JP4463663B2 - Ferritic steel material excellent in high temperature steam oxidation resistance and method of use thereof - Google Patents

Description

  The present invention relates to a ferritic steel material excellent in high-temperature strength and high-temperature steam oxidation resistance, and particularly to a ferritic steel material suitable for applications exposed to high-temperature steam such as a heat exchanger of a polymer electrolyte fuel cell. It is.

In recent years, due to problems such as the depletion of fossil fuels represented by petroleum and global warming due to CO ₂ emissions, there has been a demand for practical applications of new systems that replace conventional power generation systems and internal combustion engines. As one of them, a polymer electrolyte fuel cell (PEFC) that is promising as a power source for automobiles, home cogeneration systems, and the like has attracted attention.

  Although the battery operating temperature of PEFC is as low as about 70 to 100 ° C., it is necessary to rapidly raise the operating temperature to the operating temperature at the time of starting in order to shorten the starting time. In addition, it is important to improve the energy efficiency of the system in order to cover energy in a limited installation space. Furthermore, in a type in which hydrogen is supplied by reforming a hydrocarbon-based fuel such as natural gas or gasoline, it is necessary to operate the reformer at a high temperature of about 800 ° C. For this reason, the necessity of the high temperature heat exchanger which has durability at the high temperature of 500-800 degreeC is increasing in the use for a motor vehicle, and a cogeneration system.

  FIG. 1 schematically shows the structure of an orthogonal heat exchanger as an example of a high-temperature heat exchanger. The fins 2 made of corrugated plates are alternately sandwiched between the flat plates 1 so that the fluid passage directions are orthogonal to each other. Each plate 1 and fin 2 are usually brazed using Ni solder or the like. The plate 1 is made of a metal plate having a thickness of about 0.1 to 2 mm, and the fin 2 is made of a metal plate having a thickness of about 0.01 to 1 mm. Conventionally, a high Cr / high Ni heat resistant stainless steel such as SUS310S or an incoloy 800 series iron-based high alloy has been used for these metal members.

In a fuel cell system (PEFC type), combustion exhaust gas of a burner generated in a reformer is cooled by a heat exchanger, and the heat is used for a humidifier that humidifies hydrogen gas, for example. The combustion exhaust gas of the burner is as high as 500 to 800 ° C., and the gas component contains a large amount of H ₂ O (water vapor) and O ₂ , H ₂ , CO ₂ , CO, HC, etc. are mixed. On the other hand, the fluid receiving the heat is also rich in water and water vapor. Therefore, the heat exchanger for this application must have excellent durability in an environment exposed to high-temperature steam and hydrogen. Moreover, since it is frequently started up and stopped once a day in automobile applications and also in a household cogeneration system, it is desired to be made of a material having a low thermal expansion coefficient.

  Since SUS310S, which is a conventional material, is austenitic, it is inherently more resistant to high-temperature steam than ferritic steel types and has a relatively high high-temperature strength. However, there are many problems such as high material cost and large thermal expansion coefficient. Incoloy 800 series is even more expensive. Cost reduction is one of the important key points for popularizing fuel cell systems for automobiles and household cogeneration systems, so heat exchangers can be made of inexpensive ferritic steel grades. desired. Ferritic steel is also advantageous in terms of thermal expansion coefficient.

Patent Document 1 below describes ferritic stainless steel considering the environment of a petroleum fuel reformer exposed to a high-temperature steam atmosphere. This employs means for increasing the contents of Si and Al in order to improve the high temperature steam oxidation resistance.
Patent Document 2 also shows an example in which ferritic stainless steel is applied to a fuel reformer. This improves steam oxidation resistance by containing 2.5% or more of Al.

  On the other hand, Patent Document 3 describes ferritic stainless steel for heat exchangers. However, use in a high-temperature steam atmosphere is not assumed, and a method for realizing a ferritic steel material that can endure as a high-temperature heat exchanger of a fuel cell system is unknown.

JP 2003-160840 A JP 2003-286005 A JP 7-292446 A

As described above, the cost reduction of the high temperature heat exchanger is indispensable for the spread of the fuel cell system. Conventional SUS310S and Incoloy 800 series alloys are expensive, and there is a strong demand for replacement with inexpensive materials. In addition, it is desired to use a ferritic steel type having a small thermal expansion coefficient.
The ferritic steels disclosed in Patent Documents 1 and 2 are intended to improve steam oxidation resistance. However, since the Al content is high, the brazing property is inferior. Therefore, these are not optimal materials for heat exchangers. In addition, since the Si and Al contents are high, the cost is still too high for use as a heat exchanger in an automotive fuel cell system.
On the other hand, even if the ferritic steel of Patent Document 3 is used as it is in a high-temperature steam atmosphere, sufficient durability cannot be expected.

  In view of such a current situation, an object of the present invention is to develop and provide a steel material imparted with excellent durability that can be used in a high-temperature steam atmosphere of 500 to 800 ° C. using an inexpensive ferritic steel type.

As a result of various studies, the inventors have found that even in the case of an inexpensive ferritic stainless steel, durability in a high-temperature steam atmosphere can be remarkably improved by any of the following methods.
[Method A] The surface of the ferritic steel material is used by exposing it to a high-temperature steam atmosphere in a state where polishing strain is applied.
[Method B] A surface of the ferritic steel material is subjected to a heat treatment in a specific oxidizing atmosphere in a state where polishing distortion is applied, and this is used by being exposed to a high-temperature steam atmosphere.

Method A is to quickly form a stable protective film at the initial stage when exposed to the actual use environment due to the action of polishing strain, and this protective film has excellent high-temperature steam oxidation resistance over a long period thereafter. It bears.
In Method B, a protective film exhibiting excellent high-temperature steam oxidation resistance is formed in advance by the action of polishing distortion, and this is used by exposing it to an actual high-temperature steam oxidation atmosphere.

Needless to say, Method A is advantageous in terms of cost if the steel material after polishing can be used as it is. However, in the case of a heat exchanger member, after being subjected to a brazing process (for example, Ni brazing), it is provided for actual use. Ni brazing is generally performed by keeping the entire component at a high temperature of about 1000 ° C. In this case, the applied polishing distortion is removed, and the high temperature steam oxidation resistance is lost. Method B is extremely useful for solving this problem. This is because the heat treatment during brazing can also serve as a treatment for forming a protective film.
The present invention has been completed based on these findings.

That is, as a means for achieving the above-mentioned object along the method B, as a steel material imparted with high temperature steam oxidation resistance, in mass%, C: 0.03% or less, Si: more than 0.1 to 1.5%, Mn: 1.5% or less, Cr: 9 to 24%, S: 0.01% or less, N: 0.03% or less, Al: 0 to 1.0%, Mo: 0 to 3.0%, Nb : 0 to 0.80%, Ti: 0 to 0.50%, Cu: 0 to 2.0%, with the balance being Fe and unavoidable impurities, with the surface imparting polishing strain,
For example, a heat treatment for holding for 10 to 3600 seconds in a reduced pressure atmosphere of 900 to 1250 ° C. in which the atmosphere is evacuated or replaced with an inert gas so that the oxygen partial pressure becomes 1 × 10 ⁻⁵ to 2 × 10 ⁻¹ Pa.
There is provided a ferritic steel material excellent in high-temperature steam oxidation resistance obtained by applying the above.

  Here, Al, Mo, Nb, Ti, and Cu are arbitrarily added elements. The lower limit of 0% means that the element is not added and the content is not more than the measurement limit by an analysis method in a normal steelmaking field.

  As means for imparting the polishing strain, i) means for polishing and finishing with a number of 50 or more as defined in JIS R 6001, or ii) means for performing No. 4 polishing finish as defined in JIS G 4305, Can be adopted.

As these steel materials excellent in high temperature steam oxidation resistance, particularly when subjected to a high temperature steam test for 200 hours in an atmosphere of 70% by volume H ₂ O + 30% by volume CH ₄ and 600 ° C., the mass before and after the test What has high-temperature steam oxidation resistance that increases to 0.2 mg / cm ² or less is provided.
As an application of the steel material, a heat exchanger member that comes into contact with high-temperature steam at 500 to 800 ° C can be mentioned.

  Moreover, as a usage mode of the steel material according to the method A, the steel material having the above-described chemical composition and having a surface subjected to polishing strain is used by being exposed to an environment in contact with high-temperature steam at 500 to 800 ° C. Provided is a method for using a ferritic steel material excellent in water vapor oxidation.

Furthermore, as a use mode of the steel material according to the method B, after applying polishing strain to the surface of the steel material having the above chemical composition, the atmosphere is evacuated or replaced with an inert gas so that the oxygen partial pressure is 1 × 10 ^−5. A steel material that has been heat-treated for 10 to 3600 seconds in an atmosphere at 900 to 1250 ° C. that is adjusted to 2 × 10 ⁻¹ Pa is used by being exposed to an environment that is in contact with high-temperature steam at 500 to 800 ° C. A method of using a ferritic steel material having excellent high-temperature steam oxidation is provided.

The present invention has the following merits.
(1) Since the material is an inexpensive ferritic steel, it can contribute to cost reduction of various structures exposed to high-temperature steam.
(2) Since ferritic steel has a small coefficient of thermal expansion, it is advantageous for applications where the temperature is repeatedly increased and decreased.
(3) Excellent resistance to high temperature steam oxidation can be realized even in a structure made through high temperature heating (Ni brazing or the like) like a heat exchanger for a fuel cell.
(4) The brazing property is good because the method of increasing Al is not used.
Therefore, the present invention can greatly contribute to the spread of fuel cell systems for automobile and household cogeneration systems.

  Ferritic steel has a smaller coefficient of thermal expansion than austenite, and is excellent in thermal fatigue characteristics. However, when exposed to a high temperature atmosphere containing a large amount of water vapor, oxidation generally proceeds more easily than austenitic steel. That is, the amount of oxidation increases in the temperature range of 500 to 600 ° C., and a large amount of reddish brown scale (commonly known as “red scale”) is generated. This oxidation leads to holes in the member, and the generated red scale causes troubles such as clogging of the piping system.

  The mechanism of steam oxidation has not been fully elucidated. However, it has been found that steam oxidation can be suppressed by stabilizing the initial oxide film formed on the surface of the stainless steel material when exposed to a high-temperature steam atmosphere. The combined addition of Si and Al disclosed in Patent Document 1 and the increase in Al disclosed in Patent Document 2 are means for realizing stabilization of such an oxide film.

The inventors of the present invention have studied a method of forming an initial film that exhibits excellent protective action in a high-temperature steam atmosphere without depending on the addition of Si or Al. As a result, it was found that a technique of exposing to a “high temperature steam atmosphere as a use environment” or “oxidizing high temperature atmosphere” with polishing distortion on the surface is extremely effective.
When the surface of the ferritic steel is mechanically polished, a number of dislocations and slip bands are formed on the metal surface layer, and polishing strain can be introduced to a depth of about 50 to 100 μm from the surface layer. It is considered that the distortion in the surface layer portion promotes the diffusion of Cr and Si to the surface layer during high-temperature heating, and as a result, forms a protective oxide film on the steel material surface layer at the very initial stage of oxidation.

[Chemical composition of steel]
The alloy components of the ferritic steel targeted in the present invention will be described.
Cr is an important component for ensuring the corrosion resistance and oxidation resistance of steel. In order to improve the high temperature steam oxidation resistance at around 600 ° C., 9 mass% or more of Cr is required. However, if it exceeds 24 mass%, the workability and the low-temperature toughness deteriorate, and 475 ° C embrittlement tends to occur, which is not preferable. In order to ensure high corrosion resistance as stainless steel, the Cr content is preferably 11% by mass or more, and more preferably 14% by mass or more. Further, when importance is attached to low temperature toughness and impact resistance due to vibration, the Cr content is preferably 20% by mass or less.

  C and N are components that improve high-temperature strength, particularly creep properties, but if added excessively to ferritic steel, workability and low-temperature toughness may be significantly degraded. 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. Therefore, in the present invention, both C and N are limited to 0.03 mass% or less.

  Mn is a component that improves the scale peel resistance of the ferritic steel. However, if it exceeds 1.5% by mass, the steel material becomes hard, and the workability and low temperature toughness are reduced.

  S is a component that adversely affects hot workability and weld hot crack resistance. It also serves as a starting point for abnormal oxidation. For this reason, S content is restrict | limited to 0.01 mass% or less.

  Si is an effective component for forming a protective film that suppresses the progress of high-temperature steam oxidation. That is, it is thought that it concentrates in the oxide film itself by the heat treatment after polishing, and also effectively acts to stabilize the Cr-based oxide formed in the film. Such an effect is brought about by the Si content exceeding 0.1% by mass, but it is preferable to ensure a Si content of 0.2% by mass or more. However, if Si is added in excess of 1.5% by mass, workability, particularly ductility, is remarkably lowered, and low-temperature toughness is also lowered. Moreover, it becomes easy to produce wrinkles on the steel material surface, and manufacturability also deteriorates.

  Al is effective in improving the steam oxidation property, but deteriorates the brazing property. Excessive Al content also degrades weldability. Therefore, when adding Al, it is necessary to carry out in the content range of 1.0 mass% or less. More preferably, it is less than 0.6 mass%.

  Mo improves the high-temperature strength and heat fatigue resistance by solid solution strengthening, and can be added as necessary when importance is attached to these characteristics. However, since excessive Mo addition hardens the steel material, it is necessary to add Mo in a range of 3.0% by mass or less.

  Nb and Ti further improve the high-temperature strength of the ferritic steel by precipitation strengthening, and can be added as necessary. In order to sufficiently exhibit the above action, it is desirable that the content be 0.05 mass% or more in the case of Nb and 0.03 mass% or more in the case of Ti. However, if these elements are added excessively, the steel material becomes hard, so it is necessary to add Nb in a range of 0.80% by mass or less and Ti in a range of 0.50% by mass or less. Nb and Ti may be added in combination within the above range.

  Since Cu further improves the high-temperature strength of ferritic steel by precipitation strengthening or solid solution strengthening, it can be added as necessary. In order to fully exhibit the said effect | action, it is desirable to set it as 0.1 mass% or more content. However, when Cu is added excessively, the steel material becomes hard, so when adding Cu, it is necessary to carry out within a range of 2.0 mass% or less.

Other components are not particularly defined, but it is desirable to reduce general impurity elements such as P, O, and Ni as much as possible. P is allowed up to 0.04% by mass, O up to 0.02% by mass and Ni up to 0.6% by mass. In order to secure a high level of workability and weldability, P, O, and Ni may be more strictly regulated.
Elements such as W, Ta, V, Zr effective for improving heat resistance and B, Mg, Co, etc. effective for improving hot workability can be added as required.

  After imparting polishing strain to the surface of a ferritic steel material having the above chemical composition, it is exposed to a high-temperature steam atmosphere that is the environment in which it is used (Method A) or held in advance in an oxidizing high-temperature atmosphere (Method B). ), Imparts high temperature steam oxidation resistance. Hereinafter, these methods will be described.

[Method A]
First, a steel material (for example, a steel plate) is manufactured by a normal method, and polishing distortion is applied to the surface. Specifically, polishing distortion can be introduced by polishing and finishing with a number of 50 or more as defined in JIS R 6001. If the polishing count is too high (fine eyes), the productivity will be inferior. Polishing with different counts may be performed sequentially. Moreover, sufficient polishing distortion is imparted by applying No. 4 polishing finish defined in JIS G 4305.

Next, by exposing the steel material having such a polishing strain to a high-temperature steam atmosphere of 500 to 800 ° C., which is the use environment, the diffusion of Cr and Si to the surface is significantly faster than in the case where the polishing is not performed. Become. As a result, an initial oxide film enriched with Cr and Si is rapidly formed. This Cr, Si-based oxide film significantly suppresses subsequent steam oxidation. For example, when a high temperature steam resistance test is performed for 200 hours in an atmosphere of 70% by volume H ₂ O + 30% by volume CH ₄ and 600 ° C., no red scale is observed on the surface, and the mass increase before and after the test A material exhibiting excellent high temperature steam oxidation resistance such as 0.2 mg / cm ² or less is obtained.

Here, whether or not the mass increase is 0.2 mg / cm ² or less when held for 200 hours is whether or not abnormal oxidation can be prevented in the high-temperature steam atmosphere, that is, the continuous “red scale” This is a standard for evaluating whether or not the occurrence of a problem can be prevented. That is, if the mass increase after being exposed to the 600 ° C. high temperature steam atmosphere for 200 hours is 0.2 mg / cm ² or less, the ferritic steel material has a high temperature steam atmosphere of 500 to 800 ° C. It has become clear from numerous experiments that it has durability that does not cause abnormal oxidation. On the contrary, a steel material whose mass increase exceeds 0.2 mg / cm ² under the above conditions is maintained for a longer time or at a high temperature such as 700 ° C. or 800 ° C. even if abnormal oxidation does not occur in 200 hours. As a result, abnormal oxidation often occurs, and it cannot be determined that the steel material has stable high-temperature steam oxidation resistance. In this sense, it can be said that the numerical value “mass increase 0.2 mg / cm ² ” in the experiment under the above conditions is a critical value that determines whether or not abnormal oxidation occurs.

  In Method A, the polished steel material is applied to a steam atmosphere without being subjected to a heat treatment in a high-temperature reducing atmosphere (for example, a 100% hydrogen atmosphere at 400 ° C. or higher) so that polishing distortion of the surface layer is not removed after polishing. Is essential.

[Method B]
First, a steel material into which polishing strain is introduced is prepared by the same method as in the method A. Next, the steel material is subjected to heat treatment for 10 to 3600 seconds in an oxidizing atmosphere having an oxygen partial pressure of 1 × 10 ⁻⁵ to 2 × 10 ⁻¹ Pa and 900 to 1250 ° C. By this heat treatment, a Cr, Si-based oxide film similar to the above can be rapidly formed.

  This heat treatment condition is a condition that can be sufficiently used for the heat treatment during Ni brazing. That is, there is a great merit of the method B in that the Ni brazing treatment can be used simultaneously as a heat treatment for imparting high temperature steam oxidation resistance. Therefore, Method B is very suitable for members that require Ni brazing, such as high temperature heat exchanger members for fuel cell systems.

Ni wax can be melted by setting the heating temperature to 900 ° C. or more and the holding time to 10 seconds or more. However, when it becomes 1250 degreeC or more, the crystal grain of steel materials will coarsen and intensity | strength will fall remarkably. Further, heating for a long time of 3600 seconds or more is uneconomical.
If the oxygen partial pressure is less than 1 × 10 ⁻⁵ Pa, sufficient oxygen is not supplied to form a protective film on the surface. On the other hand, the higher the oxygen partial pressure, the better the oxide film is formed. However, when Ni brazing is selected as the heat treatment method of Method B, the oxygen partial pressure is set to 2 × to achieve good brazing. It is necessary to set it as 10 < ^-1 > Pa or less. That is, when the oxygen partial pressure exceeds 2 × 10 ⁻¹ Pa, the Ni brazing material and the material are oxidized before the brazing melt, and good bonding cannot be performed.
This heat treatment atmosphere needs to be an oxidizing atmosphere. Such an atmosphere can be easily obtained by evacuating the air, but it can also be obtained by substituting with an inert gas such as N or Ar.

In this way, the steel material on which the protective film has been formed in advance has already been freed from polishing distortion, but when exposed to a high-temperature steam atmosphere at 500 to 800 ° C. in this state, the progress of steam oxidation is remarkably suppressed and abnormal. Oxidation is prevented. Specifically, in the same manner as described above, when a high temperature steam resistance test was performed for 200 hours in an atmosphere of 70% by volume H ₂ O + 30% by volume CH ₄ and 600 ° C., the mass increase before and after the test was 0.2 mg / kg. It exhibits excellent high-temperature steam oxidation resistance such as cm ² or less.

  After the ferritic steel having the composition shown in Table 1 is melted in a 30 kg vacuum melting furnace, a cold rolled annealed steel sheet having a thickness of 1.5 mm is manufactured through rough rolling, hot rolling, annealing, pickling, cold rolling, and finish annealing. did.

Of the cold-rolled annealed steel plates, the following three types of finishing materials were prepared using steel plates 1 to 5 having a chemical composition in the range specified in the present invention.
[1] A cold-rolled annealed steel sheet having a No. 2D finish defined in JIS G 4305.
[2] Polished steel sheet with No. 4 finish specified in JIS G 4305.
[3] Polished steel sheet polished with No. 400 according to JIS R 6001.
About these [1]-[3] finishing materials, high temperature steam oxidation resistance was investigated by the following two methods.
i) A method of directly holding in a high-temperature steam atmosphere (600 ° C., 70% by volume H ₂ O + 30% by volume CH ₄ ) for 200 hours.
ii) After performing a heat treatment of holding at 1150 ° C. for 3 minutes under a reduced pressure atmosphere in which the atmosphere is evacuated and the oxygen partial pressure is 2 × 10 ⁻⁵ Pa (total pressure is 1 × 10 ⁻⁴ Pa) , A method of holding in a high-temperature steam atmosphere (600 ° C., 70% by volume H ₂ O + 30% by volume CH ₄ ) for 200 hours.

The high temperature steam atmosphere in the above method i) ii) considers the usage environment of the high temperature heat exchanger of the automotive fuel cell system. The high-temperature steam oxidation resistance was evaluated as follows in both i) and ii) by the change in mass per unit area of the test piece before and after being held in the high-temperature steam atmosphere.
○: Mass increase is 0.2 mg / cm ² or less ×: Mass increase is more than 0.2 mg / cm ^{2 In} this case, ○ is evaluated that abnormal oxidation occurs in a high-temperature steam atmosphere at 500 to 800 ° C. It can be determined that it exhibits excellent durability (described above) and is determined to be acceptable. The results are shown in Table 2.

As can be seen from Table 2, in the case of the 2D finishing material into which no polishing strain was introduced, the high temperature steam oxidation resistance could not be improved.
On the other hand, the No. 4 finish and the No. 400 polishing finish were able to impart excellent high temperature steam oxidation resistance in both the methods i) and ii). In this case, i) corresponds to the “method A”, and ii) corresponds to the “method B”.

For the five steel types of steel 4, steel 5, steel 6 to steel 8 in Table 1, cold rolling, annealing and pickling were further performed using the cold-rolled annealed steel plate (plate thickness 1.5 mm), and the plate thickness was set to 0.00. A steel plate having a thickness of 5 mm and a steel plate having a thickness of 0.2 mm was prepared, and a No. 4 polishing finish defined in JIS G 4305 was applied to these surfaces as a final finish. This 0.2 mm thick plate is processed into a corrugated shape by press working to form a “fin”, and a 0.5 mm thick plate is designated as a “plate”. A plate type heat exchanger was prepared. The distance between both plates is about 1.5 mm. The plate and fin were joined by Ni brazing. BNi-5 is used as the Ni brazing, and the heating conditions during brazing are 1080 under a reduced pressure atmosphere in which the atmosphere is evacuated and the oxygen partial pressure is 4 × 10 ⁻² Pa (total pressure is 2 × 10 ⁻¹ Pa). After holding at 10 ° C. for 10 minutes, the temperature was further raised and held at 1160 ° C. for 10 minutes. A durability test piece having a width of 10 mm and a length of 50 mm was cut out from the obtained plate-type heat exchanger. In this case, a corrugated cut of the fin appears in a cross section perpendicular to the width direction.

The endurance test was performed by a method in which a cycle of “retaining 600 ° C. × 25 minutes in air containing 10% water vapor → water cooling for 10 minutes” was repeated 1000 cycles. Appearance observation and microscopic observation were performed on the test piece after the durability test, and the presence or absence of deformation or cracking of the plate and the fin was examined.
With regard to the plate, focusing on the phenomenon that the plate bends outward or inward at the end face of the plate due to thermal fatigue, it was evaluated as follows, and the evaluation was accepted.
○: The plate has a deformation angle of 10 ° or less. ×: The plate has a deformation angle of more than 10 °. The fins were evaluated as follows depending on the presence or absence of cracks caused by thermal fatigue, and the evaluation was “good”.
○: No cracks due to thermal fatigue are observed ×: Cracks due to thermal fatigue are observed Further, regarding high temperature steam oxidation resistance, the plate portion and fin portion after the endurance test are generally observed and The evaluation was evaluated as pass.
○: No erosion exceeding 5 μm in depth X: No erosion exceeding 5 μm in depth is shown in Table 3.

As can be seen from Table 3, in the heat exchangers using steels 4 and 5, the plate deformation and fin cracks were not found, and erosion exceeding a depth of 5 μm due to steam oxidation was not observed.
On the other hand, in the examples using steel 6 (equivalent to SUS304), steel 7 (equivalent to SUS316L), steel 8 (equivalent to SUS310S), which are austenitic steels, erosion exceeding a depth of 5 μm due to steam oxidation is observed. In addition, the plate portion was greatly deformed. In Steel 6 and Steel 7, cracks due to thermal fatigue occurred in the fin portion.

The perspective view which represented typically the structure of the orthogonal type heat exchanger.

Explanation of symbols

1 plate 2 fins

Claims (7)

In mass%, C: 0.03% or less, Si: more than 0.1 to 1.5%, Mn: 1.5% or less, Cr: 9 to 24%, S: 0.01% or less, N: 0 0.03% or less, Al: 0 to 1.0%, Mo: 0 to 3.0%, Nb: 0 to 0.80%, Ti: 0 to 0.50%, Cu: 0 to 2.0%, For the steel material whose balance is made of Fe and inevitable impurities and whose surface is subjected to polishing distortion,
A heat treatment in which an oxygen partial pressure of 1 × 10 ⁻⁵ Pa or higher and 900 to 1250 ° C. is maintained for 10 to 3600 seconds;
Ferritic steel material with excellent high-temperature steam oxidation resistance obtained by applying. In mass%, C: 0.03% or less, Si: more than 0.1 to 1.5%, Mn: 1.5% or less, Cr: 9 to 24%, S: 0.01% or less, N: 0 0.03% or less, Al: 0 to 1.0%, Mo: 0 to 3.0%, Nb: 0 to 0.80%, Ti: 0 to 0.50%, Cu: 0 to 2.0%, For the steel material having the chemical composition composed of Fe and unavoidable impurities in the balance and polished and finished with a number 50 or higher as defined in JIS R 6001,
Heat treatment for holding for 10 to 3600 seconds in an atmosphere of 900 to 1250 ° C. in which the atmosphere is evacuated or replaced with an inert gas so that the oxygen partial pressure becomes 1 × 10 ⁻⁵ to 2 × 10 ⁻¹ Pa.
Ferritic steel material with excellent high-temperature steam oxidation resistance obtained by applying. In mass%, C: 0.03% or less, Si: more than 0.1 to 1.5%, Mn: 1.5% or less, Cr: 9 to 24%, S: 0.01% or less, N: 0 0.03% or less, Al: 0 to 1.0%, Mo: 0 to 3.0%, Nb: 0 to 0.80%, Ti: 0 to 0.50%, Cu: 0 to 2.0%, For the steel material having the chemical composition composed of Fe and inevitable impurities and having a No. 4 polishing finish defined in JIS G 4305,
Heat treatment for holding for 10 to 3600 seconds in an atmosphere of 900 to 1250 ° C. in which the atmosphere is evacuated or replaced with an inert gas so that the oxygen partial pressure becomes 1 × 10 ⁻⁵ to 2 × 10 ⁻¹ Pa.
Ferritic steel material with excellent high-temperature steam oxidation resistance obtained by applying. 70% by volume H ₂ O + 30% by volume CH ₄ , when subjected to a high temperature steam resistance test held for 200 hours in an atmosphere of 600 ° C., the mass increase before and after the test is 0.2 mg / cm ² or less. 3. A ferritic steel material excellent in high temperature steam oxidation resistance according to 3.   The high-temperature steam oxidation resistance according to claim 1, wherein the ferritic steel material excellent in high-temperature steam oxidation resistance is used for a heat exchanger member that contacts high-temperature steam at 500 to 800 ° C. Ferritic steel.   In mass%, C: 0.03% or less, Si: more than 0.1 to 1.5%, Mn: 1.5% or less, Cr: 9 to 24%, S: 0.01% or less, N: 0 0.03% or less, Al: 0 to 1.0%, Mo: 0 to 3.0%, Nb: 0 to 0.80%, Ti: 0 to 0.50%, Cu: 0 to 2.0%, A ferritic steel material excellent in high-temperature steam oxidation resistance, wherein the balance is made of Fe and inevitable impurities, and the steel material having a surface subjected to polishing strain is exposed to an environment in contact with high-temperature steam at 500 to 800 ° C. how to use. In mass%, C: 0.03% or less, Si: more than 0.1 to 1.5%, Mn: 1.5% or less, Cr: 9 to 24%, S: 0.01% or less, N: 0 0.03% or less, Al: 0 to 1.0%, Mo: 0 to 3.0%, Nb: 0 to 0.80%, Ti: 0 to 0.50%, Cu: 0 to 2.0%, After applying polishing strain to the surface of the steel material, the balance of which is Fe and inevitable impurities, the atmosphere is replaced with vacuum exhaust or inert gas, and the oxygen partial pressure becomes 1 × 10 ⁻⁵ to 2 × 10 ⁻¹ Pa. A ferritic material excellent in high-temperature steam oxidation resistance, which is used by exposing a steel material subjected to heat treatment in an atmosphere of 900 to 1250 ° C. for 10 to 3600 seconds to be exposed to an environment in contact with high-temperature steam at 500 to 800 ° C. How to use steel.

Images