JP2000034523A - Continuous heat treatment method for metallic strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat treatment method capable of keeping a prescribed heat treatment temp. even at the time of changing the size (thickness, width) of a metallic strip and heat treatment speed and dealing with either of acceleration or deceleration particularly in the case of quickly changing the heat treatment speed. SOLUTION: In this heat treatment method an auxiliary heating zone 21 high in the responsiveness of an induction heating or an electric heating system and fast in heating rate is disposed at the inlet side of a heating zone 11 of a radiant tube heating system. At the time of normal operation, the operation is performed so that the output of the auxiliary heating zone is regulated to almost half to the max. output or increased/decreased in the proportion with the ratio of the present heat treatment quantity to the max. heat treatment capacity in the heating zone and/or the ratio of the present speed to the max. speed of the heat treatment apparatus. At the time of changing the metallic strip size, heat treatment speed or heat treatment temp., the output of the auxiliary heating zone is instantially increased/decreased so as to keep or change the prescribed heat treatment temp. of the metallic strip, and then the output of the auxiliary heating zone is gradually returned to the output at the time of normal operating by adjusting the heating capacity of the heating zone.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for heat-treating a metal strip, and more particularly, to a metal strip size (thickness and width), a heat-treatment rate,
An object of the present invention is to provide a heat treatment method that is excellent in controllability and responsiveness of a metal zone temperature when a heat treatment temperature is changed.

[0002]

2. Description of the Related Art FIG. 3 shows a schematic diagram of a continuous heat treatment furnace. The metal strip (plate) 20 is heated to a predetermined heat treatment temperature (soaking temperature) in the heating zone 11, kept in the soaking zone 12 for several tens of seconds, and then cooled sequentially in the primary to tertiary cooling zones 13 to 15. Is done. The outline of the heat treatment process is as shown in FIG.

[0003] In a heating zone of a continuous heat treatment furnace, heating by a radiant tube (radiant tube heating method) is widely used. FIG. 5 shows a schematic view of a radiant tube. A burner 2 is installed at the end of a heat-resistant steel tube called a radiant tube 1 processed into a W-shaped or U-shaped, and C
Supplying fuel such as O gas and combustion air to burn,
The metal strip is indirectly heated by radiant heat from the radiant tube surface.

However, the heating by the radiant tube is indirect heating using radiation, so that the controllability and responsiveness of the metal strip temperature are poor.
There is a problem that a predetermined heat treatment temperature cannot be secured when changing the width or the heat treatment speed, or that it takes time to change the heat treatment temperature.

For example, when the metal strip size is changed from a thin material to a thick material in the continuous heat treatment furnace shown in FIG. 3, when the size change point passes the heating orifice side, the sheet temperature decreases in accordance with the increase in the sheet cross-sectional area. Occurs. Similarly, when increasing the line speed, the heating outlet side sheet temperature decreases due to a decrease in the furnace time due to the increase in the line speed.

Conversely, when the metal strip size is changed from a thick one to a thin one, or when the line speed is reduced, the sheet temperature on the heating strip side rises. Usually, the sheet temperature on the heating strip side is
Feedback control is performed by a sheet thermometer installed on the same exit side, but the sheet temperature controllability and responsiveness are poor for the reasons described above, and it takes time to return the sheet temperature to the predetermined heat treatment temperature. The predetermined heat treatment cannot be performed.

To solve this problem, Japanese Patent Laid-Open Publication No.
JP-A-19336 and JP-A-57-94524 disclose that an auxiliary heating zone using an induction heating system is provided at the entrance or exit of the radiant tube heating zone to improve the controllability and responsiveness of the metal zone temperature. Methods for improvement are described.

For example, as shown in FIG. 1, in a continuous annealing furnace for a steel sheet provided with an auxiliary heating zone 21 by an induction heating system on the heating zone entrance side, a case where the sheet size is changed from a thin material to a thick material is considered. In this case, as shown in FIG. 6A, when the size change point passes through the auxiliary heating zone, the output of the auxiliary heating zone is increased to increase the heating amount, thereby preventing a decrease in the sheet temperature in the downstream heating zone. I do. A predetermined heat treatment temperature can be maintained by gradually lowering the output of the auxiliary heating zone in accordance with the subsequent increase in the heating zone output (furnace temperature). FIG.
As shown in (b), when changing the plate size from a thick one to a thin one, the above operation may be reversed.

FIG. 7 shows a case where the line speed is similarly increased and decreased. As described above, by using the induction heating device having good temperature controllability and responsiveness as a supplement, it is possible to easily and quickly change the metal strip size, the line speed, and the heat treatment temperature.

[0010]

However, in using the above method, it is necessary to determine in advance whether it is necessary to increase or decrease the output of the auxiliary heating zone by changing the metal strip size, the line speed, and the heat treatment temperature. You need to judge. However, while the schedule for changing the metal band size and heat treatment temperature is determined in advance,
The line speed is changed as needed by the operator while monitoring the line. Especially, when a trouble occurs, a rapid deceleration operation may be performed, and it is difficult to predict in advance.

For example, if a sudden deceleration is performed while the operation is performed in a state where the output of the auxiliary heating zone is low, even if the output of the auxiliary heating zone is cut, the increase in the sheet temperature in the heating zone cannot be covered. , Exceeds the predetermined heat treatment temperature. Conversely, when the operation is performed in a state where the output of the auxiliary heating zone is high, the output of the auxiliary heating zone becomes insufficient even if the speed is further increased, and the temperature falls below a predetermined heat treatment temperature.

The present invention has been made in view of the above circumstances, and an object thereof is to always cope with both acceleration and deceleration by setting the output of the auxiliary heating zone during normal operation to about 50%. It is an object of the present invention to provide a heat treatment method that can perform the heat treatment.

[0013]

The above problem is solved by the following heat treatment method. (1) In a method of performing a continuous heat treatment by passing a metal strip through a heating zone of a radiant tube heating system (radiant tube) and an auxiliary heating zone having good responsiveness and a high heating rate disposed on the entrance side of the heating zone. During normal operation, the output of the auxiliary heating zone is set to approximately half of the maximum output (usually 40 to 60%), and the auxiliary heating zone is changed when the metal strip size (thickness, width), heat treatment speed, or heat treatment temperature is changed. The output is instantaneously increased or decreased to maintain or change to a predetermined metal strip heat treatment temperature, and then the auxiliary heating zone output is gradually returned to the output during steady operation according to the adjustment of the heating capacity of the heating zone. Heat treatment method for metal strips. (2) In a method of continuously heat-treating a metal strip in a heating zone of a radiant tube heating system (radiant tube) and an auxiliary heating zone arranged on the entrance side of the heating zone with good responsiveness and high heating speed, The output of the auxiliary heating zone during steady operation is increased or decreased in proportion to the heating zone output ratio at that time (the ratio of the current heat treatment amount to the maximum heat treatment capacity of the heating zone) and / or the speed ratio (the ratio of the current speed to the maximum speed of the heat treatment apparatus). Then, the output of the auxiliary heating zone is gradually returned to the output during normal operation in accordance with the adjustment of the heating capacity of the heating zone.

According to the above method (1), the output of the auxiliary heating zone during normal operation is set to about 50%, thereby accelerating (increasing the output of the auxiliary heating zone) and decelerating (decreasing the output of the auxiliary heating zone).
Can always correspond to both.

The above (2) is an improved method of (1). If the heating zone output ratio and speed ratio at a certain point in time are small, there is a high possibility that the speed will be increased by the next operation. Set low. Conversely, if the heating zone output ratio and the speed ratio are large, the possibility of deceleration by the next operation is high, so the auxiliary heating zone output is set high. By doing so, the capacity of the auxiliary heating zone can be effectively used, and it is possible to cope with a large speed change.

[0016]

Embodiments of the present invention will be described below. FIG. 1 shows a continuous heat treatment furnace for a metal strip (plate) for carrying out the present invention, which has a configuration in which an auxiliary heating zone 21 by an induction heating device 22 is installed on the entrance side of the heating zone 11 in the conventional apparatus shown in FIG. I have. A plate thermometer is installed on each outgoing side,
The output of each band is feedback controlled so that the plate temperature of each band becomes the target plate temperature.

The target plate temperature of the heating zone is determined by the following process. (1) Calculation of metal strip heat treatment amount, heating zone output ratio, speed ratio From the size of the metal band during heat treatment, line speed, etc., calculate the current steel sheet heat treatment amount, heating zone output ratio, and speed ratio using the following formulas. I do.

[0018]

(Equation 1)

[0019]

(Equation 2)

[0020]

(Equation 3) (2) The auxiliary heating zone output ratio is determined from the heating zone output ratio and / or the speed ratio.
As shown in -1), the constant value is set to a substantially half value (normally 0.4 to 0.6).

In the method according to the second aspect, the auxiliary heating zone output ratio is determined from the heating zone output ratio and / or the speed ratio by using the equation (4-2). That is, the higher the heating zone output ratio and speed ratio, the higher the auxiliary heating zone output ratio. Various methods are conceivable as a method of determining the auxiliary heating zone output ratio, and the method is not necessarily limited to Expression (4-2).

[0022]

(Equation 4) (3) auxiliary heating home use-side target plate temperature determining metal strip size of the steady state operation, the line speed, the heat treatment temperature (h _1, w
₁ , V ₁ , T _SS1 ), the auxiliary heating band target side sheet temperature at the time of steady operation is determined using equation (5). Thereafter, the feedback control of the output of the auxiliary heating zone is performed so that the temperature of the auxiliary heating zone plate becomes the same target temperature _TlH1 , so that the auxiliary heating zone _exit side _sheet temperature eventually converges to _TlH1 . At this time, the target plate temperature in the heating zone and the soaking zone is T _SS1 of the heat treatment temperature.

[0023]

(Equation 5) (4) the metal strip size, line speed, determine metal strip size of auxiliary heating home use-side target plate temperature Immediately heat treatment temperature, line speed, the heat treatment temperature (h _1, w
₁ , V ₁ , T _SS1 ) to (h ₂ , w ₂ , V ₂ , T _SS2 )
The target sheet temperature on the exit side of the auxiliary heating zone immediately after the change to (6)
And instantaneously (immediately) change the auxiliary heating zone target plate temperature from T _lH to T _lH2 . In addition, as a method of determining the heating zone exit side target plate temperature, various other methods can be considered,
It is not necessarily limited to equation (6). At this time, the target plate temperature of the heating zone and the soaking zone is determined by the heat treatment temperature T.
_SS2 .

[0024]

(Equation 6) (5) metal strip size, line speed, determine metal strip size of the auxiliary heating home use-side target root temperature during steady operation of the modified heat treatment temperature, line speed, the heat treatment temperature (h _2, w
₂ , V ₂ , T _SS2 ), and determines the auxiliary heating band side target sheet temperature at the time of steady operation after being changed to equation (7). Then, the target plate temperature of the auxiliary heating zone is gradually changed from T _lH to T _lH2, and the rate of change at this time is a low value of about 1 to 5 ° C. per minute in consideration of the slow response of the heating zone. And At this time, the target plate temperature of the heating zone and the soaking zone is T _SS2 of the heat treatment temperature.
It is.

[0025]

(Equation 7)

T _lH2 : target sheet temperature on the exit side of auxiliary heating zone _{during normal} operation after changing the metal strip size, line speed, heat treatment temperature [° C.] The above process is an example in which an induction heating device is used as a heating method. However, direct current heating, plasma heating, etc.
It is clear that the response is good, the output can be changed stepwise almost instantaneously, and the same effect can be obtained by another heating method with a high heating speed. FIG. 2 shows a configuration in a case where an energization heating method is employed as an auxiliary heating zone. In this case, electric power is supplied to the metal strip via the energizing rolls 23 and 23 ', and heating is performed by Joule heat.

[0027]

EXAMPLE A simulation (numerical calculation) of changes in sheet temperature, furnace temperature, and the like when the present invention is applied to a continuous annealing furnace for a tin-plated steel sheet having the configuration shown in FIG. 1 is shown. The simulation conditions are as follows.

Heating zone maximum heat treatment capacity: Mmax = 100 [ton / h] Maximum speed of heat treatment apparatus: V = 1000 [mpm] Constant: α = 0.5, β = 0.6 Correction coefficient: Κ = 0.9 ~ 1.1 Heat treatment temperature (auxiliary heating zone, target plate temperature in soaking zone): T _SS =
630 [° C.] Maximum output value of the auxiliary heating zone: Pmax = 3000 [kW] Efficiency of the induction heating device: η = 0.8 FIG. 8 shows the sheet temperature when the sheet thickness was changed from 0.20 mm to 0.23 mm, Changes in furnace temperature and auxiliary heating zone output are shown. By increasing the amount of heating by increasing the output of the auxiliary heating zone when passing the sheet thickness change point, the sheet temperature on the heating zone exit side is maintained at a predetermined heat treatment temperature.

FIG. 9 shows a plate thickness of 0.20 mm to 0.17 m.
The graph shows changes in the sheet temperature, the furnace temperature, and the output of the auxiliary heating zone when the temperature is changed to m. In this case, contrary to FIG. 8, the output of the auxiliary heating zone is lowered so that the sheet temperature on the heating outlet side is maintained at a predetermined heat treatment temperature.

FIGS. 10 and 11 show a case where the line speed is 800 mp.
m to 900 mpm and 700 mp
In this case, the output of the auxiliary heating zone is appropriately controlled, and the predetermined heat treatment temperature can be maintained.

In addition, shortly after passing the sheet thickness change point and immediately after the speed change, a slight temperature fluctuation of about ± 10 ° C. is observed in the heating zone and the solitary zone. It doesn't matter.

[0032]

By applying the present invention to a metal strip heat treatment apparatus, a predetermined heat treatment temperature can be maintained even when the metal strip size (thickness, width) or the heat treatment rate is changed. When the heat treatment speed is suddenly changed, it is possible to cope with both acceleration and deceleration.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a continuous heat treatment furnace (induction heating device) used for carrying out the present invention.

FIG. 2 is a diagram showing an example of a continuous heat treatment furnace (electric heating device) used for carrying out the present invention.

FIG. 3 is a diagram showing an example of a conventional continuous heat treatment furnace.

FIG. 4 is a diagram showing an example of a heat treatment cycle for a metal strip.

FIG. 5 is a view showing a radiant tube.

FIG. 6 is a diagram showing a conventional continuous heat treatment method (in the case of changing the thickness).

FIG. 7 is a diagram showing a conventional continuous heat treatment method (in the case of changing the speed).

FIG. 8 is a view showing a continuous heat treatment method (in the case of increasing the thickness) according to the present invention.

FIG. 9 is a diagram showing a continuous heat treatment method according to the present invention (in the case of a reduction in sheet thickness).

FIG. 10 is a continuous heat treatment method according to the present invention (in the case of speed increase).
FIG.

FIG. 11 is a continuous heat treatment method according to the present invention (in the case of deceleration).
FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 radiant tube 2 burner 3 recuperator 11 heating zone 12 isotropy 13 primary cooling zone 14 secondary cooling zone 15 tertiary cooling zone 20 metal strip (plate) 21 auxiliary heating zone 22 induction heating device 23 energizing roll

Claims (2)

[Claims] 1. A method for continuously heat-treating a metal strip in a heating zone of a radiant tube heating type and an auxiliary heating zone disposed on the entrance side of the heating zone and having a good response and a high heating rate. Occasionally, the output of the auxiliary heating zone is operated at approximately half of the maximum output, and the output of the auxiliary heating zone is instantaneously increased or decreased when the metal strip size, heat treatment speed, or heat treatment temperature is changed, and is maintained at a predetermined metal strip heat treatment temperature. And then gradually returning the output of the auxiliary heating zone to the output during normal operation according to the adjustment of the heating capacity of the heating zone. 2. A method for continuously heat-treating a metal strip in a heating zone of a radiant tube heating type and an auxiliary heating zone arranged on the entrance side of the heating zone and having a high responsiveness and a high heating rate. The auxiliary heating zone output at the time is increased or decreased in proportion to the ratio of the current heat treatment amount to the heating zone maximum heat treatment capacity and / or the ratio of the current speed to the maximum speed of the heat treatment apparatus,
Thereafter, the output of the auxiliary heating zone is gradually returned to the output during normal operation in accordance with the adjustment of the heating capacity of the heating zone.