The present invention relates to a process for improvement in mechanical property
particularly the percentage reduction in area (% RA) of hot rolled high carbon wire rod
in Wire Rod Mill for instant cold drawing operation of the hot rolled high carbon wire
rod.
BACKGROUND OF THE INVENTION
High carbon steel rods of thicker sections (>6 mm diameter) for wire drawing purpose
requires to be stored after rolling for about 10 to 15 days for achieving its desired
ductility (reduction in area >30 %) in Wire Rod Mill for cold drawing application.
High carbon steels rod (>6 mm diameter) possesses very low RA, normally < 5% just
after rolling, which is not suitable for wire drawing operation. For wire drawing
operation, the wire rod should possess > 30 % reduction in area (RA). So, in normal
practice, the wire rod coils are stored for a long duration (10 to 15 days) to achieve the
required reduction in area (>30 %) for further processing into wire. Long storage of coil
not only causes inventory loss but also has storing and handling issues.
High carbon steel wire rods produced from hot rolling are drawn to the required final
diameter. During this operation, hydrogen produces a harmful effect on steel ductility.
It is known from earlier analysis that Phenomenon like hydrogen embrittlement is
responsible for such a degradation of mechanical properties in high carbon steel wire
rods. Hydrogen once enters into the steel structure; it can cause an embrittling effect,
as the loss of macroscopic ductility or cracking. The effect can be observed during
tensile testing of the specimens

as a drastic decrement in % Reduction in Area (RA) values. It is well known that after a
few days at room temperature, the high diffusion coefficient of hydrogen in iron allows
that element to be released from of the steel rod but the presence of defects such as
vacancies, voids, grain boundaries, dislocations, etc. can act as hydrogen traps, strongly
reducing the rate by which hydrogen is released from the steel rods.
High Carbon steel wire rods (>6 mm diameter) cannot be processed immediately after
rolling for wire drawing due to its brittle nature caused by trapped hydrogen as found
by the laboratory studies. It becomes suitable for drawing after storing for 10 to 15
days when RA improves from 5 to 30 %. Storage of coils for long time increases the
costs of inventory and production lead time. Many attempts were made in past to
overcome this problem but could not succeed.
The present method of storing the high carbon wire rod coil and waiting for long time
for the desired RA is a chronic pain area for wire manufactures. Lots of inventory of
material is blocked and the need of checking the material properties time to time is a
cumbersome method of passing the material for wire drawing purpose. Present practice
high carbon wire rods (diameter > 6 mm) coils after hot rolling is stored for 15 days to
achieve the required property (RA).
Several attempts have been made earlier to shorten the storage time of the coil, but
never succeeded.

OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process for improvement
of percentage reduction in area (% RA) of hot rolled high carbon wire rod without
external heat-treatment for instant cold drawing operation of the hot rolled high carbon
wire rod, in which the RA is improved from 6% to >30% within 24h.
Another object of the invention is to propose a process for improvement in mechanical
property particularly the percentage reduction in area (% RA) of hot rolled high carbon
wire rod in Wire Rod Mill to allow instant cold drawing operation of the hot rolled high
carbon wire rod.
A still another object of the invention is to propose a process for improvement in
mechanical property particularly the percentage reduction in area (% RA) of hot rolled
high carbon wire rod in Wire Rod Mill for instant cold drawing operation of the hot
rolled high carbon wire rod, which utilizes the residual heat present in the hot rolled
coils for improvement of RA.
A further object of the invention is to propose a process for improvement in mechanical
property particularly the percentage reduction in area (% RA) of hot rolled high carbon
wire rod in Wire Rod Mill for instant cold drawing operation of the hot rolled high
carbon wire rod, in which the coils are insulated after hot-rolling for improving the RA.

SUMMARY OF THE INVENTION
The present inventors experimented to identity the reason of low RA instantly after hot
rolling conducting laboratory studies. It was found that the low RA is due to hydrogen
present in the wire rod which gradually decreases after storage. Various experiments at
laboratory scale indicated the time temperature relationship for diffusing out the
interstitial hydrogen to achieve the desired RA. Then, it was concluded that the solution
to the prior art problem is to diffuse the hydrogen out from the high carbon wire rod
without requiring any additional facilities that is to say, the coils after rolling and
compaction have sufficient heat to remove the hydrogen in a short period and hence
this heat was utilized by covering the coils with some insulating cloths at the WRM,
which exhibited a drastic improvement in RA.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Surface of brittle and ductile fractured wire samples
Figure 2 Scanning electron fractrography of the brittle fracture surface
Figure 3 Scanning electron fractrography of the brittle fracture surface
Figure 4 %RA obtained when heated to a defined temperature and cooled in
furnace at laboratory
Figure 5 Decrease of hydrogen (ppm) in rod sample with increase in storage
time
Figure 6(a) Partially wrapped coil after compactor
Figure 6(b) Fully wrapped coil at coil storage yard of WRM plant
Figure 7 Results obtained in plant trail

DETAIL DESCRIPTION OF THE INVENTION
The present invention was carried out by modifying the existing rolling process at the
Wire Rod Mill of Tata Steel. The conventional procedure of rolling wire rods includes
carrying out the rolling process at temperature greater than 500 deg C. Towards the
end of the rolling process when the coil inspection takes place, and just before the wire
rods are wrapped in the form of a coil by the process of compaction, they have
temperatures around 200 deg C at the outer end of the coil. However, after compaction
this temperature falls rapidly as the wire rod coils are kept for storage.
Present experiment was conducted on high carbon steel wire rod to measure the
reduction in area immediately after rolling and after various ageing time. Temperature
at various stages after finish rolling was measured (for 5mm and 12mm diameter rods),
which is shown below in the table 1.



The central idea of this invention is to utilize the heat present inherently in the wire rod
coils towards the end of the rolling process, instead of employing external heating
sources. This trapped heat will help achieve the desired modification of the mechanical
properties in lesser time than what is required during normal storage of the coils.
In accordance with the invention, the process after compaction of the wire rod coils was
modified. In order to trap the heat within the coils, bags/ covers designed from
insulating material were wrapped around the wire rod coils (Fig. 6). General
temperature after coil compactor is in the range of 100-250 C and this temperature is
quite enough to accelerate the diffusion process for hydrogen. A trial has been
successfully conducted by insulating the hot coils with ceramic cloths after compactor at
Wire Rod Mill (WRM).
The insulating materials were used in two different designs to establish this experiment.
One was to wrap the coils directly by using the cloth roll of insulating material. The
second approach included creating a bag beforehand using the insulating materials,
which had the dimensions slightly larger than the size of compacted wire rod coils. Both
these designs established the invention. For both the designs, two coils each were
wrapped and their properties studied.
For comparison, two coils were also observed following the conventional approach, i. e.
storage in the open without using any form of insulating material wrappings. All the 6
coils were stored for an entire day (~ 24 hours), and then their mechanical properties

were tested. Table 2 shows the mechanical properties obtained for all the coils studied
for this experiment.

The testing of mechanical properties of the coils after one day of storage establishes
that trapping the heat within the coils leads to achieving the desired Reduction in area
in a much quicker time as compared to when the coils are stored in open. From table 2,
it can be observed that the coils that were covered using the insulating materials
achieved an average Reduction in Area (RA) of 37.7%, whereas the uncovered coils
could only manage a RA value of 20%.This finding substantiates the central idea of the
invention. A RA value of >35% is required to carry out drawing process from the wire
rods.

To further validate that the wire rods are ready for drawing process, analysis of Fracture
Surface (Fig.1) was done for the tested wire rods. The rod on the left hand side is the
uncovered coils following the conventional route and shows brittle fracture, whereas that on the
right hand side is the one from the coils that were wrapped using the insulating materials
in accordance with the invention, and shows ductile fracture at the cross section.
Features of the fracture surfaces of the tensile specimens after testing were analysed in
the scanning electron microscope used to characterize the microstructure. Fig.2
represents the microscopic characteristics of the brittle fracture (RA=5%) surface of the
samples and Figure 3 shows the corresponding microstructure of the ductile samples
(RA=30%). This is further evidence that modification of the process in accordance with
the invention has led to desired change in properties.
The main change in fracture appearance between the two was the disappearance of the
radial fragile region. This shows the observed increase in ductility and presents the
evidence of the role of hydrogen release in the enhancement of the mechanical
properties of the high carbon wire rod during storage.
Another study was conducted by simulating furnace cooling at different temperature
and RA was measured to find out the temperature from where the desired RA can be
achieved in slow cooling. The result is shown in Figure 4.
Mathematical support:
Mathematical calculation of Hydrogen release rate can be expressed by the Equation-1.

Where,

Ct = mean Hydrogen concentration at time‘t’,
C∞ = Hydrogen concentration at t = ∞
Co = initial Hydrogen concentration
De = diffusion co-efficient
d = diameter of wire in mm.
t = time.
Following data are used while doing the calculation for Ct
d= 12mm, De=2.3×10-5mm2s-1= 13.8×10-4mm2min-1, CO = 1.85ppm, C∞ =
0.56ppm
It is experimentally found that the Hydrogen concentration decreases to a level of
0.6ppm and then becomes almost constant. From the above formula we can
estimate the time to reach the concentration of 0.6ppm
(Ct-C∞)\0.72 (CO-C∞) = exp (-22.2×De ×t\d2) …………………………..(2)
Here, Ct =0.6ppm, C∞ =0.56ppm, CO= 2 ppm, d = 12mm, De=13.8×10-4mm2min-1.
Putting these value‘t’ comes out to be equal to 255 hr. (11 days).
Now the problem with high carbon wire rod at WRM is the storage time, which is
related with the rate of hydrogen release. Therefore, how to diffuse the hydrogen
present in the steel at faster rate is to be innovated. Hence, the idea is to wrap the coil
just after auto compaction by an heat insulating material so that the residual heat
contained by the coil can be kept and a much slower rate of cooling is possible than a

unwrapped condition where the coil is in direct contact with open air atmosphere and
hence the rate of diffusion of internal hydrogen is increased substantially..
Solution:
A trial has been successfully conducted by insulating the hot coils with ceramic cloths at
around 150-170 deg. C after compactor at Wire Rod Mill (WRM).
Results:
Result showed improvement in RA from 5 % to 35% within 24 h suitable for wire
drawing. This solution does not require any capital investment and no external heat
energy.

WE CLAIM :
1. A process for improvement of percentage reduction in area (% RA) of hot rolled
high carbon wire rod without external heat-treatment for instant cold drawing
operation of the hot rolled high carbon wire rod, the improvement is
characterized by comprising :-
allowing the hot rolled high carbon wire rod coils exiting from the compactor of
the wire Rod Mill, the average temperature of the coils after the compactor
ranging between 100°C to 250°C;
instant wrapping around the hot coils insulating material to trap the heat within
the coils; and
causing the residual heat to accelerate the diffusion process for the hydrogen
content such that the percentage reduction in area (RA) of the hot-rolled high
carbon wire rod is improved to an average value of 37, which interalia allows
immediate commencing of cold-rolling of the wire rods without wait-in-storage
for about 15 days after the hot-rolling.
2. The process as claimed in claim 1, wherein the insulating of the hot rolled wire
rods comprises wraping with cloth roll of insulating materials.
3. The process as claimed in claim 1 wherein the step of insulating comprises
producing bags of insulating materials having sizes larger than the compacted
wire rod coils and immediately covering the coils upon exit from the compactor.