WO2006109664A1 - Ferritic heat-resistant steel - Google Patents

Abstract

Disclosed is a heat-resistant steel which is excellent in high-temperature long-term creep strength and creep-fatigue strength. Specifically disclosed is a heat-resistant steel having a composition consisting of, in mass%, 0.01-0.13% of C, 0.15-0.50% of Si, 0.2-0.5% of Mn, not more than 0.02% of P, not more than 0.005% of S, more than 8.0% but less than 12.0% of Cr, 0.1-1.5% of Mo, 1.0-3.0% of W, 0.1-0.5% of V, 0.02-0.10% of Nb, not more than 0.015% of sol. Al, 0.005-0.070% of N, 0.005-0.050% of Nd, 0.002-0.015% of B, and the balance of Fe and impurities. As some of the impurities, less than 0.3% of Ni, less than 0.3% of Co and less than 0.1% of Cu are contained in the heat-resistant steel. This ferritic heat-resistant steel contains Nd inclusions at a density of not less than 10,000 inclusions/mm3. This steel may further contain one or more elements selected from Ta, Hf, Ti, Ca and Mg in addition to the above-described components.

Description

 Specification

 Ferritic heat resistant steel

 Technical field

 [0001] The present invention relates to a ferritic heat resistant steel. More specifically, it 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.

 Background art

 [0002] 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.

 [0003] 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 fluorescent steel is characterized by excellent thermal fatigue resistance and low cost compared to austenitic stainless steel because of its high thermal conductivity and low coefficient of thermal expansion. In addition, there are a number of advantages, such as less descaling and less stress corrosion cracking.

 [0004] From the late 1980s to the 1990s, ASME P 91 steel was put to 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.

 [0005] Currently, reduction of CO emissions is required for environmental protection. To that end

 2

 Even in electric boilers, higher temperature and pressure are required. The AS ME P92 steel, which is currently in practical use, must be used as a thicker part in order to be used at higher temperatures, for example, about 630 ° C.

[0006] In a thermal power plant, since starting and stopping are frequently performed, especially in a thick member, the cleave fatigue strength is important. The ASME P92 steel has significantly higher creep strength than the ASME P91 steel, but the creep fatigue strength is equivalent. Higher temperature and height In order to put the pressure boiler into practical use, it is essential to improve the creep fatigue strength of ASME P92 steel.

 [0007] Patent Documents 1 and 2 disclose inventions of heat-resisting steels containing 8 to 14% Cr. Patent Document 3 discloses an invention of a heat resistant steel containing 8 to 13% Cr. However, the inventions disclosed in these documents have not been made for the purpose of improving the creep fatigue strength of heat-resistant steel. The steels of these inventions are not steels that utilize the effective action of Nd inclusions described later.

 Patent Document 1: JP 2001-192781 A

 Patent Document 2: JP 2002-224798 A

 Patent Document 3: Japanese Patent Laid-Open No. 2002-235154

 Disclosure of the invention

 Problems to be solved by the invention

[0008] An object of the present invention is to provide a ferritic heat resistant steel that is excellent in high-temperature long-term creep strength and excellent in creep fatigue strength.

 Means for solving the problem

FIG. 1 is a diagram showing an example of a strain waveform in a creep fatigue test. (A) in the figure shows the PP waveform, which is a waveform that applies strain at high speed so that creep strain does not occur on both the tension side and the compression side. (B) shows the CP waveform. This is a waveform that loads the strain with the tensile side at a low speed and the compression side at a high speed to introduce tensile creep strain.

 [0010] 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 in boilers, nuclear power generation facilities, and chemical industrial facilities is evaluated by conducting creep fatigue tests in the entire strain range of 0.4 to 1.5%.

[0011] Since the equipment such as the boiler is used for a long time under high temperature and pressure, each member is subjected to creep strain and receives a CP-type load. In general, the actual machine is constructed to reduce the generated strain in order to ensure the creep fatigue life of members used under high temperature and pressure. Therefore, the high Cr ferritic steels used in these facilities are It is necessary to ensure the creep fatigue life in the total strain range of the creep fatigue test, that is, the total strain of about 0.5%, which is the low strain region of 0.4 to 1.5%.

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, at 600 ° C, the CP waveform shown in Fig. 1 has a total strain range of 0.5. / ₀ was subjected to a creep fatigue test, life revealed no approximately 3000 cycles much different either. In other words, although the creep strength of P92 steel was higher than that of P91 steel, the creep fatigue strength was not improved. From these results, it is considered that P92 steel contains some cause for the creep fatigue strength not improving, in other words, the cause for the decrease in creep fatigue strength. Therefore, the present inventors conducted extensive studies to improve the creep fatigue strength of P92 steel.

 [0013] First, the following (a) was examined for the effect of a small amount of δ ferrite due to the prejudice of alloy elements, which is considered to be the cause of the creep fatigue strength not improving.

 [0014] (a) Investigation of the effect of δ ferrite

 In addition to the components contained in the conventional 9Cr ferritic heat-resisting steel, Ρ92 steel is rich in ferrite-forming elements (Mo, W, Nb, V, etc.). Therefore, a very small amount of δ ferrite may remain at the grain boundary. In order to completely eliminate δ-ferrite, materials with a small amount of Cu, N and Co (these are austenite forming elements) were prepared in Ρ92 steel, 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, which was a tendency to decrease compared to P92 steel.

 [0015] From the above results, the creep fatigue strength of the P92 steel is not improved because the creep fatigue strength decreases when an excessive austenite forming element is contained rather than δ ferrite. It was revealed.

 Next, for the purpose of clarifying the contribution of grain boundaries to the creep fatigue strength, the following adjustment (b) was performed.

[0017] (b) Adjustment of the effect of prior austenite grain size on creep fatigue strength of P92 steel P92 steel was processed at normalizing temperatures of 1050 ° C and 1200 ° C, and the prior austenite grain size Was changed to about 25 zm and 125 zm. Next, after tempering to a tensile strength of about 710 MPa by tempering, a creep fatigue test was conducted. The test temperature was 600 ° C and the total strain range was 0.5%.

 [0018] As a result of the above test, the life of a coarse-grained steel with a grain size of 125 x m was about 2300 cycles, while the life at a normal grain size of 25 zm was about 3000 cycles. For this reason, it became clear that in the case of coarse-grained steel, the creep fatigue life is reduced even if the strength is equivalent to that of fine-grained steel.

 [0019] (c) Elucidation of the reason why fine-grained steel has higher creep fatigue strength

 As can be seen from the test results in (b) above, the reason why the fine-grained steel has higher creep fatigue strength was considered.

 [0020] —In general, it is said that the creep characteristics at high temperatures 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 the sample with a particle size of 25 μΐη is about 6000 hours, whereas the break time of the sample with a particle size of 125 μm is about 9000 hours. The creep strength is higher in the case of grains. 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.

 [0021] In fine-grained steel, the area of grain boundaries increases. As the grain boundary area increases, segregation of impurity elements such as P, S, As, and Sn, especially S, may be suppressed. Therefore, we considered the segregation of S at the grain boundaries.

 [0022] 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 laboratory production, it is difficult to eliminate segregation by reducing S in the usual melting method because S is unavoidable from alloying elements.

 In general, temper embrittlement is known as a phenomenon caused by segregation such as S. Tempering brittleness occurs when martensite is tempered in a certain temperature range of around 600 ° C. It is known that trace amounts of Mo are effective in reducing this.

[0024] If the creep fatigue phenomenon correlates with the segregation of S, it is considered that the Mo content and the creep fatigue properties have some correlation. Therefore, the Mo content is 0.01%, 0.07%, 0.13 The creep fatigue strength (test temperature is 600 ° C and the total strain range is 0.5%) when the ratio was changed to%, 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. When the Mo content was small (0.01% and 0.07%), the creep fatigue strength was about 2000 cycles. Decreased. From this, it became clear that Mo has made a certain contribution to the taper 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.

 [0025] 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 the S trapped as MnS becomes free and segregates at the grain boundary during the creep fatigue test at high temperature, it is considered that this S force creep fatigue property is adversely affected.

 [0026] (d) S fixation

 If the prayer of S that has become free as described above adversely affects creep fatigue properties, it is possible to increase the creep fatigue strength by including an element that traps S more firmly in addition to Mn. it is conceivable that.

 [0027] Thus, the effect of Ca, Mg, Nd, La, and Ce on the creep fatigue strength that may form sulfides was examined.

 [0028] As a result, it has been clarified that when 0.025% of Nd is contained, Nd inclusions fix S in addition to MnS. The Nd inclusion means “Nd oxide” and “composite inclusion of Nd oxide and sulfide”. “Nd oxide and sulfide complex inclusions” fix S directly. On the other hand, “Nd oxide” also indirectly fixes S by segregating S around it. As an example of Nd inclusions, Fig. 3 shows "composite inclusions of Nd oxide and sulfide" observed in Nd-containing steel.

 [0029] As described above, a steel containing Nd that directly and indirectly fixes S 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 was revealed that the fatigue life improved dramatically to about 7000 cycles.

[0030] Further, the creep fatigue life (test temperature is 600 ° C, total strain range 0.5%) of steel containing Ca, Mg ヽ La and Ce alone is a force of about 3000 to 4000 cycles. It was revealed that the steel containing the component together with Nd has a life of 6000 to 7000 cycles, and the creep fatigue life is dramatically improved.

[0031] (e) Addition of Nd and Cu, M or Co

 As described in (a) above, creep fatigue strength tended to decrease in steels containing trace amounts of the austenite-forming elements Cu, M or Co. In order to clarify this phenomenon further, we evaluated the fatigue life of a steel containing a trace amount of Cu, M or Co in a steel containing a trace amount of Nd.

[0032] As a result, the creep fatigue life of steel containing a small amount of Cu, N or Co along with Nd is about 4000 cycles, and the creep fatigue properties are improved compared to steel containing no Nd. However, the creep fatigue life was found to be significantly inferior to that of the steel containing Nd alone.

 [0033] From the above examination, the following conclusions (1) to (4) can be obtained.

 (1) Mo of 0.1% or more contributes to creep fatigue properties.

 [0034] (2) Most of S is a force S fixed as MnS, and part of S becomes free during high-temperature fatigue tests and segregates at the grain boundaries, reducing the creep fatigue strength.

[0035] (3) By including Nd, fixing S as an oxide of Nd or a composite inclusion of oxide and sulfide, and fixing a part as MnS, the creep fatigue strength is greatly increased. It will be improved. The effect is remarkable when the density of Nd inclusions is 10,000 / mm ³ or more. The “Nd inclusion” is a general term for the above “Nd oxide” and “combined inclusion of Nd oxide and sulfide”.

 [0036] (4) Cu, Ni, and Co, which are austenite forming elements, decrease the creep fatigue strength. This tendency is also observed in steels containing a small amount of Nd. This phenomenon occurs because Cu, Ni, and Co promote the phenomenon that S, which is fixed as MnS, becomes free during the creep fatigue test.

 [0037] The present invention made based on the above examination results is summarized in the following heat-resistant steel. Hereinafter, “%” regarding the component content means “% by mass”.

[0038] (1) C: 0.01 to 0.13%, Si: 0.15 to 0.50%, Μη: 0.2 to 0.5%, Ρ: 0.02% or less, S: 0.005% or less, Cr: Over 8.0%, 12.0% Less than, Μο: 0.1 ~ 1-5%, W: 1.0 ~ 3.0%, V: 0.1 ~ 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: 0.002 to 0.015%, with the balance being Fe and impurities, Ferrite with Ni power of less than 0.3% of impurities, Co of less than 0.3%, and Cu of less than 0.1%, including Nd inclusions, and the density of the Nd inclusions is 10000 / mm ³ or more Heat resistant steel.

 [0039] (2) Instead of a part of Fe, containing at least one of Ta: 0.04% or less, Hf: 0.04% or less, and Ti: 0.04% or less Ferritic heat resistant steel.

 [0040] (3) The above (1) or (2) characterized by containing one or two of Ca: 0.005% or less and Mg: 0.005% or less instead of a part of Fe ) Ferritic heat resistant steel.

 [0041] (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 the impurities is 0.04% or less.

 [0042] (5) Creep in CP waveform at 600 ° C under conditions of 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% The ferritic heat resistant steel according to any one of the above (1) to (4), characterized by having a fatigue life of 5000 cy or more.

 Brief Description of Drawings

 [0043] FIG. 1 is a diagram showing 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] FIG. 3 is a view showing “composite inclusions of Nd oxide and sulfide” observed in Nd-containing steel.

 Best Mode for Carrying Out the Invention

[0044] 1. Chemical composition

 First, the effects of the components constituting the heat-resistant steel of the present invention and the reasons for limiting the content will be described.

 [0045] C: 0.01 to 0.13%

C stabilizes the structure of steel as an austenite stabilizing element. Also, MC carbide or M (C, N) carbonitride is formed, contributing to the improvement of creep strength. M in MC and M (C, N) is an alloying element. However, if the content of C is less than 0.01%, the above effects cannot be obtained sufficiently, and the amount of δ ferrite may increase and the strength may be lowered. On the other hand, the C content is If it exceeds 0.13%, not only the workability and weldability will deteriorate, but also the agglomeration and coarsening of the carbide will occur from the beginning of use, and the creep strength will be reduced for a long time. Therefore, the C content must be limited to 0.13% or less. More desirable lower and upper limits are 0.08% and 0.11%, respectively.

 [0046] Si: 0.15-0.50%

 Si is contained as a deoxidizing element in 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 decreases significantly, so the upper limit is made 0.50%. Especially when steam oxidation resistance is important, the lower limit of Si content is preferably 0.25%.

 [0047] Μη: 0 · 2 ~ 0.5%

 Μη contributes as a deoxidizing element and an austenite stabilizing element. Also, MnS is formed and S is fixed. 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 0.2-0.5%. A more preferred lower limit is 0.3%.

 [0048] P: 0.02% or less, S: 0.005% or less

 Impurities P and S deteriorate the hot workability, weldability, creep strength, and creep fatigue strength of steel, so the lower the content, the better. However, since significant cleaning of the steel results in a significant cost increase, the upper limit is set to 0.02% for P and 0.005% for S.

 [0049] 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. In addition, Cr forms carbides and improves creep strength. In order to obtain these effects, the content power needs to exceed 0.0%. 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%.

 [0050] Mo: 0.1.5%

Mo contributes to the improvement of creep strength as a solid solution strengthening element. Furthermore, as a result of detailed examination of the correlation between the Mo content and the taper fatigue strength, 0.1% or more of Mo is considered to be the creep fatigue property. It has been found that this contributes to the improvement of the creep strength and that when the content exceeds 1.5%, the creep strength decreases for a long time. Therefore, the appropriate Mo content is 0.1-1.5%. More preferred lower and upper limits are 0.3% and 0.5, respectively. / ₀ .

 [0051] W: 1.0-3.0%

 W contributes to the improvement of creep strength as a solid solution strengthening element. In addition, a part of it dissolves in Cr carbide, which suppresses the agglomeration and coarsening of the carbide and contributes to the creep strength. However, those effects are small at less than 1.0%. On the other hand, if the Mo content strength exceeds ˜3.0%, the formation of δ ferrite is promoted and the creep strength is reduced. Therefore, the appropriate range of W content is 1.0-3.0%. A more preferable lower limit is an amount exceeding 1.5%, and a more preferable upper limit is 2.0%.

 [0052] V: 0.1-0.5%

 V contributes to the improvement of creep strength by solid solution strengthening action and by forming fine carbonitrides. In order to exert its effect, its content needs to be 0.1% or more. On the other hand, if the V content force exceeds SO.5%, the formation of δ ferrite is promoted and the creep strength is reduced, so 0.5% should be the upper limit. More preferred lower and upper limits are 0.15% and 0.25%, respectively.

 [0053] Nb: 0.02-0.10%

 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.

 [0054] A1: 0.015% or less

 A1 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%.

 [0055] N: 0.005-0.070%

N, like C, is effective as an austenite stabilizing element. N is nitride or Increases the high temperature strength of the steel by precipitating carbonitrides. 0 to bring out the effect.

It is necessary to contain 005% or more. On the other hand, if the content of N is excessive, the creep strength decreases due to the coarsening of nitrides and carbonitrides, which only causes blowholes during melting and causes welding defects. Therefore, the upper limit of N content is 0.

Should be 070%. A more preferable lower limit of the N content is 0.020%.

[0056] 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%. More preferred, the upper limit of the content is 0.040%.

[0057] B: 0.002 to 0.015%

 B enhances hardenability and plays an important role in ensuring high temperature strength. The effect is 0.002

When it exceeds 0.015%, weldability and long-term creep strength are lowered.

 [0058] 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 unavoidable from mixing with the melted raw material. Therefore, in the present invention, Ni and Co are suppressed to less than 0.3% and Cu is suppressed to less than 0.1%, respectively. Within the above range, the adverse effect on creep fatigue strength is small.

[0059] Group 1 ingredients: Ta, H «¾ and Ti

 These are components to be added one or more as required. Appropriate contents in each case are as follows.

[0060] Ta: 0.04% or less, Hf: 0.04% or less, Ti: 0.04% or less

Ta, H 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 0.0 4% is recommended.

[0061] Group 2 ingredients: Ca and Mg

 These are also components that are added in one or two as required. Appropriate contents in each case of addition are as follows.

[0062] Ca: 0.005% or less, Mg: 0.005% or less

 All of these elements 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. The effect becomes significant at 0.0005% or more, so the lower limit of the content is preferably 0.0005%. However, if the content exceeds SO.005%, the creep strength decreases, so 0.005% should be the upper limit.

[0063] 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, there is no significant effect on the properties such as creep strength and creep ductility, so up to 0.04% is allowed.

[0064] 2. Nd inclusions

One of the features of the steel of the present invention is that Nd inclusions are contained at a density of 10000 / mm ³ or more.

 As described above, the Nd inclusions observed in the steel of the present invention are “Nd oxide” and “Nd oxide and sulfide composite inclusion”. Specifically, Nd 0, Nd 0 S, Nd 0 SO

 2 3 2 2 4 2 2 4

Nd O S, etc.

 twenty two

[0066] The diameter of the Nd inclusion is about 0.3 μπ! Nl inclusions are usually observed in steels containing a small amount of Nd. However, in steels rich in Co, Ni and Cu, MnS increases and Nd inclusions decrease significantly. And when the density of Nd inclusions is less than 10,000 Zm m ³ , the improvement of creep fatigue strength is not recognized. Therefore, the density of Nd inclusions must be 10000 / mm ³ or more.

 [0067] 3. Manufacturing method

The steel according to the present invention can be produced by a production facility 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 scouring in a furnace such as an electric furnace or converter, and by deoxidation and inclusion of alloy elements. In particular, when strict component adjustment is required, the molten steel should be subjected to an appropriate treatment such as vacuum treatment before adding the alloy elements.

[0068] A method for introducing 10,000 Nd inclusions of Zmm ³ or more into steel is as follows.

 In other words, sufficient deoxidation is carried out with C, Si, Mn, A1, etc. in advance from ironmaking to steelmaking. This is because when the oxygen content in the molten steel is high, the yield of Nd-added metal deteriorates. After this, in the case of the ingot method, Nd inclusions are produced by adjusting the composition other than Nd before pouring into the ingot and adding Nd just before pouring. In the case of continuous forging, Nd inclusions are produced by adjusting the composition other than Nd before introducing molten steel to the tundish and then adding Nd to the tundish. By final adjustment of Nd alone, an appropriate amount of Nd inclusions can be generated. Forged slabs, billets or ingots are further processed into steel pipes and steel plates.

 [0069] When producing a seamless steel pipe, for example, a billet is extruded, rolled with an inclined roll-type piercer, or a large-diameter forged pipe is formed by an Ernolt pipe method. Just do it. In the manufacture of steel pipes, the dimensions can be adjusted by cold working as necessary. The steel pipe that has been produced is appropriately heat-treated and then subjected to surface treatment such as shot peening and pickling as necessary.

 [0070] As the steel sheet, there are a hot-rolled steel sheet and a cold-rolled steel sheet. A hot-rolled steel sheet can be obtained by hot rolling a slab, and a cold-rolled steel sheet can be obtained by cold rolling this hot-rolled steel sheet.

 [0071] 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 steels with codes A to M and steels with codes 15 to 20, Nd was added immediately before pitting after thorough deoxidation with C, Si, Mn and A1. Nd was added to steel No. 21 from the start of melting, and Nd was added to steel No. 22 after only deoxidation with carbon (C).

[0072] These ingots were hot forged and hot-rolled into 20 mm thick plates. Then 1050 ° C After maintaining at the temperature of 1 hour, it was air-cooled (AC). Furthermore, a tempering treatment was carried out by holding at 760 ° C to 780 ° C for 3 hours and air cooling (AC). Test specimens were taken 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 adjusted under the following conditions.

[0073] (1) Creep rupture test

 Test piece: Diameter 6.0 mm, distance between gauge points: 30 mm, test temperature: 600 ° C, load stress: 160 MPa, test item: breaking time (h).

[0074] (2) Creep fatigue test

 Test piece: Diameter 10 mm, distance between gauge points: 25 mm, test temperature: 600 ° C (in air) Strain waveform: CP waveform, total strain range Δ ε = 0.5%,

 t

 Strain rate: tension side; 0.01% / sec, compression side; 0.8% / sec

 Test item: Creep fatigue life N (cycle)

[0075] (3) Investigation of distribution of Nd inclusions

Material strength as hot-worked After cutting out, polishing, and corroding, an extraction replica was prepared by C vapor deposition, and observed with an electron microscope at a magnification of 2000, and EDX analysis (Energy Di spersive X-Ray Analysis) Thus, inclusions were identified, the number of Nd inclusions (pieces / mm ² ) was quantified, and the value was multiplied by 3/2 to convert it to a precipitation density (pieces / mm ³ ). The observation was performed in 10 fields, and the average value was defined as the precipitation density.

[0076] 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 the comparative steel.

[0077] [Table 1] table 1

Chemical composition (mass%, balance: Fe and impurities)

[0078] [Table 2]

 Table 2

[0079] As shown in Table 2, compared with ASME P91 steel with code 1, ASME P92 steel with code 2 and code 6 has a clearly higher creep strength with a longer creep rupture time. However, the creep fatigue life is almost the same. That is, ASME P92 steel shows no significant improvement in creep fatigue life.

 [0080] The steels with codes 3 to 5 containing trace amounts of Cu, M or Co have a creep strength of

Although it is the same level as steel No. 2, there is a clear decrease in creep fatigue life.

[0081] Effects on creep rupture strength and creep fatigue strength of steels with symbols 2, 6, 7, 8, and 9 As a result of investigating the effect of Mo, steels of code 7 and code 8 with low Mo content have inferior creep fatigue strength compared to steels of code 2 and code 6. In addition, the steel with number 9 having a high Mo content also has poor creep fatigue strength.

[0082] The steels of code 10 to code 13 containing a small amount of La, Ce, Ca or Mg have the same level of tapping strength and creep fatigue strength as the steel of code 2, and the improvement in the properties was recognized. I can't help.

 [0083] On the other hand, the steels from code A to code M that satisfy the conditions specified in the present invention have significantly improved force S and creep fatigue life, which have the same creep rupture time as steel of code 2.

 [0084] Steel No. 14 whose Nd content falls below the range defined by the present invention is insufficient in improving the creep fatigue strength. On the other hand, steel No. 15 containing excessive Nd has low creep strength.

 [0085] Steels with codes 16 to 18 containing trace amounts of Nd and the austenite-forming elements Cu, N, and Co have the same creep strength as the steel with code 2 and the creep fatigue strength is also code 2 Compared to other steels, there is a slight improvement. However, the creep fatigue strength is clearly inferior when compared to steels with symbols A to M that do not contain Cu, M, or Co or have low contents thereof.

 [0086] The force containing Nd within the range specified by the present invention The steel of No. 19 and No. 20 where Mo is outside the range specified by the present invention, creep fatigue compared with those containing no Nd Life is 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 specified in the present invention.

 [0087] The steels of reference numerals 21 and 22 have a chemical composition within the range defined by the present invention, but the distribution density of Nd inclusions does not satisfy the range defined by the present invention. In these, since Nd was added without sufficient deoxidation, a very coarse Nd oxide was formed, the density of Nd inclusions was significantly reduced, and the creep fatigue life was low.

 Industrial applicability

[0088] 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 has excellent effects as a steel tube for heat exchange, steel plates for pressure vessels, and materials for turbines used in fields such as thermal power generation, nuclear power generation and chemical industry. Demonstrate and extremely useful in industry.

Claims

The scope of the claims % By mass, C: 0.01-0.13%, Si: 0.15-0.50%, Mn: 0.2-0.5%, P: 0.02% or less, S: 0.005% or less, Cr: more than 8.0% and less than 12.0%, Μο: 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: 0.002 to 0.015%, the balance is Fe and impurity power, of which Ni is less than 0.3%, Co is less than 0.3%, Cu is less than 0.1%, Nd intervening Ferritic heat-resisting steel with a density of 10000 inclusions / 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 or 2, which contains one or two of Ca: 0.005% or less and Mg: 0.005% or less in mass% instead of a part of Fe. Heat resistant steel.  4. The refractory heat resistant steel according to claim 1, wherein the total amount of rare earth elements excluding Nd in the impurities is 0.04 mass% or less.  When the strain rate is 0.01% Zsec on the tension side and 0.8% / sec on the compression side, and the creep fatigue life of the CP waveform at 600 ° C at 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, wherein the ferritic heat-resistant steel is provided.