DEVELOPMENT OF HIGH STRENGTH STEEL FOR LIGHTER
INDIAN DOMESTIC LPG CYLINDER
INTRODUCTION TO THE FIELD OF INVENTION
The present invention relates to high strength LPG steel alloy and a LPG gas
storage cylinder manufactured thereof with yield strength greater than 295 MPa for
fabrication of lighter Indian domestic cylinders (33.3 litres). The steel sheets
manufactured form the said high strength LPG steel alloy has reduced thickness
resulting in weight reduction in cylinder. The LPG gas storage cylinder manufactured
thereof has increased cylinder efficiency, ultimate tensile strength, fracture toughness
and fire resistance.
PRIOR ART AND DRAWBACKS
Carbon - Manganese steel is generally used for the existing 2.9 mm thick LPG
cylinders with YS 245 Mpa min. The composition of these steels is similar to low-
carbon mild steels, except they have more carbon and manganese to increase the
strength to the desired level.
One approach to achieving yield strengths between 280 and 550 MPa (40 and
80 KSI) is to use high-strength, low-alloy steels, also known as microalloyed steels.
This family of steels usually has a microstructure of fine-grained ferrite that has been
strengthened with carbon and/or nitrogen precipitates of titanium, vanadium, or
niobium (columbium). Adding manganese, phosphorus, or silicon further increases the
strength. These steels can be formed successfully when users know the limitations of
the higher-strength, lower-formability trade-off.
The fundamental property of this high strength steel for gas cylinders is that it
can withstand high pressure in different in-use conditions. To withstand high pressure,
it is advantageous to have high strength steel, where specifically high-strength
hardening during deep drawing permits a significant thickness reduction compared with
the conventional hot rolled steels used for this application, with no impact on the usual
processing steps. Combined with good ductility at low temperatures, weldability, good
fatigue and non-ageing properties, it is the safest choice of material to use for
lightweight steel cylinders. Furthermore, the environmental impact of these recyclable
lightweight steel cylinders from both manufacturing and use is significantly reduced
compared with conventional steel cylinders, making them a very sustainable
alternative.
In India, LPG grade steel in 2.9 mm thickness is regularly being produced
against IS 6240 specification titled " Hot rolled steel plate sheet and strip for the
manufacture of low pressure liquefiable gas cylinders" for fabrication of 33.3 litre
domestic cylinder and Torodial & Cylindrical auto cylinders. The mechanical properties
are comprised of Yield Strength: 240 MPa min, Ultimate Tensile Strength: 350 - 450
MPa and % Elongation: 25 min. Major market of LPG is domestic sector and home
delivery is to the tune of 3 millions cylinder / day (i.e. 900 million / year) and there is
steady growth @ 8 % in LPG consumption in India
The document US 4578113 discloses steel alloy particularly suited to gas
storage cylinder manufacture, and a gas storage cylinder manufactured thereof which
exhibits remarkably improved performance over conventional gas storage cylinders.
The paper in the journal Engineering Fracture Mechanics, Volume 30, Issue 6,
1988, Pages 877-894, describes fracture mechanics criteria employed in the
development of fracture-safe, 155-175 ksi in high-strength steel gas cylinders. A new
steel composition was developed to obtain the toughness required to satisfy leak-
before-break and plastic fracture mode criteria. Cylinders were fabricated from the new
steel, and tests were conducted to characterize their flaw tolerance capability. Leak-
before-break and plastic fracture mode performance was verified. Flawed cylinder tests
were also conducted on conventional 3AA 4130X steel cylinders (105-125 ksi strength
level) to characterize their flaw tolerance capability. The flaw tolerance capability of the
high-strength cylinders was found to be equal to that of the conventional low strength
steel cylinders at respective service stresses.
The document US 5387392 discloses high strength, high toughness steel grade
and gas cylinder thereof. A high strength, high toughness steel comprising from about
0.32 to about 0.36 percent by weight carbon, from about 0.40 to about 0.60 percent by
weight manganese, from about 0.15 to about 0.35 percent by weight silicon, from
about 0.80 to about 1.10 percent by weight chromium, from about 0.55 to about 0.70
percent by weight molybdenum, from about 0.01 to about 0.05 percent by weight
aluminum, from about 0.002 to about 0.004 calcium, no more than about 0.015 percent
by weight phosphorus, no more than about 0.008 percent by weight sulfur, with the
balance being iron.
The document US 4461657 discloses steel alloy particularly suited to gas
storage cylinder manufacture, and a gas storage cylinder manufactured thereof which
exhibits remarkably improved performance over conventional gas storage cylinders.
This invention relates to gas storage cylinders and the steel of which they are made
and more particularly to a novel gas storage cylinder which exhibits improved cylinder
efficiency, ultimate tensile strength, fracture toughness, and fire resistance over gas
storage cylinders which are currently available.
However, such modified steels used for high strength have the problem of the
so-called delayed fracture, i.e., a sudden fracturing occurs during use over a long
period of time in fastened state. In this regard, developments have been conducted
with a view to solving the just-mentioned problem, in the present invention. Hence, in
the present invention high strength steel with YS > 295 MPa for fabrication of lighter
Indian domestic cylinders has been developed.
By using high strength steel the gas cylinders can become lighter than the
conventional cylinders, keeping the excellent properties when it comes to safety and
recycling. Another benefit is also that the production is cost effective which both
cylinder manufacturers and end users can benefit from. There are many additional
advantages, such as average life span and safety aspects, as they are generally quite
heavy to handle and transport. Alternative way of light weight cylinders is by using
aluminum and composite materials, which are easy to handle but they have other
disadvantages such as price levels and difficulties to recycle. It will be helpful in
producing thinner gauge gas cylinder and therefore reduce weight of domestic / auto
and industrial cylinders. Benefit with these lighter cylinders includes more number of
cylinders per ton of steel and ease of handling and transportation.
BACKGROUND OF INVENTION
High-strength low-alloy steel is a type of alloy steel that provides better
mechanical properties or greater resistance to corrosion than carbon steel. High-
strength low-alloy steels vary from other steels in that they are not made to meet a
specific chemical composition but rather to specific mechanical properties. They have
carbon content between 0.05-0.25% to retain formability and weldability. Other
alloying elements include up to 2.0% manganese and small quantities of copper,
nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare
earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for
strengthening purposes. These elements are intended to alter the microstructure of
carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine
dispersion of alloy carbides in an almost pure ferrite matrix. This eliminates the
toughness-reducing effect of a pearlitic volume fraction yet maintains and increases
the material's strength by refining the grain size, which in the case of ferrite increases
yield strength by 50% for every halving of the mean grain diameter. Precipitation
strengthening plays a minor role, too. Their yield strengths can be anywhere between
250-590 megapascals (36,000-86,000 psi). Because of their higher strength and
toughness high-strength low-alloy steel steels usually require 25 to 30% more power to
form, as compared to carbon steels.
Copper, silicon, nickel, chromium, and phosphorus are added to increase
corrosion resistance. Zirconium, calcium, and rare earth elements are added for
sulfide-inclusion shape control which increases formability. These are needed because
most high-strength low-alloy steels have directionally sensitive properties. Formability
and impact strength can vary significantly when tested longitudinally and transversely
to the grain. Bends that are parallel to the longitudinal grain are more likely to crack
around the outer edge because it experiences tensile loads. This directional
characteristic is substantially reduced in high-strength low-alloy steels that have been
treated for sulfide shape control.
High-strength low-alloy steels are usually 20 to 30% lighter than carbon steel
with the increased strength. High-strength low-alloy steels are also more resistant to
rust than most carbon steels because of their lack of pearlite - the fine layers of ferrite
(almost pure iron) and cementite in pearlite. High-strength low-alloy steel usually have
densities of around 7800 kg/m3.
Due to safety aspects there are several obvious advantages to produce LPG
Light weight cylinders in steel. Due to recycling aspects there are advantages to
produce LPG cylinders in steel. And above all, due to economical aspects there are
advantages to use high strength steel in Light weight LPG cylinders. The cylinders
themselves become lighter of course; hence less material is needed to produce the
same amount of cylinders. On top of that there are also savings to do during
production. The secret behind light weight cylinders is simply to use thinner but
stronger steel. Steel grades, with improved technical properties, are an important part
of the product development.
OBJECTIVE OF THE INVENTION
Most LPG cylinders in In India are of LPG grade steel in 2.9 mm thickness is
regularly being produced against IS 6240 specification. Cylinders conforming to this
Specification are considered safe and exhibit good fracture toughness at the allowed
tensile strengths.
With increasing transportation costs, there has arisen a need for an improved
gas storage cylinder. In particular there has arisen a need for a gas storage cylinder
which has much better cylinder efficiency .However; any such increase in cylinder
efficiency cannot be at the expense of cylinder fracture toughness at the usable tensile
strengths.
Since tensile strength and fracture toughness are, to a large extent,
characteristic of the material of which the cylinder is made, it would be highly desirable
to have a materia! to construct a gas storage cylinder which has improved cylinder
efficiency while also having improved tensile strength and fracture toughness.
It is therefore an object of this invention is to develop of high strength steel alloy
for stress relieving, normalizing and a gas storage cylinder manufactured thereof with
YS > 295 MPa for fabrication of lighter Indian domestic cylinders (33.3, litres).
It is a further object of this invention to provide a steel alloy for stress relieving,
normalizing and a gas cylinder manufactured thereof which has reduction in thickness
from 2.9 mm to 2.6/2.4 resulted in 10-17 % weight reduction in cylinder.
It is another object of this invention to provide a steel alloy for stress relieving,
normalizing and a gas storage cylinder manufactured thereof which has increased
cylinder efficiency over that of conventional gas storage cylinders.
It is another object of this invention to provide a steel alloy for stress relieving,
normalizing and a gas storage cylinder manufactured thereof which has increased
ultimate tensile strength over that of conventional gas storage cylinders.
It is yet another object of this invention to provide a steel alloy for stress
relieving, normalizing and a gas storage cylinder manufactured thereof which has
increased temper resistance over that of conventional gas storage cylinders.
It is yet another object of this invention to provide a steel alloy for stress
relieving, normalizing and a gas storage cylinder manufactured thereof which has
increased temper resistance over that of conventional gas storage cylinders.
It is a still further object of this invention to provide a steel alloy for stress
relieving, normalizing and a gas storage cylinder manufactured thereof which has
increased fracture toughness over that of conventional gas storage cylinders.
It is a still further object of this invention to provide a steel alloy for stress
relieving, normalizing and a gas storage cylinder manufactured thereof which is lighter
than the conventional cylinders, keeping the excellent properties when it comes to
safety and recycling.
It is a still further object of this invention to provide a steel alloy for stress
relieving, normalizing and a gas storage cylinder manufactured thereof is cost effective
which both cylinder manufacturers and end users can benefit from.
There are many additional advantages, such as average life span and safety
aspects, as they are generally quite heavy to handle and transport. Alternative way of
light weight cylinders is by using aluminum and composite materials, which are easy to
handle but they have other disadvantages such as price levels and difficulties to
recycle.
It will be helpful in producing thinner gauge gas cylinder and therefore reduce
weight of domestic / auto and industrial cylinders. Benefit with these lighter cylinders
includes more number of cylinders per ton of steel and ease of handling and
transportation.
DETAILED DESCRIPTION OF THE INVENTION
Identification of Basic features and their relevance
As per international standards, formed cylinders need to be subjected to
normalizing treatment to refine the structure within weld-affected area and modification
of residual stresses. It is comprised of heating the formed cylinders to 890 C - 920 C
and then air cooling, which results in drop in values of yield and tensile strength (more
in Nb microalloyed than that in without microalloying). Considering the drop, alloy
chemistry was designed (Table 1) and hot rolled into 2.6 mm thickness.
However, as per Indian standards, formed cylinders needs to be subjected to
stress relieving treatment for removal of residual stresses generated during forming. It
is comprised of heating the formed cylinders to 680 C-700 C and then furnace cooling.
Accordingly, chemistry was on the leaner side (as shown in Table 1) for the steel to be
processed through stress relieving heat treatment. Small amount of boron was added
for nitrogen tie up in steel.
Aim was to achieve higher strength and elongation values in the formed
cylinders (YS: 295 MPa min, UTS: 440 MPa and % El : 26 min) as compared to
standard Indian specification (IS 6240 ) , processed through both the heat treatment
processes. The processing route involves Basic Oxygen Furnace (LD) - Ladle Refining
- Continuous Casting - Hot Strip Mill route.
Results
Mechanical properties of the formed of hot rolled coils (Table 2) to be subjected
for further processing to stress relieving and normalizing of formed cylinders are shown
in table 2.
Table 2: Mechanical properties of the formed of hot rolled coils

Fabrication of lighter cylinder
The newly developed material of 2.6 mm nominal thickness was taken up for
trial at LPG cylinder manufacturer premises. Drawing load was observed to be 120-125
bar, same as IS 6240 material. The depth of the halves after trimming 175 mm.
Readings were taken at exactly 25 mm interval. Cylinder halves were taken up for
detailed thickness measurements. The final thickness at the thinnest point is in the
range of 2.19 - 2.20 mm after taking into account the scale loss during normalizing. As
per international standard EN 1442, the minimum thickness for this grade is 1.74 mm.
Weight of 2.9 mm IS6240 grade cylinder halve vary in the range of 5.90 - 5.93kg.
Weight of 2.6 mm new grade cylinder halves was in the range of 5.448 - 5.505 kg. The
cylinders were subjected to stress relieving at 700° C for 1/2 hr and normalizing at a
temperature of 890° C and the properties conformed to minimum specified values (YS
> 295 MPa min, UTS :450 - 560 MPa and % El: 20 min) even after normalizing
treatment.
Test results of formed cylinders
Subsequent tests after normalizing at a temperature of 890° C were successfully
conducted. Burst Pressure was found to be 92 bars and volumetric expansion at burst
was 28 %. Macro examination of weld was found OK.
Introduction of new Indian standard for high strength LPG grade steel
The continual developmental efforts on high strength LPG grades have seen
fruition in the form of a new Indian Standard IS 15914:2011 on "High tensile strength
flat rolled steel plate, sheet and strip for the manufacture of welded gas cylinder"
issued by BIS. With introduction of this standard, it is now possible to reduce the tare
weight of the cylinders with improved yield of cylinders per ton, which is a win-win
situation for producers, fabricators and the society at large.
Dent Resistance
Impact during handling of LPG cylinder can be evaluated by Dent Resistance
which is proportional to YS (yield strength) x t2 (thickness). Dent resistance of LPG
steel conforming to IS 6240 (YS> 240 MPa) with thickness 2.9 mm is found to be
comparable with that of High strength LPG grade steel conforming to new specification
(YS > 295 MPa min) in 2.4 mm thickness (Table 3).
Table 3: Dent Resistance of conventional and high strength LPG grade
Salient Features of Innovation
> Cost effective innovative alloy designed considering stress relieving /
normalizing heat treatment after forming of cylinders.
The steel alloy of this invention contains from 0.06 to 0.08 weight percent
carbon for stress relieving of high strength LPG steel and 0.10 to 0.12 weight percent
carbon for normalizing of high strength LPG steel and a LPG gas cylinder
manufactured thereof. Carbon is the single most important element affecting the
hardness and tensile strength of a quench and tempered martensitic steel. The steel
alloy of this invention has increased fire resistance over that of the heretofore
commonly used cylinder steel. Carbon content above 0.50 weight percent can lead to
quench cracking. Thus, the defined ranged for carbon concentration ensures sufficient
carbon for the desired tensile strength after tempering while assuring a low enough
carbon content and as-quenched hardness to preclude cracking during the cylinder
quenching operation to produce martensite. Carbon, in the amount specified, also
contributes to hardenability and helps to assure that the cylinder will have a fully
martensitic structure.
Consequently, the steel alloy should contain a sufficient quantity of elements
such as manganese, silicon, boron, and the (ike to assure adequate hardenability.
The steel alloy of this invention preferably contains from 1.0 to 1.2 weight
percent manganese for stress relieving of high strength LPG steel and 1.2 to 1.4
weight percent manganese for normalizing of high strength LPG steel and a LPG gas
cylinder manufactured thereof. This defined amount, in combination with the other
specified elements and amounts of the invention, enables the steel alloy of this
invention to have sufficient hardenability to provide a fully martensitic structure at
quench rates which do not lead to quench cracking. This is important in order to obtain
an optimum combination of strength and fracture toughness. The manganese also
serves to tie up sulfur in the form of manganese sulfide inclusions rather than as iron
sulfide. Iron sulfide is present in steels as thin films at prior austenite grain boundaries
and is extremely detrimental to fracture toughness. The steel alloy of this invention
generally has sulfur 0.015 maximum as shape controlled calcium or rare earth
containing oxy-sulfides. However, it is difficult to assure that absolutely all sulfur is
incorporated into this type of inclusion. The presence of manganese in the amount
specified addresses this problem and frees the invention from potentially hazardous
iron sulfide films.
The steel alloy of this invention preferably contains from 0.10 to 0.15 weight
percent Silicon for stress relieving of high strength LPG steel and 0.15 to 0.2 weight
percent silicon for normalizing of high strength LPG steel and a LPG gas cylinder
manufactured thereof. The silicon is present as a deoxidant which will promote the
recovery of subsequent aluminum, calcium or rare earth additions. Silicon also
contributes to temper resistance and, consequently, improves the fire resistance of the
cylinder. Further, silicon is one of the elements which contribute to hardenability.
The steel alloy of this invention preferably contains 0.02 minimum weight
percent aluminum for stress relieving of high strength LPG steel and for normalizing of
high strength LPG steel and a LPG gas cylinder manufactured thereof. Aluminum is
present as a deoxidant and for its beneficial effect on inclusion chemistry.
The steel alloy of this invention contains not more than 0.025 weight percent for
stress relieving of high strength LPG steel and for normalizing of high strength LPG
steel and a LPG gas cylinder manufactured thereof.
Table 4: Chemical Composition of steel (wt %)
> Reduction in thickness from 2.9 mm (IS 6240) to 2.6/2.4 resulted in 10-17 %
weight reduction in cylinder.
> Specification for hot rolled steel suitable for stress relieving and
normalizing heat treatment of formed cylinder

Gas storage cylinders are fabricated from the steel alloy of this invention in any
effective manner known to the art. Those skilled in the art of gas storage cylinder
fabrication are familiar with such techniques and no further description of cylinder
fabrication is necessary here.
The steel alloy of this invention is extremely well suited for use in the fabrication
of gas storage cylinders intended to store gases. By such use one can now produce a
far more efficient cylinder than was heretofore possible. The steel alloy and gas
cylinder manufactured thereof of this invention simultaneously exhibit significantly
better fracture toughness at higher ultimate tensile strengths and also improved fire
resistance than any heretofore known steel alloy. This combination of qualities is
uniquely well suited for gas storage cylinders.
The present invention may, of course, be carried out in other specific ways
than those herein set forth without departing from the spirit and essential
characteristics of the invention. The present embodiments are, therefore, to be
considered in all respects as illustrative and not restrictive and any changes coming
within the meaning and equivalency range of the appended claims are to be embraced
therein.
We claim:
1. A high strength LPG steel alloy for stress relieving and a gas storage cylinder
manufactured thereof comprising:
a) from about 0.06 to about 0.08 percent by weight carbon;
b) from about 1.0 to about 1.2 percent by weight manganese;
c) from about 0.10 to about 0.15 percent by weight silicon;
d) no more than about 0.015 percent by weight sulphur;
e) no more than about 0.025 percent by weight phosphorus;
f) no less than about 0.02 percent by weight aluminium; and
g) the balance being 20 ppm boron.
2. A high strength LPG steel alloy for normalizing and a gas storage cylinder
manufactured thereof comprising:
a) from about 0.10 to about 0.12 percent by weight carbon;
b) from about 1.2 to about 1.4 percent by weight manganese;
c) from about 0.15 to about 0.20 percent by weight silicon;
d) no more than about 0.015 percent by weight sulphur;
e) no more than about 0.025 percent by weight phosphorus;
f) no less than about 0.02 percent by weight aluminium; and
h) the balance being 20 ppm boron.
3. A high strength LPG steel alloy for stress relieving and a gas storage cylinder
manufactured thereof, as claimed in claim 1 and 2, where in, the said steel is
having yield strength greater than 295 MPa for fabrication of lighter LPG
cylinders.
4. A high strength LPG steel alloy for stress relieving and a gas storage cylinder
manufactured thereof, as claimed in claim 1 and 2, where in, the said steel
sheet is reduced in thickness from 2.9 mm to 2.6/2.4 resulted in 10-17 % weight
reduction in cylinder.
5. A high strength LPG steel alloy for stress relieving and a gas storage cylinder
manufactured thereof, as claimed in claim 1 and 2, whereby increased cylinder
efficiency, ultimate tensile strength, fracture toughness and fire resistance are
attained.
6. A high strength LPG steel alloy for stress relieving and a gas storage cylinder
manufactured thereof substantially as herein described.
7. A high strength LPG steel alloy for normalizing and a gas storage cylinder
manufactured thereof substantially as herein described.

ABSTRACT

The present invention relates to high strength LPG steel alloy and a LPG gas
storage cylinder manufactured thereof with yield strength greater than 295 MPa for
fabrication of lighter Indian domestic cylinders (33.3 litres). Further the steel sheets
manufactured form the said high strength LPG steel alloy have reduced thickness
resulting in weight reduction in cylinder. The chemistry and properties of hot rolled
steel has been designed separately for formed cylinders subjected to stress
relieving / normalizing. The cylinder manufacturer will get more cylinders per ton of
steel.