JP2001073065A - High strength low Cr ferritic heat resistant steel with excellent temper brittleness resistance - Google Patents

Abstract

(57) [Summary] [Problem] Temper embrittlement and creep embrittlement during long-term use are suppressed, and the creep strength at high temperatures of 400 ° C to 625 ° C is stable and high, equivalent to existing low Cr heat resistant steel. Provided is a low Cr ferrite heat-resistant steel having the above good toughness, weldability and corrosion resistance. SOLUTION: In weight%, C: 0.01 to 0.25%, C
r: 0.5-3%, V: 0.05-0.5%, Mo:
0.05 to 2.5%, N: 0.01% or less, Ti: 0 to 0%
0.5%, P: 0.03% or less and S: 0.015%
Including, the structure after normalization and quenching is a lower bainite single phase structure, or includes at least 20% by volume of a lower bainite structure, and the remainder is one or both of a martensite structure and an upper bainite structure. Heat resistant steel which is a mixed structure containing

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to temper embrittlement suitable for heat exchangers and piping steel pipes, heat-resistant valves and connection joints used in the fields of the chemical industry, nuclear power and the like.
The present invention relates to a low Cr ferritic heat-resistant steel that suppresses creep embrittlement during long-term use at a high temperature of 0 ° C. or higher.

[0002]

2. Description of the Related Art Heat-resistant steels used at a high temperature of 400 ° C. or more are broadly classified into “austenitic heat-resistant steels” and “ferritic heat-resistant steels”. These steel types are appropriately selected and used.

[0003] The latter heat-resistant ferritic steel is generally a few%.
And alloy alloy elements such as W, Mo, Ni and Co as necessary, and the structure is δ-ferrite, tempered martensite or tempered bainite.

[0004] Among the heat-resistant ferritic steels, low Cr ferritic steel is superior in oxidation resistance, high temperature corrosion resistance and high temperature strength to carbon steel because it contains Cr. Also,
It has a small coefficient of thermal expansion and is excellent in toughness, weldability and thermal conductivity. Furthermore, low Cr heat resistant steel has the advantage that it is cheaper than austenitic stainless steel and high Cr ferritic steel.

As typical examples of the low Cr ferrite heat resistant steel having many advantages, STBA20 (0.5Cr-0.5Mo) and STBA22 (1.0C
r-0.5Mo), STBA23 (1.25Cr-0.5Mo), STBA2
4 (2.25Cr-1.0Mo) and STBA25 (5.0Cr-0.5Mo).

[0006] However, low Cr ferritic steel is inferior in high temperature strength to heat resistant austenitic steel and high Cr ferritic steel.

For the purpose of improving the high-temperature strength, low-Cr ferrite steels to which precipitation strengthening elements V, Nb, Ti and Ta are added are disclosed in JP-A-55-6458 and JP-A-57-131349.
A number of proposals have been made in Japanese Patent Publication No. JP-B-61-16345 and JP-B-61-34501.

Further, 1Cr-1Mo-0.25V steel which is a material for turbine and 2.25Cr-1Mo-Nb steel which is a structural material for fast breeder reactors are well known as precipitation strengthened low Cr ferritic steels. .

However, the following problems may occur when the strength of a low Cr ferritic heat resistant steel is increased by such a precipitation strengthening element.

Most of the precipitation strengthening elements such as V, Nb and Ti are dissolved in the matrix during normalizing, but fine carbonitrides are precipitated by tempering at a predetermined temperature after normalizing. And be strengthened. However, since carbonitride precipitates at high density in the grains, only the inside is strengthened and the prior austenite grain boundaries are relatively weakened. Therefore, the so-called temper embrittlement occurs in which the impact fracture surface transition temperature increases by 50 ° C. or more after tempering or during long-time use.

In general, temper embrittlement is caused by P, which tends to segregate at grain boundaries.
It is considered that the amount can be suppressed by reducing the amount of impurities such as S, Sb, and Sn. However, V, Nb
In the case where the inside of the grains is strengthened by precipitation and Ti or the like, the intragranular strength exceeds the grain boundary strength even if the grain boundary segregation element is reduced. Further, when the precipitates are finely dispersed, relaxation of the residual stress during tempering becomes difficult to occur, so that a non-uniform distribution of the residual stress occurs, and cracks easily occur at the grain boundary which is the embrittlement region.

The high temperature strength of the low Cr ferritic heat resistant steel is
If the tempering embrittlement can be increased without causing it, the following advantages can be obtained.

[0013] 1) Conventionally, austenitic stainless steel or high Cr ferritic steel has been used in applications where high-temperature strength is required in a usage environment where high-temperature corrosion is not so severe. Inexpensive low Cr ferritic steel can be used, and the characteristics of low Cr ferritic steel, for example, excellent weldability can be utilized.

2) By increasing the strength, it is possible to reduce the thickness of the member, thereby improving the thermal conductivity, improving the thermal efficiency of the plant, and the thermal fatigue load associated with starting and stopping the plant. Can be reduced.

3) By making the members thinner, the plant can be made compact and the manufacturing cost can be reduced.

[0016]

SUMMARY OF THE INVENTION An object of the present invention is to suppress temper embrittlement and creep embrittlement during long-term use, and to creep at a high temperature such as 400 ° C. to 625 ° C., which is the operating temperature of a practical boiler. The strength is stable and high, and the existing low Cr
Precipitation-hardened low Cr with a Cr content of 3% or less that has good toughness, weldability and corrosion resistance equal to or higher than heat-resistant steel
An object of the present invention is to provide a ferritic heat-resistant steel.

[0017]

The gist of the present invention is as follows.

C: 0.01 to 0.25% by weight, C
r: 0.5 to 3%, V: 0.02 to 0.5%, Mo:
0.01-2.5%, Ti: 0-0.05%, N: 0.
01% or less, P: 0.03% or less and S: 0.015
% Or less, and the metal structure is a lower bainite single phase structure, or a mixed structure including at least 20% by volume of a lower bainite structure and the balance including one or both of a martensite structure and an upper bainite structure. High-strength, low-Cr ferritic heat-resistant steel with excellent tempering embrittlement resistance.

The present inventors have studied the chemical composition and structure of precipitation-strengthened low Cr ferritic heat resistant steel having a Cr content of 3% or less in order to prevent temper embrittlement and creep embrittlement during long-term use. Various experiments were conducted. In particular, systematically investigate the relationship between temper embrittlement susceptibility and creep strength by observing the morphology of carbonitride precipitation and the volume ratio of the martensite structure and the upper or lower bainite structure in the lower structure by observation with a transmission electron microscope. did. As a result, the following findings were obtained.

A) When a ferrite structure is included, the structure containing ferrite must be avoided before and after tempering, since the toughness becomes the most unfavorable.

B) On the other hand, also in the case of the martensite structure or the upper bainite single phase structure, the toughness is poor.

C) After tempering, when a lower bainite single-phase structure or a mixed structure containing 20% or more by volume of lower bainite structure and the remainder is one or both of martensite structure and upper bainite structure Has the best toughness value and has a structure that is stable at high temperatures for a long time.

D) The intragranular precipitates observed after tempering are:
These are carbides and nitrides containing V, Nb and Ti as main components. These precipitates have a single-phase structure of lower bainite or a mixed structure containing 20% or more by volume of lower bainite, and the remainder contains one or both of a martensite structure and an upper bainite structure. In such a case, since it is uniformly distributed, tempering embrittlement hardly occurs.

E) As the proportion of upper bainite increases,
After tempering, these precipitates are arranged in a row along the lath interface, and the contribution to high strength is reduced. On the other hand, when the ratio of martensite increases, these precipitates become finer,
In addition, because of high-density precipitation, residual stress is not sufficiently reduced even after tempering. As a result, temper embrittlement susceptibility increases.

F) Temper brittleness is drastically improved by adding Mo. However, when Mo precipitates, its effect disappears, so it is necessary to maintain Mo in a solid solution state. In the case of steel containing no V, Mo precipitates as Mo ₂ C carbide, and the amount of dissolved Mo cannot be secured. On the other hand, when V is added, VC preferentially precipitates, so that Mo ₂ C
No longer precipitates. As a result, a large amount of Mo in the solid solution state remains, and the susceptibility to temper embrittlement decreases.

G) The temper embrittlement susceptibility decreases due to the reduction of dissolved N. However, when the amount of N exceeds 0.004%, temper embrittlement is suppressed by adding a small amount of Ti to reduce the amount of solute N as TiN.

[0027]

Next, the reason why the metal structure and the chemical composition are limited in the present invention will be described in detail. In addition,
All percentages in the chemical compositions shown below are by weight.

Lower bainite single phase structure: The bainite transformation is a type of non-diffusion transformation that does not involve the diffusion of alloy elements that change from an austenitic structure to a ferrite crystal structure by shear deformation. However, in the bainite transformation, C
Since the diffusion of atoms occurs, cementite precipitates simultaneously with the transformation, and the bainite structure is a mixed structure of ferrite and cementite.

The bainite structure is divided into lower bainite and upper bainite based on the difference in the form of precipitation of cementite. Among them, the lower bainite structure is a structure in which relatively fine cementite is uniformly dispersed in bainite grains.

When the lower bainite structure accounts for 20% by volume or more in the structure at the time of quenching from a temperature of ₃ or more Ac, the difference between the fracture surface transition temperature after quenching and the fracture surface transition temperature after tempering (△ T) can be suppressed to 40 ° C. or less at the maximum. On the other hand, when the lower bainite structure is less than 20%, ΔT is 4
Since the temperature may exceed 0 ° C., the lower bainite structure after quenching is set to 20% by volume or more. Therefore, the lower bainite may have a monolayer structure of 100% or a mixed structure. In the case of using a mixed tissue, the following structure is required. Mixed structure: In the case of a mixed structure, tempering embrittlement is prevented by forming a mixed structure containing 20% by volume or more of lower bainite and the remainder containing one or both of a martensite structure and an upper bainite structure. be able to.

In the upper bainite structure, relatively coarse cementite exists in a plate shape at the interface between bainite grains. When the upper bainite is less than 80% by volume, the maximum T can be suppressed to 40 ° C. or less, but when the upper bainite is 80% or more,
T may exceed 40 ° C. Therefore, even when the lower bainite structure and the upper bainite structure are mixed, the volume ratio of the upper bainite structure needs to be less than 80%.

The martensitic structure is a structure produced by a typical non-diffusion transformation, and does not involve the diffusion of C as in the bainite transformation. Therefore, the structure after quenching is a lath-like structure including high-density dislocations or a structure in which fine cementite is distributed in martensite grains by self-tempering. When this martensite is less than 80% by volume, ΔT can be suppressed to 40 ° C. or less at the maximum.
% Or more, ΔT may exceed 40 ° C. Therefore, even when the lower bainite structure and the martensite structure are mixed, the volume ratio of the martensite structure needs to be less than 80%.

Further, even when the mixed structure of the lower bainite structure, the upper bainite structure and the martensite is used, the total amount of the upper bainite and the martensite must be less than 80% by volume in order to keep ΔT at most 40 ° C. or less. Need to be

The volume of each structure was determined by preparing a thin film sample from the quenched material before tempering and the tempered material, and using a transmission electron microscope.
Observe at 000 times and determine the volume of each tissue by the following method,
The average value of 10 visual fields is used.

That is, the area ratio of each structure is obtained from the quenched material or the material tempered after quenching, and the obtained ratio is directly used as the volume ratio. This is because the area ratio directly becomes the volume ratio.
In addition, the area ratio can be measured directly by a microphotograph. Generally, a quenched material is used to determine the volume ratio, but a tempered material may be used. That is, as a result of repeatedly observing a large number of materials, it was confirmed that the respective structures could be discriminated even with the tempered material, and that the volume ratios obtained from the quenched material and the tempered material almost matched.

The identification of each organization is performed according to the following criteria.

1) In the case of a quenched material Lower bainite structure: A region where plate-like cementite is precipitated in bainite grains in a specific direction. Martensite structure: lath structure containing dislocations at a high density or spindle-shaped cementite is uniformly dispersed in martensite grains 2) In the case of a tempered material Lower bainite structure: When observed at a magnification of 10,000 using a transmission electron microscope,
Region in which precipitates with a diameter of 0.5 μm or more are precipitated in the lath inside the grain and on the grain boundary Tempered upper bainite structure: Precipitate with a diameter of 0.5 μm or more precipitates on the lath interface in the grain and the grain boundary. Tempered martensite structure: A region in which precipitates with a diameter of 0.5 μm or more are deposited only at the grain boundaries. The above precipitates with a diameter of 0.5 μm or more are M ₂₃ C
₆ type is one which particularly coarse in M ₇ C ₃ type and M ₆ C type carbides.

When the hardenability is insufficient, the ferrite structure precipitates with the former austenite grain boundary as a nucleus. Precipitation of the ferrite structure adversely affects not only the tempering but also the as-quenched toughness, so the precipitation of the ferrite structure must be suppressed. When observed by a transmission electron microscope, the ferrite structure can be confirmed as a region having a very low dislocation density in both the quenched material and the tempered material.

In order to obtain the above metal structure, the following heat treatment may be performed according to the steel composition.

In order to make the lower bainite single phase structure, a cooling rate that does not involve the ferrite nose shown in the continuous cooling transformation diagram of each steel but passes through the bainite nose may be selected. Further, mixed organization can be achieved depending on the position of the bainite nose. For example, in order to obtain a mixed structure with martensite, a cooling rate that passes through the tip of the bainite nose may be selected. On the other hand, in order to obtain a mixed structure of the upper bainite, a cooling rate that slightly contacts the ferrite nose is selected. In order to form a mixed structure of lower bainite + upper bainite + martensite, it is sufficient to hold the alloy immediately above the M _S point for several minutes and then quench it below the M _S point.

Hereinafter, the chemical composition will be described.

C: 0.01 to 0.25% C forms carbides with Cr, Fe, V, etc., contributes to high-temperature strength, and stabilizes the structure itself as an austenite stabilizing element. It is also important for balance control of martensite, lower bainite, and upper bainite according to the composition.

If the C content is less than 0.01%, the precipitation amount of carbides is insufficient, and the hardenability is lowered to deteriorate the strength and toughness. On the other hand, if it exceeds 0.25%, carbides are excessively precipitated, and the steel is hardened remarkably, impairing workability and weldability.
Therefore, the range of the C content is set to 0.01 to 0.25%. Desirably, it is 0.05 to 0.15%.

Cr: 0.5-3% Cr is an essential element for improving oxidation resistance and high-temperature corrosion resistance. If the Cr content is less than 0.5%, these effects cannot be obtained. On the other hand, if it is 3% or more, the economic efficiency is reduced and the advantage of the low Cr ferritic steel is reduced. Therefore, the range of the Cr content is set to 0.5% or more and 3% or less.

V: 0.02 to 0.5% V is an important element for forming MX type fine precipitates.
That is, V combines with C to form a fine VC and greatly contributes to high strength. Further, since Mo is fixed, Mo, which has an effect on temper embrittlement resistance, does not precipitate as carbides, and the amount of dissolved Mo is secured.

However, if it is less than 0.02%, the precipitation amount of VC is small, and the above effects cannot be obtained. On the other hand, 0.
If it is contained in excess of more than 5%, the VC becomes coarse, and the strength and toughness are impaired. Therefore, the V content was set to 0.02 to 0.5%. Desirably, 0.05-0.
25%.

Mo: 0.01 to 2.5% Mo has a solid solution strengthening effect. In addition, Mo solid solution significantly improves temper embrittlement resistance. However, if the Mo content is less than 0.01%, this effect cannot be obtained. Meanwhile, 2.
If it exceeds 5%, the effect is saturated, and on the contrary, the weldability and toughness are impaired. Therefore, the range of the Mo content is set to 0.01 to 2.5%. Desirably 0.05 to
1.5%, and more preferably 0.1 to 1%.

Ti: 0 to 0.05% Ti is an element to be contained as necessary, and is preferably contained when the N content is large, for example, when it exceeds 0.006%. It is effective in suppressing back embrittlement.
When it is contained, the content is preferably 0.002% or more. Further, in a case where coarsening of crystal grains becomes a problem due to local heating such as in a welded portion, coarsening can be prevented by adding Ti. However, if the content exceeds 0.05%, the toughness is rather deteriorated. Therefore, the upper limit of the Ti content is 0.05%.
% Is preferable. Desirably 0.03% or less,
More preferably, it is 0.02% or less.

N: 0.01% or less N combines with V, Nb and Ti to form fine carbonitrides, and contributes to improvement of creep strength and improvement of toughness by refinement of crystal grains. However, since solid solution N promotes tempering embrittlement, if the N content exceeds 0.01% and becomes large, the toughness is significantly deteriorated. Desirably, it is 0.006% or less.

P: 0.03% or less, S: 0.015% or less P and S are unavoidable impurity elements, and both promote temper embrittlement. For this reason, it is desirable to make it as low as possible. The allowable upper limit of P is 0.03%, and the allowable upper limit of S is 0.0
15%. Desirable upper limit of P is 0.015%, and upper limit of S is 0.005%.

The steel of the present invention is required to have at least the above chemical composition, but can further contain alloying elements as described below.

Nb: 0.002 to 0.2% Nb combines with C like V to form an MX-type precipitate and contributes to the improvement in creep strength. Sometimes V in MX
And Nb form a solid solution, MX becomes finer,
Coarsening is suppressed to prevent a long-term decrease in creep strength. Further, it is effective for making the crystal grains fine and improving the toughness. However, if the Nb content is less than 0.002%, the above effects cannot be obtained. On the other hand, if it exceeds 0.2%, the steel is hardened remarkably and the toughness, weldability and workability are impaired. Therefore, the range of the Nb content is preferably set to 0.002 to 0.2%. Desirably, it is 0.005 to 0.1%.

Ta: 0.002 to 0.2% Ta fixes solid solution N similarly to Ti and is effective in improving temper embrittlement resistance. However, if the Ta content is less than 0.002%, the above effects cannot be obtained. On the other hand, if it exceeds 0.2%, the steel is hardened remarkably and the toughness, weldability and workability are impaired. Therefore, the range of the Ta content is 0.002 to
It is better to be 0.2%. Desirably 0.005-0.
03%.

W: 0.02% to 5% W has a solid solution strengthening effect similarly to Mo. Further, it has an effect of improving temper embrittlement resistance. However, when the W content is 0.0
If less than 2%, this effect cannot be obtained. On the other hand, if it exceeds 5%, the effect is saturated, and coarse precipitates are deposited to impair weldability and toughness. Therefore, the range of the W content is preferably set to 0.02 to 5%. Preferably, 2.
5% or less.

B: 0.0001 to 0.01% B is an element effective for securing stable strength for a long time even at high temperatures. However, when the B content is 0.0001% or less, this effect cannot be obtained. On the other hand, if it exceeds 0.01%, precipitation of coarse carbides on the prior austenite grain boundaries is promoted, which causes a reduction in strength and toughness. Therefore, the range of the B content is preferably 0.0001 to 0.01%.

Co: 0.01 to 0.5% Co is an austenite stabilizing element and has a solid solution strengthening action. However, if the Co content is less than 0.01%, this effect cannot be obtained. On the other hand, if it exceeds 0.5%, the high-temperature creep strength decreases. In addition, excessive addition is not preferable from the viewpoint of economy. Therefore, the Co content is preferably set to 0.01 to 0.5%. Preferably 0.2
% Or less.

Ni: 0.01 to 0.5% Ni is an austenite stabilizing element and contributes to improvement in toughness. However, if the Ni content is less than 0.01%, this effect cannot be obtained. On the other hand, if it exceeds 0.5%, the high temperature creep strength and the toughness are deteriorated. In addition, excessive addition is not preferable from the viewpoint of economy. Therefore, the Ni content is preferably set to 0.01 to 0.5%. Desirably, it is 0.2% or less.

Cu: 0.01 to 0.5% Cu is an austenite stabilizing element and contributes to improvement in thermal conductivity. However, when the Cu content is 0.01%
If it is less than this, this effect cannot be obtained. On the other hand, if it exceeds 0.5%, the high temperature creep strength and the toughness are deteriorated. Therefore, the Cu content is preferably set to 0.01 to 0.5%.
Desirably, it is 0.2% or less.

Al: 0.001 to 0.05% Al is an element effective as a deoxidizing agent for molten steel. However, if the Al content is less than 0.001%, the deoxidizing effect cannot be obtained. On the other hand, when it exceeds 0.05%, creep strength and workability are impaired. Therefore, the range of the Al content is 0.0
The content is preferably set to 01 to 0.05%. Desirably 0.01
5% or less.

Si: 0.5% or less Si is an element that improves the steam oxidation resistance of steel. However, Si promotes tempering embrittlement, and when the Si content exceeds 0.5%, the toughness after tempering is significantly deteriorated.
Therefore, the upper limit of the Si content is preferably set to 0.5%. Desirably, it is 0.3% or less.

Mn: 1% or less Mn improves hot workability by the desulfurization and deoxidation effects during melting. Furthermore, hardenability is improved. However, when the content exceeds 1%, the toughness after quenching is significantly deteriorated. Therefore, the upper limit of the Mn content is preferably set to 1%. Desirably, it is 0.8% or less.

Regarding the heat treatment, it is desirable to quench the steel as it is cast or after normalizing the hot-worked steel at three or more Ac points, and then water-cooling or air-cooling. This is A
Tempering may be performed at c1 point or less, or the quenched material may be directly provided to the product without tempering. Furthermore, if the welded HAZ has the structure specified in the present invention,
It has strength and temper embrittlement resistance comparable to that of the base metal.

The heat-resistant steel of the present invention also has an effect of reducing the susceptibility to SR embrittlement during stress relaxation heat treatment (SR) performed after welding or the like, and is suitable as a member requiring welding.

[0064]

EXAMPLES Each steel having the chemical composition shown in Tables 1 and 2 was used for 15 samples.
An ingot obtained by melting and forging in a 0 kg vacuum melting furnace was forged and rolled at 1200 to 1000 ° C. to obtain a steel plate having a thickness of 20 mm. After normalizing these steel sheets at a temperature of Ac ₃ or higher, quenching was performed at various quenching temperatures and cooling rates within the following ranges in order to change the structure after quenching. In addition, the cooling rate was changed by furnace cooling, air cooling, oil cooling, water cooling, and the like.

Hardening temperature: 1100 to 950 ° C., cooling rate: 0.02 to 2 ° C./sec

[0066]

[Table 1]

[0067]

[Table 2]

A thin film sample for transmission electron microscope observation was prepared from each quenched steel sheet by electrolytic polishing, and the volume fractions of martensite, lower bainite, upper bainite, and ferrite in the steel were measured using transmission electron microscope observation. did.
The volume ratio was obtained by observing 10 visual fields at a magnification of 10,000 times with a transmission electron microscope, and calculating the average value thereof.

A Charpy impact test specimen was prepared from a part of each quenched steel sheet.

The remaining steel sheets were tempered at various temperatures and holding times within the temperature range from 600 ° C. to the Ac1 temperature to produce Charpy impact test pieces.

The Charpy impact test specimen was 10 × 10 × 5
A JIS No. 4 test piece with a 5 (mm) and 2 mm notch was used.

A Charpy impact test was performed using these test pieces to determine a ductile-brittle fracture transition temperature vTs.
The difference ΔT between the fracture surface transition temperature vTs of the quenched material and the fracture surface transition temperature vTs of the tempered material was determined and used as an evaluation index of the temper embrittlement susceptibility. The fracture surface transition temperature of the tempered material is that of a material (tempered embrittlement material) tempered under the tempering condition in which toughness deterioration is the most remarkable (vTs becomes the maximum value).

Tables 3 and 4 show the results.

[0074]

[Table 3]

[0075]

[Table 4]

The evaluation index ΔT of the temper embrittlement susceptibility is ≦ 40.
° C, the temper brittleness was evaluated as good, and ΔT> 40 ° C, the temper embrittlement resistance was evaluated as poor.

In order to measure the high-temperature strength, each quenched steel sheet was tempered at a temperature in the range of 750 or 770 ° C., and a room temperature tensile strength was adjusted to a predetermined value, and then a creep test was performed. Creep test is 6mm in diameter, GL
Is 30 mm, and the maximum length is 10,000 at 500 ° C.
h, and 500 ° C x 800 by regression calculation
The average creep rupture strength at 0 h was determined.

Tables 3 and 4 show the evaluation results. 1 to 1
Four steels are the present invention steels. In addition, AP steel and 1'-5
'Steel is the comparative steel. The comparative steels 1 'to 5' are comparative steels whose chemical compositions are within the range specified in the present invention example, but the volume fraction of lower bainite is out of the range specified in the present invention.

As is evident from Tables 3 and 4, ΔT was suppressed to 40 ° C. or less in all of the steels of the present invention having a structure containing 20 to 100% of lower bainite. For this reason, the toughness value after tempering is also good. further,
Creep strength at 500 ° C x 8000h time is 245MP
When it is not less than a, the high temperature strength is also good.

FIG. 1 is based on Tables 3 and 4. The figure shows the relationship between the amount of lower bainite and the index ΔT of temper embrittlement susceptibility. As is clear from FIG. 1, the lower bainite ratio and ΔT have a unique relationship. When the lower bainite ratio is 20% or more, ΔT is 40 ° C. or less, and the tempering embrittlement resistance is good.

On the other hand, in the comparative steels not having the structure specified in the present invention, it can be seen that any of tempering embrittlement resistance, toughness after tempering, and high-temperature strength is unsatisfactory.

[0082]

The heat-resistant steel of the present invention is excellent in temper embrittlement resistance and creep embrittlement resistance, has high creep strength at a high temperature of 400 ° C. or more, and is exposed to a high temperature for a long period of time. It is suitable for structural materials or structural members that require heat treatment for removing residual stress after processing, and exhibits excellent effects.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between temper embrittlement susceptibility and lower bainite.

Claims (1)

[Claims] (1) C: 0.01 to 0.25% by weight, C
r: 0.5 to 3%, V: 0.02 to 0.5%, Mo:
0.01-2.5%, Ti: 0-0.05%, N: 0.
01% or less, P: 0.03% or less and S: 0.015
% Or less, and the metal structure is a lower bainite single phase structure, or a mixed structure including at least 20% by volume of a lower bainite structure and the balance including one or both of a martensite structure and an upper bainite structure. A high-strength, low-Cr ferritic heat-resistant steel having excellent tempering embrittlement resistance.