JPS6296620A - Preheating method and equipment for continuous heat treatment of metal strip - Google Patents

Abstract

PURPOSE:To improve heat efficiency and productivity of annealing equipment, by raising the temp. of exhaust gas of heating zone with combustion exhaust gas, etc., then utilizing it for preheating steel strip in its continuous annealing equipment. CONSTITUTION:Steel strip is preheated, then heated and annealed in reducing atmosphere by a radiant tube 1 at continuous annealing of steel strip. Since temp. of exhaust gas exhausted from the tube 1 is low as 300-500 deg.C, it is introduced to a combustion furnace 10 through a duct 9. A fuel 11 and an air 12 preheated at a preheater 19 are burnt by a burner 13 at the furnace 10 and sent as 700-1,100 deg.C high temp. combustion gas to a heat exchanger 14 together with exhaust gas from a duct 9 to heat and supply heating medium to hollow rolls 17-1-17-n for preheating steel strip to the vicinity of annealing temp. at heating zone, and exhaust gas from the exchanger 14 is utilized for preheating combustion air for the furnace 10. heat efficiency including to heating zone after preheating is markedly improved and productivity of continuous annealing is improved.

Description

Detailed Description of the Invention (Industrial Application Field) In this specification, regarding preheating on the heating zone entry side in continuous heat treatment of metal steel I-+3 tubes (hereinafter referred to as steel tubes and IJ tubes), combustion exhaust gas in the heating zone This paper describes the results of research and development on a method and device for preheating the strip to a high temperature by making more effective use of sensible heat.

The configuration of continuous heat treatment equipment for strip, such as continuous annealing equipment for copper strip, usually consists of a heating zone, a soaking zone, and a cooling zone. The steel strip is heated to 600 to 850°C in the heating zone, but radiant tube heating is employed because it is necessary to create a reducing atmosphere inside the heating zone.

Sensible heat from the combustion exhaust gas from the radiant tube is recovered in the form of preheating of combustion air or fuel gas to save energy. Furthermore, in order to recover the sensible heat of the flue gas, the flue gas is blown directly onto the copper strip, or the sensible heat of the flue gas is transferred to the air or atmospheric gas through a heat exchanger to make it high temperature, and this hot air is then directed to the copper strip. Efforts are being made to utilize combustion exhaust gas, such as by spraying.

(Prior art) JP-A-57-41330 and JP-A-58-7372
7 publications, the sensible heat of the combustion exhaust gas in the heating zone is recovered into a heat medium through a heat exchanger, the heat medium is guided to a hollow roll with a flow path for the heat medium inside, and a copper band is attached to the roll. A method of preheating a copper strip by winding the copper strip is disclosed.

In this method, the sensible heat of combustion exhaust gas at 300 to 350°C is recovered to a heat medium at a temperature of about 150°C, where the acid dew point is not a problem.
If you preheat the copper strip by heating the steel strip to 130-140℃
It is possible to raise the temperature to a certain degree, and the exhaust heat recovery rate can be increased.

However, when preheating to about 130 to 140°C, there is a limit to the rate of temperature increase because the main means of bringing the steel strip to a predetermined temperature of 600 to 850°C in the heating zone is radiant heating using a radiant coil. However, this method has the disadvantage of requiring a significantly longer heating zone, especially in the case of high-speed threading required to increase production.

(Problems to be Solved by the Invention) By increasing the temperature of the combustion exhaust gas on the heating zone entrance side and using it in stages, it is possible to improve thermal efficiency, achieve high temperature preheating, and prevent meandering, making the equipment more compact. The purpose of this invention is to improve productivity and productivity.

(Means for solving the problem) The above purpose is to guide the combustion exhaust gas of the radiant tube to a higher temperature combustion furnace when heat treating metal slabs in a radiant tube type heating zone. The interior of a hollow roll that mixes with the combustion gas of the combustion furnace and heats a heat medium through a heat exchanger using the sensible heat of this mixed exhaust gas, and that the heat medium is disposed in multiple throws on the inlet side of the heating zone. A metal strip characterized in that the metal strip is sequentially circulated and distributed around the hollow roll, and the metal strip is gradually preliminarily heated to a high temperature close to the processing temperature in a heating zone while the metal strip is being passed through the hollow roll. This is advantageously achieved by the preheating method (first invention) for continuous heat treatment of metal strips, and the high temperature mixture is achieved by mixing the combustion exhaust gas of the radiant tube disposed in the heating zone of the continuous heat treatment equipment for metal strips with higher temperature combustion gas. A combustion furnace that obtains exhaust gas, a heat exchanger that transfers sensible heat from the mixed exhaust gas to and from a heat medium, and a metal (IJ) tube whose wrapping is threaded under internal circulation of the heat medium through the heat exchanger. A preheating device (second invention) for continuous heat treatment of U-tubes is suitable.

Now, the present invention will be specifically explained with reference to FIG. 1, which shows a preheating device for an IJ bulb.

The radiant tube 1 in the heating zone is equipped with a burner 2, a preheater 4 that heats combustion air 3 using sensible heat of exhaust gas, and a hot air communication pipe 5, and supplies combustion air 3 and fuel 6 to the burner 2. Combustion gas from burner 2 is transferred to radiant tube 1
The inside of the heating zone is heated by radiation. The temperature of the combustion gas flowing through the radiant tube 1 when it is exhausted from the preheater 4 is about 500°C.

Next, this combustion exhaust gas is collected from the exhaust gas duct 7 into the exhaust gas header 8 and guided to the combustion furnace 10 via the duct 9. Note that the exhaust gas duct 7 is connected to each radiant tube 1 in the heating zone, and introduces the combustion exhaust gas from each radiant tube 1 into the exhaust gas header 8, respectively.

Next, fuel 11 and preheated combustion air 12 are supplied to the combustion furnace 10, and the burner 13 generates combustion gas having a higher temperature than the combustion exhaust gas, which is mixed with the combustion exhaust gas. By this operation, the combustion exhaust gas at about 500°C is turned into a mixed exhaust gas heated to about 700 to 1100°C near the exit of the combustion furnace 10.

This mixed exhaust gas is guided to a heat exchanger 14, where the sensible heat of the mixed exhaust gas is transferred to and received from a high boiling point heat medium 15.

Examples of the heating medium include thermo-oil, metallic sodium, and molten salt, and nitrate-based molten salts are preferable to chloride-based molten salts from the viewpoint of corrosion.

The heat medium 15 was circulated and supplied from the pump 16 to the inside of a plurality of hollow rolls 17-1 to 17-n for heating the IJ tube, and was wound around the outer periphery of the rolls 17-1-n.
Heat the J tube.

Furthermore, the gas leaving the heat exchanger 14 after heat transfer is transferred to the fan 18.
After the residual heat is provided to a preheater 19 that preheats the combustion air supplied to the combustion furnace 10, it is sucked in by an exhaust fan 20 and radiated into the atmosphere through a chimney 21.

Examples of means for gradually heating the +J tube to a high temperature include adjusting the flow rate of the heating medium to the roll, and adjusting the angle of the strip around the hollow roll.

For example, as shown in FIG. 2, rolls 17-1 to 17-7
The flow path of the heat medium supplied to the rollers is devised, and the temperature is increased from roll 17-1 to roll 17-7 in order, and the rolls are passed through.
Gradually heat the IJ tube.

Further, in the example shown in FIG. 3 in which two heat exchangers are connected in tandem, the sensible heat of the mixed exhaust gas from the combustion furnace 10 is transferred to and received from the heat medium 15 by the first heat exchanger 14a, and the first heat exchanger 14a
The sensible heat of the gas that has passed through is further transferred to and received from the heat medium 15 in the second heat exchanger 14b.

The heat medium from the first heat exchanger 14a is transferred to the high temperature roll group 17a.
, the heat medium from the second heat exchanger 14b is transferred to the low temperature roll group 1.
7b respectively, and the strips are heated to approximately 250°C,
The temperature can then be raised to about 450°C.

In addition, in order to heat the strip to over 500℃, it is necessary to raise the temperature of the heating medium inside the roll to a very high temperature, but because the difference between the strip temperature and the shell temperature of the roll becomes large, the crown of the roll becomes an inverted crown. This has the disadvantage that the strip becomes more meandering.

Furthermore, nitrate-based heat carriers may decompose at temperatures above 550°C and solidify at 140°C, so
control within the range of This control is easily performed by adjusting the fuel of the combustion furnace to adjust the temperature of the heated gas that transfers heat to and from the heat medium.

Here, continuous annealing equipment equipped with a strip preheating device is installed.
It is shown in Figure 4.

First, the strip S is preheated to about 200 to 500°C using a heating device 22 equipped with a plurality of heating rolls, and then heated to a target temperature of 600 to 850°C in a heating zone 23 equipped with a radiant tube 1. Continue strip S
is passed through the soaking zone 24, passes through the first cooling zone 25, the overaging zone 26, and the second cooling zone 27, and completes the predetermined annealing.

Note that the roll is, for example, a hollow roll 28 as shown in FIG.
A system in which the supply pipe 29 and the discharge pipe 30 are connected to each other, and the supply pipe 29 and the discharge pipe 30 are supported by a rotary joint pipe 31 and a bearing 32, or Utility Model Application No. 59-64963, Japanese Patent Application No. 60-21334 Use the rolls etc. described in the book. .

Further, from the viewpoint of thermal efficiency, it is preferable for the combustion furnace used in the preheating device shown in FIG. 3 to have a configuration as shown in FIG. That is, the combustion furnace 10 and the heat exchanger 14a.

14b are arranged in series, the mixed exhaust gas from the combustion furnace 10 is passed through, and the heat medium for high temperature rolls, the heat medium for low temperature rolls, and the combustion air are sequentially heated by the heat exchanger.

(Function) Next, when heating a strip at 50°C to 750°C, Table 1 shows a comparison of the heat balance when preheating and heating were performed by the following three methods. The thickness of the strip is:
A plate having a diameter of 0.85 mm and a width of 1200 a+m was used, and the plate threading speed was 300 m/min.

(1) Heating with a radiant tube (Comparative Example 1), (
2) Wrapping the strip around a roll heated by the sensible heat of the combustion exhaust gas, preheating it to 140°C, then heating with a radiant tube (Comparative Example 2), (3) Combustion exhaust gas and combustion gas from the combustion furnace The strip is wrapped around a roll heated by the sensible heat of the mixed exhaust gas and heated to 450°C.
From Table 1, it can be seen that the thermal efficiency of the compatible example is excellent.
By performing preheating with rolls using the sensible heat of the mixed exhaust gas using combustion exhaust gas, and then performing normal radiant tube heating, it is possible to efficiently heat the strip.

Furthermore, regarding the meandering during strip threading, the results of an experiment conducted under the following conditions are shown in FIG. In the figure, the horizontal axis shows the difference between the heat medium temperature and the strip temperature, and the vertical axis shows the meandering amount per roll.

Threading speed: 100-300m/min Strip entry steepness = 0.5-1.0% tension crab 0.5-1.5kg
/mm" Metal strip entrance temperature: 50℃ Heat medium temperature: 200 to 600℃ Among the above conditions, the amount of meandering on the roll exit side was investigated by varying the heat medium temperature and threading conditions, and the amount of meandering was found to be It was found that the difference between the heat medium temperature and the IJ tube temperature is dominant, and as shown in FIG. 7, when the difference exceeds 300°C, meandering becomes severe.

It is therefore advantageous to reduce the difference between the heating medium temperature and the strip temperature, and gradual heating is required rather than rapid heating.

Furthermore, the heat transfer mechanism is heat conduction in which heat is transferred directly from the roll surface to the IJ tube, and it is possible to heat at a higher rate than in the radiant tube method, which uses radiation and convection heat transfer.

(Example) An embodiment according to the heating device shown in FIG. 3 will be described.

First, as a strip, the plate thickness is 0.85 mm and the width is 120 mm.
Using a 0 am general cold rolled steel plate, the steel plate was annealed by passing it through the plate at a line speed of 3QQm/min under the following conditions.

Roll outer diameter: 1500mm Roll material: Iron Number of rolls: 8 on low temperature side 8 on high temperature side Heat medium: Molten salt (NaNO+ -NaNO* -K
The combustion exhaust gas (500℃) of the radiant tube 1 and the combustion gas from the combustion furnace 10 are mixed to form a 4-mixed exhaust gas of 1050℃, and the sensible heat of this mixed exhaust gas is 1 heat exchanger 14
45 at the input side of the heat exchanger 14a.
The heat medium 15 at 0° C. is heated to 550° C. and then circulated and supplied to the high temperature roll group 17a.

The temperature of the heated gas leaving the heat exchanger 14a is approximately 650°C,
After the second heat exchanger 14b transfers sensible heat to and receives heat from the heat medium 15, the temperature becomes approximately 350°C. At the inlet side of the heat exchanger 14b, the heat medium 15' at about 300° C. is heated to 400° C. by giving and receiving sensible heat, and is circulated and supplied to the low temperature roll group 17b.

The strip is wound around the high-temperature roll group 17a and the low-temperature roll group 17b that have been heated as described above, as shown in FIG. Group 17a could be preheated to 450°C.

The heating state of the strip is shown in FIG. 9 together with Comparative Example 3 in which the preheating temperature by the roll was as low as 140° C. and Comparative Example 4 in which a radiant tube was used.

The temperature of the gas exiting the second heat exchanger 14b on the right is approximately 350°C.
The heat was further recovered in a preheater 19 for preheating the combustion air used in the combustion furnace IO until the temperature reached about 270°C, and then the gas was released into the atmosphere from the chimney 21 via the exhaust fan 20.

In addition, the atmosphere in the preheating device was made into a reducing atmosphere, as in the subsequent heating zone, since the metal strip temperature becomes high due to roll preheating.

(Effects of the Invention) According to the first invention, by raising the temperature of the combustion exhaust gas in the heating zone and using it, it is possible to achieve a significant improvement in thermal efficiency including the heating zone after preheating, and also to prevent high temperature preheating and meandering. By achieving this, equipment can be made more compact and productivity can be improved.

Further, according to the second invention, it is possible to provide an apparatus suitable for carrying out the first invention.

Furthermore, since reducing atmosphere gas can be used by employing roll heating, there is no risk of deterioration of the quality of the strip surface, and since the roll temperature can be easily adjusted, it can sufficiently respond to changes in line speed.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the heating device, Fig. 2 is an explanatory diagram of the heat medium piping, Fig. 3 is an explanatory diagram of the heating device, Fig. 4 is an explanatory diagram of the continuous annealing equipment, and Fig. 5 is an explanatory diagram of the roll. , FIG. 6 is an explanatory diagram of the heating medium heating device, FIG. 7 is a graph showing the meandering amount with respect to the temperature difference between the heating medium and the strip, FIG. 8 is an explanatory diagram showing the winding of the strip around a roll, and FIG. The figure is a graph showing heating time and strip temperature. 1... Radiant tube 7... Exhaust duct 10... Combustion furnace 12... Combustion air 14... Heat exchanger 17.
...Roll 19...Preheater Fig. 5 Fig. 6

Claims (1)

[Claims] 1. When heat treating a metal strip in a radiant tube type heating zone, the combustion exhaust gas of the radiant tube is guided to a higher temperature combustion furnace and mixed with the combustion gas of the combustion furnace,
A heat medium is heated via a heat exchanger by the sensible heat of this mixed exhaust gas, and the heat medium is sequentially circulated through hollow rolls arranged in multiple stages on the inlet side of the heating zone. A preheating method for continuous heat treatment of a metal strip, characterized in that the metal strip is preliminarily heated to a high temperature close to the treatment temperature in a heating zone while the metal strip is wound around a roll and passed through. 2. The method according to 1, characterized in that the recovery of sensible heat from the mixed exhaust gas by a heat exchanger spans two stages. A combustion furnace that obtains a high-temperature mixed exhaust gas by mixing it with higher-temperature combustion gas, a heat exchanger that transfers sensible heat of the mixed exhaust gas to and from a heat medium, and under internal circulation of the heat medium through the heat exchanger, A preheating device for continuous heat treatment of a metal strip, comprising: a hollow roll for heating a metal strip that is wound and passed through. 4. The apparatus according to 2, wherein the combustion furnace is equipped with a preheater that preheats the combustion air using residual heat of the mixed exhaust gas after heat exchange.