JP4085898B2

JP4085898B2 - Melting method of low carbon high manganese steel

Info

Publication number: JP4085898B2
Application number: JP2003185295A
Authority: JP
Inventors: 英寿松野; 孝彦前田; 剛村井; 良輝菊地; 栄司櫻井
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-06-27
Filing date: 2003-06-27
Publication date: 2008-05-14
Anticipated expiration: 2023-06-27
Also published as: JP2005015890A

Description

【０００１】
【発明の属する技術分野】
本発明は、真空脱ガス設備の真空槽内の溶鋼に酸素ガス等の酸素源を供給し、溶鋼に対して真空脱炭処理を施して低炭素高マンガン鋼を溶製する方法に関し、詳しくは、マンガンの酸化を抑え、効率良く脱炭することによって安価に低炭素高マンガン鋼を溶製する方法に関するものである。
【０００２】
【従来の技術】
近年、鉄鋼材料は、その用途の多様化に伴い、より苛酷な環境下で使用されることが多くなり、材料特性の高性能化が従来にも増して求められている。このような状況下、構造物の軽量化を目的として、高い引張強さと高い加工性とを両立させた低炭素高マンガン鋼が開発され、ラインパイプ用鋼板や自動車用鋼板等として使用されるようになった。ここで、低炭素高マンガン鋼とは、炭素濃度が０．０５質量％以下で、マンガン濃度が１．０質量％以上の鋼のことである。
【０００３】
溶鋼中のマンガン濃度を調整するために用いる安価なマンガン源は、マンガン鉱石や高炭素フェロマンガンであり、低炭素高マンガン鋼を溶製する際に、転炉での溶銑の脱炭精錬時に転炉内にマンガン鉱石を投入してマンガン鉱石を還元したり、転炉からの出鋼時に高炭素フェロマンガンを溶鋼に添加することによって、溶鋼中のマンガン濃度を所定値まで上昇させることは可能であるが、これらの安価なマンガン源を使用した場合には、転炉出鋼時点で十分に炭素濃度を低減させることができず、溶鋼中炭素濃度が高くなる、或いは、高炭素フェロマンガン等に含有される炭素に起因して溶鋼中炭素が上昇する等により、低炭素高マンガン鋼の炭素濃度の上限値を超えるため、炭素を溶鋼から除去する必要がある。
【０００４】
溶鋼中の炭素を効率良く除去する方法として、ＲＨ真空脱ガス装置等の真空脱ガス設備を用いて、未脱酸状態の溶鋼を高真空処理して脱炭する、或いは、真空処理下で酸素ガス等の酸素源を溶鋼に添加して脱炭する真空脱炭処理が知られている。しかしながら、低炭素高マンガン鋼を真空脱炭処理した場合には、マンガンが多量に含有されているため、酸素は溶鋼中の炭素と反応するのみならず、マンガンとも反応し、マンガンが酸化ロスしてマンガンの歩留まりが悪化するばかりでなく、溶鋼中のマンガン濃度の制御が非常に困難となる。
【０００５】
従って、この問題を避けるために、低炭素高マンガン鋼の溶製においては、マンガン源を脱ガス処理中に添加する方法が行われており、この場合、低炭素高マンガン鋼の炭素濃度の許容範囲が低く且つ狭いこともあって、炭素含有量の少ない電解マンガン等のマンガン源を使用せざるを得ず、これらのマンガン源は非常に高価であるため、溶製コストの上昇を余儀無くされていた。
【０００６】
この問題点を解消すべく、特許文献１には、マンガン濃度が１質量％以上の極低炭素高マンガン鋼を真空脱ガス設備により真空脱炭処理する際に、真空脱ガス設備の真空槽内の圧力を５ｋＰａ以上４０ｋＰａ以下に保持し、且つ溶鋼表面に上吹きランスから不活性ガスを吹き付けて真空脱炭処理する方法が提案されており、又、特許文献２には、真空脱ガス設備の真空槽内の圧力を２．５ｋＰａ〜１４ｋＰａに調整しつつ、上吹きランスから吹き付ける酸素ガスが溶鋼に当たる火点近傍にＣａＯ、ＣａＣＯ₃ 、或いはＣａ（ＯＨ）₂ の何れか１種以上を吹き付ける又は添加して高マンガン鋼を真空脱炭処理する方法が提案されている。
【０００７】
【特許文献１】
特開平５−１８６８１８号公報
【０００８】
【特許文献２】
特開２００２−２５６３２８号公報
【０００９】
【発明が解決しようとする課題】
しかしながら、特許文献１及び特許文献２には以下の問題点がある。即ち、特許文献１の方法では、脱炭のための酸素源は、溶鋼中の溶存酸素だけのため、炭素濃度の高い領域では酸素の供給律速となり、脱炭速度が遅くなるために処理時間が長くなり、効率的に処理することができない。ここで、溶鋼中の溶存酸素とは、溶解酸素とも呼び、溶鋼中の酸素の中で酸化物等の化合物になっていない酸素であり、溶鋼中の全酸素量から酸化物等の化合物形態の酸素量を差し引いたものである。
【００１０】
又、特許文献２では、粉体切り出し装置等のＣａＯ、ＣａＣＯ₃ 等を供給するための専用の装置が必要であり、設備コストが上昇する。更に、ＣａＯ、ＣａＣＯ₃ 等を添加することにより、処理中の溶鋼の温度低下が大きくなり、それを補償するために転炉終点温度を上昇させる等の対策が必要であり、これに伴う製造コストの上昇を余儀無くされる。
【００１１】
本発明は上記事情に鑑みてなされたもので、その目的とするところは、真空脱ガス設備を用いて溶鋼に真空脱炭処理を施して低炭素高マンガン鋼を溶製する際に、マンガンの酸化を抑えて効率良く脱炭し、安価に且つ容易に低炭素高マンガン鋼を溶製することのできる方法を提供することである。
【００１２】
【課題を解決するための手段】
本発明者等は、上記課題を解決すべく鋭意検討・研究を重ねた。以下に、検討・研究結果を説明する。
【００１３】
安価マンガン源を使用し、真空脱炭処理を施すことによって低炭素高マンガン鋼を溶製する場合には、脱炭すべき溶鋼中の炭素量が多いため、溶鋼中の溶存酸素だけでは効率的に脱炭することができない。従って、効率的に脱炭処理するためには、真空脱ガス設備の真空槽内の溶鋼に酸素ガス等の酸素源を供給しつつ脱炭する必要がある。酸素源としては、処理中の溶鋼温度を低下させないことから判断して酸素ガスが好ましい。
【００１４】
真空槽内の溶鋼に酸素ガスを供給して溶鋼を真空脱炭する場合、マンガンを含有しない溶鋼或いはマンガンの含有量が少ない溶鋼では、供給した酸素ガスは溶鋼中に溶解する以外は主に炭素と反応するだけであるため、酸素ガスの供給速度を高めることによって脱炭速度を高めることができる。しかしながら、マンガンを１質量％以上含有する溶鋼の場合には、供給した酸素ガスは炭素以外にマンガンとも反応するため、脱炭速度を高めて脱炭を効率的に行うためには、酸素とマンガンとの反応を抑制しなければならない。酸素と炭素との反応式を下記の（３）式に、又、酸素とマンガンとの反応式を下記の（４）式に示す。
【００１５】
【数１】

【００１６】
【数２】

【００１７】
これらの式からも分かるように、溶鋼中の炭素濃度が高い領域では（３）式の反応が優先的に起こり、マンガンはさほど酸化されないが、炭素濃度が低い領域ではマンガンの酸化が進行する。これは試験でも確認された。更に、本発明者等は、試験を重ねることによって、炭素濃度が同一であっても、溶鋼中の溶存酸素濃度に依存してマンガンの酸化速度が異なることを見出した。即ち、同じ炭素濃度から酸素ガスの供給を開始して脱炭処理しても、溶存酸素濃度に依存してマンガンの酸化速度が異なることが分かった。溶存酸素濃度が低い場合にはマンガンの酸化速度が遅く、一方、溶存酸素濃度が高い場合にはマンガンの酸化速度が速くなり、その臨界となる溶存酸素濃度は０．０１質量％であることを見出した。これは、溶存酸素濃度が０．０１質量％より高い場合には、Ｍｎ−Ｏ平衡が支配的になってマンガンが酸化されやすくなり、一方、溶存酸素濃度が０．０１質量％以下の場合には、Ｃ−Ｏ平衡が支配的になって炭素の酸化が優先するためであり、この現象は溶鋼中の炭素、マンガン、酸素の挙動から確認されている。
【００１８】
又、炭素濃度の減少に伴い、酸素ガスの供給量に対して炭素の供給が追いつかなくなり、溶鋼中炭素の物質移動律速領域となることが分かった。更に、物質移動律速となる炭素濃度領域は、溶鋼中炭素濃度が０．０４質量％以下の範囲であることを定量化することができた。従って、溶鋼中炭素濃度が０．０４質量％以下の範囲では、酸素ガス供給量を低下させることにより、マンガンの酸化を一層抑制することができることが分かった。
【００１９】
酸素とマンガンとの反応を抑制する第２の方法として、上記（３）式及び（４）式からも類推されるように、酸素ガスにＡｒガス等の不活性ガスを加えることによって真空槽内雰囲気のＣＯガス分圧（以下「Ｐco」と記す）を低下させることで、（４）式に対して（３）式が優先的に進行し、マンガンの酸化を抑制可能であることが分かった。実際、溶鋼中炭素濃度が低下した真空脱炭処理後半に、不活性ガスとしてＡｒガスを加えた酸素ガス−Ａｒガスの混合ガスを真空槽内の溶鋼に吹き付けたところ、マンガンの酸化が抑制された。このＰcoの低下に起因するマンガン酸化の抑制効果は、真空脱炭処理の後半ほど顕著であることを確認した。不活性ガスとしては、Ａｒガス等の希ガスの他に窒素ガス等の非酸化性のガスを用いることができる。但し、窒素ガスを用いた場合には、溶鋼中の窒素濃度が上昇するので、溶製する低炭素高マンガン鋼の窒素濃度規格によって使用量の制限や使用の可否等を考慮する必要がある。
【００２０】
この場合、真空脱炭処理の処理前半では、混合ガス濃度比が０．１〜１．０の範囲で脱炭効率が良く、更に混合ガス濃度比が０．２〜０．５のときに最も効率良く脱炭することができた。この理由として、混合ガス濃度比が０．１未満の場合には、Ｐcoの低減効果が小さくなるためにマンガンの酸化を抑制する効果が小さくなり、逆に、混合ガス濃度比が１．０を越える範囲では、酸素ガス供給量が不足して脱炭反応が遅くなってしまうため、好ましくない。ここで、混合ガス濃度比とは、不活性ガス濃度／酸素ガス濃度である。
【００２１】
又、真空脱炭処理の処理後半では、混合ガス濃度比が０．３〜３．０の範囲で脱炭効率が良く、更に混合ガス濃度比が０．５〜２．０のときに最も効率良く脱炭することができた。この理由として、処理前半と同様に、混合ガス濃度比が０．５未満の場合には、Ｐcoの低減効果が小さくなるためにマンガンの酸化を抑制する効果が小さくなり、逆に、混合ガス濃度比が３．０を越える範囲では、酸素ガス供給量が不足して脱炭反応が遅くなってしまうため、好ましくない。尚、真空脱炭処理の処理前半と処理後半とは、真空脱炭処理の正確な１／２の期間を意味するものではなく、前半部及び後半部を意味するものである。
【００２２】
酸素とマンガンとの反応を抑制する第３の方法として、真空脱ガス設備の真空槽内への酸素ガスの供給量と、取鍋から真空槽内へ環流する溶鋼量（以下、「環流量」と記す）との比を適正値に制御することで、マンガンの酸化を抑制可能であることが分かった。具体的には、下記の（１）式で定義される環流量（Ｑ：ｔ／ｍｉｎ）に対する酸素ガス供給量（ＦO₂：Ｎｍ³ ／ｍｉｎ）の比（ＦO₂／Ｑ）を０．１５〜０．３０の範囲内とすること、即ち、環流量（Ｑ）と、上吹きランスからの酸素ガス供給量（ＦO₂）とを、下記の（２）式の範囲内とすることによって、効率良く脱炭することが可能であることが分かった。但し、（１）式及び（２）式において、Ｑは溶鋼の環流量（ｔ／ｍｉｎ）、Ｇは環流用Ａｒガス流量（Ｎｌ／ｍｉｎ）、ｄは浸漬管内径（ｍ）、Ｐ１は環流用Ａｒガス吹き込み点の圧力（ｋＰａ）で、通常は１気圧（１０１．３ｋＰａ）、Ｐ２は真空槽内圧力（ｋＰａ）、ＦO₂は上吹きランスからの酸素ガスの供給量（Ｎｍ³ ／ｍｉｎ）である。
【００２３】
【数３】

【００２４】
【数４】

【００２５】
比（ＦO₂／Ｑ）が０．１５未満の場合には、脱炭速度が遅くなって効率的でなく、一方、比（ＦO₂／Ｑ）が０．３０を越える場合には、酸素ガス供給量が過剰になり、酸素が炭素以外にマンガンとも反応するため、好ましくない。この場合、溶鋼中の炭素濃度の低下に伴って比（ＦO₂／Ｑ）を小さくすることで、効率良く脱炭できることも確認した。
【００２６】
又、当然ではあるが、上述した脱炭処理開始時の溶存酸素濃度の制御、Ａｒガス等の不活性ガスの混合による真空槽内雰囲気ガスのＰcoの低減、及び、比（ＦO₂／Ｑ）の制御の３つの手段を組み合わせることで、マンガンの酸化がより一層抑制されることも確認した。
【００２７】
本発明は、上記検討・研究結果に基づきなされたもので、第１の発明に係る低炭素高マンガン鋼の溶製方法は、真空脱ガス設備の真空槽内の溶鋼に酸素源を供給しつつ、溶鋼に対して真空脱炭処理を施して低炭素高マンガン鋼を溶製する際に、真空脱炭処理前の溶鋼中の溶存酸素濃度を０．０１質量％以下とすることを特徴とするものである。
【００２８】
第２の発明に係る低炭素高マンガン鋼の溶製方法は、第１の発明において、前記酸素源の供給量を、溶鋼中の炭素濃度の減少に伴って連続的又は段階的に低減することを特徴とするものである。
【００２９】
第３の発明に係る低炭素高マンガン鋼の溶製方法は、第２の発明において、前記酸素源の供給量を、溶鋼中の炭素濃度が０．０４質量％以下になった時点から低減することを特徴とするものである。
【００３０】
第４の発明に係る低炭素高マンガン鋼の溶製方法は、第１ないし第３の発明の何れかにおいて、前記酸素源として酸素ガスを用いることを特徴とするものである。
【００３１】
第５の発明に係る低炭素高マンガン鋼の溶製方法は、第４の発明において、前記酸素ガスに不活性ガスを混合することを特徴とするものである。
【００４０】
第６の発明に係る低炭素高マンガン鋼の溶製方法は、第１ないし第５の発明の何れかにおいて、真空脱炭処理前の溶鋼中炭素濃度を０．２質量％以下とすることを特徴とするものである。
【００４１】
【発明の実施の形態】
以下、本発明の実施の形態を説明する。高炉から出銑された溶銑を溶銑鍋やトーピードカー等の溶銑保持・搬送用容器で受銑し、次工程の脱炭精錬を行う転炉に搬送する。通常、この搬送途中で、溶銑に対して脱硫処理や脱燐処理等の溶銑予備処理が施されており、本発明においては、低炭素高マンガン鋼の成分規格上からは溶銑予備処理が必要でない場合でも、安価なマンガン源としてマンガン鉱石を転炉内に添加するため、転炉脱炭精錬におけるマンガン鉱石の歩留まりを上昇させる観点から、溶銑予備処理を実施することが好ましい。
【００４２】
転炉精錬はマンガン源としてマンガン鉱石を添加しつつ、必要に応じて少量の生石灰等を媒溶剤として用い、酸素を上吹き又は底吹きして溶銑の脱炭精錬を行う。転炉内に添加したマンガン鉱石のみでは、溶鋼のマンガン濃度が目的とする低炭素高マンガン鋼の成分規格範囲に不足する場合には、転炉から取鍋等の溶鋼保持容器への出鋼時に高炭素フェロマンガン等の安価な合金鉄系マンガン源を所定量添加し、溶鋼のマンガン濃度を成分規格と同等のレベルまで上昇させる。この場合、安価マンガン源を使用することによるコストメリットを十分に発揮させるため、出鋼後の溶鋼保持容器内の溶鋼中のマンガン濃度を、少なくとも低炭素高マンガン鋼の成分規格値の９０％以上まで確保することが好ましい。
【００４３】
マンガン鉱石や高炭素フェロマンガン等の安価なマンガン源を使用するため、溶鋼中の炭素濃度は必然的に高くなるが、それでも、マンガン濃度を調整した後の出鋼後の溶鋼中の炭素濃度を０．２質量％以下に抑えることが好ましい。溶鋼の炭素濃度が０．２質量％を越えると、次工程の真空脱ガス設備における真空脱炭処理に長時間を費やし、真空脱ガス設備の生産性の低下のみならず、真空脱炭処理時間の延長による温度補償として出鋼時の溶鋼温度を高くする必要が生じ、これに起因する鉄歩留まりの低下や耐火物損耗量の増大等によって製造コストが上昇するため、好ましくない。
【００４４】
次いで、この溶鋼をＲＨ真空脱ガス装置又はＤＨ真空脱ガス装置、ＶＯＤ炉等の真空脱ガス設備に搬送し、真空脱炭処理を実施する。真空脱ガス設備の代表的な設備はＲＨ真空脱ガス装置であり、以下、真空脱ガス設備としてＲＨ真空脱ガス装置を用いて精錬する例で説明する。
【００４５】
図１に、本発明を実施する際に用いたＲＨ真空脱ガス装置の例を示す。図１はＲＨ真空脱ガス装置の概略縦断面図であり、図１において、１はＲＨ真空脱ガス装置、２は取鍋、３は溶鋼、４はスラグ、５は真空槽、６は上部槽、７は下部槽、８は上昇側浸漬管、９は下降側浸漬管、１０は環流用ガス吹き込み管、１１はダクト、１２は原料投入口、１３は上吹きランスであり、真空槽５は上部槽６と下部槽７とから構成され、又、上吹きランス１３は上下移動が可能となっており、この上吹きランス１３からは酸素ガス及びＡｒガス等の不活性ガスと酸素ガスとの混合ガスが真空槽５内の溶鋼３の湯面に吹き付けられるようになっている。
【００４６】
ＲＨ真空脱ガス装置１では、搬送された取鍋２を昇降装置（図示せず）にて上昇させ、上昇側浸漬管８及び下降側浸漬管９を取鍋２内の溶鋼３に浸漬させる。そして、環流用ガス吹き込み管１０から上昇側浸漬管８内に環流用Ａｒガスを吹き込むと共に、真空槽５内をダクト１１に連結される排気装置（図示せず）にて排気して真空槽５内を減圧する。真空槽５内が減圧されると、取鍋２内の溶鋼３は、環流用ガス吹き込み管１０から吹き込まれるＡｒガスと共に上昇側浸漬管８を上昇して真空槽５内に流入し、その後、下降側浸漬管９を経由して取鍋２に戻る流れ、所謂、環流を形成してＲＨ真空脱ガス精錬が施される。
【００４７】
溶鋼３の環流が形成され、溶鋼３に対してＲＨ真空脱ガス精錬が施されると、真空槽５内では溶鋼３中の炭素と溶存酸素との反応が生じ、溶鋼３中の炭素はＣＯガスとなって排ガスと共に真空槽５からダクト１１を介して排出され、溶鋼３は真空脱炭処理される。更に、上吹きランス１３から酸素ガス或いは不活性ガスと酸素ガスとの混合ガスが吹き込まれ、溶鋼３の脱炭反応が促進される。
【００４８】
この真空脱炭処理中に溶鋼３中のマンガンの酸化を抑制して脱炭反応を行うために、（１）：真空脱炭処理前の溶鋼３中の溶存酸素濃度を０．０１質量％以下に調整する、（２）：上吹きランス１３から吹き付ける酸素ガスとＡｒガス等の不活性ガスとの混合ガスの混合ガス濃度比（不活性ガス濃度／酸素ガス濃度）を、真空脱炭処理の前半に比較して真空脱炭処理の後半で高くする、（３）：前述した（１）式で定義される溶鋼３の環流量（Ｑ）と上吹きランス１３からの酸素ガス供給量（ＦO₂）とが前述した（２）式の範囲内となるように、溶鋼３の環流量（Ｑ）又は上吹きランス１３からの酸素ガス供給量（ＦO₂）を調整する、の３種類の内の少なくとも１つを実施する。
【００４９】
この場合、真空脱炭処理前の溶鋼３中の溶存酸素濃度を０．０１質量％以下に調整するために、溶鋼３にＡｌ等の脱酸剤を添加してもよい。但し、溶存酸素濃度が低下し過ぎると脱炭反応が起こらず、Ａｌが無駄になるため、溶存酸素濃度が０．００３質量％よりも低下しないように、Ａｌ等の脱酸剤の添加量を調整することが好ましい。
【００５０】
又、上吹きランス１３からの酸素ガスの供給量を、溶鋼３中の炭素濃度の減少に応じて連続的又は段階的に低減することが好ましい。換言すれば、上吹きランス１３からの酸素ガス供給量（ＦO₂）と環流量（Ｑ）との比（ＦO₂／Ｑ）を、溶鋼３中の炭素濃度の減少に伴い、連続的又は段階的に低減することが好ましい。脱炭反応が進行して溶鋼３中の炭素濃度が低下すると、酸素ガスの供給量に対して炭素の供給が追いつかなくなり、溶鋼３中の炭素の物質移動律速領域となり、マンガンの酸化が起こるが、溶鋼３中の炭素濃度の減少に応じて酸素ガスの供給量を減じることで、マンガンの酸化を抑制することができる。炭素の物質移動律速となる炭素濃度領域は、溶鋼３中の炭素濃度が０．０４質量％以下の範囲であるので、特に、溶鋼中炭素濃度が０．０４質量％以下の範囲で、酸素ガスの供給量を低下させることが好ましい。
【００５１】
又、酸素源として溶鋼３に吹き付けるガスは酸素ガス単体ではなく、酸素ガスにＡｒガス等の不活性ガスを混合することが好ましい。不活性ガスを混合することによって真空槽５内の雰囲気ガスのＰcoが低下し、脱炭反応が優先的に起こり、マンガンの酸化を抑制することができる。
【００５２】
酸素ガスと不活性ガスとの混合ガスを吹き込む場合には、混合ガスの混合ガス濃度比（不活性ガス濃度／酸素ガス濃度）を、溶鋼３中の炭素濃度が高い真空脱炭処理の前半では０．１〜１．０とし、炭素濃度が低下する真空脱炭処理の後半では０．３〜３．０とすることが好ましい。真空脱炭の後半では炭素の物質移動律速になりやすいが、不活性ガス濃度を高めて真空槽５内の雰囲気ガスのＰcoを低下させることにより、炭素の物質移動律速に移行する時期を遅らせ、マンガンの酸化を抑制することができる。
【００５３】
このようにして真空脱炭処理を施しつつ、溶鋼３中の炭素濃度が、目的とする低炭素高マンガン鋼の成分規格値以下になるまで真空脱炭処理を継続し、溶鋼３の炭素濃度が成分規格値以下の所定の値になったなら、上吹きランス１３からの酸素ガスの吹き込みを停止すると共に原料投入口１２から溶鋼３にＡｌ等の強脱酸剤を添加して溶鋼３を脱酸処理する。Ａｌ等の強脱酸剤の添加により溶鋼３中の溶存酸素濃度は急激に低下し、真空脱炭処理が終了する。
【００５４】
真空脱炭処理の終了後も更に数分間程度の環流を継続し、必要に応じてＡｌ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｒ、Ｃｕ、Ｎｂ、Ｔｉ等の成分調整剤を原料投入口１２から溶鋼３に投入して溶鋼３の成分を調整した後、真空槽５を大気圧に戻してＲＨ真空脱ガス精錬を終了し、低炭素高マンガン鋼を溶製する。ここで、低炭素高マンガン鋼とは、炭素濃度が０．０５質量％以下で、マンガン濃度が１．０質量％以上の鋼のことである。
【００５５】
以上説明したように、本発明によれば、真空脱ガス設備を用いた真空脱炭処理を施して低炭素高マンガン鋼を溶製する際に、溶鋼３中のマンガンの酸化を抑制しつつ効率良く脱炭することが可能となり、その結果、マンガン鉱石や高炭素フェロマンガン等の安価なマンガン源を原料として使用することが可能となり、製造コストが削減されるのみならず、従前の真空脱ガス設備であっても容易に低炭素高マンガン鋼を溶製することが可能となる。
【００５６】
尚、上記説明ではＲＨ真空脱ガス装置１について説明したが、上記に準じて実施することにより、ＤＨ真空脱ガス装置やＶＯＤ炉等の他の真空脱ガス設備にも適用することができる。
【００５７】
【実施例】
高炉から出銑された溶銑に対して脱硫処理、脱燐処理の溶銑予備処理を施し、この溶銑を用いて転炉精錬し、低炭素高マンガン鋼を溶製する試験（試験番号１〜３１）を実施した。転炉ではマンガン源としてマンガン鉱石を添加してマンガン濃度を上昇させ、得られた２５０トンの溶鋼を未脱酸のまま取鍋に出鋼した。出鋼時の溶鋼成分は、炭素が０．１５〜０．２０質量％、珪素が０．０５質量％以下、マンガンが１．２〜１．５質量％、燐が０．０１質量％以下、硫黄が０．００３質量％以下であった。この溶鋼をＲＨ真空脱ガス装置に搬送し、真空脱炭処理条件を種々変更して低炭素高マンガン鋼を溶製した。
【００５８】
ＲＨ真空脱ガス装置では、環流用Ａｒガス流量を１５００〜５５００Ｎｌ／ｍｉｎ、真空槽の到達真空度を０．７〜６．７ｋＰａ、上吹きランスからの酸素ガス供給量（「送酸速度」と呼ぶ）を４００〜３０００Ｎｍ³ ／ｈ、上吹きランスからの酸素ガスに混合する不活性ガスとしてＡｒガスを用い、このＡｒガス流量を０〜１５００Ｎｍ³ ／ｈの範囲で変更し、酸素ガス吹き込み時間（「送酸時間」と呼ぶ）を２０分（試験番号１〜８、試験番号２０〜３１）及び２５分（試験番号９〜１９）の２水準とした。用いたＲＨ真空脱ガス装置の浸漬管の内径（ｄ）は０．６ｍである。一部の試験では、真空脱炭処理の途中で上吹きランスからの吹き込み条件（「送酸条件」と呼ぶ）を変更した。真空脱炭処理の途中で送酸条件を変更した試験では、変更時期に溶鋼から分析試料を採取し、溶鋼成分を分析した。又、真空脱炭処理開始前の溶鋼中溶存酸素濃度を０．０１質量％以下にした試験では、真空脱炭処理に先立ち、Ａｌを添加して溶鋼を脱酸した。表１に各試験操業における送酸条件、環流条件及び試験結果を示す。表１に示す環流量は前述の（１）式を用いて算出した数値である。
【００５９】
【表１】

【００６０】
表１に示すように、本発明に係る溶製方法である、（１）：真空脱炭処理前の溶鋼中の溶存酸素濃度が０．０１質量％以下であること、（２）：上吹きランスから吹き付ける酸素ガスと不活性ガスとの混合ガスの混合ガス濃度比（不活性ガス濃度／酸素ガス濃度）が真空脱炭処理の前半に比較して真空脱炭処理の後半で高くなること、（３）：溶鋼の環流量（Ｑ）に対する上吹きランスからの酸素ガス供給量（ＦO₂）の比（ＦO₂／Ｑ）が０．１５以上０．３０以下の範囲内であること、の３種類の溶製方法の何れをも満足しない試験（試験番号６〜８、試験番号１５〜１９、試験番号２７〜３１）では、脱Ｍｎ量が多く、脱Ｍｎ量／脱炭量の値が１．０以上であり、２．０を越える試験も発生した。
【００６１】
これに対して、上記３種類の溶製方法の内の少なくとも１種類以上を満足する試験（試験番号１〜５、試験番号９〜１４、試験番号２０〜２６）では、脱Ｍｎ量が少なく、脱Ｍｎ量／脱炭量の値が１．０未満であり、０．５未満の試験も発生した。即ち、本発明方法によってマンガンの酸化を抑制しつつ溶鋼中の炭素を効率良く除去できることが確認された。又、真空脱炭処理前の溶鋼中の溶存酸素濃度を０．０１質量％以下に調整した試験では、真空脱炭処理後まで溶存酸素濃度は０．０１質量％以下の範囲に維持されていた。尚、表１の備考欄には、本発明方法の範囲内の試験には本発明例と表示し、それ以外の試験には比較例と表示した。
【００６２】
【発明の効果】
本発明によれば、真空脱ガス設備を用いた真空脱炭処理を施して低炭素高マンガン鋼を溶製する際に、溶鋼中のマンガンの酸化を抑制しつつ効率良く脱炭することが可能となり、その結果、安価なマンガン源を原料として使用することが可能となり、製造コストが削減されるのみならず、従前の真空脱ガス設備であっても容易に低炭素高マンガン鋼を溶製することができ、工業上有益な効果がもたらされる。
【図面の簡単な説明】
【図１】本発明を実施する際に用いたＲＨ真空脱ガス装置の概略縦断面図である。
【符号の説明】
１ＲＨ真空脱ガス装置
２取鍋
３溶鋼
４スラグ
５真空槽
８上昇側浸漬管
９下降側浸漬管
１０環流用ガス吹き込み管
１３上吹きランス[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for supplying low oxygen high manganese steel by supplying an oxygen source such as oxygen gas to molten steel in a vacuum tank of a vacuum degassing facility and subjecting the molten steel to vacuum decarburization. The present invention relates to a method for melting low carbon high manganese steel at low cost by suppressing manganese oxidation and efficiently decarburizing.
[0002]
[Prior art]
In recent years, with the diversification of applications, steel materials are often used in a harsher environment, and higher performance of material properties is demanded more than ever. Under these circumstances, low carbon high manganese steel that has both high tensile strength and high workability has been developed for the purpose of reducing the weight of the structure, and it seems to be used as a steel plate for line pipes and steel plates for automobiles. Became. Here, the low carbon high manganese steel is a steel having a carbon concentration of 0.05% by mass or less and a manganese concentration of 1.0% by mass or more.
[0003]
Inexpensive manganese sources used to adjust the manganese concentration in the molten steel are manganese ore and high-carbon ferromanganese. When the low-carbon high-manganese steel is produced, it is converted during the decarburization and refining of the hot metal in the converter. It is possible to raise the manganese concentration in the molten steel to a predetermined value by introducing manganese ore into the furnace and reducing the manganese ore, or by adding high carbon ferromanganese to the molten steel when steel is output from the converter. However, when these inexpensive manganese sources are used, the carbon concentration cannot be sufficiently reduced at the time of the steel leaving the converter, the carbon concentration in the molten steel becomes high, or high carbon ferromanganese etc. Since the carbon in the molten steel rises due to the contained carbon and exceeds the upper limit of the carbon concentration of the low carbon high manganese steel, it is necessary to remove the carbon from the molten steel.
[0004]
As a method for efficiently removing carbon in the molten steel, decarburization is performed by using a vacuum degassing facility such as an RH vacuum degassing apparatus to decarburize the undeoxidized molten steel by high vacuum, or oxygen under vacuum treatment. A vacuum decarburization process is known in which an oxygen source such as gas is added to molten steel for decarburization. However, when low-carbon high-manganese steel is vacuum decarburized, it contains a large amount of manganese, so oxygen not only reacts with carbon in the molten steel, but also reacts with manganese, resulting in oxidation loss of manganese. This not only deteriorates the yield of manganese, but also makes it very difficult to control the manganese concentration in the molten steel.
[0005]
Therefore, in order to avoid this problem, in the melting of low carbon high manganese steel, a method of adding a manganese source during the degassing process is performed. In this case, the carbon concentration of the low carbon high manganese steel is allowed. Since the range is low and narrow, it is necessary to use manganese sources such as electrolytic manganese with a low carbon content, and these manganese sources are very expensive, which necessitates an increase in melting costs. It was.
[0006]
In order to solve this problem, Patent Document 1 discloses that when an ultra-low carbon high manganese steel having a manganese concentration of 1% by mass or more is vacuum decarburized by a vacuum degassing facility, Is maintained at a pressure of 5 kPa or more and 40 kPa or less, and an inert gas is blown onto the surface of the molten steel from an upper blowing lance, and vacuum decarburization treatment is proposed. While adjusting the pressure in the vacuum chamber to 2.5 kPa to 14 kPa, spray one or more of CaO, CaCO ₃ , or Ca (OH) _{2 in} the vicinity of the fire point where the oxygen gas blown from the upper blow lance hits the molten steel, or A method of vacuum decarburizing high manganese steel by adding it has been proposed.
[0007]
[Patent Document 1]
JP-A-5-186818 [0008]
[Patent Document 2]
Japanese Patent Laid-Open No. 2002-256328
[Problems to be solved by the invention]
However, Patent Document 1 and Patent Document 2 have the following problems. That is, in the method of Patent Document 1, since the oxygen source for decarburization is only dissolved oxygen in molten steel, the oxygen supply rate is limited in a region where the carbon concentration is high, and the decarburization rate becomes slow, so the processing time is reduced. It becomes long and cannot be processed efficiently. Here, the dissolved oxygen in the molten steel, also called dissolved oxygen, is oxygen that is not a compound such as an oxide in the oxygen in the molten steel, and is in the form of a compound such as an oxide from the total amount of oxygen in the molten steel. The amount of oxygen is subtracted.
[0010]
Further, Patent Document 2, CaO powder cutout device, requires a special device for supplying CaCO _3, etc., the equipment cost is increased. Furthermore, by adding CaO, CaCO ₃ or the like, the temperature drop of the molten steel during processing becomes large, and measures such as raising the converter end point temperature are necessary to compensate for it, and the manufacturing costs associated with this increase. Forced to rise.
[0011]
The present invention has been made in view of the above circumstances, and the object of the present invention is that when a low-carbon high-manganese steel is melted by subjecting the molten steel to vacuum decarburization using a vacuum degassing facility, An object of the present invention is to provide a method capable of efficiently decarburizing while suppressing oxidation and melting a low carbon high manganese steel easily and inexpensively.
[0012]
[Means for Solving the Problems]
The present inventors have intensively studied and studied to solve the above problems. The results of the examination and research are explained below.
[0013]
When low-carbon, high-manganese steel is melted by vacuum decarburization using an inexpensive manganese source, the amount of carbon in the molten steel to be decarburized is large, so it is efficient only with dissolved oxygen in the molten steel. Can not be decarburized. Therefore, in order to efficiently decarburize, it is necessary to decarburize while supplying an oxygen source such as oxygen gas to the molten steel in the vacuum tank of the vacuum degassing equipment. As the oxygen source, oxygen gas is preferable because it does not lower the molten steel temperature during processing.
[0014]
When oxygen gas is supplied to the molten steel in the vacuum chamber and the molten steel is vacuum decarburized, in the case of molten steel that does not contain manganese or molten steel with a low manganese content, the supplied oxygen gas is mainly carbon except that it dissolves in the molten steel. The decarburization rate can be increased by increasing the oxygen gas supply rate. However, in the case of molten steel containing 1% by mass or more of manganese, the supplied oxygen gas reacts with manganese in addition to carbon. Therefore, in order to increase the decarburization rate and perform decarburization efficiently, oxygen and manganese The reaction with must be suppressed. The reaction formula of oxygen and carbon is shown in the following formula (3), and the reaction formula of oxygen and manganese is shown in the following formula (4).
[0015]
[Expression 1]

[0016]
[Expression 2]

[0017]
As can be seen from these formulas, the reaction of formula (3) preferentially occurs in the region where the carbon concentration in the molten steel is high, and manganese is not oxidized so much, but the oxidation of manganese proceeds in the region where the carbon concentration is low. This was confirmed in the test. Furthermore, the present inventors have found that the oxidation rate of manganese differs depending on the dissolved oxygen concentration in the molten steel even if the carbon concentration is the same by repeating the test. That is, it has been found that even if oxygen gas supply is started from the same carbon concentration and decarburized, the oxidation rate of manganese differs depending on the dissolved oxygen concentration. When the dissolved oxygen concentration is low, the oxidation rate of manganese is slow. On the other hand, when the dissolved oxygen concentration is high, the oxidation rate of manganese is high, and the critical dissolved oxygen concentration is 0.01% by mass. I found it. This is because when the dissolved oxygen concentration is higher than 0.01% by mass, the Mn—O equilibrium becomes dominant and manganese is easily oxidized, whereas, when the dissolved oxygen concentration is 0.01% by mass or less. This is because the CO equilibrium is dominant and the oxidation of carbon is prioritized, and this phenomenon is confirmed from the behavior of carbon, manganese, and oxygen in molten steel.
[0018]
Further, it was found that as the carbon concentration decreased, the supply of carbon could not catch up with the supply amount of oxygen gas, and became a mass transfer rate limiting region of carbon in molten steel. Furthermore, it was possible to quantify that the carbon concentration region in which the mass transfer rate is controlled is that the carbon concentration in the molten steel is in the range of 0.04% by mass or less. Therefore, it was found that the oxidation of manganese can be further suppressed by reducing the oxygen gas supply amount in the range where the carbon concentration in the molten steel is 0.04% by mass or less.
[0019]
As a second method for suppressing the reaction between oxygen and manganese, as inferred from the above formulas (3) and (4), an inert gas such as Ar gas is added to the oxygen gas so that the inside of the vacuum chamber It was found that by reducing the CO gas partial pressure of the atmosphere (hereinafter referred to as “Pco”), the expression (3) preferentially proceeds with respect to the expression (4), and the oxidation of manganese can be suppressed. . In fact, in the latter half of the vacuum decarburization process when the carbon concentration in the molten steel decreased, oxygen gas-Ar gas mixed gas with Ar gas added as an inert gas was sprayed onto the molten steel in the vacuum chamber, and oxidation of manganese was suppressed. It was. It was confirmed that the effect of suppressing manganese oxidation due to the decrease in Pco was more remarkable in the second half of the vacuum decarburization treatment. As the inert gas, a non-oxidizing gas such as nitrogen gas can be used in addition to a rare gas such as Ar gas. However, when nitrogen gas is used, since the nitrogen concentration in the molten steel increases, it is necessary to consider the amount of use and the availability of use depending on the nitrogen concentration standard of the low carbon high manganese steel to be melted.
[0020]
In this case, in the first half of the vacuum decarburization treatment, the decarburization efficiency is good when the mixed gas concentration ratio is in the range of 0.1 to 1.0, and the most when the mixed gas concentration ratio is 0.2 to 0.5. Efficient decarburization was possible. The reason for this is that when the mixed gas concentration ratio is less than 0.1, the effect of suppressing oxidation of manganese is reduced because the effect of reducing Pco is reduced, and conversely, the mixed gas concentration ratio is 1.0. If it exceeds the range, the oxygen gas supply amount becomes insufficient and the decarburization reaction becomes slow, which is not preferable. Here, the mixed gas concentration ratio is an inert gas concentration / oxygen gas concentration.
[0021]
In the latter half of the vacuum decarburization process, the degassing efficiency is good when the mixed gas concentration ratio is in the range of 0.3 to 3.0, and the most efficient when the mixed gas concentration ratio is 0.5 to 2.0. I was able to decarburize well. The reason for this is that, similarly to the first half of the treatment, when the mixed gas concentration ratio is less than 0.5, the effect of suppressing oxidation of manganese is reduced because the Pco reduction effect is reduced. If the ratio exceeds 3.0, the oxygen gas supply amount is insufficient and the decarburization reaction becomes slow, which is not preferable. Note that the first half and the second half of the vacuum decarburization treatment do not mean an exact half period of the vacuum decarburization treatment, but mean the first half and the second half.
[0022]
As a third method of suppressing the reaction between oxygen and manganese, the amount of oxygen gas supplied into the vacuum chamber of the vacuum degassing equipment and the amount of molten steel circulating from the ladle into the vacuum chamber (hereinafter referred to as “annular flow rate”) It was found that the oxidation of manganese can be suppressed by controlling the ratio to the appropriate value. Specifically, the ratio (FO ₂ / Q) of the oxygen gas supply rate (FO ₂ : Nm ³ / min) to the ring flow rate (Q: t / min) defined by the following formula (1) is 0.15. By setting the flow rate (Q) and the oxygen gas supply amount (FO ₂ ) from the top blowing lance within the range of the following formula (2): It was found that decarburization can be performed efficiently. However, in the formulas (1) and (2), Q is the flow rate of molten steel (t / min), G is the flow rate of Ar gas for reflux (Nl / min), d is the inner diameter of the dip tube (m), and P1 is the return flow The pressure at the point of blowing Ar gas (kPa), usually 1 atm (101.3 kPa), P2 is the pressure inside the vacuum chamber (kPa), FO ₂ is the amount of oxygen gas supplied from the top blowing lance (Nm ³ / min) ).
[0023]
[Equation 3]

[0024]
[Expression 4]

[0025]
If the ratio (FO ₂ / Q) is less than 0.15, the decarburization rate is slow and not efficient, while if the ratio (FO ₂ / Q) exceeds 0.30, oxygen gas Since the supply amount becomes excessive and oxygen reacts with manganese in addition to carbon, it is not preferable. In this case, it was also confirmed that decarburization can be efficiently performed by reducing the ratio (FO ₂ / Q) as the carbon concentration in the molten steel decreases.
[0026]
Naturally, the control of the dissolved oxygen concentration at the start of the decarburization process described above, the reduction of the Pco of the atmosphere gas in the vacuum chamber by mixing with an inert gas such as Ar gas, and the ratio (FO ₂ / Q) It was also confirmed that the oxidation of manganese was further suppressed by combining the three means for controlling the above.
[0027]
The present invention has been made on the basis of the above examination and research results. The method for melting low-carbon high-manganese steel according to the first invention is to supply an oxygen source to the molten steel in the vacuum tank of the vacuum degassing equipment. When the low-carbon high-manganese steel is melted by subjecting the molten steel to vacuum decarburization, the dissolved oxygen concentration in the molten steel before the vacuum decarburization is 0.01% by mass or less. Is.
[0028]
The method for melting low-carbon, high-manganese steel according to the second invention is the first invention, wherein the supply amount of the oxygen source is reduced continuously or stepwise as the carbon concentration in the molten steel decreases. It is characterized by.
[0029]
The method for melting low-carbon high-manganese steel according to the third invention is the second invention, wherein the supply amount of the oxygen source is reduced from the time when the carbon concentration in the molten steel becomes 0.04% by mass or less. It is characterized by this.
[0030]
The method for melting low-carbon high-manganese steel according to the fourth invention is characterized in that, in any of the first to third inventions, oxygen gas is used as the oxygen source.
[0031]
The method for melting low-carbon high-manganese steel according to the fifth invention is characterized in that, in the fourth invention, an inert gas is mixed with the oxygen gas.
[0040]
The method for melting low-carbon high-manganese steel according to the sixth invention is that, in any of the first to fifth inventions, the carbon concentration in the molten steel before vacuum decarburization treatment is 0.2% by mass or less. It is a feature.
[0041]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below. The hot metal discharged from the blast furnace is received in a hot metal holding / conveying vessel such as a hot metal ladle or torpedo car and transferred to a converter for decarburization and refining in the next step. Usually, hot metal pretreatment such as desulfurization treatment and dephosphorization treatment is applied to the hot metal during the conveyance, and in the present invention, no hot metal pretreatment is required in terms of the component specifications of the low carbon high manganese steel. Even in this case, in order to add manganese ore to the converter as an inexpensive manganese source, it is preferable to carry out hot metal pretreatment from the viewpoint of increasing the yield of manganese ore in converter decarburization refining.
[0042]
In converter refining, manganese ore is added as a manganese source, and a small amount of quick lime or the like is used as a medium solvent as needed, and oxygen is top blown or bottom blown to perform decarburization refining of hot metal. If only the manganese ore added to the converter is insufficient for the manganese concentration of the molten steel to fall within the intended component specification range of the low-carbon high-manganese steel, the steel is discharged from the converter to the molten steel holding vessel such as a ladle. A predetermined amount of an inexpensive alloy iron-based manganese source such as high carbon ferromanganese is added to raise the manganese concentration of the molten steel to a level equivalent to the component standard. In this case, in order to fully demonstrate the cost merit by using an inexpensive manganese source, the manganese concentration in the molten steel in the molten steel holding container after steel is at least 90% of the component standard value of the low carbon high manganese steel It is preferable to secure up to.
[0043]
Since cheap manganese sources such as manganese ore and high-carbon ferromanganese are used, the carbon concentration in the molten steel inevitably increases.However, the carbon concentration in the molten steel after steel output after adjusting the manganese concentration is still high. It is preferable to suppress to 0.2 mass% or less. If the carbon concentration of the molten steel exceeds 0.2% by mass, it will take a long time for the vacuum decarburization process in the vacuum degassing equipment in the next process, not only the productivity of the vacuum degassing equipment will be reduced, but also the vacuum decarburization processing time. As a temperature compensation by extending the length of the steel, it is necessary to increase the molten steel temperature at the time of steel output, and this causes an increase in manufacturing cost due to a decrease in iron yield and an increase in the amount of refractory wear.
[0044]
Next, the molten steel is transported to a vacuum degassing facility such as an RH vacuum degassing apparatus, a DH vacuum degassing apparatus, or a VOD furnace, and vacuum decarburization processing is performed. A typical equipment of the vacuum degassing equipment is an RH vacuum degassing apparatus. Hereinafter, an example of refining using an RH vacuum degassing equipment as the vacuum degassing equipment will be described.
[0045]
FIG. 1 shows an example of an RH vacuum degassing apparatus used in carrying out the present invention. FIG. 1 is a schematic longitudinal sectional view of an RH vacuum degassing apparatus. In FIG. 1, 1 is an RH vacuum degassing apparatus, 2 is a ladle, 3 is molten steel, 4 is a slag, 5 is a vacuum tank, and 6 is an upper tank. , 7 is a lower tank, 8 is an ascending side dip pipe, 9 is a descending side dip pipe, 10 is a reflux gas blowing pipe, 11 is a duct, 12 is a raw material inlet, 13 is an upper blowing lance, The upper blower lance 13 can be moved up and down, and the upper blower lance 13 includes oxygen gas, an inert gas such as Ar gas, and oxygen gas. The mixed gas is sprayed on the surface of the molten steel 3 in the vacuum chamber 5.
[0046]
In the RH vacuum degassing apparatus 1, the conveyed ladle 2 is raised by an elevating device (not shown), and the ascending side dip tube 8 and the descending side dip tube 9 are immersed in the molten steel 3 in the ladle 2. Then, Ar gas for recirculation is blown into the ascending-side dip tube 8 from the recirculation gas blowing tube 10, and the vacuum chamber 5 is evacuated by an exhaust device (not shown) connected to the duct 11. The inside is depressurized. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 in the ladle 2 ascends the rising side dip tube 8 together with Ar gas blown from the reflux gas blowing tube 10 and flows into the vacuum chamber 5, and then A flow returning to the ladle 2 via the descending side dip tube 9, that is, a so-called recirculation flow is formed and RH vacuum degassing is performed.
[0047]
When a reflux of the molten steel 3 is formed and the RH vacuum degassing refining is performed on the molten steel 3, a reaction between the carbon in the molten steel 3 and dissolved oxygen occurs in the vacuum tank 5, and the carbon in the molten steel 3 is CO 2. It becomes gas and is discharged from the vacuum tank 5 through the duct 11 together with the exhaust gas, and the molten steel 3 is vacuum decarburized. Further, oxygen gas or a mixed gas of inert gas and oxygen gas is blown from the top blowing lance 13, and the decarburization reaction of the molten steel 3 is promoted.
[0048]
In order to suppress the oxidation of manganese in the molten steel 3 during this vacuum decarburization treatment and perform the decarburization reaction, (1): the dissolved oxygen concentration in the molten steel 3 before the vacuum decarburization treatment is 0.01% by mass or less. (2): The mixed gas concentration ratio (inert gas concentration / oxygen gas concentration) of the mixed gas of the oxygen gas blown from the top blowing lance 13 and the inert gas such as Ar gas is adjusted in the vacuum decarburization process. Increased in the second half of the vacuum decarburization process compared with the first half. (3): The flow rate (Q) of the molten steel 3 defined by the above-described formula (1) and the oxygen gas supply amount (FO) from the top blowing lance 13 ₂ ) adjust the ring flow rate (Q) of the molten steel 3 or the oxygen gas supply amount (FO ₂ ) from the top blowing lance 13 so that it falls within the range of the above-mentioned formula (2). Implement at least one of the following:
[0049]
In this case, a deoxidizer such as Al may be added to the molten steel 3 in order to adjust the dissolved oxygen concentration in the molten steel 3 before the vacuum decarburization treatment to 0.01% by mass or less. However, since the decarburization reaction does not occur if the dissolved oxygen concentration is too low and Al is wasted, the amount of deoxidizer such as Al is added so that the dissolved oxygen concentration does not decrease below 0.003 mass%. It is preferable to adjust.
[0050]
Moreover, it is preferable to reduce the supply amount of the oxygen gas from the top blowing lance 13 continuously or stepwise according to the decrease in the carbon concentration in the molten steel 3. In other words, the ratio (FO ₂ / Q) of the oxygen gas supply amount (FO ₂ ) and the ring flow rate (Q) from the top blowing lance 13 is continuously or stepwise as the carbon concentration in the molten steel 3 decreases. It is preferable to reduce it. When the decarburization reaction proceeds and the carbon concentration in the molten steel 3 decreases, the supply of carbon cannot keep up with the supply amount of oxygen gas, and the mass transfer rate-determining region of carbon in the molten steel 3 is produced, and manganese oxidation occurs. The oxidation of manganese can be suppressed by reducing the supply amount of oxygen gas according to the decrease in the carbon concentration in the molten steel 3. Since the carbon concentration region in which the mass transfer rate of carbon is controlled is in the range where the carbon concentration in the molten steel 3 is 0.04% by mass or less, particularly in the range where the carbon concentration in the molten steel is 0.04% by mass or less, the oxygen gas It is preferable to reduce the supply amount.
[0051]
Moreover, it is preferable that the gas sprayed on the molten steel 3 as an oxygen source is not oxygen gas alone but an inert gas such as Ar gas is mixed with the oxygen gas. By mixing the inert gas, the Pco of the atmospheric gas in the vacuum chamber 5 is lowered, the decarburization reaction takes place preferentially, and the oxidation of manganese can be suppressed.
[0052]
When a mixed gas of oxygen gas and inert gas is blown, the mixed gas concentration ratio (inert gas concentration / oxygen gas concentration) of the mixed gas is set to the first half of the vacuum decarburization process in which the carbon concentration in the molten steel 3 is high. 0.1 to 1.0, and preferably 0.3 to 3.0 in the second half of the vacuum decarburization process in which the carbon concentration decreases. In the latter half of vacuum decarburization, the mass transfer rate of carbon tends to be controlled, but by increasing the inert gas concentration and lowering the Pco of the atmospheric gas in the vacuum chamber 5, the timing for shifting to the mass transfer rate of carbon is delayed, Manganese oxidation can be suppressed.
[0053]
While performing the vacuum decarburization treatment in this way, the vacuum decarburization treatment is continued until the carbon concentration in the molten steel 3 is equal to or less than the component standard value of the target low carbon high manganese steel, and the carbon concentration of the molten steel 3 is increased. When a predetermined value equal to or less than the component standard value is reached, the blowing of oxygen gas from the top blowing lance 13 is stopped and a strong deoxidizer such as Al is added to the molten steel 3 from the raw material inlet 12 to remove the molten steel 3. Acid treatment. By adding a strong deoxidizer such as Al, the dissolved oxygen concentration in the molten steel 3 rapidly decreases, and the vacuum decarburization process is completed.
[0054]
After completion of the vacuum decarburization treatment, the recirculation is continued for about several minutes, and if necessary, component modifiers such as Al, Si, Mn, Ni, Cr, Cu, Nb, Ti are supplied from the raw material inlet 12 to the molten steel 3 And the components of the molten steel 3 are adjusted, the vacuum chamber 5 is returned to atmospheric pressure, the RH vacuum degassing refining is completed, and the low carbon high manganese steel is melted. Here, the low carbon high manganese steel is a steel having a carbon concentration of 0.05% by mass or less and a manganese concentration of 1.0% by mass or more.
[0055]
As described above, according to the present invention, when low-carbon high-manganese steel is melted by performing vacuum decarburization processing using a vacuum degassing facility, efficiency is suppressed while suppressing oxidation of manganese in the molten steel 3. As a result, it becomes possible to use cheap manganese sources such as manganese ore and high-carbon ferromanganese as raw materials, which not only reduces the manufacturing cost but also the conventional vacuum degassing. Even with equipment, it becomes possible to easily melt low carbon high manganese steel.
[0056]
In the above description, the RH vacuum degassing apparatus 1 has been described. However, the RH vacuum degassing apparatus 1 can be applied to other vacuum degassing equipment such as a DH vacuum degassing apparatus and a VOD furnace by carrying out according to the above.
[0057]
【Example】
A test (test numbers 1-31) in which the hot metal discharged from the blast furnace is subjected to desulfurization treatment and dephosphorization hot metal pretreatment, and the hot metal is used for refining the converter to produce low carbon high manganese steel. Carried out. In the converter, manganese ore was added as a manganese source to increase the manganese concentration, and the obtained 250 tons of molten steel was put into a ladle without being deoxidized. Molten steel components at the time of steel removal are 0.15 to 0.20 mass% for carbon, 0.05 mass% or less for silicon, 1.2 to 1.5 mass% for manganese, 0.01 mass% or less for phosphorus, Sulfur was 0.003 mass% or less. The molten steel was transported to an RH vacuum degassing apparatus, and low-carbon high-manganese steel was melted under various vacuum decarburization treatment conditions.
[0058]
In the RH vacuum degassing apparatus, the Ar gas flow rate for reflux is 1500 to 5500 Nl / min, the ultimate vacuum of the vacuum chamber is 0.7 to 6.7 kPa, and the oxygen gas supply amount from the top blowing lance (“acid feed rate” and 400 to 3000 Nm ³ / h, Ar gas is used as the inert gas mixed with the oxygen gas from the top blowing lance, the Ar gas flow rate is changed in the range of 0 to 1500 Nm ³ / h, and the oxygen gas blowing time (Referred to as “acid delivery time”) was set to two levels of 20 minutes (test numbers 1 to 8, test numbers 20 to 31) and 25 minutes (test numbers 9 to 19). The inner diameter (d) of the dip tube of the used RH vacuum degasser is 0.6 m. In some tests, the blowing conditions from the top blowing lance (called “acid feeding conditions”) were changed during the vacuum decarburization process. In the test in which the acid sending conditions were changed during the vacuum decarburization treatment, an analytical sample was taken from the molten steel at the time of change, and the molten steel components were analyzed. In the test in which the dissolved oxygen concentration in the molten steel before the start of the vacuum decarburization treatment was 0.01% by mass or less, the molten steel was deoxidized by adding Al prior to the vacuum decarburization treatment. Table 1 shows the acid feeding conditions, reflux conditions and test results in each test operation. The ring flow rate shown in Table 1 is a numerical value calculated using the above-described equation (1).
[0059]
[Table 1]

[0060]
As shown in Table 1, it is a melting method according to the present invention, (1): the dissolved oxygen concentration in the molten steel before vacuum decarburization is 0.01% by mass or less, (2): top blowing The mixed gas concentration ratio of the mixed gas of oxygen gas and inert gas blown from the lance (inert gas concentration / oxygen gas concentration) is higher in the second half of the vacuum decarburization process than in the first half of the vacuum decarburization process, (3): The ratio (FO ₂ / Q) of the oxygen gas supply rate (FO ₂ ) from the top blowing lance to the ring flow rate (Q) of the molten steel is in the range of 0.15 to 0.30. In tests (test numbers 6-8, test numbers 15-19, test numbers 27-31) that do not satisfy any of the three types of melting methods, the amount of de-Mn is large, and the value of de-Mn amount / decarburization amount is high. Tests of 1.0 or more and exceeding 2.0 were also generated.
[0061]
On the other hand, in tests (test numbers 1 to 5, test numbers 9 to 14, test numbers 20 to 26) satisfying at least one of the above three types of melting methods, the amount of de-Mn removal is small, The value of the amount of de-Mn / the amount of decarburization was less than 1.0, and a test of less than 0.5 occurred. That is, it was confirmed that carbon in molten steel can be efficiently removed while suppressing oxidation of manganese by the method of the present invention. Moreover, in the test which adjusted the dissolved oxygen concentration in the molten steel before vacuum decarburization processing to 0.01 mass% or less, the dissolved oxygen concentration was maintained in the range of 0.01 mass% or less until after vacuum decarburization processing. . In the remarks column of Table 1, the present invention example is displayed for tests within the scope of the method of the present invention, and the comparative example is displayed for other tests.
[0062]
【The invention's effect】
According to the present invention, it is possible to efficiently decarburize while suppressing oxidation of manganese in molten steel when performing vacuum decarburization processing using a vacuum degassing equipment to melt low carbon high manganese steel. As a result, it becomes possible to use an inexpensive manganese source as a raw material, not only reducing the manufacturing cost, but also easily melting low carbon high manganese steel even with conventional vacuum degassing equipment. And has an industrially beneficial effect.
[Brief description of the drawings]
FIG. 1 is a schematic longitudinal sectional view of an RH vacuum degassing apparatus used in carrying out the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 RH vacuum degassing apparatus 2 Ladle 3 Molten steel 4 Slag 5 Vacuum tank 8 Ascending side dip tube 9 Descending side dip tube 10 Recirculation gas blowing tube 13 Top blowing lance

Claims

While supplying an oxygen source to the molten steel in the vacuum tank of the vacuum degassing equipment, when the molten steel is vacuum decarburized to produce low carbon high manganese steel, A method for producing a low-carbon high-manganese steel, wherein the dissolved oxygen concentration is 0.01% by mass or less.

The method for melting low-carbon high-manganese steel according to claim 1, wherein the supply amount of the oxygen source is reduced continuously or stepwise as the carbon concentration in the molten steel decreases.

The method for melting low-carbon, high-manganese steel according to claim 2, wherein the supply amount of the oxygen source is reduced from the time when the carbon concentration in the molten steel becomes 0.04 mass% or less.

The method for melting low carbon high manganese steel according to any one of claims 1 to 3, wherein oxygen gas is used as the oxygen source.

The method for melting low-carbon high-manganese steel according to claim 4, wherein an inert gas is mixed with the oxygen gas.

The method for melting low-carbon, high-manganese steel according to any one of claims 1 to 5, wherein a carbon concentration in the molten steel before vacuum decarburization treatment is 0.2 mass% or less .