FIELD OF THE INVENTION
The present Invention relates to a cost effective low carbon high formable hot rolled
steel composition and a process for its manufacturing. More particularly, the present
invention is directed to a low carbon aluminium killed formable steel composition
comprising high silicon along with boron in hot rolled steel adapted to ensure
improved cold formability favouring application in production of LPG cylinders,
structural components for automobiles and the like. Advantageously, combined
silicon and aluminium additions favour effectively reducing strain aging index than
aluminium alone ensuring softer hot rolled product with higher plasticity.
Importantly, due to change in ferro alloy additions such as replacing medium Carbon
Ferro Manganese with Silico Manganese cost of which is lower than the former and
restricting aluminium addition in view of higher silicon content in steel, the cost of
production is substantially reduced. The high silicon and boron containing hot rolled
cold formable grade steel with higher UTS, low YS/UTS ratio, higher percent
elongation, higher hole expansion ratio and lower strain aging index favouring
Improved strain distribution during press forming and thus enabling cost effective
application in a variety of industries such as the LPG cylinder manufacturer,
automobile structural components/wheel rims and other deep drawing applications
including prefabricated structural.
BACKGROUND ART
The manufacturing facility at Bokaro steel limited of the applicants regularly produce
high formable hot rolled steels for consumption by manufacturer of Liquefied
Petroleum Gas (LPG) cylinder and structural components of automobiles. The
production capacity of nearly 1,00,000 T is directed to meeting the demand for
manufacturing domestic as well as auto cylinder and similar other applications. It has
been experienced in the prior applications that presence of interstitial solutes,
usually carbon and nitrogen dissolved in ferrite is detrimental to the cold formability
of hot rolled strip. Aluminium is conventionally used to fix free nitrogen in steel but
an optimum temperature management is required for this in hot strip mill 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. To avoid such disadvantage, use of titanium/boron are known in the
existing art for fixing of nitrogen and thereby improving cold formability of steel. In
these grades, silicon is generally perceived to be detrimental due to formation of
silica inclusion and restricted to 0.04 % max. Additions of Ti/B make the cold
formable quality hot rolled steel substantially expensive.
In low carbon steel, addition of Boron converts the free nitrogen into Boron nitride
(BN) which leads to improvement in formability. The objective is to allow Boron just
to combine with carbon and nitrogen in steel, thereby reducing dissolved 'C' and 'N'
and results in softer hot rolled product with higher plasticity, but not to contribute
anyway in increasing hardenability of steel.
There has been thus a strong need in the art to substitute Ti addition in conventional
LPG grade steel with any low cost additive such as silicon which would on one hand
partially substitute Al addition and on the other hand be able to ensure desired
superior strength and cold deformation properties in the resulting hot rolled steel
product. An innovative approach has been attempted in the present work, wherein
higher silicon along with boron has been added in steel as a low cost alternative to Ti
added LPG grade steel products.
OBJECTS OF THE INVENTION
The basic object of the present invention is directed to providing a cost effective low
carbon high formable hot rolled still composition comprising high silicon and boron
addition ensuring desired high strength and cold deformation properties in steel
products required for production of LPG cylinders and the like.
A further object of the present invention is directed to developing a low cost low
carbon high formable hot rolled still composition wherein addition of high silicon and
boron, restricting aluminium addition and substituting medium carbon Fe-Mn with
low cost Si-Mn and Ca-Fe by Ca-Si result in substantial reduction in the cost of steel
produced.
A Still further object of the present invention is directed to providing a high formable
hot rolled still composition comprising high silicon and boron addition ensuring
desired high strength and cold deformation properties wherein combined silicon and
aluminium additions are much more effective in desired reduction of strain aging
index than aluminium alone.
A still further object of the present invention is directed to providing a high formable
hot rolled still composition comprising high silicon and boron addition wherein
selective alloying with silicon and boron with restricted aluminium addition helps to
achieve improved mechanical properties such as higher UTS, low YS/UTS ratio, low
strain aging index, higher percentage elongation as compared to existing LPG Ti
grade steel.
A still further object of the present invention is directed to providing a high formable
hot roiled still composition comprising high silicon and boron addition wherein
selective alloying with silicon and boron with restricted aluminium addition favor
achieving good expansion value as a measure of formablility characteristics of the
steel product.
A still further object of the present invention is directed to providing a high formable
hot rolled still composition comprising high silicon and boron addition wherein lower
YS/UTS ratio favours an improved strain distribution during press forming, enabling
more complex component to be formed without excess localized thinning.
A still further object of the present invention is directed to providing a high formable
hot rolled still composition comprising high silicon and boron addition wherein higher
silicon addition favoured ease of casting due to increased flowability and better
desulphurization.
A still further object of the present invention is directed to providing a high formable
hot rolled still composition comprising high silicon and boron addition wherein
addition of boron is controlled 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 which helps achieving lower strain aging index.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to a cost effective low
carbon high formable hot roiled steels composition suitable for LPG cylinder and the
like comprising
C: 0.06-0.10 wt%,
Mn: 0.35-0.45 wt%,
Si: 0.1-0.2 wt%,
B: 5-25 ppm,
S:0.02 max
P: 0.025 max
Al: 0.02 min
and rest is Fe.
Another aspect of the present invention is directed to a cost effective low carbon high
formable hot rolled steels composition that comprises high silicon and boron,
restricting aluminium and substituting medium carbon Fe-Mn with low cost Si-Mn and
Ca-Fe by Ca-Si .
A still further aspect of the present invention is directed to a cost effective low
carbon high formable hot rolled steels composition wherein addition of boron is
effective in bringing out carbon and nitrogen from solid solution to form boron
carbides and carbonitrides as precipitates, favoring lower strain aging index of steel.
According to yet another advantageous aspect of the present invention is directed to
a cost effective low carbon high formable hot rolled steels composition wherein said
high silicon content is adapted to favour ease of casting due to increased flowability
and better desulphurisation at SRU.
A still further aspect of the present Invention is directed to a cost effective low
carbon high formable hot rolled steels having mechanical properties comprising
UTS in the range of 375-430 MPa;
YS/UTS in the range of 0.8- 0.9;
Percentage Elongation in the range of 41-45 %;
Hole Expansion ratio In the range of 60-65 %;
Strain Aging Index/Ratio in the range of 8-10 ;
According to yet another aspect of the present invention is directed to a method for
production of cost effective low carbon high formable hot rolled steels suitable for
LPG cylinder and the like comprising the steps of
(i) providing heats with selective starting composition comprising
C: 0.06-0.10 wt%,
Mn: 0.35-0.45 wt%,
Si: 0.1-0.2 wt%,
B: 5-25 ppm,
S:0.02 wt% max
P: 0.025 wt% max
Al: 0.02 wt% min
and rest is Fe.
and rest is Fe.
(ii) processing the heats having above selective composition through Ladle Furnace;
(iii) adding selectively Sillco Manganese In the range of 1400-1600 Kg ,
(iv) followed by limited addition of Aluminium in the leval of 400-500 Kg ;
(v) maintaining the amount of High Carbon Ferro Manganese (HC FeMn) at 100-200
kg;
(vi) casting the heats thus produced continuously;
(vii) processing the Concast slabs into 2.9X1160 mm size by adhering to finishing
temperature (870°C +10°C) and coiling temperature (610°C+10°C), to obtain steel
with desired properties suitable for LPG Cylinder and the like.
The present invention and its objects and advantages is described in greater details
with reference to the following non limiting exemplary embodiment.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO EXAMPLE
The present Invention is directed to providing a cost effective low carbon aluminium
killed formabie steel composition comprising addition of Silicon and boron in order to
ensure desired strength, elongation and cold formability characteristics in the
resulting hot rolled steel product for application in LPG cylinders or automobile
structural components.
The invention is based on the principle that Boron is added in low carbon steel to
convert the free nitrogen into Boron nitride (BN) which leads to improvement in
formability. The invention allows boron addition only to combine with carbon and
nitrogen in steel to thereby reducing dissolved 'C and "N' and results in softer hot
rolled product with higher plasticity but do not allow to contribute anyway in
increasing hardenability of steel. One of the requirements for cold formabie steel has
been found to be lower Strain Aging Index (SAI). Attempt is made to introduce
combined silicon and aluminium additions which is much more effective in reducing
strain aging than aluminium alone.
The objects and advantages of the invention would be clearly under stood from the
following embodiments according to the present invention by way of an illustrative
example.
EXAMPLE I:
A cost effective low carbon high formabie hot rolled steel adapted for production of
LPG cylinders and the like, has been produced following the steps of:
(1) Heats are prepared with a selective starting composition comprising
C: 0.06-0.10 wt%,
Mn: 0.35-0.45 wt%,
Si: 0.1-0.2 wt%,
B: 5-25 ppm,
S:0.02 wt% max
P: 0.025 wt% max
Al: 0.02 wt% min
and rest is Fe.
(2) The above heat has been processed through Ladle Furnace;
(3) Silico Manganese is added in lieu of MC Fe-Mn , in the range of 1400-1600 Kg,
(4) Aluminium addition is restricted to 400-500 Kg,in view of higher silicon content in
steel.
(5) Amount of High Carbon Ferro Manganese (HC FeMn) is maintained at 100-200 kg
(6) Heats so produced are continuously cast;
(7) Concast slabs are processed into 2.9X1160 mm size by adhering to finishing
temperature (870°C +10°C) and coiling temperature (610°C+10°C).
Following the above schedule of additions, on account of the changes in ferro alloy
addition by substituting MC Fe-Mn by Si-Mn and Ca-Fe by Ca-Si, cost reduction to
the tune of Rs 550 /T has been achieved.
Testing and Results:
The effect of silicon and boron addition on the mechanical properties and strain aging
behaviour of LPG steel have been assessed. Mechanical properties of LPG grade with
modified chemistry is found to be comparable with that of LPG Titanium.
UTS value (375-430 MPa) of LPG-B with higher Si has been found to be slightly
higher than that of LPG Ti (370-415 MPa), which has resulted in lower YS/UTS
values, that favours an improved strain distribution during press forming, enabling
more complex component to be formed without excess localized thinning. Percentage
elongation value of newly developed LPG-B with higher Si grade is also slightly better
(41-45 %) than that of LPG Ti (36-42 %). Hole expansion values of newly developed
LPG was 63 % which is comparable with that of LPG Ti. Higher percentage of total
elongation coupled with lower YS/UTS ratio and good hole expansion value is a
measure of formability characteristics of the steel.
The strain aging index as a measure of carbon and nitrogen in solution of silicon-
boron treated LPG steel as of present invention is found to be 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.
The details of the properties achieved in the developed steel grade vis-a-vis that of
the conventional Ti LPG grade steel is given in following Table 1:
UTS in the range of 375-430 MPa;
YS/UTS in the range of 0.8- 0.9;
Percentage Elongation in the range of 41-45 %;
Hole Expansion ratio in the range of 60-65 %;
Strain Aging Index/Ratio in the range of 8-10 ;
Table-I
Large scale trials has been conducted with colls of newly developed LPG grade steel
for LPG cylinder manufacturing and the material has performed very well at the
application end and found to be comparable to that of LPG with lower Si (~ 0.04%)
with significant cost saving.
It is thus possible by way of the present invention to providing an alloy composition
of steel grades with boron and higher silicon adapted to ensure improved formability
in terms of lower Strain Ageing Index, lower Yield Strength/Ultimate Tensile Strength
ratio and higher hole expansion ratio. The high silicon steel composition further
favour ease of casting due to increased flowability and better desulphurisation. The
present invention is thus directed to providing a low cost low carbon aluminium killed
formable steel composition comprising addition of boron and high Silicon in order to
ensure desired strength, elongation and cold formability characteristics in the
resulting hot rolled steel product for application in production of LPG cylinders or
automobile structural components and the like.

We Claim:
1. A cost effective low carbon high formable hot rolled steels composition suitable for
LPG cylinder and the like comprising
C: 0.06-0.10 wt%,
Mn: 0.35-0.45 wt%,
Si: 0.1-0.2 wt%,
B: 5-25 ppm,
S:0.02 wt% max
P: 0.025 wt% max
Al: 0.02 wt% min
and rest is Fe.
2. A cost effective low carbon high formable hot rolled steels composition as claimed
in claim 1 comprising high silicon and boron, restricting aluminium and substituting
medium carbon Fe-Mn with low cost Si-Mn and Ca-Fe by Ca-Si .
3. A cost effective low carbon high formable hot rolled steels composition as claimed
in anyone of claims 1 or 2 wherein addition of boron is effective in bringing out
carbon and nitrogen from solid solution to form boron carbides and carbonitrides as
precipitates, favoring lower strain aging index of steel.
4. A cost effective low carbon high formable hot rolled steels composition as claimed
In anyone of claims 1 to 3 wherein said high silicon content is adapted to favour
ease of casting due to increased flowability and better desulphurisation at SRU.
5. A cost effective low carbon high formable hot rolled steels as claimed in anyone of
claims 1 to 4 having mechanical properties comprising
UTS in the range of 375-430 MPa;
YS/UTS in the range of 0.8- 0.9;
Percentage Elongation in the range of 41-45 %;
Hole Expansion ratio in the range of 60-65 %;
Strain Aging Index/Ratio in the range of 8-10 ;

5. A method for production of cost effective low carbon high formable hot rolled
steels suitable for LPG cylinder and the like as claimed in anyone of claims 1 to 4
comprising the steps of
(i) providing heats with selective starting composition comprising
C: 0.06-0.10 wt%,
Mn: 0.35-0.45 wt%,
Si: 0.1-0.2 wt%,
B: 5-25 ppm,
S: 0.02 wt% max
P: 0.025 wt% max
Al: 0.02 wt% min
and rest is Fe.
(ii) processing the heats having above selective composition through Ladle Furnace;
(iii) adding selectively Silico Manganese in the range of 1400-1600 Kg;
(iv) followed by limited addition of Aluminium in the level of 400-500 Kg;
(v) maintaining the amount of High Carbon Ferro Manganese (HC FeMn) at 100-200
kg;
(vi) casting the heats thus produced continuously;
(vii) processing the Concast slabs into 2.9X1160 mm size by adhering to finishing
temperature (870°C +10°C) and coiling temperature (610°C+10°C), to obtain steel
with desired properties suitable for LPG Cylinder and the like.
7. A cost effective low carbon high formable hot rolled steels composition suitable for
LPG cylinder and the like and its process for manufacture substantially as
herelndescribed and illustrated with reference to the accompanying examples.

A cost effective low carbon high formable hot rolled steel composition and a process
for Its manufacturing is disclosed. More particularly, a low carbon aluminium killed
high formable steel grade is claimed comprising high silicon along with boron in hot
rolled steel adapted to ensure improved cold formability favouring application in
production of LPG cylinders and the like. Advantageously, combined silicon and
aluminium additions favour effectively reducing strain aging index ensuring softer
hot rolled product with higher plasticity. Replacing medium Carbon Ferro Manganese
addition with Silico Manganese and restricting aluminium addition, the cost of
production is substantially reduced. The high silicon and boron containing hot rolled
cold formable steel is having higher UTS, low YS/UTS ratio, higher percent
elongation, higher hole expansion ratio and lower strain aging index cost effective
application in a variety of industries such as the LPG cylinder manufacturer,
automobile structural components/wheel rims and other deep drawing applications
including prefabricated structural.