TECHNICAL FIELD
Present disclosure relates in general to a field of material science and metallurgy. Particularly,
but not exclusively, the present disclosure relates to a high strength hot rolled steel. Further
embodiments of the disclosure disclose a method for manufacturing the high strength hot rolled
steel, which exhibits tensile strength greater than 540 MPa.
BACKGROUND OF THE DISCLOSURE
Steel is an alloy of iron, carbon, and other alloying elements. Because of its high tensile strength
and low cost, steel is considered as a major component in wide variety of applications. Some
of the applications of the steel includes construction, ship building tools, automobiles,
machines, bridges, and numerous other applications. The steel obtained from steel making
process may not possess all the desired properties. Therefore, the steel may be subjected to
secondary processes such as various heat treatment processes for controlling material
properties to meet various needs in the intended applications.
With rising concerns over global environmental problems and demand from automotive
industry for higher collision safety of vehicles, impose conflicting requirements on materials
used for the vehicle bodies. The vehicle bodies are required to be stronger yet lighter at the
same time. Conventionally, steels possessing tensile strength of less than 350 MPa has been
adapted in automotive applications. With evolution and advancements, advanced vehicles in
terms of load bearing capacity, crash worthiness and the like have been developed. To meet
these advancements, demand towards steel possessing high strength (i.e., high tensile strength)
has been in rise. However, steels with high tensile strength are less ductile, which affects
formality of such steels. Advanced High Strength Steels [AHSS] having tensile strength in a
range of 500-600 MPa have been adapted in components such as wheel disk and rim, to meet
the advancements in the vehicles. However, such steels are not suitable in manufacturing long
members of the vehicle, which involves several bending and welding operations, as the AHSS
steel possess poor ductile properties.
One of the patents CN103667908B discloses a method for producing a hot-rolled high-strength
steel sheet with a tensile strength of 540 MPa and used for structural and reinforcing
components. The steel composition of this patent comprises of C, Mn, Ti and Nb as main
strengthening elements. When Mn acts as an important solid solution strengthener, it can also
lead to banding issues in the steel if added in higher amounts. This demands for precise control

of the process, adding to higher manufacturing costs. Moreover, these bands act as a crack
initiators during bending operations.
Further, CN103602890B discloses a method for producing a hot-rolled high-strength steel
sheet which is precipitation strengthening and has a tensile strength greater than 540 MPa. The
patent also discloses regarding the stretchability of the material which is an important aspect
required while manufacturing automotive components. The steel contains high levels of Si -
0.1 – 1.2 % which might result in formation of fayalite on the steel. Removing fayalite is
essential and would require an additional step like pickling.
Another patent publication, US20180209007A1 discloses a method for producing a hot-rolled
high-strength steel sheet with tensile strength no less than 540 MPa. This patent proposes,
addition of higher amounts of Mn, Si, Nb and V. The larger amount of these elements might
render the steel to be costly and other segregation issues along the centerline of the hot rolled
product.
The present disclosure is directed to overcome one or more limitations stated above or any
other limitation associated with the prior arts.
SUMMARY OF THE DISCLOSURE
One or more shortcomings of the prior art are overcome by method as disclosed and additional
advantages are provided through the method as described in the present disclosure.
Additional features and advantages are realized through the techniques of the present
disclosure. Other embodiments and aspects of the disclosure are described in detail herein and
are considered a part of the claimed disclosure.
In one non-limiting embodiment, there is provided a method for manufacturing a high strength
hot rolled steel sheet is disclosed. The method starts with casting a steel slab of a composition
in weight percentage including: carbon (C) at about 0.05% to about 0.09%, manganese (Mn)
at about 0.8% to about 0.9%, silicon (Si) at about 0.05% to about 0.15%, titanium (Ti) at about
0.015% to 0.025%, aluminum (Al) at about 0.05%, niobium (Nb) at about 0.025% to 0.035%,
sulphur (S) at about 0.01%, phosphorous (P) at about 0.03%, nitrogen (N) at about 0.01%, and
the balance being Iron (Fe) optionally along with incidental elements. Upon casting, the steel
slab is heated to a first predetermined temperature for a first predetermined time, for dissolution

of niobium and homogenization. The steel slab is then subjected to a hot rolling process, to
induce at least 50% plastic strain below no-recrystallization temperature to for a steel sheet,
such that finish rolling is carried out at a second predetermined temperature. The method
further includes cooling the steel sheet to a third predetermined temperature at a predetermined
cooling rate to form the high strength hot rolled steel sheet. The high strength hot rolled steel
sheet comprises primarily a primarily a ferrite and small amount of pearlite microstructure.
In an embodiment, high strength hot rolled steel sheet exhibits tensile strength greater than 540
MPa and yield strength greater than 470 MPa.
In an embodiment, the high strength hot rolled steel sheet comprises microstructure represented
by, in area%, the ferrite of about 95% and pearlite less than 5%.
In an embodiment, the first predetermined temperature ranges from about 1220 °C to 1240 °C
and the first predetermined time is about 2 to 3 hours.
In an embodiment, the second predetermined temperature is a finish rolling temperature
ranging from about 870 °C to 900 °C.
In an embodiment, no-recrystallization temperature is a temperature below which
recrystallization stops and active strain build up occurs in the microstructure. The active strain
build-up facilitates precipitation assisted transformation aiding in grain refinement.
In an embodiment, the third predetermined temperature ranges from about 560 °C to 590 °C.
In an embodiment, cooling is performed on a run-out table and the predetermined cooling rate
is about 5 °C/s to 8 °C/s.
In another non-limiting embodiment of the disclosure, a high strength hot rolled steel sheet is
disclosed. The steel sheet comprises a composition in weight percentage including: carbon (C)
at about 0.05% to about 0.09%, manganese (Mn) at about 0.8% to about 0.9%, silicon (Si) at
about 0.05% to about 0.15%, titanium (Ti) at about 0.015% to 0.025%, aluminum (Al) at about
0.05%, niobium (Nb) at about 0.025% to 0.035%, sulphur (S) at about 0.01%, phosphorous (P)
at about 0.03%, nitrogen (N) at about 0.01%, and the balance being Iron (Fe) optionally along
with incidental elements. The high strength hot rolled steel sheet comprises primarily a
primarily a ferrite and small amount of pearlite microstructure.

In an embodiment, the high strength hot rolled steel sheet comprises microstructure represented
by, in area%, the ferrite of about 95% and pearlite less than 5%. The ferrite microstructure
includes a grain size of less than 4 µm.
It is to be understood that the aspects and embodiments of the disclosure described above may
be used in any combination with each other. Several of the aspects and embodiments may be
combined together to form a further embodiment of the disclosure.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In
addition to the illustrative aspects, embodiments, and features described above, further aspects,
embodiments, and features will become apparent by reference to the drawings and the
following detailed description.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
The novel features and characteristics of the disclosure are set forth in the appended
description. The disclosure itself, however, as well as a preferred mode of use, further
objectives, and advantages thereof, will best be understood by reference to the following
detailed description of an illustrative embodiment when read in conjunction with the
accompanying figures. One or more embodiments are now described, by way of example only,
with reference to the accompanying figures wherein like reference numerals represent like
elements and in which:
Figure. 1 is a flowchart illustrating a method for manufacturing a high strength hot rolled steel
sheet, in accordance with an embodiment of the present disclosure.
Figures. 2a and 2b illustrates an optical microscopy image of a high strength hot rolled steel
sheet, manufactured by the method of the present disclosure, in accordance with an
embodiment of the present disclosure.
Figure. 2c illustrates an optical microscopy image of the steel sheet manufactured by a
conventional method.
Figures. 3a and 3b illustrates a graphical representations of deformation below no-
recrystallization temperature of steel processed by method of present disclosure and
conventional method, respectively.

Figure. 4 illustrates a graphical representation of dissolution temperature estimation of
Nb(CN) precipitates from Thermo-Calc calculations, in accordance with an embodiment of the
present disclosure.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled
in the art will readily recognize from the following description that alternative embodiments of
the methods illustrated herein may be employed without departing from the principles of the
disclosure described herein.
DETAILED DESCRIPTION
The foregoing has broadly outlined the features and technical advantages of the present
disclosure in order that the detailed description of the disclosure that follows may be better
understood. Additional features and advantages of the disclosure will be described hereinafter
which form the subject of the description of the disclosure. It should also be realized by those
skilled in the art that such equivalent methods do not depart from the scope of the disclosure.
The novel features which are believed to be characteristic of the disclosure, as to method of
operation, together with further objects and advantages will be better understood from the
following description when considered in connection with the accompanying figures. It is to
be expressly understood, however, that each of the figures is provided for the purpose of
illustration and description only and is not intended as a definition of the limits of the present
disclosure.
In the present document, the word "exemplary" is used herein to mean "serving as an example,
instance, or illustration." Any embodiment or implementation of the present subject matter
described herein as "exemplary" is not necessarily to be construed as preferred or advantageous
over other embodiments.
While the disclosure is susceptible to various modifications and alternative forms, specific
embodiments thereof has been shown by way of example in the drawings and will be described
in detail below. It should be understood, however that it is not intended to limit the disclosure
to the particular forms disclosed, but on the contrary, the disclosure is to cover all
modifications, equivalents, and alternative falling within the spirit and the scope of the
disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a
non-exclusive inclusions, such that a method that comprises a list of acts does not include only
those acts but may include other acts not expressly listed or inherent to such method. In other
words, one or more acts in a method proceeded by “comprises… a” does not, without more
constraints, preclude the existence of other acts or additional acts in the method.
Embodiments of the present disclosure discloses a high strength hot rolled steel sheet and a
method for manufacturing or producing the high strength hot rolled steel sheet. With evolution
and advancements, vehicles developed are advanced in terms of load bearing capacity, crash
worthiness and the like. To meet these advancements, demand towards steel with high strength
(i.e., high tensile strength) has been in rise. However, steels with high tensile strength are less
ductile, which affects formality of such steels. Advanced High Strength Steels [AHSS] having
tensile strength in a range of 500-600 MPa have been adapted in components such as wheel
disk and rim, to meet the advancements in the vehicles. However, such steels are not suitable
in manufacturing long members of the vehicle, which involves several bending and welding
operations, as the AHSS steel possess poor ductile properties. Accordingly, the method for
manufacturing high strength hot rolled steel sheet with tensile strength greater than 540 MPa,
yield strength greater than 470 MPa and good ductility property and formed by a simpler
method is described in the present disclosure. The high strength hot rolled steel sheet may be
widely employed in automotive applications to manufacture long members and load bearing
components, as high strength hot rolled steel sheet possess desired ductility.
In the method of manufacturing the high strength hot rolled steel sheet, a first step may include
casting the steel of composition in weight percent including carbon (C) at about 0.05% to about
0.09%, manganese (Mn) at about 0.8% to about 0.9%, silicon (Si) at about 0.05% to about
0.15%, titanium (Ti) at about 0.015% to 0.025%, aluminum (Al) at about 0.05%, niobium (Nb)
at about 0.025% to 0.035%, sulphur (S) at about 0.01%, phosphorous (P) at about 0.03%,
nitrogen (N) at about 0.01%, and the balance being Iron (Fe) optionally along with incidental
elements or impurities, in form of a steel slab. The casted steel slab is then heated to a first
predetermined temperature ranging from about 1220 ˚C to 1240˚C for a predetermined time
period ranging from2 to 3 hours. Upon heating the steel slab, the steel slab may be deformed
in a hot rolling process. During hot rolling process least 50% of plastic strain is induced below
no-recrystallization temperature to form a steel sheet, such that finish rolling is carried out at a
second predetermined temperature ranging from 870 °C to 900 °C. In an embodiment, the hot

rolling process may be performed in a roughing mill and a tandem finishing mill. Upon
completion of the hot rolling process, the steel sheet may be subjected to cooling, which is a
performed on a run-out table (ROT), to cool the steel sheet to a third predetermined temperature
at a predetermined cooling rate, to form a high strength hot rolled steel. In an embodiment, the
third predetermined temperature may range from about 560 °C to 590 °C and the cooling rate
may range from about 5 °C/s to 8 °C/s.
In an embodiment, the high strength hot rolled sheet exhibits, tensile strength of more than 540
MPa, yield strength of more than 470 MPa and elongation of about 28%. The high strength hot
rolled steel sheet includes ferrite microstructure of about 95% with ferrite grain size of
approximately 5 µm, and pearlite less than 5%. These properties favour the high strength hot
rolled steel sheet to be used in manufacturing automotive components such as but not limiting
to long members, weight bearing components for heavy automobiles.
Henceforth, the present disclosure is explained with the help of figures for a method of
manufacturing a high strength hot rolled steel sheet. However, such exemplary embodiments
should not be construed as limitations of the present disclosure since the method may be used
on other types of steels where such need arises. A person skilled in the art may envisage various
such embodiments without deviating from scope of the present disclosure.
Figure. 1 is an exemplary embodiment of the present disclosure illustrating a flowchart
depicting a method for manufacturing a high strength hot rolled steel sheet. In the present
disclosure, mechanical properties such as strength, tensile strength, yield strength and
elongation (ductility) of the final microstructure of the steel sheet may be improved. The high
strength hot rolled steel sheet produced by the method of the present disclosure, includes less
alloying content and microstructure including ferrite and pearlite. The method is now described
with reference to the flowchart blocks and is as below. The order in which the method is
described is not intended to be construed as a limitation, and any number of the described
method blocks can be combined in any order to implement the method. Additionally, individual
blocks may be deleted from the methods without departing from the scope of the subject matter
described herein.
At block 101, a steel of desired alloying composition may be made in a Linz-Donawitz (LD)
convertor. In embodiment, the steel may have composition in weight percent [wt%] including
carbon (C) at about 0.05% to about 0.09%, manganese (Mn) at about 0.8% to about 0.9%,

silicon (Si) at about 0.05% to about 0.15%, titanium (Ti) at about 0.015% to 0.025%, aluminum
(Al) at about 0.05%, niobium (Nb) at about 0.025% to 0.035%, sulphur (S) at about 0.01%,
phosphorous (P) at about 0.03%, nitrogen (N) at about 0.01%, and the balance being Iron (Fe)
optionally along with incidental elements.
At block 102, the method may include casting the steel in form of but not limiting to a slab. In
an embodiment, liquid steel with the above-mentioned composition and range of alloying
elements may be continuously casted into a slab. The liquid steel of the above-mentioned
composition may be continuously casted either in a conventional continuous caster or a thin
slab caster. Further, the method includes a step or stage of heating the steel slab to a first
predetermined temperature for a first predetermined time to achieve dissolution of niobium
[best seen in Figure. 4] and homogenization [as shown in block 103]. In an embodiment, the
first predetermined temperature ranges from about 1220˚C to 1240˚C, and the first pre-
determined time is in the range of 2 to 3 hours.
Once the steel slab is heated as per block 103, the steel slab may be subjected to a hot rolling
process as shown in block 104. In an embodiment, the hot rolling process is carried out such
that, at least 50% of plastic strain is induced below no-recrystallization temperature in the finish
rolling [i.e., change in deformation schedule] to form a steel sheet, such that the finish rolling
is carried out at a second predetermined temperature. In an embodiment, the no-
recrystallization temperature may be a temperature at which recrystallization stops and active
strain build-up occurs in the microstructure. The active strain build-up facilitates precipitation
assisted transformation aiding in grain refinement, thus improving strength of the steel. In an
embodiment, the second temperature is in a range of [(A3 + 40 °C) < second predetermined
temperature < (A3 + 80 °C)]. A3 being the temperature at which the austenite just starts to
transform into ferrite. In an embodiment, the hot rolling process may be performed in a
roughing mill and a tandem finish mill of a seven stand hot strip mill (HSM).
The heated steel slab may be initially passed through the roughing mill, which may consists of
one or two roughing stands in which the steel slab may be hot rolled back and forth few times
repeatedly between at least two rollers, to deform the steel slab i.e., to increase length of the
steel slab by reducing thickness and without altering width of the steel slab. Upon deforming
the steel slab in the roughing mill, the steel slab may be transferred into the tandem finish mill,
where the steel slab is continuously rolled in each of the seven stands, to further thickness
reduction, preform surface finishing and dynamic recrystallization.

In an embodiment, during finish rolling, carbon may readily combine with other micro-alloying
elements such as Nb and may form Nb(CN) precipitate. This Nb(CN) precipitate may aid in
strengthening, by retarding recrystallization in the final stands.
Upon subjecting the steel slab to the hot rolling process, the steel sheet may be subjected to
cooling to a third predetermined temperature at a predetermined cooling rate, to form a high
strength steel sheet [as seen in block 105]. In an embodiment, cooling may be performed on a
run-out table and the predetermined cooling rate may be about 5 °C/s to 8 °C/s. Further, in an
embodiment the third predetermined temperature may be about 560 °C to 590 °C. The third
predetermined temperature is a coiling temperature of the steel sheet, which aids in formation
of any more ferrite and facilitates grain precipitation when the steel sheet is held continuously
at this temperature.
In an embodiment, the steel processed by the method of the present disclosure, results in
microstructural changes to form the high strength steel. The high strength hot rolled steel sheet
includes ferrite microstructure of about 95% and pearlite less than 5%.
The ferrite microstructure includes grain size of less than 5 µm. This ensures exceptional
strength and hardness of the steel.
In an embodiment, the high strength hot rolled steel sheet exhibits, tensile strength of more
than 540 MPa and yield strength of more than 470 MPa.
In an embodiment, the high strength hot rolled steel sheet can be used to manufacture
automotive components such as but not limiting to long member, weight bearing components
for heavy automobiles.
Figures. 2a and 2b are exemplary embodiments of the disclosure, which illustrates
microstructures of high strength hot rolled steel sheets having composition A and B
respectively, and formed using method of the present disclosure. The microstructures include
about 95% ferrite and 5% pearlite. The ferrite is solid solution strengthened with Si and Mn
and precipitation strengthened with fine Nb(CN) precipitates. The grain size of the ferrite in
the microstructure of the high strength steel is less than 5 µm. Ferrite microstructure aids in
imparting ductility and pearlite aids in achieving desired tensile strength.

Hence, the final microstructure of the high strength hot rolled steel sheet formed by the method
described above, exhibits high combination of yield and tensile strength. This makes the steel
sheet to exhibit high strength. These properties make the high strength hot rolled steel sheet
suitable for, but not limiting to automobile applications for manufacturing long members and
weight bearing components for heavy vehicles.
The high strength hot rolled steel sheet is a hot rolled product with changes in deformation
scheme. This makes the method of present disclosure efficient and production efficient.
The following portions of the present disclosure provides details about the proportion of each
alloying element in a composition of the steel and their role in enhancing properties.
Carbon (C) may be added in the range of about 0.05 wt% - 0.09 wt%. Carbon is one of the
most effective and economical strengthening elements. Carbon readily combines with other
micro-alloying elements like Nb and forms Nb (CN) precipitates which help in strengthening
during rolling by retarding recrystallization in the final stands of finish rolling in HSM.
However, to avoid weldability issues and peritectic reaction during continuous casting, carbon
content below 0.4% is preferable.
Manganese (Mn) may be added in the range of about 0.5 wt% to 0.015 wt%. Manganese not
only imparts solid solution strengthening to the ferrite, but it also lowers the austenite to ferrite
transformation temperature, thereby refining the ferrite grain size. Mn beyond 0.5 wt% might
induce centerline segregation during continuous casting and will also reduce the cost
effectiveness of the steel.
Silicon (Si) may be added in the range of about 0.5 wt% to about 0.015 wt%. Like manganese,
silicon is a very efficient solid solution strengthening element. Silicon less than 0.015 wt% is
preferable to prevent excessive formation of surface scales.
Phosphorus (P) may be added at about 0.03wt% maximum. Phosphorus is considered
detrimental in steel. Phosphorous content of less than 0.03wt% is preferable as higher
phosphorus levels can lead to reduction in toughness and weldability due to segregation of P
into grain boundaries.

Sulphur (S) may be added about 0.01 wt% maximum. Like phosphorus, sulphur is also
considered detrimental in steel. Sulphur content of less than 0.01 wt% is preferable as higher
sulphur levels deteriorates formability.
Nitrogen (N) may be added about 0.01 wt% maximum. Nitrogen level below 0.01wt%,
positively affects ageing stability and toughness in the heat-affected zone of the weld seam, as
well as resistance to inter-crystalline stress-corrosion cracking.
Aluminium (Al) may be added about 0.05wt% maximum. Like phosphorus, aluminium is also
considered detrimental in steel. It limits growth of austenite grains. Aluminium content of less
than 0.01wt% is preferable, as higher aluminium levels can lead to issues during coating.
Titanium (Ti) may be added about 0.05wt% maximum. Titanium improves strength by limiting
austenite grain size during reheating. Titanium forms carbides and nitrides which are stable at
high temperature.
Niobium (Nb) may be added in the range of 0.025wt% to 0.035wt%. Niobium is an effective
microalloying element for grain refinement even when it is added in very small amounts.
Niobium in austenite lowers the austenite to ferrite transformation kinetics thereby reducing
the ferrite grain size. However it is to be ensured that the niobium remains in solution and
doesn’t precipitate in the form of Nb(CN) or NbC before the rolling commences. The limit has
been set keeping in the mind the dissolution kinetics and holding duration in the re-heating
furnace.
Example:
Embodiments of the present disclosure will now be described with an example of a particular
compositions of the steel. Experiments have been carried out for a specific composition of the
steel formed by using method of the present disclosure. The composition of the steel for which
the tests are carried out is as shown in below table 1.
I I Chemical composition (wt. %)
Sl.No. _^^^^_^^_^^_^^_^^__
C Mn I Si I Al I Ti I N I S I P I Nb
A 0.08 088 0.08 0.04 0.015 0.0060 0.005 0.015 0.032~

B 0.087 0.91 0.087 0.042 0.020 0.0070 0.007 0.017 0.027
B1 0.087 091 0.087 0.042 0.020 0.0070 0.007 0.017 0.027
Different compositions of steel A and B were processed by the method of the present disclosure
to obtain high strength hot rolled steel sheet, and steel having composition B1 has been
processed by conventional techniques. Table2, indicates the deformation schedule adapted for
each composition of steel A, B and B1.
Transfer Final F1 F2 F3 F4 F5 F6 F7
Bar Thickness entry
Steel Thickness entry entry entry entry entry entry
(mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm)
A 40 8 40 29 29 20 15 11 9
B 50 10 50 35 24 18 14 14 11
B1 50 12 50 37 37 28 21 17 14
The processed steel may be subjected to testing to determine mechanical properties of the high
strength hot rolled steel sheet. As an example, tensile testing may be performed using Instron
machine as per ASTM standard. Accordingly, table 3 illustrates mechanical properties of steel
having compositions A, B and C.
Steel YS UTS % Pearlite Grain
MPa Mpa Elongation (%)
A 526 583 29 5 3
B 491 570 26 5 4

5
B1 440 540 28 8 8
Table-3
It should be understood that the experiments are carried out for particular compositions of the
steel and the results brought out in Table 3 are for the compositions shown in Table - 1.
However, the said compositions should not be construed as a limitation to the present disclosure
as it could be extended to other compositions of the steel as well.
As apparent from Tables 2 and 3, the steels A and B processed by the method of the present
disclosure, exhibit improved mechanical and microstructural properties than the steel B1,
which is processed by conventional method. Also, from Figures. 3a and 3b and table-4 it is
evident that, more than 50% deformation has been induced in steel A below no-recrystallization
temperature, when compared to that of the steel B1, which is processed by the conventional
techniques.
I I I I % ε below
Steel ε below Tnr ε above Tnr ε total Tnr
"Steel A 0.916 0.693 1.61 56.89
"Steel B1 0.56 0.867 1.427 39.21
Table 4
Equivalents:
With respect to the use of substantially any plural and/or singular terms herein, those having
skill in the art can translate from the plural to the singular and/or from the singular to the plural
as is appropriate to the context and/or application. The various singular/plural permutations
may be expressly set forth herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein, and especially
in the appended claims (e.g., bodies of the appended claims) are generally intended as "open"
terms (e.g., the term "including" should be interpreted as "including but not limited to," the
term "having" should be interpreted as "having at least," the term "includes" should be
interpreted as "includes but is not limited to," etc.). It will be further understood by those
within the art that if a specific number of an introduced claim recitation is intended, such an

intent will be explicitly recited in the claim, and in the absence of such recitation no such intent
is present. For example, as an aid to understanding, the following appended claims may
contain usage of the introductory phrases "at least one" and "one or more" to introduce claim
recitations. However, the use of such phrases should not be construed to imply that the
introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular
claim containing such introduced claim recitation to inventions containing only one such
recitation, even when the same claim includes the introductory phrases "one or more" or "at
least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be
interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite
articles used to introduce claim recitations. In addition, even if a specific number of an
introduced claim recitation is explicitly recited, those skilled in the art will recognize that such
recitation should typically be interpreted to mean at least the recited number (e.g., the bare
recitation of "two recitations," without other modifiers, typically means at least two recitations,
or two or more recitations). Furthermore, in those instances where a convention analogous to
"at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense
one having skill in the art would understand the convention (e.g., "a system having at least one
of A, B, and C" would include but not be limited to systems that have A alone, B alone, C
alone, A and B together, A and C together, B and C together, and/or A, B, and C together,
etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is
used, in general such a construction is intended in the sense one having skill in the art would
understand the convention (e.g., "a system having at least one of A, B, or C" would include but
not be limited to systems that have A alone, B alone, C alone, A and B together, A and C
together, B and C together, and/or A, B, and C together, etc.). It will be further understood by
those within the art that virtually any disjunctive word and/or phrase presenting two or more
alternative terms, whether in the description, claims, or drawings, should be understood to
contemplate the possibilities of including one of the terms, either of the terms, or both
terms. For example, the phrase "A or B" will be understood to include the possibilities of "A"
or "B" or "A and B."
While various aspects and embodiments have been disclosed herein, other aspects and
embodiments will be apparent to those skilled in the art. The various aspects and embodiments
disclosed herein are for purposes of illustration and are not intended to be limiting, with the
true scope and spirit being indicated by the following claims.

Referral I Description
Numerals
101-105 Flowchart blocks
101 Casting stage
102 Heating stage
103 Hot rolling stage
104 Cooling stage

We Claim:
1. A method for manufacturing a high strength hot rolled steel sheet, the method
comprising:
casting a steel slab of a composition comprising in weight percentage (wt.%)
of:
carbon (C) at about 0.05% to about 0.09%,
manganese (Mn) at about 0.8% to about 0.9%,
silicon (Si) at about 0.05% t0 about 0.15%
titanium (Ti) at about 0.015% to 0.025%,
aluminum (Al) at about 0.05%,
niobium (Nb) at about 0.025% to 0.035%,
sulphur (S) at about 0.01%,
phosphorous (P) at about 0.03%,
nitrogen (N) at about 0.01%, and
the balance being Iron (Fe) optionally along with incidental elements;
heating, the steel slab to a first predetermined temperature for a first
predetermined time, for dissolution of niobium and homogenization;
subjecting, the steel slab to a hot rolling process, to induce at least 50% plastic
strain below no-recrystallization temperature during finish rolling to form a steel
sheet, such that finish rolling is carried out at a second predetermined temperature;
and
cooling, the steel sheet to a third predetermined temperature at a predetermined
cooling rate, to form the high strength hot rolled steel;
wherein, the high strength hot rolled steel sheet comprises primarily a ferrite and
pearlite microstructure.
2. The method as claimed in claim 1, wherein the high strength hot rolled steel sheet
exhibits tensile strength greater than 540 MPa.
3. The method as claimed in claim 1, wherein the high strength hot rolled steel sheet
exhibits yield stress of greater than 470 MPa.

4. The method as claimed in claim 1, wherein the microstructure of the hot rolled steel
sheet is microstructure represented by, in area%, the ferrite of about 95% and pearlite
less than 5%.
5. The method as claimed in claim 1, wherein the first predetermined temperature ranges
from about 1220 °C to 1240 °C and the first predetermined time is about 2 to 3 hours.
6. The method as claimed in claim 1, wherein the hot rolling process is performed in a
roughing mill and a tandem finishing mill.
7. The method as claimed in claim 1, wherein the second predetermined temperature is a
finish rolling temperature ranging from about 870 °C to 900 °C.
8. The method as claimed in claim 1, where no-recrystallization temperature is a temperature
below which recrystallization stops and active strain build up occurs in the
microstructure.
9. The method as claimed in claim 8, wherein active strain build-up facilitates precipitation
assisted transformation aiding in grain refinement.
10. The method as claimed in claim 1, wherein the third predetermined temperature ranges
from about 560 °C to 590 °C.
11. The method as claimed in claim 1, wherein cooling is performed on a run-out table.
12. The method as claimed in claim 1, wherein the predetermined cooling rate is about 5°C/s
to 8 °C/s.
13. A high strength hot rolled steel sheet, comprising:
composition in weight percentage of:
carbon (C) at about 0.05% to about 0.09%,
manganese (Mn) at about 0.8% to about 0.9%,
silicon (Si) at about 0.05% t0 about 0.15%
titanium (Ti) at about 0.015% to 0.025%,
aluminum (Al) at about 0.05%,
niobium (Nb) at about 0.025% to 0.035%,
sulphur (S) at about 0.01%,
phosphorous (P) at about 0.03%,

nitrogen (N) at about 0.01%, and
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the steel sheet comprises ferrite and pearlite microstructure.
14. The high strength hot rolled steel sheet as claimed in claim 13, wherein the microstructure
of the steel sheet is microstructure represented by, in area%, the ferrite of about 95% and
pearlite less than 5%.
15. The high strength hot rolled steel sheet as claimed in claim 13, wherein the microstructure
includes a grain size of less than 4 µm.
16. The high strength hot rolled steel sheet as claimed in claim 13, wherein the high strength
hot rolled steel sheet exhibits tensile strength greater than 540 MPa.
17. The high strength hot rolled steel sheet as claimed in claim 13, wherein the high strength
hot rolled steel sheet exhibits yield stress greater than 470 MPa.
18. An automotive component made of a high strength hot rolled steel sheet as claimed in
claim 13.