JPH059555A - Oxygen blowing method of molten metal in degassing vessel under reduced pressure - Google Patents

Abstract

(57) [Abstract] [Purpose] Efficient decarburization of molten steel under reduced pressure in a RH degassing tank or VOD ladle, or blowing to selectively heat a steel bath. Achieve the law. [Structure] When blowing oxygen gas onto the molten steel bath surface in the RH degassing tank or the VOD ladle, the area of the nozzle throat part 3 and the nozzle discharge hole are used by using a flared Laval Lance nozzle 1. The degree of vacuum determined by the product of the ratio of the nozzle primary side pressure and the secondary side pressure for obtaining the Mach number determined by 2 and the throat section pressure for obtaining the Mach number of 1 in the nozzle throat section 3 Decarburization is mainly performed by blowing under the pressure of 1, and conversely, heat raising by secondary combustion is mainly performed by blowing under a pressure of less than the vacuum degree.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to oxygen blowing of molten metal under reduced pressure, and more particularly to a method for controlling decarburization reaction by an oxygen top blowing lance and heating of a metal bath.

[0002]

2. Description of the Prior Art When decarburizing a steel bath with an oxygen top blowing lance under reduced pressure, a nozzle tip of the top blowing oxygen lance is made into a Laval shape to improve decarburization efficiency.
57-137415, JP-A-52-43717, JP-A-50-104714
And Japanese Patent Application Laid-Open No. 2-54714.

The prior art disclosed in Japanese Patent Laid-Open No. 57-137415 and the like decarburizes by designing the Laval type nozzle so that a predetermined Mach number can be obtained at the upper limit of the pressure in the tank during blowing. Is an efficient method. However, since the pressure inside the tank changes from a normal pressure of 760 Torr to a vacuum of about 0.1 Torr, it is difficult to decide which pressure should be used as the design pressure. In addition, according to the present method, among the actions of decarburization and heating, only decarburization is efficiently performed, but heating by secondary combustion of carbon monoxide gas cannot be efficiently performed. Furthermore, since the degree of vacuum is not controlled, the efficiency of the Laval nozzle cannot be achieved as designed, and the design Mach number fluctuates when the oxygen pressure is changed, which tends to deviate from the design operation range of the Laval nozzle.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and efficiently decarburizes under reduced pressure,
Another object of the present invention is to provide an oxygen blowing technique capable of selectively achieving the two heating of the metal bath by the secondary combustion of carbon monoxide.

[0005]

According to the present invention, an upper blowing lance is provided in a degassing vessel of a vacuum degassing apparatus, and oxygen gas is blown from the upper blowing lance onto a molten metal bath surface in the vessel to discharge molten metal into oxygen. A molten metal in a degassing vessel under reduced pressure, characterized in that, in the method of blowing, a Laval lance nozzle having a divergence is used, and the following steps (A) and (B) are selectively carried out individually or in combination. This is the oxygen blowing method.

(A) The ratio of the nozzle primary side pressure to the secondary side pressure "(secondary side pressure) / (primary side pressure)" for obtaining the Mach number determined by the nozzle throat area and the nozzle discharge hole area. A step of mainly performing decarburization by performing blowing under a pressure of a vacuum degree or higher determined by the product of the "throat pressure" for obtaining a Mach number of 1 in the nozzle throat. (B) The ratio of the nozzle primary side pressure to the secondary side pressure “(secondary side pressure) / (primary side pressure)” for obtaining the Mach number determined by the nozzle throat area and the nozzle discharge hole area and the nozzle throat A step of mainly performing heat rise by secondary combustion by performing blowing under a pressure lower than the vacuum degree determined by the product of the "throat pressure" for obtaining a Mach number of 1.

In the method of the present invention, the degassing vessel of the vacuum degassing apparatus is provided with an upper blowing lance, and oxygen gas is blown from the upper blowing lance onto the surface of the molten metal bath in the vessel to blow the molten metal with oxygen. A method for blowing oxygen from a molten metal in a degassing vessel under reduced pressure in which the following two steps (A) and (B) are selectively performed using a divergent Laval lance nozzle.

(A) The ratio of the nozzle throat area and the nozzle discharge hole area for obtaining the Mach number calculated from the ratio of the required degree of vacuum in the container and the "throat pressure""(discharge hole area) / ( Throat part area) "or larger process for decarburization by using a Laval lance nozzle. (B) The ratio of the nozzle throat area and the nozzle discharge hole area for obtaining the Mach number calculated from the ratio between the required degree of vacuum in the container and the "throat pressure""(discharge hole area) / (throat area) ) ”A process that mainly performs heat rise by secondary combustion by using a Laval Lance nozzle having a smaller ratio.

[0009]

[Operation] In order to carry out decarburization efficiently, it is effective to maintain the straightness of the oxygen jet and allow oxygen to reach the metal bath surface as fast as possible. On the other hand, in order to efficiently raise the temperature of the molten metal bath, the oxygen jet is diffused while maintaining a certain degree of decarburization efficiency, and the CO gas generated by decarburization from the molten metal bath and the oxygen gas are efficiently contacted. Let
Secondary combustion of CO gas is effective.

As a characteristic of the Laval-type lance, as shown in FIG. 1, in the lance 1 in which a nozzle throat portion 3 having a diameter smaller than the diameter of the nozzle discharge hole 2 is provided in the middle of the Laval lance nozzle, the lance original pressure is jetted out. The flow velocity of the nozzle throat portion 3 becomes sonic when the ambient pressure is about twice or more, and the flow rate is linearly proportional to the lance source pressure and the cross-sectional area of the nozzle throat portion 3. Mach number (ratio between the flow velocity and the discharge velocity when the nozzle throat flow velocity becomes sonic) is determined only by the lance source pressure and the atmospheric pressure, and the following equation is established.

[0011]

[Equation 1]

This relationship is illustrated in FIG. Also,
The shape of the Laval lance nozzle is determined by the target Mach number M ₂ of the nozzle discharge hole and is represented by the following equation.

[0013]

[Equation 2]

This relationship is illustrated in FIG. on the other hand,
The flow velocity W _S at the throat portion is shown by the following equation.

[0015]

[Equation 3]

Therefore, the flow velocity at the throat portion is determined only by the inlet temperature when the same gas type is considered. On the other hand, the relationship between the gas flow rate Q to be supplied and the throat section flow velocity W _S is shown below.

[0017]

[Equation 4]

From equation (4),

[0019]

[Equation 5]

From equations (3) and (5),

[0021]

[Equation 6]

## EQU3 ## From equations (3) and (6),

[0023]

[Equation 7]

From the equation (7), the throat pressure P
_S is seen to be determined by the set gas flow rate Q and the nozzle inlet gas temperature T _1, the throat cross-sectional area A _S. By substituting the Mach number M ₂ determined by the throat cross-sectional area A _S and the nozzle discharge port cross-sectional area A ₂ in formula (2) into equation (1), "Design nozzle (secondary pressure) / design nozzle (primary pressure) ", (P ₂ / P ₁₎ is calculated. In this Laval lance nozzle, the primary side pressure P ₁ is higher than the throat pressure P _S determined by the equation (7). That is, P ₁ ≧
If P _S , the gas discharge flow can maintain supersonic velocity. Therefore, by setting P ₂ ′ that satisfies P ₂ ′ ≧ (P ₂ / P ₁ ) × P _S as the degassing vessel internal pressure during operation, it is possible to maintain the blowing by the supersonic gas jet and to perform efficient decarburization. Is possible.

Even if the nozzle primary pressure = the throat portion pressure, that is, P ₁ = P _S , the pressure P of the nozzle discharge hole is P.
_In the relationship between ₂ and P ₂ ′ in the degassing vessel, P ₂ > P ₂ ′ can be established. In this state, the gas discharged from the Laval Lance nozzle is not completely expanded, and at the moment when it exits the nozzle, the gas decompresses and vibrates and expands in a straight line so as to become equal to the degassing container internal pressure P ₂ ′. Therefore, the supersonic jet is disturbed and spreads.
Therefore, the contact interface area between the flammable gas such as CO gas in the degassing container and the oxygen gas increases, the secondary combustion is promoted, and the heat is efficiently raised.

Required degassing vessel determined by metallurgical requirements
Inner vacuum degree P₂When there exists, it is calculated by equations (3) and (6)
This pressure P₂And throat pressure P_SRatio with (P₂/
P_S) Using the formula (1)
₂To calculate. This Mach number M₂From formula (2) throw
Cross-sectional area of nozzle / cross-sectional area of nozzle discharge port (A_S/ A₂) Wanted
It Designed this Laval lance Source pressure P₁Operate with
Then, (P₂/ P _S) = (P₂′ / P₁),rubber
Lurans secondary pressure is P₂’ (A₂′ /
A_S) ≧ (A₂/ A_S) In the lance, the secondary side design pressure
P₂″ Is P₂″ <P₂′, The required vacuum P₂Than
Blowing by supersonic jet can be secured up to low pressure,
Decarburization with high efficiency is possible.

In the case of a Laval lance nozzle in which (A ₂ ′ / A ₂ ) <(A ₂ / A _S ) with respect to the required degassing container pressure P ₂ , the nozzle outlet when the lance source pressure P ₁ = P _S The pressure P ₂ ′ is P ₂ ′> P ₂ . Therefore, the supersonic oxygen gas jet goes straight to the surroundings while expanding in a vibrational manner from the moment of being discharged from the nozzle into the degassing container. Therefore, the jet is turbulent, the contact interface area between the flammable gas such as CO gas in the degassing container and the oxygen gas is increased, and the heat is raised by the efficient secondary combustion.

[0028]

[Example] Using an RH degassing equipment with a circulation flow rate capable of circulating 230 tons / heat of molten steel that has been primary decarburized and refined in a converter in 1 minute in 2 minutes, oxygen is blown by top blowing at an oxygen flow rate of 18.7 Nm ³ / min I made a fortune. Example 1 A laval lance nozzle having an opening ratio (A ₂ / A _s ) of 8.5 was used. The critical pressure P ₂ ′ for obtaining the supersonic jet of this lance is 23 Torr. The atmospheric pressure was fixed at 30 Torr by controlling the exhaust capacity by controlling the number of stages of steam booster and / or controlling the leak air flow rate.

Example 2 A Laval lance nozzle having an opening ratio (A ₂ / A _S ) of 4.5 was used. Critical pressure P to obtain supersonic jet with this lance
₂ 'is 56 torr. The tank pressure was set to 30 as in Example 1.
The atmospheric pressure was fixed to Thor. Example 3 The operation was carried out by controlling the atmospheric pressure at 30 Torr and fixing the atmospheric pressure. The Laval lance nozzle corresponding to 30 Torr has an aperture ratio of 6.8, but at this time, a lance with an aperture ratio of 8.5 was used.

Example 4 The operation was carried out by controlling the atmospheric pressure at 30 Torr and fixing the atmospheric pressure. The Laval Lance nozzle has an aperture ratio of 6.8, which corresponds to 30 Torr. At this time, a Laval Lance nozzle with an aperture ratio of 4.5 was used. Decarburization (1) and (3) in the above examples, and temperature rise (2) and (4) are mainly.
Have the same operating conditions. Lance source pressure is 2
kg / cm ² -G, oxygen blowing time was 20 minutes.

Comparative Example As a comparative example, a straight type single hole lance was used, and the operation was carried out at 50 to 30 Torr. The processing time was 20 minutes. Table 1 shows a comparison of the operation results.

[0032]

[Table 1]

As described above, in Examples (1) and (3) of the method of the present invention aimed at improving the decarburization efficiency, decarburization was extremely good and could be reduced to 25 ppm. Moreover, the effect of suppressing the increase of total Fe in the slag was also brought about by the increase of the utilization efficiency of oxygen. In Examples (2) and (4) directed to raising the temperature of molten steel, a large deceleration was obtained by the shock wave generated in the space after discharge in the case of the straight nozzle due to the shock wave in the Laval, and the soft blow was made more The temperature drop was improved by 13 ° C compared to the comparative example. Compared with the comparative example, the increase of total Fe in the slag was not seen so much.

In the above-mentioned embodiment, an example using the RH degassing equipment is described, but a metal bath is housed in a VOD ladle or a vacuum vessel, and oxygen blowing is performed on this metal bath by using an upper blowing lance. It goes without saying that the present invention can be applied to other degassing equipment for carrying out.

[0035]

According to the method of the present invention, the operating pressure is controlled with respect to the design pressure determined by the Laval lance nozzle. Or
By selecting and selecting Laval Lance for operating pressure, decarburization efficiency and molten metal heating rate can be selectively improved,
Moreover, after the treatment, T. Fe decreases or hardly increases as compared with the process method.

Therefore, the processing time is shortened, and Al and Si for heating
And the like, and the refining oxygen unit consumption can also be reduced.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a Laval lance nozzle according to the present invention.

FIG. 2 is a characteristic diagram showing a relationship between an outlet Mach number (M ₂ ) of a nozzle discharge hole and [atmospheric pressure (P ₂ ) / lance original pressure (P ₁ )].

FIG. 3 is a characteristic diagram showing the relationship between the Mach number (M ₂ ) at the outlet of the discharge hole of the nozzle and [the cross-sectional area of the throat portion (A ₁ ) / the cross-sectional area of the discharge hole (A ₂ )].

[Explanation of symbols]

1 Laval lance nozzle 2 nozzle discharge holes 3 Nozzle throat

Claims (2)

[Claims] 1. A method for blowing oxygen into a degassing vessel of a vacuum degassing apparatus, the method comprising blowing oxygen gas onto a molten metal bath surface in the vessel from the top blowing lance, and blowing the molten metal with oxygen. The method for blowing oxygen of molten metal in a degassing vessel under reduced pressure, which comprises selectively performing the following steps (A) and (B) individually or in combination by using the Laval lance nozzle described in 1. (A) The ratio of the nozzle primary side pressure to the secondary side pressure “(secondary side pressure) / (primary side pressure)” for obtaining the Mach number determined by the nozzle throat area and the nozzle discharge hole area and the nozzle throat Decarburization mainly by carrying out blowing under a pressure higher than the vacuum degree determined by the product of the "throat pressure" for obtaining a Mach number of 1. (B) The ratio of the nozzle primary side pressure to the secondary side pressure “(secondary side pressure) / (primary side pressure)” for obtaining the Mach number determined by the nozzle throat area and the nozzle discharge hole area and the nozzle throat A step of mainly performing heat rise by secondary combustion by performing blowing under a pressure lower than the vacuum degree determined by the product of the "throat pressure" for obtaining a Mach number of 1. 2. A method for blowing oxygen into a degassing vessel of a vacuum degassing apparatus, wherein a blowing lance is provided in the degassing vessel, and oxygen gas is blown onto the molten metal bath surface in the vessel from the blowing lance. The oxygen blowing method for molten metal in a degassing vessel under reduced pressure according to claim 1, wherein the following two steps (A) and (B) are selectively performed by using the Laval lance nozzle. (A) The ratio of the nozzle throat area and the nozzle discharge hole area to obtain the Mach number calculated from the ratio between the required degree of vacuum in the container and the "throat pressure""(discharge hole area) / (throat area) ) ”The above steps mainly perform decarburization by using the Laval Lance nozzle. (B) The ratio of the nozzle throat area and the nozzle discharge hole area for obtaining the Mach number calculated from the ratio between the required degree of vacuum in the container and the "throat pressure""(discharge hole area) / (throat area) ) ”A process that mainly performs heat rise by secondary combustion by using a Laval Lance nozzle having a smaller ratio.