[0001] The present invention relates to a steel wire
of special steel and a wire rod of special steel
suitable for a machine part having a tensile strength
of not less than 1200 MPa nor more than 1500 MPa,
manufacturing methods thereof, and so on.
BACKGROUND ART
[0002] Automobile parts and various industrial
machine parts each having a shaft shape such as a
bolt, a torsion bar, and a stabilizer have been
manufactured from a wire rod. Then, in recent years,
automobiles and various industrial machines have
required a high-strength machine part having a
tensile strength of 1200 MPa or more with the aim of
reduction in weight and reduction in size.
[0003] However, with the achievement of high
strength of a machine part, what is called a hydrogen
embrittlement, in which due to the effect of hydrogen
penetrated into a steel material, a machine part is
fractured by stress smaller than that to be expected
originally, has become noticeable. The hydrogen
embrittlement appears in various forms. For example,
in a bolt used for an automobile, a building, and so
on, a phenomenon in which after a while since the
bolt is fastened, fracture occurs suddenly, called
delayed fracture, sometimes occurs.
[0004] Then, various examinations for improving
hydrogen embrittlement resistance of a high-strength
- 1 -
• part have been conducted. With regard to a bolt
being one example of the high-strength machine part,
there has been known a technique utilizing pearlite
after wire drawing, as one of techniques improving
delayed fracture resistance (Patent Literatures 1 to
4) .
[0005] However, even by these conventional
techniques, it is difficult to improve the hydrogen
embrittlement resistance in the high-strength machine
part having a tensile strength of 1200 MPa or more.
Further, a steel wire and a wire rod suitable for
such a machine part are not also invented.
CITATION LIST
PATENT LITERATURE
[0006] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2005-281860
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2001-348618
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2004-307929
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2008-261027
Patent Literature 5: Japanese Laid-open Patent
Publication No. 11-315349
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2002-69579
Patent Literature 7: Japanese Laid-open Patent
Publication No. 2000-144306
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007] The present invention has an object to
- 2 -
•
provide a steel wire of special steel and a wire rod
of special steel that have high strength and are
capable of improving hydrogen embrittlement
resistance, manufacturing methods thereof, and so on.
SOLUTION TO PROBLEM
[0008] The gist of the present invention is as
follows.
[0009] (1)
A steel wire of special steel containing:
in mass%;
c: 0.35% to 0.85%;
Si: 0.05% to 2.0%;
Mn: 0.20% to 1.0%; and
AI: 0.005% to 0.05%,
a P content being 0.030% or less,
a S content being 0.030% or less, and
a balance being composed of Fe and inevitable
impurities, wherein
when a C content is represented by (C%), in a
case of (C%) being not less than 0.35% nor more than
0.65%, a volume fraction of pearlite is 64 x (C%) +
52% or more, and in a case of (C%) being greater than
0.65% and 0.85% or less, the volume fraction of
pearlite is not less than 94% nor more than 100%, and
a structure of the other portion is composed of one
or two of proeutectoid ferrite or bainite,
in a region up to a depth of 1.0 mm from a
surface of the steel wire, a volume fraction of
pearlite block having an aspect ratio of 2.0 or more
is not less than 70% nor more than 95%, and a volume
fraction of pearlite having an angle between an axial
- 3 -
direction of the steel wire and a lamellar direction
It of the perlite on a cross section parallel to the
axial direction of 40° or less is 60% or more with
respect to all pearlite, and
a tensile strength is 1200 MPa or more and less
than 1500 MPa.
(2 )
The steel wire of special steel-according to (1),
wherein, in mass%, a N content is 0.0050% or less.
( 3 )
The steel wire of special steel according to (1)
or (2), further containing, in mass%, one or two of
Cr: 0.02% to 1.0% and Ni: 0.02% to 0.50%.
( 4 )
The steel wire of special steel according to any
one of (1) to (3), further containing, in mass%, one
or two or more of Ti: 0.002% to 0.050%, V: 0.01% to
o. 20%, 0 r Nb: O. 005 % toO. 100%.
( 5 )
The steel wire
one of (1) to (4),
0.0001% to 0.0060%.
( 6 )
of special steel according to any
further containing, in mass%, B:
The steel wire of special steel according to any
one of (1) to (5), further containing, in mass%, one
or two or more of Ca: 0.001% to 0.010%, Mg: 0.001% to
0.010%, or Zr: 0.001% to 0.010%.
[0010] (7)
A wire rod of special steel containing:
in mass%;
C: 0.35 to 0.85%;
- 4 -
• Si: 0.05 to 2.0%;
Mn: 0.20 to 1.0%;
P: 0.030% or less;
S: 0.030% or less; and
AI: 0.005 to 0.05%,
a balance being composed of Fe and inevitable
impurities, wherein
when a C content is represented by (C%), in a
case of (C%) being not less than 0.35% nor more than
0.65%, a volume fraction of pearlite is 64 x (C%) +
52% or more, and in a case of (C%) being greater than
0.65% and 0.85% or less, the volume fraction of
pearlite is not less than 94% nor more than 100%, and
a structure of the other portion is composed of one
or two of proeutectoid ferrite and bainite.
( 8 )
The wire rod of special steel according to (7),
wherein, in mass%, a N content is 0.0050% or less.
( 9 )
The wire rod of special steel according to (7) or
(8), further containing, in mass%, one or two of Cr:
0.02% to 1.0% and Ni: 0.02% to 0.50%.
( 10)
The wire rod of special steel according to any
one of (7) to (9), further containing, in mass%, one
or two or more of Ti: 0.002% to 0.050%, V: 0.01% to
0.20%, or Nb: 0.005% to 0.100%.
(11 )
The wire rod of special steel according to any
one of (7) to (10), further containing, in mass%, B:
0.0001% to 0.0060%.
- 5 -
(12 )
1l The wire rod of special steel according to any
one of (7) to (11), further containing, in mass%, one
or two or more of Ca: 0.001% to 0.010%, Mg: 0.001% to
0.010%, or Zr: 0.001% to 0.010%.
[0011] (13 )
A manufacturing method of a steel wire of special
steel comprising:
performing hot rolling of a billet with a
temperature of finish rolling being not lower than
800°C nor higher than 950°C so as to obtain a steel
material having a grain size number of austenite
grains being 8 or more;
next, immersing the steel material having a
temperature of not lower than 750°C nor higher than
950°C in a first molten salt bath having a temperature
of not lower than 400°C nor higher than 600°C and
isothermally holding the steel material for not
shorter than 5 seconds nor longer than 150 seconds;
next, immersing the steel material in a second
molten salt bath having a temperature of not lower
than 500°C nor higher than 600°C and isothermally
holding the steel material for not shorter than 5
seconds nor longer than 150 seconds; and
next, performing wire drawing with a total
reduction of area of not less than 25% nor more than
80% on the wire rod at room temperature, wherein
the steel material contains:
in mass%;
C: 0.35% to 0.85%;
Si: 0.05% to 2.0%;
- 6 -
Mn: 0.20% to 1.0%; and
• Al: 0.005% to 0.05%,
a P content being 0.030% or less,
a S content being 0.030% or less, and
a balance being composed of Fe and inevitable
impurities.
( 14 )
The manufacturing method of the steel wire of
special steel according to (13), wherein a reduction
of area at the final of the wire drawing is not less
than 1% nor more than 15%.
[0012] (15 )
A manufacturing method of a wire rod of special
steel comprising:
performing hot rolling of a billet with a
temperature of finish rolling being not lower than
800°C nor higher than 950°C so as to obtain a steel
material having a grain size number of austenite
grains being 8 or more;
next, immersing the steel material having a
temperature of not lower than 750°C nor higher than
950°C in a first molten salt bath having a temperature
of not lower than 400°C nor higher than 600°C and
isothermally holding the steel material for not
shorter than 5 seconds nor longer than 150 seconds;
and
next, immersing the steel material in a second
molten salt bath having a temperature of not lower
than 500°C nor higher than 600°C and isothermally
holding the steel material for not shorter than 5
seconds nor longer than 150 seconds, wherein
- 7 -
•
the steel material contains:
in mass%;
c: 0.35% to 0.85%;
Si: 0.05% to 2.0%;
Mn: 0.20% to 1.0%; and
AI: 0.005% to 0.05%,
a P content being 0.030% or less,
a S content being 0.030% or less, and
a balance being composed of Fe and inevitable
impurities.
[0013] (16)
A machine part containing:
in mass%;
c: 0.35% to 0.85%;
Si: 0.05% to 2.0%;
Mn: 0.20% to 1.0%; and
AI: 0.005% to 0.05%;
a P content being 0.030% or less;
a S content being 0.030% or less; and
a balance being composed of Fe and inevitable
impurities, wherein
when the C content is represented by (C%), in a
case of (C%) being not less than 0.35% nor more than
0.65%, a volume fraction of pearlite is 64 x (C%) +
52% or more, and in a case of (C%) being greater than
..
0.65% and 0.85% or less, the volume fraction of
pearlite is not less than 94% nor more than 100%, and
a structure of the other portion is composed of one
or two of proeutectoid ferrite and bainite,
in a region up to a depth of 1.0 mm from a
surface of the machine part, a volume fraction of
- 8 -
•pearlite block having an aspect ratio of 2.0 or more
is not less than 70% nor more than 95%, and a volume
fraction of pearlite having an angle between an axial
direction of the machine part and a lamellar
direction of the perlite on a cross section parallel
to the axial direction of 40° or less is 60% or more
with respect to all pearlite, and
a tensile strength is 1200 MPa or more and less
than 1500 MPa.
ADVANTAGEOUS EFFECTS OF INVENTION
[0014] According to the present invention, it is
possible to significantly improve hydrogen
embrittlement resistance while obtaining high
strength. Further, in significantly improving the
hydrogen embrittlement resistance, particularly, a
significant increase in manufacturing cost is also
not needed.
BRIEF DESCRIPTION OF DRAWINGS
[0015] [Fig. 1] Fig. 1 is a view illustrating a
relationship between an axial direction and a
lamellar direction; and
[Fig. 2] Fig. 2 is a view illustrating a
relationship between a tensile strength and an area
ratio of pearlite.
DESCRIPTION OF EMBODIMENTS
[0016] The present inventors investigated effects of
components and structures on hydrogen embrittlement
resistance of a high-strength machine part having a
tensile strength of 1200 MPa or more in detail, and
found components and structures for obtaining the
excellent hydrogen embrittlement resistance.
- 9 -
Further, as a result of repeated examinations of a
tJ method for obtaining the components and the
structures based on metallurgical knowledge, the
following facts became clear. Incidentally, the unit
"%" of content of each of the components in the
following explanation means "mass%."
[0017] First, a structure of a machine part will be
explained.
[0018] It is effective to elongate pearlite block in
a surface portion of a machine part in an orientation
parallel to the surface in order to obtain an
excellent hydrogen embrittlement resistance.
Further, it is also effective to align an orientation
of a lamellar layer of pearlite having a layer
structure of ferrite and cementite with the
orientation parallel to the surface. Here, the
pearlite block, of which the detail will be described
later, is a unit of pearlite made of ferrite and
cementite having an aligned orientation, in general.
[0019] Concretely, in a case when in a region up to
a depth of 1.0 mm from the surface (surface portion),
a volume fraction of pearlite block having an aspect
ratio of 2.0 or more is 70% or more with respect to
all pearlite, the hydrogen embrittlement resistance
improves significantly. Pearlite block having a
small aspect ratio, namely one that is not
sufficiently elongated does not contribute to the
hydrogen embrittlement resistance very much, so it is
preferable to suppress a ratio of the pearlite block
having a small aspect ratio. Here, the aspect ratio
of pearlite block is a ratio indicated by the major
- 10 -
.. axis dimension/minor axis dimension of the pearlite
block .
[0020] Further, in a case when a volume fraction of
pearlite having an angle between a lamellar direction
and an axial direction on a cross section parallel to
the axial direction of 40° or less in the surface
portion is 60% or less with respect to all pearlite,
the hydrogen embrittlement resistance improves
significantly.
[0021] Further, though the range of a C content will
be described later, when the C content is represented
by (C%), in a case of (C%) being not less than 0.35%
nor more than 0.65%, the volume fraction of pearlite
is 64 x (C%) + 52% or more, and in a case of (C%)
being greater than 0.65% and 0.85% or less, the
volume fraction of pearlite is not less than 94% nor
more than 100%, and a structure of the other portion
is composed of one or two of proeutectoid ferrite or
bainite, the hydrogen embrittlement resistance
improves significantly. Pealite has an effect of
improving the hydrogen embrittlement resistance.
Then, in a case when the volume fraction of pearlite
is less than 64 x (C%) + 52%, the sufficient hydrogen
embrittlement resistance cannot be obtained.
Further, structures such as ferrite and bainite other
than pearlite may be a starting point of fracture and
thus a working crack is likely to occur in cold
forging. Incidentally, in a case when structures
other than pearlite exist, the structures may be
proeutectoid ferrite and/or bainite. When martensite
is contained as one of the structures other than
- 11 -
•
pearlite, a crack is likely to occur in cold forging
and the hydrogen embrittlement resistance
deteriorates.
[0022] As above, the structure of the machine part
is specified, and thereby it is possible to improve
the hydrogen embrittlement resistance significantly.
Then, in a case when the machine part is a bolt, it
is possible to improve delayed fracture resistance
significantly. Further, such a machine part is
suitable for automobile parts and various industrial
machine parts, and further may be used as a machine
part for building.
[0023] Further, for obtaining the machine part such
as a bolt, for example, a wire rod of special steel
is made from a billet having a special steel
composition, a steel wire of special steel is made
from the wire rod of special steel, and forming work
of the steel wire of special steel is performed.
Then, in order to obtain the machine part excellent
in hydrogen embrittlement resistance as described
above, for example, it is preferable to make the
structure of the steel wire of special steel to be
the structure as described above and to perform
forming work such as cold forging without performing
a heat treatment such as spheroidizing. As compared
with a method in which softening the steel wire of
special steel is performed by a heat treatment such
as spheroidizing to perform working, there is
sometimes a case that the above method has difficulty
in performing cold working slightly, but is more
advantageous in terms of a reduction in cost due to
- 12 -
• the omission of a heat treatment, securing of the
excellent hydrogen embrittlement resistance, and the
like.
[0024] Next, there will be explained the components
contained in the machine part and a billet used for
manufacturing the machine part. The billet contains
c: 0.35% to 0.85%, Si: 0.05% to 2.0%, Mn: 0.20% to
1.0%, and Al: 0.005% to 0.05%, and a P content is
0.030% or less, a S content is 0.030% or less, and a
balance is composed of Fe and inevitable impurities.
Then, the composition of each of the wire rod, the
steel wire, and the machine part made from the billet
is also the same.
[0025] C is contained for securing a predetermined
tensile strength. When the C content is lower than
0.35%, it is difficult to secure the tensile strength
of 1200 MPa or more. On the other hand, when the C
content is higher than 0.85%, the strength
corresponding to the C content cannot be obtained and
cold forgeability deteriorates. Thus, the C content
is 0.35% to 0.85%. Incidentally, for obtaining
higher tensile strength, the C content is preferably
0.40% or higher, and is more preferably higher than
0.6%. Further, for obtaining better cold
forgeability, the C content is preferably 0.60% or
lower.
[0026] Si functions as a deoxidizing element, and
has an effect of increasing the tensile strength by
solid solution strengthening. When the Si content is
lower than 0.05%, these effects are insufficient. On
the other hand, when the Si content is higher than
- 13 -
2.0%, these effects are saturated and ductility
~ during hot rolling deteriorates, and thus a flaw is
likely to occur. Thus, the Si content is 0.05% to
2.0%. Incidentally, for obtaining higher tensile
strength, the Si content is preferably 0.20% or
higher. Further, for obtaining better workability by
decreasing a rolling load during the hot rolling, the
Si content is preferably 0.50% or lower.
[0027] Mn has an effect of increasing the tensile
strength of the steel after pearlite transformation.
When the Mn content is lower than 0.20%, this effect
is insufficient. On the other hand, when the Mn
content is higher than 1.0%, this effect is
saturated. Thus, the Mn content is 0.20% to 1.0%.
[0028] Al functions as a deoxidizing element.
Morever, Al has an effect of improving cold
workability by forming AIN to function as a pinning
particle to make crystal grains refined. Further, Al
has an effect of suppressing dynamic strain aging by
decreasing solid solution N, and also has an effect
of improving the hydrogen embrittlement resistance.
When the Al content is lower than 0.005%, these
effects are insufficient. On the other hand, when
the Al content is higher than 0.05%, these effects
are saturated and a flaw is likely to occur during
hot rolling. Thus, the Al content is 0.005% to
0.05%.
[0029] P and S are segregated at grain boundaries so
as to deteriorate the hydrogen embrittlement
resistance. Then, in a case when the content of each
of them is higher than 0.030%, the deterioration of
- 14 -
• the hydrogen embrittlement resistance is noticeable.
Thus, the P content and the S content are each 0.030%
or lower, and are each preferably 0.015% or lower.
[0030] Moreover, N may sometimes deteriorate the
cold workability by dynamic strain aging and
deteriorate also the hydrogen embrittlement
resistance. Therefore, a N content is preferably
small, is particularly preferably 0.005% or lower,
and is more preferably 0.004% or lower.
[0031] Incidentally, the billet, the wire rod, the
steel wire, and the machine part may also contain one
or two of Cr: 0.02% to 1.0% or Ni: 0.02% to 0.50%.
Moreover, the billet, the wire rod, the steel wire,
and the machine part may also contain one or two or
more of Ti: 0.002% to 0.050%, V: 0.01% to 0.20%, or
Nb: 0.005% to 0.100%. Moreover, the billet, the wire
rod, the steel wire, and the machine part may also
contain B: 0.0001% to 0.0060%.
[0032] Cr has an effect of increasing the tensile
strength of the steel after pearlite transformation.
When the Cr content is lower than 0.02%, this effect
is insufficient. On the other hand, when the Cr
content is higher than 1.0%, martensite is likely to
be formed, the cold workability deteriorates, and the
material cost is increased. Thus, the Cr content is
preferably 0.02% to 1.0%. For securely obtaining the
effect, the Cr content is more preferably 0.10% or
higher. Moreover, for suppressing the formation of
martensite, the Cr content is more preferably 0.50%
or lower.
[0033] Ni has an effect of increasing a toughness of
- 15 -
• a steel. When the Ni content is less than 0.02%,
this effect is insufficient. When the Ni content is
higher than 0.50%, martensite is likely to be formed,
the cold workability deteriorates, and the material
cost is increased. Thus, the Ni content is
preferably 0.02% to 0.50%. Incidentally, for
securely obtaining this effect, the Ni content is
more preferably 0.05% or higher. Moreover, for
suppressing the formation of martensite, the Ni
content is more preferably 0.20%.
[0034] Ti functions as a deoxidizing element, and
has an effect of increasing the tensile strength, the
yield strength, and the proof stress by causing Tie
to precipitate and has an effect of improving the
cold workability by decreasing solid solution N.
When the Ti content is lower than 0.002%, these
effects are insufficient. On the other hand, when
the Ti content is higher than 0.050%, these effects
are saturated and the hydrogen embrittlement
resistance deteriorates. Thus, the Ti content is
preferably 0.002% to 0.050%.
[0035] V has an effect of increasing the tensile
strength, the yield strength, and the proof stress by
causing VC being carbide to precipitate and has an
effect of improving the hydrogen embrittlement
resistance. When the V content is lower than 0.01%,
these effects are insufficient. On the other hand,
when the V content is higher than 0.20%, the material
cost is increased drastically. Thus, the V content
is preferably 0.01% to 0.20%.
[0036] Nb has an effect of increasing the tensile
- 16 -
•
strength, the yield strength, and the proof stress by
causing NbC being carbide to precipitate. When the
Nb content is lower than 0.005%, this effect is
insufficient. When the Nb content is higher than
0.100%, this effect is saturated. Thus, the Nb
content is preferably 0.005% to 0.10%.
[0037] B has an effect of improving the cold
workability and the hydrogen embrittlement resistance
by suppressing formation of grain boundary ferrite
and grain boundary bainite, and has an effect of
increasing the tensile strength after the pearlite
transformation. When the B content is lower than
0.0001%, these effects are insufficient. On the
other hand, when the B content is higher than
0.0060%, this effect is saturated. Thus, the B
content is preferably 0.0001% to 0.0060%.
[0038] Moreover, the billet, the wire rod, the steel
wire, and the machine part may also contain one or
two or more of Ca: 0.001 to 0.010%, Mg: 0.001 to
0.010%, and Zr: 0.001 to 0.010%. These elements each
function as a deoxidizing element and have an effect
of improving the hydrogen embrittlement resistance by
forming sulfides such as CaS and MgS to fix solid
solution S.
[0039] Further, the billet, the wire rod, the steel
wire, and the machine part each may contain 0
inevitably, and 0 may exist as oxides such as Al and
Ti. Then, as an 0 content is higher, coarse oxides
are likely to be formed and a fatigue fracture is
likely to occur. Therefore, the 0 content is
preferably 0.01% or less.
- 17 -
[0040] Next, there will be explained a manufacturing
~ method of a wire rod of special steel suitable for
manufacturing the machine part and the steel wire of
special steel as described above.
[0041] In the manufacturing method, hot rolling of
the billet containing the above-described components
is performed so as to obtain a steel material, next
the steel material is immersed in a first molten salt
bath to be held isothermally, and next the steel
material is immersed in a second molten salt bath to
be held isothermally. In the hot rolling, the
temperature of finish rolli~g is not lower than 800°C
nor higher than 950°C, and a grain size number of
austenite grains of the steel material is made 8 or
more. Moreover, the temperature of the first molten
salt bath is not lower than 400°C nor higher than
600°C, and the immersion into the first molten salt
bath is performed when the temperature of the steel
material is not lower than 750°C nor higher than
950°C, and a period of time for the isothermal holding
is not shorter than 5 seconds nor longer than 150
seconds. Further, the temperature of the second
molten salt bath is not lower than 500°C nor higher
than 600°C, and a period of time for the isothermal
holding is not shorter than 5 seconds nor longer than
150 seconds.
[0042] The temperature of the finish rolling affects
the grain size of austenite grains before the
pearlite transformation to occur thereafter, and when
the temperature of the finish rolling is higher than
950°C, fine grains with a grain size number of 8 or
- 18 -
• more are not likely to be obtained. On the other
hand, when the temperature of the finish rolling is
lower than BOO°C, a load during rolling is extremely
high and industrial mass production is difficult.
Thus, the temperature of the finish rolling is BOO°C
to 950°C. When mass productivity is considered, the
temperature of the finish rolling is preferably B50°C
or higher.
[0043] Moreover, when austenite grains before the
pearlite transformation are less than B, due to the
effect of coarse grains, a crack is likely to occur
during wire drawing and cold forging thereafter.
Thus, the grain size number of austenite grains is B
or more, and is preferably 10 or more.
[0044] In the present invention, by the isothermal
holding in the first molten salt bath, the
temperature of the steel material is rapidly lowered
to the temperature close to a starting temperature of
the pearlite transformation, and in the subsequent
isothermal holding in the second molten salt bath,
the pearlite transformation is caused to occur in the
steel material.
[0045] When the temperature of the steel material
when being immersed into the first molten salt bath
is lower than 750°C, ferrite is more likely to be
formed during the isothermal holding in the first or
second molten salt bath. On the other hand, when the
temperature is higher than 950°C, time is taken for
lowering the temperature. That is, time is taken for
lowering the temperature of the steel material close
to a starting temperature of the pearlite
- 19 -
transformation. Therefore, there is sometimes a case
~ that the pearlite transformation is not completed
during the isothermal holding in the second molten
salt bath and the structure such as bainite and/or
martensite is formed. Thus, the temperature of the
steel material when the steel material is immersed
into the first molten salt bath is 750°C to 950°C.
[0046] Moreover, when the temperature of the first
molten salt bath is lower than 400°C, bainite is
formed. On the other hand, when the temperature of
the first molten salt bath is higher than 600°C,
reaching to the starting temperature of the pearlite
transformation is delayed. Thus, the temperature of
the first molten salt bath is 400°C to 600°C.
Further, in a case when the temperature of the second
molten salt bath is 500°C to 600°C, the pearlite
transformation is completed for an extremely short
period of time. Thus, the temperature of the second
molten salt bath is 500°C to 600°C.
[0047] Further, when the period of time for the
isothermal holding in the first molten salt bath and
the second molten salt bath is shorter than 5
seconds, the temperature of the steel material cannot
be sometimes controlled sufficiently. On the other
hand, when the period of time for the isothermal
holding is longer than 150 seconds, a reduction in
productivity sometimes is noticeable. Thus, the
period of time for the isothermal holding in the
molten salt baths is 5 seconds to 150 seconds.
[0048] Incidentally, the same effect may be obtained
even though facilities such as a lead bath and a
- 20 -
•
fluidized bed are used in place of the molten salt
bath, but when a load on the environment and the
manufacturing cost are considered, the method of
using molten salt is extremely excellent.
[0049] Then, the wire rod of special steel obtained
by such processes has the above-described
composition, and in the case when (C%) is not less
than 0.35% nor more than 0.65%, the volume fraction
of pearlite is 64 x (C%) + 52% or more, and in the
case when (C%) is greater than 0.65% and 0.85% or
less, the volume fraction of pearlite is not less
than 94% nor more than 100%, and the structure of the
other portion is composed of one or two of
proeutectoid ferrite and bainite.
[0050] Also as for the wire rod of special steel, in
the case when the volume fraction of pearlite is less
than 64 x (C%) + 52%, the sufficient hydrogen
embrittlement resistance cannot be obtained.
Further, a structure other than pearlite such as
ferrite and bainite functions as a starting point of
fracture and a working crack is likely to occur in
the cold forging. Thus, it is important that in the
case of (C%) being not less than 0.35% nor more than
0.65%, the volume fraction of pearlite is 64 x (C%) +
52% or more, and in the case of (C%) being greater
than 0.65% and 0.85% or less, the volume fraction of
pearlite is not less than 94% nor more than 100% also
in the wire rod of special steel. Further, when
martensite is contained in the wire rod of special
steel as the structure other than pearlite, a crack
is likely to occur not only in the cold forging but
- 21 -
•
also in the wire drawing.
[0051] Incidentally, the volume fraction of pearlite
may be measured by an optical microscope observation
or electron microscope observation of the wire rod of
special steel, and may be obtained from an area ratio
in an arbitrary visual field. Further, the state of
austenite grains may be fixed in a manner that a
sample of the steel material immediately after the
rolling is taken to be quenched, and the grain size
may be measured by the method of JIS G0551 using the
sample after the quenching.
[0052] As above, in the manufacturing method of the
wire rod of special steel, the temperature control is
performed with the two molten salt baths immediately
after the hot rolling, utilizing remaining heat of
hot rolling. Then, according to the method, even
though addition of expensive alloy elements is
suppressed, the wire rod of special steel having the
high volume fraction of pearlite can be obtained.
That is, the high property can be obtained
inexpensively.
[0053] Then, in a case when a steel wire of special
steel having the structure as described above is made
from the wire rod of special steel manufactured in
the manner, wire drawing is performed under
predetermined conditions.
[0054] A total reduction of area in the wire drawing
is not less than 25% nor more than 80%. In a case
when the total reduction of area in the wire drawing
is lower than 25%, the elongation of pearlite block
is insufficient to thus make it impossible to obtain
- 22 -
•
the sufficient hydrogen embrittlement resistance. On
the other hand, when the total reduction of area is
higher than 80%, a working crack is likely to occur
in the cold forging. Thus, the total reduction of
area in the wire drawing is 25% to 80%.
Incidentally, the total reduction of area is
preferably 30% or more for promoting the elongation
of pearlite block. Further, for further suppressing
a working crack, the total reduction of area is
preferably 65% or less.
[0055] Further, the number of times of the wire
drawing is not limited in particular, and one time
may be accepted, or a plurality of times may also be
accepted. In the case when the wire drawing is
performed a plurality of times, the reduction of area
in the final wire drawing (final pass) is preferably
not less than 1% nor more than 15%. This is because
it is possible to further elongate the pearlite block
in the surface portion and to further align the
lamellar direction and the axial direction. When the
reduction of area in the final pass is lower than 1%,
it is likely to be difficult to uniformly apply
strain to a circumferential direction. On the other
hand, when the reduction of area in the final pass is
higher than 15%, the above-described effect cannot be
obtained easily. Thus, the reduction of area in the
final wire drawing in the case when the wire drawing
is performed a plurality of times is preferably 1% to
15%.
[0056] Further, the wire drawing is performed at
room temperature. Here, the room temperature may
- 23 -
correspond to -20°C to 50°C, but there is sometimes a
• case that during the wire drawing, the steel wire is
increased to about 100°C or so in temperature due to
heat generation by working.
[0057] By the wire drawing performed under such
conditions, the steel wire of special steel having
the desired strength and excellent hydrogen
embrittlement resistance can be obtained. That is,
there can be obtained a steel wire in which the
volume fraction of pearlite block having an aspect
ratio of 2.0 or more is 70% or more with respect to
all pearlite in a region up to a depth of 1.0 mm from
the surface, and the volume fraction of pearlite
having an angle between the axial direction and the
lamellar direction in the region to a depth of 1.0 mm
from the surface on the cross section parallel to the
axial direction of 40° or less is 60% or more with
respect to all pearlite.
[0058] As described above, in the case when the
volume fraction of pearlite block having an aspect
ratio of 2.0 or more is 70% or more with respect to
all pearlite in the region to a depth of 1.0 mm from
the surface, the excellent hydrogen embrittlement
resistance can be obtained. However, when this
volume fraction is higher than 95%, the cold
forgeability deteriorates. That is, the cold forging
is likely to be difficult to be performed. For this
reason, in the region to a depth of 1.0 mm from the
surface, the volume fraction of pearlite block as
above is 70% to 95% with respect to all pearlite.
Inciden~ally, for obtaining the more excellent
- 24 -
hydrogen embrittlement resistance, this volume
., fraction is preferably 80% or more. The reason why
the aspect ratio of pearlite block used for the
evaluation of the volume fraction is set to 2.0 or
more is because the pearlite block that is not
elongated sufficiently, of which the aspect ratio is
less than 2.0, does not contribute to the hydrogen
embrittlement resistance very much.
[0059] Further, as described above, in the case when
the volume fraction of pearlite having an angle
between the lamellar direction and the axial
direction in the region to a depth of 1.0 mm from the
surface on the cross section parallel to the axial
direction of 40° or less is 60% or more with respect
to all pearlite, the excellent hydrogen embrittlement
resistance can be obtained. Pearlite contributing to
the improvement of hydrogen embrittlement resistance
is one of which the angle is 40° or less mainly.
Thus, the angle of pearlite used for the evaluation
of the volume fraction is 40° or less. Further, in
the case when the volume fraction of pearlite having
the angle of 40° or less is less than 60%, the effect
of improving the hydrogen embrittlement resistance is
not sufficient. Thus, on the cross section parallel
to the axial direction, such a volume fraction of
pearlite is 60% or more with respect to all pearlite.
Incidentally, for obtaining the more excellent
hydrogen embrittlement resistance, the volume
fraction is preferably 70% or more.
[0060] Incidentally, the pearlite block described
here is a unit of pearlite composed of ferrite and
- 25 -
•
cementite having a misorientation within 15 degrees,
and the misorientation may be obtained from a crystal
orientation map of ferrite measured with an electron
back scattered diffraction (EBSD: electron back
scattered diffraction) apparatus. Further, the
aspect ratio of pearlite block is the ratio of the
major axis to the minor axis of the pearlite block,
and as for the steel wire of special steel after the
wire drawing, the aspect ratio is substantially equal
to a ratio of a dimension in the axial direction to a
dimension in a direction perpendicular to the axial
direction (a radial direction). Further, the
lamellar direction may be measured through an
electron microscope observation on the cross section
parallel to the axial direction.
[0061] Then, in a case when the machine part is made
from the steel wire of special steel manufactured in
this manner, for maintaining the above-described
microstructure, for example, the forming work such as
the cold forging is performed at room temperature of,
for example, -20°C to 50°C without performing a heat
treatment such as spheroidizing. Incidentally, in
the cold forging, there is sometimes a case that the
steel wire of special steel is increased to 300°C or
so in temperature due to heat generation by the
working.
[0062] Incidentally, the tensile strength of the
machine part to be targeted by the present invention
is not less than 1200 MPa nor more than 1500 MPa.
When the tensile strength is lower than 1200 MPa, a
hydrogen embrittlement is not likely to occur, and
- 26 -
thus the present invention is not required to be
.. applied. On the other hand, when the tensile
strength is higher than 1500 MPa, the forming work by
the cold forging is difficult to be performed, and
thus the manufacturing cost is increased.
[0063] Incidentally, the machine part manufactured
in this manner has the high strength and excellent
hydrogen embrittlement resistance, but is preferably
held for not shorter than 10 minutes nor longer than
60 minutes at 200°C to 600°C to thereafter be cooled,
for example, for improving other mechanical
properties. By performing such a process, it is
possible to improve the yield strength, the yield
ratio, the ductility, and so on.
[0064] As above, in a series of processes, the
material having the chemical composition adjusted so
as to turn the structure into pearlite is used, and
by a method of immersing the material in the molten
salt baths with utilizing remaining heat of hot
rolling, the material is made into the steel wire of
special steel having the structure of almost complete
pearlite. Then, this steel wire of special steel is
subjected to the wire drawing at room temperature
under the specific conditions to perform adjustment
of pearlite having the high strength and hydrogen
embrittlement resistance, and is formed into the
machine part. Thereafter, a heat treatment at a
relatively low temperature for recovering the
ductility may be performed according to need. As a
result, it is possible to significantly improve the
hydrogen embrittlement resistance of the machine part
- 27 -
• having a tensile strength of not less than 1200 MPa
nor more than 1500 MPa inexpensively. Further, as
the wire drawing, heavy wire drawing such as a
conventional technique is not required to be
performed.
EXAMPLE
[0065] Next, experiments conducted by the present
inventors will be explained. The conditions and so
on in these experiments are examples employed for
confirming the applicability and effects of the
present invention, and the present invention is not
limited to these examples.
[0066] First, billets each being a steel type
containing components presented in Table 1 were made.
Then, under the conditions presented in Table 2, the
billets were each subjected to the hot rolling
including the finish rolling, the isothermal holding
in the first molten salt bath, and the isothermal
holding in the second molten salt bath, and wire rods
each having a wire diameter (7.0 mm to 15.0 mm)
presented in Table 2 were obtained. Incidentally,
the first molten salt bath and the second molten salt
bath were disposed in a rolling line, and what is
called an in-line process was performed. Further,
after the hot rolling~ sampling was performed and the
grain size number of austenite grains before the
pearlite transformation was measured. Results of the
measurement are also presented in Table 2.
[0067]
[Table 1]
- 28 -
TABLE 1
- ---------------------------------------------------------------------------------------
•
N
~
STEEL
C Si Mn P S AI- N Cr Ni Mo V Nb Ti B OTHER REMARKS
TYPE
A 0.36 0.24 0.72 0.008 0.023 0.034 0.0024 0.010
B 0.38 0.23 0.65 0.015 0.006 0.038 0.0039
C 0.42 0.20 0.51 0.018 0.003 0.008 0.0028 0.48 0.030 Ca:0.0024
D 0.44 0.32 0.74 0.009 0.007 0.015 0.0029 0.14 0.015 0.0011
E 0.44 0.08 0.46 0.013 0.011 0.027 0.0029 0.20 0.15
F 0.46 0.32 0.72 0.015 0.016 0.027 0.0027 0.050
G 0.46 0.09 0.45 0.012 0.009 0.025 0.0029 0.18
H 0.48 0.21 0.72 0.011 0.014 0.011 0.0026 Mg:0.0015
I 0.48 1.24 0.41 0.016 0.013 0.035 0.0028 0.21 0.30 0.020 0.0009
J 0.52 0.21 0.73 0.014 0.012 0.028 0.0024 0.04
K 0.59 0.24 0.77 0.011 0.004 0.037 0.0035
L 0.67 0.22 0.71 0.009 0.005 0.025 0.0034
M 0.69 0.21 0.65 0.009 0.004 0.019 0.0036
0.47 0.17 0.80 0.017 0.032 0.032 0.0061 1.20 0.30
COPARATIVE
N EXAMPLE
0 0.29 0.52 1.10 0.012 0.016 0.030 0.0053
COPARATIVE
EXAMPLE
P 0.75 0.22 0.72 0.011 0.009 0.027 0.0045
Q 0.79 0.24 0.77 0.008 0.005 0.026 0.0046
• TABLE 2
w
o
TEMPERATURE
TEMPERATURE HOLDING TIME TEMPERATURE HOLDING TIME GRAIN SIZE TENSILE
TOTAL REDUCTION TEMPERATURE HOLDING
PRESENCE/
STEEL DIAMETER OF FINISH
OF FIRST IN FIRST OF SECOND IN SECOND UMBER OF stRENGTH
REDUCTION
OF AREA OF HEAT TIME OF
STANDARD TYPE (mrnl ROLLING MOLTEN MOLTEN MOLTEN MOLTEN AUSTENITE OF WIRE OF AREA AT FINAL TREATMENT HEAT OFABDSREANWCEING REMARKS
("C)
SALT BATH SALT BATH SALT BATH SALT BATH BEFORE ROD
(\) DRAWING
("C)
TREATMENT
("C) (sec) ("C) (sec) TRANSFORMATION (MPa) (\) (min) CRACK
1 A 15.0 880 540 40 540 70 8.9 712 68.0 12.7 400 30 NO CRACK EXAMPLE'
2 B 7.0 930 550 30 550 53 8.8 753 54.2 11. 5 380 30 NO CRACK EXAMPLE
3 C 15.0 880 530 43 540 78 9.2 762 66.9 9.8 450 30 NO CRACK EXAMPLE
4 0 14.5 860 560 32 560 55 10.9 788 58.0 9.8 - - NO CRACK EXAMPLE
5 0 14.5 860 - - - - 7.3 688 58.0 19.6 - - NO CRACK
COMPARATIVE
EXAMPLE
6 E 14.0 910 530 36 550 65 10.4 787 ll...Z 18.2 500 30 NO CRACK
COMPARATIVE
EXAMPLE
7 E 14.0 910 530 36 550 65 10.4 787 45.2 22.2 500 30 NO CRACK EXAMPLE
8 E 14.0 910 530 36 550 65 10.4 787 45.2 12.1 500 30 NO CRACK EXAMPLE
9 E 14 .0 910 ' 530 36 550 65 10.4 787 45.2 12.1 - - NO CRACK EXAMPLE
10 F 14 .5 910 540 40 550 70 10.3 824 58.4 20.6 430 30 NO CRACK EXAMPLE
11 G 14.5 910 570 47 580 80 10.1 761 52.5 19.2 540 30 NO CRACK EXAMPLE
12 H 14 .5 890 530 54 550 95 11.2 843 58.4 20.6 400 30 NO CRACK EXAMPLE
13 H 14.5 890 - - - - 7.1 691 58.4 20.6 400 30 NO CRACK
COMPARATIVE
EXAMPLE
llBA!Ul!G COMPARATIVE 14 H 12.5 900 480 ~ 550 15 11.8 880 50.9 - - - CRACK EXAMPLE
~
15 I 12.5 900 500 36 560 65 10.6 843 44.2 20.4 380 30 NO CRACK EXAMPLE
16 J 7.0 930 530 22 560 40 10.2 817 50.8 9 •. 5 480 30 NO CRACK EXAMPLE
17 J 7.0 930 lli 22 560 40 10.9 1062 38.0 - - - 0IClARAlUCIKiG COEMXPAAMRPALTEIVE
0CCJlRll&ll
18 K 13.5 910 550 32 550 40 9.9 967 43.0 10.9 - - NO CRACK EXAMPLE
19 K 8.0 930 550 22 550 40 10 .. 6 983 53.5 11.8 400 30 NO CRACK EXAMPLE
20 L 7,0 930 540 36 550 65 11.6 1076 28.6 7.4 - - NO CRACK EXAMPLE
21 M 14.5 890 530 51 550 90 9.2 1077 lL.5. 14.5 350 30 NO CRACK
COMPARATIVE
EXAMPLE
22 M 7.0 930 530 30 550 50 9.7 1112 az...o. 21.3 350 30 NO CRACK
COMPARATIVE
EXAMPLE
23 II 12.5 910 - - - - 7.5 870 - - - - NO CRACK
COMPARATIVE
EXAMPLE
24 Q 13.0 910 540 35 550 55 9.9 683 29.8 29.8 350 30 NO CRACK
COMPARATIVE
EXAMPLE
25 P 13.0 910 540 35 550 55 10.4 1145 ll..Jl 24.0 350 30 NO CRACK
COMPARATIVE
EXAMPLE
26 Q 13.0 910 540 35 550 55 9.7 1214 ~ 10.5 400 30 NO CRACK
COMPARATIVE
EXAMPLE
t-3 0
OJ 0
0' m
f-' CO
CD
N
.....- ...-----~.=. -.~.-.,=.-.""'~~;"W""',~ »5...%"""" t OM AM Ml.:mu kg; iCMl.AJk4L, ok) ;;WhiCkW k,3M!)iI£2P;"-,,·Ui,k.#. OM.} A,.A ok) ," kJi%.4#'$1..,U ;;:g;;g;;::;;;;:;;;;;:UP4 gAM" ;J4il&VJM@GM.
[0069] After the wire rods were made, the wire
4t drawing with a reduction of area presented in Table 2
was performed and steel wires were obtained.
Further, in standards 1 to 3, 6 to 8, 10 to 13, 15 to
16, 19, 21 to 22, and 24 to 26, a heat treatment
imitated from a heat treatment after a cold forging
was performed. Results of the heat treatment are
also presented in Table 2.
[0070] Further, as for each of the wire rods, the
type of metal structure and the volume fraction of
pearlite were measured. Results of the measurement
are presented in Table 3. Incidentally, in the
section of "METAL STRUCTURE" in Table 3, "P"
represents pearlite, "B" represents bainite, "F"
represents ferrite, and "M" represents martensite.
Further, in Table 3, "LOWER LIMIT OF VOLUME FRACTION
OF PEARLITE" indicates the value of 64 x (C%) + 52%
in the case when (C%) is 0.65% or less, and the value
is 94% in the case when (C%) is higher than 0.65%.
[0071]
[Table 3]
- 31 -
•
TABLE 3
VOLUME FRACTION
VOLUME FRACTION
LOWER LIMIT OF PEARLITE
OF VOLUME
VOLUME OF PEARLITE
HAVING ANGLE
STEEL METAL FRACTION OF BLOCK HAVING
STANDARD
TYPE STRUCTURE
FRACTION OF
PEARLITE ASPECT RATIO
BETWEEN LAMELLAR REMARKS
PEARLITE DIRECTION AND
(%)
(% ) OF 2.0
AXIAL DIRECTION OF
OR MORE (%)
40° OR LESS (%)
1 A P,B,F 75.0 87 71 71 EXAMPLE
2 B P,B,F 76.3 90 77 73 EXAMPLE
3 C P,B,F 78.9 88 72 72 EXAMPLE
4 D P,B,F 80.2 94 82 73 EXAMPLE
5 D P.. F 80.2 68 76 41 COMPARATIVE EXAMPLE
6 E P,B,F 80.2 92 62 48 COMPARATIVE EXAMPLE
7 E P,B,F 80.2 92 81 84 EXAMPLE
8 E P,B,F 80.2 92 83 66 EXAMPLE
8 E P,B,F 80.2 90 79 64 EXAMPLE
10 F P,B,F 81. 4 93 88 73 EXAMPLE
11 G P,B,F 81.4 92 87 75 EXAMPLE
12 H P,B,F 82.7 93 86 74 EXAMPLE
13 H P.. F 82.7 76 68 53 COMPARATIVE EXAMPLE
14 H P,B,M,F 82.7 67 74 63 COMPARATIVE EXAMPLE
15 I P,B,F 82.7 91 80 69 EXAMPLE
16 J P,B,F 85.3 92 88 82 EXAMPLE
17 J P,B,M 85.3 42 54 47 COMPARATIVE EXAMPLE
18 K P,B,F 89.8 95 88 72 EXAMPLE
19 K P,B,F 89.8 95 91 88 EXAMPLE
20 L P,B 94.0 98 73 82 EXAMPLE
21 M P 94.0 100 41 ti COMPARATIVE EXAMPLE
22 M P 94.0 100 :n 74 COMPARATIVE EXAMPLE
23 N M 82.1 - - - COMPARATIVE EXAMPLE
24 0 P.. F.. B 70.6 67 43 44 COMPARATIVE EXAMPLE
25 P P 94.0 100 77 57 COMPARATIVE EXAMPLE
26 Q P 94.0 100 52 43 COMPARATIVE EXAMPLE
~~~'!I~"!l"f'!~l~"""''1Y!'~"~, <,,ii''''\If\I''''' T',.M %% 1iii"",.},,%~H,; .3-, ". )(,i!ZW,{·,'Wi!ii.;;;;:;jii4i'1\W'!"%Ni!IIiJ!I'I\#.JtiPWm¥k;;;;;;:W,d&@@jiiiQ ,·M; "EiW,.2,JP ,X .lA"kARY%\Sk.Ai!%JIiAA';i;:;jlif,;;: )3s;;g::;A~_::IMJ" 4),K{ 0 }"j ytJ))P-,f,·"C"IFti __ , AA_S"_~,;;_,.J,gJ.;_U)$,*M,&!f,A,MJ.,,,'; iF\4( 4JbJ!AiE, J A';':!iWP) M\Nliik.A.- .,k; , ..liPfA,.- U AI;;:,;$.JP."., .{,-"£WiVe. {L, 0 ;, 4 f"'I\i!I'i!f.,.
•
a scanning electron microscope (SEM), and due to the
area ratio on a microscopic observation surface being
equal to the volume fraction of the structure, each
of the area ratios obtained by image analysis was set
to be the volume fraction of each of the structures.
Further, in the measurement of the area ratio, on a
cross section parallel to the axial direction of each
of the steel wires, a region having a size of 125 pm
x 95 pm in a surface portion was photographed at a
magnification of 1000 times and the area ratio of
pearlite was obtained by image analysis.
[0073] As for each of the steel wires, the volume
fraction of pearlite block having an aspect ratio of
2.0 or more was measured. Further, on each of the
cross sections parallel to the axial direction, the
volume fraction of pearlite having an angle between
the lamellar direction and the axial direction in the
surface portion of 40° or less was also measured.
Results of the measurement are also presented in
Table 4. Incidentally, the type of structure of each
of the steel wires is the same as that of each of the
wire rods.
[0074] For identification of the pearlite block, an
EBSD apparatus was used. That is, on each of the
cross sections parallel to the axial direction, a
crystal orientation map of ferrite in a region having
a size of 275 pm x 165 pm in the surface portion was
obtained with an EBSD apparatus, and from the crystal
orientation map, a boundary having a misorientation
of 15 degrees or more was set to a boundary of the
pearlite block. Then, the aspect ratio of the
- 33 -
•
pearlite block having a circle-equivalent diameter of
1.0 pm or more among the pearlite blocks each
surrounded by the boundary was obtained.
[0075] Further, on each of the cross sections
parallel to the axial direction, in the measurement
of the volume fraction of pearlite having an angle
between the lamellar direction and the axial
direction in the surface portion of 40° or less,
based on a SEM photograph at a magnification of 5000
times obtained by photographing a region in the
surface portion, the region was subjected to image
analysis. Concretely, as illustrated in Fig. 1, a
region of which an angle between the lamellar
direction and the axial direction (misorientation)
was 40° or less was obtained from the SEM photograph
and an area of the region was subjected to image
analysis. Each of the white arrows in Fig. 1
indicates the lamellar direction.
[0076] After each of the steel wires was made, the
properties (the tensile strength, the hydrogen
embrittlement resistance, and the cold forgeability)
of the steel wire after being subjected to the
processes presented in Table 3 were evaluated.
Results of the evaluation are presented in Table 4.
[0077] In the evaluation of the tensile strength, a
9A test piece of JIS Z2201 was made from each of the
steel wires, and a tensile test based on the test
method of JIS Z2241 was performed. Incidentally, the
tensile strength of the machine part made from each
of these steel wires is equal to that of the steel
wire.
- 34 -
[0078] In the evaluation of the hydrogen
~ embrittlement resistance, each of the steel wires was
formed into a bolt, and diffusible hydrogen of 0.5
ppm was contained in each of samples by electric
field hydrogen charge, and then Cd plating was
performed so that hydrogen might not be released into
the atmosphere from the sample during the test.
Thereafter, a load of 90% of a maximum tensile load
was loaded in the atmosphere and the existence or
absence of fracture after 100 hours was confirmed.
Then, one having had no fracture caused therein was
evaluated to be "excellent" and one having had
fracture caused therein was evaluated to be "poor."
[0079] In the evaluation of the cold forgeability, a
sample having a diameter of 5.0 mm and a length of
7.5 mm was made from each of the steel wires by
machining, and edge surfaces were held by molds each
having a groove therein concentrically and a
compression test was performed. Then, one having had
no working crack caused therein when the steel wire
was worked at a compression ratio of 50% was
evaluated to be "excellent" and one having had a
working crack caused therein was evaluated to be
"poor."
[0080]
[Table 4]
- 35 -
• TABLE 4
STEEL
TENSILE HYDRGEN
STANDARD TYPE STRENGTH EMBRITTLEMENT FORGCEOALBDILITY REMARKS
(MPa) RESISTANCE
1 A 1207 EXCELLENT EXCELLENT EXAMPLE
2 B 1220 EXCELLENT EXCELLENT EXAMPLE
3 C 1243 EXCELLENT EXCELLENT EXAMPLE
4 D 1262 EXCELLENT EXCELLENT EXAMPLE
5 D 1083 POOR POOR
COMPARATIVE
EXAMPLE
6 E 1050 POOR POOR
COMPARATIVE
EXAMPLE
7 E 1245 EXCELLENT EXCELLENT EXAMPLE
8 E 1216 EXCELLENT EXCELLENT EXAMPLE
9 E 1256 EXCELLENT EXCELLENT EXAMPLE
10 F 1220 EXCELLENT EXCELLENT EXAMPLE
11 G 1286 EXCELLENT EXCELLENT EXAMPLE
12 H 1222 EXCELLENT EXCELLENT EXAMPLE
13 1178 POOR POOR
COMPARATIVE
H EXAMPLE
14 1273
COMPARATIVE
H - -
EXAMPLE
15 I 1235 EXCELLENT EXCELLENT EXAMPLE
16 J 1366 EXCELLENT EXCELLENT EXAMPLE
17 1420
COMPARATIVE
J - -
EXAMPLE
18 K 1235 EXCELLENT EXCELLENT EXAMPLE
19 K 1405 EXCELLENT EXCELLENT EXAMPLE
20 L 1276 EXCELLENT EXCELLENT EXAMPLE
21 1232 POOR POOR
COMPARATIVE
M EXAMPLE
22 M 1591 EXCELLENT POOR
COMPARATIVE
EXAMPLE
23 1521 POOR
COMPARATIVE
Ii -
EXAMPLE
24 Q 1026 POOR EXCELLENT COMPARATIVE
EXAMPLE
25 1280 EXCELLENT POOR
COMPARATIVE
P EXAMPLE
26 1332 POOR POOR
COMPARATIVE
Q EXAMPLE
[0081] In Table 2, the standards 5 and 13 correspond
to a conventional manufacturing method in which
cooling is performed on a Stelmor without performing
an isothermal transformation process after coiling,
and the volume fraction of pearlite of each of them
- 36 -
•
falls outside the range of the present invention. In
the standard 14, the holding time in the first molten
salt bath is shorter than the lower limit of the
present invention. In this case, martensite is mixed
in the metal structure and the volume fraction of
pearlite falls outside the range of the present
invention. In the standard 17, the temperature of
the first molten salt bath is lower than the lower
limit of the present invention. In this case,
martensite is mixed in the metal structure and the
volume fraction of pearlite falls outside the range
of the present invention. In the standards 6, 21,
25, and 26, the reduction of area in the wire drawing
is less than the lower limit of the present
invention. In this case, the volume fraction of
pearlite having an aspect of 2.0 or more, or the
volume fraction of pearlite having an angle between
the lamellar direction and the axial direction of 40°
or less falls outside the range of the present
invention. In the standard 23, Cr and Mo are
contained and the steel type of N having a
composition falling outside the range of the present
invention was used. Further, after coiling, the
processes with the use of the first and second molten
salt baths were not performed and cooling was
performed on a Stelmor, and the wire rod was thus
manufactured, and thereafter the wire rod was heated
to 880°C and was subjected to oil quenching and
hardening, and next was subjected to tempering at
580°C. As a result, the obtained structure is
tempered martensite to thus fall outside the range of
- 37 -
the present invention.
.. [0082] As for the grain size number of austenite
grains before the pearlite transformation presented
in Table 2, in both the standards 4 and 12 each
satisfying the condition of the present invention,
the grain size number is 10 or more. In contrast to
this, in the standards 5, 13, and 23 each having the
manufacturing condition falling outside the range of
the present invention, the grain size number is less
than 8, and it is found from Table 4 that they
deteriorate in the cold forgeability or hydrogen
embrittlement resistance. In the standards 14 and 17
each containing martensite, wire breakage or a crack
occurred during the wire drawing. That is, wire
drawability was poor.
[0083] In all the standards 5, 13, 23, and 24 in
which the volume fraction of pearlite falls outside
the range of the present invention, the hydrogen
embrittlement resistance is poor. Further, in all
the standards 6, 13, 21, 23, 24, and 26, in which the
volume fraction of pearlite having an aspect ratio of
2.0 or more falls outside the range of the present
invention, the hydrogen embrittlement resistance is
poor. In the standards 5, 6, 13, 21, 23, 24, 25, and
26, in which the area ratio of pearlite having an
angle between the lamellar direction and the axial
direction of 40° or less falls outside the range of
the present invention, the hydrogen embrittlement
resistance and/or the cold forgeability are/is poor.
Further, in the standard 22, in which the volume
fraction of pearlite having an aspect ratio of 2.0 or
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more is higher than the upper limit of the present
.. invention, the cold forgeability is poor.
[0084] From the above, it is found that the machine
part according to the present invention is excellent
in hydrogen embrittlement resistance and cold
forgeability.
[0085] Fig. 2 illustrates the relationship between a
tensile strength TS and the area ratio of pearlite
having an angle between the axial direction and the
lamella from the axial direction of 40° or less. It
is found that in the standards each satisfying the
range of the present invention, the delayed fracture
resistance and the cold forgeability are both
excellent.
INDUSTRIAL APPLICABILITY
[0086] It is possible to utilize the present
invention in industries related to, for example,
automobile parts, various industrial machine parts,
building parts, and so on.

• [Claim 1]
CLAIMS
A steel wire of special steel containing:
in mass%;
C: 0.35% to 0.85%;
Si: 0.05% to 2.0%;
Mn: 0.20% to 1.0%; and
AI: 0.005% to 0.05%,
a P content being 0.030% or less,
a S content being 0.030% or less, and
a balance being composed of Fe and inevitable
impurities, wherein
when a C content is represented by (C%), in a
case of (C%) being not less than 0.35% nor more than
0.65%, a volume fraction of pearlite is 64 x (C%) +
52% or more, and in a case of (C%) being greater than
0.65% and 0.85% or less, the volume fraction of
pearlite is not less than 94% nor more than 100%, and
a structure of the other portion is composed of one
or two of proeutectoid ferrite or bainite,
in a region up to a depth of 1.0 mm from a
surface of the steel wire, a volume fraction of
pearlite block having an aspect ratio of 2.0 or more
is not less than 70% nor more than 95%, and a volume
fraction of pearlite having an angle between an axial
direction of the steel wire and a lamellar direction
of the perlite on a cross section parallel to the
axial direction of 40° or less is 60% or more with
respect to all pearlite, and
a tensile strength is 1200 MPa or more and less
than 1500 MPa.
[Claim 2] The steel wire of special steel according
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to claim 1, wherein, in mass%, a N content is 0.0050%
or less. • [Claim 3] The steel wire of special steel according
to claim 1, further containing, in mass%, one or two
of Cr: 0.02% to 1.0% and Ni: 0.02% to 0.50%.
[Claim 4] The steel wire of special steel according
to claim 1, further containing, in mass%, one or two
or more of Ti: 0.002% to 0.050%, V: 0.01% to 0.20%,
or Nb: 0.005% to 0.100%.
[Claim 5] The steel wire of special steel according
to claim 1, further containing, in mass%, B: 0.0001%
to 0.0060%.
[Claim 6] The steel wire of special steel according
to claim 1, further containing, in mass%, one or two
or more of Ca: 0.001% to 0.010%, Mg: 0.001% to
0.010%, or Zr: 0.001% to 0.010%.
[Claim 7] A wire rod of special steel containing:
in mass%;
C: 0.35 to 0.85%;
Si: 0.05 to 2.0%;
Mn: 0.20 to 1 . 0%;
P: 0.030% or less;
S: 0.030% or less; and
Al: 0.005 to 0.05%,
a balance being composed of Fe and inevitable
impurities, wherein
when a C content is represented by (C%), in a
case of (C%) being not less than 0.35% nor more than
0.65%, a volume fraction of pearlite is 64 x (C%) +
52% or more, and in a case of (C%) being greater than
0.65% and 0.85% or less, the volume fraction of
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pearlite is not less than 94% nor more than 100%, and
4t a structure of the other portion is composed of one
or two of proeutectoid ferrite and bainite.
[Claim 8] The wire rod of special steel according to
claim 7, wherein, in mass%, a N content is 0.0050% or
less.
[Claim 9] The wire rod of special steel according to
claim 7, further containing, in mass%, one or two of
Cr: 0.02% to 1.0% and Ni: 0.02% to 0.50%.
[Claim 10] The wire rod of special steel according
to claim 7, further containing, in mass%, one or two
or more of Ti: 0.002% to 0.050%, V: 0.01% to 0.20%,
or Nb: 0.005% to 0.100%.
[Claim 11] The wire rod of special steel according
to claim 7, further containing, in mass%, B: 0.0001%
to 0.0060%.
[Claim 12] The wire rod of special steel according
to claim 7, further containing, in mass%, one or two
or more of Ca: 0.001% to 0.010%, Mg: 0.001% to
0.010%, or Zr: 0.001% to 0.010%.
[Claim 13] A manufacturing method of a steel wire of
special steel comprising:
performing hot rolling of a billet with a
temperature of finish rolling being not lower than
800°C nor higher than 950°C so as to obtain a steel
material having a grain size number of austenite
grains being 8 or more;
next, immersing the steel material having a
temperature of not lower than 750°C nor higher than
950°C in a first molten salt bath having a temperature
of not lower than 400°C nor higher than 600°C and
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•
isothermally holding the steel material for not
shorter than 5 seconds nor longer than 150 seconds;
next, immersing the steel material in a second
molten salt bath having a temperature of not lower
than 500°C nor higher than 600°C and isothermally
holding the steel material for not shorter than 5
seconds nor longer than 150 seconds; and
next, performing wire drawing with a total
reduction of area of not less than 25% nor more than
80% on the wire rod at room temperature, wherein
the steel material contains:
in mass%;
C: 0.35% to 0.85%;
Si: 0.05% to 2.0%;
Mn: 0.20% to 1.0%; and
AI: 0.005% to 0.05%,
a P content being 0.030% or less,
a S content being 0.030% or less, and
a balance being composed of Fe and inevitable
impurities.
[Claim 14] The manufacturing method of a steel wire
of special steel according to claim 13, wherein a
reduction of area at the final of the wire drawing is
not less than 1% nor more than 15%.
[Claim 15] A manufacturing method of a wire rod of
special steel comprising:
performing hot rolling of a billet with a
temperature of finish rolling being not lower than
800°C nor higher than 950°C so as to obtain a steel
material having a grain size number of austenite
grains being 8 or more;
- 43 -
next, immersing the steel material having a
~ temperature of not lower than 750°C nor higher than
950°C in a first molten salt bath having a temperature
of not lower than 400°C nor higher than 600°C and
isothermally holding the steel material for not
shorter than 5 seconds nor longer than 150 seconds;
and
next, immersing the steel material in a second
molten salt bath having a temperature of not lower
than 500°C nor higher than 600°C and isothermally
holding the steel material for not shorter than 5
seconds nor longer than 150 seconds, wherein
the steel material contains:
in mass%;
C: 0.35% to 0.85%;
Si: 0.05% to 2 . 0%;
Mn: 0.20% to 1 . 0%; and
AI: 0.005% to 0.05%,
a P content being 0.030% or less,
a S content being 0.030% or less, and
a balance being composed of Fe and inevitable
impurities.
[Claim 16] A machine part containing:
in mass%;
C: 0.35% to 0.85%;
Si: 0.05% to 2 . 0%;
Mn: 0.20% to 1 . 0%; and
AI: 0.005% to 0.05%;
a P content being 0.030% or less;
a S content being 0.030% or less; and
a balance being composed of Fe and inevitable
- 44 -
•
impurities, wherein
when the C content is represented by (C%), in a
case of (C%) being not less than 0.35% nor more than
0.65%, a volume fraction of pearlite is 64 x (C%) +
52% or more, and in a case of (C%) being greater than
0.65% and 0.85% or less, the volume fraction of
pearlite is not less than 94% nor more than 100%, and
a structure of the other portion is composed of one
or two of proeutectoid ferrite and bainite,
in a region up to a depth of 1.0 mm from a
surface of the machine part, a volume fraction of
pearlite block having an aspect ratio of 2.0 or more
is not less than 70% nor more than 95%, and a volume
fraction of pearlite having an angle between an axial
direction of the machine part and a lamellar
direction of the perlite on a cross section parallel
to the axial direction of 40° or less is 60% or more
with respect to all pearlite, and
a tensile strength is 1200 MPa or more and less
than 1500 MPa.
Dated this 15/02/2013
NEHA SRIVASTAVA
OF REM FRY & SAGAR
ATTORNEY FOR THE APPLICANTS
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