JP7114998B2 - austenitic stainless steel - Google Patents

Description

The present invention relates to austenitic stainless steel.

In recent years, interest in environmental problems such as global warming has increased. Therefore, power plants are required to reduce CO ₂ emissions during operation. In order to reduce CO ₂ emissions, for example, in coal-fired power plants, steam is heated to a higher temperature and a higher pressure to increase power generation efficiency. As a result, the amount of fuel consumed can be reduced for the same power generation amount.

Boiler steel tubes are used for superheater tubes and reheater tubes of boilers in power plants. As the temperature and pressure of steam increases, boiler steel pipes are required to have not only high-temperature strength, especially creep rupture strength, but also resistance to high-temperature oxidation caused by steam (steam oxidation resistance). be done.

Techniques for improving the steam oxidation resistance of steel pipes have been proposed as follows.

(A) Technology for Refining Steel Structure The austenitic stainless steel pipe disclosed in Patent Document 1 has an average grain size number of no. 6 or less coarse grain structure and the grain size number of the inner surface steel is No. It has a fine grain structure of 7 or more. C+N in the fine grain layer is 0.15% or more.

(B) Technology for spraying the surface layer In the method for preventing oxidation of austenitic stainless steel disclosed in Patent Document 2, after the final heat treatment in the manufacturing process or after hot rolling in the manufacturing process by hot finishing, austenitic stainless steel A fluid particle blast peening process is performed on a stainless steel surface.

In the method for preventing oxidation of austenitic stainless steel disclosed in Patent Document 3, particles made of carbon steel, alloy steel or stainless steel are used after the final heat treatment in the manufacturing process or after hot rolling in the manufacturing process by hot finishing. Then, spray peening is performed with a fluid at a predetermined spray pressure and spray amount.

In the method for treating a stainless steel tube disclosed in Patent Document 4, a stainless steel tube removed from an existing boiler is subjected to solution heat treatment and then chemically cleaned for the purpose of descaling the inner surface. After that, shot blasting is applied to the inner surface of the tube for the purpose of descaling and forming a cold-worked layer.

(C) Technology for imparting cold work with a high workability In the method for manufacturing a steel pipe for a boiler disclosed in Patent Document 5, the inner surface of a ferritic heat-resistant steel pipe containing 5 to 30% by mass of Cr is super-stretched. Apply sonic impact treatment.

The steel pipe disclosed in Patent Document 6 contains 8 to 28% by mass of Cr, and the Vickers hardness at a depth of 20 μm from the inner surface of the steel pipe is t/2 (t: wall thickness of the steel pipe). It has a high working layer such that the Vickers hardness at the position is 1.5 times or more.

(D) Technology for Improving Steam Oxidation Resistance of Heat-Resistant Ferritic Steel In the heat-resistant ferritic steel processing method disclosed in Patent Document 7, heat-resistant ferritic steel containing 9.5 to 15% by mass of Cr is normalized at a temperature of 900° C. or higher and tempered at _a temperature below the A1 transformation point, and then particles are sprayed onto the steel surface to form a shot-worked layer.

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

However, the fine-grained steel disclosed in Patent Document 1 may have low steam oxidation resistance in a high-temperature environment of 700° C. or higher. Similarly, the steels produced by the methods disclosed in Patent Documents 2 to 6 may also have low steam oxidation resistance in a high temperature environment of 700° C. or higher. Since the ferritic steel disclosed in Patent Document 7 has low high temperature strength, it is difficult to use in a high temperature environment of 700° C. or higher.

Power plants are expected to operate at about 800° C. in the future for the purpose of further improving power generation efficiency. Therefore, steels used in power plants are required to have excellent steam oxidation resistance even in high-temperature environments of 700° C. or higher.

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

The present invention has been made to solve the above problems, and the gist thereof is the following austenitic stainless steel.

(1) An austenitic stainless steel having scale formed on at least part of the surface of the base material,
The scale is, in order from the base material side,
a first inner layer scale mainly composed of Cr ₂ O ₃ ;
a second inner layer scale mainly composed of a Cr—Mn-based spinel-type oxide;
including a third inner layer scale mainly composed of Fe—Cr spinel-type oxide and an outer layer scale mainly composed of Fe ₃ O ₄ ,
The chemical composition of the base material is, in mass %,
C: 0.01% to 0.20%,
Si: 1.0% or less,
Mn: 0.1-0.7%,
P: 0.050% or less,
S: 0.010% or less,
N: 0.10 to 0.50%,
Cr: 16.0 to 26.0%,
Ni: 18.0% or more and less than 35.0%,
Nb: 0.01 to 1.0%,
B: 0.0005 to 0.010%,
Mo: 0-5.0%,
W: 0 to 10.0%,
Ti: 0 to 1.0%,
Al: 0-0.3%,
Ca: 0-0.1%,
Mg: 0-0.1%,
Co: 0 to 10.0%,
Cu: 0-4.0%,
V: 0 to 1.0%,
Zr: 0 to 0.2%,
Hf: 0-0.2%,
Ta: 0 to 1.0%,
REM: 0-0.1%,
Sn: 0 to 0.010%,
Sb: 0 to 0.010%,
Pb: 0 to 0.001%,
As: 0 to 0.001%,
Bi: 0 to 0.001%,
Balance: Fe and impurities, and satisfying the following formula (i),
the chemical composition of the second inner layer scale satisfies the following formula (ii),
The average grain size of the base material is No. 4 or less,
Austenitic stainless steel.
0.5<Mo+W/2<8.0 (i)
_CrI2 >40× _MnI2 (ii)
However, the meaning of each symbol in the above formula is as follows.
Mo: Mo content of the base material (% by mass)
W: W content of the base material (% by mass)
Cr _I2 : Cr content of the second inner layer scale (% by mass)
_MnI2 : Mn content in the second inner layer scale (% by mass)

(2) where the thickness of the second inner layer scale is t _I2 (μm) and the thickness of the third inner layer scale is t _I3 (μm), the following equations (iii) and (iv) are satisfied. ,
The austenitic stainless steel according to (1) above.
_tI2 < _tI3 (iii)
_tI2 + _tI3 ≤ 20 (iv)

According to the present invention, it is possible to obtain an austenitic stainless steel having excellent steam oxidation resistance and creep rupture strength even in a high temperature environment of 700° C. or higher.

It is a cross-sectional SEM photograph of a base material and a scale.

The present inventors investigated and studied the steam oxidation resistance and creep rupture strength of austenitic stainless steel in a steam oxidation atmosphere at 700° C., and obtained the following findings.

In order to improve steam oxidation resistance, it is effective to increase the Cr content in the steel so that a film containing chromia (Cr ₂ O ₃ ) is uniformly formed on the surface of the steel.

However, if the Cr content is excessive, the stability of the austenitic structure is lowered, and the creep rupture strength may be deteriorated. Therefore, it is necessary to limit the Cr content to a predetermined value or less.

Therefore, the present inventors have studied a method for achieving both steam oxidation resistance and creep rupture strength. It has been found that it is possible to improve

The reason is presumed as follows.

When austenitic stainless steel is exposed to a high-temperature steam environment, outer layer scale and inner layer scale are formed on the surface of the base material. At this time, if the Mn content in the steel is high, the formation of spinel-type oxides containing Cr and Mn is promoted in the region of the inner layer scale on the base metal side.

On the other hand, when the Mn content in the steel is low, the formation of spinel-type oxides containing Cr and Mn in the inner scale is suppressed, and a uniform chromia layer is formed at the boundary between the scale and the base metal. Become so. That is, it is believed that the formation of spinel-type oxides containing Cr and Mn inhibits the formation of chromia.

In addition to reducing the Mn content in the base material, heat treatment is performed in advance under a predetermined environment to suppress the amount of spinel-type oxides containing Cr and Mn in the inner layer scale and promote the formation of chromia. This makes it possible to improve steam oxidation resistance.

The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

1. Chemical Composition of Base Material The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.01-0.20%
Carbon (C) is an element contained as an impurity. However, in heat-resistant steels, C generally forms carbides and increases creep strength. If the C content is less than 0.01%, this effect cannot be obtained. On the other hand, when C is contained excessively, coarse carbides are formed and the creep strength is lowered. Therefore, the C content should be 0.01 to 0.20%. The C content is preferably 0.02% or more, more preferably 0.03% or more. Also, the C content is preferably 0.18% or less, more preferably 0.15% or less.

Si: 1.0% or less Silicon (Si) is inevitably contained. Si deoxidizes steel. However, if the Si content is too high, the hot workability of the steel is reduced. Therefore, the Si content should be 1.0% or less. The Si content is preferably 0.9% or less, more preferably 0.8% or less. In addition, since excessive reduction causes an increase in cost, the Si content is preferably 0.01% or more.

Mn: 0.1-0.7%
Manganese (Mn), like Si, deoxidizes steel. Furthermore, Mn combines with S to form MnS, which enhances the hot workability of steel. This effect cannot be obtained if the Mn content is less than 0.1%. On the other hand, if the Mn content is too high, it promotes the formation of Cr--Mn spinel-type oxide and lowers the steam oxidation resistance. Mn further promotes the formation of sigma (σ) phase and reduces the hot workability of steel. Therefore, the Mn content is set to 0.1 to 0.7%. The Mn content is preferably 0.15% or more, more preferably 0.2% or more. Also, the Mn content is preferably 0.65% or less, more preferably 0.6% or less.

P: 0.050% or less Phosphorus (P) is an impurity. P reduces the hot workability and ductility of steel. Therefore, the P content should be 0.050% or less. The P content is preferably 0.030% or less, preferably as low as possible.

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

N: 0.10-0.50%
Nitrogen (N) combines with Nb to form nitrides and increases the high temperature creep strength of the material. If the N content is too low, the above effect cannot be obtained. On the other hand, when nitrides are heated at a high temperature for a long time, they agglomerate and become coarse. Coarse nitrides reduce the creep strength of steel. Therefore, the N content should be 0.10 to 0.50%. The N content is preferably 0.12% or more, more preferably 0.15% or more. Also, the Nb content is preferably 0.45% or less, more preferably 0.40% or less.

Cr: 16.0-26.0%
Chromium (Cr) increases the steam oxidation resistance of steel. Cr forms a chromia (Cr ₂ O ₃ ) film near the surface of steel in a high-temperature steam environment of 700° C. or higher. By forming a uniform chromia film on the steel surface, the steam oxidation resistance of the steel is enhanced. If the Cr content is too low, the above effects cannot be obtained. On the other hand, if the Cr content is too high, the stability of the structure will decrease and the creep strength will decrease. Therefore, the Cr content should be 16.0 to 26.0%. The Cr content is preferably 18.0% or more, more preferably 20.0% or more. Also, the Cr content is preferably 25.0% or less, more preferably 24.0% or less.

Ni: 18.0% or more and less than 35.0% Nickel (Ni) stabilizes austenite. Ni further enhances the steam oxidation 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, not only do these effects saturate, but also the hot workability deteriorates, increasing the manufacturing cost. In addition, the raw material cost increases due to the Ni content more than necessary. Therefore, the Ni content should be 18.0% or more and less than 35.0%. The Ni content is 19.0%, more preferably 20.0%. A preferable upper limit of the Ni content is 34.0%, more preferably 33.0%.

Nb: 0.01-1.0%
Niobium (Nb) combines with N to form nitrides. Nb further combines with Ni and Fe to form a γ″ phase (Ni ₃ Nb) and a Laves phase (Fe ₂ Nb), respectively. to increase the high-temperature creep strength of the material.If the Nb content is too low, the above effects cannot be obtained.On the other hand, if the Nb content is too high, the toughness and hot workability of the steel are reduced.Therefore, the Nb content The amount is 0.01 to 1.0%, the Nb content is preferably 0.05% or more, more preferably 0.10% or more, and the Nb content is 0.90%. It is preferably 0.80% or less, more preferably 0.80% or less.

B: 0.0005 to 0.010%
Boron (B) increases the strength of the grain boundary by segregating at the grain boundary. This suppresses slip deformation at high temperatures and increases creep strength. If the B content is too low, the above effect cannot be obtained. On the other hand, if the B content is too high, the segregation of grain boundaries becomes excessive due to the welding heat cycle in the heat affected zone during welding, and the melting point of the grain boundaries decreases, increasing the liquation cracking susceptibility. Therefore, the B content should be 0.0005 to 0.010%. The B content is preferably 0.0008% or more, more preferably 0.001% or more. Also, the B content is preferably 0.008% or less, more preferably 0.006% or less.

Mo: 0-5.0%
W: 0-10.0%
0.5<Mo+W/2<8.0 (i)
Molybdenum (Mo) and tungsten (W) increase the high-temperature strength of steel by solid-solution strengthening in the structure or precipitation strengthening by precipitating fine intermetallic compounds. If the content of these elements is too low, the above effect cannot be obtained.

On the other hand, if the content of these elements is too high, not only the effect will saturate, but also the hot workability will deteriorate. Therefore, the median value of formula (i) should be more than 0.5% and less than 8.0%. The median value of formula (i) is preferably 0.8% or more, more preferably 1.0% or more. The median value of formula (i) is preferably 7.8% or less, more preferably 7.5% or less.

Ti: 0-1.0%
Titanium (Ti) combines with N to form nitrides and increases the high temperature creep strength of the material. However, if the Ti content is too high, nitrides form coarsely, deteriorating stress corrosion cracking resistance, high-temperature strength, workability and weldability. Therefore, the Ti content should be 1.0% or less. The Ti content is preferably 0.8% or less, more preferably 0.5% or less. In order to obtain the above effect, the Ti content is preferably 0.001% or more, more preferably 0.002% or more.

Al: 0-0.3%
Aluminum (Al) acts as a deoxidizing element during manufacturing and cleans the steel. However, if the Al content is too high, a large amount of non-metallic inclusions are formed, and stress corrosion cracking resistance, high-temperature strength, workability, toughness, and structural stability at high temperatures are lowered. Therefore, the Al content is set to 0.3% or less. The Al content is preferably 0.2% or less, more preferably 0.1% or less. In order to obtain the above effects, the Al content is preferably 0.005% or more, more preferably 0.01% or more.

Ca: 0-0.1%
Calcium (Ca) is added as a deoxidizing finish. However, too high a Ca content reduces stress corrosion cracking resistance, high temperature strength, workability and toughness. Therefore, the Ca content should be 0.1% or less. The Ca content is preferably 0.05% or less, more preferably 0.01% or less. In order to obtain the above effects, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more.

Mg: 0-0.1%
Magnesium (Mg) contributes to the improvement of high-temperature strength and corrosion resistance when added in a very small amount. However, too high a Mg content reduces strength, toughness, corrosion resistance and weldability. Therefore, the Mg content should be 0.1% or less. The Mg content is preferably 0.05% or less, more preferably 0.01% or less. In order to obtain the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more.

Co: 0-10.0%
Cobalt (Co) stabilizes the metal structure and contributes to the improvement of high-temperature strength. However, if the Co content is too high, the effect saturates, leading to an increase in cost. In addition, when the same melting furnace is used to manufacture other steels, Co contamination is caused. Therefore, the Co content should be 10.0% or less. The Co content is preferably 8.0% or less, more preferably 7.0% or less. To obtain the above effects, the Co content is preferably 0.1% or more, more preferably 0.5% or more.

Cu: 0-4.0%
Copper (Cu) precipitates as a fine Cu phase that is stable at high temperatures, and contributes to the improvement of long-term strength in a temperature range of 650° C. or lower. However, too high a Cu content reduces strength, workability and creep ductility. Therefore, the Cu content is set to 4.0% or less. The Cu content is preferably 3.0% or less, more preferably 2.0% or less. In order to obtain the above effects, the Cu content is preferably 0.001% or more, more preferably 0.005% or more.

V: 0-1.0%
Vanadium (V) combines with N to form nitrides and increases the high temperature creep strength of the material. However, if the V content is too high, nitrides form coarsely, deteriorating stress corrosion cracking resistance, high-temperature strength, workability and weldability. Therefore, the V content should be 1.0% or less. The V content is preferably 0.8% or less, more preferably 0.5% or less. To obtain the above effects, the V content is preferably 0.01% or more, more preferably 0.05% or more.

Zr: 0-0.2%
Zirconium (Zr) combines with N and O to form Zr nitride or Zr oxide. These compounds act as fine precipitation nuclei of carbonitrides, improving the high-temperature creep strength. However, if the Zr content is too high, a large amount of Zr nitrides or Zr oxides are produced, degrading hot workability and weldability. Therefore, the Zr content should be 0.2% or less. The Zr content is preferably 0.05% or less, more preferably 0.01% or less. In order to obtain the above effects, the Zr content is preferably 0.0001% or more, more preferably 0.0005% or more.

Hf: 0-0.2%
Hafnium (Hf) enhances the effect of adding Ta, REM and Zr. However, if the Hf content is too high, the amount of non-metallic inclusions increases, reducing strength, workability, toughness and weldability. Therefore, the Hf content should be 0.2% or less. The Hf content is preferably 0.05% or less, more preferably 0.01% or less. In order to obtain the above effects, the Hf content is preferably 0.0001% or more, more preferably 0.0005% or more.

Ta: 0-1.0%
Tantalum (Ta) promotes the refinement of carbonitrides and contributes to the improvement of high-temperature long-term strength and the stabilization of the structure. However, if the Ta content is too high, the amount of precipitates produced increases, leading to a decrease in toughness. Therefore, the Ta content should be 1.0% or less. The Ta content is preferably 0.8% or less, more preferably 0.6% or less. In order to obtain the above effect, the Ta content is preferably 0.0001% or more, more preferably 0.0005% or more.

REM: 0-0.1%
Rare earth elements (REMs) form sulfides and enhance the hot workability of steel. REM also forms oxides to enhance corrosion resistance, creep strength and creep ductility. However, if the REM content is too high, excessive oxide formation will occur, reducing the hot workability and weldability of the steel. Therefore, the REM content should be 0.1% or less. The REM content is preferably 0.05% or less, more preferably 0.01% or less. To obtain the above effect, the REM content is preferably 0.0001% or more, more preferably 0.0005% or more.

Note that REM is a generic term for a total of 17 elements including Sc, Y and lanthanoids, and REM content refers to the total content of one or more elements in REM. REM is generally contained in misch metal. For this reason, for example, REM may be added in the form of misch metal, and the amount of REM may be adjusted so as to fall within the above range.

Sn: 0-0.010%
Sb: 0-0.010%
Pb: 0-0.001%
As: 0-0.001%
Tin (Sn), antimony (Sb), lead (Pb), and arsenic (As) are elements that may be mixed from scraps of steel raw materials. Although the content of these elements is preferably as small as possible, considering industrial production, Sn and Sb should be 0.010% or less, and Pb and As should be 0.001% or less.

Bi: 0 to 0.001%
Bismuth (Bi) is an element that is not normally mixed, but may be mixed from scraps of steel raw materials. Bi is an element harmful to high-temperature strength and stress corrosion cracking resistance, so it must be reduced as much as possible, and is made 0.001% or less.

The balance of the austenitic stainless steel according to the invention is Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or manufacturing environment as raw materials when steel is manufactured industrially, and are within a range that does not adversely affect the austenitic stainless steel of the present invention. permissible in

2. Scale As shown in FIG. 1, scale including an outer layer scale and an inner layer scale is formed on at least a portion of the surface of the base material. The inner layer scale can be divided into a first inner layer scale, a second inner layer scale and a third inner layer scale in order from the base material side.

The first inner layer scale is a thin layer formed mainly of Cr ₂ O ₃ and formed at the boundary between the scale and the base material. For example, the first inner layer scale contains 80% or more Cr ₂ O ₃ by volume fraction. The second inner layer scale is a layer mainly composed of a Cr--Mn system spinel-type oxide, and contains, for example, 80% or more of (Cr, Mn) ₃ O ₄ by volume. The third inner layer scale is a layer mainly composed of Fe—Cr system spinel type oxide, and contains, for example, 80% or more of (Fe, Cr) ₃ O ₄ in volume fraction. Further, the outer layer scale is a layer mainly composed of Fe ₃ O ₄ and contains, for example, 80% or more of Fe ₃ O ₄ in volume fraction.

As described above, the uniform formation of the first inner layer scale mainly composed of Cr ₂ O ₃ improves the steam oxidation resistance. Here, increasing the ratio of the Cr content to the Mn content in the second inner layer scale increases the protection of the second inner layer scale. As a result, the second inner layer scale is formed thinner, promoting the formation of the first inner layer scale. On the other hand, when the ratio of the Mn content to the Cr content in the second inner layer scale is high, the protective properties of the scale decrease, the oxygen permeability increases, and the formation of the first inner layer scale is inhibited.

Therefore, in order to obtain steel with excellent resistance to steam oxidation, the chemical composition of the second inner layer scale must satisfy the following formula (ii).
_CrI2 >40× _MnI2 (ii)
However, the meaning of each symbol in the above formula is as follows.
Cr _I2 : Cr content of the second inner layer scale (% by mass)
_MnI2 : Mn content in the second inner layer scale (% by mass)

Further, the Cr--Mn system spinel-type oxide in the second inner layer scale contains a large amount of Cr, while the Fe--Cr system spinel-type oxide in the third inner layer scale contains a large amount of Fe. It is Therefore, if the thickness of the second inner scale layer is greater than the thickness of the third inner scale layer, Cr is consumed excessively, and the first inner scale layer mainly composed of Cr ₂ O ₃ is difficult to form uniformly. Furthermore, if the total thickness of the second inner layer scale and the third inner layer scale is excessive, the scale is likely to flake off.

Therefore, it is preferable to satisfy the following formulas (iii) and (iv).
_tI2 < _tI3 (iii)
_tI2 + _tI3 ≤ 20 (iv)
However, the meaning of each symbol in the above formula is as follows.
t _I2 : Thickness of the second inner layer scale (μm)
t _I3 : Thickness of the third inner layer scale (μm)

The chemical composition and thickness of the scale shall be measured by the following method. First, a test piece is cut out so that the cross section of the steel can be observed, and then buffed with diamond particles having a particle size of 3 μm or less. Then, using a scanning electron microscope (SEM), a backscattered electron image of a cross section in an arbitrary range of 1 mm is observed to identify the layer structure of the scale. At this time, the type of oxide constituting each layer is confirmed by Raman spectroscopy.

After that, the Cr and Mn contents are quantitatively analyzed by energy dispersive X-ray spectroscopy (EDX) in the region identified as the second inner layer scale mainly composed of Cr--Mn system spinel oxide. Quantitative analysis shall be performed at 5 or more arbitrary points, and the average value shall be adopted.

In the present invention, the thicknesses of the second inner layer scale and the third inner layer scale are the respective thicknesses (μm) at the position where the total thickness of the scales is maximum within the above range.

3. Average Grain Size In order to obtain the desired creep rupture strength, it is necessary to promote recrystallization of the austenite structure in the base metal and coarsen the structure. Specifically, in the austenitic stainless steel according to the present invention, the average grain size of the base material is set to No. 4 or less. If the average grain size exceeds No. 4, the required creep rupture strength cannot be obtained. The method for measuring the average grain size conforms to JIS G0551.

4. Manufacturing Method A method for manufacturing an austenitic stainless steel according to the present invention will be described. In addition, the austenitic stainless steel of the present invention is not limited to those produced by the following method.

First, molten steel having the chemical composition described above is produced. A well-known degassing treatment is performed on the manufactured molten steel as necessary.

Next, the molten steel is made into a continuously cast material by a continuous casting method. Continuously cast materials are, for example, slabs, blooms, billets, and the like. Molten steel may be made into ingots by an ingot casting method. Continuously cast material or ingots can be forged, hot worked, and cold worked by well-known methods to form austenitic stainless steel materials of arbitrary shapes. Arbitrary shapes are, for example, steel pipes, steel plates, steel bars, wire rods, and forged steel.

The manufactured austenitic stainless steel material is heat-treated at 1100 to 1300° C. for 1 to 60 minutes in an atmosphere having an oxygen partial pressure of 1.32×10 ⁻⁷ to 1.17×10 ⁻⁵ atm. implement. The heat treatment promotes recrystallization of the austenite structure in the base material, improves the creep rupture strength, forms suitable scales on at least part of the surface of the base material, and improves steam oxidation resistance. Improve.

If the water vapor concentration in the heat treatment atmosphere is less than 1.32×10 ⁻⁷ atm, suitable scale cannot be formed on the surface of the base material, and the resistance to water vapor oxidation decreases. On the other hand, when it exceeds 1.17×10 ⁻⁵ atm, coarse scales are formed on the surface of the base material, and the resistance to steam oxidation is lowered.

On the other hand, if the heat treatment temperature is less than 1100° C., recrystallization of the austenite structure becomes insufficient, resulting in a decrease in creep rupture strength. On the other hand, when the heat treatment temperature exceeds 1300° C., the scale formed becomes too thick, and the resistance to steam oxidation deteriorates. The heat treatment temperature is preferably 1120° C. or higher, more preferably 1150° C. or higher. Also, the heat treatment temperature is preferably 1280° C. or lower, more preferably 1260° C. or lower.

Furthermore, if the heat treatment time is less than 1 minute, the recrystallization of the austenite structure is insufficient and the creep rupture strength is lowered. On the other hand, when the heat treatment time exceeds 60 minutes, the formed scale becomes too thick, resulting in deterioration of steam oxidation resistance. The heat treatment time is preferably 2 minutes or longer, more preferably 3 minutes or longer. Also, the heat treatment time is preferably 45 minutes or less, more preferably 30 minutes or less.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

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

Using the molten steel of each test number shown in Table 1, ingots were produced. The ingot was hot worked to produce an austenitic stainless steel sheet.

Next, two test pieces each having a thickness of 3 mm, a width of 10 mm, and a length of 25 mm and one test piece having a thickness of 8 mm, a width of 10 mm, and a length of 50 mm were prepared from the manufactured stainless steel plate. #600 finishing was performed by wet polishing on the front and back surfaces of the obtained test piece. Heat treatment was performed under the conditions shown in Table 2 for the polished test pieces. The two test pieces with a thickness of 3 mm were used as test pieces for observation and steam oxidation test, respectively, and the test piece with a thickness of 8 mm was used as a test piece for creep rupture test.

After that, each specimen for observation after heat treatment obtained from each steel plate was cut, embedded in resin, and the cross section was subjected to buffing finish using diamond particles having a particle diameter of 3 μm or less. Then, the layer structure of the scale was specified by observing a backscattered electron image of the cross section in an arbitrary range of 1 mm using SEM. At this time, the types of oxides forming each layer were confirmed by performing Raman spectroscopy.

After that, the Cr and Mn contents were measured by energy dispersive X-ray spectroscopy (EDX) in the region identified as the second inner layer scale mainly composed of Cr--Mn system spinel-type oxide. The measurement was performed at arbitrary 10 points in the field of view, and the average value was taken as the Cr content (Cr _I2 ) and Mn content (Mn _I2 ) of the second inner layer scale.

In addition, the thickness (t _I2 ) of the second inner layer scale and the thickness (t _I3 ) of the third inner layer scale were obtained at the position where the total thickness of the scale within the field of view is maximum.

Furthermore, using the same observation test piece, the average crystal grain size of the base material of each test piece was measured. Specifically, after the observation surface was mechanically polished, it was corroded using an etchant to reveal the crystal grain boundaries of the observation surface. In 10 fields on the corroded surface, the average grain size of each field was determined. The area of each field of view is approximately 0.75 mm ² . Using the obtained average crystal grain size, the average crystal grain size was obtained using the following formula based on JIS G0551.
Index G (ASTM) = -3.2877 - 6.6439 log ₁₀ L
Here, the index G represents the average crystal grain size, and L represents the average crystal grain size (mm).

Subsequently, the following steam oxidation test was performed on the steam oxidation test specimen after heat treatment obtained from each steel plate. The test piece was inserted into a horizontal tubular heating furnace while being suspended and held by a jig. Then, a steam oxidation test was carried out at 700° C. for 2000 hours in a steam atmosphere with a dissolved oxygen content of 100 ppb. Then, the corrosion weight gain (mg/cm ² ) was obtained from the difference in weight before and after the test.

Furthermore, a test piece for creep rupture test was taken from the center of the thickness direction of the heat-treated test piece for creep rupture test obtained from each steel plate so that the rolling direction was the longitudinal direction. The test piece was a round bar with a diameter of 6 mm and a gauge length of 30 mm. Using this test piece, a creep rupture test was performed in an air atmosphere at 700 to 800° C., and the creep rupture strength at 700° C. for 1.0×10 ⁴ hours was determined using the Larson-Miller parameter method.

These results are summarized in Table 3. In the present invention, when the corrosion weight gain is 20.0 mg/cm ² or less, the steam oxidation resistance is judged to be excellent, and when the creep rupture strength is 125 MPa or more, the creep rupture strength is judged to be excellent. It was decided to.

As shown in Table 3, test no. Nos. 1 to 18 satisfy all the requirements of the present invention, and therefore are excellent in both creep rupture strength and steam oxidation resistance. However, test No. Test No. 16 has a high oxygen partial pressure during heat treatment. Test No. 17 has a high heat treatment temperature. In No. 18, the heat treatment time is long and is outside the preferred manufacturing conditions. Therefore, the thickness of the scale formed was out of the preferable range, and the steam oxidation resistance was lower than Test No. 1. The results were slightly inferior to those of 1 to 15.

On the other hand, Test No. Nos. 19 to 21 are comparative examples in which the chemical composition of the base material satisfies the requirements of the present invention, but the production conditions are inappropriate and the formed scale does not satisfy the requirements. Specifically, Test No. In No. 19, the oxygen partial pressure during heat treatment was too low, so the ratio of the Cr content to the Mn content in the second inner layer scale was low, resulting in poor steam oxidation resistance. Also, test no. Test No. 20 has a low heat treatment temperature. In No. 21, since the heat treatment time was short, recrystallization of the austenite structure was insufficient, resulting in poor creep rupture strength.

Furthermore, test no. Nos. 22 to 25 are comparative examples in which the chemical composition of the base material is outside the scope of the present invention. Specifically, Test No. Test No. 22 has a high Mn content. Since No. 23 had a low Cr content, the ratio of Cr content to Mn content in the second inner layer scale was low, resulting in poor steam oxidation resistance. Test no. Since the Cr content of No. 24 was high, the stability of the structure was lowered, resulting in poor creep rupture strength. And test no. Since No. 25 contained neither Mo nor W, the effect of solid-solution strengthening was not obtained, resulting in inferior creep rupture strength.

According to the present invention, it is possible to obtain an austenitic stainless steel having excellent steam oxidation resistance and creep rupture strength even in a high temperature environment of 700° C. or higher.

Claims (2)

An austenitic stainless steel having scales formed on at least part of the surface of the base material,
The scale is, in order from the base material side,
a first inner layer scale mainly composed of Cr ₂ O ₃ ;
a second inner layer scale mainly composed of a Cr—Mn-based spinel-type oxide;
including a third inner layer scale mainly composed of Fe—Cr spinel-type oxide and an outer layer scale mainly composed of Fe ₃ O ₄ ,
The chemical composition of the base material is, in mass %,
C: 0.01% to 0.20%,
Si: 1.0% or less,
Mn: 0.1-0.7%,
P: 0.050% or less,
S: 0.010% or less,
N: 0.10 to 0.50%,
Cr: 16.0 to 26.0%,
Ni: 18.0% or more and less than 35.0%,
Nb: 0.01 to 1.0%,
B: 0.0005 to 0.010%,
Mo: 0-5.0%,
W: 0 to 10.0%,
Ti: 0 to 1.0%,
Al: 0-0.3%,
Ca: 0-0.1%,
Mg: 0-0.1%,
Co: 0 to 10.0%,
Cu: 0-4.0%,
V: 0-0.5 %,
Zr: 0 to 0.2%,
Hf: 0-0.2%,
Ta: 0 to 1.0%,
REM: 0-0.1%,
Sn: 0 to 0.010%,
Sb: 0 to 0.010%,
Pb: 0 to 0.001%,
As: 0 to 0.001%,
Bi: 0 to 0.001%,
Balance: Fe and impurities, and satisfying the following formula (i),
the chemical composition of the second inner layer scale satisfies the following formula (ii),
The average grain size of the base material is No. 4 or less,
Austenitic stainless steel.
1.0≦ Mo+W/2<8.0 (i)
_CrI2 >40× _MnI2 (ii)
However, the meaning of each symbol in the above formula is as follows.
Mo: Mo content of the base material (% by mass)
W: W content of the base material (% by mass)
Cr _I2 : Cr content of the second inner layer scale (% by mass)
_MnI2 : Mn content in the second inner layer scale (% by mass) When the thickness of the second inner layer scale is t _I2 (μm) and the thickness of the third inner layer scale is t _I3 (μm), the following equations (iii) and (iv) are satisfied:
The austenitic stainless steel according to claim 1.
_tI2 < _tI3 (iii)
_tI2 + _tI3 ≤ 20 (iv)

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