Note: Descriptions are shown in the official language in which they were submitted.
2148870
The present invention relates to a rail excellent
in damage resistance against a rolling fatigue damage
which occur in the head top and corner portions and
determine the service life of a rail in a railroad.
Railroad transportation has a higher transportation
efficiency than other transportation systems. High-
speed railroad transportation has been demanded, and
railroad schedules have been overcrowded year by year.
The loads on rails have become severer year by year.
For this reason, the rolling fatigue and damage have
increased on the rail head top surface of a straight
rail portion and the head corner portions of a curved
rail portion.
The characteristics currently required for rails
has been higher, and demand has arisen for a rail
excellent in damage resistance against the rolling
fatigue.
A heat-treated rail is known as such a rail, as
disclosed in Jpn. Pat. Appln. KOKOK~ Publication
No. 55-2388. The head portion of the rail is heated
again to a high temperature and cooled from a predeter-
mined temperature range, and the head portion is cooled
at a rate of 10.5C/sec to 15C/sec in the temperature
range of 700C to 650C, thereby obtaining the
heat-treated rail. As this heat-treated rail has a head
portion with a high hardness, it exhibits excellent
characteristics suitable for use in railroads having
`~ 2148870
- 2 -
high-axle load.
Jpn. Pat. Appln. KOKAI Publication No. 2-282448
discloses a rail in which the C content is slightly
lower than that in a conventional rail to set a
difference in hardness between the head top and corner
portions of the rail, thereby improving the resistance
to rolling fatigue damage.
In a railroad having a high-axle load, counter-
measures such as coating of a lubricant to suppress rail
abrasion and use of a high-strength rail have been
taken. For this reason, fatigue is accumulated in rails
to increase the rolling fatigue damage on the head top
surface of a straight rail portion and the head corner
portions of a curved rail portion. A countermeasure
against this drawback has not been proposed yet.
A11 the causes of damage on the head top surface
are not necessarily clarified yet. According to one of
the causes, the rail surface is rapidly heated and
cooled by wheel slippage during traction of a freight
car having a high-axle load and acceleration or
deceleration of a train, and a martensite layer called a
white layer is formed on the surface of the rail. Since
this white layer is very hard and brittle, cracks occur
and develop at the boundary with the matrix to cause
a rolling fatigue damage.
The cause of the rolling fatigue damage at the gage
corner portion is estimated as follows. Although the
` 21~8870
contact condition is very severe and the shearing stress
acting on the interior of the rail is large, the posi-
tion where the shearing stress acts does not change by
abrasion, so that the stress is concentrated on a
specific area, thereby causing a rolling fatigue damage.
The hardness of the white layer is generally deter-
mined by the C content. As the C content is higher, the
white layer has a higher hardness and becomes brittler.
The conventional rails including the one disclosed in
Jpn. Pat. Appln. KOKOKU Publication No. 55-2388 have C
contents as high as 0.65% to 0.85%. For this reason,
when a white layer is formed, such a rail has a higher
hardness, resulting in inconvenience. In addition,
a conventional rail has a pearlite structure whose
microstructure is fine, and a soft ferrite layer and
a hard cementite layer constitute a lamellar structure.
For this reason, the conventional rail is hardly
abraded. Once a white layer is formed on the rail, it
cannot be removed by abrasion upon contact between the
rail and the wheels of a train during train traveling.
The white layer becomes the start point of a damage.
To solve the above problem, the present inventors
have studied on a rail in which a white layer has a low
hardness or a rail in which a white layer can be removed
by abrasion upon contact between the rail and wheels
during train traveling even if the white layer is
formed. Since the hardness of the white layer is
214~7~
-- 4 --
uniquely determined by the C content, it is very
effective to reduce the C content so as to reduce the
hardness of the white layer. However, when the C
content is simply reduced, the resultant rail has a low
strength and suffers a large plastic flow on its
surface. This leads to another damage such as a
creaking crack.
To the contrary, for example, Jpn. Pat. Appln.
KOKAI Publication No. 2-282448 discloses a rail having
a low C content. Only the head corner portions of this
rail are heat-treated to obtain a high hardness. That
is, since the corner portions have a high hardness, a
damage such as a creaking crack caused by a low strength
can be prevented, and at the same time the white layer
on the head top portion can be removed by abrasion.
In actual train traveling in a railroad, however,
the contact between the rails and wheels is not uniform,
and the wheels may be brought into contact with portions
near the high-hardness head corner portions. In this
case, the wheel is not brought into contact with the
head top portion, so removal of the white layer by an
increased abrasion amount, which is the primary purpose,
cannot be achieved. It is therefore difficult to remove
the white layer, and the rolling fatigue damage cannot
be effectively prevented.
The present invention has been made in considera-
tion of the above situation, and has as its object to
21 18~ D
provide a high-abrasion bainite rail excellent in
resistance to rolling fatigue damage.
According to the first aspect of the present
invention, there is provided a high-abrasion bainite
rail excellent in resistance to rolling fatigue damage,
the rail essentially consisting of 0.15 to 0.55 wt% of
C, 0.05 to 1.0 wt% of Si, 0.1 to 2.5 wt% of Mn, 0.03 wt%
or less of P, 0.03 wt% or less of S, 0.1 to 3.0 wt% of
Cr, 0.005 to 2.05 wt% of Mo, and the balance of iron and
unavoidable impurities, a head portion of the rail has a
uniform bainite structure, and a hardness at any posi-
tion of a head top portion and a head corner portion of
the rail falls within the range of 240 to 400 in Vickers
hardness Hv.
According to the second aspect of the present
invention, there is provided a high-abrasion bainite
rail excellent in resistance to rolling fatigue damage
the rail essentially consisting of 0.15 to 0.55 wt% of
C, 0.05 to 1.0 wt% of Si, o.l to 2.5 wt% of Mn, 0.03 wt%
or less of P, 0.03 wt% or less of S, 0.1 to 3.0 wt% of
Cr, 0.005 to 2.0 wt% of Mo, at least one element
selected from the group consisting of 0.005 to 0.01 wt%
of Nb, 0.005 to 0.05 wt% of V, and 0.001 to 0.01 wt% of
Ti, and a balance of iron and inevitable impurities,
a head portion of the rail has a uniform bainite
structure, and a hardness at any position of a head top
portion and a head corner portion of the rail falls
219~87~
within the range of 240 to 400 in Vickers hardness Hv.
According to the third aspect of the present
invention, there is provided a high-abrasion bainite
rail excellent in resistance to rolling fatigue damage,
the rail essentially consisting of 0.15 to 0.55 wt% of
C, 0.05 to 1.0 wt% of Si, 0.1 to 2.5 wt% of Mn, 0.03 wt%
or less of P, 0.03 wt% or less of S, 0.1 to 3.0 wt% of
Cr, 0.005 to 2.05 wt% of Mo, at least one element
selected from the group consisting of 0.05 to 2.0 wt% of
CU and 0.05 to 2.0 wt% of Ni, and a balance of iron and
inevitable impurities, a head portion of the rail has a
uniform bainite structure, and a hardness at any posi-
tion of a head top portion and a head corner portion of
the rail falls within the range of 240 to 400 in Vickers
hardness Hv.
According to the fourth aspect of the present
invention, there is provided a high-abrasion bainite
rail excellent in resistance to rolling fatigue damage,
the rail essentially consisting of 0.15 to 0.55 wt% of
C, 0.05 to 1.0 wt% of Si, 0.1 to 2.5 wt% of Mn, 0.03 wt%
or less of P, 0.03 wt% or less of S, 0.1 to 3.0 wt% of
Cr, 0.005 to 2.0 wt% of Mo, at least one element
selected from the group consisting of 0.005 to 0.01 wt%
of Nb, 0.005 to 0.05 wt% of v, and o.ool to 0.01 wt% of
Ti, at least one element selected from the group con-
sisting of 0.05 to 2.0 wt% of Cu and 0.05 to 2.0 wt% of
Ni, and a balance of iron and inevitable impurities,
21488~0
a head portion of the rail has a uniform bainite
texture, and a hardness at any position of a head top
portion and a head corner portion of the rail falls
within the range of 240 to 400 in Vickers hardness Hv.
This invention can be more fully understood from
the following detailed description when taken in con-
junction with the accompanying drawings, in which:
FIG. 1 is a graph showing the influences of the
matrix structure and hardness on the abrasion loss
ratio;
FIG. 2 is a graph showing the influences of the C
content on the hardness and thickness of a white layer;
FIG. 3 is a graph showing the influences of the
microalloy on the thickness of a white layer; and
FIG. 4 is a graph showing the influences of the
components on the hardness distribution from the rail
head top surface to the interior of a rail.
The present inventors have considered the conven-
tional drawbacks described above and found that if both
the head top portion and the head corner portion of a
rail had a uniform hardness enough not to cause a
failure by a plastic flow, the rail had a fatigue
strength and hardness equal to those of a conventional
rail, and the entire microstructure of the head portion
was a bainite structure to increase the abrasion wear
1.3 to 3.0 times that of the conventional rail, a white
layer can be removed and an excellent damage resistance
` 214~870
i~ .
to rolling fatigue damage can be obtained without a
complicated heat-treatment process. The present
inventors also found that the addition of an alloy
element in a very small amount suppresses formation of
a white layer and uniforms the hardness from the head
portion to the interior of the rail although the
abrasion wear remained equal to of a rail whose entire
head portion had a bainite structure, thereby greatly
prolonging the service life of the rail.
The present invention is made based on the above
findings. According to the first embodiment of the
present invention, there is provided a high-abrasion
bainite rail excellent in resistance to rolling fatigue
damage, wherein the rail essentially consists of 0.15 to
0.55 wt% of C, 0.05 to 1.0 wt% of Si, 0.1 to 2.5 wt~ of
Mn, 0.03 wt% or less of P, 0.03 wt% or less of S, 0.1 to
3.0 wt% of Cr, 0.005 to 2.05 wt% of Mo, and the balance
of iron and inevitable impurities, a head portion of the
rail has a uniform bainite structure, and a hardness at
any position of a head top portion and a head corner
portion of the rail falls within the range of 240 to 400
in a Vickers hardness Hv.
According to the second embodiment of the present
invention, there is provided a high-abrasion bainite 25 rail excellent in resistance to rolling fatigue damage,
in addition to the composition of the first embodiment,
wherein the rail further contains at least one element
` 2148870
selected from the group consisting of 0.005 to 0.01 wt%
of Nb, 0.005 to 0.05 wt% of v, and 0.001 to 0.01 wt%
of Ti.
According to the third embodiment of the present
invention, there is provided a high-abrasion bainite
rail excellent in resistance to rolling fatigue damage
failure, in addition to the composition of the first
embodiment, wherein the rail contains at least one
element selected from the group consisting of 0.05 to
2.0 wt% of Cu and 0.05 to 2.0 wt% of Ni.
According to the fourth embodiment of the present
invention, there is provided a high-abrasion bainite
rail excellent in resistance to rolling fatigue damage,
in addition to the composition of the second embodiment,
wherein the rail contains at least one element selected
from the group consisting of 0.05 to 2.0 wt% of Cu and
0.05 to 2.0 wt% of Ni.
The present invention will be described in detail
below.
First of all, experimental examples as the basis of
the present invention will be described below.
FIG. 1 shows the results obtained by checking the
influences of the structure and hardness of the matrix
on the abrasion loss ratio. Steel samples had composi-
tions obtained by variously changing the contents of C,
Si, Mn, Cr, Cu, Ni, Mo, v, Nb, and Ti in the component
range shown in Table 1 and were 16-mm thick steel plates
~ 21~8B70
- 10 --
obtained by hot rolling. Some of the steel plates were
cooled with air. Nishihara type abrasion test pieces
each having an outer diameter of 30 mm and a width of 8
mm were sampled from these steel plates. An abrasion
test was performed under the conditions that the contact
load set in a simulation of contact between a rail and a
wheel was 135 kg, the slip ratio was -10%, and no lubri-
cant was used. The abrasion loss amount of each sample
was measured upon 100,000 revolutions. In evaluation of
each sample, the abrasion wear of a conventional rail
was measured, and the abrasion loss ratio of the
abrasion loss amount of each rail sample to that of the
conventional rail was used. Note that in the column of
structure in Table 1, P represents pearlite, and B
represents bainite.
Table 1
Chemical Components (wt%) Micro-
Accelerated struc-
No. C Si Mn Cr Cu Ni Mo V Nb Ti Cooling ture Hv
A-l 0.68 0.30 0.65 - - - - - - - not performed P 210
A-2 0.70 0.25 0.75 - 0.05 0.05 - - - - not performed P 250
A-3 0.70 0.29 0.95 - - - 0.01 - - - not performed P 260
A-4 0.75 0.24 0.850.05 - 0.10 - - 0.009 - performed P 290
A-5 0.73 0.19 0.940.15 0.05 0.05 - 0.03 - 0.006 performed P 310
A-6 0.72 0.17 0.870.13 - - - 0.01 0.007 performed P 340
A-7 0.77 0.18 0.980.22 - - - 0.02 - - performed P 370
A-8 0.09 0.10 0.402.95 - - - - - - not performed B 213
A-9 0.12 0.95 2.100.75 - - O.03 - - - not performed B 220
A-10 0.15 0.45 0.452.70 - - 0.04 0.02 - - performed B 242
A-ll 0. 0 0.25 0.501.50 1.050.50 0.02 - - 0.007 performed B 250
A-12 0.30 0.30 2.100.71 - - 0.03 - 0.008 - performed B 290
A-13 0.30 0.06 0.450.31 - - 1.25 - - - not performed B 300
A-14 0.54 0.97 0.401.40 0.700.40 0.03 - 0.006 not performed B 380
A-15 0.56 0.31 2.400.80 - 0.30 0.02 - - - not performed B 410
21A8870
- 12 -
As can be understood from FIG. 1, the abrasion loss
amounts are reduced with increases in hardness of the
matrices. The conventional rail has a hardness Hv of
240 or more, and this hardness does not pose any serious
problem as to the creaking crack at the head corner
portion of the rail. The creaking crack is caused by
plastic deformation of the rail corner portion, and the
plastic deformation is defined by only the strength
(hardness) of the corner portion. For this reason, it
is readily understood that no problem is posed if the
lower hardness (Hv) limit is 240 or more. In matrices
having the same hardness, the abrasion loss amount of
the pearlite structure is larger than that of the
bainite structure. Judging from this fact, to obtain
a larger abrasion wear than that of the conventional
rail to obtain an excellent damage resistance, the head
portion must have a bainite structure.
Judging from various examinations of the prevent
inventors, it is necessary to assure an abrasion loss
ratio of 1.3 or more to remove a fatigue layer by
abrasion. From the viewpoint of the planned service
life of a rail, when the abrasion loss amount exceeds
3.0 times that of the conventional rail, the thickness
of the rail head portion is greatly reduced before the
use period reaches the planned service life, and the
rail cannot be used. For this reason, the upper limit
of the abrasion loss ratio is 3Ø The hardness Hv of
` 21~B870
a currently used conventional rail having a pearlite
structure is about 240 to 260. It is understood from
FIG. 1 that the hardness of the bainite structure whose
abrasion loss ratio is 1.3 to 3.0 is 240 to 400 (Hv).
Therefore, when the hardness Hv of the bainite structure
as the object of the present invention is assured to
fall within the range of 240 to 400, it is seen from
FIG. 1 the resultant rail is satisfactory in various
respects such as removal of the fatigue layer by
abrasion, the service life determined by the reduction
of the thickness of the rail head portion, and creaking
cracks at the rail head corner portions.
FIG. 2 shows the influences of the C content of the
matrix on the hardness and thickness of a white layer
formed on the surface of the head portion of a rail.
Steel samples had components in which the C content was
changed within the component range of Table 2, and 16-mm
thick steel plates were obtained by hot rolling.
Columnar pieces each having a diameter of 3 mm were
sampled from these steel plates. Each columnar piece
was set on the side of the Nishihara type rolling
fatigue tester where a rail sample was attached. Each
columnar piece was spontaneously brought into contact
with a wheel test piece to simulate formation of a white
layer caused by slippage in a real rail subjected to
rapid heating and rapid cooling performed by forced
slip. The hardness and thickness of each white layer
21~8870
- 14 -
were evaluated.
Table 2
(wt%)
No. C Si Mn Cr Mo
B-l 0.10 0.30 0.40 1.24 0.41
B-2 0.20 0.29 0.41 0.86 O.S7
B-3 0.28 0.25 0.39 1.95 0.03
B-4 0.39 0.10 1.85 0.35 1.22
B-5 0.51 0.50 0.45 1.58 0.28
B-6 0.70 0.30 0.65 0.55 0.74
As can be understood from FIG. 2, the thickness of
the white layer is reduced with a decrease in C content.
The reason for this is estimated that the critical
cooling rate in formation of a white layer, is shifted
to the higher cooling rate with a decrease in C content,
so a white layer is not formed in the interior where the
cooling rate is low. The hardness of the white layer
greatly depends on the C content, and is reduced with a
decrease in C content. If a large difference in hard-
ness is present between the white layer and the matrix,the difference becomes resistance to removal of the
white layer by abrasion. It is therefore estimated that
the hardness of the white layer must fall within the
range of 1.75 times the hardness of the matrix. Since
the upper limit of the matrix hardness is set to
400 (Hv) from the viewpoint of the abrasion loss ratio,
as shown in FIG. 1, the hardness of the white layer must
2148870
- 15 -
fall within the range of 700 (Hv) or less. Therefore,
the upper limit of the C content is 0.55%.
FIG. 3 shows the influences of a microalloy on the
thickness of a white layer formed on the surface of a
head top portion of a rail. Steel samples had composi-
tions obtained by changing v, Nb, and Ti on the basis of
0.4 wt% of C in the component range of Table 3, and
16-mm thick steel plates were obtained by hot rolling.
Columnar pieces each having a diameter of 3 mm were
sampled from these steel plates as shown in FIG. 2.
Each columnar piece was set on the side where a rail
sample was attached in the Nishihara type rolling
fatigue tester. Each columnar piece was spontaneously
brought into contact with a wheel test piece to simulate
formation of a white layer caused by slippage in a real
rail subjected to rapid heating and rapid cooling per-
formed by forced slip. The thickness of each white
layer was evaluated.
`i 2148870
- 16 -
Table 3
(wt%)
No C Si Mn Cr Mo V Nb Ti
C-l 0.40 0.30 0.45 2.10 0.05 0.005 - -
C-2 0.41 0.28 0.39 2.30 0.03 0.017
C-3 0.39 0.15 0.40 1.97 0.11 0.028
C-4 0.42 0.62 0.41 2.00 0.07 0.041
C-5 0.41 0.31 0.43 2.13 0.04 0.049
C-6 0.39 0.30 0.39 2.25 0.10 0.102
C-7 0.40 0.28 0.40 1.99 0.08 - 0.006
C-8 0.38 0.19 0.45 2.08 0.06 - 0.009
C-9 0.42 0.41 0.38 2.20 0.02 - 0.016
C-10 0.40 0.50 0.41 2.15 0.04 - - 0.004
C-ll 0.39 0.30 1.90 0.91 0.54 - - 0.009
15C-12 0.40 0.31 2.30 0.71 1.15 - - 0.018
C-13 0.41 0.29 2.40 0.72 1.07 0.039 0.005
C-14 0.41 0.16 2.41 0.80 1.11 0.041 0.010
C-15 0.42 0.46 2.37 0.75 1.21 0.038 0.019
C-16 0.39 0.40 1.99 0.95 1.09 0.040 - 0.007
C-17 0.40 0.30 2.15 0.84 1.02 0.041 - 0.015
As can be apparent from FIG. 3, when v, Nb, and Ti
are added, the thicknesses of white layers are reduced.
v, Nb, and Ti form carbides, and these carbides are not
decomposed in rapid heating. As a result, the C content
in the form of a solid solution decreases to shift the
critical cooling rate in formation of a white layer
toward the higher cooling rate, and no white layer is
21A887~
formed in the interior where the cooling rate is low.
By adding the above elements, the thickness of each
white layer is reduced. As shown in FIG. 3, this effect
is most enhanced in the case of V. Nb and T follow V
because they have high carbide production performance.
However, this effect is saturated at 0.05% or more for V
and 0.01% or more for Nb or Ti. When Nb and Ti are
added using a V-added system, as indicated by a solid
mark in FIG. 3, it is readily understood that the hard-
ness and thickness of white layer are reduced largerthan a case in which V is added singly. Therefore, Nb
and Ti are preferably added together with V.
FIG. 4 shows the influences of the matrix compo-
nents on the hardness distribution from the surface of
the rail head portion to a position 30 mm deep from the
surface. Steel samples had compositions obtained by
changing Cu, Ni, V, Nb, and Ti in the component range of
Table 4 and were hot-rolled for simulation in the form
of a rail head portion. The Vickers hardnesses were
measured from the surface of the center of the head por-
tion to a position 30 mm deep from the surface.
21~887~
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214887~
,
- 19 -
As can be understood from FIG. 4, a decrease in
hardness of the interior is reduced by adding v, Nb, and
Ti serving as precipitation hardening elements. This
decrease in hardness is also reduced by adding a combi-
nation of Cu, Ni and Mo for increasing the hardenabilityof bainite. However, since the decrease in hardness of
the interior of the rail is saturated even if v, Nb, and
Ti are added in a large amount, the upper limit of the
addition amount is 0.05% for V, 0.01% for Nb, and 0.01%
for Ti. On the other hand, the addition of Cu, Ni, and
Mo in a large amount is not preferable because a marte-
nsite layer is formed on the surface of a rail.
The reason why the chemical components, the
microstructure, and the hardness are defined will be
lS described below.
[Chemical Component]
C: 0.15% to 0.55%
C is an element which greatly influences the hard-
ness of a white layer and the hardness of a rail itself.
When the C content exceeds 0.55%, the hardness of the
white layer on the rail head top portion excessively
increases, and the difference between the hardness of
the white layer and the hardness of the matrix exces-
sively increases to form the start point of a shearing
damage. On the other hand, when the C content is less
than 0.15%, the strength of the matrix excessively
decreases to accelerate a plastic flow. A damage occurs
2148870
- 20 -
centered on the plastic flow. Therefore, the C content
is defined to fall within the range of 0.15% to 0.55%.
Si: 0.05% to 1.0%
Si is an element which serves as an deoxidiser and
which is dissolved in the matrix to increase the
strength of the matrix. When the Si content is less
than 0.05%, the addition effect of Si cannot be
obtained. When the Si content exceeds 1.0%, the matrix
becomes brittle. In addition, hard SiO2 is dispersed in
the matrix and becomes the start point of shearing.
Therefore, the Si content is defined to fall within the
range of 0.05% to 1.0%.
Mn: 0.1% to 2.5%
Mn is an element is dissolved in the matrix to
improve hardenability to srengthen a material. When the
Mn content is less than 0.10%, the addition effect of Mn
is not obtained. When the Mn content exceeds 2.5%,
martensite tends to be produced in a segregated portion,
thus forming the start point of a damage. Therefore,
the Mn content is defined to fall within the range of
0.1% to 2.5%.
P: 0.03% or less
P degrades toughness and is thus defined to fall
within the range of 0.03% or less.
S: 0.03% or less
S is mainly present in the form of an inclusion in
steel. When the S content exceeds 0.03%, the amount of
21~8870
- 21 -
the inclusion excessively increases to make the matrix
brittle. Therefore, the S content is defined to fall
within the range of 0.03% or less.
Cr: 0.1% to 3.00%
Cr is an element for improving the hardenability of
bainite. Cr is a very significant element to obtain a
high-strength rail element in which a microstructure is
set in the form of a bainite structure. If the Cr con-
tent is less than 0.1%, the hardenability of bainite is
poor, and the microstructure cannot be converted into a
uniform bainite structure. When the Cr content exceeds
3.0%, martensite tends to be produced, which forms the
start point of a damage. Therefore, the Cr content is
defined to fall within the range of 0.1% to 3.0%.
Mo: 0.005% to 2.0%
Mo is an effective element for improving the
hardenability of bainite by dissolving in the matrix and
imparting a high strength and high abrasion. If the Mo
content is less than 0.005%, the addition effect of Mo
is not obtained. When the Mo content exceeds 2.0%, mar-
tensite tends to be produced, which forms the start
point of a damage. Therefore, the Mo content is defined
to fall within the range of 0.005% to 2.0%.
V: 0.005% to 0.05%
V is combined with C in the matrix and precipitates
upon rolling. For this reason, V allows an increase in
precipitation strength to the interior of the head
~ 2148870
portion to prolong the service life of a rail. In
addition, v forms a precipitate with C in rapid heating
of the surface of the head portion during traveling or
slip of a wheel. As this precipitate is not decomposed,
the content of dissolved C in the rapidly heated portion
is reduced. The hardness of the white layer is
decreased to very effectively suppress a shearing
damage. If the v content is less than 0.005%, the
addition effect of V is not properly exhibited. If the
v content exceeds 0.05%, the effect is saturated.
Therefore, the V content is defined to fall within the
range of 0.005% to 0.05%.
Nb: 0.005% to 0.01%
Ti: 0.001% to 0.01%
Nb and Ti are combined with C in the matrix as in V
and precipitates upon rolling. For this reason, Nb and
Ti allow an increase in precipitation strength to the
interior of the head portion to prolong the service life
of a rail. In addition, Nb and Ti form a precipitate
with C in rapid heating of the surface of the head
portion during traveling or slip of a wheel. As this
precipitate is not decomposed, the content of dissolved
C in the rapidly heated portion is reduced. The
hardness of the white layer is increased to very effec-
tively suppress a shearing damage. The addition effectof Ti and Nb is enhanced in use together with V. When
the Nb and Ti contents are less than 0.005% and less
2148870
- 23 -
than 0.001%, respectively, the addition effect of Nb and
Ti is not enhanced. Even if the Nb and Ti contents
exceed 0.01% respectively, the effect is saturated. In
addition, the precipitate is coarsened to cause another
damage. Therefore, the Nb and Ti contents are defined
to fall within the ranges of 0.005% to 0.01% and 0.001%
to 0.01%, respectively.
At least one element selected from the group con-
sisting of V, Nb and Ti is added.
Cu: 0.05% to 2.0%
Ni: 0.05 to 2.0%
Both Cu and Ni are elements for effectively
improving the hardenability of bainite in the form of
solid solutions in the matrix, thereby effectively
strengthening a material. If the Cu and Ni contents are
less than the lower limits of the above ranges, the
addition effect cannot be obtained. If the Cu and Ni
contents exceed the upper limits of the above ranges,
martensite tends to be produced, which forms the start
point of a shearing damage. Each of the Cu and Ni con-
tents is defined to fall within the range of 0.05% to
2.0%. At least one of these elements is added.
[Microstructure]
According to the present invention, the rail head
portion is constituted by a uniform bainite structure.
The bainite structure has a higher dislocation density
than the pearlite structure of the conventional rail to
214&870
- 24 -
obtain a high-strength rail. For this reason, bainite
steel can have a lower C content than that of pearlite
steel. In the pearlite structure a hard carbide
(cementite) in an oriented lamellar structure and is
hardly abraded. In contrast to this, since the carbide
is dispersed as fine carbide particles in the matrix of
the bainite structure, the carbide can be removed upon
matrix abrasion. Therefore, the bainite structure has
higher abrasion rate than the pearlite structure, i.e.,
is readily abraded.
When a fatigue layer is accumulated near the sur-
face of a rail or when a crack is formed and develops in
the boundary between the white layer and the base
material, a rolling fatigue damages occur. In either
case, the accumulated fatigue layer or the produced
white layer can be abraded by contact between the rails
and the wheels during train traveling, thereby
suppressing the damage. When the rail head portion is
constituted by a uniform bainite structure having a pre-
determined hardness and easily susceptible to abrasion,as described above, the rail has a high resistance to
rolling fatigue damage. To effectively remove the white
layer by abrasion, the abrasion wear must be larger than
that of a plain rail (complying with JIS). Therefore,
the head structure must be a bainite structure having
high abrasion rate.
When the abrasion wear is largely excessive, the
2148870
- 25 -
thickness of the rail head portion is greatly reduced,
and a necessary service life cannot be assured. In
practice, if the abrasion wear is 1.3 to 3.0 times that
of the conventional rail, a necessary service life can
be assured, and at the same time the resistance to roll-
ing fatigue damage can be excellent. If a rail is manu-
factured using steel having a composition of the present
invention and has a bainite structure having a hardness
falling within the following range, the rail has an
abrasion wear 1.3 to 3.0 times that of the plain rail,
and no problem of short life is posed by a decrease in
thickness of the head portion of the rail. The bainite
structure can be obtained by cooling a rail element with
air or rapidly cooling it after rolling.
The abrasion wear of a rail is most preferably
evaluated by an abrasion wear measured when a rail is
actually laid. A value obtained in a comparative test
using a Nishihara type abrasion tester upon simulation
of a contact condition of an actually laid rail is also
effective.
[Hardness]
The hardness Hv of the head top portion and the
head corner portions at any position must fall within
the range of 240 to 400. If the head top portion has a
bainite structure, the abrasion wear is large. For this
reason, the hardness of the head top portion may be
equal to that of the head corner portion without posing
21A8~ 0
- 26 -
any problem. If the hardness Hv of the head corner por-
tions falls within the range of 240 to 400, the plastic
flow is almost equivalent to that of the conventional
rail. No damage occurs having the plastic flow as a
starting point. When the hardness Hv of the head corner
portion exceeds 400, the abrasion wear is reduced to
cause inconvenience. In addition, to obtain a hardness
Hv of 400 or more in the bainite structure, the contents
of alloy elements must be increased, resulting in an
economical disadvantage. Therefore, the upper limit of
the hardness Hv is defined as 400.
The surface region of the head portion of the rail
is constituted by a head top portion and head corner
portions. As long as the head corner portions and the
head top portion have a hardness falling within the
above range, they may have substantially the same hard-
ness or different hardnesses.
The present invention will be described in detail
by way of its examples.
(Example 1)
Steel samples having compositions, microstructure,
and hardnesses shown in Table 5 were subjected to an
abrasion test. In this abrasion test, test pieces each
having an outer diameter of 30 mm and a width of 8 mm
were sampled from the steel samples, and wheel test
pieces each having the same size as that of each rail
test piece were sampled from the material of railroad
`~ 21q887~
- 27 -
wheels. Each rail test piece was brought into contact
with each wheel test piece using a Nishihara type abra-
sion tester under the conditions that the contact load
was 135 kg, the slip ratio was -10%, and a lubricant was
not used, which conditions were reported as the contact
conditions in an existing railroad. In this case, the
ratio of the abrasion loss amount of each rail test
piece to that of a plain rail material as a control was
calculated, and abrasion of the rail test piece was
evaluated using this abrasion loss ratio. Results are
summarized in Table 5. Whether an accelerated cooling
in the manufacture of the steel samples was performed or
not performed was also summarized in Table 5.
As shown in Table 5, as for samples E-4, E-7, and
E-9 each having a pearlite structure, the hardnesses Hv
of samples E-4 and E-9 are 250 and 350, respectively,
which satisfy the range of the present invention, but
have abrasion loss ratios as low as 1.00 and 0.05,
respectively. Although sample E-7 has a high abrasion
loss ratio of 1.40, it has a hardness Hv of 200 which is
lower than the lower limit of the range of the present
invention. Therefore, samples E-4, E-7, and E-9 are
impractical.
To the contrary, samples E-l, E-2, E-3, E-5, E-6,
and E-8 exhibit bainite structures. Among them all,
sample E-2 has a Cr content of 0.05% which is lower than
the lower limit of the range of the present invention.
21g8~70
- 28 -
For this reason, sample E-2 has a hardness Hv as low as
230 and an excessively high abrasion loss ratio of 3.58.
Sample E-6 has an Mn content of 3.05% which exceeds the
upper limit of the range of the present invention.
Sample E-6 has a hardness Hv as high as 420 and an
abrasion loss ratio of 1.20 lower than 1.3 which is the
lower limit of the appropriate abrasion wear range.
Samples E-l, E-3, E-5, and E-8 having bainite structure
and components all of which fall within the range of the
present invention have hardnesses falling within the
range of the present invention. These samples have
appropriate abrasion loss amounts falling within the
range of 1.3 to 3Ø
Table 5
Abrasion Loss
Chemical Components (wt%) Ratio (with
Micro- respect to
Accelerated struc- Hardness conventional
No. ` C Si Mn Cr Mo cooling ture (Hv) rail) Remarks
Present
E-l 0.31 0.29 0.44 2.41 0.02 not performed Bainite 300 1.69 Invention
Comparative
E-2 0.15 0.40 0.50 0.05 1.15 performed Bainite 230 3.58 Example
Present
E-3 0.18 0.15 2.00 0.70 0.09 performed Bainite 275 2.02 Invnetion
Comparative
E-4 0.75 0.18 1.00 0.20 0.03 performed Pearlite 350 0.95 Example
Present
E-5 0.16 0.30 0.46 0.35 1.25 not performed Bainite 250 2.85 Invention
Comparative _~
E-6 0.50 0.35 3.05 0.80 0.03 performed Bainite 420 1.20 Example c~
Comparative
E-7 0.65 0.50 0.50 0.50 1.04 not performed Pearlite 200 1.40 Example
Present
E-8 0.44 0.15 2.49 0.72 0.03 performed Bainite 370 1.35 Invention
Comparative
E-9 0.70 0.30 0.95 - - not performed Pearlite 250 1.00 Example
2148870
-
- 30 -
(Example 2)
Steel samples having compositions, microstructure,
and hardnesses shown in Table 6 were subjected to an
abrasion test following the same procedures as in
Example 1. Abrasion loss ratios are also summarized in
Table 6. Whether an accelerated cooling in the manufac-
ture of the steel samples was performed or not performed
was also summarized in Table 6. Note that all the steel
samples exhibited bainite structure.
As shown in Table 6, samples F-l, F-4, and F-7
whose Mn, Cr, or Mn contents are lower than the lower
limits of the ranges of the present invention have low
hardnesses and abrasion loss ratios as high as 3.0 or
more. Samples F-3, F-6, and F-9 whose Mn, Cr, or Mo
contents are higher than the upper limits of the ranges
of the present invention have very high hardnesses and
abrasion loss ratios of less than 1.3. To the contrary,
samples F-2, F-5, and F-8 whose Mn, Cr, and Mo contents
fall within the ranges of the present invention have
hardnesses which fall within the ranges of the present
invention, and abrasion lose ratios of 1.3 to 3.0 which
are suitable values.
Table 6
Abrasion Loss
Che~ical Components (wt%) Ratio (with
Micro- respect to
Accelerated struc- Hardness conventional
No. C Si Mn Cr Mo cooling ture (Hv) rail) Remarks
Comparative
F-l 0.16 0.10 0. 2.80 0.09 not performed Bainite 180 4.51 Example
Present
F-2 0.20 0.25 2.30 0.90 0.35 performed Bainite 300 1.82 Invention
Comparative
F-3 0.34 0.31 4.68 1.05 0.05 performed Bainite 415 0.94 Example
Comparative
F-4 0.44 0.43 0.79 0.01 1.02 performed Bainite 194 4.28 Example
Present
F-5 0.19 O.Sl 1.35 0.75 0.04 performed Bainite 320 1.43 Invention
Comparative ~o
F-6 0.23 0.61 1.80 4.71 0.04 performed Bainite 423 1.15 Example -~
Comparative
F-7 0.38 0.79 0.16 0.42 - not performed Bainite 218 3.74 Example
Present
F-8 0.40 0.75 0.67 0.35 1.22 not performed Bainite 285 2.68 Invention
Comparative
F-9 0.41 0.81 3.44 5.84 2.51 performed Bainite 426 1.04 Example
21q8870
- 32 -
(Example 3)
Steel samples having compositions, microstructure,
and hardnesses shown in Table 7 were subjected to an
abrasion test following the same procedures as in
Example 1. Abrasion loss ratios are also summarized in
Table 7. In Example 3, in addition to the abrasion loss
ratios, the thicknesses of white layers were also
measured. The thickness of each white layer was
measured as follows. A columnar test piece having a
diameter of 3 mm was sampled from each steel sample, the
columnar test piece was set on the side of the Nishihara
type rolling fatigue tester where a rail sample was
attached, and a wheel test piece was spontaneously
brought into contact with the rail test piece to cause
forced slip to rapidly heat and cool the test piece,
thereby simulating formation of a white layer. The
thickness of the resultant white,layer was measured.
Results are also summarized in Table 7. Whether an
accelerated cooling in the manufacture of the steel
samples was performed or not performed was also summa-
rized in Table 7. Note that the contents of components,
i.e., C, Si, Mn, and Cr of the steel samples for samples
G-l, G-3, G-5, G-7, and G-9 were set equal to those for
samples G-2, G-4, G-6, G-8, and G-10, respectively.
Also note that the contents of components, i.e., V, Nb,
and Ti of the steel samples for samples G-l, G-3, G-5,
G-7, and G-9 were set to fall within the ranges of the
~_ 21~8870
- 33 -
present invention, while those for samples G-2, G-4,
G-6, G-8, and G-10 were set to fall outside the ranges
of the present invention.
As can be apparent from Table 7, samples G-l to
G-10 have bainite structure, and hardnesses and abrasion
loss ratios which fall within the ranges of the present
invention. The thicknesses of white layers hardly
change even if v, Nb, and Ti are added exceeding the
ranges of the present invention. No effectiveness is
found even if V, Nb, and Ti are added exceeding the
ranges of the present invention, only resulting in an
increase in cost.
Table 7
Chemical Co~ponents (wt%) Micro-
Accelerated struc-
No. C Si Mn Cr Mo V Nb Ti cooling ture
G-l 0.31 0.29 0.44 0.61 1.25 0.021 - - not performed Bainite
G-2 0.30 0.30 0.45 0.59 1.27 0.085 - - not performed Bainite
G-3 0.18 0.15 2.00 0.71 0.35 0.019 - - performed Bainite
G-4 0.19 0.18 2.02 0.72 0.33 0.077 - - performed sainite
G-5 0.16 0.30 1.55 1.35 0.62 0.0270.007 - performed Bainite
G-6 0.15 0.30 1.54 1.35 0.64 0.0280.051 - performed Bainite
G-7 0.29 0.50 0.50 2.34 0.42 0.043 - 0.002 not performed Bainite
G-8 0.30 0.51 0.51 2.33 0.43 0.041 - 0.034 not performed Bainite
G-9 0.33 0.25 0.43 2.86 0.51 0.026 0.006 0.006 not performed Bainite
G-10 0.34 0.23 0.41 2.89 0.50 0.028 0.008 0.017 not performed Bainite
(Continued)
Table 7
Hardness Abrasion Thickness of
No. (Hv) Loss Ratio White Layer (~m) Remarks
G-l 357 1.41 170 Present Invention
G-2 356 1.42 169 Comparative Example
G-3 326 1.67 135 Present Invention
. G-4 329 1.65 133 Comparative Example
G-5 344 1.54 91 Present Invention
G-6 346 1.55 90 Comparative Example
G-7 310 2.01 125 Present Invention
G-8 309 1.98 123 Comparative Example O
G-9 315 1.54 142 Present Invention
G-10 318 1.56 143 Comparative Example
, 214~870
- 36 -
(Example 4)
Steel samples having compositions, microstructure,
and hardnesses shown in Table 8 were subjected to an
abrasion test following the same procedures as in
Example 1. Abrasion loss ratios are also summarized in
Table 8. Whether an accelerated cooling in the manufac-
ture of the steel samples was performed or not performed
was also summarized in Table 8. The microstructures of
the steel samples whose Hv exceeded 400 were partial
mixed structures of bainite and martensite. Note that
M + B in the column of microstructure in Table 8
represents a mixed structure of martensite and bainite.
As shown in Table 8, samples H-l, H-3, H-5, H-7,
H-9, H-ll, and H-13 having at least one of the Cu and Ni
contents satisfying the ranges of the present invention
have hardnesses Hv of 271 to 335 all of which fall
within the range of the present invention. These
samples have abrasion loss ratios of 1.43 to 2.84 all of
which satisfy the appropriate range. However, samples
H-2, H-6, and H-10 in all of which the Cu contents of
the alloy elements exceed the range of the present
invention and samples H-4, H-8, H-12, and H-14 whose Ni
contents exceed the range of the present invention have
hardnesses Hv of 400 or more and abrasion loss ratios of
less than 1.3.
Table 8
Che~ical Components (wt%) Micro-
Accelerated struc-
No. C Si Mn Cr Cu Ni Mo Cooling ture
H-l 0.16 0.10 0.41 2.800.44 - 0.44 not performed Bainite
H-2 0.20 0.25 0.50 0.802.25 - 1.31 performed M + B
H-3 0.34 0.31 0.68 1.05 - 0.70 1.00 performed Bainite
H-4 0.44 0.43 0.79 0.94 - 3.20 1.05 performed M + B
H-5 0.19 0.51 1.35 0.750.08 - 0.94 performed Bainite
H-6 0.23 0.61 1.80 0.712.05 - 0.77 performed Bainite
H-7 0.38 0.79 2.43 0.760.50 0.50 0.31 not performed Bainite
H-8 0.40 0.75 0.67 1.031.20 2.51 0.29 not performed Bainite ~
H-9 0.4I 0.81 0.44 0.841.49 - 0.15 performed Bainite oo
H-10 0.22 0.32 0.56 1.142.51 - 0.13 performed M + B -~
H-ll 0.29 0.45 0.81 1.06 - 0.36 0.67 performed Bainite
H-12 0.32 0.92 2.12 0.92 - 3.03 0.64 not performed Bainite
H-13 0.30 0.28 0.61 0.911.54 0.98 1.15 not performed Bainite
H-14 0.35 0.27 0.51 0.821.58 2.88 1.08 not performed Bainite
(Continued)
Table 8
Hardness Abrasion Loss Ratio (with
No. ~Hv) respect to conventional rail) Remarks
H-l 295 1.81 Present Invention
H-2 430 1.02 Comparative Example
H-3 290 1.55 Present Invention
H-4 442 0.95 Comparative Example
H-5 320 1.43 Present Invention
H-6 423 1.15 Comparative Example
H-7 271 2.84 Present Invention
H-8 412 1.17 Comparative Example
H-9 335 1.44 Present Invention
H-10 445 0.92 Comparative Example
H-ll 280 2.25 Present Invention
H-12 401 1.04 Comparative Example
H-13 290 2.12 Present Invention
H-14 398 1.25 Comparative Example
* M + B represents a mixing texture of martensite and bainite.
2148870
- 39 -
(Example 5)
Steel samples having the compositions in Table 9
were rolled into a rail shape, and air cooling or
accelerated cooling was performed after rolling. The
surface hardnesses Hv of these steel samples were meas-
ured at the head top surfaces of the respective rolled
material with a load of 10 kg. The internal hardness Hv
of each sample at a position 30 mm deep from the head
top surface was measured with a load of 10 kg. Abrasion
test samples as in Example 1 were sampled from head por-
tions of the rolled materials (bainite layers for steel
samples each having a mixed structure of martensite and
bainite) to evaluate abrasion loss ratios according to
the same test method as in Example 1. The resultant
values are summarized in Table 10. Note that whether
an accelerated cooling in the manufacture of the steel
samples was performed or not performed was also
summarized in Table 9. Table 10 shows surface layer
structures, and M + B in this column represents
martensite + bainite.
Samples I-l and I-2 were compared with each other
in the range of higher V contents while the C, Si, Mn,
Cr, and MO contents are set equal in these samples.
Although sample I-2 has a v content exceeding the range
of the present invention, sample I-2 has an internal
hardness almost equal to that of sample I-l. No effec-
tiveness was found even when v was excessively added.
~: 21~8870
-
- 40 -
Samples I-3, I-5, and I-7 have C, Si, Mn, Cr, and
Mo contents almost equal to those of samples I-4, I-6,
and I-8, respectively, so as to check the influences
derived when Nb and Ti are added singly or in a
combination. Samples I-4, I-6, and I-8, in which Nb and
Ti are added exceeding the ranges of the present
invention, exhibited decreases in internal hardness
almost equal to those of samples I-3, I-5, and I-7 whose
components fall within the ranges of the present
invention. It is found that the effect obtained upon
addition of Nb and Ti in large amounts is saturated.
Samples I-9, I-ll, I-13, and I-15 have C, Si, Mn,
Cr, and Mo contents almost equal to those of samples
I-10, I-12, I-14, and I-16, respectively. These samples
lS were checked when Cu and Ni were added singly or in a
combination. Samples I-10, I-12, I-14, and I-16 whose
Cu and Ni contents exceed the ranges of the present
invention have decreases in internal hardness almost
equal to those of samples I-9, I-ll, I-13, and I-15
whose Cu and Ni contents fall within the ranges of the
present invention but have martensite-bainite mixed
structures in which martensite which becomes the start
point of a damage is formed on the surface layer.
The influence of the Mn content in sample I-17 was
compared with that in sample I-18. Sample I-17 whose Mn
content falls within the range of the present invention
had a surface layer hardness Hv of 379 which satisfies
`~ 2148~7~
- 41 -
the range of the present invention. Sample I-17 had a
small decrease in internal hardness and an abrasion loss
ratio of 1.37 which was an appropriate value. However,
martensite was produced in the surface layer in sample
I-18 because of a high Mn content, and the surface layer
was extremely hardened.
The influence of the C content in sample I-l9 was
compared with that in sample I-20. Sample I-l9 whose C
content falls within the range of the present invention
had a surface layer structure and a surface hardness
which fall within the ranges of the present invention.
Sample I-l9 had an appropriate abrasion loss ratio.
However, since sample I-20 has a high C content,
martensite was produced in the surface layer, and the
surface layer was extremely hardened.
The influence of v, Nb, and Ti contents in sample
I-21 was compared with that in sample I-22. When if V,
Nb, and Ti was added exceeding the amounts defined in
the present invention, the internal hardness was almost
equal to that of the steel of the present invention. It
was confirmed that the effect obtained upon addition of
these elements in large amounts was saturated.
Table 9
(wt~)
Presence/Absence of
No. C Si Mn Cr Cu Ni Mo V Nb Ti Accelerated Cooling
I-l 0.44 0.25 2.48 0.73 - - 0.31 0.048 - - performed
I-2 0.45 0.24 2.46 0.74 - - 0.30 0.105 - - performed
I-3 0.34 0.31 0.51 2.03 - - 0.45 0.0400.008 - not performed
I-4 0.33 0.33 0.52 2.01 - - 0.47 0.0390.039 - not performed
I-5 0.24 0.51 1.82 0.75 - - 0.37 0.045 - - performed
I-6 0.23 0.52 1.80 0.77 - - 0.36 0.044 - 0.006 performed
I-7 0.19 0.40 1.10 1.53 - - 0.15 0.0220.0070.049 performed ~
I-8 0.18 0.42 1.12 1.50 - - 0.14 0.0210.0210.009 performed CO
I-9 0.38 0.79 0.68 1.021.20 - 0.44 0.015 - 0.019 performed O
I-10 0.40 0.77 0.67 1.054.20 - 0.43 0.014 - - performed
I-11 0.27 0.45 0.81 1.22 - 1.97 0.98 0.022 - - not performed
(Continued)
Table 9
(wt%)
Presence/Absence of
No. C Si Mn Cr Cu Ni Mo V Nb Ti Accelerated Cooling
I-12 0.28 0.44 0.83 1.20 - 4.68 1.01 0.021 - - not performed
I-13 0.31 0.20 2.02 0.81 0.15 0.20 0.47 0.012 - - performed
I-14 0.32 0.20 2.01 0.83 0.10 3.81 0.48 0.011 - - performed
I-15 0.16 0.13 0.42 1.95 0.20 0.50 0.65 0.018 - - performed
I-16 0.17 0.13 0.41 1.98 8.23 0.49 0.63 0.017 - - performed
I-17 0.42 0.06 0.31 0.31 0.15 0.35 1.25 0.0090.0050.001 not performed '
I-18 0.40 0.08 6.04 0.32 0.13 0.33 1.24 0.0080.0070.001 not performed
I-l9 0.30 0.21 0.50 2.11 0.08 0.13 0.35 0.0310.0080.006 performed
I-20 0.78 0.20 0.49 2.13 0.06 0.12 0.32 0.0280.0070.007 performed
I-21 0.31 0.35 0.41 0.27 0.58 0.43 1.45 0.0410.0090.002 not performed
I-22 0.32 0.36 0.42 0.25 0.56 0.45 1.47 0.0400.0800.054 not performed
Table 10
Surface Layer Surface Internal Abrasion
No. Texture * Hardness Hv Hardness Hv Loss Ratio Remarks
I-l Bainite 387 358 1.34 Present Invention
I-2 Bainite 388 357 1.33 Comparative Example
I-3 Bainite 354 338 1.65 Present Invention
I-4 Bainite 358 338 1.64 Comparative Example
I-5 Bainite 326 297 1.84 Present Invention
I-6 Bainite 325 298 1.85 Comparative Example
I-7 Bainite 285 261 2.05 Present Invention
I-8 Bainite 284 260 2.04 Comparative Example ~
I-9 Bainite 375 342 1.41 Present Invention CO
I-10 M + B 495 345 1.42 Comparative Example c~
I-ll Bainite 324 295 1.76 Present Invention
(Continued)
Table 10
Surface Layer Surface Internal Abrasion
No. Texture * Hardness Hv Hardness Hv Loss Ratio Remarks
I-12 M + B 457 297 1.78 Comparative Example
I-13 Bainite 346 319 1.69 Present Invention
I-14 M + B 479 320 1.67 Comparative Example
I-15 Bainite 253 206 2.97 Present Invention
I-16 M + B 412 208 2.95 Comparative Example
I-17 Bainite 379 348 1.37 Present Invention
I-18 M + B 502 347 1.39 Comparative Example
I-l9 Bainite 350 317 1.64 Present Invention
I-20 M + B 614 380 0.81 Comparative Example 9
I-21 Bainite 361 342 1.41 Present Invention
I-22 Bainite 362 344 1.42 Comparative Example
* M + B represents a mixing texture of martensite and bainite
