FIELD OF INVENTION
The present invention relates to an interstitial free (IF) steal product for
application in automobile industries. More particularly the invention relates to an
improved steel grade with high formability and weldable quality to form steel
sheet for manufacturing of automotive body panels. The invention further relates
to a process to manufacture steel sheet from an improved steel grade with high
formability and weldable quality.
BACKGROUND OF INVENTION
The modern car manufacturing involves highly formable IF steel grades, which
are suitable for the fabrication of complicated shaped automobile body
components. While normal interstitial free steel grades can be used conveniently
for this purpose, it is even better if the drawability of IF steel grades can be
improved further.
The steel making of IF steel often encounters problems related to mould level
fluctuation.
Interstitial free steels are already known to be appropriate materials for
automotive stamping applications. The aluminium content of IF steel is usually
0.02-0.04%, which is just sufficient for 'killing' the steel.
Figure 1, shows the mould level fluctuation in a prior art IF steel sample.

Average mechanical properties of IF steel with existing chemistry is given in the
following table.

The worldwide trend is to process interstitial free steel grades through vacuum
degassing, followed by casting.
OBJECTS OF INVENTION
It is therefore an object of the present invention to propose an improved steel
grade having high formability and weldable quality adaptable for manufacturing
of automotive body panels, which enhances the drawability characteristics of the
steel over prior art.
Another object of the present invention is to propose an improved steel grade
having high formability and weldable quality adaptable to existing steel making
process with ease of casting and improved mould level fluctuations.
A further object of the present invention is to propose a process to manufacture
steel sheets from the improved steel grade, which is suitable to produce
automotive body panels through batch annealing route.

SUMMARY OF INVENTION
Accordingly, in a first aspect of the invention, there is provided an improved
steel grade with high formability and weldable quality to form steel sheet for
manufacturing of automotive body panels, comprising the compositions of:

In a second aspect of the invention, there is provided a process to manufacture
steel sheet from the improved steel grade of the invention the process
comprising the steps of:
• producing a slab from the liquid steel grade with high formability and
weldable quality;
• charging the slabs to a reheat furnace, maintained at a slab drop out
temperature between 1150-1200°C;
• finish rolling of the slab at a temperature range between 900-910°C;
• coiling the rolled strip at a temperature around 700°C;
• cold rolling reduction of the coiled strip at a rate of 75-80%;

• batch annealing the cold rolled strip at a hot spot temperature of about
700°C and further at a cold spot temperature of 690°C; and
• implementing a skin pass rolling @ 0.3% temper elongation.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Graphically represents casting stability of existing steel grade
used for automotive applications.
Figure 2 - Graphically represents casting stability of the improved steel
grade sheet produced according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, there is provided in a first aspect an improved steel
grade for automotive stamping applications. This grade has ultra low carbon
based steel composition, with suitable addition of aluminium in order to achieve
superior drawability and higher weldability.
According to the invention, the processing schedules, including steel making, hot
rolling, cold rolling and annealing, have been determined so as to achieve
desired drawability. The steel grade is also adequately weldable.

(a) Oxygen steel making;
(b) Vacuum degassing through RH degassing route;
(c) Alloying addition in usual sequence; and
(d) Slab casting
According to a second aspect of the invention, a physical processing of the
invented steel grade is provided, which comprises the steps of :-
(a) Charging the slabs to a reheat furnace;
(b) Slab drop out @ around-1200°C;
(c) Finish rolling @ higher than Ar3 temperature - 900°C±10°C;
(d) Coiling @ 700°C±10°C; and
(e) Cold rolling @75-85% reduction
(f) Batch annealing @ hot spot temperature 720°C and cold spot
temperature 690°C
The improvement in casting stability of the steel sheet is indicated in Figure 2.
The mechanical properties of the new interstitial free steel are given in the
following chart.

(a) Chemical parameters: P and Mn - Increase strength of the steel
(b) Low Al content - can lead to problem of mould level fluctuation
(c) Physical parameters: Slab drop out temperature, finish rolling
temperature, coiling temperature, cold reduction and batch annealing
temperature, all have significant effects on final mechanical properties of
all the steel grades.
Thus, the interstitial free steel sheet produced according to the invention for
automotive application has an improved r-bar value of minimum 2.0 and
improved steel making by the addition of 0.1% aluminium.
The present invention leads to the following advantages :
a) Chemical composition (extra aluminium addition) is suitable for
reducing problems related to steel making
b) Further improvement of formability.

WE CLAIM :
1. An improved steel grade with high formability and weldable quality to
form steel sheet for manufacturing of automotive body panels, comprising
the compositions of:

2. The improved steel grade as claimed in claim 1, wherein the mechanical
properties of the steel comprise :

3. A process to manufacture steel sheet from the improved steel grade as
claimed in claim 1, the process comprising the steps of:

• producing a slab from the liquid steel grade with high formability and
weldable quality;
• charging the slabs to a reheat furnace, maintained at a slab drop out
temperature between 1150-1200°C;
• finish rolling of the slab at a temperature range between 900-910°C;
• coiling the rolled strip at a temperature around 700°C;
• cold rolling reduction of the coiled strip at a rate of 75-80%;
• batch annealing of the cold rolled strip at a hot spot temperature of about
700°C and further at a cold spot temperature of 690°C; and
• implementing a skin pass rolling @ 0.3% temper elongation.
4. An improved steel grade with high formability and weldable quality to
form steel sheet for manufacturing of automotive body panels as
substantially described and illustrated herein with reference to the
accompanying drawings.

The present invention describes an improved steel grade with high formability
and weldable quality to form steel sheet for manufacturing of automotive body
panels, comprising the compositions of:

The invention further discloses a physical process to produce a Steel sheet from
the improved steel grade.