JP2016065280A - Heat resistant steel and steam turbine rotor component part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat resistant steel and a steam turbine component part capable of achieving enhancement of long term creep rupture life and excellent in high temperature property, durability or the like.SOLUTION: The heat resistant steel of the embodiment contains, by mass%, C: 0.05 to 0.13%, Si: 0.01 to less than 0.10%, Mn: over 0.15% and 0.50% or less, Ni: 0.01% to less than 0.40%, Cr: 9 to 11.5%, Mo: 0.01 to 0.50%, V: 0.15 to 0.30%, Co: over 1.5 to 4.0%, W: over 2.6% to 4.0% or less, N: 0.010 to less than 0.025%, Nb: 0.01% to 0.15%, B: 0.005 to 0.015%, Re: 0.001 to 0.300% with the balance Fe and inevitable impurities.SELECTED DRAWING: None

Description

  Embodiments of the invention relate to heat resistant steel and steam turbine components.

  Thermal power generation systems tend to increase the steam temperature of the steam turbine in order to further increase the power generation efficiency. As a result, the high temperature characteristics required for heat resistant steels used in steam turbines become even more severe.

  Until now, many proposals have been made as heat resistant steels used in steam turbines. However, even these heat-resistant steels do not necessarily have sufficient long-term creep rupture strength. In order to contribute to further improvement in power generation efficiency as a heat-resistant steel used in a steam turbine, it is necessary to further improve the long-time creep rupture life.

JP-A-8-176749 Japanese Patent No. 4284010

  The problem to be solved by the present invention is to provide heat resistant steel and steam turbine components that can improve the creep rupture life for a long time and are excellent in high temperature characteristics and durability.

  The heat-resisting steel of the embodiment is mass%, C: 0.05 to 0.13%, Si: 0.01 to less than 0.10%, Mn: more than 0.15% and 0.50% or less, Ni : 0.01% to less than 0.40%, Cr: 9 to 11.5%, Mo: 0.01 to 0.50%, V: 0.15 to 0.30%, Co: over 1.5 4.0%, W: more than 2.6% and 4.0% or less, N: 0.010 to less than 0.025%, Nb: 0.01% to 0.15%, B: 0.005 -0.015%, Re: 0.001-0.300% is contained, and the remainder consists of Fe and inevitable impurities.

  In an embodiment according to the present invention, the inventors have made it possible to improve the efficiency of power generation in a power generation system, improve the long-term durability of a steam turbine, etc. As a result of diligent research to improve the long-term creep rupture life of heat-resistant steel used for steam turbine components, in order to improve the above characteristics, the Mn content, Ni content and Co content should be appropriate. It was found that it is effective to make it easier.

  The heat-resisting steel of the embodiment is mass%, C: 0.05 to 0.13%, Si: 0.01 to less than 0.10%, Mn: more than 0.15% and 0.50% or less, Ni : 0.01% to less than 0.40%, Cr: 9 to 11.5%, Mo: 0.01 to 0.50%, V: 0.15 to 0.30%, Co: over 1.5 4.0%, W: more than 2.6% and 4.0% or less, N: 0.010 to less than 0.025%, Nb: 0.01% to 0.15%, B: 0.005 -0.015%, Re: 0.001-0.300% is contained, and the remainder consists of Fe and inevitable impurities.

  The reason for limiting each component range in the heat resistant steel of the above-described embodiment will be described. In the following description, “%” representing a composition component is “% by mass” unless otherwise specified.

  Examples of inevitable impurities in the heat-resistant steel include P and S.

(1) C (carbon)
C secures hardenability and promotes martensitic transformation, forms M ₂₃ C ₆ type carbide with Fe, Cr, Mo, etc. in the alloy, and MX type charcoal with Nb, V, N, etc. It is an indispensable element for forming nitrides and increasing high temperature creep strength by precipitation strengthening. C contributes to improvement in yield strength and is an indispensable element for suppressing the formation of δ ferrite and BN. In order to exhibit these effects, it is necessary to contain 0.05% or more of C. On the other hand, if the C content exceeds 0.13%, the agglomeration and coarsening of carbides and carbonitrides easily occur and the high temperature creep rupture strength decreases. Therefore, the C content is determined to be 0.05 to 0.13%. For the same reason, the C content is preferably 0.08 to 0.12%.

(2) Si (silicon)
Si is an element effective as a deoxidizer for molten steel. When the Si content is 0.10% or more, segregation inside the steel ingot increases and the tempering embrittlement sensitivity becomes extremely high. And notch toughness is impaired, and by maintaining for a long time at high temperature, the change of a precipitate form is promoted and toughness deteriorates with time. On the other hand, when Si contains a trace amount, segregation decreases conversely, so that it is necessary to contain 0.01% or more of Si in order to exert these effects. Therefore, the Si content is determined to be 0.01 to less than 0.10%. For the same reason, the Si content is set to 0.0%.
It is preferable to set it as 3 to less than 0.10%.

(3) Mn (manganese)
Mn is an element that is effective as a deoxidizing agent and a desulfurizing agent at the time of dissolution, and also effective in improving hardenability and improving strength. In order to exhibit this effect, it is necessary to contain 0.15% or more of Mn. On the other hand, when the Mn content exceeds 0.50%, Mn combines with S to form non-metallic inclusions of MnS, lowering the toughness, promoting toughness deterioration with time, and high-temperature creep rupture. Reduce strength. Therefore, the Mn content is more than 0.15% and 0.50% or less. For the same reason, the Mn content is preferably more than 0.15 and not more than 0.30%.

(4) Ni (nickel)
When the Ni content is 0.4% or more, the agglomeration and coarsening of carbides and Laves phases are promoted, and the high temperature creep rupture strength is reduced. Therefore, the Ni content is set to less than 0.4%. On the other hand, since Ni is an element that suppresses the formation of δ ferrite and is useful for improving toughness, it is preferably contained in an amount of 0.01% or more. Therefore, the Ni content is determined to be 0.01 to less than 0.40%. For the same reason, the Ni content is preferably 0.01 to 0.30%.

(5) Cr (chrome)
Cr is an indispensable element for improving oxidation resistance and high-temperature corrosion resistance and for increasing high-temperature creep rupture strength by precipitation strengthening with M ₂₃ C ₆ type carbide. In order to exert these effects, it is necessary to contain 9% or more of Cr. On the other hand, as the Cr content increases, the tensile strength at room temperature and the short-time creep rupture strength on the high stress side increase, but on the other hand, the long-term creep rupture strength on the low stress side tends to decrease. This is considered to be a cause of the bending phenomenon of long creep rupture life on the low stress side. When the Cr content increases, the disappearance of the MX type carbonitride accelerates in a long time region, and the aggregation and coarsening of the M ₂₃ C ₆ type carbide accelerates, so that the substructure (microstructure) of the martensite structure This is due to significant changes and significant subgraining of the underlying tissue. These tendencies increase rapidly when the Cr content exceeds 11.5%. Therefore, the Cr content is determined to be 9 to 11.5%. For the same reason, the Cr content is preferably 9 to 11%.

(6) Mo (molybdenum)
Mo is dissolved in the alloy to strengthen the matrix, and fine carbides and a fine Laves phase are generated to improve the high temperature creep rupture strength. Mo is an element effective for suppressing temper embrittlement. In order to exhibit these effects, it is necessary to contain Mo 0.01% or more. On the other hand, if the Mo content exceeds 0.50%, δ ferrite is generated, and the toughness is significantly reduced. Therefore, the Mo content is determined to be 0.01 to 0.50%. For the same reason, the Mo content is preferably 0.01 to 0.40%.

(7) V (Vanadium)
V is an element effective for forming fine carbides and carbonitrides and improving high temperature creep rupture strength. In order to exhibit this effect, it is necessary to contain V 0.15% or more. On the other hand, when the content of V exceeds 0.30%, excessive precipitation of carbides occurs, leading to a decrease in high-temperature creep rupture strength. Therefore, the V content is determined to be 0.15 to 0.30%. For the same reason, the V content is preferably 0.17 to 0.27%.

(8) Co (Cobalt)
Co suppresses the formation of δ ferrite and improves high temperature tensile strength and high temperature creep rupture strength by solid solution strengthening. This is because the Ac ₁ transformation point is hardly changed by the addition of Co. In order to exhibit these effects, it is necessary to contain Co exceeding 1.5%. On the other hand, Co reduces the solid solubility limit of W, thereby promoting aggregation and coarsening of the Laves phase and the μ phase and causing a decrease in creep strength for a long time. Therefore, from the viewpoint of suppressing the decrease in creep strength for a long time, the Co content is set to 4.0% or less. For the same reason, the Co content is preferably more than 1.5 and not more than 3.0%.

(9) W (tungsten)
W suppresses aggregation and coarsening of M ₂₃ C ₆ type carbide. W is an element effective for improving high-temperature tensile strength and high-temperature creep rupture strength by solid-solution strengthening in the alloy by dissolving in the alloy to disperse and precipitate the Laves phase on the lath boundary. In order to exhibit these effects, it is necessary to contain 2.6% or more of W. On the other hand, if the W content exceeds 4.0%, δ ferrite and a coarse Laves phase are likely to be generated, ductility and toughness are lowered, and high-temperature creep rupture strength is also lowered. Therefore, the W content is set to be over 4% and over 2.6%. For the same reason, it is preferable that the W content is more than 2.6% and not more than 3.5%.

(10) N (nitrogen)
N combines with C, Nb, V, etc. to form carbonitrides and improves high temperature creep rupture strength. If the N content is less than 0.010%, sufficient tensile strength and high temperature creep rupture strength cannot be obtained. On the other hand, N has a strong connection with B, and when the N content is 0.025% or more, BN nitride is generated, which makes it difficult to produce a sound ingot, and hot working Decreases in ductility and toughness. Moreover, since the content of the solid solution B effective for the high temperature creep rupture strength decreases due to the precipitation of the BN phase, the high temperature creep rupture strength decreases. Therefore, the N content is determined to be 0.010 to less than 0.025%. For the same reason, the N content is preferably 0.012 to 0.020%.

(11) Nb (Niobium)
Nb is effective in improving the tensile strength at room temperature, forms fine carbides and carbonitrides, and improves high temperature creep rupture strength. Moreover, Nb produces | generates fine NbC, promotes refinement | miniaturization of a crystal grain, and improves toughness. Part of Nb also has the effect of precipitating MX type carbonitride compounded with V carbonitride to improve the high temperature creep rupture strength. In order to exhibit these effects, it is preferable to contain Nb 0.01% or more. On the other hand, when the Nb content exceeds 0.15%, coarse carbides and carbonitrides are precipitated, and ductility and toughness are lowered. Therefore, the Nb content is determined to be 0.01 to 0.15%. For the same reason, the Nb content is preferably 0.01 to 0.10%.

(12) B (boron)
B increases hardenability and improves toughness when added in a small amount. In addition, B has the effect of suppressing aggregation and coarsening of carbide, carbonitride, and Laves phase in the martensite packet, martensite block, martensite lath and martensite lath of the austenite grain boundary and its substructure for a long time at high temperature. Have. Further, B is an element effective for improving the high temperature creep rupture strength by being added in combination with W, Nb or the like. In order to exhibit these effects, it is necessary to contain B 0.005% or more. On the other hand, if the B content exceeds 0.015%, B and N are combined to precipitate a BN phase, hot workability is impaired, and high temperature creep rupture ductility and toughness are greatly reduced. In addition, precipitation of the BN phase reduces the content of solid solution B effective for high-temperature creep rupture strength, so that the high-temperature creep rupture strength decreases. Therefore, the B content is determined to be 0.005 to 0.015%. For the same reason, the B content is preferably 0.007 to 0.012%.

(13) Re (Rhenium)
Re is an element effective for improving the solid solution amount of W in the matrix as well as strengthening the solid solution by dissolving in the alloy. In addition, Re contributes to maintaining high-temperature creep characteristics for a long time by suppressing the diffusion of W at high temperatures and delaying the precipitation of Laves phases and aggregation coarsening. Element. In order to exhibit these effects, it is preferable to contain Re 0.001% or more. Further, when the Re content is 0.300% or less, these effects can be remarkably exhibited. Therefore, the Re content is determined to be 0.001 to 0.300%. For the same reason, the Re content is preferably 0.001 to 0.25%.

(14) P (phosphorus), S (sulfur)
P and S are classified as inevitable impurities in the heat resistant steel of the embodiment. These inevitable impurities are preferably made to have a residual content as close to 0% as possible.

  The heat-resisting steel having the above-described composition component range is suitable as a material constituting a component of a steam turbine, for example. Examples of the component parts of the steam turbine include forged parts such as moving blades and high-temperature turbine rotors (high-pressure turbine rotor, medium-pressure turbine rotor, high-medium-pressure turbine rotor, etc.). The high temperature turbine rotor may be provided with a turbine disk.

  All the parts of the component parts of the steam turbine described above may be made of the above heat resistant steel, or some parts of the component parts may be made of the above heat resistant steel.

  In addition, since the heat resistant steel having the above compositional component range can improve the creep rupture life for a long time, the high temperature turbine rotor (high pressure turbine rotor, medium pressure turbine rotor, etc.) of a steam turbine can be used by using this heat resistant steel. By configuring the components of the steam turbine such as the high and medium pressure turbine rotor, the moving blade, and the turbine disk, it is possible to provide the components of the steam turbine having high reliability even in a high temperature environment.

  The heat-resisting steel having the above compositional component range is excellent in long-term creep rupture life particularly in a high-temperature region. Specifically, for example, a component such as a rotor blade, a turbine rotor, or a turbine disk that is used under a high temperature of 566 to 610 ° C. at the steady state maximum steam temperature, or a steady state maximum steam temperature. It is suitable for heat-resistant steel constituting components such as a moving blade, a turbine rotor, and a turbine disk used at a high temperature of 593 to 630 ° C.

  Here, the heat-resistant steel of the embodiment and a method for manufacturing a component (forged product) of a steam turbine manufactured using the heat-resistant steel will be described.

  The heat resistant steel of the present embodiment is manufactured as follows, for example.

  The raw materials necessary for obtaining the compositional components constituting the heat-resistant steel are melted in a melting furnace such as an arc electric furnace or a vacuum induction electric furnace, and are refined and degassed. Thereafter, the molten metal is poured into a mold of a predetermined size and solidified over time to form a steel ingot. The ingot that has been solidified is heated to 1100 to 1250 ° C., subjected to forging (hot working), and then subjected to quenching and tempering. Through such processes, heat-resistant steel is manufactured.

  Components such as a moving blade and a turbine rotor of a steam turbine are manufactured as follows, for example.

  First, the raw materials necessary to obtain the composition components constituting the heat-resistant steel, which constitute the components of the steam turbine, are melted in a melting furnace such as an arc electric furnace or a vacuum induction electric furnace, and are refined and degassed. I do. Thereafter, the molten metal is poured into a mold of a predetermined size and solidified over time to form a steel ingot. In addition, when pouring in a vacuum environment, since vacuum degassing is performed, the gas component in the steel ingot is further reduced, leading to a reduction in non-metallic inclusions.

  The ingot that has been solidified is heated to 1100 to 1250 ° C. and subjected to forging (hot working) to the shape of each component such as a turbine blade and a turbine rotor by a large press. After the forging process, a quenching process and a tempering process are performed. Through these steps, components such as turbine blades and turbine rotors of a steam turbine are manufactured.

  Here, it is preferable to set the heating temperature in the forging process to a temperature range of 1100 to 1250 ° C. If the temperature is lower than 1100 ° C., the hot workability of the material cannot be sufficiently obtained, and the center part of the component part Forging effect is not sufficient, or may cause forging cracks during forging deformation. When the temperature exceeds 1250 ° C., coarsening of crystal grains and non-uniformity of crystal grains become prominent. This is because the deformation due to the non-uniformity becomes a cause of crystal grain coarsening or non-uniformity during the quenching process performed after forging.

  Here, the quenching process and the tempering process will be described.

(Quenching process)
Most of the carbides and carbonitrides generated in the material by quenching heating are once dissolved in the matrix, and then the carbides and carbonitrides are finely and uniformly precipitated in the matrix by subsequent tempering treatment. As a result, the high temperature creep rupture strength can be improved, and the creep rupture ductility and toughness can be improved.

  The quenching temperature is preferably set in a temperature range of 1040 to 1120 ° C. When the quenching temperature is less than 1040 ° C., solid solution of the relatively coarse carbide and carbonitride precipitated by the forging process is not sufficient in the matrix, and the coarse undissolved solution after the subsequent tempering treatment. Remains as carbide or non-solid carbonitride. Therefore, it is difficult to obtain good high temperature creep rupture strength, ductility and toughness.

  In the heat-resistant steel according to the embodiment, Nb is added in order to improve the high temperature creep rupture strength by finely depositing MX type carbonitride in the matrix. In order to fully exhibit the effect of Nb, it is necessary to completely dissolve Nb in the austenite matrix when heated to the quenching temperature. However, when the quenching temperature is less than 1040 ° C., Nb remains as a coarse carbonitride generated during solidification in an undissolved state, and the effect of improving the creep rupture strength cannot be sufficiently obtained, and ductility and toughness are not obtained. It may cause a decrease in fatigue strength. In order to dissolve such coarse carbonitrides once and effectively precipitate them in large quantities as fine carbonitrides after quenching, it is necessary to set the quenching temperature to 1040 ° C. or higher.

  On the other hand, when the quenching temperature exceeds 1120 ° C., a δ ferrite phase is generated in the austenite phase, and the crystal grains are coarsened, and the ductility and toughness are greatly reduced.

  In the quenching process, after quenching, the forging material is preferably cooled at a cooling rate of air cooling or higher in order to obtain a predetermined microstructure. As a cooling rate in the quenching treatment, it is more preferable that the quenching is performed at a cooling rate of 100 ° C./hour or more after quenching. As a cooling method for obtaining a cooling rate in this range, for example, oil cooling can be employed.

  The cooling rate described above can be a cooling rate at a portion where the cooling rate is the smallest in the forging material. Further, when the forging material is a turbine rotor or the like, the cooling rate may be defined as, for example, the cooling rate at the center of the forging material. In this case, the central portion of the forging material refers to the center in the axial direction on the central axis, for example.

  Further, if the forging material is made of a structure having a predetermined thickness, the central portion of the forging material may be the central portion of the thickness.

(Tempering treatment)
The tempering process decomposes the residual austenite structure generated by the above-described quenching process to form a tempered martensite structure, and uniformly disperses and precipitates carbides and carbonitrides in the matrix and restores the dislocation structure to an appropriate level. . This provides the necessary high temperature creep rupture strength, rupture ductility and toughness.

  This tempering treatment is preferably carried out twice. The first tempering process (first-stage tempering process) is intended to decompose the retained austenite structure. From the viewpoint of removing as much as possible the retained austenite after quenching, the tempering process is performed at 540 to 600 ° C. It is preferably performed in the temperature range.

  That is, most of the metal structure subjected to the quenching treatment is transformed into a martensite and becomes a quenched martensite structure, but a part of the austenite structure remains. By the first stage tempering process, the quenched martensite structure becomes a tempered martensite structure, and by the first stage tempering process, the retained austenite structure becomes a quenched martensite structure and the retained austenite structure disappears.

  If the temperature of the first stage tempering treatment is less than 540 ° C., the residual austenite structure is not sufficiently decomposed, and the transformation from the residual austenite structure to the quenched martensite structure may be insufficient. On the other hand, when the temperature of the first stage tempering treatment exceeds 600 ° C., the precipitation site of MX type carbonitride decreases, and coarse MX type carbonitride tends to precipitate. Further, carbides and carbonitrides are preferentially precipitated in the martensite structure rather than in the retained austenite structure, and the precipitates are dispersed and precipitated unevenly, resulting in a decrease in high temperature creep rupture strength.

  The second tempering process (second stage tempering process) aims to obtain the necessary high temperature creep rupture strength, rupture ductility and toughness by making the entire material a tempered martensite structure. It is preferable to be performed in a temperature range of ˜730 ° C.

  That is, the retained austenite structure partially remaining in the material is transformed into a quenched martensite structure by the first stage tempering process, but the ductility and toughness are inferior in this state, so the second stage tempering process is further performed. Therefore, it is necessary to have a tempered martensite structure.

  The first stage tempering process and the second stage tempering process can be performed continuously.

  If the temperature of the second stage tempering treatment is less than 650 ° C., precipitates such as carbides and carbonitrides do not precipitate in a stable state, so that high-temperature creep rupture strength, ductility, and toughness cannot obtain predetermined characteristics. If the temperature of the second stage tempering treatment is less than 650 ° C., the transformation of the quenching martensite structure to the tempered martensite structure does not proceed sufficiently, and the precipitation of carbides and carbonitrides does not sufficiently reach the equilibrium state. However, when creep progresses at a high temperature exceeding 600 ° C., aggregation and coarsening become remarkable.

  On the other hand, when the temperature of the second stage tempering treatment exceeds 730 ° C., coarse precipitation of carbides and carbonitrides occurs, and the required high temperature creep rupture strength cannot be obtained. On the other hand, when the temperature of the second stage tempering process exceeds 730 ° C., the tempering process proceeds excessively, and the strength characteristics such as tensile strength and proof stress do not satisfy required values.

  Among the above temperature ranges, in particular, when the second-stage tempering treatment is performed at 650 to 690 ° C., a heat resistant steel exhibiting excellent creep strength under high stress conditions can be obtained, 690 to 730 ° C. When the second-stage tempering treatment is performed, a heat-resisting steel exhibiting excellent creep strength under low stress conditions can be obtained. As described above, in the temperature range of 650 to 730 ° C., by appropriately selecting the treatment temperature and performing the second stage tempering treatment, it is possible to obtain a heat resistant steel suitable for the intended use.

  In the second-stage tempering process, after the second-stage tempering, the forging material is cooled at a cooling rate of 20 to 60 ° C./hour so as not to generate strain in the stress concentration part such as a shape change portion during cooling. It is preferable.

  The cooling rate in the second stage tempering process can be defined in the same manner as described in the quenching process.

  Note that the method for producing the components of the steam turbine is not limited to the above-described method.

  According to the heat-resistant steel of the above-described embodiment, since the creep rupture life can be improved by setting the chemical composition constituting the heat-resistant steel to the above-described range, using this heat-resistant steel, for example, steam It has high reliability even in high-temperature environments by configuring steam turbine components such as turbine high-temperature turbine rotors (high-pressure turbine rotor, medium-pressure turbine rotor, high-medium-pressure turbine rotor, etc.), rotor blades, and turbine disks. Steam turbine components can be provided. Therefore, by applying these components, the steam temperature can be increased in the steam turbine, and the power generation efficiency can be improved.

  Below, it demonstrates that the heat-resistant steel of one embodiment which concerns on this invention is excellent in the high temperature creep rupture life.

(sample)
Table 1 shows chemical composition components of each sample (sample 1 to sample 56) used for the material property evaluation. Sample 1 to sample 41 are examples of the heat resistant steel of the embodiment according to the present invention, and sample 42 to sample 56 are heat resistant steels that are not in the chemical composition range of the heat resistant steel of the embodiment according to the present invention. It is a comparative example.

  These samples were formed as follows.

  Raw materials constituting each sample were melted in a vacuum induction melting furnace (VIM), degassed, and poured into a mold. And it solidified in the metal mold | die and produced the 20kg steel ingot.

  Subsequently, each solidified ingot was heated to 1200 ° C., and forging was performed at a working ratio of 3 forging ratio. Subsequently, a quenching process, a first stage tempering process, and a second stage tempering process were performed.

  Here, the forging ratio refers to the cross-sectional area of the forged object perpendicular to the direction in which the forged object is elongated before the forging process, and the direction in which the forged object is elongated after the forging process. Is divided by the cross-sectional area of the forging object perpendicular to.

  In the quenching treatment, the steel ingot was heated and held at a temperature of 1070 ° C. for 5 hours, and then the steel ingot was cooled at a cooling rate of 100 ° C./hour (cooling rate at the center of the steel ingot). In the first stage tempering treatment, the steel ingot after the quenching treatment was heated and held at a temperature of 570 ° C. for 20 hours. In the second stage tempering treatment, the steel ingot after the first stage tempering treatment was heated and held at a temperature of 680 ° C. for 20 hours, and then the steel ingot was cooled at a cooling rate of 50 ° C./hour. Here, the cooling rate after the second-stage tempering treatment was the cooling rate at the center of the steel ingot.

(Tensile test)
Using the samples 1 to 56 described above, a tensile test was performed at 25 ° C. The test piece was produced from each steel ingot described above.

  The tensile test was performed according to JIS Z 2241 (metal material tensile test method). Table 2 shows the results (tensile strength) of the tensile test for each sample.

(Charpy impact test)
A Charpy impact test was performed at 25 ° C. using Samples 1 to 56 described above. The test piece was produced from each steel ingot described above.

  The Charpy impact test was carried out according to JIS Z 2242 (Charpy impact test method for metal materials). Table 2 shows the results (Charpy impact absorption energy) of the Charpy impact test for each sample.

(Creep rupture test)
Using the samples 1 to 56 described above, a creep rupture test was performed under the conditions of 625 ° C. and 20 kgf / mm ² . The test piece was produced from each steel ingot described above.

  The creep rupture test was carried out according to JIS Z 2271 (Creep of metal material and creep rupture test method). Table 2 shows the result (creep rupture life) of the creep rupture test for each sample.

  As shown in Table 2, Samples 1 to 41 satisfy the tensile strength of 930 MPa or more, Charpy impact absorption energy of 14 J or more, and creep rupture life of 10000 h or more, and have excellent balance properties. Is greatly inferior in either characteristic.

(Effect of quenching temperature and tempering temperature)
The effects of quenching and tempering temperatures on creep rupture life were investigated.

  Here, a steel ingot having the composition shown in Sample 10 was used, and quenching treatment and tempering treatment were performed under the following conditions. As quenching temperatures in the quenching treatment, four conditions of 1020 ° C., 1070 ° C., 1100 ° C., and 1150 ° C. were performed, and the respective quenching temperatures were heated and held for 5 hours. After heating and holding for 5 hours, cooling was performed at a cooling rate of 100 ° C./hour (cooling rate at the center of the steel ingot).

  As the first stage tempering temperature in the first tempering treatment, three conditions of 530 ° C., 570 ° C., and 610 ° C. were performed, and each first stage tempering temperature was heated and held for 20 hours.

  As the second stage tempering temperature in the second tempering treatment, four conditions of 630 ° C., 680 ° C., 710 ° C., and 770 ° C. were performed, and the respective second stage tempering temperatures were heated and held for 20 hours. After heating and holding for 20 hours, cooling was performed at a cooling rate of 50 ° C./hour. Here, the cooling rate after the second tempering treatment is the cooling rate at the center of the steel ingot.

Then, a test piece is prepared from each steel ingot, a creep rupture test is performed under the conditions of 625 ° C., 20 kgf / mm ² , 650 ° C., and 15 kgf / mm ^{2 in} the same manner as described above, and the creep rupture life is evaluated. did. Table 3 shows the test results of the creep rupture test.

  As shown in Table 3, in the samples heat-treated with a quenching temperature of 1070 ° C. or 1100 ° C., a first stage tempering temperature of 570 ° C., and a second stage tempering temperature of 680 ° C. or 710 ° C., It can be seen that the creep rupture life is long and the creep rupture properties are superior compared to the sample heat-treated under the conditions.

  Thus, it can be seen that the creep rupture properties are affected by the heat treatment conditions of the quenching treatment and the tempering treatment, and by applying appropriate heat treatment conditions, a heat resistant steel excellent in the creep rupture properties can be obtained.

In addition, the heat-treated at 680 ° C. in the second stage tempering treatment has particularly excellent creep rupture life under the creep conditions of 625 ° C. and 20 kgf / mm ^2. It can be seen that those subjected to heat treatment at 650 ° C. show particularly excellent characteristics in terms of creep rupture life under creep conditions of 650 ° C. and 15 kgf / mm ² . Therefore, it can be seen that by appropriately selecting the heat treatment conditions, it is possible to obtain a heat-resistant steel having more excellent characteristics suitable for each application.

  According to at least one embodiment described above, it is possible to improve the long-term creep rupture life.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

Claims (4)

  In mass%, C: 0.05 to 0.13%, Si: 0.01 to less than 0.10%, Mn: more than 0.15% and 0.50% or less, Ni: 0.01% to 0 Less than 40%, Cr: 9 to 11.5%, Mo: 0.01 to 0.50%, V: 0.15 to 0.30%, Co: more than 1.5, 4.0%, W : More than 2.6% and 4.0% or less, N: 0.010 to less than 0.025%, Nb: 0.01% to 0.15%, B: 0.005 to 0.015%, Re : Heat-resistant steel containing 0.001 to 0.300%, the balance being Fe and inevitable impurities.   The quenching temperature is 1040 to 1120 ° C, the first stage tempering treatment temperature is 540 to 600 ° C, and the second stage tempering treatment temperature is 650 to 730 ° C. Heat-resistant steel described in 1.   The heat-resistant steel according to claim 1 or 2, wherein a cooling rate after the quenching is set to 100 ° C / hour or more. A steam turbine component having at least a predetermined portion made of the heat-resistant steel according to any one of claims 1 to 3.