2
FIELD OF THE INVENITON
The present invention relates to the development of a set of steel
properties. More particularly the invention relates to development of
a family of high strength interstitial free steel sheet with 340-440
MPa UTS formable and weldable quality, adaptable to mainly or
manufacturing automotive body panels.
BACKGROUND OF THE PRESENT INVENTION
The modern automobile industry is driven by some important factors,
such as
(a) Energy saving, or less fuel consumption
(b) Less emission
(c) Improved passenger safety
(d) Low cost manufacturing, etc.
In order to satisfy all these basic conditions, the modern car
manufacturing involves more high strength steel grades, which
should also be combined with certain degree of formability.

3
Interstitial free steel grades (both mild and high strength grades) are
usually processed through continuous annealing route. However,
due to the lack of continuous annealing facilities in India, the steel
manufacturers are compelled to process interstitial free steels
through batch annealing route.
While soft interstitial free steel grades can be manufactured
conveniently through batch annealing route, it is difficult to process
the high strength versions on the same family because of the
complex physical metallurgy involved in processing of these grades.
The present invention gives a solution to this problem. Chemical
compositions and combined with strength and adequate resistance
against secondary work embrittlement. These grades are suitable for
batch annealing process. Therefore, continuous annealing facility is
not required.

4
PRIOR ART
Interstitial free steels are known to be excellent materials for
automotive stamping applications. The high strength versions of this
family offer a vital range of high strength formable steels of various
strength levels, such as 340, 390 or 440, MPa UTS. More
importantly, these are non-aging grades.
This grade has never been manufactured anywhere else in India
before.
Drawbacks connected with hither to known processes and
advantage of the present invention
The worldwide trend is to process high strength formable steel
grades through continuous annealing line, which is the best route for
producing interstitial free high strength steels. In India, there is no
continuous annealing line for processing uncoated steel sheet.

5
Therefore, the steel manufacturers are compelled to produce high
strength formable grades through batch annealing only.
Manufacturing of interstitial free based high strength steels through
batch annealing is extremely difficult due to the complex metallurgy
of those grades. Therefore, the present invention leads to the
following advantages:
a) Chemical composition is suitable for processing though batch
annealing route
b) combination of strength and substantial formability
c) adequate weldability due to ultra low carbon composition
OBJECTS OF TH INVENTION
It is therefore an object of the present invention to propose a set of
improved grades of steel sheet having high strength, formable and
weldable quality adaptable to manufacture of automotive body panels
which eliminates the disadvantages of prior art.

6
Another object of the present invention is to propose a set of
improved grades of steel sheet having high strength, formable and
weldable quality adaptable to manufacture of automotive body panels
which is applicable and usable for a wide range of automotive
applications.
A further object of the present invention is to propose a set of
improved grades of steel sheet having high strength, formable and
weldable quality adaptable to manufacture of automotive body panels
which are suitable for producing through batch annealing route.
SUMMARY OF THE INVENTION
The invented product is a family of steel grades for automotive
stamping applications. All the grades have ultra low carbon based
steel compositions, strengthened by suitable additions of phosphorus
and manganese, in order to achieve various strength levels.

7
The processing schedules, including steel making, hot rolling, cold
rolling and annealing, have been determined appropriately so as to
achieve desired drawability and strength. The steel grades are also
adequately weldable.
Chemical composition

%C %Mn %5 %P %Si %AI %Ti %Nb N B
<0.0035 0.5-0.95 <0.01 0.02-0.065 < 0.015 0.04-0.05 0.05-0.06 0.012-0.02 <30ppm 4-11ppm
Physical processing
a) Hot charging of slabs to reheat furnace for grades IFHS-309 and
IFHS-440. Hot charging is not absolutely necessary for IFHS-340.
Hot charging desirable for high P content (more than 0.03%).
b) Slab drop out @ around-1200°C
c) Finish rolling @ higher than Ar3 temperature - 900 - 910°C
d) Coiling @ 550 - 560°C

8
e) Batch annealing @ hot spot temperature-720°C and cold spot
temperature 690°C
Test results (two results are given below for each grade of the IFHS
family):

SteelGrades Width(mm) Thickness(mm) YS(MPa) UTS (MPa) %EI r-bar
IFHS- 1260 0.8 198 375 42 1.78
340 1245 1.2 185 360 41 1.72
Desirable properties 145-245 340 min 40 min 1.4 min
IFHS- 1025 1.0 237 415 37 1.55
390 1025 0.9 232 411 37 1.6
Desirable properties 195-295 390 min 33 min 1.4 min
IFHS- 1150 1.8 293 444 36 1.52
440 1150 1.4 310 446 37 1.42
Desirable Properties 235-345 440 min 33 min 1.3 min
Secondary work embrittlement property: OK upto - 30°C (tested for
all grades)

9
Variables
(a) Chemical parameters: P and Mn - Increase strength of the
steel
(b) 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.

10
WE CLAIM
1. An improved interstitial free steel grades for high strength having
340-f440 MPa UTS formable and weldable quality to form steel
sheet adaptable mainly for manufacturing of automotive body
panels comprising;
- a steel having following chemical composition

%C < 0.0035
%Mn 0.5-0.95
%S <0.01
%P 0.02-0.065
%Si < 0.015
%AI 0.04-0.05
%Ti 0.05-0.06
%Nb 0.012-0.02
N (ppm) <30
B (ppm) 4-11

11
2. The process of operating the steel as claimed in claim 1 to form
sheet comprising:-
- manufacturing of slab from the liquid steel;
- hot charging of slabs to reheat furnace, for grades IFHS-390 and
IFHS-440
- Finish @ higher than Ar3 temperature 900-910°C;
- Coiling @ 550-560°C
- Cold rolling reduction @ 70-75% (higher the better)
- Batch annealing @ hot spot temperature 720°C and cold spot
temperature 690°C and
- Skin pass rolling @ 0.5% temper elongation

12
3. An improved interstitial free steel grades for high strength having
340-400 MPa UTS formbale and weldable quality of steel as
substantially described and illustrated herein.
4. The process of manufacturing the steel grades to form steel sheet
as substantially described and illustrated herein.

The invented product is a family of steel grades for automotive
stamping applications. All the grades have ultra low carbon based
steel compositions, strengthened by suitable additions of phosphorus
and manganese, in order to achieve various strength levels.
The processing schedules, including steel making, hot rolling, cold
rolling and annealing, have been determined appropriately so as to
achieve desired drawability and strength. The steel grades are also
adequately weldable.
Chemical composition
%C %Mn %5 %P %Si %AI %Ti %Nb N B
<0.0035 0.5-0.95 <0.01 0.02-0.065 < 0.015 0.04-0.05 0.05-0.06 0.012-0.02 <30ppm 4-11ppm
Physical processing
a) Hot charging of slabs to reheat furnace for grades IFHS-309 and
IFHS-440. Hot charging is not absolutely necessary for IFHS-340.
Hot charging desirable for high P content (more than 0.03%).
b) Slab drop out @ around-1200°C
c) Finish rolling @ higher than Ar3 temperature - 900 - 910°C
d) Coiling @ 550 - 560°C
e) Batch annealing @ hot spot temperature-720°C and cold spot
temperature 690°C
Test results (two results are given below for each grade of the IFHS
family):
SteelGrades Width(mm) Thickness(mm) YS(MPa) UTS (MPa) %EI r-bar
IFHS- 1260 0.8 198 375 42 1.78
340 1245 1.2 185 360 41 1.72
Desirable properties 145-245 340 min 40 min 1.4 min
IFHS- 1025 1.0 237 415 37 1.55
390 1025 0.9 232 411 37 1.6
Desirable properties 195-295 390 min 33 min 1.4 min
IFHS- 1150 1.8 293 444 36 1.52
440 1150 1.4 310 446 37 1.42
Desirable Properties 235-345 440 min 33 min 1.3 min
Secondary work embrittlement property: OK upto - 30°C (tested for
all grades)