【発明の詳細な説明】
本発明は鋼の希釈脱炭法において、脱炭効率を
より一層向上させるための方法に関するものであ
る。
一般に溶鋼中〔Ｃ〕はその温度、含有〔Cr〕
％が一定であれば溶鋼中炭素と接触する気相中の
COガス分圧の低いほど脱炭が進行する。このCO
ガス分圧（以下Pcoと略す）を低くするために溶
鋼と接触する界面を真空にしたり、希釈ガスを酸
素ガスと共に吹込んだりして、Pcoを低下させて
いるのである。
希釈脱炭法においては大量の酸素ガスと希釈ガ
スとが炉底又は炉底近傍の側壁から溶鋼中に吹込
まれるため、希釈ガスと溶鋼との接触界面は非常
に大きくなり、希釈ガス中にCOが移行し、Pco
は希釈され脱炭が容易に進行する。しかし前述し
た如く溶鋼中の温度、〔Cr〕％と平衡するPcoが
達成されれば脱炭は停止する。脱炭を更に効率よ
く行うためには希釈ガスを更に大量に吹込めば理
論的には可能であるが、高圧（10〜20Kg／cm2）の
希釈ガスを大量に炉底から吹込むことは操業上の
問題で不可能である。即ち溶鋼のスプラツシユ増
大、及びガスによる冷却作用が大きくなりかえつ
て脱炭には逆効果となる。又スプラツシユ等の問
題を回避しようとすれば相対的に酸素の吹込量を
低下させることが必要であり、精錬時間の延長を
もたらし、生産性を著しく阻害し好ましくない。
一方、すでに述べたように大量のガスを炉底部
より溶鋼中に吹込むため、溶鋼上面の“おどり”
が激しく該溶鋼上面における気相部と溶鋼との接
触界面もあまりガスを吹込まない真空脱炭よりも
かなり大きなものとなることは容易に推定出来
る。この気相部のPcoを下げてやれば接触界面に
おいてPcoの平衡がくずれるので脱炭の進行が容
易になることが考えられる。しかし旧法ではこの
気相部のPcoは溶鋼中から抜け出て来た希釈ガス
とCOガスで占められているため基本的には界面
における希釈ガス中のPcoと同じである。
本発明者はこの気相部のPcoを更に低下させれ
ば接触界面における酸素の脱炭に費やされる比率
すなわち脱炭酸素効率が上昇するであろうことを
推定し気相を雰囲気（Pco）コントロールする実
験を種々行つた結果気相部に雰囲気希釈ガスを吹
込むことにより脱炭がより一層進行することを見
い出した。
以下本発明を具体的な実施例にそつて詳細に説
明する。
第１図は炉底部側壁に設けた２重管羽口の外管
から保護ガスとしてアルゴンを、内管から酸素ガ
スと希釈ガスとしてのアルゴンとの混合ガスを溶
鋼中に吹込むAOD炉を用いて、脱炭期にAOD炉
頂部より雰囲気希釈ガスとしてN2とAr又は空気
を、圧力３〜10Kg／cm2の範囲で炉内気相部に吹込
んだ結果を示す。N2とArの場合も、空気の場合
も底吹酸素量との比率が向上するにつれて、脱炭
酸素効率の改善が見られ、その比率が20％でほぼ
飽和している。
すなわち本発明の要旨は、２重管羽口の外管か
ら保護ガスを、内管から希釈ガスと酸素ガスとの
混合ガスを溶鋼中に吹込むことにより脱炭精錬を
行う方法において、炉内の気相部に雰囲気希釈ガ
スを溶鋼中に吹込まれる酸素ガス量の20％以上吹
込むことを特徴とする脱炭精錬方法である。
本発明において炉内の気相部に吹込む雰囲気希
釈ガスとしてはアルゴン等の不活性ガスの他の窒
素ガス、二酸化炭素ガス、空気、炭化水素等Pco
を低下可能なガスであれば周知のあらゆるガスが
使用可能である。
２重管羽口の外管から吹込む保護ガスとしては
アルゴン等の不活性ガス、炭酸ガス、窒素ガス、
水蒸気等を、また内管から吹込む希釈ガスとして
はアルゴン等の不活性ガス、窒素ガス等周知の他
のガスを使用可能である。また鋼種はステンレス
鋼の他に希釈脱炭を行う他の鋼種にも適用可能で
ある。
第１図においてN2とArよりも空気の方が効果
が大きいのは空気中の酸素が、発生するCOと反
応し、CO2となることによる発熱の影響が溶鋼温
度の低下を防止するとともにCO→CO2となるこ
とによつてもPcoの低下を促進するため不活性ガ
スを用いたときよりも効果が大きいものと推定さ
れる。
次に具体例を示す。18Cr―8Ni（SUS304）ス
テンレス鋼を炉底側壁に設置された２重管羽口の
内管から酸素ガスとアルゴンを、外管からアルゴ
ンを吹込んで希釈脱炭精錬する際に、脱炭期にお
いても底吹酸素量23m3／Ｔ―溶鋼の20％に相当す
る５m3／Ｔ―溶鋼の空気を炉頂より気相部に圧力
５〜６Kg／cm2で吹込んだ。その結果をまとめて表
―１に示す。表―１より明らかな如く脱炭酸素効
率の向上により脱炭期の時間、及び要した還元剤
原単位はいずれも92〜93％に低下し、又耐火物の
溶損量は時間短縮の効果により従来の96％に低下
した。
尚本発明の論理的説明の中から容易に推定可能
な如く気相部と溶鋼との界面を出来るだけ広くか
つ有効に利用するために溶鋼上に浮遊するスラグ
量が少なければ少ないほど効率的である。従つて
炉内に装入する粗溶鋼中の成分、特にスラグ形成
元素である〔Si〕％を0.15％以下に規定し、かつ
持込スラグ量を10Kg／ｔ―粗溶鋼以下に規定する
ことは効果をより一層大きくする。
【表】DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for further improving decarburization efficiency in a steel dilution decarburization method. In general, [C] in molten steel is determined by its temperature and content [Cr].
% in the gas phase that comes into contact with the carbon in the molten steel.
The lower the CO gas partial pressure, the more decarburization progresses. This CO
In order to lower the gas partial pressure (hereinafter abbreviated as Pco), Pco is lowered by creating a vacuum at the interface that contacts the molten steel, or by blowing diluent gas together with oxygen gas. In the dilution decarburization method, a large amount of oxygen gas and diluent gas are injected into the molten steel from the bottom of the furnace or the side wall near the bottom of the furnace, so the contact interface between the dilution gas and the molten steel becomes very large, and the CO migrates and Pco
is diluted and decarburization progresses easily. However, as mentioned above, decarburization stops when a Pco that is in equilibrium with the temperature and [Cr]% in molten steel is achieved. In order to make decarburization more efficient, it is theoretically possible to inject a larger amount of diluent gas, but it is not possible to inject a large amount of high-pressure (10 to 20 kg/cm 2 ) diluent gas from the bottom of the furnace. This is not possible due to operational issues. That is, the splash of molten steel increases and the cooling effect of the gas increases, which has the opposite effect on decarburization. Furthermore, in order to avoid problems such as splash, it is necessary to relatively reduce the amount of oxygen blown, which is undesirable as it prolongs the refining time and significantly impedes productivity. On the other hand, as mentioned above, since a large amount of gas is injected into the molten steel from the bottom of the furnace, the ``dancing'' on the top surface of the molten steel occurs.
It can be easily estimated that the contact interface between the gas phase portion and the molten steel on the upper surface of the molten steel is considerably larger than that in vacuum decarburization in which less gas is blown into the molten steel. It is thought that if Pco in this gas phase is lowered, the equilibrium of Pco at the contact interface will be disrupted, making it easier for decarburization to proceed. However, in the old method, the Pco in this gas phase is occupied by the dilution gas and CO gas that have escaped from the molten steel, so it is basically the same as the Pco in the dilution gas at the interface. The present inventor estimates that if the Pco of this gas phase is further reduced, the ratio of oxygen spent on decarburization at the contact interface, that is, the decarburization oxygen efficiency, will increase, and the gas phase is controlled by the atmosphere (Pco). As a result of various experiments, it was found that decarburization progressed further by blowing atmospheric dilution gas into the gas phase. The present invention will be described in detail below with reference to specific examples. Figure 1 shows an AOD furnace in which argon is injected into the molten steel from the outer pipe of a double-tube tuyere installed on the side wall of the furnace bottom as a protective gas, and a mixed gas of oxygen gas and argon as a diluting gas is injected into the molten steel from the inner pipe. The results are shown below, in which N 2 and Ar or air were blown into the gas phase in the furnace at a pressure in the range of 3 to 10 kg/cm 2 from the top of the AOD furnace as an atmosphere diluent gas during the decarburization period. In the case of N 2 and Ar as well as in the case of air, as the ratio of bottom-blown oxygen increases, the decarburization oxygen efficiency improves, and the ratio is almost saturated at 20%. That is, the gist of the present invention is to provide a method for decarburizing refining by injecting a protective gas from the outer pipe of a double-tube tuyere and a mixed gas of diluent gas and oxygen gas from the inner pipe into the molten steel. This decarburization refining method is characterized by injecting at least 20% of the amount of oxygen gas injected into the molten steel into the gas phase of the molten steel. In the present invention, the atmosphere diluting gas to be blown into the gas phase in the furnace includes inert gas such as argon, nitrogen gas, carbon dioxide gas, air, hydrocarbons, etc.
Any known gas can be used as long as it is capable of lowering the temperature. The protective gas injected from the outer pipe of the double-pipe tuyere includes inert gas such as argon, carbon dioxide gas, nitrogen gas,
Water vapor or the like can be used, and other well-known gases such as inert gas such as argon, nitrogen gas, etc. can be used as the diluent gas blown from the inner tube. In addition to stainless steel, other steel types that undergo dilution decarburization can also be used. In Figure 1, air has a greater effect than N 2 and Ar because the oxygen in the air reacts with the generated CO and the heat generated by the formation of CO 2 prevents the temperature of the molten steel from decreasing. The change from CO to CO 2 also promotes the reduction of Pco, so it is estimated that the effect is greater than when an inert gas is used. A specific example is shown next. When 18Cr-8Ni (SUS304) stainless steel is diluted and decarburized by blowing oxygen gas and argon through the inner tube of the double-tube tuyere installed on the side wall of the furnace bottom, and argon from the outer tube, during the decarburization period. A bottom-blown oxygen amount of 23 m 3 /T - 5 m 3 /T, which corresponds to 20% of the molten steel, was blown into the gas phase from the top of the furnace at a pressure of 5 to 6 kg/cm 2 . The results are summarized in Table 1. As is clear from Table 1, due to improved decarburization oxygen efficiency, the decarburization period time and the required reducing agent consumption rate were both reduced to 92-93%, and the amount of erosion of refractories was reduced due to the effect of shortening the time. This decreased to 96% of the previous level. As can be easily deduced from the logical explanation of the present invention, in order to utilize the interface between the gas phase and the molten steel as widely and effectively as possible, the smaller the amount of slag floating on the molten steel, the more efficient it is. be. Therefore, it is impossible to specify the composition of the crude molten steel charged into the furnace, especially the slag-forming element [Si]%, to be 0.15% or less, and to specify the amount of slag brought in to be 10 kg/t - crude molten steel or less. Make the effect even bigger. 【table】
【図面の簡単な説明】[Brief explanation of the drawing]
第１図は炉底部側壁に設けた２重管羽口の外管
から保護ガスとしてアルゴンを、内管から酸素ガ
スと希釈ガスとしてのアルゴンとの混合ガスを溶
鋼中に吹込むAOD炉を用いて、脱炭期に炉頂部
より雰囲気希釈ガスとしてN2とAr又は空気を、
圧力３〜10Kg／cm2の範囲で炉内気相部に吹込んだ
結果を示す図である。
Figure 1 shows an AOD furnace in which argon is injected into the molten steel from the outer tube of a double-tube tuyere installed on the side wall of the furnace bottom as a protective gas, and a mixed gas of oxygen gas and argon as a diluent gas is injected into the molten steel from the inner tube. During the decarburization period, N 2 and Ar or air are introduced as atmosphere dilution gas from the top of the furnace.
It is a figure which shows the result of blowing into the gas phase part in a furnace in the pressure range of 3-10Kg/cm <2> .