STEEL FOR WHEEL
Technical Field
roo011
The present invention relates to steel for wheel, and more particularly to
steel for wheel preferable as steel for high-hardness wheels for railways
excellent in wear resistance, rolling contact fatigue resistance and spalling
resistance.
[00021
Spalling is the phenomenon in which the tread of a wheel which is
heated and rapidly cooled by emergency brake or the like is transformed into
brittle martensite called a white layer, a crack propagation with the white
layer as an origin, and a brittle fracture occurs to cause peel. Spalling is
sometimes called "thermal crack".
Background Art
[00031
In recent years, with increase in traveling distances and increase in
loading capacities on a global scale, there is a need for railway wheels
(hereinafter, also called "wheels") having longer life than the conventional
wheels.
[OOO~]
The damage factors on wheel tread mainly include three phenomena
that are (i) wear, (id rolling contact fatigue and (iii) spalling, and especially in
recent years, the number of wheels that are damaged by wear associated with
increase in traveling distances and rolling contact fatigue associated with
increase in loading capacities has been increasing. Rolling contact fatigue is
sometimes called "shelling". Though a crack that is caused by spalling is
sometimes called "shelling", the occurrence of the crack due to formation of a
white layer is defined herein as "spalling".
[00051
High temperature rolling contact fatigue (Thermal mechanical shelling,
hereinafter called "TMS") that occurs with rise in the wheel temperature
during braking is currently said to be the cause of a wheel damage. With this,
the wheels ensuring high temperature strength are required. For example, in
the Class-D standard of M R (Association of American Railroads), the yield
strength at 538OC (1000°F') is defined to be 345 MPa or more.
[0006]
Recently, in order to suppress crack occurrence on a wheel tread, it is
required to ensure the minimum ductility, and different countries have
different definitions. For example, in the Russia GOST10791 Grade 3
standard, the elongation is defined to be 8% or more, in the China TBIT 2708
CL60 definition, the elongation is defined to be 10% or more, in the Europe EN
13262 ER9 standard, the elongation is defined to be 12% or more, in the Class-
D definition-of AAR, the elongation is defined to be 14% or more, and the like.
[00071
It is empirically known that wear resistance and rolling contact fatigue
resistance are properties contrary to spalling resistance. It is urgently
required to develop the steel for wheel that is excellent in balance of wear
resistance, rolling contact fatigue resistance and spalling resistance, further
includes both high strength at high temperature and high ductility, and can
give long life to wheels.
[0008]
For example, the technology relating to wheels are disclosed in the
following documents.
[00091
JP50-104717A discloses "high toughness steel for railway wheel" adding
v.
Loo 101
JP2001-158940A discloses "rim or monoblock wheel for a wheel set of a
railway vehicle" that is excellent in wear resistance, fracture resistance and
thermal crack resistance.
[00111
JP2005-350769A discloses "railway wheel" that makes shelling
resistance and flat (spalling) resistance as thermal crack resistance compatible
by decreasing the content of C, and making the tread portion have a bainitic
microstructure, a tempered martensitic microstructure, or a mixed
microstructure of bainite and tempered martensite.
[00121
JP2004-315928A discloses "high carbon railway wheel excellent in wear
resistance and thermal crack resistance" in which the content of C is increased
to 0.85 to 1.20%.
[00131
JP9-202937A discloses "railway wheel excellent in wear resistance and
thermal crack resistance" that is an monoblock type railway wheel formed of
steel having a chemical composition consist of C: 0.4 to 0.75%, Si: 0.4 to 0.95%,
Mn: 0.6 to 1.2%, Cr: 0 to less than 0.2%, P: 0.03% or less and S: 0.03% or less,
and the balance consisting of Fe and impurities, wherein the region up to at
least the depth of 50 mm fiom the surface of the wheel tread consists of a
pearlitic microstructure, and the manufacturing method thereof.
[00141
US7559999B and US7591909B disclose "steel for railway wheel" which
are increased in strength by containing 0.01 to 0.12% and 0.009 to 0.013% of
Nb respectively, and are improved in rolling contact fatigue resistance and
spalling resistance.
[00151
JP57-143469A discloses a wheel steel containing V or Nb. According to
the invention, it is described that rolling contact fatigue resistance property
can be ensured without thermal refining.
[00161
JP6-279927A discloses high strength rail using steel with pearlite grains
refined by Ti deoxidation. According to the invention, it is described that
ductility and toughness can be improved.
[00171
JP6-279918A discloses the material that is improved in rolling contact
fatigue resistance property by defining the size of an alumina cluster.
Loo 181
US6783610B discloses the wheel steel with high TMS resistance, which
is improved in high temperature strength by increasing the contents of Si, Cr
and Mo.
Disclosure of the Invention
Problems to be Solved by the Invention
[00201
The steel disclosed in JP50-104717A has low wear resistance, because
the content of C is as low as 0.50 to 0.60%. Accordingly, the steel cannot have
sufficient performance to increase in the loading capacity of recent years.
[00211
The steel disclosed in JP2001-158940A has low wear resistance, because
the content of C is as low as 0.45 to 0.55%. Accordingly, the steel cannot have
sufficient performance to the increase in the loading capacity of recent years,
either.
100221
The wheel disclosed in JP2005-350769A has the tread portion consisting
of a bainitic microstructure, a tempered martensitic microstructure, or a mixed
microstructure of bainite and tempered martensite. Therefore, though the
wheel has high strength, the wheel has lower wear resistance, as compared
with the case of a tread portion consisting of a pearlitic microstructure, and
obtaining wear resistance equal to or beyond a conventional freight car wheel
steel is difficult. Namely, as compared with a pearlitic microstructure that is
excellent in work hardening property, and showing the behavior with lamellas
thereof being rearranged in parallel to the surface with progress of wear, the
amount of wear become large in a bainitic microstructure and a tempered
martensitic microstructure (for example, refer to Sadahiro Yamamoto:
"Technique of Improving Wear resistance of Steel by Microstructure Control -
Microstructure Control Technique for Wear Resistance Steel having
Weld~bility-", the 161th and 162th Nishiyama memorial Seminar, Heisei 8
(1996), edited by The Iron and Steel Institute of Japan, p.221).
100231
The steel of the wheel disclosed in JP2004-315928A is difficult to apply
to the wheels which are produced by the treatment peculiar to wheels and
called "tread quench method". As an example of the wheel, the schematic
diagram of "monoblock wheel" is shown in Figure 1. In the case of a wheel,
heat treatment of cooling the rim portion is applied from the outer
circumference of the wheel in order to give a compression residual stress to the
rim portion after the entire wheel is heated. In the cooling treatment, the
region near the rim portion is rapidly cooled, but the cooling speed of the hub
portion is low. Therefore, when the steel of the wheel described in the
document is heat-treated by a tread quench method, there is the possibility of
hyper-eutectoid cementite precipitating in the austenite grain boundary of the
hub portion. Hyper-eutectoid cementite has the same effect as coarse
inclusions, and extremely reduces toughness and fatigue life (for example,
refer to Yukitaka Murakami: Influence of micro defects and inclusions (2004),
p. 182 "Yokendo").
100241
The wheel disclosed in JP9-202937A is insufficient in hardness in some
cases. Accordingly, the wheel cannot always have sufficient performance to
the increase in the loading capacity of recent years.
100251
The steel for railway wheel disclosed in US7559999B contains as much
Mo as 0.20 to 0.30% of Mo. Therefore, the microstructure with low wear
resistance such as a bainitic microstructure or a degenerate pearlitic
microstructure is easily formed, and favorable wear resistance is hardly
obtained. In addition, the steel always contains 0.01 to 0.12% of Nb. Coarse
inclusions are sometimes formed in the steel containing Nb, and this
extremely reduces toughness and fatigue life similarly to the aforementioned
hyper-eutectoid cementite.
100261
The steel for railway wheel disclosed in US7591909B also always
contains 0.009 to 0.013% of Nb. As described above, coarse inclusions are
sometimes formed in the steel containing Nb, and this extremely reduces
toughness and fatigue life similarly to hyper-eutectoid cementite.
100271
The steel for railway wheel disclosed in JP57-143469A contains 0.15% or
more of Cr. In the steel with a high Cr content, the microstructure with low
wear resistance such as a bainitic microstructure is easily formed if the cooling
speed is high. In this invention, formation of these microstructures is
prevented by adopting a low cooling speed of air-blast cooling or the like to the
temperature region from 700°C to 500°C after hot forging. However, with the
slow cooling, sufficient hardness is not obtained, and the steel cannot have
sufficient performance to the increase in the loading capacity of recent years.
Further, when the cooling speed is high, a bainitic microstructure is formed in
the rim portion, and wear resistance is declined.
[00281
In the steel disclosed in JP6-279927A, coarse inclusions containing Ti
are sometimes formed depending on the production process. This extremely
reduces toughness and fatigue life similarly to the hyper-eutectoid cementite
mentioned above.
[00291
The steel disclosed in JP6-279918A is considered to have sufficient
hardness, and a high rolling fatigue resistance property, but no attention is
paid to spalling resistance.
[00301
The steel for wheel disclosed in US6783610B contains 0.08% or more of
Mo, and therefore, is high in high temperature strength and excellent in TMS
resistance, but ductility is not taken into consideration. Further, addition of
excessive Mo tends to break the lamellar microstructure of pearlite, and wear
resistance tends to be declined. Further, in the case of the steel containing
excessive Mo, it is difficult to ensure wear resistance because it is easy to form
a bainitic microstructure in the rim portion, if the cooling speed is high.
[00311
The present invention is made to solve the problems described above,
and has an objective to provide a steel for wheel that is excellent in balance of
wear resistance, rolling contact fatigue resistance and spalling resistance,
further includes both high yield strength at high temperature and high
ductility, and can give a long life to the wheel.
Means for Solving the Problems
too321
As the result of the present inventors variously studied wear resistance,
rolling contact fatigue resistance, spalling resistance, .high temperature
strength and ductility, the matters of the following (a) to (el were revealed.
[00331
(a) Wear resistance is improved more when the microstructure of the
steel materia1.i~m ade a pearfitic microstructure, and as the hardness is
higher.
(b) Rolling contact fatigue resistance is improved more as the hardness
is higher irrespective of the microstructure.
(c) Spalling resistance is improved more as hardenability is lower.
(d) High temperature strength is improved more as the contents of Si
and V are larger.
(e) Ductility is improved more as the Si content is larger, and the content
of V is smaller.
[00341
From the above, the present inventors have reached the conclusion that
in order to solve the aforementioned problems, the steel needs to be developed,
in which a pearlitic microstructure is obtained by heat treatment, hardness is
high and hardenability is low, and the contents of Si and V are optimized.
[00351
Hereinafter, an example of the contents that the present inventors
studied will be described in detail.
[0036]
First, the present inventors evaluated the influences which the
respective elements have on hardness and hardenability, by a Jominy epd
quench test (hereinafter, called "Jominy test") which is similar to the tread
quench of an actual wheel in the heat treatment conditions.
Lo0371
Steels 1 to 24 that have the chemical compositions shown in Table 1
were melted in a vacuum furnace on a laboratory scale and ingots were
produced. From each of the ingots, a round bar 35 mm in diameter, a round
bar 160 mm in diameter and a round bar 70 mm in diameter were produced by
hot forging. For steel 1, a round bar 220 mm in diameter was also produced
in order to produce "rail test specimen" for a rolling contact fatigue test that
will be described later.
[00381
Note that steel 1 in Table 1 corresponds to the steel for railway wheel of
"Class C" in the M-107IM-207 standard of AAR (Association of American
Table 1
[OO~O]
From the round bar 35 mm in diameter, a Jominy test specimen was
taken, after austenitization at 900°C for 30 minutes under air atmosphere, end
quench was performed, parallel cutting of 1.0 mm was performed next, and
measurement of Rockwell C hardness (hereinafter, also called "HRC") was
performed.
[00411
The HRC at the position 40 mm from the water-cooled end (hereinafter,
called "40 mm hardness") was measured, and the influence which each of the
elements has on the value was evaluated. As a result, it has been found out
that "40 mm hardness" has a linear relationship with Fnl expressed by the
following formula (1) as follows, as shown in Figure 2. Further, it has been
found out that when Fnl exceeds 43, as steel 23 and steel 24, bainitic
microstructures are formed at least in part, and the linear relationship is not
established.
100421
Note that the HRC at the position 40 mm from the water-cooled end was '
measured because a wheel is machined directly under the tread after heat
treatment, and is sometimes used by repeating machining after the start of
use, and the property of the steel in the interior with lower hardness than the
surface sigmiicantly influences the life of the wheel.
100431
In Figure 2, steel 1 corresponding to the steel for railway wheel of "Class
C" of AAR is shown by mark "A". Note that the microstructure at the
position 40 mm fkom the water-cooled end was mirror-polished, thereafter,
etched with nital, observed with an optical microscope, and was assessed.
100441
Fnl =2.7 + 29.5 x C + 2.9 x Si+ 6.9 xMn+ 1 0 . 8 C~r + 30.3 xMo + 44.3
x v . . . (1)
C, Si, Mn, Cr, Mo and V in formula (1) mean the contents in mass% of the
elements.
[00451 .
In Table 2, the measurement values of the "40 mm hardness" described
above and Fnl expressed by the formula (1) are organized and shown.
Lo0461
The hardenability was evaluated by measuring the distance from the
water cooled end in the unit of millimeter where the martensitic
microstructure fraction is 50% (hereinafter, called "M50%") from Jominy
hardness, based on the hardness in the case of the martensitic microstructure
fraction being 50% described in the ASTM A255 standard. As a result, it has
been found out that "M50%" has a correlation with Fn2 expressed by the
following formula (2) as follows, as shown in Figure 3. Note that in Figure 3,
steel 1 is shown by the mark "A".
[00471
Fn2 = exp(0.76) x exp(0.05 x C) x exp(1.35 x Si) x exp(0.38 x Mn) x
exp(0.77 x Cr) x exp(3.0 x Mo) x exp(4.6 x V) . . (2)
C, Si, Mn, Cr, Mo and V in formula (2) also mean the contents in mass% of the
elements. The terms "exp(0.05 x C)" and the like mean exponent
representation of "e0.06 C" and the like. Note that "e" is "Napier's constant"
that is one of mathematical constants, and is used as a base of natural
logarithm.
[00481
In Table 2, the measurement values of "M50%" described above and Fn2
expressed by formula (2) are organized and shown.
roo491
Table 2
[00501
Next, the present inventors investigated the relationship of the rolling
contact fatigue resistance and wear resistance, and Fnl expressed by the
formula (1) by using steels 1 to 24 shown in the Table 1.
[00511
1
2
3
4
6
6
7
8
9
10
11
12
13
14
16
16
17
18
19
20
21
22
23
24
M5O%
(mm)
6.6
8.3
13.6
16.6
10.6
6.3
14.2
16.2
15.7
15.5
3.8
4.6
4.6
6.8
6.4
9.2
16.0
21.8
16.5
14.9
7.1
11.2
29.7
46.9
40 mm
hardness
(HRC)
31.1
32.3
36.6
39.3
38.6
37.3
38.9
37.8
39.6
35.0
26.4
32.6
34.1
37.2
35.8
34.8
38.9
42.1
41.0
32.4
39.7
38.7
42.5
42.6
Fnl
31.2
32.4
37.1
38.7
38.1
37.8
38.7
36.6
39.2
34.4
26.2
32.3
34.0
37.1
34.6
34.9
38.9
42.6
41.2
33.7
40.0
40.2
47.2
49.6
Fn2
6.1
10.1
16.0
16,6
11.2
7.2
16.7
14.5
14.3
14.6
3.3
6.0
5.6
7.7
6.3
11.3
16.8
21.3
16.4
13.8
6.9
11.4
28.6
43.7
'Obg fatigue
contact life
(cycle)
1,830,898
2,191,426
3,283,349
4,453,779
4,114,188
3,676,793
4,342,381
3,688,978
4,480,886
2,867,978
814,399
2,138,325
2,683,220
3,602,246
3,089,949
2,786,671
4,268,560
5,621,776
5,092,738
2,085,608
4,687,370
4,266,610
6,892,201
6,675,736
amount of wear
(g)
0.320
0.312
0.268
0.242
0.241
0.249
0.225
0.247
0.241
0.279
0.384
0.312
0.286
0.263
0.263
0.279
0.242
0.201
0.223
0.327
0.227
0.236
0.314
0.312
Namely, the test specimen that was the round bar 160 mm in diameter
which was cut into a length of 100 mm, thereafter, was heated at a
temperature of 900°C for 30 minutes, and was oil quenched was produced for
each of the steels.
[00521
For steels 1 to 24, the test specimens in the configurations shown in
Figure 4(a) were firstly taken as "wheel test specimens" for use in a rolling
contact fatigue test, from the regions in the centers of the test specimens
produced as described above.
[00531
For steel 1, the test specimen that is the round bar 220 mm in diameter
which was cut into a length of 100 mm, thereafter, was heated at 900°C for 30
minutes, and thereafter, was oil quenched was produced, and from the central
portion of the test specimen, the test specimen in the configuration as shown
in Figure 4(b) was also taken as "rail test specimen" for use in a rolling contact
fatigue test.
[00541
Similarly, for steels 1 to 24, the test specimens that were the round bars
70 mm in diameter which were cut into lengths of 100 mm, thereafter, were
heated at 900°C for 30 minutes, and thereafter, were oil-quenched were
produced. From the regions in the centers of the test specimens, the test
specimens in the configuration as shown in Figure 5(a) were taken as "wheel
test specimens" for use in wear test.
[00551
For steel 1, the round bar test specimen 100 mm in length and 70 mm in
diameter for which the heat treatment similar to the wheel test specimens
described above was carried out was produced, and from the region in the
center thereof, the test specimen in the colfiguration shown in Figure 5(b) was
also taken as "rail test specimen" for use in wear test.
100561
First, a rolling contact fatigue test was carried out by the method
schematically shown in Figure 6 with use of the wheel test specimens shown in
Figure 4(a) of the steels 1 to 24, and the rail test specimen shown in Figure
4(b) of steel 1.
100571
The conditions of the rolling contact fatigue test were the Hertzian
stress: 1100 MPa, the slip ratio: 0.28%, the revolutions: 1000 rpm at the wheel
side, and 602 rpm at the rail side, and the test was carried out under water
lubrication. The test was carried out while the acceleration was monitored
with a vibration accelerometer, and the number of cycles in which 0.5G was
detected was evaluated as the rolling contact fatigue life. Note that 0.5 G was
set as the reference, because as the result of evaluating the relationship of the
detection acceleration and the damage state in the preliminary test in advance,
obvious occurrence of peel onto the contact surface was able to be confirmed in
the case of the acceleration exceeding 0.5 G.
[0058]
In Table 2, the rolling contact fatigue life is shown in combination.
Further, in Figure 7, the relationship of the rolling contact fatigue life and Fnl
expressed by the formula (1) is shown.
100591
Note that "2.E + 06" in the Figure 7 and the like mean "2.0 x 106" and
the like. In Figure 7, steel 1 is also shown by the mark "A".
[0060]
As shown in Figure 7, it has been found out that the rolling contact
fatigue life has a correlation with Fnl expressed by the formula (I), and if Fnl
is 32 or more, the rolling contact fatigue life can be that of steel 1
corresponding to the steel for railway wheel of "Class C" of AAR or more.
[0061]
Further, the wear test was carried out by the method schematically
shown in Figure 8 with use of the wheel test specimens shown in Figure 5(a) of
steels 1 to 24, and the rail test specimen shown in Figure 5(b) of steel 1. Note
that in the wear test, the Nishihara-type wear testing machine was used.
Lo0621
The specific test conditions were the Hertzian stress: 2200 MPa, the slip
ratio: 0.8%, and the revolutions: 776 rpm at the wheel side, and 800 rpm at the
rail side, and the test was carried out under dry condition. After the test was
performed up to the number of cycles of 5 x 106 times, the amount of wear was
obtained from the mass difference of the test specimen before and after the test.
Lo0631
In Table 2, the amount of wear is shown in combination. Further, in
Figure 9, the relationship of the amount of wear and Fnl expressed by the
formula (1) is shown. In Figure 9, steel 1 is shown by the mark "A".
LO0641
It has been found out that as long as the microstructure is a pearlitic
microstructure, the amount of wear decreases in proportion to Fnl expressed
by the formula (11, and if Fnl is 32 or more, the amount of wear can be made
that of steel 1 or less, as shown in Figure 9.
Lo0651
When Fnl exceeds 43, a bainitic microstructure is formed at least in
part as described above. It can be confirmed that when a bainitic
microstructure is contained, the amount of wear does not decrease even if Fnl
increases, and the wear resistance is inferior as compared with the case of the
microstructure in which pearlite predominates.
roo661
Kanetaka et al. report in Railway Technical Research Institute Report,
Vol. 19 (2005) No9, p.17 that as the thickness of the quenched layer called a
white layer is larger, the crack depth becomes larger, and sp&g (meaning
"spalling" herein, though described as "shelling" in the sentences) is likely to
occur.
[00671
Thus, the present inventors also studied the influence which
hardenability has on spalling in detail.
From the report of Kanetaka et al., it is predicted that as hardenability
becomes larger, the thickness of the white layer increases more, a crack occurs
and spalling occurrence life declines, and therefore, the relationship of the
hardenability and crack initiation life in the case of the white layer being
formed was investigated.
[00691
More specifically, "wheel test specimens" in the configuration shown in
Figure 4(a) of steel 1, steel 2, steel 5, steel 11, steel 12 and steel 14 described in
Table 1, and "rail test specimen" in the configuration shown in Figure 4(b) of
steel 1 were used. The thick white layer leading to spaUing was formed on
the test surfaces of "wheel test specimens" by YAG laser, after which, the
rolling contact fatigue test was carried out, and crack initiation life (spalling
resistance) was investigated. The conditions of the YAG laser heating were
the output power of laser: 2500 W, and the feed rate: 1.2 mlmin, and the white
layer was air-cooled after the laser heating.
[00701
Note that the conditions of the rolling contact fatigue test were the
Hertzian stress: 1100 MPa, the slip ratio: 0.28%, the revolutions: 100 rpm at
the wheel side, and 60 rpm at the rail side, and the test was carried out under
water lubrication. Note that the test was stopped every 200 cycles up to the
number of rolling cycles of 2000 times, and every 2000 cycles when the number
of rolling cycles exceeds 2000 cycles, and presence and absence of a crack on
the surfaces of the test specimens were visually checked.
[00711
As a result, it has been found out that the thickness of the white layer
increases with increase in Fn2 expressed by the formula (2) which is correlated
with "M50%" that is an index of hardenability, and with this, crack initiation
life abruptly decreases, as shown in Figures 10 and 11.
[00721
Furthermore, it has been found out that when Fn2 exceeds 25, the crack
initiation life is reduced so extremely that a crack can be already confirmed by
the first visual inspection (namely, the visual inspection at the number of
rolling cycles of 200 times).
100731
Form the result described above, the present inventors have concluded
that if the chemical composition of a steel is set so that'Fn2 expressed by the
formula (2) is 25 or less, extreme reduction in crack initiation life, that is
spalling occurrence life, can be avoided.
[00741
Next, the present inventors melted steel 1, and steels 25 to 36 of Table 1
in the vacuum furnace on a laboratory scale, produced ingots, produced round
bars 70 mm in diameter by hot forging from the respective ingots, heated and
oil quenched the round bars, and took the high temperature tensile test
specimens 6 mm in diameter of GL 25 mm in accordance with the ASTM ES
standard, and the normal temperature tensile test specimens 12.5 mm in
diameter of GL 50 mm in accordance with the ASTM E370 standard, from the
internal pearlitic microstructure portions.
[00751
With use of these test specimens, the tensile test at 538°C (1000°F) was
carried out in accordance with the ASTM E21 standard, and the influences
which the components have on the high temperature yield strength were
investigated. Further, a normal temperature tensile test was carried out in
accordance with the ASTM E370 standard. The results thereof are shown in
Table 3. Further, Figures 12 and 13 respectively show the diagrams in which
the results of the high temperature yield strength and the normal temperature
elongation are organized with the V contents.
100761
Table 3
high temperature
1 yield strength No. 1 normal
temperature
elongation
As shown in Table 3, and Figures 12 and 13, the high temperature yield
strength is improved more as the V content is higher, and the normal
temperature elongation is improved more as the V content is lower. It has
been found out that in particular, the steel with the Si content being 0.4% or
more (described as high Si in the drawings) is higher in both the high
temperature yield strength and the normal temperature elongation than the
steel with the Si content being less than 0.4% (described as low Si in the
drawings).
[00781
From the above study, in order to obtain sufficient high temperature
yield strength and normal temperature elongation, it is effective to contain
0.4% or more of Si, and contain V in the range of 0.02% to 0.12%.
[00791
The present invention is completed based on the above described finding,
and a gist thereof is in steels for wheel shown in the following 0 and (B).
[OO~O]
0 A steel for wheel comprising: in mass%, C: 0.65 to 0.84%; Si: 0.4 to
1.0%; Mn: 0.50 to 1.40%; Cr: 0.02 to 0.13%; S: 0.04% or less and V: 0.02 'to
0.12%, wherein Fnl expressed by the following formula (1) is 32 to 43, and Fn2
expressed by the following formula (2) is 25 or less, the balance being Fe and
impurities, and P, Cu and Ni in the impurities are P: 0.05% or less, Cu: 0.20%
or less, and Ni: 0.20% or less:
Fnl = 2.7 + 29.5 x C + 2.9 x Si + 6.9 x Mn + 10.8 x Cr + 30.3 x Mo + 44.3
x v . . . (1)
Fn2 = exp(0.76) x exp(0.05 x C) x exp(1.35 x Si) x exp(0.38 x Mn) x
exp(0.77 x Cr) x exp(3.0 x Mo) x exp(4.6 x V') - . (2)
where each symbol of element in formulas (1) and (2) means content
(mass%) of each element.
roo811
(B) The steel for wheel according to claim 1, comprising, in mass%, Mo:
0.07% or less in place of a part of Fe, and a total content of V and Mo is 0.02 to
0.12%.
[00821
"Impurities" refers to components which are mixed into a steel material
from raw materials such as ores and scraps, etc. or by other causes while the
steel material is commercially manufactured.
Advantageous Effects of Inventio~i
[00831
The steel for wheel of the present invention is excellent in balance of
wear resistance, rolling contact fatigue resistance and spalling resistance, and
can give a long life to wheel. As compared with the wheel with the steel for
railway wheel of "Class C" of AAR, the wheel with the steel for wheel of the
present invention has the amount of wear at the same extent or less and
decreased by 30% at the largest, and the rolling contact fatigue life equivalent
to or more and increased by 3.2 times at the largest, and has a low risk of
occurrence of spalling. Further, the steel for wheel of the present invention
includes both high temperature strength and ductility, and therefore, has a
low risk of occurrence of TMS and a crack on the tread. Accordingly, the steel
for wheel of the present invention is extremely favorable for use as a railway
wheels that are used under extremely harsh environments where the traveling
distances increase and the loading capacities increase.
Brief Description of Drawings
[0084]
Figure 1 is a view schematically explaining "monoblock wheel" as an
example of a wheel.
Figure 2 is a diagram organizing and showing a relationship of "40 mm
hardness" that is Rockwell C hardness at a position 40 mm from a water
cooled end and "Fnl" expressed by the formula (11, with respect to steels 1 to
24. "Bainite" in the drawing indicates that a bainitic microstructure is
formed in part.
Figure 3 is a diagram organizing and showing a relationship of "M50%"
that is a distance from a water-cooled end in a unit of millimeter where a
martensitic microstructure fraction becomes 50% and "Fn2" expressed by the
formula (2), with respect to steels 1 to 24.
Figure 4 is a view showing configurations of "wheel test specimen" and
"rail test specimen" used in a rolling contact fatigue test. (a) in the drawing
shows "wheel test specimen", and (b) shows "rail test specimen". Note that
the unit of the dimensions in the drawing is "mm".
Figure 5 is a view showing configurations of "wheel test specimen" and
"rail test specimen" used in wear test: (a) in the drawing shows "wheel test
specimen", and (b) shows "rail test specimen". Note that the unit of the
dimensions in the drawing is "mm".
Figure 6 is a view schematically explaining a method for a rolling
contact fatigue test using the wheel test specimen shown in Figure 4(a), and
the rail test specimen shown in Figure 4(b).
Figure 7 is a diagram organizing and showing a relationship of rolling
contact fatigue life and "Fnl" expressed by the formula (1). "Bainite" in the
drawing indicates that a bainitic microstructure is formed in part.
Figure 8 is a view schematically explaining a method for wear test using
the wheel test specimen shown in Figure 5(a) and the rail test specimen shown
in Figure 5(b).
Figure 9 is a diagram organizing and showing a relationship of amount
of wear and "Fnl" expressed by the formula (1). "Bainite" in the drawing
indicates that a bainitic microstructure is formed in part.
Figure 10 is a diagram organizing and showing a relationship of a
thickness of a white layer and "Fn2" expressed by the formula (2), with respect
to each of steel 1, steel 2, steel 5, steel 11, steel 12 and steel 14.
Figure 11 is a diagram organizing and showing a relationship of crack
initiation life and "Fn2" expressed by the formula (2), with respect to each of
steel 1, steel 2, steel 5, steel 11, steel 12 and steel 14.
Figure 12 is a diagram organizing a result of high temperature yield
strength with V content.
Figure 13 is a diagram organizing a result of normal temperature
elongation with the V content.
Figure 14 is a view explaining equipment used in an example for
performing so-called "tread quench" for a wheel.
Figure 15 is a view explaining a measurement position of Brinell
hardness of the wheel produced in the example.
Figure 16 is a view' explaining a position where a microstructure of a rim
portion of the wheel produced in the example was examined.
Figure 17 is a view explaining a position where a microstructure of a
hub portion of the wheel produced in the example was examined.
Figure 18 is a view explaining a position where wear test specimen, a
rolling contact fatigue test specimen and a Jominy test specimen were taken
from the wheel produced in the example. With the positions shown by "a", "b"
and "c" in the drawing as the references, the wear test specimen, the rolling
contact fatigue test specimen and the Jominy test specimen were taken
respectively.
Mode for Carrying Out the Invention
COO851
Hereinafter, respective requirements of the present invention will be
described in detail. Note that "%" of a content of each element means
"mass%".
[00861
C: 0.65 to 0.84%
C increases hardness, and improves wear resistance and rolling contact
fatigue resistance. Further, C in this range has a small influence on
. .
hardenability, and can increase hardness without reducing spalling resistance
so much. When the content of C is below 0.6596, sufficient hardness cannot be
obtained, an area fiaction of ferrite further increases, and wear resistance is
reduced. When the content of C exceeds 0.84%, coarse hyper-eutectoid
cementite is formed in a wheel hub portion, and sometimes extremely reduced
toughness and fatigue life, which is not favorable in safety. Therefore, the
content of C is set at 0.65 to 0.84%. The content of C is preferably set at
0.68% or more, and is preferably set at 0.82% or less.
[00871
Si: 0.4 to 1.0%
Si is an element that increases hardness by decreasing the lamellar
spacing of pearlite, and solid-solution strengthening ferrite in a pearlitic
microstructure, and further increases high temperature strength and ductility.
When the content of Si is below 0.4%, the aforementioned effects are
insufficient, and it is difficult to obtain high temperature strength and
ductility. When the content of Si exceeds 1.0%, toughness is reduced,
hardenability is further increased and spalling resistance is also reduced.
Therefore, the content of Si is set at 0.4 to 1.0%. However, in order to
increase hardness, high temperature strength and ductility by Si, the content
thereof is preferably set at 0.5% or more in particular, and is more preferably
set at 0.65% or more. Meanwhile, Si increases hardenability, and therefore,
the content thereof is preferably set at 0.90% or less.
[00881
Mn: 0.50 to 1.40%
Mn is an element that increases hardness by decreasing lamellar
spacing of pearlite, and solid-solution strengthening ferrite in a pearlitic
microstructure. Mn also has an effect of forming MnS to trap S in the steel,
and suppressing grain boundary embrittlement. When the content of Mn is
less than 0.50%, the aforementioned effects, above all, the trapping effect of S
becomes insufficient. When the content of Mn exceeds 1.40%, a bainitic
microstructure is formed to reduce wear resistance, hardenability is further
increased, and spalling resistance is also reduced. Therefore, the content of
Mn is set at 0.50 to 1.40%. The content of Mn is preferably set at 1.20% or
less.
[00891
Cf: 0.02 to 0.13%
Cr has the effect of sigmficantly increasing the hardness of pearlite by
decreasing lamellar spacing of the pearlite. When the content of Cr is less
than 0.02%, these effects are not sufficient. When the content of Cr exceeds
0.13%, carbides are difficult to dissolve into austenite at the time of heating,
and depending on the heating conditions, there arise the possibility of
undissolved carbides being formed to reduced hardness, toughness, fatigue
strength and the like. Further, when a heat-treated wheel is produced, a
bainitic microstructure with low wear resistance is easily formed directly
under a tread. Further, hardenability is increased, and spalling resistance is
reduced. Therefore, the content of Cr is set at 0.02 to 0.13%. The content of
Cr is preferably set at 0.05% or more, and is preferably set at 0.12% or less.
[OO~O]
S: 0.04% or less
S is an impurity normally contained in steel, and has a small influence
on hardness and hardenability, but has the effect of improving machinability.
Therefore, S may be positively contained, but excessive S reduced toughness of
steel. Therefore, the content of S is set at 0.04% or less. The content of S is
preferably set at 0.03% or less. Note that the effect of improving
machinability is remarkable when the content of S is 0.005% or more.
[0091]
V: 0.02 to 0.12%
V precipitates on ferrite in pearlite as a V carbide, and has the effect of
sigmficantly increasing the hardness of the pearlite. Further, V has the effect
of increasing yield strength at a high temperature. When the content of V is
less than 0.02%, these effects are not suEcient. When V exceeding 0.12% is
contained, normal temperature elongation is reduced, in addition to which,
hardenability is increased, and spalling resistance is reduced. Therefore,
when V is contained, the content thereof is set at 0.02 to 0.12%. The content
of V is preferably set at 0.07% or less, and is more preferably set at 0.05% or
less.
Fnl (refer to formula (1)): 32 to 43
When Fnl is less than 32, wear resistance and rolling contact fatigue
resistance are hardly improved as compared with the case of using the steel for
railway wheel of "Class C" of AAR, and depending on the case, wear resistance
and rolling contact fatigue resistance become lower than "Class C". Therefore,
steel with Fnl being less than 32 is difficult to use as the steel of a railway
wheel used under extremely harsh environments in which the traveling
distances increase and loading capacities increase. When Fnl exceeds 43, it
becomes difficult to obtain a microstructure consisting principally of pearlite,
and wear resistance is reduced. Further, hardness increases too much, and
therefore, ductility and toughness are reduced. Therefore, Fnl is set to be in
a range of 32 to 43. Fnl is preferably 37 or less, and is more preferably 36 or
less.
[00931
Fn2 (refer to formula (2)): 25 or less
When Fn2 exceeds 25, hardenability becomes high, which leads to
reduced in spalling resistance. Fn2 is preferably 20 or less, and is more
preferably 15 or less.
[0094]
Note that when En2 is less than 3, it becomes di£€icult to make Fnl
expressed by the formula (1) 32 or more. Therefore, Fn2 is preferably 3 or
more.
100951
One of steels for wheel of the present invention contains the above
described elements, the balance consists of Fe and impurities, and the contents
of P, Cu and Ni as the impurities should be limited to a certain range. The
range of the contents of the respective elements and the reason of limitation
are as follows.
[00961
P: 0.05% or less
P is an impurity contained in steel. When the content of P exceeds
0.05%, toughness is reduced. Accordingly, the content of P in the impurities
is set at 0.05% or less. The content of P, which is more preferable, is 0.025%
or less.
[00971
Cu: 0.20% or less
Cu is an impurity contained in steel. When the content of Cu exceeds
0.20%, the occurrence of a surface defect at the production time increases,
hardenability is further increased, and spalling resistance is reduced.
Accordingly, the content of Cu in the impurities is set at 0.20% or less. The
content of Cu which is more preferable is 0.10% or less.
[0098]
Ni: 0.20% or less
Ni is an impurity contained in steel. When the content of Ni exceeds
0.20%, hardenability is increased and spalling resistance is reduced.
Accordingly, the content of Ni in the impurities is set at 0.20% or less. The
content of Ni which is more preferable is 0.10% or less.
[0099]
The steel for wheel of the present invention may contain Mo in place of
part of Fe, in accordance with,necessity. The range of the content of Mo and
the reason of limitation are as follows.
~01001
Mo: 0.07% or less
Mo has an effect of increasing the hardness of pearlite, and has the effect
of increasing yield strength at a high temperature, similarly to V. When the
content of Mo exceeds 0.07%, it become easy to form a bainitic microstructure
directly under a tread to reduced wear resistance when a heat-treated wheel is
produced, hardenability is further increased, and spalling resistance is reduced.
Therefore, when Mo is contained, the content thereof is set at 0.07% or less.
The content of Mo is preferably set at 0.02% or more.
[01011
Especially when both V and Mo are contained, the total content (V + Mo)
is set at 0.02 to 0.12%. The upper limit that is more preferable is 0.07%, and
the upper limit that is far more preferable is 0.05%.
10 1021
The steel for wheel of the present invention may contain Al in
accordance with necessity. A range of the content of Al and the reason of
limitation are as follows.
[01031
Al: 0.20% or less
Al may be contained, because Al has the effect of refining grains to
improve toughness. However, if the content of Al exceeds 0.20%, coarse
inclusions increase, and reduced toughness and fatigue strength. Accordingly,
when Al is contained, the content thereof is set at 0.20% or less. The Al
content is preferably set at 0.08% or less. The effect of improving toughness
is remarkable when the Al content is 0.002% or more. In particular, the Al
content is preferably set at 0.011% or more.
[01041
The microstructure of the wheel with the steel for wheel of the present
invention desirably has 90% or more of the area fraction of the pearlitic
- microstructure with respect to the rim portion, and the most desirably has
100% of pearlitic microstructure. The reason is that the microstructures
other than the microstructure of pearlite, such as the microstructures of ferrite
and bainite have low wear resistance, and therefore, the total area fraction of
the microstructures other than the microstructure of pearlite is desirably 10%
or less. Further, the microstructure in which hyper-eutectoid cementite does
not precipitate is desired. The reason thereof is that precipitation of hypereutecticoid
cementite reduce toughness.
[01051
With respect to the hub portion, the microstructure is desirably similar
to that of the rim portion, and it does not especially become a problem if the
area fraction of the microstructures other than pearlite exceeds 10%.
However, the micrastructure in which hyper-eutectoid cementite does not
precipitate is desirable. The reason thereof is that there is the case in which
precipitation of the hyper-eutecticoid cementite causes extreme reduced of
toughness and fatigue life, and at least formation of the hyper-eutecticoid
cementite that can be observed by an optical microscope has to be avoided.
[0106l
The wheel which adopts the steel for wheel of the present invention w
can be produced by sequentially performing treatments described in the
following to <3>, for example. After the treatment of <3>, temper
treatment may be performed.
[01071
Melt and casting of steel:
After steel is melted by an electric furnace, a converter or the like, the
steel is cast into an ingot. Note that the ingot may be any one of a cast piece
by continuous casting, and an ingot molded in a mold.
[01081
<2> Forming into wheel:
In order to form the steel into a predetermined wheel configuration, the
steel is formed by a proper method such as hot forging and machining directly
from the ingot, or after the ingot is formed into end steel pieces. Note that
the steel may be directly formed into a wheel configuration by casting, but hot
forging is desirably performed.
[01091
<3> Quench:
A quench method give a compression residual stress to the rim portion,
such as "tread quench method" is adopted. Note that the heating
temperature on the occasion of quench is preferably set at ACQ point to (ACQ
point + 250°C). When the heating temperature is less than ACQ point, the
steel is not transformed into austenite, and pearlite with high hardness cannot
be obtained by cooling after heating in some cases, whereas when the heating
temperature exceeds ( h 3 point + 250°C), the grains coarsen and toughness is
reduced in some cases, which is not preferable in performance of a wheel. .
[01101
Cooling after heating is preferably performed by a proper method such
as water cooling, oil cooling, mist cooling, and air cooling so as to obtain the
desirable microstructure described above for the wheel, with the size of the
wheel, the facility and the like taken into consideration.
[01111
Hereinafter, the present invention will be described more specifically
according to examples, but the present invention is not limited to these
examples.
Examples
Lo1 121
After steels 37 to 63 of Table 4 were melted in an electric furnace, the
steels were cast into molds 513 mm in diameter to produce ingots, the
respective ingots were each cut into a length of 300 mm, and were heated to
1200°C, after which, the respective ingots were subjected to hot forging by a
normal method to produce wheels 965 mm in diameter. The wheels each has
the configuration of "AAR TYPE: B-38" described in the M-107/M-207 standard
of AAR.
C01131
Table 4
Steel
No.
Chemical C mpositi m (mass %, thc
Fnl
0.014
0.017
0.014
0.016
0.014
0.013
0.013
0.016
0.016
0.014
0.016
0.014
0.014
0.016
0.016
0.014
0.016
0.014
0.016
0.016
0.014
0.014
0.016
0.016
0.013
0.014
0.016
claimec
0.011
0.008
0.011
0.012
0.009
0.008
0.012
0.012
0.009
0.011
0.008
0.009
0.034
0.008
0.012
0.009
0.008
0.009
0.011
0.010
0.010
0.008
0.011
0.009
0.011
0.008
0.011
* mea s it does not satisfy th range.
Next, after the respective wheels were heated at 900°C for two hours,
the respective wheels were heat-treated by the method which cools the wheels
by injecting water from nozzles while rotating the wheels (so-called "tread
quench") with use of the equipment shown in Figure 14. After the heat
treatment, temper treatment (treatment of cooling the wheels in air
atmosphere after keeping the wheels at 500°C for two hours) was carried out.
[01151
With respect to the wheels produced like this, a hardness test of the rim
portions, microstructure examination of the rim portions and the hub portions,
wear test, a rolling contact fatigue test and a Jominy test were carried out.
The results are shown in Table 5. For the respective tests, the test result of
steel 37, which corresponds to the steel for railway wheel of "Class C" of AAR
was used as the reference.
[OI 161
[I] Hardness test of the rim portions:
For each of the steels, the Brine11 hardness (hereinafter, called "HBW")
in the position 40 mm from the tread of the tread central portion of the rim
portion was measured, as shown in Figure 15.
[01171
[2] Microstructure examination of the rim portions
For each of the steels, the microstructure in the position 40 mm from the
tread of the tread central portion of the rim portion was examined, as shown in
Figure 16. Note that the tread central portion was etched with nital, and the
microstructure was observed with an optical microscope under magnification
of 400 times, and was identified.
[0118]
Note that when the microstructure contains ferrite or bainitic
microstructure, the area fraction thereof was measured, and when the
microstructure contains 5% or more of ferrite or bainitic microstructure, it is
recognized as a microstructure that contains ferrite and bainite. When the
microstructure contains ferrite or bainite, "P + F" or "P + B" was described in
Table 5 which will be described later.
[0119]
131 Microstructure examination of the hub portion:
For each of the steels, the microstructure in the central position of the
hub portion was examined, as shown in Figure 17. Note that the hub portion
was etched with nital, and the microstructure is observed similarly to the rim
portion.
[01201
[41 Wear test:
For each of the steels, "wheel test specimen" for use in wear test
specimen (the configuration shown in Figure 5(a)) was taken, with the position
40 mm from the tread of the tread central portion of the rim portion (position
shown by "a" in the drawing) as the reference as shown in Figure 18. With
use of these " wheel test specimens" and "rail test specimen" of steel 1, the
wear test was performed under the conditions of the Hertzian stress: 2200
, MPa, the slip ratio: 0.8%, and the revolutions: 776 rpm at the wheel side, and
800 rpm at the rail side by the Nishihara-type wear testing machine, and the
test was carried out under dry condition. After the test was performed up to
the number of cycles of 5 x 105 times, the amount of wear was obtained from
the mass difference of the test specimen before and after the test.
[01211
[51 Rolling contact fatigue test:
For each of the steels, "wheel test specimen" for use in the rolling contact
fatigue test specimen (the configuration shown in figure 4(a)) was taken with
the position 40 mm from the tread of the tread central portion of the rim
portion (position shown by "b" in the drawing) as the reference as shown in
Figure 18. With use of these "wheel test specimens" and "rail test specimen"
of steel 1, the rolling contact fatigue test was performed under the conditions
of the Hertzian stress: 1100 MPa, the slip ratio: 0.28%, the revolutions: 1000
rpm at the wheel side and 602 rpm at the rail side, and under water
lubrication, and the number of cycles of detection of 0.5 G with an
accelerometer was set as the rolling contact fatigue life, and evaluated.
Lo1221
[6] Jominy test:
For each of the steels, a Jominy test specimen was taken with the
position 40 mm from the tread of the tread central portion of the rim portion
(position shown by "c" in the drawing) as the reference as shown in Figure 18,
and was austenitized at 900°C for 30 minutes under air atmosphere, after
which, end quench was performed, parallel cutting of 1.0 mm was performed
next, the hardness clistribution.up to the position 50 mm from the water-cooled
end was measured, and "M50%" was obtained.
Lo1231
171 High temperature tensile test
For each of the steels, in accordance with the ASTM E21 standard, the
tensile test at 538°C (1000°F) was carried out, and the high temperature yield
strength was measured.
LO1241
[8] Normal temperature tensile test
For each of the steels, the normal temperature tensile test was carried
out in accordance with the ASTM E370 standard, and the normal temperature
elongation was measured.
LO1251
Table 5
Microstructure Microstructure
Rolling High
of rim portion of hub portion Amount Normal
Steel of nm fatigue Hyper- Hyper- ofwear M60% yield
temperature temperature
NO. portion life (mm) strength elongation
QIB~) Phase eutectic Phase eutectic (g) (ncle)
8 8 m a )
(%I
37 318 P- None P None 0.318 1,865,869 5.2 297 15.2
38 342 P+F None P+F None 0.338 2,785,401 10.9 424 15.8
39 366 P None P Present 0.253 3,328,765 6.2 381 9.8
40 358 P None P None 0.264 3,194,736 14.8 365 14.7
41 382 P None P None 0.247 3,917,628 10.2 392 12.8
42 404 P+B None P None 0.325 5,857,203 20.5 394 10.3
43 354 P None P None 0.281 2,998,457 8.5 351 15.0
44 363 P None P None 0.270 3,328,475 11.2 394 12.2
45 305 . P None P None 0.331 1,643,892 7.4 346 16.3
46 412 P+B None P+B None 0.319 5,893,821 18.7 477 9.5
47 389 P None P None 0.271 4,875,209 29.3 48 1 10.3
48 391 P None P None 0.234 5,013,493 16.8 403 12.7
49 352 P None P None 0.275 3,137,515 10.2 385 13.2
50 353 P None P None 0.268 3,174,892 9.3 384 12.9
51 351 P None P None 0.279 3,084,952 9.8 385 12.5
52 359 P None P None 0.279 3,074,662 9.4 376 12.7
53 346 P ~ o k e P None 0.292 2,543,938 9.5 346 15.8
54 348 P None P None 0.284 2,647,392 10.1 366 15.3
55 377 P None P None 0.249 3,758,295 14.2 389 12.2
56 322 P None P None 0.308 2,093,584 5.4 334 15.4
57 360 P None P None 0.269 3,147,213 7.5 375 10.4
58 357 P None P None 0.266 3,218,493 12.8 359 15.4
59 332 P None . P None 0.301 2,184,572 9.5 352 15.8
60 301 P None P None 0.335 1,548,329 6.9 346 16.2
61 349 P None P None 0.281 2,794,738 10.8 388 14.1
62 369 P None . P None 0.260 3,514,847 12.9 40 1 12.8
63 387 P None P None 0.243 4,095,948 14.9 433 10.2
[01261
As shown in Table 5, steels 37 to 39, 42, 45 to 47, 56, 57, 60 and 63
which do not satisfy the conditions defined by the present invention were
inferior as compared with steels 40, 41, 43, 44, 48 to 55, 58, 59, 61 and 62
which satisfy the conditions defined by the present invention, in any one or
more tests of the wear test, .the rolling contact fatigue test, the Jominy test,
the high temperature tensile test and the normal temperature tensile test.
Industrial Applicability
[01271
The steel for wheel of the present invention is excellent in balance of
wear resistance, rolling contact fatigue resistance and spalling resistance, and
can give a long life to the wheel. The wheel adopting the steel for wheel of the
present invention has the amount of wear decreased by 30% at the largest, and
the rolling contact fatigue life becomes as long as 3.2 times at the largest, and
a low risk of spalling occurrence, as compared with the wheel adopting the
steel for railway wheel of "Class C" of AAR. Further, the wheel adopting the
steel for wheel of the present invention includes both high temperature
strength and ductility, and therefore, has a low risk of occurrence of TMS and
a crack on the tread. Accordingly, the steel for wheel of the present invention
is extremely preferable for use as the railway wheels that are used under
extremely harsh environments in which the traveling distances increase, and
the loading capacities increase.
We claim:
1. A steel for wheel comprising: in mass%,
C: 0.65 to 0.84%;
Si: 0.4 to 1.0%;
Mn: 0.50 to 1.40%;
Cr: 0.02 to 0.13%;
S: 0.04% or less and
V: 0.02 to 0.12%,
wherein Fnl expressed by the following formula (1) is 32 to 43, and
Fn2 expressed by the following formula (2) is 25 or less,
the balance being Fe and impurities, and
P, Cu and Ni in the impurities are
P: 0.05% or less,
Cu: 0.20% or less, and
Ni: 0.20% or less:
Fnl = 2.7 + 29.5 x C + 2.9 x Si + 6.9 x Mn + 10.8 x Cr + 30.3 xMo + 44.3
x v . . . (1)
Fn2 = exp(0.76) x exp(0.05 x C) x exp(1.35 x Si) x exp(0.38 x Md x
exp(0.77 x Cr) x exp(3.0 x Mo) x exp(4.6 x V) . - - (2)
where each symbol of element in the formulas (1) and (2) means content
(mass%) of each element.
2. The steel for wheel according to claim 1, comprising, in mass%, Mo: 0.07%
or less in place of a part of Fe, and a total content of V and Mo is 0.02 to
0.12%.
3. The steel for wheel according to claim 1 or 2, comprising, in mass%, Al:
0.20% or less in place of a part of Fe.