FIELD OF THE INVENTION
The present invention discloses a coated steel bead wire product with superior
bend and torsion property through control of patenting and wire drawing
conditions. The invention further relates to a process of producing a coated steel
bead wire product with superior bend and torsion property.
BACKGROUND OF THE INVENTION
High carbon steel wires are increasingly getting importance for different
applications (like tire bead, tire cord etc) due to good combination of tensile
strength - elongation, torsion and bend property. Bay et al (US Patent 6264759)
disclosed the development of high carbon steel wire with superior longitudinal
cracking by reducing the amount of pro-eutectoid ferrite phase in the wire rod.
Development of high tensile wire with superior longitudinal cracking resistance
through alloy engineering and microstructure control has been disclosed by Makii
et al in US Patent 6322641. US Patent 6402858 described the importance of
cleanliness of steel on the wire quality, particularly used for rope, cord and bead
application. According to that patent the authors have claimed that the
development of steel having very low inclusion content leads to excellent cold
workability with superior fatigue resistance. There is no prior art which teaches
the bend property of high carbon coated steel wire used for making tire bead
and cord applications. However, Indian Patent Application NO.316/KOL/2011
dated 10th March 2011 by the present authors, discloses drawn wire product with
superior bend properties without coating.
Fine wires made using high carbon steel is used in the automobile industries
(truck, bus, aircraft tires) for making tire. Figure 1 shows a photograph of a
typical tire where the bead area along with other components has been
indicated. The bead of the tire generally helps to reinforce the tire with the rim.
So that the failure time can be extended and as a consequence the safety of the
passenger can be increased. The wires used for tire bead application are
basically made of high carbon steel. The wires are covered with a thin layer of
brass coating. The bead can be made using few wires which are embedded in
rubber. In most instances, depending upon the type of tire being reinforced, the
property requirement of the wire varies. Hence, it is desirable for the steel wire
generally used as reinforcing material must exhibit good combination tensile,
torsion and bend properties.
Converting the steel wire rod into a fine wire suitable for making tire bead
component involves several processing stages such as pre-drawing of an initial
wire rod followed by austenitizing, patenting, drawing, stress relieving and
coating.
During wire drawing step, the diameter of the wire reduces from its original
diameter to a smaller diameter. During drawing the patented wires are allowed
to pass through a number of dies and finally fine wires are produced. Generally,
the dies used for drawing made of either tungsten carbide or diamond. Drawing
leads to decrease in the diameter of the wire as a result the strength
characteristics of the metal changes. Cold drawing can be done by using either
wet or dry lubricants. Formation of a wire with desired properties may include
multiple drawing steps both prior to and after patenting of the wire.
Patenting or low temperature heat-treatment of wire from austenite phase is
carried out to obtain a microstructure which imparts combination of high tensile
strength and ductility . As a result, wires are able to withstand a large reduction
in area during wire drawing and finally gives high tensile strength and good
toughness. Industrial patenting (heat-treatment) is a continuous process and
consists of first heating the steel at high temperature to form homogeneous
austenite, and then dipping the wire in a liquid media at lower temperature
which is predetermined. This treatment leads to transformation of austenite
phase into pearlite phase. It is always desirable to achieve a fully pearlitic
structure in the patented wire after this heat-treatment to achieve desired
properties in the finished wire product.
Combination of superior bend properties along with optimum torsion and tensile
strength remain the prime objective while making a tire bead. The steel wire
which is used in the tire in the form of bead is allowed to release the internal
stress by stress relieving treatment and finally subjected to coating. It is quite
important to design the microstructure of patented wire by controlling patenting
heat-treatment parameters, drawing condition, and stress relieving treatment
parameters to improve the bend value of the final coated wire.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a coated wire product with
superior bend and torsion properties adaptable for making tires for automobile
industries
Another object of the invention is to propose a process for producing a coated
wire product with superior bend and torsion properties adaptable to manufacture
of automobile tires.
SUMMARY OF THE INVENTION
According to the invention chemical composition of the steel is selected to be
0.5-1.1 %C, 0.35-0.9%Mn, 0.15-0.5 Si, 0.006-0.01%N, 0.09-0.025 % S and
0.01-0.035% P (all values in wt%). The steel is cast and rolled into 5.5 mm wire
rod followed by pickling. The wire rod is cold drawn to constitute a pre-drawn
wire followed by austenitization (high temperature heat-treatment) and
patenting (low temperature heat-treatment) in liquid lead media set at
predetermined temperature. The wire coming out from low temperature heat-
treatment bath is subjected to surface treatment followed by cold wire drawing
to achieve a fine wire with diameter less than 1.0 mm. A plurality of drawn wires
are wound to form a spool and further subjected to another low temperature
heat-treatment below 500°C using a lead bath called 'stress relieving annealing',
followed by a metal coating. The coated wires are finally wound in the form of a
spool. Bend and torsion properties of the coated wires are evaluated.
Characterization of the developed product is carried out systematically.
According to the invention, patenting heat-treatment condition, drawing
parameters and stress relieving parameters are identified to produce a wire with
superior bend and torsion properties after coating. The resulting microstructures
of the heat-treated materials characterized to establish the optimum process
parameters to get a metal coated wire with improved value. Thus, the invention
provides a metal coated wire with bend value 14 numbers with torsion value
minimum 65 in industrial production conditions.
BRIEF DESCRIPTION OF THE ACCC0MPANYIN6 DRAWINGS
Figure 1 - a schematic different components of a standard automobile fibre
including breed area.
Figure 2 - a flow diagram showing different steps in the production of tire bead
wires.
Figure 3 - microstructure of the inventive wire with improved bend value.
Figure 4 - shows a comparative bend values of the prior art and inventive
product.
Figure 5 - shows the microstructure of the wire to obtain high bend value.
DETAIL DESCRIPTION OF THE INVENTION
Production process of tire bead wires in an industrial line is schematically shown
in figure 2 where several steps are indicated. The initial wire is a pre-drawn wire
which is nothing but a drawn wire with diameter greater than 2 mm. The feed
wire (step 1) is allowed to pass through a high temperature furnace where the
material goes into austenitization treatment through step 2. Then the wires
coming from step are allowed to pass through step 3 (called patenting or low
temperature heat-treatment) followed by step 4 called wire drawing and step 5
(stress relieving & coating). The microstructure of the newly developed wire
product is depicted in figure 3 which shows typical drawn pearlitic structure
which is composed of ferrite and cementite. Interestingly, the microstructure of
the developed wire is fragmented and flattened in nature which is believed to be
responsible for the improvement in bend property of the coated wire according
to the present invention. In the developed wire the bend values obtained is as
high as 14 which is significantly higher [Fig. 4] than the bend values obtained
(reverse bend value 10) in a produced wire using similar grade wire rod
according to prior art. It is clearly evident from figure 4 that the present
invention leads to 3 to 4 points increase in the bend value as compared to the
wires produced using conventional operating condition. A typical pearlitic
microstructure of the patented wire (step 3 heat-treated) leading to high bend
values after final processing is shown in figure 5. It is evident that the newly
heat-treated wire giving high bend values after final processing consists of
pearlitic microstructure with fine lamellar spacing [Fig. 5a] ( at step 3) as
compared to the conventionally patented microstructure [Fig.5b] which consists
of pearlitic microstructure with coarse lamellar spacing. Hence, it is established
that in the present invention the refinement of lamellar spacing of the wire
according to step 3 and different kind of morphology in the final wire leads to an
improvement in bend values of the invented coated wire. Although it has been
described that the patenting condition (step 3 condition) playing a significant role
in improving the bend values of the final processed wire, however, it is worth to
mention that the wire drawing condition (step 4) also partly contributing in
enhancing the bend value of the coated wire.
According to the invention, the wire is hot roiled where the reheating
temperature was in the range of 1100 - 1200°C and lying head temperature in
the range of 815 to 910°C .
The hot rolled wire rod is pre-drawned where the diameter of the wire is reduced
in the range 2.8 to 3.4 mm.
The pre-drawn wire is heated at high temperature in the austenite range where
the temperature of austentitization is kept in the range 950 to 1150°C.
The heat-treatment condition in step 3 is maintained in the range 560 to 645°C
and heat-treatment time was 12-40 seconds.
During wire drawing special lubrication is given.
The wire drawing speed in the range 6 to 25 meter per second is maintained.
A combination of die angle is used during metalcoating.
The drawn wire is subjected to stress relieving annealing where the temperature
of during stress relieving annealing in step 5 was maintained in the range 390 to
470C°C and the annealing time in the range 2 to 30 seconds.
Present invention is useful for the making tire for automobile industries especially
for bus, truck tire manufacturing companies.
The present invention is simple as well as quite unique and can be implemented
in the existing production line without extra investment and efforts. The claimed
invention enables to produce high carbon steel wire with superior bends as well
as torsion and as a result the life and safety of the tires expected to be
enhanced.
WE CLAIM:
1. A coated wire product with superior bend values of at least 14 with a
torsion value of at least 65 for making tires of automobiles from a steel
grade with composition range 0.6-1.1 %C, 0.5-0.9%Mn, 0.10-0.5 Si,
0.006-0.01%N, 0.009-0.025% S and 0.01-0.035% P, balance Fe (all
values in wt%.
2. A process for producing a coated wire product with superior bend and
torsion adaptable to manufacture of automobile tires, the steps of:
a) selecting a steel grade with composition range of 0.6-1.1% c, 0.5-
0.9% mn, 0.10-0.5 Si, 0.006-0.01% N, 0.009 - 0.025% S, 0.01-
0.035% P, and balance Fe, all values being in wt%;
b) producing a wire rod from the steel slab casted out of the selected
steel grade;
c) hot-rolling the wire rod under a reheating temperature between
1100-1200°C, and lying head temperature in the range of 815 to
910°C;
d) pre-drawing the hot-rolled wire rod to reduce the diameter in the
in the range 2.8. to 3.4 mm;
e) austenitizing the pre-drawn wire rod under an austentitization
temperature range of 950 to 1150°C;
f) patenting the austentized wire rod in a temperature range of 560
to 645°C for a period of 12-40 seconds;
g) drawing the patented wire rod under application of lubricant by
maintaining a drawing speed in the range of 6 to 25 meter per
second;
h) subjecting the drawn wire to stress relieving annealing at a
temperature range of 390 to 470°C for a period of 2 to 30 seconds;
and
i) metal coating the wire in a die with combination of die angle.
3. The process as claimed in claim 2, wherein the microstructure of
of the wire product exhibits pearlitic structure composed of ferrite and
cementite, fragmented and flattened.
4. The process as claimed in claim 2 or 3, wherein the microstructure of
finally processed wire exhibits pearlitic with fine lamella spacing.

This invention relates to a coated wire product with superior bend values of at
least 14 with a torsion value of at least 65 for making fibres of automobiles from
a steel grade with composition range 0.6-1.1 %C, 0.5-0.9%Mn, 0,10-0.5, Si,
0.06-0.01N, 0.009-0:25% S and 0.01-0.035 %P, balance Fe (all values in wt %.