FIELD OF THE INVENTION
The present invention relates to a high strength formable hot rolled steel product and
a process for production of such steel product in an underpowered mill where
maximum load in the finishing strand is limited to 25 Mega Newton (MN). More
particularly, the invention is directed to producing low carbon low alloy steel with
high strength e.g. UTS >500 MPa accompanied with lower YS/UTS ratio (<0.85) and
good hole expansion ratio (λ>140%), and reasonably good ductility [elongation
>15% ] to favour ability of the sheared edges of the steel sheets/plates to undergo
forming operations without cracking. Importantly, the steel grade according to the
invention is having the phases of ferrite and pearlite in the microstructures. Such
homogeneous micro structures simultaneously provide higher hole expansion ratio
(λ>140%) in the resulting steel and thereby making this grade suitable for a variety
of applications including electric poles, tubes, welded pipes, high mast lighting poles,
wind poles, camera poles etc.
BACKGROUND OF THE INVENTION
The demand for steel grades with increased strength levels and well balanced
formability has strongly been increasing over the years in a wide range of application
fields such as electric poles, tubes, welded pipes and the like. It is experienced in the
art that producing high strength products are always associated with the difficulty of
higher loading of the rolling mills for their processing. Moreover, higher strength
always impose a limit on the hole expansion ratio which is the indicating parameter
for the formability of the steel products. Depending on the requirement of the
properties, steel grade was to be developed based on the plant's capability.
Necessary changes in alloy design (low C, low Nb and high Si) and processing
conditions (slab reheating, finishing & coiling temperatures) had to be incorporated
to achieve the desired properties. The changes in the chemistry and processing
parameters was supposed to lead to successful rolling within the mill capability (25
MN max.) of the plant. The process would involve process steps/ parameters along
with a selective steel chemistry and alloy design so as to produce steel sheets of
desired characteristics by hot rolling having combined high level of strength and
elongation.

There has been thus a need in the art to developing a high strength steel product
without overloading the mill, particularly difficult to cast grades by optimizing the
casting and hot rolling parameters. The requirement of the properties of the steel
grade has to be achieved based on the plant's capability. This need necessitated on
one hand complying to alloy design such as maintaining low C, low Nb and high Si
and on the other hand controlling processing conditions e.g. slab reheating, finishing
& coiling temperatures to achieve the desired properties. The changes in the
chemistry and processing parameters led to successful rolling within the mill
capability (25 MN max.) of the plant. The process would involve process steps/
parameters along with a selective steel chemistry and alloy design so as to produce
steel sheets of desired characteristics by hot rolling having combined high level of
strength and elongation which the existing steel grades could not provide.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to providing a High
Strength (UTS >500 MPa) hot rolled formable grade steel product in an
underpowered mill and a process for its production capable of ensuring high YS, UTS,
high hole expansion ratio and reasonably good elongation so as to favour a number
of application where high degree of forming is required apart from high strength.
A further object of the present invention is directed to providing a High Strength
(UTS >500 MPa) hot rolled formable grade steel product and process for its
production wherein micro alloying with niobium along with reduction in carbon and
increase of silicon would contribute to the excellent combination of properties
(YS/UTS < 0.85) and hole expansion ratio (A >140%).
A further object of the present invention is directed to providing a High Strength
(UTS >500 MPa) hot rolled formable grade steel product in an underpowered mill
and process for its production which can be rolled with maximum load in the finishing
strand limited to 25 Mega Newton (MN).
A still further object of the present invention is directed to providing High Strength
(UTS >500 MPa) hot rolled formable grade steel product in an underpowered mill
and process for its production wherein alloy design comprises low C, low Nb and high

Si and process conditions are controlled e.g. slab reheating, finishing & coiling
temperatures to achieve the desired properties.
A still further object of the present invention is directed to providing High Strength
(UTS >500 MPa) hot rolled formable grade steel product in an underpowered mill
wherein steel grade would have the phases of ferrite and pearlite in the
microstructures to enable achieving the desired properties in end products.
A still further object of the present invention is directed to providing High Strength
(UTS >500 MPa) hot rolled formable grade steel product in an underpowered mill
wherein the selective steel chemistry and processing parameters led to successful
rolling within the mill capability (25 MN max.) of the plant.
A still further object of the present invention is directed to providing a High Strength
(UTS >500 MPa) hot rolled formable grade steel product in an underpowered mill
wherein the steel produced would meet requirements in a wide range of application
fields such as electric poles, tubes, welded pipes, high mast lighting poles, wind
poles, camera poles and the like.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a high strength hot rolled
steel comprising:
low carbon upto 0.10 wt% alongwith alloying elements in combination with
0.015 to 0.025 wt% Nb and homogeneous microstructure providing for hole
expansion ratio A >140% .
Another aspect of the present invention is directed to said high strength hot rolled
steel comprising:
C : 0.10 max preferably 0.08 to 0.09 by wt %;
Mn : 1.20 max preferably 0.90 to 1.10 by wt %;
Si : 0.15 to 0.35 by wt %;
S : 0.020 max by wt %;

P : 0.025 max by wt %;
Al : 0.020 min by wt %;
Nb : 0.015 to 0.025 by wt %.
A still further aspect of the present invention is directed to a high strength hot rolled
steel comprising UTS >500MPa and with good ductility such as total elongation of
15% min. in a gauge length of 50 mm.
A still further aspect of the present invention is directed to a high strength hot rolled
steel, wherein said homogeneous microstructure comprises ferrite and pearlite
phases.
According to yet another aspect of the present invention is directed to a process for
producing high strength hot rolled steel 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 Nb
is added as ferroniobium 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 hours;
vi) subjecting the slabs to hot rolling with a finishing temperature between
870 to 890 °C and coiling at 670 to 690 °C.
A still further aspect of the present invention is directed to said process 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.

Yet another aspect of the present invention is directed to said process which
comprises for rolling in combination of various thickness and width maintaining the
permissible mill load within 25 MN.
The various objects and advantages of the present invention is described in further
details with reference to the following non limiting illustrative drawings and example.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 : is the optical micrograph of the high strength hot rolled steel according to
the present invention showing ferrite and pearlite phases.
Figure 2 : is the photograph of welded pipes produced using the steel according to
the invention.
Figure 3 : is the photograph of high mast lighting poles produced using the steel
according to the invention.
Figure 4 : is the photograph of wind poles produced using the steel according to the
invention.
Figure 5 : is the photograph of camera poles produced using the steel according to
the invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention relates to developing a hot rolled steel products with high
strength (UTS >500 MPa) combined with low YS/UTS ratio (<0.85) and reasonably
good hole expansion ratio (A>140%) to ensure good forming properties 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 as
steel sheets.
The basic feature of this product is achieving high level of Ultimate Tensile Strength
(UTS) and Yield Strength (YS) in combination with good elongation property and hole
expansion ratio in a steel with ferrite and pearlite phases in the microstructure. The

advantageous feature of the present invention is illustrated with reference to the
following example I.
EXAMPLE I :
According to an embodiment of the present invention a steel grade has been
produced with a composition of 0.09C-0.91Mn-0.215Si-0.014S-0.017P-0.036AI
(wt%) with the addition of 0.020 wt% Nb. It is made in the basic oxygen furnace
through LD steel making process, followed by secondary refining in ladle furnace (LF)
where Nb addition is made in the form of ferroniobium. The molten steel is degassed
in Vacuum Arc Degassing (VAD) unit and is continuously cast into 210 mm thick
slabs. These slabs are reheated to 1260 °C in reheating furnaces for 4 hours, and hot
rolled into 3.6 mm nominal thickness with a finishing temperature of 880 °C and
coiling at 680 °C.
The process according to the present invention to produce the high strength (UTS
>500 MPa) hot rolled steel grade represented by lower YS/UTS ratio (<0.85) and
higher hole expansion ratio (A>140%) for good forming properties in the steel sheet
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 Nb is
added as ferroniobium so as to obtain steel having the composition:
C : 0.10 max preferably 0.08 to 0.09 by wt %;
Mn : 1.20 max preferably 0.90 to 1.10 by wt %;
Si : 0.15 to 0.35 by wt %;
S : 0.020 max by wt %;
P : 0.025 max by wt %;
Al : 0.020 min by wt %;
Nb : 0.015 to 0.025 by wt %.
iii) degassing the molten steel in Vacuum Arc Degassing (VAD) unit;
iv) continuously casting said steel into slabs 210 mm thick;

5) reheating the slabs to 1250-1270 °C in reheating furnaces for 4 hours;
6) subjecting the slabs to hot rolling with a finishing temperature between 870 to
890 °C and coiling at 670 to 690 °C.
7) carrying out rolling in different combinations of thicknesses and width considering
the mill load not exceeding 25 MN, such as 5.9x1250; 3.6x1040; 3.2x1040;
4.8x1040; 5.0x1030; 3.7x1030; 5.5x1030; 3.1x1030; 3.8x1030; 6.3x1410;
6.0x1410; 5.6x1060 (all dimensions are in mm).
The steel samples so obtained are tested for microstructure and physical strength
properties.
Accompanying Figure 1 illustrates the optical micrograph of the high strength hot
rolled steel according to the present invention showing ferrite and pearlite phases.
Accompanying Figure 2 to 5 illustrates the application photographs such as welded
pipes, high mast lighting poles, wind poles and camera poles made from this steel.
The physical testing of the hot rolled steel so produced demonstrated high strength
such as UTS> 500 MPa, YS> 310 MPa and YS/UTS ratio <0.85. The hole expansion
ratio found to be of the order of A >140% combined with good ductility e.g. total
elongation of 15% min. in a gauge length of 50 mm. During rolling the mill load was
found 19-22 MN which is well within the permissible limit of 25 MN.
It is thus possible by way of the present invention to providing a high strength steel
(UTS >500 MPa) with good forming properties in an underpowered mill (load = 25
MN max). Micro alloying with niobium along with reduction in carbon and increase of
silicon has contributed to the excellent combination of properties (YS/UTS <0.85)
and hole expansion ratio (A>140%) thereby making this grade suitable for a variety
of applications including electric poles, tubes, welded pipes, high mast lighting poles,
wind poles, camera poles etc.

We Claim:
1. A high strength hot rolled steel comprising:
low carbon upto 0.10 wt% alongwith alloying elements in combination with
0.015 to 0.025 wt% Nb and homogeneous microstructure providing for hole
expansion ratio λ >140% .
2. A high strength hot rolled steel as claimed in claim 1 comprising
C : 0.10 max preferably 0.08 to 0.09 by wt %;
Mn : 1.20 max preferably 0.90 to 1.10 by wt %;
Si : 0.15 to 0.35 by wt %;
S : 0.020 max by wt %;
P : 0.025 max by wt %;
Al : 0.020 min by wt %;
Nb : 0.015 to 0.025 by wt %.
3. A high strength hot rolled steel as claimed in anyone of claims 1 or 2
comprising UTS >500MPa and with good ductility such as total elongation of
15% min. in a gauge length of 50 mm.
4. A high strength hot rolled steel as claimed in anyone of claims 1 to 3, wherein
said homogeneous microstructure comprises ferrite and pearlite phases.
5. A process for producing high strength hot rolled steel as claimed in anyone of
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 Nb
is added as ferroniobium 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 hours;
vi) subjecting the slabs to hot rolling with a finishing temperature between
870 to 890 °C and coiling at 670 to 690 °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 process as claimed in anyone of claims 5 or 6 comprising for rolling in
combination of various thickness and width maintaining the permissible mill
load within 25 MN.
8. A high strength hot rolled steel and a process for its manufacture
substantially within permissible mill load hereindescribed with reference to the
accompanying figures and examples.

A high strength hot rolled steel (UTS>500 MPa) product with high formability and a
process for production of such steel product is disclosed wherein maximum load in
the finishing strand is limited to 25 Mega Newton (MN). More particularly, the
invention is directed to producing low carbon low alloy steel with high strength e.g.
UTS >500 MPa accompanied with lower YS/UTS ratio (<0.85) and good hole
expansion ratio (λ>140%), and good ductility (>15%) to favour ability of the
sheared edges of the steel sheets/ plates to undergo forming operations without
cracking. The steel grade is having the phases of ferrite and pearlite in the
microstructures. Such homogeneous micro structures simultaneously provide higher
hole expansion ratio (λ>140%) in the resulting steel and thereby making this grade
suitable for a variety of applications including electric poles, tubes, welded pipes,
high mast lighting poles, wind poles, camera poles etc.