U.S. patent application number 14/767516 was filed with the patent office on 2016-01-14 for high-strength steel wire material exhibiting excellent cold-drawing properties, and high-strength steel wire.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO(KOBE STEEL, LTD.). Invention is credited to Tomonobu ISHIDA, Nao YOSHIHARA.
Application Number | 20160010196 14/767516 |
Document ID | / |
Family ID | 51623558 |
Filed Date | 2016-01-14 |
United States Patent
Application |
20160010196 |
Kind Code |
A1 |
ISHIDA; Tomonobu ; et
al. |
January 14, 2016 |
HIGH-STRENGTH STEEL WIRE MATERIAL EXHIBITING EXCELLENT COLD-DRAWING
PROPERTIES, AND HIGH-STRENGTH STEEL WIRE
Abstract
Provided are: a high-strength steel wire material which exhibits
excellent cold-drawing properties and a high prescribed strength; a
high-strength steel wire produced from this high-strength steel
wire material; and a zinc-plated high-strength steel wire. This
high-strength steel wire material is characterized by respectively
including 0.80-1.3% of C, 0.1-1.5% of Si, 0.1-1.5% of Mn, more than
0% but not more than 0.03% of P, more than 0% but not more than
0.03% of S, 0.02-0.2% of Ti, 0.01-0.10% of Al, and 0.001-0.006% of
N, the remainder comprising iron and unavoidable impurities. The
high-strength steel wire material is further characterized in that
the relationship in formula (1), namely
0.05%.gtoreq.[Ti*].gtoreq.(0.0023 [C]) (with the caveat that
[Ti*]=(the total amount of Ti-the total amount of Ti in compounds
having a size of at least 0.1 .mu.m), and [C] represents the carbon
content (in mass %)), is satisfied.
Inventors: |
ISHIDA; Tomonobu; (Kobe-shi,
JP) ; YOSHIHARA; Nao; (Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO(KOBE STEEL, LTD.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi, Hyogo
JP
|
Family ID: |
51623558 |
Appl. No.: |
14/767516 |
Filed: |
March 10, 2014 |
PCT Filed: |
March 10, 2014 |
PCT NO: |
PCT/JP2014/056103 |
371 Date: |
August 12, 2015 |
Current U.S.
Class: |
428/659 ;
420/100; 420/104; 420/120; 420/121; 420/126; 420/93; 420/99 |
Current CPC
Class: |
C21D 8/065 20130101;
C22C 38/14 20130101; C22C 38/02 20130101; C21D 9/52 20130101; C22C
38/24 20130101; C21D 2211/004 20130101; C22C 38/08 20130101; C22C
38/001 20130101; C22C 38/04 20130101; C22C 38/30 20130101; C22C
38/16 20130101; C23C 2/06 20130101; C23C 2/38 20130101; C22C 38/06
20130101; C22C 38/28 20130101; C22C 38/002 20130101; C21D 8/06
20130101; C22C 38/26 20130101; C22C 38/12 20130101; C22C 38/32
20130101; C22C 38/54 20130101 |
International
Class: |
C23C 2/38 20060101
C23C002/38; C22C 38/30 20060101 C22C038/30; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/24 20060101
C22C038/24; C22C 38/00 20060101 C22C038/00; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/08 20060101
C22C038/08; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/32 20060101
C22C038/32; C22C 38/16 20060101 C22C038/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2013 |
JP |
2013-067465 |
Claims
1. A high-strength steel wire rod having good rod drawability, the
steel wire rod comprising: C: 0.80 to 1.3% by mass; Si: 0.1 to 1.5%
by mass; Mn: 0.1 to 1.5% by mass; P: more than 0% and 0.03% or less
by mass; S: more than 0% and 0.03% or less by mass; Ti: 0.02 to
0.2% by mass; Al: 0.01 to 0.10% by mass; N: 0.001 to 0.006% by
mass; and iron and inevitable impurities, wherein a relationship of
Formula (1) is satisfied:
0.05%.gtoreq.[Ti*].gtoreq.(0.0023.times.[C]) (1) where [Ti*]
represents "a total amount of Ti-an amount of Ti in compounds
having a size of 0.1 .mu.m or more", and [C] represents carbon
content (by mass percent)).
2. The high-strength steel wire rod according to claim 1, wherein a
microstructure of the steel wire rod includes a pearlite phase
having an area ratio of 90% or more, and proeutectoid cementite has
a maximum length of 15 .mu.m or less.
3. The high-strength steel wire rod according to claim 1, wherein
an amount of dissolved N is more than 0% and 0.0005% or less.
4. The high-strength steel wire rod according to claim 2, wherein
the amount of dissolved N is more than 0% and 0.0005% or less.
5. The high-strength steel wire rod according to claim 1, further
comprising one or more of B, Cr, V, and M, M representing at least
one element selected from the group consisting of Ni, Cu, Mo, Co,
and Nb, in the proportions of B: more than 0% and 0.010% or less by
mass, Cr: more than 0% and 0.5% or less by mass, V: more than 0%
and 0.2% or less by mass, Ni: more than 0% and 0.5% or less by
mass, Cu: more than 0% and 0.5% or less by mass, Mo: more than 0%
and 0.5% or less by mass, Co: more than 0% and 1.0% or less by
mass, and Nb: more than 0% and 0.5% or less by mass.
6. A high-strength steel wire that is produced through wire-drawing
of the high-strength steel wire rod according to claim 1.
7. A high-strength steel wire that is produced through wire-drawing
of the high-strength steel wire rod according to claim 5.
8. A high-strength galvanized steel wire produced by performing
hot-dip galvanization on the high-strength steel wire according to
claim 6, wherein the high-strength galvanized steel wire has a
tensile strength TS equal to or higher than tensile strength TS*
defined by Formula (2): TS*=-87.3D+2234 (MPa) (2) where D
represents wire diameter (mm) of the high-strength galvanized steel
wire.
9. A high-strength galvanized steel wire produced by performing
hot-dip galvanization on the high-strength steel wire according to
claim 7, wherein the high-strength galvanized steel wire has a
tensile strength TS equal to or higher than tensile strength TS*
defined by Formula (2): TS*=-87.3D+2234 (MPa) (2) where D
represents wire diameter (mm) of the high-strength galvanized steel
wire.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-strength steel wire
that is useful as a material for a galvanized steel wire for use in
a rope for a bridge or the like, and a high-strength steel wire rod
to produce such a high-strength steel wire. In particular, the
invention relates to a high-strength steel wire rod having good
workability for wire-drawing without heat treatment after
rolling.
BACKGROUND ART
[0002] A steel wire or a steel wire strand, which is subjected to
hot-dip galvanization to improve corrosion resistance, is used for
a rope for use in a bridge. As a material for such a steel wire,
for example, JIS G 3548 describes a steel wire having a wire
diameter of 5 mm and a tensile strength TS of about 1500 to 1700
MPa. A carbon steel described in JIS G 3506 is mainly used as a
material steel for the steel wire.
[0003] A steel wire as a material for the hot-dip galvanized steel
wire is demanded to be reduced in manufacturing cost, and demanded
to be advantageously reduced in steel usage and improved in the
degree of freedom of bridge design by increasing strength of the
steel wire. In other words, development of a steel wire, which has
high strength and is manufactured at low cost, is demanded.
[0004] The galvanized steel wire is typically manufactured in the
following manner. First, a wire rod (steel wire rod) fabricated
through hot rolling is placed in a ring shape on a cooling conveyer
for pearlite transformation, and is then wound up into a coil to
yield a wire rod coil. Subsequently, the wire rod coil is subjected
to patenting treatment for higher strength and uniform
microstructure. The patenting treatment is a type of heat
treatment, in which a wire rod is typically heated to about
950.degree. C. using a continuous furnace for austenization, and is
then dipped in a refrigerant such as a lead bath maintained at
about 500.degree. C. to produce a fine and homogeneous pearlite
phase.
[0005] Subsequently, the wire rod is subjected to cold
wire-drawing, so that a steel wire having a predetermined strength
is produced through a work hardening function of pearlite steel.
Subsequently, the steel wire is dipped in a galvanizing bath
maintained at about 450.degree. C. for galvanization, so that a
galvanized steel wire is produced. The galvanized steel wire may be
further subjected to finish drawing. A parallel wire strand (PWS)
as a bundle of such steel wires or a galvanized steel wire strand
as a strand of such steel wires is used as a cable for a
bridge.
[0006] In such a series of manufacturing steps, the patenting
treatment causes an increase in manufacturing cost. Although the
patenting treatment is effective in increasing strength of a wire
rod and providing uniform quality thereof, it is disadvantageous
not only in increasing manufacturing cost but also in environmental
issues such as CO.sub.2 emission and use of an environment-load
substance. The hot-rolled wire rod could be advantageously drawn to
be formed into a product (i.e., a steel wire) without heat
treatment. Drawing the hot-rolled wire rod without heat treatment
is called "rod drawing".
[0007] To achieve high strength of a wire rod subjected to rod
drawing, hypereutectoid steel having a large C content must be used
to compensate strength reduction caused by omitting the patenting
treatment. However, proeutectoid cementite is gradually
precipitated with an increase in C content, which disadvantageously
reduces wire-drawability. It is therefore desired to achieve a wire
rod having a good property that enables rod drawing with reduced
influence of the proeutectoid cementite even if the C content is
increased to increase strength (such a property is referred to as
"rod drawability").
[0008] There have been provided various techniques for improving
wire-drawability. For example, PTL 1 provides a technique for
improving wire-drawability through cooling in a molten salt bath
after hot rolling. Such a technique is referred to as direct
patenting treatment. However, the direct patenting treatment using
the molten salt bath leads to higher manufacturing cost and worse
maintainability of equipment than air blast cooling. In addition,
wire-drawability of the produced steel is low, about 80% in area
reduction ratio, and a strength level of the wire (steel wire) is
only about 180 to 190 kgf/mm.sup.2 (1764 to 1862 MPa).
[0009] PTL 2 discloses a technique for increasing strength of a
wire rod by controlling a cooling condition after hot rolling so
that the patenting treatment is omitted. However, the steel
produced by such a technique has a low wire-drawability, about 50%
in area reduction ratio, and the wire has a considerably low
strength level, about 1350 to 1500 MPa.
CITATION LIST
Patent Literature
[0010] PTL 1: Japanese Unexamined Patent Application Publication
No. Hei 4(1992)-289128.
[0011] PTL 2: Japanese Unexamined Patent Application Publication
No. Hei 5(1993)-287451.
SUMMARY OF INVENTION
Technical Problem
[0012] An object of the invention, which has been achieved in light
of such circumstances, is to provide a high-strength steel wire rod
having good rod drawability and predetermined high strength, and a
high-strength steel wire produced of such a high-strength steel
wire rod, and a high-strength galvanized steel wire.
Solution to Problem
[0013] The high-strength steel wire rod of the invention, by which
the above-described object is achieved, contains C: 0.80 to 1.3%
(by mass percent (the same applies to the following for the
components)), Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, P: more than 0% and
0.03% or less, S: more than 0% and 0.03% or less, Ti: 0.02 to 0.2%,
Al: 0.01 to 0.10%, and N: 0.001 to 0.006%, the remainder consisting
of iron and inevitable impurities, where the relationship of
Formula (1) is satisfied.
0.05%.gtoreq.[Ti*].gtoreq.(0.0023.times.[C]) (1)
[0014] where [Ti*] represents "the total amount of Ti-the amount of
Ti in compounds having a size of 0.1 .mu.m or more", and [C]
represents the carbon content (by mass percent)).
[0015] The amount of Ti in compounds having a size of 0.1 .mu.m or
more refers to the amount of Ti in compounds in the residue
collected through filtration with a mesh having an opening of 0.1
.mu.m.
[0016] In the high-strength steel wire rod of the invention, it is
preferred that its microstructure includes a pearlite phase in an
area ratio of 90% or more, and proeutectoid cementite has a maximum
length of 15 .mu.m or less. It is further preferred that the amount
of dissolved N in the wire rod is more than 0% and 0.0005% or
less.
[0017] The chemical composition of the high-strength steel wire rod
further effectively contains, as necessary, (a) B: more than 0% and
0.010% or less, (b) Cr: more than 0% and 0.5% or less, (c) V: more
than 0% and 0.2% or less, and (d) at least one element selected
from the group consisting of Ni: more than 0% and 0.5% or less, Cu:
more than 0% and 0.5% or less, Mo: more than 0% and 0.5% or less,
Co: more than 0% and 1.0% or less, and Nb: more than 0% and 0.5% or
less. The properties of the wire rod are further improved depending
on a type of the component to be contained.
[0018] The invention also includes a high-strength steel wire
produced through wire-drawing (for example, a drawing process) of
the high-strength steel wire rod as described above. A
high-strength galvanized steel wire, which is produced by
performing hot-dip galvanization on the high-strength steel wire,
preferably has a tensile strength TS equal to or higher than the
tensile strength TS* defined by Formula (2).
TS*=-87.3D+2234 (MPa) (2)
[0019] where D represents wire diameter (mm) of the high-strength
galvanized steel wire.
Advantageous Effects of Invention
[0020] According to the invention, a high-strength steel wire rod
having good rod drawability and high strength is provided by
strictly defining the chemical composition of the wire rod in
consideration of a precipitation state of fine TiC. The steel wire
produced of such a high-strength steel wire rod is greatly useful
as a material for a hot-dip galvanized steel wire or a steel wire
strand as a material for a rope for use in a bridge and the
like.
DESCRIPTION OF EMBODIMENTS
[0021] To solve the above-described problems, the inventors have
made investigations on a relationship between a microstructure of a
wire rod and wire-drawability. In particular, the inventors also
have made investigations on a precipitation mechanism of
proeutectoid cementite in hypereutectoid steel. As a result, it has
been found that precipitation of proeutectoid cementite can be
suppressed by precipitating fine TiC in the vicinity of a grain
boundary. In particular, a fine TiC having a size of 0.1 .mu.m or
less is most effective, and a sufficient amount of such fine TiC
must be precipitated. As the C content in steel increases,
cementite is more readily precipitated, and thus a larger amount of
fine TiC is required. Such an effect is less likely to be given by
coarse TiC; hence, the fine TiC is necessary to be precipitated as
much as possible. It is therefore extremely important to
appropriately control the precipitated amount and size distribution
of TiC.
[0022] The fine TiC having a size of 0.1 .mu.m or less is
precipitated in the vicinity of an austenite grain boundary as
described above. This makes it possible to decrease grain boundary
energy and thus suppress precipitation of proeutectoid cementite.
Although great effort and cost are required to directly determine
the fine TiC, such determination can be made in a simplified manner
by using electroextraction residue measurement. Specifically, while
the entire amount of Ti exists in steel in a form of compounds such
as TiC and TiN, TiN has a size of about 5 to 10 .mu.m. Hence, the
amount of Ti in compounds having a size of 0.1 .mu.m or more, more
specifically the amount of Ti in compounds in the residue collected
through filtration with a mesh having an opening of 0.1 .mu.m is
measured and subtracted from the total amount of Ti in steel. The
value obtained from such a subtraction is denoted as [Ti*], which
represents the amount of fine TiC passing through the mesh. The Ti
in compounds means Ti that exists in a form of a compound.
[0023] As the C content in steel increases, the proeutectoid
cementite is more readily precipitated, and thus a larger amount of
fine TiC is required. Based on such a relationship, the [Ti*] is
required in the amount of 0.0023.times.[C] or more, preferably
0.0023.times.[C]+0.001% or more, and more preferably
0.0023.times.[C]+0.005% or more, where [C] represents the C
content. On the other hand, if a large amount of fine TiC is
precipitated, grain boundaries become brittle, and toughness of the
wire rod is degraded, causing longitudinal cracking during
wire-drawing. From such a point, the upper limit of [Ti*] is 0.05%
or less, preferably 0.03% or less, and more preferably 0.01% or
less.
[0024] The steel wire rod of the invention must satisfy the basic
composition of the wire rod. In addition, the chemical composition
of the steel wire rod must be appropriately adjusted to
appropriately control a precipitation state of TiC. From such a
point, the range of each chemical component of the wire rod is
determined based on the following reason.
(C: 0.80 to 1.3%)
[0025] C is an element that is effective in increasing strength.
Increased C content increases strength of a cold-rolled steel wire.
The C content must be 0.80% or more to ensure the target strength
level of the invention. However, if the C content is excessive,
proeutectoid cementite is precipitated in grain boundaries, which
impairs wire-drawability. From such a point, the C content must be
1.3% or less. The lower limit of the C content is preferably 0.84%
or more, and more preferably 0.90% or more. The upper limit thereof
is preferably 1.2% or less, and more preferably 1.1% or less.
(Si: 0.1 to 1.5%)
[0026] Si is an effective deoxidizer, and exhibits an effect of
decreasing the amount of oxide-based inclusion in steel. In
addition, Si increases strength of the wire rod, and exhibits an
effect of suppressing cementite granulation along with thermal
history during hot-dip galvanization, and thus suppressing a
reduction in strength. Si must be contained 0.1% or more so as to
effectively exhibit such effects. However, an excessive Si content
degrades toughness of the wire rod; hence, the Si content must be
1.5% or less. The lower limit of the Si content is preferably 0.15%
or more, and more preferably 0.20% or more. The upper limit thereof
is preferably 1.4% or less, and more preferably 1.3% or less.
(Mn: 0.1 to 1.5%)
[0027] Mn greatly improves hardenability of steel, and thus
exhibits an effect of lowering a transformation temperature during
air blast cooling, and increasing strength of a pearlite phase. Mn
must be contained 0.1% or more so as to effectively exhibit such
effects. However, Mn is an element that is easily segregated, and
if Mn is excessively contained, hardenability of a portion, in
which Mn is segregated, is excessively enhanced, and a supercooled
phase such as martensite may be formed. In consideration of such
influences, the upper limit of the Mn content is 1.5% or less. The
lower limit of the Mn content is preferably 0.2% or more, and more
preferably 0.3% or more. The upper limit thereof is preferably 1.4%
or less, and more preferably 1.3% or less.
(P; More than 0% and 0.03% or Less, S: More than 0% and 0.03% or
Less)
[0028] P and S are each segregated in prior austenite grain
boundaries and thus make the grain boundaries brittle, leading to a
degradation in fatigue characteristics. It is therefore basically
preferred that the content of each of P and S is as low as
possible, but the upper limit of the content is defined to be 0.03%
or less in terms of industrial production. Each content is
preferably 0.02% or less, and more preferably 0.01% or less. P and
S are each an impurity that is inevitably contained in steel, and
it is difficult to decrease the content thereof to 0% in terms of
industrial production.
(Ti: 0.02 to 0.2%)
[0029] Ti is an element that is extremely important for the wire
rod of the invention, which finely precipitates in a form of TiC in
the vicinity of a grain boundary, and thus exhibits an effect of
suppressing precipitation of proeutectoid cementite. The effect is
due to a function of fixing C in a form of TiC in the vicinity of a
grain boundary, and a function of relaxing grain boundary energy to
hinder cementite nucleation by the fine TiC of 0.1 .mu.m or less in
size. In addition, as with Al, Ti exhibits an effect of refining
crystal grains and an effect of improving toughness through nitride
formation. Ti must be contained 0.02% or more so as to effectively
exhibit such effects. However, if the Ti content is excessive, TiC
is excessively precipitated, which makes the grain boundary
brittle, leading to degradation in toughness. From such a point,
the Ti content must be 0.2% or less. The lower limit of the Ti
content is preferably 0.03% or more, and more preferably 0.04% or
more. The upper limit thereof is preferably 0.18% or less, and more
preferably 0.16% or less.
(Al: 0.01 to 0.10%)
[0030] Al has a strong deoxidizing function, and exhibits an effect
of decreasing the amount of oxide-based inclusion in steel. In
addition, Al promisingly exhibits an effect of refining crystal
grains due to pinning of nitride and an effect of decreasing the
amount of dissolved N. Al must be contained 0.01% or more so as to
exhibit such effects. However, if the Al content is excessive, the
amount of Al-based inclusion such as Al.sub.2O.sub.3 increases,
which disadvantageously increases wire breaking rate during
wire-drawing. The Al content must be 0.10% or less in order to
prevent such a disadvantage. The lower limit of the Al content is
preferably 0.02% or more, and more preferably 0.03% or more. The
upper limit thereof is preferably 0.08% or less, and more
preferably 0.06% or less.
(N: 0.001 to 0.006%)
[0031] N is dissolved in steel as an interstitial element and
induces embrittlement due to strain aging, which degrades toughness
of the wire rod. The upper limit of the N content (total N) in
steel is therefore 0.006% or less. However, such a disadvantage is
provided only by dissolved N that is dissolved in steel. A nitrogen
precipitate that is precipitated in a form of nitride, i.e., N in
compounds has no bad influence on toughness. Hence, the amount of
dissolved N that is dissolved in steel is desirably controlled
separately from N in steel (total N). The amount of dissolved N is
preferably 0.0005% or less, and more preferably 0.0003% or less. On
the other hand, it is difficult to decrease the amount of dissolved
N in steel to less than 0.001% in terms of industrial production;
hence, the lower limit of the N content in steel is 0.001% or more.
The upper limit of the N content in steel is preferably 0.004% or
less, and more preferably 0.003% or less.
[0032] The components defined in the invention are as described
above. The remainder consists of iron and inevitable impurities.
The inevitable impurities may include elements that are introduced
depending on starting materials, other materials, and situations of
production facilities, etc. The wire rod further effectively
contains the following elements singly or in appropriate
combination as necessary: (a) B: more than 0% and 0.010% or less,
(b) Cr: more than 0% and 0.5% or less, (c) V: more than 0% and 0.2%
or less, and (d) at least one element selected from the group
consisting of Ni: more than 0% and 0.5% or less, Cu: more than 0%
and 0.5% or less, Mo: more than 0% and 0.5% or less, Co: more than
0% and 1.0% or less, and Nb: more than 0% and 0.5% or less. The
properties of the wire rod are further improved depending on a type
of the component to be contained. The reason for defining the range
of each of the elements to be contained is as follows.
(B: More than 0% and 0.010% or Less)
[0033] B hinders formation of proeutectoid ferrite or proeutectoid
cementite, and thus exhibits an effect of facilitating control of a
microstructure to be formed into a homogeneous pearlite phase. In
addition, B fixes N in steel in a form of BN, and thereby
suppresses strain aging and improves toughness of the wire rod. B
is preferably contained 0.0003% or more so as to effectively
exhibit such effects. The B content is more preferably 0.0005% or
more, and further preferably 0.0008% or more. However, if the B
content is excessive, a compound with iron (B-constituent) is
precipitated, which induces cracking during hot rolling; hence, the
upper limit of the B content is preferably 0.010% or less. The
upper limit of the B content is more preferably 0.008% or less, and
further preferably 0.006% or less.
(Cr: More than 0% and 0.5% or Less)
[0034] Cr reduces the lamellar spacing of pearlite, and thus
exhibits an effect of improving strength or toughness of the wire
rod. In addition, as with Si, Cr exhibits an effect of suppressing
reduction in strength of the wire rod during galvanization.
However, when the Cr content is excessive, the effects wastefully
reach saturation; hence, the appropriate Cr content is preferably
0.5% or less. The Cr content is preferably 0.001% or more and more
preferably 0.05% or more so that the effects of Cr are effectively
exhibited. The upper limit of the Cr content is more preferably
0.4% or less, and further preferably 0.3% or less.
(V: More than 0% and 0.2% or Less)
[0035] V forms fine carbide/nitride (carbide, nitride, and
carbonitride) and thus exhibits an effect of increasing strength
and an effect of refining crystal grains. In addition, V fixes
dissolved N and thus promisingly suppresses aging embrittlement.
The V content is preferably 0.001% or more and more preferably
0.05% or more so that the effects of V are effectively exhibited.
However, when the V content is excessive, the effects wastefully
reach saturation; hence, the appropriate V content is preferably
0.2% or less. The V content is more preferably 0.18% or less, and
further preferably 0.15% or less.
(At least one element selected from the group consisting of Ni:
more than 0% and 0.5% or less, Cu: more than 0% and 0.5% or less,
Mo: more than 0% and 0.5% or less, Co: more than 0% and 1.0% or
less, and Nb: more than 0% and 0.5% or less)
[0036] Ni is an element that is effective in improving toughness of
the steel wire subjected to wire-drawing. The Ni content is
preferably 0.05% or more and more preferably 0.1% or more so that
the effect of Ni is effectively exhibited. However, when the Ni
content is excessive, the effect wastefully reaches saturation;
hence, the appropriate Ni content is preferably 0.5% or less, more
preferably 0.4% or less, and further preferably 0.3% or less.
[0037] Cu and Mo are each an element that is effective in improving
corrosion resistance of the steel wire. The content of each of Cu
and Mo is preferably 0.01% or more and more preferably 0.05% or
more so that such an effect is effectively exhibited. However, if
the Cu content is excessive, Cu reacts with S and forms CuS that is
segregated in grain boundaries, causing flaws during a wire-rod
manufacturing process. Hence, the upper limit of the Cu content is
preferably 0.5% or less, more preferably 0.4% or less, and further
preferably 0.3% or less.
[0038] Mo is also an element that is effective in improving
corrosion resistance of the steel wire as with Cu. However, if the
Mo content is excessive, a supercooled phase is readily formed
during hot rolling, and ductility is degraded. Consequently, the
upper limit of the Mo content is preferably 0.5% or less, more
preferably 0.4% or less, and further preferably 0.3% or less.
[0039] Co reduces the amount of proeutectoid cementite, and thus
exhibits an effect of facilitating control of a microstructure to
be formed into a homogeneous pearlite phase. However, when Co is
excessively contained, the effect wastefully reaches saturation.
The upper limit of the Co content is therefore preferably 1.0% or
less, more preferably 0.8% or less, and further preferably 0.5% or
less. The Co content is preferably 0.05% or more, more preferably
0.1% or more, and further preferably 0.2% or more so that the
effect of Co is effectively exhibited.
[0040] As with Ti, Nb forms nitride and thus contributes to
refining crystal grains. In addition, Nb fixes dissolved N and thus
promisingly suppresses aging embrittlement. However, when Nb is
excessively contained, the effects wastefully reaches saturation.
The upper limit of the Nb content is therefore preferably 0.5% or
less, more preferably 0.4% or less, and further preferably 0.3% or
less. The Nb content is preferably 0.05% or more, more preferably
0.1% or more, and further preferably 0.2% or more so that the
effects of Nb are effectively exhibited.
[0041] The high-strength steel wire rod of the invention preferably
has a microstructure mainly including a pearlite phase (for
example, 90% or more in area ratio), but is allowed to partially
(10% or less in area ratio) contain another phase (for example,
proeutectoid ferrite or bainite).
[0042] In the invention, length of the proeutectoid cementite is
further preferably controlled. The proeutectoid cementite, which is
precipitated on a side near the center with reference to a position
of D/4 (D: diameter of the wire rod) of the wire rod, causes
cracking during wire-drawing, and thus causes cuppy break. The
cementite (lamellar cementite) that forms a lamellar structure of
pearlite has a property of rotating in response to wire-drawing and
orienting in a longitudinal direction of the wire rod. However, the
proeutectoid cementite cannot rotate in synchronization with a
surrounding phase, and cracking occurs at an interface between the
phases. The dominant factor of such rotation is the length of the
proeutectoid cementite. If the length (maximum length) of the
proeutectoid cementite exceeds 15 .mu.m, the cementite is less
likely to rotate, causing cracking. However, a short proeutectoid
cementite easily rotates, and does not significantly impair the
wire-drawability. From such a point, the length (maximum length) of
the proeutectoid cementite is preferably 15 .mu.m or less, more
preferably 13 .mu.m or less, and further preferably 10 .mu.m or
less. The lower limit of the length of the proeutectoid cementite
may be, for example, but not limited to, about 0.1 .mu.m.
[0043] The high-strength steel wire rod of the invention has good
rod drawability and high strength. For example, the wire rod of the
invention is allowed to have a tensile strength of 1100 MPa or
more, and preferably 1200 MPa or more. The upper limit of the
tensile strength is typically, but not limited to, about 1500
MPa.
[0044] The high-strength steel wire rod of the invention should be
manufactured according to a usual manufacturing condition while a
billet having a chemical composition adjusted as described above is
used. However, as described below, there is a preferred
manufacturing condition to appropriately adjust the microstructure
or the like of the wire rod.
[0045] In a typical manufacturing process of a high-carbon steel
wire rod, a billet having a predeterminately adjusted chemical
composition is heated and austenized. The austenized billet is
hot-rolled into a wire rod having a predetermined wire diameter,
and is then cooled on a cooling conveyer, during which the
austenite phase is transformed into a pearlite phase. In this
process, a fine austenite phase is produced along with dynamic
recrystallization during the hot rolling, and when TiC is
precipitated concurrently with such recrystallization, the TiC can
be finely dispersed in the vicinity of a grain boundary. Last four
passes (four passes from the last pass to the last pass but three)
of rolling most greatly affect the crystal grain size, and the area
reduction strain over the last four passes is denoted as .epsilon..
When the area reduction strain .epsilon. is adjusted to be 0.4 or
more, the crystal grains can be sufficiently refined, and TiC can
be finely dispersed. The area reduction strain .epsilon. is
represented by .epsilon.=ln(S.sub.1/S.sub.2) (S.sub.1: cross
section of a wire rod on an inlet side of a mill roll, S.sub.2:
cross section of a wire rod on an outlet side thereof). The lower
limit of the area reduction strain c is preferably 0.42 or more,
and more preferably 0.45 or more. The upper limit thereof is
preferably 0.8 or less, and more preferably 0.6 or less.
[0046] The finely precipitated TiC is progressively coarsened
during the cooling step after rolling. At this time, an important
requirement is placing temperature of the wire rod. The placing
temperature is controlled at 850 to 950.degree. C., thereby a
desired precipitation state of TiC is preferably provided. If the
placing temperature exceeds 950.degree. C., TiC is coarsened. If
the placing temperature is lower than 850.degree. C., TiC is
excessively fine. The upper limit of the placing temperature is
more preferably 940.degree. C. or lower, and further preferably
930.degree. C. or lower. The lower limit of the placing temperature
is more preferably 870.degree. C. or higher, and further preferably
880.degree. C. or higher.
[0047] The wire rod is cooled by air blast cooling in the cooling
step after rolling. At this time, if cooling rate (average cooling
rate) is too high, bainite or the like is easily contained, which
prevents formation of a microstructure mainly including a pearlite
phase. From such a point, average cooling rate in a range of the
placing temperature is preferably 20.degree. C./sec or less, more
preferably 18.degree. C./sec or less, and further preferably
14.degree. C./sec or less. In light of reducing precipitation of
the proeutectoid cementite, the lower limit of the average cooling
rate is preferably 3.degree. C./sec or more, more preferably
4.degree. C./sec or more, and further preferably 5.degree. C./sec
or more.
[0048] The high-carbon steel wire rod (high-strength steel wire
rod) of the invention is good in rod drawability, and thus provides
a high-strength steel wire, which exhibits desired properties such
as strength and a torsion value, through wire-drawing. Such a
high-strength steel wire is typically used as a high-strength
galvanized steel wire after its surface is subjected to hot-dip
galvanization. The wire rod of the invention is good in rod
drawability, and thus can be drawn without wire breaking even if an
area reduction ratio is, for example, but not limited to, more than
80%, and furthermore 83% or more. The upper limit of the area
reduction ratio is, for example, but not limited to, 95% or less.
The hot-dip galvanization of the steel wire should be performed for
about 15 sec to 1 min in a hot-dip galvanization bath at, for
example, 350.degree. C. or higher (preferably 400.degree. C. or
higher) and 550.degree. C. or lower (preferably 500.degree. C. or
lower). The steel wire subjected to wire-drawing such as a drawing
process has higher strength with a smaller wire diameter thereof.
The tensile strength TS of the high-strength galvanized steel wire
is preferably equal to or higher than the tensile strength TS*
defined by Formula (2), more preferably equal to or higher than
TS*+50 (MPa), and further preferably equal to or higher than
TS*+100 (MPa). The relationship of the Formula (2) is
experimentally obtained.
TS*=-87.3D+2234 (MPa) (2)
[0049] where D represents wire diameter (mm) of the high-strength
galvanized steel wire.
[0050] This application claims the benefit of Japanese Priority
Patent Application JP 2013-67465 filed on Mar. 27, 2013, the entire
contents of which are incorporated herein by reference.
[0051] Although the invention is now described in detail with an
example, the invention should not be limited thereto, and
modifications or alterations thereof may be made within the scope
without departing from the gist described before and later, all of
which are included in the technical scope of the invention.
Example
[0052] Billets (cross section 155.times.155 mm in size) having
chemical compositions (steel types A to S) listed in Table 1 were
prepared. The billets were each hot-rolled into a predetermined
wire diameter, placed in a ring shape on a cooling conveyer,
subjected to control cooling with air blast cooling for pearlite
transformation, and wound in a coil shape, so that hot-rolled wire
rod coils were produced. In Table 1, "-" represents "not
contained".
TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %) type C
Si Mn Al P S N Ti Cr V B Mo Cu Co Ni Nb A 1.05 0.40 0.30 0.035
0.010 0.010 0.0042 0.06 -- -- -- -- -- -- -- -- B 0.92 0.90 0.50
0.040 0.011 0.006 0.0037 0.02 -- -- -- -- -- -- -- -- C 0.98 0.60
0.70 0.030 0.008 0.008 0.0053 0.08 0.15 -- -- -- -- -- -- -- D 0.88
0.60 0.70 0.033 0.010 0.010 0.0044 0.07 0.20 -- 0.0015 -- -- -- --
-- E 1.05 0.70 0.85 0.070 0.010 0.011 0.0032 0.13 -- 0.07 -- -- --
-- -- -- F 0.97 0.62 0.51 0.060 0.007 0.010 0.0046 0.08 -- --
0.0020 -- -- -- -- -- G 0.84 0.43 1.20 0.040 0.010 0.020 0.0051
0.10 -- -- -- -- -- -- -- -- H 1.02 0.60 0.70 0.030 0.020 0.008
0.0048 0.09 0.20 -- 0.0022 -- -- -- -- -- I 0.90 0.50 0.81 0.090
0.007 0.010 0.0052 0.09 -- -- -- -- 0.07 -- -- -- J 1.20 0.40 0.60
0.050 0.008 0.012 0.0031 0.05 -- -- -- -- -- -- 0.20 -- K 0.85 0.24
0.61 0.020 0.006 0.008 0.0042 0.16 0.15 0.20 -- -- -- 0.20 -- -- L
1.30 0.69 0.51 0.080 0.010 0.007 0.0058 0.18 0.20 -- -- -- -- -- --
0.21 M 0.80 0.25 0.50 0.020 0.015 0.011 0.0036 0.08 -- -- 0.0012 --
-- -- -- -- N 0.93 1.43 1.50 0.030 0.010 0.010 0.0052 0.13 -- -- --
0.20 -- -- -- -- O 1.10 0.20 0.80 0.050 0.008 0.013 0.0047 0.07
0.30 -- -- -- -- -- -- -- P 0.72 0.39 0.68 0.070 0.010 0.010 0.0018
0.05 -- -- -- -- -- -- -- 0.10 Q 1.40 0.40 0.58 0.060 0.008 0.011
0.0037 0.03 -- -- -- -- -- -- -- 0.10 R 0.96 0.61 0.59 0.050 0.008
0.011 0.0044 0.01 -- -- -- -- -- -- -- -- S 0.89 0.69 0.70 0.080
0.008 0.010 0.0053 0.25 -- -- -- -- -- -- -- -- * The remainder:
iron and inevitable impurities other than P and S
[0053] For each of the produced hot-rolled wire rods, an unsteady
portion of an edge (i.e., an end of the hot-rolled wire rod) was
cut off, and then a good edge was sampled for evaluation of the
hot-rolled wire rod. Specifically, the hot-rolled wire rod was
evaluated in the following manner on wire diameter, [Ti*], amount
of dissolved N, maximum length of proeutectoid cementite,
microstructure, and tensile strength TS. In Table 2, "heating
temperature" represents furnace temperature before hot rolling, and
area reduction strain c represents the total area reduction strain
c over the last four passes (four passes from the last pass to the
last pass but three) of rolling. In addition, "average cooling
rate" represents an average cooling rate from start of placing to a
point of 800.degree. C. For test No. 5, however, an average cooling
rate from start of placing to a point of 750.degree. C. was
obtained.
(Evaluation of Distribution State of Tic and Amount of Dissolved
N)
[0054] [Ti*] and the amount of dissolved N were evaluated with
electroextraction residue measurement. In this measurement, a 10%
acetylacetone solution was used for extraction while a 0.1 .mu.m
mesh was used. The amount of Ti in compounds in a residue was
measured by inductively coupled plasma (ICP) emission spectrometry,
the amount of N in compounds and the amount of B in compounds
therein were measured by absorption photometry, and the amount of
AIN therein was measured by a bromoester method. The sample amount
was 3 g for the bromoester method, and 0.5 g for each of the
emission spectrometry and the absorption photometry. Since the
precipitation state of TiC is changed only after heating treatment
at 1000.degree. C. or higher, the precipitation state may be
determined for a steel wire subjected to a drawing process or
hot-dip galvanization. Using such values, the amount of [Ti*] was
determined based on "[Ti*]=total Ti amount-amount of Ti in
compounds having a size 0.1 .mu.m or more", and the amount of
dissolved N was determined based on "amount of dissolved N=total N
amount-amount of N in compounds".
(Evaluation of Tensile Strength TS and Microstructure of Hot-Rolled
Wire Rod)
[0055] An edge sample of each hot-rolled wire rod was subjected to
a tensile test to determine the tensile strength TS of the
hot-rolled wire rod. At this time, the average for three tests
(n=3) was obtained. A similar edge sample was buried in a resin and
observed by a scanning electron microscope (SEM) to evaluate a
state of the proeutectoid cementite. The sample was observed along
a section (cross section) perpendicular to a longitudinal direction
of the wire rod. The proeutectoid cementite was observed on a side
near the center with reference to a position of D/4 (D: diameter of
the wire rod) in the section, and the maximum length of the
proeutectoid cementite was measured. When an end of the
proeutectoid cementite was split into a plurality of branches, a
value of the sum of the lengths of the branches was obtained.
[0056] Table 2 shows the fabrication conditions of the samples and
evaluation results. Table 2 also shows a value of 0.0023.times.[C]
of each hot-rolled wire rod (C is the C content of the hot-rolled
wire rod).
TABLE-US-00002 TABLE 2 Hot-rolled wire rod Hot-rolling condition
Wire Maximum Heating Area Placing diameter of length of temper-
reduction temper- Average hot-rolled Amount of proeutectoid Test
Steel ature strain ature cooling rate wire rod [Ti *] 0.0023
.times. [C] dissolved N cementite Micro- TS No. type (.degree. C.)
.epsilon. (--) (.degree. C.) (.degree. C./sec) (mm) (mass %) (mass
%) (mass %) (.mu.m) structure (MPa) 1 A 1100 0.41 900 8 14.0 0.0070
0.0024 00003 10 P 1293 2 B 1050 0.47 850 8 13.0 0.0040 0.0021
0.0003 8 P 1266 3 C 1100 0.43 900 8 13.5 0.0102 0.0023 0.0002 7 P
1306 4 C 1100 0.47 1000 8 13.0 0.0017 0.0023 0.0005 18 P 1267 5 C
1100 0.43 800 8 13.5 0.0600 0.0023 0.0003 10 P 1306 6 C 1100 0.27
870 8 13.0 0.0018 0.0023 0.0003 17 P 1287 7 C 1100 0.60 870 25 13.0
0.0014 0.0023 0.0003 7 P + B 1341 8 D 1000 0.51 850 14 8.0 0.0041
0.0020 0.0004 3 P 1251 9 E 1000 0.46 900 11 10.0 0.0103 0.0024
0.0002 7 P 1421 10 F 1150 0.51 920 14 8.0 0.0064 0.0022 0.0004 7 P
1383 11 G 1150 0.51 850 14 8.0 0.0035 0.0019 0.0002 1 P 1277 12 H
1000 0.47 880 8 13.0 0.0321 0.0023 0.0005 11 P 1321 13 I 1000 0.46
850 12 9.0 0.0261 0.0021 0.0001 5 P 1259 14 J 1150 0.45 900 17 6.4
0.0037 0.0028 0.0003 13 P 1423 15 K 1100 0.46 900 18 6.0 0.0076
0.0020 0.0001 4 P 1279 16 L 1100 0.46 900 18 6.0 0.0042 0.0030
0.0011 13 P 1463 17 M 1100 0.43 870 7 16.0 0.0037 0.0018 0.0002 1 P
1216 18 N 1150 0.47 880 8 13.0 0.0079 0.0021 0.0001 6 P 1237 19 O
1150 0.48 940 14 8.0 0.0206 0.0025 0.0015 11 P 1357 20 P 1100 0.42
820 8 13.0 0.0102 0.0017 0.0005 0 P 1067 21 Q 1100 0.54 820 8 13.0
0.0201 0.0032 0.0004 22 P 1403 22 R 1100 0.43 850 8 13.0 0.0014
0.0022 0.0004 18 P 1256 23 S 1100 0.42 880 14 8.0 0.0700 0.0020
0.0001 2 P 1121
[0057] Each of the hot-rolled wire rods produced as above was
formed into a predetermined wire diameter by cold wire-drawing, and
was then dipped for about 30 sec in a hot-dip galvanization bath at
about 440 to 460.degree. C. to yield a hot-dip galvanized steel
wire. The tensile strength TS of the wire (hot-dip galvanized steel
wire) was evaluated by a tensile test. At this time, the average
for three tests (n=3) was obtained. In addition, a torsion value
was measured by a torsion test, and presence of longitudinal
cracking was determined from observation of a fracture pattern. For
the torsion value, the number of times of torsion before break was
normalized with a chuck-to-chuck distance of 100 mm, and the
average for three tests (n=3) was calculated. In the three torsion
tests, a sample showing at least one longitudinal crack was
determined to be sample with longitudinal cracking.
[0058] Table 3 shows the results of evaluation of the hot-dip
galvanized steel wire, the evaluation being made on wire diameter,
an area reduction ratio after cold wire-drawing, tensile strength
TS, tensile strength TS* obtained by the Formula (2), and presence
of longitudinal cracking.
TABLE-US-00003 TABLE 3 Galvanized steel wire Wire Area Torsion
value Presence of Test Steel diameter reduction ratio TS TS* (the
number longitudinal No. type (mm) (%) (MPa) (MPa) of times)
cracking 1 A 5.2 86.2 2103 1780 34 Not present 2 B 5.1 84.6 2034
1789 34 Not present 3 C 5.2 85.2 2140 1780 32 Not present 4 C Wire
breaking 5 C 5.2 85.2 2104 1780 13 Present 6 C Wire breaking 7 C
Wire breaking 8 D 2.9 86.9 2203 1981 42 Not present 9 E 3.7 86.3
2274 1911 31 Not present 10 F 2.8 87.8 2301 1990 46 Not present 11
G 2.9 86.9 2206 1981 36 Not present 12 H 5.1 84.6 2140 1789 44 Not
present 13 I 3.3 86.6 2168 1946 32 Not present 14 J 2.3 87.1 2301
2033 31 Not present 15 K 2.4 84.0 2268 2024 33 Not present 16 L 2.2
86.6 2311 2042 22 Not present 17 M 5.8 86.9 2312 1728 43 Not
present 18 N 5.2 84.0 2097 1780 37 Not present 19 O 3.2 84.0 2234
1955 21 Not present 20 P 4.5 88.0 1820 1841 34 Not present 21 Q
Wire breaking 22 R Wire breaking 23 S 3.2 84.0 2031 1955 11
Present
[0059] The following consideration can be made from such results.
Specifically, Test Nos. 1 to 3 and 8 to 19 each satisfy all the
requirements defined in the invention, in which a pearlite phase
occupies at least 90% (by area percent) of the microstructure
thereof. In addition, any defect such as wire breaking is not found
during wire-drawing, and good wire strength and good torsion
characteristics are shown after hot-dip galvanization treatment.
Among them, each of Test Nos. 16 and 19 has a slightly large amount
of dissolved N and a slightly low torsion value.
[0060] In contrast, Test Nos. 4 to 7 and 20 to 23 are examples that
each do not satisfy one of the requirements defined in the
invention (or do not satisfy a further preferred requirement), in
which a defect such as wire breaking is found during wire-drawing,
or wire strength or torsion characteristics is/are bad after
hot-dip galvanization treatment.
[0061] Among them, for Test No. 4, the placing temperature is as
high as 1000.degree. C., and the amount of [Ti*] is small (i.e.,
TiC is coarsened, and the maximum length of the proeutectoid
cementite exceeds 15 .mu.m.); hence, the proeutectoid cementite
cannot be suppressed, and wire breaking occurs during wire-drawing.
For Test No. 5, the placing temperature is as low as 800.degree.
C., and the amount of [Ti*] is excessively large (i.e., TiC is
excessively refined); hence, grain boundaries become brittle, and
longitudinal cracking occurs.
[0062] For Test No. 6, the area reduction strain .epsilon. over the
last four passes is small, and crystal grains are not sufficiently
refined, and thus the amount of [Ti*] is small (i.e., TiC is not
refined. Furthermore, the maximum length of the proeutectoid
cementite exceeds 15 .mu.m.); hence, the proeutectoid cementite
cannot be suppressed, and wire breaking occurs during wire-drawing.
For Test No. 7, cooling rate is high, and the microstructure
includes a mixed phase of pearlite and bainite (area ratio of
bainite: 40%); hence, drawability is degraded, and wire breaking
occurs during wire-drawing.
[0063] Test No. 20 is an example using steel (steel type P) having
a low C content, for which strength is low. Test No. 21 is an
example using steel (steel type Q) having an excessively high C
content, for which the proeutectoid cementite cannot be suppressed,
and wire breaking occurs.
[0064] Test No. 22 is an example using steel (steel type R) having
a low Ti content, for which the proeutectoid cementite cannot be
suppressed, and wire breaking occurs. Test No. 23 is an example
using steel (steel type S) having an excessively high Ti content,
for which the amount of [Ti*] is excessive, and longitudinal
cracking occurs.
INDUSTRIAL APPLICABILITY
[0065] The wire rod of the invention has good rod drawability and
high strength. Hence, the wire rod is preferred as a material for a
hot-dip galvanized steel wire or a steel wire strand as a material
for a rope for use in a bridge and the like, which is thus
extremely useful in industry.
* * * * *