JPS6317896B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a strip temperature control method for a continuous annealing cooling furnace. More specifically, the present invention relates to a method for controlling strip temperature in a continuous annealing cooling furnace equipped with a roll cooling device and a gas jet cooling device.

Generally, a cold strip (cooled steel sheet) that has been rolled to a predetermined thickness in a cold rolling facility is heat treated in a continuous annealing facility to impart required mechanical properties. Continuous annealing equipment performs heat treatment by passing the strip continuously through the following steps: heating, soaking, slow cooling, cooling, overaging, and final cooling. , soaking furnace,
It consists of a slow cooling furnace, a cooling furnace, an overaging furnace, and a final cooling furnace.

Figure 1 is a diagram showing an example of the heat cycle of a strip heat-treated in a continuous annealing facility;
FIG. 2 is a conceptual diagram showing an example of the overall configuration of continuous annealing equipment. The strip 1 passes at a constant speed through the heating furnace 2, the soaking furnace 3, the slow cooling furnace 4, the cooling furnace 5, the overaging furnace 6, and the final cooling furnace 7 in a meandering manner, as shown in FIG. It is heated and cooled through a heat cycle such as that described above, and is sent out after undergoing the necessary heat treatment.

Among the above-mentioned furnaces, the cooling furnace 5 is required to cool the strip at a high cooling rate of, for example, 50 to 100 DEG C./s, as shown in FIG. This cooling method includes a method using a gas jet that blows an inert gas, a method using water spray or a fog, and a method using a cooling roll with a refrigerant flowing inside. Among these, when using water spray or fog, the strip surface oxidizes when it comes into contact with water, so surface cleaning treatment such as pickling is required, making the equipment complex and large-scale. requires the cost of inert gas as well as the power cost for forming the gas jet. Therefore, in general, as a method of cooling cold strips in continuous annealing equipment cooling furnaces, it is considered advantageous to bring the high-temperature strip into contact with a cooling roll and perform cooling by contact heat transfer.

However, in this roll cooling method, the refrigerant flows in from one axial direction of the hollow metal cooling roll and flows out from the other, so the refrigerant temperature tends to be uneven in the strip width direction, and when water is used as the refrigerant ( Water is usually used due to cost reasons, etc.) If the strip temperature is high, the water may boil, and heat exchange between the strip and the refrigerant will not occur evenly, resulting in a temperature difference in the width direction of the strip. This causes defects in the product's shape. This drawback is exacerbated by uneven contact between the cooling roll and the strip if the original strip plate sent to the continuous annealing equipment has a defective shape or uneven thickness in the width direction, resulting in an extremely defective product. This occurs and cannot be corrected even by cold straightening in the post-process, resulting in a defective product.

In order to solve the above-mentioned problems in the cooling furnace of continuous annealing equipment, the conditions of the cooling roll and refrigerant are set so as to limit the strip temperature drop per cooling roll within the required range. Attempts have been made to provide cooling. For example, Japanese Patent Application Laid-open No. 59-23826 (Japanese Patent Application No. 57-130457)
This is what was disclosed.

However, even in this method, if the strip is cooled in the cooling furnace using only the cooling rolls as described above, it is not possible to sufficiently reduce the amount of strip temperature drop per cooling roll.
Further, if it is attempted to reduce this, the number of cooling rolls must be increased and the refrigerant conditions must be precisely controlled for each roll, resulting in a disadvantage. Therefore, we installed a gas jet device upstream of the roll cooling device that performs cooling using cooling rolls, increasing the overall capacity of the cooling furnace and reducing the amount of strip temperature drop per cooling roll. Cooling methods have been considered.

In other words, the amount of strip temperature drop △T _S ℃ due to the cooling roll in the roll cooling method is calculated by the following formula:
It is given by (1).

△T _S = CK ( _S − _W ) ...(1) where, K: Heat transfer rate between the strip and the refrigerant (Kcal/m ² h℃) _S : Average temperature of the part of the strip in contact with the cooling roll (℃) _W : Average temperature of refrigerant (℃) C: Constant determined by the winding angle of the strip on the roll, roll diameter, strip thickness, and line speed (m ² h℃/Kcal) In this case, Strip temperature drop △T _S
can be taken as the amount of temperature drop per roll,
Also, although the numerical value is obviously different, it can be considered that it is the amount of temperature drop of the entire roll cooling device made up of a plurality of cooling rolls.

As is clear from this equation (1), the lower the strip temperature T _S is, the smaller ΔT _S becomes. Therefore, as mentioned above, it is possible to keep the strip temperature drop within the required range and avoid the drawbacks of the roll cooling method. can.
Therefore, as mentioned above, it is considered advantageous in practice to provide a gas jet device upstream of the roll cooling device to lower the strip temperature to a certain degree before starting roll cooling.

FIG. 3 is a side sectional view showing an example of a cooling furnace conventionally used for carrying out such a combined roll cooling and gas jet cooling method. In this cooling furnace 5, the strip 1 passes through a gas jet cooling device 8 provided immediately after the entrance of the cooling furnace 5, is blown with an inert gas, is cooled to a required temperature, and then enters a roll cooling device 9. In the roll cooling device 9, the strip 1 is wound around and brought into contact with a plurality of cooling rolls 10, exchanges heat with the refrigerant in the cooling rolls 10, is cooled to a required temperature, and is sent out of the cooling furnace 5. . Note that the reference numeral 11 in the figure is a deflector roll that deflects the strip 1 in a desired direction.

FIG. 4 is a sectional view of the cooling roll 10. The cooling roll 10 is made of a metal material and has a hollow shape, and a refrigerant such as water is fed from one shaft end of the roll as shown by the arrow in the figure, and the other shaft It is sent out from the end and circulates inside the cooling roll 10. multiple cooling rolls 10
The strips 1 are arranged in a staggered manner as shown in the figure, and the strips 1 are alternately wound around the outer periphery of each strip at a required angle and come into contact with each other, thereby exchanging heat with the refrigerant and cooling them.
Each cooling roll 10 is provided with a drive device 12 that moves it in the horizontal direction, and the operation of this drive device 12 moves the cooling roll 10 forward and backward to adjust the winding angle at which the strip 1 contacts the cooling roll 10. will be held. Note that both shaft ends of the cooling roll 10 are respectively connected to a refrigerant supply pipe and a refrigerant delivery pipe via rotary joints (not shown).

On the other hand, in the gas jet cooling device 8, the cooling gas (preferably using an inert gas such as nitrogen or a reducing gas) pressurized by the blower 13 is passed through plenum chambers 14 provided on both sides of the strip 1 passage. The water enters the plenum chamber 14 and is sprayed onto both sides of the strip 1 from a number of nozzles 15 provided in the portion facing the strip 1 to cool the strip. The amount of gas jet cooling is adjusted by controlling the opening degree of the damper 18 using a plenum chamber pressure detector 16 and a pressure regulator 17. Temperature detectors 19 provided at the inlet of the cooling furnace 5 and the inlet and outlet of the roll cooling device 9 detect the strip temperature at each location and issue a control signal to control the amount of gas jet cooling and the cooling roll 10. cooling furnace 5 by controlling the winding angle etc.
This is to manage the cooling conditions. By controlling the amount of jet cooling and the amount of roll cooling in this manner, it is possible to provide the optimum heat cycle to strips of various steel types and plate sizes. That is, if the plate thickness changes between the leading strip and the trailing strip, control can be performed using these control means.

By the way, since the strip in continuous annealing equipment is normally threaded at a high speed of about 200 to 400 m/min, the time in the cooling furnace 5 is only about a few seconds. On the other hand, the response to control of the winding angle of the cooling roll is extremely slow due to its mechanism and the heat capacity of the roll shell, and for example, it takes 120 seconds or more to change from 60° to 120°. This means that the roll outer surface temperature cannot quickly respond to changes in conditions. On the other hand, the response of the gas jet pressure is considerably more sensitive than this, and a normal change takes about 10 seconds. Therefore, in the heat treatment of steel types where the plate thickness is relatively thick and the strip temperature drop △T _S due to roll cooling is not a particular problem statically, the roll winding angle is fixed and the gas jet pressure is It is more advantageous to perform control only by using only one method, and a good yield can be obtained. However, when the thickness of the trailing strip becomes thinner than that of the leading strip, the strip temperature cannot be immediately responded to due to the delay in the response of the cooling roll as described above, and the strip temperature decreases temporarily as described below. This causes T _S to become large, leading to poor strip shape.

Figure 5 is an example of a diagram showing appropriate conditions for the plate thickness calculated in advance for the operation of a cooling furnace equipped with a roll cooling device and a gas jet cooling device as described above. Gas jet preset curve, Figure b shows the relationship between the plate thickness and the strip temperature _TSG at the inlet of the roll cooling device, Figure c shows the relationship between the plate thickness and the temperature _R of the outer surface of the cooling roll in contact with the strip. It shows. Also,
Figure d shows the plate thickness and the strip temperature drop △T _S
In relation to this, the broken line in the figure indicates the maximum allowable limit value that does not cause defects in the shape of the strip.

Now, FIG. 6 shows that the conventional operation control method as described above is performed based on the preset curve of FIG.
FIG. 3 is a diagram showing changes in cooling furnace conditions when the strip thickness changes from 1.0 mm to 0.8 mm. In this case, immediately after the welding point between the leading strip (thickness 1.0 mm) and the trailing strip (thickness 0.8 mm) passes through the cooling furnace, the gas jet pressure setting value is changed from P ₁ to P ₂ based on Figure 5a. It is something that changes.

According to the bottom curve in the figure, the strip temperature drop △T _S due to roll cooling temporarily exceeds the allowable value of the strip shape stability limit (indicated by the broken line), which may lead to deterioration of the strip shape. It has been shown that there is. This is because, as mentioned above, the strip temperature T _SG at the inlet of the roll cooling device changes quickly in response to the quick response of the gas jet pressure, whereas the change in the roll outer surface temperature _R shows that the response of the cooling roll is slow. This is due to the large delay. In this case, due to the heat capacity of the roll shell, the strip temperature drop △
It is appropriate for T _S to be transiently expressed by the following equation (2) using the average temperature _R of the outer surface of the cooling roll instead of the average temperature _W of the refrigerant in the above equation (1).

△T _S = CK' ( _S - _R ) ...(2) Here, K': Reciprocal of thermal resistance between the strip and the roll surface (Kcal/m ² h°C) This method was developed to solve the problem that occurs when the thickness of the trailing strip becomes thinner than the thickness of the preceding strip in the operation control method of a cooling furnace. The present invention provides a method for controlling a strip temperature in a cooling furnace for continuous annealing equipment using a method for changing a gas jet pressure set value without exceeding an allowable value.

That is, the method of the present invention includes a roll cooling device that cools the strip by bringing the strip into contact with the outer peripheral surface of a cooling roll through which a refrigerant is circulated, and a nozzle that cools both sides of the strip at a stage before the roll cooling device. When cooling the strip in a continuous annealing equipment cooling furnace equipped with a gas jet cooling device that cools the strip by blowing gas, if the thickness of the trailing strip becomes thinner than that of the leading strip, the leading strip At the same time as the welding point between the strip and the trailing strip passes the exit of the cooling furnace, first set the gas jet cooling amount (corresponding to the gas jet pressure) so that the strip temperature at the gas jet cooling device exit (roll cooling device inlet) is constant. The above objective is achieved by lowering the set value of the gas jet cooling amount to a calculated predetermined value over a predetermined time so that the strip temperature at the outlet of the cooling furnace becomes the predetermined value. This is what made it possible.

In the present invention, when changing the thickness of the strip as described above, at the same time that the welding point between the leading strip and the trailing strip passes through the outlet of the cooling furnace 5, the gas jet pressure setting value is changed to the previous value of _P1.
to P′, which has a smaller amount of change than P ₂ ,
Thereafter, it is changed linearly or curved from P' to _P2 , which is an appropriate target value for the trailing strip, over a predetermined period of time. (See Figure 8 below.) Here, the timing at which the gas jet pressure setting value is changed from _P1 to P' is set earlier than the time when the welding point passes the outlet of the cooling furnace 5 (for example, according to Japanese Patent Application Laid-Open No. 56-77340). As disclosed in Japanese Patent Application No. 54-155384), as mentioned above, the strip temperature _TSG at the inlet of the roll cooling device 9 rises too quickly and the response of the cooling roll 10 cannot follow it. This is not preferable because the strip temperature drop ΔT _S due to roll cooling becomes large. Furthermore, if this is delayed, the period during which the strip 1 is supercooled becomes longer, leading to a deterioration in yield due to poor strip material. Therefore, in the present invention, the timing for changing from P ₁ to P ₂ is set to be the same time as the welding point passes the outlet of the cooling furnace 5.

The above intermediate pressure setting value P' is set as follows so as not to lower the average roll outer surface temperature _R , for example, so that the strip temperature _TSG at the inlet of the roll cooling device 9 is equal to that during the preceding strip passing. Find it by method. That is, when the _TSG of the leading strip and the trailing strip are equal, the following equation (3) holds statically.

d ₂ / d ₁ = α ₂ / α ₁ ...(3) where, d ₁ : Thickness of leading strip (mm) d ₂ : Thickness of trailing strip (mm) α ₁ : Gas jet in leading strip Heat transfer coefficient (Kcal/m ² h°C) α ₂ : Gas jet heat transfer coefficient in trailing strip (Kcal/m ² h°C) By the way, the relationship between gas jet pressure and gas jet heat transfer coefficient is usually expressed by the following equation (4). It is expressed as

P=aα ^b +c ……(4) Here, α: Gas jet heat transfer coefficient (Kcal/
m ² h°C) P: gas jet pressure (mmH ₂ O) a, b, c: constants Therefore, from the above equations (3) and (4), P' can be calculated using the following equation (5).

P' = (d ₂ / d ₁ ) ^b (P ₁ - c) + c ... (5) Also, the time T _P required to change from the intermediate pressure set value P' to the target pressure set value P ₂ , the temperature difference ( _S − _R ) is not large (i.e. △
The evaluation criteria are that T _S ) is below the allowable value) and that the specified heat cycle is reached as quickly as possible.
The relationship with the amount of plate thickness change may be determined in advance by organizing experimental data or theoretical analysis.

FIG. 7 is a side sectional view showing one embodiment of a cooling furnace for applying the method of the present invention, which corresponds to FIG. The gas jet pressure is set as described above by inputting the signals from the welding point detector 20 and line speed detector 21 (both shown in Figure 2) installed at the A computing unit 22 is provided which computes the value P ₀ and supplies it to the gas jet pressure regulator 17.

FIG. 8 is a graph showing the results of changing the gas jet pressure set value (and therefore the gas jet cooling amount set value) according to the method of the present invention. From this figure, it can be seen that according to the method of the present invention, the strip temperature drop ΔT _S due to roll cooling does not exceed the allowable value for strip shape stability.

As is clear from the above explanation, according to the method of the present invention, when the trailing strip becomes thinner than the leading strip, the strip temperature drop due to roll cooling temporarily exceeds the allowable value, resulting in poor strip shape, and even worse. It is possible to operate the continuous annealing equipment stably without causing problems such as breakage.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of a heat cycle in continuous annealing equipment, Fig. 2 is a conceptual diagram showing an example of the overall configuration of continuous annealing equipment, and Fig. 3 is a side sectional view showing an example of a conventional cooling furnace. Fig. 4 is a sectional view showing an example of a cooling roll, Fig. 5 is a diagram showing an example of appropriate conditions for plate thickness in a cooling furnace, and Fig. 6 is an example of response when plate thickness changes using the conventional method. FIG. 7 is a side cross-sectional view showing an embodiment of a cooling furnace to which the method of the present invention is applied, and FIG. 8 is a diagram showing an example of the response when the plate thickness changes according to the method of the present invention. In the drawing, 1 is a strip, 5 is a cooling furnace, 8 is a gas jet cooling device, 9 is a roll cooling device, 10
is a cooling roll.

Claims (1)

[Claims] 1. A roll cooling device that cools the strip by bringing the strip into contact with the outer circumferential surface of a cooling roll through which a refrigerant is circulated, and a device that cools the strip by spraying cooling gas from a nozzle onto both sides of the strip at a stage before the roll cooling device. In a strip temperature control method in a continuous annealing cooling furnace equipped with a gas jet cooling device, when the thickness of the trailing strip is thinner than that of the leading strip, the welding point between the leading strip and the trailing strip is cooled. At the same time as passing through the outlet of the furnace, the gas jet cooling amount set value is first lowered to a predetermined value calculated so that the strip temperature at the outlet of the gas jet cooling device becomes constant, and then the gas jet cooling amount set value is lowered to the strip temperature at the outlet of the cooling furnace. 1. A strip temperature control method for a cooling furnace for continuous annealing equipment, characterized in that the strip temperature is lowered over a predetermined period of time to a target value calculated so that the strip temperature reaches a predetermined temperature.