JP4873325B2 - In-furnace atmosphere control method for heating furnace - Google Patents

Description

  The present invention relates to a method for controlling a furnace atmosphere (oxygen concentration) of a heating furnace for heating a steel material. In particular, the present invention is a heating furnace capable of stabilizing the furnace atmosphere even when the furnace temperature is quickly lowered, and thus maintaining the quality of the steel material and preventing smoke generation. The present invention relates to a furnace atmosphere control method.

  Conventionally, heating furnaces for steel materials such as cast slabs and steel slabs have been used to heat steel materials to a temperature suitable for rolling when the steel materials are subjected to ingot rolling or hot rolling.

  A heating furnace generally has a pre-tropical zone, a heating zone, and a soaking zone in order from the steel material charging side. Since the heating furnace needs to raise the temperature of the steel material promptly with emphasis on productivity, a radiant tube is not used as in the heat treatment furnace, but a burner is provided at least in the heating zone and the soaking zone. The furnace is heated and held at a predetermined temperature by directly mixing and burning the fuel and combustion air with the installed burner. The steel material introduced into the pre-tropical zone from the charging door moves on the conveyance path, passes through the heating zone and the soaking zone, and is heated to a predetermined temperature while being transported from the extraction door on the soaking zone to the outside of the furnace. . In addition, the exhaust gas produced by the combustion in the burner is discharged out of the furnace through a flue provided on the entrance side of the pre-tropical zone. At that time, the recuperator provided in the flue preheats the combustion air supplied to the burner to recover the heat in the exhaust gas. Furthermore, a damper for controlling the furnace pressure is also arranged in the flue.

  Here, when the furnace pressure of the heating furnace decreases, air flows in from the outside of the furnace, and the oxygen concentration in the furnace increases, thereby increasing the decarburization layer and oxide scale layer formed on the surface of the steel material, There is a possibility that the quality of the steel material is deteriorated such that the surface skin becomes rough. For this reason, conventionally, various methods for appropriately controlling the furnace pressure have been proposed (see, for example, Patent Document 1).

On the other hand, the furnace temperature of the heating furnace is controlled by the flow rate of fuel supplied to the burner. The flow rate of the combustion air supplied to the burner is the air flow ratio determined from the viewpoint of the flow rate of the fuel supplied to the burner and the combustion efficiency (the flow rate of the combustion air supplied to the burner and the flow rate of the fuel And the ratio). More specifically, a flow rate adjustment valve is provided in a pipe for supplying combustion air to the burner, and the flow rate of the combustion air is adjusted by adjusting the opening of the flow rate adjustment valve. When the air-fuel ratio fluctuates and the flow rate of combustion air becomes excessive, the oxygen concentration in the furnace increases, so that it is formed on the surface of the steel material as in the case where the furnace pressure decreases. There is a risk of degrading the quality of the steel material, such as an increase in the decarburized layer and oxide scale layer, and rough surface. On the other hand, if the air-fuel ratio fluctuates and the flow rate of combustion air becomes too low, there is a risk that smoke may be generated due to incomplete combustion. For this reason, conventionally, various methods for appropriately controlling the air-fuel ratio have been proposed (see, for example, Patent Document 2).
JP 2002-220620 A JP-A-60-129594

  However, the conventional method of controlling the furnace pressure and the air-fuel ratio cannot avoid the possibility that the quality of the steel material deteriorates or smoke is generated.

  For example, when the in-furnace time of the steel material becomes longer depending on the state of rolling operation of the steel material, the decarburized layer and the oxide scale layer may increase, and the quality of the steel material may be deteriorated. In order to avoid this, the furnace temperature may need to be quickly reduced. Moreover, depending on the material of the steel material, the heating temperature may have to be kept low. For this reason, it is necessary to change the furnace temperature promptly in order to process various steel materials without lowering the productivity by using the same heating furnace in a situation where the production volume is increasing recently. In such a case, in order to quickly lower the furnace temperature, the flow rate of the fuel supplied to the burner may be reduced, but the combustion supplied to the burner according to this in order to obtain an appropriate air-fuel ratio. The air flow rate must also be reduced.

  However, there is a problem that the flow rate of the combustion air cannot be appropriately controlled in a low flow rate range below a predetermined value due to a problem in mechanical characteristics of the flow rate adjusting valve. For example, in a test conducted by the present inventors, if the opening degree of the flow rate adjustment valve is less than 10%, the opening degree of the flow rate adjustment valve and the flow rate are not proportional. I could not. For this reason, the flow rate of the combustion air becomes excessive with respect to the flow rate of the fuel to be supplied, and as a result, the formation of the decarburized layer and the oxide scale layer becomes remarkable, leading to the deterioration of the quality of the steel material or the reverse supply. There is a risk that smoke will be generated when the flow rate of combustion air becomes too small relative to the flow rate of fuel. Therefore, there is a limit to quickly changing the furnace temperature.

  The present invention has been made to solve such problems of the prior art, and even when the furnace temperature is quickly lowered, the atmosphere in the furnace can be stabilized, and consequently the quality of the steel material can be improved. It is an object of the present invention to provide a furnace atmosphere control method for a heating furnace that can maintain or prevent smoke generation.

  In order to solve the above problems, the present invention is a method for controlling the atmosphere in a furnace having a pretropical zone, a heating zone and a soaking zone, and supplying combustion air to a burner provided at least in the soaking zone A first flow rate adjusting valve is provided in the main pipe for performing the operation, and a second flow rate adjusting valve is provided in the bypass pipe having a smaller diameter than the main pipe so as to communicate with the main pipe, and the combustion air supplied to the burner When the flow rate is controlled to be less than a predetermined value, the opening degree of the first flow rate adjustment valve is set to 0%, and the flow rate of the combustion air is controlled only by adjusting the opening degree of the second flow rate adjustment valve. A furnace atmosphere control method for a heating furnace is provided.

  According to the method of the present invention, the first flow rate adjusting valve is provided in the main pipe for supplying combustion air to the burner provided at least in the soaking zone, and communicates with the main pipe rather than the main pipe. A second flow rate adjustment valve is provided in the small diameter bypass pipe. Accordingly, the flow rate of the combustion air supplied to the burner provided in the soaking zone that is likely to affect the increase in the decarburization layer and the oxide scale layer at least because of the high temperature is that of the first and second flow control valves. It can be adjusted by adjusting the opening degree. For example, if the opening degree of the second flow rate adjustment valve provided in the small diameter bypass pipe is set to 0% and only the opening degree of the first flow rate adjustment valve provided in the main pipe is adjusted, the main pipe It becomes the same as the conventional method using only. That is, when the opening degree of the first flow rate adjustment valve is less than a predetermined value (for example, less than 10%), the opening degree of the first flow rate adjustment valve and the flow rate of the combustion air flowing through the main pipe are not proportional. The flow rate of combustion air cannot be controlled properly.

  Therefore, in the method according to the present invention, when the flow rate of the combustion air supplied to the burner is controlled to be less than a predetermined value, the opening degree of the first flow rate adjustment valve is set to 0%, It is characterized by controlling the flow rate of combustion air only by adjusting the opening. When the opening degree of the second flow rate adjustment valve provided in the bypass pipe is less than a predetermined value, the opening degree of the second flow rate adjustment valve and the combustion flow through the bypass pipe are the same as in the case of the first flow rate adjustment valve. It is no longer proportional to the air flow. However, since the bypass pipe has a smaller diameter than the main pipe, even if the second flow rate adjustment valve cannot control the flow rate of the combustion air properly with the same degree of opening as the first flow rate adjustment valve, the bypass pipe is controlled. The lower limit of the possible flow rate of combustion air is smaller than that of the main pipe. Therefore, according to the present invention, the flow rate of combustion air can be appropriately controlled even in a low flow rate region.

  According to the furnace atmosphere control method for a heating furnace according to the present invention, the flow rate of combustion air can be appropriately controlled even in a low flow rate region, so even if the furnace temperature is quickly lowered, The atmosphere can be stabilized, and as a result, the quality of the steel material can be maintained and smoke generation can be prevented.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings as appropriate.
FIG. 1 is a schematic diagram showing a schematic configuration of a heating furnace and its incidental equipment to which the furnace atmosphere control method according to the present invention is applied. FIG. 2 is an enlarged schematic view showing a part of the configuration shown in FIG. 1 (the part indicated by A in FIG. 1). As shown in FIG. 1, the heating furnace 100 has a pretropical zone, a heating zone, and a soaking zone in order from the charging side of the steel material M. Generally, the pre-tropical furnace temperature is controlled to 500 to 700 ° C, the furnace temperature in the heating zone is controlled to 900 to 1150 ° C, and the soaking furnace temperature is controlled to 1050 to 1250 ° C.

  The heating furnace 100 is provided with a burner 1 at least in the heating zone and in the soaking zone. In the example shown in FIG. 1, a burner 1a is provided in the upper area of the heating zone, a burner 1b is provided in the lower area of the heating zone, a burner 1c is provided in the upper area of the soaking zone, and a burner 1d is provided in the lower zone of the soaking zone. Then, the furnace and the combustion air are directly mixed and burned by the installed burner 1 (1a to 1d), whereby the inside of the heating furnace 100 is heated and held at the predetermined temperature as described above. The steel material M introduced into the pre-tropical zone from the charging door (not shown) moves in the direction of the arrow X on the conveyance path 2, passes through the heating zone and the soaking zone, and is extracted from the soaking zone (see FIG. (Not shown) while being carried out of the furnace.

  In addition, the exhaust gas produced by the combustion in the burner 1 is discharged out of the furnace from the flue 3 provided on the pre-tropical entry side. At that time, the recuperator 4 provided in the flue 3 preheats the combustion air supplied to the burner 1 to recover the heat in the exhaust gas. A damper 5 for controlling the furnace pressure is disposed in the flue 3, and a furnace pressure gauge 6 is disposed in the pre-tropical zone. The furnace pressure of the heating furnace 100 is controlled by opening and closing the damper 5 based on the furnace pressure measurement value by the furnace pressure gauge 6. That is, when the furnace pressure measurement value by the furnace pressure gauge 6 is lower than the preset target furnace pressure, the damper 5 is closed, and when the furnace pressure measurement value is higher than the target furnace pressure, the damper 5 is opened.

  As shown in FIGS. 1 and 2, the furnace atmosphere control method according to the present invention is applied to at least the soaking zone (both soaking zone and heating zone in this embodiment) of the heating furnace 100 having the above-described configuration. A first flow rate adjusting valve 11 is provided in a main pipe 10 (10a to 10d) for supplying combustion air to the provided burner 1, and a bypass pipe that communicates with the main pipe 10 and has a smaller diameter than the main pipe 10. The second flow rate adjusting valve 21 is provided at 20 (20a to 20d). In the present embodiment, the inner diameter of the bypass pipe 20 is 3/7 of the inner diameter of the main pipe 10, that is, the cross-sectional area of the bypass pipe 20 is 18% of the cross-sectional area of the main pipe 10.

  Here, if the cross-sectional area of the bypass pipe 20 with respect to the main pipe 10 is too small, the opening of the first flow control valve 11 provided in the main pipe 10 becomes small, and the flow rate of the combustion air is reduced to the first flow control valve. 11 cannot be controlled properly by adjusting the opening degree of 11, the control cannot smoothly and continuously shift to the control by adjusting the opening degree of the second flow rate adjusting valve 21 provided in the bypass pipe 20. On the other hand, if the cross-sectional area of the bypass pipe 20 with respect to the main pipe 10 is too large, the lower limit of the flow rate of combustion air that can be controlled by adjusting the opening of the second flow rate control valve 21 provided in the bypass pipe 20 becomes large (main Because the difference from the lower limit of the flow rate of combustion air that can be controlled by adjusting the opening of the first flow rate control valve 11 provided in the pipe 10 is reduced), the flow rate of combustion air in the low flow rate region cannot be controlled appropriately. . Therefore, it is desirable that the cross-sectional area of the bypass pipe 20 is 15 to 25% of the cross-sectional area of the main pipe 10.

  The combustion air is supplied from a blower (not shown), preheated with heat recovered from the exhaust gas flowing through the flue by the recuperator 4, and then branched from the main pipe 10 (10a to 10d). Sent to.

  In the furnace atmosphere control method according to the present invention, when the flow rate of the combustion air supplied to the burner 1 is controlled to be less than a predetermined value, the opening of the first flow rate adjustment valve 11 is set to 0%, The flow rate of the combustion air is controlled only by adjusting the opening degree of the second flow rate adjusting valve 21.

In the present embodiment, as shown in Table 1 below, the flow rate of combustion air supplied to the burner 1 (that is, the flow rate V1 on the outlet side of the first flow rate adjusting valve 11 and the outlet side of the second flow rate adjusting valve 21). The maximum flow rate (that is, the flow rate obtained when both the first flow rate adjustment valve 11 and the second flow rate adjustment valve 21 are fully opened, and is indicated by 100% in Table 1). Is set to 0%, the flow rate of the combustion air is controlled only by adjusting the opening degree of the second flow rate adjustment valve 21. It should be noted that the opening of the flow rate adjustment valve is intended to actually flow with respect to the maximum flow rate of combustion air that can flow through the pipe provided with this flow rate adjustment valve (the flow rate obtained when the flow rate adjustment valve is fully opened). It means the ratio of the flow rate of combustion air. For example, setting the opening degree of the first flow rate adjustment valve 11 to 100% means that the first flow rate adjustment valve 11 is fully opened, and the flow rate V1 on the outlet side of the first flow rate adjustment valve 11 is maximized. Means. Setting the opening degree of the first flow rate adjustment valve 11 to 0% means that the first flow rate adjustment valve 11 is fully closed, and that the flow rate V1 on the outlet side of the first flow rate adjustment valve 11 becomes zero. means. Setting the opening of the first flow rate adjusting valve 11 to 10% means that the flow rate V1 on the outlet side of the first flow rate adjusting valve 11 is set to 10% with respect to the maximum flow rate.

In the present embodiment, as described above, by setting the inner diameter of the bypass pipe 20 to 3/7 of the inner diameter of the main pipe 10, the maximum flow rate of the combustion air on the outlet side of the first flow control valve 11 is 16000 Nm ³ / h, and the maximum flow rate of the combustion air on the outlet side of the second flow rate adjusting valve 21 is 2900 Nm ³ / h. By applying the control method shown in Table 1 above, the flow rate of the combustion air supplied to the burner 1 is about 290 Nm ³ / h (confirmed with an orifice flow meter provided near the second flow rate adjustment valve 21. It was found that it was possible to control the flow rate as intended up to the low flow rate range. Then, when the oxygen concentration in the furnace was confirmed with an oxygen concentration meter 7 arranged in the soaking zone, the oxygen concentration was reduced to about 1 to 2% when the flow rate of combustion air supplied to the burner 1 was set to a low flow rate range. Was found to be adjustable.

On the other hand, adjustment by a conventional method using only the first flow rate adjustment valve 11 provided in the main pipe 10 (in this case, the maximum flow rate of the combustion air on the outlet side of the first flow rate adjustment valve 11 is set to 16000 Nm ³ / h. In this case, it was possible to control the flow rate as intended only in the flow range of about 1600 Nm ³ / h (confirmed with an orifice flow meter provided in the vicinity of the first flow rate adjustment valve 11). And when the oxygen concentration in the furnace was confirmed with the oxygen concentration meter 7 arranged in the soaking zone, when the flow rate of the combustion air supplied to the burner 1 was set to a low flow rate region, the oxygen concentration increased to 4% or more. In some cases, it was found that the oxygen concentration could not be adjusted stably low.

  As described above, according to the furnace atmosphere control method for a heating furnace according to the present invention, since the flow rate of combustion air can be appropriately controlled even in a low flow rate region, the furnace temperature is quickly reduced. Even so, the furnace atmosphere (oxygen concentration) can be stabilized. As a result, it is possible to maintain the quality of the steel material M and prevent smoke generation.

FIG. 1 is a schematic diagram showing a schematic configuration of a heating furnace and its incidental equipment to which the furnace atmosphere control method according to the present invention is applied. FIG. 2 is an enlarged schematic view showing a part of the configuration shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Burner 2 ... Conveyance path 3 ... Flue 4 ... Recuperator 5 ... Damper 6 ... Furnace pressure gauge 7 ... Oxygen concentration meter 10 ... Main piping 11 ... 1st flow control valve 20 ... Bypass piping 21 ... 2nd flow control valve 30 ... Blower 100 ... Heating furnace

Claims (1)

A method for controlling the furnace atmosphere of a heating furnace having a pre-tropical zone, a heating zone and a soaking zone,
A first flow rate adjusting valve is provided in a main pipe for supplying combustion air to a burner provided at least in the soaking zone, and a second pipe is connected to the main pipe and has a smaller diameter than the main pipe. Provide a flow control valve,
When the flow rate of combustion air supplied to the burner is controlled to be less than a predetermined value, the opening of the first flow rate adjustment valve is set to 0%, and the opening amount of the second flow rate adjustment valve is only adjusted. A furnace atmosphere control method for a heating furnace, characterized by controlling a flow rate of combustion air.

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