FIELD OF THE INVENTION
This invention relates to the production of interstitial free (IF) steels.
This invention further relates to the process for the production of
Interstitial free (IF) steels by severe plastic deformation (SPD) method by
multi-axial forging (MAF) at room temperature to produce ultrafine grains
of the order of few hundreds of nanometers and to increase the strength
many folds than the initial material.
BACKGROUND OF THE INVENTION:
Interstitial-free (IF) steels constitute an important class of industrial
materials. In recent times it has been extensively employed in the
automobile industries. These are a typical class of extra-low carbon
steels, where the amounts of interstitial elements are present in the ppm.
level. Typically the total interstitial content are in the region <0.0030 wt.
% C and <0.0040 wt% N. In IF steels either titanium or niobium or both
are important alloying additions. These elements stabilize the carbon in
the steel by forming carbide precipitates and hence, prevent the existence
of any solute interstitial atoms, therefore, IF steels are also non-aging. IF
steels provide very high levels of formability, as indicated by the ratio of
width to thickness strains during forming (r ≥1.8) and are adopted to

fabricate car body panels like rear floor pan, front and rear inners and
spare wheel well. However, the future of these steels lies in the improving
their strength along with restoration of appreciable amount of ductility
and at the same instance improving the crack resistance of the fabricated
automotive parts. Grain size refinement of interstitial-free steels to
submicron level leads to an obvious increase in strength along with an
optimum amount of toughness in the material. It also improves the
fatigue resistance and causes a significant drop in the superplastic
temperature of the material.
In recent years, a number of innovative severe plastic deformation (SPD)
techniques have been developed for deforming metals to a very high
degree of plastic strains with the aim of producing greatly refined grain
structures without entailing requirements of exotic alloy additions or
costly thermomechanical treatments. These include severe plastic torsion
straining (SPTS) multi-axial forging (MAF), accumulative roll bonding
(ARB) and equal channel angular extrusions (ECAE). Severe plastic
deformation processes can produce materials with a grain size of the
order of 100-1000 nm. A distinct advantage of these processes is that it
can be scaled up to produce large billets in industry and is relatively

simple and also a cheap process. The common novel feature of these
processes is that the net shape of the final product remains essentially
the same as the starting material after any given number of passes, so
there is no constraint on the strain that is build up in the material. In
comparison to conventional metal working process, like rolling,
extrusion, effective strains greater than 4 can only be obtained in
filaments which have few structural applications.
MAF is basically a plane stain compression applied to all the three axes
one after another for completion of one cycle. It involves abrupt changes
in strain path. The process can be repeated for a number of cycles in
order to obtain desired microstructure.
In the paper entitled formation of sub-micron and nanocrystalline grain
structure by severe plastic deformation' by P.B. Pragnell et al.
(Proceedings of 22nd Riso International Symposium on Materials Science,
pp. 105-126 (2001)). The definition of submicron or nanocrystalline grain
size has been proposed as a structure where (a) average spacing of the
high angle grain boundaries (HAGBs), having misorientation angle
greater than 15°, must be less than 1 micron in all orientations, and (b)

the proportions of HAGB area with respect to total boundary area in the
material must be greater than 70%.
Multiaxial forging, multiple forging or 'abc' deformation process, as it has
been variously called, was originally developed by G.A Salishchev. This
method is very effective in producing sub-micron grain size in metals and
alloys and the processing temperature lies typically between ~0.1-0.5Tm,
where Tm represents the melting temperature. The principle of multiaxial
forging assumes multiple repeats of a free forging operation with a
change of the axis of applied load after every forging operation. Although
the heterogeneity of strain developed in the material is much more in
multiaxial forging than that during ECAE or HPT, compared to other SPD
processes the triumph of this process lies in its simplicity both in terms
of its principle and tooling associated with it. This technique has
tremendous potential to be converted from laboratory scale to producing
billets at large industrial scale.
Until now multiaxial forging has been conducted on titanium,
magnesium and nickel based alloys. In the papers entitled
"Submicrocrystalline and Nanocrystalline Structure Formation in

Materials and Search for Outstanding Superplastic Properties" by
Gennady A. Salishchev, Oleg Valiakhmetov, V.A. Valitov, S.K. Mukhtarov
published in Materials Science Forum, vol. 170-172, 1994, ppl21 and
"Diffusion and Related Phenomenon in bulk Nanostructured Materials"
by G.A. Salishshev, O.R. Valiahmetov, R.M. Galeev and S.P. Malysheva
published in Russian Metally, vol. 4, 1996, pp86, nanostructured pure
titanium were fabricated using multiple forging. In the US patent entitled
"Method of processing titanium alloys "PCT/US97/18642, WO 9817836
Al, 1998 by O.A. Kaibyshev, G.A. Salishchev, R.M. Galeyev, R.Ya.
Lutfullin, O.R. Valiakhmetov and paper "Production of
submicrocrystalline structure in large-scale Ti-6A1-4V billet by warm
severe deformation processing" by S.V. Zherebtsov, G.A. Salishchev, R.M.
Galeyev, O.R. Valiakhmetov, S.Yu. Mironov, S.L. Semiatin published in
Scripta materialia, vol. 51, 2004, ppll47, a two-phase Ti 6A1-4V alloy
has been successfully experimented to produce bulk submicron grain
size. Nanostructured magnesium alloy (Mg-6%Zr) and high strength
nickel base alloys has been fabricated and reported respectively in the
papers entitled "Formation of Submicrocrystaline Structure in Materials
During Dynamic Recrystallization" by O. Kaibyshev, R. Kaibyshev and G.
Salishchev published in Materials Science Forum, vol. 113-115, pp. 423

and "submicrocrystalline and nanocrystalline structure formation in
materials and search for outstanding superplastic properties" by G.A.
Salishchev, O.R. Valiakhmetov, V.A. Valitov, S.K. Mukhtarov, published
in Materials Science Forum, vol. 170-172, 1994, pp. 121. In all the
cases, the forging operations were carried out at elevated temperatures
so that the process is associated with dynamic recrystallization. The
average grain size obtained after the deformation process is of the order
less than 500 nm.
Researchers had been trying to generate materials of submiron grain size
of IF steel by different SPD processes. ECAE is one of the attractive
methods of imparting severe straining in one pass without distorting the
material geometry. In this process, the die comprises of two
interconnected channels-entry and exit channels, intersecting at an
angle of 90°, 120° or 135°. The material is fed through the entry channel
and forced out through the exit channel. The processing conditions and
the properties derived henceforth has been reported in some of the
papers entitled "Effect of processing route on microstructure and texture
development in equal channel angular extrusion of interstitial-free steel"
by Saiyi Li, Azdiar A. Gazder, I.J. Beyerlein, E.V. Pereloma, C.H.J. Davies

published in Acta Materialia, vol. 54, 2007, pp. 1087-1100 and "On the
strength of boundaries in submicron IF Steel" by J. De Messemaeker, B.
Verlinden, J. Van Humbeeck published in Materials Letters, vol. 58,
2004, pp. 3782-3786.
In some papers entitled "Nanoscale crystallographic analysis of ultrafine
grained IF steel fabricated by ARB process" by N. Tsuji, R. Ueji, Y.
Minamino published in Scripta materialia, vol. 47, 2002, pp. 69-76 and
"Ultra-fine grain bulk steel produced by accumulative roll bonding (ARB)
process" by N. Tsuji, Y. Saito, H. Utsunomiya and S. Tanigawa published
in Scripta materialia, vol. 40, 1999, pp. 795-800, IF steel has been
processed via a different SPD technique called ARB. In ARB, strips of
metal sheets are rolled by stacking them on top of each other, which get
adhered after being rolled at a fairly high rolling speed. The process is
generally carried out at higher temperature and results in the formation
of sub-micron grain sizes.
SPD of IF steel is possible by ECAE at room temperature and by ARB at
higher temperature. These methods only help in understanding the
scientific benefits achieved by grain refinement to submicron levels and
development of preferred texture. However, none of these methods are
feasible enough to be scaled up to be used for industrial applications.

OBJECTS OF THE INVENTION:
It is therefore an object of this invention is to propose a process for the
production of interstitial free (IF) steels, which is simple.
It is a further object of this invention is to propose a process for the
production of interstitial free (IF) steels, which is cost-effective.
Another object of this invention is to propose a process for the
production of interstitial free (IF) steels, which has sufficient strength to
meet the future demands of the automobile industry.
Yet another object of this invention is to propose a process for the
production of interstitial free (IF) steels, which can be scaled up to meet
industry requirements.
These and other objects and advantages of the invention will be apparent
from the ensuing description, when read in conjunction with the
accompanying drawings

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig 1. schematic diagram of the multi-axial forging apparatus.
Fig 2. Schematic representation of the billet and the reference direction it
was compressed along its three axes with reference to the die axes.
Fig 3. Photograph of the billet in its initial condition and after undergoing
forging up to four cycles.
Fig 4. Optical Micrograph of the starting material.
Fig 5 Inverse pole figure (IPF) maps and pattern quality maps of IF steel
superimposed after 1 cycle and 4 cycles.
Fig 6 Stress strain curves of the starting material, material after 1st
cycle, 2nd cycle and 4 cycle.
BRIEF DESCRIPTION OF THE INVENTION:
This invention relates to Interstitial free (IF) steel of ultra-fine grain size,
having high strength and ductility and a process for the manufacture
thereof.

In accordance with this invention, is proposed a process for severe
plastic deformation of IF steel to produce ultra-fine grain size having
sufficient strength to meet the future demands of the automobile
industries.
Interstitial- free (IF) steel with submicron grains size has been produced
for the first time using the technique of multiaxial forging (MAF). The
experiments have been conducted using an indigenously defined MAF
die, which resulted in an effective true stain of ~ 0.7 per compression
along an axis i.e., a true strain of -2.1 per cycle. An MAF apparatus (1)
has been designed (Fig. 1) that consists of a base plate (1). Two die
panels (3), plunger and adapters to fit with the hydraulic press machine.
These are made up of H-13 tool steel. The grain structure of the coarse
grained IF steel billet is refined to an ultra-fine grain size by repeated
forging along the three axes one after another a number of times. The
resulting ultra-fine grained IF steel has strength exceeding that of HSLA
steel with an appreciable amount of ductility.
By way of a preferred embodiment and without implying any limitation
on the scope of the invention, the material used for the process was a
titanium-stabilized IF steel, the composition of which is given below in


A detailed description of the MFA process has been shown through the
schematic representation in Fig 2. In the figure, a,b and c-axes
represents the external or the die coordinate system while x,y and z axes
represent the initial reference system of the sample. The billet used for
the experiment had a square cross-section of 20mm x 20 mm and a
height of 40 mm. The billet is first kept with its longest dimension, here
the z-axis, parallel to the load axis in between the two die panels. The
load is then applied by means of the plunger, having a crosshead speed
of ~lmm/s. The load is applied till the height of the billet reduces to half
of its original height. As the second axis, y, is constrained by the die
walls, the occurs an equivalent flow of the material along the third axis
i.e. the x-axis. The sample is then given a clockwise rotation, first about
the a-axis of the die reference system as shown by rotation '1' and then a
second rotation '2' again in a clockwise sense as depicted in Fig 2. These

subsequent rotations again bring the longest dimension of the billet,
which in this case is the prior y-axis, parallel to the direction of the
applied load. After the load is applied and the billet strained to half its
width, the same sequence of rotations, as described earlier, is followed.
This time third axis i.e. the x-axis of the initial billet has the longest
dimension and final deformation is given to the sample along this axis.
Thus, the billet is compressed subsequently along all the three axis,
which constitutes one cycle of operation. Since there is no significant
change in the dimension of the sample after each pressing, this process
in principle can be repeated for any number of pressings or cycles. In
accordance with preferred embodiment, the given billet sample is forged
up to four cycles. During pressing, in order to reduce frictional effects, is
a lubricant of molybdenum disulphide (M0S2) powder mixed with grease,
is applied at the billet-tooling interface. It is to be mentioned here that
unlike the previous experiments of isothermal forgings that were carried
out at higher temperatures, the pressings in the current experiments
were conducted at room temperature.
A photograph of the billet in its initial condition and after undergoing
forging till four cycles is shown in Fig 3. Fig 4 shows the optical

micrograph of the initial material having an average grain size of -225
μm. After the first cycle only there is a drastic reduction in the grain size
to submicron level (-260 nm). A further refinement of the grains to ~ 220
nm took place at the end of four cycles. Fig 5 shows a representative
microstructure in the form of Inverse poly figure (IPE) maps obtained
after the first and fourth cycle measured by electron back scattered
diffraction (EBSD) using field emission gun-scanning electron microscope
(FEG-SEM). Thus after four cycles of the MAF, the grain size reduced by
three orders of magnitude.
The billets were then evaluated for their properties and the results are
provided hereinbelow.
To test the mechanical properties of the multi-axially forged samples,
miniature tensile sample, as per ASTM specification, were extracted from
the billets. The tensile tests were carried out on an INSTRON universal
testing machine. The stress-strain curves for the starting material and
after one, two and four cycles are given in Fig 6. Table 2 shows the
values of the values of the yield stress and ultimate tensile stress for all
the samples. The yield strength of the initial material was -105 MPa.

There occurred a dramatic increase in the yield strength after first pass
by almost about five-fold. After four cycles yield strength increases to
-600 MPa. An acceptable range of tensile ductility of nearly 5% was also
observed in all the cases. Thus, a reasonably good combination of
strength and ductility resulted after the process of multi-axial forging.