FIELD OF THE INVENTION
The present invention relates to a hot rolled steel product with high strength (Yield
Strength(YS >500MPa) combined with high stretch flangeability or edge formability
and a process for production of such steel product. More particularly, the invention is
directed to producing low carbon low alloy steel with high strength e.g. YS>500MPa
accompanied with high hole expansion ratio, (?>200%), and good ductility to favour
ability of the sheared edges of the steel sheets/plates to undergo flanging operation
without cracking. The steel grade is obtained with high YS and UTS having single
phase of ferrite wherein the strength is mainly derived from very small precipitates
of titanium carbide/carbosulphide. Such homogeneous micro structure
simultaneously provide high hole expansion ratio in the resulting steel and thereby
high stretch flangeability, making this grade of steel suitable for a variety of
applications including automakers for fabricating automobile underbody parts.
BACKGROUND OF THE INVENTION
It is well known in the art of fabricating industrial products that various components
are made from punched steel sheets involving flanging at the punched/sheared
edges. Stretch flangeability, which is sometimes also called edge formability, is an
important property of steel material in this regard which refers to the ability of
sheared edges of the steel sheets /plates to undergo flanging operation without
cracking. Such operations are routinely carried out by automakers in order to
fabricate underbody parts of an automobile. This edge formability property is
particularly an essential requirement for high strength steel sheets/plates whose
sheared edges have a tendency to crack during flanging. The available steel grades
with higher strength e.g. YS>500MPa do not provide high stretch flangeability to
meet the requirement of industries.
There has been therefore a need in the art to developing a grade of steel which
would ensure a combination of high strength e.g. YS and UTS, with high edge
formability by way of simultaneously achieving high hole expansion ratio and
ductility, such that the components fabricated from the sheared steel sheets by
flanging the sheared edges do not crack and demonstrate consistently higher stretch
flangeability. The present invention also aims to develop a process for producing
such grade of steel having the said high stretch flangeability property while also

ensuring high YS and/or UTS. The process would involve process steps/parameters
along with a selective steel chemistry and alloy addition so as to produce steel
sheets of desired characteristics by hot rolling having combined high level of strength
and edge formability.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to provide a grade of steel
having a combination of high strength in the range of YS>500MPa and high stretch
flangeability adapted to suit application in automobile and the like industry, and its
process of manufacture.
A further object of the present invention is directed to providing a grade of steel
having combination of high strength and high stretch flangeability so that the
sheared edges of steel sheets/plates produced therefrom undergoes flanging without
cracking.
Another object of the present invention is directed to provide a grade of steel having
high strength as well as high edge formability by virtue of selective alloy chemistry,
preferred micro structure and resulting high hole expansion ratio and good ductility.
A still further object of the present invention is directed to provide a grade of steel
having combination of high strength and high stretch flangeability wherein selective
titanium addition favour achieving high strength by way of very small precipitates of
titanium carbide/carbosulphide in a single phase of soft ferrite.
A still further object of the present invention is directed to provide a grade of steel
having combination of high strength and high stretch flangeability wherein the
process involves converting steel slabs produced into sheets of desired thickness by
hot rolling following controlled reheating, finishing and coiling temperature to ensure
desired properties in the steel products.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to a high strength hot
rolled steel composition comprising :
low carbon upto 0.07 wt% alongwith with alloying elements in combination with
0.07-0.08 wt% Ti having homogeneous microstructure providing for hole
expansion ratio ?>200%.
A further aspect of the present invention is directed to said high strength hot rolled
steel composition comprising
C: 0.04 to 0.07 preferably 0.06 by wt %;
Mn: 0.7 to 0.9 preferably 0.8 by wt%;
Si: 0.25 to 0.35 preferably 0.3 by wt.%;
S: 0.008 max. by wt.%.;
Al: 0.04 max. by wt.%;
Ti : 0.07-0.08 preferably 0.08 by wt. %;
N : 60 ppm max.
A still further aspect of the present invention is directed to said high strength hot
rolled steel comprising YS>500MPa and with good ductility such as total elongation
of 22%min. in a gauge length of 50mm.
A still further object of the present invention is directed to a high strength hot rolled
steel, wherein said homogeneous microstructure comprises of small precipitates of
titanium carbide and /or carbosulphide in single phase of ferrite.
According to yet another aspect of the present invention is directed to a process for
producing high strength hot rolled steel grade as described above, comprising the
steps of
(i) producing steel in Basic Oxygen Furnace through LD process of steel making;
(ii) carrying out secondary refining of said steel in ladle furnace(LF) where Ti is
added as ferrotitanium so as to obtain molten steel

(iii) degassing the molten steel;
(iv) continuously casting said steel into slabs;
(v) reheating the slabs to 1250-1270°C in reheating furnaces for 4 to 4.5 hours
preferably about 4.5 hours;
(vi) subjecting the slabs to hot rolling with a finishing temperature between 890-
900°C and coiling at 570 to 590°C preferably about 580°C.
Also in said process for producing the high strength hot rolled steel said degassing
of the molten steel is carried out in Vacuum Arc Degassing(VAD) unit and continuous
casting into slabs is done to obtain slabs of 210 mm thick.
The objects and advantages of the present invention is described in greater details
with reference to the following accompanying non limiting drawings and example.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the optical micrograph of the high strength hot rolled steel with high
stretch flangeability according to the present invention showing the single phase
ferrite.
Figure 2: is the scanning electron image of the representative sample of steel
according to the present invention wherein the microstructure shows the single
phase ferrite.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWINGS
The present invention relates to a hot rolled steel products with high strength (Yield
Strength(YS>500MPa) combined with high strength flangeability(?>200%), or edge
formability and a process for production of such steel product. The process of
manufacture of this grade of steel involves selective alloying particularly to produce
hot rolled steel with YS>500MPa in a low carbon(C=0.07max) steel with minimum
alloy addition with the objective of achieving high strength and high edge formability.
The basic feature of this product is a high level of Yield stress(YS) and Ultimate
tensile strength(UTS) in a steel with a single phase of ferrite. Since ferrite is a soft
phase, the strength is mainly derived from small precipitates of titanium
carbide/carbosulphide. Such a homogeneous microstructure gives rise to high X
values which indicate that the material possesses high stretch flangeability. The
advantageous features of the present invention is illustrated with reference to the
following example.
EXAMPLE I:
According to an embodiment of the present invention a steel grade has been
produced with a composition of 0.06C-0.76Mn-0.33Si-0.008S-0.024P-0.04AI(wt%)
with the addition of 0.07-0.08wt% Ti. It is made in basic oxygen furnace through LD
steel making process, followed by secondary refining in ladle furnace(LF) where Ti
addition is made in the form of ferrotitanium. The molten steel is degassed in
Vacuum Arc Degassing(VAD) unit and is continuously cast into 210mm thick slabs.
These slabs are reheated to 1260°C in reheating furnaces for 4.5 hours, and hot
rolled into 3.2 mm nominal thickness with a finishing temperature between 890-
900°C and coiled at 590°C.
The process according to the present invention to produce the high
strength(YS>500MPa) hot rolled steel grade with high stretch flangeability
represented by the hole expansion ratio of ?>200% thus comprises the steps of:
(i) producing steel in Basic Oxygen Furnace through LD process of steel making;
(ii) carrying out secondary refining of said steel in ladle furnace(LF) where Ti is
added as ferrotitanium so as to obtain steel having the composition:
C: 0.04 to 0.07 preferably 0.06 by wt %;
Mn: 0.7 to 0.9 preferably 0.8 by wt%;

Si: 0.25 to 0.35 preferably 0.3 by wt.%;
S: 0.008 max. by wt.%.;
Al: 0.04 max. by wt.%;
Ti : 0.07-0.08 preferably 0.08 by wt. %;
N : 60 ppm max.
(iii) degassing the molten steel in Vacuum Arc Degassing(VAD) unit;
(iv) continuously casting said steel into slabs 210 mm thick;
(v) reheating the slabs to 1250-1270°C in reheating furnaces for 4-4.5 hours;
(vi) subjecting the slabs to hot rolling into 3.2 mm nominal thickness with a finishing
temperature between 890-900°C and coiling at 570-590°C.
The steel sample so obtained are tested for microstructure and physical strength
properties.
Accompanying Figure 1 illustrates the optical micrograph of the high strength hot
rolled steel with high stretch flangeability according to the present invention showing
the single phase ferrite.
Accompanying Figure 2 illustrates scanning electron image of the representative
sample of steel according to the present invention wherein the microstructure shows
the single phase ferrite.
The above figures show the homogeneous microstructure comprising very fine
precipitates of titanium carbide/carbosulphide (not resolved by scanning electron
microscopy) dispersed in single phase of ferrite, which gives rise to high ? values
indicating that the resulting steel material possesses high stretch flangeability.
The physical testing of the hot rolled steel so produced demonstrated high strength
such as Yield strength (YS) >500MPa and UTS> 600 MPa. The hole expansion ratio

found to be of the order of ?>200% combined with good ductility e.g. total
elongation of 22%min. in a gauge length of 50mm.
It is thus possible by way of the present invention to providing a high
strength(YS>500MPa) hot rolled steel with high stretch flangeability(hole expansion
ratio, ?>200%) so that the sheet/plates obtained thereof has the sheared edges
capable of undergoing flanging operation without cracking. The combination of high
strength and high edge formability properties make this grade of steel suitable for
application in fabrication of automobile underbody parts and the like favoring wide
scale application and use in various industry.

We Claim:
1. A high strength hot rolled steel composition comprising :
low carbon upto 0.07 wt% alongwith with alloying elements in combination with
0.07-0.08 wt% Ti having homogeneous microstructure providing for hole
expansion ratio ?>200%.
2. A high strength hot rolled steel composition as claimed in claim 1 comprising
C: 0.04 to 0.07 preferably 0.06 by wt %;
Mn: 0.7 to 0.9 preferably 0.8 by wt%;
Si: 0.25 to 0.35 preferably 0.30 by wt.%;
S: 0.008 max. by wt.%.;
Al: 0.04 max. by wt.%;
Ti : 0.07-0.08 preferably 0.08 by wt. %;
N : 60 ppm max.
3. A high strength hot rolled steel as claimed in anyone of claims 1 or 2 comprising
YS>500MPa and with good ductility such as total elongation of 22% min. in a gauge
length of 50mm.
4. A high strength hot rolled steel as claimed in anyone of claims 1 to 3, wherein said
homogeneous microstructure comprises very fine precipitates of titanium carbide
and /or carbosulphide in single phase of ferrite.
5. A process for producing high strength hot rolled steel grade as claimed in claims 1
to 4, comprising the steps of
(i) producing steel in Basic Oxygen Furnace through LD process of steel making;
(ii) carrying out secondary refining of said steel in ladle furnace(LF) where Ti is
added as ferrotitanium so as to obtain molten steel
(iii) degassing the molten steel;
(iv) continuously casting said steel into slabs;

(v) reheating the slabs to 1250-1270°C in reheating furnaces for 4 to 4.5 hours
preferably about 4.5 hours;
(vi) subjecting the slabs to hot rolling with a finishing temperature between 890-
900°C and coiling at 570 to 590 preferably about 580°C.
6. A process as claimed in claim 5 wherein said
degassing the molten steel is carried out in Vacuum Arc Degassing(VAD) unit
and continuous casting into slabs is done to obtain slabs of 210 mm thick
7. A high strength hot rolled steel having high strength and high stretch flangeability
and a process for its manufacture substantially as hereindescribed with reference to
the accompanying figures and example.

The invention relates to a hot rolled steel grade with high strength (Yield
Strength(YS)>500MPa) combined with high strength flangeability or edge formability
and a process for its production. More particularly, the invention is directed to
producing low carbon low alloy steel with high strength e.g. YS>500MPa
accompanied with high hole expansion ratio, (λ>200%), and good ductility such as
to favour ability of the sheared edges of the steel sheets/plates to undergo flanging
operation without cracking. The steel grade with high YS and UTS is having single
phase of ferrite wherein the strength is mainly derived from very small precipitates
of titanium carbide and/or carbosulphide. Such homogeneous micro structure
simultaneously provide high hole expansion ratio in the resulting high strength steel
and thereby ensure high stretch flangeability, making these grade of steel suitable
for a variety of applications including automakers for fabricating automobile
underbody parts.