U.S. patent application number 15/615433 was filed with the patent office on 2017-12-07 for copper containing rail steel.
The applicant listed for this patent is CF&I Steel L.P., D/B/A Evraz Rocky Mountain Steel, Colorado School of Mines, CF&I Steel L.P., D/B/A Evraz Rocky Mountain Steel. Invention is credited to Emmanuel DE MOOR, Glenn EAVENSON, Jarred FROMAN, Joe KRISTAN, Greg LEHNHOFF, David MATLOCK, Mark RICHARDS.
Application Number | 20170349986 15/615433 |
Document ID | / |
Family ID | 60482158 |
Filed Date | 2017-12-07 |
United States Patent
Application |
20170349986 |
Kind Code |
A1 |
FROMAN; Jarred ; et
al. |
December 7, 2017 |
Copper Containing Rail Steel
Abstract
Steel railroad rails including carbon, manganese, silicon and
greater than 0.45 wt % to 1 wt % copper are provided having greater
hardness and yield strength than standard steel rails containing
less than 0.45 wt % copper. As an example, the ultimate tensile
strength of the steel rails is from 1170 MPa to 1725 MPa. As an
additional example, the hardness of the steel rails measured 2 mm
from the running surface of the rail is from 35 to 50 on the
Rockwell C scale.
Inventors: |
FROMAN; Jarred; (Golden,
CO) ; EAVENSON; Glenn; (Pueblo, CO) ;
RICHARDS; Mark; (Pueblo, CO) ; LEHNHOFF; Greg;
(Pueblo, CO) ; KRISTAN; Joe; (Pueblo, CO) ;
MATLOCK; David; (Golden, CO) ; DE MOOR; Emmanuel;
(Golden, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Colorado School of Mines
CF&I Steel L.P., D/B/A Evraz Rocky Mountain Steel |
Golden
Pueblo |
CO
CO |
US
US |
|
|
Family ID: |
60482158 |
Appl. No.: |
15/615433 |
Filed: |
June 6, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62346836 |
Jun 7, 2016 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/42 20130101; C21D 2211/009 20130101; C21D 9/04 20130101;
C22C 38/04 20130101 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C21D 9/04 20060101 C21D009/04; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04 |
Claims
1. A steel rail comprising from 0.7 to 0.95 wt % of C, from 0.8 to
1.2 wt % of Mn, from 0.26 to 0.80 wt % of Si, from greater than
0.45 to 2.0 wt % of Cu and from less than or equal to 0.35 wt % of
Cr wherein the balance is composed of Fe and less than 1 wt %
additional alloying elements and impurities.
2. The steel rail of claim 1 comprising from 0.6 to 1.0 wt %
Cu.
3. The steel rail of claim 1 comprising from 0.8 to 1.0 wt %
Cu.
4. The steel rail of claim 1, further comprising less than or equal
to 0.25 wt % Ni.
5. The steel rail of claim 1, further comprising less than or equal
to 0.05 wt % Mo.
6. The steel rail of claim 1, further comprising from 0.005 to
0.105 wt % Ti.
7. The steel rail of claim 1, further comprising less than or equal
to 0.025 wt % S.
8. The steel rail of claim 1, further comprising less than or equal
to 0.01 wt % Al.
9. The steel rail of claim 1, wherein the ultimate tensile strength
of the steel rail is from 1170 MPa to 1725 MPa.
10. The steel rail of claim 1, wherein the hardness of the steel
rail measured 2 mm from the running surface of the rail is from 35
to 50 on the Rockwell C scale.
11. A steel rail comprising from 0.9 to 1.1 wt % of C, from 0.8 to
1.2 wt % of Mn, from 0.26 to 0.80 wt % of Si, from greater than
0.45 to 2.0 wt % of Cu and from less than or equal to 0.35 wt % of
Cr wherein the balance is composed of Fe and less than 1 wt %
additional alloying elements and impurities.
12. The steel rail of claim 11 comprising from 0.6 to 1.0 wt %
Cu.
13. The steel rail of claim 11 comprising from 0.8 to 1.0 wt %
Cu.
14. The steel rail of claim 11, further comprising less than or
equal to 0.25 wt % Ni.
15. The steel rail of claim 11, further comprising less than or
equal to 0.05 wt % Mo.
16. The steel rail of claim 11, further comprising from 0.005 to
0.105 wt % Ti.
17. The steel rail of claim 11, further comprising less than or
equal to 0.025 wt % S.
18. The steel rail of claim 11, further comprising less than or
equal to 0.01 wt % Al.
19. The steel rail of claim 11, wherein the ultimate tensile
strength of the steel rail is from 1170 MPa to 1725 MPa.
20. The steel rail of claim 11, wherein the hardness of the steel
rail measured 2 mm from the running surface of the rail is from 35
to 50 on the Rockwell C scale.
21. A method for manufacturing a steel rail, the method comprising
the steps of: a) preparing a steel comprising the elements in a
range from 0.7 to 0.95 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to
0.80 by wt % of Si, from greater than 0.45 to 2.0 wt % of Cu and
less than or equal to 0.35 wt % of Cr, wherein the balance of the
steel is composed of Fe and less than 1 wt % additional alloying
elements and impurities; b) hot rolling the steel to have a rolling
finishing temperature in a range from 800.degree. C. to
1200.degree. C. and thereby forming a rail; and c) cooling the rail
at a selected cooling rate in a range from 0.1.degree. C./sec to
20.degree. C./sec beginning substantially at said rolling finishing
temperature and continuing at least until pearlite
transformation-completion temperature.
22. The method of claim 21 wherein the steel comprises from 0.9 to
1.1 wt % of C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
application 62/346,836, filed Jun. 7, 2017 which is incorporated by
reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] Efficient railroad transportation systems require that
railroad rails withstand the demands of high-axle loads,
acceleration and deceleration friction and stress, and high usage.
As schematically illustrated in the cross-sectional view of FIG.
1A, a common type of rail comprises a head 10, a base 18, and a web
16 between the head and base. The centerline is indicated as 15.
The head 10 comprises a top surface 14 and a left and right upper
gage corner 12a, 12b. The train wheel (not shown) contacts the rail
1 about the head 10. Rolling contact fatigue damage and wear occurs
on the top surface of the head of straight rail and typically the
top and one of the two upper gage corners of curved rail and is a
constant maintenance issue requiring periodic rail replacement.
[0003] Fracture toughness, or toughness, is a term used in the art
to describe steel's resistance to crack propagation. Hardness is a
term used in the art to describe steel's resistance to deformation
such as surface indentation or abrasion. Yield strength is another
term used in the art to describe a material's resistance to
deformation. A steel having a high hardness is less prone to wear
and abrasion. A steel having a high yield strength is more
resistant to deformation under the applied forces of wheels in
service. Ideal steel for rail would be one that has a high
toughness, a high yield strength and a high hardness.
[0004] In its simplest form, steel is composed of a mixture of iron
(Fe) and carbon (C) containing less than about 2 wt % carbon.
During the production process, the mixture may be cooled from about
1000.degree. C. to below about 723.degree. C. Upon slow cooling, a
mixture of iron and carbon with eutectoid composition of
approximately 0.8 wt % carbon transforms at or below approximately
723.degree. C. into a structure of alternating lamellae of ferritic
iron, and very hard iron carbide, known as cementite (Fe.sub.3C).
The resulting microstructural morphology of alternating lamellae of
ferritic iron and cementite is called pearlite, which is the most
common steel microstructure for use in rail. Eutectoid steel with a
pearlitic microstructure is characterized as having a high tensile
strength and hardness.
[0005] An iron and carbon mixture having less than about 0.8 wt %
of C results in pearlitic steel that is hypo-eutectoid. That is,
when the iron and carbon mixture is cooled from approximately
1000.degree. C. to the equilibrium eutectoid transition temperature
at approximately 723.degree. C., some of the mixture transforms
into ferrite. At and below approximately 723.degree. C., the
remaining iron and carbon can transform into pearlite. Therefore,
hypo-eutectoid steel can comprise pearlite and pro-eutectoid
ferrite.
[0006] Iron and carbon mixtures having more than about 0.8 wt % of
C are referred to as hyper-eutectoid. That is, when the iron and
carbon mixture is cooled from approximately 1000.degree. C. to
approximately 723.degree. C., some of the mixture can transform
into cementite. At and below approximately 723.degree. C., the
remaining iron and carbon can transform into pearlite. Therefore,
hyper-eutectoid steel can be comprised of pearlite and
pro-eutectoid cementite. Steel compositions having an increasing
amount of wt % of C above about 0.8 wt % will produce steel having
an increasing amount of cementite in the pro-eutectoid form as well
as more cementite in the pearlite morphology. This tends to produce
steel of increasing hardness and decreasing toughness.
Hyper-eutectoid pearlitic steel is characterized as being very hard
and therefore wear resistant, but less tough and ductile compared
to eutectoid and hypo-eutectoid steels.
[0007] Since it is desirable for railroad rails to be substantially
pearlitic, proeutectoid ferrite and proeutectoid cementite are not
desired. Consequently, rail quenching techniques, such as forced
air, can be applied to accelerate the cooling rate through the
temperature ranges where proeutectoid ferrite or cementite form,
thus reducing the amount of proeutectoid ferrite or cementite.
[0008] The presence of additional alloying elements in steel can
have an effect on the eutectoid equilibrium conditions. For
example, the presence of additional alloying elements can affect
the eutectoid carbon content and/or the eutectoid transformation
temperature. In addition, alloying elements can affect the rate of
formation and the structure of pearlite, pro-eutectoid ferrite and
pro-eutectoid cementite.
[0009] Railroad rail would benefit from being made from steel
having both high toughness and high hardness. Increasing amounts of
carbon along with alloying agents and adjustment of manufacturing
processing parameters have been used in an attempt to retain the
toughness of a hypo-eutectoid steel yet increase the hardness.
Steels alloyed with manganese, silicon and optionally chromium
(e.g., 0.74 to 0.86 wt % C, 0.75 to 1.25 wt % Mn, 0.1 to 0.6 wt %
Si and 0.30 wt % max Cr) are used for standard grade rail steel.
Rail steel compositions with higher amounts of carbon have been
described in U.S. Pat. No. 7,288,159 (Cordova) and U.S. Pat. No.
8,361,246 (Ueda). In addition, rail steels with titanium additions
have been described in U.S. Pat. No. 7,217,329 (Cordova). The
cooling rate at which the steel is cooled from a high roll-forming
temperature through the eutectoid temperature and finally to
ambient temperature has a dramatic effect on the formation of the
pearlitic structure, in which higher cooling rates produce a finer
lamellar structure compared to low cooling rates. A finer structure
pearlite is expected to increase yield strength and hardness.
BRIEF SUMMARY
[0010] In some aspects, the present disclosure provides steel
railroad rails including carbon, manganese, silicon and copper. In
other aspects, methods for manufacturing such steel rails are
provided. The amount of copper in rail steel is generally limited.
However, copper is commonly present in steel scrap used to produce
rail steel. The ability to produce useful steel rails including
higher than standard amounts of copper can provide a cost advantage
by requiring less dilution of lower grade scrap with higher grade
scrap, pig iron and/or direct reduced iron. Inclusion of greater
than 0.45 wt % to 1%, 1.5% or 2% copper can increase the hardness
and yield strength of the steel as compared to a steel including a
lesser amount of copper.
[0011] In embodiments, the steel portion of the steel rail
comprises from 0.65 to 1.1 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26
to 0.80 wt % of Si, from greater than 0.45 to 2.0 wt % of Cu and
less than or equal to 0.35 wt % of Cr. In further embodiments, the
steel portion of the steel rail comprises from 0.7 to 0.95 wt % of
C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from greater
than 0.45 to 1.0 wt % of Cu and less than or equal to 0.35 wt % of
Cr. In further embodiments, the steel portion of the steel rail
comprises from 0.9 to 1.1 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to
0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and less
than or equal to 0.35 wt % of Cr. In additional embodiments, the
steel rail comprises less than or equal to 0.25 wt % Ni, less than
or equal to 0.050 wt % Mo, from 0.005 to 0.105 wt % Ti, less than
or equal to 0.025 wt % S and/or less than or equal to 0.01 wt % Al.
In some embodiments, when the carbon content is 0.7 to 0.95 wt % of
C, the vanadium content is 0.0 to 0.02 wt %. In some embodiments,
when the carbon content is 0.9 to 1.1 wt % of C, the vanadium
content is 0.0 to 0.2 wt %. In further embodiments, the steel
comprises from 0.6 to 1.0 wt % Cu, from 0.7 to 1.0 wt % Cu, from
0.8 to 1.0 wt % Cu, or from 0.85 to 1.0 wt % Cu. In embodiments,
the balance of the steel composition comprises iron and less than 1
wt % additional alloying elements and/or impurities. In some
embodiments, at least one additional alloying element present in
the composition is present in an amount from 0.01 to 0.25 wt % or
0.01 to 0.20 wt % and is selected from the group consisting of Ni,
Mo, Ti or N. In further embodiments, the total amount of impurities
or other trace elements is less than 0.2 wt % or less than 0.1 wt
%. In some embodiments, elements present in trace amounts include,
but are not limited to, phosphorus, sulfur and combinations
thereof. In additional embodiments, hydrogen is present in amounts
of 2 ppm or less. It is noted that mill scale (e.g., a mixture of
FeO and Fe.sub.3O.sub.4) is present at the surface of the rail in
some embodiments; the above composition ranges do not encompass
such surface oxides. It is also noted that near surface
compositions may differ from the bulk composition due to carbon
depletion from the billet, bloom, or ingot form of the steel prior
to roll forming from furnace atmospheric chemical reactions with
the steel depleting the near surface carbon content.
[0012] In embodiments, the steel portion of the rail is
characterized by a substantially pearlitic microstructure. In
examples, the microstructure of the steel is fully pearlitic or
comprises less than 10% pro-eutectoid ferrite or pro-eutectic
cementite. In additional examples, ferrite regions of the pearlite
microstructure include precipitates (e.g., copper containing
precipitates). Inclusions (e.g., sulfides and oxides) that result
from the steelmaking process may also be present in the steel rail.
In some examples, the interlamellar spacing is less than 500 nm as
measured at a depth of 6 mm from the running surface or less than
300 nm as measured at a depth of 6 mm from the running surface.
[0013] In some embodiments, the ultimate tensile strength of the
steel rails is from 1170 MPa to 1725 MPa or 1375 MPa to 1450 MPa.
In additional embodiments, the yield strength of the steel rail is
from 850 MPa to 1600 MPa or 925 MPa to 1000 MPa. In an embodiment
the yield strength is an 0.2% offset yield strength. In further
embodiments, the hardness of the steel can decrease with distance
from the rail surface. In some embodiments, the hardness (as
measured from a polished cross-section) 2 mm below the top surface
of the rail surface (e.g., 14 in FIG. 1A) is from 35 to 50, 38.3 to
47, 40 to 45 or 42.5 to 44 on the Rockwell C scale (HRC).
[0014] In some embodiments, the present disclosure provides methods
for manufacturing a steel rail, the methods comprising the steps
of:
[0015] preparing a steel comprising the elements in a range from
0.65 to 1.1 wt % of C, from 0.8 to 1.2 wt % of Mn, from 0.26 to
0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and from
less than or equal to 0.35 wt % of Cr, wherein the balance of the
steel is composed of Fe and less than 1 wt % additional alloying
elements and impurities;
[0016] hot rolling the steel to have a rolling finishing
temperature in a range from 850.degree. C. to 1000.degree. C. and
thereby forming a rail; and
[0017] cooling the rail at a selected cooling rate in a range from
0.1.degree. C./sec to 20.degree. C. /sec beginning substantially at
said rolling finishing temperature and continuing at least until
600.degree. C.
[0018] In further embodiments, the steel composition is as
described for the steel rail above. For example, the steel
comprises from 0.7 to 0.95 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26
to 0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and
less than or equal to 0.35 wt % of Cr. In further a further
example, the steel portion of the steel rail comprises from 0.9 to
1.1 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from
greater than 0.45 to 1.0 wt % of Cu and less than or equal to 0.35
wt % of Cr.
[0019] In further embodiments, the steel composition is as
described for the steel rail above. For example, the steel
comprises from 0.7 to 0.95 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26
to 0.80 wt % of Si, from 0.5 to 1.0 wt % of Cu and less than or
equal to 0.35 wt % of Cr. In a further example, the steel portion
of the steel rail comprises from 0.9 to 1.1 wt % of C, 0.8 to 1.2
wt % of Mn, 0.26 to 0.80 wt % of Si, 0.5 to 1.0 wt % of Cu and less
than or equal to 0.35 wt % of Cr.
[0020] In some embodiments, the steel is in the form of a bloom or
billet prior to hot rolling. In some embodiments, the bloom or
billet is heated before being passed to the rolling mill(s). In
further embodiments, the hot rolling process involves multiple
passes of hot reduction. In embodiments, the total rolling
reduction is from 85 to 95%. In embodiments, the cross-sectional
area of the bloom or billet prior to rolling exceeds value of 8
times the cross-sectional area of the finished rail. The hot
rolling process involves a finishing stage; in embodiments the
temperature during this finishing stage, known as the rolling
finishing temperature, is from 800.degree. C. to 1100.degree. C.
After hot rolling, the steel rail is cooled to develop a pearlitic
or substantially pearlitic microstructure. In embodiments, the
endpoint of the controlled cooling process is a temperature above
ambient or room temperature at which the transformation of the
microstructure to pearlite is complete or nearly complete. This
temperature may be termed the pearlite transformation completion
temperature. In embodiments, the rail is cooled from the rolling
finishing temperature to below 600.degree. C. In embodiments, the
cooling rate is 0.1.degree. C./sec to 20.degree. C./sec or
1.degree. C./sec to 10.degree. C./sec.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A and 1B show rail cross-sections. FIG. 1A is a
schematic illustration of a cross-section of a common type of
railroad rail. FIG. 1B is a schematic illustration of a rail
section providing nominal dimensions of one commonly used rail
geometry.
[0022] FIGS. 2A, 2B, 2C, and 2D illustrate representative scanning
electron microscope secondary electron micrographs of various
alloys and a plot of inter-lamellar spacing as a function of copper
content. FIG. 2A is a micrograph of Alloy 7Cu below the running
surface of the rail. FIG. 2B is a micrograph of Alloy 38Cu below
the running surface of the rail. FIG. 2C is a micrograph of Alloy
85Cu below the running surface of the rail. FIG. 2D is a plot of
inter-lamellar spacing as a function of copper content.
[0023] FIGS. 3A and 3B illustrate hardness testing schematics. FIG.
3A is a sectioning diagram. FIG. 3B is an indentation map.
[0024] FIG. 4 shows average hardness data as a function of distance
from the running surface for three copper containing alloys.
[0025] FIG. 5 shows a tensile specimen blank sectioning
schematic.
[0026] FIG. 6 illustrates 0.2% offset yield strength and ultimate
tensile strength as a function of copper content for six
alloys.
[0027] FIG. 7 shows dry twin disc wear testing results for three
copper containing alloys.
[0028] FIG. 8 is a micrograph showing a 5 mm size bar of the
contact surface after RCF testing as described in the
description.
[0029] FIG. 9 is a graph of conformal contact pressure versus
cycles to failure RCF results showing bands of RCF life with the
general trend of increased RCF cycles to failure with decreased
contact pressures. Steels containing higher amounts of Cu exhibited
enhanced resistance to deformation, with reduced plastic
deformation of the sample crown and increased conformal contact
stresses compared to lower strength, lower Cu content alloys tested
at the same nominal maximum Hertzian contact pressure.
DETAILED DESCRIPTION OF THE INVENTION
[0030] In the accompanying drawings, like reference numerals
designate like parts.
[0031] In some embodiments, the amount of a given non-pearlitic
phase is specified to be less than a certain amount. In an
embodiment, the amount of a given phase may be determined from a
polished cross-section of the sample. As an example, the amount of
the phase is determined from area percentage of the phase in the
polished cross-section.
[0032] In some embodiments, the range of chemical components for
steel is as follows:
[0033] In embodiments, the amount of carbon (C) is from 0.65 to 1.1
wt %, from greater than 0.65 to 1.1 wt %, from 0.7 to 1.1 wt %,
from 0.7 to 0.95 wt %, from 0.9 to 1.1 wt % or from greater than
0.9 to 1.1 wt % of C. Carbon, as explained above, contributes to
the hardness of the steel. The amount of carbon directly determines
if the steel will have hypo-eutectoid properties (i.e., pearlite
with ferrite), eutectoid properties (i.e., pearlite only), or
hyper-eutectoid properties (i.e., pearlite with cementite). The
higher the amount of carbon, the steel is generally of higher
hardness, but undesirable pro-eutectoid cementite, which reduces
ductility and toughness, is more difficult to control.
[0034] In some embodiments, the amount of manganese (Mn) is from
0.8 to 1.2 wt %. Manganese, like silicon, is used to deoxidize the
steel matrix. Further, manganese improves the steel's hardness. As
the amount of manganese is increased, the manganese can segregate
from the matrix, which is detrimental to the resulting steel's
toughness.
[0035] In some embodiments, the amount of silicon (Si) is from 0.26
to 0.8 wt %. Silicon is used to deoxidize the steel matrix and
improves the strength of the resulting steel. An amount of silicon
approaching 1.0 wt %, in conjunction with the other specified
alloying levels, is predicted to result in non-pearlitic
microstructures.
[0036] In some embodiments, the amount of copper (Cu) is from
greater than 0.45 to 1.0 wt %, from 0.6 to 1.0 wt %, from 0.7 to
1.0 wt %, from 0.8 to 1.0 wt %, from 0.85 to 1.0 wt %, from 0.7 to
0.9 wt %, from greater than 0.45 to 1.5 wt %, from 0.6 to 1.5 wt %,
from 0.8 to 1.5 wt %, from greater than 0.45 to 2 wt %, from 0.6 to
2 wt %, or from 0.8 to 2 wt %.
[0037] In some embodiments, the amount of copper (Cu) is from 0.5
to 1.0 wt %, from 0.6 to 1.0 wt %, from 0.7 to 1.0 wt %, from 0.8
to 1.0 wt %, from 0.85 to 1.0 wt %, from 0.7 to 0.9 wt %, from 0.5
to 1.5 wt %, from 0.6 to 1.5 wt %, from 0.8 to 1.5 wt %, from 0.5
to 2 wt %, from 0.6 to 2 wt %, or from 0.8 to 2 wt %.
[0038] In some embodiments, the amount of chromium (Cr) has an
upper limit. In further embodiments, the amount of chromium is less
than or equal to about 0.35 wt %. Chromium can improve the strength
of the resulting steel by making the lamellae of the pearlite
thinner.
[0039] In some embodiments, the amount of nickel (Ni) is less than
or equal to 0.25 wt %.
[0040] In some embodiments, the amount of molybdenum (Mo) is less
than about 0.050 wt %. Molybdenum in a quantity up to 0.050 wt % is
utilized for the hardenability characteristics of the resulting
alloy.
[0041] In some embodiments, the amount of titanium (Ti) is from
0.005 to 0.105 wt %. Titanium is used to control austenitic grain
growth in the hot rolling process, providing a finer grain size in
the final product.
[0042] In some embodiments, the amount of vanadium (V) is up to
0.02 wt % or up to 0.20 wt %. As examples, when the carbon content
is 0.7 to 0.95 wt %, the vanadium content is 0.0 to 0.02 wt % or
when the carbon content is 0.9 to 1.1 wt % of C, the vanadium
content is 0.0 to 0.2 wt %. Vanadium improves the hardness and
strength of the resulting steel.
[0043] In some embodiments, the amount of aluminum (Al) is less
than or equal to 0.01 wt %.
[0044] In some embodiments, the amount of sulfur (S) is less than
or equal to 0.025 wt %. Sulfur is an inevitable impurity that is
detrimental to the toughness of the resulting steel. It has been
determined that as much as 0.025 wt % of S is acceptable for the
steel of this invention, and therefore, is used as an upper
limit.
[0045] In some embodiments, the amount of phosphorus (P) is less
than about 0.025 wt %. Phosphorus is an inevitable impurity that is
detrimental to the toughness of the resulting steel. It has been
determined that as much as 0.025 wt % of P is acceptable for the
steel of this invention, and therefore, is used as an upper
limit.
[0046] In some embodiments, the amount of hydrogen (H) is less than
or equal to 3 ppm, less than or equal to 2 ppm, less than or equal
to 1.5 ppm or less than or equal to 1 ppm.
[0047] There are four predominant production methods used in the
art to cool rail. They are air cooling, air/water cooling, oil
submersion, and aqueous polymer submersion. Any method may be used
in the present invention as long as the prescribed controlled rate
of cooling is obtained.
[0048] The air/water cooling technique presents a mixture of air
and water to the rail, cooling the rail in a dual process of heat
of vaporization of the water and forced convection of the air. The
cooling rates can be adjusted by controlling the air and water
temperatures and flowrates, the form of the water such as droplets,
mist or fog, and the timing and duration of the exposure to the
quench media.
[0049] The oil submersion technique is where the rail is submerged
into a tank of oil. The cooling rates can be adjusted by
controlling the oil temperature, oil formulation, fluid flow and
agitation, and duration of the immersion step or steps.
[0050] The aqueous polymer submersion technique is where the rail
is submerged into a tank of aqueous polymer. The aqueous polymer
has a high vaporization temperature effectively preventing boiling
at the rail surface and producing a more uniform cooling
environment. The cooling rates can be adjusted by controlling the
polymer formulation and concentration, controlling fluid flow and
agitation, and duration of the immersion step or steps.
[0051] The air cooling technique uses pressurized gas,
traditionally atmospheric air but not limited to air, to cool the
rail thru conduction of the rail heat to the gas. The cooling rates
can be adjusted by controlling the air temperature, flow rates, and
duration of the exposure to the cooling media.
[0052] In one embodiment in accordance with the method of
manufacturing the rail, controlled-rate in-line forced-air cooling
is performed. In-line cooling consists of cooling the rail
immediately after it is rolled. This is in contrast to re-heating
previously cooled rail to the desirable temperature at a different
location off of the rolling line and cooling it using the desired
cooling rate. In-line cooling is preferable in terms of
manufacturing efficiency and achieving mechanical properties in the
head of the rail.
[0053] In some embodiments, steel having the composition as
described above is roll-formed at a temperature range of
approximately 800-1200.degree. C. to a net shape of the finished
rail, in accordance with known roll-forming techniques. The
roll-formed rail enters a heat treating unit which controls the
cooling rate of the rail, and is otherwise known as a head
hardening unit or line-slack quench (LSQ) unit. The rail is cooled
at a controlled rate in a range from 0.1.degree. C./sec to
20.degree. C./sec using a forced air quench operation. The rail is
cooled at this rate until the rail reaches a desired temperature.
In embodiments, rail is cooled at this rate until the rail reaches
a temperature below 600.degree. C.
[0054] A head hardening unit apparatus suitable for use in the
manufacture of rail in accordance with the present invention
comprises a conveyor and an air-handling system. Rail is placed
into the air cooling position with the conveyor and is then
heat-treated (cooled) with air. The air-handling system comprises a
series of nozzles strategically placed around the rail from which
air is discharged under pressure. The air handling apparatus
controls the cooling rate of the rail by controlling the air
pressure. After heat-treatment, the rail is moved out of position
in the head hardening unit to the next processing step.
[0055] Every formulation or combination of components described or
exemplified can be used to practice the invention, unless otherwise
stated. Specific names of compounds are intended to be exemplary,
as it is known that one of ordinary skill in the art can name the
same compounds differently. When a compound is described herein
such that a particular isomer or enantiomer of the compound is not
specified, for example, in a formula or in a chemical name, that
description is intended to include each isomers and enantiomer of
the compound described individual or in any combination. One of
ordinary skill in the art will appreciate that methods, device
elements, starting materials and synthetic methods other than those
specifically exemplified can be employed in the practice of the
invention without resort to undue experimentation. All art-known
functional equivalents, of any such methods, device elements,
starting materials and synthetic methods are intended to be
included in this invention. Whenever a range is given in the
specification, for example, a temperature range, a time range, or a
composition range, all intermediate ranges and subranges, as well
as all individual values included in the ranges given are intended
to be included in the disclosure.
[0056] As used herein, "comprising" is synonymous with "including,"
"containing," or "characterized by," and is inclusive or open-ended
and does not exclude additional, unrecited elements or method
steps. As used herein, "consisting of" excludes any element, step,
or ingredient not specified in the claim element. As used herein,
"consisting essentially of" does not exclude materials or steps
that do not materially affect the basic and novel characteristics
of the claim. Any recitation herein of the term "comprising",
particularly in a description of components of a composition or in
a description of elements of a device, is understood to encompass
those compositions and methods consisting essentially of and
consisting of the recited components or elements. The invention
illustratively described herein suitably may be practiced in the
absence of any element or elements, limitation or limitations which
is not specifically disclosed herein.
[0057] The terms and expressions which have been employed are used
as terms of description and not of limitation, and there is no
intention in the use of such terms and expressions of excluding any
equivalents of the features shown and described or portions
thereof, but it is recognized that various modifications are
possible within the scope of the invention claimed. Thus, it should
be understood that although the present invention has been
specifically disclosed by preferred embodiments and optional
features, modification and variation of the concepts herein
disclosed may be resorted to by those skilled in the art, and that
such modifications and variations are considered to be within the
scope of this invention as defined by the appended claims.
[0058] In general the terms and phrases used herein have their
art-recognized meaning, which can be found by reference to standard
texts, journal references and contexts known to those skilled in
the art. The following definitions are provided to clarify their
specific use in the context of the invention.
[0059] All references cited herein are hereby incorporated by
reference to the extent not inconsistent with the disclosure
herewith. All patents and publications mentioned in the
specification are indicative of the levels of skill of those
skilled in the art to which the invention pertains.
[0060] Although the description herein contains many specificities,
these should not be construed as limiting the scope of the
invention but as merely providing illustrations of some of the
presently preferred embodiments of the invention. For example, thus
the scope of the invention should be determined by the appended
claims and their equivalents, rather than by the examples
given.
[0061] The invention may be further understood by the following
non-limiting examples.
THE EXAMPLES
[0062] Six alloys with copper levels varying from 0.07 percent by
weight or (wt %) to 0.85 wt % were produced to the same rail
section following the same processing procedures used for all rail
production at Evraz Pueblo. Processing consisted of heating the
round cast blooms with dimensions of 311 mm (12.25 in) diameter by
5.89 m (19.33 ft) length in a natural gas walking beam re-heat
furnace, a total rolling reduction of 88.7% of hot reduction to the
final nominal dimensions shown in FIG. 1B (dimensions in inches), a
final rolling temperature above 750.degree. C., and forced cooling
through pearlitic transformation in a production head hardening
unit.
[0063] Tables 1A and 1B show the chemical composition of selected
copper containing alloys in weight percentage except as noted.
TABLE-US-00001 TABLE 1A Alloy C wt % Mn wt % Si wt % Cu wt % Ni wt
% Cr wt % 7 Cu 0.93 1.02 0.38 0.07 0.04 0.23 11 Cu 0.94 1.02 0.37
0.11 0.05 0.23 22 Cu 0.91 1.02 0.36 0.22 0.07 0.24 29 Cu 0.92 1.03
0.34 0.29 0.09 0.24 38 Cu 0.93 1.01 0.33 0.38 0.08 0.22 85 Cu 0.93
1.05 0.32 0.85 0.07 0.21
TABLE-US-00002 TABLE 1B Alloy Mo wt % Ti wt % N wt % P wt % S wt %
H ppm 7 Cu 0.007 0.007 0.0060 0.007 0.012 0.8 11 Cu 0.012 0.007
0.0071 0.007 0.007 0.9 22 Cu 0.017 0.008 0.0108 0.010 0.011 1.5 29
Cu 0.018 0.009 0.0082 0.009 0.009 1.4 38 Cu 0.018 0.010 0.0109
0.012 0.010 0.9 85 Cu 0.019 0.010 0.0081 0.013 0.009 1.0
[0064] FIGS. 2A, 2B and 2C, show representative scanning electron
microscope secondary electron micrographs. The microscope
accelerating voltage was 10 kV. The white layers are cementite and
the dark layers are ferrite. FIGS. 2A, 2B and 2C are from
transverse specimens below the running surface of the rail head
(14?). FIG. 2D shows a plot of inter-lamellar spacing as a function
of copper content below the running surface using a linear
intercept method. The plot shows a significant difference between
the 7 Cu and the 85 Cu alloys.
[0065] Full head specimens of 22.2 mm (0.875 in) thickness were
sectioned from the 7 Cu, 38 Cu, and 85 Cu alloys according to the
schematic in FIG. 3A. Both cross-sectional surfaces were machined
flat to remove the rough surface from the initial sectioning
operation and ensure parallel surfaces. The surface to be tested
was given an additional 120 grit grinding step. Rockwell C scale
hardness testing was performed. A 4 mm.times.2 mm grid of hardness
tests was performed on the rail head cross section, providing
hardness readings at 2 mm depth intervals. After data collection,
each specimen was re-machined to remove 2 mm (0.08 in) and retested
for hardness. This process was repeated five times per specimen,
providing a minimum of 35 hardness measurements per depth
increment. FIG. 3B illustrates the testing locations.
[0066] FIG. 4 displays the average hardness data as a function of
distance from the running surface. The average hardness data were
obtained from the seven hardness traverse locations shown
schematically in FIG. 3B.
[0067] Tensile properties were measured on samples machined from
254 mm (10 in) lengths of full rail cross-section. Longitudinal
tensile specimens with a gage diameter of 8.75 mm (0.350 in) were
sectioned from the gauge corner of the head (FIG. 5). The
centerline of the tensile specimens corresponded to a depth of 6 mm
(0.236 in) from the rail head surface. Five tensile specimens were
machined from each alloy, stress relieved at 93.3.degree. C.
(200.degree. F.) for two hours, and tested in accordance with
ASTM-A370 and ASTM-E8 on a 489 kN (110 kip) tensile frame. Prior to
testing each specimen was marked with a 35 mm (1.4 in) gauge punch
and ground with 180 grit sandpaper to ensure a consistent surface
roughness. The tensile test was performed at a stressing rate of
345 MPa (50 ksi) per minute until the 0.2% offset yield strength
had been determined. The program then switched to cross-head speed
control at a rate of 12.7 mm (0.5 inch) per minute through specimen
fracture. A 35 mm (1.4 in) extensometer was used to determine the
0.2% offset yield strength.
[0068] Five tensile specimens per alloy were prepared and tested.
FIG. 6 illustrates the 0.2% offset yield and ultimate tensile
strengths as a function of copper content for the six alloys. The
data suggest an increase in yield and tensile strength level with
increasing copper content.
[0069] Charpy impact specimens were tested at room temperature to
assess the influence of copper on the dynamic fracture behavior of
the steels. Eight specimens per alloy were prepared for testing at
room temperature. Due to the inherently high strength and fully
pearlitic microstructure of the alloys evaluated, a 2 mm U-notch
was employed rather than the typical 2 mm V-notch or 5 mm U-notch
as referenced in ASTM-E23. The Charpy blanks were sectioned out of
the gauge corner and machined to 10 mm.times.10 mm.times.55 mm with
the long dimension parallel to the rolling direction. A 1 mm radius
U-notch, parallel to the transverse direction, on the surface
closest to the running surface, was introduced by broaching. The
specimens were tested on a 406.7 J (300.0 ft-lbf) capacity Charpy
impact test frame at an ambient temperature of 22.7.degree. C.
(72.9.degree. F.) and 80% relative humidity. Energy absorbed during
the test was recorded and fracture surfaces were evaluated visually
for percent shear.
[0070] Table 2 summarizes the observed average room temperature
(22.7.degree. C.) Charpy U-notch (2 mm) impact energies for the six
experimental alloys along with the calculated standard deviations
based on eight replicates for each condition. Measured impact
toughness values ranged between 9.0 J and 10.8 J and the data do
not exhibit a relationship with Cu content. The impact toughness
values indicate that all specimens fractured in a brittle manner, a
conclusion that is consistent with analysis of the fracture
surfaces. All samples exhibited brittle fracture without the
presence of discernable shear lips.
TABLE-US-00003 TABLE 2 Impact Energy Standard Deviation Alloy
(Joule) (Joule) 7 Cu 10.7 1.45 11 Cu 9.5 1.59 22 Cu 9.4 1.67 29 Cu
9.0 1.47 38 Cu 10.8 1.79 85 Cu 10.7 1.82
[0071] Table 3 shows average K.sub.1C Fracture Toughness for the
alloys listed. The 7 Cu alloy had the highest average K.sub.1C at
38.6 MPa Im while the 38 Cu alloy had the lowest average K.sub.1C
at 34.2 MPa m. This difference is most likely the result of normal
variation experienced when testing the same grade of steel. The
data do not demonstrate that Cu alloying influences the fracture
toughness of the material.
TABLE-US-00004 TABLE 3 Avg. K.sub.1c Std. Dev. Alloy (MPa m) (MPa
m) 7 Cu 38.6 1.25 11 Cu 36.0 0.88 22 Cu 36.4 0.73 29 Cu 36.7 0.46
38 Cu 34.2 0.71 85Cu 35.8 0.40
[0072] Fatigue crack growth rate was also assessed. The results
indicated that the copper level had no discernable influence on
fatigue crack growth rate.
[0073] Dry, twin-disc wear testing was also performed on 7 Cu, 38
Cu and 85 Cu alloys. A summary of the results is displayed in FIG.
7 where a slight decrease in wear rate with increasing copper
content is shown. Mass loss was measured periodically after an
initial 1,000 cycle run-in period for disc-on-disc rail wear
samples for copper levels of 0.07 wt %, 0.38 wt %, and 0.85 wt %.
Samples were tested at 1300 MPa with 10% slip based upon surface
velocities of the two discs. Uncertainty bars represent standard
deviation of the data sets. The wear rates of the 0.07 wt % Cu and
0.85 wt % Cu alloys were compared for the 10,000-25,000 cycle
period
[0074] Surface roughness measurements were also obtained on three
wear samples of each of the three alloys. The study was performed
to reveal any difference in the depth of gouging or the size of
flaking that is contributing to the difference in wear rate.
Samples from all three copper containing steels show flaking on the
surface. The average surface roughness (Ra) was from 1 mm to 3 mm
and the maximum peak-to-valley height (Rt) was from approximately
20 .mu.m to 140 .mu.m. The surface roughness of the samples was
comparable between the three alloys.
[0075] The influence of Cu on the anticipated performance of the
rail was simulated through rolling-sliding contact fatigue (RCF)
testing on a twin-disc tribometer. The 0.07-Cu, 0.38-Cu, and
0.85-Cu materials were selected for the twin-disc testing.
[0076] Rail samples were machined from the head of the rail. The
samples were then ground to a 1.77 in. (45 mm) diameter with a
0.394 in. (10 mm) wide running surface and a 0.984 in. (25 mm)
crown radius. The rail samples were tested against a 1.819 in.
(46.2 mm) diameter, 0.787 in. (20 mm) wide cylindrical wheel sample
made from quench and tempered 4140 with a hardness of approximately
50 HRC.
[0077] An industrially available top-of-rail friction modifier was
applied at the point of contact of the rail and wheel samples by a
time controlled micro pump as a means of controlling the friction
and preventing wear that would inhibit the formation of RCF cracks.
The use of the friction modifier resulted in a coefficient of
traction of approximately 0.1.
[0078] The RCF tests were conducted under constant load conditions,
where the force between the rail and the proxy wheel samples
resulted in a nominal maximum Hertzian contact pressure, P.sub.0,
of between 406 and 493 ksi (2800 and 3400 MPa). The RCF tests were
performed with a 20% slip ratio, which is the ratio of the
difference between the surface velocities to the average of the
surface velocities, where the wheel was driving and the rail was
braking (wheel velocity>rail velocity). The tests were monitored
by use of an eddy current sensor. The tests were terminated when
the signal from an eddy current probe surpassed a threshold value,
which was established with a machined reference flaw. If the sample
achieved a 1,000,000 cycle count without the eddy current signal
exceeding the signal threshold the test was arrested as a
runout.
[0079] An example of the contact surface after testing is shown in
FIG. 8. In all tested samples, the sample surface exhibited a dark
contact band. The profile of the material shows that the material
in the contact band had deformed, resulting in a larger contact
radius and lower contact stress. Sharp shoulders exist at the
transition from the as-machined crown to the edge of the contact
band. The width of the contact band was measured by use of a 3D
imaging mosaic technique on a digital microscope, and it was
verified that the discolored contact band edge aligned with the
deformed shoulders in the profile measurement. The measured contact
band width was used in conjunction with the applied force to
estimate the actual sustained contact pressure the material was
subjected to after plastic deformation. This calculated contact
pressure is referred to herein as the conformal contact pressure,
whereas the initial contact pressure based upon as-machined
specimen geometry for linear elastic contact is referred to as the
nominal maximum Hertzian contact pressure.
[0080] The conformal contact pressure versus cycles to failure RCF
results are shown in FIG. 9. The results show bands of RCF life
with the general trend of increased RCF cycles to failure with
decreased contact pressures. Due to the increased strength from the
Cu alloying, the higher Cu containing steels exhibited enhanced
resistance to deformation, which resulted in reduced plastic
deformation of the sample crown and increased conformal contact
stresses in comparison to lower strength, lower Cu content alloys
tested at the same nominal maximum Hertzian contact pressure.
Additionally, the RCF results in FIG. 9 show an increase in RCF
life (cycles to failure) with increasing Cu content for a similar
conformal contact pressure.
* * * * *