U.S. patent application number 15/123125 was filed with the patent office on 2017-03-09 for austenitic heat-resistant steel.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Takeo MIYAMURA, Shigenobu NAMBA.
Application Number | 20170067139 15/123125 |
Document ID | / |
Family ID | 54055354 |
Filed Date | 2017-03-09 |
United States Patent
Application |
20170067139 |
Kind Code |
A1 |
MIYAMURA; Takeo ; et
al. |
March 9, 2017 |
AUSTENITIC HEAT-RESISTANT STEEL
Abstract
An austenitic heat-resistant steel containing, by mass,
0.05-0.16% of C, 0.1-1% of Si, 0.1-2.5% of Mn, 0.01-0.05% of P,
less than 0.005% of S, 7-12% of Ni, 16-20% of Cr, 2-4% of Cu,
0.1-0.8% of Mo, 0.1-0.6% of Nb, 0.1-0.6% of Ti, 0.0005-0.005% of B,
0.001-0.15% of N, and 0.005% or less of Mg and/or 0.005% or less of
Ca, the amounts of Nb and Ti being 0.3% or above in total, with the
remainder being made up by Fe and unavoidable impurities. The
cumulative number density of a precipitate has a particle diameter
of over 0 nm to 100 urn being 0.1-2.0/.mu.m.sup.2, the precipitate
particle diameter corresponding to half of the cumulative number
density in the distribution of the cumulative number density and
the precipitate particle diameter is 70 nm or less, the average
hardness is 160 Hv or less, and the grain size number is 7.5 or
above.
Inventors: |
MIYAMURA; Takeo; (Hyogo,
JP) ; NAMBA; Shigenobu; (Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
54055354 |
Appl. No.: |
15/123125 |
Filed: |
March 4, 2015 |
PCT Filed: |
March 4, 2015 |
PCT NO: |
PCT/JP2015/056433 |
371 Date: |
September 1, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/54 20130101;
C22C 38/42 20130101; C22C 38/00 20130101; C22C 38/48 20130101; C21D
6/004 20130101; C22C 38/002 20130101; C22C 38/04 20130101; C22C
38/005 20130101; C22C 38/02 20130101; C22C 38/06 20130101; C21D
9/0068 20130101; C21D 2211/001 20130101; C22C 38/44 20130101; C22C
38/58 20130101; C22C 38/001 20130101; C22C 38/50 20130101; C21D
2211/004 20130101 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C22C 38/48 20060101 C22C038/48; C22C 38/44 20060101
C22C038/44; C21D 6/00 20060101 C21D006/00; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C22C 38/50 20060101
C22C038/50; C22C 38/42 20060101 C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 5, 2014 |
JP |
2014-042889 |
Claims
1. An austenitic heat-resistant steel, comprising: C: 0.05 to 0.16%
by mass; Si: 0.1 to 1% by mass; Mn: 0.1 to 2.5% by mass; P: 0.01 to
0.05% by mass; S: 0.005% by mass or less (not including 0% by
mass), Ni: 7 to 12% by mass; Cr: 16 to 20% by mass; Cu: 2 to 4% by
mass; Mo: 0.1 to 0.8% by mass; Nb: 0.1 to 0.6% by mass; Ti: 0.1 to
0.6% by mass; B: 0.0005 to 0.005% by mass; N: 0.001 to 0.15% by
mass; and at least one of Mg: 0.005% by mass or less (not including
0% by mass) and Ca: 0.005% by mass or less (not including 0% by
mass), with the remainder being Fe and unavoidable impurities,
wherein a total of a content of Nb and a content of Ti is 0.3% by
mass or more, a cumulative number density of a precipitate whose
particle diameter falls within a range of more than 0 nm up to 100
nm is 0.1 to 2.0 Number/.mu.m.sup.2, a precipitate particle
diameter corresponding to a half of the cumulative number density
in a distribution of the cumulative number density and the
precipitate particle diameter is 70 nm or less, an average hardness
is 160 Hv or less, and a grain size number is 7.5 or more.
2. The austenitic heat-resistant steel according to claim 1,
further comprising at least one of Zr: 0.3% by mass or less (not
including 0% by mass), a rare earth element: 0.15% by mass or less
(not including 0% by mass) and W: 3% by mass or less (not including
0% by mass).
Description
TECHNICAL FIELD
[0001] The present invention relates to an austenitic
heat-resistant steel.
BACKGROUND ART
[0002] In general, a high-temperature process at a few hundred
degrees or higher is employed in energy-related instruments such as
boilers, reactors and the like, and a heat-resistant material
having an excellent creep strength even in high-temperature
environments is needed.
[0003] In order that such a heat-resistant material could exhibit
an excellent creep strength in high-temperature environments, there
are a method of adding an element capable of dissolving in a solid
steel in a high-temperature environment to realize an effect of
solute strengthening, a method of adding an element capable of
precipitating in a high-temperature environment to form a
precipitate in a high-temperature environment thereby realizing an
effect of precipitation strengthening, a method of growing crystal
grains to be coarse to thereby prevent boundary sliding etc.
[0004] Among these, the method of growing crystal grains to be
coarse interferes with formation of a Cr.sub.2O.sub.3 protective
film and therefore may have a risk of worsening steam oxidation
resistance.
[0005] For realizing solute strengthening, the amount of the
element to be added must be increased. When the amount of the
element to be added is increased, it may have some negative
influences on other various basal characteristics than creep
strength.
[0006] In addition, when the amount of the added element is large,
the material cost may increase and there may be a possibility of
detracting from economic efficiency. Accordingly, the method of
employing the solute strengthening method for a heat-resistant
material could not be said to be desirable as a method of realizing
an intended strength.
[0007] On the other hand, it is conventional that, according to the
method of realizing a precipitation strengthening effect,
dislocation movement to be accompanied by deformation can be
strongly inhibited and therefore a creep strength can be greatly
improved. Here, many heat-resistant members are produced in a
process of softening heat treatment, cold working and final heat
treatment in that order. In these treatments, in order to form
large amounts of precipitates in a practical high-temperature
environment or during a creep test, the elements to precipitate in
the practical environment or during the creep test must be
previously dissolved in solid through high-temperature heating in
the final heat treatment followed by rapid cooling. In order that
larger amounts of precipitated components could be dissolved in
solid, such a final heat treatment must be carried out a
temperature as high as possible, in which, however, crystal grains
may grow to be coarse and, as a result, there may be a possibility
of a risk of steam oxidation resistance.
[0008] Under the situation, Patent Document 1 discloses a method
for producing an austenitic stainless steel having a high creep
strength, having a fine-grained texture and excellent corrosion
resistance, which includes a cold-processing step for an austenitic
stainless steel containing one or more of Ti: 0.15 to 0.5% by mass
and Nb: 0.3 to 1.5% by mass, wherein the steel is heated at a final
softening temperature set to be higher than 1200.degree. C. and up
to 1350.degree. C., then cooled at a cooling rate of 500.degree.
C./hr or more, thereafter cold-worked by 20 to 90%, further
thereafter heated at 1070 to 1300.degree. C. and at a temperature
lower by 30.degree. C. or more than the final softening
temperature, and processed for final heat treatment for cooling at
a cooling rate not lower than air cooling.
[0009] The method disclosed in Patent Document 1 is for
precipitating only small amounts of a part of the elements to be
precipitated in the practical environment or during the creep test
in the stage of the above-mentioned final heat treatment to thereby
prevent the crystal grains from growing to be coarse by the
boundary pinning effect of the precipitates. In other words, in the
method disclosed in Patent Document 1, the softening heat treatment
temperature before the cold processing is increased by a certain
level or more relative to the final heat treatment, so that the
difference in the solid solute amount corresponding to the
temperature difference is thereby precipitated. In that manner, by
specifically designing the two heat treatment temperatures, both
improvement of the creep strength by high-temperature heat
treatment and formation of a texture containing large quantities of
fine crystal grains (fine crystal grain texture) have been
realized.
CITATION LIST
Patent Document
[0010] Patent Document 1: JP-B H05-69885
SUMMARY OF INVENTION
Technical Problem
[0011] However, a production plant for use in practical production
has an upper limit temperature. When the softening heat treatment
temperature is increased up to the plant upper limit temperature,
and when a difference is provided between the two heat treatment
temperatures like in the method disclosed in Patent Document 1, the
final heat treatment temperature must be set lower than the plant
upper limit. However, lowering the final heat treatment temperature
may result in reduction in the amount of the precipitate to be
formed in a practical environment or during a creep test, and
therefore, as a result, there is a possibility that the creep
strength could not be fully increased. In particular, the invention
disclosed in Patent Document 1 is to realize excellent steam
oxidation resistance by providing a fine crystal grain texture and
to realize an excellent creep strength by precipitating a small
amount of precipitates to provide the boundary pinning effect.
However, as described above, it is considered that realizing the
pinning effect by lowering the final heat treatment temperature
would use forwardly but scarify the precipitates that are to be
formed in a practical environment or during a creep test.
[0012] In particular, in a steel material such as KA-SUS321J1HTB
steel, KA-SUS321J2HTB steel or the like using Ti as a precipitating
element, the presence or absence of fine precipitates of Ti
carbides may have a great influence on the high-temperature
strength of the steel. Naturally, in these steel materials, the
temperature range in which Ti dissolves in solid covers high
temperatures, and therefore the softening heat treatment
temperature may often reach the upper limit owing to limitations on
plants in many cases. Accordingly, for the purpose of providing a
temperature difference between the softening heat treatment
temperature and the final heat treatment temperature, the final
heat treatment temperature must be inevitably lowered, and as the
case may be, therefore, the amount of the Ti solute to precipitate
in a practical environment or during a creep test could not be
secured.
[0013] In consideration as above, in principle, it is presumed
that, in conventional techniques, the precipitation strengthening
that may be obtained from steel material components could not be
sufficiently utilized. Of many heat-resistant members, the creep
strength serves as a constraining factor to determine the thickness
of the member, and therefore it is considered that, with the
increase in the creep strength thereof, the member can be thinned
and the cost thereof can be reduced. At present, however, it could
hardly be said that an austenitic heat-resistant steel could have a
sufficient creep strength, and it may be said that the situation of
the steel is such that the thickness reduction could bring about
cost reduction thereof.
[0014] On the other hand, when a fine crystal grain texture of an
austenitic heat-resistant steel is taken as a premise for
maintaining steam oxidation resistance thereof, and when the method
disclosed in Patent Document 1 is applied thereto, the final heat
treatment temperature must be made low. However, as described
above, when the final heat treatment temperature is lowered, the
solute amount of the precipitating element lowers. Accordingly, the
precipitation strengthening effect could not be maximized, and it
may be presumed that the creep strength increasing effect could not
be sufficiently expressed.
[0015] The present invention has been made in consideration of the
situation as above, and its object is to provide austenitic
heat-resistant steel having an excellent creep strength while
maintaining a fine crystal grain texture.
Solution to Problem
[0016] Heretofore, a creep strength problem has been solved by
specifically noting the solute amount of a precipitating element
that depends on the temperature in heat treatment. Therefore, in
general, it has been considered that, when the final heat treatment
temperature is lowered, the solute amount of a precipitating
element may reduce and therefore the amount of fine precipitates
that would newly precipitate in a practical environment or during a
creep test may reduce to thereby lower the creep strength.
[0017] Given the situation, according to the method disclosed in
Patent Document 1, the temperature difference between the softening
heat treatment and the final heat treatment is defined to be
30.degree. C. or more and a part of elements to be precipitated are
made to be precipitated in the final heat treatment to thereby
prevent the crystal grains from growing to be coarse. However, as
described above, the precipitates to be precipitated according to
this operation are the precipitates that should naturally
precipitate in a practical environment or during a creep test to
contribute toward increasing the creep strength of the steel.
Specifically, there is a probability that, of the austenitic
stainless steel produced according to the method disclosed in
Patent Document 1, the creep strength could not be sufficiently
increased by the proportion corresponding to the precipitate formed
through precipitation of the precipitating element for preventing
the crystal grains from growing to be coarse.
[0018] The present inventors have assiduously studied the
possibility whether the precipitates formed in the final heat
treatment could directly act on the improvement of the creep
strength of steel. As a result, the inventors have found that the
precipitates that are formed by controlling the addition amount and
the solute amount of the precipitating element to fall within a
specific range and by carrying out the final heat treatment under a
specific heat treatment condition where the precipitated grain size
and the precipitation amount contained in the steel are defined to
fall within a specific range (concretely, by carrying out the final
heat treatment at a lower temperature than before) can improve
creep strength.
[0019] In other words, the present inventors have found that the
precipitates formed through final heat treatment under a specific
heat treatment condition can contribute toward improvement of creep
strength directly as fine grain precipitates. This finding
indicates that the precipitates provide a more excellent creep
strength than conventional precipitates that are formed in
high-temperature heat treatment, and is beyond the concept of the
conventional technology.
[0020] In addition, the inventors have found that, since the final
heat treatment is carried out under the above-mentioned specific
heat treatment condition (at a lower temperature than before), the
fine crystal grain texture can be kept as such and the steam
oxidation resistance can be maintained.
[0021] The reason why the good creep strength can be attained even
though the final heat treatment is carried out under a specific
heat treatment condition (that is, even though the final heat
treatment is carried out at a lower temperature than before) would
be as follows.
[0022] This time, the present inventors have found that, in an
austenitic heat-resistant steel, the precipitates formed through
final heat treatment can more effectively prevent creep deformation
than the precipitates formed during a creep test. In general, the
precipitates formed during a creep test of an austenitic
heat-resistant steel are formed along dislocation that is
introduced along with deformation. Dislocation concentrates in the
vicinity of grain boundaries, and therefore the distribution of the
precipitates would be uneven.
[0023] As opposed to this, the precipitates formed in final heat
treatment in production of an austenitic heat-resistant steel are
formed uniformly in the grains. Accordingly, it is considered that
the precipitates formed in the final heat treatment could more
efficiently prevent the dislocation movement accompanied by creep
deformation throughout the grains from the initial stage of
deformation. For these reasons, it is presumed that, when the final
heat treatment is carried out under the specific heat treatment
condition as mentioned above, a good creep strength can be
realized. This finding is beyond the conventional conception of the
solute amount of a precipitating element that depends on the
temperature of heat treatment.
[0024] The austenitic heat-resistant steel which is achieved based
on the above findings and solves the above problems includes: C:
0.05 to 0.16% by mass; Si: 0.1 to 1% by mass; Mn: 0.1 to 2.5% by
mass; P: 0.01 to 0.05% by mass; S: 0.005% by mass or less (not
including 0% by mass); Ni: 7 to 12% by mass; Cr: 16 to 20% by mass;
Cu: 2 to 4% by mass; Mo: 0.1 to 0.8% by mass; Nb: 0.1 to 0.6% by
mass; Ti: 0.1 to 0.6% by mass; B: 0.0005 to 0.005% by mass; N:
0.001 to 0.15% by mass; and at least one of Mg: 0.005% by mass or
less (not including 0% by mass) and Ca: 0.005% by mass or less (not
including 0% by mass), with the remainder being Fe and unavoidable
impurities, and a total of a content of Nb and a content of Ti is
0.3% by mass or more, and in the austenitic heat-resistant steel, a
cumulative number density of a precipitate whose particle diameter
falls within a range of more than 0 nm up to 100 nm is 0.1 to 2.0
Number/.mu.m.sup.2, a precipitate particle diameter corresponding
to a half of the cumulative number density in a distribution of the
cumulative number density and the precipitate particle diameter is
70 nm or less, an average hardness is 160 Hv or less, and a grain
size number is 7.5 or more.
[0025] Having the constitution as above, the austenitic
heat-resistant steel in the present invention contains steel
material components each falling within the above-mentioned range
and can be provide with a precipitate that may be formed through
final heat treatment under a specific heat treatment condition. The
precipitate is so controlled that the diameter of the precipitated
particles contained in the steel and the precipitation amount
thereof each could fall within a specific range, and the
precipitate directly contributes toward improving the creep
strength of the steel directly as a fine precipitate after the
precipitation. The fine precipitate is, as described above, able to
improve more the creep strength than a precipitate formed through
precipitation by final heat treatment at a higher temperature as in
before, Further, in addition thereto, since the final heat
treatment is carried out under a specific heat treatment condition,
concretely, at a lower temperature than before, the steel can have
a fine grain texture and can have excellent steam oxidation
resistance.
[0026] It is preferred that the austenitic heat-resistant steel in
the present invention further includes at least one of Zr: 0.3% by
mass or less (not including 0% by mass), a rare earth element:
0.15% by mass or less (not including 0% by mass) and W: 3% by mass
or less (not including 0% by mass).
[0027] When the austenitic heat-resistant steel in the present
invention contains Zr within the above-mentioned range, the
high-temperature strength thereof can be improved by precipitation
strengthening. When the austenitic heat-resistant steel in the
present invention contains the rare earth element within the
above-mentioned range, the oxidation resistance of the stainless
steel can be improved. Further, when the austenitic heat-resistant
steel in the present invention contains W within the
above-mentioned range, the high-temperature strength thereof can be
improved by solute strengthening.
Advantageous Effects of Invention
[0028] The austenitic heat-resistant steel in the present invention
contains steel material components each falling within the
above-mentioned range, in which the precipitate is so controlled
that the precipitated particle diameter and the precipitation
amount each could fall within a specific range, and therefore the
steel can have an excellent creep strength while maintaining a fine
crystal grain texture.
BRIEF DESCRIPTION OF DRAWING
[0029] [FIG. 1] This is a graph for explaining the obtainment of a
precipitated particle diameter corresponding to a half of a
cumulative number density in the distribution of the cumulative
number density and the precipitate particle diameter. The
horizontal axis indicates the precipitated particle diameter (nm),
and the vertical axis indicates the cumulative number density
(Number/.mu.m.sup.2).
DESCRIPTION OF EMBODIMENTS
[0030] [Austenitic Heat-Resistant Steel]
[0031] An embodiment of the austenitic heat-resistant steel in the
present invention (embodiment of carrying out the present
invention) is described in detail hereinunder.
[0032] The austenitic heat-resistant steel of this embodiment
contains, as steel material components: C: 0.05 to 0.16% by mass;
Si: 0.1 to 1% by mass; Mn: 0.1 to 2.5% by mass; P: 0.01 to 0.05% by
mass; S: 0.005% by mass or less (not including 0% by mass); Ni: 7
to 12% by mass; Cr: 16 to 20% by mass; Cu: 2 to 4% by mass; Mo: 0.1
to 0.8% by mass; Nb: 0.1 to 0.6% by mass; Ti: 0.1 to 0.6% by mass;
B: 0.0005 to 0.005% by mass; N: 0.001 to 0.15% by mass; and at
least one of Mg: 0.005% by mass or less (not including 0% by mass)
and Ca: 0.005% by mass or less (not including 0% by mass), with the
remainder being Fe and unavoidable impurities, and a total of a
content of Nb and a content of Ti is 0.3% by mass or more.
[0033] Preferably, the austenitic heat-resistant steel of this
embodiment further contains at least one of Zr: 0.3% by mass or
less (not including 0% by mass), a rare earth element: 0.15% by
mass or less (not including 0% by mass) and W: 3% by mass or less
(not including 0% by mass).
[0034] As can be seen from the above-mentioned steel material
components, the austenitic heat-resistant steel of this embodiment
is similar to KA-SUS321J2HTB steel using Ti as a precipitating
element (18 mass% Cr-10 mass% Ni-3 mass% Cu--Ni, Ti steel).
[0035] In the austenitic heat-resistant steel of this embodiment
containing the above-mentioned steel material components, the
cumulative number density of the precipitate whose particle
diameter falls within a range of more than 0 nm up to 100 nm is 0.1
to 2.0 Number/.mu.m.sup.2, the precipitate particle diameter
corresponding to a half of the cumulative number density in the
distribution of the cumulative number density and the precipitate
particle diameter is 70 nm or less, the average hardness is 160 Hv
or less, and the grain size number is 7,5 or more. In this
description, the precipitated particle diameter is one calculated
as a circle-corresponding diameter of the precipitated particle
(precipitate).
[0036] Here, the reason why the precipitate as so controlled that
the precipitated particle size and the precipitation in the steel
each fall within a specific range can be formed through final heat
treatment under a specific heat treatment condition is as already
described hereinabove in the section of Solution to Problem. The
above-mentioned average hardness and the grain size number can be
controlled by controlling the heat treatment temperature. The
specific heat treatment condition and heat treatment temperature
will be described hereinunder.
[0037] As described above, the precipitate formed under a specific
heat treatment condition contributes toward improving creep
strength, as being a fine precipitate. In addition, under a
specific heat treatment condition, the crystal grains can keep a
fine crystal grain texture. Accordingly, the austenitic
heat-resistant steel of this embodiment can be excellent in steam
oxidation resistance.
[0038] The steel material components of the austenitic
heat-resistant steel of this embodiment and the reason why the
precipitated particle diameter and the precipitation amount to be
contained in the steel are defined each to fall within a specific
range are described below.
[0039] As described above, the austenitic heat-resistant steel of
this embodiment is similar to KA-SUS321J2HTB that uses Ti as a
precipitating element. In KA-SUS321J2HTB, the steel material
components described below each exhibit the effect as described
below, and when their content falls outside a predetermined content
range, there may occur some inconveniences. [0040] [C: 0.05 to
0.16% by mass]
[0041] C has an effect of forming a carbide to improve
high-temperature strength. In this embodiment, for obtaining the
effect of improving high-temperature strength, C is contained in an
amount of 0.05% by mass or more. However, when the C content is
excessive to be more than 0.16% by mass, coarse carbides are formed
to fail in improving high-temperature strength.
[0042] The lower limit of the C content is preferably 0.08% by
mass, more preferably 0.09% by mass. The upper limit of the C
content is preferably 0.15% by mass, more preferably 0.13% by mass.
[0043] [Si: 0.1 to 1% by mass]
[0044] Si has a deoxidizing effect in a molten steel and
effectively acts for improving oxidation resistance. In this
embodiment, for obtaining both the deoxidizing effect and the
effect of improving oxidation resistance in a molten steel, Si is
contained in an amount of 0.1% by mass or more. However, the case
where the Si content is excessive and is more than 1% by mass is
unfavorable as often causing embrittlement of the steel
material.
[0045] The lower limit of the Si content is preferably 0.2% by
mass, more preferably 0.3% by mass. The upper limit of the Si
content is 0.7% by mass, more preferably 0.5% by mass. [0046] [Mn:
0.1 to 2.5% by mass]
[0047] Mn has a deoxidizing effect in a molten steel. In this
embodiment, for obtaining the deoxidizing effect in a molten steel,
Mn is contained in an amount of 0.1% by mass or more. However, the
case where the Mn content is more than 2.5% by mass is unfavorable
as promoting growth of carbide precipitates to be coarse.
[0048] The lower limit of the Mn content is preferably 0.2% by
mass, more preferably 0.3% by mass. The upper limit of the Mn
content is 2.0% by mass, more preferabl 1.8% by mass. [0049] [P:
0.01 to 0.05% by mass]
[0050] P has an effect of improving high-temperature strength. In
this embodiment, for improving high-temperature strength, P is
contained in an amount of 0.01% by mass or more. However, when the
P content is excessive to be more than 0.05% by mass, it may
detract from weldability.
[0051] The lower limit of the P content is preferably 0.015% by
mass, more preferably 0.02% by mass. The upper content of the P
content is 0.04% by mass, more preferably 0.03% by mass. [0052] [S:
0.005% by mass or less (not including 0% by mass)]
[0053] S is an unavoidable impurity. When the S content is
excessive to be more than 0.005% by mass, it degrades hot
processability. In this embodiment, for preventing degradation of
hot processability, the S content is limited to be 0.005% by mass
or less. The S content is preferably smaller.
[0054] The upper limit of the S content is preferably 0.002% by
mass, more preferably 0.001% by mass. [0055] [Ni: 7 to 12% by
mass]
[0056] Ni has an effect of stabilizing an austenitic phase. In this
embodiment, stabilizing the austenitic phase, Ni is contained in an
amount of 7% by mass or more. However, when the Ni content is more
than 12% by mass, it causes cost increase of the steel
material.
[0057] The lower limit of the Ni content is preferably 9% by mass,
more preferably 9.5% by mass. The upper limit of the Ni content is
preferably 11.5% by mass, more preferably 11% by mass. [0058] [Cr:
16 to 20% by mass]
[0059] Cr has an effect of improving oxidation resistance and
corrosion resistance of a steel material. In this embodiment, for
improving the oxidation resistance and the corrosion resistance of
the steel material, Cr is contained in an amount of 16% by mass or
more. However, when the Cr content is more than 20% by mass, the
steel material may be thereby embrittled.
[0060] The lower limit of the Cr content is preferably 17.5% by
mass, more preferably 18% by mass. The upper limit of the Cr
content is preferably 19.5% by mass, more preferably 19% by mass.
[0061] [Cu: 2 to 4% by mass]
[0062] Cu has an effect of forming a precipitate in a steel to
improve high-temperature strength. In this embodiment, for
improving high-temperature strength, Cu is contained in an amount
of 2% by mass or more. However, when the Cu content is excessive to
be more than 4% by mass, the effect may be saturated.
[0063] The lower limit of the Cu content is preferably 2.5% by
mass, more preferably 2.8% by mass. The upper limit of the Cu
content is preferably 3.5% by mass, more preferably 3.2% by mass.
[0064] [Mo: 0.1 to 0.8% by mass]
[0065] Mo has an effect of improving corrosion resistance. In this
embodiment, for improving corrosion resistance, Mo is contained in
an amount of 0.1% by mass or more. However, when the Mo content is
excessive to be more than 0.8% by mass, the steel material may be
thereby embrittled.
[0066] The lower limit of the Mo content is preferably 0.2% by
mass, more preferably 0.3% by mass. The upper limit of the Mo
content is preferably 0.6% by mass, more preferably 0.5% by mass.
[0067] [Nb: 0.1 to 0.6% by mass] [0068] [Ti: 0.1 to 0.6% by mass]
[0069] [The total of the Nb content and the Ni content is 0.3% by
mass or more.]
[0070] Nb and Ti are, when precipitated as a carbonitride (carbide,
nitride or carbonitride), able to improve high-temperature
strength. In addition, the precipitate prevents crystal grains from
growing to be coarse and promotes Cr diffusion. Owing to Cr
diffusion, the elements exhibits an effect of subsidiarily
improving corrosion resistance (steam oxidation resistance), and
therefore, these can be said to be a part of most important
elements in the present invention.
[0071] In this embodiment, for forming a precipitate of Nb and Ti
to improve high-temperature strength and for exhibiting the effect
of improving steam oxidation resistance, Nb is contained in an
amount of 0.1% by mass or more and Ti is contained in an amount of
0.1% by mass or more. By containing both Nb and Ti, the resultant
precipitate can more effectively contribute toward improving
high-temperature strength.
[0072] However, these must be contained in such that the total of
the Nb content and the Ti content is 0.3% by mass or more, and if
not, a minimum required precipitate amount could not be
secured.
[0073] The lower limit of the Nb content is preferably 0.2% by
mass. The lower limit of the Ti content is preferably 0.15% by
mass. The lower limit of the total of the Nb content and the Ti
content is preferably 0.35% by mass.
[0074] On the other hand, when the Nb content is excessive to be
more than 0.6% by mass, and when the Ti content is excessive to be
more than 0.6% by mass, the precipitate may grow to be coarse in
any case, thereby lowering toughness.
[0075] The upper limit of the Nb content and the Ti content is each
preferably 0.4% by mass, more preferably 0.3% by mass. [0076] [B:
0.0005 to 0.005% by mass]
[0077] B has an effect of promoting formation of an
M.sub.23C.sub.6-type carbide (where M is a carbide-forming element)
to improve high-temperature strength. In this embodiment, for
improving high-temperature strength, B is contained in an amount of
0.0005% by mass or more. However, when the B content is excessive
to be more than 0.005% by mass, it lowers weldability.
[0078] The lower limit of the B content is preferably 0.001% by
mass, more preferably 0.0015% by mass. The upper limit of the B
content is preferably 0.004% by mass, more preferably 0.003% by
mass. [0079] [N: 0.001 to 0.15% by mass]
[0080] N has an effect of improving high-temperature strength by
solute strengthening. In this embodiment, for improving
high-temperature strength, N is contained in an amount of 0.001% by
mass or more. However, when the N content is excessive to be more
than 0.15% by mass, it causes formation of coarse Ti nitride and Nb
nitride to worsen toughness.
[0081] The lower limit of the N content is preferably 0.002% by
mass, more preferably 0.003% by mass. The upper limit of the N
content is preferably 0.08% by mass, more preferably 0.04% by mass.
[0082] [At least one of Mg: 0.005% by mass or less (not including
0% by mass) and Ca: 0.005% by mass (not including 0% by mass)]
[0083] Mg and Ca each act as a desulfurizing/deoxidizing element
and have an effect of improving hot processability of a steel
material. Depending on the content of S that is contained as an
unavoidable impurity, Ca and Mg are preferably contained each in a
range of 0.005% by mass or less.
[0084] Preferably, the upper limit of Ca and Mg is 0.002% by mass
each. [0085] [Zr: 0.3% by mass or less (not including 0% by
mass)]
[0086] Zr is an optional component and has an effect of improving
high-temperature strength by precipitation strengthening. However,
when the Zr content is excessive to be more than 0.3% by mass, a
coarse intermetallic compound may be thereby formed to lower
high-temperature ductility.
[0087] The upper limit of the Zr content is preferably 0.25% by
mass.
[0088] However, when Zr is contained, it increases the cost of a
steel material, and therefore, the component may be optionally
contained. [0089] [Rare earth element: 0.15% by mass or less (not
including 0% by mass)]
[0090] Rare earth elements are optional components and have an
effect of improving oxidation resistance of stainless steel.
[0091] In other words, when a rare earth element is optionally
contained, an oxidation scale can be prevented from forming.
However, when the rare earth element content is excessive to be
more than 0.15% by mass, grain boundaries may partly dissolve in a
high-temperature environment, therefore unfavorably detracting from
hot processability.
[0092] The upper limit of the rare earth element content is
preferably 0.1% by mass, more preferably 0.05% by mass.
[0093] Here, rare earth elements are one or more elements selected
from Sc and Y, and 15 kinds of lanthanoid elements typified by La,
Ce and Ne, that is, 17 kinds of elements in total. The rare earth
element content is the total content of one or more elements
selected from those 17 kinds of elements. [0094] [W: 3% by mass or
less (not including 0% by mass)]
[0095] W is an optional component, and has an effect of improving
high-temperature strength by solute strengthening. However, when
the W content is excessive to be more than 3% by weight, coarse
intermetallic compounds are formed to lower high-temperature
ductility.
[0096] The upper limit of the W content is preferably 2.5% by mass,
more preferably 2.0% by mass.
[0097] The steel material components described above each exhibit
the effect as described above, when contained in steel, but at the
same time, these cause cost increase. Consequently, the content of
each component may be determined depending on the necessary
strengthening amount and the acceptable cost thereof. [0098] [The
remainder being Fe and unavoidable impurities.]
[0099] The remainder is Fe and other unavoidable impurities.
Examples of the other unavoidable impurities include, for example,
Al, Sn, Zn, Pb, As, Bi, Sb, Te, Se, In, etc.
[0100] Preferably, the amounts of the unavoidable impurities are as
small as possible, and as rough indication thereof, it is
recommended that the amount of Al is 0.01% by mass or less, Sn is
0.005% by mass or less, Zn is 0.01% by mass or less, Pb is 0.002%
by mass or less, As is 0.01% by mass or less, Bi is 0.002% by mass
or less, Sb is 0.002% by mass or less, Te is 0.01% by mass or less,
Se is 0.002% by mass or less, and In is 0.002% by mass or less.
[0101] [The Average hardness is 160 Hv or less.]
[0102] In addition to the compositional range as specified above,
and for securing the solute amount of the element to precipitate in
a practical environment or during a creep test, in this embodiment,
the average hardness (Vickers hardness) is defined to be 160 Hv or
less. When the average hardness is more than 160 Hv, the solute
amount of the element to precipitate in a practical environment or
during a creep test could not be secured, and if so, therefore, the
creep strength lowers. For controlling the average hardness to be
160 Hv or less, for example, the steel is heat-treated at a
temperature of 1150.degree. C. or higher and then cooled in water
to easily attain the numeral range, though depending on the
above-mentioned compositional formulation thereof.
[0103] Preferably, the upper limit of the average hardness is 140
Hv. Also preferably, the lower limit of the average hardness is 100
Hv, more preferably 110 Hv.
[0104] The Vickers hardness may be measured, for example, according
to JIS Z 2244:2009. [0105] [The cumulative number density of the
precipitate whose particle diameter falls within a range of more
than 0 nm up to 100 nm is 0.1 to 2.0 Number/.mu.m.sup.2.] [0106]
[The precipitate particle diameter corresponding to a half of the
cumulative number density in the distribution of the cumulative
number density and the precipitate particle diameter is 70 nm or
less.]
[0107] The cumulative number density of the precipitate whose
particle diameter falls within a range of more than 0 nm up to 100
nm is defined to be 0.1 to 2.0 Number/.mu.m.sup.2, and the
precipitate particle diameter corresponding to a half of the
cumulative number density in the distribution of the cumulative
number density and the precipitate particle diameter is defined to
be 70 nm or less, whereby the creep strength can be enhanced.
[0108] Specifically, regarding the precipitate to form in the final
heat treatment, while the amount of the precipitate having a size
of 100 nm or less is controlled to be not more than a specific
level, the precipitate particle diameter corresponding to a half of
the cumulative number density is controlled to be 70 nm or less,
that is, the precipitates are kept fine, and accordingly, the creep
strength can be thereby enhanced.
[0109] The lower limit of the cumulative number density is
preferably 0.3 Number/.mu.m.sup.2, more preferably 0.4
Number/.mu.m.sup.2.
[0110] The upper limit of the precipitate particle diameter
corresponding to a half of the cumulative number density is
preferably 60 nm, more preferably 50 nm. The lower limit of the
precipitate particle diameter corresponding to a half of the
cumulative number density is more than 0 nm.
[0111] A method for measuring the precipitated particle diameter
and the cumulative number density is described below. [0112] [The
grain size number is 7.5 or more.]
[0113] When the grain size number is 7.5 or more, the metal texture
is in a sufficiently fine state, and can be said to be a fine
crystal grain texture. Accordingly, the steel of the type can
maintain steam oxidation resistance.
[0114] For controlling the grain size number to be 7.5 or more, the
steel may be processed for final heat treatment under a specific
heat treatment condition to be mentioned below.
[Final Heat Treatment Under Specific Heat Treatment Condition]
[0115] For controlling the particle size and the precipitation
amount of the precipitated particles contained in steel to fall
within a specific range, and for controlling the grain size number
to be 7.5 or more, the steel may be subjected to final heat
treatment under a condition under which the coarsening factor for
the precipitate could be 2000.degree. C.min or less, on the premise
of the above-mentioned steel material composition and the hardness
range. This "condition under which the coarsening factor for the
precipitate could be 2000.degree. C.min or less" is the
above-mentioned specific heat treatment condition.
[0116] The coarsening factor for precipitate is an index of
indicating the influence of heat on the growth of precipitate to be
coarse grains, and is a value calculated by integrating a
temperature of 900.degree. C. or higher at which the precipitate
growth goes on relative to the temperature history during the heat
treatment, with respect to time. The coarsening factor must include
not only the retention time in heat treatment but also the heating
time at 900.degree. C. or higher and the cooling time. In this
connection, the coarsening factor for a conventional austenitic
heat-resistant steel which contains Ti as a precipitating element
and whose high-temperature strength has been sufficiently
increased, such as KA-SUS321J2HTB steel, is about 3000 to
7000.degree. C.min. As opposed to this, for the austenitic
heat-resistant of this embodiment, the coarsening factor is
2000.degree. C.min or less, as described above. As the lower limit
of the coarsening factor, it is preferably larger than 473.degree.
C.min, more preferably 500.degree. C.min or more, even more
preferably 821.degree. C.min. or more.
[0117] When the above-mentioned coarsening factor is satisfied, the
highest end-point temperature and the retention time can be
controlled in accordance with the limitations on equipment. Here,
for forming a precipitate like in a conventional technique, the
precipitating element must be dissolved in solid by carrying out
the softening heat treatment at a temperature higher by 30.degree.
C. or more than in the final heat treatment. Specifically, a
temperature lower by 30.degree. C. than in the softening heat
treatment is the upper limit temperature for the above-mentioned
final heat treatment.
[Method for Measuring Precipitated Particle Diameter and Cumulative
Number Density]
[0118] For judging whether or not the coarsening factor could
satisfy the above-mentioned condition, it is necessary to quantify
the number density and the size distribution of the precipitate.
This can be carried out by taking a microscopic image showing the
dispersion of precipitate particles on the cross section of a steel
material, and analyzing the image for quantification of the data.
The microscopic image may be taken, for example, by photographing
the surface of an electrolytically-polished steel material with a
scanning electron microscope. In a case where the precipitated
particles are fine, a transmission electron microscope may be used
in place of the scanning electron microscope. From the viewpoint of
quantification accuracy, it is recommended that at least 200
precipitated particles are quantitatively analyzed, and the data of
more than 0 nm to 100 nm are arranged through histogram at
intervals of 10 nm.
[0119] Briefly, as in the graph shown in FIG. 1, the cumulative
number density (Number/.mu.m.sup.2) at intervals of 10 nm is
plotted on the vertical axis, and the precipitated particle
diameter (nm) is on the horizontal axis, in which "the cumulative
number density of the precipitate whose particle diameter falls
within a range of more than 0 nm up to 100 nm" that is defined in
the present invention can be understood from the numerical value
falling between 90 nm and 100 nm on the horizontal axis. Regarding
"the precipitate particle diameter corresponding to a half of the
cumulative number density of the precipitate whose particle
diameter falls within a range of more than 0 nm to 100 nm", the
case shown in the drawing is referred to. In the drawing, the point
between 50 nm and 60 nm and the point between 60 nm and 70 nm are
connected to give a line, and the precipitate particle diameter can
be understood as the numeral value on the horizontal axis on which
the resultant line crosses the line extended from a half of the
numeral value falling between 90 nm and 100 nm.
[0120] Of the austenitic heat-resistant steel of this embodiment as
described above, the steel material components are defined to fall
within the above-mentioned range, and the precipitated particle
diameter and the precipitation amount in the steel are defined to
fall within a specific range, and therefore the steel can have an
excellent creep strength while maintaining a fine crystal grain
texture.
[0121] Heretofore, crystal grains have been tried to be refined at
the sacrifice of the precipitation amount formed in a practical
environment or during a creep test, but in the austenitic
heat-resistant steel of this embodiment, the precipitation that has
been heretofore sacrificed can be made to contribute toward
increasing the creep strength. Accordingly, even in the case where
the temperature in heat treatment has an upper limit owing to
limitations on equipment, etc., the precipitation strengthening
effect can be maximized. Consequently, from an austenitic
heat-resistant steel using Ti as a precipitating element therein, a
heat-resistant stainless steel whose creep strength is further
increased while the fine crystal grain texture thereof is kept as
such can be produced. The austenitic heat-resistant steel of this
embodiment can have an increased creep strength, and therefore the
thickness of a heat-resistant member to be formed of the steel can
be thinned more than before, and the present invention can realize
cost reduction of heat-resistant members.
EXAMPLES
[0122] Next, the contents of the present invention are described
concretely with reference to Examples that exhibit the effect of
the present invention and Comparative Examples not exhibiting
it.
[0123] Various steel materials of steel material components Nos. A
to F shown in Table 1 were melted individually in a vacuum melting
furnace (VIF) to give ingots of 20 kg each, and each ingot was
hot-forged to have a size of 130 mm width.times.20 mm
thickness.
[0124] Subsequently, this was softened by heat treatment at
1250.degree. C., and cold-rolled to give an original steel material
having a thickness of 13 mm. Of the steel materials Nos. A to F
shown in Table 1, Nos. A to E are similar to so-called
KA-SUS321J2HTB steel, and are steel materials satisfying the
chemical component composition defined in the present invention. As
opposed to these, No. F is a steel material overstepping the
chemical component composition defined in the present
invention.
[0125] In Table 1, the numerical values given an underline and
expressed by an italic are those not satisfying the requirements in
the present invention.
TABLE-US-00001 TABLE 1 Steel Material Component No. A B C D E F
Chemical C 0.12 0.14 0.10 0.10 0.13 0.11 Component Si 0.35 0.44
0.65 0.54 0.34 0.55 Composition Mn 1.58 1.44 1.10 1.58 1.80 0.98 (%
by mass) P 0.027 0.032 0.022 0.024 0.025 0.029 S 0.001 0.004 0.002
0.002 0.001 0.001 Ni 10.3 9.9 10.9 11.1 11.4 8.2 Cr 18.8 19.2 18.3
18.2 17.9 19.7 Cu 3.0 3.3 2.8 2.7 2.9 3.1 Mo 0.32 0.15 0.23 0.33
0.49 0.29 Nb 0.22 0.28 0.12 0.17 0.32 0.07 Ti 0.20 0.25 0.21 0.20
0.18 0.09 B 0.002 0.002 0.005 0.002 0.003 0.002 N 0.012 0.008 0.014
0.013 0.006 0.010 Mg 0.001 <0.001 0.002 0.001 0.001 <0.001 Ca
<0.001 <0.001 0.001 <0.001 0.001 <0.001 Nb + Ti 0.42
0.53 0.33 0.37 0.50 0.16 Others -- -- W: 0.5 rare earth Zr: 0.10 --
(Ce): 0.018 rare earth (Ce): 0.012 *) in the steel material Nos. A
to F, the remainder includes Fe and unavoidable impurities.
[0126] Each original steel material was heat-treated at a varying
heating temperature of 1040 to 1215.degree. C. for a varying period
of time of 0.5 to 10 minutes to vary the coarsening factor
[.degree. C.min] for the precipitate, thereby preparing steel
materials of Nos. 1 to 31 shown in Table 2. These steel materials
were analyzed for the Vickers hardness thereof, the cumulative
number density of the precipitate therein whose particle diameter
falls within a range of more than 0 nm up to 100 nm, the
precipitate particle diameter corresponding to a half of the
cumulative number density in the distribution of the cumulative
number density and the precipitate particle diameter, the grain
size number, and the creep rupture time, in the manner as mentioned
below. The measured results are shown in Table 2 along and the
coarsening factors therein.
[0127] In Table 2, the numerical values given an underline and
expressed by an italic are those not satisfying the requirements in
the present invention. [0128] (1) Vickers hardness [Hv]
[0129] Regarding the Vickers hardness, each steel material of Nos.
1 to 31 was tested in a Vickers hardness test according to JIS Z
2244:2009 to measure the hardness thereof. The load in the Vickers
hardness test was 10 kg. Those having a Vickers hardness of 160 Hv
or less were evaluated as excellent in average hardness, while
those more than 160 Hv were evaluated as poor in average hardness.
[0130] (2) Cumulative number density of precipitate whose particle
diameter falls within a range of more than 0 nm up to 100 nm
[.mu.m/cm.sup.2] [0131] (3) Precipitate particle diameter
corresponding to a half of the cumulative number density in the
distribution of the cumulative number density and the precipitate
particle diameter [.mu.m]
[0132] For the cumulative number density (2) and the precipitate
particle diameter corresponding to a half of the cumulative number
density (3), a picture of the surface of each steel material that
had been electrolytically polished was taken using a scanning
electron microscope at a magnification of 6000 times, at least 200
or more precipitated particles were analyzed on the image, and from
the resultant data, a graph as shown in FIG. 1 was drawn, in which
the distribution of the cumulative number density and the
precipitate particle diameter was calculated.
[0133] At this time, an image in which substances of 20 nm in size
could be recognized at a magnification of 6000 times was obtained,
and in this Example, it was confirmed that any other finer
precipitates than those did not exist in the image by the use of a
transmission electron microscope. [0134] (4) Grain size number
[0135] For the grain size number, the texture of the steel material
of Nos. 1 to 31 was microscopically observed according to JIS G
0551:2013 to measure the crystal grain number. Those having a
crystal grain number of 7.5 or more were considered as having
passed the test, while those with less than 7.5 were considered as
having failed in the test. [0136] (5) Creep rupture time [hr]
[0137] Regarding the creep rupture time, a test piece was prepared
from each steel material of Nos. 1 to 31 according to JIS Z
2271:2010, and tested to measure the time. Those having taken a
creep rupture time of 650 hours or more were evaluated as excellent
in creep strength, while those with less than 650 hours were
evaluated as poor in creep strength.
TABLE-US-00002 TABLE 2 Precipitate particle diameter corresponding
to a half Cumulative of the number cumulative density of number
precipitate density of whose precipitate particle whose particle
diameter diameter falls falls within within a range a range Steel
of more of more Creep material Coarsening Vickers than 0 nm than 0
nm Grain rupture component factor hardness up to 100 nm up to 100
nm size time No. No . (.degree. C. min.) (Hv) (Number/.mu.m.sup.2)
(nm) number (hr) Remarks 1 Remarks 2 1 A 3534 128 0.010 84 6.5 819
Comparative Example *1 2 A 2519 132 0.039 78 7.0 705 Comparative
Example *1 3 A 2381 136 0.118 73 7.5 613 Comparative Example 4 A
1742 146 0.227 68 8.0 687 Example 5 A 1343 152 1.309 44 8.5 702
Example 6 A 4774 143 0.036 74 7.5 576 Comparative Example 7 A 1603
144 0.395 52 8.5 656 Example 8 A 866 139 0.487 50 8.5 693 Example 9
A 473 167 1.251 46 8.0 337 Comparative Example *2 10 B 2476 127
0.109 85 7.5 636 Comparative Example 11 B 1612 130 0.165 65 8.0 701
Example 12 B 1434 139 0.596 47 8.5 745 Example 13 B 4598 139 0.055
77 7.5 610 Comparative Example 14 B 1577 141 0.414 56 8.5 668
Example 15 C 2316 145 0.089 77 7.5 596 Comparative Example 16 C
1576 139 0.145 61 7.5 683 Example 17 C 1879 142 0.106 68 7.5 661
Example 18 C 1083 153 0.418 51 8.0 685 Example 19 D 5846 125 0.013
92 6.0 823 Comparative Example *1 20 D 1941 129 0.126 68 7.5 669
Example 21 D 1698 129 0.194 56 7.5 703 Example 22 D 4633 143 0.045
81 7.5 582 Comparative Example 23 D 821 135 0.341 49 8.0 673
Example 24 E 2943 122 0.077 91 6.0 794 Comparative Example *1 25 E
1758 127 0.372 65 7.5 674 Example 26 E 1661 131 0.422 50 8.0 703
Example 27 E 2093 141 0.262 73 7.5 630 Comparative Example 28 E 935
138 0.674 49 8.0 691 Example 29 F 1858 125 0.021 67 6.5 617
Comparative Example *3 30 F 1137 133 0.065 52 7.0 573 Comparative
Example *3 31 F 697 146 0.087 44 7.5 495 Comparative Example *3 *1
in Remarks 2 indicates that, in the steel material, the crystal
grains grew coarsely. *2 in Remarks 2 indicates that the Vickers
hardness was low and the steel material could not secure the solute
amount. *3 in Remarks 2 indicates that the steel material was
outside the definition in the present invention.
[0138] As shown in Table 2, it was confirmed that the steel
materials of Nos. 4, 5, 8, 11, 12, 14, 16, 17, 18, 20, 21, 23, 25,
26 and 28 that exhibited the desired effects in the present
invention took a creek rupture time of 650 hours or more, and all
the materials had a creep rupture strength more excellent than the
comparative examples. In addition, it was confirmed that all these
steel materials of Nos. 4, 5, 7, 8, 11, 12, 14, 16, 17, 18, 20, 21,
23, 25, 26 and 28 contained fine crystal grains (that is, these had
a fine crystal grain texture) (all in Examples).
[0139] It is presumed that these Examples having a fine crystal
grain texture can have good steam oxidation-resistant
characteristics.
[0140] In particular, Nos. 4 and 7, Nos. 11 and 14, Nos. 16 and 18,
Nos. 20 and 23, and Nos. 25 and 28 are Examples in which the heat
treatment temperature for the former was lower than that for the
latter. Concretely, Nos. 4 and 7, Nos. 11 and 14, and Nos. 25 and
28 are Examples in which the temperature was lowered by 20.degree.
C.; Nos. 16 and 18 are Examples in which the temperature was
lowered by 10.degree. C.; and Nos. 20 and 23 are Examples in which
the temperature was lowered by 30.degree. C.
[0141] Among these, from the results of Nos. 16 and 18, Nos. 20 and
23, and Nos. 25 and 28, it is found that the latter number sample
took a longer creep rupture time as compared with the former number
sample that was heat-treated at a higher temperature. This finding
indicates a possibility that the creep strength enhancing effect
realized in the present invention would differ from the effect in
the conventional knowledge that notes the solute amount of a
precipitating element and means that "a higher heat treatment
temperature gives the higher creep strength".
[0142] On the other hand, as shown in Table 2, the steel materials
of Nos. 1, 2, 19 and 24 are Comparative Examples in which the
crystal grains grew coarsely since the heat treatment condition
(the precipitate coarsening factor) was inappropriate.
Specifically, these steel materials could not realize even a fine
crystal grain texture that was attained according to a conventional
technique (for example, in the invention described in Patent
Document 1). Consequently, it is presumed that the steel materials
of Nos. 1, 2, 19 and 24 could not obtain good moisture
oxidation-resistant characteristics.
[0143] The steel material of No. 9 is Comparative Example in which
the precipitate coarsening factor of the material was too low, and
therefore the precipitated component could not be sufficiently
dissolved in solid. The steel material of No. 9 had a fine crystal
grain texture, but it was confirmed that the Vickers hardness
(average hardness) thereof was outside the definition in the
present invention, and the creep rupture time was short.
[0144] The steel materials of Nos. 29 to 31 are Comparative
Examples in which the chemical component compositions are outside
the definition in the present invention.
[0145] Of those, in the steel materials of Nos. 29 and 30, the
crystal grains were coarse and contained some favorable element in
point of creep strength, but for the creep strength of both the
steel materials, the time was shorter than 650 hours, that is, as
compared with Examples, the steel materials could have only an
insufficient strength.
[0146] The steel material of No. 31 had a grain size number of 7.5
and had a good fine crystal grain texture, but for the creep
strength thereof, the time was shorter than 650 hours, that is, as
compared with Examples, the steel material could have only an
insufficient strength.
[0147] The steel materials of Nos. 3, 6, 10, 13, 15, 22 and 27 had
a good fine crystal grain texture having a gain size number of 7.5
or more. However, these steel materials of Nos. 3, 6, 10, 13, 15,
22 and 27 could not satisfy at least one of the cumulative number
density of the precipitate whose particle diameter falls within a
range of more than 0 nm up to 100 nm, and the precipitate particle
diameter corresponding to a half of the cumulative number density
in the distribution of the cumulative number density and the
precipitate particle diameter, and therefore, as compared with
Examples, these were poor in point of the creep rupture time (all
Comparative Examples).
[0148] From the above, it was confirmed that the steel materials
satisfying the definition in the present invention (the steel
materials of Examples) were excellent in creep strength in point of
having a fine crystal grain texture as compared with the steel
materials not satisfying the definition in the present invention
(the steel materials of Comparative Examples).
[0149] While the present invention has been described in detail and
with reference to specific embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope
thereof.
[0150] The present application is based on a Japanese Patent
Application No, 2014-042889 filed on Mar. 5, 2014, the contents of
which are herein incorporated by reference.
INDUSTRIAL APPLICABILITY
[0151] The austenitic heat-resistant steel in the present invention
exhibits an excellent creep strength even in a high-temperature
environment, and is therefore useful for energy-related instruments
such as boilers, reactors and the like. The steel has an excellent
creep strength even in a high-temperature environment.
* * * * *