FIELD OF THE INVENTION
The present invention relates to the development of a AL-killed rephosphorised
high strength sheet steel. More particularly the invention relates to development
of a grade of 390 MPa UTS formable and weldable steel sheet, adaptable to
mainly for manufacturing automotive body panels, internal panels (structural).
BACKGROUND OF THE 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.
The formable high strength steels are usually expensive (high strength interstitial
free steel), and the manufacturing steps requires special facility like continuous
annealing .
The present invention gives a solution to this problem. Chemical composition and
processing route are such that the manufacturing of this steel grade is less
expensive. Moreover, this grade is exclusively suitable for hot dip galvanizing.

Prior art
Al-killed drawing quality or extra deep drawing quality steels are known to be
excellent materials for automotive stamping applications. Through the drawability
is inferior to that of interstitial free steels, they find substantial application due to
their formability, combined with the cost advantage. The rephosphorised
versions of this family of steels offer a vital alternative to high strength levels, of
390 MPa UTS. More importantly, the drawability values of rephosphorised Al-
killed grades are comparable with those of IFHS grades, while IFHS is a non-
aging grade.
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. Therefore, the steel manufacturers are
compelled to produce high strength formable grades through batch annealing
only.
Manufacturing of interstitial free based high strength steels trough batch
annealing is extremely difficult due to the complex metallurgy of those grades.
Therefore, Al-killed rephosphorised grades which is processed though hot dip
galvanizing offer great advantage in terms of high strength, easy processing and
expenditure.

The main advantages are:
a) Can be produced through hot dip galvanized route
b) Combination of strength and reasonable formability
c) An economic option for automotive industry
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved grade
of steel sheet having 390 MPa UTS, formable and weldable quality adaptable to
manufacture of automotive body panels which eliminates the disadvantages of
prior art.
Another object of the present invention is to propose an improved grade of steel
sheet having 390 MPa UTS, formable and weldable quality adaptable to
manufacture of automotive body panels which is applicable and usable for a wide
range of automotive application.
A further object of the present invention is to propose an improved grade of
steel sheet having 390 MPa UTS, formable and weldable quality adaptable to
manufacture of automotive body panels which is suitable for producing through
hot dip galvanized route.
A still further object of the present invention is to propose an improved grade of
steel sheet having 390 MPa UTS, formable and weldable quality adaptable to
manufacture of automotive body panels which is cost savings.

An yet further object of the present invention is to propose an improved grade of
steel sheet having 390 MPa UTS, formable and weldable quality adaptable to
manufacture of automotive body panels which saves energy comsumption.
SUMMARY OF THE INVENTION
The invented product is a steel grade for automotive stamping applications. It is
a low carbon based steel composition, strengthened by suitable additions of
manganese and suitable micro alloying elements.
The processing schedules, including steel making, hot rolling, cold rolling and
galvanizing have been determined appropriately so as to achieve desired
drawability and strength. The material is also adequately weldable.
Chemical composition
C% Mn% S% P% Si% Al% N(ppm) MA%
0.12 1.5 0.02 0.025 0.05 0.02 60 0.025
max max max max max min max
Physical processing
a) hot charging of slabs to reheat furnace
b) slab drop out at around 1250°C

c) finish rolling at higher than Ar3 temperature - 900-910°C
d) coiling at less than 620°C
e) galvanizing at temperature < 800°C
Test results

Variables
(a) C, P, Mn & microalloying elements: Increase strength of the steel.
(b) Slab drop out temperature, finish rolling temperature and coiling
temperature, as mentioned above.
(c) galvanizing temperature, as mentioned above.
(d) Skin pass elongation.

WE CLAIM
1. An improve steel for 390 MPa UTS, formable and weldable quality
to form steel sheet adaptable to mainly for manufacturing of
automotive body panels and internal panels comprising:
- an improve steel having follows composition
% C =0.12 max
% Mn =1.50 max
% S = 0.02 max
% P = 0.025 max
% Si = 0.05 max
% Al = 0.02 max
N (ppm) = 60 max
% MA = 0.025 max
2. The process of operating the steel as claimed in claim 1 to form
sheet comprising:-
- manufacturing of slab from the liq steel;
- hot charging of slabs to reheat furnace;
- slab drop out at around - 1250°C
- finish rolling at higher than Ar3 temperature
- coiling at less than 620°C
- galvanizing at temperature < 800°C

3. An improved steel for 390 MPa UTS, formable and weldable quality
of steel as substantially described and illustrated herein.
4. The process of operating the steel to form steel sheet as
substantially described and illustrated herein.

The invented product is a steel grade for automotive stamping applications. It is a low carbon based steel composition, strengthened by suitable additions of manganese and suitable micro alloying elements.
The processing schedules, including steel making, hot rolling, cold rolling and
galvanizing have been determined appropriately so as to achieve desired drawability and strength. The material is also adequately weldable.