Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
INTERNAL HIGH HARDNESS TYPE PEARLITIC RAIL WITH EXCELLENT
WEAR RESISTANCE, ROLLING CONTACT FATIGUE RESISTANCE, AND
DELAYED FRACTURE PROPERTY AND METHOD FOR PRODUCING SAME
Technical Field
The present invention relates to an internal high
hardness type pearlitic rail with excellent wear resistance
and rolling contact fatigue (RCF) resistance and a method
for producing the same. Specifically, the present invention
relates to an internal high hardness type pearlitic rail
having excellent wear resistance, rolling contact fatigue
resistance, and delayed fracture properties and achieving
longer operating life of rails used under severe high-axle
load conditions like foreign mining railways in which
-freight cars are heavy and high curve lines are often
present, and to a method for producing the internal high
hardness type pearlitic rail.
Background Art
In high-axle load railways mainly transporting mineral
ores, a load on an axle of a freight car is significantly
higher than that of a passenger car, and the use environment
of rails is also severe. Rails used in such an environment
have been mainly composed of steel having a pearlitic
structure from the viewpoint of significant concern of wear
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resistance. To enhance the efficiency of railway transport,
progress has recently been made in increasing carrying
capacity. Thus, there is a need for further improvement in
wear resistance and rolling contact fatigue resistance.
High-axle load railways are used to indicate railways in
which trains and freight cars have a large carrying capacity
(for example, a carrying capacity of about 150 ton or more
per freight car).
In recent years, various studies have been conducted in
order to further improve wear resistance. For example, in
Japanese Unexamined Patent Application Publication Nos. 8-
109439 and 8-144016, theC content is increased to more than
0.85% and 1.20% by mass or less. In Japanese Unexamined
Patent Application Publication Nos. 8-246100 and 8-246101,
the C content is increased to more than 0.85% to 1.20% by
mass or less and a rail head is subjected to heat treatment.
In this way, for example, a technique for improving wear
resistance by increasing the C content to increase the
cementite ratio has been used.
Meanwhile, rails placed in curved sections of high-axle
load railways are subjected to rolling stress due to wheels
and slip force due to centrifugal force, causing severe wear
of rails and fatigue damage due to slippage. As described
above, in the case where the C content is simply more than
0.85% and 1.20% by mass or less, a proeutectoid cementite
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structure is formed depending on heat treatment conditions,
and the amount of a cementite layer in a brittle lamellar
pearlitic structure is also increased; hence, rolling
contact fatigue resistance is not improved. Japanese
Unexamined Patent Application Publication No. 2002-69585,
thus, discloses a technique for inhibiting the formation of
proeutectoid cementite by addition of Al and Si to improve
rolling contact fatigue resistance. The addition of Al,
however, causes the formation of an oxide acting as a
starting point of fatigue damage, for example. It is thus
difficult to satisfy both wear resistance and rolling
contact fatigue resistance of a rail having a pearlitic
structure.
To improve the operating life of rails, in Japanese
Unexamined Patent Application Publication No. 10-195601, a
portion located from the surface of corners and of the top
of the head of the rail to a depth of at least 20 mm have a
hardness of 370HV or more, thereby improving the operating
life of the rail. In Japanese Unexamined Patent Application
Publication No. 2003-293086, by controlling a pearlite block,
a portion located from the surface of corners and of the top
of the head of the rail to a depth of at least 20 mm have a
hardness of 300HV to 500HV, thereby improving the operating
life of the rail.
Further strengthening of a rail increases the risk of
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causing a delayed fracture. In Japanese Unexamined Patent
Application Publication Nos. 8-109439, 8-144016, 8-246100,
8-246101, 2002-69585, 10-195601, and 2003-293086, the effect
of preventing the delayed fracture is not sufficient.
As a technique for preventing a delayed fracture of a
rail composed of pearlitic steel (hereinafter, referred to
as a "pearlitic rail"), for example, Japanese Patent No.
3648192 and Japanese Unexamined Patent Application
Publication No. 5-287450 disclose a technique for improving
delayed fracture properties by subjecting high-strength
pearlitic steel to heavy drawing. In the case of applying
the technique to rails, disadvantageously, the use of heavy
drawing increases the production cost of rails.
The control of the figure and volume of A-type
inclusions disclosed in Japanese Unexamined Patent
Application Publication Nos. 2000-328190 and 6-279928,
Japanese Patent No. 3323272, and Japanese Unexamined Patent
Application Publication No. 6-279929 is also known to be
effective as a technique for improving delayed fracture
properties. In Japanese Unexamined Patent Application
Publication Nos. 2000-328190 and 6-279928, Japanese Patent
No. 3323272, and Japanese Unexamined Patent Application
Publication No. 6-279929, however, the figure and volume of
A-type inclusions are controlled in order to improve the
toughness and ductility of rails. For example, in Japanese
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Unexamined Patent Application Publication No. 6-279928, A-
type inclusions are controlled so as to have a size of 0.1
to 20 m and in such a manner that the number of the A-type
inclusions is 25 to 11,000 per square millimeter, thereby
improving the toughness and ductility of a rail. Thus, this
technique does not necessarily provide satisfactory delayed
fracture properties.
The use environment of pearlitic rails, however, has
been increasingly severe. To improve the operating life of
pearlitic rails, there has been a challenge for higher
hardness, the expansion of the range of quench hardening
depth, and improvement in delayed fracture properties. To
solve the problems, the present invention has been
accomplished. The optimization of the addition of Si, Mn,
Cr, V, and N, optimizations of a quench hardenability index
(hereinafter, referred to as "DI") and a carbon equivalent
(hereinafter, referred to as "Ceq"), and keeping the values
of [%Mn]/[%Cr] and [%V]/[%N], where [%Mn] represents the Mn
content, [%Cr] represents the Cr content, [%V] represents
the V content, and [%N] represents the N content, within
proper ranges increase the hardness of a portion located
from the surface of a rail head to a depth of at least 25 mm,
as compared with hypoeutectoid-, eutectoid-, and
hypereutectoid-type pearlitic rails in the related art,
thereby providing an internal high hardness type pearlitic
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rail with excellent wear resistance, rolling contact fatigue
resistance, and delayed fracture properties. The present
invention also provides a preferred method for producing the
internal high hardness type pearlitic rail.
Disclosure of Invention
To overcome the foregoing problems, the inventors have
produced pearlitic rails with different proportions of Si,
Mn, Cr, V, and N and have conducted intensive studies on the
structure, hardness, wear resistance, rolling contact
fatigue resistance, and delayed fracture properties. As a
result, the inventors have found that in the case where the
value of [%Mn]/[%Cr], which is calculated from the Mn
content [%Mn] and the Cr content [%Cr], is greater than or
equal to 0.3 and less than 1.0 and where the value of
[%V]/[%N], which is calculated from the V content [%V] and
the N content [%N], is in the range of 8.0 to 30.0, the
spacing of the lamella (lamellar spacing) of a pearlite
layer (hereinafter, also referred to simply as a "lamella")
is reduced, and the internal hardness of a rail head that is
defined by the Vickers hardness of a portion located from a
surface layer of the rail head to a depth of at least 25 mm
is greater than or equal to 380Hv and less than 480Hv,
thereby improving wear resistance, rolling contact fatigue
resistance, and delayed fracture properties. Furthermore,
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the inventors have found that in the case where the quench
hardenability index (i.e., the DI value) is in the range of
5.6 to 8.6, the carbon equivalent (i.e., the Ceq value) is in
the range of 1.04 to 1.27, and the value of [%Si] + [%Mn] +
[%Cr], which is calculated from the Mn content [%Mn], the Cr
content [%Cr], and the Si content [%Si], is in the range of
1.55% to 2.50% by mass, the effect of improving wear
resistance and rolling contact fatigue resistance can be
stably maintained.
The present invention has been accomplished on the
basis of these findings.
According to the present invention, an internal high
hardness type pearlitic rail with excellent wear resistance,
rolling contact fatigue resistance, and delayed fracture
properties has a composition containing 0.73% to 0.85% by
mass C, 0.5% to 0.75% by mass Si, 0.3% to 1.0% by mass Mn,
0.035% by mass or less P, 0.0005% to 0.012% by mass S, 0.2%
to 1.3% by mass Cr, 0.005% to 0.12% by mass V, 0.0015% to
0.0060% by mass N, and the balance being Fe and incidental
impurities, in which the value of [%Mn]/[%Cr] is greater
than or equal to 0.3 and less than 1.0, where [%Mn]
represents the Mn content, and [%Cr] represents the Cr
content, and the value of [%V]/[%N] is in the range of 8.0
to 30.0, where [%V] represents the V content, and [%N]
represents the N content, and in which the internal hardness
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of a rail head is defined by the Vickers hardness of a
portion located from a surface layer of the rail head to a
depth of at least 25 mm and is greater than or equal to
380Hv and less than 480Hv.
In the internal high hardness type pearlitic rail of
the present invention, preferably, the value of DI
calculated from expression (1) is in the range of 5.6 to 8.6,
and the value of Ceq calculated from expression (2) is in the
range of 1.04 to 1.27,
DI = (0.548 [%C] 112) x (1 + 0.64 [%Si]) x (1 + 4.1[%Mn]) x (1 +
2.83[%P]) x (1 - 0.62[%S]) x (1 + 2.23[%Cr]) x (1 +
1.82[%V]) (1); and
Ceq = [%C] + ([%Si] /11) + ([%Mn] /7) + ([%Cr] /5.8) + [%V]
(2)
where [%C] represents the C content, [%Si] represents the Si
content, [%Mn] represents the Mn content, [%P] represents
the P content, [%S] represents the S content, [%Cr]
represents the Cr content, and [%V] represents the V content
of the composition.
Preferably, the value of [%Si] + [%Mn] + [%Cr] is in
the range of 1.55% to 2.50, where [%Si] represents the Si
content, [%Mn] represents the Mn content, and [%Cr]
represents the Cr content of the composition. Preferably,
the composition further contains one or two or more selected
from 1.0% by mass or less Cu, 1.0% by mass or less Ni,
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0.001 to 0.05% by mass Nb, and 0.5% by mass or less Mo.
In the internal high hardness type pearlitic rail of
the present invention, preferably, the lamellar spacing of a
pearlite layer in the portion located from the surface layer
of the rail head to a depth of at least 25 mm is in the
range of 0.04 to 0.15 m.
Furthermore, according to the present invention, a
method for producing an internal high hardness type
pearlitic rail with excellent wear resistance, rolling
contact fatigue resistance, and delayed fracture properties
includes hot-rolling a steel material having the composition
described above to form a rail in such a manner that the
finishing rolling temperature is in the range of 850 C to
950 C, and then slack-quenching the surface of the railhead
from a temperature equal to or higher than a pearlite
transformation starting temperature to 400 C to 650 C at a
cooling rate of 1.2 to 5 C/s.
According to the present invention, a pearlitic rail
having excellent wear resistance, rolling contact fatigue
resistance, and delayed fracture properties can be stably
produced compared with pearlitic rails in the related art.
This contributes to longer operating life of pearlitic rails
used for high-axle load railways and to the prevention of
railway accidents, providing industrially beneficial effects.
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Brief Description of Drawings
[Figs. lA and 1B] Figs. 1A and 1B show a Nishihara-type
rolling contact test piece used for evaluation of wear
resistance, Fig. 1A is a plan view, and Fig. 1B is a side
view.
[Fig. 2] Fig. 2 is a cross-sectional view of a rail
head and illustrates positions where Nishihara-type rolling
contact test pieces are taken.
[Figs. 3A and 3B] Figs. 3A and 3B show a Nishihara-type
rolling contact test piece used for evaluation of rolling
contact fatigue resistance, Fig. 3A is a plan view, and Fig.
3B is a side view.
[Fig. 4] Fig. 4 is a cross-sectional view of a rail
head and illustrates a position where a slow strain rate
technique (SSRT) test piece is taken.
[Fig. 5] Fig. 5 is a cross-sectional view of the shape
and dimensions of the SSRT test piece.
[Fig. 6] Fig. 6 is a graph showing the relationship
between the [%V]/[%N] value and the rate of improvement in
delayed fracture susceptibility.
Reference Numerals
1 Nishihara-type rolling contact test piece taken
from pearlitic rail
la Nishihara-type rolling contact test piece taken
from surface layer of rail head
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lb Nishihara-type rolling contact test piece taken
from inside of rail head
2 tire specimen
3 rail head
4 SSRT test piece
Best Modes for Carrying Out the Invention
The reason for limitations for the conditions of an
internal high hardness type pearlitic rail of the present
invention including the composition will be described.
C: 0.73% to 0.85% by mass
C is an essential element to form cementite in a
pearlitic structure to ensure wear resistance. The wear
resistance is improved as the C content is increased. At a
C content of less than 0.73% by mass, however, it is
difficult to provide high wear resistance compared with heat
treatment-type pearlitic rails in the conventional art. A C
content exceeding 0.85% by mass results in the formation of
proeutectoid cementite in austenite grain boundaries during
transformation after hot rolling, thereby significantly
reducing rolling contact fatigue resistance. Thus, the C
content is set in the range of 0.73% to 0.85% by mass and
preferably 0.75% to 0.85% by mass.
Si: 0.5% to 0.75% by mass
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Si is an element serving as a deoxidizer and
strengthening. a pearlitic structure and needed in an amount
of 0.5% by mass or more. A Si content exceeding 0.75% by
mass results in a deterioration in weldability due to high
bond strength of Si with oxygen. Further more, high quench
hardenability of Si facilitates the formation of a
martensitic structure in a surface layer of the internal
high hardness type pearlitic rail. Thus, the Si content is
set in the range of 0.5% to 0.75% by mass and preferably
0.5% to 0.70% by mass.
Mn: 0.3% to 1.0% by mass
Mn reduces a pearlite transformation starting
temperature to reduce a lamellar spacing. Thus, Mn
contributes to higher strength and higher ductility of the
internal high hardness type pearlitic rail. An excessive
amount of Mn added reduces the equilibrium transformation
temperature of pearlite to reduce the degree of supercooling,
increasing the lamellar spacing. A Mn content of less than
0.3% by mass does not result in a sufficient effect. A Mn
content exceeding 1.0% by mass facilitates the formation of
a martensitic structure, so that hardening and embrittlement
occur during heat treatment and welding, thereby readily
reducing the quality of the material. Furthermore, even if
the pearlitic structure is formed, the equilibrium
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transformation temperature is reduced, thereby increasing
the lamellar spacing. Thus, the Mn content is set in the
range of 0.3% to 1.0% by mass and preferably 0.3% to 0.8% by
mass.
P: 0.035% by mass or less
A P content exceeding 0.035% results in a deterioration
in ductility. Thus, the P content is set to 0.035% by mass
or less and preferably 0.020% by mass or less.
S: 0.0005% to 0.012% by mass
S is present in steel mainly in the form of A-type
inclusions. A S content exceeding 0.012% by mass results in
a significant increase in the amount of the inclusions and
also results in the formation of coarse inclusions, thereby
reducing cleanliness of steel. A S content of less than
0.0005% by mass leads to an increase in steelmaking cost.
Thus, the S content is set in the range of 0.0005% to 0.012%
by mass and preferably 0.0005% to 0.008% by mass.
Cr: 0.2% to 1.3% by mass
Cr is an element that increases the equilibrium
transformation temperature of pearlite to contribute to a
reduction in lamellar spacing and that further increases the
strength by solid-solution hardening. However, a Cr content
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of less than 0.2% by mass does not result in sufficient
internal hardness. A Cr content exceeding 1.3% by mass
results in excessively high quench hardenability, forming
martensite to reduce wear resistance and rolling contact
fatigue resistance. Thus, the Cr content is set in the
range of 0.2% to 1.3% by mass, preferably 0.3% to 1.3% by
mass, and more preferably 0.5% to 1.3% by mass.
V: 0.005% to 0.12% by mass
V forms a carbonitride that is dispersively
precipitated in a matrix, improving wear resistance and
delayed fracture properties. At a V content of less than
0.005% by mass, the effect is reduced. A V content
exceeding 0.12% by mass results in an increase in alloy cost,
thereby increasing the cost of the internal high hardness
type pearlitic rail. Thus, the V content is in the range of
0.005% to 0.12% by mass and preferably 0.012% to 0.10% by
mass.
N: 0.0015% to 0.0060% by mass
N forms a nitride that is dispersively precipitated in
a matrix, improving wear resistance and delayed fracture
properties. At a N content of less than 0.0015% by mass,
the effect is reduced. A N content exceeding 0.0060% by
mass results in the formation of coarse nitrides in the
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internal high hardness type pearlitic rail, thereby reducing
rolling contact fatigue resistance and delayed fracture
properties. Thus, the N content is in the range of 0.0015%
to 0.060% by mass and preferably 0.0030% to 0.0060%.
[%Mn]/[%Cr]: greater than or equal to 0.3 and less than 1.0
Mn and Cr are additive elements in order to increase
the hardness of the internal high hardness type pearlitic
rail. In the case where an appropriate balance between the
Mn content [%Mn] and the Cr content [%Cr] is not achieved,
however, martensite is formed in a surface layer of the
internal high hardness type pearlitic rail. Note that the
units of [%Mn] and [%Cr] are percent by mass. When the
value of [%Mn]/[%Cr] is less than 0.3, the Cr content is
high. This facilitates the formation of martensite in the
surface layer of the internal high hardness type pearlitic
rail due to high quench hardenability of Cr. When the value
of [%Mn]/[%Cr] is 1.0 or more, the Mn content is high. This
also facilitates the formation of martensite in the surface
layer of the internal high hardness type pearlitic rail due
to high quench hardenability of Mn. In the case where the
Mn content and the Cr content are set in the above ranges
respectively and where the value of [%Mn]/[%Cr] is greater
than or equal to 0.3 and less than 1.0, the internal
hardness of the head of the rail (hardness of a portion
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located from the surface layer of the head of the internal
high hardness type pearlitic rail to a depth of at least 25
mm) can be controlled within a range described below while
the formation of martensite in the surface layer is being
prevented. Thus, the value of [%Mn]/[%Cr] is greater than
or equal to 0.3 and less than 1.0 and preferably in the
range of 0.3 to 0.9.
[%V]/[%N]: 8.0 to 30.0
V and N are important elements that form a V-based
nitride serving as a hydrogen-trapping site. To form the V-
based nitride, the amounts thereof added must be controlled.
The units of [%V] and [%N] are percent by mass. At a
[%V]/[%N] value of less than 8.0, the V-based nitride is not
sufficiently formed, thereby reducing the number of the
hydrogen-trapping sites. Thus, it is unlikely that delayed
fracture properties will be significantly improved. At a
[%V]/[%N] value exceeding 30.0, the amount of V added is
increased to increase the alloy cost, thereby increasing the
cost of the internal high hardness type pearlitic rail.
Furthermore, it is unlikely that delayed fracture properties
will be significantly improved. Thus, the [%V]/[%N] value
is in the range of 8.0 to 30.0 and preferably 8.0 to 22Ø
Internal hardness of rail head (hardness of portion located
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from surface layer of head of internal high hardness type
pearlitic rail to depth of at least 25 mm): greater than or
equal to 38OHv and less than 48OHv
An internal hardness of the rail head of less than
380Hv results in a reduction in wear resistance, thereby
reducing the operating life of the internal high hardness
type pearlitic rail. An internal hardness of the rail head
of 480Hv or more results in the formation of martensite,
thereby reducing the rolling contact fatigue resistance of
the internal high hardness type pearlitic rail. Thus, the
internal hardness of the rail head is greater than or equal
to 380Hv and less than 480Hv. The reason the internal
hardness of the rail head is defined by the hardness of the
portion located from the surface layer of the head of the
internal high hardness type pearlitic rail to a depth of at
least 25 mm is as follows: at a depth of less than 25 mm,
the wear resistance of the internal high hardness type
pearlitic rail is reduced with increasing distance from the
surface layer of the rail head toward the inside, reducing
the operating life. Preferably, the internal hardness of
the rail head is greater than 390Hv and less than 480Hv.
DI: 5.6 to 8.6
The value of DI is calculated from expression (1)
described below.
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DI = (0.548 [%C] 112) x (1 + 0.64 [%Si]) x (1 + 4. 1 [%Mn]) x (1 +
2.83[%P]) x (1 - 0.62[%S]) x (1 + 2..23[%Cr]) x (1 +
1.82[%V]) (1)
where [%C] represents the C content, [%Si] represents the Si
content, [%Mn] represents the Mn content, [%P] represents
the P content, [%S] represents the S content, [%Cr]
represents the Cr content, and [%V] represents the V content.
Note that the units of [%C], [%Si], [%Mn], [%P], [%S], [%Cr],
and [%V] are percent by mass.
The DI value indicates quench hardenability and is used
as an index to determine whether quench hardenability is
good or not. In the present invention, the DI value is used
as an index to prevent the formation of martensite in the
surface layer of the internal high hardness type pearlitic
rail and to achieve a target value of the internal hardness
of the rail head. The DI value is preferably maintained
within a suitable range. At a DI value of less than 5.6,
although a desired internal hardness is provided, the
internal hardness is close to the lower limit of the target
hardness range. Thus, it is unlikely that the wear
resistance, rolling contact fatigue resistance, and delayed
fracture properties will be further improved. A DI value
exceeding 8.6 results in an increase in the quench
hardenability of the internal high hardness type pearlitic
rail, facilitating the formation of martensite in the
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surface layer of the rail head. Thus, the DI value is
preferably in the range of 5.6 to 8.6 and more preferably
5.6 to 8.2.
Ceq: 1.04 to 1.27
The value of Ceq is calculated from expression (2)
described below.
Ceq = [%C] + ([%Si] /11) + ([%Mn] /7) + ([%Cr] /5.8) + [%V]
(2)
where [%C] represents the C content, [%Si] represents the Si
content, [%Mn] represents the Mn content, [%Cr] represents
the Cr content, and [%V] represents the V content. Note
that the units of [%C], [%Si], [%Mn], [%Cr], and [%V] are
percent by mass.
The Ceq value is used to estimate the maximum hardness
and weldability from proportions of the alloy components
added. In the present invention, the Ceq value is used as an
index to prevent the formation of martensite in the surface
layer of the internal high hardness type pearlitic rail and
to achieve a target value of the internal hardness of the
rail head. The Ceq value is preferably maintained within a
suitable range. At a Ceq value of less than 1.04, although a
desired internal hardness is provided, the internal hardness
is close to the lower limit of the target hardness range.
Thus, it is unlikely that the wear resistance and rolling
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contact fatigue resistance will be further improved. A Ceq
value exceeding 1.27 results in an increase in the quench
hardenability of the internal high hardness type pearlitic
rail, facilitating the formation of martensite in the
surface layer of the rail head. Thus, the Ceq value is
preferably in the range of 1.04 to 1.27 and more preferably
1.04 to 1.20.
[%Si] + [%Mn] + [%Cr]: 1.55% to 2.50
When the sum of the Si content [%Si], the Mn content
[%Mn], and the Cr content [%Cr] (= [%Si] + [%Mn] + [%Cr]) is
less than 1.55, it is difficult to satisfy an internal
hardness of the rail head greater than or equal to 380Hv and
less than 480Hv. When the sum exceeds 2.50, a martensitic
structure is formed because of high quench hardenability of
Si, Mn, and Cr. This is liable to cause a reduction in
ductility and toughness. Thus, the value of [%Si] + [%Mn] +
[%Cr] is preferably in the range of 1.55 to 2.50 and more
preferably 1.55 to 2.30. The units of [%Si], [%Mn], and
[%Cr] are percent by mass.
The composition described above may further contain one
or two or more selected from 1.0% by mass or less Cu, 1.0%
by mass or less Ni, 0.001% to 0.05% by mass Nb, and 0.5% by
mass or less Mo, as needed.
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Cu: 1.0% by mass or less
Like Cr, Cu is an element that further increases the
strength by solid-solution hardening. To provide the effect,
the Cu content is preferably 0.005% by mass or more. A Cu
content exceeding 1.0% by mass, however, is liable to cause
Cu cracking. Thus, in the case where Cu is added, the Cu
content is preferably 1.0% by mass or less and more
preferably 0.005% to 0.5% by mass.
Ni: 1.0% by mass or less
Ni is an element that increases the strength without
reducing ductility. Furthermore, the addition of Ni
together with Cu suppresses Cu cracking. Thus, when Cu is
added, preferably, Ni is also added. To provide the effect,
the Ni content is preferably 0.005% or more. The Ni content
exceeding 1.0% by mass, however, results in an increase in
quench hardenability, forming martensite. This is liable to
cause a reduction in wear resistance and rolling contact
fatigue resistance. In the case where Ni is added, thus,
the Ni content is preferably 1.0% by mass or less and more
preferably 0.005% to 0.5% by mass.
Nb: 0.001% to 0.05% by mass
Nb is combined with C in steel to precipitate as a
carbide during and after rolling and contributes to a
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reduction in pearlite colony size. This leads to
significant improvement in wear resistance, rolling contact
fatigue resistance and ductility and significant
contribution to longer operating life of the internal high
hardness type pearlitic rail. To provide the effects, a Nb
content of 0.001% by mass or more is preferred. At a Nb
content of less than 0.001% by mass, the effect is not
sufficiently provided. At a Nb content exceeding 0.05% by
mass, the effect of improving wear resistance and rolling
contact fatigue resistance is saturated, the effect is not
worth the amount added. In the case where Nb is added, thus,
the Nb content is preferably in the range of 0.001% to 0.05%
by mass and more preferably 0.001% to 0.03% by mass.
Mo: 0.5% by mass or less
Mo is an element that increases the strength by solid-
solution hardening. To provide the effect, the Mn content
is preferably 0.005% by mass or more. A Mo content
exceeding 0.5% by mass is liable to cause the formation of a
bainitic structure and to reduce wear resistance. In the
case where Mo is added, thus, the Mo content is preferably
0.5% by mass or less and more preferably 0.005% to 0.3% by
mass.
Lamellar spacing of pearlite layer in portion located from
CA 02687438 2009-11-13
- 23 -
surface layer of rail head to depth of at least 25 mm: 0.04
to 0.15 m
A reduction in the lamellar spacing of a pearlite layer
increases the hardness of the internal high hardness type
pearlitic rail, which is advantageous from the viewpoint of
improving wear resistance and rolling contact fatigue
resistance. A lamellar spacing exceeding 0.15 m does no
result in sufficient improvement in these properties. Thus,
the lamellar spacing is preferably 0.15 m or less. On the
other hand, for reducing the lamellar spacing to less than
0.04 m, a technique for reducing the lamellar spacing by
improving quench hardenability is to be used. This is
liable to cause the formation of martensite in the surface
layer, thereby adversely affecting rolling contact fatigue
resistance. Thus, the lamellar spacing is preferably 0.04
m or more.
The present invention also includes a pearlitic rail
containing other trace elements in place of part of the
balance Fe in a composition according to the present
invention to the extent that the effect of the present
invention is not substantially affected. Here, examples of
impurities include P and 0. A P content of up to 0.035% by
mass is allowable as described above. An 0 content of up to
0.004% by mass is allowable. Furthermore, in the present
invention, a Ti content of up to 0.0010% is allowable, Ti
CA 02687438 2009-11-13
- 24 -
being contained as an impurity. In particular, Ti forms an
oxide to reduce rolling contact fatigue resistance, which is
a basic property of the rail. Thus, the Ti content is
preferably controlled so as to be up to 0.0010%.
The internal high hardness type pearlitic rail of the
present invention is preferably produced by hot-rolling a
steel material with a composition according to the present
invention to form a rail shape in such a manner that the
finishing rolling temperature is in the range of 850 C to
950 C, and slack-quenching at least the head of the rail
article from a temperature equal to or higher than a
pearlite transformation starting temperature to 400 C to
650 C at a cooling rate of 1.2 to 5 C/s. The reason for a
finishing rolling temperature (roll finishing temperature)
of 850 C to 950 C, a cooling rate of the slack quenching of
1.2 to 5 C/s, and a cooling stop temperature of 400 C to
650 C is described below.
Finishing rolling temperature: 850 C to 950 C
In the case of a finishing rolling temperature of less
than 850 C, rolling is performed to a low-temperature
austenite range. This not only introduces processing strain
in austenite grains but also causes a significantly high
degree of extension of austenite grains. The introduction
CA 02687438 2009-11-13
- 25 -
of dislocation and an increase in austenite grain boundary
area result in an increase in the number of pearlite
nucleation sites. Although the pearlite colony size is
reduced, the increase in the number of pearlite nucleation
sites increases a pearlite transformation starting
temperature, thereby increasing the lamellar spacing of the
pearlite layer to cause a significant reduction in wear
resistance. Meanwhile, a finishing rolling temperature
exceeding 950 C increases the austenite grain size, thereby
increasing the final pearlite colony size to cause a
reduction in rolling contact fatigue resistance. Thus, the
finishing rolling temperature is preferably in the range of
850 C to 950 C.
Cooling rate from temperature equal to or higher than
pearlite transformation starting temperature: 1.2 to 5 C/s
A cooling rate of less than 1.2 C/s results in an
increase in pearlite transformation starting temperature,
thereby increasing the lamellar spacing of the pearlite
layer to cause a significant reduction in wear resistance
and rolling contact fatigue resistance. Meanwhile, a
cooling rate exceeding 5 C/s results in the formation of a
martensitic structure, thereby reducing ductility and
toughness. Thus, the cooling rate is preferably in the
range of 1.2 to 5 C/s and more preferably 1.2 to 4.6 C/s.
CA 02687438 2009-11-13
- 26 -
Although the pearlite transformation starting temperature
varies depending on the cooling rate, the present invention,
the pearlite transformation starting temperature is referred
to as an equilibrium transformation temperature in the
present invention. In the composition range of the present
invention, the cooling rate within the above range may be
used at 720 C or higher.
Cooling stop temperature: 400 C to 650 C
In the case of the composition and the cooling rate
according to the present invention, in order to obtain a
uniform pearlitic structure at a cooling rate of 1.2 to
C/s, it is preferable to ensure a cooling stop
temperature of at least about 70 C lower than the
equilibrium transformation temperature. A cooling stop
temperature of less than 400 C, however, results in an
increase in cooling time, leading to an increase in the cost
of the internal high hardness type pearlitic rail. Thus,
the cooling stop temperature is preferably in the range of
400 C to 650 C and more preferably 450 C to 650 C.
Next, methods for measuring and evaluating wear
resistance, rolling contact fatigue resistance, delayed
fracture properties, the internal hardness of the rail head,
and the lamellar spacing will be described.
CA 02687438 2009-11-13
- 27 -
(Wear resistance)
With respect to wear resistance, most preferably, the
internal high hardness type pearlitic rail is actually
placed and evaluated. In this case, disadvantageously, it
takes a long time to conduct a test. In the present
invention, thus, evaluation is made by a comparative test
performed under simulated real conditions of rail and wheel
contact with a Nishihara-type rolling contact test machine
that can evaluate wear resistance in a short time. A
Nishihara-type rolling contact test piece 1 having an
external diameter of 30 mm is taken from the rail head. The
test is performed by contacting the test piece 1 with a tire
specimen 2 and rotating them as shown in Fig. 1. Arrows in
Fig. 1 indicate rotational directions of the Nishihara-type
rolling contact test piece 1 and the tire specimen 2. With
respect to the tire specimen, a round bar with a diameter of
32 mm is taken from the head of a standard rail (Japanese
industrial standard rail) described in JIS E1101. The round
bar is subjected to heat treatment so as to have a Vickers
hardness of 390HV (load: 98 N) and a tempered martensitic
structure. Then the round bar is processed so as to have a
shape shown in Fig. 1, resulting in the tire specimen. Note
that the Nishihara-type rolling contact test piece 1 is
taken from each of two portions of a rail head 3 as shown in
Fig. 2. A piece taken from a surface layer of the rail head
CA 02687438 2009-11-13
- 28 -
3 is referred to as a Nishihara-type rolling contact test
piece la. A piece taken from the inside is referred to as a
Nishihara-type rolling contact test piece lb. The center of
the Nishihara-type rolling contact test piece lb, which is
taken from the inside of the rail head 3, in the
longitudinal direction is located at a depth of 24 to 26 mm
(mean value: 25 mm) below the top face of the rail head 3.
The test is performed in a dry state at a contact pressure
of 1.4 GPa, a slip ratio of -10%, and a rotation speed of
675 rpm (750 rpm for the tire specimen) The wear amount at
100,000 rotations is measured. A heat-treated pearlitic
rail is employed as reference steel used in comparing wear
amounts. It is determined that the wear resistance is
improved when the wear amount is at least 10% smaller than
that of the reference steel. Note that the rate of
improvement in wear resistance is calculated from {(wear
amount of reference steel - wear amount of test piece)/(wear
amount of reference steel)} x 100.
(Rolling contact fatigue resistance)
With respect to rolling contact fatigue resistance, the
Nishihara-type rolling contact test piece 1 having an
external diameter of 30 mm and a curved contact surface with
a radius of curvature of 15 mm is taken from the rail head.
A test is performed by contacting the test piece 1 with the
CA 02687438 2009-11-13
- 29 -
tire specimen 2 and rotating them as shown in Fig. 3.
Arrows in Fig. 3 indicate rotational directions of the
Nishihara-type rolling contact test piece 1 and the tire
specimen 2. Note that the Nishihara-type rolling contact
test piece 1 is taken from each of two portions of a rail
head 3 as shown in Fig. 2. The tire specimen and each
portion where the Nishihara-type rolling contact test piece
1 is taken are the same as above; hence, the description is
omitted. The test is performed under an oil-lubricated
condition at a contact pressure of 2.2 GPa, a slip ratio of
-20%, and a rotation speed of 600 rpm (750 rpm for the tire
specimen). The surface of each test piece is observed every
25,000 rotations. The number of rotations at the occurrence
of a crack with a length of 0.5 mm or more is defined as
rolling contact fatigue life. A heat-treated pearlitic rail
is employed as reference steel used in comparing rolling
contact fatigue life. It is determined that the rolling
contact fatigue resistance is improved when the rolling
contact fatigue life is at least 10% longer than that of the
reference steel. Note that the rate of improvement in
rolling contact fatigue resistance is calculated from
{(number of rotations at occurrence of fatigue damage of
test piece - number of rotation at occurrence of fatigue
damage of reference steel)/(number of rotations at
occurrence of fatigue damage of reference steel)} x 100.
CA 02687438 2009-11-13
- 30 -
(Delayed fracture property)
As shown in Fig. 4, a slow strain rate technique (SSRT)
test piece 4 having the center 25.4 mm below the top face of
the rail head 3 is taken. The SSRT test piece 4 has
dimensions and a shape shown in Fig. 5. The test piece is
subjected to three triangle mark finish, except for screw
sections and round sections. Parallel sections are polished
with emery paper (up to #600). The SSRT test piece is
mounted on an SSRT test apparatus and then subjected to an
SSRT test at a strain rate of 3.3 x 10-6 Is and a temperature
of 25 C in the atmosphere, obtaining elongation Eo of the
SSRT test piece in the atmosphere. An SSRT test piece is
subjected to an SSRT test in a 20 mass% ammonium thiocyanate
(NH4SCN) solution at a strain rate of 3.3 x 10-6 Is and a
temperature of 25 C, obtaining elongation E1 of the SSRT
test piece in the ammonium thiocyanate solution. Delayed
fracture susceptibility (i.e., DF) used as an index to
evaluate delayed fracture properties is calculated from DF
(%) = 100 x (1 - E1/E0). It is determined that the delayed
fracture properties are improved when the rate of
improvement.in delayed fracture susceptibility is at least
10% higher than that of a reference steel (i.e., a heat
treatment-type pearlitic rail having a C content of 0.68% by
mass). Note that the rate of improvement in delayed
CA 02687438 2009-11-13
- 31 -
fracture susceptibility is calculated from {(delayed
fracture susceptibility of test piece - delayed fracture
susceptibility of reference steel)/(delayed fracture
susceptibility of reference steel} x 100.
(Internal hardness of rail head)
The Vickers hardness of a portion located from the
surface layer of the rail head of to a depth of 25 mm is
measured at a load of 98 N and a pitch of 1 mm. Among all
hardness values, the minimum hardness value is defined as
the internal hardness of the rail head.
(Lamellar spacing)
Random five fields of view of each of a portion (at a
.depth of about 1 mm) close to the surface layer of the rail
head and a portion located at a depth of 25 mm are observed
with a scanning electron microscope (SEM) at a magnification
of 7,500X. In the case where a portion with the minimum
lamellar spacing is present, the portion is observed at a
magnification of 20,000X, and the lamellar spacing in the
field of view is measured. In the case where no small
lamellar spacing is observed in a field of view at a
magnification of 7,500X or where the cross-section of a
lamellar structure is not perpendicular to a lamellar plane
but is obliquely arranged, the measurement is performed in
CA 02687438 2009-11-13
- 32 -
another field of view. The lamellar spacing is evaluated by
the mean value of the lamellar spacing measurements in the
five fields of view.
EXAMPLES
(Example 1)
Steel materials with compositions shown in Table 1 were
subjected to rolling and cooling under conditions shown in
Table 2 to produce pearlitic rails. Cooling was performed
only at heads of the rails. After termination of the
cooling, the pearlitic rails were subject to natural cooling.
The resulting pearlitic rails were evaluated for Vickers
hardness, lamellar spacing, wear resistance, rolling contact
fatigue resistance, and delayed fracture properties. Table
3 shows the results. The finishing rolling temperature
shown in Table 2 indicates a value obtained by measuring a
temperature of the surface layer of a side face of each rail
head on the entrance side of a final roll mill with a
radiation thermometer. The cooling stop temperature
indicates a value obtained by measuring a temperature of the
surface layer of a side face of each rail head on the exit
side of a cooling apparatus with a radiation thermometer.
The cooling rate was defined as the rate of change in
temperature between the start and end of cooling.
The values of [%V]/[%N] were calculated from the V
CA 02687438 2009-11-13
- 33 -
content and the N content in 1-B to 1-N shown in Table 1.
Fig. 6 shows the relationship between the resulting
[%V]/[%N] values and the rate of improvement in delayed
fracture susceptibility shown in Table 3.
The results demonstrated the following: In the case
where the [%Mn]/[%Cr] value was greater than or equal to 0.3
and less than 1.0 and where the [%V]/[%N] value was in the
range of 8.0 to 30.0, the portion located from the surface
layer of the rail head to a depth of at least 25 mm had a
hardness greater than or equal to 380Hv and less than 480Hv,
so that the wear resistance and the rolling contact fatigue
resistance were improved, and the delayed fracture
properties are improved by 10% or more. In each of 1-F and
1-I, the [%V]/[%N] value exceeded 30. In this case, further
significant improvement in delayed fracture properties was
not achieved.
(Example 2)
Steel materials with compositions shown in Table 4 were
subjected to rolling and cooling under conditions shown in
Table 5 to produce pearlitic rails. Cooling was performed
only at heads of the rails. After termination of the
cooling, the pearlitic rails were subject to natural cooling.
Like Example 1, the resulting pearlitic rails were evaluated
for Vickers hardness, lamellar spacing, wear resistance,
CA 02687438 2009-11-13
34 -
rolling contact fatigue resistance, and delayed fracture
properties. Table 6 shows the results.
The results demonstrated the following: In each of 2-B
to 2-L and 2-V to 2-X, in the case where the amounts of Si,
Mn, Cr, V, and N added were optimized, the [%Mn]/[%Cr] value
was greater than or equal to 0.3 and less than 1.0, the
[%V]/[%N] value was in the range of 8.0 to 30.0, and one or
two or more components selected from Cu, Ni, Nb, and Mo were
added in proper amounts, the wear resistance, rolling
contact fatigue resistance, and delayed fracture properties
were improved. Among these examples, in each of 2-B to 2-H
and 2-V to 2-X, i.e., in the case where of a DI value of 5.6
to 8.6 and a Ceq of 1.04 to 1.27, the wear resistance and the
rolling contact fatigue resistance were improved compared
with 2-I to 2-L. In 2-U, i.e., in the case of adding Ti,
the rolling contact fatigue resistance was reduced.
According to the present invention, a pearlitic rail
having excellent wear resistance, rolling contact fatigue
resistance, and delayed fracture properties compared with
pearlitic rails in the related art can be stably produced.
This contributes to longer operating life of pearlitic rails
used for high-axle load railways and to the prevention of
railway accidents, providing industrially beneficial effects.
Industrial Applicability
CA 02687438 2009-11-13
- 35 -
According to the present invention, a pearlitic rail
having excellent wear resistance, rolling contact fatigue
resistance, and delayed fracture properties compared with
pearlitic rails in the related art can be stably produced.
This contributes to longer operating life of pearlitic rails
used for high-axle load railways and to the prevention of
railway accidents, providing industrially beneficial effects.
CA 02687438 2009-11-13
- 36 -
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CA 02687438 2011-12-29
- 37 -
TABLE 2
Finishing Cooling stop
Steel No. rolling temperature Cooling rate Remarks
temperature ( C) (C/s)
( C)
1-A 900 500 2.0 Reference
material
1-B 900 500 1.6
1-C 950 550 2.3 Example
1-D 900 450 2.2
1-E 850 600 3.2
1-F 900 550 1.4
1-G 950 500 2.2 Comparative
1-H 950 550 1.9 example
1-1 850 500 1.6
1-J 900 500 2.6
1-K 950 550 3.2
1-L 900 500 2.3 Example
1-M 850 450 2.5
1-N 900 550 3.3
CA 02687438 2009-11-13
- 38 -
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CA 02687438 2011-12-29
- 39 -
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CA 02687438 2009-11-13
- 40 -
TABLE 5
Finishing Cooling stop
rolling Cooling rate
Steel No. temperature temperature ( C/s) Remarks
( C)
2-A 900 500 2.0 Reference
material
2-B 900 500 2.3
2-C 900 500 1.9
2-D 950 550 1.3
2-E 900 500 2.2
2-F 900 500 1.9
2-G 950 500 2.3 Example
2-H 900 600 2.0
2-1 950 500 2.0
2-J 850 550 2.1
2-K 950 450 2.8
2-L 950 550 2.0
2-M 900 500 2.1
2-N 900 550 2.0
2-P 950 500 2.2
2-Q 900 500 2.3 Comparative
2-R 850 450 3.1 example
2-S 850 650 2.4
2-T 850 550 3.2
2-U 900 600 2.2
2-V 850 550 2.6
2-W 900 500 2.4 Example
2-X 900 600 2.6
CA 02687438 2009-11-13
- 41 -
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