FIELD OF THE INVENTION
The present invention relates to the development of batch annealed high
strength P free IF steel sheet. More particularly the invention relates to
development of a high strength of minimum UTS level of 340 MPA and improved
formable and weldable steel grade with improved drawability adaptable mainly
for manufacturing automotive body panels.
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

In order to satisfy all these basic conditions, the modern car manufacturing
involves in development of more high strength steel grades, which should also
be combined with certain degree of formability.
Interstitial free (IF) 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
of the same family, because of the complex physical metallurgy involved in
processing of these grades. It is now possible to manufacture IFHS (interstitial
free high strength) grade through batch annealing, with appreciable drawability.
However, the drawability of IFHS (interstitial free high strength) grades is always
less than that of soft IF steel, even if it is manufactured through continuous
annealing route.

The present invention is related with development of a new chemical
composition IFHS (interstitial free high strength) grade with minimum UTS of,
340 MPa. The drawability of this batch annealed steel is quite high, and in fact
very close to that of soft IF steel. Chemical composition and processing route
have been designed in a way as to develop excellent drawability, combined with
strength and adequate resistance against secondary work embrittlement.
Prior art drawbacks
The crux of the present invention is the development of a new chemical
composition. Usually, the IFHS (interstitial free high strength) steels are
strengthened by the addition of P, which is considered as a very effective solid
solution strengthener. Mn is added to IFHS (interstitial free high strength)
steels, as a supporting strengthening element.

Drawbacks connected with hither to known
processes and advantage of the present invention
Addition of P leads to a few problems, some of which are listed below:
• Segregation of P at grain boundaries leads to secondary work
embrittlement effect.
• B is added to control this effect, but B itself affects drawability in a
deleterious manner.
• P has a strong affinity towards the stabilizing elements like Ti, thereby
forming FeTiP type precipitates, this process drastically affects the
stabilization of steel, as well as the drawability.
• The above effects can be controlled considerably through CAL processing,
but the processing of IFHS is extremely difficult through BAF.

Objective of the invention
It is therefore an object of the present invention to propose a new concept of
chemical composition, in which P will not be added at all as a strengthening
element in IFHS steel.
Another object of the invention is to obtain steel without any secondary work
embrittlement due to non addition of P and therefore avoiding any need to B
addition; however, to be on safe side, a few steel makers add a small quantity of
B (2-3 ppm) even in the soft IF grades, in order to make these grades suitable
for using in the extreme cold service conditions, this practice may be included in
this case also, under mutual agreement between the supplier and customer.
Yet another object of the invention is to secure any chance of precipitate
formation like FeTiP.

A still another object of the invention is to conveniently process through batch
annealing route.
A further object of the invention is to obtain the steel very close to that of high
quality soft IF steel.
A still further object of the invention is to result poor dent resistance of soft IF
steel and also at the same time developing high strength steel grades.
The present invention has a potentiality to replace the soft IF (interstial free)
grade in many applications, due to the high Lankford value of the processed
material.
SUMMARY OF THE INVENTION
The invented product is a steel grade for automotive stamping applications. The
grade has ultra low carbon based steel composition, strengthened by suitable
addition of manganese, in order to achieve a minimum strength level of 340 MPa
(UTS).

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.
Illustration of the invention
Chemical composition of the proposed steel is stated as below:

Physical processing
(a) Hot charging in steel making furnace is not required in this grade, since
there is no added P.
(b) Slab drop out around - 1200° C.
(c) Finish rolling higher than Ar3 temperature - 900 - 910° C.
(d) Coiling at 700° C.

(e) cold rolling thee hot rolled strip at 73-77 % reduction
(f) Batch annealing is carried out with hot spot temperature 720° C and cold
spot temperature 690° C.
Test results (two results are given below):

Secondary work embrittlement property: OK upto -30° C .
Variables
(a) Chemical parameters: Mn - Increases strength of the steel; Nb also adds
to strength to some extent.

(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.
The invention as herein narrated with an exemplary embodiment should not be
read and construed in a restrictive manner as various modifications, alterations
and adaptations are possible within the scope and ambit of the invention as
defined in the appended claims.

WE CLAIM
1. A method of producing phosphorus free interstitial free high strength (IFHS),
formable and weldable steel sheet / strip with improved drawability comprising
the steps of:
- preparing a steel slab of IFHS grade without phosphorus addition during
melting, having a composition in weight % of C - < 0.0030, Mn - 0.5 - 0.8, S - <
0.01, P - < 0.012, Si - < 0.015, Al - 0.04 - 0.05, Ti - 0.05 - 0.06, Nb - 0.04 -
0.05, N - (ppm) - < 30; and the balance is Fe.
- reheating the slab at 1150-1200°C.
- finish rolling the steel strip/ sheet at temperature within 900 - 910°C, above Ar3
temperature of the Steel;
- coiling the strip/ sheet at 700° C;
- cold rolling the coiled strip/ sheet with 73 - 77% reduction;
- batch annealing the cold rolled sheet/ strip maintaining hot spot temperature
720° C and cold spot temperature 690° C and
- skin pass rolling the annealed sheet / strip with 0.5 % temper elongation.

2. A method of producing IFHS steel sheet / strip for automotive body panel as
claimed in claim 1, wherein secondary embrittlement of the steel is avoided due
to non-addition of P during steel making and also avoiding any BORON addition
to control secondary work embrittlement.
3. A method as claimed in claims 1 or 2, wherein Boron is added in the steel in
case the steel sheet / strip is used in extreme cold service condition.
4. A method of producing IFHS steel strip/ sheet as claimed in claim 1, wherein
the Manganese content is usually maintained at 0.5 - 0.6 %, but for increased
UTS Manganese content is kept at 0.7 - 0.8%.

5. A steel strip/ sheet having composition in weight % C < 0.0030, Mn - 0.5 -
0.8, S - < 0.01, P < 0.012%, AL - 0.04 -0.05, Ti - 0.05 - 0.06, Nb - 0.04 - 0.05,
N (ppm) - < 30, and the balance is Fe wherein the steel strip/ sheet is produced
by a method as claimed in claim 1.
6. The steel sheet/ strip as claimed in claim 5, wherein steel sheet resulted has
YS - 145 - 245 MPa and UTS - 340 - 360 MPa.
7. The steel sheet/ strip as claimed in claim 5, wherein the steel has high
drawability having % elongation of minimum 40 and minimum r-bar of 1.4.

ABSTRACT

TITLE: A METHOD OF PRODUCING PHOSPHORUS FREE INTERSTITIAL FREE
HIGH STRENGTH (IFHS) FORMABLE AND WELDABLE STEEL SHEET / STRIP
WITH IMPROVED DRAWABILITY
This invention relates to a method of producing phosphorus free interstitial free
high stength (IFHS), improved drawable and weldable steel sheet / strip
comprising the steps of: preparing a steel slab of IFHS grade without
phosphorous addition during melting having a composition in weight % of C - <
0.0030, Mn - 0.5 - 0.8, S - < 0.01, P - < 0.012, Si - < 0.015, Al - 0.04 - 0.05, Ti
- 0.05 - 0.06, Nb - 0.04 - 0.05, N - (ppm) - < 30; reheating the slab at 1150 -
120° C; finish rolling the steel strip / sheet at temperature within 900 - 910° C
above Ar3 temperature of the steel; coiling the strip / sheet at 700° C; cold
rolling the coiled strip / sheet with 73 - 77 % reduction; batch annealing the cold
rolled sheet / strip maintaining hot spot temperature 720° C and cold spot
temperature 690° C and skin pass rolling the annealed sheet / strip with 0.5 %
temper elongation.