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DESCRIPTION
RAIL MANUFACTURING METHOD AND RAIL MANUFACTURING APPARATUS
Technical Field
[0001]
The present invention relates to a method and an apparatus
for manufacturing a pearlitic steel rail with excellent
ductility obtained by performing rough rolling, finish rolling,
and heat treatment of a heated bloom and particularly relates
to a method and an apparatus for manufacturing a rail having
ductility improved by refining the pearlite block or colony
size.
Background Art
[0002]
A rail in which the structure of a head portion forms a
pearlite structure is generally manufactured by the following
manufacturing method.
First, abloom cast by a continuous casting method is heated
to 1100 C or more, and then hot-rolled into a predetermined rail
shape by rough rolling and finish rolling. A rolling method
in each rolling process is performed combining caliber rolling
and universal rolling. Herein, the rolling is performed in a
plurality of passes in the rough rolling or in a plurality of
passes or a single pass in the finish rolling.
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[0003]
Then, crops at end portions of the hot-rolled rail are sawn.
The length of the hot-rolled rail is 50 to 200 m. Therefore,
when a heat treatment apparatus has a length limitation, the
rail is sawn into a predetermined length, e.g., 25 m,
simultaneously with the sawing of the crops.
Furthermore, when the rail is required to have wear
resistance, the rail is subjected to heat treatment by the heat
treatment apparatus (heat treatment process) subsequent to the
hot-rolling process . Herein, the wear resistance improves when
the heat treatment start temperature is higher. Therefore, a
re-heating process of heating the rail may be provided before
the heat treatment process. In the heat treatment process, the
rail is fixed with a restraining device, such as a clamp, and
then a head portion, a foot portion, and, as necessary, an web
portion are forcibly cooled using a cooling medium, such as air,
water, and mist. In the heat treatment process, the forcible
cooling is usually performed until the temperature of the head
portion reaches 650 C or less.
[0004]
Thereafter, the restraint of the rail by the clamp is
released, and then the rail is conveyed to a cooling bed. On
the cooling bed, the rail is cooled until the temperature reaches
100 C or less.
For example, a rail to be used under severe environments,
such as mining sites of natural resources, such as coal, is
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demanded to have high wear resistance and high toughness.
Therefore, when the rail to be used under severe environments
is manufactured, the above-described heat treatment process is
required. However, when the rail manufactured by the process
described above is subjected to processing, such as bending
processing, for example, later, the processing becomes
difficult to achieve in some cases because the rail is
excessively hardened when subjected to heat treatment, so that
the ductility decreases. Therefore, a rail with high hardness
and excellent ductility has been demanded.
[0005]
For example, Patent Document 1 discloses a method including
setting the rolling temperature in finish rolling in a
temperature range of Ar3 transformation point to 900 C, and then
performing accelerated cooling of a rail to at least 550 C at
a cooling rate of 2 to 30/sec within 150 sec after the end of
the finish rolling to thereby increase the ductility of the rail.
Moreover, Patent Document 2 discloses a method including
performing rolling at an area reduction ratio of 10% or more
in a temperature range 800 C or less in hot-rolling to thereby
improve the ductility of a rail.
Citation List
Patent Documents
[0006]
[Patent Document 1] JP 2013-14847 A
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[Patent Document 2] JP 62-127453 A
Summary of Invention
Problems to be Solved
[0007]
However, the method described in Patent Document 1 has had
a problem in that the temperature control for a foot portion
of a rail is not performed, and therefore the ductility of the
foot portion does not improve.
The method described in Patent Document 2 has had a problem
in that the temperature adjustment conditions in rolling for
a foot portion of a rail are not specified, and therefore the
ductility of the foot portion does not improve.
Then, the present invention has been made focusing on the
problems described above. It is an object of the present
invention to provide a method and an apparatus for manufacturing
a rail having high ductility in both a head portion and a foot
portion.
Solution to the Problem
[0008]
In order to achieve the object, a method for manufacturing
a rail according to one aspect of the present invention includes
hot-rolling a heated steel rail material, adjusting the
temperature by cooling the hot-rolled steel rail material,
processing the steel rail material subjected to the temperature
adjustment into a rail shape by means of temperature-adjusted
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rolling at an area reduction ratio of 20% or more, and, in adjusting
the temperature of the steel rail material, cooling the surface
portions of the steel rail material corresponding to a head portion
and a foot portion of the rail shape to 500 C or more and 1,000 C or
less.
[0008a]
According to an embodiment, there is provided a rail
manufacturing method comprising: hot-rolling a heated steel rail
material; adjusting a temperature by cooling the hot-rolled steel
rail material; and processing the steel rail material subjected to
the temperature adjustment into a rail shape by means of temperature-
adjusted rolling at an area reduction ratio of 20% or more, wherein,
after the processing the steel rail material subjected to the
temperature adjustment into a rail shape temperature-adjusted
rolling, heat-treating the rail until a surface temperature of the
head portion of the rail reaches 600 C or less at an average cooling
rate of 1 C/s or more and 10 C/s or less, in the adjusting a
temperature of the steel rail material, a surface portions of the
steel rail material corresponding to a head portion and a foot
portion of the rail shape are cooled so that the temperatures of the
surface portions reach 500 C or more and 1,000 C or less, before the
heat-treating the rail, re-heating the rail to 730 C or more when the
surface temperature of the head portion of the rail is 730 C or less.
[0009]
An apparatus for manufacturing a rail according to one aspect of
the present invention has at least one first rolling mill rolling a
steel rail material, a cooling device adjusting a temperature by
cooling the steel rail material rolled with the first rolling mill,
and at least one second rolling mill processing the steel rail
material subjected to the temperature adjustment into a rail shape by
means of temperature-adjusted rolling at an area reduction ratio of
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20% or more, in which the cooling device cools the surface portions
of the steel rail material corresponding to a head portion and a foot
portion of the rail shape so that the temperatures of the surface
portions reach 500 C or more and 1,000 C or less.
[0009a]
According to an embodiment, there is provided a rail
manufacturing apparatus comprising: at least one first rolling mill
rolling a steel rail material; a cooling device adjusting a
temperature by cooling the steel rail material rolled with the at
least one first rolling mill; at least one second rolling mill
processing the steel rail material subjected to the temperature
adjustment into a rail shape by means of temperature-adjusted rolling
at an area reduction ratio of 20% or more, a heat treatment device
for heat-treating the rail until a surface temperature of the head
portion of the rail reaches 600 C or less at an average cooling rate
of 1 C/s or more and 10 C/s or less, and a reheat treatment device for
before the heat treating heating the rail to 730 C or more when the
surface temperature of the head portion of the rail is 730 C or less,
wherein the cooling device cools a surface portions of the steel rail
material corresponding to a head portion and a foot portion of the
rail shape so that the temperatures of the surface portions reach
500 C or more and 1,000 C or less.
Advantageous Effects of the Invention
[0010]
According to the method and the apparatus for manufacturing
a rail according to the present invention, a rail having high
ductility in both a head portion and a foot portion is able to
be manufactured.
Brief Description of the Drawings
[0011]
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FIG. 1 is a schematic view illustrating an apparatus for
manufacturing a rail according to one embodiment of the present
invention;
FIG. 2 is a cross-sectional view illustrating a rough
cooling device of one embodiment of the present invention;
FIG. 3 is a schematic view illustrating a heat treatment
apparatus of one embodiment of the present invention;
FIG. 4 is a cross-sectional view illustrating each portion
of a rail;
FIG. 5 is an explanatory view illustrating collection
positions of tensile test pieces evaluated in Examples; and
FIG. 6 is an explanatory view illustrating positions where
a Brinell hardness test evaluated in Examples is carried out.
Description of Embodiments
[0012]
Hereinafter, aspects for carrying out the present
invention (hereinafter also referred to as "embodiment") are
described in detail with reference to the drawings. In the
following description, % for chemical composition means % by
mass.
<Configuration of manufacturing apparatus>
First, a manufacturing apparatus 1 of a rail 9 according
to one embodiment of the present invention is described with
reference to FIG. 1 to FIG. 4. The rail manufacturing apparatus
1 according to this embodiment is a rolling line having a heating
furnace 2, a roughing mill 3A, a finishing mill 3B, a rough
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of
cooling device 4, a finish cooling device 5, a re-heating device
6, a heat treatment apparatus 7, and a cooling bed 8.
[0013]
The rail 9 is manufactured by rolling and heat-treating
a steel rail material, such as a continuously cast bloom, by
the manufacturing apparatus 1. As illustrated in FIG. 4, the
rail 9 extends in the width direction viewed in a cross section
perpendicular to the longitudinal direction and =has a head
portion 91 and a foot portion 93 facing each other in the vertical
direction and an web portion 92 connecting the head portion 91
disposed on the upper side and the foot portion 93 disposed on
the lower side and extending in the vertical direction. As the
rail 9, steel containing the following chemical composition is
usable, for example.
[0014]
C: 0.60% or more and 1.05% or less
C (carbon) is an important element which forms cementite
to increase hardness and strength and increases wear resistance
in a pearlitic steel rail. However, when the content is less
than 0.60%, these effects are low. Therefore, the lower limit
is preferably set to 0.60% and more preferably set to 0.70% or
more. On the other hand, excessive content of C causes an
increase in the cementite amount, and therefore an increase in
hardness and strength is expectable but, contrarily, the
ductility decreases. The increase in the C content extends the
temperature range of a y+0 zone and promotes softening of a weld
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heat affected zone. Considering these adverse effects, the
upper limit of the C content is preferably set to 1.05% and more
preferably set to 0.97% or less.
[0015]
Si: 0.1% or more and 1.5% or less
Si (silicon) is added as a deoxidizer and for reinforcing
the pearlite structure. When the content is less than 0.1%,
these effects are low. Therefore, the Si content is preferably
0.1% or more and more preferably 0.2% or more. On the other
hand, excessive content of Si promotes decarburization and
promotes the generation of surface flaws of the rail 9, and
therefore the upper limit of the Si content is preferably set
to 1.5% and more preferably 1.3% or less.
[0016]
Mn: 0.01% or more and 1.5% or less
Mn (manganese) has an effect of lowering the pearlite
transformation temperature and densifying the pearlite lamella
intervals, and therefore Mn is effective for maintaining high
hardness up to a rail inner region. When the content is less
than 0.01%, the effect is low. Therefore, the Mn content is
preferably 0.01% or more and more preferably 0.3% or more. On
the other hand, when the Mn content exceeds 1.5%, the equilibrium
transformation temperature (TE) of pearlite is lowered and
martensite transformation easily occurs in the structure.
Therefore, the upper limit of the Mn content is preferably set
to 1.5% and more preferably set to 1.3% or less.
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4
[0017]
P: 0.035% or less
When the content of P (phosphorus) exceeds 0.035%, the
toughness and the ductility are lowered. Therefore, the P
content is preferably suppressed to 0.035% or less and more
preferably limited to 0.025% or less. When special refinement
and the like are performed in order to reduce the P content as
much as possible, the cost increase in smelting is caused.
Therefore, the lower limit is preferably set to 0.001%.
[0018]
S: 0.030% or less
S (sulfur) extends in the rolling direction to form coarse
MnS reducing ductility and toughness. Therefore, the S content
is preferably suppressed to 0.030% or less and more preferably
suppressed to 0.015% or less. In order to reduce the S content
as much as possible, the cost increase in smelting, such as an
increase in smelting processing time and a flux, is remarkable.
Therefore, the lower limit is preferably set to 0.0005%.
[0019]
Cr: 0.1% or more and 2.0% or less
Cr (chromium) increases the equilibrium transformation
temperature (TE) and contributes to the reduction in the
pearlite lamella intervals to increase the hardness and the
strength. Furthermore, the use of Cr in combination with Sb
is effective for inhibition of the generation of a decarburized
layer. Therefore, when Cr is compounded, the content is
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k
4
preferably set to 0.1% or more and more preferably set to 0.2%
or more. On the other hand, when the Cr content exceeds 2.0%,
a possibility of the generation of welding defects increases,
the quenching properties increase, and the generation of
martensite is promoted. Therefore, the upper limit of the Cr
content is preferably set to 2.0%, and more preferably set to
1.5% or less.
The total content of Si and Cr is desirably set to 2.0%
or less. This is because, when the total content of Si and Cr
exceeds 2.0%, the adhesiveness of a scale increases, and
therefore the peeling of the scale may be inhibited and
decarburization may be promoted.
[0020]
Sb: 0.005% or more and 0.5 or less
When a steel rail material is heated with a heating furnace,
Sb (antimony) has a remarkable effect of preventing
decarburization during the heating. In particular, when Sb is
added together with Cr, an effect of reducing a decarburized
layer is demonstrated when the Sb content is 0.005% or more.
Therefore, when Sb is compounded, the content is preferably
0.005% or more and more preferably 0.01% or more. On the other
hand, when the Sb content exceeds 0.5%, the effect is saturated.
Therefore, the upper limit is preferably set to 0.5% and more
preferably set to 0.3% or less.
In addition to the chemical composition described above,
one or two or more elements of Cu: 0.01% or more and 1.0% or
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less, Ni: 0.01% or more and 0.5% or less, Mo: 0.01% or more and
0.5% or less, V: 0.001% or more and 0.15% or less, and Nb: 0.001%
or more and 0.030% or less may be compounded.
[0021]
Cu: 0.01% or more and 1.0% or less
Cu (copper) is an element capable of further increasing
the hardness by solid solution strengthening. Cu is effective
also for decarburization control. In order to expect the effect,
the Cu content is preferably 0.01% or more and more preferably
0.05% or more. On the other hand, when the Cu content exceeds
1.0%, surface cracks due to embrittlement in continuous casting
and rolling is easily generated. Therefore, the upper limit
of the Cu content is preferably set to 1.0% and more preferably
set to 0.6% or less.
[0022]
Ni: 0.01% or more and 0.5% or less
Ni (nickel) is an element effective for increasing
toughness and ductility. Moreover, by adding Ni in combination
with Cu, Ni is an element effective also for preventing Cu cracks.
Therefore, it is preferable to add Ni when adding Cu. However,
when the Ni content is less than 0.01%, these effects are not
obtained. Therefore, the lower limit is preferably set to 0.01%
and more preferably set to 0.05% or more. On the other hand,
when the Ni content exceeds 0.5%, hardenability excessively
increases and the generation of a martensite is promoted.
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Therefore, the upper limit is preferably set to 0.5% and more
preferably set to 0.3% or less.
[0023]
Mo: 0.01% or more and 0.5% or less
Mo (molybdenum) is an element effective for increasing
strength. When the content is less than 0.01%, the effect is
low. Therefore, the lower limit is preferably set to 0.01% and
more preferably set to 0.05% or more. On the other hand, when
the Mo content exceeds 0.5%, hardenability increases and a
martensite is generated, and therefore the toughness and the
ductility extremely decrease. Therefore, the upper limit of
the Mo content is preferably set to 0.5% and more preferably
set to 0.3% or less.
[0024]
V: 0.001% or more and 0.15% or less
V (vanadium) is an element which forms VC, VN, or the like
and is minutely precipitated into ferrite to contribute to an
increase in the strength through precipitation strengthening
of the ferrite. Moreover, V functions also as a trap site of
hydrogen, and thus an effect of preventing delayed fracture is
also expectable. To that end, the V content is preferably
0.001% or more and more preferably 0.005% or more. On the other
hand, when V is added in a proportion exceeding 0.15%, the alloy
cost extremely increases while the effects are saturated.
Therefore, the upper limit is preferably set to 0.15% and more
preferably set to 0.12% or less.
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[0025]
Nb: 0.001% or more and 0.030% or less
Nb (niobium) increases the non-recrystallization
temperature of austenite and is effective for reducing the
pearlite colony or block size by introduction of processing
strain into the austenite in rolling. Therefore, Nb is an
effective element for an improvement of ductility and toughness.
In order to obtain the effect, the Nb content is preferably
0.001% or more and more preferably 0.003% or more. On the other
hand, when the Nb content exceeds 0.030%, Nb carbonitride is
crystallized in a solidification process in casting of a steel
rail material to reduce cleanliness. Therefore, the upper
limit is preferably set to 0.030% and more preferably set to
0.025% or less.
[0026]
The remainder other than the components described above
includes Fe (iron) and inevitable impurities. As the
inevitable impurities, the mixing of N (nitrogen) up to 0.015%,
the mixing of 0 (oxygen) up to 0.004%, and the mixing of H
(hydrogen) up to 0.0003% are acceptable. In order to prevent
a reduction in rolling fatigue properties due to hard AIN or
TiN, the Al content is desirably set to 0.001% or less and the
Ti content is desirably set to 0.001% or less.
[0027]
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The heating furnace 2 is a continuation type or batch type
heating furnace and heats steel rail materials, such as a
continuously cast bloom, to a predetermined temperature.
The roughing mill 3A is a universal mill which hot-rolls
a steel material at a predetermined area reduction ratio and
two or more of the roughing mills 3A are provided. In the example
illustrated in FIG. 1, the manufacturing apparatus 1 has n pieces
of roughing mills 3A1 to 3An. The rough cooling device 4 is
provided between a k-th roughing mill 3Ak and a (K+1) -th roughing
mill 3Ak+1 among the roughing mills 3A1 to 3An along the
conveyance direction of the rail 9.
[0028]
The finishing mill 3B is a universal mill which further
hot-rolls the rough-rolled rail 9 to thereby finally process
the same into a target rail shape. In this embodiment, the area
reduction ratio of the rail 9 to be rolled from the (k+1)-th
roughing mill 3Ak+1 to the finishing mill 3B as the rolling
process after the rough cooling device 4 is set to 20% or more.
Herein, the area reduction ratio in this embodiment shows the
area reduction ratio of a cross-sectional area perpendicular
to the longitudinal direction of the steel rail material and
shows the ratio of the reduction in the cross-sectional area
during the rolling to the cross-sectional area before the
rolling of the bloom and the like.
[0029]
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The rough cooling device 4 has a head portion cooling nozzle
41, a foot portion cooling nozzle 42, a head portion thermometer
43, a foot portion thermometer 44, a conveyance table 45, guides
46a and 46b, and a control unit 47 as illustrated in FIG. 2.
The head portion cooling nozzle 41 cools the head portion
91 of the rail 9 by ejecting a cooling medium to the head portion
91. The foot portion cooling nozzle 42 cools the foot portion
93 of the rail 9 by ejecting a cooling medium to the foot portion
93. The cooling medium ejected from the head portion cooling
nozzle 41 and the foot portion cooling nozzle 42 is spray water.
The head portion cooling nozzle 41 and the foot portion cooling
nozzle 42 are provided above the head portion 91 and the foot
portion 93, respectively, on the y-axis positive direction side
and eject a cooling medium to each of the head portion 91 and
the foot portion 93 with an inclination with respect to the y
axial direction. Moreover, two or more of the head portion
cooling nozzles 41 and the foot portion cooling nozzles 42 are
provided along the z axis direction perpendicular to the x-y
plane as the longitudinal direction of the rail 9.
[0030]
The head portion thermometer 43 and the foot portion
thermometer 44 are noncontact thermometers which measure the
surface temperature of each of the head portion 91 and the foot
portion 93 of the rail 9, respectively, to which the cooling
medium is ejected and are provided facing the head portion 91
and the foot portion 93, respectively, in the x axis direction.
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=
The measurement results of the head portion thermometer 43 and
the foot portion thermometer 44 are transmitted to the control
unit 47.
The Conveyance table 45 is a conveyance roll extending in
the x axis direction and two or more of the conveyance tables
45 are provided side by side along the z axis direction. The
guides 46a and 46b are plate-like members and are provided
extending in the z axis direction. The guides 46a and 46b are
individually disposed on the upper side relative to the
conveyance table 45 on the y-axis positive direction side and
on both end sides in the longitudinal direction of the conveyance
table 45. Furthermore, the guides 46a and 46b are further
provided with openings 461a and 461b at the positions where the
head portion thermometer 43 and the foot portion thermometer
44 are disposed, respectively.
[0031]
The control unit 47 controls the conditions of the cooling
medium ejected from the head portion cooling nozzle 41 and the
foot portion cooling nozzle 42 based on the measurement results
of the head portion thermometer 43 and the foot portion
thermometer 44 to thereby cool the rail 9 to a predetermined
surface temperature. The ejection conditions of the cooling
medium include the ejection amount, the ejection pressure, the
moisture amount, the ejection time, and the like of the cooling
medium, for example.
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The rough cooling device 4 of the configuration described
above is provided between the k-th roughing mill 3Ak and the
(k+1)-th roughing mill 3Ak+1 among the plurality of roughing
mills 3A located side by side in the rolling direction of the
rail 9 and controls the surface temperature of the head portion
91 and the foot portion 93 of the rail 9 to be rolled with the
k-th roughing mill 3Ak.
[0032]
The finish cooling device 5 is provided immediately before
the finishing mill 3B and controls the surface temperature of
the head portion 91 and the foot portion 93 of the rail 9 to
be rolled with the finishing mill 3B. The finish cooling device
5 has the same configuration as that of the rough cooling device
4 illustrated in FIG. 2.
The rail 9 is conveyed and rolled with an overturned state
as illustrated in FIG. 2 when rolled or cooled with the roughing
mills 3A, the rough cooling device 4, the finish cooling device
5, and the finishing mill 3B.
[0033]
The re-heating device 6 is an induction heating type
heating device and heats the head portion 91 of the rail 9 to
a predetermined temperature.
The heat treatment apparatus 7 has head portion cooling
headers 71a to 71c, a foot portion cooling header 72, a head
portion thermometer 73, and a control unit 74 as illustrated
in FIG. 3. The head portion cooling headers 71a to 71c are
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provided facing each of the head top surface and both head side
surfaces of the head portion 91 and cool the head portion 91
by ejecting a cooling medium to the head top surface and both
the head side surfaces. The foot portion cooling header 72 is
provided facing the underside of the foot portion 93 and cools
the foot portion 93 by ejecting a cooling medium to the underside
of the foot. For the cooling medium ejected from the head
portion cooling headers 71a to 71c and the foot portion cooling
header 72, air, water, mist, and the like are used. Two or more
of the head portion cooling headers 71a to 71c and the foot
portion cooling headers 72 are provided side by side along the
longitudinal direction of the rail 9. The head portion
thermometer 73 is a non-contact-type thermometer and measures
the surface temperature of the head portion 91. The temperature
measurement results of the head portion thermometer 73 are
transmitted to the control unit 74. The control unit 74
controls the ejection conditions of the cooling medium ejected
from the head portion cooling headers 71a to 71c and the foot
portion cooling header 72 according to the temperature
measurement results of the head portion thermometer 73 to
thereby control the cooling rate of the rail 9. The heat
treatment apparatus 7 of the configuration described above cools
the rail 9 at a predetermined cooling rate until the surface
temperature reaches a predetermined surface temperature. The
heat treatment apparatus 7 has a clamp (not illustrated). The
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clamp is a device restraining the foot portion of the rail 9
by holding the same.
The cooling bed 8 is a device which naturally cools the
rail 9 and contains, for example, a base supporting the rail
9.
[0034]
<Rail manufacturing method>
Next, a method for manufacturing the rail 9 according to
one embodiment of the present invention is described.
First, a bloom which is a steel rail material cast by a
continuous casting method is carried into the heating furnace
2 to be heated to reach 1100 C or more.
Subsequently, the heated steel rail material is rolled to
have an almost rail shape by the roughing mills 3Aa to 3Ak on
the upstream side in the conveyance direction relative to the
rough cooling device 4. Hereinafter, a steel material in the
hot-rolling process is also referred to as a steel rail material.
[0035]
Furthermore, the steel rail material rolled with the
roughing mills 3Aa to 3Ak is cooled (temperature adjustment)
with the rough cooling device 4 until the surface temperature
of portions corresponding to the head portion 91 and the foot
portion 93 of the rail 9 reaches 500 C or more and 1000 C or less.
Herein, the control unit 47 controls the ejection amount, the
ejection pressure, the moisture amount, the ejection time, and
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the like of the cooling medium to thereby cool the steel rail
material.
When the steel rail material is heated to 1100 C or more,
the entire structure is transformed into austenite. In the
austenite structure of 1000 C or more, the grain boundary easily
moves and re-crystallization occurs, so that the crystal grains
are coarsened. On the other hand, when the rolling is performed,
strain is generated in the crystal grains, and thus the crystal
grains are divided, and then refined. Herein, when the
temperature in the rolling is 1000 C or less, the
re-crystallization and the coarsening of the crystal grains are
difficult to occur. Therefore, by setting the temperature of
the steel rail material in the rolling to 1000 C or less, the
coarsening of the crystal grains refined by the rolling is
difficult to occur.
[0036]
When the steel rail material is cooled with the rough
cooling device 4, the temperature adjustment is preferably
performed until the surface temperature of the portions
corresponding to the head portion 91 and the foot portion 93
reach 500 C or more and 730 C or less. When the steel rail
material is cooled to 730 C or less, the structure partially
causes pearlite transformation. Therefore, the structure of
the steel rail material has a two phase structure containing
untransformed austenite and pearlite. When the austenite and
the pearlite are compared with each other, the yield strength
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of the austenite is lower, and therefore most of strain is
introduced in the austenite grains and the structure in the
rolling is refined as compared with the case where the structure
in the rolling is an austenite single phase. The colony size
and the block size of the pearlite as the final structure are
affected by the crystal grain diameter of the austenite which
is the structure before transformation. Therefore, when the
austenite grains are coarse, the colony size and the block size
of the pearlite are also coarsened, and therefore the ductility
decreases. On the other hand, when the austenite grains are
fine, the colony size and the block size of the pearlite are
refined, and therefore the ductility improves.
[0037]
When the temperature of the rail 9 in the rolling reaches
less than 500 C, the structure completely causes pearlite
transformation, and therefore the austenite grains are not
present. Therefore, the colony size and the block size of the
pearlite do not become smaller, and thus an improvement of
ductility cannot be expected.
The phenomenon described above occurs irrespective of
portions of the rail 9. Therefore, by performing the rolling
after the temperature adjustment is performed in the portions
corresponding to the head portion 91 and the foot portion 93,
toughness and ductility is improved.
[0038]
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e
Thereafter, the steel rail material subjected to the
temperature adjustment with the rough cooling device 4 is
further rolled with the roughing mills 3Ak+1 to 3An.
Subsequently, the steel rail material rough-rolled with
the roughing mills 3A1 to 3An is cooled with the finish cooling
device 5 as necessary, and then rolled with the finishing mill
32 to be formed into the rail 9 of a desired shape. The rolling
in the roughing mills 3Ak+1 to 3An and the finishing mill 3B
after the temperature adjustment is also referred to as
temperature-adjusted rolling. The area reduction ratio of the
steel rail material to be subjected to the temperature-adjusted
rolling is 20% or more. By setting the area reduction ratio
to 20% or more, strain can be generated also in the steel rail
material, and therefore the inside structure of the rail 9 can
be refined. On the other hand, when the area reduction ratio
is less than 20%, a large number of strains are generated in
the surface of the steel rail material but the number of strains
generated inside the steel rail material decreases. Therefore,
the refinement of the inside structure of the rail 9 becomes
difficult to achieve, so that a ductility improvement degree
decreases.
[0039]
Furthermore, the rail 9 hot-rolled with the roughing mills
3A and the finishing mill 3B is conveyed to the re-heating device
6 to be heated until the surface temperature of the head portion
91 reaches 730 C or more and 900 C or less.
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CA 02962031 2017-03-21
Thereafter, the heated rail 9 is conveyed to the heat
treatment apparatus 7 to be forcibly cooled (heat treatment)
with the heat treatment apparatus 7 in the state of being
restrained by the clamp until the surface temperature of the
head portion 91 reaches 600 C or less. Herein, the control unit
74 calculates the cooling rate of the rail 9 from the temperature
measurement results of the head portion thermometer 73, and then
controls the ejection conditions of the cooling medium ejected
from the head portion cooling headers 71a to 71c so that the
average cooling rate is 1 C/s or more and 10 C/s or less.
Moreover, the control unit 74 controls the ejection conditions
of the cooling medium ejected from the foot portion cooling
header 72 in such a manner as to be the same as any one of the
ejection conditions of the cooling medium ejected from the head
portion cooling headers 71a to 71c.
[0040]
When the surface temperature of the head portion 91 is less
than 730 C before the heat treatment, the structure partially
or entirely causes pearlite transformation. Before the heat
treatment, the rail 9 is naturally cooled and the cooling rate
is low. Therefore, the pearlite lamella intervals is coarse.
Therefore, by performing re-heating so that the surface
temperature of the head portion 91 reaches 730 C or more before
the heat treatment, the pearlite structure is reversely
transformed to the austenite structure, and thus the lamella
structure is able to be formed again. On the other hand, when
- 23 -
CA 02962031 2017-03-21
the surface temperature of the head portion 91 is higher, the
hardening of the decarburized layer on the surface and the
hardening due to an improvement of the cooling rate inside the
rail are achieved, so that the wear resistance is improved.
However, when the surface temperature of the head portion 91
exceeds 900 C, the effect described above is lowered.
Furthermore, when the surface temperature of the head portion
91 exceeds 1000 C, the re-crystallization and the coarsening
of the austenite grains occur, which is not preferable.
Therefore, considering the saving of the energy required for
the re-heating and the wear resistance improvement effect, the
upper limit of the surface temperature in the re-heating before
the heat treatment is preferably set to 900 C.
[0041]
In order to achieve high wear resistance properties, the
reduction in the pearlite lamella intervals is effective. In
order to reduce pearlite lamella intervals, heat treatment at
a high cooling rate is required. Therefore, the heat treatment
is preferably performed at a surface temperature and at an
average cooling rate within the ranges mentioned. When the
cooling rate is less than 1 C/s, the pearlite lamella intervals
are coarse and the wear resistance decreases. On the other hand,
when the cooling rate exceeds 10 C/s, the structure after
transformation is such as bainite and martens ite that are poor
in toughness and ductility, and this is not preferable. In this
embodiment, the average cooling rate is a cooling rate
- 24 -
CA 02962031 2017-03-21
* =
determined from the temperature changes and the heat treatment
time from the start of the heat treatment to the end of the heat
treatment. Therefore, the thermal history from the start of
the heat treatment to the end of the heat treatment also includes
the heat generation of the phase transformation heat and
isothermal-holding by patenting treatment. When the surface
temperature of the head portion 91 at the end of the heat
treatment exceeds 600 C, the lamella structure is partially
spheroidized after the end of the heat treatment, and therefore
the lamella intervals are coarse and the wear resistance
decreases.
Subsequently, the rail 9 subjected to accelerated cooling
is conveyed to the cooling bed 8, and then naturally cooled until
the temperature reaches about 100 C or less. After the cooling
on the cooling bed 8, shape correction of the rail 9 is performed
as necessary when the rail 9 is bent or the like. By passing
through the processes described above, the rail 9 excellent in
ductility and wear resistance is manufactured.
[0042]
<Modification>
Hitherto, the preferable embodiments of the present
invention are described in detail with reference to the
accompanying drawings but the present invention is not limited
to such examples. It is apparent that a person who has ordinary
knowledge in the technical field to which the present invention
belongs can perceive various changes or modifications within
- 25 -
CA 02962031 2017-03-21
the scope of the technical thoughts described in claims. It
should be understood that these changes or modifications also
naturally belong to the technical scope of the present
invention.
[0043]
For example, the cooling method in the rough cooling device
4 and the finish cooling device 5 is spray cooling employing
spray water for the cooling medium in the embodiment described
above but the present invention is not limited to the example.
For example, mist cooling as spray cooling employing mist as
a cooling medium or mixed cooling of mist cooling and air blast
cooling employing mist and air as a cooling medium may be used
for the cooling method in the rough cooling device 4 and the
finish cooling device 5. Alternatively, natural cooling,
immersion cooling, air blast cooling, water column cooling, and
the like may be performed in place of the spray cooling with
the rough cooling device 4 and the finish cooling device 5. In
the natural cooling and the air blast cooling, the cooling rate
is low, and therefore the time until the rail 9 is cooled to
a predetermined temperature is prolonged. Therefore, when the
rolling pitch is to be increased, other cooling methods, such
as spray cooling, immersion cooling, and water column cooling,
are able to be employed. However, the cooling rate in the water
column cooling is excessively high, and therefore the cooling
rate is difficult to adjust. Furthermore, when the rail 9 is
conveyed with an overturned state, water is stored in the web
- 26 -
CA 02962031 2017-03-21
portion 92 of the rail 9, resulting in the generation of a portion
with an excessively high cooling rate. Therefore, the
structure may be transformed to a structure having low toughness
and ductility, such as bainite and martensite. On the other
hand, the spray cooling has advantages in that a somewhat high
cooling rate is able to be secured and the cooling portion is
easily localized. Therefore, the spray cooling is preferably
used for the cooling method in the rough cooling device 4 and
the finish cooling device 5.
[0044]
Furthermore, the temperature-adjusted rolling is
performed in the rolling pass after the roughing mill 3Ak+1 in
the embodiment described above but the present invention is not
limited to the example. The temperature-adjusted rolling may
be performed after any roughing mill 3A insofar as an area
reduction ratio of 20% or more is able to be secured. Herein,
the rough cooling device 4 is provided immediately before the
roughing mill 3A with which the temperature-adjusted rolling
is started. The temperature-adjusted rolling may be performed
in finish rolling by the finishing mill 3B. Herein, the rough
cooling device 4 may not be provided in the rail manufacturing
apparatus 1 and the temperature adjustment may be performed only
the finish cooling device 5. When the temperature-adjusted
rolling is performed in finish rolling, the finish rolling needs
to be performed at a large area reduction ratio of 20% or more,
and therefore the shape of the rail 9 may deteriorates.
- 27 -
CA 02962031 2017-03-21
Therefore, the temperature-adjusted rolling is preferably
performed in the rolling with some of the roughing mills 3A and
with the finishing mill 3B.
[0045]
Furthermore, the roughing mills 3A and the finishing mill
3B are universal mills in the embodiment described above but
the present invention is not limited to the example. For
example, the roughing mills 3A and the finishing mill 3B may
be caliber rolling mills. In a universal rolling method,
rolling from a plurality of directions is achieved as compared
with a caliber rolling method, and therefore the rolling load
can be reduced. In particular, in the present invention, a
rolling operation capable of obtaining a large area reduction
ratio at a low temperature is performed, and therefore rolling
is performed under an overload and the load to the rolling mills
becomes high, so that the risk of a facility trouble becomes
high. Therefore, at least any one of the roughing mills 312 and
two or more of the finishing mills 3B is preferably a universal
mill.
Furthermore, two or more of the finishing mills 3B may be
provided.
[0046]
Furthermore, the re-heating device 6 is the induction
heating type heating device in the embodiment described above
but the present invention is not limited to the example. For
example, the re-heating device 6 may be a burner type heating
- 28 -
CA 02962031 2017-03-21
device. In the induction heating type re-heating device 6, the
size of the facility is able to be made small as compared with
the burner type. Therefore, the induction heating type
re-heating device 6 is preferable when disposed in-line.
The re-heating device 6 heats the head portion 91 in the
embodiment described above but the present invention is not
limited to the example. For example, the re-heating device 6
may have a configuration of heating the entire rail 9. When
the rail 9 is used, portions contacting wheels are worn out,
and therefore particularly the head portion 91 is required to
have wear resistance. Therefore, a configuration of re-heating
only the head portion 91 in re-heating is economically excellent
because energy required for the heating is able to be reduced.
[0047]
Furthermore, the re-heating is performed with the
re-heating device 6 after the hot-rolling in the embodiment
described above but the re-heating with the re-heating device
6 may not be performed. Herein, the hot-rolled rail 9 is
conveyed to the heat treatment apparatus 7, and then
heat-treated with the heat treatment apparatus 7. Even when
the re-heating is not performed, the ductility improvement
effect of the head portion 91 and the foot portion 93 is able
to be obtained. However, when the temperature of the rail 9
after the end of the hot-rolling (after the end of the
temperature-adjusted rolling) is low, the hardness decreases
as compared with the case where the temperature is high. In
- 29 -
CA 02962031 2017-03-21
addition to the re-heating, the heat treatment with the heat
treatment apparatus 7 is also omissible. Herein, the
hot-rolled rail 9 is conveyed to the cooling bed 8, and then
cooled until the temperature reaches about 100 C or less. Even
when the re-heating and the heat treatment are not performed,
the ductility improvement effect of the head portion 91 and the
foot portion 93 is able to be obtained. However, the hardness
decreases as compared with the case where the re-heating and
the heat treatment are performed.
[0048]
<Advantageous effects of embodiment>
(1) The method for manufacturing the rail 9 according to the
embodiment described above includes hot-rolling a heated steel
rail material, adjusting the temperature by cooling the
hot-rolled steel rail material, processing the steel rail
material subjected to the temperature adjustment into a rail
shape by means of temperature-adjusted rolling at an area
reduction ratio of 20% or more, and, in adjusting the temperature
of the steel rail material, cooling the surface portions of the
steel rail material corresponding to a head portion and a foot
portion of the rail shape so that the temperatures of the surface
portions reach 500 C or more and 1,000 C or less.
According to the configuration described above, crystal
grains are able to be divided and refined while preventing the
coarsening of the crystal grains due to the re-crystallization
in the austenite temperature range in the temperature-adjusted
- 30 -
CA 02962031 2017-03-21
rolling. Therefore, the toughness and the ductility of the head
portion 91 and the foot portion 93 of the rail 9 are able to
be increased.
[0049]
(2) After performing the temperature-adjusted rolling, the
rail 9 is heat-treated until the surface temperature of the head
portion of the rail 9 reaches 600 C or less at an average cooling
rate of 1 C/s or more and 10 C/s or less.
According to the configuration described above, the
pearlite lamella intervals of the head portion 91 of the rail
9 can be refined and the wear resistance is able to be increased.
Moreover, the spheroidization of the lamella structure after
the end of the heat treatment is able to be prevented, and
therefore the wear resistance improves.
[0050]
(3) Before heat-treating the rail 9, the rail is re-heated
to 730 C or more when the surface temperature of the head portion
of the rail 9 is less than 730 C.
According to the configuration described above, the
pearlite structure is able to be reversely transformed to the
austenite structure, so that the lamella structure is able to
be re-created again. Therefore, the hardness and the wear
resistance of the rail 9 are able to be increased.
(4) In re-heating the rail 9, only the head portion 91 of
the rail 9 is re-heated.
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CA 02962031 2017-03-21
According to the configuration described above, the energy
required for the heating is able to be reduced as compared with
the case where the entire rail 9 is re-heated.
[0051]
(5) The apparatus 1 for manufacturing the rail 9 according
to the embodiment has at least one first rolling mills 3A1 to
3AK rolling a steel rail material, a cooling device 4 adjusting
a temperature by cooling the steel rail material rolled with
the first rolling mills 3A1 to 3AK, and at least one second
rolling mills 3AK+1 to 3An and 3B processing the steel rail
material subjected to the temperature adjustment into a rail
shape by means of temperature-adjusted rolling at an area
reduction ratio of 20% or more, in which the cooling device 4
cools the surface portions of the steel rail material
corresponding to the head portion 91 and the foot portion 93
of the rail shape so that the temperatures of the surface
portions reach 500 C or more and 1,000 C or less.
According to the configuration described above, the same
effects as those obtained in (1) are able to be obtained.
Examples
[Example 1]
[0052]
Next, Examples 1 performed by the present inventors are
described.
In Examples 1, rails 9 were manufactured using the rail
manufacturing apparatus 1 described in FIG. 1 under various
- 32 -
CA 02962031 2017-03-21
= .
chemical composition conditions and rolling conditions, and
then the total elongation of the manufactured rails 9 was
measured.
Table 1 shows the chemical composition of the rail 9 used
in Examples 1. The remainder includes iron and inevitable
impurities. Table 2 shows the rolling conditions and the
measurement results of the total elongation in Examples 1.
[0053]
- 33 -
CA 02962031 2017-03-21
=
[Table 1]
Composition C[%] Si[%] Mn[%] P[%] S[%] Cr[%] Sb[%] Al[%] Ti[%] Others
A 0.83 0.52 0.51 0.015 0.008 0.192 0.0001 0.0005 0.001
B 0.83 0.52 1.11 0.015 0.008 0.192 0.0001 0.0005 0.001
C 1.03 0.52 1.11 0.015 0.008 0.192 0.0001 0.0005 0.001
D 0.84 0.54 0.55 0.018 0.004 0.784 0.0001 0.0000 0.002 V[%]:
0.058
E 0.82 0.23 1.26 0.018 0.005 0.155 0.0360 0.0001 0.001
Cu [%]
0.11,
F 0.83 0.66 0.26 0.015 0.005 0.896 0.1200 0.0005 0.001
Ni[%]:
0.12,
Mo[%]: 0.11
G 0.82 0.55 1.13 0.012 0.002 0.224 0.0001 0.0000 0.000
Nb[0]:
0.009
- 34 -
. .
[0054]
[Table 2]
At start of
In temperature-adjusted
temperature-adjusted
Total Total
Time from
rolling
Compositi temperature-adjusted Number of
Temperature rolling elongation elongation
Condition adjustment temperature-
adjusted Head portion Foot portion .. of head .. of foot
on rolling to end of Head portion Foot
portion
method rolling passes
area reduction area reduction portion portion
finish rolling [s] temperature
temperature
ratio
ratio [8] [8]
[ C] [ C]
[8]
[8]
Ex. 1-1 A , Spray cooling 20 4 950
900 30 30 14 13
Ex. 1-2 B Spray cooling 5 4 950
900 30 30 14 13
Ex. 1-3 c Spray cooling 30 5 950
900 30 30 12 12
Ex. 1-4 A Natural cooling 20 2 950
900 30 30 14 13
Ex. 1-5 , A Airblastcooling 10 2 950
900 30 30 14 13
Ex. 1-6 A Spray cooling , o 1 950 900
30 30 14 13
Ex. 1-7 A Spray cooling 1 2 950
900 30 30 14 13
g
Ex. 1-8 A Spray cooling 20 3 500
500 30 30 20 19
Ex. 1-9 , A Natural cooling 5 2 950
900 30 30 14 13 2
.
,..
Ex. 1-10 A Airblast cooling 30 5 950
900 30 30 14 13 .
Ex. 1-11 A Spray cooling 30 3 990
900 30 30 12 13 .
w
f Ex. 1-12 A Spray cooling 20 4 , 850
900 30 30 15 13 r
Ex. 1-13 A Spray cooling 10 3 750
900 30 30 15 13 .
r
Ex. 1-14 A , Spray cooling 0 1 650
900 30 30 18 13 'j
Ex. 1-15 A Spray cooling 30 4 500
900 30 30 20 13 0
w
Ex. 1-16 A Spray cooling 40 4 , 950
990 30 30 14 12 I:,
Ex. 1-17 A Spray cooling 30 3 950
950 30 30 14 12 r
Ex. 1-18 , A Spray cooling 20 4 950
750 30 30 14 14
Ex. 1-19 A Spray cooling 20 5 950
650 30 30 14 17
Ex. 1-20 A Spray cooling 15 4 950
500 30 30 14 19
Ex. 1-21 A Natural cooling 30 6 950
900 20 30 12 13
Ex. 1-22 A Natural cooling 40 5 950
900 25 30 13 13
_ .
Ex. 1-23 A Spray cooling 20 4 950
900 30 20 14 12
Ex. 1-24 A Natural cooling 50 6 950
900 , 30 30 14 12
Ex. 1-25 , C Spray cooling 25 4 950
900 30 30 14 13
Ex. 1-26 E Spray cooling 25 3 950
900 30 30 14 13
Ex. 1-27 F Spray cooling 25 5 950
900 30 30 14 13
Ex. 1-28 G Spray cooling 25 4 950
900 30 30 14 13
Comp. Ex. 1-1 A , Spray cooling 20 2 850 1020
30 30 14 11
Comp. Ex. 1-2 A Spray cooling 60 4 850 900
30 10 14 10
Comp. Ex. 1-3 A Spray cooling 20 2 400 900
30 30 10 13
Comp. Ex. 1-4 A Spray cooling 45 2 1020 900
30 30 11 13
Comp. Ex. 1-5 A Spray cooling 15 4 950 900
10 30 10 13
- 35 -
CA 02962031 2017-03-21
[0055]
In Examples 1, first, a continuously cast bloom was
heated with the heating furnace 2 until the temperature
reached 1100 C. The chemical composition of the bloom used
in Examples I was any one of the composition A to the
composition G of Table 1 as shown in Table 2.
Subsequently, the heated bloom was collected from the
heating furnace 2, and then hot-rolled with the roughing mills
3A and the finishing mill 3B. For the roughing mills 3A, a
plurality of rolling mills in which a universal mill and a
caliber rolling mill were combined was used. The rail 9
during the rolling was rolled and conveyed with an overturned
state. When the hot-rolling was performed, the temperature
adjustment was performed until the surface temperatures of
the head portion 91 and the foot portion 93 reached 500 C or
more and 1000 C or less with either the rough cooling device
4 or the finish cooling device 5. The temperature adjustment
method, the time from the start of the temperature-adjusted
rolling to the end of the hot-rolling, and the number of
temperature-adjusted rolling passes are individually shown
in Table 2. The temperature-adjusted rolling refers to
hot-rolling after the temperature adjustment was performed.
[0056]
As shown in Table 2, in Examples 1, the temperature
adjustment was performed by any one of the spray cooling, air
blast cooling, and naturally cooling methods. The surface
temperatures of the head portion 91 and the foot portion 93
were adjusted by adjusting the water amount density and the
- 36 -
CA 02962031 2017-03-21
=
cooling time in the case of the spray cooling or by controlling
the cooling time without using the rough cooling device 4 and
the finish cooling device 5 in the case of the natural cooling.
[0057]
The number of the temperature-adjusted rolling passes
shown in Table 2 shows the number of rolling passes after the
temperature adjustment was performed by any one of the methods
described above. For example, the number of times of the
temperature-adjusted rolling passes was 1 time indicates that,
after the temperature adjustment, only the finish rolling was
performed and the number of times of the temperature-adjusted
rolling passes was n (1-12) times indicates that, after the
temperature adjustment, n-1 times of rough rolling and one
finish rolling were performed. When the number of times of
the temperature-adjusted rolling passes was 1 time, the
temperature adjustment was performed using the finish cooling
device 5. When the number of times of the
temperature-adjusted rolling passes was n times, the
temperature adjustment was performed using the rough cooling
device 4.
[0058]
After the hot-rolling was performed, the rail 9 was
forcibly cooled with the heat treatment apparatus 7. The
surface temperatures of the head portion 91 and the foot
portion 93 in starting the forcible cooling were set as shown
in the conditions shown in Table 2. When the forcible cooling
was performed, the average cooling rate was set to 3 C/s. The
cooling was performed until the surface temperature reached
- 37 -
CA 02962031 2017-03-21
400 C. When the forcible cooling was performed, mist was used
for a cooling medium. In Examples 1, the re-heat treatment
employing the re-heating device 6 was not performed after the
hot-rolling.
[0059]
Subsequently, the forcibly cooled rail 9 was conveyed
to the cooling bed 8, the temperature was reduced to 100 C
or less by cooling, and then the rail was straightened. After
the rail 9 was manufactured in the processes described above,
test pieces were collected from four places of an end portion,
the 1/4 position, the 1/2 position, and the 3/4 position in
the longitudinal direction of the rail 9, and then various
physical properties were measured. As illustrated in FIG.
5, a sample 9a was collected from the head portion 91 and a
sample 9b was collected from the foot portion 93 of the test
pieces collected at each position in the longitudinal
direction. The sample 9a is a JIS No. 4 test piece collected
from a position having a distance d2 = 12.7 mm from the upper
end of the head portion 91 and having a distance dl = 24.6
mm from the center in the width direction. The sample 9b is
a JIS No. 4 test piece collected from a position having a
distance d3 = 12.7 mm from the lower end of the foot portion
93 and at the center in the width direction.
[0060]
In Examples 1, as examples different in the chemical
composition, the temperature adjustment method, the number
of temperature-adjusted rolling passes, the surface
temperature, and the area reduction ratio, rails 9 were
- 38 -
CA 02962031 2017-03-21
manufactured under 28 kinds of conditions of Examples 1-1 to
1-28, and then the total elongation was evaluated.
Moreover, as shown in Table 2, rails 9 were manufactured
as comparative examples under the same conditions as those
of Examples 1-1 to 1-28, and then the total elongation was
evaluated also for Comparative Examples 1-1 to 1-5 with the
surface temperature and the area reduction ratio in the
temperature-adjusted rolling outside the ranges of the
embodiment described above. The total elongation values
shown in Table 2 show the average value of the four samples,
i.e., the sum of one sample collected from each of the test
pieces collected from each of the four places.
[0061]
It was confirmed that the total elongations of the head
portion 91 and the foot portion 93 were 12% or more as the
target total elongation under all the conditions of Examples
1-1 to 1-28. It was also confirmed that, in Examples 1-14,
1-15, 1-19, and 1-20 in which the surface temperature of
either the head portion 91 or the foot portion 93 was 730 C
or less in the temperature-adjusted rolling, the elongation
of the head portion 91 or the foot portion 93 with a low surface
temperature was as high as 17% or more. Furthermore, it was
confirmed that, in Example 1-8 in which the surface
temperatures of both the head portion 91 and the foot portion
93 in the temperature-adjusted rolling were 730 C or less,
the total elongations of the head portion 91 and the foot
portion 93 were as high as 19% or more.
[0062]
- 39 -
CA 02962031 2017-03-21
On the other hand, in Comparative Example 1-1 in which
the surface temperature of the foot portion 93 in the
temperature-adjusted rolling exceeded 1000 C and Comparative
Example 1-2 in which the area reduction ratio of the foot
portion 93 in the temperature-adjusted rolling was less than
20%, the elongation of the foot portion 93 was less than 12%
and decreased as compared with those of Examples 1-1 to 1-28.
In Comparative Examples 1-3 and 1-4 in which the surface
temperature in the temperature-adjusted rolling was less than
500 C or exceeded 1000 C and Comparative Example 1-5 in which
the rolling reduction of the head portion 91 in the
temperature-adjusted rolling was less than 20%, the
elongation of the head portion 91 was less than 12% and
decreased as compared with those of Examples 1-1 to 1-28.
[Example 2]
[0063]
Next, Examples 2 performed by the present inventors are
described.
In Examples 2, influences on the total elongation, the
hardness, and the surface structure depending on the heat
treatment conditions were confirmed by varying the chemical
composition and the conditions in the temperature-adjusted
rolling and the heat treatment. Table 3 shows the chemical
composition, the surface temperature in
temperature-adjusted rolling, the conditions of heat
treatment (forcible cooling), the measurement results of the
total elongation, the measurement results of the hardness,
- 40 -
CA 02962031 2017-03-21
, .
and the observation results of a head portion surface
structure in Examples 2.
[0064]
- 41 -
[Table 3]
In temperature-adjusted
In heat treatment
Total elongation Hardness
rolling
¨
Head
Head
Headportionsurface
Condition Composition Head portion Foot portion Start Cooling End
Head Foot portion
portion
structure
temperature temperature temperature rate temperature portion portion inner
surface
rc] [ C] [ c] ] C/s] m] [%1 [5]
[HE] region
[HE]
Ex. 2-1 A 950 900 890 3 400 14 13
410 380 Fine pearlite
Ex. 2-2 A 850 900 800 3 400 15 ,
13 408 370 Fine pearlite
Ex. 2-3 A 650 900 630 3 400 18 13
380 360 Coarse pearlite
Ex. 2-4 A 950 950 890 3 , 400 , 14
13 410 380 Fine pearlite
Ex. 2-5 A 950 750 890 3 400 14 14
410 380 Fine pearlite
Ex. 2-6 A 950 650 5 3 400 14 17
410 380 Fine pearlite
Ex. 2-7 A 950 900 890 0.5 400 14 13
375 345 Coarse pearlite g
Ex. 2-8 A 950 900 890 1 400 14 13
390 355 Fine pearlite
2
Ex. 2-9 A 950 900 890 5 400 14 13
420 385 Fine pearlite .
0,
Ex. 2-10 A 950 900 890 10 400 14 13
440 400 Fine pearlite
Partially w
H
Ex. 2-11 A 950 900 890 3 650 14 13
380 355 spheroidized ,..,
pearlite r
,
1
Ex. 2-12 A 950 900 890 3 500 14 13
400 370 Fine pearlite 0
w
I
Partially
Ex. 2-13 A 950 900 - - 14 13
350 340 spheroidized r
pearlite
Ex. 2-14 B 950 900 890 0.5 400 14 13
430 380 Fine pearlite
Ex. 2-15 B 950 900 890 3 400 14 13
465 395 Fine pearlite ,
Ex. 2-16 C 950 900 890 0.5 400 12 12
460 395 Fine pearlite
Ex. 2-17 C 950 900 890 3 400 12 12
485 410 Fine pearlite
Ex. 2-18 D 950 900 890 3 400 14 13
485 410 Fine pearlite
Ex. 2-19 E 950 900 890 3 400 14 13
410 375 Fine pearlite
Ex. 2-20 F 950 900 890 3 400 14 13
420 377 Fine pearlite
Ex. 2-21 G 950 900 890 3 400 14 13
435 382 Fine pearlite
Partially
Comp. Ex. 2-1 A 950 900 890 15 400 3 13
690 410
martensite
Partially
Comp. Ex. 2-2 B 950 900 890 15 400 3 13
720 420
martensite
Partially
Comp. Ex. 2-3 C
I 950 900 890 15 400 3 12
740 435
martensite
¨ 42 ¨
CA 02962031 2017-03-21
[0065]
In Examples 2, as the temperature-adjusted rolling,
rolling in four passes in total containing three universal
mills and one caliber rolling mill was performed so that the
area reduction ratios of the head portion 91 and the foot
portion 93 were 30%. The surface temperatures of the head
portion 91 and the foot portion 93 in the temperature-adjusted
rolling and the start temperature, the cooling rate, and the
end temperature in the heat treatment were set as shown in
the conditions shown in Table 3. When the heat treatment was
performed, air was used for a cooling medium under the
condition where the cooling rate was 3 C/s or less and a
mixture of air and mist was used for a cooling medium under
the condition where the cooling rate exceeded 3 C/s. The
other manufacturing conditions were the same as those of
Examples 1.
[0066]
With respect to the total elongation of the rail 9, test
pieces were collected, and then the total elongation was
measured by the same method as that of Examples 1. With
respect to the hardness of the rail 9, a sample 9c was collected
from a position of the head portion surface illustrated in
FIG. 6 and a sample 9d was collected from a position inside
the head portion from the test pieces of about 20 mm thickness
sawn from four places of an end portion, the 1/4 position,
the 1/2 position, and the 3/4 position in the longitudinal
direction of the rail 9. The sample 9c was collected from
the center of the upper end surface of the head portion 91
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, .
of the test pieces polished in order to remove surface
unevenness. The sample 9d was collected from a position at
the center in the width direction and having a distance d4
- 20 mm from the upper end of the head portion 91 of the test
pieces polished in order to remove surface unevenness. Next,
the hardness of the collected samples 9c and 9d was measured
by a Brinell hardness test. With respect to the surface
structure, the surface structure of the collected samples 9c
was observed.
[0067]
In Examples 2, as examples different in the chemical
composition, the surface temperature in the
temperature-adjusted rolling, and conditions in the heat
treatment, rails 9 were manufactured under 21 kinds of
conditions of Examples 2-1 to 2-21, and then the total
elongation and the hardness were measured and further the
surface structure was observed. In Example 2-13, the heat
treatment was not performed and the rail 9 after the
hot-rolling was conveyed to the cooling bed 8, and then cooled
until the temperature reached 100 C or less. After the rail
9 reached 100 C or less, the rail was straightened.
[0068]
Also in Comparative Examples 2-1 to 2-3 in which the
cooling rate in the heat treatment exceeded the ranges of the
embodiment described above, rails 9 were manufactured as
comparative examples under the same conditions as those of
Examples 2-1 to 2-21, and then the total elongation and the
hardness were measured and further the surface structure was
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observed as shown in Table 3. The values of the total
elongation and the hardness shown in Table 3 show the average
value of the four samples individually collected from the test
pieces collected from the four places.
[0069]
It was confirmed that, in Examples 2-1 to 2-21 in which
the heat treatment was performed at a cooling rate of 0.5 C/s
or more and 10 C/s or less, the total elongations of the head
portion 91 and the foot portion 93 were 12% or more as the
target total elongation in all the conditions.
In Examples 2-2 and 2-3, the surface temperature of the
head portion 91 in the temperature-adjusted rolling was lower
than that in other conditions, the surface temperature in
starting the heat treatment was also low and the total
elongation of the head portion 91 was 15% or more, which was
higher than that in other conditions. However, in Examples
2-2 and 2-3, the hardness of the head portion 91 was 380 HB
or less, which was lower than that in Example 2-1.
[0070]
In Examples 2-1, 2-7 to 2-10, and 2-14 to 2-21 in which
the conditions except the cooling rate in the heat treatment
were the same and, further, in Examples 2-14 to 2-21 in which
the composition is different, the hardness of the surface and
inside of the head portion 91 improved when the cooling rate
was higher. In Examples 2-1, 2-7 to 2-10, and 2-14 to 2-21
and Comparative Examples 2-1 to 2-3 in which the conditions
except the cooling rate in the heat treatment were the same
and, further, in Comparative Examples 2-1 to 2-3 in which the
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cooling rate exceeded 10 C/s, the cooling rate was excessively
high, and therefore the structure was partially transformed
into a martensite and the total elongation was as very low
as 3%.
[0071]
In Examples 2-1, 2-11, and 2-12 in which the conditions
except the end temperature in the heat treatment were the same,
the hardness of the surface and inside of the head portion
91 improved when the cooling stop temperature was lower. In
Example 2-11 in which the end temperature in the heat
treatment was set to 650 C, the pearlite structure was
partially spheroidized.
In Example 2-13 in which the heat treatment was not
performed, the total elongations of the head portion 91 and
the foot portion 93 were 12% or more but the hardness of the
surface and inside of the head portion 91 was the lowest in
all the conditions. In Example 2-13, the pearlite structure
was partially spheroidized.
[Example 3]
[0072]
Next, Examples 3 performed by the present inventors are
described.
In Examples 3, in order to confirm influences on the
hardness and the surface structure by re-heat treatment,
re-heating was performed before the heat treatment with
respect to the condition of Example 2-3 in which the hardness
was low. In Examples 3, manufacturing conditions other than
the surface temperature of the head portion 91 in the
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temperature-adjusted rolling and performing re-heating were
the same as those of Example 2-3. Table 4 individually shows
the chemical composition, the surface temperature in the
temperature-adjusted rolling, the conditions in the
re-heating and the heat treatment, the measurement results
of the total elongation, the measurement results of the
hardness, and the observation results of the head portion
surface structure in Example 3. The total elongation values
and the hardness shown in Table 4 show the average value of
the four samples, i.e., the sum of one sample collected from
each of the test pieces collected from each of the four places.
[0073]
- 47 -
[Table 4]
In temperature-adjusted Total
Re-heating In heat treatment
Hardness
rolling
elongation
Head
Head
Head
portion
Condition Composition Head portion Foot portion Start Cooling
End Head Foot portion
Presence
portion surface
temperature temperature Position temperature
rate temperature portion portion inner
or absence
surface structure
: C] [ C] [0C) [ C/s]
[CO] 1%-] [%] region
[HB]
[FIB]
Not
Coarse
Ex. 3-1 A 650 900 630 3 400 18
13 380 360
performed
pearlite
Coarse
Ex. 3-2 A 650 900 Performed Entire 700 3 400 18
13 380 360
pearlite
Fine
Ex. 3-3 A 950 900 Performed Entire 750 3 400 18
13 400 365
pearlite g
Fine
0
Ex. 3-4 A 950 900 Performed Entire 890 3 400 18
13 410 380 .
pearlite 0,
Fine
.
Ex. 3-5 A 950 900 Performed Entire 950 3 400 18
13 410 380 w
r
pearlite
,
,..,
Only head
Coarse .
Ex. 3-6 A 650 900 Performed 700 3 400 18
13 380 360 r
portion
pearlite ,
1
0
w
Ex. 3-7 A 950 900 Performed Only head Fine750 3 400
18 13 400 365 1
portion
pearlite
r
Only head
Fine
Ex. 3-8 A 950 900 Performed 890 3 400 18
13 410 380
portion
pearlite
Only head
Fine
Ex. 3-9 A 950 900 Performed 950 3 400 18
13 410 380
portion
pearlite
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[0074]
In Examples 3, the head portion 91 or the entire rail
9 was re-heated with the re-heating device 6 after the
hot-rolling. The re-heating device 6 is an induction heating
type heating device and is able to heat the head portion 91
or the entire rail 9 according to the conditions shown in Table
4. The surface temperature of the head portion 91 after the
re-heating is the start temperature in the heat treatment
shown in Table 4.
In Examples 3, rails 9 were manufactured under 9 kinds
of conditions of Examples 3-1 to 3-9 different in the surface
temperature of the head portion 91 in the
temperature-adjusted rolling and the re-heating conditions,
and then the total elongation and the hardness were measured
and further the surface structure was observed. A method for
collecting samples for the total elongation and the hardness
and a method for collecting samples for observing the surface
structure are the same as those of Examples 2. Example 3-1
is the condition in which the re-heating was not performed
and has the same manufacturing conditions as those of Example
2-3.
[0075]
As shown in Table 4, it was confirmed that, in all the
conditions of Examples 3-1 to 3-9, the total elongations of
the head portion 91 and the foot portion 93 were 12% or more
as the target total elongation.
In Example 3-1 in which the re-heating was not performed,
the surface temperature in starting the temperature-adjusted
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rolling was low, and therefore the surface temperature of the
head portion 91 in starting the heat treatment was as low as
630 C and the hardness of the surface and inside of the head
portion 91 was low.
[0076]
In Examples 3-2 and 3-6, the re-heating was performed
and the surface temperature of the head portion 91 in starting
the heat treatment was set to 700 C but the surface temperature
was as low as 730 C or less, and therefore the hardness of
the surface and inside of the head portion 91 was low as in
Example 3-1.
It was confirmed that, in Examples 3-3 to 3-5 in which
the entire rail 9 was re-heated and Examples 3-7 to 3-9 in
which only the head portion 91 was re-heated, the hardness
improved by 20 HE or more on the surface of the head portion
91 and 5 HB or more inside the head portion 91 as compared
with Examples 3-2 and 3-6 in which the temperature after the
re-heating was low. Moreover, it was confirmed that there
is no difference in the hardness improvement effect of the
head portion 91 between the case where the entire rail 9 was
re-heated and the case where only the head portion 91 was
re-heated. Furthermore, it was confirmed that there is no
difference in the hardness of the head portion 91 when
Examples 3-4, 3-5, 3-8, and 3-9 are compared, and therefore
there is no difference in the hardness improvement effect by
re-heating when the surface temperature after the re-heating
was 900 C or more.
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It was confirmed from the results described above that
the rail 9 having high ductility in both the head portion 91
and the foot portion 93 is able to be manufactured according
to the method and the apparatus for manufacturing a rail
according to the present invention.
Reference Signs List
[0077]
1: manufacturing apparatus
2: heating furnace
3A, 3A1 to 3An: roughing mill
3B: finishing mill
4: rough cooling device
41: head portion cooling nozzle
42: foot portion cooling nozzle
43: head portion thermometer
44: foot portion thermometer
45: conveyance table
46a, 46b: guide
461a, 461b: opening
5: finish cooling device
6: re-heating device
7: heat treatment apparatus
71a to 71c: head portion cooling header
72: foot portion cooling header
73: head portion thermometer
74: control unit
8: cooling bed
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, .
9: rail
91: head portion
92: web portion
93: foot portion
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