FIELD OF THE INVENTION:
The present invention relates to high strength formable quality hot rolled steel having
tensile strength given by YS: 460 MPa min, favouring applications in automobile industry
especially for light weight crash resistant car body. Importantly, the present invention is
directed to achieving improved forming properties along with high strength in steel by
selective addition of Si in Nb micro-alloyed steel. Moreover, such alloy properties
achieved corresponding to the grade of steel with 0.02-0.03 % Nb and 0.2-0.3 % Si has
been found to achieve all-round improvement in forming properties like lower YS/UTS
ratio in the range of 0.86-0.87 and higher elongation in the range of 30-32%. The
properties like high strength accompanied with high formability of the hot rolled alloy
steel sheet according to the invention is thus capable of wide industrial application auto
car body/components manufacturing or similar other applications where reduced weight
and higher strength and formability would favor lowering engine power requirements
based on lower car body weigh, improved fuel efficiency per unit gross weight and crash
resistant properties.
BACKGROUND ART
The globally competitive design configurations in the automotive industry all over the
world has been constantly pursuing development of new materials for reduced weight of
car body to reduce power requirement and fuel economy based on per unit gross/pay
load and improved crash performance of passenger cars. High strength steels owing to
their higher strength with moderate forming capacity favour application of thinner gauge
sheets, reducing overall weight of car and saving in fuel consumption. It has been
established that more than 20 % of the weight of the car can be taken away by using
high strength steel on a single component. In the past, ULSAB AVC (Ultra Light Steel
Auto Body Advance Vehicle Concept) promoted use of high strength steel to make lighter
and safer vehicles. Conventionally hot rolled high strength formable grades are produced
with higher Nb (> 0.04 %) and restricted Si (< 0.05 %). In the present work, innovative
alloy chemistry i.e, lower niobium and higher silicon contents (Table 1) was designed and
heats were made through BOF-LF-CC route. Traditionally, silicon has been effectively
being used for the development of advance class of high strength steels (AHSS) like Dual
Phase and TRIP (Transformation Induced Plasticity) steels because of its scavenging and

solid solution strengthening effect. However, potential of silicon has not been exploited in
hot rolled micro alloyed steel yet.
There has been thus an unfulfilled need to develop an alloy steel through the hot rolled
micro alloying rout whereby the higher formability of the steel sheet shall be induced in
terms of lowering YS/UTS ratio and improved percent elongation while simultaneously
enhancing the yield and the ultimate tensile strength properties. The limitations of the
prior art has been overcome by developing in line with the global trend, high strength
formable quality hot rolled steel (YS: 460 MPa min) at one of the steel plants of the
applicants. Innovative alloys were designed with lower niobium and higher silicon
contents and processed by following optimized hot rolling parameters through thermo
mechanical simulation studies.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide a high strength hot rolled
steel having high formablility comprising selective Si content (wt%) in niobium micro
alloyed steel and a process for its manufacture with optimized hot rolling parameters.
A further object of the present invention is directed to high strength formable hot rolled
Nb microalloyed steel involving selective Si such that it ensure properties of the resulting
rolled sheet steel have YS 460 MPa (min) with reduced YS/UTS and improved percent
elongation favoring high formability.
A still further object of the present invention is directed to developing said high strength
formable hot rolled Nb microalloyed steel wherein the process of manufacture and the
selective Si and Nb content ensure desired preferred combination of strength and
formable properties by inducing finer ferrite grain in microstructure of the resulting steel
as compared to the conventional grade.
A still further object of the present invention is directed to developing said high strength
formable hot rolled Nb micro alloyed steel wherein the hole expansion ratio {X), which is
commonly used to measure the formability of hot rolled steel, is appreciably higher in
case of newly developed steel as compared to that of conventional high strength steel.

A still further object of the present invention is directed to developing said high strength
fornnabie hot rolled Nb micro alloyed steel wherein lower YS/UTS ratio of hot rolled steel
sheet produced favours an improved strain distribution during press forming, enabling
more complex parts to be formed and thus favoring application in light weight automobile
car components/body parts building having higher strength.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided high
strength (460 MPa min.) formable hot rolled steel composition comprising:
C= 0.06-0.08 wt %;
Mn.= 0.9-1.1 wt %;
S = 0.02 max wt %;
P = 0.025 max wt %;
Si = 0.2-0.3 wt %;
Al = 0.02 min wt %;and
Nb. - 0.02-0.03 wt %.
Importantly, the above disclosed high strength (460 MPa min.) formable hot rolled steel
composition include the mechanical properties of :
YS = 425-460 MPa; and
UTS = 500-530 MPa.
Preferably also, the above high strength (460 MPa min.) formable hot rolled steel
composition of the invention have the mechanical properties comprising:
%EI (GL:50mm) = 30-32;
YS/UTS ratio = 0.86-0.87; and

Hole Expansion Ratio (λ)= 43-45 %.
Advantageously, the high strength (460 MPa min.) formable hot rolled steel have a
Ferrite grain size which is finer of about 5.4 μm.
According to another aspect of the invention there is provided a process for the
manufacture of High strength (460 MPa min.) formable hot rolled steel comprising the
step of selective obtaining the same involving a selective steel composition comprising:
C= 0.06-0.08 wt %;
Mn.= 0.9-1.1 wt %;
S = 0.02 max wt %;
P = 0.025 max wt %;
Si = 0.2-0.3 wt %;
Al = 0.02 min wt %; and
Nb. = 0.02-0.03 wt %.
The details of the invention, its objects and advantages are explained hereunder in
relation to following non-limiting exemplary illustrations:
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the illustration of the Optical micrograph of conventional grade of steel (with
lower Si and higher Nb) produced through BOF-LF-CC route
Figure 2: is the illustration of the Optical micrograph of newly developed grade of steel
(with higher Si and lower Nb) produced through BOF-LF-CC route, according to the
present invention with optimum process parameters for hot rolling providing desired
microstructure resulting into good combination of high strength and formability
properties.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention is directed to innovative alloy steel composition comprising lower
niobium and higher silicon contents and a processed for its manufacture using optimized
hot rolling parameters standardized through thermo mechanical simulation trials. The
present invention aims to achieve improved forming properties with high strength by
selective addition of Si in Nb micro alloyed steel and thereby achieving microstructure of
the hot rolled steel sheet having finer ferrite grains as compared to the conventional.
Silicon has been effectively used in the art for the development of Advance class of High
Strength Steels (AHSS) like Dual Phase and TRIP (Transformation Induced Plasticity)
steels because of its scavenging and solid solution strengthening effect. The present
invention attempts to explore the potential of higher silicon addition in production of hot
rolled microalloyed steel for achieving desired combination of properties of higher
strength and improved formability for automobile and similar other industrial
applications. It has been observed that the chemical composition of such variety
conventionally is constituted of a chemical composition comprising on wt percent basis as
follows:
Conventional High strength steel grade:
C: 0.06-0.08 % wt; Mn: 1.0-1.2 % wt.; S: 0.02% wt. max; P: 0.025% wt. max; Si:
0.05% wt. max; Al: 0.02% wt. min; Nb: 0.04-0.06%wt.
The tensile properties demonstrated on testing of the conventional high strength steel
grade as given above, are as follows:
YS: 415-440 MPa;
UTS : 450-470 MPa;
%Elongation (GL:50mm): 25-28;
YS/UTS ratio: 0.94-0.95;
Hole Expansion Ratio ( A,):32-34 %;

The process according to the present invention to develop the high strength formable
silicon added micro alloyed steels involved subjecting Continuously cast slabs having
desired starting composition on wt percent basis as :C- 0.06-0.08, Mn- 0.9-1.1, S- 0.02
max, P- 0.025 max. Si- 0.2-0.3, Al- 0.02 min, Nb- 0.02-0.03, to be rolled into 3.8 mm
hot rolled coils in hot strip mill by adhering to finish rolling temperature in the range of
860°C - 870 °C, and coiling temperature in the range of 630°C - 640 °C respectively,
decided based on the analysis of results generated through simulation studies. The
chemical compositions (wt %) of newly developed steel grade are given in the following
Table 1.
Table 1:
Chemical composition (wt %) of Nb micro-alloyed steels:

The tensile properties of hot rolled sheet materials of newly developed steel are tested
and observed values are summarized in Table 2 below. It can be observed that the yield
and tensile strength values of conventional steel (YS : 440 MPa and UTS : 470 MPa) are
lower than that of newly developed steel.

The properties achieved for the developed grade of steel with 0.02-0.03 % Nb and 0.2-
0.3 % Si were found to be of special interest because of its all-round improvement in
forming properties, like lower YS/UTS ratio (0.86-0.87) and higher elongation (30-32 %).
In general, lower YS/UTS ratio of sheet material favours an improved strain distribution
during press forming, enabling more complex parts to be formed. Higher percentage

elongation coupled with lower YS/UTS ratio is a measure of formability characteristics of
steels. The hole expansion ratio (λ), which is commonly used to measure the formability
of hot rolled steel, was found to be appreciably higher in the range of 43-45 % in case of
newly developed Si added Nb microalloyed steel grade as compared to that of
conventional high strength steel in the range of 32-34%.
To investigate and ascertain the reason behind such good combination of properties
demonstrated by the newly developed steel grade according to the invention,
microstructural evaluation were carried out. Reference is invited to the accompanying
Figure 1 which is the Optical micrograph of conventional high strength steel grade
having lower Si and higher Nb content showed a grain size of the ferrite of the order of
around 6.2 μm. Figure 2 shows that Ferrite grain size in newly developed steel is around
5.4 μm. It is an interesting observation that in case of steel with still higher Si, Nb
content was further 40 % lower (0.026 %) and yet there was no appreciable change in
ferrite grain size, indicating positive role of Si in Nb towards precipitation of Nb(CN) and
grain refinement.
It is thus possible by way of the present invention to develop a high strength and high
formable grade of microalloyed hot rolled steel and a process for its manufacture with
controlled finish rolling and coiling temperatures, such that the steel sheet developed is
adapted to an improved strain distribution during high degree of press forming, enabling
more complex parts to be formed. The developed grade of steel produced following the
process according to the present invention ensure a lower YS/UTS ratio and relatively
higher hole expansion ratio (λ) because of selective Si addition along with Nb
microalloying favoring finer ferritic microstructure of the HR sheet steel obtained and
thus favoring application of the same in light weight automobile car components/body
parts building having higher strength and similar other industrial application where
strength compared to weight is of critical consideration.

WE CLAIM:
1. High strength (460 MPa min.) formable hot rolled steel composition comprising:
C= 0.06-0.08 wt %;
Mn.= 0.9-1.1 wt %;
S = 0.02 max wt %;
P = 0.025 max wt %;
Si = 0.2-0.3 wt %;
Al = 0.02 min wt %;and
Nb. = 0.02-0.03 wt %.
2. High strength (460 MPa min.) formable hot rolled steel composition as claimed in
claim 1 wherein the mechanical properties comprise:
YS = 425-460 MPa; and
UTS = 500-530 MPa.
3. High strength (460 MPa min.) formable hot rolled steel composition as claimed in
anyone of claims 1 or 2 wherein the mechanical properties comprise:
%EI (GL:50mm) = 30-32;
YS/UTS ratio = 0.86-0.87; and
Hole Expansion Ratio (λ)=43-45 %.
4. High strength (460 MPa min.) formable hot rolled steel composition as claimed in
anyone of claims 1 to 3 wherein Ferrite grain size is finer about 5.4 μm.
5. A process for the manufacture of High strength (460 MPa min.) formable hot rolled
steel as claimed in anyone of claims 1 to 4 comprising the step of selectively obtaining
the same involving a selective steel composition comprising:

C= 0.06-0.08 wt %;
Mn.= 0.9-1.1 wt %;
S = 0.02 max wt %;
P = 0.025 max wt %;
Si = 0.2-0.3 wt %;
Al = 0.02 min wt %;and
Nb. = 0.02-0.03 wt %.
6. High strength (460 MPa min.) formable hot rolled steel composition and its process of
manufacture substantially as hereindescribed and illustrated with reference to the
accompanying figures.


Dated this 25th day of September, 2008

A high strength formable quality hot roiled steel having tensile strength given by YS: 460
MPa min, favoring applications in automobile industry for crash resistant car body. The
present invention is directed to achieving improved forming properties along with high
strength in steel by selective addition of Si in Nb micro-alloyed steel. Moreover, such
alloy properties achieved corresponding to the grade of steel with 0.02-0.03 % Nb and
0.2-0.3 % Si has been found to achieve all-round improvement in forming properties like
lower YS/UTS ratio in the range of 0.86-0.87 ,higher elongation in the range of 30-32%
and Hole Expansion Ratio (λ) of 43-45 %. The properties like high strength accompanied
with high formability of the hot rolled alloy steel sheet is thus capable of wide industrial
application including auto car body/components manufacturing.