JPH09143565A - Method for controlling atmosphere in heating furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the development of scale on a steel material to be heated and to reduce burnt loss quantity by avoiding the rising of oxygen concn. in a furnace caused by the air entering from an ejecting hole. SOLUTION: At the time of outputting a door opening signal Ts1 by opening an ejecting door 2, air-fuel ratio manipulation variable Δm decided by the relation among a furnace pressure Po , oxygen concn. C0 in the furnace, fuel flow rate F0 and door opening time T0 is calculated with a calculating means 10, and this value is outputted to an air-fuel ratio control means 11. The air-fuel ratio control means 11 executes a changing operation so as to lower the air-fuel ratio according to the air-fuel ratio manipulation variable Δm calculated with the calculating means 10. At the time of outputting a door closing signal Ts2 by closing the ejecting door 2, the air-fuel ratio control means 11 executes a returning back operation for returning back the air-fuel ratio to the original air-fuel ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heating furnace, and more particularly, to an atmosphere control method for a rotary hearth type heating furnace.

[0002]

2. Description of the Related Art Round billets, which are materials for seamless steel pipes,
Usually, after being heated up to about 1250 ° C. by a rotary hearth type heating furnace (hereinafter, abbreviated as rotary hearth furnace), it is transferred to a rolling line and made into a pipe. By the way, not only in rotary hearth furnaces but also in general heating furnaces, it is better that the oxygen concentration in the furnace is low in order to reduce the burnout due to the scale generation of the steel to be heated. The current situation is that there is a lot of burnout due to the rise.

The main cause of the increase in the oxygen concentration in the furnace is that the outside air enters through the openings such as the extraction port of the steel to be heated. In the pipe making line, since the material billet is also large, the manipulator for taking the billet into the furnace is also large, and the extraction port is necessarily large, and since the manipulator itself is also quite large, this manipulator is The amount of air pushed into the furnace cannot be ignored.
Therefore, in the rotary hearth furnace, the invasion of the atmosphere and the decrease of the furnace pressure due to the opening and closing of the extraction door are the most important causes of the increase of the oxygen concentration in the furnace. The mechanism by which the oxygen concentration rises when the extraction door opens is thought to be that the combustion pressure in the furnace leaks out of the furnace due to the opening of the extraction door, causing the furnace pressure to drop and the atmosphere to enter the furnace. .

As a conventional technique for preventing the invasion of the atmosphere from the extraction port and suppressing the increase of the oxygen concentration in the furnace, for example, an air curtain is formed at the extraction port, or JP-A-62-139820. As disclosed in the official gazette, the furnace pressure is increased to prevent atmospheric invasion from the extraction port, but in the former case, if the extraction port becomes larger than a certain size, the effect of the air curtain is almost eliminated. In the latter case, when the furnace pressure is increased in the latter, the sensors for material detection and the like provided near the furnace door are burned by flames when the furnace door is opened, so the furnace pressure cannot be raised above a certain pressure, Therefore, with such a pressure setting, it is not possible to satisfactorily prevent air from entering when the furnace door is opened.

In addition, in general heating furnaces, many air-fuel ratio controls and furnace pressure controls for controlling the oxygen concentration in the furnace have been proposed. Based on this, feedback control of the opening of the in-furnace damper is performed to raise or lower the air-fuel ratio, etc. It cannot be maintained at a level.

In the rotary hearth furnace, Japanese Patent Laid-Open No. 1-162 is used.
As described in Japanese Patent No. 718, the draft pressure at the flue inlet is set with respect to the total fuel supply amount so that the ratio of combustion gas and invading air becomes the most effective state for heating, and the total fuel supply amount A method has been proposed to correct the control disturbance caused by a sudden change by changing the oxygen concentration in the furnace, but it does not actively suppress the increase in oxygen concentration in the furnace due to opening and closing the door, and Since the sample gas outlet for the oxygen concentration is usually used for combustion management of the entire furnace, it is installed in the most downstream (furnace bottom) of the combustion gas in most cases. The oxygen concentration in the region is unknown, and scale generation cannot be effectively suppressed.

[0007]

By the way, the rotary hearth furnace has a long effective furnace length, and the combustion control zone is usually divided into a plurality of zones as shown in FIG. FIG. 6 is a graph showing the relationship between the oxygen concentration in the furnace and the furnace pressure in the zone closest to the extraction door (NO. 5 zone in FIG. 5) in each combustion control zone and the opening / closing operation of the extraction door. 7 is a graph chart obtained, and as is clear from FIG. It can be seen that the fluctuations in the oxygen concentration and furnace pressure in the 5 zones are due to the opening / closing operation of the extraction door. Then, the present inventor further conducted a study to find out NO. When the fuel flow rate (generated exhaust gas amount) in the five zones was measured, it was found that the magnitude of the fluctuation of the oxygen concentration in the furnace changed depending on the fuel flow rate.

Here, as a result of the inventors of the present invention having arranged these relations and conducted intensive studies, as a result, the air-fuel ratio was determined from the relations between the oxygen concentration in the reactor near the extraction port, the reactor pressure, the fuel flow rate, and the time when the extraction door was opened. By calculating the air-fuel ratio manipulated variable for the control system and performing feed-forward control to change the air-fuel ratio according to the manipulated variable when the extraction door is opened, the oxygen concentration in the furnace due to the intruding air from the extraction port We have found that the rise can be avoided.

The present invention has been made on the basis of such knowledge, and by preventing the oxygen concentration in the furnace from increasing when the extraction door is opened and closed, it is possible to suppress the generation of scale of the steel material to be heated and reduce the burnout. It is an object of the present invention to provide a method for controlling an atmosphere of a heating furnace that can be performed.

[0010]

In order to achieve such an object, an atmosphere control method for a heating furnace according to the present invention is an oxygen concentration detecting means for detecting the oxygen concentration in the furnace near the extraction port, and a furnace near the extraction port. A furnace pressure detecting means for detecting the pressure, a fuel flow rate detecting means for detecting the fuel flow rate in the vicinity of the extraction port, and a time measuring means for measuring the opening time of the extraction door, and the oxygen concentration when the extraction door is opened. The in-furnace oxygen concentration detection value obtained by the detection means, the furnace pressure detection value obtained by the furnace pressure detection means, the fuel flow rate detection value obtained by the fuel flow rate detection means, and the time measurement means The air-fuel ratio manipulated variable determined by the relationship with the calculated time is calculated, and the calculated value is output to the air-fuel ratio control system to change the air-fuel ratio, and when the extraction door is closed, the air-fuel ratio is restored. To operate Characterized in that it was.

Here, the air-fuel ratio changing / restoring operation may be provided with a predetermined time lag with respect to the opening / closing signal of the extraction door depending on the characteristics of the heating furnace.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION An example of an embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a schematic diagram for explaining an atmosphere control method for a rotary hearth furnace which is an example of an embodiment of the present invention, and FIG. 2 is a schematic diagram for the rotary hearth furnace. In FIG. 1, reference numeral 4 indicates the NO. 3 closest to the extraction door 2 in the seven combustion control zones 3a to 3g (see FIG. 2) of the rotary hearth furnace 1. 7 Furnace pressure gauge (furnace pressure detecting means) provided in the zone 3g to detect the furnace pressure P ₀ ; Reference numeral 5 is an oxygen concentration meter (oxygen concentration detecting means) provided in the zone 3g to detect the oxygen concentration C ₀ in the furnace; 6 is a burner provided on the wall of the zone 3g,
7 is a flow rate detector (fuel flow rate detecting means) for detecting the fuel flow rate F ₀ of the zone 3g, 8 is a charging door for charging the round billet, which is a material of the seamless steel tube, into the furnace 1, and 9 is an extraction. The controller controls combustion of the burner 6 when the door 2 is opened. The controller 9 controls the furnace pressure P ₀ and the furnace oxygen concentration C
₀ , the fuel flow rate F _0, and a calculation means 10 for calculating an air-fuel ratio manipulated variable Δm described later based on the opening time T ₀ of the extraction door 2 obtained by a timer (not shown), and an air-fuel ratio manipulated variable Δm.
And an air-fuel ratio control means 11 for controlling the air-fuel ratio in accordance with the above.

Next, the operation of the controller 9 will be described.
For example, when the round billet is extracted, the extraction door 2 is opened and the door is opened.
Issue Ts₁Is output, the calculation means 10 causes the furnace pressure P₀, Furnace
Internal oxygen concentration C₀, Fuel flow rate F₀And door opening time T₀connection of
Air-fuel ratio manipulated variable Δm = f (P₀,
C₀, F₀, T₀) Is calculated, and this is used as the air-fuel ratio control means 1
Output to 1. Then, the air-fuel ratio control means 11 is
According to the air-fuel ratio manipulated variable Δm calculated by the stage 10, the
The operation of changing the air-fuel ratio is performed so as to reduce the fuel ratio. to this
The sky entering from the extraction port 2a when the extraction door 2 is opened
The oxygen concentration in the furnace in zone 3g is increased by the air.
Avoid. The extraction door 2 is closed and the door closing signal Ts _TwoIs output
Then, the air-fuel ratio control means 11 returns the original air-fuel ratio.
Perform a return operation.

In the above embodiment, the rotary hearth furnace is taken as an example of the heating furnace, but the present invention is not limited to this, and the present invention may be applied to other general heating furnaces.

[0015]

EXAMPLE FIG. 3 is a graph showing the relationship among the oxygen concentration in the furnace, the furnace pressure and the opening / closing operation of the extraction door when the atmosphere of the rotary hearth furnace according to the present invention is not controlled, and FIG. 4 is related to the present invention. FIG. 6 is a graph showing the relationship between the oxygen concentration in the furnace, the furnace pressure, and the opening / closing operation of the extraction door when the atmosphere control is performed.

First, the case where the atmosphere control is not performed will be described. As shown in FIG. 3, if the extraction door 2 is not opened, the oxygen concentration in the furnace is about 1%, which means that the combustion is very good. When the extraction door 2 is opened and the extraction is started, the furnace pressure P ₀ immediately changes from +0.8 mmH ₂ O to a minimum of −0.5 mmH.
_It rapidly decreased to ₂ O, and at the same time, the oxygen concentration C ₀ in the furnace rose to a maximum of 5%.

On the other hand, by the atmosphere control according to the present invention, the air-fuel ratio is changed at the timing of -0.7 seconds with respect to the door open signal Ts ₁ , and the timing of +1.3 seconds with respect to the door close signal Ts ₂ . Fig. 4 shows the result when the air-fuel ratio was restored to the original value by.
As is apparent from FIG. 4, the furnace pressure P ₀ still sharply decreases with the opening operation of the extraction door 2, but the in-furnace oxygen concentration C ₀ is controlled to a level as low as 2% at the maximum as compared with the conventional 5%. You can see that Further, such atmosphere control reduced the billet yield by Δ1%, and halved the number of maintenance steps for external surface flaws on the external scale.

Incidentally, the air-fuel ratio was 1.05 before the change and 0.75 after the change, and the opening time T ₀ of the extraction door 2 was 10 seconds. Note that this embodiment is an example of a certain combustion flow rate, and it goes without saying that the change margin of the air-fuel ratio changes depending on the combustion flow rate and the like.

[0019]

As is apparent from the above description, according to the present invention, it is possible to prevent the oxygen concentration in the furnace from rising due to the air entering from the extraction port, so that the scale of the steel to be heated is not generated. It is possible to obtain an effect that the amount of burn-off can be suppressed well and the amount of burn-off can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining an atmosphere control method for a rotary hearth furnace that is an example of an embodiment of the present invention.

FIG. 2 is a schematic view of a rotary hearth furnace.

FIG. 3 is a graph showing the relationship between the oxygen concentration in the furnace, the furnace pressure, and the opening / closing operation of the extraction door when the atmosphere control of the rotary hearth furnace according to the present invention is not performed.

FIG. 4 is a graph showing the relationship between the oxygen concentration in the furnace, the furnace pressure, and the opening / closing operation of the extraction door when the atmosphere control of the rotary hearth furnace according to the present invention is performed.

FIG. 5 is an explanatory diagram for explaining a rotary hearth furnace.

FIG. 6 is a graph showing the relationship among the oxygen concentration in the furnace, the furnace pressure, and the opening / closing operation of the extraction door in the conventional rotary hearth furnace.

[Explanation of symbols]

1 ... rotary hearth furnace 2 ... extraction door T ₀ ... door opening time 2a ... spout 4 ... furnace pressure gauge (furnace pressure detecting means) P ₀ ... furnace pressure 5 ... oximeter (oxygen concentration-detecting means) C ₀ ... furnace Internal oxygen concentration 7 ... Flow rate detector (fuel flow rate detection means) F ₀ ... Fuel flow rate Δm ... Air-fuel ratio manipulated variable 10 ... Calculation means 11 ... Air-fuel ratio control means

Claims (1)

[Claims] 1. An oxygen concentration detecting means for detecting an in-furnace oxygen concentration near the extraction port, a furnace pressure detecting means for detecting a furnace pressure near the extraction port, a fuel flow rate detecting means and an extraction for detecting a fuel flow rate near the extraction port. A timer provided with a time measuring means for measuring the time when the door is open, and when the extraction door is opened, the in-reactor oxygen concentration detection value obtained by the oxygen concentration detecting means, and the furnace obtained by the furnace pressure detecting means The air-fuel ratio control amount is calculated by calculating the air-fuel ratio manipulated variable determined by the relationship between the pressure detection value, the fuel flow rate detection value obtained by the fuel flow rate detection means, and the time obtained by the timing means. The method for controlling the atmosphere of a heating furnace is characterized in that the air-fuel ratio is changed by outputting to the air-fuel ratio, and when the extraction door is closed, the air-fuel ratio is restored.