TECHNICAL FIELD
[0001] The present invention relates to a highcarbon
steel sheet with improved formability and a
method of manufacturing the same.·
BACKGROUND ART
[0002] A high-carbon steel sheet is used for various
steel products, which are a driving system component
for automobile such as a chain, a gear and a clutch,
a saw, a knife, and others. When the steel products
are manufactured, forming and heat treatments of a
high-carbon steel sheet are performed. As the
forming, punching, tensile forming, compressing,
shearing, and so on are performed, and as the heat
treatment, quenching, tempering, carburizing,
nitriding, soft-nitriding, and so on are performed.
A strength of a high-carbon steel sheet is higher
than that of a mild steel sheet, and therefore a
metal mold used for forming of a high-carbon steel
sheet is more easily worn than a metal mold used for
forming of a mild steel sheet. Further, a highcarbon
steel sheet cracks more easily than a mild
steel sheet during forming.
[0003] For suppressing the wearing of a metal mold,
improving lubricity on a surface of a high-carbon
(
steel sheet is effective, and for suppressing the
- l -
cracking during forming, softening of a high-carbon
steel sheet is effective. Thus, some techniques have
been proposed aiming at an .. imprOvement in lubricity
and softening (Patent Literatures 1 to 5).
[0004] However, these prior techniques cause a
significant increase in cost, and therefore are not
preferred.
[0005] Although a carbon steel sheet aiming at an
improvement in punchability has been described in
Patent Literature 6 and a high-carbon steel sheet
aiming at an improvement in formability has been
described in Patent Literature 7, it is not possible
for them to obtain sufficient formability.
CITATION LIST
PATENT LITERATURE
[0006] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2010-174252
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2009-215612
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2011-168842
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2010-255066
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2000-34542
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2000-265240
I
Patent Literature 7: Japanese Laid~open Patent _
- 2 -
Publication No. 10-147816
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007] An object of the present invention is to
provide a high-carbon steel sheet capable of
obtaining excellent formability while avoiding a
significant increase in cost, and· a method of
manufacturing the same.
SOLUTION TO PROBLEM
[0008] The present inventors conducted earnest
studies repeatedly to solve the above-described
problem, and consequently found out that it is
important that a high-carbon steel sheet contains a
specific amount of B, that a coefficient of microfriction
of ferrite on a surface is a specific one,
and that form of cementite is a specific one.
Further, it was also found out that, in order to
manufacture such a high-carbon steel sheet, it is
important to perform hot-rolling and annealing under
specific conditions while assuming hot-rolling and
annealing as what is called a consecutive process.
Then, the inventors of the present application
devised the following various aspects of the
invention based on these findings.
[0009] (1)
A high-carbon steel sheet, including:
a chemical composition represented by, in mass%:
(
C: 0.30% to 0.70%,
- 3 -
Si: 0. 07% to 1.00%,
Mn: 0.20% to 3.00%,
Ti: 0.010% to 0.500%,
Cr: 0. 01% to 1.50%,
B: 0.0004% to 0.0035%,
P: 0.025% or less,
Al: 0.100% or less,
s : 0.0100% or less,
N: 0.010% or less,
Cu: 0.500% or less,
Nb: 0.000% to 0.500%,
Mo: 0.000% to 0.500%,
V: 0.000% to 0.500%,
W: 0.000% to 0.500%,
Ta: 0.000% to 0.500%,
Ni: 0.000% to 0.500%,
Mg: 0.000% to 0.500%,
Ca: 0.000% to 0.500%,
Y: 0.000% to 0.500%,
Zr: 0.000% to 0.500%,
La: 0.000% to 0.500%,
Ce: 0.000% to 0.500%, and
balance: Fe and impurities; and
a structure represented by:
.a spheroidized ratio of cementite: 80% or more;
and
an .average diameter of cementite: 0.3 pm to 2.2
pm, wherein
I
- 4 -
a coefficient of micro-friction of ferrite on a
surface of the steel sheet is less than 0.5.
[0010] (2)
The high-carbon steel sheet according to (1),
wherein
in the chemical composition,
Nb: 0.001% to 0.500%,
Mo: 0.001% to 0.500%,
V: 0.001% to 0.500%,
W: 0.001% to 0.500%,
Ta: 0.001% to 0.500%,
Ni: 0.001% to 0.500%,
Mg: 0.001% to 0.500%,
Ca: 0.001% to 0.500%,
Y: 0.001% to 0.500%,
Zr: 0.001% to 0.500%,
La: 0.001% to 0.500%, or
Ce: 0.001% to 0.500%, or
any combination thereof is satisfied.
[0011] (3)
A method crf 'manufacturing a high-carbon steel
sheet, including:
hot-rolling of a slab so as to obtain a hotrolled
steel sheet;
pickling of the hot-rolled steel sheet; and
annealing of the hot-rolled steel sheet after the
pickling,
the slab including a chemic11 composition
- 5 -
-------- ----"-- ------
represented by, in mass%:
C: 0.30% to 0.70%,
Si: 0.07% to 1.00%,
Mn: 0.20% to 3.00%,
Ti: 0.010% to 0.500%,
Cr: 0.01% to 1.50%,
B: 0.0004% to 0.0035%,
P: 0.025% or less,
Al: 0.100% or less,
s: 0.0100% or less,
N: 0.010% or less,
Cu: 0.500% or less,
Nb: 0.000% to 0.500%,
Mo: 0.000% to 0.500%,
V: 0.000% to 0.500%,
W: 0.000% to 0.500%,
Ta: 0.000% to 0.500%,
Ni: 0.000% to 0.500%,
Mg: 0.000% to 0.500%,
Ca: 0.000% to 0.500%,
Y: 0.000% to 0.500%,
Zr: 0.000% to 0.500%,
La: 0.000% to 0.500%,
Ce: 0.000% to 0.500%, and
balance: Fe and impurities, wherein
in the hot-rolling,
the slab is heated at a temperature of lOOO"C or
(
more and less than 1150"C,
- 6 -
a finish rolling temperature is 830°C or more and
950°C or less, and
a coiling temperature is 450°C or more and 700°C
or less, and
the annealing comprises:
retaining the hot-rolled steel sheet at a
temperature of 730°C or more and 770°C or less for 3
hours or more and 60 hours or less; and
then cooling the hot-rolled steel sheet down to
650°C at a cooling rate of l°C/hr or more and 60°C/hr
or less.
[0012] (4)
The method of manufacturing the high-carbon steel
sheet according to (3), wherein
in the chemical composition,
Nb: 0.001% to 0.500%,
Mo: 0.001% to 0.500%,
V: 0.001% to 0.500%,
W: 0.001% to 0.500%,
Ta: 0.001% to 0.500%,
N:c: 0.001% to 0.500%,
Mg: 0.001% to 0.500%,
Ca: 0.001% to 0.500%,
y: 0.001% to 0.500%,
Zr: 0.001% to !0.500%,
La: 0.001% to 0.500%, or
Ce: 0.001% to 0.500%, or
I
any combination thereof is satisfied.
- 7 -
ADVANTAGEOUS EFFECTS OF INVENTION
[0013] According to the present invention, a B
content, a coefficient of micro-friction of ferrite
on a surface and others are appropriate, thereby
making it possible to obtain excellent formability
while avoiding a significant increase in cost.
BRIEF DESCRIPTION OF DRAWINGS
[0014] [Fig. 1] Fig. 1 is a chart illustrating a
relationship between a coefficient of micro-friction
of ferrite and a B content;
[Fig. 2] Fig. 2 is a chart illustrating a
relationship between a coefficient of micro-friction
of ferrite and a number of pressing until a flaw
occurs;
[Fig. 3A] Fig. 3A is a micrograph showing a
surface of a high-carbon steel sheet before measuring
a coefficient of micro-friction;
[Fig. 3B] Fig. 38 is a micrograph showing the
surface of the high-carbon steel sheet after
measuring the coefficient of micro-friction;
[Fig. 4] Fig. 4 is a schematic diagram
illu~trating changes in temperature from hot-rolling
to cooling;
[Fig. SA] Fig. SA is a schematic diagram
illustrating a structure at time tA;
[Fig. SB] Fig. SB is a schematic diagram
illustrating a structure at time tB;
(
[Fig. SC] Fig. SC is a schematic diagram
- 8 -
illustrating a structure at time tc;
[Fig. 50] Fig. 50 is a schematic diagram
illustrating a structure at time t 0 ;
[Fig. SE] Fig. SE is a schematic diagram
illustrating a structure at time tE;
[Fig. 6A] Fig. 6A is a schematic diagram
illustrating a structure when a slab heating
temperature is high than 1150°C;
[Fig. 68] Fig. 68 is a schematic diagram
illustrating a structure when the slab heating
temperature is lower than 1000°C;
[Fig. 6C] Fig. 6C is a schematic diagram
illustrating a structure when an annealing retention
temperature is lower than 730°C;
[Fig. 60] Fig. 60 is a schematic diagram
illustrating a structure when the annealing retention
temperature is higher than 770°C or an annealing
retention is longer than 60 hours;
[Fig. 6E] Fig. 6E is a schematic diagram
illustrating a structure when the annealing retention
is shorter than 3 hours;
[Fig. 6F] Fig. 6F is a schematic diagram
illustrating a structure when a cooling rate is less
than 1 °C/hr;
[Fig. ·6G] Fig. 6G is a schematic diagram
illustrating a structure when the cooling rate is
greater than 60°C/hr; and
[Fig. 7 l Fig. 7 is
I
a chart illustrating a
- 9 -
----- ----·----~-----------"-
relationship between a coefficient of micro-friction
of ferrite and a B content for a part of inventive
examples in a first experiment 6r a third experiment.
DESCRIPTION OF EMBODIMENTS
[0015] Hereinafter, there will be explained an
embodiment of the present invention.
[0016] First, chemical compositions of a high-carbon
steel sheet according to the embodiment of the
present invention and a slab (steel ingot) used for
manufacturing the same will be explained. Although
details will be described later, the high-carbon
steel sheet according to the embodiment of the
present invention is manufactured by going through
hot-rolling of the slab, annealing, and the like.
Accordingly, the chemical compositions of the highcarbon
steel sheet and the slab are appropriate for
the above-stated processes in addition to properties
of the high-carbon steel sheet. In the following
description, ft%" being a unit of content of each
element contained in the high-carbon steel sheet and
the slab used for manufacturing the same means
ftmass%" unless otherwise mentioned. The high-carbon
steel sheet according to the embodiment and the slab
used for manufacturing the same include a chemical
composition represented by C: 0.30% to ,0.70%, 'Si:
0 . 0 7% to 1 . 0 0%, Mn: 0 . 2 0% to 3 . 0 0%, T i : 0 . 0 1 0% to
0.500%, Cr: 0.01% to 1.50%, B: 0.0004% to 0.0035%, P:
(
0.025%'or less, Al: 0.100% or less, S: 0.0100% or
- 10 -
~-~-~~------~-------··--··-.. ---------··-~-~--··----------
less, N: 0.010% or less, Cu: 0.500% or less, Nb:
0.000% to 0.500%, Mo: 0.000% to 0.500%, V: 0.000% to
0.500%, W: 0.000% to 0.500%, Ta: 0.000% to 0.500%,
Ni: 0.000% to 0.500%, Mg: 0.000% to 0.500%, Ca:
0.000% to 0.500%, Y: 0.000% to 0.500%, Zr: 0.000% to
0.500%, La: 0.000% to 0.500%, Ce: 0.000% to 0.500%,
and balance: Fe and impurities. As the impurities,
ones contained in raw materials such as ore and
scrap, and ones contained 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 mix in by 0.003% or more. If
the content is 0.03% or less, none of them hinder the
effect of the embodiment, and thus may be tolerated
as impurities. Further, 0 may be tolerated as an
impurity up to 0.0025%. 0 forms oxide, and when
oxides aggregate and become coarse, sufficient
formability is not obtained. Therefore, the 0
content is the lower the better. However, it is
technically difficult to decrease the 0 content to
less than 0.0001%.
[0017] (C: 0.30% to 0.70%)
C bonds to Fe to form cementite having a small
friction coefficient, and thus is an important
element when securing macro-lubricity of the highcarbon
steel sheet. When the C content is less than
0.30%, the amount of cementite is insufficient,
resulting in that sufficient lub!icity cannot be
- 11 -
obtained and adhesion to a metal mold occurs during
forming. Thus, the C content is 0.30% or more, and
preferably 0.35% or more. When the C content is
greater than 0.70%, the amount of cementite is
excessive, resulting in that a crack originating from
the cementite occurs easily during forming. Thus,
the C content is 0.70% or less, and preferably 0.65%
or less.
[0018] (Si: 0.07% to 1.00%)
Si operates as a deoxidizer, and is effective for
suppressing excessive coarsening of cementite during
annealing. When the Si content is less than 0.07%,
the effect by the above-described operation cannot be
obtained sufficiently. Thus, the Si content is 0.07%
or more, and preferably 0.10% or more. When the Si
content is greater than 1.00%, the ductility of
ferrite is low and a crack originating from
transgranular fracture of ferrite occurs easily
during forming. Thus, the Si content is 1.00% or
less, and preferably 0.80% or less.
[0019] (Mn: 0.20% to 3.00%)
Mn is important for controlling pearlite
transformation. When the Mn content is less than
0.20%, the effect by the above-described operation
cannot be obtained sufficiently. That is, when the
Mn content is less than 0.20%, pearlite
transformation occurs in cooling after dual-phase
I annealing and a spheroidized ratio of cementite
- 12 -
becomes insufficient. Thus, the Mn content is 0.20%
or more, and preferably 0.25% or more. When the Mn
content is greater than 3.00%, the ductility of
ferrite is low and a crack originating from
transgranular fracture of ferrite occurs easily
during forming. Thus, the Mn content is 3.00% or
less, and preferably 2.00% or less.
[0020] (Ti: 0.010% to 0.500%)
Ti forms a nitride in molten steel, and effective
for preventing formation of BN. When the Ti content
is less than 0.010%, the effect by the abovedescribed
operation cannot be obtained sufficiently.
Thus, the Ti content is 0.010% or more, and
preferably 0.040% or more. When the Ti content is
greater than 0.500%, a crack originating from a
coarse oxide of Ti occurs easily during forming.
This is because during continuous casting, coarse
oxides of Ti are formed to get involved inside the
slab. Thus, the Ti content is 0. 500% or less, and
preferably 0.450% or less.
[0021] (Cr: 0.01% to 1.50%)
Cr has a high affinity with N, effective for
suppressing formation of BN, and effective also for
controlling pearlite transformation. When the Cr
content is less than 0.01%, the effect by the abovedescribed
operation cannot be obtained sufficiently.
Thus, the Cr content is 0.01% or more, and preferably
0.05% or more.
(
When the Cr content is greater than
- 13 -
1.50%, spheroidizing of cementite during annealing is
hindered and coarsening of cementite is suppressed
drastically. Thus, the Cr content is 1.50% or less,
and preferably 0.90% or less.
[0022] (B: 0.0004% to 0.0035%)
B lowers the coefficient of micro-friction of
ferrite on the surface of the high-carbon steel
sheet. B segregates to and concentrates at an
interface between ferrite and cementite during laterdescribed
annealing and suppresses peeling at the
interface during forming, and B is also effective for
preventing a crack. When the B content is less than
0.0004%, the effect by the above-described operation
cannot be obtained sufficiently. Thus, the B content
is 0.0004% or more, and preferably 0.0008% or more.
When the B content is greater than 0.0035%, a crack
originating from boride such as carbide of Fe and B
occurs easily during forming. Thus, the B content is
0.0035% or less, and preferably 0.0030% or less.
[0023] Fig. 1 is a chart illustrating a relationship
between a co~fficient of micro-friction of f~Yrite
and a B content. As illustrated in Fig. 1, when the
B content is 0.0004% or more, the coefficient of
micro-friction of ferrite is significantly low as
compared to the case when it is less than 0.0004%.
It may be inferred that the reason why wearing of a
metal mold can be suppressed
micro-friction of ferrite is
as a coefficient of
(
lower is because a hard
- 14 -
film of B is formed on a surface of a high-carbon
steel sheet, as will be described later. Further, it
may be inferred that the operation that B segregated
to and concentrated at an interface between ferrite
and cementite improves strength of the interface,
suppresses cracking of a high-carbon steel sheet, and
suppresses wearing of a metal mold caused by cracking
is also a reason for the above.
[0024] (P: 0.025% or less)
P is not an essential element and is contained as
an impurity in the steel sheet, for example. P
strongly segregates to the interface between ferrite
and cementite, and thereby the segregation of B to
the interface is hindered and peeling at the
interface is caused. Therefore, the P content is the
smaller the better. When the P content is greater
than 0.025%, adverse effects are particularly
prominent. Thus, the P content is 0.025% or less.
Decreasing the P content takes refining cost, and it
requires a considerable refining cost to decrease the
P content to less than 0.0001%. Thus, ·th'e P content
may be 0.0001% or more.
[0025] (Al: 0.100% or less)
Al operates as a deoxid±zer in steelmaking and is
effective for fixing N, but is not an essential
element of the high-carbon steel sheet and is
contained
example.
as an impurity in the steel sheet, for
(
When the Al content is greater than 0.100~L
- 15 -
the ductility of ferrite is low and a crack
originating from transgranular fracture of ferrite
occurs easily during forming, and strength is
excessive to cause an increase in forming load.
Thus, the Al content is set to 0.100% or less. When
the Al content of the high-carbon steel sheet is less
than 0.001%, fixation of N sometimes may be
insufficient. Thus, the Al content may be 0.001% or
more.
[0026] (S: 0.0100% or less)
S is not an essential element and is contained as
an impurity in the steel sheet, for example. S forms
coarse non-metal inclusions such as MnS to impair
formability. Therefore, the S content is the smaller
the better. When the S content is greater than
0.0100%, adverse effects are particularly prominent.
Thus, the S content is 0.0100% or less. Decreasing
the S content takes refining cost, and it requires a
considerable refining cost to decrease the S content
to less than 0.0001%. Thus, the S content may be
0.0001% or more.
[0027] (N: 0.010% or less)
N is not an essential element and is contained as
an impurity in the steel sheet, for example. N
lowers an amount of solid-solution B due to formation
of BN so as to cause adhesion to the metal mold,
cracking during forming, and the like.
(
the N content is the smaller the better.
- 16 -
Therefore,
When the N
content is greater than 0.010%, adverse effects are
particularly prominent. Thus, the N content is set
to 0.010% or less. Decreasing .. the N content takes
refining cost, and it requires a considerable
refining cost to decrease the N content to less than
0.001%. Thus, theN content may be 0.001% or more.
[0028] (Cu: 0.000% to 0.500%)
Cu is not an essential element and is mixed from
scrap or the like to be contained as an impurity in
the steel sheet, for example. Cu causes an increase
in strength and brittleness in hot working.
Therefore, the Cu content is the smaller the better.
When the Cu content is greater than 0.500%, adverse
effects are particularly prominent. Thus, the Cu
content is 0.500% or less. Decreasing the Cu content
takes refining cost, and it requires a considerable
refining cost to decrease the Cu content to less than
0.001%. Thus, the Cu content may be 0.001% or more.
[0029] Nb, Mo, V, W, Ta, Ni, Mg, Ca, Y, Zr, La, and
Ce are not essential elements, and are optional
~lements that may be appropriate~y contained in the
high-carbon steel sheet and the slab up to a specific
amount.
[0030] (Nb: 0.000% to 0.500%)
Nb forms a n~tride and is effective for
suppressing formation of BN. Thus, Nb may be
contained. However, when the Nb content is greater
than 0.500%, the ductility of fefrite is low to make
- 17 -
it impossible to obtain sufficient formability.
Thus, the Nb content is 0.500% or less. In order to
securely obtain the effect by the above-described
operation, the Nb content is preferably 0.001% or
more.
[0031] (Mo: 0.000% to 0.500%)
Mo is effective for improving hardenability.
Thus, Mo may be contained. However, when the Mo
content is greater than 0.500%, the ductility of
ferrite is low to make it impossible to obtain
sufficient formability. Thus, the Mo content is
0.500% or less. In order to securely obtain the
effect by the above-described operation, the Mo
content is preferably 0.001% or more.
[0032] (V: 0.000% to 0.500%)
V forms a nitride and is effective for
suppressing formation of BN similarly to Nb. Thus, V
may be contained. However, when the V content is
greater than 0.500%, the ductility of ferrite is low
to make it impossible to obtain sufficient
formability. Thus, the V cbntent is 0.500% or less.
In order to securely obtain the effect by the abovedescribed
operation, the V content is preferably
0.001% or more.
[0033]: (W: 0.000% to 0.500%)
W is effective for improving hardenability
similarly to Mo. Thus, W may be contained. However,
(
when the W content is greater than 0.500%, the
- 18 -
ductility of ferrite is low to make it impossible to
obtain sufficient formability. Thus, the W content
is 0.500% or less. In order to securely obtain the
effect by the above-described operation, the W
content is preferably 0.001% or more.
[0034] (Ta: 0.000% to 0.500%)
Ta forms a nitride and is effective for
suppressing formation of BN similarly to Nb and V.
Thus, Ta may be contained. However, when the Ta
content is greater than 0.500%, the ductility of
ferrite is low to make it impossible to obtain
sufficient formability. Thus, the Ta content is
0.500% or less. In order to securely obtain the
effect by the above-described operation, the Ta
content is preferably 0.001% or more.
[0035] (Ni: 0. 000% to 0. 500%)
Ni is effective for improving toughness and
improving hardenability. Thus, Ni may be contained.
However, when the Ni content is greater than 0.500%,
the coefficient of micro-friction of ferrite is high
to cause adhesion to the metal mold easily. Thus,
the Ni content is 0.500% or less. In order to
securely obtain the effect by the above-described
operation, the Ni content is preferably 0.001% or
mo:re.
[0036] (Mg: 0. 000% to 0. 500%)
Mg is ~effective for controlling the form of
sulfide. T h us, Mg may b e conta1. ne( d'' However, Mg
- 19 -
forms oxide easily, and when the Mg content is
greater than 0.500%, sufficient formability cannot be
obtained due to a crack originating from the oxide.
Thus, the Mg content is 0.500% or less. In order to
securely obtain the effect by the above-described
operation, the Mg content is preferably 0.001% or
more.
[0037] (Ca: 0.000% to 0.500%)
Ca is effective for controlling the form of
sulfide similarly to Mg. Thus, Ca may be contained.
However, Ca forms oxide easily, and when the Ca
content is greater than 0.500%, sufficient
formability cannot be obtained due to a crack
originating from the oxide. Thus, the Ca content is
0.500% or less. In order to securely obtain the
effect by the above-described operation, the Ca
content is preferably 0.001% or more.
[0038] (Y: 0.000% to 0.500%)
Y is effective for controlling the form of
sulfide similarly to Mg and Ca. Thus, Y may be
contained. However, Y forms oxide easily, and when
the Y content is greater than 0.500%, sufficient
formability cannot be obtained due to a crack
originating from the oxide. Thus, the Y content is
0.500% or less. In order to securely obtain the
effect by the above-described operation, the Y
content is preferably 0.001% or more.
(
[0039] (Zr: 0.000% to 0.500%)
- 20 -
Zr is effective for controlling the form of
sulfide similarly to Mg, Ca, and Y. Thus, Zr may be
contained. However, Zn forms oxide easily, an~~hen
the Zr content is greater than 0.500%, sufficient
formability cannot be obtained due to a crack
originating from the oxide. Thus, the Zr content is
0.500% or less. In order to securely obtain the
effect by the above-described operation, the Zr
content is preferably 0.001% or more.
[0040] (La: 0.000% to 0.500%)
La is effective for controlling the form of
sulfide similarly to Mg, Ca, Y, and Zr. Thus, La may
be contained. However, La forms oxide easily, and
when the La content is greater than 0.500%,
sufficient formability cannot be obtained due to a
crack originating from the oxide,. Thus, the La
content is 0.500% or less. In order to securely
obtain the effect by the above-described operation,
the La content is preferably 0.001% or more.
[ 0041] (Ce: 0.000% to 0.500%)
Ce is effective for controlling the form of
sulfide similarly to Mg, Ca, Y, Zr, and La. Thus, Ce
may be contained. However, Ce forms oxide easily,
and when the Ce content is greater than 0.500%,
sufficient formability cannot be obtained due to a
crack originating from the oxide,. Thus, the Ce
content is 0.500% or less. In ordeT.to securely
I
obtain the effect by the above-described operation, _
- 21 -
-----------~----------------·-··-
the Ce content is preferably 0.001% or more.
[0042] Thus, Nb, Mo, V, W, Ta, Ni, Mg, Ca, Y, Zr, La
and Ce are_ optional elements, and it is pref_erred
that "Nb: 0.001% to 0.500%," "Mo: 0.001% to 0.500%,"
"V: 0.001% to 0.500%," "W: 0.001% to 0.500%," "Ta:
0.001% to 0.500%," "Ni: 0.001% to 0.500%," "Mg:
0.001% to 0.500%," "Ca: 0.001% to 0.500%," "Y: 0.001%
to 0.500%," "Zr: 0.001% to 0.500%," "La: 0.001% to
0.500%," or "Ce: 0.001% to 0.500%," or any
combination thereof be satisfied.
[0043] Next, the coefficient of micro-friction of
ferrite on the surface of the high-carbon steel sheet
according to the embodiment is explained. The
coefficient of micro-friction of ferrite on the
surface of the high-carbon steel sheet according to
the embodiment is less than 0.5.
[0044] (Coefficient of micro-friction of ferrite on
the surface: less than 0.5)
The coefficient of micro-friction of ferrite on
the surface closely relates to adhesion of the highcarbon
steal sheet to the metal mold during .forming.
When the coefficient of micro-friction of ferrite is
0.5 or more, micro-adhesion occurs between the highcarbon
steel sheet and the metal mold during forming
using the metal mold. :As a result, when forming such
as punching is performed with several thousands to
_several tens of thousands of shots by using the metal
I
mold, adhesive matters are accumulated on the metal
- 22 -
--------------
mold during the forming, and a flaw occurs on either
the metal mold or the high-carbon steel sheet or on
the both and formability deteriorates. .Thus, the
coefficient of micro-friction of ferrite is less than
0 . 5 . From the viewpoint of formability, the
coefficient of micro-friction is the lower the
better. The coefficient of micro-friction often
tends to be 0.35 or more, though it depends on a
method of manufacturing the high-carbon steel sheet
and others.
[0045] Fig. 2 is a chart illustrating a relationship
between a coefficient of micro-friction of ferrite
and a number of pressing (shots) until a flaw occurs
on a metal mold or a high-carbon steel sheet in punch
forming of high-carbon steel sheets. As illustrated
in Fig. 2, when the coefficient of micro-friction is
less than 0.5, the number of pressing until a flaw
occurs is significantly high as compared to the case
when it is 0.5 or more.
[0046] A coefficient of micro-friction may be
measured Using a nanoindenter. That is~ a kinetic
friction force F to occur when a diamond indenter
loads a normal load P of 10 pN onto a surface of a
high-carbon steel sheet and is moved horizontally is
obtained. A moving speed then is l pro/second, for
example. A coefficient of micro-friction p (kinetic
friction coefficient) is calculated by Expression ( l)
below.
I
"TI-900 Triboindenter" made by Omicron, Inc.
- 23 -
may be used as a nanoindenter, for example.
F pP Expression(l)
[ 0 0.4 7 l Fig. 3A is a micrograph showing a surface of
a high-carbon steel sheet before measuring a
coefficient of micro-friction, and Fig. 38 is a
micrograph showing the surface of the high-carbon
steel sheet after measuring the coefficient of microfriction.
Fig. 3A and Fig. 38 each show an example
of a 10 pm x 10 pm visual field. As shown in Fig. 3A
and Fig. 38, ferrite 31 and cementite 32 exist in the
visual field example. Further, as shown in Fig. 38,
measurement flaws 33 caused by horizontal movement of
the diamond indenter exist after the measurement.
The coefficient of micro-friction of cementite was
0.4 or less.
[0048] Next, a structure of the high-carbon steel
sheet according to the embodiment is explained. The
high-carbon steel sheet according to the embodiment
includes a structure represented by a spheroidized
ratio of cementite: 80% or more and an average
diameter of cementite: 0.3 pm to 2.2 pm.
[0049] (Spheroidized ratio of cementite: 80% or
more)
Stress concentration sometimes originates from
cementite during forming, and stress is likely to
concentrate locally in acicular cementite
particularly. When the spheroidized ratio of
cementite is less than 80%, acl. cu( l ar cement.l te, in
- 24 -
--------------
which stress is likely to concentrate, is contained
in large amounts, and thus stress concentration
occurs easily and peeling occurs at an interface
between ferrite and cementite, rssulting in that
sufficient formability cannot be obtained. Thus, the
spheroidized ratio of cementite is 80% or more, and
preferably 85% or more. From the viewpoint of
formability, the spheroidized ratio of cementite is
preferred to be as higher as possible, and may be
100%. However, when the spheroidized ratio of
cementite is attempted to become 100%, productivity
could decrease, and the spheroidized ratio of
cementite is preferably 80% or more and less than
100% from the viewpoint of productivity.
[0050] (Average diameter of cementite: 0.3 pm to 2.2
pm)
The average diameter of cementite closely relates
to the degree of the stress concentration to
cementite. When the average diameter of cementite is
less than 0.3 pm, an Orowan loop is formed by
- dislocation occurred during forming with respect to
cementite, and thereby a dislocation density in the
vicinity of cementite increases and voids occur.
Thus, the average diameter of cementite is 0.3 pm or
more, and preferably 0.5 pm or more. When the
average diameter of cementite is greater than 2.2 pm,
dislocations occurred during forming are accumulated
in large amounts, local stress cdncentration is
- 25 -
generated and a crack occurs. Thus, the average
diameter of cementite is 2.2 ~m or less, and
preferably 2.0 ~m or less.
[0051] The spheroidized ratio and the average
diameter of cementite may be measured by structure
observation using a scanning electron microscope. In
preparing of a sample for structure observation, an
observation surface is mirror finished by wet
polishing with an emery paper and polishing with
diamond abrasive grains having a size of 1 ~m, then
the observation surface is etched with an etching
solution of 3 vol% of nitric acid and 97 vol% of
alcohol. An observation magnification is between
3000 times to 10000 times, for example, 10000 times,
16 visual fields where 500 or more grains of
cementite exist on the observation surface are
selected, and structure images of them are taken.
Then, an area of each cementite in the structure
image is measured by using image processing software.
"Win ROOF" made by MITANI Corporation may be used as
an image processing software,~ for example. Any
cementite grain having an area of 0.01 ~m 2 or less is
excluded from the target of evaluation in order to
suppress an influence of measurement error by noiBe
in the:measuring. 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 ~btained is obtained, thereby taking this
- 26 -
diameter as the average diameter of cementite. The
average area of cementite is a value obtained by
dividing the total area of cemerttite as the
evaluation target by the number of grains of
cementite in question. Further, any cementite having
a ratio of major axis length to minor axis length of
3 or more is assumed as an acicu~ar cementite, any
cementite having the ratio of less than 3 is assumed
as a spherical cementite grain, and a value obtained
by dividing the number of spherical cementite by the
number of all cementite is taken as the spheroidized
ratio of cementite.
[0052] Next, a method of manufacturing the highcarbon
steel sheet according to the embodiment is
explained. The manufacturing method includes hotrolling
of a slab including the above chemical
composition so as to obtain a hot-rolled steel sheet,
pickling of the hot-rolled steel sheet, and
thereafter annealing of the hot-rolled steel sheet.
In the hot-rolling, the slab is heated at a
temperature of l000°C or more and less than 1150°C, a
finish rolling temperature is 830°C or more and 950°C
or less, and a coiling temperature is 450°C or more
and 700°C or less. In the annealing, the hot~rolled
steel sheet is retained at a temperature of 730°C or
more and 770°C or less for 3 hours or more and 60
hours or less, and then, the hot-ro'lled steel sheet
(
is'cooled down to 650°C at a cooling rate of l°C/hr gr
- 27 -
more and 60°C/hr or less. An atmosphere of the
annealing may be one containing hydrogen by 75 vol%
or more at a temperature higher than 400°C, for
example, but is not limited to that.
[0053] Here, an outline of changes in the steel
sheet from the hot-rolling to the cooling is
explained. Fig. 4 is a schematic diagram
illustrating changes in temperature. Fig. SA to Fig.
5E are schematic diagrams illustrating changes in
structure.
[0054] In an example illustrated in Fig. 4, hotrolling
Sl includes slab heating Sll, finish rolling
Sl2, and coiling S13, and annealing S3 includes hightemperature
retention S31 and cooling S32. Pickling
S2 is performed between the hot-rolling Sl and the
annealing S3, and after cooling S4 is performed the
annealing S3.
[0055] At a time tA after completion of the slab
heating Sll, B atoms 13 segregate to an interface
between austenite 12 and austenite 12, as illustrated
in Fig. SA. At a time t 8 after completion of the
high-temperature retention S31, the structure of the
steel sheet contains ferrite 11 and the austenite 12,
as illustrated in Fig. 5B. Further, the B atoms 13
segregate to an interface between the .ferrite 11 and
the austenite 12. Some of the B atoms 13 are present
also on a surface 15 of the steel sheet, and the B
(
atoms 13 present on the surfac~ of the steel sheet
- 28 -
are bonded to each other by covalent bonding 14.
Although not illustrated in Fig. 5B, cementite is
also contained. in the structure of the steel sheet
and some of the B atoms 13 segregate also to an
interface between the ferrite 11 and the cementite.
At a time tc in a middle of the cooling S32, the ratio
of the ferrite 11 increases and the ratio of the
austenite 12 decreases as compared to the structure
illustrated in Fig. 5B, as illustrated in Fig. 5C,
and the interface between these two phases moves due
to the increasing and decreasing or the ratios.
Also, the B atoms 13 present on the surface of the
steel sheet increase with the movement of the
interface. Further, at a time tn when the cooling S32
has advanced, the ratio of the ferrite 11 increases,
the ratio of the austenite 12 decreases, and the B
atoms 13 present on the surface of the steel sheet
increase as compared to the structure illustrated in
Fig. 5C, as illustrated in Fig. 5D. Then, at a time
tE when the temperature of the steel sheet has reached
650°C 1 the austenite 12 disappears and the surface 15
of the steel sheet is covered with many of the B
atoms 13, as illustrated in Fig. 5E. Since the B
atoms 13 are bonded to each other by the covalent
bonding 14, they are crystallized. The structure
illustrated in Fig. 5E does not change also during
the cooling S4, and is maintained even when the
I temperature of the steel sheet has reached room
- 29 -
-~--------
temperature, for example, a temperature of less than
[0056] Acc.ording to the manufacturing method, the
surface 15 of the steel sheet is covered with many of
the B atoms 13 bonded to each other by the covalent
bonding 14, and thereby the coefficient of microfriction
of ferrite on the surface 15 can be less
than 0.5.
[0057] (Slab heating temperature: lOOO"C or more and
less than 1150"C)
[0058] When the slab heating temperature is higher
than 1150"C, oxygen easily diffuses into the inside of
the slab from the surface of the slab to bond to B in
the slab. That is, as illustrated in Fig. 6A, the B
atoms 13 are consumed due to bonding to 0 atoms 16.
Therefore, even though a process thereafter is
performed appropriately, it is not possible to obtain
a good surface covered with crystals of B, resulting
in that the coefficient of micro-friction of ferrite
on the surface cannot be less than 0.5. Thus, the
slab heating temperature is 1150"C or less, ~nd
preferably 1140"C or less. When the slab heating
temperature is lower than lOOO"C, micro-segregation
-and/or macro-segregation formed during casting cannot
be eliminated, and as illustrated in Fig. 6B,
solidification segregations of the B atoms 13 remain.
The solidification segregations of the B atoms 13
I
cannot be eliminated even though a process thereafter
- 30 -
is performed appropriately, and therefore, it is not
possible to obtain a good surface covered with
crystals of B, resulting in that the coefficient of
micro-friction of ferrite on the surface cannot be
less than 0.5. Further, when the slab heating
temperature is lower than 1000°C, regions where Cr
atoms and/or Mn atoms segregate and concentrate also
remain in the high-carbon steel sheet. Therefore,
even though a process thereafter is performed
appropriately, cracks occur from the regions during
forming, thus making it impossible to obtain good
formability. Thus, the slab heating temperature is
1000°C or more, and preferably 1030°C or more.
[0059] (Finish rolling temperature: 830°C or more and
950°C or less)
When the finish rolling temperature is higher
than 950°C, coarse scales are generated until
completion of coiling on a run out table (ROT), for
example. The coarse scales can be removed by
pickling, but traces of large irregularities are
left, resulting in that adhesion to ~the metal mold
sometimes occurs during forming due to the traces.
Further, when coarse scales are generated, irregular
flaw is caused on the surface of the steel sheet in
the coiling, resulting in that due to the !law,
adhesion to the metal mold sometimes occurs during
forming. Thus, the finish rolling temperature is
95 0°C or less, and preferably
I
94 0°C or less. When the
- 31 -
--------
finish rolling temperature is lower than 830°C,
adhesiveness of scales generated until completion of
co_iling to the steel sheet is extremely high, thus
making it difficult to remove the scales by pickling.
The scales may be removed by performing strong
pickling, but the strong pickling makes the surface
of the steel sheet rough, resulting in that adhesion
to the metal mold sometimes occurs during forming.
Further, when the finish rolling temperature is lower
than 830°C, recrystallization of austenite is not
completed by the coiling, so that anisotropy of the
hot-rolled steel sheet increases. The anisotropy of
the hot-rolled steel sheet is carried over even after
annealing, and thus sufficient formability cannot be
obtained. Thus, the finish rolling temperature is
830°C or more, and preferably 840°C or more.
[0060] (Coiling temperature: 450°C or more and 700°C
or less)
When the coiling temperature is higher than
700°C, coarse lamellar pearlite is formed in the hotrolled
steel sheet to hinder spheroidizing of
cementite during annealing, resulting in that the
spheroidized ratio of 80% or more cannot be obtained.
Thus, the coiling temperature is 700°C or less.
Further, when the coiling temperature is highe~ than
570°C, coarse scales are generated until completion of
coiling. Therefore, adhesion to the metal mold
I
sometimes occurs during forming for a reason similar
- 32 -
to the case where the finish rolling temperature is
higher than 950°C. Thus, the coiling temperature is
preferably 570°C or less, and further preferably 550°C
or less. When the coiling temperature is lower than
450°C, adhesiveness of scales generated until
completion of coiling to the steel sheet is extremely
high, thus making it difficult to·remove the scales
by pickling. The scales may be removed by performing
strong pickling, but the strong pickling makes the
surface of the steel sheet rough, resulting in that
adhesion to the metal mold sometimes occurs during
forming. Further, when the coiling temperature is
lower than 450°C, the hot-rolled steel sheet becomes
brittle and the hot-rolled steel sheet may crack when
a coil is uncoiled in pickling, resulting in that a
sufficient yield cannot be obtained. Thus, the
coiling temperature is 450°C or more, and preferably
460°C or more.
[0061] A rough-rolled bar may be heated near an
inlet of a finishing mill in order to ensure
:qualities in a longitudinal d~rection and a width
direction of a hot-rolled coil obtained by coiling(to
reduce variation of quality or the like). An
apparatus to be used for the heating and a method of
the heating are nbt limited in particular, but
heating by high-frequency induction heating is
desirably performed. A preferred temperature range
(
of the heated rough-rolled bar is between 850°C and
- 33 -
Temperatures less than 850°C are close to a
transformation temperature from austenite to ferrite,
and therefore, when the temperature of the heated
rough bar is lower than 850°C, heat generation and
heat absorption due to transformation and reverse
transformation sometimes occur, resulting in that
temperature controlling is unstable and it is
difficult to uniformize a temperatures in the
longitudinal direction and the width direction of the
hot-rolled coil. Therefore, if the rough-rolled bar
is heated, the heating temperature is preferably 850°C
or more. Increasing the temperature of the roughrolled
bar to temperature higher than 1100°C takes
excessive time, and the productivity decreases.
Therefore, if rough-rolled bar is heated, the heating
temperature is preferably 1100°C or less:
[0062] (Annealing retention temperature: 730°C or
more and 770°C or less)
When the annealing retention temperature is lower
than 730°C, the austenite 12 is not formed
sufficiently, and as illustrated in Fig. 6C, although
a large number of interfaces between the ferrite 11
and the ferrite 11 exist, sites where the B atom 13
segregates are insufficient. Therefore, even though
a process thereafter is performed appropriately, a
good surface covered with crystali of B cannot be
obtained, resulting in that the coefficient of microfriction
of ferrite
(
on the surface cannot be less
- 34 -
----------
than 0.5. Further, when the annealing retention
temperature is lower than 730°C, segregation of the B
atom 13 to the interface between the ferrite 11 and
cementite does not occur easily, and therefore,
segregating the B atoms 13 sufficiently takes an
extremely long time, which is about 100 hours, and
the productivity decreases. Thus, the annealing
retention temperature is 730°C or more, and preferably
735°C or more. When the annealing retention
temperature is higher than 770°C, as illustrated in
Fig. 6D, the B atoms 13 concentrate and coarse
crystals of B are formed in the vicinity of the
triple point of the ferrite 11, the austenite 12, and
the surface of the steel sheet. When coarse crystals
of B are formed, even though a process thereafter is
performed appropriately, the thickness of a film of
the crystals of B varies greatly, resulting in that
the coefficient of micro-friction of ferrite on the
surface cannot be less than 0.5. Further, when the
annealing retention temperature is higher than 770°C,
thermal expansion of the hot-rolled steel sheet
coiled in a coil shape is large, and the hot-rolled
steel sheet itself sometimes rubs together during
annealing to· cause abrasions on the surface. The
appearance of the surface is impained and the yield
is decreased by the abrasions. Thus, the annealing
retention temperature is 770°C or less, and preferably
765°C or less.
(
- 35 -
[0063] (Annealing retention time: 3 hours or more
and 60 hours or less)
When the anneaLing retention time is less than .3
hours, as illustrated in Fig. 6E, the B atoms 13 do
not sufficiently segregate to the interface between
the ferrite 11 and the austenite 12, and therefore,
even though a process thereafter is performed
appropriately, a good surface covered with crystals
of B cannot be obtained, resulting in that the
coefficient of micro-friction of ferrite on the
surface cannot be less than 0.5. Further, when the
annealing retention time is less than 3 hours,
cementite does not become coarse sufficiently,
resulting in that the average diameter of cementite
cannot be 0.3 pm or more. Thus, the annealing
retention time is 3 hours or more, and preferably 5
hours or more. When the annealing retention time is
greater than 60 hours, the coefficient of microfriction
of ferrite on the surface cannot be less
than 0.5 for a reason similar to the case where the
annealing retention temperature is higher than 770"C~
Further, when the annealing retention time is greater
than 60 hours, cementite becomes coarse excessively,
resulting in that the average diameter of cementite
cannot be 2.2 pm or less. Thus, the annealing
retention time is 60 hours or less, and preferably 40
hours or. less.
(
[0064] (Cooling rate down to 6~o~c:
- 36 -
l"C/hr or more
and 60°C /hr or less)
When the cooling rate down to 650°C is less than
1 °C/hr, as illustrated in Fig. 6F, crystals of B are
formed excessively during cooling and the crystals of
B form a projection on the surface of the high-carbon
steel sheet. Once a projection is formed, the
thickness of the film of the crystals of B varies
greatly, resulting in that adhesion to the metal mold
occurs during forming and a flaw occurs on the metal
mold. Further, when the cooling rate down to 650°C is
less than 1°C/hr, sufficient productivity cannot be
obtained. Thus, the cooling rate down to 650°C is
1°C/hr or more, and preferably 2°C/hr or more. When
the cooling rate down to 650°C is greater than
60°C/hr, a decrease rate of the austenite 12 is
excessive, and as illustrated in Fig. 6G, the
sufficient covalent bonding 14 cannot be caused
between the B atoms 13, resulting in that the
coefficient of micro-friction of ferrite on the
surface cannot be less than 0.5. Further, when the
cooling rate ddwn to 650°C is greater than 60°C/hr,
pearlite is formed from the austenite 12 during
cooling to hinder spheroidizing of cementite,
resulting in that the spheroidized ratio of 80% or
more cannot be obtained. Thus, the cooling rate down
to 650°C is 60°C/hr or less, and 50°C/or less.
[0065] According to the embodiment, excellent
lubricity can be obtained, and t~~refore it is
- 37 -
~~~~~~~~~------
possible to suppress adhesion of the high-carbon
steel sheet to the metal mold and suppress wearing of
the metal mold. Further, according to the
embodiment, it is also possible to suppress cracking
during forming.
[0066] It should 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
[0067] 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 spirit of the invention.
[0068] (First experiment)
In a first experiment, hot-rolling of a slab
(Steel type A to Y, BK) including a chemical
.composition listed in Table 1 was performed, thereby
(
obtaining a hot-rolled steel sheet having a thickness
- 38 -
-------
of 4 mm. In the hot-rolling, the slab heating
temperature was ll30°C, the time thereof was 1 hour,
the finish rolling temperature ~as 850°C, and the
coiling temperature was 520°C. Then, cooling was
performed down to a temperature of less than 60°C, and
pickling using sulfuric acid was performed.
Thereafter, annealing of the hot-rolled steel sheet
was performed to then obtain a hot-rolled annealed
steel sheet. In the annealing, the hot-rolled steel
sheet was retained for 15 hours at 750°C, and then was
cooled down to 650°C at a cooling rate of 30°C/hr.
Subsequently, cooling was performed down to a
temperature of less than 60°C. In this manner,
various high-carbon steel sheets were manufac~ured.
Blank fields in Table 1 indicate that the content of
the element is less than a detection limit, and the
balance is Fe and impurities. For example, the Cr
content of Steel type BK may be regarded as 0.00%.
An underline in Table 1 indicates that the numeric
value is out of the range of the present invention.
[0069].·- ·[Table 1]
(
- 39 -
STEEL CHEMICAL COMPONENT (MASS%) Aol Ao3
TYPE c REMARKS Si Mn p s AI N Ti Cc B Nb Mo V Co W To Ni M Co Y Zc Lo Co ("C) ("C)
A 0.32 0.78 1.21 0.023 0.0062 0.088 0.003 0.203 0.11 0.0022 732 850
INVENTIVE.
EXAMPLE
B 0.38 0.41 0.61 0.014 0.0064 0.035 0.002 0.332 0.39 0.0029 737 810
INVENTIVE
EXAMPLE
c 0.41 0.32 1.40 0.013 0.0051 0.085 0.004 0.157 0.77 0.0018 731 811
JNVENTJVE
' EXAMPLE
D 0.47 0.08 0.27 0.015 0.0085 0.072 0.006 0.282 1.22 0.0015 743 781
INVENTIVE
EXAMPLE
E 0.53 0.19 1_67 0.010 0.0039 0.060 0.003 0.348 0.54 0.0011 714 756
JNVENTIVE
EXAMPLE
F 0.61 0.74 1.86 O.ot7 0.0097 0.048 0.006 0.046 1.32 0.0023 748 788
INVENTIVE
EXAMPLE
G 0.64 0.97 0.99 0.002 0.0056 0.082 0.003 0-499 0.05 0.0005 735 766
INVENTIVE
EXAMPLE
H 0.64 0.47 2.92 0.021 0.0003 O.Q11 0.008 0.163 0.47 0.0009 715 758
INVENTIVE
EXAMPLE
I 0.67 0.60 1.48 0.024 0.0041 0.053 0.006 0.421 0.88 0.0008 740 755
INVENTIVE
EXAMPLE
J ~ 0.16 2.46 0.005 0.0006 0.027 0.002 0.042 0.88 0.0011 709 818
COMPARATIVE
EXAMPLE
K 0.31 0.17 2.34 0.006 0.0056 0.049 0.011 0.061 0.67 0.0016 711 819
COMPARATIVE
EXAMPLE
L 0.32 0.55 1.15 0.002 0.0017 0.110 0.005 0.194 0.29 0.0019 731 840
COMPARATIVE
EXAMPLE
M 0.32 0.27 0.42 0.022 0.0094 0.006 0.005 0.057 1.46 0.0036 747 828
COMPARATIVE
EXAMPLE
N 0.36 0.60 0.18 0.015 0.0045 0.008 0.009 0.138 0.57 0.0011 750 838
COMPARATIVE
EXAMPLE
0 0.39 0.61 2.70 0.026 0.0003 0.024 0.002 0.056 1.20 0.0033 728 814
COMPARATIVE
EXAMPLE
p 0.40 1.05 1.97 0.001 0.0095 0.070 0.006 0.445 0.36 0.0024 740 823
COMPARATIVE
EXAMPLE
Q 0.40 0.67 0.58 0.006 0.0012 0.033 0.002 0.381 0.21 0.0001 740 814
COMPARATIVE
EXAMPLE
R 0.41 021 1.74 0.006 0.0092 0.098 0.002 0.435 1.00 0.0002 725 782
COMPARATNE
EXAMPLE
s 0.49 0.05 0.53 o.ot8 0.0053 0.086 0.002 0.321 0.91 0.0014 730 764
COMPARATIVE
EXAMPLE
T 0.53 0.35 1.72 0.003 0.0122 0.055 0.008 0.201 0.42 0.0034 m 778
COMPARATNE
EXAMPLE
u 0.65 0.78 3.10 0.005 0.0052 0.011 0.004 0.332 0.02 0.0013 709 756
COMPARATIVE
EXAMPLE
v 0.70 0.58 0.60 O.ot9 0.0048 0.046 0.002 0.136 1.55 0.0029 758 775
COMPARATIVE
EXAMPLE
w 0.78 0.91 2.01 0.005 0.0046 0.076 0.009 0.487 1.37 0.0028 747 751
COMPARATIVE
EXAMPLE
X 0.42 0.24 0.58 0.011 0.0062 0.024 0.005 0.006 0.88 0.0022 738 808
COMPARATIVE
EXAMPLE
y 0.47 0.99 0.51 0.019 0.0093 0.091 0.009 0.519 1.06 0.0030 760 804
COMPARATIVE
EXAMPLE
BK 0.37 0.38 1.24 O.Dl8 0.0088 0.063 0.007 0.121 0.0022 721 816
COMPARATIVE
EXAMPLE
[0070] Then, the coefficient of micro-friction of
.. ferrite, and the spheroidized ratio artd the average
diameter of cementite of each of the high-carbon
steel sheets were measured. A friction coefficient
of cementite was also measured in measuring the
coefficient of .micro-friction of ferrite. Results of
them are listed in Table 2. An underline in Table 2
indicates that the
present invention.
item is out of the
I
- 40 -
range of the
[0071] Further, evaluation of adhesion suppressive
performance and evaluation of crack sensitivity of
each of the high-carbon steel sheets were performed
as formability evaluation. In the evaluation of
adhesion suppressive performance, a draw bead test
was performed. That is, an indentation bead with a
tip having a 20 mm radius R was pressed against the
high-carbon steel sheet with a load of 10 kN and was
pulled out. Then, presence or absence of adhesive
matter on the tip of the indentation bead was
observed, and one with presence of adhesive matter
was evaluated as X and one with no presence was
evaluated as 0. The presence of adhesive matter in
this test indicates that in press forming with
several thousands to several tens of thousands of
shots, an adhesive matter occurs early on the metal
mold to deteriorate formability. In the evaluation
of crack sensitivity, a compression test was
performed. That is, a cylindrical test piece having
a 10 mm diameter and a 4 mm height was cut out from
-the high-carbon steel sheet so that a height
direction of the test piece was parallel to a sheet
thickness direction, and the test piece was
compressed to 1 mm in height. Then, an appearance
observation and a sectional structure observation
were performed, and then one in which cracking
appeared in the appearance during compression or
. h' 1 after compression and one ln w lC h a crac k o f 1 mm or
- 41 -
more was present in the sectional structure
observation were evaluated as X, and one other than
the above was evaluated as 0. Results of them are
also listed in Table 2.
[0072] [Table 2]
COEFFICIENT COEFFICIENT SPHEROIDIZED AVERAGE
SAMPLE ADHESION STEEL OF MICRO- OF MICRO- RATIO OF DIAMETER OF CRACK
No. TYPE FRICTION FRICTION CEMENTITE CEMENTITE SUPPRESSIVE SENSITIVITY
REMARKS
OF FERRITE OF CEMENTITE (%) ( m)' PERFORMANCE
1 A 0.40 0.24 80.4 0.76 0 0 INVENTIVE
EXAMPLE
2 B 0.43 0.25 80.4 1.13 0 0 INVENTIVE
EXAMPLE
3 c 0.40 0.31 86.5 0.62 0 0 INVENTIVE
EXAMPLE
4 D 0.42 0.23 83.8 0.86 0 0 INVENTIVE
EXAMPLE
5 E 0.42 0.32 95.5 0.69 0 0 INVENTIVE
EXAMPLE
6 F 0.41 0.33 90.0 0.42 0 0 INVENTIVE
EXAMPLE
7 G 0.49 0.23 85.4 0.96 0 0 INVENTIVE
EXAMPLE
8 H 0.44 028 98.7 0.52 0 0 INVENTIVE
EXAMPLE
9 1 0.42 0.33 94.8 0.56 0 0 INVENTIVE
EXAMPLE
10 ,! 0.42 0.32 92.6 0.56 X 0 COMPARATIVE
EXAMPLE
11 lS 0.72 0.25 91.4 0.59 X X
COMPARATIVE
EXAMPLE
12 b 0.42 0.23 81.7 0.85 0 X
COMPARATIVE
EXAMPLE
13 M 0.42 0.29 85.9 0.73 0 X
COMPARATIVE
EXAMPLE
14 M 0.41 0.30 75.6 1.04 0 X
COMPARATIVE
EXAMPLE
15 Q 0.42 0.31 90.9 0.36 0 X
COMPARATIVE
EXAMPLE
16 !:'. 0.41 0.23 82.4 0,54 0 X
COMPARATIVE
EXAMPLE
17 Q 0.79 0.24 87.7 1.05 X X
COMPARATIVE
EXAMPLE
18 B 0.79 0.33 92.5 •. . 0.55 X X
COMPARATIVE
- EXAMPLE COMPARATIVE
19 !i 0.44 0.27 85.2 2.56 0 X EXAMPLE
20 I 0.41 0.31 91.2 0.71 0 X
COMPARATIVE
EXAMPLE
21 1! 0.41 0.28 98.0 0.52 0 X
COMPARATIVE
EXAMPLE
22 Y.. 0.44 0.31 65.8 0.26 0 X
COMPARATIVE
EXAMPLE
23 '!:1 0.45 0.25 98.9 0.30 0 X
COMPARATIVE
EXAMPLE
' ' COMPARATIVE
24 X 0.62 0.28 85.3 0.75 X X EXAMPLE
25 y 0.42 0.27 82.2 0.59 0 X
COMPARATIVE
EXAMPLE
26 BK ·. 0.69 0.29 80.6 0.92 X 0 COMPARATIVE
EXAMPLE
I
- 42 -
[0073] As listed in Table 2, Sample No. 1 to Sample
No. 9 were each within the range of the present
invention, thus being able to obtain good adhesion
suppressive performance and crack sensitivity.
[0074] On the other hand, in Sample No. 10, the C
content of Steel type J was too low, and thus the
amount of cementite was insufficient, sufficient
lubricity was not able to be obtained, and adhesion
to the metal mold occurred during forming. In Sample
No. 11, the N content of Steel type K was too high,
and thus BN precipitated, the amount of solidsolution
B was insufficient, the coefficient of
micro-friction of ferrite was low, and adhesion and
cracking during the compression test occurred. In
Sample No. 12, the Al content of Steel type L was too
high, and thus the ductility of ferrite was low and a
crack originating from transgranular fracture of
ferrite occurred during the compression test. In
Sample No. 13, the B content of Steel type M was too
high, and thus boride was formed and a crack
originating from the boride is occurred during-the
compression test. In Sample No. 14, the Mn content
of Steel type N was too low, and thus pearlite
transformation occurred during cooling in the
annealing, the spheroidized ratio o~ cementite was
low, and a crack originating from acicular cementite
occurred during the compression test. In Sample No.
i 15,· the P content of Steel type 0 was too high, and
- 43 -
thus segregation of B to the interface between
ferrite and cementite was hindered and cracking
occurred during the compression test. In Sample No.
16, the Si content of Steel type P was too high, and
thus the ductility of ferrite was low and a crack
originating from transgranular fracture of ferrite
occurred during the compression test. In Sample No.
17 and Sample No. 18, each B content of Steel type Q
and Steel type R was too low, and thus the
coefficient of micro-friction of ferrite was low and
adhesion and cracking during the compression test
occurred. In Sample No. 19, the Si content of Steel
type S was too low, and thus cementite became coarse
excessively during annealing and a crack originating
from the coarse cementite occurred during the
compression test. In Sample No. 20, the S content of
Steel type T was too high, and thus coarse sulfides
being non-metal inclusions were formed and a crack
originating from the coarse' sulfide occurred during
the compression test. In Sample No. 21, the Mn
content of Steel type U was too high, and thus the
ductility of ferrite was low and a crack originating
from transgranular fracture of ferrite occurred
during the compression test. In Sample No. 22, the
Cr content of Steel type V was too high, and thus
spheroidizing of cementite during annealing was
hindered, coarsening of cementite was suppressed, and
a crack originating from micro a6icular cementite
- 44 -
occurred during the compression test. In Sample No.
23, the C content of Steel type W was too high, and
thus the amount of cementite was excessive and B
crack originating from the cementite occurred during
the compression test. In Sample No. 24, the Ti
content of Steel type X was too low, and thus BN
precipitated, the amount of solid-solution B was
insufficient, the coefficient of micro-friction of
ferrite was low, and adhesion and cracking during the
compression test occurred. In Sample No. 25, the Ti
content of Steel type Y was too high, and thus coarse
oxides of Ti were formed and a crack originating from
the coarse oxide of Ti occurred during the
compression test. In Sample No. 26, the Cr content
of Steel type BK was too low, and thus BN
precipitated, the amount of solid-solution B was
insufficient, the coefficient of micro-friction of
ferrite was low, and adhesion to the metal mold
occurred during forming.
[0075] (Second experiment)
In a se~ond experiment, hot-rolling of a slab
(Steel type Z to BJ) including a chemical composition
listed in Table 3 was performed, thereby obtaining a
hot-rolled steel sheet having a thickness of 4 mm.
In the hot-rolling, the slab heating temperature was
1130°C, the time thereof was 1 hour, the finish
rolling temperature was 850°C,
temperature was 520°C. Then,
and the coiling
(
cooling was performed
- 45 -
down to a temperature of less than 60"C, and pickling
using sulfuric acid was performed. Thereafter,
annealing of the hot-rolled steel sheet was~performed
to then obtain a hot-rolled annealed steel sheet. In
the annealing, the hot-rolled steel sheet was
retained for 15 hours at 750"C, and then was cooled
down to 650"C at a cooling rate of 30"C /hr.
Subsequently, cooling was performed down to a
temperature of less than 60"C. In this manner,
various high-carbon steel sheets were manufactured.
Blank fields in Table 3 indicate that the content of
the element is less than a detection limit, and the
balance is Fe and impurities. An underline in Table
3 indicates that the numeric value is out of the
range of the present invention.
[ 007 6] [Table 3]
i
- 46 -
STEEL CHEMICAL COMPONENT (MASS~)
TYPE c s; Mo ' s A; " T; Cc 8 "" Mo v Co w To "' M c. y a "' Co
z 0.34 0.52 2.38 0.015 0.0005 0.006 0.007 0.220 1.09 0.0017 0.004
AA 0.35 0.81 2.20 0.013 0.0010 0.080 0.004 0.301 OE3 0.0020 0.1!12 0.003 0.032 0.047
Ae 0.37 022 0.31 0.022 0.0017 O.o24 0.007 0.078 0.99 0.0011 0.100 0.326 0.163 0.101 0.005 0.002 0.004
AC 0.39 0.11 0.42 0.022 0.0059 O.D76 0.005 0.192 0.18 0.0020 0.355 0.395 0.056 0.330 0.114 0.371 0242 0.311
AD 0.44 0.57 '"' 0.019 0.0024 11030 0.002 0.189 0.72 0.0013 0.108 0.286 0.114 0.225 0.263 0.467 0.272 0.481
AE 0.47 0.37 0.49 O.ot8 0.0027 0.008 0.001 0.119 0.46 0.0008 0.313 0.044 0.081 0.441 0.129 0.122 0.339 0.003
AF 0.50 0.26 0.90 0.005 0.0048 0.041 0.003 O.o35 0.29 0.0031 0.037 0.002 o_ooz
AG 0.52 0.65 2.05 0.007 0.0077 0.016 0.001 0.471 1.411 0.0011 0.040 0.258 0.271 0.183 0.002
AH 0.57 0.13 0.51 0.002 0.0062 0.066 0.003 0.169 O.D2 0.0025 0.350 0.130 0.383 0.287 0.327 0.007 0.329 0.278
A> 0.57 0.89 2.67 0.001 0.0041 0.020 0.009 O.D19 0.91 0.0016 0.080 0.4115 0.005 0.491 0.490 0.443
'" 0.62 0.43 0.78 0.009 0.0078 0.097 0.003 0.365 0.63 0.0033 0.156 0.235 0.002 0.1!17
AK 0.66 0.6!1 "'' 0.006 0.0054 0.093 0.001 0.255 0.20 0.0010 0.377 0.049 0.430 O.D23 0.005 0.268 O.ot1 0.006 0.199
AC 0.66 0.63 1.86 0.003 0.0071 0.079 0.00! 0.011 1.38 0.0007 0.004 0.019 0.003 0.459
AM m 0.27 2.88 0.023 0.0081 {}.018 0.006 0.021 0.113 0.0031 0.081 0.2.32 0.454 0.086 0.!80 0.281 0.128 0.027
AN 0.34 0.87 2.26 0.006 0.0055 0.064 0.001 0.418 1.27 0.0004 0.082 0.506 0.126
AO 0.34 0.59 0.99 0.023 0.0021 o.oeo 0.005 0.296 0.54 0.0033 0.316 0.191 0.447 0.185 Q&lQ
Ae 0.35 0.27 0.97 0.004 0.0051 0.091 0.007 0.311 0.91 0.0006 ~ 0.236
AO 0.36 0.9!1 2.37 0.022 0.0092 0.026 0.006 omo 0.30 0.0003 0.183 0.471 0.126 0.09!1 0.107
AR 0.37 0.78 2.77 O.Q23 0.0029 0.064 0.002 0.062 0.35 0.0024 0.511 0.473 0.291
AS 0.38 0.63 o.n 0.022 0.0001 0.091 0.002 0.352 0.27 0.0016 0.196 O.Q38 0.044 0.005 O.ot 1 0.255 O.ot3
AT 0.41 0.71 0.43 0.001 0.0045 0.062 0.004 0.231 0.08 0.0029 0.264 0.160 0.278 0.398 0.425 0.448 0.290 O.o29 _Q,ill_
AU 0.42 0.77 >26 O.OH 0.0021 O.o27 0.003 0.031 1.18 0.0038 0.001 0.380 0.340 0.496 0.039
AV 0.43 0.08 2.79 tl.020 0.0036 0.011 0.008 0.135 0.71 0.0005 0$23
AW 0.44 0_9{} oso 0.009 01)001 0.005 0.004 0.426 >24 110021 0.457 .M1l Jl-367 0.186 0.1.20
AX '" 0.54 02Z 0.001 O.Otl55 0.014 0.002 0.027 0.02 0.0034 0.227 0.008 0.454 0.029 0.189 0.177 Q&M 0.192
AY 0.47 0.93 2.42 0.021 tl.0012 0.058 0.004 0.477 1.70 0.0024 0.245 0.257 0.064 0.062 0.221 0Jl53
AZ 0.61 0.08 ill 0.005 0.0010 0.023 0.004 0.016 1.00 0.0032 0.304 . 0.40Q 0.394 0.014 0.045 0.137 0.471 0.082 0.433
BA 0.51 0.75 2.!18 0.008 0.0095 0.074 0.003 0.220 0.26 0.0023 0.0511 0.365 0.016 0.192 M.Q2 0.138 O.o35
88 "' 0.19 0.82 0.002 0.0091 0.014 0.003 0.282 0.89 0.0024 0.373 0.287 0.303 0.429 0.410 0.208 0.001 O.D13 0.054 0.507
BC Q.54 0.85 2.66 0.002 Qm.Q1 0.023 0.008 0.312 0.41 0.0028 0.468 0.340 O.D79 0.024 0.403 0.214 0.022
BD 0.65 026 2.16 O.o11 0.0023 0.092 0.001 0.167 ,., 0.0004 0.097 0.030 0.512 0.085
BE 0.58 0.36 0.86 Q.014 0.0073 0.069 0.006 0.009 0.83 0.0016 0.029 0.004 O.ot8 0.074 0.042
BF 0.62 ""' 0.93 O.Q15 0.0100 0.0110 0.002 O.Q27 0.28 0.0020 0.007 0.034 0.201 0.281 0.138 (1.344 0.117 --- BG 0.66 0.19 1.34 .QJW! 0.0044 O.OHl 0.004 0.464 o.so 0.0035 0.235 0.203 0.012 0.147 0.017 0.052
BH 0.68 0.98 1.94 0.004 0.0094 0.062 0.004 O.D70 "' 0.0006 0.057 0.150 0.045 'b>11
s; 0.68 0.23 2.17 0.019 0.0072 0.006 0.005 0.197 0.70 0.0011 0.475 0.098 0.263 0.427 0.253 0.316 0.455 0.509
'" 0.71 0.17 0.52 0.003 0.0064 0.008 0.008 O.o46 0.20 0,0021 0.101 0.193 O.ot3 0.015 0.177 0.022
[0077] Then, in the same manner as in the first
experiment, the coefficient of micro-friction of
ferrite, and the spheroidized ratio and the average
diameter of cementite of each of the high-carbon
steel sheets were measured, and further, the
evaluation of adhesion suppressive performance and
the evaluation of crack
, , I sensltlvlty were
47
performed.
Ao; Ao3 (oC) (oC) REMARKS
"' "" INVENTIVE
EXAMPLE
m "" lNVENTNE
EXAMPLE
'" "" INVENTIVE
EXAMPLE
m Z9B
INVENTIVE
EXAMPLE
Z33 805
INVENTIVE
EXAMPLE
"' m INVENTIVE
EXAMPLE
m "' lNVENTNE
EXAMPLE
"' m INVENTIVE
EXAMPLE
m m INVENTIVE
EXAMPLE
ZZ9 "' INVENTIVE
EXAMPLE
ZJ3 Z5Z
lNVENTNE
EXAMPLE
m '"' INVENTIVE
EXAMPLE
244 "' INVENTIVE
EXAMPLE
"' '" COMPARATIVE
EXAMPLE
Z40 "" COMPARATIVE
EXAMPLE
m 825
COMPARATJVE
EXAMPLE
no "" COMPARATIVE
EXAMPLE
no 846 COMPARATIVE
EXAMPLE
"' 836 COMPARATIVE
EXAMPLE
"' an COMPARATIVE
EXAMPLE
na '" COMPARATIVE
EXAMPLE
"' 839 COMPARATIVE
EXAMPLE
699 no COMPARATIVE
EXAMPLE
'" soz COMPARATIVE
EXAMPLE
"' '" COMPARATIVE
EXAMPLE ,,. T"'
COMPARATIVE
EXAMPLE
Z43 Z90
COMPARATIVE
EXAMPLE
"" "' COMPARATJVE
EXAMPLE
234 no COMPARATIVE
EXAMPLE
"' '"' COMPARATIVE
EXAMPLE
m no COMPARATIVE
EXAMPLE
m "' COMPARATJVE
EXAMPLE
"' 758
COMPARATIVE
EXAMPLE
"' "' COMPARATIVE
EXAMPLE
Z46 "' COMPARATIVE
EXAMPLE
"' "' COMPARATIVE
EXAMPLE
"' "' COMPARATIVE
EXAMPLE
Results of them are listed in Table 4. An underline
in Table 4 indicates that the item is out of the
range of the present invention.
[0078] [Table 4]
I
- 48 -
COEFFICIENT COEFFICIENT SPHEROIDIZED AVERAGE ADHESION
SAMPL E STEEL OF MICRO~ OF MICRO- RATIO OF DIAMETER 0 F SUPPRESSIVE CRACK
No. TYPE FRICTION FRICTION CEM~~TITE CEMENTITE PERFORMAN C SENSITIVITY
REMARKS
OF FERRITE OF CEMENTITE (% (,m) E
31 z 0.43 0.28 90.8 0.45 0 0 INVENTIVE
EXAMPLE
32 AA 0.43 0.23 85.2 0.41 0 0 INVENTIVE
EXAMPLE
33 AB I 0.41 0.27 80.2 0.98 0 0 INVENTIVE
EXAMPLE
34 AC 0.45 0.32 81.4 2.05 0 0 INVENTIVE
EXAMPLE
35 AD 0.41 0.28 86.4 0.62 0 0 INVENTIVE
EXAMPLE
36 AE 0.43 0.31 82.3 1.24 0 0 INVENTIVE
EXAMPLE
37 AF 0.42 0.23 84.5 1.02 0 0 INVENTIVE
EXAMPLE
38 AG 0.43 0.30 91.2 0.31 0 0 INVENTIVE
EXAMPLE
39 AH 0.42 0.28 82.2 1.71 0 0 INVENTIVE
EXAMPLE
40 AI 0.42 0.31 94.1 0.31 0 0 INVENTIVE
EXAMPLE
41 AJ 0.43 0.26 91.3 0.89 0 0 INVENTIVE
EXAMPLE
42 AK 0.40 0.34 93.9 0.64 0 0 INVENTIVE
EXAMPLE
43 AL 0.41 0.24 87.2 0.39 0 0 INVENTIVE
EXAMPLE
44 AM 0.43 0.31 90.5 0.45 X 0 COMPARATIVE
EXAMPLE
45 AN 0.45 0.30 85.4 0.36 X 0 COMPARATIVE
EXAMPLE
46 AO 0.45 0.25 83.0 0.75 0 X COMPARATIVE
EXAMPLE
47 AP 0.44 0.26 86.3 0.64 0 X COMPARATIVE
EXAMPLE
48 AQ 0.58 0.25 85.2 0.54 X X
COMPARATIVE
EXAMPLE
49 AR 0.42 0.31 89.1 0.42 0 X COMPARATIVE
EXAMPLE
50 AS 0.40 0.23 79.0 1.39 0 X
COMPARATIVE
EXAMPLE
51 AT 0.44 0.33 85.7 1.35 0 X COMPARATIVE
EXAMPLE
52 [ill 0.44 0.32 84.0 0.43 0 X COMPARATlVE
EXAMPLE
53 AV 0.73 0.25 98.1 0.56 X 0 COMPARATIVE
EXAMPLE
54 f>!!i 0.44 0.23 80.9 0.52 0 X COMPARATIVE
EXAMPLE
55 AX 0.42 0.27 84.7 1.82 0 X
COMPARATIVE
EXAMPLE
56 AY 0.41 0.30 64.0 0.24 0 X
COMPARATIVE
EXAMPLE
57 AZ 0.43 0.23 67.4 1.05 0 X
COMPARATIVE
EXAMPLE
58 !lA 0.43 0.28' 92.9 0.57 0 X
COMPARATIVE
EXAMPLE
59 BB 0.42 0.31 89.6 0.80 0 X
COMPARATIVE
EXAMPLE
60 !lQ 0.42 0.22 92.0 0.42 0 X
COMPARATIVE
EXAMPLE
61 BO 0.46 0.23 95.0 0.46 0 X COMPARATIVE
EXAMPLE
62 BE 0.69 0.27 89.6 0.58 X X
COMPARATIVE
EXAMPLE
63 BF 0.43 i 0.31 90.6 2.32 0 X
COMPARATIVE
EXAMPLE
64 BG 0.44 0.22 96.7 0.73 0 X
COMPARATIVE
EXAMPLE
85 BH 0.42 0.32 85_0 0.37 0 X COMPARATIVE
EXAMPLE
66 m 0.41 0.29 98.9 0.63 0 X
COMPARATIVE
EXAMPLE
67 BJ 0.42 0.25 88.3 1.64 ( 0 X
COMPARATIVE
EXAMPLE
- 49 -
[0079] As listed in Table 4, Samples No. 31 to No.
43 were each within the range of the present
invention, thus being able to obtain good adhesion
suppressive performance and crack sensitivity.
[0080] On the other hand, in Sample No. 44, the C
content of Steel type AM was too low, and thus the
amount of cementite was insufficient, sufficient
lubricity was not able to be obtained, and adhesion
to the metal mold occurred during forming. In Sample
No. 45, the Cu content of Steel type AN was too high,
and thus a flaw occurred during hot-rolling and
adhesion originating from the flaw occurred. In
Sample No. 46, the Ca content of Steel type AO was
too high, and thus coarse oxides of Ca were formed
and a crack originating from the coarse oxide of Ca
occurred during the compression test. In Sample No.
47, the Mo content of Steel type AP was too high, and
thus the ductility of ferrite was low and a crack
originating from transgranular fracture of ferrite
occurred during the compression test. In Sample No.
48, the B content of Steel type AQ was too low, and
thus the coefficient of micro-friction of ferrite was
low and adhesion and cracking during the compression
test occurred. In Sample No. 49, the Nb content of
Steel ~ype AR was too high, and thus the ductility of
ferrite was low and a crack originating from
transgranular fracture of ferrite occurred during the
I compression test. In Sample No. 50, the Mn content
- 50 -
of Steel type AS was too low, and thus pearlite
transformation occurred during cooling in the
annealing, the spheroidized ratio of cementite was
low, and a crack originating from acicular cementite
occurred during the compression test. In Sample No.
51, the Ce content of Steel type AT was too high, and
thus coarse oxides of Ce were formed and a crack
originating from the coarse oxide of Ce occurred
during the compression test. In Sample No. 52, the B
content of Steel type AU was too high, and thus
boride was formed and a crack originating from the
boride occurred during the compression test. In
Sample No. 53, the Ni content of Steel type AV was
too high, and thus the coefficient of micro-friction
of ferrite was high and adhesion occurred. In Sample
No. 54, the V content of Steel type AW was too high,
and thus the ductility of ferrite was low and a crack
originating from transgranular fracture of ferrite
occurred during the compression test. In Sample No.
55, the Zr content of Steel type AX was too high, and
thus coarse oxides of Zr were formed and a crack
originating from the coarse oxide of Zr occurred
during the compression test. In Sample No. 56, the
Cr content of Steel type AY was too high, and thus
spheroidizing of cementite during annealing was
hindered, coarsening of cementite was suppressed, and
a crack originating from micro acicular cementite
occurred during the compression t'est. In Sample No.
- 51 -
57, the Mn content of Steel type AZ was too low, and
thus pearlite transformation occurred during cooling
in the annealing, the spheroidized ratio of cementite
was low, and a crack originating from acicular
cementite occurred during the compression test. In
Sample No. 58, the Y content of Steel type BA was too
high, and thus coarse oxides of ~ were formed and a
crack originating from the coarse oxide of Y occurred
during the compression test. In Sample No. 59, the
La content of Steel type BB was too high, and thus
coarse oxides of La were formed and a crack
originating from the coarse oxide of La occurred
during the compression test. In Sample No. 60, the S
content of Steel type BC was too high, and thus
coarse sulfides being non-metal inclusions were
formed and a crack originating from the coarse
sulfide occurred during the compression test. In
Sample No. 61, the W content of Steel type BD was too
high, and thus the ductility of ferrite was low and a
crack originating from transgranular fracture of
ferrite occurred during the compression test. In
Sample No. 62, the Ti content of Steel type BE was
too low, and thus BN precipitated, the amount of
solid-solution B was insufficient, the co~fficient of
micro-friction of ferrite was lnw, and adhesion and
cracking during the compression test occurred. In
Sample Nn. 63, the Si content of Steel type BF was
too low, and thus cementite beca~e coarse excessively
- 52 -
and a crack originating from the coarse cementite
occurred during the compression test. In Sample No.
64, the P content of Steel type BG was too high, and
thus segregation of B to the interface between
ferrite and cementite was hindered and cracking
occurred during the compression test. In Sample No.
65, the Ta content of Steel type BH was too high, and
thus the ductility of ferrite was low and a crack
originating from transgranular fracture of ferrite
occurred during the compression test. In Sample No.
66, the Mg content of Steel type BI was too high, and
thus coarse oxides of Mg were formed and a crack
originating from the coarse oxide of Mg occurred
during the compression test. In Sample No. 67, the C
content of Steel type BJ was too high, and thus the
amount of cementite was excessive and a crack
originating from the cementite occurred during the
compression test.
[0081] Fig. 1 illustrates the relationship between
the coefficient of micro-friction of ferrite and the
B content of Samples No. 1 to No. 25 and No. 31 to
No. 67 except for Samples No. 11, No. 51, No. 53, and
No. 62. As illustrated in Fig. 1, when the B content
is 0:0004% or more, the coefficient o~ micro-friction
of ferrite is significantl~ low as compared to the
case when it is less than 0.0004%.
[0082] (Third experiment)
In a third experiment, hot-rblling and annealing
- 53 -
were performed under various conditions on the steel
types that were within the range of the present
invention (Steel types A to I and Steel types Z to
AL) out of the steel types used in the first
experiment and the steel types used in the second
experiment so as to manufacture high-carbon steel
sheets. Conditions of them are listed in Table 5 to
Table 7. An underline in Table 5 to Table 7
indicates that the numeric value is out of the range
of the present invention.
[0083] [Table 5]
I
- 54 -
CONDITIONS IN HOT -ROLLING CONDITIONS IN ANNEALING
SAMPLE STEEL SLAB HEATING FINISH ROLLING COILING RETENTION RETENTION COLLING
REMARKS
No. TYPE TEMPERATURE TEMPERATURE TEMPERATURE TEMPERATURE TIME RATE
('C) ('C) ('C) ('C) (hr) ('C/hr)
71 A 1116 871 540 786 51.4 56.7 COMPARATIVE
EXAMPLE
72 B 1149 841 646 745 18.4 58.6 INVENTIVE
EXAMPLE
73 c 1113 879 706 747 9.7 26.7 COMPARATIVE
EXAMPLE
74 D 1006 940 681 757 34.7 33.6 INVENTIVE
EXAMPLE
75 E 1149 889 605 756 2.2 13.9 COMPARATIVE
EXAMPLE
76 F 993 902 554 759 3.6 30.7 COMPARATIVE
EXAMPLE
77 G 1068 891 684 742 26.8 3.8 INVENTIVE
EXAMPLE
78 H 1044 874 685 736 56.2 53.2 INVENTIVE
EXAMPLE
79 I 1083 845 590 748 46.3 40.7 INVENTIVE
EXAMPLE
80 z 1120 914 616 751 6.5 1.3 INVENTIVE
EXAMPLE
81 AA 1122 880 714 765 43.0 58.9 COMPARATIVE
EXAMPLE
82 AB 1113 844 583 752 55.3 2.7 INVENTIVE
EXAMPLE
83 AC 1088 863 695 749 7.6 38.0 INVENTIVE
EXAMPLE
84 AD 1065 850 547 741 18.3 68.1 COMPARATIVE
EXAMPLE
85 AE 1095 904 680 750 36.5 48.6 INVENTIVE
EXAMPLE
86 AF 1118 949 521 776 57.9 7.5 COMPARATIVE
EXAMPLE
87 AG 1024 859 435 766 58.7 5.5 COMPARATIVE
EXAMPLE
88 AH 1078 874 620 754 18.8 21.3 INVENTIVE
EXAMPLE
89 AI 1028 861 615 753 3.1 21.2 INVENTIVE
EXAMPLE
90 AJ 1136 915 689 767 46.8 27.6 INVENTIVE
EXAMPLE
91 AK 1098 936 645 760 37.1 8.1 INVENTIVE
EXAMPLE
92 AL 1099 901 691 754 29.7 12.7 INVENTIVE
. : EXAMPLE
[0084] [Table 6]
(
- 55 -
CONDITIONS IN HOT ROLLING CONDITIONS IN ANNEALING
SAMPLE STEEL SLAB HEATING FINISH ROLLING COILING RETENTION RETENTION COLLING REMARKS
No. TYPE TEMPERATURE TEMPERATURE TEMPERATURE TEMPERATURE TIME RATE
('C) ('C) ('C) ("C) (hr) ("C/hr)
1 01 A 1104 873 527 ill 38.3 32.0
COMPARATIVE
EXAMPLE
102 B 1060 983 560 757 56.8 17.3
COMPARATIVE
EXAMPLE
103 c 1129 865 625 732 4.9 42.7
INVENTIVE
EXAMPLE
104 D 111.:i 863 548 767 12.6 30.7
COMPARATIVE
EXAMPLE
105 E 1109 875 632 749 4.6 40.7
INVENTIVE
EXAMPLE
106 F. 1088 865 677 768 23.0 2.3
INVENTIVE
EXAMPLE
107 G 1142 869 536 736 5.5 62.3
COMPARATIVE
EXAMPLE
108 H 1064 848 640 739 14.8 29.0
INVENTIVE
EXAMPLE
109 I 1064 847 621 745 7.8 24.5
INVENTIVE
EXAMPLE
110 z 1007 878 656 747 48.2 2.9
INVENTIVE
EXAMPLE
111 AA 1051 943 699 768 6.2 1.2
INVENTIVE
EXAMPLE
112 AB 11.@ 908 526 760 6.2 7.3
COMPARATIVE
EXAMPLE
113 AC 1131 823 532 761 47.9 16.2
COMPARATIVE
EXAMPLE
114 AD 1047 847 605 730 22.5 25.2
INVENTIVE
EXAMPLE
115 AE 1080 862 648 752 26.8 31.7
INVENTIVE
EXAMPLE
116 AF 885 666 741 17.0 42.8
INVENTIVE
1102 EXAMPLE
117 AG 1050 932 601 770 32.6 1.9
INVENTIVE
EXAMPLE
118 AH 1025 875 540 718 26.3 41.3
COMPARATIVE
EXAMPLE
119 AI 936 625 750 17.1 58.1
INVENTIVE
1078 EXAMPLE
120 AJ 1079 881 671 764 9.7 28.7
INVENTIVE
EXAMPLE
121 867 641 735 27.8 4.5
INVENTIVE
AK 1084 EXAMPLE
122 890 531 ', ~ . 742 4.5 9.3
INVENTIVE
AL 1116 EXAMPLE
[0085] [Table 7]
(
- 56 -
..
' '------
CONDITIONS IN HOT ROLLING CONDITIONS IN ANNEALING
SAMPLE STEEL SLAB HEATING FINISH ROLLING COILING RETENTION RETENTION COLLING
No. TYPE TEMPERATURE TEMPERATURE TEMPERATURE TEMPERATURE TIME RATE
("C) ("C) ("C) (oC) (hr) (°C/hr)
131 A 1096 870 511 764 54.5 10.2
132 B 1097 886 479 768 18.7 0.4
133 c 1107 835 502 762 6.1 56.6
134 D 1022 857 464 755 4.8 45.2
135 E 1087 801 453 743 17.6 54,7
136 F 1069 858 576 761 34.0 49.8
137 G 1032 931 444 738 35.9 16.5
138 H 1096 843 497 749 62.6 42.7
139 I 1046 895 536 754 48.1 24.5
140 z 1123 920 489 755 2.6 58.5
141 M 1082 865 495 731 34.8 33.3
142 AB 1058 924 482 749 26.5 24.3
143 AC 1123 904 524 743 35.3 35.6
144 AD 1077 877 498 741 10.4 0.8
145 AE 1008 939 574 753 22.1 28.7
146 AF 1034 962 482 751 41.0 7.4
147 AG 1133 916 457 732 4.5 24.0
148 AH 1037 884 561 748 59.0 9.4
149 AI .!W! 847 508 752 59.9 4.6
150 AJ 1126 933 479 748 68.3 3.3
151 AK 1138 893 598 752 26.9 3.2
. 152 AL 1063 865 584 '·'' 746 40.3 2.6 "
[0086] Then, in the same manner as in the first
experiment, the coefficient of micro-friction of
ferrite, and the spheroidized ratio and the average
diameter of cementite of each of the high-carbon
steel sheets were measured, and further, the
evaluation of adhesion suppressiJe performance and
- 57 -
I
REMARKS
INVENTIVE
EXAMPLE
COMPARATIVE
EXAMPLE
INVENTIVE
EXAMPLE
INVENTIVE
EXAMPLE
COMPARATIVE
EXAMPLE
INVENTIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
INVENTIVE
EXAMPLE
COMPARATIVE
EXAMPLE
INVENTIVE
EXAMPLE
INVENTIVE
EXAMPLE
INVENTIVE
EXAMPUE
COMPARATIVE
EXAMPLE
INVENTIVE
EXAMPLE
COMPARATIVE
EXAMPLE
INVENTIVE
EXAMPLE
INVENTIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
INVENTIVE
EXAMPLE
INVENTIVE
EXAMPLE
the evaluation of crack sensitivity were performed.
Results of them are listed in Table 8 to Table 10.
An underline in Table 8 to Table 10 indicates that
the item is out of the range of the present
invention.
[0087] [Table 8]
COEFFICIENT COEFFICIENT SPHEROIDIZED AVERAGE
SAMPLE OF MICRO- ADHESION OF MICRO- RAllO OF DIAMETER OF
SUPPRESSIVE CRACK REMARKS
No. FRICTION FRICTION CEMENTITE CEMENTITE SENSITIVITY
OF FERRITE OF CEMENTITE {%) (um) PERFORMANCE
71 0.72 0.33 86.8 2,25 X X
COMPARATIVE
EXAMPLE
72 0.43 0.31 87.9 1.15 0 0 INVENTIVE
EXAMPLE
73 0.64 0.27 78.3 0.63 X X
COMPARATIVE
EXAMPLE
74 0.44 0.28 88.3 0.90 0 0 INVENTIVE
EXAMPLE
75 0.63 0.24 84.3 0.29 X X
COMPARATIVE
EXAMPLE
76 0.59 0.24 90.3 0.42 X X
COMPARATIVE
EXAMPLE
77 0.49 0.28 80.9 0.99 0 0 INVENTIVE
EXAMPLE
78 0.44 0.31 94.3 0.54 0 0 INVENTIVE
EXAMPLE
79 0.43 0.29 93.1 0.59 0 0 INVENTIVE
EXAMPLE
80 0.48 0.28 80.8 0.98 0 0 INVENTIVE
EXAMPLE
81 0.71 0.23 75.4 2.12 X x COMPARATIVE
EXAMPLE
82 0.44 0.22 87.1 0.65 0 0 INVENTIVE
EXAMPLE
83 0.47 0.34 82.0 1.25 0 0 INVENTIVE
EXAMPLE
84 0.61 0.34 62.3 1.04 X X
COMPARATIVE
><. EXAMPLE
85 0.46 0.29 91.2 0.33 0 - 0 INVENTIVE
EXAMPLE
86 0.73 0.25 83.6 2.36 X X
COMPARATIVE
EXAMPLE
87 0.64 0.23 98.9 1.16 X 0 COMPARATIVE
EXAMPLE
88 0.47 0.30 93.0 0.92 0 0 INVENTIVE
EXAMPLE
89 0.45 0.28 94.8 0.63 0 0 INVENTIVE
EXAMPLE
90 0.45 0.27 97.1 0.43 0 0 INVENTIVE
EXAMPLE
91 0.44 0.32 96.5 0.68 0 0 INVENTIVE
EXAMPLE
92 0.45 0.28 90.4 0.4,6 0 0 INVENTIVE
EXAMPLE
- 58 -
[0088] [Table 9]
COEFFICIENT COEFFICIENT SPHEROIDIZED AVERAGE
SAMPLE OF MICRO- OF MICRO- RATIO OF DIAMETER OF ADHESION
CRACK
No. FRICTION FRICTION CEMENTITE CEMENTITE
SUPPRESSIVE
SENSITIVITY
REMARKS
OF FERRITE OF CEMENTITE (%) (um) PERFORMANCE
101 0.72 0.31 82.3 0.77 X X
COMPARATIVE
EXAMPLE
102 0.61 0.22 82.8 1.18 X 0 COMPARATIVE
EXAMPLE
103 0.42 0.26 81.3 0.6) 0 0 INVENTIVE
EXAMPLE
104 0.71 0.26 92.2 0.89 X 0 COMPARATIVE
EXAMPLE
105 0.44 0.24 95.3 0.69 0 0 INVENTIVE
EXAMPLE
106 0.42 0.30 94.9 0.45 0
.•
0 INVENTIVE
EXAMPLE
107 0.61 0.31 64.1 0.95 X X
COMPARATIVE
EXAMPLE
108 0.43 0.32 95.4 0.53 0 0 INVENTIVE
EXAMPLE
109 0.42 0.26 89.8 0.56 0 0 INVENTIVE
EXAMPLE
110 0.44 0.34 88.0 1.01 0 0 INVENTIVE
EXAMPLE
111 0.44 0.34 84.2 2.07 0 0 INVENTIVE
EXAMPLE
112 0.69 0.25 89.6 0.63 X 0 COMPARATIVE
EXAMPLE
113 0.59 0.32 85.5 1.29 X X
COMPARATIVE
EXAMPLE
114 0.47 0.23 84.0 1.03 0 0 INVENTIVE
EXAMPLE
115 0.48 0.28 92.8 0.33 0 0 INVENTIVE
EXAMPLE
116 0.45 0.30 90.0 1.73 0 0 INVENTIVE
EXAMPLE
117 0.44 0.26 99.8 0.34 0 0 INVENTIVE
EXAMPLE
118 0.63 0.25 85.9 0.83 X X
COMPARATIVE
EXAMPLE
119 0.47 0.23 . 93.!1 0.66 0 0 INVENTIVE
EXAMPI E
120 0.45 0.28 95.9 0.41 0 0 INVENTIVE
EXAMPLE
121 0.46 0.24 87.9 0.65 0 0 INVENTIVE
EXAMPLE
122 0.39 0.25 80.6 0.43 0 0 INVENTIVE
EXAMPLE
[0089] [Table 10]
(
- 59 -
COEFFICIENT COEFFICIENT SPHEROIDIZED AVERAGE
ADHESION
SAMPLE OF MICRO- OF MICRO- RATIO OF DIAMETER OF CRACK
No. FRICTION FRICTION CEMENTITE CEMENTITE SUPPRESSIVE
SENSITIVITY
REMARKS
OF FERRITE OF CEMENTITE (%) (~m) PERFORMANCE
131 0.39 0.32 83.6 0.81 0 0 INVENTIVE
EXAMPLE
132 0.68 0.33 97.2 2.45 X X
COMPARATIVE
EXAMPLE
133 0.40 0.24 89.9 0.63 0 0 INVENTIVE
EXAMPLE
134 0.40 0.24 87.2 0.86 0 0 INVENTIVE
EXAMPLE
135 0.66 0.31 88.1 0.74 X X
COMPARATIVE
EXAMPLE
136 0.44 0.24 91.6 0.45 0 0 INVENTIVE
EXAMPLE
137 0.59 0.26 86.5 0.99 X 0 COMPARATIVE
EXAMPLE
138 0.66 0.22 92.5 2.55 X X
COMPARATIVE
EXAMPLE
139 0.38 0.32 97.5 0.59 0 0 INVENTIVE
EXAMPLE
140 0.60 0.22 82.7 0.22 X X
COMPARATIVE
EXAMPLE
141 0.40 0.27 85.9 2.07 0 0 INVENTIVE
EXAMPLE
142 0.40 0.33 86.0 0.64 0 0 INVENTIVE
EXAMPLE
143 0.40 0.23 89.1 1.27 0 0 INVENTIVE
EXAMPLE
144 0.54 0.27 81.6 2.38 X X
COMPARATIVE
EXAMPLE
145 0.43 0.31 93.5 0.33 0 0 INVENTIVE
EXAMPLE
146 0.62 0.24 82.4 1.77 X 0 COMPARATIVE
EXAMPLE
147 0.37 0.31 81.0 0.30 0 0 INVENTIVE
EXAMPLE
148 0.40 0.32 90.4 0.93 0 0 INVENTIVE
EXAMPLE
149 0.72 0.33 94.5 0.68 X X
COMPARATIVE
EXAMPLE
150 0.58 0.29 84.7 2.46 X X
COMPARATIVE
EXAMPLE
151 0.43. 0.23 94.5 0.67 0 0 INVENTIVE
' .. . •· EXAMPLE _, INVENTIVE
152 0.44 0.25 80.4 0.46 0 0 EXAMPLE
[0090] As listed in Table 8, Samples No. 72, No. 74,
No. 77 to No. 80, No. 82, No. 83, No. 85, and No. 88
to No. 92 were each within the range of the present
invention, thus being able to obtain good adhesion
I suppressive performance and crack sensitivity. As
- 60 -
listed in Table 9, Samples No. 103, No. 105, No. 106,
No. 108 to No. 111, No. 114 to No. 117, and No. 120
to No. 122 wera.each also within the range of the
present invention, thus being able to obtain good
adhesion suppressive performance and crack
sensitivity. As listed in Table 10, Samples No. 131,
No. 133, No. 134, No. 136, No. 139, No. 141 to No.
143, No. 145, No. 147, No. 148, No. 151, and No. 152
were each also within the range of the present
invention, thus being able to obtain good adhesion
suppressive performance and crack sensitivity.
[0091] On the other hand, in Sample No. 71, the
annealing retention temperature was too high, and
thus volume expansion was large, a hot-rolled coil
was uncoiled to cause abrasions, and a tightening
mark caused by a tightening band also occurred.
Further, the thickness of the film of crystals of B
greatly varied and the coefficient of micro-friction
of ferrite was large. Therefore, adhesion occurred.
Further, cementite became coarse excessively and a
crack originating from the coarse cementite occu~red
during the compression test. In Sample No. 73, .the
coiling temperature was too high, and thus coarse
lamellar pearlite was formed in the hot-rolled steel
sheet, spheroidizing of cementite during annealing
was hindered, and the spheroidized ratio of cementite
was low. Further, large irregularitiss were formed
with removal of scales and the cdefficient of micro-
- 61 -
friction of ferrite was large. Therefore, adhesion
and cracking during the compression test occurred.
In Sample No. 7 5, the annealing retention time was
too short, and thus the coefficient of micro~friction
of ferrite was large and the average diameter of
cementite was small. Therefore, adhesion and
cracking during the compression test occurred. In
Sample No. 76, the slab heating temperature was too
low, and thus segregations of B, Mn, and others were
not eliminated and the coefficient of micro~friction
of ferrite was large. Therefore, adhesion and
cracking during the compression test occurred. In
Sample No. 81, the coiling temperature was too high,
and thus adhesion and cracking during the compression
test occurred similarly to Sample No. 73. In Sample
No. 84, the cooling rate was too high, and thus
pearlite transformation occurred during cooling and a
crack originating from acicular cementite occurred
during the compression test. Further, a good film of
crystals of B was not formed on the surface of the
high~carbon".steel sheet, the coefficient of 'micro~
friction of ferrite was high, and adhesion occurred.
In Sample No. 86, the annealing retention temperature
was too high, and thus adhesion and cracking during
the compression test occurred, similarly to Sample
No. 81. In Sample No. 87, the coiling temperature
was too low, and thus as a result of removal of
scales, ( the surface of the steel sheet became rough
~ 62 ~
and adhesion occurred.
[0092] In Sample No. 101, the annealing retention
temperature was too low, and thus the segregation of
B to the interface between ferrite and austenite was
suppressed, the coefficient of micro-friction of
ferrite was large, and adhesion occurred. Further,
the segregation of B to the interface between ferrite
and cementite was also suppressed and cracking
occurred during the compression test. In Sample No.
102, the finish rolling temperature was too high, and
thus large irregularities were formed with removal of
scales and the coefficient of micro-friction of
ferrite was large. Therefore, adhesion occurred. In
Sample No. 104, the slab heating temperature was too
high, and thus B atoms were oxidized during slab
heating and the coefficient of micro-friction of
ferrite was large. Therefore, adhesion occurred. In
Sample No. 107, the cooling rate was too high, and
thus pearlite transformation occurred during cooling
and a crack originating from acicular cementite
occurred: during the compression test. Further, a
good film of crystals of B was not formed on the
surface of the high-carbon steel sheet, the
coefficient of micro-friction of ferrite was high,
and adhesion occurred. In Sample No. 112, the slab
heating temperature was too high, and thus adhesion
occurred, similarly to Sample No. 104. In Sample No.
113,
(
the finish rolling temperature was too low, and
- 63 -
------~~-----
thus anisotropy of the structure was strong and a
crack originating from a nonuniform structure
occurred during the compression test .. Further, as a
result of removal of scales, the surface of the steel
sheet became rough and adhesion occurred. In Sample
No. 118, the annealing retention temperature was too
low, and thus adhesion and cracking during the
compression test occurred, similarly to Sample No.
101.
[ 0 0 93] In Sample No. 132, the cooling rate was too
low, and thus the thickness of the film of crystals
of B greatly varied and the coefficient of microfriction
of ferrite was large. Therefore, adhesion
occurred. Further, cementite became coarse
excessively and a crack originating from the coarse
cementite occurred during the compression test. In
Sample No. 135, the finish rolling temperature was
too low, and thus anisotropy of the structure was
strong and a crack originating from a nonuniform
structure occurred during the compression test .
. Further, as a result of removal of ~cales, the
surface of the steel sheet became rough and adhesion
occurred. In Sample No. 137, the coiling temperature
was too low, and thus as a result of removal of
scales, the surface of the steel sheet became rough
and adhesion occurred. In Sample No. 138, the
annealing retention time was too long, and thus
I
volume expansion was large, a hot-rolled coil was
- 64 -
uncoiled to cause abrasions, and a tightening mark
caused by a tightening band also occurred. Further,
-.the thickness of the film of crystals of B greatly
varied and the coefficient of micro-friction of
ferrite was large. Therefore, adhesion occurred.
Further, cementite became coarse excessively and a
crack originating from the coarse. cementite occurred
during the compression test. In Sample No. 140, the
annealing retention time was too short, and thus the
coefficient of micro-friction of ferrite was large
and the average diameter of cementite was small.
Therefore, adhesion and cracking during the
compression test occurred. In Sample No. 144, the
cooling rate was too low, and thus adhesion and
cracking during the compression test occurred,
similarly to Sample No. 132. In Sample No. 146, the
finish rolling temperature was too high, and thus,
large irregularities were formed with removal of
scales and the coefficient of micro-friction of
ferrite was large. Therefore, adhesion occurred. In
s-ample No. 149, the slab heating· temperature was too
low, and thus segregations of B, Mn, and others were
not eliminated and the coefficient of micro-friction
of ferrite was large. Therefore, adhesion and
cracking during the compression test occurred. In
Sample No. 150, the annealing retention time was too
long, and thus adhesion and cracking during the
compression test occurred, similirly to Sample No.
- 65 -
138.
[0094] Fig. 7 illustrates the relationship between
the coefficient of micro-friction of ferrite and the
B content in the samples out of the examples in the
first experiment or third experiment. As illustrated
in Fig. 7, when the B content is 0.0008% or more, the
coefficient of micro-friction of ferrite is much
lower as compared to the case when it is less than
0.0008%.
INDUSTRIAL APPLICABILITY
[0095] The present invention may be utilized in, for
example, manufacturing industries and application
industries of high-carbon steel sheets used for
various steel products, such as a driving system
component for automobile, a saw, a knife, and others.
(
- 66 -
CLAIMS
[Claim 1] A high-carbon steel sheet, comprising:
a chemical composition represented by' in mass%:
c: 0.30% to 0.70%,
s i: 0.07% to 1.00%,
Mn: 0.20% to 3.00%,
Ti: 0.010% to 0.500%,
Cr: 0.01% to 1.50%,
B: 0.0004% to 0.0035%,
P: 0.025% or less,
Al: 0.100% or less,
s: 0.0100% or less,
N: 0.010% or less,
Cu: 0.500% or less,
Nb: 0.000% to 0.500%,
Mo: 0.000% to 0.500%,
V: 0.000% to 0.500%,
W: 0.000% to 0.500%,
Ta: 0.000% to 0.500%,
Ni: 0.000% to 0.500%,
Mg: 0.000% to 0.500%,
Ca: 0.000% to 0.500%,
Y: 0.000% to 0.500%,
Zr: 0.000% to 0.500%,
La: 0.000% to 0.500%,
Ce: 0.000% to 0.500%, and
balance:. Fe and impurities; and
a structure represented by: I
- 67 -
a spheroidized ratio of cementite: 80% or more;
and
an average diameter of cementite: 0.3 ~m to 2.2
~m, wherein
a coefficient of micro-friction of ferrite on a
surface of the steel sheet is less than 0.5.
[Claim 2] The high-carbon steel sheet according to
claim 1' wherein
in the chemical composition,
Nb: 0.001% to 0.500%,
Mo: 0.001% to 0.500%,
V: 0.001% to 0.500%,
W: 0.001% to 0.500%,
Ta: 0.001% to 0.500%,
Ni: 0.001% to 0.500%,
Mg: 0.001% to 0.500%,
Ca: 0.001% to 0.500%,
Y: 0.001% to 0.500%,
Zr: 0.001% to 0.500%,
La: 0.001% to 0.500%, or
Ce: 0.001% to 0.500%, or
any combination thereof is satisfied.
[Claim 3] A method of manufacturing a high-carbon
steel sheet, comprising:
hot-rolling of a slab so as to obtain a hotrolled
steel sheet;
pickling of the hot-rolled steel sheet; and
annealing of the hot-rolled ~teel sheet after the
- 68 -
pickling,
the slab comprising a chemical composition
represented by," in mass%:
C: 0.30% to 0.70%,
Si: 0.07% to 1.00%,
Mn: 0.20% to 3.00%,
Ti: 0.010% to 0.500%,
Cr: 0.01% to 1.50%,
B: 0.0004% to 0.0035%,
P: 0.025% or less,
Al: 0.100% or less,
S: 0.0100% or less,
N: 0.010% or less,
Cu: 0.500% or less,
Nb: 0.000% to 0.500%,
Mo: 0.000% to 0.500%,
V: 0.000% to 0.500%,
W: 0.000% to 0.500%,
Ta: 0.000% to 0.500%,
Ni: 0.000% to 0.500%,
Mg: 0.000% to 0.500%,
Ca: 0.000% to 0.500%,
Y: 0.000% to 0.500%,
Zr: 0.000% to 0.500%,
La: 0.000% to 0.500%,
Ce: 0.000% to 0.500%, and
_balance: Fe and impurities,
in the hot-rolling,
- 69 -
where j n
(
the slab is heated at a temperature of lOOO"C or
more and less than 1150"C,
a finish rolling temperature is 830"C or more and
950"C or less, and
a coiling temperature is 450"C or more and 700"C
or less, and
the annealing comprises:
retaining the hot-rolled steel sheet at a
temperature of 730"C or more and 770"C or less for 3
hours or more and 60 hours or less; and
then cooling the hot-rolled steel sheet down to
650"C at a cooling rate of l"C/hr or more and 60"C/hr
or less.
[Claim 4] The method of manufacturing the highcarbon
steel sheet according to
in the chemical composition, claim 3' wherein