FIELD OF THE INVENTION
The present invention relates to providing high strength hot rolled steel for the
manufacture of normalized LPG gas cylinder and a process for its production. More
particularly, the present invention is directed to providing two grades of steel having
high yield strength of 265 MPa min. and 295 MPa min. respectively after
normalizing of formed cylinders. This is achieved through careful selection of steel
composition and normalizing temperature, by adopting an integrated approach for
development of suitable alloy chemistry and hot rolling parameters for the basic steel
sheet products.
BACKGROUND OF THE INVENTION
High formable hot rolled steel conforming to IS 6240 specification is regularly being
used for fabrication of Liquified Petroleum Gas (LPG) cylinder in India. There exist a
huge demand for another variant of LPG steel, High Strength LPG steel, used for
manufacturing cylinders of various shapes and exclusively servicing export market.
These are products conforming to European/Japanese standard with specific
requirements for hot rolled sheets /strips / coils made of steels intended for
manufacturing welded gas cylinders. It is also known that Steel weldments are
normalized to refine the structure within weld-affected area and modification of
residual stresses. Thus the high strength steel grade for LPG cylinder would ensure
higher strength and formability and also to retain desired properties as per
requirement of final product as the welded and normalized gas cylinder. There has
been thus a need in the art to developing steel grades/standard having higher
mechanical properties in hot rolled coils to take care of the drop in strength after
normalizing of formed LPG cylinders to cater to the need of the Indian as well as the
export market.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to providing a high strength
hot rolled steel product/grades and normalized LPG gas cylinder obtained thereof and
a process for its production.
A further object of the present invention is directed to providing a high strength hot
rolled steel product/grade for the manufacture of normalized LPG gas cylinder
wherein innovative alloy chemistry is designed (with and without niobium) to take
care of the requirement of mechanical properties after normalizing of formed
cylinders.
A further object of the present invention is directed to providing a high strength hot
rolled steel product/grade for the manufacture of normalized LPG gas cylinder
wherein the steel chemistry and optimized process parameter for hot rolling as well
as normalizing of formed cylinder so as to achieve fine grained ferrite pearlite
structure ensuring higher strength and formability.
A further object of the present invention is directed to providing a high strength hot
rolled steel product/grade for the manufacture of normalized LPG gas cylinder
enabling producing thinner gauge gas cylinder and therefore reducing the weight of
domestic/ auto and industrial cylinder.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to providing High strength
hot rolled steel normalized LPG gas cylinder comprising
steel composition comprising
C in the range of 0.10-0.12 wt%;
Mn in the range of 0.9-1.1 wt%;
Si in the range of 0.1-0.15 wt%;
S upto 0.015 wt%;
P upto 0.025 wt%;
Al atleast 0.02 min wt%;
B in the range of 0.0015- 0.0020 wt%;
with or without Nb. Upto 0.015 wt %.
A further aspect of the present invention is directed to providing High strength hot
rolled steel normalized LPG gas cylinder obtained of hot rolled coils having
mechanical properties comprising:
(i) for steel grade without Nb microalloying comprising
YS: 320 MPa min;
UTS:460 MPa min;
El: 30% min;
or
(ii) for steel grade with Nb microalloying comprising
YS:380 MPa min;
UTS:500 MPa min;
El:32% min;
A still further aspect of the present invention is directed to providing High strength
hot rolled steel normalized LPG gas cylinder having Mechanical properties
comprising,
(i) for said steel compostion without Nb microalloying YS: 265 MPA min;
UTS:410 MPa min;
El: 24% min.
and
(ii) for said steel compostion with Nb microalloying
YS: 295 MPA min;
UTS:440 MPa min;
El: 26% min.
A still further aspect of the present invention is directed to providing High strength
hot rolled steel normalized LPG gas cylinder wherein average ferrite grain size of
steel composition free of Nb is 9.6 micron and the microstructure of steel
composition with Nb have a finer ferrite pearlite structure with a ferrite grain size of
5.5 micron.
According to yet another aspect of the present invention is directed to a process for
the manufacture of high strength hot rolled steel normalized LPG gas cylinder
comprising the steps of
(a) providing said steel composition comprising
C in the range of 0.10-0.12 wt%;
Mn in the range of 0.9-1.1 wt%;
Si in the range of 0.1-0.15 wt%;
S upto 0.015 wt%;
P upto 0.025 wt%;
Al atleast 0.02 min wt%;
B in the range of 0.0015- 0.0020 wt%;
with or without 1Mb. upto 0.015 wt %.
(b) obtaining hot rolled coils there from having mechanical properties comprising:
(i) for steel grade without Nb microalloying comprising
YS: 320 MPa min;
UTS:460 MPa min;
El: 30% min;
or
(ii) for steel grade with Nb microalloying comprising
YS:380 MPa min;
UTS:500 MPa min;
El:32% min;
and
(c) producing said normalized cylinder therefrom following a normalizing
temperature in the range of 880°C to 900°C.
A still further aspect of the present invention is directed to said process for the
manufacture of high strength hot rolled steel normalized LPG gas cylinder comprises
(i) obtaining the hot rolled coils maintaining finishing and coiling temperature in the
range of 880 ± 10 °C and 680 ± 10 °C.
(ii) using said hot rolled coils of 2.4-2.6 mm thickness to fabricate LPG cylinders and
then subjecting to said normalizing treatment at the normalizing temperature range
of 880-900°C to obtain the high strength LPG gas cylinder.
A still further aspect of the present invention is directed to providing High strength
hot rolled steel normalized LPG gas cylinder wherein amount of silicon is selectively
maintained to ensure the higher strength and better formability in product.
The objects and advantages of the present invention are described in greater details
with reference to the following non limiting accompanying example and figure.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURE
Figure 1(a) & 1(b): is micrographic image of grain structure of the high strength hot
rolled steel according to the present invention showing the grain size distribution in
microstructure of a) Steel A(without niobium) and b) Steel B(with niobium).
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURE
The present invention is directed to providing high strength hot rolled steel
products/grades for the manufacture of normalized LPG gas cylinder and a process
for its production.
To meet the demand for High Strength LPG steel, used for manufacturing cylinders
of various shapes for exclusively servicing export market a new grade standard of
high strength hot rolled steel has been developed by way of the present invention.
These European/Japanese standard has specific requirements for hot rolled
sheets/strips/coils made of steels intended for manufacturing welded gas cylinders.
Steel weldments are normalized to refine the structure within weld-affected area and
modification of residual stresses. The salient feature of this product is to achieve high
yield strength of (265/295 MPa min.) after normalizing of formed cylinder. This
obviously calls for careful selection of chemistry and normalizing temperature. It has
been possible to meet the quality requirements for the two grades by adopting an
integrated approach for development of suitable alloy chemistry and hot rolling
parameters.
There was no standard from IOC for development of any alterative grade to IS 6240,
that has been regularly used for making domestic cylinders in India. Therefore,
cylinders manufactures have tied-up with different countries for exporting the
cylinders fabricated from high strength grades. In view of the need a separate
specification for theses grades has been developed in line with ISO 4978 and
approved by BIS. This grade of steel facilitates producing thinner gauge gas cylinder
and therefore reduces the weight of domestic/auto and industrial cylinder.
According to an embodiment of the present invention, the alloy steel
composition(with and without niobium) as the input material has been selectively
designed to take care of the requirement of mechanical properties after normalizing
of formed cylinders. The detail of the invention is described by way of the
accompanying Example I.
Example I:
Composition of steel material for producing the high strength hot rolled steel
products is obtained as follows:
Carbon is kept on the higher side (peritectic range) and to avoid the consequences
during casting, tailor-made casting powder is used. Casting powders are customized
to suit low, medium and high carbon grades to ensure sound casting.
Addition of boron helped in eliminating harmful effect of excess free nitrogen
introduced into steel during steel making. Small amount of silicon is added to ensure
the higher strength and better formability in product.
Medium Carbon Ferro Manganese has been replaced by Silico Manganese, cost of
which is lower than the former. Aluminium addition is also restricted in view of
higher silicon content in steel. Amount of High Carbon Ferro Manganese (HC FeMn)
are kept same. On account of these changes in ferro alloy addition, cost reduction in
production is achieved. An additional advantage observed as a result of higher silicon
is ease of casting and better desulphurization.
Following Table I shows the composition of two grades A & B of steel with Nb and
without Nb, for producing high strength hot rolled steel sheet for manufacture of
normalized LPG cylinder as per the two newly developed standards according to the
present invention:
The steel chemistry of high strength export quality grade has been attained by
adhering to above mentioned restricted chemistry and cast product obtained
therefrom has been hot rolled into coils (2.4X 1270 mm) following Basic Oxygen
Furnace (LD) - Ladle Refining - Continuous Casting - Hot Strip Mill route. Steel
Grade A is made without microalloying whereas in Steel Grade B, Nb is maintained
0.015 % max. Finishing and coiling temperatures are kept in the range of 880 + 10
°C and 680 ± 10 °C respectively.
Normalizing treatment (Laboratory study)
A study has been undertaken in laboratory to know the effect of different
normalizing temperature on HS LPG grade steel. Tensile samples of steel with and
without Niobium have been heated above AC3 temperature in muffle furnace at
different normalizing temperatures, 870 C, 890 C and 920 C to obtain complete
solution of the original structure with the formation of austenite, soaked for 3
minutes and then air cooled to room temperature. With increasing normalizing
temperature, grain coarsening from 5.5 micron to 8.3 micron was observed in Nb
microalloyed steel whereas no such affect on grain coarsening was revealed in case
of steel without Nb addition. Drop in tensile properties of steel with niobium has
been found as a result of increasing normalizing temperature.
Normalizing of formed cylinder manufacturer's premises
Large scale trials have also been conducted in customer's premises with steel A and
B. Hot rolled coils of 2.4-2.6 mm thickness have been fabricated into cylinders and
then subjected to normalizing treatment as suggested on the basis of laboratory
experiment. The supplied material fulfilled the property requirements equivalent to
grade A ( YS : 265 MPa min, UTS: 410 MPa min and % El: 24) for Non-Nb heats and
to Grade B (YS : 295 MPa min, UTS : 440 MPa min and % El: 26) for Nb bearing
heat.
Results:
Microstructure
Typical microstructures of hot strips of steel grades A and B are shown in
accompanying Figures 1 (a) & (b) respectively. Average ferrite grain size of steel A
is 9.6 micron. The microstructure of steel B revealed a finer ferrite pearlite structure
(Fig. 1 b) with a ferrite grain size of 5.5 micron.
Mechanical Properties of hot rolled coils:
Processing steps were specially designed to achieve slightly higher mechanical
properties in hot rolled coils to take care of the drop in strength after normalizing of
formed cylinders. The Mechanical properties of hot rolled coils produced according to
the above Example are illustrated in the following Table 2:
The comparative Mechanical Properties of hot rolled coils and the normalized
cylinders made therefrom are illustrated in the accompanying Table 3:
It is thus possible by way of the present invention to developing high strength hot
rolled steel for manufacture of normalized LPG gas cylinder involving innovative
alloy design (with and without niobium) for high strength LPG grade steel ensuring
required mechanical properties in the formed cylinder after normalizing treatment,
the above result is achieved by on one hand providing cost effective alloy design with
higher Carbon (0.1-0.12) and Silicon (0.10-0.15), without microalloying for the
grade HS 265 and with low Niobium (0.015 max) for HS 295. Moreover, increase
silicon level favor ease of casting with improved desulphurization at reduced cost of
production meeting the requirement of LPG cylinder manufacturer/consumer in India
as well as abroad.
We Claim:
1. High strength hot rolled steel normalized LPG gas cylinder comprising
steel composition comprising
C in the range of 0.10-0.12 wt%;
Mn in the range of 0.9-1.1 wt%;
Si in the range of 0.1-0.15 wt%;
S upto 0.015 wt%;
P upto 0.025 wt%;
Al atleast 0.02 min wt%;
B in the range of 0.0015- 0.0020 wt%;
with or without Nb. up to 0.015 wt %.
2. High strength hot rolled steel normalized LPG gas cylinder as claimed in claim 1
obtained of hot rolled coils having mechanical properties comprising:
(i) for steel grade without Nb microalloying comprising
YS: 320 MPa min;
UTS:460 MPa min;
El: 30% min;
or
(ii) for steel grade with Nb microalloying comprising
YS:380 MPa min;
UTS:500 MPa min;
El:32% min;
3. High strength hot rolled steel normalized LPG gas cylinder as claimed in anyone of
claims 1 or 2 having Mechanical properties comprising,
(i) for said steel compostion without Nb microalloying YS: 265 MPA min;
UTS:410 MPa min;
El: 24% min.
and
(ii) for said steel compostion with Nb microalloying
YS: 295 MPA min;
UTS:440 MPa min;
El: 26%imin.
4. High strength hot rolled steel normalized LPG gas cylinder as claimed in anyone of
claims 1 to 3 wherein average ferrite grain size of steel composition free of Nb is 9.6
micron and the microstructure of steel composition with Nb have a finer ferrite
pearlite structure with a ferrite grain size of 5.5 micron.
5. A process for the manufacture of high strength hot rolled steel normalized LPG
gas cylinder as claimed in anyone of claims 1 to 6 comprising the steps of
(a) providing said steel composition comprising
C in the range of 0.10-0.12 wt%;
Mn in the range of 0.9-1.1 wt%;
Si in the range of 0.1-0.15 wt%;
S upto 0.015 wt%;
P upto 0.025 wt%;
Al atleast 0.02 min wt%;
B in the range of 0.0015- 0.0020 wt%;
with or without Nb. upto 0.015 wt %.
(b) obtaining hot rolled coils there from having mechanical properties comprising:
(i) for steel grade without Nb microalloying comprising
YS: 320 MPa min;
UTS-.460 MPa min;
El: 30% min;
or
(ii) for steel grade with Nb microalloying comprising
YS:380 MPa min;
UTS:500 MPa min;
El:32% min;
And
(c) producing said normalized cylinder therefrom following a normalizing
temperature in the range of 880°C to 900°C.
6. A process for the manufacture of high strength hot rolled steel normalized LPG gas
cylinder as claimed in claim 5 comprising
(i) obtaining the hot rolled coils maintaining finishing and coiling temperature in the
range of 880 ± 10 °C and 680 + 10 °C.
(ii) using said hot rolled coils of 2.4-2.6 mm thickness to fabricate LPG cylinders and
then subjecting to said normalizing treatment at the normalizing temperature range
of 880-900°C to obtain the high strength LPG gas cylinder.
7. High strength hot rolled steel normalized LPG gas cylinder as claimed in anyone of
claims 1 to 6 wherein amount of silicon is selectively maintained to ensure the higher
strength and better formability in product.
8. High strength hot rolled steel normalized LPG gas cylinder and a process for its
manufacture substantially as hereindescribed with reference to the accompanying
figure and example.

ABSTRACT

There is disclosed high strength hot rolled steel normalized LPG gas cylinder and a
process for its production. More particularly, the present invention is directed to
providing two grades of steel with or without Nb microalloying, having high yield
strength of 265 MPa min. and 295 MPa min. respectively after normalizing of formed
cylinders. This is achieved through careful selection of steel composition, by adopting
an integrated approach for development of suitable alloy chemistry and hot rolling
parameters for the steel grades to produce HR coils of desired thickness and
selectively adopting normalizing temperature for formed cylinders obtained thereof.