U.S. patent application number 11/343516 was filed with the patent office on 2006-11-02 for austenite-type stainless steel hot-rolling steel material with excellent corrosion resistance, proof-stress, and low-temperature toughness and production method thereof.
Invention is credited to Shigeo Fukumoto, Hiroshige Inoue, Ryo Matsuhashi, Yuusuke Oikawa, Kazuhiro Suetsugu, Shinji Tsuge.
Application Number | 20060243356 11/343516 |
Document ID | / |
Family ID | 37233273 |
Filed Date | 2006-11-02 |
United States Patent
Application |
20060243356 |
Kind Code |
A1 |
Oikawa; Yuusuke ; et
al. |
November 2, 2006 |
Austenite-type stainless steel hot-rolling steel material with
excellent corrosion resistance, proof-stress, and low-temperature
toughness and production method thereof
Abstract
An austenitic stainless steel hot-rolled steel material can be
provided which has sea-water resistance and strength superior to
conventional steel. Low-temperature toughness can be maintained,
which is preferable in a structural member of speedy craft. The
steel material can include an austenitic stainless steel hot-rolled
steel material which excels in the properties of corrosion
resistance, proof stress, and low-temperature toughness. In such
austenitic stainless steel hot-rolling steel material, e.g.,
PI[=Cr+3.3(Mo+0.5W)+16N] ranges from 35 to 40, .delta. cal [=2.9
(Cr+0.3Si+Mo+0.5W)-2.6 (Ni+0.3Mn+0.25Cu+35C+20N)-18] ranges from -6
to +2, and a 0.2% proof stress at room temperature is not less than
550 MPa, Charpy impact value measured using a V-notch test piece at
-40.degree. C. is not less than 100 J/cm2, and the pitting
potential measured in a deaerated aqueous solution of 10% NaCl at
50.degree. C. (Vc'100) is not less than 500 mV (as it relates to
saturated Ag/AgCl).
Inventors: |
Oikawa; Yuusuke;
(Hikari-shi, JP) ; Tsuge; Shinji;
(Nishinomiya-shi, JP) ; Fukumoto; Shigeo;
(Shunan-shi, JP) ; Suetsugu; Kazuhiro;
(Kitakyushu-shi, JP) ; Matsuhashi; Ryo;
(Yokosuka-shi, JP) ; Inoue; Hiroshige;
(Kimitsu-shi, JP) |
Correspondence
Address: |
DORSEY & WHITNEY LLP;INTELLECTUAL PROPERTY DEPARTMENT
250 PARK AVENUE
NEW YORK
NY
10177
US
|
Family ID: |
37233273 |
Appl. No.: |
11/343516 |
Filed: |
January 30, 2006 |
Current U.S.
Class: |
148/609 ;
420/52 |
Current CPC
Class: |
C22C 38/42 20130101;
C22C 38/002 20130101; C22C 38/001 20130101; C22C 38/02 20130101;
C22C 38/04 20130101; C22C 38/004 20130101; C22C 38/44 20130101 |
Class at
Publication: |
148/609 ;
420/052 |
International
Class: |
C22C 38/44 20060101
C22C038/44; C21D 8/00 20060101 C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 2, 2005 |
JP |
P 2005-026176 |
Feb 2, 2005 |
JP |
P 2005-026177 |
Jan 20, 2006 |
JP |
P2006-012569 |
Claims
1-10. (canceled)
11. An austenitic stainless hot-rolled steel material having a
superior corrosion resistance, a proof stress, and a
low-temperature toughness, comprising, C: about 0.001 to 0.03 mass
%, Si: about 0.1 to 1.5 mass %, Mn: about 0.1 to 3.0 mass %, P:
about 0.005 to 0.05 mass %, S: about 0.0001 to 0.003 mass %, Ni:
about 15.0 to 21.0 mass %; Cr: about 22.0 to 28.0 mass %, Mo: about
1.5 to 3.5 mass %, N: about 0.15 to 0.35 mass %, and O: about
0.0005 to 0.007 mass %, wherein: a Pi value expressed by the
following formula ranges from about 35 to 40:
PI=Cr+3.3(Mo+0.5W)+16N, and a .delta. cal value expressed by the
following formula ranges from about -6 to +2:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18 a
remnant of the steel material comprising Fe and inevitable
impurities, a content of intermetallic compounds contained in the
steel material is at most about 0.5 mass %, about 0.2% proof stress
at a room temperature is at least about 550MPa, a Charpy impact
value measured using a V-notch test piece at about -40.degree. C.
is at least about 100 J/cm.sup.2, and a pitting potential measured
in a deaerated aqueous solution of about 10% NaCl at about
50.degree. C. (Vc'100) is at least about 500 mV as compared to a
saturated solution of Ag/AgCl.
12. The steel material according to claim 11, further comprising at
least one of: W: about 0.3 to 3.0 mass %, or Al: about 0.005 to 0.1
mass %.
13. The steel material according to claim 11, further comprising at
least one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1
mass %, Cu: about 0.3 to 2.0 mass %, or Sn: at most about 0.1 mass
%.
14. The steel material according to claim 11, further comprising at
least one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1
mass %, Cu: about 0.3 to 2.0 mass %, Sn: about at most about 0.1
mass %, Ca: about 0.0005 to 0.0050 mass %, Mg: about 0.0005 to
0.0050 mass %, or REM: about 0.005 to 0.10 mass %.
15. The steel material according to claim 11, further comprising at
least one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1
mass %, Cu: about 0.3 to 2.0 mass %, Sn: about at most about 0.1
mass %, Ca: about 0.0005 to 0.0050 mass %, Mg: about 0.0005 to
0.0050 mass %, REM: about 0.005 to 0.10 mass %, or B: about 0.0003
to 0.0060 mass %.
16. The steel material according to claim 11, further comprising at
least one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1
mass %, Cu: about 0.3 to 2.0 mass %, Sn: at most about 0.1 mass %,
Ca: about 0.0005 to 0.0050 mass %, Mg: about 0.0005 to 0.0050 mass
%, REM: about 0.005 to 0.10 mass %, B: about 0.0003 to 0.0060 mass
%, Ti: about 0.003 to 0.03 mass %, Nb: about 0.02 to 0.20 mass %,
Zr: about 0.003 to 0.03 mass %, V: about 0.05 to 0.5 mass %, or Ta:
about 0.01 to 0.1 mass %.
17. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance, a proof
stress, and a low-temperature toughness, wherein a content of
intermetallic compounds contained in the steel material is at most
about 0.5 mass %, about 0.2% proof stress at a room temperature is
at least about 550 MPa, a Charpy impact value measured using a
V-notch test piece at about -40.degree. C. is at least about 100
J/cm.sup.2, and a pitting potential measured in a deaerated aqueous
solution of about 10% NaCl at about 50.degree. C. (Vc'100) is at
least about 500 mV as compared to a saturated solution of Ag/AgCl,
the process comprising: performing, at a temperature of about 1200
to 1300.degree. C. for at least about one hour, a homogenizing-heat
treatment on a particular material which is at least one of a cast
steel or a semi-finished product of the steel material which
comprises: C: about 0.001 to 0.03 mass %, Si: about 0.1 to 1.5 mass
%, Mn: about 0.1 to 3.0 mass %, P: about 0.005 to 0.05 mass %, S:
about 0.0001 to 0.003 mass %, Ni: about 15.0 to 21.0 mass %; Cr:
about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %, N: about
0.15 to 0.35 mass %, and O: about 0.0005 to 0.007 mass %, wherein a
PI value expressed by the following formula ranges from about 35 to
40: PI=Cr+3.3(Mo+0.5W)+16N, and a .delta. cal value expressed by
the following formula ranges from about -6 to +2:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18 a
remnant of the steel material comprising Fe and inevitable
impurities, reheating the particular treated material at a
temperature of about 1100 to 1300.degree. C.; while rolling the
particular reheated material, maintaining a temperature of at least
about 850.degree. C., and by a first draft of at least about 50% at
a temperature of at least about 1050.degree. C. and a second draft
of at least about 10% at a temperature of about 1050 to 850.degree.
C.; and cooling the rolled particular material at an average
cooling rate of about 800 to 500.degree. C. after the rolling
procedure is performed for at least about 150.degree. C./min,
without a solution treatment.
18. An austenitic stainless hot-rolled steel material having a
superior corrosion resistance and a low-temperature toughness,
comprising, C: at most about 0.03 mass %, Si: about 0.1 to 1.5 mass
%, Mn: about 0.1 to 3.0 mass %, P: at most about 0.05 mass %, S: at
most about 0.003 mass %, Ni: about 15.0 to 21.0 mass %, Cr: about
22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %, N: about 0.15 to
0.35 mass %, Al: about 0.005 to 0.1 mass %, and O: at most about
0.007 mass %, wherein a PI value expressed by the following formula
ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N, and a .delta.
cal value expressed by the following formula ranges from about -6
to +4:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18 a
remnant of the steel material comprising of Fe and substantially
inevitable impurities, and a content of intermetallic compounds
contained in the steel material is at most about 0.5 mass %, and a
value by each element represents the content of the element
expressed in terms of mass %.
19. The steel material according to claim 18, further comprising at
least one of: Cu: about 0.1 to 2.0 mass %, Ti: about 0.003 to 0.03
mass %, Nb: about 0.02 to 0.20 mass %, V: about 0.05 to 0.5 mass %,
W: about 0.3 to 3.0 mass %, B: about 0.0003 to 0.0060 mass %, Ca:
about 0.0005 to 0.0050 mass %, Mg: about 0.0005 to 0.0050 mass %,
or REM: about 0.005 to 0.10.
20. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance and a
low-temperature toughness, wherein a content of intermetallic
compounds contained in the steel material is at most about 0.5 mass
%, the process comprising: performing, at a temperature of about
1200 to 1300.degree. C. for at least about one hour, a
homogenizing-heat treatment on a particular material which is at
least one of a cast steel or a semi-finished product of the steel
material which comprises: C: at most about 0.03 mass %, Si: about
0.1 to 1.5 mass %, Mn: about 0.1 to 3.0 mass %, P: at most about
0.05 mass %, S: at most about 0.003 mass %, Ni: about 15.0 to 21.0
mass %, Cr: about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %,
N: about 0.15 to 0.35 mass %, Al: about 0.005 to 0.1 mass %, and O:
at most about 0.007 mass %, wherein a PI value expressed by the
following formula ranges from about 35 to 40:
PI=Cr+3.3(Mo+0.5W)+16N, and a .delta. cal value expressed by the
following formula ranges from about -6 to +4:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18 a
remnant of the steel material comprising of Fe and substantially
inevitable impurities, a value by each element represents the
content of the element expressed in terms of mass %, and the heat
treatment is performed so as to reduce the content of the
intermetallic compound in the steel material.
21. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance, a proof
stress, and a low-temperature toughness, wherein a content of
intermetallic compounds contained in the steel material is at most
about 0.5 mass %, about 0.2% proof stress at a room temperature is
at least about 550 MPa, a Charpy impact value measured using a
V-notch test piece at about -40.degree. C. is at least about 100
J/cm.sup.2, and a pitting potential measured in a deaerated aqueous
solution of about 10% NaCl at about 50.degree. C. (Vc'100) is at
least about 500 mV as compared to a saturated solution of Ag/AgCl,
the process comprising: performing, at a temperature of about 1200
to 1300.degree. C. for at least about one hour, a homogenizing-heat
treatment on a particular material which is at least one of a cast
steel or a semi-finished product of the steel material which
comprises: C: about 0.001 to 0.03 mass %, Si: about 0.1 to 1.5 mass
%, Mn: about 0.1 to 3.0 mass %, P: about 0.005 to 0.05 mass %, S:
about 0.0001 to 0.003 mass %, Ni: about 15.0 to 21.0 mass %; Cr:
about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %, N: about
0.15 to 0.35 mass %, O: about 0.0005 to 0.007 mass %, and at least
one of: W: about 0.3 to 3.0 mass %, or Al: about 0.005 to 0.1 mass
%. wherein a PI value expressed by the following formula ranges
from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N, a .delta. cal value
expressed by the following formula ranges from about -6 to +2:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18,
and a remnant of the steel material comprising Fe and inevitable
impurities; reheating the particular treated material at a
temperature of about 1100 to 1300.degree. C.; while rolling the
particular reheated material at a temperature of at least about
850.degree. C., and by a first draft of at least about 1050% at a
temperature of at least about 1 050.degree. C. and a second draft
of at least about 10% at a temperature of about 1050 to 850.degree.
C.; and cooling the rolled particular material at an average
cooling rate of about 800 to 500.degree. C. after the rolling
procedure is performed for at least about 150.degree. C./min,
without a solution treatment.
22. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance, a proof
stress, and a low-temperature toughness, wherein a content of
intermetallic compounds contained in the steel material is at most
about 0.5 mass %, about 0.2% proof stress at a room temperature is
at least about 550 MPa, a Charpy impact value measured using a
V-notch test piece at about -40.degree. C. is at least about 100
J/cm.sup.2, and a pitting potential measured in a deaerated aqueous
solution of about 10% NaCl at about 50.degree. C. (Vc'100) is at
least about 500 mV as compared to a saturated solution of Ag/AgCl,
the process comprising: performing, at a temperature of about 1200
to 1300.degree. C. for at least about one hour, a homogenizing-heat
treatment on a particular material which is at least one of a cast
steel or a semi-finished product of the steel material which
comprises: C: about 0.001 to 0.03 mass %, Si: about 0.1 to 1.5 mass
%, Mn: about 0.1 to 3.0 mass %, P: about 0.005 to 0.05 mass %, S:
about 0.0001 to 0.003 mass %, Ni: about 15.0 to 21.0 mass %; Cr:
about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %, N: about
0.15 to 0.35 mass %, O: about 0.0005 to 0.007 mass %, and at least
one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1 mass %,
Cu: about 0.3 to 2.0 mass %, or Sn: at most about 0.1 mass %,
wherein a PI value expressed by the following formula ranges from
about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N, a .delta. cal value
expressed by the following formula ranges from about -6 to +2:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18,
and a remnant of the steel material comprising Fe and inevitable
impurities, reheating the particular treated material at a
temperature of about 1100 to 1300.degree. C.; while rolling the
particular reheated material maintaining a temperature of at least
about 850.degree. C., and by a first draft of at least about 50% at
a temperature of at least about 1050.degree. C. and a second draft
of at least about 10% at a temperature of about 1050 to 850.degree.
C.; and cooling the rolled particular material at an average
cooling rate of about 800 to 500.degree. C. after the rolling
procedure is performed for at least about 150.degree. C./min,
without a solution treatment.
23. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance, a proof
stress, and a low-temperature toughness, wherein a content of
intermetallic compounds contained in the steel material is at most
about 0.5 mass %, about 0.2% proof stress at a room temperature is
at least about 550 MPa, a Charpy impact value measured using a
V-notch test piece at about -40.degree. C. is at least about 100
J/cm.sup.2, and a pitting potential measured in a deaerated aqueous
solution of about 10% NaCl at about 50.degree. C. (Vc'100) is at
least about 500 mV as compared to a saturated solution of Ag/AgCl,
the process comprising: performing, at a temperature of about 1200
to 1300.degree. C. for at least about one hour, a homogenizing-heat
treatment on a particular material which is at least one of a cast
steel or a semi-finished product of the steel material which
comprises: C: about 0.001 to 0.03 mass %, Si: about 0.1 to 1.5 mass
%, Mn: about 0.1 to 3.0 mass %, P: about 0.005 to 0.05 mass %, S:
about 0.0001 to 0.003 mass %, Ni: about 15.0 to 21.0 mass %; Cr:
about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %, N: about
0.15 to 0.35 mass %, O: about 0.0005 to 0.007 mass %, and at least
one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1 mass %,
Cu: about 0.3 to 2.0 mass %, Sn: about at most about 0.1 mass %,
Ca: about 0.0005 to 0.0050 mass %, Mg: about 0.0005 to 0.0050 mass
%, or REM: about 0.005 to 0.10 mass %, wherein a PI value expressed
by the following formula ranges from about 35 to 40:
PI=Cr+3.3(Mo+0.5W)+16N, a .delta. cal value expressed by the
following formula ranges from about -6 to +2:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6 (Ni+0.3Mn+0.25Cu+35C+20N)-18,
and a remnant of the steel material comprising Fe and inevitable
impurities, reheating the particular treated material at a
temperature of about 1100 to 1300.degree. C.; while rolling the
particular reheated material maintaining a temperature of at least
about 850.degree. C., and by a first draft of at least about 50% at
a temperature of at least about 1050.degree. C. and a second draft
of at least about 10% at a temperature of about 1050 to 850.degree.
C.; and cooling the rolled particular material at an average
cooling rate of about 800 to 500.degree. C. after the rolling
procedure is performed for at least about 150.degree. C./min,
without a solution treatment.
24. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance, a proof
stress, and a low-temperature toughness, wherein a content of
intermetallic compounds contained in the steel material is at most
about 0.5 mass %, about 0.2% proof stress at a room temperature is
at least about 550 MPa, a Charpy impact value measured using a
V-notch test piece at about -40.degree. C. is at least about 100
J/cm.sup.2, and a pitting potential measured in a deaerated aqueous
solution of about 10% NaCl at about 50.degree. C. (Vc'100) is at
least about 500 mV as compared to a saturated solution of Ag/AgCl,
the process comprising: performing, at a temperature of about 1200
to 1300.degree. C. for at least about one hour, a homogenizing-heat
treatment on a particular material which is at least one of a cast
steel or a semi-finished product of the steel material which
comprises: C: about 0.001 to 0.03 mass %, Si: about 0.1 to 1.5 mass
%, Mn: about 0.1 to 3.0 mass %, P: about 0.005 to 0.05 mass %, S:
about 0.0001 to 0.003 mass %, Ni: about 15.0 to 21.0 mass %; Cr:
about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %, N: about
0.15 to 0.35 mass %, p2 O: about 0.0005 to 0.007 mass %, and at
least one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1
mass %, Cu: about 0.3 to 2.0 mass %, Sn: at most about 0.1 mass %,
Ca: about 0.0005 to 0.0050 mass %, Mg: about 0.0005 to 0.0050 mass
%, REM: about 0.005 to 0.10 mass %, or B: about 0.0003 to 0.0060
mass %, wherein a PI value expressed by the following formula
ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N, a .delta. cal
value expressed by the following formula ranges from about -6 to
+2:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18,
and a remnant of the steel material comprising Fe and inevitable
impurities, reheating the particular treated material at a
temperature of about 1100 to 1300.degree. C.; while rolling the
particular reheated material maintaining a temperature of at least
about 850.degree. C., and by a first draft of at least about 50% at
a temperature of at least about 1050.degree. C. and a second draft
of at least about 10% at a temperature of about 1050 to 850.degree.
C.; and cooling the rolled particular material at an average
cooling rate of about 800 to 500.degree. C. after the rolling
procedure is performed for at least about 150.degree. C./min,
without a solution treatment.
25. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance, a proof
stress, and a low-temperature toughness, wherein a content of
intermetallic compounds contained in the steel material is at most
about 0.5 mass %, about 0.2% proof stress at a room temperature is
at least about 550 MPa, a Charpy impact value measured using a
V-notch test piece at about -40.degree. C. is at least about 100
J/cm.sup.2, and a pitting potential measured in a deaerated aqueous
solution of about 10% NaCl at about 50.degree. C. (Vc'100) is at
least about 500 mV as compared to a saturated solution of Ag/AgCl,
the process comprising: performing, at a temperature of about 1200
to 1300.degree. C. for at least about one hour, a homogenizing-heat
treatment on a particular material which is at least one of a cast
steel or a semi-finished product of the steel material which
comprises: C: about 0.001 to 0.03 mass %, Si: about 0.1 to 1.5 mass
%, Mn: about 0.1 to 3.0 mass %, P: about 0.005 to 0.05 mass %, S:
about 0.0001 to 0.003 mass %, Ni: about 15.0 to 21.0 mass %; Cr:
about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %, N: about
0.15 to 0.35 mass %, O: about 0.0005 to 0.007 mass %, and at least
one of: W: about 0.3 to 3.0 mass %, Al: about 0.005 to 0.1 mass %,
Cu: about 0.3 to 2.0 mass %, Sn: at most about 0.1 mass %, Ca:
about 0.0005 to 0.0050 mass %, Mg: about 0.0005 to 0.0050 mass %,
REM: about 0.005 to 0.10 mass %, B: about 0.0003 to 0.0060 mass %,
Ti: about 0.003 to 0.03 mass %, Nb: about 0.02 to 0.20 mass %, Zr:
about 0.003 to 0.03 mass %, V: about 0.05 to 0.5 mass %, or Ta:
about 0.01 to 0.1 mass %, wherein a PI value expressed by the
following formula ranges from about 35 to 40:
PI=Cr+3.3(Mo+0.5W)+16N, a .delta. cal value expressed by the
following formula ranges from about -6 to +2:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+35C+20N)-18,
and a remnant of the steel material comprising Fe and inevitable
impurities, reheating the particular treated material at a
temperature of about 1100 to 1300.degree. C.; while rolling the
particular reheated material maintaining a temperature of at least
about 850.degree. C., and by a first draft of at least about 50% at
a temperature of at least about 1050.degree. C. and a second draft
of at least about 10% at a temperature of about 1050 to 850.degree.
C.; and cooling the rolled particular material at an average
cooling rate of about 800 to 500.degree. C. after the rolling
procedure is performed for at least about 150.degree. C./min,
without a solution treatment.
26. A process for producing an austenitic stainless hot-rolled
steel material having a superior corrosion resistance and a
low-temperature toughness, wherein a content of intermetallic
compounds contained in the steel material is at most about 0.5 mass
%, the process comprising: performing, at a temperature of about
1200 to 1300.degree. C. for at least about one hour, a
homogenizing-heat treatment on a particular material which is at
least one of a cast steel or a semi-finished product of the steel
material which comprises: C: at most about 0.03 mass %, Si: about
0.1 to 1.5 mass %, Mn: about 0.1 to 3.0 mass %, P: at most about
0.05 mass %, S: at most about 0.003 mass %, Ni: about 15.0 to 21.0
mass %, Cr: about 22.0 to 28.0 mass %, Mo: about 1.5 to 3.5 mass %,
N: about 0.15 to 0.35 mass %, Al: about 0.005 to 0.1 mass %, and O:
at most about 0.007 mass %, and at least one of: Cu: about 0.1 to
2.0 mass %, Ti: about 0.003 to 0.03 mass %, Nb: about 0.02 to 0.20
mass %, V: about 0.05 to 0.5 mass %, W: about 0.3 to 3.0 mass %, B:
about 0.0003 to 0.0060 mass %, Ca: about 0.0005 to 0.0050 mass %,
Mg: about 0.0005 to 0.0050 mass %, or REM: about 0.005 to 0.10,
wherein a PI value expressed by the following formula ranges from
about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N, and a .delta. cal value
expressed by the following formula ranges from about 6 to +4:
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni +0.3Mn+0.25Cu+35C+20N)-18 a
remnant of the steel material comprising of Fe and substantially
inevitable impurities, a value by each element represents the
content of the element expressed in terms of mass %, and the heat
treatment is performed so as to reduce the content of the
intermetallic compound in the steel material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a structural steel material
which excels in corrosion resistance and is used in a marine
(chloride) environment, for example; an austenite-type stainless
steel hot-rolling steel material, as a hull-structural material
which excels in strength as well as seawater resistance, and
low-temperature toughness, upon being used as a material for an
outer shell, a bulkhead, an frame, a hydrofoil, etc.; and a
production method thereof.
[0003] Priority is claimed on Japanese Patent Application No.
2005-26177, filed Feb. 2, 2005, Japanese Patent Application No.
2005-26176, filed Feb. 2, 2005, and Japanese Patent Application No.
2006-012569, filed Jan. 20, 2006, the contents of which are
incorporated herein by reference.
[0004] 2. Background Art
[0005] Hitherto, coated steel sheets to which a heavy corrosive
protection was applied were used for hull structures. The demand
for speedy craft equipped with hydrofoils etc. has increased, and
since high-speed sea water flow comes into contact with the
hydrofoils, etc., this use requires a material which excels in sea
water resistance without requiring being coated. In order to reduce
hull weight further, a material having a high strength is
required.
[0006] Although austenitic stainless steel is promising as a
material which excels in sea water resistance, in a common
production method, austenitic stainless steel is subjected to a
solution annealing treatment after hot-rolling, thereby softening
the resultant austenitic stainless steel so that the proof stress
of the austenitic stainless steel is at most 400 MPa.
[0007] The strength can be increased by performing a hot-rolling
processing under a specific temperature condition while omitting
the solution annealing treatment, which has been disclosed in many
literatures in the past (Patent document 1 (Japanese Unexamined
Patent Application, First Publication No. S. 60-208459), Patent
document 2 (Japanese Unexamined Patent Application, First
Publication No. H. 2-97649), and Patent document 3 (Japanese
Unexamined Patent Application, First Publication No. H.
4-6214)).
[0008] Among these, although Patent document 2 discloses a
production method of an austenitic stainless steel having a high
proof stress while maintaining a low-temperature toughness, the sea
water resistance is not taken into consideration in this austenitic
stainless steel while maintaining low-temperature toughness.
Although Patent document 3 discloses a production method of an
austenitic stainless steel which has a high proof stress of not
less than 500 MPa and excellent sea-water resistance, which
includes performing a heat treatment on steel which contains 0.3%
or more of N and 0.5 to 3.0% of Mo under a specific condition,
Patent document 3 fails to make any disclosure regarding
toughness.
[0009] Moreover, Patent document 4 (patent No. 2783895 official
report) and Patent document 5 (patent No. 2783896 official report)
disclose a production technique of an austenitic stainless steel
with little softening of a weld part by adding a Nb-type
element.
[0010] Cr, Mo, and N are known as elements which increase sea water
resistance, and the corrosion resistance ranking in steel is
determined by the formula: PI=Cr+3.3(Mo+0.5W)+16N as a pitting
index. When the PI value of the component shown in examples of
Patent document 3 is calculated, it is approximately 32 in the
minimum case, but as a stainless steel which gives a higher PI
value (not less than 35), SUS836L and 890L, which contain 23% or
more of Ni, are known as austenitic types, whereas SUS329J4L, which
contains 5.5 to 7.5% of Ni, is known as a two-phase type.
[0011] Since two-phase-type SUS329J4L contains a ferrite phase,
SUS329J4L has high proof stress. A two-phase stainless steel known
as a super two-phase, in which Mo and W contents are increased has
also been developed in recent years, and application thereof as a
material with high hardness and high corrosion resistance has
started. On the other hand, a high strength steel material of an
austenitic-type high corrosion resistance stainless steel having a
PI value over 35 has not yet been put in practical use.
[0012] Stainless steel is more susceptible to crevice corrosion
when it is shaped into a crevice form than when it is not shaped
i.e. flat. Therefore, in order to produce steel suitable for broad
use in hull structures and which is low-maintenance, it is required
to develop a highly corrosion-resistant steel material which is
higher than the steel material disclosed in Patent document 3.
[0013] On the other hand, the demand for a stainless steel material
for ocean-going craft which is reliable when stranded or after a
collision between shipping is increasing. Characteristics of bothe
base material and weldability are required for reliability.
Regarding the reliability of the mother base, high toughness is
required in preparation for a collision. Among Cr, Mo and N, which
increase corrosion resistance, as for Mo and Cr, it is not
sufficient to simply add, because processability in hot-rolling
will decrease remarkably due to the influence of delta ferrite
contained in cast steel or semi-finished products; in addition, in
the case of a high Cr and Mo steel, in general, the toughness of
the steel deteriorates remarkably due to the influence of an
intermetallic compound known as a a phase, and hence it is
necessary to add a large amount of Ni in order to suppress the
influence of both. However, considering the rising prices of raw
materials of Ni and Mo these days, development of a low-cost,
highly corrosion-resistant stainless steel is especially desired .
It should be noted that, two-phase steel cannot be adopted because
of its low-temperature toughness.
[0014] On the other hand, as for adding N as is disclosed in Patent
document 3, it is indeed effective for maintaining the strength,
however, excessive N causes the generation of bubbles at a welded
part, thereby it may decrease the bonding strength and reliability
of the welded part, to the contrary.
[0015] Thus, it is an object of the present invention to provide an
austenitic stainless steel hot-rolled steel material which has
sea-water resistance and strength superior to the conventional
steel, while maintaining low-temperature toughness, which is
required in a structural member of a high-speed ship, that is, to
provide an austenitic stainless steel hot-rolled steel material
which excels in the properties of corrosion resistance, proof
stress, and low-temperature toughness.
[0016] The inventors of the present invention have investigated the
strength, the toughness, and the corrosion resistance of a
sheethot-rolled plate obtained by casting, heat-rolling processing,
and heat treatment of an austenitic component system in which the N
amount is not more than 0.35% in view of weldability and the PI
value is not less than 35, in view of weldability. In particular,
the inventors of the present invention have found that the
toughness cannot be determined by only the Ni content, but is
determined by the content of intermetallic compounds, which are
contained in a steel material, having high Cr and Mo contents.
Formation of a metallographic structure as such starts from the
solidification of steel, in addition, the formation may be
generated at any steps in hot-rolling processing. So, they first
started to investigate the influence of a chemical composition on a
solidified structure, and then investigated the influence of
conditions on rough rolling of cast steel, homogenizing heat
treatment, hot working, and heat treatment. As a result, they
restricted the content of component elements the solidification
structure and the metallographic structure of a steel material to
obtain an austenitic stainless steel which can solve the problems
of the conventional technique and excels in corrosion resistance,
toughness, strength, and hot processability, the solidification
structure, the metallographic structure of a steel material,
thereby completing the austenitic stainless steel of the present
invention and the production method thereof.
SUMMARY OF THE INVENTION
[0017] A first aspect of the present invention is as follows.
[0018] That is, an austenitic stainless hot-rolled steel material
having excellent corrosion resistance, proof stress, and
low-temperature toughness, including: 0.001 to 0.03 mass % of C,
0.1 to 1.5 mass % of Si, 0.1 to 3.0 mass % of Mn, 0.005 to 0.05
mass % of P, 0.0001 to 0.003 mass % of S, 15.0 to 21.0 mass % of
Ni, 22.0 to 28.0 mass % of Cr, 1.5 to 3.5 mass % of Mo, 0.15 to
0.35 mass % of N and 0.0005 to 0.007 mass % of o, in which the PI
value expressed by the following formula (1) ranges from 35 to 40,
.delta. cal value expressed by the following formula (2) ranges
from -6 to +2, the remnant consists of Fe and inevitable
impurities, the content of intermetallic compounds contained in the
steel material is not more than 0.5 mass %, a 0.2% proof stress at
room temperature is not less than 550 MPa, the Charpy impact value
measured using a V-notch test piece at -40.degree. C. is not less
than 100 J/cm.sup.2, and the pitting potential measured in a
deaerated aqueous solution of 10% NaCl at 50.degree. C. (Vc'100) is
not less than 500 mV(vs saturated Ag/AgCl). PI=Cr+3.3(Mo+0.5W)+16N
(1)
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+32C+20N)-18
(2)
[0019] In addition to this, as second to sixth aspect of the
present invention, the following metallic elements can be
contained:
[0020] 1) (The Second Aspect of the Present Invention)
[0021] One or two selected from the group consisting of 0.3 to 3.0
mass % of W and 0.005 to 0.1 mass % of Al.
[0022] 2) (The Third Aspect of the Present Invention)
[0023] One or more selected from the group consisting of 0.3 to 3.0
mass % of W, 0.005 to 0.1 mass % of Al, 0.3 to 2.0 mass % of Cu,
and not more than 0.1 mass % of Sn.
[0024] 3) (The Fourth Aspect of the Present Invention)
[0025] One or more selected from the group consisting of 0.3 to 3.0
mass % of W, 0.005 to 0.1 mass % of Al, 0.0005 to 0.0050 mass % of
Ca, 0.0005 to 0.0050 mass % of Mg, and 0.005 to 0.10 mass % of
REM.
[0026] 4) (The Fifth Aspect of the Present Invention)
[0027] 0.3 to 3.0 mass % of W, 0.005 to 0.1 mass % of Al, 0.0005 to
0.0050 mass % of Ca, 0.0005 to 0.0050 mass % of Mg, 0.005 to 0.10
mass % of REM, and 0.0003 to 0.0060 mass % of B.
[0028] 5) (The Sixth Aspect of the Present Invention)
[0029] One or more selected from the group consisting of 0.3 to 3.0
mass % of W, 0.005 to 0.1 mass % of Al, 0.3 to 2.0 mass % of Cu,
not more than 0.1 mass % of Sn, 0.0005 to 0.0050 mass % of Ca,
0.0005 to 0.0050 mass % of Mg, 0.005 to 0.10 mass % of REM, 0.0003
to 0.0060 mass % of B, 0.003 to 0.03 mass % of Ti, 0.02 to 0.20
mass % of Nb, 0.003 to 0.03 mass % of Zr, 0.05 to 0.5 mass % of V,
and 0.01 to 0.1 mass % of Ta.
[0030] A seventh aspect of the present invention is a process for
producing an austenitic stainless hot-rolled steel material having
excellent corrosion resistance, proof stress, and low-temperature
toughness, including: performing homogenizing-heat treatment on a
cast steel or a semi-finished product of the austenitic stainless
as set forth in aspects 1 to 6 at a temperature of 1200 to
1300.degree. C. for 1 hour or more, reheating it at a temperature
of 1100 to 1300.degree. C., rolling it by a draft of not less than
50% at a temperature of not lower than 1050.degree. C. and a draft
of not less than 10% at a temperature of 1050 to 850.degree. C.,
while maintaining a temperature of not lower than 850.degree. C. in
the rolling step, allowing an average cooling rate at 800 to
500.degree. C. after the rolling to be not less than 150.degree.
C./min, and performing no solution treatment.
[0031] The present invention can provide austenitic stainless steel
having excellent sea water resistance, proof stress, and
low-temperature toughness, by restricting the component and
performing a specific heat treatment processing.
[0032] The present invention realizes an austenitic stainless steel
suitable for hull structures having a high level of sea water
resistance and proof stress and low-temperature toughness, which
are required as components for structures of high-speed ships, and
contributes to industry significantly.
[0033] Moreover, an eighth aspect of the present invention is as
follows.
[0034] An austenitic stainless hot-rolled steel material having
excellent corrosion resistance, and low-temperature toughness,
including: not more than 0.03 mass % of C, 0.1 to 1.5 mass % of Si,
0.1 to 3.0 mass % of Mn, not more than 0.05 mass % of P, not more
than 0.003 mass % of S, 15.0 to 21.0 mass % of Ni, 22.0 to 28.0
mass % of Cr, 1.5 to 3.5 mass % of Mo, 0.15 to 0.35 mass % of N,
0.005 to 0.1 mass % of Al, and not more than 0.007 mass % of O, in
which the PI value expressed by the following formula (1) ranges
from 35 to 40, .delta. cal value expressed by the following formula
(2) ranges from -6 to +4, the remnant consists of Fe and
substantially inevitable impurities, and the content of
intermetallic compounds contained in the steel material is not more
than 0.5 mass %, PI=Cr+3.3(Mo+0.5W)+16N (1)
.delta.cal=2.9(Cr+0.3Si+Mo+0.5W)-2.6(Ni+0.3Mn+0.25Cu+32C+20N)-18
(2) in which the value by each element represents the content of
the element expressed in terms of mass %.
[0035] A ninth aspect of the present invention is the austenitic
stainless hot-rolled steel material having excellent corrosion
resistance, and low-temperature toughness, as set forth in the
eighth aspect of the present invention, further including one or
more selected from the group consisting of 0.1 to 2.0 mass % of Cu,
0.003 to 0.03 mass % of Ti, 0.02 to 0.20 mass % of Nb, 0.05 to 0.5
mass % of V, 0.3 to 3.0 mass % of W, 0.0003 to 0.0060 mass % of B,
0.0005 to 0.0050 mass % of Ca, 0.0005 to 0.0050 mass % of Mg, and
0.005 to 0.10 of REM.
[0036] A tenth aspect of the present invention is a process for
producing the austenitic stainless hot-rolled steel material having
excellent corrosion resistance, and low-temperature toughness, as
set forth in the eighth or ninth aspect of the present invention,
including: performing homogenizing-heat treatment on a cast steel
or a semi-finished product after a rough heat-rolling processing at
a temperature of 1200 to 1300.degree. C. for 1 hour or more, in
order to reduce the content of the intermetallic compound in the
steel material.
[0037] The present invention realizes an austenitic stainless steel
suitable for hull structures having a high level of sea water
resistance and proof stress, which are required as components for
structures of high-speed ships, and low-temperature toughness, and
contributes to industry significantly.
DETAILED DESCRIPTION OF THE INVENTION
[0038] First, the reason for restricting the first aspect of the
present invention will be explained below.
[0039] First, the characteristics required for structural shipping
materials are specified.
[0040] As for corrosion resistance, it is necessary to withstand
sea water even without a heavy duty corrosion-resistant coating
being applied thereto, and those characteristics necessary for
satisfying such corrosion resistance were investigated to obtain
the following results.
[0041] That is, although a usual pitting electric potential is
measured in 30.degree. C.-3.5% NaCl, the water temperature often
reaches 50.degree. C., in consideration of sea water resistance in
the tropics, and further, sea water is often condensed in a gappy
structure so that the NaCl concentration may increase to be higher
than the 3.5% of normal sea water, and it was revealed that if the
pitting electrical potential (Vc'100) measured in a deaerated
50.degree. C.-3.5% NaCl aqueous solution is not less than 500 mV,
then there are no problems in terms of practical use. It should be
noted that saturated Ag/AgCl was used as a reference electrode.
[0042] As for impact resistance, since it becomes a problem
conversely in cold areas, it is specified that a Charpy impact
value should be not less than 100 J/cm.sup.2 at -40.degree. C., at
which it is recognized in general that no problems occur in
ships.
[0043] As for hardness, it is preferably as high as possible, for
reducing weight. The present invention can provide a steel material
having a high strength with a 0.2% proof stress of not less than
550 MPa at room temperature, provided that the above corrosion
resistance and the impact strength are satisfied.
[0044] Next, the reason for restricting the components in the
present invention will be explained.
[0045] The content of C is restricted to not more than 0.03%, in
order to maintain the corrosion resistance of stainless steel. If
the content of C exceeds 0.03%, then Cr carbide will be generated
and corrosion resistance and toughness will deteriorate. However,
if the content of C is reduced extremely, then the cost for
refining increases, and hence the lower limit is specified as
0.001%. Preferably, it is 0.01 to 0.03%.
[0046] Si is added at not less than 0.1% for deoxidation. However,
if the content of Si exceeds 1.5%, then toughness will deteriorate.
Therefore, the upper limit is specified as 1.5%. Preferably it
ranges from 0.2 to 1.0%.
[0047] Mn is added at not less than 0.1% for deoxidation. However,
if the content of Mn exceeds 3.0%, then corrosion resistance and
toughness will deteriorate. Therefore, the lower limit is specified
as 3.0%. The preferable range is from 0.2 to 1.5%.
[0048] P is restricted to not more than 0.05%, because P
deteriorates the hot-rolling processability and toughness. However,
if the content of P is remarkably decreased, then refining cost
increases, and hence the lower limit is specified as 0.005%.
Preferably, it ranges from 0.01 to 0.03%.
[0049] S is restricted to not more than 0.003%, because S
deteriorates the hot-rolling processability, toughness, and
corrosion resistance. However, if the content of S is remarkably
decreased, then refining cost increases, and hence the lower limit
is specified as 0.0001%. Preferably, it ranges from 0.0005 to
0.001%.
[0050] Since Ni stabilizes an austenitic configuration, and
improves the corrosion resistance against various acids and
toughness further, Ni is added at not less than 15.0%. On the other
hand, since Ni is an expensive metal, the content of Ni is
restricted to not more than 21.0% from the viewpoint of cost.
[0051] Cr is contained at not less than 22.0% in order to secure
basic corrosion resistance. On the other hand, if Cr is contained
at over 28.0%, then an intermetallic compound is likely to be
deposited to deteriorate toughness. For this reason, the content of
Cr is specified within a range of not less than 28.0% to not more
than 22.0%.
[0052] Mo is a very effective element which raises the corrosion
resistance of stainless steel additionally, and is contained at not
less than 1.5% in the present invention. On the other hand, Mo is a
very expensive element and Mo promotes deposition of an
intermetallic compound with Cr, and hence the upper limit of Mo is
specified as not more than 3.5%. Preferably the content of Mo
ranges from 2.0 to 3.0%.
[0053] N is an effective element which intercrystallizes into an
austenite phase to increase hardness and corrosion resistance. For
this reason, N is contained at not less than 0.15%. Although N can
be intercrystallized into a base material by up to 0.4%, the upper
limit of the content of N is specified as 0.35%, because N raises
the sensitivity of generation of bubbling when performing welding.
Preferably, the content of N is not more than 0.30%.
[0054] O is an important element which constitutes an oxide which
represents a nonmetallic inclusion, and excessive content of O
deteriorates toughness, on the other hand, if a coarse cluster-like
oxide is generated, then it cause surface cracking. For this
reason, the upper limit of the content of O is restricted to
0.007%. Moreover, if the content of O is decreased remarkably, then
the cost for refining increases, and hence the lower limit is
specified to 0.0005%. Preferably, the content of O ranges from
0.001 to 0.004%.
[0055] PI value expressed by the above formula (1): A pitting index
is an index of corrosion resistance of stainless steel to a
chloride environment, and it was possible to obtain required
characteristics by restricting the PI value to not less than 35. As
stainless steel having a PI value of more than 40, SUS836L etc. are
exemplary, but the content of Ni thereof is not less than 24%, and
it is very expensive. In the present invention, since the target is
an austenitic stainless steel which has corrosion resistance
corresponding to cost, the upper limit of the PI value is specified
as 40. It should be noted that, in the present invention which
contains no W, the value of W in formula (1) is set to 0.
[0056] The .delta. cal expressed by the above formula (2) is an
index which indicates the quantity of the delta ferrite which
appears in the solidified configuration of austenitic stainless
steel, and in order to reduce solidification crack sensitivity or
to make a configuration fine, generally it is controlled to
approximately 0 to 7%. However, in steel having a high content of
Cr as in the present invention, delta ferrite in a solidified
configuration changes into an intermetallic compound during the
hot-rolling production step, and remains in a steel material as a
by-product, thereby deteriorating toughness. For this reason, the
upper limit of .delta. cal is restricted to +2 so that delta
ferrite might decrease. If .delta. cal exceeds this value, then it
becomes difficult to obtain high toughness even when devising in
the hot-rolling production step. On the other hand, the side in
which .delta. cal is small (minus) means that the delta ferrite
content becomes substantially 0%, as a result, the above effect is
saturated and an excess of Ni content will be contained, and hence
the lower limit is restricted to -6, in view of cost. Preferably,
.delta. cal ranges from -3 to +1. It should be noted that in the
present invention without containing W and Cu, the value of W or Cu
in formula (2) is set to 0.
[0057] The content of intermetallic compound which is contained in
steel materials is an important factor which dictates the toughness
of the austenitic stainless steel material in the present
invention. An intermetallic compound is a compound which contains
Cr, Mo, or W, as main ingredients and is known as .sigma. phase and
.chi. phase, The content of this compound can be measured by
performing alkali electrolytic etching of the micro configuration
and observing it with an approximately 400-power optical
microscope. The inventors of the present invention have found that
if this content as an average value of observation of the
cross-section of a steel material exceeds 0.5%, then Charpy
absorbed energy of the steel material becomes less than 100
J/cm.sup.2, and specified the upper limit thereof to be 0.5%.
[0058] The reason for restricting the second aspect of the present
invention will be explained.
[0059] W is an element which raises the corrosion resistance of
stainless steel additionally as well as Mo, and W can be contained
by an amount ranging from 0.3 to 3.0% in the present invention
steel for this purpose.
[0060] Al is an important element for deoxidation of steel, and in
order to reduce oxygen in steel, Al is contained by at amount of
not less than 0.005%. On the other hand, Al is an element having a
relatively large affinity to N, and hence if an excess of Al is
added, then AlN is generated to deteriorate the toughness of
stainless steel. Although the degree of deterioration of toughness
depends on the N content, if the Al content exceeds 0.1%, then the
toughness deteriorates significantly, and hence the upper limit of
Al content is specified as 0.1%.
[0061] The reason for restricting the third aspect of the present
invention will be explained.
[0062] Cu is an element which raises the corrosion resistance of
stainless steel against an acid additionally, and Cu can be
contained for this purpose. It is preferable to add Cu in an amount
of not less than 0.3%, whereas if Cu in an amount of more than 2.0%
is added, the effect in line with the cost is saturated, and hence
the upper limit is specified as 2.0%.
[0063] Although Sn also raises the corrosion resistance of steel,
an excess of Sn causes hot-rolling processing cracking, and hence
the upper limit is specified as 0.1%. Preferably, the lower limit
of Sn content is specified as 0.005%.
[0064] The reason for restricting the fourth aspect of the present
invention will be explained.
[0065] Each of Ca, Mg, and REM(s) is an element which improves the
hot-rolling processability of steel, and one or more of them are
added for this purpose. Excessive addition of each of them
deteriorates the hot-rolling processability adversely, and hence
the upper limit and the lower limit thereof are specified as
follows. That is, the content of each of Ca and Mg ranges from
0.0005 to 0.0050%, and the content of REM ranges from 0.005 to
0.10%. Here, REM represents the total content of a lanthanide
series rare-earth element such as La, Ce, etc.
[0066] Furthermore, the PV value specified by the following formula
(3) is set to be not more than 0. This formula is one that
clarifies the required amount Ca, Mg, and REM to be added based on
the existing amount of S, and it is possible to add exactly by
making the PV value to be not more than 0, thereby improving the
hot-rolling processability further. PV=S+O-0.8Ca-0.3Mg-0.3REM-30
(3)
[0067] The reason for restricting the fifth aspect of the present
invention will be explained.
[0068] As for B, by adding it in an amount of not less than
0.0003%, it becomes possible to increase grain boundary strength
and improve the hot-rolling processability. However, since
excessive addition of B deteriorates the hot-rolling processability
to the contrary due to an excessively deposited boride, the upper
limit of the B content is specified as 0.0060%.
[0069] The reason for restricting the sixth aspect of the present
invention will be explained.
[0070] Ti is an element which forms an oxide, a nitride, and
sulfide with a very small amount thereof, and makes the crystal
grain of steel fine, and Ti is an element which can be
advantageously used in the steel material of the present invention.
In order to reduce the intermetallic compound content in steel
materials, it is effective to restrict the upper limit value of
.delta. cal and perform homogenizing heat treatment of
semi-finished products. Among these, in the latter method, heat
treatment at a high temperature of approximately 1250.degree. C.
will be performed for several hours, if a proper amount of Ti is
contained therein, then growth of crystal grain during the heat
treatment at a high temperature as such can be effectively
suppressed. For this purpose, it is necessary to add Ti in an
amount of not less than 0.003%. On the other hand, Ti is an element
which has a very high nitride-forming power, and hence if Ti in an
amount of over 0.03% is contained in the steel material of the
present invention which contains N, then coarse TiN will
deteriorate the toughness of the steel. For this reason, Ti content
is specified in the range of 0.003 to 0.03%. Preferably, the Ti
content ranges from 0.005 to 0.02%, in the case in which Ti is
contained.
[0071] Nb forms carbide to fix C, thereby suppressing formation of
Cr carbide to increase corrosion resistance and toughness. In
addition, Nb forms nitride to suppress the growth of crystal grain,
thereby converting steel material into fine grains to increase the
strength. For improving corrosion resistance and increasing
strength, Nb in an amount of not less than 0.02% can be contained.
However, if Nb in an amount of over 0.2% is added, then a large
amount of carbon nitride of Nb is deposited during the hot-rolling
processing step to deteriorate the hot-rolling recrystallization
and a coarse configuration will remain in a steel material as a
product, and hence the upper limit of Nb content is specified as
0.2%. Preferably Nb content ranges from 0.05% to 0.15%.
[0072] V is an element that forms a carbon nitride as well as Nb,
and V can be added in order to maintain corrosion resistance and
toughness. Although V is contained in an amount of not less than
0.05% for this purpose, if V in an amount of over 0.5% is
contained, then a coarse V series carbon nitride will be generated,
and toughness will deteriorate conversely. Therefore, the upper
limit of V is restricted to 0.5%. Preferably, V content ranges from
0.1 to 0.3%.
[0073] Although Zr and Ta can inhibit the negative influence on the
corrosion resistance of C or S by addition, if Zr or Ta is added
excessively, then deterioration of toughness will occur, and hence
Zr content is restricted to 0.003 to 0.03% and Ta content is
restricted to 0.01 to 0.1%.
[0074] The reason for restricting the seventh aspect of the present
invention will be explained.
[0075] In order to increase the toughness of steel materials in the
present invention, the amount of intermetallic compound contained
in the steel material is restricted to not more than 0.5%, however,
solidifying heat treatment after the final heat-rolling step must
be omitted in order to obtain high proof stress. Therefore, as for
an intermetallic compound, it is necessary to reduce the
intermetallic compound contained in a cast steel, and to prevent
formation of the intermetallic compound during the hot-rolling step
as far as possible.
[0076] First, as the technique for reducing the intermetallic
compound in the cast steel, it is necessary to combine the
controlling of .delta. cal with the homogenizing heat treatment to
the cast steel of steel described in this aspect. In the case in
which there is no solidification segregation in the target steel
materials of the present invention, the temperature at which an
intermetallic compound is generated is approximately not higher
than 1000.degree. C. However, in the semi-finished product which is
accompanied with segregation of ingredients caused by
solidification, it becomes necessary to perform a production step
for diffusing the segregation and homogenizing it in order to
reduce the content of an intermetallic compound in the
semi-finished product. Although the temperature and the time of
this homogenizing heat treatment will change slightly,
corresponding to chemical composition such as solidifying rate and
cross-sectional area of the cast steel, the degree of hot-rolling
processing when processing into a semi-finished product, and
.delta. cal, etc., the temperature required is not lower than
1200.degree. C., because the rate is limited by diffusion of Cr,
Mo, Ni, etc. On the other hand, if the temperature exceeds
1300.degree. C., then oxidized scale will be generated more than
usually As for the time, it is preferable that the time be as long
as possible, and at least one hour is necessary. Moreover, this
purpose can be attained by performing a soaking at 1200.degree. C.
for one hour or more during heating of the semi-finished product
for rolling a product. As mentioned above, it is specified to
perform homogenizing heat treatment for one hour or more at a
temperature ranging from 1200 to 1300.degree. C. Taking the effect
and the economical efficiency into consideration, a preferable
range of soaking time ranges from 2 to 20 hours.
[0077] As for the rolling condition, it consists of the rough
rolling stage in which re-heating is performed at a temperature
ranging from 1100 to 1300.degree. C. and making the total
compaction amount at a temperature of not lower than 1050.degree.
C. to be not less than 50%, and the successive finishing rolling
stage in which the total compaction amount at a temperature ranging
from 1050 to 850.degree. C. is made to be not less than 10%. The
rough rolling stage is a stage in which the solidification
structure is mainly destroyed, to obtain a uniform recrystallized
solidification, whereas the finishing rolling step is a step of
introducing the processing strain by the rolling and for increasing
the strength after the rolling processing. In addition, all of the
rolling processing is performed at a temperature of not lower than
850.degree. C., thereby preventing re-deposition of the
intermetallic compound. And further, a controlled cooling is
performed at an average cooling rate of not less than 150.degree.
C./min from 800 to 500.degree. C. after the rolling processing,
thereby inhibiting the re-deposition of the intermetallic compound
and the recovery of the processing strain which was introduced in
the finishing rolling step.
[0078] The reason for restricting the condition will be explained
further in detail.
[0079] In order to make it possible to perform a rolling processing
which makes the total compaction amount to be not less than 50% at
a temperature of not lower than 1050.degree. C., to reduce
deformation resistance, and to make it easy to perform the rolling
processing, it is necessary to heat the steel ingot to not lower
than 1100.degree. C. However, if it is heated over 1300.degree. C.,
then the grain boundary will be fused to cause cracks during the
hot-rolled processing, and hence the heating temperature is
restricted to be within a range of 1100 to 1300.degree. C.
[0080] In the rough rolling stage, in order to destroy the
solidification structure and to obtain a uniform recrystallized
structure, it is necessary to make the total compaction amount at a
temperature of not lower than 1050.degree. C. to be not less than
50%. If the rolling temperature is lower than 1050.degree. C. or
the total compaction amount is less than 50%, then it is not
possible to obtain uniform recrystallized structure.
[0081] In the finishing rolling stage, in order to acquire the
target proof stress of 550 MPa, it is necessary to perform a
finishing rolling by which the total compaction amount at a
temperature of 1050.degree. C. to 850.degree. C. in the component
range which is restricted in the present invention should be not
less than 10%. In addition, if a rolling processing is performed at
a temperature over 1050.degree. C., then recrystallization will
occur, and as a result compressing strain cannot be accumulated, so
that sufficient strength cannot be obtained, whereas if a rolling
processing is performed at a temperature lower than 850.degree. C.,
then deposition of the intermetallic compound will be promoted to
deteriorate toughness remarkably. Therefore, it is necessary to
perform the rolling processing during all of the rolling
processing, while maintaining the temperature to be not lower than
850.degree. C. Finally, high hardness can be maintained by omitting
solution heat treatment.
EXAMPLE 1
[0082] Example 1 will be explained below. The chemical constitution
of a test piece of steel is shown in Table 1. It should be noted
that, the content of inevitable impurity elements other than the
components indicated in Table 1 is the same level as in standard
stainless steel. Moreover, as to the portions where no contents are
shown for the components shown in Table 1, this means that the
content is the same level as in an impurity level. Moreover, REM in
Tables represents lanthanoid series rare earth elements, and the
content indicates the total of these elements. These steel samples
were melted in a 50 kg-vacuum induction furnace in a laboratory and
cast into a flat steel ingot having a thickness of approximately
100 mm. TABLE-US-00001 TABLE 1 STEEL No. C Si Mn P S Ni Cr Mo Cu A
EXAMPLE1 0.021 0.49 0.48 0.020 0.0005 17.91 25.15 2.31 0.12 B
EXAMPLE1 0.019 0.46 0.32 0.023 0.0003 18.23 24.65 2.46 0.21 C
EXAMPLE2 0.018 0.52 0.52 0.014 0.0005 17.98 25.02 2.46 0.05 D
EXAMPLE3 0.022 0.49 0.52 0.022 0.0006 18.43 24.88 2.46 0.45 E
EXAMPLE3 0.015 0.71 1.71 0.026 0.0002 19.25 26.35 1.92 0.22 F
EXAMPLE4 0.021 0.48 0.52 0.022 0.0009 18.25 25.36 2.53 0.30 G
EXAMPLE4 0.022 0.46 0.52 0.021 0.0008 20.21 24.96 2.46 0.31 H
EXAMPLE4 0.022 0.48 0.49 0.022 0.0003 19.23 24.64 3.33 0.28 I
EXAMPLE5 0.021 0.47 0.52 0.022 0.0007 18.33 24.66 2.48 0.32 J
EXAMPLE6 0.019 0.45 0.53 0.023 0.0008 18.23 24.65 2.46 0.32 K
EXAMPLE6 0.019 0.49 0.49 0.022 0.0004 20.42 27.31 1.68 0.31 L
EXAMPLE6 0.024 0.49 0.49 0.021 0.0003 19.53 24.61 2.11 1.82 M
EXAMPLE6 0.019 0.49 0.85 0.019 0.0013 18.89 25.29 2.52 0.32 N
EXAMPLE6 0.021 0.50 0.84 0.019 0.0005 16.77 25.21 2.10 0.85 a
COMPARATIVE 0.035 0.35 0.25 0.008 0.0009 17.91 24.68 2.35 0.21
EXAMPLE b COMPARATIVE 0.006 1.75 2.11 0.027 0.0006 18.93 24.67 2.64
0.06 EXAMPLE c COMPARATIVE 0.025 0.44 3.31 0.018 0.0010 17.59 27.19
1.65 0.58 EXAMPLE d COMPARATIVE 0.018 0.51 0.51 0.055 0.0003 18.21
23.77 2.33 0.91 EXAMPLE e COMPARATIVE 0.017 0.40 0.55 0.012 0.0033
17.64 24.21 2.58 1.22 EXAMPLE f COMPARATIVE 0.022 0.36 0.61 0.026
0.0008 14.55 22.91 2.53 0.46 EXAMPLE g COMPARATIVE 0.008 0.33 1.21
0.013 0.0006 20.12 27.55 1.22 0.25 EXAMPLE h COMPARATIVE 0.028 0.53
0.43 0.022 0.0006 18.88 22.84 3.68 0.66 EXAMPLE i COMPARATIVE 0.033
0.15 0.35 0.026 0.0004 20.55 25.35 2.33 0.24 EXAMPLE j COMPARATIVE
0.022 0.47 0.53 0.025 0.0008 18.21 24.96 2.46 0.33 EXAMPLE k
COMPARATIVE 0.021 0.26 1.85 0.022 0.0009 15.61 21.89 2.12 0.31
EXAMPLE l COMPARATIVE 0.021 0.48 0.51 0.024 0.0007 18.55 28.22 2.01
0.31 EXAMPLE m COMPARATIVE 0.022 0.49 0.52 0.023 0.0008 21.40 24.95
2.47 0.33 EXAMPLE n COMPARATIVE 0.024 0.45 0.52 0.024 0.0006 18.22
24.94 2.47 0.31 EXAMPLE o COMPARATIVE 0.022 0.51 0.18 0.037 0.0028
19.56 25.61 2.64 0.35 EXAMPLE p COMPARATIVE 0.019 0.53 0.46 0.021
0.0003 18.36 24.91 2.28 0.36 EXAMPLE q COMPARATIVE 0.018 0.56 0.53
0.021 0.0006 16.95 22.21 3.39 1.66 EXAMPLE r COMPARATIVE 0.026 1.22
0.66 0.016 0.0003 17.91 23.96 2.31 0.33 EXAMPLE s COMPARATIVE 0.021
0.48 0.53 0.023 0.0008 18.23 24.89 2.46 0.32 EXAMPLE t COMPARATIVE
0.022 0.47 0.54 0.024 0.0007 18.33 23.33 2.45 0.35 EXAMPLE u
COMPARATIVE 0.023 0.46 0.55 0.023 0.0008 18.25 24.98 2.48 0.32
EXAMPLE v COMPARATIVE 0.013 0.48 1.21 0.013 0.0005 16.36 22.64 2.93
0.28 EXAMPLE w COMPARATIVE 0.024 0.55 0.78 0.019 0.0005 17.21 23.51
2.33 0.33 EXAMPLE STEEL No. Cu Sn Nb Ti V Zr Ta W Al A EXAMPLE1
0.12 B EXAMPLE1 0.21 1.05 C EXAMPLE2 0.05 0.018 D EXAMPLE3 0.45
0.032 E EXAMPLE3 0.22 0.08 0.051 F EXAMPLE4 0.30 0.026 G EXAMPLE4
0.31 0.71 0.028 H EXAMPLE4 0.28 0.034 I EXAMPLE5 0.32 0.022 J
EXAMPLE6 0.32 0.012 0.023 K EXAMPLE6 0.31 0.121 0.011 L EXAMPLE6
1.82 0.28 2.10 0.023 M EXAMPLE6 0.32 0.021 0.020 N EXAMPLE6 0.85
0.035 0.018 a COMPARATIVE 0.21 EXAMPLE b COMPARATIVE 0.06 0.009
EXAMPLE c COMPARATIVE 0.58 EXAMPLE d COMPARATIVE 0.91 0.012 EXAMPLE
e COMPARATIVE 1.22 0.006 0.061 EXAMPLE f COMPARATIVE 0.46 0.111
EXAMPLE g COMPARATIVE 0.25 EXAMPLE h COMPARATIVE 0.66 1.35 EXAMPLE
i COMPARATIVE 0.24 0.035 EXAMPLE j COMPARATIVE 0.33 0.014 0.002
EXAMPLE k COMPARATIVE 0.31 0.075 0.020 EXAMPLE l COMPARATIVE 0.31
EXAMPLE m COMPARATIVE 0.33 0.026 EXAMPLE n COMPARATIVE 0.31 0.140
EXAMPLE o COMPARATIVE 0.35 0.05 0.65 EXAMPLE p COMPARATIVE 0.36
0.033 EXAMPLE q COMPARATIVE 1.66 0.024 EXAMPLE r COMPARATIVE 0.33
EXAMPLE s COMPARATIVE 0.32 0.046 0.026 EXAMPLE t COMPARATIVE 0.35
0.284 1.11 0.026 EXAMPLE u COMPARATIVE 0.32 0.88 0.024 EXAMPLE v
COMPARATIVE 0.28 0.037 EXAMPLE w COMPARATIVE 0.33 0.12 0.064
EXAMPLE STEEL No. B Ca Mg REM 0 N .delta. cal PI PV A EXAMPLE1
0.0055 0.235 0.9 36.5 30.0 B EXAMPLE1 0.0063 0.266 -0.8 38.8 36.0 C
EXAMPLE2 0.0041 0.266 -0.5 37.4 16.0 D EXAMPLE3 0.0025 0.240 -1.4
36.8 1.0 E EXAMPLE3 0.0030 0.281 -2.9 37.2 2.0 F EXAMPLE4 0.0022
0.0035 0.238 1.0 37.5 -3.6 G EXAMPLE4 0.0033 0.0025 0.242 -4.8 38.1
-6.9 H EXAMPLE4 0.009 0.0019 0.257 -2.4 39.7 -35.0 I EXAMPLE5
0.0033 0.0026 0.235 -1.3 36.6 3.0 J EXAMPLE6 0.0008 0.0033 0.0035
0.247 -1.6 36.7 -3.3 K EXAMPLE6 0.0023 0.0036 0.245 -1.6 36.8 -8.4
L EXAMPLE6 0.0020 0.0025 0.173 -0.6 37.8 -18.0 M EXAMPLE6 0.0008
0.021 0.0040 0.235 -0.9 37.4 -46.4 N EXAMPLE6 0.0042 0.0020 0.315
-1.5 37.2 -38.6 a COMPARATIVE 0.0022 0.193 0.6 35.5 1.0 EXAMPLE b
COMPARATIVE 0.0006 0.0034 0.264 -2.5 37.6 5.2 EXAMPLE c COMPARATIVE
0.0009 0.0022 0.252 1.9 36.7 2.0 EXAMPLE d COMPARATIVE 0.0008
0.0028 0.265 -5.6 35.7 -1.4 EXAMPLE e COMPARATIVE 0.0012 0.0009
0.237 -0.9 36.5 2.4 EXAMPLE f COMPARATIVE 0.0028 0.0022 0.0026
0.279 1.0 35.7 -2.6 EXAMPLE g COMPARATIVE 0.0026 0.295 -3.8 36.3
2.0 EXAMPLE h COMPARATIVE 0.0034 0.155 0.9 39.7 10.0 EXAMPLE i
COMPARATIVE 0.035 0.0052 0.121 -0.8 35.0 -79.0 EXAMPLE j
COMPARATIVE 0.0036 0.0081 0.242 -0.6 37.0 30.2 EXAMPLE k
COMPARATIVE 0.0019 0.0008 0.0032 0.184 -1.9 31.8 -6.6 EXAMPLE l
COMPARATIVE 0.0022 0.0029 0.271 5.3 39.2 -11.6 EXAMPLE m
COMPARATIVE 0.0023 0.0030 0.240 -8.8 36.9 -10.4 EXAMPLE n
COMPARATIVE 0.0024 0.0032 0.235 -0.5 36.9 -11.2 EXAMPLE o
COMPARATIVE 0.0067 0.0032 0.222 0.5 38.9 -23.6 EXAMPLE p
COMPARATIVE 0.0071 0.0032 0.262 -2.4 36.6 -16.3 EXAMPLE q
COMPARATIVE 0.0030 0.111 0.0032 0.212 -1.5 36.8 -349.0 EXAMPLE r
COMPARATIVE 0.0071 0.0032 0.222 -2.0 35.1 5.0 EXAMPLE s COMPARATIVE
0.0025 0.0032 0.241 -0.7 36.9 -10.0 EXAMPLE t COMPARATIVE 0.0026
0.015 0.0033 0.173 -0.5 36.0 -55.8 EXAMPLE u COMPARATIVE 0.0025
0.0031 0.239 -0.6 37.0 -11.0 EXAMPLE v COMPARATIVE 0.0025 0.0030
0.263 -2.0 36.5 -2.5 EXAMPLE w COMPARATIVE 0.0025 0.245 -3.1 35.1
0.0 EXAMPLE : VALUE WITHOUT THE RANGE OF THE PRESENT INVENTION
[0083] A steel sheet having a thickness ranging from 12 to 22 mm
was produced by performing cogging, homogenizing heat treatment,
and product rolling, using the above sample steel. In the cogging,
the sample steel was soaked at 1180.degree. C. for two hours, and
thereafter the sample steel was rolled to 65 mm thickness. Then the
resultant semi-finished products were subjected to homogenizing
heat treatment under the conditions shown in Tables 2 and 3. Some
of the steel chips were not subjected to the homogenizing heat
treatment. Each piece of steel was ground to 60 mm to obtain the
material for use in product rolling, and thereafter the resultant
material for use in product rolling was subjected to hot-rolling
processing to obtain a hot-rolled steel material. It should be
noted that the steel material immediately after being hot-rolled
which was in a temperature state of not less than 800.degree. C.
was cooled to a temperature of not higher than 500.degree. C. by
performing spray cooling. Some of the steel sheets were subjected
to a solution heat treatment under the condition of 1100.degree.
C..times.20 min with cooling by water, after soaking.
TABLE-US-00002 TABLE 2 REDUCTION REDUCTION AT A INTERMETALLIC
HOMOGENIZING RE-HEATING AT 1050.degree. C. TEMPERATURE STEEL
COMPOUND HEAT TEMPERATURE OR MORE OF 1050 TO No. No. CONTENT (%)
TREATMENT (.degree. C.) (%) 850.degree. C.(%) 1 A EXAMPLE 0.35
1250.degree. C. .times. 4 h 1200 60 20 2 B EXAMPLE 0.05
1250.degree. C. .times. 4 h 1200 60 20 3 C EXAMPLE 0.10
1250.degree. C. .times. 4 h 1200 60 20 4 D EXAMPLE 0.20
1250.degree. C. .times. 4 h 1200 60 20 5 E EXAMPLE 0.05
1250.degree. C. .times. 4 h 1200 60 20 6 F EXAMPLE 0.30
1250.degree. C. .times. 4 h 1200 60 20 7 F EXAMPLE 0.35
1220.degree. C. .times. 2 h 1200 60 20 8 F EXAMPLE 0.25
1200.degree. C. .times. 20 h 1200 60 20 9 F EXAMPLE 0.40
1250.degree. C. .times. 4 h 1250 60 20 10 F EXAMPLE 0.30
1250.degree. C. .times. 4 h 1200 75 15 11 F EXAMPLE 0.30
1250.degree. C. .times. 4 h 1200 60 12 12 F COMPARATIVE 0.95 UNDONE
1200 60 20 EXAMPLE 13 F COMPARATIVE 0.75 1150.degree. C. .times. 5
h 1200 60 20 EXAMPLE 14 F COMPARATIVE 0.80 1200.degree. C. .times.
15 m 1200 60 20 EXAMPLE 15 F COMPARATIVE 0.45 1250.degree. C.
.times. 4 h 1050 0 68 EXAMPLE 16 F COMPARATIVE 0.35 1250.degree. C.
.times. 4 h 1200 60 7 EXAMPLE 17 F COMPARATIVE 0.60 1250.degree. C.
.times. 4 h 1200 60 20 EXAMPLE 18 F COMPARATIVE 0.85 1250.degree.
C. .times. 4 h 1200 60 20 EXAMPLE 19 F COMPARATIVE 0.00
1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE 20 G EXAMPLE 0.20
1250.degree. C. .times. 4 h 1200 60 5 21 H EXAMPLE 0.25
1250.degree. C. .times. 4 h 1200 60 20 22 I EXAMPLE 0.30
1250.degree. C. .times. 4 h 1200 60 20 23 J EXAMPLE 0.20
1250.degree. C. .times. 4 h 1200 60 20 COOLING RATE AT A ROLLING
TEMPERATURE Vc' 100 FINISHING OF 800 SOLUTION EAR PROOF (mV vs
STEEL TEMPERATURE TO 500.degree. C. HEAT CRACK STRESS vE-40.degree.
C. Saturated No. No. (.degree. C.) (.degree. C./min) TREATMENT (mm)
(MPa) (J/cm.sup.2) Ag/AgCl) 1 A EXAMPLE 900 250 UNDONE 5 710 130
630 2 B EXAMPLE 900 250 UNDONE 8 730 147 750 3 C EXAMPLE 900 250
UNDONE 5 726 187 700 4 D EXAMPLE 900 250 UNDONE 3 711 201 685 5 E
EXAMPLE 900 250 UNDONE 5 733 203 705 6 F EXAMPLE 900 250 UNDONE 0
713 199 720 7 F EXAMPLE 900 250 UNDONE 0 720 167 715 8 F EXAMPLE
900 250 UNDONE 0 710 195 720 9 F EXAMPLE 900 250 UNDONE 0 706 163
710 10 F EXAMPLE 900 250 UNDONE 0 716 200 710 11 F EXAMPLE 900 250
UNDONE 0 590 225 725 12 F COMPARATIVE 900 250 UNDONE 0 729 63 625
EXAMPLE 13 F COMPARATIVE 900 250 UNDONE 0 724 74 645 EXAMPLE 14 F
COMPARATIVE 900 250 UNDONE 0 725 66 630 EXAMPLE 15 F COMPARATIVE
900 250 UNDONE 0 735 70 685 EXAMPLE 16 F COMPARATIVE 900 250 UNDONE
0 538 281 730 EXAMPLE 17 F COMPARATIVE 800 250 UNDONE 0 730 55 550
EXAMPLE 18 F COMPARATIVE 900 75 UNDONE 0 716 57 535 EXAMPLE 19 F
COMPARATIVE 900 250 DONE 0 345 350 780 EXAMPLE 20 G EXAMPLE 900 250
UNDONE 0 725 202 740 21 H EXAMPLE 900 250 UNDONE 0 724 217 775 22 I
EXAMPLE 900 250 UNDONE 0 712 193 645 23 J EXAMPLE 900 250 UNDONE 0
729 164 680 : VALUE WITHOUT THE RANGE OF THE PRESENT INVENTION
[0084] TABLE-US-00003 TABLE 3 REDUCTION REDUCTION AT A
INTERMETALLIC HOMOGENIZING RE-HEATING AT 1050.degree. C.
TEMPERATURE COMPOUND HEAT TEMPERATURE OR MORE OF 1050 TO No. STEEL
No. CONTENT (%) TREATMENT (.degree. C.) (%) 850.degree. C.(%) 24 K
EXAMPLE 0.15 1250.degree. C. .times. 4 h 1200 60 20 25 K EXAMPLE
0.20 1250.degree. C. .times. 4 h 1200 60 35 26 K EXAMPLE 0.30
1250.degree. C. .times. 4 h 1200 60 20 27 K EXAMPLE 0.05
1250.degree. C. .times. 4 h 1200 60 20 28 K EXAMPLE 0.05
1250.degree. C. .times. 4 h 1200 60 20 29 K COMPARATIVE 1.00 UNDONE
1200 60 20 EXAMPLE 30 K COMPARATIVE 0.70 1150.degree. C. .times. 5
h 1200 60 20 EXAMPLE 31 K COMPARATIVE 0.70 1200.degree. C. .times.
15 m 1200 60 20 EXAMPLE 32 K COMPARATIVE 0.45 1250.degree. C.
.times. 4 h 1350 60 20 EXAMPLE 33 K COMPARATIVE 0.40 1250.degree.
C. .times. 4 h 1050 0 68 EXAMPLE 34 K COMPARATIVE 0.30 1250.degree.
C. .times. 4 h 1200 60 7 EXAMPLE 35 K COMPARATIVE 0.75 1250.degree.
C. .times. 4 h 1200 60 20 EXAMPLE 36 K COMPARATIVE 0.75
1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE 37 K COMPARATIVE
0.00 1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE 38 L EXAMPLE
0.30 1250.degree. C. .times. 4 h 1200 60 20 39 M EXAMPLE 0.35
1250.degree. C. .times. 4 h 1200 60 20 40 N EXAMPLE 0.25
1250.degree. C. .times. 4 h 1200 60 20 COOLING RATE AT A ROLLING
TEMPERATURE Vc' 100 FINISHING OF 800 SOLUTION EAR PROOF (mV vs
STEEL TEMPERATURE TO 500.degree. C. HEAT CRACK STRESS vE-40.degree.
C. Saturated No. No. (.degree. C.) (.degree. C./min) TREATMENT (mm)
(MPa) (J/cm.sup.2) Ag/AgCl) 24 K EXAMPLE 900 250 UNDONE 0 740 183
700 25 K EXAMPLE 900 250 UNDONE 0 745 182 695 26 K EXAMPLE 860 250
UNDONE 0 751 167 695 27 K EXAMPLE 970 250 UNDONE 0 723 197 700 28 K
EXAMPLE 900 500 UNDONE 0 741 193 715 29 K COMPARATIVE 900 250
UNDONE 0 743 53 645 EXAMPLE 30 K COMPARATIVE 900 250 UNDONE 0 741
60 655 EXAMPLE 31 K COMPARATIVE 900 250 UNDONE 0 740 63 655 EXAMPLE
32 K COMPARATIVE 900 250 UNDONE 70 720 95 605 EXAMPLE 33 K
COMPARATIVE 900 250 UNDONE 0 755 66 705 EXAMPLE 34 K COMPARATIVE
900 250 UNDONE 0 545 270 715 EXAMPLE 35 K COMPARATIVE 800 250
UNDONE 0 752 52 585 EXAMPLE 36 K COMPARATIVE 900 75 UNDONE 0 743 47
570 EXAMPLE 37 K COMPARATIVE 900 250 DONE 0 358 304 815 EXAMPLE 38
L EXAMPLE 900 250 UNDONE 0 652 197 735 39 M EXAMPLE 900 250 UNDONE
0 700 191 720 40 N EXAMPLE 900 250 UNDONE 0 758 183 645 : VALUE
WITHOUT THE RANGE OF THE PRESENT INVENTION
[0085] TABLE-US-00004 TABLE 4 REDUCTION REDUCTION AT A
INTERMETALLIC HOMOGENIZING RE-HEATING AT 1050.degree. C.
TEMPERATURE STEEL COMPOUND HEAT TEMPERATURE OR MORE OF 1050 TO No.
No. CONTENT (%) TREATMENT (.degree. C.) (%) 850.degree. C.(%) 41 a
COMPARATIVE 0.40 1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE 42
b COMPARATIVE 0.35 1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE
43 c COMPARATIVE 0.45 1250.degree. C. .times. 4 h 1200 60 20
EXAMPLE 44 d COMPARATIVE 0.05 1250.degree. C. .times. 4 h 1200 60
20 EXAMPLE 45 e COMPARATIVE 0.20 1250.degree. C. .times. 4 h 1200
60 20 EXAMPLE 46 f COMPARATIVE 0.50 1250.degree. C. .times. 4 h
1200 60 20 EXAMPLE 47 g COMPARATIVE 0.13 1250.degree. C. .times. 4
h 1200 60 20 EXAMPLE 48 h COMPARATIVE 0.95 1250.degree. C. .times.
4 h 1200 60 20 EXAMPLE 49 i COMPARATIVE 0.40 1250.degree. C.
.times. 4 h 1200 60 20 EXAMPLE 50 j COMPARATIVE 0.30 1250.degree.
C. .times. 4 h 1200 60 20 EXAMPLE 51 k COMPARATIVE 0.15
1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE 52 l COMPARATIVE
0.90 1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE 53 m
COMPARATIVE 0.00 1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE 54
n COMPARATIVE 0.25 1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE
55 o COMPARATIVE 0.30 1250.degree. C. .times. 4 h 1200 60 20
EXAMPLE 56 p COMPARATIVE 0.10 1250.degree. C. .times. 4 h 1200 60
20 EXAMPLE 57 q COMPARATIVE 0.20 1250.degree. C. .times. 4 h 1200
60 20 EXAMPLE 58 r COMPARATIVE 0.15 1250.degree. C. .times. 4 h
1200 60 20 EXAMPLE 59 s COMPARATIVE 0.30 1250.degree. C. .times. 4
h 1200 60 20 EXAMPLE 60 t COMPARATIVE 0.30 1250.degree. C. .times.
4 h 1200 60 20 EXAMPLE 61 u COMPARATIVE 0.30 1250.degree. C.
.times. 4 h 1200 60 20 EXAMPLE 62 v COMPARATIVE 0.15 1250.degree.
C. .times. 4 h 1200 60 20 EXAMPLE 63 w COMPARATIVE 0.10
1250.degree. C. .times. 4 h 1200 60 20 EXAMPLE COOLING RATE AT A
ROLLING TEMPERATURE Vc' 100 FINISHING OF 800 SOLUTION EAR PROOF (mV
vs STEEL TEMPERATURE TO 500.degree. C. HEAT CRACK STRESS
vE-40.degree. C. Saturated No. No. (.degree. C.) (.degree. C./min)
TREATMENT (mm) (MPa) (J/cm.sup.2) Ag/AgCl) 41 a COMPARATIVE 900 250
UNDONE 5 695 199 455 EXAMPLE 42 b COMPARATIVE 900 250 UNDONE 10 726
65 730 EXAMPLE 43 c COMPARATIVE 900 250 UNDONE 8 705 77 490 EXAMPLE
44 d COMPARATIVE 900 250 UNDONE 50 728 75 550 EXAMPLE 45 e
COMPARATIVE 900 250 UNDONE 45 700 53 435 EXAMPLE 46 f COMPARATIVE
900 250 UNDONE 0 752 71 520 EXAMPLE 47 g COMPARATIVE 900 250 UNDONE
6 644 310 405 EXAMPLE 48 h COMPARATIVE 900 250 UNDONE 9 644 30 765
EXAMPLE 49 i COMPARATIVE 900 250 UNDONE 0 621 195 470 EXAMPLE 50 j
COMPARATIVE 900 250 UNDONE 12 718 35 510 EXAMPLE 51 k COMPARATIVE
900 250 UNDONE 0 685 222 205 EXAMPLE 52 l COMPARATIVE 900 250
UNDONE 33 740 20 750 EXAMPLE 53 m COMPARATIVE 900 250 UNDONE 0 715
215 660 EXAMPLE 54 n COMPARATIVE 900 250 UNDONE 0 713 30 655
EXAMPLE 55 o COMPARATIVE 900 250 UNDONE 42 703 150 445 EXAMPLE 56 p
COMPARATIVE 900 250 UNDONE 38 731 156 430 EXAMPLE 57 q COMPARATIVE
900 250 UNDONE 56 699 164 405 EXAMPLE 58 r COMPARATIVE 900 250
UNDONE 60 705 142 565 EXAMPLE 59 s COMPARATIVE 900 250 UNDONE 0 713
61 695 EXAMPLE 60 t COMPARATIVE 900 250 UNDONE 0 666 52 585 EXAMPLE
61 u COMPARATIVE 900 250 UNDONE 0 720 30 695 EXAMPLE 62 v
COMPARATIVE 900 250 UNDONE 0 735 70 650 EXAMPLE 63 w COMPARATIVE
900 250 UNDONE 0 696 21 550 EXAMPLE : VALUE WITHOUT THE RANGE OF
THE PRESENT INVENTION
[0086] The steel plate produced under the above condition was cut
into JIS. No.4 tension test pieces and JIS. No.4 V notch Charpy
test pieces from a direction perpendicular to the direction of
rolling processing. Using the resultant test pieces, 0.2% offset
proof stress and impact strength at -40.degree. C. were measured,
and further the surface of the test piece was ground with a #600
grinder and then pitting electrical potential (Vc'100) was measured
in a deaerated 10% NaCl aqueous solution held at 50.degree. C.
Moreover, test pieces for micro structure observation were cut out,
and each of the resultant test pieces was planished and thereafter
was subjected to 10% KOH electrolytic etching to reveal
intermetallic compound therefrom so as to be observed by an optical
microscope, thereby measuring the content. The content was measured
by performing point counting in each of ten fields of view with
400.times. magnification at a depth of each of 1/4, 1/2, and 3/4 of
thick, and then calculating all average values, and the resultant
value was determined as the content of the intermetallic compound
of the steel material. The obtained results are shown in Tables
2-4.
[0087] The hot-rolling processability was evaluated relatively by
judging the generation of an ear crack during the product rolling.
It was confirmed that the steel material corresponding to Example 5
or 6 (steel Nos. F to N) developed no ear cracks and exhibited
excellent hot-rolling processability, with the exception of the
case in which the reheating temperature was excessively high. On
the other hand, it was confirmed that each of the steel materials
corresponding to each Example other than Examples 5 and 6 developed
ear cracks of approximately 5 to 10 mm per one side, so that the
yield was decreased slightly. The lengths of ear cracks are shown
in Tables 2 to 4.
[0088] As is clear from the results shown in Tables 1 and 2 to 4,
as to the steel material which satisfies the steel composition
which is within the scope of the present invention, the
intermetallic compound content, production condition, all of the
corrosion resistance, the proof stress, and Charpy impact value
satisfy the specified conditions.
[0089] As can be seen from the above examples, it is clarified that
the steel material of the present invention is an austenitic
stainless steel material which excels in corrosion resistance,
toughness, and strength.
[0090] The present invention realizes an austenitic stainless steel
suitable for the hull structures of ships, having excellent
performance required for structural members of high-speed ships,
such as sea water resistance, proof stress, and low-temperature
toughness at a high level, and hence the contributions of the
present invention to industry are significant.
[0091] Next, the reason for limitation of the eighth aspect of the
present invention will be explained.
[0092] The content of C is restricted to not more than 0.03%, in
order to secure the corrosion resistance of the stainless steel. If
the content of C exceeds 0.03%, then Cr carbide will be generated
and corrosion resistance and toughness will deteriorate.
[0093] The content of Si is not less than 0.1% for deoxidation.
However, if the content of Si exceeds 1.5%, then toughness will
deteriorate. Therefore, the upper limit thereof is restricted to
1.5%. The content of Si preferably ranges from 0.2 to 1.0%.
[0094] The content of Mn is not less than 0. 1% for deoxidation.
However, if the content of Mn exceeds 3.0%, then corrosion
resistance and toughness will deteriorate. Therefore, the upper
limit thereof is restricted to 3.0%. The content of Mn preferably
ranges from 0.2 to 1.5%.
[0095] The content of P is restricted to not more than 0.05%
because P deteriorates hot-rolling processability and toughness.
The content of P is preferably not more than 0.03%.
[0096] The content of S is restricted to not more than 0.003%
because S deteriorates hot-rolling processability, toughness, and
corrosion resistance. The content of S is preferably not more than
0.001%.
[0097] The content of Ni is not less than 15.0% because Ni
stabilizes an austenitic phase, and improves resistance to various
acids and toughness. On the other hand, Ni is an expensive metal,
and hence the content of Ni is restricted to not more than 21.0%
from the viewpoint of cost.
[0098] The content of Cr is not less than 22.0% for securing basic
corrosion resistance. On the other hand, if the content of Cr
exceeds 28.0%, then an intermetallic compound will likely be
deposited to deteriorate toughness. For this reason, the content of
Cr is restricted to not less than 22.0% and not more than
28.0%.
[0099] The content of Mo is not less than 1.5% in the present
invention, because Mo is a very effective element which increases
corrosion resistance of stainless steel additionally. On the other
hand, Mo is a very expensive element and which accelerates the
deposition of intermetallic compounds, as well as Cr, and hence the
upper limit of the content of Mo is restricted to not more than
3.5%. The content of Mo preferably ranges from 2.0 to 3.0%.
[0100] N is an effective element which is intercrystallized into an
austenitic phase to increase strength and corrosion resistance. For
this reason, the content of N is not less than 0.15%. Although it
is possible to make N be intercrystallized into the base material
up to 0.4% in the steel material of the present invention, the
upper limit of the content of N is determined as 0.35% in order to
increase sensitivity to generation of bubbling during welding. The
content of N is preferably not more than 0.30%.
[0101] Al is an important element for deoxidation of steel, and
hence the content of Al is not less than 0.005% in order to reduce
oxygen in steel. On the other hand, Al is an element having a
comparatively high chemical affinity with N, and if the content of
Al is excessive, then AlN is generated to deteriorate toughness of
the stainless steel. Although the degree thereof depends on the
content of N, if the content of Al exceeds 0.1%, then toughness
will deteriorate significantly, and hence the upper limit of the
content of Al is determined to be 0. 1%.
[0102] O is an important element which constitutes an oxide which
is a representative nonmetallic inclusion, and excessive addition
of O deteriorates toughness, on the other hand if a coarse
cluster-like oxide generates, then it causes surface cracking. For
this reason, the upper limit of the content of O is determined as
0.007%. The content of O is preferably not more than 0.004%.
[0103] The PI value expressed by the above formula (1): A pitting
index is an index of corrosion resistance of stainless steel to a
chloride environment, and it is necessary to set the PI value to be
not less than 35 at least, in order to acquire the corrosion
resistance corresponding to the purpose. As a stainless steel of
which the PI value exceeds 40, SUS836L etc., is exemplary, however,
such a stainless steel contains Ni in an amount of not less than
24% and hence is very expensive. In the present invention, since
the aim is to provide austenitic stainless steel which has
corrosion resistance corresponding to cost, the upper limit of PI
value is determined to be 40. Note, the value of W in formula (1)
is set to 0 in the present invention which does not contain W.
[0104] The .delta. cal expressed by the above formula (2) is an
index indicating the quantity of the delta ferrite which appears in
the solidified configuration of austenitic stainless steel, and the
.delta. cal is in general controlled to be approximately 0 to 7% in
order to reduce solidification crack sensitivity or to make the
configuration fine. However, as in the stainless steel of the
present invention having a high content of Cr, delta ferrite in the
solidified configuration changes into an intermetallic compound
during the hot-rolling production process and it remains in the
steel material as a by-product, thereby deteriorating toughness.
For this reason, the upper limit of the .delta. cal is restricted
to +4 so that delta ferrite will decrease. If the .delta. cal
exceeds this value, then it becomes impossible to acquire high
toughness even if elaborating a plan in the hot-rolling production
process. On the other hand, if the .delta. cal is shifted to a
smaller (minus) side, then it means that the delta ferrite content
becomes substantially 0%, and as a result the above effect will be
saturated, in addition, the content of Ni becomes excessive, and
hence the lower limit of the .delta. cal is determined to be -6
from the viewpoint of cost. The .delta. cal value preferably ranges
from -3 to +3. Note, the value of W or the value of Cu in formula
(2) is set to 0 in the present invention which does not contain W
or Cu.
[0105] The content of intermetallic compounds contained in the
steel material is an important factor which determines the
toughness of the austenitic stainless steel material in the present
invention. An intermetallic compound is a compound which contains
Cr, Mo, or W as a main ingredient and is called .sigma. phase and
.psi. phase. The content of this compound can be measured by
subjecting a micro configuration to an alkaline electrolytic
etching and then observing the resultant micro configuration
through an optical microscope of approximately 400.times. power.
The inventors of the present invention have found that if this
content as an average value of a stainless steel cross sectional
observation exceeds 0.5%, then the Charpy absorbed energy of the
steel material becomes less than 100 J/cm.sup.2, and as a result,
they determined the upper limit of the content to be 0.5%.
[0106] The reason for restriction of the ninth aspect of the
present invention will be explained.
[0107] Cu is an element which increases the corrosion resistance of
stainless steel additionally against an acid, and the content of Cu
may be not less than 0.1% for this purpose. Even if the content Cu
exceeds 2.0%, the effect corresponding to cost will be saturated,
and hence the upper limit of the content of Cu is set to be
2.0%.
[0108] Ti is an element which forms an oxide, a nitride, and
sulfide with a very small amount thereof, thereby refining the
crystal grain of the steel, and hence Ti is an element which may be
positively utilized in the steel of the present invention. In order
to reduce the intermetallic compound content in the steel material,
it is effective to restrict the upper limit of the .delta. cal
value and to perform homogenizing heat treatment of the
semi-finished products. Among these, in the latter method, although
a heat treatment is performed for several hours at a high
temperature of approximately 1250.degree. C., if Ti of a proper
amount is contained, then the growth of the crystal grain at such a
high temperature can be suppressed. For this purpose, Ti in an
amount of not less than 0.003% needs to be contained. On the other
hand, Ti is an element which has a very high nitride producing
ability, and if the content of Ti exceeds 0.03% in the steel of the
present invention which contains N, then coarse TiN will
deteriorate the toughness of the steel. For this reason, the
content of Ti is determined to be within the range of 0.003 to
0.03%. The content of Ti preferably ranges from 0.005 to 0.02%.
[0109] Nb forms carbide to fix C, so that generation of Cr carbide
is suppressed, thereby increasing corrosion resistance and
toughness. Moreover, Nb forms nitride to suppress growth of crystal
grain, thereby converting the steel material into fine particles to
increase strength. In order to improve corrosion resistance and to
increase strength, not less than 0.02% of Nb can be added. However,
if more than 0.2% of Nb is added, then a large amount of
carbo-nitride of Nb will be deposited during the hot-rolling
processing to deteriorate hot-rolling recrystallization, thereby
maintaining a coarse configuration in the steel material as a
product, and hence the upper limit of the content of Nb is
determined to be 2%. The content of Nb preferably ranges from 0.05
to 0.15%.
[0110] V is an element which generates a carbo-nitride as well as
Nb, and can be added in order to secure corrosion resistance and
toughness. Although not less than 0.05% of V should be contained
for this purpose, if more than 0.5% of V is contained, then coarse
V series carbo-nitride will be generated, so that toughness will
deteriorate conversely. Therefore, the upper limit of the content
of V is restricted to 0.5%. Preferably, the content of V ranges
from 0.1 to 0.3%.
[0111] W is an element which raises the corrosion resistance of
stainless steel additionally as well as Mo, and 0.3 to 3.0% of W
can be contained in the stainless steel of the present invention
for this purpose.
[0112] Furthermore, each of B, Ca, Mg, and REM(s) is an element
which improves the hot-rolling processability, and one or more of
these is added for this purpose. If any of these is added in
excess, then it deteriorates the hot-rolling processability, and
hence the upper limit and the lower limit of content thereof are
determined as follows. The content of B ranges from 0.0003 to
0.0060%, each of the content of Ca and Mg ranges from 0.0005 to
0.0050%, and the content of REM ranges from 0.005 to 0.10%. Here,
REM is defined to be the total of the content of lanthanide series
rare-earth elements such as La, Ce, etc.
[0113] The reason for restriction of the tenth aspect of the
present invention will be explained.
[0114] In order to raise the toughness of steel materials in the
present invention, the amount of intermetallic compound which is
contained in the steel material is restricted to be not more than
0.5%. To achieve this, a chemical composition formula known as
.delta. cal, which forecasts delta ferrite amount contained in a
solidification structure configuration, and homogenizing heat
treatment which is performed on a semi-finished product specified
in this claim are exemplary. When there is no solidifying
segregation in the target steel material of the present invention,
the temperature at which an intermetallic compound is generated is
approximately not higher than 1000.degree. C. However, reduction of
the content of an intermetallic compound in the semi-finished
product accompanied by component segregation by solidification
necessitates a production step for diffusing segregation so as to
be homogenized. Although each of the temperature and the time for
performing homogenizing heat treatment changes slightly, depending
on chemical composition such as solidifying rate, cross-sectional
area of a cast steel, degree of hot-rolling processing upon being
shaped into a semi-finished product, .delta. cal, etc., each of the
temperature and the time for performing homogenizing heat treatment
is limited by diffusion of Cr, Mo, Ni, etc., and hence it
necessitates a temperature of not lower than 1200.degree. C. On the
other hand, if the temperature exceeds 1300.degree. C., then
oxidized scales will be generated extraordinarily.
[0115] Moreover, although it is preferred that the time be as long
as possible, at least 1 hour is needed. Moreover, this purpose can
also be attained by performing soaking at 1200.degree. C. for not
less than 1 hour in heating of the semi-finished product for
rolling the product. Because of the above reason, homogenizing heat
treatment of not less than 1 hour at 1200-1300.degree. C. is
specified. In view of effect and economical efficiency, the soaking
time preferably ranges from 2 to 20 hours.
EXAMPLE 2
[0116] Example 2 will be explained below. The chemical composition
of a sample steel is shown in Table 5. Note, the content of
inevitable impurity elements other than the components indicated in
Table 5 is the same grade as in standard stainless steel. Moreover,
the portion which shows no content of the components shown in Table
5 indicates the same grade as in impurities. In addition, REM in
Table 5 means lanthanide series rare-earth elements, and the
content thereof indicates the total content of each of those
elements.
[0117] Each of these steels was melted in a 50 kg vacuum induction
furnace of a laboratory, and each of them was cast into a flat
steel ingot having a thickness of approximately 100 mm.
TABLE-US-00005 TABLE 5 CONTENT (mass %) STEEL No. KIND C Si Mn P S
Ni Cr Mo Cu Nb Ti 0 EXAMPLE7 0.019 0.51 0.45 0.023 0.0005 18.03
25.01 2.48 1 EXAMPLE8 0.021 0.49 0.48 0.020 0.0007 17.91 25.24 2.50
0.15 2 EXAMPLE8 0.018 0.52 0.52 0.014 0.0012 17.98 25.02 2.46 0.008
3 EXAMPLE9 0.022 0.49 0.52 0.022 0.0008 18.43 24.88 2.46 0.29 0.096
4 EXAMPLE9 0.021 0.48 0.52 0.022 0.0007 17.38 25.36 2.53 0.30 0.034
0.006 5 EXAMPLE9 0.022 0.46 0.52 0.021 0.0008 20.21 24.96 2.46 0.31
0.103 0.015 6 EXAMPLE9 0.022 0.48 0.49 0.022 0.0003 19.23 24.32
3.33 0.28 7 EXAMPLE9 0.021 0.47 0.52 0.022 0.0007 18.33 24.66 2.48
0.32 0.005 8 EXAMPLE9 0.019 0.45 0.53 0.023 0.0008 18.23 24.65 2.46
0.32 0.003 9 EXAMPLE9 0.021 0.48 0.51 0.023 0.0008 18.52 24.03 2.46
0.32 0.006 10 EXAMPLE9 0.019 0.49 0.49 0.022 0.0004 20.42 27.31
1.68 0.31 0.042 0.006 11 EXAMPLE9 0.024 0.49 0.49 0.021 0.0003
19.53 25.61 2.11 1.82 0.012 12 EXAMPLE9 0.019 0.49 0.85 0.019
0.0013 18.89 25.29 2.52 0.32 0.076 0.004 13 EXAMPLE9 0.021 0.50
0.84 0.019 0.0005 16.77 24.66 2.10 0.85 0.006 14 EXAMPLE9 0.021
0.26 1.85 0.022 0.0009 19.53 24.33 3.*45 0.31 0.152 0.022 15
EXAMPLE9 0.018 0.56 0.53 0.021 0.0006 16.95 22.21 3.39 1.66 0.007
21 COMPARATIVE 0.021 0.48 0.53 0.023 0.0008 18.23 24.89 2.46 0.32
0.046 STEEL EXAMPLE 22 COMPARATIVE 0.022 0.47 0.54 0.024 0.0007
18.33 25.00 2.45 0.35 0.284 STEEL EXAMPLE 23 COMPARATIVE 0.023 0.46
0.55 0.023 0.0008 18.25 24.98 2.48 0.32 STEEL EXAMPLE 24
COMPARATIVE 0.024 0.45 0.52 0.024 0.0006 18.22 24.94 2.47 0.31
STEEL EXAMPLE 25 COMPARATIVE 0.022 0.47 0.53 0.025 0.0008 18.21
24.96 2.46 0.33 STEEL EXAMPLE 26 COMPARATIVE 0.021 0.48 0.51 0.024
0.0007 17.01 25.11 2.88 0.31 STEEL EXAMPLE 27 COMPARATIVE 0.022
0.49 0.52 0.023 0.0008 21.40 24.95 2.47 0.33 STEEL EXAMPLE CONTENT
(mass %) STEEL No. KIND V W Al B Ca Mg REM O N .delta. cal PI 0
EXAMPLE7 0.023 0.0027 0.275 -1.1 37.6 1 EXAMPLE8 0.020 0.0046 0.235
1.7 37.3 2 EXAMPLE8 0.018 0.0041 0.266 -0.5 37.4 3 EXAMPLE9 0.06
0.032 0.0023 0.0018 0.240 -1.3 36.8 4 EXAMPLE9 0.026 0.0022 0.0023
0.238 3.2 37.5 5 EXAMPLE9 0.028 0.0025 0.0025 0.242 -5.8 37.0 6
EXAMPLE9 0.15 0.034 0.0035 0.0019 0.257 -3.3 39.4 7 EXAMPLE9 0.35
0.022 0.0025 0.0026 0.235 -0.8 37.2 8 EXAMPLE9 1.05 0.023 0.0008
0.0033 0.0035 0.247 -0.1 38.5 9 EXAMPLE9 2.10 0.024 0.0024 0.0029
0.241 -0.9 39.5 10 EXAMPLE9 0.011 0.0023 0.0023 0.0036 0.245 -1.6
36.8 11 EXAMPLE9 0.28 0.023 0.0020 0.0025 0.173 -0.7 35.3 12
EXAMPLE9 0.020 0.0008 0.050 0.0040 0.235 -0.9 37.4 13 EXAMPLE9 0.07
0.018 0.0042 0.0020 0.315 -3.1 36.6 14 EXAMPLE9 0.020 0.0019 0.0008
0.0032 0.184 -1.1 38.7 15 EXAMPLE9 0.12 0.024 0.0030 0.0032 0.188
-0.2 36.4 21 COMPARATIVE 0.026 0.0025 0.0032 0.241 -0.7 36.9 STEEL
EXAMPLE 22 COMPARATIVE 0.026 0.0026 0.0033 0.242 -0.9 37.0 STEEL
EXAMPLE 23 COMPARATIVE 0.88 0.024 0.0025 0.0031 0.239 -0.6 37.0
STEEL EXAMPLE 24 COMPARATIVE 0.140 0.0024 0.0032 0.240 -0.8 36.9
STEEL EXAMPLE 25 COMPARATIVE 0.002 0.0036 0.0081 0.242 -0.6 37.0
STEEL EXAMPLE 26 COMPARATIVE 0.023 0.0022 0.0029 0.237 4.5 38.4
STEEL EXAMPLE 27 COMPARATIVE 0.026 0.0023 0.0030 0.240 -8.8 36.9
STEEL EXAMPLE : VALUE WITHOUT THE RANGE OF THE PRESENT
INVENTION
[0118] The sample steel was subjected to cogging, homogenizing heat
treatment, and product rolling. In the cogging, the sample steel
was soaked at 1180.degree. C. for two hours, and thereafter the
sample steel was rolled to 65 mm thickness. Then the resultant
steel chips were subjected to homogenizing heat treatment at a
temperature ranging from 1220 to 1280.degree. C. Some of the steel
chips were not subjected to the homogenizing heat treatment. Each
piece of steel was ground to 60 mm to obtain the material for use
in product rolling. In the product rolling, the sample was soaked
at 1220.degree. C. for 1 to 2 hours, and thereafter was rolled
under the condition of a finishing temperature of 850 to
950.degree. C. to obtain a steel sheet having a thickness of 12 mm.
It should be noted that the steel material immediately after being
hot-rolled which was in a temperature state of not less than
800.degree. C. was cooled to a temperature of not higher than
300.degree. C. by performing spray cooling. The final solution heat
treatment was performed under a condition of cooling with water
after performing soaking at 1100.degree. C. for 20 min. Moreover,
some steel sheets were not subjected to the solution heat
treatment.
[0119] The steel plate produced under the above condition was cut
into JIS. No.4 tension test pieces and JIS. No.4 V notch Charpy
test pieces from a direction perpendicular to the direction of
rolling processing. Using the resultant test pieces, 0.2% offset
proof stress and impact strength at -40.degree. C. were measured.
Moreover, test pieces for micro configuration observation were cut
out, and each of the resultant test pieces was planished and
thereafter was subjected to 10% KOH electrolytic etching to reveal
the intermetallic compound therefrom so as to be observed by an
optical microscope, thereby measuring the content. The content was
measured by performing point counting in each of ten fields of view
with 400.times. magnification at a depth of each of 1/4, 1/2, and
3/4 of thickness, and then calculating all the average values, and
the resultant value was determined as the content of the
intermetallic compound of the steel material. The obtained results
are shown in Table 6. TABLE-US-00006 TABLE 6 ROLLING INTERMETALLIC
HOMOGENIZING FINISHING SOLUTION EAR STEEL COMPOUND HEAT TEMPERATURE
HEAT YS TS vE-40.degree. C. CRACK NO. KIND CONTENT (%) TREATMENT
(.degree. C.) TREATMENT (MPa) (MPa) (J/cm.sup.2) (mm) 0 EXAMPLE
0.02 1250.degree. C. .times. 4 h 950 DONE 392 782 540 6 0 EXAMPLE
0.05 1250.degree. C. .times. 4 h 850 DONE 396 793 491 9 0 EXAMPLE
0.17 1250.degree. C. .times. 4 h 950 UNDONE 747 965 195 6 0
COMPARATIVE 0.75 UNDONE 850 UNDONE 892 1054 68 12 EXAMPLE 1 EXAMPLE
0.05 1250.degree. C. .times. 4 h 950 DONE 340 751 503 5 1 EXAMPLE
0.10 1250.degree. C. .times. 4 h 850 DONE 344 762 452 8 1 EXAMPLE
0.20 1250.degree. C. .times. 4 h 950 UNDONE 722 943 183 5 1
COMPARATIVE 1.2 UNDONE 850 UNDONE 841 1020 35 10 EXAMPLE 2 EXAMPLE
0.03 1250.degree. C. .times. 4 h 950 DONE 352 766 542 5 2 EXAMPLE
0.06 1250.degree. C. .times. 4 h 850 DONE 357 771 482 7 2 EXAMPLE
0.18 1250.degree. C. .times. 4 h 950 UNDONE 736 954 163 5 2
COMPARATIVE 0.8 UNDONE 850 UNDONE 882 1065 62 10 EXAMPLE 3 EXAMPLE
0.02 1250.degree. C. .times. 4 h 950 DONE 388 779 558 0 3 EXAMPLE
0.05 1250.degree. C. .times. 4 h 850 DONE 394 782 509 0 3 EXAMPLE
0.15 1250.degree. C. .times. 4 h 950 UNDONE 732 947 214 0 3
COMPARATIVE 0.7 UNDONE 850 UNDONE 865 1033 70 0 EXAMPLE 3
COMPARATIVE 0.55 UNDONE 900 UNDONE 812 982 93 0 EXAMPLE 3 EXAMPLE
0.42 UNDONE 950 UNDONE 742 949 116 0 4 EXAMPLE 0.45 1220.degree. C.
.times. 1 h 950 UNDONE 356 768 105 0 4 EXAMPLE 0.36 1250.degree. C.
.times. 2 h 950 UNDONE 356 766 114 0 4 EXAMPLE 0.23 1250.degree. C.
.times. 4 h 950 UNDONE 358 765 135 0 4 EXAMPLE 0.15 1250.degree. C.
.times. 20 h 950 UNDONE 352 764 167 0 4 EXAMPLE 0.22 1280.degree.
C. .times. 2 h 950 UNDONE 347 762 133 0 5 EXAMPLE 0.02 1250.degree.
C. .times. 4 h 850 DONE 393 785 564 0 5 EXAMPLE 0.08 1250.degree.
C. .times. 4 h 950 UNDONE 745 960 265 0 6 EXAMPLE 0.05 1250.degree.
C. .times. 4 h 850 DONE 352 753 508 0 6 EXAMPLE 0.12 1250.degree.
C. .times. 4 h 950 UNDONE 748 962 213 0 7 EXAMPLE 0.04 1250.degree.
C. .times. 4 h 850 DONE 348 753 526 0 7 EXAMPLE 0.16 1250.degree.
C. .times. 4 h 950 UNDONE 738 958 216 0 8 EXAMPLE 0.06 1250.degree.
C. .times. 4 h 850 DONE 362 772 492 0 8 EXAMPLE 0.19 1250.degree.
C. .times. 4 h 950 UNDONE 771 982 165 0 9 EXAMPLE 0.08 1250.degree.
C. .times. 4 h 850 DONE 388 795 421 0 9 EXAMPLE 0.32 1250.degree.
C. .times. 4 h 950 UNDONE 788 994 132 0 10 EXAMPLE 0.03
1250.degree. C. .times. 4 h 850 DONE 352 765 502 0 10 EXAMPLE 0.10
1250.degree. C. .times. 4 h 950 UNDONE 740 936 165 0 11 EXAMPLE
0.05 1250.degree. C. .times. 4 h 850 DONE 324 711 513 0 11 EXAMPLE
0.17 1250.degree. C. .times. 4 h 950 UNDONE 735 925 164 0 12
EXAMPLE 0.05 1250.degree. C. .times. 4 h 850 DONE 362 776 501 0 12
EXAMPLE 0.18 1250.degree. C. .times. 4 h 950 UNDONE 762 975 168 0
13 EXAMPLE 0.05 1250.degree. C. .times. 4 h 850 DONE 375 783 523 0
13 EXAMPLE 0.17 1250.degree. C. .times. 4 h 950 UNDONE 775 983 185
0 14 EXAMPLE 0.11 1250.degree. C. .times. 4 h 850 DONE 342 741 481
0 14 EXAMPLE 0.32 1250.degree. C. .times. 4 h 950 UNDONE 726 932
135 0 15 EXAMPLE 0.09 1250.degree. C. .times. 4 h 850 DONE 348 738
475 0 15 EXAMPLE 0.28 1250.degree. C. .times. 4 h 950 UNDONE 749
974 140 0 21 COMPARATIVE 0.25 1250.degree. C. .times. 4 h 950
UNDONE 721 926 85 0 EXAMPLE 22 COMPARATIVE 0.26 1250.degree. C.
.times. 4 h 950 UNDONE 765 904 76 0 EXAMPLE 23 COMPARATIVE 0.24
1250.degree. C. .times. 4 h 950 UNDONE 754 967 83 0 EXAMPLE 24
COMPARATIVE 0.28 1250.degree. C. .times. 4 h 950 UNDONE 735 954 92
0 EXAMPLE 25 COMPARATIVE 0.26 1250.degree. C. .times. 4 h 950
UNDONE 713 941 78 0 EXAMPLE 26 COMPARATIVE 0.94 1250.degree. C.
.times. 4 h 950 UNDONE 735 943 45 0 EXAMPLE 27 COMPARATIVE 0.06
1250.degree. C. .times. 4 h 950 UNDONE 742 926 198 0 EXAMPLE :
VALUE WITHOUT THE RANGE OF THE PRESENT INVENTION
[0120] The hot-rolling processability was evaluated relatively by
judging the generation of an ear crack during the product rolling.
It was confirmed that the steel material corresponding to Example 9
(steel Nos. 3 to 15) developed no ear cracks and exhibited
excellent hot-rolling processability, On the other hand, it was
confirmed that each of the steel materials corresponding to each
Example other than Examples 1 and 2 developed ear cracks of
approximately 5 to 12 mm per one side, so that the yield was
decreased slightly. The lengths of ear cracks are shown in Table 6.
That is, although there is a slight problem in the hot-rolling
processability of steel Nos. 0 to 2, in the thick steel which was
produced to have the content of an intermetallic compound of not
more than 0.5%, each Charpy impact value at -40.degree. C. exceeds
100 J/cm.sup.2. As to the steel Nos. 3 to 15, which are those in
which Al, B, Ca, Mg, REM are contained in order to improve
hot-rolling processability, no ear cracks occurred. Moreover, in
Examples of the present invention produced so as to have the
content of an intermetallic compound of not more than 0.5%, each
Charpy impact value at -40.degree. C. exceeds 100 J/cm.sup.2.
[0121] Next, in each of the comparative examples of steel Nos. 21
to 27, the content of Ti is less than 0.03%, the content of Nb is
more than 0.2%, the content of V is more than 0.5%, the content of
Al is more than 0. 1%, the content of O is more than 0.007%, the
content of .delta.Fe is more than 3%, and the content of Ni is more
than 21% (.delta.Fe<-6%), i.e. each is out of the scope of the
present invention, and the comparative examples other than No. 27
have poor impact property. Although the comparative example of
steel No.27 excels in impact property, it has a high content of Ni
and hence deviates from the purpose of the present invention.
[0122] As is clear from the results shown in Tables 5 and 6, each
of the steel materials which satisfy the steel composition and
intermetallic compound content within the scope of the present
invention has a PI value, which is an index of corrosion
resistance, of not less than 35, and exhibits high strength and a
Charpy impact value of not less than 100 J/cm.sup.2.
[0123] As can be seen from the above examples, it is clarified that
the steel material of the present invention is an austenitic
stainless steel material which excels in corrosion resistance,
toughness, and hot-rolling processability.
[0124] As mentioned above, although preferred embodiments of the
present invention are explained, the present invention is not
limited thereto. Additions, abbreviations, substitutions, and other
changes are possible, as long as do not deviate from the spirit of
the present invention. The scope of the present invention is not
limited by the above explanation and is limited by only the scope
of attached claims.
[0125] The present invention realizes an austenitic stainless steel
suitable for the hull structures of ships, having excellent
performance required for structural members of high-speed ships,
such as sea water resistance, proof stress, and low-temperature
toughness at a high level, and hence the contributions of the
present invention to industry are significant.
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