[Technical Field of the Invention]
[0001] The present invention relates to a steel wire rod.
[Related Art]
[0003] In an aluminum stranded cable used for a power line or the like, strands of steel
wire are used as a core to reinforce the strength. This stranded cable is generally
called "aluminum conductor steel-reinforced cable". Hereinafter, the aluminum conductor
steel-reinforced cable will be abbreviated as "ACSR".
[0004] In many cases, steel wire used as a core of ACSR is produced by cold drawing a piano
wire rod such as SWRS72B or SWRS82B specified in the JIS standard. In order to produce
ACSR, as necessary, a heat treatment called patenting or aluminum plating treatment
is performed before or during cold drawing.
[0005] In ACSR, not only aluminum wire but also steel wire as a core are electrically charged.
Therefore, in a case where the electrical resistivity of steel wire increases, the
total electrical resistivity of ACSR increases, and thus the amount of heat generated
during power transmission is increased. As a result, the transmission efficiency decreases.
[0006] In addition, in a case where the strength of steel wire is low, it is necessary that
the amount of steel wire having a higher electrical resistivity than aluminum is increased
to reinforce the strength. However, since steel wire is also electrically conductive,
as a result, the total electrical resistivity of ACSR is increased.
[0007] Therefore, steel wire having a low electrical resistivity and a high strength, and
a steel wire rod having a low electrical resistivity and a high strength as a material
of the steel wire, are required.
[0008] In general, the electrical resistivity of steel increases as the amounts of elements
in the steel increase. Therefore, in steel disclosed in Patent Document 1, by reducing
the amounts of major elements such as C, Mn, or Cr, the electrical resistivity is
reduced. However, in this steel, the C content and the Si content in the steel are
adjusted to be low in order to reduce the electrical resistivity and to improve cold
forgeability. Therefore, the tensile strength is insufficient.
[0009] In addition, in a steel sheet for spring disclosed in Patent Document 2, by adjusting
the C content, the Si content, and the Mn content in the steel to be lower than a
value obtained from a predetermined expression, the electrical resistivity of the
steel is reduced. However, since the metallographic structure is not optimized in
this steel sheet for spring and does not include Cr, the tensile strength cannot be
increased. Therefore, a balance between the securing of strength and the reduction
in electrical resistivity is not sufficient.
[0010] Further, in hypereutectoid steel wire having high strength and high toughness disclosed
in Patent Document 3, by specifying the contents of elements such as C, Si, or Mn
in the steel and a metallographic structure thereof, the tensile strength and drawability
are secured. However, in the hypereutectoid steel wire having high strength and high
toughness, the Si content is 0.5% or more, and a metallographic structure is not optimized
to reduce the electrical resistivity. Therefore, the electrical resistivity is high.
[0011] Patent Document 4 discloses a high carbon steel wire rod in which the spheroidizing
heat treatment time is reduced by increasing the Cr content in a carbide. However,
in this high carbon steel wire rod, the Cr content in the carbide is 6.0 mass% or
more. Therefore, a reduction in electrical resistivity and an increase in tensile
strength cannot be achieved at the same time.
[Prior Art Document]
[Patent Document]
[0012]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No.
2003-226938
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No.
2004-156120
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No.
H6-271937
[Patent Document 4] PCT International Publication No. WO 2012/144630
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0013] The present invention has been made in consideration of the above-described circumstances,
and an object thereof is to provide a steel wire rod having a high strength and a
low electrical resistivity.
[Means for Solving the Problem]
[0014] In a steel wire rod according to the present invention, in particular, the Si content
which increases the electrical resistivity is limited, and the Cr content in cementite
in pearlite is increased within a range where the electrical resistivity and the tensile
strength are well-balanced, and conversely, the Cr content in ferrite in pearlite
is reduced. As a result, an increase in electrical resistivity is prevented. In addition,
in the steel wire rod according to the present invention, by reducing the average
lamellar spacing of pearlite, the tensile strength is increased, and the reduction
in the electrical resistivity and the improvement of the tensile strength are achieved
at the same time.
[0015] The steel wire rod according to the present invention is a material before performing
cold drawing, and includes a hot rolled wire rod and a steel obtained by performing
a heat treatment on the hot rolled wire rod.
[0016] In order to solve the above-described problems and to obtain a steel wire rod having
a high strength and a low electrical resistivity, the present inventors performed
a detailed investigation and study of the chemical composition, the metallographic
structure, and the distribution state of alloy elements in the steel wire rod. As
a result, the following findings (a) to (c) were made.
- (a) The electrical resistivity of the steel wire rod at room temperature does not
substantially change due to cold drawing. Therefore, components of the steel wire
rod, a metallographic structure before cold drawing, and a state where alloy elements
are present in the steel wire rod have large effects on the electrical resistivity
and the tensile strength after cold drawing.
- (b) In a case where the Si content and the Cr content are increased, the strength
of the steel wire rod increases due to solid solution strengthening. However, an increase
in the Si content causes a significant increase in the electrical resistivity. On
the other hand, an increase in the Cr content also causes an increase in the electrical
resistivity, but Cr has a smaller effect than Si. In addition, when Cr is solid soluted
in ferrite, Cr increases the electrical resistivity. Therefore, by increasing the
Cr content in cementite and decreasing the Cr content in ferrite, an increase in electrical
resistivity can be suppressed. In other words, by increasing the Cr content in cementite
in pearlite and decreasing the Cr content in ferrite in pearlite, an increase in the
electrical resistivity can be suppressed.
In addition, by controlling the Cr content in cementite according to the Si content
contributing to solid solution strengthening, the improvement of the tensile strength
and the reduction in the electrical resistivity can be achieved at the same time with
high efficiency.
Hereinafter, "the Cr content in cementite in pearlite" will also be referred to simply
as "the Cr content in cementite", and "the Cr content in ferrite in pearlite" will
also be referred to simply as "the Cr content in ferrite".
- (c) In order to achieve the improvement of the tensile strength of the steel wire
rod and the reduction in the electrical resistivity of the steel wire rod at the same
time, it is effective that the metallographic structure of the steel wire rod includes
pearlite. Further, when the metallographic structure of the steel wire rod is pearlite
having a lamellar structure of ferrite and cementite, the tensile strength can be
improved by reducing the average lamellar spacing. On the other hand, the average
lamellar spacing of pearlite has little effect on the electrical resistivity. Therefore,
in order to achieve the improvement of the tensile strength and the reduction in the
electrical resistivity at the same time, it is preferable that the average lamellar
spacing is reduced.
[0017] The present invention has been made based on the above findings, and the scope thereof
is as follows.
- (1) According to an aspect of the present invention, a steel wire rod includes, as
a chemical composition, by mass%, C: 0.8% to 1.1%, Si: 0.02% to 0.30%, Mn: 0.1% to
0.6%, Cr: 0.3% to 1.5%, Al: 0.01% to 0.05%, N: limited to 0.008% or less, P: limited
to 0.03% or less, S: limited to 0.02% or less, and optionally includes one or more
selected from the group consisting of Mo: 0.20% or less, V: 0.15% or less, Ti: 0.050%
or less, Nb: 0.050% or less, and B: 0.0030% or less, and a remainder of Fe and impurities;
in which a metallographic structure includes a pearlite and an area ratio of the pearlite
is 85% or more; an average lamellar spacing of the pearlite is 50 nm to 100 nm; and
when the Si content is represented by [%Si] by mass%, the Cr content in a cementite
in the pearlite is represented by [%Crθ] by mass%, and the Cr content in a ferrite
in the pearlite is represented by [%Crα] by mass%, [%Si], [%Crθ], and [%Crα] satisfy
the following expression (a),
- (2) The steel wire rod according to (1) may include, as the chemical composition,
by mass%, one or more selected from the group consisting of Mo: 0.02% to 0.20%, V:
0.02% to 0.15%, Ti: 0.002% to 0.050%, Nb: 0.002% to 0.050%, and B: 0.0003% to 0.0030%.
- (3) In the steel wire rod according to (1) or (2), a tensile strength TS of the steel
wire rod may be 1350 MPa or more, and an absolute value of the tensile strength TS
of the steel wire rod may be 64 times or more an absolute value of an electrical resistivity
p expressed in units of µΩ·cm of the steel wire rod.
[Effects of the Invention]
[0018] According to the aspects (1) to (3), a steel wire rod having a high strength and
a low electrical resistivity can be provided. In particular, the steel wire rod according
to the aspect is suitable as a material of steel wire which is used for reinforcing
the strength of a power line and reduces power loss.
[0019] In addition, steel wire, which is obtained by cold drawing the steel wire rod according
to the aspect and optionally performing the aluminum plating treatment after cold
drawing, has a high strength and a low electrical resistivity. Therefore, when ACSR
is produced using this steel wire, a predetermined strength can be secured and the
electrical resistivity can be further reduced in the ACSR. Therefore, the contribution
to the industry is remarkable.
[Embodiment of the Invention]
[0020] A steel wire rod according to an embodiment will be described.
[0021] First, the reason for limiting a chemical composition of the steel wire rod according
to the embodiment will be described. In the following description, % represents mass%.
C: 0.8% to 1.1%
[0022] C is an element which is effective for allowing a metallographic structure of the
steel wire rod to include pearlite and to thereby improve the tensile strength.
[0023] In a case where the C content is less than 0.8%, it is difficult to stably impart
a high tensile strength of, for example, 1350 MPa to the steel wire rod. Therefore,
the lower limit of the C content is set to 0.8%. In order to obtain more uniform pearlite
and to improve the tensile strength, the C content is preferably 0.9% or more and
more preferably 1.0% or more.
[0024] On the other hand, when the C content is excessively high, the steel wire rod is
hardened, and it causes deterioration in drawability. In particular, when the C content
is higher than 1.1%, since it is difficult to industrially stably suppress the formation
of cementite precipitating along a prior austenite grain boundary, that is, proeutectoid
cementite, drawability deteriorates significantly. Therefore, the upper limit of the
C content is set to 1.1%.
Si: 0.02% to 0.30%,
[0025] Si is an element which is effective for improving the strength of the steel wire
rod by solid solution strengthening and is also necessary as a deoxidizer.
[0026] When the Si content is less than 0.02%, these effects are not sufficient. Therefore,
the lower limit of the Si content is set to 0.02%. In addition, in order to secure
the strength by solid solution strengthening and to exhibits the deoxidation effect
more stably, the Si content is preferably 0.05% or more.
[0027] On the other hand, when the Si content increases, the electrical resistivity increases.
In particular, when the Si content is higher than 0.30%, the improvement of the tensile
strength and the reduction in the electrical resistivity cannot be achieved at the
same time. Therefore, the upper limit of the Si content is set to 0.30%. In order
to obtain a lower electrical resistivity, the Si content is preferably 0.20% or less
and more preferably 0.10% or less.
Mn: 0.1% to 0.6%
[0028] Mn is an element which has effects of improving the strength of the steel wire rod
and preventing hot brittleness by fixing S as MnS in the steel wire rod.
[0029] When the Mn content is less than 0.1%, these effects are not sufficient. Therefore,
the lower limit of the Mn content is set to 0.1%. Further, in order to secure the
strength and further prevent hot brittleness, the Mn content is preferably 0.2% or
more and is more preferably 0.3% or more.
[0030] On the other hand, when the Mn content increases, the electrical resistivity increases.
In particular, when the Mn content is higher than 0.6%, the improvement of the tensile
strength and the reduction in the electrical resistivity cannot be achieved at the
same time. Therefore, the upper limit of the Mn content is set to 0.6%. In order to
obtain a lower electrical resistivity, the Mn content is preferably 0.5% or less and
is more preferably 0.4% or less.
Cr: 0.3% to 1.5%
[0031] Cr has an effect of reducing the average lamellar spacing of pearlite to improve
the tensile strength of the steel wire rod. In addition, when Cr is solid soluted
in ferrite in pearlite, Cr increases the electrical resistivity. Therefore, an effect
of suppressing an increase in electrical resistivity can be obtained by increasing
the Cr content in cementite such that the Cr content in ferrite is relatively reduced.
[0032] When the Cr content is lower than 0.3%, a sufficient tensile strength of the steel
wire rod cannot be secured, and the Cr content in cementite cannot be increased. Therefore,
in order to achieve the improvement of the tensile strength and the reduction in the
electrical resistivity at the same time, it is necessary that the Cr content is 0.3%
or more. In order to obtain a higher tensile strength, the Cr content is preferably
0.4% or more and more preferably 0.5% or more.
[0033] On the other hand, when the Cr content is higher than 1.5%, the drawability of the
steel wire rod deteriorates. Therefore, the upper limit of the Cr content is set to
1.5%. In order to further suppress deterioration in drawability, the Cr content is
preferably 1.0% or less and more preferably 0.8% or less.
Al: 0.01% to 0.05%
[0034] Al is an element which has a deoxidation effect and is necessary to reduce the oxygen
content in the steel wire rod.
[0035] When the Al content is lower than 0.01%, this effect is not sufficient. Therefore,
the lower limit of the Al content is set to 0.01%. In order to improve the deoxidation
effect, the Al content is preferably 0.02% or more.
[0036] On the other hand, Al is an element which forms a hard oxide-based inclusion and
deteriorates the ductility of the steel wire rod. In particular, when the Al content
is higher than 0.05%, a coarse oxide-based inclusion is likely to be formed. Therefore,
the drawability of the steel wire rod deteriorates significantly. Therefore, the upper
limit of the Al content is set to 0.05%. In order to further limit deterioration in
the drawability of the steel wire rod, the Al content is preferably 0.04% or less
and more preferably 0.03% or less.
[0037] In the steel wire rod according to the embodiment, it is necessary to limit N, P,
and S as described below.
N: 0.008% or less
[0038] N is an element which is pinned to dislocations in the steel to deteriorate drawability
during cold drawing. In particular, when the N content is higher than 0.008%, the
drawability deteriorates significantly. Accordingly, the N content is limited to 0.008%
or less. The N content is preferably 0.005% or less and more preferably 0.004% or
less.
[0039] In addition, the lower limit of the N content may be 0%. However, in consideration
of the current refining technique and the production costs, the lower limit of the
N content is preferably 0.0001%.
P: 0.03% or less
[0040] P is an element which segregates in a grain boundary and deteriorates drawability.
In particular, when the P content is higher than 0.03%, the drawability deteriorates
significantly. Accordingly, the P content is limited to 0.03% or less. The P content
is preferably 0.02% or less and more preferably 0.01% or less.
[0041] In addition, the lower limit of the P content may be 0%. However, in consideration
of the current refining technique and the production costs, the lower limit of the
P content is preferably 0.001%.
S: 0.02% or less
[0042] As in P, S is an element which deteriorates drawability. In particular, when the
S content is higher than 0.02%, the drawability deteriorates significantly. Accordingly,
the S content is limited to 0.02% or less. The S content is preferably 0.01% or less.
[0043] In addition, the lower limit of the S content may be 0%. However, in consideration
of the current refining technique and the production costs, the lower limit of the
S content is preferably 0.001%.
[0044] Hereinabove, the basic chemical composition of the steel wire rod according to the
embodiment has been described, and the remainder thereof includes iron and impurities.
Here, "impurities" described in the expression "the remainder of Fe and impurities"
refer to elements which are unavoidably incorporated from raw materials such as ore
or scrap or unavoidably incorporated in various production environments, when the
steel is industrially produced.
[0045] However, in addition to the base elements, the steel wire rod according to the embodiment
optionally includes one or more elements selected from the group consisting of Mo,
V, Ti, Nb, and B instead of a portion of Fe in the remainder.
Mo: 0.20% or less
[0046] The addition of Mo is optional, and the lower limit of the Mo content is 0%.
[0047] However, due to the addition of Mo, an effect of improving a balance between the
tensile strength and the electrical resistivity of the steel wire rod can be stably
obtained. In order to obtain this effect, it is preferable that 0.02% or more of Mo
is added. More preferably, the Mo content is 0.05% or more.
[0048] On the other hand, when the Mo content is higher than 0.20%, a martensite structure
is likely to be formed in the steel, and the drawability may deteriorate. Therefore,
the upper limit of the Mo content is preferably 0.20%. More preferably, the upper
limit of the Mo content is 0.10%.
V: 0.15% or less
[0049] The addition of V is optional, and the lower limit of the V content is 0%.
[0050] However, V has an effect of forming a carbide or a carbonitride in the steel wire
rod to reduce the pearlite block size. Therefore, due to the addition of V, the drawability
can be improved. In order to obtain this effect, it is preferable that 0.02% or more
of V is added. More preferably, the V content is 0.05% or more.
[0051] On the other hand, when the V content is higher than 0.15%, a coarse carbide or carbonitride
is likely to be formed in the steel wire rod, and the drawability may deteriorate.
Therefore, the upper limit of the V content is preferably 0.15%. More preferably,
the upper limit of the V content is 0.08%.
Ti: 0.050% or less
[0052] The addition of Ti is optional, and the lower limit of the Ti content is 0%.
[0053] However, Ti has an effect of forming a carbide or a carbonitride in the steel wire
rod to reduce the pearlite block size. Therefore, due to the addition of Ti, the drawability
can be improved. In order to obtain this effect, it is preferable that 0.002% or more
of Ti is added. More preferably, the Ti content is 0.005% or more.
[0054] On the other hand, when the Ti content is higher than 0.050%, a coarse carbide or
carbonitride is likely to be formed in the steel wire rod, and the drawability may
deteriorate. Therefore, the upper limit of the Ti content is preferably 0.050%. More
preferably, the upper limit of the Ti content is 0.030%.
Nb: 0.050% or less
[0055] The addition of Nb is optional, and the lower limit of the Nb content is 0%.
[0056] However, Nb has an effect of forming a carbide or a carbonitride in the steel wire
rod to reduce the pearlite block size. Therefore, due to the addition of Nb, the drawability
can be improved. In order to obtain this effect, it is preferable that 0.002% or more
of Nb is added. More preferably, the Nb content is 0.005% or more.
[0057] On the other hand, when the Nb content is higher than 0.050%, a coarse carbide or
carbonitride is likely to be formed in the steel wire rod, and the drawability may
deteriorate. Therefore, the upper limit of the Nb content is preferably 0.050%. More
preferably, the upper limit of the Nb content is 0.020%.
B: 0.0030% or less
[0058] The addition of B is optional, and the lower limit of the B content is 0%.
[0059] However, B has an effect of binding to N, which is solid soluted in the steel wire
rod, to form BN and to thereby reduce solid solution N. Therefore, due to the addition
of B, the drawability can be improved. In order to obtain this effect, it is preferable
that 0.0003% or more of B is added. More preferably, the B content is 0.0007% or more.
[0060] On the other hand, when the B content is higher than 0.0030%, a coarse carbide is
likely to be formed in the steel wire rod, and the drawability may deteriorate. Therefore,
the upper limit of the B content is preferably 0.0030%. More preferably, the upper
limit of the B content is 0.0020%.
[0061] Next, a metallographic structure of the steel wire rod according to the embodiment
will be described.
[0062] The metallographic structure of the steel wire rod according to the embodiment includes
pearlite in which ferrite and cementite form a layered lamellar structure. The main
metallographic structure of the steel wire rod according to the embodiment is pearlite.
Here, "main metallographic structure" refers to a metallographic structure having
an area ratio of 85% or more in a C cross section which is perpendicular to a longitudinal
direction of the steel wire rod or in a L cross section which is parallel to the longitudinal
direction of the steel wire rod. The area ratio of pearlite can be obtained by subtracting
an area ratio of a non-pearlite structure from 100%. The area ratio of pearlite is
85% or more and is preferably 90% or more and more preferably 95% or more. The area
ratio of pearlite may be 100%.
[0063] The remainder in the metallographic structure of the steel wire rod according to
the embodiment, that is, a microstructure other than pearlite is a non-pearlite structure
including proeutectoid ferrite, bainite, degenerate-pearlite, proeutectoid cementite,
or the like. When the area ratio of the non-pearlite structure is higher than 15%,
the drawability deteriorates. Therefore, the area ratio of the non-pearlite structure
is 15% or less. The area ratio of the non-pearlite structure is preferably 10% or
less and more preferably 5% or less. The area ratio of the non-pearlite structure
may be 0%.
[0064] The area ratio of pearlite can be obtained as follows.
[0065] For example, as shown in Examples described below, a C cross section of a sample
of the steel wire rod, which is perpendicular to a longitudinal direction of the steel
wire rod, is mirror polished and then is etched with nital.
[0066] Next, an arbitrary area of the sample etched with nital was imaged using a SEM at
a magnification to 5000-fold in ten visual fields. Here, the area per visual field
is 3.6×10
-4 mm
2.
[0067] Using a SEM image obtained from each of the visual fields, the area ratio of pearlite
per visual field can be obtained using a typical image analysis method.
[0068] By averaging the obtained area ratios of pearlite in the ten visual fields, the area
ratio of pearlite in the steel wire rod can be obtained.
Average Lamellar Spacing of Pearlite: 50 nm to 100 nm
[0069] The tensile strength of the steel wire rod can be improved by reducing the average
lamellar spacing of the pearlite, as above described. The average lamellar spacing
has little effect on the electrical resistivity. Therefore, in order to achieve the
improvement of the tensile strength of the steel wire rod and the reduction in the
electrical resistivity at the same time, it is necessary to reduce the average lamellar
spacing. When the average lamellar spacing of pearlite is higher 100 nm, the effect
of improving the tensile strength is not sufficient. Therefore, in the steel wire
rod according to the embodiment, in order to obtain this effect, the average lamellar
spacing of pearlite is set to 100 nm or less. The average lamellar spacing of pearlite
is preferably 75 nm or less.
[0070] On the other hand, in order to adjust the average lamellar spacing of pearlite to
be less than 50 nm, it is necessary that the transformation is completed at a low
temperature. However, when the transformation is completed at a low temperature, the
area ratio of the non-pearlite structure such as bainite is higher than 15%, and the
drawability of the steel wire rod deteriorates. Therefore, the average lamellar spacing
of pearlite is set to 50 nm or more. The average lamellar spacing of pearlite is preferably
55 nm or more.
[0071] The average lamellar spacing of pearlite can be measured as follows. For example,
as shown in Examples described below, a C cross section of a sample of the steel wire
rod is polished and is etched such that pearlite appears. Next, the C cross section
on which pearlite appears is imaged with a scanning electron microscope (SEM) in multiple
visual fields to obtain metallographic structure images of the sample. Using the obtained
metallographic structure images, the average lamellar spacing of pearlite can be measured.
[0072] Specifically, the measurement can be performed using the following method. First,
metallographic structure images in ten visual fields are obtained using a SEM. In
the obtained metallographic structure image in each of the ten visual fields, plural
positions where five lamellar spacings can be measured are selected in a range of
the visual field where lamellar orientations are aligned. At each of the selected
multiple positions, a straight line perpendicular to lamellar is drawn to measure
the length of five lamellar spacings. Next, two positions in order from the smallest
length of five spacings are selected from the selected multiple positions. At each
of the selected two positions, the measured length of five lamellar spacings is divided
by 5 to obtain the lamellar spacing at the position. That is, the lamellar spacings
at two positions can be obtained from each visual field. The average value of lamellar
spacings obtained as described above at 20 positions in total in the ten visual fields
can be obtained as "the average lamellar spacing of pearlite" of the sample.
[0073] As described above, in order to improve the tensile strength of the steel wire rod,
it is effective to reduce the average lamellar spacing of pearlite. In order to reduce
the average lamellar spacing of pearlite as described above, it is preferable that
pearlitic transformation is completed at a low temperature of about 600°C after setting
a cooling rate in a cooling process after hot rolling to be 50 °C/sec or faster.
[0074] In pearlite, electricity flows mainly through a ferrite portion. In addition, when
Cr is solid soluted in ferrite in pearlite, Cr has an effect of increasing the electrical
resistivity. Therefore, if the Cr content in ferrite in pearlite can be reduced, the
electrical resistivity can also be reduced. That is, by concentrating Cr into cementite
in pearlite such that the Cr content in ferrite in pearlite is relatively reduced,
an increase in the electrical resistivity of the steel wire rod can be suppressed.
Cr is an element which is likely to be concentrated into cementite in pearlite. Therefore,
by controlling heat treatment conditions, the Cr content in cementite in pearlite
can be increased, and the Cr content in ferrite in pearlite can be reduced.
[0075] In addition, Si is an element contributing to solid solution strengthening. Therefore,
when the Si content in the steel wire rod increases, the strength of the steel wire
rod can be improved. On the other hand, however, when the Si content in the steel
wire rod increases, the electrical resistivity increases. Therefore, according as
the Si content increases, a high tensile strength and a low electrical resistivity
can be achieved at the same time by increasing the Cr content in cementite in pearlite.
[0076] In the steel wire rod according to the embodiment, in order to obtain these effects,
it is important that the Si content, the Cr content in cementite in pearlite, and
the Cr content in ferrite in pearlite satisfy the following expression (1), by mass%.
Here, in the following expression (1), the Si content is represented by [%Si] by mass%,
the Cr content in cementite in pearlite is represented by [%Crθ] by mass%, and the
Cr content in ferrite in pearlite is represented by [%Crα].
[0077] In the steel wire rod according to the embodiment, by controlling the Cr content
in cementite in pearlite according to the Si content contributing to solid solution
strengthening such that the expression (1) is satisfied, the improvement of the tensile
strength and the reduction in the electrical resistivity can be achieved at the same
time.
[0078] The Cr content in cementite in pearlite, that is, [%Crθ] in the expression (1) can
be obtained, for example, by chemically analyzing a residue extracted by electrolysis.
Specifically, the Cr content in cementite in pearlite can be obtained using the following
method. First, the steel wire rod according to the embodiment is cut into a size which
is suitable for electrolysis, and electrolysis is performed at a current density of
250 to 350 A/m
2 using a 10% AA electrolytic solution as general conditions of electrolytic polishing
so as to extract a solution. Next, the extracted solution was filtered through a filter
having a mesh size of 0.2 µm to obtain a residue. The filtrate, that is, the residue
can be obtained by performing a general chemical analysis thereon. Here, as the general
chemical analysis, for example, a method of dissolving the residue in an acidic solution
and analyzing the solution using ICP atomic emission spectroscopy is included. In
the steel wire rod according to the embodiment, metal elements included in cementite
in pearlite are Fe, Mn, and Cr. Among these, Fe and Cr are less likely to be extracted
from a microstructure other than cementite, and Mn is more likely to form MnS rather
than cementite. Therefore, the Cr content in cementite in pearlite, that is, [%Crθ]
can be calculated using the following expression (2). Here, in the following expression
(2), the Cr content in the residue, the Fe content in the residue, and the Mn content
in the residue are represented by [%Residue Cr], [%Residue Fe], and [%Residue Mn]
by mass%, respectively, and the S content in the steel wire rod is represented by
[%S] by mass%.
[0079] In addition, in order to achieve the improvement of the tensile strength and the
reduction in the electrical resistivity, it is preferable that the Cr content in cementite
in pearlite is 0.80% to 5.80% by mass%.
[0080] Defining that C is less likely to be solid soluted in ferrite, the Cr content in
ferrite in pearlite can be calculated based on, for example, the Cr content in the
steel wire rod, that is, [%Cr], the Cr content in cementite in pearlite, that is,
[%Crθ], and the volume fraction of cementite obtained from the C content, that is,
[φθ]. Specifically, since C is less likely to be solid soluted in ferrite, it is generally
known that the volume fraction of cementite in pearlite can be obtained from the following
expression (3). In the following expression (3), the C content is represented by [%C]
by mass%, and the volume fraction of cementite in pearlite is represented by [φθ0].
[0081] In the following expression (3), the coefficient 0.149 can be obtained from 6.69
mass% of C in the composition of cementite and 7.68 g/cm
3 of the density of cementite.
[0082] Next, the volume fraction of ferrite in pearlite, that is, [φα] is obtained from
the following expression (4).
[0083] In this way, the Cr content in ferrite in pearlite, that is, [%Crα] can be calculated
using the following expression (5).
[0084] As described above, in order to suppress an increase in the electrical resistivity
of the steel wire rod, it is effective to concentrate Cr into cementite. In this way,
in order to concentrate Cr into cementite, it is preferable that a wire rod is held
at the temperature range after the pearlitic transformation from austenite is completed,
and Cr is concentrated into cementite. However, after the pearlitic transformation
is completed, when the holding time at the temperature range increases, cementite
in pearlite is spheroidized, and the strength of the steel wire rod may deteriorate.
[0085] It is preferable that the tensile strength TS of the steel wire rod according to
the embodiment is 1350 MPa or more. In addition, an absolute value of the tensile
strength TS of the steel wire rod is 64 times or more an absolute value of an electrical
resistivity p expressed in units of µΩ·cm.
[0086] When the steel wire rod according to the embodiment is applied to a core of ACSR,
if the strength of the steel wire rod is low, it may be necessary that the amount
of the steel wire rod is increased to reinforce the strength. In this case, it is
assumed that the total electrical resistivity of ACSR is increased. Therefore, the
tensile strength TS of the steel wire rod according to the embodiment is preferably
1350 MPa or more, more preferably 1400 MPa or more, and still more preferably 1500
MPa or more. By setting the tensile strength TS of the steel wire rod to 1350 MPa
or more, for example, in a case where the diameter of the steel wire rod is 11 mm
to 5 mm, when the cold drawing is performed at a true strain of 1.6 as a general drawing
amount, the tensile strength of the steel wire rod after cold drawing can be set to
1900 MPa or more.
[0087] In addition, in the steel wire rod according to the embodiment, from the viewpoint
of realizing a high strength and a low electrical resistivity of the steel wire rod
at the same time, it is preferable that a relationship between the absolute value
of the tensile strength TS and the absolute value of the electrical resistivity p
expressed in units of µΩ·cm satisfies the following numerical value range.
[0088] Regarding a wire rod which is produced under general hot rolling conditions by using
steel having a chemical composition of SWRS72B or SWRS82B specified in JIS G 3502
as a core of general ACSR, the absolute value of the tensile strength TS thereof is
about 55 times the absolute value of the electrical resistivity p. The unit of tensile
strength of the wire rod is MPa, and the unit of electrical resistivity is µΩ·cm.
[0089] Therefore, as described above, with reference to a case where the absolute value
of the tensile strength TS of the wire rod is 55 times the absolute value of the electrical
resistivity p expressed in units of µΩ·cm, that is, when the value of the tensile
strength TS which is 55 times the absolute value of the electrical resistivity p is
set as a reference value, it is preferable that the absolute value of the tensile
strength TS of the steel wire rod according to the embodiment is 64 times or more
the absolute value of the electrical resistivity ρ thereof expressed in units of µΩ·cm
so as to be 15% or more of the reference value. In addition, it is more preferable
that the absolute value of the tensile strength TS of the steel wire rod according
to the embodiment is 67 times the absolute value of the electrical resistivity ρ thereof
expressed in units of µΩ·cm so as to be 20% or more of the reference value.
[0090] In this way, by setting the tensile strength TS of the steel wire rod to 1350 MPa
or more and setting the absolute value of the tensile strength TS of the steel wire
rod to be 64 times the absolute value of the electrical resistivity ρ expressed in
units of µΩ·cm, the strength of the steel wire rod can be increased, and the electrical
resistivity can be reduced. As a result, in a case where the steel wire rod is applied
to a core of ACSR, the reinforcement number can be reduced. Further, an increase in
the total electrical resistivity of ACSR can be suppressed, the heat generation during
power transmission can be suppressed, and a stable transmission efficiency can be
secured.
[0091] In the steel wire rod according to the embodiment, the absolute value of the electrical
resistivity p expressed in units of µΩ·cm is not particularly limited. That is, in
the steel wire rod according to the embodiment, it is preferable that the absolute
value of the tensile strength TS and the absolute value of the electrical resistivity
ρ thereof expressed in units of µΩ·cm satisfy the following expression (6).
[0092] By setting a steel wire rod so as to satisfy the expression (6), the tensile strength
of the steel wire rod can be secured to be significantly higher than that in the prior
art. As a result, when the steel wire rod is applied to a core of ACSR to reinforce
the strength, the reinforcement number can be reduced, and an increase in the total
electrical resistivity of ACSR can be suppressed.
[0093] In addition, the lower the electrical resistivity p of the steel wire rod according
to the embodiment, the better, and the higher the tensile strength TS, the better.
[0094] By satisfying the chemical composition and the metallographic structure as described
above, a steel wire rod in which the improvement of the strength and the reduction
in the electrical resistivity can be achieved at the same time can be obtained. In
order to obtain the above-described steel wire rod, the steel wire rod may be produced
using a production method described below. Next, a preferable method of producing
the steel wire rod according to the embodiment will be described.
[0095] The steel wire rod according to the embodiment can be produced as follows. The method
of producing the steel wire rod described below is an example of a method of obtaining
the steel wire rod according to the embodiment. The present invention is not limited
to the following procedure and method, and any method which can realize the configuration
of the present invention can be adopted.
[0096] First, steel having the above-described chemical composition is melted and continuously
cast to produce a billet, and the billet is hot rolled. After continuous casting,
blooming may be performed. When the obtained billet is hot rolled, the central part
of the billet is heated to 1000°C to 1100°C using a general method, and the finishing
temperature is set to be 900°C to 1000°C. After finish rolling, the hot rolled wire
rod is primarily cooled to 700°C or less by using a combination of water cooling and
wind cooling with air. During this primary cooling, the average cooling rate is preferably
50 °C/sec or faster. After the primary cooling, in order to complete pearlitic transformation,
the wire rod is secondarily cooled to 590°C to 620°C by being immersed into a nitrate
molten salt at 500°C to 530°C. After the secondary cooling, the wire rod is held in
the molten salt at 550°C to 570°C for 30 seconds to 50 seconds. As a result, Cr can
be concentrated into cementite. Next, water is sprayed to the wire rod so as to remove
the molten salt, the wire rod is tertiarily cooled to room temperature, and is wound.
In addition, the winding may be performed immediately after primary cooling or secondary
cooling.
[0097] In addition, during this secondary cooling, the average cooling rate is preferably
30 °C/sec or faster. In addition, when the wire rod is held after secondary cooling,
it is preferable that the wire rod is held in a range of 600°C to 550°C for 30 seconds
to 50 seconds. For example, a lead bath or a fluidized bed furnace may be used. In
a case where a lead bath is used, the wire rod is not necessarily cooled to 700°C
during primary cooling and secondary cooling and holding may be performed in the same
lead bath. In this case, it is preferable that the wire rod is held in a lead bath
at 550°C to 600°C for 35 seconds to 60 seconds.
[0098] As a method of cooling and holding the wire rod after finish rolling, cooling and
holding can be performed using only a lead bath. For example, when the temperature
of the wire rod after finish rolling is in a range of 900°C to 700°C, in a case where
the wire rod is immersed into a lead bath at 640°C to 500°C, the average cooling rate
of the wire rod is 100 °C/sec to 200 °C/sec.
[0099] In addition, when the temperature of the wire rod after finish rolling is in a range
of 700°C to 620°C, in a case where the temperature of a lead bath is 590°C to 600°C,
the average cooling rate of the wire rod is 40 °C/sec to 50 °C/sec, in a case where
the temperature of a lead bath is 550°C to 560°C, the average cooling rate of the
wire rod is 60 °C/sec to 70 °C/sec, and in a case where the temperature of a lead
bath is 490°C to 500°C, the average cooling rate of the wire rod is 90 °C/sec to 100
°C/sec.
[0100] In addition, the finishing temperature during the hot rolling refers to the surface
temperature of the steel wire rod immediately after finish rolling. Further, the average
cooling rate during cooling after finish rolling refers to the cooling rate of the
surface of the steel wire rod.
[Examples]
[0101] Hereinafter, the effects of the steel wire rod according to the embodiment will be
described in more detail using examples of the steel wire rod according to the present
invention. However, conditions of the examples are merely exemplary to confirm the
feasibility and the effects of the present invention, and the present invention is
not limited to these examples. The conditions can be appropriately changed within
a range where the gist of the present invention can be adapted without departing from
the scope of the present invention as long as the object of the present invention
can be accomplished. Therefore, the present invention can employ various conditions,
and these conditions are included in the technical features of the present invention.
[0102] 50kg of each of steels A to Y having a chemical composition shown in Table 1 were
melted in vacuum melting furnace and were cast into ingots. The chemical composition
of Steel V satisfy SWRS82B of the JIS standard.
[0103] Each of the ingots was heated at 1250°C for 1 hour and was hot-forged under a condition
of a finishing temperature 950°C or more until the diameter reached 15 mm. Next, the
ingot was naturally cooled to room temperature to obtain a hot-forged material. This
hot-forged material was cut to obtain a cut material having a diameter of 10 mm and
a length of 1000 mm.
[0104] Next, each of the obtained cut materials was heated in a nitrogen atmosphere at 1050°C
for 15 minutes such that the temperature of the center of the cut material was 1000°C
or more. Next, the cut material was hot rolled under a condition of a finishing temperature
of 950°C to 1000°C until the diameter thereof reached 7 mm. As a result, a wire rod
was obtained. Further, in a state where the temperature of the wire rod was 900°C
or more, the wire rod was immersed into a lead bath under conditions shown in Table
2 and was held. Next, the wire rod was taken out from the lead bath and was naturally
cooled to room temperature. As a result, a steel wire rod was obtained.
[0105] When the temperature of the wire rod after finish rolling was in a range of 900°C
to 700°C, in a case where the temperature of the lead bath was 640°C to 500°C, the
average cooling rate of the wire rod was 100 °C/sec to 200 °C/sec.
[0106] In addition, when the temperature of the wire rod after finish rolling was in a range
of 700°C to 620°C, in a case where the temperature of a lead bath was 590°C to 600°C,
the average cooling rate of the wire rod was 40 °C/sec to 50 °C/sec, in a case where
the temperature of a lead bath was 550°C to 560°C, the average cooling rate of the
wire rod was 60 °C/sec to 70 °C/sec, and in a case where the temperature of a lead
bath was 490°C to 500°C, the average cooling rate of the wire rod was 90 °C/sec to
100 °C/sec.
[0107] For comparison, some of the obtained cut materials were hot rolled to obtain wire
rods until the diameters thereof reached 7 mm. Next, these wire rods were cooled to
room temperature by naturally cooling in the air or by blowing wind to them using
a fan without immersing them in a molten salt or a lead bath. As a result, steel wire
rods were obtained. When the wire rod is naturally cooled in the air, in a case where
the temperature of the wire rod after finish rolling was 900°C to 700°C, the average
cooling rate was 7 °C/sec to 8 °C/sec, and in a case where the temperature of the
wire rod after finish rolling was 700°C to 620°C, the average cooling rate was 4 °C/sec
to 5 °C/sec. When the wire rod is cooled by blowing wind thereto using a fan, in a
case where the temperature of the wire rod after finish rolling was 900°C to 700°C,
the average cooling rate was 12 °C/sec to 14 °C/sec, and in a case where the temperature
of the wire rod after finish rolling was 700°C to 620°C, the average cooling rate
was 6 °C/sec to 7 °C/sec.
[0108] The steel wire rods of Test Nos. 1 to 48 which were produced under the above-described
respective conditions were evaluated by performing the following tests.
[0109] Regarding each of the steel wire rods, a C cross section perpendicular to a longitudinal
direction of the steel wire rod was mirror polished and was etched with nital.
[0110] In order to obtain an area ratio of pearlite, an arbitrary area of the sample etched
with nital was imaged using a SEM at a magnification to 5000-fold in ten visual fields.
The area per visual field was 3.6×10
-4 mm
2.
[0111] Next, the area ratio of pearlite was obtained from each of the images of the visual
fields using a typical image analysis method. The obtained average value of the area
ratios of pearlite in the ten visual fields was set as the area ratio of pearlite
in the steel wire rod.
[0112] In order to obtain an average lamellar spacing of pearlite, an arbitrary area of
the sample etched with nital was imaged using a SEM at a magnification to 10000-fold
in ten visual fields. The area per visual field was 9.0×10
-5 mm
2.
[0113] Next, in each of the images of the visual fields, a range where lamellar orientations
of pearlite are aligned was selected. Next, at each of a position having the smallest
lamellar spacing and a position having the second smallest lamellar spacing where
five lamellar spacings can be measured, a straight line perpendicular to lamellar
was drawn to measure the length of five lamellar spacings. Then, the obtained length
of the five lamellar spacings was divided by 5. As a result, the lamellar spacing
of pearlite at each position was obtained. The obtained average value of lamellar
spacings at 20 positions in total in the ten visual fields was set as the average
lamellar spacing of pearlite of the steel wire rod.
[0114] Each of the steel wire rods was cut into a diameter of 6 mm, and electrolysis is
performed at a current density of 250 to 350 A/m
2 using a 10% AA electrolytic solution as general conditions of electrolytic polishing
so as to extract a solution. Here, the 10% AA electrolytic solution was a methanol
solution including 10 vol% of acetyl acetone and 1 mass% of tetramethylammonium chloride.
Next, the extracted solution was filtered through a filter having a mesh size of 0.2
µm to obtain a residue, and this residue was dissolved in an acidic solution. By analyzing
this solution using ICP atomic emission spectroscopy, the Cr content in the residue
[%Residue Cr], the Fe content in the residue [%Residue Fe], and the Mn content in
the residue [%Residue Mn] were obtained. The Cr content in cementite in pearlite,
that is, [%Crθ] was calculated using the following expression (A). In this case, the
following conditions were defined: "metal elements in cementite were substantially
Fe, Mn, and Cr"; "Fe and Cr were not extracted from a microstructure other than cementite";
and "in a case where S is included in the steel, Mn formed MnS prior to cementite".
Here, in the following expression (A), the Cr content, the Fe content, and the Mn
content in the residue are represented by [%Residue Cr], [%Residue Fe], and [%Residue
Mn] by mass%, respectively, and the S content in the steel wire rod is represented
by [%S] by mass%.
[0115] In addition, the Cr content in ferrite was calculated as follows. First, the volume
fraction [φθ] of cementite in pearlite was obtained from the following expression
(B), and the volume fraction [φα] of ferrite in pearlite was obtained from the following
expression (C). Next, the Cr content in ferrite [%Crα] was calculated from the following
expression (D).
[0117] From the central part of the C cross section of each of the steel wire rods, two
tensile test pieces having a parallel body with a diameter of 3.2 mm (a circle having
a radius of 1.6 mm around the center of the C cross section) and a length of 18 mm
were obtained by cutting. Using these tensile test pieces, a tensile test was performed
at room temperature using a method according to JIS Z 2241 to measure the tensile
strengths TS thereof, and the measured average value of the tensile strengths TS was
set as the tensile strength TS of the steel wire rod. Here, the unit of the tensile
strength TS is MPa.
[0118] As a test piece for measuring the electrical resistivity p, a rectangular test piece
having a size of 3.0 mm×4.0 mm×60 mm was obtained from the central part of each of
the steel wire rods, and the electrical resistivity of the test piece was measured
using a typical four-terminal method at a temperature of 20°C. The unit of the obtained
electrical resistivity ρ is µΩ·cm.
[0119] The obtained evaluation results are shown in Tables 3 and 4. Here, in Tables 3 and
4, [%Crθ] and [%Crα] represent "the Cr content in cementite in pearlite" and "the
Cr content in ferrite in pearlite" by mass%, respectively.
[0120] In addition, in Tables 3 and 4, a case where the expression (1) was satisfied was
determined as "pass" and shown as "O", and a case where the expression (1) was not
satisfied was determined as "fail" and shown as "X".
[0121] In addition, the tensile strength TS of the steel wire rod expressed in units of
MPa, the absolute value of the electrical resistivity p expressed in units of µΩ·cm,
and a value which is 64 times the absolute value of the electrical resistivity p are
shown in Tables 3 and 4.
[0122] In addition, in Tables 3 and 4, a case where the absolute value of the tensile strength
TS was 64 times or more the absolute value of the electrical resistivity ρ expressed
in units of µΩ·cm was determined as "good" and shown as "O", and a case where the
absolute value of the tensile strength TS was less than 64 times than the absolute
value of the electrical resistivity ρ was determined as "bad" and shown as "X".
[0123] In Tables 3 and 4, Test Nos. 1, 3, 6, 7, 9 to 11, 14, 17, 18, 20 to 22, 24, 26, 27,
30, 33, 34, 36, and 44 to 47 did not satisfy at least one of the technical features
specified in the present invention including the chemical composition, the metallographic
structure, the average lamellar spacing of pearlite, the relationship between the
Si content, the Cr content in cementite in pearlite, and the Cr content in ferrite
in pearlite. In addition, in Test Nos. 45 and 47, the drawability was low.
[0124] On the other hand, Test Nos. 2, 4, 5, 8, 12, 13, 15, 16, 19, 23, 25, 28, 29, 31,
32, 35, 37 to 43, and 48 satisfied all of the technical features specified in the
present invention including the chemical composition, the metallographic structure,
the average lamellar spacing of pearlite, the relationship between the Si content,
the Cr content in cementite in pearlite, and the Cr content in ferrite in pearlite.
[0125] [Table 1]
[0126] [Table 2]
Table 2
CONDITION No. |
BATH TEMPERATURE (°C) |
HOLDING TIME (sec) |
I |
640 |
45 |
II |
600 |
35 |
III |
560 |
45 |
IV |
500 |
60 |
V |
600 |
20 |
VI |
600 |
60 |
VII |
600 |
180 |
VIII |
NATURALLY COOLING IN AIR |
IX |
WIND COOLING USING FAN |
[0127] [Table 3]
[0128] [Table 4]
[Industrial Applicability]
[0129] According to the present invention, a steel wire rod having a high strength and a
low electrical resistivity can be obtained, which remarkably contributes to the industry.