FIELD OF THE INVENTION
The present invention relates to the development of galvannealed IF steel sheet, with
extra Al addition. More particularly the invention relates to development of a formable
and weldable steel grade, 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 these basic conditions, the modern car manufacturing involves in
development of steel grades with excellent formability. This reduces the necessity of
using thicker steel sheets for manufacturing certain steel components.
Interstitial free (IF) steel grades are usually processed through batch annealing and
continuous annealing routes. In a galvannealing process, the continuously annealed
steel sheet first passes through a zinc bath, followed by a galvannealing furnace
section. In this process, the zinc coating on the steel substrate is subjected to a heat
treatment, typically at around 540-5600C for 10-12 sec, thereby forming Fe-Zn
intermetallic compound layers on the steel substrate [1].
The drawability or Lankford Parameter value of fully processed IF steels is generally
expected to be around 1.9-2.0. This ensures convenient manufacturing of automobile
body panels with substantial intricacy in shapes. In the present day, there is a
requirement of further enhancement of drawability, in order to manufacture components
with even more difficult shapes, with thinner steel gauges. This is the driving force of the
present invention.

Objects of the invention
It is therefore an object of the present invention to propose a new chemical composition,
for IF (interstitial free) steel including additional amount of Al.
Another object of the invention is to conveniently process the proposed composition
through galvannealing route.
Yet another object of the invention is to obtain a high Lankford value in the steel, which
is higher than the normal IF (interstitial free) steel.
A further object of the invention is to obtain a chemical composition which would lead to
a better castability, so as to ensure proper cleanliness of steel.
SUMMARY OF THE INVENTION
Drawability is an important property for interstitial free steels, which make them suitable
for applications in automotive industry. Usually, the Lankford parameter value, which
gives a quantitative measurement of drawability, for galvannealed IF steel is around 1.8
to 1.9. The present invention is related with development of a new chemical composition
of galvannealed IF (interstitial free) steel grade with minimum UTS of 270MPa, and with
higher drawability (minimum Lankford parameter value of 2.1), compared to that of
existing galvannealed IF steel. Chemical composition and processing route have been
designed in a way as to develop excellent drawability.
The chemical composition has been designed to achieve a higher level of aluminium,
around 0.16 to 0.20%. The processing includes reheating the slab at 1100–12000C;
finish rolling the steel strip / sheet at temperature within 900-9100C, above Ar3
temperature of the steel; coiling the strip / sheet at 7000C; cold rolling the coiled strip /
sheet with 77-84 % reduction; processing through a continuous galvannealing line with

strip annealing temperature in the range 780 to 8600C and galvannealing temperature in
the range 530 to 5700C.
The invented product is a steel grade for automotive stamping applications. The grade
has ultra-low carbon based steel composition, suitably alloyed with Ti and Nb in order to
completely stabilize interstitial elements like N and C.
The processing schedules, including steel making, hot rolling, cold rolling and
galvannealing, have been determined appropriately so as to achieve desired drawability
and strength. The steel grade is also adequately weldable.
Illustration of the invention
In the present invention, the term ‘castability’ refers to the steadiness of mould level
during casting. An example is shown in Figure 2, picked up from the existing process,
depicting the mould level fluctuation. On the other hand, Figure 3 depicts an
improvement in casting stability in a steel melt with high Al composition.

Physical processing is briefly described below:
1. After the steel making, the cast slabs are taken to slab yard, from where these slabs
are picked up and placed inside the reheating furnace, which is used for soaking the
slab at a high temperature before hot rolling.
2. The residence time inside the furnace is at least 2 hr 45 min so as to ensure proper
soaking of the steel slabs. The slab drop out temperature is in the range of about
1100-12000C.

3. The next step is roughing, which is actually the first stage of hot rolling. The slab
undergoes a massive thickness reduction. The slab is passed through a pair of rolls
for 5-7 times, and the thickness comes down from 210 mm to 30 mm. This is now
called transfer bar. The roughing mill exit temperature is in the range of 1050-
10800C.
4. After roughing operation, the transfer bar goes into finish rolling mill, in which the 30
mm thick transfer bar undergoes thickness reduction in a 6-7 stand tandem mill. The
final thickness is anywhere in between 2-5 mm, as per requirement. The finish mill
entry temperature is around 10300C and the finish mill exit temperature is in the
range of 900-9100C, to ensure a temperature higher than Ar3 temperature. At this
range of temperature, the material undergoes hot deformation in austenite phase,
but the deformed austenite does not undergo recrystallization. This is very important
for developing the proper texture of the material.
5. After hot rolling, the steel strip is subjected to cooling on the run out table. The
temperature is brought down to about 7000C quickly, and then it is coiled in the
down coiler. This is known as coiling temperature, which has a significant influence
on the precipitation coarsening in IF / IFHS steels. Coiling at a lower temperature for
this grade of steel is not suitable for achieving properties.
6. The hot rolled strip is cooled down to ambient temperature, which may take 2-3
days, depending on season. These hot rolled coils are then fed into the entry section
of a pickling line. Pickling is a process in which the hot rolled strip is passed through
a series of tanks containing acid solutions of different concentrations, so as to obtain
clean surface which is free from oxide scales. Any presence of oxide scale on
surface would be embedded in the material during subsequent cold rolling, which is
considered as a serious defect.
7. After acid pickling, the material is subjected to cold rolling process, in which the hot
rolled strip is again reduced to a smaller thickness, in about 5 passes. The process

is carried out in a tandem rolling mill also. The thickness reduction is about 80% on
an average, to ensure the best results.
8. The cold rolled material is subjected to heat treatment. The present method is for
achieving a good crystallographic texture and drawability by processing through a
galvannealing line, in which the heat treatment section is basically a continuous
annealing facility.
9. The ranges of physical parameters as per the current invention are:

(a) Slab drop out around – 1100-12000C
(b) Finish rolling higher than Ar3 temperature – 900-9100C
(c) Coiling at 700-7100C
(d) Cold rolling the hot rolled strip at 77-84 % reduction
(e) Processing through a continuous galvannealing line with strip annealing
temperature in the range 780 to 8600C and galvannealing temperature in the
range 530 to 5700C.
The steel processed through above mentioned process results in steel that has %
elongation of at least 44% and r-bar value of at least 1.4.
Test results (two results are given below):
Chemical composition:

Mechanical properties:


The crystallographic texture is depicted by ODF plot (Bunge notation) as shown in
Figure 1. This plot indicates that a strong γ fibre texture was developed in this steel,
which is beneficial for drawability.
Intensity levels: 0.8, 1.0, 1.3, 1.6, 2.0, 2.6, 3.2, 4.0, 5.0, 6.4
Maximum intensity: 6.89
Variables
(a) Chemical parameters: Different chemical elements play specific roles.
C – Carbon should be as low as possible. Increase in carbon content may lead to
inadequate stabilization and poor drawability.
Mn and P – Increase strength of the steel unnecessarily. Both should be as low as
possible.
B – Boron is added to prevent the secondary work embrittlement effect. However, in
absence of P, this is not so important. B also deteriorates drawability. Therefore,
either B addition should be avoided, or, considering sub-zero climate condition, it
may be added in a very small quantity (2-4 ppm).
Ti and Nb – These microalloying elements are used for ensuring stabilization of
steel, by fixing nitrogen, sulfur and carbon atoms. A lower quantity of Ti and Nb will
lead to inadequate stabilization, while a higher quantity will lead to increase in
recrystallization temperature, which is detrimental for texture and drawability. Nb
also adds to strength to some extent, but 0.01 to 0.02 wt.%Nb does not influence the
strength significantly.
Al – Mainly used for killing the steel. In normal IF / IFHS steel, Al content is about
0.04%. However, in present invention, the Al quantity is more than 0.1%. Al should
be added in the range 0.16 to 0.20%.

Coarsening of precipitates in interstitial free steel facilitates the sharpening of
ND//<111>fibre texture, which results in high drawability. It has been observed that
addition of Al in excess quantity helps in early coarsening of precipitates like
carbides, thereby sharpening the ND//<111>fibre texture.
(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.
The newly developed steel has been subjected to actual industrial trial in the premises
of a leading automobile company. The following results may be noted:
(a) Improved drawability: The improvement of drawability could be understood from
the lab test results as shown in the table of mechanical properties. In application
trial, this material performed satisfactorily, while manufacturing fuel tanks of
motorcycles.
(b) Weldability: There was no deterioration in weldability, as the newly developed
material passed through the welding operation during application trial without any
issue.
Figure 3 clearly depicts an improvement in casting stability in a steel melt with high Al
composition as per the current invention.
The invention as herein narrated with an exemplary embodiment should not be read
and constructed 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.

References
1. M. Guttmann, "Diffusive Phase Transformations in Hot Dip Galvanizing",
Materials Science Forum, Vols.155-156, pp. 527-548, 1994. Online since. May 1994.
2. S. Hoile, Materials Sc. & Tech., Vol. 16 (2000), pp. 1079-1093.
3. H. Kang and K. Chin, POSCOTechnical Report, Vol. 10 (2007), pp. 83-92.

WE CLAIM
1. A method of producing interstitial free (IF), formable and weldable steel sheet / strip
comprising the steps of:
- preparing a steel slab of IF (interstitial free) grade with extra aluminum addition at RH
degassing stage, having a composition in weight % of C < 0.0030, Mn< 0.1, S< 0.01,
P < 0.01, Si < 0.01, Al – 0.16-0.20, Ti – 0.02-0.06, Nb – 0.01-0.02, N (ppm) < 35, B
(ppm) < 10;
- reheating the slab at 1100–12000C;
- finish rolling the steel strip / sheet at temperature within 900-9100C, above Ar3
temperature of the steel;
- coiling the strip / sheet at 7000C;
- cold rolling the coiled strip / sheet with 77-84 % reduction;
- Processing through a continuous galvannealing line with strip annealing temperature
in the range 780 to 8600C; and galvannealing temperature in the range 530 to 5700C

2. A method of producing IFHS (interstitial free high strength) steel sheet / strip as claimed
in claim 1, wherein steel sheet has YS – 120-195 MPa and UTS >270 MPa.
3. A method of producing IFHS (interstitial free high strength) steel sheet / strip for
automotive body panel as claimed in claim 1, with a chemical composition that leads to
improvedcastability due to presence of extra Al during steel making and also better steel
cleanliness by ensuring improved castability.
4. A method as claimed in claim 1, wherein the steel has high drawability having %
elongation of minimum 44 and minimum r-bar of 1.4.
5. A steel sheet / strip produced according to method of claims 1,with a completely ferritic
microstructure.