【発明の詳細な説明】[Detailed description of the invention]
本発明は含クロム溶鋼の精錬方法に係り、詳し
くは、少なくとも鋼浴下面から酸素と不活性ガス
を含む混合ガスまたは酸素または不活性ガスを吹
込んで脱炭する際の脱炭末期または仕上精錬を行
なう際の空気による窒素の吸収を有効に防止して
精錬できる含クロム溶鋼の精錬方法に係る。
現在、ステンレス鋼等の含クロム溶鋼の脱炭プ
ロセスとしてAOD法が広く用いられている。こ
のAOD法では、第１図に示す如く、精錬容器１
の底部に設けられた二重羽口２から不活性ガスと
酸素の混合ガス３を含クロム溶鋼４中に底吹き
し、炭素−クロムの平衡条件を規制するPco値を
低下させ、これによつて、クロム（以下、Crと
いう。）を優先酸化させることなく、脱炭を行な
つている。その後は、更に仕上げ精錬が行なわ
れ、このときには、けい素などの還元剤や、石
灰、ホタル石等の造滓剤が添加され、二重羽口２
からは不活性ガスのみを吹込んで溶鋼とスラグと
を撹拌して精錬が行なわれる。
また、AOD法の如く酸素を底吹きして脱炭す
る場合、その脱炭機構はガス吹込時に羽口近傍に
おいてCrが酸化されてクロム酸化物が生成され、
このクロム酸化物が溶鋼中に上昇するガス気泡の
界面で炭素と反応して脱炭が進行すると云われて
いる。
従つて、脱炭酸素効率は、炭素（以下、Ｃとい
う。）のガス気泡界面への移動量とガス気泡界面
上の脱炭反応量のバランスで決まり、脱炭反応の
進行にともなつて溶鋼中のＣ量が減少することに
よりバランスがくずれ、羽口近傍のCr分の酸化
反応が優勢になる。このため、低Ｃ領域に近づく
に従つて、脱炭酸素効率が低下する。例えば、
Cr18％を含む溶鋼に酸素を底吹きして脱炭処理
したときの溶鋼中のＣ量と脱炭酸素効率を示す
と、第２図に示す通りであつて、第２図から明ら
かな通り、溶鋼中のＣ量の低下にともなつて脱炭
酸素効率は低下し、とくに、C1.0％以上の高炭素
領域ではその低下割合は比較的緩やかであるが、
C1.0％以下の低Ｃ領域では低下割合は著しくな
り、C0.40％以下になると、脱炭酸素効率は極端
に低下し、底吹きガスから生成される排ガス量も
著しく低下していく。また、仕上精錬期では不活
性ガス量は溶鋼の撹拌に供するのみであるため、
排ガス量も少ない。
この溶鋼からの排ガス量の低下および吹込みガ
ス量の低下は、必然的に精錬容器中の炉内圧の低
下を招来し、外部からの容器内への空気の侵入を
助長する。このため、通常のAOD操業条件では、
脱炭末期および仕上精錬期には溶鋼中に侵入空気
より窒素が多量に吸収され、暫次窒素含有量が増
加する傾向になる。高純度のフエライト鋼等で
は、溶鋼中の窒素含有量は100PPm以下に制御す
る必要があつて、脱炭末期および仕上精錬期に空
気による窒素吸収が制御できる精錬方法が望まれ
ている。
本発明は上記欠点の解決を目的とし、具体的に
は、脱炭酸素効率が著しく低下する脱炭末期およ
び仕上精錬期において、空気侵入による窒素吸収
が効果的に制御して精錬でき、高純度のステンレ
ス鋼が製造できる含Cr溶鋼の精錬方法を提案す
る。
すなわち、本発明方法は少なくとも鋼浴下面か
ら酸素と不活性ガスの混合ガスまたは酸素または
不活性ガスを吹込んで、含クロム溶鋼を炭素量が
所定値に達するまで脱炭し、さらに炉頂よりの上
吹ランスを併用して鋼浴上または炉内に不活性ガ
スを上吹きして精錬することを特徴とする。
以下、本発明方法について詳しく説明する。
まず、含Cr溶鋼に対して少なくとも鋼浴下面
から酸素ならびに不活性ガスの混合ガスまたは酸
素または不活性ガスを個別的に吹込み、溶鋼中の
Ｃ量が所定値に達し脱炭酸素効率が著しく低下す
るまで脱炭する。
この脱炭処理は第１図に示す如く、AOD法に
よる底吹きによつて行なうこともできるが、第３
図に示す如く底吹きと上吹きを併用しても行なう
ことができる。
すなわち、第３図は後者の如く底吹きと上吹き
とを併用して脱炭処理する場合の説明図であつ
て、精錬容器１の底部に第１図で示すAOD法と
同様に二重羽口２が設けられるが、炉頂部には上
吹ランス５がランス昇降装置６によつて昇降自在
に設けられている。
従つて、精錬容器１中の含Cr溶鋼４には、羽
口２から不活性ガスと酸素の混合ガス３が底吹き
されると同時に、上吹ランス５から酸素７が上吹
きされて脱炭処理が行なわれる。なお、不活性ガ
ス、酸素を個別的に底吹きすることができる。ま
た、この場合、上吹ランス５の鋼浴面からのラン
ス高さ（以下、ランス高さという）や、上吹き酸
素量は含Cr溶鋼中のＣ量の推移、つまり、含Cr
溶鋼中における脱炭反応の進行度合に応じて制御
して精錬される。このように、精錬すると、容量
の大きな精錬容器を用いなくとも、吹込酸素量を
大きくして脱炭速度を速めることができると共
に、酸素吹込みによる脱炭反応時の生成熱も有効
に利用できる。
まず、羽口２からの底吹き混合ガス３の慣性エ
ネルギーによつて鋼浴表面からスプラツシユが発
生するが、このスプラツシユは上吹ランス５から
の酸素ジエツト７の下降エネルギーによつておさ
えられ、このため、スプラツシユの盛上り高さは
低くおさえられ、精錬容器の容量を大きくなくと
も吹込酸素量を増やすことができる。更に詳しく
説明すると、例えば、第２図に示す如く、Cr18
％程度の溶鋼でC0.4％以上の如き領域では、吹込
酸素量に比例して脱炭速度が速められ、精錬時間
の短縮ならびに作業能率の改善の上から多量の酸
素を吹込んで、脱炭速度を速めるのが好ましい。
しかし、AOD法の如く底吹きのみにより酸素
を吹込む場合は、酸素吹込量を増加させると、炉
壁耐火物の溶損を招来するほか、鋼浴表面から飛
散するスプラツシユの発生量が増大すると共に、
スプラツシユの発生高さが高くなる。
従つて、これに対応するよう、精錬容器の容量
を大きくして酸素吹込量の増大をはかる必要があ
り、精錬容器の大型化は築炉のための耐火物量の
増大、耐火物溶損量の増大によつて、耐火物原単
位の著しい高騰となる。この点から、通常、
AOD法は、0.7〜1Nm3O2／ｔ／分程度に設計さ
れた精錬容器で脱炭されていて、吹込酸素量を増
大することがむずかしい。
この点、上記の如く混合ガスによつて底吹きす
ると同時に、上吹ランスによつて上吹きする場合
は、例えば、2Nm3O2／ｔ／分の如く吹込酸素量
を増大しても、鋼浴表面から発生するスプラツシ
ユの発生量や高さを低くおさえることができるの
で全く支障がなく、脱炭反応時の生成熱も上吹き
酸素によつて有効に利用できる。
また、酸素を上吹きする場合、その上吹きラン
ス高さや、上吹き酸素量を溶鋼中のＣ量の推移に
合わせて制御するのが好ましく、とくに、上吹き
ランスのランス高さや上吹き酸素量は脱炭初期か
ら終期までハードブローの条件からソフトブロー
の条件に連続的に変化させるのが好ましい。
すなわち、精錬反応からみると、上吹きランス
からの酸素ジエツトの運動量が増大するほど、所
謂ハードブローの条件になり、脱炭反応効率が向
上する。これに反し、酸素ジエツトの運動量が低
い場合は、所謂ソフトブローの条件となり、上吹
き酸素の鋼浴表面に達する割合が少なくなるが、
この上吹き酸素によつて脱炭反応で生成放出され
るCOガスが有効に再燃焼され、この再燃焼によ
つて、CO＋１／２O2→CO2の式により反応熱が生成
され、反応熱は有効に鋼浴面に伝達されて鋼浴温
度が上昇し、脱炭反応は改善される。
上記の如く、含Cr溶鋼を酸素の底吹き処理す
る場合の脱炭反応は、ガス吹込羽口近傍に生成さ
れるクロム酸化物とＣとの上昇ガス気泡の界面に
おける反応バランスによつて決まり、脱炭反応が
進行し末期にいたると、溶鋼中のＣ量の減少によ
りバランスがくずれてCr分の酸化反応が優勢に
なり、第２図に示す如く、脱炭酸素効率が低下す
る。従つて、上吹きランスのランス高さ、上吹き
酸素量は、例えば、Cr18％程度を含む溶鋼の場
合、例えば、C1.0％以上の高炭素領域では上吹き
ランスの条件はハードブローの条件としてＣを直
接酸化し、COを再燃焼するが、C1.0％以下の低
炭素領域では、溶鋼中のＣの直接酸化を起こさせ
ることなく、主としてCOの再燃焼を行なう条件、
所謂ソフトブローの条件となる。
次に、上記の如く溶鋼中のＣが所定値に達する
まで脱炭し、さらに、上吹きランスからアルゴン
その他の不活性ガスを炉内または鋼浴表面に吹込
む。この上吹きランスからアルゴンその他の不活
性ガスを炉内または鋼浴表面に吹込む工程が本発
明においては、きわめて重要な意義を有するもの
である。このように不活性ガスを吹込むと、脱炭
末期に十分な排ガス量が確保でき、空気の侵入が
防止でき、侵入空気によつての溶鋼中への窒素の
吸収が十分に防止でき、高純度のステンレス鋼が
製造できる。
すなわち、脱炭が進行し、例えば、C0.40％
（18％Crの溶鋼の場合）の如く所定値までＣ量が
低減し、それ以下の領域に達すると、上記の如
く、底吹きガスから生成される排ガス量も著しく
低下する。この排ガス量の低下は必然的に精錬容
器中の炉内圧の低下を招来し、外部からの空気の
侵入を助長する。しかし、この点、上記の如く、
上吹きランスによつて不活性ガスを導入すると、
脱炭末期であつても十分な排ガス量が確保でき、
空気の侵入は十分に防止でき、溶鋼中の窒素含有
量は100PPm以下の如くきわめて低く制御でき
る。
この脱炭末期の不活性ガス吹込みは、常法の
AOD法で溶製する場合でも、更に、上吹き併用
により溶製する場合であつても、酸素効率が著し
く低い領域において充分な排ガス量が確保でき、
空気による窒素吸収が制御できて、きわめて低い
窒素含有量の鋼が製造でき、きわめて有効であ
る。
次に、実施例について説明する。
まず、18％Crならびに1.5％Ｃを含む溶鋼55ト
ンを用いて、超低炭素、低窒素のSUS−430鋼を
目標（Ｃ＋Ｎ＜200PPm）として脱炭溶製処理を
行なつた。この場合、本発明法によつて脱炭精錬
する場合には、C1.5％〜0.40％以下まで酸素を底
吹きと共に上吹きを行なつて脱炭し、その後は上
吹きランスからはアルゴンのみを上吹きし、最後
の仕上げ精錬では底吹きもアルゴンのみを用い
た。なお、この条件は第１表に示す通りであつ
た。
これに対し、比較のために、従来例のAOD法
によつて上記のところと同じ組成の溶鋼を溶製し
た。この比較例では第１表に示す如く、C1.5％か
らC0.40％まで混合ガス（アルゴン＋酸素）を底
吹きし、その後も、混合ガスを底吹きし、最後の
仕上げ精錬の場合はアルゴンのみを底吹きした
が、C0.40％以後では本発明法の如くアルゴンの
上吹きは行なわなかつた。
以上の通りに、本発明法と比較例とによつて溶
製し、溶製後の溶鋼中の最終窒素含有量
（PPm）、最終Ｃ含有量（PPm）のほか、C0.40％
までの脱炭速度（％／分）や、精錬終了時までに
金属酸化物の還元に消費されたSi原単位を示す
と、第１表の通りであつた。
The present invention relates to a method for refining chromium-containing molten steel, and more particularly, the present invention relates to a method for refining chromium-containing molten steel, and more specifically, the final stage of decarburization or final refining during decarburization by blowing a mixed gas containing oxygen and an inert gas or oxygen or an inert gas from at least the bottom surface of a steel bath. The present invention relates to a method for refining chromium-containing molten steel that can effectively prevent absorption of nitrogen by air during refining. Currently, the AOD method is widely used as a decarburization process for chromium-containing molten steel such as stainless steel. In this AOD method, as shown in Figure 1, the refining vessel 1
A mixed gas 3 of inert gas and oxygen is bottom-blown into the chromium-containing molten steel 4 from a double tuyere 2 provided at the bottom of the tuyere, thereby lowering the Pco value that regulates the carbon-chromium equilibrium condition. Therefore, decarburization is performed without preferentially oxidizing chromium (hereinafter referred to as Cr). After that, further finishing smelting is performed, and at this time, reducing agents such as silicon and slag-forming agents such as lime and fluorspar are added, and the double tuyere 2
From there, only inert gas is blown in to stir the molten steel and slag to perform refining. In addition, when decarburizing by bottom-blowing oxygen as in the AOD method, the decarburization mechanism is such that Cr is oxidized near the tuyere during gas injection and chromium oxide is generated.
It is said that decarburization progresses as this chromium oxide reacts with carbon at the interface of gas bubbles rising into the molten steel. Therefore, decarburization oxygen efficiency is determined by the balance between the amount of carbon (hereinafter referred to as C) transferred to the gas bubble interface and the amount of decarburization reaction on the gas bubble interface, and as the decarburization reaction progresses, molten steel As the amount of C inside decreases, the balance is disrupted, and the oxidation reaction of the Cr component near the tuyere becomes dominant. Therefore, as the low C region approaches, the decarburization oxygen efficiency decreases. for example,
When molten steel containing 18% Cr is decarburized by bottom-blowing oxygen, the amount of C in molten steel and the decarburization oxygen efficiency are shown in Figure 2, and as is clear from Figure 2, As the amount of C in molten steel decreases, the decarburization oxygen efficiency decreases, and the rate of decrease is relatively gradual, especially in the high carbon region of C1.0% or more.
In the low C region of C1.0% or less, the rate of decrease becomes remarkable, and when C falls below 0.40%, the decarburization oxygen efficiency decreases extremely and the amount of exhaust gas generated from the bottom-blown gas also decreases significantly. In addition, in the final refining stage, the amount of inert gas is only used for stirring the molten steel.
The amount of exhaust gas is also low. This decrease in the amount of exhaust gas from the molten steel and the decrease in the amount of blown gas inevitably leads to a decrease in the pressure inside the furnace in the refining vessel, which encourages air to enter the vessel from the outside. Therefore, under normal AOD operating conditions,
At the final stage of decarburization and final refining, a large amount of nitrogen is absorbed into the molten steel from the invading air, and the nitrogen content tends to increase temporarily. For high-purity ferrite steel, etc., it is necessary to control the nitrogen content in molten steel to 100 PPm or less, and a refining method that can control nitrogen absorption by air at the final stage of decarburization and the final refining stage is desired. The purpose of the present invention is to solve the above-mentioned drawbacks. Specifically, in the final stage of decarburization and in the final refining stage, where the oxygen efficiency of decarburization is significantly reduced, nitrogen absorption due to air entry can be effectively controlled to achieve high purity. We propose a refining method for Cr-containing molten steel that can produce stainless steel. That is, the method of the present invention decarburizes chromium-containing molten steel by injecting a mixed gas of oxygen and inert gas or oxygen or inert gas from at least the bottom of the steel bath until the carbon content reaches a predetermined value, and then decarburizes the chromium-containing molten steel from the top of the furnace. It is characterized by the use of a top blowing lance to top blow inert gas onto the steel bath or into the furnace for refining. The method of the present invention will be explained in detail below. First, a mixed gas of oxygen and inert gas or oxygen or inert gas is individually blown into the Cr-containing molten steel from at least the bottom of the steel bath until the amount of C in the molten steel reaches a predetermined value and the decarburization oxygen efficiency is significantly improved. Decarburize until it drops. This decarburization treatment can be carried out by bottom blowing using the AOD method as shown in Figure 1, but
As shown in the figure, bottom blowing and top blowing can be used in combination. That is, FIG. 3 is an explanatory diagram of the latter case where bottom blowing and top blowing are used together for decarburization treatment, and a double blade is installed at the bottom of the refining vessel 1 as in the AOD method shown in FIG. A mouth 2 is provided, and a top blowing lance 5 is provided at the top of the furnace so that it can be raised and lowered by a lance lifting device 6. Therefore, a mixed gas 3 of inert gas and oxygen is blown from the bottom of the chromium-containing molten steel 4 in the refining vessel 1 from the tuyere 2, and at the same time, oxygen 7 is blown from the top from the top blowing lance 5 to decarburize the steel. Processing is performed. Note that inert gas and oxygen can be individually blown from the bottom. In this case, the height of the top blowing lance 5 from the steel bath surface (hereinafter referred to as the lance height) and the amount of top blowing oxygen are determined by the change in the amount of C in the Cr-containing molten steel, that is, the Cr-containing molten steel.
Refining is controlled according to the progress of the decarburization reaction in molten steel. In this way, by refining, the decarburization rate can be increased by increasing the amount of oxygen blown without using a large-capacity smelting vessel, and the heat generated during the decarburization reaction due to oxygen injection can also be effectively used. . First, a splash is generated from the surface of the steel bath due to the inertial energy of the bottom-blown mixed gas 3 from the tuyere 2, but this splash is suppressed by the downward energy of the oxygen jet 7 from the top-blowing lance 5. Therefore, the height of the splash can be kept low, and the amount of oxygen blown can be increased without increasing the capacity of the refining vessel. To explain in more detail, for example, as shown in Figure 2, Cr18
% of molten steel with a carbon content of 0.4% or more, the decarburization speed is increased in proportion to the amount of blown oxygen, and in order to shorten refining time and improve work efficiency, a large amount of oxygen can be blown in to decarburize. It is preferable to increase the speed. However, when oxygen is injected only by bottom blowing as in the AOD method, increasing the amount of oxygen injected leads to melting of the furnace wall refractories and increases the amount of splash generated from the steel bath surface. With,
Increases the height of splashes. Therefore, in order to cope with this, it is necessary to increase the capacity of the refining vessel and increase the amount of oxygen blown into it.Increasing the size of the refining vessel increases the amount of refractory for furnace construction and reduces the amount of refractory corrosion. This increase will result in a significant rise in the unit consumption of refractories. From this point on, usually
In the AOD method, decarburization is carried out in a refining vessel designed to produce about 0.7 to 1 Nm 3 O 2 /t/min, and it is difficult to increase the amount of blown oxygen. In this regard, when bottom blowing is performed using a mixed gas as described above and top blowing is performed simultaneously using a top blowing lance, even if the amount of oxygen blown is increased to, for example, 2Nm 3 O 2 /t/min, the steel Since the amount and height of the splash generated from the bath surface can be kept low, there is no problem at all, and the heat generated during the decarburization reaction can also be effectively used by the top-blown oxygen. In addition, when top-blowing oxygen, it is preferable to control the top-blowing lance height and the top-blowing oxygen amount in accordance with the change in the amount of C in the molten steel. In particular, the top-blowing lance height and the top-blowing oxygen amount are It is preferable to continuously change from hard blowing conditions to soft blowing conditions from the initial stage to the final stage of decarburization. That is, from the perspective of the refining reaction, the greater the momentum of the oxygen jet from the top blowing lance, the more conditions are met for so-called hard blowing, and the efficiency of the decarburization reaction improves. On the other hand, if the momentum of the oxygen jet is low, this will result in a so-called soft blow condition, and the proportion of top-blown oxygen reaching the steel bath surface will be small;
CO gas generated and released in the decarburization reaction is effectively re-combusted by this top-blown oxygen, and through this re-combustion, reaction heat is generated according to the formula CO + 1/2 O 2 → CO 2 , and the reaction heat is This is effectively transmitted to the steel bath surface, raising the steel bath temperature and improving the decarburization reaction. As mentioned above, the decarburization reaction when Cr-containing molten steel is bottom-blown with oxygen is determined by the reaction balance at the interface of the rising gas bubbles between chromium oxide and C generated near the gas injection tuyeres. As the decarburization reaction progresses to its final stage, the balance is lost due to the decrease in the amount of C in the molten steel, and the oxidation reaction of the Cr component becomes dominant, resulting in a decrease in the decarburization oxygen efficiency, as shown in FIG. Therefore, the lance height of the top blowing lance and the amount of top blowing oxygen are, for example, in the case of molten steel containing about 18% Cr. For example, in the high carbon range of 1.0% or more, the conditions of the top blowing lance are hard blowing conditions. However, in the low carbon region of less than 1.0% C, the conditions are such that CO is mainly reburned without causing direct oxidation of C in molten steel.
This is a condition for what is called a soft blow. Next, as described above, the molten steel is decarburized until the C content reaches a predetermined value, and then argon or other inert gas is blown into the furnace or the surface of the steel bath from the top blowing lance. The step of blowing argon or other inert gas into the furnace or onto the surface of the steel bath from this top blowing lance has extremely important significance in the present invention. By blowing inert gas in this way, it is possible to secure a sufficient amount of exhaust gas at the final stage of decarburization, prevent air from entering, and sufficiently prevent nitrogen from being absorbed into the molten steel by the intruding air. We can produce high purity stainless steel. In other words, decarburization progresses and, for example, C0.40%
When the amount of C is reduced to a predetermined value as shown in (in the case of 18% Cr molten steel) and reaches a region below that value, the amount of exhaust gas generated from the bottom blowing gas also decreases significantly as described above. This decrease in the amount of exhaust gas inevitably causes a decrease in the pressure inside the furnace in the refining vessel, which promotes the intrusion of air from the outside. However, in this regard, as mentioned above,
When inert gas is introduced by a top blowing lance,
Sufficient amount of exhaust gas can be secured even in the final stage of decarbonization.
Intrusion of air can be sufficiently prevented, and the nitrogen content in molten steel can be controlled to an extremely low level of 100 PPm or less. This inert gas injection at the final stage of decarburization is carried out using the conventional method.
Even when melting using the AOD method or using top blowing in combination, a sufficient amount of exhaust gas can be secured in areas where oxygen efficiency is extremely low.
It is extremely effective because nitrogen absorption by air can be controlled and steel with extremely low nitrogen content can be produced. Next, examples will be described. First, using 55 tons of molten steel containing 18% Cr and 1.5% C, a decarburization process was carried out with the goal of producing ultra-low carbon, low nitrogen SUS-430 steel (C+N<200PPm). In this case, when decarburizing and refining using the method of the present invention, decarburization is performed by bottom blowing oxygen and top blowing to below C1.5% to 0.40%, and then only argon is used from the top blowing lance. Top blowing was carried out, and in the final refining, only argon was used for bottom blowing. The conditions were as shown in Table 1. On the other hand, for comparison, molten steel having the same composition as above was produced by the conventional AOD method. In this comparative example, as shown in Table 1, a mixed gas (argon + oxygen) is bottom blown from 1.5% C to 0.40% C, and after that, the mixed gas is bottom blown, and in the case of the final refining. Only argon was blown from the bottom, but argon was not blown from the top as in the method of the present invention when the carbon content was 0.40% or higher. As described above, in addition to the final nitrogen content (PPm) and final C content (PPm) of molten steel produced by the method of the present invention and the comparative example, the final nitrogen content (PPm) and final C content (PPm) of the molten steel after melting were determined to be 0.40% C.
Table 1 shows the decarburization rate (%/min) and the unit Si consumed for reduction of metal oxides until the end of refining.
【表】
第１表で対比して示したところから明かな通
り、本発明法の如く、脱炭末期および仕上精錬期
において、アルゴンを上吹きして排ガス量を増加
させた場合は、最終の窒素分は大巾に低下するこ
とが判る。
以上詳しく説明した通り、本発明法は、含Cr
溶鋼をＣ量が所定値に達するまで脱炭し、さらに
炉頂よりの上吹きランスを併用して鋼浴上または
炉内に不活性ガスを上吹きして精錬する方法であ
るから、脱炭末期および仕上精錬期においても十
分な排ガス量が確保でき、容器内に大気が流入し
ないので空気の侵入によつて、窒素分が溶鋼中に
入るのを防止できる。従つて、極低窒素の含クロ
ム溶鋼、とくに、SUS−430鋼の如きステンレス
鋼も容易に溶製できる。[Table] As is clear from the comparison shown in Table 1, when the amount of exhaust gas is increased by top blowing argon at the final stage of decarburization and the final refining stage as in the method of the present invention, the final It can be seen that the nitrogen content drops significantly. As explained in detail above, the method of the present invention is suitable for Cr-containing
This is a method of decarburizing molten steel until the amount of C reaches a predetermined value, and then refining by blowing inert gas over the steel bath or into the furnace using a top blowing lance from the top of the furnace. A sufficient amount of exhaust gas can be ensured even in the final and final refining stages, and since air does not enter the container, nitrogen can be prevented from entering the molten steel due to air intrusion. Therefore, extremely low nitrogen chromium-containing molten steel, especially stainless steel such as SUS-430 steel, can be easily produced.
【図面の簡単な説明】[Brief explanation of drawings]
第１図は従来例のAOD法で使用する精錬容器
の配置図、第２図は含Cr溶鋼の底吹き脱炭時の
溶鋼中のＣ量と脱炭酸素効率との関係を示すグラ
フ、第３図は底吹きと同時に上吹きを行なうこと
ができる精錬容器の配置図である。
符号１……精錬容器、２……二重羽口、３……
混合ガス、４……含Cr溶鋼、５……上吹きラン
ス、６……ランス昇降装置、７……酸素。
Figure 1 is a layout diagram of a refining vessel used in the conventional AOD method, Figure 2 is a graph showing the relationship between the amount of C in molten steel and the decarburization oxygen efficiency during bottom blowing decarburization of Cr-containing molten steel. Figure 3 is a layout diagram of a refining vessel that can perform bottom blowing and top blowing at the same time. Code 1... Refining vessel, 2... Double tuyere, 3...
Mixed gas, 4... Cr-containing molten steel, 5... Top blowing lance, 6... Lance lifting device, 7... Oxygen.