FIELD OF THE INVENTION:
The present invention relates to a cryogenic steel alloy grade with lean chemistry
for room temperature and low temperature products. The invention is specifically
related to manufacture storage structure for LNG (-165°C), Liquid Oxygen (-
186°C), Hydrogen Sulphide (-45°C), Ethane/Ethylene (-105°C),
Propane/Propylene (-50°C), Ammonia (-35 °C), Butane (-10°C), LPG (-50°C),
Liquid Carbon dioxide (-20°C) gases.
BACKGROUND OF THE INVENTION:
There is a great need to store and transport pressurized, volatile fluids at
cryogenic temperatures, there is a further need of containers for safely and
economically storing and transporting volatile fluids with high vapor pressure,
(such as methane, ethane, and propane). Since there are very high risks safety
issues associated with transportation and storage of these gases and volatile
liquids and hence, the containers should be made up of steel grades which can
withstand at very low temperatures. Steels used for such low-temperature
applications should possess high strength, ductility and toughness. However, at
cryogenic temperature, steel alloys loose strength and become brittle. Nickel
when used as alloying element contributes towards good low temperature
properties but nickel is an expensive metal and also very scarce.

Among various common alloying elements, Manganese is a good substitute for
Nickel. Mn is comparatively less expensive and available. Also, Mn has
metallurgical similarity to Nickel in terms of microstructure and phase relationship
of Iron based alloys.
The steels that are used for cryogenic applications have very expensive alloying
elements such as high content of Nickel, Chromium, Molybdenum, Columbium
and they need very complex heat treatment to achieve desired cryogenic
properties. When it comes to rebar applications such as cold storage structures
such expensive alloy elements can cause the rebar price very high and since the
heat treatment is complex they cannot be used for rapid production of huge
quantity.
In light of the above problem, there is need of steel alloy grades that has a lean
chemistry and possesses high strength at low temperatures.
OBJECTS OF THE INVENTION:
It is therefore an object of the invention to develop a cryogenic steel alloy grade
with lean chemistry for room temperature and low temperature products.

Another object of the invention is to propose a cryogenic steel alloy grade with
lean chemistry applicable for room temperature and low temperature products,
which being ambient air cooled eliminates thermo mechanical treatment (TMT
treatment).
A further object of the invention is to propose cryogenic steel allow grade
exhibiting predominantly upper bainitic with some lower bainitic.
A still another objective of the invention is to develop cost effective solution for
manufacture alloy steel for low temperature applications.
Another objective of the invention is to develop an alloy steel for low
temperature applications that is essentially free of Nickel.
SUMMARY OF THE INVENTION
A cryogenic alloy steel grade with lean chemistry applicable is produced by
processing a steel composition consisting of:


The developed steel alloy has 500MPAa yield strength and 800 MPa UTS with
40% total elongation at room temperature. The notch strength ratio (Notch
sensitivity ratio NSR) at room temperature is 3.1.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a microstructure of the steel grade according to the invention.
Figure 2 shows a photographic view of a notched sample formed of inventive
steel, and an unnotched sample with cup and cone fracture at liquid nitrogen
test temperature.
Figure 3 shows a fractograph emphasizing the cleavage fracture of a notched
specimen at liquid nitrogen temperature.
Figure 4 - shows a fractograph emphasizing dimpled fracture appearance at
liquid nitrogen test temperature.
Figure 5 - shows a graphical representation of stress - strain behaviors of a
notched and an unnotched sample.
Figure 6 - shows a process flow chart of producing the inventive steel grade.

Table 1 - shows chemical composition of air-cooled hot-rolled sheet used in the
invention.
DETAIL DESCRIPTION OF THE INVENTION
The alloy according to the invention has lean chemistry as given in the table 1
below:
Table 1: chemical composition of air-cooled hot-rolled sheet used in the
invention.

The above specified composition after reheating the billet and subsequent
forging and rolling and air cooling provides excellent room temperature and
cryogenic properties. Such process can be used for rapid and bulk manufacturing
of rebars, plates, pipes to reduce the cost of production.

The heat is forged and rolled as per the flow chart shown in Figure 6. Before
rolling, the billet is heated to 1250°C and forging is completed above 800°C. The
plate thickness is reduced further to 16mm thick using hot rolling. Just like
forging the plates are hot rolled and finish rolling temperature was above 800°C.
Both after forging and rolling, the slabs are left in the ambient air. The samples
are analyzed after forging and rolling. The target chemistry is achieved.
The microstructures of the billet after forging, rolling and ambient air cooling is
shown in Figure 1. The microstructure contains predominantly upper bainite with
some lower bainite. The upper bainite is coarse where as lower bainite is very
fine. They are visible throughout the structures in the forms of plates (Sheaves).
Titanium addition is intended to tie up the carbon atoms so titanium carbide
forms and boron is added to decorate the grain boundaries so the pearlites so
not shoot out from the grain boundaries. Boron also helps in slip transfer across
grain boundaries which in turn improve ductility. Small quantity of Niobium is
added as a carbonitride former meaning the nitrogen and residual carbon gets
further tied up and thereby improves the strength by precipitation hardening.
Manganese and Silicon is added to the alloy as solid solution strengthener.
Tensile specimens with notch and unnotched are made and subsequently tested
at room temperature and at liquid nitrogen temperature. Notched specimens had
45 degree notch all around in the middle of the tensile specimens made as per
the ASTME8 standard with a 0.25 mm root radius. The notched specimens failed

in a brittle manner both in room temperature and in liquid nitrogen test
temperature. However, the unnotched specimens failed in a cup and cone
fracture mode in both situations suggesting that the material is ductile in room
temperature and at liquid nitrogen temperature.
This fact is further substantiated with the fractography. Figure 2 shows the
macrophotograph of the specimens and shows the specimens failed in cup and
cone fracture mode in un-notched and brittle manner is notched mode. The SEM
fractograph as depicted in Figure 3 and 4, shows the dimpled fracture of un-
notched specimens at liquid nitrogen temperature. However, the notched
specimens fracture was in a cleavage manner as is shown in Figure 4.
The notch sensitivity ratio (NSR) is measured from the obtained stress strain
diagram which is shown in Figure 5. The NSR values are shown and it is way
above the value (slightly greater than 1) that is required to be notch tough. The
value for room temperature is 3.1 and at liquid nitrogen temperature it is 1.9.
Around 500MPa yield strength and 800MPa UTS and total elongation of 40% is
obtained at room temperature. The notched yield strength of 1550MPa is
obtained at room temperature. The yield strength at liquid nitrogen temperature
for un-notched samples was 800MPa and UTS around 960MPa and total
elongation around 45% were obtained. The notched yield strength at liquid

nitrogen temperature turned out to be around 1550 which is quite similar to the
room temperature notched yield strength.
Thus the specification proves the efficacy of this alloy for cryogenic product
application services such as rebars, plates of various thickness, wirerods/wires,
pipelines.

WE CLAIM:
1. A cryogenic alloy steel having 500MPa yield strength and 800 MPa ultimate
tensile strength (UTS) with 40% total elongation at room temperature, the
cryogenic alloy steel comprising:

balance being Iron.
2. The cryogenic alloy steel as claimed in claim 1, wherein the notch strength
ratio (Notch sensitivity ratio NSR) at room temperature is 3.1.
3. The cryogenic alloy steel as claimed in claim 1, wherein the notch sensitivity
ratio (NSR) at liquid nitrogen temperature is approximately 1.9.
4. The cryogenic alloy steel as claimed in claim 1, wherein the yield strength
and ultimate tensile strength of the cryogenic alloy steel at liquid nitrogen
temperature are 800MPa and 950MPa respectively with 45% total elongation.

5. The cryogenic alloy steel as claimed in claim 1, wherein the microstructure of
the composition exhibits predominantly upper bainite with minimum of lower
bainite.
6. The cryogenic alloy steel as claimed in claim 5, wherein the upper bainite is
Coarse and the lower bainite is fine.
7. The cryogenic alloy steel as claimed in claim 1, wherein the cryogenic alloy
steel is essentially free of Nickel.
8. An alloy steel for low temperature applications, prepared by a process
comprising the steps of:
providing an alloy steel composition comprising of 0.093 wt%
carbon, 1.55 wt% Manganese, 0.011 wt% Phosphorous, 0.240
wt% , Silicon, 0.028 wt% Niobium, 20 ppm Boron, 0.002 wt%
Aluminum, 0.015 wt% Titanium, 72 ppm Nitrogen and balance
being Iron;
heating the alloy steel composition to 1250°C; and
subjecting the alloy steel composition to forging and rolling at
temperature above 800°C and cooling in air.

9. The alloy steel as claimed in claim 8, wherein the process of preparation of
the alloy steel does not involve thermo mechanical treatment (TMT
treatment) and can be made with ambient temperature cooling.
10. The alloy steel as claimed in claim 8, wherein the alloy steel has 500MPa
yield strength and 800 MPa ultimate tensile strength (UTS) with 40% total
elongation at room temperature.
11. The alloy steel as claimed in claim 8, wherein the alloy steel has are 800MPa
yield strength and 950MPa ultimate tensile strength with 45% total elongat-
ion at liquid nitrogen temperature.

ABSTRACT

The invention relates to a cryogenic alloy steel grade with lean chemistry
applicable for room temperature and low temperature products produced by
processing a steel composition consisting of:

wherein the alloy has 500MPa yield strength and 800 MPa UTS with 40% total
elongation at room temperature, the notch strength ratio (Notch sensitivity
radio, NSR) at room temperature being 3.1.