U.S. patent application number 13/256977 was filed with the patent office on 2012-01-12 for duplex stainless steel sheet with excellent press-formability.
Invention is credited to Masaharu Hatano, Eiichiro Ishimaru, Ken Kimura, Akihiko Takahashi.
Application Number | 20120009433 13/256977 |
Document ID | / |
Family ID | 42739787 |
Filed Date | 2012-01-12 |
United States Patent
Application |
20120009433 |
Kind Code |
A1 |
Hatano; Masaharu ; et
al. |
January 12, 2012 |
DUPLEX STAINLESS STEEL SHEET WITH EXCELLENT PRESS-FORMABILITY
Abstract
The present invention provides a duplex stainless steel sheet
with excellent press-formability which defines the ingredients of
the steel, Ni balance, and austenite phase rate to obtain a 0.2%
proof stress and Erichsen value equal to that of SUS304 or another
austenitic stainless steel, that is, a duplex stainless steel sheet
which contains, by mass %, C: 0.05% or less, Si: 0.5 to 3%, Mn: 1
to 5%, Cr: 16 to 21%, Ni: 1 to 6%, Cu: 0.5 to 3%, and N: 0.07% or
less, has an Ni-bal. value given by the following formula <1>
of -7.5 to -3.5, has a balance of Fe and unavoidable impurities,
has an austenite phase rate of 50% to 95%, and has a balance of
ferrite phases: Ni-bal.=30(C+N)+Ni+0.5Mn+0.3Cu-1.1(Cr+1.5Si)+8.2
<1>
Inventors: |
Hatano; Masaharu; (Tokyo,
JP) ; Takahashi; Akihiko; (Tokyo, JP) ;
Ishimaru; Eiichiro; (Tokyo, JP) ; Kimura; Ken;
(Tokyo, JP) |
Family ID: |
42739787 |
Appl. No.: |
13/256977 |
Filed: |
March 17, 2010 |
PCT Filed: |
March 17, 2010 |
PCT NO: |
PCT/JP2010/055147 |
371 Date: |
September 16, 2011 |
Current U.S.
Class: |
428/577 |
Current CPC
Class: |
C22C 38/58 20130101;
C22C 38/42 20130101; C21D 2211/001 20130101; C21D 6/004 20130101;
C21D 2211/005 20130101; C21D 8/0236 20130101; C22C 38/34 20130101;
C21D 9/46 20130101; Y10T 428/12229 20150115; C22C 38/001
20130101 |
Class at
Publication: |
428/577 |
International
Class: |
B21C 1/00 20060101
B21C001/00 |
Claims
1. A duplex stainless steel sheet with excellent press-formability
characterized by containing, by mass %, C: 0.05% or less, Si: 0.5
to 3%, Mn: 1 to 5%, Cr: 16 to 21%, Ni: 1 to 6%, Cu: 0.5 to 3%, and
N: 0.07% or less having an Ni-bal value given by the following
formula <1> of -7.5 to -3.5, having a balance of Fe and
unavoidable impurities, having an austenite phase rate of 50% to
95%, and having a balance of ferrite phases:
Ni-bal=30(C+N)+Ni+0.5Mn+0.3Cu-1.1(Cr+1.5Si)+8.2 formula
<1>
2. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 1 characterized in that said steel further
contains, by mass %, one or more of: Mo: 1% or less, Nb: 0.5% or
less, V: 0.5% or less, Ti: 0.5% or less, Sn: 1% or less, Sb: 1% or
less, W: 1% or less, and Al: 0.1% or less.
3. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 1 or 2 characterized in that said steel
further contains, by mass %, one or more of B: 0.01% or less, Ca:
0.01% or less, Mg: 0.01% or less, La: 0.3% or less, Ce: 0.3% or
less, Zr: 0.3% or less, and Y: 0.3% or less.
4. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 1 or 2 characterized in that a 0.2% proof
stress in a tensile test is less than 400 MPa and an elongation at
break is 35% or more.
5. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 3 characterized in that a 0.2% proof stress
in a tensile test is less than 400 MPa and an elongation at break
is 35% or more.
6. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 1 or 2 characterized in that a formed height
found by an Erichsen test (Erichsen value) is 11 mm or more.
7. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 3 characterized in that a formed height found
by an Erichsen test (Erichsen value) is 11 mm or more.
8. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 4 characterized in that a formed height found
by an Erichsen test (Erichsen value) is 11 mm or more.
9. A duplex stainless steel sheet with excellent press-formability
as set forth in claim 5 characterized in that a formed height found
by an Erichsen test (Erichsen value) is 11 mm or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to duplex stainless steel
sheet with excellent press-formability such as stretchability.
BACKGROUND ART
[0002] Austenitic stainless steel such as SUS304 is excellent in
balance of corrosion resistance and workability, so is being used
for a broad range of applications such as for kitchen appliances,
household electric appliances, and electronic equipment. In
general, austenitic stainless steel is much higher in elongation at
break compared with ferritic stainless steel or duplex stainless
steel, is excellent in stretchability, and is often preferred for
press-formability of steel sheet. However, austenitic stainless
steel contains large amounts of the rare and expensive Ni, so has
problems in general applicability and economy in the future.
[0003] In the past, as an alternative to austenitic stainless
steel, duplex stainless steel which conserves on the amount of Ni
has been known. PLTs 1 to 3 disclose high strength duplex stainless
steels for automotive use which contain Ni: 1 to 7%, Si: over 1 to
5%, N: 0.04 to 2%, and Cr: 17 to 22% and which have Mn, Cu, etc.
added to adjust the Ni balance value and raise the Young's modulus.
These duplex stainless steels are characterized by high Si and low
Ni and are provided with a high strength of a 0.2% proof stress of
over 500 MPa and a high elongation.
[0004] In recent years, austenitic-ferritic stainless steel which
further conserves on the content of Ni and has a relatively large
amount of N added to give a high ductility has been reported. PLT 4
and PLT 5 disclose austenitic-ferritic stainless steels with
excellent formability which restrict the amount of Ni to 3% or less
and adjust the C+N and ingredient balance in the austenite phases
to obtain a high ductility. As art related to this, PLT 6 discloses
austenitic-ferritic stainless steel with excellent stretchability
and crevice corrosion resistance which restricts the amount of Ni
to 1% or less and the amount of Mn to 2% or less and adds an amount
of N in 0.05 to 0.6% in range. In the examples of the above
publication, the amount of Ni is reduced by adding an amount of N
in at least 0.08% or more.
[0005] Recently, PLT 7 has disclosed ferritic-austenitic stainless
steels with excellent corrosion resistance and workability which
make the upper limit of the amount of N 0.15% and thereby lower the
amount of Ni. These stainless steels set Cr+3Mo+10N-Mn.gtoreq.18%
from the viewpoint of the corrosion resistance and define the size,
aspect ratio, and intergranular distance of austenite grains from
the viewpoint of the workability. The steels disclosed in the above
publication have less than 50% of austenite phases and are mainly
comprised of ferrite phases.
[0006] The steels disclosed in the above PLTs use quite a bit of N
to conserve Ni and raise the strength. Much research is being
performed on the effects of N on the mechanical properties of
stainless steel and other ferrous metal materials. Addition of N
has a large effect on the rise of the 0.2% proof stress. For
example, in NPLT 1, if adding over 0.1% of N to the Fe--Cr--Ni--Mn
alloy, at ordinary temperature, the 0.2% proof stress greatly
exceeds 400 MPa. In actuality, the steels disclosed in PLTs 1 to 3
have 0.2% proof stresses of over 500 MPa. PLTs 4 to 7 do not
describe a 0.2% proof stress, but from NPLT 1, it is easy to deduce
that the value is over 400 MPa.
[0007] As explained above, when the 0.2% proof stress exceeds 400
MPa, generally the value becomes one over 100 MPa higher if
compared with SUS304 and other austenitic stainless steel. For this
reason, in the press-formability of the steel sheet, with current
press machines, the power is insufficient, so forming becomes
difficult and the problem of wear and damage to the dies becomes
feared. In other words, at the present time, how to obtain
press-formable steel sheet no different from SUS304 and other
austenitic stainless steel sheet in duplex stainless steel
conserving Ni has not yet been clarified.
CITATION LIST
Patent Literature
[0008] PLT 1: Japanese Patent Publication (A) No. 62-47461
[0009] PLT 2: Japanese Patent Publication (A) No. 62-47462
[0010] PLT 3: Japanese Patent Publication (A) No. 62-47463
[0011] PLT 4: Japanese Patent Publication (A) No. 2006-169622
[0012] PLT 5: Japanese Patent Publication (A) No. 2006-183129
[0013] PLT 6: Japanese Patent Publication (A) No. 2006-200035
[0014] PLT 7 WO2009/017258
Non-Patent Literature
[0015] NPLT 1: 190th Nishiyama Commemorative Technical Course,
November 2006, the Iron and Steel Institute of Japan, p. 60
[0016] NPLT 2: Nippon Stainless Steel Technical Reports, No. 21
(1986), p. 3 to 5
SUMMARY OF INVENTION
Technical Problem
[0017] The present invention has as its object to provide duplex
stainless steel sheet with excellent press-formability provided
with a 0.2% proof stress and Erichsen value equivalent to those of
SUS304 and other austenitic stainless steel by controlling the
steel ingredients, the Ni balance, and the austenite phase
rate.
Solution to Problem
[0018] The inventors engaged in intensive research on the effects
of ingredients, the Ni balance, and the austenite phase rate on the
0.2% proof stress and Erichsen value of duplex stainless steel
conserving Ni so as to solve the above problem and thereby
completed the present invention:
[0019] The gist of the present invention is as follows:
[0020] (1) A duplex stainless steel sheet with excellent
press-formability characterized by containing, by mass %, [0021] C:
0.05% or less, [0022] Si: 0.5 to 3%, [0023] Mn: 1 to 5%, [0024] Cr:
16 to 21%, [0025] Ni: 1 to 6%, [0026] Cu: 0.5 to 3%, and [0027] N:
0.07% or less having an Ni-bal value given by the following formula
<1> of -7.5 to -3.5, having a balance of Fe and unavoidable
impurities, having an austenite phase rate of 50% to 95%, and
having a balance of ferrite phases:
[0027] Ni-bal.=30(C+N)+Ni+0.5Mn+0.3Cu-1.1(Cr+1.5Si)+8.2 formula
<1>
[0028] (2) A duplex stainless steel sheet with excellent
press-formability as set forth in (1) characterized in that said
steel further contains, by mass %, one or more of: [0029] Mo: 1% or
less, [0030] Nb: 0.5% or less, [0031] V: 0.5% or less, [0032] Ti:
0.5% or less, [0033] Sn: 1% or less, [0034] Sb: 1% or less, [0035]
W: 1% or less, and [0036] Al: 0.1% or less.
[0037] (3) A duplex stainless steel sheet with excellent
press-formability as set forth in (1) or (2) characterized in that
said steel further contains, by mass %, one or more of [0038] B:
0.01% or less, [0039] Ca: 0.01% or less, [0040] Mg: 0.01% or less,
[0041] La: 0.3% or less, [0042] Ce: 0.3% or less, [0043] Zr: 0.3%
or less, and [0044] Y: 0.3% or less.
[0045] (4) A duplex stainless steel sheet with excellent
press-formability as set forth in (1) or (2) characterized in that
a 0.2% proof stress in a tensile test is less than 400 MPa and an
elongation at break is 35% or more.
[0046] (5) A duplex stainless steel sheet with excellent
press-formability as set forth in (3) characterized in that a 0.2%
proof stress in a tensile test is less than 400 MPa and an
elongation at break is 35% or more.
[0047] (6) A duplex stainless steel sheet with excellent
press-formability as set forth in (1) or (2) characterized in that
a formed height found by an Erichsen test (Erichsen value) is 11 mm
or more.
[0048] (7) A duplex stainless steel sheet with excellent
press-formability as set forth in (3) characterized in that a
formed height found by an Erichsen test (Erichsen value) is 11 mm
or more.
[0049] (8) A duplex stainless steel sheet with excellent
press-formability as set forth in (4) characterized in that a
formed height found by an Erichsen test (Erichsen value) is 11 mm
or more.
[0050] (9) A duplex stainless steel sheet with excellent
press-formability as set forth in (5) characterized in that a
formed height found by an Erichsen test (Erichsen value) is 11 mm
or more.
[0051] In the following explanation, the inventions according to
the steels of the above (1) to (9) will be called "present
inventions". Further, the inventions of (1) to (9) combined will
sometimes be called "the present invention".
Advantageous Effects of Invention
[0052] According to the present invention, it is possible to
provide a duplex stainless steel sheet with excellent
press-formability which defines the ingredients of the steel, Ni
balance, and austenite phase rate to obtain a 0.2% proof stress and
Erichsen value equal to that of SUS304 or another austenitic
stainless steel. The remarkable effect is exhibited that the duplex
stainless steel sheet of the present invention can be
pressed-formed no different from SUS304 or other austenitic
stainless steel sheet and Ni can be conserved.
BRIEF DESCRIPTION OF DRAWINGS
[0053] FIG. 1 is a view showing the relationship between the
Erichsen value and ingredients.
[0054] FIG. 2 is a view showing the relationship between the
Erichsen value and Ni balance.
DESCRIPTION OF EMBODIMENTS
[0055] The inventors engaged in intensive research on the effects
of ingredients, the Ni balance, and the austenite phase rate on the
0.2% proof stress and Erichsen value of duplex stainless steel
conserving Ni so as to solve the above problem and thereby
completed the present invention. Below, representative experiment
findings will be explained.
[0056] Table 1 shows representative ingredients of the test steel.
Duplex stainless steels of these ingredients were vacuum melted and
used to produce 5 mm thick hot rolled sheets. The hot rolled sheets
were annealed at 1050.degree. C. and then pickled to produce 0.6 mm
thick cold rolled sheets. The cold rolled sheets were annealed at
1050.degree. C. The cold rolled annealed sheets were measured for
austenite (.gamma.) phase rate and were used for a JIS No. 13B
tensile test and Erichsen test.
TABLE-US-00001 TABLE 1 Chemical ingredients (mass %) C Si Mn Cr Ni
Cu N Ni-bal A 0.015 1.9 4.0 17.1 4.9 1.9 0.025 -5.1 B 0.028 0.1 3.4
21.3 1.6 0.5 0.086 -8.5 C 0.030 0.3 4.6 20.5 3.7 0.5 0.150 -3.3
Ni-bal = 30(C + N) + 0.5Mn + 0.3Cu + Ni - 1.1(Cr + 1.5Si) + 8.2
[0057] The .gamma.-phase rate was found by measurement of a phase
map identifying the fcc and bcc crystal structures by the EBSP
method at the sheet cross-sections. The JIS No. 13B tensile test
obtains a tensile test piece from the rolling direction, sets the
tensile speed at 10 mm/min (range prescribed in JIS Z 2241), and
measures the 0.2% proof stress (0.2% PS), tensile strength (TS),
and elongation at break (EL). The Erichsen test obtains a 90 mm
square test piece, performs the Method B based on JIS Z 2247
(wrinkle pressure of 1 ton), and measures the deformed height when
a crack runs through the sheet thickness (Erichsen value).
[0058] Table 2 shows the mechanical properties, Erichsen value
(Er), and .gamma.-phase rate (.gamma.) obtained from sheets of
typical test sheet ingredients compared with ferrite (.alpha.) and
.gamma. single phase SUS430LX and SUS304 steels. As will be
understood from Table 2, the steel A has an Erichsen value no
different from SUS304. On the other hand, the steels B and C to
which N has been added have high elongations, but have much higher
0.2% proof stresses compared with SUS304 and have Erichsen values
equal to or lower than that of .alpha.-based SUS430LX.
TABLE-US-00002 TABLE 2 0.2% PS TS Er .gamma. N/mm.sup.2 N/mm.sup.2
EL % mm vol % A 320 700 47 12.5 75 B 427 700 46 9.5 30 C 535 790 47
10.0 70 S430LX 270 400 34 10.0 0 S304 300 700 51 12.8 100 Er:
Erichsen test value
[0059] Normally, it is known that the Erichsen value rises
proportionally with the elongation at break of a material. However,
as explained above, high strength duplex stainless steel to which N
is added does not necessarily have an Erichsen value commensurate
with high elongation. That is, the steels B and C to which N is
added sometimes cannot give a high workability in a mode of
deformation envisioning press-formation different from a tensile
test. To clarify the reasons for this, the inventors examined parts
of the steels A, B, and C after the tensile test and Erichsen test
near the fractured parts for detailed microstructure by an optical
microscope and scan electron microscope (SEM). As a result, they
obtained the following discoveries explaining the experimental
findings described in Table 2.
[0060] [a] The fractured parts after the tensile test all had
necking along with the reduction of sheet thickness. On the other
hand, after the Erichsen test, the steels B and C low in Erichsen
value fractured without almost any necking.
[0061] [b] Near the fractured parts of the steels B and C after the
Erichsen test, large number of fine voids were formed from near the
boundaries of different phases of .gamma./.alpha.. The state where
these fine voids formed starting points for progression of cracks
through the .alpha.-phases or .gamma./.alpha.-phases boundaries was
observed.
[0062] [c] N concentrates at the .gamma.-phases and raises the
strength and work hardening. Therefore, if adding N, it can be
easily predicted that the difference in strength between the
.gamma.-phases and the .alpha.-phases would increase along with the
working degree. The results of observation of [b] are believed to
be due to the difference in strength between the .alpha.-phases and
the .gamma.-phases.
[0063] [d] The steel A with a high Erichsen value is greatly
suppressed in formation of fine voids from the .alpha./.gamma.
boundary. It was confirmed that the .alpha.-phases follow the large
deformation ability .gamma.-phases and thereby are greatly necked
and fracture in the same way as a tensile test.
[0064] [e] The ingredients of the steel A feature low addition of N
and Si. By reducing N, the strength and work hardening of the
.gamma.-phases fall. Si selectively forms a solid solution in the
.alpha.-phases and raises the strength and work hardening of the
.alpha.-phases. In this way, it is believed that by reducing the
difference in strength of the .alpha.-phases and .gamma.-phases and
making the high deformation ability .gamma.-phases the main phases,
a good Erichsen value is obtained.
[0065] [f] Based on the thinking of the above [e], the inventors
took note of the amount of N and the amount of Si and investigated
in detail the range of ingredients giving a high Erichsen value.
Cr, Ni, Mn, and Cu were adjusted to a range of -9 to -2 by the Ni
balance. FIG. 1 shows the results. In the figure, an Erichsen value
of 11 mm or more is indicated by ".largecircle." while a value of
less than 11 mm is indicated by ".times.", An Erichsen value of 11
mm is difficult to reach with .alpha.-based stainless steel and was
made a threshold value close to .gamma.-based stainless steel. As
will be understood from FIG. 1, it was learned that a high Erichsen
value of 11 mm or more is obtained when the N is made 0.07% or less
and adding Si in 0.5 to 3% in range.
[0066] [g] The inventors arranged the Erichsen values of FIG. 1 by
the Ni-bal. The results are shown in FIG. 2. Here, the Ni-bal is
defined as 30(C+N)+Ni+0.5Mn+0.3Cu-1.1(Cr+1.5Si)+8.2. The Ni-bal is
often used as a parameter relating to the production of the
.gamma.-phases and .alpha.-phases. FIG. 2 also shows the Erichsen
values of the .gamma.-based stainless steel by the same parameter.
In .gamma.-based stainless steel, there is a range of ingredients
giving a good Erichsen value. The reason is the rise in the
elongation due to work induced martensite transformation of the
.gamma.-phases (transformation induced plasticity: TRIP). From this
study, the inventors discovered a balance of ingredients in which
both the elongation and the Erichsen value effectively rise due to
the TRIP phenomenon similar to that of .gamma.-based stainless
steel in the range of ingredients described in [e] and [f] (range
of Ni-bal). That is, the inventors obtained the new discovery that
a high Erichsen value is obtained in an Ni-bal of -7.5 to -3.5 in
range, more preferably -6 to -4 in range.
[0067] The present inventions of (1) to (4) were completed based on
the discoveries of [a] to [g].
[0068] Below, the different requirements of the present invention
will be explained in detail. Note that the indications of "%" in
the contents of the ingredients mean "mass %".
(A) The reasons for limitation of the ingredients will be explained
below.
[0069] C raises the .gamma.-phase rate and concentrates in the
.gamma.-phases to raise the stability of the .gamma.-phases.
Therefore, it effectively acts to adjust the Ni-bal to express the
press-formability targeted by the present invention. To obtain the
above effect, 0.001% or more is preferably contained. However, if
over 0.05%, the strength of the .gamma.-phases rises and
facilitates increased sensitization due to grain boundary
precipitation of the carbides leading to a drop in corrosion
resistance. For this reason, the upper limit is made 0.05%,
preferably 0.03% or less.
[0070] Si selectively forms a solid solution at the .alpha.-phases,
raises the strength and work hardening of the .alpha.-phases, and
reduces the difference in strengths of the .alpha.-phases and the
.gamma.-phases to express the press-formability targeted by the
present invention. It is an essentially added element for this.
Further, it has the action of raising the stability of the
.alpha.-phases and suppressing the martensite transformation in the
cooling process after annealing. If undergoing martensite
transformation, the .alpha.-phases become hard phases and the
workability is remarkably impaired. To obtain the effect on the
workability targeted by the present invention, as shown also in
FIG. 1, 0.5% or more is added. However, over 3% addition invites an
increase in the hardening of the .alpha.-phases and a drop in the
workability. For this reason, the upper limit is made 3%. The
preferable range is 1.5 to 2.5%.
[0071] Mn raises the .gamma.-phase rate and concentrates at the
.gamma.-phases to raise the stability of the .gamma.-phases.
Therefore, it effectively acts to adjust the Ni-bal to express the
press-formability targeted by the present invention. To obtain the
above effect, 1% or more is added. However, if over 5%, in addition
to a drop in the corrosion resistance, the strength of the
.gamma.-phases rises and a drop in the press-formability is
invited. For this reason, the upper limit is made 5%. From the
viewpoints of the workability and the corrosion resistance, the
preferable range is 2 to 4.5%, more preferably 3 to 4%.
[0072] Cr is an element forming .alpha.-phases and also has the
action of securing the corrosion resistance and adjusting the
stability of the .gamma.-phases to express the press-formability
targeted by the present invention. Further, Cr, like Si, suppresses
the martensite transformation of the .alpha.-phases in the cooling
process after annealing. Therefore, to secure the stability of the
.alpha.-phases and the action on the corrosion resistance etc., the
content is made 16% or more. However, if over 21%, making the
.gamma.-phases the main phases becomes difficult. This invites a
drop in the workability targeted by the present invention. For this
reason, the upper limit is made 21% or less. From the viewpoints of
the workability and corrosion resistance, the preferable range is
16.5 to 18.5%.
[0073] Ni is an effective element forming .gamma.-phases and
effectively acts to adjust the Ni-bal to express the
press-formability targeted by the present invention. To obtain this
effect, 1% or more is added. However, if over 6%, it cannot be said
that the Ni is conserved and a rise in the material costs is
incurred. For this reason, the upper limit is made 6%. From the
viewpoints of the workability and costs, the preferable range is 2
to 5%, more preferably 2.5 to 4.5%.
[0074] Cu is an effective element forming .gamma.-phases in the
same way as Ni and Mn and effectively acts to adjust the Ni-bal to
express the press-formability targeted by the present invention. To
obtain this effect, 1% or more is added. Further, it is also an
element effective for improvement of the corrosion resistance by
composite addition with Ni. To obtain this effect, 0.5% or more is
added. However, if over 3%, a drop in the manufacturability and a
rise in the material costs are incurred. For this reason, the upper
limit is made 3%. From the viewpoints of the performance and
manufacturability, the preferable range is 1.5 to 2.5%.
[0075] N, like C and Ni, is an effective element for forming
.gamma.-phases. It effectively acts to adjust the Ni-bal and
expresses press-formability targeted by the present invention. For
this reason, 0.001% or more is preferably contained. On the other
hand, it has the action of raising the strength of the
.gamma.-phases and the work hardening and enlarging the difference
of strength of the .gamma.-phases and .alpha.-phases. For this
reason, when actively utilizing the N in this way, this leads to a
drop in the press-formability targeted by the present invention.
Therefore, as shown in FIG. 1 as well, the upper limit is made
0.07%. From the viewpoint of the workability targeted by the
present invention, the preferable range is 0.02 to 0.06%.
[0076] Next, the optional ingredients of the present invention will
be explained.
[0077] Mo may be suitably added for improving the corrosion
resistance. To obtain the effect of improvement of the corrosion
resistance, 0.1% or more is preferably added. However, if over 1%,
the economy is liable to be impaired. For this reason, when added,
the content is made 1% or less. From the viewpoints of the
corrosion resistance and economy, the preferable range when added
is respectively 0.2 to 0.8%.
[0078] Nb, V, and Ti improve the corrosion resistance and express
effects similar to Si. That is, by solution strengthening of the
.alpha.-phases, the strength difference of the .alpha.-phases and
the .gamma.-phases is reduced to improve the press-formability and
suppress the martensite transformation of the .alpha.-phases in the
cooling process after annealing. These may be suitably added to
obtain the above effects. When added, the contents are preferably
respectively 0.05% or more. However, if over 0.5%, the economy is
liable to be impaired. For this reason, when added, the contents
are respectively made 0.5% or less. From the viewpoints of the
above effects and economy, the preferable ranges when added are
respectively 0.1 to 0.3%.
[0079] Sn, Sb, and W may be suitably added for improving the
corrosion resistance. To obtain the effect of improvement of the
corrosion resistance, it is preferable to respectively add 0.01% or
more. However, if over 1%, the hot workability and other aspects of
the manufacturability are sometimes impaired. For this reason, when
added, the contents are respectively made 1% or less. From the
viewpoints of the corrosion resistance and manufacturability, the
preferable ranges when added are respectively 0.1 to 0.6%.
[0080] Al is a powerful deoxidizing agent and may be suitably
added. To obtain the above effect, 0.001% or more is preferably
added. However, if over 0.1%, nitrides are formed and surface flaws
or a drop in corrosion resistance is liable to be incurred. For
this reason, when added, the content is made 0.1% or less. From the
viewpoints of the above effects and manufacturability, the
preferable range when added is 0.005 to 0.05%.
[0081] B, Ca, and Mg may be suitably added for improving the hot
workability. To obtain the above effect, preferably 0.0002% or more
are respectively added. However, if over 0.01%, the corrosion
resistance sometimes remarkably falls. For this reason, when added,
the contents are respectively made 0.01% or less. From the
viewpoints of the above effects and manufacturability, the
preferable ranges when added are 0.0005 to 0.01%.
[0082] La, Ce, Zr, Y, and other rare earth metals (REM) also have
actions of improving the hot workability in the same way as B, Ca,
and Mg. Therefore, they may be suitably added. To obtain these
effects, 0.001% or more are preferably respectively added. However,
if over 0.3%, sometimes the economy is impaired. For this reason,
when added, the contents are made 0.3% or less. From the viewpoints
of the above effects and economy, the preferable ranges when added
are 0.002 to 0.1%.
[0083] Further, in addition to the above ingredients, P, S, and O
(oxygen) may also be included as unavoidable impurities. P, S, and
O are elements harmful to the hot workability and corrosion
resistance. P is preferably made 0.1% or less, more preferably
0.05% or less. S is preferably made 0.01% or less, more preferably
0.005% or less, still more preferably less than 0.002%. O is
preferably made 0.01% or less, more preferably 0.005% or less,
still more preferably less than 0.002%.
[0084] In addition to the above ranges of ingredients, a parameter
relating to the production of the .gamma.-phases and .alpha.-phases
defined by the Ni-bal of the following formula <1> from the
amounts of C, N, Ni, Mn, Cu, Cr, and Si is prescribed in range to
obtain the press-formability targeted by the present invention. The
Erichsen value made the parameter of the press-formability, from
the results of FIG. 2, reaches the target value of the Erichsen
value of 11 mm of the present invention in the range of an Ni-bal
of -7.5 to -3.5. For this, the contents of the elements are
adjusted so as to give an Ni-bal of -7.5 to -3.5 in range.
Preferably, from the results of the study of FIG. 2, the value is
made -6 to -4 in range so that the Erichsen value becomes a maximal
value.
Ni-bal.=30(C+N)+Ni+0.5Mn+0.3Cu-1.1(Cr+1.5Si)+8.2 <1>
(B) The metal microstructure will be explained below:
[0085] The duplex stainless steel sheet of the present invention
has the ingredients and Ni-bal explained in section (A) and defines
the .gamma.-phase rate for improving the press-formability. The
.gamma.-phase rate has a general correlation with the Ni-bal. That
is, the .gamma.-phase rate tends to increase along with the rise of
the Ni-bal. However, the .gamma.-production ability in the final
annealing temperature region explained later does not necessarily
correspond straight with the coefficients of the elements in the
Ni-bal. For this reason, to obtain a press-formability targeted by
the present invention, it is necessary to define both the Ni-bal
and the .gamma.-phase rate.
[0086] The .gamma.-phase rate, as explained above, can be found by
the EBSP method. The EBSP method, for example, as described in
Microscope; Seiichi Suzuki, vol. 39, no. 2, 121 to 124, designates
crystal data of the .gamma.-phases (fcc) and .alpha.-phases (bcc)
and displays a phase distribution map color coding the individual
phases. Due to this, it becomes possible to find the .gamma.-phase
rate. Further, it is possible to obtain a grasp of the state of
dispersion of the .gamma.-phases and the .alpha.-phases. For
example, the samples are examined from the cross-sections in the
sheet thickness directions under a measurement ratio of 500.
[0087] The lower limit of the .gamma.-phase rate is made 50% for
securing the press-formability targeted by the present invention.
To reduce the 0.2% proof stress and effectively express the
press-formability, the rate is preferably 60% or more. On the other
hand, if the .gamma.-phase rate exceeds 95%, large amounts of Ni,
Mn, and Cu have to be added. This is a problem from the viewpoints
of conserving Ni and economy. Furthermore, differentiation from
.gamma.-based stainless steel is also not easy. For this reason,
the upper limit is made 95%. From the viewpoint of conserving Ni
and economy, the preferable range is 60 to 80%.
[0088] NPLT 2 reports the metal microstructure of duplex stainless
steel characterized by a high Si and low Ni in relation to
development of PLTs 1 to 3. These steels aim at a rise of the 0.2%
proof stress to secure strength for automotive use as explained in
the Background Art. In general, the 0.2% proof stress of the
.gamma.-phases is smaller than that of the .alpha.-phases. For this
reason, to raise the 0.2% proof stress, it is preferable to provide
a duplex microstructure wherein the .alpha.-phases are the main
phases. The metal microstructure shown in NPLT 2 is a duplex
stainless steel with an amount of Cr over 17% where the
.alpha.-phases constitute the main phases (53.3 to 75.0% .alpha.).
Therefore, the metal microstructure of the present invention
targeted by the press-formability of the steel sheet differs from
the metal microstructure of the steel disclosed in PLTs 1 to 3.
[0089] As explained above, the duplex stainless steel sheet of the
present invention is mainly comprised of .gamma.-phases and has a
balance of .alpha.-phases. If the amount of Cr or the amount of Si
is low, the .alpha.-phases sometimes undergo martensite
transformation in the cooling process after annealing. Martensite
phases may also be unavoidably included to an extent not
obstructing the press-formability targeted by the present
invention.
[0090] The form of dispersion of the .alpha.-phases when making
.gamma. the main phases is not particularly limited. From the
viewpoint of the press-formability, the .alpha.-phases are
preferably finely dispersed. Specifically, the less than 50 .mu.m
.alpha.-phases are preferably dispersed in fibrous shapes or grain
shapes in the sheet thickness direction.
(C) The mechanical properties and Erichsen value will be explained
below.
[0091] The duplex stainless steel sheet of the present invention
has the ingredients and Ni-bal explained in section (A) and defines
the .gamma.-phase rate explained in section (B) so as to improve
the press-formability. The mechanical properties and Erichsen value
of the steel sheets satisfying these provisions are preferably the
following values so as to enable a press-formability no different
from that of SUS304 or other austenitic stainless steel sheet.
[0092] The 0.2% proof stress is preferably made less than 400 MPa
to make it an extent no different from SUS304 or other austenitic
stainless steel. When 400 MPa or more, if envisioning an actual
press, there is a fear of insufficient power of the press or wear
and damage to the die. More preferably, the value is made 350 MPa
or less. The lower limit is not particularly defined, but if
considering amount of C+N or amount of alloy added, the more
preferable range is 250 to 350 MPa.
[0093] The elongation at break is preferably 35% or more to obtain
a high Erichsen value as explained in the explanation of the test
steels A, B, and C in Table 2 and [g]. The value is more preferably
40% or more, still more preferably 45% or more.
[0094] The Erichsen value is important as a parameter of the
press-formability such as the stretchability. As explained in the
test method and [f], to obtain an extent of press-formability no
different from austenitic stainless steel targeted by the present
invention, the value is preferably 11 mm or more, more preferably
12 mm or more. The upper limit is not particularly provided, but
making it over 15 mm is difficult under conditions prescribed in
Method B of JIS Z 2241 (wrinkle pressure of 1 tons).
(D) The method of production will be explained below.
[0095] So long as satisfying the ingredients and Ni-bal explained
in section (A) and the .gamma.-phase rate explained in section (B),
the method of production is not particularly limited.
[0096] The final cold rolling and final annealing conditions have
an effect on the .gamma.-phase rate and the dispersed state of the
microstructure. The reduction rate of the cold rolling is
preferably 40% or more from the viewpoint of fine dispersion of the
.alpha.-phases as the second phase. The final annealing is
preferably heated to 950 to 1150.degree. C. in range in order to
main the .gamma.-phases the main phases. If over 1150.degree. C.,
the amount of production of the .alpha.-phases increases and the
microstructure is liable to coarsen. If less than 950.degree. C.,
the recrystallization and softening of the .gamma.-phases are
liable to become insufficient. The cooling after annealing is
preferably a cooling rate of air cooling or more (about 3.degree.
C./sec or more) for suppressing martensite transformation of the
.alpha.-phases in the case of a small amount of Cr, amount of Si,
etc.
EXAMPLES
[0097] Below, examples of the present invention will be
explained.
[0098] Duplex stainless steels having the ingredients shown in
Table 3 were smelted and hot rolled to produce 4.0 to 5.0 mm thick
hot rolled sheets. Steel No. 1 to Steel No. 22 have ingredients and
Ni-bal's prescribed by the present invention. Steel Nos. 23 and 24
have the ingredients prescribed by the present invention, but have
Ni-bal's outside the present invention. Steel Nos. 25 to 27 have
Ni-bal's prescribed by the present invention, but have ranges of
ingredients outside of the present invention. These hot rolled
sheets were annealed and pickled, then cold rolled to 0.7 mm
thickness and final annealed at 1050.degree. C.
TABLE-US-00003 TABLE 3 Steel No. C Si Mn Cr Ni Cu N Ni-bal Others
Remarks 1 0.015 1.9 4.0 17.1 5.2 1.9 0.005 -5.4 Inv. ex. 2 0.015
1.9 4.0 17.1 5.2 1.9 0.005 -5.4 B: 10 ppm, Ca: 5 ppm Inv. ex. 3
0.003 1.9 3.7 17.1 3.9 1.8 0.028 -6.6 Inv. ex. 4 0.019 1.9 1.3 17.2
4.8 1.8 0.028 -6.5 Inv. ex. 5 0.020 2.3 4.2 17.0 1.5 2.7 0.068 -7.2
Inv. ex. 6 0.021 1.5 3.0 16.2 2.5 1.0 0.051 -5.6 Inv. ex. 7 0.021
2.8 3.8 16.3 3.0 1.8 0.060 -6.5 Inv. ex. 8 0.025 1.7 4.2 16.8 1.2
1.8 0.045 -7.2 Inv. ex. 9 0.046 2.0 3.5 16.8 2.0 0.6 0.040 -7.1
Inv. ex. 10 0.025 1.7 3.8 16.8 4.7 2.3 0.045 -3.7 Inv. ex. 11 0.025
1.7 3.8 16.8 4.7 2.3 0.045 -3.7 Mo: 0.3, Ce: Inv. ex. 0.01, Zr:
0.01, Y: 0.01 12 0.015 0.6 3.0 18.2 3.0 2.0 0.025 -6.5 Inv. ex. 13
0.015 0.6 3.0 18.2 3.0 2.0 0.025 -6.5 Nb: 0.1, V: 0.1 Inv. ex. 14
0.015 2.1 4.8 18.0 3.8 1.9 0.030 -6.9 Inv. ex. 15 0.015 1.9 3.0
18.0 5.7 1.5 0.015 -6.2 Inv. ex. 16 0.015 1.1 3.9 19.2 4.8 2.0
0.025 -6.2 Inv. ex. 17 0.015 0.7 3.8 20.5 5.2 2.0 0.020 -6.8 Al:
0.05 Inv. ex. 18 0.015 0.7 3.8 20.5 5.2 2.0 0.020 -6.8 Inv. ex. 19
0.015 1.9 3.0 18.0 5.7 1.5 0.015 -6.2 Inv. ex. 20 0.010 1.8 3.7
17.6 4.8 2.1 0.025 -5.8 Sb: 0.1 Inv. ex. 21 0.010 1.8 3.7 17.6 4.8
2.1 0.025 -5.8 W: 0.1, La: 0.05 Inv. ex. 22 0.010 1.8 3.7 17.6 4.8
2.1 0.025 -5.8 Ti: 0.1, Sn: Inv. ex. 0.1, Mg: 5 ppm 23 0.020 0.8
4.2 16.9 4.2 1.9 0.040 *-3 Comp. ex. 24 0.020 1.9 4.2 19.5 4.0 1.9
0.040 *-7.9 Comp. ex. 25 0.030 0.6 3.5 20.5 1.7 0.5 *0.14 -6.6
Comp. ex. 26 0.030 0.8 3.5 *15.5 1.7 0.5 0.020 -5.1 Comp. ex. 27
0.030 0.7 4.7 *21.5 3.5 2.5 0.060 -7.3 Comp. ex. Ni-bal. = 30(C +
N) + 0.5Mn + 0.3Cu + Ni - 1.1(Cr + 1.5Si) + 8.2 *means outside
target of the present invention
[0099] From the obtained cold rolled annealed sheets, various test
pieces were taken and measured for .gamma.-phase rate or used for
JIS No. 13B tensile tests and Erichsen tests. The measurement and
test methods were as follows. The 0.2% proof stress, tensile
strength, elongation, Erichsen value, and .gamma.-phase rate were
evaluated. The results of evaluation are shown in Table 4.
TABLE-US-00004 TABLE 4 0.2% PS TS Er .gamma. Steel No N/mm.sup.2
N/mm.sup.2 EL % mm vol % Remarks 1 280 650 49 12.3 70 Inv. ex. 2
275 650 50 12.8 70 Inv. ex. 3 310 690 44 11.9 55 Inv. ex. 4 320 690
46 12.1 55 Inv. ex. 5 380 740 43 11.3 50 Inv. ex. 6 330 700 52 13.2
65 Inv. ex. 7 340 720 43 11.4 55 Inv. ex. 8 320 700 44 11.5 50 Inv.
ex. 9 350 700 43 11.4 50 Inv. ex. 10 340 730 53 13.5 80 Inv. ex. 11
350 720 49 12.8 75 Inv. ex. 12 300 680 46 12.2 55 Inv. ex. 13 320
700 49 12.6 55 Inv. ex. 14 290 670 44 11.6 50 Inv. ex. 15 310 710
47 12.2 60 Inv. ex. 16 290 690 45 11.8 55 Inv. ex. 17 360 650 39
11.1 50 Inv. ex. 18 360 650 42 11.3 50 Inv. ex. 19 330 680 44 11.7
55 Inv. ex. 20 290 670 50 12.9 68 Inv. ex. 21 290 670 51 13.1 72
Inv. ex. 22 310 670 48 12.9 68 Inv. ex. 23 330 600 34* 10.5* 80
Comp. ex. 24 320 650 38 10.5* 45* Comp. ex. 25 450* 720 48 10* 40*
Comp. ex. 26 500* 650 25* 9* 60 Comp. ex. 27 370 680 38 9.5* 35*
Comp. ex. S430LX 270 400 34 10.0 0 -- S304 300 700 51 12.8 100 --
Target 0.2% PS: less than 400 MPa, EL: 35% or more, Erichsen value:
11 mm or more *means outside target of the present invention
[0100] Steel Nos. 1 to 22 have 0.2% proof stresses of less than 400
MPa, elongations of 35% or more, and Erichsen values higher than
the 11 mm targeted by the present invention. Further, the
.gamma.-phase rates are 50% or more. These are mainly .gamma.-phase
duplex stainless steel sheets. Due to this, it is learned that by
satisfying both the ingredients and Ni-bal range prescribed in the
present invention, the Erichsen values become higher than SUS430LX
and become no different from or more than SUS304 and other
.gamma.-based stainless steels.
[0101] Nos. 23 and 24 have an elongation of less than 35% or a
.gamma.-phase rate of less than 50% and failed to reach Erichsen
values of 11 mm or more targeted by the present invention. Due to
this, it is learned that even if satisfying the ingredients
prescribed in the present invention, when outside the Ni-bal range,
the Erichsen value targeted by present invention is not
reached.
[0102] Nos. 25 to 27 have 0.2% proof stresses of over 400 MPa or
.gamma.-phase rates of less than 50% and failed to reach Erichsen
values of 11 mm or more targeted by the present invention. Due to
this, it is learned that even if satisfying the Ni-bal range
prescribed in the present invention, when outside the ranges of
ingredients, the Erichsen value targeted by present invention is
not reached.
* * * * *