Patent Grant
11401591
The present disclosure relates to a method for selecting a rail steel and a wheel steel that is capable of suppressing fatigue damage in a rail and a railway wheel used in a railway track and of extending the service life of both the rail and the wheel by controlling the ratio of the yield strength at a head portion of the rail to the yield strength at a rim portion of the wheel.
In heavy haul railways mainly built to transport ore, the load applied to the axle of a freight car is much higher than that in passenger cars, and rails and wheels are used in increasingly harsh environments. For rails and wheels used under such circumstances, conventional rail steels primarily have a pearlite structure from the viewpoint of the importance of wear resistance and have a yield strength of 800 MPa or less, which may vary depending on the operating environment. Similarly, wheel steels having a yield strength of 500 MPa or less are conventionally used for railway wheels.
In recent years, however, in order to improve the efficiency of transportation by railway, the loading weight on freight cars is becoming larger and larger, and consequently, there is a need for further improvement of durability of rail steels and wheel steels. It is noted that heavy haul railways are railways where trains and freight cars haul large loads (loading weight is about 150 tons, for example).
Under such circumstances, for example, JP2004315928A (PTL 1) proposes a wheel for high-carbon railway vehicles in which wear resistance and thermal crack resistance are improved by increasing the C content to 0.85% to 1.20%. JP2013147725A (PTL 2) proposes a method for reducing the wear of rails and wheels by controlling the ratio of the rigidity of the rail steel and the hardness of the wheel steel.
PTL 1: JP2004315928A
PTL 2: JP2013147725A
On the other hand, as described above, since the operating environments of rails and wheels are becoming more severe, rails and wheels suffer from fatigue damage. In particular, in curve sections of a heavy haul railway, it is required to suppress fatigue damage resulting from the rolling stress exerted by wheels and the sliding force due to centrifugal force.
However, in the technique described in JP2004315928A (PTL 1), although the wear resistance and the thermal crack resistance of the wheel are improved to some extent, the C content is as high as 0.85% to 1.20%, which makes it difficult to improve fatigue damage resistance. This is because as a result of steel containing a large amount of C, a proeutectoid cementite structure is formed depending on heat treatment conditions and the amount of cementite phase contained in a pearlite lamellar structure increases.
Further, in PTL 2, since attention is paid only to the relationship between the rail and the hardness of the wheel (Vickers hardness), although it is possible to suppress wear, it is difficult to suppress fatigue damage.
It would thus be helpful to provide a method for selecting a rail steel and a wheel steel that is capable of suppressing fatigue damage in a rail used in a railway track and of a railway wheel, and that can extend the service life of both the rail and the wheel.
In order to address the above issues, we made rail steels and wheel steels with varying contents of C, Si, Mn, and Cr, and extensively investigated the relationship between yield strength and fatigue damage resistance. Our investigations revealed that by setting the ratio YSR/YSW of the yield strength YSR at a head portion of a rail and the yield strength YSW at a rim portion of a wheel to 0.85 or more and 1.95 or less, it is possible to suppress the fatigue damage in the rail and the wheel.
The present disclosure is based on the findings described above and has the following primary features.
1. A method for selecting a rail steel and a wheel steel comprising: selecting a rail steel and a wheel steel to be used as a rail and a wheel on an actual track, respectively, the rail steel having a chemical composition containing, by mass %, C: 0.70% or more and less than 0.85%, Si: 0.10% to 1.50%, Mn: 0.40% to 1.50%, and Cr: 0.05% to 1.50%, with the balance of Fe and inevitable impurities, the wheel steel having a chemical composition containing, by mass %, C: 0.57% or more and less than 0.85%, Si: 0.10% to 1.50%, Mn: 0.40% to 1.50%, and Cr: 0.05% to 1.50%, with the balance of Fe and inevitable impurities, such that the rail comprises a head portion having a yield strength YSR of 830 MPa or more, the wheel comprises a rim portion having a yield strength YSW of 580 MPa or more, and a ratio YSR/YSW of the yield strength YSR at the head portion of the rail to the yield strength YSW at the rim portion of the wheel falls within a range of:
0.85≤YSR/YSW≤1.95 (1).
The method for selecting a rail steel and a wheel steel according to 1. above, wherein the chemical composition of the rail steel further contains, by mass %, at least one selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, V: 0.30% or less, Nb: 0.05% or less, Mo: 0.5% or less, W: 0.5% or less, Al: 0.07% or less, Ti: 0.05% or less, and B: 0.005% or less.
The method for selecting a rail steel and a wheel steel according to 1. or 2. above, wherein the chemical composition of the wheel steel further contains, by mass %, at least one selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, V: 0.30% or less, Nb: 0.05% or less, Mo: 0.5% or less, W: 0.5% or less, Al: 0.07% or less, Ti: 0.05% or less, and B: 0.005% or less.
According to the present disclosure, by using a rail steel and a wheel steel having predetermined chemical compositions and by controlling the ratio of the yield strength of the resulting rail to that of the resulting wheel, it is possible to suppress the fatigue damage in the rail and the wheel, lengthening the service life of both.
Detailed description is given below. In the present disclosure, it is important that a rail steel and a wheel steel have the above-described chemical compositions. The reasons for limiting the chemical compositions as stated above are described first. The unit of the content of each component is “mass %”, but it is abbreviated as “%”.
[Chemical Composition of Rail Steel]
C: 0.70% or More and Less than 0.85%
Si: 0.10% to 1.50%
Mn: 0.40% to 1.50%
Cr: 0.05% to 1.50%
The rail steel in one embodiment of the present disclosure has a chemical composition containing the above components with the balance of Fe and inevitable impurities. Examples of the inevitable impurities include P and S, and up to 0.025% of P and up to 0.025% of S are allowable. On the other hand, a lower limit for the P content and the S content may be 0% without limitation, yet the lower limit is more than 0% in industrial terms. In addition, since excessively reducing the contents of P and S leads to an increase in the refining cost, the P content and the S content are preferably 0.0005% or more. The chemical composition of the rail steel of the present disclosure preferably consists of the above components and the balance of Fe and inevitable impurities, or alternatively, in addition to these, optional components as specified below. However, rail steels containing other trace elements within a range not substantially affecting the action and effect of the present disclosure are also encompassed by the present disclosure.
Optionally, the chemical composition of the rail steel may further contain, by mass %, at least one selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, V: 0.30% or less, Nb: 0.05% or less, Mo: 0.5% or less, W: 0.5% or less, Al: 0.07% or less, Ti: 0.05% or less, and B: 0.005% or less.
V: 0.30% or Less
Cu: 1.0% or Less
Ni: 1.0% or Less
Nb: 0.05% or Less
Mo: 0.5% or Less
W: 0.5% or Less
Al: 0.07% or Less
B: 0.005% or Less
Ti: 0.05% or Less
[Chemical Composition of Wheel Steel]
C: 0.57% or More and Less than 0.85%
Si: 0.10% to 1.50%
Mn: 0.40% to 1.50%
Cr: 0.05% to 1.50%
The wheel steel in one embodiment of the present disclosure has a chemical composition containing the above components with the balance of Fe and inevitable impurities. Examples of the inevitable impurities include P and S, and up to 0.030% of P and up to 0.030% of S are allowable. On the other hand, a lower limit for the P content and the S content may be 0% without limitation, yet it is more than 0% in industrial terms. In addition, since excessively reducing the contents of P and S leads to an increase in the refining cost, the P content and the S content are preferably 0.0005% or more. The chemical composition of the wheel steel of the present disclosure preferably consists of the above components and the balance of Fe and inevitable impurities, or alternatively, in addition to these, optional components as specified below. However, wheel steels containing other trace elements within a range not substantially affecting the action and effect of the present disclosure are also encompassed by the present disclosure.
Optionally, the chemical composition of the wheel steel may further contain, by mass %, at least one selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, V: 0.30% or less, Nb: 0.05% or less, Mo: 0.5% or less, W: 0.5% or less, Al: 0.07% or less, Ti: 0.05% or less, and B: 0.005% or less.
V: 0.30% or Less
Cu: 1.0% or Less
Ni: 1.0% or Less
Nb: 0.05% or Less
Mo: 0.5% or Less
W: 0.5% or Less
Al: 0.07% or Less
B: 0.005% or Less
Ti: 0.05% or Less
[Yield Strength Ratio YSR/YSW]
The fatigue damage resistance of the rail steel and of the wheel steel depends on the yield strength of each. It is thus believed that the fatigue damage in the rail and the wheel can be suppressed by increasing the yield strength. However, if the ratio of the yield strength of the rail steel to the yield strength of the wheel steel is not in an appropriate range, the fatigue damage resistance is rather lowered due to the accumulation of fatigue layers. If the YSR/YSW ratio is below 0.85, the yield strength of the rail steel is too low, the yield strength of the wheel steel is too high, or both. If the yield strength of the rail steel is low, the fatigue damage resistance of the rail steel itself decreases, and the rail steel is consequently prone to fatigue damage. Also, if the yield strength of the wheel steel is high, fatigue layers accumulate in the rail steel as the counterpart material, which ends up causing fatigue damage to occur in the rail steel easily. If the YSR/YSW ratio is beyond 1.95, the yield strength of the wheel steel is too low, the yield strength of the rail steel is too high, or both. When the yield strength of the wheel steel is low, the fatigue damage resistance of the wheel steel itself decreases, and the wheel steel is consequently prone to fatigue damage. Also, if the yield strength of the rail steel is high, fatigue layers accumulate in the wheel steel as the counterpart material, which ends up causing fatigue damage to occur in the wheel steel easily. Therefore, the YSR/YSW ratio is set to 0.85 or more and 1.95 or less. The YSR/YSW ratio is preferably 0.86 or more. The YSR/YSW ratio is preferably 1.90 or less.
[Yield Strength YSR at Head Portion of Rail]
When a rail is produced by hot rolling a steel raw material into a rail shape and cooling it, the yield strength YSR at the head portion of the rail can be adjusted by controlling the heating temperature before hot rolling and the cooling rate in cooling after hot rolling. In other words, since the yield strength YSR becomes higher as the heating temperature becomes higher and the cooling rate after hot rolling becomes higher, the heating temperature and the cooling rate may be adjusted for the targeted YSR.
[Yield Strength YSW at Rim Portion of Wheel]
When a wheel is formed by hot working such as hot rolling and hot forging, the yield strength YSW at the rim portion of the wheel can be adjusted by controlling the heating temperature before hot working and the cooling rate in cooling after hot working. In other words, since the yield strength YSW becomes higher as the heating temperature becomes higher and the cooling rate after hot rolling becomes higher, the heating temperature and the cooling rate may be adjusted for the targeted YSW.
[Steel Microstructure of Rail Steel and Wheel Steel]
Also, in the wheel steel, the steel microstructure of the rim portion of the wheel is preferably a pearlite structure. This is because a pearlite structure has excellent fatigue damage resistance as compared with the tempered martensite structure and the bainite structure as described above.
In order to make the steel microstructure of the head portion of the rail steel into a pearlite structure, the steel raw material is heated to 1000° C. to 1300° C. and then hot rolled. Then, air cooling is performed to 400° C. at a cooling rate of 0.5° C./s to 3° C./s.
Further, in order to make the steel microstructure of the rim portion of the wheel steel into a pearlite structure, the steel material is heated to 900° C. to 1100° C. and then hot forged. Then, air cooling is performed to 400° C. at a cooling rate of 0.5° C./s to 3° C./s.
We evaluated the effect of the yield strength ratio YSR/YSW on the occurrence of fatigue damage. Evaluation of fatigue damage is desirably carried out by using rails and wheels on an actual track, yet this process requires an extremely long test time. Therefore, in the examples below, the occurrence of fatigue damage was evaluated using test specimens fabricated from a rail steel and a wheel steel, respectively, and carrying out tests simulating a set of actual contact conditions between the rail and the wheel using a two-cylinder testing machine. At that time, the rail steel specimen and the wheel steel specimen were produced under a set of conditions simulating the head portion of the rail and the rim portion of the wheel, respectively. The specific production conditions and test methods are as follows.
Similarly, 100 kg of steels having the chemical compositions in Table 2 were each subjected to vacuum melting and hot rolled to a thickness of 80 mm. Each rolled material thus obtained was cut to a length of 150 mm, heated to 900° C. to 1100° C., and hot rolled to a final sheet thickness of 12 mm. Then, air cooling was performed to 400° C. at a cooling rate of 0.5° C./s to 3° C./s, and then allowed to cool. At this time, the yield strength of the finally obtained wheel steel was controlled by adjusting the heating temperature and the cooling rate before the hot rolling.
Yield Strength
Steel Microstructure
Fatigue Damage
It can be seen from the results in Table 3 that, the fatigue damage in a rail and a wheel can be effectively suppressed by selecting a rail steel and a wheel steel such that their chemical compositions and yield strength ratio YSR/YSW satisfy the conditions disclosed herein. On the other hand, it will be appreciated that in those combinations not satisfying the conditions of the present disclosure, peeling occurs in a short time and fatigue damage tends to occur easily.
0.68
0.86
0.05
1.52
0.35
1.52
0.01
1.52
0.56
0.86
0.05
1.52
0.35
1.52
0.01
1.52
R1-14
0.74
R1-15
2.02
R1-16
0.81
R1-17
2.04
R1-18
0.82
R1-19
1.96
R1-20
0.84
R1-21
2.01
W1-14
1.98
W1-15
0.83
W1-16
1.96
W1-17
0.70
W1-18
2.00
W1-19
0.79
W1-20
1.97
W1-21
0.70
0.84
1.96
Again, it can be seen from these results that the fatigue damage in a rail and a wheel can be effectively suppressed by selecting a rail steel and a wheel steel such that their chemical compositions and yield strength ratio YSR/YSW satisfy the conditions disclosed herein. In addition, as described in PTL 2, it is found that even with the use of a combination of a rail steel and a wheel steel in which the ratio HR/HW of the hardness HR of the rail steel to the hardness HW of the wheel steel is 1.00 or more and 1.30 or less is used, the fatigue damage resistance of the rail and the wheel is inferior if the yield strength of the rail steel is less than 830 MPa, the yield strength of the wheel steel is less than 580 MPa, and the yield strength ratio YSR/YSW is out of the range of 0.85 to 1.95 disclosed herein. It is also understood that the fatigue damage resistance of the wheel is inferior when the wheel steel has a steel microstructure other than pearlite.
823
772
569
533
1.96
0.84
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2015-244419 | Dec 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/087276 | 12/14/2016 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2017/104719 | 6/22/2017 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3726724 | Davies et al. | Apr 1973 | A |
| 7972451 | Ueda | Jul 2011 | B2 |
| 20090053095 | Robles Hernandez | Feb 2009 | A1 |
| 20120015212 | Karimine et al. | Jan 2012 | A1 |
| 20150147224 | Yamamoto et al. | May 2015 | A1 |
| 20150152516 | Kimura | Jun 2015 | A1 |
| Number | Date | Country |
|---|---|---|
| 2907609 | Dec 2017 | CA |
| H0892645 | Apr 1996 | JP |
| 2004315928 | Nov 2004 | JP |
| 2012030242 | Feb 2012 | JP |
| 2013147725 | Aug 2013 | JP |
| 2013224472 | Oct 2013 | JP |
| 2013231212 | Nov 2013 | JP |
| 2015504484 | Feb 2015 | JP |
| 2010116680 | Oct 2010 | WO |
| 2013079438 | Jun 2013 | WO |
| WO-2013161026 | Oct 2013 | WO |
| Entry |
|---|
| Lewis, R. Olofsson, U.. (2009). Wheel-Rail Interface Handbook—6.6.1 Use of Friction Modifiers. Woodhead Publishing. Retrieved from https://app.knovel.eom/hotlink/pdf/id:kt0079EUP1/wheel-rail-interface/use-friction-modifiers (Year: 2009). |
| D. Markov, Laboratory tests for wear of rail and wheel steels,Wear,vols. 181-183, Part 2, 1995, pp. 678-686, ISSN 0043-1648,https://doi.org/10.1016/0043-1648(95)90184-1. (https://www.sciencedirect.com/science/article/pii/0043164895901841) (Year: 1995). |
| Feb. 28, 2017, International Search Report issued in the International Patent Application No. PCT/2016/087276. |
| Jul. 15, 2019, Office Action issued by the Canadian Intellectual Property Office in the corresponding Canadian Patent Application No. 3,006,945. |
| Sergey M. Zakharov, Wheel and Rail Performance, The Asian Journal, 2006, vol. 13, No. 1. |
| Jan. 12, 2021, Office Action issued by the Canadian Intellectual Property Office in the corresponding Canadian Patent Application No. 3,006,945. |
| Number | Date | Country | |
|---|---|---|---|
| 20190249280 A1 | Aug 2019 | US |
