ORIGINAL
STEEL FOR WHEEL
TECHNICAL FIELD
[0001]
The present invention relates to steel for wheel. More particularly, the present
invention relates to steel for wheel that is suitable as a material for a high-hardness wheel
for railroad, the material being excellent in wear resistance, rolling contact fatigue
resistance, and spalling resistance.
[0002]
Spalling is a phenomenon that a portion of wheel, when heated and rapidly cooled
by emergency braking or the like, is transformed into brittle martensite called a white layer,
from which a crack propagates, and the white layer is subjected to brittle fracture and peels
off. This phenomenon is also called a "thermal crack" in some cases.
BACKGROUND ART
[0003]
In recent years, with a worldwide increase in running distance and loading capacity,
a wheel for railroad (hereinafter, also referred to as a "wheel") having a service life longer
than before has been demanded.
[0004]
There are three phenomena that mainly cause damages to a wheel: (i) wear, (ii)
rolling contact fatigue, and (iii) spalling. Especially in recent years, an increasing number
of wheels have been damaged by wear resulting from the increase in running distance and
by rolling contact fatigue resulting from the increase in loading capacity. The rolling
contact fatigue is sometimes called "shelling". Although the crack caused by spalling is
also called "shelling" in some cases, in this description, the generation of crack caused by
the formation of white layer is defined as "spalling".
[0005]
- 2 -
Empirically, it is known that the wear resistance and the rolling contact fatigue
resistance are properties that are incompatible with the spalling resistance. A pressing
need is to develop steel for wheel that is excellent in balance between the wear resistance.
rolling contact fatigue resistance and the spalling resistance, and can give a long life to the
wheel.
[0006]
For example, Patent Documents 1 to 7 disclose techniques concerning a wheel.
[0007]
Patent Document 1 discloses a "steel for high-toughness railroad wheel" containing
V.
[0008]
Patent Document 2 discloses a "rim or monoblock wheel for wheel set of railway
rolling stock" that is excellent in wear resistance, fracture resistance and thermal crack
resistance.
[0009]
Patent Document 3 discloses a "wheel for rolling stock" in which the C content is
decreased, and the tread part thereof has a structure composed of bainitic structure,
tempered martensitic structure, or a mixed structure of bainite and tempered martensite,
whereby both of the shelling resistance and the spalling resistance serving as thermal crack
resistance are improved.
[0010]
Patent Document 4 discloses a "high carbon rail vehicle wheel having excellent
wear resistance and thermal crack resistance" in which the C content is increased to 0.85 to
1.20%.
[0011]
Patent Document 5 discloses a "wheel for rolling stock excellent in wear resistance
and thermal crack resistance" of an monoblock type formed of steel having a chemical
composition consisting of C: 0.4 to 0.75%, Si: 0.4 to 0.95%, Mn: 0.6 to 1.2%, Cr: more
- 3 -
than 0% and less than 0.2%. P: 0.03% or less, and S: 0.03% or less, the balance beinn Fe
and impurities, wherein the region at least to a depth oi 50 mm from the surface oi a wheel
tread part is formed of a pearlitic structure, and the document also discloses the production
of the wheel.
[0012]
Patent Documents 6 and 7 disclose "railroad wheel steels" in which the strength is
increased and the rolling contact fatigue resistance and spalling resistance are improved by
containing 0.01 to 0.12% and 0.009 to 0.013%, respectively, of Nb.
[0013]
[Patent Document 1] JP50-104717A
[Patent Document 2] JP2001-158940A
[Patent Document 3] JP2005-350769A
[Patent Document 4] JP2004-315928A
[Patent Document 5] JP9-202937A
[Patent Document 6] US7559999A
[Patent Document 7] US7591909A
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0014]
The steel disclosed in Patent Document 1 has a low hardness because the C content
thereof is as low as 0.50 to 0.60%. Therefore, this steel does not have sufficient rolling
contact fatigue resistance, and cannot respond to the recent increase in loading capacity.
[0015]
The steel disclosed in Patent Document 2 has a low hardness because the C content
thereof is as low as 0.45 to 0.55%. Therefore, this steel also does not have sufficient
rolling contact fatigue resistance, and cannot respond to the recent increase in loading
capacity.
- 4 -
[0016J
The wheel disclosed in Patent Document 3 is a wheel in which the tread part thereof
has a structure composed of bainitic structure, tempered martensitic structure, or a mixed
structure of bainite and tempered martensite. Therefore, despite its high strength, the
wear resistance is lower than the case where the tread part is formed of pearlitic structure,
and it is difficult to obtain wear resistance higher than that of a wheel material for a
conventional freight car. That is, as compared with the pearlitic structure that is excellent
in work hardening property and further shows a behavior in which the lamella thereof is
rearranged in parallel to the surface as amount of wear increases, the bainitic structure and
the tempered martensitic structure provide a large amount of wear (for example, refer to
Sadahiro Yamamoto, "Technique for Improving Wear Resistance of Steel by Structure
Control - Structure Control Technique for Wear-Resistant Steel having Weldability -"
161 st-162nd Nishiyama Memorial Seminar, Heisei 8th year, edited by The Iron and Steel
Institute of Japan, p.221).
[0017]
The steel for the wheel disclosed in Patent Document 4 has difficulty in being
applied to a wheel manufactured by treatment unique to wheel called a "tread quenching
process". Figure 1 is a schematic view of an "monoblock wheel" shown as one example
of wheel. In the case of wheel, after the whole of the wheel has been heated, heat
treatment for cooling the rim part thereof from the outer surface of wheel is performed to
give a compressive residual stress to the rim part. In this cooling treatment, the rim part •
and the vicinity thereof are cooled rapidly, but the hub part thereof suffers from low
cooling rate. Therefore, in the case where the steel for the wheel disclosed in this Patent
Document is heat-treated by the tread quenching process, hypereutectoid cementite may
precipitate at the austenite grain boundary of the hub part. The hypereutectoid cementite
acts in the same way as that of coarse inclusions, and greatly reduces the toughness and the
fatigue life (for example, refer to Takayoshi Murakami, "Influence of Minute Defects and
Inclusions (2004)", p. 182 "Yokendo").
- 5 -
J
[0018]
1 he wheel disclosed in Patent Document 5 may have insufficient hardness.
Therefore, this wheel is not always capable of responding to the recent increase in loading
capacity.
[0019]
The railroad wheel steel disclosed in Patent Document 6 contains as much as 0.20
to 0.30% of Mo. Therefore, a structure having low wear resistance such as bainitic
structure or degenerate-pearlitic structure is liable to be produced, so that it is difficult to
obtain good wear resistance. Moreover, this steel always contains 0.01 to 0.12% of Nb.
In the steel containing Nb, coarse inclusions may be formed, so that the coarse inclusions
extremely reduce the toughness and the fatigue life similarly to the above-described
hypereutectoid cementite.
[0020]
Similarly, the railroad wheel steel disclosed in Patent Document 7 always contains
0.009 to 0.013% of Nb. As described above, in the steel containing Nb, coarse inclusions
may be formed, so that the coarse inclusions extremely reduce the toughness and the
fatigue life similarly to the hypereutectoid cementite.
[0021]
The present invention has been made to solve the above-described problems, and
accordingly an objective thereof is to provide steel for wheel that is excellent in balance
between the wear resistance, rolling contact fatigue resistance and the spalling resistance,
and can give a long life to the wheel.
Means for Solving the Problems
[0022]
The present inventors conducted various studies on the wear resistance, rolling
contact fatigue resistance, and spalling resistance, and resultantly found the following
items (a) to (c).
- 6 -
[0023]
(a) The wear resistance increases when the structure of steel material is pearlitie
structure and the hardness is increased.
[0024]
(b) The rolling contact fatigue resistance increases when the hardness is increased
regardless of structure.
[0025]
(c) The spalling resistance increases when the hardenability is decreased.
[0026]
From these findings, the present inventors arrived at a conclusion that in order to
solve the above-described problems, steel in which the pearlitie structure can be provided
by tread quenching, and moreover, the hardness is high and the hardenability is low only
needs to be developed.
[0027]
Hereunder, one example of studies conducted by the present inventors is explained
in detail.
[0028]
The present inventors evaluated the influence of elements on the hardness and
hardenability by the Jominy end quenching test (hereinafter, referred to as the "Jominy
test") in which the heat treatment conditions thereof are similar to those of the tread
quenching of actual wheel.
[0029]
First, ingots were produced by melting steels 1 to 24 having chemical compositions
given in Table 1 in a vacuum furnace on the laboratory scale.
[0030]
Next, for each of the steels, a 35-mm diameter round bar, a 160-mm diameter round
bar, and a 70-mm diameter round bar were produced from the ingot by hot forging.
[0031]
- 7 -
Further, for steel 1. a 220-mm diameter round bar was also produced to prepare a
~ "rail test specimen" (or the later-described rolling contact fatigue test.
|0()321
Steel 1 in Table 1 corresponds to a railroad wheel steel of "Class C" in M-107/M-
207 standards of AAR (Association of American Railroads).
[0033]
Table 1
Steel C lemical composition (mass%) Balance: Fe and impurities
C i Si I Mn I P I S 1 Cu 1 Ni I Cr I Mo I V I Al
1 0.69 0.29 0.82 0.013 0.007 0.01 0.02 0.08 0.01 0.01 0.056
2 0.69 0.80 0.76 0.011 0.007 - 0.03 0.10 0.01 0.01 0.050
3 0.70 0.82 0.82 0.004 0.008 - 0.04 0.09 0.01 0.10 0.034
4 0.77 0.83 0.82 0.004 0.007 - 0.04 0.09 0.01 0.09 0.036
5 0.86 0.85 0.83 0.004 0.008 - 0.04 0.10 0.01 0.01 0.037
6 0.88 0.50 0.61 0.004 0.008 - 0.04 0.25 0.01 0.01 0.034
7 0.74 0.80 0.39 0.004 0.008 - 0.04 0.45 0.01 0.09 0.033
8 0.71 0.79 0.77 0.014 0.007 0.01 0.02 0.10 0.01 0.09 0.036
9 0.80 0.78 0.76 0.014 0.006 0.01 0.02 0.10 0.01 0.09 0.039
10 0.63 0.79 0.79 0.014 0.009 0.01 0.02 0.10 0.01 0.09 0.037
11 0.72 0.23 0.18 0.003 0.001 0.01 0.01 - 0.01 - 0.031
12 0.72 0.25 0.77 0.003 0.001 0.01 0.01 0.19 0.01 - 0.021
13 0.76 0.25 0.78 0.003 0.001 0.01 0.01 - 0.09 - 0.031
14 0.75 0.25 0.79 0.004 0.001 0.01 0.01 - 0.20 - 0.035
15 0.74 0.23 0.74 0.003 0.001 0.01 0.01 - 0.01 0.09 0.026
16 0.74 0.79 0.80 0.004 0.001 0.01 0.01 - 0.01 0.05 0.034
17 0.75 0.86 0.79 0.004 0.001 0.01 0.01 0.15 0.15 - 0.041
18 0.75 0.81 0.82 0.003 0.001 0.01 0.02 0.42 0.07 0.07 0.023
19 0.74 0.65 0.82 0.004 0.001 0.01 0.02 0.54 0.11 - 0.032
20 0.74 1.10 0.82 0.003 0.001 0.01 0.01 - 0.01 - 0.041
21 0.74 0.23 2.10 0.004 0.001 0.01 0.01 - 0.01 - 0.038
22 0.73 0.25 0.81 0.003 0.001 0.01 0.01 - 0.01 0.21 0.028
23 0.75 0.79 0.83 0.003 0.001 0.01 0.02 0.82 0.18 - 0.026
24 1 0.74 1 0.84 | 0.82 | 0.004 [ 0.001 | 0.01 [ 0.02 [ 0.54 j 0.25 | 0.08 j 0.031 |
[0034]
A Jominy test specimen was sampled from the 35-mm diameter round bar. The
test specimen was austenitized at 900°C for 30 minutes in the atmosphere, and thereafter
was subjected to end quenching. Then, the measurement of Rockwell C hardness
(hereinafter, also referred to as "HRC") was made by 1.0-mm parallel cutting.
[0035]
- 8 -
The HRC at a position 40 mm distant from the water cooling end (hereinafter,
referred to as the "40-mm hardness") was measured, and the influence of elements on the
measurement value was evaluated. As the result, it was found that the "40-mm hardness"
has a proportional relationship with Fnl expressed by Formula (1) as shown in Figure 2.
Further, it was found that, similarly to steels 23 and 24, if Fnl exceeds 43, bainitic
structure is formed at least in a portion, and the proportional relationship does not hold.
[0036]
The reason why the HRC at the position 40 mm distant from the water cooling end
was measured is that the wheel is manufactured by being machined after tread quenching.
Also, the reason for this is that the wheel having been used is subsequently re-profiled, and
is sometimes used by repeating re-profiled work, and the properties of steel in the interior
having a hardness lower than that of the surface exerts a great influence on the service life
of wheel.
[0037]
In Figure 2, steel 1 corresponding to the railroad wheel steel of "Class C" of AAR
was indicated by a mark "A". The structure was decided by observation under an optical
microscope by mirror grinding the position 40 mm distant from the water cooling end and
thereafter corroding that position with nital.
[0038]
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 ...
(!)
in which C, Si, Mn, Cr, Mo and V each mean the content in percent by mass of the element.
[0039]
Table 2 gives the measurement value of the "40-mm hardness" and Fnl expressed
by Formula (1) that are put in order.
[0040]
The hardenability was evaluated, based on the hardness in the case where the
martensitic structure fraction described in ASTM A255 standards was 50%, by measuring
- 9 -
a distance in millimeter units from the water cooling end at which distance the martensitic
^ structure fraction is 50% (hereinafter, referred to as "M50%") from the Jominy hardness.
As the result, it was found that "M50%" is correlated with Fn2 expressed by Formula (2)
as shown in Figure 3. In Figure 3 as well, steel 1 was indicated by a mark "•*•".
[0041]
Fn2 = 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)
in which C, Si, Mn, Cr, Mo and V also each mean the content in percent by mass of the
element. The term of "exp (0.05 x C)" and the like mean exponential expressions such as
"e 3xc". The letter of "e" is one of mathematical constants: "Napier's number", and is
used as the base of natural logarithm.
[0042]
Table 2 gives the measurement value of the "M50%" and Fn2 expressed by
Formula (2) that are put in order.
[0043]
y"
/
-10-
Table 2
W I Steel I 40-mm | Fnl | M 50% | Fn2 | Rolling | Wear j
hardness (mm) | contact amount j
(HRC) fatigue life (g)
(cycle)
1 31.1 31.2 5.6 5.1 1830898 0.320
2 32.3 32.4 8.3 10.1 2191425 0.312
3 36.6 37.1 13.6 16.0 3283349 0.268
4 39.3 38.7 16.5 15.5 4453779 0.242
5 38.6 38.1 10.6 11.2 4114188 0.241
6 37.3 37.8 6.3 7.2 3675793 0.249
7 38.9 38.7 14.2 16.7 4342381 0.225
8 37.8 36.6 16.2 14.5 3688978 0.247
9 39.6 39.2 15.7 14.3 4480886 0.241
10 35.0 34.4 15.5 14.6 2867978 0.279
11 25.4 26.2 3.8 3.3 814399 0.384
12 32.6 32.3 4.6 5.0 2138325 0.312
13 34.1 34.0 4.5 5.5 2583220 0.286
14 37.2 37.1 6.8 7.7 3602246 0.253
15 35.8 34.6 5.4 6.3 3089949 0.263
16 34.8 34.9 9.2 11.3 2785571 0.279
17 38.9 38.9 15.0 16.8 4258560 0.242
18 42.1 42.6 21.8 21.3 5621775 0.201
19 41.0 41.2 16.5 15.4 5092738 0.223
20 32.4 33.7 14.9 13.8 2085608 0.327
21 39.7 40.0 7.1 6.9 4587370 0.227
22 38.7 40.2 11.2 11.4 4266610 0.235
23 42.5 47.2 29.7 28.5 5892201 0.314
24 j 42.6 I 49.6 | 46.9 1 43.7 | 5675736 | 0.312
Fnl = 2.7 + 29.5 x C + 2.9 x Si + 6.9 x M n + 10.8 x Cr + 30.3 x M o + 44.3 x V
Fn2 = 0.76 x exp (0.05 x C) x exp (1.35 x Si) x exp (0.38 x M n ) x exp (0.77 x Cr) x exp (3.0 x
Mo) x exp (4.6 x V ) |
[0044]
Next, the present inventors examined the relationship between the rolling contact
fatigue resistance, wear resistance and Fnl expressed by Formula (1) by using steels 1 to
24 given in Table 1.
[0045]
That is, for each of the steels, after the 160-mm diameter round bar had been cut
into a length of 100 mm, a test specimen was produced by oil quenching after heating at
900°C for 30 minutes.
[0046]
- 1 1 -
For steels 1 to 24. first, from the central portion of the test specimen thus produced.
^ a test specimen having the configuration, shown in Figure 4(a) was sampled as a "wheel
test specimen" used for the rolling contact fatigue test.
[0047]
For steel 1, after the 220-mm diameter round bar had been cut into a length of 100
mm, a test specimen was produced by oil quenching after heating at 900°C for 30 minutes.
From the central portion of this test specimen, a test specimen having the configuration,
shown in Figure 4(b) was sampled as a "rail test specimen" used for the rolling contact
fatigue test.
[0048]
Similarly, for, steels 1 to 24, after the 70-mm diameter round bar had been cut into a
length of 100 mm, a test specimen was produced by oil quenching after heating at 900°C
for 30 minutes. From the central portion of this test specimen, a test specimen having the
configuration, shown in Figure 5(a) was sampled as a "wheel test specimen" used for the
wear test.
[0049]
For steel 1, a 70-mm diameter round bar specimen having a length of 100 mm was
produced by being subjected to heat treatment similar to that of the above-described wheel
test specimen, and from the central portion thereof, a test specimen having the
configuration, shown in Figure 5(b) was sampled as a "rail test specimen" used for the
wearing test.
[0050]
First, the rolling contact fatigue test was conducted by the method schematically
shown in Figure 6 using the wheel test specimens shown in Figure 4(a) of steels 1 to 24
and the rail test specimen shown in Figure 4(b) of steel 1.
[0051]
The conditions of the rolling contact fatigue test were Hertzian stress: 1100 MPa,
slip ratio: 0.28% and rotational speed: 1000 rpm on the wheel side and 602 rpm on the rail
-12-
side, and the test was conducted under water lubrication. The test was carried out while
the acceleration was monitored by using a vibration accelerometer, and the number o1
cycles in which 0.5G was detected was evaluated as the rolling contact fatigue life. The
reason why 0.5G was based upon is that as the result of evaluation of the relationship
between the detected acceleration and the damaged state carried out by a preliminary test,
it could be confirmed that a part of the contact surface was peeled off apparently when
0.5G was exceeded.
[0052]
Table 2 additionally gives the rolling contact fatigue life. Also, Figure 7 shows the
relationship between the rolling contact fatigue life and Fnl expressed by Formula (1).
[0053]
In Figure 7, "2.E+06" and the like mean "2.0 x 106" and the like. In Figure 7 as
well, steel 1 was indicated by a mark "A".
[0054]
It was found that, as shown in Figure 7, the rolling contact fatigue life is correlated
with Fnl expressed by Formula (1), and if Fnl is 34 or more, the rolling contact fatigue
life increases by 40% or more as compared with the rolling contact fatigue life of steel 1
corresponding to the railroad wheel steel of "Class C" of AAR.
[0055]
Further, the wearing test was conducted by the method schematically shown in
Figure 8 using 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. For the wearing test, a Nishihara-type wear
testing machine was used.
[0056]
The specific test conditions were Hertzian stress: 2200 MPa, slip ratio: 0.8%, and
rotational speed: 776 rpm on the wheel side and 800 rpm on the rail side. After the test
had been carried out until a number of cycles of 5 x 105, the amount of wear was
determined from the difference in mass of test specimen before and after the test.
- 1 3 -
[0057]
Tabic 2 additionally gives the amount of wear. Also, Figure 0 shows the
relationship between the amount of wear and Fnl expressed by Formula (1). In Figure 9
as well, steel 1 was indicated by a mark " ^".
[0058]
It was found that, as shown in Figure 9, as far as the structure is pearlitic structure,
the amount of wear decreases in proportion to Fnl expressed by Formula (1), and if Fnl is
34 or more, the amount of wear decreases by 10% or more as compared with the amount of
wear of steel 1, and therefore the wear resistance is improved.
[0059]
On the other hand, if Fnl exceeds 43, bainitic structure is formed at least in a
portion as described above. It could be confirmed that in the case where the bainitic
structure is contained, even if Fnl increases, the amount of wear does not decrease, and the
wear resistance is poorer than the case of the structure composed mainly of pearlite.
[0060]
In Japan Railway & Technical Review, Vol. 19 (2005) No. 9, p. 17, Kanetaka et al.
reported that as the thickness of the quenched layer called a white layer increases, the crack
depth increases, and therefore spalling (though being described as "shelling" in the Review,
this means "spalling") becomes liable to occur.
[0061]
Accordingly, the present inventors also conducted detailed studies on the influence
of hardenability on spalling.
[0062]
From the report of Kanetaka et al., it is presumed that with an increase in
hardenability, the thickness of the white layer increases, and cracks initiate, so that the
spalling life is decreased. Therefore, the present inventors examined the relationship
between the hardenability and the crack initiation life in the case where the white layer was
formed.
-14-
[0063]
^ F Specifically, the "wheel test specimens" each having the configuration, shown in
Figure 4(a) of steels 1. 2, 5, 11, 12 and 14 described in Table 1, and the "rail test specimen"
having the configuration, shown in Figure 4(b) of steel 1 were used. A thick white layer
leading to spalling was formed on the test surface of the "wheel test specimen" by YAG
laser, and subsequently the rolling contact fatigue test was conducted to examine the crack
initiation life (spalling resistance). The conditions of YAG laser heating were output
power of laser: 2500 W and feed rate: 1.2 m/min, and the test specimen was air cooled
after laser heating.
[0064]
The conditions of the rolling contact fatigue test were Hertzian stress: 1100 MPa,
slip ratio: 0.28% and rotational speed: 100 rpm on the wheel side and 60 rpm on the rail
side, and the test was conducted under water lubrication. Until the number of cycles
reached 2000 cycles, the test was stopped in every 200 cycles and in the case where the
number of cycles exceeds 2000 cycles, the test was stopped in every 2000 cycles to
visually check the presence of crack on the surface of the test specimen.
[0065]
As the result, it was found that, as shown in Figures 10 and 11, with an increase in
Fn2 expressed by Formula (2) that is correlated with "M50%", which is the index of
hardenability, the thickness of white layer increases, and accordingly the crack initiation
life decreases suddenly.
[0066]
Further, it was found that when Fn2 exceeds 25, the crack initiation life decreases
extremely to such a degree that cracks can be noticeable even in the first visual inspection
(that is, in the visual inspection at the time when the number of cycles reaches 200 cycles).
[0067]
-15-
From the above-described results, the present inventors concluded that if the
chemical composition of steel is made such that Fn2 expressed by Formula (2) is 25 or less.
the spalling life can be prevented from decreasing extremely.
[0068]
The present invention has been completed based on the above-described findings,
and the gist thereof is the steels for wheel described in the following items (1) to (3).
[0069]
(1) Steel for wheel having a chemical composition consisting of, by mass percent,
C: 0.65 to 0.84%, Si: 0.02 to 1.00%, Mn: 0.50 to 1.90%, Cr: 0.02 to 0.50%, V: 0.02 to
0.20%, and S: 0.04% or less, in which Fnl expressed by Formula (1) is 34 to 43, and Fn2
expressed by Formula (2) is 25 or less, the balance being Fe and impurities, the impurities
containing 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 = 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)
in Formulas (1) and (2), C, Si, Mn, Cr, Mo and V each mean the content in percent by
mass of the element.
[0070]
(2) The steel for wheel described in item (1), wherein 0.20% or less of Mo is
contained by mass percent in lieu of a part of Fe.
[0071]
(3) The steel for wheel described in item (1) or (2), wherein 0.20% or less of Al is
contained by mass percent in lieu of a part of Fe.
[0072]
The impurities referred to herein are elements that mixedly enter from the ore and
scrap used as raw materials for steel, the environment of the manufacturing process, or the
like when the steel material is manufactured industrially.
-16-
ADVANTAGE OF THE INVENTION
[0073]
The steel for wheel in accordance with the present invention is excellent in balance
between the wear resistance, rolling contact fatigue resistance and the spalling resistance,
and can give a long life to the wheel. Specifically, for the wheel that uses the steel for
wheel in accordance with the present invention as a material, the amount of wear decreases
by 10 to 35% and the rolling contact fatigue life increases to 1.4 to 3.2 times as compared
with the wheel that uses the railroad wheel steel of "Class C" of AAR, and spalling is less
liable to occur. Therefore, the steel for wheel in accordance with the present invention is
extremely suitable as a material for a railroad wheel used in a very harsh environment of
increased running distance and increased loading capacity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0074]
Figure 1 is a schematic view for explaining an "monoblock wheel" as one example
of wheel.
Figure 2 is a graph systematically showing the relationship between "40-mm
hardness", which is the Rockwell C hardness at a position 40 mm distant from the water
cooling end, and "Fnl", which is expressed by Formula (1), for steels 1 to 24. In this
figure, "bainite" indicates that bainitic structure is formed in portions of steel.
Figure 3 is a graph systematically showing the relationship between "M50%",
which is the distance in millimeter units from the water cooling end at which distance the
martensitic structure fraction is 50% from the Jominy hardness, and "Fn2", which is
expressed by Formula (2), for steels 1 to 24.
Figure 4 is views showing the configurations of a "wheel test specimen" and a "rail
test specimen" used in the rolling contact fatigue test, Figure 4(a) showing the "wheel test
-17-
specimen", and Figure 4(b) showing the "rail test specimen". In this figure, the units of
^ J dimensions are "mm".
Figure 5 is views showing the configurations of a "wheel test specimen'' and a "rail
test specimen" used in the wearing test, Figure 5(a) showing the "wheel test specimen",
and Figure 5(b) showing the "rail test specimen". In this figure, the units of dimensions
are "mm".
Figure 6 is a schematic view for explaining a method of the 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 graph systematically showing the relationship between rolling contact
fatigue life and "Fnl" expressed by Formula (I). In this figure, "bainite" indicates that
bainitic structure is formed in portions of steel.
Figure 8 is a schematic view for explaining a method of the wearing 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 graph systematically showing the relationship between the amount of
wear and "Fnl" expressed by Formula (1). In this figure, "bainite" indicates that bainitic
structure is formed in portions of steel.
Figure 10 is a graph systematically showing the relationship between the thickness
of a white layer and "Fn2" expressed by Formula (2) for steels 1, 2, 5, 11, 12 and 14.
Figure 11 is a graph systematically showing the relationship between crack
initiation life and "Fn2" expressed by Formula (2) for steels 1, 2, 5, 11, 12 and 14.
Figure 12 is a view for explaining a device used in examples to subject a wheel to
so-called "tread quenching".
Figure 13 is a view for explaining a measurement position of Brinell hardness of a
wheel manufactured in examples.
Figure 14 is a view for explaining a position at which the microstructure of the rim
part of a wheel manufactured in examples was examined.
-18-
Figure 15 is a view lor explaining a position at which the microstructure of the huh
^ ^ part of a wheel manufactured in examples was examined
Figure 16 is a view for explaining a position from which a wearing test specimen, a
rolling contact fatigue test specimen, and a Jominy test specimen were sampled from a
wheel manufactured in examples. The wearing test specimen, the rolling contact fatigue
test specimen, and the Jominy test specimen were sampled on the basis of the positions
indicated by "a", "b" and "c", respectively, in this figure.
BEST MODE FOR CARRYING OUT THE INVENTION
[0075]
The requirements of the present invention are explained in detail. An ideogram of
"%" of the content of each element means "mass percent".
[0076]
C: 0.65 to 0.84%
Carbon (C) increases the hardness, and improves the wear resistance and the rolling
contact fatigue resistance. Further, C is an element capable of increasing the hardness
without decreasing the spalling resistance because the increase in hardenability resulting
from the increase in content is slight. If the C content is lower than 0.65%, a sufficient
hardness cannot be obtained, and further the area fraction of ferrite increases, so that the
wear resistance decreases. On the other hand, if the C content exceeds 0.84%,
hypereutectoid cementite is formed in the hub part of the wheel, so that the toughness and
the fatigue life are decreased extremely, which is unfavorable in terms of safety.
Therefore, the C content was 0.65 to 0.84%. The C content is preferably 0.68%) or more
and also 0.82%> or less.
[0077]
Si: 0.02 to 1.00%
Silicon (Si) is an element that increases the hardness by decreasing the spaces
between the lamellas of pearlite and by solid-solution strengthening the ferrite in pearlitic
-19-
^. structure. If the Si content is lower than 0.02%, the above-described effect is insufficient.
On the other hand, if the Si content exceeds 1.00%. the toughness decreases, and further,
the hardenability increases and the spalling resistance also decreases. Therefore, the Si
content was 0.02 to 1.00%. The hardness increasing effect of Si is not very large,
therefore a lot of Si must be added in order to obtain high hardness steel. Consequently,
the hardenability is easy to increase. Accordingly, the Si content is preferably 0.90% or
less. The Si content is much preferably 0.50% or less, further preferably 0.40% or less.
[0078]
Mn: 0.50 to 1.90%
Manganese (Mn) is an element that increases the hardness by decreasing the spaces
between the lamellas of pearlite and by solid-solution strengthening the ferrite in pearlitic
structure. Further, Mn has action for restraining the grain boundary embrittlement by
capturing S in the steel to form MnS. If the Mn content is lower than 0.50%, the abovedescribed
effects, especially the S capturing effect, are insufficient. On the other hand, if
the Mn content exceeds 1.90%, bainitic structure is formed and thereby the wear resistance
is reduced, and further, the hardenability increases and thereby the spalling resistance is
also reduced. Therefore, the Mn content was 0.50 to 1.90%. The Mn content is
preferably 1.40% or less.
[0079]
Cr: 0.02 to 0.50%
Chromium (Cr) achieves an effect of remarkably increasing the hardness of pearlite
by decreasing the spaces between the lamellas of pearlite. If the Cr content is lower than
0.02%), the effect is insufficient. On the other hand, if the Cr content exceeds 0.50%,
carbides are less liable to form a solid solution in austenite at the time of heating, and
undissolved carbides are formed depending on the heating conditions, so that the hardness,
toughness, fatigue strength, and the like may be decreased. Further, the hardenability
increases and the spalling resistance decreases. Therefore, the Cr content was 0.02 to
0.50%o. The Cr content is preferably 0.05%) or more and also 0.45% or less.
-20-
[0080]
^ V: 0.02 to 0.20%
Vanadium (V) achieves an effect of remarkably increasing the hardness of pearlite
by precipitating in the ferrite in pearlite as V carbides. If the V content is lower than
0.02%, the effect is insufficient. On the other hand, even if V has a content exceeding
0.20%, by the ordinary heat treatment, the hardness is saturated and the cost is increased,
and additionally, the hardenability is increased and the spalling resistance is decreased.
Therefore, the V content was 0.02 to 0.20%. The V content is preferably 0.03% or more
and also 0.15% or less.
[0081]
S: 0.04% or less
Sulfur (S) is an impurity contained in the steel. Also, in the case where S is
contained positively, although the influence on the hardness and the hardenability is small,
an effect of improving the machinability is achieved. If the S content exceeds 0.04%, the
toughness decreases. Therefore, the S content was 0.04% or less. The S content is
preferably 0.03%) or less. In order to achieve the effect of improving the machinability,
the S content is preferably 0.005% or more.
[0082]
Fnl:34to43
For the steel for wheel in accordance with the present invention, Fnl expressed by
Formula (1) must be 34 to 43.
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)
in which C, Si, Mn, Cr, Mo and V each mean the content in percent by mass of the element.
[0083]
If Fnl is less than 34, the wear resistance and the rolling contact fatigue resistance
are scarcely improved as compared with the railroad wheel steel of "Class C" of AAR and
in some cases, the resistances become lower than those of the railroad wheel steel of "Class
-21 -
C". Therefore, it is difficult to use this steel as the material for the railroad wheel used in
a very harsh environment of increased running distance and increased loading capacity.
[0084]
On the other hand, if Fnl exceeds 43, the structure composed mainly of pearlite
becomes less liable to be obtained, and the wear resistance decreases. Further, since the
hardness increases too much, the toughness decreases.
[0085]
When Fnl is 34 or more, the rolling contact fatigue resistance is improved by 40 %
or more as compared with the railroad wheel steel of "Class C" of AAR. The resistance is
improved by 50 % or more when Fnl is 35 or more, and is improved by 70 % or more
when Fnl is 36 or more. Fnl is preferably 43 or less.
[0086]
Fn2: 25 or less
For the steel for wheel in accordance with the present invention, Fn2 expressed by
Formula (2) must be 25 or less.
Fn2 - 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)
in which C, Si, Mn, Cr, Mo and V each mean the content in percent by mass of the element.
[0087]
If Fn2 exceeds 25, the hardenability increases, and the spalling resistance decreases.
Fn2 is preferably 20 or less, and further preferably 15 or less.
[0088]
If Fn2 is less than 3, it is difficult to make Fnl expressed by Formula (1) 34 or more.
Therefore, Fn2 is preferably 3 or more.
[0089]
One of the steels for wheel in accordance with the present invention has a chemical
composition consisting of the above-described elements and the balance consisting of Fe
and impurities.
-22-
^ [0090]
In the present invention, the impurities must contain P: 0.05% or less. Cu: 0.20% or
less, and Ni: 0.20% or less.
[0091J
Hereunder, these elements are explained.
[0092]
P: 0.05% or less
Phosphorus (P) is an impurity contained in the steel. If the P content exceeds
0.05%o, the toughness decreases. Therefore, the content of P in the impurities was 0.05%
or less. The P content is preferably 0.025%) or less.
[0093]
Cu: 0.20% or less
Copper (Cu) is an impurity contained in the steel. If the Cu content exceeds
0.20%o, the number of surface defects formed at the manufacturing time increases, and
furthers, the hardenability increases and the spalling resistance decreases. Therefore, the
content of Cu in the impurities was 0.20%) or less. The Cu content is preferably 0.10% or
less.
[0094]
Ni: 0.20% or less
Nickel (Ni) is an impurity contained in the steel. If the Ni content exceeds 0.20%,
the hardenability increases and the spalling resistance decreases. Therefore, the content
of Ni in the impurities was 0.20% or less. The Ni content is preferably 0.10% or less.
[0095]
As another chemical composition of the steel for wheel in accordance with the
present invention, the following content of Mo may be contained as necessary in lieu of a
part of Fe.
[0096]
Mo: 0.20%o or less
- 2 3 -
Molybdenum (Mo) may be contained because it has action for increasing the
hardness of peaiiite. However, if the Mo content exceeds 0.20%, bainitic structure is
formed, and the wear resistance decreases, and further, the hardenability increases and the
spalling resistance decreases. Therefore, the content of Mo in the case of being contained
was 0.20% or less. The content of Mo in the case of being contained is preferably 0.07%
or less.
[0097]
On the other hand, in order to stably achieve the above-described effects of Mo, the
Mo content is preferably 0.02% or more.
[0098]
As still another chemical composition of the steel for wheel in accordance with the
present invention, the following content of Al may be contained as necessary in lieu of a
part of Fe.
[0099]
Al: 0.20% or less
Aluminum (Al) may be contained because it achieves an effect of refining the grain
size and thereby improving the toughness. However, if the Al content exceeds 0.20%>, the
amount of coarse inclusions increases, whereby the toughness and the fatigue strength are
decreased. Therefore, the content of Al in the case of being contained was 0.20%> or less.
The content of Al in the case of being contained is preferably 0.15% or less.
[0100]
On the other hand, in order to stably achieve the effect of refining the grain size of
Al, the Al content is preferably 0.002% or more.
[0101]
For the micro-structure of the wheel using the steel for wheel in accordance with the
present invention as a material, it is desirable that the area fraction of pearlitic structure of
the rim part thereof be 95% or more, and a 100% pearlitic structure is most desirable.
The reason for this is that a structure other than pearlite, such as ferritic structure and
-24-
bainitic structure, has a low wear resistance, and it is desirable that the total area traction of
structures other than pearlitc be 5% or less. Further, a structure in which hypereutectoicl
cementite does not precipitate is desirable. The reason for this is that the precipitation ol'
hypereutectoid cementite decreases the rolling contact fatigue resistance.
[0102]
It is desirable that the hub part of the wheel have the same structure as that of the
rim part, although no special problem arises even if the area fraction of structures other
than pearlite exceeds 5%. However, a structure in which hypereutectoid cementite does
not precipitate is desirable. The reason for this is that in some cases, the precipitation of
hypereutectoid cementite leads to an extreme decrease in toughness and fatigue life. At
least, the formation of hypereutectoid cementite observed under an optical microscope
must be avoided.
[0103]
The wheel using the steel for wheel in accordance with the present invention as a
material can be manufactured by successively performing the treatment described, for
example, in the following items (1) to (3). After the treatment of item (3), tempering
treatment may be performed.
[0104]
(1) Melting and casting of steel
After being melted in an electric furnace, a converter, or the like, the material is cast
into an ingot. The ingot may be a cast piece obtained by continuous casting, or may be an
ingot cast in a mold.
[0105]
(2) Forming into wheel
To obtain a predetermined wheel configuration, the ingot is formed into a wheel
directly, or by an appropriate method, such as hot forging or machining, after once having
been fabricated into a slab. The ingot may be formed into a wheel configuration, directly
by casting, but it is desirable that the ingot be hot forged.
- 2 5 -
[0106]
^ ^ (3) Quenching
A quenching method in which a compressive residual stress occurs in the rim pan.
such as the "tread quenching process", is employed. The heating temperature in
quenching is preferably AC3 point to (AC3 point + 250°C). If the heating temperature is
lower than AC3 point, the structure is not transformed into austenite, and in some cases.
pearlite having a high hardness cannot be obtained by cooling after heating. On the other
hand, if the heating temperature exceeds (Ac3 point + 250°C), the grain size are coarsened,
whereby the toughness is sometimes decreased, which is unfavorable in terms of wheel
performance.
[0107]
The cooling after heating is preferably performed by an appropriate method such as
water cooling, oil cooling, mist cooling, or air cooling, given the wheel size, the facility,
and the like so that the above-described structure can be obtained in the wheel.
[0108]
Hereunder, the present invention is explained more specifically by using examples.
However, the present invention is not limited to these examples.
Embodiment
[0109]
After steels 25 to 46 having chemical components given in Table 3 had been melted
in an electric furnace, the molten steels each were poured into a mold having a diameter of
513 mm to produce ingots.
[0110]
Steels 28, 29, 31, 33, 34, and 38 to 46 are steels of example embodiments of the
present invention in which the chemical composition is within the range defined in the
present invention. On the other hand, steels 25 to 27, 30, 32, and 35 to 37 are steels of
-26-
comparative examples in which the chemical composition deviates from the conditions
^ ^ defined in the present invention
[0111]
Among the steels of comparative examples, steel 25 is steel corresponding to the
railroad wheel steel of "Class C" of AAR.
[0112]
Table 3
Steel Chemical composition (mass%) Balance; Fe and impurities
C 1 Si 1 Mn 1 P I S 1 Cu I Ni I Cr I Mo I V i Al I Fnl I Fn2~
25 0.69 0.29 0.78 0.012 0.008 0.01 0.02 0.09 - *- 0.024 *30.25 4.7
26 *0.58 0.31 0.82 0.014 0.009 0.02 0.03 0.07 - 0.16 0.024 34.21 10.1
27 *0.87 0.29 0.81 0.011 0.008 0.02 0.02 0.07 - *- 0.015 35.55 4.7
28 0.72 0.98 0.83 0.016 0.009 0.03 0.02 0.18 - 0.06 0.023 37.11 17.3
29 0.73 0.25 1.39 0.014 0.009 0.03 0.03 0.11 - 0.08 0.019 39.28 8.3
30 0.71 0.97 0.88 0.017 0.010 0.02 0.03 0.43 0.02 0.09 0.027 41.77 *
25.6
31 0.73 0.28 0.82 0.013 0.012 0.03 0.02 0.17 0.07 0.09 0.018 38.65 9.4
32 0.71 0.32 0.83 0.014 0.009 0.02 0.02 0.07 *0.30 *- 0.014 40.15 12.1
33 0.75 0.31 0.83 0.013 0.011 0.02 0.02 0.14 - 0.04 0.015 34.74 6.2
34 0.71 0.29 0.81 0.014 0.008 0.02 0.03 0.09 - 0.11 0.016 35.92 7.9
35 0.66 0.29 0.68 0.013 0.012 0.03 0.02 0.05 - 0.03 0.023 *29.57 5.1
36 0.79 0.31 0.80 0.011 0.009 0.02 0.03 0.20 0.10 0.18 0.015 *45.59 16.5
37 0.71 *1.02 0.82 0.015 0.007 0.03 0.02 0.07 0.10 0.10 0.017 40.48 *
27.1
38 0.74 0.80 0.83 0.014 0.008 0.03 0.02 0.44 0.02 0.11 0.037 42.81 22.1
39 0.75 0.48 0.82 0.014 0.009 0.02 0.03 0.42 - 0.07 0.031 39.51 11.1
40 0.76 0.21 0.81 0.015 0.008 0.03 0.02 0.39 - 0.10 0.034 39.96 8.6
41 0.76 0.81 0.79 0.013 0.032 0.03 0.02 0.18 - 0.11 0.042 39.74 17.1
42 0.72 0.48 0.83 0.012 0.037 0.03 0.03 0.31 - 0.08 0.024 37.95 10.7
43 0.73 0.48 0.80 0.014 0.012 0.09 0.02 0.28 - 0.09 - 38.16 10.8
44 0.71 0.47 0.83 0.015 0.010 0.02 0.08 0.26 - 0.09 - 37.53 10.6
45 0.73 0.47 0.80 0.017 0.008 0.02 0.02 0.30 - 0.09 0.092 38.35 10.8
46 | 0.72 1 0.47 | 0.81 | 0.014 [ 0.011 [ 0.03 | 0.02 \ 0.10 | 0.05 | 0.05 | - | 35.70 | 9.0
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
Fn2 = 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)
* mark indicates that chemical composition deviates from conditions defined in the present invention. |
[0113]
The ingot of each steel was cut into a length of 300 mm, and after having been
heated to 1200°C, the cut ingot was hot forged by the ordinary method, whereby a wheel
-27-
having a diameter of 965 mm was manufactured. This wheel has the configuration, of
^ "AAR TYPI-:B-38" described in M-107/M-207 standards of AAR.
[0114]
Next, after having been heated at 900°C for 2 hours, each of the wheels was heat
treated by a method in which, using a device shown in Figure 12, the wheel is cooled by
spraying water from nozzles while the wheel is rotated (so-called "tread quenching
process")
[0115]
After the above-described heat treatment, tempering treatment (treatment in which
the wheel is held at 500°C for 2 hours and thereafter is cooled in the atmosphere) was
performed.
[0116]
On the wheel thus manufactured, the hardness test of the rim part, the
microstructure examination of the rim part and hub part, the wearing test, the rolling
contact fatigue test, and the Jominy test were conducted. In each test, the test results in
test sign A using steel 25 were based upon.
[0117]
[1] Hardness test of rim part
For each steel, as shown in Figure 13, Brinell hardness (hereinafter, referred to as
"HB W") at a position 40 mm distant from the tread in the tread central portion of the rim
part was measured.
[0118]
[2] Microstructure examination of rim part
For each steel, as shown in Figure 14, microstructure at a position 40 mm distant
from the tread in the tread central portion of the rim part was examined. The position was
corroded with nital and the microstructure was observed under an optical microscope at
x400 magnification.
[0119]
-28-
In the case where the microstructure contained ferritic or bainitic structure, the area
^ ^ fraction thereof was measured. If the area fraction was 5% or more, the microstructure
was recognized as a microstructure containing ferrite or bainite. In the case where ferrite
or bainite was contained. "P + F" or "P + B" was written in Table 4 described later.
[0120]
[3] Microstructure examination of hub part
For each steel, as shown in Figure 15, microstructure at a position in the center of the hub
part was examined. The position was corroded with nital and the microstructure was
observed in the same way as in the rim part.
[0121]
[4] Wearing test
For each steel, as shown in Figure 16, on the basis of the position 40 mm distant
from the tread in the tread central portion of the rim part (the position indicated by "a" in
Figure 16), a "wheel test specimen" (a test specimen having the configuration, shown in
Figure 5(a)) used for the wearing test was sampled.
[0122]
By using the "wheel test specimen" of each of steels 25 to 46 and the beforementioned
"rail test specimen" of steel 1, the wearing test using the Nishihara-type wear
testing machine was conducted under the same conditions as those for steels 1 to 24 to
determine the amount of wear.
[0123]
Specifically, the wearing test was conducted under the conditions of Hertzian stress:
2200 MPa, slip ratio: 0.8%, and rotational speed: 776 rpm on the wheel side and 800 rpm
on the rail side, and after the test had been carried out until a number of cycles of 5 x 10',
the amount of wear was determined from the difference in mass of test specimen before
and after the test.
[0124]
[5] Rolling contact fatigue test
-29-
For each steel, as shown in Figure 16, on the basis of the position 40 mm distant
from the tread in the tread central portion of the rim part (the position indicated by "b" in
Figure 16), a "wheel test specimen" (a test specimen having the configuration, shown in
Figure 4(a)) used for the rolling contact fatigue test was sampled.
[0125J
By using this "wheel test specimen", the rolling contact fatigue test was conducted
under the same conditions as those for steels 1 to 24 to determine the rolling contact
fatigue life.
[0126]
Specifically, by using the "wheel test specimen" of each of steels 25 to 46 and the
before-mentioned "rail test specimen" of steel 1, the rolling contact fatigue test was
conducted under the conditions of Hertzian stress: 1100 MPa, slip ratio: 0.28%, rotational
speed: 1000 rpm on the wheel side and 602 rpm on the rail side, and under water
lubrication, and the number of cycles in which 0.5G was detected by an accelerometer was
evaluated as the rolling contact fatigue life.
[0127]
[6] Jominy test
For each steel, as shown in Figure 16, on the basis of the position 40 mm distant
from the tread in the tread central portion of the rim part (the position indicated by "c" in
Figure 16), a Jominy test specimen was sampled, and the Jominy test was conducted under
the same conditions as those for steels 1 to 24 to determine the "M50%".
[0128]
Specifically, the test specimen was austenitized at 900°C for 30 minutes in the
atmosphere, and thereafter was subjected to end quenching. Then, the hardness
distribution to a position 50 mm distant from the water cooling end was measured after
1.0-mm parallel cutting, and thereby the "M50%" was determined by the same method as
described above.
[0129]
-30-
Table 4 systematically uives the test results.
[01?0]
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[0131J
As is apparent from Table 4, for test signs D. E. G. [. J. and N to V of example
embodiments of the present invention in which steel whose chemical composition satisfies
the conditions defined in the present invention is used, the wear resistance and the rolling
contact fatigue resistance were excellent as compared with test sign A, which is the basis
using steel 25 corresponding to the railroad wheel steel of "Class C" of AAR.
[0132]
All of the values of Fn2 of steels used for test signs of example embodiments of the
present invention were smaller than 25. Therefore, it is presumed that the hardenability is
low and the spalling resistance is excellent.
[0133]
In contrast, for test sign B of comparative example in which steel 26 whose C
content is as low as 0.58%, deviating from the conditions defined in the present invention,
is used, 5% or more of ferrite was formed in the structure, and the amount of wear was
large.
[0134]
For test sign C of comparative example in which steel 27 whose C content is as high
as 0.87% and which does not contain V, deviating from the conditions defined in the
present invention, is used, hypereutectoid cementite was observed in the hub part.
[0135]
For test sign F of comparative example in which steel 30 whose Fn2 is as high as
25.6, deviating from the conditions defined in the present invention, is used, the
hardenability was high. Therefore, it is presumed that the spalling resistance is poor.
[0136]
For test sign H of comparative example in which steel 32 whose Mo content is as
high as 0.30% and which does not contain V, deviating from the conditions defined in the
present invention, is used, 5% or more of bainite was formed in the structure, so that the
amount of wear was large.
- 3 3 -
[0137] I
^P For test sign K of comparative example in which steel 35 whose Fnl is as low a- 1
29.57. deviating from the conditions defined in the present invention, is used, the hardness
of the rim part was as low as 308 in HBW, so that the amount of wear was large. Further,
the rolling contact fatigue life was also short.
[0138]
For test sign L of comparative example in which steel 36 whose Fnl is as high as
45.59, deviating from the conditions defined in the present invention, is used, 5% or more
of bainite was formed in the structure, so that the amount of wear was large.
[0139]
For test sign M of comparative example in which steel 37 whose Si content and Fn2
are as high as 1.02% and 27.1, respectively, deviating from the conditions defined in the
present invention, is used, the hardenability was high. Therefore, it is presumed that the
spalling resistance is poor.
INDUSTRIAL APPLICABILITY
[0140]
The steel for wheel in accordance with the present invention is excellent in balance
between the wear resistance, rolling contact fatigue resistance and the spalling resistance,
and can give a long life to the wheel. Specifically, for the wheel that uses the steel for
wheel in accordance with the present invention as a material, the amount of wear decreases
by 10 to 35% and the rolling contact fatigue life increases to 1.4 to 3.2 times as compared
with the wheel that uses the railroad wheel steel of "Class C" of AAR, and spalling is less
liable to occur. Therefore, the steel for wheel in accordance with the present invention is
extremely suitable as a material for a railroad wheel used in a very harsh environment of
increased running distance and increased loading capacity.
-34-

We claim:
•
[Claim 1]
Steel for wheel having a chemical composition consisting of, by mass percent, C:
0.65 to 0.84%, Si: 0.02 to 1.00%, Mn: 0.50 to 1.90%, Cr: 0.02 to 0.50%, V: 0.02 to 0.20%,
and S: 0.04% or less, in which Fnl expressed by Formula (1) is 34 to 43, and Fn2
expressed by Formula (2) is 25 or less, the balance being Fe and impurities, and this
impurities containing P: 0.05% or less, Cu: 0.20% or less, and Ni: 0,2JO%^rJess*..........,....,
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 = 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)
m Formulas (1) and (2% C, Si, Mn, Cr, Mo and V each mean the concent in percent b'y.
mass of the element.
[Claim 2]
The steel for wheel according to claim 1, wherein 0.20% or less of Mo is contained
by mass percent in lieu of a part of Fe.
[Claim 3]
The steel for wheel according to claim 1 or 2 wherein 0.20% or less of Al is
contained by mass percent in lieu of a part of Fe.