KR20070103081A - 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/mm^3. 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

Ferritic Heat Resistant Steel {FERRITIC HEAT-RESISTANT STEEL}

The present invention relates to a ferritic heat resistant steel. More particularly, the present invention relates to a ferritic heat resistant steel having excellent high temperature long time creep strength and creep fatigue strength. The heat resistant steel of the present invention is suitable for heat exchange steel pipes, steel plates for pressure vessels, turbine materials and the like used in high temperature and high pressure environments such as boilers, nuclear power plants and chemical industrial facilities.

As heat resistant steel used in high temperature, high pressure environments, such as a boiler, nuclear power plant, and a chemical industrial facility, high temperature creep strength, creep fatigue strength, corrosion resistance, oxidation resistance, etc. are generally calculated | required.

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, the high Cr ferritic steel has a high thermal conductivity and a low thermal expansion rate, and thus has excellent heat fatigue fatigue characteristics and low price as compared with the austenitic stainless steel. In addition, there are numerous advantages, such as difficulty in scale peeling and no stress corrosion cracking.

From the late 1980s to the 1990s, ASME P91 steel has been put into practical use as a high-strength ferritic heat-resistant steel, and has been used in supercritical pressure boilers having a steam temperature of 566 ° C or higher. In recent years, steel of ASME P92, which has increased creep strength, has been put into practical use, and an ultra-supercritical pressure boiler having 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, the high temperature high pressure is also demanded also about a thermal power boiler. The strength of ASME P92, which is currently in practical use, in a higher temperature range, for example about 630 ° C., and in order to use it, a thick member must be used.

In a thermal power plant, since start and stop are frequently performed, creep fatigue strength becomes important especially as a thick member. The steel of the ASME P92 significantly increased the creep strength compared with the steel of the ASME P91, but the creep fatigue strength was the same. In order to make practical use of a high temperature high pressure boiler further, improvement of the creep fatigue strength is indispensable in the steel of ASME P92.

In patent documents 1 and 2, invention of the heat-resistant copper containing 8-14% Cr is disclosed. In addition, Patent Document 3 discloses an invention of a heat resistant steel containing 8 to 13% of Cr. However, the invention disclosed in this document is not made for the purpose of improving the creep fatigue strength of heat resistant steel. Although the steel of this invention may contain Nd, it is not steel which utilized the effective effect | action of the Nd inclusion mentioned later.

[Patent Document 1: Japanese Patent Laid-Open No. 2001-192781]

[Patent Document 2: Japanese Patent Laid-Open No. 2002-224798]

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

[Problems that the invention tries to solve]

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

[Means for solving the problem]

1 is a diagram illustrating an example of a strain waveform of a creep fatigue test. As shown in (a) of the same drawing, a PP waveform is a waveform which was deformed at high speed so that creep deformation did not occur on both the tension side and the compression side. Shown in (b) are CP waveforms. This is a waveform in which the tension side is deformed at low speed and the compression side at high speed in order to introduce tensile creep deformation.

When the life under the PP waveform is compared with the life under the CP waveform, the life under the CP waveform that is creep damaged is shorter. Generally, the service life of the heat resistant steel used in the high temperature, high pressure environment of a boiler, a nuclear power plant, and a chemical industry is evaluated by carrying out a creep fatigue test in 0.4-1.5% of total deformation range.

In order to use a facility such as a boiler for a long time under high temperature and high pressure, creep deformation occurs in each member, and a CP-type load is applied. Moreover, in order to ensure the creep fatigue life of the member used under high temperature, high pressure, in a real apparatus, the structure which reduces a generation distortion is selected normally. Therefore, in the high Cr ferritic steel used in such a facility, the creep fatigue life under the CP waveform is about 0.5% of total deformation of the creep fatigue test, that is, the area of low deformation within 0.4 to 1.5%. It is necessary to secure.

The 100,000-hour creep strength at 600 ° C of the steels of ASME P91 and P92 is about 98 MPa and 128 MPa, respectively, and the P92 steel is high strength. However, at 600 ° C., a creep fatigue test of 0.5% of the total strain range was conducted with the CP waveform of FIG. 1, and all of the service life was found to be not significantly different from about 3000 cycles. In other words, although the creep strength of the P92 steel was improved than that of the P91 steel, the creep fatigue strength was not improved. From this result, it is thought that P92 steel has some cause which creep fatigue strength does not improve, in other words, the cause which a creep fatigue strength falls. Here, the inventors studied diligently so as to improve the creep fatigue strength of the P92 steel.

First, the following (a) was examined about the influence of the trace amount (delta) ferrite resulting from segregation of the alloying element which is considered as a cause which does not improve creep fatigue strength.

(a) Investigation of the influence of δ ferrite

In addition to the component contained in the conventional 9 Cr ferritic heat resistant steel, P92 steel contains many ferrite formation elements (Mo, W, Nb, V, etc.). Therefore, there is a possibility that an extremely small amount of δ ferrite remains at the grain boundary. For the purpose of completely erasing δ ferrite, a material containing a small amount of Cu, Ni, or Co (these are austenite forming elements) in P92 steel was prepared, and creep fatigue strengths were compared. The test temperature was 600 degreeC and the total deformation range was 0.5%. As a result, the tendency of the service life to fall rather than about 1600-2100 cycle and P92 steel was observed.

From the above results, it was found that the creep fatigue strength of the P92 steel did not improve due to δ ferrite, but rather, when the excessive austenite forming element was contained, the creep fatigue strength was lowered.

Next, the following investigation (b) was carried out for the purpose of clarifying the contribution to the grain boundary to the creep fatigue strength.

(b) Investigation of the Effect of Old Austenitic Particle Size on the Creep Fatigue Strength of P92 Steel

The annealing temperature was treated with steel of P92 at 1050 ° C and 1200 ° C to change the former austenite grain size to about 25 µm and 125 µm. Next, after tempering by tempering so that tensile strength might be about 710 Mpa, the creep fatigue test was done. The test temperature was 600 degreeC and the total deformation range was 0.5%.

As a result of the above test, the life of the granulated steel having a particle size of 125 m was about 2300 cycles while the life at a normal particle size of 25 m was about 3000 cycles. As a result, in the case of granulated steel, it has been found that the creep fatigue life decreases even if the strength is equivalent to that of the fine grain steel.

(c) Elucidation of why the fine-grained steel has a high creep fatigue strength.

As seen from the test results of the above (b), the reason why the fine-grained steel had a high creep fatigue strength was considered.

Generally, the creep characteristic at high temperature tends to be excellent in the case of granulation. Here, the creep strength in 600 degreeC and 160 Mpa of the sample used by the test of (b) was investigated. As a result, the breaking time of the sample having a particle size of 25 μm is about 6000 hours, and the breaking time of the sample having a particle size of 125 μm is about 9000 hours. As described above, creep strength is high in the case of granulation. From this result, it turned out that the improvement of the creep fatigue strength in a fine grained steel cannot be demonstrated with tensile strength and creep strength.

In the fine grained steel, the area of the grain boundary increases. When the area of the grain boundary increases, it is considered that segregation of impurity elements such as P, S, As and Sn, in particular S, is suppressed. Here, the segregation of S at the grain boundary was considered.

Usually, a 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 degree lower than 0.001%. Even in the manufacture of a laboratory, since incorporation of S from an alloying element cannot be avoided, segregation is difficult to be eliminated by reduction of S in a conventional solvent method.

Generally, temper brittleness is known in the phenomenon which causes segregation, such as S. Tempering brittleness occurs when the martensite is tempered at a certain temperature range around 600 ° C., but a small amount of Mo is known to be effective to reduce it.

If creep fatigue phenomena correlate with segregation of S, it is thought that Mo content and creep fatigue characteristics have some correlation. Here, 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 Mo content was 0.13% and 0.33%, the lifetime was about 3000 cycles, but when Mo content was small (0.01% and 0.07%), it became about 2000 cycles, and creep fatigue strength fell. From this, Mo was found to make a constant contribution to creep fatigue strength. When the Mo content was further increased to 1.83%, the creep fatigue life became about 2500 cycles, and the tendency for the fatigue characteristic to decrease was observed.

Next, the presence state of S in steel was investigated. As a result, as shown in Fig. 2, S was found to exist in the form of MnS. If S trapped as MnS becomes free and segregates at grain boundaries during the creep fatigue test at a high temperature, it is considered that this S adversely affects the creep fatigue characteristics.

(d) fixing of S

If the segregation of free S as described above adversely affects the creep fatigue characteristics, it is considered that it is possible to increase the creep fatigue strength by adding an element that traps S more firmly in addition to Mn.

Here, the influence on the creep fatigue strength of Ca, Mg, Nd, La, and Ce, which may form sulfides, was examined.

As a result, when 0.025% of Nd was contained, it was found that in addition to MnS, the Nd inclusions fixed S. This Nd inclusion means "oxide of Nd" and "composite inclusion of Nd oxide and sulfide". "Composite inclusion of oxide and sulfide of Nd", as described, directly fixes S. On the other hand, "Nd oxide" also indirectly fixes S by segregating around it. 3 shows "composite inclusions of oxides and sulfides of Nd" observed in Nd-containing steels as an example of Nd inclusions.

As described above, when a steel containing Nd that directly or indirectly fixes S is creep fatigue tested under the above-described conditions, that is, a test temperature of 600 ° C. and a total deformation range of 0.5%, the fatigue life is dramatically increased to about 7000 cycles. Improvements were found.

In addition, the creep fatigue life (test temperature is 600 ° C., 0.5% of total strain range) of the steel containing Ca, Mg, La, and Ce alone, respectively, is about 3000 to 4000 cycles, but the steel contains Nd as the component. In Esau, a lifespan of 6000 to 7000 cycles was found, resulting in a dramatic improvement in creep fatigue life.

(e) Complex addition of Nd with Cu, Ni or Co

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

As a result, the creep fatigue life of steels containing trace amounts of Cu, Ni, or Co together with Nd was about 4000 cycles, while the creep fatigue characteristics were improved compared to steels containing no Nd, but compared with steels containing Nd alone. The creep fatigue life was found to drop significantly.

In the above examination, the conclusion from (1) to (4) can be obtained as follows.

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

(2) Although most of S is fixed as MnS, a part of S becomes free during the fatigue test at high temperature and segregates at grain boundaries, thereby reducing the creep fatigue strength.

(3) By containing Nd, fixing S as an oxide of Nd or as a composite inclusion of an oxide and a sulfide, and fixing a portion as MnS, creep fatigue strength is greatly improved. This effect is remarkable when the density of Nd inclusions is 10000 pieces / Pa or more. In addition, "Nd inclusion" is a generic term for said "oxide of Nd" and "composite inclusion of Nd oxide and sulfide".

(4) Cu, Ni, and Co which are austenite forming elements reduce creep fatigue strength. This tendency is also recognized for steels containing a small amount of Nd. This phenomenon is considered to be because Cu, Ni, and Co promote the phenomenon that S fixed as MnS becomes free during the creep fatigue test.

This invention made based on the said examination result makes the following heat resistant steel the summary. Hereinafter,% regarding component content means the 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 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 of which is made up of Fe and impurities, wherein Ni is less than 0.3%, Co is less than 0.3%, and Cu is less than 0.1%, containing Nd inclusions, and the density of the Nd inclusions Ferritic heat-resistant steel of 10000 pieces / ㎣ or more.

(2) The ferritic heat-resistant steel according to the above (1), which contains one or more of Ta: 0.04% or less, Hf: 0.04% or less, and Ti: 0.04% or less instead of a part of Fe.

(3) The ferritic heat-resistant steel according to the above (1) or (2), which contains one or two of Ca: 0.005% or less and Mg: 0.005% or less instead of a part of Fe.

(4) The total amount of rare earth elements other than Nd in the impurity is 0.04% or less. Some ferritic heat-resistant steels from (1) to (3).

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

1 is a diagram illustrating an example of a strain waveform of a creep fatigue test.

2 is a diagram showing sulfides observed in ASME P92 steel.

FIG. 3 is a view showing a "composite inclusion of an oxide and a sulfide of Nd" observed in an Nd-containing steel. FIG.

[One. 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% to 0.13%]

C stabilizes the structure of the steel as an austenite stabilizing element. Moreover, MC carbide or M (C, N) carbonitride is formed and contributes to the improvement of creep strength. M of MC and M (C, N) is an alloying element. However, at less than 0.01% of C, the above effects cannot be sufficiently obtained, and the amount of δ ferrite increases, which may lower the strength. On the other hand, when the content of C exceeds 0.13%, not only workability and weldability deteriorate, but also coagulation coarsening of carbides occurs from the beginning of use, which causes a decrease in creep strength for a long time. Therefore, C content needs to be limited to 0.13% or less. More preferable minimum and upper limits are 0.08% and 0.11%, respectively.

[Si: 0.15 to 0.50%]

Si is contained as a deoxidation element of steel, or is an element necessary in order to improve the water vapor oxidation performance. The lower limit is made 0.15% which does not impair the water vapor oxidation performance. On the other hand, since the fall of creep strength is remarkable when content of Si exceeds 0.50%, an upper limit shall be 0.50%. In particular, when water vapor oxidation is important, the lower limit of the amount of Si is preferably 0.25%.

[Mn: 0.2-0.5%]

Mn contributes as a deoxidation element and an austenite stabilization element. Moreover, MnS is formed and S is fixed. In order to acquire such an effect, 0.2% or more of containing is required. On the other hand, when it exceeds 0.5%, a creep strength will fall. Therefore, appropriate content of Mn is 0.2 to 0.5%. Moreover, a preferable minimum is 0.3%.

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

P and S, which are impurities, deteriorate hot workability, weldability, creep strength, creep fatigue strength, and the like of steel, so the lower the content, the more preferable. However, the remarkable cleanliness of steel causes a significant cost increase, so the allowable upper limit is made 0.02% in P and 0.005% in S.

[Cr: more than 8.0% and less than 12.0%]

Cr is an indispensable element in order to ensure corrosion resistance and oxidation resistance at high temperatures of the steel of the present invention, particularly water vapor oxidation characteristics. In addition, Cr forms carbide to improve creep strength. In order to acquire such an effect, the content needs to exceed 8.0%. However, when Cr content becomes excessive, since creep strength will fall for a long time, it was made into less than 12.0%. More preferable minimum is 8.5%, and further more preferable upper limit is less than 10.0%.

[Mo: 0.1% to 1.5%]

Mo contributes to the improvement of creep strength as a solid solution strengthening element. In addition, as a result of examining the correlation between Mo content and creep fatigue strength in detail, it has been found that 0.1% or more of Mo contributes to the improvement of creep fatigue properties, and when the content exceeds 1.5%, the creep strength decreases for a long time. . Therefore, 0.1-1.5% of content of Mo is suitable. More preferable minimum 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. In addition, a part is solid-dissolved in Cr carbide, which suppresses coarsening of carbide and contributes to creep strength. However, at less than 1.0%, such an effect is small. On the other hand, when the Mo content exceeds 3.0%, the production of δ ferrite is promoted, resulting in a decrease in creep strength. Therefore, the appropriate range of W content is 1.0 to 3.0%. The minimum with more preferable amount is over 1.5%, and a further more preferable upper limit is 2.0%.

[V: 0.1 to 0.5%]

V forms a fine carbonitride further by a solid solution strengthening action, and contributes to the improvement of creep strength. In order to exhibit the effect, it is necessary to make the content 0.1% or more. On the other hand, if the V content exceeds 0.5%, the production of δ ferrite is accelerated and the creep strength is lowered. Therefore, 0.5% should be the upper limit. More preferable minimum and upper limits are 0.15% and 0.25%, respectively.

[Nb: 0.02 to 0.10%]

Nb forms fine carbonitrides and contributes to improvement of creep strength for a long time. In order to exert the effect, it is required to contain 0.02% or more. However, if the content is too large, the production of δ ferrite is promoted, causing a decrease in creep strength for a long time. Therefore, appropriate content of Nb is 0.02 to 0.10%. More preferable minimum and upper limits are 0.04% and 0.08%, respectively.

[A1: 0.015% or less]

Although A1 is used as a deoxidizer of molten steel, when the content exceeds 0.015%, creep strength will be lowered, and therefore an upper limit should be suppressed to 0.015% or less. More preferably, the upper limit is 0.010%.

[N ： 0.005-0.070%]

N is effective as an austenite stabilizing element like C. In addition, N precipitates nitride or carbonitride and increases the high temperature strength of the steel. In order to exert the effect, it is required to contain 0.005% or more. On the other hand, when N content becomes excessive, it will not only produce a blowhole at the time of melt | dissolution, or cause a welding defect, but will also cause the creep strength fall by coarsening of nitride and carbonitride. Therefore, the upper limit of N content should be 0.070%. Furthermore, the minimum of content of N is 0.020%.

[Nd ： 0.005-0.050%]

Nd greatly improves the creep fatigue strength as described above. In order to exhibit the effect, 0.005% or more of containing is required. However, if it exceeds 0.050%, coarse nitride will be formed and creep strength will be lowered, so the upper limit should be 0.050%. The upper limit of more preferable content is 0.040%.

[B: 0.002-0.015%]

B plays an important role in improving hardenability and securing high temperature strength. The effect becomes remarkable when it becomes 0.002% or more, but when it exceeds 0.015%, weldability and creep strength for a long time will fall.

[Ni: less than 0.3%, Co: less than 0.3%, Cu: less than 0.1%]

Such austenite stabilizing elements lower the creep fatigue strength even in a trace amount as described above. However, there are cases where trace amounts of Ni, Co, and Cu cannot be mixed from the melting raw materials. Herein, in the present invention, Ni and Co are each less than 0.3%, and Cu is less than 0.1%. If it is the said range, the bad influence in creep fatigue strength is small.

[Components of First Group: Ta, Hf and Ti]

These are components added with 1 type, or 2 or more types as needed. 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 needed because they form fine carbonitrides and contribute to the improvement of creep strength. In order to fully exhibit the effect, the content of 0.005% or more is preferable, respectively. However, when the respective content exceeds 0.04%, the effect is saturated, but rather, the creep strength is degraded. Therefore, the upper limit of each content is good to set it as 0.04%.

[Components of Second Group: Ca and Mg]

These are also the components added 1 type or 2 types as needed. Appropriate content in the case of adding is as follows.

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

All of these elements improve the hot workability of steel. Therefore, in the case where it is particularly desired to improve the hot working of the steel, either one or both of them are contained in combination. Since the effect is remarkable at 0.0005% or more, respectively, the lower limit of the content is preferably 0.0005%. However, if the content exceeds 0.005% in both cases, the creep strength is lowered, so the upper limit should be 0.005%.

[Rare earth elements other than 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 other than Nd is 0.04% or less, the content of up to 0.04% is allowed because it does not significantly affect the characteristics such as creep strength and creep ductility.

[2. Nd inclusions]

One of the characteristics of the steel of the present invention is that the Nd inclusion is contained at a density of 10000 particles / cc or more.

The Nd inclusions observed in the steel of the present invention are "oxides of Nd" and "composite inclusions of oxides and sulfides of Nd" as described above. Specifically, the _{Nd 2 O 3, Nd 2 O} 2 S 4, Nd 2 O 2 SO 4, Nd 2 O 2 S or the like.

Although the diameter of an Nd inclusion varies about 0.3 micrometer-about 1 micrometer, Nd inclusion is normally observed in the steel containing trace amount Nd. However, in steels containing much Co, Ni, and Cu, MnS increases, and Nd inclusions decrease markedly. And when the density of an Nd inclusion becomes less than 10000 piece / Pa, the improvement of creep fatigue strength will not be seen. Therefore, the amount of Nd inclusions should be 10000 pieces / cc or more.

[3. Manufacturing method]

The steel of the present invention can be produced by production facilities that are usually used industrially. That is, in order to obtain the steel of the chemical composition prescribed | regulated by this invention, what is necessary is to refine | purify by furnaces, such as an electric furnace and a converter, and to adjust a component by deoxidation and containing of alloying elements. In particular, when strict component adjustment is required, a method in which molten steel is subjected to a suitable treatment such as vacuum treatment may be adopted before adding an alloying element.

The method introduce | transduced in 10000 piece / Pa or more Nd inclusions is as follows. That is, in advance, sufficient deoxidation is performed by C, Si, Mn, Al, etc. in the step from steelmaking to steelmaking. It is because the yield of Nd addition will worsen when there is much oxygen content in molten steel. Then, in the case of the ingot method, a composition other than Nd is adjusted before injecting into an ingot, and Nd is added by adding Nd immediately before injecting. In the continuous casting method, the composition other than Nd is adjusted until the molten steel is introduced at the time of tundish, and then Nd is added by adding Nd at the time of tundish. By final adjustment of only Nd, an appropriate amount of Nd inclusions can be produced. The cast slabs, beads or ingots are also processed into steel pipes or steel sheets or the like.

When producing a seamless steel pipe, for example, a vitret may be extruded, rolled, rolled, or inclined roll type, or a large diameter forged tube by an Ehrhardt production method. About manufacture of a steel pipe, you may cold-process as needed and can make a dimension uniform. The steel pipe manufactured is heat-processed suitably, and, as needed, surface-treats, such as shot peening and pickling.

Examples of the steel sheet include hot rolled steel sheets and cold rolled steel sheets. 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.

EXAMPLE

Using a vacuum induction melting furnace, steel having a chemical composition shown in Table 1 was dissolved to obtain a 50 kg ingot having a diameter of 144 mm. Codes A to M are steels of the present invention and codes 1 to 22 are comparative steels. About the steel of code | symbol A-M, and the steel of code | symbol 15-20, after deoxidation by C, Si, Mn, and Al was fully performed, Nd was added just before injecting. Nd was added to the steel of code | symbol 21 from the start of melt | dissolution, and after carrying out only the deoxidation by carbon (C) in the steel of code | symbol 22, Nd was added.

Such an ingot was hot forged, hot rolled to obtain a 20 mm thick plate. Then, after hold | maintaining at the temperature of 1050 degreeC for 1 hour, it air-cooled (AC). Furthermore, the tempering process which hold | maintained at 760 degreeC-780 degreeC for 3 hours, and air-cooled (AC) was performed. The test piece was extract | collected from such a board so that the longitudinal direction of a test piece might become a rolling direction, and the creep rupture test, the creep fatigue test, and the distribution of Nd inclusion were investigated on condition of the following.

(1) creep rupture test

Test piece: Diameter 6.0mm, distance between gages: 30mm, Test temperature: 600 degreeC, Load stress: 160 Mpa, Test item: Break time (h).

(2) creep fatigue test

Test piece: Diameter 10mm, mark distance: 125mm, test temperature: 600 degrees Celsius (in air)

Strain waveform: CP waveform, full strain range Δε _t = 0.5%,

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

Test Item: Creep Fatigue Life N _f (cycle)

(3) Survey of distribution of Nd inclusions

The specimen was cut out from the raw material while hot, polished, and corroded to produce an extraction replica by C deposition, followed by electron microscope observation at 2000 times, and by inclusion of EDX analysis (Energy Dispersive X-Ray Analysis). Was classified, and the number of Nd inclusions (pieces / mm 2) was quantified, and the value was converted to the power by a third power to convert the precipitate density (pieces / kV). Moreover, it observed in 10 views and made the average value the precipitation density.

Table 2 shows the creep rupture test results, creep fatigue test results, and Nd inclusion distribution test results of the inventive steel and the comparative steel.

TABLE 1

TABLE 2

As shown in Table 2, compared with the steel of ASME P91 of code | symbol 1, the steel of ASME P92 of code | symbol 2 and code | symbol 6 has long creep rupture time, and is clearly high creep strength. However, the creep fatigue life is almost equal. That is, no significant improvement in creep fatigue life is seen in the steel of ASME P92.

Although the steel of the code | symbols 3 to 5 which contained trace amount Cu, Ni, or Co has the creep strength about the same as the steel of code | symbol 2, a clear fall is observed in creep fatigue life.

The influences of Mo on creep rupture strength and creep fatigue strength with steels 2, 6, 7, 8, and 9 were investigated.In the steels of code 7 and 8 with a small Mo content, code 2 and 6 Creep fatigue strength is lower than steel. Moreover, the strength creep fatigue strength of code | symbol 9 with many Mo content is inferior.

In the steel from 10 to 13 containing a small amount of La, Ce, Ca or Mg, the improvement of the characteristics is not recognized as the degree of the steel of 2 as well as the creep strength and the creep fatigue strength.

On the other hand, the steel from Code A to Code M satisfying the conditions specified in the present invention has a creep rupture time approximately equal to that of Code 2, but the creep fatigue life is remarkably improved.

The steel of code | symbol 14 whose Nd content is out of the range prescribed | regulated by this invention is insufficient in improvement of creep fatigue strength. On the other hand, the steel of code | symbol 15 which contained Nd excessively has low creep strength.

Steels 16 to 18 containing a small amount of Nd and austenite forming elements of Cu, Ni, or Co had a creep strength similar to that of code 2, and the creep fatigue strength was slightly improved compared to the steel of code 2. However, the creep fatigue strength is clearly inferior in comparison with steels containing no Cu, Ni, or Co, or having a lower content of these, from A to M.

Although Nd is contained in the range prescribed | regulated by this invention, the steel of code | symbol 19 and code | symbol 20 whose Mo is out of the range prescribed | regulated by this invention has a high creep fatigue life compared with the thing which does not contain Nd. However, the creep fatigue strength is clearly inferior compared to the steel from the symbols A to M having a Mo content in the range defined by the present invention.

In steels 21 and 22, the chemical composition is within the range defined by the present invention, but the distribution density of the Nd inclusion does not satisfy the range defined by the present invention. In these, since deoxidation is not performed sufficiently and Nd is added, considerable coarse Nd oxide is formed, the density of Nd inclusions falls remarkably, and 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 excellent effects as a heat exchanger steel pipe, a pressure vessel steel plate, and a turbine material used in fields such as thermal power generation, nuclear power generation, and chemical industry, and is extremely advantageous in industry.

Claims (5)

In mass%, C: 0.01 to 0.13%, Si: 0.1 to 0.50%, Mn: 0.2 to 0.5%, P: 0.02% or less, S: 0.005% or less, Cr: 8.0% or more 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 O. 050%, B: 0.002-0.015%, remainder consists of Fe and an impurity, Ni is less than 0.3%, less than 0.3% of Co, less than 0.1% of Cu, and contains Nd inclusions, The ferritic heat-resistant steel whose density of the said Nd inclusion is 10000 piece / Pa or more. The method according to claim 1, A ferritic heat resistant steel containing at least one of Ta: 0.04% or less, Hf: 0.04% or less, and Ti: 0.04% or less by mass% instead of a part of Fe. The method according to claim 1 or 2, A ferritic heat-resistant steel containing, in place of a part of Fe, one or two of Ca: 0.005% or less and Mg: 0.005% or less. The method according to any one of claims 1 to 3, A ferritic heat-resistant steel, wherein the total amount of rare earth elements other than Nd in impurities is 0.04 mass% or less. The method according to any one of claims 1 to 4 The strain rate is 0.01% / sec on the tension side and 0.8% / sec on the compression side, and the creep fatigue life in the CP waveform at 600 ° C is 5000 cycles or more under the condition that the total strain range is 0.5%. System heat resistant steel.

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