Note: Descriptions are shown in the official language in which they were submitted.
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NSC-T752
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=
HIGH-CARBON STEEL WIRE ROD OF HIGH DUCTILITY
FIELD OF THE INVENTION
This invention relates to high-carbon steel wire rod
of high post-hot-rolling ductility having a
metallographic structure mainly of pearlite.
Specifically, this invention relates to piano wire or
high-carbon steel wire complying with JIS, more
particularly to hot-rolled wire of high-carbon steel
that, as the final product steel wire, is a fine wire of
a diameter of around 0.1 to 2 mm usable, for example, in
steel cord, saw wire, hose wire, fine rope and the like.
DESCRIPTION OF THE RELATED ART
Steel cords and other reinforcing wires used to
reinforce rubber products such as tires, conveyor belts
and heavy-duty hoses are manufactured from high-carbon
steel wire rods. The high-carbon steel wire rods are
manufactured by hot rolling, followed by descaling and
then borax coating or Bonde coating to provide a carrier
coating, whereafter processing to a steel wire of 0.8 to
1.2 mm is optionally conducted by use of intermediate
patenting. As termed with respect to the present
invention, the hot-rolled steels are called "wire rods"
and the steels of smaller diameter than the hot-rolled
steels fabricated by subsequent processing are called
"steel wires."
When the steel wires are to be used for steel cord,
the patenting is followed by brass plating and then
further drawing to steel wires of 0.15 to 0.35 mm
diameter, whereafter the wires are stranded into steel
cord that is embedded in a rubber product for use.
Research is being continued on, for example, improvement
of workability in the secondary processing step and
improvement of the abrasion property of the drawing dice.
Japanese Patent Publication (A) No. H3-60900, for
example, teaches a wire rod whose C content is 0.59 to
0.86%, tensile strength is 87.5 x C equivalent + 27 2
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(kg/mm2) (C equivalent = C + Mn/5), and area accounted for
by coarse pearlite in the wire rod metallographic
structure as measured under a microscope at x500 is
adjusted to -60 x C equivalent + 69.5 3 (%). This wire
rod is directed to enabling the drawing dice to have
excellent service life and increases dice service life by
specifying tensile strength and controlling the volume
fraction of coarse pearlite to within a certain range.
Although this patent publication focuses on coarse
pearlite structure with an eye to improving drawing dice
service life, it teaches nothing whatsoever regarding
relationship with the cause of breakage after direct
drawing, which is the issue dealt with by the present
invention.
Japanese Patent Publication (A) No. 2000-6810
teaches a high-carbon steel wire rod excellent in wire
drawability wherein 90% or greater of the metallographic
structure is pearlite structure, and the pearlite has an
average lamellar spacing of 0.1 to 0.4 Km and an average
colony diameter of 150 Km or less. The fact is, however,
that the colony diameter obtained by ordinary hot rolling
is smaller than 150 Km, and an improvement in breakage
property cannot necessarily be expected because the
ductility obtained when the colony diameter is controlled
to 150 Km or less is inconsistent.
Japanese Patent No. 3681712 teaches a high-carbon
steel wire rod excellent in drawability wherein 95% or
greater of the wire rod metallographic structure is
pearlite structure, the pearlite has an average nodule
diameter (P) of 30 Km or less and an average lamellar
spacing (S) of 100 nm or greater, and the value of F
obtained by the equation
F = 350.3 / + 130.3 / 'VP - 51.7
is F > 0, where P is represented in Km and S in nm.
The invention taught by this patent publication
controls the lamellar spacing and nodule size by
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incorporating a cooling process for isothermal holding
during Stelmor cooling at the time of hot rolling.
However, in ordinary Stelmor cooling the cooling is
continuous, so that the range of lamellar spacing values
is wide and the range of nodule size values also becomes
wide. In such a case, good workability cannot be obtained
no matter how small the average values are made, and what
is more, a problem of attendant internal defects arises.
Moreover, the patented invention is directed to obtaining
a wire rod excellent in high-speed drawability by varying
the cooling conditions after wire rod rolling so as to
adjust the structure into the range of F defined by the
foregoing equation. This is problematic, however, because
bringing the structure into the range of the equation
requires use of special heat treatment that is generally
difficult to implement.
SUMMARY OF THE INVENTION
Owing to the importance of good economy in secondary
processing, recent years have seen an increasing need for
the development of wire rod that resists occurrence of
internal defects during drawing and wire rod that even
when processed with a relatively large amount of working
during primary drawing does not experience an increase in
breakage thereafter.
The present invention relates to high-carbon steel
wire rod utilized as piano wire rod, hard steel wire rod
and the like for use in finely drawn applications such as
steel cord, belt cord, rubber hose wire, rope wire and
the like, and in light of the foregoing circumstances,
provides high-carbon steel wire rod of high ductility
that is excellent in post-hot-rolling drawability,
resists occurrence of internal defects at the time of
drawing, and enables omission of intermediate patenting.
The inventors achieved the present invention based
on the results of in-depth research regarding pearlite
structure hot-rolled wire rod whose secondary
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processability is unaffected by omission of intermediate
patenting. A summary of the invention follows:
1) A high-carbon steel wire rod of high ductility,
which is a high-carbon steel wire rod having a carbon
content of 0.7 mass% or greater, wherein 95% or greater
of the wire rod metallographic structure is pearlite
structure and maximum pearlite block size at a core of a
cross-section perpendicular to the wire rod axis is 65 m
or less.
2) A high-carbon steel wire rod of high ductility
according to 1), having a tensile strength in a range of
{248 + 980 x (C mass%)} 40 MPal and a reduction of area
of {72.8 - 40 x (C mass%) %} or greater.
3) A high-carbon steel wire rod of high ductility
according to 1) or 2), wherein an average pearlite block
size at the core of the cross-section perpendicular to
the wire rod axis is 10 m or greater and 30 m or less.
4) A high-carbon steel wire rod of high ductility
according to any of 1) to 3), wherein the wire rod
metallographic structure includes pro-eutectoid ferrite
at a volume percentage of 2% or less.
5) A high-carbon steel wire rod of high ductility
according to any of 1) to 4), wherein the wire rod
comprises, in mass%, C: 0.7 to 1.1%, Si: 0.1 to 1.0%, Mn:
0.1 to 1.0%, P: 0.02% or less, S: 0.02% or less, and a
balance of Fe and unavoidable impurities.
6) A high-carbon steel wire rod of high ductility
according to 5), wherein the wire rod further comprises,
in mass%, one or more of Cr: 0.05 to 1.0%, Mo: 0.05 to
1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, V: 0.001 to
0.1%, Nb: 0.001 to 0.1%, Ti: 0.005 to 0.1%, B: 0.0005 to
0.006%, 0: 18 to 30 ppm, and N: 0 to 40 ppm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows correspondence between (a) cracks
occurring during drawing in the case of conducting
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ordinary Stelmor processing and (b) pearlite block size.
FIG. 2 is shows change in pearlite block size
between the surface and core of a rolled wire rod.
DETAILED DESCRIPTION OF THE INVENTION
The inventors discovered that when steel wire is
drawn from wire rod to the thickness at which final
patenting is conducted without conducting intermediate
patenting, the steel wire may at first sight appear not
to decline in ductility with increasing amount of
working, but defects nevertheless occur internally and
are promoted during the ensuing patenting and the drawing
thereafter, sometimes leading to breakage.
Also in the case where severe working (i.e., working
in terms of true strain equal to or exceeding 2) is
conducted during primary drawing, it is necessary to
ensure that the patenting and other ensuing processes are
not affected by controlling the wire rod metallographic
structure so as to prevent occurrence of internal defects
in primary drawing to the utmost, and also to conduct
primary drawing that minimizes occurrence of defects.
The inventors observed the internal defect sites
after primary drawing and studied the associated
conditions, which are complexly affected by numerous
factors such as the mechanical properties, processing
conditions and wire rod structure. As a result, they
discovered that among these conditions, it is the
pearlite block size of the pearlite structure at the core
of the wire rod, as measured with an EBSP (Electron Back
Scatter Pattern) analyzer, that characterizes the
structure readily experiencing internal defects. A
measurement method using an ordinary light microscope
cannot accurately ascertain the pearlite block size and
therefore does not enable determination of the structure
that impairs workability. An EBSP analyzer must therefore
be used to measure the pearlite block size.
Pearlite block size was measured with a system using
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a TSL (TexSEM Laboratories) EBSP analysis unit in
combination with a Hitachi thermal FE-SEM (model 5-
4300SE). The pearlite block was measured with the EBSP
analyzer as the region with the same ferrite crystal
orientation, in accordance with the definition given by
Takahashi et al. in The Journal of the Japan Institute of
Metals, Vol. 42 (1978), p702. Since measurement using the
structure observed with a light microscope or the
secondary electron image obtained by SEM observation was
found to be extremely difficult, the pearlite block size
was determined from the ferrite crystal orientation map
obtained by EBSP analysis. Differently from in the
ferrite single phase of low-carbon steel, countless small
angle boundaries are present in the ferrite crystal
grains of pearlite steel, even after patenting.
An investigation was therefore made regarding an
appropriate threshold angle above which the grain
boundaries that have an orientation difference of 15
degrees or greater and can be recognized as ordinary
crystal grain boundaries account for around 90% or
greater of all grain boundaries. The best results were
obtained when the grain boundaries were defined as those
obtained using a boundary orientation difference of 9
degrees or greater. Units constituted by boundaries
having orientation differences of 9 degrees or greater
were therefore defined as pearlite block grains.
Through an extensive study of ways to control the
pearlite block size, the inventors discovered that
occurrence of coarse pearlite blocks can be prevented by
control of oxygen amount along with control of post-
rolling finish-rolling temperature so as to carry out
Stelmor cooling with the y grain size in a granulated
state on the finish rolling exit side. When the y grains
are of mixed grain size, pearlite transformation occurs
more readily at small y grain regions, in which case the
pearlite transformation nuclei are present
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heterogeneously, so that pearlite blocks grow easily to
make the grain size large.
In order to make the y grain size after finish
rolling small, the steel is required to have an oxygen
content of 18 ppm or greater, preferably 20 ppm or
greater. However, increasing oxygen content increases the
amount of inclusions and causes formation of large
inclusions. As this degrades ductility, the upper limit
of oxygen content is defined as 30 ppm.
When ordinary continuous cooling is used, the
pearlite block size varies from the surface layer toward
the center of the wire rod. And, as shown in FIG. 2, the
pearlite block size varies at locations outward from the
center also in the case where the ordinary Stelmor
cooling process is conducted. In FIG. 2, each pearlite
block size shown is the average of values measured at
eight locations. Since the pearlite block size at the
core differs greatly even when the average value is the
same, the inventors studied what criteria should be used
for the control in the case of continuous cooling. They
learned that the pearlite lamellae are also coarse at the
core region where the pearlite block size is large and
that the coarse pearlite portions become starting points
of breakage during drawing. Therefore, in order not to
leave any defects following primary drawing, it is
necessary to control the maximum value of the pearlite
block size to 65 m or less. An investigation of the
relationship between the pearlite block size and the
breakage index of the final drawn wire showed that making
the pearlite block size at the core 65 m or less
improves drawability and enables reduction of wire
breakage in the ensuing drawing process.
The reasoning behind specification of the average
value of the pearlite block grains will now be explained.
Owing to the use of continuous cooling, the pearlite
block grains are present in a mixture of sizes. If the
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average pearlite block size is determined by simple
averaging based on the measurement of pearlite block size
made in this mixed condition, the numerous small pearlite
blocks present will make the average value so small that
it does not reflect the breakage property. The Johnson-
Saltykov method of calculating the average diameter of
particle groups of mixed particle size was therefore used
to determine the average value of the obtained pearlite
block size as the average of values at 8 sites in each of
the wire rod surface layer, 1/4 diameter region and core
region (1/2 diameter region), i.e., at a total of 24
sites. Details regarding the Johnson-Saltykov method can
be found in Quantitative Microscopy, R.T. DeHoff and F.N.
Rhines, Ed., McGraw Hill Publishers, New York, NY, 1968,
p169.
When the obtained average value is 10 m or less,
achievement of pearlite structure of 95% or greater is
difficult and the volume percentage of ferrite in the
pearlite structure becomes 2% or greater. The average
pearlite block size therefore needs to be made 10 m or
greater. Moreover, if the average value exceeds 30 m,
the probability of coarse blocks being included is very
high in the case of continuous cooling, so that the
average must be controlled to 30 m or less.
At a tensile strength of less than {248 + 980 x (C
mass%) - 40 MPa}, the lamellar spacing of the pearlite
structure becomes so large as to make attainment of good
workability impossible. The tensile strength must
therefore be controlled to not less than {248 + 980 x (C
mass%) - 40 MPa}. At a tensile strength of greater than
{248 + 980 x (C mass%) + 40 MPa}, large work hardening
makes post-drawing strength high so that ductility
declines. The tensile strength must therefore be
controlled to not greater than {248 + 980 x (C mass%) +
40 MPa}.
Reduction of area is preferably controlled to not
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less than {72.8 - 40 x (C mass%)}. At a reduction of area
of less than 40%, internal defects occur readily during
wire drawing. In order to keep the reduction of area from
falling below 40%, the volume fraction of pro-eutectoid
ferrite observed inside the wire rod obtained by Stelmor
cooling is controlled to 2% or less. When present at a
volume fraction exceeding 2%, the pro-eutectoid ferrite
tends to act as starting points of internal defects
during drawing and as starting points of internal defects
during tensile testing. Pro-eutectoid ferrite is
therefore controlled to 2% or less. Pro-eutectoid ferrite
becomes a problem in the carbon content region below
0.85 mass%. In the carbon content region of 0.85 mass%
and greater, pro-eutectoid ferrite is generally held to
2% or less owing to the presence of abundant carbon
content.
The reasons for limiting the components of the steel
of the high-carbon steel wire rod according to the
present invention will now be explained. All contents are
expressed in mass%.
C is an element that effectively enhances strength.
For obtaining a high-strength steel wire, C content must
be made 0.7% or greater. However, when C content is
excessive, ductility tends to be lowered by ready
precipitation of pro-eutectoid cementite. The upper limit
of C content is therefore specified as 1.1%.
Si is an element required for deoxidation of the
steel. Since the deoxidation effect is insufficient at
too low a content, Si is added to a content of 0.1% or
greater. Moreover, Si increases post-patenting strength
by dissolving into the ferrite phase in the pearlite
formed after heat treatment. But it also impairs heat
treatability. It is therefore kept to a content of 1.0%
or less.
P easily segregates and P concentrating at the
segregation sites dissolves into the ferrite to lower
workability. P content is therefore controlled to 0.02%
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or less.
S, if contained in a large amount, lowers the
ductility of the steel by forming much MnS. It is
therefore controlled to a content of 0.02% or less.
Mn is added to a content of 0.1% or greater in order
to impart hardenability to the steel. However, heavy
addition of Mn excessively prolongs transformation time
during patenting. Addition is therefore limited to 1.0%
or less.
Cr is added to enhance steel strength. When
included, it is added to a content at which this effect
is exhibited, namely to a content of 0.05% or greater,
and to a content of 1.0% or less, namely to a content
that does not give rise to a decrease in steel wire
ductility.
Mo is added to enhance steel strength. When
included, it is added to a content at which this effect
is exhibited, namely to a content of 0.05% or greater,
and to a content of 1.0% or less, namely to a content
that does not give rise to a decrease in steel wire
ductility.
Cu is added to enhance corrosion resistance and
corrosion fatigue property. When included, it is added to
a content at which these effects are manifested, namely
to a content of 0.05% or greater. However, heavy addition
tends to cause brittleness during hot rolling, so the
upper limit is defined as 1.0%.
Ni has an effect of increasing steel strength. When
included, it is added to a content at which the effect of
addition is manifested, namely to a content of 0.05% or
greater. However, since excessive addition lowers
ductility, Ni content is held to 1.0% or less.
V has an effect of increasing steel strength. When
included, it is added to a content at which the effect of
addition is manifested, namely to a content of 0.001% or
greater. However, excessive addition lowers ductility, so
the upper limit is defined as 0.1%.
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Nb has an effect of increasing steel strength. When
included, it is added to a content at which the effect of
addition is manifested, namely to a content of 0.001% or
greater. However, excessive addition lowers ductility, so
the upper limit is defined as 0.1%.
B has an effect of refining y grain size during
austenitization, and by this, of improving reduction and
other ductility properties. Therefore, when included, B
is added to a content at which its effect is manifested,
namely to a content of 0.0005% or greater. However,
addition to a content exceeding 0.006% makes the
transformation time at the time that transformation is
effected by heat treatment too long. The upper limit of B
content is therefore defined as 0.006%.
As the production method for obtaining the high-
carbon steel wire rod of high ductility according to the
present invention, it is preferable in hot rolling a
billet having the aforesaid chemical composition to
conduct the hot rolling, then carry out coiling at 850 to
900 C within 10 seconds, and thereafter conduct Stelmor
cooling or direct patenting by immersion in 500 to 570 C
molten salt.
EXAMPLES
The chemical compositions of specimen steels used in
prototyping are shown in Table 1. Steels No. 1 to No. 18
are of compositions controlled in accordance with the
invention. Steels No. 19 and No. 20 are Comparative
Steels. Comparative Steel 19 is lower in oxygen content
than the Invention Steels and Comparative Steel 20 is
higher in oxygen content than the Invention Steels.
The steels were prepared in a full-scale furnace to
have the compositions shown in Table 2 and continuously
cast into bloom of 500 x 300 mm cross-sectional
dimensions. The bloom was thereafter reheated and rolled
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with a billeting mill to obtain a 122 mm-square billet.
The steel was then reheated to the y region, hot rolled to
5.5 mm-diameter wire rod, finish rolled, controlled to a
coiling temperature of 850 to 900 C in 10 seconds, and
continuously subjected to Stelmor cooling divided into
four zones. The wire rod manufacturing conditions are
shown in Table 2. Table 2 also shows the mechanical
properties and the maximum and average values of the
measured pearlite block sizes of the wire rods obtained
under the manufacturing conditions shown in the same
Table.
Wire rods No. 1, No. 2 and, No. 6 to No. 21 in Table
2 were manufactured in accordance with the invention.
Wire rods No. 3 to No. 5, No. 22 and No. 23 were
manufactured for comparison.
In Table 2, the symbol C) indicates that when, for
the purpose of investigating primary drawability, the
wire rod was drawn from the diameter of 5.5 mm to a
diameter of 1.0 mm with the die approach angle at 20
degrees, neither breakage nor abnormality in the tensile
tests conducted at the individual passes occurred. In
addition, for the purpose of investigating secondary
drawability, the wire rod was drawn from the diameter of
5.5 mm to a diameter of 1.56 mm, brass plated and further
drawn from the diameter of 1.56 mm to a diameter of
0.2 mm, whereafter the 0.2 mm-diameter wire was subjected
to drawing under a weight of 100 kg or greater to
determine the wire breakage index. When the wire breakage
index was good, it was designated by the symbol (). In
Table 2, the symbol X indicates that the result for the
item concerned was unsatisfactory.
The invention wire rods No. 1, No. 2, and No. 6 to
No. 21 exhibited good results for both primary
drawability and secondary drawability.
Comparative wire rod No. 3, made with a comparative
steel, had a maximum pearlite block size value
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exceeding 65 m owing to the high finishing temperature
and therefore exhibited poor results for both primary
drawability and secondary drawability.
Comparative wire rod No. 4 had a maximum pearlite
block size value exceeding 65 m owing to the high
coiling temperature and therefore exhibited poor results
for both primary drawability and secondary drawability.
Comparative wire rod No. 5 had a tensile strength
(TS) below the invention range because the air flow in
Stelmor cooling was at a moderate level. In this case,
too, poor results were exhibited for both primary
drawability and secondary drawability.
Comparative wire rod No. 22 was made of a steel of a
chemical composition whose oxygen content was below the
invention range. The maximum value of the pearlite block
size at the core region of the wire rod was greater than
that defined by the invention.
Comparative wire rod No. 23 was made of a steel of a
chemical composition whose oxygen content was below the
invention range. Although the maximum value of the
pearlite block size at the core region of the wire rod
met the requirement of the invention, the total amount of
inclusions was large owing to the high oxygen content and
the secondary drawability was therefore low.
,
Table 1
Steel C Si Mn P S Cr N 0
Other Remark
No.
1 0.72 0.19 0.49 0.010 0.009 - 21 23
Invention
2 0.82 0.18 0.51 0.010 0.007 21 24 -
Invention
3 0.92 0.19 0.51 0.008 0.008 - 19 23 -
Invention
4 0.92 0.19 0.31 0.009 0.009 0.21 19 24 -
Invention
0.96 0.19 0.31 0.008 0.009 0.22 20 22 -
Invention
6 1.02 0.19 , 0.31 0.009 0.009 0.19 19
23 Invention
7 0.92 0.90 0.32 0.009 0.008 0.19 29 21
B:0.002 Invention
8 1.02 0.90 0.60 0.009 , 0.009 0.1 29 23
- Invention n
9 1.02 0.90 0.32 0.009 0.009 0.1 34 23
Mg:0.05,B:0.0025 Invention
0.82 0.19 0.21 0.010 0.008 - 26 28 Mo:0.1
Invention o
K.)
11 0.82 0.20 0.49 0.011 0.008 - 24 18
Cu:0.1 Invention M
H
12 0.82 0.20 0.48 0.009 0.007 23 22
Ni:0.1 Invention
w
13 0.82 0.21 0.49 0.009 0.006 - 26 24
V:0.07 Invention CO
p
14 0.82 0.19 0.49 0.009 0.005 - 28 26
Nb:0.05 Invention K.)
1
o
0.82 0.19 0.49 0.015 0.004 - 21 25 -
Invention o
(1)
16 0.82 0.20 0.30 0.010 0.008 0.15 34 25
V:0.07,B:0.002 Invention
17 0.82 0.19 0.50 0.010 0.009
- 22 23 Ti:0.002,B:0.002
Invention H
(1....)
1
18 0.82 0.20 0.55 0.012 0.008 - 21 22
Invention o
19 0.82 0.21 0.30 0.009 0.008 - 38 17 -
Comparative
0.82 0.20 0.32 0.010 0.008 - 23 45
Comparative
Table 2
Average
Wire Finishing Coiling Max pearlite
Steel No. Air flow TS RA
block size
pearlite Primary Secondary
Re mark
Rod temp temp
(see Table 1) (Stelmor vane
opening) (MPa) (%) block size drawing drawing
No. ( C) ( C) (pm)
(Pm)
1 1 1048 , 890 All-100 1020 46
54 28 0 0 Invention
2 2 1045 880 All-100 , 1032 44 58
29 C) C) Invention
3 2 1120 890 All-100 1032, 42 67
26 X X Comparative
4 2 1052 900 All-100 1101 41 67
27 X x Comparative
.
2 1049 890 50-50-100-100 1018 38 78 36 X
X Comparative
0
6 3 1038 880 All-100 1124 39 43
23 C) CD Invention
7 4 1040 880 All-100 1132 38 54
25 C) C) Invention o
K.)
8 5 1065 880 All-100 1190 36 57
28 0 C) Invention m
H
-.3
_ 9 6 1043 880 All-100 1220 34 58
26 C) CD Invention w
co
7 1066 880 All-100 1116 38 62 25
CD CD Invention H
K.)
11 8 , 1059 880 All-100 1215 37 61
25 C) C) Invention I o
o
12 9 1072 880 All-100 1253 36 64
26 0 C) Invention 1---, co
_
(1)
, 13 10 1041 880 All-100 1063 43 56
27 , C) C) Invention cn p
(1,...)
14 11 1062 880 All-100 1074 44 59
28 C) CD Invention 1
o
12 1053 880 All-100 1076 42 58 24
C) C) Invention
16 13 1052 880 All-100 1058 41 57
25 0 C) Invention
_ 17 14 1063 880 All-100 1062 41 62
24 C) C) Invention
18 15 1037 880 All-100 1088 45 63
27 C) C) Invention
_
19 16 1039 880 All-100 õ 1087 43 61
26 0 0 Invention
_
17 1047 880 All-100 1071 44 57 26
C) C) Invention
21 18 1061 880 All-100 1066 43 54
28 , C) C) Invention
22 19 1054 880 All-100 1054 41 72
31 X X Comparative
23 20 1067 880 All-100 _1076 39 66
28 0 X Comparative
,
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The high-carbon steel wire rod of high ductility
according to the present invention enables manufacture of
excellent extra fine wire of high fatigue strength that
is capable of reducing the weight and prolonging the
service life of rubber products.
