DESCRIPTION
TITLE OF INVENTION: HIGH-CARBON STEEL SHEET AND
METHOD OF MANUFACTURING THE SAME
TECHNICAL FIELD
[0001] The present invention relates to a highcarbon
steel sheet having an improved fatigue
characteristic after quenching and tempering and a
method of manufacturing the same.
BACKGROUND ART
[0002] A high-carbon steel sheet is used for
automobile drive-line components, such as chains,
gears and clutches. When an automobile drive-line
component is manufactured, cold-working as shaping
and quenching and tempering are performed of the
high-carbon steel sheet. Weight reduction of
automobile is currently in progress, and for driveline
components, weight reduction by strength
enhancement is also considered. For example, to
achieve strength enhancement of parts such as driveline
components undergone quenching and tempering,
adding carbide-forming elements represented by Ti,
Nb, Mo or increasing the content of C is effective.
[0003] Patent Literature 1 describes a method of
manufacturing a mechanical structural steel intended
for achieving both high hardness and high toughness,
Patent Document 2 describes a method of manufacturing
a rough-formed bearing intended for omission of
spheroidizing, or the like, and Patent Literatures 3
and 4 describe methods of a manufacturing high-carbon
steel sheet intended for improvement of punching
property. Patent Literature 5 describes a mediumcarbon
steel sheet intended for improvement of cold
workability and quenching stability, Patent
Literature 6 describes a steel material for bearing
element part intended for improvement of
machinability, Patent Literature 7 describes a method
of manufacturing a tool steel intended for omission
of normalizing, and Patent Literature 8 describes a
method of manufacturing a high-carbon steel sheet
intended for improvement of formability.
[00041 On the other hand, the high-carbon steel
sheet is required to have a good fatigue property,
for example, a rolling contact fatigue property after
quenching and tempering. However, the conventional
manufacturing methods described in Patent Literatures
1 to 8 cannot achieve a sufficient fatigue property.
CITATION LIST
PATENT LITERATURE
[0005] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2013-072105
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2009-108354
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2011-012317
Patent ~iterature 4: Japanese Laid-open Patent
Publication No. 2011-012316
Patent Literature 5: International Publication
Pamphlet No. W02013/035848
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2002-275584
Patent Literature 7: Japanese Laid-open Patent
Publication No. 2007-16284
Patent Literature 8: Japanese Laid-open Patent
Publication No. 2-101122
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006] It is an object of the present invention to
provide a high-carbon steel sheet capable of
achieving an excellent fatigue property after
quenching and tempering and a method of manufacturing
the same.
SOLUTION TO PROBLEM
[0007] The present inventors carried out dedicated
studies to determine the cause of that a good fatigue
property is not obtained in a conventional highcarbon
steel sheet after cold-working and quenching
and tempering. Consequently, it was found that
during the cold-working a crack and/or a void
(hereinafter the crack and the void may be
collectively referred to as a "void") occurs in
cementite and/or iron-carbon compound (hereinafter
the cementite and the iron-carbon compound may be
collectively referred to as "cementite"), thereby
decreasing formability and causing a crack to develop
from the void. Further, it was also found that,
while the cementite exists in ferrite grains and
ferrite grain boundaries, a void occurs much more
easily in cementite in a ferrite grain boundary than
in cementite in a ferrite grain.
[0008] The present inventors further carried out
dedicated studies to solve the above causes, and
consequently found that the fatigue property can be
improved significantly by setting the amounts of Mn
and Cr contained in cementite to appropriate ranges
and setting the size of ferrite to an appropriate
range. In the conventional manufacturing methods
described in Patent Literatures 1 to 8, these matters
were not considered, and thus a sufficient fatigue
property cannot be obtained. Moreover, it was also
found that, in order to manufacture such a highcarbon
steel sheet, it is important to set conditions
of hot-rolling, cold-rolling and annealing to
predetermined conditions while assuming these rolling
and annealing as what is called a continuous process.
Then, based on these findings, the present inventors
have devised the following various embodiments of the
invention. Note that the "cementite" in the present
specification and claims means cementite and ironcarbon
compound which are not contained in pearlite
and are distinguished from pearlite, except in any
part where it is clarified as a concept including
cementite contained in pearlite.
[0009] (1) A high-carbon steel sheet including a
chemical composition represented by, in mass%:
C: 0.60% to 0.90%;
Si: 0.10% to 0.40%;
Mn: 0.30% to 1.50%;
N: 0.0010% to 0.0100%;
Cr: 0.20% to 1.00%;
P: 0.0200% or less;
S: 0.0060% or less;
Al: 0.050% or less;
Mg: 0.000% to 0.010%;
Ca: 0.000% to 0.010%;
Y: 0.000% to 0.010%;
Zr: 0.000% to 0.010%;
La: 0.000% to 0.010%;
Ce: 0.000% to 0.010%; and
balance: Fe and impurities; and
a structure represented by:
a concentration of Mn contained in cementite: 2%
or more and 8% or less,
a concentration of Cr contained in cementite: 2%
or more and 8% or less,
an average grain diameter of ferrite: 10 pm or
more and 50 pm or less,
an average particle diameter of cementite: 0.3 pm
or more and 1.5 pm or less, and
a spheroidized ratio of cementite: 85% or more.
[0010] (2) The high-carbon steel sheet according to
(I), wherein in the chemical composition,
Mg: 0.001% to 0.010%,
Ca: 0.001% to 0.010%,
Y: 0.001% to 0.010%,
Zr: 0.001% to 0.010%,
La: 0.001% to 0.010%, or
Ce: 0.001% to 0.010%, or any combination thereof
is satisfied.
[0011] (3) A method of manufacturing a high-carbon
steel sheet, including:
hot-rolling of a slab to obtain a hot-rolled
sheet;
pickling of the hot-rolled sheet;
annealing of the hot-rolled sheet after the
pickling to obtain a hot-rolled annealed sheet;
cold-rolling ofthe hot-rolled annealed sheet to
obtain a cold-rolled sheet; and
annealing of the cold-rolled sheet, wherein
the slab has a chemical composition represented
by, in mass%:
C: 0.60% to 0.90%;
Si: 0.10% to 0.40%;
Mn: 0.30% to 1.50%;
P: 0.0200% or less;
S: 0.0060% or less;
~ l 0:.0 50% or less;
N: 0.0010% to 0.0100%;
Cr: 0.20% to 1.00%;
Mg: 0.000% to 0.010%;
Ca: 0.000% to 0.010%;
Y: 0.000% to 0.010%;
Zr: 0.000% to 0.010%;
La: 0.000% 'to' 0.010%;
Ce: 0.000% to 0.010%; and
balance: Fe and impurities, and
in the hot-rolling,
a finishing temperature of finish-rolling is
800°C or more and less than 950°C, and
a coiling temperature is 450°C or more and less
than 550°C,
a reduction ratio in the cold-rolling is 5% or
more and 35% or less,
annealing of the hot-rolled sheet includes:
heating the hot-rolled sheet to a first
temperature of 450°C or more and 550°C or less, a
heating rate from 60°C to the first temperature being
3O0C/hour or more and 15O0C/hour or less;
then holding the hot-rolled sheet at the first
temperature for one hour or more and less than 10
hours;
then heating the hot-rolled sheet at a heating
rate of 5OC/hour or more and 8O0C/hour or less from
the first temperature to a second temperature of 670°C
or more and 730°C or less; and
then holding the hot-rolled sheet at the second
temperature for 20 hours or more and 200 hours or
less,
the annealing of the cold-rolled sheet includes:
heating the cold-rolled sheet to a third
temperature of 450°C or more and 550°C or less, a
heating rate from 60°C to the third temperature is
3O0C/hour or more and 15O0C/hour or less;
then holding the cold-rolled sheet at the third
temperature for one hour or more and less than 10
hours;
then heating the cold-rolled sheet at a heating
rate of 5OC/hour or more and 80°C/hour or less from
the third temperature to a fourth temperature of 670°C
or more and 730°C or less; and
then holding the cold-rolled sheet at the fourth
temperature for 20 hours or more and 200 hours or
less.
[0012] (4) The method of manufacturing the highcarbon
steel sheet according to (3),
wherein in the chemical composition,
Mg: 0.001% to 0.010%,
Ca: 0.001% to 0.010%,
Y: 0.001% to 0.010%,
Zr: 0.001% to 0.010%,
La: 0.001% to 0.010%, or
Ce: 0.001% to 0.010%, or any combination thereof
is satisfied.
ADVANTAGEOUS EFFECTS OF INVENTION
[0013] According to the present invention,
concentrations of Mn and Cr contained in cementite
and so on are appropriate, and thus a fatigue
property after quenching and tempering can be
improved.
BRIEF DESCRIPTION OF DRAWINGS
[0014] [Fig. 11 Fig. 1 is a chart illustrating a
relationship between a concentration of Mn contained
in cementite and a rolling contact fatigue property.
[Fig. 21 Fig. 2 is a chart illustrating a
relationship between the concentration of Mn in
cementite and a number of voids by crack of
'cementite.
[Fig. 31 Fig. 3 is a chart illustrating a
relationship between a number of voids by crack of
cementite and the rolling contact fatigue property.
[Fig. 41 Fig. 4 is a chart illustrating a
relationship between a concentration of Cr contained
in cementite and the rolling contact fatigue
property.
[Fig. 51 Fig. 5 is a chart illustrating a
relationship between the concentration of Cr
contained in cementite and a number of voids by crack
of cementite.
[Fig. 61 Fig. 6 is a chart illustrating a
relationship between a holding temperature in hotrolled
sheet annealing and the concentrations of Mn
and Cr contained in cementite.
DESCRIPTION OF EMBODIMENTS
[0015] Hereinafter, embodiments of the present
invention will be described.
[0016] First, chemical compositions of a high-carbon
steel sheet according to an embodiment of the present
invention and a slab (steel ingot) used for
manufacturing the same will be described. Although
details will be described later, the high-carbon
steel sheet according to the embodiment of the
present invention is manufactured through coldrolling
of the slab, hot-rolled sheet annealing,
cold-rolling, annealing of cold-rolled sheet, and so
on. Therefore, the chemical compositions of the
high-carbon steel sheet and the slab are ones in
consideration of not only properties of the highcarbon
steel sheet but these processes. In the
following description, " % " which is a unit of content
of each element contained in the high-carbon steel
sheet and the slab used for manufacturing the same
means "mass%" unless otherwise specified. The highcarbon
steel sheet according to this embodiment and
the slab used for manufacturing the same have a
chemical composition represented by C: 0.60% to
0.90%, Si: 0.10% to 0.40%, Mn: 0.30% to 1.50%, N:
0.0010% to 0.0100%, Cr: 0.20% to 1.00%, P: 0.0200% or
less, S: 0.0060% or less, Al: 0.050% or less, Mg:
0.000% to 0.010%, Ca: 0.000% to 0.010%, Y: 0.000% to
0.010%, Zr: 0.000% to 0.010%, La: 0.000% to 0.010%,
Ce: 0.000% to 0.010%, and balance: Fe and impurities.
As the impurities, impurities contained in raw
materials, such as ore and scrap, and impurities
mixed in during a manufacturing process are
exemplified. For example, when scrap is used as a
raw material, Sn, Sb or As or any combination thereof
may be mixed in by 0.001% or more. However, when the '
content is 0.02% or less, none of them hinder the
effect of this embodiment, and hence may be tolerated
as impurities. 0 may be tolerated as an impurity up
to 0.004%. 0 forms an oxide, and when oxides
aggregate and become coarse, sufficient formability
cannot be obtained. Thus, the 0 content is the lower
the better, but it is technically difficult to
decrease the 0 content to less than 0.0001%.
Examples of the impurities also include Ti: 0.04% or
less, V: 0.04% or less, Cu: 0.04% or less, W: 0.04%
or less, Ta: 0.04% or less, Ni: 0.04% or less, Mo:
0.04% or less, B: 0.01% or less, and Nb: 0.04% or
less. The amount of these elements contained is
preferred to be as small as possible, but it is
technically difficult to decrease them to less than
0.001%.
[0017] (C: 0.60% to 0.90%)
C is an effective element for strength
enhancement of steel, and is particularly an element
that increases a quenching property. C is also an
element that contributes to improvement of fatigue
property after quenching and tempering. When the C
content is less than 0.60%, pro-eutectoid ferrite or
pearlite is formed in a prior austenite grain
boundary during quenching, resulting in a decrease in
fatigue property after quenching and tempering.
Therefore, the C content is 0.060% or more,
preferably 0.65% or more. When the C content is more
than 0.90%, a large amount of retained austenite
exists after quenching. The retained austenite is
decomposed into ferrite and cementite during
tempering, and a large strength difference occurs
between the tempered martensite or bainite and the
ferrite and cementite formed by decomposition of the
retained austenite after tempering, resulting in a
decrease in fatigue property after quenching and
tempering. Therefore, the C content is 0.90% or
Less, preferably 0.85% or less.
[0018] (Si: 0.10% to 0.40%)
Si operates as a deoxidizer, and is also an
effective element for improvement of fatigue property
after quenching and tempering. When the Si content
is less than 0.10%, the effect by the above operation
cannot be obtained sufficiently. Therefore, the Si
content is 0.10% or more, preferably 0.15% or more.
When the Si content is more than 0.40%, the amount
and the size of Si oxides formed as inclusions in
steel increase, and the fatigue property after
quenching and tempering decreases. Therefore, the Si
content is 0.40% or less, preferably 0.35% or less.
[0019] (Mn: 0.30% to 1.50%)
Mn is an element contained in cementite and
suppressing generation of void during cold-working.
When the Mn content is less than 0.30%, annealing for
causing cementite to contain a sufficient amount of
Mn takes a very long time, which significantly
decreases productivity. Therefore, the Mn content is
0.30% or more, preferably 0.50% or more. When the Mn
content is more than 1.50%, Mn contained in cementite
becomes excessive, making cementite difficult to
dissolve during heating for quenching, resulting in
an insufficient amount of C solid-dissolved in
austenite. Consequently, the strength after
quenching decreases, and the fatigue property after
quenching and tempering also decreases. Therefore,
the Mn content is 1.50% or less, preferably 1.30% or
less.
[0020] (N: 0.001 to 0.010%)
N is combined with A 1 to generate AlN, and is an
effective element for grain refinement of austenite
during heating for quenching. When the N content is
less than 0.001%, the effect by the above operation
cannot be obtained sufficiently. Therefore, the N
content is 0.001% or more, preferably 0.002% or more.
When the N content is more than 0.010%, austenite
grains become excessively small, which decreases the
quenching property and facilitates generation of proeutectoid
ferrite and pearlite during cooling of
quenching, resulting in a decrease in fatigue
property after quenching and tempering. Therefore,
the N content is 0.010% or less, preferably 0.008% or
less.
[0021] (Cr: 0.20% to 1.00%)
Cr is an element contained in cementite and
suppressing generation of void during cold-working,
similarly to Mn. When the Cr content is less than
0.20%, annealing for causing cementite to contain a
sufficient amount of Cr takes a very long time, which
significantly decreases productivity. Therefore, the
Mn content is 0.20% or more, preferably 0.35% or
more. When the Cr content is more than 1.00%, Cr
contained in cementite becomes excessive, making
cementite difficult to dissolve during heating for
quenching, resulting in an insufficient amount of C
solid-dissolved in austenite. Consequently, the
strength after quenching decreases, and the fatigue
property after quenching and tempering also
decreases. Therefore, the Cr content is 1.00% or
less, preferably 0.85% or less.
100221 ( P : 0.0200% or less)
P is not an essential element and is contained
as, for example, an impurity in steel. P is an
element which decreases the fatigue property after
quenching and tempering, and/or decreases toughness
after quenching. For example, when toughness
decreases, a crack easily occurs after quenching.
Thus, the P content is the smaller the better. In
particular, when the P content is more than 0.0200%,
adverse effects become prominent. Therefore, the P
content is 0.0200% or less, preferably 0.0180% or
less. Decreasing the P content takes time and cost,
and when it is attempted to decrease it to less than
0.0001%, the time and cost increase significantly.
Thus, the P content may be 0.0001% or more, or may be
0.0010% or more for further reduction in time and
cost.
[0023] (S: 0.0060% or less)
S is not an essential element and is contained
as, for example, an impurity in steel. S is an
element forming a sulfide such as MnS, and decreasing
the fatigue property after quenching and tempering.
Thus, the S content is smaller the better. In
particular, when the S content is more than 0.0060%,
adverse effects become prominent. Therefore, the S
content is 0.0060% or less. Decreasing the S content
takes time and cost, and when it is attempted to
decrease it to less than 0.0001%, the time and cost
increase significantly. Thus, the S content may be
0.0001% or more.
[0024] (Al: 0.050% or less)
A1 is an element which operates as a deoxidizer
at the stage of steelmaking, but is not an essential
element of the high-carbon steel sheet and is
contained as, for example, an impurity in steel.
When the A1 content is more than 0.050%, a coarse A1
oxide is formed in the high-carbon steel sheet,
resulting in a decrease in fatigue property after
quenching and tempering. Therefore, the A1 content
is 0.050% or less. When the A1 content of the highcarbon
steel sheet is less than 0.001%, it is
possible that deoxidation is insufficient.
Therefore, the A1 content may be 0.001% or more.
[0025] Mg, Ca, Y, Zr, La and Ce are not essential
elements, and are optional elements which may be
appropriately contained in the high-carbon. steel
sheet and the slab up to .a predetermined amount.
[0026] (Mg: 0.000% to 0.010%)
Mg is an effective element for controlling the
form of sulfide, and is an effective element for
improvement of fatigue property after quenching and
tempering. Thus, Mg may be contained. However, when
the Mg content is more than 0.010%, a coarse Mg oxide
is formed, and the fatigue property after quenching
and tempering decreases. Therefore, the Mg content
is 0.010% or less, preferably 0.007% or less. In
order to reliably obtain the effect by the above
operation, the Mg content is preferably 0.001% or
more.
[0027] (Ca: 0.000% to 0.010%)
Ca is an effective element for controlling the
form of sulfide, and is an effective element for
improvement of fatigue property after quenching and
tempering, similarly to Mg. Thus, Ca may be
contained. However, when the Ca content is more than
0.010%, a coarse Ca oxide is formed, and the fatigue
property after quenching and tempering decreases.
Therefore, the Ca content is 0.010% or less,
preferably 0.007% or less. In order to reliably
obtain the effect by the above operation, the Ca
content is preferably 0.001% or more.
LO0281 (Y: 0.000% to 0.010%)
Y is an effective element for controlling the
form of sulfide, and is an effective element for
improvement of fatigue property after quenching and
tempering, similarly to Mg and Ca. Thus, Y may be
contained. However, when the Y content is more than
0.010%, a coarse Y oxide is formed, and the fatigue
property after quenching and tempering decreases.
Therefore, the Y content is 0.010% or less,
preferably 0.007% or less. In order to reliably
obtain the effect by the above operation, the Y
content is preferably 0.001% or more.
[0029] (Zr: 0.000% to 0.010%)
Zr is an effective element for controlling the
form of sulfide, and is an effective element for
improvement of fatigue property after quenching and
tempering, similarly to Mg, Ca and Y. Thus, Zr may
be contained. However, when the Zr content is more
than 0.010%, a coarse Zr oxide is formed, and the
fatigue property after quenching and tempering
decreases. Therefore, the Zr content is 0.010% or
less, preferably 0.007% or less. In order to
reliably obtain the effect by the above operation,
the Zr content is preferably 0.001% or more.
[0030] (La: 0.000% to 0.010%)
La is an effective element for controlling the
form of sulfide, and is an effective element for
improvement of fatigue property after quenching and
tempering, similarly to Mg, Ca, Y and Zr. Thus, La
may be contained. However, when the La content is
more than 0.010%, a coarse La oxide is formed, and
the fatigue property after quenching and tempering
decreases. Therefore, the La content is 0.010% or
less, preferably 0.007% or less. In order to
reliably obtain the effect by the above operation,
the La content is preferably 0.001% or more.
[0031] (Ce: 0.000% to 0.010%)
Ce is an effective element for controlling the
form of sulfide, and is an effective element for
improvement of fatigue property after quenching and
tempering, similarly to Mg, Ca, Y and Zr. Thus, Ce
may be contained. However, when the Ce content is
more than 0.010%, a coarse Ce oxide is formed, and
the fatigue property after quenching and tempering
decreases. Therefore, the Ce content is 0.010% or
less, preferably 0.007% or less. In order to
reliably obtain the effect by the above operation,
the Ce content is preferably 0.001% or more.
[0032] Thus, Mg, Ca, Y, Zr, La and Ce are optional
elements, and it is preferred that "Mg: 0.001% to
0.010%", "Ca: 0.001% to 0.010%", "Y.. 0.001% to
0.010%", "Zr: 0.001% to 0.010%", "La: 0.001% to
0.010%", or "Ce: 0.001% to 0.010%", or any
combination thereof be satisfied.
[0033] Next, the structure of the high-carbon steel
sheet according to this embodiment will be described.
The high-carbon steel sheet according to this
embodiment has a structure represented by a
concentration of Mn contained in cementite: 2% or
more and 8% or less, a concentration of Cr contained
in cementite: 2% or more and 8% or less, an average
grain diameter of ferrite: 10 pm or more and 50 pm or
less, an average particle diameter of cementite
particles: 0.3 pm or more and 1.5 pm or less, and a
spheroidized ratio of cementite particles: 85% or
more.
[0034] (Concentration of Mn and concentration of Cr
contained in cementite: both 2% or more and 8% or
less)
Although details will be described later, Mn and
Cr contained in cementite contribute to suppression
of generation of void in cementite during coldworking.
The suppression of generation of void
during cold-working improves the fatigue property
after quenching and tempering. When the
concentration of Mn or Cr contained in cementite is
less than 2%, the effect by the above operation
cannot be obtained sufficiently. Therefore, the
concentration of Mn and the concentration of Cr
contained in cementite are 2% or more. When the
concentration of Mn or Cr contained in cementite is
more than 8%, solid-dissolvability of C from
cementite to austenite during heating for quenching
decreases, the quenching property decreases, and a
structure with low strength compared to pro-eutectoid
ferrite, pearlite, quenched martensite or bainite
disperses. As a result, the fatigue property after
quenching and tempering decreases. Therefore, the
concentration of Mn and the concentration of Cr
contained in cementite is 8% or less.
[ 0 0 3 5 ] Here, a study carried out by the present
inventors on the relationship between the
concentration of Mn contained in cementite and the
fatigue property will be described.
[ 0 0 3 6 ] In this study, high-carbon steel sheets v7ere
manufactured through hot-rolling, hot-rolled sheet
annealing, cold-rolling and cold-rolled sheet
annealing under various conditions. Then, with
respect to each high-carbon steel sheet, the
concentration of Mn and the concentration of Cr
contained in cementite were measured by using an
electron probe micro-analyzer (FE-EPMA) equipped with
a field-emission electron gun made by Japan Electron
Optics Laboratory. Next, the high-carbon steel sheet
was subjected to cold-rolling with a reduction ratio
of 35% simulating cold-working (shaping), and the
high-carbon steel sheet was held for 2 0 minutes in a
salt bath heated to 900°C and quenched in oil at 80°C.
Subsequently, the high-carbon steel sheet was
subjected to tempering by holding for 60 minutes in
an atmosphere at 180°C, thereby producing a sample for
fatigue test.
[ 0 0 3 7 ] Thereafter, a fatigue test was performed, and
void in cementite after cold-working was observed.
In the fatigue test, a rolling contact fatigue tester
was used, the surface pressure was set to 3 0 0 0 MPa,
and the number of cycles until peeling occurs was
counted. In the observation of void, a scanning
electron microscope (FE-SEM) equipped with a fieldemission
electron gun made by Japan Electron Optics
Laboratory was used, and the structure of a region
having an area of 1 2 0 0 pm2 was photographed at
magnification of about 3 0 0 0 times at 2 0 locations at
equal intervals in a thickness direction of the highcarbon
steel sheet. Then, the number of voids
generated by cracking of cementite (hereinafter may
also be simply referred to as "the number of voids")
was counted in a region having an area of 24000 urn2 in
total, and the total number of these voids was
divided by 12 to calculate the number of voids per
2000 urn2. In this embodiment, the average particle
diameter of cementite is 0.3 vm or more and 1.5 vm or
less, and thus the magnification for the observation
thereof is preferably 3000 times or more, or even a
higher magnification such as 5000 times or 10000
times may be chosen depending on the size of
cementite. Even when the magnification is more than
3000 times, the number of voids per unit area (for
example, per 2000 )lm2) is equal to that when it is
3000 times. Voids may also exist in the interface
between cementite and ferrite, but the influence of
such voids on the fatigue property is quite small as
compared to the influence of voids generated by
cracking of cementite. Thus, such voids are not
counted.
[0038] The sample subjected to measurement using FEEPMA
or FE-SEM was preparedas follows. First, an
observation surface was mirror polished by buffing
with a wet emery paper and diamond abrasive
particles, and then dipped for 20 seconds at room
temperature (20°C) in a picral (saturated picric acid-
3 vol% of nitric acid-alcohol) solution, so as to let
the structure appear. Thereafter, moisture on the
observation surface was removed with a hot air dryer
and the like, and then the sample was carried into a
specimen exchange chamber of the FE-EPMA and the FESEM
within three hours in order to prevent
contamination.
[0039] Their results are illustrated in Fig. 1, Fig.
2 and Fig. 3 . Fig. 1 is a chart illustrating a
relationship between a concentration of Mn contained
in cementite and a rolling contact fatigue property.
Fig. 2 is a chart illustrating a relationship between
a concentration of Mn contained in cementite and the
number of voids. Fig. 3 is a chart illustrating a
relationship between the number of voids and the
rolling contact fatigue property. The results
illustrated in Fig. 1 to Fig. 3 are of samples in
which the concentration of Cr contained in cementite
is 2% or more and 8% or less.
[0040] From Fig. 1, it can be seen that the rolling
contact fatigue property is significantly high when
the concentration of Mn contained in cementite is in
the range of 2% or more and 8% or less. From Fig. 2,
it can be seen that generation of voids is suppressed
when the concentration of Mn contained in cementite
is in the range of 2% or more and 8% or less. From
Fig. 3, it can be seen that the fatigue property is
quite high in the case where the number of voids per
2000 pm2 is 15 or less, as compared to the case where
it is more than 15. From the results illustrated i.n
Fig. 1 to Fig. 3, it is conceivable that when the
concentration of Mn contained in cementite is 2% or
more and 8% or less, the cementite becomes less
breakable during cold-working (shaping) and
generation of voids is suppressed, and thus
development of cracking at a void is suppressed in
the fatigue test after subsequent quenching and
tempering, resulting in an improvement of fatigue
property.
[0041] The present inventors have also studied the
relationship between the concentration of Cr
contained in cementite and the rolling contact
fatigue property and the number of voids. Their
results are illustrated in Fig. 4 and Fig. 5. Fig. 4
is a chart illustrating a relationship between the
concentration of Cr contained in cementite and the
rolling contact fatigue property. Fig. 5 is a chart
illustrating a relationship between the concentration
of Cr contained in cementite and the number of voids.
The results illustrated in Fig. 4 and Fig. 5 are of
samples in which the concentration of Mn contained in
cementite is 2% or more and 8% or less. As
illustrated in Fig. 4 and Fig. 5, similarly to the
relationship between the concentration of Mn
contained in cementite and the rolling contact
fatigue property or the number of voids illustrated
in Fig. 1 and Fig. 2, it was found that an excellent
rolling contact fatigue property is obtained when the
concentration of Cr contained in cementite is 2% or
more and 8% or less.
[0042] The reason why Mn and Cr contained in
cementite contribute to suppression of generation of
voids during cold-working is not clear, but it can be
assumed that mechanical properties, such as tensile
strength and ductility, of cementite are improved by
Mn and Cr contained in cementite.
[0043] (Average grain diameter of ferrite: 10 pm or
more and 50 pm or less)
The smaller the ferrite, the more the ferrite
grain boundary area increases. When the average
grain diameter of ferrite is less than 10 pm,
generation of void during cold-working in cementite
on the ferrite grain boundary becomes significant.
Therefore, the average grain diameter of ferrite is
10 pm or more, preferably 12 pm or more. When the
average grain diameter of ferrite is more than 50 pm,
a matted surface is generated on a surface of the
steel sheet after shaping, which disfigures the
surface. Therefore, the average grain diameter of
ferrite is 50 pm or less, preferably 45 pm or less.
[0044] The average grain diameter of ferrite can be
measured by the FE-SEM after the above-described
mirror-polishing and etching with a picral are
performed. For example, an average area of 200
grains of ferrite is obtained, and the diameter of a
circle with which this average area can be obtained
is obtained, thereby taking this diameter as the
average grain diameter of ferrite. The average area
of ferrite is a value obtained by dividing the total
area of ferrite by the number of ferrite, here 200.
[0045] (Average particle diameter of cementite: 0.3
pm or more and 1.5 pm or less)
The size of cementite largely influences the
fatigue.property after quenching and tempering. When
the average particle diameter of cementite is less
than 0.3 pm, the fatigue property after quenching and
tempering decreases. Therefore, the average particle
diameter of cementite is 0.3 vm or more, preferably
0.5 pm or more. When the average particle diameter
of cementite is more than 1.5 pm, voids are generated
dominantly in coarse cementice during cold-working,
and the fatigue property after quenching and
tempering decreases. Therefore, the average particle
diameter of cementite is 1.5 pm or less, preferably
1.3 pm or less.
[0046] (Spheroidized ratio of cementite: 85% or
more)
The lower the spheroidized ratio of cementite,
the more the locations where a void is easily
generated, for example acicular portions or the like,
increase. When the spheroidized ratio of cementite
is less than 85%, the void during cold-working in
cementite is significantly generated. Therefore, the
spheroidized ratio of cementite is 85% or more,
preferably 90% or more. The spheroidized ratio of
cementite is preferred to be as hjgh as possible, but
in order to make it loo%, the annealing takes a very
long time, which increases the manufacturing cost.
Therefore, in view of the manufacturing cost, the
spheroidized ratio of cementite is preferably 99% or
less, more preferably 98% or less.
[0047] The spheroidized ratio and the average
particle diameter of cementite can be measured by
micro structure observation with the FE-SEM. In
production of a sample for micro structure
observation, after the observation surface was mirror
polished by wet polishing with an emery paper and
polishing with diamond abrasive particles having a
particle size of 1 pm, etching with the abovedescribed
picral solution is performed. The
observation magnification is set between 1000 times
to 10000 times, for example 3000 times, 16 visual
fields where 500 or more particles of cementite are
contained on the observation surface are selected,
and a structure image of them is obtained. Then, the
area of each cementite in the structure image is
measured by using image processing software. As the
image processing software, for example, "WinROOF"
made by MITANI Corporation can be used. At this
time, in order to suppress the influence of
measurement error by noise, any cementite particle
having an area of 0.01 or less is excluded from
the target of evaluation. Then, the average area of
cementite as an evaluation target is obtained, and
the diameter of a circle with which this average area
can be obtained is obtained, thereby taking this
diameter as the average particle diameter of
cementite. The average area of cementite is a value
obtained by dividing the total area of cementite as
the evaluation target by the number of cementite.
Further, any cementite particle having a ratio of
major axis length to minor axis length of 3 or more
is assumed as an acicular cementite particle, any
cementite particle having the ratio of less than 3 is
assumed as a spherical cementite particle, and a
value obtained by dividing the number of spherical
cementite particles by the number of all cementite
particles is taken as the spheroidized ratio of
cementite.
[ 0 0 4 8 ] Next, a method of manufacturing the highcarbon
steel sheet according to this embodiment will
be described. This manufacturing method includes
hot-rolling of a slab having the above chemical
composition to obtain a hot-rolled sheet, pickling of
this hot-rolled sheet, thereafter annealing of the
hot-rolled sheet to obtain a hot-rolled annealed
sheet, cold-rolling of the hot-rolled annealed sheet
to obtain a cold-rolled sheet, and annealing of the
cold-rolled sheet. In the hot-rolling, the finishing
temperature of finish-rolling is 800°C or more and
less than 95OoC, and the coiling temperature is 450°C
or more and less than 550°C. The reduction ratio in
the cold-rolling is 5% or more and 35% or less. In
the hot-rolled sheet annealing, the hot-rolled sheet
is heated to a first temperature of 45OoC or more and
550°C or less, then the hot-rolled sheet is held at
the first temperature for one hour or more and less
than 10 hours, then the hot-rolled sheet is heated at'
a heating rat'e of 5'C/hour or more and 80°C/hour or
less from the first temperature to a second
temperature of 670°C or more and 730°C or less, and
then the hot-rolled sheet is held at the second
temperature for 20 hours or more and 200 hours or
less. When,the hot-rolled sheet is heated to the
first temperature, the heating rate from 60°C to the
first temperature is 30°C/hour or more and 150°C/hour
or less. In the cold-rolled sheet annealing, the
cold-rolled sheet is heated to a third temperature of
450°C or more and 550°C or less, then the cold-rolled
sheet is held at the third temperature for one hour
or more and less than 10 hours, then the cold-rolled
sheet is heated at a heating rate of 5OC/hour or more
and 80°C/hour or less from the third temperature to a
fourth temperature of 670°C or more and 730°C or less,
and then the cold-rolled sheet is held at the fourth
temperature for 20 hours or more and 200 hours or
less. When the cold-rolled sheet is heated to the
third temperature, the heating rate from 60°C to the
third temperature is 3O0C/hour or more and 15O0C/hour
or less. Both of the annealing of the hot-rolled
sheet and the annealing of the cold-rolled sheet may
be considered as including two-stage annealing.
[0049] (Finishing temperature of the finish-rolling
of hot-rolling: 800°C or more and less than 950°C)
When the finishing temperature of the finishrolling
is less than 800°C, deformation resistance of
the slab is high, the rolling load increases, the
abrasion amount of the reduction roll increases, and
productivity decreases. Therefore, the finishing
temperature of the finish-rolling is 800°C or more,
preferably 810°C or more. When the finishing
temperature of the finish-rolling is 950°C or more,
scales are generated during the hot-rolling, and the
scales are pressed against the slab by the reduction
roll and thereby form scratches on a surface of the
obtained hot-rolled sheet, resulting in a decrease in
productivity. Therefore, the finishing temperature
of the finish-rolling is less than 950°C, preferably
920°C or less. The slab can be produced by continuous
casting for example, and this slab may be subjected
as it is to hot-rolling, or may be cooled once, and
then heated and subjected to hot-rolling.
[0050] (Coiling temperature of the hot-rolling: 450°C
or more and less than 550°C)
The coiling temperature is preferred to be as low
as possible. However, when the coiling temperature
is less than 450°C, embrittlement of the hot-rolled
sheet is significant, and when the coil of the hotrolled
sheet is uncoiled for pickling, a crack or the
like occurs in the hot-rolled sheet, resulting in a
decrease in productivity. Therefore, the coiling
temperature is 450°C or more, preferably 470°C or
more. When the coiling temperature is 550°C or more,
the structure of the hot-rolled sheet does not become
fine, and it becomes difficult for Mn and Cr to
diffuse during the hot-rolled sheet annealing, making
it difficult to make cementite contain a sufficient
amount of Mn and/or Cr. Therefore, the coiling
temperature is less than 550°C, preferably 530°C or
less.
[0051] (Reduction ratio in the cold-rolling: 5% or
more and 35% or less)
If the reduction ratio in the cold-rolling is
less than 5%, even when the cold-rolled sheet is
annealed subsequently, a large amount of nonrecrystallized
ferrite remains thereafter. Thus, the
structure after the cold-rolled sheet annealing
becomes a non-uniform structure in which
recrystallized parts and non-recrystallized parts are
mixed, the distribution of strain generated inside
the high-carbon steel sheet during the cold-working
also becomes non-uniform, and voids are easily
generated in cementite which is largely distorted
Therefore, the reduction ratio in the cold-rolling is
5% or more, preferably 10% or more. When the
reduction ratio is more than 35%, nucleation rate of
recrystallized ferrite increases, and the average
grain diameter of ferrite cannot be 10 pm or more.
Therefore, the reduction ratio in the cold-rolling'is
35% or less, preferably 30% or less.
[0052] (First temperature: 450°C or more and 550°C or
less)
In this embodiment, while the hot-rolled sheet is
held at the first temperature, Mn and Cr are diffused
into cementite, so as to increase the concentrations
of Mn and Cr contained in cementite. When the first
temperature is less than 450°C, the diffusion
frequency of Fe as well as substitutional soliddissolved
elements such as Mn and Cr decreases, and
it takes a long time for making cementite contain
sufficient amounts of Mn and Cr, resulting in a
decrease in productivity. Therefore, the first
temperature is 450°C or more, preferably 480°C or
more. When the first temperature is more than 550°C,
it is not possible to make cementite contain
sufficient amounts of Mn and Cr. Therefore, the
first temperature is 550°C or less, preferably 520°C
or less.
[0053] Here, a study carried. out by the present
inventors on the relationship between the first
temperature and the concentrations of Mn and Cr
contained in cementite will be described. In this
study, it was held for nine hours at various
temperatures, and the concentrations of Mn and Cr
contained in cementite were measured. Results of
this are illustrated in Fig. 6. The vertical axis of
Fig. 6 represents the ratios of the concentrations of
Mn and Cr to values when the holding temperature is
700°C. From Fig. 6, it can be seen that both the
concentrations of Mn and Cr become high particularly
in the vicinity of 500°C.
100541 (Holding time at the first temperature: one
hour or more and less than 10 hours)
The concentrations of Mn and Cr contained in
cementite are closely related to the holding time at
the first temperature. When this time is less than
one hour, it is not possible to make cementite
contain sufficient amounts of Mn and Cr. Therefore,
this time is one hour or more, preferably 1.5 hours
or more. When this time is more than 10 hours,
increases of the concentrations of Mn and Cr
contained in cementite become small, which takes time
and cost in particular. Therefore, this time is 10
hours or less, preferably seven hours or less.
[0055] (Heating rate from 60°C to the first
temperature: 30°C/hour or more and 150°C or less)
In the annealing of hot-rolled sheet, for
example, it is heated from room temperature, and if
the heating rate from 60°C to the first tempe'rature is
less than 30°C/hour, it takes a long time to increase
in temperature, resulting in a decrease in
productivity. Therefore, this heating rate is
30°C/hour or more, preferably 60°C/hour or more. When
this heating rate is more than 150°C/hour, the
temperature difference between an inside portion and
an outside portion of the coil of the hot-rolled
sheet becomes large, and scratches and/or deformation
of coiling shape occurs due to an expansion
difference, resulting in a decrease in yield.
Therefore, this heating temperature is 150°C/hour or
less, preferably 120°C/hour or less.
[0056] (Second temperature: 670°C or more and 730°C
or less)
If the second temperature is less than 67OoC,
cementite does not become coarse during annealing of
the hot-rolled sheet, and pinning energy remains
high. This hinders grain growth of ferrite during
annealing of the cold-rolled sheet later, and it
takes a very long time to make the average grain
diameter of ferrite be 10 pm or more, resulting in a
decrease in productivity. Therefore, the second
temperature is 67OoC or more, preferably 690°C. When
the second temperature is more than 730°C, austenite
is partially formed during annealing of the hotrolled
sheet, and pearlite transformation occurs in
cooling after holding at the second temperature. The
pearlite structure formed at this time exerts strong
pinning force on the grain growth of ferrite during
annealing of the cold-rolled sheet later, and thus
grain gro~~tohf ' ferrite is hindered. Therefore, the
second temperature is 730°C or less, preferably 720°C
or less.
[0057] (Holding time at the second temperature: 20
hours or more and 200 hours or less)
When the holding time at the second temperature
is less than 20 hours, cementite does not become
coarse, and pinning energy remains high. This
hinders grain growth of ferrite during the coldrolled
sheet annealing later, an amount of cementite
existing on a ferrite grain boundary increases unless
cold-rolled sheet annealing for a long time is
performed, and voids are generated during coldworking,
resulting in a decrease in fatigue property.
Thus, this time is 20 hours or more, preferably 30
hours or more. When this time is more than 200
hours, it significantly decreases in productivity.
Therefore, this time is 200 hours or less, preferably
180 hours or less.
[0058] (Heating rate from the first temperature to
the second temperature: 5OC/hour or more and 8OoC/hour
or less)
By holding the hot-rolled sheet to the first
temperature, Mn and Cr can be diffused in cementite,
but the concentrations of Mn and Cr contained in
cementite vary among plural particles of cementite.
This variation of concentrations of Mn and Cr can be
alleviated during heating from the first temperature
to the second temperature.
[0059] The heating rate is preferred to be as low as
possible in order to alleviate the variation of
concentrations of Mn and Cr. However, when the
heating rate from the first temperature to the second
temperature is less than 5OC/hour, it significantly
decreases in productivity. Thus, this heating rate
is 5OC/hour or more, preferably 10°C/hour or more.
When this heating rate is more than 80°C/hour, it is
not possible to sufficiently alleviate the variation
of concentrations of Mn and Cr. This causes
cementite with low concentrations of Mn and/or Cr to
exist, and voids are generated during cold-working,
resulting in a decrease in fatigue property.
Therefore, this heating rate is 8OoC/hour or less,
preferably 6!i°C/hour or less.
[ 0 0 6 0 ] Here, a structural change that occurs during
heating from the first temperature to the second
temperature will be described. Here, it ts assumed
that, after the holding at the first temperature,
cementite with low concentrations of Mn and Cr (first
cementite) and cementite with high concentrations of
Mn and Cr (second cementite) exist. In either of the
first cementite and the second cementite, a local
equilibrium state is maintained in the vicinity of
the interface between cementite and a parent phase
(ferrite phase), and the concentrations of Mn and Cr
contained in this cementite do not change unless
flowing-in or flowing-out of alloy elements newly
occur.
[0061] When the hot-rolled sheet is heated after
held at the first temperature, and the frequency of
diffusion of atoms is increased thereby, C is
discharged from cementite to a ferrite phase. Since
the Mn and Cr have an operation to attract C, the
amount of C discharged from the second cementite is'
small, and the amount of C discharged from the first
cementite is large. On the other hand, C discharged
to the ferrite phase is attracted to the second
cementite with high concentrations of Mn and Cr, and
adheres to an outer skin of the second cementite,
thereby forming new cementite (third cementite).
[0062] The third cementite which is just formed does
not substantially contain Mn and Cr, and thus
attempts to contain Mn and Cr in concentrations
illustrated in Fig. 4. However, the diffusion rate
of Mn and Cr in cementite is affected by mutual
attraction with C, and is quite slow compared to that
in the ferrite phase. Thus, Mn and Cr contained in
the adjacent second cementite do not easily diffuse
to the third cementite. Therefore, in order to
maintain the distribution equilibrium, the third
cementite is supplied with Mn and Cr from the ferrite
phase, resulting in that the third cementite contains
Mn and Cr in about the same concentrations as those
of the second cementite. Further, the first
cementite also increases in concentrations of Mn and
Cr along with the discharge of C, and thus contains
Mn and Cr in about the same concentrations as those
of the second cementite. In this manner, the
variation of concentrations of Mn and Cr among plural
cementite particles is alleviated. Therefore, in
view of the variation of concentrations of Mn and Cr,
the heating rate is preferred to be as low as
possible, and when the heating rate is excessively
high, it is not possible to sufficiently alleviate
the variation of concentrations of Mn and Cr.
[0063] (Third temperature: 450°C or more and 550°C or
less)
In this embodiment, while the cold-rolled sheet
is held at the third temperature, Mn and Cr are
diffused through cementite, so as to increase the
concentrations of Mn and Cr contained in cementite.
When the third temperature is less than 450°C,
productivity decreases similarly to when the first
temperature is less than 450°C. Thus, the third
temperature is 45OoC or more, preferably 480°C or
more. When the third temperature is more than 550°C,
similarly to when the first temperature is more than
55OoC, it is not possible to make cementite contain
sufficient amounts of Mn and Cr. Therefore, the
third temperature is 550°C or less, preferably 520°C
or less.
[0064] (Holding time at the third temperature: one
hour or more and less than 10 hours)
The concentrations of Mn and Cr contained in
cementite are closely related to the holding time at
the third temperature. When this time is less than
one hour, it is not possible to make cementite
contain sufficient amounts of Mn and Cr. Therefore,
this time is one hour or more, preferably 1.5 hours
or more. When this time is more than 10 hours,
increases of the concentrations of Mn and Cr
contained in cementite become small, which takes time
and cost in particular. Therefore, this time is 10
hours or less, preferably seven hours or less.
[0065] (Heating rate from 60°C to the third
temperature: 30°C/hour or more and 150°C or less)
In the cold-rolled sheet annealing, for example,
heating from room temperature is performed, and if
the heating rate from 60°C to the third temperature is
less than 30°C/hour, praductivity decreases similarly
to when the heating rate from 60°C to the first
temperature is less than 3O0C/hour. Therefore, this
heating rate is 30°C/hour or more, preferably
60°C/hour or more. When this heating rate is more
than 150°C/hour, the temperature difference between an
inside portion and an outside portion of the coil of
the hot-rolled sheet becomes large, and scratches
and/or deformation of coiling shape occurs due to an
expansion difference, resulting in a decrease in
yield. Therefore, this heating temperature is
15O0C/hour or less, preferably 120°C/hour or less.
[0066] (Fourth temperature: 670°C or more and 730°C
or less)
In this embodiment, while the cold-rolled sheet
is held at the fourth temperature, a distortion
introduced by the cold-rolling is used as driving
force to control the average grain diameter of
ferrite to 10 pm or more by nucleation-type
recrystallization, recrystallization in situ or
distortion-induced grain boundary migration of
ferrite. As described above, when the average grain
boundary of ferrite is 10 pm or more, excellent
formability can be obtained. When the fourth
temperature is less than 670°C, non-recrystallized
ferrite remains after cold-rolled sheet annealing,
and the average grain diameter of ferrite does not
become 10 or more, with which excellent formability
cannot be obtained. Therefore, the fourth
temperature is 670°C or more, preferably 690°C. When
the fourth temperature is more than 730°C, austenite
is partially generated during the cold-rolled sheet
annealing, and pearlite transformation occurs in
cooling after holding at the fourth temperature.
When the pearlite transformation occurs, the
spheroidized ratio of cementite decreases, and voids
are easily generated during cold-working, resulting
in a decrease in fatigue property. Therefore, the
fourth temperature is 73OoC or less, preferably 720°C
or less.
[0067] (Holding time at the fourth temperature: 20
hours or more and 200 hours or less)
When the holding time at the fourth temperature
is less than 20 hours, non-recrystallized ferrite
remains after cold-rolled sheet annealing, and the
average grain diameter of ferrite does not become 10
or more, with which excellent formability cannot be
obtained. Thus, this time is 20 hours or more,
preferably 30 hours or more. When this time is more
than 200 hours, it significantly decreases in
productivity. Therefore, this time is 200 hours or
less, preferably 180 hours or less.
COO681 The atmosphere of the hot-rolled sheet
annealing and the atmosphere of the cold-rolled sheet
annealing are not particularly limited, and these
annealings can be performed in, for example, an
atmosphere containing nitrogen by 95 vol% or more, an
atmosphere containing hydrogen by 95 vol% or more, an
air atmosphere, or the like.
[0069] According to this embodiment, a high-carbon
steel sheet can be manufactured in which the
concentration of Mn contained in cementite is 2% or
more and 8% or less, the concentration of Cr
contained in cementite is 2% or more and 8% or less,
the average grain diameter of ferrite is 10 pm or
more and 50 pm or less, the average particle diameter
of cementite is 0.3 pm or more and 1.5 pm or less,
and the spheroidized ratio of cementite is 85% or
more and 99% or less. In this high-carbon steel
sheet, generation of void from cementite during coldworking
is suppressed, and a high-carbon steel sheet
with an excellent fatigue property after quenching
and tempering can be manufactured.
[0070] It shodld be noted that all of the abovedescribed
embodiments merely illustrate concrete
examples of implementing the present invention, and
the technical scope of the present invention is not
to be construed in a restrictive manner by these
embodiments. That is, the present invention may be
implemented in various forms without departing from
the technical spirit or main features thereof.
EXAMPLE
[0071] Next, examples of the present invention will
be described. Conditions in the examples are
condition examples employed for confirming
feasibility and effect of the present invention, and
the present invention is not limited to these
condition examples. The present invention can employ
various conditions as long as the object of the
present invention is achieved without departing from
the spir'it of the invention.
[0072] (First experiment)
In a first experiment, hot-rolling of a slab
(steel type A to AT) having a chemical composition
illustrated in Table 1 and a thickness of 250 mm was
performed, thereby obtaining a coil of a hot-rolled
sheet having a thickness of 2.5 mm. In the hotrolling,
the heating temperature of slab was 114O0C,
the time thereof was one hour, the finishing
temperature of finish-rolling was 880°C, and the
coiling temperature was 510°C. Then, the hot-rolled
sheet was pickled while it was uncoiled, and the hotrolled
sheet after the pickling was annealed, thereby
obtaining a hot-rolled annealed sheet. The
atmosphere of the hot-rolled sheet annealing was an
atmosphere of 95 vol% hydrogen-5 vol% nitrogen.
Thereafter, cold-rolling of the hot-rolled annealed
sheet was performed with a reduction ratio of 18%,
thereby obtaining a cold-rolled sheet. Subsequently,
the cold-rolled sheet was annealed. The atmosphere
of the cold-rolled sheet annealing was an atmosphere
of 95 vol% hydrogen-5 vol% nitrogen. In the hotrolled
sheet annealing and the cold-rolled sheet
annealing, the hot-rolled sheet or the cold-rolled
sheet was heated from room temperature, the heating
rate from 60°C to 495OC was set to 85OC/hour, the
sheet was held at 495OC for 2.8 hours, heating from
49S°C to 710°C was performed at a heating rate of
6S°C/hour, the sheet was held at 710°C for 65 hours,
and thereafter cooled to room temperature by furnace
cooling. Various high-carbon steel sheets were
produced in this manner. Blank fields in Table 1
indicate that the content of this element is less
than a detection limit, and the balance is Fe and
impurities. An underline in Table 1 indicates that
this numeric value is out of the range of the present
invention.
LO0731 [Table 11
TAELE 1
[0074] Then, the average grain diameter of ferrite,
the average particle diameter of cementite, the
spheroidized ratio of cementite, and the
concentrations of Mn and Cr contained in cementite of
each high-carbon steel sheet were measured. The
micro structure observation was performed by the
above method. Further, cold-rolling simulating coldworking
and quenching and tempering were performed by
the above method, and counting of voids per 2000 pm2
and a fatigue test with respect to rolling contact
fatigue were performed. Results of them are
illustrated in Table 2. An underline in Table 2
indicates that this numeric value is out of the range
of the present invention.
l o 0 7 5 1 [Table 21
[ 0 0 7 6 ] As illustrated in Table 2, samples No. 1 to
No. 15 and No. 35 to No. 40 were within the range of
the present invention, and hence succeeded to obtain
an excellent rolling contact fatigue property.
Specifically, peeling did not occur even when
manipulating loads of one million cycles were applied
in the fatigue test with respect to rolling contact
fatigue.
[0077] On the other hand, in sample No. 16, the Mn
content of steel type P was too low, and thus the
concentration of Mn contained in cementite was too
low. There were many voids, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 17, the Mn content of steel type Q was too high.
Thus, the concentration of Mn contained in cementite
was too high, and a sufficient rolling contact
fatigue property was not obtained. In sample No. 18,
the Si content of steel type R was too low. Thus,
cementite became coarse during tempering after
quenching, and a sufficient rolling contact fatigue
property was not obtained. Further, the average
grain diameter of ferrite was too large. Thus, a
matted surface was generated when the cold-rolling
simulating cold-working was performed, which
disfigured the surface. In sample No. 19, the C
content of steel type S was too high. Thus, there
was a large amount of retained austenite after
quenching, and a fatigue fracture occurred from the
retained austenite. Consequently, there were many
voids, and a sufficient rolling contact fatigue
property rsias not obtained. In sample No. 20, the Si
content of steel type T was too high. Thus, a coarse
Si oxide was generated, a fatigue fracture occurred
from this Si oxide, and a sufficient rolling contact
fatigue property was not obtained. In sample No. 21,
the Mn content of steel type U was too low. Thus,
the concentration of Mn contained in cementite was
too low, there were many voids, and a sufficient
rolling contact fatigue property was not obtained.
In sample No. 22, the S content of steel type V was
too high. Thus, a coarse sulfide was generated, a
fatigue fracture occurred from the sulfide, and a
sufficient rolling contact fatigue property was not
obtained. In sample No. 23, the Cr content of steel
type W was too low. Thus, the concentration of Cr
contained in cementite was too low, there were many
voids, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 24, the N
content of steel type X was too high. Thus, pinning
force of austenite by A1N was too large, austenite
grains became excessively fine and pearlite was
formed during cooling of quenching, and a fatigue
fracture occurred from this pearlite. Consequently,
a sufficient rolling contact fatigue property was not
obtained. In sample No. 25, the P content of steel
type Y was too high. Thus, a crack occurred during
quenching, a fatigue fracture occurred from this
crack, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 26, the C
content of steel type Z was too low. Thus, pearlite
was formed during quenching, a fatigue fracture
occurred from this pearlite, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 2 7 , t h e Mn c o n t e n t of s t e e l t y p e AA was t o o h i g h .
Thus, t h e c o n c e n t r a t i o n of Mn c o n t a i n e d i n c e m e n t i t e
was t o o h i g h , and a s u f f i c i e n t r o l l i n g c o n t a c t
f a t i g u e p r o p e r t y was not o b t a i n e d . In sample No. 28,
t h e A1 c o n t e n t of s t e e l t y p e AB was t o o h i g h . Thus,
a c o a r s e A 1 o x i d e was g e n e r a t e d , a f a t i g u e f r a c t u r e
o c c u r r e d from t h i s A1 o x i d e , and a s u f f i c i e n t r o l l i n g
c o n t a c t f a t i g u e p r o p e r t y was n o t o b t a i n e d . In sample
No. 2 9 , t h e C r c o n t e n t of s t e e l t y p e AC was t o o low.
Thus, t h e c o n c e n t r a t i o n of C r c o n t a i n e d i n c e m e n t i t e
was t o o low, t h e r e were many v o i d s , and a s u f f i c i e n t
r o l l i n g c o n t a c t f a t i g u e p r o p e r t y was n o t o b t a i n e d .
I n sample No. 30, t h e C r c o n t e n t of steel t y p e AD was
t o o h i g h . Thus, t h e c o n c e n t r a t i o n of C r c o n t a i n e d i n
c e m e n t i t e was t o o h i g h , and a s u f f i c i e n t r o l l i n g
c o n t a c t f a t i g u e p r o p e r t y was n o t o b t a i n e d . In sample
No. 31, t h e S i c o n t e n t of s t e e l t y p e AE was t o o h i g h .
Thus, a c o a r s e S i o x i d e v7as g e n e r a t e d , a f a t i g u e
f r a c t u r e o c c u r r e d from t h i s S i o x i d e , and a
s u f f i c i e n t r o l l i n g c o n t a c t f a t i g u e p r o p e r t y was n o t
o b t a i n e d . In sample No. 32, t h e C c o n t e n t of s t e e l
t y p e AF was t o o h i g h . Thus, t h e r e was a l a r g e amount
of r e t a i n e d a u s t e n i t e a f t e r quenching, and a f a t i g u e
f r a c t u r e o c c u r r e d from t h e r e t a i n e d a u s t e n i t e .
C o n s e q u e n t l y , t h e r e were many v o i d s , and a s u f f i c i e n t
r o l l i n g c o n t a c t f a t i g u e p r o p e r t y was n o t o b t a i n e d .
In sample No. 33, t h e C c o n t e n t of s t e e l t y p e AG was
t o o low. Thus, p e a r l i t e was formed d u r i n g quenching,
a fatigue fracture occurred from this pearlite, and a
sufficient rolling contact fatigue property was not
obtained. In sample No. 34, the Cr content of steel
type AH was too high. Thus, the concentration of Cr
contained in cementite was too high, and a sufficient
rolling contact fatigue property was not obtained.
[ 0 0 7 8 ] In sample No. 41, the Ca content of steel
type A0 was too high. Thus, a coarse Ca oxide was
generated, a fatigue fracture occurred from this Ca
oxide, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 42, the Ce
content of steel type AP was too high. Thus, a
coarse Ce oxide was generated, a fatigue fracture
occurred from this Ca oxide, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 43, the Mg content of steel type AQ was too high.
Thus, a coarse Mg oxide was generated, a fatigue
fracture occurred from this Mg oxide, and a
sufficient rolling contact fatigue property was not
obtained. In sample No. 44, the Y content of steel
'type AR was too high. Thus, a coarse Y oxide was
generated, a fatigue fracture occurred from this Y
oxide, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 45, the Zr
content of steel type AS was too high. Thus, a
coarse Zr oxide was generated, a fatigue fracture
occurred from this Zr oxide, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 46, the La content of steel type AT was too high.
Thus, a coarse La oxide was generated, a fatigue
fracture occurred from this La oxide, and a
sufficient rolling contact fatigue property was not
obtained.
[0079] (Second experiment)
In a second experiment, hot-rolling, hot-rolled
sheet annealing, cold-rolling and cold-rolled sheet
annealing of particular steel types (steel types A,
B , C , D , E , F , G , H , I , J , K , L , M , N , O , A I , A J , A K ,
AL, AM and AN) selected from the steel types used in
the first experiment were performed under various
conditions, thereby producing high-carbon steel
sheets. These conditions are illustrated in Table 3,
Table 4, Table 5 and Table 6. An underline in Table
3 to Table 6 indicates that this numeric value is out
of the range of the present invention. Conditions
not described in Table 3 to Table 6 are the same as
those in the first experiment.
[0080] [Table 31
TABLE 3
100821 [ T a b l e 51

[0084] Then, the average grain diameter of ferrite,
the average particle diameter of cementite, the
spheroidized ratio of cementite, and the
concentrations of Mn and Cr contained in cementite of
each high-carbon steel sheet were measured, and
moreover, counting of voids and a fatigue test with
respect to rolling contact fatigue were performed,
similarly to the first experiment. Results of them
are illustrated in Table 7 and Table 8. An underline
in Table 7 and Table 8 indicates that this numeric
value is out of the range of the present invention.
[ 0 0 8 5 ] [Table 71
100'861 [ T a b l e 81
TABLE 8
SAMPLE
No.
93
94
STEEL
TYPE
A
B
STRUCTURE
FERRITE
AVERAGE
G W N DIAMETER
(urn)
48.0
9.4
NOTE
INVENTION EXAMPLE
COMPARATIVE EXAMPLE
PROPERTY
NUMBER OF
VOIDS
4.6
21.7
CEMENTr'E
NUMBER OF
CYCLES
15752497
79674
CONCENTRATION
OF Cr
(%)
6.73
5.08
CONCENTRATION
OF Mn
(%)
3.91
2.09
I SPHEROIDIZED PARTICLE
DIAMETER
(urn)
0.79
0.55
RATIO
(%)
92.9
90.8
[0087] As illustrated in Table 7 and Table 8,
samples No. 51, No. 52, No. 54 to No. 58, No. 60 to
No. 62, No. 66, No. 67, No. 71, No. 74, No. 76, No.
77, No. 80, No. 83, No. 84, No. 86, No. 89 to No. 91,
No. 93, No. 99 to No. 101, No. 104 to No. 110, and
No. 112 were within the range of the present
invention, and hence succeeded to obtain an excellent
rolling contact fatigue property. Specifically,
peeling did not occur even when manipulating loads of
one million cycles were applied in the fatigue test
with respect to rolling contact fatigue.
[0088] On the other hand, in sample No. 53, the
heating rate from the third temperature to the fourth
temperature was too high. Thus, the temperature
difference between a center portion and a
circumferential edge portion of the cold-rolled sheet
coil was too large, and scratches due to a thermal
expansion difference occurred. Further, the
concentration of Cr contained in cementite was too
low, there were many voids, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 59, the holding time at the second temperature
was too short. Thus, the average grain diameter of
ferrite was small, there were many voids, and a
sufficient rolling contact fatigue property was not
obtained. In sample No. 63, the heating rate from
60°C to the first temperature was too low, and thus
productivity was quite low. In sample No. 64, the
heating rate from the first temperature to the second
temperature was too high. Thus, the temperature
difference between a center portion and a
circumferential edge portion of the cold-rolled sheet
coil was too large, and scratches due to a thermal
expansion difference occurred. Further, the
concentration of Cr contained in cementite was too
low, there were many voids, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 65, the third temperature was too low. Thus, the
concentration of Cr contained in cementite was too
low, there were many voids, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 68, the coiling temperature was too high. Thus,
the concentrations of Mn and Cr contained in
cementite and the spheroidized ratio of cementite
were too low, there were many voids, and a sufficient
rolling contact fatigue property was not obtained.
In sample No. 69, the fourth temperature was too
high. Thus, ferrite and cementite grew excessively.
Further, pearlite was formed, and the spheroidized
ratio of cementite was low. Consequently, there were
many voids, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 70, the
coiling temperature was too low, the hot-rolled sheet
became brittle, and a crack occurred when it is
uncoiled for pickling.
[0089] In sample No. 72, the coiling temperature was
too high. Thus, the concentrations of Mn and Cr
contained in cementite and the spheroidized ratio of
cementite were too low, there were many voids, and a
sufficient rolling contact fatigue property was not
obtained. In sample No. 73, the first temperature
was too high. Thus, the concentration of Mn
contained in cementite was too low, there were many
voids, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 75, the
holding time at the third temperature was too short.
Thus, the concentrations of Mn and Cr contained in
cementite were too low, there were many voids, and a
sufficient rolling contact fatigue property was not
obtained. In sample No. 78, the holding time at the
first temperature was too short. Thus, the
concentrations of Mn and Cr contained in cementite
were too low, there were many voids, and a sufficient
rolling contact fatigue property was not obtained.
In sample No. 79, the second temperature was too
high. Thus, pearlite was formed, and the average
grain diameter of ferrite was too small.
Consequently, there were many voids, and a sufficient
rolling contact fatigue property was not obtained.
In sample No. 81, the reduction ratio of cold-rolling
was too low. Thus, non-recrystallized ferrite
existed, uniformity of the structure was low, and a
large distortion locally occurred when cold-rolling
simulating cold-working was performed. Consequently,
many cracks of cementite occurred, there were many
voids, and a sufficient rolling contact fatigue
propert'y was not obtained.
In sample No. 82, the finishing temperature of
finish-rolling was too low. Thus, abrasion of the
reduction roll was significant, and productivity was
low. In sample No. 85, the heating rate from 60°C to
the first temperature was too low, and thus
' productivity was quite low. In sample No. 87, the
heating rate from 60°C to the first temperature tias
too high. Thus, the temperature difference between a
center portion and a circumferential edge portion of
the hot-rolled sheet coil was too large, and
scratches due to a thermal expansion difference
occurred. In sample No. 88, the coiling temperature
was too low, the hot-rolled sheet became brittle, and
a crack occurred when it is uncoiled for pickling.
In sample No. 92, the heating rate from 60°C to the
third temperature was too high. Thus, the
temperature difference between a center portion and a
circumferential edge portion of the cold-rolled sheet
coil was too large, and scratches due to a thermal
expansion difference occurred.
[0090] In sample No. 94, the reduction ratio of
cold-rolling was too high. Thus, the average grain
diameter of ferrite was too small, there were many
voids, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 95, the
second temperature was too low. Thus, cementite is
fine after hot-rolled sheet annealing, and the
average grain diameter of ferrite was too small.
Consequently, there were many voids, and a sufficient
rolling contact fatigue property was not obtained.
In sample No. 96, the finishing temperature of
finish-rolling was too high. Thus, scales occurred
excessively during the hot-rolling, and scratches'due
to the scales occurred. In sample No. 97, the third
temperature was too high. Thus, the concentrations
of Mn and Cr contained in cementite were too low,
there were many voids, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 98, the fourth temperature was too low. Thus,
the average grain diameter of ferrite was too small,
there were many voids, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 102, the holding time at the fourth temperature
was too short. Thus, the average grain diameter of
ferrite was too small, there were many voids, and a
sufficient rolling contact fatigue property was not
obtained. In sample No. 103, the third temperature
was 'too high. Thus, the concentration of Mn
contained in cementite was too low, there were many
voids, and a sufficient rolling contact fatigue
property was not obtained. In sample No. 111, the
third temperature was too low. Thus, the
concentration of Cr contained in cementite was too
low, there were many voids, and a sufficient rolling
contact fatigue property was not obtained. In sample
No. 113, the first temperature was too high. Thus,
the concentrations of Mn and Cr contained in
cementite were too low, there were many voids, and a
sufficient rolling contact fatigue property was not
obtained.
INDUSTRIAL APPLICABILITY
[0091] The present invention can be used in, for
example, manufacturing industries and application
industries of high-carbon steel sheets used for
various steel products, such as drive-line components
of automobiles.

CLAIMS
[Claim 11 A high-carbon steel sheet comprising:
a chemical composition represented by, in mass%:
C: 0.60% to 0.90%;
Si: 0.10% to 0.40%;
Mn: 0.30% to 1.50%;
N: 0.0010% to 0.0100%;
Cr: 0.20% to 1.00%;
P: 0.0200% or less;
S: 0.0060% or less;
Al: 0.050% or less;
Mg: 0.000% to 0.010%;
Ca: 0.000% to 0.010%;
Y: 0.000% to 0.010%;
Zr: 0.000% to 0.010%;
La: 0.000% to 0.010%;
Ce: 0.000% to 0.010%; and
balance: Fe and impurities; and
a structure represented by:
a concentration of Mn contained in
cementite: 2% or more and 8% or less,
a concentration of Cr contained in
cementite: 2% or more and 8% or less,
an average grain diameter of ferrite: 10 pm
or more and 50 pm or less,
an average particle diameter of cementite:
0.3 pm or more and 1.5 pm or less, and
a spheroidized ratio of cementite: 85% or
more.
[Claim 21 The high-carbon steel sheet according to
claim 1, wherein in the chemical composition,
Mg: 0.001% to 0.010%,
Ca: 0.001% to 0.010%,
Y: 0.001% to 0.010%,
Zr: 0.001% to 0.010%,
La: 0.001% to 0.010%, or
Ce: 0.001% to 0.010%, or any combination thereof
is satisfied.
[Claim 31 A method of manufacturing a high-carbon
steel sheet, comprising:
hot-rolling of a slab to obtain a hot-rolled
sheet;
pickling of the hot-rolled sheet;
annealing of the hot-rolled sheet after the
pickling to obtain a hot-rolled annealed sheet;
cold-rolling of the hot-rolled annealed sheet to
obtain a cold-rolled sheet; and
annealing of the cold-rolled sheet, wherein
the slab comprises a chemical composition
represented by, in mass%:
C: 0.60% to 0.90%;
Si: 0.10% to 0.40%;
Mn: 0.30% to 1.50%;
P: 0.0200% or less;
S: 0.0060% or less;
Al: 0.050% or less;
N: 0.0010% to 0.0100%;
Cr: 0.20% to 1.00%;
Mg: 0.000% to 0.010%;
Ca: 0.000% to 0.010%;
Y: 0.000% to 0.010%;
Zr: 0.000% to 0.010%;
La: 0.000% to 0.010%;
Ce: 0.000% to 0.010%; and
balance: Fe and impurities, and
in the hot-rolling,
a finishing temperature of finish-rolling is
800°C or more and less than 950°C, and
a coiling temperature is 450°C or more and
less than 550°C,
a reduction ratio in the cold-rolling is 5% or
more and 35% or less,
the annealing of the hot-rolled sheet comprises:
heating the hot-rolled sheet to a first
temperature of 450°C or more and 550°C or less, a
heating rate from 6OoC to the first temperature being
3O0C/hour or more and 15O0C/hour or less;
then holding the hot-rolled sheet at the
first temperature for one hour or more and less than
10 hours;
then heating the hot-rolled sheet at a
heating rate of 5OC/hour or more and 8O0C/hour or less
from the first temperature to a second temperature of
670°C or more and 730°C or less; and
then holding the hot-rolled sheet at the
second temperature for 20 hours or more and 200 hours
or less,
the annealing of the cold-rolled sheet comprises:
heating the cold-rolled sheet to a third
temperature of 450°C or more and 55OoC or less, a
heating rate from 60°C to the third temperature is
30°C/hour or more and 150°C/hour or less;
then holding the cold-rolled sheet at the
third temperature for one hour or more and less than
10 hours;
then heating the cold-rolled sheet at a
heating rate of 5OC/hour or more and 8O0C/hour or-Pess
from the third temperature fo a fourth temperature of
670°C or more and 730°C or less; and
then holding the cold-rolled sheet at the
fourth temperature for 20 hours or more and 200 hours
or less.
[Claim 41 The method of manufacturing the highcarbon
steel sheet according to claim 3,
wherein in the chemical composition,
Mg: 0.001% to 0.010%,
Ca: 0.001% to 0.010%,
Y: 0.001% to 0.010%,
Zr: 0.001% to 0.010%,
La: 0.001% to 0.010%, or
Ce: 0.001% to O.010%, or any combination thereof
is satisfied.