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JP2008179848A - Steel for gear having superior impact fatigue resistance and contact fatigue strength, and gear using the same - Google Patents

Steel for gear having superior impact fatigue resistance and contact fatigue strength, and gear using the same Download PDF

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JP2008179848A
JP2008179848A JP2007013247A JP2007013247A JP2008179848A JP 2008179848 A JP2008179848 A JP 2008179848A JP 2007013247 A JP2007013247 A JP 2007013247A JP 2007013247 A JP2007013247 A JP 2007013247A JP 2008179848 A JP2008179848 A JP 2008179848A
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gear
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fatigue strength
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JP4938474B2 (en
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Kazuaki Fukuoka
和明 福岡
Kunikazu Tomita
邦和 冨田
Tetsuo Shiragami
哲夫 白神
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JFE Bars and Shapes Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for a carburized gear suitable for use in automobiles and various types of industrial machines, which is required to have excellent impact fatigue characteristics and superior contact fatigue strength, and to provide a gear using the same. <P>SOLUTION: The steel has a composition which comprises, by mass%, 0.15 to 0.30% C, more than 0.50 but less than 1.50% Si, 0.50% or more Mn, 0.015% or less P, 0.30% or less Cu, less than 2.00% Cr, less than 0.50% Mo, 0.006 to 0.110% Al, less than 0.02% Ti, 0.0004% or more B, 0.0065% or less N, at least one element of Nb and V, as needed, while satisfying the expression (1): I(=14/27×Al+14/10.8×B-N)≥0.03, wherein Al, B and N represent contents (mass%), and the balance Fe with unavoidable impurities. Furthermore, the steel has such an S value after having been carburized, quenched and tempered as to satisfy the expression (2); 170≤S=E/2xD-P/2≤250, wherein (E) represents the effective depth (mm) of a hardened layer, (D) represents the hardness (HV) of the inner part and (P) represents the depth (μm) of an oxidized layer in a grain boundary. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

本発明は、歯車用鋼およびそれを用いた歯車に関し、特に優れた耐衝撃疲労特性と面疲労強度が要求される自動車および各種産業機械用として好適なものに関する。   The present invention relates to a steel for gears and a gear using the same, and particularly relates to a steel suitable for automobiles and various industrial machines that require excellent impact fatigue resistance and surface fatigue strength.

自動車等に用いられている歯車は、近年、省エネルギー化による車体重量の軽量化に伴い小型化が要求されているが、一方ではエンジンの高出力化により負荷が増大しているため、耐久性の向上が課題とされている。   In recent years, gears used in automobiles and the like have been required to be downsized with the reduction of the weight of the vehicle body due to energy saving, but on the other hand, the load has increased due to the high output of the engine. Improvement is an issue.

一般的に歯車の耐久性は、歯の耐衝撃破壊、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定される。   In general, the durability of a gear is determined by impact resistance fracture of a tooth, bending fatigue fracture of a tooth root, and surface pressure fatigue fracture of a tooth surface.

衝撃的な応力がかかる部分、例えば自動車のデファレンシャル等で使用される歯車では、高い衝撃荷重により破壊が早期に起こる場合があるため、衝撃特性の向上が種々検討されている(特許文献1〜5)。   In gears used in impact stresses such as automobile differentials, destruction may occur early due to high impact loads, and various improvements in impact characteristics have been studied (Patent Documents 1 to 5). ).

すなわち、特許文献1には、浸炭層の靭性を向上するためにMoを添加し、浸炭層の粒界強度を低下させるMn、Cr、Pを少なくすることおよびMo/(10Si+100P+Mn+Cr)により求まる値の下限を規定することと、浸炭硬化層深さの範囲を規定することにより衝撃特性を向上させることが提案されている。   That is, in Patent Document 1, Mo is added in order to improve the toughness of the carburized layer, Mn, Cr, and P are decreased to reduce the grain boundary strength of the carburized layer, and Mo / (10Si + 100P + Mn + Cr) is obtained. It has been proposed to improve impact properties by defining a lower limit and by defining a range of carburized hardened layer depth.

特許文献2には焼入れの冷却速度範囲を成分組成に応じた適正範囲に制御することにより、歯車の内部をマルテンサイトとベイナイトの混合組織として靭性を向上させることが提案されている。   Patent Document 2 proposes that the toughness is improved by controlling the quenching cooling rate range to an appropriate range according to the component composition so that the inside of the gear is a mixed structure of martensite and bainite.

特許文献3には、特許文献2と同様にミクロ組織を規定するもので、ミクロ組織をマルテンサイトと、内部の靭性を向上させるトルースタイトの混合組織とし、MnとCrの添加量の範囲を規定し、Mo添加量を規制してトルースタイトの量を制限することで内部硬度の低下を抑える方法が提案されている。   In Patent Document 3, the microstructure is defined in the same manner as Patent Document 2, and the microstructure is a mixed structure of martensite and troostite that improves internal toughness, and the range of addition amounts of Mn and Cr is defined. And the method of suppressing the fall of internal hardness by restrict | limiting Mo addition amount and restrict | limiting the amount of troostite is proposed.

さらに、特許文献4には特許文献3記載の成分組成にMoを添加した鋼が提案されている。特許文献5には成分組成においてMn、Cr、Moの複合添加量を制限して鋼材の硬さを抑え、冷間鍛造性を損なうこと無く衝撃特性を向上させた傘歯車用鋼材が提案されている。
特公平7−100840号公報 特許3094856号公報 特許第3329177号公報 特許第3733504号公報 特許第3319684号公報
Further, Patent Document 4 proposes a steel in which Mo is added to the component composition described in Patent Document 3. Patent Document 5 proposes a steel material for bevel gears in which the composite additive amount of Mn, Cr and Mo is limited in the component composition to suppress the hardness of the steel material and the impact characteristics are improved without impairing the cold forgeability. Yes.
Japanese Examined Patent Publication No. 7-100840 Japanese Patent No. 3094856 Japanese Patent No. 3329177 Japanese Patent No. 3733504 Japanese Patent No. 3319684

しかしながら、特許文献1記載の方法では、衝撃特性を向上出来たとしても、高価な合金であるMoを多量に添加させるか、Moを多く入れない場合には浸炭時間を大幅に延長させることが必要で、製品コストまたは製造コストの大幅な増加を招いてしまう。   However, in the method described in Patent Document 1, even if the impact characteristics can be improved, it is necessary to add a large amount of Mo, which is an expensive alloy, or to significantly extend the carburizing time if a large amount of Mo is not added. As a result, the product cost or the manufacturing cost is greatly increased.

特許文献2記載の方法では、ミクロ組織中にベイナイト組織を含むので靭性を向上させて衝撃値を高めることは可能である。   In the method described in Patent Document 2, since the bainite structure is included in the microstructure, it is possible to improve the toughness and increase the impact value.

しかし、内部にベイナイト組織が含まれると、内部硬さは低下するために歯車が衝撃で変形しやすくなり、衝撃力が繰り返されると破損することが懸念される。   However, if a bainite structure is contained inside, the internal hardness is reduced, so that the gears are easily deformed by impact, and there is a concern that the gear may be damaged when the impact force is repeated.

特許文献3記載の方法では、MnとCrの複合添加量を指定し、Mo添加量を規制するので、表層付近で発生する粒界酸化が多くなり、Mn,Crの酸化物が形成されるために焼入れ性が低下し、表層に不完全焼入れ層が形成される。   In the method described in Patent Document 3, since the composite addition amount of Mn and Cr is specified and the addition amount of Mo is regulated, the grain boundary oxidation generated near the surface layer increases, and oxides of Mn and Cr are formed. Therefore, the hardenability is lowered and an incompletely hardened layer is formed on the surface layer.

そのため、内部硬度が確保出来たとしても表層の硬さ低下による表層からの破壊が発生しやすくなり、結果的に衝撃疲労を含むすべての疲労強度が低下してしまう。   Therefore, even if the internal hardness can be ensured, the surface layer is likely to be broken due to a decrease in the surface layer hardness, and as a result, all fatigue strengths including impact fatigue are reduced.

特許文献4記載の方法の場合、Moを添加してもトルースタイトにより歯車内部の硬度低下が発生するため、衝撃特性が向上したとしても内部起因の曲げ疲労などの疲労強度が低下する。特許文献5記載の方法の場合、歯車は熱間鍛造で整形されるので硬度が低く、衝撃以外の疲労強度が低下する。   In the case of the method described in Patent Document 4, even if Mo is added, the hardness inside the gear is reduced due to the true tooth. Therefore, even if the impact characteristics are improved, the fatigue strength such as internal bending fatigue is reduced. In the case of the method described in Patent Document 5, since the gear is shaped by hot forging, the hardness is low, and fatigue strength other than impact is reduced.

そこで、本発明は、歯車としての特性を劣化させずに、優れた衝撃疲労強度と面疲労強度が得られる低コストな歯車用鋼およびそれを用いた歯車を提供することを目的とする。   Therefore, an object of the present invention is to provide a low-cost steel for gears that can obtain excellent impact fatigue strength and surface fatigue strength without deteriorating the characteristics as a gear, and a gear using the same.

本発明者等は上記課題を解決するため、生産コストとして安価な、従来からの方法による浸炭処理により、優れた衝撃疲労強度と面疲労強度が得られる低コストな成分組成の歯車用鋼について鋭意検討を行った。   In order to solve the above-mentioned problems, the present inventors have earnestly studied a steel for gears having a low-cost component composition that can provide excellent impact fatigue strength and surface fatigue strength by carburizing by a conventional method, which is inexpensive as a production cost. Study was carried out.

その結果、歯車の場合、繰返される衝撃応力に対して靭性を向上させる他に衝撃力による変形を起こさない耐力を有することが必要で、耐力が低い場合、靭性を向上させても実際の耐久性が劣る結果になることを知見し、以下の成分設計指針を得た。
1.衝撃疲労特性の向上には旧オーステナイト粒界の強化が最も重要であり、Pを低減して旧オーステナイト粒界の脆化を抑制する。
2.さらに鋼中にBを固溶させて旧オーステナイト粒界に優先的に偏析させ、Pの粒界偏析を抑制する。
3.固溶Bを鋼中に存在させる場合、Bとの結合力の強いNをTiまたはAlで結合させるが、溶製時に析出するTiNは比較的大きく、鋭利で硬質な介在物のため、疲労の起点となり易く、面疲労強度および曲げ疲労強度が低下する。
4.Alを添加してB,Nとの平衡関係を利用してN固定をした場合、BN,AlNが鋼中に析出する。AlNは微細なため、結晶粒は微細化し、衝撃疲労強度が向上する。また、AlNは微細なために疲労の起点にはならない。よって、固溶Bの確保はAl−B−Nバランスの調整によるものを主とし、Ti添加によるものを従とする。
5.浸炭処理時の粒界酸化を抑制するSi量には最適範囲が存在する。
6.面疲労強度と衝撃疲労強度を共に向上させるには粒界酸化酸化と内部の硬さおよび浸炭硬度分布のバランスが大切であり、最も最適な範囲が存在する。
As a result, in the case of gears, in addition to improving toughness against repeated impact stresses, it is necessary to have a proof strength that does not cause deformation due to impact force. When the proof strength is low, even if toughness is improved, actual durability Was found to be inferior, and the following component design guidelines were obtained.
1. Strengthening of prior austenite grain boundaries is most important for improving impact fatigue properties, and P is reduced to suppress embrittlement of prior austenite grain boundaries.
2. Further, B is dissolved in the steel to preferentially segregate at the prior austenite grain boundaries, thereby suppressing P grain boundary segregation.
3. When solid solution B is present in the steel, N, which has a strong binding force with B, is bonded with Ti or Al. However, TiN that precipitates during melting is relatively large, sharp and hard inclusions, It tends to be a starting point, and surface fatigue strength and bending fatigue strength are reduced.
4). When Al is added and N is fixed using the equilibrium relationship with B and N, BN and AlN precipitate in the steel. Since AlN is fine, crystal grains are refined and impact fatigue strength is improved. Further, since AlN is fine, it does not become a starting point of fatigue. Therefore, the solid solution B is secured mainly by adjusting the Al—B—N balance, and subordinated by adding Ti.
5. There is an optimum range for the amount of Si that suppresses grain boundary oxidation during carburizing.
6). In order to improve both surface fatigue strength and impact fatigue strength, the balance between grain boundary oxidation and internal hardness and carburized hardness distribution is important, and the most optimal range exists.

本発明は得られた知見に更に検討を加えてなされたものですなわち本発明は、
1.mass%で、C:0.15〜0.30%、Si:0.50%超え〜1.50%未満、Mn:0.50%以上、P:0.015%以下、Cu:0.30%以下、Cr:2.00%以下、Mo:0.50%以下、Al:0.006〜0.110%以下、Ti:0.02%未満,B:0.0004%以上、N:0.0065%以下を含有し、(1)式を満足する残部がFeおよび不可避的不純物からなる成分組成を有し、浸炭焼入れ・焼戻し後に得られる有効硬化層深さ、内部硬さ、粒界酸化層深さについて(2)式のS値を満足する耐衝撃疲労特性、面疲労強度に優れた歯車用鋼。
I(=14/27×Al+14/10.8×B−N)≧0.03・・・(1)
但し、(1)式中のAl,B,Nは含有量(mass%)を示す。
170≦S(=E/2×D-P/2)≦250 ・・・(2)
但し、E:有効硬化層深さ(mm)、D:内部硬度(HV)、P:粒界酸化層深さ(μm)を示す。
2.更に、Nb:0.050%以下,V:0.030〜0.200%のいずれか1種以上を添加する1記載の耐衝撃疲労特性、面疲労強度に優れた歯車用鋼。
3.1または2に記載の鋼材を用いて、機械加工あるいは鍛造後に機械加工を行い歯車形状とした後、浸炭焼入れ処理を施して得られる、耐衝撃疲労特性、面疲労強度に優れた歯車。
4.3の工程により歯車形状とした後、さらに歯面にショットピーニングあるいは研磨加工を施した、耐衝撃疲労特性、面疲労強度に優れた歯車。
The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. In mass%, C: 0.15 to 0.30%, Si: more than 0.50% to less than 1.50%, Mn: 0.50% or more, P: 0.015% or less, Cu: 0.30 %: Cr: 2.00% or less, Mo: 0.50% or less, Al: 0.006 to 0.110% or less, Ti: less than 0.02%, B: 0.0004% or more, N: 0 .0065% or less, the balance satisfying the formula (1) has a composition composed of Fe and inevitable impurities, and the effective hardened layer depth, internal hardness, and grain boundary oxidation obtained after carburizing and tempering Gear steel with excellent impact fatigue resistance and surface fatigue strength that satisfies the S value of equation (2) for layer depth.
I (= 14/27 × Al + 14 / 10.8 × B−N) ≧ 0.03 (1)
However, Al, B, and N in the formula (1) indicate the content (mass%).
170 ≦ S (= E / 2 × DP / 2) ≦ 250 (2)
However, E: Effective hardened layer depth (mm), D: Internal hardness (HV), P: Grain boundary oxide layer depth (micrometer) is shown.
2. Further, the gear steel excellent in impact fatigue resistance and surface fatigue strength according to 1, wherein any one or more of Nb: 0.050% or less and V: 0.030-0.200% is added.
3. A gear excellent in impact fatigue resistance and surface fatigue strength, obtained by machining the steel material described in 1 or 2 after machining or forging to obtain a gear shape and then carburizing and quenching.
A gear having excellent impact fatigue resistance and surface fatigue strength, which is formed into a gear shape by the process of 4.3 and further subjected to shot peening or polishing on the tooth surface.

本発明によれば、優れた衝撃疲労強度と面疲労強度を備え、且つ従来鋼と同等の曲げ疲労強度を備えた歯車が得られ産業上極めて有用である。   According to the present invention, a gear having excellent impact fatigue strength and surface fatigue strength and a bending fatigue strength equivalent to that of conventional steel is obtained, which is extremely useful industrially.

以下に本発明の成分元素の限定理由について述べる。説明において%はmass%とする。
C:0.15〜0.30%
Cは強度確保のために必要であり、その量は内部硬さを決定する。その量が0.15%に満たないと表面硬さが300HV以下にまで低下し、歯車としての強度を確保できない。また、0.30%より多すぎると素材が過剰に硬くなり加工性が低下し、生産コストが上昇するので0.15%以上、0.30%以下とする。
The reasons for limiting the constituent elements of the present invention will be described below. In the explanation,% is mass%.
C: 0.15-0.30%
C is necessary for securing the strength, and its amount determines the internal hardness. If the amount is less than 0.15%, the surface hardness decreases to 300 HV or less, and the strength as a gear cannot be secured. On the other hand, if it is more than 0.30%, the material becomes excessively hard, the workability is lowered, and the production cost is increased, so the content is made 0.15% or more and 0.30% or less.

Si:0.50%超え〜1.50%
Siは酸化されやすい元素であり、浸炭表面での結晶粒界での酸化を助長させ、不完全焼入層を生成しやすいが、0.50%を超えると表面酸化が結晶粒界だけでなく結晶粒の内部でも酸化は進行し、全体に均一に酸化される。
Si: more than 0.50% to 1.50%
Si is an easily oxidizable element that promotes oxidation at the grain boundaries on the carburized surface and tends to form an incompletely hardened layer. However, if it exceeds 0.50%, surface oxidation occurs not only at the grain boundaries. Oxidation proceeds even inside the crystal grains and is uniformly oxidized as a whole.

そのため、脆い粒界酸化層の切り欠き効果による亀裂の発生や進展が少なくなり、さらには均一な酸化層は歯車のなじみ性を向上させる。また、焼戻し軟化抵抗が向上するために面疲労強度を向上させる。   For this reason, the generation and propagation of cracks due to the notch effect of the brittle grain boundary oxide layer is reduced, and the uniform oxide layer improves the conformability of the gear. Moreover, since the temper softening resistance is improved, the surface fatigue strength is improved.

しかし、1.50%以上になるとその効果が飽和し、更なる添加は合金コストがかさむだけになるため、0.50超え〜1.50%とする。   However, when the amount is 1.50% or more, the effect is saturated, and further addition only increases the alloy cost. Therefore, the content is set to exceed 0.50 to 1.50%.

Mn:0.50%以上
Mnは焼き入れ性を高める元素であるが添加量が0.50%未満では焼入れ性が確保できない。よってMn添加量は0.50%以上に限定した。
Mn: 0.50% or more Mn is an element that enhances hardenability, but if the added amount is less than 0.50%, hardenability cannot be ensured. Therefore, the amount of Mn added is limited to 0.50% or more.

P:0.015%以下
Pは粒界に偏析して粒界を脆化させ粒界強度を低下させる。添加量が増えるほど疲労の亀裂の発生・伝播が起こりやすくなるため0.015%以下とする。
P: 0.015% or less P segregates at the grain boundary, embrittles the grain boundary, and lowers the grain boundary strength. As the added amount increases, the generation and propagation of fatigue cracks are more likely to occur, so 0.015% or less.

Cu:0.30%以下
Cuは圧延や鍛造等の熱間加工性を低下させ、歯車等に加工した場合に加工精度が悪くなり疲労特性が低下するので0.30%以下とする。
Cu: 0.30% or less Cu decreases the hot workability such as rolling and forging, and when processed into a gear or the like, the processing accuracy deteriorates and the fatigue characteristics deteriorate, so the content is made 0.30% or less.

Cr:2.00%以下
Crは焼入れ性向上元素であるとともに、焼戻し軟化抵抗を高める元素である。しかしその添加量が2.00%超えの場合は軟化抵抗を高める効果は飽和し、焼入れ性が高くなりすぎるため歯車内部の靭性が劣化し、衝撃値が低くなるばかりでなく曲げ疲労強度が低下する。よって、Cr添加量は2.00%以下とする。
Cr: 2.00% or less Cr is an element improving the temper softening resistance as well as a hardenability improving element. However, when the added amount exceeds 2.00%, the effect of increasing the softening resistance is saturated, and the hardenability becomes too high, so the toughness inside the gear is deteriorated, not only the impact value is lowered but also the bending fatigue strength is lowered. To do. Therefore, the Cr addition amount is set to 2.00% or less.

Mo:0.50%以下
Moは焼入れ性を向上させるのに有効な元素である。その効果は添加量が増えるほど高くなるが高価な元素のため、0.50%以下とする。
Mo: 0.50% or less Mo is an element effective for improving the hardenability. The effect becomes higher as the addition amount increases, but is 0.50% or less because of an expensive element.

Al:0.006〜0.110%
Alは固溶Bの確保と脱酸のため添加する。脱酸作用は添加量が0.006%未満ではその効果が得られず、また多く添加しすぎると溶製時において鋳造異常等の恐れがあるため0.006〜0.110%とする。
Al: 0.006 to 0.110%
Al is added for securing solid solution B and deoxidation. The effect of deoxidation is not obtained if the addition amount is less than 0.006%, and if too much is added, there is a risk of casting abnormalities at the time of melting, so 0.006 to 0.110%.

Ti:0.02%未満
固溶Bを確保するためにNを他の元素と結合させる必要がある。TiはBよりもNとの結合力が強いため、有効な元素であり、後述するAlーB−Nバランスによる効果を補完するように添加する。但し、0.02%以上添加すると粗大なTiNが生成し、疲労亀裂の起点となりやすいために、疲労強度が低下する。よってTiは0.02%未満とする。
Ti: It is necessary to combine N with other elements in order to secure a solid solution B of less than 0.02%. Ti is an effective element because it has a stronger bonding force with N than B, and is added so as to complement the effect of the Al-B-N balance described later. However, when 0.02% or more is added, coarse TiN is generated and tends to become a starting point of fatigue cracks, so that the fatigue strength decreases. Therefore, Ti is made less than 0.02%.

B:0.0004%以上
Bは鋼中に固溶して粒界偏析し焼入れ性を向上させ、低Si化による焼入れ性の低下を補う。また、Pの粒界偏析を妨げ、粒界強度を向上させて疲労特性を改善するため0.0004%以上とする。
B: 0.0004% or more B dissolves in the steel and segregates at the grain boundaries to improve the hardenability and compensate for the decrease in hardenability due to low Si content. Moreover, in order to prevent grain boundary segregation of P, improve grain boundary strength, and improve fatigue characteristics, the content is made 0.0004% or more.

N:0.0065%以下
NはBと結合してBNを生成するので固溶B確保のためにはNは低いほど良い。しかし、AlとNとBの平衡関係においてNが0.0065%以下であれば固溶Bの確保は可能となるため、0.0065%以下とする。
N: 0.0065% or less Since N combines with B to form BN, the lower N is better for securing solid solution B. However, if N is 0.0065% or less in the equilibrium relationship of Al, N, and B, solid solution B can be secured, so the content is made 0.0065% or less.

I(=14/27×Al+14/10.8×B−N):0.03以上
本パラメータは、粒界に偏析して、衝撃疲労強度を向上させる固溶Bを確保するためのAl,B,Nのバランスの指数で、本パラメータの値が0.03未満の場合は鋼中に固溶するBの確保が出来なくなるため0.03以上とする。
I (= 14/27 × Al + 14 / 10.8 × B−N): 0.03 or more This parameter is Al, B for segregating at the grain boundaries to ensure solid solution B that improves impact fatigue strength. , N balance index, and if this parameter value is less than 0.03, B cannot be secured in the steel, so it is set to 0.03 or more.

以上が本発明の基本成分組成であるが、更に特性を向上させる場合、Nb、Vの一種または二種以上を添加する。   The above is the basic component composition of the present invention. When further improving the characteristics, one or more of Nb and V are added.

Nb:0.050%以下
Nbは結晶粒を微細化させて粒界を強化して疲労強度を向上させるが、その効果は0.050%で飽和する。よってNbを添加する場合は0.050%以下に限定する。
Nb: 0.050% or less Nb refines crystal grains and strengthens grain boundaries to improve fatigue strength, but the effect is saturated at 0.050%. Therefore, when adding Nb, it limits to 0.050% or less.

V:0.030〜0.200%
Vには、浸炭後の内部強度を上昇させて全体の疲労強度を向上させる。その効果は0.030%以上で得られるが0.200%を超えると飽和するのでVを添加する場合は0.030〜0.200%とする。
V: 0.030-0.200%
V increases the internal fatigue strength after carburizing to improve the overall fatigue strength. The effect is obtained at 0.030% or more, but when it exceeds 0.200%, it is saturated, so when V is added, the content is made 0.030 to 0.200%.

尚、不可避不純物としてのSおよび酸素の含有量は、コストが許す範囲内で出来るだけ低いほうが望ましい。また、被削性を向上させるために必要に応じて、S、Pb、Se、Ca等の快削元素を含有させてもよい。   The contents of S and oxygen as unavoidable impurities are preferably as low as possible within the range allowed by the cost. Moreover, you may contain free-cutting elements, such as S, Pb, Se, and Ca, as needed in order to improve machinability.

170≦S(=E/2×D-P/2)≦250
(但し、E:有効硬化層深さ(mm)、D:内部硬度(HV)、P:粒界酸化層深さ(μm))
本パラメータSは、優れた耐衝撃疲労特性とローラーピッチング(面疲労)特性を付与するためのもので、170以上、250以下とする。表1のNo.1〜3の丸棒鋼を用いて衝撃疲労試験片とローラーピッチング試験片を作成し、種々の条件で浸炭焼入れ・焼戻しを行い、粒界酸化層深さ,内部硬度,有効硬化層深さの異なる衝撃疲労試験片を作成し、衝撃疲労試験とローラーピッチング試験を実施した。表2と図1に結果を示す。
170 ≦ S (= E / 2 × DP / 2) ≦ 250
(However, E: Effective hardened layer depth (mm), D: Internal hardness (HV), P: Grain boundary oxide layer depth (μm))
This parameter S is for imparting excellent impact fatigue resistance and roller pitting (surface fatigue) characteristics, and is set to 170 or more and 250 or less. No. in Table 1 Create an impact fatigue test piece and a roller pitching test piece using 1 to 3 round steel bars, carburize and temper under various conditions, and have different grain boundary oxide layer depth, internal hardness, and effective hardened layer depth. An impact fatigue test piece was prepared, and an impact fatigue test and a roller pitching test were performed. The results are shown in Table 2 and FIG.

Figure 2008179848
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衝撃疲労試験結果とローラーピッチング(面疲労)特性は、E:有効硬化層深さ(mm)、D:内部硬度(HV)、P:粒界酸化層深さ(μm)によるパラメータS(=E/2×D-P/2)で整理され、パラメータSの値が、170以上、250以下で衝撃疲労強度2.5N・m以上の優れた衝撃疲労強度が得られ、150以上、300以下で面疲労強度3000MPa以上の優れた面疲労強度が得られることが判明した。   Impact fatigue test results and roller pitting (surface fatigue) characteristics are as follows: E: effective hardened layer depth (mm), D: internal hardness (HV), P: grain boundary oxide layer depth (μm) parameter S (= E / 2 × DP / 2), and when the parameter S value is 170 or more and 250 or less, an excellent impact fatigue strength of 2.5 N · m or more is obtained, and 150 or more and 300 or less. It has been found that excellent surface fatigue strength of 3000 MPa or more can be obtained.

すなわち、パラメータSの値が170以上、250以下であれば衝撃疲労強度と面疲労強度が共に向上するが、範囲を外れた場合は衝撃疲労強度と面疲労強度のいずれか一方もしくは両方が低下する。   That is, if the value of the parameter S is 170 or more and 250 or less, both the impact fatigue strength and the surface fatigue strength are improved, but if out of the range, either or both of the impact fatigue strength and the surface fatigue strength are reduced. .

有効硬化層深さを0.80mm以上、粒界酸化層を13μm以上、内部硬度を424Hv以上とすると、回転曲げ疲労強度650MPa以上の優れた回転曲げ疲労特性が得られ、自動車および各種産業機械用として、より好適である。   When the effective hardened layer depth is 0.80 mm or more, the grain boundary oxide layer is 13 μm or more, and the internal hardness is 424 Hv or more, excellent rotational bending fatigue characteristics with a rotational bending fatigue strength of 650 MPa or more can be obtained. For automobiles and various industrial machines As such, it is more preferable.

尚、パラメータSを算出する際のE、D、Pにはそれぞれの単位系における数値自体を用いる。   In addition, the numerical value itself in each unit system is used for E, D, and P when calculating the parameter S.

本発明に係る歯車用鋼から歯車を作成する場合は、常法により溶解鋳造してビレットとし、熱間圧延後、歯車としての予備成形を行う。   When producing a gear from the steel for gear according to the present invention, it is melt cast by a conventional method to form a billet, and after hot rolling, preforming as a gear is performed.

次に、機械加工、あるいは鍛造後に機械加工を行い歯車形状とした後、浸炭焼入れ処理を施し、必要に応じて更に歯面にショットピーニングあるいは研磨加工を施して最終製品とする。浸炭焼入れ処理は、浸炭温度900〜1050℃、焼入れ温度800〜900℃とし、焼戻しは120〜250℃の範囲とする。   Next, it is machined or machined after forging to form a gear shape, and then carburized and quenched, and if necessary, the tooth surface is further subjected to shot peening or polishing to obtain a final product. The carburizing and quenching treatment is performed at a carburizing temperature of 900 to 1050 ° C, a quenching temperature of 800 to 900 ° C, and tempering within a range of 120 to 250 ° C.

表3に示す化学成分を有する鋼を溶解し供試鋼とした。表中のNo.1〜16は本発明鋼で、本発明範囲内の成分組成を有し、No.17〜27は本発明範囲外の成分組成を有する比較鋼である。また、No.28は従来鋼であるJISSCM822肌焼鋼である。   Steels having chemical components shown in Table 3 were melted and used as test steels. No. in the table. Nos. 1 to 16 are steels of the present invention, which have a component composition within the scope of the present invention. Reference numerals 17 to 27 are comparative steels having a component composition outside the scope of the present invention. No. 28 is JISSCM822 case hardening steel which is a conventional steel.

溶製された上記の本発明鋼,比較鋼,従来鋼のインゴットを熱間圧延により直径32mmの丸棒鋼に調製し、得られた丸棒鋼に対し焼準処理を実施した。   The ingots of the present invention steel, the comparative steel, and the conventional steel prepared as above were prepared by hot rolling into a round bar steel having a diameter of 32 mm, and the obtained round bar steel was subjected to a normalization treatment.

焼準処理後の丸棒から20mmφの丸棒、ローラーピッチング疲労試験片,衝撃疲労試験片を採取した。丸棒および各疲労試験片に対して、図2に示す条件で浸炭焼入れ・焼き戻しを施した後、表面硬度,内部硬度,有効硬化層深さの調査、および衝撃疲労試験と回転曲げ疲労試験を実施した。以下にそれぞれの調査内容について詳細に説明する。   A 20 mmφ round bar, a roller pitching fatigue test piece, and an impact fatigue test piece were collected from the round bar after the normalizing treatment. After carburizing and tempering the round bar and each fatigue test piece under the conditions shown in Fig. 2, investigation of surface hardness, internal hardness, effective hardened layer depth, impact fatigue test and rotational bending fatigue test Carried out. The details of each survey are described below.

粒界酸化層深さ、有効硬化層深さ、内部硬度調査
発明鋼、比較鋼および従来鋼の20φ丸棒を用いて、浸炭焼入れ焼戻しをした後、切断し、最大となる粒界酸化層深さを光学顕微鏡にて測定した。また、断面の硬度分布を測定し、ビッカース硬さで550HVの得られる深さを調査し「有効硬化層深さ」とした。さらに表層より5mm深さ位置の硬度を「内部硬度」と規定してビッカース硬度計を用いて測定した。
Intergranular oxide layer depth, effective hardened layer depth, internal hardness investigation invention steel, comparative steel and conventional steel 20φ round bar, carburizing quenching and tempering, then cutting, maximum grain boundary oxide layer depth The thickness was measured with an optical microscope. Further, the hardness distribution of the cross section was measured, and the depth at which 550 HV was obtained in terms of Vickers hardness was investigated, and was set as “effective hardened layer depth”. Further, the hardness at a depth of 5 mm from the surface layer was defined as “internal hardness” and measured using a Vickers hardness tester.

衝撃疲労特性調査
直径32mm径の丸棒鋼より各鋼材を用いて図3に示す試験片を作製し、図2に示す条件の浸炭焼入れ焼戻しをした後、落錘型衝撃疲労試験機により、繰返し数200回で破壊する衝撃エネルギーを調査した。
Impact Fatigue Properties Investigation Test pieces shown in FIG. 3 were prepared from each round steel bar having a diameter of 32 mm, carburized and tempered under the conditions shown in FIG. The impact energy destroyed at 200 times was investigated.

面疲労特性調査
直径32mmの丸棒鋼から、図4に示す平行部直径26mmの試験片を採取し、図2に示す条件で浸炭焼入れ・焼戻し処理を行った。それらの試験片を用いてローラーピッチング疲労試験を行い、10回を疲労限度として面疲労強度を評価した。
Test specimens having a parallel part diameter of 26 mm shown in FIG. 4 were collected from a round steel bar having a surface fatigue characteristic of 32 mm in diameter and subjected to carburizing and tempering under the conditions shown in FIG. A roller pitching fatigue test was performed using these test pieces, and the surface fatigue strength was evaluated with 10 7 times as the fatigue limit.

表4に上記調査結果を示す。本発明鋼は有効硬化層深さが0.90mm以上、粒界酸化層が10μm以上、内部硬度は417Hv以上で、面疲労強度は3000MPa以上、衝撃疲労強度は2.7N・m以上が得られ、比較鋼No.17〜No.27と従来鋼(JISSCM822肌焼鋼)より優れていた(No.28の従来鋼(JISSCM822肌焼鋼)は面疲労強度2700MPa、衝撃疲労強度は2.0N・m)。   Table 4 shows the survey results. The steel according to the present invention has an effective hardened layer depth of 0.90 mm or more, a grain boundary oxide layer of 10 μm or more, an internal hardness of 417 Hv or more, a surface fatigue strength of 3000 MPa or more, and an impact fatigue strength of 2.7 N · m or more. Comparative steel No. 17-No. 27 and the conventional steel (JISSCM822 case-hardened steel) (No. 28 conventional steel (JISSCM822 case-hardened steel) had surface fatigue strength of 2700 MPa and impact fatigue strength of 2.0 N · m).

すなわち、比較鋼No.17はC含有量が本発明範囲より低いために、内部硬度が低くなりすぎた。そのために面疲労強度が低下した。   That is, comparative steel No. No. 17 had an internal hardness that was too low because the C content was lower than the range of the present invention. As a result, the surface fatigue strength decreased.

比較鋼No.18はSi含有量が本発明の範囲よりも低いために粒界酸化層が深い。そのため衝撃疲労強度、面疲労強度が低下した。   Comparative steel No. No. 18 has a deep grain boundary oxide layer because the Si content is lower than the range of the present invention. Therefore, impact fatigue strength and surface fatigue strength were reduced.

比較鋼No.19はMn含有量が本発明の範囲より低いために焼入れ性が低すぎる。よって有効硬化層深さが浅く、内部硬度が低いため、衝撃疲労強度および面疲労強度が低下した。   Comparative steel No. No. 19 has a Mn content lower than the range of the present invention, so that the hardenability is too low. Therefore, since the effective hardened layer depth was shallow and the internal hardness was low, impact fatigue strength and surface fatigue strength were reduced.

比較鋼No.20はP含有量が本発明範囲より高いために粒界強度が不足し、衝撃疲労強度が低下した。   Comparative steel No. No. 20 had a P content higher than the range of the present invention, so the grain boundary strength was insufficient, and the impact fatigue strength was lowered.

比較鋼No.21はCu含有量が本発明範囲よりも高いために、試験片加工精度が悪くなった。そのため、衝撃疲労強度と面疲労強度のいずれも低下した。   Comparative steel No. In No. 21, the Cu content was higher than the range of the present invention, so that the processing accuracy of the test piece was deteriorated. Therefore, both impact fatigue strength and surface fatigue strength decreased.

比較鋼No.22はCr含有量が本発明の範囲より高いために焼入れ性が高くなりすぎている。そのため、衝撃疲労強度が低下した。   Comparative steel No. Since the Cr content is higher than the range of the present invention, the hardenability is too high. As a result, the impact fatigue strength decreased.

比較鋼No.23はV含有量が本発明範囲よりも低いために内部の硬さが低下した。その結果、面疲労強度が低下した。   Comparative steel No. In No. 23, since the V content was lower than the range of the present invention, the internal hardness decreased. As a result, the surface fatigue strength decreased.

比較鋼No.24はB含有量が本発明範囲よりも低いために、粒界強化作用が得られず、有効硬化層深さも浅くなり、面疲労強度および衝撃疲労強度が低下した。   Comparative steel No. In No. 24, since the B content was lower than the range of the present invention, the grain boundary strengthening action was not obtained, the effective hardened layer depth became shallow, and the surface fatigue strength and the impact fatigue strength decreased.

比較鋼No.25はTi含有量が本発明範囲よりも高いために、ローラーピッチング疲労試験および衝撃疲労試験において粗大なTiN起因の疲労破壊が多く発生した。そのため、面疲労強度および衝撃疲労強度が低下した。   Comparative steel No. In No. 25, since the Ti content was higher than the range of the present invention, a large amount of coarse fatigue failure due to TiN occurred in the roller pitting fatigue test and the impact fatigue test. Therefore, surface fatigue strength and impact fatigue strength were reduced.

比較鋼No.26はN添加量が本発明範囲よりも高い。その結果、固溶Bが確保できず、有効硬化層深さが浅くなり、衝撃疲労強度が低下した。   Comparative steel No. No. 26 has an N addition amount higher than the range of the present invention. As a result, the solid solution B could not be secured, the effective hardened layer depth became shallow, and the impact fatigue strength decreased.

比較鋼No27はI値が本発明範囲よりも低い。その結果、固溶Bの確保が出来ず、有効硬化層深さが浅くなり、内部硬度も低くなりすぎたので衝撃疲労強度と面疲労強度が低下した。   Comparative steel No. 27 has an I value lower than the range of the present invention. As a result, the solid solution B could not be secured, the effective hardened layer depth became shallow, and the internal hardness was too low, so that the impact fatigue strength and the surface fatigue strength were lowered.

Figure 2008179848
Figure 2008179848

Figure 2008179848
Figure 2008179848

衝撃疲労試験結果とローラーピッチング(面疲労)特性に及ぼすS(=E/2×D-P/2)値の影響を示す図。The figure which shows the influence of S (= E / 2xDP / 2) value which exerts on an impact fatigue test result and roller pitting (surface fatigue) characteristic. 浸炭焼入れ・焼戻し処理条件を示す図。The figure which shows the carburizing quenching and tempering process conditions. 衝撃疲労試験片の形状を示す図。The figure which shows the shape of an impact fatigue test piece. ローラピッチング疲労試験片の形状を示す図。The figure which shows the shape of a roller pitching fatigue test piece.

Claims (4)

mass%で、C:0.15〜0.30%、Si:0.50%超え〜1.50%未満、Mn:0.50%以上、P:0.015%以下、Cu:0.30%以下、Cr:2.00%以下、Mo:0.50%以下、Al:0.006〜0.110%以下、Ti:0.02%未満,B:0.0004%以上、N:0.0065%以下を含有し、(1)式を満足する残部がFeおよび不可避的不純物からなる成分組成を有し、浸炭焼入れ・焼戻し後に得られる有効硬化層深さ、内部硬さ、粒界酸化層深さについて(2)式のS値を満足する耐衝撃疲労特性、面疲労強度に優れた歯車用鋼。
I(=14/27×Al+14/10.8×B−N)≧0.03・・・(1)
但し、(1)式中のAl,B,Nは含有量(mass%)を示す。
170≦S(=E/2×D-P/2)≦250 ・・・(2)
但し、E:有効硬化層深さ(mm)、D:内部硬度(HV)、P:粒界酸化層深さ(μm)を示す。
In mass%, C: 0.15 to 0.30%, Si: more than 0.50% to less than 1.50%, Mn: 0.50% or more, P: 0.015% or less, Cu: 0.30 %: Cr: 2.00% or less, Mo: 0.50% or less, Al: 0.006 to 0.110% or less, Ti: less than 0.02%, B: 0.0004% or more, N: 0 .0065% or less, the balance satisfying the formula (1) has a composition composed of Fe and inevitable impurities, and the effective hardened layer depth, internal hardness, and grain boundary oxidation obtained after carburizing and tempering Gear steel with excellent impact fatigue resistance and surface fatigue strength that satisfies the S value of equation (2) for layer depth.
I (= 14/27 × Al + 14 / 10.8 × B−N) ≧ 0.03 (1)
However, Al, B, and N in the formula (1) indicate the content (mass%).
170 ≦ S (= E / 2 × DP / 2) ≦ 250 (2)
However, E: Effective hardened layer depth (mm), D: Internal hardness (HV), P: Grain boundary oxide layer depth (micrometer) is shown.
更に、Nb:0.050%以下,V:0.030〜0.200%のいずれか1種以上を添加する請求項1記載の耐衝撃疲労特性、面疲労強度に優れた歯車用鋼。   Furthermore, the steel for gears excellent in the impact fatigue resistance property and surface fatigue strength of Claim 1 which adds any 1 or more of Nb: 0.050% or less and V: 0.030-0.200%. 請求項1または2に記載の鋼材を用いて、機械加工あるいは鍛造後に機械加工を行い歯車形状とした後、浸炭焼入れ処理を施して得られる、耐衝撃疲労特性、面疲労強度に優れた歯車。   A gear excellent in impact fatigue resistance and surface fatigue strength obtained by machining the steel material according to claim 1 or 2 after machining or forging to obtain a gear shape and then carburizing and quenching. 請求項3の工程により歯車形状とした後、さらに歯面にショットピーニングあるいは研磨加工を施した、耐衝撃疲労特性、面疲労強度に優れた歯車。   A gear excellent in impact fatigue resistance and surface fatigue strength obtained by forming a gear shape by the process of claim 3 and further subjecting the tooth surface to shot peening or polishing.
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* Cited by examiner, † Cited by third party
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WO2015098106A1 (en) * 2013-12-27 2015-07-02 新日鐵住金株式会社 Carburized-steel-component production method, and carburized steel component
JP2015134948A (en) * 2014-01-17 2015-07-27 Jfe条鋼株式会社 Case hardened steel and machine structural component
JP2021028412A (en) * 2019-08-09 2021-02-25 日本製鉄株式会社 Steel for carburized gear, carburized gear, and manufacturing method of carburized gear

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2014101565A (en) * 2012-11-22 2014-06-05 Jfe Bars & Shapes Corp Method of manufacturing case hardened steel and parts omissible of normalizing after hot-forging and excellent in high temperature carburizability
WO2015098106A1 (en) * 2013-12-27 2015-07-02 新日鐵住金株式会社 Carburized-steel-component production method, and carburized steel component
JP6098732B2 (en) * 2013-12-27 2017-03-22 新日鐵住金株式会社 Manufacturing method of carburized steel parts and carburized steel parts
JPWO2015098106A1 (en) * 2013-12-27 2017-03-23 新日鐵住金株式会社 Manufacturing method of carburized steel parts and carburized steel parts
JP2015134948A (en) * 2014-01-17 2015-07-27 Jfe条鋼株式会社 Case hardened steel and machine structural component
JP2021028412A (en) * 2019-08-09 2021-02-25 日本製鉄株式会社 Steel for carburized gear, carburized gear, and manufacturing method of carburized gear
JP7323791B2 (en) 2019-08-09 2023-08-09 日本製鉄株式会社 Carburized gear steel, carburized gear, and method for manufacturing carburized gear

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