JP4229609B2 - Carburized and hardened gear and manufacturing method thereof - Google Patents

Carburized and hardened gear and manufacturing method thereof Download PDF

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Publication number
JP4229609B2
JP4229609B2 JP2001391679A JP2001391679A JP4229609B2 JP 4229609 B2 JP4229609 B2 JP 4229609B2 JP 2001391679 A JP2001391679 A JP 2001391679A JP 2001391679 A JP2001391679 A JP 2001391679A JP 4229609 B2 JP4229609 B2 JP 4229609B2
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quenching
carburized
less
mass
carburizing
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JP2003193137A (en
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秀雄 蟹澤
誠司 伊藤
孝男 谷口
一雅 塚本
巧治 大林
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Aisin AW Co Ltd
Nippon Steel Corp
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Aisin AW Co Ltd
Nippon Steel Corp
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Description

【0001】
【発明の属する技術分野】
本発明は、浸炭焼入れして用いられる機械構造用部品、特に自動車等の動力伝達部品用に適用できる高い疲労強度を有し、かつ、浸炭焼入れによる寸法変化の少ない浸炭焼入歯車等の浸炭焼入部品及びその製造方法に関するものである。
【0002】
【従来の技術】
機械構造用部品、例えば自動変速機の動力伝達用部品である歯車などは、その曲げ疲労強度、ピッチング強度、耐摩耗性を向上するために表面硬化処理、なかでもその効果の大きい浸炭雰囲気中におけるガス浸炭と油等による焼入れを行う浸炭焼入れが広範に行われている。しかし、従来の浸炭用鋼(例えばJIS−SCr420、SCM420、SNCM420)にガス浸炭を施すと、ガス中に含まれている酸化性ガス(CO2、H2O)により、浸炭部表面に粒界酸化層が生じ、これに付随して不完全焼入れ組織(トルースタイト)が生成する。このため、表面硬さが低下し、疲労強度の向上効果が不十分となっている。そこで、本発明者は粒界酸化層及び不完全焼入れ層の生成防止を目的とした浸炭用鋼を特開平1−47838号公報で提案した。
【0003】
しかし、最近益々進展している自動車の環境対策より、燃費低減を図るために部品の軽量化を促進することが要求され、更に強度が高く、寸法精度が高い部品が望まれている。このため、従来の浸炭用鋼を用いた浸炭焼入部品以上の十分な強度と寸法精度を備えた浸炭焼入部品の開発が望まれている。
【0004】
浸炭焼入部品を高疲労強度化するには、粒界酸化層を減らし、表層の不完全焼入れ層を抑制することが広く用いられているが、これには、▲1▼Siを低減し、▲2▼MoやNiを多く含有した鋼材を用いることや、▲3▼焼入れ時の油の温度を下げて冷却速度を早くする方法がある。しかしながら、Siを低減すると十分な強度が得られず、また、MoやNiを多量添加すると高価な鋼材となるとともに、少なからず焼入れ性を高めることとなり、焼入れ歪みを増大させる問題が生じてくる。同じように、焼入れ油の温度を下げて冷却速度を高める方法も、焼入れ歪みの増大の問題を抱えている。
【0005】
浸炭焼入れは、高温の浸炭雰囲気中で長時間加熱し、その後、冷却する処理であるので、この処理に際しては、相変態に伴う体積変化、変態応力や熱応力が発生して、熱処理歪みは不可避的に発生する。しかも、上記浸炭焼入れ処理は、部品加工の最終段階で行われるため、上記熱処理歪みはそのまま部品に残され寸法精度が劣化する。また、浸炭焼入部品には、必要に応じてショットピーニング処理が行われているが、浸炭硬化層中に存在する残留オーステナイトの加工誘起変態による変形が生じ、寸法精度の劣化が生じる。このような寸法精度の劣化は、部品の性能に直接影響を及ぼし、部品使用時に騒音や振動の発生等を招く。
【0006】
このような問題を解決するために、浸炭焼入れの際に、拘束治具を用いる方法や、浸炭処理後に矯正を行う等の方法はあるが、いずれも多大な費用と労力を要する問題がある。そのため、高強度化とともに、焼入れ歪みの小さい浸炭部品が強く要望されている。
【0007】
【発明が解決しようとする課題】
そこで、本発明は、上記従来技術の問題点を解決し、浸炭焼入れして用いられる歯車等の機械構造部品、なかでも自動車等の動力伝達部品用に適用できる高い疲労強度と、浸炭焼入れによる寸法変化が少ない浸炭焼入歯車及びその製造方法を提供することを目的とする。
【0008】
【課題を解決するための手段】
本発明者らは、上述した従来技術の問題点に対して広範な研究を行った結果、鋼材成分と浸炭焼入れ方法が、疲労強度及び浸炭歪み等に及ぼす影響に関して、次のような知見を得た。
【0009】
(1) 従来の粒界酸化層を低減し、表面硬度を高める多くの鋼材は、通常のガス浸炭・油焼入では、内部組織はベイナイトとマルテンサイトの混在組織となり、そのため硬度が必要以上に上がり、焼入歪みが大きかった。また、浸炭硬化層の残留オーステナイト量が20〜25%と多いため、本鋼材をショットピーニング処理した場合、残留オーステナイトの加工誘起変態による変形量が大きいため、更に歪みは大きくなった。
【0010】
(2) 現行のSCM420を浸炭後、緩速冷却した場合、表面硬度は通常の油焼入れと同等の値を得られるものの、内部組織はフェライト相を多数含むベイナイトで、硬度が低くなった。
【0011】
(3) 現行に比べて焼入性を高めた鋼材を、浸炭後に緩速冷却した場合、内部の非浸炭部は硬度の過剰な増加を抑制したベイナイトとなり、併せて浸炭層の残留オーステナイト量は少なかった。
【0012】
(4) 高い焼入れ性を有する鋼材から試作した歯車を、ガス冷却することによって、現行の焼入れ性であるSCM420を油焼入れした歯車に比べ、歯車の寸法精度のバラツキが大幅に改善できた。
【0013】
こうした知見を基に、本発明者らは浸炭用鋼の疲労強度の向上には、現行の浸炭用鋼材に比べ高い焼入れ性の鋼材を、油冷却に比べ遅い緩速冷却することによって、粒界酸化層を抑制しつつ、残留オーステナイトを15%以下に抑制することにより、表面硬度を高めながら、ショットピーニング処理時に、残留オーステナイトの加工誘起変態により変形を低減できることを明らかにした。更に、高い焼入れ性の鋼材でありながら、緩速冷却により、内部のマルテンサイトの生成を抑制でき、焼入れ歪みと靭性において、優れた特性を有することを見いだした。
【0014】
そこで、前述の高強度化の知見に基づいて、更に研究検討を重ね、以下のような具体的な達成手段を明らかにし、本発明に至った。
【0015】
(1) 高価な合金元素の多用無しに粒界酸化層及び不完全焼入れ層の生成を防止するために、浸炭処理としては酸化防止雰囲気での浸炭処理、特に真空浸炭処理を前提とする。
【0016】
(2) 鋼材成分としては、ピッチング強度及び靭性の向上を狙いに、高Si化による焼戻し軟化抵抗を高め、転動疲労時の組織変化の抑制を図る。併せて、MoとBの添加により、衝撃特性の向上を図る。
【0017】
(3) 更に結晶粒の細粒化及び、MnSの形態制御により強度の異方性を低減する。
【0018】
(4) 鋼材の焼入れ性を高めつつ、高温から緩速冷却することにより、内部組織をHv350以下のベイナイトとし、浸炭層の残留オ−ステナイト量を15%以下とする。
【0019】
(5) (4)を達成するため、緩速冷却の条件は、焼入急冷度Hを0.01〜0.08cm-1とする。
【0020】
なお、上記焼入急冷度Hとは、grossmannらによって提唱され、広く普及している焼入れの強さを表す指標であって、被処理物である鋼の熱伝導率をγ(kcal/mh℃)、焼入れ雰囲気における上記鋼の表面伝達係数をα(kcal/ h℃)とした場合、H=0.5×(α/γ)にて定義されるものである。
【0021】
こうした知見をもとに、本発明者らは粒界酸化層及び表層の不完全焼入れ組織の発生を防止しつつ、高温硬度や靭性を高めつつ、かつ、焼入れ歪みとショットピ−ニング処理後の変形を低減することにより、歯元部の曲げ疲労強度と耐ピッチング性の優れ、かつ歪みの低減を可能とした歯車等の浸炭焼入部品の製造方法及び浸炭焼入部品を開発したのである。
【0022】
即ち、本発明の要旨は、次の通りである。
【0023】
(1) 質量%で、
C:0.1〜0.5%、
Si:0.4〜1.0%、
Mn:0.3〜1.0%、
Cr:0.1〜1.0%、
P:0.003〜0.015%、
S:0.005〜0.03%、
Al:0.01〜0.06%、
N:0.005〜0.03%
を含むとともに、
Mo:0.3〜1.3%、
Ni:0.1〜1.0%
の1種以上を含有し、残部Fe及び不純物よりなる合金鋼を素材として用い、
該素材を所望の歯車形状に成形した後、1〜30hPaに減圧した減圧雰囲気下において浸炭することによって表面炭素量が0.6〜1.5質量%になるように浸炭層を形成し、該浸炭処理後に窒素ガス冷却により下記(1)式で定義する焼入急冷度Hが0.01〜0.08(cm−1)となる焼入処理を行って表面圧縮残留応力を300〜800MPaとすることを特徴とする浸炭焼入歯車の製造方法。
焼入急冷度H=0.5×(α/γ)・・・(1)
ここで、
α:焼入雰囲気における上記鋼の表面の熱伝達係数(kcal/m h℃)
γ:上記鋼の熱伝導率(kcal/mh℃)
である。
【0024】
(2) 合金鋼が、更に他の元素として、質量%で、
V:0.05〜1.5%、
Nb:0.02〜0.2%、
Ti:0.01〜0.2%
の1種以上を含有することを特徴とする上記(1)に記載の浸炭焼入歯車の製造方法。
【0025】
(3) 合金鋼が、更に他の元素として、質量%で、
B:0.0005〜0.005%、
Ti:0.005〜0.1%
を含有することを特徴とする上記(1)または(2)に記載の浸炭焼入歯車の製造方法。
【0027】
) 合金鋼が、更に他の元素として、質量%で、
Ca:0.01%以下、
Mg:0.01%以下、
Zr:0.05%以下、
Te:0.1%以下
よりなる群から選択される、少なくとも1種以上を含有することを特徴とする上記(1)〜()のいずれかに記載の浸炭焼入歯車の製造方法。
【0034】
) 上記(1)〜()のいずれか1項において、上記焼入処理はオーステナイト領域の温度から300℃までの間、上記焼入急冷度Hの範囲において焼入を行うことを特徴とする浸炭焼入歯車の製造方法。
【0038】
質量%で、
C:0.1〜0.5%、
Si:0.4〜1.0%、
Mn:0.3〜1.0%、
Cr:0.1〜1.0%、
P:0.003〜0.015%、
S:0.005〜0.03%、
Al:0.01〜0.06%、
N:0.005〜0.03%
を含むとともに、
Mo:0.3〜1.3%、
Ni:0.1〜1.0%
の1種以上を含有し、残部Fe及び不純物よりなる合金鋼からなり、浸炭層の表面炭素量が0.6〜1.5質量%であり、表面からの粒界酸化層深さが3μm以下であり、表面の圧縮残留応力が300〜800MPaであり、浸炭層の表面硬度が700〜900Hv、浸炭層の内側の非浸炭部の内部硬度が250〜350Hvであり、さらに、浸炭層の残留オーステナイト面積率が15%以下であることを特徴とする浸炭焼入歯車。
【0039】
合金鋼が、更に他の元素として、質量%で、
V:0.05〜1.5%、
Nb:0.02〜0.2%、
Ti:0.01〜0.2%
の1種以上を含有することを特徴とする上記(6)に記載の浸炭焼入歯車。
【0040】
合金鋼が、更に他の元素として、質量%で、
B:0.0005〜0.005%、
Ti:0.005〜0.1%
を含有することを特徴とする上記(6)または(7)に記載の浸炭焼入歯車。
【0041】
合金鋼が、更に他の元素として、質量%で、
Ca:0.01%以下、
Mg:0.01%以下、
Zr:0.05%以下、
Te:0.1%以下
よりなる群から選択される、少なくとも1種以上を含有することを特徴とする上記(6)〜(8)のいずれかに記載の浸炭焼入歯車。
【0042】
【発明の実施の形態】
以下に本発明を詳細に説明する。
【0043】
最初に、本発明で特定される浸炭焼入部品の製造方法について述べる。
【0044】
本発明の浸炭焼入部品は、浸炭焼入れ処理により製造されるものであり、特定の内部硬度を得るために、C:0.1〜0.5%、Mn:0.3〜1.0%、Cr:0.1〜1.0%を含有させることとした。このような高い焼入れ性を有する鋼材を通常の浸炭、油焼入を行うと、内部組織はマルテンサイトとベイナイトの混在した組織となり、硬さが高く、熱処理歪みも大きい。このため、本発明では、ガス等による緩速冷却により、焼入急冷度Hを0.01〜0.08cm-1とする。焼入急冷度Hが0.01cm-1未満の場合は、焼入れ不足となり、所定の焼入れ組織、特性が確保できず、部材は強度不足となる。焼入急冷度Hが0.08cm-1を超える場合は、焼入(急冷)が過剰で、組織変態応力や熱応力の発生が増大し、焼入歪みが大きくなり、部品精度が低下する。
【0045】
同時に、ガス冷却により、浸炭硬化層の残留オーステナイトを面積率で15%以下に低減する。この低い残留オーステナイトにより、たとえ浸炭後ショットピーニング処理を施しても、加工誘起変態によるマルテンサイトへの変化を抑制でき、歪みの小さい浸炭焼入歯車等の浸炭焼入部品となる。
【0046】
また、浸炭焼入部材は、その内部組織がベイナイトであることが好ましい。これにより、強度と靭性を合わせ持った部品を得ることができる。
【0047】
粒界酸化層が3μm以下でマルテンサイト組織主体の浸炭層とから形成される浸炭焼入部品は、酸化防止雰囲気、特に真空浸炭をベースに達成される。即ち、浸炭処理は、1〜30hPaに減圧した減圧雰囲気下において行うことが必要である。これにより、上記酸化防止雰囲気を減圧により容易に得ることができ、浸炭時の粒界酸化を十分に防止することができる。ここで上記減圧雰囲気の減圧の値が1hPa未満は、酸化抑制に対しては過剰であるし、減圧のための装置も高減圧仕様となり、コスト上昇してしまうという問題がある。一方、30hPaを超える場合には、酸化抑制効果の低下、浸炭炉内における煤の発生等の問題がある。
【0048】
また、浸炭処理は、不活性ガスを主成分とする雰囲気下において行うことも好ましい。この場合にも、上記酸化防止雰囲気を容易に形成することができる。不活性ガスとしては、例えば、窒素ガス、アルゴンガス等がある。
【0049】
また、浸炭層の表面炭素量が0.6〜1.5質量%となるように浸炭処理を行うことが必要である。浸炭層の表面炭素濃度は浸炭焼入材の表面硬さに影響し、浸炭層の表面炭素量が0.6質量%未満の場合には、表面硬さが不足するという問題があり、一方、1.5質量%を超えると炭化物の析出量が多くなって基地の焼入性が顕著に低下し、表面硬さが不足するという問題がある。
【0050】
粒界酸化層は一般浸炭焼入層より強度が低く、粒界酸化層が3μmを超えて深くまで生じた場合には、部材の強度不足を引き起こすおそれがある。
【0051】
また、粒界酸化物生成時に、その周辺の合金元素も化学的な化学物反応により粒界酸化物に取り込まれる。これにより、粒界酸化物周辺の浸炭焼入層における焼入性向上元素が上記粒界酸化物に取り込まれて消費され、浸炭焼入層そのものの焼入不足が生じ、硬度不足、強度不足を引き起こすおそれがある。このため、本発明では粒界酸化層を3μm以下とした。
【0052】
また、表面の圧縮残留応力は300MPa〜800MPaであることが必要である。即ち、素材の成分組成、上記浸炭時の酸化防止雰囲気、加熱温度、加熱時間等を調整することにより、上記表面の圧縮残留応力を300MPa以上にすることが必要である。これにより、表面近傍の引張作用応力を部材の表面近傍の圧縮残留応力により、作用応力を緩和することができ、特に動的強度(面疲労強度、曲げ疲労強度、ねじり疲労強度)を向上することができる。一方、上記表面圧縮残留応力が800MPaを超える場合には、マルテンサイト量を多くするため、焼入処理時の冷却速度を限度を超えて大きくしなければならない。そのため、大きな焼入歪みが生じてしまい、部材の寸法精度が確保できないという問題が生じる。
【0053】
また、焼入処理はオーステナイト領域の温度から300℃までの間、焼入急冷度H0.01〜0.08(cm-1)の範囲において焼入を行うことが好ましい。これにより、十分な焼入効果を得ることができる。一方、オーステナイト領域の温度から300℃までの間の冷却における上記焼入急冷度Hが0.01(cm-1)未満の場合は、焼入れ不足となり、所望の焼入組織、特性が確保できず、部材強度不足となる。また、オーステナイト領域の温度から300℃までの間の冷却における上記焼入急冷度Hが0.08(cm-1)を超える場合には、急冷が過剰で組織変態応力及び熱応力が増大し、焼入歪みが大きくなり部品精度が低下するおそれがある。
【0054】
また、焼入処理は窒素ガス冷却により焼入れする必要がある。窒素ガスによる冷却は、焼入急冷度Hを比較的容易に確保することができると共に、焼入時における安全性の確保、量産操業時における入手容易性、コスト、取り扱い容易性などの点から採用すべきである。
【0057】
また、浸炭層は表面硬度を700〜900Hvとすることが必要である。浸炭層の表面硬度が700Hv未満の場合には、部材表面近傍での応力集中に対し、強度を確保できない問題が考えられる。また、最表面での耐摩耗性不足という問題も考えられる。一方表面硬度が900Hvを超える場合、表層にセメンタイト等の炭化物の生成が考えられ、かえって強度不足、特に靭性の低下という問題が考えられる。そして、上記表面硬度とするためには、浸炭層の表面炭素量は、0.6〜1.5%とすることが必要である
【0058】
また、非浸炭部の内部硬度が250Hv未満の場合には強度不足、特に静的強度不足という問題が考えられる。一方、内部硬度が350Hvを超えると、組織の変態率から考えて、350Hvを確保するよう焼入処理するときに大きな変態応力が発生し、これが大きな焼入歪みとなって部品精度低下の要因となるという問題があるので、本発明では内部の非浸炭部の硬度(Hv)を250〜350Hvとした。
【0059】
即ち、特定の成分に調整され、高い焼入れ性を有した鋼材を、上記に述べたように酸化防止雰囲気における浸炭、特に真空浸炭と緩速冷却、特にガス冷却とを行うことにより、本発明の浸炭焼入部品を得ることができ、部材に作用する付加荷重により発生する部材への作用応力と部材形状の凹凸、穴等に伴って発生する部材表面近傍、応力集中とを合成した部材へ付加される応力分布に対し、静的強度(引張強度、曲げ強度、ねじり強度等)と動的強度(面疲労強度、曲げ疲労強度、ねじり疲労強度等)を表面から内部(心部)まで確保することができる。
【0060】
次に、鋼材の化学成分について述べる。
【0061】
Cは浸炭部品として必要な強度と靭性、特に芯部(非浸炭部)の強度と靭性を確保するために添加する元素であるが、0.1%未満ではこのような効果を十分に得ることができず、0.5%超では焼入前硬度が高くなりすぎ、加工コストの上昇及び靭性が低下して脆くなり、また、浸炭焼入後の非浸炭内部の組織変態率上昇により、変態応力が増大し、大きな焼入歪みにより部品精度の低下の要因となるので、その含有量を0.1%以上0.5%以下とする。
【0062】
Siは浸炭用鋼の焼き戻し軟化抵抗を高め、高温硬度を向上する元素であり、含有量が0.4%未満ではその効果が不十分であり、1.0%超ではその効果はほぼ飽和するので、その含有量を0.4%以上1.0%以下とする。
【0063】
Mnは鋼に強度、靭性、焼入れ性を与えるのに必要な元素であるが、1.0%超では熱間圧延後の冷却においてベイナイトやマルテンサイトの硬質な組織になり、その後の切削等の二次加工には適さなくなるために1.0%以下とする。しかしMnの添加量が0.3%未満では焼入れ性の効果が十分でなく、その含有量は0.3%以上とする。
【0064】
Crは鋼の機械的性質、焼入れ性、耐摩耗性の向上に寄与するが、この元素も1.0%超では、熱間圧延後の冷却においてベイナイトやマルテンサイトの硬質な組織になり、その後の切削等の二次加工には適さなくなるために1.0%以下とする。しかし、Crの添加量が0.1%未満では焼入れ性の効果が十分でなく、その含有量は0.1%以上とする。
【0065】
Pは粒界に極めて偏析しやすく延性を阻害したり遅れ破壊強度を低下する元素である。このため、上記の悪影響を回避するには0.015%以下に保つことが必要であり、また0.003%未満では延性向上効果はほぼ飽和し、かつ低減のためのコストが著しく増加するため、0.003%以上0.015%以下と定めた。
【0066】
Sは被削性を高めるのに必要な元素であり、0.005%以上含有させる。しかし、0.03%を超えて含有させると鋼中の介在物量を増加し、冷間での塑性加工性に悪影響を及ぼすため上限を0.03%とする。
【0067】
Alは結晶粒の粗大化を防止し高延性を図るために添加し、その効果を得るには0.01%以上の添加が必要であり、一方0.06%超ではその効果が飽和する。従って0.01%以上0.06%以下とする。
【0068】
Nは浸炭時にAlと結合し、組織を微細化する目的で添加する。しかし、その効果を得るには0.005%以上の添加が必要であり、一方、0.03%超ではその効果が飽和する。従って0.005%以上0.03%以下とする。
【0069】
Mo及びNiは鋼に所定の焼入れ性を与え、強度及び靭性を向上させるのに必要な元素であり、1種以上添加する。本発明では靭性や低サイクル疲労強度の向上をねらいに、Moは0.3%以上、Niは0.1%以上を含有させる。しかし、Moは1.3%、Niは1.0%を超えて含有させてもその効果は飽和して経済性を損なうためそれぞれ上限を1.3%、1.0%とする。
【0070】
V、Ti及びNbは炭窒化物を生成し、浸炭結晶粒を微細化に効果のある元素であり、任意に添加することができる。その効果を得るにはVで0.05%以上、Nbで0.02%以上及びTiで0.01%以上の含有が必要である。しかし、Vで1.5%を超えて、Nbで0.2%を超えて、Tiで0.2%を超えて含有しても効果は飽和するため、上限をVで1.5%、Nbで0.2%、Tiで0.2%とする。
【0071】
Bは粒界に偏析し、粒界強度を高める効果がある。その効果を得るにはBで0.0005%以上含有させる。一方、0.005%を超えると靭性を低下させるので、上限は0.005%とする。また、Bを有効に機能させるには、炭窒化物となっていない、固溶Bの存在が必要である。一方、フリーNが存在すると、BはBNとなりBとしての本来の機能が損なわれる。そのため、Bを添加する際は、同時にTiを添加し、NをTiNとして固定することが必要である。その効果を得るには、Tiは0.005%以上0.1%以下を必要とするが、Nの固定効果と結晶粒の微細化をともに得るには、Ti:0.11〜0.2%の含有が必要である。
【0072】
Ca、Mg、Zr及びTeは、歯車の歯元曲げ疲労破壊や軸部品のスプラインの底の疲労破壊に対して、MnSの延伸を抑制し、いっそう曲げ疲労強度を向上する元素である。即ち、MnSの延伸抑制効果を与えるために、Caで0.01%以下、Mgで0.01%以下、Zrで0.05%以下及びTeで0.1%以下を含有させる。しかし、各元素で前記量を超えて含有させても、その効果は飽和して経済性を損なうため上限とする。
【0073】
なお、本発明の浸炭焼入部品は、浸炭焼入処理後にショットピ−ニング処理を施しても、その特性は、何ら損なうことはない。特に、歯曲げ疲労強度は、浸炭ままに比べ、更に強度が向上することを確認している。従って、浸炭焼入部品の強度向上を目的として、浸炭焼入処理後に、ショットピーニング処理を行うことができる。
【0074】
【実施例】
以下に実施例をあげて更に説明する。
【0075】
表1に示す化学成分の鋼(鋼A1〜B8及び鋼B1〜B8)を溶製したのち造塊し、次に分塊圧延、棒鋼圧延して直径70mmの丸棒を製造した。
【0076】
続いて、70mmφの丸棒を熱間鍛造により120mmφに延伸し、925℃で焼きならし処理した後、機械加工によりピッチ半径54mm、歯数27、モジュール4、歯幅9mm、軸穴半径35mm(等価丸棒径10.5mmφ)の試験用平歯車を作成した。一方、ピッチング試験片は棒鋼から試験面の直径が26mmの二円筒式のローラーピッチング疲労試験片を機械加工により製作した。
【0077】
次に、各試験片及び歯車を、3種類の製造方法(製法1〜3)に振り分けて処理した。
【0078】
「製法1」は、ガス浸炭と油焼入れを特徴とするもので、浸炭ガス雰囲気中で930℃×5時間加熱→850℃×1時間拡散→130℃油焼入れ→180℃×1時間焼戻しの条件で浸炭焼入れ、焼戻しを行うものである。この場合の焼入急冷度Hは0.15(cm-1)である。
【0079】
「製法2」は、真空浸炭とガス冷却を特徴とするもので、真空浸炭で930℃×5時間加熱→850℃×1時間拡散→窒素ガス冷却→180℃×1時間焼戻しの条件で浸炭焼入れ、焼戻しを行うものである。この場合の焼入急冷度Hは0.05(cm-1)である。
【0080】
「製法3」は、上記製法2における窒素ガス冷却を130℃油焼入に変更したものであり、この場合の焼入急冷度Hは0.15(cm-1)である。
【0081】
そして、上記製法により処理した各試験片及び歯車について、測定及び試験等を行った。
【0082】
各試験片に対しては次のような試験を行った。
【0083】
まず直径25mmの丸棒試験片について、ビッカース硬度計により横断面の硬さ分布(内部硬度)を調べた。また、浸炭焼入材の表層硬さ(表面硬度)は、表面から0.02mmの位置において測定した。更に、これと同等の位置においてトルースタイトの面積率を、走査型電子顕微鏡写真を画像解析することにより測定した。
【0084】
また、粒界酸下層は、光学顕微鏡にて、表層金属組織から最大の酸化層深さを測定した。
【0085】
また、表面炭素濃度は、X線マクロアナライザーにて表面から50μmの位置を測定した。
【0086】
また、残留オーステナイト面積率は、X線回折装置においてCo−Kα線を用いて部材表面を測定した。
また、表面残留応力は、X線応力測定器にてFe−Kα線を用いて半価幅中点法により測定した。
【0087】
また、歯車については、以下のようにギヤ精度及び寸法精度を評価した。
【0088】
ギヤ精度を評価するに当たっては、専用の精密ギヤ精度測定機を用い、左右歯面それぞれにギヤの圧力各方向の誤差量とネジレ角方向の誤差量を測定した。また、歯溝高さを全周測定し、最大値から最小値を差し引いた値を歯溝の振れとして算出した。
【0089】
寸法精度を評価するに当たっては、歯車の互いに向かい合った2つの歯溝にボールを入れ、その外周寸法を専用のO.B.D測定器を用いて測定した。
【0090】
その結果を表2、表3に示す。
【0091】
表2、表3に示すように、比較例である鋼種B1〜B8は曲げ疲労強度あるいは面疲労強度が低く、また、油冷却による部品は焼入れ歪による精度のばらつきが大きく、実用上問題が多い。
【0092】
鋼種B1〜B4はガス浸炭時の粒界酸化生成による不完全焼入組織ができており、それに伴い表面硬度も低いため各強度が低い。また、油冷却により、焼入れがガス冷却よりも急激であり、冷却むらも大きいため、焼入歪みによる精度のばらつきが大きくなった。
【0093】
また、鋼種B7、B8は、鋼材の焼入れ性に対して油冷却での焼入れが強すぎ、内部の硬度が上昇しすぎた。また表面の組織変態と内部の組織変態の割合差が小さくなり、即ち、表面硬度と内部硬度の差が小さくなり、結果的に表面圧縮残留応力が小さくなったため、各強度が低くなった。また、油冷却により、焼入れがガス冷却よりも急激であり、冷却むらも大きいため、焼入歪みによる精度のばらつきが大きくなった。
【0094】
一方、本発明例の鋼種A1〜A8では、何れも表面硬度が高く、内部硬度も適切な値を示し、また歪みも小さくに抑制でき、高強度と低歪みを両立できていることが明らかである。
【0095】
従って、本実施例からも、特定の合金鋼を素材として用い、酸化防止雰囲気において浸炭処理によって浸炭層を形成した後、上記特定の焼入急冷度Hの条件下において焼き入れした場合には、焼入歪みを十分に抑制しつつ高強度化を図ることができることがわかる。
【0096】
【表1】

Figure 0004229609
【0097】
【表2】
Figure 0004229609
【0098】
【表3】
Figure 0004229609
【0099】
【発明の効果】
以上説明してきたように、本発明による浸炭焼入歯車は、自動車の動力伝達部品として使用して、高い歯元の疲労強度及び耐ピッチング性を達成しつつ、歯車の熱処理歪みを低減することが可能である。従って、自動車の高出力化に対して歯車の大型化を行う必要がなく、更に浸炭を前提とした軸部品の高強度化にも適用でき、かつ、歯車を拘束治具による焼入れや浸炭後の矯正を省略できる、という優れた効果を有するもので、その工業上の効果は極めて顕著なものがある。[0001]
BACKGROUND OF THE INVENTION
The present invention has a high fatigue strength that can be applied to machine structural parts used for carburizing and quenching, particularly power transmission parts such as automobiles, and carburizing and quenching of carburized and hardened gears and the like that have little dimensional change by carburizing and quenching. The present invention relates to an input part and a manufacturing method thereof.
[0002]
[Prior art]
Mechanical structural parts, such as gears that are parts for power transmission of automatic transmissions, are surface-hardened to improve their bending fatigue strength, pitching strength, and wear resistance. Carburizing and quenching, in which gas carburizing and quenching with oil or the like, are widely performed. However, when conventional carburizing steel (for example, JIS-SCr420, SCM420, SNCM420) is subjected to gas carburizing, the oxidizing gas (CO2, H2O) produces a grain boundary oxide layer on the surface of the carburized part, and an incompletely hardened structure (truthite) is generated along with this. For this reason, surface hardness falls and the improvement effect of fatigue strength is inadequate. In view of this, the present inventor has proposed a carburizing steel in Japanese Patent Laid-Open No. 1-47838 for the purpose of preventing the formation of a grain boundary oxide layer and an incompletely quenched layer.
[0003]
However, more and more recent environmental measures for automobiles are required to promote weight reduction of parts in order to reduce fuel consumption, and parts with higher strength and higher dimensional accuracy are desired. For this reason, development of carburizing and quenching parts having sufficient strength and dimensional accuracy over carburizing and quenching parts using conventional carburizing steel is desired.
[0004]
In order to increase the fatigue strength of carburized and hardened parts, it is widely used to reduce the grain boundary oxide layer and to suppress the incomplete quenching layer on the surface layer. (2) There are a method of using a steel material containing a large amount of Mo and Ni, and (3) a method of increasing the cooling rate by lowering the oil temperature during quenching. However, when Si is reduced, sufficient strength cannot be obtained, and when a large amount of Mo or Ni is added, an expensive steel material is obtained, and the hardenability is improved to some extent, resulting in a problem of increasing the quenching strain. Similarly, the method of increasing the cooling rate by lowering the temperature of the quenching oil also has a problem of increasing quenching distortion.
[0005]
Since carburizing and quenching is a process of heating in a high-temperature carburizing atmosphere for a long time and then cooling, the volume change accompanying the phase transformation, transformation stress and thermal stress are generated during this treatment, and heat treatment distortion is inevitable. Will occur. In addition, since the carburizing and quenching process is performed at the final stage of part processing, the heat treatment distortion remains in the part as it is, and the dimensional accuracy deteriorates. Further, the carburized and hardened parts are subjected to shot peening treatment as necessary, but deformation due to processing-induced transformation of residual austenite existing in the carburized hardened layer occurs, resulting in deterioration of dimensional accuracy. Such deterioration in dimensional accuracy directly affects the performance of the component, and causes noise and vibration when the component is used.
[0006]
In order to solve such problems, there are a method of using a restraining jig and a method of correcting after carburizing treatment at the time of carburizing and quenching, but all have problems that require a great deal of cost and labor. For this reason, there is a strong demand for carburized parts with high strength and low quenching distortion.
[0007]
[Problems to be solved by the invention]
  Therefore, the present invention solves the above-mentioned problems of the prior art, and has high fatigue strength applicable to machine structural parts such as gears used by carburizing and quenching, and especially power transmission parts such as automobiles, and dimensions by carburizing and quenching. Little changeCarburized and hardened gearsAnd it aims at providing the manufacturing method.
[0008]
[Means for Solving the Problems]
As a result of extensive research on the above-mentioned problems of the prior art, the present inventors have obtained the following knowledge regarding the effects of steel components and carburizing and quenching methods on fatigue strength and carburizing strain. It was.
[0009]
(1) Many steel materials that reduce the grain boundary oxide layer and increase the surface hardness of conventional steel materials in ordinary gas carburizing / oil quenching become a mixed structure of bainite and martensite. As a result, the quenching distortion was large. Moreover, since the amount of retained austenite of the carburized hardened layer is as large as 20 to 25%, when the steel material was shot peened, the amount of deformation due to work-induced transformation of the retained austenite was large, and thus the strain was further increased.
[0010]
(2) When the current SCM420 was carburized and then slowly cooled, the surface hardness was equivalent to that of ordinary oil quenching, but the internal structure was bainite containing many ferrite phases, and the hardness was low.
[0011]
(3) When steel material with improved hardenability compared with the current steel is cooled slowly after carburizing, the internal non-carburized part becomes bainite that suppresses excessive increase in hardness, and the amount of retained austenite in the carburized layer is There were few.
[0012]
(4) By gear-cooling a gear made from a steel material having high hardenability, variation in the dimensional accuracy of the gear can be greatly improved as compared to a gear in which SCM420, which is the current hardenability, is oil-hardened.
[0013]
Based on these findings, the present inventors have improved the fatigue strength of carburizing steel by using a steel material that has a high hardenability compared to the current carburizing steel material and slowly cooling it slower than oil cooling. It was clarified that by suppressing the retained austenite to 15% or less while suppressing the oxide layer, deformation can be reduced by processing-induced transformation of the retained austenite during shot peening treatment while increasing the surface hardness. Furthermore, it has been found that although it is a high hardenability steel material, it can suppress the formation of internal martensite by slow cooling and has excellent characteristics in quenching strain and toughness.
[0014]
Then, based on the knowledge of increasing the strength described above, further research and studies were conducted, the following specific achievement means were clarified, and the present invention was achieved.
[0015]
(1) In order to prevent the formation of a grain boundary oxide layer and an incompletely quenched layer without excessive use of expensive alloy elements, the carburizing process is premised on a carburizing process in an antioxidant atmosphere, particularly a vacuum carburizing process.
[0016]
(2) As a steel material component, with the aim of improving the pitching strength and toughness, the temper softening resistance by increasing the Si content is increased, and the structural change during rolling fatigue is suppressed. In addition, by adding Mo and B, the impact characteristics are improved.
[0017]
(3) Further, the strength anisotropy is reduced by making the crystal grains finer and controlling the morphology of MnS.
[0018]
(4) By increasing the hardenability of the steel material and slowly cooling from a high temperature, the internal structure is bainite having a Hv of 350 or less and the amount of retained austenite in the carburized layer is 15% or less.
[0019]
(5) In order to achieve (4), the condition for slow cooling is a quenching quenching degree H of 0.01 to 0.08 cm.-1And
[0020]
  The quenching quenching degree H is an index that represents the strength of quenching, which is proposed by grossmann et al. And is widely used. The thermal conductivity of steel to be treated is γ (kcal / mh ° C.). ), Α (kcal /m 2 h °), it is defined by H = 0.5 × (α / γ).
[0021]
Based on these findings, the present inventors have prevented the occurrence of incompletely quenched structures in the grain boundary oxide layer and the surface layer, while increasing the high-temperature hardness and toughness, and quenching distortion and deformation after shot pinning treatment. In this way, we have developed a carburizing and quenching part manufacturing method and carburizing and quenching part, such as gears, which have excellent bending fatigue strength and pitching resistance at the tooth root part and can reduce distortion.
[0022]
That is, the gist of the present invention is as follows.
[0023]
  (1) In mass%,
C: 0.1 to 0.5%
Si: 0.4 to 1.0%,
Mn: 0.3 to 1.0%
Cr: 0.1 to 1.0%,
P: 0.003 to 0.015%,
S: 0.005 to 0.03%,
Al: 0.01 to 0.06%,
N: 0.005 to 0.03%
Including
Mo: 0.3 to 1.3%,
Ni: 0.1 to 1.0%
Using one or more of the following as a raw material, an alloy steel consisting of Fe and impurities,
The material is desiredGear shapeAfter molding intoUnder reduced pressure atmosphere reduced to 1-30 hPaBy carburizing inSo that the surface carbon content is 0.6-1.5 mass%After forming the carburized layer,Defined by equation (1) below with nitrogen gas coolingHardening quenching degree H is 0.01-0.08 (cm-1) Quenching treatmentThe surface compressive residual stress is set to 300 to 800 MPa.Carburizing and quenching characterized bygearManufacturing method.
Quenching rapid cooling H = 0.5 × (α / γ) (1)
here,
α: Heat transfer coefficient of the surface of the steel in a quenching atmosphere (kcal / m 2 h ℃)
γ: Thermal conductivity of the steel (kcal / mh ° C.)
It is.
[0024]
  (2) Alloy steel, as another element, in mass%,
V: 0.05-1.5%
Nb: 0.02 to 0.2%,
Ti: 0.01 to 0.2%
Containing one or more ofIt is characterized byCarburizing and quenching as described in (1) abovegearManufacturing method.
[0025]
  (3) Alloy steel, as another element, in mass%,
B: 0.0005 to 0.005%,
Ti: 0.005 to 0.1%
ContainsIt is characterized byCarburizing and quenching as described in (1) or (2) abovegearManufacturing method.
[0027]
  (4) Alloy steel, as another element, in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
Zr: 0.05% or less,
Te: 0.1% or less
Containing at least one selected from the group consisting ofIt is characterized byAbove (1) to (3) Carburizing and quenchinggearManufacturing method.
[0034]
  (5) Above (1)-(4), The quenching treatment is performed in the range of the quenching quenching degree H from the temperature of the austenite region to 300 ° C.gearManufacturing method.
[0038]
  (6)% By mass
C: 0.1 to 0.5%
Si: 0.4 to 1.0%,
Mn: 0.3 to 1.0%
Cr: 0.1 to 1.0%,
P: 0.003 to 0.015%,
S: 0.005 to 0.03%,
Al: 0.01 to 0.06%,
N: 0.005 to 0.03%
Including
Mo: 0.3 to 1.3%,
Ni: 0.1 to 1.0%
Of carbon steel, the balance carbon and the surface carbon content of the carburized layer is 0.6 to 1.5 mass%, the depth of the grain boundary oxide layer from the surface is 3 μm or less The surface residual compressive stress is 300 to 800 MPa, the surface hardness of the carburized layer is 700 to 900 Hv, the internal hardness of the non-carburized part inside the carburized layer is 250 to 350 Hv, and the residual austenite of the carburized layer A carburized and hardened gear having an area ratio of 15% or less.
[0039]
  (7)Alloy steel, as another element, in mass%,
V: 0.05-1.5%
Nb: 0.02 to 0.2%,
Ti: 0.01 to 0.2%
The carburized and hardened gear according to (6) above, which contains at least one of the following.
[0040]
  (8)Alloy steel, as another element, in mass%,
B: 0.0005 to 0.005%,
Ti: 0.005 to 0.1%
The carburized and hardened gear according to (6) or (7) above, characterized in that
[0041]
  (9)Alloy steel, as another element, in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
Zr: 0.05% or less,
Te: 0.1% or less
The carburized and hardened gear according to any one of (6) to (8) above, which contains at least one selected from the group consisting of:
[0042]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
[0043]
First, a method for manufacturing a carburized and hardened part specified in the present invention will be described.
[0044]
The carburized and quenched parts of the present invention are manufactured by carburizing and quenching, and in order to obtain a specific internal hardness, C: 0.1 to 0.5%, Mn: 0.3 to 1.0% Cr: 0.1 to 1.0% was contained. When a steel material having such a high hardenability is subjected to normal carburization and oil quenching, the internal structure becomes a structure in which martensite and bainite are mixed, and has high hardness and large heat treatment distortion. For this reason, in the present invention, the quenching quenching degree H is set to 0.01 to 0.08 cm by slow cooling with gas or the like.-1And Quenching quenching H is 0.01cm-1If it is less than the range, quenching is insufficient, the predetermined quenching structure and characteristics cannot be secured, and the member is insufficient in strength. Quenching quenching degree H is 0.08cm-1If it exceeds 1, quenching (rapid cooling) is excessive, the occurrence of structural transformation stress and thermal stress increases, the quenching distortion increases, and the component accuracy decreases.
[0045]
At the same time, the residual austenite of the carburized hardened layer is reduced to 15% or less by area ratio by gas cooling. Due to this low retained austenite, even if shot peening is performed after carburizing, changes to martensite due to work-induced transformation can be suppressed, and carburized and hardened parts such as carburized and hardened gears with low distortion can be obtained.
[0046]
Moreover, it is preferable that the internal structure of the carburized and quenched member is bainite. Thereby, a component having both strength and toughness can be obtained.
[0047]
  A carburized and hardened part formed of a carburized layer mainly composed of a martensite structure with a grain boundary oxide layer of 3 μm or less is achieved based on an antioxidant atmosphere, particularly vacuum carburizing. That is, the carburizing process is performed in a reduced pressure atmosphere reduced to 1 to 30 hPa.is necessary. Thereby, the said antioxidant atmosphere can be easily obtained by pressure reduction, and the grain boundary oxidation at the time of carburizing can fully be prevented. Here, if the value of the reduced pressure in the reduced pressure atmosphere is less than 1 hPa, it is excessive for suppressing the oxidation, and the apparatus for reducing the pressure has a high reduced pressure specification, resulting in an increase in cost. On the other hand, when it exceeds 30 hPa, there exist problems, such as a fall of an oxidation inhibitory effect and generation | occurrence | production of soot in a carburizing furnace.
[0048]
The carburizing treatment is also preferably performed in an atmosphere containing an inert gas as a main component. Also in this case, the antioxidation atmosphere can be easily formed. Examples of the inert gas include nitrogen gas and argon gas.
[0049]
  Carburizing treatment may be performed so that the surface carbon content of the carburized layer is 0.6 to 1.5 mass%.is necessary. The surface carbon concentration of the carburized layer affects the surface hardness of the carburized quenching material, and when the surface carbon content of the carburized layer is less than 0.6 mass%, there is a problem that the surface hardness is insufficient, If the amount exceeds 1.5% by mass, the amount of precipitated carbide increases, and the hardenability of the base is remarkably lowered, resulting in insufficient surface hardness.
[0050]
The grain boundary oxide layer is lower in strength than a general carburized and quenched layer, and if the grain boundary oxide layer is deeper than 3 μm, the strength of the member may be insufficient.
[0051]
In addition, when the grain boundary oxide is generated, the surrounding alloy elements are also taken into the grain boundary oxide by a chemical chemical reaction. As a result, the hardenability improving elements in the carburized quenching layer around the grain boundary oxide are taken in and consumed by the grain boundary oxide, resulting in insufficient quenching of the carburized quenching layer itself, insufficient hardness, and insufficient strength. May cause. For this reason, in this invention, the grain boundary oxide layer was 3 micrometers or less.
[0052]
  The compressive residual stress on the surface should be 300 MPa to 800 MPa.is necessary. That is, by adjusting the component composition of the material, the oxidation-preventing atmosphere during carburization, the heating temperature, the heating time, etc., the compressive residual stress on the surface can be increased to 300 MPa or more.is necessary. As a result, the tensile stress in the vicinity of the surface can be mitigated by the compressive residual stress in the vicinity of the surface of the member, and in particular, the dynamic strength (surface fatigue strength, bending fatigue strength, torsional fatigue strength) should be improved. Can do. On the other hand, when the surface compressive residual stress exceeds 800 MPa, in order to increase the amount of martensite, the cooling rate during the quenching process must be increased beyond the limit. For this reason, a large quenching distortion occurs, resulting in a problem that the dimensional accuracy of the member cannot be ensured.
[0053]
Also, the quenching treatment is performed between the austenite region temperature and 300 ° C., and the quenching quenching degree H 0.01 to 0.08 (cm-1It is preferable to perform quenching in the range of Thereby, sufficient hardening effect can be acquired. On the other hand, the quenching quenching degree H in the cooling from the temperature of the austenite region to 300 ° C. is 0.01 (cm-1), The quenching is insufficient, the desired quenching structure and characteristics cannot be ensured, and the member strength is insufficient. Further, the quenching quenching degree H in the cooling from the temperature of the austenite region to 300 ° C. is 0.08 (cm-1In the case of exceeding), rapid quenching is excessive, and the structure transformation stress and thermal stress increase, and the quenching distortion increases, which may reduce the accuracy of the parts.
[0054]
  In addition, the quenching process needs to be quenched by cooling with nitrogen gas. Cooling with nitrogen gas can ensure the quenching quenching degree H relatively easily, as well as ensuring safety during quenching, availability during mass production, cost, and ease of handling. Should.
[0057]
  The carburized layer should have a surface hardness of 700 to 900 Hv.is necessary. When the surface hardness of the carburized layer is less than 700 Hv, there may be a problem that the strength cannot be secured against the stress concentration near the member surface. In addition, there may be a problem of insufficient wear resistance on the outermost surface. On the other hand, when the surface hardness exceeds 900 Hv, the formation of carbides such as cementite on the surface layer is conceivable. On the other hand, there is a problem that the strength is insufficient and particularly the toughness is lowered. And in order to set it as the said surface hardness, the surface carbon content of a carburized layer shall be 0.6 to 1.5%.is necessary.
[0058]
Further, when the internal hardness of the non-carburized portion is less than 250 Hv, there may be a problem that the strength is insufficient, particularly the static strength is insufficient. On the other hand, when the internal hardness exceeds 350 Hv, considering the transformation rate of the structure, a large transformation stress is generated when quenching to ensure 350 Hv, and this causes a large quenching distortion, which causes a decrease in component accuracy. In the present invention, the hardness (Hv) of the internal non-carburized portion is set to 250 to 350 Hv.
[0059]
That is, the steel material adjusted to a specific component and having high hardenability is subjected to carburization in an antioxidant atmosphere, particularly vacuum carburization and slow cooling, particularly gas cooling, as described above. Carburized and hardened parts can be obtained and added to the member that combines the stress applied to the member due to the additional load acting on the member, the unevenness of the member shape, the surface of the member generated by holes, etc., and the stress concentration Ensures static strength (tensile strength, bending strength, torsional strength, etc.) and dynamic strength (surface fatigue strength, bending fatigue strength, torsional fatigue strength, etc.) from the surface to the inside (heart) against the stress distribution be able to.
[0060]
Next, the chemical composition of steel will be described.
[0061]
C is an element added to ensure the strength and toughness required for carburized parts, particularly the strength and toughness of the core (non-carburized part), but if it is less than 0.1%, such an effect can be sufficiently obtained. However, if it exceeds 0.5%, the hardness before quenching becomes too high, the processing cost increases and the toughness decreases and becomes brittle, and the structure transformation rate inside the non-carburized interior after carburizing quenching increases. Since the stress increases and a large quenching strain causes a decrease in component accuracy, the content is set to 0.1% to 0.5%.
[0062]
Si is an element that increases the temper softening resistance of carburizing steel and improves the high-temperature hardness. If the content is less than 0.4%, the effect is insufficient, and if it exceeds 1.0%, the effect is almost saturated. Therefore, the content is made 0.4% to 1.0%.
[0063]
Mn is an element necessary for imparting strength, toughness, and hardenability to steel, but if it exceeds 1.0%, it becomes a hard structure of bainite or martensite in cooling after hot rolling, and subsequent cutting, etc. In order not to be suitable for secondary processing, the content is set to 1.0% or less. However, if the added amount of Mn is less than 0.3%, the effect of hardenability is not sufficient, and the content is made 0.3% or more.
[0064]
Cr contributes to improving the mechanical properties, hardenability, and wear resistance of steel, but if this element also exceeds 1.0%, it becomes a hard structure of bainite or martensite in cooling after hot rolling, and then In order not to be suitable for secondary processing such as cutting, the content is set to 1.0% or less. However, if the addition amount of Cr is less than 0.1%, the effect of hardenability is not sufficient, and the content is 0.1% or more.
[0065]
P is an element that is extremely segregated at the grain boundary and impairs ductility or lowers the delayed fracture strength. For this reason, in order to avoid the above-mentioned adverse effects, it is necessary to keep it at 0.015% or less, and if it is less than 0.003%, the effect of improving ductility is almost saturated and the cost for reduction increases remarkably. 0.003% to 0.015%.
[0066]
S is an element necessary for improving machinability, and is contained in an amount of 0.005% or more. However, if the content exceeds 0.03%, the amount of inclusions in the steel increases and adversely affects cold plastic workability, so the upper limit is made 0.03%.
[0067]
Al is added to prevent coarsening of crystal grains and to achieve high ductility. To obtain the effect, addition of 0.01% or more is necessary, while when it exceeds 0.06%, the effect is saturated. Therefore, it is 0.01% or more and 0.06% or less.
[0068]
N is added for the purpose of bonding with Al during carburizing and refining the structure. However, in order to obtain the effect, 0.005% or more of addition is necessary. On the other hand, if it exceeds 0.03%, the effect is saturated. Therefore, the content is 0.005% or more and 0.03% or less.
[0069]
Mo and Ni are elements necessary for imparting predetermined hardenability to the steel and improving the strength and toughness, and one or more of them are added. In the present invention, for the purpose of improving toughness and low cycle fatigue strength, Mo is contained in an amount of 0.3% or more, and Ni is contained in an amount of 0.1% or more. However, even if Mo is contained in an amount exceeding 1.3% and Ni is contained in an amount exceeding 1.0%, the effect is saturated and the economic efficiency is impaired, so the upper limits are set to 1.3% and 1.0%, respectively.
[0070]
V, Ti, and Nb are elements that generate carbonitrides and are effective in refining carburized crystal grains, and can be arbitrarily added. In order to obtain the effect, it is necessary to contain 0.05% or more of V, 0.02% or more of Nb, and 0.01% or more of Ti. However, since the effect is saturated even if V exceeds 1.5%, Nb exceeds 0.2%, and Ti exceeds 0.2%, the upper limit is 1.5% for V. Nb is 0.2% and Ti is 0.2%.
[0071]
B segregates at the grain boundaries and has the effect of increasing the grain boundary strength. In order to obtain the effect, the B content is 0.0005% or more. On the other hand, if it exceeds 0.005%, the toughness is lowered, so the upper limit is made 0.005%. Moreover, in order to make B function effectively, the presence of the solid solution B which is not carbonitride is required. On the other hand, if free N exists, B becomes BN and the original function as B is impaired. Therefore, when adding B, it is necessary to simultaneously add Ti and fix N as TiN. In order to obtain the effect, Ti needs to be 0.005% or more and 0.1% or less. To obtain both the N fixing effect and the refinement of crystal grains, Ti: 0.11 to 0.2 % Content is required.
[0072]
Ca, Mg, Zr, and Te are elements that suppress the elongation of MnS and further improve the bending fatigue strength against the tooth root bending fatigue failure of the gear and the fatigue failure of the spline bottom of the shaft part. That is, in order to give the effect of suppressing the stretching of MnS, 0.01% or less of Ca, 0.01% or less of Mg, 0.05% or less of Zr, and 0.1% or less of Te are contained. However, even if it contains more than the said quantity with each element, since the effect will be saturated and economical efficiency will be impaired, it is set as an upper limit.
[0073]
In addition, even if the carburizing and quenching part of the present invention is subjected to a shot peening process after the carburizing and quenching process, the characteristics are not impaired at all. In particular, it has been confirmed that the tooth bending fatigue strength is further improved as compared with carburization. Therefore, the shot peening process can be performed after the carburizing and quenching process for the purpose of improving the strength of the carburized and quenched parts.
[0074]
【Example】
Examples will be described further below.
[0075]
Steels having the chemical components shown in Table 1 (steel A1 to B8 and steel B1 to B8) were melted and then ingot, and then rolled into pieces and rolled into a steel bar to produce a round bar having a diameter of 70 mm.
[0076]
Subsequently, a 70 mmφ round bar was stretched to 120 mmφ by hot forging, and after normalizing at 925 ° C., the pitch radius was 54 mm, the number of teeth was 27, the module was 4, the tooth width was 9 mm, and the shaft hole radius was 35 mm ( A spur gear for testing having an equivalent round bar diameter of 10.5 mmφ was prepared. On the other hand, a pitching test piece was manufactured by machining a two-cylinder roller pitching fatigue test piece having a diameter of 26 mm from a steel bar.
[0077]
Next, each test piece and a gear were distributed and processed into three types of manufacturing methods (manufacturing methods 1-3).
[0078]
“Manufacturing method 1” is characterized by gas carburizing and oil quenching, and is 930 ° C. × 5 hours heating in a carburizing gas atmosphere → 850 ° C. × 1 hour diffusion → 130 ° C. oil quenching → 180 ° C. × 1 hour tempering conditions Carburizing and quenching and tempering. The quenching quenching degree H in this case is 0.15 (cm-1).
[0079]
“Manufacturing method 2” is characterized by vacuum carburizing and gas cooling, and carburizing and quenching under conditions of vacuum carburizing at 930 ° C. for 5 hours → 850 ° C. × 1 hour diffusion → nitrogen gas cooling → 180 ° C. × 1 hour tempering. Tempering. The quenching quenching degree H in this case is 0.05 (cm-1).
[0080]
“Manufacturing method 3” is obtained by changing the nitrogen gas cooling in the above manufacturing method 2 to 130 ° C. oil quenching, and the quenching quenching degree H in this case is 0.15 (cm-1).
[0081]
And about each test piece and gear processed by the said manufacturing method, a measurement, a test, etc. were performed.
[0082]
Each test piece was tested as follows.
[0083]
First, for a round bar test piece having a diameter of 25 mm, the hardness distribution (internal hardness) of the cross section was examined with a Vickers hardness tester. The surface hardness (surface hardness) of the carburized and quenched material was measured at a position 0.02 mm from the surface. Further, the area ratio of troostite was measured by image analysis of a scanning electron micrograph at the same position.
[0084]
The grain boundary acid lower layer was measured for the maximum oxide layer depth from the surface metallographic structure with an optical microscope.
[0085]
The surface carbon concentration was measured at a position of 50 μm from the surface with an X-ray macroanalyzer.
[0086]
Moreover, the retained austenite area ratio measured the member surface using the Co-K (alpha) ray in the X-ray-diffraction apparatus.
Further, the surface residual stress was measured by a half-value width midpoint method using an Fe-Kα ray with an X-ray stress measuring instrument.
[0087]
Moreover, about the gearwheel, the gear precision and dimensional precision were evaluated as follows.
[0088]
In evaluating the gear accuracy, the error amount in each direction of the gear pressure and the error amount in the twist angle direction were measured on the left and right tooth surfaces using a dedicated precision gear accuracy measuring machine. Further, the height of the tooth gap was measured over the entire circumference, and the value obtained by subtracting the minimum value from the maximum value was calculated as the tooth gap runout.
[0089]
In evaluating the dimensional accuracy, a ball is placed in two tooth spaces facing each other of the gear, and the outer peripheral dimensions are set to a dedicated O.D. B. Measurement was performed using a D measuring instrument.
[0090]
The results are shown in Tables 2 and 3.
[0091]
As shown in Tables 2 and 3, steel types B1 to B8, which are comparative examples, have low bending fatigue strength or surface fatigue strength, and oil-cooled parts have a large variation in accuracy due to quenching strain, and have many practical problems. .
[0092]
Steel types B1 to B4 have an incompletely hardened structure due to grain boundary oxidation during gas carburization, and the surface hardness is low accordingly, so each strength is low. In addition, due to oil cooling, quenching was more rapid than gas cooling, and the cooling unevenness was also large, resulting in a large variation in accuracy due to quenching distortion.
[0093]
Steel types B7 and B8 were too hard to be hardened by oil cooling with respect to the hardenability of the steel material, and the internal hardness was too high. Further, the difference in the ratio between the surface structural transformation and the internal structural transformation was reduced, that is, the difference between the surface hardness and the internal hardness was reduced, and as a result, the surface compressive residual stress was reduced. In addition, due to oil cooling, quenching was more rapid than gas cooling, and the cooling unevenness was also large, resulting in a large variation in accuracy due to quenching distortion.
[0094]
On the other hand, in the steel types A1 to A8 of the present invention examples, it is clear that the surface hardness is high, the internal hardness also shows an appropriate value, the strain can be suppressed to a small value, and both high strength and low strain can be achieved. is there.
[0095]
Therefore, also from this example, using a specific alloy steel as a raw material, after forming a carburized layer by carburizing treatment in an antioxidant atmosphere, and then quenching under the condition of the specific quenching quenching degree H, It can be seen that the strength can be increased while sufficiently suppressing the quenching distortion.
[0096]
[Table 1]
Figure 0004229609
[0097]
[Table 2]
Figure 0004229609
[0098]
[Table 3]
Figure 0004229609
[0099]
【The invention's effect】
  As explained above, carburizing and quenching according to the present invention.Gears are used as power transmission parts for automobilesIt is possible to reduce the heat treatment distortion of the gear while achieving high tooth root fatigue strength and pitting resistance. Therefore, it is not necessary to increase the size of the gear to increase the output of the automobile, it can also be applied to increase the strength of the shaft parts on the premise of carburizing, and the gear is hardened by a restraining jig or after carburizing. It has an excellent effect that correction can be omitted, and its industrial effect is extremely remarkable.

Claims (9)

質量%で、
C:0.1〜0.5%、
Si:0.4〜1.0%、
Mn:0.3〜1.0%、
Cr:0.1〜1.0%、
P:0.003〜0.015%、
S:0.005〜0.03%、
Al:0.01〜0.06%、
N:0.005〜0.03%
を含むとともに、
Mo:0.3〜1.3%、
Ni:0.1〜1.0%
の1種以上を含有し、残部Fe及び不純物よりなる合金鋼を素材として用い、
該素材を所望の歯車形状に成形した後、1〜30hPaに減圧した減圧雰囲気下において浸炭することによって表面炭素量が0.6〜1.5質量%になるように浸炭層を形成し、該浸炭処理後に窒素ガス冷却により下記(1)式で定義する焼入急冷度Hが0.01〜0.08(cm−1)となる焼入処理を行って表面圧縮残留応力を300〜800MPaとすることを特徴とする浸炭焼入歯車の製造方法。
焼入急冷度H=0.5×(α/γ)・・・(1)
ここで、
α:焼入雰囲気における上記鋼の表面の熱伝達係数(kcal/m h℃)
γ:上記鋼の熱伝導率(kcal/mh℃)
である。
% By mass
C: 0.1 to 0.5%
Si: 0.4 to 1.0%,
Mn: 0.3 to 1.0%
Cr: 0.1 to 1.0%,
P: 0.003 to 0.015%,
S: 0.005 to 0.03%,
Al: 0.01 to 0.06%,
N: 0.005 to 0.03%
Including
Mo: 0.3 to 1.3%,
Ni: 0.1 to 1.0%
Using one or more of the following as a raw material, an alloy steel consisting of Fe and impurities,
After forming the material into a desired gear shape , carburizing in a reduced pressure atmosphere reduced to 1 to 30 hPa to form a carburized layer so that the surface carbon content is 0.6 to 1.5% by mass , After the carburizing treatment, a quenching treatment in which the quenching quenching degree H defined by the following formula (1) is 0.01 to 0.08 (cm −1 ) by cooling with nitrogen gas is performed, and the surface compressive residual stress is 300 to 800 MPa. A method for manufacturing a carburized and hardened gear .
Quenching rapid cooling H = 0.5 × (α / γ) (1)
here,
α: Heat transfer coefficient of the steel surface in the quenching atmosphere (kcal / m 2 h ° C)
γ: Thermal conductivity of the steel (kcal / mh ° C.)
It is.
合金鋼が、更に他の元素として、質量%で、
V:0.05〜1.5%、
Nb:0.02〜0.2%、
Ti:0.01〜0.2%
の1種以上を含有することを特徴とする請求項1に記載の浸炭焼入歯車の製造方法。
Alloy steel, as another element, in mass%,
V: 0.05-1.5%
Nb: 0.02 to 0.2%,
Ti: 0.01 to 0.2%
One or more types of these are contained, The manufacturing method of the carburizing hardening gear of Claim 1 characterized by the above-mentioned.
合金鋼が、更に他の元素として、質量%で、
B:0.0005〜0.005%、
Ti:0.005〜0.1%
を含有することを特徴とする請求項1または2に記載の浸炭焼入歯車の製造方法。
Alloy steel, as another element, in mass%,
B: 0.0005 to 0.005%,
Ti: 0.005 to 0.1%
The method for manufacturing a carburized and hardened gear according to claim 1 or 2, characterized by comprising :
合金鋼が、更に他の元素として、質量%で、
Ca:0.01%以下、
Mg:0.01%以下、
Zr:0.05%以下、
Te:0.1%以下
よりなる群から選択される、少なくとも1種以上を含有することを特徴とする請求項1〜のいずれかに記載の浸炭焼入歯車の製造方法。
Alloy steel, as another element, in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
Zr: 0.05% or less,
Te: It contains at least 1 sort (s) or more selected from the group which consists of 0.1% or less, The manufacturing method of the carburizing hardening gear in any one of Claims 1-3 characterized by the above-mentioned.
請求項1〜のいずれか1項において、上記焼入処理はオーステナイト領域の温度から300℃までの間、上記焼入急冷度Hの範囲において焼入を行うことを特徴とする浸炭焼入歯車の製造方法。In any one of claims 1-4, during the quenching process to 300 ° C. from the temperature of the austenite region, carburized gear and performing quenching in the above range quenching quenching intensity H Manufacturing method. 質量%で、% By mass
C:0.1〜0.5%、C: 0.1 to 0.5%
Si:0.4〜1.0%、Si: 0.4 to 1.0%,
Mn:0.3〜1.0%、Mn: 0.3 to 1.0%
Cr:0.1〜1.0%、Cr: 0.1 to 1.0%,
P:0.003〜0.015%、P: 0.003 to 0.015%,
S:0.005〜0.03%、S: 0.005 to 0.03%,
Al:0.01〜0.06%、Al: 0.01 to 0.06%,
N:0.005〜0.03%N: 0.005 to 0.03%
を含むとともに、Including
Mo:0.3〜1.3%、Mo: 0.3 to 1.3%,
Ni:0.1〜1.0%Ni: 0.1 to 1.0%
の1種以上を含有し、残部Fe及び不純物よりなる合金鋼からなり、浸炭層の表面炭素量が0.6〜1.5質量%であり、表面からの粒界酸化層深さが3μm以下であり、表面の圧縮残留応力が300〜800MPaであり、浸炭層の表面硬度が700〜900Hv、浸炭層の内側の非浸炭部の内部硬度が250〜350Hvであり、さらに、浸炭層の残留オーステナイト面積率が15%以下であることを特徴とする浸炭焼入歯車。Of carbon steel, the balance carbon and the surface carbon content of the carburized layer is 0.6 to 1.5 mass%, the grain boundary oxide layer depth from the surface is 3 μm or less The surface residual compressive stress is 300 to 800 MPa, the surface hardness of the carburized layer is 700 to 900 Hv, the internal hardness of the non-carburized portion inside the carburized layer is 250 to 350 Hv, and the residual austenite of the carburized layer A carburized and hardened gear having an area ratio of 15% or less.
合金鋼が、更に他の元素として、質量%で、Alloy steel, as another element, in mass%,
V:0.05〜1.5%、V: 0.05-1.5%
Nb:0.02〜0.2%、Nb: 0.02 to 0.2%,
Ti:0.01〜0.2%Ti: 0.01 to 0.2%
の1種以上を含有することを特徴とする請求項6に記載の浸炭焼入歯車。The carburized and hardened gear according to claim 6, comprising at least one of the following.
合金鋼が、更に他の元素として、質量%で、Alloy steel, as another element, in mass%,
B:0.0005〜0.005%、B: 0.0005 to 0.005%,
Ti:0.005〜0.1%Ti: 0.005 to 0.1%
を含有することを特徴とする請求項6または7に記載の浸炭焼入歯車。The carburized and hardened gear according to claim 6 or 7, characterized in that
合金鋼が、更に他の元素として、質量%で、Alloy steel, as another element, in mass%,
Ca:0.01%以下、Ca: 0.01% or less,
Mg:0.01%以下、Mg: 0.01% or less,
Zr:0.05%以下、Zr: 0.05% or less,
Te:0.1%以下Te: 0.1% or less
よりなる群から選択される、少なくとも1種以上を含有することを特徴とする請求項6〜8のいずれかに記載の浸炭焼入歯車。The carburized and hardened gear according to any one of claims 6 to 8, comprising at least one selected from the group consisting of:
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