JP6364992B2 - Fin tube - Google Patents

Description

  The present invention relates to a fin tube.

  Conventionally, a combined cycle power plant has been put to practical use as a highly efficient power plant. In a combined cycle power plant, power is generated by combining a gas turbine and a steam turbine. Specifically, first, the gas turbine is rotated using high-temperature gas obtained by burning natural gas to generate power. The heat of the exhaust gas discharged from the gas turbine is recovered by an exhaust heat recovery boiler (hereinafter referred to as HRSG (Heat Recovery Steam Generator)). HRSG generates steam using the heat of the collected exhaust gas and supplies it to the steam turbine. A steam turbine is turned using this steam, and power is also generated in the steam turbine. Thus, in a combined cycle power plant, power generation efficiency can be increased by generating power in a gas turbine and a steam turbine.

  In HRSG, exhaust gas is allowed to flow outside the heat transfer tube, and steam is generated in the heat transfer tube using heat obtained from the exhaust gas. Conventionally, fin tubes have been used as HRSG heat transfer tubes. The fin tube has a cylindrical tube and fins provided on the outer surface of the tube. In the fin tube, since the surface area can be increased by the fin, the heat of the exhaust gas flowing outside the fin tube can be efficiently absorbed.

  As the fin tube as described above, a type in which the fin is integrally formed with the tube by rolling the outer peripheral portion of the tube (hereinafter referred to as an integrally formed type), and a type in which the fin is welded to the outer surface of the tube (hereinafter referred to as the following). Called welding type). In the integrally formed fin tube, the strength of the boundary between the fin and the tube can be sufficiently secured. However, there are limitations on the shape and material of the fin that can be molded, and there is a limit to the expansion of the surface area of the fin tube.

  On the other hand, in the welded type fin tube, the fin can be formed separately from the tube, so that the flexibility of the shape and material of the fin is great. For this reason, it is easy to increase the surface area of the fin tube, and the heat absorption efficiency can be increased. For example, Patent Document 1 describes a fin tube including a metal tube and a metal fin. Patent Document 1 describes that carbon steel, molybdenum, stainless steel, or the like is used as a tube material, and mild steel is used as a fin.

Japanese Patent Laid-Open No. 9-72682

  The power generation efficiency of a combined cycle power plant can be improved, for example, by increasing the efficiency of steam generation in HRSG. The generation efficiency of steam in HRSG can be improved by increasing the amount of heat absorbed from the exhaust gas in the heat transfer tube. Therefore, in recent years, HRSG using high-temperature exhaust gas has been put into practical use. In such HRSG, the heat transfer tube is used in a high temperature environment of, for example, 500 ° C. or higher.

  As described above, the weld-type fin tube can easily increase the surface area and increase the heat absorption efficiency. Therefore, when a welding type fin tube is used as the heat transfer tube of HRSG, the power generation efficiency of the power plant can be further improved.

  By the way, a plant that burns natural gas and generates power using a gas turbine has a shorter start-up time than a plant that burns coal and generates power using a steam turbine. For this reason, a combined cycle power plant using a gas turbine may be operated only during a time period when the power demand is high and stopped during a time period when the power demand is low, or may be operated only during weekdays and stopped during a holiday. By operating the combined cycle power plant in this way, it is possible to meet the power demand without waste.

  However, when the power plant is started / stopped as described above, the temperature of the heat transfer tube greatly fluctuates. Here, in general, in a welded type fin tube, a desired performance (for example, performance according to the use environment of the fin tube) can be obtained by freely combining the materials of the tube and the fin. However, when the tube and the fin are made of different materials, there is a difference in the coefficient of thermal expansion between the tube and the fin. In this case, the fin and the tube constrain each other's thermal deformation, and a thermal stress is generated at the joint between the tube and the fin. For this reason, when the power plant is repeatedly started and stopped, thermal stress repeatedly acts on the joint between the fin and the tube. In particular, when the heat transfer tube is used in a high temperature environment of 500 ° C. or higher, the thermal stress acting on the boundary portion also increases. For this reason, there exists a possibility that a fin may peel from a tube.

  The present invention has been made to solve such problems, and an object thereof is to provide a fin tube excellent in heat resistance and heat fatigue resistance.

  The inventors of the present invention conducted extensive research to improve heat resistance and heat fatigue resistance of fin tubes used in high temperature environments. As a result, the following knowledge was obtained.

  When steam is generated in the tube, stress is applied to the tube by pressure (internal pressure) received from the steam. Therefore, when the fin tube is used for a long time in a high temperature environment, the stress caused by the internal pressure continues to act on the tube for a long time. For this reason, the tube needs to have sufficient resistance (creep strength) against the stress that continues to act for a long time as described above. In other words, it is necessary to use a material that can exhibit sufficient creep strength in a high temperature environment for the tube.

  As described above, when a welding type fin tube is used in a high temperature environment, a large thermal stress acts on the joint between the fin and the tube. As a result of the study by the present inventors, when the coefficient of thermal expansion of the fin is lower than that of the tube, stress in a direction in which the fin and the tube are pressed against each other acts on the boundary between the fin and the tube in a high temperature environment. I understood that. In addition, the inventors' research has shown that such stress does not significantly affect fin peeling.

  On the other hand, when the coefficient of thermal expansion of the fin is higher than that of the tube, it has been found that stress in the radial direction of the tube, that is, the direction in which the fin is pulled away from the tube acts at the boundary between the fin and the tube. And it turned out that this stress has big influence on peeling of a fin. Therefore, in order to suppress the peeling of the fin in a high temperature environment, it is necessary to reduce the stress in the direction in which the fin acting on the boundary portion is separated from the tube.

  When the fin tube is used in a high temperature environment of 500 ° C. or higher, the strength of the fin and the tube is lower than that at the normal temperature. Thereby, when using a fin tube in the environment of 500 degreeC or more, peeling of a fin becomes easy to generate | occur | produce compared with the case where it uses in an environment of less than 500 degreeC. Therefore, in order to prevent the peeling of the fin, it is necessary to suppress the generation of the stress (force in the direction of pulling the fin away from the tube) particularly in a high temperature environment of 500 ° C. or higher.

  As a result of further studies by the present inventors, it has been found that when the force (thermal stress) in the direction of pulling the fin away from the tube increases, the fin and the tube are plastically deformed. And when this plastic deformation resulting from a thermal stress is repeated, it turns out that it will be in a so-called low cycle fatigue state, and the fatigue life with respect to the thermal cycle of a fin tube will fall. In order to prevent the fin tube from being in a low cycle fatigue state as described above, it is necessary to suppress the deformation of the fin and the tube so that the fin and the tube are not plastically deformed.

  The present invention has been completed based on the above findings, and the gist of the present invention is the following fin tube.

(1) It has a tube and fins,
The chemical composition of the tube is mass%,
C: 0.03-0.13%,
Si: 0.5% or less,
Mn: 0.1 to 2%,
Ni: 8-15%,
Cr: 15-22%,
Cu: 2 to 6%,
Nb: 0.3 to 1.5%,
N: 0.005 to 0.2%,
B: 0.0001-0.2%
Balance: Fe and impurities,
The fin tube which satisfy | fills the following (i) Formula and (ii) Formula when the temperature of the said tube and the said fin is 500 degreeC.
α _f (T _f −T ₀ ) −α _t (T _t −T ₀ ) ≦ σ _Yt / E _t (i)
α _f (T _f −T ₀ ) −α _t (T _t −T ₀ ) ≦ σ _Yf / E _f (ii)
In the above formula,
α _f is the coefficient of thermal expansion (1 / ° C.) of the fin,
T _f is the temperature (° C.) of the fin,
T ₀ is room temperature (° C.)
α _t is the coefficient of thermal expansion (1 / ° C) of the tube,
T _t is the tube temperature (° C),
σ _Yt is the 0.2% proof stress (MPa) of the tube,
E _t is the Young's modulus (MPa) of the tube,
σ _Yf is the yield stress (MPa) of the fin,
E _f represents the Young's modulus (MPa) of the fin, respectively.
However, when the fin material does not exhibit the yield phenomenon, σ _Yf indicates the 0.2% proof stress (MPa) of the fin.

(2) The fin tube according to (1), wherein the tube and the fin are welded.

(3) The fin tube according to (1) or (2), wherein the tube and the fin are made of different steel materials.

(4) The fin tube according to any one of (1) to (3), wherein the fin has a higher coefficient of thermal expansion than the tube.

  According to the present invention, even when a fin tube having a difference in thermal expansion coefficient between the tube and the fin is used in a high-temperature environment, peeling of the fin can be sufficiently suppressed.

Sectional drawing which shows the fin tube which concerns on one embodiment of this invention Graph showing the relationship between temperature and 0.2% proof stress Graph showing the relationship between temperature and Young's modulus Graph showing the relationship between temperature and coefficient of thermal expansion The figure which shows the experimental result of this invention example 1, 2 and a comparative example The figure which shows the analysis result of this invention example 1 and 2 and a comparative example

  The present invention will be described in detail below. In the description of the chemical composition to be described later, “%” display of the content of each element means “mass%”.

1. Fin Tube Structure FIG. 1 is a cross-sectional view showing a fin tube 10 according to an embodiment of the present invention. The fin tube 10 includes, for example, a tube 12 and a fin 14. The tube 12 is made of a steel material. As the fin 14, various metal materials (for example, heat resistant steel, stainless steel, etc.) can be used.

  As shown in FIG. 1, in the fin tube 10 according to the present embodiment, a plurality of annular fins 14 are provided on the outer peripheral surface of a cylindrical tube 12. The plurality of fins 14 are arranged at substantially equal intervals in the axial direction of the tube 12. The fin 14 is joined to the outer peripheral surface of the tube 12 by welding, for example. Instead of the plurality of fins 14, spiral fins may be provided on the outer peripheral surface of the tube 12.

2. Chemical composition of tube (steel material) The chemical composition of the steel material used as the tube 12 in the present invention is as follows.

C: 0.03-0.13%
C is an element effective for securing the tensile strength and creep rupture strength required when the tube is used in a high temperature environment. In the present invention, since the steel material contains N as described later, the C content may be low. However, in order to exhibit the above effects, the C content needs to be 0.03% or more. The minimum with preferable C content is 0.05%. On the other hand, if the C content exceeds 0.13%, the amount of undissolved carbide after solutionization becomes excessive, and mechanical properties such as ductility and toughness deteriorate. Therefore, the upper limit of the C content is 0.13%. The upper limit with preferable C content is 0.12%.

Si: 0.5% or less Si is an element effective as a deoxidizing material and effective in improving oxidation resistance. However, when the Si content exceeds 0.5%, weldability and hot workability deteriorate. Further, as described later, in the present invention, the steel material contains N. For this reason, when a steel material contains a large amount of Si, the amount of nitride deposited when the tube is used in a high temperature environment increases. As a result, the toughness and ductility of the tube are reduced. Therefore, the Si content is 0.5% or less. The upper limit with preferable Si content is 0.3%.

Mn: 0.1 to 2%
Mn is an element having a deoxidizing action like Si. Furthermore, Mn has the effect | action which fixes S and improves the hot workability of steel materials. In order to obtain these effects, the Mn content needs to be 0.1% or more. A preferable lower limit of the Mn content is 0.2%. On the other hand, when the Mn content exceeds 2%, precipitation of intermetallic compounds such as σ phase is caused, and the high temperature strength and mechanical properties of the steel material are lowered. Therefore, the Mn content is 2% or less. The upper limit with preferable Mn content is 1.7%.

Ni: 8-15%
Ni is an essential component for securing a stable austenite structure. The optimum Ni content is determined by the content of ferrite-forming elements such as Cr and Nb and austenite-generating elements such as C and N contained in the steel. In the present invention, if the Ni content is less than 8%, it becomes difficult to stabilize the austenite structure. Therefore, the Ni content is 8% or more. A preferable lower limit of the Ni content is 8.5%. On the other hand, if the Ni content exceeds 15%, the above effect is saturated and the economic efficiency is lowered. Therefore, the Ni content is 15% or less. A preferable upper limit of the Ni content is 13%.

Cr: 15-22%
Cr is an element necessary for improving the oxidation resistance and corrosion resistance of the tube in a high temperature environment. However, if the Cr content is less than 15%, a sufficient effect cannot be obtained. Therefore, the Cr content is 15% or more. A preferable lower limit of the Cr content is 17%. The above effect becomes remarkable as the Cr content increases, but when the Cr content exceeds 22%, the austenite structure becomes unstable. Therefore, the Cr content is 22% or less. The upper limit with preferable Cr content is 20%.

Cu: 2 to 6%
Cu contributes greatly to the improvement in creep rupture strength by coherent precipitation as a fine Cu phase in the austenite matrix during use of the tube in a high temperature environment. In order to exhibit the above effects, the Cu content needs to be 2% or more. A preferable lower limit of the Cu content is 2.5%. On the other hand, when the Cu content exceeds 6%, creep rupture ductility and workability deteriorate. Therefore, the Cu content is 6% or less. The upper limit with preferable Cu content is 4.5%.

Nb: 0.3 to 1.5%
Nb is an element having an effect of improving creep rupture strength by dispersion precipitation strengthening of fine carbonitride. In order to exhibit the above effects, the Nb content needs to be 0.3% or more. The minimum with preferable Nb content is 0.35%. On the other hand, when the Nb content exceeds 1.5%, the weldability and workability of the steel material deteriorate. Moreover, the amount of undissolved carbonitride is excessive, and the mechanical properties of the steel material are deteriorated. Therefore, the Nb content is 1.5% or less. The upper limit with preferable Nb content is 1.3%.

N: 0.005 to 0.2%
N, like C, is an element that is effective in improving tensile strength and creep rupture strength. However, if the N content is less than 0.005%, a sufficient effect cannot be exhibited. Therefore, the N content is 0.005% or more. The minimum with preferable N content is 0.01%. On the other hand, if the N content exceeds 0.2%, the post-aging toughness decreases due to a large amount of precipitation of nitride. Therefore, the N content is 0.2% or less. The upper limit with preferable N content is 0.15%.

B: 0.0001 to 0.2%
B is an element having an effect of improving the high temperature strength of the steel material by carbonitride dispersion strengthening and grain boundary strengthening. However, if the B content is less than 0.0001%, a sufficient effect cannot be obtained. Therefore, the B content is 0.0001% or more. A preferable lower limit of the B content is 0.001%. On the other hand, if the B content exceeds 0.2%, the weldability is deteriorated. Therefore, the B content is 0.2% or less. The upper limit with preferable B content is 0.1%.

  The steel material used as the tube 12 according to the present invention includes, in addition to the above components, one type selected from Co, Mo, W, Ti, V, Ca, Mg, Zr and REM as described below as necessary. The above can be contained.

Co: 0 to 5%
Co is an austenite-forming element like Ni and Cu, and contributes to the improvement of creep strength by increasing the stability of the austenite structure. However, if the Co content exceeds 5%, the above effect is saturated and the economic efficiency is lowered. Therefore, the Co content when contained is 5% or less. In order to exhibit the above effects, the Co content is preferably 0.05% or more.

Mo: 0 to 5%
Mo is an element that contributes to the improvement of the creep strength of the steel material at a high temperature by dissolving in the matrix. However, if the Mo content exceeds 5%, the above effect is saturated and the economic efficiency is lowered. Moreover, the stability of the austenite structure is lowered, and the hot workability of the steel material is deteriorated. Therefore, the Mo content in the case of inclusion is 5% or less. In order to exhibit the above effects, the Mo content is preferably 0.05% or more.

W: 0-5%
W, like Mo, is an element that contributes to the improvement of creep strength at high temperatures. However, if the W content exceeds 5%, the above effects are saturated and the economic efficiency is lowered. In addition, the stability of the austenite structure is reduced. Therefore, the W content in the case of inclusion is 5% or less. In order to exhibit the above effects, the W content is preferably 0.05% or more.

Ti: 0 to 0.5%
Ti is an element that contributes to the improvement of the creep strength of a steel material at a high temperature by combining with C or N and precipitating in the grains as fine carbides or carbonitrides. However, when the Ti content exceeds 0.5%, the creep ductility of the steel material is lowered. Therefore, when Ti is contained, the Ti content is 0.5% or less. In order to exhibit the above effects, the Ti content is preferably 0.002% or more.

V: 0 to 0.5%
V, like Ti, is an element that forms fine carbides or carbonitrides and contributes to the creep strength of steel. However, when the V content exceeds 0.5%, the creep ductility of the steel material is lowered. Therefore, the V content when contained is 0.5% or less. In order to exhibit the above effects, the V content is preferably 0.01% or more.

Ca: 0 to 0.2%
Ca is an element having an effect of improving the hot workability of the steel material. However, if the Ca content exceeds 0.2%, Ca and O (oxygen) are combined to significantly reduce the cleanliness and deteriorate the hot workability of the steel material. For this reason, when Ca is contained, the Ca content is 0.2% or less. In order to exhibit the above effects, the Ca content is preferably 0.0001% or more.

Mg: 0 to 0.2%
Mg, like Ca, is an element that has the effect of improving the hot workability of steel. However, when the Mg content exceeds 0.2%, Mg and O (oxygen) are combined to significantly reduce the cleanliness and deteriorate the hot workability of the steel material. For this reason, Mg content in the case of making it contain shall be 0.2% or less. In order to exhibit the above effects, the Mg content is preferably 0.0001% or more.

Zr: 0 to 0.2%
Zr is an element that contributes to grain boundary strengthening and improves the creep strength of the steel material, and has an effect of fixing S and improving the hot workability of the steel material. However, if the Zr content exceeds 0.2%, the hot workability of the steel material is deteriorated. For this reason, the Zr content in the case of inclusion is 0.2% or less. In order to exhibit the above effects, the Zr content is preferably 0.0001% or more.

REM: 0 to 0.2%
REM (rare earth element) is an element having an effect of improving the hot workability of a steel material. However, when the REM content exceeds 0.2%, REM and O (oxygen) are combined to remarkably reduce the cleanliness and deteriorate the hot workability of the steel material. For this reason, the REM content in the case of making it contain shall be 0.2% or less. In order to exhibit the above effect, the REM content is preferably 0.0001% or more. Note that REM is a general term for 17 elements including Y and Sc combined with 15 lanthanoid elements. In the present invention, one or more of these 17 elements can be contained in the steel material. The REM content means the total content of these elements.

  The steel material used as the tube 12 in the present invention contains the above-mentioned elements, and the balance is composed of Fe and impurities. “Impurity” means a component that is mixed due to raw materials such as ore and scrap and other factors when industrially producing steel materials.

3. Although there is no restriction | limiting in particular about the chemical composition of the material used as the fin 14, when the fin 14 consists of steel materials, it is preferable to contain 10 to 22% of Cr. The reason will be described below.

  For example, when the fin tube 10 is used as a heat transfer tube of HRSG, the fin 14 is exposed to high temperature exhaust gas for a long time. For this reason, it is important that the fins 14 have excellent oxidation resistance and corrosion resistance. Addition of Cr is effective in improving the oxidation resistance and corrosion resistance of the fin 14. However, if the Cr content is less than 10%, the oxidation resistance and corrosion resistance cannot be sufficiently improved. Therefore, the Cr content of the steel material used as the fin 14 is 10% or more. A preferable lower limit of the Cr content is 10.5%. On the other hand, if the Cr content exceeds 22%, the workability of the steel material decreases. Therefore, the Cr content of the steel material used as the fins 14 is 22% or less. The upper limit with preferable Cr content shall be 21%.

4). Mechanical properties of the fin tube The steel material used as the tube 12 in the present invention does not exhibit a yield phenomenon. On the other hand, as the fin 14, a material exhibiting a yield phenomenon can be used, or a material not exhibiting a yield phenomenon can be used.

In the present invention, the materials of the tube 12 and the fin 14 are at least 500 ° C. (under the environment where the temperature of the tube 12 and the fin 14 is 500 ° C.), the following equations (i) and (ii) Fulfill.
α _f (T _f −T ₀ ) −α _t (T _t −T ₀ ) ≦ σ _Yt / E _t (i)
α _f (T _f −T ₀ ) −α _t (T _t −T ₀ ) ≦ σ _Yf / E _f (ii)
In the above formula,
α _f is the coefficient of thermal expansion (1 / ° C.) of the fin,
T _f is the temperature (° C.) of the fin,
T ₀ is room temperature (° C.)
α _t is the coefficient of thermal expansion (1 / ° C) of the tube,
T _t is the tube temperature (° C),
σ _Yt is the 0.2% proof stress (MPa) of the tube,
E _t is the Young's modulus (MPa) of the tube,
σ _Yf is the yield stress (MPa) of the fin,
E _f represents the Young's modulus (MPa) of the fin, respectively.
However, when the fin material does not exhibit the yield phenomenon, σ _Yf indicates the 0.2% proof stress (MPa) of the fin. Yield stress refers to the lower yield point.
The room temperature is the temperature of the atmosphere around the fin tube before the fin tube is heated, and is generally about 20 ° C. In the present embodiment, in the above formulas (i) and (ii), the room temperature is 20 ° C., for example.

  In addition, 0.2% yield strength, yield stress, and Young's modulus can be calculated | required by the test based on JISZ2241, for example, and a thermal expansion coefficient can be calculated | required by the test based on JISZ2285, for example.

  In the above formulas (i) and (ii), the left side represents the difference in thermal expansion strain between the tube 12 and the fin 14. In the equation (i), the right side represents the maximum strain in the elastic deformation region of the tube 12, and in the equation (ii), the right side represents the maximum strain in the elastic deformation region of the fin 14. When the value on the left side in the equations (i) and (ii) is positive, it means that the thermal expansion of the fin 14 is larger than the thermal expansion of the tube 12. In this case, stress in the direction in which the fin 14 is pulled away from the tube 12 (stress in the radial direction of the tube 12) acts on the boundary between the tube 12 and the fin 14. If the value on the left side in the equations (i) and (ii) increases, the tube 12 and the fin 14 are plastically deformed.

  Here, the stress acting on the boundary between the tube 12 and the fin 14 varies depending not only on the material of the tube 12 and the fin 14 but also on the size and shape. However, regardless of the material, size, and shape of the tube 12 and the fin 14, the stress acting on the boundary is maximized in the following cases. That is, when there is a difference in thermal expansion between the tube 12 and the fin 14, only one of the tube 12 and the fin 14 is strained to eliminate the difference.

  In the present invention, the equations (i) and (ii) are defined so that the plastic deformation of the tube 12 and the fin 14 can be prevented even in the above case. Specifically, by satisfying the equation (i), even if the strain based on the above-described difference in thermal expansion occurs only in the tube 12, the strain of the tube 12 can be suppressed within the elastic deformation region. Moreover, even if the distortion based on the said thermal expansion difference arises only in the fin 14 by satisfy | filling (ii) Formula, the distortion of the fin 14 can be suppressed in an elastic deformation range. Therefore, in the fin tube 10 of the present invention, plastic deformation of the tube 12 and the fin 14 can be prevented even under a high temperature environment of 500 ° C. Thereby, even when a thermal stress repeatedly acts on the boundary portion, it is possible to prevent a so-called low cycle fatigue state. That is, the heat resistance and heat fatigue resistance of the fin tube 10 can be sufficiently ensured. As a result, peeling of the fins 14 can be sufficiently suppressed.

  In addition, it is preferable to satisfy | fill Formula (i) and (ii) when the temperature of the tube 12 and the fin 14 will be 550 degreeC, and to satisfy | fill Formula (i) and (ii) when it will be 600 degreeC. More preferably, it is more preferable to satisfy the formulas (i) and (ii) at 700 ° C.

  The present invention can be suitably used for a fin tube made of a material in which the tube and the fin are different. In particular, the present invention exhibits an excellent effect in a fin tube in which the coefficient of thermal expansion of the fin is higher than the coefficient of thermal expansion of the tube. In other words, the present invention can be effectively used in a fin tube in which the value of the left side of equations (i) and (ii) is greater than 0 under the high temperature environment as described above.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  First, three tubes (outer diameter 38 mm, wall thickness 5.1 mm) made of steel materials having the chemical composition shown in Table 1 below were prepared. Further, fins 1, 2, and 3 (thickness 1 mm, height 12.7 mm) made of steel materials having the chemical composition shown in Table 1 were prepared. The tube was manufactured by hot extrusion, and the fin was manufactured by cold rolling. The fins 1, 2, and 3 were each spirally welded to the outer peripheral surface of the tube to produce three fin tubes. In addition, the steel materials used as the fins 1, 2, and 3 in this example do not show a yield phenomenon.

  2 to 4 show the 0.2% proof stress, Young's modulus, and thermal expansion coefficient of the tube and fins 1, 2, and 3 for each temperature. The 0.2% proof stress and Young's modulus were determined by a test based on JIS Z 2241, and the thermal expansion coefficient was determined by a test based on JIS Z 2285. Table 2 below shows the values of the formulas (i) and (ii) calculated based on the values shown in FIGS. Hereinafter, the fin tube provided with the fins 1 is referred to as the present invention example 1, the fin tube including the fins 2 is referred to as the present invention example 2, and the fin tube including the fins 3 is referred to as the comparative example. As shown in Table 2, the fin tubes of Examples 1 and 2 of the present invention satisfied the formulas (i) and (ii) even under conditions of 500 ° C. or higher, but the fin tubes of the comparative examples were 500 ° C. or higher. Under the conditions of (i), the formulas (i) and (ii) were not satisfied.

  The fin tubes of Invention Examples 1 and 2 and Comparative Example were subjected to a thermal cycle test. In the thermal cycle test, the fin tube was heated from the outside in a furnace. The temperature conditions were a maximum temperature of 620 ° C. and a minimum temperature of 100 ° C. The number of test cycles was 3000. The cycle time was 50 minutes. After the heat cycle test, the condition of the tubes and fins was confirmed. As a result, in the inventive examples 1 and 2 and the comparative example, the degree of oxidation was slight and the sound state was maintained.

  Also, the fin tube not subjected to the thermal cycle test and the fin tube after the thermal cycle test are cut, and the joining length of the welded portion between the fin and the tube (the length parallel to the axial direction of the tube) is the thermal cycle test. We observed how it changed before and after. More specifically, it was examined how much the joining length after the thermal cycle test was relative to the joining length (initial value) in the initial state before the thermal cycle test. And it was judged that the fin tube whose joining length after a heat cycle test is 50% or more of the joining length before a heat cycle test has excellent heat resistance and heat fatigue resistance. As a result, as shown in FIG. 5, in the fin tubes of Invention Examples 1 and 2, the joining length after the thermal cycle test was about 80 to 90% with respect to the joining length before the thermal cycle test. . That is, the fin tubes of Invention Examples 1 and 2 had excellent heat resistance and heat fatigue resistance. On the other hand, in the fin tube of the comparative example, the joining length after the thermal cycle test was about 44% with respect to the joining length before the thermal cycle test. That is, the fin tube of the comparative example did not have excellent heat resistance and heat fatigue resistance.

  Separately from the above thermal cycle test, a finite element method analysis (FEM analysis) was performed. Specifically, for the fin tubes of the present invention examples 1 and 2 and the comparative example having the above-described configuration, the thermal stress acting on the vicinity of the welded portion due to the heat cycle load was evaluated. In the FEM analysis, considering the symmetry of the fin tube shape, a model targeting a region corresponding to one pitch of the fin in the tube axis direction at a portion of 1/36 (angle 10 °) of the entire circumference in the circumferential direction. Evaluation was performed. In the FEM analysis, thermal stress generated when the tube and the fin were heated from the initial state at room temperature (20 ° C.) to 620 ° C. was evaluated.

  The results (stress distribution) obtained by FEM analysis are shown in FIG. In FIG. 6, the stress in the radial direction of the tube, that is, the direction in which the fin is peeled from the tube is indicated by a positive value.

  In the fin tube of Example 1 of the present invention, the coefficient of thermal expansion of the fin is lower than that of the tube. Therefore, as shown in FIG. 6, the stress in the direction in which the fin and the tube are pressed against each other (compressive stress) ) Acted. In the fin tube of the present invention example 2, since the thermal expansion coefficient of the fin is higher than that of the tube, a stress (tensile stress) in a direction in which the fin is peeled off from the tube acts on the boundary portion between the fin and the tube. However, the absolute value of the stress acting on the boundary portion was very small. As described above, in the fin tubes of Examples 1 and 2 of the present invention, the force in the direction in which the fins were peeled off from the tube hardly worked in a high temperature environment. On the other hand, in the fin tube of the comparative example, a large stress was applied to the boundary portion between the fin and the tube in the direction of peeling the fin from the tube.

  From the above results, according to the fin tubes of Examples 1 and 2 of the present invention that satisfy the expressions (i) and (ii) in an environment of 500 ° C. or higher, the fin tubes act in a direction in which the fins are peeled off in a high temperature environment. It was found that stress can be suppressed and excellent heat resistance and heat fatigue resistance can be secured. The number of heat cycles applied to the fin tube in an actual power plant varies depending on the operating conditions. However, if a large temperature fluctuation such as the above heat cycle test is applied once a week, the above 3000 heat cycles are performed in the power plant. This corresponds to a heat cycle applied for about 60 years. Therefore, the fin tube of the present invention can be used without impairing performance even during such long-term use.

  According to the present invention, even when the fin tube is used in a high temperature environment, the peeling of the fin can be sufficiently suppressed. Therefore, the fin tube which concerns on this invention can be used suitably as a heat exchanger tube (for example, HRSG heat exchanger tube) used in a high temperature environment.

10 Fin tube 12 Tube 14 Fin

Claims (3)

With tubes and fins,
The chemical composition of the tube is mass%,
C: 0.03-0.13%,
Si: 0.5% or less,
Mn: 0.1 to 2%,
Ni: 8-15%,
Cr: 15-22%,
Cu: 2 to 6%,
Nb: 0.3 to 1.5%,
N: 0.005 to 0.2%,
B: 0.0001-0.2%
Balance: Fe and impurities,
The fin tube which satisfy | fills the following (i) Formula and (ii) Formula when the temperature of the said tube and the said fin is 500 degreeC.
α _f (T _f −T ₀ ) −α _t (T _t −T ₀ ) ≦ σ _Yt / E _t (i)
α _f (T _f −T ₀ ) −α _t (T _t −T ₀ ) ≦ σ _Yf / E _f (ii)
In the above formula,
α _f is the coefficient of thermal expansion (1 / ° C.) of the fin,
T _f is the temperature (° C.) of the fin,
T ₀ is room temperature (° C.)
α _t is the coefficient of thermal expansion (1 / ° C) of the tube,
T _t is the tube temperature (° C),
σ _Yt is the 0.2% proof stress (MPa) of the tube,
E _t is the Young's modulus (MPa) of the tube,
σ _Yf is the yield stress (MPa) of the fin,
E _f represents the Young's modulus (MPa) of the fin, respectively.
However, the coefficient of thermal expansion of the fin is higher than the coefficient of thermal expansion of the tube , and σ _Yf indicates the 0.2% proof stress (MPa) of the fin when the material of the fin does not exhibit the yield phenomenon.   The fin tube according to claim 1, wherein the tube and the fin are welded.   The fin tube according to claim 1, wherein the tube and the fin are made of different steel materials.

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