U.S. patent application number 11/754537 was filed with the patent office on 2007-12-06 for wire rod excellent in wire-drawing workability and method for producing same.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Takuya Kochi, Takeshi Kuroda, Tomotada Maruo, Shogo Murakami, Hidenori Sakai, Hiroshi Yaguchi.
Application Number | 20070277913 11/754537 |
Document ID | / |
Family ID | 38294103 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070277913 |
Kind Code |
A1 |
Kochi; Takuya ; et
al. |
December 6, 2007 |
WIRE ROD EXCELLENT IN WIRE-DRAWING WORKABILITY AND METHOD FOR
PRODUCING SAME
Abstract
Disclosed are a wire rod and a method therefor. The wire rod is
excellent in wire-drawing workability, insusceptible to wire break
in spite of an increase in wire-drawing rate, and reduction of
area, and capable of extending a die life by suppressing die wear.
The wire rod is made of steel containing C: 0.6 to 1.1%, Si: 0.1 to
2.0%, Mn: 0.1 to 1%, P: not more than 0.20%, S: not more than
0.20%, N: not more than 0.006%, Al: not more than 0.03%, and O: not
more than 0.003%, the balance including Fe, and unavoidable
impurities. Further, the wire rod comprises a pearlite structure
wherein an area ratio of a second-phase ferrite is not more than
11.0%, and a pearlite lamellar spacing is not less than 120
.mu.m.
Inventors: |
Kochi; Takuya; (Kobe-shi,
JP) ; Kuroda; Takeshi; (Kakogawa-shi, JP) ;
Sakai; Hidenori; (Kakogawa-shi, JP) ; Maruo;
Tomotada; (Kobe-shi, JP) ; Murakami; Shogo;
(Kobe-shi, JP) ; Yaguchi; Hiroshi; (Kobe-shi,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
651-8585
|
Family ID: |
38294103 |
Appl. No.: |
11/754537 |
Filed: |
May 29, 2007 |
Current U.S.
Class: |
148/598 ;
148/320 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/06 20130101; C22C 38/02 20130101; C21D 8/065 20130101 |
Class at
Publication: |
148/598 ;
148/320 |
International
Class: |
C22C 38/00 20060101
C22C038/00; C21D 9/52 20060101 C21D009/52 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2006 |
JP |
2006-157622 |
Claims
1. A wire rod made of steel comprising: C: 0.6 to 1.1% (mass %,
applicable to all components referred to hereunder); Si: 0.1 to
2.0%; Mn: 0.1 to 1%; P: not more than 0.020% (0% exclusive); S: not
more than 0.020% (0% exclusive); N: not more than 0.006% (0%
exclusive); Al: not more than 0.03% (0% exclusive); and O: not more
than 0.003% (0% exclusive), the balance being Fe and unavoidable
impurities, said wire rod comprising a pearlite structure wherein
an area ratio of a second-phase ferrite is not more than 11.0%, and
a pearlite lamellar spacing is not less than 120 .mu.m.
2. The wire rod according to claim 1, further comprising not more
than 1.5% Cr (0% exclusive).
3. The wire rod according to claim 1, further comprising not more
than 1% Cu (0% exclusive), and/or not more than 1% Ni (0%
exclusive).
4. The wire rod according to claim 1, further comprising at least
one element selected from the group consisting of not more than
0.30% (0% exclusive) V, not more than 0.1% (0% exclusive) Ti, not
more than 0.10% (0% exclusive) Nb, not more than 0.5% (0%
exclusive) Mo, and not more than 0.1% (0% exclusive) Zr.
5. The wire rod according to claim 1, further comprising at least
one element selected from the group consisting of not more than 5
ppm (0 ppm exclusive) Mg, not more than 5 ppm (0 ppm exclusive) Ca,
and not more than 1.5 ppm (0 ppm exclusive) REM.
6. The wire rod according to claim 1, further comprising not more
than 15 ppm B.
7. A method for producing a wire rod, comprising the steps of:
heating a steel product meeting requirements for chemical
components, specified in any of claims 1 to 6, to a temperature in
a range of 900 to 1250.degree. C.; hot rolling the steel product at
a temperature not lower than 780.degree. C., and finish-rolling the
same at a temperature not higher than 1100.degree. C. to be thereby
formed into a wire rod; water-cooling the wire rod to a temperature
range of 750 to 950.degree. C. before coiling the same up to be
placed on conveying equipment; cooling the wire rod at an average
cooling rate of not less than 20.degree. C./sec within 20 sec from
coiling of the wire rod to thereby drop temperature of the wire rod
to a minimum value point (T1) in a temperature range of 550 to
630.degree. C.; and subsequently heating the wire rod to thereby
raise the temperature of the wire rod up to a maximum value point
(T2) in a temperature range of 580 to 720.degree. C., higher in
value than the minimum value point (T1), within 50 sec from the
coiling of the wire rod.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a wire rod excellent in
wire-drawing workability, out of which a drawn wire product, such
as a steel cord, beading wire, PC steel wire, spring steel, can be
efficiently produced with high productivity, and a method for
producing the same.
[0002] In most cases of producing a drawn wire product, such as a
steel cord, a drawing process is applied to a wire rod serving as
material for the wire product in order to make adjustment in size
and quality (physical properties), and therefore, it is extremely
useful from the viewpoint of enhancement in productivity, and so
forth to improve wire-drawing workability of the wire rod. In this
connection, if improvement on wire drawing workability is
implemented, this will not only improve productivity, due to an
increase in drawing rate and a decrease in the number of drawing
passes, but also provide many benefits such as reduction in wear
and tear of draw dies.
[0003] Accordingly, in a pertinent technical field, researches on
enhancement in the wire drawing workability of the wire rod have
been under way. For example, in Japanese Unexamined Patent
Application Publication (JP-A) No. 91912/2004, there has been
disclosed a technology for improving the wire-drawing workability
by focusing attention on size of a pearlite block, a quantity of
pro-eutectoid cementite formed, an average thickness of cementite,
Cr concentration in cementite, and so forth, and by optimizing
them.
[0004] Further, in JP-A-295930/1996, there has been disclosed that
the wire-drawing workability is improved by controlling an area
ratio of upper bainite formation, and growth size of intergranular
bainite. In JP-A-130258/1987, there has been disclosed a technology
for improving resistance to wire break, and a die life by
controlling an amount of total oxygen, and nonviscous inclusion
composition, in steel.
[0005] However, a rise in wire-drawing rate, and an increase in
reduction of area per one pass cause degradation in ductility of a
drawn wire product, and deterioration in die life. Accordingly, in
order to further enhance productivity in the pertinent technical
field, there is still a demand for a wire rod excellent in the
wire-drawing workability, capable of achieving excellent resistance
to wire-break, and enhancement of the die life even in harsh
wire-drawing conditions of high wire-drawing rate, and large
reduction of area.
SUMMARY OF THE INVENTION
[0006] Under circumstances described as above, the invention has
been developed, and it is therefore an object of the invention to
provide a wire rod excellent in wire-drawing workability,
insusceptible to wire break in spite of an increase in wire-drawing
rate, and reduction of area, and capable of extending a die life by
suppressing die wear, and a method for producing the same.
[0007] According to one aspect of the invention, a wire rod that
has succeeded in achieving the object is made of steel containing
C: 0.6 to 1.1% (mass %, applicable to all components referred to
hereunder), Si: 0.1 to 2.0%, Mn: 0.1 to 1%, P: not more than 0.020%
(0% exclusive), S: not more than 0.020% (0% exclusive), N: not more
than 0.006% (0% exclusive), Al: not more than 0.03% (0% exclusive),
and O: not more than 0.003% (0% exclusive), the balance including
Fe, and unavoidable impurities, and further, the wire rod comprises
a pearlite structure wherein an area ratio of a second-phase
ferrite is not more than 11.0%, and a pearlite lamellar spacing is
not less than 120 .mu.m.
[0008] The wire rod according to the aspect of the invention may
contain not more than 1.5% Cr for higher strength, and may further
contain not more than 1% Cu, and/or not more than 1% Ni, for
suppression of decarburization.
[0009] The wire rod according to the aspect of the invention
preferably further contains at least one element selected from the
group consisting of not more than 0.30% V, not more than 0.1% Ti,
not more than 0.10% Nb, not more than 0.5% Mo, and not more than
0.1% Zr from the viewpoint of refinement of the metal
microstructure, and suppression of transformation into ferrite.
[0010] The wire rod according to the aspect of the invention may
further contain at least one element selected from the group
consisting of not more than 5 ppm Mg, not more than 5 ppm Ca, and
not more than 1.5 ppm REM in order to soften oxides and enhance the
wire drawing workability. Still further, the wire rod according to
the invention may contain not more than 15 ppm B in order to
enhance hardenability.
[0011] In accordance with another aspect of the invention, there is
provided a method for producing a wire rod, comprising the steps of
heating a steel product meeting requirements for chemical
components, as described hereinbefore, to a temperature in a range
of 900 to 1250.degree. C., hot rolling the steel product at a
temperature not lower than 780.degree. C., and finish-rolling the
same at a temperature not higher than 1100.degree. C. to be thereby
formed into a wire rod, water-cooling the wire rod to a temperature
range of 750 to 950.degree. C. before coiling the same up to be
placed on conveying equipment, cooling the wire rod at an average
cooling rate of not less than 20.degree. C./sec within 20 sec from
the coiling of the wire rod to thereby drop temperature of the wire
rod to a minimum value point (T1) in a temperature range of 550 to
630.degree. C., and subsequently heating the wire rod to thereby
raise the temperature of the wire rod up to a maximum value point
(T2) in a temperature range of 580 to 720.degree. C., higher in
temperature than T1, within 50 sec from the coiling of the wire
rod.
[0012] The inventor, et al. have found out to their surprise that a
wire rod excellent in wire-drawing workability, insusceptible to
wire break, and capable of extending a die life by suppressing die
wear, can be obtained by specifying the respective contents of C,
Si, Mn, P, S, N, Al, and O while controlling the area ratio of the
second-phase ferrite and the pearlite lamellar spacing. With the
use of the wire rod described, it will become possible to increase
a wire-drawing rate, and reduction of area, thereby enabling
productivity to be further enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Embodiments of the present invention will be described in
detail based on the following figures, wherein:
[0014] FIG. 1 is a SEM photograph of a location at D/4 on the
cross-sectional face of a wire rod according to an embodiment of
the invention (D: diameter of the wire rod); (the SEM photograph
used for explaining about the structure of a second-phase
ferrite),
[0015] FIG. 2 is another SEM photograph of a location at D/4on the
cross-sectional face of the wire rod (D: diameter of the wire rod);
(the SEM photograph used for explaining about a method of finding a
pearlite lamellar spacing), and
[0016] FIG. 3 is a schematic representation showing a treatment
pattern adopted in a method for producing the wire rod according to
the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] A wire rod according to the invention has features lying in
requirements for chemical components thereof, and requirements for
a metal microstructure thereof (an area ratio of a second-phase
ferrite, and a pearlite lamellar spacing). Accordingly, the
chemical components of the wire rod (a steel product) are first
described hereinafter.
C: 0.6 to 1.1% (mass %, applicable to all components referred to
hereunder)
[0018] Carbon is an element intensely affecting strength of the
wire rod, and in order to secure strength required of a steel cord,
beading wire, PC steel wire, and so forth, as targets for which the
invention has been developed, addition of not less than 0.6% C is
required. On the other hand, if carbon content is excessive, there
occurs degradation in ductility, so that an upper limit of the
carbon content is set to 1.1%. The carbon content is preferably in
a range of 0.8 to 1.0%.
Si: 0.1 to 2.0%
[0019] For the purpose of deoxidation, in particular, Si is added
to a wire rod to be subjected to intense drawing, and addition of
not less than 0.1% Si is required. Further, because Si also
contributes to enhancement in strength of the wire rod due to solid
solution hardening, an addition amount thereof is increased as
necessary. However, an excessive increase in the strength due to
excessive addition of Si will cause deterioration in wire-drawing
workability. Furthermore, the excessive addition of Si will cause
promotion of decarburization, to which attention should be given.
For those reasons, with the invention, an upper limit of silicon
content is set to 2.0% to prevent deterioration in wire-drawing
workability, and promotion of decarburization. The silicon content
is preferably in a range of 0.15 to 1.8%.
Mn: 0.1 to 1%
[0020] Addition of not less than 0.1% Mn is required for the
purpose of deoxidization, and locking a deleterious element S in
the form of MnS to thereby render S harmless. Further, Mn also acts
so as to stabilize carbide in steel. However, because excessive Mn
content will cause occurrence of segregation, and supercooled
structures, thereby causing deterioration in wire drawing
workability, an upper limit of Mn content is set to 1%. The Mn
content is more preferably in a range of 0.15 to 0.9%.
P: not more than 0.020% (0% exclusive)
[0021] P is an element deleterious to wire drawing workability, in
particular, and because excessive P content causes degradation in
tenacity and ductility of a wire rod, an upper limit of P content
is set to 0.020%. The P content is more preferably not more than
0.015%, and further preferably, not more than 0.010%.
S: not more than 0.020% (0% exclusive)
[0022] S as well is an element deleterious to wire drawing
workability, in particular. If Mn is contained, S can be locked in
the form of MnS, as described above, however, excessive S content
causes an increase in amount as well as size of MnS, thereby
resulting in degradation of ductility, an upper limit of S content
is set to 0.020%. The S content is more preferably not more than
0.015%, further preferably, not more than 0.010%.
N: not more than 0.006% (0% exclusive)
[0023] N is an element contributing to an increase in strength due
to age hardening. A preferable lower limit of N content is 0.001%.
However, because the N content causes degradation in ductility, an
upper limit thereof is set to 0.006%. The upper limit is preferably
not more than 0.004%, more preferably, not more than 0.003%.
Al: not more than 0.03% (0% exclusive)
[0024] Al is an element effective as a deoxidizer, and further, is
combined with N to form AlN, which contributes to refinement of a
metal microstructure. A preferable lower limit of Al content is
0.0003%. However, if the Al content is excessive, this will cause
coarse oxides to be formed, thereby resulting in deterioration of
wire drawing workability, and an upper limit thereof is therefore
set to 0.03%. The upper limit thereof is preferably not more than
0.01%, more preferably not more than 0.005%.
O: not more than 0.003% (0% exclusive)
[0025] If oxygen content in steel is high, the coarse oxides are
prone to be easily formed, thereby resulting in deterioration of
wire drawing workability, and an upper limit of the oxygen content
is therefore set to 0.003%. The upper limit thereof is preferably
not more than 0.002%, more preferably not more than 0.0015%.
[0026] With the wire rod according to the invention, the chemical
components described as above represent basic components, and the
balance includes in effect Fe, and unavoidable impurities, however,
the wire rod may contain the following elements if needs be.
Cr: not more than 1.5%
[0027] Cr is an element effective for rendering the wire rod higher
in strength, and a preferable lower limit of Cr content is 0.01%.
However, since excessive addition of Cr causes supercooled
structures prone to be formed to thereby cause deterioration in
wire drawing workability, an upper limit of the Cr content is set
to 1.5%. The upper limit thereof is preferably not more than
1.0%.
Cu: not more than 1%
[0028] Cu is an element acting so as to enhance corrosion
resistance besides acting so as to suppress decarburization in a
surface layer, and therefore, Cu can be added as necessary. A
preferable lower limit of Cu content is 0.01%. However, because
excessive addition of Cu will not only render the wire rod
susceptible to cracking upon hot working, but also adversely affect
wire drawing workability due to formation of supercooled
structures, an upper limit of the Cu content is set to 1%. The
upper limit thereof is preferably not more than 0.8%.
Ni: not more than 1%
[0029] Ni is an element effective for suppressing decarburization
in the surface layer, and enhancement in corrosion resistance, as
with the case of Cu, and therefore, Ni can be added as necessary. A
preferable lower limit of Ni content is 0.01%. However, because
excessive addition of Ni will cause deterioration in the wire
drawing workability due to formation of supercooled structures, an
upper limit of the Ni content is set to 1%. The upper limit thereof
is preferably not more than 0.8%.
V: not more than 0.30%
[0030] V is an element contributing to refinement of the metal
microstructure by forming carbide in carbon steel. Further, because
V in solid solution state will enhance hardenability, and suppress
transformation into ferrite, V can be added as necessary. A
preferable lower limit of V content is 0.0010%. However, because
excessive addition of V will cause deterioration in the wire
drawing workability due to formation of supercooled structures, an
upper limit of the V content is set to 0.3%. The upper limit
thereof is preferably not more than 0.25%.
Ti: not more than 0.1%
[0031] Ti is an element contributing to the refinement of the metal
microstructure, and the suppression of transformation into ferrite
as with the case of V, and Ti can therefore be added as necessary.
A preferable lower limit of Ti content is 0.0010%. However, because
excessive addition of Ti will cause deterioration in wire drawing
workability, an upper limit of the Ti content is set to 0.1%. The
upper limit thereof is preferably not more than 0.08%.
Nb: not more than 0.10%
[0032] Nb is an element contributing to the refinement of the metal
microstructure, and the suppression of transformation into ferrite
as with the case of V, and Nb can therefore be added as necessary.
A preferable lower limit of Nb content is 0.020%. However, because
excessive addition of Nb will cause deterioration in wire drawing
workability, an upper limit of the Nb content is set to 0.10%. The
upper limit thereof is preferably not more than 0.08%.
Mo: not more than 0.5%
[0033] Mo is an element contributing to the refinement of the metal
microstructure, and the suppression of transformation into ferrite
as with the case of V, and Mo can therefore be added as necessary.
A preferable lower limit of Mo content is 0.05%. However, because
excessive addition of Mo will cause deterioration in wire drawing
workability, an upper limit of the Mo content is set to 0.5%. The
upper limit thereof is preferably not more than 0.3%.
Zr: not more than 0.1%
[0034] Zr is an element contributing to the refinement of the metal
microstructure, and the suppression of transformation into ferrite
as with the case of V, and Zr can therefore be added as necessary.
A preferable lower limit of Zr content is 0.010%. However, because
excessive addition of Zr will cause deterioration in wire drawing
workability, an upper limit of the Zr content is set to 0.1%. The
upper limit thereof is preferably not more than 0.05%.
Mg: not more than 5 ppm
[0035] Mg is an element acting so as to soften oxides to thereby
enhance wire drawing workability, and Mg can therefore be added as
necessary. A preferable lower limit of Mg content is 0.1 ppm.
However, because excessive addition of Mg will cause oxides thereof
to undergo a change in quality to thereby rather deteriorate wire
drawing workability, an upper limit of the Mg content is set to 5
ppm. The upper limit thereof is preferably not more than 2 ppm.
Ca: not more than 5 ppm
[0036] Ca is an element acting so as to soften oxides as with the
case of Mg, and Ca can therefore be added as necessary. A
preferable lower limit of Ca content is 0.3 ppm. However, because
excessive addition of Ca will cause deterioration in wire drawing
workability, an upper limit of the Ca content is set to 5 ppm. The
upper limit thereof is preferably not more than 2 ppm.
REM: not more than 1.5 ppm
[0037] REM acts so as to soften oxides as with the case of Mg, and
REM can therefore be added as necessary. A preferable lower limit
of REM content is 0.1 ppm. However, because excessive addition of
REM will cause deterioration in wire drawing workability, an upper
limit of the REM content is set to 1.5 ppm. The upper limit thereof
is preferably not more than 0.5 ppm.
B: not more than 15 ppm
[0038] B is an element capable of enhancing hardenability, and
addition of B enables the transformation into ferrite to be
suppressed. A preferable lower limit of B content is 3 ppm.
However, because excessive addition of B causes supercooled
structures prone to be easily formed, thereby adversely affecting
wire drawing workability, an upper limit of the B content is set to
15 ppm. The upper limit thereof is preferably not more than 12
ppm.
[0039] Next, the metal microstructure of the wire rod according to
the invention is described hereinafter. The wire rod according to
the invention has a feature in that the area ratio of the
second-phase ferrite is not more than 11.0%. Herein, "the
second-phase ferrite" according to the invention refers to ferrite
in a region without pearlite (lamellar structure of ferrite and
cementite) formed therein, as indicated by respective arrows in
FIG. 1 showing an SEM photograph of a cross-sectional face of the
wire rod. Further, because there are times when it is difficult to
distinguish the second-phase ferrite from pearlite, "the
second-phase ferrite" according to the invention is more
specifically defined as "BCC--Fe crystal grains in a region
surrounded by a boundary differing in misorientation angle by not
less than 10 degrees from the periphery of the region, an area
ratio of cementite present in the respective BCC--Fe crystal grains
being not more than 6%".
[0040] "The area ratio of the second-phase ferrite", according to
the invention, refers to an area ratio (%) of the second-phase
ferrite to an observed visual field of the cross-sectional face of
the wire rod, magnified 500 to 1500 times by the scanning electron
microscope (SEM), that is, (an area of the second-phase ferrite
within the observed visual field/an area of the observed visual
field in whole).times.100. In this case, the area of the
second-phase ferrite can be found by use of image analysis
software, for example, {Image-Pro (Ver 4.0)} developed by Media
Cybernetics. Further, since there occurs variation in the area
ratio of the second-phase ferrite by the visual field as observed,
a mean value of several values thereof, found by observing not less
than eight visual fields selected at random, is adopted as a value
of the area ratio of the second-phase ferrite, according to the
invention.
[0041] The inventor, et al. have found out that a wire rod
excellent in resistance to wire-break can be obtained by
controlling the area ratio of the second-phase ferrite of the wire
rod to not more than 11%, preferably 10.0%, and more preferably not
more than 9%. A mechanism causing the above is not clearly known,
however, the mechanism can be presumed to be as follows. The
invention, however, is not to be limited in scope to the mechanism
presumed as described hereunder.
[0042] With a carbon steel wire rod to be subjected to drawing,
such as the wire rod according to the invention, the primary
constituent of the metal microstructure thereof is pearlite,
however, in general, there also exists the region of the
second-phase ferrite, without pearlite formed therein. It is
presumed that strain concentration occurs to the second-phase
ferrite lower in strength than pearlite when drawing is applied, so
that voids are prone to occur thereto. The voids each can become a
starting point of wire break. Accordingly, it is reasoned that
resistance to wire break can be enhanced by decreasing the
second-phase ferrite that is low in strength and is susceptible to
the strain concentration.
[0043] Further, the wire rod according to the invention has a
feature in that the same comprises a pearlite structure wherein the
pearlite lamellar spacing is not less than 120 .mu.m, preferably
not less than 140 .mu.m, and more preferably not less than 170
.mu.m. The wire rod according to the invention can at times include
bainite, and/or martensite besides the second-phase ferrite,
however, pearlite is the primary constituent of the metal
microstructure thereof. In the case of bainite, and/or martensite
being in existence, a ratio of a total area of those
microconstituents is preferably not more than 5%, more preferably
not more than 2%, and still more preferably, bainite, and
martensite do not in effect exist.
[0044] With the invention, "the pearlite lamellar spacing" refers
to a thickness of a lamellar layer in pearlite, composed of a pair
of a ferrite layer and a cementite layer, in pearlite. However,
since there occurs variation in the pearlite lamellar spacing by
the position for observation of the metal microstructure, what has
been found in the following manner is defined as a value of "the
pearlite lamellar spacing" according to the invention.
[0045] First, not less than six photographs of the cross-sectional
face of the wire rod, as observed and magnified 3000 to 10,000
times by the SEM, are taken. As shown in FIG. 2, in a colony (a
region where the ferrite layers and the cementite layers, in
pearlite, are aligned in same direction) in the respective
photographs taken by the SEM, a line segment orthogonal to the
ferrite layers and the cementite layers is drawn, and the pearlite
lamellar spacing in the colony as "a length of the line segment/the
number of lamellar layers within the line segment" is found on the
basis of the length of the line segment, and the number of the
lamellar layers within the line segment. Then, by finding the
pearlite lamellar spacings within not more than five pieces of the
colonies, respectively, in the respective photographs taken by the
SEM, the respective pearlite lamellar spacings within not less than
thirty pieces of the colonies altogether are worked out, and a mean
value thereof is defined as a value of "the pearlite lamellar
spacing" according to the invention.
[0046] A mechanism whereby the resistance to the wire-break of the
wire rod is enhanced if the pearlite lamellar spacing is not less
than 120 .mu.m is not clearly known, however, the mechanism can be
presumed to be as follows. The invention, however, is not to be
limited in scope to the mechanism presumed as described hereunder.
Even if the second-phase ferrite exists in the wire rod, the strain
concentration occurring to the second-phase ferrite will be
mitigated in the case that a difference in strength between the
second-phase ferrite and microstructure on the periphery thereof is
small, so that occurrence of voids likely to cause wire-break is
presumed to be checked. Further, it is reasoned that if the
pearlite lamellar spacing becomes wider, strength of pearlite
becomes lower, and a difference in strength between pearlite and
the second-phase ferrite is rendered relatively small, so that this
will probably contribute to enhancement in the resistance to the
wire-break of the wire rod.
[0047] However, if the pearlite lamellar spacing becomes
excessively wide, it is deemed that a likelihood of occurrence of
the voids will become greater on the contrary. An upper limit of
the pearlite lamellar spacing is therefore preferably not more than
350 .mu.m, more preferably not more than 300 .mu.m, and still more
preferably, not more than 280 .mu.m.
[0048] With the invention, a location on the cross-sectional face,
adopted for observation by the SEM, in order to find "the area
ratio of the second-phase ferrite", and "the pearlite lamellar
spacing", is specified as a location at D/4 on the cross-sectional
face of the wire rod (D: diameter of the wire rod). The reason for
this is to extract average data on the metal microstructure of the
wire rod. Parts in the surface layer are subjected to effects of
decarburization and central parts are subjected to effects of
segregation and so forth, so that variations in the data, at those
locations, tend to increase.
[0049] The wire rod according to the invention can be produced by,
for example, a method described hereinafter (refer to FIG. 3). The
wire rod according to the invention, however, is not limited to
that produced by the method described hereinafter. First, a steel
product meeting the requirements for the chemical components is
heated up to 900 to 1250.degree. C. to be subsequently hot rolled
at a temperature not lower than 780.degree. C., and a
finish-rolling temperature is controlled to not higher than
1100.degree. C. This is because heating is insufficient with a
heating temperature lower than 900.degree. C., and conversely, if
the heating temperature exceeds 1250.degree. C., decarburization in
the surface layer spreads, so that scales capable of adversely
affecting the wire-drawing workability tend to become harder to
peel off. Further, if a rolling temperature is lowered,
decarburization in the surface layer is similarly promoted, and a
lower limit temperature for hot rolling is therefore set to
780.degree. C. Conversely, if the finish-rolling temperature
exceeds 1100.degree. C., this will render it difficult to control
transformation of the metal microstructure by cooling and
reheating, to be executed in a subsequent process step, so that an
upper limit of the finish-rolling temperature is set to
1100.degree. C.
[0050] A wire rod formed after the finish-rolling is water-cooled
to a temperature range of 750 to 950.degree. C., and is coiled up
on conveying equipment, such as a Stelmor conveyer, to be then
placed thereon. Temperature control executed after water-cooling is
important for control of the transformation of the metal
microstructure, taking place thereafter, and control of scales. If
an ultimate temperature at the time of water-cooling is below
750.degree. C., this will at times cause the supercooled structures
to be formed in the surface layer, thereby adversely affecting the
wire-drawing workability, and on the other hand, if the ultimate
temperature exceeds 950.degree. C., this will cause loss in
deformability of scales, so that scales will peel off in the course
of transportation, thereby creating a cause for rusting.
[0051] It is of particular importance from the viewpoint of
obtaining the wire rod meeting the requirements for the metal
microstructure, excellent in the wire-drawing workability, to cool
the wire rod at an average cooling rate of not less than 20.degree.
C./sec within 20 sec from the coiling of the wire rod to thereby
drop temperature of the wire rod to a minimum value point (T1) in a
temperature range of 550 to 630.degree. C. before raising the
temperature of the wire rod up to a maximum value point (T2) in a
temperature range of 580 to 720.degree. C., higher in value than
T1, within 50 sec from the coiling of the wire rod. A reference
time for "within 20 sec from the coiling" is a point in time when a
rolled wire rod is coiled up in ring-like fashion to be placed on
the conveying equipment, such as the conveyer. Further, since the
wire rod is continuously coiled up, and is continuously cooled,
there occurs time lag between the top part of the wire rod, coiled
up, and the bottom part thereof, to be coiled up, with respect to a
time when the wire rod is placed, and a time when the wire rod is
cooled, respectively, however, respective measurements on time from
the coiling up to the cooling are started upon the coiling of the
respective part of the wire rod.
[0052] Since it is presumed that the second-phase ferrite prone to
undergo strain concentration is formed at a relatively high
temperature before pearlite transformation, formation of the
second-phase ferrite can be suppressed by rapidly cooling the wire
rod at the average cooling rate of not less than 20.degree. C./sec
down to a temperature region where ferrite is hard to be formed
within 20 sec from the coiling of the wire rod. Further, because
pearlite transformation nuclei in massive amounts are formed due to
such rapid cooling, advantageous effect of the metal microstructure
being micronized can be gained. However, if a cooling rate is
excessively high, this will raise the risk of an increase in
strength differential within the wire rod, due to localized
formation of supercooled structures, and so forth, thereby causing
deterioration in the wire drawing workability. Accordingly, the
average cooling rate is preferably set to not more than 50.degree.
C./sec. Herein, "the average cooling rate", according to the
invention, refers to a cooling rate found on the basis of a
temperature difference between the wire rod temperature upon the
coiling thereof (that is, the wire rod temperature after
water-cooling) and T1, and a cooling time length required for the
wire rod temperature at the time of the coiling to drop down to
T1.
[0053] Further, if the wire rod is cooled down only to the minimum
value point (T1) in excess of 630.degree. C. in such a cooling
process step as described above, it is not possible to sufficiently
suppress the formation of the second-phase ferrite, so that coarse
grains having adverse effects on the wire-drawing workability
become prone to be easily formed. On the contrary, if the wire rod
is excessively cooled down to the minimum value point (T1) below
550.degree. C., this will lead to an increase in strength
differential within the wire rod, due to the formation of the
supercooled structures, and so forth.
[0054] After the wire rod is cooled down to T1 in the temperature
range during the cooling process step, the wire rod is reheated to
thereby cause the pearlite transformation to occur. On this
occasion, by increasing the wire rod temperature to a high
temperature at 580.degree. C. or higher, the pearlite lamellar
spacing can be rendered wider. Further, it is presumed that the
higher a transformation temperature, the wider the pearlite
lamellar spacing can become, however, ductility becomes excessively
low at the transformation temperature in excess of 720.degree. C.,
raising the risk of the wire drawing workability undergoing
deterioration contrary to expectation.
[0055] It is deemed possible that the pearlite lamellar spacing can
be rendered wider by slowly cooling the wire rod as usual, or
holding the wire rod at a constant temperature without rapidly
cooling the same after the coiling thereof on the conveying
equipment. However, there is a likelihood that the metal
microstructure becomes coarser because a rate at which the pearlite
transformation nuclei are formed is low in a high temperature
region, thereby causing adverse effects on the wire drawing
workability. Hence, the wire rod whose metal microstructure is
fine, and has a wide pearlite lamellar spacing can be provided by a
production method according to the invention, comprising a step of
rapidly cooling a wire rod once after coiling thereof on a
conveying equipment before reheating the same, thereby causing
pearlite transformation to proceed in a high temperature
region.
WORKING EXAMPLES
[0056] The invention is more specifically described hereinafter
with reference to working examples. It is to be pointed out,
however, that the invention be not limited in scope by the working
examples described hereunder, and that various changes and
modifications may be obviously made in the invention in light of
teachings described hereinbefore and hereinafter without departing
from the spirit and scope thereof.
1. Production of Wire Rods
[0057] Hot rolled wire rods Nos. 1 to 29, each 5.5 mm in diameter,
were produced under various conditions shown in Table 2, with the
use of steel products S1 to S16, having chemical compositions shown
in Table 1, respectively. More specifically, the steel products
each were heated to a range of 978 to 1205.degree. C. in a heating
furnace to be hot rolled at a rolling temperature not lower than
807.degree. C., and to be finish-rolled at a temperature not higher
than 1068.degree. C., thereby being formed into the respective wire
rods. The wire rods each were water-cooled to a temperature in a
range of 798 to 948.degree. C., and were subsequently coiled up and
placed on the Stelmor conveyer (a cooling bed) to be continuously
cooled. In the course of cooling on the Stelmor conveyer,
temperature of the wire rod was lowered to the minimum value point
(T1) in a temperature range of 515 to 682.degree. C. within 20 sec
from the coiling of the wire rod. An average cooling rate during
this time period was in a range of 13 to 99.degree. C./sec.
Subsequently, the temperature of the wire rod was raised from T1 up
to the maximum value point (T2) in a temperature range of 584 to
705.degree. C. Further, some of the wire rods were continuously and
slowly cooled from T1 without being heated up from T1.
TABLE-US-00001 TABLE 2 minimum value maximum value point T1 point
T2 wire rod average time from time from heating lowest hot-roll
finish-rolling temperature after cooling coiling of coiling of wire
temperature temperature temperature water-cooling rate the wire
temperature the wire temperature rod steel product (.degree. C.)
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C./sec) rod
(sec) (.degree. C.) rod (sec) (.degree. C.) 1 S1 1151 933 989 940
78 5.0 550 18.0 688 2 S1 1150 930 991 948 14 19.0 682 29.0 701 3 S2
1148 945 993 845 75 3.5 583 17.0 691 4 S2 1150 941 990 847 24 10.0
607 15.0 678 5 S2 1151 940 992 850 35 7.0 605 15.0 684 6 S3 1175
946 1068 911 93 3.5 586 15.0 692 7 S3 1172 944 1059 908 18 18.0 584
28.0 689 8 S4 1150 940 1033 908 94 3.5 579 12.0 664 9 S4 1154 938
1031 912 97 3.5 573 12.0 599 10 S4 1151 935 1030 921 99 4.0 525
11.0 662 11 S5 1022 855 982 910 75 4.0 610 10.0 701 12 S5 1020 857
977 915 51 5.5 642 11.0 703 13 S5 1018 861 978 908 16 19.0 604 28.0
697 14 S6 1025 843 972 914 70 5.0 564 18.0 645 15 S6 1024 840 981
908 76 5.0 528 17.0 584 16 S6 1031 851 988 916 54 5.0 646 slow
cooling (T1 > T2) 17 S7 1020 807 934 798 61 3.5 585 10.0 705 18
S7 1022 811 930 805 58 3.5 602 slow cooling (T1 > T2) 19 S8 978
813 933 823 56 3.5 627 10.0 698 20 S9 1055 905 976 921 50 7.0 571
18.0 685 21 S10 1054 912 975 917 48 7.0 581 17.0 683 22 S11 1151
977 1064 922 47 7.0 593 17.0 701 23 S11 1148 971 1058 914 57 7.0
515 20.0 645 24 S11 1152 972 1045 912 13 18.0 678 24.0 688 25 S12
1205 968 1037 915 45 7.0 600 14.0 675 26 S13 1148 941 991 905 59
5.5 581 12.0 664 27 S14 1145 940 994 902 58 5.5 583 12.0 665 28 S15
1147 952 987 908 55 5.5 606 12.0 657 29 S16 1142 951 992 905 54 5.5
608 12.0 661
[0058] TABLE-US-00002 TABLE 1 Chemical components 1 (basic
components) of steel products (mass %) Steel Products C Si Mn P S
Al N O S1 0.60 0.20 0.49 0.007 0.008 0.0011 0.0021 0.0012 S2 0.61
0.19 0.48 0.005 0.007 0.0005 0.0024 0.0011 S3 0.70 0.20 0.50 0.005
0.006 0.0005 0.0024 0.0010 S4 0.72 0.19 0.83 0.006 0.005 0.0278
0.0032 0.0013 S5 0.80 0.20 0.50 0.006 0.004 0.0007 0.0028 0.0010 S6
0.81 0.20 0.51 0.015 0.014 0.0010 0.0030 0.0011 S7 0.81 0.19 0.50
0.005 0.007 0.0004 0.0027 0.0010 S8 0.82 0.20 0.50 0.004 0.004
0.0008 0.0022 0.0009 S9 0.89 1.77 0.49 0.005 0.004 0.0010 0.0031
0.0011 S10 0.90 0.22 0.49 0.006 0.005 0.0009 0.0025 0.0010 S11 0.92
0.19 0.49 0.004 0.005 0.0006 0.0027 0.0011 S12 1.05 0.18 0.49 0.006
0.005 0.0005 0.0025 0.0010 S13 0.81 2.30 0.50 0.005 0.005 0.0350
0.0031 0.0015 S14 0.81 0.20 1.55 0.011 0.022 0.0254 0.0037 0.0014
S15 0.81 0.21 0.49 0.021 0.013 0.0279 0.0075 0.0015 S16 1.20 0.19
0.50 0.008 0.007 0.0244 0.0050 0.0014 Chemical components 2 (basic
components) of steel products (mass %) Steel mass % mass ppm
Products Cr Cu Ni V Ti Nb Mo Zr B Mg Ca REM S1 0.01 0.02 0.01
0.0021 0.0710 -- -- -- 11 0.1 1.1 -- S2 -- 0.01 -- -- -- -- -- --
-- -- -- -- S3 -- -- -- 0.0022 0.0010 0.0300 -- 0.0240 8 0.2 -- --
S4 0.02 0.01 0.02 0.0018 -- -- -- -- -- 0.1 1.0 0.1 S5 0.01 -- 0.30
0.0017 -- 0.0510 -- -- 3 0.1 0.7 -- S6 0.78 -- -- 0.1100 -- -- 0.21
-- -- 0.2 1.2 0.1 S7 -- 0.01 0.02 -- -- -- -- -- -- 0.1 1.1 0.1 S8
0.01 -- -- -- -- -- -- -- -- 0.1 -- -- S9 0.31 0.21 0.20 0.2170
0.0011 -- -- -- -- 0.1 0.8 -- S10 0.01 -- -- -- -- -- -- -- -- --
-- -- S11 0.20 0.18 -- -- -- -- -- -- -- 0.1 1.0 0.1 S12 0.22 0.11
0.01 -- -- -- -- -- 10 0.1 1.4 0.1 S13 0.70 0.21 0.15 0.3110 -- --
-- -- -- 0.1 0.7 -- S14 0.51 0.10 0.11 -- -- -- -- -- -- -- -- --
S15 -- -- -- -- -- 0.1050 -- -- 11 -- -- -- S16 1.60 -- -- -- -- --
-- -- -- -- -- --
2. Measurement on Area Ratio of Second-Phase Ferrite, and Pearlite
Lamellar Spacing
[0059] As to the respective wire rods obtained as above, the area
ratio of the second-phase ferrite, and the pearlite lamellar
spacing were measured as follows:
[0060] First, the wire rods each were cut, and resin was embedded
therein such that the cross-sectional face of the wire rod can
serve as an evaluation face, wet polishing by use of an emery paper
and diamond powders was applied to the cross-sectional face
thereof, and a metal microstructure of the cross-sectional face
thereof was exposed by etching with Picral, thereby preparing a
specimen for observation. Then, the metal microstructure of the
wire rod, at the location corresponding to D/4 on the
cross-sectional face of the wire rod (D: diameter of the wire rod)
was observed by the SEM
[0061] In measuring the area ratio of the second-phase ferrite,
respective SEM photos of not less than 8 visual fields, as
magnified 500 to 1500 times by the SEM, were taken. On the basis of
the respective SEM photos obtained, the area ratio of the
second-phase ferrite was worked out by carrying out image analysis
with the use of the image analysis software, {Image-Pro (Ver 4.0)},
thereby having found a mean value of the area ratios of the
second-phase ferrite, worked out according to those SEM photos,
respectively. Measurement results are shown in Table 3.
[0062] In measurement of the pearlite lamellar spacing, respective
SEM photographs of not less than 6 visual fields of the
cross-sectional face of each of the wire rod, as magnified 3000 to
10,000 times by the SEM, were taken. On the basis of the respective
SEM photos obtained, the respective pearlite lamellar spacings
within not more than five pieces of the colonies were found,
thereby having worked out a mean value of the pearlite lamellar
spacings as found from not less than thirty pieces of the colonies
altogether. Measurement results are shown in Table 3.
3. Evaluation on Wire Drawing Workability
[0063] With respect to the respective wire rods, wire drawing
workability was evaluated as follows.
[0064] First, chemical descaling (acid cleaning) or mechanical
descaling (MD), shown in Table 3, as a descaling treatment to
provide a pretreatment for wire-drawing, was applied to the
respective wire rods (5.5 mm in diameter). In the case of acid
cleaning, the respective wire rods were cleaned in hydrochloric
acid to be subsequently treated with a phosphate. In the case of
mechanical descaling (MD), bending stress was imparted to the
respective wire rods with the use of a bending roller provided
alongside a wire drawing machine to thereby remove scales, and
subsequently, borax was applied to the respective wire rods. The
respective wire rods after removal of the scales by the acid
cleaning or the mechanical descaling were subjected to wire-drawing
using a Na-based lubricant.
[0065] Thereafter, dry wire-drawing with the use of a
continuous-wire-drawing machine was applied to the respective wire
rods on the following wire drawing conditions (1) to (3),
respectively, until a final diameter is reduced to 0.9 mm. The
higher a wire-drawing rate is, and the less the number of dies is,
that is, according as the wire-drawing condition turns from (1) to
(3), the higher productivity of drawing will become, however, the
wire-drawing condition will become severer.
The wire-drawing condition (1): final wire-drawing rate at 600
m/min, the number of dies; 14 pieces
The wire-drawing condition (2): final wire-drawing rate at 800
m/min, the number of dies; 14 pieces
The wire-drawing condition (3): final wire-drawing rate at 800
m/min, the number of dies; 12 pieces
[0066] Wire-drawing under the respective wire-drawing conditions
was applied to 50 tons each of the wire rods, and evaluation was
made on whether or not a wire-break occurs, and an extent of
die-wear, as criteria for the wire drawing workability. As to the
evaluation on the extent of the die-wear, symbol (X) indicates the
case where any of the dies were broken in the course of
wire-drawing, symbol (.DELTA.) indicates the case where none of the
dies were broken in the course of drawing 50 tons each of the wire
rods, but the dies were worn out, requiring replacement after the
wire-drawing, and symbol (.largecircle.) indicates the case where
none of the dies were broken, and there is no necessity of
replacing the dies, due to the wear thereof, after the wire-drawing
of 50 tons each of the wire rods. Further, symbol (-) indicates the
case where the evaluation on the extent of the die-wear was not
applicable due to occurrence of wire-break. Measurement results are
shown in Table 3. TABLE-US-00003 TABLE 3 area ratio pearlite
wire-drawing wire-drawing wire-drawing of second- lamellar
condition (1) condition (2) condition (3) wire steel phase ferrite
spacing descaling wire die wire die wire die rod product
(%)*.sup.1) (.mu.m) treatment break life break life break life 1 S1
7.8 215 MD No .smallcircle. No .smallcircle. No .smallcircle. 2 S1
15.5 201 MD Yes -- Yes -- Yes -- 3 S2 9.1 232 acid cleaning No
.smallcircle. No .smallcircle. No .smallcircle. 4 S2 10.2 202 acid
cleaning No .smallcircle. No .smallcircle. Yes -- 5 S2 9.4 214 acid
cleaning No .smallcircle. No .smallcircle. No .smallcircle. 6 S3
6.5 227 MD No .smallcircle. No .smallcircle. No .smallcircle. 7 S3
10.8 210 MD No .smallcircle. No .smallcircle. Yes -- 8 S4 2.7 192
MD No .smallcircle. No .smallcircle. No .smallcircle. 9 S4 3.7 128
MD No .smallcircle. No .smallcircle. No .smallcircle. 10 S4 2.2 118
MD Yes -- Yes -- Yes -- 11 S5 3.4 285 MD No .smallcircle. No
.smallcircle. No .smallcircle. 12 S5 12.1 305 MD Yes -- Yes -- Yes
-- 13 S5 10.2 275 MD No .smallcircle. No .smallcircle. Yes -- 14 S6
1.2 148 MD No .smallcircle. No .smallcircle. No .smallcircle. 15 S6
1.5 75 MD Yes -- Yes -- Yes -- 16 S6 11.5 127 MD Yes -- Yes -- Yes
-- 17 S7 3.6 303 acid cleaning No .smallcircle. No .smallcircle. No
.smallcircle. 18 S7 5.2 115 acid cleaning Yes -- Yes -- Yes -- 19
S8 4.7 245 acid cleaning No .smallcircle. No .smallcircle. No
.smallcircle. 20 S9 4.1 199 MD No .smallcircle. No .smallcircle. No
.smallcircle. 21 S10 4.7 180 MD No .smallcircle. No .smallcircle.
No .smallcircle. 22 S11 5.0 231 MD No .smallcircle. No
.smallcircle. No .smallcircle. 23 S11 1.0 108 MD Yes -- Yes -- Yes
-- 24 S11 11.2 198 MD Yes -- Yes -- Yes -- 25 S12 2.2 177 MD No
.smallcircle. No .smallcircle. No .smallcircle. 26 S13 4.7 172 MD
Yes -- Yes -- Yes -- 27 S14 3.9 147 MD Yes -- Yes -- Yes -- 28 S15
4.7 184 MD Yes -- Yes -- Yes -- 29 S16 3.5 141 MD Yes -- yes -- Yes
-- *.sup.1)Remarks: With respect to all the wire rods except for
the wire rods Nos. 23, and 27, the remaining metal microstructure
is, in effect, pearlite; the metal microstructure of the wire rod
No. 23: 94% pearlite, 5% martensite; the metal microstructure of
the wire rod No. 27: 93.1% pearlite, 3% martensite
[0067] It has turned out from the results shown in Table 3 that the
respective wire rods Nos. 1, 3, 5, 6, 8, 9, 11, 14, 17, 19, 20, 21,
22 and 25, meeting the requirements for the chemical components as
well as the metal microstructure, according to the invention, had
no wire-break, and little die wear even when processed under the
severe wire-drawing condition (3). Accordingly, it is evident that
those wire rods each had outstandingly excellent wire drawing
workability.
[0068] It is also shown that the respective wire rods Nos. 4, 7,
and 13, meeting the requirements for the chemical components as
well as the metal microstructure, according to the invention, had
no wire-break, and little die wear when processed under the wire
drawing conditions (1) and (2), respectively. Accordingly, those
wire rods as well each had excellent wire drawing workability.
However, those wire rods each had wire-break when processed under
the wire drawing condition (3). It is deemed that this was due to a
relatively high area ratio of the second-phase ferrite.
[0069] In the cases of the respective wire rods that did not meet
the requirements for the area ratio of the second-phase ferrite,
according to the invention, Nos. 2, 12, 16, and 24, and in the
cases of the respective wire rods that did not meet the
requirements for the pearlite lamellar spacing, according to the
invention, Nos. 10, 15, 18, and 23, wire-break occurred thereto
even when processed under the mild wire-drawing condition (1)
although all those wire rods met the requirements for the chemical
components, according to the invention.
[0070] Meanwhile, in the cases of the wire rods that did not meet
the requirements for the chemical components, more specifically, in
the cases of the wire rod No. 26 whose Si content, and Al content
are outside respective specified ranges, the wire rod No. 27 whose
Mn content, and S content are outside respective specified ranges,
the wire rod No. 28 whose P content, N content, and Nb content are
outside respective specified ranges, and the wire rod No. 29 whose
C content is outside a specified range, wire-break occurred thereto
even when processed under the mild wire-drawing condition (1)
although all those wire rods met the requirements for the metal
microstructure, according to the invention.
[0071] As described in the foregoing, the wire rod with excellent
resistance to wire-break, causing little die-wear, and excelling in
the wire-drawing workability can be obtained by adequately
controlling the requirements for the metal microstructure thereof
(the area ratio of the second-phase ferrite, and the pearlite
lamellar spacing) and the requirements for the chemical components
thereof.
* * * * *