JP2004115868A - Method for manufacturing sintered member - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、焼結部材の製造方法に関し、詳しくは、高い寸法精度を有し、かつ原料粉末のロットによる寸法のロット間バラツキを低減する焼結部材の製造方法に関する。
【０００２】
【従来の技術】
焼結部材は、原料粉末を成形して粉末成形体を形成し、この粉末成形体を溶融点以下の温度で焼結させることで焼結体となし、必要に応じてこの焼結体の寸法矯正や熱処理を施して製造されている。粉末成形体を焼結した焼結体は、焼結時に生じた膨張あるいは収縮により寸法が変化するために、予め寸法変化量を見込んで金型寸法を補正することが一般的に行われている。しかし、製造工程中の様々な要因によって焼結体の寸法変化量はバラツクために、さらに寸法矯正工程あるいは仕上げ加工が必要となり、コストアップの要因となっている。
【０００３】
工業生産における焼結体の寸法変動には、ロット内変動とロット間変動とがある。ロット内変動は比較的小さいが、ロット間での寸法変動はロット内変動に比べて大きい場合が多いので、焼結部材の寸法精度を大きく低下させることとなる。
【０００４】
焼結体の寸法変化におよぼす製造工程中の変動要因としては、原料粉末の微粉量や組成（特に炭素量）、成形密度、焼結温度あるいは焼結時間などを挙げることができる。
【０００５】
原料鉄粉中の微粉量が、焼結工程における収縮量を変動させる要因の一つであることはよく知られている。例えば、特開平４−２１０４０２号公報（特許文献１）では、原料鉄粉の粒度を調製することにより、焼結の際の寸法変化を低減して焼結体の寸法精度を向上する焼結部材の製造方法について開示している。すなわち、平均粒径４０〜２００μｍの鉄粗紛と平均粒径２０μｍ以下の鉄微粉を５〜３０重量％混合した原料粉末を用いることとしている。
【０００６】
ここでは、微粉量を原料鉄粉の製造ロットに係わらずある一定の値の範囲に管理することが必要となる。このためには、原料粉末を何らかの方法で分級して配合し直すという原料粉末の前処理工程が必要となる。さらに、そのためには相当量の在庫を保有しなければならないという問題も生じる。
【０００７】
また、焼結時の寸法変化量は、添加する炭素量によっても変動する。通常、焼結部材の機械的性質を向上するために、原料鉄粉に一定量の炭素を配合して原料粉末としている。しかし、一方で原料の鉄粉中には不純物として酸素が含まれており、この配合された炭素の一部は、焼結の際に鉄粉を還元するために消費される。従って、原料鉄粉に含まれる酸素量によって、焼結部材中の炭素量がばらつき、結果的に焼結部材の寸法の変動が生じる。
【０００８】
特に、焼結後に機械的性質などを改善するために熱処理を施す場合には、さらに熱処理工程での寸法変動が生じるために、最終的に得られる焼結部材の寸法は、焼結工程とこの熱処理工程とで発生する寸法変動の累積となるためより大きな寸法バラツキを呈することとなる。
【０００９】
生産の場においては、添加炭素量のロットごとの調整や、あるいは、焼結温度、焼結時間といった製造条件などの管理によって焼結部材の寸法バラツキを低減することも試みられているが、工程全体の管理レベルを従来以上に厳しくする必要があり必ずしも容易なことではない。
【００１０】
【特許文献１】
特開平４−２１０４０２号公報
【００１１】
【発明が解決しようとする課題】
本発明は以上の実状に鑑みてなされたもので、従来の工程及び管理レベルを大きく変更することなく、高い寸法精度を有し、かつ原料粉末のロットによる寸法のロット間バラツキを低減することのできる焼結部材の製造方法を提供することである。
【００１２】
【課題を解決するための手段】
本発明の焼結部材の製造方法は、少なくとも原料鉄粉および炭素粉末を配合して原料粉末を調製する原料粉末調製工程と、原料粉末を成形して粉末成形体を形成する成形工程と、粉末成形体を焼結させて焼結体とする焼結工程と、焼結体を熱処理する熱処理工程と、を有する焼結部材の製造方法において、粉末成形体の焼結工程における寸法変化と炭素量との相関と、焼結体の熱処理工程における寸法変化と炭素量との相関とが、一方は正の相関を呈し、他方は負の相関を呈する特性を有することを特徴とする。
【００１３】
ここで、原料鉄粉は少なくともＣｒを２〜４重量％含有する鉄合金粉末である焼結部材の製造方法を第１の製造方法とする。
【００１４】
また、原料粉末が原料鉄粉と炭素粉末のほかに、さらに銅粉末を原料粉末全体を１００重量％として、０．７〜１．３重量％含有する混合粉末である焼結部材の製造方法を第２の製造方法とする。
【００１５】
なお、いずれの方法においても、原料粉末は炭素粉末を原料粉末全体を１００重量％として０．５〜１．５重量％含むことが望ましい。
【００１６】
【発明の実施の形態】
本発明者は、焼結後熱処理を行う焼結部材について、ある成分系の原料粉末を使用すると、炭素量の変化による焼結時の寸法変化の傾向と、熱処理時の寸法変化の傾向とが逆転することに着目した。すなわち、一般に熱処理を伴う低合金系の焼結材料は焼結時に収縮するが、その収縮量は炭素量が少ないほど収縮量が大きく、炭素量が多くなれば収縮量は小さくなる。つまり、炭素量の増加に伴って焼結体の寸法変化は正の勾配をもつこととなる。また、熱処理時においても同様に炭素量の増加に従い焼結部材の寸法変化は正の勾配をもつ。このため、焼結工程と熱処理工程とを経た焼結部材は、両工程の寸法変化が累積された大きな寸法変化を生じることになる。従って、焼結工程と熱処理工程とで生じる炭素量による寸法変化の傾向を相互に打消し合うことのできる合金系の原料鉄粉を用いれば、得られる焼結部材の寸法変化は低減され、結果として、寸法バラツキの小さい焼結部材を得ることができるのである。
【００１７】
従来技術になる原料鉄粉の各工程間で生じる寸法変化の炭素量による影響について調査した。前述の通り、通常は、原料鉄粉に対して一定量の炭素が添加されるが、炭素は原料鉄粉中の酸素によって消耗するため、焼結部材中の炭素量はバラツクこととなる。また、実際の生産ラインにおいてはロットが混合することによる不均一さによってもバラツクことがある。そこで、あるロットの原料鉄粉に添加量を変化させて炭素を添加して焼結部材中の炭素量を変化させることで、実際の原料鉄粉のロット間バラツキに対する寸法変化のバラツキを調査した。
【００１８】
その一例を図５に示す。原料鉄粉は水アトマイズ法で得られたＦｅ−１．５％Ｍｏ粉末である。図５では、縦軸は寸法変化を工程間の寸法変化率（％）として示し、また、横軸は原料粉末中の配合炭素量（％）を示している。配合炭素量は０．８、１．０、１．２重量％（以下、配合に関する％は重量％を示す。）の３水準とした。図中のＧ−Ｄ（◆）は成形金型に対する粉末成形体の寸法変化率を示す。同様に、Ｓ−Ｇ（■）は粉末成形体に対する焼結体の、Ｓ−Ｄ（△）は成形金型に対する焼結体の、Ｈ−Ｓ（×）は焼結体に対する熱処理後の熱処理体（焼結部材）の、Ｈ−Ｄ（＊）は金型に対する熱処理後の焼結部材の寸法変化率を示している。例えば、配合炭素量が０．８％（焼結部材中の含有炭素量は０．７重量％であった）の場合には、粉末成形体は金型に対して約０．２６％膨張しており、焼結することにより焼結体は、粉末成形体に対して０．０９％収縮し、熱処理を施すことで熱処理体（焼結部材）は、焼結体より０．０４％膨張している。成形金型を基準にすると熱処理後の焼結部材は、約０．２％膨張したことが分る（焼結後に寸法矯正は行っていない）。この原料鉄粉の場合には、焼結工程（■）でも、また、熱処理工程（×）でも寸法の変化率は炭素量の増加に比例して増加する正の勾配を示しているので、最終的に得られる焼結部材では、焼結工程と熱処理工程との炭素量による寸法変化率が累積されたＨ−Ｄ（＊）となり大きな寸法バラツキを生じることが分る。
【００１９】
しかし、ここで例えば、焼結時の焼結体の寸法変化が、炭素量の増加に対して減少する負の勾配をもつ合金系を選択して、熱処理時の炭素量の増加による寸法変化の正の勾配を打消すことができれば、熱処理後の焼結部材の寸法変化は、炭素量のバラツキに係わらず極めて小さいものとすることができる。
【００２０】
原料鉄粉として、鉄−クロム系合金粉末（Ｆｅ−３．０％Ｃｒ−０．３％Ｍｏ−０．３％Ｖ）を用いて図５と同様に炭素量による各工程間での寸法変化の挙動を調査した。結果を図１に示す。符号は図５と同様としている。ここで示した鉄合金粉末では、■で示される焼結体の粉末成形体に対する寸法変化率が、炭素量の増加に伴って焼結体の収縮量が増加する負の勾配をもっている。一方、焼結体を熱処理した場合には、×のように炭素量の増加に伴って熱処理体の収縮量は減少し右肩上がりの正の勾配を示している。従って、焼結後に熱処理して得られる焼結部材の寸法は、両工程の寸法変化率が相殺されて、基準となる金型寸法に対する寸法変化率が、＊印で示されるように、配合炭素量が０．８〜１．２％の間では−０．０２〜＋０．０４％と極めて小さいもであることが分る。つまり、原料鉄粉のロットによって焼結部材の炭素量がばらついたとしても、最終的に得られる焼結部材の金型寸法に対する寸法変化率は、０．０１±０．０３％以内であり、極めて寸法精度が高くまたばらつきも小さい焼結部材の得られることが分る。
（第１の焼結部材の製造方法）
本発明の第１の焼結部材の製造方法は、少なくとも原料鉄粉および炭素粉末を配合して原料粉末を調製する原料粉末調製工程と、原料粉末を成形して粉末成形体を形成する成形工程と、粉末成形体を焼結させて焼結体とする焼結工程と、焼結体を熱処理する熱処理工程と、を有する焼結部材の製造方法において、原料粉末は焼結工程における粉末成形体の炭素量と寸法変化との相関と、熱処理工程における焼結体の炭素量と寸法変化との相関とが、一方は正の相関を呈し、他方は負の相関を呈する特性を有することを特徴とする。
【００２１】
ここで、原料鉄粉は少なくともＣｒを２〜４重量％含有する鉄合金粉末である焼結部材の製造方法を第１の製造方法とする。
【００２２】
原料鉄粉は、直接還元法やアトマイジング法などの方法により得られる少なくともＣｒを２〜４重量％含有する鉄合金粉末を用いることができる。ここで、Ｃｒの含有量が２重量％未満では、焼結時の炭素量による寸法変化率の変化の傾向が熱処理工程における寸法変化率の変化を相殺するのに充分ではない。また、４重量％を越えると焼結時の炭素量による寸法変化率の変化の絶対値が大きくなり過ぎるために適当ではない。好ましくは、２．４〜３．６重量％である。本鉄合金粉末には、所望によりＣｒ以外のＭｏやＶなどの合金元素を含有することができる。これら元素の含有量は、通常低合金系鉄粉として用いられる原料鉄粉に含有される程度であれば問題はないが、Ｍｏは０．２〜０．６重量％、Ｖは０．１〜０．６重量％が好ましい。
【００２３】
鉄合金粉末の粒度は通常の焼結部材の製造に用いられるものであれば特に限定はないが、鉄合金粗粉は２０〜２００μｍ程度で、４５μｍ以下（３２５メッシュ）の微粉を５〜２０重量％含有するものが望ましい。鉄合金粗分が２００μｍ以上では焼結体の高い密度が得られない。また、微粉を２０重量％以上含んでいると密度は高くなるが焼結後の寸法変化が大きいために適当ではない。
【００２４】
配合炭素は、通常用いられる天然黒鉛または人造黒鉛を使用することができる。黒鉛の粒度は特に制限はないが、平均で数μｍ〜数十μｍのものが適当である。
炭素配合量は、厳密には原料鉄粉の含有する酸素量によって異なるが、通常は、原料粉末全体を１００重量％として０．５〜１．５重量％であることが望ましい。配合炭素量が０．５％未満では焼結部材の所望の機械的性質を得ることが困難であり、１．５％を越えると靭性が低下するために好ましくない。より好ましくは０．８〜１．２％である。
【００２５】
鉄合金粉末と配合炭素との混合は、通常の方法で行えばよい。また、混合に際してステアリン酸亜鉛、アミド系ワックスなどの潤滑剤を添加してもよい。また、バインダ等により偏析防止処理を施してもよい。なお、潤滑剤は原料粉末に配合しないで、金型に塗布してもよい。
【００２６】
上記で得られた原料粉末を成形金型に充填して圧縮成形する。成形条件は特に制約されないが、単軸圧縮成形で構造用部材の場合には、面圧４００〜８００ＭＰａで行うことが好ましい。面圧が４００ＭＰａ以下では焼結体の十分な密度が得られず、８００ＭＰａ以上では設備的な制約が生じる可能性がある。しかし、用途によりさらなる密度向上が必要な場合はこの限りではない。
【００２７】
焼結は真空雰囲気、保護雰囲気、あるいは非浸炭性還元雰囲気で行われるのが望ましい。浸炭性雰囲気の場合には、雰囲気のバラツキによって、焼結部材の炭素量が変動するため、効果が損われる場合がある。真空焼結の場合には、加熱前の真空度は１０−３ｔｏｒｒ以下とすることが望ましい。焼結温度は一般に１１００〜１３００℃で、保持時間は１０〜６０分間が好ましい。焼結温度が１１００℃以下では、焼結粉末間のネックの成長が遅いために強度が得られにくい。また、１３００℃以上では焼結設備に制約が生じることがある。焼結保持時間については、１０分以下ではネックの成長が不足するため強度が得られ難く、一方、６０分以上では生産性が低下するため適当ではない。
【００２８】
また、焼結時の昇温速度には特に制約はないが、原料粉末の組成によっては、昇温速度が寸法変化に大きく影響を及ぼすことがあるので、条件を安定させることが重要である。さらに、冷却条件は、２０℃／分前後が一般的であるが、生産性や組織制御の必要性に応じて自由に選択すればよい。
【００２９】
焼結時間のバラツキを抑制するためには、バッチ式の真空焼結炉よりも、メッシュベルトなどによる連続炉を用いて段積みを避けることが望ましい。これは、焼結が行われる高温域では、伝熱は輻射伝熱が支配的であるために、段積みを行うと内部のワークへの昇温遅れが発生し、結果としてワークの積載位置により焼結時間に差異が生じるためである。
【００３０】
焼結後必要に応じて寸法矯正を行うことができる。寸法矯正は通常の方法により行うことができ特に制約はない。
【００３１】
また、焼結後に熱処理を施して機械的性質等の向上などを図ることができる。熱処理の方法としては、高周波焼入れ焼戻し、浸炭焼入れ焼戻し、光輝焼入れ焼戻しなどの一般的な硬化処理が可能であり、条件に特に制約はない。
【００３２】
焼入れは通常の方法で行うことができるが、寸法精度の面では真空熱処理炉を使用することが望ましい。ＲＸガスなどの雰囲気炉では、カーボンポテンシャルの変動により寸法がばらつくので好ましくない。熱処理の温度は、８００〜９００℃で、２０〜６０分の均熱処理が好ましい。均熱温度が８００℃以下では不完全焼入れとなり、９００℃以上では残留オーステナイトが増加する。また、均熱時間が、２０分以下では不完全焼入れとなり、６０分以上では生産性を阻害することがあるので好ましくない。均熱処理後に７０〜２００℃で油焼入れを行うとよい。焼入れ冷媒は油に限らず水や加圧ガスなどを使用することもできる。焼入れ処理後、１５０〜２００℃、もしくは３５０℃以上で、２０〜１２０分間の焼戻し処理を施すことが好ましい。処理温度が１５０℃以下では脆性に問題を生じ、２００〜３５０℃では熱脆性が発生するため適当ではない。また、処理時間が２０分以下では焼戻しが不十分であり、１２０分以上では生産性を阻害することがあるので好ましくない。
【００３３】
さらに、後処理としては、封孔、防錆などが目的の水蒸気処理や残留応力付与目的のショットピーニングなども適用することができる。
（第２の焼結部材の製造方法）
本発明の第２の焼結部材の製造方法は、第１の製造方法の原料粉末に加えて、さらに銅粉末を原料粉末全体を１００重量％として０．７〜１．３重量％含有する混合粉末とする方法である。
【００３４】
原料鉄粉は直接還元法やアトマイジング法などの方法により得られる純鉄（純度：９９％以上）または、Ｎｉ、Ｍｏ、Ｃｒなどの合金元素を３重量％未満含有する低合金粉末を用いることができる。ここで、Ｎｉ、Ｍｏ、Ｃｒなどの合金元素の含有量が３重量％以上では、銅粉末の混合効果が低減されるので好ましくない。
【００３５】
また、銅粉末としては、電解銅粉末、アトマイズ銅粉末などを例示することが出来る。銅粉末の純度は、９９％以上が好ましく、原料粉末全体を１００重量％として０．７〜１．３重量％含有することが望ましい。銅粉末の含有量が０．７重量％未満では焼結時の寸法変化の炭素依存性を熱処理時の炭素依存性で打消すのに充分ではなく、１．３重量％を越えると焼結時の寸法変化率の絶対値が大きくなりすぎるために適当ではない。
【００３６】
原料鉄粉の粒度は通常の焼結部材の製造に用いられるものであれば特に限定はないが、鉄粗粉は２０〜２００μｍ程度で、４５μｍ以下（３２５メッシュ）の微粉を５〜２０重量％含有するものが望ましい。鉄粗分が２００μｍ以上では焼結体の高い密度が得られない。また、微粉を２０重量％以上含んでいると密度は高くなるが焼結後の寸法変化が大きいために適当ではない。
【００３７】
一方、銅粉末は、５〜５０μｍ程度の大きさのものが好ましい。５μｍ未満では粉末が高価であるためにコスト高となり、５０μｍを越えると銅の偏析を生じやすくなるので好ましくない。
【００３８】
配合炭素は、通常用いられる天然黒鉛または人造黒鉛を使用することができる。黒鉛の粒度は特に制限はないが、平均で数μｍ〜数十μｍのものが適当である。
【００３９】
原料鉄粉と銅粉末及び配合炭素との混合は、通常の方法で行えばよい。また、混合に際してステアリン酸亜鉛、アミド系ワックスなどの潤滑剤を添加してもよい。また、バインダ等により偏析防止処理を施してもよい。なお、潤滑剤は原料粉末に配合しないで、金型に塗布してもよい。
【００４０】
この第２の焼結部材の製造方法は、前記第１の製造方法のうちで鉄合金粉末を原料鉄粉と銅粉末との混合粉末とした以外の工程は、第１の製造方法と同様に行うことができる。すなわち、原料粉末の調整、粉末成形工程、焼結工程、熱処理工程などは、第１の製造方法と同様に行えばよい。
【００４１】
【実施例】
以下、実施例を用いて本発明をより具体的に説明する。
（実施例１）
原料粉末として、組成がＦｅ−３．０％Ｃｒ−０．３％Ｍｏ−０．３％Ｖ（川崎製鉄製）で、粗粉の粒度が４５〜２５０μｍで、粒度が４５μｍ（３２５メッシュ）以下の微粉を９．８〜１９．３重量％含有している鉄−クロム合金粉末を準備した。
【００４２】
この原料鉄合金粉末に、平均粒径約５μｍの天然黒鉛（日本黒鉛製）を各々０．８、１．０、１．２％配合してＶ型ミキサーで３０分間混合して３水準の試料を調製した。各試料粉末から任意に約２２ｇのサンプルを１０個採取して、超硬合金製の金型（内径：２５．２３０５ｍｍ、厚さ：４５ｍｍ）に充填し、面圧：９００ＭＰａで加圧成形した。この時、ステアリン酸亜鉛をエチルアルコールに懸濁させたものを金型内面に塗布し自然乾燥さて潤滑剤とした。
【００４３】
加圧成形後の粉末成形体は、６±０．１ｍｍの厚さであった。
【００４４】
このようにして得られた３０個（１０個×３水準）の粉末成形体の直径を、２０±０．５℃の恒温室内でレーザースキャンマイクロメータを用いて１個につき２点測定しその平均値を求めた。
【００４５】
次に、上記で得られた３０個の粉末成形体を室温で真空度１０−３ｔｏｒｒ以下（１０−４ｔｏｒｒ台）の真空焼結炉で、１２５０±１℃、３０±０．５分間の焼結を行った。なお、昇温速度は２０℃／ｍｉｎであった。焼結保持後、１２５０℃〜３００℃までは、窒素ガスファン制御で平均３０℃／ｍｉｎの冷却速度で冷却した。その後空気中で放冷して３０個の焼結体を得た。得られた焼結体の直径を前記の粉末成形体と同様の方法で測定して平均値を求めた。
【００４６】
得られた焼結体に、さらに、真空焼入れ炉で、８５０℃、３０分の均熱処理を施し８０℃の焼入れ油中に投入して焼入れ処理を行った。この時、常温から均熱温度までの昇温速度は１５℃／ｍｉｎとした。引続いて、大気炉中で１８０℃、６０分の焼戻し処理を施して熱処理体（焼結部材）を得た。上記と同様に測定して熱処理体の直径の平均値を得た。なお、このようにして得られた焼結部材（熱処理体）の炭素含有量は、配合炭素量が０．８％の試料では０．７％、１．０％の試料では０．９％であり、１．２％の試料では１．１％であった。
【００４７】
結果を図１に示す。前述したように、本実施例の鉄合金粉末を用いた場合には、■で示される焼結体の粉末成形体に対する寸法変化率は、配合炭素量の増加に伴い焼結体の収縮量が増加する負の勾配をもっている。ところが、焼結体を熱処理した場合には、×のように炭素量の増加に伴って熱処理体の収縮量は減少し右肩上がりの正の勾配を示している。従って、焼結後熱処理して得られた焼結部材の寸法は両者の寸法変化率が相殺されて、基準となる金型寸法に対する寸法変化率は、＊印で示されるように、配合炭素量が０．８〜１．２％の間では、−０．０２〜＋０．０４％（すなわち０．０１±０．０３％）と極めて小さいもであることが分る。
（試験例１）
原料粉末として平均粒径１５０μｍ以下の純鉄（ＡＳＣ１００．２９ ヘガネス社製）に、平均粒径１０μｍ以下の電解銅粉（ＣＥ１１１０ 福田金属箔粉社製）を添加して、さらに平均粒径約５μｍの天然黒鉛（日本黒鉛製）を配合した試料について実施例１と同様に各工程間での試料の寸法変化率を求めた。
【００４８】
電解銅粉の添加量は、０、１．０、２．０％の３水準とし、電解銅粉の各添加量に対して天然黒鉛を０、０．３、０．６、０．９、１．２％配合した５水準の試料を準備した。すなわち、電解銅粉の添加量と配合炭素量とが異なる１５種類の試料を準備した。これらの各試料にさらに潤滑剤としてステアリン酸亜鉛を０．８％加えて、各々Ｖ型ミキサーで３０分間混合して各試料粉末を作成した。この各試料粉末から任意に約２２ｇのサンプルを１０個採取して、超硬合金製の金型（内径：２５．２３０５ｍｍ、厚さ：４５ｍｍ）に充填し、面圧：５８８ＭＰａで加圧成形した。
【００４９】
加圧成形後の粉末成形体は、６±０．１ｍｍの厚さであった。
【００５０】
以上のようにして得られた１５０個（１０個×１５水準）の粉末成形体の直径を、２０±０．５℃の恒温室内でレーザースキャンマイクロメータを用いて１個につき２点測定しその平均値を求めた。
【００５１】
次に、上記で得られた１５０個の粉末成形体を１００％窒素雰囲気のバッチ炉中で、１１２０±１℃、３０±０．５分間の焼結を行った。なお、昇温速度は２０℃／ｍｉｎであった。焼結保持後、１２５０℃〜３００℃までは、窒素ガスファン制御で平均３０℃／ｍｉｎの冷却速度で冷却した。その後空気中で放冷して１５０個の焼結体を得た。得られた焼結体の直径を前記の粉末成形体と同様の方法で測定した。
【００５２】
得られた焼結体を、さらに、真空焼入れ炉で、８５０℃、３０分の均熱処理を行い８０℃の焼入れ油中に投入して焼入れ処理を施した。この時、常温から均熱温度までの昇温速度は１５℃／ｍｉｎとした。引続いて、大気炉中で１８０℃、３０分の焼戻し処理を行って熱処理体（焼結部材）を得た。上記と同様に測定して熱処理体（焼結部材）の直径寸法を得た。結果を図２、３及び４に示す。なお、このようにして得られた焼結部材（熱処理体）の炭素含有量は、配合炭素量が０、０．３、０．６、０．９、１．２％の各試料について、各々０．２、０．５、０．８および１．１％であった。
【００５３】
図２は、焼結体の寸法変化、すなわち粉末成形体に対する焼結体の寸法変化率と炭素量との関係を示している。図中◆は、原料粉末中に銅粉末を添加しなかった場合であり、■は原料粉末全体を１００％として、銅粉末を１．０％添加した場合であり、△は、同様に２．０％添加した場合である。銅粉末を添加しない場合（◆）には、炭素量０で焼結体は粉末成形体に対して０．１３％収縮し炭素量の増加に伴い収縮量は減少して、炭素量が１．２％では０．０６％の膨張となっており、炭素量の増加に伴って寸法の変化率は正の勾配をもつことが分る。しかし、銅粉末を１．０％添加すると（■）炭素量が０％でも寸法変化率は０．１１％と粉末成形体に対して膨張する傾向にあり、炭素量が０．６％までは寸法変化率も増加し、その後寸法変化率は減少して炭素量が１．２％では０．０６％と銅粉末無添加の場合とほぼ同様の値となった。また、銅粉末を２．０％添加した場合（△）には、炭素量が０％でも寸法変化率は０．３１％と粉末成形体に対して大きく膨張する傾向にあり、炭素量の増加に従って寸法変化率も膨張する方向に増加し、炭素量が０．６％では寸法変化率は最大の０．３７％となった。その後寸法変化率は減少して炭素量が１．２％では０．０６％と銅無添加の場合とほぼ同等の値となった。この現象は、Ｆｅ−Ｃｕ−Ｃ系の成形粉末体の焼結時に生じる銅膨張と呼ばれる現象が炭素の増加によって抑制されるためと考えられる。
【００５４】
一方、熱処理工程での寸法変化率、すなわち焼結体に対する熱処理後の焼結部材（熱処理体）の寸法の変化率は、図３に示すように銅粉末の添加量に係わらず、炭素量の増加に伴いほぼ一定の正の勾配をもって変化することが分る。従って粉末成形体に対する焼結部材の寸法変化率は、図２と図３とを加算したものであるから、図４に示す通りとなる。図４から純鉄粉末に銅粉末を１．０％添加した場合には、配合炭素量の０．８〜１．２％の変動に対して焼結部材（熱処理体）の寸法変化率は、０．１１〜０．１７％（０．１４±０．０３％）と極めて小さくなることが分る。
【００５５】
すなわち、この銅粉末を含有する原料粉末を使用すれば、焼結部材中の炭素量が変動しても、従来工程の条件や管理レベルを大きく変更することなく高い寸法精度を有し、かつ原料粉末のロットによる寸法のロット間バラツキの極めて小さい焼結部材を得ることができる。
（比較例１）
原料粉末としてＦｅ−１．５％Ｍｏ（ヘガネス製）、鉄粗粉の粒度が４５〜２５０μｍで、粒度が４５μｍ（３２５メッシュ）以下の鉄微粉を５〜３０重量％含有している原料鉄粉を準備した。
【００５６】
以下実施例１と同様にして３０個の焼結部材（熱処理体）を得た。実施例１と同様に各工程間での直径寸法の変化率を測定してプロットし、図５を得た。
配合炭素量が０．８〜１．２％の範囲では、熱処理体の金型寸法に対する寸法変化率は、０．２１〜０．３７％（０．２９±０．０８％）にまで大きくバラツクことが分る。
【００５７】
【発明の効果】
本発明の焼結部材の製造方法は、炭素量による寸法変化の挙動が、焼結工程と熱処理工程において逆転するような原料粉末を使用する。このため、焼結工程と熱処理工程とを経過することによって得られる焼結部材の寸法は、両工程での寸法変化の挙動の相違により相殺されて、炭素量のバラツキがあったとしても焼結部材の寸法バラツキは極めて小さい範囲に抑制される。よって、本発明の焼結部材の製造方法によれば、通常の製造工程および管理レベルを大きく変更することなく、高い寸法精度を有し、かつ原料粉末のロットによる寸法のロット間バラツキを低減した焼結部材を安定して得ることができる。
【図面の簡単な説明】
【図１】鉄合金粉末（Ｆｅ−３％Ｃｒ−０．３％Ｍｏ−０．３％Ｖ）の各工程間における寸法変化率と炭素量との関係を示す図である。
【図２】銅粉末の各添加量について、粉末成形体に対する焼結体の寸法変化率と炭素量との関係を示した図である。
【図３】銅粉末の各添加量について、焼結体に対する熱処理体（焼結部材）の寸法変化率と炭素量との関係を示した図である。
【図４】銅粉末の各添加量について、図２と図３とから粉末成形体に対する熱処理体（焼結部材）の寸法変化率と炭素量との関係を求めた図である。
【図５】従来技術になる原料鉄粉（Ｆｅ−１．５％Ｍｏ）の各製造工程ごとの寸法変化率と炭素量との関係を示した図である。
【符号の説明】
Ｇ−Ｄ（◆）：成形金型に対する粉末成形体の寸法変化率（％）
Ｓ−Ｇ（■）：粉末成形体に対する焼結体の寸法変化率（％）
Ｓ−Ｄ（△）：成形金型に対する焼結体の寸法変化率（％）
Ｈ−Ｓ（×）：焼結体に対する熱処理後の熱処理体の寸法変化率（％）
Ｈ−Ｄ（＊）：金型に対する熱処理後の熱処理体（焼結部材）の寸法変化率（％）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a sintered member, and more particularly, to a method for manufacturing a sintered member having high dimensional accuracy and reducing variation between lots of the size of a raw material powder.
[0002]
[Prior art]
The sintered member is formed by molding the raw material powder to form a powder compact, and then sintering the powder compact at a temperature equal to or lower than a melting point to form a sintered compact. Manufactured by straightening and heat treatment. Since the dimensions of a sintered body obtained by sintering a powder compact change due to expansion or shrinkage generated during sintering, it is common practice to correct the mold dimensions in advance in consideration of the amount of dimensional change. . However, the amount of dimensional change of the sintered body varies due to various factors during the manufacturing process, so that a dimensional correction step or a finishing process is required, which causes an increase in cost.
[0003]
The dimensional fluctuation of the sintered body in industrial production includes intra-lot fluctuation and inter-lot fluctuation. Although the variation within the lot is relatively small, the dimensional variation between lots is often greater than the variation within the lot, so that the dimensional accuracy of the sintered member is greatly reduced.
[0004]
Factors that change the dimensions of the sintered body during the manufacturing process include the amount and composition of the raw material powder (particularly the amount of carbon), the molding density, the sintering temperature and the sintering time.
[0005]
It is well known that the amount of fine powder in the raw iron powder is one of the factors that cause the amount of shrinkage in the sintering process to fluctuate. For example, Japanese Patent Application Laid-Open No. 4-210402 (Patent Document 1) discloses a sintered member in which the dimensional change during sintering is reduced and the dimensional accuracy of the sintered body is improved by adjusting the particle size of the raw iron powder. Is disclosed. That is, a raw material powder obtained by mixing 5 to 30% by weight of iron coarse powder having an average particle size of 40 to 200 μm and iron fine powder having an average particle size of 20 μm or less is used.
[0006]
Here, it is necessary to control the amount of the fine powder within a certain range regardless of the production lot of the raw iron powder. For this purpose, a raw material powder pretreatment step of classifying the raw material powder by some method and remixing it is required. In addition, there is a problem that a considerable amount of stock must be held for that purpose.
[0007]
The dimensional change during sintering also varies depending on the amount of carbon added. Usually, in order to improve the mechanical properties of the sintered member, a certain amount of carbon is blended into the raw iron powder to obtain the raw powder. However, on the other hand, the iron powder as a raw material contains oxygen as an impurity, and a part of the compounded carbon is consumed to reduce the iron powder during sintering. Therefore, the amount of carbon in the sintered member varies depending on the amount of oxygen contained in the raw iron powder, and as a result, the size of the sintered member varies.
[0008]
In particular, when heat treatment is performed after sintering to improve the mechanical properties, etc., the dimensions of the finally obtained sintered member are changed by the sintering process and the sintering process. Since the dimensional fluctuations generated during the heat treatment process are accumulated, a larger dimensional variation is exhibited.
[0009]
At the production site, attempts have been made to reduce the dimensional variation of sintered members by adjusting the amount of added carbon for each lot or by controlling production conditions such as sintering temperature and sintering time. The overall management level needs to be stricter than before, which is not always easy.
[0010]
[Patent Document 1]
JP-A-4-210402
[0011]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and has a high dimensional accuracy without significantly changing the conventional process and control level, and is intended to reduce the lot-to-lot variation in the size of the raw material powder lot. It is to provide a method for manufacturing a sintered member that can be performed.
[0012]
[Means for Solving the Problems]
The method for producing a sintered member of the present invention includes a raw material powder preparing step of preparing a raw material powder by blending at least a raw iron powder and a carbon powder; a forming step of forming the raw material powder to form a powder compact; In a method for manufacturing a sintered member having a sintering step of sintering a compact to form a sintered body and a heat treatment step of heat-treating the sintered body, a dimensional change and a carbon content in the sintering step of the powder compact are provided. Is characterized in that one has a characteristic of exhibiting a positive correlation and the other has a characteristic of exhibiting a negative correlation in the correlation between the dimensional change in the heat treatment step of the sintered body and the carbon content.
[0013]
Here, the method for producing a sintered member in which the raw iron powder is an iron alloy powder containing at least 2 to 4% by weight of Cr is defined as a first production method.
[0014]
Also, a method for producing a sintered member in which the raw material powder is a mixed powder containing 0.7 to 1.3% by weight, in addition to the raw material iron powder and the carbon powder, and further containing copper powder as 100% by weight of the whole raw material powder. This is the second manufacturing method.
[0015]
In any method, the raw material powder desirably contains carbon powder in an amount of 0.5 to 1.5% by weight based on 100% by weight of the whole raw material powder.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors have found that, for a sintered member to be subjected to heat treatment after sintering, when a raw material powder of a certain component system is used, the tendency of dimensional change during sintering due to a change in the amount of carbon and the tendency of dimensional change during heat treatment are reduced. We focused on reversing. That is, a low-alloy sintered material that generally undergoes heat treatment shrinks during sintering, and the shrinkage decreases as the amount of carbon decreases, and decreases as the amount of carbon increases. That is, the dimensional change of the sintered body has a positive gradient with an increase in the amount of carbon. Similarly, during the heat treatment, the dimensional change of the sintered member has a positive gradient as the amount of carbon increases. For this reason, the sintered member that has undergone the sintering step and the heat treatment step causes a large dimensional change in which the dimensional changes in both steps are accumulated. Therefore, if an alloy-based raw material iron powder capable of mutually canceling the tendency of the dimensional change due to the amount of carbon generated in the sintering step and the heat treatment step is used, the dimensional change of the obtained sintered member is reduced, and as a result, As a result, a sintered member having small dimensional variation can be obtained.
[0017]
The influence of the carbon content on the dimensional change occurring between each step of the raw material iron powder according to the prior art was investigated. As described above, usually, a fixed amount of carbon is added to the raw iron powder, but since carbon is consumed by oxygen in the raw iron powder, the carbon amount in the sintered member varies. Further, in an actual production line, there may be a variation due to non-uniformity due to mixing of lots. Therefore, by changing the amount of addition to the raw material iron powder of a certain lot and adding carbon to change the amount of carbon in the sintered member, the variation of the dimensional change with respect to the actual lot-to-lot variation of the raw material iron powder was investigated. .
[0018]
An example is shown in FIG. The raw iron powder is Fe-1.5% Mo powder obtained by a water atomizing method. In FIG. 5, the vertical axis shows the dimensional change as the dimensional change rate (%) between the steps, and the horizontal axis shows the carbon content (%) in the raw material powder. The blended carbon amount was set to three levels of 0.8, 1.0, and 1.2% by weight (hereinafter,% relating to the blending indicates% by weight). GD (◆) in the figure indicates the dimensional change rate of the powder compact with respect to the molding die. Similarly, SG (■) is a sintered body for the powder compact, SD (△) is a sintered body for the molding die, and HS (×) is a heat treatment after the heat treatment for the sintered body. HD (*) of the body (sintered member) indicates a dimensional change rate of the sintered member after the heat treatment to the mold. For example, when the carbon content is 0.8% (the carbon content in the sintered member is 0.7% by weight), the powder compact expands about 0.26% with respect to the mold. By sintering, the sintered body shrinks by 0.09% with respect to the powder compact, and by performing heat treatment, the heat-treated body (sintered member) expands by 0.04% from the sintered body. ing. Based on the molding die, it can be seen that the sintered member after the heat treatment expanded about 0.2% (dimension correction was not performed after sintering). In the case of this raw material iron powder, the dimensional change rate shows a positive gradient that increases in proportion to the increase in the amount of carbon in both the sintering step (■) and the heat treatment step (×). It can be seen that the sintering member obtained in a practical manner has a large dimensional variation due to the accumulated HD-D (*) of the dimensional change rate due to the amount of carbon between the sintering step and the heat treatment step.
[0019]
However, for example, here, for example, an alloy system having a negative gradient in which the dimensional change of the sintered body during sintering decreases with an increase in the amount of carbon is selected, and the dimensional change due to the increase in the amount of carbon during the heat treatment is selected. If the positive gradient can be canceled, the dimensional change of the sintered member after the heat treatment can be made extremely small regardless of the variation in the amount of carbon.
[0020]
Using iron-chromium alloy powder (Fe-3.0% Cr-0.3% Mo-0.3% V) as the raw material iron powder, the dimensional change between each process depending on the amount of carbon as in FIG. Behavior was investigated. The results are shown in FIG. The reference numerals are the same as in FIG. In the iron alloy powder shown here, the dimensional change rate of the sintered body with respect to the powder compact represented by ■ has a negative gradient in which the shrinkage of the sintered body increases with an increase in the amount of carbon. On the other hand, when the sintered body was heat-treated, the shrinkage amount of the heat-treated body decreased with an increase in the amount of carbon, as shown by x, and showed a positive slope rising to the right. Therefore, the dimensions of the sintered member obtained by heat treatment after sintering are different from those of the standard mold dimensions, as indicated by the mark *, because the dimensional change rates in both steps are offset. It can be seen that when the amount is between 0.8 and 1.2%, it is extremely small, such as -0.02 to + 0.04%. In other words, even if the carbon content of the sintered member varies depending on the lot of the raw iron powder, the dimensional change rate with respect to the mold size of the finally obtained sintered member is within 0.01 ± 0.03%, It can be seen that a sintered member having extremely high dimensional accuracy and small variation can be obtained.
(Method of manufacturing first sintered member)
The first method for producing a sintered member of the present invention includes a raw material powder preparing step of preparing a raw material powder by blending at least a raw iron powder and a carbon powder, and a forming step of forming a raw material powder to form a powder compact. And a sintering step of sintering the powder compact to form a sintered compact, and a heat treatment step of heat-treating the sintered compact, wherein the raw material powder is a powder compact in the sintering step. The correlation between the carbon content and the dimensional change of the sintered body and the correlation between the carbon content and the dimensional change of the sintered body in the heat treatment step is characterized in that one has a positive correlation and the other has a negative correlation. And
[0021]
Here, the method for producing a sintered member in which the raw iron powder is an iron alloy powder containing at least 2 to 4% by weight of Cr is defined as a first production method.
[0022]
As the raw material iron powder, an iron alloy powder containing at least 2 to 4% by weight of Cr obtained by a method such as a direct reduction method or an atomizing method can be used. Here, if the Cr content is less than 2% by weight, the tendency of the change in dimensional change due to the amount of carbon during sintering is not sufficient to offset the change in dimensional change in the heat treatment step. On the other hand, if it exceeds 4% by weight, the absolute value of the change in the dimensional change due to the amount of carbon during sintering becomes too large, which is not appropriate. Preferably, it is 2.4 to 3.6% by weight. The iron alloy powder may optionally contain alloy elements such as Mo and V other than Cr. The content of these elements is not problematic as long as it is contained in the raw material iron powder usually used as a low-alloy iron powder, but Mo is 0.2 to 0.6% by weight and V is 0.1 to 0.1%. 0.6% by weight is preferred.
[0023]
The particle size of the iron alloy powder is not particularly limited as long as it is used for the production of ordinary sintered members, but the iron alloy coarse powder is about 20 to 200 μm, and the fine powder of 45 μm or less (325 mesh) is 5 to 20 weight. % Is desirable. If the iron alloy coarse content is 200 μm or more, a high density of the sintered body cannot be obtained. Further, when the content of the fine powder is 20% by weight or more, the density becomes high, but the dimensional change after sintering is large, which is not suitable.
[0024]
As the compounded carbon, natural graphite or artificial graphite which is usually used can be used. The particle size of the graphite is not particularly limited, but is suitably several μm to several tens μm on average.
Strictly speaking, the amount of carbon varies depending on the amount of oxygen contained in the raw material iron powder. However, it is usually desirable that the amount be 0.5 to 1.5% by weight based on 100% by weight of the whole raw material powder. If the carbon content is less than 0.5%, it is difficult to obtain the desired mechanical properties of the sintered member, and if it exceeds 1.5%, the toughness is undesirably reduced. More preferably, it is 0.8 to 1.2%.
[0025]
Mixing of the iron alloy powder and the compounded carbon may be performed by a usual method. Further, at the time of mixing, a lubricant such as zinc stearate and amide wax may be added. Further, a segregation preventing treatment may be performed by a binder or the like. The lubricant may be applied to a mold without being mixed with the raw material powder.
[0026]
The raw material powder obtained above is filled in a molding die and compression molded. The molding conditions are not particularly limited. However, in the case of a structural member by uniaxial compression molding, it is preferable to perform the surface pressure at 400 to 800 MPa. If the surface pressure is 400 MPa or less, a sufficient density of the sintered body cannot be obtained, and if the surface pressure is 800 MPa or more, there is a possibility that facility restrictions may occur. However, this is not the case when the density needs to be further improved depending on the application.
[0027]
Sintering is preferably performed in a vacuum atmosphere, a protective atmosphere, or a non-carburizing reducing atmosphere. In the case of a carburizing atmosphere, the effect may be impaired because the carbon content of the sintered member fluctuates due to variations in the atmosphere. In the case of vacuum sintering, the degree of vacuum before heating is 10 -3 It is desirable to set it to torr or less. The sintering temperature is generally 1100 to 1300 ° C, and the holding time is preferably 10 to 60 minutes. When the sintering temperature is 1100 ° C. or lower, it is difficult to obtain strength because the growth of the neck between the sintered powders is slow. If the temperature is higher than 1300 ° C., the sintering equipment may be restricted. If the sintering holding time is less than 10 minutes, it is difficult to obtain strength due to insufficient growth of the neck, while if it is more than 60 minutes, the productivity is lowered, which is not appropriate.
[0028]
The rate of temperature rise during sintering is not particularly limited. However, depending on the composition of the raw material powder, the rate of temperature rise may greatly affect dimensional change, so it is important to stabilize the conditions. Further, the cooling condition is generally around 20 ° C./min, but may be freely selected according to the productivity and the necessity of controlling the structure.
[0029]
In order to suppress the variation in the sintering time, it is preferable to avoid the stacking using a continuous furnace such as a mesh belt rather than a batch type vacuum sintering furnace. This is because in the high temperature range where sintering is performed, radiant heat transfer is dominant, so when stacking is performed, the temperature rise to the internal work occurs, and as a result, depending on the work loading position This is because a difference occurs in the sintering time.
[0030]
After sintering, dimension correction can be performed if necessary. Dimension correction can be performed by a usual method, and there is no particular limitation.
[0031]
In addition, heat treatment can be performed after sintering to improve mechanical properties and the like. As a heat treatment method, general hardening treatments such as induction hardening and tempering, carburizing and quenching and tempering, and bright quenching and tempering can be performed, and the conditions are not particularly limited.
[0032]
Although quenching can be performed by a usual method, it is desirable to use a vacuum heat treatment furnace in terms of dimensional accuracy. Atmosphere furnaces such as RX gas are not preferable because dimensions vary due to fluctuations in carbon potential. The temperature of the heat treatment is preferably 800 to 900 ° C., and the soaking is preferably performed for 20 to 60 minutes. If the soaking temperature is 800 ° C. or less, incomplete quenching occurs, and if it is 900 ° C. or more, retained austenite increases. If the soaking time is less than 20 minutes, incomplete quenching occurs, and if it is more than 60 minutes, productivity may be impaired, which is not preferable. It is preferable to perform oil quenching at 70 to 200 ° C. after soaking. The quenching refrigerant is not limited to oil, but may be water or pressurized gas. After the quenching treatment, it is preferable to perform a tempering treatment at 150 to 200 ° C. or 350 ° C. or more for 20 to 120 minutes. If the processing temperature is 150 ° C. or lower, there is a problem in brittleness, and if it is 200 to 350 ° C., thermal brittleness occurs, which is not appropriate. If the treatment time is less than 20 minutes, the tempering is insufficient, and if the treatment time is more than 120 minutes, the productivity may be impaired, which is not preferable.
[0033]
Further, as the post-treatment, steam treatment for the purpose of sealing and rust prevention, shot peening for the purpose of imparting residual stress, and the like can be applied.
(Method of manufacturing second sintered member)
According to the second method for producing a sintered member of the present invention, in addition to the raw material powder obtained in the first production method, a mixed powder containing 0.7 to 1.3% by weight of copper powder based on 100% by weight of the whole raw material powder. This is a method of making powder.
[0034]
As raw material iron powder, use pure iron (purity: 99% or more) obtained by a method such as a direct reduction method or an atomizing method or a low alloy powder containing less than 3% by weight of alloying elements such as Ni, Mo, and Cr. Can be. Here, when the content of alloy elements such as Ni, Mo, and Cr is 3% by weight or more, the mixing effect of the copper powder is reduced, which is not preferable.
[0035]
Examples of the copper powder include an electrolytic copper powder and an atomized copper powder. The purity of the copper powder is preferably 99% or more, and preferably 0.7 to 1.3% by weight based on 100% by weight of the whole raw material powder. When the content of the copper powder is less than 0.7% by weight, the carbon dependence of dimensional change during sintering is not enough to be canceled by the carbon dependence during heat treatment, and when the content exceeds 1.3% by weight, sintering occurs. Is not appropriate because the absolute value of the dimensional change rate becomes too large.
[0036]
The particle size of the raw iron powder is not particularly limited as long as it is used for the production of ordinary sintered members, but the iron coarse powder is about 20 to 200 μm, and the fine powder of 45 μm or less (325 mesh) is 5 to 20% by weight. What is contained is desirable. If the iron content is 200 μm or more, a high density of the sintered body cannot be obtained. Further, when the content of the fine powder is 20% by weight or more, the density becomes high, but the dimensional change after sintering is large, which is not suitable.
[0037]
On the other hand, the copper powder preferably has a size of about 5 to 50 μm. If it is less than 5 μm, the cost is high because the powder is expensive, and if it exceeds 50 μm, segregation of copper is likely to occur, which is not preferable.
[0038]
As the compounded carbon, natural graphite or artificial graphite which is usually used can be used. The particle size of the graphite is not particularly limited, but is suitably several μm to several tens μm on average.
[0039]
The mixing of the raw iron powder, the copper powder and the compounded carbon may be performed by a usual method. Further, at the time of mixing, a lubricant such as zinc stearate and amide wax may be added. Further, a segregation preventing treatment may be performed by a binder or the like. The lubricant may be applied to a mold without being mixed with the raw material powder.
[0040]
This second sintered member manufacturing method is the same as the first manufacturing method except that the iron alloy powder in the first manufacturing method is a mixed powder of raw iron powder and copper powder. It can be carried out. That is, the adjustment of the raw material powder, the powder molding step, the sintering step, the heat treatment step, and the like may be performed in the same manner as in the first manufacturing method.
[0041]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples.
(Example 1)
As the raw material powder, the composition is Fe-3.0% Cr-0.3% Mo-0.3% V (manufactured by Kawasaki Iron & Steel), the coarse powder has a particle size of 45 to 250 µm, and the particle size is 45 µm (325 mesh) or less. An iron-chromium alloy powder containing 9.8 to 19.3% by weight of fine powder was prepared.
[0042]
0.8%, 1.0%, and 1.2% of natural graphite (manufactured by Nippon Graphite), each having an average particle size of about 5 μm, were mixed with the raw material iron alloy powder and mixed with a V-type mixer for 30 minutes to obtain a three-level sample. Was prepared. Ten samples of about 22 g were arbitrarily collected from each sample powder, filled into a cemented carbide mold (inner diameter: 25.2305 mm, thickness: 45 mm), and pressure-molded at a surface pressure of 900 MPa. At this time, a suspension of zinc stearate in ethyl alcohol was applied to the inner surface of the mold and air-dried to obtain a lubricant.
[0043]
The powder compact after pressing was 6 ± 0.1 mm thick.
[0044]
The diameter of the 30 (10 × 3) powder compacts thus obtained was measured at two points per one using a laser scan micrometer in a constant temperature room at 20 ± 0.5 ° C., and the average was measured. The value was determined.
[0045]
Next, the 30 powder compacts obtained above were baked at room temperature with a degree of vacuum of 10. -3 Torr or less (10 -4 In a vacuum sintering furnace (torr unit), sintering was performed at 1250 ± 1 ° C. for 30 ± 0.5 minutes. The rate of temperature rise was 20 ° C./min. After the sintering was maintained, cooling was performed at an average cooling rate of 30 ° C./min from 1250 ° C. to 300 ° C. by controlling a nitrogen gas fan. Then, it was left to cool in air to obtain 30 sintered bodies. The diameter of the obtained sintered body was measured by the same method as that for the powder compact, and the average value was obtained.
[0046]
The obtained sintered body was further subjected to a soaking treatment at 850 ° C. for 30 minutes in a vacuum quenching furnace, and was put into quenching oil at 80 ° C. to perform a quenching treatment. At this time, the rate of temperature rise from normal temperature to soaking temperature was 15 ° C./min. Subsequently, tempering treatment was performed at 180 ° C. for 60 minutes in an atmospheric furnace to obtain a heat-treated body (sintered member). The measurement was performed in the same manner as above to obtain the average value of the diameter of the heat-treated body. The carbon content of the sintered member (heat treated body) thus obtained was 0.7% for the sample with 0.8% carbon content and 0.9% for the sample with 1.0% carbon content. Yes, 1.1% for the 1.2% sample.
[0047]
The results are shown in FIG. As described above, when the iron alloy powder of the present example is used, the dimensional change rate of the sintered body with respect to the powder compact of the sintered body indicated by ■ is such that the shrinkage amount of the sintered body is increased with an increase in the amount of blended carbon. It has an increasing negative slope. However, when the sintered body was heat-treated, the shrinkage amount of the heat-treated body decreased with an increase in the amount of carbon as shown by x, and showed a positive slope rising to the right. Therefore, the dimensions of the sintered member obtained by heat treatment after sintering are offset by the dimensional change rate of both, and the dimensional change rate with respect to the reference mold dimension is indicated by the * Is between 0.8 and 1.2%, which is extremely small, such as -0.02 to + 0.04% (that is, 0.01 ± 0.03%).
(Test Example 1)
As raw material powder, electrolytic copper powder (CE1110, manufactured by Fukuda Metal Foil Powder Co., Ltd.) having an average particle size of 10 μm or less is added to pure iron (ASC100.29, Höganäs) having an average particle size of 150 μm or less. In the same manner as in Example 1, the dimensional change of the sample between the respective steps was determined for the sample containing the natural graphite (manufactured by Nippon Graphite).
[0048]
The addition amount of the electrolytic copper powder is set to three levels of 0, 1.0, and 2.0%, and natural graphite is added to each addition amount of the electrolytic copper powder by 0, 0.3, 0.6, 0.9, Five samples of 1.2% were prepared. That is, 15 types of samples having different amounts of electrolytic copper powder and different amounts of carbon were prepared. 0.8% of zinc stearate was further added to each of these samples as a lubricant, and each was mixed with a V-type mixer for 30 minutes to prepare each sample powder. Ten samples of about 22 g were arbitrarily collected from each of the sample powders, filled into a cemented carbide mold (inner diameter: 25.2305 mm, thickness: 45 mm), and pressed at a surface pressure of 588 MPa. .
[0049]
The powder compact after pressing was 6 ± 0.1 mm thick.
[0050]
The diameter of 150 (10 × 15) powder compacts obtained as described above was measured at two points per one using a laser scan micrometer in a constant temperature room at 20 ± 0.5 ° C. The average was determined.
[0051]
Next, the 150 powder compacts obtained above were sintered in a batch furnace in a 100% nitrogen atmosphere at 1120 ± 1 ° C. for 30 ± 0.5 minutes. The rate of temperature rise was 20 ° C./min. After the sintering was maintained, cooling was performed at an average cooling rate of 30 ° C./min from 1250 ° C. to 300 ° C. by controlling a nitrogen gas fan. Then, it was left to cool in the air to obtain 150 sintered bodies. The diameter of the obtained sintered body was measured in the same manner as in the powder compact.
[0052]
The obtained sintered body was further subjected to a soaking treatment at 850 ° C. for 30 minutes in a vacuum quenching furnace, and then put into a quenching oil at 80 ° C. for quenching. At this time, the rate of temperature rise from normal temperature to soaking temperature was 15 ° C./min. Subsequently, tempering treatment was performed at 180 ° C. for 30 minutes in an air furnace to obtain a heat-treated body (sintered member). The measurement was performed in the same manner as above to obtain the diameter of the heat-treated body (sintered member). The results are shown in FIGS. In addition, the carbon content of the sintered member (heat-treated body) obtained in this manner was determined for each of the samples having the blended carbon amounts of 0, 0.3, 0.6, 0.9, and 1.2%. 0.2, 0.5, 0.8 and 1.1%.
[0053]
FIG. 2 shows the dimensional change of the sintered body, that is, the relationship between the dimensional change rate of the sintered body with respect to the powder compact and the carbon content. In the figure, ◆ indicates the case where no copper powder was added to the raw material powder, ■ indicates the case where 1.0% of the copper powder was added with the total amount of the raw material powder being 100%, and △ indicates that 2. This is the case where 0% is added. When no copper powder is added (◆), the sintered body shrinks by 0.13% with respect to the powder compact at a carbon amount of 0, and the shrinkage decreases with an increase in the carbon amount. At 2%, the expansion is 0.06%, and it can be seen that the dimensional change rate has a positive slope as the carbon content increases. However, when 1.0% of copper powder is added, (■) even when the carbon content is 0%, the dimensional change rate tends to be 0.11% and expands with respect to the powder compact. The dimensional change rate also increased, and thereafter, the dimensional change rate decreased, and when the carbon content was 1.2%, the value was 0.06%, which was almost the same value as when copper powder was not added. When 2.0% of copper powder is added (△), even if the carbon content is 0%, the dimensional change rate is 0.31%, which tends to greatly expand with respect to the powder compact. , The dimensional change rate also increased in the expanding direction, and when the carbon content was 0.6%, the dimensional change rate reached the maximum of 0.37%. After that, the dimensional change rate decreased, and when the carbon content was 1.2%, it was 0.06%, which was almost the same value as in the case where copper was not added. This phenomenon is considered to be because a phenomenon called copper expansion, which occurs during sintering of the Fe—Cu—C-based molding powder, is suppressed by increasing carbon.
[0054]
On the other hand, the dimensional change rate in the heat treatment step, that is, the dimensional change rate of the sintered member (heat treated body) after the heat treatment with respect to the sintered body, is independent of the amount of the copper powder, as shown in FIG. It can be seen that it changes with a substantially constant positive slope with increasing. Therefore, the dimensional change rate of the sintered member with respect to the powder compact is the sum of FIGS. 2 and 3 and is as shown in FIG. From FIG. 4, when 1.0% of copper powder is added to pure iron powder, the dimensional change rate of the sintered member (heat-treated body) with respect to the variation of 0.8 to 1.2% of the blended carbon amount is as follows. It can be seen that it is extremely small as 0.11 to 0.17% (0.14 ± 0.03%).
[0055]
That is, if the raw material powder containing the copper powder is used, even if the amount of carbon in the sintered member fluctuates, it has high dimensional accuracy without greatly changing the conditions and control level of the conventional process, and It is possible to obtain a sintered member in which the lot-to-lot variation in the dimensions depending on the powder lot is extremely small.
(Comparative Example 1)
Fe-1.5% Mo (made by Höganäs) as a raw material powder, iron coarse powder having a particle size of 45 to 250 μm, and a raw iron powder containing 5 to 30% by weight of iron fine powder having a particle size of 45 μm (325 mesh) or less Was prepared.
[0056]
Thereafter, in the same manner as in Example 1, 30 sintered members (heat-treated bodies) were obtained. As in Example 1, the rate of change of the diameter dimension between the steps was measured and plotted to obtain FIG.
When the blended carbon content is in the range of 0.8 to 1.2%, the dimensional change rate of the heat-treated body with respect to the mold size greatly varies from 0.21 to 0.37% (0.29 ± 0.08%). I understand.
[0057]
【The invention's effect】
In the method for manufacturing a sintered member according to the present invention, a raw material powder whose behavior of dimensional change due to the amount of carbon is reversed in a sintering step and a heat treatment step is used. For this reason, the dimensions of the sintered member obtained by passing through the sintering step and the heat treatment step are offset by the difference in the behavior of the dimensional change in both steps, and even if there is a variation in the amount of carbon, The dimensional variation of the members is suppressed to an extremely small range. Therefore, according to the method for manufacturing a sintered member of the present invention, high dimensional accuracy is achieved without significantly changing the normal manufacturing process and control level, and variation between lots of the dimensions of the raw material powder is reduced. A sintered member can be obtained stably.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the dimensional change rate and the carbon content between steps of an iron alloy powder (Fe-3% Cr-0.3% Mo-0.3% V).
FIG. 2 is a graph showing a relationship between a dimensional change rate of a sintered body with respect to a powder compact and an amount of carbon for each addition amount of copper powder.
FIG. 3 is a diagram showing a relationship between a dimensional change rate of a heat-treated body (sintered member) with respect to a sintered body and a carbon amount for each addition amount of copper powder.
FIG. 4 is a diagram showing a relationship between a dimensional change rate of a heat-treated body (sintered member) and a carbon content with respect to a powder compact from FIG. 2 and FIG. 3 for each addition amount of copper powder.
FIG. 5 is a diagram showing a relationship between a dimensional change rate and a carbon amount in each production process of a raw material iron powder (Fe-1.5% Mo) according to a conventional technique.
[Explanation of symbols]
GD (◆): dimensional change rate (%) of the powder compact with respect to the molding die
SG (■): dimensional change rate (%) of the sintered body with respect to the powder compact
SD (△): dimensional change rate (%) of the sintered body with respect to the molding die
HS (x): dimensional change rate (%) of the heat-treated body after heat-treating the sintered body
HD (*): dimensional change rate (%) of the heat-treated body (sintered member) after heat-treating the mold
Claims (4)
該原料粉末を成形して粉末成形体を形成する成形工程と、
該粉末成形体を焼結させて焼結体とする焼結工程と、
該焼結体を熱処理する熱処理工程と、
を有する焼結部材の製造方法において、
該粉末成形体の該焼結工程における寸法変化と炭素量との相関と、該焼結体の該熱処理工程における寸法変化と炭素量との相関とが、一方は正の相関を呈し、他方は負の相関を呈する特性を有することを特徴とする焼結部材の製造方法。A raw material powder preparation step of preparing a raw material powder by blending at least the raw material iron powder and the carbon powder,
A molding step of molding the raw material powder to form a powder compact,
Sintering the powder compact to form a sintered body,
A heat treatment step of heat treating the sintered body;
In the method for producing a sintered member having
The correlation between the dimensional change and the amount of carbon in the sintering step of the powder compact and the correlation between the dimensional change and the amount of carbon in the heat treatment step of the sintered body show one positive correlation, and the other A method for producing a sintered member, having a characteristic exhibiting a negative correlation.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4825200B2 (en) * | 2004-06-14 | 2011-11-30 | ホガナス アクチボラゲット | Powder metallurgy parts and manufacturing method thereof |
| JP2015004099A (en) * | 2013-06-20 | 2015-01-08 | 住友電工焼結合金株式会社 | METHOD FOR PRODUCING Fe-Cu-C BASED SINTERING MATERIAL |
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2002
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4825200B2 (en) * | 2004-06-14 | 2011-11-30 | ホガナス アクチボラゲット | Powder metallurgy parts and manufacturing method thereof |
| JP2015004099A (en) * | 2013-06-20 | 2015-01-08 | 住友電工焼結合金株式会社 | METHOD FOR PRODUCING Fe-Cu-C BASED SINTERING MATERIAL |
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