JP2005054216A - Steel material superior in machinability and fatigue characteristics, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably provide a steel material having both of high torsion-fatigue strength and adequate machinability. <P>SOLUTION: This steel material comprises, by mass%, 0.35-0.7% C, 0.30-1.1% Si, 0.2-2.0% Mn, 0.005-0.25% Al, 0.005-0.1% Ti, 0.05-0.6% Mo, 0.0003-0.006% B, 0.06% or less S, 0.02% or less P, 0.2% or less Cr, one or more elements selected among 0.2% or less Te, 0.2% or less Se, 0.3% or less Sn, 0.2% or less Zr, 0.02% or less Ca, 0.03% or less REM, 0.6% or less Pb, 0.3% or less Bi, 0.2% or less Sb, 0.1% or less W and 0.04% or less N, and the balance Fe with unavoidable impurities; has a base metal structure containing at least a bainitic structure and a martensitic structure in an amount of 10 vol.% or more in total; and contains former austenite grains with sizes of 12 μm or less through the whole thickness direction of an induction-hardened layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００６０】
【表２】

【００６１】
表２から明らかなように、本発明で規定した成分組成範囲を満足し、かつ本発明の高周波焼入れ条件を満たす条件で製造した鋼材はいずれも、硬化層の旧オーステナイト粒径が全厚にわたって１２μｍ 以下を満たしており、その結果 ７００ ＭＰａ以上の高いねじり疲労強度を得ることができた。また、被削性にも優れていることが分かる。
なお、Ｎｏ．１２ は、母材組織がマルテンサイトを多く含む組織であるため、工具寿命が２６９０ｓと短めであるが、ＣａおよびＨｆを含有しているため、同様に母材組織がマルテンサイトを多く含む組織である比較例Ｎｏ．１１ に比べると、工具寿命は大幅に向上していることが分かる。
また、Ｎｏ．１５， Ｎｏ．４３， Ｎｏ．４４ を比較すると、焼入れ回数を１回から２回に増やした場合において、２回目の焼入れ深さの方が浅い場合（Ｎｏ．４３）には、１回しか施さなかった場合よりもねじり疲労強度はむしろ低下するのに対し、２回目の焼入れ深さを深くした場合（Ｎｏ．４４）には、１回しか施さなかった場合に比べてねじり疲労強度は向上した。なお、Ｎｏ．４４ では、硬化層厚方向で、表面から硬化層厚の ４／５位置で最も旧オーステナイト粒径が大きく、３．５ μｍ であったが、表層近傍（表面から硬化層厚の １／５位置）では旧オーステナイト粒径は ２．６μｍ であり、表層の粒径が微細化していることが、疲労強度の向上に寄与したものと考えられる。
【００６２】
これに対し、Ｎｏ．１， Ｎｏ．２は、硬化層の旧オーステナイト粒径は微細で、高いねじり疲労強度を有するものの、被削性向上元素を含有していないため、その他成分および母材組織がほぼ同一の発明例であるＮｏ．３〜Ｎｏ．７に比較して工具寿命が短い。
また、Ｎｏ．８， Ｎｏ．９は、硬化層の旧オーステナイト粒径は微細で、高いねじり疲労強度を有するものの、やはり被削性向上元素を含有していないため、その他成分および母材組織がほぼ同一の発明例である Ｎｏ．１３〜Ｎｏ．１５ に比較して工具寿命が短い。
Ｎｏ．１１ は、硬化層の旧オーステナイト粒径は微細で、高いねじり疲労強度を有するものの、やはり被削性向上元素を含有していないため、その他成分および母材組織がほぼ同一の発明例である Ｎｏ．１２に比較して工具寿命が短い。
Ｎｏ．１７ は、加工後の冷却速度が小さいため、ベイナイトとマルテンサイトの合計組織分率が低く、結果として高周波焼入れ後の硬化層粒径が粗大となっており、ねじり疲労強度が低い。
Ｎｏ．３０ は、硬化層粒径は微細であるものの、Ｃ含有量が本発明の範囲より高いため、粒界強度の低下を招き、そのためねじり疲労強度が劣っている。また、被削性にも劣っていた。
Ｎｏ，３１， ３２， ３３ は、それぞれＣ， Ｓｉ， Ｍｏの含有量が本発明の適正範囲よりも低いため、硬化層粒径が粗大となり、ねじり疲労強度が劣っている。
Ｎｏ．３４ はＢ含有量が低く、またＮｏ．３５ はＭｎ含有量が、Ｎｏ．３６ はＳおよびＰ含有量が、Ｎｏ．３７ はＣｒ含有量が、それぞれ本発明の適正範囲を超えているため、いずれも粒界強度の低下を招き、ねじり疲労強度が劣っている。
Ｎｏ．３８ は、Ｔｉ含有量が本発明の適正範囲を超えているため、ねじり疲労強度が劣っており、逆にＮｏ．４１ はＴｉ含有量が低いため、硬化層粒径が粗大となり、ねじり疲労強度が劣っている。
Ｎｏ．３９ は、高周波焼入れ時の加熱温度が高すぎるため硬化層の粒径が粗大となり、一方Ｎｏ．４０ は、高周波焼入れ時の加熱温度が低すぎるため硬化層が形成されず、いずれもねじり疲労強度に劣っている。
Ｎｏ．４２ は、Ｓｉ量が本発明の下限に満たない０．２８ｍａｓｓ％の場合であるが、この例のように、Ｓｉ量が本発明の下限をわずかでも下回る場合には、硬化層全厚にわたって１２μｍ 以下の粒径を得ることができず、その結果、ねじり疲労強度に劣っている。
【００６３】
【発明の効果】
かくして、本発明によれば、高いねじり疲労強度を有する鋼材を安定して得ることができ、その結果、自動車用部材の軽量化の要求に対し偉功を奏する。[0001]
BACKGROUND OF THE INVENTION
The present invention has a hardened layer formed by induction hardening on the surface, is suitable for application to automobile drive shafts, constant velocity joints, and the like, and is a steel material as a material excellent in machinability and post-quenching fatigue properties and actually hardened. The present invention relates to a steel material having excellent fatigue characteristics and a method for producing the same.
[0002]
[Prior art]
Conventionally, mechanical structural members such as automobile drive shafts and constant velocity joints are processed by hot forging, hot cutting, further cutting, cold forging, etc. into a predetermined shape and induction hardening. By tempering, it is common to ensure torsional fatigue strength, which is an important characteristic as a machine structural member.
On the other hand, in recent years, there is a strong demand for weight reduction of automobile members due to environmental problems, and further improvement in torsional fatigue strength of automobile members is required from this viewpoint.
[0003]
In order to improve the torsional fatigue strength, for example, it is conceivable to increase the quenching depth by induction quenching. However, even if the quenching depth is increased, the fatigue strength is saturated at a certain depth.
Further, improvement of the grain boundary strength is also effective for improving the torsional fatigue strength. From this viewpoint, a technique for refining the prior austenite grain size by dispersing TiC has been proposed (for example, see Patent Document 1). )
[0004]
In the technique described in Patent Document 1 described above, since fine TiC is dispersed in a large amount during induction hardening heating, the prior austenite grain size is refined, so that TiC is solutionized before quenching. It is necessary to adopt a process of heating to 1100 ° C. or higher in the hot rolling process. Therefore, there is a problem that it is necessary to increase the heating temperature during hot rolling, resulting in poor productivity.
Further, even with the technique disclosed in the above-mentioned Patent Document 1, there is still a problem in that it cannot sufficiently meet the recent demand for torsional fatigue strength.
[0005]
Furthermore, in Patent Document 2, the ratio (CD / R) of the hardened layer depth CD and the radius R of the induction-hardened shaft component is limited to 0.3 to 0.7, and then this CD / R and induction hardening are performed. It is defined by the austenite grain size γf from the subsequent surface to 1 mm, the average Vickers hardness Hf up to (CD / R) = 0.1 as it is induction-hardened, and the average Vickers hardness Hc at the axial center after induction hardening. There has been proposed a shaft component for a mechanical structure in which the torsional fatigue strength is improved by controlling the value A within a predetermined range in accordance with the C amount.
However, in this component, since the prior austenite grain size over the entire thickness of the hardened hardened layer is not taken into consideration, it is still impossible to sufficiently meet the recent demand for torsional fatigue strength.
[0006]
By the way, as a solution to the above-mentioned problems, the inventors previously optimized the chemical composition, structure, quenching conditions of steel, and the prior austenite grain size over the entire thickness of the hardened layer after quenching. A steel material capable of obtaining fatigue strength was developed and proposed as Patent Document 3.
This patent document 3 was developed based on the novel knowledge as described below.
[0007]
(1) The torsional fatigue strength is significantly improved by quenching the steel adjusted to an appropriate chemical composition and setting the prior austenite grain size over the entire thickness of the quenched and hardened layer to 12 μm or less. Specifically, with regard to chemical composition, especially by adding Si and Mo within an appropriate range, the number of austenite nucleation sites during induction hardening increases, and the growth of austenite grains is suppressed. The grain size of the quenched and hardened layer is effectively refined, and as a result, the torsional fatigue strength is significantly improved. In particular, by adding 0.30 mass% or more of Si, a hardened layer having a particle size of 12 μm or less can be obtained over the entire thickness of the hardened layer after induction hardening.
[0008]
(2) When the structure of the base material, that is, the structure before quenching, is a structure in which the bainite structure and / or the martensite structure is contained in a specific fraction, the bainite structure or the martensite structure is compared with the ferrite-pearlite structure. Since the carbide is a finely dispersed structure, the area of the ferrite / carbide interface, which is the nucleation site of austenite, increases during quenching heating, and the generated austenite becomes finer. As a result, the grain size of the quenched and hardened layer becomes fine, which improves the grain boundary strength and increases the torsional fatigue strength.
[0009]
(3) As described above, by using a steel material having a chemical composition and a structure adjusted, and appropriately controlling induction hardening conditions (heating temperature, time, number of times of quenching), the hardened layer particle size is remarkably refined, Grain boundary strength is improved. Specifically, by setting the heating temperature to 800 to 1000 ° C. and the heating time to 5 seconds or less, fine particles having a particle size of 12 μm or less can be stably obtained over the entire thickness of the cured layer. Furthermore, by repeating the quenching process under the above conditions twice or more, a finer hardened layer particle size can be obtained as compared with one quenching.
[0010]
[Patent Document 1]
JP 2000-154819 A (Claims, paragraph [0008])
[Patent Document 2]
JP-A-8-53714 (Claims)
[Patent Document 3]
Japanese Patent Application No. 2003-9349 (Claims)
[0011]
[Problems to be solved by the invention]
The present invention is a further improvement of the technique disclosed in Patent Document 3, and aims to propose a steel material that has excellent machinability in addition to excellent torsional fatigue strength, together with its advantageous manufacturing method. .
[0012]
[Means for Solving the Problems]
That is, the gist configuration of the present invention is as follows.
1. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less
And including
Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
Mg: 0.02 mass% or less and
Hf: 0.1 mass% or less
1 or 2 or more selected from among them, the balance is composed of Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and these bainite structure and martensite A steel material excellent in machinability and fatigue properties after induction hardening, characterized in that the total structure fraction of the structure is 10% or more.
[0013]
2. Said 1 WHEREIN: The said steel materials are further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: 0.1 mass% or less and
V: 0.5 mass% or less
A steel material excellent in machinability and fatigue properties after induction hardening, characterized by having a composition containing one or more selected from among them.
[0014]
3. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less
And including
Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
Mg: 0.02 mass% or less and
Hf: 0.1 mass% or less
1 or 2 or more selected from among them, the balance is composed of Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and these bainite structure and martensite structure A steel material having excellent fatigue characteristics, wherein the total structure fraction of the site structure is 10% or more, and the prior austenite grain size of the hardened layer after induction hardening is 12 μm or less over the entire thickness of the hardened layer.
[0015]
4). In the above 3, the steel material is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: 0.1 mass% or less and
V: 0.5 mass% or less
A steel material excellent in fatigue characteristics, characterized by having a composition containing one or more selected from among the above.
[0016]
5). 3. A steel material excellent in fatigue characteristics according to 3 or 4, wherein the thickness of the hardened layer after induction hardening is 2 mm or more.
[0017]
6). C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less
And including
Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
Mg: 0.02 mass% or less and
Hf: 0.1 mass% or less
A steel material containing one or more selected from among the above, with the balance being Fe and an inevitable impurity composition, is cooled at a rate of 0.2 ° C./s or more after hot working. A method for producing a steel material excellent in machinability and fatigue properties after induction hardening.
[0018]
7). In the above 6, the steel material is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: 0.1 mass% or less and
V: 0.5 mass% or less
A method for producing a steel material having excellent machinability and fatigue properties after induction hardening, wherein the composition contains one or more selected from among them.
[0019]
8). C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less
And including
Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
Mg: 0.02 mass% or less and
Hf: 0.1 mass% or less
A steel material containing one or more selected from the above, with the balance being Fe and inevitable impurities, is cooled at a rate of 0.2 ° C./s or higher after hot working, and then cut. A method for producing a steel material excellent in fatigue characteristics, characterized by performing induction hardening under conditions of a heating temperature of 800 to 1000 ° C.
[0020]
9. In the above 8, the steel material is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: 0.1 mass% or less and
V: 0.5 mass% or less
The manufacturing method of the steel material excellent in the fatigue characteristic characterized by becoming the composition containing 1 type, or 2 or more types selected from these.
[0021]
10. 8. The method for producing a steel material with excellent fatigue characteristics according to 8 or 9, wherein after the cutting, induction hardening is repeated a plurality of times, and a heating temperature at the final induction hardening is set to 800 to 1000 ° C.
[0022]
11. 10. The method for producing a steel material excellent in fatigue characteristics according to 10, wherein a heating temperature at the time of induction hardening is set to 800 to 1000 ° C. for all of the plurality of induction hardenings.
[0023]
12 Any one of said 8-11 WHEREIN: The heating time in the said heating temperature range shall be 5 second or less per induction hardening, The manufacturing method of the steel material excellent in the fatigue characteristic characterized by the above-mentioned.
[0024]
13. Any one of said 8-12, The thickness of the hardened layer surface of the steel materials by induction hardening is 2 mm or more, The manufacturing method of the steel materials excellent in the fatigue characteristics characterized by the above-mentioned.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
First, the reason why the steel material and the component composition of the steel material are limited to the above range in the present invention will be described.
C: 0.35-0.7 mass%
C is an element having the greatest influence on hardenability, and increases the hardness and depth of the hardened hardened layer and effectively contributes to the improvement of torsional strength. However, in order to ensure the torsional strength required when the content is less than 0.35 mass%, the quench hardening depth must be dramatically increased. Since it becomes difficult to generate a structure, 0.35 mass% or more is added. On the other hand, when the content exceeds 0.7 mass%, the grain boundary strength is lowered, and accordingly, the torsional fatigue strength is lowered, and the machinability, cold forgeability and fire cracking resistance are also lowered. For this reason, C was limited to the range of 0.35 to 0.7 mass%. Preferably it is the range of 0.4-0.6 mass%.
[0026]
Si: 0.30 to 1.1 mass%
Si has the effect of increasing the number of nucleation sites of austenite during quenching heating, suppressing grain growth of austenite, and reducing the grain size of the quenched hardened layer. Moreover, carbide | carbonized_material production | generation is suppressed and the fall of the grain boundary strength by carbide | carbonized_material is suppressed. Furthermore, it is an element useful for the generation of a bainite structure, and these improve the torsional fatigue strength.
Thus, Si is a very important element in the present invention, and it is essential to contain 0.30 mass% or more. This is because if the Si amount is less than 0.30 mass%, fine particles having a particle size of 12 μm or less cannot be obtained over the entire thickness of the cured layer, regardless of how the manufacturing conditions and quenching conditions are adjusted. . However, if the amount of Si exceeds 1.1 mass%, the hardness increases due to the solid solution hardening of the ferrite, leading to a decrease in machinability and cold forgeability. Therefore, Si was limited to the range of 0.30 to 1.1 mass%. Preferably it is the range of 0.40-1.0 mass%.
[0027]
Mn: 0.2 to 2.0 mass%
Mn is an essential component for improving the hardenability and ensuring the hardening depth during quenching, so it is actively added. However, if the content is less than 0.2 mass%, the effect of addition is poor. It was set to 0.2 mass% or more. Preferably it is 0.3 mass% or more. On the other hand, when the amount of Mn exceeds 2.0 mass%, the retained austenite after quenching increases, and on the contrary, the surface hardness decreases, and as a result, the torsional fatigue strength decreases. It should be noted that if Mn is contained in a large amount, the base material is hardened, which may be disadvantageous in machinability. Therefore, the content is preferably set to 1.2 mass% or less. More preferably, it is 1.0 mass% or less.
[0028]
Al: 0.005 to 0.25 mass%
Al is an element effective for deoxidation. Moreover, it is an element useful also in refine | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating. However, if the content is less than 0.005 mass%, the effect of addition is poor. On the other hand, if the content exceeds 0.25 mass%, the effect is saturated, and rather, a disadvantage that causes an increase in the component cost occurs. Was limited to the range of 0.005 to 0.25 mass%. Preferably it is the range of 0.05-0.10 mass%.
[0029]
Ti: 0.005 to 0.1 mass%
Ti combines with N mixed as an unavoidable impurity to prevent B from becoming BN and the effect of improving the hardenability of B to disappear, and has the effect of sufficiently exerting the effect of improving the hardenability of B. . In order to obtain this effect, it is necessary to contain at least 0.005 mass%. However, if it exceeds 0.1 mass%, a large amount of TiN is formed, which becomes the starting point of fatigue fracture. Since the torsional fatigue strength is significantly reduced, Ti is limited to the range of 0.005 to 0.1 mass%. Preferably it is the range of 0.01-0.07 mass%. Furthermore, it is preferable to satisfy Ti (mass%) / N (mass%) ≧ 3.42 from the viewpoint of securely fixing N and improving the hardenability with B to obtain a bainite and martensite structure. is there.
[0030]
Mo: 0.05-0.6 mass%
Mo promotes the formation of a bainite structure, thereby minimizing the austenite grain size during quenching and heating and reducing the grain size of the quenched hardened layer. Moreover, it has the effect | action which refines | miniaturizes the particle size of a hardening hardening layer by suppressing the grain growth of austenite at the time of quenching heating. Furthermore, since it is an element useful for improving hardenability, it is used for adjusting hardenability. In addition, Mo is an element that suppresses the formation of carbides and effectively prevents a decrease in grain boundary strength due to carbides.
Thus, Mo is a very important element in the present invention. If the content is less than 0.05 mass%, the particle size over the entire thickness of the hardened layer can be adjusted no matter how the manufacturing conditions and quenching conditions are adjusted. Cannot be made into fine grains having a size of 12 μm or less. However, if the content exceeds 0.6 mass%, the hardness of the rolled material is remarkably increased, resulting in a decrease in workability. Therefore, Mo is limited to the range of 0.05 to 0.6 mass%. Preferably it is the range of 0.1-0.6 mass%.
[0031]
B: 0.0003 to 0.006 mass%
B has an effect of promoting the formation of a bainite structure or a martensite structure. B also has the effect of improving the torsional strength by improving the hardenability by adding a small amount and increasing the quenching depth during quenching. Further, B preferentially segregates at the grain boundary, reduces the concentration of P segregating at the grain boundary, improves the grain boundary strength, and thus has an effect of improving torsional fatigue strength.
For this reason, in the present invention, B is positively added. However, when the content is less than 0.0003 mass%, the effect of addition is poor, while when the content exceeds 0.006 mass%, the effect is saturated, Rather, in order to increase the component cost, B is limited to the range of 0.0003 to 0.006 mass%. Preferably it is the range of 0.0005-0.004 mass%.
[0032]
S: 0.06 mass% or less
S is a useful element that improves the machinability by forming MnS in the steel, but if it exceeds 0.06 mass%, it segregates at the grain boundary and lowers the grain boundary strength. The amount was limited to 06 mass% or less. Preferably it is 0.04 mass% or less.
[0033]
P: 0.02 mass% or less
P segregates at the austenite grain boundaries and lowers the grain boundary strength, thereby lowering the torsional fatigue strength. In addition, there is a harmful effect that promotes burning cracks. Therefore, it is desirable to reduce the content of P as much as possible, but it is allowed up to 0.02 mass%.
[0034]
Cr: 0.2 mass% or less
Cr stabilizes the carbide and promotes the formation of residual carbide, lowers the grain boundary strength, and deteriorates the torsional fatigue strength. Therefore, it is desirable to reduce the Cr content as much as possible, but it is acceptable up to 0.2 mass%. Preferably it is 0.05 mass% or less.
[0035]
In the present invention, in addition, one or more selected from Te, Se, Sn, Zr, Ca, REM, Pb, Bi, Sb, W, N, Mg, and Hf are used as machinability improving components. Contain.
Te ≦ 0.2 mass%, Se ≦ 0.2 mass%
Te and Se each combine with Mn to form MnTe and MnSe, which improves the machinability by acting as a chip breaker. However, if the content exceeds 0.2 mass%, the effect is saturated and the component cost is increased. Therefore, both are contained at 0.2 mass% or less. In addition, a suitable range is 0.1 mass% or less. In order to improve machinability, it is preferable to contain 0.003 mass% or more in the case of Te and 0.003 mass% or more in the case of Se.
[0036]
Ca ≦ 0.02 mass%, REM ≦ 0.03 mass%, Zr ≦ 0.2 mass%
Ca, REM and Zr each form a sulfide together with MnS, which improves the machinability by acting as a chip breaker. However, even if Ca, REM, and Zr are contained in amounts exceeding 0.02 mass%, 0.03 mass%, and 0.2 mass%, respectively, the effect is saturated and the component cost is increased. It was supposed to be contained in a range. More preferably, Ca ≦ 0.01 mass%. REM ≦ 0.015 mass% and Zr ≦ 0.1 mass%. In order to improve machinability, Ca is 0.0001 mass% or more, REM
Is preferably 0.0001 mass% or more, and Zr is preferably contained in 0.003 mass% or more.
[0037]
Pb ≦ 0.6 mass%, Bi ≦ 0.3 mass%
Since both Pb and Bi improve machinability by melting, lubrication and embrittlement during cutting, they can be added for this purpose.
However, the addition of Pb: 0.6 mass% and Bi: 0.3 mass% not only saturates the effect, but also increases the component cost. More preferably, it is the range of Pb ≦ 0.3 mass% and Bi ≦ 0.2 mass%. In order to improve machinability, it is preferable to contain Pb in an amount of 0.01 mass% or more and Bi in an amount of 0.01 mass% or more.
[0038]
Sn ≦ 0.3 mass%, Sb ≦ 0.2 mass%, W ≦ 0.1 mass%
Sn, Sb and W are all elements that improve machinability by the embrittlement effect. However, even if Sn: 0.3 mass%, Sb: 0.2 mass%, and W: 0.1 mass% are added, the effect is saturated and the cost increases, which is economically disadvantageous. For this reason, they are included in the above ranges. More preferably, the ranges are Sn ≦ 0.15 mass%, Sb ≦ 0.1 mass%, and W ≦ 0.05 mass%. In order to improve the machinability, it is preferable to contain Sn 0.005 mass% or more, Sb 0.0005 mass% or more, and W 0.005 mass% or more.
[0039]
N ≦ 0.04mass%
Since N has an action of embrittlement of chips at the time of cutting, N has a function of improving chip disposal and tool life in addition to machinability. However, when added over 0.04 mass%, internal defects and surface flaws of the steel ingot are generated, so the content was set to 0.04 mass% or less. More preferably, it is 0.02 mass% or less. In order to improve machinability, N is preferably contained in an amount of 0.0060 mass% or more.
[0040]
Mg ≦ 0.02 mass%, Hf ≦ 0.1 mass%
Both Mg and Hf are not only deoxidizing elements, but also serve as a stress concentration source and have an effect of improving machinability, and can be added as necessary. However, if added excessively, the effect is saturated and the component cost is increased. More preferably, the ranges are Mg ≦ 0.01% and Hf ≦ 0.05%. In order to improve machinability, it is preferable to contain Mg in an amount of 0.0001 mass% or more and Hf in an amount of 0.005 mass% or more.
[0041]
The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Cu: 1.0 mass% or less
Cu is effective in improving hardenability, and is solid-solved in ferrite, and the torsional fatigue strength is improved by this solid solution strengthening. Further, by suppressing the formation of carbides, the decrease in grain boundary strength due to carbides is suppressed, and the torsional fatigue strength is improved. However, if the content exceeds 1.0 mass%, cracking occurs during hot working, so 1.0 mass% or less is added. In addition, Preferably it is 0.5 mass% or less.
[0042]
Ni: 3.5 mass% or less
Ni is an element that improves the hardenability, and is used when adjusting the hardenability. Moreover, it is an element which suppresses the production | generation of a carbide | carbonized_material and suppresses the fall of the grain boundary strength by a carbide | carbonized_material, and improves torsional fatigue strength. However, Ni is an extremely expensive element, and if added in excess of 3.5 mass%, the cost of the steel material increases, so the addition is made 3.5 mass% or less. In addition, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added at less than 0.05 mass%, it is desirable to add 0.05 mass% or more. Preferably it is 0.1-1.0 mass%.
[0043]
Co: 1.0 mass% or less
Co is an element that suppresses the formation of carbides, suppresses a decrease in grain boundary strength due to carbides, and improves torsional strength and torsional fatigue strength. However, Co is an extremely expensive element, and if added in excess of 1.0 mass%, the cost of the steel material increases, so the addition is made 1.0 mass% or less. In addition, since addition of less than 0.01 mass% has a small effect of suppressing the decrease in grain boundary strength, it is desirable to add 0.01 mass% or more. Preferably it is 0.02-0.5 mass%.
[0044]
Nb: 0.1 mass% or less
Nb not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering softening resistance, and torsional fatigue strength is improved by these effects. However, even if the content exceeds 0.1 mass%, the effect is saturated, so 0.1 mass% is the upper limit. It should be noted that addition of 0.005 mass% or more is desirable because addition of less than 0.005% has little effect of improving precipitation strengthening action and tempering softening resistance. Preferably it is 0.01-0.05 mass%.
[0045]
V: 0.5 mass% or less
V combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering and softening resistance. These effects improve torsional fatigue strength. However, since the effect is saturated even if it contains exceeding 0.5 mass%, it shall be 0.5 mass% or less. In addition, since the improvement effect of a torsional fatigue strength is small when added less than 0.01 mass%, it is desirable to add 0.01 mass% or more. Preferably it is 0.03-0.3 mass%.
[0046]
The preferred component composition range has been described above, but in the present invention, it is not sufficient to limit the component composition to the above range, and adjustment of the matrix structure is also important.
That is, in the present invention, the structure of the base material, that is, the structure before quenching (corresponding to the structure other than the hardened layer after induction hardening) has a bainite structure and / or a martensite structure, and these bainite structure and martensite structure. The total tissue fraction of the site organization needs to be 10% or more in volume fraction (vol%). The reason for this is that the bainite structure or martensite structure is a structure in which carbides are finely dispersed compared to the ferrite-pearlite structure, so the area of the ferrite / carbide interface, which is an austenite nucleation site, is increased during quenching heating. This is because the generated austenite is refined, and thus contributes effectively to refine the grain size of the quenched and hardened layer. And grain boundary intensity | strength rises and the torsional fatigue strength improves by refinement | miniaturization of the particle size of a hardening hardening layer.
Here, it is more preferable that the total structure fraction of the bainite structure and the martensite structure is 20 vol% or more.
[0047]
The remaining structure other than the bainite structure or martensite structure may be ferrite, pearlite, or the like, and is not particularly defined.
Also, regarding the refinement of the grain size of the hardened layer after quenching, the martensite structure has the same effect as the bainite structure, but from an industrial point of view, the bainite structure is better than the martensite structure. Since the addition amount of the alloy element is small and it can be produced at a low cooling rate, it is advantageous in production.
[0048]
In the present invention, it is also important to adjust the prior austenite particle size of the hardened layer after induction hardening. That is, regarding the hardened layer after induction hardening, the prior austenite grain size needs to be 12 μm or less over the entire thickness. This is because if the grain size over the entire thickness of the quenched and hardened layer exceeds 12 μm, sufficient grain boundary strength cannot be obtained, and satisfactory improvement in torsional fatigue strength cannot be expected. In addition, Preferably it is 10 micrometers or less, More preferably, it is 5 micrometers or less.
[0049]
Here, the measurement of the prior austenite grain size over the entire thickness of the hardened hardened layer is performed as follows.
In the steel material of the present invention after induction hardening, the outermost steel layer of the induction-hardened portion has a martensite structure of 100% in area ratio. And as it goes from the surface to the inside, the area of 100% martensite structure continues at a certain depth, but the area ratio of the martensite structure decreases rapidly from a certain depth.
In the present invention, the depth region from the steel material surface until the area ratio of the martensite structure is reduced to 98% is defined as the hardened layer in the induction-quenched portion.
And about this hardened layer, the average prior austenite particle size in each position of 1/5 position, 1/2 position, and 4/5 position of hardened layer thickness is measured from the surface, and all the average prior austenite particle diameters are 12 micrometers or less. In this case, it is assumed that the prior austenite grain size over the entire thickness of the quenched and hardened layer is 12 μm or less.
The average prior austenite particle size was measured from 400 times (1 field area: 0.25 mm × 0.225 mm) to 1000 times (1 field area: 0.10 mm × 0.09 mm) using an optical microscope. 5 views are observed at each position, and the average particle diameter is measured by an image analyzer.
[0050]
Furthermore, in the present invention, the thickness of the hardened layer by induction hardening is preferably 2 mm or more. This is because when the desired properties depend only on the structure near the extreme surface layer such as the rolling fatigue life, a moderate effect can be obtained even if the thickness of the hardened layer is about 1 mm. This is because when the fatigue strength is a problem, the hardened layer thickness is preferably as thick as possible. Therefore, a more preferable hardened layer thickness is 2.5 mm or more, and more preferably 3 mm or more.
[0051]
Next, the manufacturing conditions of the present invention will be described.
The steel material adjusted to a predetermined component composition is subjected to steel bar rolling or hot forging, and then subjected to cold rolling or cold forging as necessary, followed by cutting and induction hardening to obtain a product.
In the present invention, the base material structure has the above-described bainite structure and / or martensite structure, and the total structure fraction of these bainite structure and martensite structure is 10 vol% or more. The material steel before quenching needs to be cooled at a rate of 0.2 ° C./s or higher after being processed into a predetermined shape by hot working such as rolling and forging. This is because when the cooling rate is less than 0.2 ° C./s, it becomes difficult to obtain a bainite or martensite structure, and the total structure fraction of these structures may not reach 10 vol%. is there. The preferable range of the cooling rate after hot working is 0.3 to 30 ° C./s.
In addition, it is preferable to perform a hot working in the temperature range over 900 degreeC-1150 degreeC. If the temperature is 900 ° C. or lower, the necessary bainite structure and / or martensite structure cannot be obtained. On the other hand, if it exceeds 1150 ° C., the heating cost increases, which is economically disadvantageous.
[0052]
In addition, the cutting process in this invention is not restrict | limited in particular, All processes, such as a conventionally well-known outer periphery turning, drill cutting, and milling, can be applied.
[0053]
Next, in the present invention, induction hardening is performed in order to obtain the above-described cured layer, and the heating temperature range during this induction hardening needs to be 800 to 1000 ° C. This is because when the heating temperature is less than 800 ° C., the austenite structure is not sufficiently generated, and as a result of insufficient generation of the hardened layer structure described above, sufficient torsional fatigue strength cannot be ensured. When the heating temperature exceeds 1000 ° C., the growth of austenite grains is promoted and becomes coarse, and the grain size of the hardened layer becomes coarse, so that the torsional fatigue strength is also lowered. A more preferable heating temperature range is 800 to 950 ° C.
[0054]
When the above-described induction hardening is repeated a plurality of times, at least the final induction hardening may be performed at a heating temperature of 800 to 1000 ° C. Here, when induction hardening is repeated a plurality of times, it is most desirable to set the heating temperature to 800 to 1000 ° C. for all induction hardening. Further, by performing repeated quenching twice or more, a finer cured layer particle size can be obtained as compared with the single quenching.
When induction hardening is repeated a plurality of times, it is preferable that at least the final quenching depth by induction hardening be equal to or greater than the previous quenching depth by induction hardening. This is because the crystal grain size of the hardened layer is most strongly affected by the last induction hardening, so if the hardening depth by the last induction hardening is smaller than the previous hardening depth by the induction hardening, This is because the average crystal grain size over the entire thickness is rather increased and the torsional fatigue strength tends to decrease.
[0055]
Further, in the present invention, the induction hardening is preferably performed with the heating time in the above heating temperature range being 5 seconds or less. This is because when the heating time is set to 5 seconds or less, the austenite grain growth can be further suppressed as compared with the case where the heating time exceeds 5 seconds, and a very fine hardened layer particle size can be obtained. . A more preferable heating time is 3 seconds or less.
[0056]
【Example】
Steel materials having the composition shown in Table 1 were melted by a converter and cast into continuous slabs. The slab size was 300 × 400 mm. The slab was rolled into a 150 mm square billet through a breakdown process, and then rolled into a steel bar having a diameter of 24 to 60 mm. The finishing temperature of rolling was over 900 ° C. as a suitable temperature from the viewpoint of bainite or martensite structure formation. Cooling after hot rolling was performed under the conditions shown in Table 2.
Next, a machinability test was performed on the steel bar.
The machinability test was performed by a peripheral turning test using a carbide tool (P10), cutting speed: 200 m / min, feed: 0.25 mm / rev, cutting depth: 2.0 mm, and no lubrication. Judged by. The tool life was evaluated based on the total cutting time until the carbide flank friction reached 0.2 mm 2.
[0057]
Furthermore, a torsional test piece having a notch with a parallel part: 20 mmφ and a stress concentration coefficient α = 1.5 is prepared from the hot rolled steel bar, and an induction hardening apparatus with a frequency of 15 kHz is used for this torsional test piece. After quenching under the conditions shown in Table 2, tempering was performed at 170 ° C. for 30 minutes using a heating furnace, and then a torsional fatigue test was performed.
The torsional fatigue test is performed using a torsional fatigue tester with a maximum torque of 4900 N · m (= 500 kgf · m 2), changing the stress conditions by both swings. 1 × 10 ⁵ The stress that gives the service life of each cycle was evaluated as the fatigue strength.
Table 2 also shows the results of investigation on machinability and torsional fatigue strength.
[0058]
In addition, for the torsional test piece prepared under the same conditions, the average hardened layer particle size (old austenite particle size) obtained over the base material structure of the steel material, the hardened layer thickness after quenching, and the total thickness of the hardened layer was measured using an optical microscope. Measured.
Table 2 also shows these results.
Here, as described above, the hardened layer thickness was from the steel surface to the depth at which the area ratio of the martensite structure was reduced to 98%. Moreover, about what performed induction hardening several times, the hardening layer thickness after each hardening was measured. Furthermore, as for the hardened layer particle size, the average prior austenite particle size at each of the 1/5 position, 1/2 position and 4/5 position of the hardened layer thickness from the surface was measured, and the maximum value thereof was shown. In addition, about what performed induction hardening several times, the average prior-austenite particle size after final hardening was measured.
[0059]
[Table 1]

[0060]
[Table 2]

[0061]
As is apparent from Table 2, all the steel materials manufactured under the conditions satisfying the component composition range defined in the present invention and satisfying the induction hardening conditions of the present invention have an austenite grain size of the hardened layer of 12 μm over the entire thickness. The following conditions were satisfied, and as a result, a high torsional fatigue strength of 700 MPa or more could be obtained. Moreover, it turns out that it is excellent also in machinability.
In addition, No. No. 12 is a structure in which the base material structure contains a lot of martensite, so the tool life is as short as 2690 s. However, since it contains Ca and Hf, the base material structure is a structure containing a lot of martensite as well. Some Comparative Example No. Compared to 11, it can be seen that the tool life is greatly improved.
No. 15, no. 43, no. 44, when the number of times of quenching was increased from one to two, when the second quenching depth was shallower (No. 43), the torsional fatigue strength was greater than the case where the quenching was performed only once. However, the torsional fatigue strength was improved when the second quenching depth was increased (No. 44) as compared to the case where the quenching was performed only once. In addition, No. 44, the old austenite grain size was the largest at 4/5 position of the cured layer thickness from the surface in the cured layer thickness direction, which was 3.5 μm. ), The prior austenite grain size is 2.6 μm, and it is considered that the finer grain size of the surface layer contributed to the improvement of fatigue strength.
[0062]
In contrast, no. 1, no. No. 2 is an example of the invention in which the prior austenite grain size of the hardened layer is fine and has high torsional fatigue strength, but does not contain a machinability improving element, so that the other components and the base material structure are substantially the same. 3-No. Compared to 7, tool life is short.
No. 8, no. No. 9 is an example of the invention in which the prior austenite grain size of the hardened layer is fine and has high torsional fatigue strength, but also contains no machinability improving element, so other components and the base material structure are almost the same. . 13-No. Compared to 15, tool life is short.
No. No. 11 is an example of the invention in which the prior austenite grain size of the hardened layer is fine and has high torsional fatigue strength, but also contains no machinability improving element, so other components and the base material structure are almost the same. . Compared to 12, tool life is short.
No. No. 17, since the cooling rate after processing is low, the total structural fraction of bainite and martensite is low, and as a result, the hardened layer particle size after induction hardening is coarse, and the torsional fatigue strength is low.
No. No. 30 has a fine particle size of the hardened layer, but the C content is higher than the range of the present invention, so that the grain boundary strength is lowered, and therefore the torsional fatigue strength is inferior. Moreover, it was inferior to machinability.
In Nos. 31, 32, and 33, since the contents of C, Si, and Mo are lower than the appropriate range of the present invention, the hardened layer particle size is coarse and the torsional fatigue strength is inferior.
No. No. 34 has a low B content. No. 35 has a Mn content of No. 35. No. 36 has S and P contents of No. 36. No. 37 has a Cr content exceeding the appropriate range of the present invention, so that both cause a decrease in grain boundary strength and inferior torsional fatigue strength.
No. No. 38 is inferior in torsional fatigue strength because the Ti content exceeds the appropriate range of the present invention. Since No. 41 has a low Ti content, the hardened layer has a coarse particle size and inferior torsional fatigue strength.
No. In No. 39, the heating temperature during induction hardening is too high, and the particle size of the hardened layer becomes coarse. In No. 40, since the heating temperature at the time of induction hardening is too low, a hardened layer is not formed, and all are inferior in torsional fatigue strength.
No. 42 is a case where the amount of Si is 0.28 mass%, which is less than the lower limit of the present invention. However, as in this example, when the amount of Si is slightly below the lower limit of the present invention, 12 μm over the entire thickness of the hardened layer. The following particle sizes cannot be obtained, and as a result, the torsional fatigue strength is inferior.
[0063]
【The invention's effect】
Thus, according to the present invention, a steel material having a high torsional fatigue strength can be stably obtained, and as a result, it is excellent for the demand for reducing the weight of automobile members.

Claims (13)

C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
One or more selected from Mg: 0.02 mass% or less and Hf: 0.1 mass% or less, with the balance being composed of Fe and inevitable impurities, and the base material structure is a bainite structure and A steel material having a martensite structure and having excellent machinability and fatigue characteristics after induction hardening, wherein the total structure fraction of the bainite structure and the martensite structure is 10% or more. In Claim 1, the said steel materials are further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: 0.1 mass% or less and V: 0.5 mass% or less, a composition containing one or more selected from the following, machinability and fatigue properties after induction hardening Excellent steel material. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
It contains one or more selected from Mg: 0.02 mass% or less and Hf: 0.1 mass% or less, with the balance being composed of Fe and inevitable impurities, and the base material structure is a bainite structure And / or has a martensite structure and the total structure fraction of the bainite structure and martensite structure is 10% or more, and the prior austenite grain size of the hardened layer after induction hardening is 12 μm over the entire thickness of the hardened layer. A steel material with excellent fatigue characteristics, characterized by: In Claim 3, The said steel materials are further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A steel material excellent in fatigue characteristics, characterized by having a composition containing one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less. The steel material excellent in fatigue characteristics according to claim 3 or 4, wherein the thickness of the hardened layer after induction hardening is 2 mm or more. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
A steel material containing one or more selected from Mg: 0.02 mass% or less and Hf: 0.1 mass% or less, with the balance being a composition of Fe and inevitable impurities, after hot working, A method for producing a steel material excellent in machinability and fatigue properties after induction hardening, characterized by cooling at a rate of 0.2 ° C./s or more. In Claim 6, the said steel raw material is further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: 0.1 mass% or less and V: 0.5 mass% or less, a composition containing one or more selected from the following, machinability and fatigue properties after induction hardening A superior method for manufacturing steel materials. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Zr: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Pb: 0.6 mass% or less,
Bi: 0.3 mass% or less,
Sb: 0.2 mass% or less,
W: 0.1 mass% or less,
N: 0.04 mass% or less,
A steel material containing one or more selected from Mg: 0.02 mass% or less and Hf: 0.1 mass% or less, with the balance being a composition of Fe and inevitable impurities, after hot working, Cooling at a rate of 0.2 ° C./s or higher, and then cutting, followed by induction hardening at a heating temperature of 800 to 1000 ° C. Steel manufacturing method. In Claim 8, the steel material is further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
The manufacturing method of the steel material excellent in the fatigue characteristics characterized by becoming the composition containing 1 type (s) or 2 or more types chosen from Nb: 0.1 mass% or less and V: 0.5 mass% or less. 10. The method for producing a steel material having excellent fatigue characteristics according to claim 8, wherein after the cutting, induction hardening is repeated a plurality of times, and a heating temperature at the final induction hardening is set to 800 to 1000 ° C. The method for producing a steel material with excellent fatigue characteristics according to claim 10, wherein the heating temperature during induction hardening is set to 800 to 1000 ° C. for all of the plurality of induction hardenings. The method for producing a steel material with excellent fatigue characteristics according to any one of claims 8 to 11, wherein the heating time in the heating temperature range is 5 seconds or less per induction hardening. The method for producing a steel material having excellent fatigue characteristics according to any one of claims 8 to 12, wherein the thickness of the hardened layer on the surface of the steel material by induction hardening is 2 mm or more.