JPH0459913A - Oxygen blowing method into molten metal under reduced pressure - Google Patents

Abstract

PURPOSE:To selectively achieve improvement of decarbonizing efficiency or improvement of raising temp. to molten metal by controlling the ratio of original pressure of a lance to vacuum degree in comparison with Mach number decided with the ratio of cross-sectional area at throat part to that at discharging hole in a Laval type lance. CONSTITUTION:In the method of oxygen-blowing to the molten metal under reduced pressure by blowing the oxygen gas on the molten metal surface in a vessel from a water cooled top blowing lance, by using the Laval type lance nozzle having widening at the top part, while measuring the vacuum degree in the vessel and oxygen supplying pressure and oxygen flow rate to the lance, at least one of the vacuum degree, oxygen supplying pressure or oxygen flow rate is controlled so that the ratio of the oxygen supplying pressure and the vacuum degree becomes the constant. Further, either of an operation in which the oxygen supplying pressure is constant and decarbonizing reaction having good efficiency is executed by controlling the vacuum degree so as to become the pressure to be the ratio of the oxygen pressure and the vacuum degree developing the value larger than the Mach number calculated from the ratio of the area of nozzle throat part to the area of nozzle discharging hole, or an operation in which the raising temp. to the molten metal is efficiently executed by controlling the vacuum degree so as to become the pressure to be the ratio developing the value smaller than the Mach number, is selectively executed.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to oxygen blowing of molten metal under reduced pressure.
In particular, it relates to a method for controlling the decarburization reaction using an oxygen top blowing lance and the heating up of a metal bath.

<Conventional Technique> When decarburizing a steel bath using an oxygen top-blowing lance under reduced pressure, the technique disclosed in JP-A-54-137421 is a method for increasing the decarburization efficiency in the low-grade region. be. Furthermore, Japanese Patent Application Laid-Open No. 57-237415 proposed a method of increasing decarburization efficiency by making the nozzle tip of the top-blown oxygen lance into a Laval shape.
There is a technique disclosed in Japanese Patent Publication No. 61-57886.

Technique 44i disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 54-137421
When decarburizing a high Crigtail in a vacuum, the effective open cross-sectional area of the lance nozzle is changed according to the C'a degree in 18fiiA, and decarburization is performed to an extremely low Cf of 4 degrees. be. However, in this method, when considering actual operation, it is impossible to change the lance tip diameter during operation with a single single-tube lance, and if a two-tube lance or a multi-tube lance is used, the equipment becomes complicated. Furthermore, the use of a type that includes a valve body and its drive system at the tip of the lance tip to change the cross-sectional area has the problem of requiring an excessively large cooling structure due to high heat.

The technique disclosed in JP-A No. 57-137415 is a method for efficiently decarburizing by designing a Laval type nozzle so that a predetermined Matsuha number can be obtained at the upper limit of the pressure inside the tank during blowing. However, the pressure inside the tank is 760 Torr.
Since the pressure varies from normal pressure of r to vacuum of about 0.1 orr, it is difficult to determine which pressure should be used as the design pressure. Moreover, according to the nature of the decarburization and heat raising effects, only the decarburization can be performed efficiently, but the heat raising cannot be performed efficiently.

Furthermore, since the degree of vacuum is not controlled, the efficiency of the Laval nozzle cannot be achieved as designed, and when the oxygen pressure is changed, the designed Matsuha number fluctuates, making it easy to deviate from the designed operating range of the Laval nozzle.

<Problems to be Solved by the Invention> The present invention solves the above-mentioned problems of the prior art and provides oxygen blowing that can selectively achieve either efficient decarburization at reduced pressure or heating of a steel bath. This was done to provide technology.

<Means for Solving the Problems> The present invention consists of: (1) A water-cooled top-blowing lance is provided in the degassing tank of a vacuum degassing device, and oxygen gas is blown from the water-cooled top-blowing lance onto the molten metal bath surface in the tank to melt the metal. In the method of oxygen blowing metal, a Laval-type lance nozzle with a flared tip is used to measure the degree of vacuum in the tank, the oxygen supply pressure to the lance, and the oxygen flow rate, and the ratio between the oxygen supply pressure and the degree of vacuum. A method for oxygen blowing molten metal under reduced pressure, which is characterized by controlling at least one of the degree of vacuum, oxygen supply pressure, or oxygen flow rate so that the A water-cooled top blowing lance is installed inside the tank, and oxygen gas is supplied from this water-cooled LX blowing lance into the tank? 8 In the method of blowing molten metal on the surface of a fused metal bath with oxygen, the oxygen supply pressure is kept constant, and the ratio between the pressure and the degree of vacuum is calculated from the ratio of the nozzle throat area and the nozzle discharge opening area. The degree of vacuum is controlled so that the pressure is at a ratio that can generate a value larger than the Matsuha number, and an efficient decarburization reaction is performed, and the pressure is adjusted to a ratio that generates a value smaller than the Matsuha number. This is an oxygen blowing method for molten metal under reduced pressure, which is characterized by selectively performing an operation to efficiently raise the temperature of molten metal by controlling the degree of vacuum. In the method of blowing the molten metal with oxygen by equipping the gas tank with a water-cooled top blowing lance and blowing oxygen gas from the water-cooled top blowing lance onto the surface of the molten metal bath in the tank, the ratio between the oxygen supply pressure and the degree of vacuum is An operation to efficiently perform a decarburization reaction by controlling the oxygen flow rate or oxygen supply pressure to a pressure that can generate a value larger than the Matsuha number calculated by the ratio of the nozzle throat area and the nozzle discharge opening area. under reduced pressure, characterized by selectively performing an operation to efficiently raise the temperature of molten metal by controlling the oxygen flow rate or oxygen supply pressure such that the ratio of This is a method of oxygen blowing molten metal.

<Function> To achieve efficient decarburization, it is effective to maintain the straightness of the oxygen jet and allow the oxygen to reach the steel bath surface as fast as possible.

On the other hand, in order to raise the temperature of the molten metal bath efficiently,
It is effective to diffuse the oxygen jet while maintaining a certain degree of decarburization efficiency, to bring the CO gas generated by decarburization from the molten metal bath into efficient contact with the oxygen gas, and to perform secondary combustion of the CO gas.

By the way, as a characteristic of the Laval type lance, in a lance that has a nozzle (called a throat) with a smaller diameter than the lance discharge port diameter in the middle of the lance nozzle, if the lance source pressure is about twice the atmospheric pressure from which the jet is ejected, the nozzle throat The partial flow velocity becomes the sonic velocity, and the flow rate is linearly proportional to the lance source pressure and the nozzle throat cross-sectional area. and the discharge speed) is determined only by the lance source pressure and the atmospheric pressure, and the following equation holds.

M.

W! Unit Sl Here, M2: Nozzle discharge Romanoha number, W2: Nozzle discharge flow velocity, W, Nozzle throat flow velocity (=sonic velocity), Pl: Lance source pressure, P2: Atmospheric pressure, K: Specific heat ratio.

This relationship is illustrated in FIG. 1.

Further, the shape of the Laval nozzle is determined by the target Matsuha number M2 of the nozzle discharge port, and is expressed by the following equation.

Unit Sl here, D, varnish throat diameter, D: Nozzle discharge opening diameter, A, cross-sectional area of the throat part, A: Cross-sectional area of nozzle outlet.

This relationship is illustrated in FIG. 2.

When using a lancet tube designed in this way, the first
From Fig. 2, the following can be said.

■ Atmospheric pressure P2 and oxygen supply pressure (= lance source pressure) P
By keeping the ratio constant, a nozzle installed at a certain Matsuha number can produce an oxygen jet flow with almost no spread at supersonic speeds at a Matsuha number M2, as shown on the curved surface in Figure 1. When blowing is carried out using zero jets, the steel bath can be reached with the energy loss of the oxygen jet suppressed and the scratching force suppressed, which allows the oxygen jet to penetrate deeper and improve the stirring power, resulting in decarburization. The reaction can be carried out efficiently.

■ Matsuha number Mz calculated by the ratio of P and P
' is the actual Laval nozzle A, and Matsuha number M calculated by A, M t ' ≧M! If this is done (region (B) in FIG. 1), the jet will generate pressure vibrations at the outlet of the discharge port while maintaining its straightness. This is a case where P2' ≦P2 for the pressure Pz' calculated backward from Mz and P+ in equation (1), and P, '≧P1 for Fゝ1' calculated backward from M2 and P2. This is the case.

In either case, some of the energy of the oxygen jet is taken away by the pressure oscillations, but straightness is maintained by the laval, so secondary combustion of CO gas hardly occurs, and oxygen reaches the steel bath efficiently. However, decarburization can be carried out efficiently.

■ Contrary to the case of ■, if we set M 2 '< M 2 (region (C) in Figure 1), the jet will be at the speed of sound at the Laval throat and at the exit of the Laval. It generates a shock wave within the Laval and decelerates. Therefore, energy is lost, and the discharge condition becomes a wide-angle spreading flow similar to discharge from a straight nozzle, so that soft blow is achieved and secondary combustion of CO gas can be performed efficiently.

Based on the above knowledge, it was concluded that the following method is effective for efficient decarburization, vacuuming, and heating of molten metal. In other words, when performing oxygen blowing of molten metal using a Laval nozzle under reduced pressure, in order to decarburize efficiently, ■ Maintain the ma.7 h number as the designed value under a constant exhaust capacity. The lance source pressure is changed to follow the atmospheric pressure that changes during operation. When the flow rate is controlled by a flowing silk valve, the flow rate itself is changed and the flow rate source pressure is changed to follow it.

Alternatively, the exhaust capacity may be controlled at a constant flow rate.

(2) The degree of vacuum is controlled by controlling the exhaust capacity so that M2' ≧M2.

■ Also, control the lance source pressure or the oxygen flow rate itself.

In order to efficiently heat up the molten metal, M t
At least one of the degree of vacuum, lance source pressure, and oxygen flow rate is controlled so that '< M z.

<Example> Using RH degassing equipment with a circulation flow rate that can circulate molten steel of 230 T/ch that has been primary decarburized and refined in a converter in 2 minutes, oxygen blowing was performed at an oxygen flow rate of 15 N%/min by top blowing. went.

(1) Aperture ratio A z / A s = 2.45, calculation M
, = ], , 100 To
The atmospheric pressure was fixed at rr. At that time, the lance source pressure is 1
kg/ci-G. The processing time was 20 minutes.

(2) Aperture ratio A, /As = 2.45, calculation M! =
Using L and S Laval-type single-hole lances, the exhaust capacity was controlled and operated between 100 and 50 Torr. The lance source pressure was 1 kg/cd-G, and the treatment time was 20 minutes.

(3) Aperture ratio A, /As = 4.94, calculation M! =
Using a 2.0 Laval type single-hole lance, the exhaust capacity was controlled and operated between 100 and 50 Torr. The lance source pressure was Ikg/crl-G, and the processing time was 20 minutes.

(4) As a comparative example, a process method straight type single hole lance was used and operated at 100 to 50 TorrO. The processing time was 20 minutes.

The operational results are compared and shown in Table 1.

As shown in Table 1 and above, in Examples (1) and (2) of the present invention method aimed at improving decarburization efficiency,
2 even in relatively poor vacuum conditions of 0 Torr.
A post-treatment carbon concentration of 20 to 25 carbon atoms was achieved in 0 minutes, and the decarburization efficiency was improved compared to the conventional comparative example.

Furthermore, the increase in oxygen utilization efficiency also had the effect of suppressing increases in T and Fe.

In Example (3), which is aimed at raising the temperature of molten steel, a large deceleration is obtained by the shock wave generated in the Laval in the space after discharge in the case of a straight nozzle,
Because the blow was made softer, the temperature drop was improved by 13°C compared to the process method. Compared to the comparative example, no significant increase in T and Fe was observed.

In this example, the degree of vacuum in the tank was controlled, but it is clear from theory that the same results can be obtained even if the lance source pressure or the oxygen flow rate is controlled.

<Effects of the Invention> According to the method of the present invention, by controlling the ratio between the lance source pressure and the degree of vacuum with respect to the Matsuha number determined by the ratio of the throat section of the Laval-type lance to the cross-sectional area of the discharge port, ■ decarburization efficiency can be improved. (2) It is possible to selectively improve the degree of heating of molten metal, and moreover, after treatment, T and Fe decrease or hardly increase compared to the process method.

Therefore, the processing time is shortened, the consumption of A/Si for heating is reduced, and the consumption of oxygen for refining can also be reduced.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the relationship between nozzle discharge Romanha number (M2) and [atmospheric pressure (P/Lance source pressure (PI)), and Figure 2 is a characteristic diagram showing the relationship between nozzle discharge Romanha number (M2) and It is a characteristic diagram showing the relationship between [throat section cross-sectional area (AI)/discharge port cross-sectional area (A-section)].

Claims (1)

[Claims] 1. A water-cooled top-blowing lance is provided in the degassing tank of the vacuum degassing device, and oxygen gas is blown from the water-cooled top-blowing lance onto the surface of the molten metal bath in the tank to oxygen-blow the molten metal. In this method, a Laval-type lance nozzle with a flared tip is used to measure the degree of vacuum in the tank, the oxygen supply pressure to the lance, and the oxygen flow rate, so that the ratio between the oxygen supply pressure and the degree of vacuum is constant. A method for oxygen blowing molten metal under reduced pressure, the method comprising controlling at least one of the degree of vacuum, oxygen supply pressure, or oxygen flow rate. 2. In a method in which a water-cooled top-blowing lance is provided in the degassing tank of a vacuum degassing device, and oxygen gas is blown from the water-cooled top-blowing lance onto the surface of the molten metal bath in the tank to oxygen-blow the molten metal, the oxygen supply pressure is kept constant, and the degree of vacuum is controlled so that the ratio of the pressure to the degree of vacuum becomes a pressure that can generate a value larger than the Mach number calculated from the ratio of the nozzle throat area and the nozzle discharge opening area. We selected two methods: one to perform an efficient decarburization reaction, and the other to efficiently raise the temperature of the molten metal by controlling the degree of vacuum so that the pressure is at a ratio that generates a value smaller than the Mach number. A method for oxygen blowing molten metal under reduced pressure. 3. In a method in which a water-cooled top-blowing lance is provided in the degassing tank of a vacuum degassing device, and oxygen gas is blown from the water-cooled top-blowing lance onto the surface of the molten metal bath in the tank to oxygen-blow the molten metal, the oxygen supply pressure The oxygen flow rate or oxygen supply pressure is controlled so that the ratio between Selectively perform an operation to efficiently perform decarburization reaction and an operation to efficiently heat up the molten metal by controlling the oxygen flow rate or oxygen supply pressure to a ratio that generates a value smaller than the Mach number. A method for oxygen blowing molten metal under reduced pressure, characterized by: