JP5105822B2 - Heat transfer material for heat collection and manufacturing method thereof - Google Patents

Description

  The present invention relates to an exhaust heat recovery device introduced to improve energy efficiency in a system using thermal energy, such as a Stirling engine, a fuel cell, a micro gas engine, a micro gas turbine, an exhaust gas power generation system, and other cogeneration systems. The present invention relates to a heat transfer material such as a heat collecting plate, a tube, and a fin, which is used for collecting heat from a high temperature gas containing water vapor in a high temperature heat collecting apparatus such as a heat exchanger, and a method for producing the same.

In recent years, due to problems such as the depletion of fossil fuels typified by petroleum and the global warming phenomenon due to CO ₂ emissions, there has been a demand for practical use of a new system that replaces the conventional power generation system. As part of this, various cogeneration systems such as new power generation systems, Stirling engines, fuel cells, and exhaust gas power generation used for distributed power sources or automobile power sources are being put into practical use.

In these systems, exhaust gas of about 800 ° C. is generally discharged, and energy efficiency is improved by using heat generated from the exhaust gas. However, since these exhaust gases are generally gas generated by combustion of a gas burner, an engine, etc., the atmosphere contains a large amount of water vapor. There may be an atmosphere in which oxygen, hydrogen, CO ₂ , CO, HC, and the like are mixed with other components. Also, heating and cooling are usually repeated frequently.

  In order to increase the energy efficiency of the above system, it is essential to recover and use the generated heat as efficiently as possible. As a heat transfer material for recovering heat with high efficiency, it is required to have high thermal conductivity and excellent durability at high temperatures and resistance to heating and cooling.

  Examples of the material having high thermal conductivity include copper-based and aluminum-based alloys. However, these can be used in a temperature range from room temperature to about 300 ° C., but the deterioration due to oxidation becomes larger at a higher temperature range, and traces sometimes disappear at 800 ° C.

  Examples of metal materials that can withstand an 800 ° C. water vapor environment include stainless steel, iron-base heat-resistant high alloy, and Ni-base heat-resistant high alloy. With these materials, it is possible to achieve a power generation efficiency of 18 W / m · K or more, which is a guideline that can sufficiently ensure energy efficiency as the initial performance. However, no material has yet been found that maintains good initial heat transfer characteristics in a steam environment at a temperature of 800 ° C. and is excellent in fatigue characteristics and cost.

JP 7-292446 A JP 2003-328088 A

  In view of cost and mass productivity, stainless steel is considered advantageous as a heat transfer material for collecting heat applied to the system. Patent Document 2 describes a ferritic stainless steel that is unlikely to cause abnormal oxidation in a high-temperature steam oxidation atmosphere and that can suppress an increase in oxidation. However, Patent Document 2 does not show a material having a coating that maintains a high thermal conductivity of 18 W / m · K or higher in a high-temperature steam environment. At present, a stainless steel material having satisfactory characteristics as a heat collecting member has not yet been developed.

  In view of such a current situation, the present invention intends to develop and provide a heat transfer material for heat collection having durability that maintains the initial good heat transfer characteristics in a steam environment of 800 ° C. level.

As a result of detailed studies, the inventors have made a ferritic stainless steel base material limited to a specific composition and formed an oxide film with a low spinel oxide concentration on the surface thereof. The inventors have found that the heat conductivity maintenance characteristics are extremely good in the environment, and have completed a heat transfer material suitable for the above system.
The “heat transfer material” is a material that plays a role of transferring heat from a high-temperature side substance in contact with the material to a low-temperature side substance by a heat flow generated in the material.

The heat transfer material for heat collection of the present invention is based on a ferritic stainless steel having the following composition.
In mass%, C: 0.03% or less, Si: 2.0% or less, Mn: 1.5% or less, S: 0.008% or less, Cr: 11-25%, Al: 0.05-5 A composition that satisfies 0.0 %, preferably 0.2-6.0%, N: 0.03% or less, the balance substantially Fe, and the following formula (1).
Cr + 3Si + 15Al> 22.0 (1)

Furthermore, it can be set as the composition which satisfy | fills one or both of the following i) and ii) as elements other than the above.
i) Y: not more than 0.1%, REM (rare earth element): not more than 0.1%, Ca: not more than 0.01%, Zr: not more than 0.5%.
ii) One or more of Nb: 0.8% or less, Ti: 0.5% or less, Mo: 4.0% or less, Cu: 4.0% or less, and W: 4.0% or less are contained.

  Here, “the balance is substantially Fe” means that mixing of elements other than those specified is permitted within a range that does not impair the effects of the present invention, and “the balance consists of Fe and inevitable impurities”. Is included. The content of the element expressed in mass% is substituted for the element symbol in the formula (1).

This stainless steel substrate has a thickness of 0.01 on the surface on the side in contact with the high-temperature gas containing water vapor, in which the spinel oxide concentration is suppressed to 15% by mass or less (including 0% by mass, that is, not detected). It has an oxide film of 10 μm. Those having a thermal conductivity of 18 W / m · K or more at 800 ° C. through this oxide film are suitable.
The thickness of the oxide film can be measured by cross-sectional observation with an SEM (scanning electron microscope), and the thickness of each part may be in the range of 0.01 to 10 μm. The concentration of the spinel oxide can be performed by X-ray diffraction as described later.

  Such an oxide film is formed by, for example, exposing a steel base material whose composition is adjusted to the above composition to a gas at 700 to 1000 ° C. containing 5 to 60% by volume of water vapor and 1 to 20% by volume of oxygen, for example for 1 hour or more. It can be formed by subjecting to treatment. Before the oxidation treatment, it is more effective to perform a pre-oxidation treatment by exposing it to an air atmosphere at 700 to 900 ° C. for 1 hour or less. As a base material for use in the oxidation treatment or preliminary oxidation treatment, it is preferable to use a substrate that is polished and finished in a range of # 100 to 600 with a count defined in JIS R6001.

  According to the present invention, a material capable of maintaining a high thermal conductivity when used for a long time in a high-temperature steam environment at a temperature of 800 ° C. has been provided. Since this material is based on ferritic stainless steel, it is excellent in cost and mass productivity. Therefore, the present invention contributes to the practical application and performance improvement of each of the cogeneration systems that recover and effectively use the thermal energy from the exhaust gas, leading to improvement of environmental problems.

  Stainless steel containing 11% by mass or more of chromium is a metal material with a slow growth of the oxide film to be produced and less material deterioration at high temperatures. Suitable as thermal material. In the case of general stainless steel, the initial performance is usually about 18 to 30 W / m · K at 800 ° C. as the initial performance. However, when exposed to a combustion exhaust gas atmosphere containing a large amount of water vapor, there is a problem that the water vapor oxidation proceeds and the heat transfer performance is greatly deteriorated accordingly.

The inventors have studied in detail the cause of the deterioration of the heat transfer performance of the stainless steel material due to the occurrence of steam oxidation in a high temperature atmosphere. As a result, the following was found.
In a high temperature atmosphere where water vapor is present, two layers of oxides are usually formed on the stainless steel surface. A layer mainly composed of a porous spinel oxide ((Fe, Cr) ₃ O ₄ , MnCr ₂ O _4, etc.) having poor protection on the outer side, and a dense corundum oxide (Cr ₂ O ₃ ) on the inner side. Alternatively, a layer mainly composed of Al ₂ O ₃ is formed. In addition, dense SiO ₂ -based oxides may be formed in these layers.

  As the oxide grows, voids are likely to be formed between the spinel oxide layer and the corundum oxide layer, and cracks are likely to occur due to repeated heating and cooling. As a result of these phenomena, an air layer with very poor thermal conductivity or a layer close to a vacuum (separation layer) is formed between the two layers. As a result, the thermal conductivity through the oxide film of the material is greatly reduced. When the surface layer cross section of the stainless steel material whose thermal conductivity has actually decreased due to steam oxidation is observed microscopically, the formation of the separation layer as described above is observed.

As a method for solving this problem, the inventors have studied the existence ratio of a porous spinel oxide in the oxide film and constructed a dense corundum oxide or an oxide film mainly composed of SiO ₂ as a technique for solving this problem. Has been found to be extremely effective. Specifically, when the proportion of the spinel oxide in the oxide film formed on the steel surface is reduced to 15% by mass or less, a decrease in thermal conductivity is remarkably suppressed in a high-temperature steam environment at an 800 ° C. level. Even if it is repeatedly exposed to such an atmosphere for a long period of time, a good thermal conductivity of 18 W / m · K or more at 800 ° C. can be maintained.

The abundance ratio of the spinel oxide in the oxide film can be determined by X-ray diffraction (using Cu-Kα rays) of the oxide film. Here, using the diffraction intensity I _S of the spinel oxide, the diffraction intensity I _C , I _A of the corundum oxide, and the diffraction intensity I _Si of SiO ₂ ,
I _S / (I _S + I _C + I _A + I _Si ) × 100
The value obtained by converting this to mass% is defined as the existence ratio of the spinel oxide in the oxide film.
Here, I _S is the height of the maximum diffraction peak corresponding to (Fe, Cr) ₃ O ₄ or MnCr ₂ O ₄ , I _C is the height of the maximum diffraction peak corresponding to Cr ₂ O ₃ , and I _A is The maximum diffraction peak height corresponding to Al ₂ O ₃ and I _Si is the maximum diffraction peak height of SiO ₂ . The maximum diffraction peak is a diffraction peak on the (hkl) plane having the highest diffraction intensity among the diffraction peaks of each oxide. However, the peak of the (311) plane (plane spacing d = about 0.25 nm) which is the maximum diffraction peak of the spinel oxide is the second largest diffraction peak ((110) of Cr ₂ O _{3 in} the corundum oxide). And the (104) plane of Al ₂ O ₃ ). When both the spinel oxide and the corundum oxide are identified, the height of the other main peak in the corundum oxide (Cr ₂ O ₃ , Al ₂ O ₃ ) indicates that (110) of Cr ₂ O ₃ The peak height of the plane and the peak height of the (104) plane of Al ₂ O ₃ are calculated, and the calculated peak of the (110) plane of Cr ₂ O ₃ from the measured peak height in the vicinity of 0.25 nm. I _S is determined by subtracting the height and the calculated peak height of the (104) plane of Al ₂ O ₃ . When only one type of Cr ₂ O ₃ and Al ₂ O ₃ is identified (existence is confirmed), the calculated peak height of the identified type may be subtracted from the measured peak height.

In order to form such an oxide film with little spinel oxide, a corundum type oxide (Cr ₂ O ₃ or Al ₂ O ₃ ) or a dense oxide scale mainly composed of SiO ₂ is used at the earliest stage of oxidation. It came to the conclusion that it is most effective to make it the outermost layer. As a result, the outward diffusion of Fe and Mn, which are elements that easily generate spinel-based oxides, to the outermost layer side (outward direction) is suppressed, and a single-layer structure that is substantially composed of corundum-based oxides. An oxide film is formed. As a result, a separation layer between the spinel-based oxide and the corundum-based oxide is not generated, and a decrease in thermal conductivity can be minimized.

In order to form such a corundum-based oxide-based oxide film having a single layer structure, it is important to use steel whose components are adjusted as follows as the stainless steel of the base material. That is, an increase in Cr, Si or an increase in Al is effective as a component necessary for suppressing the formation of spinel oxide. As these contents, it is necessary to satisfy the following formula (1).
Cr + 3Si + 15Al> 22.0 (1)
In the composition range out of the formula (1), a spinel oxide mainly composed of Fe or Mn is easily formed at the very initial stage of oxidation, and it is difficult to stably prevent a decrease in thermal conductivity during use.

  By subjecting the ferritic stainless steel satisfying the formula (1) to an oxidation treatment of, for example, 700 to 1000 ° C. gas containing 5 to 60% by volume of water vapor and 1 to 20% by volume of oxygen, the above-described process is performed at an early stage. An oxide film having a single layer structure is formed, and outward diffusion of Fe and Mn is suppressed. The film thickness of the oxide film needs to be in the range of 0.01 to 10 μm. If it is too thin, when it is subsequently used in a high-temperature steam atmosphere, the formation of a spinel oxide may not be suppressed depending on the conditions. If it is too thick, scale peeling tends to occur, which is counterproductive to maintaining thermal conductivity. Although it is desirable that the oxidation treatment time is approximately 1 h or longer, if the actual environment matches the oxidation treatment conditions in the above range, the oxidation treatment can be performed by use (operation) in the actual environment. The water vapor concentration in the oxidation treatment atmosphere is more preferably 5 to 20% by volume, and the oxygen concentration is more preferably 5 to 10% by volume.

  In this manner, an oxide film having a spinel oxide content of 15% by mass or less is formed, and the heat transfer material for heat collection of the present invention is constructed. Thereafter, even if the surface on which the oxide film is formed is exposed to various high-temperature steam atmospheres, the oxide film structure having a thickness of 0.01 to 10 μm in which the spinel oxide concentration is suppressed to 15% by mass or less. Is maintained, and the durability of the heat transfer material for heat collection of the present invention lasts for a long period of time. When the actual use conditions match the above-described oxidation treatment conditions, the stainless steel material of the base material can be used for actual use, and can also serve as an oxidation treatment for obtaining the material of the present invention.

Prior to this oxidation treatment, it is more effective to perform a preliminary oxidation treatment in a non-steam oxidation atmosphere such as air. As a pre-oxidation treatment condition, a method of exposing a steel substrate whose composition has been adjusted to an air atmosphere of 700 to 900 ° C. for 1 hour or less can be employed. It is preferable to secure a holding time of 0.05 h or more. When this preliminary oxidation treatment is performed, even when the Al content is as low as less than 0.2%, the predetermined oxide film can be formed by oxidation treatment in a water vapor atmosphere, and the application range of steel is expanded. . However, the contents of Cr, Si, and Al need to be ensured so as to satisfy the above-described expression (1). It is preferable that the spinel oxide content in the oxide film is 10% by mass or less after the preliminary oxidation treatment is completed. Moreover, it is preferable that the oxide film thickness is 0.05-5.0 micrometers. If it is more than that, the effect of the pre-oxidation treatment is not sufficiently exhibited. When the formula (1) is not satisfied, a spinel oxide is likely to be generated even in the atmosphere, and it is difficult to suppress the amount of the spinel oxide to 10% by mass or less by pre-oxidation treatment.
Further, as a means for increasing the diffusion rate of Cr and Al to the surface layer, it is effective to finish the surface layer within the range of counts # 100 to 600 defined in JIS R6001.

  The base stainless steel may be prepared according to a normal ferritic stainless steel plate manufacturing method. The plate thickness varies depending on the application, but it is usually applicable to many applications in the range of about 0.15 to 2 mm. Pre-oxidation treatment or oxidation treatment can be performed in the state of steel plate material, but if the actual usage environment meets the above oxidation treatment conditions, the base material or the material that has been subjected to pre-oxidation treatment is directly applied. By subjecting it to actual use, oxidation treatment can be carried out. In that case, the heat transfer material for heat collection of the present invention is constructed in the initial stage of actual use.

  This heat transfer material is used by exposing the surface on which the oxide film is formed to a high-temperature steam atmosphere, and the heat absorbed from the atmosphere gas through the oxide film passes through the base material to the low-temperature substance (member (Sometimes fluid).

Hereinafter, the constituent elements of steel constituting the substrate will be described.
C and N are components that improve high-temperature strength, particularly creep properties, but if added excessively to ferritic stainless steel, workability and low-temperature toughness are significantly reduced. In addition, carbonitrides are easily generated by reaction with Ti and Nb, and solid solution Ti and solid solution Nb effective in improving high-temperature strength are reduced. In the present invention, the C and N contents are both limited to 0.03% by mass or less.

  Si is an alloy component effective for stabilizing the Cr-based oxide, and contributes to improvement of steam oxidation resistance and heat transfer performance. For that purpose, it is desirable to secure a Si content of 0.15% by mass or more. However, when an excess of Si exceeding 2.0% by mass is included, workability, particularly ductility, is remarkably lowered, and low-temperature toughness is also lowered. Moreover, it becomes easy to produce | generate a flaw on the steel surface, and manufacturability also falls.

  Mn is a component that improves the scale peel resistance of ferritic stainless steel, and is effective when contained in an amount of 0.1% by mass or more. However, if the Mn content exceeds 1.5% by mass, the steel material becomes hard, and the workability and low temperature toughness deteriorate.

  S is a component that adversely affects hot workability and weld hot cracking resistance, and also serves as a starting point for abnormal oxidation. Therefore, the S content is preferably as low as possible. In the present invention, the S content is limited to 0.008% by mass or less.

  Cr is an alloy component necessary for imparting corrosion resistance and oxidation resistance necessary for stainless steel. In order to ensure the steam oxidation resistance at around 800 ° C., it is necessary to contain 11 mass% or more of Cr. However, addition of Cr exceeding 25% by mass is not preferable because it decreases the workability, low temperature toughness and 475 ° C. embrittlement resistance of ferritic stainless steel.

  Al stabilizes Cr-based oxides and also becomes alumina-based oxides, contributing to the formation of a corundum type single layer. For this reason, Al is an important element that suppresses a decrease in thermal conductivity during use. In order to sufficiently bring out this effect, it is desirable to secure an Al content of 0.05% by mass or more. In particular, when the addition amount of Cr is low, it is necessary to increase the addition amount of Al. When the Al content is 0.2% by mass or more, a single spinel oxide concentration of 15% by mass or less is obtained only by oxidation in a predetermined high-temperature steam atmosphere without performing the above-described pre-oxidation treatment. An oxide layer can be formed. On the other hand, the addition of Al also has an adverse effect of lowering the thermal conductivity of steel (base material itself). If the Al content exceeds 6.0% by mass, the initial properties of the material are 18 W / m · K at 800 ° C. The above thermal conductivity may not be obtained. In addition, workability and low temperature toughness are also significantly reduced. The Al content is more preferably 4.5% by mass or less.

  Since Y, REM (rare earth element), Ca, and Zr are all dissolved in the oxide film and exhibit an effect of strengthening the oxide film, one or more of these elements can be added as necessary. . In order to sufficiently obtain the above action, the content of Y: 0.001% by mass or more, REM: 0.001% by mass or more, Ca: 0.001% by mass or more, Zr: 0.03% by mass or more is ensured. Thus, it is effective to add one or more of these. However, when these elements are added excessively, the steel material is not only excessively hardened, but surface flaws are liable to occur during production, leading to an increase in production cost. Therefore, when adding these elements, it is performed in the range of Y: 0.1% or less, REM: 0.1% or less, Ca: 0.01% or less, and Zr: 0.5% or less.

  Nb and Ti further improve the high-temperature strength of ferritic stainless steel by precipitation strengthening and Mo, Cu and W by solid solution strengthening, respectively, so one or more of these elements may be added as necessary. it can. In order to fully exhibit the action, Nb: 0.05% by mass or more, Ti: 0.03% by mass or more, Mo: 0.1% by mass or more, Cu: 0.1% by mass or more, W: 0.00% It is effective to add one or more of these so as to ensure a content of 1% by mass or more. However, if excessive Nb, Ti, Mo, W is included, the steel material is excessively hardened, and if excessive Cu is included, the hot workability is reduced. Therefore, when these elements are added, Nb: 0.8% or less, Ti: 0.5% or less, Mo: 4.0% or less, Cu: 4.0% or less, W: 4.0% or less.

  Other components are not particularly defined in the present invention, but it is preferable to reduce general impurity elements such as P, O, and Ni as much as possible. In the present invention, mixing is allowed in the ranges of P: 0.04 mass% or less, O: 0.02 mass% or less, and Ni: 0.6 mass% or less. When ensuring a high level of workability and weldability, the contents of P, O, and Ni can be more strictly regulated. Further, Ta effective for improving heat resistance is allowed to be contained in an amount of 1.0 mass% or less, and V is contained in an amount of 1.0 mass% or less. B, which is effective for improving hot workability, is allowed to be contained in an amount of 0.01% by mass or less, Mg is 0.05% by mass or less, and Co is 0.2% by mass or less.

  In addition, since heat transfer performance will fall if scale peeling arises, it is advantageous in the heat-collecting material for heat collection to use the ferritic stainless steel for which a scale peeling does not produce easily as a base material rather than austenitic stainless steel.

  Various stainless steels having the compositions shown in Table 1 were melted in a vacuum melting furnace and cast into ingots. After roughly rolling the ingot, No. 2D finish cold-rolled annealed steel sheet (plate thickness: 1.0 mm) defined in JIS G4305 was manufactured through hot rolling, annealing, pickling, cold rolling, and finish annealing. .

A disk-shaped sample having a plate thickness of 1.0 mm and a diameter of 10 mm was prepared from the cold-rolled annealed steel sheet by cutting, and this was used as a base material. For each steel type, the surface of some of the base materials was dry-polished with a # 400 count defined in JIS R6001, and then subjected to a pre-oxidation treatment at 800 ° C. for 30 minutes in the air. Thereafter, a gas having a composition of 20 vol% H ₂ O + 5 vol% O ₂ + balance N ₂ is flowed through the base material not subjected to the pre-oxidation treatment (the surface remains the No. 2D finish) and the material subjected to the pre-oxidation treatment. However, 10 cycles (total heating and holding time of 1000 hours) of a steam oxidation test in which “temperature rising → 800 ° C. × 100 h holding → cooling to room temperature” was taken as one cycle were performed. As for the examples of the present invention shown in Table 2 below, this steam oxidation test also serves as an “oxidation treatment” for forming an oxide film defined by the present invention, and is used by continuing the test up to 10 cycles at the same time. The durability at is evaluated.

(Measurement of thermal conductivity)
About the base material (cold-rolled annealing steel plate) and the sample after the steam oxidation test, the thermal conductivity through the oxide film was measured by a laser flash method according to JIS R1611. On one side of the disk sample, the oxide film was removed by polishing, laser light was applied from the surface with the oxide film, and the temperature of the back surface was measured. The sample was held in a vacuum vessel of 1 × 10 ⁻⁷ Pa. The thermal conductivity at 800 ° C. was measured by this method, and those with 18 W / m · K or higher were evaluated as “good” and those with less than 18 W / m · K were evaluated as “poor”. All steel types were evaluated as ○ in the sample of the base material. The results of the sample after the steam oxidation test are shown in Table 2.

[Measurement of oxide film thickness]
For the sample after the steam oxidation test, the thickness of the oxide film was examined by SEM observation of the cross-sectional structure near the surface. A film having a film thickness in the range of 0.01 to 10 μm was evaluated as ◯ (good), and a film part having a film thickness exceeding 10 μm was evaluated as x (bad). The results are shown in Table 2. In addition, there was no sample in which a film portion of less than 0.01 μm was observed.
Further, for the sample subjected to the pre-oxidation treatment, the thickness of the oxide film after the pre-oxidation treatment was examined by GDS (glow discharge emission spectroscopic analyzer). That is, the depth direction analysis was performed by GDS, and the depth at which the oxygen detection intensity decreased to ½ of the peak value was calculated from the relationship between the sputtering time and the sputtering rate, and this was defined as the oxide film thickness. As a result, all had an oxide film in the range of 0.05 to 5.0 μm.

[Measurement of spinel oxide concentration in oxide film]
The sample after the steam oxidation test and the sample after the preliminary oxidation test were further subjected to the X-ray diffraction of the oxide film by the method described above, so that the spinel oxide concentration in the oxide film was measured. Asked. As a result, for samples after the steam oxidation test, those having a spinel oxide concentration of 15% by mass or less were evaluated as ◯ (good), and those exceeding 15% by mass were evaluated as x (bad). Moreover, about the sample after a preliminary oxidation test, the thing whose spinel type oxide density | concentration is 10 mass% or less was evaluated as (circle) (good), and the thing exceeding 10 mass% was evaluated as x (bad). The results are shown in Table 2.
In addition, about the sample after a steam oxidation test, the quantitative analysis of the element in an oxide film was performed with the EDX apparatus at the time of the above-mentioned SEM observation. As a result, it was confirmed that the content of each element in the oxide film is consistent with the value of the spinel oxide concentration determined by X-ray diffraction.

As can be seen from the results in Table 2, all of the samples of the present invention have an oxide film in which the spinel oxide concentration is suppressed to 15% by mass or less after the water vapor test. The equivalent of heat transfer material for heat was built. They have a thermal conductivity at 800 ° C. of 18 W / m · K or more after 1000 hours of testing, and have excellent durability as a heat transfer material for heat collection used by being exposed to a high-temperature steam atmosphere. Is.
In particular, steel grades Nos. 1 to 9 were able to form an oxide film having a spinel oxide concentration of 15% by mass or less even without pre-oxidation treatment by setting the Al content of the base material to 0.2% by mass or more. .

  On the other hand, the samples of steel types Nos. 21 to 24 used the base material having a composition not satisfying the formula (1), so that even when pre-oxidation treatment was performed, the samples in the oxide film were exposed to high-temperature steam. The spinel oxide concentration exceeded 15% by mass, and the thermal conductivity after the high temperature oxidation test was lowered. As a result of investigation, the oxide film formed on these samples has a two-layer structure, and formation of a separation layer was observed between them. 80% by mass or more of the outer layer was occupied by the spinel oxide. Steel type No. 25 uses austenitic SUS310S, which is a general-purpose heat-resistant stainless steel, as a base material, and the thermal conductivity decreased due to the occurrence of oxide scale peeling during the test.

Claims (8)

A heat transfer material for collecting heat from a high-temperature gas containing water vapor, in mass%, C: 0.03% or less, Si: 2.0% or less, Mn: 1.5% or less, S: 0.00. 008% or less, Cr: 11-25%, Al: 0.05-6.0 %, N: 0.03% or less, balance Fe and inevitable impurities , and steel having a composition satisfying the following formula (1) And a base material, and on the surface of the base material in contact with the gas, an oxide film having a thickness of 0.01 to 10 μm with a spinel oxide concentration suppressed to 15% by mass or less is used for heat collection. Thermal material.
Cr + 3Si + 15Al> 22.0 (1)   The steel further contains one or more of Y: 0.1% or less, REM: 0.1% or less, Ca: 0.01% or less, Zr: 0.5% or less. Heat transfer material for heat collection as described in 1.   The steel further contains one or more of Nb: 0.8% or less, Ti: 0.5% or less, Mo: 4.0% or less, Cu: 4.0% or less, and W: 4.0% or less. The heat transfer material for heat collection according to claim 1 or 2, wherein the heat transfer material is used.   The heat transfer material for heat collection according to any one of claims 1 to 3, wherein the steel has an Al content of 0.2 to 6.0%.   The heat transfer material for heat collection according to any one of claims 1 to 4, wherein the heat conductivity through the oxide film is 18 W / m · K or more at 800 ° C. The collection according to claim 4 , wherein the composition-adjusted steel substrate is exposed to a gas at 700 to 1000 ° C containing 5 to 60% by volume of water vapor and 1 to 20% by volume of oxygen to form an oxide film. Manufacturing method for heat transfer material.   After subjecting the composition-adjusted steel base material to a pre-oxidation treatment in which it is exposed to an air atmosphere at 700 to 900 ° C. for 1 h or less, 700 to 1000 ° C. containing 5 to 60% by volume of water vapor and 1 to 20% by volume of oxygen. The manufacturing method of the heat-transfer material for heat collection in any one of Claims 1-5 which forms an oxide film by exposing to this gas.   The method for producing a heat transfer material for heat collection according to claim 6 or 7, wherein the composition-adjusted base material is polished and finished within a range of counts # 100 to 600 defined in JIS R6001.