阅读说明：本技术 一种用于钢管淬火炉热处理的控制方法 (Control method for heat treatment of steel pipe quenching furnace ) 是由 吕立华 邓龙 周炜 潘飞 于 2018-06-15 设计创作，主要内容包括：本发明涉及一种用于钢管淬火炉热处理的控制方法,所述控制方法在一级控制系统和MES之间设置二级模型控制系统,所述二级模型控制系统包括物料跟踪模块、温度跟踪模块、温度设定模块和空燃比优化模块,物料跟踪模块实现炉内钢管的位置跟踪,温度跟踪模块实现炉内钢管的温度计算,温度设定模块实现炉内温度的优化设定,空燃比优化模块实现空燃比的优化设定。解决现有的热处理过程由于缺乏二级控制,对于小批量、多品种的情况,就要凭经验进行空炉,操作起来特别困难,产品质量不稳定,能源消耗大的问题。(The invention relates to a control method for heat treatment of a steel pipe quenching furnace, which is characterized in that a secondary model control system is arranged between a primary control system and an MES (manufacturing execution system), the secondary model control system comprises a material tracking module, a temperature setting module and an air-fuel ratio optimization module, the material tracking module realizes position tracking of a steel pipe in the furnace, the temperature tracking module realizes temperature calculation of the steel pipe in the furnace, the temperature setting module realizes optimal setting of the temperature in the furnace, and the air-fuel ratio optimization module realizes optimal setting of the air-fuel ratio. The method solves the problems that the prior heat treatment process is lack of secondary control, and the empty furnace is required to be carried out by experience under the conditions of small batch and various varieties, so that the operation is very difficult, the product quality is unstable, and the energy consumption is high.)

1. A control method for heat treatment of a steel pipe quenching furnace is characterized by comprising the following steps: the control method is characterized in that a secondary model control system is arranged between a primary control system and the MES, the secondary model control system comprises a material tracking module, a temperature setting module and an air-fuel ratio optimization module, and the control method comprises the following steps:

(1) tracking the material in real time: tracking the position of the tube blank in the furnace in real time according to the received charging signal, the received tapping signal and the walking beam moving signal;

(2) and (3) carrying out real-time tracking calculation on the temperature of the tube blank in the furnace: calculating furnace temperatures corresponding to different position points in the furnace according to the positions of thermocouples in the furnace and the measured temperatures of the thermocouples in the furnace, calculating surface heat flow of the tube blank according to the furnace temperatures, and calculating the temperature distribution of the tube blank in the furnace according to a heat conduction equation of the tube blank;

(3) optimally setting the furnace temperature: according to the current production rhythm and the process furnace temperature, the tapping temperature and the soaking time of the tube blank are predicted, and then the process furnace temperature is adjusted by combining the tapping temperature and the soaking time required by the tube blank process, so that the predicted tapping temperature and the predicted soaking time of the tube blank in different control sections meet the process requirement, and the dynamic adjustment of the furnace temperature is realized;

(4) air-fuel ratio optimization setting: establishing a corresponding relation table of different gas heat values and air-fuel ratios, if the fluctuation of the gas heat values exceeds a set threshold value, looking up the table according to the current gas heat values to obtain the corresponding air-fuel ratios, and taking the air-fuel ratios obtained by looking up the table as control set values;

if the fluctuation of the gas heat value is in a given range, calculating an air-fuel ratio correction value according to the oxygen content in the quenching furnace, and subtracting the air-fuel ratio correction value from the air-fuel ratio obtained by table lookup to be used as a control setting value.

2. The control method according to claim 1, wherein in the step (1), if the secondary model control system receives the charging signal and the charging position is empty, the tube blank is charged into the quenching furnace, and tube blank charging statistics are carried out; if the secondary model control system receives the walking beam moving signal, the secondary model control system updates the position information of the steel billet in the furnace according to the moving distance of the walking beam; and if the secondary model control system receives the tapping signal and the steel tapping position of the tube blank in the furnace, the secondary model control system carries out tube blank tapping statistics.

3. The control method according to claim 1, wherein in the step (2), when furnace temperatures at different positions are calculated, if no thermocouple exists on the furnace entering side, the measured temperature of the flue gas before heat exchange is used for substitution; if no thermocouple is arranged on the tapping side, according to the specific conditions of the type and the arrangement mode of the burner, subtracting a reasonable temperature deviation from the thermocouple measured temperature closest to the position on the tapping side in the furnace to obtain the temperature of the furnace gas on the tapping side; if the quenching furnace is not in a box-shaped structure, a transition area with large furnace gas temperature change exists between different control sections, the specific position of the transition area in the furnace, the starting point temperature and the end point temperature of the transition area can be determined by combining a 'toweling' temperature test in the furnace, and the temperature is measured by adopting a thermocouple in the furnace which is closest to the point in the same control section.

4. The control method according to claim 1, wherein in the step (2), the furnace gas temperature corresponding to the position p in the furnace is calculated by using the following linear equation:

wherein, T_cTo furnace gas temperature corresponding to position p, p₁、p₂The positions of 2 thermocouples or flue gas inflection points nearest to the front and back of the position p, and T1 and T2 are the temperatures of 2 thermocouples or flue gas inflection points nearest to the front and back of the position p.

5. The control method according to claim 1, wherein in the step (4), the air-fuel ratio correction value is calculated by:

where Δ La represents an air-fuel ratio correction value,X^Trespectively representing the residual oxygen amount measured at the time K and the heating furnace control target residual oxygen amount, K_p、K_IRespectively a proportional adjustment coefficient and an integral adjustment coefficient.

6. The control method according to claim 1, wherein in the step (3), if the predicted tapping temperature of the product is lower than the target temperature or the soaking time is shorter than the target soaking time, the process temperature is appropriately increased, otherwise, the process temperature is appropriately decreased, and then the tapping temperature and the soaking time of the product are predicted again by using the adjusted process temperature until the predicted value reaches the range of the process requirement.

7. The control method according to claim 6, wherein in the step (3), the method for predicting the tapping temperature and the soaking time of the tube blank comprises the following steps: and (3) calculating the remaining in-furnace time of the tube blank in the furnace according to the current production rhythm, taking the process furnace temperature as the measured temperature of the thermocouple, and calculating the tapping temperature and the soaking time of the tube blank when the tube blank is heated by the remaining in-furnace time and finally reaches the tapping position by adopting the method in the step (2), namely the predicted tapping temperature and soaking time.

8. The control method according to claim 1, wherein in the step (3), when the adjustment range of the furnace temperature exceeds the limit range of the process requirement, an alarm is given, namely:

(1) when the predicted tapping temperature reaches the upper limit of the process requirement, but the predicted soaking time does not reach the soaking time of the process requirement, an alarm is given to prompt that the production rhythm is too fast;

(2) when the predicted tapping temperature is at the lower limit of the process requirement, but the soaking time exceeds the soaking time of the process requirement, an alarm is given to prompt that the production rhythm is too slow.

Technical Field

The invention relates to the technical field of metallurgical production, relates to a heat treatment process, and particularly relates to a control method for heat treatment of a steel pipe quenching furnace.

Background

The heat treatment process is a key link for controlling the product performance, has strict requirements on the temperature of furnace gas, the time in the furnace and the atmosphere in the furnace, and hopes that the steel pipe has a uniform austenite state, and the surface is less oxidized and decarburized when the steel pipe is discharged out of the quenching furnace. These process requirements must be achieved by precisely controlling the heat treatment process. At present, the control is generally carried out according to a process schedule which comprises furnace gas temperature, tooth positions and beats of each section, and the process schedule is classified according to products and specifications. In the actual production process, the operator manually inputs the process set value on the L1 according to the specification of the product to be produced in the plan, which is only suitable for the stable mass production and requires the stable equipment state. For the conditions of small batch and various varieties, the empty furnace is required to be carried out by experience, the operation is particularly difficult, the product quality is unstable, and the energy consumption is high.

In the heat treatment process of the steel pipe production line in China, because a secondary model control system is not arranged, a manufacturing execution system MES and a primary control system are always in a separated state, and the automation level is low. With the development of intelligent manufacturing technology, the integrated operation of an information system and an equipment control system is urgently needed.

The information system and the operation system cannot be connected, and the biggest problem is that in actual production, there are many uncertainties, especially uncertainty of materials, such as: the production is not carried out according to the planned sequence on the site, so that the material tracking is abnormal. In short, under the condition of different product specifications, how to ensure the effectiveness of material tracking, how to ensure the accuracy of temperature model calculation, how to accurately control the atmosphere in the furnace and how to ensure the stability of production are the problems that the model control system must solve.

The thesis "steel pipe quenching and tempering furnace mathematical model optimization control research" establishes a steel pipe quenching furnace and tempering furnace mathematical model. The focus of the paper is the establishment of a heat conduction model and the optimization of the tempo, however, the paper cannot provide an effective solution for many uncertainties existing in the actual production.

In an intelligent heat treatment workshop, seamless butt joint of an information system and an operation system can be stably realized, and management and control integration of a steel pipe heat treatment production process is realized. Therefore, new modeling techniques must be developed to meet the requirements of advanced manufacturing.

Disclosure of Invention

The invention aims to provide a control method for heat treatment of a steel pipe quenching furnace, which can realize seamless connection of MES and production equipment by arranging a secondary model control system between the MES and a primary control system, realize integrated control of the MES, the secondary model control system and the primary control system, realize dynamic tracking of materials in the furnace, realize optimal setting of temperature, realize accurate control of atmosphere in the furnace and realize accurate calculation of blank temperature. The method is used for solving the problems that the prior heat treatment process is lack of secondary control, and the empty furnace is required to be carried out by experience under the conditions of small batch and various types, so that the operation is very difficult, the product quality is unstable, and the energy consumption is high.

In order to achieve the purpose, the scheme of the invention is as follows: a control method for heat treatment of a steel pipe quenching furnace is characterized in that a secondary model control system is arranged between a primary control system and an MES (manufacturing execution system), the secondary model control system comprises a material tracking module, a temperature setting module and an air-fuel ratio optimization module, and the control method comprises the following steps:

(1) tracking the material in real time: tracking the position of the tube blank in the furnace in real time according to the received charging signal, the received tapping signal and the walking beam moving signal;

(2) and (3) carrying out real-time tracking calculation on the temperature of the tube blank in the furnace: calculating furnace temperatures corresponding to different position points in the furnace according to the positions of thermocouples in the furnace and the measured temperatures of the thermocouples in the furnace, calculating surface heat flow of the tube blank according to the furnace temperatures, and calculating the temperature distribution of the tube blank in the furnace according to a heat conduction equation of the tube blank;

(3) optimally setting the furnace temperature: according to the current production rhythm and the process furnace temperature, the tapping temperature and the soaking time of the tube blank are predicted, and then the process furnace temperature is adjusted by combining the tapping temperature and the soaking time required by the tube blank process, so that the predicted tapping temperature and the predicted soaking time of the tube blank in different control sections meet the process requirement, and the dynamic adjustment of the furnace temperature is realized;

(4) air-fuel ratio optimization setting: establishing a corresponding relation table of different gas heat values and air-fuel ratios, if the fluctuation of the gas heat values exceeds a set threshold value, looking up the table according to the current gas heat values to obtain the corresponding air-fuel ratios, and taking the air-fuel ratios obtained by looking up the table as control set values;

if the fluctuation of the gas heat value is in a given range, calculating an air-fuel ratio correction value according to the oxygen content in the quenching furnace, and subtracting the air-fuel ratio correction value from the air-fuel ratio obtained by table lookup to be used as a control setting value.

Further, in the step (1), if the secondary model control system receives a charging signal and the charging position is empty, the tube blank is loaded into the quenching furnace, and statistics on the tube blank loading is carried out; if the secondary model control system receives the walking beam moving signal, the secondary model control system updates the position information of the steel billet in the furnace according to the moving distance of the walking beam; and if the secondary model control system receives the tapping signal and the steel tapping position of the tube blank in the furnace, the secondary model control system carries out tube blank tapping statistics.

Further, in the step (2), when furnace temperatures at different positions are calculated, if no thermocouple is arranged at the furnace entering side, the measured temperature of the flue gas before heat exchange is used for substitution; if no thermocouple is arranged on the tapping side, according to the specific conditions of the type and the arrangement mode of the burner, subtracting a reasonable temperature deviation from the thermocouple measured temperature closest to the position on the tapping side in the furnace to obtain the temperature of the furnace gas on the tapping side; if the quenching furnace is not in a box-shaped structure, a transition area with large furnace gas temperature change exists between different control sections, the specific position of the transition area in the furnace, the starting point temperature and the end point temperature of the transition area can be determined by combining a 'toweling' temperature test in the furnace, and the temperature is measured by adopting a thermocouple in the furnace which is closest to the point in the same control section.

Further, in the step (2), the furnace gas temperature corresponding to the furnace position p is calculated by adopting the following linear equation:

wherein, T_cTo furnace gas temperature corresponding to position p, p₁、p₂The positions of 2 thermocouples or flue gas inflection points nearest to the front and back of the position p, and T1 and T2 are the temperatures of 2 thermocouples or flue gas inflection points nearest to the front and back of the position p.

Further, in step (4), the air-fuel ratio correction value is calculated by:

where Δ La represents an air-fuel ratio correction value,respectively representing the residual oxygen amount measured at the time K and the heating furnace control target residual oxygen amount, K_p、K_IRespectively a proportional adjustment coefficient and an integral adjustment coefficient.

Further, in the step (3), if the predicted product tapping temperature is lower than the target temperature or the soaking time is shorter than the target soaking time, the process temperature is properly increased, otherwise, the process temperature is properly reduced, and then the product tapping temperature and the soaking time are predicted again by using the adjusted process temperature until the predicted value reaches the range of the process requirement.

Further, in the step (3), the method for predicting the tapping temperature and the soaking time of the tube blank comprises the following steps: and (3) calculating the remaining in-furnace time of the tube blank in the furnace according to the current production rhythm, taking the process furnace temperature as the measured temperature of the thermocouple, and calculating the tapping temperature and the soaking time of the tube blank when the tube blank is heated by the remaining in-furnace time and finally reaches the tapping position by adopting the method in the step (2), namely the predicted tapping temperature and soaking time.

Further, in the step (3), when the adjustment range of the furnace temperature exceeds the limit range of the process requirement, an alarm is given, namely:

(3) when the predicted tapping temperature reaches the upper limit of the process requirement, but the predicted soaking time does not reach the soaking time of the process requirement, an alarm is given to prompt that the production rhythm is too fast;

when the predicted tapping temperature is at the lower limit of the process requirement, but the soaking time exceeds the soaking time of the process requirement, an alarm is given to prompt that the production rhythm is too slow. The invention achieves the following beneficial effects: according to the invention, the second-level model control system is arranged between the MES and the first-level control system, so that seamless connection between the MES and production equipment can be realized, integrated control of the MES, the second-level model control system and the first-level control system is realized, dynamic tracking of materials in the furnace can be realized, optimal setting of temperature is realized, accurate control of atmosphere in the furnace is realized, and accurate calculation of blank temperature is realized.

Drawings

FIG. 1 is a schematic diagram of a heat treatment production system;

FIG. 2 is a flow chart of furnace temperature optimization settings;

FIG. 3 is a graph of furnace temperature in a quench furnace;

FIG. 4 is a graph of the temperature of all the tube blanks in the furnace;

FIG. 5 is a graph of predicted product discharge temperature information.

Detailed Description

The invention is described in further detail below with reference to the figures and specific examples.

As shown in FIG. 1, the invention arranges a secondary model control system (L2) between a Manufacturing Execution System (MES) and a primary control system (L1) to realize seamless connection between an information system and an operating system.

A secondary model control system:

the two-stage model control system comprises a communication module, a material tracking module, a temperature setting module and an air-fuel ratio optimization module.

And 1, a communication module is in communication connection with the MES and a primary control system L1, obtains production plan information and heat treatment process information from the MES, obtains quenching furnace instrument information and actuating mechanism action information from L1, and provides necessary data for calculation of a secondary model control system.

And the material tracking module is used for realizing real-time tracking of the position of the steel pipe in the quenching furnace by detecting a charging signal at the charging side of the quenching furnace, a walking beam moving signal and a discharging signal at the discharging side of the quenching furnace. The method comprises the following specific steps:

if a charging signal sent by the primary control system is received and the charging position is empty, a charging command is sent to the primary control system, the primary control system controls the tube blank to be charged into the quenching furnace, and the material tracking module carries out tube blank charging statistics;

if the walking beam moving signal sent by the primary control system is received, the secondary model control system updates the position information of the steel billet in the furnace according to the moving distance of the walking beam, and if the steel tapping signal sent by the primary control system is received and the steel tapping position of the steel billet in the furnace is reached, the material tracking module carries out steel tapping statistics on the steel billet.

And 3, the temperature tracking module receives the temperature measured by the thermocouple in the furnace sent by the primary control system, calculates the furnace gas temperature of different position points in the furnace according to the position of the thermocouple in the furnace and the temperature detected by the thermocouple, calculates the surface heat flow of the billet steel according to the furnace gas temperature, calculates the temperature distribution of the billet steel in the furnace according to the heat conduction equation of the billet steel, and sends the obtained temperature distribution condition of each billet steel to the temperature setting module.

The furnace gas temperature corresponding to the position p in the furnace is calculated by adopting the following linear equation:

wherein, T_cTo furnace gas temperature corresponding to position p, p₁、p₂The positions of 2 thermocouples or flue gas inflection points nearest to the front and back of the position p, and T1 and T2 are the temperatures of 2 thermocouples or flue gas inflection points nearest to the front and back of the position p.

And 4, the temperature setting module is used for predicting the tapping temperature and the soaking time of the tube blank according to the tube blank temperature of the temperature tracking module by taking the process furnace gas temperature and the production rhythm given by MES as basic values, dynamically adjusting the furnace gas temperature according to the tapping temperature and the soaking time required by the process, realizing optimal setting of the furnace temperature, and sending the adjusted furnace temperature set value to the primary control system so as to meet the heating quality of the quenching furnace.

5, an air-fuel ratio optimization module receives the in-furnace gas heat value, the gas flow, the air flow and the residual oxygen amount sent by the primary control system in real time, and sets the air-fuel ratio by adopting feed-forward control when the fluctuation of the gas heat value exceeds a given range, namely the difference value between the gas heat value at the previous moment and the gas heat value at the current moment is greater than or equal to a set threshold value; and setting the air-fuel ratio by adopting feedback control when the fluctuation of the gas heat value is in a given range.

The air-fuel ratio optimizing module sends the set value of the air-fuel ratio to the primary control system through the communication module, and timely and accurate control of the atmosphere in the quenching furnace is achieved.

The method for setting the air-fuel ratio by feedforward control comprises the following steps: establishing a mapping table of different gas heat values and air-fuel ratios, interpolating values from the mapping table to obtain a corresponding air-fuel ratio La according to the current heat value, and adopting the La as an air-fuel ratio set value;

the method for setting the air-fuel ratio by feedback control comprises the following steps: the air-fuel ratio correction value Δ La is calculated, and (La — Δ La) is used as the air-fuel ratio setting value.

Where Δ La represents a feedback correction value of the air-fuel ratio,respectively representing the measured residual oxygen amount at the moment K and the control target residual oxygen amount of the heating furnace, K_p、K_IRespectively a proportional adjustment coefficient and an integral adjustment coefficient.

The control method of the invention comprises the following steps:

1, tracking the material in real time:

and tracking the position of the tube blank in the furnace in real time according to the received charging signal, the received tapping signal and the walking beam moving signal.

The specific method comprises the following steps: and if the second-stage receives the charging signal and the charging position is empty, the tube blank is loaded into the quenching furnace, and statistics on the tube blank loading is carried out. And if the second stage receives the walking beam moving signal, the second stage model control system updates the position information of the steel billet in the furnace according to the moving distance of the walking beam. And if the secondary level receives the steel tapping signal and the tube blank is at the steel tapping position in the furnace, performing tube blank steel tapping statistics by the secondary level.

2, real-time tracking calculation is carried out on the temperature of the tube blank in the furnace:

calculating furnace temperatures corresponding to different position points in the furnace according to the positions of thermocouples in the furnace and the measured temperatures of the thermocouples in the furnace, calculating surface heat flow of the tube blank according to the furnace temperatures, and calculating the temperature distribution of the tube blank in the furnace according to a heat conduction equation of the tube blank.

The heat conduction equation involved is expressed as:

T(r)＝T₀(R₁≤r≤R₂,t＝0)

wherein r is a radial coordinate variable m; t is a time variable, s; t is the temperature of the steel tube, K; lambda is the heat conductivity coefficient W/(m.K) of the steel pipe; rho is the density of the steel pipe, kg/m³(ii) a c is the specific heat capacity of the steel pipe, J/(kg. K); q. q.s_in,q_outIs the heat flow density of the inner and outer surfaces of the steel pipe, W/m²；T₀Initial temperature, K; r₁,R₂Respectively the inner and outer diameters m of the steel tube.

The heat flux q of the inner and outer surfaces of the steel pipe_in、q_outThe calculation expression is:

wherein phi is_in,φ_outRespectively the comprehensive radiation coefficients of the inner surface and the outer surface of the steel pipe; σ is a physical constant; t is_cIs the furnace gas temperature, K; t is_in,T_outThe internal and external surface temperatures of the steel tube, K, respectively.

The related furnace gas temperature calculation method comprises the following steps: according to the thermocouple position and its measured temperature, furnace type structure and other specific conditions, linear equation is adopted to calculate furnace gas temperature T_c. Generally speaking, if there is no thermocouple on the side of entering the furnace, the temperature measurement of the flue gas before heat exchange can be used for substitution; if the furnace outlet side is not provided with a thermocouple, according to the specific conditions of the type and the arrangement mode of the burner, subtracting a reasonable temperature deviation from the thermocouple measured temperature closest to the position of the furnace outlet side in the furnace to be used as the furnace gas temperature of the furnace outlet side; if the quenching furnace is not in a box-shaped structure, a transition area with large furnace gas temperature change exists between different control sections, the specific position of the transition area in the furnace, the starting point temperature and the end point temperature of the transition area can be determined by combining a 'toweling' temperature test in the furnace, and the temperature is measured by adopting a thermocouple in the furnace which is closest to the point in the same control section.

The furnace gas temperature corresponding to the position p in the furnace is calculated by adopting the following linear equation:

wherein p1, p2, T1 and T2 are the positions and temperatures of the 2 thermocouples or flue gas inflection points nearest to the front and back of the position p, respectively.

3, optimally setting the furnace temperature:

and (2) predicting the tapping temperature and the soaking time of the tube blank according to the current production rhythm and the process furnace temperature and the real-time temperature of the tube blank obtained in the step (2), adjusting the process furnace temperature according to the tapping temperature and the soaking time of different control sections of the tube blank required by the process, if the tapping temperature of the predicted product is lower than the tapping temperature required by the process or the soaking time is shorter than the soaking time required by the process, properly increasing the process furnace temperature, otherwise, properly reducing the process furnace temperature, predicting the tapping temperature and the soaking time of the product again by using the adjusted process furnace temperature, so that the predicted tapping temperature and the soaking time of the tube blank in different control sections meet the requirements of the tapping temperature and the soaking time required by the process, taking the adjusted process furnace temperature, carrying out weighted calculation to serve as a new furnace temperature set value of the control section, and realizing the dynamic adjustment of the furnace temperature.

In order to ensure the product quality, the temperature regulation window in the steel pipe heat treatment process is narrow, and products with process temperature difference exceeding the control precision requirement are generally not allowed to be heated at the same control section at the same time. Therefore, the optimal setting of the furnace temperature generally realizes the dynamic adjustment of the furnace temperature within a certain range on the basis of product temperature tracking according to the fluctuation of a control process, and when the adjustment range exceeds the process limit requirement, an alarm is given:

(1) when the predicted tapping temperature reaches the upper limit of the process requirement and the soaking time does not reach the process requirement, an alarm is given to prompt that the production rhythm is too fast;

(2) and if the predicted tapping temperature is at the lower limit of the process requirement, but the soaking time exceeds the range of the process requirement, an alarm is given to prompt that the production rhythm is too slow.

4, air-fuel ratio optimization setting:

if the gas calorific value H at the previous moment_oldAnd the heat value H of the gas at the current moment_newIs greater than or equal to a threshold value k_vCalculating the air-fuel ratio according to a table look-up of the gas heat value, and performing feedforward setting control; if the coal gas fluctuation is in a given range, the air-fuel ratio correction amount is calculated according to the oxygen content, and feedback setting control is carried out.

The method for calculating the air-fuel ratio according to the feedforward control comprises the steps of establishing mapping tables of different gas heat values and air-fuel ratios, and interpolating values from the mapping tables to obtain the corresponding air-fuel ratio La according to the current heat value;

the method for calculating the air-fuel ratio correction value according to the feedback control comprises the following steps:

where Δ La represents a feedback correction value of the air-fuel ratio,respectively representing the measured residual oxygen amount at the moment K and the control target residual oxygen amount of the heating furnace, K_p、K_IRespectively a proportional adjustment coefficient and an integral adjustment coefficient.

Therefore, the air-fuel ratio setting is made using La when the gas calorific value fluctuates drastically, and the air-fuel ratio setting is made using (La — Δ La) when the gas calorific value is substantially stable.