JPS60135530A - Continuous annealing method of steel strip - Google Patents

Abstract

PURPOSE:To prevent decrease in thermal efficiency for heating owing to indirect heating in the stage of annealing continuously a cold rolled steel strip by indirect heating using radiant tubes by making use of the heat retained by the waste combustion gas in the radiant tubes in preheating the steel strip. CONSTITUTION:A cold rolled steel strip 1 is first passed through the inside of a preheating furnace 12 and after the strip is preheated by the high-temp. air blown from a plenum chamber 18, the strip is passed through the inside of an indirect heating furnace 2 in which a reducing atmosphere is maintained. The strip is thus subjected to bright annealing by the radiant heat from many radiant tubes. The waste combustion gas in the tubes 6 is collected in a duct 23 and is introduced into a heat exchanger 7 where the gas heats the atmosphere gas in the furnace 12. The heated gas is fed to the plenum chamber 18 of the furnace 12. The waste combustion gas of the tubes 6 cooled by the heat exchanger 7 is fed by a blower 8 into a chimney 9. The decrease in the thermal efficiency of heating by the furnace 2 is prevented by utilizing the waste heat in the furnace 12.

Description

[Detailed Description of the Invention] Technical Field Continuous furnace annealing equipment, especially when at least a preheating furnace is provided before an indirect heating furnace, is used to evaluate the amount of heat retained in the combustion waste gas in the radiant tube used in the heating furnace. Effective use is discussed in this specification.

The technical content relates to a method for continuous furnace annealing of copper strips that can more advantageously achieve the necessary temperature increase in a shorter heating furnace without deteriorating the surface properties of the copper strips, and is based on the above-mentioned continuous furnace annealing equipment. It is located in a technology field that is celebrated.

Background Technology Traditionally, indirect heating furnaces using radiant tubes have been widely used as a heating method for continuous furnace annealing of cold rolled steel strips. ' In this method, a large number of radiant tubes are arranged in a heating furnace, and the tubes are heated by radiation from the surface of the radiant tubes based on combustion of fuel inside the tubes.

Since this indirect heating furnace is filled with a reducing atmosphere, it has the characteristic that the -band is heated in a non-oxidized state. The temperature of the furnace should be set to 900℃.
Since the limit is to raise the temperature to about
There were 0 flaws.

Several proposals have been made in recent years to avoid the above-mentioned drawbacks of this indirect heating system.

In other words, first, by adopting a direct heating method as opposed to an indirect method, or by using a combination of a direct heating method and an indirect heating method, thermal efficiency can be improved, the continuous furnace annealing line can be shortened and compact, and the The purpose is to improve the plate temperature control ml f4.

However, the direct flame heating method is a method in which the copper strip is rapidly heated by bringing a flame into direct contact with the copper strip. It has already been used in continuous hot-dip galvanizing lines, etc., and in recent years, its adoption as a heating method in continuous annealing furnaces for cold-rolled steel strips has come under consideration. 5 also essentially includes the following problems.

In other words, with the indirect heating method, the copper strip is heated in a reducing atmosphere, so the surface of the steel sheet is annealed while maintaining its bright state, whereas with the direct heating method, the surface of the steel sheet after annealing is The properties deteriorate, and when < 10, chemical conversion treatment performed as a pre-treatment for painting, etc., deteriorates significantly.

The reason why the surface properties of the steel sheet deteriorate with this direct flame heating method is that the formation of a thick oxide film is unavoidable, and even if the sheet is then passed through a furnace in a reducing atmosphere to reduce The film is not completely removed and becomes uneven, resulting in an uneven film during pre-painting treatments such as chemical conversion treatment. In addition, since the combustion waste gas is in direct contact with the steel plate, S compounds contained in the combustion waste gas,
It has become clear that trace amounts of impurities such as 0 chemical compounds (4) adhere to the surface of steel sheets, and that this also causes uneven film formation during pre-painting treatment. Ta.

Contents of the basic experiment First, we clarified the factors that cause the surface quality of the copper strip to deteriorate when direct flame heating is used as the heating method for the strip. According to this, direct-fired furnace output (1
J! In addition to the effects of I steel strip temperature and the 1° air ratio in direct-fired furnaces, we have found that impurities, especially S compounds, contained in the combustion waste gas of direct-fired furnaces have a large impact on surface properties.

Generally, if the outlet temperature of a direct-fired furnace is too high, or if the air ratio exceeds 1.0 and combustion is performed on the excess air side, the surface of the steel sheet will be severely oxidized, and the oxide film formed on the surface of the steel sheet will be It is already well known that even if reduction is performed in the radiant tube heating furnace that followed the direct-fired furnace, the oxide film layer is not sufficiently removed, resulting in a significant deterioration of chemical conversion properties. However, the inventors have clarified that in addition to these factors, impurities, particularly S compounds, are present in the combustion waste gas of a direct-fired furnace, and when the concentration thereof is high, the chemical conversion properties deteriorate.

Figure 1 shows the S concentration in the combustion waste gas of a direct-fired furnace (S to H2).
An example of the results of chemical conversion treatment tests with various conversion (1) 1) m )) and steel strip temperature on the exit side of the direct-fired furnace is shown below.

In this test, the temperature of the steel strip on the exit side of the direct-fired furnace was up to 600°C, the air ratio was about 0.9, and in both cases, after heating in the direct-fired furnace, H, 7%, N, 98 %),
Reduction was performed at 700°C or higher for 80 seconds.

When a copper strip is heated to 500°C or higher in a direct-fired furnace, in any case, as is known in the art, an oxide film is formed violently, and even if it is reduced, the chemical conversion properties deteriorate ( ×mark)
. In addition, the S concentration in the combustion waste gas of the direct-fired furnace is 1100p.
At temperatures above p, the chemical conversion properties deteriorate (triangle, x marks) even if the temperature of the area on the exit side of the direct-fired furnace is lower than 500°C.

This is probably because the S concentration in the combustion exhaust gas is 100'! “p
If the temperature exceeds pm, S compounds contained in the combustion waste gas will adhere to the copper band intensely during temperature rise in the direct-fired furnace, and even after reduction, this adhering 8 minutes will prevent the formation of crystal nuclei during chemical conversion treatment. It has become clear that the chemical conversion treatment property is deteriorated. 5 In general, the fuel gas used in heating furnaces in steel plants is mainly composed of so-called coke gas produced in coke ovens, etc., and of course the effect on the surface properties of steel sheets in direct-fired furnaces is taken into consideration. However, in practice, the S concentration in the combustion gas fluctuates depending on the state of the desulfurization treatment, and even when the S concentration is low, Since it is unavoidable that the 8 minute particles adhere to the surface of the steel plate, there is a possibility that the surface treatment properties will be deteriorated.

Purpose of the Invention Based on the above-mentioned investigation into the cause of the deterioration of the surface quality of copper strips due to the direct heating method, the amount of heat retained in the combustion waste gas in the indirect heating furnace 20 (7) using radiant tubes has been investigated. It is an object of the present invention to propose a continuous annealing method for steel strip based on a novel heating method that can achieve effective sheet temperature control by making effective use of.

Structure of the Invention The above objects are advantageously achieved by a procedure consisting of the following points.

Combustion waste gas from a large number of radiant tubes installed in an indirect heating furnace is individually introduced into the heat exchanger together with the atmospheric gas in the preheating furnace installed at the front of the heating furnace (see step 1 above). The heat exchange between the heat exchanger cools the waste gas to below the heat-resistant temperature of the waste gas suction proir and the waste gas flue before dissipating it, while the high temperature atmospheric gas heated by the heat exchanger is transferred to the heating furnace). A continuous annealing method for a copper strip, which comprises blowing onto a steel strip passing through the furnace in a preheating furnace installed at the front, and subjecting the thus preheated copper strip to indirect heating by the radiant tube.

As an embodiment of the present invention, the amount of high-temperature gas blown onto the steel strip in a preheating furnace is determined based on the thickness of the copper strip, the width of the copper strip, the one-pass speed, the gas temperature blown onto the steel strip, and the temperature of the heating furnace. To control the plate temperature on the outlet side of the preheating furnace by adjusting it according to the target plate temperature on the outlet side and the plate temperature on the outlet side of the heating furnace, and to adjust the plate temperature on the outlet side of the heating furnace to the target plate temperature, and to maintain the required high temperature in the preheating furnace. A method of dissipating an excess amount of high-temperature atmospheric gas from a heat exchanger to the atmosphere by bypassing a preheating furnace with respect to the amount of atmospheric gas, and further passing the excessive amount of high-temperature atmospheric gas through a second heat exchanger. Heat recovery should be carried out, and after the copper strip passing through the preheating furnace is connected to a steel plate that has a different plate thickness, plate width, or target plate temperature on the exit side of the heating furnace, During unsteady heating until the steel strip passes through the heating furnace, the copper strip passing speed is determined according to the temperature of the high-temperature atmospheric gas that is blown onto the steel strip in the preheating furnace, and the target plate temperature on the exit side of the heating furnace in both leading and trailing zones. It is also possible to perform control to keep the deviation from the target plate temperature on the outlet side of the heating furnace in both zones, which occurs transiently during this period, within the common range of the upper limit value and lower limit value of the plate temperature on the heating outlet side in both zones. More suitable.

Regardless of the direct heating method or the indirect heating method using radiant tubes, the treatment of high temperature combustion waste gas 1 and the recovery of waste heat are also major issues.

This is because if you try to dispose of high-temperature combustion waste gas as it is, you will not only lose a huge amount of heat, but also use a suction fan if it is necessary to suck the waste gas into the flue, chimney, and back space. etc. are exposed to high temperatures,
This is because all of the equipment needs to be expensive equipment with high heat resistance.

Conventionally, the typical 1O method for processing this combustion waste gas is to cool the gas through a heat exchanger, use water as the heat exchange medium, and raise the temperature of this water to recover the waste heat. However, in this method, the combustion waste gas is cooled in a heat exchanger through which water is passed, so the moisture in the gas condenses, and the S compounds in the 1" exhaust gas condense into this moisture. There was a problem in that it melted and caused corrosion of the pipes, impairing the durability of the heat exchanger.

In addition, as another method of utilizing waste gas, a preheating furnace is installed in front of the heating furnace, and a 2" gas of combustion waste gas is introduced into this preheating furnace to cool the steel strip before it is passed through the heating furnace. Attempts have also been made to heat the copper strip by 1,000 yen, but as mentioned above, in the method of directly exposing the steel strip to the combustion waste gas, impurities, typically 80% in the combustion waste gas, may cause surface treatment of the copper strip. A continuous annealing furnace for cold-rolled steel strip 5 is not suitable because there is a risk of deterioration in properties.

In this invention, the problem of the surface properties of the steel sheet and the problem of waste heat recovery can be solved all at once, and furthermore, the controllability of the sheet temperature can be advantageously completed. Note that the atmospheric gas in the preheating furnace used as a heat medium may be other than air, such as nitrogen or a mixture of nitrogen and hydrogen, and the case of air will be described below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 illustrates a continuous 1" annealing line suitable for application of the method of the present invention.

2 is a copper strip, 2 is an indirect heating furnace, 8 is a hearth roll, 4 is an in-furnace thermometer, 5 is a plate thermometer on the outlet side of the heating furnace, 6 is a radiant tube, and 7 is a heat exchanger, and 8 is a suction blower, 9 is a chimney, '.

10 is a waste gas pressure gauge, 11 is a waste gas temperature gauge,
12 is a preheating furnace.

This preheating furnace 12 is provided with a plenum chamber 18 in which a hot air circulation pipe 18 and a hot air circulation fan 14 are connected via a fan inlet bypass valve 15, a fan outlet bypass valve 16, and a hot air regulating valve 17X. In the figure, 19 is a hot air thermometer, 20 is a Blenheim pressure gauge, 21 is a preheating furnace exit plate temperature gauge, 22 is a waste gas flue, and 28 is a waste gas duct.

Radiant tubes 6 to 1 in the indirect heating furnace 2 ([The high-temperature waste gas discharged is introduced into the heat exchanger 7 through the waste gas duct 28 for heat exchange, and is then transferred to the waste gas flue 22 and suction blower. I12 after cooling to below the heat resistant temperature of 8.
It passes through a pulling fan 8 and is disposed of through a chimney 9.

In the heat exchanger 7, the air in the preheating furnace 12 is introduced into the heat exchanger 7 as a refrigerant to cool the high-temperature waste gas, and after being wetted by heat exchange with the high-temperature waste gas, a hot air circulation fan is used. 14 and sent to a plenum chamber 211 (12) 18 installed in the preheating furnace 12. From this plenum chamber 8, high temperature air is applied to the steel strip 1 passing through the preheating furnace 12. Spray on it and use it to preheat it.

The steel strip 1 is fed into the indirect heating furnace 2 after being preheated as described above in the preheating furnace 12, and further heated until the thickness δ is 0.
8 translation
The sheet temperature at the exit side of the heating furnace 2 was 75 at a sheet passing speed of 0 m/min.
If 0°C is required, radiant tube 6 in heating furnace 2
The gas burned (about 2800 Kcal/Mm8) is about 21
o o Nm8/Hr+), while the calorific value of the waste gas from the heating furnace 2 is about 5X 10 K calzIr, at about 600°C, the waste gas i is about 80000 Nm/[(r).

When this waste gas is introduced into the heat exchanger 7 and heat exchanged with the air circulating in the preheating furnace 12, the waste gas reaches a temperature of about 500°C after II heat exchange, and then passes through the flue 22 and is dissipated from the chimney 9. It was discharged.

On the other hand, the amount of air circulating in the preheating furnace 12 is approximately 85,000.
Nm/)Ir, and by heat exchange, approximately 2.5X 1
06K cal/Hr heat exchanger 7
The air temperature on the inlet side of 20 is about 800°C, and the air temperature on the outlet side is about 400°C.
The steel strip was heated to approximately 2'00°C and was blown onto the surface of the steel sheet from the plenum chamber 18 of the preheating furnace 12, thus heating the steel strip to approximately 2'00°C.

By installing this preheating furnace 12, approximately aooo ”K c
It has become possible to save fuel for al/T.

Here, in the preheating furnace 12, the amount of high-temperature air blown onto the copper strip can be controlled by measuring the pressure in the plenum chamber 18 with a plenum pressure gauge 20.

In addition, when the temperature of the copper strip on the exit side of the preheating furnace is measured by the exit plate thermometer 21 of the preheating furnace 12, and control is performed to reduce the amount of air blown onto the steel strip in the preheating furnace 12,
This can be easily achieved by controlling the hot air regulating valve 17 in the closing direction and controlling the fan inlet side bypass valve 15 and the fan outlet 15 side bypass valve 16 in the open direction in conjunction with each other.

By adopting this method, even if the amount of air blown onto the steel strip in the preheating furnace 12 is reduced, the air scum discharged from the radiant tube heating furnace 2, the suction fans 8 and 1 in the heat exchanger 7 It is possible to constantly control the temperature to be below the heat-resistant temperature of the flue 22.

Further, the suction fan 8 is controlled based on the pressure and temperature measured by the exhaust gas pressure gauge 10 and exhaust gas thermometer 11 of the heat exchanger 7.

In this way, it is possible to effectively utilize the retained heat of the high-temperature waste gas generated in the radiant tube in the indirect heating furnace 2, and to preheat the 11114 zone 1 without deteriorating its surface quality. That is, because the combustion waste gas does not come into direct contact with the surface of the copper strip due to heat exchange with the above-mentioned air, it is possible to always maintain good surface quality of the steel sheet.

Furthermore, this method has a particularly fast response to the heating of the steel strip in the preheating furnace 12, and focusing on this, we have used this method as a factor to constantly control the strip temperature of the strip on the exit side of the heating furnace 1'' to the target temperature. Can be used.

That is, by controlling the amount of hot air introduced into the preheating furnace 12 in FIG. 2, the temperature of the outlet plate of the preheating furnace 12 can be controlled.

Of course, the indirect heating furnace 2 also has a furnace temperature control that allows you to change the furnace temperature. A liJ system is provided, and a speed control system capable of changing the passing speed of the steel strip 1 is also provided.

Conventionally, for example, when a direct fire heating method and a radiant tube heating method are used together, the method of controlling the plate temperature is to measure the plate temperature at the exit side of the heating furnace using a radiant tube, and depending on the deviation from the target temperature, the plate temperature is controlled by the radiant tube. Feedback control is performed by a combination of internal temperature control of the heating furnace and fuel flow rate control of the heating furnace using direct fire.

However, since this method uses feedback control, there is a delay in control, and the plate temperature cannot be controlled accurately. Since the settling time of step (5) is very long, there are drawbacks such as the plate temperature at the exit side of the radiant tube heating furnace deviating from the target temperature for a long time.

On the other hand, FIG. 8 shows a specific control example of a control system adapted to the present invention. '.

The preheating furnace 12 is equipped with a plate temperature control device 25 that can change the plate temperature on the exit side of the preheating furnace.
The amount of air blown onto the steel strip can be changed by adjusting the opening of the hot air control valve 17 and the bypass valves 15 and 16, so that the plate temperature at the exit side of the preheating furnace can be changed.

The valve opening degree is controlled by feedback of the pressure signal of the plenum chamber 18 measured by the blennium pressure gauge 20, and the set value of this plenum pressure is directly set from the main calculator 28 via the plate temperature control device 10. In some cases, the main calculator 28 sets the target plate temperature at the inlet side of the preheating furnace to the plate temperature control device 25, and in other cases, it is set by feedback from the output signal of the plate thermometer 21 on the outlet side of the preheating furnace.

A furnace temperature control device 26 is installed in the heating furnace 2, and the furnace temperature is controlled by the furnace temperature set value from the main calculator 28, and the furnace temperature is controlled by the furnace exit side target plate temperature from the main calculator 28. Device 2
6, and may be controlled by feedback from the output signal of the plate thermometer 5 on the outlet side of the heating furnace.

In addition, stepped points are used to track and calculate points where one or more of the steel strip thickness, width, or target plate temperature on the exit side of the heating furnace differs (hereinafter referred to as service points). position calculator 29
The passage information of the stepped points is sent to the main n unit 28.

The response of the furnace temperature in the heating furnace 2 is very slow, and the response of control of the outlet plate temperature in the preheating furnace 12 is fast. By controlling the preheating furnace using equation (1), it was possible to stabilize the temperature of the plate on the outlet side of the heating furnace.

T8P=f (D, W, TsH, TA,
TH, V) - (11 where, D: Thickness W of the plate passing through the preheating furnace: Width TSH' Target plate temperature on exit side of the heating furnace TA: Hot air temperature (actual) TH: Furnace temperature of the heating furnace (〃) ■: Passage speed (〃)7υ T8. Target plate temperature on exit side of second preheating furnace (11 formula) D, W, T0n, TA, 'rH
The data are collected in the main calculator 28, and the target plate temperature TSP on the exit side of the preheating furnace is calculated in the main calculator 28 according to equation (1).
is calculated and set in the plate temperature control device θ25.

The plate temperature control device 25 controls the plate temperature so that the target plate temperature TSP is reached.
Controls the wind blowing to the M band in the preheating furnace. Alternatively, the main calculator 28 can calculate and set the preheating furnace plenum chamber pressure (PA) according to the following equation (2): l1lPA=g (D,
W, TsH, TA, TH, V) - (Z) This method controls the preheating furnace to stabilize the temperature of the plate on the exit side of the heating furnace depending on operating conditions such as heating furnace In THs hot air temperature TAS passing speed V By controlling the preheating furnace with quick response, it is possible to always stabilize the plate temperature at the exit side of the heating furnace.

FIG. 4 shows a comparison between a conventional method (radiant tube heating furnace) and a preheating furnace controlled by the method of the present invention.

(19) For example, if the passing speed v0 is V during one period from point A to point B,
, then in the conventional method, between A and B,
The plate temperature on the outlet side of the heating furnace was supposed to rise from TSHI to TSH2 as shown by the broken line, but according to the method of this invention, when ■□ decreases to -v2, the change can be expressed by equation (1). Alternatively, according to equation (2), the plate temperature on the outlet side of the heating furnace is reduced from TSPI to TSP2 by reflecting the operation of the preheating furnace 12, and as a result, the plate temperature on the outlet side of the heating furnace remains at TSHI and is stable as shown by the solid line. .

In addition, even if the furnace temperature rises from THI to TH2, the effect remains unchanged and TSHI increases according to the conventional method.
However, according to the method of the present invention, by reducing the temperature of the plate at the exit side of the preheating furnace from TSPI to 'rspa by 15, the temperature at the outlet side of the heating furnace can be reduced by 15%. It is possible to always keep it constant as shown by the solid line in the figure.

In Figure 4, D ” L, 8 fights, ■□ = 800
m/min.

'I'H1=880℃, T8P1=200℃, 'I'8
1 (1=750'C17), Vs "" 25
0 vmln and z0(20), the conventional method results in TSH2 = 780°C, but with the method of this invention, TSP2 = 40°C.
If so, it can be operated as TSHI.

On the other hand, even if TH2 = 845°C, the conventional method would result in TSH8 = 770°C, but by the method of the invention, TSP8 = 100°C.
It can be operated as Hl.

Next, we will discuss 10 unsteady times when steel strips that differ in one or more of the plate thickness, plate width, or target plate temperature at the exit side of the heating furnace are connected, and the stepped point passes through the furnace. The preheating furnace 12 can be controlled as follows.

In Fig. 5, plate thickness, passing speed v1, heating furnace furnace temperature TH1, preheating furnace exit side plate temperature TSP1, heating furnace exit side plate temperature TS
From a stable state at H0, plate thickness DgS through 15 overspeed v8, heating furnace furnace 'tnTH2, preheating furnace outlet side plate temperature 'I'S
The control when the P2 heating furnace exit side plate TSH0 shifts to a stable state will be explained by taking as an example.

First, t□ minutes before the plate thickness change point reaches the entrance side of the preheating furnace, the heating furnace furnace temperature TH1 is started to be changed by 2O toward the next set value 'I'H2. At the same time, the heating furnace outlet plate temperature is the target plate 1rMT.
The plate temperature on the outlet side of the preheating furnace is sequentially controlled according to equation (1) or equation (2) so that sHo is achieved.

From the point H when the plate temperature on the outlet side of the preheating furnace becomes the minimum until the plate of plate thickness D2 enters the preheating furnace, the following formula (3) is used to prevent the plate temperature on the outlet side of the heating furnace 5 from increasing. , to control the passing speed.

V = h, (D, W, TSHO, TH, TSHU, T
SHL) - (81 In this case, the equation is when the preheating furnace is completely closed, and D, W, TSHO, TH, TSHU, T
SHL is the value of the plate with a thickness of 10D1, (8)
The reason why TSHU and TSHL are included in the formula is that depending on these values, it is possible to adopt a simple operation method of operating as before without changing ■. .

Subsequently, when the plate with the plate thickness D2 enters the preheating furnace, the 1" preheating furnace is fully opened, and the plate temperature on the exit side of the preheating furnace is increased from 1 to J. At the same time, the following equation (4) is used instead of equation (8). ), the passing speed V is adjusted so that the plate temperature on the exit side of the heating furnace reaches the target plate temperature TSHO.
is calculated and set, and then, according to equation (3), the calculation of the passage speed (2) is repeated and set until the heating furnace furnace temperature TH rises and reaches the target furnace temperature 2゛TH2.

V=h, (D, W, TSHO, TA, TH, TSHU
, TSHL )...(4) In the case shown in Figure 5,
After the plate of plate thickness D2 is inserted, the preheating furnace is controlled to be fully open, and assuming that the wind h1 blowing on the S copper strip in the preheating furnace is at its maximum (calculating ■ using formula 81). Become.

In the conventional method, as shown by the broken line in the diagram of the plate temperature at the exit side of the heating furnace, there is no control in the preheating furnace, so even if the passing speed is controlled, the target l is not reached before and after the plate thickness change point.

There were many cases where the deviation from the plate temperature was very large and the plate temperature did not fall within the upper and lower limits.

For example, the change in plate thickness is 80%, that is, Dg-1,8X
In the case of D□, assuming that 710 degrees of heat is generated at a passing speed of 400 m/min, in the conventional method, the deviation of the plate temperature on the exit side of the heating furnace before and after the plate 1゛ thickness change point (the temperature difference between Q and R in Figure 5) is 60 degrees.
C to 80 C, which may exceed the upper and lower limits, but by controlling the preheating furnace according to the present invention, this deviation can be almost eliminated and ideal control can be achieved. 2O (28) Figure 5 shows an example of control when the plate thickness becomes thicker.
The control method is the same when the plate thickness becomes thinner or when the target plate temperature on the outlet side of the heating furnace is changed, and the plate temperature on the outlet side of the preheating furnace is controlled to maintain the plate temperature on the outlet side of the heating furnace at a constant target value. If possible, perform (1) control of the preheating furnace using equation 5 or (2) to fully open or close the preheating furnace, and maintain the outlet plate temperature of the heating furnace at a constant target value. If this becomes impossible, the exit side of the heating furnace 1G can be kept constant by calculating and setting the passing speed (■) according to equation (8) or (4).

In FIG. 5, D, = 0.8, D, = 1.
0 visits, Vl ”8 0 0 m/min, vg = 2
5 0 m/m1ns TH1=880℃, TH2=8
45°C, TSP1= 200'C1'['5P2=
In the case of 200°C, TSHO = 750°C, and T0 = 8 minutes 15, under the conventional control method that does not control the preheating furnace, the Q point of the plate temperature on the exit side of the heating furnace rises to 770°C, and the plate thickness increases to l, Qsm. The temperature at point R, which has changed to , drops to 780°C. In addition, in this invention, the fan outlet side bypass valve 16 and the fan 20 (24) inlet side bypass valve 15 in FIG. When reducing the amount of air blown, the method shown in FIG. 6 can be used to recover heat from the hot air discharged from the fan outlet side bypass valve 16.

In the figure, 80 is a cleaning scrubber tank, 31 is a cleaning dip tank, 82 is a hot water heat exchanger, 88 is a recirculation tank, 84 is a pump, 85 is a spray pipe, 36 is a hot air pipe, 87 is a hot air discharge pipe, 38
Heat exchange side hot water piping, 1089 heat exchange side hot water piping, 4
0 is the level, 41 is the hot water control valve, 42 is the temperature gauge,
43 is an airless control valve, 44 is air piping, 45 is wet water circulation piping, and 46 is hot water return piping.

The hot air flows into the water heat exchanger 82, and then the hot air exhaust pipe 87
It is discharged through the

On the other hand, the waste heat of the hot air is recovered by increasing the temperature of the water. (The hot water produced by heat exchange can be used, for example, in a cleaning line that is usually installed at the entrance of a continuous annealing furnace.

FIG. 6 shows an example of use in a cleaning line. The steel strip 1 passes through a cleaning scrubber tank 80 installed at 5 into the spray pipe 8 and is subsequently immersed in a cleaning dip tank 81 for cleaning.

Sprayed hot water and cleaning dip tank 8
1 hot water is supplied 10 by a pump 84 and passes through a hot water return pipe 46 to a recirculation tank 88.
Return to

The temperature of the hot water in the recirculation tank 83 is measured by the thermometer 42, and the temperature is regulated by adjusting the steam control valve 48. 1' temperature is usually 80℃
~90°C. Further, the level of the recirculation tank is measured by a level meter 40 and adjusted by adjusting a hot water control valve 41.

The temperature of the supplied dry water is raised by heat exchange when the fan outlet side bypass valve 1620 is in the open direction and the hot air flows into the hot water heat exchanger 82 . By supplying hot water whose temperature has been raised through this heat exchange, it has become possible to save steam used to adjust the temperature of the recirculation tank 88.

At the royalty already mentioned, 85000 Nm8/H
rX 400°C hot air is sent to a hot water heat exchanger, and 350
When the hot water is cooled to .degree. C. and discharged, the hot water supplied to the cleaning line can be heated to about 32.degree. C. and then supplied.

Effects of the Invention According to the present invention, the slow responsiveness to the temperature control operation of an indirect heating furnace using a radiant tube can be advantageously utilized by utilizing the sensible heat of combustion waste gas in the radiant tube. Appropriate annealing is achieved by the continuous annealing method, improving the surface properties of the cold-rolled steel strip without deterioration.

[Brief explanation of the drawing]

Figure 1 shows graph 2° (2°) showing experimental data on chemical conversion treatability.
7) Fig. 2 is a skeleton diagram of the continuous annealing apparatus used to carry out the present invention; Fig. 8G; i is a sheet temperature control system diagram according to the present invention; Fig. 4 is a time chart of control factors; Fig. 5 is a similar time chart for the connecting steel strip, and Fig. 6 is a skeleton diagram of another example. 'Ji'

Claims (1)

[Scope of Claims] 1 Combustion waste gas discharged from a large number of radiant tubes installed in an indirect heating furnace is transferred to a heat exchanger together with atmospheric gas in a preheating furnace installed at the front of the heating furnace. The high-temperature atmosphere heated by the heat exchanger is cooled down to below the heat-resistant temperature of the waste gas suction prower and the waste gas flue through heat exchange with the atmospheric gas, and then dissipated. A steel characterized in that gas is blown onto the copper strip passing through the preheating furnace installed at the front of the heating furnace, and the preheated strip is indirectly heated by the radiant tube. Method 1 for continuous annealing of obi. East: The amount of high-temperature atmospheric gas blown onto the copper strip in the preheating furnace is determined based on the plate thickness, width, passing speed, atmospheric gas temperature blown onto the copper strip, furnace temperature of the heating furnace, and outlet target plate temperature. Adjust the temperature by 2 degrees depending on the temperature to control the plate temperature on the exit side of the preheating furnace.
1. The method according to l, which adjusts the plate temperature at the exit side of the heating furnace 1 to a target plate temperature. & The method according to 2, wherein an excess amount of high-temperature atmospheric gas with respect to the required amount of high-temperature atmospheric gas in the preheating furnace is dissipated into the atmosphere from the heat exchanger 5, bypassing the preheating furnace. 8. The method according to 8, wherein heat is recovered from the excess high-temperature atmospheric gas by passing it through a second heat exchanger. (v) The connection part with a copper strip that has a different plate thickness, plate width, or target plate temperature at the exit side of the heating furnace from the time it is introduced into the preheating furnace until it passes through the heating furnace. During unsteady heating, the copper strip passing speed is adjusted according to the temperature of the high-temperature 15 atmosphere gas blown onto the steel strip in the preheating furnace and the target plate temperature of each of the leading and trailing copper strips on the exit side of the heating furnace. In addition, 2Ll control is also carried out to keep the deviation from the target plate temperature on the heating furnace exit side of both copper strips that occurs transiently during this period within the common range of the upper and lower limit values of the heating outlet side plate temperatures of both copper strips. 1. The method according to g, 8 or 4.