FIELD OF THE INVENTION
The present invention relates to a method of producing fully pearlitic low carbon
hypoeutectoid steel through alloy design and processing.
BACKGROUND OF THE INVENTION
Typically, a fully pearlitci steel is obtained at a point, when the chemistry of the
steel reaches the eutectoid composition. This could be a point as can be seen
from a normal Fe-C binary phase diagram, when the carbon level reaches at
0.8% level, or a composition range, where pearlite, forms over a range of C
concentration. Thermodynamically it is also possible to develop a fully pearlitic
structure in hypoeutectoid steel by controlling the transformation and chemical
composition [1]. The shaded area in Figure 1 shows a region of the Fe-C phase-
diagram in which it is possible to get the y to pearlite transformation directly
without forming any proeutectoid phase. The extrapolated ACM line of figure 1
shows a composition of the austenite in equilibrium with cementite. At any point
within the shaded region, austenite will transform into cementite without forming
any proeutectoid ferrite. However, direct formation of pearlite in this way
becomes more and more difficult with lowering of C because of the following two
reasons. Firstly, with lowering of C, a lower temperature is required for the direct
austenite to pearlite transformation to begin; however steel with lower C has
higher Bs temperature as a result of which it might so happen that before the
austenite to pearlite transformation initiates, bainite formation would start.
Secondly, at lower temperature, the transformation kinetics will be very slow, so
a very long holding time is required for the transformation to take place.

Though Houin et al [2] have produced fully pearlitic microstructure with C
ranging from 0.2 0.8 wt% by varying the cooling rate and discussed various
properties of pearlite in detail, the publication fails to clearly describe and
analyse how it would be possible to maintain such precise cooling rates by
avoiding formation of ferrite and bainite.
In view of the prior art disadvantages, it is necessary to propose a new steel
chemistry so that a fully pearlitic transformation can take place in a proeutectoid
steel under slow cooling condition.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process to prdocue a fully
pearlitic microstructure in hypoeutectoid steel.
Another object of the invention is to propose a process to produce a fully
pearlitic microstructure in hypoeutectoid steel, which eliminates an isothermal
holding treatment for this transformation.
A still another object of the invention is to propose a process to produce a fully
pearlitic microstructure in hypoeutectoid steel, which is implementable without
employing a very high and precise cooling rate.
A further object of the invention is to propose a process to produce a fully
pearlitic microstructure in hypoeutectoid steel, which is enabled to provide a

steel grade having tensile strength of minimum 850 MPa with an elongation
value of minimum 8% in as heat treated condition.
A still further objection of the invention is to propose a process to produce a fully
pearlitic microstructure in hypoeutectoid steel, which is capable to provide a
minimum reduction of area for example, 20% during tensile testing of the
produced steel grade.
SUMMARY OF THE INVENTION
Accordingly, there is provided a process under paraequilibrium condition to
produce a fully pearlitic microstructure (<98%) in hypoeutectoid steel,
characterized in that the alloying elements such as herein described remaining
fixed, the fully pearlitic microstructure is developed under minimum 0.35 wt%
carbon without any isothermal holding treatment, and in that an elongation value
for minimum 8% with a tensile strength of minimum 850 MPa, is achieved for
the developed Steel grade.
BRIEF DESCRIPTION OF THE ACCOMPANYING TABLES/DRAWINGS
Table 1 Targeted and actual chemical composition in wt% for the two
steels.
Table 2 The results of EPMA analysis on the proeutectoid ferrite phase, the
value of alloying elements on the ferrite phase is an average of 12
individual readings.

Figure 1 Schematic of Fe-C phase diagram showing the Hultgren
extrapolation
Figure 2 Equilibrium phase diagram developed using Thermo-Calc software
for the alloy developed. The paraequilibrium A3 line is shown as a
broken line.
Figure 3 Optical microstructure of the decarburized layer of the developed
steel showing proeutectoid film of ferrite formed at the prior
austenite grain boundary (a) and the fully pearlitic inside region
showing continuous and discontinuous pearlite lamellae (b).
Figure 4 SEM microstructure of the decarburized layer of the Steel showing
fine film of ferrite at the prior austenite grain boundary (left) and
the SEM photo of the fully pearlitic region showing continuous and
discontinuous pearlite lamellae (b).
Figure 5 The concentration profiles of (a) Cr and (b) Mn at a and y interface
at different times at 777°C for Steel 1.
Figure 6 The engineering stress-strain diagram for the newly developed
Steel.
Figure 7 Optical microstructure of a sample Steel for comparison purpose,
showing the formation of proeutectoid ferrite at the grain boundary
regions.

Figure 8 SEM microstructure of the sample Steel showing the formation of
proeutectoid ferrite at the grain boundary regions both in the
decarburized region (a) as well as inside of the steel (b),
Figure 9 Calculated equilibrium phase diagram along with calculated
Paraequilibrium AC3 line for the homogenized sample steel.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The inventors have recognized that when a transformation occurs under
equilibrium condition, all the elements diffuse from a phase with lower solubility
to a phase where their solubility is higher and the elements redistribute
themselves. The equilibrium consideration exists when the diffusion rates of all
the elements are comparable. If it now happens that one component (say,
Component 1) diffuses at a much faster rate than the other (say, component 2)
then the adjoining two phases are in local equilibrium with respect to the fast
diffusing element (component 1), the other element, i.e. component 2, being the
slow diffuser, diffuses too slowly to redistribute themselves at all3. This kind of
equilibrium is called "Paraequilibrium" and normally happens at a temperature
lower than the equilibrium temperature.
During the formation of pearlitic under equilibrium condition, C along with other
elements (Cr, Mn etc) diffuse from the parent austenite phase to the
cementite and thus from solute depleted ferrite sandwiched between two
cementite plates.

Accordingly, the inventors have experimented whether the transformation if
allowed to proceed under Paraequilibrium restriction, a pearlite at a temperature
lower than equilibrium temperature can be obtained. In the following section, a
process is described where pearlite can be produced at lower than equilibrium
temperatures, following paraequilibrium restriction.
Figure 2 shows a calculated phase diagram for Steel 1 with 1.6% Si, 2.0% Mn,
1% Cr (all in wt%). It is clear from the equilibrium diagram (Figure 1) that due
to the additions of alloying elements, the eutectoid carbon composition has been
reduced to 0.59 wt%. If now the paragequilibrium restriction is imposed allowing
only C to diffuse, the eutectoid carbon concentration comes down to as low as
0.38%. It is thus expected, if steel with 0.4 wt% C is made and is allowed to
transform under paraequilibrium restriction, it will be possible to produce pearlite
even with hypoeutectoid carbon content.
According to the invention, two steels with chemical compositions as mentioned
in Table 1 have been made in an air induction furnace and the ingots were cast
in a sand mould. The ingots were then forged to break the cast and dendritic
structures and then reheated to 1200°C for 48 hours. The exact chemical
compositions were analysed after homogenizing treatment and is presented in
Table 1. The C concentration of Steel 2 was intentionally kept low to investigate
the effect of further lowering C under the paraequilibrium condition on pearlite
transformation.
Figure 3 and 4 demonstrates the microstructure of the Steel 1 obtained after
furnace cooling from 1200°C. Figures 3a and 4a show the decarburized region of

the sample and the proeutectoid ferrite at the prior austenite grain boundary.
Beyond this decarburized layer, the microstructure is fully pearlitic. It can be
seen from Figure 3b and 4b that proeutectoid ferrite has not formed at all (<2%
in volume) which can not be obtained had the transformation followed
equilibrium phase diagram where 37% proeutectoid ferrite should have been
formed.
The only possible mechanism through which such a transformation (y to pearlite
in Steel 1) can occur is by paraequilibrium which shifts the eutectoid carbon to
0.39 wt%, as described earlier. This can be seen from the phase diagram
presented in Figure 2. To prove that proeutectoid ferrite formation indeed
followed the paraequilibrium, detailed analysis of kinetics of ferrite
transformation has been done at 777°C [3]. It is known that at temperatures
above the paraequilibrium line the γ→α transformation has to occur following the
"Partitioning Local Equilibrium (PLE)" mechanism which requires the partitioning
of substitutional alloying elements like Mn, Cr and Si between austenite and
ferrite phases. One way of evaluating the changes of partitioning is by
calculating the diffusion distances of these alloying elements at the
transformation temperature. The diffusion distance, z, is expressed as z=2D/v
[4] where D is the diffusivity of the solute concerned and v is the velocity of the
interface. If the diffusion distances of the substitutional elements are very small
then it can be assumed that the growth can occur by paraequilibrium. To access
the diffusion distances for various substitutional elements, austenite grid size
was taken to be 500 nm and the same for ferrite was taken as 1 nm. The z was
calculated at 777°C which is above the paraequilibrium A3 line but below

equilibrium A3 temperature (Point A in figure 2). At this temperature, ferrite was
allowed to grow following the "No Parition Local Equilibrium (NPLE)" mechanism
and the growth of proeutectoid ferrite was tracked. The diffusion fields for
various alloying elements were also tracked from which Cr and Mn fields are
shown in Figure 5. It can be seen from figure 4 that the diffusion distance for Cr
and Mn at the start of the transformation are unrealistically small. Consequently,
the transformation is suppressed until the paraequilium phase boundary is
reached (0.7 wt% C AND 752°C) when diffusion of substitution elements are not
thermodynamically necessary for the growth of proeutectoid ferrite.
Under such circumstances, the alloy is made in such a way that even though the
sample is cooled very slowly between equilibrium A3 and paraequilibrium A3
temperatures, the condition is not favourable for PLE growth of proeutectoid
ferrite as a result of which a very little or not ferrite formation takes place
between paraequilibrium and equilibrium A3 temperature (figure 2). The
transformation of austenite to proeutectoid ferrite, in this case, is suppressed to
such a low temperature where y→pearlite transformation becomes possible since
the austenite is supersaturated with both the alloying, elements and the amount
of C with respect to both ferrite and cementite.
As shown, the proeutectoid ferrite formation in this steel is only possible by
paraequilibrium in this condition. This can be indicted by the examination of the
ferrite, which exhibits in the decarburized layer a very little redistribution of
substitutional alloying elements like Mn and Cr. The same was checked with
Electron Probe Micro-Analysis (EPMA) of the ferrite formed at the decarburized

layer very close to the fully pearlitic region. By selecting this area for analysis, it
was ascertained that the EPMA analysis was done on a region where the
composition was not very different from the bulk. An instrument was used for
this purpose with an accelerating voltage of 15 kV, beam current 20 mA and with
a beam diameter of 1 urn. 100% pure Mn and Cr were used to calibrate the
instrument. Table 1 shows the result of the analysis. It is clear from these
experiments that the substitutional alloying elements like Mn and Cr remain
almost same in the ferrite phase as well as in the bulk which is possible when
y→α transformation takes place following the paraequilibrium. The EPMA results
thus confirm that the paraequilibrium condition was rightly considered here.
Figure 6 shows the engineering stress-strain diagram of the inventive steel
grade. The yield and the tensile strength of this steel is found to be 482 and 903
MPa, respectively and the total elongation is about 10%. In fully pearlitic steel,
with about 0.8 wt%C, tensile strength can go as high as 1300 MPa. It has also
been observed that the reduction in area during tensile testing for the produced
steel, was about 23% where as commercially rolled high C steel which
transforms at 620-640°C, the amount of reduction in area is about 40% [8]. The
reason behind the lower tensile properties of the steel can be attributed to its
higher transformation temperature (eutectoid temperature ~ 750°C, Figure 2)
which leads to a higher pearlite spacing (~750°C, Figure 2) which leads to a
higher pearlite spacing (~0.32 urn). It is already known that the larger pearlite
spacing will result lower strength and ductility [5,6,7].
Figure 7 represents the optical microstructures of the Steel 2 from the inner layer

of a 60 mm thick homogenized specimen. The microstructures show the
formation of proeutectoid ferrite at the grain boundaries under the identical
condition at which Steel 1 was treated. The SEM microstructure provided in
figure 8 taken at both the decarburized region (Figure 8a) as well as inside the
material (Figure 8b) also reveled the formation of pearlite in the grain boundary
regions. The formation of this grain boundary ferrite indicates that the steel has
not achieved the eutectoid condition even under paraequilibrium condition.
Image analysis result, carried out on 15 frames of optical microstructures,
revealed the presence of 7.6± 1.3 wt% ferrite at the grain boundary regions.
Similar kind of observation is also apparent from the calculated phase diagram as
presented in figure 9. From this diagram (figure 9), under equilibrium and
paraequilibrium condition, the amounts of ferrite which can be expected from
mass balance by applying lever rule are 40 wt% and 11wt% respectively. Thus,
the image analysis result corresponds very well with the theoretical calculation
and it could be concluded that through the pearlitic transformation in the second
heat also followed the paraequilibrium mechanism yet it will not be possible to
form fully pearlitic structure in the second chemistry. This indicates there is
limiting value of carbon in these steels, with other similar composition beyond
which fully pearlitic microstructure (ferrite < 4%) cap not be produced by the
process followed here. Our first steel gives the limiting value of carbon with
some other alloying elements below which fully pearlitic transformation is not
possible.

References:
1. J.W. Christian, The Theory of Transformations in Metals and Alloys, 2002,
Pergamon.
2. J.P. Houin, A. Simon and G. Beck, Trans ISIJ, 21,1981, 726
3. D.E. Coates, Met Trans. Vol. 3,1972, PP. 1203-1212
4. DICTRA, Thermo-Cale Software AB, Stockholm Technology Park,
Bjomasvagen 21, SE-11347, Stockkholm, Sweeden.
5. D.E. Coates, Metall. Trans, 4, 1973, 1077
6. A.R. Marder and B.L. Bramfitt, Met. Trans.A 6A, 1975, 2009
7. A.R. Marder and B.L. Bramfitt, Met Trans.A 7A, 1976, 365
8. S. Jaiswal and I.D. Mclvro, Ironmaking and Steelmaking, 16,1989, 49.

WE CLAIM
1. A process to produce a fully pearlitic microstructure (<98%) under
paraequilibrium condition) in hypoeutectoid steel, characterized in that
the other alloying elements such as herein described remaining fixed, the
fully pearlitic microstructure developed under minimum 0.35 wt% carbon
without any isothermal holding treatment, and in that an elongation value
of minimum 8% with a tensile strength of minimum 850 MPa, is achieved
for the developed steel grade.
2. The process as claimed in claim 1, wherein the chemical composition of
the starting steel is carbon, silicon, Manganese, Chromium, and aluminum
is in weight percentage 0.37,1.65,1.84, 0.92, and 0.018 respectively.

The invention relates to a process under paraequilibrium condition to produce a
fully pearlitic microstructure (<98%) in hypoeutectoid steel, the other alloying
elements such as herein described remaining fixed, the fully pearlitic
microstructure is developed under minimum 0.35 wt% carbon without any
isothermal holding treatment, and in that an elongation value of minimum 8%
with a tensile strength of minimum 850 MPa, is achieved for the developed Steel
grade.
{Figure 4}