The present invention relates to a high-strength rail that is used in cargo
railways and has excellent wear resistance and internal fatigue damage resistance.
Priority is claimed on Japanese Patent Application No. 2015-011006, filed on
January 23,2015, the content of which is incorporated herein by reference.
[Related Art]
[0002]
With economic development, natural resources such as coal have been newly
developed. Specifically, mining in regions with severe natural environments which
have not yet been developed have been promoted. Along with this, the railroad
enviromnent of cargo railways used to transport resources has become increasingly
severe. As the result, rails have been required to have more wear resistance than ever.
Further, in cargo railways, recently, railway transport has become more
overcrowded. Therefore, there is a concern that fatigue damage will occur from the
inside of a rail head portion (position at a depth of 20 to 30 rnm from the outer surface
of the head portion in a shape of an unused rail).
From this background, there has been a demand for development of highstrength
rails with improved wear resistance and internal fatigue damage resistance.
[0003]
In order to improve the wear resistance of rail steel, high-strength rails
described in Patent Documents 1 and 2, for example, have been developed. The main
characteristics of these rails are the hardness of steel being increased by refining
- 1 -
lamellar spacing in a pearlite structure using a heat treatment in order to improve the
wear resistance and the increased volume ratio of cementite in lamellar of a pearlite
structure due to an increase in the amount of carbon of steel.
Specifically, Patent Document I discloses that a rail with excellent wear
resistance is obtained by performing accelerated cooling on a rail head portion which is
rolled or re-heated at a cooling rate of I 0C/sec to 4 °C/sec from the temperature of an
austenite region to a range of 850°C to 500°C.
In addition, Patent Document 2 discloses that a rail having excellent wear
resistance can be obtained by increasing the volume ratio of cementite in lamellar of a
pearlite structure using hyper-eutectoid steel (C: greater than 0.85% and 1.20% or less).
[0004]
In the technologies disclosed in Patent Documents 1 and 2, the wear
resistance of a certain region can be improved by refining the lamellar spacing in the
pearlite structure in order to improve the hardness and increase the volume ratio of
cementite in lamellar of the pearlite structure.
However, in the rails disclosed in Patent Documents 1 and 2, internal fatigue
damage cannot be suppressed.
[0005]
In consideration of the above-described problems, high-strength rails as
described in Patent Documents 3 and 4, for example, have been suggested. The main
characteristics of these rails are control ofpearlitic transformation by adding a small
amount of alloy or the hardness of the inside of a head portion being improved by
precipitating a small amount of alloy in a pearlite structure in order to improve internal
fatigue damage resistance, in addition to improvement of wear resistance.
Specifically, Patent Document 3 discloses that the hardness of the inside of
the head portion is improved by adding B to hyper-eutectoid steel (C: greater than
0.85% and 1.20% or less) so that the pearlitic transformation temperature in the inside
of the head portion is controlled. Further, Patent Document 4 discloses that the
hardness of the inside of the head portion is improved by adding V and N to hypereutectoid
steel (C: greater than 0.85% and 1.20% or less) and precipitating V
carbonitrides in the pearlite structure.
[0006]
In Patent Document 3 or 4, the wear resistance is improved by increasing the
volume ratio of cementite in lamellar of the pearlite structure and the hardness of the
inside of the head portion is improved by controlling the pearlitic transformation
temperature in the inside of the head portion or strengthening precipitation of the
pearlite structure so that the internal fatigue damage resistance of a certain region can
be improved. However, in techniques of Patent Documents 3 and 4, since the
chemical composition are based on hyper-eutectoid steel (C: greater than 0.85% and
1.20% or less) having a large amount of carbon, the toughness of the pearlite structure
is low and brittle cracks may occur in the inside of the head portion. Accordingly, in
the use of rails in a severe railroad environment which has been required in recent
years, sufficient characteristics were not able to be obtained and thus further
improvement of the internal fatigue damage resistance has been a problem. In
addition, in the techniques of Patent Documents 3 and 4, there has been a problem in
that the hardness is not sufficiently improved due to a change in production conditions
and thus the internal fatigue damage resistance may be decreased.
[0007]
In consideration of such problems, for example, Patent Document 5 suggests a
new high-strength rail with improved wear resistance and internal fatigue damage
- 3 -
resistance which are required for a rail. The main characteristics are the amount of
carbon being reduced to improve the toughness of the pearlite structure and a small
amount of alloy being added to improve the internal fatigue damage resistance so that
the hardness of the inside of the head portion is improved by precipitation hardening.
Specifically, in Patent Document 5, the hardness of the inside of the head portion is
improved by controlling Mn content and the Cr content and adding V and N based on
eutectoid steel (C: 0. 73% to 0.85%) having a pearlite structure with excellent
toughness.
[0008]
However, in the technique disclosed in Patent Document 5, abnormal
structures such as a bainite or a martensite harmful to the wear resistance are generated
depending on the production conditions even when the Mn content and the Cr content
are controlled. Further, even when V and N are added and the ratio between V and N
is controlled, the particle size or the distribution of a V nitride is not sufficiently
controlled, an increase in hardness of the inside of the head portion becomes excessive,
and a sufficient increase in hardness of the inside of the head portion is not obtained
and, accordingly, the internal fatigue damage occurs. Therefore, the objectives are
prevention of generation of abnormal structures, improvement of wear resistance,
stable generation ofV-based precipitates, and improvement of internal fatigue damage
resistance.
[0009]
As described above, a high-strength rail which can be used in cargo railways
in a severe railroad environment and has excellent wear resistance and internal fatigue
damage resistance has not been provided.
[Prior Art Document]
- 4 -
[Patent Document]
[001 0]
[Patent Document I] Japanese Examined Patent Application, Second
Publication No. S63-023244
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. H08-144016
[Patent Document 3] Japanese Patent (Granted) Publication No. 3445619
[Patent Document 4] Japanese Patent (Granted) Publication No. 3513427
[Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. 2009-108396
[Disclosure of tbe Invention]
[Problems to be solved by tbe Invention]
[0011]
The present invention has been made in consideration of the above-described
problems and an object of the present invention is to provide a rail with improved wear
resistance and internal fatigue damage resistance which are required for a rail used in
cargo railways particularly in a severe railroad environment.
[Means for Solving the Problem]
[0012]
(I) According to an aspect of the present invention, there is provided a rail
including, in terms of mass%: C: 0.75% to 0.85%; Si: 0.!0% to 1.00%; Mn: 0.30% to
1.20%; Cr: 0.20% to 0.80%; V: 0.01% to 0.20%; N: 0.0040% to 0.0200%; Mo: 0% to
0.50%; Co: 0% to 1.00%; B: 0% to 0.0050%; Cu: 0% to 1.00%; Ni: 0% to 1.00%; Nb:
0% to 0.0500%; Ti: 0% to 0.0500%; Mg: 0% to 0.0200%; Ca: 0% to 0.0200%; REM:
0% to 0.0500%; Zr: 0% to 0.0200%; Al: 0% to 1.00%; P :S 0.0250%; S :S 0.0250%; and
- 5 -
Fe and impurities as a remainder, the following Expressions 1 and 2 are satisfied, a
structure of a range between an outer surface of a head portion as an origin and a depth
of 25 mm includes 95% or greater of a pearlite structure and a hardness of the structure
is in a range ofHv 350 to 480, 50 to 500 V carbonitride having an average grain size of
5 to 20 nm are present per 1.0 !1m2 of an area to be inspected in a transverse cross
section at a position having the depth of 25 mm from the outer surface of the head
portion, and the value obtained by subtracting the hardness of the position having the
depth of 25 mm from the outer surface of the head portion from the hardness of a
position having a depth of 2 mm from the outer surface of the head portion is in a
range of Hv 0 to Hv 40.
1.00 < Mn/CcS 4.00 · ·· Expression a
0.30 :<: 0.25 x Mn + Cr :<: 1.00 -- · Expression b
Here, the symbols of elements described in the Expressions a and b indicate
the amount of each element in terms of mass%.
(2) In the rail according to (l ), when a number of carbon atoms is defined as
CA and a number of nitrogen atoms is defined as NA in the V carbonitride, the ratio
CAIN A which is a ratio of CA to NA may be 0.70 or less.
(3) The rail according to (I) or (2) may include, in terms of mass%, at least
one selected from the group consisting of: Mo: 0.01% to 0.50%; Co: 0.01% to 1.00%;
B: 0.0001% to 0.0050%; Cu: O.Dl% to 1.00%; Ni: O.Dl% to 1.00%; Nb: 0.0010% to
0.0500%; Ti: 0.0030% to 0.0500%; Mg: 0.0005% to 0.0200%; Ca: 0.0005% to
0.0200%; REM: 0.0005% to 0.0500%; Zr: 0.0001% to 0.0200%; andAl: 0.0100% to
1.00%.
[Effects of the Invention]
[0013]
- 6 -
According to the aspect of the present invention, the wear resistance and the
internal fatigue damage resistance of the rail can be improved by controlling the
composition of alloy, structures, number ofV carbonitride of rail steel (steel serving as
the material of the rail), controlling the hardness of the surface ofthe head portion or
the inside of the head portion, controlling a difference in hardness between the surface
of the head portion and the inside of the head portion, and controlling the composition
ofV carbonitride. Further, when such a rail is used, the service life ofthe rail in a
case of being used in cargo railways can be greatly improved.
[Brief Description of the Drawings]
[0014]
FIG. 1 is a diagram showing the relationship ofthe isothermal transformation
temperature, the hardness, and the metallographic structure.
FIG. 2 is a diagram showing the relationship between the values ofMn/Cr
defined in Expression 1 and the metallographic structure.
FIG. 3 is a diagram showing the relationship between the values of 0.25 x Mn
+ Cr defined in Expression 2 and the hardness of a rail head portion.
FIG. 4 is a diagram showing the relationship between the number (piece/j.tm2
)
ofV carbonitride having a grain size of 5 to 20 nm per unit area (1.0 j.tm2
) and the
hardness of the rail head portion.
FIG. 5 is a diagram showing the relationship between a ratio (CAIN A) of the
number of carbon atoms (CA) to the number of nitrogen atoms (NA) of the
carbonitrides and the presence or absence of fine cracks in the periphery of the V
carbonitride during a rolling contact fatigue test.
FIG. 6 is a diagram showing names of each position on the cross section of the
head portion and a region, for which the pearlite structure is required, of the rail
- 7 -
according to the present embodiment.
FIG. 7 is a view showing a position of machining wear test specimens.
FIG. 8 is a view showing the outline of the wear test.
FIG. ·9 is a view showing the outline of the rolling contact fatigue test.
[Embodiments of the Invention]
[0015]
Hereinafter, a rail having excellent wear resistance and internal fatigue
damage resistance according to an embodiment of the present invention (hereinafter,
also referred to as the rail according to the present embodiment) will be described in
detail. Hereinafter, "mass%" in the composition is simply described as "%".
[0016]
The rail according to the present embodiment has the following characteristics.
(i) The rail has a predetermined chemical composition and satisfies
expressions of 1.00 < Mn/Cr :S 4.00 and 0.30 :<:; 0.25 x Mn + Cr :S 1.00
(ii) A structure to a depth of25 mm from an outer surface of a head portion as
the origin includes 95% or greater of a pearlite structure and the Vickers hardness of
the structure is in a range of Hv 350 to 480.
(iii) 50 to 500 V carbonitride having an average particle size of 5 to 20 nm are
present per 1.0 f..11I12 of an area to be inspected in a transverse cross section at a position
having a depth of25 mm from the outer surface of the head portion as the origin.
(iv) The value obtained by subtracting. the hardness of the position at a depth
of 25 mm from the outer surface of the head portion as the origin from the hardness of
the position at a depth of 2 mm from the outer surface of the head portion as the origin
is in a range ofHv 0 to Hv 40.
(v) When the number of carbon atoms is setto·CAand the number of nitrogen
- 8 -
atoms is set to NA in the V carbo nitride, the ratio CAIN A which is the ratio of CA to
NA is preferably 0. 70 or less.
[0017]

.In the rail according to the present embodiment, it is necessary that 95% or
greater (area ratio) of the area at a depth of 25 mm from the outer surface of the head
portion as the origin be set to the pearlite structure.
[0018]
First, the reason for setting the area ratio ofthe pearlite structure to 95% or
greater will be described.
In the rail head portion that comes into contact with wheels, wear resistance is
considered to be the most important thing to ensure. As the result of investigation of
the relationship between the metallographic structure and the wear resistance
conducted by the present inventors, it was confirmed that the pearlite structure has the
best wear resistance. Further, the hardness (strength) of the pearlite structure is easily
obtained even when the amount of alloy elements is small and the internal fatigue
damage resistance thereof is excellent. Therefore, for the purpose of improving the
wear resistance and the internal fatigue damage resistance, the area ratio of the pearlite
structure is limited to 95% or greater. When the area ratio of pearlite structure is less
than 95%, the wear resistance and the infernal fatigue damage resistance are not
sufficiently improved.
[0019]
Next, the reason for limiting the required range ofthe metallographic structure
(structure including pearlite) including the pearlite having an area ratio of95% or
- 9 -
greater to a range to at least a depth of 25 mm from the outer surface of the head
portion (surface of head corner portions and a head top portion) as the origin will be
described.
[0020]
When the range of the structure including the pearlite is less than a depth of
25 mm from the outer surface of the head portion as the origin, if the wear at the time
of use is considered, the region is not sufficient as the region for which the wear
resistance or the internal fatigue damage resistance of the rail head portion is required,
and the wear resistance and the internal fatigue damage resistance cannot be
sufficiently improved. As the result, the rail service life is difficult to sufficiently
Improve. Therefore, it is preferable that a range to a depth of about 30 mm from the
outer surface of the head portion as the origin is set to the structure having the pearlite
in order to further improve the wear resistance and the internal fatigue damage
resistance.
[0021]
FIG. 6 shows the names of each position on the cross section of the head
portion of the rail and the region, for which a structure including the pearlite is
required, of the rail according to the present embodiment. First, the rail head portion
indicates a portion upper than the portion constricted which is located in the center of
the rail in the height direction when the rail is seen from the cross section as denoted
by the reference numeral 3 of FIG 6. Further, the rail head portion 3 includes a head
top portion I and head corner portions 2 positioned on both ends of the head top
portion 1. One head corner portion 2 is a gauge corner (G. C.) portion mainly coming
into contact with wheels. Further, the outer surface of the head portion indicates both
of the surface of the head top portion 1 facing the upper side when the rail is upright
- 10 -
and the surfaces of the head comer portions 2, in the rail head portion 3. The
positional relationship between the head top portion 1 and the head comer portions 2 is
that the head top portion I is positioned in approximately the center of the rail head
portion in the width direction and the head corner portions 2 are positioned on both
sides of the head top portion 1.
[0022]
The range to a depth of25 mm from the surface of the head comer portions 2
and the head top portion 1 (outer surface of the head portion) as the origin is referred to
as a head surface portion (3a, hatched portion). As shown in FIG 6, when a structure
(metallographic structure including the pearlite at an area ratio of95% or greater)
including the pearlite with a predetermined hardness is disposed on the head surface
portion 3a to a depth of 25 mm from the surface of the head comer portions 2 and the
head top portion 1 (outer surface of the head portion), the wear resistance and the
internal fatigue damage resistance of the rail are improved.
[0023]
Therefore, it is preferable that the structure including the pearlite is disposed
on the head surface portion 3a in which wheels and the rail are mainly in contact and
the wear resistance and the internal fatigue damage resistance are required. These
characteristics are not required in a portion other than the head surface portion, the area
ratio of the pearlite structure in a portion other than the head surface portion may or
may not be 95% or greater.
[0024]
Moreover, when the area ratio of the pearlite structure is 95% or greater, a
small amow1t of a pro-eutectoid ferrite, a pro-eutectoid cementite, a bainite structure,
or a martensite structure other than the pearlite structure may be mixed into the
- II -
metallographic structure of the head surface portion 3a of the rail according to the
present embodiment by 5% or less in terms of the area ratio. Even if these structures
are mixed into the metallographic structure, when the area ratio thereof is 5% or less,
the wear resistance of the surface of the head portion and the internal fatigue damage
resistance of the inside of the head portion are not adversely and greatly affected. In
other words, in the metallographic structure of the rail head portion of the rail
according to the present embodiment, 95% or greater of the head surface portion in
terms of the area ratio may be the pearlite structure and it is preferable that 98% or
greater of the metallographic structure of the head surface portion of the rail head
portion is set to the pearlite structure in order to sufficiently improve the wear
resistance or the internal fatigue damage resistance. The area ratio of pearlite
structure may be 100%.
[0025]
The area ratio of pearlite structure in a range between the outer surface of the
head portion as the origin and a depth of 25 mm can be acquired according to the
following method. That is, the area ratio of the pearlite structure can be determined
by observing the metallographic structure in the visual field of an optical microscope
of 200 magnifications and determining the area of each metallographic structure.
Further, 10 or more visual fields (10 sites) are used as the visual fields of the optical
microscope described above and the average value of the area ratios can be used as the
area ratio of the observed portion.
A method of evaluating the metallographic structure is as follows.
Pre-processing: 3% nita! etching treatment after diamond polishing performed
on sample
Observation of structure: optical microscope (200 magnifications)
- 12 -
Visual fields: 10 or more
Determination of structure: determination is made based on textbooks of
metallography (for example, "Introduction to Structures and Properties of metallic
materials and Heat Treatment Utilizing Materials and Structure Control": The Japan
Society for Heat Treatment), SEM observation in a case where structure is unclear
. Determination of ratio: the area of each structure is measured, the area ratio in a visual
field is calculated, and the average value of the entire visual field is set to a
representative value of the portion. Further, the area ratio of a structure can be
obtained by enclosing a predetermined structure with a continuous line based on the
above-described determination of a structure, acquiring the area of a region in the line
according to image analysis, and calculating the ratio of the area thereof to the area of ·
the entire observation visual fields.
In the rail according to the present embodiment, when the area ratio of the
pearlite structure of a position at a depth of 2 mm from the outer surface of the head
portion as the origin and a position at a depth of 25 mm from the outer surface of the
head portion as the origin is respectively 95% or greater, it can be said that 95% or
greater of the metallographic structure in a range between the outer surface of the head
portion as the origin and at least a depth of25 mm is the pearlite structure.
[0026]

Next, the reason for limiting the hardness of the structure including the
pearlite in the rail according to the present embodiment to a range ofHv 350 to 480
will be described.
The hardness of the metallographic structure including the pearlite required
for ensuring the wear resistance and the internal fatigue damage resistance of the rail
- 13 ..
was examined by the present inventors. Specifically, a rail in which the hardness of
the rail head portion is changed was produced for trial by performing rolling and a heat
treatment using a steel material ( eutectoid steel) containing chemical compositions
which are 0.80%C, 0.50%Si, 0.70%Mn, 0.50%Cr, 0.0150%P, and 0.0120%S. Further,
in the trial rail, the relationship between the hardness of the rail head portion and the
wear resistance and the surface damage resistance and the relationship between the
hardness and the internal fatigue damage resistance were investigated by performing a
wear test using test pieces machined from the rail head portion and a rolling contact
fatigue test using an actual raiL As the result, in order to ensure the wear resistance,
the surface damage resistance, and the internal fatigue damage resistance ofthe rail
head portion, it was confirmed that the hardness of the metallographic structure
including the pearlite in a range between the outer surface of the head portion as the
origin and a depth of25 mm needs to be controlled to be in a range ofHv 350 to 480.
When the hardness ofthe structure including the pearlite is less than Hv 350,
wear progresses and the wear resistance required for the rail head portion is difficult to
ensure. Further, in the inside of the head portion, fatigue cracks occur and propagate
and the internal fatigue damage resistance is degraded. Further, when the hardness of
ofthe structure including the pearlite is greater than Hv 480, in the surface of the head
portion, fme cracks occur in the outer surface of the head portion which comes into
contact with wheels and the surface damage resistance becomes difficult to ensure due
to embrittlement of the srructure including the pearlite. For this reason, the hardness
of the structure including the pearlite is limited to be in a range ofHv 350 to 480.
[0027]
The hardness of the structure including the pearlite is measured by performing
measurement on 10 or more points (10 sites) in a measurement position (for example, a
position at a depth of 2 mm from the outer surface of the head portion as the origin)
and employing the average value as the hardness of the position. In the rail of the
present embodiment, the area ratio of the pearlite structure is 95% or greater, but other
structures (pro-eutectoid cementite, pro-eutectoid ferrite, martensite, bainite, and the
like) are present at an area ratio of5% or less. Therefore, the hardness of the
structure including the pearlite may not be a representative value when the
measurement is performed on one point.
Conditions for measuring the hardness are described below.
Device: Vickers hardness tester (load of 98 N)
Collection of test pieces for measurement: machining sample out from
transverse cross section of rail head portion
Pre-processing: polishing transverse cross section with diamond abrasive
grains having average grain size of 1 lllil
Measurement method: carried out in conformity with JIS Z 2244
Measurement: 10 points or more
Hardness: the average value of measured point is set as a representative value
at a depth position
[0028]
. -In the rail according to the present embodiment, when ilie hardness of a
position at a depth of2 mm from the outer surface of the head portion as the origin and
the hardness of a position at a depth of 25 1mn from the outer surface of the head
portion are respectively in a range ofHv 350 to 480, it can be said iliat the hardness of
the range at least at a depth of25 mm from the outer surface of the head portion as ilie
- 15 -
origin is in a range ofHv 350 to 480.
[0029]
and
Next, the reason for limiting a difference (value obtained by subtracting the
hardness of the position at a depth of 25 mm from the outer surface of the head portion
as the origin from the hardness of the position at a depth of2 mm from the outer
surface of the head portion as the origin) in hardness between the surface of the head
portion and the inside of the head portion to a range ofHv 0 to Hv 40 and the reason
for limiting the number of V carbonitride having an average grain size of 5 to 20 nm to
a range of 50 to 500 per 1.0 11m2 of an area to be inspected in a transverse cross section
at a position having a depth of 25 mm from the outer surface of the head portion as the
origin will be described.
[0030]
A rolling contact fatigue test is performed by the present inventors using a
rolling contact fatigue testing machine shown in FIG. 9 under conditions in which the
shape of a test piece is set to a 141 lbs rail 8 with an entire length of 2 m, the type of a
\vheel 9 is set to an AAR type having a diameter of 92bmin, the radial load is set to be
in a range of 50 to 300 kN, the thrust load is set to 20 kN, lubrication is made by oil
which is intermittently supplyed, and the maximum number of repetition is set to 2
million, in rails of the related art. After the test, the state of occurrence offatigue
damage in the inside of the head portion is investigated in details.
- 16 -
As the result, it was confirmed that cracks occur in the inside of the head
portion. Since the cracks in the inside of the head portion greatly affect the basic
performance of the rail, it is necessary to prevent occurrence of cracks in order to
ensure safety. The present inventors examined a method of preventing occurrence of
cracks.
[0031]
For the purpose of reducing strain concentration on the inside of the head
portion which occurs due to the contact with wheels, the present inventors examined a
method of further improving the hardness of the inside of the head portion, decreasing
a difference in hardness between the surface of the head portion and the inside of the
head portion, and adjusting the material strength in the cross section of the head
portion to be as uniform as possible. Further, generation ofV carbonitride
precipitated in ferrite of the pearlite structure is considered to be effective for
improving the hardness of the inside of the head portion and control ofV carbonitride
which are easily precipitated in ferrite of the pearlite structure is examined.
[0032]
The precipitates in the inside ofthe head portion and the hardness of the head
portion were investigated by performing a hot rolling and a heat treatment for
promoting the generation of V carbonitride on a steel in which the V content is
changed by a range ofO.Ol% to 0.20% and theN content is changed by a range of
0.0040% to 0.0200% based on the steel material (eutectoid steel) having chemical
compositions of0.80%C, 0.50%Si, 0.50%Mn, 0.40%Cr, 0.0150%P, and 0.0120%S.
The heat treatment is performed with accelerated cooling and controlled cooling after
hot rolling is finished. The test conditions arc as follows.
[0033]
- 17 -
[Actual rail rolling, heat treatment test]
Chemical compositions of steel
0.80%C, 0.50%Si, 0.50%Mn, 0.40%Cr, 0.0150%P, 0.0120%S, V: 0.01% to
0.20%, and N: 0.0040% to 0.0200% (remainder is formed of Fe and impurities)
·Rail shape
order:
141 lbs (weight: 70 kg/m)
[0034]
· Conditions for hot rolling and heat treatment
Final rolling temperature (outer surface of head portion): 950°C
Conditions for heat treatment: heat treatment is performed in the following
( 1) rolling;
(2) natural air cooling; and
(3) accelerated cooling and controlled cooling.
Conditions for accelerated cooling (outer surface of head portion): performing
cooling to temperature range of800°C to 590°C at cooling rate of3°C/sec
Conditions for controlled cooling (outer surface of head portion): holding
temperature range of 580°C to 640°C for 100 to 200 sec after accelerated cooling is
stopped and then performing air cooling
Holding of temperature during controlled cooling: temperature is controlled
by repeatedly perfonning and stopping accelerated cooling. according to recuperation
from inside of rail
[0035]
[Method of investigating V carbo nitride]
· Pre-processing: machining samples from transverse cross section of rail, and
- 18 -
performing thin film processing or replica collection (method of exposing precipitates
by electrolytic etching or chemical etching and peeling precipitates off using film)
·Collection position: inside of head portion (position at depth of25 mm from
outer surface of head portion as origin)
· Measurement method
Device: transmission electron microscope
Magnifications: 50000 to 500000
Number of visual fields for observation: 20 visual fields
Selection of precipitates: The precipitates generated in ferrite of the pearlite
structure are identified with a transmission electron microscope (TEM) using a thin
film or a replica sample. The V carbonitride are determined by performing
composition analysis on the precipitates using an energy dispersive X -ray spectroscopy
device (EDX) or performing element analysis through crystal structural analysis of an
electron beam diffraction image using a TEM. During the determination, a
precipitate from which carbon or nitrogen, in addition to V, is simultaneously detected
is set to a target of evaluation, in each of the precipitates. The precipitates as an
evaluation target contain at least V and carbon, V and nitrogen, or V, carbon, and
nitrogen and may contain other alloy elements.
Measurement of grain size of precipitates: The area of precipitates serving as
the above-described evaluation target is acquired and the average grain size is
calculated using the diameter of a circle correspon
[0044]
From the viewpoint of further improving the safety, measures to improve the
characteristics at the time of long-term use are examined by the present inventors. As
the result of detailed observation on the rail after the fatigue test, it was confirmed that
fine cracks occasionally occur in the periphery ofV carbonitride. The present
inventors examined the method of eliminating these fine cracks.
[0045]
Here, the relationship between the composition ofV carbonitride and fine
cracks occurring in the periphery thereof is detailed investigated by the present
inventors. The investigation method is as follows.
[0046]
[Method of investigating fine cracks]
· Preparation of sample
The rail is machined and a sample is prepared from a position at a depth of 25
mm from the outer surface of the head portion in the inside of the head portion as the
ongm. · Pre-processing: polishing cross section with diamond advasive grains
· Observation method
Device: scanning electron microscope
Magnifications: I 0000 to 100000
Observation position: detailed observation on periphery ofV carbonitride
having average grain size of 5 to 20 nm
(The method of measuring the average grain size is the same as described
above.)
[0047]
[Method of investigating composition ofV carbonitride]
· Position for collecting samples: inside of head portion (position at depth of
- 25 -
25 mm from the outer surface of head portion as origin)
·Pre-processing: needle sample is processed (10 11m x 10 11m x 100 11m)
according to focused ion beam (FIB) method
· Measuring device: three-dimensional atom probe (3DAP) method
· Measurement method
A voltage is applied to the needle sample to release metal ions and the metal
ions are detected using a coordinate detector. The type of element is identified based
on the ion flight time and the element position or the number of atoms in three
dimensions is specified based on the detected coordinates.
Voltage: DC, pulse (pulse rate of20% or greater)
Sample temperature: 40 K or lower
· Calculation ratio of number of carbon atoms to number of nitrogen atoms of
V carbonitride
The number of carbon atoms and the number of nitrogen atoms of V
carbonitride are calculated based on the information of the element positions or the
amounts thereof described above. The number of carbon atoms and the number of
nitrogen atoms contained in V carbonitride are respectively counted from the results of
3DAP. The ratio (CA/NA) of the nmnber of carbon atoms (CA) to the number of
nitrogen atoms (NA) is calculated from the results.
· Number oftimes of measurement: 5 or more points are measured and the
average value is set to the representative value.
[0048]
As the result of investigation, it was confirmed that the state of occurrence of
cracks is greatly changed by the combination ofthe number of carbon atoms and the
number of nitrogen atoms ofV carbonitride. Further, as the result of detailed
- 26 -
investigation, it was found that occurrence of fine cracks and the number of carbon
atoms (CA) and the number of nitrogen atoms (NA) ofV carbonitride are correlated
and the hardness of V carbo nitride tends to be increased and the amount of cracks to
occur in a parent phase in the periphery thereof tends to be increased when the amount
of carbides is increased. As the result of further investigation, as shown in FIG. 5, it
was confirmed that fine cracks are eliminated by controlling the ratio (CA/NA) of the
number of carbon atoms (CA) to the number of nitrogen atoms (NA) to 0.70 or less.
[0049]
From these results, it was found that, preferably, the number ofV carbonitride
are controlled and the composition of V carbonitride as the origin of cracks is
controlled in order to suppress and prevent cracks in tl1e inside of the head portion and
occurrence of fme cracks and further improve ilie basic performance of the rail.
[0050]

The reason for limiting the chemical compositions of rail steel (steel serving
as the material of the rail) in the rail according to the present embodiment will be
described in detail.
[0051]
C: 0.75% to 0.85%
C is an element effective for promoting pearlitic transformation and ensuring
weai resistance. When the C content is less than 0. 75%, iri the present chemical
composition, the minimum strength and wear resistance required for the rail cannot be
maintained. Further, a pro-eutectoid ferrite is generated and the wear resistance is
greatly degraded. Further, a soft pro-eutectoid ferrite in which fatigue cracks easily
occur in the inside of the head portion is likely to be generated and internal fatigue
- 27 -
damage resistance is likely to be generated. Meanwhile, when the C content is
greater than 0.85%, the toughness of the pearlite structure is degraded, brittle cracks
occur in the inside of the head portion, and the internal fatigue damage resistance is
degraded. Further, the pro-eutectoid cementite is likely to be generated in the inside
of the head portion, fatigue cracks occur from the interface between the pearlite
structure and the pro-eutectoid cementite, and then the internal fatigue damage
resistance is likely to be generated. Therefore, the C content is adjusted to be in a·
range of0.75% to 0.85%. In order to stabilize generation of the pearlite structure and
improve the internal fatigue damage resistance, it is preferable that the C content is
adjusted to be in a range of 0.80% to 0.85%.
[0052]
Si: 0.10% to LOO%
Si is an element which is dissolved in solid in ferrite of the pearlite structure,
increases the hardness (strength) of the rail head portion, and improves the wear
resistance. However, when the Si content is less than 0.1 0%, these effects cannot be
sufficiently obtained. Meanwhile, when the Si content is greater than LOO%, a large
amount of surface cracks are generated at the time of hot rolling. In addition, the
hardenability is significantly increased, the martensite structure is likely to be
generated in the rail head portion so that the wear resistance is degraded. Therefore,
the Si content is adjusted to be in a range ofO.lO% to 1.00%. It is preferable that the
Si content is adjusted to be in a range of 0.20% to 0.80% in order to further stabilize
generation of the pearlite stmcture and further improve· the wear resistance or the
internal fatigue damage resistance.
[0053]
· Mn: 0.30% to 1.20%
- 28 -
Mn is an element which increases the hardenability, stabilizes pearlitic
transformation, refines the lamellar spacing ofthe pearlite structure, and ensures the
hardness of the pearlite structure so that the wear resistance or the internal fatigue
damage resistance is further improved. However, when the Mn content is less than
0.30%, the wear resistance is not improved. Further, a soft pro-eutectoid ferrite in
which fatigue cracks easily occur in the inside of the head portion is generated and the
internal fatigue damage resistance is difficult to ensure. Meanwhile, when the Mn
content is greater than 1.20%, the hardenability is significantly increased, and the
martensite structure is generated in the rail head portion so that the wear resistance or
the surface damage resistance is degraded. Therefore, the Mn addition content is
adjusted to be in a range of 0.30% to 1.20%. It is preferable that the Mn content is
adjusted to be in a range of0.40% to 1.00% in order to stabilize generation of the
pearlite structure and improve the wear resistance or the internal fatigue damage
resistance.
[0054]
Cr: 0.20% to 0.80%
Cr is an element which refines the lamellar spacing of the pearlite structure
and improves the hardness (strength) of the pearlite structure by increasing the
equilibrium transformation temperature and increasing the supercooling degree.
Further, the refining of the lamellar spacing and the improvement of the hardness of
the pearlite structure contribute to improvement' of wear resistance and internal fatigue
damage resistance. However, when the Cr content is less than 0.20%, the effects
described above are small and the effects of improving the hardness of rail steel cannot
be obtained. Meanwhile, when the Cr content is greater thim 0.80%, the hardenability
is ·Significantly increased, the bainite structure or the martensite structure is generated
- 29 -
in the rail head portion, and thus the wear resistance or the surface damage resistance is
degraded. Therefore, the Cr content is set to be in a range of0.20% to 0.80%. It is
preferable that the Cr content is set to be in a range of 0.40% to 0.75% in order to
stabilize generation of the pearlite structure and improve the wear resistance or the
internal fatigue damage resistance.
[0055]
V: 0.01% to 0.20%
Vis an element which is precipitated as a V carbonitride during a cooling
process after hot rolling, increases the hardness (strength) of the pearlite structure
using precipitation hardening, and improves the internal fatigue damage resistance in
the inside of the head portion. However, when the V content is less than 0.01 %, the
number of fine carbonitrides to be precipitated in ferrite of the pearlite structure is
small and the hardness (strength) of the inside of the head portion is not improved.
Meanwhile, when the V content is greater than 0.20%, the number offme V
carbonitride becomes excessive, the hardness of the inside ofthe head portion is more
increased than the hardness of the surface of the head portion, and the strain of the rail
which is generated from the external force due to the contact with wheels or the like is
concentrated on a region having a low hardness on the surface of the head portion.
As the result, fme cracks occur in the surface of the head portion and the surface
damage resistance is degraded. Therefore, the V content is set to be in a range of
0.01% to 0.20%. It is preferable thattheV content is set to be in arange of0.03% to
0.10% in order to stabilize generation of the pearlite structure and improve the internal
fatigue damage resistance.
[0056]
N: 0.0040% to 0.0200%
- 30 -
N is an element which promotes precipitation ofV carbonitride during the
cooling process after hot rolling when N and V are added at the same time. When V
carbonitride is precipitated, the hardness (strength) of the pearlite structure is increased
and the intemal fatigue damage resistance is improved. However, when theN content
is less than 0.0040%, the number of fine carbonitrides to be precipitated in ferrite of
the pearlite structure is small and t.lJ.e hardness (strength) ofthe inside of the head
portion is not improved. Meanwhile, when theN content is greater than 0.0200%, it
becomes difficult for N to be solid-soluted in steel. In this case, bubbles as the origin
of fatigue damage are generated so that the internal fatigue damage is likely to occur.
Therefore, theN content is set to be .in a range of 0.0040% to 0.0200%. It is
preferable that theN content is setto be in arange of0.0060% to 0.0150% in order to
stabilize generation of the pearlite structure and improve the internal fat.igue damage
resistance.
[0057]
P: 0.0250% or less
Pis an element (impurity) which is unavoidably contained in steel and the
content thereof can be controlled by performing refining in a converter. It is
preferable that the P content is small. However, when the P content is greater than
0. 0250%, the pearlite structure is embrittled and brittle cracks occur in the inside of the
head portion so that the internal fatigue damage resistance is degraded. Therefore,
the P content is limited to 0.0250% or less. The lower limit of the P content is not
limited, but the lower limit thereof at the time of actual production is approximately
0.0050% when desulfurization capacity during the refining process is considered.
[0058]
S: 0.0250% or less
- 31 -
S is an element (impurity) which is unavoidably contained in steel and the
content thereof can be controlled by performing desulfurization in a cupola pot. It is
preferable that the S content is small. However, when the S content is greater than
0.0250%, inclusions of coarse MnS-based sulfides are likely to be generated, fatigue
cracks occur in the inside of the head portion due to stress concentration on the
periphery of the inclusions, and thus the internal fatigue damage resistance is degraded.
Therefore, the S content is limited to 0.0250% or less. The lower limit of the S
content is not limited, but the lower limit thereof at the time of actual production is
approximately 0.0050% when desulfurization capacity during the refining process is
considered.
[0059]
Basically, the rail according to the present embodiment contains the abovedescribed
chemical elements and the remainder is formed of Fe and impurities.
However, in place of a part of Fe in the remainder, the remainder may further contain
at least one selected from the group consisting ofMo, Co, B, Cu, Ni, Nb, Ti, Mg, Ca,
REM, Zr, and Al, in ranges described below, for the purpose of improving the wear
resistance and the internal fatigue damage resistance due to an increase in hardness
(strength) of the pearlite structure, improving the toughness, preventing a heat affected
zone of welded joint from being softened, and controlling distribution of the hardness
in the cross section in the inside of the head portion. Specifically, Mo increases the
equilibrium transformation pomt, refines the lamellar spacing of the pearlite structure,·
and improves the hardness. Co refines the lamellar structure on the wear surface and
increases the hardness of the wear surface. B reduces cooling rate dependence ofthe
pearlitic transformation temperature to make distribution of the hardness in the cross
section of the rail head portion uniform, Cu is dissolved in solid in ferrite of the
- 32 -
pearlite structure and increases the hardness. Ni improves the toughness and hardness
of the pearlit structure e and prevents the heat affected zone of the welded joint from
being softened. Nb and Ti improve the fatigue strength of the pearlite structure by
precipitation hardening of a carbide and a nitride generated during a hot rolling and a
cooling process carried out after the hot rolling. Further, Nb and Ti make a carbide or
a nitride be stably generated at the time of re-heating and prevent the heat affected
zone of the welded joint from being softened. Mg, Ca, and REM finely disperse
MnS-based sulfides and decrease the internal fatigue damage occurring from
inclusions. Zr suppresses formation of a segregating zone of a cast slab or bloom
central portion and suppresses generation of a pro-eutectoid cementite or the
martensite by increasing the equiaxed crystal ratio ofthe solidification structure.
Consequently, these elements may be contained in order to obtain the above-described
effects. In addition, even if the amount of each element is equal to or smaller than the
range described below, the characteristics of the rail according to the present
embodiment are not damaged. Further, since these elements are not necessarily
contained, the lower limit tl1ereof is 0%.
[0060]
Mo: 0.01% to 0.50%
Mo is an element which refines the lamellar spacing of the pearlite structure
and improves the hardness (strength) of the pearlite structure so that the wear
resistance and the internal fatigue damage resistance are improved by increasing the
equilibrium transformation temperature and increasing the supercooling degree.
However, when the Mo content is less than 0.01%, the effects described above are
small and the effects of improving the hardness of rail steel cannot be obtained.
Meanwhile, when the Mo content is greater than 0.50%, the transformation rate is
- 33 -
significantly decreased, the martensite structure with low toughness is generated in the
rail head portion, and thus the wear resistance is degraded. Therefore, it is preferable
that the Mo content is set to be in a range of 0.01% to 0.50% when Mo is contained.
[0061]
Co: 0.01% to 1.00%
Co is an element which is dissolved in solid in ferrite of the pearlite structure,
refines the lamellar structure of the pearlite structure directry beneath the rolling
'
surface resulting from the contact with wheels, and increases the hardness (strength) of
the pearlite structure so that the wear resistance and the internal fatigue damage
resistance are improved. However, when the Co content is less than 0.01 %, the
refining of the lamellar structure is not promoted and thus the effects of improving the
wear resistance or the internal fatigue damage resistance cannot be obtained.
Meanwhile, when the Co content is greater than 1.00%, the above-described effects are
saturated and the lamellar structure in accordance with tl1e content cannot be refined.
Further, economic efficiency is decreased due to an increase in alloying addition cost.
Therefore, it is preferable that the Co content is set to be in a range ofO.Ol% to 1.00%
when Co is contained.
[0062]
B: 0.0001% to 0.0050%
B is an element which forms iron borocarbides (Fe23(CB)6) in austenite grain
boundaries and reduces cooling rate dependence of the pearlitic transfomiation
temperature by promoting pearlitic transformation. Further, B is an element which
imparts more uniform distribution of the hardness to a range from the outer surface of
the head portion to the inside thereof and increases the service life of the rail.
However, when the B content is less than 0.0001%, the effects described above are not
- 34 -
sufficient and improvement of distribution of the hardness in the rail head portion is
not recognized. Meanwhile, when B content is greater than 0.0050%, coarse iron
borocarbides are generated, brittle fracture is generated, and the toughness of the rail is
degraded. Therefore, it is preferable that the B content is set to be in a range of
0.0001% to 0.0050% when B is contained.
[0063]
Cu: O.Ol%to 1.00%
Cu is an element which is dissolved in solid in ferrite of the pearlite structure
and improves the hardness (strength) resulting from solid solution strengthening. As
the result, the wear resistance and the internal fatigue damage resistance are improved.
However, when the Cu content is less than 0.01 %, the effects cannot be obtained.
Meanwhile, when the Cu content is greater than 1.00%, the martensite structure is
generated in the rail head portion due to significant improvement ofhardenability and
thus the wear resistance is degraded. Therefore, it is preferable that the Cu content is
set to be in a range ofO.Ol% to 1.00% when Cu is contained.
[0064]
Ni: 0.01% to 1.00%
Ni is an element which improves the toughness of the pearlite structure and
improves the hardness (strength) resulting from solid solution strengthening. As the
result, the wear resistance and the internal fatigue damage resistance are improved.
Further, Ni rs an element which is finely precipitated in the welded neat affected zone
as an interrnctallic compound ofNi3 Ti in the form of a composite with Ti and
suppresses softening due to precipitation strengthening. In addition, Ni is an element
which suppresses embrittlement of grain boundaries in steel containing Cu. However,
when the Ni content is less than 0.01%, these effects are extremely smalL· Meanwhile,
- 35 -
when the Ni content is greater than 1.00%, the martensite structure is generated in the
rail head portion and the wear resistance is degraded due to significant improvement of
hardenability. Therefore, it is preferable that the Ni content is set to be in a range of
0.01% to 1.00% when Ni is contained.
[0065]
Nb: 0.0010% to 0.0500%
Nb is an element which is precipitated as a Nb carbide and/or a Nb nitride
during a cooling process after hot rolling, increases the hardness (strength) of the
pearlite structure by precipitation hardening, and improves the wear resistance and the
internal fatigue damage resistance. Further, Nb is an element effective for preventing
the heat affected zone of the welded joint from being softened by being stably
generated as a Nb carbide or a Nb nitride from a low temperature range to a high
temperature range, in the heat affected zone re-heated to a temperature range lower
than or eqnal to the Ac1 point. However, when the Nb content is less than 0.0010%,
these effects cannot be sufficiently obtained and improvement of the hardness
(strength) of the pearlite structure is not recognized. Meanwhile, when Nb content is
greater than 0.0500%, precipitation hardening resulting from the Nb carbide or the Nb
nitride becomes excessive, the pearlite structure is embrittled, and then the internal
fatigue damage resistance of the rail is degraded. Therefore, it is preferable that the
Nb content is set to be in a range ofO.OOlO% to 0.0500% when Nb is contained.
[0066]
Ti: 0.0030% to 0.0500%
Ti is an element which is precipitated as a Ti carbide and/or a Ti nitride during
a cooling process after hot rolling, increases the hardness (strength) of the pearlite
structure by precipitation hardening, and improves the wear resistance· and the internal
- 36 -
fatigue damage resistance. Further, Ti is an element effective for preventing the
welded joint from being embrittled by refining the structure of the heat affected zone
heated to the austenite region because the precipitated Ti carbide or Ti nitride is not
dissolved at the time of re-heating during welding. However, when the Ti content is
less than 0.0030%, these effects are small. Meanwhile, when the Ti content is greater
than 0.0500%, a Ti carbide and a Ti nitride which are coarse are generated, and fatigue
cracks occur and the internal fatigue damage resistance is degraded due to the stress
concentration. Therefore, it is preferable that the Ti content is set to be in a range of
0.0030% to 0.0500% when Ti is contained.
[0067]
Mg: 0.0005% to 0.0200%
Mg is an element which is bonded to S to form a sulfide. MgS fmely
disperses MnS so that stress concentration is relaxed and the internal fatigue damage
resistance is improved. However, when the Mg content is less than 0.0005%, these
effects are small. Meanwhile, when the Mg content is greater than 0.0200%, a coarse
oxide of Mg is generated, and fatigue cracks occur and the internal fatigue damage
resistance is degraded due to the stress concentration. Therefore, it is preferable that
the Mg content is set to be in a range of 0.0005% to 0.0200% when Mg is contained.
[0068]
Ca: 0.0005% to 0.0200%
Ca is an element which has a strong bonding force to S and forms CaS
(sulfide). CaS finely disperses MnS so that the stress concentration is relaxed and the
internal fatigue damage resistance is improved. However, when theCa content is less
than 0.0005%, these effects are small. Meanwhile, when theCa content is greater
than 0.0200%, a coarse oxide ofCa is generated, and fatigue cracks occur and the
- 37 -
internal fatigue damage resistance is degraded due to the stress concentration.
Therefore, it is preferable that the Ca content is set to be in a range of 0.0005% to
0.0200% when Ca is contained.
[0069]
REM: 0.0005% to 0.0500%
REM is a deoxidation and desulfurization element and generates oxysulfide
(REM202S) of REM serving as a nucleus that generates Mn sulfide-based inclusions
when REM is contained. Further, since the melting point of the oxysulfide
(REM20 2S) is high, stretching of the Mn sulfide-based inclusions after hot rolling is
suppressed. As the result, when REM is contained, MnS is finely dispersed, the stress
concentration is relaxed, and the internal fatigue damage resistance is improved.
However, when the REM content is less than 0.0005%, REM becomes insufficient as
the nucleus that generates MnS-based sulfides and the effects are small. Meanwhile,
when the REM content is greater than 0.0500%, oxysulfide (REM20 2S) of hard REM
is generated, and fatigue cracks occur and the internal fatigue damage resistance is
degraded due to the stress concentration. Therefore, it is preferable that the REM
content is set to be in a range of 0.0005% to 0.0500% when REM is contained.
[0070]
Further, REM is a rare earth metal such as Ce, La, Pr, or Nd. The content
described above is obtained by limiting the total amount of REM. When the total
amount ofthe contents is in the above-described range, the same effects are obtained
even when the form is either of a single element or a combination of elements (two or
more kinds).
[0071]
Zr: 0.0001% to 0.0200%
- 38 -
Zr is bonded to 0 and generates a Zr02 inclusion. Since this Zr02 inclusion
has excellent lattice matching performance with y-Fe, the ZrOz inclusion becomes a
solidified nucleus of high carbon rail steel in which y-Fe is a solidified primary phase
and suppresses formation of a segregation zone in a central part of a cast slab or bloom
by increasing the equiaxed crystal ratio of the solidification structure. In this manner,
Zr is an element which suppresses generation of the martensite structure generated in a
segregation portion ofthe rail. However, when the Zr content is less than 0.0001%,
the number of ZrOz-based inclusions to be generated is small and the inclusions do not
sufficiently exhibit effects as solidified nuclei. In this case, the martensite is likely to
be generated in the segregation portion, and accordingly, improvement of the internal
fatigue damage resistance of the rail cannot be expected. Meanwhile, when the Zr
content is greater than 0.0200%, a large amount of coarse Zr-based inclusions are
generated, and fatigue cracks occur and the internal fatigue damage resistance is
degraded due to the stress concentration. Therefore, it is preferable that the Zr
content is set to be in a range ofO.OOOl% to 0.0200% when Zr is contained.
[0072]
AI: 0.0100% to 1.00%
AI is an element which functions as a deoxidizer. Further, AI is an element
which moves the eutectoid transformation temperature to a high temperature side,
contributes to increasing the hardness (strength) of the pearlite structure, and thus
improves the wear resistance or the internal fatigue damage resistance of the pearlite
structure. However, when the AI content is less than 0. 0100%, the effects thereof are
small. Meanwhile, when the AI content is greater than 1.00%, it becomes difficult for
AI to be dissolved in steel and thus coarse alumina-based inclusions are generated.
Since these coarse Al-based inclusions are the origin oftl1e fatigue cracks, the internal
- 39 -
fatigue damage resistance is degraded. Further, an oxide is generated at the time of
welding so that the weldability is significantly degraded. Therefore, it is preferable
that the AI content is set to be in a range of 0.0100% to 1.00% when AI is contained.
[0073]

In the rail according to the present embodiment, it is necessary that the value
of the ratio (Expression 1) ofthe Mn content (Mn) to the Cr content (Cr) be set to
greater than 1.00 and 4.00 or less in addition to the content of each element. The
reason therefor will be described below.
[0074]
The present inventors examined a method of preventing generation of
abnormal structures such as martensite structure or bainite structure by stably
generating the pearlite structure having a high hardness. Specifically, the present
inventors examined the influence of the contents of Mn and Cr which are basic alloy
element on generation of abnormal structures.
[0075]
First, two kinds oftest pieces of steel, which are steel (Mn steel) having a Mn
content of 1.0% and steel (Cr steel) having a Cr content of 1.0% are produced based on
a steel (eutectoid steel) having the composition of0.80%C, 0.50%Si, Mn, Cr,
0.0150%P, and 0.0120%S, an isothermal transformation heat treatment is performed on
the test pieces, and the relationship otthe transformation temperature, the hardness,
and the metallographic structure is investigated. The conditions for the test are as
follows.
[0076]
[Isothermal transformation heat treatment experiment]
- 40 -
· Conditions for isothermal transformation heat treatment
Heating temperature and time: 1 000°C x 5 min
Conditions for cooling: cooling from heating temperature to isothermal
transformation temperature at cooling rate of 30°C/sec
Conditions for isothermal transformation: isothermal transformation
temperature of 500°C to 600°C, holding time oflOO to 1000 sec
After isothermal transformation: accelerated cooling (cooling to 50°C at
cooling rate of30°C/sec)
[0077]
· Conditions for evaluating hardness and metallographic structure
Observation of structure
Pre-processing: 3% nita! etchiug treatment after diamond polishiug performed
on cross section
Observation of structure: usiug optical microscope
Measurement of hardness
Device: Vickers hardness tester (load of 98 N)
Pre-processing: diamond polishing performed on cross section
[0078]
FIG. 1 shows the relationship of the isothermal transformation temperature,
the hardness, and the metallographic structure.
[0079]
In Mn steel (1.0%Mn), it was confirmed that the pcarlitic transformation is
stabilized to a low temperature range compared to that of Cr steel (1.0%Cr) and the
pearlitic transfom1ation easily occurs. That is, it was confirmed that generation ofthe
bainite ham1ful to the wear resistance is suppressed in Mn steel (l.O%Mn) compared to
- 41 -
Cr steel (l.O%Cr). When Cr steel is compared to Mn steel, the hardness of the
pearlite structure of the Cr steel tends to be higher than that of Mn steel at the same
transformation temperature.
[0080]
From these results, it was understood that the balance between the Mn content
and the Cr content is important in order to obtain a pearlite structure having a high
hardness, and it is preferable to design elements to which Cr is supplementarily added
to ensure the hardness while the Mn content which stabilizes generation of the pearlite
structure is adjusted to be larger than the Cr content.
[0081]
Next, the present inventors examined the optimum balance between the Mn
content and the Cr content. The total of the Mn content and the Cr content is set to
1.4% and a test piece of steel in which the Mn content and the Cr content are changed
is produced based on steel ( eutectoid steel) having the composition of 0.80%C,
0.50%Si, Mil, Cr, 0.0150%P, and 0.0120%S. Further, the balance between Mn and Cr
and the relationship between the hardness and the metallographic structure are
examined by performing a continuous cooling heat treatment, in which the cooling of
the surface of the head portion (position at a depth of 2 mm from the outer surface of
the head portion as the origin) in actual rails is reproduced, on this test piece. The test
conditions are as follows.
[0082]
[Continuous cooling heat treatment experiment]
· Chemical composition of steel
0.80%C, 0.50%Si, Mn: 0.05% to 1.40%, Cr: 0.05% to 1.40%, 0.0150%P, and
0.0120%S (remainder is Fe and impurities)
- 42 -
· Conditions for continuous cooling heat treatment
Heating temperature and time: I 000°C x 5 min
Conditions for cooling: cooling from heating temperature to 50°C at cooling
rate of 3°C/sec (simulating cooling conditions in surface of head portion)
[0083]
· Conditions for evaluating hardness and metallographic structure
Observation of structure
Pre-processing: 3% nita! etching treatment after diamond polishing performed
on cross section
Observation of structure: observation using optical microscope
Measurement of hardness
Device: Vickers hardness tester (load of 98 N)
Pre-processing: diamond polishing performed on cross section
[0084]
FIG. 2 shows the relationship between the value ofMn/Cr obtained from the
Mn content and the Cr content and the metallographic structure. As shown in FIG 2,
when the value ofMn/Cr is 1.00 or less, the Cr content becomes excessive, and the
bainite harmful to the wear resistance and the martensite harmful to the wear resistance
or the surface damage resistance are generated. Meanwhile, when the value of Mn/Cr
is greater than 4.00, the Mn content becomes excessive, and the martensite harmful to
the wear resistance or the surface damage resistance is generated.
[0085]
From these results, it was found that the Mn/Cr value needs to be controlled to
greater than 1.00 and 4.00 or less (that is, the expression of 1.00 < Mn/Cr :S 4.00 is
satisfied) in order to suppress generation of the bainite ham1ful to the wear resistance
- 43 - j ~·
and the martensite harmful to the wear resistance or the surface damage resistance and
stably obtain the pearlite structure having a high hardness, in the surface of the head
portion.
[0086]

Next, in the rail according to the present embodiment, the reason for limiting
the total value of the Mn content (Mn) and the Cr content (Cr) to a range of0.30 to
1.00 (0.30 <:: 0.25 x Mn + Cr <:: 1.00) will be described.
[0087]
As described above, Mn and Cr affect ease of pearlitic transformation and the
hardness of the pearlite structure. For this reason, the present inventors investigated
the relationship between the Mn content and the Cr content and the hardness of the
pearlite structure on the premise that the value ofMn/Cr is set to greater than 1.00 and
4.00 or less. Specifically, a test piece of steel in which the Mn content is changed to
be in a range of0.20% to 1.20% and the Cr content is changed to be in a range of
0.20% to 0.80% is produced based on a steel (eutectoid steel) containing chemical
compositions of0.80%C, 0.50%Si, Mn, Cr, 0.0150%P, and 0.0120%S. Further, the
relationship between the Mn content and the Cr content and the hardness thereof was
investigated by performing a continuous cooling heat treatment, in which the cooling
of the surface oftl1e head portion (position at a depth of2 mm from the outer surface
ofthe head portion as the origin) and the cooling of the inside ofthehead portion
(position at a depth of25 mm from the outer surface offue head portion as the origin)
are reproduced, on these test pieces. The test conditions are as follows.
[0088]
[Continuous cooling heat treatment experiment]
- 44 -
· Chemical composition
0.80%C, 0.50%Si, Mn: 0.20% to 1.20%, Cr: 0.20% to 0.80%, 0.0150%P, and
0.0120%S (remainder is formed ofFe and impurities)
· Conditions for continuous cooling heat treatment
Heating temperature and time: I 000°C x 5 min
Conditions fDr cooling in order to reproduce cooling the surface of head
portion: cooling from heating temperature to 50°C at cooling rate of 3°C/sec
Conditions for cooling in order to reproduce cooling the inside of head
portion:.cooling from heating temperature to 50°C at cooling rate of 1 °C/sec
[0089]
· · Conditions for evaluating hardness and metallographic structure
Observation of structure
Pre-processing: 3% nital etching treatment after diamond polishing performed
on cross section
Observation of structure: observation using optical microscope
Measurement of hardness
Device: Vickers hardness teater (load of98 N)
Pre-processing: diamond polishing performed on cross section
[0090]
As the result of analyzing the relationship between the amount of alloys and
the hardness of the structure including the pearlite of steel on which the experiment of
the continuous cooling heat treatment is performed, it was confirmed that both of the
hardness of the structure including the pearlite of the surface of the head portion and
the hardness of the structure including the pearlite of the inside of the head portion are
correlated with arelational expression formed of the Mn content and the Cr content.
- 45 -
FIG. 3 shows the relationship between the hardness and the value of 0.25 x Mn + Cr
(Expression 2) including the Mn content and the Cr content.
[0091]
As shown in FIG. 3, when the value of (0.25 x Mn + Cr) is controlled to 1.00
or less, the hardness of the structure including the pearlite in the surface of the head
portion can be set to Hv 480 or less, which is the value in which the surface damage
resistance can be ensured. Meanwhile, when the value of (0.25 x Mn + Cr) is
controlled to 0.30 or greater, the hardness of the structure including the pearlite in the
inside of the head portion can be set to Hv 350 or greater, which is the value required
for ensuring the wear resistance or the internal fatigue damage resistance. Therefore,
the hardness which satisfies the wear resistance and the internal fatigue damage
resistance can be ensured as the hardness of the structure including the pearlite in the
rail head portion according to the present embodiment by controlling the chemical
composition such that the value of (0.25 x Mn + Cr) is in a range of 0.30 to 1.00 (that
is, an expression of 0.30 :":: 0.25 x Mn + Cr :":: 1.00" is satisfied).
[0092]
As shown in FIG. 3, when the value of 0.25 x Mn + Cr is less than 0.30, the
required hardness (Hv 350 or greater) of the pearlite structure that ensures the internal
fatigue damage resistance in the inside ofthe head portion is difficult to ensure.
Further, when the value of 0.25 x Mn + Cr is greater than 1.00, the hardness of the
pearlite structure in the surface ofthe head portion becomes excessive (greater than Hv
480), the pearlite structure is embrittled, fme cracks occur in the outer surface of the
head portion which comes into contact with wheels, and thus the surface damage
resistance becomes difficult to ensure.
[0093]
- 46 -
In the rail according to the present embodiment, the wear resistance and the
internal fatigue damage resistance of the rail in a case of being used in cargo railways
can be improved and the service life can be greatly improved by controlling alloy
composition of rail steel, structures, the hardness of the surface of the head portion or
the inside oftl1e head portion, the number ofV carbonitride, and a difference in
hardness between the surface of the head portion and the inside of the head portion and
further controlling the composition of V carbonitride.
[0094]
Next, a preferable production method of the rail according to the present
embodiment will be described.
When the rail according to the present embodiment includes the above-described
chemical compositions, structures, and the like, the effects thereof can be obtained
regardless of the production method. However, since the rail according to the present
embodiment is stably obtained, it is preferable that fue production method including
the following process is used.
[0095]
The rail according to the present embodiment can be produced by performing
smelting the steel in a melting furnace such as a converter or an electric furnace which
is typically used, performing casting according to an ingot-making and blooming
method or a continuous casting method on the molten steel having chemical
compositions adjusted to have the above-describeifranges to obtain a slab or bloom,
performing hot rolling on the slab or bloom so as to be formed in a rail shape, and
performing a heat treatment after hot rolling.
In these series of processes, it is necessary to control the conditions for hot
rolling and the conditions for fue heat treatment after hot rolling according to the
- 47 -
required hardness of the rail head portion in order to control the hardness of the rail
head portion. As the conditions for hot rolling and the conditions for the heat
treatment after hot rolling, it is preferable that the hot rolling and the heat treatment are
performed under the following conditions in order to maintain the pearlite structure
and control the structure of the rail head portion and the hardness of the surface of the
head portion .or the inside of the head portion. Further, the temperature of the surface
of the head portion and the temperature of the outer surface of the head portion are
substantially the same as each other.
[0096]
· Conditions for hot rolling
Final hot rolling temperature of outer surface of head portion: 900°C to
1000°C
Final reduction (area reduction ratio): 2% to 20%
[0097]
· Conditions for heat treatment after hot rolling (outer surface of head
portion): performing natural air cooling for 60 to 180 sec after hot rolling and then
performing accelerated cooling and controlled cooling
Accelerated cooling (outer surface of head portion)
Cooling rate: 2°C/sec to 8°C/sec
Start temperature: 750°C or higher, stop temperature: 580°C to 640°C
Controlled cooling (outer surface ofhead portion)
Holding temperature of outer surface of head portion to range of 580°C to
640°C for I 00 to 200 sec after stopping of accelerated cooling and then performing air
cooling
Temperature holding during controlled cooling: temperature is controlled to
- 48 -
predetermined range by repeatedly performing and stopping accelerated cooling
according to recuperation from inside of rail
[0098]
In a case of prevent fine cracks occurring in the periphery of carbo nitrides by
controlling the ratio (CA/NA) of the number of carbon atoms (CA) to the number of
nitrogen atoms (NA) ofV carbonitride, it is preferable that the conditions for
accelerated cooling and the conditions for controlled cooling are changed into the
following conditions.
[0099]
·Conditions for heat treatment (outer surface of head portion): performing
natural air cooling for 60 to 180 sec after rolling and then performing accelerated
cooling and controlled cooling
Accelerated cooling (outer surface of head portion):
Cooling rate: 2°C/sec to 8°C/sec
Start temperature: 750°C or higher, stop temperature: 6l0°C to 640°C
Controlled cooling (outer surface of head portion):
Holding temperature of outer surface of head portion to range of 61 0°C to
640°C for l 00 to 200 sec after stopping of accelerated cooling and then performing air
cooling
Temperature holding during controlled cooling: temperature is controlled to
predetermined range by repeatedly pcrtorming and stopping accelerated cooling
according to recuperation from inside of rail
[01 00]
First, the reason why it is preferable that the final hot rolling temperature
(outer surface ofthe head portion) is set to be in a range of900°C to 1000°C will be
- 49 -
described.
When the final hot rolling temperature (outer surface of the head portion) is
lower than 900°C, refining of austenite grains after hot rolling becomes significant.
In this case, the hardenability is greatly degraded and the hardness of the rail head
portion is unlikely to be ensured in some cases. Further, when the final hot rolling
temperature (outer surface of the head portion) is higher than I 000°C, austenite grains
after hot rolling become coarse, the hardenability is excessively increased, and the
bainite harmful to the wear resistance is easily generated in the rail head portion.
Therefore, it is preferable that the fmal hot rolling temperature (outer surface ofthe
head portion) is set to be in a range of900°C to 1000°C.
[0101]
Next, the reason why it is preferable that the fmal reduction (reduction of
area) is set to be in a range of2% to 20% will be described.
When the fmal reduction (reduction of area) is less than 2%, austenite grains
after hot rolling become coarse, the hardenability is excessively increased, the bainite
harmful to the wear resistance is easily generated in the rail head portion, the grain size
of the pearlite structure becomes coarse, and the ductility or the toughness required for
the rail cannot be ensured in some cases. Meanwhile, when the final reduction
(reduction of area) is greater than 20%, refining of austenite grains after hot rolling
becomes significant, the hardenability is greatly degraded, and the hardness of the rail
head portion is unlikely to be ensured. Therefore, it is preferable that the final
reduction (reduction of area) is set to be in a range of 2% to 20%.
[0102]
The conditions for hot rolling of the rail head portion is not particularly
limited. It is sufficient to control the final hot rolling temperature through caliber or
- 50 ..
universal rolling of a typical rail in order to ensure the hardness of the rail head portion.
As a hot rolling method, for example, a method described in Japanese Unexamined
Patent Application, First Publication No. 2002-226915 may be used such that the
pearlite structure is mainly obtained. That is, rough hot rolling is performed on a slab
or bloom, intermediate hot rolling is performed over a plurality of passes using a
reverse mill, and then finish rolling is performed two passes or more using a
continuous mill. The temperatures may be controlled to be in the above-described
temperature range at the time of the final hot rolling of the finish hot rolling.
[0103]
Next, the reason why it is preferable that the cooling rate of accelerated
cooling (outer surface of the head portion) is set to be in a range of2°C/sec to 8°C/sec.
When the cooling rate is less than 2°C/sec, the pearlitic transformation is
started in a high temperature region on the way of the accelerated cooling. As the
result, in the chemical composition of the rail according to the present embodiment, a
portion having a hardness ofless than Hv 350 is generated in the head surface portion
of the rail head portion, and the wear resistance or the internal fatigue damage
resistance required for the rail is unlikely to be ensured in some cases. Meanwhile,
when the cooling rate is greater than 8°C/sec, in the chemical composition of the rail
according to the present embodiment, the bainite structure or the martensite structure is
generated in the head surface portion and thus the wear resistance or the touglmess of
the rail maybe degraded. Therefore, it is preferable that the cooling rate is set to· be
in a range of2°C/sec to 8°C/sec.
[0104] .
Next, the reason why it is preferable that the start temperature of accelerated
cooling is set to 750°C or higher lmd the stop temperature thereof is set to be in a range
of 5 80°C to 640°C will be described.
When the start temperature of accelerated cooling of the outer surface of the
head portion is lower than 750°C, the pearlite structure is occasionally generated in a
high temperature region before accelerated cooling. In this case, a predetermined
hardness is not obtained and the wear resistance or the surface damage resistance
required for the rail is unlikely to be ensured. Further, in steel having a relatively
large amount of carbon, there are concerns that the pro-eutectoid cementite is
generated, the pearlite structure is embrittled, and the toughness of the rail is degraded.
Therefore, it is preferable that the temperature of the outer surface of the rail head
portion at the time of starting accelerated cooling is set to 750°C or higher.
[0105]
In addition, when the stop temperature of accelerated cooling is higher than
640°C, the pearlitic transformation is started in a high temperature region immediately
after cooling and a large amount of the pearlite structure having a low hardness is
generated. As the result, the hardness of the head portion cannot be ensured and the
wear resistance or the surface damage resistance required for the rail is unlikely to be
ensured in some cases. Further, when the stop temperature of accelerated cooling is
set to lower than 5 80°C, there is a case that a large amount of the bainite structure
harmful to the wear resistance is generated immediately after cooling. In this case,
the wear resistance required for the rail is unlikely to be ensured. Therefore, it is
preferable that the stop temperature of accelerated cooling is set to be in a range of
580°C to 640°C.
[0106]
Next, the reason for limiting conditions preferable for controlled cooling will
be described. This process greatly affects the number ofV carbonitride and the
- 52 -
difference in hardness between the surface ofthe head portion and the inside of the
head portion.
First, the reason why it is preferable that the holding temperature after
accelerated cooling is set to be in a range of 580°C to 640°C will be described.
When the holding temperature is higher than 640°C, in the chemical
composition of the rail according to the present embodiment, the pearlitic
transformation is started in a high temperature region immediately after cooling and a
large amount of the pearlite structure having a low hardness is generated. As the
result, the hardness of the head portion cannot be ensured, and the wear resistance or
the surface damage resistance required for the rail is unlikely to be ensured. Further,
there is a concern that the V carbonitride generated in the inside of the head portion
become coarse and the amount of precipitation strengthening is decreased so that the
hardness cannot be improved. Meanwhile, the holding temperature is set to lower
than 580°C, a large amount of the bainite structure harmful to the wear resistance is
generated immediately after cooling. As the result, there is a concern that the wear
resistance required for the rail is unlikely to be ensured. Further, generation of V
carbonitride is suppressed so that the number of fine V carbonitride cannot be ensured
m some cases. In this case, the hardness of the inside of the head portion is not
improved and the internal fatigue dan1age resistance is unlikely to be improved.
Therefore, it is preferable that the holding temperature after accelerated cooling is set
to be in a range of 580°C to 640°C. '·
[0107]
Next, the reason why the time of holding the temperature is preferably set to
be a range of 100 to 200 sec will be described.
When the holding time is longer than 200 sec, tempering of the pearlite
- 53 -
structure progresses during the holding and the pearlite structure is softened. As the
result, the hardness of the inside of the head portion cannot be ensured and the wear
resistance or the internal fatigue damage resistance required for the rail is unlikely to
be ensured. Further, generation of V carbonitride becomes insufficient and
improvement of the hardness of the inside ofthe head portion cannot be expected.
Meanwhile, when the holding time is set to shorter than 100 sec, generation ofV
carbonitride is not sufficient and the number of fine V carbonitride cannot be ensured.
As the result, the hardness of the inside of the head portion cannot be improved and
thus the internal fatigue damage resistance is unlikely to be improved. Therefore, it is
preferable that the time of holding the temperature after accelerated cooling is set to be
in a range of 100 to 200 sec.
[0108]
The method of holding the temperature during controlled cooling is not
particularly limited. It is preferable to perform cooling that controls recuperation
generated from the inside of the rail head portion by repeatedly performing the cooling
and stopping ofthe outer surface of the rail head portion using air injection cooling,
mist cooling, mixed injection cooling of water and air, or a refrigerant obtained by
combining these. Specifically, it is preferable that accelerated cooling is stopped on a
low temperature side in a temperature region where the temperature is held, cooling is
started after looking ahead the recuperation generated from the inside of the rail head
portion, and cooling is stopped before the temperature reaches tlie lower limit of a
predetermined temperature range. Further, it is preferable thatthis temperature
control is repeatedly performed to control the holding time. In a case where the
amount of recuperation is small, it is also effective to perform heating using an IH coil
or the like.
- 54 -
[0109]
In a case of controlling the ratio (CA/NA) ofthe number of carbon atoms
(CA) to the number of nitrogen atoms (NA) ofV carbonitride for the purpose of
controlling the number of V carbonitride and controlling the difference in hardness
between the surface of the head portion and the inside of the head portion, the stop
temperature of cooling and the holding temperature thereafter may be set to be in a
range of 61 ooc to 640°C during the above-described controlled cooling.
[0110]
When the accelerated cooling is performed to lower than 61 0°C, the amount
of carbides in V carbonitride is increased, the ratio (CA/NA) of the number of carbon
atoms (CA) to the number of nitrogen atoms (NA) ofV carbonitride is unlikely to be
controlled, and fine cracks occurring in the periphery of the V carbonitride are unlikely
to be prevented. Therefore, in a case of controlling the ratio (CA/NA), it is preferable
that the temperature during temperature holding after accelerated cooling is set to be in
a range of 610°C to 640°C.
[Olll]
The refrigerant for the heat treatment of the rail head portion is not
particularly limited. In order to control the hardness so as to impart the wear
resistance and the internal fatigue damage resistance, it is preferable to control the
cooling rate of the rail head portion at the time of the heat treatment using air injection
cooling, mist cooling, mixed injection cooling of water ana air, or a combination of
these.
[Examples]
[0112]
Next, examples of the present invention will be described.
- 55 -
Tables I to 6 show the chemical compositions and characteristics of rails of
the present invention. Tables 1 to 6 show values of chemical compositions, values of
Mn/Cr to be calculated from the values of the chemical compositions (mass%), and
values of0.25 x Mn +Cr. In the microstructure of the head portion in Tables 3 and 4,
the "pearlite" indicates that the area ratio of the pearlite structure is 95% or greater and
a small amount of a pro-eutectoid ferrite, a pro-eutectoid cementite, a bainite structure,
or a martensite structure may be mixed at an area ratio of 5% or less.
[0113]
Tables 7 to 9 show the chemical compositions and characteristics of rails for
comparison. Tables 7 to 9 show values of chemical compositions, values of Mn!Cr to
be calculated from the values of the chemical compositions (mass%), and values of
0.25 x Mn +Cr. In the microstructure of the head portion in Table 8, the "pearlite"
indicates that the area ratio of the pearl it structure e is 95% or greater and a small
amount of a pro-cutectoid ferrite, a pro-cutectoid cementite, a bainite structure, or a
martensite structure may be mixed by an area ratio of 5% or less. Meanwhile, when a
structure other than the pearlite structure is described, this means that the structure is
included at an area ratio of greater than 5%.
[0114]
The outline of the production process and the production conditions of
examples and comparative examples of the present invention, listed in Tables 1 to 6
and Tables 7 to 9, is as follows.
[0115]
· Outline of entire process
· Entire process is performed in the following order:
(1) melting steel;
- 56 -
(2) chemical composition adjustment;
(3) casting (bloom or slab);
(4) re-heating (1250°C to 1300°C);
( 5) hot rolling; and
(6) heat treatment (accelerated cooling or controlled cooling).
[0116]
Further, the outline ofthe production conditions of examples and comparative
examples of the present invention is as follows.
[0117]
· Conditions for hot rolling
Final hot rolling temperature (outer surface of head portion): 900°C to 1000°C
Final reduction (reduction of area): 2% to 20%
· Conditions for heat treatment (outer surface of head portion): performing
natural air cooling after hot rolling and then performed accelerated cooling and
controlled cooling.
Accelerated cooling (outer surface of head portion):
Cooling rate: 2°C/sec to 8°C/sec
Start temperature of accelerated cooling: 750°C or higher
Stop temperature of accelerated cooling: 580°C to 640°C
Controlled cooling (outer surface of head portion):
Holding temperature in temperature "iange of 580°C to 640°C for 100 to 200
sec after stopping accelerated cooling and then performing air cooling
[0118]
In this case, in regard to A20, A22, A24, A26, and the like of Tables 1 to 6, the
conditions for accelerated cooling and controlled cooling after hot rolling were set as
- 57 -
follows in order to control the ratio (CA/NA) of the number of carbon atoms (CA) to
the number of nitrogen atoms (NA) of V carbonitride and prevent fine cracks occurring
in the periphery of carbonitrides.
[0119]
- Conditions for heat treatment (outer surface of head portion): performing
natural air cooling after hot rolling and then performed accelerated cooling and
controlled cooling.
Accelerated cooling (outer surface of head portion):
Cooling rate: 2°C/sec to 8°C/sec
Start temperature of accelerated cooling: 750°C or higher
Stop temperatrue of accelerated cooling: 61 0°C to 640°C
Controlled cooling (outer surface of head portion):
Holding temperature in temperature range of6!0°C to 640°C for 100 to 200
sec after stopping accelerated cooling and then performing air cooling
[0120]
In the above-described manner, steel Nos. AI to A44 (rails of examples of the
present invention) and steel Nos. Bl to 823, 862, and 872 (rails of comparative
examples) listed in Tables I to 9 were produced.
Rails AI to A44 of the present invention are rails in which the values of
chemical compositions, the values ofMn/Cr and the values of0.25 x Mn + Cr formed
of the values of chemical compositions (mass%), the microstructure of the head
portion, and the hardness of the head portion are in the ranges of the present
application of the invention. Meanwhile, rails 8 I to 815, 862, and 872 (17 lines) of
comparative examples are rails in which the contents of C, Si, Mn, Cr, P, S, V, and N
and the number of V carbonitride having an average grain size of 5 to 20 nm in the
- 58 -
inside of the head portion are out of the ranges of the present application of the
invention. Further, rails B 16 to B23 of comparative examples are rails in which the
values ofMn/Cr or the values of0.25 x Mn + Cr are out of the ranges of the present
application of the invention.
[0121]
Further, rails (Cl to C24) listed in Tables 10 and, 11 were produced by
changing various conditions for hot rolling and conditions for the heat treatment
(conditions for accelerated cooling and conditions for controlled cooling) using a
bloom or slab having the same chemical compositions as those of the rail of the present
invention.
[0122]
According to the following method, observation of the microstructure of the
head portion, measurement of the number ofV carbonitride having a grain size of5 to
20 nm, the hardness of the head portion, the difference in hardness between the surface
of the head portion and the inside of the head portion, and CAIN A, a wear test, and a
rolling contact fatigue test were performed on these ralls A1 to A44, B 1 to B23, and Cl
to C24. The results are listed in Tables l to 11.
[0123]
[Observation of microstructure of head portion]
The metallographic structure in the visual field of an optical microscope of
200 magnifications was observea on I 0 or more positions at a depth of 2 rinn from the
outer surface of the head portion as the origin and 10 or more positions at a depth of 25
mm from the outer surface of the head portion as the origin, the area ratio of each
metallographic structure was determined, and then the average value of the area ratio
was used as the area ratio of the observed portion.
- 59 -
- ~ ' i
[0124]
[Number ofV carbonitride having grain size of 5 to 20 nm]
Samples were machined from a position at a depth of 25 mm from the outer
surface of the head portion of the transverse cross section as the origin, thin film
processing or replica collection was performed, and then observation was performed
using a transmission electron microscope at a magnification of 50000 to 500000 times.
Further, each of the observed precipitate was analyzed, only the V carbonitride
(precipitates at least containing V and carbon, V and nitrogen, or V and carbon and
nitrogen) were selected, the.area thereof was acquired, and the average grain size was
calculated using a diameter of a circle corresponding to the area. Further, the average
value was acquired by performing observation of 20 visual fields, counting the number
ofV carbonitride having a predetermined diameter, and converting the number ofV
carbonitride to the number per unit area.
[0125]
[Measurement ofCA/NA]
A needle sample was processed (I 0 fllil x 10 fllil x 100 pm) according to
focused ion beam (FIB) method from a position at a depth of 25 mm from the outer
surface of the head portion as the origin, and the number of carbon atoms and the
number of nitrogen atoms contained in V carbo nitride were counted according to a
three-dimensional atom probe (3DAP) method. From the results, the ratio (CA/NA)
of the number of carbon atoms (CA) to the number of nitrogen atoms (NA) was
calculated. CA/NA were measured at 5 or more points and the average value was
used as the representative value. At this time, the voltage was set to DC and pulse
(pulse rate of 20% or greater) and the sample temperature was set to 40 K or lower.
[0126]
- 60 -
[Measurement of hardness of head portion and difference in hardness between
surface of head portion and inside of head portion]
A sample was machined from the transverse cross section of the rail head
portion, the transverse cross section was polished with diamond abrasive grains having
an average grain size of 1 !LID, and measurement was performed on arbitrary 20 sites at
a depth of 2 mm from the outer surface of the head portion and arbitrary 20 sites at a
depth of25 mm from the outer surface of the head portion at a load of98 N using a
Vickers hardness tester in conformity with JIS Z 2244. Further, the average value of
the hardness of 20 sites in each depth position was set to the hardness of the position.
[0127]
[Wear test]
Tester: Nishihara type wear testing machine (see FIG 8)
Shape of specimen (rail material4): disc-shaped test piece (outer diameter: 30
mm, thickness: 8 mm)
Position for machining test specimen: position at depth of 2 mm under outer
surface of head portion (surface of head portion (see FIG. 7))
Test load: 686 N (contact pressure of 640 MPa)
Slip ratio: 20%
Opposite specimen (wheel material 5): pearlitic steel (Hv 380)
Atmosphere: in air
Cooling: forced cooling using compressed air injection from air nozzle 6 for
cooling (flow rate: l 00 Nl/min)
Number of repetition: 700000
Acceptance or rejection criteria: As the result of performing the wear test and
evaluation of wear resistance at actual tracks, it was confirmed that the wear resistance
- 61 -
at actual tracks was poor when the wear amount was greater than 1.30 g.
Consequently, rails having a wear amount of greater than 1.30 g were determined to
have poor wear resistance.
[0128]
[Rolling contact fatigue test]
Tester: rolling contact fatigue tester (see FIG. 9)
Shape oftest piece
RailS: 14llbs rail x 2m
Wheel9: AAR type (diameter of920 mm)
Load
Radial: 50 to 300 kN
Thrust: 20 kN
Lubrication: oil (intermittently supplied)
Number of repetition: 2000000 in maximum
Acceptance or rejection criteria: The number of repetition at the time of
occurrence of cracks on the outer surface of the rail head portion and cracks in the
inside of the head portion was determined as t11e lifetime of the rail. In a case where
occurrence of cracks was not found respectively on the outer surface of the rail head
portion and in the inside ofthe head portion after 2000000 repetitions, it was
determined that the damage resistance was excellent. The presence or absence of
cracks in the inside of the head portion was confirmed by detecting flaws using
ultrasonic testing (UST) during the test. In this case, since fine cracks having a crack
length of up to 2 mm do not greatly deteriorate the damage resistance, the presence of
fine cracks was accepted.

[Document Type]
What is claimed is:
Claims
I. A rail comprising, in terms of mass%:
C: 0.75% to 0.85%;
Si: 0.10% to 1.00%;
Mn: 0.30% to 1.20%;
Cr: 0.20% to 0.80%;
V: O.Ol%to0.20%;
N: 0.0040% to 0.0200%;a structure of a range between an outer surface of a head portion as an origin
and a depth of25 mm includes 95% or greater of a pearlite structure and a hardness of
the structure is Hv 350 to 480,
50 to 500 V carbonitride having an average grain size of 5 to 20 nm are
present per 1.0 f.Lm2 of an area to be inspected in a transverse cross section at a position
having the depth of25 mm from the outer surface of the head portion, and
the value obtained by subtracting a hardness of the position having the depth
of 25 mm from the outer surface of the head portion from a hardness of a position
having a depth of2 mm from the outer surface of the head portion is Hv 0 to Hv 40,
1.00 < Mn/Cr :S 4.00 · · · Expression 1,
0.30 :S 0.25 x Mn + Cr :S 1.00 ···Expression 2,
here, the symbols of elements described in the Expressions l and 2 indicate
the content of each element in terms of mass%.
2. The rail according to claim 1,
wherein when a number of carbon atoms is defined as CA and a number of
nitrogen atoms is defined as NA in the V carbonitride, a ratio CA/NA which is a ratio
of CA to NA is 0. 70 or less.
3. The rail according to claim 1 or 2, comprising, in terms of mass%, at least
·one selected from the group consisting of:
Mo: 0.01% to 0.50%;
Co: O.Ol%to 1.00%;
B: 0.0001% to 0.0050%;
Cu: 0.01% to !:00%;Ni: O.Ol%to 1.00%;
Nb: 0.0010% to 0.0500%;
Ti: 0.0030% to 0.0500%;
Mg: 0.0005% to 0.0200%;
Ca: 0.0005% to 0.0200%;
REM: 0.0005% to 0.0500%;
Zr: 0.0001% to 0.0200%; and
AI: 0.0100% to 1.00%.