JPH06256893A - High toughness low alloy steel excellent in high temperature strength - Google Patents

Abstract

PURPOSE:To obtain low allay steel for the rotor of a gas turbine, disk or the like having the maximum using temp. and strength by specifying the compsn. of Ni-Cr-Mo-V steel. CONSTITUTION:This low alloy steel contains, by weight, 0.15 to 0.35% C, <=0.15% Si, >=1.2% Mn, 0.5 to 2.0% Ni, 2.0 to 2.5% Cr, 0.5 to 1.5% Mo, 0.5 to 1.0% W and 0.2 to 0.45% V and furthermore contains one or more kinds among <=0.1% Nb, <=0.1% Ta and <=0.05% N, and the balance Fe. This low alloy steel has the maximum using temp. of 400 to 480 deg.C higher than that of the conventional 3.5Ni-Cr-Mo-V steel by 100 to 180 deg.C and has the strength level equal to that of the conventional Cr-Mo-steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a turbine, particularly a low alloy steel for a rotor material or a disk material of a gas turbine, and more specifically, a gas turbine rotor or a disk excellent in creep rupture strength and toughness at high temperature. Low alloy steel for forming a.

[0002]

2. Description of the Related Art Conventionally, 3.5 Ni-Cr-Mo-V steel (low alloy steel) represented by Table 1 and Table 2 are representatively used as materials for discs of gas turbines and jet engines. Forged products such as Cr-Mo-V steel (low alloy steel), 12% Cr-based stainless steel represented by Table 3 and Fe-based heat resistant alloy represented by Table 4 have been mainly used. It was

[0003]

[Table 1]

[Table 2]

[Table 3]

[Table 4] 3.5 Ni-Cr- containing several% of Ni as typified in Table 1
Mo-V steel (low alloy steel) disc material has a relatively high strength [0.2% proof stress (hereinafter simply referred to as "proof stress") 70 to 120 kg /
mm ² ] and high toughness (V notch at 25 ° C, Charpy impact absorption energy 5 to 10 Kg · m or more), melting, forging, heat treatment are relatively easy, cost is low, and easy to obtain. is there. However, when the operating temperature (which means the metal temperature of the disk) rises above 300-350 ° C,
This disk material enters the creep region and becomes complicated due to the need to consider creep in designing the material strength, and the strength such as tensile strength and proof strength decreases over time (called "softening phenomenon"), Further, when used in the temperature range of 350 to 500 ° C. for several hundreds to tens of thousands of hours, the toughness is remarkably reduced due to temper brittleness. These phenomena are destined to be found in a disk material which is mainly made of a low alloy steel containing several% or more of Ni and which has been tempered and tempered to improve its strength and toughness.

Further, Cr-Mo-V represented by Table 2 is used.
Steel (low alloy steel) is low in cost and easy to obtain, similar to the above-mentioned 3.5 Ni-Cr-Mo-V steel.
-No remarkable softening phenomenon and temper brittleness like V steel,
Moreover, since it does not enter the creep region until the operating temperature reaches 430 to 480 ° C, the operating temperature is higher than that of 3.5 Ni-Cr-Mo-V steel.
It can be raised by 100-200 ° C. However, its toughness is not as excellent as that of 3.5 Ni-Cr-Mo-V steel, and especially when trying to increase tensile strength or proof stress, the toughness is significantly reduced (for example, proof stress of 70-80 kg / mm ² or more). If the temperature is adjusted to a high strength level, the V-notch Charpy impact energy at 25 ° C will be 1 to 2 Kg · m or less). Moreover, even at a temperature of about 400 ° C, it will enter the creep region and the notch weakening will occur in this creep temperature region ( Notch creep rupture strength is weaker than smooth creep rupture strength). Therefore, when this Cr-Mo-V steel is used as a disk material, it is not possible to perform tempering (quenching, tempering) to increase the strength so much that it is generally more difficult than 3.5 Ni-Cr-Mo-V steel. The strength level is kept low, and the proof stress at room temperature is usually 60 to 70 kg / mm ² or less.

Further, the 12% Cr type stainless steel represented by Table 3 contains a large amount of Cr, so that
Compared with both low alloy steels such as 3.5 Ni-Cr-Mo-V steel and Cr-Mo-V steel, it has excellent corrosion resistance and oxidation resistance, but not only contains a large amount of Cr, but also for the purpose of improving high temperature strength. Nb, W, Mo,
Since several% of expensive alloy elements such as V are contained, the cost is 2-3 times higher than those of the above low alloy steels, and despite the large number of such alloy elements, strength at high temperature and room temperature The toughness in No. 1 exhibits the same behavior as the above Cr-Mo-V steel, and a remarkable improvement cannot be expected.

Further, the Fe-base heat-resistant alloys represented by Table 4 are excellent in corrosion resistance and oxidation resistance because they contain a large amount of Ni and Cr and enter the creep region at a temperature of 500 to 580 ° C or higher. Therefore, high temperature strength can be obtained, and since a uniform austenite structure does not cause brittle fracture, it is not necessary to consider toughness, and strength can be increased by solutionizing and aging treatment.
It is brought about by the precipitation hardening of the intermetallic compound of the phase [Ni ₃ (Al ・ Ti)], so it can be tempered (quenched and tempered to give a tempered martensite structure like the above-mentioned low alloy steel and 12% Cr stainless steel. Or it becomes a bainite structure). However, Ni, Cr,
It contains a large amount of expensive alloy elements such as Mo, and
Since it contains highly active alloying elements such as Al and Ti, ordinary melting methods cannot be applied, and more advanced melting technology such as vacuum high frequency melting or vacuum arc melting is required, so the cost is considerably high. 5-10 of the cost of low alloy steel
Double. Also, as turbines have recently become larger in capacity, the required turbine disk weight has also increased to 5 to 6 tons or more, and at the present time it is possible to manufacture such large forged products for Fe-based heat-resistant alloy disks. Is quite difficult.

On the other hand, in recent years, gas turbines have come to have higher efficiency and larger capacity, and in the former case, it is necessary to raise the turbine inlet gas temperature in order to improve the thermal efficiency, and therefore the disk temperature also rises. , 3.5 Ni-Cr above
-It is difficult to suppress the maximum operating temperature of Mo-V steel to 300 to 350 ° C or lower. In the latter case, the size of the disk required for design becomes large, and the unit weight of the forged disk product is 5 to 6 Problems such as more than tons have come up.

[0008]

In view of the above points, the present invention can be easily obtained at a cost equivalent to that of the low alloy steels such as 3.5 Ni-Cr-Mo-V steel and Cr-Mo-V steel described above. It is possible, and the maximum operating temperature is 400 to 480 ℃ and the above-mentioned 3.5 Ni-Cr-Mo-V.
It is an object of the present invention to provide a novel low alloy steel for discs which can be heated to 100 to 180 ° C. higher than that of steel and has a strength level close to that of the above Cr-Mo-V steel. If the metal temperature of the disk can be raised to 400 to 480 ℃, the thermal efficiency can be greatly improved, and low alloy steel can be easily manufactured even for large forged products of 5 to 6 tons or more. It is possible to solve the above-mentioned recent problem that the cost does not increase.

[0009]

The present invention is based on weight,
Carbon 0.15-0.35%, silicon 0.15% or less, manganese 1.2%
Contains 0.5-2.0% nickel, 2.0-2.5% chromium, 0.5-1.5% molybdenum, 0.5-1.0% tungsten, 0.2-0.45% vanadium, 0.1% niobium, 0.1% tantalum and 0.05% nitrogen. The present invention relates to a high-toughness low-alloy steel excellent in high-temperature strength, which is characterized by containing an alloy composition containing at least one selected from the following, with the balance being iron and inevitable impurity elements.

The above alloy composition in the present invention is based on 2.25 Cr-Mo steel, which has been widely used in boilers and chemical machinery, and is conventionally used in high pressure rotor materials for steam turbines or disk materials for gas turbines. Cr-Mo-
It was found based on the results of various tests with reference to the alloy composition of V steel. In particular, in the low alloy steel of the present invention, V is 0.2 to 0.45%, preferably 0.15 to 0.25% to make the crystal grains finer, thereby enhancing the strength and preventing the deterioration of the toughness, and Ni. 0.5 to 0.5
2.0%, the amount of Cr added is 2.0 to 2.5% (that is, the conventional C
(about twice as much as r-Mo-V steel) improves the bainite hardenability during refining, thereby improving the room temperature tensile properties and high temperature creep properties.

The reasons for limiting the alloy composition and the content of the low alloy steel of the present invention will be described below. Carbon (C): C is an essential element for increasing the hardenability and ensuring the yield strength and toughness, and in order to develop the required yield strength and toughness for the rotor material and disk material of the gas turbine. 0.15% or more is required, but if added in a too large amount, the toughness is rather adversely affected and the workability is deteriorated, so the content was made 0.15 to 0.35%. Silicon (Si): Si is an element effective as a deoxidizer for molten steel. However, when a large amount is added, the deoxidation product SiO ₂ remains in the steel and impairs the cleanliness of the steel, reduces toughness and creep rupture elongation (ductility), and further The content is set to 0.15% or less because it promotes temper brittleness. In recent years, vacuum carbon deoxidizing method and electroslag remelting method have been applied, and
The need for deoxidation is no longer needed and the Si content can be reduced. Manganese (Mn): Mn is an element effective as a deoxidizing agent and a desulfurizing agent for molten steel, and is an element effective for increasing hardenability and strength. However, if too much is added, the toughness and ductility are impaired, so the maximum content is 1.2%. Nickel (Ni): Ni is an element effective in increasing the hardenability of steel and increasing the strength and toughness at room temperature, and is particularly effective in improving the toughness. In addition, these effects are
When the contents of both Cr elements are high, the synergistic effect remarkably increases. However, if added in a too large amount, Ni impairs high temperature strength (creep strength, creep rupture strength) and promotes temper embrittlement, so the content was made 0.5 to 2.0%. Chromium (Cr): Cr is the most important element as an additive element of ordinary low alloy steel. When added to steel, Cr improves corrosion and oxidation resistance, increases hardenability, and improves tensile properties at room temperature. In addition, these effects are
When the content of both elements is large, it increases remarkably due to the synergistic effect. Further, Cr is an element effective for improving high temperature strength such as creep strength and creep rupture strength. However, 2.5%
If it exceeds the above range, the toughness is improved, but it is difficult to make a large improvement in the high temperature strength, and it is not necessary to add a large amount. Further, since the hardenability (bainite hardenability) is improved as described above, the content thereof is set to 2.0 to 2.5% in consideration of the mass effect of the disc material. Molybdenum (Mo): Mo, like Cr, is an important element as an additive element of ordinary low alloy steel. When added to steel, Mo
Is effective in increasing the strength (tensile strength, proof stress) at room temperature by increasing the hardenability and increasing the temper softening resistance during tempering. Further, Mo is a very effective element for improving high temperature strength such as creep strength and creep rupture strength as a solid solution strengthening element and as a precipitation strengthening effect element through the formation of carbides. Furthermore, when Mo is added in an amount of about 0.5% or more, it is a very effective element because it prevents temper embrittlement of steel. However, even if added in a too large amount, the effect is saturated, rather the toughness is impaired, and the cost is high because it is an expensive element. Therefore, the content
It was set to 0.5 to 1.5%. Tungsten (W): W is a solid solution strengthening element and is a very effective element for improving high temperature strength such as creep strength and creep rupture strength. However, if added in too large an amount, a phenomenon such as solidification segregation, which is not preferable as a large-sized casting product, occurs, so the content was made 0.25 to 1.0%. Vanadium (V): V is an element effective in improving strength (tensile strength, proof stress) at room temperature, similar to Mo. It is an important element that increases the high temperature strength such as creep strength and creep rupture strength as a solid solution strengthening element and as a precipitation hardening action element through the formation of carbides. Further, V is effective for improving the toughness by making the crystal grains finer if the addition amount of V is within a certain range (0.2 to 0.45%). However, if added too much, the toughness is impaired, and the cost is high because it is an expensive element, so its content should be 0.2-0.45%.
And Niobium (Nb): Nb is an element that is effective for increasing room temperature strength such as tensile strength and proof stress and high temperature strength such as creep strength and creep rupture strength as well as V, and at the same time refines the crystal grains. It is a very effective element for improving toughness, but if the content is less than 0.01%, its effect is not sufficient. In the low alloy steel of the present invention, addition of Nb does not expect much increase in strength, but rather expects improvement in toughness due to grain refinement, and if too much is added, a large amount of Nb carbonitride is formed. However, it is rather ineffective because it impairs toughness. Therefore, the content is set to 0.1% or less. Tantalum (Ta): Since it has almost the same effect as Nb, T
The content of a was set to 0.1% or less. Nitrogen (N): N is an element effective for increasing the tensile strength and creep rupture strength as a precipitation hardening action element through the formation of nitrides, and if the content is less than 0.01%, the effect obtained is On the other hand, on the other hand, if added in excess of 0.05%, the toughness deteriorates, so the content was made 0.05% or less. However, regarding Nb, Ta and N, any one or more of them is to be contained. Others: P, S, Cu, etc. are unavoidable as impurity elements mixed in by the raw materials of steel making, but it is desirable that these are as low as possible (however, if the raw materials are carefully selected, the cost will be high) Generally, P 0.015% Below, S 0.0
10% or less and Cu 0.50% or less are desirable. Other impurity elements include Al, Sn, Sb, Pb and As.

[0012]

[Examples 1-6] Low alloy steels (alloys A to F) of the present invention having alloy compositions shown in Table 5 were melted in a laboratory-scale vacuum melting furnace to produce a 50 Kg steel ingot.

[Table 5] Small forgings were manufactured from these steel ingots by heating and forging (upset 1 / 2.8U, forging of 3.7S) assuming a real gas turbine disk. Then, the forged material is subjected to a preliminary heat treatment (for example, 10
It was subjected to 50 ° C. air cooling and 650 ° C. air cooling). The forged material was heat-treated by simulating the quenching and cooling rates of the central portion and the surface portion, assuming a disc material having a diameter of 1500 mm and a thickness of 200 mm. That is, after heating at 950 ° C. to completely austenite, the quenching cooling rate of the disk center portion (average of 950 to 300 ° C.) is about 5 ° C./min and the quenching cooling rate of the disk surface layer portion (950 to 300 ° C.). The average strength is about 20 ° C / min, and the tempering is performed at 660 ° C respectively to obtain the strength required for gas turbine disk material design, that is, 0.2% proof stress at room temperature is ~ 80Kg / mm. ^The test materials A to F were adjusted to have a value of 2. In addition to these low alloy steels of the present invention, the current Cr-Mo-V steel is melted in the same manner as described above (alloy G: 1Cr-1Mo-1 / 4V).
Steel) and a similar treatment to prepare a gas turbine disk material (comparative material G), and the properties of both materials were compared.

Table 6 shows the results of the tensile test and the impact test performed on the test materials A to F and the comparative material G. Also, the creep rupture strength of the test materials A to F and the comparative material G
Organize by mirror parameter, and the result is Creep Data Sheet No. 9A (data of conventional high pressure rotor material Cr-Mo-V steel) is shown in FIG. That is, FIG. 1 shows the Larson-Miller parameter T (20 + log t) × 10 ⁻³ [where T is the creep test temperature (° K) and t is the creep rupture time (hr)] and the stress (Kg / mm ² ) and the sign of the data is as follows. Specimen A: ○ surface layer, ● central portion Specimen material B: ◇ surface portion, ◆ central portion Specimen material C: □ surface portion, ■ central portion Specimen material D: △ surface portion, ▲ central portion Material E: ▽ surface part, ▼ central part Specimen material F: ☆ surface part, ★ central part Comparative material G: # surface part, * central part

[Table 6] Table 6 As is apparent, yield strength at room temperature none of the test pieces and the comparative material has a 80 Kg ^f / mm ² or more intensity levels, and has a sufficient strength as the gas turbine disc material. Further, in the creep rupture test performed using the smooth-notch combination test piece shown in FIG. 2, it is smooth and ruptured under any of the test conditions, and the notch strengthening is good. Moreover, the creep rupture strengths of the steel test materials A to F according to the present invention exceed the upper limits of the comparative material G and the conventional high pressure rotor material Cr-Mo-V steel data band. This is 480 ℃ × 10 ⁵ h
Compared with the creep rupture strength, the steel according to the present invention is σ = 36Kg / mm ^2, the conventional Cr-Mo-V steel as compared with σ = 30Kg / mm ^2, and the conventional Cr-Mo-V steel High temperature strength is improved. The elongation and drawing also satisfied the conditions of 16% or more for elongation and 45% or more for drawing, which are required for general disk materials.

On the other hand, regarding the impact characteristics, the target value of 50% FATT of the gas turbine disk material is + 80 ° C or less,
In all cases, the test materials A to F relating to the steel of the present invention are below the target value, and it is understood that they have sufficient toughness. On the other hand, the comparative material G which is the current 1Cr-1Mo-1 / 4V steel
Has a high FATT at the center of 110 ° C, and there is a problem in the toughness as a gas turbine disk material in terms of reliability, and improvement is needed. On the other hand, in the FATT of the central portion of the steel of the present invention, the test material A is + 72 ° C, and the test material B + 68
℃, test material C + 66 ℃, test material D + 45 ℃, test material E + 48
℃, test material F + 47 ℃, lower than the target value, toughness has been greatly improved.

As is clear from the above, the yield strength at room temperature is 80 kg / mm ² , excellent at room temperature strength and improved toughness, and the creep rupture strength is about 1.2 times higher than that of conventional high pressure rotor materials. It has excellent strength level properties.

An allowable stress diagram estimated from the above results is shown in FIG. In the figure, curve 1 shows the yield strength / 1.5,
Curve 2 shows the tensile strength / 2.5 and curve 3 shows the 10 ⁵ hr creep rupture strength. As is clear from FIG. 3, the allowable stress can be set without entering the creep region up to about 480 ° C., and the allowable stress can be set to 10 Kg / mm ² higher than that of the conventional Cr-Mo-V steel.

Further, the range of C + V content and Nb + N content of the steel of the present invention estimated from the above results is shown in FIG.
Shown in. That is, it is as follows. 0.5 ≤ C + V content (%) ≤ 0.7 0.03 ≤ Nb + N content (%) ≤ 0.07

[0018]

As described above, the present invention is characterized by room temperature strength,
It is possible to obtain a rotor material and a disk material which are excellent in high temperature strength and toughness, have higher reliability than conventional ones, and are suitable for a larger gas turbine disk. The low alloy steel according to the invention is also used as a steam turbine rotor material in some applications.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the Larson-Miller parameter of creep rupture strength of a disk material of the present invention and stress.

FIG. 2 is a cross-sectional view of a creep rupture test piece (smooth-notch combination type) used for Larson-Miller parameter measurement.

FIG. 3 is a graph showing an estimated allowable stress diagram.

FIG. 4 is a diagram showing the ranges of C + V content and Nb + N content according to the steel of the present invention.

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[Procedure amendment]

[Submission date] May 24, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction content]

[0009]

The present invention is based on weight,
Carbon 0.15-0.35%, silicon 0.15% or less, manganese 1.2%
It contains nickel 0.5-2.0%, chromium 2.0-2.5%, molybdenum 0.5-1.5%, tungsten 0-1.0%, vanadium 0.2-0.45%, and also niobium 0.1% or less, tantalum 0.1% or less and nitrogen 0.05%. The present invention relates to a high-toughness low-alloy steel excellent in high-temperature strength, which is characterized by containing an alloy composition containing at least one selected from the following, with the balance being iron and inevitable impurity elements.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction content]

The reasons for limiting the alloy composition and the content of the low alloy steel of the present invention will be described below. Carbon (C): C is an essential element for increasing the hardenability and ensuring the yield strength and toughness, and in order to develop the required yield strength and toughness for the rotor material and disk material of the gas turbine. 0.15% or more is required, but if added in a too large amount, the toughness is rather adversely affected and the workability is deteriorated, so the content was made 0.15 to 0.35%. Silicon (Si): Si is an element effective as a deoxidizer for molten steel. However, when a large amount is added, the deoxidation product SiO ₂ remains in the steel and impairs the cleanliness of the steel, reduces toughness and creep rupture elongation (ductility), and further The content is set to 0.15% or less because it promotes temper brittleness. In recent years, vacuum carbon deoxidizing method and electroslag remelting method have been applied, and
The need for deoxidation is no longer needed and the Si content can be reduced. Manganese (Mn): Mn is an element effective as a deoxidizing agent and a desulfurizing agent for molten steel, and is an element effective for increasing hardenability and strength. However, if too much is added, the toughness and ductility are impaired, so the maximum content is 1.2%. Nickel (Ni): Ni is an element effective in increasing the hardenability of steel and increasing the strength and toughness at room temperature, and is particularly effective in improving the toughness. In addition, these effects are
When the contents of both Cr elements are high, the synergistic effect remarkably increases. However, if added in a too large amount, Ni impairs high temperature strength (creep strength, creep rupture strength) and promotes temper embrittlement, so the content was made 0.5 to 2.0%. Chromium (Cr): Cr is the most important element as an additive element of ordinary low alloy steel. When added to steel, Cr improves corrosion and oxidation resistance, increases hardenability, and improves tensile properties at room temperature. In addition, these effects are
When the content of both elements is large, it increases remarkably due to the synergistic effect. Further, Cr is an element effective for improving high temperature strength such as creep strength and creep rupture strength. However, 2.5%
If it exceeds the above range, the toughness is improved, but it is difficult to make a large improvement in the high temperature strength, and it is not necessary to add a large amount. Further, since the hardenability (bainite hardenability) is improved as described above, the content thereof is set to 2.0 to 2.5% in consideration of the mass effect of the disc material. Molybdenum (Mo): Mo, like Cr, is an important element as an additive element of ordinary low alloy steel. When added to steel, Mo
Is effective in increasing the strength (tensile strength, proof stress) at room temperature by increasing the hardenability and increasing the temper softening resistance during tempering. Further, Mo is a very effective element for improving high temperature strength such as creep strength and creep rupture strength as a solid solution strengthening element and as a precipitation strengthening effect element through the formation of carbides. Furthermore, when Mo is added in an amount of about 0.5% or more, it is a very effective element because it prevents temper embrittlement of steel. However, even if added in a too large amount, the effect is saturated, rather the toughness is impaired, and the cost is high because it is an expensive element. Therefore, the content
It was set to 0.5 to 1.5%. Tungsten (W): W, when present in the alloy composition of the low alloy steel of the present invention, is a very effective element for improving high temperature strength such as creep strength and creep rupture strength as a solid solution strengthening element. However, if added in too large an amount, undesired phenomena such as solidification segregation will occur as a large casting product.
The content was set to 1.0% or less. Vanadium (V): V is an element effective in improving strength (tensile strength, proof stress) at room temperature, similar to Mo. It is an important element that increases the high temperature strength such as creep strength and creep rupture strength as a solid solution strengthening element and as a precipitation hardening action element through the formation of carbides. Further, V is effective for improving the toughness by making the crystal grains finer if the addition amount of V is within a certain range (0.2 to 0.45%). However, if added too much, the toughness is impaired, and the cost is high because it is an expensive element, so its content should be 0.2-0.45%.
And Niobium (Nb): Nb is an element that is effective for increasing room temperature strength such as tensile strength and proof stress and high temperature strength such as creep strength and creep rupture strength as well as V, and at the same time refines the crystal grains. It is a very effective element for improving toughness, but if the content is less than 0.01%, its effect is not sufficient. In the low alloy steel of the present invention, addition of Nb does not expect much increase in strength, but rather expects improvement in toughness due to grain refinement, and if too much is added, a large amount of Nb carbonitride is formed. However, it is rather ineffective because it impairs toughness. Therefore, the content is set to 0.1% or less. Tantalum (Ta): Since it has almost the same effect as Nb, T
The content of a was set to 0.1% or less. Nitrogen (N): N is an element that acts as a precipitation hardening action through the formation of nitrides and is an element effective for increasing the tensile strength and creep rupture strength. On the other hand, on the other hand, if added in excess of 0.05%, the toughness deteriorates, so the content was made 0.05% or less. However, regarding Nb, Ta and N, any one or more of them is to be contained. Others: P, S, Cu, etc. are unavoidable as impurity elements mixed in by the raw materials of steel making, but it is desirable that these are as low as possible (however, if the raw materials are carefully selected, the cost will be high) Generally, P 0.015% Below, S 0.0
10% or less and Cu 0.50% or less are desirable. Other impurity elements include Al, Sn, Sb, Pb and As.

Claims (1)

[Claims] 1. Carbon 0.15 to 0.35% by weight, silicon 0.
15% or less, manganese 1.2% or less, nickel 0.5 to 2.0
%, Chromium 2.0-2.5%, molybdenum 0.5-1.5%, tungsten 0.5-1.0%, vanadium 0.2-0.45%, and selected from niobium 0.1% or less, tantalum 0.1% or less and nitrogen 0.05% or less. A high-toughness low-alloy steel excellent in high-temperature strength, characterized by having an alloy composition containing at least one kind and the balance being iron and unavoidable impurity elements.