FIELD OF INVENTION
The present invention relates to a method of producing fully pearlitic low carbon
hypoeutectoid steel through alloy design and processing.
BACKGROUND OF INNOVATION
Typically, fully pearlitic steel is obtained 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 at 0.8%C level or even can be a range of composition
where pearlite forms over a range of C concentration. Thermodynamically it is
also possible to develop fully pearlitic structure in hypoeutectoid steel by
controlling the transformation temperature 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 γ→pearlite transformation directly without forming any
proeutectoid phase. The extrapolated ACM line of figure 1 shows the composition
of the austenite in equilibrium with cementite, at any point within the shaded
region, austenite will transform into cementite without forming any 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 B2 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 would be required for the transformation to take place.

Though Houin et al [2] have produced with C ranging from 0.2 0.8 wt% by
varying the cooling rate and discussed various properties of pearlite in detail, it
was not very clearly described and analysed how was it possible to maintain such
precise cooling rates to avoid formation of ferrite and bainite.
In view of the prior art disadvantages, it is necessary to develop a new steel
chemistry so that pearlitic transformation can take place in a proeutectoid steel
under slow cooling condition.
OBJECTS OF THE INNOVATION
It is therefore an object of the invention to propose a process to develop a fully
pearlitic microstructure in hypoeutectoid steel.
Another object of the invention is to propose a process to develop 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 develop a fully
pearlitic microstructure in hypoeutectoid steel, which can be implemented
without employing a very high and precise cooling rate.
A further object of the invention is to propose a process to develop a fully
pearlitic microstructure in hypoeutectoid steel, which is enabled to develop a
steel grade having tensile strength of minimum 850 MPa with an elongation
value of minimum 8%.

A still further object of the invention is to propose a process to develop a fully
pearlitic microstructure in hypoeutectoid steel, which is capable to provide a
reduction of area of minimum 20% during tensile testing of the developed 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 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 for minimum 8% with a tensile strength of minimum 850 MPa,
is achieved for the developed Steel grade.
BRIEF DESCRIPTIONS 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 the α→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 boundry
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
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 pearlite under equilibrium condition, C along with other
elements (Cr, Mn etc) diffuse from the patent austenite phase to the
cementite and thuse form solute e depleted ferrite sandwiched between two
cementite plates. If this transformation is allowed to proceed under
Paraequilibrium restriction, then it is expected to obtain pearlite at a temperature
lower than equilibrium temperature. In the following section, we will describe a
process 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 paraequilibrium restriction is imposed allowing
only C to diffuse, the eutectoid C 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 could be possible to produce pearlite even
with hypoeutectoid carbon content.
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 lower C
under the paraequilibrium condition on pearlite transformation.
Figures 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→pearlitein
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 y→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 chances of partitioning is by calculating the diffusion
distances of these alloying elements at the transformation temperature. The
diffusion distance 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 assess 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 distances for Cr and Mn at the start of the
transformation are unrealistically small. Consequently, the transformation is
suppressed until the paraequilibrium phase boundary is reached (0.37 wt% C
and 752°C) when diffusion of substitution elements are not thermodynamically
necessary for the growth of proeutectoid ferrite.

Under such circumstances, it can be said that the alloy design was made in such
a way that even though the sample was 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 there will be a very little or no
ferrite formation between paraequilibrium and equilibrium A3 temperature (figure
2). The transformation of austenite to proeutectoid ferrite, in this case, will be
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 mechanism in this condition, the examination of the ferrite in the
decarburized layer must show 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 choosing 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 this newly developed
steel. 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 developed
steel, during this work 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 (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 11 wt% respectively. Thus,
the image analysis result corresponds very well with the theoretical calculation
and it could be concluded that though 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%) can not be produced by the
process followed here.
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, Stockholm, 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 under paraequilibrium condition to produce a fully pearlitic
microstructure (<98%) in hypoeutectoid steel, characterized in that 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.
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,
characterized in that 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.