JP2021102802A - Turbine rotor material - Google Patents

Abstract

To provide a turbine rotor material that can achieve both of easiness of quenching treatment and resistance to stress corrosion cracking.SOLUTION: A turbine rotor material has a carbon content of from 0.18 mass% to 0.28 mass% (inclusive), a silicon content of from 0 mass% to 0.25 mass% (inclusive), a manganese content of from 0.10 mass% to 1.20 mass% (inclusive), a nickel content of from 0.50 mass% to 1.20 mass% (inclusive), a chromium content of from 1.80 mass% to 2.80 mass% (inclusive), a molybdenum content of from 0.40 mass% to 1.50 mass% (inclusive), a vanadium content of more than 0.35 mass% and not more than 0.45 mass%, and a tungsten content of from 0.40 mass% to 1.10 mass% (inclusive), with the balance being made up of iron and unavoidable impurities.SELECTED DRAWING: None

Description

The present disclosure relates to turbine rotor materials.

In recent years, effective use of geothermal energy has been studied as a stable and sustainable renewable energy. As a method of utilizing geothermal energy as energy, power generation using steam superheated by geothermal heat as a heat source (hereinafter referred to as geothermal power generation) is typically known.

The steam used as a heat source often contains a corrosive gas typified by hydrogen sulfide. Therefore, the turbine rotor of the generator used in geothermal power generation is required to have corrosion resistance against corrosive gas contained in steam.

In particular, corrosive gas is likely to be concentrated in a portion of the turbine casing in which drying and wetting are repeated alternately due to a temperature change accompanying the start and stop of the plant. Further, the blade groove of the turbine rotor is stress-concentrated during operation. Therefore, as described above, the phenomenon of stress corrosion cracking is likely to occur in the blade groove of the turbine rotor in the portion where the corrosive gas is likely to be concentrated. When stress corrosion cracking occurs, it leads to damage to the turbine rotor. Therefore, as a material for the turbine rotor, a material that is less likely to cause stress corrosion cracking is required.

In recent years, there has been a demand for larger turbine rotors for the purpose of improving the efficiency of geothermal power generation and increasing the amount of power generation. As the size of the turbine rotor increases, it tends to be difficult to perform uniform quenching from the surface to the center of the rotor during manufacturing of the turbine rotor.

Therefore, in recent years, an alloy suitable for a turbine rotor for geothermal power generation has been studied in place of the 1% CrMoV steel developed as a material for a conventional turbine rotor for geothermal power generation (see, for example, Patent Document 1). ).

An alloy material for a turbine rotor may be simply referred to as a "turbine rotor material" in the following description.

In addition to turbine rotors for geothermal power generation, high- and low-pressure integrated turbine rotors that generate electricity using steam produced by steam power generation boilers and exhaust heat recovery boilers for gas turbine combined power generation are also being increased in size. ing. Even for high- and low-voltage integrated turbine rotors, it is necessary to ensure the ease of quenching when the size is increased.

Further, in the low pressure portion of the high / low pressure integrated turbine rotor, there is a portion where drying and wetting are alternately repeated as in the geothermal turbine rotor. The trace components of water used for steam power generation and gas turbine combined cycle are controlled, and the amount of corrosive gas components contained in the generated steam is extremely small. Examples of the trace component include Na ion, Cl ion, sulfate ion, silica and the like. However, it has been confirmed that stress corrosion cracking occurs in the blade groove as in the case of geothermal turbines, such as when water management is inappropriate or when seawater used for cooling steam leaks.

Further, in order to enable stable use for a long period of time, it is preferable that the high and low pressure integrated turbine rotor also has high resistance to the above-mentioned stress corrosion cracking.

Japanese Unexamined Patent Publication No. 2012-225222

The demand for larger turbine rotors is increasing year by year. In Patent Document 1, a large rotor having a body diameter of 1600 mm is assumed, but in recent years, a larger turbine rotor has been demanded. In order to cope with such an increase in the size of the turbine rotor, there is still room for improvement in the turbine rotor material that has both easy quenching treatment and resistance to stress corrosion cracking.

The present disclosure has been made in view of such circumstances, and an object of the present invention is to provide a turbine rotor material capable of achieving both ease of quenching treatment and corrosion resistance.

In order to solve the above problems, the turbine rotor material according to the present disclosure has a carbon content of 0.18% by mass or more and 0.28% by mass or less, and a silicon content of 0% by mass or more and 0.25% by mass or less. , Manganese content is 0.10% by mass or more and 1.20% by mass or less, Nickel content is 0.50% by mass or more and 1.20% by mass or less, and Chromium content is 1.80% by mass or more. 80% by mass or less, molybdenum content of 0.40% by mass or more and 1.50% by mass or less, vanadium content of more than 0.35% by mass and 0.45% by mass or less, tungsten content of 0.40 It is by mass% or more and 1.10% by mass or less, and the balance is composed of iron and unavoidable impurities.

According to the present disclosure, it is possible to provide a turbine rotor material capable of achieving both ease of quenching treatment and corrosion resistance.

FIG. 1 is a schematic view showing a test piece used for the SCC test. FIG. 2 is a schematic diagram showing a test method for the SCC test. FIG. 3 is a schematic view showing a crack generated in a test piece in the SCC test.

The turbine rotor material according to the present disclosure has a carbon content of 0.18% by mass or more and 0.28% by mass or less, a silicon content of 0% by mass or more and 0.25% by mass or less, and a manganese content of 0. 1% by mass or more and 1.20% by mass or less, nickel content is 0.50% by mass or more and 1.20% by mass or less, chromium content is 1.80% by mass or more and 2.80% by mass or less, molybdenum content The ratio is 0.40% by mass or more and 1.50% by mass or less, the vanadium content is more than 0.35% by mass and 0.45% by mass or less, and the tungsten content is 0.40% by mass or more and 1.10% by mass. The rest is an alloy consisting of iron and unavoidable impurities.

Conventionally known turbine rotor materials pose various problems when the turbine rotor is enlarged.

For example, assuming that a large turbine rotor having a diameter of about 1900 mm is manufactured, if 1% CrMoV steel is used as the turbine rotor material, it is difficult to perform uniform quenching from the surface to the center of the rotor in the quenching process. Specifically, since a ferrite phase (soft phase) appears in the central portion of the rotor where the cooling rate slows down during the quenching process, it is easy to lose strength and toughness.

On the other hand, the turbine rotor material of the present disclosure having the above composition is an alloy material that is easy to quench and has excellent corrosion resistance.

The "quenching process" means a process of heating the turbine rotor material to a temperature equal to or higher than the transformation point temperature and then rapidly cooling the material. Turbine rotor materials exhibit an austenite phase at temperatures above the transformation point, but in the process of rapid cooling, they transform into fine needle-like structures called martensite and bainite, increasing their strength.

"Easy quenching" means that a softened phase called a ferrite phase is unlikely to occur even when cooled at a slower rate. Here, more specifically, it means the property that a ferrite phase is unlikely to occur in the central portion of the hardened turbine rotor material.

Further, in the following description, "the property of facilitating quenching" may be referred to as "hardenability".
“Highly hardenable” means that it has a strong property of facilitating the quenching process.

Further, "stress corrosion cracking resistance" means resistance to stress corrosion cracking. More specifically, "stress corrosion cracking resistance" means resistance to a phenomenon in which tensile stress is applied to a material and the material is cracked due to the corrosive action of a specific environment.
Hereinafter, the turbine rotor materials of the present disclosure will be described in order.

(About carbon content)
The carbon (C) content in the turbine rotor material of the present disclosure is 0.18% by mass or more and 0.28% by mass or less.

In general, C is useful for enhancing hardenability of steel and ensuring mechanical strength. In the turbine rotor material of the present disclosure, the C content is 0.18% by mass or more, and the required hardenability and mechanical properties can be obtained.
In particular, the content of C is preferably 0.20% by mass or more.

On the other hand, low alloy steels to which C is excessively added tend to have reduced toughness.

Further, when the C content of the low alloy steel is high, a large amount of chromium (Cr) carbide is likely to be generated when the low alloy steel is tempered, and the amount of Cr dissolved in the matrix of the low alloy steel is likely to decrease. .. In this case, the stress corrosion cracking resistance of the low alloy steel tends to deteriorate.

Further, C generally tends to be easily concentrated in the molten metal during solidification in the ingot-forming step. Therefore, if the low alloy steel contains more C than necessary, the content of C in the ingot tends to increase from the initial stage of solidification to the final stage of solidification when the ingot is continuously produced in the ingot formation process. As a result, when the C content is high, it becomes difficult to obtain a low alloy steel having homogeneous characteristics.

On the other hand, in the turbine rotor material of the present disclosure, when the C content is 0.28% by mass or less, sufficient toughness and high stress corrosion cracking resistance can be ensured. Further, when the C content is 0.28% by mass or less, it becomes easy to manufacture a turbine rotor material having homogeneous characteristics.

In particular, the content of C is preferably 0.25% by mass or less.

The upper limit value and the lower limit value of the C content in the turbine rotor material of the present disclosure can be arbitrarily combined. The content of C is preferably 0.20% by mass or more and 0.25% by mass or less.

(About silicon content)
The content of silicon (Si) in the turbine rotor material of the present disclosure is 0% by mass or more and 0.25% by mass or less.

Si is useful as a deoxidizer in low alloy steels. On the other hand, when Si is added in a large amount to low alloy steel, the toughness tends to decrease.

Therefore, the Si content in the turbine rotor material of the present disclosure is 0.25% by mass or less, preferably 0.15% by mass or less. The minimum value of the Si content is 0% by mass.

(About manganese content)
The content of manganese (Mn) in the turbine rotor material of the present disclosure is 0.10% by mass or more and 1.20% by mass or less.

Mn is useful for ensuring the hardenability of low alloy steels. Further, Mn is useful as a deoxidizer for low alloy steel and suppresses cracking during hot forging. In the turbine rotor material of the present disclosure, the Mn content is 0.10% by mass or more, and these effects are effectively exhibited.

In particular, the Mn content is preferably 0.50% by mass or more.

On the other hand, Mn tends to reduce the toughness of low alloy steel. In the turbine rotor material of the present disclosure, the Mn content is 1.20% by mass or less, and the required toughness can be obtained.

In particular, the Mn content is preferably 1.00% by mass or less.

The upper limit value and the lower limit value of the Mn content in the turbine rotor material of the present disclosure can be arbitrarily combined. The Mn content is preferably 0.50% by mass or more and 1.00% by mass or less.

(About nickel content)
The content of nickel (Ni) in the turbine rotor material of the present disclosure is 0.50% by mass or more and 1.20% by mass or less.

Ni is useful for ensuring the hardenability of low alloy steels. Ni also enhances the toughness of low alloy steels. In the turbine rotor material of the present disclosure, the Ni content is 0.50% by mass or more, and these effects are effectively exhibited.

On the other hand, as the content of Ni in the low alloy steel increases, the stress corrosion cracking resistance of the low alloy steel tends to decrease. In the turbine rotor material of the present disclosure, the Ni content is 1.20% by mass or less, and the required stress corrosion cracking resistance can be ensured.

(About chromium content)
The content of chromium (Cr) in the turbine rotor material is 1.80% by mass or more and 2.80% by mass or less.

Cr is useful for ensuring the hardenability of low alloy steels. Also, Cr combines with C to produce carbides. The resulting Cr carbides improve the strength of the low alloy steel. Further, Cr has an effect of suppressing stress corrosion cracking by being dissolved in the matrix. In the turbine rotor material of the present disclosure, the Cr content is 1.80% by mass or more, and these effects are effectively exhibited.

In particular, the Cr content is preferably 2.00% by mass or more.

On the other hand, when the content of Cr in the low alloy steel increases, the cracks tend to grow faster when stress corrosion cracking occurs in the low alloy steel. This is because, as a result of the increased corrosion resistance of the low alloy steel, once a crack occurs in the low alloy steel, the tip of the crack is not corroded and maintains a sharp pointed shape, and stress concentration on the tip of the crack is maintained. caused by.

That is, in low alloy steel, a high Cr content is advantageous in order to suppress the occurrence of stress corrosion cracking itself, and a high Cr content is disadvantageous in order to suppress the growth of the generated cracks. become.

In the turbine rotor material of the present disclosure, the Cr content is 2.80% by mass or less, the growth of cracks at the time of cracking is suppressed, and desired stress corrosion cracking resistance cracking characteristics can be obtained.

In particular, the Cr content is preferably 2.50% by mass or less.

The upper limit value and the lower limit value of the Cr content in the turbine rotor material of the present disclosure can be arbitrarily combined. The Cr content is preferably 2.00% by mass or more and 2.50% by mass or less.

(About molybdenum content)
The content of molybdenum (Mo) in the turbine rotor material of the present disclosure is 0.30% by mass or more and 1.20% by mass or less.

Mo is useful for ensuring the hardenability of low alloy steels. Mo also combines with C to produce carbides. The resulting Mo carbides improve the strength of the low alloy steel. In the turbine rotor material of the present disclosure, the Mo content is 0.50% by mass or more, and these effects are effectively exhibited.

On the other hand, Mo tends to decrease the toughness of the low alloy steel as the content of the low alloy steel increases. Further, since Mo is an expensive element, the material cost tends to increase as the content of Mo in the turbine rotor material increases.

Furthermore, Mo tends to inhibit the formation of vanadium (V) carbides when tempered as the content in low alloy steels increases. As will be described later, the carbide of V has an effect of enhancing stress corrosion cracking resistance.

In the turbine rotor material of the present disclosure, the Mo content is 1.20% by mass or less, and the required toughness can be ensured. Further, in the turbine rotor material of the present disclosure, since the content of Mo is 1.20% by mass or less, it is difficult to inhibit the formation of carbides of V.

In particular, the Mo content is preferably 1.00% by mass or less.

The upper limit value and the lower limit value of the Mo content in the turbine rotor material of the present disclosure can be arbitrarily combined. The Mo content is preferably 0.50% by mass or more and 1.00% by mass or less.

(About vanadium content)
The content of vanadium (V) in the turbine rotor material of the present disclosure is more than 0.35% by mass and 0.43% by mass or less.

V is useful for ensuring the hardenability of low alloy steels. V also combines with C to produce carbides. The resulting V carbides improve the strength of the low alloy steel.

Further, V is precipitated as V-carbide (matched V-carbide) matched to the matrix at the time of tempering in the quenching treatment. The resulting V-carbide traps hydrogen generated by corrosion of the material and improves stress corrosion cracking resistance.

Further, the V-carbide that did not dissolve in the matrix during quenching suppresses the grain growth (coarseness) of the matrix of the turbine rotor material, and improves toughness and stress corrosion cracking resistance.

In the generally known V-added steel, the V content is 0.15% by mass or more and 0.3% by mass or less. On the other hand, in the turbine rotor material of the present disclosure, the V content exceeds 0.35% by mass in order to sufficiently secure the amount of V carbide so that it can play the above two roles.

On the other hand, V tends to decrease the toughness as the content in the low alloy steel increases. In the turbine rotor material of the present disclosure, the V content is 0.43% by mass or less, and the required toughness can be ensured.

Generally, stress corrosion cracking of low alloy steel is considered to occur by the following mechanism.
First, when low alloy steel is exposed to a corrosive environment, hydrogen is generated by the reaction between the low alloy steel and a corrosive substance (corrosion reaction). The generated hydrogen is incorporated into the matrix of low alloy steel. Hydrogen in the matrix diffuses into the matrix and accumulates in the low alloy steel where stress is concentrated. There are various theories about the behavior of hydrogen involved in stress corrosion cracking, but it is thought that, for example, the accumulated hydrogen accelerates the generation and growth of cracks by reducing the atomic bonding force of the matrix metal.

On the other hand, the turbine rotor material of the present disclosure has a large amount of matched V carbide in the material structure by containing V at the above-mentioned content. It is believed that this matched V-carbide traps hydrogen invading the structure of the turbine rotor material and suppresses the accumulation of hydrogen at the tip of the crack. As a result, even when the turbine rotor material is exposed to a corrosive environment such as geothermal steam with stress acting on it, it is considered that the generation and growth of cracks are delayed, that is, the stress corrosion cracking resistance is enhanced.

(About tungsten content)
The content of tungsten (W) in the turbine rotor material of the present disclosure is 0.40% by mass or more and 1.10% by mass or less.

W is useful for increasing the creep strength of low alloy steels. Further, as the content of W in the low alloy steel increases, the stress corrosion cracking resistance of the low alloy steel tends to be improved. In particular, in the turbine rotor material of the present disclosure, when the W content is 0.40 mass or more, the effect of improving the stress corrosion cracking resistance is effectively exhibited.

On the other hand, as the content of W in the low alloy steel increases, the hardenability tends to decrease and the toughness tends to decrease. In the turbine rotor material of the present disclosure, the W content is 1.10% by mass or less, and the required hardenability and toughness can be ensured.

In particular, the W content is preferably 0.80% by mass or less.

(About niobium content)
The turbine rotor material of the present disclosure may contain niobium (Nb) in an amount of 0.01% by mass or more and 0.15% by mass or less.

Nb is useful for ensuring the hardenability of low alloy steels. Nb also combines with C to produce carbides. The generated carbide of Nb does not completely dissolve at the quenching temperature of the low alloy steel, and has a function of suppressing coarsening of matrix crystal grains as in V. In order to impart these functions, the Nb content of the turbine rotor material of the present disclosure is preferably 0.01% by mass or more.

On the other hand, the toughness of Nb tends to decrease as the content of Nb in the low alloy steel increases. In the turbine rotor material of the present disclosure, the content of Nb is 0.15% by mass or less, and toughness can be ensured even when Nb is added.

(About phosphorus and sulfur content)
The phosphorus (P) content in the turbine rotor material of the present disclosure is 0% by mass or more and 0.02% by mass or less.

The sulfur (S) content in the turbine rotor material of the present disclosure is 0% by mass or more and 0.01% by mass or less.

Both P and S are impurities brought in from the steelmaking raw material, and are harmful impurities that form phosphide and sulfide in the steel material and significantly reduce the toughness of the steel material. Therefore, the lower the P content and the S content, the better the characteristics are expected.

Therefore, the content of P is preferably 0.01% by mass or less, more preferably undetectable (0% by mass), with an upper limit of 0.02% by mass.

Similarly, the content of S is preferably 0.005% by mass or less, more preferably undetectable (0% by mass), with an upper limit of 0.01% by mass.

(About unavoidable impurities)
In the turbine rotor material of the present disclosure, in addition to the above-mentioned elements, steel materials such as nitrogen (N), oxygen (O), copper (Cu), arsenic (As), antimony (Sb), and tin (Sn) can be used. Regarding unavoidable impurities that are generally contained, the presence of unavoidable impurities shall be allowed at the content rate that is unavoidably contained.

(Ratio of molybdenum content to vanadium content)
As described above, when the amount of Mo is large, the formation of vanadium (V) carbide, which has the effect of enhancing stress corrosion cracking resistance, is likely to be inhibited when tempered.

Therefore, in the turbine rotor material of the present disclosure, by making the ratio of the Mo content to the V content less than twice, the amount of matching V carbides in the turbine rotor material is secured and the stress corrosion cracking resistance is improved. It is easy to secure and is preferable.

(Preferable aspect)
The turbine rotor material of the present disclosure has a carbon content of 0.20% by mass or more and 0.25% by mass or less, a silicon content of 0% by mass or more and 0.25% by mass or less, and a manganese content of 0.50. Mass% or more and 1.00% by mass or less, nickel content of 0.50% by mass or more and 0.90% by mass or less, chromium content of 2.00% by mass or more and 2.50% by mass or less, molybdenum content Is 0.50% by mass or more and 1.00% by mass or less, the vanadium content is more than 0.35% by mass and 0.40% by mass or less, and the tungsten content is 0.40% by mass or more and 0.80% by mass or less. It is preferable that the balance is composed of iron and unavoidable impurities.

(Hardenability)
As described above, the turbine rotor material of the present disclosure has high hardenability.
The hardenability of the turbine rotor material of the present disclosure is evaluated by determining the cooling rate of the central part of the turbine rotor material at the time of quenching by simulation, and evaluating the test piece subjected to the same cooling as the obtained result by the pro-eutectoid ferrite ratio.

The ferrite structure looks whiter than the surrounding structure when the metal structure is magnified and observed under a microscope. Therefore, the presence of the ferrite structure can be grasped by microscopic observation.

The primordial ferrite ratio is the ratio (percentage) of the area of the structure that appears white in the photograph to the area of the entire imaging range of one photograph in the magnified photograph in which the metal structure is imaged at a magnification of 100 times using an optical microscope. To do.

In the simulation to obtain the cooling rate of the central part of the turbine rotor material, the simulation conditions assuming the turbine rotor material equivalent to φ1900 mm are set. The simulation can be performed using general thermal analysis software, and the specific heat, thermal conductivity, and transformation point of each metal constituting the turbine rotor material are used as physical constants.

(Stress corrosion cracking resistance)
Further, the turbine rotor material of the present disclosure has high stress corrosion cracking resistance.

The stress corrosion cracking resistance of the turbine rotor material of the present disclosure is confirmed by the test (SCC test) described in "Proceedings 23rd NZ Geothermal Workshop 2001 p137-142".
FIG. 1 is a schematic view showing a test piece used for the SCC test. FIG. 2 is a schematic diagram showing a test method for the SCC test. FIG. 3 is a schematic view showing a crack generated in a test piece in the SCC test.

First, using the turbine rotor material of the present disclosure, a plurality of test pieces shown in FIG. 1 are produced. The test piece is 108 mm × 8 mm × 5 mm (machining accuracy ± 0.02 mm), and has a depth of 1,25 mm, an angle of 45 °, and a V notch with R = 0.2 at the tip in the center of the 108 mm × 5 mm surface in the longitudinal direction. Has.

Next, a plurality of prepared test pieces are inserted into a test tank in which geothermal steam has flowed, and the test pieces are exposed to geothermal steam. At that time, as shown in FIG. 2, the test piece A is supported at three points by using the jig X. Further, a deflection amount equivalent to 0.2% proof stress, which is appropriate as a turbine rotor material, is applied near the V notch of the test piece.

The test piece is then periodically removed from the test chamber. As shown in FIG. 3, cracks occur in the test piece from the tip of the V notch toward the inside of the test piece depending on the degree of stress corrosion cracking.

Next, the taken-out test piece is cut and the crack length (L) is measured. The crack propagation velocity is calculated from the correspondence between the crack length and the test time. The obtained crack propagation velocity is used as an index of stress corrosion cracking resistance. Turbine rotor materials with a slower crack propagation rate are evaluated to have higher stress corrosion cracking resistance.

(Toughness)
Further, the turbine rotor material of the present disclosure preferably exhibits high toughness in order to ensure the strength of the turbine rotor to be manufactured.

Toughness is evaluated by performing a Charpy impact test at room temperature according to JIS Z 2242.

(Manufacturing method of turbine rotor material)
The turbine rotor material of the present disclosure can be manufactured by a manufacturing process similar to that of a normal turbine rotor material manufacturing method.
First, in a melting furnace, the above-mentioned bullions of each element are melted and mixed to obtain a molten metal having the above-mentioned chemical composition.

Then, if necessary, the molten metal is degassed and inclusions are removed, and then poured into a mold to be cooled.

Next, after processing into the shape of the turbine rotor, quenching treatment is performed to obtain a turbine rotor material.

In addition, the turbine rotor material of the present disclosure is manufactured as follows, for example.
First, iron and alloy components as raw materials are melted and refined in an arc furnace or the like, and molten metal whose ratio of each metal is adjusted to the same ratio as the content in the alloy is poured into a mold and solidified to form an ingot.

After that, the obtained ingot is hot-forged and hot-molded into a shape close to that of a turbine rotor, and then a normalizing process for promoting homogenization of the structure, a quenching process for obtaining the required mechanical properties, and a tempering process. And so on. As a result, the turbine rotor material of the present disclosure is manufactured.

The above-mentioned methods of melting, hot forging, and heat treatment can use commonly known methods used in the production of low alloy steel, and are not particularly limited.

(Action effect)
In the turbine rotor material of the present disclosure, by increasing the V content as compared with the generally known V-added steel, the matching V carbide generated in the structure is increased, and the stress corrosion cracking resistance is enhanced.

Further, the amount of the matched V carbide is defined in a suitable range in consideration of the strength and toughness of the turbine rotor material.

Therefore, according to the turbine rotor material having the above configuration, both the ease of quenching treatment and the stress corrosion cracking resistance can be achieved at the same time.

<Additional notes>
The turbine rotor material of the present disclosure is grasped as follows, for example.

[1] The turbine rotor material according to the first aspect has a carbon content of 0.18% by mass or more and 0.28% by mass or less, a silicon content of 0% by mass or more and 0.25% by mass or less, and manganese. The content is 0.10% by mass or more and 1.20% by mass or less, the nickel content is 0.50% by mass or more and 1.20% by mass or less, and the chromium content is 1.80% by mass or more and 2.80% by mass. Hereinafter, the molybdenum content is 0.40% by mass or more and 1.50% by mass or less, the vanadium content is more than 0.35% by mass and 0.45% by mass or less, and the tungsten content is 0.40% by mass or more. It is 1.10% by mass or less, and the balance is composed of iron and unavoidable impurities.

According to the above aspect, by increasing the V content as compared with the generally known V-added steel, the number of matched V carbides generated in the structure is increased, and the stress corrosion cracking resistance is enhanced. Further, the amount of the matched V carbide is defined in a suitable range in consideration of the strength and toughness of the turbine rotor material.
Therefore, according to the turbine rotor material having the above configuration, both the ease of quenching treatment and the stress corrosion cracking resistance can be achieved at the same time.

[2] The turbine rotor material according to the second aspect has a niobium content of 0.01% by mass or more and 0.15% by mass or less.

According to the above aspect, by containing niobium at the above-mentioned content rate, the Nb carbide is dissolved in the matrix, and the effect of improving hardenability can be expected. In addition, the toughness of the turbine rotor material can be ensured because the matrix crystal grains are prevented from becoming coarse due to heating accompanying quenching.

[3] The turbine rotor material according to the third aspect has a molybdenum content of 2 times or less the vanadium addition rate.

According to the above aspect, by setting the ratio of the Mo content to the V content to 2 times or less, it becomes easy to secure the amount of matched V carbides in the turbine rotor material and to secure the stress corrosion cracking resistance.

The present disclosure will be described below with reference to examples, but the present disclosure is not limited to these examples.

For the turbine rotor materials of each Example and Comparative Example, 50 kg steel ingots having the chemical components shown in Tables 1 and 2 were prepared using a vacuum induction heating furnace. At the time of preparing the ingot, VCD (Vacuum Carbon Degassing) was carried out.

Next, the obtained ingot was hot forged with a forging ratio of 3 or more and 16 or less, and then the ingot was heated to 1010 ° C. and subjected to a normalizing treatment.

Next, after heating the ingot to 930 ° C., as a quenching treatment, two types of cooling were performed to imitate the surface layer portion and the central portion of the turbine rotor material having a diameter of 1900 mm. The cooled steel ingot that imitates the surface layer portion is referred to as "steel ingot A", and the cooled steel ingot that imitates the central portion is referred to as "steel ingot B".

Prior to the quenching process, it was assumed that the turbine rotor material equivalent to φ1900 mm was quenched, and the cooling rates of the surface layer portion and the central portion of the turbine rotor material were obtained by simulation. The cooling rate of the obtained surface layer portion was set as the cooling condition of "steel ingot A", and the cooling rate of the central portion was set as the cooling condition of "steel ingot B". The simulation can be performed using general thermal analysis software, and the specific heat, thermal conductivity, and transformation point of each metal constituting the turbine rotor material are used as physical constants.

Next, each of the steel ingots A and B was heated to a temperature such that the yield strength was 730 ± 20 MPa and tempered to obtain a turbine rotor material.

The turbine rotor material obtained from the steel ingot A was used for the evaluation of stress corrosion cracking property described later.
The turbine rotor material obtained from the ingot B was used for evaluation of the proeutectoid ferrite ratio and toughness described later.

The composition of Comparative Example 1 satisfies the components of the DIN standard material (2.2CrMoNiWV8-8).

The following evaluations were performed on each of the obtained turbine rotor materials.

(Evaluation 1: Hardenability)
The proeutectoid ferrite ratio of the above-mentioned ingot B was determined, and the hardenability was evaluated.

The ferrite structure looks whiter than the surrounding structure when the metal structure is magnified and observed under a microscope. Therefore, the presence of the ferrite structure can be grasped by microscopic observation.

The primordial ferrite ratio is the ratio (percentage) of the area of the structure that appears white in the photograph to the area of the entire imaging range of one photograph in the magnified photograph in which the metal structure is imaged at a magnification of 100 times using an optical microscope. did.

The turbine rotor material having a proeutectoid ferrite ratio of "0" was evaluated as a non-defective product.
Further, the turbine rotor material having an amount of proeutectoid ferrite larger than 0 was evaluated as a defective product.

The evaluation results are shown in Tables 3 and 4.

(Evaluation 2: Toughness)
The toughness of the turbine rotor material was evaluated by performing a Charpy impact test at room temperature (20 ° C.) according to JIS Z 2242 using a test piece of the turbine rotor material obtained from steel ingot B. The Charpy impact test was performed three times using a test piece provided with a 2 mm V notch, and the average value of the measured values at three points was used as the evaluation result of toughness.

The turbine rotor material with the required shock absorption energy of 50 J or more was evaluated as a good product.
Further, the turbine rotor material having a shock absorption energy of less than 50 J was evaluated as a defective product.

The evaluation results are shown in Tables 3 and 4.

(Evaluation 3: Stress corrosion cracking resistance)
For the turbine rotor material judged to be non-defective in Evaluation 1 and Evaluation 2, the stress corrosion cracking resistance was evaluated using the test piece of the turbine rotor material obtained from the steel ingot A.

The stress corrosion cracking resistance of the turbine rotor material was confirmed by the test (SCC test) described in "Proceedings 23rd NZ Geothermal Workshop 2001 p137-142".

First, using the turbine rotor material, a plurality of test pieces shown in FIG. 1 were produced. The test piece is 108 mm × 8 mm × 5 mm (machining accuracy ± 0.02 mm), and has a depth of 1,25 mm, an angle of 45 °, and a V notch with R = 0.2 at the tip in the center of the 108 mm × 5 mm surface in the longitudinal direction. Has.

Next, a plurality of prepared test pieces were inserted into a test tank in which geothermal steam was flowed, and the test pieces were exposed to geothermal steam. At that time, as shown in FIG. 2, the test piece A was supported at three points using the jig X. Further, a deflection amount equivalent to 0.2% proof stress, which is appropriate as a turbine rotor material, was loaded near the V notch of the test piece.

Next, the test piece was periodically taken out from the test tank, the taken out test piece was cut, and the crack length (L) was measured. The crack propagation velocity is obtained from the correspondence between the crack length and the test time, and is used as an index of stress corrosion cracking resistance. It was evaluated that the turbine rotor material with a slower crack propagation rate has higher stress corrosion cracking resistance.

In this example, the crack propagation velocity of each test piece was evaluated using the ratio (hereinafter referred to as relative ratio) of the DIN standard material of Comparative Example 1 to the crack propagation velocity.

The evaluation results of stress corrosion cracking resistance were evaluated to be equivalent to those of Comparative Example 1 for materials having a relative ratio of 0.75 or more and 1.25 or less in consideration of variations in data.
Further, the material having a relative ratio smaller than 0.75 was evaluated as a non-defective product because the crack propagation rate was significantly slower than that of Comparative Example 1.
Further, the material having a relative ratio larger than 1.25 was evaluated as a defective product because the crack propagation speed was significantly faster than that of Comparative Example 1.

The evaluation results are shown in Tables 3 and 4.
In Tables 3 and 4, the evaluation results are shown as follows.
⊚: Relative ratio of crack propagation velocity is less than 0.50 〇: Relative ratio of crack propagation velocity is 0.50 or more and less than 0.75 Δ: Relative ratio of crack propagation velocity is 0.75 or more and 1.25 or less ×: Crack Relative ratio of propagation speed exceeds 1.25

Further, in the "Comprehensive evaluation" column shown in Tables 3 and 4, the materials evaluated as non-defective products in evaluations 1 to 3 are "○", and the materials evaluated as defective products in any one of evaluations 1 to 3 Is described as "x".

As a result of the evaluation, it was found that Examples 1 to 10 contained in the turbine rotor material of the present disclosure are materials having both stress corrosion cracking resistance and hardenability.

Further, it was found that the turbine rotor materials of Examples 2 to 5, 7 and 8 had a Mo content of 2 times or less of the V content and were excellent in stress corrosion cracking resistance.

From the above results, it was confirmed that the turbine rotor material of the present disclosure is useful.

Claims (3)

Carbon content is 0.18% by mass or more and 0.28% by mass or less,
Silicon content is 0% by mass or more and 0.25% by mass or less,
Manganese content is 0.10% by mass or more and 1.20% by mass or less,
Nickel content is 0.50% by mass or more and 1.20% by mass or less,
Chromium content is 1.80% by mass or more and 2.80% by mass or less,
Molybdenum content is 0.40% by mass or more and 1.50% by mass or less,
Vanadium content exceeds 0.35% by mass and 0.45% by mass or less,
The tungsten content is 0.40% by mass or more and 1.10% by mass or less.
Turbine rotor material with the balance consisting of iron and unavoidable impurities. The turbine rotor material according to claim 1, wherein the content of niobium is 0.01% by mass or more and 0.15% by mass or less. The turbine rotor material according to claim 1 or 2, wherein the molybdenum content is not more than twice the vanadium addition rate.

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