Molten steel refining method

Abstract

The present invention provides a method for refining molten steel by using an RH type vacuum degassing device to increase the circulation flow rate. The stirring power energy density ε of the molten steel in the vacuum tank satisfies the following formula (where G is the circulation gas flow rate, T is the molten steel temperature, and ρ is the stirring power energy density ε of the molten steel in the vacuum tank). L : density of molten steel, g: acceleration due to gravity, H0: height from the nozzle position of the circulating gas to the steel bath surface of the vacuum tank in a static state, P: pressure in the vacuum tank, P0: atmospheric pressure, h V : The height from the static state of the vacuum tank steel bath surface to the base, L: The height from the lower end of the immersion tube to the base, h G : Height from the lower end of the immersion tube to the position of the circulating gas blowing nozzle, l: Depth of molten steel immersed in the immersion tube, D U : inner diameter of the riser. ) is used to determine the immersion depth l of the immersion pipe in the molten steel or the circulation gas flow rate G. ε=[371GT×ln{1+(ρ L gH0 / P)}] / W V , W V =(π·D V 2 / 4)×H0×ρ L / 1000, H0=h V +L-h G , h V =(P0-P) / (ρ L g)+l-L,1.35×10 5 ×D U / W V <ε<2.1×10 4 .

Description

Technical Field

[0001] The present invention relates to a method for refining molten steel using an RH type vacuum degassing device. Background Art

[0002] Various methods are known for ladle refining and vacuum degassing molten steel, including VOD and VTD. With the increasing demand for higher-grade steel and the increasing demand for these steels, the types and quantities of steel requiring vacuum degassing are trending upwards. Consequently, there is a strong desire to reduce steelmaking costs by shortening the time required for this process, improving degassing capacity, and lowering converter temperatures. These demands have led to the widespread use of RH (Rheinstahl-Heraeus) vacuum degassing equipment for vacuum degassing.

[0003] like Figure 1 As shown, the RH-type vacuum degassing apparatus 1 includes an ascending immersion pipe 8 and a descending immersion pipe 9. The ascending immersion pipe 8 and the descending immersion pipe 9 are immersed in the molten steel 3 in the ladle 2. The degassing vacuum tank 5 is depressurized by exhausting gas from the exhaust port via a pressure reducing device (not shown) through a pipe 11, thereby drawing the molten steel 3 upward. Furthermore, a circulating gas is drawn into the ascending immersion pipe 8, which is equipped with the circulating gas intake pipe 10, through the circulating gas intake pipe 10. An inert gas such as argon is often used as the circulating gas. The buoyancy of the gas causes the molten steel 3 to rise and be introduced into the degassing vacuum tank 5. The molten steel 3 is then lowered from the descending immersion pipe 9, circulating the molten steel 3 for degassing.

[0004] Refining using this RH-type vacuum degassing apparatus includes decarburization under vacuum (hereinafter referred to as "vacuum decarburization") and degassing using hydrogen or nitrogen. Increasing the recirculation flow rate is effective in accelerating the decarburization rate in vacuum decarburization and the degassing rate during degassing, and various methods for increasing the recirculation flow rate have been proposed.

[0005] For example, Patent Document 1 proposes a method of blowing an inert gas heated to 200° C. to 1000° C. at a pressure of 0.5 MPa or more into molten steel for circulation purposes.

[0006] In addition, Patent Document 2 proposes the following method: an outer immersion tube is provided by extending the degassing tank downward and opening downward, and an inner immersion tube is concentrically arranged inside the outer immersion tube and opening in the vertical direction, so as to form an ascending flow path for the molten steel by blowing argon gas from a circulation gas suction port provided in the inner immersion tube to cause the molten steel to ascend. On the other hand, the area between the inner and outer immersion tubes is used as a descending flow path for the molten steel, thereby forming an ascending flow path and a descending flow path with large cross-sectional areas, thereby increasing the circulation amount of the molten steel.

[0007] The circulation flow rate of the degasser is generally often calculated using the following formula (A) disclosed in Non-Patent Document 1.

[0008] Qc=K×G 1 / 3 ×D 4 / 3 ×{ln(P0 / P)} 1 / 3 / (ρ l / 1000)……(A)

[0009] Among them, Qc: calculated circulation flow of molten steel (molten steel m 3 / min), G: Circulation gas flow (Nm 3 / sec), D: inner diameter of the immersion tube (m), P: pressure in the vacuum tank (Pa), P0: atmospheric pressure (101325Pa), ρ l :Steel density (kg / m 3 ).

[0010] K is a fitting parameter derived from experimental results under various operating conditions. Non-Patent Document 2 reports that for molten steel conditions, K = 446.3. Equation (A) shows that the exponent of the immersion tube inner diameter D is greater than the exponent of the circulation gas flow rate G. Therefore, increasing the immersion tube inner diameter is more effective than increasing the circulation gas flow rate to increase the calculated molten steel circulation rate Qc. Generally, increasing the immersion tube inner diameter to increase the molten steel circulation rate is known to be an effective method for improving degassing reaction efficiency.

[0011] The inner diameter of the immersion pipe is limited by the size of the degassing tank. Therefore, in most cases, increasing the inner diameter of the immersion pipe requires also expanding the degassing tank. However, the size of the degassing tank is constrained by the ladle and associated equipment. Therefore, when simply expanding the degassing tank uniformly while maintaining a perfect circular shape is difficult due to the equipment, the following approach is adopted: the degassing tank is shaped into an elliptical shape that expands only in the direction of circulation, that is, from the riser to the downcomer. The immersion pipe is then expanded in a manner corresponding to the expansion in the longitudinal direction.

[0012] Patent Document 3 proposes a degassing tank structure with an elliptical cross-section and a pair of circulation pipes arranged along the long axis. Using this technology in a degassing tank for vacuum refining eliminates stagnation in the molten steel flow, preventing stagnation of the molten steel and accumulation of slag, thereby increasing the decarburization rate.

[0013] Patent Document 4 proposes a method of providing an ultrasonic vibrator on the inner peripheral surface above the installation position of a circulation gas suction pipe provided in an ascending-side immersion pipe to miniaturize the bubbles of the inert gas.

[0014] Prior art literature

[0015] Patent Literature

[0016] Patent Document 1: Japanese Patent Application Laid-Open No. 2007-31820

[0017] Patent Document 2: Japanese Patent Application Laid-Open No. 08-269534

[0018] Patent Document 3: Japanese Patent Application Laid-Open No. 04-272120

[0019] Patent Document 4: Japanese Patent Application Laid-Open No. 02-173205

[0020] Non-licensed literature

[0021] Non-patented document 1: Tatsuro Kuwahara et al.: Iron and Steel, Vol. 73 (1987) PS176

[0022] Non-patentable document 2: Tatsuro Kuwahara et al.: ISIJ, Vol. 28 (1988) P305 Summary of the Invention

[0023] However, the above-mentioned prior art has the following problems.

[0024] The method disclosed in Patent Document 1 requires equipment for preheating the inert gas, which causes a problem of high processing costs.

[0025] Furthermore, the method disclosed in Patent Document 2 requires an outer impregnation tube and an inner impregnation tube, which not only complicates the apparatus but also requires a circulating gas pipe to be passed through the degassing tank to the inner impregnation tube. Therefore, it is not possible to remove only the inner impregnation tube from the degassing tank for replacement. When replacing the impregnation tube, the entire lower tank also needs to be replaced, resulting in a significant increase in refractory costs.

[0026] Furthermore, in the technology disclosed in Patent Document 3, in order to make the degassing tank into an elliptical structure, it is necessary to manufacture a new iron sheet for the degassing tank, which has the problem of consuming a lot of cost and time for introduction.

[0027] Furthermore, the method disclosed in Patent Document 4 requires an ultrasonic vibrator, an ultrasonic wave emitting device, and the like, which not only complicates the apparatus but also inevitably increases the apparatus cost and the immersion pipe cost.

[0028] The present invention has been completed in view of such circumstances, and its purpose is to provide a method for refining molten steel. When using an RH type vacuum degassing device for molten steel refining, no new equipment investment is required and the circulation flow rate can be increased without increasing the processing cost. DETAILED DESCRIPTION

[0029] To address the above-mentioned issues, the inventors conducted various experiments focusing on the effects of operating conditions and the geometry of RH-type vacuum degassing devices on the flow within the degassing tank. As a result, they discovered that the energy of the circulating gas blown into the riser is primarily dissipated within the vacuum tank bath. By modifying the operating conditions, the amount of energy dissipated can be reduced, thereby increasing the amount of circulating gas. The present invention was developed in light of the above-mentioned circumstances, and its gist is as follows.

[0030] The method for refining molten steel of the present invention, which advantageously solves the above-mentioned problems, is characterized in that, in the method for refining molten steel using an RH type vacuum degassing device, the stirring power energy density ε of the molten steel in the vacuum tank shown in the following equations (1) to (4) satisfies the following equation (5) (wherein, ε is the stirring power energy density of the molten steel in the vacuum tank (watt / ton), G is the circulation gas flow rate (Nm 3 / sec), T: molten steel temperature (K), ρ L :Steel density (kg / m 3 ), g: acceleration due to gravity (9.8 m / sec 2 ), W V : Mass of molten steel in the vacuum tank (ton), D V : Vacuum tank inner diameter (m), H0: Height from the circulating gas blowing nozzle position to the static state of the vacuum tank steel bath surface (m), P: Vacuum tank internal pressure (Pa), P0: atmospheric pressure (101325Pa), h V : Height from the static state of the vacuum tank steel bath surface to the base (m), L: Height from the lower end of the immersion tube to the base (m), h G : Height from the lower end of the immersion tube to the position of the circulating gas blowing nozzle (m), l: Depth of immersion tube in molten steel (m), D U : inner diameter of the riser (m). ) is used to determine the immersion depth l of the immersion tube in the molten steel or the circulation gas flow rate G.

[0031] ε=[371GT×ln{1+(ρ L gH0 / P)}] / W V ……(1)

[0032] W V =(π·D V 2 / 4)×H0×ρ L / 1000……(2)

[0033] H0=h V +L-h G ……(3)

[0034] h V =(P0-P) / (ρ L g)+l-L……(4)

[0035] 1.35×10 5 ×D U / W V <ε<2.1×10 4 ……(5)

[0036] It should be noted that in the molten steel refining method of the present invention, it is considered to be a more preferable solution to determine the immersion depth l of the immersion pipe in the molten steel or the circulation gas flow rate G so that the above-mentioned stirring power energy density ε satisfies the following formula (6).

[0037] 1.35×10 5 ×D U / W V <ε<1.0×10 4 ……(6)

[0038] According to the present invention, when the RH type vacuum degassing device is used for molten steel refining, no new equipment investment is required, and processing costs are not increased. The circulation flow rate can be increased, which helps to shorten the processing time.

[0039] Figure 1 It is a schematic longitudinal sectional view showing an example of an RH type vacuum degassing device.

[0040] Figure 2 It is an enlarged cross-sectional view of the RH type vacuum degassing device showing the concept of the present invention.

[0041] Figure 3 This is a graph showing the relationship between the normalized circulation rate of liquid in a vacuum tank and the stirring power energy density ε in a water model experiment.

[0042] Figure 4 The stirring power energy E in the RH type vacuum degassing device with different immersion pipe diameters and vacuum tank inner diameters and the measured molten steel circulation flow Q calculated using formula (A) are expressed as follows: E Calculate the circulation flow Q relative to the molten steel C Ratio Q E / Q C A diagram of the relationship.

[0043] Figure 5 It indicates the diameter of the rising pipe D U With the minimum stirring power energy E min A diagram of the relationship.

[0044] Figure 6 The measured circulation flow rate Q of the molten steel relative to the stirring power energy density ε of the molten steel in the vacuum tank during actual operation E Calculate the circulation flow Q with molten steel C Ratio Q E / Q CA diagram of the relationship.

[0045] Hereinafter, the present invention will be described based on preferred embodiments. First, the results of the present invention's studies will be described. Figure 1 This is a schematic longitudinal sectional view showing an example of an RH type vacuum degassing apparatus used in a molten steel refining method according to an embodiment of the present invention.

[0046] Figure 1 In the figure, reference numeral 1 denotes an RH-type vacuum degassing device, 2 denotes a ladle, 3 denotes molten steel, 4 denotes slag, 5 denotes a vacuum tank, 6 denotes an upper tank, 7 denotes a lower tank, 8 denotes an ascending immersion pipe (ascending pipe), 9 denotes a descending immersion pipe (downer), 10 denotes a circulating gas suction pipe, 11 denotes a pipeline, 12 denotes a raw material inlet, and 13 denotes a top-blowing lance. The vacuum tank 5 comprises an upper tank 6 and a lower tank 7. Furthermore, the top-blowing lance 13, which injects oxygen and solvent into the molten steel within the vacuum tank, is located above the vacuum tank 5 and is movable up and down within the vacuum tank 5.

[0047] In the RH vacuum degassing apparatus 1, a ladle containing molten steel 3 is raised using a lifting device (not shown), and an ascending dip tube 8 and a descending dip tube 9 are immersed in the molten steel 3 within the ladle. Furthermore, an exhaust device (not shown) connected to a pipe 11 exhausts the interior of the vacuum tank 5, thereby reducing its pressure. Recirculation gas is blown into the ascending dip tube 8 from a recirculation gas intake pipe 10. As the pressure within the vacuum tank 5 is reduced, the molten steel 3 within the ladle rises proportionally to the difference between atmospheric pressure and the pressure (vacuum level) within the vacuum tank, flowing into the vacuum tank. Simultaneously, due to the gas lift effect of the recirculation gas blown in from the recirculation gas intake pipe 10, the molten steel 3, along with the recirculation gas, rises through the ascending dip tube 8 and flows into the vacuum tank 5. Thereafter, the molten steel 3 flows back to the ladle 2 via the descending dip tube 9, thus forming a recirculation process and performing RH vacuum degassing and refining. The molten steel 3 is exposed to a reduced-pressure atmosphere in the vacuum tank, and gas components in the molten steel move into the atmosphere in the vacuum tank, whereby a degassing reaction of the molten steel 3 proceeds.

[0048] In the water model experiment simulating the RH type vacuum degassing device, the study was conducted to increase the circulation flow of molten steel by changing the operating conditions in various ways. Here, the scheme of using the water model is for the following reasons. Molten steel is heavier than water, but it is also more viscous, and the dynamic viscosity of molten steel and water is almost the same. Therefore, when water is used for simulation with a total size (size ratio 1.0), the two dimensionless numbers of Froude number and Reynolds number can be matched with respect to the molten steel. That is, in the simulation method using the total size of water, the flow of molten steel can be reproduced with respect to the influence of gravity, inertia and viscosity. As a result, it was found that by controlling the stirring power energy density ε of the liquid in the vacuum tank brought by the circulating gas blown into the riser within an appropriate range, the circulation flow can be efficiently increased.

[0049] The stirring power energy density ε of the molten steel in the vacuum tank is represented by the following equations (1) to (4).

[0050] ε=[371GT×ln{1+(ρ L gH0 / P)}] / W V ……(1)

[0051] W V =(π·DV 2 / 4)×H0×ρ L / 1000……(2)

[0052] H0=h V +L-h G ……(3)

[0053] h V =(P0-P) / (ρ L g)+l-L……(4)

[0054] Where, ε: stirring power energy density of molten steel in vacuum tank (watt / ton),

[0055] G: Circulation gas flow (Nm 3 / sec),

[0056] T: molten steel temperature (K),

[0057] ρ L :Steel density (kg / m 3 ),

[0058] g: acceleration due to gravity (9.8 m / sec 2 ),

[0059] W V : Mass of molten steel in the vacuum tank (ton),

[0060] D V : Inner diameter of vacuum tank (m),

[0061] H0: The height from the circulating gas blowing nozzle position to the static state of the vacuum tank steel bath surface (m),

[0062] P: pressure inside the vacuum tank (Pa),

[0063] P0: atmospheric pressure (101325Pa),

[0064] h V : Height from the static vacuum tank steel bath surface to the base (m),

[0065] L: Height from the lower end of the immersion pipe to the base (m),

[0066] h G : Height from the lower end of the immersion pipe to the position of the circulating gas blowing nozzle (m),

[0067] l: Immersion depth of the immersion tube in molten steel (m).

[0068] Figure 2 It is an enlarged cross-sectional view of the RH type vacuum degassing device showing the concept of the present invention. Figure 2 The symbols of the dimensions of the RH type vacuum degassing apparatus used in the above (1) to (4) are shown in the figure.

[0069] In the formula (4), the immersion depth l of the immersion pipe in the molten steel is defined by the following formula (B).

[0070] l=l L -l FB -l LV ...(B)

[0071] in,

[0072] l L : Distance from the top of the ladle to the bottom of the ladle (m):

[0073] l FB : The distance from the top of the ladle to the surface of the molten steel in the ladle (m),

[0074] l LV : The distance from the lower end of the immersion tube to the bottom of the ladle (m).

[0075] Among them, l FB The height of the molten steel surface can be measured by using a molten steel level gauge, or by immersing a metal rod in the molten steel in the ladle and measuring the length of the dissolved portion. LV , calculated based on the relative distance between the ladle and the vacuum tank obtained by the control system.

[0076] In the water model experiment, the bath depth of the water in the vacuum tank was changed at various levels, and the circulation flow rate at each level was calculated by measuring the flow velocity in the downcomer. Figure 3 Figure 2 shows the relationship between the normalized circulation rate of the liquid in the vacuum tank and the stirring power energy density ε in a water model experiment. The normalized circulation rate is the ratio relative to the minimum level of circulation rate. The experimental results show that, when the circulation gas flow rate is constant, the circulation rate increases as the stirring power energy density ε of the liquid in the vacuum tank decreases.

[0077] As mentioned above, the reason for the change in the circulation flow rate is that when the stirring power energy density ε of the molten steel in the vacuum tank is small, the bath surface disturbance becomes smaller, and the ratio of energy consumed as energy to change the molten steel interface becomes smaller, so that the ratio of energy that contributes to circulation in the energy of the circulating gas increases relatively, and the circulation flow rate increases.

[0078] In addition, even when the stirring power energy density ε of the molten steel in the vacuum tank is sufficiently small, the stirring power density ε of the molten steel in the vacuum tank is relatively small relative to the inner diameter D of the riser. U (m), when the stirring power energy E (watt) represented by the following formula (C) is small, the air lift pump effect cannot be fully exerted, and the circulation flow rate is reduced.

[0079] E=[371GT×ln{1+(ρ L gH0 / P)}](=ε·W V )……(C)

[0080] Figure 4 The figure shows the stirring power energy E and the measured circulation flow rate Q of molten steel obtained using formula (A) in the RH type vacuum degassing device with different immersion pipe diameters and vacuum tank inner diameters. E (Steel molten m 3 / min) relative to the calculated circulation flow rate Q of molten steel C Q E / Q C Among them, the measured circulation flow rate of molten steel Q E During the treatment, copper is added from the vacuum tank as a tracer, and the uniform mixing time τ (sec) is measured. The uniform mixing time τ obtained is calculated using the relationship described below. In formula (A), the constant K is calculated as K = 446.3. In the range where the stirring power energy E is above a certain value, as the stirring power energy E decreases, the stirring power energy density ε of the molten steel in the vacuum tank also decreases, so the energy efficiency increases and the circulation volume increases. On the other hand, if the stirring power energy E is a certain value E min Below, relative to the rising pipe diameter D U , the air lift pump effect is insufficient, resulting in poor circulation, Q E / Q C Here, E min Defined as the minimum stirring power energy, the rising pipe diameter D U With E min The relationship is shown in Figure 5 According to Figure 5 The rising pipe diameter D is obtained from the relationship U With E min The conditions of the stirring power energy E required for normal circulation in the RH type vacuum degassing device are defined as the following formula (7) by using the proportional constant approximated by the proportional relationship.

[0081] 1.35×10 5 ×D U ≤E……(7)

[0082] In addition, if ε=E / W is used V , then formula (7) is transformed into the following formula (8).

[0083] 1.35×10 5 ×D U / W V <ε……(8)

[0084] In addition, for the same RH type vacuum degassing device, the circulation flow rate was measured under various conditions and the Q E / Q C The results show that the stirring power energy density ε of the molten steel in the vacuum tank is less than 2.1×10 4 In the case of Q E / Q C Significantly increased, greater than 1.1. The stirring power energy density ε of the molten steel in the vacuum tank is compared with Q E / Q C The relationship is shown in Figure 6 Among them, Figure 6 The conditions that do not satisfy formula (8) are excluded.

[0085] Based on this result, as a condition for increasing the circulation flow rate ε, equation (9) can be obtained:

[0086] ε<2.1×10 4 ……(9)

[0087] According to equations (8) and (9), equation (5) can be obtained as a condition for increasing the energy efficiency of the circulating gas and the stirring power energy density ε required to increase the circulation amount.

[0088] 1.35×10 5 ×D U / W V <ε<2.1×10 4 ……(5)

[0089] In addition, if the stirring power energy density ε of the molten steel in the vacuum tank is further reduced within the range satisfying formula (7), Q E / Q C Further increase, when ε is less than 1.0×10 4 Therefore, the value of ε is more preferably less than 1.0×10 4 If the above conditions are expressed in a formula, we get formula (6).

[0090] 1.35×10 5 ×DU / W V <ε<1.0×10 4 ……(6)

[0091] The parameters used to control the stirring power energy density ε in the vacuum tank within the range of formula (5) or formula (6) are the circulating gas flow rate G, the vacuum degree P, and the immersion depth l of the immersion tube in the molten steel, excluding the device size. However, when the vacuum degree is lowered, the degassing reaction rate, which is the purpose of the present invention, decreases or becomes zero. Therefore, it is preferred to change the circulating gas flow rate G or the immersion depth l of the immersion tube in the molten steel for control.

[0092] As described above, according to the present invention, the circulation rate of molten steel can be increased without requiring new equipment investment and without increasing processing costs.

[0093] Example

[0094] 300 tons of molten steel blown in a converter was vacuum refined using an RH vacuum degassing device. At this time, ε was calculated using equations (1) to (4) based on the device size and operating conditions, and the immersion depth l of the immersion pipe in the molten steel was adjusted within the range of 0.3m to 0.9m to satisfy equations (5) or (6). The vacuum tank cross-sectional area S was used as the degassing tank. A =3.14m 2 , inner diameter of riser D U = Degassing tank (A tank) or vacuum tank cross-sectional area S of 0.6m A =3.8m 2 , inner diameter of riser D U =0.8m degassing tank (B tank). In addition, for the operating conditions, the vacuum degree P is 133Pa, the circulating gas flow rate G is 0.020Nm 3 / sec、0.027Nm 3 / sec、0.037Nm 3 / sec or 0.050Nm 3 In addition, copper was added from the vacuum tank as a tracer during the circulation, and the uniform mixing time τ (sec) was measured. The actual circulation flow rate Q of the molten steel was calculated from the obtained uniform mixing time τ. E Uniform mixing time τ and measured circulation flow rate Q of molten steel E The relationship is represented by the following formulas (D), (E) and (F).

[0095] τ=800×ε －0.45 ...(D)

[0096] ε L =8.33×10 －3 ×ρQ E v2 / W L ...(E)

[0097] v=Q E / (15πD 2 )……(F)

[0098] Among them, ε L : stirring power density of molten steel in the ladle (watt / ton), v: molten steel flow rate in the downcomer (m / sec), W L : The amount of molten steel in the ladle (ton).

[0099] Then use formula (A) to calculate the circulation flow rate Q of molten steel C , calculate Q for each charge E / Q C The constant K in formula (A) is K = 446.3. The composition of the molten steel used is C: 0.04-0.06 mass%, Si: 0.05 mass% or less, Mn: 0.3 mass% or less, P: 0.02 mass% or less, S: 0.003 mass% or less, and the temperature of the molten steel before treatment is 1640-1670°C.

[0100] The experimental results are shown in Table 1. Regardless of the differences in various operating conditions and device sizes, in the region satisfying equation (5), the measured circulation flow rate Q of the molten steel is E Calculate the circulation flow Q relative to the molten steel C The ratio is 1.1 times or more, which is a good result. And within the range of satisfying formula (6), the circulation flow is further increased compared to the case of satisfying only formula (5), and Q can be obtained. E / Q C A better result of more than 1.2 times.

[0101] [Table 1]

[0102]

[0103] Industrial applicability

[0104] The method for refining molten steel of the present invention can optimize the circulation flow rate of the RH type vacuum degassing device, thereby enabling efficient vacuum decarburization and vacuum degassing, and is therefore industrially useful.

[0105] Explanation of symbols

[0106] 1 RH type vacuum degassing device

[0107] 2 Ladle

[0108] 3 Molten Steel

[0109] 4 Slag

[0110] 5 Vacuum tank

[0111] 6 Upper slot

[0112] 7 Lower slot

[0113] 8 Ascending side immersion pipe (ascending pipe)

[0114] 9 Downward side dip pipe (downcomer)

[0115] 10 Gas suction pipe for circulation

[0116] 11 Pipeline

[0117] 12 Raw material input port

[0118] 13 Top Blowing Spray Gun

Claims

1. A method for refining molten steel, characterized in that, in a method for refining molten steel using an RH type vacuum degassing apparatus, the immersion depth l of the immersion pipe in the molten steel or the circulation gas flow rate G is determined so that the stirring power energy density ε of the molten steel in the vacuum tank represented by the following equations (1) to (4) satisfies the following equation (5), ε=[371GT×ln{1+(ρ L gH0 / P)}] / W V ……(1) W V =(π·D V 2 / 4)×H0×ρ L / 1000……(2) H0=h V +L-h G ……(3) h V =(P0-P) / (ρ L g)+l-L……(4) 1.35×10 5 ×D U / W V <ε<2.1×10 4 ……(5), in, ε: Stirring power energy density of molten steel in vacuum tank (watt / ton), G: Circulation gas flow (Nm 3 / sec), T: molten steel temperature (K), ρ L :Steel density (kg / m 3 ), g: acceleration due to gravity (9.8 m / sec 2 ), W V : Mass of molten steel in the vacuum tank (ton), D V : Inner diameter of vacuum tank (m), H0: The height from the circulating gas blowing nozzle position to the static state of the vacuum tank steel bath surface (m), P: pressure inside the vacuum tank (Pa), P0: atmospheric pressure (101325Pa), h V : Height from the static vacuum tank steel bath surface to the base (m), L: Height from the lower end of the immersion pipe to the base (m), h G : Height from the lower end of the immersion pipe to the position of the circulating gas blowing nozzle (m), l: immersion depth of the immersion tube in molten steel (m), D U : Inner diameter of riser (m).

2. The method for refining molten steel according to claim 1, wherein The immersion depth l of the immersion pipe in the molten steel or the circulation gas flow rate G is determined in such a way that the stirring power energy density ε satisfies the following formula (6): 1.35×10 5 ×D U / W V <ε<1.0×10 4 ……(6)。

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