BACKGROUND OF THE INVENTION
[0001] The present invention relates to a precipitation hardening martensitic stainless
steel having excellent texture stability, mechanical properties, and corrosion-resistance;
and also a long blade for a steam turbine using the same.
[0002] Recently, in terms of energy saving (e.g., the conservation of fossil fuel) and the
prevention of global warming (e.g., the suppression of CO
2 gas production), it has been demanded to improve the efficiency of a thermal power
plant (e.g., efficiency improvement in a steam turbine). One of the effective measures
for improving the efficiency of the steam turbine is to increase the size of a steam
turbine long blade. In addition, an increase in the size of the steam turbine long
blade is also expected to have secondary effects such as shortening a facility construction
period, and reducing the resulting cost by decreasing the number of casings.
[0003] A long blade material having both excellent mechanical properties and corrosion-resistance
is required to improve the reliability of the steam turbine. A precipitation hardening
martensitic stainless steel has a large amount of Cr and a small amount of C and thus
has excellent corrosion-resistance. However, a balance between strength and toughness
thereof is poor (refer to, e.g.,
JP-A-2005-194626).
SUMMARY OF THE INVENTION
[0004] An object of the invention is to provide precipitation hardening martensitic stainless
steel having excellent mechanical properties and corrosion-resistance.
[0005] The precipitation hardening martensitic stainless steel of the present invention
contains, by mass, 0.1% or less of C; 0.1% or less of N; 10.0% ∼15.0% of Cr; 10.0%∼15.0%
of Ni; 0.5%∼ 2.5% of Mo; 1.0%∼3.0% of Al, 1.0% or less of Si; 1.0% or less of Mn,
and the rest is Fe and inevitable impurities.
[0006] According to the present invention, it is possible to provide precipitation hardening
martensitic stainless steel having excellent texture stability, mechanical properties,
and corrosion-resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007]
FIG. 1 is a perspective view schematically illustrating an example of a steam turbine
long blade of the present invention;
FIG. 2 is a schematic view illustrating an example of a low-pressure stage rotor of
the present invention;
FIG. 3 is a schematic view illustrating an example of a low-pressure stage turbine
of the present invention; and
FIG. 4 is a schematic view illustrating an example of a power plant of the present
invention;
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0008] Hereinafter, the function and amount of the constituent elements contained in precipitation
hardening martensitic stainless steel according to the present invention will be described.
[0009] In the following description, the amount of constituent elements is represented by
mass ratio (%).
[0010] Carbon (C) forms a Cr carbide, and there are problems that toughness is reduced due
to excessive precipitation of carbides and corrosion resistance deteriorates due to
reduction in a Cr concentration near a grain boundary. Further, C remarkably reduces
an end point of the martensitic transformation temperature. Consequently, the amount
of C needs to be suppressed. The amount of C may be preferably 0.1% or less, and more
preferably 0.05% or less.
[0011] Nitrogen (N) forms TiN and AlN to reduce fatigue strength, and also adversely affects
toughness. For this purpose, the amount of N added needs to be suppressed. The amount
may be preferably 0.1% or less, and more preferably 0.05% or less.
[0012] Chromium (Cr) is an element that forms a passive film on the surface and thus contributes
to an improvement in the corrosion-resistance. The lower limit of addition is set
to 10.0% so that the corrosion resistance may be sufficiently secured. On the other
hand, when Cr is excessively added, a harmful phase is precipitated and the mechanical
property remarkably deteriorates, and therefore, the upper limit thereof is set to
15.0%. From above, the amount of Cr added needs to be set to 10.0 to 15.0%. The amount
may be preferably 11.0 - 14.0%, and particularly preferably 12.0 - 13.0%.
[0013] Nickel (Ni) is an element that suppresses a formation of δ ferrite and also contributes
to an improvement in the strength through the precipitation hardening of Ni-Al compounds.
Moreover, Ni also improves hardenability and toughness. To sufficiently achieve effects
described above, the lower limit of addition needs to be set to 10.0%. On the other
hand, when the amount added is more than 15.0%, a harmful phase is precipitated. As
a result, an intended mechanical property is not obtained. Judging from the above
points, the amount of Ni added needs to be set to 10.0 ∼ 15.0%. The amount may be
more preferably 11.0 ∼ 14.0%, and particularly preferably 12.0 ∼ 13.0%.
[0014] Molybdenum (Mo) is an element that improves corrosion resistance. To obtain the intended
corrosion resistance, the amount of Mo added of at least 0.5% is needed. On the other
hand, when the amount of Mo added is more than 2.5%, a formation of a harmful phase
is assisted and conversely the properties deteriorate. Judging from the above points,
the amount of Mo added needs to be set to 0.5 - 2.5%. The amount may be more preferably
1.0 ∼ 2.0%, and particularly preferably 1.25 ∼ 1.75%.
[0015] Aluminum (Al) is an element that forms a Ni-Al compound and contributes to precipitation
hardening. To sufficiently exert precipitation hardening, the amount of Al added of
at least 1.0% or more is needed. When the amount of Al added is more than 3.0%, the
mechanical property is reduced due to the excessive precipitation of the Ni-Al compounds
and the formation of harmful phases. Judging from the above points, the amount of
Al added needs to be set to 1.0 ∼ 3.0%. The amount may be more preferably 1.5 ∼ 2.5%,
and particularly preferably 1.75 - 2.25%.
[0016] Silicon (Si) is a deoxidizing agent, and the amount of Si added may be preferably
set to 1.0% or less. When the amount is more than 1.0%, there arises a problem that
δ ferrite is precipitated. The amount of Si added may be more preferably 0.5% or less,
and may be particularly preferably 0.25% or less. It is possible to omit the addition
of Si by applying carbon vacuum deoxidization and electro slag remelting. In that
case, no addition of Si is preferable.
[0017] Manganese (Mn) is added as a deoxidizing agent and a desulfurizing agent. When the
amount of Mn added is more than 1.0%, harmful phases are excessively formed and necessary
strength is not obtained. Therefore, the amount of Mn added needs to be set to 1.0%
or less. When a solution is dissolved by using a method of vacuum induced melting
or vacuum arc remelting, Mn need not be added. The amount of Mn added may be more
preferably 0.5% or less, and particularly preferably 0.25% or less.
[0018] As another element, tungsten (W) exerts an effect of improving the corrosion resistance
as well as Mo. W further improves this effect through the addition in combination
with Mo. In this case, to prevent harmful phases from being precipitated, the sum
of the amount of Mo and W added needs to be the same as the amount of Mo added alone.
[0019] Further, niobium (Nb) is an element that forms carbides and contributes to an improvement
in the strength; however, deteriorates productivity. For this purpose, in the case
of adding Nb, the amount of Nb added needs to be set to 1.0% or less. Alternatively,
it is possible to substitute tantalum (Ta) for Nb. In the case of adding Nb and Ta
in combination, the sum of the amount of Nb and Ta added needs to be the same as the
amount of Nb added alone. Though addition of these elements is not indispensable,
this leads to more significant precipitation hardening.
[0020] Inevitable impurities in the present invention are components originally contained
in the raw materials or incorporated during production and so on, and refer to components
that are unintentionally added. Inevitable impurities include P, S, Sb, Sn, and As,
and at least one of them may be contained in the precipitation hardening martensitic
stainless steel of the present invention.
[0021] Since reduction in P and S makes it possible to improve toughness without impairing
tensile strength, it is preferable that they are minimized. In terms of improving
the toughness, it is preferable that P is set to be 0.5% or less and S is set to be
0.5% or less. It is particularly preferable that P is set to be 0.1% or less and S
is set to be 0.1% or less.
[0022] The reduction in As, Sb, and Sn makes it possible to improve toughness. For this
purpose, it is preferable that the above elements are minimized, and it is preferable
that As, Sb, and Sn are set to be 0.1% or less, respectively. It is particularly preferable
that As, Sb, and Sn are set to be 0.05% or less, respectively.
[0023] Next, the heat treatment of the present invention will be described.
[0024] In the present invention, it is necessary to perform a solution treatment in which
heating and maintenance at 800 to 1050°C, preferably at 850 to 1000°C are followed
by rapid cooling. The solution treatment in the present invention refers to a heat
treatment for both dissolving components relating to the formation of precipitates
such as Al and Ni into the texture and obtaining a martensite texture at the same
time. The martensite texture is a kind of matrix of steel and is a texture excellent
in the balance between strength and toughness. After the solution treatment, it is
necessary to perform an aging treatment of maintenance of heating at 450 to 650°C
and thereby gradual cooling. The aging treatment of the present invention refers to
a heat treatment to obtain excellent strength by finely precipitating Ni-Al compounds
and so on in the texture, performed after the solution treatment.
[0025] Further, when residual austenite is expected to be reduced, a sub-zero treatment
may be performed. In the sub-zero treatment, it is necessary to keep the residual
austenite for at least four hours or more at -70°C or less and raise it up to room
temperature in the atmosphere by using organic solvents such as dry ice and isopentane.
[0026] The application of the alloy of the present invention to steam turbine long blades
will be described. Works such as machining and straightening can be performed after
the aging treatment. However, when the above works are performed immediately after
the solution treatment in which Ni-Al compounds are not yet precipitated, a high working
efficiency is expected because of good workability.
[0027] In the steam turbine long blade to which the alloy of the present invention is applied,
stellite plates of a Co-based alloy may be joined to a front end portion of the blade
by TIG welding. This is a measure to protect the steam turbine long blade from erosion
damaging the blade by the high-speed collision of condensed steam. Other techniques
for the installation of stellite plates include silver soldering, plasma transfer
arc, and laser build-up welding. Other measures to protect the steam turbine long
blade from erosion include modification of the surface with a titanium nitride coating,
etc. It is also possible to achieve erosion resistance by repeating more than once
heat treatments of heating the surface of the front end portion of the blade to the
Ac3 transformation temperature or higher and then cooling it to room temperature by
air cooling to a grain size number 6 or finer, followed by the aging treatment of
the entire blade to increase only the surface hardness of the front end portion of
the blade. Since the alloy of the present invention has a certain degree of erosion
resistance, the above measure against erosion may be omitted under conditions that
the erosion is not severe.
FIG. 1 illustrates the steam turbine long blade (10) to which the alloy of the present
invention is applied. The long blade includes a blade profile portion (1) that receives
steam, a blade root portion (2) that implants the blade into a rotor, a stub (4) for
integrating with an adjacent blade by screwing, and a continuous cover (5). The blade
root portion of the steam turbine long blade is an axial entry type having an inverted
Christmas-tree shape. The blade joins a stellite plate as an example of an erosion
shield (3). Other techniques for the installation of stellite plates include silver
soldering, plasma transfer arc, and laser build-up welding. It is also possible to
modify the surface with a titanium nitride coating, etc. Since the alloy of the present
invention has a certain degree of erosion resistance, the above measure against erosion
may be omitted under conditions that the erosion is not severe.
FIG. 2 illustrates a low-pressure stage rotor (20) to which the long blade of the
present invention is applied. This low-pressure stage rotor has a double-flow structure,
and long blades are symmetrically placed in a long-blade implantation portion (21)
over several stages. The above-described long blade is placed in the final stage.
FIG. 3 illustrates a low-pressure stage steam turbine (30) to which the low-pressure
stage rotor of the present invention is applied. A steam turbine long blade (31) rotates
by receiving steam guided by a nozzle (32). The rotor is supported by a bearing (33).
FIG. 4 illustrates a power plant (40) to which the low-pressure stage steam turbine
of the present invention is applied. High-temperature and high-pressure steam produced
in a boiler (41) works in a high-pressure stage turbine (42), and is then reheated
in the boiler. The reheated steam works in a medium-pressure stage turbine (43) and
then works in a low-pressure stage turbine (44). The work produced in the steam turbine
is converted into electricity by a power generator (45). The steam coming out from
the low-pressure stage turbine is guided to a condenser (46).
[0028] Hereinafter, examples will be described.
EXAMPLES
[Example 1]
(Preparation of Sample)
[0029] Test samples were prepared to evaluate a relationship between the chemical composition
of the precipitation hardening martensitic stainless steel of the present invention,
tensile strength thereof, 0.02% yield strength, Charpy impact absorption energy, pitting
potential, and micro-texture. Table 1 illustrates a chemical composition of each test
sample.
[0030] First, raw materials were melted by using a high-frequency vacuum melting furnace
(5.0×10
-3 Pa or less, 1600°C or higher) to obtain compositions listed in Table 1. The obtained
ingot was hot-forged by using a press forging machine and a hammer forging machine,
and formed into a square bar having a width×thickness×length of 100 mm×30 mm×1000
mm. Next, the square bar was cut and processed to a width×thickness×length of 50 mm×30
mm×120 mm, thereby giving stainless steel starting materials.
[0031] Next, the stainless steel starting materials were subjected to various heat treatments
by using a box electric furnace. Alloys 1 to 13 of the invention were maintained at
925°C for 1 hour as a solution heat treatment, followed by rapid water cooling of
immersing in room-temperature water. Subsequently, the alloys were maintained at an
arbitrary temperature of 450 to 650°C for 2 hours as an aging heat treatment, followed
by air cooling for removing them to room-temperature air.
[0032] Evaluation tests for tensile strength, Charpy impact absorption energy, pitting potential,
and micro-texture observation were performed to respective samples obtained above.
The following summarizes each evaluation test.
(Test Method)
[0033] A test piece (distance between evaluation points: 30 mm, outer diameter: 6 mm) was
prepared from each of the samples obtained above and subjected to a tensile test at
room temperature in accordance with JIS Z 2241. The determination criteria of tensile
strength and 0.02% yield strength are as follows. A tensile strength and 0.02% yield
strength of 1500 MPa or more and 1000 MPa or more, respectively, were rated as "acceptable",
while those of less than these values were rated as "unacceptable". Further, an elongation
and drawing of 10% or more and 30% or more, respectively, were rated as "acceptable",
while those of less than these values were rated as "unacceptable".
[0034] For the measurement of Charpy impact absorption energy, a test piece having a 2-mm
V notch was prepared from each of the samples obtained above and subjected to a Charpy
impact test at room temperature in accordance with JIS Z 2242. The determination criteria
of Charpy impact absorption energy are as follows. A Charpy impact absorption energy
of 20 J or more was rated as "acceptable", while that of less than this value was
rated as "unacceptable".
[0035] For the evaluation of pitting potential, a plate-like test piece (15 mm in length,
15 mm in width, and 3 mm in thickness) was prepared from each of the samples obtained
above, and coated with an insulator so that an area of a measurement surface becomes
equal to 1.0 cm
2. The evaluation was performed under the following conditions: test solution: 3.0%
NaCl solution, temperature of the solution: 30°C, sweep rate: 20 mV/min. The determination
criteria of pitting potential are as follows. A pitting potential of 150 mV or more
was rated as "acceptable", while that of less than this value was rated as "unacceptable."
[0036] The determination criteria of micro-texture are as follows. Those having a martensite
texture in which the amounts of δ ferrite and residual austenite precipitated are
1.0% or less and 10% or less in an area ratio, respectively, were rated as "acceptable".
Others were rated as "unacceptable". The amount of δ ferrite precipitated was measured
in accordance with a point counting method described in JIS G 0555. The amount of
residual austenite precipitated was measured by X-ray diffraction.
(Test Results)
[0037] In alloys 1 to 13 of the invention, mechanical properties of tensile strength, 0.02%
yield strength, elongation, drawing, and impact absorption energy were also rated
as "acceptable". Also for pitting potential, preferable results were obtained. In
addition, the δ ferrite phase and residual austenite in the metal texture were within
a target range, and they were thus confirmed to have a martensite texture.
[0038] In any of the comparative alloys 1 to 12, all aimed values of the respective properties
were not satisfied. In the comparative alloys 1 to 8, effects on principal components
such as Cr, Ni, Mo, and Al were studied. Among them, the comparative alloy 5 is a
sample in which the amount of Al added is high, and in which tensile strength and
0.02% yield strength were high; however, elongation, drawing, and impact absorption
energy were remarkably lower than their aimed values. The reason is considered that
the amount of a reinforcing phase precipitated is excessive. On the other hand, in
the comparative alloy 6, the amount of Al added is low, and tensile strength and 0.02%
yield strength were lower than their aimed values. Further, a large amount of residual
austenite was precipitated in the texture. Further, in the comparative alloys 9 to
12, effects on impurity elements were studied. The comparative alloy 9 is a sample
in which the amount of C added is high, and its tensile strength, 0.02% yield strength,
elongation, and impact absorption energy were lower than their aimed values. Further,
pitting potential was also lower than its aimed value. The reason is considered that
a Cr concentration near a grain boundary is reduced through the formation of Cr carbides
and corrosion resistance is deteriorated. In addition, a large amount of residual
austenite was precipitated in the texture. The comparative alloy 12 is a sample in
which the amount of N added is high, and its elongation, drawing, and impact absorption
energy were lower than their aimed values. Further, a large amount of residual austenite
was precipitated in the texture.

[Example 2]
[0039] The following describes the steam turbine long blade to which the alloy of the present
invention is applied. In this embodiment, by using Alloy 1 illustrated in Table 1
of materials of the present invention, an axial-entry-type steam turbine long blade
having a blade length of 48 inch was produced. As a method for producing the long
blade, vacuum carbon deoxidation was first performed in a high vacuum state of 5.0×10
-3 Pa or less to deoxidize molten steel by a chemical reaction of C+O→CO. Subsequently,
the steel was formed into an electrode bar by extend forging. Electro-slag remelting
was thus performed to give a high-grade steel ingot by self-dissolving the electrode
bar by Joule heat generated upon the application of current at the time when this
electrode bar was immersed in molten slag, and then coagulating it in a water-cooled
die. Next, the steel ingot was hot-forged, and then press-forged by using a 48-inch
blade die. After that, as a solution treatment, the resulting product was heated and
maintained at 925°C for 2.0 hours, followed by forced cooling of rapid cooling by
using a fan. Then, the product was formed into a predetermined shape through a cutting
step and then, as an aging treatment, heated and maintained at 525°C for 4.0 hours,
followed by air cooling. As the final finishing, the curve was eliminated and the
surface was polished, thereby giving a 48-inch long blade.
[0040] Test pieces were collected from the front end, center, and root portions of the steam
turbine long blade obtained by the above steps, respectively, and subjected to evaluation
tests in the same manner as in Example 1. A direction of the collected test pieces
is the direction of the length of the blade.
[0041] The micro-texture of each portion was a uniform martensite texture. Neither residual
austenite nor δ ferrite was observed. In addition, regardless of the positions where
the test pieces were collected, aimed values of the desired tensile strength, 0.02%
yield strength, impact absorption energy, and pitting potential were achieved.
[0042] The precipitation hardening martensitic stainless steel of the present invention
has excellent mechanical properties and corrosion-resistance, and thus can be applied
to the steam turbine long blade. In addition, it can also be applied to a blade for
a gas turbine compressor, and so on.
1. A precipitation hardening martensitic stainless steel comprising, by mass, 0.1% or
less of C; 0.1% or less of N; 10.0% ∼ 15.0% of Cr; 10.0% ∼ 15.0% of Ni; 0.5% ∼ 2.5%
of Mo; 1.0% ∼ 3.0% of Al; 1.0% or less of Si; 1.0% or less of Mn, and the rest is
Fe and inevitable impurities.
2. The precipitation hardening martensitic stainless steel according to claim 1, further
comprising at least one member selected from Nb and Ta in an amount of 1.0% or less
by mass.
3. The precipitation hardening martensitic stainless steel according to claim 1 or claim
2, further comprising W, a total amount of Mo and W being the same as an amount of
Mo added alone.
4. The precipitation hardening martensitic stainless steel according to any one of claims
1∼3, wherein the inevitable impurities are at least one member selected from P, S,
Sb, Sn, and As.
5. The precipitation hardening martensitic stainless steel according to any one of claims
1∼4, wherein a temperature range of a solution treatment is 800 to 1050°C and a temperature
range of an aging treatment is 450 to 650°C.
6. A steam turbine long blade (10) using the precipitation hardening martensitic stainless
steel of any one of claims 1∼5.
7. The steam turbine long blade (10) according to claim 6, wherein a stellite plate made
of a Co-based alloy is joined to a front end portion of the blade (10).
8. A turbine rotor (20) comprising the steam turbine long blade (10) of claim 7.
9. A steam turbine (30) comprising the turbine rotor (20) of claim 8.