JP2002363636A - Method for smelting molten steel in rh vacuum degassing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform efficient smelting by circulating molten steel under an adequate condition taking into consideration not only the inside diameter of a riser side immersion pipe and the flow rate of an inert gas for circulation but also the inside diameter and the number of installation of gas-blowing nozzles when smelting the molten steel by using an RH vacuum degassing apparatus. SOLUTION: The molten steel is circulated by blowing the inert gas for circulation through an adjustment so that an inequality (1): 20<n/[[ρg /(ρl -ρg )]<1/2> ×(G/d)×D<-3/2> ]<100 is satisfied in terms of the inside diameter of the riser side immersion pipe, the number of the gas-blowing nozzles installed in the immersion pipe, the inside diameter of the gas-blowing nozzles, and the flow rate of the inert gas for circulation which is blown from the gas-blowing nozzles, where n: number of the gas-flowing nozzles, ρg : density (kg/m<3> ) of the inert gas for circulation, ρl : density (kg/m<3> ) of molten steel, G: flow rate (m<2> /sec) of the inert gas for circulation, d: inside diameter (m) of the gas- blowing nozzle, D: inside diameter (m) of the riser side immersion pipe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for refining molten steel in an RH vacuum degassing apparatus.

[0002]

2. Description of the Related Art An RH vacuum degassing apparatus is provided with a vacuum chamber and two immersion pipes provided below the vacuum chamber (an ascending immersion pipe and a descending immersion pipe). When refining molten steel using this RH vacuum degassing device, two immersion tubes are immersed in the molten steel accommodated in the ladle, the inside of the vacuum chamber is depressurized, and the molten steel is pulled up into the immersion tube. Then, an inert gas such as Ar is blown from a gas blowing nozzle provided on the rising side immersion pipe, the molten steel is raised by the principle of a gas lift pump, sent into a vacuum chamber, vacuum-processed in the vacuum chamber, and lowered. It is returned to the ladle from the side dip tube. Thus, the vacuum refining is continuously performed by circulating the molten steel between the ladle and the vacuum tank.

In order to increase the processing capacity and the processing efficiency of this RH vacuum degassing apparatus, it is necessary to increase the annular flow rate of molten steel per unit time.

It has been empirically known that the annular flow rate of molten steel depends on the inner diameter of the dip tube, the flow rate of the inert gas for reflux, the pressure difference between the vacuum chamber and the atmosphere, and the like. An increase in the reflux rate has been attempted by enlarging or increasing the flow rate of the inert gas for reflux. However, the enlargement of the inner diameter of the immersion tube involves a great deal of equipment remodeling, which leads to an increase in equipment cost, and even if it is expanded, there is a natural limit due to the size of the vacuum chamber. Although increasing the flow rate of the inert gas for reflux is effective, inert gas such as Ar is expensive, and increasing the flow rate above a certain limit conversely decreases the flow rate of the molten steel.

[0005] Apart from these countermeasures, there has been proposed a method of increasing the annular flow rate by controlling the flow of molten steel in a dip tube. For example, JP-A-3-36209 discloses that
There is disclosed a method in which the discharge direction of a gas injection nozzle installed on a rising side immersion pipe is inclined from the direction toward the center of the immersion pipe to a measurement direction to generate a swirl flow in molten steel ascending flow in the immersion pipe. Japanese Patent Application Laid-Open No. -1319 discloses a method of circulating molten steel by arranging a discharge direction of a gas blowing nozzle installed in an ascending side immersion pipe upward by 20 to 50 ° from a horizontal direction. According to these methods, the annular flow rate can be increased without increasing the inner diameter of the immersion tube or increasing the inert gas flow rate for reflux.However, since both methods have a special direction of the gas injection nozzle, Significant skills are required for refractory enforcement and fabrication.

[0006]

A plurality of gas blowing nozzles are usually installed in the ascending-side immersion pipe.
It is clear that the number of gas blowing nozzles also affects the molten steel ring flow rate. That is, even if the flow rate of the inert gas for reflux is the same, if the number of gas blowing nozzles is too small, gas bubbles become large, and gas blow-through occurs, impeding the recirculation of molten steel. If the number is too large, the discharge speed of the gas from the gas blowing nozzle becomes slow, and the blown gas rises along the inner wall of the immersion pipe,
Not only is there no effect on the reflux of the molten steel, but it also promotes the erosion of the inner wall of the immersion tube. Similarly, from the viewpoint that the inner diameter of the gas blowing nozzle also affects the gas discharge flow rate from the gas blowing nozzle, it is apparent that the inner diameter of the gas blowing nozzle affects the flow rate of the molten steel ring.

As described above, the number and the inner diameter of the gas blowing nozzles are important factors for the recirculation of the molten steel. When the flow rate of the molten steel is increased, the flow rate of the inert gas for the recirculation and the inner diameter of the immersion pipe are reduced. It should be similarly considered, but there has been no report such as the above-mentioned gazette considering the number of gas blowing nozzles and the inner diameter thereof from the viewpoint of increasing the flow rate of molten steel ring.

[0008] The present invention has been made in view of the above circumstances,
When refining molten steel using an RH vacuum degassing device, the purpose is to consider not only the inner diameter of the ascending immersion pipe and the flow rate of the inert gas for reflux, but also the inner diameter and the number of installed gas injection nozzles. The molten steel under the appropriate conditions
An object of the present invention is to provide a refining method capable of performing efficient refining.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems. The results of the study are described below.

[0010] As described above, if the number of gas injection nozzles is too small, the gas flow per nozzle will increase too much and gas will flow through, obstructing the recirculation of molten steel. On the other hand, if the number of gas blowing nozzles is too large, the gas flow per nozzle will be too small, and the gas will propagate along the side wall of the immersion pipe, and will not effectively act on the reflux of the molten steel.

Judging from these phenomena, when the molten steel is recirculated by the principle of the gas lift pump, the horizontal reach of the recirculating inert gas blown from the gas injection nozzle may affect the recirculation of the molten steel. I can analogy.

It has been proposed in Document 1 that the horizontal gas arrival distance of the inert gas is represented by the following equation (2) (Reference 1: Iron and Steel, 1979, A133). However,
In the equation (2), L: distance reached in the horizontal direction (m), d:
Inner diameter of gas injection nozzle (m), ρ _g : density of inert gas for reflux (kg / m ³ ), ρ _l : density of molten steel (kg / m ³ ),
V: Gas flow velocity at the gas injection nozzle outlet (m / sec
), G: gravity acceleration (m / sec ² ).

[0013]

(Equation 1)

In this case, the gas flow velocity (V) at the outlet of the gas injection nozzle can be expressed by the following equation (3). In the equation (3), G is the flow rate of the inert gas for reflux (m ³ / sec), and n is the number of gas blowing nozzles.

[0015]

(Equation 2)

By substituting equation (3) into equation (2) and rearranging, the following equation (4) is obtained.

[0017]

(Equation 3)

[0018] It is considered that if the horizontal distance (L) of the inert gas determined in this way is too large or too small with respect to the inner diameter (D) of the ascending side immersion pipe, the molten steel reflux will be inhibited. Can be Therefore, a test was carried out in which the ratio (D / L) of the inner diameter (D) of the ascending side immersion pipe to the horizontal distance (L) was variously changed, and the effect of this ratio (D / L) on the molten steel reflux was examined. investigated.

Here, the ratio (D / L) of the inner diameter (D) of the rising side immersion tube to the horizontal distance (L) can be expressed by the following equation (5).

[0020]

(Equation 4)

The ratio (D / L) is accurately expressed by the above equation (5), but the following equation (6) is obtained by simplifying the equation (5) by removing the right side of the equation (3) to the power of 3/2 and removing coefficients. ) Holds a proportional relationship with Z expressed by the equation.

[0022]

(Equation 5)

Therefore, when refining molten steel using an RH vacuum degassing apparatus, the inner diameter (D) of the ascending-side immersion pipe, the number (n) and inner diameter (d) of gas injection nozzles, the flow rate of the inert gas for recirculation, The uniform mixing time of the molten steel was measured by changing (G) variously, and the influence of the calculated value Z calculated by the equation (6) on the uniform mixing time was investigated. The uniform mixing time is measured by adding a trace element to the molten steel and measuring the change in concentration.
The time was from the time when the element was added to the time when the concentration became constant. In this case, the shorter the uniform mixing time, the more efficiently the molten steel is circulated. Table 1 shows the molten steel reflux test conditions. In Table 1, the condition in which the inner diameter (D) of the immersion tube, the inner diameter (d) of the nozzle, and the gas flow rate (G) are the same is displayed as one level, and the relationship between the calculated value Z and the number of nozzles is specified.

[0024]

[Table 1]

FIG. 1 shows the result of investigation on the relationship between the calculated value Z calculated by the equation (6) and the uniform mixing time. In this case, the calculated value Z is expressed as Ar, as the density ρ _g of the reflux inert gas.
Using the density of 1.786 kg / m ³ , the density of molten steel ρ _l
Was calculated as 7000 kg / m ³ . As shown in FIG.
Although the absolute value of the uniform mixing time has a difference at each level, the number of nozzles is large and the calculated value Z is 100 at any level.
In the above cases and when the number of nozzles was small and the calculated value Z was 20 or less, it was found that the uniform mixing time was long.
That is, the inner diameter (D) of the ascending side immersion pipe, the number of gas injection nozzles (n), and the inner diameter of the gas injection nozzle (d)
By adjusting the gas flow rate (G) of the inert gas for reflux to the range satisfying the following formula (1), the molten steel is refluxed, whereby the uniform mixing time can be shortened, and the molten steel can be efficiently removed. It has been found that reflux is possible.

[0026]

(Equation 6)

The present invention has been made on the basis of the above-described examination results. The method for refining molten steel in the RH vacuum degassing apparatus according to the present invention is arranged in the inner diameter (D) of the ascending side immersion pipe and the ascending side immersion pipe. The number (n) of the gas blowing nozzles, the inner diameter (d) of the gas blowing nozzles, and the gas flow rate (G) of the recirculating inert gas blown from the gas blowing nozzles satisfy the range of the above formula (1). This is characterized in that the inert gas for reflux is blown into the ascending-side immersion pipe so as to reflux the molten steel.

[0028]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 2 is a schematic vertical sectional view of an RH vacuum degassing apparatus used in carrying out the present invention.

As shown in FIG. 2, the RH vacuum degassing device 1
Comprises a vacuum tank 5 composed of an upper tank 6 and a lower tank 7, an ascending immersion pipe 8 and a descending immersion pipe 9 provided below the lower tank 7, and the upper tank 6 includes an exhaust device (not shown). And an inlet 12 for the raw material, and a gas injection nozzle 10 is provided in the ascending immersion pipe 8. Ar is blown into the rising side immersion pipe 8 from the gas blowing nozzle 10 as an inert gas for reflux. Although only one gas injection nozzle 10 is shown in FIG. 2, a plurality of (n) gas injection nozzles 10 are provided around the ascending-side immersion pipe 8 in the circumferential direction thereof. It is installed as a horizontal direction toward the center of the. Here, the inner diameter of the ascending side immersion tube 8 is D (m).
And the inner diameter of the gas injection nozzle 10 is d (m). From the viewpoint of uniformly raising the molten steel 3 in the ascending-side immersion pipe 8 in the circumferential direction, it is desirable that the gas injection nozzles 10 be installed at equal intervals in the circumferential direction of the ascending-side immersion pipe 8 if possible. In the figure, the discharge direction of the gas injection nozzle 10 is set to the horizontal direction toward the center of the rising side immersion pipe 8, but it may be set to improve the upper square, or may be inclined from the direction toward the center to the horizontal direction.

In the RH vacuum degassing apparatus 1 having such a configuration, when the refining method according to the present invention is performed,
First, molten steel 3 refined in a converter or an electric furnace is tapped into ladle 2, and ladle 2 containing molten steel 3 is conveyed directly below vacuum tank 5. Slag 4 generated by refining in a converter, an electric furnace, or the like is partially mixed in the ladle 2 and covers the molten metal surface of the molten steel 3.

Next, the ladle 2 is moved by a lifting device (not shown).
Is raised, and the ascending-side immersion pipe 8 and the descending-side immersion pipe 9 are immersed in the molten steel 3 in the ladle 2. Then, the blowing flow rate from the gas blowing nozzle 10 into the rising side immersion pipe 8 is G
(M ³ / sec) before and after Ar was blown,
The inside of the vacuum chamber 5 is evacuated by an exhaust device to reduce the pressure inside the vacuum chamber 5. When the pressure in the vacuum chamber 5 is reduced, the molten steel 3 in the ladle 2 rises along the rising side immersion pipe 8 together with Ar blown from the gas blowing nozzle 10 and flows into the vacuum chamber 5, and then flows into the falling side immersion. The flow returning to the ladle 2 via the pipe 9, forming a so-called reflux, is subjected to RH vacuum degassing refining. The Ar flow rate blown from the gas blowing nozzle 10 was initially set to G (m ³ / sec).
It does not need to be performed, and may be performed after the reflux of the molten steel 3 reaches a steady state.

At this time, the inner diameter (D) of the ascending side immersion tube 8
And the number (n) of gas blowing nozzles 10 and gas blowing
Inner diameter (d) of the gas injection nozzle 10 and the gas injection nozzle 1
The gas flow rate of the inert gas for recirculation blown from 0 (G)
Are adjusted so as to satisfy the range of the above-described expression (1).
You. In calculating equation (1), the density ρ of Ar for reflux is
_g Is 1.786 kg / m^Three , Density ρ of molten steel 3_l Is 7000kg
/ m^Three It is good.

Specifically, when the inner diameter (D) of the ascending side immersion pipe 8, the number (n) of the gas injection nozzles 10, and the inner diameter (d) of the gas injection nozzles 10 are already determined in terms of equipment, the circulation The gas flow rate (G) of the inert gas for use is adjusted to satisfy the expression (1).

In order to increase the flow rate (G) of the inert gas for reflux, if the equipment is modified to increase the inner diameter (D) of the ascending immersion pipe 8, the desired gas injection flow rate (G) The number (n) of the gas blowing nozzles 10 and the inner diameter (d) of the gas blowing nozzles 10 are set within a range satisfying the expression (1) in consideration of the inner diameter (D) of the rising side immersion pipe 8 to be enlarged. Set. However, considering the inside diameter (d) of the gas injection nozzle 10 and the insertion of the molten steel 3,
Since it cannot be made too large, in that case, it is desirable to deal with the number of nozzles (n). The maximum value of the inner diameter (d) of the gas injection nozzle 10 may be set to about 5 mm.

Based on the use of the molten steel 3 to be treated, the molten steel 3 is circulated under these conditions, and RH such as dehydrogenation, decarburization, denitrification, etc.
Vacuum degassing and refining, and if the molten steel 3 is in a non-deoxidized state, add a necessary amount of metal Al for deoxidizing the molten steel 3 from the raw material inlet 12 to the molten steel 3 as necessary. Molten steel 3
And, if necessary, a component modifier such as C, Si, Mn or the like is added to the molten steel 3 from the raw material inlet 12 to adjust the components, and then the vacuum chamber 5 is returned to the atmospheric pressure to perform RH vacuum desorption. End gas refining.

By refining the molten steel 3 in the RH vacuum degassing apparatus 1 as described above, the molten steel 3 can be efficiently recirculated. As a result, the refining time can be reduced, the components to be removed can be reduced, and the recirculation can be performed. It is possible to reduce the amount of Ar used, extend the life of the immersion tube, and the like.

[0037]

EXAMPLE Using the RH vacuum degassing apparatus shown in FIG. 2, the inner diameter of the ascending side immersion pipe, the number and inner diameter of the gas injection nozzles, and the Ar flow rate for reflux were changed under the same molten steel reflux test conditions as in Table 1 described above. Then, a test for decarburizing and refining about 250 tons of undeoxidized molten steel having a carbon concentration of 0.03 to 0.04 mass%, which had been discharged from the converter, was subjected to a total of four levels of 28 heats. The decarburization time was set to 20 minutes in all tests, and the ultimate degree of vacuum in the vacuum chamber during that time was set to 60 to 270 Pa, and the influence of molten steel reflux conditions on the ultimate carbon concentration of molten steel during decarburization refining was investigated.

FIG. 3 shows the results of the investigation. The horizontal axis in FIG. 3 is the calculated value Z according to the above-described equation (6). As shown in FIG. 3, the absolute value of the attained carbon concentration was particularly low at Level 2 where the Ar flow rate for reflux was increased and at Level 4 where the inner diameter of the ascending-side immersion pipe was increased, as compared with Levels 1 and 3. However, when the calculated value Z on the horizontal axis is more than 20 and less than 100 in any level test, the reached carbon concentration is stably low and the decarburization reaction is promoted. Do you get it.

[0039]

According to the present invention, when refining molten steel using the RH vacuum degassing apparatus, the inner diameter of the rising side immersion pipe, the number of gas injection nozzles, the inner diameter of the gas injection nozzle, Since the molten steel is recirculated by adjusting the gas flow rate of the inert gas to a predetermined range, the molten steel can be efficiently recirculated, thereby shortening the refining time, reducing the components to be removed, and recirculating the inert gas. It is possible to achieve a reduction in the amount of gas used, a longer life of the immersion tube, and the like, which brings about an industrially beneficial effect.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a calculated value Z and a uniform mixing time.

FIG. 2 is a schematic vertical sectional view of an RH vacuum degassing apparatus used in carrying out the present invention.

FIG. 3 is a diagram showing a relationship between a calculated value Z and a reached carbon concentration.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 RH vacuum degassing apparatus 2 Ladle 3 Molten steel 5 Vacuum tank 8 Upside dipping tube 9 Downside dipping tube 10 Gas injection nozzle

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Eiji Sakurai, 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Inside Nihon Kokan Co., Ltd. (72) Yoshiyuki Nakamura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Inside the Honko Co., Ltd. (72) Inventor Atsushi Tsunoda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term in the Nippon Kokan Co., Ltd. 4K013 BA02 BA08 BA09 BA11 CA02 CA21 CC01 CE01 CE05 CE06 CE09 EA19 EA20 EA28 EA30

Claims (1)

[Claims] 1. An inner diameter (D) of a rising side immersion tube, the number of gas blowing nozzles (n) arranged in the rising side immersion tube,
The inner diameter (d) of the gas injection nozzle and the gas flow rate (G) of the recirculating inert gas injected from the gas injection nozzle are adjusted so as to satisfy the range of the following equation (1), and the rising side immersion pipe is used. A method for refining molten steel in an RH vacuum degassing apparatus, characterized in that the molten steel is refluxed by blowing an inert gas for reflux into the furnace. 20 <n / [[ρ _g / (ρ _l −ρ _g )] ^1/2 × (G / d) × D ^−3/2 ] <100 (1) where, in the equation (1), n: gas Number of blowing nozzles, ρ _g : density of inert gas for reflux (kg / m ³ ), ρ _l :
Density of molten steel (kg / m ³ ), G: flow rate of inert gas for reflux (m ³ / sec), d: inner diameter of gas injection nozzle (m),
D: Inner diameter (m) of the ascending side dip tube.