JPH10330831A - Method for denitrifying molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an extra-low nitrogen molten steel having the same grade as the conventional method in shorter treating time than the conventional method by blowing gas into molten steel from the side wall of a vacuum lower vessel in an RH degassing equipment. SOLUTION: While supplying desulrizing agent into the molten steel 2a in an uptake tube 4, the molten steel in the lower extending direction of the uptake tube 4 or the molten steel 2a' in the upper extending direction thereof, inert gas and/or CO gas are blown into the molten steel 2b in the vacuum vessel from the lower vessel side wall of the vacuum vessel 6. Desirably, the dephosphorizing agent contains CaO and the injecting speed WCa O' (kg/min/ton) of this CaO content satisfies 1.75×10<-3> CS Q/W<=WCa O'<=10/t [wherein, CS is S concn. (ppm) in the molten steel before desulfurizing, Q is the circulating quantity (ton/min) of the molten steel, W is the molten steel quantity (ton) and (t) is desulfurize-treating time (min)]. Further, the blowing speed Gside (Nl/min) of the inert gas and/or the CO gas blown from the side wall part is controlled so as to satisfy 100<=Gside /A<=4000 [wherein, Gside is the gas blowing quantity (Nl/min) from the side wall part of the vacuum vessel and A is the inner part cross sectional area (m<2> ) of the lower vessel].

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently denitrifying molten steel using an RH vacuum degassing apparatus.

[0002]

2. Description of the Related Art With the increasing demand for higher performance and higher quality of steel materials, it has been desired to reduce impurity elements in steel to the utmost. For this reason, technology for purifying and purifying steel in the molten steel stage is required, and nitrogen, one of the impurity elements in steel, is reduced to improve workability of steel. There must be. From such a background, extremely low nitrogen steel in which N has been removed from molten steel using an RH degassing apparatus has been produced.

For example, Japanese Patent Application Laid-Open No. 63-57715 discloses that a mixed gas of CO and Ar is blown into molten steel to obtain a low-nitrogen steel in order to obtain a denitrification reaction area in a vacuum degassing apparatus. A method (hereinafter referred to as Prior Art 1) is disclosed.

Further, Japanese Patent Laid-Open Publication Nos.
Nos. 18 and 3-61319 disclose a method of blowing low-nitrogen steel by blowing a mixed gas of CO and Ar into molten steel from a vacuum chamber side wall in order to obtain a denitrification reaction area in a vacuum degassing apparatus. (Hereinafter referred to as Prior Art 2).

On the other hand, since S in molten steel is a surface active element, S is adsorbed on the reaction interface where the denitrification of molten steel proceeds, thereby preventing denitrification. On the other hand, for example, Japanese Unexamined Patent Publication No. 63-161
No. 113 discloses a method of performing a denitrification after lowering the S concentration that hinders the denitrification reaction of molten steel in advance (hereinafter referred to as prior art 3).

[0006]

However, in the prior arts 1 and 2, when the S concentration in the molten steel is high, the denitrification reaction is hindered, so that nitrogen cannot be reduced efficiently. According to the prior art 3, since the S that interferes with the denitrification is removed in advance and then the denitrification treatment is performed, an extremely low nitrogen steel can be obtained.
However, since the treatment time is long, the heat loss of the molten steel is large, and an extra energy cost is required.

Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide a method for denitrifying molten steel by which molten steel can be efficiently treated by an RH degassing apparatus and the nitrogen concentration can be reduced to the limit. It is in.

[0008]

Means for Solving the Problems From the above-mentioned viewpoints, the present inventors have intensively studied to develop a method for denitrification of molten steel. In order to promote the denitrification reaction of molten steel, it is necessary to increase the interfacial area of the denitrification reaction, reduce the concentration of surface active elements in the molten steel, and reduce the amount of active elements adsorbed on the interface of the denitrification reaction. It is important to minimize the disturbance of the denitrification reaction by the surface active elements as much as possible, and to increase the supply rate of the undenitrified molten steel to the denitrification reaction interface.

In order to maximize the area of the denitrification reaction zone by blowing gas into the molten steel in the RH vacuum degassing tank, the gas is blown from the side wall of the lower tank of the vacuum tank rather than blown from the rising pipe for refluxing the molten steel. A larger amount of gas bubbles can be formed in the molten steel because a larger amount of gas can be blown from the molten steel. Therefore, a wider gas-liquid interface can be provided as the denitrification reaction interface. In this case, the position of the gas injection nozzle is a side wall part avoiding the position near the downcomer pipe,
If gas is blown from there, it will not obstruct the molten steel reflux.

[0010] It is more advantageous for denitrification that the denitrification is performed in a state where the amount of the surface active element, particularly the S atom, adsorbed on the thus formed denitrification reaction interface is small. For this purpose, the desulfurizing agent is added to the molten steel inside the riser pipe or to a place where the molten steel is strongly stirred, such as the molten steel surface in the vacuum chamber immediately after flowing out of the riser pipe, and the denitrification gas is supplied to the molten steel thus desulfurized. Good to blow. As described above, the denitrification gas is blown from the side wall of the lower tank of the vacuum tank. In this way, the molten steel desulfurized in the vacuum chamber is recirculated to the ladle and can be denitrified before the desulfurization effect is reduced by the undesulfurized molten steel in the ladle. It is possible to denitrify with less adsorbed S atoms. Therefore, the denitrification of the molten steel is performed efficiently.

On the other hand, if a large amount of the reflux gas or the carrier gas for injecting the desulfurizing agent into the riser tube is blown in too much, the gas will blow through the riser tube, and the molten steel will not be sufficiently refluxed. Therefore, the reflux gas or the carrier gas for blowing the desulfurizing agent from below the riser or the riser is limited to a gas flow rate necessary for preventing clogging of the gas and the desulfurizing agent blowing nozzle and ensuring the function of blowing the desulfurizing agent.

With the above arrangement, the area of the denitrification reaction in the above items (1) to (4) is increased, the interference of the denitrification reaction by surface active elements, particularly S atoms, is eliminated, and the undenitrification reaction to the denitrification reaction interface is prevented. The supply speed of molten steel can be increased. Therefore, the denitrification reaction of molten steel can be promoted.

[0013] The present invention has been made based on the above-mentioned findings.
In a method of denitrifying molten steel by injecting gas into molten steel in a vacuum tank from a side wall of a vacuum tank of a H vacuum degassing apparatus, a desulfurizing agent is added to molten steel in a riser pipe, molten steel in a downward extension direction of a riser pipe, or riser. It is characterized in that an inert gas and / or a CO gas is blown into the molten steel in the vacuum chamber from the side wall of the vacuum chamber while supplying the molten steel in the upwardly extending direction of the pipe.

According to a second aspect of the present invention, there is provided a method for denitrification of molten steel according to the first aspect of the present invention, wherein a desulfurizing agent containing CaO is used, and the supply of the desulfurizing agent is performed by blowing the amount of CaO contained in the desulfurizing agent. The velocity W _CaO ′ is controlled so as to satisfy the following equation (1), and the blowing of the inert gas and / or the CO gas blown into the molten steel from the side wall of the vacuum chamber is performed by the inert gas and / or CO gas. It is characterized in that the blowing speed G _side of the CO gas amount is controlled so as to satisfy the following equation (2).

[0015] _{^{1.75 × 10 -3 C S Q /}} W ≦ W CaO '≦ 10 / t ---------- (1) where, W _CaO': supply rate in the desulfurizing agent CaO amount (Kg / min / to
n) C _S : S concentration of molten steel before desulfurization (ppm) Q: Ring flow rate of molten steel (ton / min) W: Quantity of molten steel (ton) t: Desulfurization treatment time (min) 100 ≦ G _side / A ≦ 4000 --- ------------------------- (2) Here, G _side : The amount of gas blown from the vacuum chamber side wall (Nl / min) A: Internal cross-sectional area of vacuum tank lower tank (m ² )

[0016]

Next, the present invention will be described with reference to the drawings. 1 and 2 are conceptual cross-sectional views of an example of an RH degassing apparatus used when carrying out the method of the present invention. FIG. 1 is a conceptual diagram of a vertical section when molten steel in a ladle is denitrified by an RH degassing device, and FIG. 2 is a view of the vacuum chamber of FIG. 1 and 2, 1 is a ladle,
2a is the circulating molten steel in the riser, 2a 'is the molten steel flowing out of the riser, 2b is the molten steel in the vacuum vessel, 4 is the riser, 5 is the descender, 6 is the vacuum vessel, and 7 is the lower part of the riser. Reference numeral 10 denotes a gas injection pipe for circulating molten steel, and reference numeral 10 denotes an alloy charging chute. As a desulfurizing agent supply device, a powder blowing lance 8 and a powder blowing lance 9 are provided. Numeral 11 denotes a gas injection nozzle whose mounting height is lower than the side wall of the vacuum chamber 6 and which is molten steel 2b in the vacuum chamber during the RH degassing process.
A plurality is provided in the circumferential direction at a position lower than the surface level, and the tip is opened inside the molten steel 2b. As shown in FIG. 2, a plurality of gas injection nozzles 11 (in this case, 8
Book) is provided.

In the RH vacuum degassing apparatus, the internal pressure of the vacuum chamber 6 is reduced to a predetermined pressure, and the reflux gas is blown from the reflux gas blowing pipe 7 for the molten steel 2 to the molten steel 2 a in the rising pipe 4. Thus, while exposing the molten steel 2 in the ladle 1 to the reduced-pressure atmosphere in the vacuum chamber 6, the molten steel 2 is raised from the riser pipe 4 and lowered from the descending pipe 5 to be circulated. Next, a powdery desulfurizing agent is blown upward from the powder blowing lance 8 by a pneumatic feed from directly below the riser 4 to the molten steel 2a, or the powdery desulfurizing agent flows out of the riser 4 from the powder blowing lance 9. A powdery desulfurizing agent is sprayed on the surface of the molten steel 2a 'by air. In addition, the desulfurizing agent may be injected into the same position by using an alloy injection chute 10 instead of the powder spraying lance 9. Thus, the molten steel 2b flowing into the vacuum chamber 6 is desulfurized. On the other hand, while desulfurizing the molten steel, the inert gas and / or C
O gas is blown into the molten steel 2b in the vacuum chamber 6. The molten steel 2b is immediately after desulfurization by the desulfurization treatment. The desulfurized molten steel refluxes and enters the ladle 1, where the entire molten steel is desulfurized and denitrified.

The desulfurizing agent desirably contains CaO from the viewpoint of achieving a large desulfurizing effect and reducing the cost. The amount of the desulfurizing agent added to the molten steel (kg / molten steel-t) is determined according to the S concentration before the desulfurization treatment and the target S concentration after the treatment.

FIG. 3 shows that in the present invention, C
Regarding the supply rate W _CaO ′ of the aO component and the gas injection amount G _side from the side wall of the vacuum chamber, a more desirable area is indicated by oblique lines. Hereinafter, W _CaO 'and G _side with reference to FIG.
Is described below.

[0020] (i) W _CaO 'supply amount W _CaO of CaO content for the desired range, during the desulfurizing agent' of, 1.75 ×
More preferably, it is 10 ^-3 C _S Q / W or more. In this case, since the supply amount of the desulfurizing agent is sufficient, the S concentration of the molten steel supplied into the vacuum chamber is sufficiently reduced, and the denitrification reaction is further promoted. More preferably, W _CaO ′ is limited to 10 / t or less.
This is because, in this case, even if the desulfurizing agent is supplied, there is no concern at all that an operational problem due to a temperature drop of the molten steel occurs.

(B) Desirable range of G _side / A ・ Amount of gas blown per _side cross-sectional area of the vacuum chamber G _side / A
A is more preferably smaller than 100 Nl / min / m ² . In this case, a sufficient increase in the area of the denitrification reaction interface is ensured, so that the denitrification reaction is further promoted.・ If the amount of gas blown into the vacuum chamber becomes too large, the denitrification reaction will be accelerated, but the amount of molten steel splash caused by the gas will increase, and this will adhere to the vacuum chamber side wall and melt out during the subsequent molten steel processing. Molten steel is contaminated, causing operational problems. From this viewpoint, it has been found that when the gas injection amount G _side / A per inner cross-sectional area of the vacuum chamber is 4000 Nl / min / m ² or less, no problem occurs, and it is more desirable. Therefore, this value was set as the upper limit.

(C) Desirable range of denitrification rate coefficient: K _N Next, the rate equation of the denitrification reaction of the molten steel is considered to be represented by the following second-order rate equation of the following equation (3).

−dN _t / dt = k ′ (A / V) N _t ² -------------------- (3) where N _t : start of denitrification N concentration at the rear time t (wt%.) k ' : apparent reaction rate constant (cm / wt% / min. ) a: denitrification interfacial area (cm ²⁾ V: molten steel volume in the vacuum chamber (cm ³ The following equation (4) derived from the equation (2) represents a denitrification reaction rate.

K _N t = (1 / N _t ) − (1 / N ₀ ) --- (4) where K _N ≡k '(A / V) where K _N : denitrification rate constant (1 / wt.% / Min) t: elapsed time of denitrification (min) N ₀ : N concentration at the start of denitrification (t = 0) (wt Therefore, in order to denitrify to a desired N concentration within a predetermined denitrification treatment time, the denitrification rate coefficient K _N must be increased to a certain value or more. In order to obtain the extremely low nitrogen molten steel to be obtained by the method of the present invention, the value of K _N is set to 7 (1 / wt.%
/ Min) or more, and more preferably 15 (1 / wt.% / Min) or more. For this purpose, the supply rate of the CaO amount in the desulfurizing agent: W _CaO ′ (kg
/ Min / ton) and the amount of gas blown from the vacuum chamber side wall:
It has been found that G _side (Nl / min) satisfies the expressions (1) and (2), respectively.

[0025]

Next, the method for denitrifying molten steel of the present invention will be described in more detail with reference to examples. The low-carbon, low-sulfur molten steel discharged from the converter at 250 t / charge into a ladle was desulfurized and degassed by an RH degassing facility. The tests of the molten steel denitrification method within the scope of the present invention (Examples 1 to 22) and the molten steel denitrification method out of the scope of the present invention (Comparative Examples 1 and 2) were conducted using the RH degassing equipment shown in FIGS. This was performed using

Tables 1 and 2 show the test conditions of Examples 1 to 22 and Comparative Examples 1 and 2.

[0027]

[Table 1]

[0028]

[Table 2]

The S concentration of molten steel before desulfurization treatment is 20-35 p
The pm and N concentrations are 25 to 40 ppm. Also, the composition and method of addition of the desulfurizing agent, the conditions of gas injection from the side wall of the lower tank of the vacuum tank, and the total flow rate of the reflux gas blown into the riser and the carry gas blown from the lower part of the riser with the desulfurizer Are as follows. However, in Comparative Example 1, the desulfurizing agent was supplied to the molten steel in the downcomer of the vacuum tank, and in Comparative Example 2, gas was not blown from the side wall of the vacuum tank lower tank.

Desulfurizing agent = CaO-CaF ₂ -MgO based flux CaO: 50 to 70 wt.%, CaF ₂ : 10 to 50 wt.%, MgO: 20 wt.% Or less (Using the powder blowing lance 8 in FIG. 1) or spraying the molten steel surface flowing out of the riser pipe (using the powder blowing lance 9) Desulfurizing agent addition rate = 8/258 to 180/255 kg / Min / ton = 0.031 to 0.706 kg / min / ton-Type of gas to be blown from the lower tank side wall = Ar gas 40 to 2000 Nl / min / (number of lances: 8)-Blown into riser Gas = Ar gas Total: 3500 (Nl / min) / 250 (ton) The above test results are also shown in Tables 1 and 2.

As is clear from Tables 1 and 2, in Examples 1 to 14, the denitrification rate constant K _N was 15
(1 / wt.% / Min) or more, and the N concentration of the molten steel falls to 20 ppm or less within a predetermined denitrification time. In Examples 15 to 22, the denitrification rate constant K _N is 7
(1 / wt.% / Min) or more, and the N concentration of the molten steel dropped to 25 ppm or less within a predetermined denitrification time, and in each case, extremely low nitrogen molten steel was efficiently obtained.

On the other hand, in Comparative Example 1, since the desulfurizing agent was supplied into the downcomer of the vacuum chamber, the denitrification rate constant K _N was 2.3 (1 / wt.% / Min).
In Comparative Example 2, gas was not blown from the side wall of the lower chamber of the vacuum chamber, and gas was blown into molten steel in the vacuum chamber.
Only the reflux gas and the carrier gas for blowing the desulfurizing agent from below the riser. Therefore, the denitrification rate constant K _N is 1.1
(1 / wt.% / Min), none of which promoted denitrification within a predetermined denitrification time.

Further, according to the method of the present invention, the denitrification treatment is performed while the desulfurization treatment is performed in the RH degassing equipment, so that the method is shorter in time than the conventional method of first desulfurizing molten steel and then denitrification treatment. It can be seen that denitrification treatment can be performed by

[0034]

As described above, according to the present invention,
With RH degassing equipment, extremely low nitrogen molten steel can be more efficiently used than before.
In addition, it is possible to provide a method for denitrification of molten steel that can be manufactured in a stable operation, and an industrially useful effect is obtained.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view of an example of an RH degassing apparatus when denitrifying molten steel in a ladle by the method of the present invention.

FIG. 2 is a view of the vacuum chamber of FIG.

FIG. 3 shows the supply rate W _CaO ′ of the CaO component in the desulfurizing agent and the gas blowing rate G from the side wall of the vacuum chamber.
It is a conceptual diagram which shows the area | region which is desirable to be filled with _side .

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2 Molten steel 2a Recirculating molten steel in a riser pipe 2a 'Molten steel flowing out of a riser pipe 2b Molten steel in a vacuum tank 3 Slag in a ladle 4 Rise pipe 5 Downcomer pipe 6 Vacuum tank 7 Recirculation gas injection pipe 8 Powder blowing Lance 9 Powder spray lance 10 Alloy injection chute 11 Gas injection nozzle

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroaki Ishikawa 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Eiji Sakurai 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Sun Honko Co., Ltd.

Claims (2)

[Claims] 1. A method for denitrifying molten steel by blowing gas into the molten steel in the vacuum chamber from a side wall of a vacuum chamber of an RH vacuum degassing apparatus, the method comprising:
Injecting inert gas and / or CO gas into the molten steel in the vacuum chamber from the side wall of the vacuum chamber while supplying the molten steel in the lower extension direction of the riser pipe or the molten steel in the upper extension direction of the riser pipe. Characteristic method of denitrification of molten steel. 2. The desulfurizing agent contains CaO, and when the desulfurizing agent is supplied, the desulfurizing agent blowing rate W _CaO ′ in terms of the CaO content is determined by the following formula (1): 1.75 × 10 ⁻³ C _S Q / W ≦ W _CaO ′ ≦ 10 / t (1) where W _CaO ′: supply rate of CaO amount in desulfurizing agent (kg / min / to
n) C _{S: S} concentration desulfurization before the molten steel (ppm) Q: molten steel ring flow (ton / min) W: amount of molten steel (ton) t: meet desulfurization treatment time (min), and the sidewall portion of the vacuum chamber When the inert gas and / or CO gas is blown into the molten steel from above, the blowing speed G _{side of the} inert gas and / or CO gas amount is expressed by the following formula (2): 100 ≦ G _side / A ≦ 4000 ---------------------------- (2) where, G _side : The amount of gas blown from the vacuum chamber side wall (Nl / min) A: The method for denitrification of molten steel according to claim 1, wherein the control is performed so as to satisfy the internal cross-sectional area (m ² ) of the lower chamber of the vacuum chamber.