JPS5847453B2 - Method of controlling billet temperature in heating furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling the temperature of a steel billet in a heating furnace.

In controlling the billet temperature in a continuous heating furnace for billets, the following two methods are required:
The point becomes an important issue.

In other words, ■ Improving the accuracy of the extraction temperature of the steel billet.

■ Improving thermal efficiency.

The above is a problem of how to bring the extraction temperature of the steel billet closer to the target value.By bringing the extraction temperature change closer to the target value, not only will the quality of the product be stabilized, but unnecessary heating will be eliminated, and the fuel source This also leads to a reduction in units.

In addition, the above item (■) is a problem of the ratio of the calorific value of the input fuel to the heat content of the extracted steel billet, which is directly linked to the reduction of the fuel consumption rate. This can be improved by appropriately distributing the flow rate (simply referred to as flow rate).

Conventionally, there are various methods of controlling the temperature of a steel billet in a heating furnace for the purposes of (1) and (2) above, but they can be roughly divided into the following two methods.

In other words, ■ a method based on furnace temperature control; ■ A method based on flow control.

However, neither of the above methods (1) and (2) could simultaneously satisfy the objectives (1) and (2) above.

In other words, method (2) above operates by feedback controlling the flow rate so that the furnace temperature reaches a certain set value, and although it can be said to be theoretically an excellent method for achieving the purpose (2) above, (2) It was not always possible to achieve the objective.

The reason for this is that since the relationship between furnace temperature and flow rate is treated like a blank box, the flow rate that has been statically determined by feedback control may be distributed unbalancedly.

There are also problems in control. That is, due to equipment constraints, it may not be possible to achieve the set furnace temperature, or the control loops of each zone may interfere.

Next, method (■) above is a method of distributing the flow rate so as to optimize the thermal efficiency of (■) above, but the flow rate itself is calculated based on a formula obtained from experience, and the furnace temperature and flow rate are calculated based on the formula obtained from experience. Since the relationship with

This invention can simultaneously satisfy the above objects and (2).

That is, the present invention provides a method for controlling the temperature of a steel billet in a heating furnace that can improve the accuracy rate of the extraction temperature of the steel billet and improve the thermal efficiency. The billet exit temperature of each zone in the heating furnace is predicted and calculated based on the dimensions of the billet to be charged, the estimated value of billet temperature change, the planned billet extraction pitch, the planned input fuel flow rate, etc. The method is characterized in that the deviation between the temperature and the target temperature at the exit of the steel billet of each zone is determined, and the planned input fuel flow rate is corrected according to the deviation, so that the billet extraction temperature is made to match the billet target extraction temperature. have

The present invention will be explained below with reference to the fourth flowchart.

The general structure of a continuous heating furnace for steel billets includes, from the billet extraction side, a soaking zone, a second heating zone, a first heating zone, and a preheating zone.

In this type of heating furnace, the moving direction of the billet and gas is opposite, so by burning as much as possible from the band near the billet extraction side, the temperature of the exhaust gas on the billet charging side is lowered.
This can increase thermal efficiency.

However, in this case, for the soaking zone, the degree of evenness, that is,
The temperature difference between the surface and inside of the billet is important for rolling and product quality.
Since it has to be within a certain range, the flow rate is limited to a value determined by that range.

Therefore, a flow rate distribution method that brings the flow rate of the second heating zone as close as possible to the maximum in terms of equipment is optimal for improving thermal efficiency.

In this invention, since it is not the furnace temperature distribution but the flow rate distribution that determines the thermal efficiency, a billet temperature control method based on flow rate control as shown in FIG. 1 is used.

In the figure, 1 is a heating furnace, 1ay1b, 1c and 1
b is a preheating zone, a first heating zone, a second heating zone, and a soaking zone.

2 is a flow rate adjustment device, 3 is a calculation device, 4 is a billet charging detector, 5 is a billet extraction detector, and 6 is a billet thermometer.

The problem here is that the second heating zone has a flow rate that is close to the lowest in terms of equipment, so the control range becomes narrower.

That is, if the flow rate in the first heating zone is erroneously lower than the normal flow rate, it will be impossible to correct it in the second heating zone due to equipment constraints.

As described above, thermal efficiency and ease of restriction are contradictory, and it is necessary to achieve a good compromise between the two, but this is determined by the accuracy of the calculation of the set flow rate.

In particular, in the first heating zone, the steel slab that will exist in that zone in the future will become the second heating zone.
When moving to the heating station, the flow rate of the second heating station must be set to a feasible value, and it is necessary to calculate backwards from a long-term future.

In order to accurately calculate the set flow rate, it is necessary to handle the heat balance relationship, and in order to handle Stefan Bornmann's thermal radiation formula, a nonlinear equation including a fourth power term must be solved.

Since this cannot be solved analytically, an approximate solution is found numerically.

The method used here is to first determine a rough flow rate for the future, remove large nonlinearities, and then linearize minute changes in the flow rate to obtain an accurate solution.

Next, the prediction calculation method according to the present invention will be explained in more detail.

First, when heating a steel billet in a heating furnace, the planned fuel injection flow rate value V1(t) t in each zone is approximately determined from the dimensions of the steel billet, the planned steel billet extraction pinch, the estimated value of the steel billet temperature, etc.
Determine V2(tJ, V3(t)...).

Since the planned fuel flow rate value does not require much accuracy,
Any commonly used method is fine.

However, it is necessary to pay attention to meaningless wide-ranging changes and oscillatory fluctuations in the scheduled fuel injection flow rate.

The reason for this is as follows.

(2) Depending on the followability of the air-fuel ratio control system, the air-fuel ratio may temporarily deteriorate when changing the flow rate set value, so it is better that the range of fluctuation is as small as possible and the frequency is infrequent.

■ Even if the flow rate is changed over a wide range, the furnace temperature changes only slowly, and since there are many slabs in one band, it is difficult to change the temperature of adjacent slabs by a wide range. It is impossible to set the target value, and only the average temperature can be controlled.Next, the estimated value of the billet temperature at the present time is used as the initial value, and the dimensions of the billet, billet extraction pinch Based on the planned input fuel flow rate, etc., the billet outlet temperature of each zone in the heating furnace is predicted and calculated from the heat balance calculation.

Next, the deviation between the billet outlet temperature of each band and the billet outlet target temperature of each strip is predicted and calculated.

In the billet exit target values for each zone, the billet exit target value in the soaking zone takes into account the billet extraction target temperature, and the billet exit target value in the second heating zone takes into account the degree of uniformity in the soaking zone. The deviations e,(t) and e2(t) are respectively calculated as values calculated backward from the target temperature for extraction of the steel billet, or as empirical values, taking into account the following.

Here, an example of a predictive calculation method for the steel billet outlet temperature will be explained.

The heating furnace is modeled as shown in FIG.

That is, ■． The heating furnace atmosphere gas side is divided into n blocks.

■． The furnace wall is divided into blocks similar to the heating furnace atmosphere gas, and further divided into m meshes to form a one-dimensional heat transfer model.

■． The steel slab is divided into β pieces in the thickness direction,
A one-dimensional vertically symmetrical heat transfer model is used.

■． The heat transfer between the steel slab and the furnace atmosphere gas, and between the furnace wall and the furnace atmosphere gas, is due to radiation, and the emissivity of the first furnace atmosphere gas block and the j-th steel slab is Φs.
The movement of the steel piece is modeled by determining ij based on the position xj of the steel piece.

Regarding the time integration in (2) above, first, the heating furnace atmosphere gas temperature that makes the heat balance O is determined from the current surface temperatures of the steel billet and the furnace wall.

Next, the temperatures of the steel slab and the furnace wall are integrated using a differential equation.

Hereinafter, the method of calculating the above item (2) will be explained in detail.

For the i-th atmospheric gas block, the unbalanced heat balance Hi is determined by the following equation.

g(T): heat loss.

The above equation (1) holds true for each block, and since the heat balance is originally O, H1-Hn is O.

However, since these are nonlinear simultaneous equations for TG, they are solved by repeated calculations using Newton's method.

As a result, the heating furnace atmosphere gas temperature is determined, and the temperature of the steel piece is integrated using the following equation.

In the above formula, ξ(T8): Heat content of the steel billet at the temperature of T5.

The temperature of the furnace wall is also integrated using the same differential equation as above.

Next, in order to averagely reduce the deviation between the billet exit temperature of each band and the billet exit target temperature of each band, which were determined as above, and to optimize the thermal efficiency, Modify the above-mentioned scheduled fuel injection flow rate.

That is, first, the deviation e1ft), e2ft)
, the weighting functions h 1(L) , h 2(t)
, h 3 (tJ) and integrate to obtain the following primary correction amount Jvl(tJ
, Jv2(t) and Jva(tJ).

If you only want to reduce the average deviation, set the setting value for each band as Vl (Ql+ A u 1 (0), V2(0
) + l u2 ((J)) and V 3 (0) + 11
v 3 (0), but in order to further improve the thermal efficiency, the predicted value of the flow rate of the second heating zone when the steel slab existing in the first heating zone passes through the second heating zone V2 ( t)+
The flow rate of the first heating zone is corrected according to the difference ε between the maximum value of A u2(t) and the equipment upper limit of the second heating zone.

Therefore, the planned fuel flow rate for each zone is V 1 (OJ +
A ut(Q , V2(0) + l u2(Q and V
3(() + 11 v 3 (0) 10β・ε.

Here, h, (t), h2(t), h3(t)
, α1, α2, α3, and β, a heating furnace is numerically simulated using the calculation method of the billet outlet temperature in each zone as described above, and the steel billet temperature in each zone is calculated based on the slight change in flow rate under average operating conditions. All you have to do is find the response of the single outlet temperature and apply the conventional linear theory.

For example, h1(t), h2(t), h3(t
), first, the impulse response p of the steel billet temperature deviation eI(tJ, e2(t)) at each outlet with respect to the flow rate is calculated.
Find t(t), p2(t), and p3(t).

That is, pl(t): e1(t), p2(t) when the impulse flow rate is input to the soaking zone; e2(t), p3(t) when the impulse flow rate is input to the second heating zone; When the impulse flow rate is input to 1 heating zone, e3(0, is.

FIG. 3 shows a block diagram of the linearized control system, and the error e' ．．
(t) (i = 1 to 3) is a feedforward control, so the impulse response including the control system and the heating furnace satisfies the equation qi(t), and qi(t) is close to the impulse. Choose hi(t) such that.

At this time, control human power ηi(t)
In order to prevent large fluctuations, hi (t) is made a smooth function with a certain width, that is, a function that averages out the errors.

Next, in order to maintain the accuracy of the predicted calculation of the steel billet outlet temperature of each band mentioned above, the parameters for the heat balance calculation are corrected based on the actual values.

The calculation method shown in this example modifies emissivity and heat loss.

The emissivity is corrected by finding the deviation between the actual value of the temperature of the steel slab extracted from the heating furnace and the predicted temperature, and uniformly multiplying the emissivity ΦSij by a coefficient according to the moving average of this deviation.

Heat loss is determined by the heating furnace atmosphere gas TG in the previous equation 1).
Substitute the solid line value into , find the heat balance imbalance Hi,
The amount of heat loss corresponds to this moving average. Correct by adding to (T2).

Note that (1') above. (2), (3) and the second
The meanings of symbols related to the figures are as follows.

Toi: heating furnace atmosphere gas temperature of the i-th block, Tsjk: temperature of the k-th mesh of the j-th steel billet, Twik: temperature of the k-th mesh of the i-th furnace wall,
Vi: fuel flow rate of the i-th block of heating furnace atmosphere gas, Φw1: emissivity of the i-th block of heating furnace atmosphere gas and the i-th furnace wall, Φsij'' block of furnace atmosphere gas and the emissivity of the Emissivity with steel billet, Xy ” Position of J-th steel billet in furnace length direction, Ssj:j
Cross-sectional area of the th steel billet, Swi: Area of the ith furnace wall, W81, ゜ Mass of the ith mesh of the j-th billet, σ
: Stefan Boltzmann constant, C, : specific heat of exhaust gas, K : thermal conductivity of steel slab, At : time step, hs : mesh spacing in thickness direction of steel slab, AH : fuel calorific value, r : volume of fuel and exhaust gas ratio, n: number of blocks of heating furnace atmosphere gas, l: number of meshes per one steel slab, m: number of meshes per one block of furnace wall, h8: number of steel slabs.

As explained above, according to the present invention, it is possible to bring the flow rate distribution of each zone drawn into the heating furnace closer to the distribution ratio with good thermal efficiency, and as a result, it is possible to increase the thermal efficiency on average, and furthermore, each steel billet By quantitatively understanding the relationship between the temperature, furnace temperature and flow rate, that is, heat balance, and performing accurate predictive calculations, it is possible to improve the accuracy rate of the extraction temperature of steel billets, which is extremely useful in industry. effect is brought about.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of this invention, FIG. 2 is a diagram showing a predictive calculation model, FIG. 3 is a block diagram of a linearized control system, and FIG. 4 is a flowchart of this invention. be. In the drawings, 1...Heating furnace, 1a...
・Pre-heating zone, 1b...First heating zone, 1c...
...Second heating zone, 1d... Soaking zone, 2...
...Flow rate adjustment device, 3...Arithmetic device, 4...
... Steel billet charging detector, 5... Steel billet extraction detector, 6... Steel billet thermometer.

Claims (1)

[Claims] 1. When heating billets in a heating furnace, determine the billet outlet of each belt in the heating furnace based on the dimensions of the billet charged into the heating furnace, estimated value of billet temperature, planned billet extraction pinch, planned fuel input flow rate, etc. Predictively calculate the temperature, find the deviation between the billet outlet temperature of each zone and the billet outlet target temperature of each zone, correct the planned input fuel flow rate according to the deviation, and adjust the billet extraction temperature to the billet exit temperature. A method for controlling the temperature of a steel billet in a heating furnace, characterized by making the temperature match a target extraction temperature.