JP2007162114A - Martensitic iron based heat resistant alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a martensitic heat resistant alloy having high temperature long time creep strength suitable for a steel tube for a heat exchanger, a steel sheet for a pressure vessel, a turbine material or the like used under high temperature-high pressure environments in a boiler, nuclear power generation equipment, chemical industry equipment or the like. <P>SOLUTION: The martensitic iron based heat resistant alloy has a composition comprising, by mass, 8 to 16% Cr, 1 to 18% Co, 0.01 to 1% Si, 0.01 to 3% Mn, 0.0005 to 0.03% B, 0.001 to 0.1% sol.Al and 0 to 0.5% Cu, further comprising either or both of ≤2% Mo and ≤5% W in the ranges satisfying inequality (1), and the balance Fe with impurities satisfying ≤0.05% C, ≤0.01% N, ≤0.05% P and ≤0.02% S, and has a modulated structure. The alloy may further comprise one or more kinds selected from Nb, V, Ni, Ta, Hf, Ti, REM, Ca and Mg. (1): Mo+(1/2)W≥0.5%, wherein Mo and W denote the respective contents (mass%). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a martensitic heat resistant alloy. More specifically, martensite systems with excellent high-temperature and long-term creep strength suitable for high-temperature, high-pressure environments such as boilers, nuclear power generation facilities, and chemical industrial facilities, which are suitable for heat exchange steel pipes, steel plates for pressure vessels, turbine materials, etc. It relates to heat resistant alloys.

  High temperature creep strength, corrosion resistance, oxidation resistance and the like are required for heat resistant materials for various parts used in high temperature and high pressure environments such as boilers, nuclear power generation facilities and chemical industrial facilities. High Cr martensitic steel, which is one of such heat resistant materials, is superior to low alloy steel in terms of strength and corrosion resistance at a temperature of 500 to 650 ° C. Further, high Cr martensitic 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. In addition, 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 with higher strength than conventional steel was put into practical use by precipitation strengthening with fine carbonitrides of Nb and V. Using this steel, supercritical boilers with a steam temperature of 566 ° C or higher have been put into practical use. In recent years, ASME P92 and P122 steel, which has been further increased in creep strength by adding W, has been put into practical use, and an ultra-supercritical boiler with a steam temperature of about 600 ° C. has also been put into practical use.

  However, even high-strength, high Cr martensitic steels have a problem of low long-term creep strength at a high temperature of about 600 ° C. As the cause of the decrease in long-term creep strength, there is a non-uniform recovery in which the structure recovery preferentially proceeds in the vicinity of the prior austenite grain boundary in the low stress and long time range.

In recent years, reduction of CO ₂ emissions has been demanded in order to protect the global environment, and in order to meet this demand, further higher temperature and pressure have been demanded in thermal power generation boilers. And in order to implement | achieve the further high temperature high pressure operation of a boiler, development of a higher intensity | strength martensitic material is calculated | required.

  Conventional martensitic heat resistant steels have been made extremely strong by solid solution strengthening and precipitation strengthening. Dislocations are responsible for deformation of martensitic heat-resistant steel, and both solid solution strengthening and precipitation strengthening are strengthened by preventing the movement of dislocations.

  Typical examples of the solid solution strengthening element include Mo and W. However, if the content of these elements is increased in order to increase the effect of solid solution strengthening, an amount of elements exceeding the solid solution limit will be precipitated. Therefore, there is a limit to strengthening only by solid solution strengthening.

From the viewpoint of precipitation strengthening, conventional steels are typically represented by M ₂₃ C ₆ type carbide, MX type carbonitride (M is a metal element, X is C and N), and Laves phase. It is strengthened by intermetallic compounds. In general, the smaller the particle spacing of the precipitate, the higher the strength. There is the following relationship between the particle diameter and the volume fraction of the particles. This volume fraction is the volume percent of particles in the entire alloy. Therefore, the material is strengthened if a large amount of fine particles are dispersed.

  In order to make effective use of precipitation strengthening more than ever, various inventions have been proposed as follows.

  Patent Document 1 (Japanese Patent Laid-Open No. 2002-226946) discloses a steel in which N is limited to less than 0.005% for the purpose of effectively utilizing B in order to suppress coarsening of carbonitrides.

  Patent Document 2 (Japanese Patent No. 3118566) discloses a martensitic iron-based heat-resistant alloy containing no carbon as a new material that does not rely on strengthening by carbides. This iron-based heat-resistant alloy adds a large amount of Mo or / and W, and precipitates fine and stable intermetallic compounds (Laves, μ, σ, etc.) up to high temperatures in the martensite matrix, lath interface and prior austenite grain boundaries. To improve the high temperature creep strength.

Japanese Patent Laid-Open No. 2002-226946 Japanese Patent No. 3118566

However, even if the type and composition of the precipitates are changed, the precipitates become coarse when used at high temperatures for thousands to tens of thousands of hours. The coarsening rate of precipitates (Ostwald growth rate) increases as the temperature increases. The growth rate of M ₂₃ C ₆ type carbide, MX type carbonitride, Laves phase and the like for strengthening the heat-resistant steel increases remarkably at a high temperature of about 600 ° C. Therefore, at a high temperature of about 600 ° C., further strengthening by the carbonitride and intermetallic compounds that have been used for strengthening is difficult.

  An object of this invention is to provide the martensitic heat-resistant alloy excellent in high temperature long-time creep strength.

  As described above, both solid solution strengthening and precipitation strengthening have limitations, and it is difficult to further strengthen the heat-resistant material by these strengthening methods. However, if it is possible to disperse a large amount of particles that are finer than the precipitates known so far and that do not become coarse, it is considered that the strength can be dramatically increased. There are the following methods (1) and (2).

(1) ODS alloy (oxide dispersion strengthened alloy)
(2) Utilization of cluster represented by GP zone It is known that the ODS alloy of (1) above can be strengthened, but it can be industrially inexpensively and stably manufactured. There are very high technical barriers to establishing a method for manufacturing large-sized members by the method.

  Regarding the cluster of (2) above, in an alloy having a composition like the alloy of the present invention, nuclei (GP zones) having the same structure as the matrix are first formed, and then a thermodynamically quasi called a cluster. A stable phase is formed and eventually changes into a thermodynamically stable precipitate. Since it takes time to shift to a thermodynamically stable state in a low temperature region, it may contribute to improvement of material properties depending on the case. However, it is considered that the high-temperature region easily changes to an equilibrium phase (precipitate) in a short time. Therefore, it is considered difficult to improve the creep strength for a long time by the cluster.

  The above methods (1) and (2) both have drawbacks. Therefore, for the purpose of searching for a strengthening method that does not rely on precipitates, a basic study was conducted using a model alloy in which precipitation of carbonitrides and intermetallic compounds was suppressed. The model alloy is an alloy in which Cr is 10.5%, Mo is 1%, W is 0.5%, Co is 1%, the balance is Fe, C is 0.003% or less, and N is 0.003% or less. In addition,% regarding content of an alloy component means "mass%" here.

  When martensite is kept at a high temperature, the dislocation density gradually decreases and softens. This is called organizational recovery. In order to infer the creep properties of alloys, it is important to grasp the structure recovery process. Therefore, after normalizing at 1100 ° C, aging is performed at 500 ° C to 650 ° C for up to 100 hours, and the structure recovery process is hardened. It was evaluated with. As a result, an increase in hardness was observed after aging at 500 ° C. In general, it is unlikely that the hardness is increased by aging treatment in a precipitate-free alloy. Then, the structure | tissue was investigated in detail using the electron microscope.

  FIG. 1 is an example of an electron microscope structure of a model alloy that has been normalized at 1100 ° C. and then heat-treated at 500 ° C. for 100 hours. In the bright field image of the electron microscope in FIG. 1 (a), a tweed regular and fine modulation structure is confirmed, and in the electron diffraction pattern of (b), satellites are observed, confirming the formation of the modulation structure. It was.

  The above-mentioned modulation structure is a structure exhibiting regular and fine “fluctuation”, and from the concentration fluctuation represented by the spinodal decomposition to the two phases (parent phase and α ′ phase) having a constant concentration amplitude. It is a structure that appears in the process until it is divided.

  The modulation structure is a phenomenon that generally occurs at a low temperature of 500 ° C. or lower, and is known as a cause of 475 brittleness that significantly lowers toughness in stainless steel. Usually, in steel materials, efforts are made not to cause 475 brittleness (that is, brittleness due to the modulation structure), and there is no example in which the modulation structure is actively utilized in the steel structure material.

  However, the modulation structure, unlike clusters, which are thermodynamically metastable phases, is thermodynamically balanced, may be significantly slower in coarsening, and dislocation motion (expansion, slip, rise) ) May be a powerful obstacle different from the normal mechanism. In other words, if the modulation structure generated in the alloy in advance is stable for a long time in the high temperature range of 575 ° C. or more, which is the use temperature of the alloy, there is a possibility of significant strengthening of the alloy in this high temperature range. it is conceivable that.

  Conventionally, there have been few research results on the active use of modulation structures in the field of martensitic heat-resisting steels. On the other hand, Patent Document 3 (Japanese Patent Laid-Open No. 6-65689) describes that adding a large amount of Mo or W causes two-phase separation / spinodal decomposition to reduce creep strength, thus avoiding spinodal decomposition. The idea is shown.

JP-A-6-65689

  In the basic study described above, the modulation structure usually found in ferritic stainless steel containing about 18% Cr was observed with a material of about 10% Cr, which is extremely epoch-making. One of the causes of this phenomenon is considered to be that the formation of precipitates, which was unavoidable in the conventional heat-resistant steel, was suppressed by limiting the contents of C and N. If a stable modulation structure can be formed in a high temperature range, it is possible to create a new material that is remarkably excellent in high temperature and long time creep strength.

  When a modulation structure is generated at 500 ° C. in the above model alloy, it is considered that the modulation structure becomes a major obstacle to the movement of dislocations and contributes to strengthening, as is apparent from an example in which the hardness is increased. However, the following two problems remain in the development of a heat-resistant alloy utilizing a modulation structure.

  (A) To suppress a decrease in toughness associated with the modulation structure.

  (B) The modulation structure should be stable even at the operating temperature of 575 ° C or higher.

  Until now, it was ferrite stainless steel that the decrease in toughness was remarkable due to the modulation structure. Therefore, it is considered that the toughness can be drastically improved by making the matrix phase martensite instead of ferrite first.

  Even in a martensitic steel, for example, in PH17-4 stainless steel, toughness decreases due to the appearance of a modulation structure. The following (1) and (2) can be considered as the reason.

  (1) The parent phase is hardened by finely precipitated Cu.

  (2) Due to the low aging temperature for causing the modulation structure to appear as low as 400 ° C., the increase in hardness due to the formation of the modulation structure is very large.

  In the present invention, as described later, for the modulation structure formation, the upper limit of the C and N content is suppressed to suppress the precipitation of carbides, intermetallic compounds, etc., but to cope with the above (1), Furthermore, it was decided to suppress precipitation including Cu. In addition, if the heat treatment for generating the modulation structure is performed at a high temperature of 500 ° C. or higher, the remarkable hardening that occurs at 400 ° C. does not occur, and the decrease in toughness can be suppressed. Various studies were conducted based on the above idea.

  In order to obtain a martensite structure, it is first necessary to form an austenite structure during normalization at 1000 to 1200 ° C. In other words, it is essential to design an alloy that enters the γ loop during normalization. In other words, an alloy design that includes an austenite-forming element that expands the γ loop and generates a modulation structure is required.

  The most effective elements for expanding the γ loop are C and N. However, C and N form a carbonitride with Cr, which is a basic element constituting the modulation structure, and hinder the formation of the modulation structure in a high temperature range (500 ° C. or higher), and thus are difficult to use. Then, utilization of Ni, Cu, and Co which are austenite forming elements other than C and N was examined.

Ni contributes to austenitization, but the Ms point and Ac ₁ transformation point are greatly lowered, so the amount of Ni is limited. Although Cu contributes to austenitization, it precipitates finely and promotes toughness reduction as in the above-mentioned PH17-4 stainless steel. Therefore, the allowable content is limited.

Co is an element that does not lower the Ms point and the Ac ₁ transformation point. Moreover, embrittlement is not induced like the above Cu. Therefore, in the present invention, martensite formation is promoted by containing Co.

  Next, in steel where Cr is significantly reduced compared to steel with 18% Cr, which is known to have a modulation structure in the parent phase, in the high temperature range of 575 ° C or higher, which is the operating temperature of the alloy, for a long time, In order to find an alloy composition capable of stably maintaining the modulation structure, various alloys were prepared, and the structure of the material heat-treated at various temperatures was observed with an electron microscope to investigate the formation of the modulation structure. As a result, the following points became clear.

  (A) By suppressing the contents of C and N, that is, suppressing the precipitation of Cr as carbonitride nitrides, and adding Mo or / and W in addition to Co, 525 The modulation structure exists stably even in a low temperature range below ℃. However, the amount of precipitation of the intermetallic compound needs to be as small as possible.

  (B) By containing B (boron), the modulation structure is stably present up to a high temperature. Even in ordinary heat-resistant steel, the effect of B is not necessarily clear, but it is presumed that it interacts with Cr, Mo or / and W, Co and the like.

  (C) By containing Nb or / and V, the modulation structure can exist more stably up to a high temperature. It is presumed that Nb or / and V, which usually precipitates by forming carbonitride, have some thermodynamically favorable effect that solid solution does not precipitate.

  By making use of the above findings (A) to (C), it has been clarified that the modulation structure can be stably present for a long time even in a high temperature range of 575 ° C. or more which is a use temperature.

  The present invention has been made on the basis of the above knowledge, and the gist thereof lies in the following martensitic iron-based heat-resistant alloy.

  (1) In mass%, Cr: 8-16%, Co: 1-18%, Si: 0.01-1%, Mn: 0.01-3%, B: 0.0005-0.03%, sol.Al: 0.001-0.1% Cu: 0 to 0.5%, Mo: 2% or less and W: 5% or less are contained within a range that satisfies the following formula (1), and the balance consists of Fe and impurities. A martensitic iron-based heat-resistant alloy characterized in that C is 0.05% or less, N is 0.01% or less, P is 0.05% or less, and S is 0.02% or less and has a modulation structure.

Mo + (1/2) W ≧ 0.5% ・ ・ ・ ・ (1)
However, Mo and W in the formula (1) indicate their contents (mass%).

  (2) The martensite system according to the above (1), which contains one or both of Nb: 0.005 to 0.1% and V: 0.05 to 0.5% in mass% instead of part of Fe Iron-based heat-resistant alloy.

  (3) The martensitic iron-based heat-resistant alloy according to (1) or (2) above, wherein Ni: 0.05 to 3% is contained in mass% instead of part of Fe.

  (4) In place of part of Fe, by mass%, Ta: 0.005-0.1%, Hf: 0.005-0.1%, Ti: 0.001-0.05% and REM: 0.005-0.1% Alternatively, the martensitic iron-based heat-resistant alloy according to any one of (1) to (3) above, which contains two or more kinds.

  (5) The above (1) to (4), characterized in that one or both of Ca: 0.001 to 0.02% and Mg: 0.001 to 0.02% are contained in mass% instead of part of Fe Any martensitic iron-based heat-resistant alloy.

  The alloy of the present invention is a heat-resistant alloy having excellent long-term creep strength at a high temperature of 575 to 650 ° C. This alloy exhibits an excellent effect when used 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.

1. Modulation Structure FIG. 2 shows the results of investigating the creep rate at 650 ° C. and 69 MPa using four types of steel having the chemical composition shown in Table 1. Steel a in Table 1 is an iron-based alloy having a ferrite matrix that does not contain Co and does not contain M ₂₃ C ₆ and MX. Steel b has M ₂₃ C ₆ dispersed in the martensite matrix, Steel c has M ₂₃ C ₆ and MX carbonitride dispersed in the martensite matrix (ASME P91 equivalent steel), Steel d It is the steel of the present invention in which the parent phase is martensite and M ₂₃ C ₆ and MX type carbonitride are substantially absent.

As shown in FIG. 2, the steels a, b, and c all show a rapid increase in creep rate from around 1000 hours. This phenomenon can be explained as follows. That is, in steel a, since there is no strengthening factor, accelerated creep appeared in a short time and broke early. In the steel b and steel c, M ₂₃ C ₆ and MX type carbonitride are aggregated and coarsened in this time zone, the strength is lowered, and the creep rate is increased. On the other hand, in steel d, the creep rate is still small even after 1000 hours, and finally starts to increase from around 6000 hours. The fact that there is no increase in the creep rate even after a long period of time means that the long-term creep characteristics are excellent. Such a very surprising phenomenon appears because the steel d has a modulation structure.

2. About the parent phase structure of the alloy The main structure of the parent phase of the alloy of the present invention is martensite. The reason why the main organization of the mother phase is martensite is as described above. In addition, “the main organization is martensite” means that martensite is 95% or more in area ratio. In addition to martensite, ferrite having a total area ratio of less than 5% may be present. It is not necessary for the alloy of the present invention to positively contain reinforcing phases such as carbides, nitrides, carbonitrides and intermetallic compounds, but rather it is better to suppress the inclusion. This is because, as described above, these become obstacles to modulation structure generation.

3. About the chemical composition of an alloy Hereinafter, the reason for selection of the chemical composition of this invention alloy is demonstrated with the effect of each component. In addition, as mentioned above,% regarding component content means the mass%.

Cr: 8-16%
Cr is an element indispensable for the formation of a modulation structure, and is essential for securing high temperature strength. It also contributes to the improvement of the steam oxidation resistance. If it is less than 8%, the modulation structure cannot exist stably at 600 ° C. or higher. However, if its content exceeds 16%, it becomes difficult to form a martensite structure, and it becomes difficult to achieve both strength and toughness. Therefore, the appropriate Cr content is 8 to 16%. More desirable is 10 to 15%.

Co: 1-18%
Co is an austenite-forming element that does not significantly lower the Ac ₁ transformation point and the Ms point, and is essential for the formation of a modulation structure. The required content varies depending on the Cr content, but even when Cr is 8%, 1% or more is necessary. On the other hand, when Co exceeds 18%, the creep strength decreases. Therefore, the proper content of Co is 1 to 18%.

Si: 0.01 to 1%
Si is added as a deoxidizing element and is also an effective element for enhancing the steam oxidation resistance. The lower limit of the content was set to 0.01% at which the effect of improving the steam oxidation resistance appeared. On the other hand, if the content exceeds 1%, the creep strength is remarkably lowered, so the upper limit was made 1%. In particular, when importance is attached to steam oxidation resistance, the lower limit of the Si content is preferably 0.15%.

Mn: 0.01-3%
Mn also contributes as a deoxidizer and as an austenite stabilizing element. Further, MnS is formed to fix S. In order to obtain these effects, 0.01% or more is necessary. However, if it exceeds 3%, the creep strength is lowered. Therefore, 0.01 to 3% is an appropriate content.

B: 0.0005-0.03%
B is effective in stabilizing the modulation structure up to a high temperature. In addition, it enhances hardenability and plays an important role in securing high temperature strength. The effect becomes remarkable at 0.0005% or more, but if it exceeds 0.03%, the weldability is impaired and the creep strength is lowered for a long time. Therefore, the appropriate content of B is 0.0005 to 0.03%.

sol.Al: 0.001 to 0.1%
Al is used as a deoxidizer for molten steel. In order to acquire the effect, it is necessary to contain 0.001% or more as sol.Al, but if it contains more than 0.1%, creep strength will be reduced. Therefore, the appropriate content as sol.Al is 0.001 to 0.1%.

Cu: 0 to 0.5%
Since Cu contributes as an austenite stabilizing element, it is contained as necessary. In order to acquire the effect, when making it contain, it is desirable to set it as 0.05% or more. However, if Cu is contained in a large amount, it precipitates finely and promotes toughness reduction.

Mo: 2% or less, W: 5% or less, but Mo + (1/2) W ≧ 0.5%
Mo or W is essential for forming the modulation structure. It also contributes to strength as a solid solution strengthening element. Since Mo and W have similar characteristics, either one may be contained alone or both may be contained in combination. In order to obtain the above effect, 0.5% or more of Mo + (1/2) W is required. When Mo or / and W are excessively contained, a large amount of coarse Laves phase precipitates, and the modulation structure is not formed and the creep strength is lowered. Therefore, the upper limit is set to 2% for Mo and 5% to W. . In particular, when emphasizing toughness, it is desirable that Mo is 1.5% or less and W is 3% or less.

  In addition to the above, it is also essential to regulate impurities C, N, P and S as described below.

C: 0.05% or less, N: 0.01% or less The alloy of the present invention secures creep strength by utilizing a modulation structure resulting from Cr concentration fluctuation. C and N form Cr and carbide, nitride or / and carbonitride, and particularly suppress the formation of a modulation structure in the heat treatment described later. However, if C is 0.05% or less and N is 0.01% or less, the effect on the characteristics is small. In particular, when importance is attached to toughness, it is desirable that C is 0.03% or less and N is 0.008% or less. More preferably, C is 0.01% or less and N is 0.005% or less.

P: 0.05% or less, S: 0.02% or less The impurities P and S adversely affect hot workability, weldability, creep strength, and the like, and therefore the content is preferably low. However, since the extreme cleaning of the steel causes a large increase in manufacturing cost, the allowable upper limit is set to 0.05% for P and 0.02% for S.

  The elements described below are components added as necessary.

Nb: 0.005 to 0.1%, V: 0.05 to 0.5%
Nb and V form MX-type carbonitrides in ordinary martensitic heat-resistant steel, and contribute to creep strength. However, in the alloy of the present invention, since the contents of C and N are suppressed, most of Nb and V are dissolved in the matrix.

  Nb and V are elements that stabilize the modulation structure, and are effective in maintaining the modulation structure up to a high temperature range of 575 ° C. or higher, which is the use temperature of the alloy. In order to obtain the effect, Nb needs to be 0.005% or more and V needs to be 0.05% or more. On the other hand, when Nb exceeds 0.1% and V exceeds 0.5%, δ ferrite is generated and the creep strength and toughness are lowered. Therefore, the content is set to 0.005 to 0.1% for Nb and 0.05 to 0.5% for V. One of these may be contained, or both may be contained.

Ni: 0.05-3%
Since Ni contributes as an austenite stabilizing element, Ni is contained as necessary. In order to acquire the effect, when it contains, it is 0.05% or more. However, if the Ni content exceeds 3%, the Ms point and Ac ₁ transformation point are greatly lowered.

Ta: 0.005-0.1%, Hf: 0.005-0.1%, Ti: 0.001-0.05%, REM: 0.005-0.1%
These elements, like Nb and V, are elements that stabilize the modulation structure, and can be contained as necessary. In order to obtain the above effects, Ta, Hf, and REM must be 0.005% or more, and Ti must be 0.001% or more. On the other hand, when Ta, Hf and REM are each 0.1% and Ti exceeds 0.05%, the creep strength of the alloy is lowered. Therefore, when included, Ta, Hf and REM are 0.005 to 0.1%, and Ti is 0.001. It is good to make it 0.05%. REM is a general term for rare earth elements including Y.

Ca: 0.001 to 0.02%, Mg: 0.001 to 0.02%
Ca and Mg enhance the crystal grain boundary by improving the grain boundary even at a very small content, and contribute to the improvement of hot workability. Accordingly, one or both of them may be contained as necessary. However, hot workability deteriorates if excessively contained. Therefore, when these elements are contained, the content is 0.001 to 0.02%, respectively.

4). Production method of the alloy of the present invention The alloy of the present invention having the chemical composition described above can be produced by a production facility that is usually used industrially. That is, in order to obtain an alloy of chemical components defined in the present invention, after melting, it is refined by AOD, VOD, LF (Ladle Furnace), etc., and the components are adjusted by deoxidation and addition of alloy elements.

  An alloy adjusted to a predetermined chemical composition is cast into a slab, billet, or steel ingot by a continuous casting method or an ingot-making method, and a steel pipe, a steel plate, or the like is produced therefrom. When manufacturing a steel plate, a hot-rolled steel plate can be obtained by hot rolling a slab.

  When producing seamless steel pipes, for example, the billet is extruded and piped, or pierced and rolled with an inclined roll type piercer, or a large forged pipe is produced by the Erhard-Pushbench process. That's fine. In some cases, hot working is performed on a slab, billet or steel ingot before pipe making. In addition, soaking may be performed at a high temperature of 1200 ° C. or higher as necessary before and after pipe making. In some cases, after pipe making, cold working is performed to adjust the dimensions. After processing into a product such as a plate or a tube, it is kept at 1000 ° C. to 1200 ° C., and then subjected to a normalizing treatment that is air-cooled or a quenching treatment that is water-cooled.

  It is necessary to make a modulation structure appear in advance in the alloy of the present invention. For this purpose, a modulation structure forming heat treatment is performed at 500 to 650 ° C. for several hours to 20 hours. It should be noted that normal high temperature tempering at around 750 ° C. is preferably not performed because it releases the transformation strain and delays the formation of the modulation structure.

  When heat treatment is performed at a temperature lower than 500 ° C., the toughness is remarkably lowered due to excessive hardening due to the formation of the modulation structure. On the other hand, even if heat treatment is performed at a temperature exceeding 650 ° C., the modulation structure is not formed. Therefore, the heat treatment temperature was set to 500 to 650 ° C. More preferred is 575 to 630 ° C.

  In addition, when the heat treatment is performed for several hours (2 to 3 hours), a modulation structure is formed. On the other hand, even if heat treatment is performed for a long time exceeding 20 hours, a large increase in the modulation structure is not recognized. Not desirable. Accordingly, the heat treatment time is preferably from several hours to about 20 hours.

  As a pretreatment for the above-mentioned heat treatment for forming a modulation structure, the modulation structure can be further refined by performing a heat treatment at a lower temperature for a short time, and the creep resistance of the alloy can be further increased. The pretreatment temperature is preferably lower than the temperature of the modulation structure forming heat treatment by 10 ° C. or more and 500 ° C. or more, and the treatment time is preferably about 30 minutes to 10 hours.

  Steel having the chemical composition shown in Table 2 was melted in a vacuum induction melting furnace to obtain 50 kg ingots each having a diameter of 144 mm. Marks A to M are the alloys of the present invention, and marks 1 to 10 are comparative alloys. These alloy ingots were hot forged and hot rolled into 20 mm thick plates. Next, after holding for 1 hour at a temperature of 1050 ° C. or higher, air cooling (AC) was performed. Further, following the heating at 525 ° C. × 5 h, a modulation structure forming heat treatment was performed by holding at 620 ° C. for 10 hours.

  Creep rupture test pieces and Charpy impact test pieces were sampled from the test pieces after the heat treatment for forming the modulation structure, and tested under the following conditions. Furthermore, the presence / absence of modulation structure formation was examined by electron microscope observation after the heat treatment for forming the modulation structure. Table 3 shows the test results of the inventive material and the comparative material.

1. Investigation of modulation structure formation After the above modulation structure formation heat treatment, a thin piece having a thickness of 0.08 mm was formed, and a thin film sample of an electron microscope was prepared by electrolytic polishing. A thin film sample is observed at a bright field of 150 to 200,000 times with an electron microscope, and whether a tweed regular and fine modulation structure is confirmed or whether satellites are observed in an electron diffraction pattern is investigated. The presence or absence of modulation structure formation was judged.

2. Conditions for creep rupture test Test piece: Diameter: 6.0 mm, distance between gauge points: 30 mm
Test temperature: 615 ° C
Load stress: 170 MPa
Test item: Break time (h)
3. Conditions for Charpy impact test Specimen: 10 mm x 10 mm x 55 mm, 2 mm V notch Test temperature: 0 ° C
Test item: Impact value (J / cm ² )

  As shown in Table 3, in the alloy of the present invention, a modulation structure was confirmed after the heat treatment at 620 ° C. Moreover, the creep rupture time is 3200 hours or more, which is much better than ASME-P91 (Mark 1 in Table 3) and P92 (Mark 2 in Table 3), which are standard steels, and there is no problem with the impact value. Is a level.

  On the other hand, the materials other than the marks 4 and 8 in the comparative alloy cannot confirm the modulation structure in the high temperature range, and the creep strength is lower than that of the alloy of the present invention. Although marks 4 and 8 can confirm the modulation structure, the content of the alloy component does not satisfy the range defined in the present invention, so that the creep strength or toughness is lower than that of the alloy of the present invention.

  For the alloy of code A, only high temperature tempering heat treatment at 780 ° C. (tempering condition: 780 ° C. × 1 h), which is performed with normal heat-resistant steel, is performed, and modulation structure forming heat treatment is not performed (mark A-2) The presence or absence of the formation of a modulation structure similar to the above, the creep rupture test and the Charpy impact test were performed. The results are shown in Table 4.

  As shown in Table 4, when only high temperature tempering at 780 ° C. is performed (A-2), modulation is caused by the disappearance of dislocations introduced by the martensitic transformation and the precipitation of trace amounts of carbides and intermetallic compounds. The formation of the structure is hindered and the creep rupture time is significantly shortened.

  The creep strength of the alloy of the present invention at 615 ° C. is significantly higher than the existing ASME-P91 and P92, and the toughness is equivalent to these existing steels. Thus, according to the present invention, a heat-resistant alloy having excellent long-term creep strength at a high temperature of 575 to 650 ° C. is obtained, and a steel tube for heat exchange, pressure used in the fields of thermal power generation, nuclear power generation, chemical industry, etc. Excellent effect when used as a steel plate for containers and a material for turbines. The present invention is an industrially extremely useful invention that contributes to the practical application of an ultrahigh pressure / high temperature boiler.

It is a figure which shows an example of the structure observation result of a model alloy. It is a figure which shows the relationship between the elapsed time in 650 degreeC and 69 MPa of various steel, and a creep rate.

Claims (5)

In mass%, Cr: 8 to 16%, Co: 1 to 18%, Si: 0.01 to 1%, Mn: 0.01 to 3%, B: 0.0005 to 0.03%, sol. Al: 0.001 to 0.1%, Cu: 0 to 0.5%, Mo: 2% or less and W: 5% or less are contained within a range that satisfies the following formula (1), the balance is Fe and impurities, and C of impurities Is a martensitic iron-based heat-resistant alloy characterized by having a modulation structure with N of 0.05% or less, N of 0.01% or less, P of 0.05% or less, and S of 0.02% or less.
Mo + (1/2) W ≧ 0.5% ・ ・ ・ ・ (1)
However, Mo and W in the formula (1) indicate their contents (mass%).   The martensitic iron group according to claim 1, wherein the martensitic iron group contains at least one of Nb: 0.005 to 0.1% and V: 0.05 to 0.5% in mass% instead of part of Fe. Heat resistant alloy.   The martensitic iron-based heat-resistant alloy according to claim 1 or 2, which contains Ni: 0.05 to 3% by mass instead of part of Fe.   Instead of part of Fe, by mass%, Ta: 0.005-0.1%, Hf: 0.005-0.1%, Ti: 0.001-0.05% and REM: 0.005-0.1% The martensitic iron-based heat-resistant alloy according to any one of claims 1 to 3, characterized by containing the above. It replaces with a part of Fe, and contains one or both of Ca: 0.001-0.02% and Mg: 0.001-0.02% by the mass%, Either of Claim 1 to 4 characterized by the above-mentioned. Martensitic iron-based heat-resistant alloy.

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