1
DESCRIPTION
STEEL WIRE ROD AND METHOD OF PRODUCING SAME
5 Technical Field
[OOO 11
The present invention relates to a steel wire rod with high strength and excellent
ductility and which is a material of steel wires such as a prestressed concrete wire, a
zinc-coated steel wire, a spring steel wire, and a bridge cable, and a method of producing
10 the same.
Priority is claimed on Japanese Patent Application No. 20 1 1-056006, filed on
March 14, 2011, an amount of which is incorporated herein by reference.
Background Art
[0002]
Commonly, a steel wire is produced by conducting wire-drawing so as to have a
predetermined wire diameter and strength by using a steel wire rod which is produced by
hot rolling and patenting treatment conducted as necessary. At a stage of a steel wire
rod, when the steel wire rod has low strength, work strain should increase in order to be
work-hardened to a predetermined strength during wire-drawing. As a result, a steel
wire produced by the wire-drawing has poor ductility. In a case where the steel wire has
poor ductility, when the steel wire is torsionally deformed, longitudinal cracking which is
called as delamination may occur along a wire-drawing direction of the steel wire at an
initial stage of deformation. Once the delamination occurs, stress may be concentrated
at a site where the delamination occurs, and fracture of the steel wire may be finally
2
promoted. In order to obtain a steel wire with high strength and excellent ductility by
suppressing the occurrence of the delamination in the steel wire, the steel wire rod needs
to have high strength and excellent ductility at a stage before the wire-drawing.
[0003]
5 Generally, it is known that, when grain size is refmed, strength is improved.
Similarly, reduction of area (RA) that is an index of ductility of the steel wire rod also
depends on austenite grain size. When the austenite grain size is refined, the reduction
of area is also improved. Therefore, the austenite grain size of the steel wire rod is to be
refined by using carbides or nitrides of Nb, By and the like as pinning particles.
10 [0004]
I For example, Patent Document 1 suggests a steel wire rod in which at least one
I selected from a group consisting of, by mass%, 0.01% to 0.1% of Nb, 0.05% to 0.1% of
Zr, and 0.02% to 0.5% of Mo is contained in a high carbon steel wire rod.
[0005]
15 In addition, Patent Document 2 suggests a steel wire rod in which the austenite
grain size is refined by containing NbC in a high-carbon steel wire rod.
However, in the steel wire rod disclosed in Patent Document 1 and Patent
Document 2, expensive elements such as Nb are added, and thus the production cost may
20 increase. Furthermore, since Nb forms coarse carbides and nitrides, these may act as
fracture origin, and thus ductility of the steel wire rod may decrease. -
[0007]
Patent Document 3 suggests a method of producing a steel wire rod having high
strength and large reduction of area by applying a direct patenting treatment (DLP: Direct
3
[OOOS]
In fact, the steel wire rod according to the production method disclosed in Patent
Document 3 obtains high strength and large reduction of area without adding the
expensive elements. However, at the present time, hrther improvement in strength and
5 ductility is required. In Patent Document 3, as described in examples thereof, in a case
of ensuring tensile strength (TS) of 1200 MPa or more, the reduction of area is less than
45%.
[0009]
In order to improve properties of the prestressed concrete wire, the zinc-coated
10 steel wire, the spring steel wire, the bridge cable, and the like in which the steel wire rod
is used as the materials, it is effective to reduce the diameter of the steel wire rod as small
as possible. Since reduction during the wire-drawing is controlled to be small by
wire-drawing the steel wire rod with small diameter, the wire-drawn steel wire is
controlled to excellent ductility. As a result, the ocdurrence of the delamination in the
15 steel wire is suppressed. Accordingly, the steel wire rod having the small diameter, high
strength, and excellent ductility (that is, large reduction of area) has been anticipated.
Specifically, in a case where the diameter is 10 rnrn or less, a steel wire rod having the
tensile strength of 1200 h4Pa or more and the reduction of area of 45% or more has been
anticipated.
20
Related Art Documents
Patent Documents
[OO 1 01
[Patent Document 11 Japanese Unexamined Patent Application, First
25 Publication No. H04-371549
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 200 1 - 13 1697
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2008-007856
5
Summary of Invention
Technical Problem
[OO 1 11
In view of the above-mentioned problems, an object of the present invention is
10 to provide a steel wire rod which has higher strength and better ductility than those of the
conventional one without adding expensive elements, specifically, tensile strength of
1200 MPa or more and reduction of area of 45% or more, and is to provide a method of
producing the same. Particularly, the present invention is to provide the steel wire rod
having the tensile strength of 1200 MPa or more and the reduction of area of 45% or
15 more, even when a diameter is 10 mm or less, and is to provide the method of producing
the same.
Solution to Problem
[OO 1 21
An aspect of the present invention employs the following.
[00 1 31
(1) A steel wire rod according to an aspect of the invention includes, as a
chemical composition, by mass%, 0.70% to 1.00% of C, 0.15% to 0.60% of Si, 0.1% to
1.0% of Mn, 0.001% to 0.005% of N, 0.005% to less than 0.050% of Ni, at least one of
25 0.005% to 0.10% of A1 and 0.005% to 0.10% of Ti, and a balance consisting of iron and -
5
unavoidable impurities, and includes, as a metallographic structure, by area%, 95% to
100% of a pearlite, wherein, when a distance fiom a peripheral surface to a center is r in
unit of mm, an average pearlite block size at a central portion which is an area fiom the
center to r x 0.99 is 1 pm to 25 pm, wherein an average pearlite block size at a surface
5 layer portion which is an area from the peripheral surface to r x 0.01 is 1 pm to 20 pm,
and wherein, when a minimum lamellar spacing of the pearlite at the central portion is S
in unit of nm, a following Expression 1 is satisfied.
S < 12r + 65 . . . (Expression 1)
(2) The steel wire rod according to (1) may hrther includes, as the chemical
10 composition, by mass %, at least one of more than 0% to 0.50% of Cr, more than 0% to
0.50% of Co, more than 0% to 0.50% of V, more than 0% to 0.20% of Cu, more than 0%
to 0.10% of Nb, more than 0% to 0.20% of Mo, more than 0% to 0.20% of W, more than
0% to 0.0030% of B, more than 0% to 0.0050% of Rare Earth Metal, more than 0.0005%
to 0.0050% of Ca, more than 0.0005% to 0.0050% of Mg, and more than 0.0005% to
(3) In the steel wire rod according to (1) or (2), when a tensile strength is TS in
unit of MPa and a reduction of area is RA in unit of %, both of a following Expression 2
and a following Expression 3 may be satisfied.
RA 2 100 - 0.045 x TS . . . (Expression 2)
20 RA 2 45 . . . (Expression 3)
(4) In the steel wire rod according to any one of (1) to (3), amounts expressed in
mass% of each element in the chemical composition may satisfy a following Expression
4.
0.005 5 A1 + Ti I 0.1 . . . (Expression 4)
6
(5) A method of producing a steel wire rod according to an aspect of the
invention includes: a casting process to obtain a cast piece consisting .of the chemical
composition according to (1) or (2); a heating process of heating the cast piece to a
temperature of 1000°C to 1 100°C; a hot-rolling process of hot-finish-rolling the cast
5 piece after the heating process by controlling a finishing temperature to be 850°C to
1000°C to obtain a hot-rolled steel; a coiling process of coiling the hot-rolled steel within
a temperature range of 780°C to 840°C; a patenting process of directly immersing the
hot-rolled steel after the coiling process in a molten salt, which is held at a temperature of
480°C to 580°C, within 15 seconds after the coiling process; and a cooling process of
10 cooling the hot-rolled steel after the patenting process to a room temperature to obtain the
steel wire rod.
Advantageous Effects of Invention
[00 141
15 According to the above aspects of the present invention, it is possible to obtain a
steel wire rod having higher strength (tensile strength of 1200 MPa or more) and better
ductility (reduction of area of 45% or more) than those of the conventional one without
adding expensive elements. As a result, the steel wire after wire-drawing is controlled
to excellent ductility, and thus occurrence of delamination in the steel wire is suppressed.
20 Specifically, it is possible to produce the stee.1 wire which has high strength and in which
fracture is suppressed.
[00 1 51
In addition, by using the above mentioned steel wire rod, it is possible to
conduct the wire-drawing of the steel wire rod which has small diameter (10 mm or less),
7 I
high strength, and excellent ductility. Accordingly, reduction of the wire-drawing is
controlled to be small, and thus the wire-drawn steel wire can be controlled to excellent
ductility. As a result, it is possible to improve properties of the steel wires such as the
prestressed concrete wire, the zinc-coated steel wire, the spring steel wire, the bridge
5 cable, and the like.
[00 161
Furthermore, according to the above aspects of the present invention, it is
possible to produce the steel wire with high strength and excellent ductility under general
hot-rolling conditions as described above. It is not necessary to adopt severe hot-rolling
10 conditions such as large rolling reduction and low rolling temperature in order to produce
the steel wire rod with high strength and excellent ductility.
Brief Description of Drawings
[00 1 71
15 FIG. 1 shows a relationship between Ni content of a steel wire rod and reduction
of area of the steel wire rod.
FIG. 2 shows a relationship between the reduction of area of the steel wire rod
and an average pearlite block size in metallographic structure at a central portion of the
steel wire rod.
20 FIG. 3 shows a relationship between a diameter of the steel wire rod and a
minimum lamellar spacing of pearlite in the metallographic structure at the central
portion of the steel wire rod.
FIG. 4 shows a relationship between tensile strength of the steel wire rod and the
reduction of area of the steel wire rod.
2 5
Description of Embodiments
[00 1 81
Hereinafter, a preferred embodiment of the present invention will be described
in detail. However, the present invention is not limited to the component disclosed in
the embodiment, and can employ various modifications as long as the conditions do not
depart from the scope of the present invention.
[00 191
The present inventors have investigated a steel wire rod having higher strength
and better ductility than those of the conventional one without adding expensive elements
and then found the following results.
[0020]
First, it is found that the steel wire rod having high strength and excellent
ductility can be obtained by adding at least one of A1 and Ti which have an effect of
suppressing coarsening of austenite grains and by adding a small amount of Ni which has
an effect of improving the strength and the ductility only when the addition is the small
amount.
[002 11
The above is derived from the fact that pearlite block size (PBS) is controlled
and that lamellar spacing of pearlite is refined in metallographic structure of the steel
wire rod. When at least one of A1 and Ti is contained, A1N or TiN appropriately
precipitates, and thus coarsening of austenite grains is suppressed at a high-temperature
region. As a result, the coarsening of the pearlite block size after pearlitic
transformation is also suppressed. In addition, when Ni is contained in the small
amount, a starting time and a finishing time of the pearlitic transformation during a
patenting treatment shift to a longer time side, and thus a pearlitic transformation
9
temperature during production of the steel wire rod substantially decreases. As a result,
both of the pearlite block size and the lamellar spacing are refined. By the above effects
the steel wire rod obtains high strength and excellent ductility.
[0022]
5 In addition, it is found that, as a production method, controlling a time after a
coiling process of coiling hot-rolled steel and before a patenting process to be very short
is effective.
[0023]
When the time after the coiling process and before the patenting process is
10 controlled to be very short, the austenite is preferentially transformed to the pearlite in
the metallographic structure, and thus the steel wire rod having a small fraction of
non-pearlite structure can be obtained. A non-pearlite structure such as upper bainite,
pro-eutectoid ferrite, degenerate pearlite, and pro-eutectoid cementite is a factor
deteriorating properties of the steel wire rod. When the fraction of the non-pearlite
15 structure is controlled to be a small value and a fraction of the pearlite is controlled to be
large, the steel wire rod obtains high strength and excellent ductility.
[0024]
Hereinafter, limitation range and reasons for the limitation of base elements of
the steel wire rod according to the embodiment will be described. In addition, % as
20 described below is mass%.
[0025]
C: 0.70% to 1.00%
C (carbon) is an element that increases the strength. When an amount of C is
less than 0.70%, the strength is insufficient, and it is difficult to obtain uniform pearlite
25 structure because precipitation of the pro-eutectoid ferrite to austenite grain boundaries is
10
promoted. On the other hand, when the amount of C is more than 1.00%, the
pro-eutectoid cementite is easily formed at a surface layer portion of the steel wire rod,
reduction of area of the steel wire rod at fracture decreases, and thus the fracture of wire
at wire-drawing tends to occur. Accordingly, the amount of C is to be 0.70% to 1.00%.
5 The amount of C is preferably 0.70% to 0.95%, and is more preferably 0.70% to 0.90%.
100261
Si: 0.15% to 0.60%
Si (silicon) is an element that increases the strength, and is a deoxidizing
element. When an amount of Si is less than 0.15%, the effects may not be obtained.
10 On the other hand, when the amount of Si is more than 0.60%, the ductility of the steel
wire rod decreases, the precipitation of the pro-eutectoid ferrite is promoted in
hyper-eutectoid steel, and it is difficult to remove surface oxide by mechanical descaling.
Accordingly, the amount of Si is to be 0.15% to 0.60%. The amount of Si is preferably
0.15% to 0.35%, and is more preferably 0.15% to 0.32%.
15 [0027]
Mn: 0.10% to 1.00%
Mn (manganese) is a deoxidizing element, and is an element that increases the
strength. Furthermore, Mn is an element that suppresses hot embrittlement by fixing S
in steel as MnS. When an amount of Mn is less than 0.10%, the effects may not be
20 obtained. On the other hand, when the amount of Mn is more than 1.00%, Mn
segregates to a central portion of the steel wire rod, martensite or bainite is formed at the
segregated portion, and thus the reduction of area and drawability decrease.
Accordingly, the amount of Mn is to be 0.10% to 1.00%. The amount of Mn is
preferably 0.10% to 0.80%.
2 5 [002 81
N (nitrogen) is an element that suppresses the coarsening of austenite grains at a
high-temperature region by forming nitrides in steel. When an amount of N is less than
0.001%, the effect may not be obtained. On the other hand, when the amount of N is
5 more than 0.005%, since the amount of nitrides excessively increases and the nitrides act
as a fracture origin, the ductility of the steel wire rod may decrease. In addition,
solid-soluted N in steel may promote age hardening after the wire-drawing.
Accordingly, the amount of N is to be 0.001% to 0.005%. The amount of N is
preferably 0.001% to 0.004%.
10 [0029]
Ni: 0.005% to less than 0.050%
Ni (nickel) is an element that improves the ductility of steel by solid-soluted in
steel. In addition, Ni is an element that suppresses the pearlitic transformation and
shifts the starting time and the finishing time of the pearlitic transformation during the
15 patenting treatment to the longer time side. Therefore, in a case where a cooling rate is
the same, a temperature further decreases before starting the pearlitic transformation in
the patenting treatment in steel which contains Ni as compared with steel which does not
contain Ni. The above indicates that the transformation temperature of the pearlitic
transformation substantially is to be a lower temperature. As a result, both of the
20 pearlite block size and the lamellar spacing of pearlite are refined. The reduction of
area of the steel wire rod is improved with refining the pearlite block size, and the
strength of the Steel wire rod is improved with refining the lamellar spacing of the
pearlite.
[0030]
When an amount of Ni is less than 0.005%, the effects may not be obtained.
12
On the other hand, when the amount of Ni is 0.050% or more, the pearlitic
transformation is excessively suppressed, the austenite remains in the metallographic
structure of the steel wire rod during the patenting treatment, and thus a large amount of
micro-martensite is formed in the metallographic structure of the steel wire rod after the
5 patenting treatment. As a result, the reduction of area of the steel wire rod decreases.
FIG. 1 shows a relationship between Ni content of the steel wire rod and the reduction of
area of the steel wire rod. As shown in the figure, when Ni content is 0.005% to less
than 0.050%, the effect of improving the reduction of area of the steel wire rod is
obtained. The amount of Ni is preferably 0.005% to 0.030%. In addition,
10 approximately 0.0005% of Ni is unavoidably contained under ordinary producing
conditions.
[003 11
Al: 0.005% to 0.10%
Al (aluminum) is a deoxidizing element. In addition, A1 is an element that
15 precipitates as AIN by bonding to N. AIN has the effects of suppressing the coarsening
of austenite grains at the high-temperature region and of suppressing the age hardening
after the wire-drawing by reducing the solid-soluted N in steel. When the coarsening of
austenite grains at the high-temperature region is suppressed, the pearlite block size in
the metallographic structure of the steel wire rod after the patenting treatment is refined.
20 As a result, the reduction of area of the steel wire rod is improved. When an amount of
A1 is less than 0.005%, the effects may not be obtained. On the other hand, when the
amount of Al is more than 0.10%, a large amount of alumina-based non-metallic
inclusions which are hard and undeformable are formed, and thus the ductility of the
steel wire rod decreases. Therefore, the amount of A1 is to be 0.005% to 0.10%. The
25 amount ofAl is preferably 0.005% to 0.050%.
Ti: 0.005% to 0.10%
Similarly to Al, Ti (titanium) is a deoxidizing element. In addition, similarly to
Al, Ti is an element that precipitates as TiN by bonding to N. TiN has the effects of
5 suppressing the coarsening of austenite grains at the high-temperature region and of
suppressing the age hardening after the wire-drawing by reducing the solid-soluted N in
steel. The pearlite block size in the metallographic structure of the steel wire rod after
the patenting treatment is refined due to TiN, and as a result, the reduction of area of the
steel wire rod is improved. When an amount of Ti is less than 0.005%, the effects may
10 not be obtained. On the other hand, when the amount of Ti is more than 0.1%, coarse
carbides are formed in the austenite, and thus the ductility may decrease. Therefore, the
amount of Ti is to be 0.005% to 0.10%. The amount ofTi is preferably 0.005% to
0.050%, and is more preferably 0.005% to 0.010%.
[0033]
15 As described above, A1 and Ti have the same operation and effect. Accordingly,
since A1 precipitates as AlN by bonding to N in a case where A1 is contained, the effects
may be obtained even when Ti is not added. Similarly, since Ti precipitates as TiN by
bonding to N in a case where Ti is contained, the effects may be obtained even when A1
is not added. Therefore, at least one of A1 and Ti may be contained. In a case where
20 both of A1 and Ti are contained, it is preferable that amounts expressed in mass% of each
element satisfy a following Expression A. When a lower limit of the Expression A is
less than 0.005, the effects may not be obtained. On the other hand, when an upper
limit of the following Expression A is more than 0.10, the alumina-based non-metallic
inclusions or Ti-based carbides are excessively formed, and thus the ductility of the steel
25 wire rod decreases. The upper limit of the following Expression A is preferably 0.05%
14
or less.
0.005 I A1 + Ti I 0.10 . . . (Expression A)
[0034]
In addition to the above mentioned base elements, the steel wire rod according
5 to the embodiment includes unavoidable impurities. Herein, the unavoidable impurities
indicate elements such as P, S, 0, Pb, Sn, Cd, and Zn which contaminate unavoidably
from auxiliary materials such as scrap and the like and from producing processes. In
the elements, P, S, and 0 may be limited to the following in order to preferably obtain the
effect. In addition, % as described below is mass%. Moreover, although a limited
10 range of the unavoidable impurities includes O%, it is industrially difficult to be stably
0%.
[0035]
P: 0.020% or less
P (phosphorous) is an impurity and is an element that causes intergranular
15 fracture by segregating to the austenite grain boundaries and by embrittling
prior-austenite grain boundaries. When an amount of P is more than 0.02%, the
influence may be promoted. Accordingly, it is preferable that the amount of P be
limited to 0.02% or less. Since it is preferable that P content is as small as possible, the
limited range includes 0%. However, it is not technically easy to control P content to be
20 0%, and also the production cost of the steel may increase in order to be stably less than
0.001%. Thus, preferable limited range of P content is 0.001% to 0.020%. More
preferable limited range of P content is 0.001% to 0.015%. Generally, in ordinary
producing conditions, P of approximately 0.020% is contained unavoidably.
[0036]
2 5 S: 0.020% or less
15
S (sulfur) is an impurity and is an element that forms the sulfides. When an
amount of S is more than 0.02%, coarse sulfides are formed, and thus the ductility of the
steel wire rod may decrease. Accordingly, it is preferable that the amount of S be
limited to 0.020% or less. Since it is preferable that S content is as small as possible,
5 the limited range includes 0%. However, it is not technically easy to control S content
to be 0%, and also the production cost of the steel may increase in order to be stably less
than 0.001%. Thus, preferable limited range of S content is 0.001% to 0.020%. More
preferable limited range of S content is 0.001% to 0.015%. Generally, in ordinary
producing conditions, S of approximately 0.020% is contained unavoidably.
10 100371
0: 0.0030% or less
0 (oxygen) is an unavoidably contained impurity and an element that forms
oxide-based inclusions. When an amount of 0 is more than 0.0030%, coarse oxides are
formed, and thus the ductility of the steel wire rod may decrease. Accordingly, it is
15 preferable that the amount of 0 be limited to 0.0030% or less. Since it is preferable that
0 content is as small as possible, the limited range includes 0%. However, it is not
technically easy to control 0 content to be 0%, and also the production cost of the steel
may increase in order to be stably less than 0.00005%. Thus, preferable limited range
of 0 content is 0.00005% to 0.0030%. More preferable limited range of 0 content is
20 0.00005% to 0.0025%. Generally, in ordinary producing conditions, 0 of
approximately 0.0035% is contained unavoidably.
[003 81
In addition to the above mentioned base elements and impurities, the steel wire
rod according to the embodiment may further include, as optional elements, at least one
25 of Cr, Co, V, Cu, Nb, Mo, W, B, REM, Ca, Mg, and Zr. Hereinafter, limitation range
16
and reasons for the limitation of the optional elements will be described. In addition, %
as described below is mass%.
[0039]
Cr: more than 0% to 0.50%
5 Cr (chromium) is an element that refines the lamellar spacing of pearlite and
improves the strength of the steel wire rod. In order to obtain the effects, it is preferable
that an amount of Cr be more than 0% to 0.5%. The amount of Cr is more preferably
0.0010% to 0.50%. When the amount of Cr is more than 0.50%, the pearlitic
transformation may be excessively suppressed, the austenite may remain in the
10 metallographic structure of the steel wire rod during the patenting treatment, and thus
supercooled structure such as the martensite and the bainite may be formed in the
metallographic structure of the steel wire rod after the patenting treatment. In addition,
it may be difficult to remove the surface oxides by the mechanical descaling.
[0040]
15 Co: more than 0% to 0.50%
Co (cobalt) is an element that suppresses the precipitation of the pro-eutectoid
cementite. In order to obtain the effect, it is preferable that an amount of Co be more
than 0% to 0.50%. The amount of Co is more preferably 0.0010% to 0.50%. When
the amount of Co is more than 0.50%, the effect may be saturated, and the cost for the
20 addition may be vain.
[0041]
V: more than 0% to 0.50%
V (vanadium) is an element that suppresses the coarsening of austenite grains at
the high-temperature region by forming fine carbonitrides and that increases the strength
25 of the steel wire rod. In order to obtain the effects, it is preferable that an amount of V
be more than 0% to 0.50%. The amount of V is more preferably 0.0010% to 0.50%.
When the amount of V is more than 0.50%, an amount of the formed carbonitrides may
increase, a size of the carbonitrides may also increase, and thus the ductility of the steel
wire rod may decrease.
5 [0042]
Cu: more than 0% to 0.20%
Cu (copper) is an element that increases corrosion resistance. In order to
obtain the effect, it is preferable that an amount of Cu be more than 0% to 0.20%. The
amount of Cu is more preferably 0.0001% to 0.20%. When the amount of Cu is more
10 than 0.20%, Cu and may segregate as CuS in the grain boundaries by reacting with S, the
ductility of the steel wire rod may decrease, and defects may occur in the steel wire rod.
[0043]
Nb: more than 0% to 0.10%
Nb (niobium) has an effect of increasing corrosion resistance. In addition, Nb
15 is an element that suppresses the coarsening of austenite grains at the high-temperature
region by forming carbides or nitrides. In order to obtain the effects, it is preferable that
an amount of Nb be more than 0% to 0.10%. The amount of Nb is more preferably
0.0005% to 0.10%. When the amount of Nb is more than 0.1%, the pearlitic
transformation may be suppressed during the patenting treatment.
20 [0044]
Mo: more than 0% to 0.20%
Mo (molybdenum) is an element that concentrates at a growth interface of the
pearlite and suppresses growth of the pearlite due to so-called solute drag effect. In
addition, Mo is an element that suppresses formation of the ferrite and reduces the
25 non-pearlite structure. In order to obtain the effects, it is preferable that an amount of
18
Mo be more than 0% to 0.20%. The amount of Mo is more preferably 0.0010% to
0.20% and further more preferably 0.005% to 0.06%. When the amount of Mo is more
than 0.20%, the growth of the pearlite may be suppressed, it may take a long time for the
patenting treatment, and a decrease in productivity may occur. In addition, when the
5 amount of Mo is more than 0.20%, coarse MozC carbides may precipitate, and thus the
drawability may decrease.
W. more than 0% to 0.20%
Similarly to Mo, W (tungsten) is an element that concentrates at the growth
10 interface of the pearlite and suppresses the growth of the pearlite due to the so-called
solute drag effect. In addition, W is an element that suppresses the formation of the
ferrite and reduces the non-pearlite structure. In order to obtain the effects, it is
preferable that an amount of W be more than 0% to 0.20%. The amount of W is more
preferably 0.0005% to 0.20% and hrther more preferably 0.005% to 0.060%. When
15 the amount of W is more than 0.2%, the growth of the pearlite may be suppressed, it may
take a long time for the patenting treatment, and the decrease in productivity may occur.
In addition, when the amount of W is more than 0.20%, coarse W2C carbides may
precipitate, and thus the drawability may decrease.
[0046]
20 B: more than 0% to 0.0030%
B (boron) is an element that suppresses the formation of the non-pearlite
precipitates such as the ferrite, the degenerate pearlite, and the bainite. In addition, B is
an element that forms carbides or nitrides, and suppresses the coarsening of austenite
grains at the high-temperature region. In order to obtain the effects, it is preferable that
25 an amount of B be more than 0% to 0.0030%. The amount of B is more preferably
19
0.0004% to 0.0025%, further more preferably 0.0004% to 0.0015%, and most preferably
0.0006% to 0.0012%. When the amount of B is more than 0.0030%, precipitation of
coarse Fe23(CB)6 carbides may be promoted, and the ductility may decrease.
[0047]
5 REM: more than 0% to 0.0050%
REM (Rare Earth Metal) is a deoxidizing element. In addition, REM is an
element that detoxifies S which is the impurity by forming sulfides. In order to obtain
the effects, it is preferable that an amount of REM be more than 0% to 0.0050%. The
amount of REM is more preferably 0.0005% to 0.0050%. When the amount of REM is
10 more than 0.0050%, coarse oxides may be formed, the ductility of the steel wire rod may
decrease, and the fracture of the wire during the wire-drawing may occur.
Herein, REM indicate a generic name of a total of 17 elements in which
scandium of the atomic number 21 and yttrium of the atomic number 39 are added to 15
elements from lanthanum of the atomic number 57 to lutetium of the atomic number 71.
15 In general, misch metal which is a mixture of the elements is supplied and added to the
steel.
[0048]
Ca: more than 0.0005% to 0.0050%
Ca (calcium) is an element that reduces alumina-based hard inclusions. In
20 addition, Ca is an element that precipitates as fine oxides. As a result, the pearlite block
size of the steel wire rod is refined, and thus the ductility of the steel wire rod is
improved. In order to obtain the effects, it is preferable that an amount of Ca be more
than 0.0005% to 0.0050%. The amount of Ca is more preferably 0.0005% to 0.0040%.
When the amount of Ca is more than 0.0050%, coarse oxides may be formed, the
25 ductility of the steel wire rod may decrease, and thus the fiacture of the wire during the
20
wire-drawing may occur. Generally, in ordinary producing conditions, Ca of
approximately 0.0003% is contained unavoidably.
[0049]
Mg: more than 0.0005% to 0.0050%
5 Mg (magnesium) is an element that precipitates as fine oxides. As a result, the
pearlite block size of the steel wire rod is refined, and thus the ductility of the steel wire
rod is improved. In order to obtain the effects, it is preferable that an amount of Mg be
more than 0.0005% to 0.0050%. The amount of Mg is more preferably 0.0005% to
0.0040%. When the amount of Mg is more than 0.0050%, coarse oxides may be
10 formed, the ductility of the steel wire rod may decrease, and thus the fracture of the wire
during the wire-drawing may occur. Generally, in ordinary producing conditions, Mg of
approximately 0.0001% is contained unavoidably.
[0050]
Zr: more than 0.0005% to 0.010%
15 Zr (zirconium) is an element that improves a fraction of equiaxial austenite and
refines the austenite grains, because Zr is crystallized as ZrO which acts as nuclei of the
austenite. As a result, the pearlite block size of the steel wire rod is refined, and thus
the ductility of the steel wire rod is improved. In order to obtain the effects, it is
preferable that an amount of Zr be more than 0.0005% to 0.010%. The amount of Zr is
20 more preferably 0.0005% to 0.0050%. When the amount of Zr is more than 0.010%,
coarse oxides may be formed, and thus the fracture of the wire during the wire-drawing
may occur.
COO5 11
Next, the metallographic structure of the steel wire rod according to the
25 embodiment will. be described.
2 1
[0052]
The steel wire rod according to the embodiment includes, the metallographic
structure, by area%, 95% to 100% of the pearlite. When a distance fiom a peripheral
surface to a center of the steel wire rod is defined as r in a unit of mm, an average pearlite
5 block size at a central portion which is an area from the center of the steel wire rod to r x
0.99 is 1 pm to 25 p.m. An average pearlite block size at a surface layer portion which
is an area fiom the peripheral surface of the steel wire rod to r x 0.01 is 1 pm to 20 p
When a minimum lamellar spacing of the pearlite at the central portion is defined as S in
a unit of nrn, a following Expression B is satisfied.
10 S < 12r + 65 . . . (Expression B)
[0053]
Pearlite: 95% to 100%
When 95% to 100% of the pearlite is contained in the metallographic structure, a
fraction of the non-pearlite structure such as the upper bainite, the pro-eutectoid ferrite,
15 the degenerate pearlite, and the pro-eutectoid cementite decreases, and thus the strength
and the ductility of the steel wire rod is improved. Although it is ideal that the
non-pearlite structure is completely suppressed by controlling the pearlite in the
metallographic structure to be loo%, in fact it is not necessary that the non-pearlite
structure is reduced to zero. In a case where 95% to 100% of pearlite is contained in the
20 metallographic structure, the strength and the ductility of the steel wire rod is sufficiently
improved.
[0054]
The metallographic structure of the steel wire rod may be observed by using a
SEM (Scanning Electron Microscope) after subjecting a sample to chemical etching with
. -
25 picric acid. An observed section may be a cross-section (L cross-section) which is
22
parallel to a longitudinal direction of the steel wire rod, metallographic micrographs of at
least five visual fields may be taken by the SEM at a magnification of 2000-fold, and an
average value of the fraction of the pearlite may be determined by an image analysis.
[0055]
Average pearlite block size at central portion of steel wire rod: 1 pm to 25 pm
The pearlite block size (PBS) is a factor affecting the ductility of the steel wire
rod or the ductility of the steel wire after the wire-drawing. When the austenite grains
are refined at the high-temperature region or the pearlitic transformation temperature
during the patenting treatment is a low temperature, the PBS is refined. In addition, the
10 ductility of the steel wire rod is improved. FIG. 2 shows a relationship between the
reduction of area of the steel wire rod and the average pearlite block size in the
metallographic structure of the central portion of the steel wire rod. As shown in the
figure, in order to sufficiently increasing and controlling the reduction of area of the steel
wire rod to be 45% or more, it is necessary for the average PBS at the central portion of
15 the steel wire rod to be 25 pm or less. The average PBS at the central portion of the
steel wire rod is preferably 20 pm or less and more preferably 15 pm or less. In
addition, although it is preferable that the PBS at the central portion of the steel wire rod
is as fine as possible, the above-described properties of the steel wire rod are satisfied as
long as the average PBS is 1 pm or more.
Average pearlite block size at surface layer portion of steel wire rod: 1 pm to 20
Pm
The surface layer portion of the steel wire rod is a region at which delamination
occurs when the steel wire is torsionally deformed. In order to suppress occurrence of
23
the delamination of the steel wire by sufficiently increasing the drawability of the steel
wire rod, the PBS at the surface layer portion of the steel wire rod is refined as compared
with that at the central portion of the steel wire rod. Accordingly, it is necessary for the
average PBS at the surface layer portion of the steel wire rod to be 20 pm or less. The
5 average PBS at the surface layer portion of the steel wire rod is preferably 15 pm or less
and more preferably 10 pm or less. In addition, although it is preferable that the PBS at
the surface layer portion of the steel wire rod is as fine as possible, the above-described
properties of the steel wire rod are satisfied as long as the average PBS is 1 pm or more.
[0057]
10 The pearlite block size of the steel wire rod may be determined by using an
EBSD (Electron Backscatter Diffraction Pattern) method. The L cross-section of the
steel wire rod which is embedded in resin may be cut and polished, EBSD measurement
may be conducted in at least three visual fields which are 150 pm x 250 pm at the central
portion and the surface layer portion of the steel wire rod, and the average pearlite block
15 size may be determined by the analysis with a method of Johnson-Saltykov in which a
region surrounded by boundaries having a misorientation of 9" is regarded as one block.
[005 81
Minimum lamellar spacing s of pearlite at central portion of steel wire rod
The lamellar spacing is a factor affecting the strength of the steel wire rod or the
20 strength of the steel wire after the wire-drawing. When the pearlitic transformation
temperature during the patenting treatment is a low temperature, the lamellar spacing is
refined. In addition, the strength of the steel wire rod increases. Accordingly, the
lamellar spacing can be controlled by adjusting the alloy elements and by changing the
pearlitic transformation temperature. In addition, a diameter of the steel wire rod also
24
affects the lamellar spacing. Since the cooling rate of the steel wire rod after hot rolling
increases with reducing the diameter of the steel wire rod, the lamellar spacing is refined.
-
FIG. 3 shows a relationship between the diameter of the steel wire rod and the minimum
lamellar spacing S of the pearlite in the metallographic structure at the central portion of
5 the steel wire rod. In the figure, results of the steel wire rods, which satisfy the
chemical composition and the metallographic structure as mentioned above, are shown as
a rhombus, and results of conventional steel wire rods are shown as a quadrangle. In
addition, in the figure, S = 12r + 65 is indicated by a straight line I. As can be seen
from the figure, the minimum lamellar spacing S of the steel wire rod, which satisfies the
10 chemical composition and the metallographic structure, is smaller than the minimum
lamellar spacing S of the conventional steel wire rod in any diameter by using the
straight line I as a border. Specifically, the minimum lamellar spacing S of the steel
wire rod according to the embodiment satisfies the above-described Expression B (S <
12r + 65). As a result, the strength of the steel wire rod further increases as compared
15 with the conventional steel wire rod.
[0059]
The minimum lamellar spacing S of the pearlite of the steel wire rod may be
observed by using the SEM. An observed section may be a cross-section (C
cross-section) which is orthogonal to the longitudinal direction of the steel wire rod, the
20 observed section which is embedded in resin may be cut and polished, metallographic
micrographs of at least five visual fields at the central portion of the steel wire rod may
be taken by the SEM at a magnification of 10000-fold, the minimum lamellar spacing in
the visual fields may be measured, and then an average value thereof may be determined.
[0060]
In addition, in the steel wire rod according to the embodiment, when tensile
2 5
strength is defined as TS in a unit of MPa and the reduction of area is defined as RA in a
unit of %, it is preferable that both of a following Expression C and a following
Expression D are satisfied. Generally, it is known that the reduction of area RA is
inversely proportional to the tensile strength TS. As described above, a steel wire rod
5 having the reduction of area of 45% or more has been anticipated at present. In addition,
in a case of a steel wire rod in which severe tensile strength TS is not required, it is
preferable that the reduction of area RA be further larger than 45%. FIG. 4 shows a
relationship between the tensile strength of the steel wire rod and the reduction of area of
the steel wire rod. In the figure, results of the above-described steel wire rod are shown
10 as a rhombus, and results of the conventional steel wire rod are shown as a quadrangle.
In addition, in the figure, RA = 100 - 0.045 x TS is indicated by a straight line 11, and RA
= 45 is indicated by a straight line 111. As can be seen from the figure, the value of the
reduction of area RA of the steel wire rod is larger than that of the conventional steel wire
rod by using the straight line I1 and the straight line I11 as a border. As mentioned above,
15 it is preferable that the value of the reduction of area RA increases as a function of the
value of the tensile strength TS so as to satisfy the following Expression C and the
following Expression D. In addition, RA > 46 is preferable, RA > 48 is more preferable,
and RA > 50 is most preferable. Although an upper limit of the reduction of area RA is
not particularly limited, the wire-drawing can be sufficiently conducted in general when
20 the reduction of area RA is 60%. Accordingly, the upper limit of the reduction of area
RA may be 60%.
RA 2 100 - 0.045 x TS . . . (Expression C)
RA 2 45 . . . (Expression D)
[0061]
When the steel wire rod satisfies the above-described chemical composition and
26
metallographic structure, the steel wire rod having higher strength and better ductility
than those of the conventional one may be obtained. In order to obtain the steel wire
rod having the metallographic structure, the steel wire rod may be produced by the
following production method.
5 100621
Next, the method of producing the steel wire rod according to the embodiment
will be described.
[0063]
In a casting process, molten steel which consists of the base elements, the
10 optional elements, and the unavoidable impurities as described above is casted to obtain a
cast piece. Although a casting method is not limited particularly, a vacuum casting
method, a continuous casting method, and the like may be employed.
[0064]
In addition, according to the necessity, a soaking, a blooming, and the like may
15 be conducted by using the cast piece after the casting process.
[0065]
In a heating process, the cast piece after the casting process is heated to a
temperature of 1000°C to 1100°C. The reason why the cast piece is heated to the
temperature range of 1000°C to 1100°C is to allow the metallographic structure of the
20 cast piece to be the austenite. When the temperature is lower than 1000°C,
transformation from the austenite to another structure may occur during the hot rolling
that is a subsequent process. When the temperature is higher than 1 100°C, austenite
grains may grow and coarsen.
[0066]
2 5 In the hot-rolling process, the cast piece after the heating process is
hot-finish-rolled so as to control a fmishing rolling temperature to be 850°C to 1000°C in
order to obtain hot-rolled steel. Here, the finish-rolling indicates rolling of a final pass
in the hot-rolling process in which plural passes of the hot rolling are conducted. The
reason why the finishing rolling temperature is the temperature range of 850°C to
5 1000°C is to control the pearlite block size (PBS). When the finishing rolling
temperature is lower than 850°C, transformation from the austenite to another structure
may occur during the hot rolling. When the finishing rolling temperature is higher than
1000°C, it is difficult to control a temperature in subsequent processes, and thus the PBS
may not be controlled. In addition, it is preferable that rolling reduction in the finish
10 rolling be 10% to less than 60%. When the rolling reduction in the finish rolling is 10%
or more, an effect of refining the austenite grains may be appropriately obtained. On
the other hand, when the rolling reduction in the finish rolling is 60% or more, load on
production facilities may be excessive, and the production cost may increase.
[0067]
15 In a coiling process, the hot-rolled steel after the hot-rolling process is coiled
within a temperature range of 780°C to 840°C. The reason why the coiling temperature
range is 780°C to 840°C is to control the PBS. When the coiling temperature is lower
than 780°C, the pearlitic transformation tends to start only at the surface layer portion
that is easily cooled. When the coiling temperature is higher than 840°C, unevenness in
20 the PBS may increase due to a difference in the cooling rate between an overlapped
portion and a non-overlapped portion during the coiling. The upper limit of the coiling
temperature is preferably lower than 800°C in order to refine the PBS and increase the
reduction of area of the steel wire rod.
100681
2 8
In a patenting process, within 15 seconds after the coiling process, the hot-rolled
steel after the coiling process is directly immersed in a molten salt (DLP) which is held at
a temperature of 480°C to 580°C. The reason why the hot-rolled steel is isothermally
maintained at the temperature range of 480°C to 580°C within 15 seconds after the
5 coiling process is to preferentially progress the pearlitic transformation. As a result, it is
possible to obtain the metallographic structure having the small fraction of the
non-pearlite structure. When the temperature of the molten salt is lower than 480°C, the
upper bainite which is soft increases, and thus the strength of the steel wire rod is not
improved. On the other hand, when the temperature of the molten salt is higher than
10 580°C, the temperature is high for the pearlitic transformation temperature, the PBS
coarsens, and the lamellar spacing also coarsens. In addition, when longer than 15
seconds, the austenite grain size may coarsen, and the fraction of the non-pearlite
structure may increase due to the formation of the pro-eutectoid cementite and the like.
It is preferable that the immersion is conducted within 10 seconds. Although it is ideal
15 that a lower limit of the number of seconds is 0 seconds, in fact it is preferable that the
lower limit is 2 seconds or longer.
[0069]
In a cooling process, the hot-rolled steel which has been subjected to the
patenting treatment and in which the pearlitic transformation has been finished is cooled
20 to room temperature after the patenting process in order to the steel wire rod. The steel
wire rod has the above-described metallographic structure.
Example
[0070]
25 Hereinafter, the effects of an aspect of the present invention will be described in
29
detail with reference to the following examples. However, the condition in the
1
examples is an example condition employed to confirm the operability and the effects of
the present invention, so that the present invention is not limited to the example condition.
The present invention can employ various types of conditions as long as the conditions
5 do not depart from the scope of the present invention and can achieve the object of the
present invention.
[007 11
Sample preparation
Examples 1 to 48 and Comparative Examples 49 to 85 with the chemical
10 composition shown in Tables 1 and 2 were casted into cast piece having a shape of 300
mm x 500 mm by using a continuous casting machine (casting process). The cast piece
was subject to blooming to a shape of a cross-section of 122 mm square. The steel
piece (cast piece) was heated to 1000°C to llOO°C (heating process). After the heating,
finish rolling was conducted so that a finishing rolling temperature was 850°C to 1000°C,
15 whereby hot-rolled steel having a wire rod diameter (diameter) shown in Tables 3 and 4
was obtained (hot rolling process). The hot-rolled steel was coiled at 780°C to 840°C
(coiling process). After the coiling, a patenting treatment was conducted (patenting
process). Some of the hot-rolled steels were subject to the patenting treatment by
immersed in a salt bath held at 480°C to 580°C within 15 seconds after the coiling.
20 After the patenting treatment, cooling to room temperature was conducted to obtain steel
wire rod (cooling process). In Tables 1 to 4, the underlined value indicates out of the
range of the present invention. In Table 1, the blank column indicates that the optional
element was not intentionally added.
[0072]
30
In addition, wire-drawing was conducted by using the produced steel wire rod.
In the wire-drawing, scale of the steel wire rod was removed by pickling, a zinc
phosphate film was applied by phosphating, the wire-drawing in which reduction per a
pass was 10% to 25% was conducted by using a die having an approach angle of lo0, and
5 whereby a high strength steel wire having a diameter of 1.5 rnm to 4.5 mm was obtained.
Work strain during the wire-drawing and the wire diameter (diameter) of the steel wire
after the wire-drawing are shown in Tables 3 and 4.
COO731
Evaluation
10 Area fraction of pearlite
The steel wire rod was embedded in resin and was polished. The steel wire rod
was subjected to chemical etching using picric acid and was observed by using a SEM.
An observed section was a cross-section (L cross-section) which is parallel to a
longitudinal direction of the steel wire rod. - In addition, grain boundary ferrite, bainite,
15 pro-eutectoid cementite, and micromartensite were regarded as a non-pearlite structure,
and a fraction of the balance was regarded as the area fraction of pearlite. Evaluation of
the area fraction of pearlite was conducted by SEM-observing total five areas including,
when the diameter of the steel wire rod was defined as D in a unit of mm, total four areas
which were obtained by rotating a 1/4D region in the L cross-section of the steel wire rod
20 by 90" around the center of the steel wire rod and one area which was the center of the
steel of a 1/2D region in the L cross-section of the steel wire rod. In the SEM
observation, metallographic micrographs with a visual field of vertically 100 pm x
horizontally 200 pm were taken at a magnification of 2000-fold, and an average value of
the area fraction of pearlite was determined by an image analysis of the metallographic
25 micrographs. A case in which the pearlite was 95% to 100% in a unit of area% was
judged to be acceptable.
Average pearlite block size
A pearlite block size (PBS) of the steel wire rod was determined by using an
5 EBSD method. The L cross-section of the steel wire rod was embedded in resin and
was polished. When a distance from a peripheral surface to the center of the steel wire
rod was r in a unit of mm, a central portion was an area from the center of the steel wire
rod to r x 0.99, and a surface layer portion was an area from the peripheral surface of the
steel wire rod to r x 0.01, the central portion and the surface layer portion were evaluated.
10 EBSD measurement was conducted in at least three visual fields which were 150 pm x
250 pm at the central portion and the surface layer portion of the steel wire rod, and an
average pearlite block size was determined by the analysis with a method of
Johnson-Saltykov in which a region surrounded by boundaries having a misorientation of
9" was regarded as one block. A case in which the average pearlite block size at the
15 central portion was I pm to 25 pm and a case in which the average pearlite block size at
the surface layer portion was 1 pm to 20 pm were judged to be acceptable.
[0075]
Minimum lamellar spacing
A minimum lamellar spacing S at the central portion of the steel wire rod was
20 observed by using the SEM. An observed section was a cross-section (C cross-section)
which was orthogonal to the longitudinal direction of the steel wire rod. Metallographic
micrographs of at least five visual fields at the central portion of the steel wire rod were
taken by the SEM at a magnification of 10000-fold, the minimum lamellar spacing in the
visual fields was measured, and then an average value thereof was determined. A case
3 2
in which the r that is a distance fiom the peripheral surface to the center of the steel wire
rod and the S satisfied S < 12r + 65 was judged to be acceptable.
[0076]
Mechanical Properties
5 Test specimens having a gauge length of 200 mm were prepared so that the
longitudinal direction of the steel wire rod and the steel wire was a tensile direction, and
tensile tests were conducted under a rate of 10 mmlmin. Average values of the tensile
strength (TS) and the reduction of area (RA) were determined fiom results of at least
three times of the tests. A case in which the tensile strength (TS) was 1200 MPa or
10 more and a case in which the reduction of area (RA) was 45% were judged to be
acceptable.
[0077]
Occurrence of delamination
Occurrence of delamination was evaluated by using the steel wire after the
15 wire-drawing. When the diameter of the steel wire was d, the steel wire after the
wire-rolling was subjected to a torsion test by using a torsion testing machine under
conditions such that a gauge length was 100 x d and a rotational speed was 10 rpm. In
addition, at least three times of the torsion tests were conducted. A case in which the at
least one occurrence of the delamination was confirmed by visual observation was
20 regarded as "occurred", and a case in which the occurrence of the delamination was not
confirmed was regarded as "not occurred". The delamination "not occurred" was
judged to be acceptable.
[0078]
The production results and the evaluation results are shown in Tables 1 to 4. In
25 Nos. 1 to 48 that were examples, the steel wire rods had excellent strength and ductility.
In addition, in the steel wires that were wire-drawn from the steel wire rod, the strength
was high strength, and the occurrence of delamination was suppressed.
[0079]
On the other hand, in Nos. 49 to 85 that were comparative examples, the steel
5 wire rods were out of the range of the present invention. In the steel wires that were
wire-drawn fiom the steel wire rod, the occurrence of delamination was confirmed.
[0080]
In Comparative Example 49, the amount of A1 + Ti was excessive, and thus RA
of the steel wire rod was insufficient. In Comparative Example 50, the amount of Cr
10 was excessive, and thus the fraction of the pearlite of the steel wire rod was insufficient.
In Comparative Example 5 1, the amount of Co was excessive, a large amount of
expensive element was contained, and the cost increased. In Comparative Example 52,
the amount of V was excessive, and thus RA of the steel wire rod was insufficient. In
Comparative Example 53, the amount of Cu was excessive, and thus RA of the steel wire
15 rod was insufficient. In Comparative Example 54, the amount of Nb was excessive, and
thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative
Example 55, the amount of Mo was excessive, and thus the fraction of the pearlite of the
steel wire rod was insufficient. In Comparative Example 56, the amount of W was
excessive, and thus the fiaction of the pearlite of the steel wire rod was insufficient. In
20 Comparative Example 57, the amount of B was excessive, and thus RA of the steel wire
rod was insufficient. In Comparative Example 58, the amount of REM was excessive,
and thus RA of the steel wire rod was insufficient. Ln Comparative Example 59, the
amount of Ca was excessive, and thus RA of the steel wire rod was insuficient. In
Comparative Example 60, the amount of Mg was excessive, and thus RA of the steel
25 wire rod was insufficient. In Comparative Example 61, the amount of Zr was excessive,
3 4
and thus RA of the steel wire rod was insufficient.
In Comparative Example 62, the amount of C was insufficient, and thus TS and
RA of the steel wire rod were insufficient. In Comparative Example 63, the amount of
C was excessive, and thus RA of the steel wire rod was insufficient.
5 In Comparative Example 64, the amount of Si was insufficient, and thus TS and
RA of the steel wire rod were insufficient. In Comparative Example 65, the amount of
Si was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 66, the amount of Mn was insufficient, and thus TS
and RA of the steel wire rod were insufficient. In Comparative Example 67, the amount
10 of Mn was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 68, the amount of N was insufficient, and thus the
average PBS at the central portion of the steel wire rod and the average PBS at the
surface layer portion of the steel wire rod were insufficient. In Comparative Example
69, the amount of N was excessive, and thus RA of the steel wire rod was insuficient.
15 In Comparative Example 70, the amount of Ni was insufficient, and thus the
average PBS at the central portion of the steel wire rod, the average PBS at the surface
layer portion of the steel wire rod, and the minimum lamellar spacing at the central
portion of the steel wire rod were insufficient. In Comparative Example 71, the amount
of Ni was excessive, and thus RA of the steel wire rod was insufficient.
20 In Comparative Example 72, the amount of A1 was insufficient, and thus the
average PBS at the central portion of the steel wire rod and the average PBS at the
surface layer portion of the steel wire rod were insufficient. In Comparative Example
73, the amount of A1 was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 74, the amount of Ti was insufficient, and thus the
25 average PBS at the central portion of the steel wire rod and the average PBS at the
35
surface layer portion of the steel wire rod were insufficient. In Comparative Example
75, the amount of Ti was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 76, the heating temperature in the heating process was
low, and thus the fraction of the pearlite of the steel wire rod was insufficient. In
5 Comparative Example 77, the heating temperature in the heating process was high, and
thus the average PBS at the central portion of the steel wire rod and the average PBS at
the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 78, the reduction of the finish rolling in the hot-rolling
process was small, and thus the average PBS at the central portion of the steel wire rod
10 and the average PBS at the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 79, the finishing rolling temperature in the hot-rolling
process was low, and thus the fraction of the pearlite of the steel wire rod was insufficient.
In Comparative Example 80, the finishing rolling temperature in the hot-rolling process
was high, and thus the average PBS at the central portion of the steel wire rod and the
15 average PBS at the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 8 1, the coiling temperature in the coiling process was
low, and thus the fraction of the pearlite of the steel wire rod was insufficient. In
Comparative Example 82, the coiling temperature in the coiling process was high, and
thus the average PBS at the central portion of the steel wire rod and the average PBS at
20 the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 83, a time after the coiling process before the patenting
process was long, and thus the fraction of the pearlite of the steel wire rod, the average
PBS at the central portion of the steel wire rod, and the average PBS at the surface layer
portion of the steel wire rod were insufficient.
2 5 In Comparative Example 84, the temperature of the molten salt in the patenting
36
process was low, and thus the fraction of the pearlite of the steel wire rod was insufficient.
In Comparative Example 85, the temperature of the molten salt in the patenting process
was high, and thus the minimum lamellar spacing at the central portion of the steel wire
rod was insufficient.
5 [008 11
[Table I]
[0082]
[Table 21
[0083]
[Table 31
[0084]
[Table 41
Industrial Applicability
15 [0085]
According to the aspects of the present invention, it is possible to obtain a steel
wire rod having higher strength and better ductility than those of the conventional one
without adding expensive elements. As a result, it is possible to produce a steel wire in
which the occurrence of delamination is suppressed and in which strength is high.
20 Accordingly, the present invention has significant industrial applicability.
TABLE 1-1
NO. CHEMI CAL COMPOSITION (mass%)
C Si Mn P S 0 Al Ti N Cr Mo
TABLE 1-2
No. CHEM I CAL GOMPOSI T I ON (mass%)
Ni Cu V Co W Nb B Mg Ga REM Zr AI+Ti
TABLE 2-1
No. GHEM I GAL COMPOS I T l ON (mass%)
C Si Mn P S 0 Al Ti N Cr Mo
TABLE 2-2
NO. CHEMICAL COMPOS I T ION (mass%)
Ni Cu V Co W Nb B Mg Ca REM Zr AI+Ti
TABLE 3-1
No. PRODUC- I ON COND I T IONS
3) (9)
I
( 1 ) HEATING PROCESS
(2) HEAT 1 NG TEMPERATURE
(3) HOT-ROLL I NG PROCESS
(4) ROLLING REDUCTION I N FINISH ROLLING
(5) F I N l SH I NG ROLL l NG TEMPERATURE
(6) DIAMETER 2r AFTER FINISH ROLLING
(7) GO1 LING PROCESS
(8) CO l L I NG TEMPERATURE
(9) PATENT I NG PROCESS
(1 0) TIME AFTER COILING (sec. )
(1 1 ) PATENT1 NG METHOD
(12) TEMPERATURE OF MOLTEN SALT
TABLE 3-2
No. (1 3)
I (2 1 )
22 -.
(1 6)
(23) (25) (29) ((26) 30) (32) (33)
(1 3) EVALUATION RESULTS OF STEEL WIRE ROD (24) REDUCTION OF AREA
(1 4) METALLOGRAPH I C STRUCTURE (25) REDUCTION OF AREA RA
(1 5) FRACTION OF PEARL1 TE (26) VALUE OF (1 00-0.045 x TS)
(1 6) AVERAGE PEARLlTE BLOCK SIZE (27) STEEL W 1 RE AFTER W I RE-DRAW I NG
(1 7) CENTRAL PORT 1 ON (28) PRODUCT 1 ON CONDITIONS
(1 8) SURFACE LAYER PORTION (29) D l AMETER AFTER W I RE-DRAW I NG
(1 9) LAMELLAR SPAC 1 NG (30) STRAIN DURING W I RE-DRAW I NG
(20) M IN 1 MUM LAMELLAR SPAC I NG AT CENTRAL PORT I ON (31 ) EVALUAT 1 ON RESULTS
(21) VALUE OF (12r+65) (32) OCCURRENCE OF DELAMINATION
(22) MECHAN I GAL PROPERT l ES (33) TENSILE STRENGTH TS
(23) TENSILE STRENGTH TS (34) NOT OCCURRED
TABLE 4-1
(1) HEATING PROCESS (9) PATENT I NG PROCESS
(2) HEAT I NG TEMPERATURE (10) TIME AFTER COILING (sec.)
(3) HOT-ROLL I NG PROCESS (1 1 ) PATENT I NG METHOD
(4) ROLLING REDUCTION I N FINISH ROLLING (1 2) TEMPERATURE OF MOLTEN SALT
(5) F I N I SH I NG ROLL I NG TEMPERATURE (36) STELMOR
(6) DIAMETER 2 r AFTER FINISH ROLLING (37) REHEAT LP
(7) COILING PROCESS
(8) GO I LING TEMPERATURE
TABLE. 4-2
EVALUAT 1 ON RESULTS OF STEEL
METALLOGRAPH 1 C STRUCTURE
FRACTION OF PEARL 1 TE
AVERAGE PEARL l TE BLOCK SIZE
CENTRAL PORT 1 ON
SURFACE LAYER PORT I ON
LAMELLAR SPACING
M 1 N I MUM LAMELLAR SPACING AT
VALUE OF (12r+65)
MECHAN 1 CAL PROPERT 1 ES
TENSILE STRENGTH TS
WIRE ROD
CENTRAL PORT I ON
REDUCT ION OF AREA
REDUCTION OF AREA RA
VALUE OF (1 00-0.045 x TS)
STEEL WIRE AFTER W I RE-DRAW I NG
PRODUCT I ON COND I T I ONS
DIAMETER AFTER W I RE-DRAW I NG
STRAIN DURING W I RE-DRAW I NG
EVALUAT I ON RESULTS
OCCURRENCE OF DELAM 1 NAT I ON
TENSILE STRENGTH TS
NOT OCCURRED
OCCURRED

CLAIMS
1. A steel wire rod comprising, as a chemical composition, by mass%:
0.70% to 1.00% of C;
0.15% to 0.60% of Si;
0.1% to 1.0% of Mn;
0.001% to 0.005% of N;
0.005% to less than 0.050% of Ni;
at least one of 0.005% to 0.10% ofAl and 0.005% to 0.10% ofTi; and
a balance consisting of iron and unavoidable impurities, and
as a metallographic structure, by area%, 95% to 100% of a pearlite,
wherein, when a distance from a peripheral surface to a center is r in unit of mm,
an average pearlite block size at a central portion which is an area from the center to r x
0.99 is 1 pm to 25 ym,
15 an average pearlite block size at a surface layer portion which is an area from the
peripheral surface to r x 0.01 is 1 pm to 20 ym, and
when a minimum lamellar spacing of the pearlite at the central portion is S in
unit of nm, a following Expression 1 is satisfied.
S < 12r + 65 . . . (Expression 1)
2. The steel wire rod according to Claim 1, further comprising, as the chemical
composition, by mass %, at least one of
more than 0% to 0.50% of Cr,
more than 0% to 0.50% of Co,
more than 0% to 0.50% of V,
more than 0% to 0.20% of Cu,
more than 0% to 0.10% of Nb,
more than 0% to 0.20% of Mo,
more than 0% to 0.20% of W,
5 more than 0% to 0.0030% of B,
more than 0% to 0.0050% of Rare Earth Metal,
more than 0.0005% to 0.0050% of Ca,
more than 0.0005% to 0.0050% of Mg, and
more than 0.0005% to 0.010% of Zr.
10
3. The steel wire-rod according to Claim 1 or 2,
wherein, when a tensile strength is TS in unit of MPa and a reduction of area is
RA in unit of %, both of a following Expression 2 and a following Expression 3 are
satisfied.
15 RA 2 100 - 0.045 x TS . . . (Expression 2)
RA 2 45 . . . (Expression 3)
4. The steel wire rod according to Claim 1 or 2,
wherein amounts expressed in mass% of each element in the chemical
20 composition satisfj a following Expression 4.
0.005 I A1 + Ti I 0.1 . . . (Expression 4)
5. A method of producing a steel wire rod, the method comprising:
a casting process to obtain a cast piece consisting of the chemical composition
25 according to Claim 1 or 2;
t L r
1 I
I
I
I
J !
a heating process of heating the cast piece to a temperature of 1000°C to
I
a hot-rolling process of hbt~finish-rollingth e cast piece after the heating process I
!
by controlling a finishing temperature to be 850°C to 1000°C to obtain a hot-rolled steel;
. . ,
a coiling process ofcoiling thehot-iolled steel withina temperature range of ' . . ,
780°C to 840°C;
a patenting process of directly immersing the hot-rolled steel after the coiling
. . . . . .
process in a molt& salt, which is held at a temperature of 4 8 0 " ~ t o58 0°C, within 15 ,
. .
seconds after the coiling process; and
a cooling process of cooling the hot-rolled steel after the patenting process to a
room temperature to obtain the steel wire rod.
- - -- --- - - ---
Dated this 10/09/20 13
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]