JPS6345452B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for preheating a metal strip in continuous heat treatment.

<Prior art> Continuous heat treatment of metal strips involves unwinding the metal strip from a coil, forming it into a strip, and successively heating, soaking, and cooling it in a continuous heat treatment furnace. Normally, as shown in FIG. 7, a heating furnace 2, a soaking furnace 3, and a cooling furnace 4 are successively connected and integrated.

The heat treatment of the metal strip 1 in the heating furnace 2 is as follows:
In order to heat the strip at room temperature to several hundred degrees Celsius, an extremely large amount of heat is supplied to the heating furnace.
The heating method includes direct heating with non-oxidizing combustion gas, or as shown in FIG. 8, filling the heating furnace 2 with a reducing gas atmosphere and inserting a radiant tube 5 into the heating furnace. The radiant tube 5 is heated by burning aerated coke oven gas 6 or the like within the radiant tube 5, and the metal strip 1 is heated by radiation from the radiant tube 5.

The former method of directly heating the metal strip 1 with non-oxidizing combustion gas has a large amount of heat transferred per unit area, but when the metal strip is heated to a high temperature of 400 to 900°C, an oxide film is formed on the surface of the strip. Therefore, the upper limit temperature for heating the strip is set at 400℃ from the viewpoint of product quality. Under such conditions, the non-oxidizing flue gas temperature is 500-1000℃,
Of the calorific value of the supplied fuel, the heat used to heat the strip is 40-60%. In addition, in the latter method of heating the radiant tube by burning coke oven gas in the radiant tube, the exhaust gas temperature is 500 to 1000℃, but the combustion air is preheated to 300 to 450℃ and mixed with the coke oven gas. Then, the exhaust gas temperature is 500-700°C, and only 55-60% of the supplied heat is used for heating the strip.

For these reasons, the following methods of preheating metal strips have been proposed for the purpose of effectively utilizing the combustion heat in the heating furnace and reducing the unit cost of fuel.

○B. A preheating method in which a flue is provided on the entrance side of the heating furnace so that the combustion exhaust gas from the heating furnace and the metal strip entering the heating furnace flow in opposite directions.

○B A method of preheating the strip by blowing combustion exhaust gas from the heating furnace onto the surface of the metal strip at the front of the heating furnace.

○B A preheating method in which a heat exchanger is interposed between the heating medium gas and the combustion exhaust gas of the heating furnace, the heating medium gas is indirectly heated, and the high temperature heating medium gas is blown onto the metal strip.

○D A method of preheating a continuous heat treatment furnace by passing the combustion exhaust gas from the heating furnace through a preheating roll placed on the inlet side of the heating furnace and wrapping a metal strip around the preheating roll (Japanese Patent Publication No. 56-52093).

○ E. The metal strip is preheated by passing the sensible heat of the combustion exhaust gas of the heating furnace through the preheating roll disposed on the inlet side of the heating furnace of the continuous heat treatment furnace and recovering it in the heating medium, and by circulating the heating medium through the preheating roll. There are methods (Japanese Patent Application Laid-Open No. 57-41330, JP-A No. 58-73727), etc.

<Problems to be Solved by the Invention> However, among the above-mentioned methods for preheating a metal strip, methods ○A, ○B, and ○C use the combustion exhaust gas of the heating furnace or the sensible heat of the combustion exhaust gas through a heat exchanger. Since the exhaust gas or heat medium is recovered as a heat medium and blown onto the surface of the metal strip, it has the disadvantage that an oxide film is formed on the surface of the strip, deteriorating its properties.

In addition, in method ○2, the combustion exhaust gas from the heating furnace is passed through the preheating roll, and the heat transfer between the preheating roll shell and the combustion exhaust gas is poor, and the equipment will be large if sufficient heat recovery is to be performed. . In addition, the response to changes in the heating furnace load is poor.

Furthermore, in method ○e, like method ○d, the thermal conductivity between the heating medium gas and the preheating roll shell is poor, and the response to load fluctuations of the heating furnace is poor. Furthermore, the limit for heat recovery from combustion exhaust gas from a heating furnace is about 150°C, where its acid dew point does not pose a problem. The temperature of the metal strip wound around the threading roll is raised by this heating medium gas to a range of about 130 to 150°C.

Therefore, even if the preheating method described in Furnaces cannot be downsized.

This invention was made in order to eliminate the drawbacks of the conventional method of preheating metal strips in continuous heat treatment. In addition to being used to raise the temperature, the metal strip can quickly respond to changes in the heating furnace load and adjust the heating temperature of the preheating roll, stably preheating the metal strip, and making the heating furnace more compact. The present invention attempts to provide a preheating method.

<Means for Solving the Problems> In order to achieve the above-mentioned object, the present invention uses a heating medium, which recovers sensible heat of the combustion exhaust gas of the heating furnace through heat exchange, in a preheating roll provided on the inlet side of the heating furnace. A method for preheating a metal strip in a continuous heat treatment in which the metal strip is passed through the sheet, and the metal strip is wound around the passing plate, preheated, and then sent to a heating furnace, the preheating roll being a plurality of low-temperature rolls disposed on the input side;
It is composed of a plurality of high-temperature rolls arranged on the exit side, and the heat medium flowing to each low-temperature roll is a heat medium recovered by heat exchange of the sensible heat of the combustion exhaust gas of the heating furnace. The heating medium is characterized in that a heating medium is used which recovers the sensible heat of a mixed gas of combustion exhaust gas from a heating furnace and exhaust gas heated through a heating device through heat exchange.

<Function> In this way, the preheating roll placed on the inlet side of the heating furnace is composed of a plurality of low temperature rolls placed on the inlet side and a plurality of high temperature rolls placed on the exit side, and each low temperature roll has a A heating medium that recovers the sensible heat of combustion exhaust gas through heat exchange is passed through each high-temperature roll, and the combustion exhaust gas from the heating furnace is heated to a predetermined temperature via a heating means, and the heating medium that has exchanged the sensible heat is passed through each high-temperature roll. In order to allow the flow of gas, in the low temperature roll group (room temperature to 150°C) and high temperature roll group (150 to 450°C) of these preheating rolls, the mixing ratio of the combustion exhaust gas of the heating furnace and the introduced gas and the operation of the heating means, Since the temperature of each roll group can be changed in response to load fluctuations in the heating furnace, stable preheating can be performed without being affected by load fluctuations in the heating furnace.

<Examples> Next, typical examples of the present invention will be described.

1 to 6 show schematic diagrams of a heating furnace and a preheating furnace to which the method of preheating a metal strip in continuous heat treatment of the present invention is applied. however,
Figures 2 to 4 show the temperature distribution diagram of the metal strip in the preheating furnace and heating furnace shown in Figure 1, the exhaust gas temperature distribution diagram, and the temperature distribution of the heat medium in the passing roll in the low-temperature preheating roll furnace and the high-temperature preheating roll furnace. It is a diagram.

The structure of the heating furnace and preheating furnace used in the method of preheating a metal strip in continuous heat treatment shown in Figs. 7 and a high-temperature preheating roll furnace 8 are arranged, and the metal strip 1 is preheated by the low-temperature preheating roll furnace 7 and the high-temperature preheating roll furnace 8 before being fed into the heating furnace 2.

In the furnace of this structure, the heating furnace 2 is
As shown in the figure, rolls 9, 9, . and is sent to the exit 2b side. Further, radiant tubes 10 are provided above and below the roll 9, and combustion air 11 and fuel gas (for example, coke oven gas) 12, which have been preheated outside the heating furnace 2, are combusted within the radiant tubes 10. Radiant tube 1
Heat 0. Then, the temperature of the fuel exhaust gas is lowered to about 500°C by an air recuperator 13, and then the gas is sent to the gas delivery paths A-1 and B-1.
The structure is such that they are fed to the high-temperature preheating roll furnace 8 and the low-temperature preheating roll furnace 7 side through them, respectively.

The high-temperature preheating roll furnace 8 has a plurality of high-temperature rolls 14, 14,
14, . . . are arranged, and the metal strip 1 sent from a low temperature preheating roll furnace 7, which will be described later, is sequentially wound around high temperature rolls 14, 14, 14, . It functions to send data to 2. The high-temperature rolls 14, 14, 14, . It is mixed with exhaust gas and heated to 700-1100°C, and the sensible heat is exchanged with the heat medium gas in the high-temperature heat medium source 17 using the high-temperature heat exchanger 16 to generate a temperature of 300-500°C.
℃, and a part of the cooled heating medium gas passes through the gas delivery path A-2 to each high temperature roll 14, 14,
14, put it in... The remaining heat transfer gas is sent to the gas delivery path B-1 of the low temperature preheating roll furnace 7 through the gas delivery path A-3.

The low temperature preheating roll furnace 7 includes a plurality of low temperature rolls 18, 18, 18, . . . in a room separated by a wall 7-1.
... are provided, and the metal strip 1 fed from the inlet side is rolled by these low-temperature rolls 18, 18,
18,... is preheated while being wrapped in sequence,
It is sent to the high temperature preheating roll furnace side. The radiant tube 10 is sent out to the gas delivery path B-1 to each low temperature roll 18, 18, 18,...
The sensible heat of the mixed gas of the combustion exhaust gas of After raising the temperature of the low-temperature heat transfer gas, it passes through the gas delivery path B-2 to the low-temperature roll 18,
Sent to 18, 18,... In this way,
The low temperature heat medium gas fed into the low temperature rolls 18, 18, 18, . . . circulates within the rolls and then returns to the low temperature heat medium source 20 again.

High temperature preheating roll furnace 8 and low temperature preheating roll furnace 7
The walls 8-1 and 7-1 are the rolls 14, 14,
14, . . . 18, 18, 18, . . . are used to prevent heat dissipation during heating and to confine atmospheric gas.

The temperature distribution of the metal strip 1 in the low-temperature preheating roll furnace 7, high-temperature preheating roll furnace 8, and heating furnace 2 described above has the temperature shown in FIG. 150-450 in the furnace
The temperature is raised to 600-900°C in the heating furnace 2.

Note that the structure of the preheating furnace is not limited to the one described above, but for example, as shown in FIG. Alternatively, the low-temperature preheating roll furnace 7 may be arranged on the entry side of the high-temperature preheating roll furnace 8, and the furnace may be filled with air as a gas atmosphere.

In addition, if necessary, the walls of the low temperature preheating roll furnace 7, the high temperature preheating roll furnace 8, and the heating furnace 2 may be integrally configured, and the internal gas atmosphere may be made into a reducing gas.
Alternatively, a separate structure may be used in which the low-temperature preheating roll furnace 7 and the high-temperature preheating roll furnace 8 are made independent from the heating furnace 2.

Next, an example of preheating a metal strip using molten salt as a heating medium is shown below.

Example 1 Metal strip to be heat treated: cold rolled steel plate (strip thickness 1mm strip width 1m) Line speed: 300rpm, preheating roll: 1.5mφ (material stainless steel) Heating heat medium: Molten salt of NaNO ₂ -KNO ₃ -NaNO ₃ Low temperature Heat medium temperature: 300℃ High temperature Heat medium temperature: 550℃ Number of preheating rolls: 8 low temperature rolls 8 high temperature rolls Strip temperature: Inlet 40℃ Low temperature preheating roll furnace outlet, 190℃ High temperature preheating roll furnace outlet, 400℃ As a heating medium, In addition to the molten salts mentioned above, NaNO ₃ −
Flame-retardant heat transfer oil such as molten salt of KNO ₃ can also be used.
Pressure water (and steam) can also be used as a low-temperature heating medium.

<Effects of the Invention> As is clear from the above description, when the continuous heat treatment method of preheating a metal strip according to the present invention is applied, the thermal efficiency can be increased from 40 to 60% according to the conventional method to 70 to 75%.

Additionally, the heating furnace that heats the metal strip from room temperature to annealing temperature (600-800°C) can be reduced to about 60-50% of the conventional heating furnace.

Further, there is an advantage that stable preheating can be performed without being affected by load fluctuations in the heating zone.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a furnace used in the method for preheating metal strips in continuous heat treatment of the present invention;
Figure 2 is a heating temperature distribution diagram of the metal strip in the furnace of Figure 1, and Figure 3 is a temperature distribution diagram of the heating medium and combustion exhaust gas flowing through the rolls in the low-temperature preheating roll furnace, high-temperature preheating roll furnace, and heating furnace. , Figures 4A and 4B are temperature distribution diagrams of the heating medium flowing through the preheating rolls of the low-temperature preheating roll furnace and the high-temperature preheating roll furnace, and Figure 5 shows the furnace used in the method of preheating metal strip in continuous heat treatment of the present invention. 6 is a schematic diagram of a heating furnace of the furnace shown in Figure 1, Figure 7 is an overall diagram of a continuous heat treatment furnace used for conventional continuous heat treatment, and Figure 8 is a diagram of a heating furnace of the furnace shown in Figure 1. FIG. 2 is a configuration diagram of an example of a heating furnace in the continuous heat treatment furnace shown in FIG. In the drawing, 1 is a metal strip, 2 is a heating furnace, and 3 is a metal strip.
is a soaking furnace, 4 is a cooling furnace, 5 and 10 are radiant tubes, 7 is a low temperature preheating roll furnace, 8 is a high temperature preheating roll furnace, 14 is a low temperature roll, 16 is a high temperature heat exchanger, 17 is a high temperature heat medium source, 18 is a high temperature roll, 19
is a low temperature heat medium source, and 20 is a low temperature heat medium source.

Claims (1)

[Scope of Claims] 1. A heating medium recovered by heat exchange from the sensible heat of the combustion exhaust gas of the heating furnace is made to flow through a preheating roll provided on the inlet side of the heating furnace, and a metal strip is wound around the preheating roll for preheating. In the method for preheating a metal strip in continuous heat treatment in which the metal strip is heated and then fed into a heating furnace, the preheating roll is composed of a plurality of low temperature rolls disposed on the input side and a plurality of high temperature rolls disposed on the output side. The heating medium distributed to each low-temperature roll is a heating medium recovered by heat exchange of the sensible heat of the combustion exhaust gas of the heating furnace, and the heating medium distributed to each high-temperature roll is heated through the combustion exhaust gas of the heating furnace and a heating device. A method for preheating a metal strip in continuous heat treatment, characterized by using a heating medium that recovers sensible heat from a mixed gas of exhaust gas through heat exchange. 2. The method of preheating a metal strip in continuous heat treatment according to claim 1, characterized in that the preheating of the metal strip by the preheating roll is performed entirely in an air or reducing gas atmosphere. 3. The metal strip is preheated by the preheating roll in an air or non-oxidizing gas atmosphere in the low-temperature preheating roll furnace, and in a reducing gas or non-oxidizing gas atmosphere in the high-temperature preheating roll furnace. A method for preheating a metal strip in continuous heat treatment according to item 1.