JP6926928B2 - Refining method of molten steel - Google Patents

Description

The present invention relates to a method for refining molten steel in which high-grade steel is efficiently melted by significantly accelerating the refining powder injected from a top-blown lance in a decompression refining apparatus for molten steel.

In recent years, the performance level required for steel products has been increasing more and more, and thorough removal of impurities in molten steel such as S and C is required. For example, with respect to S, in order to melt ultra-low sulfur steel of 10 ppm or less, a desulfurization treatment is performed by spraying a desulfurizing agent powder containing CaO together with a carrier gas on the surface of the molten steel with a recirculation type vacuum degassing device such as RH. The method of applying may be taken. Further, regarding C, a technique is known in which _{Fe 2} O ₃ powder is sprayed on the _{surface of molten steel to cause reduction of Fe 2} O _{3 by C and promote a decarburization reaction.}

Examples of the method for improving the refining efficiency in the powder top blowing treatment include a method of expanding the injection range of the powder and a method of promoting the intrusion of the powder into the molten steel. In particular, in the latter method, it is necessary to increase the injection speed of the powder to increase the inertial force, break the surface tension of the molten steel, and penetrate deeply into the molten steel. Further, for the powder top blowing treatment, a method of injecting refining powder together with a carrier gas using a supersonic nozzle having a diameter reducing portion (throat portion) and a diameter expanding portion generally called a Laval nozzle. Is widely used. However, although the carrier gas is instantly accelerated to supersonic speed by volume contraction and expansion, the refining powder without volume change does not always have the same speed as the gas when it is injected from the nozzle. At present, the powder is injected due to poor acceleration and does not efficiently penetrate into the molten steel.

In response to the above problems, Patent Document 1 discloses a vacuum refining method using a two-stage nozzle having a throat portion and a cylindrical enlarged portion. The gas jet formed by the nozzle is inferior in maximum dynamic pressure and flow velocity to the Laval nozzle, but the injection range is expanded. Therefore, it is said that the reaction efficiency is improved by injecting the powder over a wide range. However, the refining powder does not undergo volume expansion and has a high density with respect to the gas, so that the straightness is very high, and the powder is not always sprayed over a wide range like a jet. In addition, since the maximum dynamic pressure and flow velocity of the jet are greatly reduced with respect to the Laval nozzle, the inertial force of the powder falls below the surface tension of the molten steel surface without sufficient acceleration of the powder, and the powder penetrates into the molten steel. There is a concern that the reaction efficiency will be greatly reduced due to the accumulation on the surface of the molten steel.

On the other hand, in Patent Document 2, a straight inner tube is connected to the tip of a rubber nozzle to secure an acceleration distance of powder to improve the injection speed and promote the penetration of powder into molten steel for refining efficiency. A lance characterized by improving is disclosed. However, the gas expands remarkably from the throat outlet to the diameter expansion portion, and the pressure drops. Therefore, the pressure of the gas in the straight inner tube of the nozzle becomes very small, and the acceleration effect is extremely small. In addition, the above-mentioned technique does not describe the operating conditions during the powder top blowing process, and there is a concern that the refining efficiency may fluctuate greatly depending on the conditions.

Japanese Patent No. 3654216 Japanese Patent No. 3598536

As described above, in the conventional method, since the refining powder is not efficiently accelerated, it is difficult to stably exhibit high refining efficiency.

Therefore, in view of the above-mentioned problems, it is an object of the present invention to provide a method for refining molten steel, which significantly accelerates the powder, promotes the invasion of the powder into the molten steel, and enhances the refining efficiency.

The present inventors first conducted a test for investigating the velocity of the powder ejected from the nozzle under various nozzle shapes and powder blowing conditions. FIG. 1 shows an example of the internal structure of the Laval nozzle. It has been found that the powder can be significantly accelerated by increasing _{the length l t} (m) of the throat portion where the pressure is highest in the Laval nozzle based on the Laval nozzle.
Further, the nozzle front pressure P ₀ (Torr) is expressed by the equation (3) from _{the throat diameter D t} (m) and the carrier gas flow rate Q (Nm ^{3 / min).}
P ₀ = 0.089 ・ Q / D _t ²・ ・ ・ (3)

As can be seen from Eq. (3), the nozzle front pressure P ₀ increases as the throat diameter D _{t decreases.} Therefore, even if the length l _t of the throat portion is secured to some extent, if the throat diameter D _t is too large, the nozzle front pressure P ₀ becomes small, resulting in poor acceleration of the powder. From this, it was found that there is an appropriate range in the ratio _{between the length l t} of the throat portion and the diameter (throat diameter) D _{t of the throat portion.}

Furthermore, the present inventors conducted a powder top blowing test on molten steel for desulfurization under the condition that the powder injection rate was changed, and investigated the relationship between the powder reaction efficiency and the powder rate. As a result, it was found that when the powder velocity exceeds a certain critical value, the powder breaks the surface of the molten steel and invades into the molten steel, and the powder reaction efficiency is dramatically improved. Since it is necessary to increase the absolute value of the pressure inside the throat in order to accelerate the powder in the throat, the nozzle pre-pressure P is added to the ratio of the _{throat length l t} and the throat diameter D _{t. It} was found that by controlling 0 to a certain critical value or higher, the powder is accelerated to a speed at which it can penetrate into the molten steel, and the powder reaction efficiency is dramatically improved.

The present invention clarifies the lance shape and the powder injection conditions for significantly accelerating the injection speed of the refining powder, promoting the invasion into the molten steel, and dramatically improving the refining efficiency. It was done. Specifically, it is as follows.
(1) In the processing ambient pressure P _e (Torr) is the average value of the particle size the molten steel surface together with a carrier gas at a reduced pressure of less than 100Torr blows refining powder is 20 to 100 [mu] m,
The tip shape of the top-blowing lance that sprays the refining powder consists of a cylindrical throat portion and a conical diameter-expanding portion.
The throat diameter of the throat portion is 0.01 to 0.05 m, the outlet diameter of the diameter expansion portion is 0.04 to 0.1 m, and the lance height of the top blow lance is 2.6 to 4.2 m.
As the top-blown lance, a top-blown single-hole lance in which the diameter D _t (m) of the throat portion and the length l _t (m) of the throat portion satisfy the relationship of the equation (1) is used, and the carrier gas is used. characterized with Ar, supply conditions of that is (2) to (3) satisfying the condition equation, the molten steel process of refining.
l _t / D _t ≧ 5.0 ・ ・ ・ (1)
_{P 0 ≧ 540 × (l t} / D t) -0.27 ··· (2)
P ₀ = 0.089 ・ Q / D _t ²・ ・ ・ (3)
Here, P ₀ : nozzle front pressure (Torr), Q: carrier gas flow rate (Nm ³ / min).
(2) The method for refining molten steel according to (1) above, wherein when the refining powder is sprayed onto the surface of the molten steel using the top-blown lance, the condition of the equation (4) is further satisfied.
^{_{(P 0 / P e) 0.2}} (D e / H) 0.1 · (l t / W PB) 0.1 ≧ 0.6 ··· (4)
Here, _De : the outlet diameter (m) of the enlarged diameter portion, H: the distance between the lance and the molten metal surface (m), and W _PB : the powder supply rate (kg / min).

According to the present invention, when the refining powder is sprayed from the top-blown lance, the refining powder can be efficiently penetrated into the molten steel, so that the refining efficiency can be dramatically improved and it is industrial. The value is very great.

It is a figure which showed typically the nozzle shape of the top blowing lance in this invention. It is a figure which showed the relationship between the powder reaction efficiency index and the powder injection speed. It is a figure which shows the relationship between the _{nozzle front pressure P 0} to which this invention is applied, and the ratio l _t / D _t.

(1. Definition of terms in the present invention)
The present invention will be described below. The "RH vacuum degassing device" described below is a molten steel processing device having a vacuum tank, and the "powder reaction efficiency" is that the top-blown refining powder (hereinafter, powder) reacts with the molten steel. It shows the efficiency that contributed to the reduction of impurities. Further, the "powder top blowing process" is a process of refining molten steel by spraying powder together with a carrier gas from a top blowing lance installed inside a refining container such as an RH vacuum degassing device. The calculation of powder, the reaction between powder and molten steel, and the calculation of "powder reaction efficiency" based on specific examples of impurity elements will be described later.

(2. Powder injection conditions according to the present invention)
[P _e : less than 100 Torr]
Since the jet formed by the carrier gas uses the pressure gradient between the nozzle front pressure and the nozzle outlet pressure as the driving force, the lower the nozzle outlet pressure, the easier it is to form a supersonic jet. _{In addition, the lower the atmospheric pressure P e} in the vacuum chamber, the smaller the air resistance, so the attenuation of the speed at which the powder injected from the nozzle reaches the surface of the molten steel is suppressed, and the penetration efficiency of the powder into the molten steel is suppressed. Is stable. Therefore, when the ambient pressure P _e results in the above 100 Torr, a significant decrease in the carrier gas jet velocity, since it leads to a significant attenuation of the speed after the powder injection due to a significant increase in air resistance, an atmospheric pressure P _e 100 Torr Less than.

[L _t / D _t ≧ 5.0 ・ ・ ・ (1)]
When the powder velocity exceeds a certain critical value, the powder breaks the surface of the molten steel and invades the bath, dramatically improving the reaction efficiency. FIG. 2 shows the relationship between the powder reaction efficiency index investigated by the method described later and the powder injection speed at the nozzle outlet. From this survey result, it was found that the desulfurization efficiency is dramatically improved when the powder injection speed exceeds about 60 m / s.

FIG. 3 shows the results of actually measuring the powder injection speed at the nozzle outlet under the condition that the nozzle pre-pressure P _{0 was changed and arranging the powder injection speed by the ratio l t} / D _t. In FIG. 3, a circle indicates that the powder injection speed exceeds 60 m / s, and a cross indicates that the powder injection speed does not exceed 60 m / s. As shown in FIG. 3, the _{powder acceleration effect is insufficient in the range of the ratio l t} / D _t <5.0, and even if the nozzle pre-pressure P ₀ is controlled to a high level, the powder injection speed is 60 m /. Could not be more than s. Therefore, in order to fully exert the effect of the present invention, it is necessary to set the ratio l _t / D _t to 5.0 or more.

_{[P 0 ≧ 540 × (l} t / D t) -0.27 ··· (2)]
Since the powder acceleration effect differs greatly depending on the _{ratio l t} / D _t , the value of the _{nozzle pre-pressure P 0} required to exceed the critical speed naturally differs depending on the _{ratio l t} / D _t. Therefore, from the results shown in FIG. 3, the _{nozzle prepressure P 0} and the ratio l required for the powder injection speed to exceed 60 m / s under the condition that _{the ratio l t} / D _{t satisfies 5.0 or more. The condition of t} / D _t was formulated to obtain Eq. (2). Therefore, in order to obtain the effect of the present invention, it is necessary to control the nozzle front pressure P ₀ so as to satisfy the condition of the equation (2) _{according to the ratio l t} / D _{t in addition to the condition of the equation (1).} be.

_{[(P 0 / P e)} 0.2 (D e / H) 0.1 · (l t / W PB) 0.1 ≧ 0.6 ··· (4)]
Before the powder ejected from the nozzle reaches the surface of the molten steel, the velocity is attenuated due to various factors. The present inventors extracted the operating factors necessary for obtaining the effect of the present invention more stably according to the method described later, and formulated the influence thereof.

First, in relation to the nozzle front pressure P ₀ and the ambient pressure P _e, the larger the ratio between the nozzle front pressure and nozzle exit pressure as described above, since the driving force of the jet is large, a nozzle pre-pressure P ₀ It is desirable that the atmospheric pressure P _e and the ratio P ₀ / P _{e are large.} Further, an outlet diameter D _e and lance nozzle expansion portion - in relation to the water level distance (lance height) H, the lance height H increases, the powder until it reaches the correspondingly melt surface Since it is exposed to air resistance, it is desirable _{that the ratio D e / H of the outlet diameter D e} of the nozzle enlargement portion and the lance height H is also large. _{Further, due to the relationship between the length l t} of the throat portion and the powder supply speed W _PB , when the powder supply speed W _PB increases, energy consumption due to aggregation between powders increases, so that the length of the throat portion is the ratio l _t / W of l _t and a powder feed rate W _{PB PB} is large is desirable. From the above, in order to stably obtain the remarkable effect of the present invention, it is desirable that the conditions of the equation (4) are further satisfied in addition to the equations (1) and (2).

(3. Processing conditions)
In the present invention, the molten steel discharged from a smelting furnace such as a converter to a ladle is made of molten steel in a smelting container typified by an RH vacuum degassing device after finishing component adjustment such as deoxidation and alloy addition. The process is carried out. In a refining container equipped with a top-blown lance, the lance shape and operating conditions are set within the range satisfying the above conditions, and the refining powder is top-blown together with the carrier gas. Here, the cross-sectional shape of the top blowing lance used for powder top blowing does not necessarily have to be a perfect circle. When the cross-sectional shape is other than a perfect circle, the values of the throat diameter D _t and the outlet diameter De _e of the nozzle enlargement portion are converted from the cross-sectional area to the equivalent circle diameter. Further, the number of holes is not particularly limited, but if the number of holes is too large, the prepressure per hole decreases and it becomes difficult to accelerate the powder, so a single hole is desirable.

On the other hand, the type of carrier gas used for powder top blowing is not limited, but a rare gas element is desirable from the viewpoint of operational stability and processing cost, and it is more desirable to use Ar in particular. Further, if the carrier gas flow rate Q is too large, _{it becomes difficult to maintain the atmospheric pressure P e} in the vacuum chamber at a high vacuum, and the molten steel is actively scattered by the carrier gas jet, which greatly increases the equipment load. Therefore, it is desirable that the carrier gas flow rate Q is ^{less than 20 Nm 3 / min.}

Furthermore, since the present invention is not limited to desulfurization but covers all powder top blowing treatments, the type, particle size, etc. of the refining powder may be changed according to the impurities to be removed. However, if the particle size of the powder is too large, the surface area with respect to the mass decreases, and the reaction rate with the molten steel greatly decreases. Further, if the particle size of the powder is too small, the mass of the single particle is greatly reduced, and it becomes difficult to penetrate into the molten steel. Therefore, it is desirable that the particle size of the refining powder is 20 to 100 μm on average.

(4. How to check the effect)
The effect of the present invention can be evaluated by the powder reaction efficiency. First, a molten steel sample is taken before and after the powder top blowing treatment and subjected to chemical analysis to obtain an impurity concentration [% X] in the molten steel. Here, the powder reaction efficiency is evaluated by the powder reaction efficiency index of the following formula (5).
Powder reaction efficiency index = ln ([% X] _{before top blowing} / [% X] _{after top blowing} ) / powder basic unit
... (5)

Here, the powder basic unit (kg / ton) is a value obtained by dividing the total mass of the powder used in the powder top blowing treatment by the mass of molten steel. In the present invention, it can be determined that the effect of the invention was obtained when the powder reaction efficiency index was 0.100 or more, and the effect of the invention was particularly remarkable when the powder reaction efficiency index was 0.150 or more. It can be judged that it was obtained.

Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

After the molten steel that has been blown from the converter is discharged into a ladle, the ladle is transported to the RH vacuum degassing device, a vacuum tank equipped with a top-blown lance is inserted into the molten steel in the pan, and the molten steel is evacuated. The powder was sucked into the tank and the powder top blowing process was started. Examples 1 and 2 (Example of the present invention) and Comparative Examples are all for desulfurization treatment, the top-blown lance is a single-hole rubber lance, the top-blown powder is CaO, the carrier gas is Ar, the amount of molten steel is 250 ton scale, and molten steel. The temperature was 1600 to 1640 ° C. When performing the blowing on the powder processing, the length l _t of the throat portion, the throat diameter D _t, outlet diameter D _e, the ambient pressure P _e, the carrier gas flow rate Q, the powder feed rate W _PB and lance height H Was changed as an operating factor as shown in Table 1 below, and other refining conditions were as follows.
S concentration [% S] before powder top blowing treatment: 0.0020 to 0.0030% by mass
Composition of molten steel before powder top blowing treatment:
C concentration [% C]: 0.05 to 0.20% by mass
Si concentration [% Si]: 0.05 to 0.30% by mass
Mn concentration [% Mn]: 0.50 to 1.50% by mass
Al concentration [% Al]: 0.10 to 0.20% by mass
Powder top blowing processing time: 10 min

A molten steel sample is taken before and after the powder top blowing treatment, and a part of the molten steel sample is subjected to chemical analysis to obtain an S concentration [% S] before and after the treatment, and the powder reaction is carried out by the above formula (5). The efficiency index was calculated. Table 2 also shows the values of the powder reaction efficiency index under each condition. In addition, according to the above-mentioned method for confirming the effect, the one in which the effect of the invention was particularly remarkable when the powder reaction efficiency index was 0.150 or more was ⊚, and the powder reaction efficiency index was 0.100 or more, although it was not remarkable. Those in which the effect of the invention was obtained were evaluated as ◯, and those in which the effect of the invention was not obtained were evaluated as ×. The underlined items in Tables 1 and 2 show values that do not satisfy the present invention.

Ch. Of Example 1. No. Since all of the above-mentioned equations (1) to (4) were satisfied in 1 to 6, the effect of the invention was particularly remarkable.
Ch. Of Example 2 No. Nos. 7 to 11 did not satisfy the condition of the above formula (4), but all of the other conditions were satisfied, so that the effect of the invention was obtained. If the condition of Eq. (4) is not satisfied, the speed of the powder ejected from the nozzle is slightly attenuated, and the powder speed at the time of reaching the molten metal surface is slightly low, so that the effect of the invention is reduced. Conceivable. Therefore, in order to maximize the effect of the present invention, it was confirmed that it is desirable to satisfy the conditions of the formulas (4) in addition to the conditions of the formulas (1) to (3).

On the other hand, Ch. No. Nos. 12 to 16 did not satisfy some of the essential conditions, so that the effects of the invention could not be obtained. Ch. No. In No. 12, since the atmospheric pressure was 100 Torr or more, the dynamic pressure and the flow velocity of the carrier gas jet were greatly attenuated, and the air resistance of the atmosphere was large, and the powder velocity after injection was greatly reduced. It is probable that was not obtained. Therefore, it was confirmed that the atmospheric pressure during the powder top blowing treatment must always be less than 100 Torr.

Ch. No. In Nos. 13 and 14, since the ratio l _t / D _t of the top blowing lance in the powder top blowing treatment was less than 5.0, the acceleration distance of the powder was insufficient and the effect of the invention could not be obtained. _{Therefore, it was confirmed that the ratio l t} / D _t of the top blow lance must always be 5.0 or more.

Ch. No. In Nos. 15 and 16, since the nozzle pre-pressure P ₀ in the powder top blowing process did not satisfy the condition of Eq. (2), Ch. No. Similar to 13 and 14, the acceleration of the powder was insufficient, and the effect of the invention could not be obtained. Therefore, it was confirmed that the nozzle pre-pressure P ₀ must satisfy the condition of Eq. (2).

Claims (2)

In the process ambient pressure P _e (Torr) is the average value of the particle size the molten steel surface together with a carrier gas at a reduced pressure of less than 100Torr blows refining powder is 20 to 100 [mu] m,
The tip shape of the top-blowing lance that sprays the refining powder consists of a cylindrical throat portion and a conical diameter-expanding portion.
The throat diameter of the throat portion is 0.01 to 0.05 m, the outlet diameter of the diameter expansion portion is 0.04 to 0.1 m, and the lance height of the top blow lance is 2.6 to 4.2 m.
As the top-blown lance, a top-blown single-hole lance in which the diameter D _t (m) of the throat portion and the length l _t (m) of the throat portion satisfy the relationship of the equation (1) is used, and the carrier gas is used. characterized with Ar, supply conditions of that is (2) to (3) satisfying the condition equation, the molten steel process of refining.
l _t / D _t ≧ 5.0 ・ ・ ・ (1)
_{P 0 ≧ 540 × (l t} / D t) -0.27 ··· (2)
P ₀ = 0.089 ・ Q / D _t ²・ ・ ・ (3)
Here, P ₀ : nozzle front pressure (Torr), Q: carrier gas flow rate (Nm ³ / min). The method for refining molten steel according to claim 1, wherein when the refining powder is sprayed onto the surface of the molten steel using the top-blown lance, the condition of the equation (4) is further satisfied.
^{_{(P 0 / P e) 0.2}} (D e / H) 0.1 · (l t / W PB) 0.1 ≧ 0.6 ··· (4)
Here, _De : the outlet diameter (m) of the enlarged diameter portion, H: the distance between the lance and the molten metal surface (m), and W _PB : the powder supply rate (kg / min).

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