U.S. patent application number 12/698344 was filed with the patent office on 2011-08-04 for railroad rail steels resistant to rolling contact fatigue.
This patent application is currently assigned to TRANSPORTATION TECHNOLOGY CENTER, INC.. Invention is credited to Anthony J. Deardo, C. Isaac Garcia, Semih Kalay, Raymundo Ordoniez, Francisco C. Robles Hernandez, Daniel Szablewski.
Application Number | 20110189047 12/698344 |
Document ID | / |
Family ID | 44341865 |
Filed Date | 2011-08-04 |
United States Patent
Application |
20110189047 |
Kind Code |
A1 |
Szablewski; Daniel ; et
al. |
August 4, 2011 |
RAILROAD RAIL STEELS RESISTANT TO ROLLING CONTACT FATIGUE
Abstract
Railroad rail steels having a pearlitic structure and containing
0.720 to 0.860 wt % carbon; 1.000 to 1.280 wt % manganese; 0.450 to
1.000 wt % silicon; 0.010 to 0.100 wt % copper; 0.150 to 0.280 wt %
chromium; 0.0010 to 0.0500 wt % aluminum; 0.050 to 0.120 wt %
nickel; 0.100 to 0.260 wt % molybdenum; 0.100 to 0.210 wt %
vanadium; 0.0010 to 0.0065 wt % nitrogen; 0.0010 to 0.0080 wt %
phosphorus; 0.0010 to 0.0040 wt % sulfur; and 0.0100 to 0.0350 wt %
niobium with the remainder of said steel being iron, can be used to
make railway rails that are particularly resistant to rolling
contact fatigue and, hence, shelling.
Inventors: |
Szablewski; Daniel;
(Colorado Springs, CO) ; Robles Hernandez; Francisco
C.; (Houston, TX) ; Garcia; C. Isaac;
(Pittsburgh, PA) ; Kalay; Semih; (Colorado
Springs, CO) ; Deardo; Anthony J.; (Pittsburgh,
PA) ; Ordoniez; Raymundo; (Pittsburgh, PA) |
Assignee: |
TRANSPORTATION TECHNOLOGY CENTER,
INC.
Pueblo
CO
|
Family ID: |
44341865 |
Appl. No.: |
12/698344 |
Filed: |
February 2, 2010 |
Current U.S.
Class: |
420/91 |
Current CPC
Class: |
C22C 38/48 20130101;
C22C 38/42 20130101; C22C 38/46 20130101; C22C 38/44 20130101 |
Class at
Publication: |
420/91 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44 |
Claims
1. A railroad rail steel having a pearlitic structure is comprised
of: 0.720 to 0.860 wt % carbon; 1.000 to 1.280 wt % manganese;
0.450 to 1.000 wt % silicon; 0.010 to 0.100 wt % copper; 0.150 to
0.280 wt % chromium; 0.0010 to 0.0500 wt % aluminum; 0.050 to 0.120
wt % nickel; 0.100 to 0.260 wt % molybdenum; 0.100 to 0.210 wt %
vanadium; 0.0010 to 0.0065 wt % nitrogen; 0.0010 to 0.0080 wt %
phosphorus; 0.0010 to 0.0040 wt % sulfur; and 0.0100 to 0.0350 wt %
niobium with the balance being iron.
2. The railroad rail steel of claim 1 having an Austenite grain
size of 15.0 to 40.0 microns.
3. The railroad rail steel of claim 1 having interlamellar spacings
of 0.070 to 0.100 mm.
4. The railroad rail steel of claim 1 having non-metallic
inclusions having a volume fraction of less than 0.001.
5. The railroad rail steel of claim 1 having virtually no
pro-eutectoid cementite.
6. The railroad rail steel of claim 1 having a Brinell hardness of
at least 400.
7. The railroad rail steel of claim 1 having a yield strength
greater than 150.
8. The railroad rail steel of claim 1 having an ultimate tensile
strength greater than 210 ksi.
9. The railroad rail steel of claim 1 having an elongation
percentage greater than 12.0%.
10. A railroad rail steel having a pearlitic structure is comprised
of: 0.720 to 0.860 wt % carbon; 1.000 to 1.280 wt % manganese;
0.450 to 1.000 wt % silicon; 0.010 to 0.100 wt % copper; 0.150 to
0.280 wt % chromium; 0.0010 to 0.0500 wt % aluminum; 0.050 to 0.120
wt % nickel; 0.100 to 0.260 wt % molybdenum; 0.100 to 0.210 wt %
vanadium; 0.0010 to 0.0065 wt % nitrogen; 0.0010 to 0.0080 wt %
phosphorus; 0.0010 to 0.0040 wt % sulfur; and 0.0100 to 0.0350 wt %
niobium with the balance being iron and wherein a molten form of
said steel was subjected to a degassing operation.
11. The railroad rail steel of claim 10 having an Austenite grain
size of 15.0 to 40.0 microns.
12. The railroad rail steel of claim 10 having interlamellar
spacings of 0.070 to 0.100 mm.
13. The railroad rail steel of claim 10 having non-metallic
inclusions having a volume fraction of less than 0.0001.
14. The railroad rail steel of claim 10 having virtually no
pro-eutectoid cementite.
15. The railroad rail steel of claim 10 having a Brinell hardness
greater than 400.
16. The railroad rail steel of claim 10 having a yield strength
greater than 150.
17. The railroad rail steel of claim 10 having an ultimate tensile
strength greater than 210 ksi.
18. The railroad rail steel of claim 10 having an elongation
percentage greater than 12.0%.
19. A railroad rail steel having a pearlitic structure is comprised
of: 0.720 to 0.860 wt % carbon; 1.000 to 1.280 wt % manganese;
0.450 to 1.000 wt % silicon; 0.010 to 0.100 wt % copper; 0.150 to
0.280 wt % chromium; 0.0010 to 0.0500 wt % aluminum; 0.050 to 0.120
wt % nickel; 0.100 to 0.260 wt % molybdenum; 0.100 to 0.210 wt %
vanadium; 0.0010 to 0.0065 wt % nitrogen; 0.0010 to 0.0080 wt %
phosphorus; 0.0010 to 0.0040 wt % sulfur; and 0.0100 to 0.0350 wt %
niobium with the balance being iron and wherein a bloom of said
steel was subjected to thereto-mechanical processing.
20. The railroad rail steel of claim 19 having an Austenite grain
size of 15.0 to 40.0 microns.
21. The railroad rail steel of claim 19 having interlamellar
spacings of 0.070 to 0.100 mm.
22. The railroad rail steel of claim 19 having non-metallic
inclusions having less than 0.001 volume fraction.
23. The railroad rail steel of claim 19 having virtually no
pro-eutectoid cementite.
24. The railroad rail steel of claim 19 having a Brinell hardness
of at least 400.
25. The railroad rail steel of claim 19 having a yield strength
greater than 150.
26. The railroad rail steel of claim 19 having an ultimate tensile
strength greater than 210 ksi.
27. The railroad rail steel of claim 19 having an elongation
percentage greater than 12.0%.
28. A railroad rail steel having a pearlitic structure is comprised
of: 0.720 to 0.860 wt % carbon; 1.000 to 1.280 wt % manganese;
0.450 to 1.000 wt % silicon; 0.010 to 0.100 wt % copper; 0.150 to
0.280 wt % chromium; 0.0010 to 0.0500 wt % aluminum; 0.050 to 0.120
wt % nickel; 0.100 to 0.260 wt % molybdenum; 0.100 to 0.210 wt %
vanadium; 0.0010 to 0.0065 wt % nitrogen; 0.0010 to 0.0080 wt %
phosphorus; 0.0010 to 0.0040 wt % sulfur; and 0.0100 to 0.0350 wt %
niobium with the balance being iron and whose rail head region was
subjected to head hardening operations during its rail rolling
operations.
29. The railroad rail steel of claim 28 having an Austenite grain
size of 15.0 to 40.0 microns.
30. The railroad rail steel of claim 28 having interlamellar
spacings of 0.070 to 0.100 mm.
31. The railroad rail steel of claim 28 having non-metallic
inclusions having a volume fraction less than 0.001.
32. The railroad rail steel of claim 28 having virtually no
pro-eutectoid cementite.
33. The railroad rail steel of claim 28 having a Brinell hardness
of at least 400.
34. The railroad rail steel of claim 28 having a yield strength
greater than 150.
35. The railroad rail steel of claim 28 having an ultimate tensile
strength greater than 210 ksi.
36. The railroad rail steel of claim 28 having an elongation
percentage greater than 12.0%.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to railroad rail
steels. More particularly, it is concerned with those railroad rail
steels that are specifically alloyed to resist fatigue effects
including rolling contact fatigue (RCF) and shelling in the head
regions of such rails. The term "shelling" generally refers to loss
of steel material as a result of deterioration arising from
mechanical stresses. In the context of this invention, the term
shelling is often contrasted with the term "spalling." Spalling
generally refers to loss of steel material as a result of
metallurgical damage created by excessive heat that arises from the
sliding of railroad wheels over railroad rails during extreme train
braking operations. Since shelling and spalling often occur in
conjunction they are often collectively referred to as
"thermo-mechanical deterioration."
[0003] Various problems arise from each form of rail head material
loss. For example, when railway rails experience thermo-engendered
deterioration, surface cracks tend to propagate from such
deteriorated areas and cause potentially dangerous defects in
contiguous regions of the rail. Similar surface cracks are also
created as a result of mechanically generated deterioration. Aside
from their catastrophic accident causing potentials, such rail
defects are also known to cause increased rail/wheel dynamic forces
that, in turn, tend to produce consequential damage such as
accelerated track deterioration. The railroad industry is therefore
constantly looking for ways to minimize both aspects of
thermo-mechanical deterioration of rails while still preserving, as
far as possible, their wear resistance qualities.
[0004] 2. Description of the Prior Art
Re: Thermo Aspects of Thermo-Mechanical Deterioration
[0005] Railroad rails eventually wear out as a result of normal
usage. Such rails are however often prematurely retired from
service as a result of various forms of thermo-mechanical
deterioration. For example, a great deal of thermo-mechanical
deterioration is associated with metallurgical transformations of
the rail steel from the original, relatively tough, pearlitic
microstructure to more brittle microstructures such as bainite
and/or martensite--with associated loss of the
austenite/bainite/martensite steel material through spalling.
Again, thermo-mechanical deterioration is caused by the heat
generated by friction when the train's wheels skid on railroad
rails during extreme braking operation. That is to say that the
above noted brittle steel materials are produced in rails when such
frictional heat is sufficient to raise the temperature of the head
region to the austenite transformation level. Upon rapid cooling,
this austenite phase is then transformed to bainite and, in many
cases, to martensite (with different level of retained austenite).
The localized rail heating can occur in time periods as short as
one second; indeed it can occur in time periods of less than one
thousandth of a second. Thereafter, because the rest of the rail
serves as a heat sink, such very high local temperatures are then
quickly lowered. A process referred to as quenching. Thus, in skid
producing braking situations, local areas of the rail head top
surface are rapidly transformed to austenite, as the steel material
rapidly heats--and then to bainite and/or martensite as the steel
rapidly cools.
[0006] Those skilled in railroad rail manufacturing arts will
appreciate that a martensite transformation progresses only while
the steel is cooling rapidly (that is to say that more and more
discrete volumes of a parent austenite solid solution transform as
the steel cools). Martensite transformations can be prevented if
the cooling process is interrupted at a temperature above the
so-called "Start Martensite" temperature. Moreover, the amount of
martensite formed per degree of decrease in temperature is not a
constant (i.e., the number of martensite crystalline units produced
at first is small, but increases exponentially as the temperature
continues to decrease). In any case, the resulting brittle
martensite steel then tends to crack and spall away from the rail
head surface.
Re: Mechanical Aspects of Thermo-Mechanical Deterioration
[0007] The mechanical aspects of thermal-mechanical deterioration
are often referred to as "rolling contact fatigue" (or RCF). The
present invention is particularly concerned with minimizing RCF in
railway rails. Again, RCF produces the undesired form of steel
material loss known as "shelling" wherein the rolling action of a
steel railroad wheel over a steel rail produces mechanical stresses
in the rail that--in their own right--contribute to a rail's
deterioration. That is to say that rolling contact fatigue can
occur even if the rail does not experience metallographic changes
attributed to temperature effects. Rolling contact fatigue is also
associated with diminished shear fatigue strength of a rail's head
surface. This form of damage is usually considered in conjunction
with the level of subsurface shear stress being applied to the rail
head, especially in the region just below the rail head's wheel
contact surface. In any case, rolling contact fatigue is related to
both the strength of the rail surface and to the load applied to
it. And, as previously noted, the strength of the rail head surface
steel is also related to its hardness.
[0008] Modern railroad rails are being called upon to carry out
increasingly severe duties. For example freight car wheels
frequently subject the rails over which they travel to local
contact stresses in excess of 160,000 p.s.i. The relatively high
loads carried by the rails lead directly to higher levels of
rolling contact fatigue. It should also be borne in mind that
modern railroad rails must be made from relatively hard steels in
order to impart acceptable wear life characteristics. The use of
hard steels notwithstanding, the incidence of shelling type defects
in railroad rails is increasing as a result of the greater loads
they are currently called upon to carry.
[0009] And as previously discussed, if a rail is heated to high
enough temperatures, the stresses produced therein can exceed the
yield strength of that rail steel. Moreover, when such rails cool
down, residual tensile stresses may remain within the rail and
subsequently serve to further open any surface cracks that may be
present. Those skilled in the rail manufacturing arts will also
appreciate that the phenomenon of shelling due to rolling contact
fatigue is much more pronounced in rails residing on long and steep
grades, e.g., in mountainous regions, where a train's brakes are
much more heavily employed.
Re: Intimate Relationships Between Thermo and Mechanical
Deterioration
[0010] Again, those skilled in the rail manufacturing arts will
appreciate that RCF in rails often occurs in intimate conjunction
with the thermo aspects of thermo-mechanical deterioration. For
example, elevated temperatures in a steel rail serve to reduce its
ability to resist mechanical loading owing to the steel's
diminished mechanical strength above certain temperatures.
Moreover, the longer a steel rail experiences elevated
temperatures, the greater the degree of shelling that will result
from this time related circumstance. Thus, in formulating rail
steels resistant to wear, thermo deterioration and/or mechanical
deterioration, one must always appreciate that these phenomena are
often intimately related.
Re: the Wear Resistance Vs. Thermo-Mechanical Deterioration
Dilemma
[0011] Ideally, steels from which railway rails are made will,
simultaneously, have high levels of the three general properties
previously described. That is to say that such steels would be
highly wear resistant, highly resistant to thermo-generated
deterioration and highly resistant to mechanical deterioration
resulting from rolling contact fatigue. Unfortunately, to varying
degrees, these properties range from being metallurgically
antagonistic to being metallurgically incompatible. For example,
increased hardness in a steel usually implies decreased resistance
to thermo-generated deterioration. Conversely, when a steel is
alloyed to be more resistant to thermo-generated deterioration,
this usually implies that the steel will be less hard, and hence,
inherently less wear resistant.
[0012] The ability of a given alloying element to create and/or
stabilize certain metallographic phases is of great importance.
Indeed, many steel alloying elements are categorized around this
concept. For example, nickel and manganese are often referred to as
austenite-forming elements. Chromium, silicon, molybdenum, tungsten
and aluminum are frequently referred to as ferrite-forming
elements. Another group of elements known as carbide-forming
elements includes chromium, tungsten, molybdenum, vanadium,
titanium, niobium, tantalum and zirconium. In most cases however,
any given desired resistance to thermo-mechanical deterioration
through the use of alloys must be considered in the context of the
degree of sacrifice of a steel's pearlitic structure that will be
the result of the specific alloying elements employed. This remains
a very important consideration because a pearlitic microstructure
serves to impart the quality of wear resistance to steel railroad
rails.
Re: Vacuum Degassing of Rail Steels
[0013] It has been long known that liquid steels, including those
used to make railroad rails, can be further purified by exposing
them to subatmospheric pressure (commonly referred to as a
"vacuum"). In effect the presence of such vacuum conditions serves
to remove dissolved gasses formed during chemical reactions of
various elements in molten steels. Applicants' degassing processes
are specifically directed at diffusing and removing various
non-metallic inclusions such as manganese sulfide (MnS) and
aluminum oxide (Al.sub.2O.sub.3) from the molten steel--and thereby
precluding their presence in the solidified rail steel. A liquid
steel under vacuum conditions forces the gas density flux to flow
down the concentration gradient towards the vacuum. Ultimately,
this serves to reduce the porosity of Applicants' solidified
rails.
[0014] The most commonly used molten steel degassing systems
generally fall into three categories--recirculating degassers, tank
degassers and stream degassers. Recirculating degassers insert two
snorkels into a ladle of liquid steel. The steel in the ladle is
drawn into a vacuum chamber wherein argon is injected to promote
turbulence. The molten steel is then exposed to vacuum conditions
in order to remove undesired gases. The degasified molten steel is
then recirculated back into the ladle via the second snorkel.
Representative recirculation degassers are disclosed in U.S. Pat.
Nos. 2,893,860 and 3,099,699. Tank degassers are vessels into which
the ladle is sent and stirred by argon injection. The chamber is
then depressurized to remove the undesired gas. Thereafter, the
ladle is removed from the vessel. Representative tank degassers are
disclosed in U.S. Pat. Nos. 1,131,488 and 2,993,780.
Re: Literature Review
[0015] The technical and patent literature reveals that many
alloying materials have been added to (or, in the case of carbon,
taken from) a host of railroad rail steel formulations for the
purposes of striking a balance between imparting hardness (and
hence wear resistance) to a given steel while imparting, as far as
possible, resistance to thermo-mechanical deterioration. By way of
general example only, it is well known that in situations where
wear resistance is the more desirable property in a rail, high
carbon steels having carbon contents ranging from about 0.73 to
about 1.0 weight percent are preferred. Such steels are especially
hard and, hence, relatively wear resistant. Such steels are not,
however, particularly resistant to thermo-mechanical deterioration.
Conversely, it is also well known that medium carbon steels having
carbon contents ranging from approximately 0.45 to 0.55 weight
percent are more resistant to thermo-mechanical deterioration than
harder steels, but they are generally less wear resistant. It is
also common knowledge that virtually all other steel alloying
elements (other than carbon) tend to produce decreased wear
resistance in railroad rails as the concentrations of such elements
are increased.
[0016] The literature also shows that it has been a long standing
custom to consider steel alloying elements in terms of the
properties they confer upon a steel (e.g., chromium makes a steel
hard, nickel and manganese make it tough, and so on). However, it
also should be appreciated that some of these custom based
statements can lead to certain misunderstandings. For example, when
a statement to the effect "chromium makes a steel hard, and hence,
wear-resistant," is encountered, one should realize that author of
such a statement probably has in mind a steel having a relatively
high (e.g., 1.2%) carbon concentration and a relatively high (e.g.,
2.0%) chromium concentration. If, however, a steel contained the
same 2.0% chromium concentration--but only a 0.10% carbon
concentration--the hardness of that steel would be considerably
lower than that of the 1.2% carbon, 2.0% chromium steel. Similarly,
if a statement to the effect that "manganese makes a steel tough"
is encountered, one should realize that the author of such a
statement probably has a steel with a high (e.g., 13%) manganese
concentration in mind because, in fact, steels containing lower
manganese concentrations (e.g., 1.0% to 5.0% manganese), especially
in conjunction with other alloys, can have relatively higher levels
of toughness.
[0017] This all goes to say that the wear resistance versus
thermo-mechanical resistance problem has a persistent dilemmatic
quality that continues to thwart the railroad industry's attempts
to extend the useful life of railway rails. It also should be noted
that railroad rail designers have long accepted that
thermo-generated deterioration is the more intractable aspect of
the wear resistance versus thermo-mechanical deterioration
resistance dilemma. Aside from economic considerations, this
acceptance generally follows from the fact that normal rail wear is
somewhat predictable, and gradual, in nature. Conversely,
heat-producing railroad wheel skids over such rails are relatively
unpredictable. Worse yet, thermo-generated deterioration tends to
produce damage that is much more immediate and much more severe in
nature. Nonetheless, most railway rail steel compositions are still
designed toward trying to (for economic reasons) satisfy railroad
industry requirements for greater wear resistance, while "silently"
conceding that thermo deterioration due to railroad wheel skids,
and/or mechanical deterioration in its own right, will be dealt
with by: (1) physically machining the rail head region on a
scheduled basis, or (2) by machining heavily spalled rails on an
"as needed" basis, or (3) by simply scrapping heavily damaged
rails.
Re: Theoretical Considerations Regarding Steel Alloys
[0018] Thus far, alloying practices have been of somewhat limited
value in dealing with the wear resistance vs. thermo-mechanical
deterioration dilemma. For example, even though the constitution of
three component steels can theoretically be deduced from ternary
phase diagrams, they are often rather difficult to interpret. Their
practical value is also limited by the fact that they only describe
equilibrium cooling conditions. Therefore, since most modern
railroad rail steels are both heat treated during their manufacture
and contain more than three alloying components, much more complex
graphing methods (e.g., Temperature Time Transformation diagrams)
must be employed and interpreted--thus far with varying degrees of
success as far as railroad rail steels are concerned.
[0019] Indeed, it seems fair to say that even though modern steel
metallurgy is a highly skilled science, it nonetheless has certain
elements of empiricism in many circumstances wherein even
relatively minor changes in the identity and/or relative
concentrations of any given alloying element can potentially make
very significant changes in the resulting properties of a given
steel. Further complexities arise from various heat treatment
processes to which steels are usually exposed. These competing
considerations are very nicely summarized by Dr. Edgar C. Bain on
page 4 of his now somewhat dated, but still very highly regarded,
work on this subject: "Functions of the Alloying Elements in
Steel." There, he said: [0020] "The author has been forced to
conclude that it is unproductive to attempt to correlate
systematically ultimate mechanical properties directly with the
presence of the several common alloying elements without
considering the proportion of the element, the carbon content, and
above all, the heat-treatment employed and the final structure.
Thus, it would seem almost misleading to say, without
qualification, that any certain element contributes, for example,
hardness and toughness to steels without stating in what
composition and after which treatment. It is now established that
an element does not, merely by its auspicious presence alone,
contribute a property, as sugar lends sweetness, without regard for
the structure favored by the element under specific
circumstances."
[0021] This concession to empiricism in the steel making arts has
not changed much over the years since Dr. Bain's seminal work was
published. For example, in discussing the alloying of steels, the
Encyclopedia Britannica Online makes a much more up-to-date
concession to steel alloying empiricism with the statement: [0022]
"Alloying elements are added to steel in order to improve specific
properties such as strength, wear, and corrosion resistance.
Although theories of alloying have been developed, most commercial
alloy steels have been developed by an experimental approach with
occasional inspired guesses."
Re: the Patent Literature Concerning Railway Rails
[0023] The patent literature reflects the railway industry's
continued attempts to deal with the wear resistance vs. rolling
contact fatigue. For example, U.S. Pat. No. 4,575,397 describes a
wear resistant steel railroad rail comprising 0.50 to 0.85 wt. %
carbon, 0.10 to 1.00 wt. % silicon, 0.50 to 1.50 wt. % manganese,
less than 0.035 wt, % phosphorus, less than 0.035 wt. % sulfur, and
less than 0.050 wt. % aluminum.
[0024] U.S. Pat. No. 4,426,236 discloses a high strength steel rail
containing 0.65 to 0.85% C, 0.50 to 1.20% Si, 0.50 to 1.50% Mn,
0.005 to 0.050% Al, 0.004 to 0.050% of one or both of Nb and Ti,
with the balance being iron and unavoidable impurities. These rails
are designed to have a surface layer (to a depth of 10 mm or more)
that has a fine pearlite structure with a tensile strength of more
than 120 kg/mm.sup.2. They also exhibit a reduction of area of more
than 40% and a surface hardness at the surface layer of H.sub.V 350
or more. Moreover, this rail steel is particularly well suited to
welding operations.
[0025] U.S. Pat. No. 5,759,299 describes a bainitic railroad rail
steel containing 0.15 to 0.45 wt % of C, 0.05 to 1.0 wt % of Si,
0.1 to 2.5 wt % of Mn, 0.03 wt % or less of P, 0.03 wt % or less of
S, 0.1 to 3.0 wt % of Cr, 0.005 to 2.05 wt % of Mo, with the
balance being iron and incidental impurities. This steel is
particularly characterized by its resistance rolling contact
fatigue.
[0026] U.S. Pat. No. 5,676,772 describes a high-strength bainitic
steel having 0.2 to 0.5 wt % C, 0.1 to 2.0 wt % Si, 0.3 to 4.0 wt %
Mn, 0.035 wt % or less of P, 0.035 wt % or less S, and 0.3 to 4.0
wt % Cr, with the balance being Fe. This steel is particularly
characterized by its damage resistance properties.
[0027] U.S. Pat. No. 5,711,914 discloses a rail steel consisting of
0.5 to 0.75% carbon, 0.10 to 0.50% silicon, greater than 0.90 and
up to 1.70% manganese, less than 0.025% aluminum, not more than
0.05% phosphorus, a tellurium content of less than 0.004%, and a
sulfur content such that the tellurium/sulfur ratio is from 0.1 to
0.6, with the balance being iron and incidental impurities.
[0028] U.S. Pat. No. 5,762,723 describes a pearlitic rail steel
comprising 0.85 to 1.20 weight % carbon. This steel is further
characterized by having a pearlite lamella spacing of not more than
100 mm, and a ratio of cementite thickness to ferrite thickness of
at least 0.15.
[0029] U.S. Pat. No. 5,830,286 discloses a wear resistant steel
containing 0.85 to 1.20% C, 0.10 to 1.00% S, 0.40 to 1.50% Mn,
0.0005 to 0.0040% B, with the balance being iron. The rail head
region is hardened to a depth of at least 20 mm and exhibits a
pearlitic structure having a hardness of at least Hv 370. The range
in the hardness within this depth is not more than Hv 30.
[0030] U.S. Pat. No. 5,879,474 describes a method of producing a
wear and rolling contact fatigue resistant bainitic rail steel. The
method comprises of the hot rolling steps the steel composition by
weight % includes 0.05 to 0.50% carbon, 1.00 to 3.00% silicon
and/or aluminum, 0.50 to 2.50% manganese, and 0.25 to 2.50%
chromium with the balance being iron. The steel can be continuously
cooled from its rolling temperature in air, or by accelerated
cooling techniques.
[0031] U.S. Pat. No. 6,254,696 describes a bainitic steel rail
containing, by weight %, 0.15 to 0.45 percent carbon, 0.10 to 2.00
percent silicon, 0.20 to 3.00 percent manganese, and 0.20 to 3.00
percent chromium, with the remainder consisting of iron. This steel
is particularly characterized as having excellent resistance to
surface fatigue failures.
[0032] U.S. Pat. No. 6,406,569 describes a method for thermal
treatment of the rail's head region that involves immersing a rail
head (at an initial temperature of about 720.degree.C.) in a
cooling agent that contains a synthetic cooling additive, and then
withdrawing the rail head from the cooling agent upon obtaining a
head surface temperature between 450 and 550.degree.C. without
temperature equalization over the entire cross-section of the rail
head.
[0033] U.S. Pat. No. RE40,263 describes a pearlitic rail steel
having improved wear resistance and comprising of 0.85 to 1.20 wt.
% C. This steel is further characterized by the fact that its
structure is pearlitic and it has a lamella spacing that is not
more than 100 nm. The ratio of cementite thickness to ferrite
thickness in the pearlite is at least 0.15.
[0034] U.S. Pat. No. RE40,263 describes railroad rails with 0.85 to
1.20 wt. % C, 0.10 to 1.00 wt. % Si, 0.40 to 1.50 wt. % Mn and if
necessary, at least one member selected from the group consisting
of Cr, Mo, V, Nb, Co and B. The head portion of this rail is
rapidly cooled at a rate ranging from 1 degree to 10 degree C./sec
from an austenite boundary temperature to a cooling stop
temperature of 700 degrees to 500 degrees C. The hardness of the
head portion of such rails is at least Hv 320. This steel is
especially characterized by its wear and damage resistance
qualities.
[0035] In closing their comments concerning the prior art
concerning steel railroad rails, applicants would say that even
though a great deal has been learned about rolling contact fatigue
in the rail head, the fact remains that such damage mechanism
contributes in a significant way to the accelerated wear of rails.
Indeed, rolling contact fatigue problems are becoming more and more
pronounced as rails are utilized to carry heavier and heavier loads
as well as more tonnage of traffic. It is therefore an important
object of this invention to provide steels for railway rails that
have increased resistance to rolling contact fatigue (and hence
shelling) by virtue of their alloy formulations--without unduly
sacrificing their wear resistant and thermo deterioration resistant
qualities. Moreover, certain added advantages can be imparted to
these particular railroad rail steels by various thermo-mechanical
processes hereinafter more fully discussed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a Continuous Cooling Transformation (CCT) diagram
for a representative steel of this patent disclosure.
[0037] FIG. 2 is a Thermo-Mechanical Process schematic diagram for
the representative steel that was used to generated the cooling
transformation (CCT) diagram of FIG. 1.
SUMMARY OF THE INVENTION
[0038] Steel alloys characterized by their virtually
fully-pearlitic, hypo-eutectoid microstructures having the alloying
formulations given below can be used to make railway rails that are
particularly resistant to rolling contact fatigue--and hence
shelling. It also should be understood that various treatments of
the molten forms of these steels (e.g., degassing) and their solid
bloom forms (e.g., thermo-mechanical processes) may be employed
during the manufacture of such rails in order to improve their
metallurgical properties. The degassing treatments of the molten
steels may include recirculation degassing, tank degassing and/or
stream degassing in ways known to this art. The thermo-mechanical
processes that may be applied to the bloom forms of these steels
may include forging, quenching, hot working, cold working and the
like. It also should be appreciated that these thermo-mechanical
processes may be more specifically directed at the top surface
("head") regions of Applicants' railroad rails.
[0039] The steel rails of this patent disclosure are comprised
of:
[0040] 0.720 to 0.860 wt % carbon;
[0041] 1.000 to 1.280 wt % manganese;
[0042] 0.450 to 1.000 wt % silicon;
[0043] 0.010 to 0.100 wt % copper;
[0044] 0.150 to 0.280 wt % chromium;
[0045] 0.0010 to 0.0500 wt % aluminum;
[0046] 0.050 to 0.120 wt % nickel;
[0047] 0.100 to 0.260 wt % molybdenum;
[0048] 0.100 to 0.210 wt % vanadium;
[0049] 0.0010 to 0.0065 wt % nitrogen;
[0050] 0.0010 to 0.0080 wt % phosphorus;
[0051] 0.0010 to 0.0040 wt % sulfur; and
[0052] 0.0100 to 0.0350 wt % niobium
with the balance being iron.
[0053] It might also be noted here that the use of the herein
described amounts of aluminum, phosphorous and sulfur is
specifically aimed at reducing the amount of non-metallic
inclusions in the final rail product. These non-metallic inclusions
have been linked to degradation in mechanical properties (e.g.,
ductility) in the final rail products owing to the fact that they
act essentially as pre-formed cracks in the metal matrix. Examples
of some of the more common non-metallic inclusions that applicants
seek to minimize are MnS, Al.sub.2O.sub.3 (commonly known as
manganese sulfide and alumina, respectively), as well as other
complex oxides.
[0054] Once applicants' liquid steel has been degassed and cast
into blooms (e.g., bars of either round or square cross-section and
of varying lengths), said blooms are then hot rolled into final
rail products. This general process is often referred to as
Thermo-Mechanical-Processing (TMP). During applicants' TMP process,
the blooms are subjected to a number of rolling stages that vary in
number from about 12 to about 16. Each rolling stage further
reduces the cross-sectional area of the bloom and forces the
original bloom towards the final shape of the rail product. Each
rolling stage is done at a specific temperature and reduction rate.
The representative TMP schematic depicted in FIG. 2 has been
simplified to reflect two (2) reductions, each of which has the aim
of simulating a number of combined rolling stages that a bloom
experiences in the applicants' rail rolling processes. The range of
rolling parameters for the applicants' rails are as follows:
TABLE-US-00001 Reheating Temperature [.degree. C.]: 1120-1240
1.sup.st Reduction Temperature [.degree. C.]: 1070-1160 1.sup.st
Reduction [%]: 40-60 1.sup.st Cooling Rate in .degree. C. per
second [.degree. C./s]: 2.0-6.0 2.sup.nd Reduction Temperature
[.degree. C.]: 840-930 2.sup.nd Reduction [%]: 40-70 2.sup.nd
Cooling Rate [.degree. C./s]: 4.0-6.0 Coil Temperature [.degree.
C.]: 500-650 Coil Hold Time [minutes]: 5-20 Air Cooling Target:
Room Temperature
[0055] Finally, it should be understood that these
thermo-mechanical operations (e.g., hot rolling) can also be
specifically directed at the head regions of applicants' rails in
order to improve their hardness and mechanical properties.
FURTHER DESCRIPTION OF THE INVENTION
[0056] Applicants have found that the presence of the previously
described alloying elements, in the concentrations given, are
especially significant factors in imparting rolling contact fatigue
occurrence in the railroad rails at the contact zone between the
rolling wheel and the rail head surface. Another key point with
respect to these steel formulations is that a pearlitic
transformation of such steels takes place at relatively long coil
hold times, see for example the continuous cooling transformation
(CCT) diagram depicted in FIG. 1. It illustrates a representative
cooling practice used to cool down applicants' steel after bloom
rolling. The cooling can be continuous, or it can be arrested at a
certain temperature and then the rail can be held at a temperature
between 600 and 700.degree. C. in order to allow a full pearlitic
transformation to take place in the rail. Once the pearlitic
transformation is completed the rails can be cooled down naturally
to reach the room temperature.
[0057] Applicants have also found that for most practical
considerations, 900.degree. C. usually constitutes an optimum
austenitic temperature. Applicants also favor holding the subject
steels at that temperature for time periods indicated in FIG. 1
that are longer than about 1000 seconds in order to ensure proper
homogenization conditions in the blooms. Again, those skilled in
this art will appreciate that railroad wheel sliding over railroad
rail is usually a sudden process that reaches high
temperatures--but for only short time periods. In such cases,
applicants have determined that the use of niobium, serves to
retain an austenite phase in the rail. Hence, niobium plays a key
role in imparting thermal damage to the railroad rail steels of
this patent disclosure. Moreover, applicants' data also suggests
that niobium is an austenite stabilizer that serves to prevent
martensite formation by providing a decrease in the start
martensite transformation temperature.
[0058] Applicants have also found that heat treatments conducted at
900.degree. C. or higher for more than 1000 seconds of
austenization, as well as the use of the cooling rates shown in
FIG. 1 serve to transform the microstructure of this representative
steel to a virtually fully-pearlitic, hypo-eutectoid pearlitic
micro-structure. However, any sudden heating of this steel to
temperatures above 900.degree. C. can create austenite. And, as
previously noted, during the cooling processes applicants' niobium
component serves particularly well in forcing retention of the
austenite phase and thereby preventing formation of a martensite
phase independent of the cooling rate conditions. The formation of
the defined pearlitic microstructure will result in the prevention
of shelling, which of course is a major goal in the development of
the herein described rail steels.
[0059] The selection of the Carbon content affects the formation of
pro-eutectoid cementite (Fe.sub.3C) at the austenite grain
boundaries during solidification. In current hypereutectoid rail
steels the Fe.sub.3C tends to form at the grain boundaries prior to
the formation of pearlite. This is a concern since this cementite
is hard and brittle, exhibiting low ductility and low impact
toughness. As a result in hypereutectoid steels the Fe.sub.3C phase
becomes the preferred site for crack nucleation in rolling contact
fatigue. The proposed steels suppress the Fe.sub.3C formation
during rail processing. In particular, Silicon was selected as the
solute element that suppresses Fe.sub.3C formation (as Si level
increased, then Fe.sub.3C growth rate decreased). In the proposed
steels Chromium and Silicon act synergistically to improve the
mechanical properties by solid solution strengthening and by
preventing the coarsening of the Fe.sub.3C phase. Manganese
additions reduced the amount of pro-eutectoid ferrite, and lowered
the temperature at which Fe.sub.3C begins to form.
[0060] Applicants' railroad rail steels will preferably have the
following micro-structure and mechanical properties:
TABLE-US-00002 Austenite Grain Size (microns) 15.0 to 40.0
Interlamellar Spacing (microns) 0.070 to 0.100 Non-Metallic
Inclusions (vol. fraction) less than 0.001 Pro-Eutectoid Cementite
(vol. fraction) Not Present
[0061] The mechanical properties of this steel are as follows:
TABLE-US-00003 Hardness (HB) greater than 400 Yield Strength [ksi]
greater than 150 Ultimate Tensile Strength in greater than 210
kilopounds per square inch [ksi] Elongation [%] greater than
12.0
[0062] An added plus for the applicants' steels is their ability to
be made with substantially the same manufacturing processes used to
make various prior art railroad rail steels. Moreover, the relative
cost of the applicants' rail steels remains competitive--especially
given their improved rolling contact fatigue resistance
qualities.
[0063] Finally, those skilled in the steel railroad rail making
arts will appreciate that, while this invention has been described
in detail and with reference to certain specific embodiments
thereof, various changes and modifications can be made therein
without departing from the spirit and scope of this patent
disclosure.
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