JP3365753B2 - Gas supply system for bottom blow converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas supply device for a bottom-blowing converter that controls a flow rate over a wide range by using two valves, for example, a master valve and a slave valve.

[0002]

2. Description of the Related Art FIG. 4 is a flow rate control system diagram of a parent valve 2 of a bottom blowing converter 1 used for steel refining. A plurality of tuyere 3 is provided on the bottom of the converter 1, and each tuyere 3 has a parent-child valve 2
A gas such as an inert gas and oxygen is blown in through. Since the gas blown into the converter needs to be controlled in a wide range from a small flow rate to a large flow rate, it is controlled using a parent valve 2 having a child valve 5 which is a small flow valve and a parent valve 4 which is a large flow valve.

The parent-child valve 2 includes a first passage 6 connected to the tuyere 3 of the converter 1, and a child valve 5 interposed in the first passage 6.
A small flow rate detector 10 which is interposed upstream of the child valve 5 and detects a flow rate passing through the child valve 5, and a second flow rate bypassing the small flow rate detector 10 and the child valve 5 and connected to the first flow path 6. A master valve 4 interposed in the flow path 8 and the second flow path 8, and a large flow rate detector 9 that is interposed upstream of the master valve 4 and detects a flow rate passing through the master valve 4.
And a control means 11 for controlling the valve opening degree of the master valve 4 and the slave valve 5 based on the small flow rate detector 10 and the large flow rate detector 9. Inert gas and oxygen are supplied to the first flow path 5, and are blown into the converter from the tuyere 3 of the converter 1 via the slave valve 5 and the master valve 4.

The set flow rate Q is input to the control means 11,
Based on the detected outputs of the set flow rate Q, the small flow rate detector 10 and the large flow rate detector 9, the sum of the gas flow rate passing through the master valve 4 and the gas flow rate passing through the slave valve 5 should be equal to the set flow rate Q. The control means 11 controls the valve opening degrees of the master valve 4 and the slave valve 5. The valves 4 and 5 are, for example, spherical valves, and the flow rate increases in proportion to the valve opening.

FIG. 5 is a graph showing the relationship between the set flow rate Q and the valve opening degree of the child valve 5 and the valve opening degree of the master valve 4. The horizontal axis represents the input set flow rate Q and the left vertical axis. The axis is the valve opening of the slave valve 5, the right vertical axis is the valve opening of the parent valve 4, and the line 1
2 is a graph showing the relationship between the set flow rate Q and the valve opening degree of the slave valve 5, and line 13 is a graph showing the relationship between the set flow rate Q and the valve opening degree of the master valve 4.

When the set flow rate Q is small, the master valve 4 is closed, only the slave valve 5 is opened, and only the slave valve 5 is controlled so that the gas flow rate passing through the slave valve 5 becomes the set flow rate. , If the switching flow rate A corresponding to the valve opening of 80% of the child valve 5 is larger, the valve opening of the child valve 5 is maintained at 80%, and the parent valve 4
And the valve opening degree of the master valve 4 is controlled so that the sum of the gas flow rate passing through the slave valve 5 and the gas flow rate passing through the master valve 4 becomes the set flow rate Q.

The valves 4 and 5 are, as described above, sphere valves.
Although the flow rate of the passing gas increases in proportion to the valve opening degree, the passing gas flow rate does not change much in the vicinity of full opening, for example, the valve opening degree of 80% or more, and the responsiveness of the passing gas flow rate to the valve opening degree becomes dull. , So to speak, dead zone. Therefore, as shown in FIG. 5, when the switching flow rate A corresponding to the valve opening of 80% of the child valve 5 or more, the valve opening of the child valve is maintained at 80% and only the valve opening of the parent valve 4 is maintained. To control. As a result, it is possible to prevent the occurrence of a dead zone in which the control is difficult in the range of the set flow rate Q.

[0008]

As described above, the flow rate of the passing gas of the valves 4 and 5 increases in proportion to the valve opening, and it becomes difficult to control the valve near the fully open position. When the opening is less than 20%, the change in the flow rate of the passing gas in accordance with the change in the valve opening becomes too large, so to speak, it is hypersensitive, which makes control difficult.

In the above-mentioned conventional technique, it is possible to prevent the control difficulty due to the dead zone near the full opening of the child valve 5, but when the set flow rate Q is slightly larger than the switching flow rate A, the valve opening degree of the parent valve 4 is increased. Is less than 20%, which makes it difficult to control.

FIG. 6 is a graph showing a second conventional valve control method for solving such a problem. In this control method, when the set flow rate Q is larger than the switching flow rate A corresponding to the valve opening of 80% of the slave valve 5, the slave valve 5 is completely closed and only the master valve 4 is opened for control. . Assuming that the switching flow rate A corresponding to the valve opening degree of 80% of the slave valve 5 is smaller than the flow rate corresponding to the valve opening degree of 20% of the master valve 4, when the set flow rate Q is larger than the switching flow rate A, , Valve opening of master valve 4 is 20%
As described above, this makes it possible to prevent control difficulties in the region where the set flow rate Q is slightly larger than the switching flow rate A.

In the bottom blowing converter 1, gas is blown into the hot metal from the lower part of the converter 1 to stir the hot metal, but when both the master valve 4 and the slave valve 5 are closed during the gas blowing, the nozzle There is a risk of clogging the tip of the. In the above-mentioned second conventional technique, when the set flow rate Q becomes larger than the switching flow rate A, the slave valve 5 is fully closed. Therefore, when the set flow rate Q becomes smaller than the switching flow rate A with the child valve 5 fully closed, it is necessary to close the parent valve 4 and open the child valve 5, but at this time, the child valve 5 If it doesn't open due to a malfunction,
Both the master valve 4 and the slave valve 5 are closed, and the nozzle clogging as described above occurs.

As a third conventional technique, Japanese Patent Laid-Open No. 5-25
Japanese Patent Laid-Open Publication No. 0043 discloses a parent-child valve flow rate control method for performing control compensation for a deviation generated at the control limit of a parent valve to ensure control accuracy of a control system. The above problems cannot be solved.

An object of the present invention is to prevent a region in which control is difficult when a gas flow rate is controlled over a wide range using two valves, and to completely close one of the valves during gas flow rate control. It is an object of the present invention to provide a bottom blowing converter gas supply device.

[0014]

The present invention provides (a) a first gas supply from a gas source to tuyere 22 of a bottom blowing converter 20.
A flow channel 30, (b) a first valve 25 interposed in the first flow channel 30, and (c) a second flow channel 32 bypassing the first valve 25 and connected to the first flow channel 30. (D) The flow rate when fully opened is between the first flow path 32 and the first flow path 32.
A second valve 24 that exceeds the flow rate of the valve 25 when it is fully opened, and (e) a first valve that detects the flow rate of gas passing through the first valve 25.
Flow rate detector 31, and (f) second for detecting the flow rate of gas passing through the second valve 24
Responsive to the flow rate detector 33, (g) flow rate setting means for setting the flow rate Q supplied to the tuyere 22, and (h) the outputs of the first and second flow rate detectors 31, 33 and the output of the flow rate setting means. (H1) When the set flow rate Q is less than the first flow rate A corresponding to a predetermined upper limit valve opening (for example, 80% valve opening of the child valve 25) near the fully open position of the first valve 25, The opening amount of the first valve 25 is controlled so that the detected flow rate F1 of the first flow rate detector 31 corresponds to the set flow rate Q, the second valve 24 is fully closed, and (h2) the second flow rate B is set to The set flow rate Q is the sum of 1 flow rate A and a flow rate corresponding to a predetermined lower limit valve opening degree (for example, 20% valve opening degree of the parent valve 25) near the fully closed second valve 24, and the set flow rate Q is the first flow rate. When it is A or more and less than the second flow rate B, the detected flow rate F2 of the second flow rate detector 33 is the second valve 2
4 holds the opening degree of the second valve 24 so that the flow rate corresponds to the lower limit valve opening degree of the second valve 24.
Is controlled so that the sum of the detected flow rates F1 and F2 of the first and second flow rate detectors 31 and 33 becomes the set flow rate Q, and (h3) the set flow rate Q is equal to or greater than the second flow rate B. At this time, the opening degree of the first valve 25 is set to the detected flow rate F1 of the first flow rate detector 31.
So as to become the first flow rate A, and the opening degree of the second valve 24 is set to the flow rate F1, detected by the first and second detectors 31, 33.
Control means 3 for controlling so that the sum of F2 becomes the set flow rate Q
And a gas supply device for a bottom blowing converter.

For example, the first valve 25 is a small flow valve and the second valve is
The valve 24 is a large flow valve, and the vicinity of full opening above the upper limit valve opening is a dead zone where control becomes difficult, and when it is less than the lower limit valve opening, it is an area that reacts hypersensitively and becomes difficult to control. . Therefore, when the set flow rate Q is larger than the flow rate B corresponding to the lower limit valve opening degree of the second valve, the control means opens and holds the first valve up to the lower limit valve opening degree, and the passing flow rate F1 of the first valve.
Only the valve opening of the first valve is controlled so that the sum of the flow rate F2 and the passing flow rate F2 of the second valve becomes the set flow rate. By controlling in this manner, it is possible to eliminate a region in which it is difficult to control the valve in the vicinity of the fully closed first valve and in the vicinity of the fully opened second valve, and the first valve is completely closed during the gas flow rate control. Can be prevented.

[0016]

According to the present invention, when the set flow rate is larger than the sum of the flow rate corresponding to the lower limit valve opening degree of the second valve and the flow rate corresponding to the upper limit valve opening degree of the first valve, the first valve is turned on. Maintaining the upper limit valve opening, the flow rate F1 of the first valve and the flow rate F2 of the second valve
The control means controls only the valve opening degree of the second valve so that the sum of and becomes the set flow rate. As a result, it is possible to deal with a large set flow rate without causing an area that becomes difficult to control. Also in this case, it is possible to prevent the first valve from being completely closed.

According to the present invention, in the bottom blowing converter, it is possible to control the gas flow rate in a wide range by using the two valves without causing an area that is difficult to control. Further, it is possible to prevent one valve from being completely closed during the control, and it is possible to reliably prevent the nozzle from being clogged. According to the present invention, the first flow rate A of the first valve 25 is a flow rate corresponding to the lower limit valve opening degree of the second valve 24 and a predetermined lower limit valve opening degree (slave valve near the fully closed position of the first valve 25). 20% of 25
It is characterized by being larger than the sum of the flow rate corresponding to the valve opening).

When the set flow rate Q is equal to or higher than the first flow rate A, the first and second valves 25 and 24 have flow rates corresponding to the opening exceeding the lower limit valve opening of each of them, so that the accurate flow rate is obtained. Gas can be supplied to the tuyere 22.

[0020]

1 is a flow control system diagram of a parent-child valve 21 of a bottom blowing converter 20 using a valve control method according to an embodiment of the present invention. In the bottom blowing converter 20, an inert gas such as argon or nitrogen and an oxygen gas are blown from a plurality of tuyere 22 provided at the bottom of the furnace, and the carbon amount is 4
% Molten metal is refined into molten steel having a carbon content of 0.2% or more. At this time, in order to make the set value of the carbon amount of the molten steel in a wide range, it is necessary to control the gas flow rate to be blown in a wide range. For example, the control range in which the ratio between the minimum set flow rate and the maximum set flow rate is about 1:20. Need.

When a valve having a simple structure for controlling the flow rate by increasing / decreasing the valve opening degree is used as a valve to be controlled, it is difficult to highly accurately control a wide flow rate range with one valve. Is. Therefore, for example, if the flow ratio is 1: 5,
A child valve 25 for controlling a small flow range and a flow ratio of 5:20
A parent-child valve 2 using a parent valve 24 that controls a large flow range with
Control is performed at 1.

The parent-child valve 21 is provided for each tuyere 22, the inert gas and oxygen are supplied to one end side from a gas source (not shown), and the other end side is connected to the tuyere 22 of the converter 20. One flow passage 30 and a child valve 25 interposed in the first flow passage 30
A small flow rate detector 31 interposed in the first flow path 30 upstream of the child valve 25, and a second flow connected to the first flow path 30 bypassing the child valve 25 and the small flow rate detector 31. Path 32,
The master valve 24 interposed in the second flow path 32, the large flow rate detector 33 interposed in the second flow path 32 upstream of the master valve 24, and the set flow rate Q are input, and the small flow rate detector 31 and Based on the detection output of the large flow rate detector 33, the child valve 25 and the parent valve 24
And a control means 37 for controlling.

The control means 37 includes a first controller 34 for controlling the valve opening of the child valve 25, a second controller 35 for controlling the valve opening of the parent valve 24, a small flow rate detector 31, and a large flow rate detector 33. , And an arithmetic unit 36 for calculating the child valve command signal S1 and the parent valve command signal S2 based on the flow rate set value Q input from the flow rate setting means (not shown). . Each of the command signals S1 and S2 is either a flow rate command value or a valve opening command value, and for example, the command signal S for the slave valve, which is a flow rate command value for the first controller.
When 1 is input, the first controller 34 controls the valve opening degree of the slave valve 25 based on the small flow rate detector 31 so that the flow rate passing through the slave valve 25 becomes the input command value. If the command signal S1 is a valve opening command, the child valve 25 is opened up to the input valve opening and the valve opening is maintained. Such a configuration is the same in the second controller 35 as well.

In the sphere valve, the passage flow rate increases and decreases in proportion to the valve opening, but as described above, it becomes difficult to control near the fully open position and near the fully closed position. Therefore, in this embodiment, each valve 24,
25 is controlled in the range of valve opening of 20 to 80%. That is, the minimum set flow rate is the flow rate corresponding to the 20% valve opening of the child valve 25, and the maximum set flow rate is 8% of the child valve 25.
The flow rate corresponding to the 0% valve opening is added to the flow rate corresponding to the 80% valve opening of the parent valve 24. However, this is the case where the gas pressure is constant at the reference pressure, and in reality, the valve opening of the master valve is 90
It may be over%.

FIG. 2 shows the set flow rate Q, the slave valve 25 and the master valve 2.
It is a graph which shows the relationship with each valve opening degree of No. 4. In FIG. 2, the horizontal axis represents the input set flow rate Q, the left vertical axis represents the valve opening of the child valve 25, the right vertical axis represents the valve opening of the parent valve 24, and the line 40 represents the setting. It is a graph which shows the relationship between the flow rate Q and the valve opening degree of the child valve 25, and the line 41 is a graph which shows the relationship between the set flow rate Q and the valve opening degree of the parent valve 24. Also,
Reference symbol A is the first switching flow rate corresponding to the 80% valve opening of the slave valve 25, and reference symbol B is the flow rate corresponding to the 80% valve opening of the slave valve 25, and the 20% valve of the master valve 24. It is the second switching flow rate obtained by adding the flow rate corresponding to the opening degree. The switching flow rates A and B are gas flow rates that pass at each valve opening at a predetermined constant pressure that is a reference, and are set in advance. Further, the flow rate corresponding to the 80% valve opening of the child valve 25 is equal to the flow rate corresponding to the 20% valve opening of the parent valve 24.
Is larger than the value obtained by adding the flow rate corresponding to the 20% valve opening degree.

When the set flow rate Q is input to the calculator 36,
Based on the graph shown in FIG. 2 and the detectors 31 and 33, command signals S1 and S2 to the controllers 34 and 35, respectively.
To calculate.

Next, a method of calculating the command signals S1 and S2 by the arithmetic unit 36 will be described with reference to the flowchart shown in FIG. In step a1, when the set flow rate Q is input to the calculator 36, the calculator 36 compares the input set flow rate Q with the switching flow rates A and B, and sets the set flow rate Q.
Is smaller than the switching flow rate A, the process proceeds to step a2.

As can be seen from the graph of FIG. 2, when the set flow rate Q is less than the switching flow rate A, the master valve 24 is closed and controlled only by the slave valve 25. Therefore, at step a2, the master valve command signal S2 The 0% valve opening command value is input to the second controller. Based on this, the second controller 24
Keep it fully closed.

At the next step a3, the slave valve command signal S1
Is input to the controller 34 as a flow rate command value. Since the master valve 24 is fully closed, the command signal S1 input to the controller 34 becomes equal to the set flow rate Q. In response to the input command signal S1, the controller 34 feeds back the valve opening degree of the slave valve 25 based on the detection output of the small flow rate detector 31 so that the gas flow rate passing through the slave valve 25 becomes the set flow rate Q. Control.

If the set flow rate Q input to the computer 36 in step a1 is greater than or equal to the switching flow rate A and less than the switching flow rate B, the process proceeds to step a4. As can be seen from the graph of FIG. 2, when the set flow rate Q is A ≦ Q <B, the master valve 24 is set to 20
The valve opening degree is maintained at%, and only the child valve 25 is controlled. That is, in step a4, the valve opening command value of 20% is input to the second controller 35 as the master valve command signal S2. Second
In response to this, the controller 35 opens the master valve 24 to a valve opening degree of 20% and holds this state.

At the next step a5, the computer 36 causes the master valve 24 to operate.
Based on the set flow rate Q and the detection outputs of the detectors 31 and 33 so that the sum of the gas flow rate F2 passing through the sub valve and the gas flow rate F1 passing through the slave valve 25 becomes the set flow rate Q (that is, Q = F1 + F2). The slave valve command signal S1 is calculated. That is, if the detection output of the small flow rate detector 31 is F1 and the detection output of the large flow rate detector 33 is F2, S1 = Q-F1-F2
Becomes The flow rate value calculated in this way is input to the first controller 34 as the slave valve command signal S1. Based on the input command signal S1 and the detection output of the small flow rate detector 31, the first controller 34 feedback-controls the slave valve 25 so that the gas flow rate passing through the slave valve 25 matches the command signal S1.

At this time, the master valve 24 is held at a valve opening of 20%, but the gas flow rate passing through the master valve 24 fluctuates due to pressure fluctuations, and the calculator 36 causes the large flow rate detector 3 to operate.
Since the command signal S1 is calculated also based on the detection output F2 of No. 3, the command signal S1 has a value that cancels the fluctuation of the flow rate of the gas passing through the master valve 24 due to the pressure fluctuation. As a result, the gas flow rate supplied to the converter 20 is maintained at the set flow rate Q with high accuracy.

If the set flow rate Q input to the calculator 36 is equal to or larger than the switching flow rate B in step a1, the process proceeds to step a6. When the set flow rate Q is B ≦ Q, as can be seen from the graph of FIG. 2, the valve opening degree of the slave valve 25 is maintained at 80% and only the master valve 24 is controlled.

Therefore, in step a6, the command signal for the slave valve is set to S1 and a valve opening command value of 80% is input to the first controller 34. In response to this, the first controller 34 opens the child valve 25 to a valve opening of 80% and holds it.

At the next step a7, the computer 36 determines the gas flow rate passing through the child valve 25 and the gas flow rate passing through the master valve 24 based on the set flow rate Q and the detection outputs F1 and F2 of the detectors 31 and 33. Command signal S for master valve whose sum is set flow rate Q
Calculate 2. That is, S2 = Q-F1-F2 is calculated and input to the second controller 35. In response to this, the second controller 35 performs feedback control based on the detection output F2 of the large flow rate detector 33 so that the gas flow rate passing through the master valve 24 matches the command signal S2.

At this time as well, the valve opening of the sub-valve 25 is kept constant as in the above case, but the flow rate of the gas passing through the sub-valve 25 fluctuates due to the pressure fluctuation, and the calculator 36 makes this fluctuation. Since the command signal S2 for canceling is calculated,
The gas flow rate supplied to the converter 20 is maintained at the set flow rate Q with high accuracy.

As described above, each time the set flow rate Q is input to the calculating means 37, the gas flow rate to be blown by rapidly controlling the valves 24 and 25 becomes the set flow rate Q.

As can be seen from FIG. 2, in the valve control method of the present invention, the valve opening of the child valve 25 is 80% or more and the valve opening of the parent valve 24 is 20% or less regardless of the set flow rate. Since such a condition does not occur, there is no region in which control is difficult, and it is possible to control with high accuracy over a wide range.

Further, since the sub-valve 25 is not completely closed during the gas flow rate control, it is possible to surely prevent both the valves from being closed at the same time and the nozzle is clogged as in the prior art. Even when both the master valve 24 and the slave valve 25 are open at the same time, one of the valves is held at a predetermined valve opening and the feedback control is performed only by the other valve, so that both valves are opened at the same time. It is possible to prevent hunting from occurring by performing feedback control of the valve.

When the set flow rate Q is less than the switching flow rate A, control is performed only by the slave valve 25. However, the minimum value min of the set flow rate Q is a flow rate corresponding to the 20% valve opening of the slave valve 25.
The maximum value max of the set flow rate Q is a value obtained by adding the gas flow rate corresponding to the 80% valve opening degree of the slave valve 25 to the gas flow rate corresponding to the 80% valve opening degree of the parent valve 24. However, as described above, since this value is the flow rate of passage through each valve 24, 25 at a constant reference pressure, the valve opening of the slave valve 25 may be less than 20% depending on the pressure fluctuation. Or the valve opening of the parent valve 24 may be 80% or more.

In the present embodiment, the valve control method for controlling two valves having different valve capacities has been described. However, the present invention is not limited to such a case, and the case where the two valves have the same capacity. Alternatively, conversely to the present embodiment, the valve interposed in the first flow path 30 is a large-capacity master valve, and the valve interposed in the second flow path is a small-capacity slave valve. May be

Furthermore, the valve control method of the present invention is not limited to the case where two valves are connected in parallel, and for example, three or more valves are connected in parallel and the valve opening degree of each valve is increased. It can also be applied to a valve control method for controlling

[0043]

[0044]

As described above, according to the present invention, it is possible to prevent the control from being difficult when switching between the first valve and the second valve, and to control the set flow rate in a wide range with high accuracy. be able to.

Further, it is surely prevented that both valves are completely closed at the same time, so that the tuyere 22 of the bottom blowing converter is surely prevented.
It is possible to reliably prevent clogging of the tip of the nozzle. Thus, the flow rate of the gas supplied to the tuyere 22 of the bottom blowing converter is accurately set, and there is no possibility that the tuyere 22 will be blocked by the solidified steel.

[Brief description of drawings]

FIG. 1 is a flow rate control system diagram of a bottom blowing converter 20 using a valve control method according to an embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the set flow rate Q and the valve opening degrees of the slave valve 25 and the master valve 24.

FIG. 3 is a flowchart showing a method of controlling a computer 36 of a control means 37.

FIG. 4 is a flow control system diagram of a bottom blowing converter 1 using a conventional valve control method.

FIG. 5 is a graph showing the relationship between the set flow rate Q of the conventional parent-child valve 2 and the child valve 5 and the parent valve 4.

FIG. 6 is a graph showing the relationship between the set flow rate Q and the valve opening degrees of the slave valve 5 and the master valve 4 in the second conventional valve control method.

[Explanation of symbols]

20 Bottom blowing converter 21 Parent-child petition 24 Profanity 25 petals 30 First flow path 31 Small flow rate detector 32 Second channel 33 Large flow rate detector 34 First Controller 35 Second controller 36 computing machine 37 Control means

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Claims (2)

(57) [Claims] 1. A bottom blowing converter 2 for (a) gas from a gas source.
0 to the tuyere 22; (b) the first valve 25 interposed in the first flow channel 30; (c) the first valve 25 bypassing the first flow channel 30; The second flow path 32 to be connected is (d) the second flow path 32, and the flow rate at the time of full opening is the first
A second valve 24 that exceeds the flow rate of the valve 25 when it is fully opened, and (e) a first valve that detects the flow rate of gas passing through the first valve 25.
Flow rate detector 31, and (f) second for detecting the flow rate of gas passing through the second valve 24
Responsive to the flow rate detector 33, (g) flow rate setting means for setting the flow rate Q supplied to the tuyere 22, and (h) the outputs of the first and second flow rate detectors 31 and 33 and the output of the flow rate setting means. (H1) When the set flow rate Q is less than the first flow rate A corresponding to a predetermined upper limit valve opening near the fully open position of the first valve 25, the detected flow rate F1 of the first flow rate detector 31 is set to The opening degree of the first valve 25 is controlled so as to correspond to the flow rate Q, the second valve 24 is fully closed, and (h2) the second flow rate B is set to the first flow rate A and the second valve 24 is fully closed. When the set flow rate Q is greater than or equal to the first flow rate A and less than the second flow rate B, the detected flow rate of the second flow rate detector 33 is defined as the sum of the flow rates corresponding to the predetermined lower limit valve opening in the vicinity. F2 opens the second valve 24,
The opening of the second valve 24 is held and the opening of the first valve 25 is adjusted so that the flow rate corresponds to the lower limit valve opening of the second valve 24.
33 is controlled so that the sum of the detected flow rates F1 and F2 of 33 becomes the set flow rate Q. (h3) When the set flow rate Q is the second flow rate B or more, the opening degree of the first valve 25 is set to the first flow rate detection value. Flow rate F of detector 31
1 so as to be the first flow rate A, and the opening degree of the second valve 24 is set to the first and second detectors 31, 33.
And a control means 37 for controlling the sum of the detected flow rates F1 and F2 of (1) to become a set flow rate Q. 2. The first flow rate A of the first valve 25 corresponds to a flow rate corresponding to the lower limit valve opening of the second valve 24 and a predetermined lower limit valve opening near the fully closed position of the first valve 25. 2. The gas supply device for a bottom blowing converter according to claim 1, wherein the gas supply device has a flow rate larger than the sum of the flow rate and the flow rate.