[Technical Field of the Invention]
[0001]
The present invention relates to a rail having excellent breakage resistance
and fatigue resistance in high-strength rails used in cargo railways. Priority is
claimed on Japanese Patent Application No. 2015-011007, 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
were not developed yet has been promoted. Along with this, the railroad environment
of cargo railways used to transport resources has become significantly severe.
Therefore, rails have been required to have more wear resistance than ever. From this
background, there has been a demand for development of rails with improved wear
resistance.
Further, in recent years, railway transport has been further overcrowded and,
therefore, a possibility that breakage or fatigue damage is generated from rail bottom
portions has been pointed out. Consequently, for further improvement of rail service
life, there has been a demand f~r improvement of the breakage resistance aiid fatigue '·'
resistance of rails in addition to wear resistance.
[0003]
In order to improve the wear resistance of rail steel, for example, highstrength
rails described in Patent Documents 1 to 5 have been developed. Main
- 1 -
characteristics of these rails are the hardness of steel being increased by refining
pearlite lamellar spacing using a heat treatment in order to improve the wear resistance
and an increased volume rate of cementite in pearlite lamellar by increasing the
amount of carbon of steel.
[0004]
Patent Document 1 discloses that a rail with excellent wear resistance is
obtained by performing accelerated cooling on a rail head portion which is rolled or reheated
at a cooling rate of 1 °C/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).
[0005]
In disclosed technologies of Patent Documents 1 and 2, the wear resistance of
a rail head portion is improved so that a certain length of service life is increased by
refining lamellar spacing in pearlite structure in order to improve the hardness and
increasing the volume ratio of cementite in lamellar of pearlite structure. However, in
the rails disclosed in Patent Documents I and 2, the breakage resistance and the fatigue
resistance of a rail bottom portion are not examined.
[0006]
Further, for example, Patent Documents 3 to 5 disclose a method of
performing a heat treatment on a rail bottom portion for the purpose of controlling the
material of the rail bottom portion and preventing breakage originated from the rail
bottom portion. According to the technologies disclosed in these documents, it is
suggested that the service time of rails can be drastically improved.
- 2 -
[0007]
Specifically, Patent Document 3 discloses a heat treatment method of
performing accelerated cooling on the rail bottom surface at a cooling rate of I °C/sec
to 5°C/sec from a temperature range of 800°C to 450°C while performing accelerated
cooling on the rail head portion from the temperature ofthe austenite region after rail
rolling_ Further, according to the heat treatment method, it is disclosed that a rail
having improved characteristics of drop weight resistance and breakage resistance can
be provided by adjusting pearlite structure average hardness of the rail bottom portion
to HB 320 or greater.
[0008]
Patent Document 4 discloses that a rail having improved drop weight
characteristics and excellent breakage resistance can be provided by re-heating the rail
bottom portion which is rolled and subjected to a heat treatment in a temperature range
of 600°C to 750°C, spheroidizing pearlite structure, and then per.forming rapid cooling
on the rail bottom portion.
[0009]
Patent Document 5 discloses a method of setting the hardness of a foot -edge
portion to Hv 320 or greater by re-heating the foot-edge portion of a rail in a
temperature range of an Ar3 transformation point or an Arcm transformation point to
950°C, performing accelerated cooling on the foot-edge portion at a cooling rate of
0.5°C to 20°C, stopping the accelerated cooling at 400°C or higher, perfoilliing air
cooling or accelerated cooling on the foot-edge portion to room temperature, further
re-heating the foot-edge portion to a temperature range of 500°C to 650°C, and
performing air cooling or accelerated cooling on the foot-edge portion to room
temperature. It is disclosed that a rail having excellent breakage resistance can be
- 3 -
provided when this method is used because generation of fatigue damage to the footedge
portion, generation of breakage due to fatigue damage, and generation of
breakage due to brittle fractures caused by an excessively impact load, among the
breakage in the rail bottom portion, can be suppressed.
[0010]
According to the disclosed technology of Patent Document 3, since the
hardness of pearlite structure is improved by performing accelerated cooling on the rail
bottom portion, the characteristics of drop weight resistance or fatigue resistance for
which strength is mainly required are improved. However, the toughness is degraded
due to high hardness, the breakage resistance is unlikely to be improved. Further,
since a pro-eutectoid cementite harmful to the toughness is likely to be generated at the
above-described cooling rate of the accelerated cooling in a case of rail steel having a
high carbon content, the breakage resistance is unlikely to be improved from this
viewpoint.
[0011]
Further, according to the disclosed technology of Patent Document 4, since
the entire rail bottom portion is re-heated and then the rail bottom portion is rapidly
cooled, the toughness can be improved by tempering pearlite structure. However,
since the structure is softened by the tempering, the fatigue resistance is unlikely to be
improved.
[0012]
Further, according to the disclosed technology of Patent Document 5, since
the foot -edge portion of the rail is re-heated and then controlled cooling is performed,
the hardness of pearlite structure is increased and pearlite structure can be refined.
Moreover, a certain degree of toughness is obtained by tempering which is performed
- 4 -
after the cooling. However, since the hardness of the structure is increased, the
toughness is unlikely to be sufficiently improved and thus excellent breakage
resistance is difficult to obtain.
[Prior Art Document]
[Patent Document]
[0013]
[Patent Document 1] Japanese Examined Patent Application, Second
Publication No. S63-023244
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. HOS-144016
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. HO l-139724
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. H04-202626
[Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. 2008-266675
[Disclosure of the Invention]
[Problems to be solved by the Invention]
[0014]
The present invention has been made in consideration of the above-described
problems. An object of the present invention is to provide a rail having excellent
breakage resistance and fatigue resistance which are required tor rails of cargo
railways and in which generation of breakage from a bottom portion can be suppressed.
[Means for Solving the Problem]
[0015]
- 5 -
The scope of the present invention is as follows.
(1) According to an aspect of the present invention, a rail includes, as steel
composition, in terms of mass%: C: 0. 75% to 1.20%; Si: 0.10% to 2.00%; Mn: 0.10%
to 2.00%; Cr: 0% to 2.00%; 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%; V: 0% to 0.50%; Nb: 0% to 0.050%; 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%; N: 0% to 0.0200%; AI: 0% to 1.00%; P: 0.0250% or less; S: 0.0250% or
less; and Fe and impurities as a remainder.
90% or more of a metallographic structure in a range between an outer surface
of a rail bottom portion as an origin and a depth of 5 mm is a pearlite structure, and an
HC which is a surface hardness of a foot-bottom central·portion is in a range ofHv 360
to 500. An HE which is a surface hardness of a foot-edge portion is in a range of Hv
260 to 315, and the HC, the HE, and an HM which is a surface hardness of a middle
portion positioned between the foot-bottom central portion and the foot-edge portion
satisfy the following Expression l.
HC :0: HM :0: HE · · · (Expression 1 ).
(2) In the rail according to (1 ), the HM and the HC may satisfy the following
Expression 2.
HM/HC :0: 0.900 · · · (Expression 2)
(3) In the rail according to (I) or (2), the steel composition may include, in
terms of mass%, at least one selected from the group consisting ofCr: 0.01% to 2.00%,
Mo: 0.01% to 0.50%, Co: 0.01% to 1.00%, B: 0.0001% to 0.0050%, Cu: 0.01% to
1.00%, Ni: 0.01% to 1.00%, V: 0.005% to 0.50%, Nb: 0.0010% to 0.050%, 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%, N: 0.0060% to 0:0200%, and AI:
- 6 -
0.0100% to 1.00%.
[Effects of the Invention]
[0016]
According to the aspect ofthe present invention, it is possible to provide a rail
having excellent breakage resistance and the fatigue resistance, which are required for
the rail bottom portion of cargo railways, by controlling the compositions of rail steel
serving as the material of the rail, controlling the metallographic structure of the rail
bottom portion and the surface hardness of the foot-bottom central portion and the
foot -edge portion of the rail bottom portion, and controlling strain concentration on the
vicinity of the middle portion, by controlling the balance of the surface hardness of the
foot-bottom central portion, the foot-edge portion, and the middle portion.
[Brief Description of the Drawings]
[0017]
FIG. 1 is a graph showing measurement results of surface stress applied to a
rail bottom portion.
FIG. 2 is a graph showing the relationship between the surface hardness and
the fatigue limit stress range of a foot-bottom central portion of a rail.
FIG. 3 is a graph showing the relationship between the surface hardness and
the fatigue limit stress range of a foot-edge portion of a rail.
FIG. 4 is a graph showing the relationship between the surface hardness and
impact values of the foot-edge portion of a rail.
FIG. 5 is a graph showing the relationship between the surface hardness of a
middle portion and the fatigue limit stress range of a rail bottom portion of a rail.
FIG. 6 is a graph showing the relationship between the surface hardness of the
foot-bottom central portion and the middle portion and the fatigue limit stress range of
- 7 -
a rail bottom portion of a rail.
FIG. 7 is a graph showing names of each position of a rail bottom portion
according to the present embodiment and a region for which pearlite structure is
required.
FIG. 8 is a side view showing the outline of a fatigue test of a rail.
FIG. 9 is a perspective view showing a position of machining impact test
samples in a rail.
FIG. 10 is a view showing the relationship between the ratio of the surface
hardness HM (Hv) of the middle portion to the surface hardness HC (Hv) of the footbottom
central portion and the fatigue limit stress of a rail.
[Embodiments of the Invention]
[0018]
Hereinafter, a rail having excellent breakage resistance and fatigue resistance
according to an embodiment of the present invention (hereinafter, also referred to as a
rail according to the present embodiment) will be described in detail. Hereinafter,
"%" in the composition indicates mass%.
[0019]
First, the present inventors examined the details of the cause ofbrealcage
being generated from the rail bottom portion in the current cargo railways. As a
result, it was found that rail breakage is mainly divided into two types of breakage
forms based on the causes thereof. That is, the breakage is divided into two types of
breakage forms which are brittle fracture in which the foot-edge portion of the rail
bottom portion is the origin and fatigue fracture in which the foot-bottom central
portion of the rail bottom portion is the origin.
[0020]
- 8 -
Further, the occurrence of brittle fracture from the foot -edge portion as the
origin is frequently found in the outside rail of a curved line section and the occurrence
of the fatigue fracture from the foot -bottom central portion as the origin is frequently
found in the rail of a straight line section.
In addition, in the brittle fracture occurring in the foot-edge portion of the
outside rail of the curved line section, occurrence of fatigue cracks is not found.
Therefore, it is assumed that the brittle fracture occurring in the foot-edge portion of
the outside rail ofthe curved line section is breakage formed by impact stress being
applied instantaneously.
[0021]
FIG 7 is a schematic view showing the rail bottom portion according to the
present embodiment The rail bottom portion (rail bottom portion 4) according to the
present embodiment will be described with reference to FIG 7.
[0022]
The rail bottom portion 4 includes a foot-bottom central portion l, a foot-edge
portion 2 positioned on both ends of the foot-bottom central portion 1, and a middle
portion 3 positioned between the foot -bottom central portion 1 and the foot -edge
portion 2.
As shown in FIG. 7, the foot-edge portion 2 is a portion positioned in the
vicinity of the both ends of the rail bottom portion 4 in the width direction and
positioned close to an outer surface :i of the rail bottom portion. Further, as shown in
FIG. 7, the foot-bottom central portion 1 is a portion positioned in the vicinity of the
center of the rail bottom portion 4 in the width direction and positioned close to the
outer surface 5 of the rail bottom portion. Further, as shown in FIG. 7, the middle
portion 3 is a portion positioned between the foot-edge portion 2 and the foot-bottom
- 9 -
central portion 1 and positioned close to the outer surface 5 of the rail bottom portion.
More specifically, when the width dimension of the rail bottom portion 4 in FIG. 7 is
defined as W, the foot-bottom central portion 1 is in a region ofO.l W interposed
between the position of +o.os Wand the width center of the rail bottom portion 4.
Further, the foot-edge portion 2 positioned on both ends of the foot-bottom central
. portion I is in a region of 0.1 W from the end portion of the rail bottom portion 4 in the
width direction. Further, the middle portion 3 positioned between the foot-bottom
central portion 1 and the foot-edge portion 2 is in a region of 0.2 to 0.3 W from the end
portion of the rail bottom portion 4 in the width direction.
[0023]
In a case where the rail is seen from the vertical cross section in the length
direction, a portion in which the width of the rail is constricted is present in the center
of the rail in the height direction. A portion which has a width wider than the width
of the constricted portion and is positioned on a side lower than the constricted portion
is referred to as the rail bottom portion 4 and a portion which is positioned on a side
upper than the constricted portion is referred to as a rail column portion or a head
portion (not illustrated). Further, the outer surface 5 of the rail bottom portion
indicates at least the surface, among the surfaces of the rail bottom portion, facing the
lower side when the rail is upright. The outer surface 5 of the rail bottom portion may
include the side end surfaces of the rail bottom portion.
(0024]
In general, it is said that low hardness (soft) is effective for brittle fracture
generated by impact stress being applied and high hardness (full hard) is effective for
fatigue fracture. That is, contrary methods are necessary to improve these
characteristics. Therefore, it is not easy to improve these characteristics
- 10 -
simultaneously. The present inventors found that the hardness of the surface in each
position of the bottom portion needs to be suitably controlled according to the main
causes of generation of fracture, in order to suppress damage occurring in the rail
bottom portion.
[0025]
The present inventors examined the cause of occurrence of fatigue fracture
originated from the foot-bottom central portion. Specifically, the stress applied to the
surface of the bottom portion in the foot-bottom central portion from the foot -edge
portion is measured by performing an actual rail bending fatigue test assuming heavy
load railways using a rail which includes a steel composition with a LOO%C, 0.50%Si,
0.90%Mn, P: 0.0250% or less, and S: 0.0250% or less (the remainder of the steel
composition is Fe and impurities) and in which the hardness of the entire outer surface
of the rail bottom portion from one foot-edge portion to the other foot-edge portion is
set to be almost constant. The test conditions are as described below.
[0026]
• Actual rail bending fatigue test
Used rail
Shape: 141 lbs rail (weight: 70 kg/m, width of bottom portion: 152 mm)
Metallographic structure of bottom portion: pearlite
Surface hardness of bottom portion: Hv 380 to 420 (average value at depth of
I mm under surfaces between foot-edge portion and middle portion and between
middle portion and foot-bottom central portion)
[0027]
Conditions of fatigue test
Test method: 3 point bending of actual rail (span length: 0.65 m) (see FIG 8)
- 11 -
Load condition: in range of 7 to 70 tons (frequency of applied load: 5 Hz)
Test attitude: load is applied to rail head portion (tensile stress is applied to rail bottom
portion)
[0028]
Stress measurement
Measurement method: measurement using strain gauge adhering to rail
bottom portion
[0029]
FIG l shows the relationship between the distance from the center on the
surface of the rail bottom portion in the width direction and the measurement results of
stress applied to the rail bottom portion. The vertical axis in FIG. 1 shows the stress
range obtained by organizing the results of measuring the surface stress three times.
As understood from FIG 1, it was found that the stress range is greatly different for
each position site in the rail bottom portion, the maximum stress is 200 MPa, which is
the highest value and measured in the foot-bottom central portion, the stress
monotonically decreases toward the foot-edge portion from the foot-bottom central
portion, and the stress of the foot-edge portion in which restraint is less and
deformation is easily made decreases to 150 MPa. Therefore, it is suggested that the
surface hardness required for improving the fatigue resistance is different for each
position because the load stress is different for each position in the rail bottom portion.
[0030]
In order to clarifY the surface hardness required for ensuring the fatigue
resistance of each position of the rail, a plurality of rails A in which the hardness of the
foot-bottom central portion is changed and a plurality of rails Bin which the hardness
of the foot-edge p01tion is changed are produced, by the present inventors, by
- 12 -
performing hot rolling and a heat treatment on rail steel (steel serving as the material of
the rail) which contains l.OO%C, 0.50%Si, 0.90%Mn, P: 0.0250% or less, and S:
0.0250% or less and the remainder of Fe and impurities. Further, a fatigue test is
performed by reproducing the conditions of using actual tracks to the obtained rails A
and B to investigate the fatigue limit stress range. The test conditions are as follows.
[0031]

Used rail
Shape: 14llbs rail (weight: 70 kg/m, width ofbottom portion: 152 mm)
Metallographic structure of bottom portion: pearlite
[0032]
Hardness of rail
Rail A having foot-bottom central portion of which hardness is controlled:
surface hardness HC (Hv) of foot-bottom central portion: Hv 320 to 540, and surface
hardness HE (Hv) offoot-edge portion: Hv 315 (constant)
Rail B having foot-edge portion of which hardness is controlled: surface
hardness HC (Hv) of foot-bottom central portion: Hv 400 (constant), and surface
hardness HE (Hv) offoot-edge portion: Hv 200 to 340
Here, the surface hardness of the foot -bottom central portion is an average
value obtained by measuring the surface hardness (hardness of the cross section at
depths of 1 mm and 5 mm under the surface) of 20 sites shown in FrG. 7. Further, the
surface hardness of the foot -edge portion is an average value obtained by measuring
the surface hardness (hardness of the cross section at depths of 1 mm and 5 mm under
the surface) of20 sites shown in FIG. 7. In addition, Hv represents the Vickers
hardness.
- 13 -
The surface hardness between the foot -edge portion and the foot -bottom
central portion which includes the hardness HM (Hv) of the middle portion between
the foot -edge portion and the foot -bottom central portion is in a state of distribution
which monotonically increases toward the foot -bottom central portion from the footedge
portion.
[0033]
Conditions of fatigue test
Test method: 3 point bending of actual rail (span length: 0.65 m) (see FIG. 8)
Load condition: stress range is controlled (maximum load -minimum load,
minimum load is 10% of maximum load), frequency of applied load: 5 Hz
Test attitude: load is applied to rail head portion (tensile stress is applied to
bottom portion)
Controlling stress: stress is controlled using strain gauge adhering to footbottom
central portion of rail bottom portion
Number of repetition: number of repetition is set to 2 million times and
maximum stress range in case of being unfractured is set to fatigue limit stress range
[0034]
FIG. 2 shows fatigue test results of the rails A and FIG. 3 shows fatigue test
results of the rails B.
[0035]
FIG. 2 is a graph organized based on the relationship"betwecn the surface
hardness HC (Hv) and the fatigue limit stress range of the foot-bottom central portions
of the rails A. As understood from the results ofF! G. 2, it is understood that the
surface hardness HC (Hv) of the foot-bottom central portion is required to be in a
range ofHv 360 to 500 in order to ensure the fatigue limit stress range of the load
- 14 -
stress (200 MPa) or greater which is assumed to be applied to an actual rail. When
HC (Hv) is less than Hv 360, the hardness of pearlite structure is insufficient and
fatigue cracks occur. When HC (Hv) is greater than Hv 500, cracks occur due to
embrittlement of pearlite structure.
[0036]
FIG. 3 is a graph organized based on the relationship between the surface
hardness HE (Hv) and the fatigue limit stress range of the foot -edge portions of the
rails B. As understood from the results of FIG. 3, the surface hardness HE (Hv) of the
foot -edge portion is required to be Hv 260 or greater in order to suppress occurrence of
.fatigue cracks from the foot-edge portion and to ensure\the fatigue resistance (fatigue
limit stress range of a load stress of200 MPa or greater) of the rail.
[0037]
From the test results described above, it is evident that the hardness HC (Hv)
of the foot-bottom central portion is controlled to be in a range ofHv 360 to 500 and
the surface hardness HE (Hv) of the foot-edge portion is controlled to be Hv 260 or
greater in order to improve the fatigue resistance of the rail bottom portion in actual
tracks.
[0038]
Moreover, the hardness suitable for suppressing brittle fracture occurring from
the foot-edge portion as the origin is examined by the present inventors. Specifically,
. a rail in which the hardness of the foot-edge portion is changed is produced by
performing hot rolling and a heat treatment on rail steel which has C: 0.75% to 1.20%,
0.50%Si, 0.90%Mn, P: 0.0250% or less, and S: 0.0250% or less and the remainder of
Fe and impurities. Further, impact test pieces are machined from the foot-edge
portion of the obtained rail to investigate impact characteristics according to an impact
- 15 -
test in order to evaluate the breakage resistance.
The test conditions are as follows.
[0039]
[Impact test]
Used rail
Shape: 141 lbs rail (weight: 70 kg/m, width of bottom portion: !52 mm)
Metallographic structure of bottom portion: pearlite
Hardness offoot-edge portion: Hv 240 to 360
Hardness of foot-bottom central portion: Hv 360 to 500
Position of measuring hardness: The surface hardness of the foot-edge portion
from the outer surface of the rail bottom portion to sites at depths of 1 mm and 5 mm
of the foot-edge portion shown in FIG 7 is obtained by measuring the surface hardness
of 20 sites and averaging the values.
[0040]
Conditions of impact test
Shape of specimen: JIS No. 3, 2 mm U-notch Charpy impact test piece
Position of machining test pieces: foot-edge portion of rail (see FIG. 9)
Test temperature: room temperature (+20°C)
Test conditions: carried out in conformity with JIS Z2242
[0041]
FIG. 4 shows results of an impact test perfonned on the foot-edge portion.
FIG. 4 is a graph organized based on the relationship between the surface hardness and
impact values of the foot-edge portion. As shown in FIG. 4, the impact values tend to
increase when the hardness of the foot -edge portion is decreased. It is confirmed that
excellent toughness (15.0 J/cm2 or greater at 20°C) is obtained when the hardness of
- 16 -
the foot-edge portion is Hv 315 or less.
[0042]
From the results described above, in order to improve the breakage resistance
and the fatigue resistance of the rail bottom portion by suppressing the brittle fracture
occurring from the foot-edge portion and suppressing the fatigue fracture occurring
from the foot -edge portion or the foot -bottom central portion, it was found that the
surface hardness of the foot-bottom central portion needs to be controlled to be in a
range ofHv 360 to 500 and the surface hardness of the foot-edge portion is controlled
to be in a range of Hv 260 to 315.
[0043]
Further, in the rail with the hardness having the above-described range, the
relationship between the surface hardness of the middle portion positioned between the
foot-bottom central portion and the foot-edge portion and the fatigue resistance of the
rail bottom portion is verified by the present inventors. Specifically, a plurality of
rails (rails C to E) in which the surface hardness HM (Hv) of the middle portion is
changed are produced by performing hot rolling and a heat treatment on rail steel
which has l.OO%C, 0.50%Si, 0.90%Mn, P: 0.0250% or less, and S: 0.0250% or less
and the remainder of Fe and impurities and by controlling the surface hardness HC
(Hv) ofthefoot-bottom central portion and the surface hardness HE (Hv) of the footedge
portion to be constant. Further, a fatigue test is performed reproducing the
conditions of using actual tracks to the obtained trial rails C to E to investigate the
fatigue limit stress range. The test conditions are as follows.
[0044]

Used rail
- 17 -
Shape: 141lbs rail (weight: 70 kg/m, width ofbottom portion: 152 mm)
Metallographic structure of bottom portion: pearlite
[0045]
Hardness of rail
Rails C (8 pieces) having middle portion of which hardness is controlled:
surface hardness HC (Hv) of foot-bottom central portion: Hv400 (constant), surface
hardness HE (Hv) offoot-edge portion: Hv 315 (constant), and surface hardness HM
(Hv) of middle portion positioned between foot-bottom central portion and foot-edge
portion: Hv 315 to 400 (HC 2: HM 2: HE)
Rails D (2 pieces) having middle portion of which hardness is controlled:
surface hardness HC (Hv) offoot-bottom central portion: Hv 400 (constant), surface
hardness HE (Hv) offoot-edge portion: Hv 315 (constant), and surface hardness HM
(Hv) of middle portion positioned between foot-bottom central portion and foot-edge
portion: Hv 310 or Hv 290 (HM < HE)
Rails E (2 pieces) having middle portion of which hardness is controlled:
surface hardness HC (Hv) of foot-bottom central portion: Hv 400 (constant), surface
hardness HE (Hv) offoot-edge portion: Hv 315 (constant), and surface hardness HM
(Hv) of middle portion positioned between foot-bottom central portion and foot-edge
portion: Hv 405 or Hv 420 (HM > HC)
[0046]
The surface hardness ofthetoot-bottom central portion is an average value
obtained by measuring the surface hardness (hardness of the cross section at depths of
l rum and 5 mm under the surface) of 20 sites shown in Fl G. 7; the surface hardness of
the foot-edge portion is an average value obtained by measuring the surface hardness
(hardness of the cross section at depths of I mm and 5 mm under the surface) of 20
- 18 -
sites shown in FIG 7; and the surface hardness of the middle portion is an average
value obtained by measuring the surface hardness (hardness of the cross section at
depths of 1 mm and 5 mm under the surface) of20 sites shown in FIG 7.
The surface hardness between the foot -edge portion and the middle portion and the
surface hardness between the middle portion and the foot-bottom central portion are
. respectively in a state of distribution which monotonically increases or decreases.
[0047]
Fatigue test
Test method: 3 point bending of actual rail (span length: 0.65 m) (see FIG 8)
Load condition: stress range is controlled (maximum load- minimum load,
minimum load is l 0% of maximum load), frequency of applied load: 5 Hz
Test attitude: load is applied to rail head portion (tensile stress is applied to
bottom portion)
Controlling stress: stress is controlled using strain gauge adhering to footbottom
central portion of rail bottom portion
Number of repetition: number of repetition is set to 2 million times (maximum
stress range in case of being unfractured is set to fatigue limit stress range)
[0048]
FIG 5 shows the results of the fatigue test performed on the rails C (8 pieces),
the rails D (2 pieces), and rails E (2 pieces). FIG 5 is a graph organized based on the
relationship between the surface hardness HM (Hv) of the middle portion and the
fatigue limit stress range in the foot -bottom central portion of the bottom portion. In
consideration of variation in results, the test is respectively perfonned on 4 pieces for
each rail. As a result, in the rails D that satisfy HM < HE, the strain is concentrated
on the middle portion (soft portion) having a surface hardness lower than that of the
- 19 .
foot-edge portion and the fatigue fracture occurs from the middle portion. Further, in
the rails E that satisf'y HM > HC, the strain is concentrated on the boundary portion
between the foot-bottom central portion and the middle portion having a surface
hardness higher than that of the foot -bottom central portion and the fatigue fracture
occurs from the boundary portion. Further, in the rails C, the strain concentration on
the middle portion or on the boundary portion between the foot -bottom central portion
and the middle portion is suppressed so that the fatigue resistance (load stress of 200
MPa or greater) ofthe rail bottom portion is ensured.
[0049]
From the results described above, it was found that the strain concentration on
the rail bottom portion needs to be suppressed by controlling the surface hardness HC
(Hv) of the foot-bottom central portion, the surface hardness HE (Hv) of the foot-edge
portion, and the surface hardness HM (Hv) of the middle portion to satisfy the
following Expression 1 in order to improve the fatigue resistance of the rail bottom
portion.
HC 2 HM 2 HE Expression I
[0050]
The present inventors conducted research by focusing on the balance between
the hardness of the foot -bottom central portion and the middle portion in order to
further improve the fatigue resistance of the rail bottom portion. Specifically, rails F
to H in which the surface hardness HC (Hv) of the foot-bottom central portion and the
surface hardness HM (Hv) of the middle portion are changed are produced by
performing hot rolling and a heat treatment on rail steel which contains l.OO%C,
0.50%S, 0.90%Mn, P: 0.0250% or less, and S: 0.0250% or less and the remainder of
Fe and impurities and by controlling the surface hardness HE (Hv) of the foot-edge
- 20 -
portion to be constant. Further, a fatigue test is performed reproducing the conditions
of using actual tracks to the obtained trial rails F to H to investigate the fatigue limit
stress range. The test conditions are as follows.
[0051]

Used rail
Shape: 141lbs rail (weight: 70 kg/m, width of bottom portion: 152 mm)
Metallographic structure of bottom portion: pearlite
[0052]
Hardness of rail
Rails F (6 pieces) having foot-bottom central portion and middle portion, each
of which hardness is controlled: surface hardness HE (Hv) offoot-edge portion: Hv
315 (constant), surface hardness HC (Hv) of foot-bottom central portion: Hv 360, and
surface hardness HM (Hv) of middle portion positioned between foot-bottom central
portion and foot-edge portion: Hv 315 to 360 (HC;;. HM 2' HE)
Rails G (8 pieces) having foot-bottom central portion and middle portion,
each of which hardness is controlled: surface hardness HE (Hv) of foot-edge portion:
Hv 315 (constant), surface hardness HC (Hv) offoot-bottom central portion: Hv 440,
and surface hardness HM (Hv) of middle portion positioned between foot-bottom
central portion and foot-edge portion: Hv 315 to 440 (HC;;. HM 2: HE)
RailsH (II pieces) having foot-bottom central portion and middle portion,
each of which hardness is controlled: surface hardness HE (Hv) of foot-edge portion:
Hv 315 (constant), surface hardness HC (Hv) offoot-bottom central portion: Hv 500,
and surface hardness HM (Hv) of middle portion positioned between foot-bottom
central portion and foot-edge portion: Hv 315 to 500 (HC 2: HM 2: HE)
- 21 -
[0053]
The surface hardness of the foot-bottom central portion is an average value
obtained by measuring the surface hardness (hardness of the cross section at depths of
1 mm and 5 mm under the surface) of20 sites shown in FIG. 7; the surface hardness of
the foot -edge portion is an average value obtained by measuring the surface hardness
(hardness of the cross section at depths of 1 mm and 5 mm under the surface) of 20
sites shown in FIG. 7; and the surface hardness of the middle portion is an average
value obtained by measuring the surface hardness (hardness of the cross section at
depths of 1 mm and 5 mm under the surface) of 20 sites shown in FIG. 7.
,.Jhe surface hardness between the foot -edge portion and the middle portion
and the surface hardness between the middle portion and the foot -bottom central
portion are respectively in a state of distribution which monotonically increases or
decreases.
[0054]
Conditions of fatigue test
Test method: 3 point bending of actual rail (span length: 0.65 m) (see FIG. 8)
Load condition: stress range is controlled (maximum load- minimum load,
minimum load is 10% of maximum load), frequency of applied load: 5 Hz
Test attitude: load is applied to rail head portion (tensile stress is applied to
bottom portion)
Controlling stress: stress is controlled using strain gauge adhering to footbottom
central portion of rail bottom portion
Number of repetition: number of repetition is set to 2 million times (maximum
stress range in case of being unfractured is set to fatigue limit stress range)
[0055]
- 22 -
FIG. 6 shows the results of the fatigue test performed on the rails F (6 pieces),
the rails G (8 pieces), and rails H (11 pieces). FIG. 6 is a graph organized based on
the relationship between the surface hardness HM (Hv) of the middle portion and tbe
fatigue limit stress range in the bottom portion. In all rails, it was confirmed that the
fatigue resistance of the foot-bottom central portion ofthe rail bottom portion is
improved in a region in which the surface hardness HM (Hv) of the middle portion is
0.900 times or greater the surface hardness HC (Hv) of the foot-bottom central portion.
The reason for this is considered that the strain concentration on the boundary portion
between the foot-bottom central portion and the middle portion is further suppressed
due to a decrease of a difference in hardness between the foot -bottom central portion
and the middle portion.
[0056]
From the results described above, it was found that the fatigue stress of the
rail bottom portion is further improved by controlling the surface hardness HC (Hv) of
the foot-bottom central portion, the surface hardness HE (Hv) of the foot-edge portion,
and the surface hardness HM (Hv) of the middle portion to satisfy HC;::, HM;::, HE,
controlling the surface hardness HM (Hv) of the middle portion and the surface
hardness HC (Hv) of the foot-bottom central portion to satisfY the following
Expression 2, and suppressing the strain concentration on the rail bottom portion.
HM/HC ;::, 0.900 Expression 2
[0057]
Based on the fmdings described above, the rail according to the present
embodiment is a rail used for the purpose of improving breakage resistance and the
fatigue resistance of the rail bottom portion used in cargo railways so that the service
lile is greatly improved by controlling the compositions of rail steel; controlling the
- 23 -
metallographic structure of the rail bottom portion and the surface hardness ofthe footbottom
central portion and the foot-edge portion of the rail bottom portion, controlling
the balance of the surface hardness of the foot-bottom central portion, the foot-edge
portion, and the middle portion, and suppressing the strain concentration on the
vicinity of the middle portion.
[0058]
Next, the rail according to the present embodiment will be described in detail.
Hereinafter, "%" in the steel composition indicates mass%.
[0059]
(I) Reason for limiting chemical compositions (steel compositions) of rail
steel
The ceason for limiting the chemical compositions of steel in the rail according to the
present embodiment will be described in detail.
[0060]
C: 0.75%to L20%
C is an element which promotes pearlitic transformation and contributes to
improvement of fatigue resistance. However, when the C content is less than 0.75%,
the minimum strength and breakage resistance required for the rail cannot be ensured.
Further, a large amount of soft pro-eutectoid ferrite in which fatigue cracks easily
occur in the rail bottom portion is likely to be generated and fatigue damage is likely to
be generated. When the C content is greater than 1.20%, the pro-eutectoid cementite
is likely to be generated and fatigue cracks occur from the cementite between the proeutectoid
cementite and pearlite structure so that the fatigue resistance is degraded.
Further, the toughness is degraded and the breakage resistance ofthe foot-edge portion
is degraded. Therefore, the C content is adjusted to be in· a range of 0. 75% to 1.20%
- 24 -
in order to promote generation of pearlite structnre and ensure a constant level of
fatigue resistance or breakage resistance. It is preferable that the C content is
adjusted to be in a range of 0.85% to 1.10% in order to further stabilize generation of
pearlite structnre and further improve the fatigue resistance or the breakage resistance.
[0061]
Si: 0.10% to 2.00%
Si is an element which is solid-soluted in ferrite of pearlite structure, increases
the hardness (strength) ofthe rail bottom portion, and improves the fatigue resistance.
Further, Si is also an element which suppresses generation of the pro-eutectoid
cementite, prevents fatigue damage occurring from the interface between the proeutectoid
cementite and the pearlite structnre, improves the fatigue resistance,
suppresses degradation of toughness due to the generation of the pro-eutectoid ferrite,
and improves the breakage resistance of the foot-edge portion. However, when the Si
content is less than 0.1 0%, these effects carrnot be sufficiently obtained. Meanwhile,
when the Si content is greater than 2.00%, a large amount of surface cracks are
generated during hot rolling. In addition, hardenability is significantly increased, and
martensite structure with low toughness is likely to be generated in the rail bottom
portion so that the fatigue resistance is degraded. Further, the hardness is excessively
increased and thus the breakage resistance of the foot -edge portion is degraded.
Therefore, the Si content is adjusted to be in a range of 0.10% to 2.00% in order to
promote generation of pearlite structure and ensure a constant level of fatigue
resistance or breakage resistance. It is preferable that the Si content is adjusted to be
in a range of 0.20% to 1.50% in order to further stabilize generation of pearlite
structure and further improve the fatigue resistance or the breakage resistance.
[0062]
- 25 -
Mn: 0.10% to 2.00%
Mn is an element which increases the hardenability, stabilizes pearlitic
transformation, refines the lamellar spacing of pearlite structure, and ensures the
hardness of pearlite structure so that the fatigue resistance is improved. However,
when the Mn content is less than 0.1 0%, the effects thereof are small and a soft proeutectoid
ferrite in which fatigue cracks easily occur in the rail bottom portion is likely
to be generated. When pro-eutectoid ferrite is generated, the fatigue resistance is
unlikely to be ensured. Meanwhile, when the Mn content is greater than 2.00%, the
hardenability is significantly increased, and martensite structure with low toughness is
likely to be generated in the rail bottom portion so that the fatigue resistance is
degraded. Further, the hardness is excessively increased and thus the breakage
resistance of the foot -edge portion is degraded. Therefore, the Mn addition content is
adjusted to be in a range ofO.lO% to2.00% in order to promote generation of pearlite
structure and ensure a constant level of fatigue resistance or breakage resistance. It is
preferable that the Mn content is adjusted to be in a range of 0.20% to 1.50% in order
to further stabilize generation of pearlite structure and further improve the fatigue
resistance or the breakage resistance.
[0063]
P: 0.0250% or less
Pis an element which is unavoidably contained in steel. The amount thereof
can be controlled by perfi:Jnning refining in a converter. It is preferable that the P
content is small. Particularly, when tl1e P content is greater than 0.0250%, brittle
cracks occur from the tip of fatigue cracks in the rail bottom portion so that the fatigue
resistance is degraded. Further, the toughness of the foot-edge portion is degraded
and the breakage resistance is degraded. Therefore; the P content is limited to
- 26 -
0.0250% or less. The lower limit of the P content is not limited, but the lower limit
thereof during actual production is approximately 0.0050% when dephosphrization
capacity during the refining process is considered.
[0064]
S is an element which is unavoidably contained in steel. The content thereof
can be controlled by performing desulfurization in a cupola pot. It is preferable that
the S content is small. Particularly, when the S content is greater than 0.0250%,
pearlite structure is embrittled, inclusions of coarse MnS-based sulfides are likely to be
generated, and fatigue cracks occur in the rail bottom portion due to stress
concentration on the periphery of the inclusions, and thus the fatigue resistance is
degraded. Therefore, the S content is limited to 0.0250% or less. The lower limit of
the S content i5 not limited, but the lower limit thereof during actual production i5
approximately 0.0030% when desulfurization capacity during the refining process is
considered.
[0065]
Basically, the rail according to the present embodiment contains the abovedescribed
chemical compositions and the remainder of Fe and impurities. However,
instead of a part of Fe in the remainder, at least one selected from the group consisting
of Cr, Mo, Co, B, Cu, Ni, V, Nb, Ti, Mg, Ca, REM, Zr, N, and AI may be further
contained, in ranges described below, for the purpose of improving the fatigue
resistance due to an increase in hardness (strength) of pearlite structure, improving the
touglmess, preventing a heat affected zone from being softened, and controlling
distribution of the hardness in the cross section in the inside of the rail bottom portion.
Specifically, Cr and Mo increase the equilibrium transformation point, refine the
lamellar spacing of pearlite structure, and improve the hardness. Co refines the
- 27 -
lamellar structure directly beneath the rolling contact surface resulting from the contact
with wheels and increases the hardness. B reduces the cooling rate dependence of the
pearlitic transformation temperature to make distribution of the hardness in the cross
section of the rail bottom portion uniform. Cu is solid-soluted in ferrite of pearlite
structure and increases the hardness. N i improves the toughness and hardness of
pearlite structure and prevents the heat affected zone of the weld joint from being
softened. V, Nb, and Ti improve the fatigue strength of 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, V, Nb, and Ti make a
carbide or a nitride be stably generated during re-heating and prevent the heat affected
portion of the weld joint from being softened. Mg, Ca., and REM finely disperse
MnS-based sulfides, refme austenite grains, promote the pearlitic transformation, and
improve the toughness simultaneously. 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 structure by increasing the equiaxed crystal ratio of the
solidification structure. N promotes the pearlitic transformation by being segregated
in austenite grain boundaries, improves the toughness, and promotes precipitation of a
V carbide or a V nitride during a cooling process carried out after hot rolling to
improve the fatigue resistance of pearlite structure. Consequently, these elements
may be contained in ranges described below 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 tl1e present
embodiment are not damaged. Further, since these elements are not necessary, the
lower limit thereof is 0%.
[0066]
- 28 -
Cr: 0.01% to 2.00%
Cr is an element which refines the lamellar spacing of pearlite structure and
improves the hardness (strength) of pearlite structure so that the fatigue resistance is
improved by increasing the equilibrium transformation temperature and increasing the
supercooling degree. However, when the Cr 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 Cr content is greater than 2.00%, the hardenability
is significantly increased, a martensite structure with low toughness is generated in the
rail bottom portion, and thus the breakage resistance is degraded. Therefore, it is
preferable that the Cr content is set to be in a range ofO.Ol% to 2.00% when Cr is
contained.
[0067]
Mo: O.Ol%to0.50%
Similar to Cr, Mo is an element which refmes the lamellar spacing of pearlite
structure and improves the hardness (strength) of pearlite structure so that the fatigue
resistance is 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 significantly decreased, the martensite structure with
low toughness is generated in the rail bottom portion, and thus the breakage 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.
[0068]
Co: O.Olo/oto 1.00%
- 29 -
Co is an element which is solid-soluted in ferrite of pearlite structure, refines
the lamellar structure of pearlite structure directry beneath the rolling contact surface
resulting from the contact with wheels, and increases the hardness (strength) of pearlite
structure so that the fatigue resistance is 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 fatigue resistance cannot be obtained. Meanwhile, when the
Co content is greater than 1.00%, the above-described eifects are saturated and
economic efficiency is decreased due to an increase in alloying addition cost.
Therefore, it is preferable that the Co content is setto be in a range ofO.Ol% to 1.00%
when Co is contained.
[0069]
B: 0.000l%to0.0050%
B is an element which forms iron borocarbides (Fe23(CB)6) in austenite grain
boundaries and reduces cooling rate dependence of the pearlitic transformation
temperature by promoting pearlitic transformation. When the cooling rate
dependence of the pearlitic transformation temperature is reduced, more uniform
distribution of the hardness is imparted to a region from the surface to the inside of the
rail bottom portion of the rail and thus the fatigue resistance is improved. However,
when the B content is less than 0.0001%, the effects described above are not sufficient
and improvement of distribution of the hardness in the rail bottom portion is not
recognized. Meanwhile, when B content is greater than 0.0050%, coarse
borocarbides are generated and fatigue breakage is likely to occur because of the stress
concentration. 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.
[0070]
- 30 -
Cu: 0.01% to 1.00%
Cu is an element which is solid-soluted in ferrite of pearlite structure and
improves the hardness (strength) resulting from solid solution strengthening. As a
result, the fatigue resistance is improved. However, when the Cu content is less than
0.01 %, the effects cannot be obtained. Meanwhile, when the Cu content is greater
than I. 00%, martensite structure is generated in the rail bottom portion because of
significant improvement of hardenability and thus the breakage resistance is degraded.
Therefore, it is preferable that the Cu content is set to be in a range of 0.01% to 1.00%
when Cu.is contained.
[0071]
Ni: 0.01%to 1.00%
Ni is an element which improves the toughness of pearlite structure and
improves the hardness (strength) resulting from solid solution strengthening. As a
result, the fatigue resistance is improved. Further, Ni is an element which is finely
precipitated in the heat affected zone as an intermetallic 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, when the Ni content is greater than 1.00%, martensite
structure with low toughness is generated in the rail bottom portion because of
significant improvement of harden ability and thus the breakage resistance is degraded.
Therefore, it is preferable that the Ni content is set to be in a range of0.01% to 1.00%
when Ni is contained.
[0072]
V: 0.005% to 0.50%
- 31 -
Vis an element which increases the hardness (strength) of pearlite structure
using precipitation hardening of a V carbide and a V nitride generated during the
cooling process after hot rolling and improves the fatigue resistance. Further, Vis an
element effective for preventing the heat affected zone of the welded joint from being
softened by being generated as a V carbide or a V nitride in a relatively high
temperature range, in the heat affected zone re-heated to a temperature range lower
than or equal to the Acl point. However, when V content is less than 0.005%, these
effects cannot be sufficiently obtained and improvement of the hardness (strength) is
not recognized. Meanwhile, when V content is greater than 0.50%, precipitation
hardening resulting from the V carbide or the V nitride becomes excessive, pearlite
structure is embrittled, and then the fatigue resistance of the rail is degraded
Therefore, it is preferable that the V content is set to be in a range of0.005% to 0.50%
when V is contained
[0073]
Nb: 0.0010% to 0.050%
Similar to V, Nb is an element which increases the hardness (strength) of
pearlite structure using precipitation hardening of a Nb carbide and a Nb nitride
generated during the cooling process after hot rolling and improves the fatigue
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 equal to the Ac l point.
However, when the Nb content is less than 0.00 I 0%, these effects cannot be
sufficiently obtained and improvement of the hardness (strength) of pearlite structure is
not recognized. Meanwhile, when Nb content is greater than 0.050%, precipitation
- 32 -
hardening resulting from the Nb carbide or the Nb nitride becomes excessive, pearlite
structure is embrittled, and then the fatigue resistance of the rail is degraded.
Therefore, it is preferable that the Nb content is set to be in a range of 0.0010% to
0.050% when Nb is contained.
[0074]
Ti: 0.0030% to 0.0500%
Ti is an element which is precipitated as a Ti carbide or a Ti nitride generated
during the cooling process after hot rolling, increases the hardness (strength) of pearlite
structure using precipitation hardening, and improves the fatigue resistance. Further,
Ti is an element effective for preventing the weldedjoint from being embrittled by
attempting refinement of 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 0.0500o/o, a Ti
carbide and a Ti nitride which are coarse are generated and fatigue damage is likely to
occur due to the stress concentration. Therefore, it is preferable that the Ti content is
set to be in a range of0.0030% to 0.0500% when Ti is contained.
[0075]
Mg: 0.0005% to 0.0200%
Mg is an element which is bonded to S to form a sulfide (MgS). MgS finely
disperses MnS. In addition, the finely dispersed MnS becomes a nucleus of pearlitic
transformation so that the pearlitic transformation is promoted and the toughness of
pearlite structure 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 damage is likely to occur due to the stress
- 33 -
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.
[0076]
Ca: 0.0005% to 0.0200%
Ca is an element which has a strong binding force with S and forms a sulfide
(CaS). CaS ,finely disperses MnS. In addition, the finely dispersed MnS becomes a
nucleus of pearlitic transformation so that the pearlitic transformation is promoted and
the toughness of pearlite structure is improved. However, when the Ca content is less
than 0.0005%, these effects are smalL Meanwhile, when theCa content is greater
than 0. 0200%, a coarse oxide of Ca is generated and fatigue damage is likely to occur
due to the stress concentration. Therefore, it is preferable that theCa content is set to
be in a range of 0.0005% to 0.0200% when Ca is contained.
[0077]
REM: 0.0005% to 0.0500%
REM is a deoxidation and desulfurizing element and is also an element which
generates oxysulfide (REM20 2S) of REM when contained and becomes a nucleus that
generates Mn sulfide-based inclusions. Further, since the melting point of the
oxysulfide (REMz02S) is high as this nucleus, stretching of the Mn sulfide-based
inclusions after hot rolling is suppressed. As a result, when REM is contained, MnS
is finely dispersed, the stress concentration is relaxed, and the fatigue resistance is
improved. However, when the REM content is less than 0.0005%, the effects are
small and REM becomes insufficient as the nucleus that generates MnS-based sulfides.
Meanwhile, when the REM content is greater than 0.0500%, oxysulfide (REM20 2S) of
full hard REM is generated and fatigue damage is likely to.occur due to the stress
· concentration. Therefore, it is preferable that the REM content is set to be in a range
- 34 -
of 0.0005% to 0.0500% when REM is contained.
[0078]
Here, 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 all REM. When the total
amount of all REM elements is in the above-described range, the same effects are
obtained even when a single element or a combination of elements (two or more kinds)
is contained.
[0079]
Zr: 0.0001%to0.0200%
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 portion of a cast slab or
bloom and suppresses generation of martensite structure or pro-eutectoid cementite
generated in a segregation portion of the rail by increasing the equiaxed crystal ratio of
the solidification structure. However, when the Zr content is less than 0.0001%, the
number of Zr02-based inclusions is small and the inclusions do not sufficiently exhibit
effects as solidified nuclei. In this case, martensite structure or pro-eutectoid
cementite is likely to be generated in the segregation portion of the rail bottom portion,
and accordingly, improvement of the fatigue resistance of the rail cannot be expected.
Meanwhile, when the Zr content is greater than 0.0200%, a large amount of coarse Zrbased
inclusions are generated and fatigue damage is likely to occur due to the stress
concentration. Therefore, it is preferable that the Zr content is set to be in a range of
0.0001% to 0.0200% when Zr is contained.
[0080]
- 35 -
N: 0.0060% to 0.0200%
N is an element which is effective for improving toughness by promoting
pearlitic transformation from austenite grain boundaries by being segregated on the
austenite grain boundaries and refining pearlite block size. In addition, N is an
element which promotes precipitation of a carbonitride ofV during the cooling process
after hot rolling, increases the hardness (strength) of pearlite structure, and improves
the fatigue resistance when N and V are added simultaneously. However, when the N
content is less than 0.0060%, these effects are small. Meanwhile, when theN content
is greater than 0.0200%, it becomes difficult for N to be dissolved in steel. In this
case, bubbles as the origin of fatigue daruage are generated so that the fatigue daruage
is likely to occur. Therefore, it is preferable that theN content is set to be in a range
of0.0060% to 0.0200% whenN is contained.
[0081]
AI: 0.0100%to LOO%
AI is an element which functions as a deoxidizer. Further, AI is an element
which changes the eutectoid transformation temperature to a high temperature side,
contributes to increase the hardness (strength) of pearlite structure, and improves the
fatigue resistance. However, when theA! content is less than 0.0100%, the effects
thereof are small. Meanwhile, when the Al content is greater than 1. 00%, it becomes
difficult for AI to be dissolved in steel. In this case, coarse alumina-based inclusions
are generated and fatigue cracks occur from the coarse precipitates so that the fatigue
damage is likely to occur. Further, an oxide is generated during welding so that the
weldability is significantly degraded. Therefore, it is preferable that the Al content is
set to be in a range of 0.0 I 00% to I .00% when AI is contained.
[0082]
- 36 -
(2) Reason for limiting metallographic structure and required regions of
pearlite structure
In the rail according to the present embodiment, the reason for limiting 90%
or greater of the area of the metallographic structure at a depth of 5 mm from the outer
surface of the bottom portion as the origin to pearlite will be described in detail.
[0083]
First, the reason for limiting 90% or greater of the area of the metallographic
structure to pearlite will be described.
Pearlite is a structure advantageous for improving the fatigue resistance
because it is possible to obtain the strength (hardness) by pearlite structure even if the
amount of alloy element is low. Further, the strength (hardness) is easily controlled,
the toughness is easily improved, and the breakage resistance is excellent Therefore,
for the purpose of improving the breakage resistance and the fatigue resistance of the
rail bottom portion, 90% or greater of the area of the metallographic structure is
limited to pearlite.
[0084]
Next, the reason for limiting the required region of pearlite structure to the
region at a depth of 5 mm from the outer surface ofthe bottom portion as the origin
will be described.
When the required region of pearlite structure is less than a depth of 5 mm
from the outer surface of the bottom portion, the effects for improving the breakage
resistance or the fatigue resistance required for the rail bottom portion are small and
the rail service life is difficult to sufficiently improve. Therefore, 90% or greater of
the area of the metallographic structure at a depth of 5 mm from the outer surface of
the bottom portion as the origin is set to pearlite structure.
- 37 -
[0085]
FIG. 7 shows a region required for pearlite structure. As described above,
the rail bottom portion 4 includes the foot-bottom central portion I, the foot-edge
portion 2 positioned on both ends of the foot -bottom central portion 1, and the middle
portion 3 positioned between the foot -bottom central portion 1 and the foot -edge
portion 2. The outer surface 5 of the rail bottom portion indicates the entire surface
of the rail bottom portion 4 including the foot-bottom central portion I, the middle
portion 3, and the foot-edge portion 2 of the rail shown by the bold line and indicates
the surface facing down when the rail is upright. In addition, the outer surface 5 of
the rail bottom portion may include the side end surfaces of the rail bottom portion_
[0086]
When pearlite structure is disposed on the surface layer portion of the bottom
portion to a depth of 5 mm from the outer surface 5 of the rail bottom portion as the
origin, in a region from the foot-bottom central portion 1 to the foot-edge portion 2 on
both ends through the middle portion 3, the breakage resistance and the fatigue
resistance of the rail are improved. Therefore, as shown in the hatched region in FIG.
7, pearlite Pis disposed at least in a region at a depth of 5 mm from the outer surface 5
of the rail bottom portion as the origin for which improvement of the breakage
resistance and the fatigue resistance are required. In addition, other portions may be
pearlite structure or the metallographic structure other than pearlite structure. Further,
in a case where characteristics of the entire cross section of the rail are considered,
ensuring of the wear resistance is considered to be the most important particularly in
the rail head portion that comes into contact with wheels. As a result of investigation
of the relationship between the metallographic structure and the wear resistance, since
it was confirmed that pearlite structure is most excellent, it is preferable that the
- 38 -
structure of the rail head portion is pearlite.
[0087]
Moreover, it is preferable that the metallographic structure of the surface layer
portion of the rail bottom portion according to the present embodiment is the pearlite
as described above, but a small amount of pro-eutectoid ferrite, pro-eutectoid
cementite, bainite structure, or martensite structure may be mixed into pearlite
structure by I 0% or less in terms of the area ratio depending on the chemical
composition or a heat treatment production method of the rail. However, even when
these structures are mixed into pearlite structure, since the breakage resistance and the
fatigue resistance of the rail bottom portion are not greatly affected ifthe amount
thereof is small, the mixture of a small amount of pro-eutectoid ferrite, pro-eutectoid
cementite, bainite structure, or martensite structure into pearlite structure by 10% or
less in terms of the area ratio is accepted as the rail structure having excellent breakage
resistance and fatigue resistance. In other words, 90% or greater of the area ratio of
the metallographic structure of the surface layer portion of the rail bottom portion
according to the present embodiment may be pearlite. In order to sufficiently
improve the breakage resistance and the fatigue resistance, it is preferable that 95% or
greater ofthe area ratio of the metallographic structure of the surface layer portion of
the bottom portion is set to be pearlite.
The area ratio is obtained by machining test pieces from the transverse cross
section perpendicular to the outer surface of the rail bottom portion, p'Oiishing the test
pieces, showing the metallographic structure to appear through etching, and observing
the metallographic structure at respective positions of 1 mm and 5 mm from the
surface. Specifically, in observation at each position described above, the area ratio is
obtained by observing the metallographic structure in the visual field of an optical
- 39 -
microscope of200 magnifications and determining the area of each structure. As a
result of observation, when both of the area ratios of pearlite structure at positions of a
depth of I mm and a depth of 5 mm from the surface are 90% or greater, 90% or
greater of the metallographic structure at a depth of 5 mm from the outer surface of the
rail bottom portion as the origin may be determined to be pearlite structure (the area
ratio of pearlite structure at a depth of 5 mm from the outer surface of the rail bottom
portion as the origin is 90% or greater). That is, when the area ratio of each position
described above is 90%, the middle position interposed by each of the positions may
have a pearlite structure area ratio of 90% or greater.
[0088]
(3) Reason for limiting surface hardness of foot-bottom central portion
In the rail according to the present embodiment, the reason for limiting the
surface hardness of the foot-bottom central portion to a range of Hv 360 to 500 will be
described.
When the surface hardness of the foot-bottom central portion is less than Hv
360, the fatigue limit stress range cannot be ensured with respect to the load stress (200
MPa) of the foot-bottom central portion applied to the heavy load railways as shown in
FIG. 2 and thus the fatigue resistance of the rail bottom portion is degraded.
Meanwhile, when the surface hardness is greater than Hv 500, embrittlement of
pearlite structure advances, the fatigue limit stress range cannot be ensured due to
occurrence of cracks, and thus fatigue resistance of the rail bottom portion is degraded
as shown in FIG. 2. For this reason, the surface hardness of the foot-bottom central
portion is limited to a range ofHv 360 to 500.
[0089]
(4} Reason for limiting surface hardness of foot-edge portion
- 40 -
In the rail according to the present embodiment, the reason for limiting the
surface hardness of the foot-edge portion to a range ofHv 260 to 315 will be describe.
When the surface hardness of the foot-edge portion is less than Hv 260, the fatigue
limit stress range cannot be ensured with respect to the load stress (150 MPa) ofthe
foot-edge portion applied to the heavy load railways as shown in FIG. 3 and thus the
fatigue resistance of the rail bottom portion is degraded. Meanwhile, the surface
hardness is greater than Hv 315, the toughness of pearlite structure is degraded and the
breakage resistance of the rail bottom portion is degraded due to the promotion of
brittle fracture as shown in FIG. 4. For this reason, the surface hardness of the footedge
portion is limited to a range ofHv 260 to 315.
[0090]
(5) Reason for limiting relationship of surface hardness HC of foot -bottom
central portion, surface hardness HE of foot-edge portion, and surface hardness HM of
middle portion
When the surface hardness of the middle portion is set to be smaller than the
surface hardness of the foot-edge portion, as shown in FIG. 5, strain is concentrated on
the middle portion (soft portion) so that fatigue fracture occurs from the middle portion.
Further, when the surface hardness of the middle portion is set to be larger than the
surface hardness of the foot-bottom central portion, as shown in FIG 5, strain is
concentrated on the boundary portion between the foot-bottom central portion and the
middle portion so that the fatigue fracture occurs from the boundary portion.
Therefore, the relationship ofthe surface hardness HC of the foot-bottom central
portion, the surface hardness HE of the foot -edge portion, and the surface hardness
HM ofthe middle portion is limited to satisfY the following conditions .
. [0091]
- 41 -
HC~HM~HE
[0092]
(6) Reason for limiting relationship between surface hardness HC offootbottom
central portion and surface hardness HM of middle portion
When the surface hardness HC (Hv) of the foot-bottom central portion, the
surface hardness HE (Hv) of the foot-edge portion, and the surface hardness HM (Hv)
of the middle portion is controlled to be in the above-described relationship (HC ~ HM
~HE), the surface hardness HM (Hv) of the middle portion is controlled to be 0.900
times or greater the surface hardness HC (Hv) of the foot-bottom central portion, and a
difference in hardness between the foot -bottom central portion and the middle portion,
the strain concentration on the boundary portion between the foot-bottom central
portion and the middle portion is further suppressed and the fatigue resistance of the
rail bottom portion is more improved as shown in FIG 6. Therefore, the relationship
of the surface hardness HC of the foot-bottom central portion and the surface hardness
HM of the middle portion is limited to satisfY the following conditions.
[0093]
HM/HC ~ 0.900
[0094]
It is preferable that the surface hardness of the rail bottom portion is measured
under the following conditions.
[Method of measuring surface hardness of rail bottom portion]
Measurement
Measuring device: Vickers hardness tester (load of 98 N)
. Collection oftest pieces for measurement: machining sample out from
transverse cross section of bottom portion
- 42 -
Pre-processing: polishing transverse cross section with diamond grains having
average grain size of I 11m
Measurement method: carried out in conformity with JIS Z2244
[0095]
Calculation of hardness
Foot-bottom central portion: Measurement is performed on respectively 20
sites at a depth of I mm and a depth of 5 mm under the surface of the site shown in
FIG. 7 and the average value thereof is set to the hardness of each position.
Foot-edge portion: Measurement is performed on respectively 20 sites at a
depth of 1 mm and a depth of 5 mm under the surface of the site shown in FIG. 7 and
the average value thereof is set to the hardness of each position.
Middle portion: Measurement is performed on respectively 20 sites at a depth
of l mm and a depth of 5 mm under the surface of the site shown in FIG. 7 and the
average value thereof is set to the hardness of each position.
[0096]
Calculation of ratio between surface hardness of middle portion (HM) and
surface hardness of foot-bottom central portion (HC).
The ratio between the surface hardness of the middle portion (HM) and the
surface hardness of the foot-bottom central portion (HC) is calculated by setting the
value obtained by further averaging the average value of each hardness at a depth of I
mm and a depth of5 mm under the surface in each site as the surface hardness of the
foot-bottom central portion (HC) and the surface hardness of the middle portion (HM).
[0097]
(7) Metl1od of controlling hardness of rail bottom portion
The hardness of the rail bottom portion can be controlled by adjusting the hot
- 43 -
rolling conditions and the heat treatment conditions after hot rolling according to the
hardness required for the foot-bottom central portion, the foot-edge portion, and the
middle portion.
[0098]
The rail according to the present embodiment can obtain the effects thereof
regardless of the production method when the rail includes the above-described
compositions, structures, and the like. However, the effects can be obtained by the
rail steel having the above-described compositions by performing a smelting in a
melting furnace such as a converter or an electric furnace which is typically used,
performing an ingot-making and blooming method or a continuous casting method on
the molten steel and then hot rolling, and performing a heat treatment in order to
control the metallographic structure or the hardness of the rail bottom portion as
necessary.
[0099]
For example, the rail according to the present embodiment is formed in a rail
shape by casting molten steel after the compositions are adjusted to obtain a slab or
bloom, heating the slab or bloom in a temperature range of l250°C to 1300°C, and
carrying out hot rolling. Further, the rail can be obtained by performing air cooling or
accelerated cooling after hot rolling or perforn1ing accelerated cooling after hot rolling,
air cooling, and re-heating.
[0100]
In these series of processes, any one or more of production conditions from
among hot rolling conditions, the cooling rate of accelerated cooling after hot rolling,
the re-heating temperature after hot rolling, and the cooling rate of accelerated cooling
after re-heating subsequent to hot rolling may be controlled in order to adjust the
- 44 -
surface hardness of the foot-bottom central portion, the foot-edge portion, and the
middle portion.
[0101]
• Preferable hot rolling conditions and re-heating conditions
In order to ensure characteristics of the foot-edge portion with a low hardness
when compared to the hardness of the foot -bottom central portion, the final hot rolling
temperatures of the foot-bottom central portion and the foot-edge portion are
individually controlled, for example, the foot -edge portion is cooled before the final
hot rolling. As the hot rolling conditions of the actual rail, the hardness of each
position can be individually controlled by setting the final hot rolling temperature of
the foot-bottom central portion to be in a range of900°C to 1000°C (temperature of the
outer swface of the rail bottom portion) and setting the final rolling temperature of the
foot-edge portion to be in a range of800°C to 900°C (temperature of the outer surface
of the rail bottom portion).
[0102]
In order to control the hardness ofthe rail bottom portion for imparting the
breakage of the fatigue resistance, it seems enough to control the final hot rolling
temperature through caliber rolling of a typical rail. Other rolling conditions of the
.rail bottom portion may be set such that pearlite structure is mainly obtained according
to a known method. For example, with reference to a method described in Japanese
Unexamined Patent Application, First Publication No. 2002-2269!5, rough hot rolling
is performed on a slab or bloom, intermediate rolling is performed over a plurality of
passes using a reverse mill, the surface of the rail head portion and the central surface
of the bottom portion are cooled such that the temperatures thereof are respectively in a
range of 50°C to l oooc immediately after hot rolling of each pass of intermediate
- 45 -
rolling is performed, and then fmish hot rolling may be performed two passes or more
using a continuous mill. At this time, for the purpose of controlling the hardness of
the rail bottom portion, the temperatures of the foot -edge portion and the foot -bottom
central portion of the rail bottom portion may be respectively controlled to be in the
above-described range before the final hot rolling of the fmish rolling.
[0103]
Moreover, in a case where the rail bottom portion is re-heated after hot rolling,
the heating conditions may be controlled to set the heating temperature of the footedge
portion to be low by comparing to the heating temperature of the foot-bottom
central portion in order to decrease the hardness of the foot-edge portion by comparing
the hardness of the foot -bottom central portion. As the re-heating conditions of the
actual rail, the hardness of the rail bottom portion can be controlled by performing reheating
such that the re-heating temperature of the foot -bottom central portion is in a
range of950°C to 10500C (outer surface of the rail bottom portion) and the re-heating
temperature of the foot-edge portion is in a range of850°C to 950°C (outer surface of
the rail bottom portion).
[0104]
In the middle portion, it is preferable that the final hot rolling temperature or
the re-heating temperature of a portion in the vicinity of the foot-edge portion is set to
be slightly higher than that of the foot-edge portion and the final hot rolling
temperature or the re-heating temperature of a portion in the vicinity of the foot-bottom
central portion is set to be slightly lower than that of the foot -bottom central portion,
based on the conditions in confom1ity with the hot rolling conditions and the re-heating
conditions of the foot-bottom central portion and the foot-edge portion. As a result,
the target hardness can be ensured.
- 46 -
[0105]
• Conditions of accelerated cooling after hot rolling and re-heating
The method of performing accelerated cooling on the rail bottom portion is
not particularly limited. In order to impart the breakage resistance or the fatigue
resistance and control the hardness, the cooling rate of the rail bottom portion during
the heat treatment may be controlled by means of air injection cooling, mist cooling,
mixed injection cooling of water and air, or a combination of these. However, for
example, in a case where the accelerated cooling is performed after hot rolling, water
or mist is used as a refrigerant for the accelerated cooling of the foot -bottom central
portion and air is used as a refrigerant for the accelerated cooling of the foot-edge
portion in order to decrease the hardness of the foot -edge portion by comparing to the
hardness of the foot-bottom central portion so that the cooling rate of the foot-edge
portion is decreased by comparing to the cooling rate of the foot-bottom central portion.
Further, the cooling rate and the cooling temperature range are controlled based on the
temperature of the outer surface of the rail bottom portion.
In a case where the accelerated cooling is performed after hot rolling, for
example, the hardness of each portion can be controlled by performing cooling on the
foot-bottom central portion at an accelerated cooling rate of 3°C/sec to 1 0°C/sec
(cooling temperature range: 850°C to 600°C) and the foot-edge portion at an
accelerated cooling rate of 1 °C/sec to 5°C/sec (cooling temperature range: 800°C to
650°C). Further, the accelerated cooling may be performed in a temperature range of
800°C to 600°C and the cooling conditions of a temperature of lower than 600°C is not
particularly limited.
[01 06]
In a case where the re-heating and then the accelerated cooling are
- 47 -
subsequently performed after hot rolling, for example, the hardness of each portion can
be controlled by performing cooling on the foot-bottom central portion at an
·accelerated cooling rate of5°C/sec to l2°C/sec (cooling temperature range: 850°C to
600°C) and the foot-edge portion at an accelerated cooling rate of3°C/sec to 8°C/sec
(cooling temperature range: 800°C to 600°C). Further, the accelerated cooling may
be performed in a temperature range of 800°C to 600°C and the cooling conditions of a
temperature of lower than 600°C is not particularly limited.
[0107]
In the middle portion, it is preferable that the accelerated cooling rate of a
portion in the vicinity of the foot-edge portion is set to be slightly higher than that of
the foot-edge portion and the accelerated cooling rate of a portion in the vicinity of the
foot-bottom central portion is set to be slightly lower than that of the foot-bottom
portion, based on the conditions in conformity with the accelerated cooling conditions
of the foot-bottom central portion and the foot-edge portion. As a result, the target
hardness can be ensured.
[0108]
In order to decrease a difference in hardness between the middle portion and
the foot -bottom central portion for the purpose of further improving the fatigue
resistance, it is preferable that the accelerated cooling rate of the middle portion is set
to be close to the cooling rate of the foot-bottom central portion or the temperature of
finishing the accelerated cooling is set to be slightly low, specifically, the accelerated
cooling is performed to a temperature of around 600°C.
[01 09]
The hardness of the rail bottom portion can be controlled using a combination
of the above-described production conditions and the area ratio of pearlite structure can
- 48 -
be set to be 90% or greater in the metallographic structure with a predetermined range.
In the production of an actual rail, adjustment within the range of the
production conditions described above is necessary according to the composition of
rail steel. In the adjustment, the relationship between crystal grains and conditions of
hot rolling of steel, equilibrium diagrams of steel, continuous cooling transformation
diagrams (CCT diagrams), and the like described in disclosed known documents may
be referred to.
[0110]
When the fmish hot rolling temperature is controlled, .the hardness of each
portion can be differentiated and the structure can be determined by selecting the hot
rolling temperature of the foot-edge portion, the foot-bottom central portion, or the
middle portion based on the relationship between the conditions of hot rolling and the
austenite grain size. As a specific example, in the foot-edge portion expected to
decrease the hardness thereof, the austenite grain size can be reduced (grain size
number is increased) by decreasing the rolling temperature. Further, delay before hot
rolling or forced cooling of the foot -edge portion can be applied to a decrease in hot
rolling temperature of the foot-edge portion.
[Olll]
Further, when the re-heating temperature is controlled, the re-heating
temperature can be selected from the equilibrium state diagram of iron carbon. As a
specitic example, the austenite grain size is reduced by decreasing the re-heating
temperature in the foot -edge portion expected to decrease the hardness thereof. In
addition, when the temperature is extremely decreased, the metallographic structure is
not completely austenitized in some cases. For this reason, it is preferable that the
minimum heating temperature is controlled using the AI line, A3 line, and A em line as
- 49 -
the base. In order to set the re-heating temperature of the foot -edge portion to be low,
suppression of heating such as installation of a shielding plate or the like can be
applied in a case of re-heating with radiation heat. In a case of using induction
heating, the heating ofthe foot-edge portion is suppressed by adjusting the
arrangement of a plurality of coils or the heating ofthe foot-edge portion is suppressed
by adjusting the output ofinduction heating coils in the vicinity of the foot-edge
portion.
[0112]
When the cooling rate of the accelerated cooling is controlled (cooling carried
10ut as the heat treatment after the finish rolling or the re-heating is controlled), the
accelerated cooling rate can be determined from the CCT diagrams according to the
composition of the rail steel. Specifically, in order to ensure generation of pearlite
structure, it is preferable that an appropriate cooling rate of pearlite transformation is
derived from the CCT diagrams and the cooling rate is contro!led such that the target
hardness can be obtained from the range. As a specific example, it is necessary to
control the cooling rate to be low in the foot-edge portion expected to decrease the
hardness thereof by comparing to the cooling rate of the foot-bottom central portion.
[0113]
The rail according to the present embodiment can be produced by using the
above-described microstructure control method in combination with new knowledge
obtained by the present inventors.
[Examples]
[OH4]
Next, examples of the present invention will be described.·
Tables 1 to 4 show the chemical compositions and characteristics of rails in
- 50 -
examples of the present invention. Tables 1 to 4 show the values of chemical
composition, the microstructure of the bottom portion, the surface hardness of the
bottom portion, and the ratio between the surface hardness of the foot-bottom central
portion and the surface hardness of the middle portion. The remainder of the
chemical compositions is Fe and impurities. The results of the fatigue test performed
according to the method shown in FIG 8 and the results of the impact test performed
on the foot-edge portion by machining test pieces from the position shown in FIG 9
are also listed. In a case where only "pearlite" is described, the area ratio of pearlite
structure at a depth of 5 mm from the outer surface of the rail bottom portion as the
origin is 90% or greater and the microstructure of the bottom portion includes a small
amount of at least one of pro-eutectoid ferrite, pro-eutectoid cementite, bainite
structure, aud martensite structure, mixed into pearlite structure, by 10% or less in
terms of the area ratio.
[0115]
Further, Tables 5 to 9 show the values of chemical composition, the
microstructure of the bottom portion, the surface hardness of the bottom portion, and
the ratio between the surface hardness of the foot -bottom central portion and the
surface hardness of the middle portion of rails in the comparative examples. Further,
the results of the fatigue test performed according to the method shown in FIG. 8 and
the results of the impact test performed on the foot-edge portion by machining test
pieces from the position shown in FIG. 9 are also listed. In a case where only
"pearlite" is described, the area ratio of pearlite structure at a depth of 5 mm from the
outer surface of the rail bottom portion as the origin is 90% or greater and the
microstructure of the bottom portion includes a small amount of at least one of proeutectoid
ferrite, pro-eutectoid cementite, bainite structure, and martensite structure,
- 51 -
mixed into pearlite structure, by 10% or less in terms of the area ratio. In addition,
when a structure other than pearlite is described in the columns of the microstructure,
the area ratio is greater than 10% based on the entire area ratio. For example, in a
case where there is a description of"pearlite + pro-eutectoid ferrite", the area ratio of
pearlite structure is less than 90% and the main structure ofthe remainder is proeutectoid
ferrite.
[0116]
The outline of the production process and the production conditions of rails of
the present invention and rails for comparison listed in Tables 1 to 4 and Tables 5 to 9
will be described below in two ways.
[0117]
[Process of producing rails of present invention]
Rails of present invention are produced in the following order:
(1) melting steel;
(2) composition adjustment;
(3) casting (bloom);
(4) re-heating (l250°C to l300°C);
( 5) hot rolling; and
(6) air cooling or heat treatment (accelerated cooling).
Other rails of present invention are produced in the following order:
(1) melting steel;
(2) composition adjustment;
(3) casting;
( 4) re-heating;
(5) hot rolling;
- 52 -
(6) air cooling;
(7) re-heating (rail); and
(8) heat treatment (accelerated cooling).
[0118]
Further, the outline of the conditions for producing the rails ofthe present
invention listed in Tables 1 to 4 is as follows. In conditions for producing rails for
comparison in Tables 5 to 9, the rails of Comparative Examples 1 to 8 were produced
within the range of the conditions for producing the rails of the present invention.
Further, in conditions for producing rails of Comparative Examples 9 to 20, the rails
were produced under conditions, some of which were outside of the conditions for
producing the rails of the present invention.
[0119]
[Conditions for producing rails of present invention]
• Hot rolling conditions (only examples to which conditions were applied)
Final hot rolling temperature of foot-bottom central portion: 900°C to 1000°C
Final hot rolling temperature of foot-edge portion: 800°C to 900°C
• Re-heating conditions (only examples to which conditions were applied)
Re-heating temperature of foot-bottom central portion: 950°C to l050°C
Re-heating temperature of foot-edge portion: 850°C to 950°C
• Conditions for heat treatment performed on bottom portion (only examples
to which conditions were applied)
Heat treatment cooling rate immediately after hot rolling
Foot-bottom central portion: 3°C/sec to I0°C/sec (cooling temperature range:
850°C to 600°C)
Foot-edge portion: l 0 C/sec to 5°C/sec (cooling temperature range: 800°C to
- 53 -
Heat treatment cooling rate immediately after reheating
Foot-bottom central portion: 5°C/sec to l2°C/sec (cooling temperature range:
850°C to 600°C)
Foot-edge portion: 3°C/sec to 8°C/sec (cooling temperature range: 800°C to
[0120]
Further, the details of the rails of the present invention and the rails for
comparison respectively listed in Tables 1 to 4 and Tables 5 to 9 are as follows.
[0121]
(1) Rails of present invention (35 pieces)
Examples 1 to 35 of present invention: Rails in which the values of the
chemical compositions, the microstructure of the bottom portion, the surface hardness
of the bottom portion (foot-bottom central portion and foot-edge portion), and the ratio
between the surface hardness of the foot-bottom central portion and the surface
hardness of the middle portion were in the ranges of the invention of the present
application.
[0122]
(2) Rails for comparison (20 pieces)
Comparative Examples l to 8 (8 pieces): Rails in which any of the contents of
C, Si, Mn, P, and S and the microstructure of the bottom portion was out of the range
of the invention of the present application.
Comparative Examples 9 to 20 (12 pieces): Rails in which the foot-bottom
central portion of the rail bottom portion, the surface hardness of the foot-edge portion,
and the balance of the surface hardnesses of the foot-bottom central portion, the foot-
- 54 -
edge portion, and the middle portion were out of the ranges of the invention of the
present application.
Hz)
[0123]
In addition, conditions for various tests are as follows.
[Actual rail bending fatigue test (see FIG. 8)]
Test method: 3 point bending of actual rail (span length: 0.65 m, frequency: 5
Load condition: stress range was controlled (maximum load -minimum load,
minimum load was 10% of maximum load)
Test attitude: load was applied to rail head portion (tensile stress was applied
to bottom portion)
Controlling stress: stress was controlled using strain gauge adhering to footbottom
central portion of rail bottom portion
Number of repetition: 2 million times, maximum stress range in case of being
unfractured was set to fatigue limit stress range
[0124]
[Impact test]
Shape of specimen: JIS No. 3, 2 mm U-notch Charpy impact test piece
Position of machining test pieces: foot-edge portion of rail (see FIG 9)
Test temperature: room temperature (+20°C)
[0125]
[Method of measuring surface hardness of rail bottom portion]
Measurement
Measuring device: Vickers hardness tester (load of98 N)
Collection of test pieces for measurement: machining sample out from
- 55 -
transverse cross section of bottom portion
Pre-processing: polishing transverse cross section with diamond grains having
average grain size of 1 J-Lm
Measurement method: carried out in conformity with JIS Z2244
[0126]
Method of calculating hardness
Surface hardness of foot-bottom central portion: Measurement was performed
on respectively 20 sites at a depth of 1 mm and a depth of 5 mm under the surface of
the site shown in FIG. 7 and the average value thereof was set to the hardness of each
position.
Surface hardness of foot-edge portion: Measurement was performed on
respectively 20 sites at a depth of 1 rum and a depth of 5 rum under the surface of the
site shown in FIG. 7 and the average value thereof was set to the hardness of each
position.
Surfuce hardness of middle portion: Measurement was performed on
respectively 20 sites at a depth of 1 mm and a depth of 5 mm under the surface of the
site shown in FIG. 7 and the average value thereof was set to the hardness of each
position.
[0127]
Method of calculating ratio between surface hardness (HM) of middle portion
and surface hardness (HC) of foot-bottom central portion
The ratio between the surface hardness (HM) of the middle portion and the
surface hardness (HC) of the foot-bottom central portion was calculated by setting the
value obtained by further averaging the average value of each hardness at a depth of I
mm and a depth of 5 mm·under the surface in each site as the surface hardness (HC) of
- 56 -
the foot-bottom central portion and the surface hardness (HM) ofthe middle portion.
[0128]
As shown in Tables 1 to 4 and Tables 5 to 9, in the rails of the present
invention (Examples 1 to 35) compared to the rails for comparison (Comparative
Examples 1 to 8), the fatigue strength ofthe foot-bottom central portion and the
toughness of the foot-edge portion were improved and the breakage resistance and the
fatigue resistance of rails were improved by setting the contents of C, Si, Mn, P, and S
of steel to be in the limited ranges, suppressing generation of pro-eutectoid ferrite, proeutectoid
cementite, bainite structure, or marutensite structure, controlling the
inclusions or the toughness of pearlite structure, and controlling the surface hardness of
the foot-bottom central portion and the foot-edge portion of the rail bottom portion.
[0129]
In addition, in the rails of the present invention (Examples 1 to 35) compared
to the rails for comparison (Comparative Examples 9 to 20), the fatigue resistance was
improved by controlling the balance of the surface hardness of the foot -bottom central
portion and the foot -edge portion of the rail bottom portion and the surface hardness of
the middle portion.
[0130]
Further, as shown in Tables 1 to 4 and FIG. 10, the fatigue resistance of the
rails ofthe present invention (Examples 9, 10, 12, 13, 15, 16, 18, 19, 20, 21, 23, 24, 25,
26, 29, 30, 32, and 33) was further improved by controlling the surface hardness HC
(Hv) of the foot bottom central portion of the rail bottom portion and the surface
hardness (HM) (Hv) of the middle portion to satisfy the expression of HM/HC :>: 0.900
and further controlling the balance of the surface hardness.
- 57 -
[0131]
[Table 1]
Example Chemical composition (mass%)
of
invention c Si Mn p s Cr Mo Co B Cu Ni v Nb Ti Mg Ca REM Zr N AI
1 0.75 0.25 1.00 0.0150 0.0120 0.00 - - - - - - - - - - -
2 1.20 0.25 1.00 0.0150 0.0120 0.00 - - - - - - - - - - -
3 0.80 0.10 0.80 0.0180 0.0100 0.00 - - . - . - - - - - - - -
4 0.80 2.00 0.80 0.0180 0.0100 0,00 - - . - - - - - - - - -
5 0.90 0.45 0.10 0.0120 0.0080 0.00 - - - - - - - - - - - -
6 0.90 0.45 2.00
L
0.0120 0.0080 0.00 - - - - - - - - - -
1 1.00 0.75 0.75 0.0250 O.o!OO 0.00 - - - - . - - - - - -
8 uo 0.65 0.55 0.0120 0.0250 0.00 - - - - - - - - -
9 0.76 0.35 0.85 0.0140 0.0130 0.22 - . - - - - - - - -
10 0.76 0.35 0.85 0.0140 0.0130 0.22 - . - - - - - - - - - -
11 0.77 0.60 0.75 0.0200 0.0200 0.00 - - 0.20 - - - - - - - -
12 0.80 0.35 0.85 0.0190 0.0150 0.17 - - - -- 0.025 - - - - - - -
13 0.80 0.35 0.85 0.0190 0.0150 0.17 - - - - 0.025 - - - - -
14 0.80 1.60 0.25 0.0150 0.0180 0.00 - - - - 0.15 - - - - - - -
15 0.80 0.50 1.35 0.0070 0.0150 0.00 - - . - - - - - - - - -
16 0.80 0.50 1.35 0.0070 0.0150 0.00 - - - - - - - - - -
17 0.86 0.35 1.15 0.0200 0.0240 0.00 0.10 - - - - - - -
[0132]
• 58 -
[Table 2]
Example Chemical composition (mass%)
of
invention c Si Mn p s Cr Mo Co B Cu Ni v Nb Ti Mg Ca REM Zr N AI
18 0.90 0.40 0.65 0.0120 0.0180 0.65 - - - - - - - - - - -
19 0.90 0.40 0.65 0.0120 0.0180 0.65 - - - - - - - - -
20 0.90 0.50 1.10 0.0150 0.0120 0.00 - . - - - - - - - - -
21 0.90 0.50 1.10 0.0150 0.0120 0.00 - . . - - - - - - -
22 0.96 0.85 0.85 0.0120 0.0120 0.00 O.Dl - . - - - - - -
23 1.00 0.85 0.65 0.0150 0.0245 0.00 - . - - 0.0025 0.0050 - - - -
24 1.00 0.85 0.65 0.0150 0.0245 0.00 . . - - 0.0025 0.0050 - - - - -
' 25 1.00 0.45 1.00 0.0135 0.0090 0.21 . . - . - - - - - -
26 1.00 0.45 1.00 0.0135 0.0090 0.21 . - . - - - - - -
27 1.04 0.25 1.15 0.0050 0.0100 0.00 - - 0.0009 - - - - - -
28 1.04 0.85 0.75 0.0190 0.01!0 0.00 - - - - - - 0.0025 0.0015 - - -
29 1.05 0.25 1.15 0.0150 0.0070 0.00 - . - 0.050 - - - - - 0.011 -
30 1.05 0.25 l.l5 0.0150 0.0070 0.00 - . . 0.050 - - - - - - 0.011
31 1.06 0.65 0.85 0.0150 0.0030 0.00 - - . - - - - 0.0025 - -
32 1.10 0.45 0.35 0.0080 0.0080 0.00 - - - - - - - - -
33 1.10 0.45 0.35 0.0080 0.0080 0.00 - - - - - - - -
34 1.15 0.50 0.85 0.0180 0.0090 0.00 . - . - - - - - 0.0025 -
35 1.20 0.80 0.65 0.0150 0.0050 0.00 - - - - - - - - - 0.0200
- 59 -
[0133]
[Table 3]
Example of Position for observing Microstructure of bottom portion Surface hardness of bottom portion Ratio Result of Result of Special note for production Remark
invention microstructure and between fatigue test impact test method
measuring hardness surface performed on
hardness of foot-edge
foot-bottom portion (test
central temperature:
portion and 20°C)
Foot- Foot~ Middle Foot- Foot· Middle surface Fatigue Impact value
bottom edge portion bottom edge portion hardness of limit stress (J/cm2
)
central portion central portion HM (Hv) middle range of
portion portion HE (Hv) portion foot-bottom
HC (Hv) (HM/HC) central
portion
(MPa)
1 Depth of 1 mm Ut1der Pearlite Pearlite Pearlite 380 260 300 0801 215 22.0 Performing heat treatment Lower limit of
surface after hot rolling c
Depth of 5 l11m under Pearlite Pearlite Pearlite 375 260 305 . Controlllng cooling rate
surface
2 Depth ·or 1 tttm under Pearlite Pearlite Pearlite 460 280 350 0.781 230 17.0 Performing heat treatment Upper limit of
surface ·~ after hot rolling c
Depth of 5 mm under Pearlite Pearlite Pearlite 456 275 365 Controlling cooling rate
surface
3 Depth of 1 mm under Pearlite Pearlite Pearlite 400 285 325 0.824 220 210 Performing re-heat treatment Lower limit of
I
surface after hot rolling Si
Depth of 5 mm under Pearlite Pearlite Pearlite 395 280 330 Controlling cooling rate
surface
4 Depth of 1 mm under Pearlite Pearlite Pearlite 410 280 380 0.944 260 20.5 Performing re-heat treatment Upper limit of
surface after hot rolling Si
' Depth of 5 mm under Pearlite Pearlite Pearlite 400 I 27l 385 Controlling cooling rate
smfacc
5 Depth of 1 mm tUlder Pearlite Pearlite Pearlite 365 260 325 0.898 220 21.0 Cotttrolling re-heating Lower limit of
surface temperature Mn
Depth of 5 mm under Pearlite Pearlite Pearlite 364 260 330
surface
6 Depth of lmm under Pearlite Pearlite Pearlite 450 300 395 0.898 230 18.0 Controlling re-heating Upper limit of
surface temperature Mn
Depth of 5 mm under Pearlite Pearlite Pearlite 435 290 400
surface
7 Depth of lmm under Pearlite Pearlite Pearlite 430 295 385 0.894 225 16.5 Controlling finish hot rolling Upper limit of P
- 60 -
--
surface temperahrre
Depth of 5 mm under Pearlite Pearlite Pearlite 420 290 375
surface
8 Depth of 1 mm Ullder Pearlite Pearlite Pearlite 430 305 395 0.918 265 16.5 Controlling finish hot rolling Upper limit of S
surface temperature
Depth of 5 rum under Pearlite Pearlite Pearlite 425 295 390
surface
9 Depth of 1 mm under Pearlite Pearlite Pearlite 370 260 310 0.836 215 24.0 Performing heat treatment Addition of Cr
surface afier hot rolling
Depth of 5 mm under Pearlite Pearlite Pearlite 360 260 300 Controlling cooling rate
surface
10 Depth of 1 mm under Pearlite Pearlite Pearlite 370 260 360 0.986 270 24.0 Controlling finish hot rolling Addition of Cr
surface temperature + performing
Depth of 5 mm ullder Pearlite Pearlite Pearlite 360 260 360 heat treatment and cooling
surface after hot rolling
11 Depth of 1 mm under Pearlite Pearlite Pearlite 360 290 320 0.882 215 21.5 Controlling finish hot rolling Addition of Cu
surface tetnperahtre + pcrfonning
Depth of 5 rom under Pearlite Pearlite Pearlite 360 280 315 heat treatment and cooling
surface after hot rolling
12 Depth of 1 mm Wldcr Pearlite Pearlite Pearlite 420 300 335 0.796 230 20.0 Controlling finish hot rolling Addition of Cr
surface temperature +V
Depth of 5 111111 -.:Y,der Pearlite Pearlite Pearlite 415 295 330
surface
13 Depth of 1 mm under Pearlite Pearlite Pearlite 420 300 385 0.916 265 20.0 Controlling finish hot rolling Addition of Cr
surface temperature + performing +V
Depth of 5 mm under Pearlite Pearlite Pearlite 415 295 380 heat treatment and cooling
surface after hot rolling
14 Depth of 1 mm tmder Pearlite Pearlite Pearlite 380 265 325 0.860 220 220 Controlling re-heating Addition ofNi
surface temperature
Depth of 5 mm under Pearlite Pearlite Pearlite 370 260 320
surface
15 Depth of 1 rom tmder Pearlite Pearlite Pearlite 430 290 350 0.813 230 21.0 Controlling finish hot rolling None
surface temperature
Depth of 5 mm under Pearlite Pearlite Pearlite 425 285 345
surface
16 Depth of lmm under Pearlite Pearlite Pearlite 430 290 405 0.942 275 21.0 Controlling finish hot rolling None
surface temperature + performing
Depth of 5 mm under Pearlite Pearlite Pearlite 425 285 400 heat treatment and cooling
surface after hot rolling
17 Depth of 1 mm tmder Pearlite Pearlite Pearlite 445 300 420 0.944 285 19.0 Conttolllng finish hot rolling Addition of Co
surface tetnperahrre
Depth of 5 mm tlllder Pearlite Pearlite Pearlite 440 295 415
surface
- 61 -
[0134]
[Table 4]
Example of Position for observing Microstructure of bottom portion Surface hardness tlf bottom portion Ratio Result of Result of Special note for production Remark
invention microstructure and bet\veen fatigue test impact test mellwd
measuring hardness surface performed on
hardness of foot-edge
foot-bottom portion (test
; central temperature:
portio11 and 20°C)
Foot- Foot- Middle Foot· Fool· Middle surface Fatigue Impact value
bottom edge portion bottom edge portion hardness of limit stress (J/cm2
)
central portion central portion HM (Hv) middle range of
portion portion HE (Hv) portion foot-bottom
HC (Hv) (HM!HC) central
port-ion
(MPa)
18 Depth of 1 rum under Pearlite Pearlite Pearlite 460 310 405 0.885 230 18.0 Controlling finish hot rolling Addition of Cr
surface temperature
' Depth of 5 mm under Pearlite Pearlite Pearlite 455 300 405
surface I
19 Depth of l mm under Pearlite Pearlite Pearlite 460 I 310 440 0.951 285 18.0 Controlling finish hot rolling Addition of Cr
surface temperature + performing
Depth of 5 mm under Pearlite Pearlite Pearlite 455 300 430 heat treatment and
surface controlling cooling rate after
hot rolling
20 Depth of 1 mm under Pearlite Pearlite Pearlite 420 280 340 0.813 230 19.5 Performing heat treatment None
surface after hot rolling
Depth of 5 mm under Pearlite Pearlite Pearlite 410 275 335 Controlling cooling rate
surface
21 Depth of 1 mm under Pearlite Pearlite Pearlite 420 280 375 0.910 265 19.5 Controlling finish hot rolling None
surface temperature+ performing
Depth of5 111111 under Pearlite Pearlite Pearlite 410 275 380 heat treatment and
smface co1ttrollhtg cooling rate after
hot tolling
22 Depth of 1 mm under Pearlite Pearlite Pearlite 430 295 335 0.782 230 19.0 Controlling re-heating Addition of Mo
surface ' temperature
Depth of 5 mm under Pearlite Pearlite Pearlite 420 290 330
surface
23 Depth of 1111111 tffider Pearlite Pearlite Pearlite 435 290 370 0.860 235 18.5 Controlling finish hot rolling Addilion ofNb
surface - temperature +Ti
Depth of 5 111111 nnder Pearlite Pearlite Pearlite 42l 285 370
. 62 -
surface
24 Depth of 1 mm under Pearlite Pearlite Pearlite 435 290 4DO 0.924 280 18.5 Controlling finish hot rolling Addition ofNb
surface tcmperahtre + performing +Ti
Depth of 5 mm under Pearlite Pearlite Pearlite 425 285 395 heat treatment and
surface controlling cooling rate after
hot rolling
25 Depth of I mm under Pearlite Pearlite Pearlite 420 29() 350 0.837 230 18.0 Controlling finish hot rolling Addition of Cr
surface temperature
Depth of 5 mm ti'!tder Pearlite Pearlite Pearlite 410 285 345
surface
26 Depth of l nun under Pearlite Pearlite Peat! He 420 290 380 0.910 265 !R.O Control!ittg finish hot rolling Addition of Cr
surface tcmperahtre +performing
Depth of 5 rum under Pearlite Pearlite Pearlite 410 285 375 heat treatment and
surface controlling cooling rate after
hot rolling
27 Depth Of 1 mm Udder Pearlite Pearlite Pearlite 46.5 300 385 0.842 240 17.0 Performing heat treatment Addition of B
surface . after re~heating
Depth of 5 mm under Pearlite Pearlite Pearlite 450 295 385 Controlling cooling tate
surface
28 Depth of lmm under Pearlite Pearlite Pearlite 415 290 365 0.878 225 17.5 Performing heat treatment Addition of Mg
surface after hot rolling +Ca
Depth of 5 mm under Pearlite Pearlite Pearlite 405 280 355 Controlling cooling rate
surface
29 Depth of 1 mni under Penrlite Pearlite Pearlite 500 315 410 0.818 240 16.5 Controlling finish hot rolling Addition of V +
surface temperature N
Depth of .5 mm under Pearlite Pearlite Pearlite 490 305 400
surface
30 Depth of 1mm under Pearlite Pearlite Pearlite 500 31l 480 0.960 300 16.5 Controlling finish hot rolling Addition ofV +
surface temperature + performing N
Depth of .5 rum under Pearlite Pearlite Pearlite 490 305 470 heat treatment and
surface
. controlling cooling raie after
hot rolling
31 Depth of 1 rum under Pearlite Pearlite Pearlite 450 270 380 0.843 235 18.0 Performing heat treatment Addition of
surface after hot rolling REM
Depth of 5 mm under Pearlite Pearlite Pearlite 440 265 370 O::mtrolling cooling rate
surface
32 Depth of 1 mm tmdcr Pearlite Pearlite Pearlite 405 280 315 0.781 225 18.5 Performing beat treatment None
surface after hot rolling
Depth of 5 mm under Pearlite Pearlite Pearlite 395 275 310 Controlling cooling rate
surface
33 Depth of 1 mm under Pearlite Pearlite Pearlite 405 280 390 0.969 280 17.0 Controlling finish hot rolling None
surface temperature+ performing
Depth of 5 rum under Pearlite Pearlite Pearlite 395 275 385 heat treatment and
surface controlling cooling rate after
hot rolling I
. 63 .
34 Depth of 1 mm llltder Pearlite Pearlite Pearlite 475 300 360 0.761 235 17.5 Performing heat treatment Addition of Zr
surface after hot rolling
Depth of 5 mm under Pearlite Pearlite Pearlite 465 290 355 Cotttrolling cooling rate
surface
35 Depth of 1 rum u~der Pearlite Pearlite Pearlite 480 310 400 0.842 240 16.5 Controlling finish hot rolling Addition of AI
smface temperature
Depth of 5 rum under Pearlite Pearlite Pearlite 470 305 400
sutface
[0135]
[Table 5]
Chemical col:llposition (mass%)
Comparative
Example c Si Mn p s Cr Mo Co B Cu Ni v Nb Ti Mg Ca REM Zr N A1
1 0.70 0.25 1.00 0.0150 0.0120 0.00 . - - - - - - -
2 1lQ 0.25 1.00 0.0150 0.0120 ' 0.00 - - - - - - - -
3 O.SO 0.05 0.80 0.0180 0.0100 0.00 - - - - - - -
4 0.80 2.35 0.80 0.0180 0.0100 0.00 - - - - - - -
5 0.90 0.45 0.05 0.0120 0.0080 0.00 - . - - - - - - - -
6 0.90 0.45 2.50 0.0120 0.0080 0.00 - . - - - - - - - -
7 1.00 0.75 0.75 0.0300 0.0100 0.00 - . - - - - - - - - -
8 1.10 0.65 0.55 0.0120 0.0350 0.00 - . - - - - - - - - -
i 9 0.76 o.3s 0.85 0.0140 0.0130 0.22 - . - - - - - - - - -
10 0.77 0.60 0.75 0.0200 0.0200 0.00 . - - 0.20 - - - - - - -
11 0.80 0.50 1.35 0.0070 0.0150 0.00 - - . - - - - - - - -
12 1.10 0.45 0.35 0.0080 0.0080 0.00 - - - - - - - - -
13 0.90 0.40 0.65 0.0120 0.0180 0.65 - - - - -
14 0.90 0.50 1.10 0.0150 0.0120 0.00 I . - - - - - -
'
. 64 •
15 1.05 O.f5 1.15 0.0150 0.0070 0.00 . . 0.050 . . . 0.011 .
16 1.10 0.45 0.35 0.0080 0.0080 0.00 . . . . . . . . .
'
17 0.90 0.50 1.10 0.0150 0.0120 0.00 . . . . . . . .
18 1.00 0.45 1.00 0.0135 0.0090 0.21 . . . . . . . . .
19 0.76 0.15_ 0.85 0.0140 0,0130 0.22 . . . . . . . .
20 1.10 0.45 0.35 0.0080 0.0080 0.00 . . . . . .
'----
[0136]
[Table 6]
Comparative Position for Microstructure of' bottom oortion Surface hard:rtoss of bottom portion Ratio between surface hardness
Example observing Foot-bottom Foot-edge portion Middle portio11 Foot-bottom Foot-edge portion :Middle portion of foot-bottom central portion
microstructure and central portion centra! portion HC HE (Hv) HM(Hv) and surface hardness of middle
measuring hardness (Hv) portion (HM/HC)
1 Depth of 1 mm Pearlite + ~rQ- Pearlite + Qto~ Pearlite + pro- 345 240 300 0.881
under surface eutectoid ferrij:s;; cutectoid ferrite cutcctoid ferrite
Depth of5 mm Pearlite + Q!Q· Pearlite + Qto· PoarlitQ + Qto· 330 235 295
under surface eutectoid ferrite eutectoid ferrite eutectoid ferrite
2 Depth of 1 mm Pearlite + t:!rQ· Pearlite+ Qro- Pearlite + n:ro· 440 270 320 0.730
under surface eutectoid ferrite eutectoid cementite eutectoid cementite
Depth of5 mm Pearlite + Qro- Pearlite + t1to- Pearlite + Qro· 430 260 315
under surface eutectoid ferrite eutectoid cementite eutectotd cementite
3 Depth Of lmm Pearlite Pearlite + t1ro- Pearlite + nro- 390 265 330 0.851
under ilurface cutectoid cementite etttectoid cementite
Depth Of5 mm Pearlite Pearlite + t1ro- Pearlite + n:ro· 380 260 325
under surface eutectoid cementite eutectdid cementite
4 Depth of 1 mm Pearlite+ Pearlite Pearlite 540 330 450 0.836
under surface martensite
Depth of5 mm Pearlite+ Pearlite PMrlitc 530 325 445
under surface martensite
5 , Depth ~.::f 1 rum Pearlite Pearlite + tJto· Pearlite 355 250 300 0.871
under surface cutcctoid ferrite
Depth of5 mm Pearlite Pearlite + tlfO· Pearlite 345 245 310
under surface eutectoid ferrite
6 Depth ofl mm Pearlite+ Pearlite Pt1arlitc 525 330 420 0.798
under surface martensite
Depth of5 mm Pearlite+ Pearlite Pearlite 515 310 410
under surface martensite
- 65 -
7 Depth brl mm Pearlite Pearlite Po.;nrlitc 430 295 360 0.835
under surface
Depth of5 mm Pearlile Pearlite Pearlite 420 285 350
under surface
8 Depth of 1 mm Pearlite Pearlite Penrlite 430 305 345 0.806
under ~,Jrface
Depth of5 mm Pearlite Pearlite Pearlite 420 300 340
under surface
[0137]
[Table 7]
Comparative Position for Microstructure of bottom portion Surface hardness of bottom r;ortion Ratio between surface hardness
Example observing Foot-bottom Foot-edge portion Middle portion Foot·bottom Foot-edge portion Middle portion of foot-bottom central portion and
microstructure and central portion central portion HC HE (Hv) HM (Hv) surface hardness of middle
measuring hardness (Hv) portion (HM/HC)
9 Depth of 1 il1lll Pearlite Pearlite PearHte 370 250 310 0.836
under surface
Depth of 5 mn1 Pearlite Pearlite Pearlite 360 240 300
under surface
10 Depth of l mm Pearlite Pearlite Pearlite 345 290 320 0.934
under surface
Depth of 5 rum Pearlite Pear! He Pearlite 335 280 315
under .sutface
11 Depth of lmtu Pearlite Pearlite Pearlite 350 255 350 1.007
under"'surface
Depth Qf 5 mm Pearlite Pearlite Pearlite 340 245 345
under Surface
12 Depth of 1 mm Pearlite Pearlite Pearlite 405 250 315 0.776
under surface
Depth of 5 tutu Pearlite Pearlite Pearlite 400 240 310
under surface
13 ' Depth- u·r 1 mm Pearlite Pearlite Pearlite 520 310 405 0.786
~~cr surface
Depth o£5 mm Pearlite Pearlite Pearlite lli 300 405
under surface
14 Depth of 1 rum Pearlite Pearlite Pearlite 420 320 340 0.813
under surface
Depth of5 rum Pearlite Pearlite Pearlite 410 320 335
under surface
15 Depth of 1 mm Pearlite Pearlite Pearlite 530 330 410 0.768
under surface
Depth of 5 rum Pearlite Pearlite Pearlite 525 325 400
- 66 -
under surface
16 Depth of lmm Pearlite Pearlite Pearlite 505 280 315 0.619
undcr.'lurfacc
Depth of5 mm Pearlite Pearlite Pearlite 505 275 310
under surface
17 Depth of lmm Pearlite Pearlite Pearlite 420 280 435 1042
under surface
Depth o£5 mm Pearlite Pearlite Pearlite 410 275 430
tmdcr surface
18 Depth of! mm Pearlite Pearlit.c Pearlite 420 290 270 0.645
under surface
Depth of5 mm Pearlite Pearl he Pearlite 410 285 265
under surface
19 Depth of 1 mm Pearlite
I
Pearlite Pearlite 370 260 250 0.678
under surface
Depth of5 mm Pearlite
under surface
I Pearlite Pearlite 360 260 245
20 Depth of 1 mm Pearlite Pearlite Pearlite 405 280 425 1056
1U1der surface
Depth of 5 rum Pearlite Pearlite Peartite 395 275 420
tutder surface
[0138]
[Table 8]
Comparative Example Result of fatigue test Result of impact test performed on foot- Special note for production method Remark
edge portion (test temperature: 20°C)
Fatigue limit stress range of foot-bottom Impact value (J/cm2
)
central portion (MPa)
1 110 26.0 Performing heat treatment after hot rolling Lower limit of C
Generation ofpro-eutectoid ferrite Controlling cooling rate
2 135 7.8 Cdecrease in toughness) Performing heat ~treatment after hot rolling Upper limit or C
Generation ofpro-cutcctoid cementite Generation ofvro-eutectoid cementite Co11trolling cooling rate
3 140 8.0 (decrense_in tottghncss) Performing hoot treatment after re-heating Lower limit of Si
Generation of Qro-eutectoid cementite Generation of nro~eutectoid cementite Controlling cooling rate
4 22 14.0 (decrease in to-ughness) Performing heat treatment after re-heating Upper limit of Si
Generation of martensite in central nortion of Hardening or pearlite Controlling coolit1g rate
bottom oortion
5 ill 22,0 Controlling temperature of re-heating Lower limit of Mn
Generation ofyro-eutectoid ferrite in foot-edge
portion -
6 100 12.0 (decrease in toughness) Controlling temperature of re-heating Upper limit of Mn
Generation of martensite in central t,;Jortion of Bardeniug of pearlite
• 67 •
~
bottom :Qortion
7 145 9.0