Note: Descriptions are shown in the official language in which they were submitted.
~79~
HIGH STRENGTH, LOW CARBON, DUAL PHASE STEEL RODS AND
WIRES AND PROCESS FOR MAKING SAME
The present invention is directed to a process for
making high-strength, high-ductility, low-carbon steel
wires, bars and rods by cold drawing dual-phase steels.
Here, the term "dual-phass steels" refers to a class of
steels which are processed by continuous annealing,
batch annealing, or conventional hot rolling to obtain a
ferrite matrix with a dispersed second phase such as
marten site, Bennett and/or retained austenite. The
second phase is controlled to be a strong, tough and
deformable phase unlike the hard, non-deformable carbide
phase found in pyrolytic rods and wires. It must be
suitably dispersed and in sufficient volume fraction
i.e. greater than 10%, to provide a substantial contra-
button to the strength in the as-heat-treated condition
and to increase the work hardening rate during wire
drawing. Various heat treatment paths can be used to
develop the dual-phase micro structure and the morphology
depends on the particular heat treatment employed. A
preferred heat treatment is the intermediate quench
method i.e. austenitize and quench to 100% marten site
prior to annealing in the two phase my field and
quenching to a ferrite marten site structure The
invention it further directed to the high-strength, high
ductility steel wires, bars and rods produced by the
process of the present invention.
--2--
Steel wire has many known uses, such as for maXir.g
cables, chains, and springs. It is also used to make
steel belts and bead wire for tires, and steel strands
are included in multi strand electrical wire to improve
the tensile strength of the wire. In these applique-
lions, the diameter requirements range from 0.127 mm to
more than 6.35 mm with strength requirement
ranging from 1722.5 ma to as much as 2756 ma in the
smaller diameters In all of these applications, it is
important to provide a steel wire having a high tensile
strength and good ductility at the required diameter.
The oldest and most common method of producing high
strength, high ductility wire is by patenting a near
eutectoid composition pyrolytic steel. However, this
process it complex and expensive. A further disadvan-
tare of the patenting method is an inherent limitation
in the maximum wire diameter that can be produced at a
given strength level.
There is a need for steel wire and rods having higher
tensile strength and higher ductility than steel wire
and rods produced by the known methods, as well as a
more economical method for producing high strength steel
wire and rods. The present invention would replace the
conventional method of patenting pyrolytic steel to
produce wiry with a process whereby an alloy of rota-
lively simple composition is cold drawn into wire or
rods in a jingle multipas~ operation, i.e., without
intermediate annealing or patenting heat treatments.
Elimination of the patenting heat treatments in the
production of high strength steel wire should lower the
cost of producing high strength steel wire, especially
in light of the present fuel situation.
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The cold drawing process requires a low alloy steel
composition with a micro structure and morphology which
provides high initial strength, high ductility, rapid
work hardening, and good cold formability. The steel
should be capable of being cold drawn, without inter-
mediate anneals or patenting heat treatments, to the
desired diameter, tensile strength, and ductility.
A specific group of steels with a chemical composition
specifically developed to impart higher mechanical
property values is known in the art as high-strength,
low-alloy (HULA) steel. These steels contain carbon as
a strengthening element in an amount reasonably consist
tent with weld ability and ductility. Various levels and
types of alloy carbide former are added to achieve the
mechanical properties which characterize these steels.
However, the high tensile strength and high ductility
needed in many applications for steel wire and rods do
not seem to be attainable using HULA steels.
The factors governing the properties of low carbon
steels are primarily its carbon content and micro-
structure, and secondarily the residual alloy. Commonly,
low carbon steels contain silicon, manganese, or a
combination of silicon and manganese. In addition,
carbide forming elements such as, vanadium, chromium,
I niobium, molybdenum may be added.
Low carbon, dual-phase micro structured steels kirk-
terraced by a strong second phase dispersed in a soft
ferrite matrix show potential for satisfying the tensile
strength ductility, flexibility and diameter require-
mints of high strength steel wire. Furthermore, thieve potential for achieving a level of cold formability
which allows cold drawing without patenting or inter-
--4--
mediate heating. In particular, a low carbon, duplexferrite-martensite steel, disclosed in US. Patent No.
4,067,756 issued January 10, 1978, is of interest in the
present invention because it has high strength, high
ductility characteristics and is composed of inexpensive
elements. However, as generally fabricated, it has a
tensile strength of about 827 ma which is much lower
than the tensile strength required for most applications
of high strength steel wire. The process of the present
lo invention is directed to producing a high-strength steel
wire having a tensile strength of at least about 827
ma A preferred tensile strength range it 827 ma to
2687 ma, but strengths above 2756 ma may be achieved.
It is therefore an object of the present invention to
provide an improved process for making high strength,
high ductility steel wires and rods and to produce steel
wires or rods with increased tensile strength, ductile
fly, and flexibility at the desired diameter.
Another object of the present invention is to provide a
process for making high strength, high ductility steel
wires or rods comprising the step of cold drawing a
dual-phase steel composition to the required strength
and ductility without intermediate anneals or patenting
heat treatments, thereby providing complete flexibility
in choosing the wire diameter.
It is another object of the present invention to provide
a process for making high strength, high ductility steel
wires or rods that eliminates the intermediate patenting
step used in the present process for making pyrolytic
steel wire, thereby reducing the complexity, cost, and
energy consumption of the process for making high
strength steel wires and rods.
to
-5-
A further object of the present invention is to provide
a process for making high strength steel wires and rods
which is versatile, allowing for a wide range of dime-
lens, strength, and ductility properties in the final
steel wire or rod based on the choice of the initial
duplex micro structure and manipulation of the micro-
structure through appropriate thermal processing.
A further object of the present invention is to provide
high strength, high ductility steel wires or rods which
have a tensile strength at least about 827 ma.
Additional objects and advantages of the present invent
lion will become evident from the following description
taken in conjunction with the accompanying drawings.
In general, the present invention is directed to high
strength, high ductility, low carbon steel wires or rods
and the process for making the same. The process
involves cold drawing a low carbon dual-phase steel to
the desired diameter in a single multi pass operation.
The steel is characterized by a duplex micro structure
consisting essentially of a strong second phase
dispersed in a soft ferrite matrix and a micro~tructure
and morphology having sufficient cold formability to
allow reductions in cross-sectional area of up to about
39.9%.
One preferred embodiment of the invention is a high
strength high ductility, low carbon steel rod or wire
produced from a steel composition characterized by an
appropriate duplex ferrite-martensite micro structure,
for example as shown in FIG. 1, and the process for
making the same The process involves cold drawing the
duplex ferrite-martensite steel to the desired diameter
~LZ~9~7
in a single multi pass operation. In high strength
steels with a duplex ferrite-martensite micro structure,
the strong, deformable second phase consists
predominately of marten site but may contain Bennett and
retained austenite. the strong second phase is
dispersed in a soft ductile ferrite matrix; the
marten site provides the strength in the composite
whereas the ferrite provides the ductility.
FIG. 1 is an optical micro graph showing a typical low
carbon dual-phase ferrite-martensite micro structure
prior to cold drawing.
FUGUE. 2 is a transmission electron micro graph of disk
located lath marten site which comprises the strong
second phase in a dual-phase steel according to the
present invention.
FIG. 3 is a graph exemplifying a typical comparison
between a cold drawing schedule for duplex microstruc-
lure steel wire according to the present invention and a
drawing schedule for pyrolytic steel wire according to a
patenting method
According to the present invention, the high strength,
high ductility steel wires or rods are produced by a
process whereby a low carbon steel composition, kirk-
terraced by a duplex micro structure consisting essential
lye of a strong second phase dispersed in a soft ferrite
matrix, is cold drawn to the desired diameter in a
single multi pass operation. The starting steel compost-
lion prior to the cold drawing should possess duplex
micro structure and a morphology which are sufficient to
provide a level of cold formability allowing reductions
I
in cross-sectional area of up to 99.9% during cold
drawing.
The process of the invention provides an advantage over
known processes in that it eliminates the intermediate
heat treatments or patenting steps used in the known
process for making pyrolytic steel wire, and thereby
reduces the complexity, cost, and energy consumption of
the process. Furthermore, a wider range of rod and wire
diameter sizes can be produced by the process of the
invention than in the patenting method. In the patent-
in method, there is an inherent limitation in the
maximum wire diameter that can be produced at a given
strength level.
Referring to FIG. 3, the differences between the process
of the present invention and the patenting process are
shown. The solid line illustrates the cold drawing
schedule of a low carbon duplex steel wire according to
the present invention and the tensile strengths which
can be achieved at different diameters. The broken
lines indicate the drawing schedule of a pyrolytic steel
wire according to the patenting method, including the
intermediate heat treatments. In the drawing of the
pyrolytic steel wire, the intermediate heat treatments
are necessary in order to achieve the greater tensile
strength that the process of the present invention can
achieve at various diameters. These intermediate heat
treatments increase the complexity and expense of the
process for making high-strength steel wire, The
process according to the present invention does not
involve intermediate heat treatments and thus provides a
significant improvement over the known process.
The process of the present invention can produce steel
wires and rods with a wide range of tensile strength,
ductility, and diameter. The final properties of the
steel wire or rod at a given diameter are determined by
a combination of the initial micro structure, the prop-
reties of the starting steel and the amount of subset
quint reduction in cross-sectional area during the cold
drawing process. Since the micros~ructure of the steel
is easily manipulated through appropriate thermal
processing, the properties of the drawn wire can be
tailored to match the required specifications of the
desired application. The choice of alloying elements
such as silicon, aluminum, manganese, and carbide
forming elements, such as, molybdenum, niobium, vanadium
and the like, is determined by the micro structure and
properties desired. Thus, a wide range of alloys,
including many simple and inexpensive alloys, can be
used as long as they can be heat treated to the desired
dual-phase micro structure.
One preferred duplex micro structure is the ferrite-
marten site micro structure. Another preferred micro-
structure is the duplex ferrite-bainite micro structure.
In both cases, the strong second phase, either marten-
site or Bennett, is dispersed in a soft, ductile ferrite
matrix.
In one preferred embodiment of the process of the
present invention the starting steel composition con-
sits essentially of iron, from about 0.05 to 0.15
weight % carbon, and from about 1.0 to 3 0 weight %
silicon. In another preferred embodiment the starting
steel composition consists essentially of iron, from
about 0.05 to 0.15 weight percent carbon, from about 1-3
weight percent silicon, and from byway 0.05 to 0.15
weight percent vanadium. In both preferred embodiments,
the steel composition is thermally treated to form a
duplex ferrite-martensite micro structure in a fibrous
morphology. Briefly, the preferred process comprises
the steps of austenitizing the steel composition,
quenching the steel composition to transform the
austenite to substantially 100% marten site, heating the
resulting steel composition to an annealing temperature
for a time sufficient to provide the desired ratio of
austenite and ferrite, quick quenching the austenite
ferrite composition to transform the austenite to
marten site, and cold drawing the resulting dual-phase
steel which is characterized by a duplex ferrite-
marten site micro structure in a fibrous morphology to the
desired diameter in a single multi pass operation.
More specifically, the starting steel composition is
heated to a temperature, To, above the critical
temperature at which austenite forms. The temperature
range for I is from about 1050C to 1170C. The
composition is held at that temperature for a period of
time sufficient to substantially and completely Austin-
tire the steel. The resulting composition is quenched
in order to transform the austenite to substantially
100~ marten site. The composition is then reheated to an
annealing temperature, To, in the two phase (I ye
range. The air temperature range is from about 800C to
1000C. The composition it held at this temperature for
a period of time sufficient to transform the martensitic
steel composition to the desired volume ratio of ferrite
and austenite. Upon final quenching, the austenite
transforms to marten site, resulting in a strong second
phase of marten site dispersed in a soft or ductile
ferrite matrix.
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The steel composition at this point is characterized by
a unique micro structure which is a fine, isotropic,
acicular marten site in a ductile ferrite matrix. The
micro structure results due to the combination of the
double heat treatment and the presence of silicon in the
above-specified amount. The unique micro structure
maximizes the potential ductility of the soft phase
ferrite and also fully exploits the strong marten site
phase as a load carrying constituent in the duplex
micro structure. It is the micro structure as well as the
morphology of the steel composition that enables the
steel to be cold drawn to the desired wire or rod
diameter in a single multi pass operation.
Any dual-phase steel may be used in the process of the
present invention as long as a duplex micro structure and
morphology can be produced having sufficient cold
formability to allow reductions in cross-sectional area
of up to about 99.9% when the composition is cold drawn.
In particular, dual-phase ferrite marten site steels have
a greater continuous yielding behavior, higher ultimate
tensile strength, and better ductility than commercial
high strength low alloy steels, including MacWorld,
fine-grain steels. Furthermore, the high tensile to
yield ratio and high strain hardening rate in ferrite-
US marten site dual-phase steel provides excellent cold
formability.
The exact temperature, To, to which the steel compost-
lion is heated in the first austenization step is not
critical as long as it is above the temperature at which
complete austenization occurs. The exact temperature,
To, in the second heating step where the composition
is transformed to the two phases of ferrite and as-
twenty depends upon the desired volume ratio of ferrite
I
and austenite, which in turn depends upon the desired
volume ratio of ferrite-to-martensite. In general, the
desired volume ratio of ferrite and marten site depends
upon the ultimate properties desired for the steel wire
or rod. Generally, 10-40 volume percent of marten site
in the ferrite-martensite micro structure will allow the
steel composition to be cold drawn to diameters repro-
setting up to 99.9~ reductions in cross sectional area
and will still result in steel wires and rods having a
lo tensile strength at least about 827 ma. Usually,
tensile strengths in the range of 827 ma to 2687 ma
are obtained, but 2756 ma and above may also be
obtained.
The following examples will illustrate the process of
the invention more clearly, the resulting properties of
the steel wires and rods produced by the process, as
well as the flexibility of the process in allowing a
choice of alloys, tensile strengths, ductility, and
diameters.
A high strength, high ductility steel wire was made to
satisfy the requirements for bead wire used in the
manufacture of automobile tires. The bead wire requires
a tensile strength of 1860 ma with I elongation, and a
proportional limit of 1488 ma. The bead wire should be
about 0.94 mm in diameter with sufficient ductility
to pass a torsion test requiring 58 axial twists in a
203 mm length. A 5.6 mm diameter steel rod
having a composition consisting essentially of iron, 0.1
weight percent carbon, 2 weight percent silicon, and 0.1
weight percent vanadium was austenitized and rapidly
quenched to yield a substantially 100% martensitic
composition. The rod was then reheated to a temperature
of 950 C in the two phase I range and rapidly
quenched to produce a duplex ferrite-martensite micro-
structure of approximately 30 volume percent marten site
and 70 volume percent ferrite. The needle-like, act-
cuter character of the ferrite-martensite micro structure
is shown in the optical micro graph in FIG. 1. The heat
treated rod was then cold drawn through lubricated
conical dies down to a diameter of 0.94 mm in 8
passes of approximately 36% reduction in area per pass.
After a short stress relief anneal at 425C similar to
the current practice, an ultimate tensile strength of
1902 ma was achieved, thus satisfying the tensile
strength requirement of bead wire. The ductility of the
steel wire was sufficient to satisfy the twist test
requirement.
Example 2
A steel rod consisting essentially of iron, 0.1 weight
percent carbon, and 2.0 weight percent silicon was hot
rolled to a diameter of 6.4 mm. The rod was then
heated to a temperature of about 1150C for about 30
minutes to austenitize the composition. the steel was
then quenched in iced brine to transform the austenite
to substantially 100~ marten site. The rod was then
rapidly reheated to a temperature of 950C in order to
convert the structure to approximately 70% ferrite and
30% austenite~ The steel rod was then quenched in iced
brine to convert the austenite to marten site. Finally,
the rod was cold drawn to a diameter of 0.76 mm
where its tensile strength was 2460 ma, and also drawn
to a diameter of 0.61 mm where its tensile strength
was 2480 ma. Continued cold drawing may achieve
tensile strengths to 2756 ma or higher.
