ROLLED ROUND STEEL MATERIAL FOR STEERING RACK BAR, AND STEERINGRACKBAR TECHNICALFIELD [0001] The present invention relates to a rolled round steel material for a steering rack bar, and a steering rack bar. BACKGROIINDART [0002] Among automobile components, a steering rack bar (also simply referred to as "rack bar" hereinafter) used in a steering device is an important component serving as a backbone that changes the traveling direction of a car and connects right and left wheels, and if this steering rack bar is damaged, a steering wheel cannot be operated. Thus, a steel material used for a rack bar needs to be very reliable. [0003] It should be noted that a rack bar has been conventionally manufactured by perfonning thermal ref,rning, namely hardening and tempering, performing drawing processing as required thereafter, and performing punching and gear cutting tbrough cutting processing on a round steel material obtained by rolling a medium-carbon steel material, and then performing induction hardening and tempering on the tooth-shaped portions. It should be noted that the rolled round steel material means a steel material whose cross-section is processed into a circular shape by rolling, and gear cutting means forming tooth-shaped portions. [0004] In addition, it is required that a rack bar that has undergone the induction hardening does not break due to a crack in an induction hardening layer developing in a base material, even when an excessive load is applied thrreto. [000s] Furthermore, deep hole drilling is performed on the rack bar in the lengthwise direction ofa central portion in the diameter direction. [0006] Therefore, a round steel rnaterial used as the raw material for a rack bar needs to have good machinability and excellent base material impact properties (base material toughness) that resist crack development. [0007] Inventors of the present invention proposed the following steel material, for example, as a steel material used in such a steering rack bar [0008] Patent Document 1 discloses a rolled steel material for induction hardening that contains, in masso/0, C in a¡r amount of 0.38 to 0.55%, Si in an amount of 1.0% or less, Mn in an amount of 0.20 to 2.0Vo, P tt an amount of 0.020% or less, S in an amount of 0.10% or less, Cr in an amount of 0.10 to 2.0%, Al in an amount of 0.10% or less, N in an amount of 0.004 to 0.039/0, the remaining portion being constituted by Fe and impurities, and has a chemical component in which the value of fn 1 expressed by the equation [fn 1=C+(1/10)Si+(ll5)Mn+(5/22)Cr+1.65V-(5/7)S] (where C, Si, Mr, Cr, Y and S in the equation respectively express the âmounts of the elements in mass%) is 1.20 or less, the microsh'ucture þs'ing canstituted by ferrite, lamellar pearlite, and spheroidal cementite, the average crystalline particle diameter ofthe ferrite being 10 pm or less, the a¡ea percentage of lamellar pearlite with a lamella¡ interval of 200 nm or less with respect to the midostructure of the lamellar peariite being in a range of 20 to 50 %, and the number ofpieces of spheroidal cementite is 4x10s pieces/mm2 ormore. [000e] It should be noted that this rolled steel material for induction hardening may furthe¡ contain at least one selected from Cu, Ni, Mo, Ti, Nb, and V. [0010] Pateflt Document 2 discloses a rolled steel material for induction hardening that contains, in masso/0, C in an amount of 0.38 to 0.55%, Si in an amount of 1.00lo or less, I .] l l. Mn in an amount of 0.20 fo 2.0yo, P n an amount of 0.020olo or less, S in an amount of 0.10% or less, Cr in an amount of 0.10 to2.0%,A1 in an amount of 0.010 to 0.10%, N in an amount of 0.004 to 0.03o/o, the remaining portion being constituted by Fe and impurities, a¡d has a chemical component in which the value of Ceq expressed by the equation [Ceq=C+(1/10)Si+(ll5)Mn+(5/22)Cr+1.65V-(5/7)S] (where C, Si, Mn, Cr, V, and S in the equation respectively express the amounts of the elements in mass%) is 1.20 or 1ess, the total content of Si, Mn, and Cr satis$ring L2 to 3.50/o, the microstructure being constituted by ferrite, lamellar pearlite, and spheroidal cementite, and the average crystalline particle diameter of the ferrite being 10 ¡rm or less, the area percentage of lamellar pearlite with respect to the microstructure being in a range of 200lo o¡ less (including 0%), and the number of pieces of spheroidal camentite being 6x105 pieces/mm2 or more. [0011] It should be noted that this rolled steel material for induction hardening may fuithe¡ contain at least one selected from Cu, Ni, Mo, Ti, Nb, and V LIST OF PRIORART DOCUMENTS PATENTDOCUMENT [00i2] Patent Document I fP 2t1I-214130A Patent Document 2: IP 20lI-241466A DISCLOSURE OF THE IN.\,'ENTION PROBLEMS TO BE SOLVED BYTFIE INVENTION [0013] There has been increasing demand for a rolled round steel material for a steering rack bar, ard a stee¡ing rack bar having a higher base material tougbness for preventing damage and higher machinability for forming a deep hole compared to the rolled steel material proposed in Patent Documents 1 and 2. [0014] Al object ofthe present invention is to provide a rolled round steel material that can be favorably used as a raw material for a rack bar that undergoes induction hardening and a rack bar made of the same. In parficular, an object of the present invention is to provide a rolled round steel material havìng an excellent base material toughness and machinability without adding high-cost elements or performing thermal refining, and a ¡ack bar made of the same. Fufihemore, an obj ect of the present invention is to provide a rolled round steel material in which a deep hole can be easily formed in a lengthwise direction of the central portion in the diarneter direction and a rack bar that prevents a crack that has appeared fiom developing. [001s] It should be noted that a high base material toughness, which is an object of the present invention, means that in the state of a rolled steel material, the impact value obtained in a Charpy impact test at a test temperature of 25oC wíth a standard test piece (referred to as "V notch Charpy impact test piece" hereinafter) defined in JIS Z 2242 (2005), which has a width of 10 mm and a V notch with a notch angle of45 degrees, a notch depth of 2 mm, and a notch bottom radius of 0.25 mm, is 160 J/cm2 or more. If the impact value obtained with the above-described test piece at a test temperatüe of 25"C is 160 J/cm2 or more, it is possible to secure a much higher level of safery at the time of traveling in an environment in which there is a high possibiiity of a steering rack bar being damaged, that is, rough road traveling, for example. MEANS FOR SOLVING THE PROBLEMS [0016] In order to resolve the above-described issues, the inventors performed various laboratorial experiments regarding a means for obtaining a high base material toughness and securing good machinability of the central portion without performing thermal refining on medium-carbon steel. [0017] Specifically, first, the inventors studied a means for improving the base material toughness, referenced on a microstructure constituted by ferrite and lamellar pearlite. i: lì ].. l l --[ I l l l As a result, the following findings were obtained. [0018] (A) If ferrite is formed to have a fine structue and is extended in a direction parallel to the rolling di¡ection, and cementite in lamellar pearlite is formed into spheroidal cementite so that its microsù'rÌcture contains lamellar pearlite in an amount of less than a specific percentage and spheroidal cementite in a specific amount or more, a resistarce to cracks developing in the cross-section perpendicular to the rolling direction is increased, resulting in an increase in the base material toughness. [001e] Next, the ínventors studied the influence of the structüe on machinability when a deep hole is forrned, referenced on the microstmcture constituted by ferrite and lamellar pearlite. As a result, the following findings were obtained. [0020] (B) If the microstructure contains an excessive amount of spheroidal cementite, cutting resistance increases due to deterioration of chippability and machinability deteriorates. Onthe other hand, in the case where the structue contains lamellar pearlite in a specific percentage or more and spheroidal cernentite i¡ less than a specifrc amount, cutling resistance decreases due to an increase in chippability, and therefore the rolled round steel material has excellent machinability. [0021] In view of this, in order to fui1her improve both base material toughness and machinability, the inventors further studied the influence of element components. As a resul! the following findings were obtained. 100221 (C) S binds to Mn to form MrS, resulting extension in the longitudinal direction of the steel material (direction parallel to the rolling direction) and an improvement in tougbness. Moreover, if the steel material contains a specihc amount of S, cutting resisLance decreases due to improvement in chippability, and thus the machinability becorne favorable. t00231 In view of this, based on the find.ings of (A) to (C) described above, more detailed studies were performed. As a result, the following important findings were obtained. 100241 (D) A portion that requires the base material toughness for preventing damage as a rolled round steel material for a steering rack bar is a region extending from the surface of the round steel material to a position located a distance of 1/2 the radius therefrom. Therefore, in the case ofarolledround steel material having a microstructure constihrted by ferrite, lameliar pearlite, and cementite, if the microstructure in the abovedescribed region is constituted by fine ferrite that is extended in the direction parallel to the rolling direction, a specifrc percentage o¡ less of lamellar pearlite, and a specific amount or more of spheroidal cementite, a base material toughness for preventing damage can be obtained. [002s] (E) On the other hand, if the microstructure contains lamellar pearlite in an amount of a specific percentage or more and spheroidal cementite in less than a specific amount in the central porLion of the rolled round steei material constituted by ferrite, lamellar pearlite, and cementite, excellent machinability can be obtained. 100261 Furthermore, based on the findings of (A) to (E) above, in order to achieve a firrther improvement in toughness, specifically, in order that in the state of the rolled steel material, the impact value that is obiained in the Charpy impact test with a V notch Charpy impact test piece at a test temperature of25oC reached to 160 J/cm2 o¡ more, the inventors studied the influence of element components. As a result, the foilowing findings were obtained. 10027) (F) B has properties of suppressing release of stain at a high temperature and suppressing segregatioa of P and S in austenite grain boundaries at the time of induction hardening by strengthening grain boundaries. As a result, the toughness further increases. î and bar, I I l i I I I l I ! I I I í [0028] The present invention was made based on the above-described findings, comprises a rolled round steel material for a steering rack bar, and a steering rack which will be described later. [002e] (1) A rolled round steel material for a steering rack bar, comprising: in masso/0, C in an amount of 0.38 to 0.55%; Si in an amount of 1.0o% or less; Mn in an amount of 0.20 to 2.0o/o; S inanamount of 0.005 to 0.10%; Crin an amount of 0.0i fo 2.0Vo;.\l in an amount of 0.003 to 0.10%; B in an amount of 0.0005 to 0.0030%; Ti in an amount of 0.047% or less; Cu in an amount of 0 to 1.0%; Ni in an amount of 0 to 3.0%; Mo in an amount of 0 to 0.50%; Nb in an amount of 0 to 0.10%; V in an amount of 0 to 0.30%; Ca in an amount of 0 to 0.005%; and Pb in an amount of 0 to 0.30%, a remaining portion being constituted by Fe and impurities, the impurities containing P in an amount of 0.0300/o or less and N in an amount of0.008% or less, a¡d the rolled round steel material having a chemicai composition satisfring Equation (1) below, 3.4N=Ti<3.4N+0.02' . . (1) where a slmbol of an element indicates a content of the element in masso/o, in which a microstructure is constituted by ferrite, lamellar pearlite, and cementite, in a cross-section perpendicular to a rolling directior¡ in a region extending from a surface to a position located a distance of 1i2 the radius therefrom, an average particle diameter ofthe ferite is 10 pm or less, an area percentage ofthe lamellar pearlite is less than 20o/o, a ntmbu of pieces of spheroidal cementite of the cementite is 4x 105 pieces/mm2 or more, and, in a central portion, an area percentage of the lamellar pearlite is 200lo or more and a number of pieces of spheroidal cementite of the cementite is less than 4x10s pieces/mm2, and in a üoss-section that passes through a centerline of the round steel material and is parallel to the rolling direction, an average aspect ratio of the fe¡rite in a region extending. from a surface to a position located a distance of 1/2 the radius therefrom is 3 or more. [00301 (2) The rolled round steel material for a steering rack bar according to ( 1 ) above, comprising, in masso/o, at least one selected from Cu in an amount of 0.05 to 1.0%, Ni in an amount of 0.0 5 io 3.A%, and Mo in an amount of 0.05 to 0.50%. [0031] (3) The rolled round steel material for a steering rack bar according to (1) above, comprising, in masso/o, at least one selected from Nb in an amount of 0.010 to 0.10% and V in an amount of 0.01 to 0.30%. [0032] (4) The rolled round steel materiai for a steering rack bar according to (1) above, comprising, in masso/0, at least one selected from Ca in an amount of 0.0005 to 0.005% and Pb in an amount of 0.05 to 0.30%. [0033] (5) A steering rack ba¡ in which the rolled round steel material for a steering rack bar according to any of(1) to (4) above is used without thermal refrning. [0034] "Impurities" indicate substances coming from ore, which is the raw.material, scraps, the manufachrring environment, o¡ the 1ike, when a steel material is industrially manufactured. [00351 "Spheroidal cementite" indicates cementite having a ratio of a major axis L to a minor axis W (LÃV) that is 2.0 or less. [00361 "Central pofion" indicates a portion extending from the center to a position located a distance of 1/4 the radius therefrom. [0037] "Use without thermal refiníng" indicates use without performing uthermal refining", namely, hardening and tempering. ADVANTAGEOUS EFFECTS OF TI{E INVENTION [0038] It is not necessary for tlte rolled round steel material for a steering rack bar of the present invention to contain high-cost V, and even if thermal refining is not performed, the rolled round steei material has good machinability for fonning a deep hole in the central portion and a high base material toughness, or in other \Ã/o¡ds, the impact value obtained in the Charpy impact test with a V notch Charpy impact test piece at a test tempemture of 25'C in the state of the rolled round steel material is 160 J/cm2 or more, and therefore the rolled round steel material is favorably used as the raw mate¡ial fo¡ a steering rack bar. [003e] In additior¡ the steering rack bar of the present invention can be obtained by using the above-described rolled round steel material for a steering ¡ack bar without thermal refining. BRIEF DESCRIPTION OF TI{E DRAWINGS [0040] [Figure 1] FIG. 1 is a diagram schematically illustrating the direction of a notch of a V notch Charpy impact test piece collected from a steel bar in Working Example 1. [Figure 2] FIG. 2 is a diagram illustrating the shape of a test piece that is collected f¡om a steel bar that has undergone drawing processing and imitates a steering rack bar used in a tlree-point bending test in Working Exampie 2. In FIG. 2, (a) is a front view (overall view), (b) is a side view, and (c) is an enlarged view of cross-section A-A of a toothshaped portion. It should be noted that the unit ofthe dimension in FIGS. 2(a) to 2(c) is rrrûn . [Figure 3] FIG. 3. is a diagram schematically illustrating a method of the three-point bending test that was perforrned in Working Example 2. MODE FOR CARRYING OUT THE I\\TENTION [0041] Hereinafter, essential elements ofthe present invention will be described in detail. It should be noted that "Yo" of the content of each element means "masso/0" in the following description. 10 100421 1 Chemical cornposition: C: 0.38 to 0.55% C has properties of improving the strength and induction hardenability of steel and the strength of a quench-hardened layer formed by inductìon hardening. However, if the content ofC is less than 0.38%, desired effects resulting from the properties cannot be obtained. On the other hand, if the content of C exceeds 0.55o/o, lhe base material toughness decreases. Therefore, the content ofC is set to 0.38 to 0.55%. It should be noted that in order to stably obtain the above-described effects, the content of C is preferably set to 0.40% or more. Also, the content of C is preferably set to 0.510/0 ot less. [0043] Si: 1.0% or less Si is a deoxidizing elemen! and is an element for improving the strength of ferrite by solid-solution strengthening. However, ifthe content ofSi exceeds 1.0%o, the machinability decreases and it becomes diffrcult to form a deep hole. Therefore, the content of Si is set to 1.00lo or less. The content ofSi is preferably set to 0.80/0 or less. [0044] It should be noted that Al, which will be described later, also has deoxidizing properties, and therefore it is not necessary to define the lower limit of the content of Si, in particular However, in order to reliably secure the strength with the above-described solid-solution strengthening properties of Si, the content of Si is preferably set to 0.03% or more and more preferably set to 0.10o% or more. [004s] Mt: O.2O fo 2.0o/o Mn binds to S to form MnS, and has properties ofreducing cutting resistance by increasing machinability, especially chippability required when a deep hole is formed, and has effects of increasing toughness due to extended MnS suppressing crack development. In addition, Mn is an element that is effective in improvement in induction hardenability and improves the strength of ferrite by solid-solution 11 strengthening. However, if the content of Mn is less than 0.20%, desired effects resultìng from the above-described properties cannot be obtained. On the other hand, if the content of Mn exceeds 2.00/0, the machinability decreases and it becomes diffrcult to form a deep hole. Therefore, the content of Mn is set to 0.20 to 2.0o/o. It should be noted that in order to stably obtain the above-described effects with a lower cost for alloy, the content of Mn is preferably set to 0.400% or more and 1.500% or less. [0046] S: 0.005 to 0.10% S is an important element in the present invention. S binds to Ml to form MnS, and has properties of reducing cutting resistance by increasing machinability, especially chippability required when a deep hole is formed, and has effects of increasing toughness due to extended MnS suppressing crack development. However, if the content of S is less than 0.005%, such effects cannot be obtained. On the other hand, ifthe content of S increases and MnS is formed excessively, the toughness conversely decreases. Therefore, the content of S is set to 0.005 to 0.10%. It should be noted that the content of S is preferably set to 0.010% or more and more preferably set to 0.015% or more. Also, the content of S is preferably set to û.080/o or less. 100471 Cr: 0.01 Io 2.0o/o Cr is an element that is effective in improvement in induction hardenability and improves the strength of ferrite by solid-solution strengthening, and therefore it is necessary for the content of Cr to be O.jlYo or more. However, if the content of Cr exceeds 2.0'r/o, the machinability decreases and it becomes diffrcult to form a deep hole. Therefore, the content of Cr is set to A.OI fo 2.0o/o. It should be noted that the content of Cr is preferably set to 0.05% or more and more preferably set to 0.10% or more. In addition, the content ofCr is preferably set to 1.80lo or less. [0048] Al: 0.003 to 0.10% Al has deoxidizing properties. However, if the content of Al is less than 0.003%, desired effects resulting frorr the above-described properties cannot be obtained. 12 1. i, l On the other hand, if the content of Al exceeds 0.10%, induction hardenability significantly decreases, which incurs deterioration of the base material toughness as well. Therefore, the content ofAl is set to 0.003 to 0.10%. It should be noted that the content ofAl is preferably set to 0.08% or less. on the other hand, in order to stably obtain the deoxidizing effects of Al, the content ofAl is preferably set to 0.0050/o or more and more preferably set to 0.100/o or more. [004e] B: 0.0005 to 0.0030% B suppresses the release of strain at a high temperature by strengthening grain boundaries and has properties of improving induction hardenability and propefies of suppressing the segregation of P a¡d S in austenite grain boundaries at the time of induction hardening, and as a result, the toughness significantly increases. The abovedesc¡ ibed effects are significant when the content of B is 0.0005% or more. However, if the content ofB exceeds 0.0030%, the above-described effects are not further irnproved and the cost increases. Therefore, the content ofB is set to 0.0005 to 0.0030%. The content ofB is preferabiy set to 0.0010% or more and 0.0020o/o o¡ less. [0050] Ti: 0.047% or less Ti preferentíally binds to \ which is the element of impurities in the steel, and fixes N so that the fomation of BN is suppressed and B is present as solid-solution B. Therefore, Ti is an element that is effective in securing the above-described effects ofB strengthening grain boundaries, improving ìnduction hardenability, and suppressing the segregation of P and S in austenite grain boundaries at the time of induction hardening. However, ifthe content ofri exceeds 0.047%o, the base material toughness significantly dec¡eases. Therefore, the content ofTi is set to 0.0470lo or less. [00s1] Cu: 0 to 1.0% Cu has properties of improving induction hardenability and increasing base material toughness, and therefore cu may be included in order to í'prove the base material toughness. However, if the content of cu exceeds 1.0%o, the machinability 13 decreases and it becomes difficult to fonn a deep hole. Therefore, the content ofcu in the case ofbeing included is set to 1.00% or less. It should be noted that the content of Cu is preferably set to 0.800% or less. [00s2] On the other hand, in order to stably obtain the above-described base material toughness improving effects ofcu, the content ofcu is preferably set to 0.05% or more and more preferably set to 0.10% or more. [00s3] Ni: 0 to 3.0% Ni has properties of improving induction hardenabilitl' and increasing base material toughness, and therefore Ni may be inctuded in order to improve the base material toughness. However, if the content of Ni exceeds 3.0y0, fhe machinability decreases and it becomes diffrcuÌt to form a deep hole. Therefore, the content ofNi in the case of being included is set to 3.0o% or less. It should be noted that the content of Ni is preferably set to 2.0o% or less. [0054] On the other hand, in order to stably obtain the above-described base material toughness improving effects of Ni, the content ofNi is preferably set to 0.050/o or more and more preferabiy set to 0.10% or more. [005s] Mo: 0 to 0.50% Mo has properties of improving induction hardenability and increasing base material toughless, and therefore Mo may be included in order to improve the base materìal toughness. Howeve¡ if the content of Mo exceeds 0.50%, the machinability decreases and it becomes difficult to form a deep hole. Therefore, the content of Mo in the case ofbeing included is set to 0.50% or less. It should be noted that the content of Mo is preferably set to 0.4002 or less. [00s6] On the other hand, in order to stably obtain the above-described base material toughness improving effects of Mo, the content of Mo is preferably set to 0.05% or more and more preferâbly set to 0.10olo or more. [00s7] It should be noted that the rolled round steel material may contain one or more ofthe above-described Cu, Ni, and Mo. It shouldbe noted that although the totâl amount of tlrese elements may be 4-50Yo, the total amount thereof is preferably set to 3.20Vo ot less. [00s8] Nb: 0 to 0.10% Nb binds to C or N in the steel to form carbide or carbonitride, and has properties of refining crystal grains. Also, Nb has properties of improving the strength of steel. Howeve¡ if the content ofNb exceeds 0.10olo, a decrease in the toughness is incurred in addition to the effects being not further improved and the cost increasing. Therefore, the content ofNb in the case ofbeing included is set to 0.10% o¡ less. It should be noted that the content ofNb is preferably set to 0.080/0 or less. [00se] On the other hand, in order to stably obtain the crystal grain refining effects of Nb, the content of Nb is preferably set to 0.010% or more and more preferably set to 0.0150/o or more. [0060] V: 0 to 0.30% V binds to C or N in the steel to form carbide or carbonitride, and has properties of refining crystal grains. Also, V has properties of improving the strength of stee1. However, if the content of V exceeds 0.30%o, a decrease in the toughness is incurred in addition to the effects being not further improved and the cost increasing. Therefore, the content ofV in the case ofbeing included is set to 0.300/o or less. It should be noted that the content ofV is preferably set to 0.250ó or less. [0061] On the other hand in order to stably obtain the crystal grain refining effects of V, the content ofV is preferably set to 0.01% or more ard more preferably set to 0.02% or more. 15 i I L a' 100621 It should be noted that the rolled round steel material may contain one or both of Nb and V described above. It should be noted that although the total amount of these elements may be 0.4\rYo,the total amount thereof is preferably set to 0.33olo or less. [0063] Ca: 0 to 0.005% Ca has propefiies of improving the machinability of steel. Therefore, Ca may be included as required. However, ifthe content ofCa exceeds 0.005o/o, hot workability decreases, resulting in a decrease in ma¡ufacturability. Therefore, the content of Ca in the case of being included is set to 0.005% or less. The content ofCa is preferably set to 0.0035%n or less. [0064] On the other hand, in order to stably obtain the above-described machinability improving effects ofCa, the content of Ca is desirably set to 0.00050/o or more. [0065] Pb: 0 to 0.30% Similarly to Ca, Pb has properties of improving the machinability of steel. Therefore, Pb may be included as required. However, if the content of Pb exceeds O.30%o, the above-described machinability improving effects are not further improved, hot workability decreases excessively, and it becomes diffrcult to perform manufacturing. Therefore, the content ofPb in the case ofbeing included is set to 0.300lo or less. [0066] On the other hand, in order to stably obtain the above-descnbed machinability improving effects ofPb, the content ofPb is desirably 0.050/o or more. [0067] It should be noted that the rolled round steel material may contain one or both of Ca and Pb described above. The total content of these elements is preferably 0.30% or 1ess. [0068] The chemical composition of the rolled ¡ound steel material for a steering rack 16 bar of the present invention is such that the remaining portion ís constituted by Fe and impurities, the impurities contain P in an amount of 0.030% or less and N in an amount of 0.008% or less, and the following expression is satisfied. 3.4NtTi<3.4N+0.02 (1) [006e] P: 0.030% or less P is included in the steel as the impurìties, causes grain boundary segregation and center segregation, and incurs a decrease in the base material toughness, and in particular, if the content ofP exceeds 0.0300/0, the base material toughness significantþ decreases. Therefore, the content ofP is set to 0.030% or less. It should be noted that the content ofP is preferably set to 0.020% or less. [0070] N: 0.008% or less N is also included in the steel as the impurities. N has a high affrnity for B and in the case of binding to B in the steel to form B\ effects of strengthening grain boundaries, effects of improving induction hardenability, and effects of suppressing segregation ofP and S in the austenite grain boundaries at the time of induction hardening resulting from the steel containing B cannot be achieved. In particular, if the content of N exceeds 0.008% due to an increase in the content of \ the above-described effects resulting from the steel containing B cannot be obtained. Therefore, the content ofN is set to 0.0080/o o¡ less. [00711 3.4NlTis3.4N+0.02 A rolled round steel material for a steering rack bar according to the present invention is to have a chemicai composition satisffing the following expression. 3.4N