Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Title of Invention: HOT-ROLLED AND ANNEALED FERRITIC
STAINLESS STEEL SHEET AND METHOD FOR PRODUCING THE SAME
Technical Field
[0001]
The present invention relates to a hot-rolled and
annealed ferritic stainless steel sheet having excellent
workability and being suitably used for flanges and so forth
and to a method for producing the hot-rolled and annealed
ferritic stainless steel sheet.
Background Art
[0002]
In recent years, laws and regulations regarding exhaust
gases from automobiles have been increasingly tightened in
order to reduce the amount of emission of CO2, which is a
greenhouse effect gas. An improvement in fuel economy is
effective in reducing the amount of emission of CO2 in
automotive exhaust gases. Thus, studies have been made of
an increase in combustion temperature in engines.
[0003]
Exhaust gases generated by engines are released to the
atmosphere through exhaust system components, such as
exhaust gas recirculation (EGR) systems and mufflers. These
components of automotive exhaust systems are fastened with
flanges in order to prevent gas leakage. Flanges used for
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exhaust system components need to have dimensional accuracy
sufficient for fastening components.
[0004]
Hitherto, such thick-wall flanges have been composed of
plain carbon steel. In recent years, however, as a further
improvement in the fuel economy of automobiles is required,
combustion temperatures in engines and the temperatures of
exhaust gases from engines have been further increased.
Thus, flanges are required to have higher high-temperature
strength and corrosion resistance than before. From such a
background, in recent years, stainless steel, which has
higher high-temperature strength and corrosion resistance
than plain carbon steel, in particular, a high-strength
ferritic stainless steel sheet, which has a relatively low
coefficient of thermal expansion and being less likely to
generate thermal stress (for example, a thick sheet composed
of ASTM A240/240M-S40975 (11 mass%Cr-Ti-Ni steel)) (for
example, a sheet thickness of 5 mm or more), has been
increasingly used.
[0005]
However, since flanges used in exhaust systems have
thick walls (often 5 mm or more), there is a problem that
cracking may occur during punching work in producing flanges
to fail to appropriately produce flange components. A thick
ferritic stainless steel sheet excellent in punching
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workability is strongly demanded.
[0006]
In response to the demands of the market, for example,
Patent Literature 1 discloses a hot-rolled ferritic
stainless steel sheet containing, on a mass percent basis,
C: 0.015% or less, Si: 0.01% to 0.4%, Mn: 0.01% to 0.8%, P:
0.04% or less, S: 0.01% or less, Cr: 14.0% to less than
18.0%, Ni: 0.05% to 1%, Nb: 0.3% to 0.6%, Ti: 0.05% or less,
N: 0.020% or less, Al: 0.10% or less, and B: 0.0002% to
0.0020%, the balance being Fe and incidental impurities, the
Nb, C, and N contents satisfying Nb/(C + N) 16, the
CharPY
impact value being 10 J/cm2 or more at 0 C, the thickness of
the sheet being 5.0 to 9.0 mm.
Citation List
Patent Literature
[0007]
PTL 1: International Publication No. 2014/157576
Summary of Invention
Technical Problem
[0008]
The inventors produced experimentally a ferritic
stainless steel sheet having a thickness of 10 mm and
containing steel components in compliance with ASTM
A240/240M-540975 by a method disclosed in Patent Literature
1 and then produced flanges having holes with a diameter of
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20 mm by punching work with a clearance of 10%. The results
demonstrated that although no cracking occurred during the
punching in any of the flanges, the circumference and/or
center hole dimensions of each flange sometimes exceeded the
tolerance of the component, which indicated that the steel
sheet had not sufficient performance for the thick-wall
flange.
[0009]
The present invention aims to solve the foregoing
problems and provide a hot-rolled and annealed ferritic
stainless steel sheet having sufficient corrosion resistance
and excellent punching workability, a predetermined
dimensional accuracy being obtained without cracking when
the steel sheet is formed into a thick-wall flange by
punching work, and a method for producing the hot-rolled and
annealed ferritic stainless steel sheet.
Solution to Problem
[0010]
The inventors have conducted detailed studies in order
to solve the foregoing problems and have found that in order
to obtain a predetermined dimensional accuracy without
cracking during punching work, it is sufficient that the
metal microstructure of a steel sheet is controlled to be a
single ferrite phase microstructure having an average grain
size of 5 to 20 Rm.
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[0011]
Additionally, the inventors have also found that the
metal microstructure can be controlled to be a ferrite
single phase having an average grain size of 5 to 20 pm
by subjecting a ferritic stainless steel containing
appropriate composition to hot rolling and subjecting
the resulting hot-rolled steel sheet to hot-rolled steel
sheet annealing under appropriate conditions that lead
to a single ferrite phase, specifically, holding the
hot-rolled steel sheet at 600 C or higher and lower than
750 C for 1 minute to 24 hours.
[0012]
These findings have led to the present invention.
The gist of the present invention is described below.
[1] A hot-rolled and annealed ferritic stainless
steel sheet has a chemical composition containing, on a
mass percent basis, C: 0.001% to 0.020%, Si: 0.05% to
1.00%, Mn: 0.05% to 1.00%, P: 0.04% or less, S: 0.01%
or less, Al: 0.01% to 0.10%, Cr: 10.0% to 20.0%, Ni:
0.50% to 2.00%, Ti: 0.10% to 0.40%, and N: 0.001% to
0.020%, the balance being Fe and incidental impurities;
and has a metal microstructure is a single ferrite
phase microstructure having an average grain size of
to 20 pm.
[1a] In an embodiment of [1], the hot-rolled and
annealed ferritic stainless steel sheet has a chemical
composition containing, on a mass percent basis: C:
0.001% to 0.020%, Si: 0.05% to 0.40%, Mn: 0.05% to
0.60%, P: 0.04% or less, S: 0.01% or less, Al: 0.01% to
0.10%, Cr: 11.2% to 20.0%, Ni: 0.50% to 0.86%, Ti:
0.10% to 0.40%, and N: 0.001% to 0.020%, the balance
being Fe and incidental impurities.
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[2] The hot-rolled and annealed ferritic stainless
steel sheet described in [1] further contains, on a
mass percent basis, one or two or more selected from
Cu: 0.01% to
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1.00%, Mo: 0.01% to 2.00%, W: 0.01% to 0.20%, and Co: 0.01%
to 0.20%.
[3] The hot-rolled and annealed ferritic stainless
steel sheet described in [1] or [2] further contains, on a
mass percent basis, one or two or more selected from V:
0.01% to 0.20%, Nb: 0.01% to 0.10%, Zr: 0.01% to 0.20%, REM:
0.001% to 0.100%, B: 0.0002% to 0.0025%, Mg: 0.0005% to
0.0030%, and Ca: 0.0003% to 0.0030%.
[4] A method for producing the hot-rolled and annealed
ferritic stainless steel sheet described in any one of [1]
to [3] includes performing hot-rolled steel sheet annealing
in which a hot-rolled steel sheet produced through a hot-
rolling step is held at 600 C or higher and lower than 750 C
for 1 minute to 24 hours.
Advantageous Effects of Invention
[0013]
According to the present invention, a hot-rolled and
annealed ferritic stainless steel sheet having sufficient
corrosion resistance and excellent punching workability is
provided.
[0014]
The term "sufficient corrosion resistance" used in the
present invention indicates that when a steel sheet obtained
by subjecting surfaces thereof to abrasive finishing with
600-grit emery paper and then sealing end-face portions
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thereof is subjected to five cycles of a cyclic salt-spray
test specified in JIS H 8502 (one cycle of the test
consisting of salt spraying (5% by mass NaCl, 35 C, spraying
for 2 hours) -* drying (60 C, 4 hours, relative humidity:
40%) -* wetting (50 C, 2 hours, relative humidity: 95%)),
the percentage of rusted area of the surfaces of the steel
sheet (= rusted area/total area of steel sheet x 100[%]) is
25% or less.
[0015]
The punching workability is evaluated as follows:
Samples measuring 100 mm x 100 mm for testing are taken from
a hot-rolled and annealed steel sheet. Five test specimens
are produced by punching work with a crank press equipped
with an upper die (punch) and a lower die (die) so as to
form a hole having a diameter of 20 mm (tolerance: 0.1 mm)
in the middle portion of each of the samples, the upper die
having a cylindrical edge for punching, the lower die having
a hole with a diameter of 20 mm or more. The punching work
is performed by selecting the hole diameter of the lower die
in accordance with the thickness of the test specimens in
such a manner that the clearance between the upper die and
the lower die is 10%. Here, the relationships among the
clearance (C) [%], the hole diameter of the die (the inside
diameter of the die) (Dd) [mm], the diameter of the punch
(Dp) [mm], and the sheet thickness (t) [mm] are represented
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by formula (1) below.
C = (Dd -Dp) + (2 x t) x 100 formula (1)
The "excellent punching workability" in the present
invention indicates that, with respect to the test specimens
thus obtained, when the visual inspection of the appearance
of each test specimen and the measurement of the diameter of
the hole in the middle portion of the test specimen with a
digital caliper are performed, no cracks are observed, and
the hole diameter of each of the five test specimens after
the punching work is in the range of 19.9 to 20.1 mm.
Description of Embodiments
[0016]
Embodiments of the present invention will be described
below.
[0017]
A hot-rolled and annealed ferritic stainless steel
sheet of the present invention has a chemical composition
containing, on a mass percent basis, C: 0.001% to 0.020%,
Si: 0.05% to 1.00%, Mn: 0.05% to 1.00%, P: 0.04% or less, S:
0.01% or less, Al: 0.01% to 0.10%, Cr: 10.0% to 20.0%, Ni:
0.50% to 2.00%, Ti: 0.10% to 0.40%, and N: 0.001% to 0.020%,
the balance being Fe and incidental impurities, in which a
metal microstructure is a single ferrite phase
microstructure having an average grain size of 5 to 20 Rm.
[0018]
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The present invention will be described in detail below.
[0019]
The inventors have produced flanges having holes with a
diameter of 20 mm by punching work with a clearance of 10%
using various ferritic stainless steel sheets having 10 mm
in thick in conformity with ASTM A240/240M-S40975 (a
chemical composition containing, on a mass percent basis, C
< 0.03%, Si 1.00%, Mn < 1.00%, P 0.040%, S 0.030%, Cr:
10.5% to 11.7%, Ni: 0.50% to 1.00%, N 0.03%, and Ti: 6 x
(C + N) to 0.74%, the balance being Fe and incidental
impurities). The results revealed that although no cracking
occurred during the punching in any of the sheets, the
circumference and/or center hole dimensions of each flange
sometimes exceeded the tolerance of the component.
[0020]
The inventors have studied in detail the reason why the
levels of the dimensional accuracy in the punching work
varied greatly, depending on the steel sheets, and have
found that the dimensions of the component after the
punching work tended to be smaller than the tolerance when
the steel sheet subjected to the punching work had an
average grain size of less than 5 WU and that the dimensions
of the component after the punching work tended to be larger
than the tolerance when the steel sheet had an average grain
size of more than 20 m. The inventors have found out that
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the reasons why sufficient dimensional accuracy is not
stably obtained in the punching work are that an excessively
small average grain size results in a small percentage of a
sheared surface in the punching work due to an excessively
hard steel sheet and that an excessively large average grain
size results in the occurrence of large rollover or burrs in
the punching work.
[0021]
The inventors have conducted intensive studies on a
method for producing a ferritic stainless steel sheet having
a metal microstructure composed of a single ferrite phase
having an average grain size of 5 to 20 pm from the
viewpoints of steel composition, a hot-rolling process, and
a hot-rolled steel sheet annealing process. The following
method has been found to be effective: Steel composition,
particularly Cr and Ni contents, are controlled to
appropriate ranges. In a hot-rolling step, an austenite
phase and a ferrite phase are formed, and then hot rolling
is performed. Subsequently, the hot-rolled sheet is
annealed in an appropriate temperature range in a single
ferrite phase temperature region.
[0022]
A hot-rolled steel sheet annealing step is performed by
performing holding the hot-rolled steel sheet in the
appropriate temperature range in the single ferrite phase
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temperature region, specifically at 600 C or higher and
lower than 750 C for 1 minute to 24 hours. Regarding the
ferrite phase and a martensite phase that have been present
in the metal microstructure after the hot rolling, the hot-
rolled steel sheet annealing brings recrystallization of the
ferrite phase and transformation of the martensite phase
into the ferrite phase, thereby providing a single ferrite-
phase microstructure. An annealing temperature of the hot-
rolled sheet of lower than 600 C results in insufficient
recrystallization of the ferrite phase and insufficient
transformation of the martensite phase into the ferrite
phase, thereby easily forming punching cracks due to
excessive hardening of the steel sheet. An annealing
temperature of higher than 750 C results in excessively
coarse grains having an average grain size of more than 20
m to easily cause large rollover and burrs during punching
work, thereby failing to obtain a predetermined dimensional
accuracy during the punching work. A holding time of less
than 1 minute results in insufficient recrystallization of
the ferrite phase and insufficient transformation of the
martensite phase into the ferrite phase, thereby easily
forming punching cracks due to excessive hardening of the
steel sheet. A holding time of more than 24 hours results
in excessively coarse grains having an average grain size of
more than 20 m to easily cause large rollover and burrs
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during punching work, thereby failing to obtain the
predetermined dimensional accuracy during the punching work.
Accordingly, in the present invention, it is necessary to
perform the hot-rolled steel sheet annealing in which
holding is performed in a temperature range of 600 C or
higher and lower than 750 C for 1 minute to 24 hours.
[0023]
As described above, in the present invention, the metal
microstructure is a single ferrite phase microstructure, and
the single ferrite phase microstructure has an average grain
size of 5 to 20 pm. The average grain size is preferably 7
pm or more, more preferably 10 pm or more. The average grain
size is preferably 18 pm or less, more preferably 15 pm or
less.
[0024]
The average grain size can be determined as follows: A
test piece for microstructure observation is taken from the
middle portion of the sheet in the width direction. A
section thereof in the rolling direction is mirror-polished.
Then measurement and analysis are performed in the field of
view including the entire thickness by a SEM/EBSD method.
Boundaries with a misorientation of 15 or more are defined
as grain boundaries. The average grain size can be
determined on the basis of an area method.
[0025]
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The thickness of the hot-rolled and annealed ferritic
stainless steel sheet of the present invention is preferably,
but not necessarily, 5.0 mm or more, more preferably 8.0 mm
or more because the sheet thickness is desirably a sheet
thickness that can be used for a thick-wall flange. The
sheet thickness is preferably 15.0 mm or less, more
preferably 13.0 mm or less.
[0026]
The chemical composition of the hot-rolled and annealed
ferritic stainless steel sheet of the present invention will
be described below.
Hereinafter, the component contents are given in units
of "%", which indicates "% by mass", unless otherwise
specified.
[0027]
C: 0.001% to 0.020%
When C is contained in an amount of more than 0.020%,
the workability and the corrosion resistance in a weld zone
deteriorate markedly. A lower C content is more preferable
from the viewpoints of the corrosion resistance and the
workability. To obtain a C content of less than 0.001%, it
takes long time to perform refining, which is not preferred
in terms of production. Thus, the C content is in the range
of 0.001% to 0.020%. The C content is preferably 0.003% or
more, more preferably 0.004% or more. The C content is
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preferably 0.015% or less, more preferably 0.012% or less.
[0028]
Si: 0.05% to 1.00%
Si is an element that concentrates at an oxide layer
formed during welding, is thus effective in improving the
corrosion resistance in a weld zone, and is also effective
as a deoxidizing element in the steelmaking process. These
effects are obtained at a Si content of 0.05% or more and
are enhanced as the Si content increases. However, a Si
content of more than 1.00% results in an increase in rolling
load and significant formation of scales in the hot-rolling
step to induce the increases of surface defects and
production costs and thus is not preferred. Accordingly,
the Si content is 0.05% to 1.00%. The Si content is
preferably 0.10% or more, more preferably 0.15% or more.
The Si content is preferably 0.60% or less, more preferably
0.40% or less.
[0029]
Mn: 0.05% to 1.00%
Mn is an austenite-forming element and effective in
increasing the amount of austenite formed during heating
before rolling in the hot-rolling step. Additionally, Mn
acts as a deoxidizer. To obtain these effects, a Mn content
of 0.05% or more is needed. However, a Mn content of more
than 1.00% results in the promotion of the precipitation of
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MnS acting as a starting point of corrosion to deteriorate
the corrosion resistance. Accordingly, the Mn content is
0.05% to 1.00%. The Mn content is preferably 0.10% or more,
more preferably 0.15% or more. The Mn content is preferably
0.60% or less, more preferably 0.30% or less.
[0030]
P: 0.04% or Less
P is an element that is unavoidably contained in steel
and is also an element harmful to the corrosion resistance
and workability; thus, the P content is preferably minimized.
In particular, a P content of more than 0.04% results in a
significant deterioration in workability due to solution
hardening. Accordingly, the P content is 0.04% or less. The
P content is preferably 0.03% or less.
[0031]
S: 0.01% or Less
As with P, S is also an element that is unavoidably
contained in steel and is also an element harmful to the
corrosion resistance and workability; thus, the S content is
preferably minimized. In particular, a S content of more
than 0.01% results in a significant deterioration in
corrosion resistance. Accordingly, the S content is 0.01%
or less. The S content is preferably 0.008% or less. The S
content is more preferably 0.003% or less.
[0032]
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Al: 0.01% to 0.10%
Al is an effective deoxidizer. Al has a higher affinity
for nitrogen than Cr; thus, Al is effective in precipitating
nitrogen in the form of aluminum nitride instead of chromium
nitride to suppress sensitization when nitrogen enters a
weld zone. These effects are obtained at an Al content of
0.01% or more. However, an Al content of more than 0.10%
results in the deterioration of the penetration
characteristics during welding to deteriorate the welding
workability and thus is not preferred. Accordingly, the Al
content is in the range of 0.01% to 0.10%. The Al content
is preferably 0.02% or more, more preferably 0.03% or more.
The Al content is preferably 0.06% or less, more preferably
0.04% or less.
[0033]
Cr: 10.0% to 20.0%
Cr is the most important element for ensuring the
corrosion resistance of stainless steel. If the Cr content
is less than 10.0%, the corrosion resistance in an
automobile exhaust gas atmosphere cannot be ensured
sufficiently. If the Cr content is more than 20.0%, an
amount of an austenite phase formed in the hot-rolling step
is insufficient even if a predetermined amount of Ni is
contained; thus, the effect of reducing the size of grains
in the metal microstructure in the hot-rolling step is not
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sufficiently obtained to lead to an average grain size of
more than 20 m after the hot-rolled steel sheet annealing,
thereby failing to obtain the predetermined dimensional
accuracy during the punching work. Accordingly, the Cr
content is in the range of 10.0% to 20.0%. The Cr content
is preferably in the range of 10.0% to 17.0%. The Cr
content is more preferably 10.5% or more, even more
preferably 11.2% or more. The Cr content is more preferably
12.0% or less, even more preferably 11.7% or less.
[0034]
Ni: 0.50% to 2.00%
Ni is an austenite-forming element and effective in
increasing the amount of austenite formed during heating
before rolling in the hot-rolling step. In the present
invention, an austenite phase is formed during heating in
the hot-rolling step by controlling the Cr and Ni contents
to predetermined values. The formation of the austenite
phase results in the grain refinement of the coarse-grained
metal microstructure formed during casting. Additionally,
the dynamic and/or static recrystallization of the austenite
phase occurs in the hot rolling to lead to further grain
refinement of the metal microstructure after the hot rolling,
thereby contributing to the grain refinement of the metal
microstructure after the hot-rolled steel sheet annealing.
These effects are obtained at a Ni content of 0.50% or more.
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A Ni content of more than 2.00% results in an excess of
dissolved Ni. This is liable to cause the formation of
punching cracks due to excessive hardening of the hot-rolled
and annealed steel sheet. Accordingly, the Ni content is
0.50% to 2.00%. The Ni content is preferably 0.60% or more,
more preferably 0.70% or more, even more preferably 0.75% or
more. The Ni content is preferably 1.50% or less, more
preferably 1.00% or less.
[0035]
Ti: 0.10% to 0.40%
Ti preferentially combines with C and N and thus is
effective in suppressing the precipitation of chromium
carbonitride, reducing the recrystallization temperature,
and suppressing a deterioration in corrosion resistance due
to sensitization caused by the precipitation of chromium
carbonitride. To obtain these effects, Ti needs to be
contained in an amount of 0.10% or more. However, a Ti
content of more than 0.40% results in the formation of
coarse titanium carbonitride in a casting step to
significantly deteriorate the toughness of the steel sheet
and to cause surface defects and thus is not preferred in
terms of production. Accordingly, the Ti content is 0.10%
to 0.40%. The Ti content is preferably 0.15% or more, more
preferably 0.20% or more. The Ti content is preferably
0.35% or less. The Ti content is more preferably 0.30% or
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less. Preferably, the Ti content is such that the Ti
content satisfies the following formula Ti/(C + N) 8
(where, in the formula, Ti, C, and N are the amounts of the
relevant elements contained (% by mass)) in view of the
corrosion resistance in a weld zone.
[0036]
N: 0.001% to 0.020%
At a N content of more than 0.020%, the workability and
the corrosion resistance in a weld zone deteriorate markedly.
A lower N content is more preferable from the viewpoint of
the corrosion resistance. It takes long time to perform
refining to reduce the N content to less than 0.001%. Such
refining leads to an increase in production cost and a
deterioration in productivity, which is not preferred.
Accordingly, the N content is in the range of 0.001% to
0.020%. The N content is preferably 0.005% or more, more
preferably 0.007% or more. The N content is preferably
0.015% or less. The N content is more preferably 0.012% or
less.
[0037]
The present invention provides a ferritic stainless
steel featured by containing the foregoing essential
components and the balance being Fe and incidental
impurities. If necessary, one or two or more selected from
Cu, Mo, W, and Co, and/or one or two or more selected from V,
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Nb, Zr, REM, B, Mg, and Ca may be further contained in
ranges described below. The advantageous effects of the
present invention are not impaired when the elements are
contained in amounts of less than the lower limits. Thus,
when the elements are contained in amounts of less than the
lower limits, the elements are regarded as incidental
impurities.
[0038]
Cu: 0.01% to 1.00%
Cu is an element particularly effective in improving
the corrosion resistance of the base metal and a weld zone
in an aqueous solution or when slightly acidic water
droplets adhere thereto. The effect is obtained at a Cu
content of 0.01% or more and is enhanced as the Cu content
increases. However, a Cu content of more than 1.00% may
result in a deterioration in hot workability to induce
surface defects. Furthermore, a difficulty may lie in
descaling after annealing. Accordingly, when Cu is
contained, the Cu content is preferably in the range of
0.01% to 1.00%. The Cu content is more preferably 0.10% or
more, even more preferably 0.30% or more. The Cu content is
more preferably 0.60% or less, even more preferably 0.45% or
less.
[0039]
Mo: 0.01% to 2.00%
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Mo is an element that markedly improves the corrosion
resistance of stainless steel. The effect is obtained at a
Mo content of 0.01% or more and is improved as the Mo
content increases. However, a Mo content of more than 2.00%
may result in an increase in rolling load during hot rolling
to deteriorate the productivity and an excessive increase in
the strength of the steel sheet. Since Mo is an expensive
element, a large amount of Mo contained results in an
increase in production cost. Accordingly, when Mo is
contained, the Mo content is preferably 0.01% to 2.00%. The
Mo content is more preferably 0.10% or more, even more
preferably 0.30% or more. The Mo content is more preferably
1.40% or less, even more preferably 0.90% or less.
[0040]
W: 0.01% to 0.20%
As with Mo, W is effective in improving the corrosion
resistance. The effect is obtained at a W content of 0.01%
or more. However, a W content of more than 0.20% may result
in an increase in strength to lead to a deterioration in
productivity due to, for example, an increase in rolling
load. Accordingly, when W is contained, the W content is
preferably in the range of 0.01% to 0.20%. The W content is
more preferably 0.05% or more. The W content is more
preferably 0.15% or less.
[0041]
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Co: 0.01% to 0.20%
Co is an element that improves the toughness. The
effect is obtained when Co is contained in an amount of
0.01% or more. A Co content of more than 0.20% may result
in a deterioration in workability. Accordingly, when Co is
contained, the Co content is preferably in the range of
0.01% to 0.20%.
[0042]
V: 0.01% to 0.20%
V combines with C and N in the form of carbonitride to
suppress sensitization during welding, improving the
corrosion resistance in the weld zone. The effect is
obtained at a V content of 0.01% or more. A V content of
more than 0.20% may result in significant deteriorations in
workability and toughness. Accordingly, the V content is
preferably 0.01% to 0.20%. The V content is more preferably
0.02% or more. The V content is more preferably 0.050% or
less.
[0043]
Nb: 0.01% to 0.10%
Nb is effective in increasing the 0.2% proof stress by
refining grains and precipitating in the form of a fine
carbonitride. These effects are obtained at a Nb content of
0.01% or more. Nb is also effective in increasing the
recrystallization temperature. At a Nb content of more than
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0.10%, an excessively high annealing temperature is required
for sufficient recrystallization in the hot-rolled steel
sheet annealing; thus, a single ferrite phase microstructure
having an average grain size of 5 to 20 pm after the hot-
rolled steel sheet annealing, which is a requirement for the
present invention, is not obtained, in some cases.
Accordingly, when Nb is contained, the Nb content is
preferably in the range of 0.01% to 0.10%. The Nb content
is more preferably 0.01% to 0.05%.
[0044]
Zr: 0.01% to 0.20%
Zr combines with C and N and is effective in
suppressing sensitization. The effect is obtained at a Zr
content of 0.01% or more. A Zr content of more than 0.20%
may result in a significant deterioration in workability.
Accordingly, when Zr is contained, the Zr content is
preferably in the range of 0.01% to 0.20%. The Zr content
is more preferably in the range of 0.01% to 0.10%.
[0045]
REMs: 0.001% to 0.100%
Rare earth metals (REMs) are effective in improving
oxidation resistance and suppress the formation of an oxide
film in a weld zone (temper color due to welding) to
suppress the formation of a Cr-depleted region immediately
below the oxide film. The effect is obtained when REMs are
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contained in amounts of 0.001% or more. When REMs are
contained in amounts of more than 0.100%, the hot
workability may deteriorate. Accordingly, when REMs are
contained, REMs are preferably contained in the range of
0.001% to 0.100%. REMs are more preferably contained in the
range of 0.001% to 0.050%.
[0046]
B: 0.0002% to 0.0025%
B is an element effective in improving resistance to
secondary work embrittlement after deep drawing. The effect
is obtained at a B content of 0.0002% or more. A B content
of more than 0.0025% may result in deteriorations in
workability and toughness. Accordingly, when B is contained,
the B content is preferably in the range of 0.0002% to
0.0025%. The B content is more preferably 0.0003% or more.
The B content is more preferably 0.0006% or less.
[0047]
Mg: 0.0005% to 0.0030%
Mg is an element effective in improving the fraction of
equiaxed crystals in a slab to improve the workability and
the toughness. Regarding Ti-containing steel as in the
present invention, the coarsening of titanium carbonitride
deteriorates the toughness. Mg is also effective in
suppressing the coarsening of titanium carbonitride. These
effects are obtained at a Mg content of 0.0005% or more. A
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Mg content of 0.0030% may deteriorate the surface properties
of steel. Accordingly, when Mg is contained, the Mg content
is preferably in the range of 0.0005% to 0.0030%. The Mg
content is more preferably 0.0010% or more. The Mg content
is more preferably 0.0020% or less.
[0048]
Ca: 0.0003% to 0.0030%
Ca is an element effective in preventing nozzle
clogging due to the crystallization of Ti-based inclusions,
which tends to occur during continuous casting. The effect
is obtained at a Ca content of 0.0003% or more. However, a
Ca content of more than 0.0030% may result in the formation
of CaS to deteriorate the corrosion resistance. Accordingly,
when Ca is contained, the Ca content is preferably in the
range of 0.0003% to 0.0030%. The Ca content is more
preferably 0.0005% or more. The Ca content is more
preferably 0.0015% or less, even more preferably 0.0010% or
less.
[0049]
A method for producing a hot-rolled and annealed
ferritic stainless steel sheet of the present invention will
be described below.
[0050]
A hot-rolled and annealed ferritic stainless steel
sheet of the present invention is produced by subjecting a
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steel slab having the foregoing chemical composition to the
usual hot rolling process to produce a hot-rolled steel
sheet and then subjecting the resulting hot-rolled steel
sheet to hot-rolled steel sheet annealing in which holding
is performed at 600 C or higher and lower than 750 C for 1
minute to 24 hours.
[0051]
First, a molten steel having the foregoing chemical
composition is produced by a known method using, for example,
a converter, an electric furnace, or a vacuum melting
furnace and is formed into a steel (slab) by a continuous
casting process or an ingot casting-slabbing process.
[0052]
The slab is heated at 1,050 C to 1,250 C for 1 to 24
hours and then subjected to hot rolling. Alternatively, the
as-cast slab is directly subjected to hot rolling before the
temperature of the slab after casting is decreased to a
temperature bellow the foregoing temperature range. In the
present invention, the technique and conditions of the hot
rolling are not particularly limited. When a coiling
process is performed at an excessively low temperature, the
hot-rolled steel sheet may be significantly hardened to make
it difficult to perform the operation of the subsequent
process. Thus, the coiling process is preferably performed
at 550 C or higher.
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[0053]
Hot-Rolled Steel Sheet Annealing: Holding at 600 C or Higher
and Lower Than 750 C for I Minute to 24 Hours
In the present invention, after the completion of the
hot-rolling step, the hot-rolled steel sheet annealing is
performed. In the hot-rolled steel sheet annealing, a
rolled deformed microstructure formed in the hot-rolling
step is recrystallized without excessively coarsening the
metal microstructure, and a martensite phase formed in the
hot-rolling step is transformed into a ferrite phase. To
obtain the effect, the hot-rolled steel sheet annealing
needs to be performed at 600 C or higher and lower than
750 C. At an annealing temperature of lower than 600 C,
insufficient recrystallization results in fine recovered
grains from the hot-rolled deformed microstructure. The
metal microstructure is excessively refined to fail to
obtain the predetermined dimensional accuracy during
punching work. Additionally, a deformed microstructure and
a martensite phase remain in the metal microstructure after
the hot-rolled steel sheet annealing. This may lead to the
formation of punching cracks due to excessive hardening of
the steel sheet even if the average grain size is within a
predetermined range. An annealing temperature of 750 C or
higher results in excessively coarse grains having an
average grain size of more than 20 m, thereby failing to
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obtain the predetermined dimensional accuracy during
punching work. At a holding time of less than 1 minute, a
deformed microstructure and a martensite phase remain in the
metal microstructure after the hot-rolled steel sheet
annealing. This easily leads to the formation of punching
cracks due to excessive hardening of the steel sheet even if
the average grain size is within a predetermined range. A
holding time of more than 24 hours results in excessively
coarse grains having an average grain size of more than 20
m, thereby failing to obtain the predetermined dimensional
accuracy during punching work. Accordingly, the hot-rolled
steel sheet annealing is performed by performing holding in
a temperature range of 600 C or higher and lower than 750 C
for 1 minute to 24 hours. The temperature of the hot-rolled
steel sheet annealing is preferably 600 C or higher, more
preferably 640 C or higher. The temperature of the hot-
rolled steel sheet annealing is preferably 700 C or lower.
The holding time is preferably 1 hour or more, more
preferably 6 hours or more. The holding time is preferably
20 hours or less, more preferably 12 hours or less. There
is no particular limitation on the process of annealing the
hot-rolled steel sheet, i.e., the hot-rolled steel sheet
annealing process. Any one of box annealing (batch
annealing) and continuous annealing may be employed.
[0054]
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The resulting hot-rolled and annealed steel sheet may
be subjected to descaling treatment using, for example, shot
blasting or pickling, as needed. To improve surface
properties, for example, grinding or polishing may be
performed. The hot-rolled and annealed steel sheet
according to the present invention may then be subjected to
cold rolling and cold-rolled steel sheet annealing.
EXAMPLES
[0055]
The present invention will be described in detail below
by examples.
[0056]
Molten stainless steels having chemical compositions
given in Table 1 were produced with a 100 kg vacuum melting
furnace. Steel ingots of those molten steels were heated at
1,100 C for 1 hour, hot-rolled to the thicknesses given in
Table 2 (see "Thickness after completion of hot rolling" in
Table 2), and subjected to simulated coiling treatment in
which the steel sheets were held at 650 C for 1 hour and
then furnace-cooled, thereby producing hot-rolled steel
sheets. Subsequently, the hot-rolled steel sheets were
subjected to hot-rolled steel sheet annealing in which the
steel sheets were held at the temperatures given in Table 2
(see "Hot-rolled steel sheet annealing temperature" in Table
2) for 8 hours and then slowly cooled, thereby producing
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hot-rolled and annealed steel sheets.
The thicknesses of the resulting hot-rolled and
annealed steel sheets were the same as the respective
thicknesses after the completion of the hot rolling.
The hot-rolled and annealed steel sheets thus obtained
were evaluated as described below.
[0057]
(1) Evaluation of Metal Microstructure
Test pieces for microstructure observation were taken
from the middle portions of the sheets in the width
direction. A section of each test piece in the rolling
direction was mirror-polished. Then measurement and
analysis were performed in the field of view including the
entire thickness by a SEM/EBSD method. Boundaries with a
misorientation of 15 or more were defined as grain
boundaries. The average grain size was determined on the
basis of an area method. A steel sheet having an average
grain size of 5 lam or more and 20 WU or less is within the
scope of the present invention. A steel sheet having an
average grain size of less than 5 WU or more than 20 m is
outside the scope of the present invention and underlined in
Table 2.
[0058]
Similarly, test pieces for microstructure observation
were taken from the middle portions of the sheets in the
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width direction. A section of each test piece in the
rolling direction was mirror-polished. The section was
etched for observation with an aqueous solution of picric
acid and hydrochloric acid to expose a metal microstructure.
The metal microstructure was observed with an optical
microscope at a magnification of x500. Whether the metal
microstructure of each steel sheet was a single ferrite
phase microstructure was determined by distinguishing a
ferrite phase from a martensite phase on the basis of the
morphology of the metal microstructure. Specifically, a
region in which a uniform and flat morphology was observed
in a grain and a relatively bright tone was observed was
regarded as a ferrite phase. A region in which surface
morphology unique to the martensite phase, such as
subboundaries and block boundaries, was observed in a grain
and which exhibited a darker tone than the ferrite phase was
regarded as a martensite phase. In the table, F indicates
that the metal microstructure was a single ferrite phase
microstructure.
[0059]
(2) Evaluation of Corrosion Resistance
Test pieces measuring 60 x 100 mm were taken from the
hot-rolled and annealed steel sheets. Surfaces of each of
the test pieces were subjected to abrasive finishing with
600-grit emery paper. Then end-face portions of each test
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piece were sealed. The test pieces were subjected to a
cyclic salt-spray test specified in JIS H 8502. One cycle
of the cyclic salt-spray test consisted of salt spraying (
5% by mass NaCl, 35 C, spraying for 2 hours) -* drying (60 C,
4 hours, relative humidity: 40%) -* wetting (50 C, 2 hours,
relative humidity: 95%). The cyclic salt-spray test was
performed five cycles. After the cyclic salt-spray test was
performed five cycles, the surfaces of each test piece were
photographed. The rusted area of the surfaces of the test
piece was measured by image analysis. The percentage of
rusted area ((rusted area of test piece/total area of test
piece) x 100 [%]) was calculated with respect to the total
area of the test piece. A percentage of rusted area of 10%
or less, which indicated that the steel sheet had
outstanding corrosion resistance, was rated as "acceptable"
(D). A percentage of rusted area of more than 10% and 25%
or less was rated as "acceptable" (D). A percentage of
rusted area of more than 25% was rated as "unacceptable" (x).
[0060]
(3) Evaluation of Punching Work
Samples measuring 100 mm x 100 mm for testing were
taken from the hot-rolled and annealed steel sheets. Five
test specimens were produced by punching work with a crank
press equipped with an upper die (punch) and a lower die
(die) so as to form a hole having a diameter of 20 mm
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(tolerance: 0.1 mm) in the middle portion of each of the
samples, the upper die having a cylindrical edge for
punching and a diameter of 20 mm, the lower die having a
hole appropriately selected in such a manner that the
clearance between the upper die and the lower die was 10%.
The relationships among the clearance (C) [%], the hole
diameter of the die (the inside diameter of the die) (Dd)
[mm], the diameter of the punch (Dp) [mm], and the sheet
thickness (t) [mm] are represented by formula (1) below.
C = (Dd -Dp) + (2 x t) x 100 formula (1)
With respect to the test specimens thus obtained, the
appearance of each test specimen was visually inspected, and
the diameter of the hole in the middle portion of the test
specimen was measured with a digital caliper. The case
where no cracks were observed and where the hole diameter of
each of the five test specimens after the punching work was
in the range of 19.9 to 20.1 mm was rated as "acceptable"
(D). The case where any one of them was cracked or had a
hole diameter of less than 19.9 mm or more than 20.1 mm was
rated as "unacceptable" (x).
[0061]
Table 2 presents the test results together with hot-
rolled steel sheet annealing conditions.
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[0062]
[Table 1]
Steel Chemical composition (% by mass)
Remarks
symbol C Si Mn P S Al Cr Ni Ti N Others
Al 0.008 0.24 0.34 0.03 0.004 0.05 10.3
0.83 0.23 0.008 Example
A2 0.006 0.24 0.30 0.02 0.004 0.04 15.7 0.85 0.25 0.007 Example
A3 0.008 0.23 0.29 0.02 0.003 0.03 19.7 0.88 0.23 0.007 Example
A4 0.007 0.27 0.27 0.03 0.004 0.06 11.5 0.86 0.25 0.008 Example
A5 0.009 0.07 0.29 0.03 0.005 0.05 11.5 0.84 0.24 0.006 Example
A6 0.008 0.11 0.30 0.04 0.004 0.03 11.3 0.82 0.26 0.009 Example
A7 0.006 0.15 0.28 0.02 0.003 0.03 11.4 0.84 0.25 0.005 Example
A8 0.008 0.58 0.32 0.03 0.004 0.04 11.6 0.81 0.25 0.008 Example
A9 0.008 0.26 0.06 0.03 0.004 0.04 11.6 0.81 0.23 0.006 Example
A10 0.007 0.23 0.10 0.03 0.003 0.03 11.4 0.83 0.21 0.009 Example
All 0.008 0.24 0.17 0.04 0.005 0.03 11.6 0.85 0.20 0.010 Example
Al2 0.006 0.27 0.97 0.02 0.003 0.06 11.6 0.90 0.22 0.006 Example
A13 0.008 0.23 0.26 0.03 0.004 0.04 11.6 0.52 0.25 0.007 Example
A14 0.007 0.26 0.27 0.03 0.003 0.06 11.7 0.98 0.24 0.008 Example
A15 0.010 0.23 0.25 0.04 0.003 0.05 11.5 1.97 0.21 0.010 Example
A16 0.008 0.28 0.25 0.02 0.004 0.04 10.2 0.51 0.24 0.007 Example
A17 0.006 0.23 0.34 0.03 0.004 0.04 11.8 0.97 0.24 0.008 Example
A18 0.009 0.25 0.35 0.03 0.005 0.01 19.6 1.98 0.23 0.011 Example
A19 0.007 0.27 0.33 0.02 0.004 0.04 11.4 0.50 0.24 0.008 Cu: 0.34, B:
0.0011 Example
A20 0.008 0.24 0.31 0.03 0.004 0.04 11.5 0.99 0.26 0.007 Mo: 0.51, Nb: 0.08
Example
A21 0.007 0.28 0.28 0.02 0.004 0.03 11.6
0.83 0.26 0.007 Cu: 0.46 Example
A22 0.009 0.23 0.35 0.03 0.004 0.06 11.6 0.88 0.23 0.008 Mo: 1.22
Example
A23 0.007 0.26 0.27 0.02 0.004 0.06 11.7
0.78 0.22 0.009 W: 0.11 Example
A24 0.008 0.25 0.31 0.03 0.003 0.04 11.6
0.88 0.23 0.007 Co: 0.15 Example
A25 0.007 0.26 0.34 0.02 0.004 0.04 11.5
0.86 0.25 0.006 V: 0.12 Example
A26 0.009 0.27 0.35 0.02 0.005 0.04 11.6
0.78 0.24 0.008 V: 0.05, Nb: 0.09 Example
A27 0.008 0.25 0.26 0.03 0.005 0.05 11.5
0.86 0.24 0.007 Zr: 0.10 Example
A28 0.008 0.24 0.34 0.03 0.004 0.04 11.6
0.86 0.25 0.009 REM: 0.008 Example
A29 0.007 0.27 0.29 0.03 0.005 0.06 11.5
0.83 0.25 0.008 B: 0.0014 Example
A30 0.006 0.27 0.32 0.03 0.004 0.04 11.5 0.80 0.23 0.006 Mg: 0.0007, Ca:
0.0005 Example
B1 0.008 0.27 0.28 0.03 0.003 0.03 11.5
0.21 0.23 0.008 Comparative example
B2 0.008 0.25 0.28 0.03 0.005 0.04 11.4
2.08 0.26 0.008 Comparative example
B3 0.009 0.28 0.26 0.03 0.003 0.05 9.4
0.79 0.22 0.007 Comparative example
B4 0.006 0.28 0.33 0.03 0.003 0.03 20.8
0.81 0.24 0.009 Comparative example
B5 0.006 0.25 0.28 0.03 0.004 0.05 11.6
0.86 0.07 0.009 Comparative example
The balance other than the component composition described above is Fe and
incidental impurities.
Underlined values are outside the scope of the present invention.
[0063]
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[Table 2]
Hot-rolled steel
Thickness after Metal Average
Steel sheet annealing = Punching Corrosion
No. symbol (*) size Remarks completion of
microstructure grain temperature workability resistance
hot rolling [mm] [Iirn]
[ C]
1 Al 10.1 652 F 11 0 0 Example
2 A2 9.9 653 F 16 0 0 Example
3 A3 10.2 655 F 19 0 0 Example
4 A4 9.8 651 F 12 0 0 Example
A5 10.0 651 F 9 0 0 Example
6 A6 9.9 653 F 11 0 0 Example
7 A7 10.2 652 F 12 0 0 Example
8 A8 10.2 649 F 17 0 0 Example
9 A9 9.9 653 F 16 0 0 Example
A10 9.9 654 F 10 0 0 Example
11 All 9.9 650 F 18 0 0 Example
12 Al2 9.8 647 F 8 0 0 Example
13 A13 10.3 655 F 5 0 0 Example
14 A14 10.0 646 F 19 0 0 Example
A15 10.0 653 F 8 0 0 Example
16 A16 10.1 655 F 18 0 0 Example
17 A17 9.8 646 F 17 0 0 Example
18 A18 10.0 653 F 9 0 0 Example
19 A19 9.9 650 F 11 0 0 Example
A20 9.8 652 F 10 0 0 Example
21 A21 9.8 645 F 13 0 0 Example
22 A22 9.9 652 F 12 0 0 Example
23 A23 10.1 646 F 13 0 0 Example
24 A24 9.8 652 F 14 0 0 Example
A25 10.0 647 F 13 0 0 Example
26 A26 10.1 649 F 12 0 0 Example
27 A27 9.9 653 F 9 0 0 Example
28 A28 10.0 645 F 11 0 0 Example
29 A29 10.2 648 F 10 0 0 Example
A30 10.2 655 F 12 0 0 Example
31 Al 5.2 652 F 10 0 0 Example
32 Al 8.1 650 F 12 0 0 Example
33 Al 13.0 645 F 16 0 0 Example
34 Al 14.7 647 F 18 0 0 Example
A13 10.0 602 F 8 0 0 Example
36 A13 9.9 747 F 17 0 0 Example
37 B1 9.8 749 F 34 x 0
Comparative example
38 B2 9.8 652 F 8 x 0
Comparative example
39 B3 10.1 650 F 11 0 x
Comparative example
B4 9.8 651 F 26 x 0 Comparative
example
41 B5 10.1 646 F 16 0 x
Comparative example
43 A26 10.0 803 F 28 x 0
Comparative example
44 A14 9.9 806 F 34 x 0
Comparative example
- Underlined values are outside the scope of the present invention.
*) F: ferrite phase
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[0064]
In Nos. 1 to 36 in which the steel compositions and the
hot-rolled steel sheet annealing conditions were within the
scope of the present invention, the predetermined punching
workability was obtained because, in addition to the
formation of an austenite phase during heating in the hot-
rolling step, recrystallization occurred by the
predetermined hot-rolled steel sheet annealing without
excessive coarsening of grains to obtain the predetermined
average grain size. The evaluation results of the corrosion
resistance of the resulting hot-rolled and annealed sheets
revealed that each sheet had a percentage of rusted area of
25% or less and had sufficient corrosion resistance.
[0065]
In particular, in each of No. 19 using Cu-containing
steel A19, No. 21 using Cu-containing steel A21, No. 20
using Mo-containing steel A20, and No. 22 using Mo-
containing steel A22, the percentage of rusted area was 10%
or less, and better corrosion resistance was provided.
[0066]
In each of No. 3 using steel A3 having a high Cr
content of 19.7% and No. 18 using steel A18 having a high Cr
content of 19.6%, a strong passivation film was formed on
the surfaces of the steel sheet; thus, the percentage of
rusted area was 10% or less, and better corrosion resistance
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was provided.
[0067]
In No. 37 using steel Bl having a Ni content of less
than that in the scope of the present invention, almost no
austenite phase was formed during heating in the hot-rolling
step to fail to obtain the effect of refining the metal
microstructure. Thus, the average grain size was more than
the scope of the present invention to fail to obtain the
predetermined punching workability.
[0068]
In No. 38 using steel B2 having a Ni content of more
than that in the scope of the present invention, although
the predetermined average grain size was obtained, the
excessive amount of dissolved Ni resulted in the excessively
hardened steel sheet to cause cracking during punching work,
thereby failing to process the steel sheet into a
predetermined shape.
[0069]
In No. 39 using steel B3 having a Cr content of less
than that in the scope of the present invention, the
predetermined corrosion resistance was not be obtained due
to the insufficient Cr content.
[0070]
In No. 40 using steel B4 having a Cr content of more
than that in the scope of the present invention, the
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excessive Cr content resulted in the decrease of the
austenite phase formed during heating in the hot-rolling
step, even though the predetermined amount of Ni was
contained. Thus, the effect of grain refinement due to the
formation of the austenite phase in the hot-rolling step was
not sufficiently obtained. Thereby, the predetermined
average grain size was not obtained, failing to obtain the
predetermined punching workability.
[0071]
In No. 41 using steel B5 having a Ti content of less
than that in the scope of the present invention, a large
amount of chromium carbonitride was precipitated during the
hot-rolled steel sheet annealing to cause sensitization,
thereby failing to obtain the predetermined corrosion
resistance.
[0072]
In No. 43 in which the hot-rolled steel sheet annealing
temperature was higher than that in the scope of the present
invention, recrystallized grains formed coarsened
significantly to fail to provide the predetermined average
grain size, thereby failing to obtain the predetermined
punching workability.
[0073]
No. 44 is an example in which steel A14 containing the
predetermined steel composition was annealed at 806 C, which
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is higher than that in the scope of the present invention,
to increase the average grain size to 34 m, which is more
than that in the scope of the present invention. Although
the predetermined steel composition was contained, the
excessively coarse grains resulted in significant rollover
and burrs during the punching work, thereby failing to
obtain the predetermined punching workability.
Industrial Applicability
[0074]
The hot-rolled and annealed ferritic stainless steel
sheet according to the present invention is particularly
suitable for applications that require high workability and
corrosion resistance, for example, flanges having burring
portions.
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