JP2004131822A

JP2004131822A - Superfine grained steel, and its production method

Info

Publication number: JP2004131822A
Application number: JP2002298910A
Authority: JP
Inventors: Shoichi Tomita; 富田　正一; Shinichi Takagi; 高木　眞一; Susumu Takahashi; 高橋　進; Terukazu Kanda; 神田　輝一
Original assignee: Kanagawa Prefecture; Kanto Yakin Kogyo Co Ltd
Current assignee: Kanagawa Prefecture; Kanto Yakin Kogyo Co Ltd
Priority date: 2002-10-11
Filing date: 2002-10-11
Publication date: 2004-04-30

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively mass-produce superfine grained steel by using steel powder with a superfine grained structure produced by a mechanical milling method without coarsening the crystal grain size thereof. <P>SOLUTION: A cold compact of iron based alloy powder with a superfine grained structure of a mean crystal grain size of <1 μm applied by mechanical milling treatment is inserted into a sealed or half-sealed forging die and is subjected to powder forging in a warm or hot temperature region where the growth of the grains thereof can be suppressed, so that the dense, superfine-grained steel is obtained. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は平均結晶粒径が２μｍ未満の超細粒綱と、その製造方法にかかるものである。
【０００２】
【従来の技術】
最近、鉄鋼材料の飛躍的な強靱化を実現する技術として、結晶粒径が１μｍあるいはそれ以下にまで微細化した、いわゆる超細粒鋼の開発が活発に行われている。超細粒鋼を得る手段の一つであるメカニカルミリング法は、平均結晶粒径が十数ｎｍにも達する極めて微細な多結晶組織を有する鋼粉末を容易に作製できる、優れた結晶粒微細化法である。このようにして得られた鋼粉末を高温で焼結すれば、超細粒鋼を大量生産することが可能になるが、このような極めて微細な組織は、高温では容易に結晶粒径が粗大化してしまうため、これを如何に抑制しつつ固化成形するかが重要な技術課題となる。
粒成長を抑制しつつ固化成形するために、
▲１▼鋼の化学組成を最適化する方法と、
▲２▼固化成形法に工夫を凝らす方法、
の両面から検討がなされている。
【０００３】
▲１▼としてはメカニカルミリング処理およびその後の熱処理によって、図１に示すように極微細な酸化物を超微細組織中に分散させる方法がある。これは、微細な酸化物が結晶粒界に生成することによって粒成長を抑制する、いわゆる粒界ピン止め効果を活用しようとするものである。例えば特開２０００−１７４０５号公報には、鋼粉末にあらかじめ適量のＳｉＯ_２、ＭｎＯ、ＴｉＯ_２、Ｃｒ_２Ｏ_３、Ａｌ_２Ｏ_３、ＣａＯ、ＴａＯ、Ｙ_２Ｏ_３などの酸化物を混合した原料粉にメカニカルミリング処理を施すことによって、これらの酸化物をいったん合金化した後、加熱時に均一微細に再析出させる手法が報告されている。
【０００４】
▲２▼については、図２に示すように粒成長が起こりにくい比較的低温域で加圧焼結する手法が試みられている。例えば、特開２０００−１７３６には超細粒鋼を安価に大量生産する方法として、メカニカルミリング処理を施した鋼粉を空間を設けた鋼片の中に充填し、内部を真空ポンプ等で長時間吸引したうえで溶接して密閉し、熱間圧延を施すことによって大型バルク材を製造する方法が開示されている。この他にも、メカニカルミリング処理を施した粉末を鉄製パイプ中に充填し、内部を真空状態にして密閉したのち、粒成長が顕著に起こらない比較的低温領域で熱間静水圧プレス（ＨＩＰ）処理を施して超細粒鋼を作製する方法や同じく比較的低温域でホットプレス装置を用いて加圧焼結する方法などが実施されている。
【０００５】
これまでに、▲１▼と▲２▼を組み合わせた方法によって超細粒鋼が作製されている。しかしながら、これらの方法のうち、特開２０００−１７３６に示される方法は鋼片中への粉末の充填作業、あるいは圧延後の鋼片の除去作業に長時間を要すると考えられ、量産性に優れる方法とは考えにくい。また、その他の方法もＨＩＰ処理やホットプレス処理そのものに長時間を要するため、製造コストが高くなり超細粒鋼を大量生産する方法として現実的ではない。
【０００６】
【発明が解決しようとする課題】
本発明は、メカニカルミリング処理を施した粉末に少量のバインダーを添加して室温にて加圧成形したのち、温間あるいは熱間の温度域で粉末鍛造することによって、超細粒鋼を安価にかつ大量に生産する技術を提案するものである。
【０００７】
【課題を解決するための手段】
本発明者らは、上記の課題を解決するために、メカニカルミリング処理を施した粉末に有機物から成るバインダーを少量混合して室温で加圧成形した成形体を所定の温度で加熱したのち、直ちに密閉、あるいは半密閉状態の型に挿入して粉末鍛造することによって、超細粒鋼である機械部品あるいはバルク材を安価にかつ大量に製造することが可能であることを見出した。
【０００８】
【発明の実施の形態】
遊星ボールミルや振動ボールミルなどによってメカニカルミリング処理を施した純鉄粉末は、一般的に粉末粒子径が数μｍから数十μｍ程度であって内部の平均結晶粒径がナノメートルオーダーの極めて微細な多結晶組織を呈している。この粉末を高温に加熱して焼結しようとすると、高温で顕著に粒成長を起こしてしまう。
【０００９】
そこで、本発明においては、所定のチタン（Ｔｉ）および酸素（Ｏ）を含有することによって加熱中にＴｉ酸化物を析出させ、その粒界ピン止め効果によって粒成長を抑制する手段を用いる。添加するＴｉとＯの添加量が少なすぎると析出するＴｉ酸化物の量が不足し十分な粒成長抑制効果を発揮しない。逆に添加量が多すぎると析出するＴｉ酸化物の量が過剰となり得られる鋼材を脆化させる。したがって、ＴｉとＯの添加量は粉末鍛造のプロセス中に粒成長を起こさない最小限の量であることが望ましい。したがって、Ｔｉの含有量は０．５ｍａｓｓ％〜３．０ｍａｓｓ％の範囲に、また、Ｏは０．２ｍａｓｓ％〜０．８ｍａｓｓ％の範囲とする。
【００１０】
本発明ではチタン酸化物はあらかじめ鉄原料粉末に混合するのではなく、チタンと酸素は別々に添加する点が特徴である。すなわち、チタンは純チタン粉末あるいはフェロチタン粉末を混合するか、さらにもし可能であればあらかじめ所定のチタンを合金化した鉄合金粉末を用いる。また、酸素は、主として鉄原料粉末に不純物として含まれる酸素を用い、もしもこれによって所定の酸素量に達しない場合には、さらに酸化鉄を適量添加するか、あるいはメカニカルミリング中の雰囲気中の酸素を利用する手法を用いる。
【００１１】
メカニカルミリング処理によってチタンと酸素が均一に合金化した過飽和のフェライト固溶体が形成される。その後の加熱中に、この過飽和固溶体から化学的に安定なチタン酸化物が析出する。析出するチタン酸化物の結晶構造はチタンと酸素の添加量に依存するだけでなく、フェライト母相と析出物との整合性に起因する界面エネルギーにも依存すると考えられる。本発明においては加熱中に母相のフェライト相と整合性の高いＮａＣｌ構造を持つ数十ナノメートル程度の極微細なＴｉＯが析出し、これが強力な粒成長抑制効果を発揮している。このように、別々に添加したチタンと酸素がいったん固溶した後、ＴｉＯとして再析出することがフェライト母相の粒成長抑制に対して重要な役割を担っており、ＴｉＯ_２など他の結晶構造のチタン酸化物をあらかじめ添加して再析出させる方法では、実現し得ない本発明の特徴である。
【００１２】
次に、本発明の合金にＣｒ、Ｖ、Ｓｉ、Ｍｏ、Ｎｉ、Ｃｕ、Ｎｂ、Ｔａ、Ｗを添加する効果について説明する。これらの元素は、本発明合金の特徴を維持しつつ、さらに特性を向上させるために添加するものである。これらの元素を添加する一つの目的は、固溶強化によって母相をより高強度化するためである。これに加えて、Ｃｒ、Ｖ、Ｓｉ、Ｍｏ、Ｗは固溶により母相であるフェライト相をより高温まで安定化させる効果がある。メカニカルミリング処理により得られた超微細粒組織はα→γ変態点以上に加熱すると急速に粒成長を起こすため、後述する粉末鍛造工程はα→γ変態点以下で実施するのが望ましい。これらの元素の効果により、鍛造温度をより高温にすることが可能となり、粉末粒子同士の結合を促進することができる。また、Ｃｒ、Ｖ、Ｍｏ、Ｎｂ、Ｔａ、Ｗは、鉄母相の再結晶温度を上昇させる効果があり、超微細粒組織をより高温まで安定化させる効果がある。さらに、Ｎｂ、Ｖなどの炭化物形成能の高い元素は、鉄母相中の微量の炭素と結合して析出強化に寄与する効果もある。Ｃｕは析出強化による高強度化に加えて、耐食性の向上にも寄与する。
以上のように、これらの合金元素は、本発明合金の特徴であるチタン酸化物を微細に析出した超細粒鋼の特性をさらに向上させるために、鉄母相に固溶して効果を発揮するものである。しかし、これらの元素を過剰に添加すると、靭性の劣化を招き、本発明合金の特徴である超微細粒組織による強靭な特性を維持できない。従って、このような効果のための各元素の添加量の上下限は、それぞれ請求項４中に示される通りである。
【００１３】
次に、本発明においてメカニカルミリング処理を施した粉末を粉末鍛造法によって固化成形する手順を図３に従って説明する。
まず、粉末鍛造に先立って、メカニカルミリング処理を施した粉末を室温で加圧成形する。メカニカルミリング処理した粉末は強度が極めて高い（ＨＶ６００〜ＨＶ９００程度）ため、従来の粉末冶金法のように成形時に粉末を塑性変形させて成形体を作製することができない。そのため、成形時の加圧力は１００ＭＰａ程度の低圧力とし、バインダーの結合力のみによって成形する必要がある。これには少量でも結合力の高いバインダー、例えばメチルセルロース水溶液などが望ましい。
【００１４】
次に、加圧成形した成形体を所定の温度で加熱する。加熱中の粉末粒子表面の酸化を抑制するために加熱炉内には不活性ガスあるいは還元性ガスを流す構造にして大気と遮断する。加熱温度は粒成長を抑制するためには低温である方が望ましい。しかし、あまり低温では粉末粒子同士の結合が十分に起こらないので緻密な焼結体を得ることができない。また、加熱温度がα→γ変態点を上回ると急速な粒成長が起こる。従って、加熱温度はα→γ変態点以下であることが望ましい。これらのことを考慮すると、加熱温度は６００℃〜９００℃の範囲が望ましい。加熱時間は成形体が均一になるために充分な時間保持すればよい。
【００１５】
次いで、加熱した成形体を加熱炉から取り出し、密閉または半密閉状の型に挿入して粉末鍛造を施す。加熱された成形体は粉末同士の結合が弱いので、粉末鍛造中に引張応力が発生すると簡単に崩壊してしまう。従って、引張応力を発生させず高い静水圧によって成形体を完全に緻密化するためには、密閉あるいは半密閉状の鍛造型を用いる必要がある。また、加熱炉から取り出した成形体を型に挿入すると成形体の温度が急激に低下してしまうため、型をおおむね１５０℃以上に予熱しておく方がよい。
このようにして得られたバルク材は、必要に応じて粉末鍛造後に熱処理を施しても良い。あるいは粉末鍛造後にさらに温間あるいは熱間の温度域で鍛造を施しても良い。熱処理や鍛造を施すことによって、粉末粒子同士の結合がより強固なものになり強度および伸びを改善する効果がある。ただし、これらの処理は結晶粒径が２μｍ以上に粗大化しない条件の範囲内で行わなければならない。
なお、これら一連の粉末鍛造工程は、本発明範囲の合金のみならず、メカニカルミリング処理によって超微細組織を付与された鉄基合金粉末であって、粉末鍛造中に結晶粒径の顕著な粗大化が抑制できるように工夫された合金であれば、いずれの合金に対しても適用可能である。
【００１６】
上記の方法により、超細粒鋼を安価にかつ大量に生産することが可能となる。図４に本発明による超細粒鋼の生産ラインの一例を示す。メカニカルミリング処理した鋼粉末は室温にて成形された後、ベルトコンベアーなどを有する連続加熱炉内に搬送される。加熱炉内を搬送され温間あるは熱間の温度域に保持された成形体は、図中の▲１▼供給装置によって鍛造型の内部に自動的に供給され直ちに鍛造させる。鍛造型は、常に１５０〜２００℃程度に予熱され、鍛造の度に離型剤が塗布される構造になっている。鍛造を終えた製品は図中の▲２▼搬出装置によって型内部から取り出され冷却帯に搬送される。必要に応じて▲２▼の搬出装置から取り出された製品は冷却帯の前方に配置された熱処理炉内を搬送されて所定の熱処理を施された後、冷却帯へと搬送される。
さらに、必要に応じて▲２▼の搬出装置から取り出された製品は二次的な鍛造を施される場合もある。
このように、製造工程は自動化された連続製造ラインとなっており、超細粒鋼を大量生産するのに適している。なお、これらの工程は不活性ガスあるいは還元雰囲気ガスに置換され大気と遮断されている。
【００１７】
実施例
まず、化学組成がＦｅ−１．０ｍａｓｓ％Ｔｉ−０．４ｍａｓｓ％Ｏになるように、カーボニル鉄粉、Ｆｅ_２Ｏ_３粉末、フェロチタン（Ｆｅ−４２．１ｍａｓｓ％Ｔｉ）粉末を配合した原料粉末を、高速遊星ボールミルを用いてメカニカルミリング処理した。メカニカルミリング処理は、容積２００ｍｌのステンレス鋼（ＳＵＳ３０４）製ミルポット内に、直径が約１０ｍｍの軸受鋼（ＳＵＪ２）製ボールと原料粉末を重量比で１０：１になるように充填し、ミルポット内部をアルゴンガスで置換して実施した。メカニカルミリング処理時間は１００時間である。
【００１８】
次に、メカニカルミリング処理した粉末をふるいにかけて、粉末粒子径が９０μｍ以下の粉末のみを採取した。採取したメカニカルミリング処理粉末約５０ｇに１．５ｍａｓｓ％メチルセルロース水溶液を６ｍａｓｓ％混合したものを、断面が１０ｍｍ×５４．５ｍｍの冷間成形用の型に充填して加圧力１００ＭＰａで成形し、直方体形状の成形体を得た。成形体はメチルセルロース水溶液に含有される水分を蒸発させるために６０℃で約３０分間乾燥した。
続いて、乾燥した冷間成形体を表１に示した加熱温度に保持され、アルゴン雰囲気で大気から遮断された加熱炉内に挿入して３０分間加熱した。
【００１９】
加熱された成形体を加熱炉から大気中に取り出した後、直ちに、１５０℃〜２００℃程度に予熱された、断面が１１ｍｍ×５６ｍｍの鍛造型内に挿入して粉末鍛造した。成形体が大気にさらされる時間、すなわち、炉からの取り出しから鍛造終了までの所要時間は約２秒程度である。この鍛造型は、図５に示すようにポンチとダイの間に１ｍｍの隙間が設けられており、鍛造時に内圧が高まるとこの隙間を通して材料が押し出されるように設計された半密閉状の鍛造型である。
さらに、粉末鍛造によって得られたバルク材のうちの一部は、アルゴン雰囲気中で８００℃〜８５０℃の温度範囲でさらに熱処理を加えた。鍛造後の熱処理条件は表１に示す通りである。
なお、得られたバルク材の結晶粒径はミクロ組織を走査型電子顕微鏡により観察して評価した。
【００２０】
表１に本発明範囲内の実施例（鋼種Ａ〜Ｅ）の室温における引張特性を示す。表１から明らかなように本発明範囲の鋼種を本発明の粉末鍛造法で固化成形した場合には、固化成形後の平均結晶粒径が２μｍ未満であり１０００ＭＰａ以上の優れた強度を発揮している。
【００２１】
なお、ここでは、加熱された成形体をいったん大気中に取り出した後、直ちに粉末鍛造を施しているが、加熱炉から粉末鍛造を経て、さらに必要に応じて熱処理を施すまでの工程をアルゴンガス雰囲気中で行うことができれば、成形体の酸化を抑制することが可能となり、より優れた特性のバルク材を効率よく作製することができると予想される。すなわち、図４に示した製造方法を用いて、本実施例と同等あるいはそれ以上の特性のバルク材を大量生産できることは明らかである。
【００２２】
比較例
本発明を比較するために、化学組成がＦｅ−１．０ｍａｓｓ％Ｔｉ−０．１５ｍａｓｓ％ＯおよびＦｅ−０．２ｍａｓｓ％Ｏである合金のバルク材を上記した実施例と全く同様の手順で作製した。これらの鋼種の引張特性を表１の鋼種ＦおよびＧに示すが、フェライト母相の粒成長抑制効果を発揮するＴｉ酸化物の析出量が不足しているために結晶粒径が粗大であり強度も本発明例と比較して著しく低くなっている。
【００２３】
【表１】

【００２４】
【発明の効果】
このように、本発明によれば、超細粒鋼を、鋼製容器への封入した上での熱間圧延あるいはＨＩＰ処理、ホットプレス処理などの長時間を要するプロセスを経ることなく安価に効率よく大量生産できる。
また、Ｃｒ、Ｖ、Ｓｉ、Ｍｏ、Ｎｉ、Ｃｕ、Ｎｂ、Ｔａ、Ｗの１種あるいは２種以上を添加した場合にも同様の効果が得られた。
【図面の簡単な説明】
【図１】従来技術による、メカニカルミリング処理による超微細組織の生成と析出物による粒成長の抑制とを示す流れ図である。
【図２】従来技術による、メカニカルミリング処理した粉末の固化成形のための数方法を示す流れ図である。
【図３】本発明による超細粒鋼の製造工程を示す流れ図である。
【図４】本発明により超細粒鋼を量産するために好適な一工程を示す説明図である。
【図５】発明を実施するために好適な半密閉鍛造型を示す説明的な断面図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an ultrafine grain having an average crystal grain size of less than 2 μm and a method for producing the same.
[0002]
[Prior art]
In recent years, as a technique for realizing a remarkable toughness of a steel material, the development of a so-called ultrafine-grained steel in which the crystal grain size is reduced to 1 μm or less has been actively performed. The mechanical milling method, which is one of the means for obtaining ultrafine-grained steel, is an excellent grain refinement that can easily produce steel powder having an extremely fine polycrystalline structure with an average crystal grain size as large as ten and several nm. Is the law. If the steel powder thus obtained is sintered at a high temperature, ultrafine-grained steel can be mass-produced. However, such an extremely fine structure easily has a large crystal grain size at a high temperature. Therefore, it is an important technical problem how to suppress the solidification and perform solidification molding.
In order to perform solidification molding while suppressing grain growth,
(1) a method of optimizing the chemical composition of steel;
(2) A method of devising the solidification molding method,
Considerations are being made from both sides.
[0003]
As a method (1), there is a method in which an ultrafine oxide is dispersed in an ultrafine structure as shown in FIG. 1 by a mechanical milling treatment and a subsequent heat treatment. This is intended to utilize a so-called grain boundary pinning effect that suppresses grain growth by generating fine oxides at crystal grain boundaries. For example, Japanese Patent Application Laid-Open No. 2000-17405 discloses that an appropriate amount of oxide such as SiO ₂ , MnO, TiO ₂ , Cr ₂ O ₃ , Al ₂ O ₃ , CaO, TaO, and Y ₂ O ₃ is mixed in advance with steel powder. A method has been reported in which these oxides are alloyed once by subjecting the raw material powder to mechanical milling, and then uniformly and finely reprecipitated upon heating.
[0004]
As for (2), as shown in FIG. 2, a method of performing pressure sintering in a relatively low temperature range in which grain growth does not easily occur has been attempted. For example, Japanese Patent Application Laid-Open No. 2000-1736 discloses a method of mass-producing ultrafine-grained steel at low cost, in which steel powder subjected to mechanical milling is filled into a steel slab having a space, and the inside is lengthened by a vacuum pump or the like. A method of manufacturing a large bulk material by sucking for a time, sealing by welding, and performing hot rolling is disclosed. In addition, the powder subjected to the mechanical milling treatment is filled in an iron pipe, and the inside thereof is evacuated and sealed. Then, the hot isostatic pressing (HIP) is performed in a relatively low temperature region in which grain growth does not significantly occur. A method of producing ultra-fine-grained steel by performing a treatment and a method of pressure sintering using a hot press device in a relatively low temperature range have also been implemented.
[0005]
Heretofore, ultrafine-grained steel has been produced by a method combining (1) and (2). However, among these methods, the method disclosed in Japanese Patent Application Laid-Open No. 2000-1736 is considered to require a long time for the work of filling powder into the steel slab or the work of removing the steel slab after rolling, and is excellent in mass productivity. It is hard to imagine a method. In addition, other methods require a long time for the HIP treatment and the hot press treatment itself, so that the production cost is increased and it is not practical as a method for mass-producing ultrafine-grained steel.
[0006]
[Problems to be solved by the invention]
The present invention provides a low-priced ultrafine-grained steel by adding a small amount of binder to a powder subjected to mechanical milling and pressing at room temperature, followed by powder forging in a warm or hot temperature range. It also proposes a technology for mass production.
[0007]
[Means for Solving the Problems]
The present inventors, in order to solve the above problems, a small amount of a binder made of an organic substance mixed with the powder subjected to mechanical milling, and then heated at room temperature and molded at a predetermined temperature, and then immediately heated, By inserting into a closed or semi-closed mold and performing powder forging, it has been found that ultrafine-grained mechanical parts or bulk materials can be produced inexpensively and in large quantities.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Pure iron powder that has been subjected to mechanical milling using a planetary ball mill, vibrating ball mill, or the like generally has a fine particle size of several μm to several tens μm and an average internal crystal grain size of nanometer order. It has a crystalline structure. If this powder is heated to a high temperature and sintered, the grain growth will be remarkable at a high temperature.
[0009]
Therefore, in the present invention, a means is used in which a predetermined amount of titanium (Ti) and oxygen (O) is contained to precipitate a Ti oxide during heating, and the grain growth is suppressed by the grain boundary pinning effect. If the added amounts of Ti and O are too small, the amount of precipitated Ti oxide will be insufficient, and a sufficient effect of suppressing grain growth will not be exhibited. Conversely, if the addition amount is too large, the amount of the precipitated Ti oxide becomes excessive and the obtained steel material is embrittled. Therefore, it is desirable that the addition amounts of Ti and O be the minimum amounts that do not cause grain growth during the powder forging process. Therefore, the content of Ti is set in the range of 0.5 mass% to 3.0 mass%, and O is set in the range of 0.2 mass% to 0.8 mass%.
[0010]
The present invention is characterized in that titanium oxide is not added to the iron raw material powder in advance, but titanium and oxygen are added separately. That is, as titanium, pure titanium powder or ferro-titanium powder is mixed, or, if possible, an iron alloy powder obtained by alloying predetermined titanium in advance. The oxygen is mainly oxygen contained as an impurity in the iron raw material powder. If the oxygen does not reach the predetermined oxygen amount, an appropriate amount of iron oxide is further added, or oxygen in the atmosphere during the mechanical milling is used. Is used.
[0011]
A supersaturated ferrite solid solution in which titanium and oxygen are uniformly alloyed by the mechanical milling process is formed. During the subsequent heating, chemically stable titanium oxide precipitates from this supersaturated solid solution. It is considered that the crystal structure of the precipitated titanium oxide depends not only on the added amount of titanium and oxygen, but also on the interface energy due to the consistency between the ferrite matrix and the precipitate. In the present invention, during heating, ultra-fine TiO of about several tens of nanometers having a NaCl structure having a high consistency with the ferrite phase of the mother phase is precipitated, and this has a strong grain growth suppressing effect. As described above, once separately added titanium and oxygen form a solid solution, re-precipitation as TiO plays an important role in suppressing grain growth of the ferrite matrix, and other crystal structures such as TiO ₂ This is a feature of the present invention that cannot be realized by a method of re-precipitating by adding titanium oxide in advance.
[0012]
Next, the effect of adding Cr, V, Si, Mo, Ni, Cu, Nb, Ta, and W to the alloy of the present invention will be described. These elements are added to maintain the characteristics of the alloy of the present invention and further improve the characteristics. One purpose of adding these elements is to increase the strength of the matrix by solid solution strengthening. In addition, Cr, V, Si, Mo, and W have the effect of stabilizing the ferrite phase, which is the parent phase, to a higher temperature by solid solution. Since the ultrafine grain structure obtained by the mechanical milling process rapidly undergoes grain growth when heated to a temperature above the α → γ transformation point, the powder forging step described below is desirably carried out at a temperature below the α → γ transformation point. Due to the effects of these elements, the forging temperature can be made higher, and the bonding between the powder particles can be promoted. Further, Cr, V, Mo, Nb, Ta, and W have an effect of increasing the recrystallization temperature of the iron matrix and have an effect of stabilizing the ultrafine grain structure to a higher temperature. Further, elements having a high carbide forming ability, such as Nb and V, have an effect of contributing to precipitation strengthening by binding to a small amount of carbon in the iron matrix. Cu contributes to improvement of corrosion resistance in addition to strengthening by precipitation strengthening.
As described above, in order to further improve the properties of ultrafine-grained steel in which titanium oxide, which is a feature of the alloy of the present invention, has been finely precipitated, these alloy elements exhibit an effect by forming a solid solution in the iron matrix. Is what you do. However, when these elements are added excessively, the toughness is deteriorated, and the tough properties due to the ultrafine grain structure characteristic of the alloy of the present invention cannot be maintained. Therefore, the upper and lower limits of the added amount of each element for such an effect are as shown in claim 4.
[0013]
Next, a procedure of solidifying and forming the powder subjected to the mechanical milling process by the powder forging method in the present invention will be described with reference to FIG.
First, prior to powder forging, powder subjected to mechanical milling is subjected to pressure molding at room temperature. Since the mechanically milled powder has extremely high strength (about HV600 to HV900), it is not possible to produce a compact by plastically deforming the powder at the time of compacting as in the conventional powder metallurgy method. Therefore, it is necessary to set the pressing force at the time of molding to a low pressure of about 100 MPa and to perform molding only by the binding force of the binder. For this, a binder having a high bonding strength even in a small amount, for example, an aqueous solution of methylcellulose is desirable.
[0014]
Next, the compact formed by pressure molding is heated at a predetermined temperature. In order to suppress the oxidation of the surface of the powder particles during heating, the heating furnace is made to have a structure in which an inert gas or a reducing gas flows so as to shut off the atmosphere. The heating temperature is desirably low to suppress grain growth. However, if the temperature is too low, the bonding between the powder particles does not occur sufficiently, so that a dense sintered body cannot be obtained. When the heating temperature exceeds the α → γ transformation point, rapid grain growth occurs. Therefore, it is desirable that the heating temperature be equal to or lower than the α → γ transformation point. Considering these facts, the heating temperature is desirably in the range of 600C to 900C. The heating time may be maintained for a time sufficient for the molded body to be uniform.
[0015]
Next, the heated compact is taken out of the heating furnace, inserted into a closed or semi-closed mold, and subjected to powder forging. Since the heated compact has a weak bond between the powders, it easily collapses when a tensile stress is generated during powder forging. Therefore, in order to completely densify the compact by high hydrostatic pressure without generating tensile stress, it is necessary to use a closed or semi-closed forging die. In addition, when the molded body taken out of the heating furnace is inserted into the mold, the temperature of the molded body rapidly decreases. Therefore, it is better to preheat the mold to about 150 ° C. or more.
The bulk material thus obtained may be subjected to heat treatment after powder forging, if necessary. Alternatively, forging may be performed in a warm or hot temperature range after powder forging. By performing the heat treatment or the forging, the bonding between the powder particles becomes stronger and the strength and the elongation are improved. However, these treatments must be performed within a range that does not increase the crystal grain size to 2 μm or more.
These series of powder forging processes include not only alloys within the scope of the present invention but also iron-based alloy powders to which an ultra-fine structure has been imparted by mechanical milling, and the crystal grain size during powder forging has been significantly increased. It is applicable to any alloy as long as it is an alloy that is devised so as to be able to suppress the above.
[0016]
According to the above method, ultrafine-grained steel can be mass-produced inexpensively. FIG. 4 shows an example of a production line for ultrafine-grained steel according to the present invention. The steel powder subjected to the mechanical milling is formed at room temperature, and then conveyed into a continuous heating furnace having a belt conveyor and the like. The molded body conveyed in the heating furnace and kept in a warm or hot temperature range is automatically supplied to the inside of the forging die by the supply device (1) in the figure and immediately forged. The forging die is always preheated to about 150 to 200 ° C., and has a structure in which a release agent is applied every time forging is performed. The product after forging is taken out from the inside of the mold by the unloading device (2) in the figure and transported to the cooling zone. If necessary, the product taken out of the unloading device (2) is transported in a heat treatment furnace arranged in front of the cooling zone, subjected to a predetermined heat treatment, and then transported to the cooling zone.
Further, the product taken out from the unloading device of (2) may be subjected to secondary forging as required.
Thus, the manufacturing process is an automated continuous manufacturing line, which is suitable for mass-producing ultrafine-grained steel. Note that these steps are replaced with an inert gas or a reducing atmosphere gas and are isolated from the atmosphere.
[0017]
Example First, carbonyl iron powder, Fe ₂ O ₃ powder, and ferro-titanium (Fe-42.1 mass% Ti) powder were blended so that the chemical composition became Fe-1.0 mass% Ti-0.4 mass% O. The raw material powder was subjected to mechanical milling using a high-speed planetary ball mill. In the mechanical milling process, a 200 ml stainless steel (SUS304) mill pot is filled with a bearing steel (SUJ2) ball having a diameter of about 10 mm and raw material powder in a weight ratio of 10: 1, and the inside of the mill pot is filled. The operation was performed by replacing with argon gas. The mechanical milling processing time is 100 hours.
[0018]
Next, the powder subjected to the mechanical milling treatment was sieved to collect only powder having a powder particle size of 90 μm or less. A mixture of 1.5 mass% aqueous methylcellulose solution and 6 mass% mixed with about 50 g of the collected mechanical milling powder was filled into a cold-forming mold having a cross section of 10 mm × 54.5 mm, and molded at a pressure of 100 MPa to form a rectangular parallelepiped. Was obtained. The molded body was dried at 60 ° C. for about 30 minutes to evaporate water contained in the aqueous solution of methyl cellulose.
Subsequently, the dried cold compact was maintained at the heating temperature shown in Table 1, inserted into a heating furnace shielded from the atmosphere in an argon atmosphere, and heated for 30 minutes.
[0019]
After the heated compact was taken out of the heating furnace into the atmosphere, it was immediately inserted into a forging die having a cross section of 11 mm × 56 mm, which was preheated to about 150 ° C. to 200 ° C., and powder forged. The time during which the molded body is exposed to the atmosphere, that is, the time required from removal from the furnace to completion of forging is about 2 seconds. In this forging die, a 1 mm gap is provided between the punch and the die as shown in FIG. 5, and a semi-hermetic forging die designed so that the material is extruded through this gap when the internal pressure increases during forging. It is.
Further, a part of the bulk material obtained by powder forging was further subjected to a heat treatment in a temperature range of 800 ° C. to 850 ° C. in an argon atmosphere. The heat treatment conditions after forging are as shown in Table 1.
The crystal grain size of the obtained bulk material was evaluated by observing the microstructure with a scanning electron microscope.
[0020]
Table 1 shows the tensile properties at room temperature of the examples (steel types A to E) within the scope of the present invention. As is clear from Table 1, when the steel type in the range of the present invention was solidified and formed by the powder forging method of the present invention, the average crystal grain size after the solidification was less than 2 μm and exhibited excellent strength of 1000 MPa or more. I have.
[0021]
In this case, powder forging is performed immediately after the heated compact is once taken out into the atmosphere.However, the process of performing powder forging from a heating furnace and further performing heat treatment as necessary is performed using argon gas. If it can be carried out in an atmosphere, it is possible to suppress the oxidation of the molded body, and it is expected that a bulk material having more excellent properties can be efficiently produced. That is, it is apparent that a bulk material having characteristics equal to or better than that of the present embodiment can be mass-produced by using the manufacturing method shown in FIG.
[0022]
Comparative Example In order to compare the present invention, a bulk material of an alloy having a chemical composition of Fe-1.0 mass% Ti-0.15 mass% O and Fe-0.2 mass% O was prepared in exactly the same procedure as in the above-described embodiment. It was produced in. The tensile properties of these steel types are shown in steel types F and G in Table 1. However, since the precipitation amount of Ti oxide which exerts the effect of suppressing the grain growth of the ferrite matrix is insufficient, the crystal grain size is coarse and the strength is low. Is also significantly lower than that of the present invention.
[0023]
[Table 1]

[0024]
【The invention's effect】
As described above, according to the present invention, the ultrafine-grained steel can be efficiently inexpensively manufactured without being subjected to a time-consuming process such as hot rolling, HIP processing, or hot pressing after sealing in a steel container. Can be mass-produced well.
Similar effects were obtained when one or more of Cr, V, Si, Mo, Ni, Cu, Nb, Ta, and W were added.
[Brief description of the drawings]
FIG. 1 is a flowchart showing generation of an ultrafine structure by mechanical milling and suppression of grain growth by precipitates according to a conventional technique.
FIG. 2 is a flow chart illustrating several methods for solidification of mechanically milled powders according to the prior art.
FIG. 3 is a flowchart showing a manufacturing process of ultrafine-grained steel according to the present invention.
FIG. 4 is an explanatory view showing one process suitable for mass-producing ultrafine-grained steel according to the present invention.
FIG. 5 is an explanatory sectional view showing a semi-hermetic forging die suitable for carrying out the invention.

Claims

Ti: 0.5 to 3.0 mass%, O: 0.2 to 0.8 mass%, the balance being a chemical composition composed of iron and unavoidable impurities, and having an average crystal grain size of 1 μm by mechanical milling. Ultrafine-grained steel having an average crystal grain size of less than 2 μm obtained by solidifying and molding an iron-based alloy powder having an ultrafine structure of less than or equal to less than 2 μm.

2. The steel material according to claim 1, wherein ferro-titanium or alloy steel powder containing a predetermined amount of Ti in advance is used as a source of Ti, and oxygen or iron oxide powder in the raw material powder or mechanical is used as a source of oxygen. An ultrafine-grained steel characterized by using oxygen in a milling treatment atmosphere.

3. The ultrafine-grained steel according to claim 1, wherein ultrafine TiO is precipitated in an iron matrix during heating of the mechanically milled powder by heating.

Cr: 0.01 to 30.0 mass%, V: 0.01 to 15.0 mass%, Si: 0.01 to 5.0 mass%, Mo: 0.01 to 5.0 mass%, Ni: 0.01 to 15.0 mass%, Cu: 0.01 to 5.0 mass%, Nb: 0.01 to 2.0 mass%, Ta: 0.01 to 2.0 mass%, W: 0.01 to 2.0 mass% The ultrafine-grained steel according to any one of claims 1, 2 and 3, wherein the steel comprises one or more kinds.

A small amount of a binder is added to an iron-based alloy powder provided with an ultrafine structure having an average crystal grain size of less than 1 μm by mechanical milling, and the compact is pressed at room temperature and heated to a warm or hot temperature range. A method for producing ultrafine-grained steel which is immediately inserted into a closed or semi-closed forging die and subjected to powder forging to be a bulk material having an ultrafine structure with an average crystal grain size of less than 2 μm.

The method for producing ultrafine-grained steel according to claim 5, wherein heat treatment or forging is performed subsequent to powder forging.

The method for producing ultrafine-grained steel according to claim 5 or 6, further comprising a conveying device, such as a belt conveyor, for pressing the molded body at room temperature from the atmosphere by an inert gas or a reducing atmosphere. Heated to a warm or hot temperature range inside the continuous heating furnace which was cut off, and then heated to a powder forging device which was placed behind this continuous heating furnace and was cut off from the atmosphere by an inert gas or a reducing atmosphere. A method for mass-producing ultrafine-grained steel, characterized in that a compacted body is automatically supplied and powder forging is performed immediately.

The method for mass-producing ultrafine-grained steel according to claim 5, claim 6, or claim 7, wherein after the powder forging, the powder forging is disposed behind the powder forging apparatus, and is removed from the atmosphere by an inert gas or a reducing atmosphere. A method for mass-producing ultrafine-grained steel, wherein heat treatment or further forging is performed using a cut-off device.

The method for producing ultrafine-grained steel according to any one of claims 1 to 4, wherein the procedure according to claims 5 to 8 is used as a solidification molding method.