Refining lance and method for refining molten steel

Abstract

To provide a top-blown lance suppressing abrasion to powder with a simple structure and excellent in refining efficiency, and to provide a method for refining molten steel using the same.SOLUTION: A refining lance for top-blowing powdery material for refining comprises a cylindrical throat part, a conical diameter reduction part positioned on the upper side of the throat part, and a conical diameter expansion part positioned on the lower side of the throat part. A diameter Dt (m) of the throat part and a length lt (m) of the throat part satisfy a relation lt / Dt≥5.0, and a relation between the diameter Dt (m) of the throat part, a pipe inner diameter Din (m) above th diameter reduction part, and a taper height H (m) of the diameter reduction part satisfies 0.100≥H / (Din / Dt)≥0.040.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a refining lance suitable for use in suppressing wear caused by refining powder, and a method for refining molten steel using the same. [Background technology]

[0002] In recent years, the performance level required for steel products has been increasing, and there is a demand for the thorough removal of impurities in molten steel such as S and C. For example, in order to produce ultra-low sulfur steel with a sulfur content of 10 ppm or less, a method of desulfurization is sometimes used in which a desulfurization agent powder containing CaO is sprayed onto the surface of molten steel together with a carrier gas in a reflux type vacuum degassing device such as an RH. In addition, a technique for reducing C is known in which Fe2O3 powder is sprayed onto the surface of molten steel, causing the reduction of Fe2O3 by C and promoting the decarburization reaction.

[0003] Such refining powders, such as desulfurization agent powder and Fe2O3 powder, are sprayed from the top lance along with carrier gas toward the surface of molten steel from the nozzle at the tip of the lance. In order to increase the refining efficiency, various improvements have been made to the shape of the internal piping of the top lance, such as providing a reduced diameter section (and a throat section) and an expanded diameter section.

[0004] On the other hand, when the refining powder is sprayed, the inside of the pipe is worn due to the collision between the refining powder and the inner wall inside the top blowing lance, and the life of the nozzle is shortened. Therefore, various techniques have been proposed to suppress the wear caused by the spraying of the refining powder. Patent documents 1 to 4 each disclose a technique for suppressing the wear caused by the refining powder. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-171248 [Patent Document 2] JP 2003-213318 A [Patent Document 3] Japanese Patent Application Publication No. 11-335722 [Patent Document 4] Patent Publication No. 2022-13818 Summary of the Invention [Problem to be solved by the invention]

[0006] However, if the internal shape of the top-blowing lance is complicated or the material is changed for the purpose of improving refining efficiency and suppressing wear, the processing cost of the top-blowing lance is high. In the lance described in Patent Document 1, the gas and the powder are supplied from separate pipes, so not only is the gas system complicated, but the manufacturing and installation of the lance are difficult. In the refining lance described in Patent Document 2, the throat wall with a small inner diameter is hard chrome plated, but it is very difficult to electroplate such a small inner wall, and the processing cost becomes high. In the refining lance described in Patent Document 3, wear-resistant members such as high Mn steel are individually installed only on the throat part, so the manufacturing cost becomes high. Furthermore, the top-blowing lance described in Patent Document 4 has a shape with multiple nozzles, so the shape is complicated and installation is difficult, and the manufacturing cost becomes high.

[0007] In view of the above problems, the present invention has an object to provide a top blowing lance having a simple structure, suppressing wear caused by powder, and having excellent refining efficiency, and a method for refining molten steel using the same. [Means for solving the problem]

[0008] In order to improve the efficiency of the refining reaction of powder in vacuum degassing equipment such as RH, it is effective to accelerate the powder by top blowing and have the powder penetrate into the molten steel. First, as a method to accelerate the powder, it is effective to use a long-throw transformer, which is an extension of the throat of the Laval nozzle, for the top blowing lance, as shown in Figure 1.

[0009] Meanwhile, the inventors of the present invention have investigated how the piping inside the top-blowing lance wears when powder is blown from above, and have found that, as shown in Figure 2(a), the powder scatters at the tapered part of the reduced diameter part at the top of the throat, and collisions with the throat wall accelerate nozzle wear. When hard powder is sprayed in particular, wear progresses significantly, and if the worn part reaches the cooling water piping inside the nozzle, problems such as water leakage occur, shortening the nozzle's lifespan and increasing the operational load due to nozzle replacement. Therefore, the inventors of the present invention have investigated an internal structure that is less susceptible to wear from powder, and have found that by increasing the taper height H of the reduced diameter part and reducing the diameter D of the inner tube, in , D t It was found that by setting the appropriate range depending on the application, the powder trajectory inside the nozzle can be straightened and the wear progression of the throat wall can be suppressed.

[0010] The present invention is as follows. [1] A refining lance for blowing refining powder from above, The nozzle has a cylindrical throat portion, a conical diameter reduction portion located on an upper side of the throat portion, and a conical diameter expansion portion located on a lower side of the throat portion, The diameter D of the throat t (m) and the length of the throat portion l t (m) satisfies the relationship of formula (1), and the diameter D t (m) and the inner diameter D of the pipe at the top of the diameter reduction section in The relationship between the taper height H(m) of the reduced diameter portion and the taper height H(m) of the reduced diameter portion satisfies formula (2). l t / D t ≧5.0 (1) 0.100≧H / (D in / D t ) ≧ 0.040 (2) [2] In a process of spraying refining powder onto a molten steel surface together with a carrier gas under a reduced pressure of less than 13.33 kPa, the shape of a top blowing lance for spraying the refining powder comprises a cylindrical throat portion, a conical diameter reduction portion located on the upper side of the throat portion, and a conical diameter expansion portion located on the lower side of the throat portion, and the diameter D of the throat portion is 1.0 mm. t (m) and the length of the throat portion l t (m) satisfies the relationship of formula (1), and the diameter D t (m) and the inner diameter D of the pipe at the top of the diameter reduction section in a top blowing lance in which the relationship between the diameter reduction section's taper height H(m) and the diameter reduction section's taper height H(m) satisfies formula (2), and the carrier gas supply conditions satisfy formulas (3) to (4). l t / D t ≧5.0 (1) 0.100≧H / (D in / D t ) ≧ 0.040 (2) ΔP0≧72×(l t / D t ) -0.27 (3) ΔP0=0.0118·Q / D t 2 (4) Where, ΔP0: nozzle pressure loss (kPa), Q: carrier gas flow rate (Nm 3 / min). Effect of the Invention

[0011] According to the present invention, it is possible to suppress wear on powder with a simple structure and perform refining with excellent refining efficiency. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram for explaining the internal structure of a long-throw transformer in which the throat portion of a Laval nozzle is extended. [Diagram 2] 1A to 1C are diagrams for explaining the powder wear mechanism in a nozzle and the concept of wear suppression. [Diagram 3] FIG. 13 is a graph showing the relationship between the index H / (Din / Dt) and the collision energy and powder velocity of the powder with the inner wall of the throat portion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining the internal structure of a long-throw transformer in which the throat of a Laval nozzle is extended. The long-throw transformer having the structure shown in FIG. 1 is used as a top-blowing lance for spraying powder together with a carrier gas, and is installed as a refining lance inside a refining vessel such as a vacuum degassing device for RH, etc. As shown in FIG. 1, the top-blowing lance according to this embodiment has a structure including a conical diameter reduction portion located on the upper side, a cylindrical throat portion, and a conical diameter expansion portion located on the lower side of the throat portion. The detailed shape of the top-blowing lance according to this embodiment will be described below.

[0014] [l t / D t ≧5.0 (1)] The length of the throat where the pressure is highest in order to accelerate the powder and improve refining efficiency. t By increasing the diameter of the throat D (m), the powder can be accelerated significantly. t If the length of the throat (m) is too large, the powder will not accelerate properly. t and throat diameter D t There is a proper range for the ratio of t / D t In the range of <5.0, the powder acceleration effect is insufficient, and the powder injection speed cannot be made high enough to ensure sufficient refining efficiency. t / D t shall be 5.0 or more.

[0015] [0.100≧H / (D in / D t ) ≧ 0.040 (2)] Next, the conditions for achieving excellent wear suppression effects are explained. Figure 2 is a diagram for explaining the powder wear mechanism in the nozzle and the concept of wear suppression. As shown in Figure 2(a), when the powder is blown upward, if the diameter reduction section located on the upper side of the throat section is short, an angle is generated in the powder trajectory, so the powder scatters at the tapered section and the nozzle wears due to collision with the throat wall. As a result, wear is likely to progress on the inner wall of the throat section. Therefore, by increasing the taper height H (m) of the diameter reduction section as shown in Figure 2(b), the angle of incidence of the powder with respect to the inner wall of the diameter reduction section becomes smaller, and the powder trajectory is rectified in the downward direction, so that wear can be reduced even on the inner wall of the throat section.

[0016] On the other hand, as shown in Fig. 2(c), the throat diameter D t If is too small, a longer taper height H is required to reduce collisions with the inner wall of the throat. Therefore, an analysis was conducted to determine the conditions for the taper height H that would suppress wear at the throat while ensuring a powder velocity with high refining efficiency. The inventors conducted a fluid analysis using Fluent (registered trademark) and found that the taper height H and the inner diameter D of the pipe at the top of the diameter reduction section were in and throat diameter D t The index H / (D in / D t It was found that there is a correlation between the powder impact energy on the inner wall of the throat and the powder impact energy.

[0017] Figure 3 shows the index H / (D in / D t ) and the impact energy of the powder on the inner wall of the throat (J / m 2 ) and the powder velocity (m / s) at the nozzle outlet. The collision energy shown in FIG. 3 is calculated using the index H / (D in / D t The inner diameter of the pipe at the top of the reduced diameter section D in affects the taper angle, and the inner diameter of the pipe D in The smaller the diameter of the throat, the larger the taper angle. tThe smaller the taper height H required to suppress wear, the larger it becomes. Based on this, the index H / (D in / D t The collision energy shown in Figure 3 was calculated using the following formula (5).

[0018]

number

[0019] (5) In formula, m p represents the powder mass (kg), and v p represents the powder velocity (m / s) at the time of collision with the inner wall of the throat. face is the collision area (m 2 ), but A face is the surface area of ​​the throat inner wall. As shown in Figure 3, the indicator H / (D in / D t The smaller the H / (D) is, the larger the collision energy tends to be. It was found that the collision energy becomes significantly larger at a boundary of around 0.04. Therefore, in equation (2), the indicator H / (D in / D t ) is set to 0.040. On the other hand, the index H / (D in / D t ) increases, the powder velocity injected from the nozzle tends to decrease. This is thought to be because the powder becomes rectified and becomes concentrated in the center of the throat, decreasing the powder acceleration efficiency in the throat. From the viewpoint of ensuring high refining efficiency, the indicator H / (D in / D t ) is set to an upper limit of 0.100.

[0020] Next, suitable conditions for blowing powder using the top blowing lance having the above-mentioned shape will be described. Note that there is no particular limitation on the elements and types of powder to be processed by refining, and the following description will be given taking desulfurization as an example.

[0021] First, the immersion tube of the RH type vacuum degassing apparatus is immersed in a ladle containing molten steel, and reduced pressure is applied in a vacuum tank. Then, while the molten steel is circulated through the immersion tube by the reflux gas, refining powder is sprayed onto the molten steel together with carrier gas from the top blowing lance. At this time, by using a top blowing lance that satisfies the above-mentioned formulas (1) and (2) and spraying the powder onto the molten steel together with carrier gas according to the conditions described below, refining with high powder reaction efficiency with impurities can be performed. Detailed powder spraying conditions are described below.

[0022] [Atmospheric pressure: less than 13.33kPa] The jet formed by the carrier gas is driven by the pressure gradient between the nozzle front pressure and the nozzle outlet pressure, so the lower the nozzle outlet pressure, the easier it is to form a supersonic jet. Also, the lower the atmospheric pressure in the vacuum chamber, the smaller the air resistance, so the attenuation of the speed of the powder injected from the nozzle until it reaches the surface of the molten steel is suppressed, and the efficiency of the powder penetrating into the molten steel is stabilized. Therefore, if the atmospheric pressure is set to 13.33 kPa or more, the carrier gas jet flow rate will be significantly reduced and the air resistance will be significantly increased, resulting in a significant attenuation of the speed after the powder is injected, so the atmospheric pressure is set to less than 13.33 kPa.

[0023] [ΔP0≧72×(l t / D t ) -0.27 (3)] [ΔP0=0.0118·Q / D t 2 (4)] ratio t / D t Since the powder acceleration effect varies greatly depending on the nozzle pressure loss ΔP0 required to ensure a high powder velocity for refining efficiency, t / D t Naturally, it will vary depending on the t / D t The nozzle pressure drop ΔP0 and ratio l required to achieve high refining efficiency under the condition that t / D tThe above conditions are formulated by equations (3) and (4). Note that Q in equation (4) is the flow rate of the carrier gas (Nm 3 If the condition of formula (3) is not satisfied, the powder velocity will be insufficient, and high refining efficiency will not be achieved.

[0024] As described above, by using a top blowing lance that satisfies the conditions of equations (1) and (2) and spraying refining powder onto molten steel under the conditions of equations (3) and (4) under a reduced atmospheric pressure of less than 13.33 kPa, secondary refining can be carried out with high refining efficiency and while suppressing internal wear of the top blowing lance. EXAMPLES

[0025] Next, an embodiment of the present invention will be described, but the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0026] The immersion tube of the RH type vacuum degassing equipment was immersed in a ladle containing molten steel with a sulfur concentration of 20 to 40 ppm, and the molten steel was desulfurized by blowing powder together with carrier gas from the top blowing lance while circulating the molten steel under reduced pressure. The top blowing lance used in the desulfurization treatment had a taper height H and an inner diameter D above the reduced diameter section, as shown in Table 1 below. in , throat diameter D t , throat length l t The powder used was CaO-Al2O3, and the carrier gas was Ar gas. The processing conditions were the ambient pressure, the carrier gas flow rate Q, and the nozzle pressure loss ΔP0 shown in Table 1.

[0027] The desulfurization k value, expressed by the following formula (6), was used as an index of refining efficiency. For each test number, the desulfurization k value was obtained for several tens of charges of the top blowing lance and the average value was calculated. A desulfurization k value of 0.120 or higher was evaluated as having excellent refining efficiency. Desulfurization k value = ln([S] PB前 / [S] PB後 ) / desulfurization agent consumption (kg / t-steel) (6) (6) In formula, [S] PB前 represents the S concentration (mass%) in the molten steel before desulfurization treatment, and [S] PB後 represents the S concentration (mass%) in molten steel after desulfurization treatment.

[0028] As an index of the degree of wear suppression, the powder was blown from the top of the top blowing lance of each test number for 100 channels, the maximum thickness of the throat was measured, and the wear rate was calculated. A wear rate of 0.05 mm / ch or less was evaluated as having successfully suppressed wear. The test results are shown in Table 1.

[0029] [Table 1]

[0030] As shown in Table 1, in Examples No. 1 to No. 6, good results were obtained in both the desulfurization k value and the wear rate. On the other hand, in Comparative Examples No. 7 and No. 8, the top blowing lance did not satisfy the condition of formula (1), and the powder was not accelerated sufficiently at the throat, and the powder velocity injected from the nozzle was insufficient, resulting in a low desulfurization k value.In Comparative Examples No. 9 and No. 10, the atmospheric pressure was higher than 13.33 kPa, and therefore the powder velocity attenuated before it reached the molten steel surface, resulting in a low desulfurization k value.

[0031] Comparative Examples No. 11 to No. 13 are the top blowing lance index H / (D in / D t In the comparative examples No. 14 and No. 15, the index H / (D in / D t) was large and did not satisfy the condition of formula (2), and the powder was concentrated in the center of the throat, which resulted in insufficient powder velocity, and the desulfurization k value was low. In comparative example No. 16, the nozzle pressure loss ΔP0 did not satisfy the condition of formula (3), so the powder acceleration was insufficient and the desulfurization k value was low.

Claims

1. A refining lance for blowing refining powder from above, The nozzle has a cylindrical throat portion, a conical diameter reduction portion located on an upper side of the throat portion, and a conical diameter expansion portion located on a lower side of the throat portion, The diameter D of the throat portion t (m) and the length of the throat portion l t (m) satisfies the relationship of formula (1), and the diameter D t (m) and the inner diameter D of the pipe at the upper part of the diameter reduction portion in The relationship between the taper height H(m) of the reduced diameter portion and the taper height H(m) of the reduced diameter portion satisfies formula (2). l t / D t ≧5.0 ・・・(1) 0.100≧H / (D in / D t )≧0.040 ・・・(2)

2. In a process of spraying a refining powder onto a molten steel surface together with a carrier gas under a reduced pressure of less than 13.33 kPa, the shape of a top blowing lance for spraying the refining powder comprises a cylindrical throat portion, a conical diameter reduction portion located on the upper side of the throat portion, and a conical diameter expansion portion located on the lower side of the throat portion, and the diameter D of the throat portion is 1.0 mm. t (m) and the length of the throat portion l t (m) satisfies the relationship of formula (1), and the diameter D t (m) and the inner diameter D of the pipe at the upper part of the diameter reduction portion in a top blowing lance in which the relationship between the diameter reduction section's taper height H(m) and the diameter reduction section's taper height H(m) satisfies formula (2), and the carrier gas supply conditions satisfy formulas (3) to (4). l t / D t ≧5.0 ・・・(1) 0.100≧H / (D in / D t )≧0.040 ・・・(2) ΔP 0 ≧72×(l t / D t ) -0.27 ・・・(3) ΔP 0 =0.0118・Q / D t 2 ・・・(4) Here, ΔP 0 : Nozzle pressure loss (kPa), Q: Carrier gas flow rate (Nm 3 / min).

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