JP3176085B2 - Sheet temperature control device in continuous annealing furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous annealing furnace for performing continuous annealing by continuously passing metal strips having different sheet thicknesses, sheet widths or target sheet temperatures at the outlet of the annealing furnace through the annealing furnace. Further, the present invention relates to an apparatus for controlling the sheet temperature of each metal strip at the outlet of the annealing furnace so as to reach the target sheet temperature as much as possible.

[0002]

2. Description of the Related Art In continuous annealing furnaces, metal strips (hereinafter, sometimes simply referred to as steel strips) including various steel strips having different sheet thicknesses, sheet widths, or target sheet temperatures at the outlet of the annealing furnace are used. Each of the steel strips connected in advance and formed continuously is continuously passed through an annealing furnace to perform continuous annealing.

[0003] In the continuous annealing, the sheet temperature of the steel strip, particularly the sheet temperature at the outlet of the annealing furnace, has a great effect on the mechanical properties and the like of the quality of the steel strip to be manufactured. It is necessary to set a target sheet temperature at the outlet of the annealing furnace and an allowable range including the target value, and control the sheet temperature of each steel strip to fall within the set allowable range.

[0004] As a conventional sheet temperature control method in a continuous annealing furnace, it is general to set the sheet passing speed and the furnace temperature of the annealing furnace according to the specifications of each steel strip. However, the response to changes in furnace temperature of an annealing furnace built with a large furnace volume and a large amount of refractories is slow, especially near the connection point of steel strips with different specifications. There is a problem that the deviation increases, the actual sheet temperature deviates from the allowable range, a large number of abnormal quality parts occur, and the yield decreases.

[0005] Since the response to the furnace temperature change is slow, the deviation between the actual sheet temperature and the target sheet temperature becomes large, and the allowable range including the target sheet temperature (target value) is minimized from the viewpoint of steel strip quality compensation. There are problems that are difficult to narrow. In order to minimize and avoid such deviations, there is a means to increase the furnace length.However, this also requires a large site and it is difficult to remodel the existing annealing furnace installed in the production line. The cost and maintenance cost are high, and energy conservation is a problem. Furthermore, in order to effectively rescue the steel strip in which the abnormal quality portion has occurred without lowering the yield, for example, it is necessary to change its destination, reroll and re-anneal to a thinner sheet thickness. However, there is a problem that an opportunity is lost and productivity is reduced.

Referring to FIG. 10, a control operation of such a prior art will be described. The furnace body of the continuous annealing furnace is provided with a burner that heats the furnace wall to heat the steel strip running in the furnace and mainly heats the radiant heat transfer from the furnace wall.
In FIG. 0 (a), such a steel strip 1 is running in the direction of arrow 2, and this steel strip 1 is welded and threaded at a connection point W between a preceding steel strip 1a and a following steel strip 1b. The passing speed is a constant value as shown in FIG. Assuming that the temperature of the succeeding steel strip 1b at the time of annealing needs to be set higher than that of the preceding steel strip 1a, the furnace temperature of the continuous annealing furnace is set to the connection point W as shown in FIG. When the burner enters the furnace, the burner burn amount, that is, the amount of heat generated per unit time is increased, and the burner burn amount changes rapidly with the entry of the connection point W of the steel strip 1. The furnace temperature for heating the succeeding steel strip 1b of the steel strip 1 is as shown in FIG.
As shown in FIG. 10C, the sheet temperature of the steel strip 1 slowly rises with a low response, and therefore, as shown in FIG. 10D, the length L ( Only after traveling m), the sheet temperature falls within the allowable range ΔT above and below the target sheet temperature T0. Therefore, the length L of the succeeding steel strip 1b deviates from the allowable range ΔT of the sheet temperature, and annealing is performed. It becomes an abnormal quality part that deviates from the target quality allowable range later, is turned into waste, and lowers the yield.

As a control method for solving such a problem, there is a method disclosed in Japanese Patent Laid-Open No. 2-258933. This control method is based on the target steel sheet temperature and the sheet passing speed of the steel strip which are set in advance, the amount of change in the furnace temperature set value of the annealing furnace, the response change amount of the furnace temperature of the annealing furnace to the change amount, and Estimate the sheet temperature change in the strip direction of the steel strip, and if the deviation between the estimated sheet temperature change and the target sheet temperature of the steel strip is outside the allowable range, control the furnace temperature of the annealing furnace and This is a method for performing a correction control of the steel sheet passing speed.

[0008]

In the conventional control method described above, the delay in the response of the plate temperature caused by the delay in the response of the furnace temperature is promoted, and based on this estimation, the compensation is made by the correction control of the passing plate speed. In addition, it is possible to improve the poor response of the sheet temperature and to reduce a part where the sheet temperature is outside the allowable range to some extent.
However, modifying the speed control of this line is effective for controlling the sheet temperature, but it has limitations, and it is easy to fluctuate the speed control and tension control of the entire line or local rolls, and the steel strip However, there is a problem of adverse effects on quality, such as generation of slip flaws on the surface.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and particularly to a portion where the sheet temperature of each metal strip annealed in a continuous annealing furnace is out of an allowable range, that is, this quality abnormality and It is an object of the present invention to provide a sheet temperature control device in a continuous annealing furnace capable of significantly reducing a portion to improve a yield.

[0010]

[0011]

[0012]

The present invention provides: (a) a first burner provided on a side wall of a furnace body for heating the inside of the furnace body to heat a metal band mainly by radiant heat transfer from the furnace wall; (B) a second burner provided upstream of the first burner of the furnace body and arranged in a plurality in the longitudinal direction and the width direction of the furnace body to heat the metal strip mainly by convective heat transfer, Including a burner main body to which fuel and combustion air are supplied, and a burner tile attached to the burner main body, the burner main body is provided with a fuel header, and a tip end of the fuel header has a rectifying piece. A nozzle is provided, and the burner tile has a second burner formed with a throat portion and a slit portion for injecting a flat flame, and (c) detecting a position of a portion of the metal strip where the temperature is to be changed. Means (d) Calculation means for calculating the combustion amounts of the first and second burners according to the sheet thickness, sheet width, and composition of the metal strip to be formed, predetermined target sheet temperature and sheet passing speed of the metal strip, and the set furnace temperature. (E) combustion control means for controlling the amount of combustion of the first and second burners in response to the output of the arithmetic means, and in response to the output of the detection means, a metal strip upstream of the portion; (E1) Before the part comes to the heating position by the second burner, the amount of combustion of the first burner is increased, and the amount of combustion of the second burner is decreased. (E2) After the part comes to the heating position by the second burner, the combustion amount of the first burner is kept at an increased constant value, and the heating amount of the second burner is suddenly increased. And combustion control means for maintaining the increased constant value. A sheet temperature control device in the continuous 燃鈍 furnace characterized and.

[0013] Further, the present invention is characterized in that the combustion control means is responsive to an output of the detection means to reduce the amount of heat of heating of the metal strip upstream of the part. After reaching the heating position, the combustion amount of the first burner is decreased, and after a predetermined time has elapsed, the combustion amount of the second burner is suddenly decreased, and maintained at the reduced constant value. It increases in accordance with the above, and is maintained at a constant value after the lapse of the predetermined time after the sudden decrease.

[0014]

According to the present invention, each of the metal strips connected and continuously passed through the furnace body of the continuous annealing furnace has a first wall mainly heated by indirect radiant heat transfer from the furnace wall. Burners (group)
And a second burner (group) mainly provided by convection heat transfer provided by the combustion flame in direct contact with or in close proximity to the metal strip, and the amount of combustion of the first and second burners. That is, the amount of heat generated for heating each metal strip per unit time is individually changed, and thus the heat quantity required for the preceding and succeeding metal strips before and after the portion where the heat quantity of heating of the metal strip is different is determined. It is possible to accurately apply and anneal, and use the second burner capable of increasing or decreasing the response speed even if the response speed of the furnace temperature change by the first burner is low. Heating is performed by giving the required heating heat over the entire length of the strip, thus greatly reducing the part where the sheet temperature of each metal strip at the furnace outlet is out of the allowable range, thereby reducing the part where the quality is abnormal as much as possible. Continuous firing of metal strip To prevent abnormal annealing in the furnace, it is possible to improve the yield.

Further, according to the present invention, the portion where the heating heat quantity of the metal strip, which is the connection point between the preceding metal strip and the following metal strip, is to be changed is detected by the detection means, and the portion is detected on the upstream side in the sheet passing direction. A second burner is disposed, a first burner is disposed downstream of the second burner, and before the portion comes to the heating position by the second burner, a metal band upstream of the second burner, that is, When the heating heat of the row metal strip is to be increased, the combustion quantity of the first burner is increased and the combustion quantity of the second burner is decreased, thus keeping the heating heat quantity of the preceding metal strip constant, After the portion reaches the heating position by the second burner, the combustion amount of the first burner is kept at the increased constant value and the heating amount of the second burner is suddenly changed, that is, the heating amount per unit time is reduced. Large change, increase And, the combustion state of the second burner, the object amount of heat of the metal strip is controlled as given metal strip. Even when the heating heat of the metal strip should be reduced,
A similar operation is performed. Since the second burner can normally perform rated combustion, the efficiency of the second burner can be improved. In this way, the combination of the heating by the first burner with poor response and the second burner with good response can be used properly, and each metal before and after the connection point between the preceding metal band and the following metal band can be used. The amount of heating heat applied to the band can be rapidly changed.

Therefore, by increasing or decreasing the heating heat of the metal strip, for example, the sheet temperature of the succeeding metal strip can be increased or decreased as compared with the preceding metal strip. In this way, the sheet temperature can be changed suddenly, so if the target metal temperature is changed or the target metal temperature is changed even if the steel type is changed between the preceding metal band and the subsequent metal band, In order to keep the sheet temperature with the succeeding metal strip constant, there is a case where the heating heat of each metal strip must be changed, and in such a case, the present invention is implemented.

[0017]

FIG. 1 is an overall system diagram of an embodiment of the present invention. The steel strip 1 formed by connecting various steel strips has the passing direction 2
And is heated and annealed in the continuous annealing furnace 3. In the continuous annealing furnace 3, the steel strip 1 is supported by the transport rolls 4 and continuously passed therethrough. In the continuous annealing furnace 3, a thermometer 5 for measuring the furnace temperature is provided, and a signal representing the detected temperature is given to a combustion control means 6, whereby the steel strip 1 is mainly heated by radiant heat transfer. First
The combustion amount of the burner 7, that is, the heating heat amount per unit time (unit kcal / h) is changed, and the combustion amount of the second burner 8 provided upstream of the first burner is also changed.

At the inlet side of the continuous annealing furnace 3, a speedometer 9 for detecting a passing speed and a steel strip 1 of a preceding steel strip 1a and a following steel strip 1b (see FIG. 8 (a) described later). Detecting means 10 for detecting the connection point W is provided. The outputs of the speedometer 9 and the detecting means 10 are given to the tracking means 11, and thus the signal indicating the position of the connection point W is output to the tracking means 11.
Is given to the combustion control means 6. The tracking means 11 detects the position of the connection point W of the preceding and following steel strips 1a, 1b as described above, and the position is detected by the second burner 8
Before and after reaching the heating position 8a, the combustion amounts of the first and second burners 7, 8 by the combustion control means 6 are changed.

The specification setting means 12 sets the specifications of the steel strips 1, that is, the steel type, the sheet thickness, the sheet width, the target sheet temperature at the outlet of the annealing furnace, the permissible range and the sheet passing speed for each steel strip 1. Are provided for the next control, and such specifications for each steel strip 1 are set in advance. The calculating means 13 includes a connecting portion W such as a leading steel strip 1a and a following steel strip 1b which are passed through.
Based on the specifications of the steel strips 1a and 1b before and after the adjacent steel strips 1a and 1b, the heating heat amounts of the steel strips 1a and 1b before and after the connection point W are calculated, and based on this, the changes in the furnace temperature and the sheet temperature are calculated. , And the outputs of the first and second burners 7 and 8, that is, the required amount of change in the amount of combustion, is calculated, and the calculation result is provided to the combustion control means 6.

FIG. 2 is a longitudinal sectional view of a main part of the continuous annealing furnace 3, and FIG. 3 is a plan view thereof. Side wall 14 of continuous annealing furnace 3
Is provided with a first burner 7 called a side burner or the like. The first burner 7 heats the entire inside of the furnace body 15 constructed of a refractory inside the furnace 3 and mainly heats the inside of the furnace body 15. The steel strip 1 is heated by radiant heat from the entire furnace wall.

The steel strip 1 which is heated while being continuously passed through the continuous annealing furnace 3 mainly has a large amount of the inside of the furnace body 15 having a large furnace volume by the first burner 7. The refractory or the like is heated, and receives the radiation (radiation) radiated from the heated refractory (furnace inner wall), that is, is indirectly transferred and heated.
Therefore, even if the succeeding steel strip 1b is rapidly raised or lowered from the boundary of the connection portion W to the target temperature based on the set specifications, the subsequent steel strip 1b has a large amount of large heat capacity. The refractory (furnace inner wall) must be raised or lowered, which is difficult because it cannot be changed rapidly and causes a considerable delay and low response.

Further, on the ceiling wall 16 of the furnace body 15, a plurality of second burners 8 called slit burners or the like are installed upstream of the first burners 7. The second burner 8 injects the burning flame so as to directly contact or approach the surface of the steel strip 1 to heat the steel strip 1 mainly by convective heat transfer by the flame. Downstream of the second burner 8, the ceiling wall 16 may be provided with another first burner (not shown) for mainly heating the steel strip 1 for radiant heat transfer. A first burner 7a may be further provided near the second burner 8. 2 and 3
1 shows an example in which a single or a single second burner 8 is arranged in the lengthwise direction of the furnace 3 over the entire width of the steel strip 1.

FIG. 4 is another simplified longitudinal sectional view of the second burner 8. The second burner 8 has a slit-shaped nozzle having a width W1, and a plurality of such second burners 8 are arranged in the width direction of the furnace body 15, for example.
It is also possible to inject a flat flame at a substantially uniform temperature over the entire width of the steel strip 1 and heat it at a uniform temperature in the width direction.

FIG. 5 is a simplified longitudinal sectional view of the second burner 8, and FIG. 6 is a sectional view taken along line VI-VI of FIG. A gas header 21 is attached to a burner main body 20 to which gas fuel is supplied from a connection port 18 and combustion air is supplied from a connection port 19, and thus gas fuel and combustion air are supplied. From the gas nozzle 22 having the rectifying piece, a flat flame is jetted from the slit part 25 to the throat part 24 of the burner tile 23, and comes into contact with or close to the surface of the steel strip 1.
Such a second burner 8 may be arranged not only above the steel strip 1 but also below it so as to heat the lower surface of the steel strip 1. When the flame from the second burner 8 comes into contact with the steel strip 1, the steel strip 1 is highly responsive,
It can be rapidly heated to the desired temperature.

FIG. 7 is a flow chart for explaining the operation of the combustion control means 6. The process proceeds from step n1 to step n2, in which set furnace temperatures Ta, Tb equal to set plate temperatures in the steady state of the preceding steel strip 1a and the following steel strip 1b are compared,

[0026]

## EQU1 ## If Ta <Tb, the routine goes to step n3, where the furnace temperature difference ΔT is calculated.

[0027]

At step n4, a furnace time constant based on the furnace temperature difference ΔT, that is, a required time H for changing the furnace temperature is calculated. In step n5, the amount of output change of the second burner 8 necessary for compensating the furnace temperature difference ΔT is calculated. Thus, in step n6, the output change ratio of the second burner 8 is determined, and the fuel to the second burner 8 is determined. The combustion amount of the second burner 8 is changed by controlling the gas flow rate. In Step n7, the furnace temperature change start timing is determined. For this purpose, whether or not the connection point W has reached the heating position 8a of the steel strip 1 by the second burner 8 is determined by the output of the tracking means 11 in response to the output of the speedometer 9 and the detection means 10 of the connection point W. Is determined.

In step n8, the timing for starting the output change of the second burner 8 is determined. Step n2 described above
At

[0029]

When Ta> Tb, the process proceeds to step n9, where the furnace temperature difference ΔT is
It is obtained in the same manner as in the above equation (2). In step n10,
Second burner 8 necessary to correct this furnace temperature difference ΔT
In step n11, a furnace time constant based on the furnace temperature difference ΔT, that is, a required furnace temperature change time H is calculated. In step n12, the second burner 8
Is determined, and the combustion amount of the second burner 8 is thus controlled.

In step n2, when the furnace temperatures of the preceding steel strip 1a and the following steel strip 1b are equal (Ta = Tb), the second
The combustion amount of the burner 8 is not changed.

Although the operation shown in FIG. 7 has been described with reference to the second burner 8, the combustion amount of the first burner 7 is also controlled by the combustion control means 6.

FIG. 8 shows the following steel strip 1 compared to the preceding steel strip 1a.
It is a figure for explaining operation when it is necessary to raise the sheet calorie of the following steel strip 1b by increasing the heating heat quantity of b. As shown in FIG. 8A, each of the preceding and following steel strips 1
a and 1b are represented by connection points W, and the passing speed is shown in FIG.
As shown in (b), it is a constant value in the passing direction 2. Before the connection point W comes to the heating position 8a of the second burner 8 by a certain length, and thus by a predetermined time,
The combustion amount of the first burner 7 increases at time t1 and the furnace temperature of the radiant heat transfer by the first burner 7 of the steel strip 1 increases with time, and the connection point W reaches the heating position 8a at time t2. After that, keep it at a constant value. On the other hand, the second burner 8
8B, the sheet temperature of the steel strip 1 by the second burner 8 is, as shown in FIG.
It is decreased with time until time t2. Therefore, the preceding steel strip 1a can be heated to a connection point W with a constant amount of heating heat. This plate temperature is as shown in FIG.

The connection point W is located at the heating position 8a of the second burner 8.
After time t2, when the combustion amount of the first burner 7 is increased, the combustion amount of the first burner 7 is maintained at the increased constant value as shown in FIG.
, And then kept at a constant value, thus increasing the amount of heat generated by the second burner 8 of the succeeding steel strip 1b. Thus, the preceding and succeeding steel strips 1a, 1b have the allowable ranges ΔTa1, ΔT around the target plate temperatures Ta, Tb.
The temperature is maintained at b1, the occurrence of abnormal quality portions in the continuous annealing furnace 3 for the steel strip 1 is prevented, and the yield is improved.

FIG. 9 shows the following steel strip 1 compared to the preceding steel strip 1a.
It is a figure for demonstrating operation | movement at the time of reducing the sheet | seat temperature of the succeeding steel strip 1b by reducing the heating heat quantity of b. As shown in FIG. 9 (a), the preceding and following steel strips 1a, 1b are connected at a connection point W, and the passing speed is constant as shown in FIG. 9 (b). . Connection point W is second burner 8
After the time t3 when the heating position 8a is reached, the first burner 7 has its calorific value reduced at the time t3 shown in FIG. 9C, and the furnace temperature becomes as shown in FIG. 9C. Then, the time is reduced by a predetermined time until a predetermined time t4. The calorific value of the second burner 8, that is, the sheet temperature of the steel strip 1 by the second burner 8 is rapidly reduced at time t3 as shown in FIG. The combustion amount is increased to time t4 so as to be given to the steel strip 1b. Thus, leading and trailing steel strip 1
a, 1b can be brought to the permissible ranges ΔTa2, ΔTb2 centered on the target sheet temperatures Ta2, Tb2, and also the occurrence of abnormal quality portions in the continuous annealing furnace 3 of the steel strip 1 can be prevented to improve the yield. it can.

As still another embodiment of the present invention, even if the sheet temperature of the preceding steel strip 1a and the succeeding steel strip 1b is constant, each steel sheet, sheet thickness, sheet width and the like may cause a difference in specifications. In some cases, it is necessary to increase or decrease the amount of heating heat of the steel strips 1a, 1b, thereby keeping the sheet temperature of each steel strip 1a, 1b constant. Even in such a case, the heat quantity of heating of the first and second burners 7, 8 is changed in the same manner as described above.

In the present invention, the first burner 7 mainly heats radiant heat transfer and the second burner 7 mainly heats convective heat transfer.
The means for heating in combination with the burner 8 is the most important, but the second burner 8 is connected to the first burner 7 in the continuous annealing furnace 3.
It is not always important to install it on the more upstream side, and even when it is installed on the contrary, the amount of heating heat of the first and second burners 7 and 8 can be changed in the same manner as described above.

The invention can be practiced not only with steel strips, but also with other metal strips.

[0038]

As described above, according to the present invention, in a continuous annealing furnace, a preceding metal band and a following metal band are connected at a connection point or the like, and this metal band is mainly heated by radiant heat transfer. And a second burner that mainly heats convective heat transfer, and by appropriately and timely changing the combustion amount of the first and second burners, the response of the first burner to the plate temperature heating. Even if the sex is low,
Due to the high responsiveness of sheet temperature heating by the second burner used in combination with this, the amount of heating heat applied to the preceding metal band and the following metal band can be changed rapidly, and therefore, the preceding metal band and the following metal band can be changed. In the vicinity of the portion where the amount of heat to be heated is to be changed, the length outside the allowable range of the target sheet temperature is set to zero or negligible, thereby preventing the occurrence of an abnormal portion of the metal strip and improving the yield. Will be able to do it.

In a production line for a continuous annealing furnace and various processing processes in which the same is incorporated, when the processing speed of the furnace and the productivity of the line are increased by increasing the passing speed of the metal strip, the above-mentioned method according to the present invention is used. The effects of the item are further promoted, and the higher the passing speed, the more effective the present invention is.

There is no need to lengthen the furnace length, and it is not necessary to reduce the quality due to the occurrence of slip flaws or the like on the surface of the metal strip while passing through a furnace or a line. There is no opportunity loss or a drop in productivity due to redressing the generated steel strip by changing its destination. The allowable range centered on the target plate temperature can be narrowed, quality control can be performed more strictly as intended, and this is preferable in terms of quality compensation.

[Brief description of the drawings]

FIG. 1 is an overall system diagram of an embodiment of the present invention.

FIG. 2 is a longitudinal sectional view of a main part of a continuous annealing furnace 3;

FIG. 3 is a plan view of the continuous annealing furnace 3;

FIG. 4 is a simplified longitudinal sectional view of a second burner 8;

FIG. 5 is a cross-sectional view of the second burner 8 as viewed from a passing direction 2;

FIG. 6 is a cross-sectional view as viewed from VI-VI in FIG. 5;

FIG. 7 is a flowchart for explaining the operation of the combustion control means 6;

FIG. 8 is a diagram for explaining an operation when the sheet metal temperature is increased by increasing the heat quantity of heating of the succeeding metal strip (steel strip) 1b as compared with the preceding metal strip (steel strip) 1a.

FIG. 9 is a view for explaining an operation when the sheet temperature is lowered by reducing the heating heat amount of the succeeding metal strip (steel strip) 1b as compared with the preceding metal strip (steel strip) 1a.

FIG. 10 is a view for explaining an operation when the sheet temperature of the succeeding steel strip 1b is increased as compared with the preceding steel strip 1a in the prior art.

[Explanation of symbols]

 Reference Signs List 1 metal strip (steel strip) 1a preceding metal strip (steel strip) 1b succeeding metal strip (steel strip) 2 passing direction 3 continuous annealing furnace 5 furnace temperature detecting means 6 combustion control means 7 first burner 8 second burner 8a Heating position by the second burner 8 9 Speedometer 10 Connection point W detection means 11 Tracking means 12 Specification setting means 13 Calculation means 14 Side wall 15 Furnace body 16 Ceiling wall

Continuation of the front page (56) References JP-A-2-25523 (JP, A) JP-A-57-94524 (JP, A) JP-B-58-43452 (JP, B2) JP-B 5-57335 (JP, A) , B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/52-11/00

Claims (2)

(57) [Claims] (A) a first burner provided on a side wall of a furnace body to heat the inside of the furnace body to heat a metal band mainly by radiant heat transfer from the furnace wall; and (b) a first burner of the furnace body. A second burner provided upstream of the burner and arranged in a plurality of lengthwise and widthwise directions of the furnace body to heat the metal band mainly by convective heat transfer, wherein fuel and combustion air are supplied A burner main body, and a burner tile attached to the burner main body, the burner main body is provided with a fuel header, and a nozzle having a rectifying piece is provided at a tip end of the fuel header. A second burner in which a throat portion and a flat flame injecting slit are formed in the tile; (c) means for detecting the position of a portion of the metal strip where the temperature is to change; and (d) heating. Metal strip thickness, Calculating means for calculating the combustion amounts of the first and second burners according to the predetermined target sheet temperature and sheet passing speed of the metal strip and the set furnace temperature according to the width and the component, and (e) output of the calculating means A combustion control means for controlling the amount of combustion of the first and second burners, in response to the output of the detection means, when the amount of heating heat of the metal strip upstream of the portion should be increased, (E1) increasing the amount of combustion of the first burner and decreasing the amount of combustion of the second burner before the part comes to the heating position by the second burner, and keeping the heating heat of the metal strip constant; (E2) After the part reaches the heating position by the second burner, the combustion amount of the first burner is kept at an increased constant value, the heating amount of the second burner is suddenly increased, and the increased constant value is obtained. Combustion control means for maintaining Sheet temperature control device in the furnace. 2. The combustion control means, in response to an output of the detection means, to reduce the amount of heat of heating of the metal strip upstream of the part, (e3) the position where the part is heated by the second burner. After that, the combustion amount of the first burner is decreased, and after a predetermined time has elapsed, the combustion amount of the second burner is suddenly decreased, and maintained at the reduced constant value, and thereafter increases with the elapse of time. 2. The sheet temperature control device for a continuous annealing furnace according to claim 1, wherein the constant value is maintained after a lapse of the predetermined time after the rapid decrease.