JPH0885848A - High chromium ferritic heat resistant steel - Google Patents

Abstract

PURPOSE: To produce a new high-Cr ferritic heat resistant steel excellent in toughness as well as in high-temp. and long-time creep strength at a welded joint by preparing a steel having a specific composition in which respective contents of Ti, Zr, Hf, Nb, and N are specified. CONSTITUTION: The high Cr ferritic heat resistant steel, which has a composition containing, 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, 2.5-8.0% Co, 0.001-0.050% sol. Al, 0.020-0.12% N, 0-0.50% Ta, 0-1.5% Ni, 0-1.00% Mo and/or 0-0.030% B, 0-0.010% Ca and/or 0-0.010% Mg, and one or >=2 kinds among 0.005-0.08% Ti, 0.005-0.15% Zr, and 0.005-0.29% Hf so that the inequality is satisfied and having the balance Fe with inevitable impurities (<=0.030% P, <=0.015% S), 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, The present invention relates to a high Cr ferritic heat-resistant steel suitable for use as a steel plate for pressure vessels and a material for turbines, which has excellent long-term creep strength and toughness of a welded joint.

[0002]

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

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.

As a countermeasure against this, it is most effective to use a single phase of martensite. For example, the amount of Cr should be reduced as proposed in JP-A-5-263196 or JP-A-5-263196. No. 5-311342, No. 5-311343, No. 5-311344, No. 5-311345, No. 5-3113
As proposed in Japanese Patent Laid-Open No. 42 etc., it is possible to add Ni, Cu, Co and the like which are austenite forming elements. However, if the Cr content is reduced, it becomes impossible to maintain a stable corundum type dense Cr ₂ O ₃ scale, so the steam oxidation resistance decreases, and a large amount of austenite-forming elements such as Ni, Cu, and Co are added. Is a decrease in strength in the welded joint, especially softening occurs in the weld heat affected zone (hereinafter simply referred to as HAZ section), resulting in a significant decrease in long-term creep strength, as well as the Ac ₁ transformation point and
By lowering the Ac ₃ transformation point, the resistance to temper softening is lowered, not only the long-term creep strength is lowered, but also the scale structure of the oxide mainly composed of Cr ₂ O ₃ is changed, so the oxidation resistance is improved. It also has the drawback that it also deteriorates.

Steel having improved toughness at the welded joint includes:
For example, as proposed in JP-A-2-294452, Mn, Ni, for the purpose of preventing δ-ferrite formation after welding,
Steel content was limited regulations such as Cu, as proposed in JP-A-6-065689, following the oxide dispersion strengthened Ta ₂
Although there are steels in which O ₅ is dispersed to improve the softening of the HAZ part, the former steel has a decrease in steam oxidation resistance or high temperature long-term creep strength due to the addition of a large amount of Mn, Ni, etc. There is concern about the problem of reduced toughness.

As described above, the high temperature long-term creep strength under high temperature and high pressure super-supercritical pressure conditions, the toughness, and the steam oxidation resistance are excellent. Cr ferritic heat-resistant steel is not found yet.

[0011]

SUMMARY OF THE INVENTION In view of the above situation, an object of the present invention is to provide high-temperature long-term creep strength, toughness and steam oxidation resistance, and particularly excellent in high-temperature long-term creep strength and toughness in a welded joint. Another object is to provide a new high Cr ferritic heat resistant steel.

[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: 0.001-0.050%, N: 0.020-0.12%, Ta: 0
~ 0.50%, Ni: 0-1.5%, Mo: 0-1.00% and
B: contains 0 to 0.030% of one or two, Ca: 0 to 0.010% and Mg: 0 to 0.010% of one or two, and further,
Ti: 0.005-0.08%, Zr: 0.005-0.15% and Hf: 0.005-
One or more of 0.29% is contained within the range that satisfies the following formula, 0.001 ≤ (14 / 47.90) Ti + (14 / 91.22) Zr + (14 / 178.49) Hf
≤ (14 / 92.91) Nb Consists of balance Fe and unavoidable impurities.
High Cr ferritic heat-resistant steel with excellent high-temperature long-term creep strength and toughness of welded joints, characterized by S of 0.030% or less and 0.015% or less, respectively.

In the above steels, Mo and B, Ca and Mg, and Si, Ta and Ni may all be unadded.

When one or two kinds of Mo and B are contained, 0.01% or more for Mo and 0.0005% for B
Above, 0.001% or more when containing one or two kinds of Ca and Mg, 0.01% or more when containing Si, 0.01% or more when containing Ta When containing Ni, 0.10
It is desirable that each be at least%.

The present inventors have made a detailed study on how the high temperature long-term creep strength and toughness in the welded joint of high Cr ferritic heat-resistant steel correspond to the chemical composition and microstructure of the material. The present invention has been made based on the following findings.

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

The deterioration of the creep strength at high temperature and long time in the welded joint portion of the high strength and high Cr ferritic heat resistant steel having the martensitic single phase structure is closely related mainly to the softening property of the HAZ portion and the creep rupture. Indicates that the strength is significantly reduced when the HAZ / base metal interface is sheared and fractured.

The shear failure at the HAZ / base metal boundary is caused by the concentration of creep strain in the HAZ softening layer, and its prevention requires local elimination of the softening layer or dispersion. To be.

It should be noted that the formation of the HAZ softening layer is not due to the formation of the δ-ferrite phase in the cooling process after welding, which is conventionally said. That is, the tempered martensite structure was partially converted into Ac ₃ by the heat input of welding.
It becomes a state where it is again subjected to thermal history above the transformation point or below the Ac ₁ transformation point, and as a result, it is reheated to a higher temperature than the normalizing (normal) and tempering (tempering) treatments, and the tempering of the base metal is performed. Although it has a completely different structure from the martensite structure, even if it is tempered and softened again as a post-welding heat treatment (PWHT), it will change to a continuously changed structure in the HAZ part, resulting in a base metal. Nearest HA
At the so-called low temperature HAZ at the Z / base material boundary, the softening becomes the largest. In this low-temperature HAZ part, it is heated to just above the Ac ₁ transformation point by welding heat input and partially undergoes γ (austenite) transformation, but γ produced at low temperature has a small solid solution amount of C, N, etc. When it is tempered again by PWHT, it becomes a relatively soft martensitic structure with a low dislocation density, which is the cause of the HAZ softening layer.

The formation of the soft martensitic structure associated with the above partial transformation is unavoidable in high Cr ferritic heat-resistant steels, and as a result, this is the greatest cause of the decrease in long-term creep strength of welded joints.

It is extremely difficult to harden the soft martensite structure itself, but stable fine precipitates that do not re-dissolve or re-precipitate even when the welding heat input in a wide temperature range, specifically, Ti However, even if the martensite structure itself has a low dislocation density and is easily recovered and softened by heating for a long time, it is possible to delay the recovery by the presence of the precipitates, if Zr, Zr, and Hf-based nitrides are generated. ,
It should be possible to prevent deterioration of the high temperature long-term creep strength and toughness of the joint.

To control the formation of Ti, Zr, and Hf-based nitrides,
It is necessary to correlate their contents with Nb and N and strictly specify the content ratio.

[0024]

The reason for limiting the alloy components of the steel 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 time, precipitation of fine carbides such as VC, NbC and TaC also progresses, which contributes to the improvement of creep strength on the long time side. However, in order to obtain the effect of this precipitation strengthening, 0.02% or more is necessary. On the other hand, if it exceeds 0.15%, it causes coagulation and coarsening of carbides from the initial stage of use,
On the contrary, since it causes a decrease in creep strength on the long side, C
The content was 0.02 to 0.15%. Preferably 0.06-0.12
%.

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 temperatures, 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 may 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, forms a dense oxide film mainly of Cr, and corrosion resistance and oxidation resistance at high temperature of the steel of the present invention, particularly 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. During high temperature use, it is finely dispersed and precipitated in the grains as a Fe ₇ W ₆ type μ phase, and long-term creep strength. It is an element that contributes to the improvement of the strength of the steel, and also partially dissolves in the Cr carbide to suppress the agglomeration and coarsening of the carbide and greatly contribute to the maintenance of the strength. However, in order to obtain the effect, 2.5% or more is required, while on the other hand, if it exceeds 4.0%, the formation of δ-ferrite is promoted and the toughness is deteriorated. Therefore, the W content is set to 2.5 to 4.0%. 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. On the other hand, even if added in excess of 0.05%, 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 necessary, while on the other hand, if it exceeds 0.15%, a coarse nitride is formed and the toughness is decreased, so the Nb content is 0.01 to 0.15%. did. Preferably 0.
It is from 04 to 0.12%.

Co: 2.5 to 8.0% Co promotes the 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 was set to 2.5 to 8.0% because the decrease in the Ac ₁ transformation point becomes remarkable and conversely the strength decreases. Preferably
It is 3.0 to 6.0%.

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: 0.020 to 0.12% N forms nitrides and carbonitrides to form creep strength,
It is an element that contributes to the improvement of toughness. However, 0.020% or more is necessary to obtain the effect, while 0.12%
If the content exceeds N, the coarsening of the nitride will proceed, and on the contrary, the toughness will be significantly reduced. Therefore, the N content was set to 0.020 to 0.12%.
It is preferably 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,
It forms a fine nitride stable against welding heat input due to the inclusion of a very small amount, and is an extremely important element especially for improving the creep strength of the welded joint at high temperature for a long time.
One or more of Zr and Hf are selected and contained, but in order to obtain the effect, all elements are 0.005
% Or more is required. However, in the case of Ti, 0.08%, Zr
0.15% in the case of Hf, and 0.29% in the case of Hf, coarse nitrides are formed, and the toughness is rapidly deteriorated.Therefore, the upper limit of the content when adding these elements alone is , 0.08%, 0.15%, 0.29%.

Further, in the steel of the present invention, in order to improve the creep strength of the welded joint at high temperature for a long time, Ti,
The complex precipitation effect of the complex essential addition of one or more of Zr and Hf and Nb is utilized. At this time, the content of these elements is such that the content of Ti and Zr is different from that of the nitride of Nb.
In order to effectively and finely precipitate Hf nitride without coarsening, it is necessary to be within the range of the following formula.

0.001 ≤ (14 / 47.90) Ti + (14 / 91.22) Zr + (14
/178.49)Hf ≤ (14 / 92.91) Nb That is, NQ1 = (14 / 47.90) Ti + (14 / 91.22) Zr + (14/17
When 8.49) Hf and NQ2 = (14 / 92.91) Nb, both NQ1 value and NQ2 value are 0.001 or more, and (NQ2
-NQ1) Value must be greater than or equal to 0 (zero).

The above formula is a formula found by the present inventors as a result of various experimental studies, and it is clear from the results shown in FIG. 1 that the strength of the welded joint is not improved outside this range.

FIG. 1 shows the results for test steels of the examples described below, in which the creep rupture time ratio and the toughness ratio of the welded joint part to the base metal part are plotted on the vertical axis, and the (NQ2-NQ1) values are plotted on the horizontal axis. It is understood that if the (NQ2-NQ1) value is less than 0, the creep strength of the welded joint is not improved or the toughness after heating for a long time is extremely deteriorated. .

P, S: The upper limits are 0.030% and 0.
015% P and S are elements that are contained in steel as unavoidable impurities and have an adverse effect on hot workability, weld toughness, etc.
From the viewpoint of ensuring hot workability, weld toughness, etc., it is desirable to be as low as possible, but 0.030% or less and 0.015% or less do not directly affect the performance of the steel of the present invention. It was set to 0.030% and 0.015%.

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

Ta: The upper limit of 0.50% Ta has a function of forming nitrides and carbonitrides to contribute to the improvement of strength and toughness, like the above Nb. It can be contained if necessary. Since the effect is obtained at 0.01% or more, when it is contained, 0.01% or more is desirable. However, if it exceeds 0.50%, coarse nitrides are formed and the toughness is decreased, so the upper limit was made 0.50%.

Ni: upper limit 1.5% Ni has the same effect as Co as an austenite forming element, and also has the effect of strengthening the martensitic structure and improving the toughness, so that the creep strength and toughness are improved and the structure is improved. Can be added for the purpose of further stabilization.

Since the effect is obtained when the content is 0.10% or more, it is desirable to set it to 0.10% or more when it is contained. However, if it exceeds 1.5%, the Ac ₁ transformation point of the steel is remarkably lowered and the strength is lowered, so the upper limit was made 1.5%.

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 temperatures over a long period of time. , And / or B may be contained if necessary. The effect is 0.01% or more for Mo,
Since B becomes remarkable at 0.0005% or more, when it is contained, Mo is preferably 0.01% or more and B is preferably 0.0005% or more. However, when Mo exceeds 1.0% and 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%.

When B is a thick material and the cooling is slow after the heat treatment, it has a function of enhancing the hardenability and contributing to the securing of high temperature strength.

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

Ca, Mg: Both upper limits are 0.010% Ca, Mg are elements that improve the hot workability of steel, and can be contained when the purpose is to improve the hot workability. . The effect is obtained with a content of 0.0005% or more, so when Ca or / and Mg is contained,
In each case, 0.0005% or more is desirable. However, if the content exceeds 0.010% in both cases, coarsening of inclusions is caused and conversely the workability and toughness are impaired, so the upper limit was made 0.010% in all cases.

[0052]

EXAMPLES 50 kinds of steels having the chemical compositions shown in Tables 1 and 2 (No. 1 to 2 are conventional steels, No. 3 to 29 are comparative steels, No. 30
~ 50 is steel of the present invention) is melted in a 50 kg vacuum induction melting furnace
A 144 mmφ ingot was produced, and the obtained ingot was hot forged and hot rolled into a plate material with a thickness of 20 mm, and then N
For o.1 and No.2 steels, 950 ℃ x 1 hour → AC (air cooling) normalizing treatment usually applied to these steels, then 750 ℃ x 1 hour → AC tempering treatment For other steels, 1050 ℃ x 1 hour → After AC normalizing treatment,
780 ° C. × 1 hour → AC tempering treatment, various test pieces were sampled and subjected to a test, and the creep strength and toughness were investigated under each method under the following conditions.

Some test pieces collected from the plate material are
A 60 ° groove process is applied and the first layer is formed using a common metal welding material.
TIG welding is performed under the conditions of 130 A, 15 V, 10 cm / min, and two or more layers are manually welded under the conditions of 160 A, 16 V, 10 cm / min to produce a welded joint, and then 740 ℃ x 2 hours → furnace cooling After the post-weld heat treatment (PWHT) of the
The creep test piece and the impact test piece shown in FIGS. 3 and 4 were sampled and subjected to the creep strength and toughness tests of the welded joint.

In Table 1, Nos. 1 and 2 are conventional high Cr ferritic heat-resistant steels, No. 1 is JIS-STBA26 and No. 2 is ASTM.
-Steel specified in A213-T91.

[0055]

[Table 1]

[0056]

[Table 2]

[Creep rupture test (base metal and joint)] Test temperature: 650 ℃ Specimen: 6.0 mmφ × GL = 30 mm Load: 100 MPa Test item: Breaking time (target; 10,000 hours or more) [Charpy impact test (Base material and joint)] Test temperature: 0 ° C Test piece: 10mm width x 10mm thickness x 55mm length -2mm V notch Test item: Impact value (target: vEo ≥ 50 J / cm ² ) Table 3 and Table 4 The results of these tests are shown.

[0058]

[Table 3]

[0059]

[Table 4]

As shown in Tables 3 and 4, the conventional steels of Nos. 1 and 2 all had a fracture time of less than 1000 hours in the creep rupture test at 650 ° C. and 100 MPa even in the base metal.
Insufficient high temperature creep characteristics at 650 ℃ or higher.

The comparative steels No. 3 to 17 and No. 29 are
Both steels show the results when some components are out of the range of the present invention, but some of the base metals have excellent high temperature creep properties, and the creep properties or toughness of the welded joints are significantly deteriorated. , None satisfy all characteristics. Further, the comparative steels of No. 18 to 28, each component is within the scope of the present invention, but since the contents of Ti, Zr, Hf and Nb do not satisfy the formula of the present invention, It is clear from FIG. 1 that although the characteristics are good, the creep characteristics or toughness of the welded joint are poor.

On the other hand, in the steels of the present invention Nos. 30 to 50, these characteristics were satisfied at the same time, and the high temperature long-time creep characteristics unprecedented in the welded joint were obtained, and the toughness was also excellent. It can be seen that an epoch-making high Cr ferritic heat resistant steel has been obtained.

[0063]

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 high temperature long-term creep properties and toughness of a welded joint used as a steel pipe, a steel plate for a pressure vessel, and a material for a turbine can be obtained. Therefore, the benefits of the present invention to the field are extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of Ti, Zr, and Hf contents on the creep strength and toughness of a welded joint.

FIG. 2 is a diagram showing sampling positions of various test pieces from a welded joint.

FIG. 3 is a view showing a Charpy impact test piece.

FIG. 4 is a diagram showing a creep test piece.

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: 0.001-0.050%, N: 0.020-0.12%, Ta: 0
~ 0.50%, Ni: 0-1.5%, Mo: 0-1.00% and
B: contains 0 to 0.030% of one or two, Ca: 0 to 0.010% and Mg: 0 to 0.010% of one or two, and further,
Ti: 0.005-0.08%, Zr: 0.005-0.15% and Hf: 0.005-
One or more of 0.29% is contained within the range satisfying the following formula, 0.001 ≤ (14 / 47.90) Ti + (14 / 91.22) Zr + (14 / 178.49) Hf
≤ (14 / 92.91) Nb Consists of balance Fe and unavoidable impurities.
High Cr ferritic heat-resistant steel with excellent high-temperature long-term creep strength and toughness of welded joints, characterized by S of 0.030% or less and 0.015% or less, respectively.