FIELD OF THE INVENTION
The present invention relates to the development of an Al-killed rephosphorised
high strength sheet steel. More particularly the invention relates to a
development of drawing quality Al-killed rephosphorised steel sheets having
Tensile Strength of 340-440 MPa UTS formable and weldable steel sheet,
adaptable to mainly for 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.
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 batch annealing process. Therefore, continuous
annealing facility is not required.
PRIOR ART
Al-killed drawing quality or extra deep drawing quality steels are
known to be excellent materials for automotive stamping
applications. Though 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 interstitial
free steels of strength levels, between 340 to 440 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 through
batch annealing is extremely difficult due to the complex metallurgy
of those grades. Therefore, Al-killed rephosphorised grades, though
slightly inferior in forming property, offer great advantage in terms of
easy processing and expenditure.
The main advantages are:
a) Can be produced through batch annealing route
b) Combination of strength and substantial formability
c) An economic option for automotive industry

OBJECTS OF TH INVENTION
It is therefore an object of the present invention to propose an
improved grade of steel sheet having 340 to 440 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 340 to 440 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 340 to 440 MPa UTS, formable and
weldable quality adaptable to manufacture of automotive body panels
which is suitable for producing through batch annealing route.

A still further object of the present invention is to propose an
improved grade of steel sheet 340 to 440 MPa UTS, formable and
weldable quality adaptable to manufacture of automotive body panels
which is cost savings.
A yet further object of the present invention is to propose an
improved grade of steel sheet having 340 to 440 MPa UTS, formable
and weldable quality adaptable to manufacture of automotive body
panels which saves energy consumption.
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 phosphorus and manganese.
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 material is also
adequately weldable.

Chemical composition
C% Mn% S% P% Si% Al% N (ppm)
0.04-0.06 0.3-1.5 0.01 0.05-0.09 0.01 0.04-0.05 35-50
max max
Physical processing
a) hot charging of slabs to reheat furnace
b) slab drop out @ around-1200°C
c) finish rolling @ higher than Ar3 temperature - 900 - 910°C
d) coiling @ 550 - 560°C
e) cold rolling @ 66 - 74%
f) batch annealing @ hot spot temperature - 700°C - 720°C and cold
spot temperature 620 - 640° (table suitably adjusted)
g) skin pass rolling @ 1.0% temper elongation

Test results (two results are given below for each grade of Al-killed
rephos family):

Secondary work embrittlement property: OK upto - 30°C (tested for
rephos - 340 and rephos - 390, rephos - 440 was not tested, but
expected to show same property due to same phosphorus content as
in rephos - 390)
Variables
(a) Chemical parameters: C, 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.

WE CLAIM:
1. Drawing quality Al-killed rephosphorised steel sheets having tensile
strength of 340-440 MPa comprising a steel having following
composition:
%C = 0.40-0.06
%Mn= 0.3-1.5
%S = 0.01 max
%P = 0.05-0.09
%Si = 0.01 max
%AI = 0.04-0.05
N (ppm) = 35-50
characterized in that Al-killed steel by addition of Phosphorus and
Manganese and subsequent heat treatment and Thermo-Mechanical
Treatment develop secondary work embrittlement properties like a
strength of 340-440 MPa, drawability and weldability.
2. The process to convert liquid steel to steel sheet comprising of
following steps:
- manufacturing of slab from the liquid sheet;
- hot charging the slab to reheat furnace;
- finish rolling @ higher than Ar3 temperature 900-910°C;
- coiling @ 550-560;
- cold rolling @ 66-74%;
- batch annealing @ hot spot temperature 700°C and cold spot
temperature 620°C; and
- skin pass rolling @ 1.0% tempered elongation;

characterized in that Al-killed steel having a ferrite microstructure after
hot rolling is being converted to a pancaked ferritic microstructure after
subsequent cold rolling and annealing treatment producing a good
secondary work embrittlement property and weldability.
3. The process to convert liquid steel to steel sheet as substantially described
and illustrated herein.

ABSTRACT

TITLE: DRAWING QUALITY AI-KILLEP REPHOSPHORISED STEEL
SHEETS HAVING TENSILE STRENGTH OF 340-440 PMa
The present invention is provided with a development of drawing quality Al-
killed rephosphorised steel sheets having tensile strength of 340-440 MPa
comprising a steel having the composition %C = 0.04-0.06, %Mn = 0.3-.15,
%S = 0.01 max, %P = 0.05-0.09, %Si = 0.01 max, %AI = 0.04-0.05, N
(ppm) = 35-50; characterized in that Al-killed steel by addition of
Phosphorus and Manganese and subsequent heat treatment and Thermo-
Mechanical Treatment develop secondary work embrittlement properties like
a strength of 340-440 MPa, drawability and weldability.