JP4609491B2 - Ferritic heat resistant steel - Google Patents

Description

  The present invention relates to a ferritic heat resistant steel. More specifically, the present invention relates to a ferritic heat resistant steel having excellent high-temperature long-term creep strength and creep fatigue strength. The heat-resistant steel of the present invention is suitable for heat exchange steel pipes, pressure vessel steel sheets, turbine materials, and the like used under high temperature and high pressure environments such as boilers, nuclear power generation facilities, and chemical industrial facilities.

  High temperature creep strength, creep fatigue strength, corrosion resistance, oxidation resistance and the like are generally required for heat resistant steels used in high temperature and high pressure environments such as boilers, nuclear power generation facilities and chemical industrial facilities.

  High Cr ferritic steel is superior to low alloy steel in terms of strength and corrosion resistance at a temperature of 500 to 650 ° C. In addition, high Cr ferritic steel has a high thermal conductivity and a low coefficient of thermal expansion, and is therefore characterized by excellent heat fatigue resistance and low cost compared to austenitic stainless steel. Furthermore, there are a number of advantages such as less scale peeling and no stress corrosion cracking.

  From the late 1980s to the 1990s, ASME P91 steel was put into practical use as a high-strength ferritic heat-resistant steel and used in supercritical pressure boilers with a steam temperature of 566 ° C or higher. In recent years, ASME P92 steel with increased creep strength has been put into practical use, and an ultra supercritical boiler with a steam temperature of about 600 ° C. has been put into practical use.

Currently, reduction of CO ₂ emissions is required for environmental protection. Therefore, further higher temperature and pressure are required in thermal power generation boilers. The ASME P92 steel, which is currently in practical use, must be used as a thick member in order to use it in a higher temperature range, for example, about 630 ° C.

  In a thermal power plant, since starting and stopping are frequently performed, especially for a thick member, creep fatigue strength is important. ASME P92 steel has significantly increased creep strength compared to ASME P91 steel, but the creep fatigue strength is equivalent. For further practical use of high-temperature and high-pressure boilers, it is essential to improve the creep fatigue strength of ASME P92 steel.

Patent Documents 1 and 2 disclose inventions of heat-resistant steel containing 8 to 14% Cr. Patent Document 3 discloses an invention of a heat resistant steel containing 8 to 13% Cr. However, the invention disclosed in these documents is not made for the purpose of improving the creep fatigue strength of the heat-resistant steel. Although the steels of these inventions may contain Nd, they are not steels utilizing the effective action of Nd inclusions described later.
Japanese Patent Laid-Open No. 2001-192781 JP 2002-224798 A JP 2002-235154 A

  An object of the present invention is to provide a ferritic heat resistant steel having excellent high-temperature long-term creep strength and excellent creep fatigue strength.

  FIG. 1 is a diagram illustrating an example of a strain waveform in a creep fatigue test. FIG. 4A shows a PP waveform, which is a waveform in which strain is applied at high speed so that creep strain does not occur on both the tension side and the compression side. Shown in (b) is a CP waveform. This is a waveform in which strain is loaded with a low speed on the tension side and a high speed on the compression side in order to introduce a creep creep strain.

  When the lifetime under the PP waveform is compared with the lifetime under the CP waveform, the lifetime is shorter in the CP waveform that undergoes creep damage. In general, the life of heat-resistant steel used in high-temperature and high-pressure environments of boilers, nuclear power generation facilities, and chemical industrial facilities is evaluated by conducting a creep fatigue test in the entire strain range of 0.4 to 1.5%.

  Since the equipment such as the boiler is used for a long time under high temperature and high pressure, each member undergoes creep strain and receives a CP-type load. Further, in general, the actual machine has a structure that reduces the generated strain in order to ensure the creep fatigue life of the member used under high temperature and high pressure. Therefore, in the high Cr ferritic steel used in these facilities, creep is performed at the total strain range of the creep fatigue test under the CP waveform, that is, a total strain of about 0.5%, which is a low strain region of 0.4 to 1.5%. It is necessary to ensure a fatigue life.

  The ASME P91 and P92 steels have a 100,000 hour creep strength at 600 ° C. of about 98 MPa and 128 MPa, respectively, and the P92 steel has higher strength. However, when a creep fatigue test was performed at 600 ° C with the CP waveform in Fig. 1 and a total strain range of 0.5%, it was revealed that the lifetime was not much different from about 3000 cycles. That is, it was found that the creep fatigue strength of the P92 steel was not improved, although the creep strength was higher than that of the P91 steel. From these results, it is considered that the steel of P92 contains some cause that the creep fatigue strength does not improve, in other words, the cause that the creep fatigue strength decreases. Therefore, the present inventors have intensively studied to improve the creep fatigue strength of P92 steel.

  First, the effect of a small amount of δ ferrite due to segregation of alloy elements, which is considered to be a cause of not improving the creep fatigue strength, was examined as follows (a).

(a) Investigation of the effect of δ ferrite
P92 steel contains a large amount of ferrite-forming elements (Mo, W, Nb, V, etc.) in addition to the components contained in the conventional 9Cr ferritic heat-resistant steel. Therefore, a very small amount of δ ferrite may remain at the grain boundary. In order to completely eliminate δ-ferrite, materials containing a small amount of Cu, Ni, or Co (these are austenite forming elements) in P92 steel were prepared, and the creep fatigue strength was compared. The test temperature was 600 ° C. and the total strain range was 0.5%. As a result, the life was about 1600-2100 cycles, and a tendency to decrease rather than P92 steel was observed.

  From the above results, it was clarified that the creep fatigue strength of P92 steel does not improve due to δ ferrite, but rather the creep fatigue strength decreases when an excessive austenite forming element is contained. .

  Next, in order to clarify the contribution of grain boundaries to creep fatigue strength, the following investigation (b) was conducted.

(b) Investigation of the effect of prior austenite grain size on the creep fatigue strength of P92 steel Treated with P92 steel at normalizing temperatures of 1050 ° C and 1200 ° C and changed the prior austenite grain size to approximately 25 µm and 125 µm. It was. Next, after tempering to a tensile strength of about 710 MPa by tempering, a creep fatigue test was performed. The test temperature was 600 ° C. and the total strain range was 0.5%.

  As a result of the above test, the life of a coarse-grained steel having a particle size of 125 μm was about 2300 cycles, while the life at a normal particle size of 25 μm was about 3000 cycles. From this, in the case of coarse-grained steel, it became clear that the creep fatigue life is reduced even if the strength is the same as that of fine-grained steel.

(c) Elucidation of the reason why the fine-grained steel has higher creep fatigue strength The reason why the fine-grained steel has higher creep fatigue strength was examined as shown in the test results of (b) above.

  In general, it is said that creep characteristics at a high temperature tend to be better in the case of coarse particles. Therefore, the creep strength of the sample used in the test (b) at 600 ° C. and 160 MPa was investigated. As a result, the break time of a sample with a particle size of 25 μm is about 6000 hours, whereas the break time of a sample with a particle size of 125 μm is about 9000 hours. The creep strength is higher. From this result, it was found that the improvement in creep fatigue strength in fine-grained steel cannot be explained by tensile strength and creep strength.

  With fine-grained steel, the grain boundary area increases. When the area of the grain boundary increases, it is considered that segregation of impurity elements such as P, S, As, and Sn, particularly S, is suppressed. Then, the segregation of S to the grain boundary was considered.

  Usually, ferritic heat resistant steel contains about 0.001% of S as an impurity. At the actual product level, it is difficult to reduce S to a level lower than 0.001%. Even in the production in the laboratory, since mixing of S from the alloy elements is inevitable, it is difficult to eliminate segregation by reducing S in a normal melting method.

  Generally, temper embrittlement is known as a phenomenon caused by segregation such as S. Temper embrittlement occurs when martensite is tempered in a certain temperature range of around 600 ° C., and it is known that a trace amount of Mo is effective in reducing this.

  If the creep fatigue phenomenon correlates with the segregation of S, it is considered that the Mo content and the creep fatigue characteristics have some correlation. Accordingly, creep fatigue strength (test temperature is 600 ° C., total strain range is 0.5%) when the Mo content is changed to 0.01%, 0.07%, 0.13%, 0.33%, and 1.83% was investigated. As a result, when the Mo content was 0.13% and 0.33%, the life was about 3000 cycles, but when the Mo content was small (0.01% and 0.07%), the creep fatigue strength was around 2000 cycles. Decreased. From this, it became clear that Mo has made a certain contribution to the creep fatigue strength. When the Mo content was further increased to 1.83%, the creep fatigue life was about 2500 cycles, and the fatigue characteristics tended to decrease rather.

  Next, the existence state of S in the steel was investigated. As a result, as shown in FIG. 2, it became clear that S exists in the form of MnS. If S trapped as MnS becomes free and segregates at the grain boundary during the creep fatigue test at a high temperature, this S is considered to adversely affect the creep fatigue properties.

(d) S fixation If the segregation of S, which has become free as described above, adversely affects creep fatigue properties, the creep fatigue strength can be increased by adding an element that traps S more firmly in addition to Mn. It is considered possible to increase

  Therefore, the effect of Ca, Mg, Nd, La and Ce on the creep fatigue strength, which may form sulfides, was investigated.

  As a result, it was found that Nd inclusions fix S in addition to MnS when 0.025% Nd is contained. The Nd inclusion means “Nd oxide” and “complex inclusion of Nd oxide and sulfide”. In other words, “the complex inclusion of Nd oxide and sulfide” directly fixes S. On the other hand, “Nd oxide” also indirectly fixes S by segregating around it. As an example of Nd inclusions, FIG. 3 shows “composite inclusions of Nd oxide and sulfide” observed in Nd-containing steel.

  As described above, a steel containing Nd directly and indirectly fixing Sd was subjected to a creep fatigue test under the above-described conditions, that is, a test temperature of 600 ° C. and a total strain range of 0.5%. It became clear that it improved dramatically with the cycle.

  In addition, the creep fatigue life of steel containing Ca, Mg, La and Ce alone (test temperature is 600 ° C, total strain range 0.5%) is about 3000 to 4000 cycles. It became clear that the contained steel had a life of 6000 to 7000 cycles, and the creep fatigue life was dramatically improved.

(e) Combined addition of Nd and Cu, Ni or Co As described in (a) above, creep fatigue strength tends to decrease in steels containing trace amounts of austenite forming elements such as Cu, Ni or Co. It was. In order to further clarify this phenomenon, the creep fatigue life of a steel containing a trace amount of Cu, Ni or Co in a steel containing a trace amount of Nd was evaluated.

  As a result, the creep fatigue life of steel containing Nd and trace amounts of Cu, Ni, or Co is about 4000 cycles, and the creep fatigue properties are improved compared to steel not containing Nd, but Nd alone It was found that the creep fatigue life was significantly inferior to that of the steel contained in.

From the above examination, the following conclusions (1) to (4) can be obtained.
(1) 0.1% or more of Mo contributes to creep fatigue properties.

  (2) Most of S is fixed as MnS, but part of S becomes free during the fatigue test at high temperature, segregates at the grain boundary, and decreases the creep fatigue strength.

(3) Creep fatigue strength is greatly improved by containing Nd, fixing S with an oxide of Nd or a composite inclusion of oxide and sulfide, and fixing a part thereof with MnS. . The effect is remarkable when the density of Nd inclusions is 10,000 / mm ³ or more. The “Nd inclusion” is a generic term for the above “Nd oxide” and “composite inclusion of Nd oxide and sulfide”.

  (4) Cu, Ni, and Co, which are austenite forming elements, lower the creep fatigue strength. This tendency is also observed in steel containing a small amount of Nd. Such a phenomenon occurs because Cu, Ni and Co promote the phenomenon that S fixed as MnS becomes free during the creep fatigue test.

  The gist of the present invention based on the above examination results is the heat resistant steel described below. Hereinafter, “%” regarding the component content means “% by mass”.

(1) C: 0.01 to 0.13%, Si: 0.15 to 0.50%, Mn: 0.2 to 0.5%, P: 0.02% or less, S: 0.005% or less, Cr: more than 8.0% and less than 12.0%, Mo: 0.1 -1.5%, W: 1.0-3.0%, V: 0.1-0.5%, Nb: 0.02-0.10%, sol.Al: 0.015% or less, N: 0.005-0.070%, Nd: 0.005-0.050%, B: 0.002 Steel containing ˜0.015%, the balance being Fe and impurities, of which Ni is less than 0.3%, Co is less than 0.3%, and Cu is less than 0.1%, including Nd inclusions, Ferritic heat resistant steel with Nd inclusion density of 10,000 / mm ³ or more.

  (2) The ferrite heat-resisting as described in (1) above, which contains at least one of Ta: 0.04% or less, Hf: 0.04% or less, and Ti: 0.04% or less instead of part of Fe steel.

  (3) Ferrite heat resistance as described in (1) or (2) above, wherein one or two of Ca: 0.005% or less and Mg: 0.005% or less are contained in place of part of Fe steel.

  (4) The ferritic heat resistant steel according to any one of (1) to (3) above, wherein the total amount of rare earth elements excluding Nd in impurities is 0.04% or less.

  (5) Creep fatigue life in CP waveform at 600 ° C at 5000 ° C or more under the condition that the strain rate is 0.01% / sec on the tension side and 0.8% / sec on the compression side and the total strain range is 0.5% The ferritic heat resistant steel according to any one of (1) to (4) above.

FIG. 1 is a diagram illustrating an example of a strain waveform in a creep fatigue test. FIG. 2 is a diagram showing sulfides observed in ASME P92 steel. FIG. 3 is a diagram showing “composite inclusions of Nd oxide and sulfide” observed in Nd-containing steel.

Best Mode for Carrying Out the Invention

1. Chemical composition First, the effect of the component which comprises the heat-resistant steel of this invention, and the reason for limitation of content are demonstrated.

C: 0.01-0.13%
C stabilizes the structure of steel as an austenite stabilizing element. In addition, MC carbide or M (C, N) carbonitride is formed, which contributes to improvement of creep strength. M in MC and M (C, N) is an alloy element. However, if the content of C is less than 0.01%, the above effect cannot be obtained sufficiently, and the amount of δ ferrite increases and the strength may be lowered. On the other hand, if the C content exceeds 0.13%, not only the workability and weldability are deteriorated, but also agglomeration of carbides occurs from the beginning of use, resulting in a decrease in creep strength for a long time. Therefore, the C content needs to be limited to 0.13% or less. More desirable lower and upper limits are 0.08% and 0.11%, respectively.

Si: 0.15-0.50%
Si is contained as a deoxidizing element of steel and is also an element necessary for improving the steam oxidation resistance. The lower limit is 0.15% which does not impair the steam oxidation resistance. On the other hand, if the Si content exceeds 0.50%, the creep strength is significantly reduced, so the upper limit is made 0.50%. In particular, when importance is attached to steam oxidation resistance, it is desirable to set the lower limit of the Si amount to 0.25%.

Mn: 0.2-0.5%
Mn contributes as a deoxidizing element and an austenite stabilizing element. Further, MnS is formed to fix S. In order to obtain these effects, a content of 0.2% or more is necessary. On the other hand, if it exceeds 0.5%, the creep strength is reduced. Therefore, the appropriate content of Mn is 0.2 to 0.5%. A more preferred lower limit is 0.3%.

P: 0.02% or less, S: 0.005% or less The impurities P and S deteriorate the hot workability, weldability, creep strength, creep fatigue strength, and the like of the steel, so the lower the content, the better. However, since significant cleaning of the steel causes a significant cost increase, the allowable upper limit is 0.02% for P and 0.005% for S.

Cr: more than 8.0% and less than 12.0%
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. Furthermore, Cr forms carbides and improves the creep strength. In order to obtain these effects, the content needs to exceed 8.0%. However, if the Cr content is excessive, the creep strength decreases for a long time, so the content was made less than 12.0%. A more preferred lower limit is 8.5%, and a more preferred upper limit is less than 10.0%.

Mo: 0.1-1.5%
Mo contributes to the improvement of creep strength as a solid solution strengthening element. Furthermore, as a result of examining the correlation between the Mo content and the creep fatigue strength in detail, it is found that 0.1% or more of Mo contributes to the improvement of creep fatigue properties, and if the content exceeds 1.5%, the long-term creep strength It has been found that this leads to a decline. Accordingly, the appropriate Mo content is 0.1 to 1.5%. More preferred lower and upper limits are 0.3% and 0.5%, respectively.

W: 1.0-3.0%
W contributes to the improvement of creep strength as a solid solution strengthening element. Furthermore, a part of the solid solution is dissolved in the Cr carbide, thereby suppressing the aggregation and coarsening of the carbide and contributing to the creep strength. However, below 1.0% their effects are small. On the other hand, if the W content exceeds 3.0%, the formation of δ ferrite is promoted and the creep strength is reduced. Therefore, the appropriate range of W content is 1.0 to 3.0%. A more preferred lower limit is an amount exceeding 1.5%, and a more preferred upper limit is 2.0%.

V: 0.1-0.5%
V contributes to the improvement of creep strength by the solid solution strengthening action and by forming fine carbonitrides. In order to exert the effect, the content needs to be 0.1% or more. On the other hand, if the V content exceeds 0.5%, the formation of δ ferrite is promoted and the creep strength is lowered, so 0.5% should be made the upper limit. More preferred lower and upper limits are 0.15% and 0.25%, respectively.

Nb: 0.02 to 0.10%
Nb contributes to the improvement of creep strength for a long time by forming fine carbonitrides. In order to exert the effect, it is necessary to contain 0.02% or more. However, if the content is too large, the formation of δ ferrite is promoted and the creep strength is lowered for a long time. Therefore, the appropriate content of Nb is 0.02 to 0.10%. More preferred lower and upper limits are 0.04% and 0.08%, respectively.

sol. Al: 0.015% or less
Al is used as a deoxidizer for molten steel, but if its content exceeds 0.015%, the creep strength decreases, so the upper limit should be kept to 0.015% or less. A more preferred upper limit is 0.010%.

N: 0.005-0.070%
N, like C, is effective as an austenite stabilizing element. N also precipitates nitrides or carbonitrides to increase the high temperature strength of the steel. In order to exhibit the effect, it is necessary to contain 0.005% or more. On the other hand, when the content of N is excessive, not only blow holes are generated during melting and welding defects are caused, but also the creep strength is reduced due to coarsening of nitrides and carbonitrides. Therefore, the upper limit of N content should be 0.070%. A more preferable lower limit of the N content is 0.020%.

Nd: 0.005 to 0.050%
As described above, Nd greatly improves the creep fatigue strength. In order to exert the effect, it is necessary to contain 0.005% or more. However, if it exceeds 0.050%, coarse nitrides are formed and the creep strength is lowered, so the upper limit should be 0.050%. A more preferable upper limit of the content is 0.040%.

B: 0.002 to 0.015%
B enhances hardenability and plays an important role in securing high temperature strength. The effect becomes remarkable at 0.002% or more, but if it exceeds 0.015%, weldability and long-term creep strength are lowered.

Ni: less than 0.3%, Co: less than 0.3%, Cu: less than 0.1% As described above, these austenite stabilizing elements lower the creep fatigue strength even with a slight content. However, trace amounts of Ni, Co, and Cu may be unavoidably mixed from the melting raw material. Therefore, in the present invention, Ni and Co are each suppressed to less than 0.3% and Cu is suppressed to less than 0.1%. If it is said range, the bad influence on creep fatigue strength is small.

Group 1 components: Ta, Hf and Ti
These are components added by one or more as required. Each appropriate content in the case of adding is as follows.

Ta: 0.04% or less, Hf: 0.04% or less, Ti: 0.04% or less
Ta, Hf, and Ti are included as necessary because they form fine carbonitrides and contribute to the improvement of creep strength. In order to fully exhibit the effect, it is desirable that each content is 0.005% or more. However, even if each content exceeds 0.04%, the effect is saturated and the creep strength is deteriorated. Therefore, the upper limit of each content is preferably 0.04%.

Group 2 components: Ca and Mg
These are also components added as one or two as required. Each appropriate content in the case of adding is as follows.

Ca: 0.005% or less, Mg: 0.005% or less These elements all improve the hot workability of steel. Accordingly, when it is desired to particularly improve the hot working of steel, either one of them is contained alone or a combination of both. Since the effect becomes remarkable at 0.0005% or more, the lower limit of the content is preferably 0.0005%. However, if the content exceeds 0.005%, the creep strength decreases, so the upper limit should be 0.005%.

Rare earth elements excluding Nd: 0.04% or less
Rare earth elements such as La and Ce may be mixed as impurities when Nd is added. However, if the total content of rare earth elements excluding Nd is 0.04% or less, it does not significantly affect the properties such as creep strength and creep ductility, so that it is allowed to contain up to 0.04%.

2. Nd inclusions One of the features of the steel of the present invention is that Nd inclusions are contained at a density of 10,000 or more per mm ³ .

As described above, Nd inclusions observed in the steel of the present invention are “Nd oxide” and “Nd oxide and sulfide composite inclusion”. Specifically, Nd ₂ O ₃ , Nd ₂ O ₂ S ₄ , Nd ₂ O ₂ SO ₄ , Nd ₂ O ₂ S and the like.

The diameter of Nd inclusions varies from about 0.3 μm to about 1 μm, but Nd inclusions are usually observed in steels containing a small amount of Nd. However, steel containing a large amount of Co, Ni and Cu increases MnS and significantly reduces Nd inclusions. When the density of Nd inclusions is less than 10,000 / mm ³ , improvement in creep fatigue strength is not recognized. Therefore, the density of Nd inclusions must be 10,000 / mm ³ or more.

3. Production Method The steel of the present invention can be produced by production equipment usually used industrially. That is, in order to obtain a steel having the chemical composition defined in the present invention, the components may be adjusted by refining with a furnace such as an electric furnace or a converter, and by deoxidation and inclusion of alloy elements. In particular, when strict component adjustment is required, a method of subjecting the molten steel to an appropriate treatment such as a vacuum treatment before adding the alloy element may be employed.

How 10,000 / mm ³ or more Nd inclusions is introduced in the steel are as follows. That is, sufficient deoxidation is performed beforehand with C, Si, Mn, Al or the like at the stage from iron making to steel making. This is because if the oxygen content in the molten steel is large, the yield of Nd addition will deteriorate. Thereafter, in the case of the ingot-making method, the composition other than Nd is adjusted before casting into the ingot, and Nd inclusions are generated by adding Nd immediately before casting. In the case of the continuous casting method, before introducing molten steel to the tundish, a composition other than Nd is adjusted, and then Nd is added to the tundish to generate Nd inclusions. By final adjustment of only Nd, an appropriate amount of Nd inclusions can be generated. The cast slab, billet or steel ingot is further processed into a steel pipe or a steel plate.

  When producing a seamless steel pipe, for example, a billet may be extruded, rolled with an inclined roll-type piercer, or a large-diameter forged pipe by an Erhard pipe method. In the production of steel pipes, the dimensions can be adjusted by cold working as necessary. The formed steel pipe is appropriately heat-treated and then subjected to a surface treatment such as shot peening or pickling as necessary.

  Steel plates include hot rolled steel plates and cold rolled steel plates. A hot-rolled steel sheet can be obtained by hot rolling the slab, and a cold-rolled steel sheet can be obtained by cold rolling the hot-rolled steel sheet.

  Steel having the chemical composition shown in Table 1 was melted using a vacuum induction melting furnace to obtain a 50 kg ingot having a diameter of 144 mm. Reference signs A to M are steels of the present invention, and reference numerals 1 to 22 are comparative steels. For the steels of codes A to M and the steels of codes 15 to 20, Nd was added immediately before casting after sufficiently deoxidizing with C, Si, Mn and Al. Nd was added to steel No. 21 from the start of dissolution, and Nd was added to steel No. 22 after only deoxidation with carbon (C).

  These ingots were hot forged and hot rolled into 20 mm thick plates. Subsequently, after holding at a temperature of 1050 ° C. for 1 hour, it was air cooled (AC). Furthermore, the tempering process which hold | maintains at 760 degreeC-780 degreeC for 3 hours and air-cools (AC) was performed. Test specimens were collected from these plates so that the length direction of the test specimen was the rolling direction, and the creep rupture test, creep fatigue test, and Nd inclusion distribution were investigated under the following conditions.

(1) Creep rupture test Specimen: Diameter 6.0mm, Gap distance: 30mm, Test temperature: 600 ℃, Load stress: 160Mpa,
Test item: Break time (h).

(2) Creep fatigue test specimen: diameter 10mm, distance between gauge points: 25mm, test temperature: 600 ℃ (in air)
Strain waveform: CP waveform, total strain range Δε _t = 0.5%,
Strain rate: tension side; 0.01% / sec, compression side; 0.8% / sec
Test item: Creep fatigue life N _f (cycle)

(3) Distribution investigation of Nd inclusions Specimens were cut out from the raw materials as they were hot-worked, polished and corroded, and then extracted replicas were produced by C deposition, followed by electron microscope observation at 2000 times and EDX analysis ( Inclusions are identified by Energy Dispersive X-Ray Analysis), the number of Nd inclusions (pieces / mm ² ) is quantified, and the value is multiplied by 3/2 to obtain the precipitation density (pieces / mm ³ ). Converted into In addition, it observed by 10 visual fields and made the average value the precipitation density.

  Table 2 shows the creep rupture test results, creep fatigue test results, and Nd inclusion distribution survey results of the steels of the present invention and comparative steels.

  As shown in Table 2, compared with the ASME P91 steel with the reference numeral 1, the ASME P92 steel with the reference numerals 2 and 6 has a long creep rupture time and clearly has a high creep strength. However, the creep fatigue life is almost the same. That is, ASME P92 steel shows no significant improvement in creep fatigue life.

  Steels 3 to 5 containing trace amounts of Cu, Ni or Co have the same level of creep strength as the steel 2 but have a clear decrease in creep fatigue life.

  When the effects of Mo on creep rupture strength and creep fatigue strength were investigated for steels of Nos. 2, 6, 7, 8 and 9, the steels of Nos. 7 and 8 having a low Mo content were used for No. 2 and No. 6 steels. Compared to steel, creep fatigue strength is inferior. Moreover, the steel of the code | symbol 9 with much Mo content also has inferior creep fatigue strength.

  In steels of code 10 to code 13 containing a small amount of La, Ce, Ca, or Mg, the creep strength and creep fatigue strength are the same as those of the steel of code 2, and no improvement in properties is observed.

  On the other hand, the steels from code A to code M that satisfy the conditions specified in the present invention have the same creep rupture time as steel of code 2, but the creep fatigue life is remarkably improved.

  The steel with the code 14 whose Nd content is below the range defined by the present invention is insufficient in improving the creep fatigue strength. On the other hand, the steel of the code | symbol 15 which contained Nd excessively has low creep strength.

  Steels with a code of 16 to 18 containing a small amount of Nd and the austenite forming elements Cu, Ni or Co have the same creep strength as that of the steel of code 2, and the creep fatigue strength is slightly higher than that of the steel of code 2. It has been improved. However, the creep fatigue strength is clearly inferior when compared to steels of codes A to M that do not contain Cu, Ni, or Co or have a low content thereof.

  The steels of Nos. 19 and 20 that contain Nd within the range specified in the present invention, but Mo is outside the range specified in the present invention, have a creep fatigue life as compared with those not containing Nd. high. However, the creep fatigue strength is clearly inferior when compared with steels with symbols A to M whose Mo content is within the range defined by the present invention.

  The steels of reference numerals 21 and 22 have chemical compositions within the range specified by the present invention, but the distribution density of Nd inclusions does not satisfy the range specified by the present invention. In these, since Nd is added without sufficiently deoxidizing, a very coarse Nd oxide is formed, the density of Nd inclusions is remarkably lowered, and the creep fatigue life is low.

The steel of the present invention is a heat-resistant steel excellent in long-term creep strength and creep fatigue strength at a high temperature of 600 to 650 ° C. This steel exhibits an excellent effect as a steel tube for heat exchange, a steel plate for a pressure vessel, and a turbine material used in fields such as thermal power generation, nuclear power generation, and chemical industry, and is extremely useful industrially.

Claims (5)

In mass%, C: 0.01 to 0.13%, Si: 0.15 to 0.50%, Mn: 0.2 to 0.5%, P: 0.02% or less, S: 0.005% or less, Cr: more than 8.0% and less than 12.0%, Mo: 0.1 to 1.5%, W: 1.0 to 3.0%, V: 0.1 to 0.5%, Nb: 0.02 to 0.10%, sol.Al: 0.015% or less, N: 0.005 to 0.070%, Nd: 0.005 to 0.050%, B: A steel containing 0.002 to 0.015%, the balance being Fe and impurities, of which Ni is less than 0.3%, Co is less than 0.3%, and Cu is less than 0.1%, and Nd inclusions A ferritic heat resistant steel including Nd inclusions having a density of 10,000 / mm ³ or more.   2. The ferrite according to claim 1, comprising, in place of a part of Fe, at least one of Ta: 0.04% or less, Hf: 0.04% or less, and Ti: 0.04% or less in mass%. Heat resistant steel.   3. The ferrite according to claim 1, wherein one or two of Ca: 0.005% or less and Mg: 0.005% or less are contained in mass% instead of part of Fe. Heat resistant steel.   The ferritic heat resistant steel according to any one of claims 1 to 3, wherein the total amount of rare earth elements excluding Nd in the impurities is 0.04 mass% or less.   The creep fatigue life of the CP waveform at 600 ° C at a strain rate of 0.01% / sec on the tension side and 0.8% / sec on the compression side and a total strain range of 0.5% is 5000 cycles or more. The ferritic heat resistant steel according to any one of claims 1 to 4, characterized in that:

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