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DESCRIPTION
HOT-ROLLED Nb-CONTAINING FERRITIC STAINLESS STEEL SHEET AND
METHOD FOR PRODUCING SAME, AND COLD-ROLLED Nb-CONTAINING
FERRITIC STAINLESS STEEL SHEET AND METHOD FOR PRODUCING SAME
TECHNICAL FIELD
[0001] The present invention relates to a hot-rolled Nb-
containing ferritic stainless steel sheet and a method for
producing same, and to a cold-rolled Nb-containing ferritic
stainless steel sheet and a method for producing same. More
particularly, the present invention relates to a hot-rolled
Nb-containing ferritic stainless steel sheet and a method for
producing same, and to a cold-rolled Nb-containing ferritic
stainless steel sheet and a method for producing same, the
stainless steel sheets being used in order to produce exhaust
pipe flange parts and exhaust pipe parts.
BACKGROUND ART
[0002] Characteristics such as corrosion resistance, heat
resistance, and strength are required for exhaust pipe flange
parts and exhaust pipe parts, and accordingly stainless steel
sheets, which are excellent in such characteristics, are used
as a material of such parts. Herein, the term "exhaust pipe
part" denotes parts through which exhaust gas can flow, in
particular exhaust manifolds, front pipes, center pipes,
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catalytic converter barrels and the like used in automobiles.
The term "exhaust pipe flange part" denotes a part that is
welded to an end of the exhaust pipe part, and that
constitutes a flange portion having the function of fastening
the exhaust pipe part to other parts.
Stainless steel sheets generally used in conventional art
are austenitic stainless steel sheets having good
manufacturability, but these are being replaced by ferritic
stainless steel sheets, which are advantageous from the
viewpoint of coefficient of thermal expansion and cost.
Examples of such ferritic stainless steel sheets include Nb-
containing ferritic stainless steel sheets.
[0003] Exhaust pipe flange parts are produced through cold
forging of a hot-rolled steel sheet. Exhaust pipe flange parts
have a hole corresponding to an end of the exhaust pipe part,
and a hole for bolt fastening, and ordinarily also undergo
cutting work. Workability is accordingly required for a hot-
rolled steel sheet that is used to produce exhaust pipe flange
parts.
Exhaust pipe parts are generally produced through
pressing of cold-rolled steel sheets and pipe processing of
cold-rolled steel sheets, followed by various work processes.
Workability is accordingly required for a cold-rolled steel
sheet that is used to produce exhaust pipe parts. Better
workability of cold-rolled steel sheets has come to be
demanded in recent years as exhaust pipe parts (in particular,
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exhaust manifolds) have become smaller. The workability of
cold-rolled steel sheets can be expressed using the Lankford
value (hereafter "r-value") as an index. Increasing the cold
rolling reduction ratio is effective herein in order to
increase the r-value.
However, Nb-containing ferritic stainless steel sheets
are prone to suffer from drops in toughness due to the
generation of Laves phases (intermetallic compounds being
mainly Fe2Nb). To begin with, ferritic stainless steel sheets
are prone to exhibit 475 C embrittlement. As a result, cracks
occur readily and it is difficult to increase the cold rolling
reduction ratio upon cold rolling of a produced hot-rolled Nb-
containing ferritic stainless steel sheet of thick gauge (5 mm
to 10 mm).
[0004] As a method for increasing the toughness of a hot-
rolled Nb-containing ferritic stainless steel sheet, for
instance Patent Document 1 proposes a method of suppressing
generation of Laves phases, through control of the total
amount of C and N so as to lie within a specific range.
As a method for increasing the workability of a cold-
rolled Nb-containing ferritic stainless steel sheet, for
instance Patent Document 2 proposes a method that involves
controlling, among others, the starting temperature and end
temperature of hot rolling finishing, as well as the annealing
temperature of a hot-rolled sheet.
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[0005] Patent Document 1: Japanese Patent Application
Laid-Open No. H10-237602
Patent Document 2: Japanese Patent Application
Laid-Open No. 2002-30346
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0006] However, the method of Patent Document 1 is aimed
at hot-rolled Nb-containing ferritic stainless steel sheets
having a sheet thickness of about 4.5 mm, and cannot
sufficiently suppress generation of Laves phases in hot-rolled
Nb-containing ferritic stainless steel sheets of thick gauge.
A further problem is that sufficient workability of a
cold-rolled Nb-containing ferritic stainless steel sheet
cannot be secured even when resorting to the method of Patent
Document 2.
It is thus an object of the present invention, arrived at
in order to solve the above problems, to provide a hot-rolled
Nb-containing ferritic stainless steel sheet having excellent
toughness and workability, and a production method thereof.
It is also an object of the present invention to provide
a cold-rolled Nb-containing ferritic stainless steel sheet
having excellent workability, and a production method thereof.
MEANS FOR SOLVING THE PROBLEMS
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[0007] As a result of diligent and assiduous research
aimed at solving the above problems, the inventors found that
the amount of Nb carbonitrides and quantity of Laves phases
can be controlled, so as to lie within proper ranges, by
holding at a temperature of 1100 C to 1000 C for 60 seconds or
longer and setting a finish hot rolling temperature to 850 C or
higher, during hot rolling of a stainless steel slab having a
specific composition, and, after hot rolling, performing
coiling at a coiling temperature of 550 C or lower, as a result
of which there is enhanced toughness of the resulting hot-
rolled Nb-containing ferritic stainless steel sheet, and
perfected the present invention on the basis of that finding.
Further, the inventors found that the r-value can be
increased to 1.2 or more by, after annealing of the hot-rolled
Nb-containing ferritic stainless steel sheet, performing cold
rolling at a reduction ratio of 70% or higher, and then
annealing the cold-rolled steel sheet, as a result of which
there is enhanced workability of the cold-rolled Nb-containing
ferritic stainless steel sheet, and perfected the present
invention on the basis of that finding.
[0008] Specifically, the present invention is a hot-rolled
Nb-containing ferritic stainless steel sheet having a
composition containing C: 0.030 mass% or less, Si: 2.00 mass%
or less, Mn: 2.00 mass% or less, P: 0.050 mass% or less, S:
0.040 mass% or less, Cr: 10.00 mass% to 25.00 mass%, N: 0.030
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mass% or less and Nb: 0.01 mass% to 0.80 mass%, with the
balance being made up of Fe and unavoidable impurities,
wherein the precipitation amount of Nb carbonitrides is 0.2
mass% or more, and the number of Laves phases having a grain
size of 0.1 pm or less is 10 or fewer per 10 pm2 of surface
area.
The present invention is also a method for producing a
hot-rolled Nb-containing ferritic stainless steel sheet, the
method including: holding at a temperature of 1000 C to 1100 C
for 60 seconds or longer, and setting a finish hot rolling
temperature to 850 C or higher, during hot rolling of a
stainless steel slab having a composition containing C: 0.030
mass% or less, Si: 2.00 mass% or less, Mn: 2.00 mass% or less,
P: 0.050 mass% or less, S: 0.040 mass% or less, Cr: 10.00
mass% to 25.00 mass%, N: 0.030 mass% or less and Nb: 0.01
mass% to 0.80 mass%, with the balance being made up of Fe and
unavoidable impurities, and, after hot rolling, performing
coiling at a coiling temperature of 550 C or lower.
[0009] The present invention is also a cold-rolled Nb-
containing ferritic stainless steel sheet having a composition
containing C: 0.030 mass% or less, Si: 2.00 mass% or less, Mn:
2.00 mass% or less, P: 0.050 mass% or less, S: 0.040 mass% or
less, Cr: 10.00 mass% to 25.00 mass%, N: 0.030 mass% or less
and Nb: 0.01 mass% to 0.80 mass%, with the balance being made
up of Fe and unavoidable impurities, wherein the precipitation
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amount of Nb carbonitrides is 0.2 mass% or more, the number of
Laves phases having a grain size of 0.1 m or less is 10 or
fewer per 10 m2 of surface area, and the r-value is 1.2 or
greater.
The present invention is also a method for producing a
cold-rolled Nb-containing ferritic stainless steel sheet, the
method including annealing the above hot-rolled Nb-containing
ferritic stainless steel sheet, and performing thereafter cold
rolling at a reduction ratio of 70% or higher, and annealing
the cold-rolled steel sheet.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0010] The present invention allows the provision of a
hot-rolled Nb-containing ferritic stainless steel sheet having
excellent toughness and workability, and a production method
thereof.
Further, the present invention allows the provision of a
cold-rolled Nb-containing ferritic stainless steel sheet
having excellent workability, and a production method thereof.
DESCRIPTION OF EMBODIMENTS
[0011] <Hot-rolled Nb-containing ferritic stainless steel
sheet>
The hot-rolled Nb-containing ferritic stainless steel
sheet of the present invention (hereafter also referred to as
"hot-rolled steel sheet" for short) has a composition
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containing C, Si, Mn, P, S. Cr, N and Nb, with the balance
being made up of Fe and unavoidable impurities. The hot-rolled
steel sheet of the present invention may have a composition
further containing one or more from among Ni, Mo, Cu, Co, Al,
W, V, Ti, Zr, B, rare earth elements and Ca.
The hot-rolled steel sheet of the present invention will
be explained in detail next.
[0012] <C: 0.030 mass% or less>
Herein C causes steel hardening and drops in the
toughness of the hot-rolled steel sheet. Accordingly, the
content of C is limited to 0.030 mass% or less. However, there
is no need to lower the content of C to the extreme, and
generally it suffices to set a C content of 0.001 mass% to
0.030 mass%, preferably 0.003 mass% to 0.025 mass% and more
preferably 0.005 mass% to 0.020 mass%.
[0013] <Si: 2.00 mass% or less; Mn: 2.00 mass% or less>
Herein Si and Mn are effective as deoxidizers, and
moreover elicit the effect of increasing resistance to
oxidation at high temperature. In particular when resistance
to oxidation at high temperature is emphasized, it is
effective to secure a content of 0.05 mass% or more of Si, and
0.05 mass% or more of Mn. However, an excessively high content
of these elements gives rise to steel embrittlement. As a
result of various studies, the content of both Si and Mn is
limited to 2.00 mass% or less. The content of both Si and Mn
may be managed to be 1.00 mass% or less, or 0.50 mass% or
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less. The lower limit of the content of Si and Mn is not
particularly restricted, and is generally 0.05 mass%,
preferably 0.1 mass% and more preferably 0.15 mass%.
[0014] <P: 0.050 mass% or less; S: 0.040 mass% or less>
Herein P and S give rise to, for example, drops in
corrosion resistance when present in large amounts.
Accordingly, the content of P is limited to 0.050 mass% or
less, and the content of S is limited to 0.040 mass% or less.
Ordinarily it suffices to set the content of P to lie in the
range of 0.010 mass% to 0.050 mass%, and the content of S to
lie in the range of 0.0005 mass% to 0.040 mass%. The preferred
content of P is 0.020 mass% to 0.040 mass% while the preferred
content of S is 0.001 mass% to 0.010 mass%. In particular
where corrosion resistance is emphasized, it is effective to
limit the content of S to 0.005 mass% or less.
[0015] <Cr: 10.00 mass% to 25.00 mass%>
Herein Cr is an important element in order to secure
corrosion resistance in stainless steel, and is also effective
in terms of enhancing resistance to oxidation at high
temperature. In order to bring about these effects, the
content of Cr must be 10.00 mass% or more. The content of Cr
is preferably 13.50 mass% or more, more preferably 17.00 mass%
or more. This is effective in terms of eliciting the above
effect. When the content of Cr is high, on the other hand,
manufacturability of thick-gauge hot-rolled steel sheet is
impaired on account of the resulting steel hardening and drop
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in toughness. As a result of various studies, the content of
Cr is limited to 25.00 mass% or less, preferably to 22.00
mass% or less and more preferably to 20.00 mass% or less.
[0016] <N: 0.030 mass% or less>
Herein N causes a decrease in toughness. Accordingly,
the content of N is limited to 0.030 mass% or less. However,
there is no need to lower the content of N to the extreme, and
generally it suffices to set the content of N to 0.001 mass%
to 0.030 mass%, preferably to 0.005 mass% to 0.025 mass%.
[0017] <Nb: 0.01 mass% to 0.80 mass%>
Herein Nb is an element effective for suppressing
boundary segregation of Cr carbonitrides (carbides /
nitrides), through fixing of C and N, and for preserving high
corrosion resistance, and high resistance to oxidation at high
temperature, in steel. Accordingly, the content of Nb must be
set to 0.01 mass% or more. It is effective to set the content
of Nb to 0.05 mass% or more, and more effective to set the
content of Nb to 0.20 mass% or more. An excessively high
content of Nb, however, promotes drops in the toughness of the
hot-rolled steel sheet, and is therefore undesirable. As a
result of various studies, the content of Nb is limited to
0.80 mass% or less, preferably to 0.60 mass% or less.
[0018] <Ni: 2.00 mass% or less>
Herein Ni has the effect of inhibiting the progress of
corrosion, and can be added as needed. In this case it is
effective to secure a Ni content of 0.01 mass% or more.
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However, a high Ni content has an adverse impact on
workability, and hence the content of Ni that is added, if
any, must be 2.00 mass% or less, preferably 1.00 mass% or
less.
[0019] <Mo: 2.50 mass% or less>
Herein Mo is an effective element in terms of enhancing
corrosion resistance, and can be added as needed. In this case
it is effective to set the content of Mo to 0.02 mass% or
more, and more effective to set the content of Mo to 0.50
mass% or more. A high content of Mo, however, affects
toughness adversely, and hence the content of Mo that is
added, if any, must be 2.50 mass% or less, preferably 1.50
mass% or less.
[0020] <Cu: 1.80 mass% or less>
Herein Cu is an element effective in terms of enhancing
low-temperature toughness, and in terms of enhancing also
high-temperature strength. Accordingly, Cu can be added as
needed. In this case it is effective to secure a Cu content of
0.02 mass% or more. However, workability rather decreases when
Cu is added in large amounts. The content of Cu that is added,
if any, must be 1.80 mass% or less, preferably 0.80 mass% or
less.
[0021] <Co: 0.50 mass% or less>
Herein, Co is an element that contributes to low-
temperature toughness, and can be added as needed. In this
case it is effective to secure a Co content of 0.010 mass% or
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more. However, excessive addition of Co results in a loss of
ductility, and hence the content of Co that is added, if any,
must be 0.50 mass% or less.
[0022] <Al: 0.50 mass% or less>
Herein Al is an effective element as a deoxidizer, and
can be added as needed. In this case, it is effective to set
an Al content of 0.005 mass% or more. However, a high Al
content is one factor underlying drops in toughness.
Therefore, if Al is contained, the content thereof is limited
to 0.50 mass% or less, and is preferably limited to 0.20 mass%
or less.
[0023] <W: 1.80 mass% or less; V: 0.30 mass% or less>
Herein W and V are effective elements in terms of
increasing high-temperature strength, and one or more of the
foregoing can be added as needed. In this case it is effective
to secure a content of 0.10 mass% or more of W and a content
of 0.10 mass% or more of V. However, steel becomes hard, which
may give rise to cracks, when the foregoing elements are added
in large amounts. The content of W that is added, if any, must
be 1.80 mass% or less, preferably 0.50 mass% or less. The
content of V that is added, if any, must be 0.30 mass% or
less, preferably 0.15 mass% or less.
[0024] <Ti: 0.50 mass% or less; Zr: 0.20 mass% or less>
Herein Ti and Zr have the effect of fixing C and N, and
are effective elements in terms of preserving high corrosion
resistance, and high resistance to oxidation at high
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temperature, in steel. Accordingly, one or both of Ti and Zr
can be added as needed. In this case it is effective to secure
a content of 0.01 mass% or more of Ti, and a content of 0.02
mass% or more of Zr. However, an excessive content of Ti
promotes loss of toughness in hot-rolled coils, and
accordingly the content of Ti that is added, if any, must be
0.50 mass% or less. Further, a high content of Zr constitutes
a hindrance to workability, and hence the content of Zr that
is added, if any, must be 0.20 mass% or less.
[0025] <B: 0.0050 mass% or less>
Herein B is an element that improves corrosion resistance
and workability by being added in small amounts, and can be
added as needed in the form of one or more types. In this case
it is effective to secure a B content of 0.0001 mass% or more.
However, an excessive B content affects hot workability
adversely, and accordingly the content of B that is added, if
any, must be 0.0050 mass% or less.
[0026] <Rare earth elements: 0.100 mass% or less; Ca:
0.0050 mass% or less>
Rare earth elements and Ca are effective elements in
terms of enhancing resistance to oxidation at high
temperature, and one or more of the foregoing can be added as
needed. In this case it is effective to secure a content of
0.001 mass% or more of rare earth elements and a content of
0.0005 mass% or more of Ca. However, toughness decreases when
these elements are added in large amounts, and hence the
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content of the rare earth elements that are added, if any,
must be 0.100 mass% or less, and that of Ca that is added, if
any, must be 0.0050 mass% or less.
[0027] <Balance: Fe and unavoidable impurities>
The balance, being components other than those above, is
made up of Fe and unavoidable impurities. The term
"unavoidable impurities" denotes herein impurity elements that
cannot be prevented from being mixed into the materials during
the production process. The unavoidable impurities are not
particularly limited.
[0028] <Precipitation amount of Nb carbonitrides: 0.2
mass% or more; Number of Laves phases having a grain size of
0.1 m or less, per 10 m2 of surface area: 10 or fewer>
Herein Nb carbonitrides (carbides / nitrides) and Laves
phases are precipitates generated as a result of a hot rolling
process. The toughness of the hot-rolled steel sheet decreases
when C and N are present in the form of a solid solution in
steel, and thus the presence of such a solid solution is
accordingly effective in allowing C and N to precipitate in
the form of Nb carbonitrides. Through precipitation of Nb
carbonitrides, moreover, the amount of Nb in solid solution
within the steel decreases, and it becomes possible to reduce
the precipitation quantity of Laves phases that reduce the
toughness of the hot-rolled steel sheet. The precipitation
amount of Nb carbonitrides must be set to 0.2 mass% or more in
order to increase the toughness of the hot-rolled steel sheet
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through a reduction in the amount of C and N in solid solution
within the steel. Further, the number of Laves phases having a
grain size of 0.1 gm or less must be 10 or fewer per 10 gm2 of
surface area.
To calculate the precipitation amount (mass%) of Nb
carbonitrides, there was used a mixed solution of 10 mass%
acetylacetone + 1 mass% tetramethylammonium chloride + 89
mass% methyl alcohol, with electrolytic extraction of a
precipitate residue at a SCE potential of -100 mV to 400 mV
with respect to a saturated calomel electrode, followed by
filtration of the extracted residue, using a 0.2 lam micropore
filter. The precipitation amount was calculated on the basis
of the ratio between the weight of the filtered residue and
total dissolution weight.
A scanning electron microscope (SEM) was used to capture
surface micrographs and measure the size of Laves phases, and
also measure the number of Laves phases having a grain size of
0.1 gm or less, per 10 gm2 of surface area. The average value
of the number of Laves phases, measured at least in five
points, was taken herein as the number of Laves phases.
[0029] <Thickness>
The thickness of the hot-rolled steel sheet of the
present invention is not particularly limited and may be set
as appropriate depending on the intended application. In a
case for instance where the hot-rolled steel sheet of the
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present invention is used to produce exhaust pipe flange parts
for automobiles, the thickness of the hot-rolled steel sheet
is ordinarily 5.0 mm to 11.0 mm preferably 5.5 mm to 9.0 mm.
In a case where the hot-rolled steel sheet of the present
invention is used to produce automotive exhaust pipe parts,
the reduction ratio must be increased at the time of cold
rolling of the hot-rolled steel sheet of the present
invention, in order to increase the r-value, which is an index
of the workability of a cold-rolled Nb-containing ferritic
stainless steel sheet (hereafter also referred to as "cold-
rolled steel sheet" for short). Therefore, the thickness of
the hot-rolled steel sheet is ordinarily set to be larger than
4.5 mm, but no larger than 10.00 mm, taking into consideration
the thickness and cold rolling reduction ratio of the cold-
rolled steel sheet that is used in order to produce automotive
exhaust pipe parts. The thickness of the hot-rolled steel
sheet is preferably 5.0 mm to 9.0 mm, more preferably 5.5 mm
to 8.0 mm.
[0030] <Method for producing a hot-rolled Nb-containing
ferritic stainless steel sheet>
The hot-rolled Nb-containing ferritic stainless steel
sheet of the present invention having the above features can
be produced through hot rolling of a stainless steel slab
having a composition identical to that of the above hot-rolled
Nb-containing ferritic stainless steel sheet, such that during
hot rolling the slab is held at a temperature of 1000 C to
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1100 C for 60 seconds or longer and a finish hot rolling
temperature is set to 850 C or higher, and after hot rolling,
coiling is performed at a coiling temperature of 550 C or
lower.
[0031] The stainless steel slab is ordinarily heated prior
to hot rolling. The heating temperature of the stainless steel
slab is not particularly limited, but is preferably 1200 C to
1300 C. When the heating temperature of the stainless steel
slab is lower than 1200 C, excessive strain derived from hot
rolling is introduced, and it is difficult to control
thereafter the structure of the steel, and moreover surface
scratches become problematic. On the other hand, a heating
temperature of the stainless steel slab in excess of 1300 C
results in structure coarsening, and there may be a failure to
obtain a hot-rolled steel sheet having the desired
characteristics.
[0032] As described above, hot rolling is carried out
after heating of the stainless steel slab. Hot rolling
ordinarily includes a plurality of rough rolling passes and a
plurality of finish hot rolling passes. Holding at a
temperature of 1000 C to 1100 C for 60 seconds or longer is
necessary, and the finish hot rolling temperature must be set
to 850 C or higher, in order to reduce precipitation of Laves
phases during hot rolling while efficiently promoting
precipitation of Nb carbonitrides. The reason for setting the
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holding temperature to be 1000 C to 1100 C is that
precipitation of Nb carbonitrides can be promoted with good
efficiency by a precipitation temperature of Nb carbonitrides
of 1100 C or lower, and in particular by setting such a holding
temperature. Precipitation of Nb carbonitrides is insufficient
when the holding temperature and the hold time lie outside the
above ranges. When the finish hot rolling temperature is lower
than 850 C, moreover, the precipitation temperature of the
Laves phases is about 800 C, and accordingly precipitation of
Laves phases cannot be reduced sufficiently.
[0033] The method for holding at a temperature of 1000 C
to 1100 C for 60 seconds or longer is not particularly limited,
and may involve lowering a passing speed, and/or introducing a
delay before finish rolling.
The timing for the holding at a temperature of 1000 C to
1100 C for 60 seconds or longer is not particularly limited, so
long as it lies within the hot rolling process, but preferably
lasts from the end of rough rolling up to the beginning of
finish hot rolling.
The finish hot rolling time is not particularly limited,
and can be set in accordance with known hot rolling methods in
the relevant technical field. The finish hot rolling time is
generally established taking into consideration a balance with
respect to the total duration of the hot rolling process, but
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the longer the finish hot rolling time, the greater the
precipitation amount of Nb carbonitrides is.
[0034] Hot rolling is followed by coiling into coils at a
coiling temperature of 550 C or lower. A coiling temperature
in excess of 550 C may result in precipitation of Laves phases
and in reduced toughness.
The precipitation amount of Nb carbonitrides in the hot-
rolled steel sheet obtained as described above is sufficiently
increased during the hot rolling process, and accordingly
Laves phases do not precipitate readily even at the
precipitation temperature of the Laves phases (around 800 C).
Accordingly, there is little need for a method of quenching
the hot-rolled steel sheet by water cooling or the like before
coiling, to shorten the transit time of the Laves phases at
the precipitation temperature.
[0035] <Cold-rolled Nb-containing ferritic stainless steel
sheet and production method thereof>
In addition to the characterizing features of the above
hot-rolled steel sheet, a further characterizing feature of
the cold-rolled steel sheet of the present invention is that
the r-value of the sheet is 1.2 or greater. As a result the
cold-rolled steel sheet of the present invention boasts
excellent workability, and, by being worked in various ways,
allows the production of automotive exhaust pipe parts such as
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exhaust manifolds, front pipes, center pipes and catalyst
converter barrels.
The cold-rolled steel sheet of the present invention
having the above characterizing features can be produced
through annealing of the above hot-rolled steel sheet,
followed by cold rolling at a reduction ratio of 70% or
higher, and annealing the cold-rolled steel sheet.
The hot-rolled steel sheet is annealed prior to cold
rolling. Annealing is carried out at a temperature such that a
recrystallized structure is obtained. The annealing
temperature is not particularly limited and may be set as
appropriate depending on the composition of the hot-rolled
steel sheet, but is ordinarily 950 C to 1150 C. There may be a
failure to obtain a recrystallized structure in some instances
when the annealing temperature is lower than 950 C. On the
other hand, crystal grains become coarser when the annealing
temperature exceeds 1150 C.
[0036] Cold rolling is carried out at a reduction ratio of
70% or more, in order to increase the r-value of the cold-
rolled steel sheet to 1.2 or more. The r-value of the cold-
rolled steel sheet is smaller than 1.2 when the reduction
ratio is lower than 70%.
The cold-rolled steel sheet is annealed after cold
rolling. Annealing is carried out at a temperature such that a
recrystallized structure is obtained. The annealing
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temperature is not particularly limited and may be set as
appropriate depending on the composition of the cold-rolled
steel sheet, but is ordinarily 1000 C to 1100 C. There may be
a failure to obtain a recrystallized structure in some
instances when the annealing temperature is lower than 1000 C.
Crystal grains become coarser and rough skin arises during
working, which may give rise to cracks, when the annealing
temperature exceeds 1100 C.
EXAMPLES
[0037] The present invention will be further explained
next by way of examples, but the invention is not meant to be
limited to these examples.
Stainless steel slabs were produced through smelting of
steels having the component compositions given in Table 1, and
the slabs were hot rolled in accordance with the conditions
given in Table 1, to yield respective hot-rolled Nb-containing
ferritic stainless steel sheets having a predetermined
thickness.
[0038]
21
Chemical composition (mass%) Hot
rolling condition
No. Slab heating Hold
time Finish rolling Coiling .. Thickness .. Classification
C Si Mn P S Ni Cu Cr Mo N Nb Ti Al
temp. ( C) (sec)
temp. ( C) temp. ( C) (mm)
_
1 0.012 014 0.27 0.027 0.002 -- -- 18.4 1.24 0.02 0.39 -- 0.02 .. 1205 .. 61
.. 860 .. 440 .. 5.0
2 0.010 r 0.78 0.30 , 0.025 0.006 0.12 -- 14.2 0.04
0.01 0.43 -- 0.01 1253 _ 63 870 400 6.0
3 0.013 0.41 0.31 0.029 0.003 -- -- 19.8 -- 0.01 0.41 -- -- .. 1240 .. 64 ..
862 .. 540 .. 5.0
4 0.015 0.54 0.28 0.028 0.004 0.32 - 22.5 -- 0.01
0.52 -- -- 1234 70 856 500 5.1 Example of the
0.014 0.56 0.21 , 0.030 0.001 -- -- , 18.5 0.05 0.01
0.42 -- -- 1221 63 900 490 ' 5.6 invention
6 0.014 0.56 0.21 0.030 0.001 -- -- 18.5 0.05 0.01 0.42 -- -- 1213 61
897 480 4.3
7 0.019 0.79 0.43 0.290 0.003 0.24 0.49 18.9 0.06 0.01 0.46 -- -- .. 1225 ..
62 .. 887 .. 485 .. 5.2
8 , 0.007 0.09 0.25 0.330 0.001 0.11 - 16.5 0.01
0.01 0.25 0.19 0.03 1201 71 901 450 8.1 ,
9 0.007 0.54 0.26 0.020 - 0.005 -- -- 18.2 0.54
0.02 0.42 -- -- 1260 49 960 530 5.0
10 0.023 0.38 0.34 0.034 0.007 -- 0.21 18 0.06 0.01 0.42 -- 0.03 1243 40
860 460 5.0 Comparative
11 0.016 0.57 0.37 0.032 0.005 -- -- 13.9 -- 0.01 0.44 -- 0.04 1246 51
855 480 5.0 example
12 0.015 0.28 0.32 0.027 0.004 0.11 -- 18.6 0.04 0.01 0.38 0.11 0.03 1231
49 865 500 5.0
(Remarks)
*1) Hold time at a temperature of 1000 C to 1100 C.
P
The balance other than the above components of the chemical composition is Fe
and unavoidable impurities. Underline denotes values outside the condition
range of the present invention. 0
L.
o
o
1-
L.
L.
1.,
o
1-
03
i
o
i
1-
,..
22
CA 03009133 2018-06-19
[0039] Next, specimens were sampled from the obtained hot-
rolled Nb-containing ferritic stainless steel sheets, and were
evaluated for precipitation amount of Nb carbonitrides, size
of Laves phases, quantity of Laves phases having a grain size
of 0.1 m or less, per 10 m2 of surface area, and toughness.
The precipitation amount of Nb carbonitrides and the
size and quantity of Laves phases were measured in accordance
with the methods described above. The SCE potential in the
measurement of the precipitation amount of Nb carbonitrides
was set to 400 mV. Toughness was evaluated on the basis of a
Charpy impact test of U-notch specimens. The admissibility of
toughness was evaluated on the basis of observable toughness
(good: 0) at a ductile-brittle transition temperature (DBTT)
of 20 C or lower.
The various evaluation results are given in Table 2.
[0040]
Precipitation amount of Nb Size of Laves Number of Laves phases
No. Toughness
Classification
carbonitrides (mass%) phases ( m) *1(phases)
1 0.21 0.08 9 0
2 0.22 0.09 7 0
3 0.22 0.08 6
4 0.26 0.06 4 0 Example of the
0.21 0.05 7 0 invention
6 0.21 0.05 7 o
7 an aw 8
8 0.21 aw 5
9 0.12 0.09 14
0.05 0.06 14 x Comparative
11 0.12 0.12 8 x example
12 0.09 0.11 11
(Remarks)
*1) Number of Laves phases having a grain size of 0.1 pm or less, per 10 p.m2
of surface area.
Underline denotes values outside the condition range of the present invention.
23
CA 03009133 2018-06-19
[0041] Table 2 reveals that hot-rolled Nb-containing
ferritic stainless steel sheets Nos. 1 to 8, produced by
holding at a temperature of 1000 C to 1100 C for 60 seconds or
longer, with the finish hot rolling temperature set to 850 C or
higher, during hot rolling of the stainless steel slab, and,
after hot rolling, by coiling at a coiling temperature of 550 C
or lower, exhibited a precipitation amount of Nb carbonitrides
of 0.2 mass% or more, 10 or fewer Laves phases having a grain
size of 0.1 gm or less, per 10 gm2 of surface area, and
excellent toughness.
By contrast, it was found that in hot-rolled Nb-
containing ferritic stainless steel sheets Nos. 9 to 12, in
which the hold time at a temperature of 1000 C to 1100 C during
hot rolling of the stainless steel slab was too short, the
precipitation amount of Nb carbonitrides was small, the
quantity of Laves phases substantial, and toughness
insufficient.
[0042] A cold forging test, a press drilling test and a
cutting test for simulation of working of an exhaust pipe
flange part were carried out on each hot-rolled Nb-containing
ferritic stainless steel sheet that was obtained. The results
revealed that hot-rolled Nb-containing ferritic stainless
steel sheets Nos. 1 to 8 exhibited good workability to a
desired shape, without occurrence of cracks or the like caused
by lack of toughness. By contrast, hot-rolled Nb-containing
24
CA 03009133 2018-06-19
ferritic stainless steel sheets Nos. 9 to 12 exhibited cracks
caused by lack of toughness.
[0043] Next, the obtained hot-rolled Nb-containing
ferritic stainless steel sheets Nos. 1 to 7 were annealed and
were thereafter cold rolled, with further annealing to yield
respective cold-rolled Nb-containing ferritic stainless steel
sheets. The production conditions involved are given in Table
3. Hot-rolled Nb-containing ferritic stainless steel sheets
Nos. 9 to 12 had low toughness and could not be cold rolled.
The r-value of the obtained cold-rolled Nb-containing
ferritic stainless steel sheets was worked out next. The r-
value was calculated in the form of an average r-value, in
accordance with Expression (1) and Expression (2) below, after
application of 14.4% strain to a JIS 135 tensile specimen of
each cold-rolled Nb-containing ferritic stainless steel sheet.
r = ln(Wo/W)/1n(to/t) (1)
where Wo denotes sheet width before tension, W denotes
sheet width after tension, to denotes sheet thickness before
tension and t denotes sheet thickness after tension.
Average r-value = (ro + 2r45 + r90)/4 (2)
where ro denotes the r-value in the rolling direction,
r45 denotes the r-value in a 45 direction with respect to the
rolling direction, and r90 denotes the r-value in a direction
perpendicular to the rolling direction.
An average r-value of 1.2 or greater translates into a
characteristic of enabling sufficient working of automotive
CA 03009133 2018-06-19
exhaust pipe parts, for which complex shapes are required.
Accordingly, it can be concluded that workability is excellent
if the average r-value is 1.2 or greater.
The evaluation results are given in Table 3.
[0044]
Annealing temp. after hot Cold rolling reduction Annealing temp.
after cold r¨
No.
Classification
rolling ( C) ratio (%) rolling ( C) value
1 1002 70 1003 1.4
2 985 75 1010 1.5
3 1050 70 1043 1.4 Example
of the
invention
4 1103 71 1085 1.3
1030 73 1045 1.4
6 1024 65 1053 1.1
Comparative example
7 1028 71 1048 1.3 Example
of the
invention
(Remarks)
Underline denotes values outside the condition range of the present invention.
[0045] As Table 3 reveals, cold-rolled Nb-containing
ferritic stainless steel sheets Nos. 1 to 5 and 7, having been
cold rolled at a reduction ratio of 70% or higher, had an r-
value of 1.2 or greater and exhibited excellent workability.
By contrast, it was found that cold-rolled Nb-containing
ferritic stainless steel sheet No. 6, having been cold rolled
at a reduction ratio lower than 70%, had an r-value smaller
than 1.2 and exhibited insufficient workability.
[0046] The above results indicate that the present
invention allows the provision of a hot-rolled Nb-containing
ferritic stainless steel sheet having excellent toughness and
workability, and a production method thereof. Further, the
present invention allows the provision of a cold-rolled Nb-
containing ferritic stainless steel sheet having excellent
workability, and a production method thereof.
26
CA 03009133 2018-06-19
. .
[0047] The present application claims the right of
priority based on Japanese Patent Application No. 2016-017883,
filed on February 2nd, 2016, the entire contents thereof are
incorporated herein by reference.
27
