JP4753504B2 - Method for producing high purity Fe-Cr, Fe-Cr-Ni alloy - Google Patents
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Description
【０００１】
【産業上の利用分野】
本発明は、精錬条件を調整して高濃度のＣｒを含む高純度のＦｅ−Ｃｒ合金もしくはＦｅ−Ｃｒ−Ｎｉ合金を製造する方法に関する。
【０００２】
【従来の技術】
高濃度のＣｒを含むＦｅ−Ｃｒ，Ｆｅ−Ｃｒ−Ｎｉ合金は耐食性、耐酸化性、加工性に優れた材料として知られている。近年、これらの合金により一層の品質の向上、材料特性の向上が求められてきている。その要求に対応する方法の一つとして耐食性や靭性、加工性に影響を及ぼすＣ，Ｎ，Ｏ，Ｓなどの不純物を低減させて高純度化する方法が挙げられる。また、合金中のＣｒ含有量を高くすることで従来にない耐熱性、耐食性が得られる。しかしながら、Ｃｒ含有量が高くなるとＣｒの相互作用によりＣ，Ｎの活量が小さくなり、極低Ｃ，Ｎの材料を溶製することが困難になる。特にフェライト系ステンレス鋼における極低Ｃ，Ｎ鋼の精錬技術に関してはこれまで数多くの技術開発がなされ、成果が得られてきた。しかしながら、従来のプロセスでは厳しい要求への対応が困難になってきている。
【０００３】
【発明が解決しようとする課題】
低Ｃ，Ｎ鋼を製造するプロセスは一般的にＶＯＤ処理が適していると言われている。現状のＶＯＤプロセスにおいては極低Ｃ，Ｎ鋼を得るために減圧下での攪拌強化や反応面積増大効果の利用を中心に対応してきた。しかしながらより一層の極低Ｃ，Ｎ鋼を経済的に製造しようとすると、さらにはＣｒ含有量が３０質量％を超えるＦｅ−Ｃｒ合金の極低Ｃ，Ｎ鋼を製造しようとすると、従来のプロセスでは容易に対応できなかった。
本発明は、このような問題を解消すべく案出されたものであり、高濃度のＣｒを含むＦｅ−Ｃｒ合金やＦｅ−Ｃｒ−Ｎｉ合金でも極めて低いＣ，Ｎ濃度でしかもＯ，Ｓをも極低レベルにした合金を低コストで製造する方法を提供することを目的とする。
【０００４】
【課題を解決するための手段】
本発明の高純度で高濃度のＣｒを含むＦｅ−Ｃｒ，Ｆｅ−Ｃｒ−Ｎｉ合金の製造方法は、その目的を達成するため、ガス攪拌手段を有する精錬炉内のＦｅ−Ｃｒ−Ｃ溶湯あるいはＦｅ−Ｃｒ−Ｎｉ−Ｃ溶湯に対して、ガス攪拌しながらＯ2吹錬し粗脱炭するとともに脱窒する第一工程、溶湯をシールしつつあるいはシールせずに取鍋内に出湯する第二工程、真空精錬炉において真空下で溶湯をガス攪拌し、および／もしくは真空下で適時Ｏ2吹錬し仕上げ脱炭、仕上げ脱窒する第三工程、真空精錬炉内をＡｒ雰囲気としガス攪拌しながらフラックスならびに脱酸剤を用いて脱硫、脱酸の仕上げ精錬を行う第四工程、フラックスで取鍋内溶湯を覆って鋳造を行う第五工程からなることを特徴とする。
【０００５】
粗脱炭、脱窒する第一工程の前あるいは同時に高塩基度フラックスを利用した脱窒を行うこともできる。この際の高塩基度フラックスとしては、（質量％ＣａＯ）／（質量％ＳｉＯ２）が１．５以上のもの、あるいはアルカリ金属の酸化物もしくは炭酸塩を５質量％以上含有しているものを使用することが好ましい。
溶湯出湯の際のシールとして、溶湯の熱でＣＯ2ガスを発生する物質および／もしくは溶融、滓化するフラックスを取鍋に入れて行うことが好ましい。
また、真空精錬炉内をＡｒ雰囲気とするためのＡｒガスとして、Ｎ2ガス含有量が０．００１体積％以下、露点が−４０℃以下の高純度乾燥Ａｒガスを使用することが好ましく、さらにＮ2ガス含有量が０．０００２体積％以下、露点が−６０℃以下の高純度乾燥Ａｒガスを使用することがより好ましい。
【０００６】
【実施の形態】
まず、上底吹転炉やＡＯＤ炉等のガス攪拌手段を有する精錬炉を用いて粗脱Ｃ、脱Ｎを行う。攪拌用ガスとしてはＡｒガスやＡｒ＋Ｏ2の混合ガスあるいは水蒸気を用いる。二重管羽口の場合、冷却ガスとして外管に炭化水素ガスを用いても良い。粗脱炭、脱窒を実施する前の高濃度のＣｒを含むＦｅ−Ｃｒ−Ｃ、Ｆｅ−Ｃｒ−Ｎｉ−Ｃ溶湯成分はＣｒ含有量８〜６５％のものを使用し、Ｃ含有量は４％以上のものが好ましい。粗脱ＣはＯ2吹錬により、Ｃ＋Ｏ→ＣＯ（ガス）の反応を利用して行う。その際、脱炭時のＯ2吹錬によって生成したＣＯガスの希釈効果、すなわちＣＯ気泡中で２Ｎ→Ｎ2（ガス）の反応によりＮ2が希釈されることを利用して脱Ｎを行う。なお、ＸはＸ成分が溶湯中に溶解していることを示し、［Ｘ］は溶湯中のＸ成分の濃度を示す。粗脱炭後の終点［Ｃ］は［Ｃ］＝０．３〜１．５質量％であり、Ｃｒ含有量が高いほど終点［Ｃ］は高いほうが良い。なお、粗脱炭期のスラグは出湯時に除滓することが好ましい。
【０００７】
この粗脱炭、脱窒工程で脱Ｎをより一層促進させるためには、高塩基度フラックスを用いることが好ましい。高塩基度フラックスとしては（質量％ＣａＯ）／（質量％ＳｉＯ2）が１．５以上の組成をもつものを使用することが好ましく、さらに２．０以上のものを使用することが望ましい。さらに脱Ｎを強化するには、高塩基度フラックスにアルカリ金属酸化物、特にリチウム，ナトリウム，バリウムの酸化物，炭酸塩を添加すると良い。フラックスにこれらの化合物が５質量％以上含まれるとフラックスの融点低下と塩基度の大幅な向上により、フラックスと溶湯との反応が大きく促進されるため、大きな脱窒効果が得られる。この高塩基度フラックスによる脱窒工程ではスラグ−メタル反応を十分に促進させるためにＡｒガスやＡｒ＋Ｏ2混合ガスあるいは水蒸気を用いたガス攪拌は必須である。
脱窒後、フラックスは復Ｎを避けるために望ましくは粗脱炭処理前に、遅くとも真空処理前には除滓する必要がある。
【０００８】
粗脱炭，脱窒後、溶湯を取鍋に出湯する。その際、溶湯中のＣｒ含有量が高いほど、出湯流と大気、溶湯落下地点で巻き込まれた空気気泡と溶湯、取鍋内溶湯表面が大気と反応することによりＮのピックアップが生じやすくなるので、溶湯流および取鍋内溶湯表面をシールする必要がある。Ｎのピックアップを防止するには、溶湯の熱でＣＯ2ガスを発生する物質および／もしくは溶融、滓化するフラックスを取鍋に入れることが好ましい。
ＣＯ2ガスを発生する物質としては、熱分解でＣＯ2を発生する炭酸マグネシウムを主成分とする固体物質が好適である。具体的には工業用炭酸マグネシウム（化学式：４ＭｇＣＯ3・Ｍｇ（ＯＨ）2・４Ｈ2Ｏ）が安価で入手し易い。また、ドライアイスも利用できる。
溶融、滓化するフラックスとしては、例えば５０質量％ＣａＯ＋５０質量％Ａｌ2Ｏ3が好適である。
【０００９】
溶湯を取鍋に移した後、真空精錬炉において真空下に溶湯をガス攪拌しながら適時Ｏ2吹錬し、仕上げ脱炭、仕上げ脱窒を行う。この工程における初期［Ｃ］は粗脱炭後の終点［Ｃ］値であり、［Ｃ］＝０．３〜１．５質量％である。初期［Ｃ］を高くする理由は、ＣによりＮの活量を高めさせ、脱Ｎを生じ易くするためである。また、Ｃは界面活性元素であるＯの濃度を低下させるため、脱Ｎを生じ易くする。
脱Ｎは真空処理により発生するＣＯ気泡中で２Ｎ→Ｎ2（ガス）の反応によりＮ2が希釈されて生じると考えられる。従って、［Ｃ］は高い方が良く、具体的には［Ｃ］≧０．３質量％の領域で真空処理することにより脱Ｎが促進される。
脱Ｎを効果的に促進させるためには真空下でのＯ2吹錬とＯ2吹錬を止めた真空処理の組み合わせを少なくとも１回は実施し、Ｃｒ含有量が高い場合は、この処理を複数回繰り返して脱炭と脱窒を行うことが好ましい。
真空度はＯ2吹錬時で２００Ｔｏｒｒ以下、Ｏ2吹錬を行わない時は１Ｔｏｒｒ以下とすることが好ましい。
［Ｃ］が約０．０１質量％に到達すればＯ2吹錬を止め、真空処理のみによる脱Ｃを行う。ガス攪拌は主に取鍋底部からＡｒガスを吹き込んで行う。Ａｒガス流量は２〜２５ＮＬ／ｍｉｎ・ｔが好ましい。
【００１０】
仕上げ脱炭、脱窒が終了すると真空排気を止め直ちにＡｒガスを真空精錬炉内に導入して高塩基度フラックを用いた脱酸，脱硫の仕上げ精錬を行う。Ａｒ気圧は大気からのＮの侵入を防止するために１ａｔｍ〜若干の加圧が望ましい。Ａｒ以上０．５ａｔｍになるとシール効果が出始める。Ａｒガスは炉内に封じ込めても良いし、精錬中流しつづけても良い。
Ａｒガス中の不純物は低くすることが好ましい。好ましくはＮ2ガス含有量が０．００１体積％以下、露点が−４０℃以下の高純度乾燥Ａｒガスを使用する。また、約３０質量％以上のＣｒ含有量の材料を溶製する場合は、ＡｒガスからのＯ，Ｎ，Ｈがピックアップし易くなるので、より一層Ａｒガスの不純物を低くする必要がある。具体的には、Ｎ2ガス含有量が０．０００２体積％以下、露点が−６０℃以下の高純度乾燥Ａｒガスを使用する。
【００１１】
さらに、ガス配管の中の水分や配管の接続部からのＮの混入もあり得るので精錬の際は前もってＡｒガス洗浄を十分に行っておく。なお、Ａｒガス中にはＨｅなどの不活性ガスは混ざっていても良い。
仕上げ精錬時の高塩基度フラックス中のＣａＯとＳｉＯ2の質量％比は（ＣａＯ／ＳｉＯ2）は１．５以上が好ましい。２．０以上にするとさらに好ましい。脱酸剤はＳｉ，Ａｌ，Ｍｎ，Ｔｉ，Ｚｒ，希土類元素のうち１種もしくは２種以上を組み合わせて用いる。仕上げ精錬時も取鍋底部からのＡｒガス攪拌は続ける。脱硫、脱酸精錬時の攪拌用Ａｒガス純度は上記のものと同様にすることが好ましい。
【００１２】
仕上げ精錬を終えた後、フラックスで取鍋内溶湯を覆った状態とする。鋳造場に移送する際、取鍋内溶湯をフラックで覆って大気と遮断すれば大気からのＮ，Ｏのピックアップを防止できる。そのため、このフラックスには低融点フラックスを用いる。例えば５０質量％ＣａＯ＋５０質量％Ａｌ2Ｏ3のフラックスが好適である。
フラックスは十分に溶融させておく必要があるため、溶湯へのフラックス添加は仕上げ脱炭、脱窒後、溶湯の温度が十分確保できる時期に行う。
【００１３】
上記のようにＡｒガス等を利用してのガス攪拌状態での脱炭、脱窒、さらにＡｒガス雰囲気下での脱炭、脱窒、脱硫、脱酸の条件を最適な条件としてそれぞれの反応効率を高めるとともに、それらを行う精錬炉から取鍋ないしは鋳造場への移送の段階での復Ｎ、復Ｏを極力防止して、精錬された合金中のＣ，Ｎ，Ｓ，Ｏ含有量を極めて低いものとすることができる。具体的には［Ｃ］＋［Ｎ］＋［Ｓ］＋［Ｏ］を０．０２重量％以下にまで低減させた高濃度のＣｒを含むＦｅ−Ｃｒ合金，Ｆｅ−Ｃｒ−Ｎｉ合金を製造することができる。
【００１４】
【実施例】
Ｃｒ含有量１０〜６５質量％、Ｃ含有量４．５〜６．５質量％の高濃度のＣｒを含むＦｅ−Ｃｒ−Ｃ溶湯やＦｅ−Ｃｒ−Ｎｉ−Ｃ溶湯７０トンを上底吹転炉に装入し、粗脱炭、脱窒を施した。Ｃｒを３０質量％以上含有する高濃度の合金においてはＣａＯとＳｉＯ2の質量比（ＣａＯ／ＳｉＯ2）を３．０とし、炭酸リチウムを５質量％含んだ高塩基性フラックスを使用して脱窒精錬を行った。脱窒精錬後、除滓した後、攪拌用としてＡｒガスを５ＮＬ／ｍｉｎ・ｔの流量で吹き込みつつ、Ｏ2流量：２．０５ＮＬ／ｍｉｎ・ｔ、Ｏ2吹錬時間：４０分で、フラックスを１５ｋｇ／ｔ−ｍｅｔａｌ添加して粗脱炭精錬を行った。
【００１５】
粗脱炭、脱窒後、溶湯を取鍋に出湯した。その際、Ｃｒ含有量が３０質量％以上の合金の場合は、Ｎのピックアップ防止剤として工業用炭酸マグネシウムを取鍋中に入れておいた。溶湯を取鍋に移した後、真空精錬炉において真空下で溶湯をガス攪拌しながらＯ2吹錬して仕上げ脱炭、仕上げ脱窒を行った。
脱窒を効果的に促進させるために真空下でのＯ2吹錬とＯ2吹錬を止めた真空処理の組み合わせを２回実施した。真空度はＯ2吹錬時で１００Ｔｏｒｒ、Ｏ2精錬を行わない時は０．４Ｔｏｒｒとした。
［Ｃ］が０．１１質量％に到達した後、Ｏ2吹錬を止め、真空処理のみによる脱炭を行った。ガス攪拌は取鍋底部からＡｒガスを吹き込んで行った。Ａｒガス流量は１８ＮＬ／ｍｉｎ・ｔとした。
【００１６】
仕上げ脱炭、脱窒が終了すると真空排気を止め直ちにＡｒガスを真空精錬炉内に導入して高塩基性フラックスを３０ｋｇ／ｔ−ｍｅｔａｌ添加して脱酸、脱硫の仕上げ精錬を行った。Ａｒ気圧は１ａｔｍとし、流しつづけた。Ａｒガス中のＮ2含有量は０．０００１体積％、露点は−７０℃である高純度乾燥Ａｒガスを用いた。
なお、Ｃｒ含有量が３０質量％以上の合金を精錬する場合、ガス配管の中の水分や配管の接続部からのＮの混入を防止するために、事前に２回のＡｒガス洗浄を２回行った。
【００１７】
仕上げ精錬時の高塩基性フラックスは、ＣａＯを１３ｋｇ／ｔ−ｍｅｔａｌ添加して（ＣａＯ／ＳｉＯ2）を２．５とした。脱酸剤としてＡｌを１３ｋｇ／ｔ−ｍｅｔａｌ添加した。高塩基度フラックスを添加する際、後工程においてＮ，Ｏのピックアップを防止するため５０質量％ＣａＯ＋５０質量％Ａｌ2Ｏ3のフラックスも添加した。添加量は５ｋｇ／ｔ−ｍｅｔａｌとした。
なお、仕上げ精錬時、取鍋底部から１０ＮＬ／ｍｉｎ・ｔの流量でＡｒガス攪拌を行った。大気解放前には取鍋内溶湯が極力大気と接することがないようにＡｒガス流量を３ＮＬ／ｍｉｎ・ｔに減らした。その後、成分調整を行い、材料の要求される特性に応じてＣｏ，Ｍｏ，Ｎｂ，Ｔｉ，Ｖ，Ｗ，Ｂ，Ｃｕ等を必要量添加した。取鍋を鋳造場に移送した。鋳造後得られた素材の成分を表１に示す。
【００１８】
比較例として、高濃度のＣｒを含むＦｅ−Ｃｒ−Ｃ溶湯を底吹きなしにＯ2吹錬のみで粗脱炭、脱窒した場合（試験番号７）、仕上げ脱炭、仕上げ脱窒工程において真空にせずＡｒ＝１ａｔｍの雰囲気下でガス攪拌＋Ｏ2吹錬とした場合（試験番号８）、仕上げ脱炭、脱窒終了後、Ａｒガスを導入せずにそのまま真空排気を続けながら高塩基度フラックスを用いて脱硫、脱酸仕上げ精錬を行った場合（試験番号９）、脱硫、脱酸仕上げ精錬終了後、フラックスを添加せずにそのまま鋳造場へ取鍋を移送し、鋳造した場合（試験番号１０）の各チャージの鋳造後の素材の成分を併せて表１に示す。
【００１９】
表１に示す結果からもわかるように、本発明の条件で製造した試験番号１〜６の場合、［Ｃ］＋［Ｎ］＋［Ｓ］＋［Ｏ］≦０．０２０質量％であり、極めて不純物含有量の少ない高純度合金が得られた。
これに対して、本発明の条件を外れた試験番号７〜１０の場合、Ｃ，Ｎ，Ｓ，Ｏ含有量が多く所期の目的を達成できていない。Ａｒガス等による攪拌を行わずに粗脱炭、脱窒を行った試験番号７では、脱炭に伴うＣＯガスを有効に利用した脱窒が効果的になされず、Ｎの低減ができていない。真空にせずＡｒ＝１ａｔｍの雰囲気下で仕上げ脱炭、仕上げ脱硫した試験番号８では、脱炭、脱窒が効果的に進行せず期待したＣ，Ｎの低減ができていない。仕上げ脱炭、仕上げ脱窒終了後、Ａｒガスを導入せずそのまま真空下で仕上げ脱硫、脱酸した試験番号９では、大気からのＮの侵入が発生したため、復Ｎし、合金中のＮ含有量が多くなっている。さらに、精錬終了後溶湯をフラックスで覆わず鋳造場に移送した試験番号１０では、移送時に大気から復Ｎ、復Ｏし、合金中のＮ，Ｏ含有量が多くなっている。
【００２１】
【発明の効果】
以上に説明したように、精錬炉および取鍋内での脱炭、脱窒処理の、攪拌、吹錬、雰囲気、フラックス使用等の条件をそれぞれの反応が効率よく行えるように組み合わせるとともに、溶湯の出湯、移送の段階で復Ｎし難い条件を設定することにより、合金中のＣ，Ｎ，Ｓ，Ｏ含有量が極めて少ない高純度の高濃度のＣｒを含むＦｅ−Ｃｒ合金あるいはＦｅ−Ｃｒ−Ｎｉ合金を低コストで製造することができた。
（以上 余白）[0001]
[Industrial application fields]
The present invention relates to a method for producing a high-purity Fe—Cr alloy or Fe—Cr—Ni alloy containing a high concentration of Cr by adjusting refining conditions.
[0002]
[Prior art]
Fe—Cr and Fe—Cr—Ni alloys containing a high concentration of Cr are known as materials having excellent corrosion resistance, oxidation resistance, and workability. In recent years, these alloys have been required to further improve quality and material properties. As one of methods for meeting the demand, there is a method of purifying by reducing impurities such as C, N, O, and S that affect corrosion resistance, toughness, and workability. Further, by increasing the Cr content in the alloy, unprecedented heat resistance and corrosion resistance can be obtained. However, when the Cr content is high, the C and N activities are reduced due to the interaction of Cr, and it becomes difficult to melt extremely low C and N materials. In particular, with regard to the refining technology of ultra-low C, N steel in ferritic stainless steel, many technical developments have been made and results have been obtained. However, it is difficult for conventional processes to meet strict requirements.
[0003]
[Problems to be solved by the invention]
It is generally said that VOD treatment is suitable for the process of producing low C, N steel. In the current VOD process, in order to obtain ultra-low C, N steel, the strengthening of stirring under reduced pressure and the use of the effect of increasing the reaction area have been dealt with. However, if an attempt is made to economically produce a further ultra-low C, N steel, and further an Fe-Cr alloy ultra-low C, N steel having a Cr content exceeding 30% by mass, the conventional process will be described. But it was not easy to deal with.
The present invention has been devised to solve such a problem. Even in a Fe—Cr alloy or a Fe—Cr—Ni alloy containing a high concentration of Cr, the O and S are reduced at a very low C and N concentration. Another object of the present invention is to provide a method for producing an alloy having a very low level at a low cost.
[0004]
[Means for Solving the Problems]
In order to achieve the object, the method for producing an Fe—Cr, Fe—Cr—Ni alloy containing high-purity and high-concentration Cr according to the present invention has a Fe—Cr—C molten metal in a refining furnace having gas stirring means. A first step in which the Fe-Cr-Ni-C molten metal is O2 blown and coarsely decarburized and denitrified while stirring the gas, and the second molten metal is discharged into the ladle with or without sealing the molten metal. Process, the third step of stirring the molten metal under vacuum in a vacuum smelting furnace and / or timely O 2 blowing and finishing decarburization and finishing denitrification in a vacuum smelting furnace, Ar gas inside the vacuum smelting furnace with Ar atmosphere However, it is characterized by comprising a fourth step of performing desulfurization and deoxidation finishing refining using a flux and a deoxidizer, and a fifth step of covering the molten metal in the ladle with the flux and casting.
[0005]
It is also possible to perform denitrification using a high basicity flux before or simultaneously with the first step of rough decarburization and denitrification. As the high basicity flux at this time, (mass% CaO) / (mass% SiO 2) having a mass of 1.5 or more, or containing 5 mass% or more of an alkali metal oxide or carbonate is used. It is preferable to do.
It is preferable that the sealing at the time of the molten metal is performed by putting a substance that generates CO 2 gas by the heat of the molten metal and / or a flux that melts and hatches into a ladle.
Moreover, it is preferable to use high-purity dry Ar gas having an N 2 gas content of 0.001% by volume or less and a dew point of −40 ° C. or less as Ar gas for making the vacuum refining furnace an Ar atmosphere. It is more preferable to use high purity dry Ar gas having an N 2 gas content of 0.0002% by volume or less and a dew point of −60 ° C. or less.
[0006]
Embodiment
First, rough de-C and N are performed using a refining furnace having gas stirring means such as an upper-bottom blow converter or an AOD furnace. As the stirring gas, Ar gas, Ar + O 2 mixed gas, or water vapor is used. In the case of a double pipe tuyere, hydrocarbon gas may be used for the outer pipe as the cooling gas. Fe-Cr-C and Fe-Cr-Ni-C molten metal containing high concentration of Cr before carrying out rough decarburization and denitrification use a Cr content of 8 to 65%, and the C content is 4% or more is preferable. Rough decarbonization is performed by O 2 blowing and utilizing a reaction of C + O → CO (gas). At that time, denitrification is performed by utilizing the dilution effect of CO gas generated by O 2 blowing during decarburization, that is, N 2 is diluted by a reaction of 2N → N 2 (gas) in CO bubbles. X indicates that the X component is dissolved in the molten metal, and [X] indicates the concentration of the X component in the molten metal. The end point [C] after rough decarburization is [C] = 0.3 to 1.5% by mass, and the higher the Cr content, the higher the end point [C]. In addition, it is preferable to remove the slag during the rough decarburization period at the time of tapping.
[0007]
In order to further promote de-N in this rough decarburization and denitrification step, it is preferable to use a high basicity flux. As the high basicity flux, it is preferable to use a flux having a composition of (mass% CaO) / (mass% SiO 2 ) of 1.5 or more, and more preferably 2.0 or more. In order to further enhance the de-N, an alkali metal oxide, particularly an oxide of lithium, sodium, barium, or carbonate may be added to the high basicity flux. When these compounds are contained in the flux in an amount of 5% by mass or more, the reaction between the flux and the molten metal is greatly promoted due to the lowering of the melting point of the flux and the substantial improvement of the basicity, so that a great denitrification effect is obtained. In the denitrification process using the high basicity flux, in order to sufficiently promote the slag-metal reaction, gas stirring using Ar gas, Ar + O 2 mixed gas or water vapor is essential.
After denitrification, the flux should be removed before the rough decarburization treatment and at the latest before the vacuum treatment to avoid recovery N.
[0008]
After rough decarburization and denitrification, pour molten metal into a ladle. At that time, the higher the Cr content in the molten metal, the easier it is for N pick-up to occur due to the reaction between the tapping stream and the atmosphere, the air bubbles and molten metal caught at the molten metal falling point, and the molten metal surface in the ladle. It is necessary to seal the molten metal flow and the molten metal surface in the ladle. In order to prevent N from being picked up, it is preferable to put a substance that generates CO 2 gas by the heat of the molten metal and / or a flux that melts and hatches into a ladle.
As the substance that generates CO 2 gas, a solid substance mainly composed of magnesium carbonate that generates CO 2 by thermal decomposition is suitable. Specifically, industrial magnesium carbonate (chemical formula: 4MgCO 3 · Mg (OH) 2 · 4H 2 O) is inexpensive and easily available. Dry ice can also be used.
As a flux that melts and hatches, for example, 50 mass% CaO + 50 mass% Al 2 O 3 is suitable.
[0009]
After the molten metal is transferred to the ladle, the molten metal is blown with O 2 in a vacuum in a vacuum smelting furnace as needed, and finish decarburization and final denitrification are performed. The initial [C] in this step is the end point [C] value after rough decarburization, and [C] = 0.3 to 1.5 mass%. The reason why the initial [C] is increased is that the activity of N is increased by C to facilitate de-N. Further, since C lowers the concentration of O, which is a surface active element, it facilitates denitrification.
It is considered that N removal is caused by dilution of N 2 by a reaction of 2 N → N 2 (gas) in CO bubbles generated by vacuum treatment. Therefore, [C] should be high. Specifically, de-N is promoted by vacuum treatment in the region of [C] ≧ 0.3 mass%.
In order to effectively promote de-N, at least one combination of O 2 blowing under vacuum and O 2 blowing stopped is performed at least once. If the Cr content is high, this treatment is performed. It is preferable to perform decarburization and denitrification by repeating a plurality of times.
Degree of vacuum 200Torr or less when O 2 blowing, when not performing the O 2 blowing is preferably set to 1Torr or less.
When [C] reaches about 0.01% by mass, O 2 blowing is stopped and de-C is performed only by vacuum treatment. Gas agitation is performed mainly by blowing Ar gas from the bottom of the ladle. The Ar gas flow rate is preferably 2 to 25 NL / min · t.
[0010]
When finish decarburization and denitrification are completed, the vacuum exhaust is stopped and Ar gas is immediately introduced into the vacuum refining furnace to perform deoxidation and desulfurization finish refining using high basicity flack. Ar pressure is preferably 1 atm to slight pressurization in order to prevent intrusion of N from the atmosphere. When Ar is 0.5 atm or more, the sealing effect starts to appear. Ar gas may be contained in the furnace, or may continue to flow during refining.
It is preferable to reduce the impurities in the Ar gas. Preferably, high purity dry Ar gas having an N 2 gas content of 0.001% by volume or less and a dew point of −40 ° C. or less is used. In addition, when melting a material having a Cr content of about 30% by mass or more, O, N, and H from Ar gas can be easily picked up, so that it is necessary to further reduce the Ar gas impurities. Specifically, high purity dry Ar gas having an N 2 gas content of 0.0002% by volume or less and a dew point of −60 ° C. or less is used.
[0011]
Furthermore, since water in the gas pipe and N from the pipe connection may be mixed, the Ar gas is sufficiently cleaned in advance before refining. Note that an inert gas such as He may be mixed in the Ar gas.
High basicity CaO mass% ratio of SiO 2 in the flux of the finish refining (CaO / SiO 2) is preferably 1.5 or more. More preferably, it is 2.0 or more. The deoxidizer is used alone or in combination of two or more of Si, Al, Mn, Ti, Zr, and rare earth elements. Ar gas stirring from the bottom of the ladle is continued during finishing refining. The purity of Ar gas for stirring during desulfurization and deoxidation refining is preferably the same as described above.
[0012]
After finishing refining, the molten metal in the ladle is covered with flux. When transporting to the foundry, the molten metal in the ladle is covered with a flack and cut off from the atmosphere, so that pickup of N and O from the atmosphere can be prevented. Therefore, a low melting point flux is used for this flux. For example, a flux of 50% by mass CaO + 50% by mass Al 2 O 3 is suitable.
Since the flux needs to be sufficiently melted, the flux should be added to the molten metal at a time when the molten metal temperature can be sufficiently secured after finish decarburization and denitrification.
[0013]
As described above, decarburization and denitrification in a gas stirring state using Ar gas or the like, and further, each reaction under the conditions of decarburization, denitrification, desulfurization, and deoxidation in an Ar gas atmosphere as optimum conditions In addition to increasing the efficiency, the N, S and O contents in the refined alloy are reduced by preventing recovery N and recovery O as much as possible at the stage of transfer from the smelting furnace to the ladle or foundry. It can be very low. Specifically, Fe-Cr alloys and Fe-Cr-Ni alloys containing high concentrations of Cr with [C] + [N] + [S] + [O] reduced to 0.02 wt% or less are manufactured. can do.
[0014]
【Example】
Top-bottomed 70 tons of Fe-Cr-C and Fe-Cr-Ni-C melts containing high concentrations of Cr with a Cr content of 10 to 65 mass% and a C content of 4.5 to 6.5 mass% The furnace was charged and subjected to rough decarburization and denitrification. In a high-concentration alloy containing 30% by mass or more of Cr, the mass ratio of CaO to SiO 2 (CaO / SiO 2 ) is set to 3.0, and desorption is performed using a high basic flux containing 5% by mass of lithium carbonate. Nitrogen refining was performed. After denitrification refining, after Jokasu while blowing Ar gas as a stirring rate of 5NL / min · t, O 2 flow rate: 2.05NL / min · t, O 2 blowing time: 40 minutes, flux The crude decarburization refining was performed by adding 15 kg / t-metal.
[0015]
After rough decarburization and denitrification, the molten metal was poured into a ladle. At that time, in the case of an alloy having a Cr content of 30% by mass or more, industrial magnesium carbonate was placed in a ladle as an N pick-up inhibitor. After the molten metal was transferred to the ladle, it was subjected to finish decarburization and finish denitrification by O 2 blowing while stirring the molten metal under vacuum in a vacuum refining furnace.
In order to effectively promote denitrification, a combination of O 2 blowing under vacuum and vacuum treatment with O 2 blowing stopped was performed twice. The degree of vacuum 100Torr in a time O 2 blowing, when not performing the O 2 refining was 0.4 Torr.
After [C] reached 0.11% by mass, O 2 blowing was stopped and decarburization was performed only by vacuum treatment. Gas stirring was performed by blowing Ar gas from the bottom of the ladle. The Ar gas flow rate was 18 NL / min · t.
[0016]
After finishing decarburization and denitrification, the vacuum exhaust was stopped and Ar gas was immediately introduced into the vacuum refining furnace, and high basic flux was added at 30 kg / t-metal, and final refining of deoxidation and desulfurization was performed. The Ar pressure was set to 1 atm and kept flowing. A high-purity dry Ar gas having an N 2 content of 0.0001% by volume and a dew point of −70 ° C. in the Ar gas was used.
In addition, when refining an alloy having a Cr content of 30% by mass or more, two Ar gas cleanings are performed twice in advance in order to prevent moisture in the gas pipe and N from entering the pipe connection. went.
[0017]
The high basic flux at the time of final refining was adjusted to (CaO / SiO 2 ) of 2.5 by adding 13 kg / t-metal of CaO. Al was added at 13 kg / t-metal as a deoxidizer. When adding a high basicity flux, a flux of 50% by mass CaO + 50% by mass Al 2 O 3 was also added in order to prevent N and O pickup in the subsequent process. The amount added was 5 kg / t-metal.
At the time of finish refining, Ar gas was stirred from the bottom of the ladle at a flow rate of 10 NL / min · t. Before the atmosphere was released, the Ar gas flow rate was reduced to 3 NL / min · t so that the molten metal in the ladle would not contact the atmosphere as much as possible. Thereafter, the components were adjusted, and necessary amounts of Co, Mo, Nb, Ti, V, W, B, Cu and the like were added according to the required characteristics of the material. The ladle was transferred to the foundry. Table 1 shows the components of the raw material obtained after casting.
[0018]
As a comparative example, in the case of rough decarburization and denitrification by only O 2 blowing without bottom blowing of Fe-Cr-C molten metal containing high concentration Cr (test number 7), in the final decarburization and final denitrification processes In the case of gas stirring and O 2 blowing in an atmosphere of Ar = 1 atm without applying a vacuum (Test No. 8), after completion of decarburization and denitrification, high basicity while continuing evacuation without introducing Ar gas When desulfurization and deoxidation finish refining is performed using flux (test number 9), after desulfurization and deoxidation finish refining, the ladle is transferred to the foundry without adding flux and cast (test) Table 1 shows the components of the material after casting of each charge of No. 10).
[0019]
As can be seen from the results shown in Table 1, in the case of test numbers 1 to 6 manufactured under the conditions of the present invention, [C] + [N] + [S] + [O] ≦ 0.020 mass%, A high purity alloy with very low impurity content was obtained.
On the other hand, in the case of test numbers 7 to 10 that deviate from the conditions of the present invention, the C, N, S, and O contents are large and the intended purpose cannot be achieved. In test number 7 in which rough decarburization and denitrification were performed without stirring with Ar gas or the like, denitrification using CO gas effectively accompanying decarburization was not effective, and N could not be reduced. . In Test No. 8 in which decarburization and final desulfurization were performed in an atmosphere of Ar = 1 atm without applying vacuum, decarburization and denitrification did not proceed effectively, and the expected C and N could not be reduced. After finishing decarburization and finish denitrification, in test No. 9 where final desulfurization and deoxidation were performed under vacuum without introducing Ar gas, N intrusion from the atmosphere occurred. The amount is increasing. Further, in test No. 10 in which the molten metal was transferred to the foundry without being covered with flux after refining, the N and O contents in the alloy increased due to recovery from the atmosphere during recovery.
[0021]
【The invention's effect】
As explained above, the conditions of decarburization and denitrification treatment in the smelting furnace and ladle are combined so that each reaction can be efficiently performed, and the molten metal Fe-Cr alloy or Fe-Cr- containing high-purity and high-concentration Cr with extremely low C, N, S, O content in the alloy by setting conditions that make it difficult to recover N at the stage of tapping and transfer Ni alloy could be manufactured at low cost.
(More margins)
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| UA104595C2 (en) | 2008-08-04 | 2014-02-25 | Ньюкор Корпорейшн | method for making a steel with low carbon low sulphur low nitrogen using conventional steelmaking Equipment |
| EP3147376B1 (en) * | 2008-08-04 | 2018-10-17 | Nucor Corporation | Low cost making of a low carbon, low sulfur, and low nitrogen steel using conventional steelmaking equipment |
| JP5696271B2 (en) * | 2008-11-06 | 2015-04-08 | スーパーピュアメタル合同会社 | Austenitic high-purity iron alloy with excellent high-temperature strength characteristics |
| JP5493061B2 (en) * | 2008-11-06 | 2014-05-14 | スーパーピュアメタル合同会社 | Austenitic high-purity iron alloy with excellent radiation damage resistance |
| JP5493060B2 (en) * | 2008-11-06 | 2014-05-14 | スーパーピュアメタル合同会社 | Austenitic high-purity iron alloy |
| US8523977B2 (en) | 2011-01-14 | 2013-09-03 | Nucor Corporation | Method of desulfurizing steel |
| JP6582904B2 (en) * | 2015-11-12 | 2019-10-02 | 東洋インキＳｃホールディングス株式会社 | Hot melt adhesive sheet for electromagnetic induction heating, adhesive structure using the same, and method for producing adhesive structure |
| CN107574285B (en) * | 2017-08-09 | 2021-06-22 | 新疆八一钢铁股份有限公司 | Method for controlling nitrogen content through smelting process of LF (ladle furnace) refining furnace |
| WO2019040704A1 (en) | 2017-08-24 | 2019-02-28 | Nucor Corporation | Improved manufacture of low carbon steel |
| CN110846550B (en) * | 2019-12-02 | 2021-08-24 | 抚顺特殊钢股份有限公司 | Intermediate frequency furnace smelting process for nitrogen-containing and niobium-containing high-temperature alloy |
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