JP3841455B2 - Method for producing high density sintered stainless steel - Google Patents
Method for producing high density sintered stainless steel Download PDFInfo
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- JP3841455B2 JP3841455B2 JP29211694A JP29211694A JP3841455B2 JP 3841455 B2 JP3841455 B2 JP 3841455B2 JP 29211694 A JP29211694 A JP 29211694A JP 29211694 A JP29211694 A JP 29211694A JP 3841455 B2 JP3841455 B2 JP 3841455B2
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- 229910001220 stainless steel Inorganic materials 0.000 title claims description 47
- 239000010935 stainless steel Substances 0.000 title claims description 47
- 238000004519 manufacturing process Methods 0.000 title claims description 12
- 238000005245 sintering Methods 0.000 claims description 56
- 230000004907 flux Effects 0.000 claims description 47
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- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 claims description 3
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- ZLMJMSJWJFRBEC-UHFFFAOYSA-N Potassium Chemical compound [K] ZLMJMSJWJFRBEC-UHFFFAOYSA-N 0.000 claims description 3
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims description 3
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- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 3
- 229910052788 barium Inorganic materials 0.000 claims description 3
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- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 claims description 3
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- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 claims description 3
- 229910052700 potassium Inorganic materials 0.000 claims description 3
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- 239000010936 titanium Substances 0.000 claims description 3
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- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 2
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Description
【０００１】
【産業上の利用分野】
本発明は、ステンレス鋼粉末およびそれを用いる焼結ステンレス鋼の製造方法に係るものである。
【０００２】
【従来の技術】
焼結ステンレス鋼は、素材としてのステンレス鋼の特性である耐食性、強磁性、非磁性等の性質を有するとともに、容易にニアネットシェープのものが得られるので、水道用メカニカルシール、各種流体用ポンプの部品、カメラマウント、時計バンド、自動販売機のコイン投入口、電磁ソレノイドポンプおよび燃料噴射ポンプ等の部品、カセットテープレコーダのヘッド、ハードディスク装置のアーム等の広範な分野で使用されている。
一般にステンレス鋼は、Ｃrの含有量が多く、脱酸材としてＳiおよびＭnをある程度含有しているので、ステンレス鋼粉末の表面は厚み１５ｎｍ程度のＳiＯ2、ＭnＯ、Ｃr2Ｏ3およびＦe2Ｏ3などの酸化被膜で覆われている。これらのうち、
Ｃr2Ｏ3およびＦe2Ｏ3の酸化被膜は、真空雰囲気あるいは還元ガス雰囲気中で焼結することにより除去することが可能である。しかし、ＳiＯ2およびＭnＯの酸化被膜は除去が困難であり、粉末表面の酸化被膜は粉末粒子が相互に拡散接合を行う上で障害となり、焼結の進行を阻害する。従って、通常の焼結工程によって高密度の焼結ステンレス鋼を得ることは困難であり、熱間鍛造もしくはＨＩＰ（熱間静水圧圧縮）処理等を行い緻密化させる必要があった。
【０００３】
【発明が解決しようとする課題】
一方、通常の焼結工程で緻密化させる方法として、例えば、金属射出成形で行なわれているようにきわめて微細な粉末を原料にして大きな表面エネルギーを利用するか、あるいはＳi、Ｆe−ＰまたはＦe−Ｂのような、低融点で共晶液相を生成する成分を添加する液相焼結による方法が開発された。しかし、微細な粉末の製造は製造効率が低くかつ高価である。また、ＳiおよびＦｅ−Ｐは多量に添加しなければ効果がなく、かつ多量に添加するとステンレス鋼の素地および特性に影響を与える点で問題があった。また、Ｆe−ＰやＦe−Ｂの粉末を添加する場合には、それらが硬いため、ステンレス鋼粉と混合した混合粉末の圧縮性を低下させたり、成形時の金型を損傷するほか、Ｆe−ＰあるいはＦe−Ｂの粉末が溶け出した後の空隙が焼結後に残存するなどの点で検討の余地があった。
【０００４】
【課題を解決するための手段】
本発明は、上記課題を解決するためになされたものである。すなわち、真空雰囲気で焼結する場合においては、ステンレス鋼粉末に、０ . ８〜４重量％のオルト硼酸、メタ硼酸および次硼酸などの硼酸類および硼酸塩から選ばれる少なくとも１種の化合物からなるフラックスを混合した混合粉末からなる圧粉体か、あるいは上記フラックスによりステンレス鋼粉末を被覆した粉末からなる圧粉体を、また、不活性ガス、窒素ガス、水素ガス、アンモニア分解ガスあるいはそれらの混合ガスなどの非酸化性ガスの雰囲気で焼結する場合においては、上記フラックスを１ . ０〜２ . ０重量％とした圧粉体を焼結することを特徴とする高密度焼結ステンレス鋼の製造方法を提供するものである。上記硼酸塩としては、オルト硼酸塩、二硼酸塩、メタ硼酸塩、四硼酸塩、五硼酸塩および八硼酸塩などがあり、それらの塩としては、リチウム、ナトリウム、カリウム、マグネシウム、カルシウム、バリウム、クロム、鉄、コバルト、ニッケル、マンガン、珪素、鉛、銅、チタンおよびアルミニウムなどの金属の硼酸塩類が例示され、また、その他の硼化物なども使用することができる。本発明はこれらの例示に限定されるものではない。
また、本発明は、上記ステンレス鋼粉末の圧粉体を、１１００〜１４００℃の温度で焼結することを特徴とする高密度焼結ステンレス鋼の製造方法を提供するものである。
【０００５】
【作用】
硼酸類および硼酸塩類から選ばれる少なくとも１種の化合物からなるフラックスとステンレス鋼粉末とを混合し、または上記フラックスによりステンレス鋼粉末を被覆して圧粉成形した後に焼結すると、フラックスは焼結温度より低い融点を有するために、焼結の際の昇温過程で溶融し、混合粉末の場合にはステンレス鋼粉末相互の隙間に毛細管現象により入り込んだ後に、また、被覆粉末の場合には直接に、溶融したフラックスがステンレス鋼粉末の表面を覆う（段階１）。
さらに、その後の温度上昇により、ステンレス鋼粉末の表面に存在するＳｉＯ２とＭｎＯの酸化被膜がフラックス中に溶解する（段階２）。これ（段階１〜２）によりステンレス鋼粉末の表面が活性になると共に、フラックスに覆われているので新たな酸化が防止される。
続いて、ステンレス鋼中に固溶しているＣｒ、ＳｉおよびＭｎも粒子表面に拡散移動して、フラックスが分解して生成されたＢ２Ｏ３と次のような反応を起こす（段階３〜４）。例えば、硼酸（オルト硼酸）の場合： ２Ｈ３ＢＯ３＝３Ｈ２Ｏ＋Ｂ２Ｏ３ 硼砂の場合： Ｎａ２Ｂ４Ｏ７＝Ｎａ２Ｏ＋２Ｂ２Ｏ３の分解反応によりＢ２Ｏ３を生成し（段階３）、Ｂ２Ｏ３はさらに以下のように反応する（段階４）。
３Ｓｉ＋８Ｃｒ＋２Ｂ２Ｏ３＝３ＳｉＯ２＋４Ｃｒ２Ｂ
３Ｍｎ＋４Ｃｒ＋Ｂ２Ｏ３＝３ＭｎＯ＋２Ｃｒ２Ｂ
【０００６】
この反応（段階４）の自由エネルギーの変化により、ステンレス鋼粉中のＣｒ、ＭｎおよびＳｉは、粒子表面、すなわち鋼とフラックスの界面まで速やかに拡散するための駆動力を獲得する。通常の焼結における拡散の駆動力は、表面曲率の差による自由エネルギーの差であるが、化学反応が起こる場合は、界面におけるＣｒ、ＭｎおよびＳｉの化学ポテンシャルが上記反応式で決まるように非常に低くなり、内部から界面への拡散の駆動力が大きくなる。この化学拡散（段階４）が焼結の緻密化に寄与する。
また、上記の段階４の生成物のＣｒ２Ｂは、ステンレス鋼粉の粒子間で鋼粉粒子の架け橋となって焼結ネックを形成する（段階５）ことにより、速やかに焼結が進む起点となる。さらに上記の段階４の生成物のＣｒ２Ｂとステンレス鋼素地のＦｅなどが反応し、Ｆｅ−Ｃｒ−Ｂを主とする低融点の液相が発生し、毛細管現象により緻密化が進行する（段階６）と共に、気孔が球状化する。一方、フラックスは焼結後ＳｉＯ２を主として、ＭｎＯおよびＮａ２Ｏを含むガラス成分に近い複合化合物、いわゆるスラグとなり気孔中に残留（段階７）する。
すなわち、フラックスの添加により、低温における酸化被膜の除去（段階１〜２）、反応拡散（段階３〜４）によるネックの急速形成（段階５）、およびＦｅ−Ｃｒ−Ｂ系液相発生による毛細管現象による緻密化（段階６）などのステップを経て、ステンレス鋼粉末の高密度焼結が可能となる。
フラックス添加によるＦｅ−Ｃｒ−Ｂ系共晶液相の発生は、粉末の全表面で均等に起こり、他方Ｆｅ−Ｂ粉添加による共晶液相は、Ｆｅ−Ｂ粉末粒子だけで起こるために、Ｆｅ−Ｂ粒子が溶融して毛細管現象により他の小さな隙間に移動した後に大きな気孔として残り、消滅させることが困難である。そのため、フラックス添加の方がＦｅ−Ｂ粉添加よりも有利である。
本発明で用いるフラックスとしては、前記のように各種の硼酸類および硼酸塩類を使用することができ、かつ、前記のような作用機序から、それらの融点は焼結温度よりも低いことが必要である。
【０００７】
フラックスの量が０.２５重量％よりも少ないと、上記酸化被膜の除去、反応拡散によるネックの急速形成（段階５）およびＦｅ−Ｃｒ−Ｂ系液相発生による毛細管現象による緻密化（段階６）の効果が十分でなく、また４重量％を越えて添加すると、圧粉体の密度低下の原因となるほか、ガス雰囲気の場合には、Ｆｅ−Ｃｒ−Ｂ系液相発生により生じる閉鎖気孔中にガスが閉じこめられた状態となり焼結後まで残留し、緻密化を阻害する。これらの理由から、フラックスの量は０.２５〜４重量％が適当である。
【０００８】
上記フラックスは、単味で添加しても昇温過程で溶融し、毛細管現象によりステンレス鋼粉末の表面を覆うようになるが、あらかじめステンレス鋼粉末の表面をフラックスで被覆する方がより効果的である。
【０００９】
また、焼結の雰囲気としては、真空雰囲気、またはＡr、Ｎe、Ｈe等の不活性ガス、Ｈ2ガス、Ｎ2ガス、アンモニア分解ガスあるいはそれらの混合ガス等の非酸化性ガスの雰囲気中であれば、上記反応が生じて緻密化を図ることができる。なお、Ｎ2ガス雰囲気およびアンモニア分解ガス雰囲気中で焼結を行うと、ステンレス鋼が窒化し、耐食性あるいは磁気特性に影響を与えるため、真空雰囲気、不活性ガス雰囲気、Ｈ2ガス雰囲気中で行うことが望ましい。さらに、真空雰囲気中では、Ｆe−Ｃr−Ｂ液相が発生して閉鎖気孔が生じても、閉鎖気孔内部にガスが封じ込められていないので、焼結の進行に従って閉鎖気孔が消滅し、珪酸塩のスラグが残留するのみとなるのでさらに好適である。
【００１０】
上記の段階４のＣｒ２Ｂの生成は８００℃付近から始まり、溶融したフラックスがステンレス鋼粉末の表面の酸化被膜を除去し、低い温度でも焼結の進行は可能になるが、γ相の低い温度領域では元素の拡散が遅いため緻密化があまり進行せず、１１００℃付近でＦｅ−Ｃｒ−Ｂを主とする金属液相が発生することにより緻密化が急激に進むようになる。このような理由から焼結温度は１１００℃以上であることが好ましい。一方、焼結温度が１４００℃を越えると、真空雰囲気の場合、ステンレス鋼の主成分であるＦｅやＣｒの蒸発が著しくなるため、却って緻密化が阻害される。従って、焼結温度は１１００〜１４００℃の範囲が好ましい。
【００１１】
なお、緻密化があまり要求されない用途の場合でも、フラックスを添加することにより急速に緻密化するので、従来に比べて焼結時間を短縮することができ、コスト低減の効果が得られる。
さらに、閉鎖気孔中に残留するスラグはガラス質であるから耐食性に悪影響はなく、適正なフラックスを用いれば、残留したスラグが快削成分となり、被削性向上の効果も期待できる。
【００１２】
【実施例】
以下に本発明の実施例を示す。
ステンレス鋼粉末としてＳＵＳ３１６Ｌ粉末を、また、フラックスとして硼酸（オルト硼酸：Ｈ3ＢＯ3）および硼砂（Ｎa2Ｂ4Ｏ7・１０Ｈ2Ｏ）を準備し、表１および表２に示す割合で配合した。その混合粉末を、ステアリン酸亜鉛で型壁潤滑した成形金型に充填し、６００ＭＰaの成形圧力で圧粉成形を行った。次に、これらの圧粉体を５００℃×３０minの条件で脱ろうした後、表１および表２に示す焼結条件で焼結を行った。
得られた焼結体について、焼結体の密度を測定し、ＳＵＳ３１６Ｌ溶製材の密度（真密度）に対する密度比を求めた。その結果を表１および表２に示す。
また、密度比と、フラックス添加量、焼結温度、焼結時間との関係を、それぞれ図１、図２および図３に示す。
【００１３】
【表１】
図１はフラックス添加量と焼結雰囲気が密度比に及ぼす影響を示すグラフであり、フラックスとして硼砂または硼酸を添加することにより、密度比が向上しており、フラックス添加による緻密化の効果が確認された。
また、フラックス添加量が増加するにつれて密度比は向上するが、真空雰囲気の場合には、密度比が一定値まで急激に向上した後に向上が抑制されるのに対し、アルゴンガス雰囲気の場合には、硼砂の添加量が１重量％のときをピークとして密度比は徐々に低下し、４重量％添加では、フラックスを添加しない場合と同程度まで低下する。これらの現象は以下のように考えられる。
すなわち、液相発生により閉鎖気孔が生じるが、真空雰囲気の場合には、閉じこめられるガスが存在しないので、閉鎖気孔の消滅が進行し、焼結体の気孔中には低密度のスラグのみが残留する。したがってフラックス添加量が多くなると残留するスラグの量も多くなるため、密度比の上昇が抑えられる。一方、アルゴンガス雰囲気の場合は、閉鎖気孔中に閉じこめられたガスが閉鎖気孔の消滅を阻害するため、緻密化が進行しないことに加えて、フラックス添加量の増加につれて残留する低密度のスラグの量が多くなるため密度比が却って減少することになる。
以上の結果から、フラックス添加の効果を得るためのフラックス添加量は、焼結雰囲気に関係なく、０.２５〜４重量％が好ましいことが確認された。
また、上記範囲内であれば、アルゴンガス雰囲気でもフラックス添加の効果が得られ、真空雰囲気を用いれば、さらに好適であることが確認された。
【００１５】
図２は焼結温度が密度比に及ぼす影響を示すグラフであり、硼砂２重量％添加と硼酸１重量％添加のいずれの場合も、焼結温度が１０００℃以下では密度比はあまり向上しないが、１１００℃付近を越えると密度比が向上し、１２００℃以上では急激に向上する傾向を示す。また密度比は１３５０〜１３７０℃付近で最大となり、それ以上の温度では密度比がやや減少している。
硼酸の融点は１８４℃であるが、硼酸は３００℃以上で水を失って酸化硼素（Ｂ2Ｏ3）となる。酸化硼素の融点は４５０℃である。また硼砂の融点は７４５℃である。従って、焼結温度が８００〜１０００℃の状態では硼砂および硼酸の何れも溶融しステンレス鋼粉末の表面を覆って、ステンレス鋼粉末表面の酸化被膜を除去し、フラックスとステンレス鋼粉が反応してＣr2Ｂが析出した状態となっている。そのため焼結の進行は可能であるが、温度が低く、元素の拡散が遅いため、焼結は速やかに進行せず、密度比があまり向上しないと考えられる。
一方、１１００℃付近を越えると、金属の液相の発生が始まるため、毛細管現象による緻密化が生じ、密度比は急激に上昇して、１３５０〜１３７０℃程度で最大となる。しかし、それ以上の温度では、真空中でステンレス鋼の主成分であるＦeやＣrの蒸発が著しくなるために、却って緻密化が妨害され、密度比がやや減少するものと考えられる。
【００１６】
さらに、これらの試料について、金属組織の観察およびＥＰＭＡ分析を行ったところ、８００℃の試料でＣrとＢの化合物を確認することができた。Ｃr2Ｂは１２００℃までは析出量が増加するが、それ以上では析出相が急激に減少し、金属の液相が発生している。この液相にはＣr、Ｆe、Ｂの他にＭo、Ｎiも検出されるため、Ｆe−Ｃr−Ｂを主とする共晶液相であると考えられる。また、フラックス中のＢは液相発生後に減少して消滅し、代わりにＳiとＭnの酸化物が増加している。これは、ステンレス鋼粉末の表面の酸化膜がフラックスに溶け、さらにフラックスが鋼中に固溶しているＳi、Ｍnと反応するためであると考えられる。
【００１７】
以上のことから、フラックスの添加による低温における酸化被膜の除去、反応拡散によるネックの急速形成、そしてＦe−Ｃr−Ｂ系液相発生による毛細管現象による緻密化のステップで進行するフラックス添加の効果が確認され、それと共に焼結温度は１１００〜１４００℃が好ましいことが確認された。
【００１８】
図３は焼結時間が密度比に及ぼす影響を示すグラフである。
焼結温度が１３５０℃の場合、焼結温度までの昇温過程で既に液相が発生することにより、かなりの緻密化が生じていることが判る。また焼結温度が１２５０℃の場合において、２０分の焼結でも６０分の焼結と比べて得られる密度比に大差がないことから、焼結による緻密化に対する要求が高くない場合にも、フラックスを添加することにより、従来の方法に比べて焼結時間を短縮する効果が得られることが判る。
【００１９】
【発明の効果】
以上のように、ステンレス鋼粉末に０.２５〜４重量％のフラックスを添加して混合した粉末を圧粉成形した成形体を、１１００〜１４００℃の温度で焼結する本発明の製造方法により、高密度焼結ステンレス鋼を製造することが可能となった。また、本発明により、従来よりも焼結時間を短縮することが可能となった。
フラックスとしては、前記のようにオルト硼酸、メタ硼酸および次硼酸などの硼酸類およびリチウム、ナトリウム、カリウム、マグネシウム、カルシウム、バリウム、クロム、鉄、コバルト、ニッケル、マンガン、珪素、鉛、銅、チタンおよびアルミニウムなどの金属の硼酸塩を使用することができ、また、焼結は各種の非酸化性ガス雰囲気中で行なうことができるが、真空雰囲気中で焼結することが好ましい。
従って、本発明は、高価な薬剤を使用したり、煩雑な処理を行なうことなく、短い時間で容易に実施することができるので、産業上有益である。
【図面の簡単な説明】
【図１】フラックス添加量および焼結雰囲気が密度比に及ぼす影響を示すグラフである。
【図２】焼結温度が密度比に及ぼす影響を示すグラフである。
【図３】焼結時間が密度比に及ぼす影響を示すグラフである。[0001]
[Industrial application fields]
The present invention relates to a stainless steel powder and a method for producing sintered stainless steel using the same.
[0002]
[Prior art]
Sintered stainless steel has properties such as corrosion resistance, ferromagnetism, and non-magnetism, which are the characteristics of stainless steel as a raw material, and can be easily obtained with a near net shape, so mechanical seals for waterworks and pumps for various fluids Parts, camera mounts, watch bands, coin insertion slots of vending machines, electromagnetic solenoid pumps, fuel injection pumps, and other parts, cassette tape recorder heads, hard disk drive arms, etc.
In general, stainless steel has a high Cr content and contains Si and Mn to some extent as deoxidizers. Therefore, the surface of the stainless steel powder has a thickness of about 15 nm of SiO 2 , MnO, Cr 2 O 3 and Fe 2 O. Covered with oxide film such as 3 . Of these,
The oxide film of Cr 2 O 3 and Fe 2 O 3 can be removed by sintering in a vacuum atmosphere or a reducing gas atmosphere. However, the SiO 2 and MnO oxide films are difficult to remove, and the oxide film on the surface of the powder becomes an obstacle to the diffusion bonding of the powder particles and inhibits the progress of sintering. Therefore, it is difficult to obtain high-density sintered stainless steel by a normal sintering process, and it has been necessary to perform densification by hot forging or HIP (hot isostatic pressing).
[0003]
[Problems to be solved by the invention]
On the other hand, as a method of densifying in a normal sintering process, for example, a very fine powder is used as a raw material, as in the case of metal injection molding, or a large surface energy is used, or Si, Fe-P or Fe A method by liquid phase sintering has been developed that adds a component that produces a eutectic liquid phase with a low melting point, such as -B. However, the production of fine powder is low in production efficiency and expensive. Further, Si and Fe—P are not effective unless added in large amounts, and there is a problem in that adding them in a large amount affects the base and properties of stainless steel. In addition, when Fe-P or Fe-B powders are added, they are hard, so that the compressibility of the mixed powder mixed with the stainless steel powder is reduced, the mold at the time of molding is damaged, and Fe There was room for study in that the voids after the -P or Fe-B powder was dissolved remained after sintering.
[0004]
[Means for Solving the Problems]
The present invention has been made to solve the above problems. That is, in the case of sintering in a vacuum atmosphere, stainless steel powder, consisting of from 0.8 to 4% by weight of orthoboric acid, at least one compound selected from boric acids and borates, such as meta-boric acid and following borate A green compact made of mixed powder mixed with flux, or a powder made of powder coated with stainless steel powder by the above flux , inert gas, nitrogen gas, hydrogen gas , ammonia decomposition gas, or a mixture thereof in the case of sintering in an atmosphere of non-oxidizing gas, such as gas, the flux from 1.0 to 2.0 wt% and the high-density sintered stainless steel which is characterized by sintering a powder compact A manufacturing method is provided. Examples of the borate include orthoborate, diborate, metaborate, tetraborate, pentaborate, and octaborate. These salts include lithium, sodium, potassium, magnesium, calcium, and barium. Examples include borates of metals such as chromium, iron, cobalt, nickel, manganese, silicon, lead, copper, titanium, and aluminum, and other borides can also be used. The present invention is not limited to these examples.
Further, the present invention is to provide a method of manufacturing a high density sintered stainless steel, characterized in that the green compact of the stainless steel powder is sintered at a temperature of 11 00 to 1,400 ° C..
[0005]
[Action]
When a flux composed of at least one compound selected from boric acids and borates is mixed with stainless steel powder, or the stainless steel powder is coated with the above flux and sintered after compacting, the flux becomes the sintering temperature. Because it has a lower melting point, it melts during the heating process during sintering, and in the case of mixed powder, after entering into the gap between stainless steel powders by capillarity, and directly in the case of coated powder The molten flux covers the surface of the stainless steel powder (step 1) .
Further, the subsequent temperature rise causes the SiO 2 and MnO oxide films present on the surface of the stainless steel powder to dissolve in the flux (step 2) . This (steps 1 and 2) activates the surface of the stainless steel powder and prevents new oxidation because it is covered with the flux .
Subsequently, Cr, Si and Mn dissolved in the stainless steel also diffuse and move to the particle surface to cause the following reaction with B 2 O 3 produced by decomposition of the flux (steps 3 to 3). 4) For example, in the case of boric acid (orthoboric acid): 2H 3 BO 3 = 3H 2 O + B 2 O 3 when the borax: the decomposition reaction of Na 2 B 4 O 7 = Na 2 O + 2B 2 O 3 produces a B 2 O 3 ( Step 3) , B 2 O 3 further reacts as follows (Step 4) .
3Si + 8Cr + 2B 2 O 3 = 3SiO 2 + 4Cr 2 B
3Mn + 4Cr + B 2 O 3 = 3MnO + 2Cr 2 B
[0006]
Due to the change in the free energy of this reaction (stage 4) , Cr, Mn and Si in the stainless steel powder acquire a driving force for quickly diffusing to the particle surface, that is, the interface between the steel and the flux. The driving force of diffusion in normal sintering is the difference in free energy due to the difference in surface curvature. However, when a chemical reaction occurs, the chemical potential of Cr, Mn and Si at the interface is determined by the above reaction formula. And the driving force for diffusion from the inside to the interface increases. This chemical diffusion (stage 4) contributes to densification of the sintering .
In addition, the Cr 2 B product of the above stage 4 is a starting point where the sintering proceeds quickly by forming a sintering neck as a bridge of the steel powder particles between the stainless steel powder particles (stage 5). It becomes. Further, the product Cr 2 B in the above step 4 reacts with Fe of the stainless steel substrate, and a low melting point liquid phase mainly composed of Fe—Cr—B is generated, and densification proceeds by capillary action ( With step 6) , the pores become spherical. Meanwhile, the flux mainly Shoyuigo SiO 2, complex compound close to the glass component including MnO and Na 2 O, residual (step 7) in the pores become a so-called slag.
That is, removal of the oxide film at low temperature (steps 1 and 2) , rapid formation of a neck by reaction diffusion (steps 3 and 4) (step 5) , and capillary due to Fe—Cr—B liquid phase generation by addition of flux Through steps such as densification by the phenomenon (stage 6) , high-density sintering of the stainless steel powder becomes possible.
The generation of the Fe—Cr—B eutectic liquid phase by the addition of the flux occurs uniformly on the entire surface of the powder, while the eutectic liquid phase by the addition of the Fe—B powder occurs only by the Fe—B powder particles. After the Fe-B particles melt and move to other small gaps by capillary action, they remain as large pores and are difficult to extinguish. Therefore, the addition of flux is more advantageous than the addition of Fe-B powder.
As the flux used in the present invention, various boric acids and borates can be used as described above, and the melting point thereof must be lower than the sintering temperature because of the mechanism of action as described above. It is.
[0007]
When the amount of the flux is less than 0.25% by weight, removal of the oxide film, rapid formation of a neck by reaction diffusion (step 5), and densification by capillary action due to generation of a Fe—Cr—B liquid phase (step 6). ) Is not sufficient, and if added over 4% by weight, the density of the green compact is reduced, and in the case of a gas atmosphere, closed pores generated by the generation of a Fe—Cr—B liquid phase The gas is confined inside and remains until after sintering, which inhibits densification. For these reasons, the amount of flux is suitably 0.25 to 4% by weight.
[0008]
Even if the flux is added as a simple substance, it melts in the temperature rising process and covers the surface of the stainless steel powder by capillary action, but it is more effective to coat the surface of the stainless steel powder with the flux in advance. is there.
[0009]
The sintering atmosphere is a vacuum atmosphere or an atmosphere of non-oxidizing gas such as inert gas such as Ar, Ne, and He, H 2 gas, N 2 gas, ammonia decomposition gas, or a mixed gas thereof. If it exists, the said reaction will arise and it can aim at densification. In addition, if sintering is performed in an N 2 gas atmosphere and an ammonia decomposition gas atmosphere, the stainless steel is nitrided and affects the corrosion resistance or magnetic properties. Therefore, the sintering is performed in a vacuum atmosphere, an inert gas atmosphere, or an H 2 gas atmosphere. It is desirable. Further, in the vacuum atmosphere, even if the Fe—Cr—B liquid phase is generated and the closed pores are generated, the closed pores disappear as the sintering progresses because the closed pores disappear, and the silicate disappears. This is more preferable because only the remaining slag remains.
[0010]
The generation of Cr 2 B in the above stage 4 starts from around 800 ° C., and the melted flux removes the oxide film on the surface of the stainless steel powder, allowing the sintering to proceed even at a low temperature, but the γ phase is low In the temperature region, the diffusion of elements is slow, so that the densification does not progress so much, and the densification proceeds rapidly by generating a metal liquid phase mainly composed of Fe—Cr—B at around 1100 ° C. For this reason, the sintering temperature is preferably 1100 ° C. or higher. On the other hand, when the sintering temperature exceeds 1400 ° C., in a vacuum atmosphere, the evaporation of Fe and Cr, which are the main components of stainless steel, becomes significant, so that densification is hindered. Therefore, the sintering temperature is preferably in the range of 1100 to 1400 ° C.
[0011]
Even in the case of applications where densification is not so required, the densification is rapidly performed by adding flux, so that the sintering time can be shortened compared to the conventional case, and the effect of cost reduction can be obtained.
Furthermore, since the slag remaining in the closed pores is glassy, the corrosion resistance is not adversely affected. If an appropriate flux is used, the remaining slag becomes a free-cutting component, and an effect of improving machinability can be expected.
[0012]
【Example】
Examples of the present invention are shown below.
SUS316L powder is prepared as a stainless steel powder, and boric acid (orthoboric acid: H 3 BO 3 ) and borax (Na 2 B 4 O 7 · 10H 2 O) are prepared as fluxes. did. The mixed powder was filled in a molding die whose mold wall was lubricated with zinc stearate, and compacted with a molding pressure of 600 MPa. Next, these green compacts were dewaxed under conditions of 500 ° C. × 30 min, and then sintered under the sintering conditions shown in Tables 1 and 2.
About the obtained sintered compact, the density of the sintered compact was measured and the density ratio with respect to the density (true density) of SUS316L melted lumber was calculated | required. The results are shown in Tables 1 and 2.
Moreover, the relationship between the density ratio, the amount of added flux, the sintering temperature, and the sintering time is shown in FIGS. 1, 2, and 3, respectively.
[0013]
[Table 1]
Fig. 1 is a graph showing the influence of the flux addition amount and the sintering atmosphere on the density ratio. By adding borax or boric acid as the flux, the density ratio is improved and the effect of densification by the flux addition is confirmed. It was done.
In addition, the density ratio is improved as the amount of added flux is increased. In the case of a vacuum atmosphere, the improvement is suppressed after the density ratio is rapidly increased to a constant value, whereas in the case of an argon gas atmosphere. The density ratio gradually decreases with a peak when the addition amount of borax is 1% by weight, and decreases to the same level as when no flux is added when 4% by weight is added. These phenomena are considered as follows.
That is, closed pores are generated due to the generation of a liquid phase, but in the case of a vacuum atmosphere, there is no gas to be confined, so the disappearance of the closed pores proceeds, and only low-density slag remains in the pores of the sintered body. To do. Therefore, since the amount of residual slag increases as the flux addition amount increases, the increase in density ratio can be suppressed. On the other hand, in the case of an argon gas atmosphere, since the gas confined in the closed pores inhibits the disappearance of the closed pores, in addition to the fact that the densification does not proceed, the low-density slag remaining as the flux addition amount increases. Since the amount increases, the density ratio decreases instead.
From the above results, it was confirmed that the flux addition amount for obtaining the effect of flux addition is preferably 0.25 to 4% by weight regardless of the sintering atmosphere.
Moreover, if it was in the said range, the effect of flux addition was acquired also in argon gas atmosphere, and it was confirmed that it is still more suitable if a vacuum atmosphere is used.
[0015]
FIG. 2 is a graph showing the influence of the sintering temperature on the density ratio. In both cases of adding 2% by weight of borax and 1% by weight of boric acid, the density ratio does not improve much when the sintering temperature is 1000 ° C. or less. When the temperature exceeds about 1100 ° C., the density ratio is improved, and when it exceeds 1200 ° C., the density ratio tends to increase rapidly. Further, the density ratio becomes maximum around 1350 to 1370 ° C., and the density ratio slightly decreases at a temperature higher than that.
Boric acid has a melting point of 184 ° C., but boric acid loses water at 300 ° C. or higher and becomes boron oxide (B 2 O 3 ). Boron oxide has a melting point of 450 ° C. The melting point of borax is 745 ° C. Therefore, when the sintering temperature is 800 to 1000 ° C., both borax and boric acid melt and cover the surface of the stainless steel powder, remove the oxide film on the surface of the stainless steel powder, and the flux reacts with the stainless steel powder. Cr 2 B is deposited. Therefore, although the progress of sintering is possible, since the temperature is low and the diffusion of elements is slow, it is considered that the sintering does not proceed quickly and the density ratio is not improved so much.
On the other hand, when the temperature exceeds about 1100 ° C., generation of a metal liquid phase starts, so that densification due to capillary action occurs, and the density ratio rapidly increases and reaches a maximum at about 1350 to 1370 ° C. However, at temperatures higher than that, the evaporation of Fe and Cr, which are the main components of stainless steel, becomes significant in vacuum, so that densification is hindered, and the density ratio is considered to decrease slightly.
[0016]
Further, when these samples were subjected to observation of the metal structure and EPMA analysis, Cr and B compounds could be confirmed in the 800 ° C. sample. The amount of precipitation of Cr 2 B increases up to 1200 ° C., but beyond this, the precipitation phase rapidly decreases and a metal liquid phase is generated. In addition to Cr, Fe, and B, Mo and Ni are also detected in this liquid phase, so it is considered to be a eutectic liquid phase mainly composed of Fe-Cr-B. Further, B in the flux decreases and disappears after the generation of the liquid phase, and Si and Mn oxides increase instead. This is considered to be because the oxide film on the surface of the stainless steel powder is dissolved in the flux, and the flux reacts with Si and Mn dissolved in the steel.
[0017]
From the above, the effect of flux addition that proceeds in the step of densification due to capillary phenomenon due to removal of oxide film at low temperature by addition of flux, rapid formation of neck by reaction diffusion, and generation of Fe-Cr-B system liquid phase. It was confirmed that the sintering temperature was preferably 1100 to 1400 ° C.
[0018]
FIG. 3 is a graph showing the effect of sintering time on the density ratio.
When the sintering temperature is 1350 ° C., it can be seen that considerable densification has occurred due to the occurrence of a liquid phase already in the process of raising the temperature to the sintering temperature. In addition, when the sintering temperature is 1250 ° C., there is no great difference in the density ratio obtained compared with the sintering for 60 minutes even when the sintering is performed for 20 minutes, so even when the demand for densification by sintering is not high, It can be seen that the effect of shortening the sintering time can be obtained by adding the flux as compared with the conventional method.
[0019]
【The invention's effect】
As described above, according to the manufacturing method of the present invention, a compact obtained by compacting a powder obtained by adding 0.25 to 4% by weight of a flux to stainless steel powder and sintering it at a temperature of 1100 to 1400 ° C. It has become possible to produce high-density sintered stainless steel. In addition, the present invention makes it possible to shorten the sintering time as compared with the prior art.
As described above, boric acids such as orthoboric acid, metaboric acid and hypoboric acid and lithium, sodium, potassium, magnesium, calcium, barium, chromium, iron, cobalt, nickel, manganese, silicon, lead, copper, titanium In addition, borates of metals such as aluminum can be used, and sintering can be performed in various non-oxidizing gas atmospheres, but sintering is preferably performed in a vacuum atmosphere.
Therefore, the present invention is industrially useful because it can be easily carried out in a short time without using an expensive drug or performing complicated processing.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of flux addition amount and sintering atmosphere on density ratio.
FIG. 2 is a graph showing the effect of sintering temperature on the density ratio.
FIG. 3 is a graph showing the influence of sintering time on the density ratio.
Claims (5)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP29211694A JP3841455B2 (en) | 1994-11-01 | 1994-11-01 | Method for producing high density sintered stainless steel |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP29211694A JP3841455B2 (en) | 1994-11-01 | 1994-11-01 | Method for producing high density sintered stainless steel |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH08134503A JPH08134503A (en) | 1996-05-28 |
| JP3841455B2 true JP3841455B2 (en) | 2006-11-01 |
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| JP29211694A Expired - Fee Related JP3841455B2 (en) | 1994-11-01 | 1994-11-01 | Method for producing high density sintered stainless steel |
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| JP5260913B2 (en) * | 2007-08-03 | 2013-08-14 | 株式会社神戸製鋼所 | Iron-based mixed powder for powder metallurgy and sintered iron powder |
| KR101522627B1 (en) * | 2014-10-16 | 2015-05-22 | 주식회사 유승 | Method for manufacturing stainless steel parts in a motor vehicle and the parts |
| WO2018084056A1 (en) * | 2016-11-02 | 2018-05-11 | コニカミノルタ株式会社 | Metal powder, powder-sintered additively manufactured object, and manufacturing method for same |
| US20230278101A1 (en) * | 2020-07-01 | 2023-09-07 | Hewlett-Packard Development Company, L.P. | Three-dimensional printing kits |
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