HIGH STRENGTH MILITARY STEEL

ACIER A HAUTE RESISTANCE DE QUALITE MILITAIRE

English Abstract

A low cost steel having high hardness, high strength, and high impact
toughness
is applicable for military articles such as armor plates, bodies of deep
penetrate bombs,
and missiles. After oil quenching, refrigerating, and low tempering, the new
steel
consisting of (% wt.): C=0.35, Cr=1.32, Mo=0.35, W = 0.52, Ni=2.66, Mn=0.85,
Cu=0.51, Si=0.83, V=0.26, Ti=0.12, the balance essentially Fe and incidental
impurities
has HRC of 55, UTS of 301 ksi, YS of 233 ksi; Charpy V-notch impact toughness
energy of 26 ft-lbs.

Claims

The Embodiments Of The Invention In Which An Exclusive Property Or Privilege
Is
Claimed Are Defined As Follows:



1. A low cost high hardness, high strength, and high impact toughness steel
comprising by weight about: 0.3 to 0.45% of C; 0.1 to 3.0% of Cr; 0.1 to 0.55%
of Mo; 0.1
to 2.0% of W; 0.1 to 3.5% of Ni; 0.1 to 1.0% of Mn; 0.1 to 1.0% of Si; 0.1 to
0.6%% of Cu;
0.02 to 0.2% of Ti or Nb; 0.1 to 0.55% of V; and a balance of Fe and
incidental impurities.

2. The low cost steel recited in claim 1 wherein after quenching and low
tempering, said steel has a hardness of Rockwell C52 to 54, an ultimate
tensile strength
of 285 to 295 ksi, a yield strength of 215 to 220 ksi, an elongation of 13 to
14 %, a
reduction of area of 48 to 50%, and Charpy v-notch impact toughness energy of
26 to 30
ft-lb at the room temperature.

3. The low cost steel recited in claim 1 wherein after quenching,
refrigerating
and low tempering, said steel has a hardness of Rockwell C54 to 56, an
ultimate tensile
strength of 290 to 305 ksi, a yield strength of 225 to 235 ksi, an elongation
of 13 to 14 %,
a reduction of area of 47 to 50%, and Charpy v-notch impact toughness energy
of 26 to
28 ft-lb at the room temperature.

4. The low cost steel recited in claim 1 wherein after quenching and a second
hardening by high tempering, said steel has a hardness of Rockwell C48 to 50,
an
ultimate tensile strength of 240 to 250 ksi, a yield strength of 225 to 235
ksi, an
elongation of 10 to 11 %, a reduction of area of 48 to 50%, and Charpy v-notch
impact
toughness energy of 20 to 22 ft-lb at the room temperature.

5. A low cost high hardness, high strength, and high impact toughness steel
has a fine dispersed tempered martensite microstructure comprised of small
packets of
martensitic laths, fine titanium carbides as centers of growth of said
martensitic lathes,
and retained austenite; boundaries of said packets are free of carbides; said
steel
comprised of by % weight about: 0.3 to 0.45% of C; 0.1 to 2.0% of W; 0.1 to
0.55% of V;
0.02 to 0.2% of Ti or Nb; at most 9.65% of sum of Cr, Mo, Ni, Mn, Si, Cu; and
a balance
of Fe and incidental impurities.

6. The low alloy steel recited in claim 5 wherein said steel comprises by


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weight of about: 0.37% of C; 1.25% of Cr; 0.51 of Mo; 0.51% of W; 3.45% of Ni;
0.82% of
Mn; 0.91 of Si; 0.52% of Cu; 0.24% of V; 0.11 % of Ti, and a the balance
essentially Fe
and incidental impurities, and after austenitizing at about 1900 F, oil
quenching,
refrigerating at about -60 F, and tempering at about 400 F, said steel has a
hardness of
about Rockwell C 54, an ultimate tensile strength of about 296 ksi, a yield
strength of
about 234 ksi, and Charpy v-notch impact energy of about 27.5 ft-lb at the
room
temperature.

7. The low alloy steel recited in claim 5 wherein said steel comprises by of
about: 0.35% of C; 1.32% of Cr; 0.35 of Mo; 0.52% of W; 2.66% of Ni; 0.83% of
Mn; 0.85
of Si; 0.51% of Cu; 0.26% of V; 0.12% of Ti; and a balance essentially of Fe
and
incidental impurities, and after austenitizing at about 1900 F, oil quenching,
refrigerating
at about -60 F, and tempering at about 450 F, said steel has a hardness of
about
Rockwell C 55, an ultimate tensile strength of about 301 ksi, a yield strength
of about 233
ksi, and Charpy v-notch impact energy of about 26 ft-lb at the room
temperature.

8. The low alloy steel recited in claim 5 wherein said steel comprises by
weight of about: 0.32% of C; 1.24% of Cr; 0.38 of Mo; 0.82% of W; 2.52% of Ni;
0.86% of
Mn; 0.87 of Si; 0.53% of Cu; 0.25% of V; 0.11% of Ti; a balance of Fe and
incidental
impurities and after austenitizing at about 1900 F, oil quenching,
refrigerating at about -60
F, and low tempering at 420 F, said steel has a hardness of about Rockwell C
55, an
ultimate tensile strength of about 298 ksi, a yield strength of about 229 ksi,
and Charpy v-
notch impact toughness energy of about 26 ft-lb at the room temperature.

9. A low cost high strength steel has a microstructure comprised of a fine
dispersed titanium, vanadium, and complex tungsten carbides in a ferrite -
martensite -
retained austenite matrix; said steel comprising by weight of about: 0.3 to
0.45% of C; 0.1
to 2.0% of W; 0.1 to 0.55% of V; 0.02 to 0.2% of Ti or Nb; at most 9.65% of
sum of Cr,
Mo, Ni, Mn, Si, Cu; and a balance of Fe and incidental impurities

10. The low cost steel recited in claim 9 wherein said steel comprises by
weight of
about: 0.37% of C; 1.61 of Cr; 0.49 of Mo; 1.23% of W; 0.54% of Ni; 0.41% of
Mn; 0.75 of
Si; 0.29% of Cu; 0.54% of V; 0.11 % of Ti; 1.61 of Cr; and a balance of Fe and
incidental
impurities, and after austenitizing at about 1900 F, oil quenching, tempering
at 1070 F
followed by tempering at 1000 F, said steel has a hardness of about Rockwell C
49, an


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ultimate tensile strength of about 250 ksi, a yield strength of about 234 ksi,
and Charpy v-
notch impact toughness energy of about 20.5 ft-lb at the room temperature.


11. The low cost steel recited in claim 9 wherein said steel comprises by
weight
of about; 0.35% of C; 1.43 of Cr; 0.52 of Mo; 1.35% of W; 0.69% of Ni; 0.43%
of Mn; 0.72
of Si; 0.31% of Cu; 0.52% of V; 0.12% of Ti; and a balance of Fe and
incidental impurities
and after austenitizing at about 1900 F , oil quenching, tempering at about
1070 F
followed by tempering at about 1000 F, said steel has a hardness of about
Rockwell C
49, an ultimate tensile strength of about 249 ksi, a yield strength of about
234 ksi, and a
room temperature Charpy v-notch impact toughness energy of about 21 ft-lb at
the room
temperature.



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Description

CA 02702515 2010-05-04

HIGH STRENGTH MILITARY STEEL
FIELD OF THE INVENTION
This invention relates to a low cost high hardness, high strength, high impact
toughness military steel and more particularly to a military steel which is an
improvement
over Eglin steel.

BACKGROUND OF THE INVENTION
Large amounts of expensive high strength, high impact toughness military
steels
are used for purposes such as bunker buster bombs, missiles, tank bodies and
aircraft
landing gears.
Eglin Steel was a joint effort of the US Air Force and Ellwood National Forge
Company program to develop a low cost replacement for the expensive high
strength and
high toughness steels, AF-1410, Aermet-100, HY-180, and HP9-4-20/30. One
application of Eglin steel was the new bunker buster bombs, e.g. the Massive
Ordnance
Penetrator and the improved version of the GBU-28 bomb known as EGBU-28.
A high-performance steel is required to survive the high impact speeds during
deep penetration. Eglin steel was planned for a wide range of other
applications, from
missile and tank bodies to machine parts.
One shortcoming of Eglin steel is the limited mechanical properties for large
manufactured products, as follows:
= Hardness (HRC), up to 48
= Ultimate tensile strength (UTS), up to 250 ksi
= Yield strength (YS) up to 210 ksi
Another shortcoming of Eglin steel is that the structural properties from
impact
tests of large articles, such as bunker buster bombs vary somewhat below the
impact test
results of smaller laboratory products. The discrepancies in results are due
to difficulties
with heat treating of Eglin steel.
The present invention overcomes the shortcomings of Eglin steel by providing a
lower cost steel that has higher mechanical properties and consistent results
from heat
treating. The improved steel has a medium carbon content, low nickel,
molybdenum, and
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CA 02702515 2010-05-04

tungsten contents, and the strong carbide forming elements vanadium and
titanium or
niobium. The new alloying concentrations of vanadium, titanium or niobium, and
tungsten
affect the conditions of melting, processing, and heat treatment and as a
result, it's
higher mechanical properties.
One benefit of the new steel is higher performances of armor plate, deep
penetrating bombs and missiles. Another benefit is that, in the alternative,
less steel is
required to match the performance of Eglin steel.
Another benefit of the invention is smaller amounts are required of the
expensive
elements nickel (Ni) and tungsten (W). The invention requires at most 3.5% of
Ni and
2% of W, versus of 5 % of Ni and 3.25% of W for Eglin Steel.

SUMMARY OF THE INVENTION
The present invention is a lower cost military steel ("new steel") with higher
levels
of hardness, strength, and impact toughness than Eglin steel. The higher
mechanical
properties are due to optimizations of the following factors:
= selections of alloying compositions that supply high hardness, strength, and
impact
toughness
= selections of critical temperatures
The hardness, strength and impact toughness of the invention was verified by
the
melting of laboratory and industrial scale ingots, processing of ingots from
the melt,
production of articles from the ingots, heat treating of the articles and
mechanical testing
of the articles.
The new steel differs from Eglin Steel by the following features:
= A microstructure of tempered dispersed lath martensite consisting of small
packets
of martensite laths grown on fine carbides and retained austenite, and packet
boundaries free of carbides after quenching, low tempering or quenching,
refrigerating, and low tempering.
= After quenching and low tempering, a Rockwell hardness of C52-54, an
ultimate
tensile strength of 285-295 ksi, a yield strength of 215-220 ksi, an
elongation of 13
- 14 %, a reduction of area of 48 - 50 %, and a Charpy V-notch impact
toughness
energy of 26 - 30 ft-lb.

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CA 02702515 2010-05-04

= After quenching, refrigerating, and low tempering, a Rockwell hardness of
C54-56,
an ultimate tensile strength of 290-305 ksi, a yield strength of 225-235 ksi,
an
elongation of 13 - 14 %, a reduction of area of 47 - 50 %, and a Charpy V-
notch
impact toughness energy of 26 - 28 ft-lb.
= After quenching and a second hardening by high tempering a microstructure
consisting of a fine dispersion of titanium carbide (TiC), vanadium carbide
(VC),
and complex tungsten carbides, (MW)XCy in a ferrite - martensite - retained
austenite matrix
= After quenching and a second hardening by high tempering, a Rockwell
hardness
of C 48 - 50, an ultimate tensile strength of 240-250 ksi, a yield strength of
225-
235 ksi, an elongation of 10 - 11 %, a reduction of area of 48 - 50 %, and a
Charpy V-notch impact toughness energy of 20-22 ft-lb
= A high ductility and high formability during hot forging or rolling
= A use of only homogenized and recrystallization annealing without
normalizing for
the low tempered new steel
= A sum of alloying elements of that is less than the sum of alloying elements
of
Eglin steel
= Cost of charge materials of the new steel is less than cost of charge
materials of
Eglin steel.
The chemical compositions and mechanical properties of the invention and Eglin
steel are compared in Fig.1 and Fig.2.

Brief Description of the Drawings
Fig. 1 compares the chemical compositions of the new steel and Eglin Steel.
Fig.2 compares the mechanical properties at room temperature of Eglin Steel
and
the invention after quenching and low tempering; after quenching,
refrigerating, and low
tempering; and after quenching and a second hardening by high tempering.

DETAILED DESCRIPTION OF THE INVENTION
The composition of the invention is comprised of: carbon (C); ferrite
stabilizing
chromium (Cr), molybdenum (Mo); silicon (Si); strong carbide forming tungsten
(W),
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CA 02702515 2010-05-04

vanadium (V), and titanium (Ti) or niobium (Nb); austenite stabilizing nickel
(Ni),
manganese (Mn), copper (Cu); iron (Fe) and incidental impurities.
The carbon (C) content of 0.30 to 0.45% wt. supports the forming of carbides
of
tungsten (W), vanadium (V), titanium (Ti) or niobium (Nb), and complex
carbides as
centers of growth of martensite laths forming the microstructure of tempered
dispersed
lath martensite with retained austenite.
The chromium (Cr) content of 1.0 to 3.0% wt. increases strength, hardenability
and
temper resistance.
The molybdenum (Mo) content of 0.1 to 0.55 % wt. improves hardenability,
eliminates reversible temper brittleness, resists hydrogen attack & sulfur
stress cracking,
and increases elevated temperature strength.
The nickel (Ni) content of 0.1 to 3.5 % wt. supplies impact toughness.
The manganese (Mn) is a strong deoxidizing, and austenite stabilizing element.
It's content is 0.1 to 1.0% wt.
The silicon (Si) strengthens the steel matrix by increasing the bonds between
atoms in a solid solution. It protects the grain boundary from the growth of
carbides,
which decrease the toughness of the new steel. The content of Si is 0.1 to 1.0
% wt.
The copper (Cu) improves corrosion resistance, ductility, and machinability.
The
preferred content of Cu is 0.1 to 0.6 % wt.
The tungsten (W) forms fine dispersed carbides, eliminates reversible temper
brittleness, and increases hardness and temperature resistance. Its content is
0.1 to
2.0% wt.
The vanadium (V) affects on the structure and properties of the new steel in
several ways. It forms finely dispersed particles of carbides in austenite
which control the
size and shape of grains by precipitating vanadium based, finely dispersed
secondary
carbides during high tempering and by affecting the kinetic and morphology of
the
austenite-martensite transformation. The concentration of V is 0.1 to 0.55 %
wt.
The titanium (Ti) and niobium (Nb) are more active carbide forming elements
than
vanadium (V). Small concentrations of the strong carbide forming titanium (Ti)
or niobium
(Nb) do not affect the kinetics of phase transformations. A basic function of
these
elements is to inhibit austenite grain growth at high temperatures during
heating. One
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CA 02702515 2010-05-04

element Ti or Nb is a part of the new steels. The concentration of Ti or Nb is
0.02 to 0.2
% wt.
The balance of the new steel is iron (Fe) and incidental impurities.
Industrial scale ingots of the new steel were initially melted in an open
induction
furnace and then were melted in an electro-arc furnace (EAF), utilizing scrap
and
conventional charge materials. From the EAF, the steel was transported to a
ladle
refining furnace (LRF). In LRF the steel was reheated, refined from
impurities, the
necessary ingredients were added, and the steel was homogenized. Thereafter,
the steel
was transported to a vacuum de-gas station to remove hydrogen and nitrogen.
Liquid
steel was poured at 2950 to 3000 OF into iron molds. Ingots were subjected to
homogenized annealing at 2100 to 2150 OF for 12 - 24 hours. Afterwards, the
ingots
were heated to 2100 to 2150 F and forged to final size blanks. The blanks were
subjected to re-crystallization annealing at 1080 to 1150 OF for 8 - 18 hours.
Some
ingots were subjected to normalizing at 1925 to 1950 OF for 8 -12 hours and
high
tempering at 1100 to 1120 OF for 8 - 12 hours to eliminate the banding
microstructure
after the severe hot forging.
After austenitizing at 1875-1925 OF and further quenching and low tempering or
quenching, refrigerating, and low tempering, a tempered martensite
microstructure
consisting essentially of martensitic lathes, fine titanium carbide, TiC as
centers of growth
of the martensitic laths, and retained austenite was formed. The boundaries of
the
packets were free of carbides.
The second hardening of the new steel by high tempering consists of heating at
950-1200 OF for 5 - 7 hours to precipitate vanadium carbide, VC and complex
tungsten
carbides, (MW)XCy as a fine dispersion.

After quenching and second hardening by high tempering, the new steel had a
microstructure consisting of fine dispersion titanium carbide, TiC, vanadium
carbide, VC,
complex tungsten carbides, (MW)X Cy in a ferrite - martensite - retained
austenite matrix.

True production cost of the new steel is difficult to assess. However, based
on
data of the London Metal Exchange (LME), dated April, 2009, cost of charge
materials of
the new steel is at most 3,150 USD per metric ton, versus of Eglin steel at
most 3,850
USD per metric ton.

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CA 02702515 2010-05-04
EXAMPLES OF THE NEW STEEL

Example I
The composition of the new steel is comprised of (%, wt): C=0.37, Cr=1.25,
Ni=3.45, Mn=0.82, Cu=0.52, V=0.24, Si=0.91, Mo=0.52, Ti=0.1 1, and a balance
of Fe
and incidental impurities.
The new steel has the following critical temperatures, upper critical
temperature
AC3, low critical temperature AC1 , and martensite start temperature MS :

AC3 =1465 OF, Act =1260 OF, MS = 440 OF.

Processing of laboratory scale ingots of the new steel consists of:
= Homogenized annealing at 2100 OF for 6 hrs and air cooling
= Hot rolling with a start temperature of 2150 IF and a finish temperature of
1850 IF and air cooling
= Recrystallization annealing at 1100 IF for 4 hrs
Test specimens of the new steel are heat treated in the following manner:
= Austenitizing at 1900 OF for 60 min.
= Oil quenching for 2.5 min. and further air cooled
= Refrigerating at -60 OF for 60 min.
= Tempering at 400 OF for 4 hrs.
The new steel has the following room temperature mechanical properties:
HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb)
54 296 234 14 50 27.5

The new steel has a tempered martensite microstructure consisting of
martensitic
laths, fine titanium carbides, TiC as centers of growth of the martensitic
laths, and at most
14% of retained austenite. The boundaries of the packets are free of carbides.

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CA 02702515 2010-05-04
Example2
The composition of the new steel is comprised of (%, wt): C=0.35, Cr=1.32,
W = 0.52, Ni=2.66, Mn=0.85, Cu=0.51, V=0.26, Si=0.83, Mo=0.35, Ti=0.12, and a
balance of Fe and incidental impurities.
The new steel has the following critical temperatures:
AC3 =1475 OF, Act =1270 OF, MS = 485 IF.

Laboratory scale ingots of the new steel are processed the same as Example 1.
Test specimens of the new steel are heat treated in the following manner:
= Austenitizing at 1900 IF for 60 min.
= Oil quenching for 2.5 min. and further air cooled
= Refrigerating at -60 IF for 60 min.
= Tempering at 450 OF for 4 hrs.
The new steel has the following room temperature mechanical properties:
HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb)
55 301 233 13.5 49 26

The microstructure of the new steel is similar to the microstructure of
Examplel
and has a retained austenite at most 11 % wt.

Example3
The composition of the new steel is comprised of (%, wt): C=0.32 , Cr=1.24,
W=0.82, Ni=2.52, Mn=0.86, Cu=0.53, V=0.25, Si=0.87, Mo=0.38, Ti=0.11, balance
essentially Fe.
The new steel had the critical temperatures:
AC3 =1470 OF, AC1 =1265 OF, MS = 455 OF.

Laboratory scale ingots of the new steel had the same processing as in
Examplel.
Test specimens of the new steel was heat treated by the following mode:
= Austenizing at 1900 IF for 60 min.
= Oil quenching for 2.5 min. and further air cooled
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CA 02702515 2010-05-04

= Refrigerating at -60 OF for 60 min.
= Tempering at 420 IF for 4 hrs.
The new steel has the following room temperature mechanical properties:
HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb)
55 298 229 13.5 49 26
The new steel has a microstructure that is similar to the microstructure of
Example1 and has a retained austenite at most 9% wt.

Example 4
The composition of the new steel is comprised of (%, wt): C=0.37, Cr=1.61,
Ni=0.54, Mn=0.41, Cu=0.29, V=0.54, Si=0.75, Mo=0.49, W=1.23, Ti=0.11, and a
balance
of Fe and incidental impurities.
The new steel has the following critical temperatures:
AC3 =1555 OF, AC1 =1345 OF, MS = 565 OF .

Processing of laboratory scale ingots of the new steel is comprised of:
= Homogenized annealing at 2100 IF for 6hrs and air cooling
= Hot rolling with a start temperature of 2150 OF and a finish temperature of
1850 IF and air cooling
= Recrystallization annealing at 1150 IF for 4 hrs
= Normalizing at 1925 IF for 4 hrs
Test specimens of the new steel was heat treated by the following mode:
= Austenitizing at 1900 OF for 60 min.
= Oil quenching for 2.5 min. and further air cooled
= Second hardening by high tempering at 1070 OF for 3 hrs. and further high
tempering at 1000 IF for 4 his.
The new steel has the following room temperature mechanical properties:
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CA 02702515 2010-05-04

HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb)
49 250 234 10 49 20.5

The new steel has a microstructure that consists essentially of a fine
dispersion of
titanium carbide, TiC, vanadium carbide, VC, complex tungsten carbides,
(MW)xCy in a
ferrite - martensite - retained austenite matrix.

Example 5
The composition of the new steel is comprised of (%, wt): C=0.35, Cr=1.43,
Ni=0.69, Mn=0.43, Cu=0.31, V=0.52, Si=0.72, Mo=0.52, W=1.35, Ti=0.12, and
balance
essentially Fe.
The new steel has the following critical temperatures:
AC3 =1560 OF, AC1 =1345 OF, MS = 580 OF.

Laboratory scale ingots of the new steel are processed the same as Example4.
Test specimens of the new steel are heat treated in the same manner as
Example4.
The new steel has the following room temperature mechanical properties:
HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb)
49 249 234 10 48 21

The new steel has a microstructure that is similar to the microstructure of
Example 4.
From the above, it is apparent that the high hardness, high strength, high
impact
toughness steel which is the subject of the invention is an important
development in the
steel making art. Although only five examples have been described, it is
obvious that
other examples of the new steel can be derived from what is claimed in the
presented
description without departing from the spirit thereof.

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