U.S. patent application number 12/452918 was filed with the patent office on 2010-05-27 for ferritic-austenitic stainless steel excellent in corrosion resistance and workability andmethod of production of same.
Invention is credited to Masaharu Hatano, Eiichiro Ishimaru, Ken Kimura, Akihiko Takahashi.
Application Number | 20100126644 12/452918 |
Document ID | / |
Family ID | 40304477 |
Filed Date | 2010-05-27 |
United States Patent
Application |
20100126644 |
Kind Code |
A1 |
Hatano; Masaharu ; et
al. |
May 27, 2010 |
FERRITIC-AUSTENITIC STAINLESS STEEL EXCELLENT IN CORROSION
RESISTANCE AND WORKABILITY ANDMETHOD OF PRODUCTION OF SAME
Abstract
The present invention relates to ferritic-austenitic stainless
steel oriented to have low Ni which is excellent in corrosion
resistance, particularly in corrosion resistance in a neutral
chloride environment, and has high "uniform elongation"--a factor
governing workability--and a method of production for the same.
There are independently provided ferritic-austenitic stainless
steels and methods of production for the same particularly having a
corrosion resistance in a neutral chloride environment satisfying
PI value(=Cr+3Mo+10N--Mn).gtoreq.18% and having a uniform
elongation satisfying -10.ltoreq.Md.ltoreq.110 (where
Md=551-462({C}+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29[Ni]-29[Cu]-18.5[Mo],
where [ ] is composition (mass %) in the austenite phase, and { }
is average composition (mass %))
Inventors: |
Hatano; Masaharu; (Tokyo,
JP) ; Takahashi; Akihiko; (Tokyo, JP) ;
Ishimaru; Eiichiro; (Tokyo, JP) ; Kimura; Ken;
(Tokyo, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
40304477 |
Appl. No.: |
12/452918 |
Filed: |
August 1, 2008 |
PCT Filed: |
August 1, 2008 |
PCT NO: |
PCT/JP2008/064260 |
371 Date: |
January 27, 2010 |
Current U.S.
Class: |
148/610 ;
148/320; 148/325; 148/332; 148/653 |
Current CPC
Class: |
C21D 8/0236 20130101;
C22C 38/42 20130101; C21D 2211/001 20130101; C21D 6/005 20130101;
C21D 8/0226 20130101; C21D 8/0263 20130101; C21D 6/004 20130101;
C21D 2211/005 20130101; C22C 38/18 20130101; C22C 38/001 20130101;
C22C 38/58 20130101 |
Class at
Publication: |
148/610 ;
148/325; 148/320; 148/332; 148/653 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/18 20060101 C22C038/18; C22C 38/38 20060101
C22C038/38; C22C 38/02 20060101 C22C038/02; C22C 38/42 20060101
C22C038/42; C22C 38/58 20060101 C22C038/58 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 2, 2007 |
JP |
2007 202016 |
Aug 29, 2007 |
JP |
2007 222259 |
Claims
1. Ferritic-austenitic stainless steel excellent in corrosion
resistance and workability characterized by containing, by mass %,
C: 0.001 to 0.1%, Cr: 17 to 25%, Si: 0.01 to 1%, Mn: 0.5 to 3.7%,
and N: 0.06% to less than 0.15%, having a pitting indicator (PI
value) shown by the following formula (a) of over 18%, having a
balance of Fe and unavoidable impurities, and having a ferrite
phase as the matrix phase and having a volume fraction of the
austenite phase of 15 to 50%. Pitting indicator(PI
value)=Cr+3Mo+10N--Mn (a)
2. Ferritic-austenitic stainless steel excellent in corrosion
resistance and workability as set forth in claim 1, characterized
by further containing, by mass %, both Ni: 0.6 to 3% and Cu: 0.1 to
3%.
3. Ferritic-austenitic stainless steel excellent in corrosion
resistance and workability as set forth in claim 1, characterized
by further containing, by mass %, one or more of Mo: 1% or less,
Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.1% or less, B: 0.01% or
less, Ca: 0.01% or less, and Mg: 0.01% or less.
4. Ferritic-austenitic stainless steel excellent in corrosion
resistance and workability as set forth in claim 1, characterized
by having a pitting potential Vc'100 in a 30.degree. C., 3.5% NaCl
aqueous solution of 0.3V (Vv.s.AGCL) or more.
5. A method of production of ferritic-austenitic stainless steel
excellent in corrosion resistance and workability comprising hot
forging or hot rolling a stainless steel ingot having steel
ingredients as set forth in claim 1 to obtain a hot rolled steel
material, annealing the hot rolled steel material, then repeating
cold working and annealing, said method of production of a steel
material characterized by performing the final annealing by heating
and holding the material at 950 to 1150.degree. C., making an
average cooling rate from the heating temperature to 200.degree. C.
3.degree. C./sec or more, and making a ferrite phase the matrix
phase and making a volume fraction of the austenite phase 15 to
50%.
6. A method of production of ferritic-austenitic stainless steel
excellent in corrosion resistance and workability comprising hot
forging or hot rolling a stainless steel ingot having steel
ingredients as set forth in claim 1 to obtain a hot rolled steel
material, annealing the hot rolled steel material, then repeating
cold working and annealing, said method of production of a steel
material characterized by performing the final annealing by heating
and holding the material at 950 to 1150.degree. C., then making an
average cooling rate until 600.degree. C. 3.degree. C./sec or more,
holding the material at a 200 to 600.degree. C. temperature region
for 1 minute or more, then making the average cooling rate from the
holding temperature to room temperature 3.degree. C./sec or more,
and making a ferrite phase the matrix phase and making a volume
fraction of the austenite phase 15 to 50%.
7. A method of production of ferritic-austenitic stainless steel
excellent in corrosion resistance and workability as set forth in
claim 5, characterized by making the ferrite phase the matrix phase
and making a volume fraction of the austenite phase 15 to 50% and
by making a pitting potential Vc'100 in a 30.degree. C., 3.5% NaCl
aqueous solution 0.3V (Vv.s.AGCL) or more.
8. Ferritic-austenitic stainless steel excellent in workability
characterized by having a volume fraction of an austenite phase of
10% to less than 50%, having an Md value calculated from the
chemical composition in the austenite phase that satisfies the
following formula (b), having a ratio of austenite grains in a
cross-section vertical to a rolling transverse direction with a
grain size of 15 .mu.m or less and a shape aspect ratio of less
than 3 accounting for 90% or more of the total number of austenite
grains, and further having at the same cross-section an average
distance between nearest austenite grains of 12 .mu.m or less:
-10.ltoreq.Md.ltoreq.110 (b) (where,
Md=551-462({C}+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29[Ni]-29[Cu]-18.5
[Mo], [ ] is the composition (mass %) in the austenite phase, and {
} is the average composition (mass %))
9. Ferritic-austenitic stainless steel excellent in workability as
set forth in claim 8 characterized by further containing, by mass
%, C: 0.002 to 0.1%, Si: 0.05 to 2%, Mn: 0.05 to 5%, P: less than
0.05%, S: less than 0.01%, Cr: 17 to 25%, and N: 0.01 to 0.15% and
having a balance of iron and unavoidable impurities.
10. Ferritic-austenitic stainless steel excellent in workability as
set forth in claim 8 characterized by further containing, by mass
%, one or more of Ni: 5% or less, Cu: 5% or less, and Mo: 5% or
less.
11. Ferritic-austenitic stainless steel excellent in workability as
set forth in claim 8 characterized by further containing, by mass
%, one or both of Nb: 0.5% or less and Ti: 0.5% or less.
12. Ferritic-austenitic stainless steel excellent in workability as
set forth in claim 8 characterized by further containing, by mass
%, one or both of Ca: 0.003% or less and Mg: 0.003% or less.
13. A method of production of ferritic-austenitic stainless steel
excellent in workability characterized by continuously casting
steel of ingredients as set forth in claim 8, heating the obtained
steel slab before hot rolling at a heating temperature T1 (.degree.
C.) of 1150.degree. C. to less than 1250.degree. C., then rolling
at 1000.degree. C. or more with reduction of a 30% or higher
reduction rate then holding for 30 sec or more for one pass or more
so as to obtain a hot rolled plate with a total rolling rate of hot
rolling of 96% or more, annealing this at a temperature of
T1-100.degree. C. to T1.degree. C., suitably thereafter cold
rolling, performing process annealing or not performing it, then
performing final annealing at 1000.degree. C. to 1100.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to ferritic-austenitic
stainless steel excellent in corrosion resistance and workability
and a method of production of the same. According to the present
invention, it is possible to produce ferritic-austenitic stainless
steel excellent in corrosion resistance and workability without
including a large amount of the expensive and rare element of Ni,
so it is believed this can contribute to resource conservation and
environmental protection.
BACKGROUND ART
[0002] Stainless steel may be broadly divided into austenitic
stainless steel, ferritic stainless steel, and two-phase
(ferritic-austenitic) stainless steel. Austenitic stainless steel
contains expensive Ni in 7% mass or more. Many types are excellent
in workability. Ferritic stainless steel contains almost no Ni and
generally has considerably lower workability than austenitic
stainless steel. On the other hand, two-phase (ferritic-austenitic)
stainless steel has a comparatively small Ni content. It is
considered that many types have an intermediate position in
workability, corrosion resistance, etc. between austenitic
stainless steel and ferritic stainless steel.
[0003] In recent years, improvements in refining technology have
enabled a shift to ultra low carbon/nitrogen content. Due to the
addition of Ti, Nb, and other stabilizing elements, corrosion
resistance and workability are being improved as well in ferritic
stainless steel. This is therefore being used in a broader range of
fields. A large factor in this is that ferritic stainless steel is
economically superior to austenitic stainless steel, which contains
a large amount of Ni. However, ferritic stainless steel is very
inferior to austenitic stainless steel with respect to workability,
particularly in material elongation and uniform elongation.
[0004] Therefore, austenitic-ferritic stainless steel, which is
positioned between austenitic and ferritic steel, has come under
the spotlight in recent years.
[0005] Conventionally, austenitic-ferritic stainless steel such as
SUS 329J4L contains over 5% of Ni. Further, it contains several
percent of Mo--which is rarer and more expensive than Ni.
Therefore, there is still a problem in terms of spread of use and
economy.
[0006] To deal with this problem, austenitic-ferritic stainless
steel using Mo as an optional additive element and limiting the
amount of Ni to more than 0.1% and less than 1% has been disclosed
in Japanese Patent Publication (A) No. 11-071643 or to 0.5% to 1.7%
has been disclosed in WO/02/27056. These austenitic-ferritic
stainless steels are directed at lower Ni content, so contain N in
an amount exceeding 0.1% and Mn in an amount substantially
exceeding 3.7%.
[0007] Japanese Patent Publication (A) No. 2006-169622 and Japanese
Patent Publication (A) No. 2006-183129 disclose austenitic-ferritic
stainless steels aimed at improvement of total elongation and deep
drawability by limiting the Ni amount to substantially 3% or less
and adjusting the austenite phase C+N and ingredient balance.
Further, Japanese Patent Publication (A) No. 10-219407 discloses
ferritic stainless steel excellent in ductility having an amount of
N of substantially less than 0.06, having a ferrite phase as the
matrix phase, and having a residual austenite phase of less than
20%.
[0008] If seen from the viewpoint of workability, these patent
citations include findings for improving the ductility of
ferritic-austenitic stainless steel, but these are all techniques
for improving the tensile elongation at break. Elongation at break
comprises uniform elongation and local elongation, so there
sometimes increasing the local elongation may result in the
elongation at break increasing. However, if the uniform elongation
does not increase, the actual workability will not improve. In the
above art, there is no mention at all of a technique for improving
the uniform elongation which is so important in actual
workability.
[0009] For example, Japanese Patent Publication (A) No. 10-219407
describes technology that uses stainless steel having a ferrite
phase as its principal constituent phase and containing a residual
austenite phase to improve the tensile elongation at break by the
TRIP phenomenon. Japanese Patent Publication (A) No. 11-071643
describes a method defining the stability of an austenite phase to
improve the tensile elongation. Japanese Patent Publication (A) No.
2006-169622 discloses the art of defining the percentage of the
austenite phase and amounts of C and N in the austenite phase to
improve the total elongation in a tensile test.
[0010] However, in Japanese Patent Publication (A) No. 10-219407,
as shown in the examples, the values of the tensile elongation at
break are 34 to 42%, that is, the elongation at break is not
necessarily high. Further, even if steel sheet or plate does not
break and "fracture" in actual shaping, it is often judged
unworkable when necking occurs. That is, rather than the
"elongation at break" in a tensile test, the uniform deformation
limit, that is, the "uniform elongation", determines the
workability, but what extent of level the uniform elongation is at
is unknown. Japanese Patent Publication 11-071643 describes a
tensile elongation at break of up to a 46% maximum. Further,
Japanese Patent Publication (A) No. 2006-169622 describes an
elongation at break of up to a 71% maximum in the examples.
However, even these citations do not describe at all the uniform
elongation which governs the actual workability.
[0011] Next, let's look at this from the viewpoint of the corrosion
resistance. Japanese Patent Publication (A) No. 2006-200035 and
Japanese Patent Publication (A) No. 2006-233308 disclose
improvements in the crevice corrosion resistance and intergranular
corrosion resistance of austenitic-ferritic stainless steels
similar to Japanese Patent Publication (A) No. 2006-169622 and
Japanese
[0012] Patent Publication (A) No. 2006-183129. Japanese Patent
Publication (A) No. 2006-200035 describes suppression of crevice
corrosion in a coastal environment exposure test for an
austenitic-ferritic stainless steel in which an amount of Mn is
limited to less than 2% and an amount of Ni of over 0.5% is
added.
[0013] On the other hand, Japanese Patent Publication (A) No.
2006-233308 describes an austenitic-ferritic stainless steel
containing Mn in over 2% and less than 4% and having an amount of
Ni of substantially less than 0.6% in which grain boundary cracking
after boiling in a sulfuric acid/cupric sulfate solution is
suppressed.
[0014] Japanese Patent Publication (A) No. 5-247594 discloses
two-phase stainless steel with improved resistance to weather under
near coastal conditions. This two-phase stainless steel contains an
amount of Mn substantially over 4% or an amount of Mn less than 4%
and an amount of Ni over 3%.
[0015] None of the above publications have descriptions suggesting
in any way corrosion resistance in a neutral chloride environment
such as the pitting potential most generally used at. In other
words, it can be said that the ingredients of ferritic-austenitic
stainless steel oriented at lower Ni which is provided with a
corrosion resistance in a neutral chloride environment equal to or
greater than SUS 304 and which is excellent in workability and the
method of production of the same have not yet been clarified.
DISCLOSURE OF THE INVENTION
[0016] The present invention was made in view of the current state
of the prior art and has as its object to provide
ferritic-austenitic stainless steel oriented at lower Ni which is
excellent in corrosion resistance, particularly corrosion
resistance in a neutral chloride environment, and which has a high
"uniform elongation", the factor governing workability, and a
method of production of the same.
[0017] The inventors engaged in intensive research to solve the
above problems and as a result discovered that by defining the
ingredients and metal structure of the steel, particularly the
balance of the ferrite phase and austenite phase, and controlling
the annealing conditions and other production conditions,
ferritic-austenitic stainless steel which has a corrosion
resistance in a neutral chloride environment equal to or greater
than SUS 304, which has excellent uniform elongation, and which is
excellent in corrosion resistance and workability can be obtained
and thereby completed the present invention.
[0018] The gist of the invention is as follows:
[0019] (1) Ferritic-austenitic stainless steel excellent in
corrosion resistance and workability characterized by containing,
by mass %,
C: 0.001 to 0.1%,
Cr: 17 to 25%,
Si: 0.01 to 1%,
Mn: 0.5 to 3.7%, and
[0020] N: 0.06% to less than 0.15%, having a pitting indicator (PI
value) shown by the following formula (a) of over 18%, having a
balance of Fe and unavoidable impurities, and having a ferrite
phase as the matrix phase and having a volume fraction of the
austenite phase of 15 to 50%.
Pitting indicator(PI value)=Cr+3Mo+10N--Mn (a)
[0021] (2) Ferritic-austenitic stainless steel excellent in
corrosion resistance and workability as set forth in (1),
characterized by further containing, by mass %, both Ni: 0.6 to 3%
and Cu: 0.1 to 3%.
[0022] (3) Ferritic-austenitic stainless steel excellent in
corrosion resistance and workability as set forth in (1) or (2),
characterized by further containing, by mass %, one or more of
Mo: 1% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.1% or
less, B: 0.01% or less, Ca: 0.01% or less, and Mg: 0.01% or
less
[0023] (4) Ferritic-austenitic stainless steel excellent in
corrosion resistance and workability as set forth in any of (1) to
(3), characterized by having a pitting potential Vc'100 in a
30.degree. C., 3.5% NaCl aqueous solution of 0.3V (Vv.s.AGCL) or
more.
[0024] (5) A method of production of ferritic-austenitic stainless
steel excellent in corrosion resistance and workability comprising
hot forging or hot rolling a stainless steel ingot having steel
ingredients as set forth in any of (1) to (3) to obtain a hot
rolled steel material, annealing the hot rolled steel material,
then repeating cold working and annealing, said method of
production of a steel material characterized by performing the
final annealing by heating and holding the material at 950 to
1150.degree. C., making an average cooling rate from the heating
temperature to 200.degree. C. 3.degree. C./sec or more, and making
the ferrite phase a matrix phase and making a volume fraction of
the austenite phase 15 to 50%.
[0025] (6) A method of production of ferritic-austenitic stainless
steel excellent in corrosion resistance and workability comprising
hot forging or hot rolling a stainless steel ingot having steel
ingredients as set forth in any of (1) to (3) to obtain a hot
rolled steel material, annealing the hot rolled steel material,
then repeating cold working and annealing, said method of
production of a steel material characterized by performing the
final annealing by heating and holding the material at 950 to
1150.degree. C., then making an average cooling rate until
600.degree. C. 3.degree. C./sec or more, holding the material at a
200 to 600.degree. C. temperature region for 1 minute or more, then
making the average cooling rate from the holding temperature to
room temperature 3.degree. C./sec or more, and making a ferrite
phase the matrix phase and making a volume fraction of the
austenite phase 15 to 50%.
[0026] (7) A method of production of ferritic-austenitic stainless
steel excellent in corrosion resistance and workability as set
forth in (5) or (6), characterized by making a ferrite phase the
matrix phase and making a volume fraction of the austenite phase 15
to 50% and by making a pitting potential Vc'100 in a 30.degree. C.,
3.5% NaCl aqueous solution 0.3V (Vv.s.AGCL) or more.
[0027] (8) Ferritic-austenitic stainless steel excellent in
workability characterized by having a volume fraction of an
austenite phase of 10% to less than 50%, having an Md value
calculated from the chemical composition in the austenite phase
that satisfies the following formula (b), having a ratio of
austenite grains in a cross-section vertical to a rolling
transverse direction with a grain size of 15 .mu.m or less and a
shape aspect ratio of less than 3 accounting for 90% or more of the
total number of austenite grains, and further having at the same
cross-section an average distance between nearest austenite grains
of 12 .mu.m or less:
-10.ltoreq.Md.ltoreq.110 (b)
(where,
Md=551-462({C}+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29[Ni]-29[Cu]-18.5[M-
o], [ ] is the composition (mass %) in the austenite phase, and { }
is the average composition (mass %))
[0028] (9) Ferritic-austenitic stainless steel excellent in
workability as set forth in (8) characterized by further
containing, by mass %,
C: 0.002 to 0.1%,
Si: 0.05 to 2%,
Mn: 0.05 to 5%,
[0029] P: less than 0.05%, S: less than 0.01%,
Cr: 17 to 25%, and
N: 0.01 to 0.15% and
[0030] having a balance of iron and unavoidable impurities.
[0031] (10) Ferritic-austenitic stainless steel excellent in
workability as set forth in (8) or (9) characterized by further
containing, by mass %, one or more of
Ni: 5% or less, Cu: 5% or less, and Mo: 5% or less.
[0032] (11) Ferritic-austenitic stainless steel excellent in
workability as set forth in any of (8) to (10) characterized by
further containing, by mass %, one or both of
Nb: 0.5% or less and Ti: 0.5% or less.
[0033] (12) Ferritic-austenitic stainless steel excellent in
workability as set forth in any of (8) to (11) characterized by
further containing, by mass %, one or both of
Ca: 0.003% or less and Mg: 0.003% or less.
[0034] (13) A method of production of ferritic-austenitic stainless
steel excellent in workability characterized by continuously
casting steel of ingredients as set forth in any of (8) to (12),
heating the obtained steel slab before hot rolling at a heating
temperature T1 (.degree. C.) of 1150.degree. C. to less than
1250.degree. C., then rolling at 1000.degree. C. or more with
reduction of a 30% or higher reduction rate then holding for 30 sec
or more for one pass or more so as to obtain a hot rolled plate
with a total rolling rate of hot rolling of 96% or more, annealing
this at a temperature of T1-100.degree. C. to T1.degree. C.,
suitably thereafter cold rolling, performing process annealing or
not performing it, then performing final annealing at 1000.degree.
C. to 1100.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a view showing the relationship of the cooling
rate of final annealing and the pitting potential of Steel No.
1.
[0036] FIG. 2 is a view showing the EBSP measurement results
classified into a BCC phase and FCC phase, where (a) shows the BCC
phase, and (b) the FCC source by a white color.
[0037] FIG. 3 is a view showing the relationship of a .gamma.
fraction and uniform elongation (u-EL).
[0038] FIG. 4 is a view showing the relationship of an Md value and
uniform elongation (u-EL).
[0039] FIG. 5 is a view showing the relationship of the ratio (X1)
of austenite grains with a grain size of 15 .mu.m or less and a
shape aspect ratio of less than 3 in all austenite grains and
uniform elongation (u-EL).
[0040] FIG. 6 is a view showing the relationship of the average
value (X2) of distance of each austenite grain to the nearest grain
and uniform elongation (u-EL).
BEST MODE FOR CARRYING OUT THE INVENTION
[0041] The inventors engaged in research on the effects of the
ingredients, phase balance, and final annealing condition on the
corrosion resistance and workability of ferritic-austenitic
stainless steel oriented toward lower Ni.
[0042] As a result, first, as a first finding, they discovered
that, from the viewpoint of corrosion resistance, it is possible to
obtain ferritic-austenitic stainless steel excellent in corrosion
resistance and workability provided with a corrosion resistance in
a neutral chloride environment equal to or greater than SUS 304,
that is, of a pitting potential Vc'100 in a 30.degree. C., 3.5%
NaCl aqueous solution of 0.3V (Vv.s.AGCL) or more, and provided
with an excellent material elongation, particularly an excellent
uniform elongation of a uniform elongation in a tensile test of 30%
or more.
[0043] Further, as a second finding, they discovered that, from the
viewpoint of workability, it is possible to obtain
ferritic-austenitic stainless steel excellent in corrosion
resistance and workability by making the conditions which the
austenite grains should be provided with: (1) a grain size which is
small and a shape which is close to spherical (is not flattened in
the rolling direction), (2) a distance between nearest austenite
grains which is narrow, and, further, (3) an austenite stability
(Md value) calculated from the chemical composition in the
austenite phase which is within an appropriate range, and thereby
completed the present invention.
[0044] First, the inventors will explain the first findings.
[0045] The inventors engaged in intensive research on the effects
of the ingredients and phase balance on the corrosion resistance
and the workability of ferritic-austenitic stainless steel oriented
to lower Ni and the effects of the final annealing conditions on
the corrosion resistance and thereby completed the present
invention. Below, they will explain representative test
results.
[0046] The inventors hot rolled a stainless steel ingot obtained by
vacuum melting ferritic-austenitic stainless steel having the
ingredients shown in Table 1 to produce 5 mm thick hot rolled
plate. They annealed the hot rolled plate at 1000.degree. C.,
pickled it, then cold rolled it to prepare 1 mm thick cold rolled
sheet. They annealed the cold rolled sheet at 1000.degree. C., then
cooled it by forced air cooling from 1000.degree. C. to 200.degree.
C. at an average cooling rate in a range of 35 to 40.degree.
C./secs. The cold rolled-annealed sheet was used for measurement of
the volume fraction of the austenite (.gamma.) phase, measurement
of the pitting potential, and a JIS 13B tensile test. As
comparative materials, 1 mm thick SUS 304 and ultralow C and N SUS
430LX were used. Note that, the pitting indicator (PI value) of
steel containing a comparatively large amount of Mn was calculated
from Cr+3Mo+10N--Mn(%).
[0047] The volume fraction of the .gamma. phase (hereinafter
described as the ".gamma. fraction") was found by measurement of a
phase map that identifies the crystal structure of fcc and bcc by
using the EBSP method at the sheet cross-section. For the pitting
potential, the Vc'100 (Vv.s.AGCL) was measured on a #500 polished
surface in a 30.degree. C., 3.5% NaCl aqueous solution. The
measurement value of the pitting potential was made the average of
n3. For the JIS 13B tensile test, a tensile test piece was taken
from the rolling direction, and the uniform elongation until
necking occurred was measured at a tension rate of 20 mm/min (range
of tension rate defined in JIS Z 2241).
[0048] Table 1 shows, in addition to the steel ingredients, the
measurement results of the .gamma. fraction, Vc'100, and uniform
elongation. As clear from Table 1, Steel No. 1 has a pitting
potential of 0.38V, a uniform elongation of 35%, and corrosion
resistance in a neutral chloride environment equal to or greater
than SUS 304. Compared with SUS 430LX with workability improved
through a shift to ultra low C and N, the uniform elongation was
greatly improved.
[0049] On the other hand, Steel Nos. 2 to 6 have uniform
elongations sufficiently higher than SUS 430LX, but have pitting
potentials equal to or less than SUS 430LX or greatly inferior
compared to SUS 304. The ingredients of steels with inferior
pitting potentials are characterized by having (i) an amount of Si
which is a high over 1% (Steel No. 2), (ii) an amount of Mn which
is a high 3.8% (Steel No. 3), (iii) an amount of N which is a high
0.15% (Steel No. 4), (iv) a pitting indicator (PI value) which is
less than 18% (Steel No. 5), and (v) an amount of N which is a high
0.16% and a .gamma. fraction which is over 50% (Steel No. 6).
TABLE-US-00001 TABLE 1 Steel Chemical ingredients (mass %) .gamma.
Vc' Uniform No. C Si Mn Cr N Ni Cu Pi fraction 100 elongation 1
0.03 0.1 3.2 21.2 0.09 1.0 0.4 19.0 30% 0.38 35% 2 0.03 1.1 3.2
21.3 0.10 1.0 0.5 19.2 25% 0.16 31% 3 0.03 0.1 3.8 22.0 0.12 0.9
0.6 19.4 35% 0.22 36% 4 0.02 0.1 3.1 21.0 0.15 0.8 0.4 19.5 35%
0.18 37% 5 0.03 0.1 3.2 20.0 0.09 1.0 0.5 17.7 20% 0.23 30% 6 0.02
0.1 3.0 21.2 0.16 1.6 0.4 19.8 55% 0.12 40% SUS 304 0.05 0.5 1.0
18.1 0.03 8.3 0.2 17.4 100% 0.33 42% SUS 430LX 0.003 0.1 0.1 16.3
0.01 -- -- 16.3 0% 0.20 19% Vv.s.AGCL --: No addition SUS430LX:
0.3% Ti content
[0050] FIG. 1 shows the relationship of the cooling rate in the
final annealing and the pitting potential of the Steel No. 1. To
obtain a pitting potential equal to or greater than SUS 304 (0.3V
or more), it is necessary to restrict the cooling rate to 3.degree.
C./sec or more. Further, as shown by the black dot in the drawing,
steel cooled by the method of holding at 500.degree. C. for 1
minute has the feature of having a pitting potential higher than
when continuously cooling by a cooling rate of 5.degree. C./sec
without holding.
[0051] To explain the test results, the inventors analyzed the
structure in detail using an optical microscope, SEM (scan electron
microscope), and TEM (transmission electron microscope).
[0052] First, they buried the cross-section of the sheet in resin
and polished it, then etched it with a potassium ferricyanide
solution (product name: Murakami's Reagent) and further
electrolytically etched it by oxalic acid for use for observation
by an optical microscope. If etching by a potassium ferricyanide
solution, the ferrite phase can be discerned by its gray color and
the austenite phase by its white color. Further, if
electrolytically etching by oxalic acid, when sensitized,
intergranular corrosion can be confirmed. Next, the inventors
analyzed the same test piece by SEM-EDS so as to analyze the metal
elements in the ferrite phase and austenite phase. Finally, they
identified the precipitate of the same test piece by the extraction
replica TEM method.
[0053] For the volume fraction of the .gamma. phase, the inventors
engaged in a detailed structural analysis by the method of
measurement of a phase map identifying the fcc and bcc crystal
structures at the sheet cross-section by the EBSP method, measured
the pitting potential by the method of measuring the Vc'100
(Vv.s.AGCL) on a #500 polished surface as an evaluation surface in
a 30.degree. C., 3.5% NaCl aqueous solution (note that the
measurement value of the pitting potential was made the average of
n3), and, according to a JIS 13B tensile test, obtained a tensile
test piece from the rolling direction and measured the uniform
elongation until necking occurred by a tension rate of 20 mm/min
(range of tension rate defined in JIS Z 2241). As a result, the
inventors arrived at the following findings explaining the test
results of Table 1 and FIG. 1.
[0054] (a) At the ferrite grain boundaries and ferritic-austenitic
grain boundaries of Steel Nos. 2, 4, and 6, intergranular corrosion
due to sensitization was confirmed. Further, at the crystal grain
boundaries, precipitation of Cr nitrides was observed. Therefore,
the reduction in the pitting potential can be interpreted as being
due to the sensitization accompanying precipitation of Cr nitrides.
That is, by increasing the amount of Si (over 1%) or the amount of
(0.15% or more), the precipitation sensitivity of Cr nitrides at
the crystal grain boundaries rises, and the pitting potential falls
in inverse proportion to the PI value of the pitting indicator.
[0055] (b) The amount of Cr and the amount of Mn relating to the PI
value differ in distribution in the ferrite phase and austenite
phase. For example, in the case of Steel Nos. 1, 2, 4, and 6, the
amount of Cr is 22 to 23% at the ferrite phase and 18 to 19% at the
austenite phase, while the amount of Mn amount is approximately 3%
at the ferrite phase and approximately 4% at the austenite phase.
Despite Steel Nos. 4 and 6 having the same degrees of amount of N,
the pitting potential of No. 6 is low. It is believed that the drop
in the pitting potential, in addition to the sensitization
mentioned in (a), is also due to the .gamma. fraction with the low
amount of Cr and high amount of Mn being a large rate of over 50%.
That is, the possibility is suggested that if allowing the
formation of a large amount of an austenite phase with a low amount
of Cr and a high amount of Mn, the corrosion resistance will become
inferior.
[0056] (c) Steel No. 3, in comparison with the other steels, has
large Mn-based sulfides exceeding 5 .mu.m in long sides scattered
in it. Due to this, the drop in pitting potential is believed to be
due to the comparatively large Mn-based sulfides formed due to the
high amount of Mn (3.8%) acting starting points of pitting.
[0057] (d) Steel Nos. 1 and 5 did not exhibit either the
above-mentioned sensitization or comparatively large Mn-based
sulfides. Therefore, the drop in the pitting potential in Steel No.
5 is believed to be largely due to the low PI value (<18%).
[0058] (e) The pitting potential of Steel No. 1 falls as the
cooling rate drops as shown in FIG. 1. When cooling rate is
5.degree. C./sec or less, while clear intergranular corrosion could
not be confirmed with electrolytic etching by oxalic acid, the
presence of some Cr nitrides at the crystal grain boundaries was
found by TEM observation. Due to this, the drop in pitting
potential is believed to be related to precipitation of Cr
nitrides.
[0059] (f) The pitting potential of Steel No. 1, as shown by the
black dot in FIG. 1, improves if temporarily holding the steel at
500.degree. C. instead of being continuously cooling it. When held
at 500.degree. C., the presence of Cr nitrides explained in the
above (e) was not seen. It is thought that the N present in an
oversaturated state near the ferritic-austenitic grain boundaries
diffuses to austenite grains with large solid solubility limits
when holding the steel at 500.degree. C. and thereby suppresses
precipitation of Cr nitrides.
[0060] (g) The uniform elongation of a material, the indicator of
workability, tends to increase as the .gamma. fraction rises as is
clear from Table 1. However, while a high uniform elongation
comparable to SUS 304 is obtained when the .gamma. fraction is over
50%, the drop in corrosion resistance is remarkable as explained in
the above (b). When the .gamma. fraction is 20 to 35%, the metal
structure becomes one where the ferrite phase is the matrix phase
and elliptic to circular austenite phases are equally dispersed in
it. Such a metal structure with the austenite phase dispersed in it
has a uniform elongation higher compared with the
ferritic-austenitic phase layer structure seen in normal SUS 329J4L
and other two-phase stainless steels.
[0061] The aspects of the invention set forth in the above (1) to
(7) were completed based on the first findings, that is, the
findings of (a) to (g).
[0062] Next, the inventors will explain the second findings.
[0063] The inventors investigated the factors of metal structure
governing uniform elongation in relation to the above problems by
producing various ferritic-austenitic stainless steels in the
laboratory, hot rolling them, then annealing and cold rolling them
to produce thin-gauge steel sheet. They investigated the
relationship of the metal structures of the obtained thin-gauge
steel sheet and the uniform elongation after the tensile tests and
as a result obtained the following findings concerning the
characteristics of austenite grain in steels with high uniform
elongation.
[0064] (h) The grain size was small and shape was close to
spherical (not flattened in the rolling direction).
[0065] (i) The distance between nearest austenite grains was
narrow.
[0066] (j) There was a suitable value for the austenite stability
calculated from the chemical composition in the austenite
phase.
[0067] The inventors will explain details of these next.
[0068] First, the inventors produced 10 types of steel having
compositions of 0.006 to 0.030% C, 0.10 to 0.85% Si, 1.0 to 3.0%
Mn, 0.022 to 0.039% P, 0.0002 to 0.0035% S, 20.1 to 21.0% Cr, and
0.08 to 0.12% N, then hot rolled them, then annealed and cold
rolled them to produce thin-gauge steel sheets. During this, they
changed the hot rolling conditions, annealing temperatures, and
other production conditions. They obtained JIS 13B tensile test
pieces from the obtained 1 mm thick thin-gauge steel sheets
parallel to the rolling direction and measured their uniform
elongations by a method based on JIS Z 2241. The tension rate was
made 10 mm/min. Further, they investigated the metal structure at a
cross-section vertical to the rolling transverse direction
(L-section) at the center position of the rolling transverse
direction of each of the thin-gauge steel sheets by EBSP and
identified the phases. They classified the data obtained by EBSP
for each crystal grain into ferrite grains (BCC phase) and
austenite grains (FCC phase) and first measured the austenite
fraction. Further, places where the crystal orientation difference
was 15.degree. or more at adjacent measurement points were deemed
crystal grain boundaries and shown by black lines. Measurement
examples are shown in FIG. 2. FIG. 2(a) shows the BCC phase, and
FIG. 2(b) the FCC phase, each shown by the white color.
[0069] Further, the inventors measured the grain sizes and aspect
ratios of the grains of the austenite grains (FCC phase) and
measured the distance between nearest grains for the austenite
grains. For the distance between nearest grains, they used the
smallest value of the distance between the center positions of
austenite grains as the distance between nearest grains of those
grains. For the center position of each crystal grain, they made
the position of 1/2L and 1/2H, where the rolling direction length
of the grain is L and the thickness direction length of the sheet
is H, the center position. They measured the distance between
nearest grains for each of 100 austenite grains and found their
average value.
[0070] Further, the inventors used EPMA to investigate the chemical
composition in the austenite grains. From the obtained chemical
composition, they calculated an Md value as an indicator of the
stability of the austenite phase. Here, Md is an indicator
expressing the austenite stability calculated by the following
formula (2). The coefficients in the formula were based on the
formula of Nohara et al. (see Journal of the ISIJ 63 (1977) p.
772). The [ ] in the formula indicates a composition measured by
EPMA for each element. However, for C, quantification in the
austenite phase is difficult by EPMA, so the average composition {
} is shown. The "average composition" referred to here expresses
the average composition contained in the steel irrespective of the
phase and is found by the combustion-infrared absorption method
described in JIS G 1211:
Md=551-462({C}+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29[Ni]-29[Cu]-18.5[Mo]
(2)
[0071] The Md value is determined by the chemical composition in
the austenite grains. Therefore, by changing the chemical
composition in the austenite grains by, for example, the annealing
temperature, annealing time, or the like, the Md value can be
adjusted.
[0072] N, Cu, Ni, and Mn concentrate at the austenite phase, that
is, are elements which have higher concentrations in the austenite
phase than concentrations in the ferrite phase, so by increasing
the amounts of addition of these, the Md value can be lowered.
Further, normally, the composition of the austenite phase is not an
equilibrium composition determined by the annealing temperature.
This is because time is needed for diffusion for the elements to be
distributed to the austenite phase and ferrite phase at a certain
annealing temperature. Therefore, increasing the holding time at
the final annealing process will enable the equilibrium composition
to be approached (the concentration in the austenite phase of N,
Cu, Ni, and Mn rising), so making the holding time longer is also
an effective method of lowering the Md value. However, with a
holding time of 30 minutes, the equilibrium composition will
substantially be reached.
[0073] C is an element lowering the Md value. By increasing the
amount of addition, the Md value can be lowered. Further, C is also
an element that concentrates in the austenite phase, however,
measurement of the concentration in the austenite phase is
difficult. In the present invention, in the formula for calculation
of the Md value, the average composition of C is used. Therefore,
the holding time of the annealing has no effect on the Md value of
the present invention.
[0074] The effects of Si and Cr on the Md value cannot be said to
be clear. That is, these elements act as minus coefficients on the
Md value, so when these elements are viewed alone, addition of
larger amounts will lower the Md value. However, when the amounts
of Si and Cr are high, the concentrations of Mn, Ni, Cu, and the
like in the austenite phase drop, so sometimes, conversely, the Md
value increases. Depending on the concentrations of Mn, Ni, Cu, and
other elements and the annealing conditions, the degrees of the
effects of Cr and Si change.
[0075] As explained above, the Md value is determined by the
chemical composition in the austenite grains. The chemical
composition in the austenite grains also changes depending on the
austenite fraction. That is, when the austenite fraction is low,
the concentration of austenite forming elements in the austenite
phase becomes higher, so the Md value tends to fall. On the other
hand, when the austenite fraction is low, the concentration of
austenite forming elements in the austenite phase becomes lower, so
the Md value rises. Further, the austenite fraction changes
depending on the temperature. In the ingredients set forth in the
present invention, the austenite fraction is the highest at
1000.degree. C. to 1150.degree. C. At temperatures higher or lower
than that, the austenite fraction decreases.
[0076] Further, the higher the absolute value of the uniform
elongation, the higher the workability, however, if the uniform
elongation is 30% or more, the level becomes higher compared to
ferritic stainless steel, while if 40% or more, it is possible to
work the steel into nearly the same shape as with austenitic
stainless steel with good workability.
[0077] First, the inventors investigated the relationship between
the volume fraction (austenite fraction) of the austenite phase and
the uniform elongation for all data. The relationship between the
austenite phase and the uniform elongation during a tensile test is
shown in FIG. 3.
[0078] There is a range of suitability of the austenite fraction
with respect to the uniform elongation. If too high or too low, the
uniform elongation will drop. To secure a uniform elongation of 30%
or more, it is necessary to make the austenite fraction 10% to less
than 50%. The range is preferably 15 to 40%.
[0079] Next, the relationship between the Md value and the uniform
elongation for data where the austenite fraction is 10% to less
than 50% is shown in FIG. 4. To obtain a good uniform elongation,
there is also a suitable range of the Md value similar to the
austenite fraction. At a -10 to +110 Md value range, the uniform
elongation is a high value of 34 to 44%. Such a high uniform
elongation cannot be obtained outside this range. However, with
just the Md value alone, the variation in uniform elongation is too
large. There may also be other structural factors that affect the
uniform elongation.
[0080] The grain size and shape of the austenite grains may also
affect the uniform elongation, so the inventors measured, for the
-10 to +110 Md value data in FIG. 4, the "ratio of austenite grains
with a grain size of 15 .mu.m or less and a shape aspect ratio of
less than 3 in the total number of austenite grains" X1 (%) was
measured and investigated its relationship to the uniform
elongation u-EL(%). The results are shown in FIG. 5. As shown in
FIG. 5, the higher this ratio, the higher the uniform elongation
tends to be. When the ratio is 90% or more, even better uniform
elongation can be obtained.
[0081] Further, the relationship between the average distance X2
(.mu.m) with the nearest grains of the austenite grains, measured
as explained above extracting data where the uniform elongation is
37% or more in FIG. 5, and the uniform elongation u-EL(%) is shown
in FIG. 6. The shorter the average value of the distance with the
nearest grains, the higher the uniform elongation. When 12 .mu.m or
less, the uniform elongation becomes extremely high.
[0082] The aspects of the invention set forth in (8) to (13) were
completed based on the above explained second findings, that is,
the findings of (h) to (j).
[0083] Below, the requirements of the present invention will be
explained in detail. Note that, the "%" indications of the contents
of the elements mean "mass %".
[0084] First, the reasons for limiting the ingredients, metal
structure, and production conditions according to the first
findings will be explained below.
[0085] C is an element raising the volume fraction of the austenite
phase and concentrating in the austenite phase to raise the
austenite phase stability. To obtain the above effects, it is
included in 0.001% or more. However, if over 0.1%, the heat
treatment temperature for making C form a solid solution becomes
remarkably high and the steel will become susceptible to
sensitization due to carbide grain boundary precipitation.
Therefore, the content is made 0.1% or less, preferably 0.05% or
less.
[0086] Cr is an element necessary to secure the corrosion
resistance. To achieve the object of the first findings of
corrosion resistance, its lower limit is made 17%. However, if over
25%, a drop in toughness and drop in elongation occur and formation
of an austenite phase in the steel becomes difficult. For this
reason, the content is made 25% or less. From the viewpoints of the
corrosion resistance, workability, and production, the content is
preferably 19 to 23%, more preferably 20 to 22%.
[0087] Si is sometimes added as a deoxidizing element. To obtain
the above effect, it is included in 0.01% or more. However, if over
1%, it becomes difficult to secure the object of the first findings
of corrosion resistance. Therefore, the content is made 1% or less.
Excessive addition leads to an increase in refining costs as well.
From the viewpoint of corrosion resistance and production, the
content is preferably 0.02 to 0.6%, more preferably 0.05 to
0.2%.
[0088] Mn is an element raising the volume fraction of the
austenite phase and concentrating in the austenite phase to raise
the austenite phase stability. Further, it is also an element
effective as a deoxidizing agent. To obtain the above effect, it is
included in 0.5% or more. However, if over 3.7%, it becomes
difficult to secure the object of the first findings of corrosion
resistance. Therefore, the content is made 3.7% or less. From the
viewpoints of corrosion resistance, workability, and production,
the content is preferably 2 to 3.5%, more preferably 2.5 to
3.3%.
[0089] N, like C, is an element raising the volume fraction of the
austenite phase and concentrating in the austenite phase to
stabilize the austenite phase. Further, it is an element forming a
solid solution with the austenite phase and raising pitting
resistance. To obtain the above effects, its lower limit is made
0.06%. However, if 0.15% or more is added, the chromium nitride
contained in the steel material will exceed 0.1 mass % and most of
the chromium nitride will precipitate at the crystal grain
boundaries, so this will become a factor forming a chrome-depleted
layer. Therefore, securing the object of the first findings of
corrosion resistance, will become difficult.
[0090] Therefore, the content is made less than 0.15%. Further,
adding N causes the occurrence of blowholes during melting and
causes the hot workability to drop. From the viewpoints of
corrosion resistance, workability, and production, the content is
preferably 0.07 to 0.14%, more preferably 0.08 to 0.12%.
[0091] The pitting indicator (PI value) at a neutral chloride
environment is calculated by the following formula (a):
Pitting indicator(PI value)=Cr+3Mo+10N--Mn(%) (a)
[0092] Note that the Cr, Mo, N, and Mn in the above formula mean
the mass % of the elements. Elements that are not contained are
treated as 0.
[0093] As described in for example "Stainless Steel Handbook 3rd
edition", p. 622, Japan Stainless Steel Association, a coefficient
of Mo to Cr of 3 and a coefficient of N to Cr of 10 were
employed.
[0094] A coefficient of Mn to Cr of for example -1 described in for
Current Advances in Materials and Processes, vol. 18 (2005), 607
was used. To obtain a corrosion resistance equal to or greater than
SUS 304 in a neutral chloride environment as targeted by the first
findings, Cr+3Mo+10N--Mn>18(%), more preferably 19% or more.
[0095] Ni is an austenite forming element and is effective in
securing the objects of the first findings of corrosion resistance
and workability. When adding it, the content is made 0.6% or more
to achieve the above effects. A content exceeding 3% leads to a
rise in material costs and makes it difficult to achieve effects
that match the costs. Therefore, when adding it, its content is 3%
or less. From the viewpoints of corrosion resistance, workability,
and economy, the content is preferably 0.7 to 2.8%, more preferably
0.9 to 2.0%.
[0096] Cu, like Mn and Ni, is an austenite forming element and is
effective in securing mainly the objects of the first findings of
corrosion resistance and workability. Particularly, it is an
element effective in improving corrosion resistance added in
combination with Ni. When adding it, it is made 0.1% or more to
obtain the above effect added in combination with Ni. A content
exceeding 3% leads to a rise in material costs and makes it
difficult to achieve effects that match the costs. Therefore, when
adding it, the content is made 3% or less. From the viewpoints of
corrosion resistance, workability, and economy, the content is
preferably 0.3 to 1%, more preferably 0.4 to 0.6%.
[0097] Mo can be appropriately added to improve the corrosion
resistance. To obtain the above effect, addition of 0.2% or more is
preferable. However, a content exceeding 1% sometimes detracts from
the economy. Therefore, when adding it, the content is made 1% or
less. From the viewpoints of corrosion resistance and economy, the
content is preferably 0.2 to 0.8%.
[0098] Ti and Nb can be appropriately added to suppress
sensitization due to C and N and thereby improve the corrosion
resistance. To obtain the above effect, addition of 0.01% or more
for each is preferable.
[0099] However, contents exceeding 0.5% sometimes detract from the
economy and also lower the austenite fraction and harden the
ferrite phase to thereby detract from the workability. Therefore,
when adding these, the content of each is respectively made 0.5% or
less. From the viewpoints of corrosion resistance and workability,
the content of each is more preferably 0.03 to 0.3%, further
preferably 0.05 to 0.1%.
[0100] Al is a powerful deoxidizing agent and can be appropriately
added. To obtain the above effect, addition of 0.001% or more is
preferable. However, a content exceeding 0.2% sometimes is a factor
in forming nitrides and causing surface damage and a drop in
corrosion resistance. Therefore, when adding it, the content is
made 0.2% or less. From the viewpoint of production and corrosion
resistance, the content is more preferably 0.005 to 0.1%.
[0101] B, Ca, and Mg can be appropriately added to improve the hot
workability. To obtain the above effect, addition for each of
0.0002% or more is preferable. However, a content for each
exceeding 0.01% sometimes causes the corrosion resistance to drop
remarkably. Therefore, when adding these, the content for each is
made 0.01% or less. From the viewpoints of hot workability and
corrosion resistance, the content for each is more preferably
0.0005 to 0.005%.
[0102] Further, the stainless steel according to the first findings
may contain, other than the above ingredients, P and S as part of
the unavoidable impurities in the ranges below. P and S are
elements harmful to hot workability and corrosion resistance. The
content of P is preferably 0.1% or less, more preferably 0.05% or
less. Excessive reduction leads to an increase in refining and
material costs, so the lower limit is preferably 0.005%. The
content of S is preferably 0.01% or less, more preferably 0.005% or
less. Excessive reduction leads to an increase in refining and
material costs, so the lower limit is preferably 0.0005%.
[0103] Next, the reasons for limitation of the metal structure will
be explained. The ferritic-austenitic stainless steel according to
the first findings has the above ingredients and is improved in
corrosion resistance and workability by having a defined volume
fraction of the austenite phase (hereinafter, ".gamma.
fraction").
[0104] The .gamma. fraction, as mentioned above, can be found by
the EBSP method. The EBSP method, as for example described in
Microscopy; Seiichi Suzuki, Vol. 39, No. 2, 121 to 124, designates
the crystal system data of the austenite phase (fcc) and ferrite
phase (bcc) and displays a phase distribution map in which a color
is given to each phase. Due to this, the state of dispersion of the
austenite phase can be understood and the .gamma. fraction found.
The test piece is a sheet cross-section, and measurement is at a
magnification of 500.times. and step size of 10 .mu.m.
[0105] The upper limit of the .gamma. fraction is made 50% or less
to secure the object of the first findings mentioned above of
corrosion resistance. To improve the uniform elongation of the
material, the lower limit of the .gamma. fraction is made 15% or
more, more preferably 20% or more. From the viewpoints of the
corrosion resistance and elongation, the content is more preferably
30 to 40% in range.
[0106] The state of dispersion of the austenite phase is not
particularly defined, but from the viewpoint of improving the
uniform elongation of the material, it is preferable that, rather
than have a ferritic-austenitic phase layer-type structure, the
ferrite phase be made the matrix phase and 100 .mu.m elliptic to
circular austenite phases be dispersed, more preferably austenite
phases less than 50 .mu.m be dispersed.
[0107] The ferritic-austenitic stainless steel having the
ingredients of the first findings and the above metal structure can
have a pitting potential, an indicator of the corrosion resistance,
of 0.3V or more and a uniform elongation, an indicator of the
workability, increased from 30% to 50% and can give a corrosion
resistance in a neutral chloride environment equal to or greater
than SUS 304 and a workability significantly higher than SUS 430LX
and close to SUS 304. The measurement conditions of the pitting
potential and uniform elongation are similar to those mentioned
above and are as follows. For the pitting potential, Vc'100
(Vv.s.AGCL) was measured on a #500 polished surface as the
evaluation surface in a 30.degree. C., 3.5% NaCl aqueous solution.
The measurement value of the pitting potential was made the average
value of n3. For the uniform elongation, according to the JIS 13 B
tensile test, a tensile test piece was taken from the rolling
direction and the uniform elongation until necking occurred was
measured at a tension rate of 20 mm/min (range of tension rate
specified in JIS Z 2241).
[0108] To achieve the objects of the first findings of corrosion
resistance and workability, in ferritic-austenitic stainless steels
having the ingredients and metal structure explained above, the
following production conditions are preferable.
[0109] The hot rolled steel material used for production is not
particularly limited so long as it has the above ingredients. The
final annealing following the cold working preferably comprises
heating and holding at 950 to 1150.degree. C. When less than
950.degree. C., recrystallization of the worked structure sometimes
is insufficient. When over 1150.degree. C., the grain size becomes
larger and sometimes the structure becomes not a
ferritic-austenitic phase layer-type structure, but one greatly
deviating from the preferred one where the ferrite phase is the
matrix phase and elliptical to circular austenite phases less than
100 .mu.m are dispersed. Further, sometimes the .gamma. fraction
decreases and good elongation is not able to be obtained. To obtain
a preferable structure for achieving corrosion resistance and
workability, the range is more preferably 980 to 1100.degree. C.,
further preferably 980 to 1050.degree. C.
[0110] The cooling following the final annealing preferably is
performed at an average cooling rate from the heating temperature
to 200.degree. C. of 3.degree. C./sec or more. When less than
3.degree. C./sec, the corrosion resistance drops due to the
sensitization based on grain boundary precipitation of Cr nitrides.
The upper limit of the cooling rate is not particularly defined,
but in the case of gas cooling, it is approximately 50.degree.
C./sec. In the case of water cooling, it is 300 to 500.degree.
C./sec. When using an industrial continuous annealing facility, the
rate is preferably 10 to 40.degree. C./sec, more preferably 25 to
35.degree. C./sec.
[0111] In the cooling process of the final annealing, it is
preferable that the steel material be held at a 200 to 600.degree.
C. temperature region for 1 minute or more. By the N present in an
oversaturated state near the crystal grain boundaries diffusing
into the austenite phase with its large solid solubility limit and
forming a solid solution t the time of holding at that temperature
region, the grain boundary precipitation of Cr nitrides leading to
a drop in pitting potential is suppressed. That is, the drop in the
corrosion resistance caused by sensitization can be suppressed.
[0112] The higher the holding temperature, the more effective for
diffusing N, however, a temperature over 600.degree. C. accelerates
grain boundary precipitation of Cr carbon nitrides. Therefore, the
upper limit is made 600.degree. C. If less than 200.degree. C., N
diffusion will take a long time, making achievement of the above
effect difficult. Therefore, the lower limit is made 200.degree.
C., more preferably in the range of 300 to 550.degree. C., further
preferably 400 to 550.degree. C.
[0113] The holding time is preferably made 1 minute or more in
order to obtain above effect. The upper limit is not particularly
defined, but when using an industrial continuous annealing
facility, a long holding time leads to a drop in productivity, so
the time is preferably 5 minutes or less, more preferably 3 minutes
or less.
[0114] According to the above method of production, it is possible
to produce ferritic-austenitic stainless steel excellent in
corrosion resistance and workability having a ferrite phase as the
matrix phase, having a volume fraction of an austenite phase of 15
to 50%, having a pitting potential Vc'100 of 0.3V (Vv.s.AGCL) or
more in a 30.degree. C., 3.5% NaCl aqueous solution, and having a
uniform elongation in a tensile test of 30% or more.
[0115] Next, the reasons for limiting the ingredients, metal
structure, and production conditions according to the second
findings will be explained below.
[0116] C is an element having a large effect on the stability of
the austenite phase. When adding over 0.100%, sometimes the uniform
elongation drops. Further, it promotes precipitation of the Cr
carbides, so causes granular corrosion. Therefore, the upper limit
was made 0.100%. Further, from the viewpoint of corrosion
resistance, the lower the C, the more preferable, however, if
taking the ability of current facilities into consideration,
lowering the amount of C to less than 0.002% would lead to a large
increase in costs, so the lower limit is preferably this value,
more preferably 0.002 to 0.8%.
[0117] Si is used as a deoxidizing element and sometimes is added
to improve the oxidation resistance. However, adding over 2.00%
will lead to hardening of the material and cause a drop in uniform
elongation, so the upper limit is preferably this value, more
preferably 1.6% or less. Further, extreme reduction of Si would
lead to an increase in costs at the time of refining, so the lower
limit was made 0.05, preferably 0.08%.
[0118] Mn concentrates at the austenite phase and has an important
role in changing the stability of the austenite phase. However,
large addition will not only lower the uniform elongation, but also
lower the corrosion resistance and hot workability, so the upper
limit was made 5.00%. Less than 0.05% causes an increase in costs
in the refining process, so this value was made the lower limit.
From the viewpoint of corrosion resistance, a lower value is
preferable, so the upper limit is more preferably 3.00%, further
preferably 2.80%.
[0119] P is an element which is unavoidably included. Further, it
is contained in Cr and other materials. Therefore, reducing it is
difficult, however, if a large amount is contained, it causes the
workability to drop, so the upper limit was made less than 0.050%.
However, the lower it is, the more preferable, so making it 0.035%
or less is preferable.
[0120] S is an element which is unavoidably included. It bonds with
Mn to form inclusions and sometimes becomes the starting point of
rust, therefore the upper limit was made less than 0.010%. The
lower it is, the more preferable from the viewpoint of corrosion
resistance, so making it 0.0020% or less is preferable.
[0121] Cr is an element necessary for securing the corrosion
resistance. An addition of 17% or more is necessary. However, large
addition causes hot working cracks and leads to an increase in
refining process costs, so the upper limit was made 25%, preferably
17 to 22%.
[0122] N, like C, is an element having a large effect on the
stability of the austenite phase. Further, if existing in a solid
solution, it has the effect of improving corrosion resistance, so
0.010 or more is added. However, if over 0.150% is added, sometimes
the uniform elongation will drop. Also, Cr nitrides will
precipitate easily and cause the corrosion resistance to conversely
drop. Therefore, the upper limit was made this value, preferably
0.03 to 0.13%.
[0123] Further, the following elements may be selectively
added.
[0124] Ni is an austenite stabilizing element and is important for
adjusting the stability of the austenite phase. Further, it has the
effect of suppressing hot working cracks, so when wishing to obtain
these effects, 0.10% or more may be added. Addition over 5.00%
causes an increase in material costs and, further, sometimes makes
achievement of an austenitic-ferritic two-phase structure
difficult, so the upper limit was made this value, preferably 3.00%
or less.
[0125] Cu, like Ni, is an austenite stabilizing element and is
important for adjusting the stability of the austenite phase.
Further, it has the effect of improving corrosion resistance, so
0.10% or more may be added. However, addition over 5.00% promotes
cracks during hot working and, further, lowers the corrosion
resistance, so the upper limit was made this value.
[0126] Mo is an element improving corrosion resistance, so may be
selectively added. Addition of 0.10% or more enables the effect of
improving corrosion resistance to be obtained, so addition of this
value or more is preferable. However, addition over 5.00% lowers
the uniform elongation and greatly increases the material costs, so
the upper limit was made this value.
[0127] Nb has the effect of preventing coarsening of the weld heat
affected zone, so to obtain good effects even when added, addition
of 0.03% or more is required. This therefore may be added with the
lower limit as this value. However, addition over 0.50% lowers the
uniform elongation, so the upper limit was made this value.
[0128] Ti, like Nb, may be added in 0.03% or more so as to prevent
coarsening of the weld heat affected zone and further make the
solidified structure finer equiaxial crystalline. However, addition
over 0.50% lowers the uniform elongation, so the upper limit was
made this value.
[0129] Some amount of Ca is sometimes added for desulfurization and
deoxidation. The effects are demonstrated with addition of 0.0002%
or more, so this may be added with the lower limit as this value.
However, addition over 0.0030% makes the steel susceptible to hot
working cracks and, further, lowers the corrosion resistance, so
the upper limit was made this value.
[0130] Mg has an effect of not only deoxidation, but also making
the solidified structure finer and therefore is sometimes added. To
obtain these effects, addition of 0.0002% or more is necessary, so
this may be added with the lower limit as this value. Further,
addition over 0.0030% causes an increase in costs in the
steelmaking process, so the upper limit was made this value.
[0131] Next, the reasons for limiting the metal structure will be
explained.
[0132] Volume fraction of austenite phase of 10% to less than 50%:
As shown by the results of the study above, to obtain a good
uniform elongation, a ratio of austenite phase of 10% or more is
required, so the lower limit was made this value. Further, the
uniform elongation does not necessarily become higher the higher
the austenite fraction. If over 50%, the uniform elongation
conversely drops, so the upper limit was made this value. It is
preferable to measure the austenite fraction with a method
classifying phases using EBSP, extracting only the austenite
grains, then measuring the area ratio. The measurement range at
this time is 200 .mu.m.times.200 .mu.m or more. In the present
invention, the austenite fraction is important as an indicator of
workability (uniform elongation). The reasons for this are thought
to be as follows. The austenite phase causes work-induced
martensite transformation during work and contributes to an
increase in the uniform elongation. If the amount transformed at
this time is small, the uniform elongation becomes smaller.
Further, the reason why the uniform elongation is low when the
austenite fraction is over 50% is not currently clear, but it is
believed that it is because deformation concentrates in the ferrite
phase which is softer in comparison to the austenite phase.
[0133] Md value calculated from chemical composition in austenite
phase of -10 to 110: In the present invention, the properties of
the austenite phase are also defined. That is, the steel is
characterized in that the Md value calculated from the chemical
composition in the austenite phase satisfies the following formula
(b):
-10Md.ltoreq.110 (b)
[0134] (where,
Md=551-462({C}+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29[Ni]-29[Cu]-18.5[Mo],
[ ] is composition (mass %) in the austenite phase, and { } is the
average composition (mass %))
[0135] The chemical composition in the austenite phase forming the
basis of the calculation of the Md is measured by EPMA. [ ] in the
above Md formula shows the composition (mass %) in the austenite
phase measured by EPMA for each element. However, for C,
measurement by the EPMA is difficult, so instead of the composition
in the austenite phase, the average composition (mass %) is shown.
When the Md value is less than -10 or over +110, a good uniform
elongation cannot be obtained, so the lower limit and upper limit
were made these values. The reason why the Md value affects the
uniform elongation is thought to be as follows: The Md value is an
indicator representing the stability of the austenite phase, that
is, it can be said to represent the amount of strain needed to
cause work-induced martensite transformation. If the amount of
strain is too small, work-induced martensite transformation
finishes at the initial steps of working and sufficient ductility
cannot be maintained at the later steps of working important for
determining success of working. Further, if the amount of strain is
too large, uniform deformation finishes before reaching that amount
of strain and work-induced martensite transformation cannot be
applied effectively. Therefore, there is a suitable range of Md
value for generating work-induced martensite transformation during
work.
[0136] Ratio of austenite grains with a grain size of 15 .mu.m or
less and a shape aspect ratio of less than 3 accounting for 90% or
more of the total number of austenite grains: As metal structural
features of the austenite grains when good uniform elongation is
obtained, the grains are fine and not flattened in the rolling
direction. Specifically, the ratio of austenite grains with a grain
size of 15 .mu.m or less and a shape aspect ratio of less than 3
account for 90% or more of the total number of austenite grains.
When there are many crystal grains with a grain size over 15 .mu.m,
the uniform elongation drops, so the upper limit was made this
value. Further, it is not particularly necessary to define the
lower limit, however, making it 1 .mu.m or less greatly increases
costs in the production process, so the lower limit is preferably
made 1 .mu.m.
[0137] Further, the shape of crystal grains is also an important
factor. In the present invention, through observation from the L
cross-section (surface parallel to the rolling direction observed
from the sheet width direction), the aspect ratio of each grain was
measured. The ratio of crystal grains with an aspect ratio of less
than 3 becomes important. At this time, when the aspect ratio is 3
or more, the uniform elongation tends to drop, so the condition of
the aspect ratio defined as a structural factor was made less than
3. The aspect ratio is measured by dividing the length of the
longest side of each grain by the length orthogonal to it.
Therefore, the lower limit of the aspect ratio is 1. The number of
crystal grains for measuring the grain size and aspect ratio is 100
or more. In the present invention, it was first discovered that the
austenite grain size and grain shape affect the uniform elongation,
but the reasons for this are currently unknown. However, it is
believed that there have an effect on the mode of deformation in
the austenite grains (dislocation density, deformation zone,
twinning presence, etc.) and that this changes the work-induced
martensite transformation behavior.
[0138] Average distance between nearest austenite grains of 12
.mu.m or less: The distance between nearest austenite grains also
affects the uniform elongation, so the average distance was made 12
.mu.m or less. If over 12 .mu.m, the uniform elongation drops, so
the upper limit was made this value. Further, the lower limit is
not particularly defined. The distance between nearest grains is
determined by defining the point where the center line of the
rolling direction length of each austenite grain intersects the
center line of the sheet thickness direction length as the center
position of a crystal grain and defining the smallest value of the
distances between center positions of grains as the distance
between nearest grains of that grain. The average of the results of
measurements for 100 crystal grains or more is defined as the
"average distance between nearest austenite grains". The reason why
the average distance between nearest austenite grains affects the
uniform elongation is thought to be as follows: In the deformation
process, when strain is introduced in a certain austenite grain,
work-induced martensite transformation occurs, and a certain degree
of strain is reached, if considering the process by which
deformation spreads, a smaller distance between austenite grains
enables work-induced martensite transformation to propagate to
several crystal grains and occurs continuously allowing extremely
high uniform elongation to be obtained. This is clear from the fact
that, in comparison to the uniform elongation of 30% or more
secured in the first findings, an even higher 40% or more can be
secured in the second findings.
[0139] Note that the uniform elongation is an important indicator
representing the workability in the present invention. The uniform
elongation is measured by taking a JIS 13 B tensile test piece
parallel to the rolling direction and following the method based on
JIS Z 2241.
[0140] Note that, the state of the ferrite grains is not
particularly defined in the second findings, however, when the
ferrite phase is coarse in grain size, the above distance between
austenite grains becomes larger, so the grain size is preferably at
average of 25 .mu.m or less. Further, when the shape aspect ratio
is large as well, the distance between austenite grains becomes
large, so the ratio is preferably less than 3.
[0141] As mentioned above, to obtain an extremely good uniform
elongation, it is necessary to control the metal structure, but
such a metal structure is not obtained only by the chemical
composition. To achieve the object of the second discovery of
workability, particularly uniform elongation and corrosion
resistance, the following production conditions are preferable.
[0142] As hot rolling material, a steel slab obtained by continuous
casting is used. The heating temperature Ti before hot rolling is
made 1150.degree. C. to less than 1250.degree. C. When less than
1150.degree. C., edge cracking occurs in hot rolling, so the lower
limit was made this value. Further, when the heating temperature is
made over 1250.degree. C., it is easy for the austenite grain size
after final annealing to become larger, the steel slab deforms
inside the heating furnace, and defects are likely to occur during
hot rolling, so the upper limit was made this value.
[0143] Further, during hot rolling, rolling at 1000.degree. C. or
more with reduction of a 30% or higher reduction rate then holding
for 30 sec or more for one pass or more is performed. To make a
metal structure for obtaining a good uniform elongation, a grain
refining process in which recrystallization is applied during hot
rolling is necessary. To cause hot recrystallization in
ferritic-austenitic stainless steel, this reduction process is
necessary. When the rolling temperature is less than 1000.degree.
C., even if holding for 30 sec or more after 1 pass of 30% or more
rolling, the austenite grain size in the metal structure after cold
rolling and annealing becomes coarser and the uniform elongation
during the tensile test becomes insufficient. Further, the
reduction rate and time between passes both greatly affect the
recrystallization behavior, but to obtain austenite grains that are
fine and have small aspect ratios after cold rolling and annealing,
it is necessary for the reduction rate during hot rolling to be 30%
or more and the holding time following it to be 30 sec or more.
[0144] Further, the total rolling rate of hot rolling was made 96%
or more. When less than 96%, the crystal grains after cold rolling
and annealing become coarser. Further, the distance between
austenite grains becomes larger, so the uniform elongation becomes
insufficient. The annealing temperature of the hot rolled sheet is
between the heating temperature T1-100.degree. C. before hot
rolling and
[0145] T1.degree. C. When lower than T1-100.degree. C., the aspect
ratio of the crystal grains after cold rolling and annealing
becomes larger. Further, when T1.degree. C. or more, the grain size
after cold rolling and annealing becomes coarser, the metal
structure of the object is not obtained, and the uniform elongation
during the tensile test drops.
[0146] Further, the cold rolling and annealing may be repeatedly
performed, i.e., so called two-time cold rolling may be performed.
It is necessary to make the process annealing temperature at this
time T1-100.degree. C. to T1.degree. C. in the same way as hot
rolled sheet annealing.
[0147] Further, the final annealing temperature is 1000.degree. C.
to 1100.degree. C. This is because when less than 1000.degree. C.,
the shape aspect ratios of the austenite and ferrite grain become
bigger, the Md value deviates from the appropriate range, and the
uniform elongation drops. Further, when over 1100.degree. C., the
.gamma. fraction drops, the Md value deviates from the appropriate
range, or the grain size becomes too large
[0148] Below, examples according to the first findings will be
explained.
Example 1
[0149] Ferritic-austenitic stainless steel 250 mm thick cast slabs
if the ingredients shown in Table 2 were produced and hot rolled
into hot rolled steel plates with 5.0 mm plate thicknesses. Steel
No. 1 to Steel No. 20 have ingredients defined in the present
invention. Steel No. 21 to 26 have ingredients deviating from the
definitions in the present invention. These hot rolled steel plates
were annealed and pickled, then cold rolled to 1 mm thicknesses and
final annealed. The final annealing was also performed under
conditions deviating from the definitions of the present invention
for comparison.
[0150] Various test pieces were taken from each of the obtained
cold rolled and annealed sheets and evaluated for the .gamma. phase
volume fraction (.gamma. fraction), pitting potential, and uniform
elongation. The .gamma. fraction was found by the EBSP method
described in paragraph 0046. For the pitting potential, the V'c100
(Vv.s.AGCL) was measured for a #500 polished surface in a
30.degree. C., 3.5% NaCl aqueous solution. The measurement value of
the pitting potential was made the average value of n3. For the
uniform elongation, a JIS 13 B test piece was taken from the
rolling direction and the value was measured at a tension rate of
20 mm/min (range of the tension rate defined in JIS Z 2241).
TABLE-US-00002 TABLE 2 Steel Chemical ingredients (mass %) No C Si
Mn Cr N Ni Cu Other PI Remarks 1 0.06 0.1 2.9 20.8 0.14 -- -- --
19.3 Inv. ex. 2 0.03 0.1 3.2 21.2 0.09 1.0 0.47 18.9 Inv. ex. 3
0.01 0.1 3.1 21.0 0.11 0.9 0.45 19.0 Inv. ex. 4 0.06 0.1 3.0 20.8
0.10 1.0 0.50 18.8 Inv. ex. 5 0.03 0.8 3.0 21.2 0.10 1.0 0.48 19.2
Inv. ex. 6 0.03 0.1 0.5 21.0 0.11 1.0 0.47 21.6 Inv. ex. 7 0.03 0.1
1.5 21.0 0.11 1.0 0.48 20.6 Inv. ex. 8 0.03 0.1 3.7 21.0 0.11 1.0
0.48 18.4 Inv. ex. 9 0.03 0.1 3.0 21.0 0.06 1.0 0.47 18.6 Inv. ex.
10 0.03 0.1 3.1 21.0 0.14 1.0 0.47 19.3 Inv. ex. 11 0.03 0.1 0.8
17.5 0.14 1.0 0.45 18.1 Inv. ex. 12 0.03 0.1 3.7 24.0 0.11 1.5 0.70
21.4 Inv. ex. 13 0.03 0.1 3.2 21.0 0.11 0.6 0.45 18.9 Inv. ex. 14
0.03 0.1 3.2 21.0 0.10 2.7 0.45 18.8 Inv. ex. 15 0.03 0.1 3.1 21.0
0.10 1.0 0.10 18.9 Inv. ex. 16 0.03 0.1 3.1 21.0 0.10 1.0 2.80 18.9
Inv. ex. 17 0.03 0.1 3.2 21.0 0.10 1.1 0.45 Mo: 0.7 20.9 Inv. ex.
18 0.03 0.1 3.2 21.0 0.10 1.1 0.45 Nb: 0.1 18.8 Inv. ex. 19 0.03
0.1 3.1 21.0 0.10 1.0 0.46 Mo: 0.3, Nb: 0.05 19.8 Inv. ex. 20 0.03
0.1 3.1 21.0 0.10 1.0 0.48 Ti: 0.1 18.9 Inv. ex. 21 0.03 0.1 3.1
21.0 0.10 1.0 0.48 Al: 0.05 18.9 Inv. ex. 22 0.03 0.1 3.1 21.0 0.10
1.0 0.48 Al: 0.03, B: 0.001 18.9 Inv. ex. 23 0.03 0.1 3.1 21.0 0.10
1.0 0.48 Ca: 0.002, Mg: 0.001 18.9 Inv. ex. 24 0.01 0.3 0.9 20.0
0.20 0.5 0.53 21.1 Comp. ex. 25 0.01 0.3 5.0 20.0 0.26 0.5 0.50
17.6 Comp. ex. 26 0.01 0.3 5.2 21.5 0.10 0.5 0.55 17.3 Comp. ex. 27
0.11 0.5 1.3 21.2 0.15 1.2 -- 21.4 Comp. ex. 28 0.04 1.1 3.1 21.3
0.10 1.0 0.48 19.2 Comp. ex. 29 0.01 0.3 0.3 16.5 0.13 1.0 0.55
17.5 Comp. ex. --: means not added. Underlines mean deviation from
the definition of the present invention
[0151] The relationship of the production conditions and the
.gamma. fraction and characteristics of the final annealed sheet
are shown in Table 3. Here, the "Cooling rate 1" shows the average
cooling rate from the annealing temperature to 200.degree. C.
However, when held during cooling, the average cooling rate from
the annealing temperature to the holding temperature is shown.
Further, the "Cooling rate 2" shows the average cooling rate from
the holding temperature to ordinary temperature when holding during
cooling.
[0152] Nos. 1 to 11 and 15 to 35 have the ingredients of the
present invention and final annealed as defined in the present
invention. These invention examples satisfy the .gamma. fraction 15
to 50% defined in the present invention and have 0.3V or more
pitting potentials and 30% or more uniform elongations. Due to
this, by subjecting the ferritic-austenitic stainless steels having
the ingredients defined in the present invention to the final
annealing defined in the present invention, a corrosion resistance
equal to or greater than SUS 304 in a neutral chloride environment
is provided and a ductility sufficiently high compared to SUS 430LX
and comparable to SUS 304 is obtained. Particularly, Nos. 9 to 11
are examples in which, as the final annealing condition, the steels
are held for approximately two minutes at a specified temperature
of the 200 to 600.degree. C. temperature region with final
annealing, then cooled from the holding temperature to room
temperature. In these examples, the pitting potential Vc'100 shows
good values.
[0153] Nos. 12 to 14 have the ingredients defined in the present
invention, but deviate from the final annealing conditions defined
in the present invention. They did not give the pitting potential
and uniform elongation of the objects of the present invention.
[0154] Nos. 36 to 41 have ingredients deviating from the definition
of the present invention. Even if subjected to final annealing
defined in the present invention, they did not give the pitting
potential and uniform elongation of the objects of the present
invention.
TABLE-US-00003 TABLE 3 Characteristics Final annealing conditions
Pitting Cooling Cooling .gamma. phase potential Uniform Steel Temp.
rate 1 Holding rate 2 volume Vc'100 elongation No. No. (.degree.
C.) (.degree. C./sec) (.degree. C.) (.degree. C./sec) fraction %
(V) (%) Remarks 1 1 1000 33 -- -- 30 0.34 32.5 Inv. ex. 2 2 1000 32
-- -- 30 0.37 33.5 Inv. ex. 3 2 1000 5 -- -- 30 0.33 33.0 Inv. ex.
4 2 1000 400 -- -- 30 0.40 32.0 Inv. ex. (Water cooling) 5 2 960 35
-- -- 20 0.35 31.0 Inv. ex. 6 2 1050 30 -- -- 35 0.36 38.0 Inv. ex.
7 2 1100 25 -- -- 35 0.35 35.0 Inv. ex. 8 2 1130 25 -- -- 30 0.33
33.0 Inv. ex. 9 2 1000 25 300 20 30 0.40 34.0 Inv. ex. 10 2 1000 25
400 20 33 0.42 35.0 Inv. ex. 11 2 1000 25 450 20 33 0.38 34.5 Inv.
ex. 12 2 900 30 -- -- 10 0.35 21.5 Comp. ex. 13 2 1180 30 -- -- 12
0.35 25.0 Comp. ex. 14 2 1000 2 -- -- 25 0.20 30.0 Comp. ex. 15 3
1000 32 -- -- 27 0.37 32.5 Inv. ex. 16 4 1000 33 -- -- 35 0.36 33.5
Inv. ex. 17 5 1000 30 -- -- 17 0.31 30.0 Inv. ex. 18 6 1000 32 --
-- 20 0.45 31.0 Inv. ex. 19 7 1000 32 -- -- 25 0.40 32.0 Inv. ex.
20 8 1000 30 -- -- 45 0.35 40.0 Inv. ex. 21 9 1000 30 -- -- 16 0.30
30.0 Inv. ex. 22 10 1000 32 -- -- 35 0.38 35.0 Inv. ex. 23 11 1000
32 -- -- 20 0.32 30.0 Inv. ex. 24 12 1000 33 -- -- 30 0.37 32.0
Inv. ex. 25 13 1000 33 -- -- 18 0.33 31.0 Inv. ex. 26 14 1000 33 --
-- 45 0.38 40.0 Inv. ex. 27 15 1000 32 -- -- 25 0.32 31.0 Inv. ex.
28 16 1000 33 -- -- 35 0.37 37.0 Inv. ex. 29 17 1000 32 -- -- 25
0.50 31.0 Inv. ex. 30 18 1000 32 -- -- 30 0.40 33.0 Inv. ex. 31 19
1000 33 -- -- 27 0.45 31.0 Inv. ex. 32 20 1000 32 -- -- 25 0.39
31.0 Inv. ex. 33 21 1000 30 -- -- 28 0.33 32.0 Inv. ex. 34 22 1000
31 -- -- 28 0.34 31.5 Inv. ex. 35 23 1000 32 -- -- 30 0.32 32.5
Inv. ex. 36 24 1050 33 -- -- 45 0.20 43.0 Comp. ex. 37 25 1050 32
-- -- 60 0.15 50.0 Comp. ex. 38 26 1050 33 -- -- 20 0.20 38.0 Comp.
ex. 39 27 1050 32 -- -- 55 0.20 35.0 Comp. ex. 40 28 1000 32 -- --
20 0.20 28.0 Comp. ex. 41 29 1000 32 -- -- 10 0.25 20.0 Comp. ex.
Cooling rate 1: Average cooling rate from annealing temperature to
200.degree. C. and, when holding during cooling, average cooling
rate from annealing temperature to holding temperature Cooling rate
2: When holding during cooling, the average cooling rate from the
holding temperature to ordinary temperature --: means continuous
cooling with no holding during cooling Underlines mean deviation
from the definition of the present invention
Example 2
[0155] Next, examples according to the second findings will be
shown.
[0156] The steels shown in Table 4 were produced, then were hot
rolled, then the hot rolled plates were annealed, cold rolled, and
final annealed to produce 1.0 mm thick thin-gauge steel sheets. In
producing the steel sheets, the metal structure can be changed by
changing the material thickness, heating temperature of hot
rolling, rolling pass schedule, rolling pass time, hot rolled sheet
annealing temperature, and final annealing temperature and time.
However, this time, the final annealing temperature was changed,
and the annealing time was made 60 seconds. The obtained product
sheets were subjected to tensile tests and the uniform elongation
was measured. Further, from the metal structure of the thin-gauge
steel sheet/L cross-section, the phases were identified by EBSP,
the grain size and shape aspect ratio were investigated, and the
distance between nearest grains of the austenite grains was
measured. The conditions were as mentioned above. The .gamma.
fraction, Md value, X1, and X2 were measured for the obtained metal
structure. The relationship with the uniform elongation was shown
in Table 5 together with the production conditions.
TABLE-US-00004 TABLE 4 Steel Chemical ingredients (mass %) No. C Si
Mn P S Cr N Ni Cu Mo Nb Ti Ca Mg Remarks 101 0.030 0.13 3.10 0.031
0.004 21.0 0.102 0.99 0.48 0.38 Inv. ex. 102 0.029 0.10 0.51 0.033
0.002 21.1 0.111 1.01 0.47 Inv. ex. 103 0.085 0.95 4.30 0.03 0.001
20.0 0.086 Inv. ex. 104 0.004 1.85 4.22 0.019 0.001 20.0 0.134 0.54
0.22 0.43 Inv. ex. 105 0.018 0.35 3.10 0.039 0.002 17.6 0.144 0.04
Inv. ex. 106 0.065 0.56 1.11 0.035 0.001 24.2 0.089 2.64 0.0006
0.0020 Inv. ex. 107 0.045 0.06 0.09 0.022 0.002 24.6 0.095 2.65
0.05 0.0004 Inv. ex. 108 0.089 1.45 4.01 0.032 0.002 19.3 0.142
2.01 0.40 Inv. ex. 109 0.078 0.98 4.75 0.03 0.001 20.8 0.085 0.0028
Inv. ex. 110 0.011 0.35 2.88 0.038 0.001 21.0 0.098 3.40 Inv. ex.
111 0.004 1.58 1.95 0.016 0.001 19.9 0.035 4.30 Inv. ex. 112 0.028
1.11 2.55 0.029 0.002 18.6 0.123 0.50 0.12 0.12 Inv. ex. 113 0.098
0.19 2.87 0.042 0.003 21.5 0.088 1.06 0.55 0.05 Inv. ex. 114 0.028
0.08 2.98 0.034 0.001 21.3 0.110 1.02 0.55 Inv. ex. 115 0.025 0.95
5.65 0.034 0.001 20.1 0.118 0.11 Comp. ex. Underlines mean
deviation from the definition of the present invention
TABLE-US-00005 TABLE 5 .gamma. Uniform Steel T.sub.1 N R T.sub.2
T.sub.3 fraction X.sub.1 X.sub.2 elongation Condition No. (.degree.
C.) (X) (%) (.degree. C.) (.degree. C.) (%) Md (%) (.mu.m) (%) 1a
101 1240 1 98 1150 1050 34 49 92 9 42 Inv. ex. 1b 1200 2 97 1220
1020 36 44 89 18 36 Comp. ex. 1c 1260 1 98 1200 1100 19 42 80 11 35
Comp. ex. 2a 102 1200 1 90 1150 1080 38 89 90 21 36 Comp. ex. 2b
1190 1 96 1180 1080 35 72 95 8 43 Inv. ex. 2c 1200 2 97 1180 1140 9
44 92 18 35 Comp. ex. 3a 103 1160 1 97 1160 980 45 78 82 5 32 Comp.
ex. 3b 1200 2 98 1100 1080 40 71 98 7 42 Inv. ex. 3c 1200 0 97 1200
1020 43 82 78 9 36 Comp. ex. 4a 104 1120 1 94 1100 1000 35 21 77 12
37 Comp. ex. 4b 1150 1 96 1140 1100 27 15 100 11 43 Inv. ex. 4c
1150 2 97 1200 1100 40 32 91 13 37 Comp. ex. 5a 105 1200 1 98 1180
1080 48 82 94 7 43 Inv. ex. 5b 1240 1 98 1000 1120 59 100 68 12 35
Comp. ex. 5c 1120 1 76 1120 1050 40 78 72 10 34 Comp. ex. 6a 106
1250 1 95 1160 1100 25 -9 90 21 36 Comp. ex. 6b 1250 2 96 1170 1100
21 -5 95 8 43 Inv. ex. 6c 1250 1 97 1260 1120 10 -19 92 14 31 Comp.
ex. 7a 107 1160 1 98 1160 900 28 13 80 6 30 Comp. ex. 7b 1160 1 96
1140 1100 12 -9 100 8 43 Inv. ex. 7c 1160 0 96 1100 1050 20 -2 78 6
35 Comp. ex. 8a 108 1100 0 94 1100 1150 2 -11 100 35 27 Comp. ex.
8b 1200 1 96 1150 1100 36 -5 98 6 44 Inv. ex. 8c 1200 1 96 1250
1100 4 5 73 19 35 Comp. ex. 9a 109 1250 2 98 1180 1080 34 78 91 8
42 Inv. ex. 9b 1250 2 98 950 1050 33 80 81 6 34 Comp. ex. 9c 1270 2
98 1200 1100 25 90 89 15 35 Comp. ex. 10a 110 1200 1 93 1160 1100
34 0 88 9 36 Comp. ex. 10b 1200 1 98 1100 1000 48 10 95 5 42 Inv.
ex. 10c 1150 1 96 1100 1110 21 -15 91 13 34 Comp. ex. 11a 111 1150
1 97 1150 900 52 89 40 5 31 Comp. ex. 11b 1200 1 97 1150 1100 24 70
97 9 42 Inv. ex. 11c 1200 0 97 1150 1050 35 60 72 12 35 Comp. ex.
12a 112 1100 0 96 1100 1000 24 64 62 5 30 Comp. ex. 12b 1180 1 97
1160 1050 30 68 98 8 43 Inv. ex. 12c 1160 1 97 1180 1080 21 78 88
13 35 Comp. ex. 13a 113 1220 1 97 1190 1080 46 38 90 8 42 Inv. ex.
13b 1200 1 97 1250 1080 45 37 80 14 35 Comp. ex. 13c 1260 0 96 1200
1100 37 25 64 9 35 Comp. ex. 14a 114 1230 1 97 1200 1080 27 52 95
10 42 Inv. ex. 14b 1220 1 97 1250 1000 34 43 87 16 35 Comp. ex. 14c
1280 2 96 1190 1070 30 40 68 10 34 Comp. ex. 15a 115 1150 1 97 1150
1050 40 56 100 11 34 Comp. ex. 15b 1170 1 98 1100 1050 41 55 90 11
32 Comp. ex. 15c 1150 1 97 1200 1080 32 34 91 13 31 Comp. ex. The
notations in Table 5 are as shown below. T1: Heating temperature
(.degree. C.) before hot rolling N: Number of times of rolling in
the hot rolling process comprising rolling at 1000.degree. C. or
more with a 30% or higher reduction rate then holding for 30 sec or
more R: Hot rolling total reduction rate (%) T2: Hot rolled sheet
annealing temperature (.degree. C.) T3: Final annealing temperature
(.degree. C.) X1: Ratio of austenite grains with a grain size of 15
.mu.m or less and a shape aspect ratio of less than 3 in total
austenite grains X2: Average value of distance of each austenite
grain from nearest grain Md: Value calculated by the formula below
from the composition (average composition only for C) in the
austenite phase Md = 551-462({C} + [N]) - 9.2[Si] - 8.1[Mn] -
13.7[Cr] - 29[Ni] - 29[Cu] - 18.5[Mo] where [ ] is composition
(mass %) in the austenite phase, and { } is average composition
(mass %).
[0157] Condition 1a is an invention example where extremely good
uniform elongation is obtained. In condition 1b, T2 does not
satisfy the present invention range, so X1 and X2 deviate from the
present invention. Further, in condition 1c, T1 does not satisfy
the present invention range, so X1 deviates from the present
invention.
[0158] In condition 2a, R does not satisfy the present invention
range, so X2 deviates from the present invention. Condition 2b is
an invention example where extremely good uniform elongation is
obtained. In condition 2c, T3 does not satisfy the present
invention range, so the .gamma. fraction and X2 deviate from the
present invention.
[0159] In condition 3a, T3 does not satisfy the present invention
range, so X1 deviates from the present invention. Condition 3b is
an invention example where extremely good uniform elongation is
obtained. In condition 3c, N does not satisfy the present invention
range, so X1 deviates from the present invention.
[0160] In condition 4a, T1 and R do not satisfy the present
invention range, so X1 deviates from the present invention.
Condition 4b is an invention example where extremely good uniform
elongation is obtained. In condition 4c, T2 does not satisfy the
present invention range, so X2 deviates from the present
invention.
[0161] Condition 5a is an invention example where extremely good
uniform elongation is obtained. In condition 5b, T2 and T3 do not
satisfy the present invention range, so the .gamma. fraction and X1
deviate from the present invention. In condition 5c, T1 does not
satisfy the present invention range, so X1 deviates from the
present invention.
[0162] In condition 6a, R does not satisfy the present invention
range, so X2 deviates from the present invention. Condition 6b is
an invention example where extremely good uniform elongation is
obtained. In condition 6c, T2 and T3 do not satisfy the present
invention range, so Md and X2 deviate from the present
invention.
[0163] In condition 7a, T3 does not satisfy the present invention
range, so X1 deviates from the present invention. Condition 7b is
an invention example where extremely good uniform elongation is
obtained. In condition 7c, N does not satisfy the present invention
range, so X1 deviates from the present invention.
[0164] In condition 8a, T1, N, R, and T3 do not satisfy the present
invention range, so the .gamma. fraction, Md, and X2 deviate from
the present invention. Condition 8b is an invention example where
extremely good uniform elongation is obtained. In condition 8c, T2
does not satisfy the present invention range, so X1 and X2 deviate
from the present invention.
[0165] Condition 9a is an invention example where extremely good
uniform elongation is obtained. In condition 9b, T2 does not
satisfy the present invention range, so X1 deviates from the
present invention. In condition 9c, T1 does not satisfy the present
invention range, so X1 and X2 deviate from the present
invention.
[0166] In condition 10a, R does not satisfy the present invention
range, so X1 deviates from the present invention. Condition 10b is
an invention example where extremely good uniform elongation is
obtained. In condition 10c, T3 does not satisfy the present
invention range, so Md and X2 deviate from the present
invention.
[0167] In condition 11a, T3 does not satisfy the present invention
range, so .gamma. fraction and X1 deviate from the present
invention. Condition 11b is an invention example where extremely
good uniform elongation is obtained. In condition 11c, N does not
satisfy the present invention range, so X1 deviates from the
present invention.
[0168] In condition 12a, T1 and N do not satisfy the present
invention range, so X1 deviates from the present invention.
Condition 12b is an invention example where extremely good uniform
elongation is obtained. In condition 12c, T2 does not satisfy the
present invention range, so X1 and X2 deviate from the present
invention.
[0169] Condition 13a is an invention example where extremely good
uniform elongation is obtained. In condition 13b, T2 does not
satisfy the present invention range, so X1 and X2 deviate from the
present invention. In condition 13c, T1 and N do not satisfy the
present invention range, so X1 deviates from the present
invention.
[0170] Condition 14a is an invention example where extremely good
uniform elongation is obtained. In condition 14b, T2 does not
satisfy the present invention range, so X1 and X2 deviate from the
present invention. In condition 14c, T1 does not satisfy the
present invention range, so X1 deviates from the present
invention.
[0171] In each of the conditions 15a, 15b, and 15c, the ingredient
systems do not satisfy the present invention range, so extremely
good uniform elongation is not obtained.
[0172] As explained above, in the invention examples, extremely
good uniform elongation is obtained. In the comparative examples,
either of the .gamma. fraction, Md value, X1, or X2 does not
satisfy the conditions, so the uniform elongation is low.
INDUSTRIAL APPLICABILITY
[0173] According to the first findings of the present invention, by
defining the steel ingredients and .gamma. fraction and controlling
the final annealing conditions, ferritic-austenitic stainless steel
excellent in corrosion resistance and workability having a
corrosion resistance equal to or greater than SUS 304 in a neutral
chloride environment can be produced.
[0174] Further, according to the second findings of the present
invention, ferritic-austenitic-based stainless thin-gauge steel
sheet excellent in workability, particularly in uniform elongation,
can be obtained without having to include a large amount of Ni.
[0175] The present invention can be applied to parts where
conventionally austenitic stainless steel sheets containing large
amounts of Ni have been used, for example, kitchen appliances, home
electric appliances, electronic equipment, etc. used in a neutral
chloride environment, and other broad fields, so from the viewpoint
of Ni resource conservation, it greatly contributes to the
environment.
* * * * *