JP2009263767A - Method for manufacturing high-strength case hardened steel part - Google Patents

Method for manufacturing high-strength case hardened steel part Download PDF

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JP2009263767A
JP2009263767A JP2009052384A JP2009052384A JP2009263767A JP 2009263767 A JP2009263767 A JP 2009263767A JP 2009052384 A JP2009052384 A JP 2009052384A JP 2009052384 A JP2009052384 A JP 2009052384A JP 2009263767 A JP2009263767 A JP 2009263767A
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steel
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fatigue strength
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JP5381171B2 (en
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Keiichi Maruta
慶一 丸田
Hideto Kimura
秀途 木村
Nobutaka Kurosawa
伸隆 黒澤
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JFE Steel Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing high-strength case hardened steel parts which, when worked to gears, are higher in bending fatigue strength of dedendum than the conventional gears, are further excellent in face pressure fatigue strength as well, and even more, enable mass production. <P>SOLUTION: The steel raw material consists of texture containing, by mass%, 0.1 to 0.35% C, 0.05 to 1.1% Si, 0.5 to 2.0% Mn, 0.5 to 2.5% Cr, ≤0.015% P, ≤0.02% S, 0.02 to 0.05% Al, 0.008 to 0.02% N, and ≤0.0015% O within a range satisfying the following formulas (1), (2), consisting of the balance Fe and inevitable impurities. In addition, the maximum diameter of oxide-based non-metallic inclusions is ≤19 μm. The steel raw material is subjected to forging or machining, then to carburizing treatment or carbonitriding treatment to make the C content of the surface layer 0.7 to 0.9%, and further to perform shot peening of an arc height: 0.1 to 0.5 mmN such that the face roughness of a surface layer is made 0.5 to 1.5 μm in arithmetic average roughness.(1): 3.0≥ä([%Si]/2)+[%Mn]+[%Cr]}≥2.4.(2): 2.5≥[%Al]/[%N]≥1.8. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

この発明は、自動車や各種産業機器等に使用される、高い曲げ疲労強度および面圧疲労強度を有する高強度歯車等に用いて好適な高強度肌焼鋼部品の製造方法に関するものである。   The present invention relates to a method for producing a high-strength case-hardened steel part suitable for use in high-strength gears having high bending fatigue strength and surface pressure fatigue strength, which are used in automobiles and various industrial equipment.

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、またエンジンの高出力化に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 as the output of the engine has increased. Yes. The durability of the 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 SCM420H、SCM822H等により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 by JIS SCM420H, SCM822H, etc., 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%.

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

しかしながら、上述した特許文献1,2にはいずれも、以下に述べるような問題があった。
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、逆に焼戻し軟化抵抗が低下して、破壊の発生が歯元から歯面側に移行する結果、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るという問題があった。
However, both of Patent Documents 1 and 2 described above have the following problems.
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 resistance to temper softening decreases and the occurrence of fracture shifts from the tooth base to the tooth surface. As a result, temper softening due to frictional heat on the tooth surface cannot be suppressed, and the surface softens. There has been a problem that it is likely to occur.

また、特許文献2では、焼戻し軟化抵抗を上げるために逆にSi等を添加し、一方粒界酸化の進行を抑制するために浸炭工法を真空浸炭あるいはプラズマ浸炭等に限定しているが、この方法は、製造コストが嵩むという不利があり、量産化には不適であった。   In Patent Document 2, Si or the like is added to increase the temper softening resistance, while the carburizing method is limited to vacuum carburizing or plasma carburizing to suppress the progress of grain boundary oxidation. The method is disadvantageous in that the production cost increases, and is not suitable for mass production.

本発明は、上記の実状に鑑み開発されたもので、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に用いて好適で、しかも量産化が可能な高強度肌焼鋼部品の有利な製造方法を提案することを目的とする。   The present invention has been developed in view of the above circumstances, and is suitable for use in high-strength gears and the like that have higher bending fatigue strength at the root than conventional gears and also have excellent surface fatigue properties. It is an object of the present invention to propose an advantageous method for producing a high-strength case-hardened steel part that can be made into a steel.

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,Crを増量することによって、焼戻し軟化抵抗を高めると共に、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,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。さらに、この酸化層は、剥離し易くなり、ショットピーニングによりほぼ剥離して無くなるために、影響がでない。
c)上記aおよびbで述べたとおり、Si,Mn,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,Crについて、その含有量を厳密に制御する必要がある。
d)ショットピーニングによる表面の圧縮残留応力の増加によって高疲労強度を得るためには、鋼中の酸素量だけでなく、酸化物系の介在物の最大値を小さくする必要がある。
e)ショットピーニングにより転動疲労寿命を向上させるためには、適性なアークハイト値とショットピーニングによって生じる表面粗さの適性値が存在する。
本発明は上記の知見に立脚するものである。
As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
a) By increasing the amount of Si, Mn, and Cr in the steel material, the temper softening resistance is increased, and if the softening due to heat generation on the gear contact surface is suppressed, the generation of cracks in the tooth surface that occurs when the gear is driven can be suppressed. it can.
b) For grain boundary oxide layers that can be the starting point of bending fatigue and fatigue cracks, by adding more than a certain amount of Si, Mn, Cr, the growth direction of the grain boundary oxide layer is changed from the depth direction to the surface density increasing direction. 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. Furthermore, this oxide layer is easy to peel off, and is almost unpeeled by shot peening, so there is no influence.
c) As described in the above a and b, Si, Mn, and Cr are effective for improving the temper softening resistance and controlling the grain boundary oxide layer. In order to achieve both of these effects, Si, Mn, It is necessary to strictly control the content of Cr.
d) In order to obtain high fatigue strength by increasing the surface compressive residual stress by shot peening, it is necessary to reduce not only the amount of oxygen in the steel but also the maximum value of oxide inclusions.
e) In order to improve the rolling fatigue life by shot peening, there is an appropriate arc height value and an appropriate value of surface roughness caused by shot peening.
The present invention is based on the above findings.

すなわち、本発明は、質量%で、C:0.1〜0.35%、Si:0.05〜1.1%、Mn:0.5〜2.0%、Cr:0.5〜2.5%、P:0.015%以下、S:0.02%以下、Al:0.02〜0.05%、N:0.008〜0.02%およびO:0.0015%以下を、下記式(1),(2)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り、かつ酸化物系非金属介在物の最大径が19μm 以下の組織になる鋼素材を、鍛造または機械加工した後、表層のC含有量を0.7〜0.9質量%とする浸炭処理または浸炭窒化処理を施し、さらにアークハイト:0.1〜0.5mmNのショットピーニングを施して、表層の面粗度を算術平均粗さRaで0.5〜1.5μmとすることを特徴とする高強度肌焼鋼部品の製造方法である。

3.0 ≧{([%Si]/2)+[%Mn]+[%Cr]}≧ 2.4 --- (1)
2.5 ≧ [%Al]/[%N] ≧ 1.8 --- (2)
但し、[%M]は、元素Mの含有量(質量%)
That is, the present invention is, in mass%, C: 0.1 to 0.35%, Si: 0.05 to 1.1%, Mn: 0.5 to 2.0%, Cr: 0.5 to 2.5%, P: 0.015% or less, S: 0.02% or less, Al: 0.02 to 0.05%, N: 0.008 to 0.02% and O: 0.0015% or less are contained within a range satisfying the following formulas (1) and (2), and the balance is Fe and inevitable impurities composition. In addition, after forging or machining a steel material having a maximum diameter of oxide-based nonmetallic inclusions of 19 μm or less, carburization or carbonitriding with a C content of 0.7 to 0.9% by mass is performed. Further, arc height: 0.1 to 0.5 mmN shot peening is performed, and the surface roughness of the surface layer is 0.5 to 1.5 μm in terms of arithmetic average roughness Ra. .
Record
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.8 --- (2)
However, [% M] is the content of element M (% by mass)

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なみならず、歯面の面圧疲労特性に優れた高強度肌焼鋼部品を、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, not only the bending fatigue characteristics of the tooth root but also the high-strength case-hardened steel parts excellent in the tooth surface fatigue characteristics can be obtained under mass production. it can.

以下、本発明を具体的に説明する。
まず、本発明において、鋼片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心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%を超えると心部の靭性が低下するので、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。
The present invention will be specifically described below.
First, the reason why the component composition of the steel slab 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 central part by quenching after carburizing treatment, 0.1% or more of C is required, but if the content exceeds 0.35%, the toughness of the core part decreases, so the C content is 0.1 to 0.35% It was limited to the range. Preferably it is 0.1 to 0.3% of range.

Si:0.05〜1.1%
Siは、本発明において最も重要な元素である。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0.05%の添加が不可欠である。好ましくは0.3%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り、過剰な添加はAc3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心部でフェライトが出現し易くなり強度の低下を招く。また、過剰な添加は浸炭前の鋼材を硬化させ、切削性を劣化させる不利もある。この点、Si量が1.1%以下であれば、上記のような弊害は生じないので、Si量は0.05〜1.1%の範囲に限定した。好ましくは0.3〜1.1%の範囲である。
Si: 0.05-1.1%
Si is the most important element in the present invention. Si is an element that increases the softening resistance in the temperature range of 200 to 300 ° C. that is considered to be reached during the rolling of gears and the like, and at least 0.05% addition is indispensable for exerting the effect. Preferably 0.3% or more is added. However, Si, on the other hand, is a ferrite stabilizing element, and excessive addition raises the Ac 3 transformation point, and ferrite tends to appear in the core with a low carbon content in the normal quenching temperature range, resulting in a decrease in strength. Invite. Excessive addition also has the disadvantage of hardening the steel before carburizing and degrading the machinability. In this respect, if the Si content is 1.1% or less, the above-described adverse effects do not occur, so the Si content is limited to a range of 0.05 to 1.1%. Preferably it is 0.3 to 1.1% of range.

Mn:0.5〜2.0%
Mnは、焼入性に有効な元素であり、少なくとも0.5%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2.0%とした。好ましくは0.5〜1.5%の範囲である。
Mn: 0.5-2.0%
Mn is an element effective for hardenability, and requires addition of at least 0.5%. 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 2.0%. Preferably it is 0.5 to 1.5% of range.

Cr:0.5〜2.5%
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0.5%に満たないとその添加効果に乏しく、一方2.5%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0.5〜2.5%の範囲に限定した。好ましくは0.5〜2.0%の範囲である。
Cr: 0.5-2.5%
Cr is an element effective for improving not only hardenability but also temper softening resistance. However, if the content is less than 0.5%, the effect of addition is poor. On the other hand, if it exceeds 2.5%, the effect of increasing softening resistance is The Cr content is limited to the range of 0.5 to 2.5% because it becomes saturated and rather easily forms a carburized abnormal layer. Preferably it is 0.5 to 2.0% of range.

P:0.015%以下
Pは、結晶粒界に偏析し、浸炭層および心部の靭性を低下させるので、その混入は低いほど望ましいが、0.015%までは許容される。
P: 0.015% 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 0.015% is acceptable.

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ので、上限を0.02%とした。
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%.

Al:0.02〜0.05%
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0.02%以上の添加を必要とするが、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0.02〜0.05%の範囲に限定した。
Al: 0.02 to 0.05%
Al is an element that combines with N to form AlN and contributes to the refinement of austenite crystal grains. To obtain this effect, 0.02% or more of addition is required, but the content is 0.05%. In order to promote the formation of Al 2 O 3 inclusions that are harmful to fatigue strength, the Al content is limited to a range of 0.02 to 0.05%.

N:0.008〜0.02%
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0.02%とする。好ましくは0.01〜0.015%の範囲である。
N: 0.008 to 0.02%
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 0.02%. Preferably it is 0.01 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。低いほど望ましいが、0.0015%までは許容される。
O: 0.0015% or less O is an element that exists as an oxide inclusion in steel and impairs fatigue strength. A lower value is preferable, but 0.0015% is acceptable.

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、Si,Mn,Cr,AlおよびNについては、次式(1),(2)の関係を満足させることが重要である。
3.0 ≧{([%Si]/2)+[%Mn]+[%Cr]}≧ 2.4 --- (1)
2.5 ≧ [%Al]/[%N] ≧ 1.8 --- (2)
但し、[%M]は、元素Mの含有量(質量%)
Although the proper composition range of the basic component of the present invention has been described above, in the present invention, it is not sufficient that each element simply satisfies the above range. For Si, Mn, Cr, Al, and N, It is important to satisfy the relationship of equations (1) and (2).
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.8 --- (2)
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2.4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、一方3.0を超えると上記の改善効果が飽和するだけでなく、加工性の劣化を招く。
また、上掲(2)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、(2)式の値が1.8に満たないと微細化効果に乏しく、一方2.5を超えると結晶粒が容易に粗大化するだけでなく、固溶Al,Nに起因して加工性の低下を招く。
The above formula (1) is a factor affecting the hardenability and temper softening resistance.If the formula (1) is less than 2.4, the improvement effect of the hardenability and temper softening resistance is not sufficient, while 3.0 If it exceeds, not only the above improvement effect is saturated, but also the workability is deteriorated.
The above formula (2) is a factor that affects the refinement of austenite crystal grains, and if the value of formula (2) is less than 1.8, the effect of miniaturization is poor, while if it exceeds 2.5, the grain becomes easy In addition to coarsening, the workability is lowered due to the solid solution Al and N.

さらに、本発明では、素材中に存在する酸化物系介在物の大きさを制御することも重要である。
すなわち、鋼中の酸素量を下げても、鋼中の酸化物の最大径が大きいと面圧疲労強度の向上は望めないため、素材中に存在する酸化物系介在物の最大径は19μm 以下に制限する。
なお、酸化物系介在物の大きさを上記の範囲に調整するには、RH脱ガス工程が重要で、このときの処理時間を50分以上とすることが好ましい。
In the present invention, it is also important to control the size of oxide inclusions present in the material.
In other words, even if the amount of oxygen in the steel is reduced, if the maximum diameter of the oxide in the steel is large, improvement in surface pressure fatigue strength cannot be expected, so the maximum diameter of oxide inclusions present in the material is 19 μm or less. Limit to.
In order to adjust the size of the oxide inclusions within the above range, the RH degassing step is important, and the treatment time at this time is preferably 50 minutes or more.

表層のC濃度:0.7〜0.9%
浸炭により表層のC含有量は高くなる。その結果、表面硬度が上がり、曲げ疲労および面圧疲労強度が向上する。しかしながら、鋼板表層のC濃度が0.7%未満ではその効果が十分ではなく、一方0.9%を超えると、表層付近の粒界にセメンタイトが析出し、疲労の起点となるため、曲げ疲労強度および面圧疲労強度の低下を招く。それ故、浸炭後の表層C量は0.7〜0.9%の範囲に限定した。
なお、表層とは、鋼板表面から50μm 深さ位置までを意味する。
Surface C concentration: 0.7-0.9%
Carburization increases the C content of the surface layer. As a result, the surface hardness is increased, and bending fatigue and surface fatigue strength are improved. However, if the C concentration of the steel sheet surface layer is less than 0.7%, the effect is not sufficient. On the other hand, if it exceeds 0.9%, cementite precipitates at the grain boundaries near the surface layer and becomes the starting point of fatigue. Reduces fatigue strength. Therefore, the surface layer C amount after carburizing is limited to a range of 0.7 to 0.9%.
The surface layer means from the steel plate surface to a depth of 50 μm.

ショットピーニングのアークハイト:0.1〜0.5 mmN
ショットピーニングは、表層付近に圧縮残留応力を付与して曲げ疲労強度および、面圧疲労強度をさらに上昇させる効果がある。この処理を行う場合、アークハイトが0.1mmN未満では圧縮残留応力の付与が少なすぎて、十分な曲げ疲労強度および面圧疲労強度の改善効果が得られず、一方0.5mmNを超えると表面状態が悪化し、曲げ疲労強度および面圧疲労強度が低下する場合があるため、ショットピーニングのアークハイトは0.1〜0.5 mmNの範囲に限定した。
Shot peening arc height: 0.1 to 0.5 mmN
Shot peening has the effect of further increasing bending fatigue strength and surface fatigue strength by applying compressive residual stress in the vicinity of the surface layer. When this treatment is performed, if the arc height is less than 0.1 mmN, compressive residual stress is applied too little, and sufficient improvement of bending fatigue strength and surface fatigue strength cannot be obtained. Since the bending fatigue strength and the contact pressure fatigue strength may be deteriorated, the arc height of shot peening is limited to the range of 0.1 to 0.5 mmN.

ショットピーニング後の表面粗さ:算術平均粗さRaで0.5〜1.5μm
表面状態が粗すぎると応力集中源として作用し、曲げ疲労強度、面圧疲労強度が低下するためRaの上限を1.5μm とする。一方、表面状態が良すぎると高面圧下での疲労試験時に油による潤滑が不十分となり面圧疲労強度が低下するため0.5μm を下限とする。
Surface roughness after shot peening: 0.5 to 1.5 μm in arithmetic average roughness Ra
If the surface condition is too rough, it will act as a stress concentration source and the bending fatigue strength and surface fatigue strength will decrease, so the upper limit of Ra is set to 1.5 μm. On the other hand, if the surface condition is too good, lubrication with oil becomes insufficient during a fatigue test under high surface pressure, and the surface pressure fatigue strength decreases, so 0.5 μm is the lower limit.

なお、本発明の鋼素材の製造条件については、特に制限はないが、好適な製造条件については次のとおりである。
前記した好適成分組成になる鋼素材を、1100℃以上に加熱後、800〜950℃の温度域で圧下率:30〜80%程度の圧延を施したのち、800〜500℃の温度域を1.0℃/s以下の速度で冷却する。
なお、素材中の酸化物系介在物の最大径を19μm 以下に抑制するには、RH脱ガス工程において、処理時間を50分以上とするのが望ましいことは、前述したとおりである。
In addition, although there is no restriction | limiting in particular about the manufacturing conditions of the steel raw material of this invention, It is as follows about suitable manufacturing conditions.
After heating the steel material having the above-mentioned preferred component composition to 1100 ° C. or higher, rolling it at a temperature of 800 to 950 ° C. and rolling at a reduction ratio of about 30 to 80%, and then setting the temperature range of 800 to 500 ° C. to 1.0 Cool at a rate of ℃ / s or less.
As described above, in order to suppress the maximum diameter of the oxide inclusions in the material to 19 μm or less, it is desirable that the treatment time is set to 50 minutes or longer in the RH degassing step.

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た。その際に、O量および酸化物の大きさを変えるために、真空度を表1に示すように変化させた。ついで、150mm角に鍛伸後、ダミービレットに溶接し、加熱温度:1100℃で熱間圧延を施し、直径:30〜50mmの棒鋼に圧延した。この圧延ままの棒鋼から回転曲げ疲労試験片と面圧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た。これらの試験片に、930℃、7時間の浸炭処理を施した。表層C含有量は、浸炭時の雰囲気のカーボンポテンシャルを変化させて制御した。浸炭後にそのまま60℃の油温中に入れ、その後に180℃,1時間の焼戻しをオイルバスにて施した。
酸化物の最大径について調べた結果を表1に併記する。
また、得られた肌焼鋼の表層C含有量、ショットピーニングアークハイト値、表面粗さ回転曲げ疲労強度、面圧疲労強度、旧オーステナイト粒径、および被削性について調べた結果を、表2に示す。
Steels having various component compositions shown in Table 1 were melted in a 100 kg vacuum melting furnace. At that time, in order to change the amount of O and the size of the oxide, the degree of vacuum was changed as shown in Table 1. Next, after forging to 150 mm square, it was welded to a dummy billet, hot rolled at a heating temperature of 1100 ° C., and rolled into a steel bar having a diameter of 30 to 50 mm. A rotating bending fatigue test piece and a roller pitching test piece for evaluating the surface pressure fatigue strength were processed from the rolled steel bar. These test pieces were carburized at 930 ° C. for 7 hours. The surface layer C content was controlled by changing the carbon potential of the atmosphere during carburizing. After carburizing, it was put in an oil temperature of 60 ° C. and then tempered at 180 ° C. for 1 hour in an oil bath.
The results of examining the maximum diameter of the oxide are also shown in Table 1.
Table 2 shows the results of examining the surface layer C content, shot peening arc height value, surface roughness rotational bending fatigue strength, surface pressure fatigue strength, prior austenite grain size, and machinability of the obtained case hardening steel. Shown in

評価方法は次のとおりである。
酸化物の最大径については、圧延材のL断面を10×10mm2に切り出した試料を30個準備し、研磨後に各試料の最大酸化物径を測定した。そして、これら30個のデータから極値統計法によって30000mm2とした場合の予測した酸化物径を最大酸化物径とした。
粒界酸化深さは、浸炭処理後の試験片の表面を倍率:400倍の光学顕微鏡で10視野観察し、各視野での最大粒界酸化深さを測定し、10視野の平均値をとった。
旧オーステナイト粒径は、焼入れ焼戻しした試験片のつかみ部から倍率:1000倍の光学顕微鏡写真を5視野撮影し、切断法により求めた。
回転曲げ疲労試験は、小野式回転曲げ疲労試験機を用い、回転数:1800rpmで実施し、107回時間の限界強度で評価した。
また、面圧疲労強度は、ローラピッチング試験により求めた。ローラピッチング試験は、すべり率:40%、油温:80℃の条件で106回時間の限界強度で評価した。
ショットピーニングは、各水準中に記してあるアークハイト値(硬さHRC46の幅:19mm、長さ:75mmの板にショットを打ち付けた際の板の反り量で規定)になるようにショット粒のサイズを変えることで、各アークハイト値での試験片表面粗さを調整した。
被削性試験については、得られた素材をφ50mmに鍛伸した丸棒を用いて外周切削試験を行った。工具はP20種を使用し、切り込み:2mm、切削速度:200mm/min、送り:0.25mm/rev、潤滑なしの条件で切削し、切削時間:900秒の段階での工具逃げ面摩耗の幅を実体顕微鏡で観察・測定した。
The evaluation method is as follows.
For the maximum oxide diameter, 30 samples were prepared by cutting the L section of the rolled material into 10 × 10 mm 2, and the maximum oxide diameter of each sample was measured after polishing. Then, the predicted oxide diameter in the case of 30000 mm 2 by the extreme value statistical method from these 30 data was taken as the maximum oxide diameter.
The grain boundary oxidation depth was measured by observing the surface of the test specimen after carburizing treatment with an optical microscope with a magnification of 400 times, measuring the maximum grain boundary oxidation depth in each field, and taking the average of the 10 fields. It was.
The prior austenite grain size was obtained by cutting an optical microscope photograph of 1000 magnifications from a grip portion of a quenched and tempered test piece and cutting it.
The rotating bending fatigue test was performed using an Ono type rotating bending fatigue tester at a rotation speed of 1800 rpm and evaluated with a limit strength of 10 7 times.
Further, the surface fatigue strength was obtained by a roller pitching test. The roller pitching test was evaluated with a limit strength of 10 6 times under the conditions of slip ratio: 40% and oil temperature: 80 ° C.
Shot peening is achieved by setting the shot grain size to the arc height value (specified by the amount of warpage of the shot when hitting a shot with a hardness HRC46 width: 19 mm and length: 75 mm). The test piece surface roughness at each arc height value was adjusted by changing the size.
As for the machinability test, a peripheral cutting test was performed using a round bar obtained by forging the obtained material to 50 mm. The tool uses P20 type, cutting: 2mm, cutting speed: 200mm / min, feed: 0.25mm / rev, cutting without lubrication, cutting time: width of tool flank wear at 900 seconds Observed and measured with a stereomicroscope.

表2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、粒界酸化深さが浅く、また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 2, all of the inventive examples obtained according to the present invention have a shallow grain boundary oxidation depth, and are superior in rotational bending fatigue strength and surface pressure fatigue strength compared to the comparative examples.

Claims (1)

質量%で、C:0.1〜0.35%、Si:0.05〜1.1%、Mn:0.5〜2.0%、Cr:0.5〜2.5%、P:0.015%以下、S:0.02%以下、Al:0.02〜0.05%、N:0.008〜0.02%およびO:0.0015%以下を、下記式(1),(2)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り、かつ酸化物系非金属介在物の最大径が19μm 以下の組織になる鋼素材を、鍛造または機械加工した後、表層のC含有量を0.7〜0.9質量%とする浸炭処理または浸炭窒化処理を施し、さらにアークハイト:0.1〜0.5mmNのショットピーニングを施して、表層の面粗度を算術平均粗さRaで0.5〜1.5μmとすることを特徴とする高強度肌焼鋼部品の製造方法。

3.0 ≧{([%Si]/2)+[%Mn]+[%Cr]}≧ 2.4 --- (1)
2.5 ≧ [%Al]/[%N] ≧ 1.8 --- (2)
但し、[%M]は、元素Mの含有量(質量%)
In mass%, C: 0.1 to 0.35%, Si: 0.05 to 1.1%, Mn: 0.5 to 2.0%, Cr: 0.5 to 2.5%, P: 0.015% or less, S: 0.02% or less, Al: 0.02 to 0.05% , N: 0.008 to 0.02% and O: 0.0015% or less in a range satisfying the following formulas (1) and (2), the balance being the composition of Fe and inevitable impurities, and an oxide-based nonmetal A steel material having a structure with a maximum diameter of 19 μm or less is forged or machined, and then subjected to carburizing or carbonitriding to make the C content of the surface layer 0.7 to 0.9 mass%, and arc height: 0.1 A method for producing a high-strength case-hardened steel part, characterized by performing shot peening of .about.0.5 mmN and adjusting the surface roughness of the surface layer to an arithmetic average roughness Ra of 0.5 to 1.5 .mu.m.
Record
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.8 --- (2)
However, [% M] is the content of element M (% by mass)
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JP2013072104A (en) * 2011-09-27 2013-04-22 Sanyo Special Steel Co Ltd Steel excellent in toughness and wear resistance
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JP2002322536A (en) * 2001-04-23 2002-11-08 Aichi Steel Works Ltd High strength gear having excellent bending strength of dedendum and pitting resistance, and producing method therefor
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JP2012197472A (en) * 2011-03-18 2012-10-18 Sanyo Special Steel Co Ltd Steel stock for machine structure with small heat treatment deformation
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JP2013040364A (en) * 2011-08-12 2013-02-28 Nippon Steel & Sumitomo Metal Corp Rolled steel bar or wire rod for hot forging
JP2013072104A (en) * 2011-09-27 2013-04-22 Sanyo Special Steel Co Ltd Steel excellent in toughness and wear resistance
JP2016204752A (en) * 2015-04-22 2016-12-08 Jfeスチール株式会社 Case-hardened steel and production method thereof

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