-2-
FIELD OF APPLICATION
The present invention relates to a method of producing cold rolled high strength
interstitial free (HS-IF) steel sheets, having excellent formability (high ā and n
values) and low planar anisotropy Δr value).
BACKGROUND OF THE INVENTION
Commercial application of interstitial free (IF) steel came into practice in early
eighties. Steel for outer panel application, particularly in automobile applications
needs to possess higher strength for reduction in thickness, without sacrificing
dent resistance and formability properties. In view of this high strength
interstitial free steel was developed with manganese and phosphorus addition as
solid solution strengthening elements. It was however, reported in literature
that high phosphorus and titanium results in Fe Ti P formation during ROT
cooling and annealing in a batch annealing furnace. These precipitates
deteriorate the texture favorable for drawability and deteriorates r value.

-3-
Conventionally produced cold rolled high strength ultra low carbon steels (IF-HS)
using batch-annealing process have shown a poor r value (<1.5) along with-
high planar anisotropy.( Δr ~ 0.3).
High planar anisotropy (Δr) is undesirable, since it leads to ear formation during
the forming operation of components causing higher rejections. Lower planar
anisotropy along with high r value is difficult to achieve in steel sheets processed
with the conventional chemistry and process parameters.
A need was therefore, felt to improve the r value for the smooth forming of
components for automobile and other applications.

-4-
SUMMARY OF THE INVENTION
The main object of the present invention is therefore, to improve the plastic
strain ratio ( r value ) by reducing the formation of Fe Ti P i.e. decreasing the
amount of Ti is steel.
Another object of the present invention is to improve the planar anisotropy (Δr)
by niobium addition.
In a preferred embodiment the present invention provides a method of
producing high strength interstitial free cold rolled steel sheets comprising the
steps of producing steel sheets of appropriate chemistry with small amounts of
Ti, Nb and B in ultra low carbon steel through LD-RH-slab caster-hot rolling and
cold rolling route; processing the cold rolled sheet in a batch annealing furnace
with appropriate annealing parameters for producing high strength and good
formability, and skin pass rolling of the steel sheet.

-5-
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure l(a) and
Figure l(b) show the tensile curves free from any kink or yield point
elongation in skin passed condition and aged condition (100 C/lh).
The present invention provides high strength interstitial free (IF-HS) cold rolled
sheets having excellent formability (high r and n-values) along with low planar
anisotropy (Δr-value). This development addresses controlling the material
chemistry, hot rolling, cold rolling and annealing parameters for producing sheets
with an excellent combination of properties (high strength, good formability and
low planar anisotropy. Commercial trial processing of steel through LD-RH-slab
caster-hot rolling-cold rolling-batch annealing-skin pass rolling route has resulted
in steel sheets with r- 1.8, (Δ r < 0.15, yield strength (YS) - 226 MPa, ultimate
tensile strength (UTS) - 365 MPa, elongation (El) = 41 % and n = 0.22. The
steel sheet of the present invention is suitable for many automobile parts and
deep drawn earing free components.

-6-
When producing axially symmetrical parts by deep-drawing operation from a
sheet blank, earing as well as variable wall thickness may be observed despite a
symmetrical operating stress. If a material exhibits uniform magnitude of
mechanical properties in all directions, the earing would be low. "Earing" is a
result of non-uniform plastic flow due to heterogeneous strain distribution over
the sheet plane. If the plastic flow properties vary with angle, the flow of the
metal will be uneven in different directions, and this will give rise to peaks and
troughs around the periphery of the drawn cup, which is called earing. And the
phenomenon is known as plastic anisotropy. Anisotropy is measured in terms
plastic strain ratio (r-value), and planar anisotropy (Δ r). These can be
measured by tensile test. This can also be measured using modul r-tester based
on the principle of resonance frequency. The r is measured on the samples
taken from all three directions i.e. rolling, transverse and diagonal directions (0°,
90°, 45° directions).

-7-
r= εw/εt where, εw and εt are true width and true thickness strain
respectively
r = (ro-2r45 + r90)/4
Δr = (r0 - 2r45 + r90) / 2,
where, the suffixes indicate direction of the sheet with respect to rolling.
In the present invention to improve the r value, a new chemistry is designed
with a small amount of Ti, Nb and B in ultra low carbon steel. Specified
chemistry is shown below in Table 1. These steels were produced through LD-
RH-slab caster-hot rolling-cold rolling route. After cold rolling, the coil was
processed through batch annealing (BAF) route and skin passed in (SPM). The
optimum processing parameters are shown in Table 2.

-8-
Table 1: Specified chemical composition in wt. %.

c Mn P Al N S Ti Nb B
0.004max 0.60max 0.06max 0.025min 0.004max 0.012max 0.020max 0.020max 0.0008max
Table 2 : Process parameters adopted for the newly developed steel

FRT, °C CT, TCM CR Ann. SPM
°C red., thk, temp, ° C def, %
% mm
900 650 70 0.5-2.0 >650 >0.8
min min min

-9-
TESTING
Samples in cold rolled-skin passed condition were evaluated for yield strength
(YS), ultimate tensile strength (UTS), elongation and n-value using Instron
Tensile Machine. Drawability ( r - value ) was evaluated using modul r tester
as well as by tensile method. Bake hardening (BH) and aging tests were
conducted to check the stabilization of carbon and nitrogen. For BH test, tensile
specimens were given an elongation of 2 %, heated at 170° C for 20 min in a
furnace and then air-cooled. The aged sample was again tested on tensile
machine. The reappearance of the yield point is noted and BH strength was
evaluated as the difference between flow stress after baking and lower YS after
2 % elongation. For aging test, the samples were heated at 100° C for 1h and
then tensile test were conducted to check the yield point elongation.
The properties of cold rolled strips in skin passed condition has been presented
in Table 3. The tensile properties of newly developed material are satisfactory
(YS: 226 MPa, UTS: 365 MPa, EI-41 %) with excellent r value of 1.8. A large

-10-
reduction in Δ r (from 0.3 to < 0.14) was also noted. The tensile properties
conform to the strength requirement of IF-HS grade in all respect. Tensile
curves for the new material in skin passed and aged condition are displayed in
Figure 1(a) and (b). Tensile curves shows free from any kink or luder strain in
skin passed condition as well as after aging at 100° C for 1 h revealing the
condition for full stabilization. Bake hardening strength values was also found to
be almost zero confirming the material to be fully stabilized.
Table 3: Properties of the steel developed

YS, UTS, El, r Δr r Δr Aging BH, MPa Remarks
MPa MPa % by modul r by by by at 100
tester modul r tensile tensile C/1h
tester test test
226 365 41 1.8 <0.15 1.8 <0.1 No Nil Fully
Kink stabilised

-11-
Commercially processed steel through LD-RH-slab caster - hot rolling-cold rolling-
batch annealing-skin pass rolling route has resulted in excellent combination of
properties (high strength, good formability and low planar anisotropy). The steel
sheet of the present invention is suitable for the automobile applications i.e. for
three wheelers, four-wheelers, light commercial vehicles and heavy commercial
vehicles. Due to high strength with an excellent formability, this may lead to a
large reduction in weight of an automobile leading to a fuel saving. These steels
may be also used for many other deep drawn parts for non-auto applications,
where low earing is desired.

The main object of the present invention is therefore, to improve the plastic
strain ratio ( r value ) by reducing the formation of Fe Ti P i.e. decreasing the
amount of Ti is steel.
Another object of the present invention is to improve the planar anisotropy (Δr)
by niobium addition.
In a preferred embodiment the present invention provides a method of
producing high strength interstitial free cold rolled steel sheets comprising the
steps of producing steel sheets of appropriate chemistry with small amounts of
Ti, Nb and B in ultra low carbon steel through LD-RH-slab caster-hot rolling and
cold rolling route; processing the cold rolled sheet in a batch annealing furnace
with appropriate annealing parameters for producing high strength and good
formability, and skin pass rolling of the steel sheet.