JP6686703B2

JP6686703B2 - Method of desulfurizing molten steel

Info

Publication number: JP6686703B2
Application number: JP2016106266A
Authority: JP
Inventors: 紀史浅原; 田中　康弘; 康弘田中
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-05-27
Filing date: 2016-05-27
Publication date: 2020-04-22
Anticipated expiration: 2036-05-27
Also published as: JP2017210671A

Description

本発明は、真空脱ガス装置を用いて行う溶鋼の脱硫方法に関するものである。 The present invention relates to a method for desulfurizing molten steel using a vacuum degassing device.

硫黄（以下、Ｓと記載する場合あり）含有量が数十ｐｐｍである低硫鋼を溶製する場合、溶銑予備処理で脱硫処理した後に転炉等で脱炭処理するのみでは、十分な低硫レベルに到達しないため、溶鋼脱硫処理が必要となる。 When producing low-sulfur steel having a sulfur (hereinafter sometimes referred to as S) content of several tens of ppm, it is sufficient to reduce the sulfur content by performing a desulfurization treatment in a hot metal pretreatment and then a decarburization treatment in a converter or the like. Molten steel desulfurization treatment is required because the sulfur level is not reached.

溶鋼脱硫処理として、溶鋼を加熱しつつスラグ精錬によりＳを除去する方法（いわゆるＬＦ法）が実用化されているが、二次精錬工程としてのＬＦ工程が必須となり、製造時間の延長や製造コストの増加を招いている。 As a molten steel desulfurization treatment, a method of removing S by slag refining while heating molten steel (so-called LF method) has been put to practical use, but an LF step as a secondary refining step is essential, which extends the production time and production cost. Is increasing.

そこで二次精錬工程として汎用されている真空脱ガス装置での脱硫処理の確立が求められている。真空脱ガス装置での脱硫処理としては、真空槽内で溶鋼表面に上方から粉体状の脱硫剤を吹き付ける方法が一般的である。脱硫剤としては、ＣａＯ系フラックスが一般的に用いられる。溶鋼表面に吹き付けられた脱硫剤は溶鋼内に侵入し、溶鋼との間で脱硫反応が生じる。ＣａＯ系フラックスは溶鋼より比重が小さく、浮力によっていずれは溶鋼表面に浮上分離するため、浮上分離するまでの間にいかに脱硫反応を進行させるかが重要である。 Therefore, it is required to establish desulfurization treatment in a vacuum degassing device that is widely used as a secondary refining process. As a desulfurization treatment in a vacuum degassing apparatus, a method of spraying a powdery desulfurizing agent from above onto the surface of molten steel in a vacuum chamber is generally used. CaO-based flux is generally used as the desulfurizing agent. The desulfurizing agent sprayed on the surface of the molten steel penetrates into the molten steel and a desulfurization reaction occurs with the molten steel. Since the CaO-based flux has a smaller specific gravity than molten steel and is eventually floated and separated on the surface of molten steel by buoyancy, it is important how to proceed the desulfurization reaction before the floating and separation.

従来、ＣａＯ系フラックスを使用する際にはＣａＦ₂（蛍石）が媒溶剤として用いられてきた。ＣａＯ系フラックスにＣａＦ₂を添加すると、脱硫能の高い液相を形成しうる。脱硫剤に液相を形成することで、脱硫剤内部でのＳの移動が容易となり、脱硫反応を迅速に進めさせる効果がある。よって脱硫剤が溶鋼表面に浮上分離するまでの時間が短くても、脱硫反応を十分に進行させることができた。 Conventionally, CaF ₂ (fluorite) has been used as a solvent when using a CaO-based flux. When CaF ₂ is added to the CaO-based flux, a liquid phase having high desulfurization ability can be formed. Forming a liquid phase in the desulfurizing agent facilitates the movement of S inside the desulfurizing agent, and has an effect of rapidly advancing the desulfurizing reaction. Therefore, the desulfurization reaction was able to proceed sufficiently even if the time until the desulfurizing agent floated and separated on the surface of the molten steel was short.

フッ素を含有するフラックスで脱硫処理を行うと、処理後のスラグにもフッ素が残留する。そのため、スラグ中のフッ素が環境に及ぼす影響を考慮し、昨今ではフッ素源を使用しない溶鋼脱硫法が望まれている。 When desulfurization treatment is performed with a flux containing fluorine, fluorine remains in the treated slag. Therefore, in consideration of the influence of fluorine in the slag on the environment, a molten steel desulfurization method that does not use a fluorine source has been demanded recently.

実質的にＦを含まないＣａＯ系フラックスによる溶鋼脱硫を行う場合、脱硫剤は溶鋼処理温度で固相が主体となり、脱硫剤内部では固相内拡散によってＳが移動することになる。固相内拡散によるＳの移動は、液相状態での移動にくらべて極めて遅く、溶鋼との脱硫反応に時間を要する。 When performing molten steel desulfurization with a CaO-based flux that does not substantially contain F, the desulfurization agent mainly has a solid phase at the molten steel treatment temperature, and S moves inside the desulfurization agent due to diffusion in the solid phase. The movement of S due to diffusion in the solid phase is extremely slow as compared with the movement in the liquid phase state, and it takes time for the desulfurization reaction with the molten steel.

したがって、実質的にＦを含まないＣａＯ系フラックスによる溶鋼脱硫では、脱硫剤がＳを十分に吸収する前に溶鋼表面に浮上分離してしまうため、脱硫剤の利用効率が低下する課題があった。 Therefore, in the molten steel desulfurization using CaO-based flux that does not substantially contain F, the desulfurization agent floats and separates on the surface of the molten steel before the S sufficiently absorbs S, so that there is a problem that the utilization efficiency of the desulfurization agent decreases. .

特許文献１は、真空脱ガス装置において実質的にＦを含まないＣａＯ系脱硫剤を用いる場合に、脱硫剤の凝集を防ぐためのＡｌ濃度と粉体供給速度の関係を規定している。脱硫剤の溶鋼中での凝集を防ぐことで、脱硫剤と溶鋼の反応界面積を確保することができ、脱硫反応を進行させることができる。しかし、Ａｌ濃度は脱硫処理中に経時変化するため、適正範囲内への制御が困難であり、結果として脱硫速度が低下する場合があった。 Patent Document 1 defines the relationship between the Al concentration and the powder supply rate for preventing the agglomeration of the desulfurizing agent when a CaO-based desulfurizing agent that does not substantially contain F is used in a vacuum degassing apparatus. By preventing the desulfurizing agent from aggregating in the molten steel, the reaction interfacial area between the desulfurizing agent and the molten steel can be secured, and the desulfurization reaction can proceed. However, since the Al concentration changes with time during the desulfurization treatment, it is difficult to control the Al concentration within an appropriate range, and as a result, the desulfurization rate may decrease.

特許文献２は、ランス高さを下げてハードブロー化することで脱硫剤の溶鋼内侵入率は向上するとしている。このとき、脱硫剤には１重量％以上のフッ化物が含まれる。 Patent Document 2 states that the penetration rate of the desulfurizing agent into the molten steel is improved by lowering the lance height and forming a hard blow. At this time, the desulfurizing agent contains 1% by weight or more of fluoride.

特開２００７−２７０１７８号公報JP, 2007-270178, A 特開平６−３２２４３１号公報JP-A-6-322431

発明者らは、実質的にＦを含まないＣａＯ系脱硫剤において、特許文献２の方法での脱硫処理を試みた。しかし特許文献２の方法では、溶鋼内に侵入するものの、溶鋼表面から溶鋼内に侵入しようとする粉体が互いに衝突しやすくなり、衝突した粉体を主体とした積層物を形成して、脱硫に寄与しない脱硫剤（粉体）が発生してしまい、脱硫速度が低下することを知見した。 The inventors attempted a desulfurization treatment by the method of Patent Document 2 in a CaO-based desulfurizing agent containing substantially no F. However, in the method of Patent Document 2, the powders that enter the molten steel tend to collide with each other from the surface of the molten steel, but the powders that collide with each other easily form a laminate, and desulfurization is performed. It has been found that a desulfurizing agent (powder) that does not contribute to the generation is generated, and the desulfurization rate decreases.

本発明は、真空脱ガス装置において脱硫剤吹付けノズルから真空容器内の溶鋼表面に脱硫剤を吹き付けて脱硫する方法において、実質的にＦを含まない脱硫剤を効率よく利用する溶鋼の脱硫方法を提供することを目的とする。 The present invention relates to a method for desulfurizing a desulfurizing agent by spraying a desulfurizing agent from a desulfurizing agent spraying nozzle to a surface of molten steel in a vacuum vessel in a vacuum degassing apparatus, which is a method for desulfurizing molten steel, which effectively utilizes a desulfurizing agent containing substantially no F. The purpose is to provide.

即ち、本発明の要旨とするところは以下のとおりである。
（１）真空脱ガス装置において溶鋼の脱硫処理を行う際に、脱硫剤吹付けノズルから真空容器内の溶鋼表面に脱硫剤を吹き付け、前記脱硫剤として実質的にＦを含まない、ＣａＯを５０質量％以上含むＣａＯ系脱硫剤（但し、７０質量％ＣａＯ−２０質量％ＣａＣｌ _２ −１０質量％ＣａＳｉ、の組成である場合を除く）を使用する方法において、真空脱ガス装置として、溶鋼鍋底部から撹拌用ガスを吹きこむ装置を使用するとともに、前記した溶鋼表面への脱硫剤の吹き付けには、先端にノズルを配置したランスを用い、前記溶鋼表面から前記ノズルの先端までの鉛直方向距離をランス高さとして、当該ランス高さを２ｍ以上とし、真空槽内圧力は１３００Ｐａ以上、６７０００Ｐａ以下とすることを特徴とする、溶鋼の脱硫方法。
（２）脱硫前の溶鋼鍋内溶鋼面に存在しているスラグのＦｅＯ成分とＭｎＯ成分の合計が１０質量％以下であることを特徴とする、上記（１）に記載の溶鋼の脱硫方法。 That is, the gist of the present invention is as follows.
(1) When performing desulfurization treatment of molten steel in a vacuum degassing device, a desulfurizing agent is sprayed from a desulfurizing agent spraying nozzle onto the surface of molten steel in a vacuum container, and CaO containing substantially no F as the desulfurizing agent is added. In a method using a CaO-based desulfurizing agent containing 70 % by mass or more ( excluding the case of having a composition of 70% by mass CaO-20% by mass CaCl ₂ -10% by mass CaSi) , a molten steel ladle bottom is used as a vacuum degassing device. While using a device for blowing a stirring gas from , to spray the desulfurizing agent to the molten steel surface described above, using a lance with a nozzle arranged at the tip, the vertical distance from the molten steel surface to the tip of the nozzle. As the lance height, the lance height is set to 2 m or more, and the pressure in the vacuum chamber is set to 1300 Pa or more and 67000 Pa or less.
(2) The desulfurization method for molten steel according to (1) above, wherein the total of FeO component and MnO component of the slag existing on the molten steel surface in the molten steel ladle before desulfurization is 10% by mass or less.

本発明に従えば、従来技術に比べて溶鋼内に実質的にＦを含まないＣａＯ系脱硫剤を均一に供給でき、積層合体による反応効率低下を防ぐことができる。 According to the present invention, the CaO-based desulfurizing agent that does not substantially contain F can be uniformly supplied into the molten steel as compared with the prior art, and the decrease in reaction efficiency due to the layered coalescence can be prevented.

溶鋼内での循環流を利用することにより、溶鋼内での反応時間を確保し、固相率の高いＦレスフラックスでも十分に反応させることができる。 By utilizing the circulating flow in the molten steel, it is possible to secure the reaction time in the molten steel and sufficiently react even the F-less flux having a high solid fraction.

真空脱ガス装置を用いて行う溶鋼の脱硫方法を示す図であり、（Ａ）は大径浸漬管方式を用いた例、（Ｂ）はＲＨ方式を用いた例である。It is a figure which shows the desulfurization method of the molten steel performed using a vacuum degassing device, (A) is an example using a large diameter immersion pipe system, (B) is an example using an RH system. 真空槽内の溶鋼面に吹き付けるガス噴流を示す図であり、（Ａ）は真空槽内圧力が高い場合、（Ｂ）は真空槽内圧力が低い場合である。It is a figure which shows the gas jet flow sprayed on the molten steel surface in a vacuum tank, (A) is when a vacuum tank internal pressure is high, (B) is a case where a vacuum tank internal pressure is low.

本発明の溶鋼の脱硫方法において、溶鋼の脱硫は真空脱ガス装置を用いて行う。溶鋼の真空脱ガス装置には種々の方式が存在する。溶鋼を真空脱ガスするに際し、溶鋼鍋全体を真空容器内に収容して真空容器内を真空とする方法（例えばＶＯＤ（Vacuum Oxygen Decarburization）法、ＶＡＤ（Vacuum Arc Degassing）法など）、あるいは真空容器の下方開口部を溶鋼鍋内の溶鋼中に浸漬して真空容器内を真空とする方法（例えば大径浸漬管方式、ＲＨ真空脱ガス装置など）がある。大径浸漬管方式真空脱ガス装置２１とは、図１（Ａ）に示すように、真空容器２の下部に一つの大径浸漬管３が設けられ、この大径浸漬管３を溶鋼鍋１に収容した溶鋼１０中に浸漬し、真空容器２内を減圧して溶鋼を真空容器内に吸い上げ、溶鋼鍋底部の撹拌用ガスを吹きこむ装置６からガスを吹き込み、吹き込んだガスによって溶鋼鍋中の溶鋼と真空容器内の溶鋼との間に攪拌流を形成することにより、溶鋼を攪拌・混合する方法である。 In the method for desulfurizing molten steel of the present invention, desulfurization of molten steel is performed using a vacuum degassing device. There are various types of vacuum degassing equipment for molten steel. When degassing molten steel in a vacuum, a method for accommodating the entire molten steel pot in a vacuum vessel and creating a vacuum in the vacuum vessel (for example, VOD (Vacuum Oxygen Decarburization) method, VAD (Vacuum Arc Degassing) method), or a vacuum vessel There is a method of immersing the lower opening of the above into the molten steel in the molten steel pot to make the inside of the vacuum vessel into a vacuum (for example, a large diameter immersion pipe system, an RH vacuum degassing device, etc.). As shown in FIG. 1 (A), the large-diameter immersion pipe type vacuum degassing device 21 is provided with one large-diameter immersion pipe 3 at the bottom of a vacuum container 2, and the large-diameter immersion pipe 3 is connected to the molten steel ladle 1 Dipped in the molten steel 10 housed in the vacuum vessel 2, decompressing the inside of the vacuum vessel 2 to suck the molten steel into the vacuum vessel, blowing gas from the device 6 for blowing stirring gas at the bottom of the molten steel pot, and blowing the gas into the molten steel pot. It is a method of stirring and mixing molten steel by forming a stirring flow between the molten steel and the molten steel in the vacuum vessel.

ここでは、溶鋼を攪拌・混合するための不活性ガスの吹き込み位置の違いによって真空脱ガス装置を分類する。真空脱ガス装置には、溶鋼鍋底部から撹拌用ガスを供給する方式（ＶＯＤ方式や大型浸漬管方式など）と、溶鋼上方から浸漬する浸漬管の途中から撹拌・混合用ガスを供給するＲＨ真空脱ガス装置がある。 Here, the vacuum degassing apparatus is classified according to the difference in the blowing position of the inert gas for stirring and mixing the molten steel. For the vacuum degassing device, a method of supplying stirring gas from the bottom of the molten steel ladle (VOD method, large immersion pipe method, etc.) and an RH vacuum that supplies stirring / mixing gas from the middle of the immersion pipe immersed from above the molten steel There is a degassing device.

ＲＨ方式真空脱ガス装置２２の場合、図１（Ｂ）に示すように、真空容器２の底部に２つの浸漬管が設けられ、一方が上昇管７、他方が下降管８となる。上昇管７の中間位置側壁のガス吹き込み口９から溶鋼中にガスを吹き込み、ガスの浮力によって上昇管７中を溶鋼が上昇し、溶鋼は真空容器２内を経由して下降管８から下降し、溶鋼鍋１中に放出される。下降管８から溶鋼鍋１に向かう溶鋼の流れが強く、その流れは溶鋼鍋１の底部に衝突し、その後は流れが分散されて弱まる。真空容器内で溶鋼内に侵入した粉体については、下降管８からの溶鋼の流れに乗って溶鋼鍋１の底部で分散され、その後、主に溶鋼鍋１の側壁に沿ってゆっくりと上昇し、溶鋼鍋内の溶鋼表面１２付近に至るとともに、溶鋼流れの弱い部分に到達する。溶鋼流れの弱い部分で粉体に働く力は、溶鋼から受ける粘性力よりも浮力の影響が大きくなり、浮上分離しやすい。溶鋼鍋の溶鋼表面１２付近で浮上分離した粉体は、溶鋼鍋内の表面に存在しているスラグ１３に取り込まれ、その後は脱硫への寄与が大幅に低下する。ＣａＦ₂を含むＣａＯ系フラックスであれば、反応速度が大きく短時間でも十分に脱硫が進行するため、問題にはならない。一方、実質的にＦを含まないＣａＯ系脱硫剤を用いた場合は、脱硫剤中のＳの移動は固相内拡散によるため、脱硫反応に時間を要するので、粉体が浮上分離するまでの時間では反応が十分に進行し難い。そのため、ＲＨ方式では、実質的にＦを含まないＣａＯ系脱硫剤を用いた際には反応効率の低下が顕著であった。 In the case of the RH type vacuum degassing device 22, as shown in FIG. 1B, two immersion pipes are provided at the bottom of the vacuum container 2, one of which serves as the ascending pipe 7 and the other serves as the descending pipe 8. Gas is blown into the molten steel from the gas blowing port 9 on the side wall at the intermediate position of the rising pipe 7, the molten steel rises in the rising pipe 7 due to the buoyancy of the gas, and the molten steel descends from the downcomer pipe 8 through the vacuum vessel 2. , Released into the molten steel ladle 1. The flow of molten steel from the downcomer pipe 8 to the molten steel ladle 1 is strong, the flow collides with the bottom of the molten steel ladle 1, and thereafter the flow is dispersed and weakened. The powder that has penetrated into the molten steel in the vacuum vessel is dispersed on the bottom of the molten steel ladle 1 along with the flow of the molten steel from the downcomer pipe 8, and then slowly rises mainly along the side wall of the molten steel ladle 1. , Near the molten steel surface 12 in the molten steel ladle, and reach a portion where the molten steel flow is weak. The force acting on the powder in the portion where the molten steel flow is weak has a greater effect of buoyancy than the viscous force received from the molten steel, and is easily floated and separated. The powder floated and separated near the molten steel surface 12 of the molten steel ladle is taken into the slag 13 existing on the surface inside the molten steel ladle, and thereafter, the contribution to desulfurization is significantly reduced. A CaO-based flux containing CaF ₂ has a large reaction rate and sufficiently desulfurizes even in a short time, so that it does not cause a problem. On the other hand, when a CaO-based desulfurizing agent that does not substantially contain F is used, the movement of S in the desulfurizing agent is due to diffusion in the solid phase, and therefore, the desulfurization reaction requires time, so that the powder is floated and separated. It is difficult for the reaction to proceed sufficiently in time. Therefore, in the RH method, when the CaO-based desulfurizing agent containing substantially no F was used, the reaction efficiency was significantly reduced.

本発明では、真空脱ガス装置として、溶鋼鍋底部から撹拌用ガスを吹きこむ装置を使用する。大径浸漬管方式、ＶＯＤ法、ＶＡＤ法などがこの分類に入る真空脱ガス装置である。ＶＯＤ方式や大型浸漬管方式のように、溶鋼鍋底部から撹拌用ガスを吹きこむ装置６を用いることで、ＲＨ方式で問題となる上記課題を解決できることを知見した。溶鋼鍋底部の撹拌用ガスを吹きこむ装置６は、当該撹拌用ガスを吹きこむ装置６から溶鋼中に吹き込まれた気泡が上昇し、真空容器内の溶鋼表面１１に到達するように配置される（図１（Ａ）参照）。 In the present invention, a device for blowing a stirring gas from the bottom of the molten steel ladle is used as the vacuum degassing device. The large diameter immersion tube system, the VOD method, the VAD method, etc. are vacuum degassing devices that fall into this category. It has been found that the above-mentioned problem, which is a problem in the RH system, can be solved by using the device 6 that blows the stirring gas from the bottom of the molten steel ladle, such as the VOD system and the large immersion pipe system. The device 6 for blowing the stirring gas at the bottom of the molten steel ladle is arranged so that the bubbles blown into the molten steel from the device 6 for blowing the stirring gas rise and reach the molten steel surface 11 in the vacuum container. (See FIG. 1A).

上記のような、溶鋼鍋底部から撹拌用ガスを吹きこむ装置６を用いる場合、撹拌用ガスを吹きこむ装置６から真空容器内の溶鋼表面１１に向けた気泡上昇流に沿って、溶鋼の上昇流１５が形成される。真空容器内の溶鋼表面１１に到達した溶鋼の上昇流１５は、真空容器内の溶鋼表面１１における上昇流の到達点から放射状に流れ、真空容器２の側壁に至り、その位置から下降に転じる。下降流１６は溶鋼鍋中において溶鋼鍋の底部に至るまで持続する下降流を形成する。すなわち、これらの装置では、溶鋼内に大きな循環流が生じる。脱硫剤吹付けノズル５から真空容器内の溶鋼表面１１に吹き付けられ、溶鋼内に侵入した粉体は、下降流１６に付随して溶鋼鍋内を下降し、溶鋼鍋底部においてガス気泡による上昇流１５に取り込まれ、真空容器内の溶鋼表面１１まで循環する。真空容器内の溶鋼表面１１では気泡の膨張によって溶鋼が激しく混合し、浮上した粉体は溶鋼表面に留まらずに溶鋼に取り込まれ、溶鋼の下降流１６とともに溶鋼鍋中を下降する。このように、粉体は溶鋼の循環流の中に留まることで反応時間を確保することができる。よって、実質的にＦを含まないＣａＯ系脱硫剤を用いた場合でも、脱硫剤中のＳの移動が固相内拡散であって脱硫反応に時間を要するものの、脱硫反応に必要な十分な時間を溶鋼中に滞在しているので、反応効率を確保できる。 When the device 6 for blowing the stirring gas from the bottom of the molten steel is used as described above, the molten steel rises along the bubble rising flow from the device 6 for blowing the stirring gas toward the molten steel surface 11 in the vacuum container. Stream 15 is formed. The ascending flow 15 of the molten steel that has reached the molten steel surface 11 in the vacuum container flows radially from the arrival point of the ascending flow on the molten steel surface 11 in the vacuum container, reaches the side wall of the vacuum container 2, and then descends from that position. The downflow 16 forms a downflow that continues in the ladle to the bottom of the ladle. That is, in these devices, a large circulating flow is generated in the molten steel. The powder sprayed from the desulfurizing agent spray nozzle 5 onto the molten steel surface 11 in the vacuum vessel and entering the molten steel descends in the molten steel ladle along with the downward flow 16 and rises due to gas bubbles at the bottom of the molten steel ladle. It is taken into 15 and circulates to the molten steel surface 11 in a vacuum container. On the molten steel surface 11 in the vacuum container, the molten steel is violently mixed due to the expansion of bubbles, and the floated powder is not retained on the molten steel surface but is taken into the molten steel and descends in the molten steel ladle with the downward flow 16 of the molten steel. In this way, the reaction time can be secured by keeping the powder in the circulating flow of molten steel. Therefore, even when a CaO-based desulfurizing agent that does not substantially contain F is used, although the movement of S in the desulfurizing agent is diffusion in the solid phase and the desulfurization reaction requires time, the sufficient time required for the desulfurization reaction is required. Since it stays in molten steel, the reaction efficiency can be secured.

なお、実質的にＦを含まないこととは、脱硫精錬後のスラグからフッ素（Ｆ）の溶出が顕著には認められないことを指すもので、本発明者らの知見では精錬後のスラグ組成においてＦが１質量％以下となる場合を指す。Ｆが０．５質量％以下であれば更に好ましい。 The term "substantially free of F" means that no significant elution of fluorine (F) is observed from the slag after desulfurization refining. According to the findings of the present inventors, the slag composition after refining Indicates the case where F is 1% by mass or less. More preferably, F is 0.5% by mass or less.

本発明においてＣａＯ系脱硫剤とは、ＣａＯを５０質量％以上含んでいる脱硫剤を指す。ＣａＯ源としてはＣａＯの他にＣａＣＯ₃やＣａ(ＯＨ)₂でもよく、完全にＣａＯに熱分解した際の質量でＣａＯ含有量を計算できる。脱硫剤中のＣａＯ以外の成分については特に限定しない。Ｍｇなどのそれ自体で脱硫能力を有する成分を含むとより好ましい。 In the present invention, the CaO-based desulfurizing agent refers to a desulfurizing agent containing 50% by mass or more of CaO. The CaO source may be CaCO ₃ or Ca (OH) ₂ in addition to CaO, and the CaO content can be calculated by the mass when completely pyrolyzed into CaO. The components other than CaO in the desulfurizing agent are not particularly limited. It is more preferable to include a component having desulfurization ability by itself such as Mg.

真空脱ガス装置を用いた溶鋼の脱硫において、真空容器２内に脱硫剤吹き込みランス４を下方に向けて挿入する。脱硫剤吹き込みランス４の先端には脱硫剤吹付けノズル５が配置されている。脱硫剤吹付けノズル５から真空容器内の溶鋼表面１１に脱硫剤を吹き付ける。本発明は、脱硫剤吹き付けに際し、真空槽内圧力を１３００Ｐａ以上、６７０００Ｐａ以下とすることを特徴とする。以下、詳細に説明する。 In desulfurization of molten steel using a vacuum degassing device, a desulfurizing agent blowing lance 4 is inserted into the vacuum container 2 so as to face downward. A desulfurizing agent blowing nozzle 5 is arranged at the tip of the desulfurizing agent blowing lance 4. The desulfurizing agent is sprayed from the desulfurizing agent spraying nozzle 5 onto the molten steel surface 11 in the vacuum container. The present invention is characterized in that the pressure in the vacuum chamber is set to 1300 Pa or more and 67000 Pa or less when the desulfurizing agent is sprayed. The details will be described below.

脱硫剤粉体はキャリアガスによって脱硫剤吹き込みランス４中を搬送され、脱硫剤吹き込みランス４先端の脱硫剤吹付けノズル５から真空容器内の溶鋼表面１１に向けてガス噴流１４として噴出される。この際、ガスはノズル内外の圧力差によって急激に膨張し、ランス近傍ではジェットコアを形成する。ノズルから鉛直下向きに吹き出されたガス噴流１４は、ジェットコアより後流では、溶鋼湯面に向かって進行するとともに水平方向に広がる。この際、粉体も水平方向に分散する。 The desulfurizing agent powder is conveyed in the desulfurizing agent blowing lance 4 by the carrier gas, and is jetted as a gas jet flow 14 from the desulfurizing agent blowing nozzle 5 at the tip of the desulfurizing agent blowing lance 4 toward the molten steel surface 11 in the vacuum container. At this time, the gas rapidly expands due to the pressure difference between the inside and outside of the nozzle, forming a jet core near the lance. The gas jet flow 14 blown vertically downward from the nozzle advances toward the surface of the molten steel and spreads horizontally in the downstream of the jet core. At this time, the powder is also dispersed in the horizontal direction.

図２には、真空槽内の溶鋼表面１１に吹き付けるガス噴流１４を示す。図２（Ａ）は真空槽内圧力が１３００Ｐａ以上と高い場合、（Ｂ）は真空槽内圧力が１３００Ｐａ未満と低い場合である。真空槽内圧力が低い場合、粉体を含むガス噴流１４の直進性が高いため、図２（Ｂ）に示すように、粉体の広がりが小さく、溶鋼表面の一部に局所的に粉体が供給されることになる。局所的に供給された粉体は、粒子同士が衝突する確率が高まる。実質的にＦを含まないＣａＯ系脱硫剤を用いた場合は、粒子同士が衝突すると溶鋼表面で積層し、合体してしまう。積層合体が生じると、粉体の比表面積（体積に対する表面積の比率）が小さくなる。その結果、粒子が単独で存在する場合よりも反応速度が低下してしまう。 FIG. 2 shows a gas jet flow 14 sprayed on the molten steel surface 11 in the vacuum chamber. FIG. 2A shows the case where the pressure inside the vacuum chamber is as high as 1300 Pa or higher, and FIG. 2B shows the case where the pressure inside the vacuum chamber is as low as less than 1300 Pa. When the pressure in the vacuum chamber is low, the straightness of the gas jet flow 14 containing the powder is high, so that the spread of the powder is small and the powder is locally distributed on a part of the molten steel surface as shown in FIG. 2 (B). Will be supplied. The powder that is locally supplied has a higher probability that the particles collide with each other. When a CaO-based desulfurizing agent that does not substantially contain F is used, when the particles collide with each other, they are stacked on the surface of the molten steel and coalesce. When layered coalescence occurs, the specific surface area (ratio of surface area to volume) of the powder becomes small. As a result, the reaction rate will be slower than if the particles were present alone.

これに対して、真空槽内圧力を高くすることで、上記課題を解決できることを知見した。すなわち、真空槽内圧力が高ければ、図２（Ａ）に示すように、粉体を含むガス噴流１４の直進性が低くなり、粉体が分散した状態で溶鋼表面に到達するため、粒子同士の衝突が生じにくくなる。よって、実質的にＦを含まないＣａＯ系脱硫剤を用いた場合でも、反応効率を確保できる。 On the other hand, it was found that the above problems can be solved by increasing the pressure in the vacuum chamber. That is, if the pressure in the vacuum chamber is high, as shown in FIG. 2 (A), the straightness of the gas jet flow 14 containing the powder becomes low, and the powder reaches the molten steel surface in a dispersed state. Collisions are less likely to occur. Therefore, the reaction efficiency can be ensured even when the CaO-based desulfurizing agent containing substantially no F is used.

上記効果を得るためには、本発明者らの知見では真空槽内圧力を１３００Ｐａ（９．８Ｔｏｒｒ）以上とするとよい。１３００Ｐａ未満だと、上記のように、ガス噴流１４の直進性が高いために粉体の広がりが小さく、脱硫反応効率が低下する。 In order to obtain the above effect, the inventors of the present invention have found that the pressure in the vacuum chamber should be 1300 Pa (9.8 Torr) or more. If it is less than 1300 Pa, as described above, the straightness of the gas jet flow 14 is high, so that the spread of the powder is small and the desulfurization reaction efficiency is reduced.

一方、真空槽内圧力を過剰に高めると、かえって脱硫反応効率が低下する現象が生じる。これは、底吹き撹拌ガスによる撹拌力が低下するためである。なおこの傾向は真空槽内圧力が１３３３２Ｐａ（１００Ｔｏｒｒ）以上で表れはじめるが、本発明者らの知見では、真空槽内圧力を６７０００Ｐａ（５０３Ｔｏｒｒ）以下であれば本発明の効果は得られる。 On the other hand, when the pressure in the vacuum chamber is excessively increased, the desulfurization reaction efficiency rather decreases. This is because the stirring force of the bottom-blown stirring gas is reduced. This tendency begins to appear when the pressure in the vacuum chamber is 13332 Pa (100 Torr) or more, but according to the knowledge of the present inventors, the effect of the present invention can be obtained when the pressure in the vacuum chamber is 67,000 Pa (503 Torr) or less.

以上のとおり、真空脱ガス装置において溶鋼の脱硫処理を行う際に、脱硫剤吹付けノズル５から真空容器内の溶鋼表面１１に脱硫剤を吹き付け、前記脱硫剤として実質的にＦを含まないＣａＯ系脱硫剤を使用する方法において、真空槽内圧力を１３００Ｐａ（９．８Ｔｏｒｒ）以上とすることにより、脱硫剤の粒子同士の衝突が生じにくくなる。また、真空脱ガス装置として、溶鋼鍋底部から撹拌用ガスを吹きこむ装置６を使用することにより、実質的にＦを含まないＣａＯ系脱硫剤を用いた場合でも、反応効率を確保できる。 As described above, when performing desulfurization treatment of molten steel in the vacuum degassing device, the desulfurizing agent is sprayed from the desulfurizing agent spraying nozzle 5 onto the molten steel surface 11 in the vacuum container, and CaO containing substantially no F as the desulfurizing agent is used. In the method using a system desulfurizing agent, by setting the pressure in the vacuum chamber to 1300 Pa (9.8 Torr) or more, collision of particles of the desulfurizing agent is less likely to occur. Further, by using the device 6 for blowing the stirring gas from the bottom of the molten steel pot as the vacuum degassing device, the reaction efficiency can be ensured even when the CaO-based desulfurizing agent containing substantially no F is used.

本発明において、脱硫前における溶鋼鍋内の溶鋼表面１２に存在しているスラグ１３のＦｅＯ成分とＭｎＯ成分の合計が１０質量％以下であると好ましい。以下詳述する。 In the present invention, it is preferable that the total of FeO component and MnO component of the slag 13 existing on the molten steel surface 12 in the molten steel pot before desulfurization is 10% by mass or less. This will be described in detail below.

上記の通り、溶鋼鍋底部から撹拌用ガスを吹きこむ装置６を使用する本発明においては、循環溶鋼流中の脱硫剤粒子は、溶鋼中を循環し、脱硫への寄与を継続するが、一定の確率で循環溶鋼流から逸脱し、溶鋼鍋浴面へ浮上し、溶鋼鍋浴面に存在するスラグ１３に取り込まれる。溶鋼鍋浴面のスラグ１３は、転炉からの出鋼時に転炉から流出した酸化精錬スラグを主体とするものであり、ＦｅＯ成分やＭｎＯ成分含有量が高く、いわゆる酸化性の高いスラグである。そのため、溶鋼鍋浴面へ浮上した脱硫剤粒子を構成する脱硫生成物は、浴面のスラグにより酸化されて溶鋼中に復硫する場合がある。 As described above, in the present invention in which the device 6 for blowing the stirring gas from the bottom of the molten steel ladle is used, the desulfurizing agent particles in the circulating molten steel flow circulate in the molten steel and continue to contribute to desulfurization. With the probability of, it deviates from the circulating molten steel flow, floats on the molten steel ladle bath surface, and is taken into the slag 13 existing on the molten steel ladle bath surface. The slag 13 on the bath surface of the molten steel ladle is mainly composed of the oxidized refining slag flowing out from the converter at the time of tapping the steel from the converter, and has a high FeO component and MnO component content, and is a so-called highly oxidizable slag. . Therefore, the desulfurization product that constitutes the desulfurizing agent particles floating on the bath surface of the molten steel ladle may be oxidized by the slag on the bath surface to be re-sulfurized in the molten steel.

そこで本発明では、脱硫前の溶鋼面に存在しているスラグは、ＦｅＯ成分とＭｎＯ成分の合計が１０質量％以下であることを好適（スラグの酸化度が低いと好適）とする。スラグ中のＦｅＯ成分やＭｎＯ成分の濃度を低減する方法としては、例えば、Ａｌ等の強脱酸元素を添加して還元する方法や、スラグにＣａＯ源を添加して希釈する方法などを用いることができる。なお実操業においては、ＦｅＯ成分量（質量％）は、Ｔ．Ｆｅ量（質量％）分析値に基づき、Ｔ．ＦｅのすべてがＦｅＯであるとしてＦｅＯ含有量を算出しても良い。 Therefore, in the present invention, it is preferable that the total amount of FeO component and MnO component in the slag existing on the molten steel surface before desulfurization is 10% by mass or less (it is preferable that the oxidation degree of the slag is low). As a method for reducing the concentration of FeO component or MnO component in the slag, for example, a method of adding a strong deoxidizing element such as Al for reduction or a method of adding a CaO source to the slag for dilution is used. You can In the actual operation, the FeO component amount (mass%) is T.O. Based on the analysis value of Fe content (mass%), the T. The FeO content may be calculated assuming that all of Fe is FeO.

本発明の効果を検証するために実施した検証試験について説明する。 A verification test carried out to verify the effect of the present invention will be described.

検証試験には、３００〜３５０ｔｏｎ／チャージ、低炭素アルミキルド鋼（脱硫前のＳ濃度が３０ｐｐｍ〜４０ｐｐｍ）の溶鋼を用いた。溶鋼鍋の内径は５ｍである。真空脱ガス装置は大径浸漬管方式（図１（Ａ）参照）（大径浸漬管３の内径：２ｍ）及びＲＨ方式（図１（Ｂ）参照）とした。なお本発明では、溶鋼鍋の内径３〜６ｍ、大径浸漬管の内径１．５〜２．５ｍ、のものを想定している。 In the verification test, molten steel of 300 to 350 ton / charge and low carbon aluminum killed steel (S concentration before desulfurization is 30 to 40 ppm) was used. The inner diameter of the molten steel ladle is 5 m. The vacuum degassing apparatus was a large diameter immersion pipe system (see FIG. 1A) (inner diameter of large diameter immersion pipe 3: 2 m) and an RH system (see FIG. 1B). In the present invention, it is assumed that the molten steel ladle has an inner diameter of 3 to 6 m and the large diameter dip tube has an inner diameter of 1.5 to 2.5 m.

ここで、真空槽内の溶鋼表面からノズル先端までの鉛直方向距離をランス高さと称する。大径浸漬管方式及びＲＨ方式の場合、真空槽内の溶鋼表面１１の高さは、真空槽内の圧力に応じて変化する。真空槽内の圧力と大気圧との差ΔＰ（ｍｍ鉄柱）は、真空槽外の溶鋼鍋内の溶鋼表面１２の位置をゼロとした真空槽内の溶鋼表面１１の高さＪ（ｍｍ）と同値である。常用される圧力単位ＰａでΔＰを表示するとΔＰ＝ρｇＪ／１０００となる。ただし、ρは溶鋼の密度（ｋｇ／ｍ³）、ｇは重力加速度（ｍ／ｓ²）である。なお、ガス撹拌による溶鋼表面の乱れは無視し、静止面と仮定して求めた。また、溶鋼鍋内の溶鋼表面に存在しているスラグ１３を考慮するとより正確に高さＪを求めることができる。 Here, the vertical distance from the molten steel surface in the vacuum chamber to the nozzle tip is called the lance height. In the case of the large diameter immersion pipe system and the RH system, the height of the molten steel surface 11 in the vacuum chamber changes according to the pressure in the vacuum chamber. The difference ΔP (mm iron column) between the pressure in the vacuum chamber and the atmospheric pressure is the height J (mm) of the molten steel surface 11 in the vacuum chamber where the position of the molten steel surface 12 in the molten steel pot outside the vacuum chamber is zero. It is the same value. If ΔP is displayed in the pressure unit Pa that is commonly used, ΔP = ρgJ / 1000. Here, ρ is the density of molten steel (kg / m ³ ) and g is the acceleration of gravity (m / s ² ). The disturbance on the molten steel surface due to gas agitation was ignored, and the surface was assumed to be a stationary surface. Further, the height J can be obtained more accurately by considering the slag 13 existing on the surface of the molten steel in the molten steel ladle.

本検証試験では、ランス高さを１〜２ｍとして脱硫処理を実施した。なお、本発明ではランス高さ１ｍ以上での操業を想定している。ランス高さが大きいほど、粉体は広がりやすく脱硫に有利であるため、好ましい。この効果は２ｍ以上で顕著であり、２．６ｍ以上でさらに顕著となる。ランス高さは通常、高々６ｍ程度である。 In this verification test, desulfurization treatment was performed with a lance height of 1 to 2 m. In the present invention, it is assumed that the lance has a height of 1 m or more. The larger the lance height, the more easily the powder spreads, which is advantageous for desulfurization, and therefore it is preferable. This effect is remarkable at 2 m or more, and is more remarkable at 2.6 m or more. The lance height is usually about 6 m at most.

大径浸漬管方式を用いた水準では、鍋底に備えた撹拌ガス吹込みプラグ（撹拌用ガスを吹きこむ装置６）から撹拌用ガスを供給した。ＲＨ方式を用いた水準では、真空槽に備えた２本の浸漬管のうち１本の浸漬管（上昇管７）のガス吹き込み口９から撹拌用ガスを供給した。いずれの水準でも、撹拌用ガスにはＡｒガスを用いた。 At the level using the large diameter immersion tube system, the stirring gas was supplied from the stirring gas blowing plug (the stirring gas blowing device 6) provided at the bottom of the pan. In the level using the RH system, the stirring gas was supplied from the gas blowing port 9 of one of the two dipping tubes (the rising pipe 7) provided in the vacuum chamber. At any level, Ar gas was used as the stirring gas.

脱硫剤は、キャリアガスとともに、脱硫剤吹き込みランス４先端の脱硫剤吹付けノズル５から真空容器内の溶鋼表面１１に向けて噴出される。ＣａＯ系脱硫剤を吹き付けるためのキャリアガスにはＡｒガスを用い、１７５ｋｇ／分にて５ｋｇ／ｔｏｎ−ｓｔｅｅｌの量を溶鋼に吹付けた。脱硫剤吹付けノズル５は、ノズル下端開口径（直径）が１２０ｍｍのラバールノズルを用いた。本発明では７５〜１５０ｍｍのものを想定している。 The desulfurizing agent is jetted together with the carrier gas from the desulfurizing agent blowing nozzle 5 at the tip of the desulfurizing agent blowing lance 4 toward the molten steel surface 11 in the vacuum container. Ar gas was used as a carrier gas for spraying the CaO-based desulfurizing agent, and an amount of 5 kg / ton-steel was sprayed onto the molten steel at 175 kg / min. As the desulfurizing agent spraying nozzle 5, a Laval nozzle having a nozzle lower end opening diameter (diameter) of 120 mm was used. In the present invention, the size of 75 to 150 mm is assumed.

ＣａＯ系脱硫剤の成分としては、実施例１〜５、比較例１〜３については８０質量％ＣａＯ＋２０質量％Ａｌ₂Ｏ₃を使用した。従来技術を用いた水準では、８０質量％ＣａＯ＋２０質量％ＣａＦ₂を使用した。なお本発明では、ＣａＯ系脱硫剤の単位時間あたりの供給量は、５０〜３００（ｋｇ／ｍｉｎ）を想定している。 As a component of the CaO-based desulfurizing agent, 80 mass% CaO + 20 mass% Al ₂ O ₃ was used in Examples 1 to 5 and Comparative Examples 1 to 3. At the level used in the prior art, 80 wt% CaO + 20 wt% CaF ₂ was used. In the present invention, the supply amount of CaO-based desulfurizing agent per unit time is assumed to be 50 to 300 (kg / min).

試験結果の評価は以下のように行った。
脱硫前のＳ濃度３０ｐｐｍ〜４０ｐｐｍを、脱硫処理によって到達Ｓ濃度５ｐｐｍ〜１３ｐｐｍとし、脱硫処理開始から脱硫処理終了までの時間を測定した。次いで、Ｓ濃度変化を時間の一次関数と仮定し、当該関数を用いて３０ｐｐｍから１５ｐｐｍに低下するのに必要な時間に換算して「脱硫処理時間」とした。そして、比較例１における脱硫処理時間を基準とし、１０％超改善したものを◎、５％以上１０％以下改善したものを○、悪化または５％未満の改善であったものを×として評価した。 The evaluation of test results was performed as follows.
The S concentration before desulfurization of 30 ppm to 40 ppm was set to the reached S concentration of 5 ppm to 13 ppm by desulfurization treatment, and the time from the start of desulfurization treatment to the end of desulfurization treatment was measured. Next, assuming that the change in the S concentration is a linear function of time, it was converted to the time required to decrease from 30 ppm to 15 ppm by using the function, and was referred to as “desulfurization treatment time”. Then, based on the desulfurization treatment time in Comparative Example 1, those having an improvement of more than 10% were evaluated as ⊚, those having an improvement of 5% or more and 10% or less were evaluated as ○, and those having a deterioration or less than 5% were evaluated as x. .

検証試験の結果を表１に示す。
Ｆを含まないＣａＯ系脱硫剤を用いた場合で対比すると、大径浸漬管方式（実施例１）では、ＲＨ方式（比較例３）に比べて脱硫処理時間が短くなる。大径浸漬管方式は溶鋼鍋底部から撹拌用ガスを吹きこむ装置６を使用しているため、吹き込んだ脱硫剤が溶鋼循環流に乗って長い時間溶鋼中に滞在しているため、Ｆを含まないので脱硫所要時間が長い脱硫剤であるにもかかわらず、十分な脱硫が行われたためと推定される。 The results of the verification test are shown in Table 1.
When compared with the case of using a CaO-based desulfurizing agent that does not contain F, the large-diameter immersion pipe method (Example 1) has a shorter desulfurization treatment time than the RH method (Comparative Example 3). The large-diameter dip tube method uses the device 6 that blows the stirring gas from the bottom of the molten steel ladle, so the blown desulfurizing agent stays in the molten steel for a long time on the molten steel circulation flow, so it contains F. Since there is no desulfurization agent, it is presumed that sufficient desulfurization was performed even though the desulfurization time was long.

また、同じ大径浸漬管方式で比較すると、真空槽内圧力が１３００Ｐａ（９．８Ｔｏｒｒ）以上（実施例１）が、４００Ｐａ（比較例１）と比較して脱硫処理時間が短くなる。また、真空槽内圧力が６７０００Ｐａ（５０３Ｔｏｒｒ）以下（実施例４）が、９００００Ｐａ（比較例２）と比較して脱硫処理時間が短くなる。真空槽内圧力が１３０００Ｐａ（９８Ｔｏｒｒ）以下（実施例５）だと、６７０００Ｐａ（実施例４）よりさらに脱硫処理時間の短縮効果が得られる。
ランス高さが高いほうが、脱硫処理時間が短くなる（実施例１、参考例３）。 Further, when compared with the same large-diameter immersion pipe system, when the pressure in the vacuum chamber is 1300 Pa (9.8 Torr) or more (Example 1), the desulfurization treatment time becomes shorter than 400 Pa (Comparative Example 1). Further, when the pressure in the vacuum chamber is 67,000 Pa (503 Torr) or less (Example 4), the desulfurization treatment time becomes shorter than that of 90000 Pa (Comparative Example 2). When the pressure in the vacuum tank is 13,000 Pa (98 Torr) or less (Example 5), the effect of shortening the desulfurization treatment time can be obtained more than 67,000 Pa (Example 4).
The higher the lance height, the shorter the desulfurization treatment time (Example 1, Reference Example 3 ).

スラグのＦｅＯ成分＋ＭｎＯ成分の割合が減少するにつれて脱硫処理時間が短くなる（実施例１、実施例２）。 As the ratio of FeO component + MnO component in the slag decreases, the desulfurization treatment time becomes shorter (Examples 1 and 2).

１溶鋼鍋
２真空容器
３大径浸漬管
４脱硫剤吹き込みランス
５脱硫剤吹付けノズル
６撹拌用ガスを吹きこむ装置
７上昇管
８下降管
９ガス吹き込み口
１０溶鋼
１１真空容器内の溶鋼表面
１２溶鋼鍋内の溶鋼表面
１３スラグ
１４ガス噴流
１５上昇流
１６下降流
２１大径浸漬管方式真空脱ガス装置
２２ＲＨ方式真空脱ガス装置
1 Molten Steel Pan 2 Vacuum Container 3 Large Diameter Immersion Tube 4 Desulfurizing Agent Blowing Lance 5 Desulfurizing Agent Blowing Nozzle 6 Device for Blowing Gas for Stirring 7 Rise Pipe 8 Downcomer 9 Gas Blowing Port 10 Molten Steel 11 Molten Steel Surface 12 in Vacuum Container Molten steel surface in molten steel ladle 13 Slag 14 Gas jet 15 Upflow 16 Downflow 21 Large diameter immersion pipe type vacuum degasser 22 RH type vacuum degasser

Claims

When performing desulfurization treatment of molten steel in a vacuum degassing device, a desulfurizing agent is sprayed from a desulfurizing agent spraying nozzle onto the surface of molten steel in a vacuum container to substantially contain no F as the desulfurizing agent , and CaO is 50% by mass or more. a method of using a CaO-based desulfurizing agent (except when 70 wt% CaO-20 wt% CaCl ₂ -10 wt% CaSi, a composition of the) comprising,
As a vacuum degassing device, while using a device that blows the stirring gas from the bottom of the molten steel ladle,
For spraying the desulfurizing agent onto the molten steel surface described above, a lance having a nozzle arranged at the tip was used, and the vertical distance from the molten steel surface to the tip of the nozzle was defined as the lance height, and the lance height was set to 2 m or more. ,
A method for desulfurizing molten steel, characterized in that the pressure in the vacuum tank is 1300 Pa or more and 67,000 Pa or less.

The desulfurization method of molten steel according to claim 1, wherein the total of FeO component and MnO component of the slag existing on the molten steel surface in the molten steel pot before desulfurization is 10% by mass or less.