JP2000026940A - HIGH Cr FERRITIC HEAT RESISTANT STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow steel to withstand use under high temp. steam by allowing it to have a specified compsn. contg. C, Mn, P, S, Cr, W, Co, V, Ta, Nb, Nd, N, B, A1, M, Mo, Si, Ca, La, Ce, Y, Hf, and the balance Fe with inevitable impurities. SOLUTION: This steel contains, by mass, 0.02 to 0.15% C, 0.05 to 1.5% Mn, <=0.03% P, <=0.015% S, 8 to 13% Cr, 1.5 to 4% W, 2 to 6% Co, 0.1 to 0.5% V, 0.01 to 0.15% Ta, 0.01 to 0.15% Nb, 0.001 to 0.2% Nd, <0.02% N, 0.0005 to 0.02% B, 0.001 to 0.05% Al, 0 to 1% Mo, 0 to 1% Si, 0 to 0.02% Ca, 0 to 0.2% La, 0 to 0.2% Ce, 0 to 0.2% Y and 0 to 0.2% Hf. In this way, it can withstand use under high temp. steam. In the case great importance is attached to toughness, the contents of Nd and N are allowed to satisfy Nd(%)<=5×N(%)+0.10(%).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high Cr ferritic heat-resistant steel, 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. C with excellent high-temperature long-time creep strength and toughness suitable for steel sheets and turbine materials
Related to r-ferritic heat-resistant steel.

[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, toughness, 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. Therefore, 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 the above severe conditions, 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.

JP-A-8-85850, JP-A-9-71
JP-A-845 and JP-A-9-71846 disclose heat-resistant steels to which Nd is added as steel for super-supercritical pressure conditions from the viewpoint of improving the high-temperature long-term creep characteristics of a welded joint.

[0007] However, Nd is an element having a strong affinity for N, and a part of Nd remains as coarse NdN inclusions, so that in high N steels, the effect of improving the creep strength of Nd cannot always be fully exhibited. There was a problem.

In order to use a high Cr ferritic steel under ultra-supercritical pressure conditions such as in a thermal power boiler or the like, it is necessary to further improve the creep strength. It is important to delay the recovery softening phenomenon of the tissue as long as possible at a high temperature for a long time.

[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 and toughness that can withstand use under high-temperature steam of 5 ° C or more.

[0010]

The gist of the present invention relating to a high Cr ferritic heat resistant steel is as follows.

(1) In mass%, C: 0.02 to 0.15%, Mn: 0.05 to 1.5%, P: 0.03% or less, S: 0.015% or less, Cr: 8 to 13%, W: 1.5 to 4%, Co: 2 to 6%, V: 0.1 to 0.5%, Ta: 0.01 to 0.15%, Nb: 0.01 to 0 .15%, Nd: 0.001 to 0.2%, N: less than 0.02%, B: 0.0005 to 0.02%, Al: 0.001 to 0.05%, Mo: 0 to 1 %, Si: 0 to 1%, Ca: 0 to 0.02%, La: 0 to 0.2%, Ce: 0 to 0.2%, Y: 0 to 0.2%, Hf: 0 to 0% High Cr ferritic heat-resistant steel containing 0.2% and the balance being Fe and unavoidable impurities and having excellent high-temperature long-term creep strength and toughness.

(2) The high Cr ferritic heat resistant steel according to (1), which satisfies the following expression.

Nd (%) ≦ 5 × N (%) + 0.1 The present inventors studied in detail the effect of N on the high-temperature long-time creep characteristics and toughness of a high Cr ferritic heat-resistant steel containing Nd. did. As a result, the following findings were obtained and the present invention was completed.

A) Nd fixes oxygen in steel as Nd oxide and suppresses a part of Nb or V as a precipitation strengthening element that precipitates fine carbides contributing to creep strength from becoming an oxide. Has the effect of doing In addition, Nd has an action of generating carbides such as NdC ₂ , and these carbides are finely and stably precipitated to a high temperature and a long time side, thereby contributing to an improvement in a high temperature and a long time creep strength. However, it has a high affinity for N (nitrogen), and coarse Nd in steel containing N.
Since N acts as an inclusion, the effect of suppressing the formation of oxides of Nb and V and the effect of precipitating precipitation by depositing fine carbides such as NdC ₂ become insufficient, and the effect of improving creep strength cannot be sufficiently exhibited. .

B) In a high Cr ferritic heat-resistant steel containing Nd, the formation of coarse NdN can be prevented by suppressing the N content in the steel to less than 0.02%. And carbides such as NdC ₂ precipitate stably up to high temperature and long time, and as a result, the recovery softening phenomenon of the martensitic structure is suppressed up to high temperature and long time, and the creep strength is greatly reduced. improves.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of the heat-resistant steel of the present invention will be described below (hereinafter,% indicates% by mass).

C: 0.02 to 0.15% C is carbide MC (M is an alloying element), M ₇ C ₃ and M ₂₃ C
Form a _6- type carbide [may be formed as carbonitride M (C, N)]. This carbide contributes to the improvement of the creep strength, and C itself stabilizes the structure as an austenite stabilizing element. If it is less than 0.02%, the precipitation of carbides is insufficient, and the amount of δ ferrite increases, so that sufficient creep strength and toughness cannot be obtained. However, if it is contained in a large amount exceeding 0.15%, the carbides are coarsened and coarsened from the beginning of use, so that the creep strength is reduced for a long time and the workability and weldability are also deteriorated. 0.15%.

Mn: 0.05-1.5% Mn is effective as an element for deoxidizing and fixing S, and also contributes as an austenite stabilizing element. To obtain those effects, 0.05% or more is required, but 1.5% or more is required.
%, The toughness is deteriorated.
%.

P: 0.003% or less, S: 0.015%
The impurities P and S are preferably lower from the viewpoints 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 an indispensable element for ensuring the corrosion resistance and oxidation resistance at high temperatures of the steel of the present invention, particularly the 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 contained in a large amount, the formation of δ-ferrite is promoted and the toughness is deteriorated, so the upper limit was made 13%.

W: 1.5-4 W is one of the main strengthening elements of the steel of the present invention. W is finely dispersed and precipitated as an intermetallic compound such as a Fe ₇ W ₆ type μ phase or an Fe ₂ W type Laves phase during use at a high temperature, and contributes to improvement in long-time creep strength. Furthermore, it also partially dissolves in the Cr carbide to suppress aggregation and coarsening of the carbide and contribute to maintenance of strength. However, if contained in a large amount, the formation of δ-ferrite is promoted.
% To 4%.

Co: 2 to 6% Co is an austenite stabilizing element and is an essential element in the steel of the present invention to which W is positively added. Co
Unlike Ni, which is the same austenitic stabilizing element, has the effect of improving the creep strength without lowering the creep strength. In order to exert these effects, the addition of 2% or more is necessary. However, if the addition exceeds 6%, the Ac ₁ transformation point of the steel decreases significantly,
Conversely, the creep strength decreases.

V: 0.1-0.5% V is an important element in the steel of the present invention and forms fine carbonitrides, which contributes to improvement in creep strength. In order to exert the effect, it is necessary to be 0.1% or more,
If the content exceeds 0.5%, the effect is saturated, so the content is set to 0.1% to 0.5%.

Ta, Nb: 0.01 to 0.15% Ta and Nb form fine carbonitrides like V,
It is an element that contributes to improvement in creep strength. In order to exhibit the effect, 0.01% or more is required for each. However, the effect is saturated even if the content exceeds 0.15%, so the content is set to 0.01% to 0.15%.

Nd: 0.001 to 0.2% Nd contributes greatly to suppressing the recovery and softening of the martensitic structure, since carbides such as NdC ₂ precipitate finely and stably even at high temperatures for a long time, and reduces the creep strength. Greatly improve. In order to exhibit the effect, it is necessary to contain 0.001% or more. However, if the content exceeds 0.2%, the toughness is deteriorated. Therefore, the content is set to 0.001% to 0.2%.

N: less than 0.02% N is effective as an austenite stabilizing element like C. However, in a steel containing Nd, as the N content increases, coarse NdN remains in the steel as inclusions. Therefore, the effect of improving the creep strength is not sufficiently exhibited, and the toughness is deteriorated. Therefore, in order to sufficiently exert the effect of Nd, the upper limit of the amount of N in steel needs to be less than 0.02%. When the toughness is particularly important, Nd
It is desirable to adjust the balance between N and the amount of N within a range satisfying the following expression.

Nd (%) ≦ 5 × N (%) + 0.10 (%) B: 0.0005-0.02% B is finely dispersed and precipitates M ₂₃ C ₆ type carbide when added in a small amount. And contributes to the improvement of high-temperature long-time creep characteristics. When the cooling after the heat treatment is slow in a thick material or the like, the quenching property is enhanced, and also plays an important role in ensuring high-temperature strength. The effect is remarkable at 0.0005% or more. However, when the content exceeds 0.02%, coarse precipitates are formed and the toughness is deteriorated. Therefore, the B content was set to 0.0005 to 0.02%.

Al: 0.001 to 0.05% Al is required to be at least 0.001% as a deoxidizing agent for molten steel. On the other hand, if it is contained in a large amount exceeding 0.05%, the creep strength is lowered, so that the content was made 0.001 to 0.05%.

Si: 0 to 1% Si is used as a deoxidizing agent for molten steel if necessary. Si
Is effective for improving steam oxidation resistance at high temperatures, but if contained in a large amount exceeding 1%, toughness is deteriorated. In particular, when importance is attached to steam oxidation resistance, the lower limit of the amount of Si is 0.10.
% Is desirable.

Mo: 0 to 1% Mo is an element to be contained as necessary, and contributes to the improvement of creep strength as a solid solution strengthening element. However, if it exceeds 1%, intermetallic compounds such as Laves phase precipitate. I do.
In an Mo-containing steel, such an intermetallic compound precipitates extremely coarsely, so that it does not contribute to improvement in creep strength, and also reduces toughness after aging. Therefore, the content of Mo is set to 0 to 1%.

Ca, La, Ce, Y, Hf: Ca is 0 to
0.02%, others are 0 to 0.2% Ca, La, Ce,
One or more of Y and Hf are contained as necessary. These elements strengthen the crystal grain boundaries even with a very small content, improve the creep strength, and contribute to the improvement of hot workability. However, since the hot workability deteriorates when added excessively, the upper limits of these elements are set to 0.02% for Ca and 0.2% for La, Ce, Y and Hf.

[0032]

EXAMPLES In a vacuum induction melting furnace, a 50 kg ingot having a chemical composition shown in Tables 1 and 2 and having a diameter of 144 mm was produced. Symbols 1 to 24 are steels of the present invention, and symbols AL are comparative steels.

[0033]

[Table 1]

[0034]

[Table 2]

These ingots were hot-forged and then hot-rolled to form steel plates having a thickness of 20 mm. Then, 1050 ° C
, And air-cooled (AC).
Tempering treatment was performed in which air was cooled (AC) while maintaining the time.
A creep rupture test piece and a Charpy impact test piece were prepared from these steel sheets, and a creep rupture test and a Charpy impact test were performed under the following conditions.

(1) Creep rupture test Specimen: Diameter 6.0 mm Distance between gauge marks 30 mm Holding temperature: 650 ° C. Load stress: 98 MPa (2) Charpy impact test Specimen: 10 mm × 10 mm × 55 mm 2 mm V notch Test temperature: 0 ° C Creep rupture time and Charpy impact value (J / mm ² ) measured in these tests are shown in Tables 3 and 4.

[0037]

[Table 3]

[0038]

[Table 4]

The creep rupture times of the symbols 1, 2, 3 and 4, 5, and 6 of the steels of the present invention having an N content of less than 0.02% are defined by the N content of the comparative steels according to the present invention. It can be seen that it is clearly improved as compared with the symbol A steel and the symbol B steel which exceed the range shown in FIG. The effect of improving the creep rupture life by reducing N is also evident in the comparison between the steels 7 to 17 of the steel of the present invention and the comparative steels C to M.

The symbols 7, 18, 20, 2 of the steel of the present invention
In the case of the steel No. 2, 24, the contents of Nd and N do not satisfy the following formula. In this case, the symbol 1 of the present invention that satisfies the following equation:
It can be seen that there is almost no difference in the creep rupture life as compared with the 7, 19, 21 and 23 steels, but the impact value is lower.
Therefore, when importance is placed on toughness, it is desirable to adjust the Nd and N contents within a range satisfying the following expression.

Nd (%) ≦ 5 × N (%) + 0.10 (%)

[0042]

The high Cr ferritic heat resistant steel of the present invention has 6
Excellent in high-temperature long-term creep strength at high temperatures of 25 ° C or higher and toughness at room temperature, and excellent for use in steel tubes for heat exchange, steel plates for pressure vessels, and turbine materials used in fields such as nuclear power generation and the chemical industry. It is extremely useful in industry.

Claims (2)

[Claims] 1. Mass%: C: 0.02 to 0.15%, Mn: 0.05 to 1.5%, P: 0.03% or less, S: 0.015% or less, Cr: 8 -13%, W: 1.5-4%, Co: 2-6%, V: 0.1-0.5%, Ta: 0.01-0.15%, Nb: 0.01-0. 15%, Nd: 0.001 to 0.2%, N: less than 0.02%, B: 0.0005 to 0.02%, Al: 0.001 to 0.05%, Mo: 0 to 1% , Si: 0 to 1%, Ca: 0 to 0.02%, La: 0 to 0.2%, Ce: 0 to 0.2%, Y: 0 to 0.2%, Hf: 0 to 0. A high Cr ferritic heat-resistant steel which is excellent in high-temperature long-time creep strength and toughness, containing 2% and the balance of Fe and inevitable impurities. 2. The high Cr ferritic heat resistant steel according to claim 1, which satisfies the following expression, wherein the high Cr long term creep strength and the toughness are excellent. Nd (%) ≦ 5 × N (%) + 0.1