JP7339526B2 - Austenitic stainless steel - Google Patents

Description

TECHNICAL FIELD The present disclosure relates to steel materials, and more particularly to austenitic stainless steel materials.

High-temperature strength is required for steel materials used in chemical plant facilities such as petroleum refining plants and petrochemical plants. Austenitic stainless steel materials are used as steel materials for these chemical plant equipment applications.

Chemical plant equipment includes multiple devices. Each device of the chemical plant equipment is, for example, a vacuum distillation device, a desulfurization device, a catalytic reforming device, and the like. These devices include furnace tubes, reactor columns, vessels, heat exchangers, piping, and the like. These devices are welded structures formed by welding steel materials.

A heating furnace tube or the like of a vacuum distillation apparatus is generally operated at 300 to 400° C., and the pressure applied to the heating furnace tube is not so high. However, when the petroleum is vaporized during heating, an insulating effect is generated by the vaporized petroleum. Due to this insulating effect, the temperature of the heating furnace tube may rise to about 700°C. Therefore, austenitic stainless steel materials used in chemical plant equipment are required to have high creep strength in a high temperature environment of 300 to 700°C. In addition, high creep ductility is also required for the austenitic stainless steel materials used in the above-described high-temperature environment in order to suppress fracture of the steel materials during operation.

International Publication No. 2018/043565 (Patent Document 1) discloses improvements in creep strength and creep ductility of austenitic stainless steel materials used in high temperature ranges. The austenitic stainless steel disclosed in this document has C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0% by mass. .040% or less, S: 0.010% or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.0% 20-1.00%, N: 0.050-0.300%, sol. Al: 0.0005-0.100%, B: 0.0010-0.0080%, Cu: 0-5.0%, W: 0-5.0%, Co: 0-1.0%, V : 0 to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, and rare earth elements: 0 ~0.10%, the balance being Fe and impurities, and having a chemical composition that satisfies B+0.004−0.9C+0.017Mo ² ≧0. The austenitic stainless steel proposed in Patent Document 1 has excellent creep strength and creep ductility by adjusting the B, Cu and Mo contents.

WO2018/043565

By the way, depending on equipment such as chemical plant equipment, there are cases where it is required to have both creep strength and creep ductility in a high-temperature environment for a longer period of time than assumed in Patent Document 1. In particular, an austenitic stainless steel material having creep strength and creep ductility with a creep rupture time of 10,000 hours or more and a creep rupture reduction of 50% or more in a creep rupture test at 700°C with a test stress of 80 MPa. It has been demanded.

An object of the present disclosure is to provide an austenitic stainless steel material capable of achieving both excellent creep strength and excellent creep ductility even when used for a long time in a high temperature environment.

The austenitic stainless steel material according to the present disclosure is
The chemical composition, in mass %,
C: 0.030% or less,
Si: 0.10 to 1.00%,
Mn: 0.2-2.0%,
P: 0.01 to 0.04%,
S: 0.0100% or less,
Cr: 15.00 to 25.00%,
Ni: 9.00 to 18.00%,
Mo: 1.0 to 5.0%,
Nb: 0.20 to 2.00%,
N: 0.050 to 0.180%,
sol. Al: 0.001 to 0.080%,
B: 0.0005 to 0.0080%,
Cu: 0 to 2.00%,
V: 0 to 1.00%,
Co: 0 to 1.0%,
Y: 0 to 1.00%,
Zr: 0 to 1.0%,
Hf: 0-0.20%,
Ta: 0 to 0.20%,
W: 0 to 5.0%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%, and
Rare earth elements other than Y: 0 to 0.100%,
The balance consists of Fe and impurities,
The grain size number is 4.0 to 9.0,
When the B concentration (% by mass) at the austenite grain boundaries in the austenitic stainless steel material is defined as [B _GB ], and the B concentration (% by mass) within the austenite crystal grains is defined as [B _BM ], the formula (1) is satisfied.
[B _GB ]/[B _BM ]≧500 (1)

The austenitic stainless steel material of the present disclosure can achieve both excellent creep strength and excellent creep ductility even when used for a long time in a high temperature environment.

FIG. 1 is a schematic diagram for explaining a method for measuring the degree of grain boundary B segregation ([B _GB ]/[B _BM ]).

The present inventors investigated an austenitic stainless steel material capable of achieving both excellent creep strength and excellent creep ductility in a high-temperature environment.

The present inventors first investigated the chemical composition of steel. When used in chemical plant facilities, etc., the steel material is also required to have polythionic acid SCC resistance. It is known that Cr and Mo are effective for polythioic acid SCC resistance. Furthermore, stabilizing austenite is effective in increasing the creep strength of steel in a high-temperature environment. Ni, Mn, and N are effective in stabilizing austenite. Furthermore, both creep strength and creep ductility can be increased by increasing grain boundary strength in a high temperature environment. B is effective for increasing the grain boundary strength.

In consideration of the above matters, the present inventors investigated the chemical composition of the austenitic stainless steel material. As a result, the chemical composition is, in mass%, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.2 to 2.0%, P: 0.01 to 0.04 %, S: 0.0100% or less, Cr: 15.00-25.00%, Ni: 9.00-18.00%, Mo: 1.0-5.0%, Nb: 0.20-2 .00%, N: 0.050-0.180%, sol. Al: 0.001 to 0.080%, B: 0.0005 to 0.0080%, Cu: 0 to 2.00%, V: 0 to 1.00%, Co: 0 to 1.0%, Y : 0-1.00%, Zr: 0-1.0%, Hf: 0-0.20%, Ta: 0-0.20%, W: 0-5.0%, Ca: 0-0. 0100%, Mg: 0 to 0.0100%, rare earth elements other than Y: 0 to 0.100%, and the balance being Fe and impurities. It was considered possible to achieve both creep strength and excellent creep ductility.

The present inventors also paid attention to the segregation of B at grain boundaries. B segregates at grain boundaries to increase grain boundary strength. A higher grain boundary strength not only increases the creep strength, but also increases the creep ductility. That is, by increasing the degree of B segregation at the grain boundaries, both creep strength and creep ductility can be increased. Therefore, the present inventors investigated a method for increasing the degree of B segregation at grain boundaries. As a result, it was found that the degree of B segregation at grain boundaries is related to the size of austenite grains. This point will be described below.

If the austenite grains are fine, the grain boundary area increases. In this case, deformation at grain boundaries is promoted in a high-temperature environment. Therefore, the creep strength is lowered. Therefore, considering the creep strength, the larger the austenite grains, the better.

However, when B segregation at grain boundaries is taken into account, the larger the austenite grains, the longer the distance until B in the grains moves to the grain boundaries. In this case, even if B diffuses, it becomes difficult for B to reach the grain boundary, and as a result, the degree of segregation of B at the grain boundary cannot be increased. Therefore, the austenite grains should be small enough to ensure a sufficiently high degree of B segregation at the grain boundaries and large enough to maintain sufficient creep strength in a high-temperature environment.

Accordingly, the present inventors have investigated the appropriate range of the grain size number in the austenitic stainless steel material having the chemical composition described above. As a result, it was found that if the grain size number is 4.0 to 9.0, the creep strength in a high-temperature environment can be sufficiently maintained while sufficiently increasing the degree of B segregation at the grain boundary.

However, even with an austenitic stainless steel material having the chemical composition described above and a grain size number of 4.0 to 9.0, it is difficult to achieve both sufficient creep strength and sufficient creep ductility in a high-temperature environment. there was a case. Therefore, the present inventors conducted further studies. As described above, both creep strength and creep ductility can be increased by sufficiently increasing the degree of B segregation at grain boundaries. Therefore, the present inventors investigated the degree of B segregation at grain boundaries. As a result, when the B concentration (% by mass) at the austenite grain boundaries in the austenitic stainless steel is defined as [B _GB ], and the B concentration (% by mass) within the austenite grains is defined as [B _BM ] If the austenitic stainless steel material having the chemical composition described above and having a grain size number of 4.0 to 9.0 further satisfies the formula (1), it has excellent creep strength and excellent creep ductility in a high-temperature environment. It was discovered that both
[B _GB ]/[B _BM ]≧500 (1)

The austenitic stainless steel material of this embodiment completed based on the above findings has the following configuration.

[1]
An austenitic stainless steel material,
The chemical composition, in mass %,
C: 0.030% or less,
Si: 0.10 to 1.00%,
Mn: 0.2-2.0%,
P: 0.01 to 0.04%,
S: 0.0100% or less,
Cr: 15.00 to 25.00%,
Ni: 9.00 to 18.00%,
Mo: 1.0 to 5.0%,
Nb: 0.20 to 2.00%,
N: 0.050 to 0.180%,
sol. Al: 0.001 to 0.080%,
B: 0.0005 to 0.0080%,
Cu: 0 to 2.00%,
V: 0 to 1.00%,
Co: 0 to 1.0%,
Y: 0 to 1.00%,
Zr: 0 to 1.0%,
Hf: 0-0.20%,
Ta: 0 to 0.20%,
W: 0 to 5.0%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%, and
Rare earth elements other than Y: 0 to 0.100%,
The balance consists of Fe and impurities,
The grain size number is 4.0 to 9.0,
When the B concentration (% by mass) at the austenite grain boundaries in the austenitic stainless steel material is defined as [B _GB ], and the B concentration (% by mass) within the austenite crystal grains is defined as [B _BM ], the formula satisfy (1),
Austenitic stainless steel material.
[B _GB ]/[B _BM ]≧500 (1)

[2]
The austenitic stainless steel material according to [1],
The chemical composition is
Cu: 0.05 to 2.00%,
V: 0.10 to 1.00%,
Co: 0.1 to 1.0%,
Y: 0.01 to 1.00%
Zr: 0.1-1.0%
Hf: 0.01 to 0.20%,
Ta: 0.01 to 0.20%, and
W: 0.1 to 5.0%, containing one or more elements selected from the group consisting of
Austenitic stainless steel material.

[3]
The austenitic stainless steel material according to [1] or [2],
The chemical composition is
Ca: 0.0005 to 0.0100%,
Mg: 0.0005 to 0.0100%, and
Rare earth elements other than Y: containing one or more elements selected from the group consisting of 0.001 to 0.100%,
Austenitic stainless steel material.

[4]
The austenitic stainless steel material according to any one of [1] to [3],
The chemical composition further satisfies formula (2),
Austenitic stainless steel material.
0.2×Mo+5×Nb+500×B>2.00 (2)

The austenitic stainless steel material of this embodiment will be described in detail below. "%" for elements means % by weight unless otherwise specified.

[About chemical composition]
The chemical composition of the austenitic stainless steel material of this embodiment contains the following elements.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the C content is over 0%. C forms M ₂₃ C ₆ type Cr carbides at grain boundaries during the use of austenitic stainless steel materials in high temperature environments. This Cr carbide lowers the polythioic acid SCC resistance of the steel material. If the C content exceeds 0.030%, the polythionic acid SCC resistance of the steel material is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.030% or less. A preferable upper limit of the C content is 0.028%, more preferably 0.024%, and still more preferably 0.022%. The C content is preferably as low as possible. However, excessive reduction of the C content increases manufacturing costs. Therefore, in view of industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.002%.

Si: 0.10-1.00%
Silicon (Si) deoxidizes steel in the steelmaking process. Si further enhances the oxidation resistance and steam oxidation resistance of steel in high temperature environments. If the Si content is less than 0.10%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 1.00%, a sigma phase (σ phase) is formed in the steel material in a high-temperature environment even if the contents of other elements are within the ranges of the present embodiment. In this case, the creep strength of the steel material is lowered in a high-temperature environment. Therefore, the Si content is 0.10-1.00%. A preferable lower limit of the Si content is 0.14%, more preferably 0.16%, and still more preferably 0.18%. A preferable upper limit of the Si content is 0.90%, more preferably 0.80%, and still more preferably 0.75%.

Mn: 0.2-2.0%
Manganese (Mn) stabilizes austenite and increases the creep strength of steel in high temperature environments. Mn also deoxidizes steel in the steelmaking process. If the Mn content is less than 0.2%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 2.0%, the σ phase is formed in the steel material in a high-temperature environment even if the content of other elements is within the range of the present embodiment. In this case, the creep strength of the steel material is lowered in a high-temperature environment. Therefore, the Mn content is 0.2-2.0%. A preferred lower limit for the Mn content is 0.3%, more preferably 0.4%. A preferable upper limit of the Mn content is 1.9%, more preferably 1.8%, and still more preferably 1.7%.

P: 0.01 to 0.04%
Phosphorus (P) segregates at grain boundaries in a high-temperature environment to increase the creep ductility of steel materials. If the P content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the P content exceeds 0.04%, the hot workability and toughness of the steel are lowered even if the content of other elements is within the range of the present embodiment. Therefore, the P content is 0.01-0.04%. A preferable lower limit of the P content is 0.02%. A preferable upper limit of the P content is 0.03%.

S: 0.0100% or less Sulfur (S) is inevitably contained. That is, the S content is over 0%. S lowers the hot workability and toughness of steel. If the S content exceeds 0.0100%, the hot workability and toughness are remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the S content is 0.0100% or less. The upper limit of the S content is preferably 0.0080%, more preferably 0.0070%, still more preferably 0.0060%, still more preferably 0.0050%. It is preferable that the S content is as low as possible. However, an excessive reduction in the S content raises the production cost of steel materials. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%.

Cr: 15.00-25.00%
Chromium (Cr) enhances the polythioic acid SCC resistance of steel materials used in high-temperature environments. Cr also enhances the oxidation and corrosion resistance of steel in high temperature environments. If the Cr content is less than 15.00%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 25.00%, the stability of austenite is lowered even if the content of other elements is within the range of the present embodiment. In this case, the creep strength of the steel material is lowered in a high temperature environment. Therefore, the Cr content is 15.00-25.00%. A preferable lower limit of the Cr content is 15.50%, more preferably 15.80%, and still more preferably 16.00%. A preferable upper limit of the Cr content is 24.00%, more preferably 23.00%, still more preferably 22.00%, still more preferably 21.00%.

Ni: 9.00-18.00%
Nickel (Ni) stabilizes austenite and increases the creep strength of steel in high temperature environments. If the Ni content is less than 9.00%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ni content exceeds 18.00%, even if the content of other elements is within the range of the present embodiment, the above effects are saturated, and the manufacturing cost increases. Therefore, the Ni content is 9.00-18.00%. A preferable lower limit of the Ni content is 9.50%, more preferably 9.80%, still more preferably 10.00%, still more preferably 10.20%, still more preferably 10.2%. 40%. A preferable upper limit of the Ni content is 17.00%, more preferably 16.00%, and still more preferably 15.00%.

Mo: 1.0-5.0%
Molybdenum (Mo) suppresses the formation and growth of M ₂₃ C ₆ type Cr carbides at grain boundaries during use of the steel material in a high temperature environment. As a result, Mo enhances the polythioic acid SCC resistance of the steel material. Mo also forms a chi phase (χ phase: Fe ₁₂ Cr ₃₆ Mo ₁₀ ) during use of the steel material in a high-temperature environment, and increases the creep strength of the steel material by precipitation strengthening. Mo further promotes the diffusion of B and increases the degree of segregation of B at grain boundaries. As a result, the creep strength and creep ductility of the steel are increased in high temperature environments. If the Mo content is less than 1.0%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content exceeds 5.0%, the stability of austenite is lowered even if the content of other elements is within the range of the present embodiment. In this case, the creep strength of the steel material in a high temperature environment is rather lowered. Therefore, the Mo content is 1.0-5.0%. The lower limit of the Mo content is preferably 1.1%, more preferably 1.3%, still more preferably 1.5%, still more preferably 1.8%. The upper limit of the Mo content is preferably 4.5%, more preferably 4.0%, still more preferably 3.8%, still more preferably 3.5%.

Nb: 0.20-2.00%
Niobium (Nb) combines with C to form MX-type Nb carbonitrides. By generating Nb carbonitrides and fixing C, the amount of dissolved C in the steel material is reduced. This enhances the polythioic acid SCC resistance of the steel material in a high temperature environment. Nb carbonitrides also increase the creep strength of steel in high temperature environments. Nb further promotes the diffusion of B and increases the degree of segregation of B at grain boundaries. As a result, the creep strength and creep ductility of the steel are increased in high temperature environments. If the Nb content is less than 0.20%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Nb content exceeds 2.00%, δ ferrite is formed even if the content of other elements is within the range of the present embodiment. In this case, the creep strength of the steel material is lowered in a high-temperature environment. Furthermore, the toughness and weldability of the steel are reduced. Therefore, the Nb content is 0.20-2.00%. The lower limit of the Nb content is preferably 0.25%, more preferably 0.30%, still more preferably 0.35%, still more preferably 0.40%. A preferred upper limit of the Nb content is 1.80%, more preferably 1.60%, still more preferably 1.40%, still more preferably 1.20%.

N: 0.050-0.180%
Nitrogen (N) dissolves in the matrix (mother phase) and stabilizes austenite. This increases the creep strength in high temperature environments. Furthermore, N forms carbonitrides in grains in a high-temperature environment to increase creep strength. That is, N increases the creep strength in a high temperature environment by both solid solution strengthening and precipitation strengthening. If the N content is less than 0.050%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the N content exceeds 0.180%, Cr nitrides (Cr ₂ N) are generated at the grain boundaries even if the content of other elements is within the range of the present embodiment. In this case, during welding, the polythionic acid SCC resistance in the weld heat affected zone (HAZ) is lowered. If the N content exceeds 0.180%, the hot workability of the steel is further deteriorated. Therefore, the N content is 0.050-0.180%. A preferable lower limit of the N content is 0.060%, more preferably 0.070%. A preferable upper limit of the N content is 0.160%, more preferably 0.150%, and still more preferably 0.130%.

sol. Al: 0.001-0.080%
Aluminum (Al) deoxidizes steel in the steelmaking process. sol. If the Al content is less than 0.001%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, sol. If the Al content exceeds 0.080%, the hot workability and toughness of the steel deteriorate even if the content of other elements is within the range of the present embodiment. Therefore, sol. The Al content is 0.001-0.080%. sol. A preferable lower limit of the Al content is 0.005%, more preferably 0.007%. sol. A preferable upper limit of the Al content is 0.070%, more preferably 0.060%, and still more preferably 0.050%. In this embodiment, sol. The Al content means the content of acid-soluble Al (sol. Al).

B: 0.0005 to 0.0080%
Boron (B) segregates at grain boundaries and increases grain boundary strength. As a result, both the creep strength and creep ductility of the steel are enhanced in high temperature environments. If the B content is less than 0.0005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the B content exceeds 0.0080%, the hot workability and weldability of the steel deteriorate even if the contents of other elements are within the ranges of the present embodiment. Therefore, the B content is 0.0005-0.0080%. A preferable lower limit of the B content is 0.0008%, more preferably 0.0010%, and still more preferably 0.0012%. A preferable upper limit of the B content is 0.0070%, more preferably 0.0060%, and still more preferably 0.0050%.

The remainder of the chemical composition of the austenitic stainless steel material according to this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the austenitic stainless steel material is industrially produced, and have an adverse effect on the austenitic stainless steel material of the present embodiment. It means what is allowed as long as it does not give

[Regarding arbitrary elements]
[Arbitrary Element Group 1]
The austenitic stainless steel material of the present embodiment further contains at least one element selected from the group consisting of Cu, V, Co, Y, Zr, Hf, Ta, and W in place of part of Fe. may All of these elements increase the creep strength of steel in high temperature environments.

Cu: 0-2.00%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu precipitates as a Cu phase in the grains in a high-temperature environment, and increases the creep strength of the steel material by precipitation strengthening. If even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 2.00%, the Cu phase is excessively precipitated even if the content of other elements is within the range of the present embodiment. In this case, the creep ductility of the steel material is lowered in a high temperature environment. Therefore, the Cu content is 0-2.00%. A preferable lower limit of the Cu content is 0.01%, more preferably 0.05%, still more preferably 0.07%, still more preferably 0.08%. A preferred upper limit of the Cu content is 1.80%, more preferably 1.60%, still more preferably 1.40%, still more preferably 1.20%.

V: 0-1.00%
Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When included, V combines with C to form V carbonitrides in high temperature environments. The produced V carbonitride increases the creep strength of the steel material in a high-temperature environment. V carbonitride further reduces solute C and enhances the polythionic acid SCC resistance of the steel material. If even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 1.00%, even if the content of other elements is within the range of the present embodiment, δ ferrite is generated in the steel material, and the creep strength, toughness, and weldability of the steel material decreases. Therefore, the V content is 0-1.00%. The preferred lower limit of the V content is more than 0%, more preferably 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.12% is. A preferable upper limit of the V content is 0.90%, more preferably 0.80%.

Co: 0-1.0%
Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When included, Co stabilizes austenite and increases the creep strength of steel in high temperature environments. If even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 1.0%, the raw material cost increases. Therefore, the Co content is 0-1.0%. The lower limit of the Co content is preferably over 0%, more preferably 0.1%, and still more preferably 0.2%. A preferable upper limit of the Co content is 0.9%, more preferably 0.8%.

Y: 0 to 1.00%
Yttrium (Y) is an optional element and may not be contained. That is, the Y content may be 0%. When included, Y promotes grain boundary segregation of B and enhances the creep strength and creep ductility of the steel in high temperature environments. If Y is contained even in a small amount, the above effect can be obtained to some extent. However, if the Y content exceeds 1.00%, inclusions such as oxides increase even if the content of other elements is within the range of the present embodiment, and the workability and weldability of the steel deteriorate. do. Therefore, the Y content is 0-1.00%. The lower limit of the Y content is preferably over 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%. A preferable upper limit of the Y content is 0.90%, more preferably 0.85%, and still more preferably 0.80%.

Zr: 0-1.0%
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When included, Zr combines with carbon and nitrogen to form Zr carbonitrides. The produced Zr carbonitride increases the creep strength of the steel material in a high temperature environment. Zr further promotes the grain boundary segregation of B and enhances the creep strength of the steel material in a high temperature environment. If even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 1.0%, the creep ductility of the steel material in a high-temperature environment is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Zr content is 0-1.0%. A preferable lower limit of the Zr content is more than 0%, more preferably 0.1%. A preferred upper limit for the Zr content is 0.9%, more preferably 0.8%.

Hf: 0-0.20%
Hafnium (Hf) is an optional element and may not be contained. That is, the Hf content may be 0%. When included, Hf combines with carbon and nitrogen to form Hf carbonitrides. The Hf carbonitrides produced increase the creep strength of the steel material in a high-temperature environment. Hf carbonitrides further reduce solute C and improve the polythionic acid SCC resistance of the steel material. If even a small amount of Hf is contained, the above effect can be obtained to some extent. However, if the Hf content exceeds 0.20%, δ ferrite is formed even if the content of other elements is within the range of the present embodiment, and the creep strength, toughness, and Weldability deteriorates. Therefore, the Hf content is 0-0.20%. A preferable lower limit of the Hf content is 0.01%, more preferably 0.02%, and still more preferably 0.03%. A preferable upper limit of the Hf content is 0.18%, more preferably 0.15%, and still more preferably 0.10%.

Ta: 0-0.20%
Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content may be 0%. When included, Ta combines with carbon and nitrogen to form Ta carbonitrides. The produced Ta carbonitride increases the creep strength of the steel material in a high-temperature environment. Ta carbonitride further reduces solute C and enhances the polythionic acid SCC resistance of the steel material. If even a small amount of Ta is contained, the above effect can be obtained to some extent. However, if the Ta content exceeds 0.20%, δ ferrite is generated even if the content of other elements is within the range of the present embodiment, and the creep strength, toughness, and Weldability deteriorates. Therefore, the Ta content is 0-0.20%. A preferable lower limit of the Ta content is more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.05%. A preferable upper limit of the Ta content is 0.18%, more preferably 0.15%, and still more preferably 0.10%.

W: 0-5.0%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W forms a solid solution in the steel material and increases the creep strength of the steel material in a high temperature environment. If even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 5.0%, the raw material cost increases. Therefore, the W content is 0-5.0%. The lower limit of the W content is preferably over 0%, more preferably 0.1%, still more preferably 0.2%, still more preferably 0.3%. A preferable upper limit of the W content is 4.5%, more preferably 4.0%, and still more preferably 3.5%.

[Second Group of Arbitrary Elements]
The austenitic stainless steel material of the present embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, and rare earth elements other than Y (REM) in place of part of Fe. These elements are optional elements, and all increase the creep ductility of the steel material in a high temperature environment.

Ca: 0 to 0.0100%,
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca fixes O (oxygen) and S (sulfur) as inclusions and enhances the hot workability of the steel material and the creep ductility in high temperature environments. If even a little Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0100%, the hot workability and creep ductility of the steel deteriorate even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Ca content is 0-0.0100%. A preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0005%, still more preferably 0.0010%. A preferable upper limit of the Ca content is 0.0090%, more preferably 0.0080%, and still more preferably 0.0060%.

Mg: 0-0.0100%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When included, Mg fixes O (oxygen) and S (sulfur) as inclusions and enhances the hot workability of the steel and creep ductility in high temperature environments. If even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0100%, the hot workability and creep ductility of the steel deteriorate even if the content of other elements is within the range of the present embodiment. Therefore, the Mg content is 0-0.0100%. A preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, still more preferably 0.0005%, still more preferably 0.0010%. A preferable upper limit of the Mg content is 0.0090%, more preferably 0.0080%, and still more preferably 0.0060%.

Rare earth elements other than Y (REM): 0 to 0.100%
Rare earth elements (REM) other than Y are optional elements and may not be contained. That is, the REM content other than Y may be 0%. When included, REMs other than Y fix O (oxygen) and S (sulfur) as inclusions and enhance the hot workability of the steel and creep ductility in high temperature environments. The above effect can be obtained to some extent if REM is contained even in a small amount. However, if the content of REM other than Y exceeds 0.100%, the hot workability and creep ductility of the steel deteriorate even if the content of the other elements is within the range of the present embodiment. Therefore, the REM content other than Y is 0 to 0.100%. A preferred lower limit for the REM content other than Y is 0.001%, more preferably 0.002%. A preferable upper limit of the REM content other than Y is 0.080%, more preferably 0.060%.

In this specification, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoid (La) with atomic number 57 to atomic number 71. One or more elements selected from the group consisting of lutetium (Lu). In addition, the REM content other than Y in this specification means the total content of the above elements excluding Y.

[Chemical composition analysis method for austenitic stainless steel]
The chemical composition of the austenitic stainless steel material of this embodiment can be determined by a well-known component analysis method. Specifically, when the austenitic stainless steel material is a steel pipe, a drill is used to drill holes at the center of the wall thickness to generate chips, which are collected. When the austenitic stainless steel material is a steel plate with a plate thickness of TH mm, a drill is used to drill a hole at a depth of TH/4 in the plate thickness direction from the surface to generate chips, and the chips are collected. When the austenitic stainless steel material is a steel bar, a drill is used to drill at the R/2 position to generate chips, which are collected. Here, the R/2 position means the central position of the radius R in the cross section perpendicular to the longitudinal direction of the steel bar.

The collected chips are dissolved in acid to obtain a solution. ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are obtained by a well-known high-frequency combustion method. Specifically, the above solution is combusted by high-frequency heating in an oxygen stream, the carbon dioxide and sulfur dioxide generated are detected, and the C content and S content are determined. The N content is determined using the well-known inert gas fusion-thermal conductivity method. The chemical composition of the austenitic stainless steel material can be determined by the above analysis method.

[Regarding the grain size number]
The austenitic stainless steel material of this embodiment further has a grain size number of 4.0 to 9.0.

The smaller the grain size number, that is, the larger the austenite grains, the smaller the total area of grain boundaries in the steel. In this case, deformation at the grain boundaries is suppressed as compared with the case where the total area of the grain boundaries is large. Therefore, the creep strength is increased. However, the larger the austenite grains, the longer the distance for B in the grains to move to the grain boundaries. In this case, even if B diffuses, it becomes difficult for it to reach the grain boundary, and as a result, the degree of segregation of B at the grain boundary cannot be increased. If _the grain size number is less than 4.0, the segregation degree of B at the grain boundary is _low . Become. On the other hand, the larger the grain size number, that is, the smaller the austenite grains, the larger the total area of grain boundaries in the steel material. In this case, deformation at the grain boundaries is promoted, so the creep strength is lowered. If the grain size number exceeds 9.0, the creep strength in a high temperature environment will be low. Therefore, the grain size number is 4.0-9.0. The preferred lower limit of the grain size number is 4.1, more preferably 4.2, still more preferably 4.4, still more preferably 4.6. The preferred upper limit of the grain size number is 8.8, more preferably 8.6, still more preferably 8.4.

[Method for measuring grain size number]
The grain size number of the austenitic stainless steel material of this embodiment can be obtained by the following method. One sample is taken from the thickness center position of the austenitic stainless steel material. When the austenitic stainless steel material is a steel pipe, a sample is taken from the thickness center position. When the austenitic stainless steel material is a steel plate, a sample is taken from the center position of the plate width and the center position of the plate thickness. When the austenitic stainless steel material is a steel bar, a sample is taken from the central position of the cross section perpendicular to the longitudinal direction.

A cross-section perpendicular to the longitudinal direction of the austenitic stainless steel material among the surfaces of the collected samples is used as an observation surface. Polish the viewing surface to a mirror finish. The observation surface after mirror polishing is corroded with 10% oxalic acid to expose the grain boundaries of austenite. Any 3 fields of view of the corroded observation surface are observed, and the grain size number is obtained based on the cutting method according to JIS G 0551 (2013). The area of each field of view is 0.75 mm ² . The arithmetic mean value of the grain size numbers in the field of view is defined as the grain size number.

[Regarding grain boundary B segregation]
In the austenitic stainless steel material of the present embodiment, the B concentration (% by mass) at the austenite grain boundaries is further defined as [B _GB ], and the B concentration (% by mass) within the austenite grains is defined as [B _BM ]. Then, the formula (1) is satisfied.
[B _GB ]/[B _BM ]≧500 (1)

The grain boundary B segregation degree is defined as [B _GB ]/[B _BM ]. Grain boundary B segregation degree indicates the segregation degree of B at the grain boundary. The higher the ratio of the grain boundary B concentration to the intragranular B concentration, that is, the larger the amount of B segregation at the grain size, the greater the grain boundary strength due to B. As a result, the austenitic stainless steel material of the present embodiment has the above-described chemical composition and has a grain size number of 4.0 to 9.0. Both ductility increases. Specifically, it has the above-described chemical composition, a grain size number of 4.0 to 9.0, and a grain boundary B segregation degree (= [B _GB ]/[B _BM ]) of 500. If it is above, the creep strength in a high-temperature environment is sufficiently increased, and the creep ductility is sufficiently increased.

The lower limit of the grain boundary B segregation degree is preferably 550, more preferably 580, still more preferably 600, still more preferably 620, still more preferably 650. The upper limit of the degree of grain boundary B segregation is not particularly limited. However, if the degree of grain boundary B segregation is excessively high, the adjustment of manufacturing conditions (operating conditions) during the steel manufacturing process becomes excessively complicated. Therefore, when considering industrial productivity, the upper limit of the grain boundary B segregation degree is preferably 6000, more preferably 5000, still more preferably 4500, still more preferably 3500, still more preferably 3000. , more preferably 2500, more preferably 2000.

[Measurement method of grain boundary B segregation degree]
Grain boundary B segregation degree can be calculated|required using electron energy loss spectroscopy (Electron Energy Loss Spectroscopy:EELS). Specifically, one sample is taken from the thickness center position of the austenitic stainless steel material. When the austenitic stainless steel material is a steel pipe, a sample is taken from the thickness center position. When the austenitic stainless steel material is a steel plate, a sample is taken from the center position of the plate width and the center position of the plate thickness. When the austenitic stainless steel material is a steel bar, a sample is taken from the central position of the cross section perpendicular to the longitudinal direction.

A cross-section perpendicular to the longitudinal direction of the austenitic stainless steel material of the collected sample surface is used as an observation surface. The observation surface of the sample is mirror-polished. Grain boundaries are identified using SEM-EBSD (Scanning Electron Microscope--Electron Back Scattering Diffraction) on the mirror-polished observation surface. After identifying the grain boundaries, a 10 μm×10 μm×30 nm thin film sample containing the grain boundaries is prepared by focused ion beam (FIB) or argon ion milling. Thin film samples are prepared one by one from five different grain boundaries (that is, a total of five thin film samples are prepared).

EELS is performed on thin film samples using an electron energy loss spectroscopy (EELS) detector attached to a Transmission Electron Microscope (TEM). Specifically, as shown in FIG. 1, line analysis of 10 nm is performed in a direction perpendicular to an arbitrary crystal grain boundary GB that is an interface between crystal grains 10 . At this time, the acceleration voltage is set to 300 kV. A line analysis range L is determined so that the grain boundary GB is positioned at the center position of the range L in which the line analysis is performed. The length of the range L is assumed to be 10 nm. EELS is performed to determine the B concentration (mass %) at each position in range L. As shown in FIG. 1, in the B concentration analysis results in the range L, the concentration of B shows a peak at the position of the grain boundary GB. Therefore, in the line analysis result, the B concentration at the top position P1 of the peak at the grain boundary GB position is defined as the grain boundary B concentration (% by mass). On the other hand, among the line analysis results, the arithmetic average value of the B concentration at any four points (P2 to P4) in the grain interior is defined as the grain interior B concentration (% by mass). The arithmetic average value of the grain boundary B concentration in 5 fields of view is defined as the grain boundary B concentration [B _GB ] (% by mass). Then, the arithmetic average value of the intragranular B concentrations in the five fields of view is defined as the intragranular B concentration [B _BM ] (% by mass). Using the obtained grain boundary B concentration [B _GB ] and grain B concentration [B _BM ], [B _GB ]/[B _BM ] is obtained.

[Preferred condition: Regarding formula (2)]
Preferably, the chemical composition of the austenitic stainless steel material of the present embodiment further satisfies formula (2).
0.2×Mo+5×Nb+500×B>2.00 (2)

Define F2=0.2*Mo+5*Nb+500*B. F2 is an index indicating the degree of B segregation at grain boundaries. Nb and Mo promote diffusion of B. Therefore, Nb and Mo promote grain boundary segregation of B. As a result, the grain boundary B segregation degree is increased, and the creep strength and creep ductility of the steel material in a high temperature environment are further increased.

If F2 is higher than 2.00, B segregation to grain boundaries is further promoted. As a result, the degree of grain boundary B segregation is further increased. As a result, the creep strength and creep ductility of the steel are further enhanced in high temperature environments. The preferred lower limit of F2 is 2.40, more preferably 2.60, still more preferably 2.80, still more preferably 3.00, still more preferably 3.10. Although the upper limit of F2 is not particularly limited, the upper limit of F2 for the above chemical composition is 15.00.

[Shape of austenitic stainless steel material of the present embodiment]
The shape of the austenitic stainless steel material of this embodiment is not particularly limited. The austenitic stainless steel material of this embodiment may be a steel pipe, a steel plate, or a steel bar. Also, the austenitic stainless steel material of the present embodiment may be a forged product.

[Uses of the austenitic stainless steel material of the present embodiment]
The austenitic stainless steel material of this embodiment is suitable for equipment that is used for a long period of time in a high temperature environment. A high-temperature environment as used herein means an environment with an average operating temperature of 300 to 700°C. Operating temperatures may exceed 700°C. Devices in such a high-temperature environment are, for example, devices in chemical plant facilities typified by petroleum refining and petrochemicals.

It should be noted that the austenitic stainless steel material of this embodiment can naturally be used for equipment other than chemical plant equipment. Equipment other than chemical plant equipment is, for example, thermal power boiler equipment (for example, boiler tubes, etc.), which is assumed to be used in a high temperature environment with an average operating temperature of 300 to 700 ° C. like chemical plant equipment. .

As described above, the austenitic stainless steel material of the present embodiment has the chemical composition described above, the grain size number is 4.0 to 9.0, and the grain boundary B segregation degree ([B _GB ]/[B _BM ]) is 500 or more. Therefore, both the creep strength and creep ductility of the steel material used for a long time in a high temperature environment can be sufficiently enhanced.

[Method for producing austenitic stainless steel according to the present embodiment]
A method for producing an austenitic stainless steel material according to this embodiment will be described below. The method for producing an austenitic stainless steel material described below is an example of the method for producing an austenitic stainless steel material according to this embodiment. Therefore, the austenitic stainless steel material having the structure described above may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the manufacturing method of the austenitic stainless steel material of the present embodiment.

The method for producing an austenitic stainless steel material according to the present embodiment comprises a step of preparing a material (preparing step), a step of hot working the material to produce an intermediate steel material (hot working step), Depending on the process, cold working is performed on the intermediate steel material after the hot working process (cold working process), and solution treatment is performed on the intermediate steel material after the hot working process or after the cold working process. and a step of performing treatment (solution treatment step). Each step will be described below.

[Preparation process]
In the preparation step, a material having the chemical composition described above is prepared. Materials may be supplied by a third party or manufactured. The material may be ingots, slabs, blooms or billets. When manufacturing the material, the material is manufactured by the following method. A molten steel having the above chemical composition is produced. Using the produced molten steel, an ingot is produced by an ingot casting method. The produced molten steel may be used to produce slabs, blooms, and billets (columnar raw materials) by continuous casting. Hot working may be performed on the produced ingots, slabs, and blooms to produce billets. For example, hot forging may be performed on an ingot to produce a columnar billet, and this billet may be used as a raw material (columnar raw material). In this case, the temperature of the raw material immediately before the start of hot forging is not particularly limited, but is, for example, 1000 to 1300.degree. A method for cooling the material after hot forging is not particularly limited.

[Hot working process]
In the hot working process, the raw material prepared in the preparation process is hot worked to manufacture an intermediate steel material. The intermediate steel material may be, for example, a steel pipe, a steel plate, or a steel bar.

When the intermediate steel material is a steel pipe, the following working is performed in the hot working process. First, a cylindrical material is prepared. A through hole is formed along the central axis of the cylindrical material by machining. An intermediate steel material (steel pipe) is manufactured by performing hot extrusion represented by the Ugine Sejournet method on a columnar material in which through holes are formed. The temperature of the raw material immediately before hot extrusion is not particularly limited. The temperature of the material immediately before hot extrusion is, for example, 1000-1300.degree. Instead of the hot extrusion method, a hot extrusion tube-making method may be implemented.

Instead of hot extrusion, piercing-rolling by the Mannesmann method may be carried out to produce a steel pipe. In this case, the round billet is pierced and rolled by a piercing machine. In the case of piercing-rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced-rolled round billet is further hot-rolled by a mandrel mill, a reducer, a sizing mill, or the like to form a mother tube. Although the cumulative area reduction rate in the hot working step is not particularly limited, it is, for example, 20 to 80%. When a steel pipe is produced by hot working, the steel pipe temperature (finishing temperature) immediately after the hot working is completed is preferably 900° C. or higher.

If the intermediate steel is a steel plate, the hot working process uses, for example, one or more rolling mills with a pair of work rolls. A raw material such as a slab is hot-rolled using a rolling mill to produce a steel plate. Heat the material before hot rolling. Hot rolling is performed on the raw material after heating. The temperature of the material immediately before hot rolling is, for example, 1000-1300.degree.

When the intermediate steel material is a steel bar, the hot working process includes, for example, a rough rolling process and a finish rolling process. In the rough rolling process, the raw material is hot worked to produce a billet. The rough rolling process uses, for example, a blooming mill. When a continuous rolling mill is installed downstream of the blooming mill, the billet after blooming is further hot-rolled using the continuous rolling mill to produce a smaller billet. may In a continuous rolling mill, for example, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a row. In the rough rolling process, materials such as blooms are manufactured into billets. The material temperature immediately before the rough rolling step is not particularly limited, but is, for example, 1000 to 1300°C. In the finish rolling process, the billet is first heated. The billet after heating is subjected to hot rolling using a continuous rolling mill to produce a steel bar. Although the heating temperature in the heating furnace in the finish rolling step is not particularly limited, it is, for example, 1000 to 1300°C.

[Cold working process]
A cold working process is performed as needed. In other words, the cold working process may not be performed. When implemented, the intermediate steel material is subjected to pickling treatment and then to cold working. If the intermediate steel material is a steel pipe or steel bar, cold working is, for example, cold drawing. If the intermediate steel material is a steel plate, cold working is, for example, cold rolling. By performing the cold working process, strain is applied to the intermediate steel material before the solution treatment process. As a result, recrystallization and grain size regulation can be performed during the solution treatment process. Although the area reduction rate in the cold working step is not particularly limited, it is, for example, 10 to 90%.

[Solution treatment step]
In the solution treatment process, the intermediate steel material after the hot working process or the cold working process is subjected to the solution treatment. In the solution treatment step, grain boundary B segregation degree is increased while suppressing coarsening of crystal grains. In the solution treatment, there are a step of raising the temperature of the intermediate steel material to the solution treatment temperature (heating step), a step of holding the intermediate steel at the solution treatment temperature after the temperature raising step (holding step), and a solution treatment after the holding step. and a step of cooling from temperature to room temperature (cooling step). The heating rate RR in the heating process, the solution treatment temperature T in the holding process and the holding time t at the solution treatment temperature T, and the average cooling rate CR _900-500 in the cooling process are set as follows. .
Heating rate RR up to 1000°C: 0.5°C/sec or more Solution treatment temperature T: 1000 to 1250°C
Holding time t at solution treatment temperature T: 2 to 60 minutes Average cooling rate from 900 ° C to 500 ° C CR _900-500 : 10 to 500 ° C / sec

[Regarding the temperature rise rate RR]
If the heating rate RR is less than 0.5° C./second, crystal grains become coarse. As a result, the grain size number is less than 4.0. If the heating rate RR is 0.5° C./second or more, coarsening of crystal grains can be suppressed. A preferable lower limit of the heating rate RR is 0.7° C./second, more preferably 0.9° C./second. A preferable upper limit of the heating rate RR is 10° C./sec.

[Regarding the solution treatment temperature T]
If the solution treatment temperature T is less than 1000° C., precipitates such as Nb carbonitrides do not sufficiently dissolve. In this case, crystal grains are excessively refined. Therefore, the creep strength in a high temperature environment is lowered. On the other hand, if the solution heat treatment temperature T exceeds 1250° C., the crystal grains become coarse and the grain size number becomes less than 4.0. Therefore, the creep ductility in a high temperature environment is lowered. If the solution treatment temperature T is 1000 to 1250° C., the precipitates can be sufficiently dissolved, and the grain size number can be 4.0 to 9.0.

[Retention time t at solution heat treatment temperature T]
If the holding time t at the solution treatment temperature T exceeds 60 minutes, the crystal grains become coarse and the grain size number becomes less than 4.0. If the holding time t at the solution treatment temperature T is 2 to 60 minutes, the precipitates can be sufficiently dissolved, and the grain size number can be maintained at 4.0 or more. The holding time t at the solution treatment temperature T is usually 2 minutes or longer.

[About average cooling rate CR _900-500 ]
In the temperature range from 900° C. to 500° C., on the premise that the grain size number is from 4.0 to 9.0, B tends to segregate at grain boundaries. Therefore, the average cooling rate CR _900-500 in this temperature range is set to 10-500° C./sec. If the average cooling rate CR _900-500 is less than 10° C./sec, B diffuses excessively, making it difficult for B to remain at grain boundaries. Therefore, the grain boundary B segregation degree [B _GB ]/[B _BM ] is less than 500. On the other hand, when the average cooling rate CR _900-500 exceeds 500° C./sec, the diffusion distance of B is too short to allow B to diffuse to grain boundaries. Also in this case, the grain boundary B segregation degree [B _GB ]/[B _BM ] is less than 500.

When the average cooling rate CR _900-500 is 10 to 500° C./sec, diffusion of B is appropriate, and B tends to segregate at grain boundaries. As a result, the grain boundary B segregation degree [B _GB ]/[B _BM ] becomes 500 or more. The average cooling rate from the solution treatment temperature T to 900° C. is slower than the average cooling rate CR _900-500 . Also, the average cooling rate from 500° C. to room temperature is slower than the average cooling rate CR _900-500 .

Through the steps described above, the austenitic stainless steel material of the present embodiment can be manufactured. The manufacturing method described above is an example of a method for manufacturing the austenitic stainless steel material of the present embodiment. Therefore, the manufacturing method of the austenitic stainless steel material of this embodiment is not limited to the manufacturing method described above. If it has the above-mentioned chemical composition, the grain size number is 4.0 to 9.0, and the grain boundary B segregation degree ([B _GB ]/[B _BM ]) is 500 or more, the austenite of the present embodiment The stainless steel material is not limited to the manufacturing method described above.

As described above, the austenitic stainless steel material of the present embodiment has the chemical composition described above, the grain size number is 4.0 to 9.0, and the grain boundary B segregation degree ([B _GB ]/[B _BM ]) is 500 or more. Therefore, it has sufficient creep strength and sufficient creep ductility even when used for a long time in a high temperature environment.

EXAMPLES Hereinafter, the effects of the austenitic stainless steel material of the present embodiment will be described more specifically by way of examples. The conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the austenitic stainless steel material of this embodiment. Therefore, the austenitic stainless steel material of this embodiment is not limited to this example of one condition.

[Manufacturing of austenitic stainless steel]
Molten steel having the chemical composition shown in Table 1 was produced.

The "Others" column in Table 1 shows the contents of the optional elements contained. For example, in the case of Test No. 1, it means that 0.08% of Cu, which is an optional element, was contained. In the case of Test No. 8, it means that the optional element V was contained at 0.15%, Y was contained at 0.11%, and Mg was contained at 0.0020%.

An ingot with an outer diameter of 120 mm and 30 kg was manufactured using molten steel. The ingot was subjected to hot forging to obtain a material (steel plate) having a thickness of 40 mm. The temperature of the ingot before hot forging was 1050° C. or higher. Furthermore, hot rolling was applied to the raw material to produce an intermediate steel material (steel plate) having a thickness of 15 mm. The material temperature before hot working (hot rolling) was 1050° C. or higher.

The intermediate steel material after hot rolling was subjected to cold rolling to produce a steel plate having a thickness of 10.5 mm, a width of 50 mm and a length of 100 mm. The steel plate after cold rolling was subjected to solution treatment. Temperature increase rate RR (°C/sec) in solution treatment, solution treatment temperature T (°C), holding time t (minutes) at solution treatment temperature T, average cooling rate in the temperature range of 900 to 500°C CR _900-500 (°C/sec) were as shown in Table 2, respectively. Through the above steps, austenitic stainless steel materials (steel plates) of each test number were manufactured.

[Evaluation test]
The following evaluation tests were performed on the austenitic stainless steel material of each test number.

[Chemical composition measurement test]
With the plate thickness of the steel plate of each test number being TH (mm), a drill was used to perform drilling at a depth of TH/4 from the surface to generate chips, and the chips were collected. The collected chips were dissolved in acid to obtain a solution. ICP-OES was performed on the solution to perform elemental analysis of chemical composition. The C content and S content were obtained by a well-known high-frequency combustion method. The N content was determined using the well-known inert gas fusion-thermal conductivity method. The chemical composition of the austenitic stainless steel material of each test number was obtained by the above analysis method. As a result, the chemical composition of the austenitic stainless steel material of each test number agreed with Table 1.

[Grain size number measurement test]
The grain size number of the austenitic stainless steel material of each test number was obtained by the following method. A sample was taken from the center position of the plate width and the plate thickness center position of the austenitic stainless steel material. A cross-section perpendicular to the longitudinal direction of the austenitic stainless steel material was used as an observation surface among the surfaces of the collected samples. The observation surface was mirror-polished. The observed surface after mirror polishing was corroded with 10% oxalic acid to expose the grain boundaries of austenite. Arbitrary 3 fields of view of the corroded observation surface were observed, and the grain size number was obtained based on the cutting method based on JIS G 0551 (2013). The area of each field of view was 0.75 mm ² . The arithmetic mean value of the grain size numbers in the field of view was defined as the grain size number. The obtained crystal grain size number is shown in the "grain size number" column of Table 2.

[Grain boundary B segregation degree measurement test]
Grain boundary B concentration [B _GB ] (% by mass) and intragranular B concentration [B _BM ] (% by mass) are obtained by the following method, and further, grain boundary B segregation degree (= [B _GB ]/[B _BM ]). A sample was taken from the center position of the plate width and the plate thickness center position of the austenitic stainless steel material. A cross-section perpendicular to the longitudinal direction of the austenitic stainless steel material was used as an observation surface among the collected sample surfaces. The observation surface of the sample was mirror-polished. Grain boundaries were identified using SEM-EBSD on the mirror-polished observed surface. After identifying the grain boundaries, a 10 μm×10 μm×30 nm thin film sample containing the grain boundaries was prepared by performing focused ion beam (FIB). Thin film samples were prepared one by one from five different grain boundaries (that is, a total of five thin film samples were prepared).

EELS was performed on thin film samples using an electron energy loss spectroscopy (EELS) detector attached to a transmission electron microscope (TEM). Specifically, as shown in FIG. 1, line analysis of 10 nm was performed in a direction perpendicular to an arbitrary crystal grain boundary GB, which is an interface between crystal grains 10 . At this time, the acceleration voltage was set to 300 kV. A range L for line analysis was determined so that the grain boundary GB was located at the center position of the range L for line analysis. The length of the range L was set to 10 nm. EELS was performed to determine the B concentration in the range L (mass %). As shown in FIG. 1, in the B concentration analysis results in the range L, the concentration of B shows a peak at the position of the grain boundary GB. Therefore, in the line analysis results, the B concentration at the top position P1 of the peak at the grain boundary GB position was defined as the grain boundary B concentration (% by mass). On the other hand, among the line analysis results, the arithmetic mean value of the B concentration at arbitrary four points (P2 to P4) in the grain interior was defined as the grain interior B concentration (% by mass). The arithmetic average value of the grain boundary B concentration in 5 fields of view was defined as the grain boundary B concentration [B _GB ] (% by mass). Then, the arithmetic average value of the intragranular B concentrations in the five fields of view was defined as the intragranular B concentration [B _BM ] (% by mass). Using the obtained grain boundary B concentration [B _GB ] and grain B concentration [B _BM ], [B _GB ]/[B _BM ] was determined. Table 2 shows the degree of grain boundary B segregation (=[B _GB ]/[B _BM ]).

[Creep strength and creep ductility evaluation test (creep rupture test)]
A creep rupture test piece conforming to JIS Z2271 (2010) was produced from the austenitic stainless steel material of each test number. A creep rupture test piece was prepared from the plate thickness center position and the plate width center position of the steel plate of each test number. The cross section perpendicular to the axial direction of the creep rupture test piece was circular, the outer diameter of the creep rupture test piece was 6 mm, and the parallel portion was 30 mm. The parallel portion was parallel to the longitudinal direction (rolling direction) of the steel plate.

Using the produced creep rupture test piece, a creep rupture test was carried out in accordance with JIS Z2271 (2010). Specifically, after heating the creep rupture test piece at 700° C., the creep rupture test was performed. The test stress was 80 MPa, and the creep rupture time (hours) and creep rupture reduction (%) were obtained.

Regarding the creep strength, it was judged that the creep strength was remarkably excellent in a high-temperature environment when the creep rupture time was 15000 hours or more (indicated by "E (EXCELLENT)" in the "creep strength" column in Table 2). When the creep rupture time was 10,000 hours to less than 15,000 hours, it was judged that the creep strength was good in a high-temperature environment (indicated by "G (GOOD)" in the "creep strength" column in Table 2). When the creep rupture time was less than 10,000 hours, it was judged that the creep strength was low in a high-temperature environment (indicated by "B (BAD)" in the "creep strength" column in Table 2). When the creep rupture time was "E" or "G", it was judged to be excellent in long-term creep strength in a high-temperature environment.

With respect to creep ductility, it was judged that creep ductility was remarkably excellent if the creep rupture reduction ratio exceeded 70% (indicated by "E (EXCELLENT)" in the "creep ductility" column in Table 2). When the creep rupture reduction was 50 to 70%, it was judged that the creep ductility was good in a high temperature environment (indicated by "G (GOOD)" in the "creep ductility" column in Table 2). When the creep rupture reduction was less than 50%, it was judged that the creep ductility was low in a high-temperature environment (indicated by "B (BAD)" in the "creep ductility" column in Table 2). When the creep rupture reduction is "E" or "G", it was determined that the creep ductility in a high temperature environment is excellent.

[Evaluation results]
Table 2 shows the evaluation results.

With reference to Tables 1 and 2, in test numbers 1 to 11, the content of each element in the chemical composition is appropriate, the grain size number is 4.0 to 9.0, and the grain boundary B segregation degree ( [B _GB ]/[B _BM ]) was 500 or more, satisfying the formula (1). Therefore, it has excellent creep strength and creep ductility in a high-temperature environment. Specifically, in a creep rupture test at 700° C. with a test stress of 80 MPa, the creep rupture time was 10000 hours or more and the creep rupture reduction was 50% or more.

Test numbers 1 to 6 and 8 to 11 further exceeded F2 of 2.0 and satisfied formula (2). Therefore, in test numbers 1 to 6 and 8 to 11, the grain boundary B segregation degree ([B _GB ]/[B _BM ]) was higher than that in test number 7, and was 650 or more. As a result, the creep rupture time was 15000 hours or longer and the creep rupture reduction exceeded 70%.

On the other hand, in Test No. 12, the average cooling rate CR _900-500 in the temperature range of 900-500° C. in the solution treatment step was too fast. Therefore, the grain boundary B segregation degree ([B _GB ]/[B _BM ]) was less than 500. As a result, both creep strength and creep ductility were low.

In Test No. 13, the heating rate RR was too slow. Therefore, the grain size number was less than 4.0. As a result, the creep strength was excellent, but the creep ductility was low.

In test number 14, the solution treatment temperature T was too low. Therefore, the grain size number exceeded 9.0. As a result, the creep ductility was excellent, but the creep strength was low.

In Test No. 15, the heating rate RR in the solution treatment step was too slow. Therefore, the grain size number was less than 4.0. As a result, the creep strength was excellent, but the creep ductility was low.

In test number 16, the solution treatment temperature T in the solution treatment step was too high. Therefore, the grain size number was less than 4.0. As a result, the creep strength was excellent, but the creep ductility was low.

In test number 17, the holding time t at the solution treatment temperature T in the solution treatment step was too long. Therefore, the grain size number was less than 4.0. As a result, the creep strength was excellent, but the creep ductility was low.

In Test No. 18, the average cooling rate CR _900-500 in the temperature range of 900-500° C. in the solution treatment step was too slow. Therefore, the grain boundary B segregation degree ([B _GB ]/[B _BM ]) was less than 500. As a result, both creep strength and creep ductility were low.

In Test No. 19, the average cooling rate CR _900-500 in the temperature range of 900-500° C. in the solution treatment step was too fast. Therefore, the grain boundary B segregation degree ([B _GB ]/[B _BM ]) was less than 500. As a result, both creep strength and creep ductility were low.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (4)

An austenitic stainless steel material,
The chemical composition, in mass %,
C: 0.030% or less,
Si: 0.10 to 1.00%,
Mn: 0.2-2.0%,
P: 0.01 to 0.04%,
S: 0.0100% or less,
Cr: 15.00 to 25.00%,
Ni: 9.00 to 18.00%,
Mo: 1.0 to 5.0%,
Nb: 0.20 to 2.00%,
N: 0.050 to 0.180%,
sol. Al: 0.001 to 0.080%,
B: 0.0005 to 0.0080%,
Cu: 0 to 2.00%,
V: 0 to 1.00%,
Co: 0 to 1.0%,
Y: 0 to 1.00%,
Zr: 0 to 1.0%,
Hf: 0-0.20%,
Ta: 0 to 0.20%,
W: 0 to 5.0%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%, and
Rare earth elements other than Y: 0 to 0.100%,
The balance consists of Fe and impurities,
The grain size number is 4.0 to 9.0,
When the B concentration (% by mass) at the austenite grain boundaries in the austenitic stainless steel material is defined as [B _GB ], and the B concentration (% by mass) within the austenite crystal grains is defined as [B _BM ], the formula satisfy (1),
Austenitic stainless steel material.
[B _GB ]/[B _BM ]≧500 (1) The austenitic stainless steel material according to claim 1,
The chemical composition is
Cu: 0.05 to 2.00%,
V: 0.10 to 1.00%,
Co: 0.1 to 1.0%,
Y: 0.01 to 1.00%
Zr: 0.1-1.0%
Hf: 0.01 to 0.20%,
Ta: 0.01 to 0.20%, and
W: 0.1 to 5.0%, containing one or more elements selected from the group consisting of
Austenitic stainless steel material. The austenitic stainless steel material according to claim 1 or claim 2,
The chemical composition is
Ca: 0.0005 to 0.0100%,
Mg: 0.0005 to 0.0100%, and
Rare earth elements other than Y: containing one or more elements selected from the group consisting of 0.001 to 0.100%,
Austenitic stainless steel material. The austenitic stainless steel material according to any one of claims 1 to 3,
The chemical composition further satisfies formula (2),
Austenitic stainless steel material.
0.2×Mo+5×Nb+500×B>2.00 (2)

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