Technical Field
[0001] The present invention relates to a high-strength steel sheet (specifically, hot-rolled
steel sheet) that is suitable as an automotive member and improved particularly in
terms of strength and fatigue resistance and a method for producing the high-strength
steel sheet.
Background Art
[0002] Reductions in CO
2 emissions have been anticipated in a worldwide framework from the viewpoints of global
environment conservation. In particular, there has been a strong demand for increases
in the mileage of automobiles. Reductions in the weights of automotive bodies have
been anticipated. It is effective to increase the strengths of steel sheets used as
materials for automotive members and reduce the thicknesses of the steel sheets for
reducing the weights of automotive bodies without reducing the strengths of the automotive
bodies. In particular, a steel sheet having a tensile strength of 980 MPa or more
has been considered as a promising material that enables the reductions in the thicknesses
of the steel sheets and thereby markedly increases the mileage of automobiles.
[0003] In order to maintain durability, which is likely to degrade with reductions in the
thicknesses of automotive parts, it is necessary to enhance the fatigue resistance
of steel sheets. Automotive parts, that is, specifically, undercarriage parts, such
as suspension parts, are subjected to cyclic loading through tires. Therefore, if
the fatigue strengths of the above parts are low, the durability of the parts may
fall below the designed durability with an increase in the distance travelled. Commonly,
an increase in the strength of a steel sheet does not always result in an increase
in the fatigue strength of the steel sheet.
[0004] For enhancing the fatigue strength of a steel sheet while increasing the tensile
strength of the steel sheet, various studies (Patent Literatures 1 to 3) have been
conducted in the related art.
[0005] Patent Literature 1 discloses a technique related to a high-strength hot-rolled steel
sheet having excellent formability and excellent fatigue resistance, which is produced
by controlling the manufacturing conditions under which hot rolling is performed,
forming ferrite as a primary phase, and controlling the shape of inclusions and the
mode in which the inclusions are dispersed.
[0006] Patent Literature 2 discloses a technique related to a high-strength hot-rolled steel
sheet having excellent stretch flangeability and excellent fatigue resistance, which
is produced by controlling the manufacturing conditions under which hot rolling is
performed, forming bainite as a primary phase, dispersing a fine hard secondary phase
therein, and controlling the amount of solute Ti.
[0007] Patent Literature 3 discloses a technique related to a high-strength hot-rolled steel
sheet having excellent formability, excellent fracture properties, and excellent fatigue
resistance, which is produced by forming ferrite as a primary phase and controlling
the number density of cementite in ferrite grains, the size of a hard secondary phase,
and the number density of inclusions.
Citation List
Patent Literature
Summary of Invention
Technical Problem
[0009] The techniques known in the related art, such as those described in Patent Literatures
1 to 3, have the following issues.
[0010] In the technique described in Patent Literature 1, a tensile strength of 980 MPa
or more cannot be achieved.
[0011] In the technique described in Patent Literature 2, a fatigue strength at which the
steel sheet is practically used as an automotive part is not studied sufficiently.
[0012] In the technique described in Patent Literature 3, a high-strength steel sheet having
excellent fatigue resistance is reportedly produced. However, fatigue resistance is
not described specifically.
[0013] Thus, a technique related to a high-strength hot-rolled steel sheet having a tensile
strength of 980 MPa or more and excellent fatigue resistance has not been established
in the related art.
[0014] Accordingly, the present invention was developed in light of the above-described
circumstances. An object of the present invention is to provide a high-strength hot-rolled
steel sheet having a tensile strength of 980 MPa or more and a markedly high fatigue
strength and a method for producing the high-strength hot-rolled steel sheet. Solution
to Problem
[0015] In order to achieve the above object, the inventors of the present invention conducted
extensive studies of a technique for enhancing the fatigue resistance of a hot-rolled
steel sheet while maintaining a tensile strength of 980 MPa or more and consequently
found the following facts. Specifically, a microstructure that includes upper bainite
as a primary phase and an adequate amount of a martensite and/or retained austenite
phase, which serves as a hard secondary phase, is formed. Furthermore, the dislocation
density in all the phases included in the surface layer region which extends from
the surface of the steel sheet to the position 1/10 of the thickness of the steel
sheet increases. Moreover, the grain sizes of all the phases are controlled. This
enables a steel sheet having a high strength of 980 MPa or more and a markedly high
fatigue strength to be formed subsequent to a heat treatment that corresponds to baking
coating.
[0016] Note that the term "upper bainite phase" refers to a microstructure phase that is
an assembly of lath-shaped ferrite grains having a misorientation of less than 15°
and includes a Fe-based carbide and/or a retained austenite phase interposed between
the lath-shaped ferrite grains. Note that the above microstructure phase does not
always include Fe-based carbide and/or retained austenite interposed between the lath-shaped
ferrite grains.
[0017] Since lath-shaped ferrite grains have a lath-like shape and the inside of the grains
has a relatively high dislocation density unlike lamellar (laminar) ferrite or polygonal
ferrite in pearlite, they can be distinguished from each other using a SEM (scanning
electron microscope) or a TEM (transmission electron microscope).
[0018] In the case where retained austenite is present between lath grains, only the lath-shaped
ferrite parts are considered as upper bainite and distinguished from retained austenite.
[0019] A martensite and/or retained austenite phase appears brighter in a SEM image than
an upper bainite phase, a lower bainite phase, or a polygonal ferrite phase. Thus,
a martensite phase and/or retained austenite phase can be distinguished from the above
microstructure phase using a SEM.
[0020] Although martensite and retained austenite phases appear to have the same degrees
of brightness with a SEM, they can be distinguished from each other using an electron
backscatter diffraction patterns (EBSD) method.
[0022] The inventors of the present invention conducted further studies on the basis of
the above findings and consequently devised the present invention. The summary of
the present invention is as follows.
- [1] A high-strength steel sheet having a chemical composition containing, by mass,
C: 0.03% to 0.15%, Si: 0.1% to 3.0%, Mn: 0.8% to 3.0%, P: 0.001% to 0.1%, S: 0.0001%
to 0.03%, Al: 0.001% to 2.0%, N: 0.001% to 0.01%, and B: 0.0002% to 0.010%, the chemical
composition further containing at least one selected from Ti: 0.01% to 0.30%, and
Nb: 0.001% to 0.10%, with a balance being Fe and incidental impurities, a microstructure
consisting of, in a surface layer region of the high-strength steel sheet extending
from a surface of the steel sheet to a position 1/10 of a thickness of the steel sheet,
75% or more and less than 98.5% by area of an upper bainite phase as a primary phase,
1.5% or more and less than 25% by area of a martensite phase and/or a retained austenite
phase as a secondary phase, and 2.0% or less by area of a remaining microstructure
phase other than the upper bainite phase, the martensite phase and/or the retained
austenite phase, wherein an average grain size of all phases included in the surface
layer region extending from the surface of the steel sheet to the position 1/10 of
the thickness of the steel sheet is 6.0 µm or less; and wherein a dislocation density
in all the phases included in the surface layer region extending from the surface
of the steel sheet to the position 1/10 of the thickness of the steel sheet is 8.0
× 1014 /m2 or more.
- [2] The high-strength steel sheet according to [1], wherein the chemical composition
further contains, by mass, at least one group selected from Groups a to c below: Group
a: at least one element selected from Cu: 0.005% to 2.0%, Ni: 0.005% to 2.0%, Cr:
0.005% to 2.5%, V: 0.001% to 0.5%, and Mo: 0.005% to 1.0%, Group b: at least one element
selected from Sb: 0.005% to 0.2%, and Sn: 0.001% to 0.05%, and Group c: at least one
element selected from Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, and REM: 0.0005%
to 0.01%.
- [3] A method for producing the high-strength steel sheet according to [1] or [2],
the method including heating a steel material having the chemical composition to a
heating temperature of 1150°C or more; rough-rolling the heated steel material into
a steel sheet; finish-rolling the steel sheet such that a total rolling reduction
achieved in a temperature range of (RC1 - 150)°C or more and RC1°C or less is 35%
or more and a finish rolling delivery temperature is (RC2 - 100)°C or more and (RC2
+ 50)°C or less; cooling the finish-rolled steel sheet such that a time interval between
an end of finish rolling to a start of cooling is 2.0 s or less, an average cooling
rate at a surface of the steel sheet is 20 °C/s or more, and a cooling stop temperature
is Trs°C or more and (Trs + 180)°C or less; coiling the cooled steel sheet such that
a coiling temperature is Trs°C or more and (Trs + 180)°C or less; performing cooling
to a temperature of (Trs - 250)°C or less at an average cooling rate of 1 °C/s or
less; and performing temper rolling at a rolling reduction of 0.1% or more and 5.0%
or less,
wherein RC1, RC2, and Trs are defined by Formulae (1), (2), and (3) below, respectively,
RC1 (°C) = 900 + 120 × C + 100 × N + 10 × Mn + 500 × Ti + 5000 × B + 10 × Cr + 50
× Mo + 1500 × Nb + 150 × V
RC2 (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50
× Mo + 750 × Nb + 150 × V
Trs (°C) = 500 - 450 × C - 35 × Mn - 15 × Cr - 10 × Ni - 20 × Mo
where each of element symbols used in Formulae (1), (2), and (3) above represents
the content (% by mass) of the element and is zero when the element is absent. Advantageous
Effects of Invention
[0023] According to the present invention, a high-strength steel sheet having a tensile
strength of 980 MPa or more and excellent fatigue resistance and a method for producing
the high-strength steel sheet can be provided.
[0024] Applying the high-strength steel sheet according to the present invention to automotive
undercarriage parts, such as a suspension, structural parts, framework parts, or truck
frame parts enhances safety, allows reductions in the weights of the automotive bodies,
and therefore produces markedly advantageous effects from the viewpoint of industry.
[0025] In the present invention, the expression "excellent fatigue resistance" means that
the ratio (fatigue limit ratio) of the fatigue strength at 2 × 10
6 cycles of plane bending performed in an alternating plane bending fatigue test relative
to the tensile strength is 0.50 or more. Brief Description of Drawings
[0026] [Fig. 1] Fig. 1 is a schematic diagram illustrating the shape of a test specimen
used for a plane bending fatigue test in the present invention.
Description of Embodiments
[0027] Embodiments of the present invention are described below. Note that the following
description is intended to be illustrative of preferable embodiments of the present
invention, and the present invention is not limited by the following embodiments.
[0028] The steel sheet has the chemical composition described below. In the following description,
the symbol "%" used as the unit of the content of an element in the chemical composition
means "% by mass" unless otherwise specified.
<C: 0.03% to 0.15%>
[0029] C is an element that effectively facilitates the formation of bainite and increases
strength by enhancing hardenability. If the C content is less than 0.03%, the above
advantageous effects are not produced to a sufficient degree, and a tensile strength
of 980 MPa or more cannot be achieved. Accordingly, the C content is 0.03% or more,
is preferably 0.04% or more, and is more preferably 0.05% or more. On the other hand,
if the C content is more than 0.15%, the amount of martensite and retained austenite
increases and, consequently, sufficiently high fatigue resistance cannot be achieved.
Accordingly, the C content is 0.15% or less, is preferably 0.14% or less, and is more
preferably 0.13% or less.
<Si: 0.1% to 3.0%>
[0030] Si contributes to an increase in steel strength by solid solution strengthening of
steel. Accordingly, the Si content is 0.1% or more, is preferably 0.3% or more, and
is more preferably 0.5% or more. However, since Si is an element that facilitates
the formation of ferrite, a Si content exceeding 3.0% causes the formation of ferrite
and degrades fatigue resistance. Accordingly, the Si content is 3.0% or less, is preferably
2.5% or less, and is more preferably 2.0% or less.
<Mn: 0.8% to 3.0%>
[0031] Mn is an element that stabilizes austenite. Mn is also an element effective for suppressing
the formation of ferrite and increasing strength. If the Mn content is less than 0.8%,
the above advantageous effects are not produced to a sufficient degree and ferrite,
etc. are formed. Consequently, a tensile strength of 980 MPa or more cannot be achieved.
Accordingly, the Mn content is 0.8% or more, is preferably 1.0% or more, and is more
preferably 1.2% or more. On the other hand, if the Mn content is more than 3.0%, the
amount of martensite and retained austenite increases and, consequently, sufficiently
high fatigue resistance cannot be achieved. Accordingly, the Mn content is 3.0% or
less, is preferably 2.8% or less, and is more preferably 2.5% or less.
<P: 0.001% to 0.1%>
[0032] Since P degrades weldability, it is desirable to minimize the P content. The maximum
P content allowable in the present invention is 0.1%. Thus, the P content is 0.1%
or less. Since a P content of less than 0.001% reduces production efficiency, the
lower limit is set to 0.001% or more.
<S: 0.0001% to 0.03%>
[0033] Since S degrades weldability, it is desirable to minimize the S content. The maximum
S content allowable in the present invention is 0.03%. Thus, the S content is 0.03%
or less. Since a S content of less than 0.0001% reduces production efficiency, the
lower limit is set to 0.0001% or more.
<Al: 0.001% to 2.0%>
[0034] Al is an element that serves as a deoxidizing agent and effectively enhances the
cleanliness of steel. If the Al content is excessively low, the advantageous effects
cannot always be produced to a sufficient degree. Accordingly, the Al content is 0.001%
or more, is preferably 0.01% or more, and is more preferably 0.02% or more. However,
since Al is an element that facilitates the formation of ferrite, an Al content exceeding
2.0% causes the formation of ferrite and reduces a fatigue strength. Accordingly,
the Al content is 2.0% or less, is preferably 1.8% or less, and is more preferably
1.6% or less.
<N: 0.001% to 0.01%>
[0035] N binds to an element that forms a nitride to precipitate in the form of a nitride
and thereby contributes to a reduction in grain size. For producing the above advantageous
effects, the N content needs to be 0.001% or more. However, since N is likely to bind
to Ti at high temperatures to form coarse nitride particles, an excessively high N
content degrades fatigue resistance. Accordingly, the N content is 0.01% or less,
is preferably 0.008% or less, and is more preferably 0.006% or less.
<B: 0.0002% to 0.010%>
[0036] B is an element that effectively facilitates the formation of upper bainite and increases
the strength of the steel sheet by segregating at prior-austenite grain boundaries
to suppress the formation of ferrite. For producing the above advantageous effects,
the B content needs to be 0.0002% or more. Therefore, the B content is 0.0002% or
more, is preferably 0.0005% or more, and is more preferably 0.0007% or more. However,
if the B content is more than 0.010%, the above advantageous effects become saturated.
Thus, the B content is 0.010% or less, is preferably 0.009% or less, and is more preferably
0.008% or less.
<One or More Elements Selected From Ti: 0.01% to 0.30% and Nb: 0.001% to 0.10%>
[0037] Ti and Nb are elements that form a carbide and effectively increase strength by precipitation
strengthening. Therefore, one or more elements selected from Ti and Nb need to be
included in the chemical composition. The lower limits for the Ti and Nb contents
are set to Ti: 0.01% or more and Nb: 0.001% or more. The Ti and Nb contents are preferably
Ti: 0.02% or more and Nb: 0.002% or more and are more preferably Ti: 0.03% or more
and Nb: 0.003% or more. However, if the Ti and Nb contents are more than Ti: 0.30%
and Nb: 0.10%, carbide particles become coarsened to degrade hardenability and, consequently,
it may become impossible to form the steel microstructure intended in the present
invention. Accordingly, the upper limits for the Ti and Nb contents are set to Ti:
0.30% or less and Nb: 0.10% or less. The Ti and Nb contents are preferably Ti: 0.25%
or less and Nb: 0.08% or less and are more preferably Ti: 0.20% or less and Nb: 0.05%
or less.
[0038] The balance includes Fe and incidental impurities.
[0039] The above are constituents of the fundamental chemical composition of the high-strength
steel sheet according to the present invention. The chemical composition of the high-strength
steel sheet may optionally contain the following elements as needed.
[0040] Cr, Ni, Cu, V, and Mo are elements that stabilize austenite and are also elements
effective for suppressing the formation of ferrite and increasing strength. In order
to produce the above advantageous effects, one or more elements selected from the
above elements are preferably included in the chemical composition. In the case where
one or more elements selected from Cr, Ni, Cu, V, and Mo are included in the chemical
composition, the contents of the above elements are preferably Cu: 0.005% to 2.0%,
Ni: 0.005% to 2.0%, Cr: 0.005% to 2.5%, V: 0.001% to 0.5%, and Mo: 0.005% to 1.0%.
If the Cr, Ni, Cu, V, and Mo contents are more than the respective upper limits described
above, martensite and retained austenite are likely to remain and, consequently, it
may become impossible to form the steel microstructure intended in the present invention.
The lower limit for the Cr content is more preferably 0.1% or more. The upper limit
for the Cu content is more preferably 0.6% or less. The lower limit for the Ni content
is more preferably 0.1% or more. The upper limit for the Ni content is more preferably
0.6% or less. The lower limit for the Cu content is more preferably 0.1% or more.
The upper limit for the Cu content is more preferably 0.6% or less. The lower limit
for the V content is more preferably 0.005% or more. The upper limit for the V content
is more preferably 0.3% or less. The lower limit for the Mo content is more preferably
0.1% or more. The upper limit for the Mo content is more preferably 0.5% or less.
[0041] Sb is an element that reduces the likelihood of elements being removed from the surface
of a steel material when the steel material is heated and thereby effectively limits
a reduction in steel strength. Accordingly, in the case where the chemical composition
contains Sb, the Sb content is preferably 0.005% to 0.2%. If the Sb content is more
than the above upper limit, embrittlement of the steel sheet may occur. The lower
limit for the Sb content is more preferably 0.01% or more. The upper limit for the
Sb content is more preferably 0.050% or less.
[0042] Sn is an element that suppresses the formation of pearlite and thereby effectively
limits a reduction in steel strength. In order to produce the above advantageous effects,
in the case where the chemical composition contains Sn, the Sn content is preferably
0.001% to 0.05%. If the Sn content is more than the above upper limit, embrittlement
of the steel sheet may occur. The lower limit for the Sn content is more preferably
0.005% or more. The upper limit for the Sn content is more preferably 0.03% or less.
[0043] Ca, Mg, and REMs are elements that effectively enhance workability by shape control
of inclusions. In order to produce the above advantageous effects, one or more elements
selected from the above elements are preferably included in the chemical composition.
In the case where the chemical composition includes one or more elements selected
from Ca, Mg, and REMs, the contents of the above elements are preferably Ca: 0.0005%
to 0.01%, Mg: 0.0005% to 0.01%, and REM: 0.0005% to 0.01%. However, if the Ca, Mg,
and REM contents are more than the respective upper limits, the amount of inclusion
increases and workability may become degraded consequently. The lower limit for the
Ca content is more preferably 0.001% or more. The upper limit for the Ca content is
more preferably 0.005% or less. The lower limit for the Mg content is more preferably
0.001% or more. The upper limit for the Mg content is more preferably 0.005% or less.
The lower limit for the REM content is more preferably 0.001% or more. The upper limit
for the REM content is more preferably 0.005% or less. Note that the term "REM (rare-earth
element)" used herein refers collectively to Sc, Y, and the 15 elements from lanthanum
(La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71. The
term "REM content" used herein refers to the total content of these elements.
[0044] The advantageous effects of the present invention are not impaired even when the
Mo, V, Cr, Ni, Cu, Sb, Sn, Ca, Mg, and REM contents are less than the respective lower
limits described above. Thus, when the contents of the above constituents are less
than the respective lower limits described above, it is considered that the chemical
composition contains the above elements as incidental impurities.
[0045] The microstructure of the high-strength steel sheet according to the present invention
is described below.
[0046] A surface layer region of the high-strength steel sheet according to the present
invention which extends from the surface of the steel sheet to the position 1/10 of
the thickness of the steel sheet has the microstructure described below. Specifically,
an upper bainite phase, the area fraction of which is 75% or more and less than 98.5%,
is the primary phase, and a microstructure phase consisting of a martensite phase
and/or a retained austenite phase, the area fraction of which is 1.5% or more and
less than 25%, is the secondary phase. Furthermore, the average grain size of all
the phases included in the surface layer region is 6.0 um or less, and the dislocation
density in all the phases is 8.0 × 10
14 /m
2 or more.
<Upper Bainite Phase: 75% or More and Less Than 98.5% by Area>
[0047] The microstructure of the high-strength steel sheet according to the present invention
includes upper bainite as a primary phase. If the area fraction of upper bainite is
less than 75%, a markedly high fatigue strength cannot be achieved. Accordingly, the
lower limit for the area fraction of upper bainite is set to 75% or more and is preferably
85% or more. However, if the upper bainite phase is 98.5% or more, the intended dislocation
density cannot be achieved. Accordingly, the upper limit for the area fraction of
upper bainite is set to be less than 98.5% and is preferably 97% or less.
<Martensite Phase and/or Retained Austenite Phase: 1.5% or More and Less Than 25%
by Area>
[0048] The microstructure of the high-strength steel sheet according to the present invention
includes a martensite phase and/or a retained austenite phase. If the martensite phase
and/or retained austenite phase is less than 1.5%, it becomes impossible to achieve
a tensile strength of 980 MPa or more and excellent fatigue resistance. On the other
hand, if the area fraction of martensite and/or retained austenite is 25% or more,
the amount of interfaces between martensite and/or retained austenite and upper bainite,
which may act as the origins of fatigue cracks, increases and, consequently, fatigue
resistance may become degraded. For the above reasons, it is necessary to limit the
total area fraction of martensite and/or retained austenite to be less than 25%. The
above total area fraction is preferably 20% or less and is more preferably 15% or
less. Note that, in the present invention, the term "martensite" refers to as-quenched
martensite.
[0049] The advantageous effects of the present invention are not impaired when the area
fraction of the remaining microstructure phase, which is a phase other than any of
upper bainite and martensite and/or retained austenite, is 2.0% or less at the maximum.
The remaining microstructure phase includes known microstructure phases, such as ferrite
and pearlite.
<Average Grain Size: 6.0 um or Less>
[0050] It is considered that fatigue cracks are formed as a result of slip deformation occurring
in the crystal grains included in the surface layer. Grain boundaries reduce the likelihood
of the slip deformation being propagated to adjacent crystal grains and consequently
delay the occurrence of cracking. That is, fatigue strength can be increased by reducing
the size of crystal grains. Reducing the grain size also contributes to an increase
in strength. Accordingly, the average grain size is 6.0 µm or less and is preferably
5.0 µm or less. However, if the average grain size is excessively small, elongation
may be reduced with an increase in strength. Accordingly, the average grain size is
preferably 2.0 µm or more. Note that the term "average grain size" used herein refers
to the average grain size of all the phases included in the surface layer region of
the steel sheet which extends from the surface of the steel sheet to the position
1/10 of the thickness of the steel sheet. In the case where the surface layer region,
which extends to the position 1/10 of the thickness, includes the remaining microstructure
phase, the remaining microstructure phase is also included in the "all the phases".
<Dislocation Density: 8.0 × 1014 /m2 or More>
[0051] Most of the fatigue cracks occur at the surface of the steel sheet. After the fatigue
cracks have grown to several tens of micrometers in length, a fatigue crack propagation
stage starts. In high-cycle fatigue, fatigue life is primarily affected by the number
of the cycles performed until the occurrence of cracking. Therefore, for increasing
a fatigue strength at 2 × 10
6 cycles, it is necessary to reduce the formation of cracks. It is important to control
the dislocation behavior of the surface layer region, which extends from the surface
of the steel sheet to the position 1/10 of the thickness. In the high-strength steel
sheet according to the present invention, the dislocations introduced to the microstructure
are pinned as a result of the heat treatment being performed in the subsequent step
to obstruct the movement of the dislocations. This prevents the movement and rearrangement
of the dislocations, delays cyclic softening, and consequently enhances fatigue resistance.
In order to produce the above advantageous effects, the dislocation density is limited
to 8.0 × 10
14 /m
2 or more. The dislocation density is preferably 1.0 × 10
15 /m
2 or more and is more preferably 1.2 × 10
15 /m
2 or more. The upper limit for the dislocation density is not set but preferably 4.0
× 10
15 /m
2 or less. Although it is most important to control the dislocation density in the
primary phase included in the surface layer region, which extends from the surface
of the steel sheet to the position 1/10 of the thickness, it is difficult to measure
the dislocation density in the primary phase only. Therefore, the dislocation density
measured in the present invention is the dislocation density in all the phases included
in the surface layer region, which extends to the position 1/10 of the thickness.
In the case where the surface layer region, which extends to the position 1/10 of
the thickness, includes the remaining microstructure phase, the remaining microstructure
phase is also included in the "all the phases".
[0052] The high-strength steel sheet according to the present invention has a tensile strength
of 980 MPa or more and a fatigue limit ratio of 0.50 or more. The term "fatigue limit
ratio" used herein refers to the ratio of the fatigue strength at 2 × 10
6 cycles of plane bending to the tensile strength. Thus, the high-strength steel sheet
according to the present invention has a high tensile strength and is capable of maintaining
safety even when the thickness of the steel sheet is reduced. The high-strength steel
sheet according to the present invention can be applied to members for trucks or automobiles.
[0053] Note that, in the present invention, the area fractions and mechanical properties
of the above microstructure phases are determined by the methods described in Examples
below.
[0054] A method for producing the high-strength steel sheet according to an embodiment of
the present invention is described below. In the following description, the symbol
"°C" used for describing temperature refers to the surface temperature of the object
(steel material or steel sheet) unless otherwise specified.
[0055] The high-strength steel sheet according to the present invention can be produced
by subjecting a steel material to the treatments (1) to (6) below in order. Each of
the steps is described below.
- (1) Heating
- (2) Hot rolling
- (3) Cooling (first cooling)
- (4) Coiling
- (5) Cooling (second cooling)
- (6) Temper rolling
[0056] Note that the steel material may be any steel material having the above-described
chemical composition. The high-strength steel sheet that is to be produced finally
has the same chemical composition as the steel material used. The steel material may
be, for example, a steel slab. The method for producing the steel material is not
limited. For example, a molten steel having the above-described chemical composition
is prepared using a known method, such as a converter, and the molten steel is formed
into a steel material by a casting method, such as continuous casting. A method other
than continuous casting, such as ingot casting-blooming rolling, may also be used.
Alternatively, steel scrap may also be used as a raw material. After the steel material
has been produced by continuous casting or the like, it may be directly subjected
to the subsequent heating step. In another case, the steel material may be cooled
to prepare warm or cold steel pieces, which are subjected to the heating step.
(1) Heating
[0057] First, the steel material is heated to a heating temperature of 1150°C or more. In
the steel material cooled to low temperatures, most of the carbonitride-forming elements,
such as Ti, are present in the form of coarse carbonitride particles in a nonuniform
manner. The presence of the coarse and nonuniform precipitates degrades various properties
(e.g., strength and fatigue resistance) commonly required for high-strength steel
sheets used for producing parts for trucks or automobiles. Therefore, it is necessary
to heat the steel material prior to hot rolling to dissolve the coarse precipitates.
Accordingly, the temperature to which the steel material is heated is 1150°C or more.
The above heating temperature is preferably 1180°C or more and is more preferably
1200°C or more. However, if the temperature to which the steel material is heated
is excessively high, slab flaws may occur. Furthermore, yields may be reduced due
to descaling. Accordingly, the temperature to which the steel material is heated is
preferably 1350°C or less, is more preferably 1300°C or less, and is further preferably
1280°C or less.
[0058] In the heating step, it is preferable to hold the temperature of the steel material
at the above heating temperature after the temperature of the steel material has been
increased to the heating temperature in order to make the temperature of the steel
material uniform. Although the amount of time (holding time) during which the temperature
of the steel material is held at the heating temperature is not limited, the holding
time is preferably 1800 seconds or more in order to increase the uniformity in the
temperature of the steel material. However, if the holding time is more than 10000
seconds, the amount of the scale generated may be increased. This increases, for example,
the possibility of entanglement of the scale in the subsequent hot rolling step and
consequently may reduce yields due to surface flaw defects. Accordingly, the holding
time is preferably 10000 seconds or less and is more preferably 8000 seconds or less.
Alternatively, subsequent to casting, the steel material that has not been hot-rolled
may be directly subjected to hot rolling (hot direct rolling) while the temperature
of the steel material is high (i.e., while the temperature of the steel material is
held to fall within the above heating temperature range).
(2) Hot Rolling
[0059] The heated steel material (or, the as-cast steel material having a high temperature)
is subjected to a hot rolling step in which rough rolling and finish rolling are performed.
The conditions under which the rough rolling is performed are not limited and may
be any conditions with which the resulting sheet bar has intended dimensions. The
steel material is rough-rolled to form a rough-rolled steel sheet bar. Prior to finish
rolling, the rough-rolled steel sheet bar may be subjected to descaling (high-pressure
water descaling) in which high-pressure water is sprayed at the entry side of the
finish rolling mill.
[0060] In the present invention, when temperatures RC1 and RC2 are defined by Formulae (1)
and (2) below, finish rolling is performed such that the total of the rolling reductions
achieved in the temperature range of equal to or more than (RC1 - 150)°C and equal
to or less than RC1°C is 35% or more. The amount of residence time during which the
temperature is held in the temperature range is not limited and may be 3 seconds or
more and 20 seconds or less. Moreover, the finish rolling delivery temperature is
set to a temperature equal to or more than (RC2 - 100)°C and equal to or less than
(RC2 + 50)°C. RC1 is an austenite 50%-recrystallization temperature estimated from
the chemical composition and RC2 is the lower limit for the austenite recrystallization
temperature which is estimated from the chemical composition. If the above total rolling
reduction achieved in the temperature range of equal to or more than (RC1 - 150)°C
and equal to or less than RC1°C is less than 35%, the average grain size increases
and, consequently, it becomes impossible to enhance fatigue resistance. Accordingly,
the total rolling reduction achieved in the temperature range of equal to or more
than (RC1 - 150)°C and equal to or less than RC1°C is 35% or more. The above total
rolling reduction is preferably 45% or more and is more preferably 60% or more.
[0061] Hot rolling is performed such that the finish rolling delivery temperature is equal
to or more than (RC2 - 100)°C and equal to or less than (RC2 + 50)°C. If the finish
rolling delivery temperature is less than (RC2 - 100)°C, ferrite is formed and, consequently,
a tensile strength of 980 MPa or more cannot be achieved. Accordingly, the finish
rolling delivery temperature is equal to or more than (RC2 - 100)°C, is preferably
equal to or more than (RC2 - 90)°C, and is more preferably equal to or more than (RC2
- 70)°C. On the other hand, if the finish rolling delivery temperature is more than
(RC2 + 50)°C, austenite grains become coarsened and the average grain size of upper
bainite increases consequently. This reduces strength. Accordingly, the finish rolling
delivery temperature is equal to or less than (RC2 + 50)°C, is preferably equal to
or less than (RC2 + 40)°C, and is more preferably equal to or less than (RC2 + 30)°C.
RC1 and RC2 are defined by Formulae (1) and (2) below.
RC1 (°C) = 900 + 120 × C + 100 × N + 10 × Mn + 500 × Ti + 5000 × B + 10 × Cr + 50
× Mo + 1500 × Nb + 150 × V
RC2 (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50
× Mo + 750 × Nb + 150 × V
where each of element symbols used in Formulae (1) and (2) represents the content
(% by mass) of the element and is zero when the element is absent.
(3) Cooling (First Cooling)
[0062] The hot-rolled steel sheet is cooled (first cooling). In the cooling step, the time
interval between the end of hot rolling and the start of cooling (cooling start time)
is limited to 2.0 s or less after the end of finish rolling. If the above cooling
start time is less than 2.0 s, austenite grains grow disadvantageously and a tensile
strength of 980 MPa or more cannot be achieved consequently. Accordingly, the cooling
start time is 2.0 s or less, is preferably 1.5 s or less, and is more preferably 1.0
s or less.
[0063] In the cooling step, if the average cooling rate at which the temperature is reduced
from the finish rolling delivery temperature to the cooling stop temperature is excessively
low, ferrite transformation may disadvantageously occur prior to upper bainite transformation
and the intended area fraction of upper bainite phase cannot be formed consequently.
Accordingly, the average cooling rate is 20 °C/s or more, is preferably 30 °C/s or
more, and is more preferably 50 °C/s or more. Although the upper limit is not set,
if the average cooling rate is excessively high, it becomes difficult to control the
cooling stop temperature and, consequently, it may become difficult to form the intended
microstructure. Therefore, the average cooling rate is preferably 500 °C/s or less,
is more preferably 300 °C/s or less, and is further preferably 150 °C/s or less. In
the cooling step, forced cooling may be performed such that the above average cooling
rate can be achieved. The cooling method is not limited. It is preferable to perform
water cooling or the like.
[0064] The cooling stop temperature is set to a temperature equal to or more than Trs°C
and equal to or less than (Trs + 180)°C. If the cooling stop temperature is less than
Trs°C, the microstructure includes lower bainite. Although lower bainite is a microstructure
phase having a high strength, it may have low fatigue resistance after subjected to
a heat treatment. Accordingly, the cooling stop temperature is set to a temperature
equal to or more than Trs°C. On the other hand, if the cooling stop temperature is
more than (Trs + 180)°C, ferrite may be generated disadvantageously. This makes it
impossible to achieve a tensile strength of 980 MPa or more. Accordingly, the cooling
stop temperature is set to a temperature equal to or less than (Trs + 180)°C. Trs
is defined using Formula (3) below.
Trs (°C) = 500 - 450 × C - 35 × Mn - 15 × Cr - 10 × Ni - 20 × Mo
where each of element symbols used in Formula (3) represents the content (% by mass)
of the element and is zero when the element is absent.
(4) Coiling
[0065] The cooled hot-rolled steel sheet is coiled at a coiling temperature equal to or
more than Trs°C and equal to or less than (Trs + 180)°C. If the coiling temperature
is less than Trs°C, lower bainite transformation may occur subsequent to coiling and,
consequently, intended martensite and/or retained austenite cannot be formed. Accordingly,
the coiling temperature is limited to a temperature equal to or more than Trs°C, is
preferably equal to or more than (Trs + 10)°C, and is more preferably equal to or
more than (Trs + 30)°C. On the other hand, if the coiling temperature is more than
(Trs + 180)°C, ferrite may be generated disadvantageously. This makes it impossible
to achieve a tensile strength of 980 MPa or more. Accordingly, the coiling temperature
is limited to a temperature equal to or less than (Trs + 180)°C, is preferably equal
to or less than (Trs + 150)°C, and is more preferably equal to or less than (Trs +
120)°C.
(5) Cooling (Second Cooling)
[0066] Subsequently, the temperature is reduced to a temperature equal to or less than (Trs
- 250)°C at an average cooling rate of 1 °C/s or less (second cooling). If the average
cooling rate at which the temperature is reduced from the coiling temperature to a
temperature equal to or less than (Trs - 250)°C is more than 1 °C/s, bainite transformation
does not occur to a sufficient degree and the amount of martensite and retained austenite
increases consequently. This makes it impossible to form the microstructure intended
in the present invention. Accordingly, the average cooling rate at which the temperature
is reduced from the coiling temperature to a temperature equal to or less than (Trs
- 250)°C is limited to 1 °C/s or less, is preferably 0.8 °C/s or less, and is more
preferably 0.5 °C/s or less. Although cooling may be performed to any temperature
equal to or less than (Trs - 250)°C, it is preferable to reduce the temperature to
about 10°C to 30°C. The steel sheet may be cooled in any form. For example, the steel
sheet may be cooled after it has been wound into a coil.
(6) Temper Rolling
[0067] The cooled steel sheet is temper-rolled at a rolling reduction of 0.1% or more and
5.0% or less. If the rolling reduction is less than 0.1%, the dislocation density
becomes insufficient and a markedly high fatigue strength cannot be achieved. Accordingly,
the rolling reduction is limited to 0.1% or more, is preferably 0.3% or more, and
is more preferably 0.5% or more. However, if temper rolling is performed at a rolling
reduction of more than 5.0%, the amount of load applied to the rolls increases. This
disadvantageously increases the number of times the rollers need to be replaced and
the manufacturing costs. Accordingly, the rolling reduction is limited to 5.0% or
less, is preferably 4.0% or less, and is more preferably 3.0% or less.
[0068] The high-strength steel sheet according to the present invention can be produced
by the above-described steps. Optionally, for example, pickling may be performed after
temper rolling in accordance with the conventional method in order to remove scales
formed on the surface of the steel sheet.
Examples
[0069] Molten steels having the compositions described in Table 1 were prepared using a
converter and formed into steel slabs by continuous casting, which were used as steel
materials.

[0070] The steel materials were heated to the respective heating temperatures described
in Table 2. The heated steel materials were subjected to a hot-rolling process consisting
of rough rolling and finish rolling to form hot-rolled steel sheets. Table 2 lists
the finish rolling delivery temperatures in the hot rolling process.
[0071] The hot-rolled steel sheets were each cooled (first cooling) with the average cooling
rate and the cooling stop temperature listed in Table 2. The cooled hot-rolled steel
sheets were coiled at the respective coiling temperatures listed in Table 2. The coiled
steel sheets were cooled (second cooling) with the respective average cooling rates
listed in Table 2 to form high-strength steel sheets. Subsequent to cooling, temper
rolling was performed at the rolling reductions listed in Table 2. Then, pickling
was performed. Pickling was performed at a temperature of 85°C using a 10-mass% aqueous
solution of hydrochloric acid. Subsequently, the steel sheets were subjected to a
heat treatment (170°C and 20 minutes) that corresponded to baking coating. Hereby,
high-strength hot-rolled steel sheets were prepared.
[Table 2]
No. |
Steel type |
Manufacturing conditions |
Remarks |
Heating |
Hot rolling |
First cooling |
Coiling |
Second cooling |
Temper rolling |
Heating temperature (°C) |
Total rolling reduction in temperature range of (RC1-150°C) or more and RC1 or less
(%) |
Finish rolling delivery temperature (°C) |
Time interval between end of hot rolling and start of cooling (s) |
Average cooling rate (°C/s) |
Cooling stop temperature (°C) |
Coiling temperature (°C) |
Average cooling rate (°C/s) |
Cooling stop temperature (°C) |
Rolling reduction %) |
1 |
A |
1250 |
40 |
790 |
1.0 |
60 |
455 |
445 |
<0.5 |
50 |
1.5 |
Invention example |
2 |
A |
1180 |
45 |
820 |
1.4 |
45 |
470 |
460 |
<0.5 |
55 |
1.6 |
Invention example |
3 |
A |
1210 |
45 |
850 |
0.5 |
45 |
465 |
455 |
<0.5 |
55 |
2.4 |
Invention example |
4 |
A |
1200 |
40 |
750 |
0.8 |
80 |
515 |
420 |
<0.5 |
35 |
1.2 |
Invention example |
5 |
A |
1240 |
50 |
800 |
1.0 |
55 |
570 |
550 |
<0.5 |
80 |
0.6 |
Invention example |
6 |
A |
1245 |
25 |
820 |
0.8 |
60 |
480 |
465 |
<0.5 |
60 |
1.5 |
Comparative example |
7 |
A |
1250 |
45 |
690 |
2.2 |
50 |
430 |
450 |
<0.5 |
35 |
2.4 |
Comparative example |
8 |
A |
1230 |
80 |
890 |
1.0 |
70 |
525 |
510 |
<0.5 |
45 |
1.1 |
Comparative example |
9 |
A |
1270 |
50 |
825 |
0.6 |
4 |
440 |
440 |
<0.5 |
85 |
0.7 |
Comparative example |
10 |
A |
1260 |
40 |
780 |
0.8 |
45 |
310 |
380 |
<0.5 |
55 |
2.0 |
Comparative example |
11 |
A |
1240 |
45 |
760 |
1.0 |
50 |
650 |
600 |
<0.5 |
75 |
2.7 |
Comparative example |
12 |
A |
1240 |
40 |
810 |
1.4 |
37 |
450 |
470 |
2 |
35 |
0.8 |
Comparative example |
13 |
A |
1230 |
50 |
800 |
12 |
40 |
475 |
465 |
<0.5 |
30 |
0.0 |
Comparative example |
14 |
B |
1230 |
45 |
850 |
1.6 |
75 |
455 |
470 |
<0.5 |
45 |
2.6 |
Invention example |
15 |
B |
1220 |
40 |
700 |
0.5 |
49 |
430 |
440 |
<0.5 |
55 |
0.9 |
Comparative example |
16 |
C |
1220 |
65 |
855 |
1.0 |
37 |
530 |
520 |
<0.5 |
75 |
0.7 |
Invention example |
17 |
C |
1190 |
75 |
950 |
0.8 |
56 |
515 |
410 |
<0.5 |
50 |
1.3 |
Comparative example |
18 |
D |
1220 |
60 |
740 |
0.6 |
92 |
450 |
445 |
<0.5 |
40 |
2.1 |
Invention example |
19 |
D |
1215 |
85 |
785 |
1.0 |
3 |
430 |
460 |
<0.5 |
35 |
0.6 |
Comparative example |
20 |
E |
1270 |
40 |
765 |
0.8 |
75 |
500 |
490 |
<0.5 |
80 |
2.5 |
Invention example |
21 |
E |
1230 |
40 |
775 |
12 |
34 |
280 |
370 |
<0.5 |
55 |
2.0 |
Comparative example |
22 |
F |
1200 |
45 |
780 |
1.0 |
62 |
520 |
450 |
<0.5 |
40 |
1.9 |
Invention example |
23 |
G |
1170 |
70 |
840 |
10 |
95 |
580 |
570 |
<0.5 |
75 |
2.9 |
Invention example |
24 |
G |
1250 |
50 |
810 |
1.4 |
55 |
485 |
420 |
<0.5 |
50 |
1.8 |
Invention example |
25 |
G |
1230 |
55 |
825 |
0.5 |
38 |
470 |
430 |
<0.5 |
70 |
2.5 |
Invention example |
26 |
G |
1260 |
45 |
785 |
0.8 |
32 |
445 |
450 |
<0.5 |
85 |
0.5 |
Invention example |
27 |
G |
1250 |
60 |
800 |
0.6 |
28 |
670 |
630 |
<0.5 |
65 |
2.4 |
Comparative example |
28 |
H |
1230 |
60 |
850 |
1.6 |
47 |
445 |
440 |
<0.5 |
70 |
1.0 |
Invention example |
29 |
H |
1230 |
50 |
820 |
1.8 |
67 |
525 |
510 |
3 |
30 |
1.0 |
Comparative example |
30 |
I |
1250 |
40 |
795 |
1.0 |
38 |
490 |
490 |
<0.5 |
55 |
1.7 |
Invention example |
31 |
J |
1240 |
65 |
930 |
0.6 |
46 |
520 |
510 |
<0.5 |
70 |
1.5 |
Invention example |
32 |
K |
1235 |
45 |
910 |
0.8 |
58 |
510 |
430 |
<0.5 |
50 |
22 |
Invention example |
33 |
L |
1210 |
50 |
875 |
1.4 |
92 |
555 |
410 |
<0.5 |
55 |
0.5 |
Invention example |
34 |
M |
1235 |
45 |
790 |
0.5 |
62 |
500 |
495 |
<0.5 |
80 |
1.2 |
Invention example |
35 |
N |
1220 |
50 |
860 |
0.8 |
38 |
460 |
430 |
<0.5 |
55 |
0.8 |
Invention example |
36 |
O |
1300 |
60 |
800 |
1.0 |
77 |
525 |
510 |
<0.5 |
35 |
0.6 |
Invention example |
37 |
P |
1270 |
75 |
780 |
1.6 |
64 |
540 |
530 |
<0.5 |
75 |
2.5 |
Invention example |
38 |
Q |
1240 |
40 |
845 |
1.0 |
70 |
460 |
440 |
<0.5 |
70 |
1.0 |
Invention example |
39 |
a |
1250 |
50 |
865 |
0.6 |
88 |
515 |
420 |
<0.5 |
35 |
25 |
Comparative example |
40 |
b |
1220 |
40 |
780 |
0.8 |
29 |
470 |
450 |
<0.5 |
45 |
1.4 |
Comparative example |
41 |
c |
1210 |
55 |
760 |
10 |
26 |
435 |
435 |
<0.5 |
50 |
22 |
Comparative example |
42 |
d |
1215 |
40 |
780 |
1.4 |
46 |
440 |
425 |
<0.5 |
25 |
2.4 |
Comparative example |
43 |
e |
1230 |
35 |
815 |
0.5 |
32 |
520 |
510 |
<0.5 |
50 |
2.1 |
Comparative example |
44 |
f |
1260 |
45 |
805 |
0.8 |
67 |
500 |
480 |
<0.5 |
85 |
3.0 |
Comparative example |
45 |
g |
1240 |
50 |
890 |
1.0 |
70 |
510 |
430 |
<0.5 |
60 |
1.6 |
Comparative example |
46 |
h |
1245 |
65 |
780 |
1.6 |
82 |
520 |
440 |
<0.5 |
60 |
1.7 |
Comparative example |
47 |
i |
1220 |
45 |
810 |
1.0 |
78 |
490 |
430 |
<0.5 |
80 |
1.6 |
Comparative example |
48 |
j |
1300 |
75 |
785 |
1.4 |
57 |
520 |
510 |
<0.5 |
50 |
4.3 |
Comparative example |
49 |
k |
1210 |
30 |
790 |
0.8 |
48 |
500 |
490 |
<0.5 |
40 |
12 |
Comparative example |
The underlined parts are outside the scope of the invention |
[0072] A test specimen was taken from each of the high-strength steel sheets, and the microstructure
and mechanical properties of the specimen were determined by the following steps.
<Microstructure>
[0073] A test specimen for microstructure observation was taken from each of the high-strength
steel sheets such that a cross section of the steel sheet which was taken in the thickness
direction so as to be parallel to the rolling direction was exposed as an observation
plane. The surface of the test specimen was ground and then corroded with an etchant
(3% nital solution) in order to cause the microstructure to appear. Subsequently,
an image of the surface layer that extended from the surface to the position 1/10
of the thickness of the steel sheet was taken using a scanning electron microscope
(SEM) at a 5000-fold magnification in 10 fields of view in order to obtain SEM images
of microstructure. The SEM images were analyzed by image processing in order to determine
the area fractions of upper bainite (UB), polygonal ferrite (F), and lower bainite
(LB). Since it is difficult to distinguish martensite (M) and retained austenite (γ)
from each other with a SEM, the area fractions and average grain sizes thereof were
determined by making identifications using an electron back scatter diffraction patterns
(EBSD) method. Table 3 lists the area fractions and average grain sizes of the above
microstructure phases. Table 3 also lists the total area fractions (M + γ) of martensite
and retained austenite.
<Tensile Test>
[0074] A JIS No. 5 test piece for tensile test (JIS Z 2201) was taken from each of the hot-rolled
steel sheets such that the tensile direction of the test piece was perpendicular to
the rolling direction. A tensile test was conducted in conformity with JIS Z 2241
at a strain rate of 10
-3 /s in order to determine tensile strength. In the present invention, an evaluation
of "Passed" was given when the tensile strength was 980 MPa or more. Table 3 lists
the results.
<Plane Bending Fatigue Test>
[0075] A test specimen having the dimensions and shape illustrated in Fig. 1 was taken from
each of the hot-rolled steel sheets such that the longitudinal direction of the test
specimen was perpendicular to the rolling direction. A plane bending fatigue test
was conducted in conformity with JIS Z 2275. The stress loading mode was such that
the stress ratio R was -1 and the frequency f was 25 Hz. The amplitude of loading
stress was changed in six stages, and the number of stress cycles applied until rupture
occurred was measured. An S-N curve was determined, and a fatigue strength (fatigue
limit) at 2 × 10
6 cycles was calculated. In the present invention, an evaluation of "excellent fatigue
resistance" was given when a fatigue limit ratio calculated by dividing the fatigue
limit by the tensile strength determined in the tensile test was 0.50 or more. Table
3 lists the results.
[Table 3]
No. |
Steel type |
Microstructure of surface layer |
Mechanical property |
Plane bending fatigue strength at 2×106 cycles, σw (MPa) |
Fatigue limit ratio, σw/TS (-) |
Remarks |
Area fraction (%) |
Average grain size (µm) |
Dislocation density (1014/m2) |
Tensile strength (MPa) |
UB |
M |
γ |
M+γ |
F |
LB |
1 |
A |
90 |
8 |
2 |
10 |
0 |
0 |
5.4 |
100 |
1137 |
603 |
0.53 |
Invention example |
2 |
A |
88 |
11 1 |
|
12 |
0 |
0 |
5.1 |
15.0 |
1080 |
562 |
0.52 |
Invention example |
3 |
A |
89 |
10 1 |
|
11 |
0 |
0 |
5.1 |
8.0 |
1190 |
690 |
0.58 |
Invention example |
4 |
A |
93 |
5 |
2 |
7 |
0 |
0 |
4.9 |
100 |
1188 |
653 |
0.55 |
Invention example |
5 |
A |
83 |
13 4 |
|
17 |
0 |
0 |
5.5 |
15.0 |
1002 |
601 |
0.60 |
Invention example |
6 |
A |
91 |
8 |
1 |
9 |
0 |
0 |
8.3 |
9.0 |
1068 |
374 |
0.35 |
Comparative example |
7 |
A |
76 |
5 |
1 |
6 |
18 |
0 |
1.8 |
3.5 |
937 |
525 |
0.56 |
Comparative example |
8 |
A |
89 |
9 |
2 |
11 |
0 |
0 |
7.5 |
12.0 |
1182 |
449 |
0.38 |
Comparative example |
9 |
A |
69 |
16 |
5 |
21 |
10 |
0 |
4.8 |
5.5 |
975 |
371 |
0.52 |
Comparative example |
10 |
A |
7 |
1 |
0 |
1 |
0 |
92 |
5.3 |
13.5 |
1006 |
507 |
0.34 |
Comparative example |
11 |
A |
0 |
0 |
0 |
0 |
100 |
0 |
5.6 |
4.5 |
940 |
342 |
0.54 |
Comparative example |
12 |
A |
53 |
45 2 |
|
47 |
0 |
0 |
5.2 |
15.0 |
1256 |
508 |
0.41 |
Comparative example |
13 |
A |
92 |
3 |
|
5 8 |
0 |
0 |
5.9 |
7.0 |
991 |
515 |
0.47 |
Comparative example |
14 |
B |
82 |
135 |
|
18 |
0 |
0 |
5.2 |
120 |
1097 |
669 |
0.61 |
Invention example |
15 |
B |
65 |
6 |
2 |
8 |
27 |
0 |
5.1 |
15.0 |
876 |
515 |
0.43 |
Comparative example |
16 |
C |
89 |
6 |
5 |
11 |
0 |
0 |
4.6 |
100 |
1068 |
555 |
0.52 |
Invention example |
17 |
C |
90 |
7 |
3 |
10 |
0 |
0 |
9.0 |
8.0 |
970 |
377 |
0.33 |
Comparative example |
18 |
D |
85 |
123 |
|
15 |
0 |
0 |
3.8 |
120 |
1163 |
593 |
0.51 |
Invention example |
19 |
D |
15 |
0 |
0 |
0 |
85 |
0 |
5.4 |
2.0 |
894 |
320 |
0.52 |
Comparative example |
20 |
E |
90 |
6 |
4 |
10 |
0 |
0 |
5.9 |
13.0 |
1171 |
644 |
0.55 |
Invention example |
21 |
E |
4 |
0 |
0 |
0 |
0 |
96 |
5.6 |
9.5 |
1087 |
465 |
0.36 |
Comparative example |
22 |
F |
82 |
162 |
|
18 |
0 |
0 |
5.3 |
8.0 |
1075 |
570 |
0.53 |
Invention example |
23 |
G |
85 |
123 |
|
15 |
0 |
0 |
5.1 |
8.0 |
1036 |
559 |
0.54 |
Invention example |
24 |
G |
85 |
132 |
|
15 |
0 |
0 |
4.9 |
14.0 |
1022 |
593 |
0.58 |
Invention example |
25 |
G |
92 |
5 |
3 |
8 |
0 |
0 |
5.8 |
14.0 |
1113 |
579 |
0.52 |
Invention example |
26 |
G |
79 |
16 5 |
|
21 |
0 |
0 |
4.7 |
100 |
1132 |
623 |
0.55 |
Invention example |
27 |
G |
0 |
0 |
0 |
0 |
100 |
0 |
7.6 |
2.5 |
870 |
391 |
0.38 |
Comparative example |
28 |
H |
83 |
11 6 |
|
17 |
0 |
0 |
4.2 |
100 |
1066 |
629 |
0.59 |
Invention example |
29 |
H |
69 |
28 3 |
|
31 |
0 |
0 |
5.9 |
12.5 |
1346 |
331 |
0.38 |
Comparative example |
30 |
I |
86 |
11 3 |
|
14 |
0 |
0 |
5.7 |
8.0 |
1105 |
575 |
0.52 |
Invention example |
31 |
J |
85 |
12 3 |
|
15 |
0 |
0 |
5.8 |
16.5 |
1079 |
669 |
0.62 |
Invention example |
32 |
K |
77 |
18 5 |
|
23 |
0 |
0 |
5.2 |
9.0 |
1070 |
578 |
0.54 |
Invention example |
33 |
L |
80 |
16 |
4 |
20 |
0 |
0 |
4.9 |
100 |
1034 |
579 |
0.56 |
Invention example |
34 |
M |
86 |
13 1 |
|
14 |
0 |
0 |
5.4 |
13.0 |
1106 |
586 |
0.53 |
Invention example |
35 |
N |
88 |
9 |
3 |
12 |
0 |
0 |
5.2 |
9.0 |
1057 |
581 |
0.55 |
Invention example |
36 |
O |
86 |
10 4 |
|
14 |
0 |
0 |
4.1 |
14.0 |
1112 |
578 |
0.52 |
Invention example |
37 |
P |
90 |
8 |
2 |
10 |
0 |
0 |
3.8 |
13.0 |
1131 |
679 |
0.60 |
Invention example |
38 |
Q |
96 |
2 |
2 |
4 |
0 |
0 |
4.2 |
8.0 |
1119 |
615 |
0.55 |
Invention example |
39 |
a |
94 |
5 |
1 |
6 |
0 |
0 |
5.5 |
120 |
970 |
534 |
0.55 |
Comparative example |
40 |
b |
91 |
6 |
3 |
9 |
0 |
0 |
5.7 |
15.0 |
1017 |
427 |
0.42 |
Comparative example |
41 |
c |
93 |
3 |
4 |
7 |
0 |
0 |
4.8 |
14.0 |
1160 |
452 |
0.39 |
Comparative example |
42 |
d |
73 |
1 |
2 |
3 |
24 |
0 |
5.8 |
4.5 |
887 |
426 |
0.48 |
Comparative example |
43 |
e |
100 |
0 |
0 |
0 |
0 |
0 |
5.5 |
3.0 |
1045 |
376 |
0.36 |
Comparative example |
44 |
f |
88 |
7 |
5 |
12 |
0 |
0 |
5.7 |
14.0 |
1200 |
564 |
0.47 |
Comparative example |
45 |
g |
48 |
45 |
7 |
52 |
0 |
0 |
4.9 |
120 |
1062 |
350 |
0.33 |
Comparative example |
46 |
h |
70 |
2 |
1 |
3 |
27 |
0 |
4.3 |
6.5 |
959 |
451 |
0.47 |
Comparative example |
47 |
i |
77 |
21 |
2 |
23 |
0 |
0 |
3.2 |
110 |
922 |
479 |
0.52 |
Comparative example |
48 |
j |
84 |
9 |
7 |
16 |
0 |
0 |
5.2 |
100 |
1214 |
364 |
0.30 |
Comparative example |
49 |
k |
66 |
32 2 |
|
34 |
0 |
0 |
4.3 |
14.0 |
1312 |
604 |
0.46 |
Comparative example |
The underlined parts are outside the scope of the invention
UB: upper bainite, M: martensite, γ: retained austenite, F: polygonal ferrite, LB:
lower bainite |
[0076] Invention Examples were all high-strength steel sheets having a tensile strength
of 980 MPa or more and excellent fatigue resistance. In contrast, Comparative Examples,
which were outside the scope of the present invention, did not have a tensile strength
of 980 MPa or more or excellent fatigue resistance.