U.S. patent application number 12/132593 was filed with the patent office on 2008-10-02 for cold-worked steels with packet-lath martensite/austenite microstructure.
This patent application is currently assigned to MMFX Technologies Corporation. Invention is credited to Grzegorz J. Kusinski, Gareth Thomas.
Application Number | 20080236709 12/132593 |
Document ID | / |
Family ID | 32329190 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080236709 |
Kind Code |
A1 |
Kusinski; Grzegorz J. ; et
al. |
October 2, 2008 |
COLD-WORKED STEELS WITH PACKET-LATH MARTENSITE/AUSTENITE
MICROSTRUCTURE
Abstract
Strain-hardened steel alloys having a high tensile strength are
prepared by cold working of alloys whose microstructure includes
grains in which laths of martensite alternate with thin films of
stabilized austenite. Due to the high dislocation density of this
microstructure and the tendency of the strains to move between the
martensite and austenite phases, the strains created by cold
working provide the microstructure with unique mechanical
properties including a high tensile strength. Surprisingly, this is
achieved without the need for intermediate heat treatments
(patenting, in the case of steel wire) of the steel between cold
working reductions.
Inventors: |
Kusinski; Grzegorz J.;
(Irvine, CA) ; Thomas; Gareth; (Cassis,
FR) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
MMFX Technologies
Corporation
Irvine
CA
|
Family ID: |
32329190 |
Appl. No.: |
12/132593 |
Filed: |
June 3, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10645833 |
Aug 20, 2003 |
|
|
|
12132593 |
|
|
|
|
60427830 |
Nov 19, 2002 |
|
|
|
Current U.S.
Class: |
148/579 ;
148/650 |
Current CPC
Class: |
C21D 2211/001 20130101;
C21D 1/185 20130101; C21D 7/10 20130101; C21D 1/18 20130101; C21D
1/19 20130101; C21D 2211/008 20130101; C21D 7/02 20130101; C21D
2211/005 20130101; C21D 8/06 20130101 |
Class at
Publication: |
148/579 ;
148/650 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Claims
1. A process for manufacturing a high-strength, high-ductility
alloy carbon steel, said process comprising: (a) cooling a solid
carbon alloy steel having a homogeneous austenite phase with all
alloying elements in solid solution so as to form a carbon steel
alloy having a microstructure consisting of laths of martensite
alternating with from about 0.5% to about 15% by volume of films of
retained austenite, and (b) cold working the carbon steel alloy
from step (a) without intermediate heat treatment to a reduction
sufficient to achieve a tensile strength of at least about 150
ksi.
2. The process of claim 1 wherein step (b) comprises cold working
said carbon steel alloy to a reduction sufficient to achieve a
tensile strength of from about 150 ksi to about 500 ksi.
3. The process of claim 1 wherein step (b) comprises cold working
said carbon steel alloy to a cross-sectional area reduction of at
least about 20% per pass.
4. The process of claim 1 wherein step (b) comprises cold working
said steel alloy to a cross-sectional area reduction of at least
about 25% per pass
5. The process of claim 1 wherein step (b) comprises cold working
said carbon steel alloy to a cross-sectional area reduction of from
about 25% to about 50% per pass.
6. The process of claim 1 wherein step (b) comprises cold working
said carbon steel alloy in a series of passes without heat
treatment between passes.
7. The process of claim 1 wherein step (b) is performed at a
temperature of about 10.degree. C. or below.
8. The process of claim 1 wherein step (b) is performed within
approximately 25 C of ambient temperature.
9. The process of claim 1 wherein said carbon steel alloy is in the
form of a rod or wire, and step (b) comprises drawing said carbon
steel alloy through a die.
10. The process of claim 1 wherein said carbon steel alloy is in
the form of a sheet, and step (b) comprises rolling said carbon
steel alloy.
11. The process of claim 1 wherein step (a) further comprises (i)
forming a carbon steel alloy composition having a martensite start
temperature of at least about 300 C, (ii) heating said carbon steel
alloy composition to a temperature sufficiently high to cause
austenitization thereof, to produce a homogeneous austenite phase
with all alloying elements in solution, and (iii) cooling said
homogeneous austenite phase through said martensite transition
range at a cooling rate sufficiently fast to achieve said
microstructure substantially avoiding carbide formation at
interfaces between said laths of martensite and said films of
retained austenite.
12. The process of claim 11 wherein said carbon steel alloy
composition having a martensite start temperature of at least about
350 C.
13. The process of claim 11 wherein said retained austenite films
are of a uniform orientation.
14. The process of claim 11 wherein said carbon steel alloy
composition consists of iron and alloying elements comprising from
about 0.04% to about 0.12% carbon, from 0% to about 11% chromium,
from 0% to about 2.0% manganese, and from 0% to about 2.0% silicon,
all by weight.
15. The process of claim 11 wherein said temperature of step (ii)
is from about 800 C to about 1150 C.
16. (canceled)
17. The process of claim 11 wherein step (iii) comprises cooling
said homogeneous austenite phase to a temperature of from about 800
C to about 1,000 C.
18. The process of claim 11 wherein step (ii) comprises heating
said carbon steel alloy composition to a temperature of from about
1,050 C to about 1,170 C, and step (iii) comprises cooling said
homogeneous austenite phase to a temperature of from about 800 C to
about 1,000 C.
19. The process of claim 11 wherein said carbon steel alloy
composition consists of iron and alloying elements comprising from
about 0.02% to about 0.14% carbon, from 0% to about 3.0% silicon,
from 0% to about 1.5% manganese, and from 0% to about 1.5%
aluminum, all by weight.
20. The process of claim 1 wherein said films of retained austenite
constitute from about 3% to about 10% by volume of said
microstructure.
21. The process of claim 1 wherein said films of retained austenite
constitute from about 0.5% to about 5% by volume of said
microstructure.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This patent application claims priority from U.S.
Provisional Patent Application Ser. No. 60/427,830, filed Nov. 19,
2002, for all purposes legally capable of being served thereby. The
contents of provisional patent application No. 60/427,830 are
incorporated herein by reference in their entirety, as are all
literature citations in this specification.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention resides in the technology of low and medium
carbon steel alloys, particularly those of high-strength and
toughness, and the cold formability of such alloys.
[0004] 2. Description of the Prior Art
[0005] An important step in the processing of high-performance
steels is cold working, which typically consists of a series of
compressions and/or expansions achieved by processes such as
drawing, extruding, cold heading, or rolling. Cold working causes
plastic deformation of the steel which produces strain hardening
while forming the steel into the shape in which it will ultimately
be used. Cold working, which in the case of steel wire is performed
by wire drawing, is typically performed in a succession of stages
with intermediate heat treatments, which in the case of steel wire
are termed "patenting."
[0006] High-strength steel wire is an example of a high-performance
steel and is useful in a variety of engineering applications
including tire cord, wire rope, and strand for pre-stressed
concrete reinforcements. The steel most commonly used in
high-strength steel wire is medium- or high-carbon steel. In the
typical procedure for forming the wire, hot-rolled rods with
pearlitic microstructures are cold drawn in several stages, with
intermediate patenting treatments to soften the pearlite for
continued cold drawing. For example, hot rolled rods of about 5.5
mm diameter might be coarse drawn in several stages to a diameter
of about 3 mm. Patenting might then be performed at 850-900.degree.
C., causing austenitization of the steel, followed by
transformation of the steel at 500-550.degree. C. to fine pearlitic
lamellae. The steel would then be pickled, in hydrochloric acid,
for example, to remove the scale formed during patenting. The
pickling would be followed by several further drawing stages to
reduce the diameter down to about 1 mm, then further patenting and
pickling. The final drawing would then be done in several stages to
the final desired diameter, which may for example be about 0.4 mm,
to achieve the desired properties, notably strength. This may be
followed by further processing such as stranding, depending on the
ultimate use.
[0007] The purpose of the initial patenting treatment is to produce
a wire rod with a fine lamellar pearlite structure, which requires
a low transformation temperature. To achieve the desired
temperature control, the process is typically performed in a molten
lead bath. In the succeeding drawing stages, the wire is drawn to
true strains (defined below) of 6-7 to obtain high strength levels
of approximately 3,000 MPa. For conventional pearlitic wires, these
high strains and strengths are attainable only by applying a series
of patenting treatments. Without these patenting treatments, the
cold drawing will cause shear cracking of the pearlitic lamellae.
Because of the need for a molten lead bath the entire process is
costly and tends to raise environmental concerns.
[0008] Cold working is also used in the production of expandable
steel tubing, i.e., tubing that is expanded on-site and in some
cases below ground.
[0009] A recent development in steel alloys is the formation of
microstructures containing both martensite and austenite phases in
an alternating configuration in which the martensite is present as
laths that are separated by thin films of austenite. The
microstructures are fused grains in which individual grains contain
several laths of martensite separated by thin austenite films with,
in some cases, an austenite shell surrounding each grain. These
structures are termed "dislocated lath martensite" structures or
"packet-lath" martensite/austenite" structures. Patents disclosing
these microstructures are as follows, each of which is incorporated
herein by reference in its entirety: [0010] U.S. Pat. No. 4,170,497
(Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an
application filed Aug. 24, 1977 [0011] U.S. Pat. No. 4,170,499
(Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an
application filed Sep. 14, 1978 as a continuation-in-part of the
above application filed on Aug. 24, 1977 [0012] U.S. Pat. No.
4,671,827 (Gareth Thomas, Nack J. Kim, and Ramamoorthy Ramesh),
issued Jun. 9, 1987 on an application filed on Oct. 11, 1985 [0013]
U.S. Pat. No. 6,273,968 B1 (Gareth Thomas), issued Aug. 14, 2001 on
an application filed on Mar. 28, 2000 While these microstructures
offer certain performance benefits, notably a high resistance to
corrosion, it has not heretofore been known that processing steps
typically used for steel alloys could be simplified or eliminated
when these microstructures are present.
[0014] Of further potential relevance to this invention are two
United States patents that disclose the cold working of steel rods
and wires without patenting. These patents are: [0015] U.S. Pat.
No. 4,613,385 (Gareth Thomas and Alvin H. Nakagawa), issued Sep.
23, 1986 on an application filed Dec. 9, 1982 [0016] U.S. Pat. No.
4,619,714 (Gareth Thomas, Jae-Hwan Ahn, and Nack-Joon Kim), issued
Oct. 28, 1986 on an application filed Nov. 29, 1984 as a
continuation-in-part of the above application filed on Aug. 6, 1984
These patents are likewise incorporated herein in their entirety.
The microstructures of the steels in these patents are considerably
different from those of the first four patents listed above.
SUMMARY OF THE INVENTION
[0017] It has now been discovered that the packet-lath
martensite/austenite microstructure is unique in its
crystallographic characteristics and how these characteristics
cause it to respond to cold working. Because of the high
dislocation density of this microstructure and the ease with which
strains in the structure can move between the martensite and
austenite phases, cold working provides the microstructure with
unique mechanical properties that include a high tensile strength.
As a result, these alloys can be cold worked without intermediate
heat treatments, while still achieving tensile strengths comparable
to the tensile strengths of conventional steel alloys that have
been processed by cold working with intermediate heat treatments.
In the case of steel wire having the packet-lath
martensite/austenite microstructure, this invention lies in the
discovery that cold drawing can be performed without intermediate
patenting treatments. In accordance with the present invention,
therefore, carbon steel alloys having the packet-lath
martensite/austenite microstructure, i.e., those whose
microstructure includes laths of martensite alternating with thin
films of retained austenite are cold formed, preferably without
intermediate heat treatments, to a reduction sufficient to achieve
a tensile strength of about 150 ksi or higher ("ksi" denotes
kilo-pounds-force per square inch), equivalent to approximately
1,085 MPa or higher ("MPa" denotes megapascals, i.e., newtons per
square millimeter). Cold working to tensile strengths of 2,000 MPa
(290 ksi) of higher is of particular interest, and indeed, tensile
strengths of 3,000 MPa (435 ksi) and as high as 4,000 MPa (580 ksi)
can be achieved by the practice of this invention. These values are
approximate; the conversion factor to the nearest thousandth is
6.895 MPa equal 1 ksi.
[0018] The benefits of this invention extend to simple packet-lath
martensite/austenite microstructures containing no ferrite or
insignificant amounts of ferrite, and also to microstructures that
include packet-lath grains fused with ferrite grains, and to
variants on these structures, including those whose packet-lath
grains are encased by austenite shells, those that are free of
interphase carbide precipitates, and those in which the austenite
films are of a uniform orientation. The discovery of the ability of
packet-lath martensite/austenite microstructures to respond to cold
working in this manner is surprising relative to the disclosures in
U.S. Pat. Nos. 4,613,385 and 4,619,714 referenced above, since the
ferrite in the microstructures of those patents has a lower yield
strength than the martensite. As a result, the ferrite will
preferentially absorb the strain introduced by the cold working,
while the martensite will not respond to the cold working until the
ferrite phase is work hardened to a level above the yield strength
of the martensite. In the microstructures addressed by the present
invention, the relatively low level of ferrite, or its absence when
no ferrite is present, will cause the martensite to absorb the
strain at an earlier stage of the cold working process. Martensite
and ferrite are distinctly different from each other in crystal
structure and hardening behavior.
[0019] These and other features, objects, advantages, and
embodiments of the invention will be better understood from the
descriptions that follow.
BRIEF DESCRIPTION OF THE FIGURES
[0020] FIG. 1 is a plot of tensile strength vs. true total strain
for two steel alloys of dual-phase packet-lath martensite/austenite
microstructure, upon cold working in accordance with this invention
in the absence of intermediate heat treatments.
[0021] FIG. 2 is a plot of tensile strength vs. true total strain
for three steel alloys of triple-phase packet-lath
martensite/austenite/ferrite microstructure and one steel alloy of
dual-phase packet-lath martensite/austenite microstructure, upon
cold working in accordance with this invention in the absence of
intermediate heat treatments.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
[0022] Cold working in the practice of this invention can be
performed by the use of techniques and equipment that have been
used for cold working in the prior art on other steel alloys and
microstructures. For alloys in the form of blooms, billets, bars,
slabs or sheets, cold working may consist of rolling the steel
between rollers or other means of compression to reduce the
thickness of and elongate the steel. When cold working is performed
by rolling, multiple reductions are achieved by multiple passes
through a rolling mill. For rod-shaped or wire-shaped workpieces,
cold working may consist of cold-drawing or extrusion through a
die. For multiple reductions, the workpiece is extruded through a
series of successively smaller dies. Tubing is achieved by drawing
the steel through a ring-shaped die with a mandrel inside the die.
For multiple passes, the tubing that has already been drawn is
further drawn through a smaller ring-shaped die with a mandrel
placed inside the tubing.
[0023] Cold working is performed at a temperature below the lowest
temperature at which recrystallization occurs. Suitable
temperatures are therefore those that do not induce any phase
change in the steel. For carbon steels, recrystallization typically
occurs at approximately 1,000.degree. C. (1,832.degree. F.), and
accordingly, cold working in accordance with this invention is
performed well below this temperature. Preferably, cold working is
performed at temperatures of about 500.degree. C. (932.degree. F.)
or less, more preferably about 100.degree. C. (212.degree. F.) or
less, and most preferably at a temperature that is within about
25.degree. C. of ambient temperature.
[0024] Cold working can be performed in a single pass or in a
succession of passes. In either case, intermediate heat treatments
(which, in the case of steel wire, are termed "patenting") may be
performed for further improvement in properties, but the properties
resulting from the cold working alone are sufficiently high that
the intermediate heat treatments are not required and are
preferably not performed. The degree of reduction per pass is not
critical to the invention and can vary widely, although the
reductions should be great enough to avoid hardening the steel so
much that the steel becomes susceptible to breakage after a small
total reduction. In most cases, preferred reductions are at least
about 20% per pass, more preferably at least about 25% per pass,
and most preferably from about 25% to about 50% per pass. The
reduction per pass is at least partially governed by such factors
as the die angle and the drawing efficiency coefficient. The larger
the die angle, the larger the minimum reduction that is required to
avoid central burst cracking. The lower the drawing efficiency
coefficient, however, the lower the maximum reduction for a steel
with a given strain hardening exponent. A compromise is typically
sought between these two competing considerations. In terms of the
tensile strength of the final product, the cold working will
preferably be performed to a tensile strength within the range of
from about 150 ksi to about 500 ksi.
[0025] The process of this invention is applicable to carbon steel
alloys having packet-lath martensite/austenite microstructures such
as those described in the patents cited above, as well as those
described in co-pending U.S. patent application Ser. Nos.
10/017,847, filed Dec. 15, 2001 (entitled "Triple-Phase
Nano-Composite Steels," inventors Kusinski, G. J., Pollack, D., and
Thomas, G.), and 10/017,879, filed Dec. 14, 2001 (entitled
"Nano-Composite Martensitic Steels," inventors Kusinski, G. J.,
Pollack, D., and Thomas, G.), both of which are incorporated herein
by reference in their entirety. To permit formation of the
packet-lath martensite/austenite microstructure, the alloy
composition will typically have a martensite start temperature
M.sub.s of about 300.degree. C. or higher, and preferably
350.degree. C. or higher. While alloying elements in general affect
the M.sub.s, the alloying element that has the strongest influence
on the M.sub.s is carbon, and achieving an alloy with M.sub.s above
300.degree. C. can be achieved by limiting the carbon content of
the alloy to a maximum of 0.35% by weight. In preferred embodiments
of the invention, the carbon content is within the range of from
about 0.03% to about 0.35%, and in more preferred embodiments, the
range is from about 0.05% to about 0.33%, all by weight. Further
alloying elements, such as molybdenum, titanium, niobium, and
aluminum, can also be present in amounts sufficient to serve as
nucleation sites for fine grain formation yet low enough in
concentration to avoid affecting the properties of the finished
alloy by their presence. The concentration should also be low
enough to avoid the formation of inclusions and other large
precipitates, which may render the steel susceptible to early
fracture. In certain embodiments of the invention, it will be
advantageous to include one or more austenite stabilizing elements,
examples of which are nitrogen, manganese, nickel, copper, and
zinc. Particularly preferred among these are manganese and nickel.
When nickel is present, the nickel concentration is preferably
within the range of about 0.25% to about 5%, and when manganese is
present, the manganese concentration is preferably within the range
of from about 0.25% to about 6%. Chromium is also included in many
embodiments of the invention, and when it is present, the chromium
concentration is preferably from about 0.5% to about 12%. All
concentrations herein are by weight.
[0026] Certain embodiments of the invention involve alloys that
include a ferrite phase in addition to the packet-lath
martensite/austenite grains (triple-phase alloys) while others
contain only the packet-lath martensite/austenite grains and do not
include a ferrite phase (dual-phase alloys). In general, the
presence or absence of the ferrite phase is determined by the type
of heat treatment in the initial austenitization stage. By
appropriate selection of the temperature, the steel can be
transformed into a single austenite phase or into a two-phase
structure containing both austenite and ferrite. In addition, the
alloy composition can be selected or adjusted to either cause
ferrite formation during the initial cooling of the alloy from the
austenite phase or to avoid ferrite formation during the cooling,
i.e., to avoid the formation of ferrite grains prior to the further
cooling of the austenite to form the packet-lath
microstructure.
[0027] As noted above, in certain cases it will be beneficial to
use alloys with packet-lath martensite/austenite microstructures in
which the austenite films in a single packet-lath grain are all of
approximately the same orientation, although the crystallographic
orientation may vary, or those in which the austenite films in a
single packet-lath grain are all of the same crystal plane
orientation. The latter can be achieved by limiting the grain size
to ten microns or less. Preferably, the grain size in these cases
is within the range of about 1 micron to about 10 microns, and most
preferably from about 5 microns to about 9 microns.
[0028] The preparation of--phase packet-lath martensite/austenite
microstructures that do not contain ferrite (i.e., "dual-phase"
microstructures) begins with the selection of the alloy components
and the combining of these components in the appropriate portions
as indicated above. The combined components are then homogenized
("soaked") for a sufficient period of time and at a sufficient
temperature to achieve a uniform austenitic structure with all
elements and components in solid solution. The temperature will be
above the austenite recrystallization temperature but preferably at
a level that will cause very fine grains to form. The austenite
recrystallization temperature typically varies with the alloy
composition, but in general will be readily apparent to those
skilled in the art. In most cases, best results will be achieved by
soaking at a temperature within the range of 800.degree. C. to
1150.degree. C. Rolling, forging or both are optionally performed
on the alloy at this temperature.
[0029] Once homogenization is completed, the alloy is subjected to
a combination of cooling and grain refinement to the desired grain
size, which as noted above may vary. Grain refinement may be
performed in stages, but the final grain refinement is generally
achieved at an intermediate temperature that is above, yet close
to, the austenite recrystallization temperature. The alloy may
first be rolled at the homogenization temperature to achieve
dynamic recrystallization, then cooled to an intermediate
temperature and rolled again for further dynamic recrystallization.
The intermediate temperature is between the austenite
recrystallization temperature and a temperature that is about 50
degrees Celsius above the austenite recrystallization temperature.
For alloy compositions whose austenite recrystallization
temperature is about 900.degree. C., and the intermediate
temperature to which the alloy is cooled is preferably between
about 9000 to about 950.degree. C., and most preferably between
about 900.degree. to about 925.degree. C. For alloy compositions
whose austenite recrystallization temperature is about 820.degree.
C., the preferred intermediate temperature is about 850.degree. C.
Dynamic recrystallization can also be achieved by forging or by
other means known to those skilled in the art. Dynamic
recrystallization produces a grain size reduction of 10% or
greater, and in many cases a grain size reduction of from about 30%
to about 90%.
[0030] Once the desired grain size is achieved, the alloy is
quenched by cooling from a temperature above the austenite
recrystallization temperature down to the martensite start
temperature M.sub.s, then through the martensite transition range
to convert the austenite crystals to the packet-lath
martensite/austenite microstructure. When ferrite crystals are
present among the austenite crystals, the conversion occurs only in
the austenite crystals. The optimal cooling rate varies with the
chemical composition, and hence the hardenability, of the alloy.
The resulting packets are of approximately the same small size as
the austenite grains produced during the rolling stages, but the
only austenite remaining in these grains is in the thin films and
in some cases in the shell surrounding each packet-lath grain. When
the thin austenite films are to be of a single variant in crystal
orientation, this is achieved by controlling the process to achieve
a grain size of less than 50 microns.
[0031] As an alternative to dynamic recrystallization, grain
refinement to the desired grain size can be accomplished by heat
treatment alone. To use this method, the alloy is quenched as
described in the preceding paragraph, then reheated to a
temperature that is approximately equal to the austenite
recrystallization temperature or slightly below, then quenched once
again to achieve, or to return to, the packet-lath
martensite/austenite microstructure. The reheating temperature is
preferably within about 50 degrees Celsius of the austenite
recrystallization temperature, for example about 870.degree. C.
[0032] Processing steps such as heating the alloy composition to
the austenite phase, cooling the alloy with controlled rolling or
forging to achieve the desired reduction and grain size, and
quenching the austenite grains through the martensite transition
region to achieve the packet-lath structure are performed by
methods known in the art. These methods include castings, heat
treatment, and hot working of the alloy such as by forging or
rolling, followed by finishing at the controlled temperature for
optimum grain refinement. Controlled rolling serves various
functions, including aiding in the diffusion of the alloying
elements to form a homogeneous austenite crystalline phase and in
the storage of strain energy in the grains. In the quenching stages
of the process, controlled rolling guides the newly forming
martensite phase into a packet-lath arrangement of martensite laths
separated by thin films of retained austenite. The degree of
rolling reduction can vary and will be readily apparent to those of
skill in the art. Quenching is preferably done fast enough to avoid
formation of detrimental microstructures including pearlite,
bainite, and particles or precipitates, particularly interphase
precipitation and particle formation, including the formation of
undesirable carbides and carbonitrides. In the packet-lath
martensite-austenite grains, the retained austenite films will
constitute from about 0.5% to about 15% by volume of the
microstructure, preferably from about 3% to about 10%, and most
preferably a maximum of about 5%.
[0033] Triple-phase alloys have a microstructure consisting of two
types of grains, ferrite grains and packet-lath
martensite/austenite grains, fused together as a continuous mass.
As in the dual-phase alloys, the individual grain size is not
critical and can vary widely. For best results, the grain sizes
will generally have diameters (or other appropriately
characteristic linear dimension) that fall within the range of
about 2 microns to about 100 microns, or preferably within the
range of about 5 microns to about 30 microns. The amount of ferrite
phase relative to the martensite-austenite phase may vary. In most
cases, however, best results will be obtained when the
martensite/austenite grains constitute from about 5% to about 95%
of the triple-phase structure, preferably from about 15% to about
60%, and most preferably from about 20% to about 40%, all by
weight.
[0034] Triple-phase alloys can be prepared by first combining the
appropriate components needed to form an alloy of the desired
composition, then soaking to achieve a uniform austenitic structure
with all elements and components in solid solution, as in the
preparation of the dual-phase alloys described above. A preferred
soaking temperature range is from about 900.degree. C. to about
1,170.degree. C. Once the austenite phase is formed, the alloy
composition is cooled to a temperature in the intercritical region,
which is defined as the region in which austenite and ferrite
phases coexist at equilibrium. The cooling thus causes a portion of
the austenite to transform into ferrite grains, leaving the
remainder as austenite. The relative amounts of each of the two
phases at equilibrium varies with the temperature to which the
composition is cooled in this stage, and also with the levels of
the alloying elements. The distribution of the carbon between the
two phases (again at equilibrium) also varies with the temperature.
The relative amounts of the two phases are not critical to the
invention and can vary. The temperature to which the composition is
cooled in order to achieve the dual-phase ferrite-austenite
structure is preferably within the range of from about 800.degree.
C. to about 1,000.degree. C.
[0035] Once the ferrite and austenite crystals are formed (i.e.,
once equilibrium at the selected temperature in the intercritical
phase is achieved), the alloy is rapidly quenched by cooling
through the martensite transition range to convert the austenite
crystals to the packet-lath martensite/austenite microstructure.
The cooling rate used during this transition is great enough to
substantially avoid any changes to the ferrite phase and to avoid
undesirable austenite decomposition. Depending on the alloy
composition and its hardenability, water cooling may be required to
achieve the desired cooling rate, although for certain alloys air
cooling will suffice. In some alloys, notably triple-phase
containing 6% Cr, the desired cooling rate is slow enough that air
cooling can be used. The considerations noted above in connection
with dual-phase alloys apply here as well.
[0036] Preferred dual-phase alloy compositions are those that
contain from about 0.04% to about 0.12% carbon, from zero to about
11.0% chromium, from zero to about 2.0% manganese, and from zero to
about 2.0% silicon, all by weight, the remainder being iron.
Preferred triple-phase alloy compositions are those that contain
from about 0.02% to about 0.14% carbon, from zero to about 3.0%
silicon, from zero to about 1.5% manganese, and from zero to about
1.5% aluminum, all by weight, the remainder being iron.
[0037] The formation of precipitates or other small particles
within the microstructure upon cooling is collectively referred to
as "autotempering." In certain applications of this invention,
whether dual-phase or triple-phase alloys, autotempering will
purposely be avoided by using a relatively fast cooling rate. The
minimum cooling rates that will avoid autotempering are evident
from the transformation-temperature-time diagram for the alloy. In
the typical diagram, the vertical axis represents temperature and
the horizontal axis represents time, while curves on the diagram
indicate the regions where each phase exists either by itself or in
combination with another phase(s). A typical such diagram is shown
in Thomas, U.S. Pat. No. 6,273,968 B1, referenced above. In such
diagrams, the minimum cooling rate is a line of descending
temperature over time which abuts the left side of a C-shaped
curve. The region to the right of the curve represents the presence
of carbides, and cooling rates that avoid carbide formation are
therefore those represented by lines that remain to the left of the
curve. The line that is tangential to the curve has the smallest
slope and is therefore the slowest rate that can be used while
still avoiding carbide formation.
[0038] The terms "interphase precipitation" and "interphase
precipitates" are used herein to denote the formation of small
alloy particles at locations between the martensite and austenite
phases, i.e., between the laths and the thin films separating the
laths. "Interphase precipitates" does not refer to the austenite
films themselves. Interphase precipitates are to be distinguished
from "intraphase precipitates," which are precipitates located
within the martensite laths rather than along the interfaces
between the martensite laths and the austenite films. Intraphase
precipitates that are about 500 .ANG. or less in diameter are not
detrimental to toughness and may in fact enhance toughness. Thus,
autotempering is not necessarily detrimental provided that the
autotempering is limited to intraphase precipitation and does not
result in interphase precipitation. The term "substantially no
carbides" is used herein to indicate that if any carbides are
present, their distribution and amount are such that they have a
negligible effect on the performance characteristics, and
particularly the corrosion characteristics, of the finished
alloy.
[0039] Depending on the alloy composition, a cooling rate that is
sufficiently high to prevent carbide formation or autotempering in
general may be one that can be achieved with air cooling or one
that requires water cooling. In alloy compositions in which
autotempering can be avoided with air cooling, air cooling can
still be done when the levels of certain alloying elements are
reduced provided that the levels of other alloying elements are
raised. For example, a reduction in the amount of carbon, chromium,
or silicon can be compensated for by raising the level of
manganese.
[0040] The processes and conditions set forth in the U.S. patents
referenced above, particularly heat treatments, grain refinements,
on-line forgings and the use of rolling mills for rounds, flats,
and other shapes, may be used in the practice of the present
invention for the heating of the alloy composition to the austenite
phase, the cooling of the alloy from the austenite phase to the
intercritical phase in the case of triple-phase alloys, and then
the cooling through the martensite transition region. Rolling is
performed in a controlled manner at one or more stages during the
austenitization and first-stage cooling procedures, for example, to
aid in the diffusion of the alloying elements to form a homogeneous
austenite crystalline phase and then to deform the crystal grains
and store strain energy in the grains, while in the second-stage
cooling, rolling can serve to guide the newly forming martensite
phase into the packet-lath arrangement of martensite laths
separated by thin films of retained austenite. The degree of
rolling reductions can vary, and will be readily apparent to those
skilled in the art. In the packet-lath martensite-austenite
crystals, the retained austenite films will constitute from about
0.5% to about 15% by volume of the microstructure, preferably from
about 3% to about 10%, and most preferably a maximum of about 5%.
The proportion of austenite relative to the entire triple-phase
microstructure will be a maximum of about 5%. The actual width of a
single retained austenite film is preferably within the range of
about 50 .ANG. to about 250 .ANG., and preferably about 100 .ANG..
The proportion of austenite relative to the entire triple-phase
microstructure will in general be a maximum of about 5%. The
rolling discussed in this paragraph is to be distinguished from the
cold working that is done in accordance with this invention after
the packet-lath martensite/austenite microstructures, whether
dual-phase or part of a triple-phase structure, have been
formed.
[0041] The following examples are offered only by way of
illustration.
Example 1
[0042] This example illustrates the deformation of a carbon steel
rod with a packet-lath martensite/austenite microstructure, by a
cold drawing process according to the present invention to an area
reduction of 99%.
[0043] The experiment reported in this example was performed on a
steel rod measuring 6 mm in diameter and having an alloy
composition of 0.1% carbon, 2.0% silicon, 0.5% chromium, 0.5%
manganese, all by weight, and the balance iron, with a
microstructure consisting of grains measuring approximately 50
microns in diameter, each grain consisting of laths of martensite
measuring approximately 100 nm in thickness alternating with thin
films of austenite measuring approximately 10 nm in thickness, with
no ferrite phases and each grain surrounded by an austenite shell
measuring approximately 10 nm in thickness. The rod was prepared by
the method described in co-pending U.S. patent application Ser. No.
10/017,879, filed Dec. 14, 2001, referenced above.
[0044] The uncoated steel rod was surface cleaned and lubricated,
then cold drawn through lubricated dies in 15 passes at a
temperature of 25.degree. C. to a diameter of 0.0095 inch (0.024
cm). At a final wire diameter of 0.0105 inch (0.027 cm),
representing a total area reduction of 99%, the wire had a tensile
strength of 390 ksi (2,690 MPa).
Example 2
[0045] This example is another illustration of the cold working of
carbon steel rods with packet-lath martensite/austenite
microstructures in accordance with the present invention. In this
example, two different alloys were used, Fe/8Cr/0.05C and
Fe/2Si/0.1C, with a microstructure consisting of grains measuring
approximately 50 microns in diameter, each grain consisting of
laths of martensite measuring approximately 150 nm in thickness
alternating with thin films of austenite measuring approximately 10
nm in thickness, with no significant ferrite phases, each grain
surrounded by an austenite shell measuring approximately 10 nm in
thickness.
[0046] The steel rods were 6 mm in diameter, and were surface
cleaned and lubricated, then cold drawn through lubricated dies in
a series of passes at a temperature of 25.degree. C. The drawing
schedule shown in Table I was used for the Fe/8Cr/0.05C alloy, and
a similar drawing schedule was used for the Fe/2Si/0.1C alloy. In
this table, A.sub.o represents the initial rod diameter and A is
the rod diameter after the particular pass.
TABLE-US-00001 TABLE I Drawing Schedule for Fe/8Cr/0.05C With
Substantially Ferrite-Free Packet-Lath Martensite Microstructure
Single Pass Total Pass Diameter True Total Strain Area Reduction
Area Reduction No. (mm) (ln(A/A.sub.o)) (%) (%) (initial) 6.000 0.0
0.0 0.0 1 4.3 0.7 48.2 48.2 2 3.4 1.1 37.0 67.3 3 2.7 1.6 37.1 79.4
4 2.2 2.0 34.0 86.4 5 1.8 2.5 36.6 91.4 6 1.4 2.9 38.5 94.7 7 1.0
3.5 45.4 97.1
[0047] Tensile strengths were measured on the starting rod and
after each pass, and the results are plotted against the true total
strain in FIG. 1, in which the squares represent the Fe/8Cr/0.05C
alloy and the diamonds represent the Fe/2Si/0.1C alloy. The Figure
shows that the tensile strengths of both alloys reach approximately
2,000 MPa by the end of the entire drawing sequence at a total area
reduction of 97%.
Example 3
[0048] This example illustrates cold working in accordance with the
present invention, using carbon steel rods with packet-lath
martensite/austenite microstructures that contain ferrite crystals
as a third phase (in addition to the laths of martensite and the
thin films of austenite, i.e., a triple-phase microstructure).
[0049] In this example, the alloy was Fe/2Si/0.1C, with a
microstructure consisting of ferrite fused with packet-lath grains
similar to those described above in Examples 1 and 2, containing
martensite laths alternating with thin films of austenite and
encased in an austenite shell. The rods were prepared by the method
described in U.S. patent application Ser. No. 10/017,847, filed
Dec. 14, 2001, referenced above, using a reheat temperature of
950.degree. C. to achieve a ferrite content of 70 volume percent of
the microstructure. The initial rod diameter was 0.220 inch (5.59
mm), and the cold working consisted of drawing the rods through
lubricated conical dies at a temperature of 25.degree. C. in 15
passes with approximately 36% reduction per pass to a final
diameter of 0.037 inch (0.94 mm).
[0050] The drawing schedule is shown in Table II, where A.sub.o
represents the initial rod diameter and A is the rod diameter after
the particular pass.
TABLE-US-00002 TABLE II Drawing Schedule for Fe/2Cr/0.1C With
Triple-Phase Microstructure Single Pass Total Pass Diameter True
Total Strain Area Reduction Area Reduction No. (mm) (ln(A/A.sub.o))
(%) (%) (initial) 6.050 0.00 0.00 0.00 1 4.580 0.56 42.69 42.69 2
3.650 1.01 36.49 63.60 3 2.910 1.46 36.44 76.86 4 2.320 1.92 36.44
85.29 5 1.870 2.35 35.03 90.45 6 1.660 2.59 21.20 92.47 7 1.320
3.04 36.77 95.24 8 1.090 3.43 31.81 96.75 9 0.910 3.79 30.30 97.74
10 0.756 4.16 30.98 98.44 11 0.624 4.54 31.87 98.94 12 0.526 4.89
28.94 99.24 13 0.437 5.26 30.98 99.48 14 0.390 5.48 20.35 99.58 15
0.359 5.65 15.27 99.65
[0051] The tensile strength of the final wire was 2760 MPa (400
ksi).
Example 4
[0052] This example is a further illustration of the cold work of
carbon steel rods whose microstructure consists of packet-lath
martensite/austenite and ferrite crystals, in accordance with the
present invention.
[0053] In this example, the alloy was Fe/2Si/0.1C as in Example 3,
with a microstructure consisting of ferrite fused with packet-lath
grains similar to those described above in Examples 1 and 2,
containing martensite laths alternating with thin films of
austenite and encased in an austenite shell. A rod of this
composition was prepared by the general method described in U.S.
patent application Ser. No. 10/017,847, filed Dec. 14, 2001,
referenced above. In this case, the rod was initially hot rolled to
a diameter of 0.25 inch (6.35 mm), then heated to 1,150.degree. C.
for about 30 minutes to austenitize the composition, then quenched
in iced brine to transform the austenite to substantially 100%
martensite, then rapidly reheated to convert the structure to
approximately 70% ferrite and 30% austenite. The rod was then
quenched in iced brine to convert the austenite to the packet-lath
martensite/austenite structure. The rod was then cold drawn in 7
passes at a reduction of 35% per pass to a final diameter of 0.055
inch (1.40 mm), resulting in a tensile strength of 1,875 MPa (272
ksi). In a parallel experiment, a rod of the same composition and
treated in the identical manner was cold drawn in 13 passes at a
reduction of 35% per pass to a final diameter of 0.015 inch (0.37
mm), resulting in a tensile strength of 2,480 MPa (360 ksi).
Example 5
[0054] This example is a still further illustration of the cold
working of carbon steel rods whose microstructure consists of
packet-lath martensite/austenite and ferrite crystals, in
accordance with the present invention, demonstrating the effect of
varying the relative amounts of packet-lath martensite/austenite
and ferrite.
[0055] The steel alloy was Fe/2Si/0.1C as in Examples 3 and 4, and
the rods were prepared as described in Example 4, using different
reheat temperatures to achieve ferrite contents of 0%, 56%, 66%,
and 75%, corresponding to contents of packet-lath
martensite/austenite contents of 100%, 44%, 35%, and 25%,
respectively, all by volume. Drawing schedules similar to that
shown in Table II were used on all four microstructures, and the
resulting tensile strengths are plotted against the true total
strain in FIG. 2, in which the squares represent the 100%
packet-lath alloy, the triangles represent the 44% packet-lath
alloy, the circles represent the 34% packet-lath alloy, and the
diamonds represent the 25% packet-lath alloy. The plot shows that
all four microstructures achieved a tensile strength well in excess
of 2,000 MPa, and those in which the packet-lath
martensite/austenite portions exceeded 25% produced higher tensile
strengths than the microstructure in which the packet-lath portion
was 25%.
[0056] The foregoing is offered primarily for purposes of
illustration. Further modifications and variations of the various
parameters of the alloy composition and the processing procedures
and conditions may be made that still embody the basic and novel
concepts of this invention. These will readily occur to those
skilled in the art and are included within the scope of this
invention.
* * * * *