FIELD OF THE INVENTION
The present invention relates to development of a metallurgical
process to improve physical properties of steel. More particularly the
present invention relates to development of a metallurgical process
and chemistry of steel to Improve formabillty of batch annealed
interstitial free high strength steels adaptable to automobile body
applications.
BACKGROUND OF THE INVENTION
Application of interstitial free high strength (IFHS) steels in the
automotive industry has increased significantly in the last couple of
decades due to their excellent deep drawability coupled with
adequate strength. Deterioration of drawability has been noticed in
this grade, as compared to normal IF steels, which is very often been
attributed to the presence of P, the principal solid solution
strengthening element in this grade. It is assumed that P
precipitates as FeTiP which leads to the deterioration of both
drawability and loss of strength. This effect is more pronounced in
batch annealed IFHS steels, as the long soaking allows sufficient time
for this precipitation to occur.

OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a
process and chemistry of steel for Improving formability of batch
annealed interstitial free high strength steels adaptable to automobile
body application which eliminates the disadvantages of prior art
Another object of the present invention Is to propose a process and
chemistry of steel for improving formability of batch annealed
interstitial free high strength steels adaptable to automobile body
application which helps to increase the formability properties of steel.
A further object of the present invention is to propose a process and
chemistry of steel for improving formability of batch annealed
interstitial free high strength steels adaptable to automobile body
application which increases strength by batch annealing process.
A still further object of the present invention is to propose a process
and chemistry of steel for improving formability of batch annealed
Interstitial free high strength steels adaptable to automobile body
application which improves steel chemistry.

SUMMARY OF THE INVENTION:
FeTiP precipitation temperature range is around 700°C and batch annealing is usually
done around this temperature. As a result, precipitation of FeTiP interferes with
recrystallization annealing resulting in poor {111} texture formation. Not only that, it
also precipitates in preference to TiC, as a result of this most of the Ti is exhausted and C
remains in solution and this adversely affects the {111} texture formation. Three different
techniques have been identified for improving the Lankford parameter (average r value)
by suppressing FeTiP precipitation in the steels.
According to this invention there is provided An improved steel chemistry and processing
for improving formability of batch annealed interstitial free high strength steels adaptable
to automobile body applications comprising:-
-an improved chemistry of steel having a composition of:-
(in wt%) (in wt%)
C: 0.02-0.005 Al: 0.02-0.07
Mn: 1.5 or less Ti: 0.1 or less
Si: 0.05 or less Nb: 0.05 or less
S: 0.05 or less B: 30 ppm or less
P: 1.5 or less N: 80 ppm or less
and the balance being Fe and other unavoidable impurities.
DETAILED DESCRIPTION OF THE INVENTION:
The IFHS steels studied contain (in wt %):
C: 0.002-0.005 Al: 0.02-0.07
Mn: 1.5 or less Ti: 0.1 or less
Si: 0.05 or less Nb: 0.05 or less
S: 0.05 or less B: 30 ppm or less
P: 1.5 or less N: 80 ppm or less

To improve the formability and therefore, the Lankford parameter,
the major goal is to produce a very large volume fraction of {111}
oriented grains during recrystallization annealing. The first and
foremost condition for this is to make the matrix free of interstiuals,
mainly C and N Two techniques have been identified to achieve this
based on:
1. The chemistry
2. The processing parameters
Chemistry:
Ti level in the steel should be just above the stoichiometric rquirment
for fixing N and C so that is does not get any chance to form FeTIP.
For 30-40 ppm carbon and 30-40 ppm N, 0.02 to 0.05 Ti is ideal.
Process Paramenters:
Processing of IFHS steels consists of hot rolling, cold rolling and
annealing. During hot rolling, the coiling temperature plays a vital
role. If IFHS steels are coiled at a much higher temperature, which

does not lie in the temperature range for the precipitation of FeTiP, much better
results will be obtained. Coiling at 580-65°C produces Ti4C2S2 which causes
carbon to be taken out of solid solution. On the contrary, when the same steel is
coiled at a somewhat lower temperature (470-570°C) no precipitation of TI4C2S2
takes place.
This is the reason why even when an IFHS steel is annealed at 670-750° (within
the precipitation regime of FeTiP) after cold rolling, coiling at a higher
temperature results in a higher average r value of > 1.9 as compared to < 1.3
when the same steel is coiled at lower coiling temperature.
Another important parameter is the temperature of annealing after cold rolling. It
has been observed that annealing at 670-750°C produces poorer {111} texture
than after annealing at 770-860°C. This is also attributed to the formation of
FeTip precipitate instead of any carbide at 670-750° annealing temperature.
However, at 770-850°C annealing temperature, Ti4C2S2 forms and presence of
FeTiP is not observed. Higher average r value (≈2.4) is obtained in the latter as
compared to in the former one.

WE CLAIM:
1. An improved steel chemistry and processing for improving formability of
batch annealed interstitial free high strength steels adaptable to automobile
body applications comprising:-
-an improved chemistry of steel having a composition of:-
(in wt%) (in wt%)
C: 0.02-0.005 Al: 0.02-0.07
Mn: 1.5 or less Ti: 0.1 or less
Si: 0.05 or less Nb: 0.05 or less
S: 0.05 or less B: 30 ppm or less
P: 1.5 or less N: 80 ppm or less
and the balance being Fe and other unavoidable impurities.
2. An improved process for improving formability of batch annealed
interstitial free high strength steel comprises:-
- Hot rolling the steel at a coiling temperature of 580°C - 650°C
- a cold rolling and
- a batch annealing at 770°C - 850°C.

3. The steel chemistry as claimed in claim 1, wherein Ti level in the steel
should be just above the stoichiometric requirement for fixing C and N
which result in better formability in IFHS steels.
4. The process as claimed in claim 2, wherein annealing temperature at
770°C - 850°C results in the formation of Ti4C2S2 precipitation,
eliminating the presence of FeTip precipitation.
5. An improved steel chemistry and processing for improving formability of
batch annealed interstitial free high strength steels adaptable to automobile
body applications as substantially illustrated and described herein.

ABSTRACT

Title: An improved steel chemistry and processing for improving formability of batch
annealed interstitial free high strength steels for automobile body applications.
An improved steel chemistry and processing for improving formability of batch annealed
interstitial free high strength steels adaptable to automobile body applications
comprising:-
-an improved chemistry of steel having a composition of:-
(in wt%) (in wt%)
C: 0.02-0.005 Al: 0.02-0.07
Mn: 1.5 or less Ti: 0.1 or less
Si: 0.05 or less Nb: 0.05 or less
S: 0.05 or less B: 30 ppm or less
P: 1.5 or less N: 80 ppm or less
and the balance being Fe and other unavoidable impurities.