FIELD OF THE INVENTION
This invention relates to a high strength steel composition. This invention further
relates to a high strength steel composition which has improved ductility for use
in automobiles.
BACKGROUND OF THE INVENTION
Design application for steel products have triggered the development of new
steels with unique properties. In general, the strength ductility relationship is
some what inverse particularly in steel gamete. To increase the strength to
weight ratio of the automobiles which will in turn lead to a decrease in the fuel
consumption as well as increase in crash-worthiness, there has been an
increased emphasis on the development of next generation advanced high
strength sheet steels (AHSS).
However, there is a need in the industry for a steel for such applications, having
a good combination of strength and ductility.
OBJECTS OF THE INVENTION
It is, therefore, an object of this invention to propose a steel composition which
has high strength.

It is a further object of this invention to propose a steel composition which has
improved ductility.
Another object of this invention is to propose a steel composition which is
manufactured using a simple route and is cost effective.
These and other objects and advantages of the invention will be apparent from
the ensuing description.
DESCRIPTION OF THE INVENTION
Thus, according to this invention is provided a steel composition having high
strength and ductility, and a process for the preparation thereof.
In accordance with this invention, a steel composition is developed, having UTS
~ 700-820 Mpa, % elongation of 20-30%, tensile strength x elongation >24 Gpa
%, sigma 3 coincidence site lattice volume fraction >7 of FCC phase after final
processing by the intercritical annealing followed by isothermal bainitic
transformation route.

The steel composition comprises mainly of Carbon, Manganese, Aluminium and
Silicon. The carbon is present in 0.18 to 0.27 wt. %, Manganese in 1.51 to 1.67
wt. %, Si in 1.19 to 1.29 wt. %, Aluminium in 0.31 to 0.45 wt. %. Trace
quantities of Phosphorous and Sulphur also from part of the composition, where
Phosphorous and Sulphur are each present in 0.010 wt. % or less.
The steel is made in the air induction furnace and subsequently forged at 900-
1200°C with different amount of deformation and temperature at each stroke to
achieve homogenized microstructure. The forged material is hot rolled and the
FRT is kept just above Ar3 temperature. The steel is then cooled. The hot rolled
material is then cold rolled up to 80%. The cold rolled material is then subjected
to inter critical annealing at various temperatures ranging from 850°C to 750°C
for 10-25 mins followed by isothermal bainitic transformation (IBT) at
temperatures ranging from 440°C to 390°C for 6-8 mins. This gives the steel the
required properties, which is possibly the next generation advanced high
strength sheet steels (AHSS) family having TRIP effect.
This invention will now be explained with the help of the following non-limiting
example.

Example:
The steel is made in an air induction furnace and forged at 1050°C. The forged
material is hot rolled and the FRT is kept just above Ar3 temperature of 877°C.
the steel is then cooled and cold rolled up to 80%. The cold rolled steel is
subjected to inter critical annealing at 820°C for 15 minutes followed by IBT at
420°C for 8 minutes. This yields a steel having the following composition:
C:0.26
Mn:1.74
Si:1.25
Al:0.38
P:0.008
S:0.008

WE CLAIM:
1. A steel composition having high strength and ductility having UTS ~
700-820 Mpa, % of elongation of 20-30%, tensile Strength x
elongation >24 Gpa %, sigma 3 coincidence site lattice volume
fraction >7 of FCC phase after final processing.
2. A steel composition having high strength and ductility comprising in
wt. %:
C: 0.18-0.27
Mn: 1.51-1.67
Si: 1.19- 1.29
Al: 0.31-0.45
P: 0.010 or less
S: 0.010 or less
3. A process for manufacturing the steel composition as claimed in Claim
1, comprising the steps of preparing steel in an air induction furnace
and forging the same, followed by hot-rolling the forged material
keeping the FRT just above Ar3 temperature to obtain the hot rolled

material, cold rolling the hot rolled material followed by inter critical
annealing and isothermal bainitic transformation to obtain the steel
composition.
4. The process for manufacturing the steel composition as claimed in
Claim 3, wherein the steel is forged at a temperature in the range of
900 to 1200°C.
5. The process for manufacturing the steel composition as claimed in
Claim 3, wherein the hot rolled material is cold rolled up to 80%.
6. The process for manufacturing the steel composition as claimed in
Claim 3, wherein inter critical annealing is effected at temperatures in
the range of 850°C to 750°C.
7. The process for manufacturing the steel composition as claimed in
Claim 3, wherein inter critical annealing is effected for 10 to 25
minutes.
8. The process for manufacturing the steel composition as claimed in
Claim 3, wherein isothermal bainitic transformation is effected at
temperatures ranging from 440°C to 390°C.

9. The process for manufacturing the steel composition as claimed in
Claim 3, wherein isothermal bainitic transformation is effected for 6 to
8 minutes.

This invention relates to a steel composition having high strength and
ductility having UTS ~ 700-820 Mpa, % of elongation of 20-30%, tensile
Strength x elongation >24 Gpa %, sigma 3 coincidence site lattice volume
fraction >7 of FCC phase after final processing.