U.S. patent application number 11/071333 was filed with the patent office on 2005-09-08 for heat-resistant austenitic stainless steel and a production process thereof.
This patent application is currently assigned to DAIDO STEEL CO., LTD.. Invention is credited to Hamano, Shuji, Shimizu, Tetsuya.
Application Number | 20050194073 11/071333 |
Document ID | / |
Family ID | 34840236 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194073 |
Kind Code |
A1 |
Hamano, Shuji ; et
al. |
September 8, 2005 |
Heat-resistant austenitic stainless steel and a production process
thereof
Abstract
To provide a heat-resistant austenitic stainless steel having
high-temperature strength and sag-resistance capable of resisting
working temperatures of not less than 550.degree. C. as well as
being low in cost, and a production process thereof. The steel
consistent with the present invention contains not more than 0.1 wt
% C, less than 1.0 wt % Si, 1.0 wt % to 10.0 wt % Mn, not more than
0.03 wt % P, not more than 0.01 wt % S, 0.01 wt % to 3.0 wt % Cu,
7.0 wt % to 15.0 wt % Ni, 15.0 wt % to 25.0 wt % Cr, 0.5 wt % to
5.0 wt % Mo, not more than 0.03 wt % Al, 0.4 wt % to 0.8 wt % N,
and the remainder substantially consisting of Fe and unavoidable
impurities.
Inventors: |
Hamano, Shuji; (Nagoya-shi,
JP) ; Shimizu, Tetsuya; (Nagoya-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
DAIDO STEEL CO., LTD.
Nagoya-shi
JP
|
Family ID: |
34840236 |
Appl. No.: |
11/071333 |
Filed: |
March 4, 2005 |
Current U.S.
Class: |
148/608 ;
148/326; 148/327 |
Current CPC
Class: |
C22C 38/42 20130101;
C22C 38/44 20130101; C22C 38/001 20130101 |
Class at
Publication: |
148/608 ;
148/326; 148/327 |
International
Class: |
C22C 038/58 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2004 |
JP |
2004-060124 |
Feb 2, 2005 |
JP |
2005-026158 |
Claims
What is claimed is:
1. A heat-resistant austenitic stainless steel comprising: not more
than 0.1 wt % C; less than 1.0 wt % Si; 1.0 wt % to 10.0 wt % Mn;
not more than 0.03 wt % P; not more than 0.01 wt % S; 0.01 wt % to
3.0 wt % Cu; 7.0 wt % to 15.0 wt % Ni; 15.0 wt % to 25.0 wt % Cr;
0.5 wt % to 5.0 wt % Mo; not more than 0.03 wt % Al; 0.4 wt % to
0.8 wt % N, and optionally at least one element selected from the
group consisting of: not more than 1.0 wt % W; and not more than
5.0 wt % Co; at least one element selected from the group
consisting of: 0.03 wt % to 0.5 wt % Ti; 0.03 wt % to 0.5 wt % Nb;
and 0.03 wt % to 1.0 wt % V; at least one element selected from the
group consisting of: 0.001 wt % to 0.010 wt % B; and 0.01 wt % to
0.10 wt % Zr; at least one element selected from the group
consisting of: 0.001 wt % to 0.010 wt % Ca; 0.001 wt % to 0.010 wt
% Mg, and the remainder substantially consisting of Fe and
unavoidable impurities.
2. The heat-resistant austenitic stainless steel according to claim
1, wherein PN is not less than 60 when expressed by the following
equation: PN=2.4Mn--Cu-0.6Ni+3Cr+0.8Mo (wt %).
3. The heat-resistant austenitic stainless steel according to claim
2, wherein the heat-resistant austenitic stainless steel is
subjected to solution treatment, cold working at a cold working
ratio of 40% to 70%, and aging treatment at 400.degree. C. to
650.degree. C. for not less than one minute.
4. The heat-resistant austenitic stainless steel according to claim
3, wherein primary hardness at room temperature is not less than 45
HRC.
5. The heat-resistant austenitic stainless steel according to claim
4, wherein room-temperature hardness after 400-hour heat treatment
at 600.degree. C. is 45 HRC, and room-temperature hardness after
400-hour heat treatment at 700.degree. C. is 40 HRC.
6. The heat-resistant austenitic stainless steel according to claim
5, wherein a residual stress ratio after a 50-hour relaxation test
at 700.degree. C. is not less than 25%.
7. The heat-resistant austenitic stainless steel according to claim
4, wherein a residual stress ratio after a 50-hour relaxation test
at 700.degree. C. is not less than 25%.
8. The heat-resistant austenitic stainless steel according to claim
3, wherein room-temperature hardness after 400-hour heat treatment
at 600.degree. C. is 45 HRC, and room-temperature hardness after
400-hour heat treatment at 700.degree. C. is 40 HRC.
9. The heat-resistant austenitic stainless steel according to claim
8, wherein a residual stress ratio after a 50-hour relaxation test
at 700.degree. C. is not less than 25%.
10. The heat-resistant austenitic stainless steel according to
claim 3, wherein a residual stress ratio after a 50-hour relaxation
test at 700.degree. C. is not less than 25%.
11. The heat-resistant austenitic stainless steel according to
claim 2, wherein primary hardness at room temperature is not less
than 45 HRC.
12. The heat-resistant austenitic stainless steel according to
claim 2, wherein room-temperature hardness after 400-hour heat
treatment at 600.degree. C. is 45 HRC, and room-temperature
hardness after 400-hour heat treatment at 700.degree. C. is 40
HRC.
13. The heat-resistant austenitic stainless steel according to
claim 2, wherein a residual stress ratio after a 50-hour relaxation
test at 700.degree. C. is not less than 25%.
14. A production process of a heat-resistant austenitic stainless
steel comprising the-steps of: applying solution treatment to the
heat-resistant austenitic stainless steel according to claim 2;
providing cold-working at a cold working ratio of 40% to 70% to the
heat-resistant austenitic stainless steel subjected to the solution
treatment; and applying aging treatment at temperatures of
400.degree. C. to 650.degree. C. for not less than one minute to
the heat-resistant austenitic stainless steel subjected to the cold
working.
15. The heat-resistant austenitic stainless steel according to
claim 1, wherein the heat-resistant austenitic stainless steel is
subjected to solution treatment, cold working at a cold working
ratio of 40% to 70%, and aging treatment at 400.degree. C. to
650.degree. C. for not less than one minute.
16. The heat-resistant austenitic stainless steel according to
claim 15, wherein primary hardness at room temperature is not less
than 45 HRC.
17. The heat-resistant austenitic stainless steel according to
claim 15, wherein room-temperature hardness after 400-hour heat
treatment at 600.degree. C. is 45 HRC, and room-temperature
hardness after 400-hour heat treatment at 700.degree. C. is 40
HRC.
18. The heat-resistant austenitic stainless steel according to
claim 15, wherein a residual stress ratio after a 50-hour
relaxation test at 700.degree. C. is not less than 25%.
19. The heat-resistant austenitic stainless steel according to
claim 1, wherein primary hardness at room temperature is not less
than 45 HRC.
20. The heat-resistant austenitic stainless steel according to
claim 1, wherein room-temperature hardness after 400-hour heat
treatment at 600.degree. C. is 45 HRC, and room-temperature
hardness after 400-hour heat treatment at 700.degree. C. is 40
HRC.
21. The heat-resistant austenitic stainless steel according to
claim 1, wherein a residual stress ratio after a 50-hour relaxation
test at 700.degree. C. is not less than 25%.
22. A production process of a heat-resistant austenitic stainless
steel comprising the steps of: applying solution treatment to the
heat-resistant austenitic stainless steel according to claim 1;
providing cold-working at a cold working ratio of 40% to 70% to the
heat-resistant austenitic stainless steel subjected to the solution
treatment; and applying aging treatment at temperatures of
400.degree. C. to 650.degree. C. for not less than one minute to
the heat-resistant austenitic stainless steel subjected to the cold
working.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a heat-resistant austenitic
stainless steel and a production process thereof, more specifically
relates to a heat-resistant austenitic stainless steel suitable for
a steel material for heat-resistant members used in exhaust systems
of an automobile engine and an aeroengine, industrial manufacturing
facilities and the like, and a production process thereof.
[0003] 2. Description of Related Art
[0004] Conventionally, for heat-resistant members used in exhaust
systems of an automobile engine and an aeroengine, industrial
manufacturing facilities and the like, high-temperature strength
and sag-resistance are required, and various metal materials are
selectively used according to working temperatures. For example, in
a relatively low-temperature range of not more than 350.degree. C.,
an austenitic stainless steel (e.g., SUS304 and SUS316), a
precipitation hardened stainless steel (e.g., SUS631J1) and the
like are used. In addition, in a high-temperature range not less
than 350.degree. C., an Fe-based superalloy (e.g., SUH660 (A286))
and an Ni-based superalloy (e.g., Inconel 718 and Inconel X750) and
the like are used.
[0005] On the other hand, improvement in engine performance and
thermal efficiency and the like have been promoted recently, so
that working temperatures of the members tend to rise. Therefore, a
heat-resistant steel material which is low in cost and more
excellent in the high-temperature strength and sag-resistance is
strongly required. Among the above-described various metal
materials, the conventional austenitic stainless steel and the
precipitation hardened stainless steel are relatively low in cost;
however, the working temperatures thereof are limited. On the other
hand, the Fe-based or Ni-based superalloy satisfies a requirement
for high-temperature strength and sag-resistance at not less than
550.degree. C., so that a heat-resistant member capable of
resisting working temperatures of up to about 700.degree. C. may be
obtained. However, the heat-resistant member made from the
superalloy causes increases in a melting cost and a process cost as
well as a raw material cost, so that there arises a problem of high
manufacturing costs.
[0006] To overcome the problem described above, various proposals
have been made heretofore. For example, Japanese Patent Application
Unexamined Publication No. Hei9-143633 discloses a martensitic
stainless steel for a heat-resistant spring consisting of 11 wt %
to 14 wt % Cr, 4.5 wt % to 7.0 wt % Ni, 1.0 wt % to 3.0 wt % Mo,
1.0 wt % to 3.0 wt % Al, 0.10 wt % to 0.20 wt % C, less than
10.times. C wt % Nb, and Fe and unavoidable impurities (see claim 1
and paragraph 0023). In this reference, it is described that when
the martensitic stainless steel having composition as above is
subjected to cold rolling at not more than 70% after solution
treatment, its 0.2% proof stress at temperatures of 350 to
550.degree. C. becomes 120 kgf/mm.sup.2 (1176 MPa) or more.
[0007] Further, Japanese Patent Application Unexamined Publication
No. 2000-239804 discloses a stainless steel wire for a
heat-resistant spring containing 0.04 wt % to 0.40 wt % C, 0.02 wt
% to 0.30 wt % N, 0.24 wt % to 0.60 wt % C+N, 1.5 wt % to 20.0 wt %
Mn, 17.0 wt % to 19.0 wt % Cr, 2.0 wt % to 12.0 wt % Ni and 0.5 wt
% to 2.0 wt % Mo as well as at least one element selected from 0.8
wt % Nb, 0.6 wt % to 1.2 wt % Si, 1.0 wt % Ti and 1.0 wt % W, and
the remainder substantially consisting of Fe and unavoidable
impurities (see paragraphs 0008 and 0009, and Tables 1 and 3). In
this reference, it is described that a stainless steel wire which
is excellent in the sag-resistance at 350 to 500.degree. C. is
obtained by increasing a solution amounts of interstitial solute
elements such as C and N, and a ferrite-stabilizing elements such
as W and Mo.
[0008] Further, Japanese Patent Application Unexamined Publication
No. 2003-73784 discloses a heat-resistant steel wire containing
0.02 wt % to 0.30 wt % C, 0.02 wt % to 3.5 wt % Si, 0.02 wt % to
2.5 wt % Mn, 20 wt % to 30 wt % Ni, 15 wt % to 25 wt % Cr, 1.0 wt %
to 5.0 wt % Ti and 0.002 wt % to 1.0 wt % Al as well as one or more
elements selected from 0.1 wt % to 2.0 wt % Nb, 0.1 wt % to 2.0 wt
% Ta and 0.1 wt % to 4.0 wt % Mo--the total content of Ti, Al, Nb
and Ta is 2.0 wt % to 7.0 wt % --, and the remainder substantially
consisting of Fe and unavoidable impurities (see claim 1 and
paragraph 0053). In this reference, it is described that by
controlling a structure of a .gamma. phase being a matrix phase, a
precipitation amount on an .gamma. phase (Ni.sub.3Ti), and a form
of a .gamma.' phase (Ni.sub.3(Al, Ti, Nb)), both of tensile
strength and high-temperature sag-resistance at 450 to 600.degree.
C. (especially at about 450.degree. C.) are attained.
[0009] Further, Japanese Patent Application Unexamined Publication
No. 2000-109955 discloses a heat-resistant stainless steel
containing 0.02 wt % to 0.30 wt % C, 0.02 wt % to 3.5 wt % Si, 0.02
wt % to 2.5 wt % Mn, 10 wt % to 50 wt % Ni, 12 wt % to 25 wt % Cr,
1.0 wt % to 5.0 wt % Ti and 0.002 wt % to 1.0 wt % Al as well as
one or more elements selected from 0.1 wt % to 3.0 wt % Nb, 0.001
wt % to 0.01 wt % B and 0.1 wt % to 4.0 wt % Mo--the total content
of Ti, Al and Nb is 3.0 wt % to 7.0 wt % (see claim 1 and paragraph
0037). In this reference, it is described that bringing the weight
percentage of an .eta. phase (Ni.sub.3Ti) and a .gamma.' phase
(Ni.sub.3(Al, Ti, Nb)) into a predetermined range allows
high-temperature tensile strength and the high-temperature
sag-resistance at temperatures close to 600.degree. C. to
improve.
[0010] Further, Japanese Patent Application Unexamined Publication
No. 2000-345268 discloses a high-heat-resistant alloy wire for a
spring containing not more than 0.1 wt % C, 18.0 wt % to 21.0 wt %
Cr, 12.0 wt % to 15.0 wt % Co, 3.5 wt % to 5.0 wt % Mo, 2.0 wt % to
4.0 wt % Ti and 1.0 wt % to 3.0 wt % Al, and the remainder
substantially consisting of Ni (see claim 1, and paragraphs 0022
and 0068). In this reference, it is described that a cold wire
drawing process is performed to obtain the alloy wire with a worked
austenitic structure where a grain size is predetermined, having
surface roughness within a predetermined range and being not more
than 5 mm in diameter, and thereby the sag-resistance at not less
than 600.degree. C. is improved.
[0011] Further, Japanese Patent Application Unexamined Publication
No. Hei8-269632 discloses a high-strength and
high-corrosion-resistant nitrogen-contained austenitic stainless
steel consisting of not more than 0.1 wt % C, not more than 1.0 wt
% Si, 5 wt % to 10 wt % Mn, not more than 0.01 wt % S, 8 wt % to 15
wt % Ni, 15 wt % to 25 wt % Cr, 0.5 wt % to 4 wt % Mo and 0.3 wt %
to 1.0 wt % N, and the remainder substantially consisting of Fe
(see claim 1, and paragraph 0024). In this reference, it is
described that dissolving nitrogen completely in solid solution
through a solution heat treatment at not more than 1100.degree. C.
allows room-temperature strength and corrosion resistance to
improve.
[0012] Furthermore, Japanese Patent Application Unexamined
Publication No. Hei9-279315 discloses an austenitic stainless steel
for a metal gasket consisting of not more than 0.1 wt % C, not more
than 1.0 wt % Si, 1.0 wt % to 10.0 wt % Mn, not more than 0.01 wt %
S, not more than 3.0 wt % Cu, 7.0 wt % to 15.0 wt % Ni, 15.0 wt %
to 25.0 wt % Cr, not more than 5.0 wt % Mo, 0.35 wt % to 0.8 wt % N
and not more than 0.02 wt % Al, and the remainder substantially
consisting of Fe (see claim 1, and paragraphs 0006 and 0029). In
this reference, it is described that by increasing a content of N,
and decreasing an Al-content restraining a solution amount of N to
less than the predetermined amount, a metal gasket excellent in
strength, high-temperature strength, sag-resistance and
high-temperature oxidation resistance is obtained.
[0013] However, the materials disclosed in the above-mentioned
Publications No. Hei9-143633 and No. 2000-239804 are prepared for
working temperatures of not more than 500.degree. C., and not more
than 550.degree. C., respectively, so that they do not satisfy
requirements for high-temperature strength and sag-resistance at
temperatures higher than 550.degree. C. Further, the amounts of
nitrogen contained in these materials are 0.3 wt % at the maximum
(see Table 1 of No. 2000-239804) On the other hand, the materials
disclosed in the above-mentioned Publications No. 2003-73784, No.
2000-109955 and No. 2000-345268 are prepared for working
temperatures of not less than 550.degree. C.; However, costs of the
materials rise up to the same as or more than that of the Fe-based
superalloy (e.g., SUH660) since improvement in heat resistance is
attempted in the respective materials by adding a large amount of
Ni or Co so that precipitation on the .gamma.' phase (Ni.sub.3Al)
is mainly reinforced.
[0014] Further, in the above-mentioned Publications No. Hei8-269632
and No. Hei9-279315, it is described that increasing the N-content
in the austenitic stainless steels allows the room-temperature
strength, the corrosion resistance, the high-temperature strength
and the like to improve. In these Publications, it is described
that those materials are useful for a component for which high
corrosion resistance is required such as a self-tapping screw, a
drill screw and a bolt used in the open air, an industrial area, a
coastal area and the like, or for the metal gasket for an internal
combustion engine; however, it is not disclosed definitely that
those materials are useful for a heat-resistant member such as a
heat-resistant spring which is used at working temperatures of not
less than 550.degree. C.
SUMMARY OF THE INVENTION
[0015] An object of the invention is to overcome the problems
described above and to provide a heat-resistant austenitic
stainless steel having high-temperature strength and sag-resistance
capable of resisting working temperatures of not less than
550.degree. C. as well as being low in cost, and a production
process thereof.
[0016] To achieve the objects and in accordance with the purpose of
the present invention, a heat-resistant austenitic stainless steel
consistent with the preferred embodiment of the present invention
contains not more than 0.1 wt % C, less than 1.0 wt % Si, 1.0 wt %
to 10.0 wt % Mn, not more than 0.03 wt % P, not more than 0.01 wt %
S, 0.01 wt % to 3.0 wt % Cu, 7.0 wt % to 15.0 wt % Ni, 15.0 wt % to
25.0 wt % Cr, 0.5 wt % to 5.0 wt % Mo, not more than 0.03 wt % Al,
0.4 wt % to 0.8 wt % N, and the remainder substantially consisting
of Fe and unavoidable impurities.
[0017] In another aspect of the present invention, a production
process of a heat-resistant austenitic stainless steel consistent
with the preferred embodiment of the present invention includes the
steps of applying solution treatment to the heat-resistant
austenitic stainless steel consistent with the present invention,
providing cold-working at a cold working ratio of 40% to 70% to the
steel subjected to the solution treatment, and applying aging
treatment at temperatures of 400.degree. C. to 650.degree. C. for
not less than one minute to the steel subjected to the cold
working.
[0018] The heat-resistant austenitic stainless steel consistent
with the preferred embodiment of the present invention is low in
cost since an addition amount of Ni is restrained. Further, an
austenitic phase is stabilized since amounts of respective alloying
elements such as Mn, Cr and Mo, which contribute to an increase in
a solution amount of N, are kept in balance, and thereby the
N-content is increased to the highest level above which N exceeds
an amount of N-solubility in molten metal under the atmosphere.
Furthermore, excellent high-temperature strength is attained
through the aging treatment after the cold working. Moreover, the
Al-content is made not more than 0.03 wt %, so that generation of
AlN which leads to decline in strength, toughness and ductility is
suppressed. Therefore, by optimizing conditions of the cold working
and the aging treatment, a heat-resistant member having
high-temperature strength and sag-resistance approximately equal to
those of an Fe-based superalloy is obtained.
[0019] Additional objects and advantages of the invention are set
forth in the description which follows, are obvious from the
description, or may be learned by practicing the invention. The
objects and advantages of the invention may be realized and
attained by the heat-resistant austenitic stainless steel and the
production process thereof in the claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] A detailed description of one preferred embodiment of a
heat-resistant austenitic stainless steel embodied by the present
invention is provided below. The heat-resistant austenitic
stainless steel consistent with the present invention is
characterized as containing elements as provided below, and the
remainder thereof substantially consisting of Fe and unavoidable
impurities. Hereinafter, description will be given on the addition
elements, ranges of addition amounts thereof, and reasons for
limitation of the ranges.
[0021] (1) C: Not More Than 0.1 wt %
[0022] C is an interstitial element which contributes to
improvement in strength. Further, C acts to improve heat resistance
by combining with Cr, Mo, W, V, Ti and Nb described later to form
carbide. Therefore, it is preferable for the heat-resistant
austenitic stainless steel to contain C so as to attain excellent
high-temperature strength and sag-resistance. Specifically, a
C-content is preferably not less than 0.001 wt %, more preferably
not less than 0.005 wt %, and still more preferably not less than
0.010 wt %.
[0023] Excessive addition of C, however, leads to decline in a
solution amount of N. Further, formation of Cr carbide decreases an
amount of Cr in solid solution in a matrix phase, and lowers
oxidation resistance. Furthermore, formation of coarse primary
carbide significantly degrades cold workability after solution
treatment, and reduces ductility, too. Therefore, the C-content is
preferably not more than 0.1 wt %, more preferably not more than
0.05 wt %, and still more preferably not more than 0.04 wt %.
[0024] (2) Si: Less Than 1.0 wt %
[0025] The present steel is characterized in that N may be
dissolved in solid solution to the maximum solution amount. Si
functions similarly to Al as a deoxidation element; however, since
Al reacts with N and generates AlN to decrease a solution amount of
N in the matrix phase, and the generated AlN significantly declines
the high-temperature strength, toughness and ductility, it is
preferable to use Si as the deoxidation element to reduce an
Al-content in the steel. Specifically, an Si-content is preferably
not less than 0.01 wt %, more preferably not less than 0.05 wt %,
and still more preferably not less than 0.10 wt %.
[0026] Excessive addition of Si, however, leads to decline in the
toughness and ductility as well as interference with forging.
Therefore, the Si-content is preferably less than 1.0 wt %, more
preferably not more than 0.7 wt %, and still more preferably not
more than 0.5 wt %.
[0027] (3) Mn: From 1.0 wt % to 10.0 wt %
[0028] Mn is an austenite-stabilizing element which contributes to
stabilization of an austenitic phase. Further, Mn is an important
element which contributes to improvement in the strength since it
significantly increases the solution amount of N. Furthermore, Mn
is effective as deoxidation and desulfurization elements.
Specifically, an Mn-content is preferably not less than 1.0 wt %,
more preferably not less than 3.0 wt %, and still more preferably
not less than 4.0 wt %.
[0029] Excessive addition of Mn, however, lowers the oxidation
resistance, degrades hot workability, and reduces the toughness and
ductility. Therefore, the Mn-content is preferably not more than
10.0 wt %, more preferably not more than 9.0 wt %, and still more
preferably not more than 8.0 wt %.
[0030] (4) P: Not More Than 0.03 wt %
[0031] P degrades the hot workability, and reduces intergranular
strength, toughness and ductility. Therefore, a P-content is
preferably small, specifically not more than 0.03 wt %. Excessive
reduction in P, however, causes a cost rise.
[0032] (5) S: Not More Than 0.01 wt %
[0033] S reduces the toughness and ductility at the time of the
cold working, and degrades the hot workability. Therefore, an
S-content is preferably small, specifically not more than 0.01 wt
%. Excessive reduction in S, however, causes a cost rise.
[0034] (6) Cu: 0.01 wt % to 3.0 wt %
[0035] Cu is an austenite-stabilizing element which contributes to
the stabilization of the austenitic phase. Further, Cu contributes
to improvement in the toughness at the time of the cold working.
Specifically, a Cu-content is preferably not less than 0.01 wt %,
and more preferably not less than 0.02 wt %.
[0036] Excessive addition of C, however, reduces the solution
amount of N as well as rises a solution temperature of Cr nitride
to increase an amount of insoluble Cr nitride at the time of
solution treatment, so that the high-temperature strength,
toughness and ductility are lowered, and the cold and hot
workability is degraded. Therefore, the Cu-content is preferably
not more than 3.0 wt %, more preferably not more than 2.5 wt %, and
still more preferably not more than 2.0 wt %.
[0037] (7) Ni: 7.0 wt % to 15.0 wt %
[0038] Ni is an austenite-stabilizing element which contributes to
the stabilization of the austenitic phase. Further, Ni contributes
to the improvement in the high-temperature strength. Specifically,
an Ni-content is preferably not less than 7.0 wt %, and more
preferably not less than 7.5 wt %, and still more preferably not
less than 8.0 wt %.
[0039] Excessive addition of Ni, however, causes a cost rise, so
that a material less expensive than an Fe-based superalloy such as
SUH660 cannot be obtained. Further, the excessive addition reduces
the solution amount of N as well as rises the solution temperature
of Cr nitride to increase the amount of the insoluble Cr nitride at
the time of the solution treatment, and thereby the
high-temperature strength, toughness and ductility are lowered and
the cold workability is degraded, significantly. Therefore, the
Ni-content is preferably not more than 15.0 wt %, more preferably
not more than 14.0 wt %, and still more preferably not more than
12.0 wt %.
[0040] (8) Cr: 15.0 wt % to 25.0 wt %
[0041] Cr significantly increases the solution amount of N, and
contributes to the improvement in the strength. Further, Cr is an
important element which improves the oxidation resistance and
corrosion resistance. Furthermore, Cr combines with C and N through
aging treatment after the cold working so as to greatly contribute
to the improvement in the high-temperature strength and the
sag-resistance. Specifically, a Cr-content is preferably not less
than 15.0 wt %, and more preferably not less than 18.0 wt %, and
still more preferably not less than 21.0 wt %.
[0042] However, excessive addition of Cr, being a
ferrite-stabilizing element brings a ferrite-austenite duplex-phase
structure, and promotes precipitation on a .sigma. phase which
leads to decline in the toughness and ductility. Therefore, the
Cr-content is preferably not more than 25.0 wt %, and more
preferably not more than 24.0 wt %.
[0043] (9) Mo: 0.5 wt % to 5.0 wt %
[0044] Mo is an element for increasing the solution amount of N,
which improves the corrosion resistance, the high-temperature
strength and the sag-resistance. Furthermore, similarly to Cr, Mo
combines with C to form the carbide, and improves the heat
resistance. Specifically, an Mo-content is preferably not less than
0.5 wt %, and more preferably not less than 0.8 wt %, and still
more preferably not less than 1.0 wt %.
[0045] Excessive addition of Mo, however, significantly degrades
the cold workability since Mo combines with C to generate the
coarse primary carbide. Further, the excessive addition lowers the
oxidation resistance, and interferes with the hot working.
Furthermore, the excessive addition lowers the toughness and
ductility to cause embrittlement. Moreover, the excessive addition,
rising the solution temperature of the Cr nitride to increase the
amount of the insoluble Cr nitride at the time of the solution
treatment, significantly lowers the high-temperature strength,
toughness and ductility as well as degrades the cold workability.
Therefore, the Mo content is preferably not more than 5.0 wt %, and
more preferably not more than 4.5 wt %, and still more preferably
not more than 4.0 wt %.
[0046] (10) Al: not more than 0.03 wt %
[0047] Similarly to Si and Mn, Al is highly effective as a
deoxidation element; however, excessive addition of Al
significantly lowers the high-temperature strength, toughness and
ductility, and further, degrades the cold workability since Al
combines with N to generate AlN, while the present steel is
characterized in that N may be dissolved in solid solution to the
maximum solution amount. Specifically, an Al-content is preferably
not more than 0.03 wt %, and more preferably not more than 0.025 wt
%, and still more preferably not more than 0.020 wt %.
[0048] (11) N: 0.4 wt % to 0.8 wt %
[0049] N is the interstitial element which is one of the most
important elements in the present invention, and highly effective
in improving the strength and the corrosion resistance and
stabilizing the austenitic phase. Further, N is highly effective in
improving the high-temperature strength and the sag-resistance
through the aging treatment after the cold working. Specifically,
an N-content is preferably not less than 0.4 wt %, and more
preferably not less than 0.42 wt %.
[0050] Excessive addition of N, however, causes N blow-hole, and
further, significantly degrades the cold workability and reduces
the toughness and ductility since the insoluble Cr nitride and a
large amount of Ti, Nb and V nitride remain in the steel at the
time of the solution treatment. Therefore, the N-content is
preferably not more than 0.8 wt %, and more preferably not more
than 0.7 wt %, and still more preferably not more than 0.6 wt
%.
[0051] Then, in addition to the above-described elements, the
heat-resistant austenitic stainless steel consistent with the
present invention may further include at least one element selected
from W and Co. Hereinafter, descriptions will be given on ranges of
addition amounts of W and Co, and reasons for limitation of the
ranges.
[0052] (12) W: Not More Than 1.0 wt %
[0053] W is an element for increasing the solution amount of N
which contributes to improvement in the high-temperature strength
and the sag-resistance. Further, similarly to Mo, W combines with C
to form carbide, and improves the heat resistance. Specifically, a
W-content is preferably not less than 0.01 wt %, and more
preferably not less than 0.05 wt %, and still more preferably not
less than 0.10 wt %.
[0054] Excessive addition of W, however, generates the coarse
primary carbide to significantly degrade the cold workability as in
the case of Mo. Further, the excessive addition interferes with the
forging, and reduces the toughness and ductility to cause
embrittlement. Therefore, the W-content is preferably not more than
1.0 wt %, and more preferably not more than 0.9 wt %, and still
more preferably not more than 0.8 wt %.
[0055] (13) Co: not more than 5.0 wt %
[0056] Co contributes to the improvement in the high-temperature
strength and the sag-resistance. Specifically, a Co-content is
preferably not less than 0.01 wt %, and more preferably not less
than 0.05 wt %, and still more preferably not less than 0.10 wt
%.
[0057] Excessive addition of Co, however, causes a cost rise, so
that a material less expensive than the Fe-based superalloy such as
SUH660 cannot be obtained. Further, the excessive addition leads to
the degradation in the cold workability. Therefore, the Co-content
is preferably not more than 5.0 wt %, and more preferably not more
than 4.5 wt %, and still more preferably not more than 4.0 wt
%.
[0058] Then, in addition to the above-described elements, the
heat-resistant austenitic stainless steel consistent with the
present invention may further include at least one element selected
from Ti, Nb and V. Hereinafter, descriptions will be given on
ranges of addition amounts of Ti, Nb and V, and reasons for
limitation of the ranges.
[0059] (14) Ti: 0.03 wt % to 0.5 wt %
[0060] Ti combines with C and N, and contributes to the improvement
in the high-temperature strength and refining of crystal grains.
Specifically, a Ti-content is preferably not less than 0.03 wt %,
and more preferably not less than 0.035 wt %, and still more
preferably not less than 0.04 wt %.
[0061] Excessive addition of Ti, however, causes a large amount of
oxide, carbide and nitride to remain in the steel to degrade the
cold workability. Further, the excessive addition decreases the
effective solution amount of N to lower the high-temperature
strength. Therefore, the Ti-content is preferably not more than 0.5
wt %, and more preferably not more than 0.4 wt %, and still more
preferably not more than 0.3 wt %.
[0062] (15) Nb: 0.03 wt % to 0.5 wt %
[0063] Similarly to Ti, Nb combines with C and N, and contributes
to the improvement in the high-temperature strength and the
refining of the crystal grains. Specifically, an Nb-content is
preferably not less than 0.03 wt %, and more preferably not less
than 0.035 wt %, and still more preferably not less than 0.04 wt
%.
[0064] Excessive addition of Nb, however, causes a large amount of
oxide, carbide and nitride to remain in the steel to degrade the
cold workability. Further, the excessive addition decreases the
effective solution amount of N to lower the high-temperature
strength. Therefore, the Nb-content is preferably not more than 0.5
wt %, and more preferably not more than 0.4 wt %, and still more
preferably not more than 0.3 wt %.
[0065] (16) V: 0.03 wt % to 1.0 wt %
[0066] Similarly to Ti and Nb, V combines with C and N, and
contributes to the improvement in the high-temperature strength and
the refining of the crystal grains. Specifically, a V-content is
preferably not less than 0.03 wt %, and more preferably not less
than 0.04 wt %, and still more preferably not less than 0.05 wt
%.
[0067] Excessive addition of V, however, causes a large amount of
oxide and nitride to remain in the steel to degrade the cold
workability. Further, the excessive addition decreases the
effective solution amount of N to lower the high-temperature
strength. Therefore, the V-content is preferably not more than 1.0
wt %, and more preferably not more than 0.9 wt %, and still more
preferably not more than 0.8 wt %.
[0068] Then, in addition to the above-described elements, the
heat-resistant austenitic stainless steel consistent with the
present invention may further include at least one element selected
from B and Zr. Hereinafter, descriptions will be given on ranges of
addition amounts of B and Zr, and reasons for limitation of the
ranges.
[0069] (17) B: 0.001 wt % to 0.010 wt %
[0070] B contributes to the improvement in the high-temperature
strength and the sag-resistance. Further, B is effective in
improving the hot workability. Specifically, a B-content is
preferably not less than 0.001 wt %.
[0071] Excessive addition of B, however, contrarily degrades the
hot workability. Therefore, the B-content is preferably not more
than 0.010 wt %, and more preferably not more than 0.008 wt %, and
still more preferably not more than 0.005 wt %.
[0072] (18) Zr: 0.01 wt % to 0.10 wt %
[0073] Zr contributes to the improvement in the high-temperature
strength and the sag-resistance. Specifically, a Zr-content is
preferably not less than 0.01 wt, and more preferably not less than
0.02 wt %, and still more preferably not less than 0.03 wt %.
[0074] Excessive addition of Zr, however, reduces the toughness and
ductility. Therefore, the Zr-content is preferably not more than
0.10 wt %, and more preferably not more than 0.09 wt %, and still
more preferably not more than 0.08 wt %.
[0075] Then, in addition to the above-described elements, the
heat-resistant austenitic stainless steel consistent with the
present invention may further include at least one element selected
from Ca and Mg. Hereinafter, descriptions will be given on ranges
of addition amounts of Ca and Mg, and reasons for limitation of the
ranges.
[0076] (19) Ca: 0.001 wt % to 0.010 wt %
[0077] Ca is effective in improving the hot workability, and is
also effective in improving machinability. Specifically, a
Ca-content is preferably not less than 0.001 wt %.
[0078] Excessive addition of Ca, however, contrarily degrades the
hot workability. Therefore, the Ca-content is preferably not more
than 0.010 wt %, and more preferably not more than 0.008 wt %, and
still more preferably not more than 0.005 wt %.
[0079] (20) Mg: 0.001 wt % to 0.010 wt %
[0080] Mg is effective in improving the hot workability.
Specifically, an Mg-content is preferably not less than 0.001 wt
%.
[0081] Excessive addition of Mg, however, contrarily degrades the
hot workability. Therefore, the Mg-content is preferably not more
than 0.010 wt %, and more preferably not more than 0.008 wt %, and
still more preferably not more than 0.005 wt %.
[0082] Among materials having the above-described composition, a
material having a PN value of not less than 60 when expressed in
the following Equation 1 is preferable for the heat-resistant
austenitic stainless steel consistent with the present
invention.
PN=2.4Mn--Cu-0.6Ni+3Cr+0.8Mo(wt %) Equation 1
[0083] In order to increase the solution amount of N in the steel,
addition amounts of the alloying elements such as Cr and Mn need to
be made proper. For Equation 1, Mn, Cu, Ni, Cr and Mo are selected
as an element which contributes to the solution amount of N, and
contribution rates of the respective elements to the solution
amount of N are obtained. When the PN value expressed by Equation 1
is not less than 60, it means that the solution amount of N capable
of satisfying a requirement for a high-temperature property is
secured. To obtain a material having an excellent high-temperature
property, the PN value is more preferably not less than 62, and
still more preferably not less than 64.
[0084] Next, a production process of the heat-resistant austenitic
stainless steel excellent in the high-temperature property will be
described. When the heat-resistant austenitic stainless steel
consistent with the present invention consisting of the
above-described composition is subjected to the solution treatment,
the cold working and the aging treatment under predetermined
conditions, a heat-resistant steel material excellent in the
high-temperature strength and the sag-resistance is obtained.
[0085] The solution treatment is applied to a forged and rolled
alloy for the purpose of uniforming the structure so that the cold
workability is secured and Cr.sub.2N precipitates while being
refined and dispersed uniformly at the time of the aging treatment.
As for the condition of the solution treatment, a condition
necessary and sufficient for uniforming the structure may be
applied. In the present invention, specifically, a temperature of
the solution treatment is preferably 1000.degree. C. to
1150.degree. C., and the time is preferably 0.1 hour to 2
hours.
[0086] After the solution treatment, the alloy is subjected to the
cold working to be formed into a shape for a desired application
such as a spring. A cold working ratio is preferably 40% to 70%.
When the cold working ratio is below 40%, an increase in the
strength by work hardening becomes small, and further, no increase
can be attained at the succeeding aging treatment. As a result,
primary hardness of 45 HRC at room temperature cannot be secured,
and a residual stress ratio in a relaxation test at 700.degree. C.
becomes 25% or less. Also in a case where the cold working ratio
rises over 70%, the residual stress ratio falls, which is not
preferable. Besides, a method for the cold working is not limited,
and various methods such as wire drawing, cold rolling and swaging
may be applied.
[0087] The aging treatment is applied to the alloy, which is
cold-worked at 40 to 70% after the solution treatment, for the
purpose of improving the strength and the sag-resistance. The aging
treatment is preferably conducted for not less than 1 minute at
400.degree. C. to 650.degree. C. Under the conditions other than
the one above, the primary hardness of 45 HRC at room temperature
cannot be secured, and the residual stress ratio at 700.degree. C.
becomes 25% or less. An upper limit of the aging treatment time is
not specified particularly; however, not more than 1 hour is
recommended to avoid a cost rise in terms of industrial use.
[0088] By subjecting the material having the above-described
composition under the above-described condition to the solution
treatment, the cold working and the aging treatment, a
heat-resistant austenitic stainless steels having the primary
hardness of 45 HRC at room temperature is obtained. By optimizing
the material composition and the treatment conditions, a
heat-resistant austenitic stainless steel having the primary
hardness of 50 HRC at room temperature is obtained.
[0089] Further, by optimizing the material composition and the
treatment conditions, a heat-resistant austenitic stainless steel
is obtained, which has hardness of not less than 45 HRC at room
temperature after 400-hour heat treatment at 600.degree. C., and
hardness of not less than 40 HRC at room temperature after 400-hour
heat treatment at 700.degree. C.
[0090] Furthermore, by optimizing the material composition and the
treatment conditions, a heat-resistant austenitic stainless steel
is obtained, which has the residual stress ratio of not less than
25% after a 50-hour relaxation test at 700.degree. C.
[0091] The heat-resistant austenitic stainless steel consistent
with the present invention is low in cost compared with the
conventional Fe-based or Ni-based superalloys since the addition
amount of Ni which causes a cost rise is restrained.
[0092] Further, the austenitic phase is stabilized, and the
excellent high-temperature strength is attained through the aging
treatment after the cold working since the amounts of the
respective alloying elements such as Mn, Cr and Mo, which
contribute to the increase of the solution amount of N, are kept in
balance so as to increase the N-content to the highest level above
which N exceeds an amount of N-solubility in molten metal under the
atmosphere. Especially, by adjusting the amounts of the respective
alloying elements so that the PN value becomes not less than 60,
the solution amount of N necessary for satisfying the requirement
for the high-temperature property may be secured. Moreover, the
generation of AlN which leads to decline in the strength, toughness
and ductility may be suppressed since the Al-content is made not
more than 0.03 wt %.
[0093] Further, by optimizing the conditions of the cold working
after the solution treatment, and of the aging treatment, the
heat-resistant austenitic stainless steel exhibits the
high-temperature strength and the sag-resistance capable of
resisting working temperatures of up to 700.degree. C. which is
approximately equal to those of the Fe-based superalloy. Therefore,
when the steel is applied to various heat-resistant members for
which the high-temperature strength and the sag-resistance are
required, an improvement in performance and thermal efficiency of
machines and the like where the heat-resistant members are
installed may be yielded while a cost rise is curbed.
EXAMPLES
[0094] Alloys having chemical compositions as listed in Table 1
attached hereto (Examples 1 to 14) were melted by using a
high-frequency induction furnace, subjected to homogenization and
heating, and made into a round bar of 24 mm in diameter by hot
forging. Then, solution treatment was conducted where the bar was
water-cooled after kept at 1100.degree. C. for 1 hour. Next, the
bar was subjected to cold working at a cold working ratio of 60% to
be formed into a round bar of 15.2 mm in diameter. Further, aging
treatment was conducted where the bar was air-cooled after kept at
500.degree. C. for 1 hour.
[0095] From the resultant material, test pieces were taken and
hardness tests (HRC) at room temperature and tensile tests (MPa) at
600.degree. C. and 700.degree. C. were conducted thereon. Further,
hardness tests (HRC) at room temperature after kept at 600.degree.
C. and 700.degree. C. for 400 hours were conducted, and residual
stress ratios (%) in relaxation tests at 700.degree. C. where
primary stress was made 530 MPa was evaluated. The material with a
greater residual stress ratio is more excellent in
sag-resistance.
[0096] Incidentally, the test procedure is as follows.
[0097] The hardness test: Rockwell hardness measurement test (based
on JIS Z2245)
[0098] The high-temperature tensile test: based on JIS G0567
[0099] The hardness test after long-time heat treatment at a high
temperature: Rockwell hardness measurement test (based on JIS
Z2245)
[0100] The relaxation test: based on JIS Z2276
[0101] Further, the same tests were conducted on SUH660
(Comparative Example 1) as a currently-used typical material. A
production process of SUH660 before cold working was the same as
that of Examples 1 to 14. Further, the cold working was conducted
at a cold working ratio of 50%, and SUH660 was formed into a round
bar of 17 mm in diameter. Furthermore, for aging treatment, the bar
was air-cooled after kept at 720.degree. C. for 4 hours.
[0102] Furthermore, the same tests were conducted on a low-Mo
material (Comparative Example 2), a high-Mo material (Comparative
Example 3), a low-Cr, low-N and low-PN material (Comparative
Example 4), a high-Cr material (Comparative Example 5), a low-Ni
material (Comparative Example 6), a high-Ni material (Comparative
Example 7), a high-Al material (Comparative Example 8), and a
high-Mn material (Comparative Example 9). Production process of
Comparative Examples 2 to 9 was the same as that of Examples 1 to
14.
[0103] Table 1 shows alloy composition of the respective materials.
Further, Table 2 shows primary hardness (HRC) after the aging
treatment, tensile strength (MPa) at 600.degree. C. and 700.degree.
C., hardness (HRC) after kept at 600.degree. C. and 700.degree. C.
for 400 hours, and the residual stress ratio (%). As demonstrated
in Tables 1 and 2, Examples 1 to 14 respectively satisfy
requirements for both the primary hardness of not less than 45 HRC
and the residual stress ratio at 700.degree. C. of not less than
25% at the same time, while Comparative Examples 1 to 9 cannot
satisfy both the requirements at the same time. In addition, Tables
1 and 2 shows that the high-temperature tensile strength and the
hardness after the long-time heat treatment at a high temperature
in Examples 1 to 14 are the same as or greater than those in
Comparative Examples 1 to 9.
1TABLE 1 C Si Mn P S Cu Ni Cr Mo W Co Example 1 0.02 0.26 9.76
0.019 0.002 0.11 7.3 24.8 4.7 -- -- Example 2 0.03 0.35 7.66 0.018
0.003 0.15 8.1 19.2 4.5 -- -- Example 3 0.02 0.20 5.98 0.019 0.002
0.05 10.1 23.2 1.9 -- -- Example 4 0.02 0.31 6.20 0.019 0.003 0.10
11.0 23.5 2.1 0.82 -- Example 5 0.08 0.10 5.09 0.020 0.006 2.12 7.9
22.2 2.5 -- 2.01 Example 6 0.04 0.89 5.02 0.019 0.004 0.18 12.1
22.1 3.1 0.48 1.11 Example 7 0.05 0.19 7.11 0.022 0.005 1.00 9.0
24.4 3.2 0.12 -- Example 8 0.01 0.56 3.99 0.022 0.005 0.27 11.5
21.6 2.1 -- -- Example 9 0.03 0.35 6.07 0.019 0.003 0.14 9.8 22.9
2.2 0.22 -- Example10 0.03 0.37 4.96 0.021 0.008 0.14 7.4 23.3 2.0
-- 3.62 Example11 0.02 0.14 6.03 0.019 0.007 0.11 10.2 22.5 1.5 --
0.52 Example12 0.02 0.20 5.92 0.021 0.002 0.09 8.1 22.0 1.9 -- 0.18
Example13 0.06 0.38 6.00 0.023 0.004 0.15 10.2 23.1 2.0 -- --
Example14 0.03 0.24 6.99 0.020 0.001 0.12 14.1 22.7 0.9 0.53 --
Comparative 0.06 0.43 0.62 0.020 0.001 0.03 24.9 14.3 1.1 -- --
Example 1 Comparative 0.02 0.15 5.90 0.018 0.002 0.21 9.9 23.0 0.1
-- -- Example 2 Comparative 0.04 0.15 6.02 0.028 0.008 0.17 10.0
22.1 6.0 -- -- Example 3 Comparative 0.03 0.25 8.94 0.017 0.008
0.16 10.2 13.1 1.9 -- -- Example 4 Comparative 0.02 0.19 5.99 0.018
0.008 0.20 9.9 27.9 1.9 -- -- Example 5 Comparative 0.04 0.15 5.10
0.018 0.008 0.11 5.1 22.9 1.8 -- -- Example 6 Comparative 0.02 0.21
4.97 0.021 0.003 0.12 17.9 23.3 3.2 -- -- Example 7 Comparative
0.02 0.16 5.12 0.017 0.005 0.18 10.1 23.0 2.0 -- -- Example 8
Comparative 0.02 0.22 12.11 0.017 0.005 0.17 9.7 22.9 2.1 -- --
Example 9 Al Ti Nb V N B Zr Mg Ca PN Example 1 0.005 -- -- -- 0.71
-- -- -- -- 97.1 Example 2 0.009 -- -- -- 0.44 0.003 -- -- -- 74.6
Example 3 0.003 -- -- 0.09 0.50 -- -- -- -- 79.3 Example 4 0.018 --
0.15 -- 0.52 -- 0.07 -- -- 80.4 Example 5 0.009 -- 0.06 0.08 0.45
-- -- 0.003 -- 74.0 Example 6 0.007 0.05 -- 0.14 0.43 -- -- -- --
73.4 Example 7 0.026 0.06 -- -- 0.58 -- -- 0.001 0.002 86.4 Example
8 0.013 0.35 -- -- 0.41 -- -- -- -- 68.9 Example 9 0.017 -- -- 0.71
0.58 -- -- -- -- 79.0 Example10 0.011 0.09 0.05 0.05 0.46 -- -- --
0.003 78.8 Example11 0.012 -- 0.36 -- 0.50 0.002 0.07 0.003 -- 76.9
Example12 0.009 -- -- -- 0.47 -- -- -- -- 76.8 Example13 0.003 0.04
-- 0.19 0.51 0.003 0.08 -- 0.001 79.0 Example14 0.020 0.14 0.05 --
0.47 0.002 0.05 0.002 0.001 77.0 Comparative 0.180 2.11 0.01 0.25
0.03 0.002 -- -- -- 30.3 Example 1 Comparative 0.009 -- -- -- 0.48
-- -- -- -- 77.1 Example 2 Comparative 0.008 -- -- -- 0.51 -- -- --
-- 79.4 Example 3 Comparative 0.009 -- -- -- 0.30 -- -- -- -- 56.0
Example 4 Comparative 0.009 -- -- -- 0.67 -- -- -- -- 93.5 Example
5 Comparative 0.011 -- -- -- 0.51 -- -- -- -- 79.2 Example 6
Comparative 0.009 -- -- -- 0.45 -- -- -- -- 73.5 Example 7
Comparative 0.059 -- -- -- 0.48 -- -- -- -- 76.6 Example 8
Comparative 0.008 -- -- -- 0.63 -- -- -- -- 93.5 Example 9
[0104]
2 TABLE 2 Primary High-temperature Hardness Tensile Hardness after
After Aging Strength Heat Treatment Residual Treatment 600.degree.
C. 700.degree. C. 600.degree. C./400 h 700.degree. C./400 h Stress
Ratio (HRC) (MPa) (MPa) (HRC) (HRC) (%) Example 1 53 1053 853 51 44
36 Example 2 51 987 796 50 42 32 Example 3 51 1001 802 50 42 35
Example 4 52 1027 844 51 43 31 Example 5 51 1003 801 50 42 31
Example 6 51 992 798 50 42 30 Example 7 52 1038 846 51 43 33
Example 8 51 987 797 50 42 30 Example 9 52 1021 849 51 43 31
Example10 51 1023 842 51 43 31 Example11 51 996 795 50 42 30
Example12 51 1004 803 50 42 33 Example13 51 1005 805 50 42 35
Example14 51 1009 809 51 43 32 Comparative 40 1098 877 43 34 30
Example 1 Comparative 42 816 589 42 38 18 Example 2 Comparative 40
791 593 40 33 22 Example 3 Comparative 40 803 611 38 32 12 Example
4 Comparative 41 800 608 43 38 23 Example 5 Comparative 43 837 626
40 34 17 Example 6 Comparative 52 954 663 45 34 12 Example 7
Comparative 51 931 624 43 32 10 Example 8 Comparative 51 893 614 41
32 15 Example 9 Comparative Example 1: SUH660 Comparative Example
2: Low Mo Comparative Example 3: High Mo Comparative Example 4: Low
Cr, N, PN Comparative Example 5: High Cr Comparative Example 6: Low
Ni Comparative Example 7: High Ni Comparative Example 8: High Al
Comparative Example 9: High Mn
[0105] Next, a material having the same composition as Example 3
was subjected to melting, forging, solution treatment, cold working
and aging treatment following the same procedure as above except
that only the cold working ratio at the time of the cold working
after the solution treatment was changed. A test piece was taken
from the obtained material, and residual stress ratios (%) thereof
under the above-described conditions were obtained. Table 3 shows
the result. As demonstrated in Table 3, in cases where the cold
working ratios are below 40%, and over 70%, the residual stress
ratios decline.
3 TABLE 3 Cold Working Residual Ratio Stress Ratio (%) (%) 30 18 40
31 50 34 60 35 70 32 80 12
[0106] Next, a material having the same composition as Example 3
was subjected to melting, forging, solution treatment, cold working
and aging treatment following the same procedure as above except
that only the condition of the aging treatment was changed. A test
piece was taken from the obtained material, and the residual stress
ratios (%) thereof under the above-described conditions were
obtained. Table 4 shows the result. As demonstrated in Table 4, in
cases where the temperatures of the aging treatment are below
400.degree. C., and over 650.degree. C., the residual stress ratios
decline.
4TABLE 4 Aging Heat Treatment Aging Heat Residual Temperature
Treatment Time Stress Ratio (.degree. C.) (h) (%) 300 1.0 18 500
0.1 34 500 1.0 35 500 4.0 35 700 1.0 24
[0107] The heat-resistant austenitic stainless steel consistent
with the present invention may be applied extensively to a
heat-resistant member for which a low cost, and the
high-temperature strength and the sag-resistance are required.
Examples of specific applications include: a heat-resistant spring
used in exhaust systems of an automobile engine and an aeroengine,
an industrial manufacturing facilities and the like; a
high-temperature bolt and the like which are typically used in the
automobile engine, the aeroengine, a generator turbine and the
like; a turbo casing; a boiler part; a part for an industrial
furnace, and the like.
[0108] Further, examples of more specific applications include: a
nozzle, a vane, a blade, a disk, a casing and a bolt of a gas
turbine, a combustor liner, a compressor disk and the like for
aviation and generator; intake and exhaust valves for automobile
engine, a rotor, a housing, a nozzle and a vane of a turbocharger,
an exhaust manifold, a front pipe, a muffler, an exhaust valve
spring, an exhaust bolt and the like for an automobile; a boiler, a
rotor, a casing, a blade, a bolt and the like for a steam turbine;
petrochemical industrial parts such as a heat exchanger, a pressure
vessel, an ethylene decomposition tube and a valve; parts for a
heat treating furnace such as a fitting, a fixture, a jig for heat
treatment, a forging mold or die, a hot reduction roll, a
continuous cast roll, a heater sheath and a radiant tube; parts for
a garbage incinerator such as a heat exchanger tube; parts for a
burner such as a nozzle and a casing; a valve for a ship diesel
engine, and the like.
[0109] The foregoing description of the preferred embodiments of
the invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and modifications and
variations are possible in the light of the above teachings or may
be acquired from practice of the invention. The embodiments chosen
and described in order to explain the principles of the invention
and its practical application to enable one skilled in the art to
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto, and their equivalents.
* * * * *