JP5927868B2 - Carburizing steel excellent in cold forgeability and method for producing the same - Google Patents

Carburizing steel excellent in cold forgeability and method for producing the same Download PDF

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JP5927868B2
JP5927868B2 JP2011260725A JP2011260725A JP5927868B2 JP 5927868 B2 JP5927868 B2 JP 5927868B2 JP 2011260725 A JP2011260725 A JP 2011260725A JP 2011260725 A JP2011260725 A JP 2011260725A JP 5927868 B2 JP5927868 B2 JP 5927868B2
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克行 一宮
克行 一宮
三田尾 眞司
眞司 三田尾
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JFE Steel Corp
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    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06Surface hardening
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    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26Methods of annealing
    • C21D1/32Soft annealing, e.g. spheroidising
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    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
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    • C22C1/02Making alloys by melting
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    • C22CALLOYS
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    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
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    • C22C38/002Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
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    • C22CALLOYS
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    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20Ferrous alloys, e.g. steel alloys containing chromium with copper
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02Pretreatment of the material to be coated
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    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20Carburising
    • C23C8/22Carburising of ferrous surfaces
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    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/80After-treatment
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/005Ferrite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/009Pearlite

Description

本発明は、自動車や各種産業機器等に供して好適な、冷間鍛造性に優れた浸炭用鋼およびその製造方法に関するものである。   The present invention relates to a carburizing steel excellent in cold forgeability suitable for use in automobiles, various industrial equipment, and the like, and a method for producing the same.

自動車等に用いられている歯車には、近年、省エネルギー化による車体重量の軽量化に伴って、サイズの小型化が要求され、歯車にかかる負荷が増大している。また、エンジンの高出力化にも伴って歯車にかかる負荷が増大している。歯車の耐久性は、主に歯元の曲げ疲労破壊ならびに歯面の面圧疲労破壊によって決まる。   In recent years, gears used in automobiles and the like have been required to be smaller in size as the weight of the vehicle body has been reduced due to energy saving, and the load on the gears has increased. Also, the load on the gears is increasing with the increase in engine output. The durability of a gear is mainly determined by the bending fatigue failure of the tooth root and the surface pressure fatigue failure of the tooth surface.

従来、歯車は、JIS G 4053(2003)においてSCM420H、SCM822H等として規定された肌焼鋼を用いて歯車材を調製し、この歯車材に浸炭等の表面処理を施して製造されていた。しかしながら、このような歯車は、高応力下での使用に耐え得るものではないことから、鋼材の変更や熱処理方法の変更、さらには表面の加工硬化処理等によって、歯元曲げ疲労強度および耐ピッチング性の向上を図っていた。   Conventionally, gears have been manufactured by preparing gear materials using case-hardened steel defined as SCM420H, SCM822H, etc. in JIS G 4053 (2003), and subjecting the gear materials to surface treatment such as carburization. However, since such gears cannot withstand use under high stress, the tooth root bending fatigue strength and pitting resistance can be reduced by changing steel materials, heat treatment methods, surface work hardening, etc. I tried to improve the sex.

例えば、特許文献1には、鋼中のSiを低減すると共に、Mn、Cr、MoおよびNiをコントロールすることにより、浸炭熱処理後の表面の粒界酸化層を低減して亀裂の発生を少なくし、また不完全焼入層の生成を抑制することにより、表面硬さの低減を抑えて疲労強度を高め、さらにCaを添加して、亀裂の発生・伝播を助長するMnSの延伸を制御する方法が開示されている。
また、特許文献2には、素材としてSiを0.25〜1.50%添加した鋼材を用いて焼戻し軟化抵抗を高める方法が開示されている。
For example, in Patent Document 1, while reducing Si in steel and controlling Mn, Cr, Mo and Ni, the grain boundary oxide layer on the surface after carburizing heat treatment is reduced, and the occurrence of cracks is reduced. Moreover, by suppressing the generation of an incompletely hardened layer, the reduction of surface hardness is suppressed to increase the fatigue strength, and further Ca is added to control the elongation of MnS that promotes the generation and propagation of cracks. Is disclosed.
Patent Document 2 discloses a method for increasing the temper softening resistance using a steel material to which Si is added in an amount of 0.25 to 1.50%.

また、棒材を冷間成形して製造される自動車等の部品素材には、高い冷間鍛造性が要求される。そのため、球状化熱処理を施して炭化物を球状化し、冷間鍛造性を高めることが行われている。
例えば、特許文献3には、圧延ままの組織制御を行い、かつ減面率28%以上の伸線引抜き加工を施した後に球状化焼鈍を行うことによって、球状化焼鈍後の硬さが低く、かつ均質な硬さの鋼材を得る方法が開示されている。
Moreover, high cold forgeability is requested | required of components raw materials, such as a motor vehicle manufactured by cold-forming a bar. Therefore, spheroidizing heat treatment is performed to spheroidize carbides to improve cold forgeability.
For example, in Patent Document 3, the hardness after spheroidizing annealing is low by performing structure control as rolled and performing spheroidizing annealing after drawing wire drawing with a reduction in area of 28% or more, And the method of obtaining the steel material of homogeneous hardness is disclosed.

特公平07−122118号公報Japanese Patent Publication No. 07-122118 特許第2945714号公報Japanese Patent No. 2945714 特許第4392324号公報Japanese Patent No. 4392324

しかしながら、上述した特許文献1,2および3に記載の技術はいずれも、以下に述べるような問題があった。
すなわち、特許文献1によれば、Siを低減することにより、粒界酸化層および不完全焼入れ層が低減するので、歯元での曲げ疲労亀裂発生を抑えることはできる。しかしながら、単純なSiの低減のみでは、焼戻し軟化抵抗が低下する。その結果、歯面での摩擦熱による焼戻し軟化を抑えることができなくなって表面が軟化するため、ピッチングが発生し易くなり、破壊の発生が歯元から歯面側に移行するという問題があった。
However, all of the techniques described in Patent Documents 1, 2, and 3 described above have the following problems.
That is, according to Patent Document 1, since the grain boundary oxide layer and the incompletely hardened layer are reduced by reducing Si, it is possible to suppress the occurrence of bending fatigue cracks at the tooth root. However, the temper softening resistance decreases only by simple Si reduction. As a result, temper softening due to frictional heat on the tooth surface cannot be suppressed, and the surface softens, so that pitching is likely to occur, and there is a problem that the occurrence of fracture shifts from the tooth base to the tooth surface side. .

特許文献2では、焼戻し軟化抵抗性を上げるために、Si量を増加させているが、これでは、冷間加工時の変形抵抗が増大してしまい、冷間鍛造の用途としては不向きとなる。   In Patent Document 2, the Si amount is increased in order to increase the temper softening resistance. However, this increases the deformation resistance during cold working, which is unsuitable for use in cold forging.

また、特許文献3では、球状化焼鈍前に伸線加工を施す、という余分な工程が必要であり、コスト増加を招くことになる。   Moreover, in patent document 3, the extra process of performing a wire-drawing process before spheroidizing annealing is required, and it causes a cost increase.

さらに、球状化熱処理後の組織や硬さには、圧延ままのミクロ組織の形態が影響する。特に、比較的粗いフェライト+パーライト組織の場合、適正な球状化組織を得るための制御範囲が狭いため、安定した組織を得ることが難しいことも問題であった。   Furthermore, the form of the microstructure as it is rolled affects the structure and hardness after the spheroidizing heat treatment. In particular, in the case of a relatively coarse ferrite + pearlite structure, the control range for obtaining an appropriate spheroidized structure is narrow, so that it is difficult to obtain a stable structure.

本発明は、上記の実状に鑑み開発されたものであり、歯元の曲げ疲労強度が従来の歯車よりも高く、さらに面圧疲労特性にも優れた高強度歯車等の素材として好適で、しかも球状化焼鈍組織を低コストで比較的容易に得ることができ、さらに冷間鍛造性に優れ、かつ量産化が可能な浸炭用鋼と、その有利な製造方法について提案することを目的とする。   The present invention has been developed in view of the above situation, and is suitable as a material for high-strength gears and the like, which has a higher bending fatigue strength at the root than that of conventional gears, and also has excellent surface fatigue properties. It is an object of the present invention to propose a carburizing steel that can obtain a spheroidized annealed structure relatively easily at low cost, has excellent cold forgeability, and can be mass-produced, and an advantageous manufacturing method thereof.

さて、発明者等は、上記の課題を解決すべく鋭意研究を重ねた結果、以下に述べる知見を得た。
a)鋼材中のSi,MnおよびCr量を適正化することによって、焼戻し軟化抵抗を高めると共に、この適正化により歯車接触面での発熱による軟化を抑えれば、歯車駆動時に生じる歯面の亀裂発生を抑制することができる。
b)曲げ疲労および疲労亀裂の起点となり得る粒界酸化層については、Si,MnおよびCrをある量以上添加することにより、粒界酸化層の成長方向が深さ方向から表面の密度増加方向に変わる。従って、起点となるような深さ方向に成長した酸化層がなくなるので、曲げ疲労および疲労亀裂の起点となり難くなる。
c)上記aおよびbで述べたとおり、Si,MnおよびCrは、焼戻し軟化抵抗の向上と粒界酸化層のコントロールに有効であるが、これらの効果を両立させるためには、Si,MnおよびCrについて、その含有量を厳密に制御する必要がある。
d)炭化物の球状化を促進し、冷間鍛造性を向上させるためには、C,Si,MnおよびCrの含有量を厳密に制御する必要がある。特に、Crの多量添加が有効である。
As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
a) By optimizing the amounts of Si, Mn, and Cr in the steel material, the resistance to temper softening is increased, and if this optimization prevents softening due to heat generation on the gear contact surface, cracks in the tooth surfaces that occur during gear drive Occurrence can be suppressed.
b) For grain boundary oxide layers that can be the origin of bending fatigue and fatigue cracks, the growth direction of the grain boundary oxide layer is changed from the depth direction to the surface density increasing direction by adding a certain amount of Si, Mn and Cr. change. Therefore, since there is no oxide layer grown in the depth direction to be a starting point, it becomes difficult to be a starting point for bending fatigue and fatigue cracks.
c) As described in the above a and b, Si, Mn and Cr are effective in improving the temper softening resistance and controlling the grain boundary oxide layer. In order to achieve both of these effects, Si, Mn and Cr It is necessary to strictly control the content of Cr.
d) In order to promote spheroidization of carbide and improve cold forgeability, it is necessary to strictly control the contents of C, Si, Mn and Cr. In particular, a large amount of Cr is effective.

e)炭化物の球状化を安定して得るためには、圧延まま組織を微細なフェライト−パーライト組織とすることが重要である。そこで、図1に示す球状化熱処理条件を高温加熱圧延材(1140℃、粗大フェライト−パーライト組織)および低温加熱圧延材(950℃加熱、微細フェライト−パーライト組織)に適用し、該熱処理後の硬さを評価してみた。この評価結果について、図2に、球状化焼鈍後の硬さに及ぼす焼鈍保持温度の影響を示す。加熱温度が高く、組織が粗大なフェライト−パーライト組織の場合は、加熱温度が低い微細フェライト−パーライト組織に比べて、全体に硬さが高く、かつビッカース硬度HV130以下の領域は、非常に狭い温度範囲でしか実現できないことがわかった。とくに焼鈍保持温度が低温の場合、低温加熱圧延材が有利となる。
なお、実験に供した鋼は後述の要件および好適条件を満たす基本成分を含有するものである。
e) In order to stably obtain spheroidization of carbides, it is important to make the microstructure as-rolled into a fine ferrite-pearlite structure. Therefore, the spheroidizing heat treatment conditions shown in FIG. 1 are applied to a high temperature heated rolled material (1140 ° C., coarse ferrite-pearlite structure) and a low temperature heated rolled material (950 ° C. heated, fine ferrite-pearlite structure). I evaluated it. About this evaluation result, FIG. 2 shows the influence of the annealing holding temperature on the hardness after spheroidizing annealing. In the case of a ferrite-pearlite structure with a high heating temperature and a coarse structure, compared to a fine ferrite-pearlite structure with a low heating temperature, the whole area is harder and the region where the Vickers hardness is HV130 or less is a very narrow temperature. It turns out that it is realizable only in the range. In particular, when the annealing holding temperature is low, a low-temperature heated rolled material is advantageous.
The steel subjected to the experiment contains basic components that satisfy the requirements and suitable conditions described later.

f)さらに、冷間鍛造性にはミクロ組織が影響するが、このミクロ組織は上記の球状化焼鈍条件に加えて焼鈍前組織の影響を強く受ける。すなわち、この焼鈍前組織について、フェライト−パーライト組織の分率とフェライト粒径に関する調査を行った。
図3に、球状化処理(765℃−8時間)後の冷間鍛造性に及ぼす球状化焼鈍前組織の影響を示すように、球状化焼鈍前組織を制御、具体的には、フェライトとパーライトとの合計の組織分率を85%以上に、かつフェライトの平均粒径を25μm以下とすることによって、優れた冷間鍛造性を有する鋼材が得られることが分かった。
なお、図3に示した実験において、限界据え込み率とは、円柱をプレス機により据え込みし、端部に割れが入ったときの据え込み率である。また、鋼の組成は上記図2の実験の場合と同じである。
本発明は上記の知見に立脚するものである。
f) Further, the microstructure is influenced by the cold forgeability, but this microstructure is strongly influenced by the structure before annealing in addition to the above spheroidizing annealing conditions. That is, the pre-annealing structure was investigated with respect to the ferrite-pearlite structure fraction and the ferrite particle size.
In FIG. 3, the structure before spheroidizing annealing is controlled, specifically, ferrite and pearlite so as to show the influence of the structure before spheroidizing annealing on the cold forgeability after spheroidizing treatment (765 ° C.-8 hours). It was found that a steel material having excellent cold forgeability can be obtained by setting the total structural fraction to 85% or more and the average grain size of ferrite to 25 μm or less.
In the experiment shown in FIG. 3, the limit upsetting rate is the upsetting rate when the column is upset by a press machine and a crack occurs at the end. The steel composition is the same as in the experiment of FIG.
The present invention is based on the above findings.

すなわち、本発明の要旨構成は次のとおりである。
1.質量%で、
C:0.1〜0.35%、
Si:0.01〜0.22%、
Mn:0.3〜1.5%、
Cr:1.35〜3.0%、
P:0.018%以下、
S:0.02%以下、
Al:0.015〜0.05%、
N:0.008〜0.015%および
O:0.0015%以下
を、下記式(1)、(2)および(3)を満足する範囲で含有し、残部はFeおよび不可避的不純物の組成であり、また鋼組織におけるフェライトとパーライトとの合計の組織分率が85%以上で、かつフェライトの平均粒径が25μm以下であり、さらに下記の条件で浸炭熱処理後の、回転数:1800rpmでの10 7 回時間強度で評価した回転曲げ疲労強度が806MPa以上で、かつ前記浸炭熱処理後のすべり率:40%、油温:80℃の条件での10 7 回時間強度で評価した面疲労強度が3250MPa以上である浸炭用鋼。

3.1≧{([%Si]/2)+[%Mn]+[%Cr]}≧2.2 ---(1)
[%C]−([%Si]/2)+([%Mn]/5)+2[%Cr]≧3.0 ---(2)
2.5 ≧ [%Al]/[%N] ≧ 1.7 ---(3)
但し、[%M]は、元素Mの含有量(質量%)
・浸炭熱処理条件
浸炭:930℃、7時間、カーボンポテンシャル:0.8%
焼戻し:180℃,1時間
That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.1 to 0.35%,
Si: 0.01 to 0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%,
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
N: 0.008 to 0.015% and O: 0.0015% or less are contained within the range satisfying the following formulas (1), (2) and (3), the balance being the composition of Fe and inevitable impurities, and the steel structure The total structural fraction of ferrite and pearlite is 85% or more and the average grain size of ferrite is 25μm or less, and after carburizing heat treatment under the following conditions , the rotational speed is 10 7 times at 1800rpm. Rotating bending fatigue strength evaluated in 1) is 806 MPa or more, and after the carburizing heat treatment , the surface fatigue strength evaluated by 10 7 times strength under the conditions of slip rate: 40% and oil temperature: 80 ° C. is 3250 MPa or more. Carburizing steel.
Record
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] − ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)
Carburizing heat treatment conditions Carburizing: 930 ° C, 7 hours, carbon potential: 0.8%
Tempering: 180 ° C, 1 hour

なお、上記の浸炭用鋼は、浸炭処理後に各種部品形状に加工する冷間鍛造に供される。この冷間鍛造に先立って球状化焼鈍を行うことが好ましいが、必要とされる加工量などに応じて、球状化焼鈍を行うことなく冷間鍛造に供してもよい。   The carburizing steel is subjected to cold forging which is processed into various parts shapes after carburizing treatment. Prior to this cold forging, it is preferable to perform spheroidizing annealing, but depending on the required amount of processing, etc., it may be subjected to cold forging without performing spheroidizing annealing.

2.前記1において、前記鋼は、さらに、質量%で、
Cu:1.0%以下、
Ni:0.5%以下、
Mo:0.5%以下、
V:0.5%以下および
Nb:0.06%以下
のうちから選んだ1種または2種以上を含有する浸炭用鋼。
2. In the above 1, the steel is further in mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
V: 0.5% or less and
Nb: Carburizing steel containing one or more selected from 0.06% or less.

3.質量%で、
C:0.1〜0.35%、
Si:0.01〜0.22%、
Mn:0.3〜1.5%、
Cr:1.35〜3.0%、
P:0.018%以下、
S:0.02%以下、
Al:0.015〜0.05%、
N:0.008〜0.015%および
O:0.0015%以下
を、下記式(1)、(2)および(3)を満足する範囲で含有し、残部はFeおよび不可避的不純物の組成になる鋼素材を、1160℃以上1220℃未満に加熱して熱間加工を施し、Ar点以上の温度域にて熱間加工を一旦終了して、450℃以下まで冷却し、次いで900℃超970℃以下の温度に再加熱して熱間加工を再開し、再加熱後における総圧下率70%以上の条件にて熱間加工を終了したのち、800〜500℃の温度域を0.1〜1.0℃/sの速度で冷却し、鋼組織におけるフェライトとパーライトとの合計の組織分率が85%以上で、かつフェライトの平均粒径が25μm以下とし、さらに下記の条件で浸炭熱処理後の、回転数:1800rpmでの10 7 回時間強度で評価した回転曲げ疲労強度が806MPa以上で、かつ前記浸炭熱処理後の、すべり率:40%、油温:80℃の条件での10 7 回時間強度で評価した面疲労強度が3250MPa以上である、浸炭用鋼の製造方法。

3.1≧{([%Si]/2)+[%Mn]+[%Cr]}≧2.2 ---(1)
[%C]−([%Si]/2)+([%Mn]/5)+2[%Cr]≧3.0 ---(2)
2.5 ≧ [%Al]/[%N] ≧ 1.7 ---(3)
但し、[%M]は、元素Mの含有量(質量%)
・浸炭熱処理条件
浸炭:930℃、7時間、カーボンポテンシャル:0.8%
焼戻し:180℃,1時間
3. % By mass
C: 0.1 to 0.35%,
Si: 0.01 to 0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%,
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
A steel material containing N: 0.008 to 0.015% and O: 0.0015% or less in a range satisfying the following formulas (1), (2) and (3), with the balance being a composition of Fe and inevitable impurities, Heated to 1160 ° C or higher and lower than 1220 ° C to perform hot working, once finished hot working in a temperature range of 3 or more points of Ar, cooled to 450 ° C or lower, then over 900 ° C to 970 ° C or lower After re-heating, the hot working is resumed, and after the hot working is finished under the condition of the total reduction ratio of 70% or more after the re-heating, the temperature range of 800-500 ° C is 0.1-1.0 ° C / s. The total structural fraction of ferrite and pearlite in the steel structure is 85% or more and the average grain size of ferrite is 25 μm or less, and after carburizing heat treatment under the following conditions, at a rotation speed of 1800 rpm Rotating bending fatigue strength evaluated by 10 7 times time strength of 806 MPa or more, slip rate after the carburizing heat treatment: 40%, oil temperature: 8 A method for producing carburized steel, wherein the surface fatigue strength evaluated by 10 7 time strength at 0 ° C is 3250 MPa or more .
Record
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] − ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)
・ Carburizing heat treatment conditions
Carburization: 930 ° C, 7 hours, carbon potential: 0.8%
Tempering: 180 ° C, 1 hour

4.前記3において、前記鋼素材は、さらに、質量%で、
Cu:1.0%以下、
Ni:0.5%以下、
Mo:0.5%以下、
V:0.5%以下および
Nb:0.06%以下
のうちから選んだ1種または2種以上を含有する浸炭用鋼の製造方法。
4). In 3 above, the steel material is further in mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
V: 0.5% or less and
Nb: A method for producing carburizing steel containing one or more selected from 0.06% or less.

本発明によれば、例えば歯車に加工した場合に、歯元の曲げ疲労特性のみならず、歯面の面圧疲労特性に優れた浸炭用鋼を、冷間鍛造を伴う工程において量産化の下で得ることができる。   According to the present invention, for example, when processed into a gear, carburizing steel excellent in not only the bending fatigue property of the tooth root but also the surface pressure fatigue property of the tooth surface is mass-produced in a process involving cold forging. Can be obtained at

球状化熱処理における熱処理条件を示す図である。It is a figure which shows the heat processing conditions in spheroidization heat processing. 球状化熱処理後の硬さに及ぼす焼鈍保持温度の影響を示す図である。It is a figure which shows the influence of the annealing holding temperature which acts on the hardness after spheroidization heat processing. 球状化処理後の冷間鍛造性に及ぼす球状化焼鈍前組織の影響を示すグラフである。It is a graph which shows the influence of the structure | tissue before spheroidizing annealing on the cold forgeability after a spheroidizing process. 球状化熱処理における熱処理条件を示す図である。It is a figure which shows the heat processing conditions in spheroidization heat processing.

以下、本発明を具体的に説明する。
まず、本発明において、鋼素材の成分組成を上記の範囲に限定した理由について説明する。なお、成分に関する「%」表示は特に断らない限り質量%を意味するものとする。
C:0.1〜0.35%
浸炭処理後の焼入れにより中芯部の硬度を高めるためには0.1%以上のCを必要とするが、含有量が0.35%を超えると芯部の靭性が低下することから、C量は0.1〜0.35%の範囲に限定した。好ましくは0.1〜0.3%の範囲である。
Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.1-0.35%
In order to increase the hardness of the core part by quenching after carburizing treatment, 0.1% or more of C is required. However, if the content exceeds 0.35%, the toughness of the core part decreases, so the amount of C is 0.1 to Limited to a range of 0.35%. Preferably it is 0.1 to 0.3% of range.

Si:0.01〜0.22%
Siは、歯車等が転動中に到達すると思われる200〜300℃の温度域における軟化抵抗を高める元素であり、その効果を発揮するためには少なくとも0.01%の添加が不可欠である。好ましくは0.03%以上を添加する。しかしながら、一方でSiはフェライト安定化元素であるので、過剰な添加はAc変態点を上昇させ、通常の焼入れ温度範囲で炭素の含有量の低い芯部でフェライトが出現し易くなり、その結果、強度の低下を招く。また、過剰な添加は浸炭前の鋼材を硬化させ、冷間鍛造性を劣化させる不利もある。この点、Si量が0.22%以下であれば、上記のような弊害は生じないため、Si量は0.01〜0.22%の範囲に限定した。好ましくは0.03〜0.22%の範囲である。
Si: 0.01 to 0.22%
Si is an element that enhances the softening resistance in the temperature range of 200 to 300 ° C. that the gears and the like are supposed to reach during rolling, and at least 0.01% addition is indispensable in order to exert the effect. Preferably 0.03% or more is added. However, on the other hand, since Si is a ferrite stabilizing element, excessive addition raises the Ac 3 transformation point, and ferrite tends to appear in the core portion having a low carbon content in the normal quenching temperature range. , Leading to a decrease in strength. Excessive addition also has the disadvantage of hardening the steel material before carburizing and degrading the cold forgeability. In this respect, when the Si content is 0.22% or less, the above-described adverse effects do not occur. Therefore, the Si content is limited to a range of 0.01 to 0.22%. Preferably it is 0.03 to 0.22% of range.

Mn:0.3〜1.5%
Mnは、焼入性に有効な元素であり、少なくとも0.3%の添加を必要とする。しかしながら、Mnは、浸炭異常層を形成し易く、また過剰な添加は残留オーステナイト量が過多となって硬さの低下を招くので、上限を1.5%とした。好ましくは0.4〜1.2%の範囲である。より好ましくは0.6〜1.2%の範囲である。
Mn: 0.3-1.5%
Mn is an element effective for hardenability, and requires addition of at least 0.3%. However, Mn tends to form an abnormal carburizing layer, and excessive addition causes an excessive amount of retained austenite and leads to a decrease in hardness, so the upper limit was made 1.5%. Preferably it is 0.4 to 1.2% of range. More preferably, it is 0.6 to 1.2% of range.

Cr:1.35〜3.0%
Crは、焼入性のみならず焼戻し軟化抵抗の向上にも有効な元素であるが、含有量が1.35%に満たないとその添加効果に乏しい。一方3.0%を超えると軟化抵抗を高める効果は飽和し、むしろ浸炭異常層を形成し易くなるので、Cr量は1.35〜3.0%の範囲に限定した。好ましくは1.35〜2.6%の範囲である。
Cr: 1.35 to 3.0%
Cr is an element effective for improving not only hardenability but also temper softening resistance. However, if its content is less than 1.35%, its addition effect is poor. On the other hand, if it exceeds 3.0%, the effect of increasing the softening resistance is saturated, and rather, it becomes easier to form a carburized abnormal layer, so the Cr content is limited to the range of 1.35 to 3.0%. Preferably it is 1.35 to 2.6% of range.

P:0.018%以下
Pは、結晶粒界に偏析し、浸炭層および芯部の靭性を低下させるため、その混入は低いほど望ましいが、0.018%までは許容される。好ましくは0.016%以下である。通常、含有量を0%とすることは難しいが、可能であれば0%として良い。
P: 0.018% or less P is segregated at the grain boundary and lowers the toughness of the carburized layer and the core. Therefore, the lower the content, the better, but 0.018% is acceptable. Preferably it is 0.016% or less. Usually, it is difficult to make the content 0%, but if possible, the content may be 0%.

S:0.02%以下
Sは、硫化物系介在物として存在し、被削性の向上に有効な元素である。しかしながら、過剰な添加は疲労強度の低下を招く要因となるため、上限を0.02%とした。被削性の観点からは0.004%以上含有させてもよい。
S: 0.02% or less S is an element that exists as sulfide inclusions and is effective in improving machinability. However, excessive addition causes a decrease in fatigue strength, so the upper limit was made 0.02%. From the viewpoint of machinability, 0.004% or more may be contained.

Al:0.015〜0.05%
Alは、Nと結合してAlNを形成し、オーステナイト結晶粒の微細化に寄与する元素であり、この効果を得るためには0.015%以上、好ましくは0.018%以上の添加を必要とする。一方、含有量が0.05%を超えると、疲労強度に対して有害なAl2O3介在物の生成を助長するため、Al量は0.015〜0.05%の範囲に限定した。好ましくは0.015〜0.037%の範囲である。
Al: 0.015-0.05%
Al is an element which combines with N to form AlN and contributes to the refinement of austenite crystal grains. To obtain this effect, 0.015% or more, preferably 0.018% or more is required to be added. On the other hand, when the content exceeds 0.05%, the formation of Al 2 O 3 inclusions harmful to fatigue strength is promoted, so the Al content is limited to a range of 0.015 to 0.05%. Preferably it is 0.015 to 0.037% of range.

N:0.008〜0.015%
Nは、Alと結合してAlNを形成し、オーステナイト結晶粒の微細化に寄与する元素である。従って、適正添加量はAlとの量的バランスで決まるが、その効果を発揮するためには0.008%以上の添加が必要である。しかし、過剰に添加すると凝固時の鋼塊に気泡が発生したり、鍛造性の劣化を招くため、上限を0.015%とする。好ましくは0.010〜0.015%の範囲である。
N: 0.008 to 0.015%
N is an element that combines with Al to form AlN and contributes to the refinement of austenite crystal grains. Therefore, the appropriate addition amount is determined by the quantitative balance with Al, but 0.008% or more of addition is necessary to exert the effect. However, if added in excess, bubbles are generated in the steel ingot during solidification and deterioration of forgeability is caused, so the upper limit is made 0.015%. Preferably it is 0.010 to 0.015% of range.

O:0.0015%以下
Oは、鋼中において酸化物系介在物として存在し、疲労強度を損なう元素であるため、低いほど望ましいが、0.0015%までは許容される。通常、含有量を0%とすることは難しいが、可能であれば0%として良い。
O: 0.0015% or less O is present as an oxide inclusion in steel and is an element that impairs fatigue strength. Therefore, the lower the content, the better, but 0.0015% is acceptable. Usually, it is difficult to make the content 0%, but if possible, the content may be 0%.

以上、本発明の基本成分の適正組成範囲について説明したが、本発明では、各々の元素が単に上記の範囲を満足するだけでは不十分で、C,Si,Mn,Cr,AlおよびNについては、次式(1),(2)および(3)の関係を満足させることが重要である。
3.1≧{([%Si]/2)+[%Mn]+[%Cr]}≧2.2 ---(1)
[%C]−([%Si]/2)+([%Mn]/5)+2[%Cr]≧3.0 ---(2)
2.5 ≧ [%Al]/[%N] ≧ 1.7 ---(3)
但し、[%M]は、元素Mの含有量(質量%)
In the above, the proper composition range of the basic component of the present invention has been described. However, in the present invention, it is not sufficient that each element simply satisfies the above range. For C, Si, Mn, Cr, Al and N, It is important to satisfy the relationships of the following expressions (1), (2) and (3).
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] − ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)

上掲(1)式は、焼入性および焼戻し軟化抵抗性に影響を与える因子で、(1)式が2.2未満では焼入性および焼戻し軟化抵抗性の改善効果が十分でなく、疲労強度が不十分となる。一方3.1を超えると上記の改善効果が飽和するだけでなく、冷間加工性の劣化を招く。
また、上掲(2)式は、炭化物の球状化の容易さに影響を与える因子であり、(2)式が3.0以上を満たすことにより球状化が容易となる。この組成と前記e、fの知見とを組合せることで、球状化焼鈍後に極めて優れた冷間鍛造性を得ることができる。
さらに、上掲(3)式は、オーステナイト結晶粒の微細化に影響を与える因子で、(3)式の値が1.7に満たないと微細化効果に乏しく、疲労強度が不十分となる。一方2.5を超えると結晶粒が容易に粗大化し疲労強度が不十分となるだけでなく、固溶Al,固溶Nに起因して加工性の低下を招く。
The above formula (1) is a factor that affects the hardenability and temper softening resistance.If the formula (1) is less than 2.2, the effect of improving the hardenability and temper softening resistance is not sufficient, and the fatigue strength is low. It becomes insufficient. On the other hand, if it exceeds 3.1, not only the above-mentioned improvement effect is saturated, but also cold workability is deteriorated.
In addition, the above formula (2) is a factor that affects the ease of spheroidizing of the carbide, and when the formula (2) satisfies 3.0 or more, the spheroidization becomes easy. By combining this composition with the knowledge of e and f, extremely excellent cold forgeability after spheroidizing annealing can be obtained.
Furthermore, the above equation (3) is a factor that affects the refinement of austenite crystal grains. If the value of the equation (3) is less than 1.7, the refinement effect is poor and the fatigue strength is insufficient. On the other hand, if it exceeds 2.5, the crystal grains easily become coarse and fatigue strength becomes insufficient, and workability is reduced due to solute Al and solute N.

以上、本発明の基本成分について説明したが、本発明では、その他にも必要に応じて、以下に述べる成分を適宜含有させることができる。
Cu:1.0%以下
Cuは、母材の強度向上に有効であるが、含有量が1.0%を超えると熱間脆性を生じ、鋼材の表面性状が劣化するため、1.0%以下とする。好適な添加量は0.01%以上である。
The basic components of the present invention have been described above. However, in the present invention, the following components can be appropriately contained as needed.
Cu: 1.0% or less
Cu is effective in improving the strength of the base material, but if the content exceeds 1.0%, hot brittleness is caused and the surface properties of the steel material deteriorate, so the content is made 1.0% or less. A suitable addition amount is 0.01% or more.

Ni:0.5%以下
Niは、母材の強度および靭性の向上に有効であるが、高価であることから0.5%以下で含有させるものとした。好適な添加量は0.01%以上である。
Ni: 0.5% or less
Ni is effective in improving the strength and toughness of the base material, but it is expensive and is therefore contained at 0.5% or less. A suitable addition amount is 0.01% or more.

Mo:0.5%以下
Moは、Niと同様、母材の強度および靭性の向上に有効であるが、高価であることから0.5%以下で含有させるものとした。含有量は0.2%以下としてもよい。好適な添加量は0.05%以上である。
Mo: 0.5% or less
Mo, like Ni, is effective in improving the strength and toughness of the base material, but is included at 0.5% or less because it is expensive. The content may be 0.2% or less. A suitable addition amount is 0.05% or more.

V:0.5%以下
Vは、Siと同様、焼戻し軟化抵抗を高めるのに有用な元素であるが、含有量が0.5%を超えると効果が飽和するため、0.5%以下で含有させるものとした。好適な添加量は0.01%以上である。
V: 0.5% or less V, like Si, is an element useful for increasing the temper softening resistance. However, if the content exceeds 0.5%, the effect is saturated. A suitable addition amount is 0.01% or more.

Nb:0.06%以下
Nbは、VやSiと同様、焼戻し軟化抵抗を高めるのに有用な元素であるが、含有量が0.06%を超えると効果が飽和することから、0.06%以下とする。好適な添加量は0.007%以上である。
鋼素材の残部組成はFeおよび不可避的不純物である。例えばBはとくに添加しないが、0.0003%未満程度であれば、不純物として含有してもよい。
Nb: 0.06% or less
Nb, like V and Si, is an element useful for increasing the temper softening resistance. However, if the content exceeds 0.06%, the effect is saturated, so 0.06% or less. A suitable addition amount is 0.007% or more.
The balance composition of the steel material is Fe and inevitable impurities. For example, B is not particularly added, but may be contained as an impurity as long as it is less than about 0.0003%.

また、以上説明した成分組成の調整に加えて、素材の球状化焼鈍前の鋼組織についても制御する必要がある。
フェライトとパーライトとの合計の組織分率:85%以上
球状化焼鈍前組織におけるベイナイト分率が高くなると、変形抵抗が高くなって冷間鍛造性が悪化するため、フェライトとパーライトとの合計の組織分率を85%以上としてベイナイト分率を下げる必要がある。なお、上限は100%としてよい。
本発明では、前記(1)式等を満たす、焼入れ性の高い鋼を用いるため、通常の製造方法では上記フェライト+パーライトの量を確保しにくいが、圧延時の加熱温度、総圧下率および冷却速度を調整することにより、フェライト+パーライト:85%以上を実現することができる。
フェライト平均粒径:25μm以下
球状化焼鈍前組織は、球状化焼鈍後の特性に大きく影響する。すなわち、球状化焼鈍前組織におけるフェライト粒径が25μm超では球状化処理後の冷鍛性が悪化する。特に、限界据え込み率への影響が大きいことから、フェライトの平均粒径は25μm以下とする。技術思想上、とくに下限を規定する必要はないが、現実的な下限としては5μm程度である。
In addition to the adjustment of the component composition described above, it is necessary to control the steel structure of the material before spheroidizing annealing.
Total structure fraction of ferrite and pearlite: 85% or more When the bainite fraction in the structure before spheroidizing annealing increases, the deformation resistance increases and the cold forgeability deteriorates, so the total structure of ferrite and pearlite It is necessary to lower the bainite fraction by setting the fraction to 85% or more. The upper limit may be 100%.
In the present invention, since steel with high hardenability that satisfies the above-mentioned formula (1) is used, it is difficult to secure the amount of ferrite + pearlite in the normal production method, but the heating temperature, total rolling reduction rate and cooling during rolling are difficult. By adjusting the speed, ferrite + pearlite: 85% or more can be realized.
Average ferrite particle diameter: 25 μm or less The structure before spheroidizing annealing greatly affects the characteristics after spheroidizing annealing. That is, if the ferrite grain size in the microstructure before spheroidizing annealing exceeds 25 μm, the cold forgeability after spheroidizing treatment deteriorates. In particular, since the influence on the limit upsetting rate is large, the average particle diameter of ferrite is set to 25 μm or less. In terms of technical thought, it is not necessary to specify a lower limit, but a practical lower limit is about 5 μm.

次に、本発明の製造条件について説明する。
本発明では、上述した好適成分組成になる鋼素材を、1160℃以上1220℃未満に加熱後、Ar点以上の温度域にて圧延を終了して一旦450℃以下まで空冷し、次いで900℃超970℃以下の温度に再加熱し、再加熱後における総圧下率70%以上の条件にて熱間圧延を終了したのち、800〜500℃の温度域を0.1〜1.0℃/sの速度で冷却することが必要である。
以下、各処理条件を上記のように限定した理由について説明する。
Next, the manufacturing conditions of the present invention will be described.
In the present invention, the steel material having the above-mentioned preferred component composition is heated to 1160 ° C. or more and less than 1220 ° C., then rolled in a temperature range of Ar 3 points or more, temporarily cooled to 450 ° C. or less, and then 900 ° C. After re-heating to a temperature below 970 ° C and hot rolling under conditions of 70% or more of the total reduction after reheating, the temperature range of 800-500 ° C is set at a rate of 0.1-1.0 ° C / s. It is necessary to cool.
Hereinafter, the reason why each processing condition is limited as described above will be described.

[鋼素材加熱温度(第1段):1160℃以上1220℃未満]
本発明では、凝固ままの状態から一度AlNを十分に固溶させておく必要があるため、鋼素材を1160℃以上の温度に加熱することとした。しかし、加熱温度が高すぎるとスケールロスや表面性状の悪化、燃料コストの増加などがあることから第1段加熱温度は1220℃未満とした。
[Steel material heating temperature (1st stage): 1160 ℃ or more and less than 1220 ℃]
In the present invention, since it is necessary to dissolve AlN sufficiently once from the solidified state, the steel material is heated to a temperature of 1160 ° C. or higher. However, if the heating temperature is too high, the first stage heating temperature is set to less than 1220 ° C. because scale loss, surface property deterioration, fuel cost increase, and the like.

[Ar点以上の温度域にて熱間加工終了後一旦450℃以下まで冷却]
この熱間加工工程、好ましくは熱間圧延工程においては、鋳造組織を壊してフェライト−パーライト組織を得るために、Ar点以上で加工を終了し、450℃以下まで冷却する。また、熱間加工は、50%以上の圧下率にて行うことが、フェライト−パーライト組織を得る観点から有利である。冷却終了温度についてはとくに下限を設ける必要はなく、再加熱コストなどを考慮して現実的な値を選定すればよい。熱間加工の圧下率の上限もとくに設ける必要は無く、設備負荷などを考慮して現実的な値を選定すればよい。
[After the hot working is completed in the temperature range of Ar 3 points or higher, it is once cooled to 450 ° C or lower]
In this hot working step, preferably the hot rolling step, in order to break the cast structure and obtain a ferrite-pearlite structure, the working is finished at 3 or more points of Ar and cooled to 450 ° C. or lower. Moreover, it is advantageous from the viewpoint of obtaining a ferrite-pearlite structure that the hot working is performed at a rolling reduction of 50% or more. There is no particular need to set a lower limit for the cooling end temperature, and a realistic value may be selected in consideration of the reheating cost. It is not necessary to provide an upper limit for the reduction ratio in hot working, and a realistic value may be selected in consideration of equipment load.

[鋼素材再加熱温度(第2段):900℃超970℃以下]
球状化焼鈍組織と低い硬さとを得るには、圧延まま組織を微細なフェライト−パーライト組織とする必要があるため、970℃以下の温度に再加熱することとした。970℃を超えるとAlNが粗大析出するのに対して、970℃以下であれば、微細析出することにより、浸炭時の粗粒化抑制にも有効である。しかし、900℃以下の加熱ではAlNの析出が十分になされないことから、第2段加熱温度は900℃超とする。好ましくは920℃以上である。
[Steel material reheating temperature (second stage): Over 900 ° C and below 970 ° C]
In order to obtain a spheroidized annealing structure and low hardness, it is necessary to re-heat the structure to a temperature of 970 ° C. or lower because the structure needs to be a fine ferrite-pearlite structure as it is rolled. When the temperature exceeds 970 ° C., AlN is coarsely precipitated, whereas when it is 970 ° C. or lower, it is effective for suppressing coarsening during carburization by fine precipitation. However, since the AlN is not sufficiently precipitated by heating at 900 ° C. or lower, the second stage heating temperature is set to over 900 ° C. Preferably it is 920 degreeC or more.

[熱間加工における総圧下率:70%以上]
再加熱後の熱間加工における総圧下率、すなわち再加熱後の加工工程における圧下率の合計が少ないと、結晶粒が粗大となって冷却後のフェライト分率が減少し、浸炭時に粗大粒が発生し易くなるだけでなく、加工材の硬さが上昇するため、70%以上とする。圧下率の上限はとくに設ける必要は無く、設備負荷などを考慮して現実的な値を選定すればよい。
なお、この圧下率は、熱間加工により得る鋼材が板の場合には厚さの減少率を、一方棒鋼や線材の場合には減面率のことをいう。
[Total rolling reduction in hot working: 70% or more]
If the total rolling reduction ratio in hot working after reheating, that is, the total rolling reduction ratio in the processing step after reheating is small, the crystal grains are coarse and the ferrite fraction after cooling is reduced. Not only is it easy to occur, but the hardness of the workpiece increases, so 70% or more. The upper limit of the rolling reduction is not particularly required, and a realistic value may be selected in consideration of the equipment load.
In addition, this rolling reduction means the reduction rate of thickness when the steel material obtained by hot working is a plate, and the area reduction rate when it is a steel bar or wire.

[500〜800℃の温度域の冷却速度:0.1〜1.0℃/s]
熱間加工後の冷却過程において、800〜500℃の温度域における冷却速度が0.1℃/sに満たないと、フェライト粒径が大きくなり、粗大なフェライト−パーライト組織となる。一方、1.0℃/sを超えると、冷却後のフェライト分率が減少して、ベイナイトとフェライト−パーライトの混合組織となる。よって、この温度域における冷却速度は0.1〜1.0℃/sの範囲に限定した。
[Cooling rate in the temperature range of 500 to 800 ° C: 0.1 to 1.0 ° C / s]
In the cooling process after hot working, if the cooling rate in the temperature range of 800 to 500 ° C. is less than 0.1 ° C./s, the ferrite particle size becomes large and a coarse ferrite-pearlite structure is formed. On the other hand, when it exceeds 1.0 ° C./s, the ferrite fraction after cooling decreases, and a mixed structure of bainite and ferrite-pearlite is obtained. Therefore, the cooling rate in this temperature range was limited to the range of 0.1 to 1.0 ° C./s.

上記製法により得られた浸炭用鋼は、望ましくは球状化焼鈍を施され、その後冷間鍛造に供される。球状化焼鈍は760〜820℃にて2〜15時間程度施すことが好ましいが、本発明はとくに740〜760℃程度の比較的低温の球状化焼鈍でも、優れた冷間鍛造性を得ることができる。なお、球状化焼鈍後の組織は、前組織の層状パーライト中の板状セメンタイトを分断・球状化させた組織である。地組織はフェライトであるが、加熱段階でオーステナイトとフェライトの二相域に保持するため、前組織を概ね継承する。
所定の部品形状に冷間鍛造された鋼は、常法により浸炭熱処理を施される。浸炭熱処理後の部材は表面がマルテンサイト(焼戻し処理した場合は焼戻しマルテンサイト)主体の組織となる。
The carburizing steel obtained by the above manufacturing method is preferably subjected to spheroidizing annealing and then subjected to cold forging. The spheroidizing annealing is preferably performed at 760 to 820 ° C. for about 2 to 15 hours, but the present invention can obtain excellent cold forgeability even at a relatively low temperature spheroidizing annealing of about 740 to 760 ° C. it can. The structure after spheroidizing annealing is a structure obtained by dividing and spheroidizing plate-like cementite in the layered pearlite of the previous structure. Although the ground structure is ferrite, it retains the two-phase region of austenite and ferrite in the heating stage, and therefore generally inherits the previous structure.
The steel that has been cold forged into a predetermined part shape is subjected to carburizing heat treatment by a conventional method. The surface of the member after the carburizing heat treatment has a structure mainly composed of martensite (tempered martensite when tempered).

表1に示す種々の成分組成になる鋼を、100kg真空溶解炉にて溶製し、鋳片を表2に示す熱間加工条件および冷却条件にて圧延を実施し、棒鋼とした。すなわち、表2に示した加熱温度で加熱して第1段熱間加工を行い、450℃以下まで冷却した後、表2に示した加熱温度、総圧下率および冷却速度条件にて、加熱、圧延および冷却の第2段熱間加工を行って、棒鋼を得た。得られた棒鋼について、組織分率およびフェライト平均粒径、冷間加工性、球状化熱処理性、浸炭部特性および疲労特性の評価を、以下の条件にて行った。   Steels having various component compositions shown in Table 1 were melted in a 100 kg vacuum melting furnace, and the slab was rolled under the hot working conditions and cooling conditions shown in Table 2 to obtain bar steel. That is, the first stage hot working is performed by heating at the heating temperature shown in Table 2, and after cooling to 450 ° C. or lower, the heating is performed at the heating temperature, the total reduction rate, and the cooling rate conditions shown in Table 2. The second stage hot working of rolling and cooling was performed to obtain a steel bar. The obtained steel bar was evaluated under the following conditions for the structural fraction, ferrite average particle size, cold workability, spheroidizing heat treatment property, carburized portion property, and fatigue property.

(1)組織分率およびフェライト平均粒径
棒鋼のL方向断面の1/4D位置を鏡面研磨したのち、ナイタールで腐食し、400倍で撮影した写真を画像解析することにより、フェライト+パーライトの組織分率(面積分率)およびフェライトの平均粒径を求めた。
(1) Structure fraction and ferrite average particle size After 1 / 4D position of the cross section in the L direction of the steel bar is mirror-polished, it is corroded with nital, and the photograph taken at 400 times is analyzed by image analysis. The fraction (area fraction) and the average particle diameter of the ferrite were determined.

(2)冷間加工性(冷間鍛造性)の評価方法
冷間加工性は、変形抵抗値および限界据え込み率の2項目で評価した。
すなわち、変形抵抗値は、圧延ままの棒鋼(直径D)の表面から1/4Dの位置から、直径:10mmおよび高さ:15mmの試験片を採取し、300t(3000kN)プレス機を用いて、70%据え込み時の圧縮荷重を測定し、日本塑性加工学会が提唱している端面拘束圧縮による変形抵抗測定方法を用いて求めた。
限界据え込み率は、変形抵抗を測定した方法で圧縮加工を行い、端部に割れが入ったときの据え込み率を限界据え込み率とした。
変形抵抗値が 918 MPa以下、限界据え込み率が76%以上であれば冷間加工性は良好であるといえる。
(2) Evaluation method of cold workability (cold forgeability) Cold workability was evaluated by two items of a deformation resistance value and a limit upsetting rate.
That is, the deformation resistance value was obtained by collecting a test piece having a diameter of 10 mm and a height of 15 mm from a position 1 / 4D from the surface of a rolled steel bar (diameter D), and using a 300 t (3000 kN) press machine. The compressive load at 70% upsetting was measured, and the deformation resistance measurement method by end face constrained compression proposed by the Japan Society for Technology of Plasticity was used.
The limit upsetting rate was defined as the upsetting rate when compression processing was performed by a method of measuring deformation resistance and a crack occurred at the end.
If the deformation resistance value is 918 MPa or less and the limit upsetting rate is 76% or more, it can be said that the cold workability is good.

(3)球状化熱処理性の評価方法
球状化熱処理性は、球状化熱処理後の硬さ、変形抵抗値および限界据え込み率の3項目にて評価した。
すなわち、上記(2)の冷間加工性の評価と同様にして、圧延ままの棒鋼(直径D)の表面から1/4Dの位置から、直径:10mmおよび高さ:15mmの試験片を採取し、この試験片に球状化熱処理を施した後、変形抵抗値および限界据え込み率を求めた。球状化熱処理は、図4に示す2条件(A)および(B)にて行い、ビッカース硬さ試験〔荷重:98N(10kgf)〕で9点測定し、平均値および最大値を求めた。球状化熱処理後の硬さの平均値がHV130未満および最大値がHV135以下であれば、冷間鍛造性に非常に優れ、かつその安定性にも優れていると言える。
また、球状化熱処理(条件(A))後の変形抵抗値が890MPa以下および限界据え込み率が80%以上であれば、冷間加工性は良好であるといえる。
(3) Evaluation method of spheroidizing heat treatment property The spheroidizing heat treatment property was evaluated by three items: hardness after spheroidizing heat treatment, deformation resistance value, and limit upsetting rate.
That is, in the same manner as the evaluation of cold workability in (2) above, a test piece having a diameter of 10 mm and a height of 15 mm was taken from a position 1 / 4D from the surface of the rolled steel bar (diameter D). After subjecting this test piece to spheroidizing heat treatment, the deformation resistance value and the limit upsetting rate were determined. The spheroidizing heat treatment was carried out under the two conditions (A) and (B) shown in FIG. 4, and 9 points were measured by a Vickers hardness test [load: 98 N (10 kgf)] to obtain an average value and a maximum value. If the average hardness after spheroidizing heat treatment is less than HV130 and the maximum value is HV135 or less, it can be said that the cold forgeability is very excellent and the stability is also excellent.
Further, if the deformation resistance value after spheroidizing heat treatment (condition (A)) is 890 MPa or less and the limit upsetting rate is 80% or more, it can be said that the cold workability is good.

(4)浸炭部特性の評価方法
浸炭部特性は、930℃、7時間、カーボンポテンシャル:0.8%の条件で浸炭を実施後、浸炭部での粗大粒発生の有無と粒界酸化深さの2項目で評価した。
すなわち、浸炭部において、粗大粒の発生がなかった場合を○、粗大粒の発生があった場合を×とした。
粒界酸化挙動は、浸炭処理後の試験片の表面を光学顕微鏡で観察し、粒界酸化深さを測定することで評価した。すなわち、倍率:400倍で光学顕微鏡観察し、各視野での最大粒界酸化深さを求め、10視野の平均値を粒界酸化深さとした。
浸炭部での粗大粒の発生がなく、粒界酸化深さが10μm 以下であれば、浸炭部特性に優れているといえる。
(4) Evaluation method of carburized zone characteristics Carburized zone characteristics are 2 of the presence of coarse grains in the carburized zone and the grain boundary oxidation depth after carburizing at 930 ° C for 7 hours and carbon potential: 0.8%. We evaluated by item.
That is, in the carburized portion, the case where there was no generation of coarse particles was marked as ◯, and the case where coarse particles were generated was marked as x.
Grain boundary oxidation behavior was evaluated by observing the surface of the test piece after carburizing treatment with an optical microscope and measuring the grain boundary oxidation depth. That is, an optical microscope observation was performed at a magnification of 400 times, the maximum grain boundary oxidation depth in each field of view was obtained, and the average value of 10 fields of view was defined as the grain boundary oxidation depth.
If there is no generation of coarse grains in the carburized part and the grain boundary oxidation depth is 10 μm or less, it can be said that the carburized part characteristics are excellent.

(5)疲労特性の評価方法
疲労特性は、回転曲げ疲労試験片と面疲労強度の2項目で評価した。
すなわち、圧延ままの棒鋼から回転曲げ疲労試験片と面疲労強度を評価するためのローラピッチング試験片とを加工し、試験に供した。これらの試験片に930℃、7時間、カーボンポテンシャル:0.8%の条件で浸炭を実施後、180℃,1時間の加熱焼戻し処理を施した。
回転曲げ疲労試験は、回転数:1800rpmで実施し、107回時間強度で評価した。
ローラピッチング試験は、すべり率:40%、油温:80℃の条件で107回時間強度で評価した。
回転曲げ疲労強度が806MPa以上で、面疲労強度が3250MPa以上であれば、疲労強度は良好であるといえる。
得られた結果を表3に示す。
(5) Evaluation Method of Fatigue Properties Fatigue properties were evaluated by two items: a rotating bending fatigue test piece and surface fatigue strength.
That is, a rotating bending fatigue test piece and a roller pitching test piece for evaluating surface fatigue strength were processed from a rolled steel bar and subjected to the test. These test pieces were carburized under conditions of 930 ° C., 7 hours, carbon potential: 0.8%, and then subjected to heat tempering treatment at 180 ° C. for 1 hour.
The rotating bending fatigue test was carried out at a rotational speed of 1800 rpm and evaluated with a strength of 10 7 times.
The roller pitching test was evaluated with a strength of 10 7 times under the conditions of slip ratio: 40% and oil temperature: 80 ° C.
If the rotational bending fatigue strength is 806 MPa or more and the surface fatigue strength is 3250 MPa or more, it can be said that the fatigue strength is good.
The obtained results are shown in Table 3.

表3に示したとおり、本発明に従い得られた発明例はいずれも、圧延ままおよび球状化熱処理後の冷間加工性に優れ、また粒界酸化深さが浅く、かつ浸炭部に粗大粒の発生もなく、さらに比較例に比べて回転曲げ疲労強度および面圧疲労強度に優れていることが分かる。   As shown in Table 3, all of the inventive examples obtained according to the present invention are excellent in cold workability after rolling and after spheroidizing heat treatment, have a shallow grain boundary oxidation depth, and have coarse grains in the carburized portion. It can be seen that there is no occurrence and that the rotating bending fatigue strength and the contact pressure fatigue strength are superior to those of the comparative example.

本発明により、冷間加工性に優れ、また回転曲げ疲労強度および面圧疲労強度に優れる浸炭用鋼の提供が可能になる。従って、例えば歯車に加工した場合に、歯元の曲げ疲労特性だけでなく、歯面の面圧疲労特性に優れた浸炭用鋼を、冷間鍛造を伴う工程において量産化の下で得ることができる。   According to the present invention, it is possible to provide a carburizing steel having excellent cold workability and excellent rotating bending fatigue strength and surface pressure fatigue strength. Therefore, for example, when machined into gears, it is possible to obtain carburizing steel excellent in not only the bending fatigue characteristics of the tooth root but also the surface pressure fatigue characteristics of the tooth surface under mass production in the process involving cold forging. it can.

Claims (4)

質量%で、
C:0.1〜0.35%、
Si:0.01〜0.22%、
Mn:0.3〜1.5%、
Cr:1.35〜3.0%、
P:0.018%以下、
S:0.02%以下、
Al:0.015〜0.05%、
N:0.008〜0.015%および
O:0.0015%以下
を、下記式(1)、(2)および(3)を満足する範囲で含有し、残部はFeおよび不可避的不純物の組成であり、また鋼組織におけるフェライトとパーライトとの合計の組織分率が85%以上で、かつフェライトの平均粒径が25μm以下であり、さらに下記の条件で浸炭熱処理後の、回転数:1800rpmでの10 7 回時間強度で評価した回転曲げ疲労強度が806MPa以上で、かつ前記浸炭熱処理後のすべり率:40%、油温:80℃の条件での10 7 回時間強度で評価した面疲労強度が3250MPa以上である浸炭用鋼。

3.1≧{([%Si]/2)+[%Mn]+[%Cr]}≧2.2 ---(1)
[%C]−([%Si]/2)+([%Mn]/5)+2[%Cr]≧3.0 ---(2)
2.5 ≧ [%Al]/[%N] ≧ 1.7 ---(3)
但し、[%M]は、元素Mの含有量(質量%)
・浸炭熱処理条件
浸炭:930℃、7時間、カーボンポテンシャル:0.8%
焼戻し:180℃,1時間
% By mass
C: 0.1 to 0.35%,
Si: 0.01 to 0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%,
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
N: 0.008 to 0.015% and O: 0.0015% or less are contained within the range satisfying the following formulas (1), (2) and (3), the balance being the composition of Fe and inevitable impurities, and the steel structure The total structural fraction of ferrite and pearlite is 85% or more and the average grain size of ferrite is 25μm or less, and after carburizing heat treatment under the following conditions , the rotational speed is 10 7 times at 1800rpm. Rotating bending fatigue strength evaluated in 1) is 806 MPa or more, and after the carburizing heat treatment , the surface fatigue strength evaluated by 10 7 times strength under the conditions of slip rate: 40% and oil temperature: 80 ° C. is 3250 MPa or more. Carburizing steel.
Record
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] − ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)
Carburizing heat treatment conditions Carburizing: 930 ° C, 7 hours, carbon potential: 0.8%
Tempering: 180 ° C, 1 hour
請求項1において、前記鋼は、さらに、質量%で、
Cu:1.0%以下、
Ni:0.5%以下、
Mo:0.5%以下、
V:0.5%以下および
Nb:0.06%以下
のうちから選んだ1種または2種以上を含有する浸炭用鋼。
The steel according to claim 1, further comprising, by mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
V: 0.5% or less and
Nb: Carburizing steel containing one or more selected from 0.06% or less.
質量%で、
C:0.1〜0.35%、
Si:0.01〜0.22%、
Mn:0.3〜1.5%、
Cr:1.35〜3.0%、
P:0.018%以下、
S:0.02%以下、
Al:0.015〜0.05%、
N:0.008〜0.015%および
O:0.0015%以下
を、下記式(1)、(2)および(3)を満足する範囲で含有し、残部はFeおよび不可避的不純物の組成になる鋼素材を、1160℃以上1220℃未満に加熱して熱間加工を施し、Ar点以上の温度域にて熱間加工を一旦終了して、450℃以下まで冷却し、次いで900℃超970℃以下の温度に再加熱して熱間加工を再開し、再加熱後における総圧下率70%以上の条件にて熱間加工を終了したのち、800〜500℃の温度域を0.1〜1.0℃/sの速度で冷却し、鋼組織におけるフェライトとパーライトとの合計の組織分率が85%以上で、かつフェライトの平均粒径が25μm以下とし、さらに下記の条件で浸炭熱処理後の、回転数:1800rpmでの10 7 回時間強度で評価した回転曲げ疲労強度が806MPa以上で、かつ前記浸炭熱処理後の、すべり率:40%、油温:80℃の条件での10 7 回時間強度で評価した面疲労強度が3250MPa以上である、浸炭用鋼の製造方法。

3.1≧{([%Si]/2)+[%Mn]+[%Cr]}≧2.2 ---(1)
[%C]−([%Si]/2)+([%Mn]/5)+2[%Cr]≧3.0 ---(2)
2.5 ≧ [%Al]/[%N] ≧ 1.7 ---(3)
但し、[%M]は、元素Mの含有量(質量%)
・浸炭熱処理条件
浸炭:930℃、7時間、カーボンポテンシャル:0.8%
焼戻し:180℃,1時間
% By mass
C: 0.1 to 0.35%,
Si: 0.01 to 0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%,
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
A steel material containing N: 0.008 to 0.015% and O: 0.0015% or less in a range satisfying the following formulas (1), (2) and (3), with the balance being a composition of Fe and inevitable impurities, Heated to 1160 ° C or higher and lower than 1220 ° C to perform hot working, once finished hot working in a temperature range of 3 or more points of Ar, cooled to 450 ° C or lower, then over 900 ° C to 970 ° C or lower After re-heating, the hot working is resumed, and after the hot working is finished under the condition of the total reduction ratio of 70% or more after the re-heating, the temperature range of 800-500 ° C is 0.1-1.0 ° C / s. The total structural fraction of ferrite and pearlite in the steel structure is 85% or more and the average grain size of ferrite is 25 μm or less, and after carburizing heat treatment under the following conditions, at a rotation speed of 1800 rpm Rotating bending fatigue strength evaluated by 10 7 times time strength of 806 MPa or more, slip rate after the carburizing heat treatment: 40%, oil temperature: 8 A method for producing carburized steel, wherein the surface fatigue strength evaluated by 10 7 time strength at 0 ° C is 3250 MPa or more .
Record
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] − ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)
・ Carburizing heat treatment conditions
Carburization: 930 ° C, 7 hours, carbon potential: 0.8%
Tempering: 180 ° C, 1 hour
請求項3において、前記鋼素材は、さらに、質量%で、
Cu:1.0%以下、
Ni:0.5%以下、
Mo:0.5%以下、
V:0.5%以下および
Nb:0.06%以下
のうちから選んだ1種または2種以上を含有する浸炭用鋼の製造方法。
In Claim 3, the steel material is further in mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
V: 0.5% or less and
Nb: A method for producing carburizing steel containing one or more selected from 0.06% or less.
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