JP3463573B2 - Manufacturing method of ultra clean ultra low sulfur steel - Google Patents

Manufacturing method of ultra clean ultra low sulfur steel

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Publication number
JP3463573B2
JP3463573B2 JP24516698A JP24516698A JP3463573B2 JP 3463573 B2 JP3463573 B2 JP 3463573B2 JP 24516698 A JP24516698 A JP 24516698A JP 24516698 A JP24516698 A JP 24516698A JP 3463573 B2 JP3463573 B2 JP 3463573B2
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JP
Japan
Prior art keywords
molten steel
desulfurization
flux
slag
ladle
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Fee Related
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JP24516698A
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Japanese (ja)
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JP2000073116A (en
Inventor
光裕 沼田
善彦 樋口
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Sumitomo Metal Industries Ltd
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Priority to JP24516698A priority Critical patent/JP3463573B2/en
Publication of JP2000073116A publication Critical patent/JP2000073116A/en
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Description

【発明の詳細な説明】 【０００１】 【発明の属する技術分野】本発明は、高清浄極低硫鋼の
溶製方法に関する。 【０００２】 【従来の技術】Ｓ（硫黄）は鋼材、特に厚板材の機械的
特性、耐腐食性を低下させる不純物であり、特にライン
パイプ用鋼では水素誘起割れの原因となる。近年の製品
要求性能はますます向上しており、一層のＳ低減が要求
されている。また、介在物も水素誘起割れの起点となる
ため、介在物低減、すなわち清浄度の向上が同時に要求
される。 【０００３】一方、製鋼工程においてＳを低減する処理
は脱硫処理と呼ばれるが、脱硫処理ではＳを低減するこ
とに加え、短時間でかつ簡便な方法で処理を完了するこ
とも重要である。これは、溶鋼温度の低下や脱硫剤の大
量消費等によるコスト悪化を抑制するためである。 【０００４】製鋼工程での脱硫処理は大きく分けて２種
類の方法がある。一つは脱硫能力の高いスラグを溶鋼表
面に形成させ、溶鋼をガスバブリング等の手法により撹
拌する方法である。攪拌によって、溶鋼とスラグ間の反
応を促進させ、溶鋼中Ｓをスラグ中へ移行させる方法で
ある。この場合のスラグ組成はＣａＯを４０〜６０重量
％含有したＣａＯ−Ａｌ2 Ｏ3 −ＳｉＯ2 系スラグが一
般的に用いられる。このスラグにＭｇＯやＣａＦ2 が含
有させる場合もある。 【０００５】もう一つの方法は、脱硫能力の高いフラッ
クスをガスとともに溶鋼中に吹き込む方法（インジェク
ション法）である。この方法では、溶鋼に吹き込まれた
脱硫フラックスが溶鋼中を浮上していく過程で溶鋼中Ｓ
と反応し、反応後のＳは溶鋼表面のスラグに吸収される
ことによって、脱硫が進行する。 【０００６】いずれの方法においても、下記(1) 式に示
す化学反応で脱硫が進行する。 【０００７】 （ＣａＯ）＋Ｓ＝（ＣａＳ）＋Ｏ (1) ただし、（ＣａＯ）はスラグまたは脱硫フラックス中の
ＣａＯ、（ＣａＳ）はスラグまたは脱硫フラックス中の
ＣａＳ、ＳおよびＯは溶鋼中のＳおよびＯである。 【０００８】(1) 式から明らかなように、脱硫をより進
行させるには、(ア) ＣａＯ量を増大させる、(イ) 溶鋼中
Ｏ濃度を低くする、(ウ) スラグまたはフラックス中のＣ
ａＳ飽和溶解度を増加させる、ことが有効である。具体
的には(ア) に対してはスラグ量または脱硫フラックス量
の増量、(イ) に対してはＡｌ、Ｃａ等での脱酸強化、
(ウ) に対してはスラグや脱硫フラックスへのＣａＦ2 や
アルミナの所定量配合、がある。また、ＣａＦ2 やアル
ミナの配合はスラグや脱硫フラックスの融点を低下させ
る効果もあるため、溶鋼中Ｓとの反応速度を早める効果
もある。 【０００９】上記に基づき、これまでに様々な脱硫方法
が提案されてきた。また、近年では、脱硫に加え清浄度
向上も目的とした技術が提案されている。 【００１０】例えば、特開平４−９９８１１号公報に
は、脱酸した溶鋼にＣａＯを吹き込み、その後Ｃａ合金
を添加する介在物微細化方法が示されている。 【００１１】特開昭５６−９８４１５号公報には、溶鋼
に浸漬させたランスからＡｒガスを吹き込み、脱硫し、
その後Ｃａ合金を添加する方法が提案されている。 【００１２】以上のように、従来技術では、製品性能確
保のため、脱硫とＣａ添加の両方が行われる場合が多
い。近年のＣａ添加方法としては、清浄度向上や工程省
略の観点からＲＨ式などの真空脱ガス装置を用いる技術
がある。 【００１３】例えば、特開平５−２３９５３８号公報に
は、真空槽内溶鋼下降管付近にＣａ添加剤を添加する方
法が開示されており、蒸発性の高いＣａを高歩留まりと
することができるとしている。 【００１４】特開平６−２０２８号公報には、ＲＨ真空
脱ガス装置において、溶鋼下降流にＡｌなどの脱酸剤と
ＣａＯ−Ａｌ2 Ｏ3 などのフラックスを同時に同一箇所
に添加する方法が示されており、これにより、清浄度を
極めて高くできるとしている。 【００１５】 【発明が解決しようとする課題】しかし、上記の従来技
術には下記に示す問題があり、実操業において適用する
のは困難であることが多い。 【００１６】第１に清浄度悪化の問題があった。前記特
開昭５６−９８４１５号公報に開示された技術のよう
に、溶鋼にＡｒガスでバブリングし、スラグとの反応を
促進させることにより脱硫を施す場合、ガスバブリング
の影響によりスラグが溶鋼に巻き込まれ、そのスラグ滴
が溶鋼中に残留し、これが介在物となってしまう。前記
特開平４−９９８１１号公報に開示された技術のよう
に、脱硫フラックスを溶鋼に吹き込む場合でも、吹き込
みガスによるスラグ巻き込みが発生する他、脱硫フラッ
クスの一部が溶鋼に残留してしまう。 【００１７】前記特開平５−２３９５３８号公報に開示
された技術のように、ＲＨ真空脱ガス装置の真空槽内の
溶鋼表面に脱硫フラックスを吹き付ける場合でも、溶鋼
に残留する脱硫フラックス量が増加してしまうという問
題があった。 【００１８】第２に処理時間の問題があった。脱硫反応
を促進するためにはスラグを完全に溶融することが必要
である。しかし、Ａｒガスでバブリングする方法は攪拌
力が不十分でバブリングを開始してもスラグの溶融に時
間を要し、脱硫反応は速やかには進行しない。 【００１９】これに比べて、脱硫フラックスを溶鋼に吹
き込む場合は、脱硫フラックスが溶鋼中Ｓと速やかに反
応し、かつ、フラックスと溶鋼との接触界面積が大きい
ため、フラックスとＳとの反応による脱硫効率は極めて
高い。 【００２０】脱硫反応後、フラックスは溶鋼表面のスラ
グに吸収されるが、スラグ組成が脱硫に適したものでな
ければ、フラックスに伴ってスラグに吸収されたＳが再
び、溶鋼に戻る反応が起きる。この現象は、前記(1) 式
に示す反応が左へ進行することであるが、溶鋼が脱硫フ
ラックス吹き込みのキャリアガスに撹拌されるため、
(1) 式の左側への反応が加速されてしまう。この溶鋼へ
Ｓの戻る反応を抑制するにはスラグ組成も脱硫に適した
組成に制御する必要がある。従って、スラグ組成制御処
理実施に伴い、処理時間の大幅な延長となる。 【００２１】ＲＨ真空脱ガス装置の真空槽内に脱硫フラ
ックスを吹き付けるときの脱硫速度は脱硫フラックスの
吹き付け速度に依存する。しかし、減圧下であることや
設備上の制約から、吹き付け速度に限界があるために、
必ずしも脱硫速度を十分にあげることができない。 【００２２】すなわち、従来の技術では(a) 十分な清浄
度向上が得られない、(b) 極低Ｓ濃度まで脱硫するのに
時間がかかりすぎる、という二つの大きな課題があっ
た。 【００２３】本発明の課題は、より短時間で溶鋼を脱硫
して、Ｓ濃度を極めて低い濃度まで低減すると同時に、
介在物のない高い清浄度を持った鋼を製造する方法を提
供することにある。 【００２４】 【課題を解決するための手段】発明者らは前述した課題
を解決するために、以下の検討をした。脱硫を効率よく
進行させるには、前述したように脱硫フラックスを用い
ることが適当である。しかし、通常の吹き込みでは、結
果的にスラグからＳが溶鋼に戻る復硫反応が起きる。そ
こで、スラグと溶鋼との復硫反応を抑制した状態で、脱
硫フラックスを添加する方法が必要である。 【００２５】上記の観点からすると、スラグと溶鋼との
反応が極めて遅いＲＨ真空脱ガス装置を用いて、溶鋼表
面に脱硫フラックスを吹き付ける方法が最も適してい
る。しかし、単に真空槽内の溶鋼表面に脱硫フラックス
を吹き付けるのみでは、前述した課題、すなわち脱硫時
間が長いこと、清浄度が悪化することなどの問題点は解
決されない。 【００２６】次に、真空槽内の溶鋼表面に脱硫フラック
スを吹き付けた場合、脱硫速度を向上させる方法を検討
した。脱硫を進行させるには前記(1) 式で述べたよう
に、脱酸を強化すればよい。 【００２７】脱酸用のＡｌ添加量を増すと、(ア) アルミ
ナ介在物が多数生成すること、(イ)脱酸を強化するには
Ａｌ濃度をかなり上昇させなければならず、この場合、
Ａｌが製品成分で許される上限を越えてしまうこと、か
らＡｌによる脱酸は不適当である。そこで、微量でも十
分に脱酸を強化できるＣａを用いて脱酸を強化すれば良
いという着想に至った。 【００２８】脱酸にＣａを用いた場合、溶鋼中のアルミ
ナ介在物を低融点・低比重の液体ＣａＯ−Ａｌ2 Ｏ3 系
介在物に形態を変化させられる。この液体介在物は、ア
ルミナ介在物に比較して、溶鋼から浮上分離しやすいた
め、清浄度向上にも有利である。また、介在物をＣａＯ
−Ａｌ2 Ｏ3 系の液体介在物に制御すると、吹き付け添
加された脱硫フラックスと溶鋼中でより合体・巨大化
し、介在物除去促進と脱硫フラックス残留防止の両方に
効果がある。 【００２９】従って、高効率で脱硫すると同時に清浄度
を向上させるには、Ｃａ脱酸された溶鋼に対し、ＲＨ真
空脱ガス装置の真空槽内の溶鋼表面に脱硫フラックスを
吹き付ければ良いと考えた。脱硫フラックスは脱硫能の
高いＣａＯ含有フラックスが適当である。 【００３０】しかし、この方法にはＣａが容易に蒸発し
てしまうという問題がある。Ｃａ脱酸を行った後に、真
空脱ガス処理を行うと、蒸発により急速にＣａ濃度が低
下するため、脱硫フラックス吹き付け中にＣａ添加効果
が低下してしまう。 【００３１】真空槽内の溶鋼に脱硫フラックスとＣａ含
有物質を吹き付けたり、溶鋼上から添加したりする場
合、Ｃａは真空槽内で速やかに蒸発してしまうため、Ｃ
ａ濃度を十分に上げることができず、脱硫効率、清浄度
共に向上しない。 【００３２】脱硫フラックスを取鍋内溶鋼に吹き込み、
Ｃａ含有物質を真空槽内溶鋼に添加した場合も同様の問
題がある。 【００３３】本発明者らは、ＲＨ真空脱ガス装置におけ
る処理中もＣａ濃度を維持する方法を検討した結果、真
空槽内の溶鋼表面に脱硫フラックスを吹き付けつつ、取
鍋内溶鋼中に連続的にＣａ含有物質を添加する方法が最
も効果的であることを見いだした。 【００３４】この方法によれば、Ｃａは大気圧下での添
加となるためＣａは十分取鍋内の溶鋼に溶解し、脱酸に
寄与すると同時に、アルミナ介在物をＣａＯ−Ａｌ2 Ｏ
3 系の液体介在物へ形態を変化させる。次に取鍋の溶鋼
は真空槽内へ移動するが、ここで、Ｃａの一部が蒸発し
ても、溶鋼は十分に脱酸されているために、脱硫フラッ
クスの脱硫効果は極めて大きくなる。さらに、真空槽内
で吹き付け添加された脱硫フラックスは溶鋼流に乗っ
て、取鍋に還流移動するが、取鍋内ではＣａ添加が行わ
れているため、その効果により、さらに脱硫が進行す
る。 【００３５】上記知見に基づいて、完成した本発明の要
旨は、「ＲＨ真空脱ガス装置の真空槽内の溶鋼表面にＣ
ａＯを含有するフラックスを吹き付けつつ、取鍋内の溶
鋼にＣａおよび／またはＣａ合金を添加することを特徴
とする高清浄極低硫鋼の製造方法」にある。 【００３６】 【発明の実施の形態】本発明の実施形態を、転炉とＲＨ
真空脱ガス装置を用いて実施する場合を例に説明する。
転炉処理終了後、溶鋼を取鍋へ出鋼する。取鍋をＲＨ真
空脱ガス装置へ移動し真空脱ガス処理を開始する。 【００３７】本発明はスラグを活用しないので、特にそ
の組成を制御する必要はないが、より脱硫効率を高める
ために、出鋼時スラグ改質を実施したり、バブリング等
でスラグ改質を実施してもよい。また、スラグ組成はＣ
ａＯ−Ａｌ2 Ｏ3 系スラグが望ましく、より好ましく
は、重量比でＣａＯ／Ａｌ2 Ｏ3 ＝０．９〜２．５、Ｆ
ｅＯ＋ＭｎＯ濃度２重量％以下としておくことが望まし
い。 【００３８】真空脱ガス処理開始後、本発明の処理を開
始する。ただし、本発明の処理の前に、ＲＨ真空脱ガス
装置にてＡｌと酸素ガスを用いた昇温処理を施しても良
い。また、Ｃａ脱酸をより強化するために、製品規格上
許される範囲でＡｌ濃度を高めておいてもよい。 【００３９】図１はＲＨ真空脱ガス装置における本発明
の処理概要を示す縦断面図である。同図において符号１
は真空槽、２は取鍋、３は溶鋼、４は上昇管、５は下降
管、６は真空槽の排気管、７はＣａＯを含むフラックス
を吹き付ける上吹きランス、８はフラックス、９はＣａ
および／またはＣａ合金を内包するワイヤ、１０はワイ
ヤ供給装置である。 【００４０】同図に示すように、上吹きランス７からフ
ラックス８を真空槽１内の溶鋼３の表面に吹き付け、同
時に取鍋２内の溶鋼３にＣａおよび／またはＣａ合金を
添加する。同図においてはＣａおよび／またはＣａ合金
はこれを内包するワイヤとして、ワイヤ供給装置１０か
ら供給される例を示している。 【００４１】フラックス８は脱硫能の高いＣａＯを含有
していればよいが、ＣａＯの含有濃度は４０重量％以上
が望ましい。この濃度未満であると、脱硫効率低下やフ
ラックス量増大に伴う処理時間延長となってしまうから
である。また、より効率を高めるためにフラックス８に
ＣａＦ2 を含有させたり、ＭｇＯを含有させてもよい。 【００４２】フラックスの吹き付け速度ＶF （ｋｇ／
（溶鋼トン・分））は０．５≦ＶF ≦１．３が望まし
い。ＶF が０．５未満となると、脱硫に要する時間が長
くなり、１．３を超えて高くなると、フラックス吹き付
け時の溶鋼からのスプラッシュ発生量が大きくなってし
まう。 【００４３】取鍋へ添加するＣａおよび／またはＣａ合
金は、金属Ｃａや、ＣａＳｉ、ＣａＡｌなどのＣａ合金
またはこれらの混合物でもよい。また、脱硫をさらに促
進させるために、Ｃａおよび／またはＣａ合金にＣａＯ
を含有するフラックスを混合してもよい。以下では、Ｃ
ａおよび／またはＣａ合金を単にＣａともいう。 【００４４】Ｃａの添加速度Ｖc （ｋｇ／（溶鋼トン・
分））はＣａ純分換算で、０．０１≦Ｖc ≦０．１が望
ましい。Ｖc が０．０１未満となると、Ｃａ濃度を脱硫
に効果を与えるに十分な濃度に上げるのに、時間がかか
りすぎてしまう。Ｖc が０．１を越えて高くなると、Ｃ
ａ添加中に一部気化したＣａ蒸気による撹拌効果が強く
なりすぎ、スラグの巻き込み、スラグ−溶鋼間の反応の
進行等の問題が生じてしまう。さらに、Ｖc が０．１を
越えて高くなると、Ｃａ濃度が高くなりすぎてＣａＳ介
在物が新たに生成し、清浄度を悪化させてしまう。 【００４５】図２は取鍋の溶鋼面位置でのＲＨ真空脱ガ
ス装置との位置関係を示す水平断面図である。同図にお
いて図１と同一部品は同一符号で表す。 【００４６】Ｃａの添加位置は取鍋内で安定的に添加で
きる位置であればよく、図２の好ましい添加領域として
示す範囲がよい。すなわち、ＲＨ真空脱ガス装置の上昇
管および下降管の直下および周辺を避けた位置である。
上昇管の直近にＣａを添加すると、Ｃａが溶鋼と十分反
応するまえに、真空槽内にＣａが移動してしまうため、
Ｃａが活発に蒸発してしまい、溶鋼中のＣａ濃度をあげ
ることができない。一方、下降管の直近にＣａを添加す
ると、Ｃａ蒸気が下降流と衝突し、溶鋼流動が減衰し、
溶鋼の撹拌も失われるため、脱硫、脱酸、介在物形態制
御等、全ての反応が不安定になる恐れがある。また、上
昇管、下降管の外側（取鍋壁との間）も作業性、反応の
均一性等の点から好ましい位置ではない。 【００４７】Ｃａの添加方法としては、連続的に添加で
きる方法であればいかなる方法でもよいが、好ましくは
Ｃａおよび／またはＣａ合金を内包した金属ワイヤを溶
鋼に送り込むワイヤ−フィ−ディング法が望ましい。こ
の方法は、吹き込み法と違いＡｒガスを用いないため、
不要なガス撹拌によるスラグ巻き込みを回避できるから
である。 【００４８】Ｃａの添加は脱硫フラックス吹き付け開始
と同時に開始すればよいが、Ｃａの効果をより高めるに
は、Ｃａ添加を脱硫フラックス吹き付け２分前程度から
開始すればより望ましい。 【００４９】処理時の真空度は１５０Ｔｏｒｒ以下が望
ましい。真空度が１５０Ｔｏｒｒを超えて高くなると、
溶鋼の環流が遅くなり、処理時間が長くなってしまうか
らである。 【００５０】本発明に従って、ＣａＯを含有するフラッ
クスを真空槽内の溶鋼に吹き付けつつ、同時に取鍋内の
溶鋼にＣａ（同図の例ではワイヤ−フィ−ディング法に
よるＣａＳｉ合金）を連続的に添加した場合の溶鋼中Ｓ
濃度の推移と清浄度の推移を調査した。併せて、従来方
法による脱硫処理の溶鋼中Ｓ濃度、清浄度を調査した。 【００５１】図３は本発明および従来方法による脱硫処
理の処理時間と溶鋼中Ｓの濃度の関係を示すグラフであ
る。 【００５２】図４は本発明および従来方法による脱硫処
理の処理時間と溶鋼中の介在物個数の関係を示すグラフ
である。同図は処理前の介在物個数を１とし、各処理時
間経過後の採取サンプルの介在物個数を指数として示し
たものである。 【００５３】図３および図４には、本発明の方法に従
い、真空槽内の溶鋼にフラックスのみを吹き付け、取鍋
溶鋼にはＣａを添加した場合（●）、真空槽内の溶鋼に
脱硫フラックスとＣａを吹き付け、取鍋には何も添加し
ない場合（▼）、大気圧下で取鍋中の溶鋼に脱硫フラッ
クスとＣａを吹き込んだ場合（■）、および下降管付近
でフラックスとＣａ合金を混合したものをワイヤフィー
ディングで添加した場合(○）を示す。 【００５４】図３および図４から明らかなように、本発
明のフラックスとＣａとを全く別の位置から同時に添加
する方法は、他法に比較して、高速で脱硫が進行すると
ともに、溶鋼中介在物が急速に除去される。 【００５５】 【実施例】転炉で脱炭後、取鍋中の溶鋼２５０トンにＡ
ｌを添加し、スラグにはＡｌ含有炭酸カルシウムを添加
し、スラグ中のＦｅＯ、ＭｎＯ濃度を合計で１．４重量
％とした。その後、取鍋をＲＨ真空脱ガス装置に移動
し、真空脱ガス処理を開始した。 【００５６】処理開始後、溶鋼にＡｌを添加し、真空槽
内の溶鋼表面に酸素ガスを吹き付けて溶鋼温度を上昇さ
せた。この昇温処理後、真空度を１Ｔｏｒｒとした。本
発明開始前の溶鋼中Ａｌ濃度は０．０４５重量％、Ｍｎ
濃度１．３重量％、Ｃ濃度０．０５重量％である。上記
の処理後、本発明例と比較例の試験を実施した。 【００５７】本発明例は以下の処理を行った。真空槽内
で溶鋼に吹き付けた脱硫フラックスは、８０重量％Ｃａ
Ｏ、１０重量％ＣａＦ2 、１０重量％ＭｇＯであり、吹
き付け速度は０．５〜１．３ｋｇ／トン／分とした。取
鍋内の溶鋼に添加したＣａとしては、３０重量％Ｃａ−
７０重量％Ｓｉなる組成のＣａＳｉ合金で、ワイヤーフ
ィーディング法により添加した。添加速度はＣａ純分換
算で０．０５〜０．１ｋｇ／（トン・分）とした。処理
時間は１０分とした。 【００５８】脱硫処理後、最終合金調整を行った後、真
空脱ガス処理を終了し、連続鋳造機によって鋳造した。
鋳造されたスラブを圧延し、ラインパイプに加工し、Ｎ
ＡＣＥ条件の耐サワーガス性能評価試験によって性能を
確認した。なお製品はＡＰＩ規格Ｘ７０である。 【００５９】表１は、処理前、処理後、製品におけるＳ
濃度、清浄度指数、Ｎ濃度、ＮＡＣＥ試験の結果を示
す。ＮＡＣＥ試験結果は、○が合格、×は不合格を示す
（本発明例：試験Ｎｏ．１〜４）。 【００６０】比較例として、Ｃａを取鍋溶鋼に添加せず
に、真空槽内溶鋼表面に脱硫フラックスを１０分間吹き
付けた場合（比較例(1) 試験Ｎｏ．５〜８）と、真空脱
ガス処理中の取鍋のスラグ中にＣａＯ／Ａｌ2 Ｏ3 ＝
１．５なる組成のスラグを上から添加し、取鍋の溶鋼に
同様の脱硫フラックスを１０分間吹き込んだ場合（比較
例(2) 試験Ｎｏ．９〜１２）の結果を表１に併せて示
す。 【００６１】表１に示すように、本発明例（Ｎｏ．１〜
４）では、Ｓ濃度は１〜２ｐｐｍまで低下したのに比
べ、同じ１０分間の処理をした比較例(1) 、(2) とも、
Ｓ濃度は６ｐｐｍ以上にとどまった。 【００６２】介在物については、本発明例が、０．１〜
０．２の介在物指数であったのに対し、比較例(1) (2)
とも介在物指数は０．５以上で清浄度は不十分であっ
た。すなわち、比較例(1) ではＣａをせず、十分な清浄
度向上の効果がないため、介在物指数が高くなったと考
えられる。比較例(2) では、取鍋内にフラックスを投入
するためのキャリアガスによって、上層スラグと溶鋼と
が攪拌され、取鍋スラグから介在物を巻き込んだため、
比較例(1) よりも介在物指数が高くなり、キャリアガス
攪拌による大気の巻き込みもあって、窒素濃度も比較例
(1) より高くなったと考えられる。 【００６３】これに対して、本発明方法では溶鋼中のＳ
濃度、酸素濃度を著しく低減できるため、処理中に脱窒
反応が進行し窒素濃度も低減できることがわかった。 【００６４】本発明の方法ではそのほか、介在物の形態
制御も同時に行われるため、脱硫後にＣａ処理を施さな
くてもよいという利点がある。 【００６５】以上のように、本発明方法による鋼を用い
れば、ラインパイプ等に加工したときにも耐サワーガス
性能が確保できることがわかった。 【００６６】 【表１】 【００６７】 【発明の効果】本発明によれば、高能率で脱硫と介在物
の低減を行うことができ、介在物の形態制御も同時に行
われるため、後工程でＣａ処理を施さなくても高清浄極
低硫の耐サワーガス鋼の製造が可能である。Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a very clean ultra-low sulfur steel. [0002] S (sulfur) is an impurity which lowers the mechanical properties and corrosion resistance of steel materials, especially thick plates, and causes hydrogen-induced cracking particularly in linepipe steels. In recent years, required performance of products has been further improved, and further reduction of S is required. In addition, inclusions also serve as starting points of hydrogen-induced cracking, so that reduction of inclusions, that is, improvement of cleanliness, is also required. [0003] On the other hand, the treatment for reducing S in the steel making process is called desulfurization treatment. In the desulfurization treatment, it is important to complete the treatment in a short time and a simple method in addition to reducing S. This is to suppress cost deterioration due to a decrease in the temperature of the molten steel, a large consumption of the desulfurizing agent, and the like. [0004] Desulfurization treatment in the steel making process is roughly classified into two types. One is a method in which slag having a high desulfurization ability is formed on the surface of molten steel, and the molten steel is stirred by a method such as gas bubbling. In this method, the reaction between the molten steel and the slag is promoted by stirring to transfer S in the molten steel into the slag. The slag composition in the CaO-Al 2 O 3 -SiO 2 system slag containing 40-60 wt% of CaO is generally used. In some cases, this slag contains MgO or CaF 2 . [0005] Another method is a method of injecting a flux having a high desulfurization ability into molten steel together with a gas (injection method). According to this method, the desulfurization flux blown into the molten steel floats in the molten steel and causes
And S after the reaction is absorbed by the slag on the surface of the molten steel, whereby desulfurization proceeds. In either method, desulfurization proceeds by a chemical reaction represented by the following formula (1). (CaO) + S = (CaS) + O (1) where (CaO) is CaO in slag or desulfurization flux, (CaS) is CaS in slag or desulfurization flux, S and O are S and M in molten steel. O. As is clear from equation (1), desulfurization can be further promoted by (a) increasing the amount of CaO, (a) decreasing the O concentration in molten steel, and (c) removing C in slag or flux.
It is effective to increase the aS saturation solubility. Specifically, for (a), the amount of slag or desulfurization flux is increased; for (a), deoxidation with Al, Ca, etc. is strengthened,
For (c), there is a prescribed amount of CaF 2 or alumina mixed with slag or desulfurization flux. Further, since the addition of CaF 2 and alumina also has the effect of lowering the melting point of slag and desulfurization flux, it also has the effect of increasing the reaction rate with S in molten steel. Based on the above, various desulfurization methods have been proposed so far. In recent years, techniques for improving cleanliness in addition to desulfurization have been proposed. For example, Japanese Patent Application Laid-Open No. 4-99811 discloses a method for refining inclusions by blowing CaO into deoxidized molten steel and then adding a Ca alloy. JP-A-56-98415 discloses that Ar gas is blown from a lance immersed in molten steel, desulfurized,
Thereafter, a method of adding a Ca alloy has been proposed. As described above, in the prior art, both desulfurization and addition of Ca are often performed in order to ensure product performance. As a Ca addition method in recent years, there is a technique using a vacuum degassing device such as an RH type from the viewpoint of improving cleanliness and omitting steps. For example, Japanese Patent Application Laid-Open No. 5-239538 discloses a method of adding a Ca additive to the vicinity of a molten steel downcomer in a vacuum chamber. I have. Japanese Patent Application Laid-Open No. 6-2028 discloses a method in which a deoxidizing agent such as Al and a flux such as CaO-Al 2 O 3 are simultaneously added to the same location in a molten steel descending flow in an RH vacuum degassing apparatus. According to this, the cleanliness can be extremely increased. [0015] However, the above-mentioned prior art has the following problems, and it is often difficult to apply it in actual operation. First, there is a problem of deterioration in cleanliness. As in the technique disclosed in JP-A-56-98415, when desulfurization is performed by bubbling molten steel with Ar gas to accelerate the reaction with slag, the slag is entrained in the molten steel due to the effect of gas bubbling. The slag droplets remain in the molten steel and become inclusions. Even when the desulfurization flux is blown into the molten steel as in the technique disclosed in Japanese Patent Application Laid-Open No. 4-99811, slag is entrained by the blown gas, and a part of the desulfurization flux remains in the molten steel. Even when the desulfurization flux is sprayed on the surface of the molten steel in the vacuum chamber of the RH vacuum degassing apparatus as in the technique disclosed in Japanese Patent Application Laid-Open No. Hei 5-239538, the amount of the desulfurization flux remaining in the molten steel increases. There was a problem that would. Second, there is a problem of processing time. In order to accelerate the desulfurization reaction, it is necessary to completely melt the slag. However, in the method of bubbling with Ar gas, the stirring power is insufficient, so that even if bubbling is started, it takes time to melt the slag, and the desulfurization reaction does not proceed quickly. On the other hand, when the desulfurized flux is blown into the molten steel, the desulfurized flux reacts quickly with S in the molten steel, and the contact area between the flux and the molten steel is large. The desulfurization efficiency is extremely high. After the desulfurization reaction, the flux is absorbed by the slag on the surface of the molten steel, but if the slag composition is not suitable for desulfurization, a reaction occurs in which S absorbed by the slag with the flux returns to the molten steel again. . This phenomenon is that the reaction shown in the above formula (1) proceeds to the left, but because the molten steel is stirred by the carrier gas injected with the desulfurization flux,
The reaction to the left side of equation (1) is accelerated. In order to suppress the reaction of returning S to the molten steel, the slag composition also needs to be controlled to a composition suitable for desulfurization. Therefore, the processing time is greatly extended with the execution of the slag composition control processing. The desulfurization speed at the time of blowing the desulfurization flux into the vacuum chamber of the RH vacuum degassing apparatus depends on the blowing speed of the desulfurization flux. However, because there is a limit to the spray speed due to reduced pressure and restrictions on equipment,
The desulfurization rate cannot always be sufficiently increased. That is, the conventional techniques have two major problems: (a) a sufficient improvement in cleanliness cannot be obtained, and (b) it takes too much time to desulfurize to an extremely low S concentration. An object of the present invention is to desulfurize molten steel in a shorter time to reduce the S concentration to an extremely low concentration,
An object of the present invention is to provide a method for producing a steel having a high cleanliness without inclusions. Means for Solving the Problems The present inventors have made the following studies in order to solve the above-mentioned problems. In order to efficiently advance desulfurization, it is appropriate to use a desulfurization flux as described above. However, the normal blowing results in a resulfurization reaction in which S returns from the slag to the molten steel. Therefore, a method of adding a desulfurization flux while suppressing the resulfurization reaction between slag and molten steel is required. In view of the above, the most suitable method is to use a RH vacuum degassing apparatus in which the reaction between slag and molten steel is extremely slow, and to blow desulfurization flux onto the surface of the molten steel. However, simply spraying a desulfurization flux onto the surface of molten steel in a vacuum chamber does not solve the above-mentioned problems, that is, problems such as a long desulfurization time and deterioration in cleanliness. Next, a method for improving the desulfurization rate when a desulfurization flux was sprayed on the surface of the molten steel in the vacuum chamber was studied. Desulfurization can be advanced by enhancing the deoxidation as described in the above equation (1). When the amount of Al added for deoxidation is increased, (a) a large number of alumina inclusions are formed, and (b) the Al concentration must be considerably increased in order to strengthen the deoxidation.
Since Al exceeds the upper limit allowed for the product component, deoxidation with Al is inappropriate. Then, the idea was reached that the deoxidation should be enhanced using Ca, which can sufficiently enhance the deoxidation even in a small amount. When Ca is used for deoxidation, the form of alumina inclusions in molten steel can be changed to liquid CaO-Al 2 O 3 inclusions having a low melting point and a low specific gravity. The liquid inclusions are easier to float and separate from the molten steel than the alumina inclusions, and thus are advantageous in improving cleanliness. In addition, the inclusion is CaO
Controlling to -Al 2 O 3 -based liquid inclusions makes the desulfurization flux added by spraying more integrated and larger in the molten steel, and is effective in both promoting removal of inclusions and preventing residual desulfurization flux. Therefore, it is considered that desulfurization flux should be sprayed on the surface of the molten steel in the vacuum chamber of the RH vacuum degassing apparatus for the desulfurization with high efficiency and at the same time to improve the cleanliness at the same time. Was. As the desulfurization flux, a CaO-containing flux having a high desulfurization ability is suitable. However, this method has a problem that Ca is easily evaporated. If vacuum degassing is performed after performing Ca deoxidation, the Ca concentration rapidly decreases due to evaporation, so that the effect of adding Ca during the desulfurization flux spraying decreases. When a desulfurization flux and a Ca-containing substance are sprayed on molten steel in a vacuum chamber or added from the molten steel, Ca evaporates quickly in the vacuum chamber.
a The concentration cannot be sufficiently increased, and neither the desulfurization efficiency nor the cleanliness is improved. The desulfurization flux is blown into the molten steel in the ladle,
The same problem occurs when a Ca-containing substance is added to molten steel in a vacuum chamber. The present inventors have studied a method for maintaining the Ca concentration even during the treatment in the RH vacuum degassing apparatus. As a result, the desulfurization flux was sprayed on the surface of the molten steel in the vacuum chamber, and the molten steel in the ladle was continuously cooled. It has been found that the method of adding a Ca-containing substance to the mixture is most effective. According to this method, since Ca is added under atmospheric pressure, Ca sufficiently dissolves in the molten steel in the ladle and contributes to deoxidation, and at the same time, alumina inclusions are reduced to CaO-Al 2 O.
Changes the form to three- system liquid inclusions. Next, the molten steel in the ladle moves into the vacuum chamber. Here, even if a portion of Ca evaporates, the desulfurization effect of the desulfurization flux becomes extremely large because the molten steel is sufficiently deoxidized. Furthermore, the desulfurization flux spray-added in the vacuum chamber rides on the molten steel flow and moves back to the ladle, but since Ca is added in the ladle, desulfurization further proceeds due to the effect. Based on the above findings, the gist of the completed present invention is as follows: "The surface of molten steel in the vacuum chamber of the RH vacuum degassing apparatus has C
A method for producing a highly clean ultra-low sulfur steel characterized by adding Ca and / or a Ca alloy to molten steel in a ladle while spraying a flux containing aO. Embodiments of the present invention will be described with reference to a converter and an RH.
The case of using a vacuum degassing apparatus will be described as an example.
After the completion of the converter process, the molten steel is tapped into a ladle. The ladle is moved to the RH vacuum degassing device to start the vacuum degassing process. Since the present invention does not utilize slag, it is not particularly necessary to control the composition thereof. However, in order to further increase the desulfurization efficiency, slag reforming at the time of tapping or slag reforming by bubbling is performed. May be. The slag composition is C
aO-Al 2 O 3 based slag is desirable, more preferably, CaO / Al 2 O 3 = 0.9~2.5 weight ratio, F
It is desirable that the concentration of eO + MnO be 2% by weight or less. After the start of the vacuum degassing process, the process of the present invention is started. However, before the treatment of the present invention, a temperature raising treatment using Al and oxygen gas may be performed in an RH vacuum degassing apparatus. Further, in order to further enhance Ca deoxidation, the Al concentration may be increased within a range permitted by product specifications. FIG. 1 is a longitudinal sectional view showing an outline of the processing of the present invention in the RH vacuum degassing apparatus. In FIG.
Is a vacuum tank, 2 is a ladle, 3 is molten steel, 4 is an ascending pipe, 5 is a descending pipe, 6 is an exhaust pipe of the vacuum tank, 7 is an upper blowing lance for blowing a flux containing CaO, 8 is a flux, and 9 is Ca
And / or a wire 10 containing a Ca alloy is a wire supply device. As shown in the figure, a flux 8 is sprayed from the top blowing lance 7 onto the surface of the molten steel 3 in the vacuum chamber 1, and simultaneously, Ca and / or a Ca alloy is added to the molten steel 3 in the ladle 2. FIG. 1 shows an example in which Ca and / or a Ca alloy is supplied from the wire supply device 10 as a wire containing the same. The flux 8 may contain CaO having a high desulfurization ability, and the CaO concentration is desirably 40% by weight or more. If the concentration is less than this, the processing time is prolonged due to a decrease in desulfurization efficiency and an increase in the amount of flux. Further, in order to further increase the efficiency, the flux 8 may contain CaF 2 or MgO. The flux blowing speed V F (kg /
(Molten steel ton-min)) is desirably 0.5 ≦ V F ≦ 1.3. If V F is less than 0.5, the time required for the desulfurization becomes longer, becomes higher than 1.3, splash generation amount from the molten steel during blowing the flux is increased. The Ca and / or Ca alloy added to the ladle may be metallic Ca, a Ca alloy such as CaSi, CaAl, or a mixture thereof. Further, in order to further promote desulfurization, CaO and / or Ca alloy is added to CaO.
May be mixed. In the following, C
a and / or a Ca alloy is also simply referred to as Ca. Ca addition rate V c (kg / (ton of molten steel
Min)) is desirably 0.01 ≦ V c ≦ 0.1 in terms of Ca pure content. If Vc is less than 0.01, it takes too much time to raise the Ca concentration to a concentration sufficient to exert an effect on desulfurization. When V c rises above 0.1, C
The stirring effect of the Ca vapor partially vaporized during the addition of a becomes too strong, causing problems such as slag entrainment and progress of reaction between slag and molten steel. Further, when V c is increased beyond 0.1, Ca concentration becomes too high CaS inclusions newly generated, are deteriorated cleanliness. FIG. 2 is a horizontal sectional view showing the positional relationship between the molten steel surface of the ladle and the RH vacuum degassing apparatus. In the figure, the same parts as those in FIG. 1 are represented by the same reference numerals. The Ca can be added at any position as long as it can be added stably in the ladle, and the range shown as a preferable addition region in FIG. 2 is good. That is, it is a position avoiding immediately below and around the riser and the downcomer of the RH vacuum degassing device.
If Ca is added in the immediate vicinity of the riser, Ca moves into the vacuum chamber before Ca sufficiently reacts with the molten steel.
Ca evaporates actively, and the Ca concentration in the molten steel cannot be increased. On the other hand, when Ca is added in the immediate vicinity of the downcomer, Ca vapor collides with the downflow, the molten steel flow is attenuated,
Since the stirring of the molten steel is also lost, all the reactions such as desulfurization, deoxidation, and inclusion morphology control may be unstable. Further, the outside of the riser pipe and the lower pipe (between the ladle wall) is not a preferable position in terms of workability, reaction uniformity, and the like. As a method of adding Ca, any method can be used as long as it can be added continuously, but a wire feeding method of feeding a metal wire containing Ca and / or a Ca alloy into molten steel is preferable. . Since this method does not use Ar gas unlike the blowing method,
This is because slag entrainment due to unnecessary gas stirring can be avoided. The addition of Ca may be started at the same time as the start of the desulfurization flux spraying. However, in order to further enhance the effect of Ca, it is more preferable to start the Ca addition about two minutes before the spraying of the desulfurization flux. The degree of vacuum at the time of processing is desirably 150 Torr or less. When the degree of vacuum rises above 150 Torr,
This is because the reflux of the molten steel becomes slow, and the processing time becomes long. According to the present invention, a flux containing CaO is sprayed onto the molten steel in the vacuum chamber, and simultaneously, Ca (in the example shown in the figure, a CaSi alloy by the wire-feeding method) is continuously applied to the molten steel in the ladle. S in molten steel when added
Changes in concentration and cleanliness were investigated. In addition, the S concentration and cleanliness in the molten steel in the desulfurization treatment by the conventional method were investigated. FIG. 3 is a graph showing the relationship between the treatment time of desulfurization treatment according to the present invention and the conventional method and the concentration of S in molten steel. FIG. 4 is a graph showing the relationship between the treatment time of desulfurization treatment according to the present invention and the conventional method and the number of inclusions in molten steel. In the figure, the number of inclusions before the treatment is set to 1, and the number of inclusions of the sample collected after each treatment time is shown as an index. FIG. 3 and FIG. 4 show that, according to the method of the present invention, when only the flux is sprayed on the molten steel in the vacuum chamber and Ca is added to the molten steel in the ladle (●), the desulfurization flux is applied to the molten steel in the vacuum chamber. And Ca are sprayed and nothing is added to the ladle (▼), desulfurization flux and Ca are blown into the molten steel in the ladle under atmospheric pressure (■), and flux and Ca alloy are The case where the mixture was added by wire feeding is shown (示 す). As is clear from FIGS. 3 and 4, the method of the present invention for simultaneously adding the flux and Ca from completely different positions allows desulfurization to proceed at a higher speed than the other methods, The entity is rapidly removed. [Example] After decarburization in a converter, A was added to 250 tons of molten steel in a ladle.
l, calcium carbonate containing Al was added to the slag, and the total concentration of FeO and MnO in the slag was 1.4% by weight. Thereafter, the ladle was moved to an RH vacuum degassing device, and a vacuum degassing process was started. After the start of the treatment, Al was added to the molten steel, and oxygen gas was blown onto the surface of the molten steel in the vacuum chamber to raise the temperature of the molten steel. After this temperature raising treatment, the degree of vacuum was set to 1 Torr. Al concentration in molten steel before the start of the present invention is 0.045% by weight, Mn
The concentration is 1.3% by weight and the C concentration is 0.05% by weight. After the above treatment, tests of the present invention example and the comparative example were performed. In the example of the present invention, the following processing was performed. The desulfurization flux sprayed on the molten steel in the vacuum chamber is 80% by weight Ca
O, 10% by weight CaF 2 , and 10% by weight MgO, and the spraying rate was 0.5 to 1.3 kg / ton / min. As Ca added to the molten steel in the ladle, 30% by weight Ca-
A CaSi alloy having a composition of 70% by weight Si was added by a wire feeding method. The addition rate was 0.05 to 0.1 kg / (ton-minute) in terms of Ca pure content. The processing time was 10 minutes. After the desulfurization treatment, the final alloy was adjusted, the vacuum degassing treatment was terminated, and casting was performed by a continuous casting machine.
The cast slab is rolled, processed into line pipe,
The performance was confirmed by a sour gas resistance performance evaluation test under ACE conditions. The product is API standard X70. Table 1 shows S before and after treatment.
The results of the concentration, cleanliness index, N concentration, and NACE test are shown. In the NACE test results, ○ indicates pass and X indicates rejection (Example of the present invention: Test Nos. 1 to 4). As a comparative example, when Ca was not added to the molten steel and a desulfurization flux was sprayed on the surface of the molten steel in the vacuum tank for 10 minutes (Comparative Example (1) Test Nos. 5 to 8), CaO / Al 2 O 3 = in slag of ladle during treatment
A slag having a composition of 1.5 was added from above, and the same desulfurization flux was blown into molten steel in a ladle for 10 minutes (Comparative Example (2), Test Nos. 9 to 12). . As shown in Table 1, the examples of the present invention (No.
In 4), the S concentration was reduced to 1 to 2 ppm, whereas in Comparative Examples (1) and (2), the same treatment was performed for 10 minutes.
The S concentration remained at 6 ppm or more. Regarding inclusions, the examples of the present invention show that
While the inclusion index was 0.2, the comparative examples (1) and (2)
In both cases, the inclusion index was 0.5 or more, and the cleanliness was insufficient. That is, in Comparative Example (1), Ca was not added, and there was no effect of sufficiently improving the cleanliness. Therefore, it is considered that the inclusion index increased. In Comparative Example (2), the upper layer slag and the molten steel were stirred by the carrier gas for introducing the flux into the ladle, and inclusions were involved from the ladle slag,
The inclusion index was higher than that of Comparative Example (1).
(1) It is considered higher. On the other hand, in the method of the present invention, S
It was found that since the concentration and the oxygen concentration could be significantly reduced, the denitrification reaction proceeded during the treatment and the nitrogen concentration could also be reduced. In addition, the method of the present invention also has the advantage that it is not necessary to perform Ca treatment after desulfurization because the form control of inclusions is also performed at the same time. As described above, it was found that the use of the steel according to the method of the present invention can ensure sour gas resistance even when processed into a line pipe or the like. [Table 1] According to the present invention, desulfurization and reduction of inclusions can be performed with high efficiency, and morphological control of inclusions is performed at the same time. It is possible to manufacture high-purity extremely low-sulfur resistant sour gas steel.

【図面の簡単な説明】 【図１】本発明の処理概要を示す縦断面図である。 【図２】取鍋の溶鋼面位置でのＲＨ真空脱ガス装置との
位置関係を示す水平断面図である。 【図３】本発明および従来方法による脱硫処理の処理時
間と溶鋼中Ｓの濃度の関係を示すグラフである。 【図４】本発明および従来方法による脱硫処理の処理時
間と溶鋼中の介在物個数の関係を示すグラフである。 【符号の説明】 １ 真空槽 ２ 取鍋 ３ 溶鋼 ４ 上昇管 ５ 下降管 ６ 排気管 ７ 上吹きランス ８ フラックス ９ ワイヤ １０ ワイヤ供給装置BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view showing an outline of processing of the present invention. FIG. 2 is a horizontal sectional view showing a positional relationship between a ladle and a RH vacuum degassing apparatus at a molten steel surface position. FIG. 3 is a graph showing the relationship between the treatment time of desulfurization treatment according to the present invention and the conventional method and the concentration of S in molten steel. FIG. 4 is a graph showing the relationship between the treatment time of desulfurization treatment according to the present invention and the conventional method and the number of inclusions in molten steel. [Description of Signs] 1 Vacuum tank 2 Ladle 3 Molten steel 4 Up pipe 5 Down pipe 6 Exhaust pipe 7 Top blowing lance 8 Flux 9 Wire 10 Wire supply device

Claims (1)

(57)【特許請求の範囲】 【請求項１】 ＲＨ真空脱ガス装置の真空槽内の溶鋼表
面にCaO を含有するフラックスを吹き付けつつ、取鍋内
の溶鋼にCaおよび／またはCa合金を内包した金属ワイヤ
を送り込むことを特徴とする高清浄極低硫鋼の製造方
法。(57) [Claim 1] Ca and / or Ca alloy is included in molten steel in a ladle while blowing a flux containing CaO on the surface of molten steel in a vacuum chamber of an RH vacuum degassing apparatus. A method for producing a highly clean ultra-low sulfur steel, comprising feeding a metal wire .
JP24516698A 1998-08-31 1998-08-31 Manufacturing method of ultra clean ultra low sulfur steel Expired - Fee Related JP3463573B2 (en)

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JP3417311B2 (en) * 1998-09-04 2003-06-16 住友金属工業株式会社 Method for producing highly clean HIC steel
JP3885387B2 (en) * 1998-10-20 2007-02-21 Ｊｆｅスチール株式会社 Method for producing ultra-low sulfur steel with excellent cleanability
JP3577988B2 (en) * 1999-04-09 2004-10-20 住友金属工業株式会社 Manufacturing method of low Al ultra low sulfur steel
JP3577989B2 (en) * 1999-04-09 2004-10-20 住友金属工業株式会社 High-speed desulfurization of molten steel
JP3646633B2 (en) * 2000-08-10 2005-05-11 住友金属工業株式会社 High cleanability ultra-low sulfur steel and its manufacturing method
CN103305659B (en) 2012-03-08 2016-03-30 宝山钢铁股份有限公司 The non-oriented electromagnetic steel sheet of excellent magnetic and calcium treating method thereof
JP6273947B2 (en) * 2014-03-20 2018-02-07 新日鐵住金株式会社 Desulfurization treatment method for molten steel
CN104232848B (en) * 2014-10-15 2016-08-17 中冶南方工程技术有限公司 Add heat preserving agent for RH operation and feed the system of silk
JP6780695B2 (en) * 2018-01-05 2020-11-04 Ｊｆｅスチール株式会社 Melting method of ultra-low sulfur low nitrogen steel

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