Note: Descriptions are shown in the official language in which they were submitted.
21S2877
-
The present invention relates to a high-toughness,
high wear-resistance rail, an more particularly to a
rail which great wear-resistant and damage-resistant
enough to serve in high-rate transportation railroads
S and heavy-duty mine railroads and which has strength,
toughness and wear-resistance great enough to serve in
cold districts. The invention also relates to a method
of manufacturing the rail.
Hitherto, rails for use in railroads have been
developed to be stronger, in terms of wear-resistance,
rolling-fatigue resistance and the like, which are
considered important properties required of the rails.
Rails for use in railroads need to be even stronger
since the railroad transportation has recently become
more speedy and more heavy-duty, applying a high
axle-load on the rails. Particularly in cold districts
such as Russia and Canada, rails having excellent
toughness are demanded. The ordinary rails at present
have but insufficient toughness; their 2 mm, U-notch
Charpy absorbed energy is as small as 10 J or less at
20C. It is difficult to increase their Charpy absorbed
energy by 5 J.
One of methods of improving the toughness is heat
treatment. Rails may be heat-treated to acquire
an increased toughness in one of the following two
alternative methods.
In the first method generally known as ~off-line
- 21~2877
heat treatment." In this method, rails made by rolling
are cooled. Thereafter, they are heated to AC3 point or
a higher temperature and subjected to accelerated
cooling. (See Jpn. Pat. Appln. KOKAI Publication
No. 63-128123.)
In the second method known as "on-line heat
treatment." In this method, rails made by rolling and
remaining at Ar3 point or a higher temperature are
subjected to accelerated cooling. (See Jpn. Pat. Appln.
KOKAI Publication No. 63-23244.)
The off-line method may increase the strength and
toughness of the rail, since the microstructure of the
steel is transformed at low temperatures by accelerated
cooling and microstructure of the steel is made finer by
repeatedly transforming the microstructure of the steel.
The method, however, is undesirable in view of heat
efficiency because the rails must be heated before they
are subjected to accelerated cooling.
The on-line method excels in heating efficiency
because the rails are subjected to accelerated cooling
immediately after having been made by rolling. However,
the method can hardly improve the toughness of rails
greatly, though strengthening the rails due to the
accelerated cooling strengthens, if the rails are of the
conventional steel composition.
Conventional rails is made of steel having fine
pearlite structure to have high strength and high
- 2152877
wear-resistance. Having pearlite structure, they cannot
have their toughness increased greatly. It is proposed
that rails be made of bainitic steel to have great
toughness. However, bainite structure is said to less
resistant to wear than fine pearlite structure, though
it is superior to fine pearlite structure in terms of
toughness.
Jpn. Pat. Appln. KOKAI Publication No. 2-282448
discloses a rail which is made of steel having C content
slightly lower than the conventional steel, the top and
corner portions of which differ in hardness, and which
has an improved resistance to rolling-fatigue damage.
Jpn. Pat. Appln. KOKAI Publication No. 5-271871 dis-
closes a rail which is made of bainitic steel having C
content slightly lower than the conventional steel so
that its top portion may be sufficiently resistant to
rolling-fatigue damage. Neither rail has an improved
toughness, however. Further, the bainitic steel rail
disclosed in Publication No. 5-271871 is disadvantageous
in that its service life will be shorten if it is used
in mine railroad and inevitably subjected to a high
axle-load. This is because it is intended to increase
wear amount of the rail to improve resistance to
rolling-fatigue damage of the rail.
As mentioned above, conventional rails of pearlite
structure can hardly have their toughness improved
greatly, whereas conventional rails of bainite structure
21S2877
is intended to increase wear amount of the rail to
improve the resistance to rolling-fatigue damage is
increased. No technique is available which can provide
rails which excels in both toughness and wear-
resistance.
The present invention has been developed in view of
the above-described circumstances, and has as its object
to provide a rail excellent in toughness, strength and
wear resistance, and method of manufacturing the same.
According to a first aspect of the present
invention, there is provided a rail of high toughness
and high wear resistance, consisting essentially of 0.2
to O.S wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of
Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3
to 4.0 wt% of Cr, and the balance of iron and inevitable
impurities, the rail having a metal structure of a
bainite structure, a hardness of 400 Hv or more at each
of a head top portion and a head corner portion thereof,
a tensile strength of 1200 MPa or more, and a 2 mm,
U-notch Charpy absorbed energy of 30 J or more at +20C.
According to a second aspect of the present
invention, there is provided a rail of high toughness
and high wear resistance, consisting essentially of 0.2
to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of
Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3
to 4.0 wt% of Cr, and the balance of iron and inevitable
impurities, the rail having a metal structure of
2152~77
a bainite structure, and a hardness of 400 Hv or more at
each of a head top portion and a head corner portion
thereof.
According to a third aspect of the invention, there
is provided a method for manufacturing a rail of high
toughness and high wear resistance, comprising the steps
of:
(a) preparing a steel consisting essentially of
0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to
4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less
of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and
inevitable impurities;
(b) hot rolling the steel to have a rolling
finishing temperature of 800 - 1000C, thereby forming
a rail stock; and
(c) cooling the rail stock at a cooling rate of
5C/sec. or less between a bainite transformation-
starting temperature or more and 400C or less.
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 relationship between
the hardnesses and the wear amounts of rails; and
FIG. 2 is a graph, showing the relationship between
the hardnesses and the wear loss ratios of rails.
As a result of having paid attention to a bainite
structure of excellent toughness, the present invention
2152877
provides a rail of high toughness, high strength and
high wear resistance by forming its microstructure of
bainite and adjusting its composition and manufacturing
conditions so as to increase its hardness.
As is aforementioned, it is known that the conven-
tional rail with a bainite structure has high toughness
but low wear resistance. FIG. 1 shows the relationship
between the amount of wear and the hardnesses of rails
having a pearlite structure, a bainite structure and a
tempering martensite structure, respectively. As is
evident from FIG. 1, the amount of wear in the bainite
structure is larger than that in the pearlite structure.
This means that in the conventional bainitic rail of
high strength, bainite is employed to enhance the resis-
tance against a rolling fatigue damage at the sacrifice
of the resistance against wear. On the other hand, in
the present invention, the bainite structure is made to
have high hardness, thereby enhancing both its toughness
and wear resistance.
According to a first embodiment of the present
invention, there is provided a rail, which essentially
consists of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si,
1.0 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt%
or less of S, 0.3 to 4.0 wt% of Cr, and the balance of
iron and inevitable impurities; and has a metal struc-
ture of a bainite structure, a hardness of 400 Hv or
more at each of a head top portion and a head corner
21~2~77
portion thereof, a tensile strength of 1200 MPa or more,
and a 2 mm, U-notch Charpy absorbed energy of 30 J or
more at +20C.
The rail constructed as above can be used in high
rate transportation railroads or heavy-duty railroads,
and in cold districts.
Preferably, the rail further contains at least
one selected from the group consisting of 0.1 to 1.0 wt%
of Ni and 0.1 to 1.0 wt% of Mo, in light of more
increasing its strength. Preferably, the rail further
contains at least one selected from the group consisting
of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of V, in
- light of more increasing its wear resistance. To
increase both the strength and the wear resistance, the
rail may contain both the above-described ingredients.
According to a second aspect of the invention,
there is provided a rail, which essentially consists of
0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to
4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less
of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and
inevitable impurities; and has a metal structure of a
bainite structure and a hardness of 400 Hv or more at
each of a head top portion and a head corner portion
thereof.
By virtue of this structure, the rail can have high
toughness and wear resistance, and be used in high-rate
transportation railroads or heavy-duty railroads.
2152877
-- 8 --
Also in this case, the rail further preferably
containing of at least one selected from the group
consisting of 0.1 to 1.0 wt% of Ni and 0.1 to 1.0 wt%
of Mo, in light of more increasing its strength.
Preferably, the rail further contains of at least one
selected from the group consisting of 0.01 to 0.1 wt% of
Nb and 0.01 to 1.0 wt% of V, in light of more increasing
its wear resistance. To increase both the strength and
the wear resistance, the rail may contain both the
above-described ingredients.
The reasons why the chemical composition, the
microstructure, the hardness, the toughness and
manufacturing conditions of the rail are set to the
above-described values will be explained.
[Re: Chemical Composition]
0.2 to 0.5 wt% of C
C is an essential element for securing sufficient
strength and wear resistance to a rail steel. If the C
content is less than 0.2 wt%, a steel of a hardness
suitable to a rail cannot be prepared at low cost.
Further, if the C content exceeds 0.5 wt%, the rail head
portion cannot have a uniform bainite structure, thereby
deteriorating the toughness. In light of the above, the
C content is set to 0.2 to 0.5 wt%.
0.1 to 2.0 wt% of Si
Si is an element which is effective not only as
a deoxidizer but also an element for increasing the
- 21~2877
strength and hence the wear resistance by solving
ferrite in the bainite structure. If the Si content is
less than 0.1 wt%, no effect can be found. If the Si
content is more than 2.0 wt%, the steel embrittles.
Therefore, the Si content is set to 0.1 to 2.0 wt%.
1.0 to 4.0 wt% of Mn
Mn is an element for increasing the strength of the
rail steel by lowering the transformation temperature of
bainite to enhance the hardenability. If the Mn content
is less than 1.0 wt%, little effect will be found. If
the Mn content is more than 4.0 wt%, a martensite
structure will easily be formed because of micro
segregation of the steel. The martensite structure
hardens or embrittles during heating and welding,
thereby degrading the steel. Therefore, the Mn content
is set to 1.0 to 4.0 wt%.
0.035 wt% or less of P
Since P reduces the toughness, its content is
limited to less than 0.035 wt%.
0.035 wt% or less of S
S is contained in the steel, mainly in the form of
inclusion. If the S content exceeds 0.035 wt%, the
amount of inclusion significantly increases and hence
the steel embrittles and degrades. To avoid this, the S
content is limited less than 0.035 wt%.
0.3 to 4.0 wt% of Cr
Cr is an element for improving the hardenability
- 2152877
-- 10 --
of bainite. Cr is a very important element in the steel
of the present invention for strengthening the bainite
structure as the metal structure. If the Cr content is
less than 0.3 wt%, the quenching properties of bainite
are degraded, and the microstructure is not formed by a
uniform bainite structure. If the Cr content exceeds
4.0 wt%, martensite may easily be formed. Therefore,
the Cr content is set to 0.3 to 4.0 wt%.
0.1 to l.0 wt% of Ni, 0.1 to l.0 wt% of Mo
Each of Ni and Mo is an element to be solved in
bainite for improving the hardenability of bainite and
strengthening the same. If the Ni or Mo content is less
than 0.1 wt%, no effect can be found. Further, even if
the Ni or Mo content exceeds 1.0 wt%, no more effect can
be expected. In light of these, it is preferable that
each of the Ni and Mo content is set to o.1 to l.0 wt%,
and at least one of them is added.
0.01 to 0.1 wt% of Nb and 0.01 to 0.1 wt% of V
Since each of Nb and V is bonded with C contained
in bainite and deposited after rolling, they are
effective elements for increasing the hardness and wear
resistance of the head portion of the rail until deep
portion by precipitation hardening, thereby elongating
the service life of the rail. If the Nb or V content is
less than 0.01 wt%, a sufficient effect cannot be
obtained. Further, even if the Nb or V content exceeds
0.1 wt%, no more effect can be expected. In light of
21S2877
the above, it is preferable that each of the Nb and V
content is set to 0.01 to 0.1 wt%, and at least one of
them is added.
[Re: Metal Structure (Microstructure)]
In the present invention, the rail has a bainite
structure. As compared with the conventional pearlitic
rail, the bainite structure has a high dislocation
density, and accordingly has high strength and high
toughness. Therefore, the amount of C can be set
smaller than that in the pearlitic rail.
[Re: Hardness]
Any portion of the head top portion and head corner
portion of the rail has a hardness of 400 Hv or more.
In the case of the bainitic rail of the invention, if
the hardness of the rail is 400 Hv or more, the amount
of wear is less than that of an ordinary rail.
The reason why the hardness is set to the above-
described values is based on the relationship between
the hardness and the amount of wear.
Although it is most desirable to estimate the
amount of wear of a rail actually used, it is also
effective to estimate the same from comparison tests in
which contact conditions of actually-used rails are
simulated using the Nishihara type wear-testing system.
The above method can estimate the wear resistance (the
relationship between the hardness and the wear loss
ratio) in a short time. Hereinbelow, estimation results
- 2152877
obtained using the method will be described.
FIG. 2 shows the influence of the hardness on the
wear loss ratio. Steel samples of various compositions
as shown in table 1 and tables 2, 4, 6, 8 and 10 (which
will be later referred to) were prepared by varying the
contents of C, Si, Mn, Cr, Ni, Mo, Nb and V. These
samples were heated at 1250C, rolled at 920C, and
acceleratedly cooled at 3C/sec. so that the rolling
finishing temperature and the cooling rate of the
invention could be satisfied, thereby forming steel
plates of a thickness of 12 mm.
Nishihara type wear test pieces having a diameter
of 30 mm and a width of 8 mm, were cut from the steel
plates, and were subjected to a wear test carried out
under the conditions of a contact load of 50 kg, a
slippage ratio of -10%, and no lubricant. The wear
loss of each test piece after 500,000 rotations was
measured. Estimation was performed on the basis of the
ratio of the wear loss of each test piece to the wear
loss of an ordinary rail.
The hardness of an ordinary rail having a pearlite
structure is about 250 Hv. As can be understood from
FIG. 2, the wear loss ratio decreases in accordance with
an increase in hardness, and at the same hardness, the
bainite structure had a greater wear loss ratio than the
pearlite structure. As regards the bainite structure,
at the same hardness, bainite containing 1.0 wt% or more
2152877
- 13 -
of Mn has a lower wear loss ratio than that containing
less than 1.0 wt% of Mn. Specifically, the wear loss
ratio of each steel in table 1 which contains less than
1.0 wt% of Mn is higher than that of each steel in
tables 2, 4, 6, 8 and 10, which contains 1.0 wt% or more
of Mn.
This is probably because an increase in Mn content
reduces the bainite transformation temperature, thereby
narrowing the width of the lath of bainite and hence
enhancing the wear resistance. Further, in the case of
containing 1.0 wt% or more of Mn, the wear loss ratio of
the bainite structure is equal to or less than that of
the usual rail when its hardness is 400 Hv or more.
Thus, if the content of Mn is 1.0 wt% or more and the
hardness is 400 Hv or more, the bainite structure of the
invention can have a wear resistance equal to or more
than that of the ordinary rail, and can be put to
practice.
Although in the invention, it suffices if the
hardness is 400 Hv or more, the hardness is preferably
500 Hv or less to effectively prevent delayed fracture.
Table 1
Chemical Composition (wt%)
Sample Hardness Wear Loss
C Si Mn P S Cr (Hv) Ratio
A-l 0.31 0.33 0.41 0.011 0.0083.03 347 2.91
A-2 0.30 0.32 0.60 0.011 0.0082.53 369 2.07
A-3 0.41 0.32 0.39 0.011 0.0081.52 390 1.55
A-4 0.29 0.32 0.82 0.010 0.007 2.50 414 1.21
- 2152877
[Re: Strength and Toughness]
In the case of using the rail of the present
invention in cold districts, it is necessary to set the
tensile strength to 1200 MPa or more, the 2 mm, U-notch
Charpy absorbed energy to 30 J or more at +20C. These
conditions are satisfied in the first embodiment of the
invention. However, it is not always necessary to
satisfy the conditions in districts other than cold
ones.
[Re: Manufacturing Conditions]
In order to form the above-described bainite
structure of a steel having a composition as above and
obtain the above-described rail properties, the steel
of the composition is hot-rolled to have a finishing
temperature of 800 - 1000C, and is cooled at a
cooling rate of 5C/sec. or less between a bainite
transformation-starting temperature or more and 400C or
less.
If the rolling finishing temperature is less than
800C, bainite transformation will start disadvan-
tageously during rolling, thereby significantly reducing
the strength. Moreover, if the rolling finishing
temperature is more than 1000C, austenite grains are
enlarged, making it difficult to secure a predetermined
toughness after hot rolling. In light of these facts,
the rolling finishing temperature is set to 800 -
1000C.
21S2~77
- 16 -
As regards the cooling rate, the bainite structure
having a desired strength and a desired toughness can be
obtained even by air cooling. If, however, the cooling
rate exceeds 5C/sec., martensite will appear and reduce
the toughness. Therefore, the cooling rate is set to
5C or less.
It is preferable to perform on-line cooling in
which the bainite structure is cooled on a rolling line
immediately after it is rolled on the same line. The
on-line cooling is advantageous in thermal efficiency,
as compared with a treatment in which the bainite
structure is cooled to a room temperature after hot
rolling and then reheated.
In the rail manufactured by the above-described
method, each of the head top portion and the head corner
portion has a hardness of 400 Hv or more, and a tensile
strength of 1200 MPa or more, and a 2 mm, U-notch Charpy
absorbed energy of 30 J or more at +20C. The rail
constructed as above can be used in high-speed transpor-
tation railroads or heavy-duty railroads, or in cold
districts.
Examples
Examples of the invention will be explained.
In the description, the drawings and the tables,
uE20 represents a 2 mm, U-notch Charpy absorbed energy
at +20C.
2152877
- 17 -
Example 1
Steel samples having compositions shown in FIG. 2
were heated to 1250C, rolled at 920C, and acceler-
atedly cooled at 3C/sec., thereby forming steel plates
of a thickness of 12 mm. The steel plates were sub-
jected to a tensile test, a Charpy impact test and a
wear test. In the wear test, test pieces having a
diameter of 30 mm and a width of 8 mm were cut from the
steel plates, and were tested under the conditions of a
contact load of 50 kg, a slippage ration of -10%, and
no lubricant. The wear loss of each test piece after
500,000 rotations was measured. The ratio of the wear
loss of each test piece to the wear loss of an ordinary
rail was calculated. Table 3 shows the mechanical
properties and the wear loss ratio of each steel sample.
As is shown in FIG. 3, a sample B-l, which has a C
content lower than the present invention, has a hardness
of 333 Hv lower than the lower limit value of the
present invention and a wear loss ratio of 2.64 higher
than the invention. Accordingly, the sample B-1 cannot
be put to practice.
Samples B-6 and B-7, which have C contents higher
than the invention and a pearlite structure, have
hardnesses and wear loss ratios falling within target
ranges of the present invention. However, they have low
toughnesses of uE20 = 20.5 J and 16.4 J.
On the other hand, samples s-2, B-3 B-4 and B-5,
21~2877
- 18 -
which satisfy the component ranges of the invention,
have strengths, toughnesses and wear loss ratios falling
within target ranges of the invention.
Table 2
(wt%)
Sample C Si Mn P S Cr
B-l 0.13 0.332.02 0.009 0.0072.01
B-2 0.21 0.332.03 0.011 0.0082.03
B-3 0.30 0.322.03 0.011 0.0082.03
B-4 0.41 0.322.02 0.011 0.0082.02
B-5 0.49 0.322.04 0.010 0.0072.00
s-6 0.60 0.321.03 0.010 0.0072.01
B-7 0.80 0.540.85 0.016 0.0092.00
21S2877
-- 19 --
Table 3
Sample TS uE20 Hard- Wear Micro-
(MPa) (J) ness Loss Structure
(Hv) Ratio
B-l 1009 93.9 333 2.64 Bainite
B-2 1224 60.9 406 0.99 Bainite
B-3 1269 51.8 417 0.62 Bainite
B-4 1331 39.4 445 0.50 Bainite
B-5 1416 33.4 464 0.41 Bainite
B-6 1088 20.5 302 1.30 Pearlite
B-7 1228 16.4 348 0.46 Pearlite
Example 2
Steel samples having compositions shown in table 4
were processed in the same manner as in Example 1, and
resultant steel plates were subjected to the tensile
test, the Charpy impact test and the wear test. All the
samples had the bainite structure. Table 5 shows the
mechanical properties and the wear loss ratio of each
steel sample. A sample C-l, which has a Mn content
lower than the present invention, has a hardness lower
than the present invention and a wear loss ratio of 3.15
higher than the present invention.
On the other hand, samples C-2, C-3, C-4, C-5, C-6,
- 2152~77
- 20 -
C-7 and C-8, which have Mn contents falling within the
range of the present invention, have hardnesses of
400 Hv or more, and wear loss ratios of less than 1.
Furthermore, they show excellent tensile strengths
of 1200 MPa or more and excellent toughnesses of
uE20 = 30 J or more. However, in the case of a sample
C-9 having a Mn content higher than the range of the
invention, it is found that the Mn effect of increasing
the wear resistance is saturated.
Table 4
(wt%)
Sample C Si Mn P S Cr
C-l 0.31 0.340.31 0.008 0.0072.51
C-2 0.31 0.341.02 0.010 0.0072.51
C-3 0.30 0.311.53 0.010 0.0072.53
C-4 0.30 0.311.99 0.010 0.0072.53
C-5 0.31 0.312.48 0.010 0.0072.52
C-6 0.32 0.303.04 0.009 0.0082.53
C-7 0.31 0.313.50 0.009 0.0072.52
C-8 0.30 0.313.99 0.009 0.0072.52
C-9 0.31 0.344.52 0.009 0.0082.53
2152~77
Table 5
Sample TS uE20 Hard- Wear
(MPa) (J) ness Loss
(HV) Ratio
C-l 1122 32.4 340 3.15
C-2 1281 63.7 403 0.99
C-3 1356 61.6 411 0.88
C-4 1428 66.8 425 0.79
C-5 1509 62.2 447 0.65
C-6 1571 60.3 468 0.53
C-7 1613 58.8 479 0.46
C-8 1652 55.4 485 0.44
C-9 1707 48.1 506 0.48
Example 3
Steel samples having compositions shown in table 6
were processed in the same manner as in Example 1, and
resultant steel plates were subjected to the tensile
test, the Charpy impact test and the wear test. All the
samples had the bainite structure. Table 7 shows the
mechanical properties and the wear loss ratio of each
steel sample. A sample D-l, which has a Cr content
lower than the present invention, has a hardness lower
21~2877
- 22 -
than the present invention and a high wear loss ratio of
2.52.
On the other hand, samples D-2, D-3, D-4, D-S, D-6,
D-7, D-8, D-9 and D-10, which have Cr contents falling
within the range of the present invention, have
hardnesses of 400 Hv or more, and wear loss ratios of
less than 1. Furthermore, they show excellent tensile
strengths of 1200 MPa or more and excellent toughnesses
of uE20 = 30 J or more. However, in the case of a
sample D-ll having a Cr content higher than the range of
the invention, it is found that the Cr effect of
increasing the wear resistance is saturated.
21S2877
Table 6
( wt%
Sample C Si Mn P S Cr
D- 1 0.40 0.31 2.04 0.009 0.008 0.10
D- 2 0.40 0.32 2.05 0.009 0.007 0.35
D- 3 0.41 0.32 2.02 0.011 0.007 0.57
D- 4 0.42 0.31 2.01 0.010 0.008 1.00
D- 5 0.40 0.33 2.01 0.011 0.007 1.51
D- 6 0.41 0.32 2.02 0.011 0.008 2.02
D- 7 0.40 0.32 2.04 0.008 0.008 2.52
D- 8 0.40 0.32 2.04 0.009 0.008 3.04
D- 9 0.42 0.32 2.03 0.010 0.007 3.49
D-10 0.41 0.32 2.03 0.010 0.007 3.98
D-ll 0.41 0.32 2.01 0.010 0.006 4.50
- 21~2877
- 24 -
Table 7
Sample TS uE20 Hard- Wear
(MPa) tJ) ness Loss
(Hv) Ratio
D- 1 1081 31.2 332 2.52
D- 2 1212 43.8 400 0.97
D- 3 1154 49.1 405 0.90
D- 4 1296 48.4 411 0.78
D- 5 1339 49.8 429 0.61
D- 6 1388 45.5 433 0.57
D- 7 1474 49.2 448 0.52
D- 8 1564 43.7 469 0.44
D- 9 1620 47.3 486 0.39
D-10 1677 46.6 502 0.36
D-ll 1701 40.2 524 0.37
Example 4
Steel samples having compositions shown in table 8
were processed in the same manner as in Example 1, and
resultant steel plates were subjected to the tensile
test, the Charpy impact test and the wear test. All the
samples had the bainite structure. Table 9 shows the
2152877
- 25 -
mechanical properties and the wear loss ratio of each
steel sample. A sample E-l, which has a composition
of the present invention and in which Ni and Mo are not
contained, has a hardness of 400 Hv or more. Further,
it shows a strength, a toughness and a wear loss ratio
which fall within the target ranges of the invention.
Steel samples E-2 and E6, which contain less than
0.1 wt% of Ni and Mo, respectively, show substantially
the same strength, toughness and wear loss ratio as the
sample D-l containing no Ni and Mo. This means that
addition of less than 0.1 wt% of Ni and Mo shows almost
no effect. Samples E-3, E-4, E-7, E-8 and E-10, which
contain 0.1 to 1.0 wt% of Ni and/or 0.1 to 1.0 wt%
of Mo, have hardnesses of 400 Hv or more, and show
excellent strengths, toughnesses and wear loss ratios.
In particular, they show strengths higher than the
sample E-l. Steel samples E-5 and E-9, which respec-
tively contain more than 1.0 wt% of Ni and Mo,
respectively, show substantially the same strength,
toughness and wear loss ratio as the samples E-4 and
E-8. This means that if the Ni or Mo content exceeds
1.0 wt%, its addition effect is saturated.
Table 8
( wt% )
Sample C Si Mn P S Cr Ni Mo
E- 1 0.41 0.32 1.02 0.011 0.007 2.02 - -
E- 2 0.40 0.31 1.04 0.011 0.007 2.02 0.05
E- 3 0.40 0.31 1.04 0.011 0.007 2.02 0.21
E- 4 0.39 0.32 1.01 0.012 0.008 2.02 0.73
E- 5 0.39 0.32 1.01 0.012 0.008 2.02 1.50
E- 6 0.40 0.31 1.02 0.012 0.007 2.01 - O .06 ~ r~
E- 7 0.40 0.31 1.02 0.012 0.007 2.01 - O .22 00
E- 8 0.41 0.31 1.02 0.010 0.007 2.01 - 0.70
E- 9 0.41 0.31 1.02 0.010 0.007 2.01 - 1.49
E-10 0.40 0.31 1.04 0.011 0.007 2.02 0.21 0.23
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Table 9
Sample TS uE20 Hard- Wear
(MPa) (J) ness Loss
(HV) Ratio
E- 1 1378 41.8 422 0.54
E- 2 1386 40.6 424 0.54
E- 3 1454 41.1 435 0.47
E- 4 1569 37.8 461 0.39
E- 5 1601 33.4 470 0.39
E- 6 1370 42.2 419 0.56
E- 7 1438 38.3 431 0.50
E- 8 1526 35.5 456 0.41
E- 9 1555 34.9 558 0.41
E-10 1481 38.6 442 0.45
Steel samples having compositions shown in table 10
were processed in the same manner as in Example 1, and
resultant steel plates were subjected to the tensile
test, the Charpy impact test and the wear test. All the
samples had the bainite structure. Table 11 shows the
mechanical properties and the wear loss ratio of each
steel sample. A steel sample E-l, which has a composi-
tion of the present invention and in which Nb and V are
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not contained, has a hardness of 400 Hv or more.
Further, it shows a strength, a toughness and a wear
loss ratio which fall within the target ranges of the
present invention.
Steel samples F-2, F-3, F-5, F-6 and F-8, which
contain 0.01 to 0.1 wt% of Nb and/or Mo, show excellent
strength, toughness and wear loss ratio. In particular,
their strength and hardness are higher than those of the
sample F-l. Steel samples F-4 and F-7, which respec-
tively contain more than o.l wt% of Nb and v, show
substantially the same strength, toughness and wear loss
ratio as the samples F-3 and F-6. This means that if
the Nb or V content exceeds 0.1 wt%, its addition effect
is saturated.
Moreover, a steel sample F-9, which have Ni, Mo, Nb
and V contents falling within the ranges of the present
invention, shows excellent strength, toughness and wear
loss ratio as compared with the sample E-10 containing
Ni and Mo and the sample F-8 containing Nb and V.
Table 10
( wt% )
Sample C Si Mn P S Cr Ni Mo Nb P
F-l 0.30 0.31 1.53 0.010 0.007 2.53 - - - -
F-2 0.32 0.32 1.51 0.012 0.008 2.52 - - 0.03
F-3 0.32 0.32 1.51 0.012 0.008 2.52 - - 0.08
F-4 0.32 0.32 1.51 0.012 0.008 2.52 - - 0.15
F-5 0.31 0.31 1.51 0.010 0.008 2.53 - - - 0.03
F-6 0.31 0.31 1.50 0.010 0.008 2.53 - - - 0.10 cn
oo
F-7 0.31 0.31 1.50 0.010 0.008 2.53 - - - O .20 ~~
F-8 0.32 0.32 1.51 0.011 0.008 2.51 - - 0.09 0.09
F-9 0.32 0.31 1.51 0.011 0.008 2.510.20 0.19 0.09 0.09
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Table 11
Sample TS uE20 Hard- Wear
(MPa) (J) ness Loss
(HV) Ratio
F-l 1356 61.6 411 0.88
F-2 1400 60.1 419 0.84
F-3 1471 63.9 441 0.53
F-4 1503 59.4 446 0.51
F-5 1406 58.2 421 0.74
F-6 1512 61.3 448 0.50
F-7 1571 55.6 464 0.47
F-8 1558 55.6 460 0.42
F-9 1634 44.8 483 0.40
Example 6
Table 12 shows steel samples G-l and G-2. A rail
stock was prepared by hot rolling the samples to the
actual shape of a rail, with the rolling finishing
temperature varied from 760 to 1030C. Thereafter, the
rail stock was cooled with the cooling conditions varied
from air cooling to accelerated cooling of 6. 5C/sec.,
thereby forming a rail. Table 13 shows the manufac-
turing conditions.
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Table 13 also shows the tensile properties, the
2 mm, U-notch Charpy absorbed energy at +20C, the
hardness and the wear loss ratio of each rail sample
manufactured as above. The wear loss ratio was
estimated by sub;ecting the wear test samples in
Example 1 extracted from the rolling material head
portion, to the same test as in Example 1.
Under conditions 1 which satisfied the cooling rate
but did not satisfy the rolling finishing temperature,
the sample showed a low tensile strength of 1068 MPa and
a high wear loss ratio of 3.11 (a hardness of 320 Hv).
Under conditions 2 - 6, 8 - 10 and 12 which satisfied
both the cooling rate and the rolling finishing
temperature, the samples showed excellent values, i.e. a
hardness of 400 Hv or more, a wear loss ratio of 1 or
less, a tensile strength of 1200 MPa or more and a
toughness of uE20 = 30 J or more.
Under conditions 7, 11 and 13 which satisfied the
rolling finishing temperature but did not satisfy the
cooling rate, the samples showed low toughnesses of
UE20 = 23.0 J, 28.5 J and 21.1 J, respectively.
Under conditions 14 and 15 which satisfied the
cooling rate but did not satisfy the rolling finishing
temperature, the samples showed low toughnesses of
uE20 = 28.9 J and 22.0 J, respectively.
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Table 12
(wt%)
Steel C Si Mn P S Cr
G-l 0.29 0.34 1.52 0.011 0.007 2.31
G-2 0.40 0.33 1.03 0.011 0.007 2.04
Table 13
Conditions Steel Rolling Cooling TS uE20 Hardness Wear Loss
No. Temperature Rate (C/s) (MPa) (J) (Hv) Ratio
( C)
1 G-l 760Air Cooling 1068 32.8 320 3.11
2 G-l 820Air Cooling 1207 84.4 401 0.99
3 G-2 820 2.9 1309 47.3 413 0.66
4 G-2 870Air Cooling 1246 46.2 406 0.68
G-l 870 2.0 1310 62.2 410 0.87
6 G-l 870 3.2 1341 56.6 419 0.83
7 G-2 870 6.3 1421 23.0 430 0.53
8 G-2 920Air Cooling 1285 52.9 408 0.63
9 G-2 920 3.1 1378 41.8 422 0.54
G-l 920 4.9 1426 55.3 436 0.65
11 G-l 920 6.5 1476 28.5 444 0.55
12 G-2 970 3.1 1391 36.4 428 0.57
13 G-l 970 6.2 1494 21.1 446 0.63
14 G-l 1030Air Cooling 1372 28.9 421 0.73
G-2 1030 2.9 1436 22.0 428 0.53
