JPS5843452B2 - Plate temperature control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention applies to continuous heating furnaces for metal steel sheets such as continuous annealing lines, especially furnaces with a fast thermal response rate such as direct-fired heating furnaces, and furnaces with a slow thermal response rate such as indirect heating furnaces. The present invention relates to a plate temperature control method for a continuous heating furnace comprising:

In a continuous annealing furnace for metal steel sheets (strips), a continuous heating furnace is used as a heating device, and a specific heat cycle is determined from the viewpoint of the quality of the metal steel sheet.

In such an annealing furnace, the plate temperature deviation from the target plate temperature on the exit side of the heating furnace has an important effect on quality.

Furnace heating methods include electric methods and gas methods, but gas heating methods are advantageous in terms of energy efficiency and cost.

There are two types of furnaces that use a gas heating method: indirect heating furnaces such as radiant tube furnaces, and direct heating furnaces.

In terms of quality, the furnace is usually configured with only an indirect furnace or a combination of a direct-fired furnace and an indirect furnace.

Next, we will explain the conventional method for controlling the plate temperature at the outlet of the heating furnace in a continuous heating furnace in which a direct-fired heating furnace is installed before the indirect heating furnace. This is about plate temperature control near the joint between steel plates with different temperatures.

= Generally, when the target plate temperature is increased, the target plate temperature can be achieved by increasing the heating furnace furnace temperature if there is a margin in the heating capacity of the furnace.

On the other hand, if there is no surplus heating capacity in the furnace, the target sheet temperature is achieved by lowering the sheet passing speed without changing the furnace temperature.

In some cases, the target plate temperature can only be achieved by combining these two methods to raise the heating furnace furnace temperature to an allowable limit and further lowering the plate threading speed.

When the set values of the heating furnace temperature and sheet threading speed are changed in this way, if the set values are appropriate, the sheet temperature at the outlet of the heating furnace will normally be the target sheet temperature.

However, there is usually a delay until the plate temperature at the outlet of the heating furnace reaches a steady value after giving a command to change the furnace temperature and strip threading speed, and in this transient state, the actual plate temperature deviates from the target value.

The main reason why this transient state exists is, firstly, when the operating end is at the furnace temperature, it is due to the thermal time constant of the furnace, and secondly, when the operating end is at the threading speed, it is due to the responsiveness of the speed control system. and the length of the heating furnace.

To explain each case in more detail, when the operating end is the furnace temperature, the actual operating end is the fuel flow rate, and the time constant (delay) when changing the furnace temperature after changing the fuel flow rate in a stepwise manner is It usually takes about 10 to 20 minutes.

If there were no limit to the fuel flow rate, if we built a furnace temperature control system that detects the furnace temperature and feeds back the changes to the flow rate, set the control parameters appropriately, and made the control period short enough, the furnace temperature could be adjusted. The time constant (delay) of the step response to the temperature reference can be reduced to approximately 1 to 3 minutes.

However, in general, there are upper and lower limits to the fuel flow rate in terms of equipment capacity, so in the worst case, the time constant is forced to be about 10 to 20 minutes.

Experiments have shown that in indirect heating furnaces, there is almost no time delay between changes in furnace temperature and changes in furnace exit plate temperature, and the response is the same as the step response of furnace temperature to fuel flow rate.

Therefore, assuming the sheet threading speed is 300 mpm, 10~
If there is a delay of 20 minutes, the board temperature will deviate from 3000 to 6000 m (
(Poor heating part) will appear.

Next, the case where the operating end is the sheet passing speed will be explained.

When changing the threading speed in steps, assuming that the furnace temperature does not change due to the speed change, the steel plate that was at the furnace inlet when the speed was changed will reach the specified plate temperature after reaching the furnace outlet. becomes stable.

However, in reality, the steel plate that was in the furnace at the time the speed was changed was threaded from the furnace entrance to the point where it currently exists at the speed before the change, so the sheet temperature will vary by that amount.

Therefore, in this case, the length of the plate temperature deviation is equal to the heating furnace length.

If both the furnace temperature and the strip threading speed have to be changed, the two types of strip temperature changes described above will occur in combination.

FIG. 1 is a graph illustrating transient changes in the heating furnace outlet plate temperature when the target plate temperature is increased and the target plate temperature is achieved by changing the plate passing speed and furnace temperature.

Figure A shows the change in the target plate temperature at the outlet of the indirect furnace, Figure B,
C shows temporal changes in the sheet passing speed and indirect furnace furnace temperature that were changed in order to achieve the target sheet temperature, and the resulting temporal changes in the indirect furnace outlet sheet temperature are shown in the same figure.

In the plate temperature deviation section a in the same figure, a□ corresponds to the length of the furnace temperature settling time, and a2 corresponds to the length outside the furnace length.

As is clear from the above, in any case, the conventional sheet temperature control method has the drawback that the sheet temperature deviates for a considerable period of time, resulting in a large number of defective steel strips.

The present invention is suitable for cases where steel plates with different thicknesses are passed through a furnace near the joint (the outlet target plate temperature is constant), or where the plate thickness remains unchanged but the outlet target plate temperature is changed, or both occur at the same time. It is an object of the present invention to provide a sheet temperature control method that suppresses transient sheet temperature deviations as much as possible even if the sheet passing speed or furnace temperature is changed accordingly.

Hereinafter, the present invention will be described in detail with reference to the drawings.

Figure 2 is a schematic configuration diagram showing a continuous heating furnace for metal steel sheets to which the present invention is applied, and 8 is a furnace with a fast thermal response speed installed at the front stage of the strip (metal steel strip) 90 in the running direction (indicated by the black arrow). (in this example, a direct-fired heating furnace), 7 is a furnace with a slow thermal response speed installed at the subsequent stage (in this example, an indirect heating furnace), and 10 is a plate temperature control device.

In the indirect heating furnace I, the response speed of the outlet plate temperature to the heat flow rate is very slow, so the heat flow rate is temporarily forced (to increase the plate temperature, it is increased by a larger amount than the appropriate amount, or conversely, it is lowered). In the case of a direct-fired heating furnace 8, it has a time constant of several minutes even if the response speed is increased by reducing the amount of heat.

The present invention utilizes the speed of thermal response of such a direct-fired furnace.

The response speed of a direct-fired furnace will be explained based on FIG. 3.

Figure 3 shows experimental results showing the changes in the furnace temperature (C in the figure) and plate temperature at the outlet of the direct-fired furnace (B in the figure) when the fuel flow rate (A in the figure) is changed stepwise in the direct-fired furnace 8. .

This furnace temperature is the ambient temperature near the furnace wall, and is almost proportional to the furnace wall temperature, and as is clear from C in the figure, shows a very slow change (increase).

On the other hand, the plate temperature increase curve is clearly divided into two parts, , and b2, as shown in Figure B.

Part b2 is a region where the temperature rises slowly as the furnace temperature rises, whereas part b2 is a region where the temperature rises rapidly regardless of the rise in furnace temperature.

The temperature rise in the portion b2 is thought to be caused by radiation from the furnace wall and heat conduction from the atmospheric gas, while the temperature rise in the portion b1 is thought to be caused by direct radiation from the flame.

The presence of part b1 is a feature of the direct-fired heating furnace 8, and by forcing the fuel flow rate, it rapidly heats up to the target plate temperature only by radiation heating, and plate temperature changes begin to occur due to changes in the furnace temperature. By gradually increasing the fuel flow rate,
The response speed of plate temperature control can be greatly increased.

Next, one embodiment of the present invention will be described based on FIG.

Here, the plate thickness (A in the same figure) increases from Hl to H2,
In order to achieve the target plate temperature, the strip threading speed (B in the same figure) must be lowered from Vl to V2 at the indirect furnace inlet, and the indirect furnace temperature (C in the same figure) must be increased from TFo to TFI. This is an example of a case.

If the speed and furnace temperature are changed as shown in FIGS. 4B and 4C, the plate temperature at the outlet of the indirect furnace changes as shown in the figure, and a plate temperature deviation C occurs transiently.

The amount of transient change in the plate temperature deviation C consists of a plate temperature deviation C1 due to a change in furnace temperature (indirect furnace) and a plate temperature deviation C2 due to a speed change.

The former can be determined quantitatively in advance if the transient response of the indirect furnace furnace temperature and the relationship between the furnace temperature and the furnace outlet plate temperature are known.

The latter can be determined in advance if the transient characteristics of the indirect furnace outlet plate temperature due to speed changes are known.

This indirect furnace plate temperature deviation C occurs when the plate temperature at the indirect furnace inlet is constant, and the relationship between the indirect furnace inlet plate temperature and indirect furnace outlet plate temperature is different. The plate temperature at the inlet of the indirect furnace, that is, the plate temperature at the outlet of the direct-fired furnace, which can cancel out the temperature deviation, is determined.

FIG. 4E shows such plate temperature changes.

Therefore, if a metal steel sheet having a plate temperature distribution as shown in the figure E is entered into the indirect furnace 7, even if the sheet passing speed and the indirect furnace temperature change as shown in the figure B p C, the indirect furnace exit plate The same figure F
The temperature can be kept constant as shown in (no discrepancy occurs).
.

Indirect furnace inlet plate temperature (direct-fired furnace outlet plate temperature) shown in Figure 4 E
This is difficult with a normal operating end because it is necessary to make a sudden change in the part where the plate thickness changes as shown in FIG.

Therefore, in the present invention, as described above, the speed of thermal response in the direct-fired heating furnace 8 is used to change the plate temperature at the indirect furnace inlet (the plate temperature at the outlet of the direct-fired furnace), thereby increasing the sheet threading speed and the temperature at the indirect furnace. Minimize as much as possible the indirect furnace outlet plate temperature deviation caused by changing the furnace temperature setting.

Returning again to FIG. 2, the control aspect of the present invention will be explained in detail.

In the figure, reference numeral 1 denotes a main calculation unit that calculates the indirect furnace furnace temperature and sheet threading speed in response to changes in the target sheet temperature and threading thickness of the indirect furnace 7, and each calculation output is regulated by the strip position tracking device 4. It is output to the furnace temperature control device 2 and the speed control device 3 at the predetermined value change timing.

A direct-fired heating furnace 8 installed upstream of the indirect furnace T is opposed to a plate temperature control device 5 whose operating end is the fuel flow rate.

6 is a new performance device required for the plate temperature control method of the present invention,
Here, the set values of the indirect furnace furnace temperature and sheet threading speed and their timing information are received from the main processing unit 1, and the plate temperature deviation (plate temperature deviation) at the outlet of the indirect furnace that occurs due to these setting changes is calculated in advance. The plate temperature change at the indirect furnace inlet plate temperature required to counteract this prediction is calculated, and the calculated result is sequentially given to the plate temperature control device 5 as a reference plate temperature at the direct-fired furnace outlet plate temperature.

In the above configuration, the contents of the calculation in the main processing unit 1 are as follows.

The relationship between the plate temperature at the outlet of the indirect furnace, the plate threading speed, and the furnace temperature is given by the following equation, for example.

In equation (1), V and Tg are known numbers, and To
Use the target plate temperature at the outlet of the indirect furnace.

The logic for determining the threading speed V and furnace temperature Tg based on equation (1) is as shown in FIG.

That is, in order to maximize the production amount, first the threading speed V (Vr
Assuming that ef) is the equipment or operational upper limit value Vmax (a), the furnace temperature Tg is calculated from equation (1) (b).

If the calculated furnace temperature Tg (Tgref) does not exceed the equipment or operational upper limit Tgmax, the standard threading speed to achieve the target outlet plate temperature To is Vm.
ax, and the furnace temperature reference is the calculated Tgref.
becomes.

However, if Tgref exceeds Tgmax, the heating capacity of the indirect furnace 1 will be insufficient if the sheet is threaded at Vmax, so the sheet threading speed must be reduced to below Vmax.

For this reason, the sheet threading speed V is calculated again from equation (1) every time Tgr e f2 Tgmax).

Therefore, the furnace temperature reference in this case is Tgmax, and the speed reference is Vr e f calculated in (e).

The timing of outputting these outputs to the control devices 2 and 3 is determined mainly for operational and quality reasons.

If the occurrence of sheet temperature deviation in a continuous heating furnace is unavoidable,
It is often preferable for the direction of the deviation to be higher than the plate temperature standard, so for example, when the plate thickness changes from normal to thick and the speed must be reduced,
The speed is changed when the strip connection portion (the plate thickness changing portion) reaches the inlet of the indirect furnace 7.

Conversely, when the plate thickness changes from thick to thin and the speed must be increased, the speed is changed when the strip connection reaches the outlet of the indirect furnace 1.

If it is necessary to change the furnace temperature standard, the timing of the change is determined in the same way.

Although the output timing from the main processing unit 1 is determined in this way, it is the strip position tracking device 4 that actually tracks the strip connection portion and notifies the device 1 of the output timing.

This device 4 counts pulse signals from a pulse oscillator (not shown) attached to the priddle rolls or other rolls installed on the front or rear sides of the indirect furnace 1, thereby controlling the welding process on the front side of the furnace 1. A point detector (not shown) is used to track the strip position by calculating the strip transfer time based on the point at which the IJ tube connection passes.

The furnace temperature control device 2 feeds back the furnace temperature measured by the furnace temperature meter 11 and automatically maintains the furnace temperature at the furnace temperature standard by manipulating the fuel flow rate X to the indirect furnace 7.
It has a PID control element.

The speed control device 3 feeds back the threading speed detected by the speed detector 12 and operates the voltage of the roll 13a, 13b drive motor (not shown) to automatically set the threading speed to the speed standard. Keep control.

Next, the calculation contents of the arithmetic unit 6 are as follows: The thickness of the preceding strip is Hl and the thickness of the following strip is H2 (Hl < H2
), and in order to keep the outlet plate temperature of the indirect furnace 7 constant, the speed must be lowered from Vl to V2, which will be explained with reference to the time chart in FIG.

In this case, the timing to change the threading speed from Vl to ■2 as shown in Figure B is when the connection J of the strip with thickness H1 and H2 as shown in Figure A comes to the entrance of indirect furnace 1. be.

At this time, all the strips existing in the furnace I have a thickness H
1.

When a strip with a thickness H1 is threaded at a speed V1, its indirect furnace exit plate temperature reaches the target value TH, but when the speed decreases from V1 to V2, the thickness H remaining in the furnace reaches the target value TH.
From then on, the strip 1 is passed through the furnace I at a low speed V2, so its exit plate temperature becomes higher than TH.

As shown in Figure C, the increase in plate temperature caused by changing the speed is expressed as (TIN).The area where JTH is maximum is the strip area with plate thickness H1 just before the plate thickness changes (strip connection part J). It is.

The plate temperature (T, +JTH) at this part is H=H,, VmV2
.

T such that equation (1) holds true when Tc is 2 frosts.

It is.

The strip length in the section where JTH>0 is equal to the indirect furnace length, but the indirect furnace outlet plate temperature during this period is the 6th. Assume that it descends linearly as shown in Figure C.

Calculating the indirect furnace inlet plate temperature necessary to keep the indirect furnace outlet plate temperature at TH at all times, using the change in the calculation device 6 & circle as ATH20, and using this as the direct-fired furnace plate temperature standard for the plate temperature control device 5. Output sequentially.

First, the plate temperature at the outlet of the indirect furnace just before the strip connection J is determined by TH
The plate temperature at the inlet of the indirect furnace required to achieve this, that is, the plate temperature at the outlet of the direct-fired furnace in Figure 6, is determined from equation (1).

In equation (1), if H2■□, To=frost, and VmV2 are used to find the furnace inlet plate temperature Ti, then Ti is □-,!
(It becomes TN.

Ti is included in both sides of equation (1), and the calculation logic is as shown in FIG. 1 (it is somewhat complicated).

In this case as well, the JTN)00 section is still the indirect furnace length Lo, and the plate temperature during that period varies linearly (
Rise.

The calculation device 6 receives information from the strip position tracking device 41 about how many meters in front of the strip connection part J the strip portion currently passing through the indirect furnace 1 is, and based on this timing, the calculation device 6 sends information to the plate temperature control device 5. For this, the current standard value for the plate temperature at the outlet of the direct-fired furnace is given.

To explain the flowchart in FIG. 7, the above (
1) Let the left side of the equation be X and the right side be Y, and calculate the furnace inlet plate temperature Ti
If you draw a graph of X and Y with the horizontal axis as shown in FIG. 9, it will look like this.

To find the temperature Ti that satisfies equation (1),
, the temperature at the intersection of the Y curves.

In block (2) of FIG. 7, variables and constants for determining the values of X and Y other than the plate temperature Ti are determined.

In this case, the plate passing speed V is V2, the plate thickness H is HHL, and the plate temperature T at the furnace exit.

＆′! , becomes TH. In block ■, for example, the first time is T.
Let i=T□ (Ti is never higher than To, so it is the highest possible value).

Let the plate temperature Ti at this time be Ti1. In block ■, calculate X and Y.

In block ■, using X and Y calculated in ■, l(Y→0
/Y1 is calculated.

If this is, for example, 0o01 or less, the calculation is assumed to have converged, and the plate temperature Ti at this time is taken as the entrance side plate temperature.

If it is higher than that, another plate temperature Ti is assumed in block (2).

The second time, for example, take the indoor temperature (=50℃) and set it to T.
Let it be i2.

Now go back to block ■ and calculate X and Y, block ■
Calculate l(Y→0/Y1.

If this does not converge, the third plate temperature Ti (this visit is counted in block ■) will be Ti in Figure 9.
IY-XI when 1 is tl, IY-X when Ti2
Let I be t2, then calculate Ti and use it as the plate temperature Ti.
shall be.

If this Ti is Ti3, then X, Y
Calculate, further calculate I(Y-X)/Yl, and if it converges, Ti3 will obtain the required plate temperature Ti1, if it does not converge (
2) Find Ti using the same method as the formula and use it as the new plate temperature Ti.

This calculation is performed until convergence.

If the calculation does not converge even after a certain number of calculations, for example 6 or more, the calculation is stopped and the final Ti is determined as the inlet plate temperature Ti.

This is done with block ■. The plate temperature control device 5 controls the outlet plate temperature of the direct-fired furnace 8 as a reference by feeding back the outlet plate temperature detected by the plate temperature meter 14 provided on the outlet side of the direct-fired furnace 8 and manipulating the fuel flow rate Y to the direct-fired furnace 8. Performs automatic control to maintain the value.

Next, another embodiment will be described with reference to FIG.

In this example, the plate temperature at the outlet of indirect furnace 7 is TR8 as shown in figure A.
From TR2! F- changed (TRI > TH□), so the indirect furnace furnace temperature was changed from TF, □ to Ti2
The case where it is lowered is shown below.

In this case, when the outlet plate temperature standard change point arrives at the outlet of the indirect furnace, if the furnace temperature standard is changed to Ti2, the furnace temperature will change to T1 after a delay time of tl (usually 3 to 20 minutes) as shown in Figure B.
It gradually descends until t2.

At this time, the outlet plate temperature has almost the same waveform as shown in Figure C.
Descend to 2 (ζT work).

Therefore, in order to compensate for this change, it is necessary to change the plate temperature at the outlet of the direct-fired furnace as shown in the figure.

To do this, when the plate temperature standard change point passes the direct-fired furnace outlet, first set the direct-fired furnace outlet plate temperature to (TN-JTN), and then set the plate temperature at the direct-fired furnace outlet for a time of t.
, so that it becomes TN.

The delay time t1 becomes a fixed value once the parameters of the indirect furnace T and its furnace temperature control system are determined, so the value is determined in advance by experiment.

(TN-, the curve from (TN) to TN is approximated by a straight line or an exponential curve.

The value of (TN JTN ) is T in equation (1).

=TR2, Tg=TF1 (H2■ is constant), the seventh
It can be easily obtained by calculating the furnace inlet plate temperature Ti using the convergence calculation logic shown in the figure.

As described above, in the metal steel sheet Q continuous heating furnace in which the furnace 8 with an extremely fast thermal response rate is placed upstream of the furnace I with a slow thermal response rate, the furnace 8 is normally used for preheating, and the furnace 7 is The plate temperature is maintained at the target plate temperature, and in an emergency such as when changing the target plate temperature at the outlet of the furnace 7 or when passing a plate thickness change point,
The distribution of the deviation (plate temperature deviation) between the target outlet plate temperature and the actual outlet plate temperature of the furnace 7 that occurs transiently before and after these change points is estimated, and the deviation distribution information is returned to cancel the deviation. According to the plate temperature control method of the present invention, which controls the target plate temperature at the outlet of the furnace 8, the furnace 8,
Since the outlet plate temperature of the strip passing through the furnace 7 is always maintained at the target plate temperature not only in the steady state but also in the above-mentioned unsteady state, the generation of defective steel strips due to transient strip temperature deviations can be prevented as much as possible. It has the advantage of being able to

[Brief explanation of the drawing]

Figures 1A-D are time charts showing transient changes in indirect furnace outlet plate temperature, Figure 2 is a block diagram showing a continuous heating furnace to which the present invention is applied, and Figures 3A-C are responses of the direct-fired furnace. 4A to 4F are time charts for explaining an embodiment of the present invention, FIG. 5 is a flowchart showing the calculation contents of the calculation device of FIG. 2, and FIGS. 6A to D
8 is a time chart mainly showing the plate temperature at the outlet of the direct-fired furnace that changes based on the calculation result of the arithmetic device, FIG. Figures A to D are time charts for explaining other embodiments of the present invention, and Figure 9 is an explanatory diagram of the calculation procedure. 1... Main processing unit, 2... Furnace temperature control device, 3... Speed control device, 4... Strip position tracking device, 5... ...Plate temperature control device, 6.
...Computational unit, T...Indirect heating furnace (furnace B
), 8... Direct-fired heating furnace (furnace A), 9... Strip, 10... Plate temperature control device, 11...
... Furnace temperature needle, 12 ... Speed detector, 13a.
...13b...Roll, 14...Plate thermometer.

Claims (1)

[Scope of Claims] 1 Furnace A with an extremely fast thermal response speed, such as a direct heating furnace equipped with an exit plate temperature control system that allows changes in the outlet plate temperature setting, to a furnace A that allows changes in the furnace temperature setting. These furnaces A are placed before furnace B, which has a slow thermal response rate, such as an indirect heating furnace equipped with a control system.
, a method for controlling the plate temperature of a continuous heating furnace for metal steel sheets, in which a speed control system is provided that can arbitrarily adjust the threading speed of the strip passing sequentially through B, and the outlet plate temperature of the furnace B is heated to a desired value by both tanks. During steady state, the furnace A is preheated while controlling the plate temperature under a predetermined plate temperature set value, and then the furnace B is set to the appropriate furnace temperature and speed settings so that the outlet plate temperature reaches the target plate temperature. At unsteady times, such as when the strip passes through the furnace at a point where the target plate temperature at the exit of furnace B changes or at a point where the plate thickness changes, the strip threading speed setting value that is optimally set to heat the preceding strip or the furnace In addition to changing the B furnace temperature setting value or both to a value suitable for the trailing strip, these Q changes cause a transient effect before and after the furnace B outlet target plate temperature change point or plate thickness change point of the strip on the furnace B exit side. The deviation between the target plate temperature at the furnace B outlet and the actual plate temperature at the furnace B outlet that occurs at each point in the longitudinal direction of the strip is estimated, and when each point passes through the furnace A, the difference between the target plate temperature at the furnace B outlet and the actual plate temperature at the furnace B outlet is estimated.
A plate temperature control method, comprising adjusting a furnace A outlet target plate temperature according to the estimation result so that the outlet temperature becomes the furnace B outlet target plate temperature.