JP2833461B2 - Metal strip temperature control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to plate temperature control in a direct heating furnace for a metal strip or the like, and in particular, to provide a method for preventing the plate temperature from deviating when a heat cycle is changed.

[0002]

2. Description of the Related Art Direct heating furnaces are often used in heating furnaces for continuous annealing lines of metal strips and hot metal plating lines in order to improve thermal efficiency and shorten the line length. Regarding the sheet temperature control of the metal strip, there is a problem in control in a transient state when a heat cycle is changed such as a change in a sheet thickness, a sheet width, a target sheet temperature or the like, and various techniques have been proposed.
In particular, regarding the temperature control of a direct heating furnace, see
These are disclosed in, for example, JP-A-55-73831, JP-A-60-70127.

[0003] In the technique described in Japanese Patent Application Laid-Open No. 55-73831, a direct heating furnace is arranged in front of an indirect heating furnace, and a plate temperature at a heating furnace outlet (in this technique, an outlet of the indirect heating furnace) is set to a predetermined value. The direct heating furnace and the indirect heating furnace are controlled so as to obtain the values. In the transient state when the heat cycle is changed, a deviation between the sheet temperature at the outlet of the indirect heating furnace and a predetermined value is estimated and calculated, and the target sheet temperature at the outlet of the direct heating furnace is set so as to cancel the deviation.

The technique described in Japanese Patent Application Laid-Open No. 60-70127 discloses a technique in which a direct heating furnace is arranged in front of an indirect heating furnace, and the sheet temperature at the heating furnace outlet (outlet of the indirect heating furnace) is set to a predetermined value. The operating conditions are controlled to be as follows. In the transient state when changing the heat cycle, adjust the line speed so that the plate temperature at the direct heating furnace outlet is constant until the furnace temperature of the indirect heating furnace is changed to the set value, or adjust the line speed. Either method of adjusting and fixing the sheet temperature at the outlet of the heating furnace directly is used.

[0005]

These techniques compensate for the difficulty of controlling the plate temperature due to the low thermal response of the indirect heating furnace with the direct heating furnace having a high thermal response. However, in practice, even in a direct heating furnace, combustion control is restricted due to control of oxygen partial pressure in the furnace atmosphere and securing of combustion stability, and at present, high thermal responsiveness cannot always be obtained.

[0006] Further, these techniques mainly consist of a control method combining both a direct heating furnace and an indirect heating furnace, and no plate temperature control using only the direct heating furnace has been proposed. For example, in Japanese Patent Application Laid-Open No. 55-73831,
`` The sheet temperature control device feeds back the sheet temperature detected by a sheet thermometer provided at the outlet of the direct heating furnace and controls the fuel flow rate to the direct heating furnace so that the sheet temperature of the direct heating furnace outlet can be set to a reference value. Perform automatic control to keep ", but the content of automatic control such as operation of fuel flow rate is not described.

[0007] Similarly, Japanese Patent Application Laid-Open No. 60-70127 also states that "the temperature of each zone of the indirect heating furnace is sequentially measured, and the target temperature (plate temperature) at the indirect heating furnace (outlet) can be secured from the measured value. Calculate the sheet temperature at the direct heating furnace (outlet) and set the sheet temperature at the direct heating furnace (outlet) to the calculated sheet temperature. " It has not been.

[0008] One of the reasons that the direct heating furnace is not controlled is that it is difficult to set the furnace temperature. Generally, it is difficult to measure the temperature of the atmosphere in the furnace, but in an indirect heating furnace, the temperature of the atmosphere is almost equal to the temperature of the furnace wall, and the furnace wall temperature may be used as the furnace temperature. On the other hand, these are greatly different in the direct heating furnace, and there is a situation that the influence of the ambient temperature cannot be reflected in the control only by measuring the furnace wall temperature.

However, regarding the breakdown of heat transfer to the metal strip in the direct heating furnace, the amount of heat transferred from the furnace wall is only about 50% of the whole. The rest is the amount of heat transferred from the furnace atmosphere, that is, the combustion gas, about 20% by gas radiation,
Convection is estimated to be about 30%. Thus, despite the fact that the heat transfer from the furnace atmosphere cannot be ignored, the heat transfer model of the prior art could not handle it.

As a result, the normal heat transfer model using the furnace wall temperature as the furnace temperature cannot be applied to the control of the direct heating furnace. Furthermore, the transient state at the time of changing the heat cycle cannot be appropriately controlled, and the sheet temperature control cannot be achieved without resorting to an inadequate method such as the setting of operating conditions by a table method.

In the prior art, the point at which the set value of the sheet temperature is changed is after the succeeding material has directly reached the outlet of the heating furnace. Therefore, when the set value is changed, the following material is contained in the furnace, and the portion is heated under the optimal heating conditions for the preceding material, so that the temperature of the following material falls within the allowable range of the sheet temperature of the following material. I often didn't enter. The length of this part of the following material is comparable to the line length of the direct heating furnace, and has reached several tens of meters as the size of the direct heating furnace increases. This size reaches several percent of the whole, and there is a problem that the yield is reduced.

The present invention proposes a heat transfer model incorporating the calculated value of the ambient temperature and the furnace wall temperature in order to solve the above-mentioned problems of the prior art. In particular, the present invention provides a sheet temperature control method capable of keeping the sheet temperature deviation of the metal strip when changing the heat cycle within an allowable range and reducing the area as much as possible even if the sheet temperature does not fit.

[0013]

According to the first aspect of the present invention, a metal strip which is continuously heated by a direct heating furnace is connected when a connection portion of the metal strip or a place where a heating condition is changed passes through the furnace. In the method of controlling the sheet temperature of the preceding material and the succeeding material preceding the part or the changed part using the heat transfer model formula, (a) first, the sheet temperature of the metal strip,
Using the heat transfer model formula including the combustion condition such as the amount of fuel input and the three values of the moving speed of the metal strip as variables, the operating conditions other than the combustion condition remain the operating conditions of the preceding material, Given the upper and lower limits of the allowable range of the sheet temperature of the following material, the combustion conditions for each are estimated and calculated. (B) If the estimated calculated combustion conditions exceed the allowable range, the combustion conditions Then, the moving speed of the metal band is estimated and calculated using the heat transfer model formula, and the set value of the moving speed of the metal band is changed based on the result. Is changed to the operating condition corresponding to the upper limit or the lower limit of the sheet temperature of the following material, the sheet temperature of the preceding material is estimated and calculated using the heat transfer model formula, and (d) Two estimated values of the sheet temperature of the preceding material corresponding to the upper or lower value of the sheet temperature of the material The temperature range between, if there is a temperature region overlapping with the allowable range of the plate temperature of the preceding material, change the target plate temperature set value of the preceding material at that temperature region, (e)
If there is no temperature range between the two calculated values of the sheet temperature of the preceding material and the allowable range of the sheet temperature of the preceding material, the temperature range between these estimated values and the allowable range of the sheet temperature And changing the set value of the target sheet temperature of the preceding material to a value between the above values.

According to a second aspect of the present invention, when the connecting portion of the metal strip or the changing position of the heating condition passes a predetermined position on the entrance side of the direct heating furnace, the set value of the target sheet temperature of the preceding material is changed, 2. The metal strip sheet temperature control method according to claim 1, wherein the set value of the target sheet temperature is changed to the target sheet temperature of the succeeding material when the sheet passes a predetermined position on the outlet side of the heating furnace.

[0015]

First, the heat transfer model of the present invention uses the sheet temperature of the metal strip,
The three values of combustion conditions, such as fuel input, and the movement speed of the metal strip are included as variables. It goes without saying that, in addition to these three variables, other variables are used as needed in the heat transfer model. A heat transfer model in a general direct heating furnace will be described. The equation representing the amount of heat transfer to the metal strip in the direct heating furnace is as follows.

[0016]

(Equation 1)

Here, ρ: density of the metal strip, cp: specific heat of the metal strip, h: thickness of the metal strip, v: moving speed of the metal strip, T: sheet temperature of the metal strip, t: time, k: Constants, C1, C2, C3: weighting factors, σ: Boltzmann constant, Tw: furnace wall temperature, Tg: ambient temperature.

The left side of this equation represents an increase in the amount of heat per unit time and unit area of the metal strip, the first term in square brackets on the right side is radiant heat transfer from the furnace wall to the metal strip, and the second term is from the atmosphere. Radiative heat transfer to the metal strip, and the third term represents convective heat transfer from the atmosphere to the metal strip. Further, the constant k is a constant reflecting the influence of the geometrical arrangement and the like on the radiant heat transfer, and varies depending on the shape of the furnace. The weighting factors C1 and C2 are determined by the emissivity of the metal band, and C3 is determined by the convective heat transfer coefficient and their respective contributions. These constants and coefficients (especially C1, C2, C3) are appropriately corrected by learning from the operation results and the like to increase the accuracy. Note that the unit of temperature uses the absolute temperature to handle radiant heat transfer.

Next, since it is difficult to measure the ambient temperature, the present invention calculates the ambient temperature by calculation. The ambient temperature Tg is expressed by an approximate expression based on the fuel input amount u, the metal strip width b, the metal strip thickness h, and the metal strip moving speed v. The specific form of the equation is that the ambient temperature Tg changes almost linearly with the fuel input u in the range where stable combustion is possible, and the effects of other factors (metal strip size, moving speed) Since it is smaller than the effect of the fuel input u, a linear equation is sufficient. The approximate expression for the ambient temperature Tg is shown below.

[0020]

(Equation 2)

Here, A0 is a constant term, and A1, A2, A3, and A4 are coefficients, each of which is determined from operation results and the like, and is appropriately corrected by learning.

Approximate expression (Equation 2) representing the ambient temperature Tg
Is put into the equation (1), the following equation is obtained.

[0023]

(Equation 3)

Equation 3 is used as a heat transfer model equation. This heat transfer model formula includes three values of the sheet temperature T of the metal strip, the fuel input amount u, and the moving velocity v of the metal strip as variables, and expresses the mutual relation therebetween. Therefore, if two of these three variables are given, the other one will be calculated by the heat transfer model equation. The estimation calculation is as follows because the heat transfer model equation and Equation 3 are differential equations. First, the sheet temperature T of the metal strip can be easily obtained by a numerical solution of the equation (3) (iterative calculation of a difference equation). Starting from the first approximation of u or v, the fuel injection amount u or the moving speed v of the metal strip is successively calculated until the numerical solution of the equation (3) for the sheet temperature T of the metal strip converges to the target sheet temperature. It is obtained by approximation.

Here, if the air ratio of the burner in the combustion of the direct heating furnace is less than 1.0, a value obtained by correcting the fuel input u by the air ratio (fuel input × air ratio, etc.) is used. Be more accurate. Further, an equation using the combustion air amount instead of the fuel input amount u may be used. Therefore, the amount of injected fuel, its correction value, the amount of combustion air, and the like are collectively referred to as combustion conditions.

A method for directly controlling the heating furnace using the heat transfer model equation (Equation 3) will be described. FIG. 1 is a flowchart showing the control method of the present invention. First, regarding the variables of the heat transfer model equation (Equation 3), those relating to the operating conditions are kept at the values of the preceding material except for the combustion conditions, and those relating to the metal strip are replaced with the values of the succeeding material. Next, the upper limit value or the lower limit value of the sheet temperature of the following material is put into the sheet temperature, and the estimation calculation of each combustion condition for the upper limit value and the lower limit value of the sheet temperature of the following material is performed. As a result, when the combustion condition is within an allowable range such as the burner capacity, the combustion condition and the moving speed of the preceding material become operating conditions corresponding to the upper and lower limits of the succeeding material sheet temperature.

As a result of the above estimation calculation, if the combustion condition exceeds the upper or lower limit of the allowable range, the combustion condition is reset to the allowable range. This time, the transfer speed of the metal strip is estimated and calculated by giving the combustion conditions such as the fuel input amount and the upper limit or lower limit of the sheet temperature of the succeeding material to the heat transfer model formula. In this way, the combustion conditions and the moving speed of the metal strip, that is, the operating conditions, that can keep the sheet temperature of the following material within the allowable range are determined.

Next, the sheet temperature of the preceding material when the operating conditions are changed to the operating conditions corresponding to the upper limit value and the lower limit value of the sheet temperature of the following material is determined. This is obtained by giving the operating conditions obtained above to the heat transfer model formula and estimating and calculating the sheet temperature by replacing the variables relating to the metal strip with the values of the preceding material.

As a result, when the obtained temperature range between the two estimated values of the sheet temperature of the preceding material overlaps with the allowable range of the sheet temperature of the preceding material, the preceding temperature range is included in the overlapping temperature region. Change the set value of the target sheet temperature of the material. Thereafter, while the predecessor material is in the furnace, by controlling with this new target sheet temperature,
The sheet temperature of the preceding material gradually approaches the new target sheet temperature.
This portion is a portion that was in the furnace when the set value of the target plate temperature was changed, and is hereinafter referred to as a transition region. At the end of the transition region, the sheet temperature of the metal strip is basically controlled at the new target sheet temperature.

If the temperature range between the two calculated values above and below the obtained sheet temperature of the preceding material does not overlap with the allowable range of the sheet temperature of the preceding material, the temperature range based on these estimated calculations and the allowable value of the sheet temperature are used. The set value of the target sheet temperature of the preceding material is changed to a value within the range.

Further, from the point in time when the connecting portion of the metal strip or the place where the heating condition is changed passes through a predetermined position on the entrance side of the direct heating furnace to the point in time when it passes through the predetermined position on the exit side of the direct heating furnace,
The target plate temperature is set based on the result of the estimation calculation. During this period, the metal strips of both the leading and trailing materials coexist directly in the heating furnace, but the set temperature of the sheet temperature is set at the outlet of the heating furnace because the sheet temperature is measured directly at the outlet of the heating furnace. Set as the sheet temperature of the material. As described above, in the present invention, both sheet temperatures are set to be within the allowable ranges, or set to such a temperature that even if it is impossible, the deviation from the respective allowable ranges is small.

[0032]

FIG. 2 is a block diagram of a control system used in the present invention. In the figure, 1 is a heat transfer model formula estimating calculation means, 2 is a combustion control means, 3 is a metal strip moving speed control means, 4 is a combustion regulator, 5 and 6 are temperature converters, 10 is a direct heating furnace, and 20 is a heating furnace. Furnace temperature detector, 30 is a burner, 40 is a plate temperature detector, 50 is a motor, 6
Reference numerals 0 and 70 denote metal belt transport rolls, respectively. The following control is performed with such a system configuration.

FIGS. 3 and 4 are flowcharts showing the control procedure of the present invention. The figure or the control steps are shown. First, the input amount u2H of the fuel corresponding to the upper limit T2H of the sheet temperature of the succeeding material is estimated by the heat transfer model formula. Specifically, the upper limit value T2H of the sheet temperature of the succeeding material is inserted into T of the heat transfer model equation (Equation 3) to obtain a solution of u, and this is set as u2H. For other numerical values, the values for the preceding material are used as the operating conditions (for example, furnace wall temperature Tw), and the values for the succeeding material are used for the metal band specifications (ρ, cp, h, b, etc.). .

When the solution of u exceeds the upper limit umax of the fuel injection amount of the combustion device, umax is used as the value of u2H. This time, the moving speed v of the metal band with respect to umax is obtained from the model formula, and is set to v2H. Specifically, umax is added to u in the heat transfer model equation (Equation 3) to find the solution of v, and it is calculated as v2H
And From the above, as the operating conditions corresponding to the plate temperature upper limit T2H, the fuel input amount u2H and the moving speed v2H of the metal strip are determined.

The operating conditions (u
If it is changed to 2H, v2H, etc.), calculate the sheet temperature of the preceding material using the heat transfer model formula. Sheet temperature T of the obtained preceding material
Is the sheet temperature of the preceding material corresponding to the upper limit T2H of the sheet temperature of the succeeding material, and is expressed as T1H *. Similarly, the fuel input amount u2L corresponding to the sheet temperature lower limit T2L of the succeeding material is estimated by the heat transfer model formula.

The amount of fuel input corresponding to the plate temperature lower limit T2L
When u2L is less than the lower limit umin of the fuel input amount of the combustion device, umin is used as the value of u2L. Similarly, the moving speed v of the metal band with respect to umin is obtained from the model formula and is set to v2L.
From the above, as the operating conditions corresponding to the plate temperature lower limit T2L, the fuel input amount u2L and the moving speed v2L of the metal strip are obtained.

[0037] Similarly, the operating condition (u2L, v2L, etc.) corresponding to the sheet temperature lower limit T2L is used to estimate and calculate the sheet temperature of the current material using a heat transfer model equation. The obtained value is the sheet temperature of the preceding material corresponding to the lower limit value T2L of the sheet temperature of the succeeding material, and T1L *
It expresses. The estimated values T1H * and T1L * of the sheet temperature of the preceding material are compared with the allowable range (T1H to T1L) of the sheet temperature of the preceding material. If T1H * or T1L * is included in the allowable temperature range of the preceding material, T1H *
Alternatively, T1L * is set as the set value T1S of the sheet temperature of the preceding material.

Estimated calculated value T1H * or T1 of sheet temperature of preceding material
If L * does not fall within the allowable range (T1H to T1L) of the sheet temperature of the preceding material, a target value T1S of the sheet temperature of the preceding material is set to a temperature between the estimated calculated value and the allowable range. If the estimated calculated value T1H * corresponding to the upper limit of the allowable range of the sheet temperature of the succeeding material is lower than the lower limit T1L of the allowable range of the sheet temperature of the preceding material, set the target temperature of the sheet temperature of the preceding material between T1H * and T1L. When the value T1S is set and the estimated calculated value T1L * is higher than the upper limit T1H of the allowable range of the sheet temperature, the target value T1S of the sheet temperature of the preceding material is set between T1H and T1L *.

FIG. 5 is a diagram showing an embodiment of setting the plate temperature.
FIG. 2 is a diagram showing a method of the present invention, and FIG. In the figure, ti indicates the time at which the connecting portion of the metal strip passes directly through the inlet of the heating furnace, and to indicates the time at which it passes through the outlet of the direct heating furnace. In the figure, T1H and T1L are the upper and lower limits of the allowable temperature of the preceding material, T1S is the set value of the temperature of the preceding material, T2H and T2L are the upper and lower limits of the allowable temperature of the following material, and T2S is the following value. The sheet temperature set values of the material, T1H * and T1L *, respectively indicate the estimated values of the sheet temperature of the preceding material corresponding to the upper and lower limits of the allowable sheet temperature of the succeeding material.

In the embodiment of the present invention, as shown in (a) of the figure, at the time (time ti) when the connecting portion of the metal strip directly passes through the inlet of the heating furnace, the sheet temperature set value T1S of the preceding material is changed. The setting is changed within the area (between T1H * and T1L) where both the allowable temperature range of the preceding material and the allowable temperature range of the preceding material corresponding to the allowable temperature range of the succeeding material overlap. I have. Then, when the connection of the metal strip reaches the outlet of the heating furnace directly (time t
o) The sheet temperature set value has been changed to the target value (T2S) of the sheet temperature of the following material. In this way, the sheet temperature of the preceding material and the sheet temperature of the succeeding material can be simultaneously controlled to be within the allowable range even at the connection portion of the metal strip.

In the prior art, as shown in (b) of the figure, until the connecting portion of the metal strip directly reaches the outlet of the heating furnace (the position where the thermometer is installed) (until time to), The temperature set value (T1S) remains at the target value of the preceding material, and thereafter, the setting is changed to the target value (T2S) of the sheet temperature of the following material. Since the sheet temperature set value T1S is higher than the estimated calculated value T1H * of the sheet temperature of the preceding material corresponding to the upper and lower limits of the allowable range of the sheet temperature of the succeeding material, the sheet temperature of the succeeding material is set at a set temperature exceeding the allowable range. Temperature controlled, overheating is inevitable.

[0042]

According to the present invention, the operating conditions are estimated and calculated by using a heat transfer model formula including three variables of the combustion conditions such as the plate temperature of the metal strip, the amount of injected fuel, and the moving speed of the metal strip as variables. By performing the estimation calculation of the sheet temperature of the metal strip using the same heat transfer model formula, it is possible to set the target sheet temperature within the range of the operating conditions such as the combustion conditions. As a result, at the connection part of the metal band, etc., the plate temperature of both the leading material and the succeeding material,
The target sheet temperature can be set so as to be within the allowable range of each sheet temperature or a value close to the allowable range of each sheet temperature within the limits of the operating conditions.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a control method of the present invention.

FIG. 2 is a configuration diagram of a control system according to the present invention.

FIG. 3 is a flowchart showing a control procedure of the present invention.

FIG. 4 is a flowchart showing a control procedure of the invention.

FIG. 5 is a diagram showing an embodiment of setting a plate temperature.

Claims (2)

(57) [Claims] 1. A metal strip which is continuously heated by a direct heating furnace, when a connection part of the metal strip or a place where the heating condition is changed passes through the furnace, a precedent material preceding the connection part or the changed part. In the sheet temperature control method for controlling the sheet temperature of the succeeding subsequent material using the heat transfer model formula, (a) first, the combustion conditions such as the sheet temperature of the metal strip, the amount of injected fuel, and the movement of the metal strip Using the heat transfer model formula including the three values of speed as variables, the operating conditions other than the combustion conditions remain the same as the operating conditions of the preceding material, and the sheet temperature is the upper limit of the allowable range of the sheet temperature of the succeeding material. Estimate and calculate the combustion conditions for each given the lower limit,
(B) If the estimated combustion condition exceeds the allowable range, give a value within the allowable range to the combustion condition, and then estimate and calculate the moving speed of the metal band using the heat transfer model equation. Based on the result, change the setting value of the moving speed of the metal band,
(C) Next, when the operating conditions are changed to the operating conditions corresponding to the upper limit value or the lower limit value of the sheet temperature of the following material, the sheet temperature of the preceding material is estimated and calculated using the heat transfer model formula. (D) In the temperature range between the two calculated values of the sheet temperature of the preceding material corresponding to the upper or lower limit of the sheet temperature of the succeeding material, the temperature range overlapping the allowable range of the sheet temperature of the preceding material. If there is, change the set value of the target sheet temperature of the preceding material within that temperature range, and (e) change the set temperature of the preceding material sheet temperature for the temperature range between the two estimated calculated values of the preceding material sheet temperature. If there is no temperature range overlapping with the allowable range, the set value of the target sheet temperature of the preceding material is changed to a value between the temperature range between these estimated calculated values and the allowable range of the sheet temperature. Metal strip temperature control method. 2. When the connecting portion of the metal strip or the change point of the heating condition passes a predetermined position on the entrance side of the direct heating furnace, the set value of the target sheet temperature of the preceding material is changed, and the exit side of the direct heating furnace is changed. 2. The metal strip sheet temperature control method according to claim 1, wherein the set value of the target sheet temperature is changed to the target sheet temperature of the following material when passing through the predetermined position.