JP6575265B2 - Austenitic stainless steel - Google Patents

Description

  The present invention relates to stainless steel, and more particularly to austenitic stainless steel.

In recent years, interest in environmental issues such as global warming has increased. Therefore, power plants are required to reduce CO ₂ emissions during operation. As a method for reducing CO ₂ emissions, for example, a coal-fired power plant employs a method of increasing the temperature and pressure of steam.

  Steel pipes for boilers are used for heater tubes and reheater tubes of power plant boilers. With the increase in steam temperature and pressure, boiler steel pipes are required to have not only high-temperature strength but also resistance to high-temperature oxidation by steam (steam oxidation resistance).

  Techniques for improving the steam oxidation resistance of steel pipes are disclosed in, for example, JP-A-58-133352 (Patent Document 1), JP-A-49-135822 (Patent Document 2), JP-A-52-8930 ( JP-A-63-54598 (Patent Document 4), JP-A-2004-132437 (Patent Document 5), and JP-A-2009-68079 (Patent Document 6).

[Technology to refine steel structure]
Patent Document 1 discloses a steel pipe whose steam oxidation resistance is improved by refining the structure. The austenitic stainless steel pipe disclosed in Patent Document 1 has a coarse grain structure having an average grain size number of 6 or less and a fine grain layer having a grain size number of 7 or more on the inner surface side. The (C + N) content of the fine-grained layer portion of the austenitic stainless steel pipe disclosed in Patent Document 1 is 0.15% or more.

[Technology for spraying the surface]
Patent Documents 2 to 4 disclose a technique for improving the steam oxidation resistance of a steel pipe by spraying the surface. Specifically, Patent Document 2 and Patent Document 3 disclose a manufacturing method for performing peening. The austenitic stainless steel disclosed in Patent Document 2 is subjected to particle spray peening by a fluid on the surface of the austenitic stainless steel after the final heat treatment or hot rolling.

  The austenitic stainless steel disclosed in Patent Document 3 is subjected to particle spray peening by a fluid at a predetermined spray pressure and spray amount on the austenitic stainless steel surface after final heat treatment or hot rolling. For the particles, carbon steel, alloy steel or stainless steel is used.

  Patent Document 4 discloses a manufacturing method for performing shot blasting. The stainless steel tube disclosed in Patent Document 4 is first removed from the existing boiler and subjected to solution heat treatment. Thereafter, the stainless steel tube is subjected to chemical cleaning for the purpose of descaling the inner surface. Furthermore, shot blasting is performed on the inner surface of the stainless steel tube for the purpose of forming a cold-worked layer.

[Technology that provides cold working with high degree of processing]
Patent Document 5 and Patent Document 6 disclose a technique for improving the steam oxidation resistance of a steel pipe by imparting a high workability cold work to the steel pipe. Specifically, in the method disclosed in Patent Document 5, ultrasonic shock treatment is performed on the inner surface of an austenitic heat-resistant steel pipe containing 5 to 30% Cr by mass.

  Patent Document 6 discloses a steel pipe containing 8 to 28% by mass of Cr and having a processed layer on the inner surface. The processed layer has a Vickers hardness at a position where the depth from the inner surface of the steel pipe is 20 μm, and the depth from the inner surface is 1.5 times or more of the Vickers hardness at a position where t / 2 (t is the thickness of the steel pipe). It has the hardness which becomes.

JP 58-133352 A JP-A-49-135822 JP 52-8930 A JP-A-63-54598 JP 2004-132437 A JP 2009-68079 A

  However, the steel pipe having a fine particle layer disclosed in Patent Document 1 may have low steam oxidation resistance in a high temperature environment of 700 ° C. or higher. Similarly, steel manufactured by the methods disclosed in Patent Documents 2 to 6 may have low steam oxidation resistance in a high temperature environment of 700 ° C. or higher.

  In the power generation plant, operation at about 800 ° C. is expected in the future for the purpose of further improving the power generation efficiency. Accordingly, steel used for power plant applications is required to have excellent steam oxidation resistance even in a high temperature environment of 700 ° C. or higher.

  An object of the present invention is to provide an austenitic stainless steel having excellent steam oxidation resistance even in a high temperature environment of 700 ° C. or higher.

The austenitic stainless steel according to the present embodiment is, in mass%, C: 0.02% or less, Si: 1.5% or less, Mn: 1.0% or less, P: 0.020% or less, S: 0.00. Contains 010% or less, Cr: 15 to less than 20%, Ni: less than 25 to 45%, Nb: 2.3 to 7.0%, and N: 0.030% or less, with the balance being Fe and impurities The expression (1) is satisfied.
Ni (sf) / Ni (cp)> 1.00 (1)
Here, Ni content (mass%) in the range from the surface of the austenitic stainless steel to a depth of 5 μm is substituted for Ni (sf) in the formula (1), and the austenitic stainless steel is substituted for Ni (cp). The average Ni content (mass%) of the entire steel is substituted.

  The austenitic stainless steel according to the present embodiment has excellent steam oxidation resistance even in a high temperature environment of 700 ° C. or higher.

FIG. 1 is a cross-sectional view including the surface of a conventional austenitic stainless steel used in a high temperature steam oxidation atmosphere. FIG. 2 is a cross-sectional view including the surface of the austenitic stainless steel of this embodiment used in a high temperature steam oxidation atmosphere.

  The present inventors investigated and examined the steam oxidation resistance of austenitic stainless steel in a steam oxidation atmosphere at 700 ° C. or higher (hereinafter referred to as a high temperature steam oxidation atmosphere), and obtained the following knowledge.

  When austenitic stainless steel is used in a high temperature steam oxidation atmosphere, oxide scale 2 is formed on the surface of the austenitic stainless steel as shown in FIG. The oxide scale 2 includes an inner layer oxide scale 3 and an outer layer oxide scale 4.

The inner layer oxide scale 3 is formed on the surface of the austenitic stainless steel 1. The inner oxide scale 3 is mainly composed of a mixed oxide of Fe and Cr. The inner layer oxide scale 3 contains, for example, 80% by volume or more of (Fe, Cr) ₃ O ₄ . The outer layer oxide scale 4 is formed on the inner layer oxide scale 3. The outer oxide scale 4 is mainly composed of an Fe-based oxide. The outer layer oxide scale 4 contains, for example, 80% by volume or more of Fe ₃ O ₄ .

  In the case of conventional austenitic stainless steel, oxygen penetrates into the surface layer of the austenitic stainless steel from the outside with the use time in the high temperature steam oxidation atmosphere. Further, Fe moves from the inside of the austenitic stainless steel to the oxide scale. Therefore, the oxide scale grows.

In the austenitic stainless steel, if the Ni content of the surface layer is made higher than the average Ni content of the whole steel, that is, if the austenitic stainless steel satisfies the formula (1), the oxide scale in the high temperature steam oxidation atmosphere Growth can be suppressed.
Ni (sf) / Ni (cp)> 1.00 (1)
Here, the Ni content (mass%) in the range from the surface of the austenitic stainless steel to the depth of 5 μm is substituted for Ni (sf) in the formula (1), and the entire austenitic stainless steel is substituted for Ni (cp). The average Ni content (% by mass) is substituted.

  When an austenitic stainless steel in which the Ni content in the surface layer is higher than the average Ni content of the entire steel is used in a high-temperature steam oxidation atmosphere, the oxide scale 20 shown in FIG. 2 is formed.

  In the oxide scale 20 of FIG. 2, a metal Ni layer 50 is formed at the interface between the inner layer oxide scale 30 and the outer layer oxide scale 40. Here, the metal Ni layer refers to a layer having an Ni content of 80% by mass or more and the balance being an oxide of Fe or Cr. The configurations of the inner layer oxide scale 30 and the outer layer oxide scale 40 are the same as the inner layer oxide scale 3 and the outer layer oxide scale 4.

  The metal Ni layer suppresses the growth of oxide scale and improves the steam oxidation resistance. The reason for this is not clear, but the following reasons can be considered. The diffusion capacity of oxygen and Fe in the metal Ni layer is smaller than the diffusion capacity of oxygen and Fe in the oxide scale. Therefore, the metal Ni layer suppresses the entry of oxygen and suppresses the movement of Fe to the oxide scale. As a result, the growth of oxide scale is suppressed and the steam oxidation resistance is enhanced.

The austenitic stainless steel according to the present embodiment completed based on the above knowledge is in mass%, C: 0.02% or less, Si: 1.5% or less, Mn: 1.0% or less, P: 0.00. 020% or less, S: 0.010% or less, Cr: 15 to less than 20%, Ni: 25 to less than 45%, Nb: 2.3 to 7.0%, and N: 0.030% or less The balance is made of Fe and impurities and satisfies the formula (1).
Ni (sf) / Ni (cp)> 1.00 (1)
Here, the Ni content (mass%) in the range from the surface of the austenitic stainless steel to the depth of 5 μm is substituted for Ni (sf) in the formula (1), and the entire austenitic stainless steel is substituted for Ni (cp). The average Ni content (% by mass) is substituted.

  The austenitic stainless steel described above may further contain an inner layer oxide scale, an outer layer oxide scale, and a metal Ni layer. The inner layer oxide scale is formed on the surface of the austenitic stainless steel. The outer oxide scale is formed on the inner oxide scale. The metal Ni layer is formed at the interface between the inner layer oxide scale and the outer layer oxide scale, and occupies 60% of the interface. The ratio of the metal Ni layer occupying the interface between the inner layer oxide scale and the outer layer oxide scale is defined as the area ratio and the Ni occupation ratio. In this case, the steam oxidation resistance is further enhanced by the metal Ni layer.

  The austenitic stainless steel is, for example, a steel pipe.

  Hereinafter, embodiments of the present invention will be described in detail. Hereinafter, unless otherwise specified, “%” of the element content means mass%.

[Chemical composition]
The austenitic stainless steel according to the present embodiment contains the following elements.

C: 0.02% or less Carbon (C) is an impurity. In heat resistant steels, C generally forms carbides and increases high temperature creep strength. However, in this embodiment, C combines with Ni, Nb, and Cr to form a carbide, and reduces the amount of precipitation of intermetallic compounds represented by the Laves phase. Furthermore, carbides aggregate and coarsen when heated at a high temperature for a long time. Coarse carbides reduce the strength of the crystal grains and grain boundaries. Therefore, the C content is 0.02% or less. A preferable C content is 0.015% or less. More preferably, it is 0.01% or less. The C content is preferably as low as possible.

Si: 1.5% or less Silicon (Si) is inevitably contained. Si deoxidizes steel. However, if the Si content is too high, the hot workability of the steel decreases. Therefore, the Si content is 1.5% or less. The Si content is preferably 0.70% or less, more preferably 0.50% or less.

Mn: 1.0% or less Manganese (Mn) is inevitably contained. Mn deoxidizes steel in the same way as Si. Further, Mn combines with S to form MnS and enhances the hot workability of steel. However, if the Mn content is too high, Mn promotes the formation of the sigma (σ) phase and reduces the hot workability of the steel. Therefore, the Mn content is 1.0% or less. A preferable Mn content is 0.70% or less, and more preferably 0.50% or less.

P: 0.020% or less Phosphorus (P) is an impurity. P decreases the hot workability and ductility of the steel. Therefore, the P content is 0.020% or less. A preferable P content is 0.010% or less. The P content is preferably as low as possible.

S: 0.010% or less Sulfur (S) is an impurity. S decreases the hot workability of steel. Therefore, the S content is 0.010% or less. A preferable S content is 0.008% or less. The S content is preferably as low as possible.

Cr: 15.0 to less than 20.0% Chromium (Cr) improves the steam oxidation resistance of steel. In a high temperature steam oxidation atmosphere, Cr forms a chromia (Cr ₂ O ₃ ) film in the vicinity of the steel surface. By forming a uniform chromia film on the surface of the steel, the steam oxidation resistance of the steel is enhanced. If the Cr content is too low, the above effect cannot be obtained. On the other hand, if the Cr content is too high, the stability of the structure is lowered and the high temperature creep strength is lowered. Therefore, the Cr content is less than 15.0 to 20.0%. In the present embodiment, as described above, when the austenitic stainless steel of the present embodiment is used in a high-temperature steam oxidation atmosphere, a metal Ni layer is formed in the oxide scale on the surface. This metal Ni layer can maintain excellent steam oxidation resistance even with a Cr content of less than 20.0%. The minimum with preferable Cr content is higher than 15.0%, More preferably, it is 16.0%, More preferably, it is 17.0%. The upper limit with preferable Cr content is 19.5%, More preferably, it is 19.0%.

Ni: 25.0 to less than 45.0% Nickel (Ni) stabilizes austenite. Ni further enhances the steam oxidation resistance and corrosion resistance of the steel. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, these effects are not only saturated, but hot workability is reduced. If the Ni content is too high, the raw material cost further increases. Therefore, the Ni content is less than 25.0-45.0%. The minimum with preferable Ni content is higher than 25.0%, More preferably, it is 26.0%, More preferably, it is 28.0%. The upper limit with preferable Ni content is 42.0%, More preferably, it is 40.0%.

Nb: 2.3-7.0%
Niobium (Nb) combines with Ni to form a γ "phase (Ni ₃ Nb). Nb further combines with Fe to form a Laves phase (Fe ₂ Nb). These intermetallic compounds (γ The “phase and the Laves phase” are precipitated in the grain boundaries and grains in a high temperature environment, thereby increasing the high temperature creep strength of the material. If the Nb content is too low, the above effect cannot be obtained. On the other hand, if the Nb content is too high, the toughness and hot workability of the steel decrease. Therefore, the Nb content is 2.3 to 7.0%. The minimum with preferable Nb content is higher than 2.3%, More preferably, it is 2.5%, More preferably, it is 2.8%. The upper limit with preferable Nb content is less than 7.0%, More preferably, it is 6.5%, More preferably, it is 6.0%.

N: 0.030% or less Nitrogen (N) is an impurity. N combines with Nb to form a nitride. Therefore, the amount of precipitation of the above-mentioned intermetallic compounds containing Nb (γ "phase and Laves phase) is reduced, and the high-temperature creep strength is reduced. Further, the nitride aggregates and becomes coarse when heated at a high temperature for a long time. Coarse nitrides reduce the high temperature creep strength of the steel, so the N content is 0.030% or less, the preferred N content is 0.010% or less, and the N content is as low as possible. Is preferred.

  The balance of the austenitic stainless steel of this embodiment is Fe and impurities. Here, the impurities are mixed from ore, scrap, or production environment as a raw material when industrially producing a steel material, and do not adversely affect the austenitic stainless steel of the present embodiment. It means what is allowed in the range.

The austenitic stainless steel of this embodiment further satisfies the following formula (1).
Ni (sf) / Ni (cp)> 1.00 (1)
Here, Ni content (mass%) in the surface layer of austenitic stainless steel is substituted for Ni (sf) in the formula (1). Further, the average Ni content (% by mass) of the entire austenitic stainless steel is substituted for Ni (cp). In the present specification, the surface layer of austenitic stainless steel means a range from the surface of the austenitic stainless steel to a depth of 5 μm.

  As shown in Formula (1), in the austenitic stainless steel of this embodiment, the Ni content on the surface is higher than the average Ni content of the entire steel. In this case, the steam oxidation resistance is enhanced in a high temperature steam oxidation atmosphere.

  The minimum with preferable Formula (1) is 1.01, More preferably, it is 1.02. Moreover, although the upper limit of Formula (1) is not specifically limited, An example of the upper limit of Formula (1) in the chemical composition of this embodiment is 2.50.

  The Ni content Ni (sf) of the surface layer described above is obtained by the following method. Of the surface of the austenitic stainless steel, Ni (sf) is measured at any five points (measurement points). An austenitic stainless steel is cut perpendicular to the surface, and a sample including the measurement points is taken. At the measurement point, the average Ni content in the range from the surface to a depth of 5 μm (that is, the surface layer) is determined by EDX (energy dispersive X-ray spectroscopy). The obtained value is defined as Ni (sf) (%).

  The average Ni content Ni (cp) of the entire steel is obtained by the following method. A test piece is collected at the center of the cross section perpendicular to the surface of the austenitic stainless steel material of the present embodiment. For example, when the austenitic stainless steel material is a steel plate, a test piece is collected from the central portion (central axis portion) of the cross section perpendicular to the rolling direction. Similarly, when the austenitic stainless steel material is a steel bar or a wire, a test piece is collected from the central shaft portion. When the austenitic stainless steel material is a steel pipe, a test piece is taken from the center of the wall thickness. Ni content is calculated | required with respect to the extract | collected test piece by the inductively coupled plasma (ICP) emission-spectral-analysis method. The obtained value is defined as the average Ni content Ni (cp) (%) of the entire steel.

  The shape of the austenitic stainless steel according to the present embodiment is not particularly limited. Examples of the austenitic stainless steel include a steel plate, a steel pipe, a steel bar, and a wire rod. When the austenitic stainless steel is a steel pipe, the austenitic stainless steel pipe is used as a piping for a boiler.

[Production method]
An example of the manufacturing method of the austenitic stainless steel of this embodiment is demonstrated. However, the manufacturing method is not particularly limited as long as the austenitic stainless steel has the above configuration.

[Intermediate material manufacturing process]
A molten steel having the above chemical composition is produced. A well-known degassing process is implemented with respect to molten steel as needed.

  An ingot is manufactured from the manufactured molten steel by an ingot-making method. The ingot is hot-worked (hot forging, etc.) to produce steel materials such as slabs, blooms and billets. Steel materials such as slabs, blooms and billets may be manufactured from the manufactured molten steel by a continuous casting method.

  An intermediate material is manufactured by hot-working the manufactured steel material. For example, a steel material is hot-rolled to produce a steel plate, a steel bar, or a wire rod. Moreover, a steel pipe is manufactured by hot extrusion, piercing and rolling. As described above, the specific method of hot working is not particularly limited, and hot working corresponding to the shape of the final product may be performed.

  Cold working may be performed on the intermediate material after hot working. When the intermediate material is a steel pipe, the cold working is, for example, cold drawing or cold rolling. When the intermediate material is a steel plate, it is cold rolling or the like.

[Heat treatment process]
A heat treatment is performed on the manufactured intermediate material. By performing the heat treatment, a metal oxide layer is formed on the surface of the intermediate material.

  The heat treatment temperature is 1000 to 1200 ° C. If the heat treatment temperature is too low, the crystal grains are not sufficiently homogenized. Moreover, Ni concentration in the surface vicinity of austenitic stainless steel becomes inadequate after the below-mentioned Ni surface concentration treatment process. As a result, Expression (1) is not satisfied. On the other hand, if the heat treatment temperature is too high, the crystal grain size is remarkably coarsened, so that the workability is lowered. Accordingly, the heat treatment temperature is 1000 to 1200 ° C.

  The heat treatment time is 5 to 60 minutes. If the heat treatment time is too short, the homogenization of crystal grains becomes insufficient. Further, the concentration of Ni near the surface of the austenitic stainless steel becomes insufficient after the Ni surface concentration treatment step. As a result, Expression (1) is not satisfied. On the other hand, if the heat treatment time is too long, the crystal grain size is remarkably coarsened, so that the workability is lowered. Therefore, the heat treatment time is 5 to 60 minutes.

  The heat treatment atmosphere is preferably an air atmosphere or an oxidizing gas atmosphere. If the atmosphere is not an oxidizing atmosphere, the above effects may not be obtained.

[Ni surface enrichment process]
Ni surface concentration treatment is performed on the intermediate material after the heat treatment. In the Ni surface concentration treatment, a bath is prepared. The intermediate material after the heat treatment is immersed in a bath to remove the metal oxide layer formed by the heat treatment. At this time, the metal oxide layer containing Fe and Cr oxides on the surface layer of the intermediate material is removed. As a result, the Ni concentration in the surface layer increases.

  The Ni surface concentration treatment temperature is 5 to 50 ° C. If the Ni surface concentration treatment temperature is too low, the surface Ni concentration is unlikely to increase with respect to the average Ni concentration of the entire steel, and the formula (1) is not satisfied. On the other hand, if the Ni surface concentration treatment temperature is too high, the Ni concentration on the surface does not increase uniformly and equation (1) is not satisfied. If the Ni surface concentration treatment temperature is 5 to 50 ° C., the Ni concentration on the surface increases uniformly, and the austenitic stainless steel after the Ni surface concentration treatment satisfies the formula (1).

  The Ni surface concentration treatment time is 0.5 to 10 hours. If the Ni surface concentration treatment time is too short, the Ni concentration on the surface is difficult to increase, and the formula (1) is not satisfied. On the other hand, if the Ni surface concentration treatment time is too long, the Ni layer concentrated on the surface is also dissolved. Therefore, the Ni concentration on the surface is difficult to increase uniformly, and equation (1) is not satisfied. When the Ni surface concentration treatment time is 0.5 to 10 hours, the Ni concentration on the surface increases uniformly, and the austenitic stainless steel after the Ni surface concentration treatment satisfies the formula (1).

  The type of bath used for the Ni surface concentration treatment is not particularly limited. A preferred bath is a mixed solution of hydrofluoric acid and one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid.

  The austenitic stainless steel of this embodiment is manufactured by the above manufacturing method. By the above heat treatment and Ni surface concentration treatment, the metal oxide layer containing the oxides of Fe and Cr is removed from the surface layer of the intermediate material. On the other hand, the surface Ni remains. As a result, in the surface layer of the austenitic stainless steel, the Fe concentration and the Cr concentration are decreased, and the Ni concentration is relatively increased to satisfy the formula (1).

  After the Ni surface concentration treatment step, a blast treatment using a projection material may be performed on the surface of the austenitic stainless steel. In the case of an austenitic stainless steel pipe, for example, blasting is performed on the inner surface of the steel pipe. In the case of an austenitic stainless steel sheet, blasting is performed on one surface. In this case, a processed layer is formed on the surface of the austenitic stainless steel that has been subjected to blasting. Therefore, the steam oxidation resistance is further increased.

[Austenitic stainless steel of this embodiment in use in a high temperature steam oxidation atmosphere]
The austenitic stainless steel described above is used, for example, in a high temperature steam oxidation atmosphere. For example, this corresponds to the case where the austenitic stainless steel according to the present embodiment is used as a boiler steel pipe for a heater pipe and a reheater pipe of a power plant.

  As shown in FIG. 2, an oxide scale 20 is formed on the surface of the austenitic stainless steel of this embodiment used in a high-temperature steam oxidation atmosphere. The oxide scale 20 includes an inner layer oxide scale 30 and an outer layer oxide scale 40.

The inner oxide scale 30 is formed on the surface of the austenitic stainless steel 10. The inner oxide scale 30 is mainly composed of a mixed oxide of Fe and Cr. The inner layer oxide scale 30 contains, for example, 80% by volume or more of (Fe, Cr) ₃ O ₄ . The outer oxide scale 40 is formed on the inner oxide scale 30. The outer layer oxide scale 40 is mainly composed of an Fe-based oxide. The outer layer oxide scale 40 contains, for example, 80% by volume or more of Fe ₃ O ₄ .

  The austenitic stainless steel 10 further includes a metal Ni layer 50 at the interface between the inner oxide scale 30 and the outer oxide scale 40. The metal Ni layer 50 contains 80% by mass or more of Ni. The Ni concentration can be measured by EDX.

  When the metal Ni layer 50 is formed, the growth of oxide scale in the high-temperature steam oxidation atmosphere is suppressed. Therefore, the steam oxidation resistance of the austenitic stainless steel is increased. The reason for this is not clear, but the metal Ni layer 50 causes O to enter the oxide scale 20 (inner layer oxide scale 30) from the outside and to the iron oxide scale 20 (outer layer oxide scale 40) from the base material. This is thought to be due to the suppression of supply.

  As described above, the formation of the metal Ni layer 50 increases the steam oxidation resistance of the austenitic stainless steel of the present embodiment.

  In order to obtain the above effect more effectively, the Ni occupation ratio of the metal Ni layer 50 is preferably 60% or more, more preferably 65% or more in terms of area ratio. In this case, the steam oxidation resistance in the high temperature steam oxidation atmosphere is further enhanced.

[Test method]
Molten steel having the chemical composition shown in Table 1 was produced using a vacuum melting furnace.

  Ingots were manufactured using the molten steels having the numbers shown in Table 1. The ingot was hot worked to produce an intermediate material (steel plate).

  The manufactured intermediate material was subjected to heat treatment and Ni surface concentration treatment shown in Table 2 to produce an austenitic stainless steel sheet.

[Ni (sf) / Ni (cp) measurement]
The austenitic stainless steel plate of each test number was cut vertically and a sample including the surface was taken. The sample was embedded in resin, and the observation surface including the cross section near the surface was polished. The Ni content of the surface layer (range from the surface to a depth of 5 μm) was determined on the observation surface after polishing using the method described above.

  Furthermore, a test piece for analysis was taken from the center of the cross section of the austenitic stainless steel plate of each test number, and the Ni content was determined by ICP emission spectroscopy, which was defined as Ni (cp).

[Steam oxidation test]
A test piece of 2 mm × 10 mm × 25 mm was taken from the austenitic stainless steel plate of each test number, and a steam oxidation test was performed.

  Specifically, the test piece was suspended from a jig. Subsequently, the test piece was suspended and charged into a horizontal tubular heating furnace. After charging, it was kept in a steam atmosphere at 700 ° C. (the amount of dissolved oxygen was 100 ppb) for 2000 hours.

  After the test, each test piece was cut. The cut specimen was embedded in the resin so that the cut surface became the observation surface. Thereafter, the observation surface was mirror-polished. The mirror-polished observation surface was observed at 1000 to 3000 times using an optical microscope observation or a scanning electron microscope (SEM), and the average thickness of the inner layer oxide scale formed on the test piece was determined. Specifically, the surface vicinity of the test piece was observed using an optical microscope observation or SEM, and the thickness of the inner layer oxide scale was measured in any 10 visual fields. As described above, the composition of the inner layer oxide scale is different from the composition of the outer layer oxide scale. Therefore, the inner oxide scale can be distinguished from the outer oxide scale. The average thickness of the inner oxide scale measured in each field was defined as the thickness of the inner oxide scale.

  Furthermore, the Ni occupation ratio of the metal Ni layer after the steam oxidation test was measured. Specifically, the mirror-polished observation surface was observed using an optical microscope or SEM. By measuring the length of the metal Ni layer existing on the interface between the inner layer oxide scale and the outer layer oxide scale on the observation surface, the ratio of the length of the metal Ni layer to the total length of the interface is defined as the Ni occupation of the metal Ni layer. Defined as rate.

[Test results]
Table 2 shows the test results.

  Referring to Table 2, the chemical compositions of the austenitic stainless steels of test numbers 1, 7, and 8 were appropriate, and the value of Ni (sf) / Ni (cp) satisfied the formula (1). As a result, after the steam oxidation test, the thickness of the inner layer oxide scale was less than 10 μm. Further, after the steam oxidation test, the Ni occupancy ratio of the metal Ni layer was 60% or more.

  On the other hand, in Test Nos. 2-6, 12 and 13, although the chemical components were appropriate, the heat treatment or Ni surface concentration treatment conditions were inappropriate, so the value of Ni (sf) / Ni (cp) was 1. It was 0.000 or less, and the formula (1) was not satisfied. Specifically, in test number 2, the heat treatment time was too short. In test number 3, the heat treatment temperature was too low. In test number 4, the heat treatment atmosphere was Ar and not an oxidizing atmosphere. In test number 5, the Ni surface concentration treatment temperature was too low. In test number 6, the Ni surface concentration treatment time was too short. In test number 12, the Ni surface concentration treatment temperature was too high. In test number 13, the Ni surface concentration treatment time was too long. Therefore, in the test numbers 2 to 6, 12, and 13, the value of Ni (sf) / Ni (cp) was 1.00 or less. As a result, the inner layer oxide scale thickness of these test numbers was thicker than test numbers 1, 7 and 8, and was 10 μm or more. Furthermore, the ratio of the metal Ni layer to the interface was less than 60%.

  The Ni content of the austenitic stainless steels of test number 9 and test number 10 was too low, and the austenitic stainless steel Cr content of test number 11 was too low. Therefore, the inner layer oxide scale thicknesses of these test numbers were thicker than the test numbers 1, 7, and 8, and were 10 μm or more. Furthermore, the ratio of the metal Ni layer to the interface was less than 60%.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

1, 10 Austenitic stainless steel 2, 20 Oxide scale 3, 30 Inner layer oxide scale 4, 40 Outer layer oxide scale 50 Metal Ni layer

Claims (3)

Austenitic stainless steel,
% By mass
C: 0.02% or less,
Si: 1.5% or less,
Mn: 1.0% or less,
P: 0.020% or less,
S: 0.010% or less,
Cr: 15 to less than 20%,
Ni: 25 to less than 45%,
Nb: 2.3 to 7.0%, and N: 0.030% or less, with the balance being Fe and impurities,
An austenitic stainless steel that satisfies formula (1).
Ni (sf) / Ni (cp)> 1.00 (1)
Here, Ni content (mass%) in the range from the surface of the austenitic stainless steel to a depth of 5 μm is substituted for Ni (sf) in the formula (1), and the austenitic stainless steel is substituted for Ni (cp). The average Ni content (mass%) of the entire steel is substituted. The austenitic stainless steel according to claim 1, further comprising:
An inner layer oxide scale formed on the surface of the austenitic stainless steel;
An outer layer oxide scale formed on the inner layer oxide scale;
An austenitic stainless steel comprising a metallic Ni layer formed at an interface between the inner layer oxide scale and the outer layer oxide scale and occupying 60% or more of the interface. The austenitic stainless steel according to claim 1 or 2,
The austenitic stainless steel is an austenitic stainless steel which is a steel pipe.

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