JP2587521B2 - Batch type annealing furnace temperature control method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to temperature control of a batch annealing furnace.

(Prior Art) Steel roll-formed by a rolling mill generates internal stress, and therefore has relatively hard and brittle properties. Thus, annealing is performed to provide predetermined mechanical properties suitable for the final use.

In particular, in the annealing of high carbon steel sheet that requires spheroidization of carbide, compared to ordinary steel annealing, the allowable annealing temperature range is narrow,
In addition, since long-time annealing is indispensable, batch-type annealing is widely performed.

As a conventional annealing temperature control method, Japanese Patent Publication No. 59-24167
There is an official gazette.

 This is shown in FIGS. 3 and 4.

In the conventional method, a top temperature sensor 1 that measures the highest coil temperature by contacting the coil at the upper end of the uppermost coil 5 in the furnace,
A berth temperature sensor 3 for measuring a base temperature is mounted on the lower end of the lowermost coil, and a furnace temperature temperature sensor 2 is mounted on the furnace wall.

Connect each sensor 1, 2, 3 to the computer 11 and the controller 10,
The furnace temperature _TF is controlled to a constant value T _F1 until the maximum temperature of the furnace coil reaches a desired value T _t (max). Next, the base temperature T _B
Most cold temperatures T _c estimated from controls the maximum temperature T _t of the coil until the desired value T _h to a constant value _{T t} (max). The T
_{After c} reaches T _n , the base temperature T _B
Is controlled to a constant value Th.

(Problems to be solved by the invention) The basic principle of annealing is that the upper and lower limits of [temperature] x [time] to be given to the material are set from metallurgical and quality requirements, and that Furnace operating conditions are determined so that a part is included, and this is achieved by [soaking temperature range] × [soaking time] in the soaking process.

To illustrate the concept in Figure 5, it is at the heating step, and the coil temperature upper limit of the soaking temperature range the lowest cold temperature T _c determined by the estimated calculation furnace coil maximum temperature T _t, than the base temperature T _B The temperature is raised to be between the coil temperature lower limit and the coolest temperature T _c
Starts the soaking process from the time when the temperature reaches the lower limit of the coil temperature.

In the soaking process, control is performed so that all of the _Tt · T _B and T _C fall within the soaking temperature range, and a predetermined time, that is, a soaking time
Control was maintained by H _t, to furnace operation to enter the cooling step is a general furnace operation conditions.

The problem of the conventional method relates to a control method in a constant base temperature section.

FIG. 3 for explaining the conventional method shows a method of switching the feedback input of one controller and giving a set value from the computer in conjunction therewith, but there are two problems.

First, regarding the method of switching the feedback input of one controller, when the furnace temperature, coil maximum temperature, and base temperature having different thermal time constants (thermal response delay times) are switched and input, the thermal time constant is changed. Feedback control of a large base temperature is a problem that the control becomes unstable.

This is because the basic principle of feedback control detects the amount of process change that appears when operating the operating end,
Since this is a closed-loop control in which the operation end is adjusted to match the desired set value, the control becomes unstable when applied to a process that takes a long time before the effect of operating the operation end appears. That's why.

FIG. 6 shows an example of measurement of the thermal time constant of the annealing furnace, which was conducted by the inventors. FIG. 6 (A) is a conceptual diagram showing the measuring method, and FIG. 6 (B) is a sensor arrangement around the coil. FIG.

As an experimental method, a thickness of 1,2 mm and a width of 100
A steel coil of 0 mm and a weight of 13.8 tons is stacked in three stages, the uppermost coil 5 is arranged around the sensor shown in FIG. 8B, and the lowermost coil is arranged with a base temperature sensor 3 contacting the lower end. And the inner cover 6 and the annealing furnace 7 as shown in FIG.
The fuel is supplied to the heating device 4 by the control valve 8, and the valve closing degree is controlled by a furnace temperature that outputs an operation output so that the measured value of the furnace temperature sensor 2 arranged in the combustion furnace 7 matches the set value. The adjustment was performed by the controller 22. Each temperature data was recorded on a recorder.

The measurement results are shown in FIG. 4C. First, the furnace temperature set value is set to 920 ° C. in a stepwise manner at the same time as the start of heating, whereby the opening of the control valve 8 is fully opened, and a large amount of fuel is injected. First, the furnace temperature rises, then the inner cover 6 is heated by the furnace temperature, and the heat transfer causes the coil ambient temperature inside the inner cover 6 to rise, and the heat transfer heats the coil 5 to cause the furnace coil ( Max) temperature rises,
Finally, it can be seen that the base temperature rises, and when the temperature after 32.5Hr is taken as 100% and the time when each temperature reaches 63.2% is expressed as a thermal time constant, the furnace temperature is 0.33Hr and the coil ambient temperature is The results were 1.0Hr, 1.5Hr at the furnace coil (maximum) temperature, and 4.5Hr at the base temperature.

The present inventors further attempted to investigate the stability of the feedback control by changing the furnace temperature, which is the feedback input of the controller 22, to the base temperature. The maximum temperature also fluctuates greatly according to this.However, after seeing the phenomenon that the base temperature, which has a large thermal time constant, hardly changes, we were worried about quality problems that would occur to the coils 5, 5 ' I have.

This merely demonstrates the basic principle of the feedback control described above.

Therefore, even if feedback control is performed by switching the input of one controller, the thermal time constant is 4.5Hr
It can be said that it is almost impossible to control this constant by feeding back a large base temperature.

Therefore, it is common practice to use the furnace temperature feedback control, which has the smallest thermal time constant and is used in the heating step, as it is, and to change the furnace temperature set value to keep the base temperature constant. Prior art.

However, considering the purpose of keeping the base temperature close to the coldest point temperature of the furnace coil constant, controlling the coil ambient temperature that directly transfers heat to the coil temperature,
It is considered that setting the base temperature to a predetermined value is the quickest response and the best means for obtaining high accuracy. However, at present, there is no known example by this method.

Next, the second point is that there is no description of a specific control method in the conventional method regarding how to give the set value to the basic control so as to be performed by the controller. A cascade setting method is adopted in which a furnace temperature set value required for controlling to a desired constant value is continuously calculated from a computer or a dedicated controller and set as a target value.

However, even if the cascade setting method is adopted, the target value obtained by continuously calculating the base temperature whose thermal time constant is as large as 4.5Hr has a large change width in a long cycle,
When this is set in cascade to the furnace temperature control, which is the basic control, the furnace temperature also fluctuates greatly as a result, and therefore, there has been a problem that it is very difficult to control the base temperature to a desired constant value in a precise manner. .

(Means for Solving the Problems) The present invention has been made in order to solve the problems of the conventional method. In the base temperature constant control which is a problem of the conventional method, "the furnace temperature control is used as the basic control, In the method of cascading the target value obtained by continuously calculating the base temperature to the furnace temperature control, it is difficult to control the base temperature to be constant. It is a temperature control method of a batch type annealing furnace made for the purpose of solving the above problem.

That is, to measure the maximum temperature T _ta the coil atmosphere furnace, and means 23 for adjusting it, the T _B of the lower end temperature or the base temperature of the lowermost coil is sampled at a constant period, the furnace coil ambient maximum temperature setpoint The present invention provides a method of controlling the lower end coil lower end temperature or the base temperature to a fixed value T _B (max) by providing means for correcting 25 and 26.

Hereinafter, the details will be described with reference to the illustrated embodiment.

FIG. 1 is an explanatory view of an annealing furnace embodying the present invention.
In the figure, reference numeral 9 denotes a base, on which the coil 5 is stacked. Above the uppermost coil, a coil ambient temperature sensor for measuring the maximum ambient temperature around the furnace coil
21. A base temperature sensor 3 for measuring the temperature by contacting the lower end of the lowermost coil is accommodated in the inner cover 6.

Further, the periphery of the inner cover 6 is covered by a combustion furnace 7. A furnace temperature sensor 2 for measuring the furnace temperature is attached to the furnace wall of the combustion 7.

The inner cover 6 is configured such that the high heat of the heating device 4 provided in the combustion furnace 7 does not directly affect the coil 5.
The inside of the inner cover 6 is filled with a slightly reducing gas or an inert gas as a holding atmosphere gas for holding the bright surface of the coil 5.

The fuel control to the heating device 4 is performed by the fuel valve 8, the valve opening of which is controlled by means of a furnace temperature controller 22 which feeds back the furnace end and issues an operation output so as to match a desired set value; The operation signal switch 24 can select a means by the coil atmosphere temperature controller 23 which feeds back the coil atmosphere temperature and outputs an operation output so as to match a desired set value.

Further, a base temperature is input, sampling is performed at a constant period, and an output from a correction calculator 25 for calculating a coil atmosphere temperature correction value for keeping the base temperature constant is switched by a correction signal switch 26 so that the coil atmosphere is changed. The temperature setting is modified.

 FIG. 2 is a heat pattern according to the present invention.

 Accordingly, the present invention will be described in detail.

First, from the start of heating, the coil atmosphere maximum temperature _Tta measured by the coil atmosphere control sensor 21 is equal to a desired value T.
until reaching _ta (max) controls the heating device 4 to hold the furnace temperature T _F which is determined by the furnace temperature measurement temperature sensor 2 to a constant value T _F1. In this case, the operation signal switch 24 selects the furnace temperature controller 22 and adjusts the fuel valve 8. This heating section is called a constant furnace temperature section.

Then, the from T _ta has reached the T _ta (max) to a base temperature T _B reaches the desired value T _B (max), the control coil ambient maximum temperature T _ta constant value T _ta (max) I do. In this case, the operation signal switch 24 is
Is selected, and the contact of the correction signal switch 26 is kept open. This heating section is referred to as a constant coil atmosphere temperature section.

In the constant coil ambient temperature section, the maximum coil temperature in the furnace
_Tt shows a rising tendency approaching the coil ambient temperature _Tta (max). However, as can be seen from the results of the temperature measurement experiment shown in FIG. 6 (C), _Tt does not exceed _Tta (max), so at least _Tta (max) is set to the furnace coil maximum temperature target value _Tt ( max)
, The maximum temperature of the furnace coil remains at its target value T _t (ma
x).

The coil ambient temperature predetermined section gradually increases base temperature T _B, to reach the desired value with a time lapse that T _B (max), and terminates the heating process. Most cold temperature T _C is the desired value T _h in furnace coils at this time (soak period target value of the most cold temperature)
However, if the relationship between the required time of the section where the coil ambient temperature is constant and T _ta (max) and T _B (max) is calculated in advance from the size, weight, material, etc. of the loading coil, or by experiment, , T _B (max) ≒ T _h can be set.

Therefore, in the coil ambient temperature constant section of the present invention,
The maximum furnace coil temperature T _t is set to the maximum furnace coil temperature target value T.
Since the temperature is gradually increased within the range of _t (max) or less, there is no period in which the temperature is maintained at the furnace coil maximum temperature target value _Tt (max).

After the base temperature T _B has reached the desired T _B (max), it will enter the soaking process, where, over the desired soaking time H _t, controls the base temperature to a constant value T _B (max). This soaking section is referred to as a constant base temperature section.

In the constant base temperature section, the operation signal switch 24 selects the coil atmosphere temperature controller 23 and adjusts the fuel valve 8. In addition, the correction signal switch 26 closes the contact so that the output of the correction calculator 25 corrects the set value of the coil ambient temperature controller 23 to reduce production.

The correction calculator 25 periodically samples the base temperature T _B that tends to increase due to the coil ambient temperature control performed as the basic control in the heat equalization process, and based on the amount of increase or the rate of increase, determines the base temperature T _B The desired T _B (max)
Calculate the coil ambient temperature correction value required to hold the temperature. When showing the embodiment with the arithmetic expression as an example, the sampling period is set to 1 hour, X (℃) = K _{[T B (℃) -T B (} max) (℃) ] X; correction value (ε) K; constants However, it was kept at 1 ≦ X <5 (° C.) to prevent excessive correction.

Constant K, the variation width of the base temperature T _B when the correct change coil ambient temperature setpoint, an adjustment determined constants on the basis of the control result to fit the desired range. It is also possible to set this as a variable constant and to learn and automatically set it based on the control result.

When the soaking process is completed, the heating device 4 is stopped, and natural cooling is performed for a while. After that, the inner cover 6 is forcibly cooled to a low temperature, the inner cover 6 is removed, the coil is taken out, and the entire process is completed.

The above control method can be easily realized by a combination of general-purpose analog instruments.In addition, it can be easily incorporated into a computer control system, which has recently been popularized, as a backup system when a computer goes down.
By adding an analog meter, higher reliability can be obtained.

(Embodiment) An annealing heat treatment was performed under the following conditions by applying the heating temperature control method of the present invention described above.

As the annealing conditions, the target coil is standard JIS, S58C and thickness 1.
A steel strip coil of 2 mm, width of 1000 mm and weight of 13.8 ton was stacked in three stages, and was subjected to the annealing pattern shown in FIG. 1 according to the heat pattern shown in FIG.

In this embodiment, in order to measure the maximum temperature of the in-furnace coil, a sensor is embedded in the uppermost coil at a position similar to the position shown in FIGS. did. The set temperature range in the soaking process was such that the lower limit temperature as the base temperature was 710 ° C, the upper limit temperature as the maximum temperature of the furnace coil was 740 ° C or less, and the required time was 11 hours. The time required for the heating step from the start of the heating to the start of the so-called soaking step in which the base temperature reaches 710 ° C. was set to 34 hours. The furnace temperature set temperature T _F1 in the heating step was 770 ° C. until the coil atmosphere temperature reached 740 ° C., and the heating established temperature value until the start of the subsequent soaking step was 740 ° C. as the coil atmosphere temperature setting. .

FIG. 7 shows a temperature chart of an example in which the annealing treatment was performed by the temperature control method of the present invention under such condition setting.

The coil ambient temperature constant interval for controlling from FIG from furnace temperature control furnace temperature constant interval by the coil ambient temperature, although furnace temperature T _F becomes a downward trend, the base temperature T _B is close to 710 ° C. of T _B (max) Show a rising trend. Furnace coil maximum temperature T ₁ of the this section has been increased to 735 ° C. from 720 ° C., T _B is 710 ° C. T
_When it reaches _B (max) and enters the soaking process, it drops.

The base temperature predetermined section of the soaking step, successively downgraded T _ta (max) of the coil ambient temperature setpoint 740 ° C. in response to an increase in T _B, at the same time the sampling correction control hourly base temperature To keep the base temperature constant, it can be seen from the figure that the base temperature T _B
Accuracy of -2 ℃ ~ + 4 against set value 710 ℃
It shows very good control in ° C.

In addition, the maximum temperature of the furnace coil gradually decreases and approaches the base temperature.
The difference between the temperature and the temperature has become smaller, and the overall temperature transition shows that the temperature is far below the soaking upper limit temperature of 740 ° C. Therefore, it is possible to expect a good annealed coil having a small variation in the quality in the coil.

On the other hand, as a comparative example, the results of annealing heat treatment performed in a control method by setting the furnace temperature in all of the heating step and the soaking step will be described.

Annealing conditions, temperature set values, etc. were all the same as in the example of the present invention. However, the heat pattern was based on FIG. 4 of the conventional method, and the furnace temperature set value T _F1 was 10 times more than that of the embodiment of the present invention.
The temperature was raised to 780 ° C. The reason for raising the temperature by 10 ° C is that in the case of furnace temperature control, when the maximum temperature of the in-furnace coil reaches 740 ° C, it is necessary to greatly reduce the furnace temperature due to the problem of the thermal time constant. This is because the start of soaking is delayed for more than 34 hours.

FIG. 8 shows a temperature chart of a comparative example performed under such conditions.

From this figure, the base temperature T _B 710 ° C. at the start of the soaking process reached in 34 hours as set, but the maximum temperature T _t of the furnace coil in the constant coil maximum temperature section before the start of the soaking process was already The set upper limit of 740 ° C. has been reached, the difference from the base temperature is 30 ° C. to 37 ° C., and the humidity fluctuation range is about 10 ° C. compared to 23 ° C. to 27 ° C. in the constant coil atmosphere temperature section of the embodiment of the present invention. large.

In a constant base temperature section of the soaking process, even if the measured base temperature changes the furnace temperature set value, there is a delay of 4 to 5 hours before the effect appears, so the furnace temperature set value is frequently changed. As a result, the actual furnace temperature value had a large fluctuation range and a large cycle change.

Therefore, the coil ambient maximum temperature _Tta and the furnace coil maximum temperature _Tt change accordingly, and the difference between the furnace coil maximum temperature and the base temperature is not so small and there is a temperature range of 20 ° C to 30 ° C.

Since this temperature range is larger than 9 ° C. to 25 ° C. in the embodiment of the present invention, in addition to the variation in the quality in the coil being increased,
There is also concern about coil seizure at long-term high temperatures.

FIG. 9 shows that the annealing coil of the above-described embodiment is
The hardness variation (Hv) surveyed hardness variation results are shown for every 100 m of coil length.

FIG. 4A shows a coil of the comparative example, and FIG. 4B shows a coil to which the present invention is applied. As is clear from the figure, the hardness variation of the coil of the comparative example is It can be seen that the coil to which the present invention is applied has σ = 3, which is much better than the temperature control of the present invention.

When this coil for hardness measurement was unwound from the coil, the seizure state in the coil was examined. As a result, seizure was found in a part of the outer side of the upper coil of the comparative example. The coil of the example was completely good.

(Effects of the Invention) According to the method of the present invention, since the furnace coil is not heated and held near the coil maximum temperature target value for a long time, the variation in coil quality is minimized. , Seizure can be prevented. Furthermore, conventionally,
Base temperature constant control, which was difficult due to a large thermal time constant, can be realized with high accuracy, and is of high industrial utility value.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an annealing furnace in which the method of the present invention is carried out, FIG. 2 is a table showing an example of a heat pattern by an annealing temperature control method of the present invention, FIG. The figure is a chart showing the heat pattern of the conventional annealing temperature control.
FIG. 6 is an explanatory diagram of the furnace operating conditions in the annealing soaking process. FIG. 6 is a diagram showing a measurement example of a thermal time constant. FIG. 6A is an annealing furnace used for the measurement, and FIG. Is a layout diagram of the temperature measurement sensor around the top coil of the three-tiered product, (C) is a chart of the measurement results of the thermal time constant, FIG. 7 is a temperature chart of the embodiment to which the present invention is applied,
FIG. 8 is a temperature chart of the comparative example, and FIG. 9 is a table showing the hardness variation in the annealing coil. FIG. 8A is a comparative example, and FIG. 8B is a hardness variation example according to the method of the present invention. 1. Top temperature sensor 2. Furnace temperature sensor 3. Base temperature sensor 4, Heating device 5, 5 'Coil 6, Inner cover 7. Combustion furnace 8, … Fuel valve 9… Base plate, 10… Controller 11… Calculator 21… Coil atmosphere temperature sensor 22… Furnace temperature controller 23… Coil atmosphere temperature controller 24… Operation signal switch, 25… … Correction calculator 26… Correction signal switch _TF … Detected furnace temperature, T _F1 … Target furnace temperature T _t … Maximum furnace coil temperature T _t (max)… Maximum furnace coil target temperature T _B …… Base temperature, T _C … Cooling temperature T _h …… Cooling temperature target value H _t …… Heat equalization time T _ta …… Coil atmosphere maximum temperature T _ta (max) …… Coil Atmosphere maximum temperature target value

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasuyuki Nishiyama 12 Nakamachi, Muroran-shi, Hokkaido Inside Nippon Steel Corporation Muroran Steel Works (56) References JP-B-59-24167 (JP, B2)

Claims (1)

(57) [Claims] In a soaking process of a batch type annealing furnace for heat-treating a plurality of stacked coils, a coil atmosphere has a maximum temperature.
Measuring the T _ta, and means 23 for adjusting it, the lower end temperature of the lowest coil or base temperature T _B is sampled at a constant period, means for correcting the furnace coil ambient maximum temperature setpoint 2
5, 26, the lower end temperature of the bottom coil or the base temperature
Temperature control method of a batch-type annealing furnace, characterized by controlling the T _B to a constant value T _B (max).