JP2002241903A - HIGH Cr FERRITIC HEAT RESISTANT STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Cr ferritic heat resistant steel which has excellent high temperature-long time creep strength, and withstands the use in a high temperature vapors of >=625 deg.C. SOLUTION: The high Cr ferritic heat resistant steel having excellent high temperature-long time creep strength has a composition containing, by mass, 0.05 to 0.15% C, <=1% Si, 8 to 13% Cr, 0.2 to 0.5% V, 0.002 to 0.2% Nb, 2 to 5% W, 0.0001 to 0.01% B and 0.001 to 0.05% Al. The steel has a metallic structure consisting of a tempered martensitic matrix, and M23C6 and intermetallic compounds having the particle size of <=0.6 μm are precipitated into martensite laths by >=0.4 pieces/μm3 in total.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Cr ferritic heat resistant steel material, and more particularly to a heat exchange steel pipe and a pressure vessel used in a high temperature and high pressure environment such as a boiler, a nuclear power plant and a chemical industrial facility. The present invention relates to a high-Cr ferritic heat-resistant steel material excellent in high-temperature long-time creep strength suitable for steel sheets, turbine materials and the like.

[0002]

2. Description of the Related Art Heat-resistant steels used in high-temperature, high-pressure environments such as boilers, nuclear power plants, and chemical industrial facilities generally require high-temperature strength, corrosion resistance, oxidation resistance, and the like.

[0003] For these uses, JIS SUS3
Austenitic stainless steel such as 21H, SUS347H steel, low alloy steel such as 2.1 / 4Cr-1Mo steel,
Further, high Cr ferritic steels of 9-12Cr type have been used. Among them, high Cr ferritic steel is 500 ℃ ~
At a temperature of 650 ° C., it is superior to low alloy steel in strength and corrosion resistance. Also, high Cr ferritic steel is
Numerous advantages, such as being less expensive than austenitic stainless steel, having high thermal conductivity and low thermal expansion coefficient, making it less likely to have thermal fatigue resistance and scale peeling, and not causing stress corrosion cracking There is.

[0004] In recent years, in order to further improve thermal efficiency in thermal power generation, boiler steam conditions have been increased to high temperatures and high pressures. That is, the supercritical pressure condition of 538 ° C.
From 246 atm, operation under ultra-supercritical conditions such as 625 ° C. and 300 atm in the future is planned. With such changes in steam conditions, the required performance of steel pipes for boilers and the like has become increasingly severe. For this reason, the conventional high Cr ferritic steel has not been able to sufficiently respond to the long-term creep strength at high temperatures as described above.

[0005] Austenitic stainless steel is capable of meeting such severe conditions as described above, but is expensive. Therefore, attempts have been made to improve the properties of high-Cr ferritic steels, which are less expensive than austenitic stainless steels.

[0006] High Cr heat resistant steel with an increased W content
Japanese Patent Application Laid-Open (JP-A) No. 3-113342), heat-resistant steel with an increased W content, and Cu added from the viewpoint of improving high-temperature steam oxidation resistance (Japanese Patent Application Laid-Open No. 3-97732). There is. -Creep strength becomes unstable when used for a long time at high temperature.

[0007] Excessive addition of W deteriorates toughness and weldability.

For the use of high Cr ferritic steels under the ultra-supercritical pressure conditions described above for the steam conditions of a thermal power boiler or the like, it is necessary to further improve the high-temperature and long-time creep strength. It is important to delay the recovery and softening of the martensitic structure to the high temperature and long time side as much as possible.

[0009]

The problem to be solved by the present invention is to
An object of the present invention is to provide a high-Cr ferritic heat-resistant steel excellent in high-temperature long-time creep strength that can withstand use under high-temperature steam of 5 ° C or more.

[0010]

Means for Solving the Problems The present inventors have proposed an intermetallic compound (Laves) precipitated in a high Cr ferritic heat resistant steel.
The following findings were obtained as a result of experiments and investigations focusing on phase) and its effect on precipitation behavior and high-temperature long-time creep characteristics.

By controlling the precipitation form of M ₂₃ C ₆ and the intermetallic compound, recovery softening of martensite is remarkably suppressed, and the high-temperature long-time creep strength is greatly improved. That is, M which usually precipitates mainly at the martensite lath interface
The martensitic structure recovery softening phenomenon of the steel in which ₂₃ C ₆ and the intermetallic compound are precipitated inside the lath is suppressed to a high temperature and a long time side, and the creep strength is greatly improved.

The present invention has been made based on such findings, and the gist thereof is as follows.

(1) In mass%, C: 0.001 to 0.1
5%, Si: 1% or less, Mn: 0.05-1.5%,
P: 0.03% or less, S: 0.015% or less, Cr: 8
１３13%, V: 0.2-0.5%, Nb: 0.002%
-0.2%, W: 2-5%, N: 0.001-0.03
%, B: 0.0001 to 0.01%, Al: 0.001
~ 0.05%, the balance consists of Fe and impurities, the metal structure consists of tempered martensite matrix, and the M
0.4 total of ₂₃ C ₆ and intermetallic compound / μm ³
High Cr ferritic heat-resistant steel with excellent high-temperature long-term creep strength precipitated as described above.

(2) Instead of part of Fe, Ta: 0.0
02% to 0.2%, one or two of Ti: 0.001% to 0.1% and Nd: 0.001% to 0.2%
The high Cr ferritic heat-resistant steel according to the above (1), containing at least one kind.

(3) Instead of part of Fe, Mo: 0.0
The high Cr ferritic heat-resistant steel material according to the above (1) or (2), containing 1 to 0.5%.

(4) Instead of part of Fe, Co: 0.0
1% -6%, Ni: 0.01% -1% and Cu: 0.
The high Cr ferritic heat-resistant steel material according to any one of the above (1) to (3), containing one or more of 01% to 2%.

(5) Instead of part of Fe, Ca: 0.0
2% or less, La: 0.2% or less, Ce: 0.2% or less,
Y: 0.2% or less and Hf: 1 of 0.2% or less
The high Cr ferritic heat-resistant steel material according to any one of the above (1) to (4), containing one or more kinds.

Here, the particle diameters of M ₂₃ C ₆ and the intermetallic compound are defined as the diameter obtained by observing a thin film with an electron microscope, calculating the area of the precipitate by image analysis, and calculating the precipitate as a sphere. I do.

[0019]

Embodiments of the present invention will be described below in detail. In addition,% display of a chemical composition shows all mass%. a) Chemical composition C: 0.001 to 0.15% C stabilizes the structure as an austenite stabilizing element and forms MC (M is an alloying element) carbide, contributing to improvement in high-temperature long-time creep strength. I do. But 0.00
If it is less than 1%, the above effects cannot be obtained, and the amount of δ ferrite increases, so that sufficient creep strength cannot be obtained. On the other hand, when it is contained in a large amount exceeding 0.15%, workability and weldability are deteriorated, and the carbides are coarsened and coarsened from an early stage of use, resulting in a decrease in high-temperature long-time creep strength. Therefore, the upper limit is set to 0.15%. Desirably, it is 0.08% to 0.12%.

Si: 1% or less Si is used as a deoxidizing agent for molten steel. If it is contained in a large amount exceeding 1%, the toughness and the creep strength are remarkably reduced, so the upper limit was made 1%. The lower limit is not particularly limited,
In order to sufficiently obtain the above-mentioned effects, the content is preferably 0.01% or more.

[0021] Si further contributes to the improvement of steam oxidation resistance at high temperatures. However, when importance is attached to steam oxidation resistance, the lower limit of the Si content is desirably 0.1%.

Mn: 0.05-1.5% Mn is effective as an element for fixing deoxidation and S, and contributes to austenite stabilization. To obtain these effects, it is necessary to contain 0.05% or more.
If it exceeds 5%, the creep strength decreases. Therefore, the Mn content is set to 0.05 to 1.5%.

P: 0.03% or less, S: 0.015% or less The impurities P and S are desirably low from the viewpoint of hot workability, weldability and toughness.
If the content is not more than 0.015%, it does not directly affect the properties of the steel of the present invention, so the upper limits are 0.03% and 0.015%, respectively.
%.

Cr: 8 to 13% Cr is a steel material of the present invention which has high corrosion resistance and oxidation resistance at high temperatures.
In particular, it is an indispensable element for securing steam oxidation resistance. Further, a carbide is formed to improve the creep strength. In addition, there is an effect of forming a dense oxide film mainly composed of Cr to improve corrosion resistance and oxidation resistance, and in order to obtain these effects, the content needs to be 8% or more. However, when a large amount is contained, the toughness is deteriorated and the long-term creep strength is reduced, so the upper limit is set to 13%. Desirably, it is 9 to 12%.

V: 0.2% to 0.5% V contributes to the improvement of creep strength by forming solid solution strengthening and forming fine carbonitrides. In order to exert the effect, it is necessary to contain 0.2% or more. Since the desired structure can be easily obtained by containing 0.2% or more, the lower limit is set to 0.2%. If the content exceeds 0.5%, the creep strength decreases, so the upper limit is set to 0.
5%.

Nb: 0.002 to 0.2% Nb forms fine carbonitrides and contributes to improvement in long-time creep strength. In order to show the effect,
The content of Nb must be not less than 0.002%, but if it is contained in a large amount, the formation of δ ferrite is promoted and the long-term creep strength and toughness are reduced.
0.002 to 0.2%.

W: 2 to 5% W contributes to the improvement of creep strength as a solid solution strengthening element and precipitates as an intermetallic compound to contribute to creep strength. Further, it partially forms a solid solution in the Cr carbide and suppresses agglomeration and coarsening of the carbide and contributes to maintenance of strength.
However, at less than 2%, the effect is small. Also,
Since the desired structure can be easily obtained by containing 2% or more, the lower limit is set to 2%. On the other hand, if the content is larger than 5%, the formation of δ ferrite is promoted. Therefore, the content of W is set to 2 to 5%.

N: 0.001 to 0.03% N is effective as an austenite stabilizing element like C. N also precipitates nitride or carbonitride and increases the high-temperature strength. 0.00 to achieve its effect
It is necessary to contain 1% or more. On the other hand, if it exceeds 0.03%, the creep strength is lowered due to coarsening of nitrides and carbonitrides. Further, if the content is 0.03% or less, a desired structure can be easily obtained, so the N content is 0.001%.
-0.03%.

B: 0.0001 to 0.01% B plays an important role in enhancing hardenability and ensuring high-temperature strength. The effect is remarkable at 0.0001% or more. However, when the content exceeds 0.01%, the weldability and the long-term creep strength are reduced.
1 to 0.01%.

Al: 0.001% to 0.05% Al is used as a deoxidizing agent for molten steel. The effect is 0.0
The content becomes remarkable when added in an amount of 01% or more. However, when the content exceeds 0.05%, the creep strength is reduced. Therefore, the upper limit is set to 0.05%.

The heat-resistant steel material of the present invention is a heat-resistant steel containing the above-mentioned various elements, with the balance being Fe and impurities, and may further contain the following various elements if necessary.

One or more of Ta, Ti and Nd These elements are elements that form fine carbonitrides and contribute to improvement in high-temperature long-time creep strength. In order to exert the effect, Ta needs 0.002% or more, and Ti and Nd each need 0.001% or more. On the other hand, even if the content of Ta exceeds 0.2%, the content of Ti exceeds 0.1%, and the content of Nd exceeds 0.2%, the effect is saturated, and the toughness and creep strength are rather deteriorated.
Therefore, when Ta, Ti and Nd are contained, 0.002 to 0.2% and 0.001 to 0.
1, and 0.001% to 0.2%. Mo: 0.01 to 0.5% Mo is contained as necessary as a solid solution strengthening element to contribute to improvement in creep strength. If you want to include
If it is less than 0.01%, the effect will not appear. 0.5%
If it is contained in excess of, the intermetallic compound precipitates coarsely, leading to a decrease in toughness and high-temperature long-time creep strength. Also, when Mo, which diffuses faster than W, is contained in a large amount, the distribution of precipitates is biased toward the lath interface, and it is difficult to obtain a desired structure. Therefore, it was set to 0.01 to 0.5%.

One or more of Co, Ni and Cu These elements are effective as austenite stabilizing elements. When a large amount of a ferrite former such as Cr, Nb, W, Mo, or V is contained, it is preferable to positively contain it. However, when the content is less than 0.01%, the effect does not appear. On the other hand, if Co is excessively added in excess of 6%, the Ac1 transformation point of the steel will be significantly reduced, and conversely, the creep strength will be reduced. When the amount of Ni exceeds 1% and the amount of Cu exceeds 2%, the high-temperature long-term creep strength is significantly reduced. Therefore, Co is 0.0
1-6%, Ni: 0.01-1%, Cu: 0.01-2
%.

One kind of Ca, La, Ce, Y and Hf
Or two or more types Ca, La, Ce, Y and Hf are contained in a trace amount.
To strengthen the grain boundaries to improve creep strength
In particular, it also contributes to improving hot workability. But excessively
When added, the hot workability decreases, so that these elements
The upper limit is 0.02% for Ca, and that for La, Ce, Y and Hf
Each was set to 0.2%. The lower limit is not limited.
However, it is preferable to set each to 0.01% or more. b) Metal structure The metal structure of the heat-resistant steel material of the present invention is such that
Other carbonitrides, intermetallic compounds and intercalations
And δ ferrite in some cases. Such a metal
Microstructure is preferred for high-temperature long-term creep strength and toughness.
Because it is. In addition, grains inside the martensite lath
M having a diameter of 0.6 μm or less₂₃C ₆And intermetallic
Compounds (Laves phase) in total of 0.4 / μm³The above precipitates
ing.

M₂₃C₆ And intermetallic compound grains
High-temperature long-time creep when the diameter is as small as 0.6 μm or less
Although the strength is improved, precipitates with a particle size exceeding 0.6 μm
Even if precipitated inside the lath, the creep strength does not improve, so
The upper limit of the diameter was 0.6 μm. Further, the precipitation density is 0.
4 pieces / μm³ If it is smaller, the increase in creep strength
Precipitation density 0.4 / μm because it is not observed³It was above.
Desirably 1 piece / μm ³Above, and the upper limit is particularly limited
It is not a thing, but the more it does not affect toughness, the better.
New This heat-resistant steel can be manufactured by the following method.
Wear.

The steel having the above-mentioned chemical composition is melted, ingot-formed, lumped, and hot-worked by a conventional method to obtain steel materials such as steel plates and steel pipes. Heat treatment for the purpose of precipitating carbides and intermetallic compounds for 3 hours or more within 650-800 ° C.
Tempering in the temperature range described above. As a result, M ₂₃ C ₆ having a particle size of 0.6 μm or less and an intermetallic compound (Laves phase) were contained in the martensite lath in a total of 0.1 μm.
A steel material having a structure of 4 / μm ³ or more is obtained.

[0037]

EXAMPLE In a vacuum induction melting furnace, steels having 34 kinds of chemical compositions shown in Tables 1 and 2 were melted, and each steel having a diameter of 144 mm was prepared.
A 0 kg ingot was used. Reference numerals 1 to 13 in Table 1 are steels of the present invention, and reference numerals A to U in Table 2 are comparative steels.

[0038]

[Table 1]

[Table 2] These ingots are hot forged and hot rolled to a thickness of 20
mm hot rolled steel sheet was manufactured.

These steel sheets were subjected to a normalizing treatment at 1050 to 1150 ° C., and a heat treatment for depositing precipitates at various temperatures and holding times within a temperature range of 575 to 650. The tempering treatment was performed by changing the temperature and the holding period within a temperature range of up to 780 ° C.

A creep rupture test specimen and a Charpy impact test specimen were prepared from the heat-treated steel sheet, and subjected to a creep rupture test and a Charpy impact test under the following conditions.

(1) Creep rupture test Specimen: diameter 6.0 mm, distance between gauge points 30 mm (taken from steel sheet so that the longitudinal direction is the rolling direction?) Test temperature: 675 ° C. Load stress: 100 MPa (2) Charpy impact Test Specimen: 10 mm × 10 mm × 55 mm 2 mm V notch (Sampling from steel sheet so that notch is perpendicular to the rolling direction?) Test temperature 0 ° C. In addition, a thin film was formed from the center in the thickness direction of the steel sheet, and an electron micrograph was taken. After photographing, the size and precipitation density of M ₂₃ C ₆ and the intermetallic compound were calculated by image analysis.

The test results are shown in Tables 1 and 2.
In these tables, the M particles having a particle size of 0.6 μm or less
_{When 23} C ₆ and the intermetallic compound were precipitated in the martensite lath in total at least 0.4 / μm ³ ,
Those that did not precipitate were rated as x.

As is clear from Table 1, the heat-resistant steel having the chemical composition and precipitate specified in the present invention has a creep rupture time of about 3,000 to 4,400 hours, excellent high-temperature long-term strength, and excellent toughness. Is also excellent. On the other hand, comparative steels whose chemical composition and precipitates do not satisfy the requirements of the present invention,
The creep rupture time is about 850-2260 hours, which is lower in strength than the steel material of the present invention. Further, even if the chemical composition is within the range specified by the present invention, such as L, M, S, and U, the steel material whose precipitate does not satisfy the specification is inferior in high-temperature long-time creep strength.

[0044]

The high Cr ferritic heat-resistant steel material of the present invention has excellent high-temperature long-term creep strength at a high temperature of 625 ° C. or more, and is used for heat exchange steel pipes and pressure vessels used in fields such as nuclear power generation and the chemical industry. It is used as a material for steel plates and turbines and exhibits excellent effects, contributing to industrial development.

Claims (5)

[Claims] (1) C: 0.001 to 0.15% by mass%
Si: 1% or less, Mn: 0.05-1.5%, P: 0.
03% or less, S: 0.015% or less, Cr: 8 to 13
%, V: 0.2-0.5%, Nb: 0.002% -0.
2%, W: 2 to 5%, N: 0.001 to 0.03%,
B: 0.0001 to 0.01%, Al: 0.001 to
M ₂ containing 0.05%, the balance being Fe and impurities, the metal structure being a tempered martensite matrix, and the M ₂ having a particle size of 0.6 μm or less inside the martensite lath.
0.4 total of ₃ C ₆ and intermetallic compound / μm ³
A high Cr ferritic heat-resistant steel excellent in high-temperature long-time creep strength characterized by being precipitated as described above. 2. Ta: 0.002% instead of part of Fe
~ 0.2%, Ti: 0.001% ~ 0.1% and N
The high Cr according to claim 1, wherein one or more of d: 0.001% to 0.2% is contained.
Ferritic heat-resistant steel. 3. The method according to claim 1, wherein Mo: 0.01 to
The high Cr ferritic heat-resistant steel material according to claim 1 or 2, which contains 0.5%. 4. A method according to claim 1, wherein Co: 0.01% to
6%, Ni: 0.01% to 1% and Cu: 0.01%
The high Cr content according to any one of claims 1 to 3, wherein the content of Cr is 1 or 2% or more.
Ferritic heat-resistant steel. 5. In place of a part of Fe, Ca: 0.02% or less, La: 0.2% or less, Ce: 0.2% or less, Y:
The high Cr ferritic heat-resistant steel material according to any one of claims 1 to 4, comprising one or more of 0.2% or less and Hf: 0.2% or less.