JP2004315893A - Method for refining molten metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for refining molten metal by which fine bubbles are developed and widely dispersed in the molten metal to efficiently remove inclusions. <P>SOLUTION: A pressure-reducing process, in which slag is floated up on the molten metal surface held in a vessel and an immersion tube is dipped into this molten metal and the molten metal is sucked into the inner part by reducing the pressure in the inner part of the immersion tube, and a pressing process, in which the molten metal in the inner part is spouted by pressing the inner part of this immersion tube, are repeated in order. Then, in at least the pressing process, gas is blown into the direction crossing with the flowing direction of the molten metal from the portion in contact with the molten metal in the inner part of the immersion tube, and a cycle time T(s) for one period time of the pressing and the pressure-reducing processes, is 1s to <15s and also out of range of the following formula. T<SB>B</SB>-0.03≤T≤T<SB>B</SB>+0.03T<SB>B</SB>=0.8×n×(D<SB>L</SB>-Do)<SP>0.5</SP>. Wherein, n: positive integer (1, 2, 3, 4, 5, ...), D<SB>L</SB>: inner diameter (m)of the vessel and Do: outer diameter (m) of the immersion tube. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００８１】
【発明の効果】
本発明により、スラグの巻き込みを抑制しつつ、溶融金属中に微細な気泡をさせるとともに該気泡を溶融金属中に広く分散させることにより効率的に介在物を除去することができ、本発明は、溶融金属の精錬には多大の寄与をする発明である。
【図面の簡単な説明】
【図１】溶融金属を模した水が入った容器の底面もしくは側面から、水中にガスを吹き込んだ状態を示す概念図であり、図１（ａ） は、静止した水に容器底面からガスを吹き込んだ状態、図１（ｂ） は、水平方向に流れる水に容器底面からガスを吹き込んだ状態、図１（ｃ） は、下方向に流れる水に容器側面からガスを吹き込んだ状態をそれぞれ示す。
【図２】水モデル試験装置の一例を示す模式図である。
【図３】図３（ａ） は、ある特定の条件で、浴面が振動することを示す模式図である。図３（ｂ） は、浴面上に浮かべた模擬スラグが巻き込まれることを示す模式図である。
【図４】加圧工程における浸漬管内部の圧力変化速度と平均気泡径ｄＢとの関係を示すグラフである。
【図５】サイクルタイムと介在物除去率ηとの関係を示すグラフである。
【図６】臨界周期Ｔｃと圧力変化速度との関係を示すグラフである。
【図７】計算周期Ｔ_Ｂ と介在物除去率が急激に悪化した周期Ｔ_Ａ との関係を示すグラフである。
【図８】本発明の方法を用いて、取鍋内の溶鋼を精錬する場合の装置構成の一例を示す模式図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a molten metal refining method capable of efficiently reducing inclusions in a molten metal.
[0002]
[Prior art]
As a method for removing inclusions contained in the molten metal, there is known a method in which a gas is blown into the molten metal to generate bubbles in the molten metal, and the inclusions are captured by the bubbles and floated to remove the inclusions. It is known that making the bubbles finer and dispersing the bubbles widely in the molten metal are effective for floating removal of minute inclusions in the molten metal.
[0003]
Inclusions can be efficiently removed by dispersing fine bubbles in the molten metal. However, it is difficult to achieve this because the wettability between the molten metal and the refractory is poor.
[0004]
From this point of view, in refining molten metal, several methods for generating fine bubbles in the molten metal and methods for dispersing the bubbles have been proposed.
[0005]
Patent Literature 1 discloses a method of blowing gas using a gas blowing device including a gas pool and a plurality of small-diameter porous plugs or annular porous bricks.
[0006]
Patent Literature 2 discloses a method for generating fine bubbles using a blowing plug made of a porous refractory, in which a gas is blown with a gas blowing amount per unit area of an operating surface of the refractory being equal to or less than a predetermined amount. ing.
[0007]
Further, Patent Literature 3 discloses a method in which a gas is blown while rotating a bubbling lance having a gas discharge portion formed of a porous refractory at a distal end portion in molten steel.
[0008]
[Patent Document 1] JP-A-59-125249
[Patent Document 2] JP-A-59-226129
[Patent Document 3] JP-A-62-192240
[0009]
[Problems to be solved by the invention]
However, the method disclosed in the above publication has the following problems.
The methods disclosed in Patent Literature 1 and Patent Literature 2 use a porous refractory to inject gas into molten steel. However, since the wettability between the molten steel and the refractory is small, each method is disclosed. The bubbles generated from the refractory grow so as to cover the surface of the refractory, and easily coalesce before detaching from the operating surface of the refractory. This phenomenon does not change even if the gas blowing amount is reduced as disclosed in Patent Document 1. Therefore, it is difficult for the method disclosed in the above publication to generate fine bubbles having a diameter of, for example, 10 mm or less in the molten metal.
[0010]
In the method disclosed in Patent Document 3, since gas is blown while rotating the bubbling lance, separation of bubbles from the operating surface of the porous refractory is promoted, and Patent Document 1 and Patent Document 2 disclose. Compared with the method, fine bubbles can be generated in the molten metal. However, the generated air bubbles quickly reach the surface and disappear along with the ascending flow around the bubbling lance, so that it is difficult to widely disperse the air bubbles in the molten metal. Therefore, the frequency of capturing the inclusions by the bubbles is reduced, and the effect of floating and removing the inclusions is reduced.
[0011]
An object of the present invention is to efficiently remove inclusions by generating fine bubbles in a molten metal and dispersing the bubbles widely in the molten metal in view of the above-described problems of the related art. It is an object of the present invention to provide a method for refining a molten metal using a method for generating fine bubbles.
[0012]
[Means for Solving the Problems]
The present inventors first performed the following water model test in order to study a method for generating fine bubbles in a molten metal.
[0013]
Gas was blown into water from a nozzle formed on the inner wall of a container containing water simulating a molten metal, and the flow state of water and the blowing direction of the gas were variously changed to investigate the state of bubble formation. Here, a water-repellent material was applied to the inner surface of the container to simulate poor wettability between the molten metal and the refractory.
[0014]
FIGS. 1A to 1C are conceptual diagrams showing a state in which gas is blown into water from the bottom or side surface of a container containing water imitating molten metal, and FIG. 1A shows a stationary state. FIG. 1 (b) shows a state in which gas is blown into the water from the bottom of the container, FIG. 1 (b) shows a state where gas is blown into the water flowing from the bottom of the container, and FIG. Are respectively shown.
[0015]
As shown in FIG. 1 (a), when gas is blown into stationary water from the bottom of the container, only the surface tension and buoyancy are applied to the bubbles in the growth process before separation, and the buoyancy is increased by the growth of the bubbles. The bubbles are separated from the nozzle only when the surface tension is exceeded, so that the bubbles dispersed in the water have a relatively large diameter.
[0016]
On the other hand, as shown in FIG. 1 (b), when gas is blown into the water flowing in the horizontal direction from the bottom of the container, a shear force due to the water flow is further applied to the bubbles in the growth process before separation. The separation of the bubbles from the nozzle is promoted, and the bubbles are separated at a faster stage than in the case of FIG. Therefore, the bubbles dispersed in the water had a smaller diameter than in the case of FIG.
[0017]
Further, as shown in FIG. 1 (c), when gas is blown into the water flowing downward from the side of the container, the shear force applied to the bubbles in the growth process before separation acts in the direction opposite to the buoyancy. Therefore, the separation of the air bubbles from the nozzle is further promoted, and the air bubbles are released at an earlier stage than in the case of FIG. Therefore, the bubbles dispersed in the water had a smaller diameter than in the case of FIG. 1 (b).
[0018]
From the above, it has been found that the mechanism shown in FIG. 1C is effective for generating fine bubbles in the molten metal.
Next, the present inventors studied to establish a practical and simple method capable of widely dispersing fine bubbles generated by the method shown in FIG. 1 (c) in a molten metal. As a result, the immersion tube is immersed in the molten metal bath, the inside of the immersion tube is depressurized to suck the molten metal, and then the inside of the immersion tube is pressurized to discharge the sucked molten metal, thereby forming the immersion tube. While forming a downward molten metal flow inside, blowing gas in a direction intersecting with the molten metal flow direction from a portion in contact with the molten metal inside the dip tube, for example, provided on the inner wall of the tip of the dip tube We conceived a method of injecting gas from the gas inlet, and performed the following water model test.
[0019]
FIG. 2 is an explanatory diagram showing an outline of the water model test device. As shown in the figure, in the water model test apparatus, a water 3 simulating a molten metal is placed in a container 2 simulating a ladle, and a water bath simulating a molten metal bath is formed. Is immersed in the water bath. The upper part of the immersion tube 1 is provided with a decompression means and a pressurization means which are respectively connected to a decompression chamber (not shown) via a decompression valve 6 and a pressurization chamber (not shown) via a pressure valve 7. Has been. The pressure inside the pressure reducing chamber is reduced in advance, and the inside of the immersion tube is quickly reduced in pressure by opening the pressure reducing valve 6.
[0020]
The pressure inside the pressurizing chamber is pressurized in advance, and the inside of the immersion tube is pressurized quickly by opening the pressurizing valve 7. Further, the pressure inside the immersion tube 1 can be detected by the pressure gauge 8, whereby the pressure change rate inside the immersion tube 1 can be determined. Further, a gas inlet 4 is provided on the inner wall at the lower end of the immersion tube 1.
[0021]
The following test was performed using the water model test apparatus shown in FIG. First, the pressure reducing valve 6 was opened and the pressure valve 7 was closed to reduce the pressure inside the immersion tube 1, and the water 3 in the container 2 was sucked into the immersion tube 1. Hereinafter, this step is also referred to as a “pressure reduction step”. Next, the pressure reducing valve 6 is closed and the pressure applying valve 7 is opened to increase the pressure inside the immersion tube 1, and the water 3 sucked into the immersion tube 1 in the previous pressure reducing step is again introduced into the container 2. Discharged. Hereinafter, this step is also referred to as a “pressurizing step”. Then, in the pressurizing step, gas was blown from a gas blowing port 4 provided on the inner surface of the lower end of the immersion tube 1.
[0022]
As a result, in the pressing step, the following phenomena (1) and (2) were confirmed.
{Circle around (1)} A downward water flow is formed inside the immersion tube, and fine bubbles 5 are formed by blowing gas from the direction crossing the water flow.
[0023]
(2) At the same time, a vortex ring is formed immediately below the immersion pipe due to the discharge of water. The aforementioned group of microbubbles is taken into this interior and descends to the vicinity of the bottom, and then the downward precipitation flow just below the immersion pipe turns at the bottom. This is accompanied by a horizontal flow, which is formed at the side, and a rising flow which is formed by turning at the side, and is widely dispersed in the water bath.
[0024]
In addition, when the pressurization in the pressurization step was performed simply by opening to the atmosphere, only a few bubbles were dispersed in the water bath at the end of the pressurization step, and the degree of dispersion was small.
[0025]
Further investigations have shown that, when the above-mentioned pressurization / decompression step is repeatedly performed, the phenomenon that the bath surface of the water contained in the container 2 repeatedly moves up and down, and the amplitude of the up and down movement is increased under certain conditions It is newly found that this occurs (FIG. 3A). Therefore, silicon oil simulating slag used in refining of molten metal was floated on a water bath of the container 2 and an experiment was conducted under conditions that cause the above phenomenon. In this case, a phenomenon was observed in which the silicon oil layer was drawn into the inside of the water bath due to the vertical vibration of the water bath surface and dispersed as silicon oil droplets in the water bath (FIG. 3B).
[0026]
These phenomena are particularly problematic in refining molten metal. That is, the phenomenon in which the molten metal overflows from the container due to the vertical movement of the bath surface decreases the yield of the molten metal and leads to an increase in manufacturing cost. In addition, slag is usually present on the molten metal in the vessel during refining of the molten metal, but when the slag droplets disperse in the molten metal due to vertical movement (also called swinging) of the bath surface outside the immersion tube, slag-based inclusions , Which leads to deterioration of quality.
[0027]
Therefore, with the slag floating on the bath surface, using the immersion tube, while blowing gas from the inner wall of the tip, repeatedly pressurizing and depressurizing the inside of the immersion tube to generate fine bubbles and widely disperse the bubbles in the bath. When dispersing, it is important to suppress the vertical movement (or swing) of the bath surface as much as possible.
[0028]
From the above, in order to generate fine bubbles in the molten metal and to widely disperse the bubbles in the molten metal, in the pressurizing step following the depressurizing step, the inside of the immersion pipe is pressed at a pressing rate of a predetermined value or more. By discharging the molten metal that has been pressed and sucked, a downward molten metal flow is formed inside the dip tube, and a direction that intersects with the molten metal flow direction from the portion that contacts the molten metal inside the dip tube. It was found that it is effective to inject gas into the container and to suppress the fluctuation of the container surface.
[0029]
Next, the present inventors, in order to establish a molten metal refining method capable of efficiently removing inclusions by utilizing such fine bubbles generated in the molten metal for the actual molten metal. A small-scale test using molten steel as the molten metal was conducted, and various conditions were examined.
[0030]
As the testing machine, a small testing machine having the same basic configuration as the water model testing machine shown in FIG. 2 and having a molten steel amount of 1000 kg (container inner diameter 0.6 m) was used. An immersion tube having an inner diameter of about 0.15 m and an outer diameter of 0.21 m was immersed in molten steel. The inside of the immersion tube was depressurized to suck molten steel into the inside of the immersion tube, and then the inside of the immersion tube was pressurized to discharge the sucked molten steel. During this time, Ar gas was blown from a gas blowing port provided at the lower end of the immersion tube.
[0031]
In this way, the process of repeatedly performing the pressure reduction step and the pressure step was performed.
First, in order to clarify the optimal range of the pressurizing speed inside the immersion tube during pressurization, the pressure was changed from 10 kPa / s to 2500 kPa / s, and the average bubble diameter dB was evaluated. Conditions other than the pressurizing speed were constant as follows.
[0032]
Gas blowing speed Q: 3.3 × 10^-5Nm³/ S
Difference ΔP between minimum pressure and maximum pressure in immersion tube: 80 kPa
Since the inside of the bath could not be observed in the test using molten steel, the diameter of the bubbles and the state of dispersion were evaluated by taking an image of the bubbles floating on the surface of the molten steel with a video camera and performing image processing.
[0033]
FIG. 4 is a graph showing the relationship between the pressure change speed inside the immersion tube during pressurization and the average bubble diameter dB. Here, the pressure change speed is the maximum instantaneous pressure change speed in the pressurizing step.
[0034]
As shown in the figure, when the maximum instantaneous pressure change rate becomes 100 kPa / s or more, the average bubble diameter dB becomes extremely small. Therefore, in order to generate fine bubbles in the molten steel, the maximum instantaneous pressure change rate is preferably set to 150 kPa / s or more. More preferably, it is 200 kPa / s or more. The upper limit of the maximum instantaneous pressure change rate is not particularly limited from the viewpoint of finer bubbles, but if the maximum instantaneous pressure change rate is excessively large, a splash of molten steel may occur. Is preferred. More preferably, it is 1500 kP / s or less.
[0035]
In this specification, the “maximum instantaneous pressure change rate” may be simply referred to as “pressure change rate”.
Next, the inventors examined the effect of the repetition cycle T (s) of depressurization and pressurization on the inclusion removal rate using a similar molten steel experimental apparatus.
[0036]
The inclusion removal ability was evaluated by an inclusion removal rate η using a total oxygen concentration (hereinafter also referred to as T. [O] (unit: ppm)) as an index of the inclusion concentration. That is, η is a value defined by equation (1).
[0037]
η = (T. [O] before processing−T. [O] after processing) / T. [O] (1)
Inclusions floating in molten steel cause problems such as deterioration of product characteristics and product flaws, and there are two types of inclusions. One is a so-called deoxidation product-based inclusion that is generated when the deoxidizer reacts with oxygen in the molten steel when the deoxidizer is added to the molten steel. The other is slag-based inclusions generated by slag existing on the upper surface of molten steel entering as slag droplets in molten steel for some reason. Therefore, the total oxygen concentration used as an index of the inclusion concentration indicates the total of the deoxidation product-based inclusions and the slag-based inclusions.
[0038]
FIG. 5 shows an instantaneous maximum pressurizing speed in the immersion tube: 800 kPa / s, and a blowing gas flow rate Q: 3.3 × 10^-5Nm³/ S, the difference between the maximum pressure and the minimum pressure in the immersion tube: constant at 80 kPa, the repetition cycle T of depressurization and pressurization is changed from 0.6 s to 15 s, and this process is performed 600 times (cycles). It is a figure showing change of inclusion removal rate eta. In both experiments, argon gas was supplied as a seal gas (or purge gas) to the space above the molten steel surface to prevent the reaction between oxygen gas in the atmosphere and the molten steel (so-called air oxidation or atmospheric oxidation). .
[0039]
As shown in FIG. 5, η rapidly increased from 1 s, gradually increased from 1 s to 2 s, and gradually decreased when 2 s or more. The reason is considered as follows.
As described above, the generated microbubble group is carried to the vicinity of the bottom surface by the vortex ring formed immediately below the immersion tube. Therefore, if T is small and the distance between the continuously formed vortex rings is not sufficient, the vortex disappears due to the interference between them and the fine bubble groups float without being transported to the bottom surface. That is, the chance of one bubble contacting the inclusions due to insufficient dispersion is reduced. The result of FIG. 5 was obtained because such interference of vortices is suppressed when T exceeds 1 s.
[0040]
The critical period Tc was determined to be 1 s under the above conditions, but it is considered that this Tc is affected by the liquid descending speed in the pipe. Therefore, the same investigation as described above was conducted under the condition that the pressure change rate (kPa / s) in the immersion tube at the time of pressurization which affects this was different. FIG. 6 shows the result.
[0041]
This is considered as follows. As the pressure change speed in the pipe is higher, the descending speed of the molten steel in the pipe is higher, and if the cycle is the same, the distance between the continuously formed vortex rings is larger, but on the other hand, the vortex velocity is larger. The former reduces interference between vortices and the latter promotes. As described above, since the two effects are opposite to each other, the effect of the in-pipe pressurizing speed becomes smaller as a result, and Tc becomes substantially constant.
[0042]
Further, as shown in FIG. 5, the reason why η gradually decreases when T is 2 s or more is that the absolute number of ejections decreases and the number of bubbles generated in the same time decreases. The figure also shows the inclusion removal rate when ordinary bubbling was performed at the same gas injection speed as in the above experiment. According to the figure, the inclusion removal rate was the same when T was 15 s or more. Or even less.
[0043]
From the above, it can be concluded that the proper range of the repetition cycle T of pressurization / decompression in the present invention is 1 s or more and less than 15 s.
Here, the period T is, for example, a time (s) from the start of pressurization to the start of next pressurization, and in the present invention, this may be referred to as a cycle time.
[0044]
However, when a slag floats, a certain cycle time T_A, A phenomenon that η suddenly decreased was observed. A sample was taken from the molten steel after the treatment, and the inclusion morphology was examined under a microscope. As a result, slag-based inclusions were observed in the sample under a condition where η was sharply reduced. Also, under these conditions, a phenomenon was observed in which the swing of the bath surface became large. Examination of the sample under conditions where η did not decrease sharply revealed that the inclusions were mainly deoxidation products.
[0045]
From the above, it has been found that the condition in which slag is floated on molten steel is an effective condition for improving the removal rate of inclusions, but it is necessary to limit the cycle time in order to sufficiently secure its advantages.
[0046]
First, the inventors found from FIG. 5 that the cycle time at which η rapidly deteriorates is represented by an integral multiple of the basic cycle time Tx. Therefore, from FIG. 5, the m-th cycle time Tm where T is 1 s or more and less than 15 s and η rapidly deteriorates is sequentially read, and Tm−Tm is read._-1, The basic cycle time Tx was calculated.
[0047]
Next, the inventors changed various conditions to change the basic cycle time Tx to the ladle diameter D._LAnd the immersion tube outer shape Do, it was found that it can be arranged by the formula (2). Therefore, it has been found that the cycle time at which η deteriorates can be expressed by an integral multiple of equation (3). Calculate the cycle time calculated from the empirical formula as T_BAnd
[0048]
Tx = 0.8 · (D_L-Do)^0.5    ... (2)
T_B= N · Tx (3)
Where T_B: Calculated cycle time (s) at which η rapidly increases, n: Positive integer (1, 2, 3,...), D_L: The inner diameter of the vessel on the bath surface (m), and Do: the outer diameter of the immersion tube on the bath surface (m).
[0049]
FIG. 7 shows the cycle time T obtained by the above equation._BAnd the cycle time T when η actually deteriorated sharply_AIt shows the relationship with. T which is the actual cycle time_AIs the calculated value T_BIs slightly different from T_BNearly within the range of ± 0.03 s.
[0050]
As described above, in the present invention, in order to suppress deterioration of cleanliness due to slag-based inclusions, the cycle time T is T_B−0.03 ≦ T ≦ T_BUse a value excluding +0.03.
[0051]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described in the case where the molten metal is molten steel. FIG. 8 is a schematic diagram showing an example of an apparatus configuration when the method of the present invention is applied to molten steel inside a ladle.
[0052]
As shown in the figure, as an example of an apparatus when the method of the present invention is applied to molten steel inside a ladle, a molten steel 13 is put in a ladle 12, a molten steel bath is formed, and a lower part is opened. An immersion tube 11 whose upper part is closed is provided so as to be able to move up and down. The figure shows a state where the lower part of the immersion pipe 11 is immersed in a molten steel bath. The operation of each member is as described above with reference to FIG.
[0053]
According to the present invention, control means (not shown) for supplying gas to the gas blowing port is provided at least in the pressurizing step. Such control means can be constituted by using a valve device as appropriate and an appropriate switch device in conjunction with the pressurizing and depressurizing means.
[0054]
Here, the depressurizing means includes a depressurizing valve 16 and a depressurizing chamber (not shown) communicating with the inside of the immersion tube through the depressurizing valve 16. And a pressurizing chamber (not shown) that communicates with the inside of the immersion pipe through the immersion pipe.
[0055]
The decompression chamber has a volume sufficiently larger than the internal volume of the dip tube, and the internal pressure is reduced in advance. By opening the pressure reducing valve 16, the internal pressure of the dip tube is rapidly reduced. It is so. Further, the pressurizing chamber does not necessarily have to have a larger volume than the internal volume of the immersion tube, but the internal pressure is preliminarily pressurized, and the pressurizing valve 17 is opened, so that The inside is quickly pressurized.
[0056]
The pressure inside the immersion tube 11 can be detected by a pressure gauge 18 so that the pressure change rate inside the immersion tube 11 can be determined. Further, at the lower end of the immersion pipe 11, a blowing gas flow passage formed inside the refractory constituting the immersion pipe is opened on the inner surface side of the immersion pipe, and is used for blowing Ar gas into molten steel. A gas inlet 14 is formed.
[0057]
In the present invention, the structure of the gas injection port itself is not particularly limited, but in order to inject gas in a direction intersecting with the downward flow, as a specific form, one or more thin tubes (eg, stainless steel, steel, May be buried at the position of the inner side wall of the immersion pipe, a gas flow passage may be provided inside the refractory during the production of the refractory, or a porous nozzle port may be provided on the inner wall of the immersion pipe. When using a bubbling lance immersed in molten steel, these nozzle ports may be provided on the side of the lance or the like.
[0058]
The method of the present invention can be carried out, for example, as follows using the apparatus shown in FIG. First, the pressure reducing valve 16 is opened and the pressure valve 17 is closed to reduce the pressure inside the immersion tube 11, and the molten steel 13 in the ladle 12 is sucked into the immersion tube 11. Next, the pressure reducing valve 16 is closed, the pressure valve 17 is opened, the pressure inside the immersion tube 11 is increased, and the molten steel 13 sucked into the inside of the immersion tube 11 in the previous step is again put into the ladle 12. Discharge. At least in the pressurizing step, an appropriate control means (not shown) is provided so that Ar gas is blown from the gas blowing port 14 provided on the inner surface of the lower end of the immersion tube 11.
[0059]
By repeatedly performing the pressure reduction step and the pressure step sequentially, fine Ar gas bubbles 15 can be generated intermittently in the molten steel bath. Further, the molten steel discharged from the immersion tube 11 in the pressurizing step forms a vortex ring slightly larger in diameter than the immersion tube, thereby transporting the bubbles to the vicinity of the bottom surface of the ladle 12. After the air bubbles reach the bottom surface of the ladle 12, the molten steel descending flow is dispersed in the horizontal direction by the horizontal flow formed by the turning, and finally, the horizontal flow is turned near the side wall and the upward flow and the action of buoyancy are generated. To rise. As described above, the bubbles are widely dispersed in the molten steel bath, and the inclusions can be efficiently removed.
[0060]
The appropriate conditions for applying the present invention to molten steel in a ladle and the reasons for limiting the conditions are described below.
First, the present invention is practiced by suspending slag on a bath surface of molten steel in a ladle. When slag is suspended, inclusions that have risen in the molten steel bath, or inclusions that come in contact with bubbles that have moved in the molten steel bath and are taken in by the bubbles and move with the bubbles are absorbed by the slag, This is because the inclusion removal efficiency is improved as compared with the case where the slag is not floated.
[0061]
It is preferable that the cycle time T satisfies the following expression.
1 ≦ T <15
If T is less than 1 s, the interference between the vortex rings that transport the generated microbubbles to the vicinity of the bottom surface is large, and they collapse before reaching the bottom surface. As a result, the microbubbles float before reaching the bottom surface. That is, the amount of inclusions removed by the bubbles is reduced. On the other hand, when T is 15 s or more, the number of bubbles discharged in a predetermined time is too small, so that an effect equivalent to or less than that of ordinary bubbling can be obtained.
[0062]
Furthermore, it is preferable that the cycle time T of the pressurization / decompression step does not satisfy the following expression.
T_B−0.03 ≦ T ≦ T_B+0.03
T_B= 0.8 · n · (D₁-D₂)^0.5
When the cycle time T satisfies the above condition, a vibration phenomenon occurs on the molten steel bath surface, and the suspended slag is dispersed in the bath to become slag-based inclusions, so that the inclusion removal efficiency is reduced. .
[0063]
Next, the maximum instantaneous pressure change rate of the immersion tube in the pressurizing step is set to 100 kPa / s or more. If the pressure change rate is less than 100 kPa / s, the descending flow rate of the molten steel in the pipe is not sufficient, and fine bubbles cannot be obtained, that is, inclusions cannot be efficiently removed.
[0064]
In order to stably generate finer bubbles, it is preferable that the pressure change rate be 200 kPa / s or more. The upper limit of the pressure change rate is not particularly limited from the viewpoint of miniaturization of bubbles, but if the pressure change rate is too large, the molten steel may be splashed. Therefore, the pressure change rate is set to 2000 kPa / s or less. It is preferable that
[0065]
Next, the difference between the maximum pressure in the pipe immediately after pressurization and the minimum pressure in the pipe at the end of depressurization, that is, the maximum pressure difference change ΔP is set to 100 kPa / s or less.
The greater the ΔP, the greater the amount of microbubble generation and therefore the greater the efficiency of inclusion removal. However, it has been found that the inclusion removal efficiency is saturated even if ΔP is increased to more than 100 kPa. On the other hand, from the viewpoint of equipment cost, it is preferable that the length of the immersion tube is short, that is, the movement amount of the molten steel is preferably small. Therefore, the maximum pressure change ΔP in the immersion tube is set to 100 kPa or less. However, a certain amount of molten steel movement is necessary in actual operation in order to effectively remove inclusions by obtaining a sufficient descending flow velocity in the tube for generating fine bubbles. In this sense, ΔP is desirably at least 5 kPa or more.
[0066]
From the above, ΔP is desirably 100 kPa or less, more desirably 5 kPa or more.
Also, the gas flow rate Q (m³(Standard state) / s) and the dip tube inner diameter Di (m), Q / Di is 6.7 × 10^-5m²/ S or more 6.7 × 10^-4m²/ S or less is desirable. If Q / Di is too small, the bubble diameter is sufficiently small, but the absolute amount of the number of bubbles is small. Therefore, even if the bubbles are widely dispersed in the molten steel, the number of inclusions that can be captured is limited. On the other hand, if the ratio Q / Di is too large, the bubble diameter becomes excessively large, and the inclusions slip due to the flow of molten steel formed around the bubbles when the bubbles float, making it difficult for both to contact each other. Alternatively, since the rising speed of the bubbles is high, the chance of contact between the two is reduced.
[0067]
Therefore, in order to remove inclusions with high efficiency, the ratio Q / Di must be 6.7 × 10^-5m²/ S or more 6.7 × 10^-4m²/ S or less. More preferably 1.3 × 10^-4m²/ S or more 5 × 10^-4m²/ S or less.
[0068]
Furthermore, the inner diameter Di of the dip tube and the inner diameter D of the ladle_LRatio Di / D_LIs preferably 0.1 or more and 0.7 or less from the viewpoint of bubble dispersibility. That is, the bubbles discharged from the immersion tube are transported to the vicinity of the bottom surface by the vortex ring formed immediately below the tube as described above. Therefore, the inner diameter Di of the immersion tube is equal to the inner diameter D of the ladle._LIf it is too large, the area through which bubbles pass will be small, and the efficiency of inclusion removal will decrease. Also, the inner diameter Di of the immersion tube is equal to the inner diameter D of the ladle._LIf the number is too small, the absolute number of bubbles is too small, and the efficiency of removing inclusions decreases.
[0069]
Therefore, in order to remove inclusions with high efficiency, Di / D_LIs preferably 0.1 or more and 0.7 or less. More preferably, it is 0.2 or more and 0.5 or less.
In the present embodiment, the case where the molten metal is molten steel has been described as an example, but the present invention is not limited to this. For example, the refining method of the present invention can be applied to any molten metal that requires a process of removing inclusions in a manufacturing process.
[0070]
Further, in the present embodiment, the case where the gas blown into the molten metal is Ar has been described as an example, but the present invention is not limited to this. The type of gas can be appropriately selected according to the target molten metal and the purpose of refining. As the type of gas, for example, Ar, N₂, H₂, He, or a mixture of two or more of these gases.
[0071]
Here, as an example of the case of “selecting appropriately according to the purpose of refining”, as for a gas whose dissolution in the molten metal adversely affects the product quality, another gas is used instead of the gas, or another gas is used. And the use of a mixed gas. As another method, there is a method of suppressing the flow rate of gas.
[0072]
The cross-sectional shape of the immersion pipe is preferably circular from the viewpoint of the construction of the refractory of the immersion pipe. However, when the immersion pipe is not circular, the inner diameter Di of the immersion pipe is S = (π / 4) from the horizontal cross-sectional area S of the immersion pipe. ) Di²The circle-equivalent inner diameter Di may be obtained using the following conversion formula.
[0073]
The “portion in contact with the molten metal inside the immersion tube” may be, for example, the portion of the inner surface of the immersion tube that is in contact with the molten metal when a gas inlet is provided in the immersion tube. When the bubbling lance is immersed in the molten metal inside the immersion tube and gas is blown from the bubbling lance, the outer surface of the bubbling lance may be a portion in contact with the molten metal. From the viewpoint of suppressing equipment costs while avoiding the complexity of the equipment, it is preferable to provide the part on the inner surface of the tip of the dip tube.
[0074]
The `` direction intersecting with the flow direction of the molten metal '' is a direction in which a shear force that promotes the separation of the bubbles due to the flow of the molten metal acts on the bubbles in the growth process before the separation, and preferably the melting direction. This is a direction substantially perpendicular to the metal flow direction.
[0075]
【Example】
[0076]
Embodiment 1
Using a device having the basic configuration shown in FIG. 8, Al deoxidation was performed, and 250 tons of molten steel was refined to remove inclusions. Slag was floated on the molten steel. CaO weight concentration in slag and Al₂O₃Concentration ratio CaO / Al₂O₃Is set to 1.5, CaO weight concentration in slag and SiO₂Concentration ratio CaO / SiO₂Was set to 3.0. In the device used for the test, the inner diameter of the ladle D_LIs 3.9 m, the inner diameter Di of the dip tube is 0.66 m 2, that is, Di / D_LWas set to 0.17. The outer diameter Do of the immersion tube was 1.16 m 2. Therefore, T_B= 0.8 × n × (D_L-Do)^0.5= 1.324 × n. At the lower end of the immersion tube, a total of four stainless steel pipes having an inner diameter of 2 mm are buried at 90 ° intervals in the circumferential direction of the immersion tube, and a gas inlet for injecting Ar gas into the molten steel. Was provided.
[0077]
After the immersion tube is immersed in the molten steel bath, a depressurizing step of depressurizing the inside of the immersion tube and sucking the molten steel into the immersion tube, and a pressurizing step of discharging the sucked molten steel by pressurizing the inside of the immersion tube. In addition to repeating the above steps sequentially, in the pressurizing step, Ar gas was blown from a gas blowing port provided at the lower end of the immersion tube. The flow rate of this Ar gas is 2.0 × 10^-4m³  (Standard condition) ・ S was the same for all conditions. Other conditions were as shown in Table 1, and the treatment was performed for 600 seconds (immersion tube method).
[0078]
For comparison, a conventional method of blowing Ar gas from a porous plug at the bottom of the ladle for 10 minutes was also performed (bubbling method).
When each refining method was applied, the total oxygen concentration of the molten steel before and after the refining was measured, and evaluation was performed using the above-described inclusion removal rate η.
[0079]
The results are summarized in Table 1.
[0080]
[Table 1]

[0081]
【The invention's effect】
According to the present invention, it is possible to efficiently remove inclusions by suppressing fine particles in the molten metal and dispersing the bubbles widely in the molten metal while suppressing slag entrainment. This is an invention that greatly contributes to the refining of molten metal.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a state in which gas is blown into water from the bottom or side surface of a container containing water simulating molten metal, and FIG. FIG. 1 (b) shows a state in which gas is blown into water flowing horizontally from the bottom of the container, and FIG. 1 (c) shows a state in which gas is blown into water flowing downward from the side of the container. .
FIG. 2 is a schematic diagram showing an example of a water model test device.
FIG. 3 (a) is a schematic view showing that a bath surface vibrates under a specific condition. FIG. 3B is a schematic diagram showing that a simulated slag floating on the bath surface is involved.
FIG. 4 is a graph showing a relationship between a pressure change rate inside a dip tube and an average bubble diameter dB in a pressurizing step.
FIG. 5 is a graph showing a relationship between a cycle time and an inclusion removal rate η.
FIG. 6 is a graph showing a relationship between a critical period Tc and a pressure change rate.
FIG. 7: Calculation cycle T_BAnd the period T at which the inclusion removal rate deteriorates sharply_A6 is a graph showing the relationship between
FIG. 8 is a schematic diagram showing an example of an apparatus configuration when refining molten steel in a ladle using the method of the present invention.

Claims (3)

A depressurizing step in which slag is floated on a molten metal bath surface accommodated in a container, and the pressure inside the dip tube immersed in the molten metal is reduced to suck the molten metal into the dip tube; A pressurizing step of discharging the molten metal inside the immersion pipe by pressurizing the pressure inside the pipe is sequentially repeated, and at least in the pressurizing step, from a portion in contact with the molten metal inside the immersion pipe. A method for refining molten metal in which a gas is blown in a direction intersecting with the flow direction of the molten metal, wherein a cycle time T (s) of one cycle of a pressure-decompression step is 1 s or more and less than 15 s, and A method for refining molten metal, wherein the method is outside the range of the following formula.
_{T B -0.03 ≦ T ≦ T B} +0.03
_{T B = 0.8 × n × (} D L -Do) 0.5
Here, n: a positive integer (1, 2, 3, 4, 5,...), D _L : container inner diameter (m), Do: immersion tube outer diameter (m). 2. The method for refining molten metal according to claim 1, wherein a pressure change amount ΔP inside the immersion tube is 100 kPa or less in at least the pressing step. 3. The method for refining molten metal according to claim 1, wherein the maximum instantaneous pressure change rate inside the immersion tube in the pressurizing step is 100 kPa / s or more.

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