JP2004300519A - Manufacturing method of hot-forged component for high-temperature carburization and hot-forged component for high-temperature carburization manufactured through the same - Google Patents

Manufacturing method of hot-forged component for high-temperature carburization and hot-forged component for high-temperature carburization manufactured through the same Download PDF

Info

Publication number
JP2004300519A
JP2004300519A JP2003094987A JP2003094987A JP2004300519A JP 2004300519 A JP2004300519 A JP 2004300519A JP 2003094987 A JP2003094987 A JP 2003094987A JP 2003094987 A JP2003094987 A JP 2003094987A JP 2004300519 A JP2004300519 A JP 2004300519A
Authority
JP
Japan
Prior art keywords
temperature
aln
carburizing
hot
precipitation
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
JP2003094987A
Other languages
Japanese (ja)
Other versions
JP4212941B2 (en
Inventor
Yasuhiro Fukuda
康弘 福田
Takumi Kozuka
巧 小塚
Isao Sumita
庸 住田
Kinsei Kino
欣成 嬉野
Tadashi Eriguchi
正 江里口
Kazuhiko Mori
和彦 森
Original Assignee
Aichi Steel Works Ltd
愛知製鋼株式会社
Toyota Motor Corp
トヨタ自動車株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Aichi Steel Works Ltd, 愛知製鋼株式会社, Toyota Motor Corp, トヨタ自動車株式会社 filed Critical Aichi Steel Works Ltd
Priority to JP2003094987A priority Critical patent/JP4212941B2/en
Publication of JP2004300519A publication Critical patent/JP2004300519A/en
Application granted granted Critical
Publication of JP4212941B2 publication Critical patent/JP4212941B2/en
Active legal-status Critical Current
Anticipated expiration legal-status Critical

Links

Images

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a hot-forged component for high-temperature carburization which inhibits the occurrence of duplex grain size and greatly shortens carburization time, particularly when the component is subjected to hot-temperature carburization after hot forging. <P>SOLUTION: A hot-rolled steel material comprising, by weight, 0.10-0.30% C, 0.05-0.50% Si, 0.30-1.50% Mn, 0.30-2.00% Cr, 0.005-0.050% Al, 0.01-0.10% Nb, 0.0080-0.0250% N, ≤0.01% V, ≤0.80% Mo if required and the balance being Fe and unavoidable impurities is heated to 1,150-1,350°C, subsequently hot-forged at a forging finishing temperature of 1,100-1,300°C so that the Nb contained in the steel substantially becomes solid solution (with number of deposited Nb of ≤0.3/μm<SP>2</SP>), cooled to 620-700°C after forging, retained at that temperature for 30 min to 5 hr, cooled to room temperature and subjected to carburization. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【0001】
【発明の属する技術分野】
本発明は、1000℃以上の高温浸炭処理での結晶粒異常成長(混粒の発生)を抑制するための高温浸炭用熱間鍛造部品の製造方法及びその方法により製造された高温浸炭用熱間鍛造部品に関する。
【0002】
【従来の技術】
自動車、建設車両、建設機器等に使用される歯車やシャフト等の動力伝達に使用される鋼部品には、浸炭処理により表面に硬化層を形成する肌焼鋼が多用される。これは、前記部品には優れた耐摩耗性と高靭性を同時に要求されるため、表面は浸炭処理により硬い組織として耐摩耗性を確保し、内部は低Cのままとして高い靭性をもたせるためである。
【0003】
最近、これらの部品の高強度化と共に大幅な製造コスト低減が大きな課題になっている。部品の製造コストは、材料自体のコストと浸炭等の熱処理コストに大きく分けることができるが、前者については、特に高価な成分元素が多量に添加されていない肌焼鋼の場合、大きなコスト低減は困難であり、後者の熱処理コストの削減方法の研究が盛んに検討されている。
【0004】
その中でも、最近検討が進められている方法は高温浸炭処理である。現在肌焼鋼の浸炭処理は、その大部分がガス浸炭処理法により行われており、所定の硬化深さを得るために、4〜20時間程度もの長時間の処理が実施されている。その結果、生産性の面でも問題になるとともに、多大なエネルギーを消費するため、改善が強く要望されていた。高温浸炭処理は、浸炭温度を高く設定して反応を促進させることにより、短時間でより多くの炭素原子を侵入及び拡散させて処理時間の短縮を図る方法で、時間短縮に最も効果的な方法として古くから知られている。
【0005】
しかしながら、高い浸炭温度での処理は、処理時間の短縮には効果的な方法であるが、一方で大きな問題が生じる。すなわち、浸炭処理後にオーステナイト粒が粗大化したり混粒が生じることである。浸炭処理後においてこのようなオーステナイト粒の粗大化や混粒が生じると、強度が低下したり、熱処理歪のバラツキが生じる。通常、浸炭処理後は研磨等の必要最小限の機械加工を施すだけであるのが普通であり、このような歪のバラツキは製品寸法不良の原因となり、問題となる。そのため、実際には浸炭温度を高めて処理時間の短縮を進めることが十分にできていないのが現状である。
【0006】
このような浸炭処理時におきる結晶粒粗大化と混粒化現象はかなり以前から知られており、様々な対策が検討され、新しい技術が提案されており、多数の特許出願がされている。
【0007】
その中でも最も良く知られている方法は、AlNを微細分散させてピン止め効果により粗大化を防止する方法であり、例えば、特許文献1、2に示される鋼が提案されている。
【0008】
また、Nbの炭窒化物は、AlNよりも高温まで固溶しにくいことから、より高温での結晶粒粗大化防止効果を期待して、Nbを添加し粗大化防止を図るという提案もされている。例えば、特許文献3、4に示される鋼がある。
【0009】
しかしながら、AlNやNb(C、N)を析出させることは、確かに結晶粒粗大化防止に効果を示すことが確認されているが、単純なAl、N、Nbの添加量の調整だけでは、十分な効果が得られないことが判明し、AlNやNb炭窒化物をよりピン止め効果の大きい状態に析出させた状態とするための熱処理を行って、粗大化防止を図るという提案もされている。例えば、特許文献5〜7に示される熱処理方法が提案されている。この3件の文献に記載された内容は、対象とする鋼成分に差異はあるが、全て浸炭処理前に600℃〜A1変態点の温度域に加熱してAlN、Nb(C、N)の析出状態を浸炭処理時に粗大化しにくい状態に変化させることを特徴とするものである。
【0010】
また、特許文献8、9には、圧延、熱間鍛造時の加熱及び仕上温度を適切に調整し、800〜500℃の間を1℃/秒以下の速度で冷却することによって、Nb(C、N)を多量に微細分散するとともに、AlNの析出量を抑制し、粗大化を抑制する技術について記載されている。
【0011】
【特許文献1】
特開昭56−75551号公報
【特許文献2】
特開昭59−123714号公報
【特許文献3】
特開昭49−125220号公報
【特許文献4】
特開平6−299241号公報
【特許文献5】
特開昭58−16022号公報
【特許文献6】
特開昭62−205229号公報
【特許文献7】
特開平10−121128号公報
【特許文献8】
再公表特許99/05333号公報
【特許文献9】
特開2001−303174号公報
【0012】
【発明が解決しようとする課題】
しかしながら、前記した今までに提案された方法には、次の問題がある。
即ち、特許文献1、2に記載されているAlNによるピン止め効果は、980℃未満の浸炭処理ではある程度の効果を得ることができるものの980℃以上の浸炭温度になると微細分散させたAlNのかなりの割合が固溶してしまいピン止め効果による結晶粒粗大化防止効果が十分に得られなくなる。従って、本発明で狙いとしている1000℃以上の高温での浸炭処理においては、その効果は非常に小さいものとなり、粗大化を完全に防止することができない。
【0013】
また、AlNに比べ高温での結晶粒安定化効果を期待してNbを添加したことを特徴とする特許文献3、4に記載の発明は、添加量についてしか検討されておらず、どのような状態に析出させた場合に大きな粗大化防止効果が得られるかについての検討がほとんどされていない。本発明者等が検討した結果によると、熱間鍛造材を浸炭処理する場合、析出状態を最適に調整しないと混粒の発生を防止することが難しいことが判明した。特にAlNの析出状態に注意する必要があるが、特許文献3、4には、その点について全く記載されていない。
【0014】
また、浸炭処理前の熱処理によって、混粒発生を防止することを特徴とする特許文献5〜7の発明についても以下の問題がある。すなわち、特許文献5は、Nbを含有しない鋼を対象とした発明であり、特許文献6もNbについて全く記載がなく、浸炭処理する前のAlN、Nb(C、N)の最適な析出状態について全く検討されていない。また、特許文献7は、Nb炭窒化物の析出状態については、かなり詳細に検討されているが、同時に存在しているAlNの影響について考慮されていない。また、熱間鍛造後にNb炭窒化物を析出させ、その後の熱処理により凝集させることが特徴となっているが、本発明者等が調査した結果、AlNが多く存在した状態で、前記熱処理を行うと、AlNの影響で炭窒化物が凝集しやすくなるとともに、AlNを多く含むNb炭窒化物が多く存在した状態となり、1000℃以下の温度での浸炭処理には効果があるが、1000℃以上の高温で浸炭処理した場合には、十分に粗大化を防止できなくなることが判明した。
【0015】
さらに、特許文献8、9には、AlNの析出量を抑制して粗大化を防止する技術について記載されているが、特許文献8は、圧延後の析出状態しか記載されておらず、特許文献9にも記載されているように、熱間鍛造後に焼準することが前提となっており、最も問題となる浸炭直前の析出状態を熱間鍛造品の場合にいかなる手段で達成すれば良いかについて、全く記載されていない。また、特許文献9は、本発明と同様に熱間鍛造後のAlN、Nb(C、N)の析出状態を鍛造条件、鍛造後の冷却条件の指定によって達成するものであるが、鍛造後の冷却条件が徐冷と記載されているものの、この文献で記載の徐冷という意味は単純な空冷に比べて遅く冷却するという意味であり、Nb(C、N)の析出に必要な時間と比較するとかなり早く(記載されている0.1〜1℃/秒の冷却速度では、本発明で指定の620〜700℃をわずか80〜800秒で通過することになる。)、温度も析出させるのに適した温度のみに限定されていないため、十分な微細析出がされない場合が多いことがわかった。
【0016】
本発明は、以上記載した問題点を解決するために成されたものであり、1050℃程度の高温で浸炭処理した場合でも異常な粒成長を防止することができ、かつ高温での処理を可能とすることで、浸炭処理の生産性を大幅に改善可能とすることを可能とする高温浸炭用熱間鍛造部品の製造方法を提供することを目的とする。
【0017】
【課題を解決するための手段】
請求項1の発明は、重量比でC:0.10〜0.30%、Si:0.05〜0.50%、Mn:0.30〜1.50%、Cr:0.30〜2.00%、Al:0.005〜0.050%、Nb:0.01〜0.10%、N:0.0080〜0.0250%、V:0.01%以下を含有し、残部Fe及び不純物元素からなる熱間圧延鋼材を1150〜1350℃に加熱後、鍛造仕上温度が1100〜1300℃となる条件で熱間鍛造して、鋼中に含有するNbがほぼ固溶した状態(析出個数が0.3個/μm以下)とし、鍛造後620〜700℃まで冷却後、その温度で30分〜5時間保持し、室温まで冷却後浸炭処理することを特徴とする高温浸炭用熱間鍛造部品の製造方法である。
【0018】
本発明において注目すべきことは、上記特定組成の肌焼鋼を用いて熱間加工後浸炭処理する際において、従来の結晶粒粗大化防止鋼のように、AlNやNbの炭窒化物を単純に微細析出させるのではなく、熱間鍛造時に高温に加熱かつ高温で鍛造を終了させて、Nb炭窒化物がほぼ固溶した状態とし、その後室温に冷却することなく620〜700℃で加熱保持することによって、Nb炭窒化物を多数析出させることにある。このような析出状態としたことにより、浸炭昇温中に既に析出しているNb(C、N)の周囲に優先的にAlNを析出させ、複合析出物にすることで、高温浸炭時でも固溶しにくく確実にピン止め効果を得られるため、混粒発生の抑制を可能にすることができる。
【0019】
従来は、AlN、Nb(C、N)が共に結晶粒微細化に効果が大きいと考えられており、両者共に微細分散させることが結晶粒の粗大化防止に効果が大きいと考えられてきた。しかしながら、AlNがNb(C、N)に比較して固溶しやすく高温での結晶粒粗大化への効果が大きく期待できないことが判明するにつれて、Nb(C、N)を中心とした微細化対策が中心に考えられるようになった。しかしながら、その場合でもAlNの存在が異常粒成長防止にとって弊害となるという考え方は、前記した特許文献8、9等ごく一部を除いて皆無であった。
【0020】
それに対し、本発明者等が詳細に調査した結果、単独のAlNが多量に析出した状態からなる鋼は、下記の点で問題があることが判明した。
▲1▼AlN、特に単独のAlNが多量に析出した鋼を浸炭処理すると、結晶粒はAlNのピンニング効果によって微細となるが、微細であるため、逆に粒成長の駆動力が大きくなる。
【0021】
▲2▼比較的低い温度で浸炭処理する場合には、浸炭時もAlNの析出物が残留してピンニング効果を得ることができるため、粗大化の駆動力は大きいものの、粗大化防止に大きな効果を得ることができる。
【0022】
▲3▼しかしながら、浸炭処理温度が高くなると、析出していたAlNの固溶が進み、AlN析出量の少ない領域が生じはじめる。その結果、部分的にピンニング効果が十分に得られない領域が生じ、粗大化の駆動力を抑制することができなくなり、部分的な異常粒成長(混粒)が発生する。
【0023】
本発明は、上記のAlNを積極的に利用した結晶粒粗大化防止技術の問題点を解決するために成されたもので、以下の知見を得ることにより成されたものである。
▲1▼熱間鍛造時に高温加熱、高温鍛造して、Nbがほぼ固溶した状態(0.3個/μm以下)とし、室温まで冷却することなく、620〜700℃の温度まで冷却、加熱保持した場合には、AlNとの複合又は単独のNb(C、N)が多量かつ微細に分散し、かつ単独のAlN析出が少ない鋼を得ることができる。
【0024】
▲2▼単独のAlN析出を抑制した本発明からなる鍛造部品(以下、本発明鋼と記載)を加熱して達成される浸炭初期粒度は、単独のAlN析出を抑制していないNb添加鋼に比べ、かなり粗粒になり、粒成長の駆動力を小さく抑えることができる。1000℃を超える温度での浸炭処理中において、Nb炭窒化物は従来のNb添加鋼でも固溶せずに存在するが、本発明鋼は、比較的粗粒となった状態を得ることができるので、従来のNb添加鋼では十分に抑制できなかった部分的な異常粒成長を確実に防止することができる。
【0025】
▲3▼単独のAlN析出の抑制を図った本発明鋼を高温浸炭処理すると、その加熱途中には、AlNが析出するが、この析出は加熱前に既に析出しているNb(C、N)の周囲に優先的に析出する。従って、浸炭処理前だけでなく浸炭昇温中においても単独のAlN析出の生成を抑制することができるため、前記した通り浸炭初期に比較的粗粒の結晶粒からなる組織となる。そして、処理が進行し、温度が1000℃を超えると、AlNの固溶が進んでいくが、このような高温状態でもNb(C、N)は残存するため、十分なピンニング効果が確保できる。また、粗粒であるため粗大化の駆動力が小さいことから、混粒の発生を防止することが可能になる。
【0026】
また、特許文献8、9では、AlNを抑制し、Nb(C、N)を多量微細析出させている点では同一であるが、本発明では、Nb(C、N)を620〜700℃という最も析出させるのに条件の良い温度領域で保持することによって確実に微細析出させており、特許文献8、9の発明に比べ確実かつ効率的に微細析出させることができるので、より確実に混粒発生を防止することができる。
【0027】
次に、請求項1の製造方法で用いられる肌焼鋼の化学成分の限定理由について説明する。
C:0.10〜0.30%
浸炭処理を行った部品に要求される強度、内部硬さを確保するためには、0.10%以上のCを含有する必要がある。しかし、0.30%を超えて含有させると内部の靱性が劣化し、さらには被削性を低下させるため、上限を0.30%とした。
【0028】
Si:0.05〜0.50%
Siは鋼の製造時において脱酸のために必要な元素であり、最低でも0.05%以上の含有が必要である。しかしながら、Siは浸炭処理時、浸炭雰囲気中の酸素と反応して酸化物を形成する。このため被処理品の表層付近は焼入性が低下し、いわゆる浸炭異常層を形成する。従って、多量に含有させると浸炭異常層の生成による悪影響が大きくなって強度が低下するとともに、被削性が低下するので、上限を0.50%とした。
【0029】
Mn:0.30〜1.50%
Mnは、必要な焼入性を確保して内部まで強度を確保するのに必要な硬さを保証するためには、0.30%以上のMnを含有する必要がある。しかしながら、多量に含有させると、残留オーステナイトが増加して、硬さ低下、内部の靭性が劣化するとともに、被削性が低下するので、上限を1.50%とした。
【0030】
P:0.035%以下
Pは製造時に混入が避けられない不純物である。本発明では特に必須の条件としては限定していないが、粒界の強度を低下させ、疲労特性を悪化させる原因となる元素であるので、その上限を0.035%以下とすることが好ましい。
【0031】
S:0.030%以下
SはPと同様に製造時に少量の混入が避けられない不純物であり、例えばMnS等のような硫化物系介在物となって存在している。しかし、この介在物は、疲労破壊の起点となったり、耐ピッチング性を低下させたり、鋼材の異方性が大きくなる原因となる元素である。従って、本発明では、必須では限定していないが、理想的には極力低減することが好ましく、上限を0.030%とした方がより好ましい。
【0032】
Cr:0.30〜2.00%
Crは、焼入性を向上させ、必要な強度を確保し、本発明により製造した鋼の性能を向上させるために必要な元素であり、0.30%以上の含有が必要である。しかしながら、多量に含有させると靭性が劣化するとともに被削性が低下するため、上限を2.00%とした。
【0033】
Al:0.005〜0.050%、
Alは、Siと同様に脱酸に必要な元素であるとともに、AlNとして存在し、ピン止め効果により浸炭処理後の異常粒成長防止に効果のある元素である。従って、最低でも脱酸に必要な量を添加する必要があり、下限を0.005%とした。しかしながら、本発明では従来の結晶粒粗大化防止鋼とは異なり、AlNの析出を抑制しており、全くピンニング効果を期待していないわけではないが、主となるピンニング効果は、Nb(C、N)により得ている。また、Alを多量に含有していると、浸炭処理前の単独のAlN析出数が増加し、浸炭初期粒径の微細化につながるので、上限を0.050%とした。
【0034】
N:0.0080〜0.0250%
Nは上述の通り、AlやNbと結合し、AlNやNb(C、N)となって鋼中に存在し、浸炭処理後の異常粒成長を防止するために効果のある元素である。この効果を十分に得るためには、0.0080%以上のNを含有させる必要がある。しかしながら、AlNやNb(C、N)の析出量には適量があり、多すぎると浸炭初期粒径が細かくなって却って異常粒成長が起きやすくなってしまうため、上限を0.0250%とした。
【0035】
Nb:0.01〜0.10%
Nbは本発明において最も重要な元素であり、炭窒化物となって鋼中に存在し、特にAlに比べ高温度での浸炭処理における結晶粒異常成長を防止する効果の大きい元素である。Nb添加量が少ない場合、特に1050℃以上の浸炭では浸炭処理前に析出していた炭窒化物の一部が固溶し、ピン止め効果に寄与するNb炭窒化物の量が不足して粗粒化抑制作用が十分に得られなくなるので、下限を0.01%とした。一方、多量に含有させると、熱間鍛造時の加熱によってNb(C、N)が十分に固溶した状態とならず、粗大なNb(C、N)の析出物が残存した状態となって、ピンニング効果が低下するので、上限を0.10%に規定した。
【0036】
V:0.01%以下
VはNbと同様に炭窒化物を形成し、ピン止め効果により結晶粒成長の防止に寄与する元素であるが、Vの炭窒化物はNbの炭窒化物に比べ高温で固溶しやすく、1000℃以上の高温浸炭の場合、浸炭加熱によって固溶して浸炭中にピン止め効果が消失し、結晶粒成長抑制効果が得られなくなるので、高温浸炭される場合には、Vよりも高温浸炭処理温度において固溶しにくい炭窒化物を形成する元素に、鋼中のC、Nを優先的に結合させておく必要がある。Vが含有していると、鋼中のC、Nの一部がVと結合し、浸炭初期粒径を微細化する作用が生じ、かつ1000℃以上の浸炭中にそれらが固溶して、ピン止め効果を消失させるので、異常粒成長を助長する。従って、高温浸炭時にはVが存在すると逆に異常粒成長が起きやすくなる。Vは積極添加しなくても鋼の製造時に使用するスクラップ等から少量混入する可能性のある元素であるため、不純物として含有するV量を少なく抑える必要があり、上限を0.01%に規制した。
【0037】
次に、請求項1の発明の製造条件の限定理由について、以下に説明する。
鍛造時の加熱温度を1150〜1350℃としたのは、鍛造時の加熱の際に圧延鋼材中にNb(C、N)を十分に固溶(具体的には析出個数で0.3個/μm以下)させて、後述の620〜700℃の温度保持(以下析出処理と記す)中に微細に析出させるためである。鍛造時の加熱温度が低く固溶が不十分になってNb(C、N)の粗大な析出物が残存していると、鍛造後の析出処理時に残存している析出物がさらに成長して大きな析出物となり、Nb(C、N)の析出物が結晶粒粗大化に寄与しなくなってしまうので、温度の下限を1150℃としてNb(C、N)を十分に固溶させておく必要がある。但しあまり温度を高くしすぎるのは、エネルギーの無駄となるので、上限を1350℃とした。鍛造仕上温度を1100〜1300℃としたのも、同じ理由である。
【0038】
本発明の鍛造部品は、熱間鍛造後620〜700℃まで冷却され、この温度域で保持することにより析出処理される。ここで、鍛造直後から析出処理温度までの冷却条件を特に指定していないのは、徐冷、空冷、放冷、加速空冷(ファン冷却)等、通常の鍛造工場で実施できるどのような条件で行っても同様の効果が得られるからである。従って、実際に実施する工場において、最も都合の良い条件を選択して実施することができる。
【0039】
以上説明したように、620〜700℃の温度域まで冷却した後、この温度域で保持し、Nb(C、N)を微細析出させる。析出処理の温度範囲の下限を620℃としたのは、この温度より低い温度で保持してもNb(C、N)の析出が効率良く進まないためであり、上限を700℃としたのは、温度が変態温度を超えて2相域に入ってしまうと、変態が析出処理の後に起きることになり、その後の冷却によってベイナイトやマルテンサイトが部分的に生成し、その影響で粗大化を防止することが難しくなるためである。
【0040】
また、析出処理の温度保持時間を30分〜5時間としたのは、析出処理に必要な時間を考慮すると、この程度の時間が最も適当であるからである。すなわち、30分より短くなると、十分に析出させることが難しくなるためであり、5時間より長くなると、十分に析出させることはできるが、生産性が低下して熱処理コストが増加するからである。
【0041】
析出処理が終了した後は、室温まで冷却する。なお、析出処理は変態点より低い温度での熱処理であるため、析出処理の終了時点で既に変態は終了している。従って、得られる組織が室温までの冷却条件に左右されることはなく、実施する場所の都合に応じて適切に選択すれば良い。但し500℃までの冷却は単独のAlNの析出する時間的余裕を与えないようにするために、25℃/分以上で冷却するのが望ましい。
以上説明した手順で熱間鍛造、析出処理を行うことにより、単独のAlNの析出数が少ない高温浸炭用熱間鍛造部品を得ることができる。
【0042】
次に、請求項2の発明のように、請求項1の製造方法で使用される肌焼鋼に加え、Moを0.80%以下含有させた鋼を用いることもできる。以下、その限定理由を記載する。
【0043】
Mo:0.80%以下
Moは、焼入性及び靱性を向上させるとともに、浸炭異常層を抑制して強度を向上させる効果を有する元素であり、必要に応じ少量添加して使用することができる元素である。しかしながら、多量に添加すると、残留オーステナイトが増加し、浸炭硬さの低下の原因になるとともに、内部の靭性、被削性を低下させるため、0.80%を上限とした。
【0044】
また、請求項3の発明は、請求項1、2の方法により製造され、AlN析出量が100ppm以下であり、かつNb(C、N)が素地中に1〜10個/μm析出していることを特徴とする高温浸炭用熱間鍛造部品である。
【0045】
本発明では、熱間鍛造後の冷却途中である620〜700℃の温度域に30分〜5時間温度を保持する析出処理を行うことによって、結晶粒粗大化防止に効果のあるAlN、Nb(C、N)のうちNb(C、N)を優先的に析出させ、AlNの析出を抑制することにより浸炭処理時の初期結晶粒度を粗粒として粗大化を防止することを特徴としている。
【0046】
なお、本発明の請求項に記載しているNb(C、N)とは、Nb(C、N)単独の析出物と、AlNとの複合析出物の両方のことであり、単独のAlNとは、Nb(C、N)は勿論他の組成を含まないAlNのことを意味している。
【0047】
AlN析出量の上限を100ppmとしたのは、前記した通り浸炭処理前に単独のAlNが多量に析出してしまうと、浸炭処理時に得られる初期結晶粒度が微細になり、かつ単独で析出したAlNは1000℃以上の高温ではかなりの量が固溶してしまい、粒成長を起こす原因となるため、Nb(C、N)を適量析出させても混粒発生の防止が難しくなるためである。但し、AlNの分析は、単独か複合かで分離して分析することは困難なため、100ppmという数字は、単独で存在しているAlNのみではない。しかしながら、本発明の製造方法を実施することにより、AlN析出量が100ppm以下であって、かつ単独のAlNがほとんどないか、非常に少ない鋼を製造することができるものである(詳細は、後述の実施例に記載)。但し、AlNの析出を抑制するには、熱間鍛造時にNb(C、N)を十分に固溶させた後、室温に冷却することなく冷却途中に前記した析出処理を行うことが必要である。また、室温に冷却した後再加熱して析出処理した場合には、単独のAlN析出数が大幅に増加するので注意が必要である。
【0048】
このような方法でNb(C、N)を多量微細析出させ、全AlN析出量を抑制し、単独AlNの析出が抑制できる理由は明確ではない。しかし、本発明では熱間鍛造時の加熱で鋼中Nbを十分固溶させているため、析出処理される温度領域においてはNbが極端な過飽和状態になっており、Nb(C、N)が析出しやすい状態となっていること、620〜700℃という温度領域がAlNに比べNb(C、N)の析出に有利な温度領域となっていること等がAlNの析出抑制に影響していると推察される。
【0049】
また、本発明では、析出処理後浸炭処理前におけるNb(C、N)の素地中の析出個数を1〜10個/μmに限定している。これは、下限を1個/μmとしたのは、必要とするピンニング効果を確保するために最低限必要な個数であるからであり、上限を10個/μmとしたのは、個数が多すぎると単独のAlN析出数を抑制しても、浸炭初期の結晶粒径は小さくなって、混粒が発生しやすくなるためである。
【0050】
なお、炭窒化物の個数は、TEM、FESEMを用いることにより容易に測定することができる。なお、使用する測定機器の精度によって同じ試験片を測定した場合の測定結果の誤差を防止するため、ここで対象とする炭窒化物は、大きさ(最も長い部分の長さ)が10nm以上のものに限定する。存在する炭窒化物のうち10nm以上の大きさの個数が、Nb(C、N)については1〜10個/μmとする。
【0051】
なお、10nmの析出物を確認するには、少なくとも5万倍、好ましくは10万倍程度に拡大して観察する(10nmの析出物が10万倍で1mmとなる。)ことが必要である。低倍率で観察すると、小さい析出物を見落とす可能性があるので、個数測定時は注意が必要である。
【0052】
【発明の実施の形態】
次に、本発明の効果を実施例を示すことにより明らかにする。表1は準備した供試鋼の化学成分を示すものである。表1に示す供試鋼のうち、1〜6鋼は本発明の条件を満足する鋼、7〜9鋼は一部の成分が本発明の条件を満足しない比較鋼、10鋼は従来鋼であるSCM420Hである。
【0053】
【表1】
【0054】
各供試鋼は、電気炉で溶解し、圧延してφ20の丸棒を製造した。そして、その丸棒から直径10mm、高さ15mmの試験片を作製した。この試験片を富士電波工機(株)製熱間加工再現試験装置(商品名サーメックマスター)にて1250℃で加熱後、仕上温度が1200℃となる条件で50%の据込み加工を行い、その後670℃となるまで空冷した。そして670℃で40分間保持するという析出処理を行った後、室温まで空冷し、さらに70%の据込み加工(高さ7.5mmから2.25mmへの加工)をアムスラー試験機で実施した。ここで、据込み加工を実施したのは、冷間歪を付与した場合の方が異常粒成長が起きやすくなるためである。この試験片を実際の浸炭処理時に相当する条件で加熱処理(950℃、1000℃、1050℃、保持時間2時間)を行い、混粒状態となっている部分がないか調査した。なお、一部の試験片は、単独のAlN析出数の差の影響を調査するため、一度室温まで空冷してから加熱するという条件で試験を実施した。
【0055】
各試験片の結晶粒異常成長の判定は、光学顕微鏡(倍率は100倍)でランダムに10視野観察することにより評価した。そして、通常異常粒成長が起きていない箇所の粒度番号は8〜10程度であるため、10視野観察した範囲内において、結晶粒度番号が4番以下となっている場合に異常粒成長が生じたとみなすこととした。なお、結晶粒度の測定は全てJISG0551の基準に準拠した方法で行った。そして、この基準で評価した結果、混粒が認められた領域の面積率が20%以上の場合を×、0%超〜20%未満のものを△、0%のものを○で示した。
【0056】
また、混粒の発生状況とAlN、Nb(C、N)の析出状態との関係を調査するために、異常粒成長発生状況を調査した試験片と同時に準備した試験片(前記した浸炭処理相当の加熱する前のもの)を用いて、Nb(C、N)の析出数(析出処理前と析出処理後の両方)を測定した。個数の測定はTEMを用いて行った。測定は、析出処理前のものは、前記した据込み加工終了後、直ちに窒素ガスで急冷することにより、析出する時間的余裕を与えずに冷却した試験片を用いて行った。析出処理後のNb(C、N)の個数の測定は、前記した析出処理後、室温まで冷却した試験片を用いて行った。AlN析出量は、化学分析(よう素メタノール溶解−蒸留中和滴定法)により求めた。
結果を表2に示す。
【0057】
【表2】
【0058】
表2から明らかなように、本発明の成分の範囲内である1〜6鋼は、1050℃という高い温度でも、異常粒成長をすることがなかった。それに対し、一部の成分が本発明の条件を満足しない比較鋼は従来鋼SCM420Hに比べれば優れた結果が得られたが、本発明鋼に比べ劣るものであった。このうち、7鋼は、ピン止め効果を得るために必要な元素であるNb含有率が低いためピン止め効果が十分に得られず1000℃以上の温度で異常粒成長が生じたものであり、8鋼はNb含有率が高いため、鍛造時の加熱によって十分に炭窒化物を固溶させることができず、その影響で1050℃での異常粒成長が防止できなかったものである。また、従来鋼SCM420Hである10鋼は、著しく劣り、950℃以上の温度で異常粒成長が発生した。
【0059】
なお、V含有率が高い9鋼は析出物数が本発明の範囲内であるにもかかわらず1050℃の加熱で異常粒が発生したが、これは組織内に析出していたV炭窒化物が1050℃の加熱により固溶してしまい、組織の一部において炭窒化物の少ない領域が生じ異常粒成長が起きたものと推定される。
【0060】
次に、前記実施例で行った条件を基本に前記供試鋼のうち、本発明の成分範囲の条件を満足する1、2鋼を使用して、熱間鍛造条件、析出処理条件を種々変化させた場合の別の実施例を示す。実験した条件は表3に示す通りである。評価した項目及び評価方法は、鍛造条件、析出処理時の温度条件を変更した以外は、前記実施例と同様である。
【0061】
【表3】
【0062】
表3から明らかなように、本発明で規定した成分範囲内の鋼であっても、加熱温度、仕上温度、析出処理温度等のいずれかの条件が本発明で規定した条件の範囲外である試験No.9〜12は、優れた結果が得られないことが分かった。このうち、試験No.9は仕上温度が低く、11は、加熱温度、仕上温度の両方が低いため、据込み加工直後のNb(C、N)の固溶が不十分となって、析出処理後のNb(C、N)析出個数が減少したものであり、No.10は析出処理温度が高いため、析出処理後の冷却時にベイナイトが生成し、その影響で混粒が生じたものであり、No.12は一度室温まで冷却し、再度加熱した影響から単独のAlN析出数が大幅に増加し、AlNによるピン止め効果がほぼ完全に消失する1050℃加熱の場合において、混粒が発生したものである。
【0063】
これに対し、本発明の条件を満足する実施例である試験No.1〜8はすべて1050℃で加熱した場合でも異常粒成長を生じないことが確認できた。
【0064】
また、この実施例では、析出処理時間が40分の場合しか示していないが、これは本実施例では非常に小さな据込み試験片を使用したため、極めて短時間に内部まで十分に加熱されるため、40分の加熱で十分すぎる析出処理効果が得られてしまうからである。しかし、実際の部品はより大きな部品が多く、部品の大きさによっては、さらに長い加熱保持を行って十分にNb(C、N)を析出させることが必要である。
【0065】
なお、以上説明した実施例は、全て小さな試験片での評価結果を示したものであるが、実際の歯車部品でも同様の試作実験を行い、同様に混粒防止効果が得られることが確認できた。
【0066】
次に、単独のAlNの析出状態を調査した別の実施例を示す。
前記した実施例では、AlNの析出量のみ示したが、この数値は、AlNとNb(C、N)との複合析出物中のAlNも含まれているので、このデータのみでは、単独のAlN析出が抑制されていると判断することはできない。そこで、前記した表2のデータ測定で試験No.5、12に使用した加熱実験直前の供試材を用い、走査型電子顕微鏡を用いて、二次電子像と反射電子像を試験片の全く同じ位置で撮影した結果を図1(試験No.5)、図2(試験No.12)に示す。
【0067】
ここで、AlNは、反射電子像に撮影されるが、二次電子像には撮影されないことがわかっているため、この2枚の写真を比較することにより、単独のAlNの有無を正確に把握することができる。なお、倍率は4枚共に50000倍である。
【0068】
まず、図2は、AlN析出量が非常に多い比較例No.12のSEM写真を示したもので、(a)が二次電子像、(b)が、反射電子像である。この図の(b)から明らかなように、AlNの析出物(丸で囲んだ部分の、薄い灰色部分)を観察することができる。そして、試験片の同じ位置で撮影した反射電子像(a)には、(b)で析出物が観察された位置と同位置には何も観察されない。このことは、単独のAlNが多数析出していることを意味している。
なお、図2(a)、(b)ともに、斜方向に2つの黒色部分が見られるが、これは窪みであってAlN析出物ではない。
【0069】
それに対し、本発明の範囲内である試験No.5のSEM写真(図1)をみると、(a)、(b)共に何も析出物が観察されないことがわかる。この傾向は、ここで示した視野以外の場所でも同様であった。この結果は前記したNo.12とは全く逆であり、単独のAlNがほとんど析出していないことを意味している。
以上説明した図は、単独のAlNの有無を明確にするため、あえてNb(C、N)が析出していない箇所を選択して撮影した。従って、Nb(C、N)が析出している箇所を選択して撮影すれば、(a)、(b)共にNb(C、N)の析出物が撮影されることは勿論である。
【0070】
また、明細書には示していないが、表3の評価に使用した他の本発明の供試材についても同様に調査したが、全く同様の結果であった。
従って、この観察結果より、本発明鋼では、単独のAlNの析出が抑制されており、その結果高温浸炭時の粒成長が抑制されていることがわかる。
【0071】
【発明の効果】
本発明による高温浸炭用熱間鍛造部品の製造方法では、Nbを少量添加した鋼を用い、高温で加熱及び熱間鍛造することにより、一旦Nb(C、N)をほぼ固溶させた状態とし、その後の冷却途中の620〜700℃の温度領域で30分〜5時間温度を保持するという析出処理によって、単独のAlNの析出を抑制しつつNb(C、N)を微細分散させ、1000℃を超える高温浸炭でも異常粒成長を防止することができる。従って、浸炭温度を高め、浸炭処理時間を大幅に短縮することが可能となり、自動車等の部品のうち浸炭処理される部品の製造コスト中の、熱処理コストを大幅に低減することが可能になるとともに品質を向上させることができる。
【図面の簡単な説明】
【図1】実施例における本発明鋼、試験No.5の炭窒化物析出状態を説明する、図面代用電子顕微鏡写真(倍率50000倍)であり、(a)が二次電子像、(b)が反射電子像である。
【図2】比較例試験No.12における、AlN析出量が増加した場合の炭窒化物析出状態を説明する、図面代用電子顕微鏡写真(倍率50000倍)であり、(a)が二次電子像、(b)が反射電子像である。
[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a hot forged part for high-temperature carburizing for suppressing abnormal grain growth (generation of mixed grains) in a high-temperature carburizing treatment at a temperature of 1000 ° C. or more, and a high-temperature carburizing hot forged part produced by the method. Related to forged parts.
[0002]
[Prior art]
For steel parts used for power transmission, such as gears and shafts used in automobiles, construction vehicles, construction equipment, and the like, case hardened steel, which forms a hardened layer on the surface by carburizing, is frequently used. This is because the parts are required to have excellent wear resistance and high toughness at the same time, so the surface is hardened by carburizing to secure the wear resistance, and the inside remains low C to give high toughness. is there.
[0003]
Recently, significant reduction in manufacturing cost as well as high strength of these parts has become a major issue. The cost of manufacturing parts can be broadly divided into the cost of the material itself and the cost of heat treatment such as carburization.However, in the former case, especially in case hardening steel to which a large amount of expensive component elements are not added, significant cost reduction is not possible. It is difficult, and studies on the latter heat treatment cost reduction method are being actively studied.
[0004]
Among them, a method being studied recently is high-temperature carburizing treatment. At present, most of the carburizing treatment of case hardening steel is performed by gas carburizing treatment, and a long time treatment of about 4 to 20 hours is performed to obtain a predetermined hardening depth. As a result, productivity has become a problem, and a great deal of energy has been consumed. The high-temperature carburizing treatment is a method that sets the carburizing temperature high and promotes the reaction, thereby invading and diffusing more carbon atoms in a short time to shorten the treatment time, and is the most effective method for reducing the time. It has been known for a long time.
[0005]
However, treatment at a high carburizing temperature is an effective method for shortening the treatment time, but causes a serious problem. That is, austenite grains are coarsened or mixed after carburizing. If such austenite grains are coarsened or mixed after carburizing, the strength is reduced or the heat treatment strain varies. Usually, after the carburizing treatment, only the minimum necessary mechanical processing such as polishing is usually performed, and such a variation in distortion causes a defective product size, which is a problem. Therefore, in reality, it has not been possible to sufficiently increase the carburizing temperature to reduce the processing time.
[0006]
The phenomenon of coarsening and mixing of grains during carburizing has been known for quite some time, various measures have been studied, new technologies have been proposed, and numerous patent applications have been filed.
[0007]
Among them, the most well-known method is a method in which AlN is finely dispersed to prevent coarsening by a pinning effect. For example, steels disclosed in Patent Documents 1 and 2 have been proposed.
[0008]
In addition, since carbonitride of Nb is harder to form a solid solution at a higher temperature than AlN, it has been proposed to add Nb to prevent coarsening at a higher temperature in expectation of an effect of preventing crystal grain coarsening at a higher temperature. I have. For example, there is steel disclosed in Patent Documents 3 and 4.
[0009]
However, it has been confirmed that the precipitation of AlN or Nb (C, N) has an effect on the prevention of coarsening of crystal grains. However, simply adjusting the addition amounts of Al, N, and Nb is not enough. It has been found that a sufficient effect cannot be obtained, and it has been proposed to perform a heat treatment to precipitate AlN or Nb carbonitride in a state having a larger pinning effect, thereby preventing coarsening. I have. For example, heat treatment methods disclosed in Patent Documents 5 to 7 have been proposed. Although the contents described in these three documents differ in the target steel components, they are all heated to a temperature range of 600 ° C. to the A1 transformation point before the carburizing treatment to form AlN, Nb (C, N). The present invention is characterized in that the precipitation state is changed to a state in which it is difficult to coarsen during carburizing treatment.
[0010]
Patent Documents 8 and 9 disclose Nb (C) by appropriately adjusting the heating and finishing temperature during rolling and hot forging, and cooling between 800 to 500 ° C. at a rate of 1 ° C./sec or less. , N) are described in detail, in which a large amount of fine particles are finely dispersed, the amount of AlN precipitated is suppressed, and coarsening is suppressed.
[0011]
[Patent Document 1]
JP-A-56-75551
[Patent Document 2]
JP-A-59-123714
[Patent Document 3]
JP-A-49-125220
[Patent Document 4]
JP-A-6-299241
[Patent Document 5]
JP-A-58-16022
[Patent Document 6]
JP-A-62-205229
[Patent Document 7]
JP-A-10-121128
[Patent Document 8]
RE-Publication No. 99/05333
[Patent Document 9]
JP 2001-303174 A
[0012]
[Problems to be solved by the invention]
However, the above-mentioned proposed methods have the following problems.
That is, although the pinning effect of AlN described in Patent Documents 1 and 2 can be obtained to a certain extent by carburizing treatment at a temperature of less than 980 ° C., the carburizing temperature of 980 ° C. or more significantly reduces the finely dispersed AlN. Is dissolved, and the effect of preventing crystal grain coarsening due to the pinning effect cannot be sufficiently obtained. Therefore, in the carburizing treatment at a high temperature of 1000 ° C. or more, which is aimed at in the present invention, the effect is very small, and the coarsening cannot be completely prevented.
[0013]
In addition, the inventions described in Patent Documents 3 and 4 in which Nb is added in expectation of a crystal grain stabilizing effect at a higher temperature than AlN have been studied only with respect to the amount of addition. Almost no studies have been made as to whether a large coarsening prevention effect can be obtained when precipitated in a state. According to the results of studies by the present inventors, it has been found that when carburizing a hot forged material, it is difficult to prevent the generation of mixed grains unless the precipitation state is adjusted optimally. In particular, attention should be paid to the precipitation state of AlN, but Patent Documents 3 and 4 do not disclose that point at all.
[0014]
Further, the inventions of Patent Documents 5 to 7, which are characterized in that generation of mixed grains is prevented by heat treatment before carburizing, also have the following problems. That is, Patent Literature 5 is an invention directed to steel containing no Nb, and Patent Literature 6 does not describe Nb at all, and describes the optimal precipitation state of AlN and Nb (C, N) before carburizing. Not considered at all. In addition, Patent Document 7 considers the deposition state of Nb carbonitride in considerable detail, but does not consider the influence of AlN that is present at the same time. Further, it is characterized in that Nb carbonitride is precipitated after hot forging and agglomerated by subsequent heat treatment, but as a result of investigation by the present inventors, the heat treatment is performed in a state where a large amount of AlN is present. In addition to the above, the carbonitride is easily aggregated under the influence of AlN, and a large amount of Nb carbonitride containing a large amount of AlN is present, which is effective for carburizing at a temperature of 1000 ° C. or lower, but is higher than 1000 ° C. It has been found that when carburizing treatment is performed at a high temperature, coarsening cannot be sufficiently prevented.
[0015]
Further, Patent Documents 8 and 9 describe a technique for suppressing the amount of AlN precipitation to prevent coarsening, but Patent Document 8 describes only the precipitation state after rolling. As described in No. 9, it is assumed that normalization is performed after hot forging, and what means should be used to achieve the most problematic precipitation state immediately before carburizing in the case of a hot forged product. Is not described at all. Patent Document 9 discloses that the precipitation state of AlN and Nb (C, N) after hot forging is achieved by designating forging conditions and cooling conditions after forging, as in the present invention. Although the cooling condition is described as slow cooling, the slow cooling described in this document means that cooling is slower than simple air cooling, and the time required for the precipitation of Nb (C, N) is compared with the time required for precipitation. Then, it is quite fast (at the stated cooling rate of 0.1-1 ° C./sec, it passes through the 620-700 ° C. specified in the present invention in only 80-800 seconds), and the temperature also precipitates. It has been found that the temperature is not limited only to the temperature suitable for the method, and that sufficient fine precipitation is not often performed.
[0016]
The present invention has been made to solve the above-described problems, and can prevent abnormal grain growth even when carburizing is performed at a high temperature of about 1050 ° C., and can be performed at a high temperature. Accordingly, it is an object of the present invention to provide a method for manufacturing a hot forged part for high-temperature carburizing, which can significantly improve the productivity of the carburizing treatment.
[0017]
[Means for Solving the Problems]
According to the first aspect of the present invention, C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.30 to 1.50%, Cr: 0.30 to 2 by weight ratio. 0.000%, Al: 0.005 to 0.050%, Nb: 0.01 to 0.10%, N: 0.0080 to 0.0250%, V: 0.01% or less, with the balance Fe And a hot-rolled steel material composed of an impurity element are heated to 1150-1350 ° C., and then hot-forged under the condition that the forging finish temperature is 1100-1300 ° C., and the Nb contained in the steel is substantially dissolved (precipitation). The number is 0.3 / μm 2 The following is a method for producing a hot forged part for high-temperature carburizing, comprising cooling to 620 to 700 ° C. after forging, holding at that temperature for 30 minutes to 5 hours, and carburizing after cooling to room temperature.
[0018]
It should be noted in the present invention that when carburizing after hot working using the case hardening steel having the above specific composition, the carbonitride of AlN or Nb is simply removed as in the conventional steel for preventing grain coarsening. Instead of precipitating finely into the steel, heat it to a high temperature during hot forging and terminate the forging at a high temperature to make the Nb carbonitride almost solid-dissolved, and then heat it to 620-700 ° C without cooling it to room temperature By doing so, a large number of Nb carbonitrides are deposited. By adopting such a precipitation state, AlN is preferentially precipitated around Nb (C, N) which has already been precipitated during the carburizing temperature rise to form a composite precipitate, so that even during high-temperature carburization, AlN is solidified. Since it is difficult to dissolve and a pinning effect can be surely obtained, it is possible to suppress generation of mixed grains.
[0019]
Conventionally, both AlN and Nb (C, N) have been considered to have a large effect on crystal grain refinement, and it has been considered that finely dispersing both of them has a large effect on preventing crystal grain coarsening. However, as it turns out that AlN is more likely to form a solid solution than Nb (C, N) and cannot greatly expect an effect on crystal grain coarsening at a high temperature, miniaturization centering on Nb (C, N) has been realized. Measures have come to be considered mainly. However, even in such a case, there is no idea that the presence of AlN is harmful to the prevention of abnormal grain growth, except for a very small portion such as Patent Documents 8 and 9 described above.
[0020]
On the other hand, as a result of a detailed investigation by the present inventors, it has been found that a steel composed of a state in which a single amount of AlN is precipitated has the following problems.
{Circle around (1)} When carburizing AlN, particularly steel in which a large amount of single AlN is precipitated, the crystal grains become fine due to the pinning effect of AlN. However, since the crystal grains are fine, the driving force for grain growth increases.
[0021]
(2) When carburizing is performed at a relatively low temperature, AlN precipitates remain even during carburizing and a pinning effect can be obtained, so that the driving force for coarsening is large, but a large effect for preventing coarsening. Can be obtained.
[0022]
{Circle around (3)} However, when the carburizing treatment temperature is increased, the solid solution of the precipitated AlN proceeds, and a region where the amount of precipitated AlN is small starts to occur. As a result, a region where the pinning effect is not sufficiently obtained partially occurs, and the driving force for coarsening cannot be suppressed, and partial abnormal grain growth (mixed grain) occurs.
[0023]
The present invention has been made in order to solve the above-described problems of the crystal grain coarsening prevention technology that positively utilizes AlN, and has been made by obtaining the following knowledge.
{Circle around (1)} High temperature heating and high temperature forging at the time of hot forging to obtain a state in which Nb is substantially dissolved (0.3 / μm). 2 When cooling and heating to a temperature of 620 to 700 ° C. without cooling to room temperature, complex with AlN or single Nb (C, N) is dispersed in a large amount and finely, and A steel with less AlN precipitation can be obtained.
[0024]
{Circle over (2)} The initial carburizing grain size achieved by heating a forged part of the present invention in which sole AlN precipitation is suppressed (hereinafter referred to as the present invention steel) is the same as that of Nb-added steel in which sole AlN precipitation is not suppressed. In comparison, the grains become considerably coarse, and the driving force for grain growth can be suppressed to a small value. During the carburizing process at a temperature exceeding 1000 ° C., Nb carbonitride exists without solid solution even in the conventional Nb-added steel, but the steel of the present invention can obtain a relatively coarse-grained state. Therefore, partial abnormal grain growth, which could not be sufficiently suppressed by the conventional Nb-added steel, can be reliably prevented.
[0025]
{Circle around (3)} When the steel of the present invention, in which the precipitation of AlN alone is suppressed, is subjected to high-temperature carburizing treatment, AlN precipitates during heating, but this precipitation is caused by Nb (C, N) already precipitated before heating. Precipitates preferentially around the Therefore, the formation of a single AlN precipitate can be suppressed not only before the carburizing treatment but also during the heating of the carburizing, and as described above, the structure is formed of relatively coarse crystal grains in the early stage of the carburizing. When the processing proceeds and the temperature exceeds 1000 ° C., the solid solution of AlN proceeds, but even in such a high temperature state, Nb (C, N) remains, so that a sufficient pinning effect can be secured. Further, since the particles are coarse, the driving force for the coarsening is small, so that it is possible to prevent the occurrence of mixed particles.
[0026]
Further, Patent Documents 8 and 9 are the same in that AlN is suppressed and a large amount of Nb (C, N) is finely precipitated, but in the present invention, Nb (C, N) is 620 to 700 ° C. By maintaining the temperature in a temperature range in which conditions are most favorable for precipitation, fine precipitation is ensured. Compared with the inventions of Patent Documents 8 and 9, fine precipitation can be carried out more reliably and efficiently, so that the mixed grains can be more reliably mixed. Occurrence can be prevented.
[0027]
Next, the reasons for limiting the chemical components of the case hardening steel used in the production method of claim 1 will be described.
C: 0.10 to 0.30%
In order to secure the strength and internal hardness required for a carburized part, it is necessary to contain 0.10% or more of C. However, when the content exceeds 0.30%, the internal toughness is deteriorated and the machinability is further reduced. Therefore, the upper limit is set to 0.30%.
[0028]
Si: 0.05 to 0.50%
Si is an element necessary for deoxidation during the production of steel, and must be contained at least 0.05% or more. However, Si reacts with oxygen in a carburizing atmosphere during carburizing to form an oxide. Therefore, the hardenability decreases near the surface layer of the article to be treated, and a so-called abnormal carburizing layer is formed. Therefore, when a large amount is contained, the adverse effect due to the formation of the abnormal carburized layer is increased and the strength is reduced, and the machinability is also reduced. Therefore, the upper limit is set to 0.50%.
[0029]
Mn: 0.30 to 1.50%
Mn must contain 0.30% or more of Mn in order to ensure the necessary hardenability and the hardness required to secure the strength to the inside. However, when contained in a large amount, the retained austenite increases, the hardness decreases, the internal toughness deteriorates, and the machinability decreases. Therefore, the upper limit was set to 1.50%.
[0030]
P: 0.035% or less
P is an impurity which cannot be avoided during production. The present invention is not particularly limited as an essential condition, but since it is an element that lowers the strength of grain boundaries and deteriorates fatigue properties, its upper limit is preferably made 0.035% or less.
[0031]
S: 0.030% or less
S, like P, is an impurity inevitably mixed in a small amount during production, and exists as a sulfide-based inclusion such as MnS. However, this inclusion is an element that becomes a starting point of fatigue fracture, lowers pitting resistance, and causes anisotropy of the steel material to increase. Therefore, in the present invention, although not necessarily limited, it is ideally preferably reduced as much as possible, and the upper limit is more preferably set to 0.030%.
[0032]
Cr: 0.30 to 2.00%
Cr is an element necessary for improving hardenability, securing necessary strength, and improving the performance of the steel produced according to the present invention, and it is necessary to contain 0.30% or more. However, if contained in a large amount, the toughness is deteriorated and the machinability is reduced. Therefore, the upper limit is set to 2.00%.
[0033]
Al: 0.005 to 0.050%,
Al is an element necessary for deoxidation like Si, and is an element that exists as AlN and is effective in preventing abnormal grain growth after carburizing due to a pinning effect. Therefore, it is necessary to add at least an amount necessary for deoxidation, and the lower limit is set to 0.005%. However, unlike the conventional steel for preventing grain coarsening in the present invention, the precipitation of AlN is suppressed and the pinning effect is not expected at all, but the main pinning effect is Nb (C, N). Further, if a large amount of Al is contained, the number of single AlN precipitates before the carburizing treatment increases, which leads to the refinement of the initial carburizing particle size. Therefore, the upper limit was made 0.050%.
[0034]
N: 0.0080 to 0.0250%
As described above, N is combined with Al and Nb, becomes AlN and Nb (C, N), exists in the steel, and is an element effective in preventing abnormal grain growth after carburizing. In order to obtain this effect sufficiently, it is necessary to contain 0.0080% or more of N. However, there is an appropriate amount of AlN or Nb (C, N) to be precipitated, and if the amount is too large, the initial carburizing particle size becomes small and abnormal grain growth tends to occur, so the upper limit is made 0.0250%. .
[0035]
Nb: 0.01 to 0.10%
Nb is the most important element in the present invention, and is present in steel as a carbonitride, which is an element having a large effect of preventing abnormal growth of crystal grains in carburizing at a high temperature, particularly in comparison with Al. When the amount of Nb added is small, particularly in carburizing at 1050 ° C. or more, a part of the carbonitride precipitated before the carburizing treatment becomes a solid solution, and the amount of Nb carbonitride contributing to the pinning effect is insufficient, and the amount of Nb is insufficient. Since the effect of suppressing granulation cannot be sufficiently obtained, the lower limit is set to 0.01%. On the other hand, if a large amount is contained, Nb (C, N) is not brought into a sufficiently solid solution state by heating during hot forging, and coarse Nb (C, N) precipitates remain. Since the pinning effect decreases, the upper limit is set to 0.10%.
[0036]
V: 0.01% or less
V is an element that forms a carbonitride like Nb and contributes to the prevention of crystal grain growth by a pinning effect. However, carbonitride of V easily forms a solid solution at a higher temperature than carbonitride of Nb. In the case of high-temperature carburization at 1000 ° C. or more, carburizing heating causes a solid solution and the pinning effect disappears during carburization, so that the effect of suppressing the growth of crystal grains cannot be obtained. It is necessary to preferentially bind C and N in steel to elements that form carbonitrides that are hardly dissolved at the processing temperature. When V is contained, a part of C and N in the steel is combined with V, an action of reducing the initial particle size of carburizing occurs, and they form a solid solution during carburizing at 1000 ° C. or more, Since the pinning effect is lost, abnormal grain growth is promoted. Therefore, when V is present during high-temperature carburizing, abnormal grain growth tends to occur. V is an element that may be mixed in a small amount from scrap used in the production of steel even if not actively added, so it is necessary to reduce the amount of V contained as an impurity, and the upper limit is restricted to 0.01%. did.
[0037]
Next, the reasons for limiting the manufacturing conditions according to the first aspect of the invention will be described below.
The heating temperature during forging was set to 1150 to 1350 ° C. because Nb (C, N) was sufficiently dissolved in the rolled steel material during heating during forging (specifically, 0.3 / μm 2 This is for the purpose of precipitating finely during the temperature holding (hereinafter referred to as precipitation treatment) of 620 to 700 ° C. to be described later. If the heating temperature during forging is low and the solid solution becomes insufficient to leave coarse precipitates of Nb (C, N), the precipitates remaining during the precipitation treatment after forging further grow. Since the precipitate becomes a large precipitate and the precipitate of Nb (C, N) does not contribute to the coarsening of the crystal grains, it is necessary to set the lower limit of the temperature to 1150 ° C. and sufficiently dissolve Nb (C, N) in solid solution. is there. However, setting the temperature too high wastes energy, so the upper limit was set to 1350 ° C. The forging finish temperature is set to 1100 to 1300 ° C. for the same reason.
[0038]
The forged part of the present invention is cooled to 620 to 700 ° C. after hot forging, and is subjected to a precipitation treatment by maintaining the temperature in this temperature range. Here, the cooling conditions from the time immediately after forging to the precipitation temperature are not specified under any conditions such as slow cooling, air cooling, standing cooling, and accelerated air cooling (fan cooling) that can be performed in a normal forging factory. This is because the same effect can be obtained even if it is performed. Therefore, the most convenient conditions can be selected and implemented in the factory where the actual implementation is performed.
[0039]
As described above, after cooling to a temperature range of 620 to 700 ° C., the temperature is maintained in this temperature range, and Nb (C, N) is finely precipitated. The reason why the lower limit of the temperature range of the precipitation treatment is set to 620 ° C. is that the precipitation of Nb (C, N) does not proceed efficiently even when the temperature is kept lower than this temperature, and the upper limit is set to 700 ° C. If the temperature exceeds the transformation temperature and enters the two-phase region, transformation will occur after the precipitation treatment, and subsequent cooling will partially produce bainite and martensite, which will prevent coarsening. It is because it becomes difficult.
[0040]
The reason why the temperature holding time of the precipitation treatment is set to 30 minutes to 5 hours is that such a time is most appropriate in consideration of the time required for the precipitation treatment. That is, if the time is shorter than 30 minutes, it becomes difficult to sufficiently precipitate, and if the time is longer than 5 hours, the precipitation can be sufficiently performed, but the productivity is reduced and the heat treatment cost is increased.
[0041]
After completion of the precipitation treatment, the mixture is cooled to room temperature. Since the precipitation treatment is a heat treatment at a temperature lower than the transformation point, the transformation has already been completed at the end of the precipitation treatment. Therefore, the obtained tissue is not affected by the cooling condition to room temperature, and may be appropriately selected according to the location of the place where the operation is performed. However, cooling to 500 ° C. is desirably performed at a rate of 25 ° C./min or more so as not to give a time margin for precipitation of a single AlN.
By performing the hot forging and the precipitation treatment according to the procedure described above, it is possible to obtain a hot forged part for high-temperature carburizing with a small number of single AlN precipitates.
[0042]
Next, as in the invention of claim 2, in addition to the case hardening steel used in the production method of claim 1, steel containing 0.80% or less of Mo can be used. Hereinafter, the reasons for the limitation will be described.
[0043]
Mo: 0.80% or less
Mo is an element that has the effect of improving hardenability and toughness, and also has the effect of suppressing the abnormal carburizing layer and improving the strength, and can be used by adding a small amount as necessary. However, when added in a large amount, retained austenite increases, which causes a reduction in carburized hardness and lowers internal toughness and machinability. Therefore, the upper limit is set to 0.80%.
[0044]
The invention according to claim 3 is produced by the method according to claims 1 and 2, wherein the amount of AlN deposited is 100 ppm or less, and Nb (C, N) is 1 to 10 particles / μm in the substrate. 2 It is a hot forged part for high-temperature carburizing characterized by being precipitated.
[0045]
In the present invention, AlN, Nb (Effective for preventing crystal grain coarsening) is subjected to a precipitation treatment in which the temperature is maintained in a temperature range of 620 to 700 ° C during cooling after hot forging for 30 minutes to 5 hours. (C, N) is characterized in that Nb (C, N) is preferentially precipitated, and the precipitation of AlN is suppressed, whereby the initial crystal grain size at the time of carburizing is coarsened to prevent coarsening.
[0046]
It should be noted that Nb (C, N) described in the claims of the present invention refers to both a precipitate of Nb (C, N) alone and a composite precipitate of AlN, and a single precipitate of AlN. Means AlN which does not contain Nb (C, N) and other components.
[0047]
The reason for setting the upper limit of the amount of precipitated AlN to 100 ppm is that, as described above, if a large amount of single AlN precipitates before the carburizing treatment, the initial crystal grain size obtained during the carburizing treatment becomes fine, and the single precipitated AlN At a high temperature of 1000 ° C. or more, a considerable amount of solid solution forms a solid solution and causes grain growth. Therefore, even if an appropriate amount of Nb (C, N) is precipitated, it is difficult to prevent generation of mixed grains. However, since it is difficult to analyze AlN separately or separately depending on the combination, the number of 100 ppm is not limited to AlN alone. However, by carrying out the production method of the present invention, it is possible to produce a steel with an AlN precipitation amount of 100 ppm or less and little or no AlN alone (details will be described later). Described in Examples). However, in order to suppress the precipitation of AlN, it is necessary to sufficiently dissolve Nb (C, N) during hot forging and then perform the above-described precipitation treatment during cooling without cooling to room temperature. . In addition, when the precipitation treatment is performed by reheating after cooling to room temperature, it is necessary to be careful because the number of single AlN precipitates is greatly increased.
[0048]
It is not clear why a large amount of Nb (C, N) is finely precipitated by such a method, the total amount of AlN is suppressed, and the precipitation of single AlN can be suppressed. However, in the present invention, since Nb in the steel is sufficiently dissolved by heating at the time of hot forging, Nb is in an extremely supersaturated state in a temperature range where the precipitation treatment is performed, and Nb (C, N) is increased. The fact that it is in a state where it is easy to precipitate and that the temperature region of 620 to 700 ° C. is a temperature region that is advantageous for Nb (C, N) precipitation as compared with AlN, etc. affects the suppression of AlN precipitation. It is inferred.
[0049]
In the present invention, the number of Nb (C, N) precipitated in the substrate after the precipitation treatment and before the carburizing treatment is 1 to 10 / μm. 2 Limited to. This means that the lower limit is 1 piece / μm 2 The reason for this is that the number is the minimum required to secure the required pinning effect, and the upper limit is 10 / μm. 2 The reason for this is that if the number is too large, the crystal grain size in the early stage of carburizing becomes small and mixed grains are likely to occur even if the number of single AlN precipitates is suppressed.
[0050]
Note that the number of carbonitrides can be easily measured by using TEM and FESEM. In addition, in order to prevent an error in the measurement result when the same test piece is measured depending on the accuracy of the measuring device used, the target carbonitride has a size (length of the longest portion) of 10 nm or more. Limited to those. The number of carbon nitrides having a size of 10 nm or more is 1 to 10 / μm for Nb (C, N). 2 And
[0051]
In addition, in order to confirm the precipitate of 10 nm, it is necessary to observe at least 50,000 times, preferably about 100,000 times (observation of a precipitate of 10 nm becomes 1 mm at 100,000 times). When observed at low magnification, small precipitates may be overlooked, so care must be taken when counting the number.
[0052]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the effects of the present invention will be clarified by showing examples. Table 1 shows the chemical components of the prepared test steels. Of the test steels shown in Table 1, 1 to 6 steels satisfy the conditions of the present invention, 7 to 9 steels are comparative steels whose components do not satisfy the conditions of the present invention, and 10 steels are conventional steels. There is a certain SCM420H.
[0053]
[Table 1]
[0054]
Each test steel was melted and rolled in an electric furnace to produce a φ20 round bar. Then, a test piece having a diameter of 10 mm and a height of 15 mm was prepared from the round bar. This test piece was heated at 1250 ° C. with a hot working reproduction test device (trade name: Cermec Master) manufactured by Fuji Denki Koki Co., Ltd., and then subjected to 50% upsetting at a finish temperature of 1200 ° C. Then, the mixture was air-cooled to 670 ° C. Then, after performing a precipitation treatment of holding at 670 ° C. for 40 minutes, the mixture was air-cooled to room temperature, and 70% upsetting (processing from 7.5 mm to 2.25 mm in height) was performed using an Amsler testing machine. Here, the upsetting is performed because abnormal grain growth is more likely to occur when cold strain is applied. This test piece was subjected to a heat treatment (950 ° C., 1000 ° C., 1050 ° C., holding time: 2 hours) under the conditions corresponding to the actual carburizing treatment, and an examination was made to see if there were any mixed particles. In addition, in order to investigate the influence of the difference in the number of precipitated AlN alone, some of the test pieces were tested under the condition that they were air-cooled once to room temperature and then heated.
[0055]
The determination of abnormal growth of crystal grains of each test piece was evaluated by observing 10 visual fields randomly using an optical microscope (magnification: 100 times). Since the grain size number of the portion where the abnormal grain growth does not usually occur is about 8 to 10, it is considered that the abnormal grain growth occurs when the crystal grain size number is 4 or less within the range observed in 10 visual fields. I decided to consider it. In addition, the measurement of the crystal grain size was all performed by the method based on the standard of JISG0551. As a result of the evaluation based on this criterion, the case where the area ratio of the region where the mixed grains were recognized was 20% or more was indicated by x, the case where the area ratio was more than 0% to less than 20% was indicated by Δ, and the case where 0% was indicated by ○.
[0056]
Further, in order to investigate the relationship between the occurrence of mixed grains and the precipitation of AlN and Nb (C, N), a test piece prepared at the same time as the test piece for which the occurrence of abnormal grain growth was investigated (corresponding to the aforementioned carburizing treatment) (Before heating), the number of Nb (C, N) deposited (both before and after the precipitation treatment) was measured. The number was measured using a TEM. The measurement before the precipitation treatment was performed using a test piece which was immediately cooled with nitrogen gas immediately after the end of the upsetting process and cooled without giving a sufficient time for precipitation. The number of Nb (C, N) after the precipitation treatment was measured using a test piece cooled to room temperature after the above-mentioned precipitation treatment. The amount of AlN deposited was determined by chemical analysis (iodine methanol dissolution-distillation neutralization titration method).
Table 2 shows the results.
[0057]
[Table 2]
[0058]
As is clear from Table 2, 1 to 6 steels within the range of the components of the present invention did not undergo abnormal grain growth even at a high temperature of 1050 ° C. On the other hand, the comparative steel in which some of the components did not satisfy the conditions of the present invention obtained excellent results as compared with the conventional steel SCM420H, but was inferior to the steel of the present invention. Among them, Steel No. 7 has a low content of Nb, which is an element necessary for obtaining the pinning effect, and cannot sufficiently obtain the pinning effect, and causes abnormal grain growth at a temperature of 1000 ° C. or more. Steel No. 8 has a high Nb content, so that it was not possible to sufficiently dissolve the carbonitride by heating during forging, so that abnormal grain growth at 1050 ° C. could not be prevented. Further, the conventional steel SCM420H, 10 steel, was remarkably inferior, and abnormal grain growth occurred at a temperature of 950 ° C. or higher.
[0059]
In addition, although the number of precipitates was within the range of the present invention, abnormal grains were generated by heating at 1050 ° C. in the 9 steels having a high V content, but this was due to the V carbonitride precipitated in the structure. Is presumed to be dissolved by heating at 1050 ° C., causing a region with a small amount of carbonitrides in a part of the structure to cause abnormal grain growth.
[0060]
Next, hot forging conditions and precipitation treatment conditions were variously changed using one or two steels satisfying the conditions of the composition range of the present invention among the test steels based on the conditions performed in the above examples. Another embodiment will be described. The experimental conditions are as shown in Table 3. The evaluated items and the evaluation method are the same as those in the above-mentioned Example except that the forging conditions and the temperature conditions during the precipitation treatment were changed.
[0061]
[Table 3]
[0062]
As is clear from Table 3, even if the steel is within the component range specified by the present invention, any of the conditions such as the heating temperature, the finishing temperature, and the precipitation temperature are out of the range specified by the present invention. Test No. Nos. 9 to 12 did not show excellent results. Test No. 9 has a low finishing temperature, and 11 has a low heating temperature and a low finishing temperature, so that the solid solution of Nb (C, N) immediately after the upsetting is insufficient, and Nb (C, No. N) The number of precipitates decreased. No. 10 has a high precipitation treatment temperature, so that bainite was generated during cooling after the precipitation treatment, and mixed grains were generated due to the effect. In the case of heating at 1050 ° C., the number of single AlN precipitates was significantly increased due to the effect of heating once and cooled down again to room temperature, and the pinning effect of AlN almost completely disappeared. .
[0063]
On the other hand, Test No. which is an example satisfying the conditions of the present invention. It was confirmed that all of Nos. 1 to 8 did not cause abnormal grain growth even when heated at 1050 ° C.
[0064]
Further, in this embodiment, the case where the precipitation treatment time is only 40 minutes is shown. However, this is because the inside is heated sufficiently in a very short time because a very small upsetting test piece is used in this embodiment. This is because the effect of the precipitation treatment which is too sufficient can be obtained by heating for 40 minutes. However, many actual parts are larger, and depending on the size of the parts, it is necessary to perform heating and holding for a longer time to sufficiently precipitate Nb (C, N).
[0065]
Although the above-described examples all show evaluation results with small test pieces, similar trial production experiments were performed on actual gear parts, and it was confirmed that the effect of preventing particle mixing was similarly obtained. Was.
[0066]
Next, another example of investigating the precipitation state of a single AlN will be described.
In the above-described embodiment, only the amount of AlN deposited is shown. However, this value includes AlN in the composite precipitate of AlN and Nb (C, N). It cannot be determined that precipitation is suppressed. Therefore, in the data measurement of Table 2 described above, Test No. Using the specimen immediately before the heating experiment used in Examples 5 and 12, a secondary electron image and a reflected electron image were taken at exactly the same position on the test piece using a scanning electron microscope, and the results are shown in FIG. 5) and FIG. 2 (Test No. 12).
[0067]
Here, it is known that AlN is photographed in a reflected electron image, but not in a secondary electron image. Therefore, by comparing these two photographs, the presence or absence of a single AlN can be accurately grasped. can do. The magnification is 50,000 times for all four sheets.
[0068]
First, FIG. 2 shows Comparative Example No. 1 in which the amount of AlN precipitated was very large. 12 shows SEM photographs, (a) is a secondary electron image, and (b) is a reflected electron image. As apparent from FIG. 3B, a precipitate of AlN (a light gray portion in a portion surrounded by a circle) can be observed. In the backscattered electron image (a) taken at the same position on the test piece, nothing is observed at the same position as the position where the precipitate was observed in (b). This means that a large number of single AlNs are precipitated.
In both FIGS. 2 (a) and 2 (b), two black portions are seen in the oblique direction, but these are depressions and not AlN precipitates.
[0069]
On the other hand, the test Nos. 5 shows that no precipitate is observed in both (a) and (b). This tendency was the same in places other than the visual field shown here. This result is the same as that of No. This is the opposite of 12 and means that almost no single AlN is deposited.
In the figures described above, in order to clarify the presence or absence of a single AlN, a place where Nb (C, N) is not deposited is selected and photographed. Therefore, if a part where Nb (C, N) is precipitated is selected and photographed, a deposit of Nb (C, N) is naturally photographed in both (a) and (b).
[0070]
Further, although not shown in the specification, other test materials of the present invention used in the evaluation of Table 3 were examined in the same manner, and the same result was obtained.
Therefore, from the observation results, it is understood that in the steel of the present invention, the precipitation of single AlN is suppressed, and as a result, the grain growth during high-temperature carburizing is suppressed.
[0071]
【The invention's effect】
In the method for producing a hot forged part for high-temperature carburizing according to the present invention, a steel to which a small amount of Nb is added is heated and hot forged at a high temperature to temporarily bring Nb (C, N) into a substantially solid solution state. Nb (C, N) is finely dispersed while suppressing precipitation of a single AlN by a precipitation treatment of maintaining the temperature in a temperature range of 620 to 700 ° C. during the subsequent cooling for 30 minutes to 5 hours, and 1000 ° C. Abnormal grain growth can be prevented even with high-temperature carburization exceeding. Therefore, it is possible to raise the carburizing temperature and to significantly shorten the carburizing time, and it is possible to greatly reduce the heat treatment cost during the manufacturing cost of the carburized part of the automobile and the like. Quality can be improved.
[Brief description of the drawings]
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 5 is a drawing-substituted electron micrograph (magnification: 50,000) illustrating the carbonitride precipitation state of No. 5, in which (a) is a secondary electron image and (b) is a reflected electron image.
FIG. 12 is a drawing substitute electron micrograph (magnification: 50,000 times) illustrating the carbonitride precipitation state when the amount of AlN precipitation increases in (12), (a) is a secondary electron image, and (b) is a reflected electron image. is there.

Claims (3)

重量比でC:0.10〜0.30%、Si:0.05〜0.50%、Mn:0.30〜1.50%、Cr:0.30〜2.00%、Al:0.005〜0.050%、Nb:0.01〜0.10%、N:0.0080〜0.0250%、V:0.01%以下を含有し、残部Fe及び不純物元素からなる熱間圧延鋼材を1150〜1350℃に加熱後、鍛造仕上温度が1100〜1300℃となる条件で熱間鍛造して、鋼中に含有するNbがほぼ固溶した状態(析出個数が0.3個/μm以下)とし、鍛造後620〜700℃まで冷却後、その温度で30分〜5時間保持し、室温まで冷却後浸炭処理することを特徴とする高温浸炭用熱間鍛造部品の製造方法。C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.30 to 1.50%, Cr: 0.30 to 2.00%, Al: 0 by weight ratio 0.005 to 0.050%, Nb: 0.01 to 0.10%, N: 0.0080 to 0.0250%, V: 0.01% or less; After the rolled steel material is heated to 1150 to 1350 ° C, hot forging is performed under the condition that the forging finish temperature is 1100 to 1300 ° C, and the Nb contained in the steel is substantially in a solid solution (the number of precipitates is 0.3 / μm 2 or less), cooling to 620 to 700 ° C. after forging, holding at that temperature for 30 minutes to 5 hours, carburizing after cooling to room temperature, and a carburizing process for high-temperature carburizing. 請求項1の製造方法で使用する鋼に加えてさらにMo:0.80%以下を含有する熱間圧延鋼材に、請求項1記載の製造方法を施すことを特徴とする高温浸炭用熱間鍛造部品の製造方法。A hot forging for high-temperature carburizing, wherein a hot-rolled steel material further containing Mo: 0.80% or less in addition to the steel used in the manufacturing method according to claim 1 is subjected to the manufacturing method according to claim 1. The method of manufacturing the part. 請求項1、2の方法により製造され、AlN析出量が100ppm以下であり、かつNb(C、N)が、素地中に1〜10個/μm析出していることを特徴とする高温浸炭用熱間鍛造部品。 3. High-temperature carburization produced by the method according to claim 1, wherein the amount of AlN precipitated is 100 ppm or less, and 1 to 10 Nb (C, N) are precipitated in the substrate. For hot forging parts.
JP2003094987A 2003-03-31 2003-03-31 Method for manufacturing hot forged parts for high temperature carburizing and hot forged parts for high temperature carburizing manufactured by the method Active JP4212941B2 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
JP2003094987A JP4212941B2 (en) 2003-03-31 2003-03-31 Method for manufacturing hot forged parts for high temperature carburizing and hot forged parts for high temperature carburizing manufactured by the method

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
JP2003094987A JP4212941B2 (en) 2003-03-31 2003-03-31 Method for manufacturing hot forged parts for high temperature carburizing and hot forged parts for high temperature carburizing manufactured by the method

Publications (2)

Publication Number Publication Date
JP2004300519A true JP2004300519A (en) 2004-10-28
JP4212941B2 JP4212941B2 (en) 2009-01-21

Family

ID=33407424

Family Applications (1)

Application Number Title Priority Date Filing Date
JP2003094987A Active JP4212941B2 (en) 2003-03-31 2003-03-31 Method for manufacturing hot forged parts for high temperature carburizing and hot forged parts for high temperature carburizing manufactured by the method

Country Status (1)

Country Link
JP (1) JP4212941B2 (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010222634A (en) * 2009-03-23 2010-10-07 Kobe Steel Ltd Case hardening steel superior in properties of reducing size of maximum crystal grain and manufacturing method therefor
JP2014189857A (en) * 2013-03-27 2014-10-06 Aisin Aw Co Ltd Method of producing composite part

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP3169580B2 (en) 1998-04-03 2001-05-28 キヤノン株式会社 Optics, demands and cameras

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010222634A (en) * 2009-03-23 2010-10-07 Kobe Steel Ltd Case hardening steel superior in properties of reducing size of maximum crystal grain and manufacturing method therefor
JP2014189857A (en) * 2013-03-27 2014-10-06 Aisin Aw Co Ltd Method of producing composite part

Also Published As

Publication number Publication date
JP4212941B2 (en) 2009-01-21

Similar Documents

Publication Publication Date Title
JP4350968B2 (en) Steel for vacuum carburizing and manufacturing method of vacuum carburized parts
EP2865778B1 (en) High-strength hot-rolled steel sheet and process for producing same
JP2007284739A (en) Steel component and its production method
JP5858204B2 (en) Steel material for hot forging, method for producing the same, and method for producing hot forged raw material using the steel material
JP4073860B2 (en) Manufacturing method of carburized steel with excellent coarsening resistance after high-temperature carburizing
WO2016158343A1 (en) Steel wire for use in bolts that has excellent cold headability and resistance to delayed fracture after quenching and tempering, and bolt
JP6182489B2 (en) Case-hardened steel that has excellent cold forgeability and can suppress abnormal grain generation during carburizing.
JP5644483B2 (en) Hot-worked steel for surface hardening
JP5397308B2 (en) Hot-worked steel for case hardening
JP2008261029A (en) High-strength hot-rolled steel sheet superior in punching workability, and manufacturing method thereof
JP4212941B2 (en) Method for manufacturing hot forged parts for high temperature carburizing and hot forged parts for high temperature carburizing manufactured by the method
JP4322093B2 (en) Method for producing hot forged parts subjected to high-pressure carburization under reduced pressure
JP5649887B2 (en) Case-hardened steel and method for producing the same
JP4617783B2 (en) Manufacturing method of hot forged parts for high temperature carburizing
KR102006093B1 (en) Progressive steel parts
JP2005163168A (en) Production method for high-temperature carburizing steel capable of omitting normalizing after hot forging
JP5440720B2 (en) Steel for carburizing or carbonitriding
JP4681160B2 (en) Manufacturing method of high temperature carburizing steel and high temperature carburizing steel manufactured by the method
JP2005256142A (en) Method for producing high temperature carburized steel excellent in grain-coarsening resistance and machinability
JP2010215961A (en) Steel sheet of boron steel superior in hardenability, and manufacturing method therefor
JP2017133052A (en) Case hardened steel excellent in coarse particle prevention property, fatigue property and machinability during carburization and manufacturing method therefor
JP2005264318A (en) Soft nitriding treated steel superior in wear resistance
JP2021147643A (en) Rough shape material for vacuum carburization and method for producing the same
WO2018061396A1 (en) Forged heat-treated product of case hardening steel
JP2014047357A (en) Steel material

Legal Events

Date Code Title Description
A621 Written request for application examination

Free format text: JAPANESE INTERMEDIATE CODE: A621

Effective date: 20050908

A977 Report on retrieval

Free format text: JAPANESE INTERMEDIATE CODE: A971007

Effective date: 20070319

A131 Notification of reasons for refusal

Free format text: JAPANESE INTERMEDIATE CODE: A131

Effective date: 20070327

TRDD Decision of grant or rejection written
A01 Written decision to grant a patent or to grant a registration (utility model)

Free format text: JAPANESE INTERMEDIATE CODE: A01

Effective date: 20081028

A01 Written decision to grant a patent or to grant a registration (utility model)

Free format text: JAPANESE INTERMEDIATE CODE: A01

A61 First payment of annual fees (during grant procedure)

Free format text: JAPANESE INTERMEDIATE CODE: A61

Effective date: 20081029

R150 Certificate of patent or registration of utility model

Ref document number: 4212941

Country of ref document: JP

Free format text: JAPANESE INTERMEDIATE CODE: R150

Free format text: JAPANESE INTERMEDIATE CODE: R150

FPAY Renewal fee payment (event date is renewal date of database)

Free format text: PAYMENT UNTIL: 20111107

Year of fee payment: 3

FPAY Renewal fee payment (event date is renewal date of database)

Free format text: PAYMENT UNTIL: 20111107

Year of fee payment: 3

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

FPAY Renewal fee payment (event date is renewal date of database)

Free format text: PAYMENT UNTIL: 20121107

Year of fee payment: 4

FPAY Renewal fee payment (event date is renewal date of database)

Free format text: PAYMENT UNTIL: 20131107

Year of fee payment: 5

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250

R250 Receipt of annual fees

Free format text: JAPANESE INTERMEDIATE CODE: R250