The present invention relates to low alloy ultra-high strength (LAUS) steels and in
particular to low alloy ultra-high strength (LAUS) steel with superior combinations of
strengths and toughness.
It is presently known in the art to produce LAUS steels (incorporating) good amount of
costly alloying elements like Ni & Mo. The use of costly alloying elements make this
class of hardenable steels quite costly. On the other hand the addition of Ni & Mo are of
vital importance in order to impart adequete toughness to this class of steel in hardened
& tempered (H & T) condition.
The composition details of some known commercial grades of LAUS steel presently
available are provided hereunder in TABLE I.
The above detailed LAUS steels presently available (TABLE-I) suffer from either of the
two disadvantages 1) the same is expensive or 2) lack in required toughness. In such
known LAUS steels poor toughness is primarily because the source of strength is
obtained from medium carbon content i.e. 0.23% - 0.5%, as shown for grades AISI (Si -
Modified), D-6a/D6ac, HY - TUF, AISI-4130, AISI 4140, AISI 4340 (Table-I). On the
other hand when toughness is improved they become expensive (BP 25) due to the
required
i) addition of costly alloying elements Ni & Mo in substantial amount,
ii) additional processing cost towards refining in order to make the steels ultra
clean.
Added to the above associated disadvantages of existing LAUS steels these steels
generally have high carbon equivalent in the range of 0.75 - 1.00 indicating poor
weldability.
It is thus the basic objective of the present invention to provide a cost effective LAUS
steel alloy composition and process for manufacture the same.
-y
Another objective of the present invention is to provide for LAUS steel which would
achieve the desired toughness but would not be expensive to obtain.
Yet further object of the present invention is directed to the provision of LAUS steel
which would have

iii) Flexibility in terms of impurity contents i.e. both S & P to the tune of 0.018% max.
Yet further object is directed to the provision of LAUS steel with good toughness from
lower content of Ni (<0.75%) & Mo(<0.30%)
Yet further object of the present invention is to provide LAUS steel which would be
simple to manufacture and easy to obtain.
Yet further object of the present invention is directed to the process of LAUS steel and a
process for producing the same which would have good combination of strength,
ductility & toughness with XS>1050 Mpa; UTS>1250 Mpa; % Elongation > 7.5; CE<0.8
and CVN > 40 J at Room Temperature.
Thus according to the present invention there is provided a synergistic LAUS steel
composition comprising :
Carbon 0.18 to 0.22% by wt.
Manganese 1.00 to 1.30% by wt.
Silicon 0.95 to 1.00% by wt.
Cromium 1.25 to 1.50% by wt.
Nickel 0.65 to 0.75% by wt.
Molybdenum 0.25 to 0.30% by wt.
Vanadium 0.04% to 0.12% by wt.
Aluminium 0.04% to 0.09% by wt.
Sulphur upto 0.018% by wt. and
Phosphorous upto .018% by wt.
Fe - Balance
According to another aspect of the present invention there is provided a process for
producing LAUS steel comprising :
A) providing the cast stock comprising alloying elements and minor constituents
comprising
Carbon 0.18 to 0.22% by wt.
Manganese 1.00 to 1.30% by wt.
Silicon 0.95 to 1.00% by wt.
Cromium 1.25 to 0.75% by wt.
Nickel 0.65 to 0.75% by wt.
Molybdenum 0.25 to 0.30% by wt.
Vanadium 0.04% to 0.12% by wt.
Aluminium 0.04% to 0.09% by wt. and minor constituents/residual elements
comprising Sulphur upto 0.018% by wt. and
Phosphorous upto .018% by wt.
B) hot working of the alloying element comprising forging/rolling ;
C) annealing and cold shaping of the stock ;
D) subjecting the cold shaped alloy to heat treatment comprising atleast the steps of
hardening followed by tempering to thereby provide for the LAUS steel having a carbon
equivalent of <0.8, UTS > 1250 Mpa, % elongation > 7.5, CVN values > 40 J at room
temperature
Preferably, the process of producing LAUS steel of this invention comprise
A) said step of providing the said cast stock of alloying elements comprising :
a) selection of raw materials to achieve the desired composition of the cast;
b) complete deoxidization of the melt with Si, Mn and Al ;
c) analyzing the melt and making final addition of alloying elements (LadleA/AD
treatment is preferred) to thereby achieve the desired chemical composition given
below:

d) Ingot casting by bottom pouring practice or casting of blooms/billets/slabs through
concast.
B) said step of hot working of the alloying elements as cast stock comprising :
initial slow heating of cast stock if the stock input temperature is below 500°C followed
by subjecting the same to step of forging above 900 C following preferably
i) initial press forging of ingots above 950°C followed by hammer forging, if
required, above 900°C.
ii) rolling preferablycomprising
a) primary rolling of ingots/as-cast stock above 1000°C and
b) rolling of blooms/billets/slabs above 900°C.
C) said step of annealing and cold shaping comprising
i) annealing of the stock comprising selecting the actual annealing temperature
depending upon the shape & size of stock, annealed hardness required, subsequent
processing required and end uses...
ii) subjecting the stock to cold shaping selected from various cold shaping operations
such as bending, machining, cutting, shearing and the like.
D) said step of heat treating of the alloy comprising :
i) step of hardening comprising
a) solution treatment of the stock at austenitizing temperature of 880-950°,
preferably under controlled furnace temperature ; and
b) quenching in oil/water
ii) step of tempering comprising :
tempering at temperatures of 200 - 350°C for a period of 1 hr - 3 hrs
Normalizing operation prior to hardening is optional and can be carried out to further
improve the mechanical properties of the steel components.
The heat treated alloy is subsequently finally shaped by
grinding/precision machining/bending/shaping etc.
In the above process of the invention the important parameters of heat treatment i.e.
soaking period & temperature for all the heat treatment operations are dependent upon
various factors as mentioned below :
i) alloy chemistry vis-a-vis AC1, AC3 etc.
ii) type of furnace.
iii) rate of heating.
iv) shape & size of the stock.
v) initial condition & the structure of the stock.
vi) final properties required / applications envisaged.
vii) critical properties for end use etc.
Also, the controlled furnace atmosphere although not a must is always preferable in
order to eliminate phenomena like scaling, decarburization, penetration of oxygen in the
surface etc.
Selection of quenchant is dependent upon the shape & size of the stock, critical aspects
of the final properties required etc.
Following the above disclosed process it is thus possible to provide LAUS steel which
would have the desired toughness and yet will be cost effective to obtain. Importantly,
the product properties achieved following the process of manufacture of the LAUS steel
of the invention include
i) CE<0.8
ii) UTS « 1400 Mpa, % ELongation » 10%, CVN » 45 J at Room Temperature.
iii) YS« 1200 Mpa
The details of the invention, its objects and advantages will be explained in greater
detail hereunder with reference to the following non-limiting exemplary embodiment:
Experimental alloys were obtained following the process of this invention with the
composition details as provided in TABLE - II hereunder:
TABLE - II

In the above disclosed experimental alloys Examples I to 4V are ail experimental LAUS
alloy compositions while the LAUS alloy composition in accordance with the inventive
composition of present invention is defined under Example IV. The process of producing
the above experimental alloy compositions Examples I to IV are detailed hereunder:
For obtaining each of the alloy compositions under Examples I to'V the following
process was followed
Processing of the alloys :
Melting & casting :
i) Selection of suitable raw materials to achieve the final chemistry.
ii) Complete deoxidization of the melt with Si, Mn and Al.
iii) Checking analysis of melt and making final addition of alloying elements
(Ladle/VAD treatment is preferred) in order to achieve the chemical composition
as detailed in Table-ll above for the respective alloy components under
Examples I to IV.
iv) Ingot casting was carried out by bottom pouring practice.
Hot working :
Initial slow heating of as cast stock was carried out for the cast stock below 500°C
which was followed by
i) Forging comprising initial press forging of ingots above 950°C followed by
hammer forging above 900°C.
ii) Rolling comprising primary rolling of ingots/as-cast stock above 1000°C followed
by rolling of blooms/billets/slabs above 900°C.
Annealing & cold shaping :
i) Annealing comprising annealing of stock of suitable size at <700°C
ii) Coid Shaping of Stock following conventional processes.
Heat treatment:
Heat treatment was carried out in two steps following Hardening & Tempering wherein
the step of hardening comprised :
a) solution treatment of the stock at austenitizing temperature (880-950°) and
b) quenching in oil, followed by
said step of tempering comprising tempering in the temperature range of 200 -
350°C for a period of 1 hr - 3 hrs.
Final Shaping : The alloys thus obtained were finally shaped by grinding/precision
machining/bending/shaping etc.
The transformation temperature values and carbon equivalent of each of the
experimental alloys obtained under Examples I to IV above were tested and the results
are shown in TABLE-MI hereunder:
TABLE - III
VALUES ON TRANSFORMATION TEMPERATURES &
CARBON EQUIVALENT

As would be evident from the above Table-Ill the alloy of the invention under Example
IV showed desirable carbon equivalent of less than 0.8.
Further the CVN values (J/cm2) of the alloy & invention (Example IV) vis-a-vis other
experimental alloys (Examples I to III) were tested and the test results are reproduced
hereunder in TABLE - IV.

It would be evident from TABLE - IV above the alloy composition of the invention under
Eixample IV achieve desired CVN values of > 40 J at room temperature thereby
providing for a cost effective LAUS steel with superior combination of strength and
impact toughness.
The rolling/forging conducted at various temperatures demonstrated good hot
workability. Apart from working on the as cast structures the suitable hot working
temperature was found to be in the range of 900 - 950°C.
Further the tensile test was carried out on the alloy of the invention (Example IV) and
the results are provided hereunder in TABLE - V.
TABLE - V

T = Tempered
As evident from TABLE - V, the tensile test revealeo achievement of desired
combination of strength, ductility and toughness of YS > 1C50 Mpa, UTS > 1250 Mpa
and % elongation > 8%.
Thus the present invention provides for cost effective LAUS steel with following
properties :
Good impact toughness (CVN>40 J at RT) & Strength (>1250 Mpa).
Adequate flexibility on residuals (S & P each 0.018 max)
The invention provides for suitable combination of cheaper alloying elements like Si,
Mn & Cr to impart adequate impact toughness and provide for substitutes of costly
alloying elements (Ni & Mo).
WE CLAIM
1. A process for producing LAUS steel comprising :
A) providing the cast stock comprising of said alloying elements and minor constituents
selected from
Carbon 0.18 to 0.22% by wt.
Manganese 1.00 to 1.30% by wt.
Silicon 0.95 to 1.00% by wt.
Cromium 1.25 to 0.75% by wt.
Nickel 0.65 to 0.75% by wt.
Molybdenum 0.25 to 0.30% by wt.
Vanadium 0.04% to 0.12% by wt.
Aluminium 0.04% to 0.09% by wt.
Sulphur upto 0.018% by wt. and
Phosphorous upto .018% by wt.
Fe - Balance
B) hot working of the alloying element comprising forging/rolling ;
C) annealing and cold shaping of the stock ;
D) subjecting the cold shaped alloy to heat treatment comprising atleast subjecting the
same to step of hardening and tempering to thereby provide for the LAUS steel
having UTS > 1250 Mpa, % elongation > 7.5% CVN values of > 40 J at room
temperature.
2. A process for producing LAUS steel as claimed in claim 1 comprising
A) said step of providing the said cast stock of alloying elements comprising :
a) selection of raw materials to achieve the desired components of the cast;
b) complete deoxidization of the melt with Si, Mn and Al;
analyzing the melt and making final addition of alloying elements to thereby achieve the
said desired chemical composition of the cast ;
c) Ingot casting of the above desired chemical composition of the cast;
B) said step of hot working of the alloying elements as cast stock comprising :
initial slow heating of cast stock if the stock input temperature is below 500°C followed
by
i) initial press forging of ingots above 950°C followed by hammer forging, if required,
above 900°C.
ii) rolling comprising
a) primary rolling of ingots/as-cast stock above 1000°C and
b) rolling of blooms/billets/slabs above 900°C.
C) said step of annealing and cold shaping comprising
i) annealing of the stock comprising selection of the actual annealing temperature
depending upon the shape & size of stock, annealed hardness required, subsequent
processing required and end users.
ii) subjecting the stock to cold shaping selected from various cold shaping operations
such as bending, machining, cutting, shearing and the like.
D) said step of heat treating of the alloy comprising :
i) step of hardening comprising
a) solution Treatment of the stock at austenitizing temperature of 880-950°,
preferably under controlled furnace temperature ; and
b) quenching
ii) step of tempering comprising :
tempering at temperatures of 200 - 350°C for a period of 1 hr - 3 hrs
3. A process as claimed in anyone of claims 1 or 2 wherein said step of heat treatment
in step D comprise the step of normalizing the cold shaped alloy prior to subjecting
the same to hardening and tempering.
4. A process as claimed in anyone of claims 1 to 3 comprising subjecting the heat
treated alloy to final shaping following any one or more of grinding/precision
machining/bending/shaping and the like.
5. A process as claimed in anyone of claims 1 to 4 wherein the quenchent is selected
depend upon the shape & size of the stock, critical aspects of the final properties
required.
6. A process as claimed in anyone of claims 1 to 5 wherein said step of quenching
comprise quenching in water/oil.
7. A process as claimed in anyone of claims 1 to 6 wherein said step of ingot casting
comprise by bottom pouring practice or casting of blooms/billets/slabs through
concast.
8. A process as claimed in anyone of claims 1 to 7 wherein said addition of alloying
elements for adjustment of the chemical composition of the alloy is carried out using
LadleA/AD treatment.
9 A process for producing LAUS steel substantially as herein described and illustrated
with reference to the accompanying examples.

A process of preparing low ultra high strength (LAUS) steel comprising:
providing the cast stock comprising alloying elements and minor constituents comprising
Carbon 0.18 to 0.22% by wt, Manganese 1.00 to 1.30% by wt, Silicon 0.95 to 1.00% by
wt, Cromium 1 25 to 0.75% by wt, Nickel 0.65 to 0.75% by wt, Molybdenum 0.25 to
0.30% by wt, Vanadium 0.04% to 0 12% by wt; Aluminium 0.04% to 0.09% by wt and
minor constituents/residual elements comprising Sulphur upto 0.018% by wt And
Phosphorous upto 018% by wt; hot working of the alloying element comprising
forging/rolling : annealing and cold shaping of the stock ; subjecting the cold shaped
alloy to heat treatment comprising atleast the steps of hardening followed by tempering to
thereby provide for the LAUS steel having a carbon equivalent of <0.8, UTS > 1250
Mpa. % elongation > 7.5, CVN values > 40 J at room temperature.
A process such that the LAUS steel produced has good combination of strength, ductility
and hardness and is cost effective.