STEEL RAIL AND METHOD OF MANUFACTURING THE SAME
5
Technical Field
[0001]
The present invention relates to a steel rail which is a steel rail used for a freight
railway for purposes of simultaneously enhancing the wear resistance and toughness of a
10 head portion.
Priority is claimed on Japanese Patent Application No. 2010-130164, filed on
June 07, 2010, the content of which is incorporated herein by reference.
Background Art
15 [0002]
With economic development, terrain in rugged natural environments that have
hitherto not been developed is being mined for natural resources such as coal.
Therefore, the track environment of a freight railway for transport of resources has
become significantly harsher, and thus there is demand of the rail for wear resistance, and
20 touglmess in cold regions, and the like at least as high as currently available. From this
background, there is demand for the development of a rail having wear resistance and
high toughness at least as high as the high-strength rail that is currently used.
[0003]
In order to improve the wear resistance of rail steel, rails as described below
25 were developed. The main characteristies of such rails are that in order to enhance wear
2
resistance, the carbon content in steel was increased, the volume ratio of a cementite
phase in pearlite lamellae was increased, and moreover hardness was controlled (for
example, refer to Patent Documents 1 and 2).
[0004]
5 In the technique disclosed in Patent Document 1, using hypereutectoid steel
(with higher than 0.85% to 1.20% of C), the volume ratio of cementite in the lamellae in
a pearlite structure is increased, thereby providing a rail having excellent wear resistance.
[0005]
In addition, in the technique disclosed in Patent Document 2, using
10 hypereutectoid steel (with higher than 0.85% to 1.20% of C), the volume ratio of
cementite in the lamellae in a pearlite structure is increased, and simultaneously, hardness
is controlled, thereby providing a rail having excellent wear resistance.
[0006]
In the techniques disclosed in Patent Documents I and 2, the volume ratio of the
15 cementite phase in the pearlite structure is increased by increasing the carbon content in
steel, and thus an increase in wear resistance to a certain level is achieved. However, in
such cases, the toughness of the pearlite structure itself is significantly degraded, and thus
there is a problem in that rail breakage is likely to occur.
[0007]
20 From this background, it was desired to provide a steel rail having excellent
wear resistance and toughness obtained by enhancing the wear resistance of a pearlite
structure and simultaneously enhancing toughness.
[0008]
In general, in order to increase the toughness of pearlite steel, it is said that
25 refinement (increasing the fineness) of a pearlile structure, specifically, refinement of the
grains of an austenite structure before pearlite transformation or refinement of a pearlite
block size is effective. In order to achieve the fine-grained austenite structure, a
reduction in rolling temperature and an increase in rolling reduction during hot rolling,
and moreover, heat treatment by low-temperature reheating after rail rolling, are
5 performed. In addition, in order to achieve the fine pearlite structure, acceleration of
pearlite transformation from the inside of austenite grains using transformation nuclei, or
the like is performe
[0009]
However, in the manufacture of rails, from the viewpoint of ensuring formability
10 during hot rolling, there are limitations on the reduction in rolling temperature and the
increase in rolling reduction, and thus sufficiently refinement of the austenite grains is
difficult to achieve. In addition, regarding the pearlite transformation from the inside of
the austenite grains using the transformation nuclei, there are problems in that controlling
the amount of transformation nuclei is difficult, the pearlite transformation from the
15 inside of the grains is not stabilized, and the like, preventing a sufficiently fine pearlite
structure from being achieved.
[0010]
From these problems, in order to fundamentally improve the toughness of a rail
having a pearlite structure, a method of performing low-temperature reheating after rail
20 rolling, and thereafter causing pearlite transformation through accelerated cooling,
thereby refinement of the pearlite structure has been used. However, in recent years,
there has been a progressive increase in the carbon content in rails in order to improve
wear resistance. In this case, there is a problem in that coarse carbides remain dissolved
in austenite grains during the low=temperature reheating heat treatment, and thus the
25 ductility or toughness of the pearlite structure is degraded after the accelerated cooling.
In addition , since the reheating is performed, there are economic problems such as high
manufacturing cost and low productivity.
[0011]
Here, there is demand for the development of a method of manufacturing a
5 high-carbon steel rail by ensuring formability during hot rolling and refinement of a
pearlite structure after the hot rolling . In order to solve the problems , methods of
manufacturing a high-carbon steel rail as described below have been developed. The
main characteristics of such rails are that in order to increase the fineness of a pearlite
structure, a property of austenite grains of high-carbon steel being more likely to
10 recrystallize at a relatively low temperature and at a small rolling reduction amount is
used. Accordingly, well-ordered fine grains are obtained by continuous rolling with a
small rolling reduction , thereby enhancing the ductility or toughness of pearlite steel (for
example, refer to Patent Documents 3, 4, and 5).
[0012]
15 In the technique disclosed in Patent Document 3, in finish rolling of a steel rail
having high-carbon steel , three or more continuous passes of hot rolling are performed
between predetermined interval time of rolling passes, thereby providing a high-ductility
and high-toughness rail.
[0013]
20 In addition, in the technique disclosed in Patent Document 4, in finish rolling of
a steel rail having high-carbon steel, two or more continuous passes of rolling are
performed between predetermined interval time of hot rolling passes, and moreover, after
performing continuous rolling, accelerated cooling is performed after the hot rolling,
thereby providing a high-wear-resistance and high-touglnress rail.
25 [0014]
5
Moreover, in the technique disclosed in Patent Document 5, in finish rolling of a
steel rail having high-carbon steel, cooling is performed between hot rolling passes, and
after performing continuous rolling, accelerated cooling is performed after the hot rolling,
thereby providing a high-wear-resistance and high-toughness rail.
5 [0015]
In the techniques disclosed in Patent Documents 3 to 5, by the temperature
during continuous hot rolling, and a combination of the number of rolling passes and
time between passes, refinement of the austenite structure to a certain level is achieved,
and thus a slight increase in toughness is acknowledged. However, the effect is not
10 acknowledged regarding fractures that occur from inclusions existing in steel as origins
or fractures that occur from a pearlite structure as an origin other than from inclusions as
origins, and toughness is not fundamentally enhanced.
Citation List
15 Patent Literature
[0016]
[Patent Document 1 ] Japanese Unexamined Patent Application, First
Publication No. H8=144016
[Patent Document 2] Japanese Unexamined Patent Application, First
20 Publication No. 148-246100
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. H7-173530
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2001-23/123 8
25 [Patent Document 5] Japanese Unexamined Patent Application, First
6
Publication No. 2002-22691.5
Summary of Invention
Technical Problem
[0017]
The present invention has been made taking the foregoing circumstances into
consideration, and an object thereof is to provide a steel rail having a head portion with
simultaneously enhanced wear resistance and toughness, required of a rail for a freight
railway in a rugged track environment.
10
Solution to Problem
[0018]
In order to accomplish the object to solve the problem, the present invention
employs the following measures.
15 (1) That is, according to an aspect of the present invention, there is provided a
steel rail including: by mass%, higher than 0.85% to 1.20% of C; 0.05% to 2.00% of Si;
0.05% to 0.50% of Mit; 0.05% to 0.60% of Cr; P'0.0150%; and the balance consisting of
Fe and inevitable impurities, wherein 97% or more of a head surface portion which is in a
range from a surface of a head corner portion and a head top portion as a starting point to
20 a depth of 10 mm has a pearlite structure, a Vickers hardness of the pearlite structure is
Hv320 to 500, and a CMn/FMn value which is a value obtained by dividing CMn [at.%]
that is a Mn concentration of a cementite phase in the pearlite structure by FMn [at.%]
that is a Mn concentration of a ferrite phase is equal to or higher than 1.0 and equal to or
less than 5.0.
25 Here, Hv represents a Vickers hardness specified in JIS Z2244. In addition,
7
at.% represents an atomic composition percentage.
[0019]
(2) In the aspect described in (1), further included are one kind or two or more
kinds selected from the group: by mass%, 0.0 1% to 0.50% of Mo; 0.005% to 0.50% of V;
5 0.001% to 0.050% of Nb; 0.01% to 1.00% of Co; 0.0001% to 0.0050% of f3; 0.01% to
1.00% of Cu; 0.01% to 1.00% of Ni; 0.0050% to 0.0500% of Ti; 0.0005% to 0.0200% of
Mg; 0.0005% to 0.0200% of Ca; 0.0001 % to 0.0100% of Zr; 0.0040% to 1.00% of Al;
and 0.0060% to 0,0200% of N.
[0020]
10 (3) According to another aspect of the present invention, there is a method of
manufacturing a steel rail which is a method of manufacturing the steel rail described in
(1) or (2). The method may employ a configuration including: performing first
accelerated cooling on a head portion of the steel rail at a temperature of equal to or
higher than an Art point immediately after hot rolling, or a head portion of the steel rail
15 reheated to a temperature of equal to or higher than the Act point+30°C for purposes of a
heat treatment, at a cooling rate of 4 to 15°C/sec from a temperature range of equal to or
higher than 750°C; stopping the first accelerated cooling at a time point when a
temperature of the head portion of the steel rail reaches 600°C to 450°C; controlling a
maximum temperature increase amount including transformation heat and recuperative
20 heat to be equal to or less than 50°C from an accelerated cooling stop temperature;
thereafter performing second accelerated cooling at a cooling rate of 0.5 to 2.0°C/see;
and stopping the second accelerated cooling at a time point when the temperature of the
head portion of the steel rail reaches 400°C or less.
8
Advantageous Effects of Invention
[0021]
According to the aspects described in (1) to (3), by controlling the structure,
hardness, and moreover CMn/FMn value of the head portion of the steel rail that has a
5 high-carbon pearlite structure to be in predetermined ranges, it is possible to
simultaneously enhance the wear resistance and toughness of the rail for a freight
railway,
Brief Description of Drawings
10 [0022]
FIG. I is a graph showing the relationship between Mn addition and impact
value in pearlite steel having a carbon content of 1.00%.
FIG. 2 is a graph showing the relationship between CMn/FMn value and impact
value in the pearlite steel having a carbon content of 1.00%.
15 FIG. 3(A) is a graph showing the relationship between accelerated cooling rate
(cooling rate of first accelerated cooling) after hot rolling or after reheating of the pearlite
steel having a carbon content of 1.00% and CMn/FMn value. FIG 3(B) is a graph
showing the relationship between accelerated cooling rate after hot rolling or after
reheating of the pearlite steel having a carbon content of 1.00% and impact value.
20 FIG. 4(A) is a graph showing the relationship between maximum temperature
increase amount after accelerated cooling after hot rolling or after reheating of the
pearlite steel having a carbon content of 1.00% and CMn/FMn value. FIG. 4(B) is a
graph showing the relationship between maximum temperature increase amount after
accelerated cooling after hot rolling or after reheating of the pearlite steel having a
25 carbon content of 1.00% and impact value.
9
FIG. 5(A) is a graph showing the relationship between accelerated cooling rate
(cooling rate of second accelerated cooling) after a temperature increase of the pearlite
steel having a carbon content of 1.00% and CMn/FMn value. FIG 5(B) is a graph
showing the relationship between accelerated cooling rate after a temperature increase of
5 the pearlite steel having a carbon content of 1.00% and impact value.
FIG 6 is an explanatory view of the head portion of a steel rail manufactured by
a method of manufacturing a steel rail according to an embodiment of the present
invention.
FIG. 7 is a diagram showing the head portion of the steel rail and is an
10 explanatory view showing a specimen collection position in wear tests shown in Tables
1=1 to 3=2.
FIG 8 is a side view showing the summary of the wear tests shown in Tables 1m1
to 3=2.
FIG. 9 is a diagram showing the head portion of the steel rail and is an
15 explanatory view showing a specimen collection position in impact tests shown in Tables
1=1 to 3=2.
FIG. 10 is a graph showing the relationship between carbon content and wear
amount of rail steels (reference numerals Al to A47) of the present invention and
comparative rail steels (reference numerals al, a3, a4, a5, a7, a8, and a12) shown in
20 Tables I=l to 2.
FIG 11 is a graph showing the relationship between carbon content and impact
value of the rail steels (reference numerals Al to A47) of the present invention and
comparative rail steels (reference numerals a
to 2.
t6, and a9 to a12) shown in Tables 1=1 9
25 FIG, 12 is a graph showing the relationship between carbon content and wear
10
amount of rail steels (reference numerals B 1 to B25) manufactured by the method of
manufacturing a steel rail according to the embodiment and rail steels (reference
numerals b 1, b3, b5 to b8, b 12, and b 13) manufactured by a comparative manufacturing
method, shown in Tables 3-1 and 3-2.
5 FIG. 13 is a graph showing the relationship between carbon content and impact
value of the rail steels (reference numerals B1 to B25) manufactured by the method of
manufacturing a steel rail according to the embodiment and rail steels (reference
numerals b2 to b6 and b9 to b 12) manufactured by the comparative manufacturing
method, shown in Tables 3-1 and 3-2.
10
Description of Embodiments
[0023]
Hereinafter, a steel rail having excellent wear resistance and toughness
according to an embodiment of the present invention will be described in detail. Here,
15 the present invention is not limited to the following description and it will be easily
understood by those skilled. in the art that the shapes and details thereof can be modified
in various forms without departing from the spirit and scope of the present invention.
Therefore, the present invention is not construed as being limited by the contents of
embodiments described as follows. Hereinafter, mass% that represents composition is
20 simply described as %.
1) C
[0024]
First, the inventors had examined a component system of steel that had an
adverse effect on the toughness of a rail. Using steels in which steel having a carbon
content of 1.00%C was contained as the base and the P content was changed, hot rolling
and_ heat treatment experiments were carried out under simulated hot rolling conditions
11
corresponding to a rail. In addition, the effect of the P content on an impact value was
examined by performing an impact test.
[0025]
As a result, it was confirmed that when the P content in a rail steel having a
5 pearlite structure with a hardness of Hv320 to 500 is reduced to 0.0150% or less, an
impact value is increased.
[0026]
Next, the inventors clarified the factors that control impact values in order to
further increase the impact value of a. rail, that is, to enhance toughness. In order to
10 investigate the origin of a fracture in a rail steel having a pearlite structure in which a
layered structure is composed of a ferrite phase and a cementite phase, specimens
subjected to the Charpy impact test were observed in detail. As a result, in many cases,
inclusions and the like were not acknowledged at the origin portions of the fracture, and
the origin was the pearlite structure,
15 [0027]
Moreover, the inventors had investigated the pearlite structure that becomes the
origin of the fracture in detail. As a result, it was confirmed that cracking occurs in the
cementite phase in the pearlite structure of the origin.
[0028]
20 Here, the inventors had investigated the relationship between the occurrence of
cracking of the cementite phase and components. Steels having a pearlite structure
which contains as the base steel that has a P content of equal to or less than 0.0150% and
a carbon content of 1.00% and which changes with the content of Mn added, were melted
for testing, and test rolling under simulated hot rolling conditions corresponding to the
25 manufacture of rails and heat treatment experiments were carried out. In addition, the
12
effect of the Mn addition on an impact value was examined by performing an impact test.
[0029]
FIG. 1 is a graph showing the relationship between Mn addition and impact
value . It was confirmed that when the Mn addition was reduced , an impact value was
5 increased , and when the Mn addition was equal to or less than 0.50%, an impact value
was significantly increased. Moreover, as a result of observing the pearlite structure at
the origin portion, it was confirmed that when the Mn addition is equal to or less than
0.50%, the number of cracks in the cementite phase was reduced.
[0030]
10 Next, the inventors had investigated the Mn content in the ferrite phase and the
cementite phase in the pearlite structure . As a result, it was confirmed that when the
Mn addition in the pearlite structure was reduced, the Mn content in the cementite phase
was particularly reduced.
[0031]
15 From these results, it became apparent that the toughness of the pearlite structure
had a correlation with the Mn addition, and when the Mn addition was reduced, the Mn
content in the cementite phase was reduced , cracking in the cementite phase at the origin
portion was suppressed , and consequently the toughness of the pearlite structure was
enhanced.
20 [0032]
Mn in the pearlite structure dissolves as a solid solution in the cementite and
ferrite phases . When the Mn concentration of the cementite phase that becomes an
origin of a fracture is suppressed, the Mn concentration of the ferrite phase is increased.
Here, the inventors had basically investigated the relationship between the balance of the
25 Mn concentrations of both the phases and toughness in a case where the Mn addition was
13
reduced.
[0033]
Steels having a pearlite structure which has a P content of equal to or less than
0.0150%, an Mn addition of 0.30%, and a carbon content of 1.00% were produced as
5 ingots in a laboratory, and test rolling under simulated hot rolling conditions
corresponding to the manufacture of rails and heat treatment experiments under various
conditions were carried out. In addition, by performing investigation of the Mn content
in the ferrite phase and the cementite phase and an impact test, the relationship between
impact value and the Mn content in the ferrite phase and the cementite phase was
10 investigated.
FIG. 2 shows the relationship between CMn/FMn value and impact value. It
was confirmed that in a case of pearlite structures having the same Mn addition, when the
CMn/FMn value was reduced, an impact value was increased, and when the CMn/FMn
value was equal to or less than 5.0, an impact value was significantly increased.
15 [0034]
From the result, it became apparent that by controlling the Mn addition of the
pearlite structure to be equal to or less than 0.50% and controlling the CMn/FMn value to
be equal to or less than 5.0, cracking in the cementite phase at the origin where an impact
was exerted was significantly reduced, and as a result, the toughness of the pearlite
20 structure was enhanced.
[0035]
Moreover, the inventors had examined a method of controlling the CMn/FMn
value in a case where the Mn addition of the pearlite structure was controlled to be equal
to or less than 0.50%. Steel having a pearlite structure in which a P content was equal
25 to or less than 0.0150%, an Mn addition of 0.30%, and a carbon content of 1.00% was
14
produced as ingots in a laboratory, and test rolling as simulated hot rolling for rails and
heat treatment experiments under various conditions were carried out. In addition, the
effect of heat treatment conditions on the relationship between CMn/FMn value and
impact value were investigated by performing investigation of CMn/FMn values and an
5 impact test.
[0036]
FIG. 3 (A) is a graph showing the relationship between accelerated cooling rate
after hot rolling or after reheating and CMn/FMn value.
FIG. 3 (B) is a graph showing the relationship between accelerated cooling rate
10 after hot rolling or after reheating and impact value.
[0037]
FIG. 4(A) is a graph showing the relationship between maximum temperature
increase amount after accelerated cooling and CMn/FMn value.
FIG. 4(B) is a graph showing the relationship between maximum temperature
15 increase amount after accelerated cooling and impact value.
[0038]
FIG 5 (A) is a graph showing the relationship between accelerated cooling rate
after a temperature increase and CMn/FMn value.
FIG 5(B) is a graph showing the relationship between accelerated cooling rate
20 after a temperature increase and impact value .
In addition , manufacturing conditions of the base of rail steels shown in FIGS. 3
to 5 are as follows, and regarding the base manufacturing conditions, manufacturing was
performed by changing only the conditions to be evaluated.
[Cooling conditions after hot rolling and reheating]
25 Cooling start temperature : 800°C, cooling rate : 7°C/sec,
15
Cooling stop temperature: 500°C, maximum temperature increase amount: 30°C
[Cooling conditions after temperature increase]
Cooling start temperature: 530°C, cooling rate: 1.0°C/sec,
Cooling stop temperature: 3 50°C
5 [0039]
For example, regarding the relationship between accelerated cooling rate after
hot rolling or after reheating and CMn/FMn value shown in FIG. 3, manufacturing in a
condition in which only the accelerated cooling rate after hot rolling or after reheating
was changed under the base manufacturing conditions was cited.
10 [0040]
As a result, it became apparent that the CMn/FMn value was significantly
changed by (1) an accelerated cooling rate after hot rolling or after reheating, (2) the
maximum temperature increase amount after accelerated cooling, and (3) an accelerated
cooling rate after a temperature increase. In addition, it was found that by controlling
15 the cooling rate and the temperature increase amount in constant ranges, an increase in
the concentration of Mn in the cementite phase was suppressed, the CMn/FMn value was
reduced, and cracking in the cementite phase in the pearlite structure at the origin portion
was consequently suppressed, resulting in a significant increase in impact value.
[0041]
20 That is, according to this embodiment, by controlling the structure, hardness,
Mn addition, and CMn/FMn value of the head portion of a steel rail that has a
high-carbon pearlite structure to be in constant ranges and by performing appropriate
heat treatments on the rail head portion, it is possible to simultaneously enhance the wear
resistance and toughness of the rail for a freight railway.
25 [0042]
16
Next, the reason for limitation in the present invention will be described in
detail.
[0043]
(1) Reason for Limitation of Chemical Components of Steel
5 The reason that the chemical components of steel in the steel rail of this
embodiment are limited to the above-described numerical ranges will be described in
detail.
[0044]
C is an element effective in accelerating pearlite transformation and ensuring
10 wear resistance. When the C content is less than 0.85%, minimum strength or wear
resistance required of a rail may not be maintained in this component system. In
addition, when the C content exceeds 1.20%, a large amount of coarse proaeutectoid
cementite structure is generated, and thus wear resistance or toughness is degraded.
Therefore, a C addition is limited to higher than 0.85% to 1.20%. In addition, in order
15 to enhance wear resistance and toughness, it is more preferable that the C content be
0.90% to 1.10%.
[0045]
Si is an essential component as a deoxidizing material. In addition, Si
increases the hardness (strength) of the rail head portion through solid solution
20 strengthening in the ferrite phase in the pearlite structure, and thus enhances wear
resistance. Moreover, Si is an element that suppresses the generation of a pro-cutectoid
cementite structure in hypereutectoid steel and thus suppresses the degradation of
toughness. However, when the Si content is less than 0.05%, those effects may not be
sufficiently expected. In addition, when the Si content exceeds 2.00%, many surface
25 defects are generated during hot rolling or oxides are generated, resulting in the
17
degradation of weldability. Moreover, hardenability significantly increases, and thus a
martensite structure which is harmful to the wear resistance or toughness of the rail is
more likely to be generated. Therefore, the Si addition is limited to 0.05% to 2.00%.
In addition, in order to increase the hardness (strength) of the rail head portion and
5 suppress the generation of the martensite structure which is harmful to wear resistance or
toughness, it is more preferable that the Si content be 0.10% to 1.30%.
[0046]
Mn is an element that increases hardenability and thus increases the fineness of a
pearlite lamellar spacing, thereby ensuring the hardness of the pearlite structure and
10 enhancing wear resistance. However, when the Mn content is less than 0.05%, those
effects are small, and it is difficult to ensure wear resistance that is needed for the rail.
In addition, when the Mn content exceeds 0.50%, the Mn concentration of the cementite
phase in the pearlite structure is increased, cracking in the cementite phase of the fracture
origin portion is exacerbated, resulting in a significant degradation in the toughness of
15 the pearlite structure. Therefore, the Mn addition is limited to 0.05% to 0.50%. In
addition, in order to suppress cracking in the cementite phase and the hardness of the
pearlite structure, it is more preferable that the Mn content be 0.10% to 0.45%.
[0047]
Cr is an element that increases an equilibrium transformation temperature and
20 consequently increases the fineness of the lamellar spacing of the pearlite structure,
thereby contributing to an increase in hardness (strength). Simultaneously, Cr
strengthens a cementite phase and thus enhances the hardness (strength) of the pearlite
structure, thereby enhancing the wear resistance of the pearlite structure. However,
when the Cr content is less than 0.05%, those effects are small, and an effect of
25 enhancing the hardness of the rail steel may not be completely exhibited. In addition,
18
when an excessive addition is performed to cause the Cr content to be higher than 0.60%,
a. bainite structure which is harmful to the wear resistance of the rail is more likely to be
generated. In addition, hardenability is increased, and thus the martensite structure
which is harmful to the wear resistance or toughness of the rail is more likely to be
5 generated. Therefore, the Cr addition is limited to 0.05% to 0.60%. In addition, in
order to enhance the hardness of the rail steel and suppress the generation of the bainite
structure or the martensite structure which is harmful to wear resistance or toughness, it
is more preferable that the Cr content be 0.10% to 0.40%.
[0048]
10 P is an element that is inevitably contained in steel. There is a correlation
between the P content and toughness. When the P content is increased, the pearlite
structure becomes embrittled due to the embrittlement of the ferrite phase, and thus
brittle fracture, that is, rail damage is more likely to occur. Therefore, in order to
enhance toughness, it is preferable that the P content be low. As a result of checking the
15 correlation between impact value and P content in a laboratory, it was confirmed that
when the P content was reduced to 0.0 150% or less, the embrittlem _ent of the ferrite phase
which was the origin of a fracture was suppressed, and thus an impact value was
significantly enhanced. From this result, the P content i s limited to be equal to or less
than 0.0150%. In addition , the lower limit of the P content is not limited . However, in
20 consideration of dephosphorizing performance in a refining process , it is thought that
about 0.0020% is the limit of the P content during actual manufacturing.
[00/1.9]
In addition, a treatment of reducing the P content not only causes an increase in
refining cost but also degrades productivity . Here, in consideration of economic
25 efficiency and in order to stably increase the impact value, it is preferable that the P
19
content be 0.0030% to 0.0100%.
[0050]
In addition, to the rail manufactured of the component composition described
above, elements Mo, V, Nb, Co, B, Cu, Ni, Ti, Ca, Mg, Zr, Al, and N may be added as
5 necessary for purposes of enhancing the hardness (strength) of the pearlite structure, that
is, enhancing wear resistance, furthermore, enhancing toughness, preventing a welding
heat-affected zone from softening, and controlling a cross-sectional hardness distribution
of the inside of the rail head portion.
[0051]
10 Here, Mo increases the equilibrium transformation point of pearlite and mainly
increases the fineness of the pearlite lamellar spacing, thereby enhancing the hardness of
the pearlite structure. V and Nb suppress the growth of austenite grains by carbides and
nitrides generated during hot rolling and a cooling process thereafter, and enhance the
toughness and hardness of the pearlite structure by precipitation hardening. In addition,
15 V and Nb stably generate carbides and nitrides during reheating and thus prevent a
heat-affected zone of a welding joint from softening. Co increases the fineness of the
lamellar structure or ferrite grain size of a wearing surface, thereby increasing the wear
resistance of the pearlite structure. B reduces the cooling rate dependence of a pearlite
transformation temperature, thereby uniformizing the hardness distribution of the rail
20 head portion. Cu dissolves as a solid solution into ferrite in the ferrite structure or the
pearlite structure, thereby increasing the hardness of the pearlite structure. Ni enhances
the toughness and hardness of the ferrite structure or the pearlite structure and
simultaneously prevents the heat-affected zone of the welding joint from softening. Ti
increases the fineness of the structure of the heat-affected zone and thus prevents the
25 embrittlement of the welding joint portion. Ca and Mg increase the fineness of the
20
5
austenite grains during rail rolling and simultaneously accelerate pearlite transformation,
thereby enhancing the toughness of the pearlite structure. Zr increases the equiaxial
crystallization rate of a solidified structure and suppresses the formation of a segregation
zone of the center portion of a slab or bloom, thereby reducing the thickness of the
pro-eutectoid cementite structure and enhancing the toughness of the pearlite structure.
Al moves a eutectoid transformation temperature to a higher temperature side and thus
increases the hardness of the pearlite structure. N accelerates pearlite transformation
due to segregation at austenite grain boundaries and increases the fineness of a pearlite
block size, thereby enhancing toughness. The effects of each of the elements are
10 described above and are the main purpose of addition.
[0052]
The reason for the limitation of such components will now be described in
detail.
Mo is an element that increases the equilibrium transformation temperature like
15 Cr and consequently increases the fineness of the lamellar spacing of the pearlite
structure, thereby increasing the hardness of the pearlite structure and enhancing the wear
resistance of the rail. However, when a Mo content is less than 0.01%, those effects are
small, and an effect of enhancing the hardness of the rail steel is not exhibited at all. In
addition, when an excessive addition is performed to cause a Mo content to be higher
20 than 0.50%, a transformation rate is significantly reduced, and thus the bainite structure
which is harmful to the wear resistance of the rail is more likely to be generated. In
addition, the martensite structure which is harmful to the toughness of the rail is
generated in the pearlite structure. Therefore, a Mo addition is limited to 0.01% to
0.50%.
25 [0053]
21
V is an element that precipitates as V carbides or V nitrides during typical hot
rolling or heat treatment performed at a high temperature and increases the fineness of
austenite grains due to a pinning effect, thereby enhancing the toughness of the pearlite
structure. Moreover, V is an element that increases the hardness (strength) of the
5 pearlite structure through precipitation hardening by the V carbides and V nitrides
generated during the cooling process after the hot rolling, thereby enhancing the wear
resistance of the pearlite structure. In addition, V is an element that generates V
carbides or V nitrides in a relatively high temperature range in a heat-affected zone that is
reheated in a temperature range of equal to or less than an Ac 1 point, and is thus effective
10 in preventing the heat-affected zone of the welding joint from softening. However,
when a V content is less than 0.005%, those effects may not be sufficiently expected, and
the enhancement of the pearlite structure in the toughness or hardness (strength) is not
acknowledged. In addition, when a V content exceeds 0.50%, the precipitation
hardening of V carbides or V nitrides excessively occurs, and thus the pearlite structure
15 becomes embrittled, thereby degrading the toughness of the rail. Accordingly, a V
addition is limited to 0.005% to 0.50%.
[0054]
Like V, Nb is an element that increases the fineness of austenite grains due to the
pinning effect of Nb carbides or Nb nitrides in a case where typical hot rolling or heat
20 treatment performed at a high temperature is performed and thus enhances the toughness
of the pearlite structure. Moreover, Nb is an element that increases the hardness
(strength) of the pearlite structure through precipitation hardening by Nb carbides and Nb
nitrides generated during a cooling process after hot rolling, thereby enhancing the wear
resistance of the pearlite structure. In addition, Nb is an element that stably generates
25 Nb carbides or Nb nitrides from a low temperature range to a high temperature range in
22
the heat-affected zone that is reheated in a temperature range of equal to or less than the
Act point, and is thus effective in preventing the heat-affected zone of the welding joint
from softening. However, when the Nb content is less than 0.001%, those effects may
not be expected, and the enhancement of the pearlite structure in the toughness or
5 hardness (strength) is not acknowledged. In addition, when the Nb content exceeds
0.050%, the precipitation hardening of the Nb carbides or Nb nitrides excessively occurs,
and thus the pearlite structure becomes embrittled, thereby degrading the toughness of
the rail. Therefore, the Nb addition is limited to 0.001% to 0.050%.
[0055]
10 Co is an element. that dissolves as a solid solution into the ferrite in the pearlite
structure and further increases the fineness of the ferrite in the pearlite structu.ire, thereby
enhancing wear resistance. However, when a Co content is less than 0.01%, refinement
of a ferrite in the pearlite structure may not be achieved, and thus the effect of enhancing
wear resistance may not be expected. In addition, when the Co content exceeds 1.00%,
15 those effects are saturated, and thus refinement of the ferrite in the pearlite structure
according to the addition content may not be achieved. In addition, economic efficiency
is reduced due to an increase in costs caused by adding alloys. Therefore, a Co addition
is limited to 0.0 1% to 1.00%.
[0056]
20 B is an element that forms iron-borocarbides (Fe23(CB)6) in austenite grain
boundaries, accelerates pearlite transformation, and thus reduces the cooling rate
dependence of the pearlite transformation temperature. Accordingly, B imparts a more
uniform hardness distribution from a head surface to the inside and thus increases the
service life of the rail. However, when a B content is less than 0.0001%, those effects
25 are not sufficient, and the improvement of the hardness distribution of the rail head
23
portion is not acknowledged, In addition, when a B content exceeds 0.0050%, coarse
iron-borocarbides are generated, and thus brittle fracture is exacerbated, resulting in the
degradation of the toughness of the rail. Therefore, a B addition is limited to 0.0001%
to 0.0050%.
5 [0057]
Cu is an element that dissolves as a solid solution into ferrite in the pearlite
structure and enhances the hardness (strength) of the pearlite structure through solid
solution strengthening, thereby enhancing the wear resistance of the pearlite structure.
However, when a Cu content is less than 0.01%, those effects may not be expected. In
10 addition, when the Cu content exceeds 1.00%, due to a significant increase in
hardenability, the martensite structure which is harmful to the toughness of the pearlite
structure is generated, resulting in the degradation of the toughness of the rail.
Therefore, a Cu content is limited to 0.01% to 1.00%.
[0058]
15 Ni is an element that enhances the toughness of the pearlite structure and
simultaneously increases the hardness (strength) thereof through solid solution
strengthening, thereby enhancing the wear resistance of the pearlite structure. Moreover,
Ni is an element that finely precipitates as an intermetallie compound of Ni3Ti with Ti at
the welding heat-affected zone and suppresses softening through precipitation hardening.
20 In addition, Ni is an element that suppresses the embrittlement of grain boundaries of
steel having Cu added. However, when the Ni content is less than 0.01%, those effects
are significantly small. In addition, when the Ni content exceeds 1.00%, the martensite
structure is generated in the pearlite structure due to the significant increase in
hardenability, resulting in the degradation of the toughness of the rail. Therefore, the Ni
25 content is limited to 0.01% to 1.00%.
24
[0059]
Ti is an element that precipitates as Ti carbides or Ti nitrides in a case where
typical hot rolling or heat treatment performed at a high temperature is performed and
increases the fineness of austenite grains due to the pinning effect, thereby being
5 effective in enhancing the toughness of the pearlite structure. Moreover, Ti is an
element that increases the hardness (strength) of the pearlite structure through
precipitation hardening by the Ti carbides and Ti nitrides generated during a cooling
process after the hot rolling, thereby enhancing the wear resistance of the pearlite
structure. In addition, Ti is a component that increases the fineness of the structure of
10 the heat-affected zone heated to an austenite range by using properties of the Ti carbides
and Ti nitrides, which precipitate during reheating for welding, not dissolving, and is thus
effective in preventing the ernbrittlement of the welding joint portion. However, when a
Ti content is smaller than 0.0050%, those effects are small. In addition, when a. Ti
content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are generated, and thus
15 brittle fracture is exacerbated, resulting in the degradation of the toughness of the rail.
Therefore, a Ti addition is limited to 0.0050% to 0.0500%.
[0060]
Mg is an element that is bonded to 0, S, Al, or the like and forms fine oxides,
suppresses the growth of crystal grains during reheating in rail rolling, and thus increases
20 the fineness of the austenite grains, thereby enhancing the toughness of the pearlite
structure. Moreover, Mg contributes to the occurrence of pearlite transformation
because MgS causes MnS to be finely distributed and thus nuclei of ferrite or cementite
form in the periphery of MnS. As a result, the fineness of the block size of pearlite is
increased, thereby enhancing the toughness of the pearlite structure. However, when
25 the Mg content is less than 0.0005%, those effects are weak. When the Mg content
25
exceeds 0.0200%, coarse oxides of Mg are generated, and thus brittle fracture is
exacerbated, resulting in the degradation of the toughness of the rail. Therefore, the Ml
content is limited to 0.0005% to 0.0200%.
[0061]
5 Ca is strongly bonded to S and forms sulfide as CaS. CaS causes MnS to be
finely distributed and causes a dilute zone of Mn to form in the periphery of MnS,
thereby contributing to the occurrence of pearlite transformation. As a result, the
fineness of the block size of pearlite is increased, so that the toughness of the pearlite
structure can be enhanced. However, when the Ca content is less than 0.0005%, those
10 effects are weak. When the Ca content exceeds 0.0200%, coarse oxides of Ca are
generated, and thus brittle fracture is exacerbated, resulting in the degradation of the
toughness of the rail. Therefore, the Ca content is limited to 0.0005% to 0.0200%.
[0062]
Zr increases the equiaxial crystallization rate of a solidified structure because a
15 Zr02 inclusion has good lattice matching with 7-Fe and thus the Zr02 inclusion becomes
a solidification nucleus of a high-carbon rail steel which is a 7-phase solidification. As
a result, the formation of a segregation zone of the center portion of a slab or bloom is
suppressed, thereby suppressing the generation of the martensite or pro-eutectoid
cementite structure generated at the rail segregation portion. However, when the Zr
20 content is less than 0.0001%, the number of Zr02-based inclusions is small, and thus a
sufficient action as a solidification nucleus is not exhibited. As a result, a martensite or
promeutectoid cementite structure is generated at the segregation portion, and thus the
toughness of the rail is degraded. In addition, when the Zr content exceeds 0.2000%, a
large amount of coarse Zr-based inclusions is generated, and thus brittle fracture is
25 exacerbated, resulting in the degradation of the toughness of the rail. Therefore, the Zr
26
content is limited to 0.0001 % to 0.2000%
[0063]
Al is an effective component as a deoxidizing material. In addition, Al is an
element that moves the eutectoid transformation temperature to a higher temperature side
5 and thus contributes to an increase in the hardness (strength) of the pearlite structure,
thereby enhancing the wear resistance of the pearlite structure. However, when the Al
content is less than 0.0040%, those effects are weak. In addition, when the Al content
exceeds 1.00%, it is difficult to cause Al to dissolve as a solid solution in steel, and thus
coarse alumina-based inclusions are generated. In addition, the coarse precipitates
10 become the origins of fatigue damage, and thus brittle fracture is exacerbated, resulting in
the degradation of the toughness of the rail. Moreover, oxides are generated during
welding, so that weldability is significantly degraded. Therefore, an Al addition is
limited to 0.0040% to 1,00%.
[0064]
15 N segregates at austenite grain boundaries and thus accelerates pearlite
transformation from the austenite grain boundaries. In addition, N mainly increases the
fineness of the pearlite block size, thereby enhancing toughness. In addition,
precipitation of VN or A1N is accelerated by simultaneously adding V and Al.
Therefore, in a case where typical hot rolling or heat treatment performed at a high
20 temperature is performed, the fineness of austenite grains are increased due to the
pinning effect of VN or A1N, thereby enhancing the tougluress of the pearlite structure.
However, when the N content is less than 0.0050%, those effects are weak. When the N
content exceeds 0.0200%, it is difficult for N to dissolve as a solid solution in steel,
bubbles that become the origins of fatigue damage are generated, and thus brittle fracture
25 is exacerbated, resulting in the degradation of the toughness of the rail. Therefore, the
27
N content is limited to 0.0050% to 0.0200%. The rail steel having the component
composition described above may be manufactured as ingots in a typical melting furnace
such as a converter furnace or an electric furnace, and the melted steel may be
manufactured as a rail by ingot casting, and blooming or continuous casting and father
5 by hot rolling.
[0065]
(2) Reason for Limitation of Metallic Structure
The reason that the metallic structure of a rail head surface portion in the steel
rail of the present invention is limited to pearlite will be described in detail.
10 [0066]
When the pro-eutectoid ferrite structure, the pro-eutectoid cementite structure,
the bainite structure, and the martensite structure are mixed with the pearlite structure,
fine brittle cracking occurs in the pro-eutectoid cementite structure and the martensite
structure having relatively low toughnesses, resulting in degradation of the toughness of
15 the rail. In addition, when the pro-eutectoid ferrite structure and the bainite structure
having relatively low hardnesses are mixed with the pearlite structure, wear is accelerated,
resulting in the degradation of the wear resistance of the rail. Therefore, for purposes of
enhancing wear resistance and toughness, a pearlite structure is preferable as the metallic
structure of the rail head surface portion. Therefore, the metallic structure of the rail
20 head surface portion is limited to the pearlite structure.
[0067]
In addition, it is preferable that the metallic structure of the rail according to this
embodiment be a pearlite single phase structure according to the above limitation.
However, depending on the component system of the rail and the heat treatment
25 manufacturing method, a small amount of the pro-eutectoid ferrite structure, the
28
pro-eutectoid cementite structure, the bainite structure, or the martensite structure which
has an area ratio of less than 3% is incorporated into the pearlite structure. However,
even though such a structure is incorporated, when the area ratio thereof is less than 3%,
the structure does not have a significant adverse effect on the wear resistance or
5 toughness of the rail head portion. Therefore, a structure other than the pearlite
structure, such as the pro-eutectoid ferrite structure, the pro-eutectoid cementite structure,
the bainite structure, or the martensite structure may bemixed with the structure of the
steel rail having excellent wear resistance and toughness as long as the area ratio of the
structure is less than 3%, that is, the structure is small in amount.
10 [0068]
In other words, 97% or higher of the metallic structure of the rail head surface
portion according to this embodiment may be the pearlite structure. In order to
sufficiently ensure the wear resistance or toughness needed for the rail, it is more
preferable that 99% or higher of the metallic structure of the head surface portion be the
15 pearlite structure. In addition, in the Microstructure column in Tables 1-1 to 3-2, a
small amount designates less than 3%.
Specifically, the ratio of the metallic structure is the value of an area ratio in a
case where a position at a depth of 4 mm from the surface of the rail head surface portion
and the position is observed using a microscope. The measurement method is as
20 described below.
Pretreatment: after rail cutting, polishing of a transverse cross-section.
Etching: 3% Nital
Observation machine: optical microscope.
Observation position: a position at a depth of 4 mm from the surface of the rail
25 head surface portion.
29
* Specific positions of the rail head surface portion are as indicated in FIG. 6.
Observation count: 10 or more points.
Structure determination method: each structure of pearlite, bainite, martensite,
pro-eutectoid ferrite, and pro-eutectoid cementite was determined through taking
5 photographs of the structures and detailed observation.
Ratio calculation: calculation of area ratio through image analysis.
[0069]
(3) Necessary Range of Pearlite Structure
Next, the reason that the necessary range of the pearlite structure for the rail
10 head portion of the steel rail of the present invention is limited to the head surface portion
of the rail steel will be described.
[0070]
FIG. 6 shows a diagram in a case where the steel rail having excellent wear
resistance and toughness according to this embodiment is viewed in a cross-section
15 perpendicular to the longitudinal direction thereof. A rail head portion 3 includes a head
top portion 1 and head corner portions 2 positioned at both ends of the head top portion 1.
One of the head corner portions 2 is a gauge corner (G C.) portion that mainly comes into
contact with wheels.
[0071]
20 A range from the surface of the head corner portions 2 and the head top portion
1 as a starting point to a depth of 10 mm is called a head surface portion (reference
numeral 3a, solid line portion). In addition, a range from the surface of the head corner
portions 2 and the head top portion 1 as the starting point to a depth of 20 mm denoted by
reference numeral 3b (dotted line portion).
25 [0072]
30
As shown in FIG. 6, when the pearlite structure is disposed in the head surface
portion (reference numeral 3a) in the range from the surface of the head corner portions 2
and the head top portion 1 as the starting point to a depth of 10 mm, wear due to contact
with wheels is suppressed, and thus the enhancement of the wear resistance of the rail is
5 achieved. On the other hand, in a case where the pearlite structure is disposed in a
range of less than 10 mm, the suppression of wear due to contact with wheels is not
sufficiently achieved, and the service life of the rail is reduced. Therefore, a necessary
depth for the pearlite structure is limited to the head surface portion having a depth of 10
mm from the surface of the head corner portions 2 and the head top portion 1 as the
10 starting point.
[0073]
In addition, it is more preferable that the pearlite structure be disposed in the
range 3b from the surface of the head corner portions 2 and the head top portion 1 as the
starting point to a depth of 20 mm, that is, at least in the dotted line portion in FIG. 1.
15 Accordingly, wear resistance in a case where the rail head portion is worn down to the
inner portion due to contact with wheels may further be enhanced, and thus the
enhancement of the service life of the rail is achieved.
[0074]
It is preferable that the pearlite structure be disposed in the vicinity of the
20 surface of the rail head portion 3 where wheels and the rail mainly come into contact
with each other, and in terms of wear resistance, the other portions may have a metallic
structure other than the pearlite structure.
[0075]
(4) Reason for Limitation of Hardness of Pearlite Structure of Head Surface Portion
25 Next, the reason that the hardness of the pearlite structure of the rail head
31
surface portion in the steel rail of this embodiment is limited to a range of Hv320 to 500
will be described.
[0076]
In this component system, when the hardness of the pearlite structure is less than
5 Hv320, the wear resistance of the rail head surface portion is degraded, resulting in a
reduction in the service life of the rail. In addition, when the hardness of the pearlite
structure exceeds Hv500, fine brittle cracking is more likely to occur in the pearlite
structure, resulting in the degradation of the toughness of the rail. Therefore, the
hardness of the pearlite structure is limited to the range of Hv320 to 500.
10 [00771
In addition, as a method of obtaining the pearlite structure having a hardness of
Hv320 to 500 in the rail head portion, as described later, accelerated cooling is preferably
performed on the rail head portion at 750°C or higher after hot rolling or after reheating.
[0078]
15 Specifically, the hardness of the head portion of the rail of this embodiment is a
value obtained when a position at a depth of 4 mm from the surface of the rail head
surface portion is measured by a Vickers hardness tester, The measurement method is
as described below.
Pretreatment: after rail cutting, polishing of a transverse cross-section.
20 Measurement method: measurement based on JIS Z 2244.
Measurer: Vickers hardness tester (a load of 98N).
Measurement point: a position at a depth of 4 mm from the surface of the rail
head surface portion
* Specific position of the rail head surface portion is as indicated in FIG. 6.
25 Measure count: it is preferable that 5 or more points be measured and the
3
average value thereof is used as a representative value of the steel rail.
[0079]
(5) Reason for Limitation of CMn/FMn Value in Pearlite Structure
[0080]
5 Next, the reason that the CMn/FMn value in the pearlite structure in the steel rail
of the present invention is limited to 5.0 or less will be described.
[0081]
When the CMn/FMn value in the pearlite structure is reduced, the Mn
concentration in the cementite phase is reduced. As a result, the toughness of the
10 cementite phase is enhanced, and thus cracking in the cementite phase at an origin that
receives an impact is reduced. Asa result of performing a laboratory test in detail, it
was confirmed that when the CMn/FMn value was controlled to be equal to or less than
5.0, cracking in the cementite phase at the origin that received an impact was
significantly reduced, and thus an impact value was significantly enhanced. Therefore,
15 the CMn/FMn value is limited to 5.0 or less. In addition, in consideration of a range of
a heat treatment condition on the premise that the pearlite structure is ensured, it is
thought that the limit of the CMn/FMn value is about 1.0 when a rail is actually
manufactured.
[0082]
20 To measure the Mn concentration of the cementite phase (CMn) and the Mn
concentration of the ferrite phase (FMn) in the pearlite structure of the rail of this
embodiment, a 3D atom probe (3DAP) method was used. The measurement method is
as described below.
Specimen collection position: a position of 4 rim from the surface of the rail
25 head surface portion
33
Pretreatment: a needle specimen is processed according to an FIB (focused ion
beam) method (101.rmx 10μmx 1001im)
Measurer: 3D atom probe (3 DAP) method
Measurement method:
Component analysis of metallic ions emitted by voltage application
using a coordinate detector
Ion flight time: kind of element, Coordinates: 3D position
Voltage: DC, Pulse (pulse ratio of 20% or higher)
Specimen Temperature: 40K or less
10 Measurement count: 5 or more points are measured and the average value
thereof is used as a representative value.
[0083]
(6) Heat Treatment Condition
First, the reason that the temperature of the head portion of the rail at which
15 accelerated cooling is started is limited to 750°C or higher will be described.
[0084]
When the temperature of the head portion is less than 750°C, a pearlite structure
is generated before accelerated cooling, and controlling the hardness of the head surface
portion by heat treatment becomes impossible, and thus a predetermined hardness is not
20 obtained. In addition, in steel with a high carbon content, a proaeutectoid cementite
structure is generated, and thus the pearlite structure becomes embrittled, resulting in the
degradation of the toughness of the rail. Therefore, the temperature of the head portion
of the steel rail at which accelerated cooling is performed is limited to 750°C or higher.
Next, in a method of performing accelerated cooling on the rail head portion at a
34
cooling rate of 4 to 15°C/sec from a temperature range of equal to or higher than 750°C
and stopping the accelerated cooling at a time point when the temperature of the head
portion of the steel rail reaches 600°C to 450°C, the reason that the accelerated cooling
stop temperature range and the accelerated cooling rate are limited to the above ranges
5 will be described.
[0085]
When accelerated cooling is stopped at a temperature of higher than 600°C,
pearlite transformation is started at a high temperature range immediately after the
cooling, and thus a large amount of coarse pearlite structure having a low hardness is
10 generated. As a result, when the hardness of the head surface portion becomes less than
Hv320, and thus it is difficult to ensure the necessary wear resistance for the rail. In
addition, when accelerated cooling to less than 450°C is performed, in the component
system, an austenite structure is not transformed at all during accelerated cooling, and a
bainite structure or a martensite structure is generated in the head surface portion,
15 resulting in the degradation of the wear resistance or toughness of the rail. Therefore,
the accelerated cooling stop temperature range is limited to a range of 600°C to 450°C.
[0086]
Next, when the accelerated cooling rate of the head portion becomes less than
4°C/sec, pearlite transformation is started during the accelerated cooling in a high
20 temperature range. As a result, the hardness of the head surface portion becomes less
than Hv320, and it is difficult to ensure the necessary wear resistance for the rail. In
addition, the diffusion of Mn is accelerated during the pearlite transformation, the Mn
concentration of the cementite phase is increased, and thus the CMn/FMn value exceeds
5.0. As a result, the occurrence of cementite cracking at a starting point portion is
accelerated, and thus the toughness of the rail is degraded. In addition, when the
accelerated cooling rate exceeds 15°C/see, in the component system, a bainite structure
or a martensite structure is generated in the head surface portion. In addition, in a case
when the accelerated cooling temperature is relatively high, high recuperative heat is
5 generated after the accelerated cooling. As a result, the diffusion of Mn is accelerated
during transformation, the Mn concentration of the cementite phase is increased, and thus
the CMn/FMn value exceeds 5.0. As a result, the wear resistance or toughness of the
rail is degraded. Therefore, the cooling rate is limited to a range of 4 to 15°C/sec.
[0087]
10 In addition, in order to stably generate a pearlite structure having excellent wear
resistance and toughness, it is preferable that the accelerated cooling rate have a range of
5 to 12°C/sec.
[0088]
Next, the reason that the maximum temperature increase amount including
15 transformation heat and recuperative heat generated after the accelerated cooling is
limited to 50°C or less from the accelerated cooling stop temperature will be described.
[0089]
In the component system, accelerated cooling is performed on the rail head
portion from a temperature range of equal to or higher than 750°C, and when the
20 accelerated cooling is stopped in a range of 600°C to 450°C, a temperature increase
including transformation heat and recuperative heat occurs after the accelerated cooling.
The temperature increase amount is significantly changed by a selection of the
accelerated cooling rate or the stop temperature, and there may be cases where the
temperature of the surface of the rail head portion is increased to about 150°C at the
36
maximum. The temperature increase amount represents the behavior of the pearlite
transformation of the head surface portion as well as the surface of the rail head portion,
and has a significant effect on the properties of the pearlite structure of the rail head
surface portion, that is, toughness (the Mn content in the cementite phase). When the
5 maximum temperature increase amount including transformation heat and recuperative
heat exceeds 50°C, the diffusion of Mn into the cementite phase during pearlite
transformation is accelerated due to a temperature increase, the Mn concentration of the
cementite phase is increased, and thus the CMn/FMn value exceeds 5.0. As a result, the
occurrence of cracking in the cementite phase at a starting point portion is accelerated,
10 and thus the toughness of the rail is degraded. Therefore, the maximum temperature
increase amount is limited to 50°C or less from the accelerated cooling stop temperature.
In addition, although the lower limit of the maximum temperature increase amount is n
limited, in order to steadily terminate the pcarlite transformation and to cause the
CMn/FMn value to reliably be equal to or less than 5.0, it is preferable that the lower
15 limit thereof be 0°C.
[0090]
Next, in a method of performing accelerated cooling at a cooling rate of 0.5 to
2.0°C/sec after the temperature increase including transformation heat and recuperative
heat and stopping the accelerated cooling at a time point when the temperature of the
20 head portion of the steel rail reaches 400°C or less, the reason that the accelerated
cooling stop temperature range and the accelerated cooling rate are limited to the above
ranges will be described.
[0091]
When accelerated cooling is stopped at a temperature of higher than 400°C,
of
37
tempering occurs in the pearlite structure after transformation . As a result, the hardness
of the pearlite structure is reduced, and thus the wear resistance of the rail is degraded.
Therefore, the accelerated cooling stop temperature is limited to a range of equal to or
less than 400° C. In addition, although the lower limit of the accelerated cooling stop
5 temperature is not limited , in order to suppress the tempering of the pearlite structure and
suppress the generation of the martensite structure at a segregation portion, it is
preferable that the lower limit thereof be 100 ° C or higher.
[0092]
In addition , tempering of a pearlite structure described here designate that the
10 cementite phase of a pearlite structure is in a separated state. When the cementite phase
is separated, the hardness of the pearlite structure is reduced , and thus wear resistance is
degraded.
[0093]
Next, when the accelerated cooling rate of the head portion becomes less than
15 0.5°C/sec, the diffusion of Mn is accelerated, a partial increase in the concentration of
Mn in the cementite phase occurs, and thus CMn/FMn value exceeds 5.0. As a result,
the occurrence of cracking in the cementite phase at a starting point portion is accelerated,
and thus the toughness of the rail is degraded . In addition, when the accelerated cooling
rate exceeds 2.0°C/see, the generation of a martensite structure at a segregation portion is
20 exacerbated , and thus the toughness of the rail is significantly degraded . Therefore, the
accelerated cooling rate is limited to a range of 0.5 to 2.0°C/sec . In addition, in terms of
suppressing an increase in the concentration of Mn in the cementite phase, it is preferable
that the accelerated cooling be performed as immediately as possible after completing the
temperature increase in an actual operation.
38
[0094]
Temperature control of the rail head portion during a heat treatment may be
performed by representatively measuring the temperature of the surface of the head
portion at the head top portion (reference numeral 1) and the head corner portion
5 (reference numeral 2) shown in FIG. 6 for the entire rail head surface portion (reference
numeral 3a).
[Examples]
[0095]
Next, Examples of the present invention will be described.
10 Tables 1 -I and 1 -2 show the chemical components and characteristics of the rail
steel of the present invention. Tables 1 1 and 1.2 show chemical component value, the
microstructure of the rail head portion, hardness, and CMn/FMn value. Moreover, the
results of a wear test performed on a specimen collected from the position shown in FIG.
7 by a method shown in FIG. 8 and the results of an impact test performed on a specimen
15 collected from the position shown in FIG. 9 are also shown.
[0096]
In addition, the manufacturing conditions of the rail steel of the present
invention shown in Tables 1 -I and 1 -2 are as described below.
[Cooling conditions after hot rolling and reheating]
20 Cooling start temperature: 800°C, cooling rate: 7°C/sec,
Cooling stop temperature: 500°C, maximum temperature increase amount: 30°C
[Cooling conditions after temperature increase]
Cooling start temperature: 530°C, cooling rate: 1.0°C/sec,
Cooling stop temperature: 350°C
39
[0097]
Table 2 shows the chemical components and characteristics of comparative rail
steels. Table 2 shows chemical component value, the microstructure of the rail head
portion, hardness, and CMn/FMn value. Moreover, the results of a wear test performed
5 on a specimen collected from the position shown in FIG. 7 by a method shown in FIG. 8
and the results of an impact test performed on a specimen collected from the position
shown in FIG 9 are also shown.
[0098]
In addition, the manufacturing conditions of the rail steel of the present
10 invention shown in Table 2 are as described below.
[Cooling conditions after hot rolling and reheating]
Cooling start temperature: 800°C, cooling rate: 7°C/sec,
Cooling stop temperature: 500°C, maximum temperature increase amount: 30°C
[Cooling conditions after temperature increase]
15 Cooling start temperature: 530°C, cooling rate: 1.0°C/sec,
Cooling stop temperature: 350°C
[0099]
Tables 3-1 and 3-2 show the manufacturing results of the method of
manufacturing a rail of the present invention and the manufacturing results of a
20 comparative manufacturing method, using the rail steels shown in Tables 1 -I and 1-2.
Tables 3-1 and 3-2 show, as the cooling conditions after hot rolling and reheating,
cooling start temperature, cooling rate, cooling stop temperature, and moreover
maximum temperature increase amount after stopping cooling, and show, as the cooling
conditions after atemperature increase, cooling start temperature, cooling rate, and
40
cooling stop temperature.
In addition, the microstructure of the rail head portion, hardness, and CMn/FMn
value. Moreover, the results of a wear test performed on a specimen collected from the
position shown in FIG 7 by a method shown in FIG. 8 and the results of an impact test
p rformed on a specimen collected from the position shown in FIG 9 are also shown,
[0100]
'ft
Table-] °1 (1/2)
Rail steel of
present invention
0.99 10.30 10.25 10.60 1 0.0130
Ni Ti MgV Ca
Note 1: The balance is composed of inevitable impurities and Fe.
*I: Microstructure and hardness the data at a position of 4 ram under the surface of
the rail head surface portion.
*2: The wear test was performed on a specimen collected from a position shown in FIG. 7 by
a method shown in FIG. 8. The experimental conditions are as described in the specification.
*3: Impact test was performed on a specimen collected from a position shown in FIG. 9.
The experimental conditions are as described in the specification.
Al 10.86 10. 25 10.40 1 0.50
0.25 0.40 0.50
0.05 0,30 10.45
2.()(1 0.310
.
.45
n.5o 00ti 35
0.50'0.50F0.35
A9 1 1.00 0 . 60 0,50 10.20
to q 1.oo 0.60 0. It J W20
All 10.86 10.50
Al'
0.d 10.20
Al 0 90 G..'r(1 0.45 p 0 . 40
I, 00 ((.91) 0.01 (1 '>0 04o
A17 0.911 0.50 0 i , o.2(0
All; 0.9 UM [U ',j 0.10
A19 0.91 (1.J.0 O "I 0,45
X17.11 0.9d U.5O 0. 30 0.20
^I t 0.9'0. 55 0.40 0.15
F ' 0.95 1 0.55 0.30 0,15
F°3 0,9.'i (1.55 0.10 0.15
0.0100
0.012.0
0.0120
0.0060
0.0060
0.01001 e 10.0
0.01001 - 10.0
0.0150 10'0
0.01
(A1
2
2fl
0.01201 ^^ -
0,01'0 a 1 U 0.00
0.0120
0.0120
0.0140
00080
0.0040
4 10.98 10 . 10 10.40 10.55 10.0130
TableI -° 1 (2/2)
1-lead portion material` 1
teal
Pearlii.e+
Hall amount of uroneutectoid ferrite
Pearlite t
Small amount morn-eut ' old cementite
Pearnte
Pearlite+
all vuuonntofmartensiW
Pearlite
r'enrlite __LLm
Pearlite
Pearlite-
Small amount of bainite
Pearlite
A U Pearlite
Pearlirc;
Snell ^moniri ofliro eutruiui 1 t [cite
Pearlite r
Small amount ofpro-eutentoid 6 i(c
Hardness
(I-Iv, 98N)
420 4.9
335
490 4.4
320 4.9
T mPM4n
value
340 1.0
415
350 p 1
0.35 12,1
1.10 18.5
0.82 16.5
0,75 16.5
0.68 .t
0.62 15.0'
4-1.U 2.1 1 0.52 13.5
430 4.9 0,61 17.
470 4.9 0.67,
390 t 4.6 F II 1.10
Pearli
Small o-'u,iuunt o ro'eutectoid fr^
Pcar'lite+
Small amount of bainite
Pearlite
Pearbte
Pe trlite
A18 H ^'inulrt^
.4.19
v^W .._
Pcndiic
m9ilr
Pearli is
Pearlite
Pearlite
Pearlite
amount of bainite
Pearlite F
Small amonnf o6bainite
1.1
1.5
2,0
380 1.0
405:
400
390
47.0
Wear test lmpa
result''2 re° !1' '
Wear amount bfpacr valu„
(g, 700000
times) (J/cm2)
1.40 19.2
0 . 83 184
084 205
097 175
0.88 16.5
^0.8't. 16.5
0,'l4 16.8
O.Y3 1g2
0.76 198.
0.69 G v 17.8
0.70 18.2
Note 1: The balance is compoooed of inevitable impurities and Fe.
1: Microstructure and hardness are data at a position of 4 mm under the surface of
the rail head surface portion.
*2: The wear test was performed on a specimen collected from a position shown in FIG. 7 by
a method shown in FIG. 8. The experimental conditions are as described in the specification.
3: Impact test was performed on a specimen collected from a position shown in FIG. 9.
The experimental conditions are as described in the specification.
'fable, --2(1/2)
1,20 1 0.70 10,10
g ^ 11 ^,11.,^r
0,0(132
0,0032
Note 1: The balance is composed of inevitable impurities and Fe.
1: Microstructure and hardness are data at a position of 4 nun under the surface of
the rail head surface portion.
''r2: The wear test was performed on a specimen collected from a position shown in FIG. 7 by
a method shown in FIG. 8. The experimental conditions are as described in the specification.
*3: Impact test was performed on a specimen collected from a position shown, in FIG, 9.
The experimental conditions are as described in the specification.
Steel
A?6 1.00 1 0. 55 1 0.45 0,25 0.0130
A)./ 1.00. 059 0,30 0.25 0,0130
0.55 0,10 A2S 1.00 10.25 0.0130
Az9 1,02 0.70 0.20 0,30 0.0100
ti30 1,02 0,70 0,20 0.30 0.0100.
A31 1,04 1.30 0.10 0.05 0,0100 ^- -
-
A32 1.04 1,30 0.10 0.05_ 0.0100 - 1)
1.33 1,05 0,35 0,45 0.30 0,0060 fl
A34 1,05 0.35 0,30 0.30 0,0060
1.35 1,05 0.35 0,10 0,30 0.0060
A36 ?,07 0,15 0.20 0,0060
A3'/ 1p0 10,30 0,30 10,0070
A 13 1.10 065 0,40 0,20 0.0060
A'Q 1,10 0.6S 0,25 0,20 0,0060
A' I t In U,65 0.10 (1,20 0,0040
A41 1 1.11 1 1,00 1 0,25 1 0.15 10,0060
A42 1 1,14 100,,60 1 0.35 1 0,25 1 0,0060
A43 1 1.14 1 0,60 1 0.35 1 0.25
A44 1 1,14 1 0.60 1 0
A45 1 1,20 1 0,70 1 0.40
A46 1 1.20 1 0.70 1 0,20
r.OU.^ti
0100
0,0200
0,0100
0.0140 1 0.0100
0,0100
Tablel --°2(2/2)
Steel
Head portion materials l Hardness
Wear test
CM&P-Ma
result*2
Wear amount
Micros"ature (I1v, 9814)
value
(Br700000
Impact test
resuk'13
Impaclvalue
(J/Cm2)
A26
timc s )
Penr)ite 435 4. .5 0.0 16,r
A27 Pc . lit, 430 29 0.G1
A..e 420 1.3 0 . 65 18,1
A29 Pmrlit. 485 1 . 9 0,5'l ` 15,9
A30 P "'lit' 4135 1.9 0.55 16.8
A31 Pearlitu 415 1.3 Q 0,0, 16.4
At Pearlite 41.5 1i 0.63 ... 17.5
A33 Peadiy 435 .4 056 15.5
A34 Pemlite 430 3 , 1 0.57 16,8
A35 Pearlila 425 0.59 18.0
A36 Pearlite 425 1,6 0.59 14,9
A37 Pearlile 430 3.1 0.511 1•:.0
A38 Pea^lite 440 4,0 n. -,o 13.0
A39 J °ht , 435 2.4 0s?,
_
14.2
A40
_A41_
A42
A43
1lr.
.. _, . rro-e"ecloid cemendte
i aeli, ,
"tea ' fprp eutectoid cementite
Pearlita+
YO.. eutectoid cemendte
430
470
__
410
410
1 . 1
2.6
gtltl
3.7
3.7
U54
0.45
_ _
0.48
0.47
153
129
12.7
13.5
A44
cPoaelite+
po eutectoid eemen to 410 1 0.46 13.6
A45
_ra tl u,no¢
oPearhteti
S uro auleclo^d cenenOto
480 1J
u
0.35
w 4
12.5
A46 Pearhtr r
11 mint of pro er toeladl ormeutite
_
470 0, 36 14.0
A47 1'earhle+
rat of ro aaleetmd cemeatile
465 1,0 038 15.0
Note 1: The balance is composed of inevitable impurities and Fe.
*1: Microstructure and hardness are data at a position of 4 trim under the surface of
the rail head surface portion.
'^2: The wear test was performed on a specimen collected from a position shown in FIG. 7 by
a method shown in FIG. 8. The experimental conditions are as described in the specification.
*3: Impact test was performed on a specimen collected from a position shown in FIG. 9.
The experimental conditions are as described in the specification.
Table 2 (1 /2)
Si IMI Cry P IMO I V
1.00 10.50
0.50 0 0.0100
0.50 0 0.0100
0.0120
0.0120
0.35 10,0060
Pl W 10.35 10,0060
4 y 1.10l0.801Vo 015
0.0080 N
O.0080 1
a9 1 1.00 10.60 G 0 .50 10.20 1 0.0250
al0 y 1.10 10.651 .80 0.201 0.0060
all 1 1.20 10. 70 10.70 ^ 0, 30 10.00610
a12 y 1.20 10.70 0 0.20 0.0040
onent (rmass%)
Note 1: The balance is composed of inevitable impurities and Fe.
1: Microstructure and hardness are data at a position of 4 mrn under the surface of
the rail head surface portion.
*2: The wear test was performed on a specimen collected from a position shown in FIG. 7
by a method shown in FIG. 8. The experimental conditions are as described in the specification.
*3: Impact test was performed on a specimen collected from a position shown in FIG, 9.
The experimental conditions are as described in the specification.
Tablet, (2/ ?,)
Weal tl.Si. "pact WGA
Head pcffion material*1 Hardness
result 7 i UW3
Steel
Microstructure
(Hv,
4n
C value
-
Wu .u
( '700000
Impact value
98N)
g, (J/cm2
.
Pearlite& 1.87
al
Pro-eutectoid ferrite
300 4.9
lar e rVe,
2L2
Peailitel- ^-,-7 ,8
415 4.9 0.45 (impact value
Pro-eutectoid cementite
reduction
Pearlite 295 4.4 1.9j 19.5
i^r^n ,par?
Pearlite+
I.08 5.6
a4 525 4.4 (impact value
aitensite (lame wear)
reduction
a5 Pearlite 315 1.0 17.0
large v; na,
6.5
a6 Pearlite 430 6.4 0.65 (impact value
redlining)
a7 Pearlite 318 2.1
1.80
16.8
larA. weal
a8
k G Iii t
375 2. 1 1.60 15.6
Pm'^ filar e wear
8.9
a9 Pearlite 435 4.9 0.61 (impact value
rediiefion)
V. I
a10 t'eMlite 440 6Y 0.50 (impact value
reduction)
all
,,..
Pearlite+
Small amount ol'pro-cuteetoid cementite
480 62 0.32
7.1
(impact value
rcdiictlon
Pearlite4
1.77 ;.0
a12
Maitensite
550 2.,,1
(larir, wear
(impact value
reduction
Note 1: The balance is composed of inevitable impurities and Fe.
1: Microstructure and hardness are data at a position of 4 mm under the surface of
the rail head surface portion.
*2: The wear test was performed on a specimen collected from a position shown in FIG. 7
by a method shown in FIG, €t. The experimental conditions are as described in the specificatio) 1.
*3: Impact test was performed on a specimen collected from a position shown in FIG, 9.
The experimental conditions are as described in the specification,
Tallle -10 /2)
Rail
Manufacturing
method of
present
invention
B23
P7?
A12
A12
B3 1 A12
34 1 A12 1 750
B5 1 A12
B6LL
) t'/
3..
139
100
I t I',.
S 1 1 (I A),'/ it
B18 A39
IS)() A39
zB 9l,,_
322 F9 800
A46
A,!
A46
Cooling
start
n
(°C) I (°C/sec) (°C)
750
rature
750
750
/So
'MO
/80
'/80
/ISO
300
800
800
Soo
)Soo
1300
900
820
Cooling
rate
4.0
5 .0
7.0
2.0
15.0
7.0
/0 'l.0
7.0
/.0
/.1)
/.0 it
/.o
/,0
/.o
/.0
0
s.c
/.0
0
15.0
7.0
820 1 7.0
7.0
500
500
500
50o
500
600
500
050
;00
;00
i00
A0
00
100
;00
5300
500
oo^
! 5300
Cooling
rate
Maximum
emperatur'e Cooling
increase start
teinoeratuo e
30
30 520 1.0 1 350
30
500 30
(°C) (°C/sec) (°C)
520 1.0 350
1.0 350
520
Cooling
stop
rnueratur'e
520 1.0 350
1,0 350
0X10 J 3)
'I/'O1.0
!O (,0
)o I m
5(10 I m
21) 0.5
5.'.0 1 ,0
';10 i,0
10 .0
^^ 10 t,o 300
?.0 ), 0 " 50
5)0 !{ 1.0 350
^^21) j 1.0 :350
0 1_0 1)0
20 1.0
500 50 515 1.0
500 30
e V W-a
1.0
500 1.0
it
as0
350
350
550
TM
10
:550
150
400
350
50
Manufacturing
No.
B1
B2
B25
Steel
Cooling conditic i^ an, llot
rolling andrehe,, ),t
Cooling
stop
noerature
820 1
,^uT
Cooling conditions after
teiri erature increase
515
5301
Note 1: The balance is composed of inevitable impurities and Fe.
* 1: Microsiiucture and hardness are data at a position of 4 mm under the surface of
the rail head surface portion.
*2: The wear test was performed on a specimen collected from a position shown in FIG. 7
by a method shown in FIG. 8. The expeririaentall conditions are as described in the specification.
*3: impact test was performed on a specimen collected front a position shown in FIG. 9.
The experimental conditions are as described in the specification.
350
350
350
liable. 1(2/2)
R8
B9
R1)
All
RI2
RIG
B17_ II
S18LL I~
Head portion material1l`1
Microstructure
Pearlitef5'
nall autotn'tt of pro-entcctoirl ferrite
Pe nlili;
f)iii, ll^t'no ro e:nicc(oid k"Iritn
Pearlite I
of iO-eutectold t(:1iltt
Pearlite+
mall amount of pro-eutectoid fei i ile-
Pearlite+
Seidl a ount of lino-eutectoid ferrite+
•v1f ulonnt f ainii.c
I irlite
'irwlitc
'rt,rlitc;
Pe; rlite
''.;+' Irte
Ii 'tt'lite
1_'c^ii InP
1'crnlu^; ^^^^^
Pearlite
Pearlite+
Small amount of martensitc^
Pearlite+
Small amount of I), n-eutectoid c'
-^- I'i nrliir:;
Sm911 auunltti lll luii (r.iliccioltl c(t^n
X17))
Wear test
result _2
Wear amot:uib u It 111 ii;t valu
(g, 700000
timed
3.1 it 0.33 18.4
38 0,%'; 13.9
U.'// I I/O '.
3.0 0,'/'; 11 Iii7,
0 I 0,/ I I ).i
7.1
!:m
y
i7)1
4+Q0
Sell rniiorutt of uio Mint oid rClirl ^rrio-P
2.1
1.3
0.(0 18. 8
o.'+'/ 16.8
o. s() 16.
O.rjr}
1).'1)
0. t)
Note 1: The balance is composed of inevitable impurities oiid Fe,
1: Microsti-trcture and hardness are data at a position of 4 tarn under the surface of'
the rail head surface portion.
The wear test was performed on a specimen collected :loin a prt ition shown in FIG.
by a method shown is FIG. 8. The experimental conditions are as described in the specification.
*3: Impact test was performed on a specimen collected from a porii it, I ' iiown in FIG. 9.
The experimental conditions are as described in the specification.
T abie3 -2(1/2)
Comparative
nanufacturing
method
7.0 500 520 R 3.0 R 350
Note 1: The balance is composed of inevitable impurities and Ve.
1: Microstructure and hardness are data at a position of 4 mY i under the surface of
the rail head surface portion.
2: The wear test was performed on a specimen collected from a position shown ht F 16. 7 by
a method shown in FIG. 8. The experimental conditions are as described in the specification,
*3: impact test was performed on a specimen collected from a position shown in FIG 9.
The experimental conditions are as described in the specification.
tooling conditions ;, f ter hot rolling
and ri,hetj iing
Cooling
start
.emperatnre
Cooling
rate
Cooling
stop
emperatur
Maximum
emperature
increase
Cooling conditions after
Cooling
start
temperature
,dire i ccepse.
Cooling
rate
(CC) N (°C/sec) I (°C) 6 (°C) (°C) (°C/sec) (°C)
bl A12 680 7.0 1 500 30 520 B 1.0 350
b2 A46 R 720 7.0 500 30 515 1.0 N 350
b3 A12 750 3.0 500 30 520 a 1.0 350
b4 A39 9 800 2.0 500 30 0 5')0 1.0 H 350
b5 A12 750 16.0 500 30 520 1 1.0 350
b6 A39 800 17.0 500 30 520 1 1.0 350
bl A16 770 7.0 440 30 460 1,0 350
b8 A16 770 7.0 650 30 680 1 . 0 350
b9 A22 780 18.0 600 80 670 1.0 350
b10 /v46 820 16.0 9 590 70 650 1.0 350
A27 1 780 N 7.0 500 1 30 1 520 0.3 1 350
Taabl-"; -2("/2)
(klv, 98N)
Pearlite+
Promeutectoid cementite
CMu/F.IVIrd
value
Wear test Impact test
result*2 result*3
Wear amount 1..gJLet value
(g, 700000
timed (7/crn)
310 2.8
138
20.0
(large wear)
420 2.1 0.42 I (ingaactvalue
reduction)
(large wear)
12.0
(impact value
reduction
8 . 5 T
360 0.65 (impact value
b9 Il Pearlite H 360
blO
Pearlite+-
420
1 Small amount of pro-eutectoid cementite
1.75
(large wear;
1.61
(large wear
e.u
(Impact value
reduction)_
(impart vn a ae
reduction)
11.0
(impart value
rlii Lion)
/1
(hl IJ )rI' i:value
reduction Y
Note 1: The balance is composed of inevitable `ntpurities and Fe.
1: Sicr°ostr°ucture and hardness are data at a position of 4 n trader the surface of
the rail head surface portion.
*2: The wear test was performed on a specimen collected from a, position shown in FIG. 7 by
a method shown in FIG. 8. The experimental conditions are as described in the specification.
* 3: impact test was performed on a specimen collected from a position shown in FIG. 9.
the experimental condition are as described in the specification,
Pearlite+
Small amount of pro-eutectoid ferrite
Pearlite+-
Pro-cutectoid cementite
P":1Ik+
Small amount of pro-eutectoid ferrite
[0105]
In addition, various test conditions are as described below.
[1] Head Portion Wear Test
Tester : Nishihara-type wear testing machine (see FIG 8)
5 Specimen shape: disk-shaped specimen (outside diameter : 30 mm, thickness: 8
mm)
Specimen collection position: 2 mm under the surface of the rail head portion
(see FIG. 7)
Test load: 686 N (contact surface pressure 64.0 MPa)
10 Slip ratio: 20%
Wheel specimen (Opposite material): pearlite steel (Vickers hardness: Hv380)
Atmosphere: in the air
Cooling: forced cooling by compressed air (flow rate: 100 L/min)
Number of cycle : 700,000 revolution
15 In addition, the flow rate of the compressed air is a. flow rate converted into a
volume at room temperature (20°C) and at the atmospheric pressure (101.3 kPa).
[0106]
[2] Head Portion Impact Test
Tester: impact tester
20 Test method: performed on the basis of JIS Z 2242
Specimen shape: JIS3 type 2 mm U notch
Specimen collection position: 2 turn tinder the surface of the rail head portion
(see FIG. 9, 4 mm under the notch position)
Test temperature: room temperature (20°C)
25 In addition, the conditions of each of the rails are as follows.
[0107]
(1) Rails of the present invention (47 rails)
Reference numerals Al to A47: rails of which the chemical component values,
the microstructures of the rail head portions, hardnesses, and CMn/FMn values are in the
5 ranges of the present invention.
[0108]
(2) Comparative rails (12 rails)
Reference numerals al to a12: rails of which the chemical component values,
the microstructures of the rail head portions, hardnesses, or CMn/FMn values are out of
10 the ranges of the present invention.
[0109]
(3) Rails manufactured by the manufacturing method of the present invention (25 rails)
Reference numerals B1 to B25: rails of which the cooling start temperatures
after hot rolling and reheating, the cooling rates, the cooling stop temperatures, the
15 maximum temperature increase amounts, the cooling rates after a temperature increase,
and the cooling stop temperatures are in the ranges of the present invention.
[0110]
(4) Rails manufactured by the comparative manufacturing method (13 rails)
Reference numerals bl to b 13: rails of which any of the cooling start
20 temperatures after hot rolling and reheating, the cooling rates, the cooling stop
temperatures, the maximum temperature increase amounts, the cooling rates after a
temperature increase, or the cooling stop temperatures is out of the ranges of the present
invention.
25 As shown in Tables 1 .- 1, 1-2, and 2, in the rail steels of the present invention
(reference numerals Al to A47), compared to the comparative rail steels (reference
numerals at to a12), by causing the chemical components C, Si, Mn, Cr, and P of the
steel to be in the limited ranges, the generation of a promeutectoid ferrite structure, a
pro-eutectoid cementite structure, a bainite structure, and a martensite structure that has
5 an adverse effect on wear resistance or toughness is suppressed, and thus a pearlite
structure having a hardness in an optimal range is obtained . In addition, by causing the
CMn/FMn value to be equal to or less than a constant value, the wear resistance or
toughness of the rail is enhanced.
[0112]
10 FIG. 10 shows the relationship between carbon content and wear amount of the
rail steels of the present invention (reference numerals Al to A47) and the comparative
rail steels (reference numerals a1 , a3, a4, aS, a7 , a8, and a 12). FIG. I I shows the
relationship between carbon content and impact value of the rail steels of the present
invention (reference numerals Al to A47) and the comparative rail steels (reference
15 numerals a2, a4 , a6, and a9 to a 12).
[0113]
As shown in FIGS. 10 and 11, in the rail steels of the present invention
(reference numerals Al to A47), compared to the comparative rail steels (reference
numerals at to a12), wear amounts are small and impact values are enhanced when the
20 carbon contents are the same. That is, at any carbon content , the wear resistance or
toughness of the rail is enhanced.
[0114]
In addition, as shown in Tables 3 -1 and 3-2, in the rail steels of the present
invention (reference numerals B 1 to 1325), compared to the comparative rail steels
25 (reference numerals bl to b13 ), by causing the cooling start temperat ures after hot
rolling and reheating, cooling rates, cooling stop temperatures, and maximum
temperature increase amounts after stopping cooling, cooling rates after a temperature
increase, and cooling stop temperatures to be in the limited ranges, the tempering of a
pro-eutectoid cementite structure, a bainite structure, a martensite structure, and a pearlite
5 structure that has an adverse effect on wear resistance or toughness is suppressed, and
thus a pearlite structure having a hardness in an optimal range is obtained. In addition,
by causing the CMn/FMn values to be equal to or less than a constant value, the wear
resistance or toughness of the rail is enhanced.
[0115]
10 FIG. 12 shows the relationship between carbon content and wear amount of the
rail steels manufactured by the manufacturing method of the present invention
(reference numerals B 1 to B25) and the rail steels manufactured by the comparative
manufacturing method (reference numerals bl, b3, b5 to b8, b12, and b13). FIG. 13
shows the relationship between carbon content and impact value of the rail steels
15 manufactured by the manufacturing method of the present invention (reference numerals
B1 to B25) and the rail steels manufactured by the comparative manufacturing method
(reference numerals b2 to b6 and b9 to b12).
[0116]
As shown in FIGS. 12 and 13, in the rail steels manufactured by the
20 manufacturing method of the present invention (reference numerals B1 to A25),
compared to the rail steels manufactured by the comparative manufacturing method
(reference numerals b 1 to b 13), wear amounts are small and impact values are entranced
when the carbon contents are the same. That is, at any carbon content, the wear
resistance or toughness of the rail is enhanced.
25
Reference Signs List
[0117]
1: head top portion
2: head corner portion
5 3: rail head portion
3a: head surface portion (range from surface of head corner portion and head top
portion as starting point to depth of 10 mm)
3b: range from surface of head conger portion and head top portion as starting
point to depth of 20 nun)
10 4: rail specimen
5: Wheel specimen (opposite material)
6: cooling nozzle

CLAIMS
1. A steel rail comprisin
by mass%,
5 higher than 0,85% to 1.20% of C;
0.05% to 2.00% of Si;
0.05% to 0.50% of Mn;
0.05% to 0.60% of Cr;
P<0.0150%; and
10 the balance consisting of Fe and inevitable impurities,
wherein 97% or more of a head surface portion which is in a. range from a
surface of a head corner portion and a head top portion as a starting point to a depth of 10
mm has a pearlite structure,
a Vickers hardness of the pearlite structure is Hv320 to 500, and
15 a CMn/FMn value which is a value obtained by dividing CMn [at.%] that is a
Mn concentration of a cementite phase in the pearlite structure by FMn [at.%] that is a
Mn concentration of a ferrite phase is equal to or higher than 1.0 and equal to or less than
5.0.
20 2. The steel rail according to claim 1, further comprising one kind or two or
more kinds selected from the group:
by mass%,
0.01% to 0.50% of Mo;
0.005% to 0.50% of V;
25 0.001 % to 0.050% of lib;
0,01% to 1.00% of Co;
0.0001% to 0.0050% of B;
0.01% to 1.00% of Cu;
0.01 % to 1.00% of Ni;
5 0.0050% to 0.0500% of Ti;
0.0005% to 0.0200% of Mg;
0.0005% to 0.0200% of Ca;
0.0001% to 0.2000% of Zr;
0.0040% to 1.00% of Al; and
10 0.0050% to 0.0200% of N.
3. A method of manufacturing the steel rail according to claim 1 or 2,
comprising:
performing first accelerated cooling on a head portion of the steel rail at a
15 temperature of equal to or higher than an Arl point immediately after hot rolling, or a
head portion of the steel rail reheated to a temperature of equal to or higher than the Ac I
point+30°C for purposes of a heat treatment, at a cooling rate of 4. to 15°C/sec from a
temperature range of equal to or higher than 750°C;
stopping the first accelerated cooling at a time point when a temperature of the
20 head portion of the steel rail reaches 600°C to 450°C;
controlling a maximum temperature increase amount including transformation
heat and recuperative heat to be equal to or less than 50°C from an accelerated cooling
stop temperature;
thereafter performing second accelerated cooling at a cooling rate of 0.5 to
)
2.0°C/see; and
stopping the second accelerated cooling at a time point when the temperature of
the head portion of the steel rail reaches 400°C or less.