[Designation of Docunleilt] SPECIFICATION
[Title of the Invention] WIRE ROD AND STEEL WIRE USING THE SAME, AND
STEEL PIECE
[Technical Field]
[OOOl]
The present invention relates to a wire rod which is a material of a highstrength
steel wire used in the field of a high-strength wire rope, a tnoorit~gw ire rope
of an offshore oil drilling platform, a PWS (Prefabricated Parallel Wire Strand) for a
bridge, a high-strength PC strand wire, or the like. Further, the present invention
relates to a steel wire manufacttired from this wire rod; and a steel piece which can be
used to manufacture the steel wire.
Priority is claimed on Japanese Patent ApplicationNo. 2012-089220, filed on
April 10,2012, the content of which is incorporated herein by reference.
[Background Art]
[0002]
A steel wire which is used in the above-described fields is required to have a
high strength (for example, a tensile strength of 2000 MPa or higher). In a wire rod
which is used for such a steel wire requiring a high strength, nlicromartensite which is
present in a center segregation portion pron~otesth e for~nationo f a microvoid.
There is a problem that this microvoid is a starting point where a chevron
crack (one of void defects which are initiated in the vicinity of a material center) is
initiated during the subsequent wire-drawing process, which causes wire breakage and
insufficient strength.
[0003]
In order to solve such a problem, Patent Docutnent 1 focuses on a segregation
peak, which forms a macrosegregation portion, and discloses a method of controlling a
segregation peak to be lower than or equal to a critical concentration tl~roughs oaking.
[0004]
In the method of controlling a segregation peak disclosed in Patent Document
1, as the carbon content becomes greater in order to improve strength, the time
required for processes which is performed to reduce a peak height of a
macrosegregation particle size of a center, for example soaking, becomes longer.
Therefore, economic disadvantages such as an increase in manufacturing cost may
arise.
In addition, as another method, Patent Document 2 discloses a technique of
reducing center segregation while continuously forming steel during casting.
[0006]
In the technique disclosed in Patent Document 2, if center segregation is
reduced while continuously fornling steel during casting, in order to exhibit an ideal
forming effect, high-accuracy estimation of a crater end point (ternlinal of a solidliquid
boundary inside a cast piece) is inevitable. If manufacturing conditions are
slightly deviated during casting, a crater end point may be positioned before or after a
forging point.
[0007]
A deviation between the crater end point and the fornling point may
deteriorate a center segregation portion. Therefore, with the teclulique disclosed in
Patent Document 2, an effect of not improving but deteriorating center segregation
may be obtained.
[OOOS]
In addition, in such a method of reducing a center segregation while
continuously forming steel disclosed in Patent Document 2, even ~vlienth is method is
performed under optimal conditions, it is difficult for a surface portion and a center
portion to simultaneously have negative segregation.
[Prior Art Document]
[Patent Document]
[0009]
[Patent Docun~ent I] Japanese Examined Patent Application, Second
PublicationNo. H06-76643
[Patent Document 21 Japauese Unexamined Patent Application, First
Publication No. H09-174213
[Disclosure of tl~Ien vention]
[Problems to be Solved by the Invention]
[0010]
The present invention has been made in order to solve the above-described
proble~ns. That is, an object of the present i~iveiitionis to provide a wire rod in which
high drawability is obtained by forming a negative segregation region in a center and
becomes a steel wire having 11ig11 strength and superior delayed fracture resistance
through wire-drawing by filrtlier forming a negative segregation region in a surface
portion. In addition, another object of the present invention is to provide a highstrength
steel wire having superior delayed fracture resistance, which is obtained from
the above-described wire rod. In addition, still another object of the present invention
is to provide a steel piece in which a negative segregation region is formed in both a
center and a surface portion.
[Means for Solving the Problems]
[OOll]
The present inventors have focused on the relationsi~ipb etween a profile
inside a cross-section of center segregation of a wire rod; and drawability and delayed
fracture resistance (of a steel wire) after drawing and intensively studied.
As an evaluatiol~m ethod, an evaluation method of processing a center
segregation portion under more strict drawing conditions than ordinary drawing
conditions is used. That is, using a die whose approach angle is 40" which is greater
than an ordinaiy approach angle of lo0, the evaluation is performed under drawing
conditions that can impart a tensile strength to the vicinity of a central axis of a wire
rod.
As a result, it was found that drawability can be in~provedb y appropriately
imparting a segregation profile of carbon inside a cross-section (horizontal section)
obtained by cutting a wire rod in a radial direction thereof, that is, by appropriately
imparting a C segregation profile.
[0012]
Further, the present inventors newly found that both drawability and delayed
fracture resistance after drawing can be simnltaneously and effectively improved if
both a surface and a center of a wire rod are simt~ltaneouslylo cally softened by
appropriately controlling the C segregation profile.
In addition, the present inventors found that, if an appropriate C segregation
profile is obtained in the stage of a steel piece, a C segregation profile of a wire rod
obtained ftoln this steel piece is not substantially changed fion~th at of the steel piece.
Furthennore, it was found that, after the wire rod is drawn to obtain a steel wire, the
diameter thereof is decreased, but the shape of the C segregation profile is not
substantially changed between the wire rod (before drawing) and the steel wire (after
drawing). That is, by obtaining the above-described C segregation profile in the stage
of a steel piece, a wire rod obtained by processing this steel piece and a steel wire
obtained by drawing the wire rod can have the same C segregation profile. As
described above, since the C segregation profile is the same before and after drawing,
even a wire rod before drawing can obtain inlproved delayed fracture resistance.
However, a wire rod before drawing has a low strength and does not cause a problem
in delayed fracture resistance. Therefore, in the present invention, delayed fracture
resistance is evaluated after drawing.
It should be noted that the same shall be applied to a case where a wire rod is
subjected to, for example, extruding or conforn~ingin stead of drawing.
[0013]
The present invention has been made based on the above-described findings,
and the summary thereof is as follows.
(1) That is, according to an aspect of the present invention, there is provided
a wire rod including, by mass%, 0.60% to 1.15% of C, 0.30% to 1.30% of Si, 0.25% to
0.90% of Mn, and a balance consisting of Fe and impurities, wherein: wlien a region
that is concentrically fonned from a surface to the inside of the wire rod and has a
cross-sectional ratio of 13% to 56% with respect to a cross-sectional area of a crosssection
of the wire rod is defined as a region I, a region that is cotice~itricallyf oniied
centering on a central axis of the wire rod and has a cross-sectional ratio of 3% to 11%
with respect to the cross-sectional area of the wire rod is defined as a region 111, and a
region that is formed between the region I and the region 111 is defined as a region 11,
the region I is a first negative segregation portion in wllich a C segregation degree wit11
respect to an average C concentration of the wire rod is 0.75 to 0.95, the region I1 is a
positive segregation portion in which the C segregation degree is 1.00 to 1.10, and the
region I11 is a second negative segregation portion in which the C segregation degree is
0.80 to 0.95; and the wire rod has a sandwich structure in which the first negative
segregation portion, the positive segregation portion, and the second negative
segregation portion are lanlinated in this order from the surface.
[0014]
(2) The wire rod according to (1) may further include, by mass%, one
selected from the gronp cot~sistingo f 0.40% or less of Cr, 0.40% or less of V, and
0.0030% or less of B.
[0015]
(3) According to another aspect of the present invention, there is provided a
steel wire ~vl~icish o btained by drawing the wire rod according to (1) or (2).
[0016]
(4) In the steel wire according to (3), wherein a tensile strength may be
higher than or equal to 2000 MPa.
[0017]
(5) According to still another aspect of the present invention, there is
provided a steel piece including, by mass%, 0.60% to 1.15% of C, 0.30% to 1.30% of
Si, 0.25% to 0.90% of Mn, and a balance consisting of Fe and impurities, wherein:
wlvhen a region that is concentrically formed from a surface to the inside of the steel
piece and has a cross-sectional ratio of 13% to 56% with respect to a cross-sectional
area of a cross-section of the steel piece is defined as a region I, a region that is
concentrically formed centering on a central axis of the steel piece and has a crosssectional
ratio of 3% to 11% with respect to the cross-sectional area of the steel piece
is defined as a region 111, and a region that is foniled between the region I and the
region I11 is defined as a region 11, the region I is a first negative segregation portion in
~vlvllicla~ C segregation degree with respect to an average C concentration of the steel
piece is 0.75 to 0.95, the region I1 is a positive segregation portion in which the C
segregation degree is 1.00 to 1.10, and the region 111 is a second negative segregation
portion in which the C segregation degree is 0.80 to 0.95; and the steel piece has a
sandwich structure in which the first negative segregation portion, the positive
segregation portion, and the second negative segregation portion are laminated in this
order from the surface.
[0018]
(6) The steel piece according to (5) may further include, by mass%, one
selected from the group consisting of 0.40% or less of Cr, 0.40% or less of V, and
0.0030% or less of B.
[Advantage of the Invention]
[a0191
When a higli carbon steel wire is manufactured using a method of the related
art, as described above, the stress acting on a center of a wire rod is changed during
drawing by several factors, tensile stress is applied to a central axis portion, and wire
breakage occurs due to a chevron crack, which may cause a large probletn in
production activity.
[0020]
011 the other hand, according to the above-described aspects of the present
invention, both regions of a surface portion and the vicinity of a center portion of a
wire rod are negative segregation regions. Therefore, the initiation of a chevron crack
can be stably suppressed, and a wire rod which is superior in drawability and delayed
fracture resistance after drawing can be obtained. Since this wire rod has high
drawability, production activity is stable, and a steel wire can be economically
produced.
In addition, according to the above-described aspects of the present invention,
the ductility of a surface is improved. As a result, a steel wire having improved
delayed fracture resistance, ~vl~icish m ore likely to occur in a higher-strength steel, can
be obtained.
In addition, according to the above-described aspects of the present invention,
a steel piece in which a negative segregatiou region is formed in both a center portion
and a surface portion can be obtained.
prief Description of the Drawing]
[0021]
FIG. 1A is a cross-sectional view illustrating a cross-section of a wire rod
~vl~icish d ivided based on a C segregation degree.
FIG 1 B is a diagram illustrating a C concentration of a region with diameter ak
including a center in a cross-section of a wire rod.
FIG. 2 is a diagram illustrating a sampling method during chemical analysis.
FIG. 3 is a diagratn illustrating C segregation distributions of a steel piece
according to an en~boditnenot f the present invention and a steel piece of the related art
in a diametrical direction.
pmbodirnents of the Invention]
[0022]
Hereinafter, a wire rod according to an embodiment of the present invention
(hereinafter also referred to as "the wire rod according to the e~libodiment"), a steel
wire according to an embodiment of the present invention (hereinafter also referred to
as "the steel wire according to the embodiment") obtained by drawing the wire rod
according to the enlbodiment, and a steel piece according to an en~bodirnenot f the
present invention (hereinafter also referred to as "the steel piece according to the
embodiment") will be described.
[0023]
FIG 1A is a cross-sectional view illustrating a cross-section of the wire rod
according to the embodiment which is divided based on a C segregation degree.
In the wire rod according to the embodiment, when a region that is
concentrically formed frorn a surface to the inside of the wire rod and has a crosssectional
ratio of 13% to 56% with respect to a cross-sectional area of a cross-section
of the wire rod is defined as a region I, a region that is concentrically formed centering
on a central axis of the wire rod and has a cross-sectional ratio of 3% to 11% with
respect to the cross-sectional area of the wire rod is defuled as a region 111, and a
region that is formed between the region I and the region I11 is defined as a region 11,
the region I is a first negative segregation portion in which a C segregation degree with
respect to a11 average C concentration of the wire rod is 0.75 to 0.95, the region I1 is a
positive segregation portion in which the C segregation degree is 1.00 to 1.10, and the
regioti.111 is a second negative segregation portion in which the C segregation degree is
0.80 to 0.95. In addition, the wire rod has a sandwich structure (laminate structure) in
which the first negative segregation portion (region I), the positive segregation portion
(region 11), and the second negative segregation portion (region 111) are laminated in
this order fro111 the surface.
The reason for limiting an area ratio and a C segregation degree of each
region in the cross-section of the wire rod according to the enibodi~nenwt ill be
described with reference to FIGS. 1A and 1B.
[0024]
(Region a-b and Region j-k in FIGS. 1A and 1B)
As illustrated in FIG lA, the region I is formed from a surface to the inside of
the wire rod (in a central axis direction of the wire rod) so as to be concentric to an
outer diameter of the wire rod.
The lower limit of the area ratio of the region I with respect to the area of the
cross-section of the wire rod is set as 13% which is a limit where there is effect of
improving delayed fracture resistance after drawing (hereinafter, in the embodiment,
all the area ratios refer to the area ratios of the respective regions to the cross-sectional
area of the cross-section of the wire rod).
Extreme softening adversely affects fatigue fractures after drawing.
Therefore, the upper limit of the area ratio of the region I is set as 56%.
[0025]
The lower limit of the C segregation degree (a io FIG. 1B) of the region I
indicating the negative segregation degree thereof is set as 0.75. This is because,
when the C segregation degree is lower than 0.75, there is an adverse effect, such as a
deterioration in fatigue strength, on other properties.
When the C segregation degree is higher than 0.95, an effect of improving the
ductility of the surface or an effect of improving delayed fracture resistance after
drawing cannot be obtained. Therefore, the upper limit of the C segregation degree of
the region I is 0.95.
[0026]
(Region b-e and Region g-j in FIG 1A (Region b-c-d-e and
Region g-h-i-j in FIG. 1B))
It is preferable that the lower limit of the area ratio of the region I1 is set as
33% from the viewpoint of securing a preferable strength in the assumption of being
used as a steel wire. In addition, an increase in the area ratio of the region I1 causes a
decrease in tlie area ratios of tlie regions I and 111, whicli may deteriorate drawability
and delayed fracture resistance after drawing. Therefore, it is preferable that the
upper lin~iot f tlie area ratio of the region I1 be set as 84%.
[0027]
The lower lin~iot f the C segregation degree (indicated by P in FIG. 1B) of the
region I1 is set as 1.00 from the viewpoint of securing a preferable strength in the
assumption of being used as a steel wire. On the other hand, the upper limit is set to
1.10 in order to suppress the formation of pro-entectoid cementite or the like and to
secure dra~vability.
[0028]
(Region e-f-g in FIGS. 1A and 1B)
The lower limit of the area ratio of the region I11 is set as 3% fro111 tlie
viewpoint of securing dra~vability.
Tlle upper limit of tlie area ratio of the region 111 is set as 11% from the
viewpoint of securing a preferable strength in tlie assumption of being used as a steel
wire.
[0029]
The lower limit of the C segregation degree of the region I11 indicating tlie
negative segregation degree thereof (indicated by y in FIG. 1B) is set as 0.80. This is
because, when a cast piece is reduced so as to form Inore negative segregation,
cracking occurs in a surface of the cast piece and in a cross-section thereof.
The upper limit of the C segregation degree of the region 111 is set as 0.95.
This is because, when the C segregation degree is higher than 0.95, drawability
deteriorates.
[0030]
The steel piece according to the embodiment has tile same sandwich structure
as that of the wire rod according to the embodiment, except that a cross-sectional shape
is a square shape or a rectangular shape. The reason for limititlg the area ratio and a
C segregation degree of each region is the same as that of tlie above-described wire rod.
By processing the steel piece according to the embodiment, the wire rod according to
the embodiment can be easily obtained.
In addition, the steel wire according to the enibodiment has also the same
sandwich structure as that of the wire rod according to the embodiment.
[003 11
Next, components will be described. In order to improve drawability and
delayed fracture resistance after drawing, it is important for the wire rod according to
the embodiment to include the above-described sandwich structure. However, when
considering the dra~vabilityt,h e delayed fracture structure after drawing, the strength in
the assumption of being used as a steel wire, and the like, it is important for the wire
rod according to tlie embodiment to furtlier include the following components. hi the
following description, "%" of each component represents "mass%".
Even after processes such as heating, rolling, and heating, chemical
components do not change. Therefore, it is only necessary that the following
chemical components are included in a steel piece. In addition, likewise, even after
drawing and the like, the chemical components do not change. Therefore, the steel
wire according to the embodiment also has the same chemical components as those of
tlie wire rod which is the material of the steel wire.
[0032]
C: 0.60% to 1.15%
C is a rnajor elelnent ~vhichc ontrols a strength of steel and is effective for
securing strength. In order to obtain a wire rod which is used for the above-described
high-strength steel wire, the lower litnit of the C content is set to 0.60%. When the C
coutent is less than 0.60%, sufficient strength may not be obtained. On the other hand,
when the C content is greater than 1.15%, in a cooling process of wire rod
manufacturing processes, it is difficult to prevent the formation of network proeotectoid
cementite it1 a surface portion or a center portion, which may significantly
deteriorate the drawability and the delayed fracture resistance. Therefore, the C
content is set to he 0.60% to 1.15%.
[0033]
Si: 0.30% to1.30%
Si is an element which is used as a deoxidizing agent. In addition, along
with an increase in the Si content, the strength is also increased by solute strengthening.
I11 particular, a direct effect of a Si increase on properties is to suppress a reduction of
the tensile strength after galvanizing in a hot dip galvanizing process.
When the Si content is less than 0.30%, a deoxidizing force is insufficient, the
surface quality of steel deteriorates. On the other hand, when the Si content is greater
than 1.30%, descaling performance deteriorates, which may deteriorate surface quality
and productivity. Therefore, the Si content is set to be 0.30% to 1.30%.
[0034]
Mn: 0.25% to 0.90%
Mn is an element wllich acts as a deoxidizing element, affects hardenability of
steel, and contributes an increase in strength. The lower limit of the Mn content is set
to 0.25% for the following reasons. When the Mn content is less than 0.25'36,
deoxidation is insufficient, the soundness of a steel surface deteriorates, and an effect
of inlproving strength is insufficient. On the other hand, when the Mn content is
greater than 0.90%, Mn concentrates in a center which is formed in the stage of a cast
piece. I11 a portion in which Mn concentrates, since transformation is delayed as
compared to the other portions, micromartensite is likely to be formed. If
micromartensite is formed, depending on the size, wire breakage occurs during
drawing, which significantly deteriorates productivity. Accordingly, the upper limit
of the Mn content is set to 0.90%.
[003 51
In order to further increase a strength and the like, the wire rod according to
the embodimeut nay furtiler il~cludea t least one of Cr, V, and B within the follolving
ranges. The wire rod according to the embodiment does not necessarily include these
elements. Therefore, it is not tiecessary that the lower limits of the contents of the
elements are particularly limited, and these lower limits are set to 0%.
[0036]
Cr: 0.40% or less
Cr is an element which is effective for increasing a strength of steel. In order
to stably obtain the effect of improving a strength, it is preferable that the Cr content is
greater than or equal to 0.10%. On the other hand, when the Cr content is greater
than 0.40%, ductility deteriorates. Therefore, when the wire rod according to the
embodiment includes Cr, the upper limit of the Cr content is set to 0.40%.
[0037]
V: 0.40% or less
V is an element which is effective for illcreasing a strength of steel. In order
to stably obtain the effect of improving a strength, it is preferable that the V content is
greater than or equal to 0.03%. On the other hand, when the V conterit is greater than
0.40%, ductility deteriorates. Therefore, lvhen the wire rod according to the
embodimet~itn cludes V, the upper limit of tile V content is set to 0.40%.
[0038]
B: 0.0030% (30 ppm) or less
B is an element xvl~ichis effective at improving hardenability and suppressing
the formation of pro-eotecitoid ferrite. In order to stably obtain such effects, it is
preferable that the B content is greater than or equal to 0.0005%. On the other hand,
B is an element which forms a nitride, and when the B content is greater than 0.0030%,
an effect of ilnproving hardenability is saturated. Further, the nitride is precipitated,
which deteriorates drawability. Therefore, xv11en the wire rod according to the
embodiment includes B, the upper limit of the B content is set to 0.0030%.
[0039]
The wire rod according to the embodiment may further include elements other
than the above-described elements as impurities within a range not impairing the
properties. The impurities include raw nlaterials suc11 as ores and scraps and
materials which are incorporated from a manufacturing environment.
[0040]
Next, a preferable method of manufacturing the wire rod according to the
etnboditnent will be described.
Tlie manufacturing method according to the elnbodilnet~its merely an
exatnple and is not limited thereto. That is, if the above-described C segregation
profile is obtained using another method instead of the following manufacturing
method, the effects of the wire rod according to the embodilnent can be obtained.
[0041]
When a steel piece is obtained from molten steel having predetermined
cl~etnicalc omponents by continuous casting, it is preferable that the tnanufacturing
method include the following processes (a) to (c) in order to easily obtain the abovedescribed
C segregation profile.
(a) Molten Steel Temperatore Controlling Process
In a moltetl steel temperature controlling process, it is preferable that a molten
steel temperature in a tundish immediately before pouring the molten steel into a
continuous casting machine is controlled such that a difference AT (degree of
superheat) between the molten steel temperature and TLL (liquidus temperature) is
lower than or equal to 25OC. When AT is lower than or equal to a structure
during solidification is likely to be equiaxial, thereby easily obtaining a desired C
segregation profile. When AT is lower than or equal to 5OC, the molten steel
temperatore approaches a solidification temperature, and tlie viscosity of the molten
steel increases. As a result, the molten steel is in a sherbet-like state, and the surface
quality of a cast piece deteriorates. Therefore, it is preferable that the lower limit of
AT is set to 5'C.
(b) Electromagnetic Stirring (EMS) Process
In an electron~agnetics tirring process, it is preferable that a magnetic field be
applied to molten steel inside a casting mold of a continuous casting machine to stir
(electromagnetically stir) the molten steel. By performing electromagnetic stirring, a
negative segregation region can be imparted to a surface, thereby easily obtaining a
desired C segregation profile.
(c) Light Reduction Process
I11 a light reduction process, a steel piece during solidification is reduced using
rolls of a continuous casting machine. By performing the light reduction, a
concentration of carbon in a center segregation portion can be reduced, thereby easily
obtaining a desired C segregation profile.
A steel piece which is obtained by performing steelmaking processes as
described above is appropriately subjected to heating, rolling, coiling, a heat treatment,
and the like according to desired mechanical properties. As a result, a wire rod
having a desired C segregation profile and desired mechanical properties can be
obtained.
[0043]
The wire rod which is obtained as above is drawn rising a well-known method,
thereby obtaining a steel wire.
[Example 11
[0044]
Examples of the present invention will be described.
Each of Steel grades A to J having chemical cotnponents shown in Table 1
was melted, followed by continuous casting under manufacturing conditions (during
steel making) shown in Table 2, thereby obtaining a 500 mm x 300 nlm cast piece
(bloom). This cast piece was heated to 1250°C for 45 minutes, followed by blooming,
thereby obtaining a 122 Inn1 x 122 tmn steel piece (billet). This steel piece was
heated under manufacturing conditions (after steel making) shown in Table 2, was
rolled according to an ordinaly method, and was coiled under conditions shown in
Table 2, thereby obtaining wire rods having a diameter of 12 mm and a diameter of 5.5
mm. The wire rods having a diameter of 12 mm and a diameter of 5.5 nun were
subjected to a heat treatment under conditions shown in Table 2.
111 Table 1, "-" represents being less than or equal to a measurement limit, and
the balance consists of Fe and impurities.
[0045]
[0047]
In the stage of a steel piece, as illustrated in FIG 2, a small block having a
thickness of 2 mm, a width of 5 mm, and a length of 10 nnn was collected from a site
of the steel piece including a fill1 section in a thickness direction thereof. The
chemical analysis of the collected small block was performed to obtain a C segregation
profile. Regarding a steel piece of Test No. 3 according to Example and a steel piece
of Test No. 14 according to Comparative Example, the obtained C segregation profiles
were illustrated in FIG. 3.
[0048]
As can be seen from FIG. 3, in Test No. 3, a sandwich structure was obtained
in which a negative segregation profile was shown in a surface portion and a center
portion, and a positive segregation profile was shown in an intermediate portion
between the surface and the center.
On the other hand, Test No. 14 showed a profile in which a center portion had
a clear positive segregation portion, and a negative segregation portion of a surface
portion had a low segregation degree.
[0049]
In addition, regarding wire rods having a diameter 12 mm obtained from the
steel pieces, C segregation profiles were also obtained. As a result, in the C
segregation profiles of the wire rods, it was confirmed that C segregation profiles had
the same sandwich structure as that in the stage of the steel pieces. Table 4 shows
area ratios and C segregation degrees of regions I to I11 obtained from the C
segregation profiles. The C segregation profiles of the wire rods having a diameter of
12 mm were obtained according to a method in which, using a vertical cross-section in
a longitudinal direction as a target surface, EPMA line analysis was performed in a
range of a diameter froni a surface to an opposite surface in a direction crossing a
center segregation portion at a right angle.
[OOSO]

Regarding the wire rods of Test No 1 to Test No. 14 obtained as above,
drawability was evaluated.
Table 3 shows wire drawing die schedules which were used to evaluate
drawability. All of the die approach angles were set as 40°, and a wire rod having a
diameter of 5.5 mm was drawn to forcedly cause Cr~ppyb reakage. A drawing strain
obtained from a die diameter itnmediate before the breakage was occurred as a
drawable limit strain. Usiug this value, drawability was evaluated. The results are
shown in Table 4. When the drawing strain is greater thau or equal to 0.88, that is,
~vhen a wire rod was drawn without breakage in three or more passes, the drawability
of the wire rod was evaluated to be superior.
[0053]

Further, the above-described wire rods having a diameter of 12 mm were
drawn using a die having an approach angle of 10" to obtain steel wires having a
diameter of 5 mm. Regarding the obtained steel wires, delayed fracture resistance
was evaluated.
A delayed fracture test was performed according to a delayed fracture test
method called a FIP test \vhicl~w as defined in a construction guideline of a PC
strncture using a high-strength PC steel (June, 2011, Japan Prestressed Concrete
Institute). In this test, a solution of 20% ammonium thiocyanate at 50°C was used, a
loading condition was a breaking strength of 70%, and an elapsed time until fracture
was occurred.
The fracture time was evalnated based on a delayed fractnre resistance index
obtained using the following method. It should be noted that, as this value is higher,
the drawability is further improved as co~nparedto a method of the related art. In
Examples, when the delayed fracture resistance index was greater than or equal to 1.5,
the delayed fracture resistance was evaluated to be superior. Table 4 shows test
results.
[0054]

As shown in the following expression (I), by using a fracture time of the
related art as a reference for comparison, a fracture time of each Test No. was divided
by a fractnre time of Test No. 14 to obtain a value as an index. This index was nondimensionalized
for the evaluation.
[0055]
Delayed Fracture Resistance Index=(Fractore Time of Each Test
No.)l(Fractnre Time of Test No. 14) . . . (1)
[0056]
In addition, a tensile strength of a steel wire was obtained. Atensile test was
performed in conformity to conditions of JIS Z 2241. The results are shown in Table
4. In the present invention, when the tensile strength was greater than or equal to
2000 MPa, the tensile strength was evaluated to be sufficient.
[0057]
As can be seen from Tables 1 to 4, the steel pieces, the wire rods, and the steel
wires which were obtained using the method according to the present invention had a
sandwich structure in which a first negative segregation portion, a positive segregation
portion, and a second negative segregation portion were laminated in this order &om
the surface, and segregation degrees thereof were desirable. Therefore, since the
initiation of a chevron crack was stably suppressed, the drawability of the wire rods
was improved. Further, in the steel wires, the strength was high, and the delayed
fiacture resistance was superior.
[0058]
On the other hand, in Test No. 6 to Test No. 14 in which a desired C
segregation profile was not obtained, the effects of improving drawability andlor
delayed fracture resistance were not obtained.
[Industrial Applicability]
[0059]
According to the present invention, both regions of a surface portion and the
vicinity of a center portion of steel are negative segregation regions. Therefore, since
the initiation of a chevron crack can be stably suppressed, a wire rod which is superior
in drawability and delayed fracture resistance after drawing can be obtained. Since
this wire rod has high drawability, production activity is stable, and the wire rod can be
econon~icallyp roduced.
[0060]
In addition, according to the present invention, the ductility of a surface is
improved. As a result, a high-strength steel wire having improved delayed fracture
resistance, which is more likely to occur in a higher-strength steel, can be obtained.
In addition, a steel piece in which a negative segregation region is formed both in a
surface portion and the vicinity of a center portion of steel can be obtained.
[Designation of Document] CLAIMS
[Claim 11
A wire rod comprising, by mass%,
0.60% to 1.15% of C,
0.30% to 1.30% of Si,
0.25% to 0.90% of Mn, and
a balance consisting of Fe and impurities,
wherein:
\v11en a region that is concentrically formed from a surface to the inside of the
wire rod and has a cross-sectional ratio of 13% to 56% with respect to a cross-sectional
area of a cross-section of the wire rod is defined as a region I,
a region that is concentrically formed centering on a central axis of the wire
rod and has a crossaectional ratio of 3% to 11% with respect to the cross-sectional
area of the wire rod is defined as a region 111, and
a region that is formed between the region I and the region I11 is defined as a
region 11,
the region I is a first negative segregation portion in which a C segregation
degree with respect to an average C concentration of the wire rod is 0.75 to 0.95,
the region I1 is a positive segregation portion in which the C segregation
degree is 1 .OO to 1.10, and
the region 111 is a second negative segregation portion in which the C
segregation degree is 0.80 to 0.95; and
the wire rod has a sandwich structure in which the first negative segregation
portion, the positive segregation portion, and the second negative segregation portion
are laminated in this order from the surface.
[Claim 21
The wire rod according to Claim 1, further cotnprising, by mass%,
one selected from the group cotisisting of
0.40% or less of Cr,
0.40% or less of V, and
0.0030% or less of B.
[Claim 31
A steel wire \vIvhich is obtained by drawing the wire rod according to Claim 1
or 2.
[Claim 41
The steel wire according to Claim 3,
~vhereina tensile strength is higher than or equal to 2000 m a .
[Claim 51
Asteel piece comprising, by mass%,
0.60% to 1.15% of C,
0.30% to 1.30% of Si,
0.25% to 0.90% of Mn, and
a balance consisting of Fe and impurities,
wherein:
when a region that is co~lcentricallyf ormed from a surface to the inside of the
steel piece and has a cross-sectional ratio of 13% to 56% with respect to a crosssectional
area of a cross-section of the steel piece is defined as a region I,
a region that is concentrically fornled centering on a central axis of the steel
piece and has a cross-sectional ratio of 3% to 11% with respect to the cross-sectional
area of the steel piece is defined as a region 111, and
tlegrec with rcspcct to o average I: co~lcer~tratiooflt~ll e steel piece js 0.)5 to 0.95, b the region II i a positive segregation porlion in which the ($ sekI regation
degree is 1.00 to 1-10, and 1 1
I I ! : ,
the regio~lI1 1 1s a seco~~l~dc gativcs egregation pottion in w"ich/the C
I
region 11,
the region I is
segregation degree is ri. SO to 0.95; and I ji the steel pieco has a satld~vichs tmcture in whicll the first n gative segregation portion, the positive seI reg ation portion, and thc second negative P
are lamitlated it1 his o de~fi o~nth e sui-ace. 'i ' , ' I ii
I ' 1 ~
a first ~lcgatives egregation polti011 in \vl~,ic!li ~C segt-egation
! !
[Claim 61 i I I /I The steel piec accordi~lgto Claim 5, fi~rtherc onlprising, by inass%, .j
I :
one selected *om tl~cgr oup consisting of 1 ' / , '
0.40% or less Lf Cr,
0.40% or less of \I, and
I