JP6631400B2 - Desulfurization method of molten steel - Google Patents

Description

  The present invention relates to a method for desulfurizing molten steel in which RH vacuum degassing equipment optimizes the conditions of top blowing and the flow of molten steel to efficiently and stably produce extremely low sulfur steel.

  In recent years, there has been an increasing demand for higher purity molten steel. In particular, hot metal desulfurization alone is not enough to produce ultra-low sulfur steel having an S concentration of 10 ppm or less, and a desulfurization process at the molten steel stage is essential. It becomes. As a desulfurization method at the molten steel stage, for example, there is a method of performing a desulfurization treatment by spraying a desulfurizing agent containing CaO together with a carrier gas onto a surface of molten steel sucked into a vacuum tank of a reflux type vacuum degassing device such as RH. .

As described above, methods of spraying a desulfurizing agent to promote desulfurization of molten steel are roughly classified into a method of promoting a chemical reaction of the desulfurizing agent and a method of improving the reaction efficiency between the sprayed desulfurizing agent powder and the molten steel. You. However, in recent years, the use of CaF ₂ having high refining ability has been regulated from the viewpoint of reducing the environmental load and the like, and it is extremely difficult to promote the chemical reaction of the desulfurizing agent to greatly promote desulfurization. Therefore, it is considered that the most practical method is to improve the reaction efficiency of the powder to promote desulfurization.

As a method for improving the reaction efficiency of powder, for example, Patent Document 1 discloses a powdery desulfurizing agent containing CaF _{2 of} 15 to 50% by weight and MgO of 20% by weight or less and containing CaO as a main component at a rate of 1 to 5. There is disclosed a desulfurization method in which a surface of molten steel in a vacuum chamber is sprayed from an upper blowing lance at a rate of 2 kg / min / ton. This method is based on the idea that if the powder is excessively supplied to shorten the desulfurization time, the residence time of the powder in the molten steel is shortened and the reaction efficiency is rather reduced, so that there is an optimum value for the supply speed. ing.

However, the reaction efficiency of the powder varies greatly depending on the flow and top blowing conditions of the molten steel, and since the equipment characteristics and reflux conditions vary greatly from facility to facility, spraying was performed only by specifying the conditions for the powder supply speed. Since the reaction efficiency of the powder is not always optimal, it is not possible to stably increase the reaction efficiency of the powder. Further, the method described in Patent Document 1 is based on the premise that CaF ₂ having high refining ability is used, and cannot be applied when a desulfurizing agent containing no F is used.

  Patent Document 2 discloses that a supply speed V of a refining agent is set to a specific condition according to a vertical distance H between a lower end of an upper blowing lance and a surface of molten steel in a vacuum chamber and a degree of vacuum P in the vacuum chamber. A method of controlling the refining agent overblowing is disclosed. According to this method, when the powder supply speed is low, the energy received by the powder is increased and the scattering loss is promoted with respect to the energy of the carrier gas jet depending on H and P. The working energy is reduced, the powder speed is reduced, and the powder is deposited in the powder injection region on the surface of the molten steel.

  However, the conditions described in Patent Literature 2 do not include the conditions related to the flow of molten steel as described above, and the reaction efficiency of the powder is not necessarily the same as that of the equipment described in Patent Literature 2 with the same index. Since it is not optimal, the reaction efficiency of the powder cannot be stably increased.

Further, Patent Document 3 discloses that a desulfurization flux containing CaO and CaF ₂ as a main component is supplied under the condition that the value of flux blowing rate (kg / min) / fluid steel ring flow rate (ton / min) is 3 kg / ton or less. A blowing desulfurization refining process is disclosed. This method is based on the idea of optimizing the balance between the powder supply rate and the dispersibility of the molten steel in the molten steel due to the reflux, and suppressing the agglomeration of the powder in the blowing region to improve the powder efficiency.

However, although the conditions described in Patent Document 3 include a parameter relating to the flow of molten steel, which is a ring flow rate, the reaction of the top-blown powder is not a reflux of the entire apparatus but a local flow in a vacuum chamber. It has a strong influence. Further, the local flow in the vacuum chamber cannot be defined only by the circulating flow rate of the entire apparatus. Therefore, depending on the operation mode, the reaction efficiency of the powder is not always optimal under the conditions described in Patent Document 3, and the reaction efficiency of the powder cannot be stably increased. In addition, this technique is based on the premise that CaF ₂ having high refining ability is used as in the method described in Patent Document 1, and cannot be applied when a desulfurizing agent containing no F is used.

JP-A-11-6009 JP 2001-271117 A JP-A-8-269533

Kashimura et al .: Proceedings of the 73rd Annual Meeting of the Japan Society of Mechanical Engineers (II) p173

  With the conventional method as described above, it has been difficult to stably produce extremely low sulfur steel regardless of the desulfurizing agent and the operating conditions of the equipment.

  Accordingly, an object of the present invention is to provide a method for desulfurizing molten steel capable of stably increasing the reaction efficiency of a powdery desulfurizing agent to produce ultra-low sulfur steel.

  The present inventors believe that the dispersibility and moving speed of the injected powder depend on the surface flow velocity of the molten steel in the vacuum chamber, and that the injection is performed by controlling the balance between the powder supply rate and the surface flow velocity. It was thought that the accumulation state of the powder in the region could be alleviated. In addition, the present inventors have attempted to reduce the accumulation of powder in the blowing region, thereby suppressing the agglomeration of the powder and facilitating the penetration of the powder into the molten steel, and newly spraying the powder together with the carrier gas. It has been found that the scattering loss caused by being repelled by the deposited powder can be reduced, and the utilization efficiency and desulfurization rate of the powder are greatly improved.

Furthermore, the present inventors have found that the phenomenon of powder accumulation in the blowing region tends to occur when the ratio W _PB / v ₁ between the powder supply speed W _PB (kg / min) and the surface flow velocity v _l is equal to or greater than a certain value. And found that the index has an optimum value at which the desulfurization efficiency and the desulfurization rate are maximized.

On the other hand, since the powder blown upward moves with the carrier gas, the powder is considered to accumulate in the area of the cavity formed by the carrier gas jet. The present inventors have found that not only the powder accumulation in the blowing region described above is reduced, but also the powder utilization efficiency and the desulfurization rate are further improved by appropriately controlling the powder accumulation. It has been found that the ratio L / D _VAC between the cavity diameter L and the vacuum chamber inner diameter D _VAC has an appropriate range.

Therefore, the present invention is to clarify the supply conditions of powder desulfurizing agent and the appropriate conditions of surface flow of molten steel for efficiently and stably producing ultra-low sulfur steel in an RH vacuum degassing apparatus. And as described below.
(1) Reflux gas is blown into molten steel by an RH vacuum degassing device to circulate the molten steel, and CaO and Al ₂ O ₃ are contained in a total of 60% by mass or more from an upper blowing lance installed inside the vacuum chamber. and, and when the CaO / Al ₂ O ₃ ratio performs desulfurization of the molten steel by blowing with a carrier gas to satisfy powdery desulfurizing agent 1.2 to 2.0, powder of the powdery desulfurizing agent The relationship between the body supply speed and the surface flow velocity of the molten steel in the vacuum chamber satisfies the conditions of equations (1) to (3), and the relation between the cavity diameter formed by the carrier gas and the inner diameter of the vacuum chamber is as follows. A method for desulfurizing molten steel, which satisfies the condition of the expression (4).
2.0 ≦ W _PB / v _l ≦ 3.5 (1)
v _l = Q / (7 · h · D _VAC ) (2)
Q = 11.4G ^1/3 D _LEG ^4/3 · {ln (P ₁ / P _VAC )} ^1/3 (3)
0.3 ≦ L / D _VAC ≦ 0.5 (4)
Here, v _l is the surface velocity of the molten steel in the vacuum chamber (m / min), h is the bath depth of the molten steel in the vacuum chamber (m), D _VAC is the inner diameter of the vacuum chamber (m), and Q is The ring flow rate of molten steel (ton / min), G is the reflux gas flow rate (Nl / min), D _LEG is the inner diameter (m) of the immersion tube, P ₁ is the pressure (Torr) at the gas injection position of the reflux gas, P _VAC Represents the pressure in the vacuum chamber (Torr), W _PB represents the powder supply rate (kg / min), and L represents the cavity diameter (m) formed by the carrier gas.

  According to the present invention, a method for desulfurizing molten steel capable of smelting ultra-low sulfur steel by stably increasing the reaction efficiency of a powdery desulfurizing agent without depending on the desulfurizing agent composition and without adding equipment Can be provided.

It is a figure for explaining a calculation method of a cavity diameter. It is a figure which shows the relationship between the ratio W _PB / v _{l of the} powder supply speed W _PB (kg / min) and the surface flow velocity v _l (m / min) and the desulfurization rate constant K _S. It is a figure which shows the relationship between the ratio W _PB / v _{l of the} powder supply speed W _PB (kg / min) and the surface flow velocity v _l (m / min) and the desulfurization k value.

  Hereinafter, the present invention will be described with reference to the drawings. The “RH vacuum degassing device” described below is a molten steel processing device having a vacuum chamber, and the “reflux process” refers to a process of circulating molten steel by the RH vacuum degassing device. In addition, “PB (Power Blowing) desulfurization treatment” refers to a treatment for desulfurizing molten steel by spraying a powdery desulfurizing agent together with a carrier gas from an upper blowing lance installed inside a vacuum chamber.

  In the present invention, molten steel discharged from a refining furnace such as a converter into a ladle is subjected to a reflux treatment in an RH vacuum degassing apparatus after completing component adjustment such as deoxidation and alloy addition. First, a vacuum tank equipped with an upper blowing lance is immersed in molten steel in a ladle, and after sucking the molten steel into the vacuum tank, the desulfurizing agent powder is blown upward together with a carrier gas from the upper blowing lance in the vacuum tank. Perform PB desulfurization treatment. The shape of the lance nozzle of the upper blowing lance in the PB desulfurization treatment is not limited, but it is preferable to use a lance nozzle having a throat portion in the nozzle and generally called a Laval nozzle.

In the present invention, the ratio W _PB / v ₁ between the powder supply speed W _PB (kg / min) and the surface flow velocity v _l (m / min) satisfies the following expression (1), and the cavity diameter L (m) The PB desulfurization treatment is performed so that the ratio L / D _VAC between the pressure and the inner diameter D _VAC (m) of the vacuum tank satisfies the following expression (4).
2.0 ≦ W _PB / v _l ≦ 3.5 (1)
0.3 ≦ L / D _VAC ≦ 0.5 (4)

First, the surface flow velocity v _l will be described. The surface flow velocity v _l (m / min) of the molten steel in the vacuum chamber is represented by the following equation (3): the molten steel ring flow rate Q (ton / min), the bath depth h (m) of the molten steel in the vacuum chamber, and the inner diameter D of the vacuum chamber. It is expressed by equation (2) using _VAC (m).
v _l = Q / (7 · h · D _VAC ) (2)
Q = 11.4G ^1/3 D _LEG ^4/3 · {ln (P ₁ / P _VAC )} ^1/3 (3)

Here, G is the reflux gas flow rate (Nl / min), D _LEG is the inner diameter of the immersion tube (m), P ₁ is the pressure at the gas injection position of the reflux gas (Torr), and P _VAC is the pressure in the vacuum chamber (Torr). Is represented. When the vacuum chamber is elliptical, the value of the short diameter of the vacuum chamber is used as the inner diameter D _VAC of the vacuum chamber.

Next, a method of calculating the cavity diameter L will be described below. FIG. 1 is a diagram for explaining a method of calculating a cavity diameter. The nozzle pre-pressure P ₀ (Torr) in the throat portion 2 of the lance nozzle 1 is determined by the gas flow rate Q _gas (Nm ³ / min) of the carrier gas, the throat diameter D _s (m), and the molar mass M _gas (kg / kg) of the carrier gas. kmol) and is represented by the following equation (5).
P ₀ = 0.55 · Q _gas / (D _s ² × M _gas ^0.5 ) (5)

The jet that emerges from the lance nozzle 1 forms a supersonic region (core) and does not diffuse. However, since the lower part of the core is a subsonic region, the flow velocity is attenuated as shown in FIG. And diffuses toward the molten steel 3. Here, with respect to the length of the core h _core (m), are the following (6) are reported in Non-Patent Document 1, and the width of the core is equal to the lance outlet diameter D _e, the core below Assuming that the jet expands at an angle of 6 °, the cavity diameter L (m) formed immediately above the molten steel by the jet is expressed by the following equation (7).
h _core = 0.608 · (P ₀ / P _VAC ) ^0.5 · D _e (6)
L = 2 (Hh _core ) tan6 ° + D _e (7)

  Here, H represents the distance (m) between the lance and the molten metal surface. As described above, the cavity diameter L can be changed by controlling, for example, the height of the lance, the nozzle diameter, the flow rate of the carrier gas, and the like.

Next, the reason why the upper limit value and the lower limit value in the above equations (1) and (4) are set will be described. To improve the desulfurization efficiency, it is necessary to control the W _PB / v _l a proper range is an indication of the powder deposition amount in the blowing region. Then, in order to confirm an appropriate range of W _PB / v _l , 250 ton molten steel was subjected to PB desulfurization treatment by an RH vacuum degassing apparatus.

The effects of the present invention were evaluated based on the desulfurization rate and desulfurized powder reaction efficiency. Before and after the PB desulfurization treatment, a molten steel sample was collected and subjected to chemical analysis to obtain an S concentration [% S] in the molten steel. Here, the desulfurization rate was evaluated by an apparent desulfurization rate constant Ks expressed by the following equation (8), and the desulfurization powder reaction efficiency was evaluated by a desulfurization k value expressed by the following equation (9).
K _S = ln ([% S] _{before PB desulfurization} / [% S] _{after PB desulfurization} ) / t (8)
Desulfurization k value = ln ([% S] _{before PB desulfurization} / [% S] _{after PB desulfurization} ) / unit of desulfurizing agent (9)

Here, t represents the time (min) of the PB desulfurization treatment, and the basic unit of the desulfurization agent is a value (kg / ton) obtained by dividing the total mass of the desulfurization agent used in the PB desulfurization treatment by the mass of the molten steel. In the present invention, those having a desulfurization rate constant K _S of 0.10 or more and a desulfurization k value of 0.30 or more were judged to have obtained the effects of the invention. The test results are shown in FIGS.

As shown in FIGS. 2 and 3, since the W _PB / v _l is large enough powder is excessively deposited desulfurizing rate constant K _S and desulfurization k value is greatly reduced, W _PB / v _l is 3. Must be 5 or less. On the other hand, if W _PB / v ₁ is too small, the amount of powder required for desulfurization is not ensured, and the desulfurization rate constant K _S is greatly reduced. Therefore, the lower limit of W _PB / v ₁ is 2.0.

Also, it is necessary to optimize powder deposition by controlling the ratio L / D _VAC of the cavity diameter L to the inner diameter D _{VAC of the} vacuum chamber. However, if the L / D _VAC is too large, the energy of the jet is dispersed. Since the powder supply speed is reduced and it becomes difficult for the powder to enter the molten steel, the upper limit of L / D _VAC is set to 0.5. On the other hand, if the L / D _VAC is too small, the powder will excessively agglomerate and the desulfurization efficiency will decrease, so the L / D _VAC has a lower limit of 0.3.

  Next, the composition of the desulfurizing agent used in the present invention will be described.

[CaO and Al ₂ O ₃ are 60% by mass or more in total]
CaO is indispensable for the desulfurization reaction of molten steel, and Al ₂ O ₃ is necessary for securing the liquid phase of the desulfurizing agent. Therefore, if the amounts of CaO and Al ₂ O ₃ are too small, there is a concern that the desulfurizing ability of the desulfurizing agent is greatly reduced, and it is necessary that the desulfurizing agent contains 60% by mass or more in total.

[CaO / Al ₂ O ₃ ratio: 1.2-2.0]
Generally higher CaO / Al ₂ O ₃ ratio (C / A) is high, desulfurization ability of CaO activity of high desulfurizing agent has been known that is high, CaO / Al ₂ O ₃ ratio is at least 1 .2 must be secured. However, if the CaO / Al ₂ O ₃ ratio is too high, the amount of the liquid phase contributing to the desulfurization reaction will be extremely reduced. Therefore, the upper limit of the CaO / Al ₂ O ₃ ratio is 2.0.

  Next, the present invention will be further described based on examples. The conditions in the examples are examples of conditions adopted to confirm the operability and effects of the present invention. It is not limited to the example conditions. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

After the molten steel after the converter blowing was discharged to the ladle, the vacuum tank provided with the upper blowing lance was inserted into the molten steel in the ladle, and the molten steel was sucked to start the PB desulfurization treatment. In both the present invention example and the comparative example, the amount of molten steel was 250 ton scale, and the temperature of molten steel was 1600 to 1640 ° C. In the PB desulfurization treatment, the powder supply speed W _PB , the molten steel ring flow rate Q, the cavity diameter L and the surface speed v _l were changed as operating factors as shown in Table 1 to control W _PB / v _l and L / D _VAC . . Other refining conditions were as follows.
Carrier gas: Ar, 500Nl / min
Lance nozzle: Laval _{(D s = 0.03m, D e} = 0.08m)
Vacuum chamber: D _VAC = 1.8m
Immersion tube: D _LEG = 0.7m
Composition of molten steel before desulfurization:
C: 0.05 to 0.20 mass%
Si: 0.05 to 0.30 mass%
Mn: 0.50 to 1.50% by mass
Al: 0.05 to 0.20 mass%
S: 0.0030 to 0.0047% by mass
PB desulfurization time: 10min
Desulfurizer basic unit: 3.0-4.5 kg / ton

After completion of the PB desulfurization treatment, a molten steel sample was collected, and a part of the sample was subjected to chemical analysis to obtain the S concentration ([% S]) before and after the treatment, and desulfurization was performed using the above-described equations (8) and (9). The rate constant K _S and desulfurization k value were calculated. The desulfurization rate constant K _S and desulfurization k value under each condition are also shown in Table 2 below.

  In Table 2, Ch. No. Since Nos. 1 to 5 satisfied all the conditions of the present invention, the effects of the present invention were obtained.

Ch. No. In Nos. 6 to 7, the effects of the invention could not be obtained because the L / D _VAC was out of the range of the expression (4). Ch. No. In _No. 6, the L / D _VAC exceeded 0.5, the energy of the carrier gas jet was dispersed, the powder speed was reduced, and the powder penetration into the molten steel was poor, so that the effects of the invention could not be obtained. Conceivable. On the other hand, Ch. No. In _No. 7, since the L / D _VAC was less than 0.3 and the powder was excessively agglomerated in the blowing region, it is considered that the effects of the invention were not obtained. Therefore, in the present invention, it was confirmed that L / D _VAC must be controlled within the range of 0.3 to 0.5.

Ch. No. In Nos. 8 to 9, W _PB / v ₁ was out of the range of the expression (1), so that the effects of the invention could not be obtained. Ch. No. 8, W _{for PB} / v _l exceeds the 3.5, since the powder blowing region is excessively deposited, the penetration of sprayed powder is inhibited, further scattering loss occurs remarkably, the effect of the invention It is considered that was not obtained. On the other hand, Ch. No. In _No. 9, since W _PB / _vl was less than 2.0, the powder reaction efficiency was high, but the amount of powder required for desulfurization was not secured, and the desulfurization rate was greatly reduced, and the effect of the present invention was reduced. Probably it was not obtained. Therefore, it was confirmed that in the present invention, it is necessary to control W _PB / v _l within a range of 2.0 to 3.5.

Ch. No. In Nos. 10 to 12, the effects of the present invention were not obtained because the composition of the desulfurizing agent was out of the range of the present invention. Ch. No. In No. 10, the total amount of CaO and Al ₂ O ₃ contributing to the desulfurization reaction was less than 60% by mass, so that the amount of CaO—Al ₂ O ₃ liquid phase necessary for desulfurization was insufficient, and the effect of the invention was not obtained. Conceivable. Therefore, it was confirmed that CaO and Al ₂ O ₃ need to be contained in the desulfurizing agent in an amount of 60% by mass or more. Ch. No. In No. 11, C / A (CaO / Al ₂ O ₃ ) was less than 1.2, so that the CaO activity was low and the desulfurization ability was remarkably reduced, so that the effects of the invention could not be obtained. On the other hand, Ch. No. In No. 12, it is considered that the C / A exceeded 2.0, so that the solid phase precipitated excessively and the amount of the liquid phase contributing to the desulfurization reaction was reduced, so that the effects of the invention could not be obtained. Therefore, it was confirmed that the C / A of the desulfurizing agent needed to be controlled within the range of 1.2 to 2.0.

  ADVANTAGE OF THE INVENTION According to this invention, it is possible to stably produce ultra-low sulfur steel by optimizing the powder supply speed and the surface flow of molten steel without using a desulfurizing agent and without adding equipment. The industrial value of the present invention is very large.

DESCRIPTION OF SYMBOLS 1 Lance nozzle 2 Throat part 3 Molten steel

Claims (1)

Reflux gas is blown into the molten steel by an RH vacuum degassing device to circulate the molten steel, and CaO and Al ₂ O ₃ are contained in a total of 60% by mass or more from an upper blowing lance installed inside the vacuum chamber, and when performing desulfurization of the molten steel CaO / Al ₂ O ₃ ratio is blown together with the carrier gas to satisfy powdery desulfurizing agent 1.2 to 2.0, a powder supply rate of the powder-like desulfurizing agent And the relationship between the surface velocity of the molten steel in the vacuum tank and the surface velocity of the molten steel satisfies the conditions of the expressions (1) to (3), and the relation between the cavity diameter formed by the carrier gas and the inner diameter of the vacuum tank is as follows (4). A method for desulfurizing molten steel, which satisfies the conditions of the formula.
2.0 ≦ W _PB / v _l ≦ 3.5 (1)
v _l = Q / (7 · h · D _VAC ) (2)
Q = 11.4G ^1/3 D _LEG ^4/3 · {ln (P ₁ / P _VAC )} ^1/3 (3)
0.3 ≦ L / D _VAC ≦ 0.5 (4)
Here, v _l is the surface velocity of the molten steel in the vacuum chamber (m / min), h is the bath depth of the molten steel in the vacuum chamber (m), D _VAC is the inner diameter of the vacuum chamber (m), and Q is The ring flow rate of molten steel (ton / min), G is the reflux gas flow rate (Nl / min), D _LEG is the inner diameter (m) of the immersion tube, P ₁ is the pressure (Torr) at the gas injection position of the reflux gas, P _VAC Represents the pressure in the vacuum chamber (Torr), W _PB represents the powder supply rate (kg / min), and L represents the cavity diameter (m) formed by the carrier gas.

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