JPH04314819A - Method for controlling decarbonization in circulating flow type vacuum degassing apparatus - Google Patents

Abstract

PURPOSE:To provide the method, which can sufficiently cope with variations of various conditions in the actual operation. CONSTITUTION:Circulating flow rate of molten steel is obtd. from blowing rate of inert gas measured with an inert gas flowmeter 7, pressure in an RH vacuum vessel 1 measured with a vacuum pressure gage 8, molten steel temp. measured by a thermometer 9 and inner diameters of submerged tubes 4 and 5 through an operation processor (Ca1), and on the other hand, carbon concn. in the molten steel is obtd. from an exhaust gas analyzer 11, and by controlling the blowing rate of inert gas through an inert gas blowing nozzle 6 based on the circulating rate from start of reducing pressure to return-back pressure in the vacuum vessel 1 and the carbon concn. in the molten steel, the carbon concn. in the molten steel 3 is controlled in the narrow range.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a decarburization control method for a recirculation vacuum degassing apparatus.

0002

BACKGROUND OF THE INVENTION In recent years, with the increasing quality of steel and the increasing demand for it, the vacuum degassing refining method, which is one of the refining techniques, particularly the RH vacuum degassing refining method, has been widely practiced. This RH method involves immersing two rising pipes and a downcomer pipe hanging down at the bottom of a vacuum chamber into molten steel in a ladle, operating a vacuum pump to suck up the molten steel into the vacuum tank, and then A circulating flow of molten steel is generated by blowing an inert gas (for example, Ar) into the molten steel, and degassing is performed in a vacuum chamber while the molten steel returns from the downcomer pipe to the ladle. In this RH method, smooth reflux of molten steel is a necessary condition, and in order to know the uniform mixing time and stirring state of molten steel during operation, it is important to accurately grasp the molten steel reflux flow rate. .

[0003] Various proposals have been made to express the recirculation flow rate. For example, in "Tetsu to Hagane" magazine, Vol. 73, No. 4, page S176, formula (1) has been obtained from experiments. . W=11.4{G0 Du 4 ln(P2 /P
1)}0.33...(1) However, W: Circulation flow rate (t/min) G0:
Circulating Ar gas flow rate (Nl/min) Du: Rising pipe inner diameter (m) P1: Vacuum chamber pressure (torr) P2: Pressure at the injection point (torr) On the other hand, as a method for measuring the circulating flow rate of molten steel, for example, JP-A-58 -16773
As in Publication No. 2, nonmetallic inclusions are mixed in the circulating molten steel, and the amount of emitted light at multiple points on the surface of the molten steel in the flow direction is measured over a certain period of time, and the molten steel velocity is determined and the circulating amount is calculated from that. A calculation method has been proposed.

0004

[Problem to be solved by the invention] Recently, in order to improve workability such as deep drawability and stretchability, ultra-low carbon steels with as low carbon concentration as possible and ultra-low carbon steels that aim to improve workability and shorten processing time, etc. have been developed. The proportion of medium- and low-carbon steel produced is increasing. Also,
The standards for ingredients are becoming stricter year by year, and it is desired to establish a method that can estimate the ingredients in molten steel with high accuracy.

[0005] The uniformity of the molten steel composition has a strong correlation with the recirculation flow rate, and in order to narrowly estimate the molten steel composition, it is necessary to grasp the stirring state (recirculation flow rate) in all states. Particularly in the case of medium-low carbon steel, the carbon concentration reaches the standard before the reflux reaches a steady state, that is, before it is sufficiently stirred.

However, the above formula (1) and the
In the estimation method according to Publication No. 167732, only the recirculation amount can be determined in a steady state or with a considerable time delay. Furthermore, in actual operation, it is difficult to accurately determine the recirculation flow rate due to various factors such as expansion and deformation of the inner diameter of the immersion pipe due to erosion of the refractory material and narrowing due to deposits as the operation continues for a long time.

[0007] Therefore, the main object of the present invention is to provide a decarburization control method for a recirculation type vacuum degassing apparatus which eliminates the above-mentioned drawbacks and can sufficiently cope with changes in various conditions during actual operation.

[0008]

[Means for Solving the Problems] The above object is a reflux method in which molten steel is introduced into a vacuum chamber through a riser pipe by blowing inert gas and returned from a downcomer pipe to reflux, and degassing is performed under vacuum conditions during this time. When performing vacuum degassing, the amount of molten metal recirculated is determined based on the amount of inert gas blown in, the vacuum conditions, the molten steel temperature, and the inner diameter of the immersion tube.On the other hand, the carbon concentration in the molten steel is determined using an exhaust gas analyzer, and the vacuum chamber is depressurized. This problem can be solved by controlling the amount of inert gas blown based on the recirculation flow rate from the start to the return pressure and the carbon concentration in the molten steel.

[0009]

[Operation] According to the present invention, the reflux flow rate of molten steel is measured using the inner diameter of the immersion pipe measured before treatment, the flow rate of the reflux inert gas measured online, the pressure inside the vacuum chamber, and the molten steel temperature. On the other hand, by estimating the carbon concentration of molten steel using the value measured online by an exhaust gas analyzer and controlling the recirculation flow rate in all states from transient to steady state, the carbon concentration in molten steel can be estimated. This allows the components to be estimated within a narrow range.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in more detail below with reference to embodiments shown in the drawings.

FIG. 1 is a diagram showing an example of the configuration of an apparatus for carrying out the method of the present invention, in which 1 is an RH vacuum chamber, 2 is a ladle, 3 is a molten steel,
4 is a rising pipe, 5 is a descending pipe, 6 is an inert gas blowing nozzle, 7 is an inert gas flow meter, 8 is a vacuum pressure gauge, 9 is a thermometer, 10 is an exhaust pipe, 11 is an exhaust gas analyzer, Cal is The arithmetic processing unit and Ar indicate an inert gas source, respectively. This structure is well known and will not be described in detail.

First, a method for estimating the circulating flow rate of molten steel using the above-mentioned apparatus will be explained.The velocity of the inert gas (argon in this case) blown from the inert gas source can be calculated using equation (2).

[0013]

[Math 1]

[0014] However, ug: bubble velocity (m/sec)
ust: Molten steel speed (m/sec) ρg: Bubble density (kg/m3)
ρst: Molten steel density (kg/m3) g: Gravitational acceleration (m/sec2)
C: drag coefficient s: cross-sectional area (m2)
vg: Bubble volume (m3) However, the bubble volume vg is expressed as in equation (3) using the molten steel temperature T measured online and the pressure inside the vacuum chamber P2, and the cross-sectional area s is expressed using the bubble volume. It can be expressed as in equation (4). vg = f1 (T, P2) ... (3) s =
f2 (vg) ... (4) From the bubble velocity ug calculated by equation (2) and the inert gas flow rate G0 (Nm3/min) measured online, the volume of the bubbles occupying the riser pipe Vg (m3) and Molten steel volume in the riser pipe Vs
t(m3) can be calculated from equations (5) and (6). Vg = f3 (ug, vg, G0) ... (5
)

[0015]

[Math 2]

[0016] However, Du: Inner diameter of rising pipe (m) h
: Height of riser pipe (m) From the molten steel volume, reaction to bubble drag, pressure difference, etc., the molten steel velocity ust is:

[0017]

[Math 3]

[0018] However, S: riser pipe cross-sectional area (m2)
p: Pressure (Pa) x: Distance from the inert gas injection point (m) Dw: Frictional force with the riser pipe wall (N) It can be calculated from equation (7). Also, the frictional force Dw is Dw
= f4 (ust, Du, ρst, ν) ... (
8) However, ν: Kinematic viscosity coefficient (m2/sec) Derived from equation (8).

Then, the bubble velocity and molten steel velocity are determined from equations (2) and (7),

[0020]

[Math 4]

On the other hand, from equation (9), the recirculation amount Q (m3/se
c) Calculate.

As described above, by solving the two differential equations (2) and (7) and the seven accompanying equations, the molten steel can be determined in all conditions from the start to the end of the RH vacuum degassing process. The amount of reflux can be determined.

On the other hand, the exhaust gas analyzer shows that C in the exhaust gas is
O concentration, CO2 concentration and exhaust gas flow rate FGAS (kg/
sec) on-line, and the decarburization amount and current carbon concentration are determined from equations (10) and (11).

[0024]

[Math 5]

CL=CL0−ΔCOUT/VρST...
(11) In general, the decarburization reaction proceeds in the form of a first-order reaction, and if the mass balance of carbon in the steel in the ladle and vacuum chamber is determined and the initial conditions are given, the relationships in equations (12) and (13) can be obtained. It will be done. CL = CL 0 exp (-Kt) ... (12)

[0026]

[Math 6]

[0027] However, CL0: Initial carbon concentration (%) K: Decarburization reaction rate constant (l/sec) Q': Molten steel circulation flow rate (m3/sec) ak: Capacity coefficient of decarburization reaction (m3/sec) (11),
The apparent decarburization reaction rate constant can be found from equations (12) and (13). It is also known that the capacity coefficient changes depending on the carbon concentration in steel, and can be expressed in the form of equation (14). ak=f(CL)...(14) From the above, by adjusting the blown inert gas based on the apparent molten steel circulation flow rate Q' and the estimated molten steel circulation flow rate Q, the optimum value can be achieved in all conditions from the start of processing to the end. It becomes possible to obtain the molten steel circulation flow rate.

[Example] Figure 2 shows 298 (ton) of molten steel in a ladle in an RH type vacuum degassing device (with an inner diameter of immersion tube of 75 (c)).
It shows the relationship between the reflux flow rate, the flow rate of the blown inert gas, and the pressure inside the vacuum chamber when treated for 30 minutes under m)). Table 1 also shows equation (1) and (9) under the operating conditions of the actual machine.
) is a comparison of the reflux amount calculated using the formula and the actual measured value. However, since equation (1) can only calculate the steady state, the recirculation amount in the present invention is also shown as the steady state value.

[0029]

[Table 1]

[0030] As shown in Table 1, according to the present invention, the recirculation flow rate value in a steady state could be estimated with high accuracy and almost equal to the actually measured value. Furthermore, in the present invention, adjustment is possible even before the steady state is reached, and it can be seen that the controllability is higher than that of the conventional method.

[0031]

As described above, according to the present invention, it is possible to grasp and control the recirculation amount in all conditions from the start to the end of RH treatment, which contributes to highly accurate estimation of molten steel components and improves composition. With improved estimation, effects such as shortening vacuum processing time and optimizing various processing agents (heating materials, cooling materials) can be obtained.

[0032] Furthermore, it is possible to accurately determine the time at which the vacuum chamber should be repaired from an extreme increase or decrease in the recirculation amount.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the method of the present invention.

FIG. 2 is an explanatory diagram showing the relationship between the inert gas flow rate and the recirculation amount.

[Explanation of symbols]

1 RH vacuum tank 2 ladle 3 molten steel 4 rising pipe 5 descending pipe 6 inert gas blowing nozzle 7
Inert gas flow meter 8 Vacuum pressure gauge 9 Thermometer 10 Exhaust pipe 11 Exhaust gas analyzer Cal Arithmetic processing unit Ar Inert gas source

Claims (1)

[Claims] Claim 1: In reflux type vacuum degassing, in which molten steel is introduced into a vacuum chamber through a riser pipe by blowing inert gas and returned from a downcomer pipe, and degassed under vacuum conditions during the reflux type vacuum degassing, the molten steel is The recirculation flow rate of molten steel is determined based on the gas injection amount, vacuum conditions, molten steel temperature, and immersion tube inner diameter.On the other hand, the carbon concentration in the molten steel is determined using an exhaust gas analyzer, and the flow rate from the start of depressurization in the vacuum chamber to the return to pressure is calculated using an exhaust gas analyzer. A decarburization control method for a recirculation type vacuum degassing apparatus, characterized by controlling the amount of inert gas blown based on the recirculation flow rate and the carbon concentration in molten steel.