JPH1161277A - Continuous heat treatment method of steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the responsiveness when the temperature of the steel sheet is changed without greatly increasing the heating cost of the steel plate. SOLUTION: In heat treating a steel sheet in a continuous annealing facility of the steel sheet provided with a radiation tube heating furnace, a direct-fired heating furnace at the inlet side of the radiation tube heating furnace, and a preheating furnace 11 using the combustion exhaust gas of the direct-fired heating furnace on the inlet side of the direct-fired heating furnace, an induction heating device 17 is installed at least between the direct-fired heating furnace and the preheating furnace, or between the direct-fired heating furnaces where a plurality of direct-fired heating furnaces 15, 19 are provided, the steel plate is mainly heated using the direct-fired heating furnace and the radiation tube heating furnace 21, and heat-treated by compensating the response delay or insufficient heating capacity by the induction heating in the case of the response delay of the temperature control in changing the heating load of the direct-fired heating furnace and the radiation tube heating furnace or in the case of insufficient heating capacity thereof. The problem of oxidation of the steel plate surface is solved, an induction heating equipment of large capacity is dispensed with, and the heating cost by induction heating is less increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous heat treatment method for a steel sheet.

[0002]

2. Description of the Related Art Continuous annealing can complete annealing in a short time and has high productivity. Therefore, continuous annealing is widely used for heat-treating a steel sheet in a continuous annealing line or a hot-dip plating line of the steel sheet.

[0003] In recent years, in order to make the installation space for continuous annealing equipment compact, a vertical furnace in which steel sheets travel while turning vertically in the annealing equipment and a direct-fired heating furnace capable of rapidly heating steel sheets are provided. In addition, in order to reduce the energy cost for heating the steel sheet, a preheating furnace for preheating the steel sheet using the high temperature combustion exhaust gas of the direct heating furnace is often provided in front of the direct heating furnace.

[0004] Fig. 8 shows an example of a main part of a continuous annealing facility for a steel sheet provided with a vertical furnace. In FIG. 8, 1 is a preheating furnace, 2
Denotes a direct heating furnace, and 3 denotes a radiation tube heating furnace.

[0005] First, the steel sheet S is preheated to a predetermined temperature in the preheating furnace 1 by using the high temperature combustion exhaust gas of the direct heating furnace 2, and then travels to the subsequent direct heating furnace 2.

In the direct-fired heating furnace 2, the steel sheet S is directly heated using a high-temperature combustion gas obtained by burning a fuel gas such as coke oven gas or natural gas, and rapidly heated to a temperature of about 720 ° C. . At the same time, while preventing the surface oxidation of the steel sheet,
Oil such as rolling oil is removed, and the subsequent radiant tube heating furnace 3
Travel to.

In the radiant tube heating furnace 3, fuel gas is burned in the radiant tube to heat and maintain the steel sheet S at a predetermined annealing temperature. The atmosphere inside the radiation tube type heating furnace 3 is kept in a reducing atmosphere, and at the same time, the oxide film on the steel sheet surface is reduced.

Next, the steel sheet S is subjected to a process such as overaging in a subsequent overaging furnace (not shown) and cooling in a cooling furnace. In the case of continuous annealing equipment in a hot-dip plating line, hot-dip plating is continuously performed.

The direct-fired heating furnace in the apparatus shown in FIG. 8 is provided with one furnace body in one pass. However, in order to improve the heating capacity, the direct-fired heating furnace is provided in a plurality of passes to provide a plurality of furnace bodies. In some cases.

In the above-described continuous annealing equipment, a predetermined heat treatment cycle is set according to the steel sheet size, the steel sheet material and the like. When changing the heat treatment cycle, it is necessary to change the combustion load of the heating furnace so that the temperature of the heating furnace follows the temperature corresponding to the required heat treatment cycle.

However, in a heating furnace, particularly a radiation tube heating furnace, the steel sheet is heated by radiant heating from the furnace wall, so that the thermal inertia is large. Therefore, the temperature of the heating furnace cannot quickly follow the set temperature, and there is a large time delay until the temperature reaches the set temperature. Until the temperature of the heating furnace reaches a predetermined set temperature range, there is a problem that a large amount of so-called off-cycle materials that cannot secure a necessary annealing temperature are generated.

When the heat treatment cycle is changed, the line speed is reduced to minimize the occurrence of the off-cycle material. However, in this case, there is a problem that productivity is reduced.

[0013]

With the use of induction heating,
The fact that the steel sheet can be heated in a short time while saving space is disclosed in Energy Saving, Vol. 45, no. 10 (199
3), p. Known as described in 42-47.
However, at the present time, continuous heating of a steel tube equipped with a radiation tube heating furnace, a direct-fired heating furnace on the entrance side of the radiation tube-type heating furnace, and a preheating furnace for preheating the steel sheet on the entrance side of the direct-fired heating furnace. In annealing equipment, heating of steel sheets using induction heating devices has not been put into practical use, and effective heating methods for steel sheets using induction heating devices have also been sufficiently studied. I can't say.

One reason for this is that the development of an efficient induction heating apparatus for large-capacity thin steel sheets was delayed,
The biggest reason is that the electricity cost when heating the steel sheet is higher than the fuel gas cost.

The present invention has been made in view of the above circumstances, and has a radiation tube type heating furnace, a direct-fired heating furnace at an inlet of the radiation tube-type heating furnace, and a direct-fired heating furnace at an entrance of the direct-fired heating furnace. In continuous annealing equipment for steel sheets equipped with a preheating furnace that preheats steel sheets, while utilizing the characteristic of induction heating, which enables heating and heating of steel sheets in a short time, without significantly increasing the heating cost of steel sheets, It is an object of the present invention to provide a method for continuously heat-treating a steel sheet, which can enhance the responsiveness when changed.

[0016]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is characterized in that a radiant tube heating furnace, a direct-fired heating furnace on the inlet side of the radiant tube-type heating furnace, and the direct-fired heating furnace are provided. When heat-treating a steel sheet in a continuous annealing equipment for a steel sheet provided with a preheating furnace using combustion exhaust gas of the direct-fired heating furnace on the inlet side of the type-heating furnace, between the direct-fired heating furnace and the preheating furnace, or When the direct-fired heating furnace includes a plurality of direct-fired heating furnaces, an induction heating device is installed on at least one of the direct-fired heating furnaces, and heating of the steel sheet is mainly performed by the direct-fired heating furnace. Performed using the radiation tube heating furnace, the response delay of temperature control at the time of heating load change in the direct heating furnace or the radiation tube heating furnace, or also the direct fire heating furnace or the radiation tube type When the heating capacity of the heating furnace is insufficient, the induction heating by the induction heating device is performed. In a continuous heat treatment method of steel plate characterized by heat-treating the response delay, or also to compensate for the lack heating capacity.

If the steel sheet is excessively preheated in the preheating furnace, the surface of the steel sheet is excessively oxidized, and the reduction of the steel sheet surface in the direct heating furnace or the radiant tube heating furnace becomes incomplete, so that the coating substrate Problems such as poor chemical conversion treatment performed as a treatment and poor plating adhesion occur when hot-dip plating is performed. When an induction heating device is installed on the inlet side of the preheating furnace and the steel sheet is heated by induction heating, the surface of the steel sheet is excessively oxidized in the preheating furnace, and the above-described problem is likely to occur.

Further, when the steel sheet is heated by induction heating on the outlet side of the direct-fired heating furnace, the steel sheet is heated near the Curie point, so that the heating efficiency of the induction heating is significantly reduced.

In the present invention, between the direct-fired heating furnace and the preheating furnace,
Alternatively, when the direct-fired heating furnace includes a plurality of direct-fired heating furnaces, at least one of the direct-fired heating furnaces, an induction heating device is disposed, and the steel sheet is induction-heated. There is no problem that the surface is excessively oxidized, and the heating efficiency of induction heating can be increased.

Further, in the present invention, the heating by induction heating is not performed as a main heating means of the steel sheet, but the heating of the steel sheet is mainly performed by using the direct heating furnace and the radiant tube heating furnace. In the case of a response delay of temperature control at the time of a heating load change in a fired heating furnace or the radiant tube heating furnace, or the heating capability of the direct fired heating furnace or the radiant tube heating furnace is insufficient, the induction heating device is used. Since the induction heating is only performed to compensate for the response delay and the shortage of the heating capacity, no large-capacity induction heating equipment is required, and the increase in heating cost due to the induction heating is small.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a continuous annealing facility having two direct-fired heating furnaces corresponding to two-pass direct-fired heating paths, in which induction is provided between a first direct-fired heating furnace and a second direct-fired heating furnace. This is a case where a heating device is provided.

In FIG. 1, 11 is a preheating furnace (PHF),
Reference numeral 15 denotes a first direct heating furnace (DFF-1), 19 denotes a second direct heating furnace (DFF-2), and 21 denotes a radiation tube heating furnace (R
TF). Before and after the preheating furnace 11, the roll chamber 10,
12. Roll chambers 14, 1 before and after the first open flame heating furnace 15
6. Roll chambers 18 and 20 before and after the second direct-fired heating furnace 19
And each roll chamber is provided with a deflector roll for changing the running direction of the roll.

Reference numeral 17 denotes an induction heating device (IH),
It is disposed between a roll chamber 16 after the direct heating furnace 15 and a roll chamber 18 before the second direct heating furnace 19. Reference numeral 22 denotes an exhaust duct of the preheating furnace, and reference numerals 23 and 24 each denote a preheating furnace, a direct-fired heating furnace, and a gas duct communicating between the direct-heating heating furnace.

FIG. 2 shows part A of FIG.
-The detailed drawing of the vicinity of the induction heating device 17-the roll chamber 18 is shown. FIG. 3 is a sectional view taken along the line AA of FIG.

In FIG. 2, reference numeral 34 denotes an induction heating coil, 3
5 is a heat insulating material, 36 is a water cooling jacket. Also, 30
a and 30b are deflector rolls, 31a and 31b are seal rolls, 32a and 32b are seal members, 33
Reference numerals a and 33b denote supply ports for the sealing gas N2 to the sealing chambers 16 and 18, respectively.

As shown in FIG. 2, the roll chambers 16, 18
Is provided with a gas seal mechanism for preventing inflow of combustion gas from the adjacent direct-fired heating furnaces 15 and 19, respectively.

The combustion gas of the second direct-fired heating furnace 19 is
The first direct-fired heating furnace 15 passes through a gas duct 24 that communicates the inlet of the direct-fired heating furnace 19 with the outlet of the first direct-fired heating furnace 15.
The combustion gas of the first direct-fired heating furnace 15 flows into the preheating furnace 11 through a gas duct 23 that communicates the inlet of the first direct-fired heating furnace 15 with the outlet of the preheating furnace 11. The gas flows out of the preheating furnace from the exhaust duct 22 on the inlet side of the furnace 11. If necessary, an afterburning chamber may be provided in the gas duct.

The sealing member 32a has a cylindrical cross section, and a steel plate S runs inside the sealing member 32a.
a, it is arranged in contact with the peripheral surface of the seal roll 31a. With such a configuration, the sealing property between the first direct-fired heating furnace 15 and the roll chamber 16 is good.

The seal between the second direct-fired heating furnace 19 and the roll chamber 18 is also provided in the same manner as described above. The deflector roll 30b, the seal roll 31b, and the seal member 32b
, The sealing property is good.

Gas supply ports 33a are provided in the roll chambers 16 and 18.
N2 gas is supplied from 33b and the roll chambers 16, 18 are supplied.
When the space between the direct heating furnaces 15 and 19 is sealed, it is possible to prevent the combustion gas of the direct heating furnace from flowing into the roll chambers 16 and 18.

The heat treatment is performed as follows using the annealing equipment shown in FIGS. The steel sheet S is preheated in the preheating furnace 11 using the combustion exhaust gas from the direct-fired heating furnaces 15 and 19, and heated to about 720 ° C. using the high-temperature combustion gas obtained by burning the fuel gas in the direct-heating heating furnaces 15 and 19. It is rapidly heated to a temperature, and at the same time, while preventing the surface oxidation of the steel sheet, oil components such as rolling oil are removed, and further, it is heated and maintained at a predetermined annealing temperature in a radiant tube heating furnace 21.

Subsequently, the steel sheet S is subjected to processing such as overaging in a subsequent overaging furnace (not shown) and cooling in a cooling furnace, and the required heat treatment is completed. In the case of continuous annealing equipment in the hot-dip plating line, hot-dip plating is further performed.

In the present invention, the induction heating device 17 is not used as the main heating means for the steel sheet, but is used as the direct-fired heating furnace 15,1.
When the combustion load conditions of the furnace 9 or the radiation tube heating furnace 21 are changed, the direct heating furnaces 15 and 19 or the radiation tube heating furnace 2 are used.
1 is used as auxiliary heating means for compensating for the transient state of the response delay of the temperature control characteristic and the insufficient heating capacity of each heating furnace by induction heating by the induction heating device 17. That is, the induction heating device 17 is not usually used, and the auxiliary heating is performed using the induction heating device 17 when changing the combustion load conditions of the radiation tube heating furnace 21 or the like or when the heating capacity of each heating furnace is insufficient. I do.

The case where the combustion load condition of the radiation tube type heating furnace is changed will be described below. When increasing the combustion load of the radiant tube furnace, a change in the combustion load (for example, a change in the heat treatment cycle such as a welding point when the size is changed) reaches the radiant tube furnace or when possible. Increasing the combustion load of the radiant tube heating furnace from before.

Also, when the point of change in the combustion load passes through the induction heating device, the induction heating device is output to instantaneously heat the steel plate to compensate for the delay in the temperature control characteristics of the radiant tube heating furnace. When the temperature of the radiant tube heating furnace approaches the set temperature and the plate temperature approaches the predetermined temperature, the induction heating device is adjusted accordingly to reduce its output.

When the combustion load of the radiant tube heating furnace is reduced, the response delay is not so large as when the combustion load of the radiant tube heating furnace is increased. The heating device can be used effectively. In this case, before the change point of the combustion load passes through the induction heating device, the radiation tube heating device is set in advance so that the radiation tube heating furnace has a predetermined temperature when the combustion load change point passes through the induction heating device. Reduce combustion load.

After the combustion load is reduced, until the change in the combustion load passes through the induction heating device, the shortage of the combustion load of the radiant tube heating furnace for the running steel sheet is adjusted by adjusting the output of the induction heating device. In addition, compensation is performed so that the temperature of the running steel sheet becomes a predetermined temperature.

The frequency of the induction heating device is as follows.
The range is preferably in the range of 30 to 100 kHz at which the thin steel sheet can be efficiently heated.

Next, a specific description will be given, with reference to FIG. 4, of a heat treatment method when the heat treatment cycle is changed in the above-described apparatus so as to increase the combustion load. FIG.
Is the case where the efficiency of the steel sheet to be heat-treated is constant 210 t / h, and the heat-treatment cycle is changed from the low-combustion load A cycle to the high-combustion load D cycle, and the frequency of the induction heating device is 30 kHz and the output is 3600 kW. , Heating efficiency is 85%.

The heat treatment cycle of the A cycle is indicated by the A cycle plate temperature (annealing temperature: 700 ° C.), and the corresponding furnace temperature of the radiation tube type heating furnace is the A cycle furnace temperature. In this case, the sheet temperature in the steady state rises to 700 ° C. along the ABCDX1 temperature rising curve, and the steel sheet is continuously soaked for a predetermined time.

The heat treatment cycle of the D cycle is indicated by the D cycle plate temperature (annealing temperature: 850 ° C.), and the corresponding furnace temperature of the radiation tube type heating furnace is the D cycle furnace temperature. In this case, the plate temperature in the steady state is 850 along the heating curve of AFG.
° C and the steel sheet is subsequently soaked for a predetermined time.

When the furnace temperature of the radiant tube heating furnace is changed from the A cycle furnace temperature to the D cycle furnace temperature without using the induction heating device, the furnace temperature of the radiant tube heating furnace is set to the D cycle furnace temperature. To raise the furnace temperature, and set the plate temperature to the X cycle plate temperature X
It is necessary to raise from 1 to G of the D cycle plate temperature.

On the other hand, when the induction heating device is used, the point of the heat treatment cycle change from the A cycle to the D cycle is that the heat treatment cycle is changed when passing through the induction heating device between the first direct heating furnace and the second direct heating. The steel sheet is instantaneously heated from B to E using the apparatus. As a result, the sheet temperature immediately after the change in the heat treatment cycle is as shown by a temperature rise curve of EH-D1-X2. In this case, in the radiation tube heating furnace, the plate temperature may be increased from X2 to G.

Comparing the time required to change the heat treatment cycle from the A cycle to the D cycle, it is necessary to raise the sheet temperature from X1 to G when induction heating is not used. However, in the case of using induction heating, it takes only time to raise the sheet temperature from X2 to G,
The required time for changing the heat treatment cycle is shortened, and the occurrence of off-cycle materials can be reduced.

It is also possible to set the furnace temperature of the radiation tube type heating furnace in advance to the D cycle furnace temperature and, if possible, to a furnace temperature higher than that, so that the furnace temperature can be raised earlier. Can be further reduced.

Further, by adjusting the output of the induction heating device so that the sheet temperature X2 falls within the lower limit of the D cycle sheet temperature control range, the generation of off-cycle materials can be substantially eliminated.

A first object of the present invention is to compensate for the response delay of temperature control when changing the combustion load of the heating furnace by induction heating and minimize the occurrence of out-of-cycle. When the increase in the line speed is restricted due to the restriction of the heating capacity, the heating capacity can be further enhanced by induction heating to further increase the heating capacity, and can be used as a means for increasing the production amount. Hereinafter, this point will be described.

The apparatus shown in FIG. 5 includes a preheating furnace (PHF), a direct-fired heating furnace (DFF), and a radiation tube heating furnace (RT). The radiation tube heating furnace is used at the maximum heating capacity. In the continuous annealing equipment operated under the condition that the combustion exhaust gas temperature of the direct-fired heating furnace cannot be increased from the current level, the space between the preheating furnace 11a and the direct-fired heating furnace 15a was modified and described with reference to FIGS. Induction heating device (1250)
kW × 30 kHz × 1). In the apparatus shown in FIG. 5, portions corresponding to the portions shown in FIG. 1 are denoted by the same reference numerals as those used in FIG. 1, and a suffix a is added to the end of the reference numeral.

In this apparatus, a general material having a thickness of 1.0 mm and a width of 1650 mm when the maximum heating capacity of the radiant tube type heating furnace and the upper limit of the combustion exhaust gas temperature of the direct fire type heating furnace are operated under the same conditions as before. Table 1 shows the line speed and production capacity before and after the use of the induction heating device in the material for deep drawing (annealing temperature: 850 ° C.) and the material for deep drawing (annealing temperature: 850 ° C.). In addition, the corresponding heat treatment cycle is shown in FIG.
It is shown in FIG. 7 (material for deep drawing).

[0051]

[Table 1]

As shown in FIGS. 6 and 7, with the installation of the induction heating device between the preheating furnace and the direct heating furnace, the sheet temperature at the entrance of the direct heating furnace can be raised by about 70 to 80 ° C. , As shown in Table 1, the production efficiency was about 10%.
It is increased by about%.

In the present invention, since the surface of the steel sheet is not excessively oxidized by the induction heating, even if a chemical conversion treatment or a hot-dip coating is performed after the heat treatment, problems such as a poor chemical conversion treatment and poor plating adhesion may occur. Does not occur. Further, the heating temperature range of the induction heating device is a temperature range below the Curie point, so that the heating efficiency is excellent.

Since the induction heating device is disposed between the roll chambers provided with the deflector rolls, the space for the annealing furnace is not increased, for example, the furnace length of the direct-fired heating furnace is increased. During normal operation without using
There is no hindrance to operation.

Further, since the induction heating device is disposed between the roll chambers provided with the deflector rolls, the fluttering of the steel plate in the thickness direction can be reduced. Furthermore, since a seal mechanism is provided between the direct-fired heating furnace and the roll chamber to prevent the combustion gas of the direct-fired heating furnace from flowing into the roll chamber, the combustion gas of the direct-fired heating furnace flows into the roll chamber. This also prevents flapping of the steel sheet. From these results, since the distance between the induction heating device and the steel plate can be reduced, the heating efficiency of induction heating can be further improved.

Further, the apparatus shown in FIG.
In this case, the induction heating device 17 is provided between the first heating furnace 11 and the first direct heating furnace 15.
For example, between the roll chamber 12 and the roll chamber 1 of the apparatus of FIG.
When the induction heating device is provided in the furnace body 13 between the four, the same effect as described above can be obtained. Also in this case, similarly to the above, the roll chamber 12 and the preheating chamber 11, and the roll chamber 14 and the first
It is desirable to provide a seal mechanism between the direct-fired heating furnace 15 for preventing inflow of combustion gas from the direct-fired heating furnace.

The apparatus shown in FIG. 1 is a case where a steel plate path traveling in a preheating furnace, a direct-fired heating furnace, or a radiant tube heating furnace is a vertical path. With respect to the above, the induction heating device described above may be provided.

In the present invention, since the installation space for the induction heating device is small, it is easy to convert existing equipment to the device of the present invention.

[0059]

According to the present invention, when the heating load of the radiant tube heating furnace is changed to change the heating furnace temperature when changing the size or material of the steel sheet, the temperature of the steel sheet can be quickly changed. Therefore, it is possible to reduce the generation of the material out of the heat treatment cycle.

Further, when the heating capacity of the heating furnace is insufficient, the heating capacity can be increased by the auxiliary heating of the induction heating, and the production amount can be increased.

In addition, induction heating is only used as an auxiliary heating means when changing the heating load of the radiant tube heating furnace or when the heating capacity of the heating furnace is insufficient, so that a large-capacity induction heating equipment is required. In addition, the induction heating does not significantly increase the heating cost. Also, since it is used at a temperature lower than the Curie point, the heating efficiency is excellent. Further, in the present invention, since there is no possibility of forming an oxide film on the surface of the steel sheet,
It does not affect chemical conversion properties or plating properties.

[Brief description of the drawings]

FIG. 1 is a diagram showing a continuous annealing facility used for describing an embodiment of the present invention.

FIG. 2 is a diagram showing details of an induction heating device of the continuous annealing equipment of FIG. 1 and equipment in the vicinity thereof.

FIG. 3 is a diagram showing a cross section of the induction heating device of FIG. 2;

FIG. 4 is a view for explaining an effect of reducing a deviation from a heat treatment cycle according to the present invention.

FIG. 5 is a diagram showing a continuous annealing facility used for explaining the effect of increasing the production amount of the present invention.

FIG. 6 is a view showing a heat treatment cycle of a general material.

FIG. 7 is a view showing a heat treatment cycle of a deep drawing material.

FIG. 8 is a view showing a conventional continuous annealing facility.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Preheating furnace 12, 14 Roll chamber 15 1st direct-fired heating furnace 16 Roll chamber 17 Induction heating device 18 Roll chamber 19 2nd direct-fired heating furnace 21 Radiation tube-type heating furnace 30a, 30b Deflector roll 31a, 31b Seal roll 32a, 32b Sealing member 34 Induction heating coil 35 Insulation material 36 Water cooling jacket S Steel plate

Claims (1)

[Claims] 1. A radiant tube heating furnace, a direct heating furnace on the entrance side of the radiation tube heating furnace, and a preheating furnace using combustion exhaust gas from the direct heating furnace on the entrance side of the direct heating furnace When the steel sheet is heat-treated in a continuous annealing facility for a steel sheet provided with: between the direct-fired heating furnace and the preheating furnace, or, if the direct-fired heating furnace includes a plurality of direct-fired heating furnaces, a direct fire An induction heating device is installed on at least one between the heating furnaces, and heating of the steel sheet is mainly performed using the direct heating furnace and the radiant tube heating furnace, and the direct heating furnace and the radiant heating are performed. In the case of a delay in response to temperature control when changing the heating load in a tubular heating furnace, or in the case of insufficient heating capacity of the direct-fired heating furnace or the radiant tube heating furnace, the induction heating by the induction heating device causes the response. Heat treatment to compensate for delay or insufficient heating capacity Continuous heat treatment method of steel plate characterized by.