JPH09256072A - Operation of continuous annealing furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently execute the change of annealing cycle in a short time by supplying the minimum quantity of combustion gas together with the low temp. air into combustion burners at the time of changing a mode from high temp. annealing cycle to low temp. annealing cycle. SOLUTION: At the time of changing a mode from a stationary operation at the high temp. annealing cycle to the low temp. annealing cycle in a continuous annealing furnace 6, firstly, fuel supply rate to the burner is limited with a flow rate control valve 15a to make the fuel supply rate absolutely minimum for keeping the combustion. At the time of changing the furnace temp. change- over valves 4a, 4b are closed and a change-over valve 4 is opened. The air for combustion fed out from a fan 5 as combustion air is supplied to the burners 12 from a piping 11 without executing the heat exchange at a recuperate 2. The max. supply rate of the air for combustion to the burners 12 is attained by fully opening a flow rate control valve 15b at the initial time of starting cooling. Thereafter, the supply rate is controlled while detecting the difference between the furnace temp. and the set temp. By this method, a large quantity of the low temp. air is supplied to the burners 12 and the dropping speed of the furnace temp. is accelerated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating a direct-fired continuous annealing furnace, and more particularly to a method for operating an annealing furnace when changing an annealing cycle.

[0002]

2. Description of the Related Art Generally, in an annealing furnace, the annealing temperature is determined by the material to be annealed, and if the material to be annealed changes, the furnace temperature of the annealing furnace must be changed. Conventionally, when changing from a high temperature annealing cycle to a low temperature annealing cycle in an annealing furnace, a method of reducing the combustion gas to the minimum and waiting for the temperature of the furnace to decrease by natural cooling has been generally used. Therefore, in the annealing furnace having a high heat retention performance, there is a problem that the change from the high temperature annealing cycle to the low temperature annealing cycle requires a long time and the production efficiency is reduced.

Further, in the heating furnace and the annealing furnace, the supply of fuel gas may be stopped and only the combustion air may be jetted from the burner into the furnace to completely cool the furnace. However, in the heating furnace and the annealing furnace, a recuperator is installed in the exhaust gas flue, and the combustion air is preheated by the sensible heat of the exhaust gas through the recuperator to improve the combustion efficiency and the thermal efficiency of the furnace. Therefore, the combustion air flows into the furnace from the burner after being heat-exchanged through the recuperator during furnace cooling as well as during furnace operation. Therefore, there is a problem that the temperature of the combustion air becomes high and the cooling effect is reduced.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above problems and provides a continuous annealing furnace capable of efficiently and quickly completing the change from the high temperature annealing cycle to the low temperature annealing cycle in a direct-fired continuous annealing furnace. The purpose is to provide a method of operation.

[0005]

Means for Solving the Problems As a result of earnest studies on an efficient furnace operating method in a direct-fired annealing furnace, the present inventors have obtained the prospect that it can be achieved by changing the temperature and flow rate of combustion air. The invention has been constructed. That is, the present invention, in the direct-fired continuous annealing furnace, when changing from the high temperature annealing cycle to the low temperature annealing cycle, the minimum amount of combustion gas that can continue combustion is supplied to the combustion burner together with the low temperature combustion air. The operating method of the continuous annealing furnace is characterized in that the furnace temperature is rapidly set to the set temperature of the low temperature annealing cycle. Further, the present invention is the method for operating a continuous annealing furnace, wherein the combustion air is air having a temperature of 250 ° C. or less, and the supply amount thereof is 8.0 or more in air ratio.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention can be suitably applied to a direct-fired heating type continuous annealing furnace. In the direct-fire heating system, fuel gas and combustion air are supplied to a burner and burned, and heat is transferred by radiation from a flame and convection by the combustion gas. FIG. 2 shows a conceptual diagram of a combustion gas / air piping system of a continuous annealing furnace capable of carrying out the method of the present invention.

The steel strip 9 is annealed in the continuous annealing furnace 6 and then cooled in the cooling zone 7. Exhaust gas 10 of continuous annealing furnace 6
Is discharged to the outside of the furnace through the flue by the exhaust gas fan 1. A boiler 3 and a recuperator 2 are provided inside the flue to collect the sensible heat of the exhaust gas. When the furnace is in steady operation, the combustion air 14 is sent from the combustion air fan 5 and enters the recuperator 2 through the pipe 11. At this time, the switching valve 4 is closed and the switching valves 4a and 4b are opened. In the recuperator 2, the combustion air 14
Is heat-exchanged with the sensible heat of the exhaust gas 10 to become high-temperature air,
The flow rate is controlled by the flow rate control valve 15b through the pipe 11 and is supplied to the burner 12. The flow rate of the fuel gas 13 is supplied to the burner 12 after the flow rate of the fuel gas 13 is limited by the flow rate control valve 15a, and the fuel gas 13 is mixed with fuel air and burned.

When the continuous annealing furnace 6 is changed from the steady operation in the high temperature annealing cycle to the low temperature annealing cycle, it is necessary to cool the furnace from the high temperature to the low temperature. When changing the furnace temperature, first, the fuel supply amount to the burner is limited by the flow rate control valve 15a to the minimum amount that allows combustion to continue. If combustion is not continued, hunting of the furnace temperature will become too large when combustion is restarted, and it will take a long time for the furnace temperature to stabilize.
It is desirable to continue combustion with a minimum amount of fuel supply.

When changing the furnace temperature, the switching valve 4
A and 4b are closed and the switching valve 4 is opened. The combustion air sent from the combustion air fan 5 is supplied to the recuperator 2
It is supplied to the burner 12 through the pipe 11 without performing heat exchange in. Since heat exchange is not performed in the recuperator 2, the supplied air has a low temperature. The air temperature is preferably as low as possible, but is preferably 250 ° C. or lower. At the beginning of cooling, the amount of supply is such that the flow control valve 15b is opened to the maximum and supplied to the burner. In addition, it is desirable to control the subsequent supply amount of the combustion air while detecting the difference between the furnace temperature and the set temperature. In addition, the air ratio is 8.0 to
It is desirable to set it to 20.0, and if it is less than 8.0, the amount of air is too small and a rapid decrease in the furnace temperature cannot be expected. Meanwhile, 2
If it exceeds 0.0, the combustion burner will misfire due to the wind pressure of the supply air, so the upper limit was made.

As a result, a large amount of low-temperature air is supplied from the burner into the furnace, and the rate of decrease in furnace temperature increases. Compared with the case of supplying high-temperature air, the rate of decrease in furnace temperature is extremely high. The supply amount of low-temperature combustion air varies depending on the burner, but in the case of a direct-fired burner, it is preferably 600 m ³ / hr or more. In the present invention, the low temperature in the combustion air is 25
0 ° C or lower is desirable. When the temperature of the combustion air exceeds 250 ° C, the cooling effect of the combustion air is small.

In the continuous annealing furnace, the amount of combustion gas was set to the minimum value at which combustion could be continued, and the temperature of the combustion air supplied from the burner and the flow rate were changed to lower the furnace temperature. The measurement results of the rate of decrease of the furnace temperature in each zone in the furnace are shown in FIG. From FIG. 1, the lowering speed is increased by lowering the temperature of the combustion air and increasing the supply amount.

[0012]

EXAMPLE A direct-fired continuous annealing furnace having a heating zone of 10 zones equipped with a direct-burning burner was changed from a high temperature annealing cycle of 1110 ° C. to a low temperature annealing cycle of 850 ° C. As an example of the present invention, the combustion gas amount is set to 15 m ³ / hr, which is the minimum combustion state of the burner, and the combustion air is
The heat was not exchanged by the recuperator and was supplied to the burner. The temperature of the combustion air at that time was 210 ° C. At that time, the supply amount of the combustion air is 9.3 (6
00 m ³ / hr).

As a conventional example, the amount of combustion gas is set to 15 m ³ / hr which is the minimum combustion state of the burner, the combustion air is heat-exchanged by a recuperator, and its supply amount is
An air ratio of 4.7 (300 m ³⁾ corresponding to the minimum combustion condition
/ Hr). The temperature of the combustion air was 380 ° C. FIG. 3 shows the change curves of the furnace temperatures of the present invention example and the conventional example.

From FIG. 3, in the example of the present invention, the time from the start of cooling to the set temperature of the low temperature annealing cycle is about 8
It is 0 min, and it can be seen that the time is shortened to about 1/2 of the conventional example.

[0015]

According to the present invention, the annealing cycle can be changed from a high temperature to a low temperature in a short time, and the remarkable effect of improving the production efficiency can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing influences of a combustion air flow rate and a combustion air temperature on a descent rate of an annealing furnace.

FIG. 2 is a fuel system conceptual diagram of fuel gas / fuel air in a continuous annealing furnace suitable for implementing the present invention.

FIG. 3 is a graph showing changes in furnace temperature in Examples and Conventional Examples.

[Explanation of symbols]

 1 exhaust gas fan 2 recuperator 3 boiler 4 switching valve 4a switching valve 4b switching valve 5 combustion air fan 6 continuous annealing furnace 7 cooling zone 9 steel strip 10 exhaust gas 11 piping 12 burner 13 combustion gas 14 combustion air 15a flow control valve 15b Flow control valve

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Tsuzuki 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Chiba Works (72) Inventor Hiroshi Saito 1 Kawasaki-cho, Chuo-ku, Chiba Chiba Chiba Steel Works, Ltd.

Claims (2)

[Claims] 1. In a direct-fired continuous annealing furnace, at the time of changing from a high temperature annealing cycle to a low temperature annealing cycle, a minimum amount of combustion gas capable of continuing combustion is supplied to a combustion burner together with low temperature combustion air. A method for operating a continuous annealing furnace, wherein the furnace temperature is rapidly set to a set temperature for a low temperature annealing cycle. 2. A method for operating a continuous annealing furnace, wherein the combustion air is air having a temperature of 250 ° C. or less, and the supply amount is 8.0 or more in an air ratio.