JP2012237049A - Heat resistant steel and steam turbine component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat resistant steel and a steam turbine component constituted by the heat resistant steel, which can suppress precipitation of a Z phase and improve creep rupture strength.SOLUTION: The heat resistant steel comprises, by mass%, 0.05-0.2 C, ≤0.1 Si, ≤0.15 Mn, 0.05-1 Ni, 8-10 Cr, 0.05-1 Mo, 0.01-0.05 V, 0.5-5 Co, 1-3 W, 0.006-0.012 N, 0.003-0.03 B, 0.15-0.5 Nb+Ta, and the balance Fe with inevitable impurities.

Description

  Embodiments of the present invention relate to a heat resistant steel excellent in creep rupture life and a steam turbine component made of the heat resistant steel.

  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.

  Many proposals have been made so far as heat-resistant steel used in steam turbines. As a heat-resistant steel used for steam turbines, it is necessary to improve the long-term creep rupture life in order to contribute to further improvement in power generation efficiency.

JP 2002-256396 A

  However, the phenomenon that the long creep rupture life is shorter than the long creep rupture life estimated from the relatively short creep rupture life under the temperature accelerated creep test condition ("bending phenomenon" or "bending phenomenon" Has been pointed out in recent heat-resisting steel research.

The MX phase and the M ₂ X phase are effective in improving the creep rupture strength, but there is a problem that the creep rupture strength decreases due to the precipitation of the Z phase that grows while consuming these constituent elements. Here, the Z phase is a composite nitride of Cr, Nb, and V.

  The problem to be solved by the present invention is to provide a heat resistant steel capable of suppressing the precipitation of the Z phase and improving the creep rupture strength, and a steam turbine component composed of the heat resistant steel.

  The heat-resisting steel of the embodiment is mass%, C: 0.05 to 0.2, Si: 0.1 or less, Mn: 0.15 or less, Ni: 0.05 to 1, Cr: 8 to 10, Mo : 0.05 to 1, V: 0.01 to 0.05, Co: 0.5 to 5, W: 1 to 3, N: 0.006 to 0.012, B: 0.003 to 0.03 , Nb + Ta (including only one of them): 0.15 to 0.5, with the balance being Fe and inevitable impurities.

  According to the heat-resistant steel and the steam turbine component according to the present invention, the precipitation of the Z phase can be suppressed and the creep rupture strength can be improved.

It is a figure which shows the relationship between the content rate of N and the mass fraction of a Z phase which is a thermodynamic calculation result for determining the composition component range of the heat resistant steel of embodiment. FIG. 3 shows the relationship between the Nb content and the Z phase mass fraction when the V content is 0.02 mass%, which is a thermodynamic calculation result for determining the compositional component range of the heat resistant steel of the embodiment. FIG. The thermodynamic calculation result for determining the composition component range of the heat resistant steel of the embodiment shows the relationship between the Nb content and the Z phase mass fraction when the V content is 0.2% by mass. FIG.

In the heat resistant steel of the embodiment, the creep rupture strength is improved by suppressing the precipitation of the Z phase that grows while consuming the constituent elements of the MX phase and the M ₂ X phase. Therefore, in the present embodiment, attention is paid particularly to V, N, and Nb as the composition components that suppress the precipitation of the Z phase, and the contents thereof are determined based on the following thermodynamic calculation. The following is a part of the thermodynamic calculation result for determining the contents of V, N, and Nb.

  FIG. 1 is a diagram showing the relationship between the N content and the mass fraction of the Z phase, which is a thermodynamic calculation result for determining the composition component range of the heat resistant steel of the embodiment. These relationships were obtained by thermodynamic calculation software (Thermo-calc, product of Thermocalc). The temperature condition in the thermodynamic calculation was 600 ° C. Here, in terms of mass%, C is 0.1, Si is 0.05, Mn is 0.1, Ni is 0.2, Cr is 9.9, Mo is 0.6, and V is 0.03. , Co was 3, W was 1.8, B was 0.01, Nb was 0.3, the N content was changed, and the balance was Fe.

  FIG. 2 is a thermodynamic calculation result for determining the compositional component range of the heat resistant steel of the embodiment, and the Nb content and the Z phase mass fraction when the V content is 0.02 mass%. It is a figure which shows the relationship. FIG. 3 is a thermodynamic calculation result for determining the compositional component range of the heat resistant steel of the embodiment, and the Nb content and the Z phase mass fraction when the V content is 0.2 mass%. It is a figure which shows the relationship. These relationships were also determined by the thermodynamic calculation software described above. The temperature condition in the thermodynamic calculation was 600 ° C. Also, here, in mass%, C is 0.1, Si is 0.05, Mn is 0.1, Ni is 0.2, Cr is 9.9, Mo is 0.6, and V is 0.02. Alternatively, 0.2, Co, W, 1.8, B, 0.01 and N were contained, the content of Nb was changed, and the balance was Fe.

  Here, the above-described thermodynamic calculation software (Thermo-calc, manufactured by Thermocalc) is a general-purpose thermodynamic equilibrium calculation tool based on the CALPHAD method, and is widely used as a de facto standard software.

As shown in FIG. 1, it can be seen that the mass fraction of the Z phase decreases as the mass fraction of N decreases. When the V content is 0.02% by mass, as shown in FIG. 2, when the Nb mass fraction is less than 7 × 10 ⁻⁴ (Nb content is less than 0.07% by mass), Nb As the mass fraction of Nb increases, the mass fraction of the Z phase increases, and when the mass fraction of Nb is 7 × 10 ⁻⁴ or more (Nb content is 0.07 mass% or more), the mass of Nb As the fraction increases, the mass fraction of the Z phase decreases. On the other hand, when the V content is 0.2% by mass, as shown in FIG. 3, as the mass fraction of Nb increases, the mass fraction of the Z phase decreases monotonously.

Comparing the results of FIG. 2 and FIG. 3, when the mass fraction of Nb is 7 × 10 ⁻⁴ or more (Nb content is 0.07 mass% or more), the V content is 0.02 mass%. When the V content is 0.2% by mass, the mass fraction of the Z phase decreases more rapidly as the mass fraction of Nb increases.

  Here, it is conceivable to reduce the mass fraction of the Z phase by reducing both V and Nb, but these elements are constituent elements of the MX phase that are effective in improving the creep rupture strength. Therefore, reducing both is expected to lead to a decrease in creep rupture strength. Then, based on the above-mentioned thermodynamic calculation result, the heat resistant steel in the embodiment which concerns on this invention shown next was obtained.

  The heat-resisting steel in the embodiment according to the present invention is mass%, C: 0.05 to 0.2, Si: 0.1 or less, Mn: 0.15 or less, Ni: 0.05 to 1, Cr: 8 to 10, Mo: 0.05 to 1, V: 0.01 to 0.05, Co: 0.5 to 5, W: 1 to 3, N: 0.006 to 0.012%, B: 0 0.003 to 0.03%, Nb + Ta (including only one of them): 0.15 to 0.5, the balance being 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.

(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 0.2%, 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 0.2%. For the same reason, the C content is more preferably 0.09 to 0.17%.

(2) Si (silicon)
Si is an element effective as a deoxidizer for molten steel. If the Si content exceeds 0.1%, segregation inside the steel ingot increases and the temper embrittlement susceptibility becomes extremely high. And notch toughness is impaired, and by maintaining at high temperature for a long time, the change of the precipitate form is promoted, and the toughness deteriorates with time. Therefore, the Si content is set to 0.1% or less. Moreover, in order to exhibit the effect as a deoxidizer of molten steel, it is preferable to contain 0.01% or more of Si. That is, the preferable Si content is set to 0.01 to 0.1%.

  Recently, the vacuum carbon deoxidation method and the electroslag remelting method are generally applied, and it is not always necessary to perform deoxidation with Si. In this case, the Si content can be suppressed to 0.05% or less. Therefore, a more preferable Si content is set to 0.01 to 0.05%.

(3) Mn (manganese)
Mn is an effective element as a deoxidizing agent or desulfurizing agent at the time of dissolution, and is also an effective element for improving hardenability and improving strength. When the Mn content exceeds 0.15%, Mn combines with S to form non-metallic inclusions of MnS, lowering the toughness, promoting toughness deterioration with time, and increasing the high temperature creep rupture strength. Reduce. Therefore, the Mn content is set to 0.15% or less. Moreover, in order to exhibit the effect as a deoxidizer or a desulfurization agent at the time of melt | dissolution, it is preferable to contain Mn 0.01% or more. That is, the preferable Mn content is set to 0.01 to 0.15%.

  Recently, refining techniques such as out-of-furnace refining have made it easier to reduce the S content, and there is no need to add Mn as a desulfurizing agent. In this case, the Mn content can be suppressed to 0.1% or less. Therefore, a more preferable Mn content is set to 0.01 to 0.1%.

(4) Ni (nickel)
Ni is an austenite stabilizing element and is effective in improving toughness. It is also effective for increasing hardenability, suppressing the formation of δ ferrite, and increasing strength and toughness at room temperature. In order to exhibit these effects, it is necessary to contain Ni 0.05% or more. On the other hand, if the Ni content exceeds 1%, the agglomeration and coarsening of carbides and Laves phases are promoted, and the high-temperature creep rupture strength is lowered or temper brittleness is promoted. Therefore, the Ni content is determined to be 0.05 to 1%. For the same reason, the Ni content is more preferably 0.1 to 0.5%.

(5) Cr (chrome)
Cr is an indispensable element for improving oxidation resistance and high-temperature corrosion resistance, and increasing high-temperature creep rupture strength by precipitation strengthening with M ₂₃ C ₆ type carbide or M ₂ X type carbonitride. In order to exhibit these effects, it is necessary to contain 8% 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 increase, but on the other hand, the long-term creep rupture strength tends to decrease. This is also considered to be a cause of the bending phenomenon of long creep rupture life. In addition, when the Cr content increases, the substructure (microstructure) of the martensite structure changes significantly over a long period of time, resulting in subgraining of the substructure and significant aggregation and coarsening of precipitates near the grain boundaries. Degradation of the microstructure proceeds, such as conversion and a marked decrease in dislocation density. These tendencies increase rapidly when the Cr content exceeds 10%. Therefore, the Cr content is determined to be 8 to 10%. For the same reason, the Cr content is more preferably 8 to 9.2%.

(6) Mo (molybdenum)
Mo forms a solid solution in the alloy to strengthen the matrix, and generates fine carbon (nitride) and a fine Laves phase 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 0.05% or more. On the other hand, if the Mo content exceeds 1%, δ ferrite is generated, and the toughness is remarkably lowered and the high temperature creep rupture strength is also lowered. Therefore, the Mo content is determined to be 0.05 to 1%. For the same reason, the Mo content is more preferably 0.2 to 0.8%.

(7) V (Vanadium)
V is an element effective in forming fine carbides and carbonitrides and improving high temperature creep rupture strength. In order to exhibit this effect, it is necessary to contain V 0.01% or more. On the other hand, if the V content exceeds 0.05%, the amount of precipitation of the Z phase (Cr, Nb, V composite nitride) increases rapidly, and the creep rupture strength decreases for a long time. Therefore, the V content is determined to be 0.01 to 0.05%. For the same reason, the V content is more preferably 0.01 to 0.025%.

(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 addition of Co hardly reduces the Ac ₁ transformation point, so that the formation of δ ferrite can be suppressed without reducing the structural stability. In order to exhibit these effects, it is necessary to contain Co 0.5% or more. On the other hand, when the Co content exceeds 5%, ductility and high-temperature creep rupture strength are lowered, and the production cost is increased. Therefore, the content ratio of Co is set to 0.5 to 5%. For the same reason, the Co content is more preferably 0.5 to 4%.

(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. These effects are significant when combined with Mo. In order to exhibit these effects, it is necessary to contain 1% or more of W. On the other hand, if the W content exceeds 3%, 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 determined to be 1 to 3%. For the same reason, the W content is more preferably 1.5 to 2%.

(10) N (nitrogen)
N forms carbonitride by combining with C, Nb, V, etc., and improves high temperature creep rupture strength. If the N content is less than 0.006%, sufficient tensile strength and high temperature creep rupture strength cannot be obtained. On the other hand, N is strongly bound to B, and when the N content exceeds 0.012%, the content of solid solution B effective for high-temperature creep rupture strength decreases, and high-temperature creep rupture strength decreases. Moreover, since N is a main constituent element of the Z phase, it is closely related to the amount of precipitation of the Z phase. If the N content exceeds 0.012%, the bending phenomenon occurs due to excessive precipitation of the Z phase. Facilitate. Therefore, the N content is determined to be 0.006 to 0.012%. For the same reason, the N content is more preferably 0.006 to 0.01%.

(11) Nb (niobium) and Ta (tantalum)
Nb and Ta may each contain only one or both. 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 Nb (C, N), 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.

  Nb also has an action of suppressing the precipitation of the Z phase, and when the Nb content is 0.15% or more, the effect of suppressing the precipitation of the Z phase is large. Furthermore, this effect is achieved when the V content is 0.05%. It becomes extremely noticeable in the following cases. As described above, the effect of suppressing the Z-phase precipitation becomes remarkable by the interaction by decreasing the V content and increasing the Nb content.

  On the other hand, when the Nb content exceeds 0.5%, coarse carbides and carbonitrides are precipitated, and ductility and toughness are lowered. Therefore, the Nb content is determined to be 0.15 to 0.5%. For the same reason, the Nb content is more preferably 0.21 to 0.3%.

  In addition, although the case where it contained only Nb was demonstrated above, Ta has the property very similar to Nb, and a part or all of Nb is substituted by Ta, and the above effects are obtained. can get. Therefore, even when only Ta is contained, the content of Ta is set to 0.15 to 0.5%, and the more preferable content is set to 0.21 to 0.3%. Further, even when both Nb and Ta are contained, the total content (Nb + Ta) of Nb and Ta is 0.15 to 0.5%, and a more preferable content is 0.21 to 0.3%. did.

(12) B (boron)
B increases hardenability and improves toughness when added in a small amount. In addition, B suppresses 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 the underlying structure for a long time at high temperature. Has an effect. 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 0.003% or more. On the other hand, if the B content exceeds 0.03%, 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 0.003 to 0.03%. For the same reason, the B content is more preferably 0.005 to 0.015%.

  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. As a component part of a steam turbine, forged parts, such as a turbine rotor, etc. are mentioned, for example. Moreover, casting parts, such as a casing of a turbine and a steam valve, are mentioned.

  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. Further, in the heat resistant steel having the compositional component range described above, since the precipitation of the Z phase is suppressed, the long-term creep rupture life can be improved. Therefore, by using this heat-resistant steel to configure the components of the steam turbine, it is possible to provide the components of the steam turbine having high reliability even in a high temperature environment.

(I) Forging 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 1200 ° C., subjected to forging treatment, and then subjected to tempering heat treatment (quenching treatment and tempering treatment). Through such processes, heat-resistant steel is manufactured.

  Components such as 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 ° C. to 1200 ° C., and a forging process (hot working) is performed to the shape of the component by a large press. After the forging process, tempering heat treatment (quenching process and tempering process) is performed. Through such a process, components such as a turbine rotor 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 ° C. to 1200 ° C. If the temperature is lower than 1100 ° C., the hot workability of the material cannot be sufficiently obtained, and the central part of the component part The forging effect in the steel may not be sufficient, or it may cause forging cracks during forging deformation. If the temperature exceeds 1200 ° C., coarsening of crystal grains and non-uniformity of crystal grains become remarkable. This is because deformation due to forging becomes non-uniform and causes crystal grain coarsening and non-uniformity during the quenching treatment of the tempering heat treatment performed after forging.

  Here, the tempering heat treatment 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. Can improve high temperature creep rupture strength, creep rupture ductility and toughness.

  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. 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 to reduce ductility and toughness.

  In the quenching process, after quenching, the forged material is preferably cooled at a cooling rate of 50 to 300 ° C./hour in the center of the forged material in order to obtain a quenched martensite structure. As a cooling method for obtaining a cooling rate in this range, for example, oil cooling can be employed.

  For example, when the forging material is a turbine rotor or the like, the central portion of the forging material refers to the center axis and the center in the axial direction. Moreover, if the forging material consists of a structure having a predetermined thickness, the center portion of the forging material means the center portion of the thickness. That is, these portions are portions where the cooling rate is the smallest in the forging material. In addition, although the cooling rate of the center part of a forge raw material is defined here, the above-mentioned cooling rate is good also as a cooling rate of the site | part where the cooling rate becomes the smallest in a forge raw material. The same applies to the tempering process.

(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 preferably performed in a temperature range of 540 to 600 ° C. for the purpose of decomposing the retained austenite structure. If the temperature of the first stage tempering treatment is less than 540 ° C., the residual austenite structure is not sufficiently decomposed. On the other hand, when the temperature of the first-stage tempering process exceeds 600 ° C., carbides and carbonitrides are more likely to precipitate preferentially in the martensite structure than in the retained austenite structure, and the precipitates are dispersed unevenly. As a result, the high temperature creep rupture strength decreases.

  In the first stage tempering treatment, after the first stage tempering, the forging material is 20 to 100 ° C./in the center of the forging material so that a large strain is not generated in the stress concentration part such as a shape change part during cooling. It is preferable to cool at the cooling rate of time. As a cooling method for obtaining a cooling rate in this range, for example, furnace cooling or air cooling can be employed.

  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 carried out in a temperature range of from ° C to 750 ° C. 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, and thus the characteristics required for high-temperature creep rupture strength, ductility and toughness cannot be obtained. On the other hand, when the temperature of the second stage tempering treatment exceeds 750 ° C., coarse precipitation of carbides and carbonitrides occurs, and the required high temperature creep rupture strength cannot be obtained.

  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. As a cooling method for obtaining a cooling rate in this range, for example, furnace cooling or the like can be employed. In addition, since the cooling in the second stage tempering process is performed at a small cooling rate by furnace cooling or the like, the temperature difference between the central portion and the outer peripheral portion of the forging material in the cooling process is small. Therefore, in the definition of the cooling rate in the second-stage tempering process, the central portion of the forging material is not limited, and for example, the cooling rate at any position in the forging material, such as the center portion or the outer peripheral portion of the forging material. It may be.

(Ii) Case of Casting Here, a heat-resistant steel according to the embodiment and a method for manufacturing a component (cast 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 melting furnace, and are refined and degassed. Then, for example, the molten metal is poured into a sand mold that is actively directional solidified and solidified over time. The cast steel material solidified and cooled to below the transformation point is taken out of the mold and subjected to high-temperature annealing at a temperature of 1000 to 1150 ° C. to recrystallize and diffuse the cast primary crystal structure and microsegregation formed during casting. Thereafter, tempering heat treatment (normalizing treatment and tempering treatment) is performed. Through such processes, heat-resistant steel is manufactured.

  The components of the steam turbine, such as the casing of the steam turbine and the casing of the steam valve, are manufactured as follows, for example.

  Here, the casing of the steam turbine, the casing of the steam valve, and the like have a cast-in weight of about 2 to 150 tons (product weight is about 1 to 50 tons), so in order to manufacture cast steel with good internal quality Requires advanced steelmaking and casting techniques.

  First, 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 melting furnace, and refining and degassing are performed. Thereafter, the molten metal is poured into a sand mold formed in accordance with the shape of the components of the steam turbine and solidified over time. In order to prevent casting defects such as shrinkage and cracks due to solidification from remaining inside the product, a casting method such as a sufficiently large feeder or padding with sufficient solidification direction is used. It is important to design in advance.

  The cast steel material solidified and cooled to below the transformation point is taken out of the mold and subjected to high-temperature annealing at a temperature of 1000 to 1150 ° C. to temporarily destroy the cast structure formed at the time of casting. In this state, the hot metal serving as the final solidified portion required at the time of casting is cut, and the padding attached to the product for directional solidification is removed.

  In the annealing treatment, after annealing, the cast steel material is preferably cooled relatively slowly at a cooling rate of 20 to 60 ° C./hour so that cracks do not occur in stress concentration sites such as shape change sites during cooling. As a cooling method for obtaining a cooling rate in this range, for example, furnace cooling or the like can be employed. In addition, since the cooling in the annealing process is performed at a small cooling rate by furnace cooling or the like, the temperature difference between the central portion and the outer peripheral portion of the cast steel material in the cooling process is small. Therefore, in the definition of the cooling rate in the annealing process, the central portion of the cast steel material is not limited. For example, the cooling rate at any position in the cast steel material, such as the central portion or the outer peripheral portion of the cast steel material. Good.

  After the annealing treatment, tempering heat treatment (normalizing treatment and tempering treatment) is performed. Through these steps, the components of the steam turbine are manufactured.

  Here, it is preferable to set the annealing temperature to a temperature range of 1000 to 1150 ° C. If the annealing temperature is less than 1000 ° C, the fracture of the cast structure formed during casting is insufficient, and the annealing temperature is 1150 ° C. If it exceeds, the crystal grains become coarse and non-uniform, and cracks are likely to occur when the hot metal is cut or the attached meat is removed.

  Here, the tempering heat treatment will be described.

(Normalization processing)
Almost all of the carbides and carbonitrides generated in the material are temporarily dissolved in the matrix by normalizing heating, and then the carbides and carbonitrides are finely and uniformly deposited in the matrix by subsequent tempering treatment. Can improve high temperature creep rupture strength, creep rupture ductility and toughness.

  The normalizing temperature is preferably set to a temperature range of 1000 to 1200 ° C. When the normalizing temperature is less than 1000 ° C., solid solution of the relatively coarse carbide or carbonitride precipitated by the casting 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. On the other hand, when the normalizing temperature exceeds 1200 ° C., the crystal grains become coarse and ductility and toughness are lowered.

  In the normalization treatment, after normalization, the cast steel material is preferably cooled at a cooling rate of 100 to 600 ° C./hour in the center of the cast steel material in order to obtain a predetermined microstructure. As a cooling method for obtaining a cooling rate in this range, for example, forced air cooling or the like can be employed.

  For example, when the cast steel material is a casing of a steam turbine, a casing of a steam valve, or the like, the center portion of the cast steel material means a center portion of the wall thickness of the casing of the steam turbine or the casing of the steam valve. That is, these portions are portions where the cooling rate becomes the smallest in the cast steel material. In addition, although the cooling rate of the center part of a cast steel raw material is defined here, the above-mentioned cooling rate is good also as a cooling rate of the site | part where a cooling rate becomes the smallest in a cast steel raw material. The same applies to the tempering process.

(Tempering treatment)
The tempering process decomposes the residual austenite structure generated by the above-mentioned normalizing 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 preferably performed in a temperature range of 500 to 700 ° C. for the purpose of decomposing the retained austenite structure. When the temperature of the first stage tempering treatment is less than 500 ° C., the residual austenite structure is not sufficiently decomposed. On the other hand, when the temperature of the first-stage tempering treatment exceeds 700 ° C., carbides and carbonitrides are preferentially precipitated in the martensite structure rather than in the retained austenite structure, and the precipitates are dispersed unevenly. As a result, the high temperature creep rupture strength decreases.

  In the first stage tempering process, after the first stage tempering, the cast steel material has a temperature of 40 to 100 ° C./hour at the center of the cast steel material so that a large strain is not generated in the stress concentration part such as a shape change part during cooling. It is preferable to cool at a cooling rate of As a cooling method for obtaining a cooling rate in this range, for example, air cooling can be employed.

  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 carried out in a temperature range of from ° C to 780 ° C. If the temperature of the second stage tempering treatment is less than 700 ° C., precipitates such as carbides and carbonitrides do not precipitate in a stable state, and thus the characteristics required for high temperature creep rupture strength, ductility and toughness cannot be obtained. On the other hand, if the temperature of the second stage tempering treatment exceeds 780 ° C., carbides and carbonitrides become coarse precipitates, and the required high temperature creep rupture strength cannot be obtained.

  In the second-stage tempering process, after the second-stage tempering, the cast steel 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. As a cooling method for obtaining a cooling rate in this range, for example, furnace cooling or the like can be employed. In addition, since the cooling in the second stage tempering process is performed at a small cooling rate by furnace cooling or the like, the temperature difference between the central portion and the outer peripheral portion of the cast steel material in the cooling process is small. Therefore, in the definition of the cooling rate in the second stage tempering process, there is no limitation on the center part of the cast steel material, for example, the cooling rate at any position in the cast steel material such as the center part or the outer peripheral part of the cast steel material. It may be.

  In addition, the method of producing the components (forged product and cast product) of the steam turbine is not limited to the method described above.

According to the heat-resistant steel of the above-described embodiment, by making the chemical composition constituting the heat-resistant steel within the above-described range, while consuming the constituent elements of the MX phase and M ₂ X phase that are effective in improving the creep rupture strength, Precipitation of the growing Z phase can be suppressed. Therefore, the creep rupture strength can be improved. In addition, when determining the chemical composition which comprises heat-resistant steel, precipitation of a Z phase can be suppressed exactly by paying attention to the content rate of V, N, and Nb especially.

  Hereinafter, it will be described that the precipitation of the Z phase is suppressed in the heat resistant steel of the embodiment.

(sample)
Table 1 shows chemical composition components of each sample (sample 1 to sample 49) used for the material property evaluation. Sample 1 to sample 30 are examples of the heat-resistant steel according to the embodiment of the present invention, and sample 31 to sample 49 are heat-resistant steel that is not in the chemical composition range of the heat-resistant steel according to the embodiment of the present invention. It is a comparative example. Table 2 shows the mass fraction of the Z phase in each sample shown in Table 1. Note that the mass fraction of the Z phase was determined by the thermodynamic calculation software described above. The temperature condition in the thermodynamic calculation was 600 ° C.

  As shown in Table 2, it can be seen that the mass fraction of the Z phase in Samples 1 to 30 is lower than the mass fraction of the Z phase in Samples 31 to 49. Samples 46 to 49 correspond to the improved 9% Cr-1% Mo steel (T91 steel) defined in ASME, and are conventionally used heat-resistant steels. It can be seen that the mass fraction of the Z phase in Sample 1 to Sample 30 is much lower than that of.

Thus, since the mass fraction of the Z phase in Samples 1 to 30 is low, precipitation of the Z phase during high-temperature creep deformation is difficult to occur for a long time, and the bending phenomenon accompanying the disappearance of the MX phase and M ₂ X phase Can be suppressed. Samples 3 to 6, Samples 10 to 12, Samples 15 to 18, Samples 22 to 24, and Samples 29 to 30 have a Z-phase mass fraction of “0”. Therefore, in these samples, the Z-phase does not precipitate at all during high temperature and long-time creep deformation, and the disappearance of strengthening phases such as the MX phase and M ₂ X phase does not occur. It is extremely effective in suppressing the bending phenomenon.

  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.

Claims (3)

  In mass%, C: 0.05 to 0.2, Si: 0.1 or less, Mn: 0.15 or less, Ni: 0.05 to 1, Cr: 8 to 10, Mo: 0.05 to 1, V: 0.01 to 0.05, Co: 0.5 to 5, W: 1 to 3, N: 0.006 to 0.012, B: 0.003 to 0.03, Nb + Ta (only one of them) In addition, the heat resistant steel is characterized by 0.15 to 0.5, the balance being Fe and inevitable impurities.   In mass%, C: 0.09 to 0.17, Cr: 8 to 9.2, V: 0.01 to 0.025, N: 0.006 to 0.01%, Nb + Ta (only one of them) The heat resistant steel according to claim 1, wherein the heat resistant steel is 0.21 to 0.3.   A steam turbine component comprising at least a predetermined portion made of the heat-resistant steel according to claim 1.

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