Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 03133206 2021-09-10
DESCRIPTION
FERRITIC STAINLESS STEEL SHEET FOR AUTOMOBILE BRAKE DISK ROTORS,
AUTOMOBILE BRAKE DISK ROTOR, AND HOT-STAMPED ARTICLE FOR
AUTOMOBILE BRAKE DISK ROTORS
TECHNICAL FIELD
[0001]
The present invention relates to a ferritic stainless steel sheet for an
automobile brake disc rotor, an automobile brake disc rotor and a hot-stamped
product for an automobile brake disc rotor, which have excellent heat
resistance and
formability, and specifically to a ferritic stainless steel sheet suitably
used for an
automobile brake disc rotor and the like that are required to have high-
temperature
strength.
BACKGROUND ART
[0002]
A disc brake is widely used as one of automobile brake systems. This disc
brake reduces a speed of an automobile by pressing a disc-like structure
connected
to a tire, called a disc rotor, between brake pads to cause friction, thereby
converting
kinetic energy to thermal energy. As a material for this disc rotor, flake
graphite cast
iron (hereinafter referred to as cast iron) is used in light of thermal
conductivity, a cost
and the like.
[0003]
Cast iron, to which an element improving corrosion resistance is not added,
is inferior in corrosion resistance and thus gathers red rust upon being left.
This red
rust conventionally is not so noticeable due to a position of the disc that is
lower than
a line of sight and a shape of a wheel. However, since aluminum is used for a
material
for the wheel and a spoke is made thinner in order to respond to a request for
improvement in fuel efficiency, the rust of the disc cannot be left ignored,
thereby
generating a need for improving corrosion resistance.
[0004]
A material having excellent corrosion resistance is exemplified by stainless
steel. Specifically, a martensitic material, i.e., 5U5410, is widely used for
a two-
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wheeled vehicle such as a motorcycle. This is because the disc rotor for the
two-
wheeled vehicle is exposed to be easily noticeable and thus corrosion
resistance is
considered as important. However, stainless steel is inferior in thermal
conductivity
to cast iron. In the two-wheeled vehicle, a brake system being exposed to have
excellent cooling performance allows use of even stainless steel with no
problem. In
a case of an automobile, since a brake system including a tire is housed in a
wheelhouse, the disc rotor is less likely to be cooled and has low thermal
conductivity,
whereby stainless steel has not been applied thereto.
[0005]
However, for recent EV, FCV, HV and the like, a "regenerative brake" that
converts kinetic energy to electrical energy during running to recover the
electrical
energy has been increasingly adopted. This application reduces frictional heat
generated by friction between the disc rotor and the pads, allowing a growing
possibility for application thereof also to stainless steel that is inferior
in thermal
conductivity to cast iron.
[0006]
Another problem that prevents application of stainless steel to the disc brake
of the automobile is formability. The disc rotor for the two-wheeled vehicle
is in a form
of a ring-shaped disc, and is produced, without requiring large machining, by
.. punching plate-shaped stainless steel. On the other hand, the current disc
rotor for
the automobile is in a form of a disc whose center is squeezed, called a hat
shape,
and is produced by casting. Machining stainless steel to be formed into this
shape
requires deep drawing. However, stainless steel used for the two-wheeled
vehicle is
martensitic stainless steel, which has extremely high hardness and entails
machining
difficulty. As one method of solution thereto, hot stamping involving pressing
at a high
temperature recently has been increasingly used. The hot stamping also allows
forming stainless steel into a hat shape with a high precision.
[0007]
Against this background, in order to meet the request for improvement in fuel
.. efficiency, the disc rotor needs to be made thinner and lighter. However,
since cast
iron is low in strength and the disc rotor is typically produced by casting,
there is a
limit to thinning thereof. In addition, since a temperature to be reached at
braking of
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the automobile is said to be near 700 degrees C at a maximum, it may be
difficult to
apply martensitic stainless steel having a heat resistance temperature of near
500
degrees C. Moreover, a temperature to be reached under driving conditions of
frequent braking on a mountain road or the like may be 300 degrees C.
[0008]
Patent Literature 1 relates to a stainless steel disc rotor for the automobile
but mainly focuses on formability not on high-temperature strength.
Furthermore,
Patent Literature 2 improves strength by martensitic phases including highly
saturated solid solution C and N but does not describe strength near 700
degrees C.
Both Patent Literatures 1 and 2 adopt a martensitic structure but are not
found to
reliably provide heat resistance near 700 degrees C.
CITATION LIST
PATENT LITERATURE(S)
[0009]
Patent Literature 1: JP 5700172 B
Patent Literature 2: JP 2016-117925 A
SUMMARY OF THE INVENTION
PROBLEM(S) TO BE SOLVED BY THE INVENTION
[0010]
The invention relates to a ferritic stainless steel sheet for an automobile
brake
disc rotor having excellent heat resistance and formability. A target
component, which
is an intended use of the invention whose problem is to be solved, is a
braking system
component for an automobile, particularly a disc rotor.
The disc rotor for the automobile having a hat shape requires formability. In
addition, since a temperature to be reached is usually about 100 degrees C in
driving
on a city road and about 300 degrees C in driving on a mountain road while
being
near 700 degrees C at a maximum, thinning thereof requires strength in an
intermediate to high temperature range. Since cast iron is molded by casting,
thinning
of the disc rotor deteriorates metal flow, which may result in a failure of
molding.
Moreover, since cast iron is low in strength, the thinning of the disc rotor
cannot
reliably secure sufficient strength as a disc rotor. Ferritic stainless steel
can be formed
into a hat shape through hot stamping with a high precision. However,
stainless steel
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having low strength cannot be thinned. Meanwhile, stainless steel having high
strength, due to requiring an excessive load in hot stamping, cannot be formed
into
a hat shape with a high precision or is likely to cause cracking. Martensitic
stainless
steel is excellent in formability in hot stamping but has a heat resistance
temperature
of about 500 degrees C, thus failing to achieve both formability and heat
resistance.
[0011]
The invention provides a ferritic stainless steel sheet for an automobile
brake
disc rotor, an automobile brake disc rotor and a hot-stamped product for an
automobile brake disc rotor, which have excellent heat resistance and
formability.
MEANS FOR SOLVING THE PROBLEM(S)
[0012]
In order to solve the above problems, the inventors have focused on and
studied in detail precipitates of a ferritic stainless steel sheet. The
precipitates may
be formed in the steel in a temperature range in which the component according
to
the invention is formed through hot stamping. The finely dispersed
precipitates can
improve strength of a material. However, presence of the precipitates before
the
forming results in excessively high strength to reduce elongation of the
steel, making
it likely for cracking to occur during the forming. Accordingly, it has been
believed that
finely forming the precipitates during the hot stamping can reliably secure
formability
and strength after the hot stamping. After intense studies to achieve the
above
objects, the inventors have obtained the following findings.
[0013]
By appropriately controlling an added amount of Si, setting a finishing
temperature after hot rolling in a range from 900 to 1100 degrees C and
setting a
winding temperature at 650 degrees C or less, a crystal grain size is
increased during
heating in the hot stamping to form the precipitates during the hot stamping.
Setting
the finishing temperature at more than 950 degrees C effectively increases the
crystal
grain size to improve strength also in an intermediate temperature range.
Since the
precipitates are finely formed in crystal grains of the steel, excellent high-
temperature
strength during use as a disc rotor can be attained. The precipitates formed
at grain
boundaries are likely to grow and coarsen. In this regard, it has been found
that the
precipitates are formed mainly in the crystal grains by appropriately
controlling the
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crystal grain size during heating in the hot stamping. The precipitates in the
crystal
grains are less likely to grow than the precipitates at the grain boundaries
and thus
are less likely to coarsen during use. The precipitates are finely formed in
the grains
during the hot stamping, thereby effectively exhibiting precipitation
strengthening.
Accordingly, a heat-resistant ferritic stainless steel sheet applicable to a
disc rotor
has been successfully provided.
[0014]
A summary of the invention for solving the above problems is as follows.
[1] A ferritic stainless steel sheet for an automobile brake disc rotor
includes: 0.001
to 0.05 mass% of C; 0.001 to 0.05 mass% of N; 0.3 to 4.0 mass% of Si; 0.01 to
2.0
mass% of Mn; 0.01 to 0.05 mass% of P; 0.0001 to 0.02 mass% of S; 10 to 20
mass%
of Cr; one or both of 0.001 to 0.5 mass% of Ti and 0.01 to 0.8 mass% of Nb;
and a
balance consisting of Fe and impurities,
through a heat treatment (hereinafter referred to as a "hot stamping pseudo
heat treatment") in which the steel sheet is heated to 1000 degrees C and
subsequently cooled by being retained in a range from 890 to 700 degrees C for
one
to ten minutes, a crystal grain size is in a range from 100 to 200 pm, and
precipitates
each having a grain size of 500 nm or less are at a density of 0.01 to 20
pieces per
square micrometer, and the ferritic stainless steel sheet is a ferritic
stainless steel for
hot stamping.
[0015]
[2] A ferritic stainless steel sheet for an automobile brake disc rotor
includes: 0.001
to 0.05 mass% of C; 0.001 to 0.05 mass% of N; 0.3 to 4.0 mass% of Si; 0.01 to
2.0
mass% of Mn; 0.01 to 0.05 mass% of P; 0.0001 to 0.02 mass% of S; 10 to 20
mass%
of Cr; one or both of 0.001 to 0.5 mass% of Ti and 0.01 to 0.8 mass% of Nb;
and a
balance consisting of Fe and impurities,
the ferritic stainless steel sheet in which a crystal grain size is in a range
from
100 to 200 pm and precipitates each having a grain size of 500 nm or less are
present
at a density of 0.01 to 20 pieces per square micrometer is in a form of a hot-
stamped
product.
[0016]
[3] A ferritic stainless steel sheet for an automobile brake disc rotor
includes: 0.001
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to 0.05 mass% of C; 0.001 to 0.05 mass% of N; 0.3 to 4.0 mass% of Si; 0.01 to
2.0
mass% of Mn; 0.01 to 0.05 mass% of P; 0.0001 to 0.02 mass% of S; 10 to 20
mass%
of Cr; one or both of 0.001 to 0.5 mass% of Ti and 0.01 to 0.8 mass% of Nb;
and a
balance consisting of Fe and impurities.
[0017]
[4] In the ferritic stainless steel sheet for an automobile brake disc rotor
with the
above arrangement, the ferritic stainless steel sheet is a ferritic stainless
steel sheet
for hot stamping.
[5] In the ferritic stainless steel sheet for an automobile brake disc rotor
with the
above arrangement, fracture elongation at 1000 degrees C is 50% or more, and
0.2%
proof strength at 700 degrees C after the hot stamping pseudo heat treatment
is 80
MPa or more.
[6] In the ferritic stainless steel sheet for an automobile brake disc rotor
with the
above arrangement, 0.2% proof strength at 700 degrees C is 80 MPa or more.
[7] In the ferritic stainless steel sheet for an automobile brake disc rotor
with the
above arrangements, the crystal grain size is in a range from 130 to 200 pm.
[8] In the ferritic stainless steel sheet for an automobile brake disc rotor
with the
above arrangement, fracture elongation at 1000 degrees C is 50% or more.
[9] In the ferritic stainless steel sheet for an automobile brake disc rotor
with the
above arrangement, 0.2% proof strength at 300 degrees C is 170 MPa or more.
[0018]
[10] The ferritic stainless steel sheet for an automobile brake disc rotor
with the
above arrangements further includes, in place of a part of Fe, at least one of
0.0001
to 0.0030 mass% of B, 0.001 to 4.0 mass% of Al, 0.01 to 3.0 mass% of Cu, 0.01
to
3.0 mass% of Mo, 0.001 to 2.0 mass% of W, 0.001 to 1.0 mass% of V, 0.01 to 0.5
mass% of Sn, 0.01 to 1.0 mass% of Ni, 0.0001 to 0.01 mass% of Mg, 0.005 to 0.5
mass% of Sb, 0.001 to 0.3 mass% of Zr, 0.001 to 0.3 mass% of Ta, 0.001 to 0.3
mass% of Hf, 0.001 to 0.3 mass% of Co, 0.0001 to 0.01 mass% of Ca, 0.001 to
0.2
mass% of REM, and 0.0002 to 0.3 mass% of Ga.
[0019]
[11] An automobile brake disc rotor is made of the stainless steel sheet with
the
above arrangements.
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[12] A hot-stamped product for an automobile brake disc rotor is made of the
stainless steel sheet with the above arrangements.
[0020]
According to the above aspects of the invention, a ferritic stainless steel
sheet
improves in heat resistance and formability to provide a material suitable for
an
automobile brake disc rotor, thereby attaining significant effects such as
reducing a
weight and improving an appearance thereof.
DESCRIPTION OF EMBODIMENT(S)
[0021]
In producing an automobile brake disc rotor using a ferritic stainless steel
sheet through hot stamping, the steel sheet is heated to about 1000 degrees C
for
the hot stamping. The steel sheet before the hot stamping is required to have
sufficient ductility for the hot stamping, which is performed at about 1000
degrees C.
Meanwhile, an automobile brake disc rotor after the hot stamping is required
to
achieve sufficient high-temperature strength.
[0022]
As described above, a temperature range for forming through the hot
stamping may allow formation of precipitates. The precipitates being finely
dispersed
in the steel can improve strength of a material. However, presence of the
precipitates
before the forming results in excessively high strength to reduce elongation,
making
it likely for cracking to occur during the forming through the hot stamping.
Accordingly,
the invention secures hot stamping formability and strength after the hot
stamping by
finely forming the precipitates during the hot stamping.
[0023]
A crystal grain size of the steel during the hot stamping is given an
attention.
In a case where the crystal grain size is small, a ratio of grain boundaries
in the steel
is high, thus resulting in formation of more precipitates at the grain
boundaries during
the hot stamping. The precipitates formed at the grain boundaries are likely
to grow
and coarsen, and thus fine precipitates are not likely to be obtained.
According to the
invention, it has been found that the precipitates are formed mainly in the
grains by
growing and appropriately controlling the crystal grain size during heating in
the hot
stamping. The precipitates in the grains are less likely to grow than the
precipitates
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at the grain boundaries and thus are less likely to coarsen during use. The
precipitates are finely formed in the grains during the hot stamping to
effectively
exhibit precipitation strengthening after the hot stamping, thereby securing
strength
after the forming.
[0024]
As described above, according to the invention, it has been found that: finely
forming the precipitates in the grains is crucial in terms of high-temperature
strength
after the hot stamping; and for this purpose, growing the crystal grain size
during the
heating in the hot stamping to a certain extent is required. Specifically, it
has been
found that refinement of the precipitates can be achieved by having the
crystal grain
size after the hot stamping in a range from 100 to 200 pm. In addition, it
also has
been found that the crystal grain size during the hot stamping is the same as
the
crystal grain size after the hot stamping. With a crystal grain size in the
above range,
the precipitates are finely formed in the grains and are not likely to grow,
from which
it is presumed that there is a corresponding relationship therebetween.
Accordingly, a metal structure in the invention has been defined by the
crystal
grain size after the hot stamping. By controlling the crystal grain size after
the hot
stamping in a range from 100 to 200 pm, the precipitates are finely formed
during the
hot stamping and are not likely to grow, thereby effectively exhibiting
precipitation
strengthening. With a crystal grain size after the hot stamping being 100 pm
or more,
the precipitates are finely formed and thus sufficient proof strength up to
near 700
degrees C has been obtained. Moreover, with a crystal grain size after the hot
stamping being 130 pm or more, sufficient proof strength has been obtained
also in
an intermediate temperature range (i.e., near 300 degrees C).
[0025]
The crystal grains in the steel grow by the heating in the hot stamping to
increase the crystal grain size. There is a tendency that the larger the
crystal grain
size before the hot stamping is, the larger the crystal grain size during and
after the
hot stamping also is. The crystal grain size after the hot stamping exceeding
200 pm
means that the crystal grain size of the steel sheet before the hot stamping
is also
large, and consequently, toughness of the steel sheet significantly reduces.
Accordingly, an upper limit of the crystal grain size after the hot stamping
has been
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set at 200 pm.
[0026]
In order to effectively exhibit precipitation strengthening after the hot
stamping, the precipitates each having a grain size of 500 nm or less are
defined to
be present at a density of 0.01 to 20 pieces per square micrometer in the
steel after
the hot stamping. With the precipitates each having a grain size of 500 nm or
less
being present at a density of 0.01 to 20 pieces per square micrometer,
sufficient proof
strength up to near 700 degrees C is obtained. With a grain size exceeding 500
nm,
precipitation strengthening is not likely to function. With a density of the
precipitates
being less than 0.01 pieces per square micrometer, precipitation strengthening
is
also not likely to function due to a small precipitation amount. A density
exceeding
pieces per square micrometer excessively increases strength, making it likely
for
cracking to occur. In view of the above, it is preferable that the
precipitates being in
the grains and each having a grain size of 500 nm or less are present at a
density of
15 0.01 to 20 pieces per square micrometer.
[0027]
In a case where a product to be evaluated is a hot-stamped product, or a final
product in a form of an automobile brake disc rotor, the crystal grain size
and the
density of the precipitates in the steel can be evaluated. Meanwhile, in a
case where
20 the product to be evaluated is a steel sheet before the hot stamping,
the crystal grain
size and the density of the precipitates in the steel may be evaluated after
subjecting
the steel sheet to a hot stamping pseudo heat treatment. The hot stamping
pseudo
heat treatment may be a heat treatment including: heating the steel sheet to
1000
degrees C; and subsequently cooling by retaining the steel sheet in a range
from 890
to 700 degrees C for one to ten minutes (e.g., two minutes).
[0028]
A basis for defining contents of respective components in the steel will be
described below.
C deteriorates formability and corrosion resistance to reduce high-
temperature elongation and high-temperature strength of a steel sheet, and
precipitates Cr carbon itrides and Nb carbon itrides to make a density of the
obtained
precipitates excessive after hot stamping. Accordingly, the C content is
preferably as
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small as possible and thus is set at 0.05% or less. The C content is
preferably 0.020%
or less, more preferably 0.0015% or less. However, since an excessive
reduction in
the C content increases a refining cost, the C content is preferably 0.001% or
more.
[0029]
Similarly to C, N deteriorates formability and corrosion resistance to reduce
high-temperature elongation and high-temperature strength of a steel sheet,
and
precipitates Cr carbonitrides and Nb carbonitrides to make a density of the
obtained
precipitates excessive after hot stamping. Accordingly, the N content is
preferably as
small as possible and thus is set at 0.05% or less. The N content is
preferably 0.020%
or less, more preferably 0.015% or less. However, since an excessive reduction
in
the N content increases a refining cost, the N content is preferably 0.001% or
more.
[0030]
Si is an element useful as a deoxidizer and is also an element improving high-
temperature strength, oxidation resistance and high-temperature salt-damage
resistance. The high-temperature strength, oxidation resistance and high-
temperature salt-damage resistance improve in line with an increase in the Si
content.
In order to improve the high-temperature strength, controlling of
precipitation is
crucial, and thus this effect is attained by finely forming precipitates in a
large amount.
Si has an effect of finely forming aging precipitates and the effect is stably
exhibited
at 0.3% or more of Si. However, excessive addition of Si: reduces ductility of
a steel
sheet at a normal temperature and a high temperature to harden a hot-rolled
steel
sheet, thereby reducing toughness; and causes refinement of a crystal grain
size and
formation of excessive precipitates during hot stamping. Accordingly, an upper
limit
of the Si content is set at 4.0%.In terms of picklability and toughness, the
Si content
is preferably in a range from 0.3% to 3.5%.In terms of productivity, the Si
content is
preferably 3.0% or less.
[0031]
Mn is an element added as a deoxidizer and contributes to increasing high-
temperature strength in an intermediate temperature range. However, addition
of Mn
at more than 2.0%: causes MnS (which does not contribute to strengthening) to
precipitate in a large amount to reduce the high-temperature strength after a
pseudo
heat treatment; and precipitates Mn oxides on a surface layer at a high
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to make it likely to cause unfavorable adhesion of scales and abnormal
oxidation. In
particular, there is a tendency that combined addition of Mn with Mo and W
causes
abnormal oxidation with respect to the Mn amount. Accordingly, an upper limit
of the
Mn content is set at 2.0%.In terms of picklability and normal-temperature
ductility in
producing a steel sheet, the Mn content is preferably in a range from 0.01% to
1.5%,
more preferably 1.0% or less.
[0032]
P is an impurity mixed mainly from a material in steelmaking refining. As the
P content increases, toughness and weldability of a steel sheet reduce. Thus,
the P
content is preferably reduced as low as possible, but the P content of less
than 0.01%
increases a production cost due to use of a low-P material. Accordingly, the P
content
in the invention is set at 0.01% or more. The P content is more preferably
0.02% or
more. Meanwhile, the P content of more than 0.05% not only causes more
significant
hardening but also deteriorates corrosion resistance, toughness and
picklability.
Accordingly, an upper limit of the P content is set at 0.05%.The P content is
more
preferably 0.04% or less.
[0033]
S is an element deteriorating corrosion resistance and oxidation resistance.
However, since an effect of improving workability through bonding of S to Ti
and C is
exhibited at 0.0001% or more of S, a lower limit of the S content is set at
0.0001%.In
terms of a refining cost, the S content is preferably 0.0010% or more.
Meanwhile,
excessive addition of S allows S to be bonded to Ti and C to reduce an amount
of
solid solution Ti and coarsen precipitates, thereby reducing toughness and
high-
temperature strength of a steel sheet. Accordingly, an upper limit of the S
content is
set at 0.02%.In terms of high-temperature oxidation properties, the S content
is
preferably 0.0090% or less.
[0034]
Cr is an essential element for ensuring oxidation resistance and corrosion
resistance in the invention. Less than 10% of Cr cannot secure, particularly,
oxidation
resistance, to further reduce proof strength at 700 degrees C after hot
stamping and
coarsen a crystal grain size. Meanwhile, since more than 20% of Cr causes a
reduction in workability and deterioration of toughness and makes the number
of
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precipitates after hot stamping excessively large, the Cr content is set in a
range from
to 20%.In terms of productivity and scale peelability, the Cr content is
preferably
in a range from 12% to 18%, more preferably 15% or less.
[0035]
5 One or both of 0.001 to 0.5% of Ti and 0.01 to 0.8% of Nb are
contained.
Ti is an element bonded to C, N and S to improve corrosion resistance,
intergranular corrosion resistance, normal-temperature ductility and deep
drawability.
Accordingly, Ti is added as required. Combined addition where Ti is added in
an
appropriate amount with Nb and Mo increases an amount of solid solution of Nb
and
10 Mo and improves high-temperature strength in hot rolling and annealing,
thereby
improving thermal fatigue properties. This effect is exhibited at 0.001% or
more of Ti
and thus a lower limit of the Ti content is set at 0.001%.Meanwhile, addition
of more
than 0.5% of Ti not only increases an amount of solid solution Ti to reduce
ductility
of a steel sheet at a normal temperature and a high temperature but also makes
the
number of precipitates after hot stamping excessive and further forms coarse
Ti
precipitates to be an initiation point of cracking in hole expansion, thereby
deteriorating press workability. Since oxidation resistance also deteriorates,
an
added amount of Ti is set at 0.5% or less. In terms of occurrence of surface
flaws
and toughness, the Ti content is preferably in a range from 0.05% to 0.2%.
[0036]
Nb is an element effective in improving high-temperature strength through
solid solution strengthening and precipitation strengthening by fine
precipitates. Nb
also has a function of fixing C and N as carbonitrides to contribute to growth
of
recrystallization texture affecting corrosion resistance and an r value of a
product
sheet. These effects are exhibited at 0.01% or more of Nb and thus a lower
limit of
the Nb content is set at 0.01%.Meanwhile, since addition of more than 0.8% of
Nb
reduces high-temperature ductility of the steel sheet and makes the number of
precipitates after hot stamping excessive to further significantly harden the
steel
sheet and deteriorate productivity, an upper limit of the Nb content is set at
0.8%.In
terms of a material cost and toughness, the Nb content is preferably in a
range from
0.3% to 0.6%.
[0037]
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In addition to the above components, a balance consists of Fe and impurities
as components in the steel. The invention may further contain the following
components in place of a part of Fe as required.
[0038]
B is an element improving secondary workability, high-temperature strength
and thermal fatigue properties of a product in pressing. B causes fine
precipitation of
Laves phases and the like and exhibits long-term stability of this
precipitation
strengthening, thereby contributing to inhibiting a reduction in strength and
improving
a thermal fatigue life. This effect is exhibited at 0.0001% or more of B.
Meanwhile,
since excessive addition of B not only causes hardening and deteriorates
susceptibility to intergranular corrosion and oxidation resistance but also
causes weld
cracking, the B content is set at 0.0030% or less. In terms of corrosion
resistance
and a production cost, the B content is preferably 0.0010% or less, more
preferably
0.0005% or less.
[0039]
Al is an element to be added as a deoxidizing element and improve oxidation
resistance. Al is also useful for improving high-temperature strength as a
solid
solution strengthening element. This effect is stably exhibited at 0.001% or
more of
Al. Meanwhile, since excessive addition of Al hardens steel to significantly
reduce
uniform elongation and toughness, an upper limit of the Al content is set at
4.0%.In
terms of occurrence of surface flaws, weldability and productivity, the Al
content is
preferably in a range from 0.01% to 2.2%.
[0040]
Cu is an element effective in improving corrosion resistance. This effect is
stably exhibited at 0.01% or more of Cu. Although Cu also improves high-
temperature
strength through precipitation strengthening by precipitation of c-Cu,
excessive
addition of Cu reduces hot workability. Accordingly, an upper limit of the Cu
content
is set at 3.0%.In terms of thermal fatigue properties, productivity and
weldability, the
Cu content is preferably 1.6% or less.
[0041]
Mo is an element effective in solid solution strengthening at a high
temperature and improves corrosion resistance and high-temperature salt-damage
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resistance. Accordingly, Mo is added at 0.01% or more as required. Since
addition of
3.0% or more of Mo significantly deteriorates normal-temperature ductility and
oxidation resistance, the Mo content is set at 3.0% or less. In terms of
thermal fatigue
properties and productivity, the Mo content is preferably in a range from 0.3%
to 0.9%.
[0042]
Similarly to Mo, W is an element effective in solid solution strengthening at
a
high temperature and forms Laves phases (Fe2W) to exhibit an effect of
precipitation
strengthening. In particular, combined addition of W with Nb and Mo forms
Laves
phases of Fe2(Nb, Mo, W), however, addition of W inhibits coarsening of the
Laves
phases to improve precipitation strengthening capability. This effect is
exhibited by
addition of 0.001% or more of W. Meanwhile, since addition of more than 2.0%
of W
increases a cost and reduces normal-temperature ductility, an upper limit of
the W
content is set at 2.0%.In terms of productivity, low-temperature toughness and
oxidation resistance, an added amount of W is preferably 1.5% or less.
[0043]
V is an element improving corrosion resistance and thus is added as required.
This effect is stably exhibited by addition of 0.001% or more of V. Meanwhile,
since
addition of more than 1% of V coarsens precipitates to reduce high-temperature
strength and deteriorate oxidation resistance, an upper limit of the V content
is set at
1%.In terms of a production cost and productivity, the V content is preferably
in a
range from 0.08% to 0.5%.
[0044]
Sn is an element improving corrosion resistance, improves high-temperature
strength in an intermediate temperature range, and thus is added as required.
These
effects are exhibited at 0.01% or more of Sn. Meanwhile, since addition of
more than
0.5% of Sn significantly reduces productivity and toughness, the Sn content is
set at
0.5% or less. In terms of oxidation resistance and a production cost, the Sn
content
is preferably 0.1% or more.
[0045]
Ni is an element improving acid resistance, toughness and high-temperature
strength and thus is added as required. These effects are exhibited at 0.01%
or more
of Ni. Meanwhile, since addition of more than 1.0% of Ni increases a cost, the
Ni
14
Date Recue/Date Received 2021-09-10
CA 03133206 2021-09-10
content is set at 1.0% or less. In terms of productivity, the Ni content is
preferably in
a range from 0.08% to 0.5%.
[0046]
Mg may be added as a deoxidizing element and is an element refining a slab
structure to contribute to improving formability. In addition, Mg oxides
become
precipitation sites of carbon itrides such as Ti(C, N) and Nb(C, N) and have
an effect
of allowing finely dispersed precipitation thereof. This effect is exhibited
at 0.0001%
or more of Mg, thereby contributing to improving toughness. However, since
excessive addition of Mg deteriorates weldability, corrosion resistance and
surface
quality, an upper limit of the Mg content is set at 0.01%.In terms of a
refining cost,
the Mg content is preferably in a range from 0.0003% to 0.0010%.
[0047]
Sb contributes to improving corrosion resistance and high-temperature
strength, and is added at 0.005% or more as required. Since addition of more
than
0.5% of Sb may excessively cause slab cracking and a reduction in ductility in
producing a steel sheet, an upper limit of the Sb content is set at 0.5%.In
terms of a
refining cost and productivity, the Sb content is preferably in a range from
0.01% to
0.3%.
[0048]
Similarly to Ti and Nb, Zr is a carbonitride forming element and an element
improving corrosion resistance and deep drawability, and thus is added as
required.
These effects are exhibited at 0.001% or more of Zr. Meanwhile, since addition
of
more than 0.3% of Zr significantly deteriorates productivity, the Zr content
is set at
0.3% or less. In terms of a cost and surface grade, the Zr content is
preferably in a
range from 0.1% to 0.2%.
[0049]
Zr, Ta and Hf are bonded to C and N to contribute to improving toughness
and thus are added at 0.001% or more as required. However, since addition of
more
than 0.3% of Zr, Ta and Hf increases a cost and significantly deteriorates
productivity,
an upper limit of each of the Zr, Ta and Hf contents is set at 0.3%.In terms
of a refining
cost and productivity, the Zr, Ta and Hf contents are each preferably in a
range from
0.01% to 0.08%.
Date Recue/Date Received 2021-09-10
CA 03133206 2021-09-10
[0050]
Co contributes to improving high-temperature strength and thus is added at
0.001% or more as required. Since addition of more than 0.3% of Co
deteriorates
toughness, an upper limit of the Co content is set at 0.3%.In terms of a
refining cost
and productivity, the Co content is preferably in a range from 0.01% to 0.1%.
[0051]
Ca may be added for desulfurization, of which effect is exhibited at 0.0001%
or more of Ca. However, since addition of more than 0.01% of Ca forms coarse
CaS
to deteriorate toughness and corrosion resistance, an upper limit of the Ca
content is
set at 0.01%.In terms of a refining cost and productivity, the Ca content is
preferably
in a range from 0.0003% to 0.0020%.
[0052]
REM may be added as required in order to improve toughness and oxidation
resistance through refinement of various precipitates, of which effect is
exhibited at
0.001% or more of REM. However, since addition of more than 0.2% of REM
significantly deteriorates castability and reduces ductility, an upper limit
of the REM
content is set at 0.2%.In terms of a refining cost and productivity, the REM
content is
preferably 0.05% or less. Specifically, REM (rare-earth elements) collectively
refers
to two elements of scandium (Sc) and yttrium (Y) and fifteen elements
(lanthanoid)
from lanthanum (La) to lutetium (Lu) according to general definition. These
elements
may be added alone or may be added in a form of a mixture.
[0053]
Ga may be added at 0.3% or less in order to improve corrosion resistance
and inhibit hydrogen embrittlement. In order to form sulfides and hydrides, a
lower
limit of the Ga content is preferably 0.0002%.In terms of productivity, a
cost, ductility
and toughness, the Ga content is preferably 0.0020% or less.
[0054]
Although other components are not specifically defined in the invention,
0.001 to 0.1% of Bi or the like may be added as required in the invention. It
should
be noted that the contents of common harmful elements such as As and Pb and
impurity elements are preferably reduced as much as possible.
[0055]
16
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CA 03133206 2021-09-10
Next, a production method will be described.
The production method of a steel sheet according to the invention includes
processes of steelmaking, hot rolling, annealing and pickling. In the
steelmaking,
steel containing the above essential components and components added as
required
is suitably melted in a converter furnace and subsequently subjected to
secondary
refining. The molten steel is formed into a slab according to a known casting
process
(continuous casting). The slab is heated to a predetermined temperature and
hot-
rolled to have a predetermined thickness through continuous rolling. The hot
rolling
is performed by rolling the slab using a hot rolling mill including a
plurality of stands,
which is followed by winding.
The annealing process after the hot rolling may be omitted.
[0056]
In order fora crystal grain size during hot stamping to fall in a range from
100
to 200 pm, a finishing temperature after the hot rolling is preferably in a
range from
900 to 1100 degrees C. Afinishing temperature of less than 900 degrees C does
not
sufficiently grow the crystal grain size of the steel sheet, consequently
failing to grow
the crystal grain size after the hot stamping to 100 pm or more. Meanwhile, a
finishing
temperature of more than 1100 degrees C excessively grows the crystal grain
size
of the steel sheet to make the crystal grain size after the hot stamping more
than 200
pm. The finishing temperature after the hot rolling is more preferably more
than 950
degrees C. Setting the finishing temperature at more than 950 degrees C allows
the
crystal grain size to grow to be 130 pm or more, thereby exhibiting an effect
of
improving strength in an intermediate temperature range.
Moreover, since a winding temperature of more than 650 degrees C reduces
toughness of the hot-rolled steel sheet, the winding temperature is preferably
650
degrees C or less.
[0057]
Next, a forming method will be described. In forming of the steel sheet
according to the invention, the hot stamping in which the steel sheet is
heated to a
predetermined temperature, formed into a hat shape at a high temperature and
subsequently cooled is used. A heating temperature is set in a range from 900
to
1000 degrees C, and the cooling is performed subsequent to the forming. In
order to
17
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CA 03133206 2021-09-10
finely form precipitates in a large amount, the cooling is performed by
retaining the
steel sheet in a range from 890 to 700 degrees C for one to ten minutes. Since
a
retention time of less than one minute does not cause sufficient precipitation
to make
an amount of precipitation strengthening small, a lower limit of the retention
time is
set at one minute. The excessively long retention time grows and coarsens
finely
formed precipitates to reduce the amount of precipitation strengthening. Since
the
excessively long retention time also significantly reduces productivity, an
upper limit
of the retention time is set at ten minutes. In terms of stability of the
precipitates, the
retention time is preferably in a range from 1.5 minutes to five minutes.
[0058]
A "ferritic stainless steel sheet for an automobile brake disc rotor in a form
of
a hot-stamped product" in the invention refers to a steel sheet obtained after
hot
stamping. In other words, the ferritic stainless steel sheet refers to a hot-
stamped
product for an automobile brake disc rotor for which a stainless steel sheet
is used.
In addition, the hot-stamped product for an automobile brake disc rotor made
of a stainless steel sheet refers to a hot-stamped product for an automobile
brake
disc rotor obtained by subjecting a steel sheet to hot stamping.
Moreover, the automobile brake disc rotor made of a stainless steel sheet
refers to an automobile brake disc rotor obtained by subjecting a stainless
steel sheet
to hot stamping and further machining.
Exam pie(s)
[0059]
Steel having chemical composition shown in Tables 1 and 2 was melted and
cast into a slab. The slab was hot-rolled under hot rolling conditions shown
in Tables
3 and 4 to prepare a 6-mm-thick hot-rolled coil. The coil was pickled. In
Table 1, Nos.
Al to A34 are steel according to the invention. In Table 2, Nos. B1 to B14 are
comparative steel and No. B15 is steel without being subjected to a heat
treatment.
Numerical values outside respective ranges of the invention are underlined.
[0060]
The thus obtained hot-rolled steel sheet (other than B15) was subjected to a
hot stamping pseudo heat treatment (hereinafter simply referred to as a
"pseudo heat
treatment") by: heating the steel sheet to 1000 degrees C; then retaining the
steel
18
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CA 03133206 2021-09-10
sheet in a range from 890 to 700 degrees C for two minutes; and subsequently
water-
cooling the steel sheet. In a case where the steel sheet after the pseudo heat
treatment had cracking, "cracking" was indicated in a column of "Quality after
Pseudo
Heat Treatment/Notes" in Table 4.
[0061]
The material that was subjected to the hot stamping pseudo heat treatment
was measured for a crystal grain size at a t/4 portion (according to JIS G
0551, and
numerical values were rounded off to the closest whole number). By setting an
image
magnification at 50 times and the number of imaging fields at five, an average
crystal
grain size of the five imaging fields was calculated. Furthermore, five fields
of the
same material that was subjected to the pseudo heat treatment were observed
under
bright field microscopy at the image magnification of 12500 times using a 200
kV field
emission transmission electron microscope (EM-2100F) produced by JEOL Ltd.,
thereby evaluating precipitates. An equivalent circle diameter of the
precipitates
observed in the bright field microscope image was measured to obtain the grain
size
of the precipitates. The precipitates each having a grain size of 500 nm or
less were
used to calculate an average density of the precipitates at the five fields.
[0062]
A high-temperature tensile test piece was taken from the material that was
__ subjected to the pseudo heat treatment such that a rolling direction
thereof was a
tensile direction. The test piece was subjected to a tensile test at 300
degrees C and
700 degrees C to measure 0.2% proof strength (according to JIS G 0567, and
numerical values were rounded off to the closest whole number). Here, at 0.2%
proof
strength at 300 degrees C of 150 MPa or more and 0.2% proof strength at 700
degrees C of 80 MPa or more, the material is applicable to a general disc
rotor and
thinning thereof is achievable. Accordingly, steel having 0.2% proof strength
at 300
degrees C of 150 MPa or more and 0.2% proof strength at 700 degrees C of 80
MPa
or more was evaluated to pass and was indicated by a mark "A" in Tables 3 and
4.
In addition, steel having 0.2% proof strength at 300 degrees C of 170 MPa or
more
and 0.2% proof strength at 700 degrees C of 100 MPa or more was evaluated to
be
particularly superior and was indicated by a mark "S". Steel other than the
above was
evaluated to fail and was indicated by a mark "X".
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CA 03133206 2021-09-10
[0063]
In order to evaluate press formability of the hot-rolled steel sheet before
the
hot stamping at a high temperature, a high-temperature tensile test piece was
taken
from the hot-rolled steel sheet such that a rolling direction thereof was a
tensile
direction. The test piece was subjected to a tensile test at 1000 degrees C to
measure
fracture elongation (according to JIS G 0567, and numerical values were
rounded off
to the closest whole number). Here, at fracture elongation at 1000 degrees C
of 50%
or more, the steel sheet can be machined into a hat shape. Accordingly, steel
having
fracture elongation at 1000 degrees C of 50% or more was evaluated to pass and
was indicated by a mark "A" in Tables 3 and 4. Moreover, steel having fracture
elongation at 1000 degrees C of 65% or more was evaluated to be particularly
superior and was indicated by a mark "S". Steel other than the above was
evaluated
to fail and was indicated by a mark "X".
[0064]
In order to evaluate toughness of the hot-rolled steel sheet, a Charpy test
piece (C direction notch) was prepared from the hot-rolled steel sheet and
subjected
to a Charpy impact test at a normal temperature. In a case where an average
impact
value of three tests was 10 J/cm2 or less, "unfavorable toughness" was
indicated in
a column of "Notes" for steel sheet quality.
[0065]
Table 1
Date Recue/Date Received 2021-09-10
CA 03133206 2021-09-10
No. Components Contained (mass%)
Si Mn P 5 Cr Ti Nb Others
Al 0.048 0.005 1.24 0.36 0.026 0.0008 13.6 0.009 0.41
A2 0.003 0.047 0.88 0.37 0.032 0.0008 14 0.009 0.42 -
A3 0.004 0.004 0.33 0.38 0.031 0.0008 13.1 0.007 0.40 -
A4 0.005 0.011 3.92 0.35 0.023 0.0006 13.5 0.009 0.44 -
A5 0.012 0.014 1.07 0.03 0.026 0.0006 13.1 0.009 0.40 -
A6 0.013 0.011 0.92 1.98 0.032 0.0007 13.2 0.009 0.41
A7 0.011 0.013 1.20 0.39 0.013 0.0008 13.9 0.008 0.44 -
A8 0.007 0.010 0.97 0.39 0.048 0.0007 13.4 0.009 0.47 -
A9 0.012 0.010 0.99 0.38 0.033 0.0002 13.8 0.009 0.41
Al 0 0.013 0.007 0.82 0.30 0.031 0.0183 13.8
0.007 0.44 -
All 0.012 0.004 1.22 0.36 0.031 0.0005 10.1 0.009 0.40 -
Al2 0.008 0.006 1.26 0.34 0.029 0.0008 19.9 0.009 0.45 -
A13 0.009 0.012 1.12 0.40 0.024 0.0008 13.6 0.002 0.47 -
A14 0.014 0.010 0.81 0.36 0.027 0.0008 13.4 0.492 0.42 -
A15 0.008 0.005 0.87 0.38 0.034 0.0009 13.9 0.009 0.02 -
A16 0.012 0.008 1.10 0.37 0.029 0.0008 13.9 0.007 0.79 -
A17 0.004 0.006 1.09 0.37 0.032 0.0008 13.5 0.001 -
w A18 0.006 0.006 1.02 0.37 0.033 0.0009 13.3 0.488 -
a)
a A19 0.008 0.006 1.22 0.31 0.034 0.0008 13.6 - 0.03 -
E
co A20 0.007 0.004 0.82 0.38 0.022 0.0005 13.9 - 0.78 -
x
A21 0.011 0.007 0.84 0.37 0.029 0.0006 13.4 0.007 0.41
A22 0.012 0.010 0.99 0.38 0.033 0.0002 13.8 0.009 0.41 B 0.0003
A23 0.012 0.010 1.02 0.38 0.033 0.0002 13.8 0.009 0.41 Al 0.02
A24 0.012 0.010 0.98 0.38 0.033 0.0002 13.8 0.009 0.41 Cu 0.04
A25 0.012 0.010 1.01 0.38 0.033 0.0002 13.8 0.009 0.41 Mo 0.20
A26 0.012 0.010 0.98 0.38 0.033 0.0002 13.8 0.009 0.41 W 0.100
A27 0.012 0.010 1.04 0.38 0.033 0.0002 13.8 0.009 0.41 V 0.122
A28 0.012 0.010 0.99 0.38 0.033 0.0002 13.8
0.009 0.41 Sn, Sb 0.05, 0.02
A29 0.012 0.010 0.98 0.38 0.033 0.0002 13.8 0.009 0.41 Ni 0.32
A30 0.012 0.010 1.02 0.38 0.033 0.0002 13.8
0.009 0.41 Mg, Ca 0.0003, 0.0005
A31 0.012 0.010 0.99 0.38 0.033 0.0002 13.8 0.009 0.41 Co 0.101
A28 0.012 0.010 0.97 0.38 0.033 0.0002 13.8
0.009 0.41 Ta, Hf 0.101, 0.01
A29 0.012 0.010 0.99 0.38 0.033 0.0002 13.8 0.009 0.41 Zr 0.012
A30 0.012 0.010 1.01 0.38 0.033 0.0002 13.8 0.009 0.41 REM 0.01
A31 0.012 0.010 0.99 0.38 0.033 0.0002 13.8 0.009 0.41 Ga 0.0051
A32 0.012 0.006 1.04 0.33 0.031 0.0003 13.5 0.009 0.41
A33 0.010 0.007 0.91 0.31 0.031 0.0003 13.5 0.009 0.42 -
A34 0.009 0.009 0.91 0.31 0.031 0.0003 13.1 0.009 0.44 -
[0066]
Table 2
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Date Recue/Date Received 2021-09-10
CA 03133206 2021-09-10
No. Components Contained (mass%)
Si Mn P 5 Cr
Ti Nb Others
B1 0.055 0.014 0.89 0.40 0.034 0.0008 13.6 0.008 0.41 -
B2 0.008 0.062 1.28 0.33 0.029 0.0006 13.2 0.010 0.44 -
B3 0.008 0.005 0.21 0.39 0.023 0.0006 13.2 0.010 0.45 -
B4 0.003 0.004 4.23 0.30 0.023 0.0009 13.5 0.008 0.45 -
B5 0.003 0.010 1.15 2.13 0.033 0.0008 13.7 0.010 0.43 -
.>) B6 0.008 0.004 0.88 0.32 0.058 0.0005 13.7 0.009 0.42 -
-r2 B7 0.008 0.013 1.14 0.38 0.031 0.0387 13.1 0.008 0.41 -
co
a B8 0.010 0.003 1.25 0.35 0.028 0.0007 9.3 0.008 0.47 -
E
B9 0.011 0.013 1.28 0.33 0.024 0.0005 20.9 0.010 0.47 -
0
B10 0.003 0.010 1.22 0.40 0.025 0.0007 13.3 0.580 0.43 -
B11 0.005 0.009 0.97 0.30 0.033 0.0006 13.9 0.010 0.87 -
B12 0.007 0.005 0.87 0.35 0.032 0.0007 13.8 0.009 0.02 -
B13 0.009 0.005 0.88 0.38 0.034 0.0009 13.9 0.010 0.41 -
B14 0.009 0.005 0.82 0.33 0.022 0.0006 13.3 0.010 0.40 -
-0
a)
2 B15 0.014 0.010 1.02 0.33 0.032 0.0006 13.3 0.009 0.42
[0067]
Table 3
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Date Recue/Date Received 2021-09-10
CA 03133206 2021-09-10
Hot Rolling Conditions Steel Sheet Quality Quality
after Pseudo Heat Treatment
Fracture Proof Proof
No. Finishing Winding Crystal Number of
Elongation Strength Strength
Temperature Temperature Grain Size Precipitates
at 1000 C Notes at 700 C at
300 C Notes
( C) ( C) (oh) Om (pieces/) (MPa) (MPa)
pm2)
Al 1100 400 S 199 6.7 A S
A2 900 630 A 103 5.7 A A
A3 900 630 A 117 19.8 S A
A4 1000 500 A 164 18 A S
A5 1100 400 A 186 12.8 A S
A6 1000 630 A 133 17 A S
A7 1000 630 A 137 9 A S
A8 1000 630 A 132 10.6 A S
A9 1000 500 A 154 14.2 A S
Al 0 1000 500 A 165 15.9 A S
All 1000 500 A 164 17.2 A S
Al2 1100 400 S 197 2.6 A S
A13 1000 400 A 163 18.2 A S
A14 1100 400 A 179 6.8 A S
A15 1100 400 S 190 18.6 A S
A16 900 630 A 112 13.8 A A
A17 900 630 A 102 19.1 S A
w A18 1000 500 A 156 14.3 A S
w
0- A19 900 630 A 112 13.7 A A
E
m A20 1000 500 A 166 7.5 A S
x
1-1-1 A21 900 630 A 107 3.1 A A
A22 1000 500 A 150 16.1 A S
A23 1000 500 A 154 16.2 A S
A24 1000 500 A 155 14.1 A S
A25 1000 500 A 140 14.0 A S
A26 1000 500 A 153 14.2 A S
A27 1000 500 A 149 16.3 A S
A28 1000 500 A 153 13.9 A S
A29 1000 500 A 154 13.2 A S
A30 1000 500 A 151 17.1 A S
A31 1000 500 A 157 14.1 A S
A28 1000 500 A 153 14.2 A S
A29 1000 500 A 155 17.1 A S
A30 1000 500 A 154 16.9 A S
A31 1000 500 A 160 13.4 A S
A32 955 550 A 131 10.2 A S
A33 955 550 A 136 13.3 A S
A34 955 600 A 148 16.6 A S
[0068]
Table 4
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Date Recue/Date Received 2021-09-10
CA 03133206 2021-09-10
Hot Rolling Conditions Steel Sheet Quality Quality after Pseudo
Heat Treatment
Fracture Crystal Proof Proof
Finishing Winding Number of
No. Elongation Grain Strength Strength
Temperature Temperature Precipitates
at 1000 C Notes Size at 700 C at 300
C Notes
(pieces/
( C) ( C) (%) (-1m) pm2) (MPa) (MPa)
B1 1100 400 X 181 25.2 A A
B2 1100 500 X 172 24.8 A A
B3 900 630 A 115 0.003 X X
unfavorable
B4 1100 400 X 81 28.2 S A
cracking
toughness
B5 1100 400 A 155 15.4 X X
B6 1100 400 A unfavorable 181 9.3 A A
toughness
w unfavorable
a) B7 1100 400 A
> 177 8.8 A A
.-
12 toughness
as 0- B8 1100 400 S unfavorable 231 14.3 X X
E toughness
0 unfavorable
B9 1000 500 X 148 21.3 S A
cracking
toughness
B10 1100 500 X 173 22.1 S A
cracking
B11 1100 500 X 187 23.4 S A
cracking
B12 1150 400 A unfavorable 253 19.1 A .. A
toughness
B13 850 400 X 88 0.002 X X
B14 1000 700 A unfavorable 170 10.5 A A
toughness
-0
U)
as
92 B15 1000 500 S 153 0.003 X X
t
D
[0069]
As is evident from Tables 1 to 4, Examples have superior 0.2% proof strength
at 700 degrees C after the hot stamping pseudo heat treatment to that of
Comparatives. It has been found that the Examples, of which finishing
temperature
of the hot-rolled steel sheet is more than 950 degrees C, have a crystal grain
size of
130 pm or more and all have particularly superior proof strength at 300
degrees C
that is evaluated to be "S". In a case where even one of 0.2% proof strength
at 300
degrees C and 700 degrees C after the pseudo heat treatment and fracture
elongation of the hot-rolled steel sheet at 1000 degrees C was evaluated to
fail and
in a case where toughness of the hot-rolled steel sheet was unfavorable,
application
thereof to a disc rotor was determined as unsuitable. Accordingly, it has been
found
that the steel defined in the invention is excellent in heat resistance and
formability.
[0070]
Comparatives B1 and B2, which respectively had C and N concentrations
exceeding the upper limit thereof, had unfavorable fracture elongation at 1000
24
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CA 03133206 2021-09-10
degrees C of the steel sheets.
Comparative B3, whose Si concentration was below the lower limit thereof,
had an insufficient number of precipitates after the pseudo heat treatment,
thereby
being low in proof strength at 300 degrees C and 700 degrees C. Comparative
B4,
whose Si concentration exceeded the upper limit thereof, had unfavorable
elongation
at 1000 degrees C of the steel sheet, an excessively small crystal grain size
and an
excessive number of precipitates after the pseudo heat treatment, thereby
causing
cracking.
Comparative B5, whose Mn concentration exceeded the upper limit thereof,
had insufficient proof strength at 300 degrees C and 700 degrees C.
Comparatives B6 and B7, which respectively had P and S concentrations
exceeding the upper limit thereof, had unfavorable toughness of the steel
sheets.
Comparative B8, whose Cr concentration was below the lower limit thereof,
had reduced high-temperature strength, resulting in unfavorable proof strength
at 300
degrees C and 700 degrees C after the pseudo heat treatment. As is evident
from an
excessively large crystal grain size after the pseudo heat treatment,
Comparative B8
also had an excessively large crystal grain size of the steel sheet, thereby
causing
unfavorable toughness of the steel sheet.
Comparatives B9, B10 and B11, which respectively had Cr, Ti and Nb
concentrations exceeding the upper limit thereof, had unfavorable fracture
elongation
at 1000 degrees C of the steel sheets and an excessive number of precipitates
during
the pseudo heat treatment, thereby causing cracking.
[0071]
As is evident from the finishing temperature of the hot rolling exceeding the
upper limit thereof and an excessively large crystal grain size after the
pseudo heat
treatment, Comparative B12 had an excessively large crystal grain size of the
steel
sheet, thereby causing unfavorable toughness of the steel sheet.
Comparative B13, whose finishing temperature of hot rolling was below the
lower limit thereof, had an excessively small crystal grain size after the
pseudo heat
treatment and an excessively small number of precipitates, consequently
obtaining
unfavorable proof strength at 300 degrees C and 700 degrees C.
Comparative B14, whose winding temperature of the hot rolling exceeded
Date Recue/Date Received 2021-09-10
CA 03133206 2021-09-10
the upper limit thereof, had unfavorable toughness of the steel sheet.
[0072]
B15, which was indicated by "Untreated" in the left column in Table 2, was
not subjected to the hot stamping pseudo heat treatment but evaluated for a
crystal
grain size, the number of precipitates and proof strength at 300 degrees C and
700
degrees C. As a result of failing to progress precipitation and obtaining an
excessively
small number of precipitates, B15 had unfavorable proof strength at 300
degrees C
and 700 degrees C.
26
Date Recue/Date Received 2021-09-10
