FIELD OF INVENTION
The present invention in general relates to a method of reducing the spheroidize
annealing cycle time for cold drawn steel wire to obtain a desired microstructure,
tensile strength and ductility to enable cold forming of card clothing wire for
textile industries. More particularly, the invention relates to an improved method
of spheroidize annealing of hypoeutectoid steel wires with reduced cycle time.
BACKGROUND OF INVENTION
Prior art practice in spheroidize annealing of wires enable to obtain a
microstructure which is substantially spheroidised; completely free of carbide
network and pearlite; and relatively soft and ductile. Because of the
microstructure and a high tensile strength of steel after wire drawing, the
spheroidisation annealing cycles subsequently used are lengthy. According to the
known spheroidisation annealing processes, steel wires are soaked at a
predefined temperature for about 19 hours to a few days to produce the desired
microstructure, tensile strength and ductility required for good cold formability.
According to the known processes the steel wire stock is heated in steps to reach
a predefined spheroidisation temperature, held at said temperature for several
hours and finally cooled down slowly by furnace cooling. Although, the
microstructure and the strength produced by the practices are acceptable by the
textile industries standard, the cycle heating and soaking practice does not
however, effectively reduce the length of annealing cycle.

Several improvements in the manufacture of cold drawn wire have been
suggested. One such improvement is disclosed in US Patent No. 3285789 to
Raymond A. Grange et al. entitled, "Method of softening steel." The
improvement is directed to heating the hypereutectoid steels to a temperature till
the steels become completely austenitic, working the steels while remaining at
these temperature, cooling the steel to ambient temperature and carrying-out a
spheriodise annealing of the processed steel within a temperature range
between to a predefined temperature (A1) and a temperature, which is at least
10°C below the predefined temperature (A1). However, the cited invention fails
to achieve a complete spheroidisation and elimination of lamellar carbides.
As the drawn steel wire contains ferrite and pearlite, the cited invention further
requires a spheroidise annealing of the wire to produce a pheroidised structure.
Another improved method directed to hypereutectoid steel and highly alloyed
steel is described in US Patent No. 3459599 issued August 5, 1969 to Raymond
E. Grange en titled "Method of Thermomechnically annealed steel." According to
this invention, hypereutectoid steels are heated to and drastically worked at a
temperature not more than 65°C below the A1 temperature. Thus, problem of
complete spheroidisation with elimination of lamellar carbides connected with the
hypereutectoid steel is not solved by this invention.
US Patent No. 4375378 en titled "Process of producing spheroidise wire rods for
cold forging" discloses a process for producing a steel

wire with minimal decarburization layer on the surface and having excellent cold
forgeability. According to this invention, the rolling process of the hot-rolled wire
rods is so controlled that a predetermined thickness of scale including a rapidly
cooled structure is provided to the wire rods. Further, the spheroidise annealing
of this wire rod reduces the heating cycle time by 4 hours from that required by
the conventional spheroidising.
Although prior art practices have been developed to spheroidise anneal the wire
rods prior to heating and cold working, the heat treatment cycles are still very
length, and this increases the cost of production of the steel wire.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method of
spheroidze annealing of hypoecutectoid steel wires with reduced cycle time,
which eliminates the prior art disadvantages.
Another object of the invention is to propose an improved method of spheroidze
annealing of hypoecutectoid steel wires with reduced cycle time, which is
enabled to decrease the tensile strength and increase reduction in area (RA) of
the steel wire.
A still another object of the invention is to propose an improved method of
spheroidze annealing of hypoecutectoid steel wires with reduced cycle time,
which generates a high formability characteristics in the treated steel wire.

Yet another object of the invention is to propose an improved method of
spheroidal annealing of hypereutectoid steel wires with reduced cycle time,
which is enabled to transform the microstructure of the steel wire to a fine, well-
dispersed spheroidize carbides in a ferritic matrix devoid of lamellar carbides.
A further object of the invention is to propose an improved method of spheroidal
annealing of hypoecutectoid steel wires with reduced cycle time, which provides
steel wire with higher durability.
SUMMARY OF THE INVENTION
Accordingly, the invention provides a method for producing hypoeutectoid steel
wires having good formability wherein said steel wires are directly heated to a
temperature range below the A1 temperature, and wires are held at that
temperature for a first-predefined time to decrease the tensile strength and
increase the reduction in area of steel wire. Further, the steel wires are held at
that temperature for a second predefined time to obtain microstructure of fine
well-dispersed spheroidal carbides in a fine ferritic matric substantially devoid of
lamellar carbides, when steel containing about 0.05 to about 0.70% Carbon,
wherein said steel wires are heated to 675°C within 5 hours and held at that
temperature for 6 hours to produce microstructure consisting of well dispersed
spheroidal carbides in a ferritic matrix, said steel wires being characterized by
low strength and good ductility.
Broadly, the invention includes the steps of heating a cold drawn hypoeutectic

steel wires for at least 5 hours to reach spheroidisation annealing temperature of
675°C, soaking at said temperature for 6 hours followed by a furnace cooling to
obtain a low tensile strength of 530 MPa maximum and 70% minimum RA
(reduction in area).
DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 schematically shows a conventional spheroidize annealing cycle
Figure 2 schematic diagram showing heating cycle in a Gleeble thermo
mechanical simulator according to the invention
Figure 3 is a reproduction of a SEM photomicrograph of a quenched sample
(heating rate 1.4°C/min) according to the invention
Figure 4 schematically represent a spheroidize annealing cycle according to
the invention
Figure 5 is SEM photograph of a test piece undergone spherodize annealing
according to conventional method
Figure 6 is SEM photograph of a test piece undergone spheroidise annealing
according to the inventive method.
Figure 7 is a graphical comparison between the technical parameters
adapted by a conventional method of spheroidize annealing and
the method of the invention.
DETAIL DESCRIPTION OF THE INVENTION
The hot-rolled plain carbon wire rods (5.5 mm dia) after pickling and borax
coating were drawn to 3.2 mm diameter on a 7 block machine with a maximum
reduction of 16 to 17% in one die using a Sodium/calcium based powered as
the dry lubricant. The drawn wires were subjected to a spheroidise annealing. In
the conventional annealing cycle, coils of drawn wire are stacked vertically one
above another in a furnace and heated to 350°C in 2.5 hours, soaked at this
temperature for 3.5 hours followed by raising the furnace temperature to 675°C
in 5 hours and held at this temperature for 8 hours. After prolonged soaking at
this temperature, the furnace is witched off. (Figure 1). During the entire heat
treatment cycle, nitrogen is continuously purged to avoid oxidation of wire
surface as well as homogenizing the temperature. The chemical composition of
the steel used for preparing card clothing wire is shown in Table 1.
In order to observe the effect of heating rate of wires in particular the cementite
particles of the steel under a constant spheroidizing temperature, the
spheroidization process has been accessed by adapting a controlled heating
experiment in a Gleeble thermo-mechanical simulator. According to the
experiment, drawn wires of 3.2 mm diameter were heated to the spheroidisation
temperature at three different stages i.e. 5.6% C/minute, 2.25°C/minute and
1.4°C/minute. As soon as the spheroidization temperature was reached, the
sample was quenched in water (Figure 2) to retain the microstructure developed
at various heating stage. The microstructure of the sample heated to the
spheroidising temperature with a heating rate of 1.4°C/min shows that carbide
particles have grown in size and the distribution became more uniform as
depicted in Figure 3.
Based on the data exhibiting the effects of heating time on the rate of
globularisation of the spheridised carbide particles, the heating rate of 1.4°C
/minute time of 5 hours was selected in place of a stage-wise heating to reach
675°C (which is spheroidization temperature), and the sample was held at his
temperature for 6 hours according to the method of invention as shown in Figure
4.
A comparison of the tensile test results of the heat treated sample of
conventional and invented spheroidisation annealing cycles are shown in Table 2.
The tensile test result of the draw wire sample is also added.

The average value of UTS is based on the results of 10 samples. To meet the
requirement of textile industries for metallic card clothing wire in spheroidised
annealed condition, the average ultimate tensile strength (UTS) of wire should
not exceed 550 MPa, with an improved reduction in area (RAO for cold
formability. It is evident from the Table 2 that the wire which has been treated
by the method of invention has the more favourable properties for example,
softer raw material easy for cold forming and profiling. It is worthwhile to
mention here that the combined effect of heating rate and the SA time on the
mechanical properties of the wire has been shown in Table - 2.
A close examination of the microstructure by SEM as shown in Figure 5,
apparently shows that in conventional cycle the spheroidisation is complete.
However, a further examination reveals that ferrite grains are outlined by the
carbides. The soaking time of 6 hours was optimum for getting a slow tensile
strength in combination of high ductility. The degree of spheroidisation is
conventional including the invented cycles is substantially the same, however,
with a little improvement in the invented cycle. The ferrite grains appear to be
smaller in conventionally spheroidise annealed wire than the ferrite grains of
steel wire heat treated by the method of invention as shown in Figure 6. From
the experimental results it is established that a reduction in heat treatment cycle
does not deteriorate the degree of spheroidisation.
After heat treating the test sample by 5 hours according to the method of
invention, the tensile strength of the steel wire had been lowered by about 60
MPa and reduction in area (RA) increased by 5% which is suitable for cold
forming.
A comparison of short treating cycle of the invention and a typical conventional
spheroidise annealing cycle is shown in Figure 7. Note that the heat treatment
cycle of the invention is considerably shorter than the typical conventional SA
cycle.
The disclosure further describes through experimental results the effect of
heating rate and holding time during spheroidization annealing on the
mechanical properties and microstructure of cold drawn card clothing wire.
According to the invention, based on the experimental results the annealing cycle
time has been optimized to achieve a combination of best mechanical properties
and cost effectiveness.
WE CLAIM
1. An improved method of spheroidize annealing of hypereutectoid steel
wires with reduced cycle time, comprising the steps of:-
- providing at least a standard length of hypereutectoid steel wire having
about 0.6 to 0.8% of carbon in a heating furnace;
- directly heating the steel wire in the furnace to a temperature range of
about 675°C by maintaining a heating rate substantially equal to
1.4°C/minute, the total heating time being five hours;
- soaking the heated steel wire within the furnace at the temperature of
about 675°C for a time period of around 6.0 hours; and
- switching off the furnace.
2. The method as claimed in claim 1, wherein the spheroidize annealing
cycle reduces the tensile strength of the steel wire by about 6%.
3. The method as claimed in claim 1, wherein the reduction-in-area (RA) of
the steel microstructure increases by at least 5%.
4. The method as claimed in claim 1, wherein the heating rate at the furnace
can be 5.6°C/minute to reach at a subcritcal temperature (675°C).
5. The method as claimed in claim 1 or 4, wherein the heating rate at the
furnace can be 2.25°C/minute to reach at the subcritical temperature.
6. The method as claimed in claim 1, wherein the steel wire after spheroidize
annealing produces a microstructure consisting of well-dispersed
spheroidical cabbies in a ferritic matrix.
7. The method as claimed in any of the preceding claims, wherein the
produced steel wire having a lower tensile strength and higher ductility.
8. The method as claimed in claim 7, wherein the tensile strength and
reduction in area (RA) of the produced steel wire is respectively in the
range of 510-530 MPa and 70-72%.
9. The method as claimed in claim 1, wherein the method reduces the cycle
time for spheroidize annealing of the steel wire without deteriorating the
degree of spheroidisation.
10.An improved method of spheroidize annealing of hypereutectoid steel
wires with reduced cycle time as substantially described and illustrated
herein with reference to the accompanying drawings.

The invention relates to an improved method of spheroidize annealing of
hypereutectoid steel wires with reduced cycle time, comprising the steps of
providing at least a standard length of hypereutectoid steel wire having about
0.6 to 0.8% of carbon in a heating furnace; directly heating the steel wire in the
furnace to a temperature range of about 675°C by maintaining a heating rate
substantially equal to 1.4°C/minute, the total heating time being five hours;
soaking the heated steel wire within the furnace at the temperature of about
675°C for a time period of around 6.0 hours; and switching off the furnace.