JP3439517B2 - Refining method of chromium-containing molten steel - Google Patents
Refining method of chromium-containing molten steelInfo
- Publication number
- JP3439517B2 JP3439517B2 JP02456494A JP2456494A JP3439517B2 JP 3439517 B2 JP3439517 B2 JP 3439517B2 JP 02456494 A JP02456494 A JP 02456494A JP 2456494 A JP2456494 A JP 2456494A JP 3439517 B2 JP3439517 B2 JP 3439517B2
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- molten steel
- refining
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Description
【発明の詳細な説明】
【０００１】
【産業上の利用分野】本発明は同一精錬容器を用いて大
気圧精錬後、真空精錬を行う含クロム溶鋼の脱炭精錬に
おいて、真空精錬時の溶鋼中の〔Ｃｒ〕の酸化を抑え、
効率よく脱炭を行い、安定した精錬を行う含クロム溶鋼
の精錬法に関する。
【０００２】
【従来の技術】従来ステンレス鋼の如く１１ｍａｓｓ％
以上のクロムを含むような含クロム溶鋼の脱炭法として
は、浴面下より酸素ガスまたは酸素ガス（以下、単に酸
素という）と不活性ガスの混合ガスを吹込むＡＯＤ法が
広く用いられている。ＡＯＤ法は脱炭が進行し、溶鋼中
の〔Ｃ〕濃度が低下してくると〔Ｃｒ〕が酸化されやす
くなることから、〔Ｃ〕濃度の低下に伴い吹込みガス中
のＡｒガスのような不活性ガスの比率を高くし、酸素の
比率を低くして、〔Ｃｒ〕の酸化を抑える方法が採られ
ている。しかし、低〔Ｃ〕濃度域では脱炭速度が低下す
るために所望の〔Ｃ〕濃度に到達するのに長時間を要
し、かつ吹込みガス中の不活性ガスの比率が高いため
に、不活性ガスの消費量が大幅に増大することから経済
的にも不利になる。
【０００３】このような低〔Ｃ〕濃度域における脱炭を
促進する方法として、真空精錬法の利用が挙げられる。
例えば、特公昭６０−１００８７号公報には、高クロム
・ステンレス鋼を０．０３ｍａｓｓ％以下の低〔Ｃ〕濃
度域まで脱炭するために、大気圧下での酸素による脱炭
を〔Ｃ〕＝０．２〜０．４ｍａｓｓ％まで行い、その後
は非酸化性ガスによる攪拌は続けるが酸素吹込みは停止
し、鋼浴上の圧力を約１０Ｔｏｒｒまで連続的に低下さ
せてボイリングを起こさせることによって所望の脱炭を
行う方法が記載されている。
【０００４】該方法は比較的高〔Ｃ〕濃度より酸素の供
給を止めるために、〔Ｃｒ〕の酸化による損失は少なく
なるが、急激な真空精錬の適用により、ＣＯガスを大量
に発生し、爆発の危険を招く。この対策として、真空吸
引をゆるやかにすれば爆発の危険はなくなるが、経過時
間が長くなって溶鋼温度が低下し、かつ反応が遅くな
る。また、圧力を１０Ｔｏｒｒ以下の高真空にすれば、
溶鋼のスプラッシュが激しくなり、合金材料投入用ホッ
パーの閉塞などの問題が生じる。
【０００５】これらの問題点を解決する方法として、特
開平３−６８７１３号公報および特開平４−２５４５０
９号公報記載の方法が提案されている。これらに記載さ
れている含クロム溶鋼の精錬方法は〔Ｃ〕濃度０．２〜
０．０５ｍａｓｓ％までは吹込みガスとして非酸化性ガ
スと酸素の混合ガスを使用し、〔Ｃ〕濃度がこの範囲内
に低下した後は、２００〜１５Ｔｏｒｒに減圧し、かつ
吹込みガスとして非酸化性ガスのみを使用するというも
のである。該方法は比較的低〔Ｃ〕濃度域まで大気圧下
で精錬を行うために、〔Ｃｒ〕の酸化損失が大きくな
る。また、真空下での脱炭は不活性ガスのみを用いるこ
とで〔Ｃｒ〕の酸化は抑えられるが、脱炭の酸素源は溶
鋼中の〔Ｏ〕あるいはスラグ中の酸素となり、酸素の供
給速度が遅くなるために脱炭速度の低下を招く。また、
溶鋼温度の制御が不可能なために、真空精錬の開始時の
溶鋼温度が低い場合には脱炭の停滞を招き、効率的な脱
炭精錬法とはいえない。
【０００６】
【発明が解決しようとする課題】本発明は、同一精錬容
器を用いて大気圧精錬後、真空精錬を行う含クロム溶鋼
の脱炭精錬において、真空精錬時に吹込むガスの条件、
真空精錬を開始する〔Ｃ〕濃度および真空精錬時の真空
度を好適な範囲に維持し、かつ溶鋼温度を制御すること
により溶鋼中の〔Ｃｒ〕の酸化を抑え、効率よく脱炭を
行い、併せて還元用Ｓｉ原単位の低減、精錬時間の短縮
および極低炭素濃度鋼の精錬を可能にする含クロム溶鋼
の精錬法を提供することを目的とするものである。
【０００７】
【課題を解決するための手段】本発明は上述の課題を有
利に解決したものであり、その要旨とするところは、同
一の精錬容器を用いて大気圧精錬後、真空精錬を行う含
クロム溶鋼の精錬法において、前記真空精錬を開始する
時の溶鋼中の〔Ｃ〕濃度を０．０６ｍａｓｓ％以上、
０．２５ｍａｓｓ％以下とし、該真空精錬の真空度を３
００Ｔｏｒｒ以下とし、かつ酸素ガス比率を５％以上、
４５％以下とする酸素ガスと不活性ガスとの混合ガスを
溶鋼中の〔Ｃ〕濃度０．０５ｍａｓｓ％以上の領域で溶
鋼に吹込み、該溶鋼の温度を１６４０℃以上に保持する
ことを特徴とする含クロム溶鋼の精錬法にある。
【０００８】以下本発明について詳細に説明する。本発
明の含クロム溶鋼の脱炭精練は、〔Ｃ〕濃度が０．０６
ｍａｓｓ％以上、０．２５ｍａｓｓ％以下の範囲におい
て、図１に例示するような精練容器を用いて行う精練方
法である。精錬容器１内で含クロム溶鋼４中に底吹き羽
口２を通して、精錬ガス５を吹き込む。また、精錬容器
１は脱着可能な排気フード３を有しており、３００Ｔｏ
ｒｒ以下の減圧が可能である。
【０００９】本発明は真空精錬を用いる含クロム溶鋼の
脱炭精錬において、真空度３００Ｔｏｒｒ以下の真空精
錬下で、吹込みガスとして酸素と不活性ガスの混合ガス
を用いることで、溶鋼温度を１６４０℃以上に保持し、
溶鋼中〔Ｃｒ〕の酸化を抑え、脱炭速度を高位に保つこ
とが可能であることに着目したものである。図２にＳＵ
Ｓ３０４ステンレス鋼を酸素比率５〜４５％の酸素ガス
とＡｒガスとの混合ガスで処理した場合の真空精錬時の
真空度と〔Ｃｒ〕酸化指数の関係を示す。なお、〔Ｃ
ｒ〕酸化指数は真空度３００Ｔｏｒｒにおける溶鋼中
〔Ｃｒ〕の平均の酸化量を１．０として指数化した値で
ある。また、真空処理の〔Ｃ〕濃度範囲は０．０５〜
０．２５ｍａｓｓ％であり、溶鋼温度は１６４０〜１７
２０℃の範囲であった。図２より真空度が３００Ｔｏｒ
ｒを超えると急激に〔Ｃｒ〕酸化量が大きくなることか
ら、真空精錬時の真空度は３００Ｔｏｒｒ以下とする必
要があることがわかる。また、真空精錬の開始時は溶鋼
のスプラッシュやボイリングが避けられないことから、
できるだけこれらを抑えるために徐々に真空度を下げて
いくことが好ましい。
【００１０】図３にＳＵＳ３０４ステンレス鋼を大気圧
精錬後、真空精錬を行った場合の真空精錬の吹込みガス
の酸素ガス比率と〔Ｃｒ〕酸化指数の関係を示す。な
お、〔Ｃｒ〕酸化指数は酸素ガス比率４５％における溶
鋼中〔Ｃｒ〕の平均の酸化量を１．０として指数化した
値である。また、真空処理時の真空度は３００Ｔｏｒｒ
以下であり、〔Ｃ〕濃度範囲は０．０５〜０．２５ｍａ
ｓｓ％であり、溶鋼温度は１６４０〜１７２０℃の範囲
であった。図３より混合ガスの酸素ガス比率が４５％を
超えると急激に〔Ｃｒ〕酸化量が大きくなることから、
酸素ガス比率は４５％以下とする必要があることがわか
る。また、酸素ガス比率５％未満では〔Ｃｒ〕酸化量は
小さいが、この場合は酸素の供給が全てスラグあるいは
溶鋼中〔Ｏ〕となるために精錬中、溶鋼温度が急激に低
下する。溶鋼温度の制御を可能にするためには５％以上
の酸素を含む混合ガスの吹込みが必要である。したがっ
て、吹き込みガスとしては酸素を５％以上、４５％以下
含む不活性ガスとの混合ガスが必要である。
【００１１】図４にＳＵＳ３０４ステンレス鋼を大気圧
精錬後、真空精錬を行った場合の真空精錬時の平均溶鋼
温度と〔Ｃｒ〕酸化指数の関係を示す。なお、〔Ｃｒ〕
酸化指数は溶鋼温度１６４０℃における溶鋼中〔Ｃｒ〕
の平均の酸化量を１．０として指数化した値である。ま
た、真空処理時の真空度は３００Ｔｏｒｒであり、
〔Ｃ〕濃度範囲は０．０５〜０．２５ｍａｓｓ％であっ
た。図４より溶鋼温度１６４０℃以上では〔Ｃｒ〕酸化
が抑えられ、安定した精錬が可能であることがわかる。
この溶鋼温度を１６４０℃以上に保持するには、真空精
錬では精錬容器からの抜熱および溶鋼表面からの抜熱が
大きいので、吹込みガスに酸素を含む混合ガスを使用す
る必要があり、その比率として、前記の５％以上、４５
％以下とする必要がある。なお、溶鋼温度は高い方が
〔Ｃｒ〕酸化は小さくなるが、高すぎると精錬容器の耐
火物の損耗が激しくなるために、溶鋼温度としては１７
２０℃以下が好ましい。
【００１２】なお、酸素を含む混合ガスの供給は、
〔Ｃ〕濃度０．０５ｍａｓｓ％未満では〔Ｃｒ〕酸化が
大きくなること、および脱炭する〔Ｃ〕量が小さいこと
から、〔Ｃ〕濃度０．０５ｍａｓｓ％以上とする必要が
ある。また、真空精錬を開始する〔Ｃ〕濃度は、高すぎ
ると真空精錬の負荷が増大して溶鋼のスプラッシュやボ
イリングによるロスが大きくなること、低すぎると真空
精錬の効果が享受できず大気圧精錬でのロスが大きくな
ることから、〔Ｃ〕濃度範囲として０．０６ｍａｓｓ％
以上、０．２５ｍａｓｓ％以下とする必要がある。
【００１３】以上より、溶鋼中の〔Ｃｒ〕の酸化を抑
え、効率よく含クロム溶鋼の精錬を行うには、〔Ｃ〕濃
度０．０６ｍａｓｓ％以上、０．２５ｍａｓｓ％以下の
範囲で真空精錬を適用し、真空度３００Ｔｏｒｒ以下
で、吹込みガスとして酸素を５％以上、４５％以下含む
不活性ガスとの混合ガスを吹込み、溶鋼温度を１６４０
℃以上とすることが効果的である。
【００１４】操業においては〔Ｃ〕濃度の時間変化は概
略予測できることから、粗溶鋼の装入時の溶鋼組成およ
び溶鋼温度を把握し、真空精錬を開始する時期を決定す
る。また、真空精錬中は〔Ｃ〕濃度および溶鋼温度を把
握し、かつ炉内状況を観察しながら、ガス吹込み条件お
よび真空度の条件を決定することが可能である。特に溶
鋼温度が低めの場合には、吹込みガスの酸素比率を高く
して温度低下を防止するような操業が可能である。該操
業方法により、溶鋼のスプラッシュの大量発生は防止可
能であり、安定した操業が可能である。
【００１５】
【作用】含クロム溶鋼の脱炭精錬では、吹込んだ酸素が
一旦式で表される反応でクロム酸化物（Ｃｒ2 Ｏ3 ）
を生成し、その後式で表される脱炭反応が進行するも
のと考えられる。なお、式の反応平衡定数Ｋは式で
表される。
２Ｃｒ＋３／２Ｏ2 （ｇ）＝（Ｃｒ2 Ｏ3 ） …
（Ｃｒ2 Ｏ3 ）＋３Ｃ＝２Ｃｒ＋３ＣＯ（ｇ） …
Ｋ＝ａcr203 ・ａc 2 ／ａcr 2 ・ＰCO 3 …
ここで、ａcr203 はスラグ中（Ｃｒ2 Ｏ3 ）の活量、ａ
c は溶鋼中〔Ｃ〕の活量、ａcrは溶鋼中〔Ｃｒ〕の活
量、ＰCOは雰囲気中のＣＯガス分圧を示す。
【００１６】Ｄ．Ｃ．Ｈｉｌｔｙらは式の関係を実験
的に求め、式を提示している。
ｌｏｇ（〔Ｃｒ〕・ＰCO／〔Ｃ〕＝−１３８００／Ｔ＋８．７６…
ここで、Ｔは溶鋼温度（Ｋ）を示す。式の関係より、
〔Ｃｒ〕濃度が一定の場合、脱炭を効率的に進めるに
は、〔Ｃ〕濃度に合わせてＰCOおよびＴを制御すればよ
いことがわかる。真空精錬では、精錬容器内の真空度Ｐ
は雰囲気中のＣＯガス分圧Ｐcoと比例関係にある。した
がって、精錬容器内の真空度の制御によってＰcoを制御
することができる。本発明者らは〔Ｃ〕濃度０．０６ｍ
ａｓｓ％以上、好ましくは０．２５ｍａｓｓ％以下の範
囲において真空精錬の適用を開始し、かつ上限の真空度
を３００Ｔｏｒｒとすることによって〔Ｃｒ〕酸化を抑
制した効率的な脱炭が可能であることを見出した。ま
た、溶鋼温度については、１６４０℃以上にすることで
効率的な脱炭が可能であることを見出した。真空精錬で
は、大気圧精錬に比べ溶鋼表面からの抜熱および精錬容
器からの抜熱が大きい。従来の方法では、真空精錬時は
不活性ガスのみの吹込みで、式の反応を進行させてい
た。式の反応は吸熱反応であり、さらに温度降下が大
きくなり、脱炭時に溶鋼温度を１６４０℃以上に保持す
るためには真空精錬の開始時の溶鋼温度を１７２０℃以
上の超高温にするか、開始〔Ｃ〕濃度を下げるしか手段
がなく、効率的ではなかった。本発明者らは、吹込みガ
スに酸素を混合することで、大きな発熱反応である式
の反応を進行させ、温度制御を可能とし、真空精錬時の
溶鋼温度を１６４０℃以上とすることを可能とした。
【００１７】
【実施例】ＳＵＳ３０４ステンレス鋼（８ｍａｓｓ％Ｎ
ｉ−１８ｍａｓｓ％Ｃｒ）で目標〔Ｃ〕濃度０．０３ｍ
ａｓｓ％以下が要求される鋼６０ｔｏｎの処理を図１に
示す実施態様で実施した。脱炭開始時の〔Ｃ〕濃度は全
て１．５ｍａｓｓ％とし、真空精錬を開始するまでは酸
素または酸素と不活性ガスの混合ガスを用いて精錬を行
った。真空精錬では開始〔Ｃ〕濃度、溶鋼温度を種々変
化させるとともに、真空度および吹込みガスの酸素比率
を変化させた。真空精錬での酸素の吹込みは〔Ｃ〕濃度
０．０５ｍａｓｓ％で終了させ、その後、Ａｒガスのみ
を吹込みながら、大気圧精錬で酸化した〔Ｃｒ〕を還元
するために、還元剤としてＦｅ−Ｓｉを添加して、目標
〔Ｃ〕濃度までの脱炭および還元を実施した。その後、
大気圧状態に戻すとともに成分調整を行い、取鍋に出鋼
した。なお、真空精錬時の全ガス供給速度は溶鋼トン当
たり０．１〜０．５Ｎｍ3 ／ｍｉｎの範囲で行った。
【００１８】表１に真空精錬の開始〔Ｃ〕濃度および真
空精錬の精錬条件の実施例を示す。本発明の実施例は、
先に示した条件を満足するようにして実施した。比較例
のＮｏ．７およびＮｏ．８は真空精錬の開始〔Ｃ〕濃度
が本発明の条件外の例、Ｎｏ．９は真空度が本発明の条
件外の例、Ｎｏ．１０およびＮｏ．１１は吹込みガスの
酸素比率が本発明の条件外の例、Ｎｏ．１２は溶鋼温度
が本発明の条件外の例である。
【００１９】実施結果を表２に示す。表中の値は本発明
例のＮｏ．１の結果を１００として、比例換算した値で
ある。本発明例ではガスコスト、還元用Ｓｉ原単位、精
錬時間および精錬コストとも低位に安定した値となって
いる。一方、比較例では表中の値のいずれかが１１０を
超えており、効率的な精錬が達成されていない。
【００２０】
【表１】
【００２１】
【表２】【００２２】
【発明の効果】本発明により、同一精錬容器を用いた含
クロム溶鋼の大気圧精錬後の真空精錬において、溶鋼温
度の制御を可能とし、溶鋼中〔Ｃｒ〕の酸化を抑え、効
率的な脱炭が可能となり、安定した精錬が達成できるた
めに、精錬コストの大幅な低減が可能となる。Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the decarburization refining of chromium-containing molten steel in which vacuum refining is performed after atmospheric refining using the same refining vessel. The oxidation of [Cr]
The present invention relates to a method for refining chromium-containing molten steel that performs decarburization efficiently and performs stable refining. [0002] Conventionally, 11 mass% like stainless steel
As a method for decarburizing chromium-containing molten steel containing chromium, an AOD method in which oxygen gas or a mixed gas of oxygen gas (hereinafter simply referred to as oxygen) and an inert gas is blown from below a bath surface is widely used. I have. In the AOD method, when the decarburization progresses and the [C] concentration in the molten steel decreases, [Cr] is easily oxidized. A method has been adopted in which the ratio of inert gas is increased and the ratio of oxygen is reduced to suppress the oxidation of [Cr]. However, in the low [C] concentration region, it takes a long time to reach the desired [C] concentration because the decarburization rate is reduced, and the ratio of the inert gas in the injected gas is high. This is economically disadvantageous because the consumption of inert gas is greatly increased. As a method of accelerating the decarburization in such a low [C] concentration region, use of a vacuum refining method can be mentioned.
For example, Japanese Patent Publication No. 60-10087 discloses a method of decarburizing high chromium stainless steel with oxygen under atmospheric pressure in order to decarburize to a low [C] concentration range of 0.03 mass% or less. = 0.2 to 0.4 mass%, after which stirring with a non-oxidizing gas is continued, but oxygen blowing is stopped, and the pressure on the steel bath is continuously reduced to about 10 Torr to cause boiling. Describes a method for performing a desired decarburization. In this method, since the supply of oxygen is stopped from a relatively high [C] concentration, the loss due to oxidation of [Cr] is reduced, but a large amount of CO gas is generated due to rapid vacuum refining. Causes explosion hazard. As a countermeasure, if the vacuum suction is made slower, there is no danger of explosion, but the elapsed time becomes longer, the temperature of the molten steel decreases, and the reaction slows down. Also, if the pressure is set to a high vacuum of 10 Torr or less,
The splash of molten steel becomes severe, causing problems such as blockage of a hopper for charging the alloy material. As a method of solving these problems, Japanese Patent Laid-Open Publication No. 3-68713 and Japanese Patent Laid-Open Publication No.
No. 9 has been proposed. The smelting method of chromium-containing molten steel described in these methods has a [C] concentration of 0.2 to
A mixed gas of a non-oxidizing gas and oxygen is used as the blowing gas up to 0.05 mass%. After the [C] concentration falls within this range, the pressure is reduced to 200 to 15 Torr, and the blowing gas is a non-oxidizing gas. Only oxidizing gas is used. In this method, refining is performed under atmospheric pressure to a relatively low [C] concentration range, so that the [Cr] oxidation loss increases. Decarburization under vacuum can suppress the oxidation of [Cr] by using only inert gas, but the oxygen source of decarburization is [O] in molten steel or oxygen in slag, and the oxygen supply rate , The decarburization speed decreases. Also,
Since it is impossible to control the temperature of the molten steel, if the temperature of the molten steel at the start of vacuum refining is low, decarburization stagnates, which is not an efficient decarburization refining method. SUMMARY OF THE INVENTION The present invention relates to a decarburization refining process for chromium-containing molten steel in which vacuum refining is performed after atmospheric refining is performed using the same refining vessel.
Starting the vacuum refining [C] concentration and the degree of vacuum during the vacuum refining are maintained in a suitable range, and controlling the temperature of the molten steel suppresses the oxidation of [Cr] in the molten steel, efficiently decarburizing, It is another object of the present invention to provide a method for refining chromium-containing molten steel that enables reduction of the reduction Si unit consumption, reduction of refining time, and refining of extremely low carbon concentration steel. SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems. The gist of the present invention is to perform vacuum refining after atmospheric pressure refining using the same refining vessel. In the refining method of chromium-containing molten steel, start the vacuum refining
[C] concentration in molten steel at the time of 0.06 mass% or more,
0.25 mass% or less, and the degree of vacuum of the vacuum refining is 3
00 Torr or less, and the oxygen gas ratio is 5% or more,
A mixed gas of oxygen gas and inert gas which is 45% or less
A method for refining chromium-containing molten steel, characterized by blowing into molten steel in a region where the [C] concentration in the molten steel is 0.05 mass% or more, and maintaining the temperature of the molten steel at 1640 ° C or more. Hereinafter, the present invention will be described in detail. The decarburization scouring of the chromium-containing molten steel of the present invention has a [C] concentration of 0.06.
This is a scouring method performed using a scouring vessel as illustrated in FIG. 1 in a range from mass% to 0.25 mass%. In the smelting vessel 1, the smelting gas 5 is blown into the chromium-containing molten steel 4 through the tuyere 2. The refining vessel 1 has a detachable exhaust hood 3 and has a capacity of 300
Decompression below rr is possible. According to the present invention, in decarburization refining of chromium-containing molten steel using vacuum refining, the molten steel temperature is raised to 1640 by using a mixed gas of oxygen and an inert gas as a blowing gas under vacuum refining at a degree of vacuum of 300 Torr or less. ℃ or more,
It focuses on the fact that the oxidation of [Cr] in molten steel can be suppressed and the decarburization rate can be kept high. Fig. 2 shows SU
The relationship between the degree of vacuum during vacuum refining and the [Cr] oxidation index when S304 stainless steel is treated with a mixed gas of an oxygen gas and an Ar gas having an oxygen ratio of 5 to 45%. Note that [C
r] Oxidation index is a value obtained by converting the average oxidation amount of [Cr] in molten steel at a degree of vacuum of 300 Torr to 1.0. The [C] concentration range of the vacuum treatment is 0.05 to
0.25 mass%, and the molten steel temperature is 1640-17.
It was in the range of 20 ° C. 2, the degree of vacuum is 300 Torr
If it exceeds r, the amount of [Cr] oxidation rapidly increases, indicating that the degree of vacuum during vacuum refining needs to be 300 Torr or less. At the beginning of vacuum refining, splash and boiling of molten steel are inevitable,
It is preferable to gradually reduce the degree of vacuum in order to suppress these as much as possible. FIG. 3 shows the relationship between the oxygen gas ratio of the blowing gas for vacuum refining and the [Cr] oxidation index when SUS304 stainless steel is subjected to vacuum refining after atmospheric refining. The [Cr] oxidation index is a value obtained by indexing the average oxidation amount of [Cr] in molten steel at an oxygen gas ratio of 45% to 1.0. The degree of vacuum during the vacuum processing is 300 Torr
And [C] concentration range is 0.05 to 0.25 ma.
ss%, and the molten steel temperature was in the range of 1640-1720 ° C. According to FIG. 3, when the oxygen gas ratio of the mixed gas exceeds 45%, the [Cr] oxidation amount sharply increases.
It is understood that the oxygen gas ratio needs to be 45% or less. When the oxygen gas ratio is less than 5%, the amount of [Cr] oxidation is small, but in this case, the supply of oxygen is entirely in the slag or molten steel [O], so that the temperature of the molten steel sharply drops during refining. In order to be able to control the temperature of molten steel, it is necessary to blow a mixed gas containing 5% or more of oxygen. Therefore, a mixed gas with an inert gas containing 5% or more and 45% or less of oxygen is required as the blowing gas. FIG. 4 shows the relationship between the average molten steel temperature and the [Cr] oxidation index during vacuum refining when SUS304 stainless steel is subjected to vacuum refining after atmospheric refining. In addition, [Cr]
The oxidation index is [Cr] in molten steel at a temperature of 1640 ° C.
Is an index value with an average oxidation amount of 1.0 being 1.0. The degree of vacuum during the vacuum processing is 300 Torr,
[C] The concentration range was 0.05 to 0.25 mass%. It can be seen from FIG. 4 that when the molten steel temperature is 1640 ° C. or higher, [Cr] oxidation is suppressed, and stable refining is possible.
In order to maintain the temperature of the molten steel at 1640 ° C. or higher, it is necessary to use a mixed gas containing oxygen as the gas to be blown, since the heat removal from the smelting vessel and the surface of the molten steel is large in vacuum refining. As a ratio, the above 5% or more, 45
% Or less. The higher the molten steel temperature, the lower the [Cr] oxidation, but if it is too high, the refractory in the refining vessel will be severely worn.
20 ° C. or lower is preferred. The supply of the mixed gas containing oxygen is as follows:
When the [C] concentration is less than 0.05 mass%, the [Cr] oxidation is large and the amount of decarburized [C] is small. Therefore, the [C] concentration needs to be 0.05 mass% or more. On the other hand, if the [C] concentration at which vacuum refining is started is too high, the load of vacuum refining increases and the loss due to splashing or boiling of molten steel increases, and if it is too low, the effect of vacuum refining cannot be enjoyed and atmospheric refining cannot be performed. From the above, the concentration in the [C] concentration range is 0.06 mass%.
As described above, the content needs to be 0.25 mass% or less. As described above, in order to suppress the oxidation of [Cr] in the molten steel and to efficiently perform the refining of the chromium-containing molten steel, the vacuum refining is performed in the range of the [C] concentration of 0.06 mass% or more and 0.25 mass% or less. At a vacuum of 300 Torr or less, a gas mixture with an inert gas containing 5% or more and 45% or less of oxygen is blown as a blowing gas, and the molten steel temperature is set to 1640.
It is effective to set the temperature to not less than ° C. In the operation, since the time change of the [C] concentration can be roughly predicted, the molten steel composition and the molten steel temperature at the time of charging the molten steel are grasped, and the timing for starting the vacuum refining is determined. During vacuum refining, it is possible to determine the gas injection conditions and the degree of vacuum while grasping the [C] concentration and the temperature of the molten steel and observing the conditions in the furnace. Particularly when the molten steel temperature is low, it is possible to increase the oxygen ratio of the blown gas to prevent the temperature from dropping. According to the operation method, it is possible to prevent a large amount of splashes of molten steel from occurring, and a stable operation is possible. In the decarburization and refining of chromium-containing molten steel, the injected oxygen is converted into chromium oxide (Cr 2 O 3 ) by a reaction represented by the following formula:
Is generated, and then the decarburization reaction represented by the formula is considered to proceed. The reaction equilibrium constant K in the equation is represented by the equation. 2 Cr + 3 / 2O 2 ( g) = (Cr 2 O 3) ... (Cr 2 O 3) +3 C = 2 Cr + 3CO (g) ... K = a cr203 · a c 2 / a cr 2 · P CO 3 ... Here, a cr203 is the activity of (Cr 2 O 3 ) in the slag, a
c is the activity of the molten steel (C), a cr the activity of the molten steel [Cr], P CO denotes a CO gas partial pressure in the atmosphere. D. C. Hilty et al. Experimentally obtained the relationship between the expressions and presented the expressions. log ([Cr] · P CO /〔C〕=-13800/Ttasu8.76... where, T is from. Expressions a relationship showing molten steel temperature (K),
It can be seen that when the [Cr] concentration is constant, PCO and T should be controlled in accordance with the [C] concentration in order to efficiently advance decarburization. In vacuum refining, the degree of vacuum P in the refining vessel
Is proportional to the CO gas partial pressure Pco in the atmosphere. Therefore, it is possible to control the P co by controlling the degree of vacuum refining vessel. The present inventors [C] concentration 0.06m
Starting the application of vacuum refining in the range of ass% or more, preferably 0.25 mass% or less, and setting the upper limit of the degree of vacuum to 300 Torr enables efficient decarburization while suppressing [Cr] oxidation. Was found. Further, it has been found that efficient decarburization is possible by setting the molten steel temperature to 1640 ° C. or higher. In vacuum refining, heat removal from the molten steel surface and heat removal from the refining vessel are greater than in atmospheric refining. In the conventional method, during vacuum refining, only the inert gas is blown to advance the reaction of the formula. The reaction of the formula is an endothermic reaction, and the temperature drop further increases. To maintain the molten steel temperature at 1640 ° C. or more during decarburization, the molten steel temperature at the start of vacuum refining is set to an ultra-high temperature of 1720 ° C. or more, There was no alternative but to reduce the starting [C] concentration, which was not efficient. The present inventors have made it possible to control the temperature by mixing oxygen into the blown gas to promote the reaction of a large exothermic reaction, thereby enabling the molten steel temperature during vacuum refining to be 1640 ° C. or higher. And EXAMPLE SUS304 stainless steel (8 mass% N
i-18 mass% Cr) and target [C] concentration 0.03m
The treatment of 60 tons of steel requiring ass% or less was carried out in the embodiment shown in FIG. The concentration of [C] at the start of decarburization was 1.5 mass%, and refining was performed using oxygen or a mixed gas of oxygen and an inert gas until vacuum refining was started. In the vacuum refining, the starting [C] concentration and the molten steel temperature were variously changed, and the degree of vacuum and the oxygen ratio of the blown gas were also changed. Injection of oxygen in vacuum refining is terminated at a [C] concentration of 0.05 mass%, and then Fe is used as a reducing agent in order to reduce [Cr] oxidized by atmospheric pressure refining while blowing only Ar gas. -Carbon was added and decarburization and reduction were performed up to the target [C] concentration. afterwards,
Atmospheric pressure was restored and the components were adjusted. The total gas supply rate during vacuum refining was in the range of 0.1 to 0.5 Nm 3 / min per ton of molten steel. Table 1 shows examples of the concentration [C] at which vacuum refining starts and the refining conditions for vacuum refining. Embodiments of the present invention
The test was performed so as to satisfy the conditions described above. No. of the comparative example. 7 and No. 7 No. 8 is an example in which the [C] concentration at the start of vacuum refining is out of the condition of the present invention. No. 9 is an example in which the degree of vacuum is out of the condition of the present invention. 10 and no. No. 11 is an example in which the oxygen ratio of the blown gas is out of the condition of the present invention. 12 is an example in which the molten steel temperature is out of the condition of the present invention. Table 2 shows the results of the operation. The values in the table are No. of the present invention. This is a value obtained by proportionally converting the result of 1 to 100. In the example of the present invention, the gas cost, the basic unit of Si for reduction, the refining time and the refining cost are all low and stable values. On the other hand, in the comparative example, any of the values in the table exceeded 110, and efficient refining was not achieved. [Table 1] [Table 2] According to the present invention, in vacuum refining of chromium-containing molten steel after atmospheric pressure refining using the same refining vessel, it is possible to control the temperature of the molten steel, suppress the oxidation of [Cr] in the molten steel, and improve the efficiency. Decarburization becomes possible and stable refining can be achieved, so that refining costs can be significantly reduced.
【図面の簡単な説明】
【図１】本発明の実施態様例の精錬容器を示す図であ
る。
【図２】本発明における真空度の上限の限定理由を示す
図である。
【図３】本発明における吹込みガスの酸素ガス比率の上
限の限定理由を示す図である。
【図４】本発明における溶鋼温度の下限の限定理由を示
す図である。
【符号の説明】
１ 精錬容器
２ 底吹き羽口
３ 排気フード
４ 溶鋼
５ ガスBRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a smelting vessel according to an embodiment of the present invention. FIG. 2 is a diagram showing the reason for limiting the upper limit of the degree of vacuum in the present invention. FIG. 3 is a diagram showing the reason for limiting the upper limit of the oxygen gas ratio of the blowing gas in the present invention. FIG. 4 is a diagram showing the reason for limiting the lower limit of the molten steel temperature in the present invention. [Description of Signs] 1 Refining vessel 2 Bottom blowing tuyere 3 Exhaust hood 4 Molten steel 5 Gas
───────────────────────────────────────────────────── フロントページの続き (56)参考文献 特開 平６−330143（ＪＰ，Ａ) 特開 平７−233408（ＪＰ，Ａ) 特開 平７−188727（ＪＰ，Ａ) 特開 平４−254509（ＪＰ，Ａ) 特開 昭57−181322（ＪＰ，Ａ) 特開 昭61−99619（ＪＰ，Ａ) 特公 昭54−6483（ＪＰ，Ｂ１) 特公 昭49−46203（ＪＰ，Ｂ１) (58)調査した分野(Int.Cl.7，ＤＢ名) C21C 7/068 C21C 7/00 C21C 7/10 ────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-6-330143 (JP, A) JP-A-7-233408 (JP, A) JP-A-7-188727 (JP, A) JP-A-4- 254509 (JP, A) JP-A-57-181322 (JP, A) JP-A-61-99619 (JP, A) JP-B-54-6483 (JP, B1) JP-B-49-46203 (JP, B1) (58) Field surveyed (Int. Cl. 7 , DB name) C21C 7/068 C21C 7/00 C21C 7/10
Claims (1)
真空精錬を行う含クロム溶鋼の精錬法において、前記真
空精錬を開始する時の溶鋼中の〔Ｃ〕濃度を０．０６ｍ
ａｓｓ％以上、０．２５ｍａｓｓ％以下とし、該真空精
錬の真空度を３００Ｔｏｒｒ以下とし、かつ酸素ガス比
率を５％以上、４５％以下とする酸素ガスと不活性ガス
との混合ガスを溶鋼中の〔Ｃ〕濃度０．０５ｍａｓｓ％
以上の領域で溶鋼に吹込み、該溶鋼の温度を１６４０℃
以上に保持することを特徴とする含クロム溶鋼の精錬
法。(57) [Claims] [Claim 1] After atmospheric pressure refining using the same refining vessel,
In the method of refining chromium-containing molten steel for performing vacuum refining, the concentration of [C] in the molten steel at the time of starting the vacuum refining is 0.06 m.
ass% or more and 0.25 mass% or less.
The degree of vacuum refining to less 300 Torr, and oxygen gas ratio of 5% or more, in the molten steel, a mixed gas of oxygen gas and inert gas to be 45% or less [C] concentration 0.05 mass%
In the above area, the molten steel is blown and the temperature of the molten steel is increased to 1640 ° C.
A method for refining chromium-containing molten steel, characterized by maintaining the above.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP02456494A JP3439517B2 (en) | 1994-02-22 | 1994-02-22 | Refining method of chromium-containing molten steel |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP02456494A JP3439517B2 (en) | 1994-02-22 | 1994-02-22 | Refining method of chromium-containing molten steel |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH07233409A JPH07233409A (en) | 1995-09-05 |
| JP3439517B2 true JP3439517B2 (en) | 2003-08-25 |
Family
ID=12141657
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP02456494A Expired - Lifetime JP3439517B2 (en) | 1994-02-22 | 1994-02-22 | Refining method of chromium-containing molten steel |
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| Country | Link |
|---|---|
| JP (1) | JP3439517B2 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2002212625A (en) * | 2001-01-17 | 2002-07-31 | Daido Steel Co Ltd | Method for decarburizing molten chromium-containing steel |
| JP4532106B2 (en) * | 2001-07-02 | 2010-08-25 | 新日本製鐵株式会社 | Method for decarburizing and refining chromium-containing molten steel |
-
1994
- 1994-02-22 JP JP02456494A patent/JP3439517B2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| JPH07233409A (en) | 1995-09-05 |
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