JP2013117056A - Method for operating heat treatment furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for operating heat treatment furnace, which can minimize fuel loss during operation stop in a steel material heat treatment furnace using a radiant tube.SOLUTION: There is provided the method for operating heat treatment furnace in which C gas or M gas is combusted by a main burner of the radiant tube to heat to a predetermined heat treatment temperature of a furnace temperature of ≥800°C for heat-treating a steel material. During the operation stop of the heat treatment furnace, the main burner is extinguished for naturally cooling the furnace temperature. When the furnace temperature is decreased to lower than 800°C, a pilot burner of the radiant tube using LPG as combustion gas is ignited. When the furnace temperature is decreased to a temperature of 600-400°C, the main burner is ignited to rise the furnace temperature.

Description

  TECHNICAL FIELD The present invention relates to a method for operating a heat treatment furnace for heat-treating a steel material, and specifically relates to a method for operating a heat treatment furnace that can reduce fuel loss during suspension or standby of heat treatment. .

  In the manufacture of steel products, various heat treatments are performed in a heat treatment furnace in order to improve mechanical properties or the like or to impart desired properties. As the heating method of the heat treatment furnace, when combustion heat is used and the oxidation of the workpiece is allowed, a direct-fire type combustion heating method with high heating efficiency is adopted, but the oxidation of the workpiece is not allowed. Sometimes, an indirect heating method using a radiant tube heating device is employed.

  In recent years, a regenerative (heat storage type) radiant tube system incorporating a heat storage heat exchanger is often used as the radiant tube heating device from the viewpoint of energy saving. The heating device includes a radiant tube, a pair of burners having a heat storage body provided at both ends of the radiant tube (main burner), and a pair of combustion air switching three-way valves connected to the pair of main burners, respectively. In general, the main burner is alternately burned. That is, the combustion air passage of each main burner is used as a passage to which combustion air is supplied during burner combustion, and is used as an exhaust passage for combustion exhaust gas during non-combustion. A regenerator is connected to each. Each regenerator recovers the sensible heat of the combustion exhaust gas that passes through it and preheats the combustion air supplied at the next combustion. The high-temperature exhaust gas causes the heat of the exhaust gas generated by the combustion of one burner to It is collected by the heat storage body of the burner.

  In such a regenerative heating apparatus, a pilot burner is usually installed in the main burner in order to reliably ignite the main burner and ensure combustion stability. In order to ensure ignition of the main burner, the pilot burner is installed in the vicinity of the fuel nozzle and the combustion air nozzle where the fuel and the combustion air of the main burner are mixed, and the fuel and the combustion air are mixed with the combustible mixture. It becomes a structure that can be burned immediately. In general, as the radiant tube, a heat-resistant alloy tube having a diameter of 90 to 200 mm, a ceramic tube, or the like is used.

  By the way, many steel heat treatment furnaces are generally operated continuously, but when the furnace is opened for periodic inspections and periodic repairs, etc. There are times when it stops or pauses (hereinafter also referred to as “operation stop”). In such a case, it is usual to stop the operation of the heating furnace in order to reduce the fuel cost. For example, in a heat treatment furnace in which a steel material is heat treated at a temperature exceeding 800 ° C., when the operation is stopped, the main burner of the radiant tube is extinguished to naturally cool the furnace.

  In addition, if the main burner misfires or fuel gas leaks from the supply piping and unburned gas remains in the radiant tube, there is a risk of explosion, so the furnace temperature is set at a predetermined value. When the temperature falls below this temperature, the pilot burner is ignited to burn those unburned gas gases.

  As the fuel for the heat treatment furnace used in the steel industry, the main burner uses a coke oven gas (C gas) produced in-house or M gas that is a mixture of C gas and blast furnace gas (B gas). LPG and C gas are often used for pilot burners. However, since LPG is more expensive than M gas, even if the amount of LPG used for the pilot burner is small, the fuel cost cannot be ignored. Therefore, when the operation of the heat treatment furnace is temporarily stopped, it is necessary to reduce the fuel loss and minimize the fuel cost.

  As a technique for reducing the fuel cost when the heating furnace is stopped or when the temperature is raised thereafter, for example, there is a technique disclosed in Patent Document 1.

JP 2007-177316 A

  However, the technique of Patent Document 1 is a technique for a heat treatment furnace that heats a steel material such as bloom, billet, and slab using a direct-fired burner that burns LNG. In addition, a technique for reducing fuel loss when the operation is stopped in a heat treatment furnace using a radiant tube has been hardly studied.

  The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to provide a heat treatment furnace capable of minimizing fuel loss when a steel material heat treatment furnace using a radiant tube is shut down. It is to propose a method of operation.

  The inventors have intensively studied to solve the above problems, and have developed the present invention having the following configuration. That is, the present invention relates to a method for operating a heat treatment furnace in which C or M gas is burned by a main burner of a radiant tube and the furnace temperature is heated to a predetermined heat treatment temperature of 800 ° C. or higher to heat treat the steel material. When the operation is stopped, the main burner is extinguished to naturally cool the furnace temperature, and when the furnace temperature falls below 800 ° C., the pilot burner of the radiant tube using LPG as the combustion gas is ignited, When the temperature is lowered to 600 to 400 ° C., the heat treatment furnace operating method is characterized in that the main burner is ignited to raise the furnace temperature.

In the operation method of the heat treatment furnace of the present invention, when the operation stop time is 22.5 hours or less, the furnace temperature is lowered to a temperature of 600 to 400 ° C. by natural cooling, and then the main burner of the radiant tube is ignited. The furnace temperature is raised to an operational heat treatment temperature of 800 ° C. or higher.
When the operation stop time is longer than 22.5 hours and shorter than 27 hours, the furnace temperature is lowered to a temperature of 600 to 400 ° C. by natural cooling, and then the main burner of the radiant tube is ignited so that the furnace temperature is 700 to 900. The temperature is raised to a temperature of ℃ and held at that temperature, and the temperature is raised to the operating heat treatment temperature 2 hours before the resumption of operation.
When the operation stop time exceeds 27 hours, the furnace temperature is lowered to a temperature of 600 to 400 ° C. by natural cooling, and then the main burner of the radiant tube is ignited so that the furnace temperature is 800 ° C. to the furnace temperature at the time of heat treatment. It is characterized by repeating the process of extinguishing the main burner of the radiant tube and naturally cooling it again after being heated up to 2 hours and held for 2 hours.

  Moreover, the operating method of the heat treatment furnace of the present invention is characterized in that the cooling rate in natural cooling when the operation is stopped is 80 ° C./hr or less.

  According to the present invention, it is possible to suppress excessive cooling of the furnace temperature when the heat treatment furnace is stopped, and to minimize fuel loss when the operation is stopped and restarted. Greatly contribute. In addition, by limiting the cooling rate when the operation is stopped, damage to the radiant tube due to thermal strain can be prevented, which contributes to improvement in productivity.

It is the figure which showed an example of the operation method in case the operation stop time of a heat processing furnace is less than 22.5 hours. It is the figure which showed an example of the operation method in case the operation stop time of a heat processing furnace is 22.5 hours or more and less than 27 hours. It is the figure which showed an example of the operation method in case the operation stop time of a heat processing furnace is 27 hours or more.

  A heat treatment furnace for steel generally has a plurality of heating zones, and heat treatment is performed by heating the furnace temperature to a temperature (heat treatment temperature) of 800 ° C. or higher, for example, about 850 to 950 ° C. On the other hand, when the operation is temporarily stopped because the heat treatment furnace is stopped or waiting, for example, the main burner of the radiant tube is usually extinguished and naturally cooled. However, when restarting the operation, it is necessary to quickly raise the furnace temperature to the temperature at the time of operation. If the furnace temperature is lowered too much, it takes a long time to increase the temperature and the fuel used increases. become.

  Therefore, in the operation method of the heat treatment furnace of the present invention, when the operation is resumed, the furnace temperature is maintained at a predetermined temperature that can be dealt with immediately, for example, a temperature of 500 ° C., and when the operation is resumed, The main burner was ignited and the furnace temperature was quickly raised to the heat treatment temperature. In addition, the minimum temperature of the furnace temperature at the time of the above operation stop is not necessarily 500 ° C., and may be changed depending on the heat insulating property and heating capacity inherent to the heat treatment furnace, but it is an intermediate temperature between the room temperature and the furnace temperature at the time of operation. It is most efficient and preferable that the temperature is within the range of 400 to 600 ° C.

  In addition, it is preferable that the cooling rate in the said natural cooling shall be 80 degrees C / hr or less. This is because if the cooling rate exceeds 80 ° C./hr, the radiant tube may be damaged due to thermal strain during cooling. Preferably it is 50 degrees C / hr or less. Here, the said cooling rate is an average cooling rate when the atmospheric temperature in the furnace of the part in which the radiant tube of a heating zone is installed cools to the furnace temperature -200 degreeC at the time of heat processing.

  Further, the pilot burner is for surely igniting the main burner and ensuring the combustion stability of the gas, and does not need to be always ignited. However, if unburned gas remains in the radiant tube, an explosion may occur. Therefore, in order to surely burn the unburned gas, it is preferable to ignite the pilot burner when the temperature in the radiant tube (equal to the furnace temperature when the operation is stopped) is lower than the ignition temperature of the combustion gas. For example, when the combustion gas of the main burner is C gas or M gas, the ignition temperature is about 730 ° C. Therefore, in the present invention, in order to ensure safety, the pilot burner is always ignited at a temperature lower than 800 ° C.

  However, when the combustion gas used for the pilot burner is LPG, it is more expensive than C gas or M gas, and it is preferable to shorten the ignition time as much as possible in order to reduce the fuel cost. Therefore, for example, when the furnace temperature is lowered to less than 800 ° C., it is better to cool the pilot burner using expensive LPG while igniting it, or the main using cheap M gas or C gas The question is whether to ignite the burner, raise the furnace temperature, and not ignite the pilot burner. In the case of natural cooling while igniting the former pilot burner, if the furnace temperature is excessively cooled, the amount of M gas and C gas required for raising the temperature to the furnace temperature during operation of 800 ° C. or higher increases. However, since the time becomes longer, the fuel cost increases. Therefore, there should be an optimum temperature for the furnace temperature to be lowered. That is, when the heating furnace is stopped and the furnace temperature falls below 800 ° C., the furnace temperature is set so that the total fuel cost of the M and C gases for the main burner and the LPG for the pilot burner is minimized. Need to control.

  In order to solve the above-mentioned problems, the inventors have studied an operation method that can reduce the fuel cost most by changing the operation pattern when the operation is stopped in the actual heat treatment furnace. As a result, when there is an optimum operation pattern depending on the operation stop time, specifically, when the operation stop time is 22.5 hours or less, as shown in FIG. An operation pattern in which the temperature is lowered to a temperature of 600 to 400 ° C. (FIG. 1 shows an example of 500 ° C.) and then the main burner of the radiant tube is ignited to raise the furnace temperature to a predetermined heat treatment temperature of 800 ° C. In the case where the operation stop time is longer than 22.5 hours and shorter than 27 hours, as shown in FIG. 2, the furnace temperature reaches 600 to 400 ° C. by natural cooling (FIG. 2 shows an example of 500 ° C.). After lowering the temperature, the main burner of the radiant tube is ignited, the furnace temperature is raised to a temperature of 700 to 900 ° C. and maintained at that temperature, and the operation pattern of raising the temperature to a predetermined heat treatment temperature 2 hours before the resumption of operation is , The above operation stop When the time exceeds 27 hours, as shown in FIG. 3, the furnace temperature is lowered to a temperature of 600 to 400 ° C. by natural cooling (FIG. 3 shows an example of 500 ° C.) and then the main burner of the radiant tube. By igniting, the furnace temperature is raised from 800 ° C. to the furnace temperature at the time of heat treatment, and after holding for 2 hours, the main burner of the radiant tube is extinguished again and the operation pattern is repeated repeatedly. It became clear that the cost could be kept the lowest.

  In a heat treatment furnace that heats the steel by heating the furnace temperature (operation heat treatment temperature) to 900 ° C with a radiant tube that uses M gas as the fuel gas for the main burner and LPG as the fuel gas for the pilot burner, In the case where the main burner of the radiant tube is extinguished and naturally cooled, the operation stop time (the time from the start of cooling until the heat treatment temperature is reheated to 900 ° C. and restarted) is 20 hours and 25 hours. 1 to 3 shown in FIGS. 1 to 3 and the operation pattern D in which the main burner of the radiant tube is simply extinguished to naturally cool the used amount of M gas and LPG at 30 hours. Compared on the basis.

  As a result, the operation method when the operation of the heat treatment furnace is stopped has the most suitable operation pattern depending on the length of the operation stop time, and this pattern has a lower fuel cost than the operation pattern D for natural cooling. Specifically, when the operation stop time of the heat treatment furnace is less than 22.5 hours, the operation pattern A as shown in FIG. 1 has an operation stop time of 22.5 hours or more and less than 27 hours. In this case, the operation pattern B as shown in FIG. 2 is adopted, and when the operation stop time is 27 hours or more, the operation pattern C as shown in FIG. It was confirmed that can be reduced most.

Claims (3)

In a method of operating a heat treatment furnace in which C or M gas is burned by a main burner of a radiant tube and the furnace temperature is heated to a predetermined heat treatment temperature of 800 ° C. or more to heat treat the steel material,
When the heat treatment furnace is shut down, the main burner is extinguished to naturally cool the furnace temperature,
When the furnace temperature drops below 800 ° C., ignite the pilot burner of the radiant tube that uses LPG as the combustion gas,
A method for operating a heat treatment furnace, characterized in that when the furnace temperature falls to a temperature of 600 to 400 ° C., the main burner is ignited to raise the furnace temperature. When the operation stop time is 22.5 hours or less, after the furnace temperature is lowered to a temperature of 600 to 400 ° C. by natural cooling, the main burner of the radiant tube is ignited and the operation heat treatment is performed at a furnace temperature of 800 ° C. or more. Rise to temperature,
When the operation stop time is longer than 22.5 hours and shorter than 27 hours, the furnace temperature is lowered to a temperature of 600 to 400 ° C. by natural cooling, and then the main burner of the radiant tube is ignited so that the furnace temperature is 700 to 900. The temperature is raised to a temperature of ℃ and held at that temperature, and the temperature is raised to the operating heat treatment temperature 2 hours before the resumption of operation.
When the operation stop time exceeds 27 hours, the furnace temperature is lowered to a temperature of 600 to 400 ° C. by natural cooling, and then the main burner of the radiant tube is ignited so that the furnace temperature is 800 ° C. to the furnace temperature at the time of heat treatment. The method of operating a heat treatment furnace according to claim 1, wherein after the temperature is raised to 2 hours and held for 2 hours, the main burner of the radiant tube is extinguished and naturally cooled again. The method for operating a heat treatment furnace according to claim 1 or 2, wherein a cooling rate in natural cooling when the operation is stopped is 80 ° C / hr or less.

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