JPH08219421A - Radiant tube - Google Patents

Abstract

PURPOSE: To reduce thermal strain and prevent the generation of deformation or crack by a method wherein two kinds or more of heat resistant alloys, having different coefficients of linear thermal expansion, are used to reduce a difference between the coefficients of linear thermal expansion of radiant tubes at a combustion burner side and an exhaust gas side. CONSTITUTION: A radiant tube, having W-shape, is constituted of a first straight tube unit 1, a second straight tube unit 2, a third straight tube unit 3, a fourth straight tube unit 4, a first bent tube unit 5, a second bent tube unit 6, a third bent tube unit 7 and connecting parts 8 through welding. Both end parts of the radiant tube are fixed rigidly to the outer surface of a furnace wall 9. Accordingly, a temperature difference is generated between the high-temperature unit of a combustion burner 10 side and the low-temperature unit of exhaust gas 11 side whereby a difference of the length of thermal expansion of the radiant tube between the first straight tube unit 1 and the fourth straight tube unit 4 is generated thereby generating a thermal stress. Due to such a reason, two kinds or more of alloys having different coefficient of linear thermal expansion are used for the material of the heat resistant alloys constituting the radiant tubes to reduce the difference of the coefficient of linear thermal expansion due to the temperature difference.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiant tube used for indirect heating of a steel sheet after cold rolling in a heat treatment furnace.

[0002]

2. Description of the Related Art Radiant tubes which are generally used in continuous annealing furnaces for cold rolled steel sheets and tin plates and various other heat treatment furnaces and heating equipment such as heating furnaces are
As shown in FIGS. 5 and 6, W-shaped and U-shaped radiant tubes are known. That is, FIG. 5 is a sectional view showing the structure of the W-shaped radiant tube. As shown in this figure, the W-shaped radiant tube includes a first straight pipe portion 1, a second straight pipe portion 2, a third straight pipe portion 3, a fourth straight pipe portion 4, a first curved pipe portion 5, and a first straight pipe portion 5. The W-shaped radiant tube of the continuous annealing furnace is composed of the second curved pipe portion 6, the third curved pipe portion 7 and the welded joint portion 8. Both end portions of the radiant tube are the furnace wall 9
Since it is firmly fixed to the outer surface of the furnace wall, the expansion and contraction can be released when thermal strain occurs due to the temperature difference between the high temperature part on the combustion burner 10 side and the low temperature part on the exhaust gas side 11. Because of this, the radiant tube may be greatly deformed or cracked, or the attachment part between the radiant tube and the outer surface of the furnace wall of the furnace wall may be damaged.In such a case, repair the incidental equipment. There are problems such as having to.

FIG. 6 is a sectional view showing the structure of a U-shaped radiant tube. As shown in this figure, the U-shaped radiant tube has a first straight pipe portion 1 and a second straight pipe portion 2
And a first bent pipe portion 5 and a welded joint portion 8 for joining them together. In this U-shaped radiant tube, both end portions are the outer wall portion of the furnace wall 9 as in the W-shaped radiant tube. Since it is firmly fixed, thermal stress damage due to thermal stress occurs due to the temperature gradient in the exhaust gas flow direction between the high temperature portion on the combustion burner 10 side and the low temperature portion on the exhaust gas side 11.

[0004]

As measures against the occurrence of deformation and cracks in the above W-shaped radiant tube and U-shaped radiant tube, the material is changed to a high creep strength material or the burner is improved in the longitudinal direction of the tube. The temperature distribution of the steel has been made uniform, local overheating has been prevented, and the support structure has been improved. Especially as a material change, Ni-Cr
This type of Ni-C
In the case of a single r-type heat-resistant alloy, even if the high creep strength is improved by changing the material, as long as there is a temperature difference between the high temperature part on the combustion burner side and the low temperature on the exhaust gas side, the first straight pipe part And the fourth straight pipe portion has a difference in thermal expansion length, and there is a problem that an effect cannot be expected with respect to thermal stress due to this temperature distribution.

As a result of intensive development by the inventors to solve these problems, two or more kinds of heat-resistant alloys constituting the radiant tube having different linear expansion coefficients are used, and the linear expansion coefficient is radiant. By arranging the tubes so that they gradually increase from the combustion burner, the difference in the linear expansion coefficient of the radiant tube is reduced and the thermal strain is reduced despite the temperature difference from the combustion burner side to the exhaust gas side. To reduce the thermal strain caused by the temperature distribution,
It is an object of the present invention to provide a radiant tube for a steel plate heat treatment furnace, which prevents deformation and cracking of the radiant tube, extends the life of the radiant tube, and reduces repair of incidental equipment.

[0006]

The means for achieving the above object are as follows. (1) A radiant tube characterized by using two or more kinds of alloys having different linear expansion coefficients in a heat resistant alloy constituting a radiant tube used for indirect heating of a heat treatment furnace for a steel sheet after cold rolling. (2) A radiant tube, wherein the heat-resistant alloy according to (1) is arranged so that the linear expansion coefficient of the heat-resistant alloy increases in order from the combustion burner of the radiant tube. (3) A radiant tube characterized in that the difference in linear expansion coefficient between the heat-resistant alloys of the adjacent radiant tubes described in (1) and (2) is 1 to 2 × 10 ⁻⁶ .

[0007]

The present invention will be described in detail below with reference to the drawings. FIG. 1 is a diagram showing the relationship between the combustion exhaust gas temperature of a W-type radiant tube and the linear expansion coefficient of the heat-resistant alloy of the radiant tube according to the present invention at room temperature. As shown in FIG. 1, in the radiant tube of the steel sheet heat treatment furnace, the exhaust gas temperature on the combustion burner side is high, while the temperature on the exhaust gas outlet side is about 200 ° C. lower than the exhaust gas temperature on the combustion burner side. Therefore, a temperature difference occurs between the combustion burner side and the exhaust gas outlet side, causing a difference in the thermal expansion length of the radiant tube between the first straight pipe portion and the fourth straight pipe portion, which causes thermal stress. This causes deformation and cracks in the radiant tube. FIG. 2 is a diagram showing the relationship between the combustion exhaust gas temperature of the U-shaped radiant tube and the linear expansion coefficient of the heat-resistant alloy of the radiant tube according to the present invention at room temperature. Also in the case of the U-shaped radiant tube, the exhaust gas temperature on the combustion burner side is high as in FIG.
In addition, there is a difference of about 130 to 150 ° C. from the outlet temperature. Therefore, it was found that the first straight pipe portion and the second straight pipe portion have different thermal expansion lengths, and thermal stress is also generated, which causes deformation and cracks of the radiant tube.

Therefore, in the present invention, it is intended to reduce the difference in linear expansion coefficient due to temperature difference by using two or more kinds of heat-resistant alloys constituting the radiant tube, which alloys have different linear expansion coefficients. Is.
Therefore, a heat-resistant alloy having the chemical composition shown in Table 1 was used. Table 2 shows the melting point and the specific gravity which are the physical properties of each heat resistant alloy. In addition, the temperature of each of these heat-resistant alloys is 70
Table 3 shows the values of the thermal expansion coefficient and the thermal conductivity at 0 ° C to 1000 ° C. Radiant tubes made of these heat-resistant alloys are sequentially combined from the combustion burner so that the linear expansion coefficient increases. For example, by combining the heat-resistant alloys A, B, C and D in this order from the combustion burner to the exhaust gas side, the difference in linear expansion coefficient can be set small. Here, it is particularly necessary to combine heat-resistant alloys such that the difference in linear expansion coefficient between the heat-resistant alloys of adjacent radiant tubes is 1 to 2 × 10 ⁻⁶ .

[0009]

[Table 1]

[0010]

[Table 2]

[0011]

[Table 3]

FIG. 3 shows the calculated values of the thermal expansion lengths of the heat-resistant alloys of the first straight pipe portion and the fourth straight pipe portion of the W type radiant tube according to the present invention and the conventional heat-resistant alloy of the single material radiant tube. It is a figure which shows the calculated value of thermal expansion length. As shown in FIG. 3, the thermal expansion lengths of the heat-resistant alloys of the first straight pipe portion and the fourth straight pipe portion are also calculated, and it can be seen that the product of the present invention has a smaller difference than the conventional product.

[0013]

EXAMPLE Using the W type radiant tube shown in FIG. 1, a heat-resistant alloy A having a linear expansion coefficient of 16 ± 1 × 10 ^{−6 at} room temperature was used for the first straight pipe part exposed to high temperature due to combustion.
Further, a heat-resistant alloy B having a linear expansion coefficient of 17 ± 1 × 10 ⁻⁶ was used for the second straight pipe portion, and a linear expansion coefficient of 17 ± 1 × was similarly used for the third straight pipe portion.
^10-6 becomes heat resistant alloy C, further in the fourth straight pipe section using a linear expansion coefficient 18 ± 1 × 10 ^-6 comprising heat resistant alloy D, respectively. In addition, the heat-resistant alloy A, second
The heat-resistant alloy B was used for the curved pipe portion, the heat-resistant alloy D was used for the third curved pipe portion, and the same heat-resistant alloy as that of the adjacent straight pipe was used for the material of each welded joint. The following test was conducted in order to confirm the effect of the radiant tube of the present invention sequentially combined from the combustion burner to the exhaust gas side in this way. Burning amount 15 × 10
Combustion was carried out using a main burner having a diameter of 150 mm and a wall thickness of 9 mm at ⁴ kcal / hr and a tube average maximum temperature of 1000 ° C. FIG. 4 shows the results. FIG. 4 shows the stress measurement values of the first straight pipe portion and the fourth straight pipe portion of the W-shaped radiant tube according to the present invention during the combustion test and the heat-resistant alloy stress measurement value of the conventional single-material radiant tube. FIG. As shown in FIG. 4, the product of the present invention has smaller thermal stress values in both the first straight pipe portion and the fourth straight pipe portion than the conventional product. Therefore, the radiant tube according to the present invention has less deformation and cracks,
As a result, it has become possible to extend the life of the radiant tube and reduce the repair of incidental equipment.

[0014]

As described above, by using two or more kinds of heat resistant alloys having different linear expansion coefficients according to the present invention,
By reducing the difference in the linear expansion coefficient of the radiant tube between the combustion burner side and the exhaust gas side and preventing distortion due to thermal stress in the radiant tube, it is possible to extend the life of the radiant tube and reduce the repair of incidental equipment. It has an extremely excellent effect.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a combustion exhaust gas temperature of a W-shaped radiant tube and a linear expansion coefficient of a heat-resistant alloy of the radiant tube according to the present invention,

FIG. 2 is a graph showing the relationship between the combustion exhaust gas temperature of a U-shaped radiant tube and the linear expansion coefficient of the heat-resistant alloy of the radiant tube according to the present invention.

FIG. 3 shows calculated values of thermal expansion lengths of the heat-resistant alloys of the first straight pipe portion and the fourth straight pipe portion of the W-shaped radiant tube according to the present invention and the thermal expansion of the heat-resistant alloy of the conventional single-material radiant tube. Figure showing the calculated length and

FIG. 4 is a stress measurement value of a heat-resistant alloy of a first straight pipe portion and a fourth straight pipe portion of a W-shaped radiant tube according to the present invention during a combustion test and a heat-resistant alloy stress measurement value of a conventional single-material radiant tube. And a diagram showing

FIG. 5 is a cross-sectional view showing the structure of a W-shaped radiant tube,

FIG. 6 is a cross-sectional view showing the structure of a U-shaped radiant tube.

[Explanation of symbols]

 1 1st straight pipe part 2 2nd straight pipe part 3 3rd straight pipe part 4 4th straight pipe part 5 1st curved pipe part 6 2nd curved pipe part 7 3rd curved pipe part 8 Weld joint 9 furnace wall 10 Combustion burner 11 Exhaust gas side

Claims (3)

[Claims] 1. A radiant tube characterized by using two or more kinds of alloys having different linear expansion coefficients in a heat-resistant alloy constituting a radiant tube used for indirect heating of a heat treatment furnace for a steel sheet after cold rolling. 2. A radiant tube characterized in that the heat-resistant alloy according to claim 1 is arranged such that the linear expansion coefficient thereof increases in order from the combustion burner of the radiant tube. 3. The difference in linear expansion coefficient between the heat-resistant alloys of the adjacent radiant tubes according to claim 1 or 2 is 1 to 2 × 10.
Radiant tube characterized by ^-6 .