FIELD OF INVENTION
The present invention relates to a graphitic steels adaptable to wire drawing
application.
BACKGROUND OF THE INVENTION
Graphitic steels are mainly used for machine structural parts.Generally as the
amount of carbon increases in steel, the carbon changes into the form of
cementite. The microstructure of carbon steel is ferrite/carbide (cementite).
However, the presence of cementite limits the cold working properties and
degrades formability. Therefore, the key apparoach is to transform
ferrite/cementite to ferrite/graphite structure, resulting in both machinability
and cold forgeability. Therefore, studies have been made to develop a steel
which maintains carbon as easily deformable graphite in a soft ferritic matrix.
Even high carbon steels can be softened to the level of low carbon level by
this approach. By transforming the ferrite/cementite structure to a
ferrite/graphite structure,both the machinability and cold forgeability can be
improved, and at the same time, after first softening by graphitisation, the
strength can be regained by dissolving graphite in to austenite and quenching
[l,2].The graphitization process during the annealing of carbon steel consists
of two steps : 1) the dissolution of cementite and 2) the nucleation of
graphite. It appears that the various approaches adopted to accelerate
graphitization, based upon alloying, can be considered to fall into two
categories, either destabilization of the cementite phase or the provision of
heterogeneous nucleation sites for the graphite. The former approach has
2

generally centered upon alloying with Si, which reduces the stability of
cementite whilst also avoiding or reducing alloying elements such as Mn and
Cr, which increase cementite stability [3-6]. Graphitization of cementite at
subcritical temperature is reported mostly in special carbon steels containing
large amounts of silicon, nickel or aluminium [7-11]. The latter approach
considers additions, like Al, Si, Ni,Ti, Zr or B which provide a variety of
nucleating sites, for example, non-metallic inclusions, such as AI2O3,SiO2 or
silicates, and nitrides and carbides such as BN, AIN, TiC, ZrN, Nb (C,N) and
V(C,N) for the nucleation of graphite [3,4,12,13].
It is generally known that graphitization readily proceeds in hypereutectoid
composition. Sato et al has reported that 1% or more silicon significantly
accelerates graphitization[14]. Similarly it has been studied that ti causes
acceleration of graphitization since the distribution of Ti in cementite is quite
low, and TiC is easily formed. Thus the graphitization is accelerated due to
dentrification by Ti rather than the effect of cementite[15].
However, graphitization of hypoeutectoid steel is difficult and the behaviour
of graphitization by alloying and heat treatment are not quite well known. In
promoting the precipitation of graphite Si and Ni are added for the
precipitation of graphite [16]. By alloying with Si (1.76-1.82 wt pet) and Al
(91.28-1.44 wt pet), in 0.38C medium carbon steel, graphitization has been
reported to be completed within 2-4 hours at 680°C [17]. However, an
addition of large amount of Si significantly lowers ductility by solid solution
hardening. Hence, the effects of Si in combination with Ni/Co on
graphitization of hypo-eutectoid steel have been examined by Sueyoshi et al
[18]. However, considering that the materials containing Ni or Co are difficult
to recycle and that such elements are relatively expensive, the addition of
these materials is not effective. The effect of cold working on the
graphitization of aluminium killed cold rolled steel has been investigated by
Okamoto [19]. It was observed that extreme reduction of P and S accelerated
graphitization in hypoeutectoid steels (C-0.06 pet and 0.5 pet) and the void

between the cracked fragments of cementite, generated during cold working,
provided enough space to form graphite in the ferrite matrix during
annealing. However, this strategy has problems in that stringent cold working
of 20% or more is required for dividing cementite under the state of poor cold
working properties, and can not be considered to be a stable manufacturing
procedure. Futhermore, a strategy has been proposed for accelerating
graphitization by addition of B, Al or rare earth metals in an hypoeutectoid
steel of 0.53% carbon[20].Although nucleation of graphite particles was
accelerated by the existence of various types of precipitate in the steel, BN is
an extremely effective nucleation site[21,22].In addition to the
crystallographic structure of the precipitates, graphite and BN also bear an
extremely close resemblance in terms of their precipitation modes[23].
Therefore,epitaxial formation and growth of graphite on the surface of BN are
easy, and BN in steel can be used very effectively as a graphite nucleation
site. The surface of BN particles provides the preferred sites for graphite
nucleation, so the distributions of graphite particles are very similar to that of
BN prior to graphitization[22]. The distribution of graphite particles, however,
clearly changes with the amount of B and N. By adding a graphitization
accelerating particle BN[24] and employing quenching and annealing
treatment, machinability and drillability by cemented carbide insert of the
steel can be improved.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a graphitic steels
adaptable to wire drawing applications which eliminates the disadvantages of
prior art.
4

Another object of the present invention is to propose a graphitic steels
adaptable to wire drawing applications which improves ductility with low yield
ratio.
A further object of the present invention is to propose a graphitic steels
adaptable to wire drawing applications which increase strength.
A still further object of the present invention is to propose a graphitic steels
adaptable to wire drawing applications which increases productivity in wire
drawing industry.
BRIEF DESCRIPTION OF THE ACCOMPANYING PHOTO FIGURES
ACCORDING TO THE INVENTION
Fig-1 FEG-SEM micrographs showing cast structure of steels.
Fig-2 FEG-SEM micrograph showing ferritic-pearlitic structure in hot rolled
Steels.
Fig-3 Optical micrographs in unetched conditions for Steel-I hot rolled and
annealed for 15 hrs and 23 hrs.
Fig-4 Optical micrographs in unetched conditions for Steel-2 hot rolled and
annealed for 15 hrs and 23 hrs.
Fig-5 Optical micrographs in unetched conditions for Steel-3 hot rolled and
annealed for 15 hrs and 23 hrs.
Fig-6 Optical micrographs in etched conditions for Steel-1 hot rolled and
annealed for 15 hrs. (top) and 23 hrs (bottom).
Fig-7 Optical micrographs in etched conditions for Steel-2 hot rolled and
annealed for 15 hrs. (top) and 23 hrs (bottom).
Fig-8 Optical micrographs in etched conditions for Steel-3 hot rolled and
annealed for 15 hrs. (top) and 23 hrs (bottom).
5

Fig-9 Representative SEM micrograph (a) along with EDS (b) of Steel 2 hot
rolled and annealed at 710°C for 23 hrs. Arrows showing graphite
precipitation on BN.
Fig-10 Percent graphite of hot rolled Steel 1-3 annealed for 15-23 hours.
Fig-11 Average No. of graphites for hot rolled Steel 1-3 annealed for 15-23
hours.
Fig-12 Yield and ultimate tensile strength of hot rolled Steel 1-3 annealed for
15-23 hours.
Fig-13 Percent elongation of hot rolled Steel 1-3 annealed for 15-23 hours.
Fig-14 Percent reduction in area of hot rolled Steel 1-3 annealed for 15-23
hours.
Fig-15 Representative optical micrograph of Steel 3 hot rolled and annealed at
710°C for 23 hrs, showing disintegration of graphite particle.
SUMMARY OF THE INVENTION
Graphitic steels are mainly used for machine structural parts. However, in the
present study, it has been shown that graphitic steels can be used for wire
drawing application with extensive drawability. It is generally seen that the
elongation in high carbon steels used for wire drawing applications is less
than 10% and the YS is close to UTS. However, by transforming the ferrite-
cementite structure to a ferrite, graphite and spheroidised cementite
structure, drawability of the wires can be increased substantially and at the
same time, after first softening by graphitization, the strength can be
regained by dissolving graphite in austenite and quenching. The graphitization
process during the annealing of carbon steel consists of two steps: 1)
dissolution of cementite and 2) nucleation of graphite. The approaches
adopted to accelerate graphitization are 1) by destabilization of the cementite
phase by alloying with Si and reducing Mn and Cr and 2) by provision of
heterogeneous nucleation sites, for example, non-metallic inclusions, such as
6

AI2O3,SiO2 or silicates, and nitrides and carbides such as BN, AIN, ZrN,
Nb(C,N) and V(C,N) for the nucleation of graphite.
It is generally known that graphitization readily proceeds in hypereutectoid
composition. However, graphitization of hypereutectoid steel is difficult and
the behaviour of graphitization by alloying and heat treatment is not quite
well known. The present work proposes a new method top create graphites in
an induction melted Si-killed hypoeutectoid graphitic steel to deliver improved
ductility with low yield ratio along with increased strength.
The composition of the steels used in this study was:
0.64-0.68 C, 0.4-0.6 Mn, 0.003-0.008 S, 0.007-0.01 P, 0.68-79 Si, 0.004-
0.006 Al, 0.0042-0.0065 B, 0.0060-0.0120 N
The cast steels were austenized at 1200°C and finish rolled in the range of
920-890°C to reduced thickness. Subsequently, hot rolled samples were
annealed at 710°C in the range of 15-25 hrs in the batch annealing furnace of
Tata Steel for graphitization. In annealed steel, graphites were found to have
nucleated on BN (as crystallographically BN and graphite both are hexagonal
in structure) and at the surface of AI2O3 using SEM-EDS and EPMA analyses.
It is generally seen that the elongation in high carbon steels is less than 10%
and the YS is close to UTS. From the present study it has been observed that
by the development of the process which leads to graphitization and
spheroidization of cementite, it is possible to achieve elongation as high as
20-25%,% RA in the range of 46-51% and the yield ratio about -0.5 (YS-
337-350, UTS-614-690 Mpa). With such a low yield ratio and high ductility, it
is expected that the steel would be highly drawable. After hot-rolling, it is
possible to anneal in roller hearth furnace for 680-710°C for 2-hrs to achieve
the desired graphitization. The desired microstructure can be attained for
suitable strength in the finished product (i. e. drawn wire) by suitable heat
treatment process.
7

The improved process will produce steels with higher drawability and which
can be used in the wire drawing application consistently with higher
productivity.
EXPERIMENTAL DATA
The composition of the steels used in this study is presented in Table I.

Steel
i
No. C Mn S P Si Al Cr B N.ppm
1 0.78 0.41 0.003 0.007 0.74 0.006 0.024 0.0038 140
2 0.68 0.6 0.003 0.01 0.68 0.006 0.04 0.0042 120
3 0.64 0.46 0.008 0.011 0.79 0.004 0.055 0.0065 60
All the steels were made by induction melting furnace and cast into 25 kg
ingots. They were cut longitudinally and were austenized at 1200°C in a
laboratory hot rolling furnace. The heated pieces were hot rolled to 5-mm
thickness with finishing temperatures of 920, 915°C and 895°C for steel 1,2
and 3 respectively, followed by air cooling. After removing scale, the hot
rolled samples were cut into suitable sizes and one set of the samples was
further normalized and hardened at 850°C for 20 mins in a controlled
atmosphere muffle furnace. Subsequently, hot rolled samples were annealed
at 710°C for 15 and 23 hrs and hot-rolled and normalized and hot rolled and
hardened and annealed to compare the effect of normalizing and hardening
treatment on tensile properties with respect to hot rolled samples. The
annealed specimens for metallographic analysis were examined in the
through thickness direction in unetched condition to analyse th evolume
fraction and the size distribution of graphite in the samples, selecting 100
different regions, using Axiovision image analysis software. The
microstructures of the as hot rolled normalized and annealed samples were
examined by optical microscope and scanning electron microscope (SEM).
8

SEM along with energy dispersive spectroscopy (EDS) was used to detect the
presence of boron, nitrogen and carbon in unetched annealed samples.
Electron probe microanalyser (EPMA) was used to detect the nucleation sites
for graphite. Tensile testing was conducted in an Instron machine 5582,
supplied by Instron Limited, Bucks HP, UK, following ASTM E 8M-04 standard.
RESULTS AND OBSERVATION
Figure 1 illustrates the SEM micrograph of cast structure of steel 1, 2 and 3
which is fully pearlitic. Figure 2 shows ferritic-pearlitic structure of hot-rolled
steels 1, 2and 3. Optical micrographs in unetched condition after annealing of
the hot rolled samples at 710°C for 15 and 23 hrs for steels 1-3 are presented
in Figs 3-5. That the graphitization of all cementite in steel is not necessarily
needed, the required properties can be obtained by a combination of the
reduction in the amount of cementite in association with partial graphitization
of cementite, and shape control of the remaining cementite. Utilizing the
structure consisting of ferrite, spheroidal cementite and graphite, the desired
properties are obtained by engineering the volume fraction, size distribution
and morphology of the graphite.
Optical micrographs of etched samples of hot rolled and annealed for 15 hrs
and hot rolled and annealed for 23 hrs are shown in Figure 6 for Steel 1,
Figure 7 for steel 2 and Figure 8 for steel 3, respectively. All the annealed
micrographs showed ferrite together with spheroidal cementite along with
"black" particles. SEM-EDS point analysis was used to confirm the "black"
particles as graphites and a confirmed peak of XC along with the presence of
B and N peaks at the same spot was obtained (Figures 9a and b). Presence of
graphite on BN precipitate in steel 1 and 2 was confirmed using SEM-EDS and
electron probe microanalysis (EPMA); however, no evidence of presence of
BN was obtained in the case of Steel 3. Besides,BN, the surface of AI2O3 also
acted as nucleation site for graphite precipitation. Evidence of graphite
9

precipitation by dissociation of cementite has also been observed. BN
precipitates were however, not detected in the graphite in polished specimens
under optical microscope. BN is precipitated finely during the hot rolling
process and in the current study using ThermocalC software (without
rejecting any phase) it has been analysed that thermodynamically BN is
stable in the temperature range of 200°-1300°C and which covers the
graphite formation temperature range of 200-800°C. In addition to the
crystallographic structure of the precipitates, graphite and BN have similar
morphologies and distribution [25], Since the surface of BN acts as the
preferred nucleation site of graphite [26], the precipitation control of BN is
quite important to control the distribution of graphite.
It is to be noted that in Steel 1 and 2, N 140 ppm and 120 ppm, respectively,
is much more than it is needed for B, 38 ppm and 42 ppm, respectively, as
per stoichiometric requirement (N-120 ppm). While in the case of Steel 3, N
(42 ppm) does not meet its stoichiometric requirement (N-91 ppm) for 65
ppm of B. Steel 1 and Steel 2 annealed at 710°C for 15 hours resulted in 0.9
and 0.74 area % of graphite (Fig. 10) and the average number of graphite
particles in this conditions are 182 and 180, respectively (Fig 11). However,
as the annealing time was increased to 23 hrs, the area % of graphite
decreased to 0.62 and 0.38 and the average No. of graphite particles also
reduced to 116 and 140, for Steel 1 and 2 respectively. The reduction in
percent graphite and average No. of graphite after annealing for 23 hours
compared to annealing for 15 hours is possibly due to the dissolution of
graphite for higher holding time of 23 hours.
The yield strength (YS) and ultimate tensile strength (UTS) for Steel 1 and 2
after graphitising annealing are 370 & 340 Mpa (YS) and 700 & 640 Mpa for
15 hrs annealing, respectively, and 350 & 335 Mpa (YS) and 650 & 620 Mpa
(UTS), for 23 hrs annealing, respectively (Fig 12). While ductility in terms of
% elongation (% El) (Fig. 13) and % reduction in area (% RA)(Fig. 14)
increased marginally from 24.4 & 24.5 (15 hrs) to 25.4 & 25.1 (23 hrs) and
10

from 41.5 & 49 (15 hrs) to 43.5 &51.5 (23 hrs) for Steel 1 and 2, respectively.
The higher ductility and lower strength after annealing for 23 hrs are
presumably due to an increase in the formation of fine spherical graphite
particles [27].
In contrast, in the case of steel 3, the % area fraction of graphite increased
to 0.85 from 0.5 while annealed at 710°C for 23 hrs compared to that for 15
hrs, respectively (Fig. 10). Similarly, the average number of graphite particles
also increased from 85 to 118 for 23 hrs and 15 hrs annealing at 710°C,
respectively (Fig. 11). The rise in area % graphite and increase in number of
graphite particles can be attributed to more and more number of dissociation
of cementite with time and nucleation of graphite on its surface along with
heterogeneous nucleation of graphite at the surface of AI2O3 and growth of
the particles after site saturation [28] after annealing for a longer period of
time, i.e., 23 hrs. The disintegration of coarse graphite particle (Fig. 15) was
also noticed after the annealing was carried out for 23 hrs, that resulted in
number of finer particles in the surrounding adjacent area of the disintegrated
coarse particle, which clearly indicates the stage prior to dissolution of coarse
graphite particles. Thus the increase in number of particles after 23 hrs of
annealing is suggested to be the outcome of the disintegration process of the
coarse graphite particle and which subsequently has generated the finer
graphite particles surrounding the disintegrated coarse particle. While YS
slightly increases to 350 (23 hrs) from 340 Mpa (15 hrs) and UTS increased
690 Mpa (23 hrs) from 670 Mpa (15 hrs), respectively (Fig.13). Accordingly,
% El (Fig. 13) and % RA (Fig. 14) reduced to 22.7% and 45.1% for 23 hrs
from 24% and 47% for 15 hours annealing, respectively. The increase in
strength and decrease in ductility after 23 hrs. annealing can be attributed to
coarsening of graphite particles after 23 hours annealing. The low YS/UTS (~
0.50-0.55) is beneficial for extensive drawability.
11

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WE CLAIM
1. Development of Hypoeutectoid graphite steel with enhanced
drawability of wires comprising:-
- a steel having composition in Wt%
C-0.64-0.68
Mn- 0.4-0.6
S- 0.003-0.008
P- 0.007-0.01
Si- 0.68-0.79
Al- 0.004-0.006
B- 0.0042-0.0065
N- 0.0060- 0.0120
- the said cast steels were autenitized at 1200°C and hot rolled;
- finished rolling in the range 920-890°C to reduce thickness;
- hot rolled samples were annealed at 710°C in the range 15-25
hr;
2. The Hypoeutectoid graphite steel as claimed in claim 1 wherein the
said steel possess elongation 20-25%, Ra in the range of 46-51% and
the yield ratio about 0.5 (YS 337-350 UPS 614-690 Mpa).

Graphitic steels are mainly used for machine structural parts. However, in the
present study, it has been shown that graphitic steels can be used for wire
drawing application with extensive drawability. It is generally seen that the
elongation in high carbon steels used for wire drawing applications is less
than 10% and the YS is close to UTS. However, by transforming the ferrite-
cementite structure to a ferrite, graphite and spheroidised cementite
structure, drawability of the wires can be increased substantially and at the
same time, after first softening by graphitization, the strength can be
regained by dissolving graphite in austenite and quenching. The graphitization
process during the annealing of carbon steel consists of two steps: 1)
dissolution of cementite and 2) nucleation of graphite. The approaches
adopted to accelerate graphitization are 1) by destabilization of the cementite
phase by alloying with Si and reducing Mn and Cr and 2) by provision of
heterogeneous nucleation sites, for example, non-metallic inclusions, such as