FIELD OF THE INVENTION
The present invention relates to a high formable hot rolled
steel composition and a process for preparing the same. More
particularly, the invention is concerned about a novel
formable hot rolled steel composition having improved
formability and reduction of hardness and amelioration in the
strain aging index and also a process for preparation of the
steel composition thereof.
BACKGROUND AND PRIOR ART
In the industries cold formability of mild, unalloyed, hot
rolled strip is seriously impaired by presence of interstitial
solutes, usually carbon and nitrogen dissolved in ferrite.
Being a strong nitride former, Aluminium is conventionally
used to fix free nitrogen in steel and thereby lower Strain
Aging Index (SAI) and improve formability of hot rolled steel.
However fixing of nitrogen by Al require an optimum
temperature management in hot strip mill ref i.e low slab
reheating temperature and high coiling temperature , which
results in non-uniform mechanical properties over the strip
length and unfavourable pickling behaviour. Additionally while
attempting very high coiling temperature, the strength level
maybe lowered and it can lead to mixed grain structure which
is undesirable for forming application.
2

US 6808678 disclosed a steel composition comprising, by
weight, carbon: not more than 0.0018%, silicon: not more than
0.020%, manganese: 0.10 to 0.30%, phosphorus: 0.010 to 0.030%,
sulfur: not more than 0.030%, aluminum: not more than 0.005%,
nitrogen: 0.0008 to 0.0050%, boron: not more than 0.0050% and
not less than 0.6 time the nitrogen content, and oxygen: 0.010
to 0.05%, and regulating the chemical composition of the steel
and regulating mainly hot rolling conditions to regulate the
form of nitrides. It disclosed a process for the manufacturing
of a non-aging, highly anti-seed and anti-black-speck steel
plate for enameling. However, this prior art pertains to steel
plate having different chemistry. It was not known that a
specific combination of boron with Fe would provide improved
formability and amelioration in the strain aging index. No
nitrogen fixation by use of boron and thereby reducing the
hardness is found in this prior art.
US 7005016 discloses a method of producing hot rolled steel
having improved formability and minimal silver formation
comprises the addition of Titanium and Boron to the molten
steel to combine with and remove the free nitrogen prior to
rolling. Titanium is added so that the amount of nitrogen
remaining after Ti addition is about 0.0005 wt % to about
0.0025 wt % and Boron is added to remove the balance of the
nitrogen by forming BN.
While the above prior art provides hot rolled steel with
improved hardness. Boron and titanium both are added to
improve hardenability and thereby hardness of the steel. There
was the need to reduce the hardness of hot rolled steel.
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Thus there is a need to provide for a novel formable hot
rolled steel composition having improved formability and
reduction of hardness and amelioration in the strain aging
index of the composition and also a process for preparation of
the steel composition.
The present inventors have found that on addition of boron in
appropriately selected proportion fixation of nitrogen can be
achieved which reduces the hardness and accounts for
improvement in high degree of formability properties of hot
rolled steel with higher % elongation.
OBJECTS OF THE INVENTION
Accordingly, one object of the present invention is to achieve
high formable hot rolled steel.
Another object of the present invention is to achieve high
percentage elongation, lower hardness and lower Strain Aging
Index (SAI)
Yet another object of the present invention is to provide high
formable hot rolled steel for fabrication of Liquified
Petroleum Gas (LPG) cylinder.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is
provided a novel improved hot rolled steel composition
comprising selectively carbon (C) , manganese (Mn), silicon
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(Si), Sulphur (S), phosphorous (P), boron (B) , Aluminium (Al)
and nitrogen and combinations thereof
wherein wt% of boron ranges from 15 to 25 ppm so as to convert
nitrogen present in the composition to a nitride compound
thereby resulting in soft hot rolled steel with high
plasticity.
Another aspect of present invention is provided a process to
prepare high formable hot rolled steel having reduced
hardness, said process comprising :
i) making steel with the composition as described
hereinabove at a high temeprature;
ii) secondary refining said composition;
iii) adding Fe-B in ladle furnace after killing by
Aluminium
iv) continuous casting of slab and
v) hot rolling.
DETAILED DESCRIPTION OF INVENTION
The present invention relates to the novel composition of hot
rolled steel. The composition of present invention comprises
Carbon 0.05 to 0.07 wt %, Manganese 0.35 to 0.45 wt %,
Silicon 0.02 to 0.04 wt %, Sulphur 0 .02 wt % max, Phosphorus
0.02 wt % max, Boron 15 to 25 ppm, Aluminium 0.03 to 0.06 wt %
and Nitrogen 40 to 80 ppm. The chemistry of the composition is
also provided in table -1 as below :
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Table - 1

c
max. Mn
max Si
max S
max P max B* in
ppm Al
min N in
ppm
0.05-
0.07 0.35-
0.45 0.02-
0.04 0.02 0.02 15-25 0.03-
0.06 40-80
* Addition will depend on nitrogen content of steel
In low carbon steel, addition of Boron converts the free
nitrogen into Boron nitride (BN) which leads to improvement in
formability with reduced strain aging index and plasticity. In
the present invention Boron was allowed to combine with carbon
and nitrogen in steel, thereby reducing dissolved C and ,N'
and results in softer hot rolled product with higher
plasticity. However addition of boron does not contribute in
increasing hardenability of steel. Rather boron was added to
fix nitrogen and to reduce the hardness.
Present invention provides a process to prepare high formable
hot rolled steel having reduced hardness, said process
comprises :
i)steel making with composition in BOF(Basic Oxygen Furnace)
The composition of Carbon 0.05 to 0.07 wt %, Manganese
0.35 to 0.45 wt %, Silicon 0.02 to 0.04 wt %, Sulphur 0 .02
wt % max, Phosphorus 0.02 wt % max, Boron 15 to 25 ppm,
Aluminium 0.03 to 0.06 wt % and Nitrogen 40 to 80 ppm are
heated to finishing temperature (870+10 C) and coiling
temperature (610+10 C).
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ii)secondary refining of the above composition of Carbon
0.05 to 0.07 wt %, Manganese 0.35 to 0.45 wt %, Silicon
0.02 to 0.04 wt %, Sulphur 0 .02 wt %, Phosphorus 0.02 wt
%, Boron 15 to 25 ppm, Aluminium 0.03 to 0.06 wt % and
Nitrogen 40 to 80 ppm in ladle firnace to reduce Sulphur and
Phosphorus.
Secondary refining comprises steps of
fine adjustment of carbon, aluminium and manganese and
addition of Fe-B in ladle furnace after killing by
Aluminium
iii) continuous casting of slab and
iv) reheating the concast slabs
v)roughing the slab and finish rolling is done in Hot Strip
Mill
vi) the strip is coiled.
Finally the product is processed into 2.9 mm thick hot rolled
coils.
Processing of non-Boron bearing conventional C-Mn LPG steel
demands higher coiling temperature (690+10 C) to facilitate
fixing of nitrogen to a greater extent by way of precipitation
of A1N.
The steel composition of the present invention is used for
making LPG cylinders and in addition to the LPG cylinders, the
steel of the present invention with high formability is well
suited for fabrication of wheels of two/three wheels,
prefabricated structural etc.
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Results :
The effect of boron addition on the mechanical properties and
strain aging behaviour of LPG steel have been assessed. The
properties of boron added LPG steel (Table given below) showed
significant improvement compared to that of conventional C-Mn
LPG steel. This has been quantified in terms of higher
elongation ranging from 38% to 40 %, lower hardness ranging
from55to 58 HRB and lower strain ageing index ranging from8%
tol3%) as compared to steel without Boron. Comparison chart of
the improvement in properties of the LPG steel with boron of
the present invention with respect to the conventionally
available C_Mn LPG steel is provided below in table - 2.
Table - 2

Properties LPG steel with
Boron Conventional C-Mn LPG
steel
Percentage
Elongation 36% -40% 30% -32%
Hardness, HRB 55-58 60-64
Strain Aging
Index, 8%-12% 14%-20%
Yield 61.2
cylinders/T 59 cylinders / T
Forming Load 180 bar 200 bar
Number of
cylinders using
one die 5000 3000
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The strain aging index (measure of carbon and nitrogen in
solution) of boron treated LPG is 30% lower compared to
conventional C-Mn LPG steel without boron. This confirms that
addition of boron has been very effective in bringing out
carbon and nitrogen from solid solution to form boron carbides
and carbonitrides as precipitates, leaving no opportunity for
boron to remain in solution.
Advantages
i)Addition of Boron (15-25 ppm) in Aluminium killed low carbon
steel with nitrogen (40-80 ppm) has resulted in high degree of
formability properties with Higher Elongation ranging from
36% - 40%, Lower hardness ranging from 55 to 58 HRB and Lower
SAI ranging from8% tol2 %.
ii) Fabrication of LPG cylinder can be done by the steel of
present invention as this shows higher yield (61.2
cylinders/ton of steel), Lower forming load (180 bar) and Less
die wearas compared to the conventional LPG cylinders.
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WE CLAIM
1. A novel improved hot rolled steel composition comprising
selectively carbon (C), manganese (Mn), silicon (Si),
Sulphur (S) , phosphorous (P), boron (B) , Aluminium (Al)
and nitrogen and combinations thereof
wherein wt% of boron ranges from 15 to 25 ppm so as to
convert nitrogen present in the composition to a nitride
compound thereby resulting in soft hot rolled steel with
high plasticity.
2. Composition as claimed in claim 1 comprising
0.05-0.07 wt % of carbon
0.03-0.45 wt % of manganese
0.02- 0.04 wt % of silicon
0.02 wt % max of sulphur
0.02 wt % of max phosphorus
0.03 - 0.06 wt % of aluminium and
40 - 80 ppm nitrogen
3. Composition as claimed in claims 1 and 2 comprising %
elongation ranging from 36 to 40.
4. Composition as claimed in claims 1 to 3 comprising
hardness ranging from 55 to 58 HRB.
5. A process to prepare high formable hot rolled steel
having reduced hardness, said process comprising :
i) steel making with the composition as as claimed in
claim 1 to a high temperature;
ii) secondary refining said composition;
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iii) adding Fe-B in ladle furnace after killing by
Aluminium
iv) continuous casting of slab and
v) hot Rolling.
6. Process as claimed in claim 5 wherein the heating
temperature comprises finishing temperature and coiling
temperature.
7. Process as claimed in claim 6, wherein the finishing
temperature ranges from 860°C to 880°C.
8. Process as claimed in claim 6, wherein the coiling
temperature ranges from 600°C to 620°C.

Dated this the 18th day of December 2007
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The present invention relates to a novel improved hot
rolled steel composition comprising selectively carbon (C),
manganese (Mn), silicon (Si), Sulphur (S), phosphorous (P),
boron (B), Aluminium (Al) and nitrogen and combinations
thereof wherein wt% of boron ranges from 15 to 25 ppm so as
to convert nitrogen present in the composition to a nitride
compound thereby resulting in soft hot rolled steel with
high plasticity. The invention also relates to a process to
prepare high formable hot rolled steel having reduced
hardness. The process comprises making steel at a high
temperature, secondary refining said composition, adding
Fe-B in ladle furnace after killing by Aluminium,
continuous casting of slab and hot rolling.