JPH0885850A - High chromium ferritic heat resistant steel - Google Patents

Abstract

PURPOSE: To produce a high Cr ferritic heat resistant steel excellent in toughness as well as in high temp. creep strength by preparing a steel having a specific composition in which respective contents of Ti, Zr, and Hf are specified. CONSTITUTION: The high Cr ferritic heat resistant steel, having a composition consisting of, by weight, 0.02-0.15% C, 0-1.0% Si, 0.05-1.5% Mn, 8.0-13.0% Cr, 2.5-4.0% W, 0.10-0.50% V, 0.01-0.15% Nb,.5-8.0% 0.001-0.050% sol. Al, 0.020-0.12% N, 0.20-3.0% Cu, 0.10-1.5% Ni, and 0-0.50% Ta, either or both of 0-1.00% Mo and 0-0.030% B, either or both of 0-0.010% Ca and 0-0.10% Mg, one or >=2 kinds among 0-0.08% Sc, 0-0.15% Y, 0-0.23% La, 0-0.23% Ce, and 0-0.24% Nd, one or >=2 kinds among 0.005-0.08% Ti, 0.005-0.15% Zr, and 0.005-0.29% Hf, and the balance Fe with inevitable impurities, is prepared.

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, more specifically, a high temperature heat resistant pressure resistant member used in a wide industrial field such as boiler, nuclear power, chemical industry, specifically, a steel pipe, High Cr with excellent high temperature creep strength and toughness, suitable for use as pressure vessel steel plate and turbine material
Ferritic heat-resistant steel.

[0002]

2. Description of the Related Art Heat resistant steels used for high temperature heat resistant and pressure resistant members for boilers, nuclear power plants, chemical industries, etc. are generally required to have high temperature strength, toughness, high temperature corrosion resistance and oxidation resistance.

Conventionally, JIS-SUS321H,
Austenitic stainless steel such as SUS347H steel, JI
Low alloy steel such as S-STBA24 (2.1 / 4Cr-1Mo steel),
9-12Cr high Cr ferritic steels such as JIS-STBA26 (9Cr-1Mo steel) have been used.

Among them, high Cr ferritic steel is 500 to 650.
In the temperature range of ℃, it is superior in strength and corrosion resistance to low alloy steels, is cheaper than austenitic stainless steels, has high thermal conductivity and small thermal expansion, and therefore has thermal fatigue resistance. It is often used because it has the advantages that it does not easily cause scale peeling and does not cause stress corrosion cracking.

In recent years, in order to further improve the thermal efficiency in thermal power generation, high temperature and high pressure steam conditions are being promoted. In the future, from supercritical pressure conditions to ultra-supercritical pressure conditions of 650 ° C. and 350 atm. Operations are planned. With such changes in operating conditions, the performance requirements for steel pipes for boilers are becoming more and more severe, and from the viewpoint of long-term creep strength, oxidation resistance, especially steam oxidation resistance, the existing high Cr Ferrite steel has reached a situation where the required performance cannot be sufficiently satisfied.

To meet this requirement, it is appropriate to use austenitic stainless steel, but since it is expensive and uneconomical, W is contained in large amount even in high Cr ferritic steel which is cheaper than austenitic stainless steel. The application of new high-Cr ferritic steel with inclusions is under consideration.

[0007] For example, Japanese Patent Laid-Open No. 3-097832 discloses a high Cr heat-resistant steel containing Cu from the viewpoint of increasing the W content and improving the high temperature oxidation resistance as compared with the prior art.
4-371551 and Japanese Patent Laid-Open No. 4-371552 disclose Mo /
In addition to the optimization of W, a high Cr heat-resisting steel has been proposed, in which high temperature strength and toughness are improved by adding Co and B together. However, since these steels contain a large amount of W, the high temperature creep strength is certainly improved, but since W is a ferrite-forming element together with Mo, Cr, etc., addition of a large amount produces δ-ferrite, There is a drawback in that the toughness is unavoidable.

The most effective measure against this is to use a single phase of martensite.
5-263196 publication, by reducing the amount of Cr,
JP-A-5-311342, JP-A-5-311343, JP-A-5-311344
No. 5-311345, No. 5-311346, etc.
It has been proposed to improve the toughness of steel by adding a large amount of austenite forming elements such as Ni, Cu and Co.

However, the steel proposed in the Japanese Unexamined Patent Publication (Kokai) No. 5-263196 is inferior in steam oxidation resistance because Mo, Ni, etc. destroy the stable scale structure of Cr ₂ O ₃ , and the latter steel. The steel proposed in JP-A-5-311342 and the like contains a large amount of Ni, Cu, etc., and therefore lowers the Ac ₁ transformation point and the Ac ₃ transformation point of the steel, so that the temper softening resistance becomes small. On the contrary, on the contrary, the strength decreases for a long time, but the addition of a large amount of these elements has a drawback that the structure of the oxide mainly composed of Cr ₂ O ₃ is changed and the steam oxidation resistance is also deteriorated.

As described above, a high Cr ferritic heat-resistant steel satisfying all the properties such as high temperature long-term creep strength and toughness under high-temperature and high-pressure ultra-supercritical pressure conditions has not yet been found.

[0011]

SUMMARY OF THE INVENTION In view of the above situation, an object of the present invention is to provide a new high Cr ferritic heat resistant steel which is excellent in high temperature long time creep strength and toughness.

[0012]

The gist of the present invention is as follows.
Cr Ferritic heat resistant steel.

% By weight, C: 0.02 to 0.15%, Si: 0 to 1.0
%, Mn: 0.05 to 1.5%, Cr: 8.0 to 13.0%, W: 2.5 to 4.0
%, V: 0.10 to 0.50%, Nb: 0.01 to 0.15%, Co: 2.5 to 8.0
%, Sol-Al: 0.001 to 0.050%, N: 0.020 to 0.12%, Cu:
0.20 to 3.0%, Ni: 0.10 to 1.5%, Ta: 0 to 0.50%, and Mo: 0 to 1.00% and B: 0 to 0.030%, 1 or 2
Species, Ca: 0 to 0.010% and Mg: 0 to 0.010%, 1 or 2 kinds, Sc: 0 to 0.08%, Y: 0 to 0.15%, La: 0 to 0.23%, C
e: 0 to 0.23% and Nd: 0 to 0.24%, 1 or 2
Including more than one species, further Ti: 0.005-0.08%, Zr: 0.005-
0.15% and Hf: 0.005 to 0.29%, one or more, and the balance Fe and unavoidable impurities. P and S in the impurities are 0.030% or less and 0.015% or less, respectively. High temperature creep strength and high toughness
Cr ferritic heat resistant steel.

In the above-mentioned steel, Sc, Y, La, Ce and Nd, Mo and B, Ca and Mg, and Si and Ta may all be unadded.

When one or more of Sc, Y, La, Ce and Nd are contained, 0.001% of each element is contained.
In the case of containing one or two kinds of Mo and B, the content of Mo is 0.01% or more, the content of B is 0.0005% or more, and when one or two kinds of Ca and Mg is contained, any of The element content should be 0.0005% or more, 0.01% or more when Si is contained, and 0% when Ta is contained.
It is desirable that each value be 01% or more.

The inventors of the present invention have studied in detail how the high temperature long-term creep strength and toughness of the high Cr ferritic heat resistant steel correspond to the chemical composition and microstructure of the steel. Therefore, the present invention has been made.

The technical new knowledge of the present invention and the design of the steel of the present invention are based on the following technical ideas.

The improvement of the creep strength of the high Cr ferritic heat-resisting steel containing Cu in addition to Co and Ni up to 100,000 hours at 600 ° C. or longer is improved by Fe ₇ W ₆ type [containing Cr and Mo. In this case, for example, the composition is Fe ₅₅ Cr ₂₂ (Mo, W) ₂₃ ], and it is most effective to have a finely dispersed precipitation structure of an intermetallic compound mainly composed of a μ phase.

Cu in high Cr ferritic steel containing Co and Ni
However, the precipitation of δ-ferrite is suppressed, but the recovery of the tempered martensite structure during high-temperature long-term creep and the resistance to softening are rather reduced, which results in long-term Causing a drastic decrease in creep strength. Also, Cu tends to segregate at grain boundaries during heating for a long time, which is one of the causes of high temperature embrittlement.

[0020] Recovery of martensitic structure of high Cr ferritic heat resistant steel containing Cu, is not easy to control the softening direct conventional V (C, N), NbN , charcoal, etc. M ₂₃ C ₆ By dispersing fine precipitates that are stable against long-term heating in addition to nitrides, it is possible to suppress the recovery of the martensitic structure and the deterioration of creep strength and toughness due to softening.

As the above-mentioned fine precipitates which are stable even after heating for a long time, nitrides of hexagonal transition metals of Ti, Zr and Hf are effective, and Ti, Zr and Hf are heated for a long time. It is possible to prevent Cu segregation to grain boundaries in the inside to prevent high temperature embrittlement.

[0022]

The reason for limiting each alloy component of the present invention will be described below.

C: 0.02 to 0.15% C may be formed as MC [carbonitride M (C, N)]. It should be noted that M indicates an alloy element, and the same applies hereinafter], M
It is an element that forms ₇ C ₃ and M ₂₃ C ₆ type carbides and greatly affects the performance of the steel of the present invention. The high Cr ferritic heat-resistant steel of the present invention is usually used by obtaining a tempered martensite structure by a normalizing (norma) + tempering (tempering) treatment. The short-term creep strength is determined,
During heating for a long period of time, precipitation of fine carbides such as VC and TaC also progresses, which contributes to the improvement of creep strength on the long side. However, 0.02% or more is required to obtain this precipitation strengthening effect, while if it exceeds 0.15%, cohesive coarsening of the carbides is caused from the initial stage of use, and conversely a decrease in creep strength on the long-term side. Therefore, the C content is 0.
It was set to 02 to 0.15%. Preferably, it is 0.06 to 0.12%.

Si: upper limit 1.0% Si is an element effective as a deoxidizing agent for molten steel and for improving steam oxidation resistance at high temperature, but addition of a large amount thereof causes deterioration of toughness. It has been added in the range of 0.01 to 1.0%. Therefore, even in the present invention, it is desirable to contain 0.01% or more when added,
When deoxidizing with a trace amount of Al having a sol-Al content of about 0.050% or less, Si need not be added. Therefore, the upper limit was set to 1.0%.

Mn: 0.05 to 1.5% Mn is added as a deoxidizing agent and a desulfurizing agent for molten steel, and is an element effective for improving short-time creep strength under high stress. However, in order to obtain the effect, 0.05% or more is required, and on the other hand, if it exceeds 1.5%, the toughness deteriorates, so the Mn content was made 0.05 to 1.5%. It is preferably 0.10 to 1.0%.

Cr: 8.0 to 13.0% Cr forms carbides to improve creep strength and forms a dense oxide film mainly of Cr, and the corrosion resistance and oxidation resistance of the steel of the present invention at high temperature, especially steam resistance. It is an element that greatly contributes to maintaining the oxidative property. However, in order to obtain the effect, 8.0% or more is required. On the other hand, if it exceeds 13.0%, the formation of δ-ferrite is promoted and the toughness is deteriorated. Therefore, the Cr content is set to 8.0 to 13.0%. . It is preferably 9.0 to 12.0%.

W: 2.5 to 4.0% W is one of the main strengthening elements of the steel of the present invention, and is finely dispersed in the grains as an intermetallic compound mainly composed of Fe ₇ W ₆ type μ phase during high temperature use. It is an element that precipitates and contributes to the improvement of creep strength for a long time, and also partially forms a solid solution in Cr carbide to suppress agglomeration and coarsening of carbide and contribute to maintenance of strength. However, in order to obtain the effect, 2.5% or more is necessary, while on the other hand, if it exceeds 4.0%, the formation of δ-ferrite is promoted and the toughness is deteriorated.
It was set to 4.0%. It is preferably 2.5 to 3.5%.

V: 0.10 to 0.50% V is an element that forms fine carbonitrides and contributes to the improvement of creep strength.

However, in order to obtain the effect, 0.10% or more is required, while even if added over 0.50%, the effect is saturated, so the V content was made 0.10 to 0.50%. It is preferably 0.15 to 0.35%.

Nb: 0.01 to 0.15% Nb is an element that forms nitrides and carbonitrides and contributes to the improvement of strength and toughness. However, in order to obtain the effect, 0.01% or more is required. On the other hand, if it exceeds 0.15%, coarse nitrides are formed, and conversely the toughness is lowered. It was 0.15%. Preferably, 0.
It is from 04 to 0.12%.

Co: 2.5 to 8.0% Co promotes precipitation of the Fe ₇ W ₆ type μ phase in the steel of the present invention, contributes to the improvement of creep strength, and stabilizes the martensite structure as an austenite forming element. Is an element that also contributes to. But to get that effect
2.5% or more is required, while if it exceeds 8.0%,
The Co content is set to 2.5 to 8.0% because the decrease in the Ac ₁ transformation point becomes remarkable and the strength decreases. Preferably,
It is 3.0 to 6.0%.

Ni: 0.10 to 1.50% Ni is an element which has the same function as Co as an austenite-forming element, and which strengthens the martensite structure and contributes to the improvement of toughness. However, in order to obtain the effect, 0.10% or more is required, while if it exceeds 1.50%, the Ac ₁ transformation point of steel is significantly lowered, and the strength is reduced, so the Ni content is 0.10 to 1.50%. And Preferably 0.20
~ 0.50%.

Cu: 0.20 to 3.0% Cu is an element which has the same effect as Co and Ni as an austenite forming element, and is extremely effective in improving the oxidation resistance at high temperatures and the corrosion resistance to coal ash and the like. However, in order to obtain the effect, 0.20% or more is required. On the other hand, if it exceeds 3.0%, the hot workability is deteriorated and high temperature embrittlement is caused. Therefore, the Cu content is set to 0.20 to 3.0%. It is preferably 0.30 to 0.80%.

Sol-Al: 0.001 to 0.050% Al is added as a deoxidizing agent for molten steel. However, in order to obtain this effect, the sol-Al content must be 0.001% or more. On the other hand, if the sol-Al content exceeds 0.050%, the creep strength will be reduced. Is 0.001 to 0.
It was 050%.

It is preferably 0.01 to 0.03%.

N: 0.01 to 0.12% N forms nitrides and carbonitrides to form creep strength,
It is an element that contributes to the improvement of toughness. However, in order to obtain the effect, 0.01% or more is necessary. On the other hand, when it exceeds 0.12%, the coarsening of the nitride progresses, and on the contrary, the toughness is significantly reduced, so the N content is 0.01 to 0.12. %. Preferably, it is 0.04 to 0.08%.

Ti, Zr, Hf: 0.005 to 0.08%,
0.005-0.15%, 0.005-0.29% Ti, Zr, and Hf are all powerful nitride-forming elements,
Stable for a long time due to the inclusion of a trace amount, it forms a fine nitride, especially the recovery of the tempered martensite structure of the present invention steel containing Cu, the softening resistance is enhanced, and the creep strength at high temperature for a long time It is an extremely important element for improving the heat treatment. One or more of Ti, Zr and Hf are selected and contained.
05% or more is required. However, in the case of Ti, 0.08%
If Zr exceeds 0.15% and Hf exceeds 0.29%, coarse nitrides are formed and the toughness deteriorates rapidly.
When adding these elements alone, the upper limit of the content is
It was set to 0.08%, 0.15%, and 0.29%, respectively.

S, P: The upper limits are 0.015%,
0.030% S and P are contained in steel as unavoidable impurities and are elements that adversely affect hot workability, weld zone toughness, etc.,
From the viewpoint of ensuring hot workability, weld zone toughness, etc., it is desirable that it is as low as possible, but if it is 0.015% or less and 0.030% or less respectively, it does not directly affect the performance of the steel of the present invention, so the upper limits are It was set to 0.015% or less and 0.030% or less.

In the steel of the present invention, in addition to the above components,
The following Ta may be contained.

Ta: Upper limit 0.50% Ta forms nitrides and carbonitrides, similar to Nb,
Since it has the effect of improving strength and toughness, it can be contained if necessary in order to obtain this effect. Since the effect is obtained at 0.01% or more, when it is contained, 0.01% or more is desirable. But 0.
If it exceeds 50%, coarse nitrides are formed, and conversely the toughness is lowered, so the upper limit was made 0.50%.

In the steel of the present invention, in addition, the following Mo and / or B may be selected and contained.

Mo and B: The upper limits are 1.0% and 0.03, respectively.
0% Mo and B are elements that have the effect of finely dispersing and precipitating M ₂₃ C ₆ type carbides, and have the effect of improving the creep strength at high temperature for a long time, so if this effect is desired, Mo and / or B can be contained if necessary. The effect is 0.01% or more for Mo, B
Is obtained with 0.0005% or more, so when it is contained, it is desirable that Mo is 0.01% or more and B is 0.0005% or more.

However, when the content of Mo exceeds 1.0% and the content of B exceeds 0.030%, coarse precipitates are formed and the toughness deteriorates. Therefore, the upper limit of Mo is 1.0% and the upper limit of B is
It was set to 0.030%.

In the steel of the present invention, in addition, the following Ca and / or Mg may be selected and contained.

Ca, Mg: The upper limits are both 0.010% Ca and Mg are elements having an action of improving the hot workability of steel, and are contained for the purpose of improving the hot workability. be able to. The effect is that the content is 0.00
When Ca or / and Mg is contained, the content is preferably 0.0005% or more because it can be obtained at 05% or more. However, if the content exceeds 0.010% in both cases, coarsening of inclusions is caused, and conversely, workability and toughness are impaired, so the upper limit was made 0.010% in all cases.

In addition to the steel of the present invention,
One or more of Sc, Y, La, Ce and Nd may be selected and contained.

Sc, Y, La, Ce, Nd: The upper limits are:
0.08%, 0.15%, 0.23%, 0.23%, 0.24% Sc, Y, La, Ce, and Nd have a function of improving the creep strength on the long-term side by finely dispersing and depositing as a nitride due to the inclusion of a trace amount. So if you want to get this effect,
If desired, one or more of Sc, Y 2, La, Ce and Nd may be selected and contained. The effect is obtained with a content of 0.001% or more, so
When it is contained, it is desirable that the content of each is 0.001% or more.

However, in the case of Sc, it exceeds 0.08%, and in the case of Y,
>0.15%,> 0.23% for La,> 0.23% for Ce,
In the case of Nd, addition of a large amount of more than 0.24% causes formation of coarse precipitates and lowers toughness.
%, 0.15%, 0.23%, 0.23%, 0.24%.

[0049]

EXAMPLES 41 kinds of steels having the chemical compositions shown in Tables 1 and 2 (No. 1 to 4 are conventional steels, No. 5 to 20 are comparative steels, No. 21)
~ 41 is the steel of the present invention) is melted in a 50 kg vacuum induction melting furnace 1
44 mmφ ingots were produced, and the obtained ingots were hot forged and hot rolled into plate materials with a thickness of 20 mm, and various test pieces were collected from these plate materials.

In Table 1, Nos. 1 to 4 are conventional high Cr ferritic heat-resistant steels, No. 1 is JIS-STBA26, No. 2 is fire ST.
BA27 (Thermal and Nuclear Technology Association Standard), No.3 is ASTM-A213-T
91 and No. 4 are steels specified in DIN-X20CrMoWV121.

[0051]

[Table 1]

[0052]

[Table 2]

Prior to the various tests, the No. 1 and No. 2 steels were normally 950 ° C. × 1 hour applied to these steels → AC (air cooling) normalizing treatment, and then 750 ° C. × 1 hour →
AC tempered, 1050 for other steels
℃ × 1 hour → After normalizing the AC, 780 ℃ × 1 hour →
AC tempering treatment was performed and various tests were performed, and the creep strength and toughness were investigated under each method under the following conditions.

Some of the test pieces of each test steel were subjected to a Charpy impact test after aging treatment at 600 ° C. for 10,000 hours in order to evaluate the long-term heat embrittlement.

[Creep rupture test] Test temperature: 650 ° C Specimen: 6.0 mm φ x GL = 30 mm Applied load: 100 MPa Test item: Breaking time (target: 10,000 hours or more) [Charpy impact test] Test temperature: 0 ° C Test piece: 10 mm width × 10 mm thickness × 55 mm length−2 mm V notch Test item: Impact value (target: vEo ≧ 50 J / cm ² ) Table 3 shows these test results.

[0056]

[Table 3]

As shown in Table 3, all of the conventional steels Nos. 1 to 4 were subjected to a creep rupture test at 650 ° C. and 100 MPa.
The fracture time is less than 1000 hours and the high temperature creep properties at 650 ° C or higher are not sufficient.

In the steels of Comparative Examples Nos. 5 to 20, some components were out of the scope of the present invention, so that the high temperature creep properties were not good, or even if the high temperature creep properties were excellent, The toughness after heating for a long time is not good, and none of them satisfy both properties at the same time.

On the other hand, the steels of the present invention Nos. 21 to 41 simultaneously satisfy all of these characteristics, and are excellent in high-temperature long-term creep characteristics, which cannot be obtained by conventional steels, and also excellent in toughness. Periodical high Cr ferritic heat resistant steel has been obtained.

[0060]

INDUSTRIAL APPLICABILITY The steel of the present invention is used in a wide range of industrial fields such as boilers, nuclear power, chemical industry, etc.
For example, a high Cr ferritic heat-resistant steel excellent in long-term creep properties and toughness, which is used as a material for steel pipes, steel plates for pressure vessels and turbines, especially after long-term heating, can be obtained. Therefore, the benefits of the present invention to the field are extremely large.

Claims (1)

[Claims] 1. In weight%, C: 0.02 to 0.15%, Si: 0 to 1.0
%, Mn: 0.05 to 1.5%, Cr: 8.0 to 13.0%, W: 2.5 to 4.0
%, V: 0.10 to 0.50%, Nb: 0.01 to 0.15%, Co: 2.5 to 8.0
%, Sol-Al: 0.001 to 0.050%, N: 0.020 to 0.12%, Cu:
0.20 to 3.0%, Ni: 0.10 to 1.5%, Ta: 0 to 0.50%, and Mo: 0 to 1.00% and B: 0 to 0.030%, 1 or 2
Species, Ca: 0 to 0.010% and Mg: 0 to 0.010%, 1 or 2 kinds, Sc: 0 to 0.08%, Y: 0 to 0.15%, La: 0 to 0.23%, C
e: 0 to 0.23% and Nd: 0 to 0.24%, 1 or 2
Including more than one species, further Ti: 0.005-0.08%, Zr: 0.005-
0.15% and Hf: 0.005 to 0.29%, one or more, and the balance Fe and unavoidable impurities. P and S in the impurities are 0.030% or less and 0.015% or less, respectively. High temperature creep strength and high toughness
Cr ferritic heat resistant steel.