JP2015200008A - austenitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel having excellent steam oxidation resistance even in high temperature environment of 700°C or more.SOLUTION: An austenitic stainless steel according to the embodiment has a chemical composition consisting of, by mass%, Cr:15 to less than 20%, Ni:25 to less than 45%, Nb:2.3 to 5.0% and Mo+W/2:4.0 to 8.0% and the balance Fe with inevitable impurities. The contents of C, Si, Mn, P, S and N of the impurities are C:less than 0.02%, Si:less than 1.0%, Mn:less than 1.0%, P:less than 0.020%, S:less than 0.010% and N:less than 0.030% respectively. The above chemical composition further satisfies the formula (1) and the formula (2). 8.0≤Nb+Mo+W/2≤12.0 (1) and 1.0≤(Mo+W/2)/Nb≤4.0 (2), where contents (mass%) of corresponding elements are assigned to element symbols in the formulae.

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. For this reason, power plants are required to reduce CO ₂ emissions during operation. In order to reduce CO ₂ emissions, for example, in a coal-fired power plant, the power generation efficiency is increased by increasing the temperature and pressure of steam.

  A boiler steel pipe is used as a superheater pipe and a reheater pipe of a boiler of a power plant. 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 have been proposed as follows.

(A) Technology for making steel structure finer The austenitic stainless steel pipe disclosed in Japanese Patent Laid-Open No. 58-133352 (Patent Document 1) has an average grain size number of No. The coarse grain structure of 6 or less and the grain size number of the inner steel are No. 7 or more fine grain structure. C + N of the fine-grained layer portion is 0.15% or more.

(B) Technology for spraying surface layer The method for preventing oxidation of austenitic stainless steel disclosed in Japanese Patent Application Laid-Open No. 49-135822 (Patent Document 2) is manufactured after the final heat treatment in the manufacturing process or by hot finishing. After the hot rolling in the process, particle spray peening is performed on the surface of the austenitic stainless steel.

  The method for preventing oxidation of austenitic stainless steel disclosed in Japanese Patent Application Laid-Open No. 52-8930 (Patent Document 3) is performed after the final heat treatment of the manufacturing process or after hot rolling of the manufacturing process by hot finishing, and then carbon steel, alloy Using particles made of steel or stainless steel, spray peening is performed with a fluid at a predetermined spray pressure and spray amount.

  The stainless steel tube processing method disclosed in Japanese Patent Application Laid-Open No. 63-54598 (Patent Document 4) is for the purpose of descaling the inner surface of a stainless steel tube taken out from an existing boiler after performing solution heat treatment. Apply chemical cleaning. Thereafter, the inner surface of the tube is subjected to descaling and shot blasting for the purpose of forming a cold-worked layer.

(C) Technology for imparting high degree of cold working In the method for producing a steel pipe for boiler disclosed in Japanese Patent Application Laid-Open No. 2004-132437 (Patent Document 5), 5-30% Cr is contained in mass%. Ultrasonic impact treatment is applied to the inner surface of the ferritic heat-resistant steel pipe.

  The steel pipe disclosed in Japanese Patent Application Laid-Open No. 2009-68079 (Patent Document 6) contains 8 to 28% by mass of Cr and has a Vickers hardness at a position where the depth from the inner surface of the steel pipe is 20 μm, t / 2 (t: thickness of the steel pipe) It has a high processing layer that becomes 1.5 times or more of the Vickers hardness at the position.

(D) Technology for improving steam oxidation resistance of ferritic heat resistant steel In the processing method of ferritic heat resistant steel disclosed in JP 2002-285236 A (Patent Document 7), 9.5 to 15% by mass% the heat resistant ferritic steel containing Cr of, and normalizing treatment at 900 ° C. or higher temperature, treated tempered at a temperature below the a ₁ transformation point, to form a shot processing layer by blowing the particles to the steel surface .

JP 58-133352 A JP-A-49-135822 JP 52-8930 A JP-A-63-54598 JP 2004-132437 A JP 2009-68079 A JP 2002-285236 A

  However, the fine-grain steel 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. The ferritic steel disclosed in Patent Document 7 has a low high-temperature strength and is difficult to use in a high-temperature environment of 700 ° C. or higher.

  In the power plant, operation at about 800 ° C. is expected in the future for the purpose of further improving the power generation efficiency. Therefore, 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%, Cr: less than 15-20%, Ni: less than 25-45%, Nb: 2.3-5.0%, and Mo + W / 2: 4.0. It contains ˜8.0%, and the balance has a chemical composition composed of Fe and impurities. Among impurities, the contents of C, Si, Mn, P, S and N are respectively C: less than 0.02%, Si: less than 1.0%, Mn: less than 1.0%, P: 0.020% Less than, S: Less than 0.010%, and N: Less than 0.030%. The chemical composition further satisfies formulas (1) and (2).
8.0 ≦ Nb + Mo + W / 2 ≦ 12.0 (1)
1.0 ≦ (Mo + W / 2) /Nb≦4.0 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula.

  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.

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

The austenitic stainless steel containing niobium (Nb), molybdenum (Mo) and tungsten (W) has excellent steam oxidation resistance compared to the austenitic stainless steel not containing these elements. Nb, Mo and W form a uniform chromia (Cr ₂ O ₃ ) film on the steel surface. Therefore, excellent steam oxidation resistance is obtained.

The reason why the steam oxidation resistance is increased by containing Nb, Mo and W is considered to be as follows. Nb, Mo, and W promote precipitation of the Fe ₂ M type Laves phase in the steel. Here, the metal “M” corresponds to Nb and W. If precipitation of the Laves phase is promoted, the chemical composition in the steel changes and the Cr activity gradient increases. As the Cr activity gradient increases, the outward flux of Cr (the movement of Cr from the inside of the steel to the surface) increases. As a result, chromia (Cr ₂ O ₃ ) is easily formed uniformly on the steel surface. When the total content of Nb, Mo and W in the steel satisfies the formula (1), the Cr activity gradient is sufficiently increased, and chromia is uniformly formed on the surface of the steel. As a result, excellent steam oxidation resistance can be obtained even in a high temperature environment of 700 ° C. or higher.
8.0 ≦ Nb + Mo + W / 2 ≦ 12.0 (1)
The content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

When not only Mo and / or W but also Nb is contained, the steam oxidation resistance is improved as compared with the case of containing only Mo and / or W. This is presumably because the amount of precipitation of the Laves phase containing Mo and / or W increases due to the inclusion of Nb. That is, the ratio of the Mo and / or W content and the Nb content is related to the amount of precipitation of the Laves phase. If the Nb, Mo and W contents in the steel satisfy the formula (2), the ratio of the Nb content and the Mo and / or W contents is appropriate, and the Laves phase is sufficiently precipitated. Therefore, the steam oxidation resistance in a high temperature environment of 700 ° C. or higher is enhanced.
1.0 ≦ (Mo + W / 2) /Nb≦4.0 (2)

The austenitic stainless steel according to the present embodiment completed based on the above knowledge is in mass%, Cr: less than 15-20%, Ni: less than 25-45%, Nb: 2.3-5.0%, and Mo + W / 2: 4.0 to 8.0%, with the balance being Fe and impurities. Among impurities, the contents of C, Si, Mn, P, S and N are respectively C: less than 0.02%, Si: less than 1.0%, Mn: less than 1.0%, P: 0.020% Less than, S: Less than 0.010%, and N: Less than 0.030%. The chemical composition satisfies the formulas (1) and (2).
8.0 ≦ Nb + Mo + W / 2 ≦ 12.0 (1)
1.0 ≦ (Mo + W / 2) /Nb≦4.0 (2)
The content (mass%) of the corresponding element is substituted for the element symbol in the formula.

  The austenitic stainless steel may include a hardened layer having a depth of 10 μm or more on the surface layer.

  In this case, Cr tends to diffuse in the processed layer. Therefore, a chromia film is easily formed on the surface.

  The austenitic stainless steel pipe of this embodiment is manufactured from the austenitic stainless steel.

  Hereinafter, the austenitic stainless steel of this embodiment will be described in detail.

[Chemical composition]
The austenitic stainless steel of this embodiment has the following chemical composition.

Cr: 15 to less than 20% Chromium (Cr) improves the steam oxidation resistance and corrosion resistance of steel. In a high temperature environment of 700 ° C. or higher, 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 decreases and the creep strength decreases. Therefore, the Cr content is 15 to less than 20%. In this embodiment, by containing Nb, Mo and W, a uniform chromia film can be formed even with a Cr content of less than 20%, and the steam oxidation resistance is improved. The minimum with preferable Cr content is higher than 15%, More preferably, it is 16%, More preferably, it is 17%. The upper limit with preferable Cr content is 19.5%, More preferably, it is 19%.

Ni: 25 to less than 45% Nickel (Ni) stabilizes austenite. Ni further enhances the steam oxidation resistance and corrosion resistance of the steel. If the Ni content is too low, the above effect cannot be obtained. On the other hand, if the Ni content is too high, the creep strength of the steel decreases. If the Ni content is too high, the production cost further increases. Therefore, the Ni content is less than 25-45%. A preferable lower limit of the Ni content is higher than 25%, more preferably 26%, and further preferably 28%. The upper limit with preferable Ni content is 40%, More preferably, it is 35%.

Nb: 2.3-5.0%
Niobium (Nb) combines with Ni to form Ni ₃ Nb. Nb further combines with Fe to form Fe ₂ Nb. These precipitates enhance the high temperature creep properties.
Nb further increases the steam oxidation resistance of the steel. Nb combines with Fe to form Fe ₂ Nb. Nb further increases the amount of precipitation of FeMo which is the σ phase and Fe ₂ W which is the Laves phase. As the amount of precipitation of the σ phase and the Laves phase increases, the chemical composition of the parent phase changes and the Cr activity gradient increases. As a result, Cr easily moves from the inside of the steel to the surface, and a uniform chromia (Cr ₂ O ₃ ) film is easily formed on the steel surface. As a result, the steam oxidation resistance of the steel increases. 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 5.0%. 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 5.0%, More preferably, it is 4.8%, More preferably, it is 4.5%.

Mo + W / 2: 4.0-8.0%
Molybdenum (Mo) and tungsten (W) combine with Fe to form a σ phase (FeMo) and a Loves phase (Fe ₂ W). Formation of the Laves phase changes the chemical composition of the parent phase. As a result, the Cr flux to the surface layer increases, and a uniform chromia film is formed on the steel surface. As a result, the steam oxidation resistance of the steel increases. If the Mo and / or W content is too low, the above effects cannot be obtained. On the other hand, if the Mo and W contents are too high, the toughness and hot workability of the steel will decrease. Therefore, the Mo + W / 2 content is 4.0 to 8.0%. Mo + W / 2 means the total content of Mo and / or W. Either one of Mo and W may be contained, or Mo and W may be contained.

  The minimum with preferable Mo + W / 2 is higher than 4.0%, More preferably, it is 4.2%, More preferably, it is 4.5%. The upper limit with preferable Mo + W / 2 is less than 8.0%, More preferably, it is 7.5%, More preferably, it is 7.0%.

  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 steel is industrially produced, and do not adversely affect the austenitic stainless steel of this embodiment. It means what is allowed in the range.

  Among the impurities of the austenitic stainless steel of this embodiment, the contents of C, Si, Mn, P, S, and N are as follows.

C: Less than 0.02% Carbon (C) is an impurity. In heat-resisting steels, C generally forms carbides and increases creep strength. However, in this embodiment, C combines with Ni, Nb, and Cr to form carbides, and reduces the amount of intermetallic compounds typified 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 crystal grains and grain boundaries. Accordingly, the C content is less than 0.02%. A preferable C content is 0.01% or less. The C content is preferably as low as possible.

Si: less than 1.0% Silicon (Si) is an impurity. Si combines with Ni to form silicide which is an intermetallic compound. Silicide is very brittle and reduces the hot workability of the steel. Accordingly, the Si content is less than 1.0%. A preferable Si content is 0.8% or less. The Si content is preferably as low as possible.

Mn: less than 1.0% Manganese (Mn) is an impurity. In general, Mn combines with S to form MnS and enhances the hot workability of steel. However, in the austenitic stainless steel of this embodiment having a low S content, Mn promotes the formation of a sigma (σ) phase and lowers the hot workability of the steel. Therefore, the Mn content is less than 1.0%. A preferable Mn content is 0.5% or less. It is preferable that the Mn content is as low as possible.

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

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

N: Less than 0.030% Nitrogen (N) is an impurity. N combines with Nb to form a nitride. Therefore, the amount of precipitation of the Nb-containing intermetallic compound is reduced. Furthermore, nitrides aggregate and coarsen when heated for a long time at a high temperature. Coarse nitrides reduce the creep strength of the steel. Therefore, the N content is less than 0.030%. A preferable N content is 0.010% or less. The N content is preferably as low as possible.

[Regarding Formula (1) and Formula (2)]
The chemical composition of the austenitic stainless steel of the present embodiment further satisfies the expressions (1) and (2).
8.0 ≦ Nb + Mo + W / 2 ≦ 12.0 (1)
1.0 ≦ (Mo + W / 2) /Nb≦4.0 (2)
The content (mass%) of the corresponding element is substituted for the element symbol in the formula.

[Regarding Formula (1)]
As described above, Nb, Mo and W form an intermetallic compound containing Fe. Thereby, Cr in steel becomes easy to move to the surface of steel, and it becomes easy to form a uniform chromia film on the surface. As a result, the steam oxidation resistance of the steel increases.

  Define F1 = Nb + Mo + W / 2. If F1 is too low, the amount of precipitation of the above-mentioned intermetallic compound is too small. As a result, the steam oxidation resistance of the steel is lowered. On the other hand, if F1 is too high, the toughness and hot workability of steel will decrease. If F1 is 8.0 or more, excellent steam oxidation resistance of steel can be obtained. Furthermore, if F1 is 12.0 or less, high toughness and hot workability can be obtained.

  The preferable lower limit of F1 is higher than 8.0, more preferably 8.2%, and further preferably 8.5%.

[Regarding Formula (2)]
If the Mo and W contents with respect to the Nb content are in an appropriate range, Nb promotes precipitation of FeMo and Fe ₂ W, and as a result, the steam oxidation resistance of the steel increases. It is defined as F2 = (Mo + W / 2) / Nb. If F2 is too low or too high, that is, if the Mo and W contents relative to the Nb content are too low or too high, the steam oxidation resistance of the steel will be low. When F2 is 1.0 to 4.0, the Mo and W contents relative to the Nb content are appropriate, and the steam oxidation resistance of the steel is increased.

  The preferable lower limit of F2 is higher than 1.0, more preferably 1.2%, and further preferably 1.5%. The upper limit with preferable F2 is less than 4.0%, More preferably, it is 3.5%, More preferably, it is 3.0%.

[Production method]
The manufacturing method of the austenitic stainless steel of this embodiment is demonstrated.

  A molten steel having the above chemical composition is produced. A well-known degassing process is implemented with respect to the manufactured molten steel as needed.

  Next, the molten steel is made into a continuous cast material by a continuous casting method. The continuous cast material is, for example, slab, bloom, billet and the like. Molten steel may be made into an ingot by the ingot-making method. A continuously cast material or an ingot is hot-worked by a known method to obtain an austenitic stainless steel material. Examples of the austenitic stainless steel material include steel pipes, steel plates, steel bars, wire rods, and forged steels.

  An austenitic stainless steel pipe is manufactured, for example, by hot extrusion by the Eugene Sejurne method.

  Solution treatment is performed on the manufactured austenitic stainless steel material. The solution treatment is performed by a known method. The solution treatment temperature (solution treatment temperature) is, for example, 1000 to 1300 ° C. The solution treatment time is, for example, 0.1 to 2 hours. A well-known aging treatment may be performed on the solution-treated austenitic stainless steel material. Through the above steps, austenitic stainless steel is produced.

  A processed layer of 10 μm or more may be formed on the surface layer of the solution-treated austenitic stainless steel material. The processed layer is formed, for example, by spraying particles (projection material) onto an austenitic stainless steel material. Examples of the spraying method include known shot peening, shot blasting, shot processing, sand blasting, sand processing, air blasting, water jet and the like. The particles (projection material) are, for example, steel, cast steel, stainless steel, glass, silica sand, alumina, and amorphous. The shape of the particles is, for example, a sphere, a cut wire, a grid, or the like. The particles may be sprayed by compressed air, centrifugal force by an impeller (impeller type), high-pressure water, ultrasonic waves, or the like. The particles may be mixed with the liquid and sprayed with compressed air or the like (liquid honing).

  The processed layer may also be formed by polishing, ball milling, grinder processing, honing processing, ultrasonic impact processing, or the like.

  In the processed layer, Cr easily diffuses. Therefore, if the austenitic stainless steel material has a processed layer as a surface layer, the formation of chromia is promoted, and the steam oxidation resistance is further enhanced.

  In order to stably ensure long-term steam oxidation resistance at high temperatures, it is preferable to form a processed layer by the above-mentioned spraying method. This is because uniform processing is possible over the entire surface layer of the austenitic stainless steel material.

[Test method]
The molten steel which has the chemical composition of the test numbers 1-18 shown in Table 1 was manufactured using the vacuum melting furnace.

  In F1 in Table 1, a value of F1 = Nb + Mo + W / 2 is described. In F2 in Table 1, the value of F2 = (Mo + W / 2) / Nb is described. Ingots were manufactured using molten steel of each test number. The ingot was hot worked to produce an austenitic stainless steel sheet.

  Solution treatment was performed on the manufactured stainless steel sheet. The solution temperature was 1120 ° C., and the solution treatment time was 1 hour.

  A test piece of 2 mm × 10 mm × 25 mm was collected from the stainless steel plate after the solution treatment. About the stainless steel plate of the test numbers 1-17, the surface of the extract | collected test piece was grind | polished with wet paper and finished. Thereafter, electrolytic polishing was performed on the surface of the test piece to remove the surface distortion of the test piece.

  About the stainless steel plate of the test number 18, after grind | polishing and finishing the surface of the extract | collected test piece with wet paper, the steel shot was implemented and the process layer was formed. The diameter of the steel shot sphere was 0.6 mm. The average thickness of the processed layer was 150 μm. For the stainless steel plates of test numbers 1 to 17, no processed layer was formed.

  The test piece after electropolishing was suspended and held in a jig and inserted into a horizontal tubular heating furnace. Then, a steam oxidation test was performed at 800 ° C. for 2000 hours in a steam atmosphere having a dissolved oxygen amount of 100 ppb.

  After the test, each test piece was cut. Then, it embedded in resin and the cross section was mirror-polished. The mirror-polished cross section was observed with an optical microscope at 300 times. Furthermore, the average thickness of the oxide scale of the test piece was determined. Specifically, the thickness of the inner layer oxide scale was measured in an arbitrary 10 visual fields near the surface of the test piece (the width of the surface of the test piece in each visual field is 1000 μm) in 500 times optical microscope observation. Here, the inner layer oxide scale means an oxide formed inside the original test piece surface. The average value of the measured thickness of the inner oxide scale was defined as the average thickness of the oxide scale.

[Test results]
Table 1 shows the average thickness of the oxide scale. Referring to Table 1, the chemical compositions of the steels with test numbers 1 to 10 and 18 were appropriate and satisfied Formula (1) and Formula (2). Therefore, the thickness of the oxide scale was less than 10 μm.

  In particular, since the stainless steel plate of test number 18 had a processed layer, the thickness of the oxide scale was thinner than those of test numbers 1-10.

  On the other hand, in test numbers 11 and 14, the Nb content was too low. Furthermore, in test number 11, F2 was too high. Therefore, the oxide scale thickness exceeded 10 μm. In test numbers 12, 13, and 16, the Mo + W / 2 content was too low. Therefore, the oxide scale thickness exceeded 10 μm.

  In test number 15, F1 was less than the lower limit of formula (1). Therefore, the oxide scale thickness exceeded 10 μm. In test number 17, F2 was less than the lower limit of formula (2). As a result, the oxide scale thickness exceeded 10 μm.

  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.

Claims (3)

% By mass
Cr: 15 to less than 20%,
Ni: 25 to less than 45%,
Nb: 2.3-5.0% and
Mo + W / 2: 4.0-8.0% is contained, the balance consists of Fe and impurities,
Among the impurities, the contents of C, Si, Mn, P, S and N are respectively
C: less than 0.02%,
Si: less than 1.0%,
Mn: less than 1.0%,
P: less than 0.020%,
S: less than 0.010%, and
N: less than 0.030%
An austenitic stainless steel having a chemical composition satisfying the formulas (1) and (2).
8.0 ≦ Nb + Mo + W / 2 ≦ 12.0 (1)
1.0 ≦ (Mo + W / 2) /Nb≦4.0 (2)
The content (mass%) of the corresponding element is substituted for the element symbol in the formula. The austenitic stainless steel according to claim 1,
An austenitic stainless steel having a surface layer with a processed layer having a depth of 10 μm or more.   An austenitic stainless steel pipe made of the austenitic stainless steel according to claim 1.