workpiece producing step), carburk'mg, isotl~ermallyIn olding, and quenching the workpiece to obhin a carburized material, during which making a carburlzing temperature (TI) 900" to 1050°6, a carbon potential at the time of carburk'mg (Cp1) 0-7% to I. 196, a carburlzing time (tl) 50 minutes or more, an 20 isotkrmal hoMlng tempemture (T2) 820°C to 870°C, a carbn potential at the time of isotkrmal "nolding (Cp2) 0.7% to 0.9% or less, and an isotl~ermahl olding time (t2) 20 to 60 mhutes, I so that in the carburizd material, the slructure at a reference position correspondkg to a I position of a 1 lnln depth fiom the oukr clrcumkrential surface of the filial form of the carburized shaft part in the axial diection of the hole aid a position of a 20 ym depth &om a 25 position corresponding .to the surfam of the hole con&ks ma&ensite, a volume mtio (RZ) of 12.0 to 35.0% of retained austenite, and a volume ratio of 3% or less of phases other than the I ma&ensite and retained austenite (carburked material producillg step), and machining a bole in the carburized material to obtain a carburbed shaft part, during whic11 making a tool feed at the time of machlniilg over 0.01 mmlrev to 0.1 30 nnmlrev, a machining speed 10 dm'm to 50 mlmin, and a depth of cut (d) 0.05 m to 0.25 mm, so that in the structure at the reference position, the volume ratio (RF) of the retained austenite becomes 20% or less and the retained austenite redudion ratio (Ay') found froin the volu~nera tio (RH) of the retained austenite before machining and the vsluane ratio @F) of the retained austenite after machining by the formula (B) becomes 35% or more (hole machining 35 step): 1.54~C+O.$BxSi+1.59xMn-tl.65xCr+1.74xM0+0.63xNi22~3(15 1 1 1.3~-0.lxSi+l5.2xMn+7.0xCr+6.7xMo+6.2xNi133.8 (2) where, the contents (mass%) of the elements are entered in the notations of the elements in formula (1 ) and formula (2) and 0 is entered in the case where the elements are not included. Ay'=(RI -RF)lRI x 1 00 (B) 5 10067j [Workpiece Producing Step] In this step, a workpiece having a desired shape close to the shape of the carburized shaft part is produced. First, a steel material having the above chemical composition is prepared. [0068] 10 (Producing of Miorkpiece) A steel material having the above chemical composition is vvoked to obtain a workpiece. For the working method, a known method can be employed. For example, the workrng metllod includes hot working, cold working, machinhg, etc. The workpiece has a shape similar to the carburized shaR part at the pants other than the hole. The diameter of the hole is made slnaller 15 than the diameter of the carburized shalt part. Note that, the difference between the radius of the hole of the carburized shaft part and the radius of the hole d the workpiece corresponds to the depth of czll (d) in the subsequent hole machining step. 10069j [Cahurized Material Pmducing Step] 20 The above owained workpiece is carburized, isothermally held, and quenched to obtain a carburized material. Due to this, in the carburlzed material, the structure at the reference position 2 1 (see FIG. 2) at a 1 mm depth &om the outer ckcuinfel-ential surface of the final form of the carburized shag part 'm the axial direction of the hole and a position of a 20 prn depth &om the position corresponding to the surface of the hole contains n~afiensitea, volume ratio (RI) of 12.0 25 to 35.0% of retained austenite, and a volume ratio of 3% or less of phases other than the ma~$ensitea nd retairmed austenite. 100701 (Carburizing and Quenching) The carburizing and quenchi~lgs tep fist perforlns carbul-izing, then performs isothemal 30 holdhg. The carburiziilg and isothermal holdillg are performed under the following conditions. [0071] (Carbur izing) Carburllzlng "relmperature (TI): 900 to I 050°C If the carburking temperature (81) is too low, the surface layer of the worlcpiece is not 35 sufficiently carburized. In this case, there is little retained austenite after carburizing and quenching and the surface layer is also Bow in hardness. For this reason, the static torsional strength and torsional fatigue strength ofthe carburized shaft part become low. 011 the other hand, ifthe carburizing temperature (TI) is too high, the austenite grains become coarser and the static torsional strength and torsional fatigue strength of the carburized shaft part fall. Therefore, the carburlzing temperature (Tl) is 900 to 1050°C. The prefemble lower limit ofthe carburizing 5 temperature (TI) is 910°C, wllile the preferable upper limit is 1000°C. [0072j Carbon potential at time of carburizing (Cp1): 0.7 to 1 . 1% If the carboll potential (Cp 1 ) is too low, the material is not suficiently carbukzed- I11 this case, there is I&tle retained austenite after carburizing and quenching and the surface layer is also 10 low in hardness. For this reason, the static torsional strength and torsional fatigue stren@h of the carburized sl~afpt art fall. On the other hand, if the carbon potential (Cpl) is too high, the hard pro-eutectoid cementik precipitating at the ti~neo fcarburizing rema'ms in over 3% even after carburizing and quenching. In this case, cracks form starting fiorn the pro-eutectoid cementite and the carburized shaft part falls in torsional fatigue strength. Further, the tool wear at tlae time 15 of machining increases and the machineability of the carburlzed material falls. Therefore, the earbon Wential (Cp 1) is 0.7 to I. I %. The cgbon potential (Cp 1 $ may be made to fluctuate in that rmge at the time of carbur king. [ooaal Carburlzing time (t 1): 60 mi11 or Inore 20 If the time ofthe carburizing (carburlzing time) (tl) is too short, the material is not sufficiently carburlzed. Therefore, the carburlzing time (tI) is 60 ~nlnuteso r more- On tlx other hand, if the casburizlng time (t1) is too bng, the productivity falls. Therefore, the upper Illnit of the carburlzing time (t1) Is preferably made 240 minutes. [0074] 25 (Isothemad holding) ARer carburizing, the material is isotherinally held. The isothermal l~oldingis performned under the following conditions. [0075) lsotherinal holding temperature (T2): 820 to 870°C 30 Ifthe isothemal holding temperature (T2) is too low, control of the atinosphere such as the carbon poten~tlalb ecomes difficult. In this case, the volutne ratio of the retained austenite is difficult to adjust. On the other hand, if the lsother~nahl olding temperature (T2) is too high, the strain occuninag at the time of quenchi~~ingc reases amad sollaetirnes quench cracking occurs. Therefore, the isotlaerl-raal holding temperature (T2) is 820 to 89O0C. 3 5 [0076] Carbon potential at time of isothermal holding (Cp2): 0.4 to 0.9% If the carbon potential at the time of isothermal holding (Cp2) is too low, the C which entered at the time of carburizing is again discharged to the outside. In this case, there is little retained austenite after carburizing and quenching and the surface layer hardness is also low. As a result, the static torsional strength and torsional fatigue strength of the carburized shaft part 5 fall. On the other hand, if the carbon potential (Cp2) is too high, hard pro-eutectoid ceme~ltite precipitates. 111 this case, cracks form stai.-ting &om the pro-eutectoid ceil~eiltitea nd the carburized shaft part falls in the torsional fatigue strength. Fudher, the tool wear at the time of inachining increases and the carburized material falls in n~achineability. Therefore, the carbon pote~ltia(lC p2) is 0.7 to 0.9%. 10 10077) Isothermal holding time (t2): 20 to 60 min If the isothermal holding time (t2) is too shod, the temperahre of the workpiece will not &come uniform and the strain occull-ing at the time of quenching increases. In this case, sometimes quench cracking occurs in the carburized material. On the other hand, if the 15 isothermal holding time (t2) is too long, the productivity falls. Therefore, the isothermal holding time (t2) is 20 to SO miilutes. [0078] (Quenching) AAer isothemal holdkg, the lu~ow~mae thod is used for quenching. The quenching can 20 lFor example be lnacle oil quenching. 18879) (Tempering) If desiring to raise the toughness of the carburlzed shaA part, it is possible to perfonn carburlzii~ga nd quenching, then perform temperhg. 25 tooe(o1 (Structure of Carburked Material After End of Carburlzed Material Producing Step) The structure at the reference position 2 1 correspondhg to a position of a I mm depth from the outer circumfere~lCiasl urfxe of the final fom of the shaft part in the axial direction of the hole and a position of a 20 pm depth fiom a position corresponding to the surface ofthe hole 30 under the above conditions contains ma&ensite, a volume ratio (R1) of 12.0 to 35.0% of retained austenite, and a volume ratio of 3% or less of phases other than the martensite and retained austenite- [OOSl] Note that, the structure at the reference position 21 corresponding to a position of a I mrn 35 depth from the outer ckcumferentaaB surface of the final form of the quenched material ofthe carburized shaft part in the axial direction ofthe hole and a position ofa 20 prn depth fiorn a positioil corresponding to the surface of the hole is observed by the following method. In the quenched material, a test piece is taken so as to include a hole surface layer part at a position of a 1 inm depth fi-om the outer circumference of the final form of the carburized shaft part in the axial direction of the hole and at a cross-section vertical to the hole axial center and so that the 5 surface vertical to the axial direction ofthe hole (horizontal cross-section) becomes the observed surface (FIG. IA-A'). The mirror polished test piece is etched by a 5% Nital solution. The etched surface is observed at three fields by a l000X power optical microscope. At this time, the reference position is made the center ofthe field (FIG. 1-1 I). In the plane of a range of 20 plnx100 prn of 10 pm in the surface direction ofthe quenched material from the center of the 10 field, 10 pm in the dirwtlon opposite to the surface of the quenched material from the center of the field, and 50 prn each in the two directions vertical to the surface dkection of the quellched material from the center of the field, the area ratios of the phases were found by the usual image analysis method. The average values of the area ratios ofthe phases obtained for three fields were defined as the volume ratios of the phases. 15 [0082] In the observation of the structure by an optical microscope, the retained austenite is included in the martensite. In other words, in observation of the structure by an optical microscope, martellsite and retained austenite cannot be diEerentiated. Tllerefore, the retained austenite volu~nera tio (RI) at the reference position corresponding to a position of a 1 rn~nd epth 20 kom the outer circumferential surface of the f i ~ ~faorlm of the carburlzed shaR part in the axial direction of the hole and a position of a 20 pm depth Ero~na position corresponding to the surface of the hole (FIG 2-2 I) is measured by the following n~ethodT. he carburized material is cut in the axial direction of the hole so as to pass through the center and bisect the hole (FIG. 2BB'). The surface of the hole is masked leaving open a hole of