FIELD OF THE INVENTION
The present invention relates to the development of batch annealed high
strength IF steel sheet, with extra Al addition. More particularly the invention
relates to development of a high strength 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 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, since there are not too
many continuous annealing facilities available in India, the steel manufacturers
are compelled to process interstitial free steels mainly 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 problem is mainly related to the presence of solid solution strengthening
elements. Generally, 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. However, there are a number of
drawbacks associated with known processes. Addition of P in IFHS steel leads to
a few problems such as segregation of P at grain boundaries leads to secondary
work embrittlement effect.Boron (B) is added to control this effect, but B itself
affects drawability in a deleterious manner.Phosphorous (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.With a stringent control
of all the above factors, the drawability of batch annealed IFHS steels can be
maintained at an appreciable level, but it is always considerably lower than that
of soft IF steel. A number of researchers have investigated on the influences of
various elements on the properties of IF and IFHS steels. A few of them are
mentioned below:
(a) In presence of Mn, the drop in ѓ value is attributed to the impeded {111}
ferrite grain orientation due to a solute drag mechanism [1].
(b) Mn leads to deterioration of ѓ value, but this is more effective with higher
amount of carbon in solid solution, and less effective when carbon content is
very low and properly stabilized [2, 3].
(c) Though P is an important strengthening element, it has detrimental effect on
ѓ value [4, 5].
(d) Sometimes, Si is also added to some extent for solid solution strengthening.
The combined effect of all the strengthening elements (Mn, P, Si) is also
detrimental on ѓ value [6].

(e) Boron needs to be added to IFHS steel, when P is there. But B also
deteriorates the ѓ value [7].
(f) It has been shown in an earlier work how the ѓ value of IFHS steel can be
increased by eliminating P, which is a principal strengthening element for this
material [8]. However, in that case (without P), only Mn can be used for solid
solution strengthening, and therefore, the strength of this IFHS steel cannot
go beyond 350 MPa, because Mn addition beyond a certain limit would again
lead to the deterioration of ѓ value.
Objective of the invention
It is therefore an object of the present invention to propose a new chemical
composition, for IFHS (interstitial free high strength) steel including additional
amount of Al.
Another object of the invention is to conveniently process the proposed
composition through batch annealing route.
Yet another object of the invention is to obtain a high Lankford value in the steel
which is close to that of high quality soft 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
The present invention is related with development of a new chemical
composition of IFHS (interstitial free high strength) grade with minimum UTS of
340 MPa, and with a higher drawability compared to that of existing batch
annealed IFHS 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.
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 phosphorus and 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 gradeis also adequately weldable.

Illustration of the invention

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. Since this material
contains some P, it is prescribed that the steel slabs are not allowed cool down
to room temperature and are taken directly into the reheating furnace. The
reason is that the segregation of P at the grain boundaries may even lead to
cracking of the slabs inside the reheating furnace, which is not desirable.The
residence time inside the furnace should be at least 2 hr 45 min so as to ensure
proper soaking of the steel slabs. The slab drop out temperature should be about
1150-12000C.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 should be around 1070-10800C.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 should be around 10300C and the finish mill exit temperature should
be 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.After hot rolling,
the steel strip is subjected to cooling on the run out table. The temperature
should be brought down to about 7000C quickly, and then it should be 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.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 considered as a serious defect.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 may be
carried out in a tandem rolling mill also. The thickness reduction should be about
80% on an average, to ensure the best result.The cold rolled material should be
subjected to heat treatment. The present method is for achieving a good
crystallographic texture and drawability by processing through batch annealing
furnace. The coils are stacked inside (4-5 coils in a stack) a bell type furnace, in
which the annealing treatment is performed in 100% hydrogen atmosphere. The
hot spot and cold spot temperatures for this process should be 7200C and 6900C,
respectively.
The prescribed ranges of physical parameters
(a) Slab drop out around – 1150-12000C
(b) Finish rolling higher than Ar3 temperature – 900-9100C
(c) Coiling at700-7100C
(d) Cold rolling the hot rolled strip at 77-82% reduction

(e) Batch annealing is carried out withhot spot temperature 7200C and cold
spot temperature 6900C
Test results (two results are given below):

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.
The improvement of castability was confirmed by the mould level plot. This plot,
as shown in Figure 2, confirms that there was no mould level fluctuation. This is
absolutely necessary in order to ensure a superior cleanliness of steel.
Chemical composition: 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. Presence of excess amount of
these elements is harmful, as they affect the texture and drawability.
B – Boron is added to prevent the secondary work embrittlement effect of P.
But it is most effective at an optimum range of 5-8 ppm, beyond which B also
deteriorates drawability.
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%. When Al is in the range 0.11 to 0.13%, it improves texture and
drawability. Further addition of Al is not much helpful.

Physical parameters such as slab drop out temperature, finish rolling
temperature, coiling temperature, cold reduction and batch annealing
temperature, all have significant effects on final mechanical properties.
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. G. Krauss, D. O. Wilshynsky and D. K. Matlock, in Proc. Int. Symp. on
Interstitial Free Steel Sheet: Processing, Fabrication and Properties, Ed. L. E.
Collins and D. L. Baragar, CIM/ICM, Ottawa (1991) pp. 1-14.
2. W. B. Hutchinson, K-I. Nilsson and J. Hirsch, in Proc. Int. Symp. on
Metallurgy of Vacuum Degassed Steel Products, Ed. R. Pradhan, TMS (1989)
pp. 109-125.
3. R. Yoda, I. Tsukatani, T. Inoue and T. Saito, ISIJ Int., Vol. 34 (1994), pp. 70-
76.
4. K-I. Nilsson and E. Johansson, in Proc. Int. Symp. on Metallurgy of Vacuum
Degassed Steel Products, Ed. R. Pradhan, TMS (1989) pp. 143-160.
5. D. P. Hoydick and T. M. Osman, in Proc. 40th MWSP Conf., ISS (1998) pp.
195-204.
6. O. Hashimoto, S. Satoh, T. Irie and N. Ohashi, in Proc. Int. Conf. on
Advances in Physical Metallurgy and Applications of Steels, The Metals Society
of Great Britain, TMS, Book 284 (1982), pp. 95-104.
7. J. Haga, N. Mizui, T. Nagamichi and A. Okamoto, ISIJ Int., Vol. 38 (1998), pp.
580-586.

8. B. Bhattacharya, A method of producing phosphorus free interstitial free high
strength (IFHS) formable and weldable steel sheet / strip with improved
drawability, Indian Patent 263079

WE CLAIM
1. A method of producing interstitial free high strength (IFHS), formable and
weldable steel sheet / strip comprising the steps of:
- preparing a steel slab of IFHS (interstitial free high strength) grade
with extra aluminum addition during melting, having a composition in
weight % of C < 0.0030, Mn – 0.5-0.6, S –< 0.01, P –0.02-0.03, Si –<
0.015, Al – 0.11-0.13, Ti – 0.05-0.06, Nb – 0.01-0.02, N – (ppm) –<
30;
- reheating the slab at 1150–12000C;
- 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 77-82 % reduction;
- batch annealing the cold rolled sheet / strip maintaining hot spot
temperature 7200C and cold spot temperature 6900C; and
- skin pass rolling the annealed sheet / strip with 0.5% temper
elongation.

2. A method of producing IFHS (interstitial free high strength) steel sheet /
strip as claimed in claim 1, wherein steel sheet has YS -145-245 MPa and
UTS > 340 MPa.
3. A method of producing IFHS (interestitial free high strength) steel sheet /
strip for automotive body pane, as claimed in claim 1, with a chemical
composition that would lead to improved castability 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 40 and minimum r-bar of 1.4.
5. A steel sheet / strip produced according to method of claims 1, with a
completely ferritic microstructure.