Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02558850 2006-09-06
DESCRIPTION
A METHOD FOR PRODUCING HIGH-CARBON STEEL RAILS
EXCELLENT IN WEAR RESISTANCE AND DUCTILITY
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates to methods for producing pearlitic
steel rails having a high content of carbon.
The present steel
rails are excellent in both wear resistance and ductility and may
be used in railroads for carrying heavy loads.
Description of the Related Art
[0002] Pearlitic steel having a high carbon content has been used
for railroads due to its excellent wear resistance. However, the
high carbon content therein also causes problems of low ductility
and toughness.
For example, steel rails having a typical carbon
content (e.g., containing about 0.6 - 0.7 mass% of carbon
described in JIS (Japan Industrial Standard) E1101-1990) have a
value according to the JIS No.3 U notch Charpy test at room
temperature of around 12 - 18 J/cm2.
These steel rails having
such typical carbon content have the problem of brittle fractures
caused by small initial defects and fatigue cracks when used in
lower temperature such as what we call it "cold range". In recent
years, the carbon content of steel rails has increased in order to
improve the wear resistance, which causes further lowering of both
the ductility and the toughness of the steel rails.
[0003] Generally, it is said that grain refinement of the pearlite
structure, more specifically having a fine-grained structure of
both austenite (before transformation to pearlite) and pearlite,
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is effective in simultaneously improving both ductility and
toughness of the steel rail.
In order to obtain a fine-grained
structure of austenite, a decrease in the temperature and an
increase in the amount of the reduction rate of the hot rolling
process are carried out. Furthermore, a reheat treatment at a low
temperature after rolling is carried out.
In order to obtain a
fine-grained structure of pearlite, an acceleration of pearlite
transformation from austenite grains by the use of seed
transformation is carried out.
[0004] However, decreasing the temperature and increasing the
amount of the reduction rate of the hot rolling process have a
limitation in terms of maintaining good formability.
This
limitation has not allowed production of a sufficiently fine-
grained austenite grain. As for effecting pearlite transformation
from austenite grain using seed transformation, it is difficult to
control the amount of seed transformation. This limitation makes
it difficult to perform stable pearlite transformation from
austenite grain.
[0005] In view of above, the following method has been used. This
method is one where a fine-grained pearlite structure is obtained
by pearlite transformation caused by rapid cooling after reheat
treatment of a steel rail at a low temperature following a rolling
process.
This method improves both the ductility and the
toughness of the pearlitic steel rail. However, the carbon rails,
in which has increased the carbon content in order to improve the
wear resistance, causes a decrease in the ductility and toughness
of the pearlite structure after the accelerated cooling process.
This problem is due to the fact that coarse carbides remain
insoluble in the austenite grains when the reheat treatment at a
low temperature is carried out.
The reheat process also
introduces economic problems since it generally increases the
production cost and decreases the productivity.
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[0006] Research and development of a production method for steel
rails having a high carbon rails, which simultaneously ensures
both formability during rolling and a fine-grained pearlite
structure after rolling has been required. In order to address
this requirement, the following production methods of high-carbon
steel rails have been developed:
(1) Japanese Laid-open Patent Hei 07-173530 discloses a
production method for steel rails with high ductility where
three or more consecutive passes of rolling at set intervals
of time from one pass to next pass is carried out in the
finish rolling process of high carbon content steel rails;
(2) Japanese Laid-open Patent Hei 2001-234238 discloses a
production method for steel rails with a high wear resistance
and a high toughness where two or more consecutive passes of
rolling at set intervals of time from one pass to the next
pass is performed, then continuous rolling and rapid cooling
are sequentially carried out in the finish rolling process of
high carbon content steel rails; and
(3) Japanese Laid-open Patent Hei 2002-226915 discloses a
production method for steel rails with a high wear resistance
and a high toughness where cooling is allowed between passes
of rolling (inter-stand), and continuous rolling and rapid
cooling are sequentially carried out in the finish rolling
process of high-carbon rails.
[0007] Features of the rails in the above Japanese Laid-open
Patent Hei 07-173530, 2001-234238, and 2002-226915 include
improved ductility and toughness of pearlitic steel by obtaining a
uniformly sized fine-grained austenite grain by continuous rolling
thereby achieving a small reduction. This takes advantage of the
fact that steel with high carbon content is easy to recrystallize
at relatively low temperatures and with only a small reduction.
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[0008] The continuous rolling methods mentioned above, which are
mainly combinations of the carbon content of steel, the
temperature of continuous hot rolling, the number of rolling
passes and the time between passes, cannot achieve a fine-grain
austenite structure.
This leads to a coarse pearlite structure
and results in a failure to improve ductility. This is especially
true for the method employing cooling between passes of rolling
(inter-stand), as the rate of grain growth immediately after
rolling is high in high carbon content steel.
Thus, the grain
growth depends remarkably on the interval of time if cooling is
carried out between rollings (inter-stand).
Therefore, a fine-
grained austenite structure is not obtained and the pearlite
structure becomes coarse.
This results in the problem of no
improvement of ductility, even if the above-mentioned methods of
continuous rolling and/or cooling between rollings (inter-stand)
are applied.
SUMMARY OF THE INVENTION
[0009] The present invention relates to a method
for manufacturing a rail that is excellent in both ductility and
wear resistance by producing a pearlite of fine-grain structure
and high hardness.
[0010]
One embodiment of the invention relates to a method for
producing a steel rail having a high content of carbon,
comprising: finish rolling the rail in two consecutive passes,
with a reduction rate per pass for a cross-section of the rail of
2-30%, wherein conditions of the finish rolling satisfy the
following relationship: S-5 CPT1; wherein the CPT1 is the value
expressed by the following expression 1
CPT1 - 800 / (C x T) (expression 1)
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wherein S is the maximum rolling interval time (seconds), and (C x
T) is defined as follows; C is the carbon content of the steel,
wherein the carbon content is more than 0.85 mass%, but less than
or equal to 1.40 mass%, based on the total mass of the steel, and
T is the maximum surface temperature (degree C) of a rail head.
This method produces a steel rail with a high content of carbon
that is excellent in wear resistance and ductility.
[0011]
Another embodiment of the invention relates to a method
for producing a steel rail with a high content of carbon,
comprising: finish rolling the rail in three or more passes, with
a reduction rate per pass for a cross-section of the rail of 2-30%,
wherein conditions of the finish rolling satisfy the following
relationship: S
CPT2, wherein the CPT2 is the value expressed
by the following expression 2,
CPT2 - 2400 / (C x T x P) (expression 2)
wherein S is the maximum rolling interval time (seconds), and (C
X T X P) is defined as follows; C is the carbon content of the
steel rail, wherein the carbon content is more than 0.85 mass%,
but less than or equal to 1.40 mass%, based on the total mass of
the steel, and T is the maximum surface temperature (degree C) of
a rail head, and P is the number of passes, which is 3 or more.
This method produces a steel rail with a high content of carbon
that is excellent in wear resistance and ductility.
[0012]
In yet another embodiment, the rail of the present
invention, in addition to the carbon, further comprises at least
one element in the following list: Si, Mn, Cr, Mo, B, Co, Cu, Ni,
Ti, Mg, Ca, Al, Zr, N, V, Nb.
The balance of the rail comprises
Fe.
Additionally, the rail further optionally comprises
impurities, which may be unavoidable.
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[0013]
In another embodiment, the chemical composition of the
rail meet the following expression: 0.30 PC a 0.04;
where PC is expressed as the following (expression 3),
PC = V(mass%) + 10xNb(mass%) + 5xN(mass %) (expression 3)
[0014]
In yet another embodiment, the methods of the present
invention further comprise: immediately after finish rolling,
cooling the surface of the rail head at a cooling rate of
2-30 C/second until the surface temperature reaches 950-750 C.
Optionally, after such cooling, when the temperature of the rail
head is more than 700 C, the methods further comprise cooling the
surface of the rail head at a cooling rate of 2-30 C/second until
the surface temperature reaches at least 600 C, and then allowing
the rail to further cool at room temperature (e.g., approximately
45 F to 95 F, preferably 65 F to 85 F)
[0015]
In another embodiment, the methods of the present
invention further comprise: after the finish rolling process, when
the temperature of the rail head is more than 700 C, cooling the
surface of the rail head at a cooling rate of 2-30 C/second until
the surface temperature reaches at least 600 C, and then allowing
the rail to further cool at room temperature (e.g., approximately
45 F to 95 F, preferably 65 F to 85 F)
[0016]
According to the present invention, it is possible with
respect to a rail to obtain a fine-grained pearlite structure with
high-hardness, to improve the ductility of the rail and to
increase the life-span of the rail. This is a result of applying
one or more of the following conditions when a high carbon content
bloom of rail is continuously finish-rolled to form a rail:
(1) The maximum interval time of rolling is controlled to be less
than the time calculated from an expression concerning the
carbon content of steel and the maximum surface temperature
of rail at rolling (rail head) or from an expression
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concerning the carbon content of steel and the maximum
surface temperature of rail at rolling (rail head) and the
number of passes,
(2) The additional amounts of V, Nb and N are controlled so as to
be within a range defined based from an expression concerning
each additional amount of V, Nb and N in order to inhibit the
growth of austenite grain caused after the continuous rolling,
(3) Immediately after the continuous rolling, the surface of the
head of the rail is rapidly cooled down at a predetermined
cooling rate in a predetermined temperature range, and
(4) Furthermore the surface of the head of the rail in an
austenite phase is rapidly cooled down at a predetermined
cooling rate in a predetermined temperature range to obtain a
pearlite structure excellent in wear resistance and ductility.
BRIEF DESCRIPTION OF DRAWINGS
[0017]
FIG.1: Shows the relationship between the maximum
temperature of the rail head ( C) and the multiple of S, C, T (sX
CXT), where S is the maximum interval time of rolling (seconds),
C is the carbon content of the steel (mass%), and T is the maximum
temperature of rail head ( C).
[0018]
FIG.2: Shows the relationship between the carbon content
(mass%) and the multiple of S, C, T (S X C X T), where S is the
maximum interval time of rolling (seconds), C is the carbon
content of the steel (mass%), and T is the maximum temperature of
the rail head ( C).
[0019] FIG.3: Shows the relationship between the maximum interval
time of rolling (second) and the multiple of S, C, T (SxCxT),
where S is the maximum interval time of rolling (seconds), C is
the carbon content of the steel, and T is the maximum temperature
of the rail head ( C).
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[0020]
FIG.4: Shows the relationship between the number of
rollings (times) and the multiple of S, C, T, P (SxCxTxP), where S
is the maximum interval time of rolling (seconds), C is the carbon
content of the steel (mass%), T is the maximum temperature of the
rail head ( C), and P is the numbers of rollings (times).
[0021]
FIG.5: An illustration explaining the different portions
of the rail. In FIG.5, 1 is the top of rail head, and 2 is the
head corner.
[0022] FIG.6:
Shows the portion of the rail where the specimen
for the tensile test is taken.
[0023] FIG.7:
Shows the relationship between the carbon content
and the total elongation value of the rail. In FIG.7,
= indicates a rail produced by the methods of the invention
without control of the expression:
0.30 V(mass%) + 10xNb(mass%) + 5xN(mass %) 0.04,
0 indicates a rail produced by the methods of the invention with
the control of the above expression (PC value, wherein
PC = V(mass%) + 10xNb(mass%) + 5xN(mass %), therefore,
0.30 PC 0.04), and
x indicates a rail produced by conventional methods.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024]
The present inventors analyzed factors that cause the
pearlite structure to be coarse, which is the reason why ductility
is not improved.
This analysis was performed by studying the
combinations of carbon content, the surface temperature at the
rail head, the reduction rate of the cross-section of the rail,
and the interval time of rolling.
After various experiments, it
was found that the grain size of austenite structure turns coarse
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after continuous hot rolling if the maximum interval time during
continuous rolling exceeds a certain value.
[0025]
The present inventors investigated why the grain size of
austenite become coarse if the maximum interval time of rolling
increases.
It was found that the growth of grains of austenite
structure have a positive correlation with the carbon content of
the steel and the maximum surface temperature of the rail head
during continuous finish rolling. In addition, it was also found
that there is a positive correlation between the growth of grains
of austenite structure and the number of passes of rolling, such
as when the number of passes of rolling is 3 or more.
[0026] Based on the above results, the present inventors carried
out an analysis of multiple correlations on the relationship
between the optimal interval time of rolling for inhibiting the
grain size of austenite from becoming coarse, the carbon content,
the maximum surface temperature of the rail head during continuous
finish-rolling, and the number of rolling passes. The result was
that the growth of austenite grain at the interval of rolling is
inhibited and a fine-grained austenite structure is obtained if
the maximum interval time of continuous rolling is equal to or
less than the value calculated by particular equations. If the
number of rolling passes is 2, the equation is one based on the
carbon content and the maximum surface temperature of the rail
head. However, if the number of rolling passes is 3 or more, the
equation is one based on the carbon content, the maximum surface
temperature of the rail head, and the number of passes of rolling.
[0027]
The present inventors also investigated a method for
inhibiting the growth of austenite grain caused after continuous
rolling by controlling precipitation.
It was found that the
precipitation of V-carbide, V-Nitride, V-carbonitride, Nb-carbide
and Nb-carbonitride generated during continuous rolling causes
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pinning of austenite grains, which inhibits the growth of
austenite grain.
In addition, the present inventors investigated
the conditions where the precipitation of V-carbide, V-Nitride,
V-carbonitride, Nb-carbide and Nb-carbonitride during the
continuous rolling can be fully controlled.
The result is that
the growth of austenite grain after continuous rolling is
inhibited enough if the amount of addition of V, Nb and N (mass%)
are controlled, respectively, so that the value calculated by an
equation based on the amount of addition of V, Nb and N (mass%)
can be within a given range.
[0028]
The present inventors further investigated a method of
inhibiting the growth of austenite grain after (finishing)
continuous rolling by applying rapid cooling immediately after the
rolling. The result is that the growth of austenite grain after
rolling can be inhibited if the surface of the rail head is cooled
down rapidly within a predetermined range of temperature and at a
predetermined cooling rate immediately after completing the
continuous rolling.
[0029]
In addition to the above, the present inventors further
investigated a method of obtaining pearlite structure excellent in
wear resistance and ductility from a fine-grained austenite
structure. The result was that a pearlite structure having high
toughness and fine-grained structure can be obtained by rapidly
cooling the surface of the rail head having an austenite phase at
a predetermined temperature range and at a predetermined cooling
rate.
The obtained pearlite structure of the rail head retains
wear resistance and ductility.
[0030] The following are explanations of various limitations
defining the present invention:
(1) The reason for the limitation on the chemical composition of
steel rails:
C (carbon) is an element for expediting pearlite
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transformation and ensuring wear resistance. If the C content is
0.85 mass% or less, it is difficult to ensure a volume ratio of
cementite in the pearlite structure, which makes it difficult to
ensure wear resistance in use for railroads carrying heavy loads.
On the other hand, if the C content exceeds 1.40 mass%, a large
amount of pro-eutectoid cementite is generated on the old
austenite grain boundary, which lowers the wear resistance and the
ductility. In view of this, the C content is limited to the range
from more than 0.85 to 1.40 mass%. Preferably, a lower limit of C
content of 0.95 mass% can highly improve the wear resistance,
which greatly improves the life-span of the rail.
[0031] With respect to a rail produced using the above-mentioned
composition, at least one of Si, Mn, Cr, Mo, B, Co, Cu, Ni, Ti, Mg,
Ca, Al, Zr, N, V and/or Nb can be further added when needed for
improving the hardness (strength) of the pearlite structure, for
improving ductility of the pearlite structure, for preventing a
heat affected zone, for instance a welding zone, from softening,
and for controlling the section hardness distribution inside the
rail head.
[0032] The reasons for the limitations by these elements are as
follows: Si is an important element as an oxygen scavenger and as
an element for increasing the hardness (strength) of the rail head
through solid-solution strengthening with a ferrite phase in the
pearlite structure.
Besides, Si is an element for inhibiting
generation of a pro-eutectoid cementite structure in a
hyper-eutectoid steel to prevent the lowering of ductility. If the
Si content is less than 0.05 mass%, these good effects cannot be
significantly expected, and if the Si content is more than 2.00
mass%, the weldability is degraded because of generation of oxide
and/or generation of a great deal of surface flaws during hot
rolling.
In addition, the hardenability is drastically increased
and a martensite structure is generated which is detrimental to
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the wear resistance and the ductility of the rail. Thus, the Si
content is limited to the range of from 0.05 to 2.00 mass%.
[0033] Mn is an element for increasing the hardenability and for
improving the wear resistance by decreasing the pearlite lamellar
spacing to ensure the hardness of the pearlite structure. If the
Mn content is less than 0.05 mass%, these effects cannot be
significantly expected, which makes it difficult to ensure the
wear resistance necessary for the rail. If the Mn content is more
than 2.00%, the hardenability is drastically increased and a
martensite structure is generated which is detrimental to the wear
resistance and the ductility of the rail.
Therefore, the Mn
content is limited to the range of from 0.05 to 2.00 mass%.
[0034]
Cr is an element capable of increasing the equilibrium
transformation temperature, which leads to decrease of the
pearlite lamellar spacing to provide high hardness (strength). Cr
is also capable of strengthening the cementite phase, which leads
to increased hardness (strength) of the pearlite to provide
improved wear resistance.
If the Cr content is less than 0.05
mass%, these effects cannot be significantly expected. If the Cr
content is more than 2.00%, the hardenability is drastically
increased and a martensite structure is largely generated which
degrades the wear resistance and the ductility of the rail.
Therefore, the Cr content is limited to the range of from 0.05 to
2.00 mass %.
[0035]
Mo is an element capable of increasing the equilibrium
transformation temperature similar to Cr, which leads to decrease
of the pearlite lamellar spacing to provide high hardness
(strength). If the Mo content is less than 0.01 mass%, these
effects cannot be expected, i.e. improvement of hardness of the
rail cannot be significantly expected. If the Mo content is more
than 0.50 mass%, the transformation rate is drastically lowered,
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which leads to generation of a martensite structure, which is
detrimental to the ductility. In view of this, the Mo amount is
limited to the range of from 0.01 to 0.50 mass%.
[0036] B (boron) is an element for forming iron-carbon boride on
the grain boundary of austenite, increasing the fineness of the
generated pro-eutectoid cementite structure, making the pearlite
transformation temperature less dependent on the cooling rate and
making hardness distribution of the rail head uniform, which
prevents degradation of rail ductility and provides a longer life.
If the B content is less than 0.0001 mass%, these effects cannot
be significantly expected, i.e. there cannot be expected
improvement with respect to generation of pro-eutectoid cementite
structure and/or rail head hardness distribution uniformity.
If
the B content is more than 0.0050 mass%, coarse iron-carbon boride
is generated on the austenite grain boundary, which greatly lowers
the rail ductility and the fatigue-damage resistance. Thus, the B
content is limited to the range of from 0.0001 to 0.0050 mass%.
[0037] Co is an element for making solid-solution with ferrite in
the pearlite structure, which improves the hardness (strength) of
the pearlite structure by solid-solution strengthening.
Co is
also an element for increasing the transformation energy of
pearlite, which improves the ductility by refining the grain of
the pearlite structure. Also, Co is an element for improving wear
resistance by refining the grain of ferrite which is formed by
wheel contact on the rail head.
If the Co content is less than
0.003 mass%, these effects cannot be significantly expected.
If
the Co content is more than 2.00 mass%, the ductility of the
ferrite phase in the pearlite structure is drastically lowered,
which causes spalling damage on the rolling contact surface and
lowers the surface damage resistance of the rail. Therefore, the
Co content is limited to the range of from 0.003 to 2.00 mass%.
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[0038] Cu is an element for making solid-solution with ferrite in
the pearlite structure, which improves the hardness (strength) of
the pearlite structure by solid-solution strengthening. If the Cu
content is less than 0.01 mass%, these effects cannot be readily
expected. If the Cu content is more than 1.00 mass%, the
hardenability is drastically increased and a martensite structure
is generated which is detrimental to the wear resistance of the
rail. Further, the ductility of the ferrite phase in the pearlite
structure is drastically lowered, which degrades the ductility of
the rail. Therefore, the Cu content is limited to the range from
0.01 to 1.00 mass %.
[0039]
Ni is an element for preventing the creation of
brittleness during hot rolling caused by adding Cu and for
increasing the hardness (strength) of the pearlitic steel by
solid-solution strengthening with ferrite.
It is also an element
for inhibiting softening in heat-affected zones, for instance
welding zones, by precipitation strengthening (fine precipitation
of Ni3Ti, intermetallic compound). If the Ni content is less than
0.01 mass%, these effects cannot be readily expected. If the Ni
content is more than 1.00 mass%, the ductility of the ferrite
phase is drastically lowered, which causes on the rolling contact
surface and lowers the surface damage resistance of the rail.
Therefore, the Ni content is limited to the range of from 0.01 to
1.00 mass%.
[0040]
Ti is a element effective in preventing creation of
brittleness of the welded joint portion by increasing the fineness
of the structure of heat affected zones which are heated up to the
austenite region taking advantage of the insolubility of Ti
nitride and Ti carbide precipitated in reheating during welding.
If the Ti content is less than 0.0050 mass%, these effects cannot
be readily expected. If the Ti content is more than 0.0500 mass%,
coarse Ti nitride and Ti carbide are generated, which greatly
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lowers the ductility and the fatigue-damage resistance of the rail.
Thus, the Ti content is limited to the range of from 0.0050 to
0.0500 mass%.
[0041]
Mg is an element effective in improving the ductility of
the pearlite structure by forming fine oxide bonding with 0
(oxygen), S (sulfur) or Al, inhibiting grain growth of crystal
grains during reheating for rail rolling and for improving the
fineness of the austenite grain. Mg is also an element effective
in improving the ductility of the pearlite structure by finely
dispersing MnS with MgO and/or MgS, forming Mn depleted zones
around MnS, expediting the generation of pearlite transformation,
and increasing the fineness of the pearlite block size as a result.
If the Mg content is less than 0.0005 mass%, these effects cannot
be readily expected. If the Mg content is more than 0.0200 mass%,
coarse Mg oxide is generated, which greatly lowers the ductility
and the fatigue-damage resistance of the rail.
Thus, the Mg
content is limited to the range of from 0.0005 to 0.0200 mass %.
[0042]
Ca is an element effective in improving the ductility of
the pearlite structure by forming sulfide CaS (Ca has a strong
bonding force with S), finely dispersing MnS with CaS, forming Mn
depleted zone around MnS, expediting the generation of pearlite
transformation, and increasing the fineness of the pearlite block
size as a result.
If the Ca content is less than 0.0005 mass%,
these effects cannot be expected. If the Ca content is more than
0.0150 mass%, coarse Ca oxide is generated, which lowers the
ductility and the fatigue-damage resistance of the rail.
Thus,
the Ca content is limited to the range of from 0.0005 to 0.0150
mass %.
[0043] Al is an important element as an oxygen scavenger. Al is
also an element for shifting the eutectoid transformation
temperature toward the side of a higher temperature and for
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shifting the amount of eutectoid carbon toward the higher side.
Al is also an element effective in inhibiting the generation of
pro-eutectoid cementite structure and in highly strengthening the
pearlite structure. If the Al content is less than 0.0100 mass%,
these effects cannot be expected. If the Al content is more than
1.00 mass%, it becomes difficult to dissolve into the steel, which
causes generation of coarse aluminum-type inclusions which can be
a source of fatigue-damage and lower the ductility and the
fatigue-damage resistance of the rail. Also, oxide is formed at
welding, which degrades weldability drastically. In view of above,
the Al content is limited to the range of from 0.0100 to 1.00
mass %.
[0044] Zr is an element for inhibiting the formation of a
segregation zone in the central region of the billet(bloom, slab)
and thereby inhibiting generation of pro-eutectoid cementite
structures generated in the segregation region of the rail. This
is made by increasing the percentage of equiaxed crystals (grains)
in the solidification structure, since Zr02 inclusions have a good
lattice match, and become solidification cores of the high carbon
content steel rail of which the primary crystal is y-Fe, which
enables to form high equi-axed crystal rate in the solidification
structure. If the Zr content is less than 0.0001 mass%, the number
of Zr02 inclusions are not enough to work as solidification cores.
Consequently a pro-eutectoid cementite structure is generated in
the segregation region, which degrades the ductility of the rail.
If the Zr content is more than 0.2000 mass%, a great amount of
coarse Zr type inclusions are generated, which also degrades the
ductility of the rail and generates fatigue damage resulting from
the coarse Zr type inclusions. This reduces the life-span of the
rail. Consequently, the Zr content is limited to the range of from
0.0001 to 0.2000 mass%.
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[0045]
N enables the inhibition of grain growth of austenite
grain by precipitating V nitride, V-carbonitride and/or
Nb-carbonitride during continuous rolling.
N is also an element
effective in increasing both the ductility and the hardness
(strength) of the pearlite structure by precipitating V nitride,
V-carbonitride and/or Nb-carbonitride during the cooling process
after continuous rolling.
Further N is an element effective in
preventing heat affected zones of welded joint parts from
softening by precipitating V nitride, V-carbonitride and/or
Nb-carbonitride in the heat affected zones which is reheated at a
temperature range below the Ad l point. In addition to the above,
N is an element effective in improving the ductility of the
pearlite structure by forming segregation on the austenite grain
boundary, which expedites pearlite transformation from the
austenite grain boundary and increases the fineness of the
pearlite block size. If the N content is less than 0.0060 mass%,
the effects mentioned above are very weak. If the N content is
more than 0.0200 mass%, it becomes difficult to dissolve N into
the steel to make a solid-solution, which generates bubbles which
can be a source of fatigue damage. In view of this, the N content
is limited to the range of from 0.0060 to 0.0200 mass%. Usually,
steel rail initially includes N as impurity by a maximum of 0.0050
mass%. Consequently, N should be added in amounts sufficient to
provide N in amounts within the range of from 0.0060 to 0.0200
mass% to expect the above effects.
[0046]
V enables the inhibition of grain growth of austenite
grain by precipitating V carbide, V nitride, and/or V-carbonitride
during continuous rolling. V is also an element effective in
increasing both the ductility and the hardness (strength) of the
pearlite structure through precipitation-hardening
by
precipitating V carbide, V nitride, and/or V-carbonitride during
the cooling process after continuous rolling. Further V is an
element effective in preventing heat affected zones of welded
17
CA 02558850 2006-09-06
joint parts from softening by precipitating V carbide, V nitride,
and/or V-carbonitride at relatively a high temperature range in
the heat affected zones, which are reheated at a temperature range
below the Ad l point. If the V content is less than 0.005 mass%,
these effects cannot be significantly expected, i.e. no
significant improvement in the ductility and the hardness of the
pearlite structure will be achieved. If the V content is more than
0.500 mass%, coarse V carbide, V nitride, and/or V-carbonitride,
which can be sources of fatigue-damage, generate and the ductility
and the fatigue damage resistance of the rail are degraded. Thus,
the V content is limited to the range of from 0.005 to 0.500 mass%.
[0047] Nb enables the inhibition of grain growth of an austenite
grain by precipitating Nb carbide, and/or Nb-carbonitride during
continuous rolling. Nb is also an element effective in increasing
both the ductility and the hardness (strength) of the pearlite
structure through precipitation-hardening by precipitating Nb
carbide, and/or Nb-carbonitride during the cooling process after
continuous rolling.
Further, Nb is an element effective in
preventing heat affected zones of welded joint parts from
softening by precipitating Nb carbide, and/or Nb-carbonitride at
temperatures ranging from low to high in the heat affected zones,
which are reheated at a temperature range below the Acl point. If
the Nb content is less than 0.002 mass%, these effects cannot be
significantly expected, i.e. no significant improvement in the
ductility and the hardness of the pearlite structure can be
expected.
If the Nb content is more than 0.050 mass%, coarse Nb
carbide, and/or Nb-carbonitride, which can be sources of
fatigue-damage, generate and the ductility and the fatigue damage
resistance of the rail are degraded.
Thus, the Nb content is
limited to the range of from 0.002 to 0.050 mass %.
18
CA 02558850 2006-09-06
[0048]
(2) The reason for the limitation on the added amount of V, Nb or
N, which enables the inhibition of the grain growth of austenite
grain after rolling is as follows. Concerning the above-mentioned
V, Nb and N, it is preferable to add these elements in amounts
such that (expression 3) below is satisfied.
The reason why the
added amount of V, Nb or N is limited to the range calculated
based on the (expression 3) below concerning V mass%, Nb mass% and
N mass% is now explained.
The reason is that in continuous
rolling of high carbon content steel rail, methods for inhibiting
grain growth of austenite grain after rolling by controlling
precipitations have been studied. As a result, it was found that
the precipitation of V-carbide, V-Nitride, V-carbonitride, Nb-
carbide and Nb-carbonitride generated during the continuous
rolling causes pinning of austenite grains, which inhibits the
growth of austenite grain.
In addition, the conditions on which
the precipitation of V-carbide, V-Nitride, V-carbonitride, Nb-
carbide and Nb-carbonitride during the continuous rolling can be
fully controlled was investigated. It was found that the
generation of the precipitation has a positive correlation with
the added amounts of V, Nb and N.
[0049] Based on the above results, the range of added amounts of
V, Nb and N needed to sufficiently inhibit the growth of austenite
grain was experimentally investigated. The investigation indicated
that the contribution rate by unit amount (mass%) of V, Nb and N
(N is added to expedite formation of V-nitride, V-,
Nb-carbonitride) was different from each other. Then, the
contribution rate was experimentally obtained and the (expression
3) below was derived.
PC = V(mass%) + 10xNb(mass%) + 5xN(mass%)
(expression 3)
19
CA 02558850 2006-09-06
Using this expression, experiments were carried out with respect
to optimal ranges for amounts of V, Nb and N. As a result it was
found that if the value of PC defined by the (expression 3) is
less than 0.04, the growth of austenite grain after continuous
rolling cannot be inhibited since the pinning force with austenite
grain was too small; and if the value of PC is more than 0.30, the
growth of an austenite grain after continuous rolling cannot be
significantly inhibited where the properties of the rail are not
adversely affected but coarse V-carbide, Nb-carbide, V-nitride, V-
carbonitride, Nb-carbonitride are generated, which degrade the
pinning force with austenite. In view of above, the PC value is
limited as described in the following expression:
0.30 PC 0.04.
[0050] As mentioned above, N is added in order to expedite the
formation of precipitation V-nitride, V-carbonitride, and
Nb-carbonitride.
Addition of N alone does not form the above
precipitations, i.e. there is no effect of inhibiting the growth
of the austenite grain.
Consequently, in order to inhibit the
growth of the austenite grain, addition of V alone, Nb alone, or
addition of a combination of V and Nb, V and N, Nb and N, or V, Nb
and N can be made. In the case wherein N alone is added, in other
words, when neither V nor Nb are added, the value of PC is
regarded as 0 (zero) mass%.
As mentioned above, steel rail
usually contains N as an impurity in an amount of about 0.0050
mass% at maximum. In order to expedite the formation of V-nitride,
V-carbonitride, and/or Nb-carbonitride, N is added so that the N
content becomes equal to or more than 0.0060 mass%. Therefore, in
the calculation of the (expression 3) above, the N content is
assumed to be 0 (zero) mass% if the N content is less than 0.0060
mass%.
. (
CA 02558850 2008-12-29
[0051]
(3) The reason for the limitation of the cross-section reduction
rate per pass is as follows. The cross-section reduction rate per
pass of the rail in the finish rolling is limited to the range of
from 2 to 30%. If the cross-section reduction rate of the rail is
more than 30%, a great amount of heat is generated, which largely
increases the temperature of the rail head surface. This causes
the austenite grain of the rail head to become coarse, which makes
it difficult to ensure the ductility of the rail. In addition, it
also becomes difficult to ensure the formability during rail
rolling. If the cross-section reduction rate per pass in the
finish rolling is less than 2%, it is not possible to obtain the
necessary strain to re-crystallize the austenite grain of the rail
head. Therefore, the austenite grain is not fine-grain, which
fails to ensure the ductility of the rail. Thus, the cross-section
reduction rate per pass in the finish rolling is limited to the
range from 2 to 30%.
[0052]
(4) The reason for the limitation of the maximum interval time
of rolling is as follows. The maximum interval time of rolling (S
in seconds) is limited to a time equal to or less than the value
calculated from the following two expressions (expression (1) and
expression (2) below). The experiment involving two passes of
continuous rolling with a 2-30% cross-sectional reduction rate per
pass is carried out with respect to high carbon content steel rail
while changing the conditions of maximum rolling interval time
(S), the carbon content of the steel (C, mass), and the maximum
surface temperature of the rail head (T, C) and the ductility
(total elongation value) of the steel rail was checked by a
tensile test. As comparison examples, steel containing the same
chemical composition is rolled with the conditions of one pass, a
rolling temperature of 950 C and a cross-section reduction rate of
10%, and the ductility (total elongation value) was checked in the
same manner.
21
CA 02558850 2006-09-06
[0053] FIG.1 shows the results of a continuous rolling experiment.
The experimental conditions were: carbon content (C, mass%) of the
steel was 1.0 mass%, the cross-section reduction rate was 2-30%
per pass, the maximum rolling interval time (S, seconds) was 0.8
seconds, the number of passes was 2, and the maximum surface
temperature of the rail head was changed (T, C).
The vertical
axis represents (SxCxT) and the horizontal axis represents the
maximum surface temperature of the rail head (T, C).
[0054]
FIG.2 shows the result of another continuous rolling
experiment. The experimental conditions were: the carbon content
(C, mass%) of the steel was changed, the cross-section reduction
rate was 2-30% per pass, the maximum rolling interval time (S,
seconds) was 0.8 seconds, the number of passes was 2, and the
maximum surface temperature of the rail head was 950 C.
The
vertical axis represents (SxCxT) and the horizontal axis
represents the carbon content (C, mass%).
[0055]
FIG.3 shows the results of another continuous rolling
experiment. The experimental conditions were: the carbon content
(C, mass%)of the steel was 1.0 mass%, the cross-section reduction
rate was 2-30% per pass, the maximum rolling interval time (S,
seconds) was changed, the number of passes was 2, and the maximum
surface temperature of the rail head was 950 C. The vertical axis
represents (SxCxT) and the horizontal axis represents the maximum
rolling interval time (S, seconds).
[0056] As shown in Figures 1-3, when the value (SxCxT) exceeds
800, improvement of the ductility (total elongation value) becomes
insufficient compared to the comparison examples. This result is
independent of the change of individual S, C, and T. When the
value (SxCxT) exceeds 900, there is no difference in improvement
of the ductility (total elongation value) compared to comparison
22
CA 02558850 2006-09-06
examples. When the value (SxCxT) becomes smaller than 800, the
ductility is drastically improved compared to comparison examples.
[0057] Next, the effect of the number of passes in continuous
rolling was evaluated. FIG.4 shows the results of another
continuous rolling experiment. The experimental conditions were:
the carbon content (C, mass%)of the steel was 1.0 mass%, the
cross-section reduction rate was 2-30% per pass, the maximum
rolling interval time (S, seconds) was 0.5 seconds, the number of
passes (P, times) was changed (3-6 passes), and the maximum
surface temperature of the rail head (T, C) was 950 C. As
comparison examples, steel containing the same chemical
composition was rolled with the conditions of one pass, a rolling
temperature of 950 C and a cross-section reduction rate of 10% per
pass, and the ductility (total elongation value) was checked in
the same manner.
The vertical axis represents (SxCxTxP) and the
horizontal axis represents the number of passes (P, times) in the
continuous finish rolling.
[0058] As shown in Figure 4, when the value (SxCxTxP) exceeds
2400, improvement of the ductility (total elongation value)
becomes insufficient compared to the comparison examples. When the
value (SxCxTxP) exceeds 2600, there is no difference with
improvement of the ductility (total elongation value) compared to
the comparison examples. When the value (SxCxTxP) becomes smaller
than 2400, the ductility is drastically improved compared to the
comparison examples.
[0059] The present inventors have studied the operation conditions
of continuous rolling to ensure the ductility (total elongation
value) using the correlations described above. In the actual
rolling process for producing a commercial rail, it is difficult
to change the carbon content of the steel (C, mass%) and the
number of passes (P, times) since the wear resistance and the
23
CA 02558850 2006-09-06
rolling formability have to be ensured.
In view of this, the
maximum rolling interval time (S, seconds) and the maximum surface
temperature of the rail head (T, C) are controlled. As mentioned
above, the maximum rolling interval time (S, seconds) and the
ductility (total elongation value) have a correlation. As S
increases, both (S xcxT) and (SxCxTxP) are increased, and the
ductility (total elongation value) is lowered.
The present
inventors came up with idea that if the maximum rolling interval
time S was kept lower than the values shown below for expressions
(1) and (2), which are determined from the relation above, then
the ductility (total elongation value) of the steel rail would be
improved. As a result of the rolling experiments for commercial
rail, it was found that in order to inhibit the growth of
austenite grain at inter-stand (standing between consecutive
passes) and to increase the fineness of austenite grain after
continuous rolling, if the number of passes is 2, the maximum
rolling interval time S has to be less than or equal to the value
CPT1 calculated from the following (expression 1) consisting of C
(mass%) of the carbon content of the steel and T ( C) of the
maximum surface temperature of a rail head during the rolling, and
if the number of passes is 3 or more, the maximum rolling interval
time S has to be less than or equal to the value CPT2 calculated
from the following (expression 2) consisting of C (mass%) of the
carbon content of the steel, T ( C) of the maximum surface
temperature of a rail head during the rolling and P (number of
times) of the number of passes.
CPT1 = 800/(C x T) ................................. (expression 1)
CPT2 = 2400/(C x T x P) ............................ (expression 2)
S(sec) CPT1, CPT2
[0060] Definitions:
24
CA 02558850 2006-09-06
The rolling interval time means the time that a blank (billet,
bloom, slab) needs to travel from one rolling stand (pass) to next
rolling stand (pass), wherein each of the rolling stands is
required to be operated with the reduction rate of 21 or mo-r-e. In
other words, if a particular rolling stand in the continuous
finish rolling process is operated with the reduction rate less
than 2%, the particular stand cannot be taken into account for
determining the rolling interval time, but rather be ignored. The
maximum rolling interval time means the longest time among the
rolling interval times. In the case of 3 passes (3 rolling stands),
for example, if the time A taken between first pass and second
pass is longer than the time B taken between second pass and third
pass, then the time A is the Maximum rolling interval time.
[0061]
The surface temperature of the rail head (T, C) is the
surface temperature of the rail head measured between each
consecutive pass. The maximum surface temperature of the rail head
is the highest temperature among those measured.
[0062]
(5) The reason for the limitation of the condition on the rapid
cooling of the rail head immediately after hot rolling is as
follows.
If the cooling rate for cooling the rail head
immediately after hot rolling is less than 20C/sec., the austenite
grains become coarse during the cooling, which degrades the
ductility of the rail head. If the cooling rate for cooling the
rail head immediately after hot rolling is more than 300C/sec., a
large amount of heat recuperation from inside the rail head
generates after the rapid cooling, which raises the temperature of
the surface of the rail head to form coarse austenite grains and
leads to degradation of the ductility. Therefore, the cooling rate
for the rail head immediately after hot rolling is limited to the
range of 2-300C/sec.
CA 02558850 2006-09-06
[0063]
As for the temperature range within which the rapid
cooling is applied, if the rapid cooling is terminated at a
temperature of more than 950 C, austenite grains may significantly
grow depending on the carbon content of the steel, which causes
coarse grains of austenite and degrades the ductility of the rail
head. If the rapid cooling is still applied after the temperature
reaches below 750 C, a large amount of heat returning from inside
the rail head may generate depending on the rate of cooling, which
raises the temperature of the surface of the rail head and
generates coarse austenite grains, which lower the ductility. In
view of this, the temperature range within which the rapid cooling
is applied is limited to the range of 950-750 C.
[0064]
(6) The reason for the limitation of the condition on rapid
cooling of the head of the rails after hot rolling is as follows.
This is a final heat treatment performed after hot rolling. When
the temperature of the rail head falls below 700 C, pearlite
transformation will commence. Therefore, if the rapid cooling on
this stage starts after the temperature of the rail head falls
below 700 C, the hardness of the rail head cannot be increased and
this will fail to improve the wear resistance. Also, depending on
the carbon content and/or alloy elements, pro-eutectoid cementite
structures are generated, which degrades the ductility of the rail
head. Therefore, the starting temperature for the rapid cooling at
the final stage after hot rolling is limited to a temperature
higher than 700 C.
[0065] As for the range of the rapid cooling rate, if the rapid
cooling rate of the surface of the rail head is less than
2'C/second, no improvement on hardness of the rail head can be
seen. Besides, pro-eutectoid cementite may be generated depending
on the carbon content and/or alloy elements, which degrades the
ductility. And, if the rapid cooling rate is more than 30 C/second,
26
CA 02558850 2006-09-06
a martensite structure is generated in the present composition
system, which significantly degrades the ductility of the rail
head.
Thus, the rapid cooling rate is limited to a range of
2-30 C/second.
[0066] As for the temperature to which the rapid cooling is
terminated, if the rapid cooling is terminated at a temperature of
more than 600 C, a large amount of heat returning from inside the
rail is generated. As a result, the temperature rise causes
pearlite transformation, which leads to the failure of hardening
the pearlite structure, i.e., failure of ensuring wear resistance.
This also causes the pearlite structure to become coarse, which
degrades the ductility of the rail head surface. Therefore, the
rapid cooling has to be performed until the temperature reaches at
least 600 C.
There is no limitation on the lower temperature.
However, 400 C is the practical lower limit considering the
requirements of ensuring the hardness of the rail head surface and
preventing the martensite structure from being formed in the
segregation region inside the rail head.
[0067]
FIG.5 shows the names of the parts of a rail. Shown are
the top of the rail head 1 and the head corner 2. The "surface
temperature of the rail head" of the present invention described
herein refers to the surface temperature at the top of head 1 and
the head corners 2, 2 in FIG.5.
By controlling the surface
temperature as discussed above, austenite grain can be
fine-grained at the rolling and the ductility of the rail is
improved.
Likewise, the temperatures relating to the heat
treatments performed immediately after or after continuous hot
rolling, such as the rapid cooling process, refer to the
temperature of surface of the top of head 1 and the head corners
2,2, or the temperature of the region within a depth of 5mm from
the head surface. By controlling the temperature of this region,
27
CA 02558850 2006-09-06
a fine-grained pearlite structure having an excellent wear
resistance can be obtained.
[0068]
In this producing method, coolant used for cooling is not
limited. However, air, mist, and a mixture of air and mist are
preferable to ensure controlled cooling.
The metal structure of
the rail head produced by the present invention should preferably
be a pearlite structure. A slight amount of pro-eutectoid ferrite
structure, pro-eutectoid cementite structure and bainite structure
may generate in the pearlite structure depending on the selection
of chemical composition and/or selection of rapid cooling
conditions. However, a slight amount of these structures in the
pearlite structure do not significantly affect the fatigue
strength or the ductility. A rail head produced using the present
invention can therefore include a slight amount of pro-eutectoid
ferrite structure, pro-eutectoid cementite structure and bainite
structure.
EXAMPLES
[0069]
TABLE 1 shows the chemical composition of the tested steel rails.
TABLE 2 shows the elements (carbon content, PC value), hot rolling
conditions, heat treatment conditions,
micro-structures,
hardnesses and total elongation values of tensile test of the
rails produced by the methods of the invention from the tested
steel rails.
TABLE 3 shows the elements (carbon content, PC value), hot rolling
conditions, heat treatment conditions,
micro-structures,
hardnesses and total elongation values of tensile test of the
rails produced by conventional methods from the tested steel rails.
[0070] The rails for the examples are as follows:
(1) Rails produced by the methods of the invention (26 rails
listed in TABLE 2 denoted by Nos. 1-26).
28
CA 02558850 2008-12-29
Rails denoted Nos. 1-4, and 6-15: chemical composition
are shown in TABLE 1 and the hot rolling conditions and heat
treatment conditions are shown in TABLE 2.
Rails denoted Nos. 5, and 16-26: chemical composition
are shown in TABLE 1 and the hot rolling conditions, reheat
treatment conditions and PC values are shown in TABLE 2.
(2) Rails produced by methods for comparison (18 rails listed in
TABLE 3 denoted by Nos. 27-44).
Rails denoted 27-44: chemical composition are shown in
TABLE 1 and the hot rolling conditions are shown in TABLE 3.
[0071]
(1) Tensile test of the head rails
Test Machine: all-purpose miniature tensile test machine,
Form of the specimen: Similar to JIS No.4,
Length of parallel part: 25mm,
Diameter of parallel part: 6mm,
The distance for measurement and evaluation of elongation: 21mm,
The portion of rails 5 where specimens were taken: 5mm below the
head of the rails (See FIG. 6),
The rate of tensile: lOmm/min,
Temperature: room temperature (approx. 20 C)
[0072] The following are explanations based on the examples.
TABLE 1 shows the chemical composition of steel for tested rail
examples.
TABLE 2 shows the conditions of hot rolling, cooling after rolling
and heat treatment after rolling and the properties of the heads
of the rails produced by the method of the present invention.
TABLE 3 shows the conditions of hot rolling, cooling after rolling
and heat treatment after rolling and the properties of the heads
of the rails produced by conventional methods.
29
CA 02558850 2006-09-06
[0073]
First, the effects of the hot rolling conditions (the
maximum interval time of rolling
values in expressions (1) and
(2) above) are explained. The chemical composition of rail 10 in
TABLE 2 and rail 32 in TABLE 3 are the same in composition as
steel G in TABLE 1, whose carbon content is 1.10 mass%. The value
of the total elongation on the tensile test of rail 10 in TABLE 2
is 2.0% higher than that of rail 32 in TABLE 3, the former and the
latter being 11.7% and 9.7% respectively.
The difference comes
from the fact that the maximum interval time of rolling of rail 10
in TABLE 2 is controlled less than expression (2) value and this
control makes the austenite structure fine-grained.
[0074] Second, the effects of the PC value are explained.
The
chemical composition of rail 10 in TABLE 2 and rail 24 in TABLE 2
are the same in composition as steel G in TABLE 1, whose carbon
content is 1.10 mass %. The value of the total elongation on the
tensile test of rail 24 in TABLE 2 is 1.0% higher than that of
rail 10 in TABLE 2, the former and the latter being 12.7% and
11.7%, respectively. The difference comes from the fact that the
PC value of rail 10 is not within the range of 0.04-0.30, while
the PC value of rail 24 is within the stated range of 0.04-0.30,
and this inhibits the growth of the austenite grains after rolling.
[0075]
On the other hand, the value of the total elongation on
the tensile test of the rail 37 in TABLE 3, whose PC value is not
within the range of 0.04-0.30, is deteriorated to 11.0%, because
the coarse V-Nb carbide, V nitride, and V-Nb carbonitride are
generated if the PC value is not within the stated range.
As
stated above, the control of the maximum interval time of the
rolling must meet expressions (1) or (2) above, and with
continuous rolling this improves the ductility of the rails.
In
addition, control of the PC value within the range of 0.04-0.30
improves the ductility of the rails.
CA 02558850 2006-09-06
[0076]
FIG.7 shows the relationship between the carbon content
and the value of the total elongation on the tensile test, which
have the most influence on the ductility.
As shown in FIG.7,
compared with rails 27-36, produced by a conventional method,
rails 1-4 and 6-15, produced by the methods of the present
invention have improved ductility on the head of the rails in any
amount of carbon content because the maximum interval time of the
rolling is controlled.
In particular, rails 5 and 16-26 have
further improved ductility on the head of rails in any amount of
carbon content because not only the maximum interval time of
rolling but also the PC value, which is calculated by the equation
based on V, Nb, N, is controlled within the required range of
0.04-0.30 in rails 5 and 16-26.
Therefore, not only is the
austenite structure made fine-grained but also the growth of the
austenite grains is inhibited.
[0077] Third, the effects of cooling after rolling are explained.
The chemical composition of rail 10 in TABLE 2 and rail 38 in
TABLE 3 are the same in composition as steel G in TABLE 1, whose
carbon content is 1.10 mass%. The value of the total elongation
on the tensile test of rail 10 in TABLE 2 is 2.9% higher than that
of rail 38 in TABLE 3, the former and the latter being 11.7% and
8.8%, respectively.
The difference comes from the fact that the
cooling rate of rail 10 in TABLE 2 is controlled within the
inventive range and this control inhibits the growth of the
austenite grains, while the cooling rate of rail 38 in TABLE 3 is
not controlled.
[0078] The chemical composition of rail 8 in TABLE 2 and rail 39
in TABLE 3 are the same as that of steel F in TABLE 1, whose
carbon content is 1.00 mass%.
The value of the total elongation
on the tensile test of rail 8 in TABLE 2 is 1.7% higher than that
of rail 39 in TABLE 3, the former and the latter being 11.2% and
9.5%, respectively.
The difference comes from the fact that the
31
CA 02558850 2006-09-06
temperature of termination of rapid cooling of rail 8 in TABLE 2
is controlled within the inventive range and this control inhibits
the growth of the austenite grains, while the temperature of
termination of rapid cooling of rail 39 in TABLE 3 is not
controlled.
[0079] The chemical composition of rail 10 in TABLE 2 and rail 40
in TABLE 3 are the same in composition as steel G in TABLE 1,
whose carbon content is 1.10 mass%. The value of the total
elongation on the tensile test of rail 10 in TABLE 2 is 3.2%
higher than that of rail 40 in TABLE 3, the former and the latter
being 11.1% and 8.5%, respectively. The difference comes from the
fact that the temperature of termination of rapid cooling of rail
10 in TABLE 2 is controlled within the inventive range and this
control inhibits the growth of the austenite grains, while the
temperature of termination of rapid cooling of rail 40 in TABLE 3
is not controlled.
[0080] Fourth, the effects of heat treatment are explained. The
chemical composition of rail 2 in TABLE 2 and rail 41 in TABLE 3
are the same in composition as steel A in TABLE 1, whose carbon
content is 0.86 mass%. The value of the total elongation on the
tensile test of rail 2 in TABLE 2 is 11.5% higher than that of
rail 41 in TABLE 3, the former and the latter being 16.5% and 5.0%,
respectively. The difference comes from the fact that the cooling
rate of rail 2 in TABLE 2 is controlled within the inventive range
and this control inhibits the generation of martensite, while the
cooling rate of rail 41 in TABLE 3 is not controlled.
[0081] The chemical composition of rail 11 in TABLE 2 and rail 42
in TABLE 3 are the same in composition as steel G in TABLE 1,
whose carbon content is 1.10 mass%. The value of the total
elongation on the tensile test of rail 11 in TABLE 2 is 4.6%
higher than that of rail 42 in TABLE 3, the former and the latter
32
CA 02558850 2006-09-06
being 11.5% and 6.9%, respectively. The difference comes from the
fact that the cooling rate of rail 11 in TABLE 2 is controlled
within the inventive range and this control inhibits the
precipitation of martensite, while the cooling rate of rail 42 in
TABLE 3 is not controlled.
[0082] The chemical composition of rail 13 in TABLE 2 and rail 43
in TABLE 3 are the same as that of steel H in TABLE 1, whose
carbon content is 1.10 mass%.
The value of the total elongation
on the tensile test of rail 13 in TABLE 2 is 5.3% higher than that
of rail 43 in TABLE 3, the former and the latter being 11.5% and
6.2%, respectively.
The difference comes from the fact that the
cooling rate of rail 13 in TABLE 2 is controlled within the
inventive range and this control inhibits the pro-eutectoid
cementite, while the cooling rate of rail 43 in TABLE 3 is not
controlled.
[0083] The chemical composition of rail 5 in TABLE 2 and rail 44
in TABLE 3 are the same in composition as steel D in TABLE 1,
whose carbon content is 0.95 mass%.
The value of the total
elongation on the tensile test of rail 5 in TABLE 2 is 5.6% higher
than that of rail 44 in TABLE 3, the former and the latter being
13.2% and 7.6%, respectively. The difference comes from the fact
that the temperature of termination of cooling of rail 5 in TABLE
2 is controlled within the inventive range and this control
inhibits the coarseness of the grain of pearlite structure, while
the temperature of termination of cooling of rail 44 in TABLE 3 is
not controlled.
[0084]
As stated above, not only is a fine-grain austenite and
pearlite structure obtained but also the stable generation of a
pearlite structure is obtained by controlling the cooling
conditions and the heat treatment conditions after rolling which
33
, -
CA 02558850 2008-12-29
causes the improvement of ductility (the total elongation value)
of the rails.
[0085]
Compared with the rails 27-44, rails 1-26 have improved
wear resistance and ductility. The mechanisms are as follows:
As for rails 1-26, the maximum interval time of rolling is
controlled to be less than the value calculated by the equation
based on the carbon content of steel, the maximum head temperature
of the rails during continuous rolling and the number of rolling
passes and rapid cooling is carried out immediately after rolling.
These controls provide a fine-grain austenite structure. In
addition, as for rails 1-26, the heat treatment process after
rolling at a suitable temperature range and at a suitable cooling
rate is carried out. These controls inhibit the generation of pro-
eutectoid cementite and martensite, which are detrimental to the
ductility of the rails.
[0086] This application claims priority to Application Nos.
JP-2004-65676 and JP-2004-285934, filed in Japan on March 9, 2004
and September 30, 2004, respectively.
[0087]
The invention being thus described, it will be obvious
that the same may be varied in many ways. Such variations are not
to be regarded as a departure from the spirit and scope of the
present invention, and all such modifications as would be obvious
to one skilled in the art are intended to be included within the
scope of the following claims.
34
TABLE 1: Chemical Composition of Steels
Steel Chemical Composition (mass%)
C Si/Mn/Cr/Mo/V/Nb/B/Co/ Cu/Ni/Ti/Mg/Ca/A1/Zr/N
A 0.86 Si:O. 25, Mn:O. 65, Cu:O. 25, Co:O. 05, Ni:O. 25
B 0.90 Si:O. 54, Mn:O. 92, Cr:O. 15.,N:0. 0120
C 0.90 Si:O. 22, Mn:O. 75, Cr:O. 21, Ti:O. 0150, B:O. 0022
D 0.95 Si:O. 70, Mn:O. 60, Cr:O. 55.,V:0. 03, Nb:O. 015
E 1.00 Si:O. 40, Mn:O. 75, Cr:O. 28
F 1.00 Si:O. 75, Mn:O. 45, Cr:O. 55
G 1. 10 Si:O. 65, Mn:O. 70, Cr:O. 25, Zr:O. 0015
H 1. 10 Si: 1. 20, Mn: 1. 15, Cr:O. 22, Ti:O. 0130, AI:O. 07
I 1.20 Si:O. 65, Mn:O. 35, Ca:O. 0025
0
J 1.40 Si:O. 25, Mn:O. 55, Mg:O. 0020, Mo:O. 03
K 0.95 Si:O. 70, Mn:O. 60, Cr:O. 55.,V:0. 03, Nb:O. 015.,N:0. 0080
co
L
1.00 Si:O. 40, Mn:O. 75, Cr:O.
28.,V:0. 02.,N:0. 0060 co
0
M 1.00 Si:O. 40, Mn:O. 75, Cr:O. 28.,V:0. 05.,N:0. 0080
(.11
N
1.00 Si:O. 40, Mn:O. 75, Cr:O.
28.,V:0. 07 0
0
0 1.00 Si:O. 75, Mn:O. 45, Cr:O. 55.,V:0. 25
0
P 1. 10 Si:O. 65, Mn:O. 70, Cr:O. 25.,V:0. 07, N:O. 0120
0
Q 1. 10 Si:O. 65, Mn:O. 70, Cr:O. 25, Nb:O. 015
R 1. 10 Si:O. 65, Mn:O. 70, Cr:O. 25, Zr:O. 0015, Nb:O.
010.,N:0. 0080
S 1. 10 Si:O. 65, Mn:O. 70, Cr:O. 25, Zr:O. 0015, V:O. 07, Nb:O. 015
T 1. 20 Si:O. 65, Mn:O. 35, Ca:O. 0025.,V:0. 06
U 1.40 Si:O. 25, Mn:O. 55, Mg:O. 0020, Mo:O. 03.,V:0. 05, Nb:O. 010
/ 1. 10 Si:O. 65, Mn:O. 70, Cr:O. 25, Zr:O. 0015.,V:0. 10, Nb:O. 04
( the balance Fe and unavoidable impurities)
=
TABLE 2 (1): Rails produced by the method of the invention
Carbon content and PC Value Hot rolling conditions
Method No. Steel C content PC value
Number of - Range of cross-
Maximum temperature
(CPT)
rollings
section reduction
(mass%) V+10xNb+5xN of rail head (T, deg. C)
800/CxT
P(times) ( /0)
1 A , 0. 86 0. 00 2 5---20
1000 0. 93
2 _ A 0. 86 0. 00 3 8-24
950 -
3 B 0. 90 0. 00 4 10-30
950 -
4 C 0.90 , 0.00 2 10-28 920 0.97
D 0. 95 0. 18 4 10-24 950
-
6 E 1.00 0.00 2 15--30 950 0.84
7 E 1.00 0.00 5 8--18 875
- n
8 F 1.00 , 0.00 , , 5
10-20 1000 - 0
I.)
9 F 1. 00 0. 00 6 2-15
900 -
u-,
G 1. 10 0. 00 4 4-25 930
- co
co
11 G 1. 10 0.00 2 10--
28 920 0. 79
0
w
12 H 1. 10 0. 00 5 2^-25
900 - "
a,
0
13 H 1.10 0.00 6 5-10 850 -
0
0,
1
Inventive
14 I 1. 20 0. 00 5 _ 10--30
900 - 0
ko
'
15 J 1.40 0.00 5 10-30 940 -
0
16 K 0. 95 0. 22 4 10-24
950 - 0,
17 L 1.00 0.05 5 8-18 875 -
18 M 1. 00 0. 09 , 5 8-18
875 -
19 N 1.00 0.07 5 8-- 18
875 -
0 1. 00 0. 25 6 2-15 900
-
21 P 1. 10 0. 13 4 4-µ25
930 -
22 CD , 1. 10 0. 15 4 4-25
930 -
23 R 1. 10 0. 14 4 4-25
930 -
_
24 S 1. 10 0. 22 4 4-25
930 -
_
T 1. 20 0. 06 5 10--30 900
-
_
26 U 1. 40 0. 16 5 10-30
940 -
* 1 :structure 2mm under the surface of the head of the rails
* 2 :hardness 2mm under the surface of the head of the rails
*3:the elongation of the specimen 5mm below the surface of the head of rails
at tensile test (See FIG.6)
_
TABLE 2 (2)
-
i
Cooling condition after rolling
Heat treatment condition after rolling
-Maximum interval
Temperature of Temperature of
(CPT2)
Cooling rate Temperature of ending Cooling rate
time of rolling (S,
starting heat
2400/OxTxP ( C/sec.) cooling ( C)
CC/sec.) ending cooling
sec.)
treatment ( C) ( C)
, _
- 0. 8 5
900 750 7 580
_ 0. 98 0. 7 6 945
710 , 2 520
0. 70 0. 6 7 900
890 5 510
- 0. 6
air cooling after rolling 760 8 530
0. 66 0. 5 8 870
740 10 480
_
- 0. 7 10
850 930 9 500
0. 55 0. 4 air.cooling after rolling
760 10 480 0
0.48 0.2 11 ' 835
770 6 510 0
0. 44 0. 3 7 , 820
730 11 450 I.)
u-,
u-,
_ 0. 59 0. 4 15 820
800 12 450 co
co
co - 0. 6 18 800
750 14 480
0
-...i , 0. 48 0. 3 air cooling after rolling
780 12 465 I.)
.
0
_ 0. 43 0. 1 20 780
780 15 500 0
0,
.
1
0. 44 0. 3 24 780
770 20 550 0
, 0. 36 0. 2 28 760
760 25 480 ,0
1
0
0. 66 0. 5 8 870
740 10 480 0,
0. 55 0. 4 air cooling after rolling
760 10 480
0. 55 0. 4 air cooling after rolling
760 10 480
_
0. 55 0. 4 air cooling after rolling
760 10 480
0. 44 0. 3 7 820
730 11 450
0. 59 0. 4 15 820
800 12 450
_ ,
0. 59 0. 4 15 820
800 12 450
_ .
0.59 0.4 15 820
800 12 450
0. 59 0. 4 15 820
800 12 450
_
0. 44 0. 3 24 780
770 20 550
0. 36 0. 2 28 760
760 25 480
L
TABLE 2 (3)
Metallurgical property of head of the rails
Metallurgy structure of head Hardness of Total elongation
of rails *1 rails * 2 (Hv) * 3 ( %)
_
pearlite 390 15. 0
pearlite 342 16. 5
pearlite 402 14. 5
pearlite 425 14. 5
pearlite 445 = 13. 2
-
pearlite 430 12. 5
0
I.)
pearlite 440 11. 9
u-,
pearlite 420 11. 2
co
co
pearlite 455 12. 4
0
(,)
co pearlite 430 11. 7
N)
0
pearlite 450 11. 5
0
0,
1
pearlite 450 10. 5
0
ko
1
pearlite 475 11. 5
0
pearlite 440 10. 1
0,
pearlite 470 9. 0
pearlite 445 14. 0
-
pearlite 450 12. 7
pearlite 455 13. 2
pearlite 460 12. 9
pearlite 470 13. 8 .
pearlite 445 12. 1
pearlite 440 12. 5
pearlite 440 12. 3
pearlite 430 12. 7
pearlite 440 11. 2
_
pearlite 470 9. 9
TABLE 3 (1): Comparison method
Carbon content and PC Value Hot rolling conditions
Method No. Steel C content (C, PC value Numbers of Range of
cross- Maximum temperature
(CPT1)
rollings section
reduction of the head of rails (T,
mass%) V+10xNb+5xN
800/CxT
P(times) (%)
C)
27 A 0. 86 0. 00 2 5-20
1000 0. 93
28 B 0. 90 0. 00 4 10-30
950 -
29 D 0. 95 0. 18 4 10-24
950 -
30 E 1.00 0.00 5 8-18
875 -
31 F 1.00 0.00 6 5-15 900
-
32 G 1. 10 0. 00 4 4-25
930 -
33 G 1. 10 0.00 2 10--28
920 0.79
34 H 1. 10 0. 00 = 6 5 --=
10 850 - o
35 I 1.20 0.00 5 10-30 900
- 0
36 J 1.40 0.00 5 10-30
940 - "
in
in
co
co
(.0 37 V 1, 10 0. 50 4 4-25
930 - in
0
I.)
0
0
Comparison 38 G 1. 10 0. 00 4 4-
25 930 - 0,
i
0
q)
39 F 1.00 0.00 5 10- I20
1000 - 0
0,
40 G 1. 10 0.00 4 4-25
930 -
41 A 0. 86 0. 00 3 8-24
950 -
42 G 1. 10 0. 00 2 10-28
920 0. 79
43 H 1. 10 0. 00 6 5-10
850 -
44 D 0. 95 0. 00 4 10-24
950
* 1 :structure 2mm under the surface of the head of the rails
*2:hardness 2mm under the surface of the head of the rails
* 3:the elongation of the specimen 5mm below the surface of the head of rails
at tensile test (See FIG.6)
TABLE 3 (2)
Cooling condition after rolling
Heat treatment condition after rolling
(CPT2) Temperature of ending
Maximum interval Cooling
Temperature of Cooling Temperature of
time of rolling (S, rate CC
starting heat rate CC ending cooling
2400/CxTxP cooling CC)
sec.) /sec.)
treatment ( C) /sec.) ( C)
- 3.0 5
900 750 7 580
0. 70 6. 0 7 900
890 5 510
0.66 1.2 8 870
740 10 480
0. 55 2. 0 air cooling
760 10 480
0.44 1.0 7 820
730 11 450
0. 59 0. 8 15 820
800 12 450
- 1.0 18
800 750 14 480
0. 43 0. 6 20. 780
780 15 500 n
0. 44 0. 9 24 . 780
770 20 550 0
0. 36 0. 4 28 760
760 25 480 I.)
u-,
u-,
co
co
0. 59 0. 4 15 820
800 12 450
0
a,.
o
I.)
0
-
0
2 (slow
0,
0.55 0.4 820
800 12 450 I
rate) _
0
li)
I
0. 48 0. 2 11 960 (high temp.)
770 6 510 0
0,
720 (low temp.-*
0. 55 0. 4 15
700 12 450
high recur.)
,
35 (high
0.98 0.7 6 945
710 520
rate)
- 0. 6 18 800
680 (low temp.) 14 480
,
0. 43 0. 1 20 780
780 1(low rate) 500
'
620 (low
0. 66 0. 5 8 870
740 10 tempe.-,
high recup.)
temp. = temperature
recup.= recuperation
TABLE 3 (3)
Metallurgical property of head of the rails
Metallurgy structure of head Hardness of
Total elongation *3(%)
of rails *1 rails * 2 (Hv)
pearlite 390 13. 6
pearlite 402 12. 4
pearlite 445 12. 1
pearlite 440 10. 0
pearlite 455 9. 5
pearlite 430 9. 7
pearlite 450 = 9. 2
n
pearlite 475 9. 0
.
0
pearlite 440 8. 2
N)
u-,
pearlite 470 7. 5
co
co
11.0 (coarse deposition,
.1,
0
1-- pearlite 430 small ductility
I.)
0
improvement)
0
1
pearlite(coarse) 430 8.8 (grain growth)
0
li)
I
0
pearlite(coarse) 420 9.5 (grain growth)
0,
pearlite(coarse) 430 8.5 (grain growth)
pearlite + martensite 560 5.0 (ductility
deteriorated)
pearlite + initial deposition of
385 6.9(ductility
deteriorated)
cementite ,
pearlite + initial deposition of
345 6.2 (ductility
deteriarated)
cementite
pearlite (coarse) 336 7.6 (ductility
deteriorated)
