Technical Field
[0001] The present invention relates to a wire rod and a steel wire using the same each
of which is used for or as prestressing steel wires and wire ropes.
Background Art
[0002] There are strong demands for concrete members to have a higher strength and a lighter
weight in civil engineering and construction areas. Prestressed concrete (hereinafter
also referred to as "PC") is well known as a way to strengthen such concrete members.
In the prestressed concrete, compression stress is applied to material concrete by
using steel wires. Such a steel wire for PC, namely, a prestressing steel wire (PC
steel wire), when having a higher strength, can contribute more satisfactorily to
a higher strength and a lighter weight of PC. There is now known a prestressing strand
including 7 wires with a diameter of 15.2 mm and having a maximum force of about 261
kN, as prescribed in Japanese Industrial Standard (JIS) G3536.
[0003] In addition to Japanese Industrial Standards, various standards (specifications)
and recommended tests are prescribed for prestressing steel wires from the viewpoint
of architectural safety. In particular, it is important to take delayed fracture resistance
into consideration when a high strength prestressing steel wire is applied. The delayed
fracture is a phenomenon where, when a steel is used for a long time under the application
of a stress, hydrogen migrated into the steel accumulates typically in a fine flaw
in the steel surface, causes a microstructure around the flaw to become brittle, and
thereby induces brittle fracture. The prestressing steel wires are used while being
always tensed and may possibly undergo delayed fracture. To prevent this, strict specifications
are prescribed on them. In particular, the prestressing steel wires are well known
to become more susceptible to delayed fracture with an increasing strength. Demands
are therefore made to develop steels that can less suffer from delayed fracture even
when having a higher strength.
[0004] Typically, Patent Literature 1 discloses a technique of improving delayed fracture
resistance of a prestressing steel wire having a carbon content of 0.6% to 1.1%. According
to the technique, a wire rod after wire drawing is subjected to blueing at a temperature
of 450°C or higher to spheroidize plate-like cementite to thereby improve the delayed
fracture resistance. The technique in Patent Literature 1, however, causes the steel
wire to have a low strength due to the spheroidization of plate-like cementite, thereby
has limitations in strength improvement, and disadvantageously fails to help the steel
wire to have a wire strength of 2000 MPa or more.
[0005] Patent Literature 2 discloses a technique of improving delayed fracture resistance
of a prestressing steel wire having a carbon content of 0.6% to 1.3%. The improvement
is achieved by imparting a compressive residual stress to a surface layer of the steel
wire and thereby forming a deformed pearlite in the surface layer. The technique in
Patent Literature 2, however, is applied to prestressing steel wires having a wire
strength of up to about 1600 MPa and may probably fail to sufficiently ensure resistance
to delayed fracture that is caused by hydrogen diffusion at higher wire strengths
of typically 2000 MPa or more.
[0006] Patent Literature 3 discloses a technique of improving delayed fracture resistance
in a tempered martensite phase of not a prestressing steel wire, but a bearing steel
having a carbon content of 0.65% to 1.20%. The improvement is achieved by dispersing
particles typically of Ti- or Al-containing nitrides having a particle size of 50
to 300 nm in an amount at a predetermined or higher so as to trap hydrogen. However,
hydrogen behaves differently in different microstructures, and dimensions, amounts,
and other factors of precipitates acting as appropriate trapping sites also differ
in different microstructures. This impedes the application of the technique in Patent
Literature 3 typically to a prestressing steel wire without modification, where the
prestressing steel wire includes a pearlite as a main phase. A manufacturing process
for such a bearing steel performs a quenching-tempering treatment after wire drawing;
whereas a manufacturing process for a prestressing steel wire performs wire drawing
after a patenting treatment. Thus, the two manufacturing processes significantly differ
from each other and employ different procedures to control precipitation typically
of nitrides.
[0007] EP 1 897 964 A1 discloses a high strength wire rod excelling in wire drawing performance and a process
for producing the same.
Citation List
Patent Literature
Summary of Invention
Technical Problem
[0009] An object of the present invention is to provide a wire rod including a pearlite
as a main phase, where the wire rod less suffers from reduction in delayed fracture
resistance even when having a high strength and is usable typically as a high strength
prestressing steel wire and wire rope that have such delayed fracture resistance as
to meet building standards.
Solution to Problem
[0010] The invention is defined in the claims.
[0011] The present inventors made investigations on inclusions having a hydrogen trap effect
in a wire rod including a pearlite as a main phase. As a result, they have found that
it is important to ensure AlN particles in an amount at a predetermined level or higher
and to ensure, of such AlN particles, AlN particles having a size of 10 to 20 µm in
an amount at a predetermined level or higher. Specifically, the present invention
provides, in an aspect, a wire rod including C in a content of 0.8% to 1.2% (in mass
percent, hereinafter the same for chemical composition); Si in a content of 0.1% to
2.0%; Mn in a content of 0.1% to 2.0%; N in a content of 0.002% to 0.010%; Al in a
content of 0.04% to 0.15%; P in a content of 0.02% or less (including 0%); and S in
a content of 0.02% or less (including 0%), with the remainder being iron and inevitable
impurities, in which the Al content and N content meet a condition specified by Expression
(1) given as follows:
where [Al] and [N] are contents (in mass percent) of Al and N, respectively; the
wire rod has a microstructure including 95 percent by area or more of a pearlite;
the wire rod has a content of AlN of 0.005% or more; and a percentage of AlN particles
having a diameter d
GM of 10 to 20 µm is 50% or more in number percent in an extreme value distribution
of maximum values of the diameters d
GM of AlN particles, where the diameter d
GM is represented by a geometrical mean (ab)
1/2 of a length "a" and a thickness "b" of an AlN particle. In a preferred embodiment,
the wire rod may have a solute nitrogen content of 0.003% or less.
[0012] In the present invention, the wire rod may further contain any of (a) at least one
element selected from the group consisting of Cr in a content of 1.0% or less (excluding
0%), Ni in a content of 1.0% or less (excluding 0%), Co in a content of 1.0% or less
(excluding 0%), Mo in a content of 1.0% or less (excluding 0%), and Cu in a content
of 0.5% or less (excluding 0%); and (b) at least one element selected from the group
consisting of B in a content of 0.005% or less (excluding 0%), Nb in a content of
0.5% or less (excluding 0%), and V in a content of 0.5% or less (excluding 0%).
The present invention also includes a steel wire obtained from the wire rod. Advantageous
Effects of Invention
[0013] The present invention adjusts the Al and N contents appropriately and controls the
total content of AlN particles and the content of AlN particles having a predetermined
size (d
GM of 10 to 20 µm) appropriately. The present invention can therefore provide a wire
rod having excellent delayed fracture resistance. The present invention, in the preferred
embodiment, adjusts a solute nitrogen content at a predetermined level or higher and
can thereby help the steel wire to have better twisting properties.
Description of Embodiments
[0014] After intensive investigations, the present inventors have found that, in a wire
rod including a pearlite as a main phase, it is effective to ensure AlN (particles)
in a predetermined content as a hydrogen trap site and to ensure AlN particles having
a size of 10 to 20 µm in a content at a predetermined level or higher.
[0015] The AlN content is specified to 0.005% or more, because the wire rod offers an increasing
hydrogen trap effect with an increasing AlN content. The AlN content is preferably
0.006% or more, more preferably 0.007% or more, and particularly preferably 0.01%
or more. Though not critical, the upper limit of the AlN content is generally about
0.04%.
[0016] A extreme value distribution of maximum values is herein employed as an index for
ensuring AlN particles having a size of 10 to 20 µm in a number at a predetermined
level or higher. Initially, a geometrical mean (ab)
1/2 of the length "a" and the thickness "b" of an AlN particle is employed as a size
of the AlN particle and is indicated as d
GM (µm). As used herein the term 'length "a"" of an AlN particle refers to the length
(dimension) of the AlN particle in the wire rod longitudinal direction; and the term
"thickness "b"" of the AlN particle refers to a dimension of the AlN particle in a
direction perpendicular to the wire rod longitudinal direction.
[0017] The term "extreme value distribution of maximum values of d
GM" refers to a distribution determined by measuring a maximum d
GM (max) among d
GM values of AlN particles present in a predetermined area; repeating this procedure
on two or more view fields; and subjecting the measured two or more d
GM (max) values to a statistical processing. The percentage of AlN particles having
a d
GM (max) of 10 to 20 µm is 50% or more (in number percent) in the extreme value distribution
in the embodiment of the present invention. If AlN particles having a size d
GM greater than 20 µm is present in a large number percent, AlN particles are present
in a smaller total number and may fail to exhibit the hydrogen trap effect sufficiently.
In addition, such AlN particles having a size d
GM less than 10 µm exhibit a lower hydrogen trap effect. AlN particles that are effective
for hydrogen trap can therefore be ensured sufficiently by controlling AlN particles
having a d
GM (max) of 10 to 20 µm to be present in a number percent of 50% or more in the extreme
value distribution.
[0018] The wire rod according to the embodiment of the present invention includes a pearlite
that occupies 95 percent by area or more of the main phase. The area percentage of
the pearlite is preferably 97% or more, and more preferably 100%.
[0019] Next, chemical compositions of the wire rod according to the embodiment of the present
invention will be illustrated below.
Carbon (C) content: 0.8% to 1.2%
[0020] Carbon (C) element effectively contributes to a higher strength. The wire rod and
a steel wire after cold working have higher strengths with an increasing carbon content.
The carbon content is therefore specified to 0.8% or more. The carbon content is preferably
0.85% or more, and more preferably 0.90% or more. However, carbon, if present in an
excessively high content, may cause aging embrittlement during cold wire drawing,
thereby cause the steel wire to have inferior toughness, and disadvantageously invite
cracking during stranding. To prevent this, the carbon content is specified to 1.2%
or less. The carbon content is preferably 1.1% or less, and more preferably 1.05%
or less.
Silicon (Si) content: 0.1% to 2.0%
[0021] Silicon (Si) element not only acts as a deoxidizer, but also effectively has actions
of helping the wire rod to have a higher strength and to offer better relaxation properties.
When hot dip galvanizing is applied to the wire rod, silicon element also offers an
action of suppressing strength reduction occurring upon galvanizing. To exhibit these
actions effectively, the Si content is specified to 0.1% or more. The Si content is
preferably 0.2% or more, and more preferably 0.4% or more. In contrast, Si, if present
in an excessively high content, may cause the wire rod to have inferior cold wire
drawability and to suffer from a higher breakage ratio. To prevent this, the Si content
is specified to 2.0% or less. The Si content is preferably 1.8% or less, and more
preferably 1.5% or less.
Manganese (Mn) content: 0.1% to 2.0%
[0022] Manganese (Mn) element not only act as a deoxidizer as with Si, but also particularly
has an action of fixing sulfur (S) in the steel as MnS and helping the steel to have
better toughness and ductility. To exhibit these actions effectively, the Mn content
is specified to 0.1% or more. The Mn content is preferably 0.15% or more, and more
preferably 0.2% or more. However, manganese element is readily segregated and, if
added in excess, may cause the formation of supercooled phases such as martensite
because of excessively increased hardenability of a region where Mn is segregated.
To prevent this, the Mn content is specified to 2.0% or less. The Mn content is preferably
1.8% or less, and more preferably 1.5% or less.
Nitrogen (N) content: 0.002% to 0.010%
[0023] Nitrogen (N) element is important for the formation of AlN that features the embodiment
of the present invention and is contained in a content of 0.002% or more. The nitrogen
content is preferably 0.0025% or more, more preferably 0.0030% or more, and particularly
preferably 0.0040% or more. However, nitrogen, if added in excess, may cause the wire
rod to have inferior twisting properties due to an increased solute nitrogen content.
This is because nitrogen dissolves as an interstitial element in the steel as with
carbon and causes embrittlement due to strain aging. To prevent this, the nitrogen
content is specified to 0.010% or less. The nitrogen content is preferably 0.0090%
or less, and more preferably 0.0080% or less.
Solute nitrogen content: 0.003% or less
[0024] Solute nitrogen causes inferior twisting properties and is preferably minimized in
amount. The solute nitrogen content is therefore preferably 0.003% or less, more preferably
0.002% or less, and furthermore preferably 0.001% or less. The solute nitrogen content
may be controlled typically by adjusting the contents of nitride-forming elements
such as Al, B, and Nb; and the nitrogen content.
Aluminum (Al) content: 0.04% to 0.15% and [Al]≤-2.1×10×[N]+0.255
[0025] Aluminum (Al) element acts as a deoxidizer and is important herein, because aluminum
is combined with nitrogen to form AlN, thereby traps hydrogen, and helps the wire
rod to have better delayed fracture resistance. The aluminum nitride AlN also effectively
contributes to grain refinement by a pinning effect. To exhibit these effects effectively,
the Al content is specified to 0.04% or more. The Al content is preferably 0.05% or
more, and more preferably 0.055% or more. In contrast, Al, if present in an excessively
high content particularly in a range of high nitrogen contents, may form coarse AlN
particles, and this may reduce the hydrogen trap effect of AlN. To prevent this, the
Al content is specified to 0.15% in terms of its upper limit and is adapted to meet
a condition specified by Expression (1) given as follows.
[Math. 1]
[0026] In Expression (1), [Al] and [N] denote contents (in mass percent) of Al and N, respectively.
Expression (1) is an expression that has been derived from many experimental examples
in which delayed fracture resistance was examined at varying nitrogen contents and
aluminum contents. When the Al content meets the condition specified by Expression
(1), the upper limit of the Al content is more strictly controlled in a range of high
nitrogen contents so as to suppress the formation of coarse AlN particles. The Al
content is preferably 0.14% or less, and more preferably 0.12% or less in terms of
its upper limit.
Phosphorus (P) content: 0.02% or less
[0027] Phosphorus (P) element is segregated at a prior austenite grain boundary, makes the
grain boundary brittle, and causes the wire rod to have inferior fatigue properties.
To prevent this, the phosphorus content is preferably minimized and is specified herein
to 0.02% or less. The phosphorus content is preferably 0.015% or less, and more preferably
0.010% or less.
Sulfur (S) content: 0.02% or less
[0028] Sulfur (S) element is segregated at a prior austenite grain boundary, makes the grain
boundary brittle, and causes the wire rod to have inferior fatigue properties, as
with phosphorus. To prevent this, the sulfur content is preferably minimized and is
herein specified to 0.02% or less. The sulfur content is preferably 0.015% or less,
and more preferably 0.010% or less.
[0029] The wire rod according to the embodiment of the present invention has a basic chemical
composition as above, with the remainder substantially being iron. However, inevitable
impurities are naturally acceptable, where the inevitable impurities are brought into
the steel under conditions typically of raw materials, facility materials, and manufacturing
facilities and are contained in the steel. According to necessity to have further
better properties such as strength, toughness, and ductility, the wire rod according
to the embodiment of the present invention may further contain any of elements as
follows.
[0030] At least one element selected from the group consisting of Cr in a content of 1.0%
or less (excluding 0%), Ni in a content of 1.0% or less (excluding 0%), Co in a content
of 1.0% or less (excluding 0%), Mo in a content of 1.0% or less (excluding 0%), and
Cu in a content of 0.5% or less (excluding 0%)
[0031] Chromium (Cr) element has actions of reducing lamellar spacing of pearlite and helping
the wire rod to have a higher strength and better toughness. To exhibit these actions
effectively, the Cr content is preferably 0.05% or more, more preferably 0.1% or more,
and furthermore preferably 0.2% or more. In contrast, Cr, if present in an excessively
high content, may cause the wire rod to have higher hardenability and thereby increase
the risk of the formation of a supercooled phase during hot rolling. To prevent this,
the Cr content is preferably 1.0% or less, more preferably 0.6% or less, and furthermore
preferably 0.5% or less.
[0032] Nickel (Ni) element helps the steel wire after wire drawing to have better toughness.
To exhibit such actions effectively, the Ni content is preferably 0.05% or more, more
preferably 0.1% or more, and furthermore preferably 0.2% or more. However, Ni, if
added in excess, may exhibit saturated effects, thus being economically useless. To
prevent this, the Ni content is preferably 1.0% or less, more preferably 0.7% or less,
and furthermore preferably 0.6% or less.
[0033] Cobalt (Co) element has actions of reducing pro-eutectoid cementite (particularly
at a high carbon content) and helping the wire rod to more readily control its microstructure
to be a homogeneous pearlite. To exhibit these actions effectively, the Co content
is preferably 0.05% or more, more preferably 0.1% or more, and furthermore preferably
0.2% or more. However, Co, if added in excess, may exhibit saturated effects, thus
being economically useless. To prevent this, the Co content is preferably 1.0% or
less, more preferably 0.8% or less, and furthermore preferably 0.6% or less.
[0034] Molybdenum (Mo) element helps the steel wire to have better corrosion resistance.
To exhibit such actions effectively, the Mo content is preferably 0.05% or more, and
more preferably 0.1% or more. However, Mo, if present in an excessively high content,
may cause the formation of a supercooled phase more readily during hot rolling and
cause the wire rod to have inferior ductility. To prevent this, the Mo content is
preferably 1.0% or less, more preferably 0.5% or less, and furthermore preferably
0.3% or less.
[0035] Copper (Cu) element helps the steel wire to have better corrosion resistance. To
exhibit such actions effectively, the Cu content is preferably 0.05% or more, and
more preferably 0.08% or more. In contrast, Cu, if present in an excessively high
content, may react with sulfur to be segregate as CuS in a grain boundary region and
thereby cause a flaw to be generated during the wire rod manufacturing. To avoid such
influence, the Cu content is preferably 0.5% or less, more preferably 0.2% or less,
and furthermore preferably 0.18% or less.
[0036] At least one element selected from the group consisting of B in a content of 0.005%
or less (excluding 0%), Nb in a content of 0.5% or less (excluding 0%), and V in a
content of 0.5% or less (excluding 0%)
[0037] Boron (B) element has actions of preventing the formation of pro-eutectoid ferrite
and pro-eutectoid cementite and helping the wire rod to readily control its microstructure
to be a homogeneous pearlite. Boron also has actions of fixing, as boron nitride (BN),
excess solute nitrogen after the precipitation of AlN, suppressing strain aging caused
by the solute nitrogen, and helping the wire rod to have better toughness. In addition,
solute boron itself has an action of helping the wire rod to have better toughness.
To exhibit such actions effectively, the boron content is preferably 0.0003% or more,
more preferably 0.0005% or more, and furthermore preferably 0.001% or more. In contrast,
boron, if present in an excessively high content, may cause the precipitation of a
compound with iron, i.e., an Fe-B compound such as FeB
2 and cause cracks upon hot rolling. To prevent this, the boron content is preferably
0.005% or less, more preferably 0.004% or less, and furthermore preferably 0.003%
or less.
[0038] Niobium (Nb) element forms a nitride with excess solute nitrogen after the precipitation
of AlN and contributes to grain refinement. In addition, the element also advantageously
fixes solute nitrogen and thereby suppresses aging embrittlement. To exhibit such
actions effectively, the Nb content is preferably 0.01% or more, more preferably 0.03%
or more, and furthermore preferably 0.05% or more. However, Nb, if present in an excessively
high content, may exhibit saturated effects, thus being economically useless. To prevent
this, the Nb content is preferably 0.5% or less, more preferably 0.4% or less, and
furthermore preferably 0.2% or less.
[0039] Vanadium (V) element forms a nitride with excess solute nitrogen after the precipitation
of AlN and contributes to grain refinement, as with Nb. In addition, vanadium also
fixes solute nitrogen and thereby suppresses aging embrittlement. To exhibit such
actions effectively, the vanadium content is preferably 0.01% or more, more preferably
0.02% or more, and furthermore preferably 0.03% or more. However, vanadium, if present
in an excessively high content, may exhibit saturated effects, thus being economically
useless. To prevent this, the vanadium content is preferably 0.5% or less, more preferably
0.4% or less, and furthermore preferably 0.2% or less.
[0040] A regular wire rod (referring to one before cold wire drawing) can be manufactured
generally by preparing a steel ingot having appropriately controlled chemical compositions
by ingot-making, and subjecting the ingot to blooming and hot rolling (where necessary,
further to a patenting treatment). It should be noted, however, that the wire rod
according to the embodiment of the present invention is intended to control the content
and particle size distribution of AlN particles appropriately, where the particle
size distribution is controlled so that the percentage of AlN particles having a size
d
GM of 10 to 20 µm be 50% or more (in number percent) in the d
GM extreme value distribution of maximum values of AlN particles. For this purpose,
it is important to appropriately control the Al and N contents within the above-specified
ranges and to appropriately control a thermal hysteresis in a temperature range in
which AlN is precipitated.
[0041] In the steel, AlN begins to be precipitated at about 1300°C or lower, precipitated
in a larger amount with a falling temperature, and completely precipitated at about
900°C. During manufacturing processes, blooming and hot rolling processes significantly
affect the precipitation behavior of AlN because the steel is exposed to temperatures
within the above-mentioned ranges in these processes. Accordingly, blooming and hot
rolling conditions should be appropriately controlled In general, cooling after blooming
is performed at a low cooling rate and thereby often causes precipitated AlN particle
to coarsen. In contrast, cooling after hot rolling is performed at a relatively high
cooling rate and thereby allows precipitated AlN particles to be fine.
[0042] Specifically, blooming may be performed at a heating temperature of 1230°C to 1280°C
and a cooling rate of 0.2°C/second or more. Blooming, when performed at a high heating
temperature and at a high cooling rate, can prevent precipitation and coarsening of
AlN particles. For this reason, the blooming temperature is preferably 1230°C or higher,
and more preferably 1240°C or higher. In contrast, blooming, if performed at an excessively
high heating temperature, may cause quenching cracks. To prevent this, the blooming
temperature is preferably 1280°C or lower, and more preferably 1270°C or lower in
terms of its upper limit. The cooling rate is preferably 0.2°C/second or more, more
preferably 0.4°C/second or more, and furthermore preferably 0.5°C/second or more.
The cooling rate is not limited in its upper limit, but is typically 1.5°C/second
or less, and preferably 1.2°C/second or less.
[0043] A billet obtained by blooming is hot-rolled, cooled down to 850°C to 950°C typically
by water cooling, and placed in the form of a coil. Fine AlN particles (having a d
GM of 10 to 20 µm) can be precipitated by placing the coil-form wire rod at a relatively
low temperature. For this reason, the placing temperature is preferably 950°C or lower,
more preferably 940°C or lower, and furthermore preferably 920°C or lower. In contrast,
placing, if performed at an excessively low temperature, may cause very fine AlN particles
to be precipitated in a large number, where such very fine AlN particles do not contribute
to hydrogen trap. To prevent this, the placing temperature is preferably 850°C or
higher, more preferably 870°C or higher, and furthermore preferably 890°C or higher.
[0044] The content and particle size distribution of AlN particles may be not appropriately
controllable typically when at least part of the blooming and hot rolling conditions
does not meet the above-specified conditions. In this case, it is also effective to
perform a patenting treatment in an appropriate temperature range after hot rolling.
The patenting treatment is preferably performed at a re-heating temperature of 880°C
to 1000°C and a patenting temperature of 530°C to 620°C. If the work after hot rolling
has a low AlN content, the re-heating temperature may be set to be relatively low
(e.g., about 880°C to about 940°C) so as to increase the amount of AlN precipitation.
If the work after hot rolling includes coarsened AlN particles, the re-heating temperature
may be set to be relatively high (e.g., 940°C to 1000°C) so as to allow the coarsened
AlN particles to be once dissolved in the steel and to be precipitated again.
[0045] The wire rod according to the embodiment of the present invention sufficiently includes
AlN particles capable of effectively acting as hydrogen trap sites, can thereby give
steel wires such as wire ropes and prestressing steel wires having excellent delayed
fracture resistance, and are useful The present invention, in another aspect, also
includes such steel wires.
Examples
[0046] The present invention will be illustrated in further detail with reference to several
working examples below. It should be noted, however, that the examples are by no means
intended to limit the scope of the invention; that various changes and modifications
can naturally be made therein without deviating from the spirit and scope of the invention
as described herein; and all such changes and modifications should be considered to
be within the scope of the invention.
[0047] Steel ingots having chemical compositions given in Table 1 were subjected to blooming,
hot rolling, and processing into wire rod coils, and some of them were further subjected
to a patenting treatment under conditions given in Table 2. Samples were sampled from
the resulting works and subjected to an extraction residue measurement to determine
the total content of AlN particles and to cross-section observation to evaluate the
distribution of AlN particles. The results are indicated in Table 2.
1. AlN Total Content and Solute Nitrogen Content Measurements
[0048] An electrolytic extraction residue measurement with a 10% acetylacetone solution
using a 0.1-µm mesh was performed as the extraction residue measurement, in which
the amount of AlN particles in the residue was measured by the bromoester method.
Independently, the solute nitrogen content was determined by measuring the content
of nitrogen compounds including AlN by indophenol absorption spectrophotometry and
subtracting the content from the total nitrogen content in the steel. The sample weights
were 3 g in the bromoester method and 0.5 g in the absorption spectrophotometry.
2. AlN Distribution Measurement
[0049] The measurement was performed in the following manner. A sample was cut out from
each wire rod in a cross section including the wire rod axis and being in parallel
to the wire rod longitudinal direction so that the total area of two areas from the
surface layer to a position of one-fourth (D/4) the diameter D of the wire rod be
140 mm
2. Specifically, the length L of the sample was determined so that L×D/4+L×D/4=L×D/2
be 140 mm
2. The size of an AlN particle having the maximum size in an observation view field
in the cross section was measured according to JIS G0555, and this measurement was
performed in arbitrary twenty (20) view fields. In the measurement, Group D and Group
DS inclusions as specified in JIS G0551 were regarded as AlN particles, and the geometrical
mean (ab)
1/2 of the length (a) and thickness (b) of each AlN particle was employed as the size
of the AlN particle.
[0050] Next, the above-obtained wire rod coils were subjected to wire drawing to give steel
wires, and the tensile strength (wire strength) of each steel wire was measured. The
steel wires were further subjected to stranding and hot stretching to give strands
having strand diameters and strand structures as given in Table 2, and the rope strength,
delayed fracture resistance, and twisting properties of each strand were measured.
The results are indicated in Table 3.
3. Steel Wire Tensile Strength (Wire Strength) Measurement
[0051] The tensile strength of each steel wire was measured according to JIS Z2241.
4. Rope Strength Measurement
[0052] As the rope strength, the maximum force of a sample in a tensile test was measured
according to JIS G3536.
5. Delayed Fracture Resistance Measurement
[0053] The delayed fracture property (delayed fracture resistance) was measured in the following
manner. Each of twelve (12) samples was immersed in a 20 percent by mass ammonium
thiocyanate solution at 50°C under a load of 0.8 p.u according to the description
in Literature 1 (fib Bulletin No. 30: Acceptance of stay cable systems using prestressing
steels, January 2005), and a time period until the sample was broken was measured.
The term "0.8 p.u" refers to a load of 80% of a breaking load. A test sample having
a minimum rupture time of 2 hours or longer and a median rupture time of 5 hours or
longer was accepted herein.
6. Twisting Properties Measurement
[0054] For the twisting properties, a test sample having a twisting number of 3 or more
(capable of twisted three times or more) according to FKK HTS-26 Standard of the FKK
Freynessit system was accepted herein.
[Table 1]
Steel type |
Chemical composition (in mass percent) *with the remainder being iron and inevitable
impurities |
Upper limit of Al content (right-hand value of Expression (1)) |
C |
Si |
Mn |
N |
Al |
P |
S |
Cr |
Cu |
Co |
Mo |
Ni |
Nb |
V |
B |
A |
0.92 |
1.18 |
0.48 |
0.0040 |
0.06 |
0.010 |
0.010 |
0.19 |
|
|
|
|
|
|
|
0.17 |
B |
0.80 |
0.61 |
0.51 |
0.0025 |
0.10 |
0.011 |
0.006 |
|
|
|
|
|
|
|
|
0.20 |
C |
0.90 |
1.19 |
0.50 |
0.0053 |
0.06 |
0.008 |
0.008 |
|
|
|
|
|
|
|
|
0.14 |
D |
0.86 |
1.21 |
0.71 |
0.0080 |
0.05 |
0.010 |
0.010 |
|
|
|
|
|
0.09 |
|
|
0.09 |
E |
1.05 |
0.30 |
1.78 |
0.0032 |
0.07 |
0.010 |
0.011 |
|
|
|
|
|
|
|
|
0.19 |
F |
0.92 |
0.81 |
0.51 |
0.0020 |
0.06 |
0.007 |
0.010 |
|
0.08 |
|
|
0.28 |
|
|
|
0.21 |
G |
0.84 |
1.82 |
0.20 |
0.0061 |
0.12 |
0.010 |
0.020 |
|
|
|
|
|
|
|
0.0048 |
0.13 |
H |
0.92 |
1.10 |
0.48 |
0.0048 |
0.07 |
0.020 |
0.008 |
0.28 |
|
|
|
|
|
|
0.0028 |
0.15 |
I |
0.90 |
0.89 |
0.81 |
0.0068 |
0.09 |
0.007 |
0.010 |
|
|
|
|
|
|
|
0.0039 |
0.11 |
J |
1.20 |
0.40 |
0.49 |
0.0096 |
0.05 |
0.008 |
0.012 |
|
|
0.18 |
0.08 |
|
0.07 |
|
|
0.05 |
K |
0.85 |
0.58 |
0.61 |
0.0042 |
0.13 |
0.006 |
0.008 |
|
|
|
|
|
|
|
|
0.17 |
L |
1.30 |
0.79 |
0.51 |
0.0072 |
0.08 |
0.010 |
0.007 |
|
|
|
|
|
|
|
|
0.10 |
M |
0.70 |
1.32 |
0.81 |
0.0036 |
0.10 |
0.015 |
0.011 |
|
|
|
|
|
|
|
|
0.18 |
N |
0.93 |
0.71 |
0.60 |
0.0028 |
0.03 |
0.010 |
0.010 |
|
|
|
|
|
|
|
|
0.20 |
O |
1.10 |
0.38 |
0.81 |
0.0029 |
0.17 |
0.008 |
0.013 |
|
|
|
|
|
|
|
|
0.19 |
P |
0.85 |
0.39 |
0.68 |
0.0010 |
0.07 |
0.010 |
0.010 |
|
|
|
|
|
|
|
|
- |
Q |
0.90 |
0.40 |
0.58 |
0.0120 |
0.06 |
0.008 |
0.011 |
|
|
|
|
|
|
|
|
- |
R |
0.96 |
0.61 |
0.59 |
0.0090 |
0.11 |
0.008 |
0.011 |
|
|
|
|
|
|
|
|
0.07 |
S |
0.96 |
1.20 |
0.30 |
0.0050 |
0.08 |
0.008 |
0.010 |
|
|
|
|
|
|
0.10 |
|
0.06 |
[Table 2]
Test number |
Steel type |
Blooming |
Hot rolling |
Patenting |
Microstructure |
Solute nitrogen content (ppm) |
AlN content (ppm) |
AlN distribution |
Blooming temperature |
Cooling rate |
Placing temperature |
Rolled wire diameter |
Heating temperature |
Patenting temperature |
(°C) |
(°C/s) |
(°C) |
(mm) |
(°C) |
(°C) |
1 |
A |
1260 |
0.5 |
950 |
18.0 |
- |
- |
P |
15 |
73 |
○ |
2 |
B |
1240 |
0.8 |
850 |
16.0 |
- |
- |
P |
3 |
64 |
○ |
3 |
C |
1260 |
0.4 |
900 |
14.0 |
- |
- |
P |
12 |
120 |
○ |
4 |
C |
1200 |
0.4 |
900 |
14.0 |
- |
- |
P |
5 |
140 |
× |
5 |
C |
1260 |
0.5 |
950 |
14.0 |
960 |
550 |
P |
11 |
121 |
○ |
6 |
C |
1250 |
0.1 |
900 |
14.0 |
- |
- |
P |
3 |
150 |
× |
7 |
C |
1230 |
0.5 |
1050 |
14.0 |
- |
- |
P |
13 |
37 |
× |
8 |
C |
1280 |
0.6 |
750 |
14.0 |
- |
- |
P |
11 |
113 |
× |
9 |
C |
1280 |
0.8 |
940 |
14.0 |
- |
- |
P |
30 |
62 |
○ |
10 |
C |
1250 |
0.5 |
1000 |
14.0 |
970 |
570 |
P |
12 |
113 |
○ |
11 |
C |
1300 |
0.5 |
Quenching cracks developed upon blooming |
12 |
C |
1260 |
0.5 |
900 |
14.0 |
900 |
500 |
P+B |
9 |
118 |
○ |
13 |
D |
1260 |
0.5 |
800 |
16.0 |
880 |
590 |
P |
2 |
72 |
○ |
14 |
E |
1250 |
0.5 |
850 |
14.0 |
910 |
540 |
P |
5 |
64 |
○ |
15 |
F |
1240 |
0.5 |
820 |
12.0 |
930 |
550 |
P |
2 |
53 |
○ |
16 |
G |
1280 |
0.8 |
850 |
10.0 |
940 |
610 |
P |
6 |
161 |
○ |
17 |
H |
1260 |
0.7 |
800 |
6.0 |
940 |
570 |
P |
1 |
138 |
○ |
18 |
I |
1270 |
0.5 |
850 |
5.5 |
1000 |
590 |
P |
2 |
223 |
○ |
19 |
J |
1260 |
0.5 |
900 |
7.0 |
940 |
540 |
P |
1 |
69 |
○ |
20 |
K |
1250 |
0.5 |
900 |
8.0 |
950 |
550 |
P |
1 |
106 |
○ |
21 |
L |
1250 |
0.5 |
900 |
14.0 |
960 |
550 |
P |
11 |
174 |
○ |
22 |
M |
1260 |
0.5 |
870 |
12.0 |
960 |
620 |
P |
3 |
97 |
○ |
23 |
N |
1240 |
0.5 |
880 |
13.0 |
960 |
560 |
P |
12 |
47 |
○ |
24 |
O |
1250 |
0.5 |
880 |
8.0 |
960 |
530 |
P |
4 |
65 |
○ |
25 |
P |
1250 |
0.5 |
820 |
13.0 |
960 |
580 |
P |
3 |
12 |
× |
26 |
Q |
1260 |
0.5 |
820 |
13.0 |
960 |
580 |
P |
40 |
206 |
× |
27 |
R |
1250 |
0.5 |
850 |
13.0 |
960 |
580 |
P |
6 |
165 |
× |
28 |
S |
1250 |
0.5 |
880 |
8.0 |
950 |
580 |
P |
8 |
123 |
○ |
[Table 3]
Test number |
Steel type |
Wire strength |
Strand diameter |
Strand structure |
Maximum force |
Minimum rupture time |
Median rupture time |
Number of twisting |
Remarks |
(MPa) |
(mm) |
(kN) |
(hour) |
(hour) |
(time) |
1 |
A |
2240 |
15.2 |
7-wire |
346 |
2.5 |
5.3 |
31 |
High strength |
2 |
B |
2169 |
15.2 |
7-wire |
335 |
3 |
5.2 |
29 |
|
3 |
C |
2176 |
15.2 |
7-wire |
336 |
2.3 |
5.8 |
32 |
|
4 |
C |
2143 |
15.2 |
7-wire |
331 |
0.6 |
2.5 |
26 |
|
5 |
C |
2137 |
15.2 |
7-wire |
330 |
3.1 |
6.3 |
28 |
|
6 |
C |
2130 |
15.2 |
7-wire |
329 |
0.5 |
2.7 |
21 |
|
7 |
C |
2143 |
15.2 |
7-wire |
331 |
0.7 |
2.5 |
22 |
|
8 |
C |
2169 |
15.2 |
7-wire |
335 |
0.5 |
2.7 |
21 |
|
9 |
C |
2169 |
15.2 |
7-wire |
335 |
2.2 |
6.3 |
8 |
|
10 |
C |
2182 |
15.2 |
7-wire |
337 |
3.5 |
6.8 |
26 |
|
11 |
C |
- |
|
12 |
C |
Numerous breaks developed upon wire drawing |
|
13 |
D |
2272 |
12.7 |
7-wire |
231 |
3.5 |
6.8 |
26 |
Good ductility |
14 |
E |
2213 |
12.7 |
7-wire |
225 |
2.2 |
6.2 |
38 |
|
15 |
F |
2207 |
21.8 |
19-wi re |
667 |
9 |
14.2 |
46 |
Large twisting number |
16 |
G |
2187 |
21.8 |
19-wire |
661 |
4 |
5.9 |
51 |
Large twisting number |
17 |
H |
2116 |
28.6 |
19-wire |
1062 |
3.2 |
6.1 |
48 |
High strength and large twisting number |
18 |
I |
2090 |
28.6 |
19-wire |
1049 |
3.5 |
5.6 |
47 |
Large twisting |
19 |
J |
2111 |
15.2 |
7-wire |
326 |
10 |
15.7 |
28 |
Good ductility |
20 |
K |
2040 |
15.2 |
7-wire |
315 |
11 |
16.3 |
25 |
21 |
L |
Numerous breaks developed upon wire drawing |
|
22 |
M |
1823 |
15.2 |
7-wire |
257 |
- |
- |
- |
- |
23 |
N |
2046 |
15.2 |
7-wire |
316 |
0.6 |
2.6 |
26 |
|
24 |
O |
Numerous breaks developed upon wire drawing |
|
25 |
P |
1998 |
28.6 |
19-wire |
1003 |
0.4 |
3.1 |
31 |
|
26 |
Q |
2096 |
28.6 |
19-wire |
1052 |
1.4 |
4.3 |
2 |
|
27 |
R |
2197 |
28.6 |
19-wire |
1103 |
0.6 |
3.6 |
16 |
|
28 |
S |
2194 |
21.8 |
19-wire |
663 |
2.5 |
5.4 |
34 |
|
[0055] Test Nos. 1 to 3, 5, 9, 10, and 13 to 20 had chemical compositions, microstructures,
AlN contents, and AlN distributions respectively meeting the conditions specified
in the present invention, thereby achieved a wire strength of 2000 MPa or more (preferably
2100 MPa or more), offered such a high strand strength as to meet a criterion prescribed
in JIS G3536, still had good delayed fracture resistance, and could give high-strength
strands that are practically workable. In addition, these test samples also met the
condition for solute nitrogen content and thereby offered excellent twisting properties.
Of the samples according to the embodiment of the present invention, Test Nos. 15
to 18 were samples particularly having a reduced solute nitrogen content and thereby
offered very excellent twisting properties; whereas Test No. 9 had a highest solute
nitrogen content and offered a smallest number of twisting among the samples according
to the embodiment of the present invention.
[0056] Test Nos. 10, 15, and 17 underwent hot rolling performed at a placing temperature
out of the range of preferred condition, but underwent an appropriate patenting treatment
thereafter, and could give wire rods meeting the conditions specified in the present
invention.
[0057] In contrast, Test Nos. 4, 6 to 8, 11, 12, and 21 to 27 were samples that failed to
meet any of the conditions specified in the present invention or were manufactured
under a condition not meeting the manufacturing conditions required for obtaining
steels according to the embodiment of the present invention.
[0058] Test No. 4 underwent blooming performed at a low heating temperature; and Test No.
6 underwent cooling performed at a low cooling rate after blooming. These samples
each suffered from precipitation of coarse AlN particles, had an AlN particle size
distribution not meeting the condition specified in the present invention, and offered
inferior delayed fracture resistance.
[0059] Test No. 7 underwent placing performed at an excessively high temperature after hot
rolling, suffered from insufficient precipitation of AlN particles during placing,
had an AlN content and an AlN particle size distribution both not meeting the conditions
specified in the present invention, and offered inferior delayed fracture resistance.
Test No. 8 underwent placing performed at an excessively low temperature after hot
rolling, suffered from excessive refinement of AlN particles, thereby had an AlN particle
size distribution not meeting the condition specified in the present invention, and
offered inferior delayed fracture resistance.
[0060] Test No. 11 underwent blooming performed at an excessively high heating temperature
and suffered from quenching cracks.
[0061] Test No. 12 underwent a patenting treatment performed at an excessively low temperature,
thereby had a microstructure including a mixture (P+B) of bainite (B) and pearlite
(P) phases, and offered inferior wire drawability. This sample had a bainite fraction
of about 20 percent by area.
[0062] Test No. 21 was a sample having an excessively high carbon content, underwent significant
aging embrittlement during wire drawing, and suffered from numerous breaks. Test No.
22 was a sample having an excessively low carbon content and failed to offer a strength
corresponding to strand B type as prescribed in JIS G3536.
[0063] Test No. 23 was a sample having an excessively low Al content, failed to include
AlN particles in a sufficient amount, and offered inferior delayed fracture resistance.
Test No. 24 was a sample having a nitrogen content within the range specified in the
present invention but being relatively low and having an excessively high Al content,
suffered from the formation of Al-containing oxides in a large amount, and suffered
from numerous breaks upon wire drawing.
[0064] Test No. 25 was a sample having an excessively low nitrogen content, failed to include
AlN particles in a sufficient amount, had an AlN particle size distribution not meeting
the conditions specified in the present invention, and offered inferior delayed fracture
resistance. Test No. 26 was a sample having an excessively high nitrogen content,
suffered from the precipitation of coarse AlN particles, and thereby offered inferior
delayed fracture resistance. Test No. 26 had a solute nitrogen content not meeting
the preferred condition in the present invention and had a smallest number of twisting
among the entire test samples.
[0065] Test No. 27 was a sample having a nitrogen content within the range specified in
the present invention, but being relatively high, and having an Al content not meeting
the condition specified by Expression (1), suffered from the precipitation of coarse
AlN particles, and offered inferior delayed fracture resistance.
1. Walzdraht, bestehend aus: in Massenprozent, im Folgenden dasselbe für die chemische
Zusammensetzung;
C in einem Gehalt von 0,8% bis 1,2%;
Si in einem Gehalt von 0,1% bis 2,0%;
Mn in einem Gehalt von 0,1% bis 2,0%;
N in einem Gehalt von 0,002% bis 0,010%;
Al in einem Gehalt von 0,04% bis 0,15%;
P in einem Gehalt von 0,02% oder weniger; und
S in einem Gehalt von 0,02% oder weniger;
und gegebenenfalls mindestens ein Element, ausgewählt aus der Gruppe, bestehend aus:
Cr in einem Gehalt von 1,0% oder weniger, 0% ausschließend;
Ni in einem Gehalt von 1,0% oder weniger, 0% ausschließend;
Co in einem Gehalt von 1,0% oder weniger, 0% ausschließend;
Mo in einem Gehalt von 1,0% oder weniger, 0% ausschließend; und
Cu in einem Gehalt von 0,5% oder weniger, 0% ausschließend;
und gegebenenfalls mindestens ein Element, ausgewählt aus der Gruppe, bestehend aus:
B in einem Gehalt von 0,005% oder weniger, 0% ausschließend;
Nb in einem Gehalt von 0,5% oder weniger, 0% ausschließend; und
V in einem Gehalt von 0,5% oder weniger, 0% ausschließend;
wobei der Rest Eisen und unvermeidbare Verunreinigungen ist;
wobei der Al-Gehalt und der N-Gehalt eine Bedingung erfüllen, die durch den wie folgt
angegebenen Ausdruck (1) spezifiziert ist:
wobei [Al] und [N] Gehalte in Massenprozent von Al bzw. N sind;
wobei der Walzdraht eine Mikrostruktur aufweist, die 95 Flächenprozent oder mehr eines
Perlits umfasst;
wobei der Walzdraht einen Gehalt an AlN von 0,005% oder mehr aufweist; und wobei ein
Anteil an AlN-Teilchen mit einem Durchmesser dGM von 10 bis 20 µm 50% oder mehr in Zahlenprozent in einer Extremwertverteilung von
Maximalwerten der Durchmesser dGM von AlN-Teilchen beträgt, wobei der Durchmesser dGM durch ein geometrisches Mittel (ab)1/2 einer Länge "a" und einer Dicke "b" eines AlN-Teilchens dargestellt wird,
wobei der Begriff "Länge "a"" eines AlN-Teilchens sich auf die Länge (Abmessung) des
AlN-Teilchens in der Walzdrahtlängsrichtung bezieht;
der Begriff "Dicke "b"" des AlN-Teilchens sich auf eine Abmessung des AIN-Teilchens
in einer Richtung senkrecht zur Walzdrahtlängsrichtung bezieht und die Größe eines
AlN-Teilchens mit der maximalen Größe in einem Beobachtungssichtfeld im Querschnitt
gemäß JIS G0555 gemessen wird, wobei die Messung in beliebigen zwanzig Sichtfeldern
durchgeführt wird und wobei Einschlüsse der Gruppe D und der Gruppe DS, wie in JIS
G0551 spezifiziert, als AlN-Teilchen in der Messung angesehen werden.
2. Walzdraht nach Anspruch 1 mit einem Gehalt an gelöstem Stickstoff von 0,003% oder
weniger.
3. Walzdraht nach Anspruch 1, weiter umfassend mindestens ein Element, ausgewählt aus
der Gruppe, bestehend aus:
Cr in einem Gehalt von 0,05% bis 1,0%;
Ni in einem Gehalt von 0,05% bis 1,0%;
Co in einem Gehalt von 0,05% bis 1,0%;
Mo in einem Gehalt von 0,05% bis 1,0%; und
Cu in einem Gehalt von 0,05% bis 0,5%.
4. Walzdraht nach Anspruch 1, weiter umfassend mindestens ein Element, ausgewählt aus
der Gruppe, bestehend aus:
B in einem Gehalt von 0,0003% bis 0,005%;
Nb in einem Gehalt von 0,01% bis 0,5%; und
V in einem Gehalt von 0,01% bis 0,5%.
5. Stahldraht, erhalten aus dem Walzdraht nach einem der Ansprüche 1 bis 4.
1. Tige de fil métallique constituée par : en pourcentage en masse, ci-après de même
pour la composition chimique ;
C en une teneur de 0,8 % à 1,2 %,
Si en une teneur de 0,1 % à 2,0 % ;
Mn en une teneur de 0,1 % à 2,0 % ;
N en une teneur de 0,002 % à 0,010 % ;
Al en une teneur de 0,04 % à 0,15 % ;
P en une teneur de 0,02 % ou moins ; et
S en une teneur de 0,02 % ou moins ;
et facultativement au moins un élément sélectionné dans le groupe constitué par :
Cr en une teneur de 1,0 % ou moins, à l'exclusion de 0 % ;
Ni en une teneur de 1,0 % ou moins, à l'exclusion de 0 % ;
Co en une teneur de 1,0 % ou moins, à l'exclusion de 0 % ;
Mo en une teneur de 1,0 % ou moins, à l'exclusion de 0 % ; et
Cu en une teneur de 0,5 % ou moins, à l'exclusion de 0 % ;
et facultativement au moins un élément sélectionné dans le groupe constitué par :
B en une teneur de 0,005 % ou moins, à l'exclusion de 0 % ;
Nb en une teneur de 0,5 % ou moins, à l'exclusion de 0 % ; et
V en une teneur de 0,5 % ou moins, à l'exclusion de 0 % ;
le reste étant du fer et des impuretés inévitables ;
la teneur en Al et la teneur en N satisfaisant une condition spécifiée par l'expression
(1) donnée ci-dessous :
où [Al] et [N] sont les teneurs en pourcentage en masse d'Al et de N, respectivement
;
la tige de fil métallique ayant une microstructure comprenant 95 pour cent en surface
ou plus d'une perlite ;
la tige de fil métallique ayant une teneur en AlN de 0,005 % ou plus ; et
un pourcentage de particules d'AlN ayant un diamètre dGM de 10 à 20 µm étant de 50 % ou plus en pourcentage en nombre dans une distribution de valeurs
extrêmes de valeurs maximales des diamètres dGM de particules d'AlN, où le diamètre dGM est représenté par une moyenne géométrique (ab)1/2 d'une longueur « a » et d'une épaisseur « b » d'une particule d'AlN, dans laquelle
le terme « longueur « a » » d'une particule d'AlN fait référence à la longueur (dimension)
de la particule d'AlN dans la direction longitudinale de la tige de fil métallique
;
le terme « épaisseur « b » » de la particule d'AlN fait référence à une dimension
de la particule d'AlN dans une direction perpendiculaire à la direction longitudinale
de la tige de fil métallique, et
la taille d'une particule d'AlN ayant la taille maximale dans un champ de vue d'observation
dans la section transversale est mesurée selon la norme JIS G0555, dans laquelle la
mesure est effectuée dans vingt champs de vue arbitraires et dans laquelle des inclusions
du groupe D et du groupe DS tel que spécifié dans la norme JIS G0551 sont considérées
comme étant des particules d'AlN dans la mesure.
2. Tige de fil métallique selon la revendication 1, présentant une teneur en azote en
solution de 0,003 % ou moins.
3. Tige de fil métallique selon la revendication 1, comprenant en outre au moins un élément
sélectionné dans le groupe constitué par :
Cr en une teneur de 0,05 % à 1,0 % ;
Ni en une teneur de 0,05 % à 1,0 % ;
Co en une teneur de 0,05 % à 1,0 % ;
Mo en une teneur de 0,05 % à 1,0 % ; et
Cu en une teneur de 0,05 % à 0,5 %.
4. Tige de fil métallique selon la revendication 1, comprenant en outre au moins un élément
sélectionné dans le groupe constitué par :
B en une teneur de 0,0003 % à 0,005 % ;
Nb en une teneur de 0,01 % à 0,5 % ; et
V en une teneur de 0,01 % à 0,5 %.
5. Fil d'acier obtenu à partir de la tige de fil métallique selon l'une quelconque des
revendications 1 à 4.