JP2019010668A - Method for manufacturing steam turbine material - Google Patents

Abstract

To provide a method for manufacturing a steam turbine material in which the strength and toughness of the steam turbine material can be increased furthermore.SOLUTION: A method for manufacturing a steam turbine material of the present invention comprises: a pre-forging heating step of heating a forging material to 820-1000°C, the forging material having a composition containing in mass%, of 0.4% or less of C, 0.2-0.6% of Mn, 0.5% or less of Si, 3.0-4.0% of N, 1.0-2.0% of Cr, 0.2-0.6% of Mo, 0.05-0.15% of V, with the balance being Fe and inevitable impurities; and a hot forging step of performing the hot forging of the forging material heated in the pre-forging heating step at a temperature lower than that in the pre-forging heating step and of 700°C or more, and making the forged material. Then, the cooling of the forged material in the hot forging step is performed, also serving as hardening.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for manufacturing a steam turbine material.

  Conventionally, forged products of 3.5NiCrMoV steel have been widely used for large rotors for low-pressure turbines (see Non-Patent Document 1). On the other hand, the size of low-pressure rotor blades for steam turbines is being increased in order to improve power generation efficiency. Since the centrifugal force generated during operation increases as the rotor blades increase in size, the strength of the rotor that supports them is also required. As a study for improving the strength and toughness of the rotor for low-pressure turbines, a chemical composition and tempering conditions have been studied (see Non-Patent Document 2).

Tomo Asai et al. “Welding rotor of steam turbine for high capacity and high temperature” Toshiba review Vol. 65 No. 8 (2010) Pages 12-14 Hiroaki Yoshioka et al. "Effects of chemical composition and tempering heat treatment on strength and impact properties of Ni-Cr-Mo-V steel" Iron and Steel Vol. 89 (2003) pp. 83-88

  As described above, the steam turbine material is required to have higher strength and toughness, but the conventional examination has not been sufficient. Although 3.5NiCrMoV steel as a steam turbine material has excellent strength and toughness, further improvement in strength and toughness is required.

  Then, an object of this invention is to provide the manufacturing method of the raw material for steam turbines which can raise the intensity | strength and toughness of the raw material for steam turbines further in view of the said problem.

  The present invention, as one aspect thereof, is a method for producing a material for a steam turbine, and is in mass%, C: 0.4% or less, Mn: 0.2 to 0.6%, Si: 0.5% or less Ni: 3.0-4.0%, Cr: 1.0-2.0%, Mo: 0.2-0.6%, V: 0.05-0.15%, the balance being Fe and inevitable A forging material having a composition composed of mechanical impurities was heated in the pre-forging heating step at a temperature lower than that of the pre-forging heating step and 700 ° C. or higher, which is heated to 820 to 1000 ° C. And a hot forging step in which the forging material is hot forged to obtain a forged material, and cooling of the forged material in the hot forging step is also performed by quenching.

  In the hot forging step, the compression rate of the forging material may be 50% or more.

  You may further include the tempering process which tempers the forging material obtained at the said hot forging process at the temperature of 550-650 degreeC.

  The forging material to be heated in the pre-forging heating process has an old austenite crystal grain number of ASTM No. It may be 4 or more.

  The forged material obtained in the hot forging step may have a disk shape, and the average crystal grain size of the disk-shaped forged material may be 10 μm or less.

  The disk-shaped forged material may have a diameter of 1 m or more and a mass of 4000 kg or more.

  According to the present invention, after performing a pre-forging heating step of heating the forging material having the predetermined composition within a predetermined temperature range, forging within a predetermined temperature range lower than the pre-forging heating step. While hot forging the material and cooling the forging material in the hot forging step, also by quenching, strain can be introduced into the forging material, and the old austenite crystal grains can be refined. It is possible to produce a steam turbine material with higher strength and toughness.

It is a metal structure photograph of the initial material (forging material) of an example and a comparative example. It is a metallographic photograph of the forging material by an Example. It is a metal structure photograph of the heat processing material by a comparative example.

  Hereinafter, an embodiment of a method for producing a steam turbine material according to the present invention will be described. In the manufacturing method of the present embodiment, a forging material having a predetermined alloy composition is heated to a temperature of 820 to 1000 ° C., and a heating temperature before the forging and a heating temperature of 700 ° C. or higher. And a hot forging step of hot forging the forging material heated at a temperature. The forging material and each process will be described in detail.

1. First, the alloy composition specified in the present invention is a steel type generally used as a rotor material for turbines, and conforms to ASTM-A470-Grade C. The reasons for limiting the composition defined in the present invention are as follows. The unit of composition is mass%.

<C: 0.4% or less>
C contributes to further precipitation of carbides by forming carbides during tempering, and therefore contains 0.4% as an upper limit. The upper limit with preferable C is 0.3%, More preferably, it is 0.28%. A preferable lower limit is 0.05%, and more preferably 0.10%. By containing C at an upper limit of 0.4%, it is possible to prevent the precipitation of excessive carbides that cause the toughness to decrease. When C is contained with 0.05% as the lower limit, toughness can be improved by precipitation of appropriate carbides.

<Mn: 0.2 to 0.6%>
Mn is added as a deoxidizer during refining and is contained in the range of 0.2 to 0.6%. The upper limit with preferable Mn is 0.5%, More preferably, it is 0.4%. A preferable lower limit is 0.23%, and more preferably 0.25%. When Mn is 0.2% or more, a sufficient deoxidation effect can be obtained. On the other hand, when Mn is 0.6% or less, a decrease in forgeability can be prevented.

<Si: 0.5% or less>
Si is added as a deoxidizer at the time of refining similarly to Mn, and contains 0.5% as an upper limit. The upper limit with preferable Si is 0.45%, More preferably, it is 0.4%. A preferable lower limit is 0.05%, and more preferably 0.10%. When Si is contained with an upper limit of 0.5%, toughness can be improved. By containing 0.05% as a lower limit, Si can be sufficiently deoxidized.

<Ni: 3.0-4.0%>
Ni has an effect of suppressing embrittlement at a low temperature, and is contained in a range of 3.0 to 4.0%. The upper limit with preferable Ni is 3.80%, More preferably, it is 3.60%. A preferable lower limit is 3.20%, and more preferably 3.40%. When Ni is 3.0% or more, cracking can be suppressed even when a high stress is applied at a low temperature. On the other hand, by being 4.0% or less, the amount of retained austenite can be reduced and the strength can be improved.

<Cr: 1.0 to 2.0%>
Cr has an effect of improving corrosion resistance and hardenability, and is contained in a range of 1.0 to 2.0%. The upper limit with preferable Cr is 1.90%, More preferably, it is 1.80%. A preferable lower limit is 1.20%, and more preferably 1.40%. When Cr is 1.0% or more, occurrence of stress corrosion cracking during use can be suppressed. On the other hand, strength and toughness can be improved by being 2.0% or less.

<Mo: 0.2 to 0.6%>
Mo has an effect of improving hardenability and is contained in a range of 0.2 to 0.6%. The upper limit with preferable Mo is 0.5%, More preferably, it is 0.45%. A preferable lower limit is 0.25%, and more preferably 0.3%. When Mo is 0.2% or more, it can be sufficiently quenched during cooling, and a desired strength can be obtained. On the other hand, sufficient hot workability can be obtained by being 0.6% or less.

<V: 0.05 to 0.15%>
V has an effect of improving hardenability and is contained in the range of 0.05 to 0.15%. The upper limit with preferable V is 0.13%, More preferably, it is 0.11%. A preferable lower limit is 0.06%, and more preferably 0.08%. When V is 0.05% or more, the hardenability can be sufficiently enhanced. On the other hand, strength and toughness can be improved by being 0.15% or less.

<The balance is Fe and inevitable impurities>
The balance is Fe and inevitable impurities. Typical inevitable impurities include P, S, Al and the like. P can be allowed within a range of 0.02% or less, S: 0.015% or less, and Al: 0.015% or less.

2. Pre-forging heating process Next, hot forging conditions will be described. The alloy having the above-described composition is referred to as “3.5NiCrMoV steel”. Forging is performed on the alloy forging material having the above-described composition. Usually, 3.5NiCrMoV steel is subjected to heat treatment of quenching and tempering using a forged material that has been forged. Conventionally, prior austenite crystal grains have been determined by the temperature and time of quenching. On the other hand, the forging process of the present invention aims to refine the prior austenite crystal grains remaining after cooling by applying high strain to the austenite structure and recrystallizing it.

  The forging material of 3.5NiCrMoV steel used in the present invention (hereinafter referred to as “forging material”) has a crystal grain number number of prior austenite crystal grains of ASTM-No. It is preferable to prepare what is 4 or more. This is because if the crystal grain size is excessively coarse, the effect of sufficient grain refinement may not be obtained at the time of forging, and this needs to be suppressed.

  And the heat processing before forging is performed using the prepared forging material. In the present invention, the heating temperature of the forging material before forging is set to 820 ° C. to 1000 ° C. in order to make the forging material into an austenite single phase. The upper limit with preferable heating temperature is 950 degreeC, More preferably, it is 900 degreeC. The minimum with a preferable heating temperature is 840 degreeC, More preferably, it is 860 degreeC. When heated in the range from room temperature to less than 820 ° C., it becomes an initial martensite single phase or two phases of martensite and austenite, the initial martensite phase does not undergo hardening by quenching after forging, and softens during heating. Although strength cannot be obtained after forging, the strength after forging can be improved by setting the forging material to an austenite single phase by setting the temperature to 820 ° C. or higher. By setting the temperature to 1000 ° C. or lower, it is possible to suppress austenite crystal grain coarsening and obtain a fine effect after forging.

3. Hot forging process Hot forging is performed using a forging material heated within the above temperature range. As a condition for hot forging, hot forging a forging material at a temperature lower than the heating temperature in the pre-forging heating step and at a temperature of 700 ° C. or higher is to refine austenite grains generated by recrystallization after forging. This is because the recrystallized grains become finer as the forging is performed at a lower temperature. The preferred hot forging temperature is preferably, for example, about 50 to 200 ° C. lower than the heating temperature before forging, more specifically, the upper limit of the hot forging temperature is 950 ° C., more preferably 900 ° C. It is. The minimum with preferable heating temperature is 750 degreeC, More preferably, it is 800 degreeC. By making the temperature during hot forging lower than the heating temperature in the pre-forging heating step, the effect of refining the prior austenite crystal grains by hot forging can be sufficiently obtained. On the other hand, by setting the temperature to 700 ° C. or higher, it is possible to prevent recrystallization from occurring due to extremely high strain required for recrystallization, and to enable recrystallization and generation of finer recrystallized grains.

  It is preferable that the compression rate of the forging material in the hot forging step is 50% or more. A preferable compression ratio is 60% or more. The upper limit of the compression ratio varies depending on the shape and size of the forged material or the capacity of the forging device to be used, but the upper limit is approximately 70%. When the compression ratio is 50% or more, the strain introduced into the material by forging increases, and strain necessary for forming recrystallized grains can be imparted. The compression rate is determined as the length of the longest part after forging / the length of the longest part before forging in the forging reduction direction.

<Disc-shaped hot forging>
Next, the hot forging method when the forged material shape is a disk shape will be described. The “forged material” referred to in the present invention is a shape after hot forging. In order to obtain a disk-shaped forging material, for example, a cylindrical forging material may be prepared. Of course, the crystal grain size number of the former austenite crystal grains of the forging material is ASTM-No. 4 or more are preferable.
When the forging material is heated before the forging, the heating temperature and the time are, for example, 820 to 1000 ° C. and 5 to 50 hours for obtaining a forging material having a diameter of 1 m or more and a weight of 4000 kg or more. It is.
And, for example, using a hot press forging device (hot forging device) capable of applying a forging load of 4000 to 50000 tons, the forging material is placed on the upper surface of the lower die, and pressed with the upper die, It is preferable to perform die forging (hot forging) using an upper die and a lower die. Even in this die forging, the compression rate is preferably 50% or more.
In order to increase the above-mentioned compression rate or increase the total strain amount, for example, a forging material with a high height is used, or the shape of the engraved surface formed in the upper die and the lower die It is preferable to use a mold having a shape that can selectively increase the compression rate and the total strain amount at a desired location.
By applying the forging load and compressibility described above, the average crystal grain can be made 10 μm or less. By making the average crystal grains 10 μm or less, both strength and toughness can be achieved by refining the crystal grains.

<Cooling method>
Cooling of the forging material in the hot forging step is cooling that also serves as quenching. In order to achieve cooling that also serves as quenching, a cooling rate that causes martensitic transformation is required. Although it depends on the size of the forged material, a cooling rate equal to or higher than that of air cooling is necessary. As specific cooling methods, various known cooling methods such as water cooling, oil cooling, blast cooling, and mist cooling can be applied. Among them, the effect of refining the crystal grains is a method of immersing in a liquid such as water cooling or oil cooling with a fast cooling rate. For example, a large forged material weighing 4000 kg or more is cooled at a high rate such as water cooling. Doing so may cause cracking or deformation. Therefore, it is preferable to apply air cooling such as blast cooling to large forged materials. Thus, the cooling that also serves as quenching is the purpose of obtaining the effect of maintaining the fine austenite crystal grains formed by forging in the hot forging process.

4). Tempering Step After the hot forging step, the forged material can optionally be tempered at 550 to 650 ° C. By performing such tempering, toughness can be increased. Since the tempering temperature is 550 ° C. or higher, carbides covering the grain boundaries can be hardly formed, so that toughness is increased. On the other hand, by setting the tempering temperature to 650 ° C. or less, it is possible to prevent the effect of tempering from becoming excessive and to obtain sufficient strength. Therefore, the temperature range of tempering shall be 550-650 degreeC.

  As described above, by implementing the method for manufacturing a turbine material according to the present embodiment to be described, it is possible to refine the prior austenite crystal grains while introducing strain into the forging material. It can be expected to improve strength and toughness compared to the material.

  C: 0.24%, Si: 0.06%, Mn: 0.30%, Cr: 1.61%, Ni: 3.37%, Mo: 0.3%, V: 0.11 in mass% The forging material of 3.5NiCrMoV steel in accordance with ASTM-A470-Grade C, which contains% and the balance of Fe and inevitable impurities, was prepared. The grain size number of the former austenite crystal grains of the forging material is ASTM-No. 4.6. For the forging material, a quenching and tempering material was used in order to make the structure uniform. Its dimensions are φ60 mm × 150 mm. FIG. 1 shows a structural photograph of the prepared forging material.

  Using the forging material, after heating and holding at 900 ° C., hot forging at a compression rate of 70% at 800 ° C. was performed. In hot forging, the forging material was placed on the upper surface of a lower mold having a flat work surface and pressed by a hot press so as to be sandwiched between upper molds having a flat work surface. After hot forging, cooling was performed by air cooling to room temperature. Thereafter, tempering was performed at 600 ° C. for 120 minutes, and cooling by air cooling was performed. As a comparative example, the forging material was heated and held at 900 ° C., then the temperature was lowered to 800 ° C., then air-cooled as it was without hot forging, and tempered at 600 ° C. for 120 minutes. Was made.

  From the position of the height and the diameter of the forged material produced, the test piece No. JIS14A tensile test piece of φ3 mm whose axial direction is perpendicular to the forging direction of the forged material. 1 was collected. Further, from the comparative material, a test piece No. 3 which is a JIS 14A tensile test piece of φ3 mm. Two were collected. In Table 1, the test piece No. 1 and no. The result of the tension test of 2 is shown. No. No. 1 is a No. 1 sample subjected to heat treatment only. It can be seen that the 0.2% yield strength and the maximum tensile strength are higher than 2.

  Moreover, the structure | tissue evaluation in the forging material which forged at 800 degreeC was implemented. FIG. 2 shows the metal structure at a position half the height and diameter of the forged material produced. It can be seen that fine crystal grains of 10 μm or less are formed on the entire surface. FIG. 3 shows the metal structure of the heat-treated material according to the comparative example. It can be seen that the crystal grain size is about 30 μm. From the above results, it can be seen that the crystal grains were refined by hot forging.

  From the above results, it is understood that by performing the pre-forging heating step and the hot forging step within a predetermined temperature range, strain can be introduced into the forging material and the prior austenite crystal grains can be refined. . In particular, it can be expected that a place where a high compression ratio and a high total strain amount are imparted is improved in strength and toughness. Moreover, the intensity | strength and toughness of an outer peripheral part can be improved by implementing the forging which gives a high distortion | strain to the outer peripheral part of a large forged material. For example, improvement in power generation efficiency can be expected by using the power generation turbine component.

Claims (6)

In mass%, C: 0.4% or less, Mn: 0.2-0.6%, Si: 0.5% or less, Ni: 3.0-4.0%, Cr: 1.0-2. 0%, Mo: 0.2-0.6%, V: 0.05-0.15%, the forging which heats the forging material which has a composition which consists of Fe and an unavoidable impurity to 820-1000 degreeC. A preheating step;
A hot forging step in which the forging material heated in the pre-forging heating step is hot forged at a temperature lower than the heating temperature of the pre-forging heating step and at a temperature of 700 ° C. or more to form a forging material,
The manufacturing method of the raw material for steam turbines which cools the forging material in the said hot forging process, and also serves as quenching.   The manufacturing method of the raw material for steam turbines of Claim 1 which sets the compression rate of the said forging raw material to 50% or more in the said hot forging process.   The manufacturing method of the raw material for steam turbines of Claim 1 or 2 which further includes the tempering process of tempering the forging material obtained at the said hot forging process at the temperature of 550-650 degreeC.   The forging material to be heated in the pre-forging heating process has an old austenite crystal grain number of ASTM No. The method for producing a steam turbine material according to any one of claims 1 to 3, wherein the number is 4 or more.   The forging material obtained in the hot forging step has a disk shape, and the average crystal grain size of the disk-shaped forging material is 10 µm or less. Method.   The method for producing a steam turbine material according to claim 5, wherein the disk-shaped forged material has a diameter of 1 m or more and a mass of 4000 kg or more.