JP2000239732A - Method for refining molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method obtaining the optimization of an immersion tube shape, the improvement and the stabilization of alloy yield, the improvement of dephosphorizing efficiency and the improvement of productivity by preventing the stickiness of skull, in molten steel refining using a bottom-blown stirring gas and an immersion tube. SOLUTION: In the case of using Rs (m) for the inner radius of the immersion tube, Hs (m) for a roof height in the immersion tube, Hh (m) for the max. height at the center part of the molten steel raised up with the stirring gas and Rh (m) for the half value of the raised up molten steel height = Hh/2, Rs/Rh is regulated to 1.25-2.75 and Hs/Hh is regulated to 5-10. The Hh and the Rh are obtd. from the measured value of a molten steel level meter or obtd. from the equation I. Equation I, Hh (g/Qo2)1/5=212 (g.do5/Qo2)-0.14(Hb/do)-1.03, and Equation II, Rh (g/Qo2)1/5=0.17 (g.do5/Qo2)0.05(Hb/do)0.59 (wherein, g: gravity acceleration = 9.8 (m/s2), Qo: bottom-blown gas floing rate (m3/s), do: bottom- blown tuyere nozzle diameter (m) and Hb: steel bath depth (m)).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining molten steel in which a dip tube is immersed in a molten steel surface in a ladle and secondary refining of the molten steel is performed inside the dip tube.

[0002]

2. Description of the Related Art In the steel industry, secondary refining for adjusting the steel composition is often performed after tapping in a converter or an electric furnace. In the secondary refining method, alloys are added and mixed while stirring molten steel by blowing inert gas under atmospheric pressure,
A method of performing desulfurization and dephosphorization by adding slag, a method of performing degassing such as degassing using a vacuum atmosphere, or degassing such as H and N are generally performed. As another method, there is a method of performing secondary refining while immersing a dip tube in the surface of molten steel and stirring the molten steel with gas.

[0003] As a method of using a stirring gas and a dip tube under atmospheric pressure, a CAS method and a SAB method are introduced in Iron and Steel Handbook (Ironmaking, Steelmaking), edited by The Iron and Steel Institute of Japan, 1979/5, page 690. ing. This is because Al and C
a, an element such as REM (rare earth element) is to be efficiently and accurately added to the ladle molten steel.

Japanese Patent Application Laid-Open No. 1-100216 discloses that a heating agent is heated from the surface of molten steel with a blowing lance while a heating agent is added to an immersion tube, and a desulfurizing agent is blown from a lower portion of the blast acid lance with another lance. It discloses that a method for heating molten steel and desulfurization with high efficiency is possible.

[0005]

However, in the method disclosed in the above-mentioned document or the method disclosed in Japanese Patent Application Laid-Open No. 1-100216, unless the inner diameter or the height of the immersion tube is properly set, the efficiency of production becomes high. There's a problem. That is, depending on the dimensions of the dip tube, (1) the added alloy, desulfurizing agent, etc. react with the slag in the ladle to lower the yield or become unstable. In the method of performing desulfurization treatment by adding a flux, phenomena such as resulfurization from the flux side to molten steel depending on the refining conditions, and (3) swelling of the metal in the immersion pipe due to the splash of molten steel occur, and the like.

[0006] The decrease in the yield of the additive alloy (1) increases the alloy cost and the manufacturing cost. If the alloy yield becomes unstable, it leads to a decrease in the efficiency of the component adjustment step, and the standard ratio of the components decreases, which is a serious problem.

[0007] The resulfurization phenomenon (2) reduces the desulfurization rate, resulting in an increase in refining costs and a decrease in productivity. If the resulfurization is significant, the target sulfur concentration cannot be reached.

[0008] As described in the above (3), if a large metal ingot adheres to the immersion pipe, frequent metal cutting work is required, leading to a decrease in productivity.

An object of the present invention is to solve these problems, and by specifying the shape of the immersion tube and the conditions for blowing the stirring gas, (1) improving the yield and stabilizing the alloy,
It is an object of the present invention to provide a method for (2) improving desulfurization or dephosphorization efficiency at the time of adding a refining flux, and (3) improving productivity by suppressing adhesion of immersed pipe metal.

[0010]

Means for Solving the Problems Molten steel smelted in a converter or an electric furnace is transported to a secondary refining process. The refining method of blowing gas for stirring was studied.

FIG. 1 is a schematic diagram showing an example of a refining method using an immersion tube under atmospheric pressure. In the figure, reference numeral 1 denotes molten steel, 2 denotes slag, 3 denotes a ladle, 4 denotes a bottom blowing tuyere, 5 denotes a dip tube, 6 denotes an exhaust port, 7 denotes a flux inlet, 8 denotes a lifting device, and 9 denotes a molten steel level. It is total. The slag 2 is inevitably mixed into the ladle when tapping from a converter or the like. In the secondary refining, the dipping tube 5 is dipped from the upper part of the molten steel 1 in the ladle 3 using the elevating device 8. The slag in the dip tube is minimized by immersing the dip tube after exposing the molten steel to the slag. Thereafter, a desulfurizing agent, an alloy element, and the like are introduced from the flux inlet 7. Furthermore, from the bottom blowing tuyere 4 provided at the ladle bottom, Ar,
The molten steel is stirred by blowing an inert gas such as N ₂ to increase the reaction efficiency between the flux and the molten steel.

The inventors have observed a state in which the surface of molten steel rises when a bottom blowing gas for stirring is blown. The molten steel level meter 9 is provided with a swing mechanism so as to two-dimensionally scan the surface of the molten steel in the immersion pipe 5 so that the position and the height of the raised molten steel can be measured.
The bare metal surface is exposed at the swelling portion of the molten steel, and contact between the ladle slag and the flux, which is a problem at the time of alloy addition, flux input, or degassing refining, can be suppressed.
The inventors have found that if the relationship between the radius Rh of the raised portion and the inner radius Rs of the immersion tube is set to a value within a specific range, the refining characteristics are greatly improved.

Further, the inventors have observed molten steel particles scattered from the raised portion. As a result, it was found that there is a correlation between the maximum height Hh of the raised portion and the amount of scattered molten steel particles. And the maximum height Hh of the raised part of the molten steel,
Height Hs (m) from the surface of the bath to the ceiling in the immersion tube
It was found that if the relationship of "free board" was set to a value within a specific range, it would be possible to suppress sticking.

The gist of the present invention based on the above findings is as follows (A) to (C).

(A) A method of refining molten steel using a dip tube while blowing a stirring gas into the molten steel from the bottom of the ladle, wherein the inner radius of the dip tube is Rs (m), and the inner radius of the dip tube ceiling is The height from the molten metal surface is Hs (m), the maximum height of the swelling of the molten steel by the stirring gas in the immersion pipe is Hh (m), and the half value radius at which the height of the swelling of the molten steel is Hh / 2 is Rh.
A method for refining molten steel, wherein Rs / Rh is 1.25 to 2.75 and Hs / Hh is 5 to 10, when (m).

(B) Using a scanning molten steel level meter provided on the ceiling of the immersion pipe, the height of the molten steel surface at each position is measured, and the center position and the height of the raised maximum height of the molten steel are measured. Hh, the average value of the distance between the center position and the position where the swelling height is Hh / 2 is determined as a half-value radius Rh, and the bottom blown gas flow rate Qo is set so as to satisfy the condition described in the above item (A). (M ³ / s) and Hs,
A method for refining molten steel, wherein at least one is changed.

(C) Hh and Rh are represented by the following (1) and (2)
A method for refining molten steel, characterized in that at least one of the bottom-blown gas flow rates Qo (m ³ / s) and Hs is changed so as to satisfy the condition described in the above item (A) by an equation.

[0018] ^{Hh (g / Qo 2) 1/5} = 212 (g · do 5 / Qo 2) -0.14 (Hb / do) -1.03 (1) Rh (g / Qo 2) 1/5 = 0.17 (g・ Do ⁵ / Qo ² ) ^0.05 (Hb / do) ^0.59 (2) where g: gravitational acceleration = 9.8 (m / s ² ), do:
Bottom blowing tuyere nozzle diameter (m), Hb: steel bath depth (m).

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors tried to quantify the shape of a raised portion of molten steel by a bottom-blown gas. Using the apparatus shown in FIG. 1, bottom blow gas was blown into molten steel stored in a 200-t ladle, and the shape of the swell was measured using a scanning type molten steel level meter 9 provided on the inner ceiling of the immersion pipe.

FIG. 2 is a graph in which the swelling height H (m) of the molten steel in the direction of the radius R (m) is plotted. The figure shows
Bottom blown gas flow rate Qo (m ³ / s), steel bath depth Hb
(M), which was measured by changing the conditions of the bottom nozzle port diameter do (m).

As shown in the drawing, the height of the swell at the center of the gas injection is Hh (m), and the radius (half-value radius, also simply referred to as the radius of the swell) at which the height of the swell is Hh / 2 is Rh ( Examining the relationship between R and H, that is, the shape of the swelling as m), it was found that the relationship can be approximately expressed by a normal distribution equation. That is, the swell height H can be expressed by the following equation (3) as a function of the radius R.
The curve derived from this equation is shown in FIG.

H / Hh = exp ｛−0.7 × (R / Rh) ² ｝ (3) When the number of gas blowing ports of the bottom blowing nozzle is not one, for example, in the case of a single tube assembly, The sum S of the cross-sectional areas of the portions through which the gas passes is determined, and S = (π / 4) · do
^The converted diameter do obtained from ² is used as the nozzle inner diameter. Further, when gas flows from the pores of the refractory like a porous plug, a simple geometrical area S of the porous plug in contact with the molten steel is determined, and converted into a gas passing area by 0.005. (S '= 0.005 · S)
Then, the converted diameter do may be obtained from S ′ = (π / 4) · do ² .

As described above, in order to define the shape of the swelling, two variables of Hh and Rh may be determined. The inventors conducted tests using a model experiment, a scouring test facility, and the like, and arranged using factors such as a gas flow rate Qo and a bottom blowing tuyere radius do. As a result, the following empirical formulas (1) and (2) were obtained.

[0024] ^{Hh (g / Qo 2) 1/5} = 212 (g · do 5 / Qo 2) -0.14 (Hb / do) -1.03 (1) Rh (g / Qo 2) 1/5 = 0.17 (g・ Do ⁵ / Qo ² ) ^0.05 (Hb / do) ^0.59 (2) where g: gravitational acceleration = 9.8 (m / s ² ), Qo:
Bottom blowing gas flow rate (m ³ / s), do: bottom blowing tuyere nozzle diameter (m), Hb: steel bath depth (m).

FIG. 3 is a graph showing the relationship between the nozzle diameter of the bottom blowing tuyere, the flow rate of the bottom blowing gas, and the swelling height of the molten steel.

FIG. 4 is a graph showing the relationship between the nozzle diameter of the bottom blow tuyere, the flow rate of the bottom blow gas, and the radius of the raised portion of molten steel.

The fitted straight lines shown in FIG. 3 and FIG.
The relationship between Equations (1) and (2) is shown, and each plot is the above-described measured data. From FIGS. 3 and 4, it can be seen that equations (1) and (2) are valid as empirical equations for Hh and Rh.

Next, assuming that Hs / Hh = 6 was constant, the effect of the ratio of the half-height radius Rh on the rise to the radius Rs in the immersion pipe on the yield of the alloy was investigated.

With the apparatus shown in FIG.
Was about 0.5 m, and tests were conducted by preparing dip tubes of various inner diameters. As an additive alloy, 0.3 kg of shot aluminum with 98% Al purity / molten steel t was added from a flux inlet, and the mixture was mixed with gas for about 3 minutes, and then a metal sample was collected and analyzed. The yield of Al was calculated from the analysis values and arranged by Rs / Rh.

FIG. 5 is a graph showing the relationship between the ratio of the radius Rs in the immersion pipe to the radius Rh of the rise of the molten steel and the Al yield.

As shown in the figure, in order to improve the Al yield (90% or more), Rs / Rh is set to 1.25 to 2.
It is understood that 75 is better. The reason is considered as follows.

When the upward flow generated by the introduction of the stirring gas in the immersion tube changes to a horizontal flow (more precisely, a surface flow along the rising shape) on the surface of the molten steel, and then collides with the inner wall of the immersion tube and changes to a downward flow. Conceivable. Therefore, Rs / R
When h is less than 1.25, the downflow is strong because the diameter of the immersion tube is too small with respect to the rise, and the added metal Al
Is completely discharged from the dip tube before being completely dissolved and trapped in the slag. Therefore, the Al addition yield decreases. on the other hand,
When Rs / Rh exceeds 2.75, the diameter of the immersion pipe is too large, so that a part of the ladle slag is always present in the immersion pipe, and the added metal Al is trapped in the ladle slag in the immersion pipe, and Al addition is performed. It is thought that the yield decreases. Preferably, Rs / Rh is between 1.5 and 2.5.

Next, the effect of the ratio of the radius in the immersion tube to the size of the swelling of the molten steel (the half-value radius Rh) on the desulfurization efficiency was investigated.

In the apparatus shown in FIG.
O-based flux _{(CaO: 60% -CaF 2:} 40%)
5 kg / t of molten steel, and desulfurizing the molten steel using a dip tube of various inner radii (concentration in molten steel before treatment: [S] = 50 p
pm). The processing time is 10 minutes. At this time, the (FeO + MnO) concentration in the ladle slag before the treatment was 6%.

FIG. 6 is a graph showing the relationship between the ratio of the inner radius Rs of the immersion tube to the bulge radius Rh of the molten steel and the desulfurization rate.

As shown in the figure, in order to improve the desulfurization rate (60% or more), Rs / Rh is 1.25 to 2.75.
It is understood that it is better to set When Rs / Rh is less than 1.25, the diameter of the immersion tube is too small as described above, and the added flux flows out of the immersion tube, so that the desulfurization rate decreases.
When Rs / Rh exceeds 2.75, (FeO + M
The ladle slag having a high concentration of nO) remains in the immersion tube and inhibits desulfurization, so that the desulfurization rate decreases. Preferably, Rs
/ Rh is 1.5 to 2.5.

The desulfurization of molten steel by a CaO-based flux is carried out by C
aO + S = CaS + O, and has a high oxygen potential (ie, a lower oxide (FeO + M
It is considered that ladle slag with a high nO) concentration inhibits desulfurization.

As shown in FIGS. 5 and 6, it was found that the preferred range of Rs / Rh was substantially the same both in terms of the yield of the alloy and the desulfurization efficiency.

Next, the effect of the height Hs of the free board on the metal in the immersion tube was investigated. Rs / Rh = 1.7
The operation was carried out for several months at a constant value of 5, and the number of times of slab cutting per month was used as an index. Hs / H
The evaluation was performed using the refractory cost index based on the condition of h = 3.

FIG. 7 is a graph showing the relationship between the ratio of the height Hs of the ceiling in the immersion pipe to the height Hh of the swelling portion of the molten steel and the cost of the slab cutting and refractory costs.

As shown in the figure, it is understood that Hs / Hh needs to be 5 or more in order to reduce the number of times of slab cutting (3 times / month or less). If Hs / Hh is less than 5, the amount of the molten steel particles scattered from the raised portion of the molten steel adheres to the ceiling of the immersion pipe, and the number of times of ingot cutting increases. Although the molten iron particles are scattered on the inner side wall of the immersion pipe, it is considered that since the distance from the molten steel surface is short, the molten iron particles are heated by the radiant heat of the molten steel, flow down along the side wall, and do not contribute to an increase in the number of slab cutting. Further, FIG. 7 shows that the refractory cost index is significantly reduced (3 or less) when Hs / Hh is 10 or less. If Hs / Hh exceeds 10, the temperature difference in the vertical direction of the refractory increases, and cracks in the brick occur frequently during operation, so that the cost of the refractory increases.
Preferably, it is 6-9.

FIG. 8 is a correlation diagram in which the evaluations of the alloy yield, desulfurization rate, metal ingot, and refractory cost are arranged by Rs / Rh and Hs / Hh. In the figure, × indicates refining conditions that have two or more major problems such as alloy yield, desulfurization rate, ingots, refractory cost, etc., and Δ indicates that there are some problems in two or more of these evaluation indices, Refining conditions in which any one has a serious problem, and ○ indicates refining conditions in which these indexes are favorable. That is, region [1] has a large decrease in alloy yield and desulfurization rate and large metal ingot, region [2] has a low alloy yield and desulfurization ratio, and region [3] has an alloy yield and low desulfurization rate. In addition, refractory costs have increased significantly. Area [4] has large ingots, area [5] has good alloy yield, desulfurization rate, ingots, refractory costs are good, and area [6] has large refractory costs. Area [7] is low alloy yield and desulfurization rate and large ingot with area [8] is alloy yield and low desulfurization rate, area [9] is alloy yield and low desulfurization rate and large refractory cost It is an area of increase.

As shown in the figure, Rs / Rh = 1.2 in order to secure the improvement of the alloy yield and the desulfurization rate, and to reduce the number of times of ingot cutting and the refractory cost index.
It is understood that it is necessary to simultaneously satisfy the condition of 5 to 2.75 and the condition of Hs / Hh = 5 to 10.

In practicing the present invention, a first method is as follows:
A sensor for measuring the swelling shape of the molten steel is provided inside the immersion pipe having the inner radius Rs and the free board height Hs, and the maximum height Hh and the half-value radius Rh of the swelling of the molten steel are directly measured.
Rs / Rh is 1.25 to 2.75, Hs / Hh is 5-1.
The gas injection amount Qo from the bottom tuyere to be 0
And adjusting one or both of the Hs changes by changing the immersion depth of the immersion tube.

The above Hh and Rh are obtained, for example, as follows. Using the scanning mechanism (swinging mechanism or electronic scanning) of the molten steel level meter 9 shown in FIG. 1, the height z (x, y) of the swelling at the plane position (x, y) of the molten steel in the immersion tube is measured. And memorize. A position (x _max , y _max ) at which the measured value z reaches the maximum value z _max is determined, and this position is set as the center position of the swell. At this time, in consideration of the fact that the height of the raised surface of the molten steel fluctuates finely in a foamed state, the measured data z (x, y) is not used as it is, but the spatial data in a mesh area of, for example, 30 mm × 30 mm is used. And an average over time. A position (x _i , y) where the height of the bulge is z _max / 2 in, for example, eight directions radially from the bulge center position (x _max , y _max ).
_j) (i, j obtains a representative) to 8 coordinate direction, the distance R _i of the center position (x _max, y _max) and (x _i, y _j), obtains the _j. The obtained z _max is Hh, and the average value of R _{i, j} is Rh.

In the second method of the present invention, Hh and Rh are obtained from the equations (1) and (2) without measuring the swelling shape of the molten steel, and the values of Rs / Rh and Hs / Hh are determined to be predetermined. This is a method of adjusting one or both of the gas blowing amount Qo and the immersion depth of the immersion tube so as to fall within the range. In both the first and second methods, the controllable factors are the amount of the bottom blown gas and the immersion depth of the immersion tube, but even if these are adjusted, Rs / Rh: 1.25 to 2.75, Hs / Hh: 5-1
When the condition of 0 is not achieved, a dip tube having a different inner diameter may be used, or the nozzle of the bottom blow gas may be replaced.

Either the first method or the second method may be selected in consideration of the reliability, measurement accuracy, and the like of the molten steel level meter.

[0048]

EXAMPLE 250 tons of hot metal was blown in a top and bottom blown converter.
After decarburization blowing to a predetermined carbon concentration (here, [C] = 0.05%), molten steel was discharged from a tapping port to a ladle. In the initial and final stages of tapping, converter slag (lower oxide concentration (FeO + Mn)
O) = 22%) flowed out to the ladle. During tapping, 2 kg of metal Al for deoxidation / molten steel t and quick lime CaO for slag making were added to the ladle.
Was added at 2 kg / t. The metal Al for deoxidation was used for reaction with dissolved oxygen in molten steel and lower oxides in slag.
As a result, the low-grade oxide in the slag was 6%.

The steel bath depth of the ladle was 3 m, and Ar gas for stirring was blown at a flow rate of 1 m ³ / min from a tuyere (inner diameter 8 mm) at the bottom of the ladle. The rise of the gas bubbles formed a bare surface of the molten steel and minimized the slag, and then immersed the immersion tube in the upper part of the molten steel.

Under these conditions, Hh and Rh are as described in (1) and (2) above.
From the formula, Hh = 0.4 m and Rh = 0.35 m.

The immersion depth of the immersion tube was 0.3 m as a vertical distance from the lower end of the immersion tube to the surface of the molten steel. The ladle bottom tuyere was filled with a refractory filling material about 5 cm in front so that it would not be blocked by molten steel insertion when tapping, and when the ladle bottom was blown, the pressure of Ar gas was increased to blow off the filling material. .

(Example 1) The inner radius of the immersion tube was set to 0.17 to
1.25 m, and the height Hs in the immersion tube is 1.2 to 4.4 m
Then, immediately after that, 1 kg of metal Al for component adjustment / molten steel t was added into the dip tube. Acid soluble Al concentration in molten steel before adding Al for component adjustment (hereinafter, referred to as
[Sol. Al] was 0.02%. Al
100 kg of metal Al with a pure content of 98% is added at 1 kg / molten steel t.
% When a yield of [sol. Al] is 0.098%. Therefore, [s] before and after the addition of Al
ol. [Al] The value obtained by dividing the increase by 0.098% and multiplying by 100 is the yield. Table 1 shows the test results.

[0053]

[Table 1]

In Table 1, No. From the data of 1, 2, 7, and 8, the ratio Rs / Rh of the radius Rs to Rh in the immersion tube is 1.25.
It was found that the yield of metal Al was reduced when the conditions of ~ 2.75 were not satisfied. Further, if the condition is not met, 10
The standard deviation of the yield of metal Al in charge (charge is one unit of molten steel processing; hereinafter, abbreviated as ch) increases to more than 10%. If this variation is large,
After the addition of the alloy, a molten steel sample is collected and analyzed to determine whether or not it is within the target components, so that the ladle cannot be transferred to the continuous casting apparatus.

Further, if the extra analysis is performed, the analysis waiting time increases, and the re-adjustment time of the components also increases, so that the temperature drop of the molten steel becomes 15 ° C. on an average of 10 channels, and the tapping is made to compensate for that. The temperature had to be increased by 15 ° C. Converter refractory wear rate is 0.1m due to increase in converter tapping temperature
It increased from about m / ch to more than 0.2 mm / ch and adversely affected the converter life.

On the other hand, in Table 1, From the data of 9, 10, and 14, the ratio Hs / Hh of the height Hs to Hh in the immersion pipe is 5
If it is less than 10, the ingot has increased, and if it exceeds 10, the refractory cost of the dip tube has increased.

From the above results, it is possible to improve the productivity by maintaining the alloy yield stably at a high level, suppressing the temperature at the converter steel tapping and the wear of refractories, and suppressing the ingot of metal in the secondary refining. In order to reduce the cost of refractories, Rs / Rh is 1.25 to 2.75.
And it turned out that Hs / Hh needs to be 5-10.

(Example 2) The inner radius Rs of the immersion tube was set to 0.17.
To 1.25 m, and the height Hs in the immersion tube is set to 1.2 to 4.4.
Then, the steel was immersed in molten steel as m, and immediately thereafter, 5 kg of a flux for desulfurization (CaO: 60% + CaF ₂ : 40%) / t was added to the immersion tube, and desulfurization treatment was performed for 10 minutes.
Sulfur concentration So before desulfurization treatment, sulfur concentration Se after desulfurization treatment
And the desulfurization rate α (%) was determined by the following equation.

Α = 100 × (So—Se) / So The sulfur concentration So before desulfurization was 48 to 52 ppm and the average value was 50 ppm. In the slag before desulfurization (Fe
O + MnO) = 6%. Table 2 shows the results.

[0060]

[Table 2]

In Table 2, No. From the data of 1, 2, 7, and 8, it was found that when Rs / Rh was out of the range of 1.25 to 2.75, the desulfurization rate was reduced.

In the table, No. From the data of 9, 10, and 14, the ratio Hs / Hh of the height Hs to Hh in the immersion pipe is 5
If it is less than 10, the ingot increases, and if it exceeds 10, the refractory cost increases.

From the above, in order to stably maintain the molten steel desulfurization rate at a high level, suppress productivity of the metal in secondary refining, increase productivity, and reduce the cost of refractories, R / Rh must be 1.
It turned out that it is necessary to make 25 / 2.75 and H / Hh 5-10.

[0064]

Industrial Applicability According to the present invention, it is possible to stably maintain the alloy yield and the desulfurization rate of molten steel at a high level, suppress the sticking of metal in secondary refining, increase productivity, and reduce the cost of refractories. Is possible.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a refining method using an immersion tube under atmospheric pressure.

FIG. 2 is a graph in which the molten steel swelling height in a radial direction in an immersion pipe is plotted.

FIG. 3 is a graph showing the relationship between the nozzle diameter of the bottom blowing tuyere, the flow rate of the bottom blowing gas, and the swelling height of molten steel.

FIG. 4 is a graph showing the relationship between the nozzle diameter of the bottom blowing tuyere, the flow rate of the bottom blowing gas, and the radius of the raised portion of molten steel.

FIG. 5: Radius Rs in immersion pipe versus radius Rh of raised portion of molten steel
6 is a graph showing the relationship between the ratio of Al and the Al yield.

FIG. 6: Radius Rs in immersion pipe versus radius Rh of raised portion of molten steel
4 is a graph showing the relationship between the ratio of the above and the desulfurization rate.

FIG. 7 is a graph showing the relationship between the ratio of the height Hs of the ceiling in the immersion pipe to the height Hh of the bulging portion of the molten steel, the metal cutting operation, and the cost of the refractory.

FIG. 8 is a correlation diagram in which the evaluation of alloy yield, desulfurization rate, metal ingot, and refractory cost are arranged by Rs / Rh and Hs / Hh.

[Explanation of symbols]

 1: molten steel 2: slag 3: ladle 4: bottom blowing tuyere 5: dip tube 6: exhaust port 7: flux inlet 8: lifting device 9: molten steel level meter

Claims (3)

[Claims] 1. A method for refining molten steel using a dip tube while blowing a stirring gas into the molten steel from the bottom of the ladle, wherein the inner radius of the dip tube is Rs (m), The height from the surface is Hs (m), the maximum height of the swelling of the molten steel due to the stirring gas in the dip tube is Hh (m), and the half-value radius at which the height of the swelling of the molten steel is Hh / 2 is Rh (m).
When Rs / Rh is set to 1.25 to 2.75, H
A method for refining molten steel, wherein s / Hh is 5 to 10. 2. A swelling height for each position of a molten steel surface is measured using a scanning molten steel level meter provided on a ceiling portion of a dip tube, and a center position and a swelling height at which the swelling height of the molten steel is maximum. Hh is determined, and the average value of the distance between the center position and the position where the swell height is Hh / 2 is determined as a half-value radius Rh, and the bottom blown gas flow rate Qo (m ^{3) is set} so as to satisfy the condition described in claim 1. / S) and Hs, wherein at least one of them is changed. 3. Hh and Rh are determined by the following equations (1) and (2), and at least one of the bottom blown gas flow rate Qo (m ³ / s) and Hs is set so as to satisfy the conditions described in claim 1. A method for refining molten steel, characterized in that ^{Hh (g / Qo 2) 1/5} = 212 (g · do 5 / Qo 2) -0.14 (Hb / do) -1.03 (1) Rh (g / Qo 2) 1/5 = 0.17 (g · do 5 / Qo ² ) ^0.05 (Hb / do) ^0.59 (2) where g: gravitational acceleration = 9.8 (m / s ² ), do:
Bottom blowing tuyere nozzle diameter (m), Hb: steel bath depth (m).