JP3196587B2 - High Cr ferritic heat resistant steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high Cr ferritic heat-resistant steel. The heat-resistant steel of the present invention is excellent in long-term creep strength at high temperatures, steam oxidation resistance, and toughness at room temperature. Therefore, the heat-resistant steel of the present invention,
It is suitable for materials for equipment operated at high temperature and high pressure, such as boilers, nuclear power plants, and chemical industry equipment, specifically, steel tubes for heat exchange or steel plates for pressure vessels, and materials for turbines.

[0002]

2. Description of the Related Art Devices such as boilers, nuclear power plants, and chemical industry devices are used for a long time under high temperature and high pressure. Therefore, the heat-resistant steel used in these devices is required to be excellent in high-temperature strength, corrosion resistance, oxidation resistance, toughness at room temperature, and the like.

[0003] In these applications, conventionally, austenitic stainless steels (for example, JIS-SUS321H and SUS347H steels) and low alloy steels (for example, JIS-ST
BA24 (2 1 / 4Cr-1Mo)), and 9
~ 12Cr high Cr ferritic steel (for example, JIS-
STBA26 (9Cr-1Mo steel)) and the like have been used. Among them, high Cr ferritic steel is 500-65.
In a temperature range of 0 ° C., it is superior to low alloy steel in strength and corrosion resistance. In addition, high Cr ferritic steel is inexpensive compared to austenitic stainless steel, has high thermal conductivity and low coefficient of thermal expansion, so it has excellent thermal fatigue resistance, and it is difficult for scale peeling to occur. There is a feature that it is equipped with. In addition, high Cr
Ferrite steels are widely used because they have the advantage of not causing stress corrosion cracking.

[0004] In recent years, in thermal power generation, steam conditions of a boiler have been increased in temperature and pressure in order to improve thermal efficiency. That is, the current supercritical pressure condition 538
It is planned to operate under ultra-supercritical conditions such as 650 ° C. and 350 atm. With such changes in steam conditions, the performance required for steel pipes for boilers has become increasingly severe. Therefore, it is a fact that the conventional high Cr ferritic steel cannot sufficiently respond to the properties such as the long-term creep strength and the oxidation resistance at high temperatures as described above, particularly the steam oxidation resistance. The steam oxidation is an oxidation phenomenon that occurs on the surface of a boiler steel pipe or the like exposed to high-temperature and high-pressure steam. When this oxidation occurs and an oxide film (scale) is formed, the scale peels off as the temperature of the boiler changes. Since the peeled scale causes troubles such as clogging in the steel pipe, prevention of steam oxidation is an important issue.

[0005] Austenitic stainless steel has a performance capable of meeting the above requirements. But,
Austenitic stainless steel is expensive,
From an economic point of view, the range of use for commercial equipment is limited. Therefore, efforts have been made to improve the properties of high Cr ferritic steels, which are less expensive than austenitic stainless steels, and to expand the usable range.

As a measure for improving the characteristics of high Cr ferritic steel, heat resistant steel containing W in conventional high Cr ferritic steel has been developed. For example, Japanese Patent Application Laid-Open No. 3-97832 discloses a high Cr ferritic steel containing higher W content than conventional ones and containing Cu in order to improve oxidation resistance at high temperatures. In addition, Japanese Unexamined Patent Publication No.
Japanese Patent No. 371551 and Japanese Patent Application Laid-Open No. 4-371552 disclose W and Mo, select an appropriate ratio of the contents of Mo and W, and contain both Co and B, so that the strength at high temperatures is increased. High Cr ferritic steel with improved toughness has been proposed.

[0007] These high Cr ferritic steels are excellent in high temperature creep strength because they contain a large amount of W.
However, since W is a ferrite-forming element together with Mo, Cr, etc., when contained in a large amount, δ-ferrite is formed in steel. As a result, there is an adverse effect that the toughness of the high Cr ferritic steel is reduced.

In order to prevent a decrease in toughness, it is effective to change the structure of the high Cr ferritic steel to a martensite structure single phase. Taking this point into consideration, Japanese Patent Application Laid-Open No. 5-263196 discloses a heat-resistant steel having a single phase of martensite by lowering the Cr content. Also, JP-A-5-31142, JP-A-5-31343, and JP-A-5-31343.
Nos. 311344, 5-31345 and 5-311
No. 346 proposes a high Cr ferritic steel in which the toughness is improved by adding Ni, Cu, Co, and the like, which are austenite forming elements, to the high Cr ferritic steel.

In the high Cr ferritic steel disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 5-263196, Mo, N
Since i and the like break the dense and stable corundum-type Cr ₂ O ₃ scale layer formed on the surface of the steel, there is a defect that the steam oxidation resistance is poor. Also, Japanese Patent Application Laid-Open
The high-Cr ferritic steel disclosed in JP-A-31342 and the like contains a large amount of Ni, Cu, and the like.
The c ₁ transformation point and the Ac ₃ transformation point are low. Therefore, since the tempering softening resistance is low, the long-time creep strength is low. In addition, when these elements are contained, the structure of the oxide mainly composed of Cr ₂ O ₃ changes, so that there is a disadvantage that the steam oxidation resistance of the high Cr ferritic steel is deteriorated.

[0010]

As described above, high carbon which satisfies all the properties of long-term creep strength, steam oxidation resistance and toughness under high temperature and high pressure ultra supercritical pressure conditions.
The reality is that r-ferritic heat-resistant steel has not yet been developed.

[0011] The present invention has been made in view of the above-mentioned circumstances, and at a high temperature exceeding 600 ° C,
It is an object of the present invention to provide a high-Cr ferritic heat-resistant steel excellent in high-temperature long-time creep strength, steam oxidation resistance and toughness at room temperature.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted research and development with the aim of developing a high Cr ferritic heat resistant steel having long-term creep strength at a temperature exceeding 600 ° C., steam oxidation resistance and toughness at room temperature. Was done. For this purpose, the relationship between the properties of high Cr ferritic steel, such as high-temperature long-time creep strength, toughness, and steam oxidation resistance, and the chemical composition and metal structure (microstructure) of the steel were studied in detail. As a result, the following new knowledge was obtained.

-Long-term creep strength and toughness-The metal structure of the high Cr ferritic steel becomes a martensite structure in which carbonitrides are precipitated by normalizing and tempering. This starting structure determines the initial strength of the steel. However, when used at a temperature higher than 600 ° C., the martensite structure recovers and softens over time, so that the creep strength is not maintained.

In order to improve the long-term creep strength of a high Cr ferritic steel even at a temperature exceeding 600 ° C., fine Fe ₇ W is contained in the metal structure during long-term use.
Type ₆ (including Cr and Mo is, for example, Fe ₅₅ C
It is extremely effective to uniformly disperse and precipitate the μ phase of r ₂₂ (Mo, W) ₂₃ ). This is because the μ phase has a function of maintaining the creep strength even after the recovery and softening of the martensite structure.

In order to disperse and precipitate a fine μ phase, W is contained in a large amount alone, or W and Mo are contained.
Is used in combination, it is effective to lower the ratio of Mo to W. The reason is that when there are many Mo,
The μ phase locally precipitates at the prior austenite grain boundary and the martensite lath interface, whereas when the amount of W is large, W
This is because the μ phase is precipitated not only in the grain boundaries but also in the grains due to the low diffusion rate of the .mu. Further, since the upper limit of the temperature at which the μ phase can stably exist is high, it is advantageous to exhibit the effect of the μ phase up to a high temperature.

In the state where the fine μ phase is dispersed and precipitated, the toughness caused by the μ phase does not decrease.

-Steam Oxidation Resistance- The scale layer formed on the surface of the high Cr ferritic steel is made of a corundum-type dense oxide mainly composed of Cr oxide, thereby suppressing steam oxidation. . However, if Mo exists in the scale,
Since the scale layer changes to a spinel-type brittle structure, the steam oxidation resistance is significantly reduced. W does not adversely affect steam oxidation resistance. Therefore, by containing W and reducing Mo, steam oxidation resistance can be improved.

N which has a very strong tendency to form oxides
By including d (neodymium) in steel, the effect of Mo on steam oxidation resistance can be reduced.

The addition of Nd changes the form of nitride precipitation at high temperatures. In particular, in steel to which Nb (niobium) is added, fine NbN is dispersed and precipitated,
N improves toughness.

In order to sufficiently exert the effect of Nd, it is important to strictly control the relationship between the Nd and O (oxygen) contents and the relationship between Nb and Nd and the O contents.

The present invention has been completed based on the above findings and is based on the following technical concept.

A) Long-term creep strength and toughness are F
It is improved by dispersing and precipitating a fine μ phase mainly composed of e ₇ W ₆ type.

B) The content of Mo, which is harmful to steam oxidation resistance, is reduced to an amount necessary for obtaining the strength of the base material. The decrease in Mo is compensated for by W.

C) By containing Nd, Mo
And to improve toughness.

D) The relationship between Nd and O content and the relationship between Nd, Nb and O content are controlled.

According to the present invention based on the above technical idea, "C: 0.02 to 0.15%, Si: 0 to 1.0%, Mn: 0.05 to 1.5%, P: 0.030% or less, S: 0.015% Cr: 8.0-13.0%, W: 1.5-3.5%, Mo: 0-1.0%, Co: 2.5-6.0%, V: 0.10-0.50%, Nb: 0.030-0.14%, Nd: 0.001-0.17%, N: 0.020 to 0.08% , B: 0 to 0.020% , O: 0.010% or less, sol.Al : 0.001 to 0.050% , and the balance is provided with a chemical composition consisting of Fe and unavoidable impurities. High Cr ferritic heat-resistant steel satisfying the expressions 1) and (2). ”0 ≦ P _Nd ≦ 0.103 (1) 0.005 ≦ P _Nb ≦ 0.03 (2) P _Nd = Nd−6. The summary is 01 × O (oxygen) P _Nb = 0.151 × Nb + 0.971 × P _Nd (element symbols indicate the content (% by weight) of each element in steel).

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION The relationship between the alloying elements contained in the high Cr ferritic heat resistant steel of the present invention (hereinafter referred to as the present invention steel) and the properties of the steel, the range of the content of each alloying element and the range thereof The reason for the limitation will be described below.

C: C is a carbide MC (M is an alloying element), M
₇ C ₃ , M ₂₃ C ₆ type carbides are formed (carbonitride M
(C, N))). This carbide has a significant effect on the properties of the steel according to the invention. The high Cr ferritic steel is usually used in its tempered martensitic structure by normalizing and tempering. When used under high temperature for a long time, V
Precipitation of fine carbides such as C and NbC proceeds. These carbides serve to maintain the creep strength for a long time. In order to obtain the effect of this carbide, 0.02% by weight
(Hereinafter, the percentage of the chemical composition is represented by% by weight) or more. On the other hand, if the C content exceeds 0.15%, when used at a high temperature, agglomeration and coarsening of carbides occur from the initial stage, and the long-term creep strength decreases. Therefore, the C content is preferably 0.02 to 0.15%. Preferably, it is 0.06 to 0.12%.

Si: Si is used as a deoxidizing agent for molten steel. In addition, they are effective elements for improving steam oxidation resistance at high temperatures. However, if it is excessive, the toughness of the steel decreases, so 1% or less is preferable. A with enough molten steel
When deoxidation is performed in 1 amount, it is not necessary to particularly include Si.

Mn: Mn is usually added to fix S as MnS and improve the hot workability of steel.
In the steel of the present invention, Mn also has the effect of improving short-time creep strength under high stress. The Mn content at which the effect can be obtained is 0.05% or more. on the other hand,
If it exceeds 1.5%, the toughness of the steel decreases. Therefore, the Mn content is set to 0.05 to 1.5%. Preferably, it is 0.10 to 1.0%.

Cr: Cr is an indispensable element for securing the corrosion resistance and oxidation resistance at high temperatures of the steel of the present invention, particularly the steam oxidation resistance. When Cr is contained, a dense oxide film mainly composed of Cr oxide is formed on the surface of the steel. This oxide film improves the corrosion resistance and oxidation resistance at high temperatures of the steel of the present invention, particularly the steam oxidation resistance.

Further, Cr has a function of forming a carbide to improve the creep strength.

In order to obtain these effects, the Cr content must be 8.0% or more. On the other hand, when the content exceeds 13.0%, δ-ferrite is easily formed, so that the toughness is reduced. Therefore, the Cr content is 8.0-13.
0%.

W: W is one of the important elements for increasing the creep strength in the steel of the present invention. W forms an intermetallic compound mainly composed of μ phase of Fe ₇ W ₆ type when steel is used at a high temperature. Since this intermetallic compound is finely dispersed and precipitated in the crystal grains, the creep strength for a long time is improved. W is also an element effective in maintaining the high-temperature strength of the steel of the present invention, since W partially dissolves in the Cr carbide and has a function of suppressing aggregation and coarsening of the carbide. In order to obtain this effect of W, the content is 1.5%
The above is necessary. On the other hand, when the content exceeds 3.5%, δ-ferrite is easily formed, and the toughness is reduced. Therefore, the W content is set to 1.5 to 3.5%.

Mo: Mo mainly functions to strengthen solid solution by dissolving in the base material and strengthen precipitation by forming precipitates. In particular, M ₂₃ C ₆ containing Mo or M
₇ C ₃ -type carbide is an element that is extremely effective in ensuring long-term creep strength because it is stable at high temperatures. However, Mo is an element harmful to steam oxidation resistance as described above. As described above, the adverse effect can be reduced by adding Nd. However, even in that case, 1.0%
Addition of a large amount exceeding the above causes a decrease in toughness. Therefore,
When Mo is contained, the upper limit is set to 1.0%. The steel of the present invention may not contain Mo.

Co: Co is Fe ₇ W in the steel of the present invention.
_It promotes precipitation of type ₆ μ phase and contributes to improvement of creep strength. Co is an austenite forming element and is an element that contributes to stabilization of the martensite structure. In order to obtain the effect, a content of 2.5% or more is required. On the other hand, if it exceeds 6.0%, the Ac ₁ transformation point of the steel is significantly reduced, and the high-temperature strength is reduced. Therefore, the Co content was set to 2.5 to 6.0%.

V: V is an element that forms fine carbonitrides and contributes to improvement in creep strength. The effect of V appears at a content of 0.10% or more. On the other hand, the content rate is 0.50
%, The effect is saturated, so the V content was set to 0.10 to 0.50%.

Nb: Nb improves the strength and toughness of steel by forming nitrides and carbonitrides. Especially Nd
When used together, fine NbN-type nitrides are dispersed and precipitated at a high temperature, thereby effectively improving toughness. To obtain the effect, the Nb content needs to be 0.03% or more. However, if it exceeds 0.14%, the effect of Nb saturates. Therefore, the Nb content is suitably 0.03 to 0.14%.

Nd: Nd has a very strong tendency to bond with O (oxygen) and forms an oxide in molten steel. When an appropriate amount of Nd is contained, fine Nd oxides are uniformly dispersed in the steel. On the other hand, when the high Cr ferritic steel is exposed to a high temperature and high pressure steam atmosphere, a film mainly composed of Cr oxide is formed on the surface thereof. At this time, the fine Nd oxide dispersed on the surface of the steel exerts a pinning effect of suppressing the growth of Cr oxide. Therefore, the growth of the scale is suppressed and the scale is densified. Therefore, when an appropriate amount of Nd is included, the steam oxidation resistance of steel is dramatically improved. In addition, Nd fixes oxygen in the steel as Nd oxide, thereby preventing Nb or the like from being combined with oxygen to form an oxide. Because Nb helps to form nitrides and carbonitrides,
It has the effect of indirectly improving the strength and toughness of steel. Such an effect is obtained because the Nd content is 0.0%.
It is in the range of 01 to 0.17%. If it exceeds 0.17%, harm of Nd appears and the toughness of the steel decreases.

Nd further forms a carbide such as NdC ₂ . This carbide has small lattice mismatch (misfit) with the matrix and is fine and stable as compared with other REM carbides. Therefore, Nd also contributes to improvement in creep strength.

N: N is one of the important elements that form nitrides and carbonitrides and contribute to the improvement of creep strength and toughness.
One. In order to obtain the effect, it is necessary to contain 0.02% or more. However, if the content exceeds 0.08%, coarsening of the nitride proceeds, and the toughness significantly decreases. Therefore, the N content is set to 0.02 to 0.08%.

B: When B is contained in a trace amount in steel, M ₂₃
C ₆ type carbide is finely dispersed and precipitated. for that reason,
High temperature and long time creep strength is improved. Further, when the cooling rate after the heat treatment is slow for a thick material or the like, it has a function of improving the hardenability and improving the high-temperature strength. In the steel of the present invention,
B may not be contained, but may be contained for the purpose of increasing the high-temperature strength. Since the effect of B becomes significant at 0.0005% or more, it is desirable to make it 0.0005% or more when it is contained. However, if it exceeds 0.020%, coarse precipitates are formed and the toughness is reduced, so the upper limit was made 0.020%.

Sol. Al: Al is mainly added as a deoxidizer for molten steel. In steel, Al as oxide
And Al which exists in a form other than oxides. Usually, the latter Al is distinguished as hydrochloric acid-soluble Al (sol. Al) on analysis. To obtain a deoxidizing effect, sol. A
l content of 0.001% or more is required. On the other hand, 0.0
If it exceeds 50%, the creep strength will decrease. Therefore, sol. Al content is 0.001 to 0.050%
And

P, S: P and S are elements contained in steel as unavoidable impurities and adversely affect hot workability, toughness of a welded portion, and the like. In any case, the content should be as low as possible. The contents of P and S were 0.03, respectively.
0% or less and 0.015% or less are desirable.

O (oxygen): O is an element contained in steel as an unavoidable impurity, and adversely affects toughness and the like when unevenly distributed as a coarse oxide. In particular, in order to ensure toughness, the lower the better, the better. When the O content is 0.010% or less, the effect on the toughness of the steel of the present invention is small, so the upper limit is set to 0.010%.

Relationship between Nd, Nb and O: As described above, N
d and Nb have an extremely strong tendency to combine with oxygen to form an oxide. In order to deposit Nb nitride, carbonitride, etc., (1) In order to secure Nd and Nb which are not bonded to oxygen at the time of depositing NbN, the following formula (1) is used.
To disperse and precipitate fine NbN, the following (2)
It is necessary to satisfy the formula.

0 ≦ P _Nd ≦ 0.103 (1) 0.005 ≦ P _Nb ≦ 0.03 (2) P _Nd = Nd−6.01 × O (oxygen) P _Nb = 0.151 × Nb + 0.0971 × P _Nd (element symbols indicate the content (% by weight) of each element) The steel of the present invention can be manufactured by manufacturing equipment and a manufacturing process which are usually used industrially. In order to obtain a steel having the chemical composition of the steel of the present invention, the steel may be refined in a furnace such as an electric furnace or a converter, and the components may be adjusted by adding a deoxidizer and an alloy element. The relationship between the above formulas (1) and (2) indicates that each alloy element is added in consideration of the O content in the molten steel after deoxidation, the addition yield of each alloy element, and the like, which are empirically determined. Can be satisfied. In particular, when strict component adjustment is required, a method of performing vacuum treatment on molten steel before adding an alloy element may be employed.

The molten steel adjusted to a predetermined chemical composition is cast into a slab, a billet or a steel ingot by a continuous casting method or an ingot making method. From these slabs, steel ingots, etc., steel pipes, steel plates, etc. are manufactured. When manufacturing a seamless steel pipe, for example, a billet may be extruded and formed.
When a steel sheet is manufactured, a hot-rolled steel sheet can be obtained by hot rolling a slab. When manufacturing a cold-rolled steel sheet, the hot-rolled steel sheet may be further cold-rolled. The obtained steel pipe and steel plate are subjected to a heat treatment such as annealing as necessary to adjust them to predetermined characteristics. When cold working such as cold rolling is performed after hot working, annealing and pickling are usually performed prior to cold working.

[0049]

EXAMPLES The performance of the steel of the present invention will be described below by way of examples. The test methods and the results in the examples are as follows.

Tables 1 and 2 show the chemical compositions of the test materials used in this test. Table 1 shows the chemical composition of the test material of the steel of the present invention, and Table 2 shows the chemical composition of the test material of the comparative steel (including the conventional steel). In Table 2, No. 48
-51 are conventional high Cr ferritic steels (conventional steels);
No. No. 48 is JIS-STBA26, No. 48. 49 is fire S
No. TBA27 (Thermal Atomic Energy Technology Association Standard), 50 is ASTM-A213-T91, No. 50. 51 is DIN-X
This is a test material having a chemical composition specified in 20CrMoWV121.

[0051]

[Table 1]

[0052]

[Table 2]

The manufacturing method of each test material is as follows.
First, the raw material was melted by a vacuum high-frequency induction furnace having a capacity of 50 kg, and the components were adjusted to a predetermined chemical composition.
m ingot. The obtained ingot was heated to a temperature of 130.
Hot forging at 0-1000 ° C, width 200mm, length 4
A test material having a thickness of 00 mm and a thickness of 25 mm was prepared. The following heat treatment was performed on each test material. No. 48 and N
o. For the 49 test materials, these steels are usually kept at 950 ° C for 1 hour, then air-cooled normalizing, and further kept at 750 ° C for 1 hour, air-cooled tempering. gave. 1050 ° C for other test materials
After holding for one hour, air-cooled normalizing treatment and 78
After holding at 0 ° C. for 1 hour, an air-cooled tempering treatment was performed.
From these test materials, test specimens for evaluating high-temperature creep strength, steam oxidation resistance and toughness of steel were collected.

The evaluation methods for high temperature creep strength, steam oxidation resistance and toughness are as follows.

[High Temperature Creep Strength] High temperature creep strength was evaluated by a creep rupture test under the following test conditions.

[0056] [Steam Oxidation Resistance] The steam oxidation resistance was evaluated by a steam oxidation test under the following test conditions.

Test environment: steam atmosphere, temperature 700 ° C. Holding time: 1000 hours Measurement item: scale layer thickness [toughness] Toughness was evaluated by a Charpy impact test under the following test conditions.

[0058] Tables 3 and 4 show the results of these tests. Table 3 shows the test results of the test materials of the steel of the present invention, and Table 4 shows the test results of the test materials of the comparative steel (including the conventional steel).

[0059]

[Table 3]

[0060]

[Table 4]

As shown in Table 3, the test material N
o. Regarding Examples 1 to 21, the creep rupture time at 650 ° C. and 98 MPa was 10,000 hours or more, and the impact value was 1
20 J / cm ² or more, and the thickness of the scale layer by steam oxidation was 93 μm or less. From these results, it was confirmed that the steel of the present invention simultaneously satisfied all the properties of high-temperature creep strength, steam oxidation resistance, and toughness.

On the other hand, as shown in Table 4, the test material No. With respect to 22 to 51, in some of the test materials, measured values of the creep rupture time, the thickness of the steam oxidation scale layer, or the impact value were as high as those of the steel of the present invention. However, no test material satisfies all three characteristics. In particular,
Test material No. For conventional steels of 48 to 51, 65
The creep rupture time under the conditions of 0 ° C. and 98 MPa is as follows:
Less than 900 hours, the high temperature creep strength is extremely low. The thickness of the scale layer in steam oxidation is also 100 μm
In many cases, and is inferior in steam oxidation resistance.
In addition, the test material No. whose content of any one of the elements is outside the range of the chemical composition of the steel of the present invention. Regarding Nos. 22 to 47, any or all of the three properties are inferior to the steel of the present invention as described above. Test material No. of comparative steel 2
7, 34 and 37-47 and 48-51 of conventional steel
Does not satisfy the above equation (1), and the test material No. 37 and 40 and 4 of conventional steel
8, 49 and 51 do not satisfy the expression (2). In this case, although some of the characteristic values were comparable to those of the steel of the present invention, any of the high-temperature creep strength, steam oxidation resistance, and toughness were significantly inferior.

As described above, it has been demonstrated that the steel of the present invention is remarkably excellent in high-temperature creep strength, steam oxidation resistance and toughness as compared with the comparative steel or the conventional steel.

[0064]

The high Cr ferritic heat-resistant steel of the present invention is excellent in long-term creep strength at high temperatures exceeding 600 ° C., steam oxidation resistance and toughness at room temperature. Therefore, it is suitable as a material for equipment operated under high temperature and high pressure, such as boilers, nuclear power plants, and chemical industry equipment, such as a steel pipe for heat exchange or a steel plate for a pressure vessel, and a material for turbine.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-330245 (JP, A) JP-A-6-179954 (JP, A) JP-A-4-1655043 (JP, A) JP-A Sho 57- 29560 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00 302 C22C 38/32

Claims (1)

(57) [Claims] (Claim 1) C: 0.02 to 0.15%, Si: 0 to 1.0%, Mn: 0.05 to 1.5%, P: 0.030% or less, S: 0.015% or less Cr: 8.0 to 13.0%, W: 1.5 ~ 3.5%, Mo: 0 ~ 1.0%, Co: 2.5 ~ 6.0%, V: 0.10 ~ 0.50%, Nb: 0.030 ~ 0.14%, Nd : 0.001 ~ 0.17% , N: 0.020 ~ 0.08% , B: 0 ~ 0.020% , O: 0.010% or less, sol.Al:0.001~0.050%, balance: chemical composition consisting of Fe and unavoidable impurities, further satisfying the following formulas (1) and (2) High Cr ferritic heat resistant steel with excellent long-term creep strength, steam oxidation resistance and toughness at high temperatures. 0 ≦ P _Nd ≦ 0.103 (1) 0.005 ≦ P _Nb ≦ 0.03 (2) P _Nd = Nd−6.01 × O (oxygen) P _Nb = 0.151 × Nb + 0.0971 × P _Nd (Element symbols indicate the content (% by weight) of each element in steel)