Method for selecting rail steel and wheel steel

Abstract

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 and the wheel steel having a specific chemical composition, such that the rail comprises a head portion having a yield strength YS.sub.R of 830 MPa or more, the wheel comprises a rim portion having a yield strength YS.sub.W of 580 MPa or more, and a ratio YS.sub.R/YS.sub.W of the yield strength YS.sub.R at the head portion of the rail to the yield strength YS.sub.W at the rim portion of the wheel falls within a range of: 0.85≤YS.sub.R/YS.sub.W≤1.95 (1).

Claims

1. A method for selecting a combination of a rail steel and a wheel steel comprising: preparing a plurality of rail steels each 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, preparing a plurality of wheel steels each 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, measuring yield strengths YS.sub.R of the rail steels and yield strengths YS.sub.W of the wheel steels, and selecting one of the rail steels and one of the wheel steels such that a combination of the selected rail steel and wheel steel satisfies the following conditions:
YS.sub.R≥830 MPa,
580 MPa≤YS.sub.W≤1000 MPa, and
1.02≤YS.sub.R/YS.sub.W≤1.95  (1).

2. The method for selecting a combination of a rail steel and a wheel steel according to claim 1, wherein the chemical composition of each 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.

3. The method for selecting a combination of a rail steel and a wheel steel according to claim 2, wherein the chemical composition of each 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.

4. The method for selecting a combination of a rail steel and a wheel steel according to claim 1, wherein the chemical composition of each 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.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 schematically illustrates a fatigue damage test method.

DETAILED DESCRIPTION

(2) 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 “%”.

(3) [Chemical Composition of Rail Steel]

(4) C: 0.70% or More and Less than 0.85%

(5) C is an element that forms cementite in a pearlite structure and has the effect of securing yield strength and fatigue damage resistance. If the C content is less than 0.70%, the yield strength decreases, making it difficult to obtain excellent fatigue damage resistance. On the other hand, when the C content is 0.85% or more, pro-eutectoid cementite is formed at austenite grain boundaries at the time of transformation after hot rolling, and the fatigue damage resistance is remarkably deteriorated. Therefore, the C content is set to 0.70% or more and less than 0.85%.

(6) Si: 0.10% to 1.50% Si is an element that is added as a deoxidizer and as a pearlite-structure-strengthening element. To obtain the addition effect of Si, the Si content needs to be 0.10% or more. On the other hand, a Si content beyond 1.50% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Therefore, the Si content is set in a range of 0.10% to 1.50%.

(7) Mn: 0.40% to 1.50% Mn is an element that contributes to achieving high yield strength of the rail by decreasing the pearlite transformation temperature to refine the lamellar spacing. When the Mn content is below 0.40%, however, this effect cannot be obtained sufficiently. On the other hand, a Mn content beyond 1.50% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Therefore, the Mn content is set in a range of 0.40% to 1.50%.

(8) Cr: 0.05% to 1.50% Cr is an element that has the effect of increasing the pearlite equilibrium transformation temperature to refine the lamellar spacing and improving the yield strength by solid solution strengthening. When the Cr content is below 0.05%, however, sufficient yield strength cannot be obtained. On the other hand, a Cr content beyond 1.50% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Therefore, the Cr content is set to 0.05% to 1.50%.

(9) 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.

(10) 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.

(11) V: 0.30% or Less V is an element that has the effect of improving the yield strength by dispersing and precipitating in the matrix by forming carbides or nitrides. On the other hand, a V content beyond 0.30% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Also, since V is an expensive element, the cost of rail steel increases. Therefore, in the case of adding V, it is preferable to set the V content to 0.30% or less. The lower limit of the V content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the V content to 0.001% or more.

(12) Cu: 1.0% or Less Like Cr, Cu is an element having the effect of improving the yield strength by solid solution strengthening. However, when the Cu content exceeds 1.0%, Cu cracking is liable to occur. Therefore, in the case of adding Cu, it is preferable to set the Cu content to 1.0% or less. The lower limit of the Cu content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Cu content to 0.001% or more.

(13) Ni: 1.0% or Less Ni is an element that has the effect of improving the yield strength without deteriorating the ductility. In addition, in the case of adding Cu, it is preferable to add Ni because Cu cracking can be suppressed by the addition of Ni in combination with Cu. When the Ni content exceeds 1.0%, however, the quench hardenability increases and martensite is formed, with the result that the fatigue damage resistance tends to decrease. Therefore, in the case of adding Ni, it is preferable to set the Ni content to 1.0% or less. The lower limit of the Ni content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Ni content to 0.001% or more.

(14) Nb: 0.05% or Less Nb bonds to C or N in the steel to form precipitates as carbides, nitrides, or carbonitrides during and after rolling, and effectively acts to increase the yield strength. Therefore, by adding Nb, the fatigue damage resistance can be greatly improved and the service life of the rail can be further extended. However, a Nb content beyond 0.05% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Therefore, in the case of adding Nb, it is preferable to set the Nb content to 0.05% or less. The lower limit of the Nb content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Nb content to 0.001% or more.

(15) Mo: 0.5% or Less Mo is an element having the effect of improving the yield strength by solid solution strengthening. However, a Mo content beyond 0.5% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Therefore, in the case of adding Mo, it is preferable to set the Mo content to 0.5% or less. The lower limit of the Mo content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Mo content to 0.001% or more.

(16) W: 0.5% or Less W is an element having the effect of improving the yield strength by solid solution strengthening. However, a W content beyond 0.5% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Therefore, in the case of adding W, it is preferable to set the W content to 0.5% or less. The lower limit of the W content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the W content to 0.001% or more.

(17) Al: 0.07% or Less Al bonds to N in the steel to form precipitates as nitrides during and after rolling, and effectively acts to increase the yield strength. Therefore, by adding Al, the fatigue damage resistance can be greatly improved and the service life of the rail can be further extended. However, when the Al content exceeds 0.07%, a large amount of oxides is produced in the steel, which ends up making the rail steel prone to fatigue damage. Therefore, in the case of adding Al, it is preferable to set the Al content to 0.07% or less. The lower limit of the Al content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Al content to 0.001% or more.

(18) B: 0.005% or Less B precipitates as nitrides during and after rolling, and effectively acts to increase the yield strength by precipitation strengthening. Therefore, by adding B, the fatigue damage resistance can be greatly improved and the service life of the rail can be further extended. However, a B content beyond 0.005% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the wheel steel, prone to fatigue damage. Therefore, in the case of adding B, it is preferable to set the B content to 0.005% or less. The lower limit of the B content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the B content to 0.0001% or more.

(19) Ti: 0.05% or Less Ti forms precipitates as carbides, nitrides, and carbonitrides during and after rolling, and effectively acts to increase the yield strength by precipitation strengthening. Therefore, by adding Ti, the fatigue damage resistance can be greatly improved and the lift of the rail can be further extended. However, when the Ti content exceeds 0.05%, coarse carbides, nitrides, or carbonitrides are formed, which ends up lowering the fatigue damage resistance of the rail. Therefore, in the case of adding Ti, it is preferable to set the Ti content to 0.05% or less. The lower limit of the Ti content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Ti content to 0.001% or more.

(20) [Chemical Composition of Wheel Steel]

(21) C: 0.57% or More and Less than 0.85%

(22) C is an element that forms cementite in a pearlite structure and has the effect of securing yield strength and fatigue damage resistance. If the C content is less than 0.57%, the yield strength decreases, making it difficult to obtain excellent fatigue damage resistance. On the other hand, if the C content is 0.85% or more, pro-eutectoid cementite is formed at austenite grain boundaries at the time of transformation after hot rolling, and the fatigue damage resistance is remarkably deteriorated. Therefore, the C content is set to 0.57% or more and less than 0.85%.

(23) Si: 0.10% to 1.50% Si is an element that is added as a deoxidizer and as a pearlite-structure-strengthening element. To obtain the addition effect of Si, the Si content needs to be 0.10% or more. On the other hand, a Si content beyond 1.50% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Therefore, the Si content is set in a range of 0.10% to 1.50%.

(24) Mn: 0.40% to 1.50% Mn is an element that contributes to achieving high yield strength of the wheel by decreasing the pearlite transformation temperature to refine the lamellar spacing. When the Mn content is less than 0.40%, however, this effect cannot be obtained sufficiently. On the other hand, a Mn content beyond 1.50% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Therefore, the Mn content is set in a range of 0.40% to 1.50%.

(25) Cr: 0.05% to 1.50% Cr is an element that has the effect of increasing the pearlite equilibrium transformation temperature to refine the lamellar spacing and improving the yield strength by solid solution strengthening. When the Cr content is below 0.05%, however, sufficient yield strength cannot be obtained. On the other hand, a Cr content beyond 1.50% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Therefore, the Cr content is set to 0.05% to 1.50%.

(26) 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.

(27) 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.

(28) V: 0.30% or Less V is an element that has the effect of improving the yield strength by dispersing and precipitating in the matrix by forming carbides or nitrides. On the other hand, a V content beyond 0.30% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Also, since V is an expensive element, the cost of the wheel steel increases. Therefore, in the case of adding V, it is preferable to set the V content to 0.30% or less. The lower limit of the V content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the V content to 0.001% or more.

(29) Cu: 1.0% or Less Like Cr, Cu is an element having an effect of improving the yield strength by solid solution strengthening. However, when the Cu content exceeds 1.0%, Cu cracking is liable to occur. Therefore, in the case of adding Cu, it is preferable to set the Cu content to 1.0% or less. The lower limit of the Cu content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Cu content to 0.001% or more.

(30) Ni: 1.0% or Less Ni is an element that has an effect of improving the yield strength without deteriorating the ductility. In addition, in the case of adding Cu, it is preferable to add Ni because Cu cracking can be suppressed by the addition of Ni in combination with Cu. When the Ni content exceeds 1.0%, however, the quench hardenability increases and martensite is formed, with the result that the fatigue damage resistance tends to decrease. Therefore, in the case of adding Ni, it is preferable to set the Ni content to 1.0% or less. The lower limit of the Ni content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Ni content to 0.001% or more.

(31) Nb: 0.05% or Less Nb bonds to C or N in the steel to form precipitates as carbides, nitrides, or carbonitrides during and after rolling, and effectively acts to increase the yield strength. Therefore, by adding Nb, the fatigue damage resistance can be greatly improved and the service life of the wheel can be further extended. However, a Nb content beyond 0.05% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Therefore, in the case of adding Nb, it is preferable to set the Nb content to 0.05% or less. The lower limit of the Nb content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Nb content to 0.001% or more.

(32) Mo: 0.5% or Less Mo is an element having an effect of improving the yield strength by solid solution strengthening. However, a Mo content beyond 0.5% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Therefore, in the case of adding Mo, it is preferable to set the Mo content to 0.5% or less. The lower limit of the Mo content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Mo content to 0.001% or more.

(33) W: 0.5% or Less W is an element having an effect of improving the yield strength by solid solution strengthening. However, a W content beyond 0.5% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Therefore, in the case of adding W, it is preferable to set the W content to 0.5% or less. The lower limit of the W content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the W content to 0.001% or more.

(34) Al: 0.07% or Less Al bonds to N in the steel to form precipitates as nitrides during and after rolling, and effectively acts to increase the yield strength. Therefore, by adding Al, the fatigue damage resistance can be greatly improved and the service life of the wheel can be further extended. However, when the Al content exceeds 0.07%, a large amount of oxides is produced in the steel, which ends up making the wheel steel prone to fatigue damage. Therefore, in the case of adding Al, it is preferable to set the Al content to 0.07% or less. The lower limit of the Al content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Al content to 0.001% or more.

(35) B: 0.005% or Less B precipitates as nitrides during and after rolling, and effectively acts to increase the yield strength by precipitation strengthening. Therefore, by adding B, the fatigue damage resistance can be greatly improved and the service life of the wheel can be further extended. However, a B content beyond 0.005% leads to an excessive increase in the yield strength, which ends up making the counterpart material, the rail steel, prone to fatigue damage. Therefore, in the case of adding B, it is preferable to set the B content to 0.005% or less. The lower limit of the B content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the B content to 0.0001% or more.

(36) Ti: 0.05% or Less Ti forms precipitates as carbides, nitrides, and carbonitrides during and after rolling, and effectively acts to increase the yield strength by precipitation strengthening. Therefore, by adding Ti, the fatigue damage resistance can be greatly improved and the service life of the wheel can be further extended. However, when the Ti content exceeds 0.05%, coarse carbides, nitrides, or carbonitrides are formed, which ends up lowering the fatigue damage resistance of the wheel. Therefore, in the case of adding Ti, it is preferable to set the Ti content to 0.05% or less. The lower limit of the Ti content is not particularly limited, yet from the viewpoint of improving the yield strength, it is preferable to set the Ti content to 0.001% or more.

(37) [Yield Strength Ratio YS.sub.R/YS.sub.W] In the present disclosure, a rail steel and a wheel steel to be used as a rail and a wheel on an actual track, respectively, having the above-described chemical compositions are selected such that the rail comprises a head portion having a yield strength YS.sub.R, the wheel comprises a rim portion having a yield strength YS.sub.W, and a YS.sub.R/YS.sub.W ratio falls within a range of:
0.85≤YS.sub.R/YS.sub.W≤1.95  (1).
In this case, the yield strength YS.sub.R of the rail is determined by collecting a tensile test specimen with a parallel portion of 0.25 inch or 0.5 inch as specified in ASTM A370 from a position as specified in AREMA Chapter 4, 2.1.3.4, and subjecting it to a tensile test. The yield strength YS.sub.W of the wheel is obtained by collecting a tensile test specimen similar to that obtained in the rail test from a position described in AAR Specification M-107/M-208, 3.1.1., and subjecting it to a tensile test.

(38) 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 YS.sub.R/YS.sub.W 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 YS.sub.R/YS.sub.W 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 YS.sub.R/YS.sub.W ratio is set to 0.85 or more and 1.95 or less. The YS.sub.R/YS.sub.W ratio is preferably 0.86 or more. The YS.sub.R/YS.sub.W ratio is preferably 1.90 or less.

(39) [Yield Strength YS.sub.R at Head Portion of Rail] Since the fatigue damage resistance of the rail itself can be further enhanced by increasing the yield strength YS.sub.R at the head portion of the rail, YS.sub.R is set to 830 MPa or more. Although no upper limit is placed on YS.sub.R, excessively increasing YS.sub.R makes it difficult to satisfy the condition of formula (1). Thus, a preferred upper limit is 1200 MPa.

(40) When a rail is produced by hot rolling a steel raw material into a rail shape and cooling it, the yield strength YS.sub.R 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 YS.sub.R 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 YS.sub.R.

(41) [Yield Strength YS.sub.W at Rim Portion of Wheel] By increasing the yield strength YS.sub.W at the rim portion of the wheel, the fatigue damage resistance of the wheel itself can be enhanced. Therefore, the YS.sub.W is set to 580 MPa or more. Although no upper limit is placed on YS.sub.W, excessively increasing YS.sub.W makes it difficult to satisfy the condition of formula (1). Thus, a preferred upper limit is 1000 MPa.

(42) When a wheel is formed by hot working such as hot rolling and hot forging, the yield strength YS.sub.W 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 YS.sub.W 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 YS.sub.W.

(43) [Steel Microstructure of Rail Steel and Wheel Steel] In the rail steel, the steel microstructure of the head portion of the rail is preferably a pearlite structure. This is because the pearlite structure has better fatigue damage resistance than the tempered martensite structure and the bainite structure.

(44) 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.

(45) 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.

(46) 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.

EXAMPLES

(47) We evaluated the effect of the yield strength ratio YS.sub.R/YS.sub.W 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.

Example 1

(48) In this case, 100 kg of steels having the chemical compositions in Table 1 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 1000° C. to 1300° 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 to obtain a rail steel. At this time, the yield strength of the finally obtained rail steel was controlled by adjusting the heating temperature and the cooling rate before the hot rolling.

(49) 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.

(50) Yield Strength The yield strength of each rail steel and wheel steel thus obtained was evaluated by a tensile test in accordance with ASTM A370. From each rail steel and wheel steel, a tensile test specimen having a parallel portion diameter of 0.25 inch (6.35 mm) as prescribed in ASTM A370 was collected and subjected to a tensile test at a tensile rate of 1 mm/min, where a 0.2% proof stress was determined from the stress-strain curve and used as the yield strength. The measured values are presented in Table 2.

(51) Steel Microstructure After polishing the surface of each obtained rail steel and wheel steel to a mirror surface, it was etched with nital, and microstructure observation was carried out at ×100 magnification.

(52) Fatigue Damage Test specimens with a diameter of 30 mm were prepared from each obtained rail steel and wheel steel with a contact surface being a curved surface having a radius of curvature of 15 mm. Then, in each combination of a rail steel and a wheel steel listed in Table 3, the occurrence of fatigue damage was evaluated using a two-cylinder testing machine. Tests were conducted at a contact pressure of 2.2 GPa and a slip rate of ˜20% under oil lubrication condition, and the number of revolutions at the time when peeling (fatigue damage) occurred was counted as presented in Table 3. The number of revolutions can be regarded as an index of fatigue damage life of the rail and the wheel. Since it takes a long time to continue the test until peeling occurs, in this example, in the case where the rail steel was peeled off at less than 1,728,000 revolutions and where the wheel steel was peeled off at less than 2,160,000 revolutions, it was judged that satisfactory fatigue damage resistance could not be obtained with that rail steel and wheel steel combination, and the test was interrupted. In this case, for members that did not peel off, the number of revolutions in Table 2 is set to “-”. On the other hand, the fatigue damage resistance was determined to be good when the number of revolutions was 1,728,000 or more for rail steels and 2,160,000 or more for wheel steels, as indicated by “no peeling” in Table 3.

(53) 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 YS.sub.R/YS.sub.W 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.

(54) TABLE-US-00001 TABLE 1 Steel Chemical composition of rail steel (mass %)* No. C Si Mn P S Cr Remarks R1-1 0.82 1.50 0.49 0.014 0.007 0.26 Conforming Steel R1-2 0.83 0.25 0.85 0.005 0.007 0.61 Conforming Steel R1-3 0.70 0.41 0.40 0.003 0.006 1.50 Conforming Steel R1-4 0.83 0.87 0.47 0.003 0.006 1.46 Conforming Steel R1-5 0.84 0.88 0.46 0.016 0.005 0.79 Conforming Steel R1-6 0.83 0.87 0.47 0.003 0.006 1.46 Conforming Steel R1-7 0.79 0.98 0.71 0.005 0.007 0.27 Conforming Steel R1-8 0.81 0.69 0.56 0.015 0.007 0.79 Conforming Steel R1-9 0.77 0.52 0.78 0.012 0.007 0.75 Conforming Steel R1-10 0.81 0.71 0.40 0.004 0.004 0.93 Conforming Steel R1-11 0.71 1.16 1.34 0.016 0.004 0.88 Conforming Steel R1-12 0.84 1.06 0.83 0.019 0.006 0.05 Conforming Steel R1-13 0.84 0.48 0.71 0.016 0.004 0.32 Conforming Steel R1-14 0.68 0.25 0.81 0.015 0.006 0.05 Comparative Steel R1-15 0.86 0.88 0.81 0.015 0.007 1.39 Comparative Steel R1-16 0.72 0.05 0.81 0.015 0.005 0.21 Comparative Steel R1-17 0.82 1.52 0.82 0.014 0.005 0.99 Comparative Steel R1-18 0.72 0.25 0.35 0.015 0.005 0.18 Comparative Steel R1-19 0.84 0.29 1.52 0.011 0.005 0.99 Comparative Steel R1-20 0.81 0.63 0.81 0.006 0.003 0.01 Comparative Steel R1-21 0.85 0.59 0.81 0.007 0.003 1.52 Comparative Steel R1-22 0.70 0.55 1.50 0.010 0.005 0.27 Conforming Steel R1-23 0.84 0.11 0.74 0.005 0.007 0.90 Conforming Steel R1-24 0.83 0.31 0.81 0.005 0.007 0.33 Conforming Steel R1-25 0.84 0.96 0.95 0.005 0.007 0.96 Conforming Steel *The balance consists of Fe and inevitable impurities.

(55) TABLE-US-00002 TABLE 2 Steel Chemical composition of wheel steel (mass %)* No. C Si Mn P S Cr Remarks W1-1 0.84 1.01 1.15 0.012 0.002 0.09 Conforming Steel W1-2 0.65 0.29 1.50 0.015 0.008 0.20 Conforming Steel W1-3 0.81 0.75 0.70 0.019 0.004 0.34 Conforming Steel W1-4 0.84 1.50 0.40 0.007 0.010 0.33 Conforming Steel W1-5 0.78 0.25 0.80 0.012 0.005 1.50 Conforming Steel W1-6 0.74 0.27 0.70 0.019 0.007 0.22 Conforming Steel W1-7 0.85 1.00 0.85 0.008 0.009 0.39 Conforming Steel W1-8 0.78 0.10 0.71 0.005 0.003 0.24 Conforming Steel W1-9 0.79 0.26 0.71 0.015 0.009 0.22 Conforming Steel W1-10 0.69 0.33 0.81 0.019 0.003 0.22 Conforming Steel W1-11 0.84 0.28 0.65 0.003 0.001 0.05 Conforming Steel W1-12 0.80 0.22 0.74 0.015 0.007 0.20 Conforming Steel W1-13 0.76 0.21 0.70 0.004 0.009 0.21 Conforming Steel W1-14 0.56 0.69 0.81 0.011 0.005 0.31 Comparative Steel W1-15 0.86 0.39 0.91 0.015 0.006 0.77 Comparative Steel W1-16 0.72 0.05 0.81 0.015 0.005 0.19 Comparative Steel W1-17 0.82 1.52 0.82 0.014 0.005 0.99 Comparative Steel W1-18 0.72 0.25 0.35 0.015 0.005 0.18 Comparative Steel W1-19 0.84 0.29 1.52 0.011 0.005 0.99 Comparative Steel W1-20 0.74 0.21 0.77 0.006 0.003 0.01 Comparative Steel W1-21 0.85 0.59 0.81 0.007 0.003 1.52 Comparative Steel W1-22 0.75 0.15 0.75 0.004 0.005 0.19 Conforming Steel W1-23 0.68 0.23 0.71 0.014 0.003 0.24 Conforming Steel W1-24 0.79 0.95 0.95 0.014 0.003 0.74 Conforming Steel W1-25 0.69 0.31 0.69 0.013 0.007 0.34 Conforming Steel *The balance consists of Fe and inevitable impurities.

(56) TABLE-US-00003 TABLE 3 Rail Wheel Yield Yield Yield strength Number of revolutions Steel Steel strength Steel Steel strength ratio when peeling occurred No. No. microstructure* YS.sub.R (MPa) No. microstructure* YS.sub.W (MPa) YS.sub.R/YS.sub.W Rail Wheel Remarks 1 R1-1 P 875 W1-12 P 709 1.23 no peeling no peeling Example 2 R1-2 P 890 W1-13 P 646 1.38 no peeling no peeling Example 3 R1-3 P 860 W1-11 P 727 1.18 no peeling no peeling Example 4 R1-4 P 1135 W1-10 P 582 1.95 no peeling no peeling Example 5 R1-5 P 948 W1-8 P 678 1.40 no peeling no peeling Example 6 R1-6 P 1135 W1-9 P 711 1.60 no peeling no peeling Example 7 R1-7 P 835 W1-7 P 983 0.85 no peeling no peeling Example 8 R1-8 P 896 W1-1 P 953 0.94 no peeling no peeling Example 9 R1-9 P 865 W1-2 P 661 1.31 no peeling no peeling Example 10 R1-10 P 907 W1-3 P 832 1.09 no peeling no peeling Example 11 R1-11 P 1006 W1-7 P 983 1.02 no peeling no peeling Example 12 R1-12 P 877 W1-4 P 922 0.95 no peeling no peeling Example 13 R1-13 P 857 W1-12 P 709 1.21 no peeling no peeling Example 14 R1-14 P 780 W1-5 P 1055 0.74 1080000 — Comparative Example 15 R1-15 P 1074 W1-23 P 532 2.02 — 472500 Comparative Example 16 R1-16 P 770 W1-1 P 953 0.81 1231200 — Comparative Example 17 R1-17 P 1083 W1-23 P 532 2.04 — 481500 Comparative Example 18 R1-18 P 781 W1-1 P 953 0.82 1299600 — Comparative Example 19 R1-19 P 1043 W1-23 P 532 1.96 — 472500 Comparative Example 20 R1-20 P 802 W1-1 P 953 0.84 1436400 — Comparative Example 21 R1-21 P 1068 W1-23 P 532 2.01 — 481500 Comparative Example 22 R1-22 P 830 W1-12 P 727 1.14 no peeling no peeling Example 23 R1-23 P 931 W1-5 P 1055 0.88 no peeling no peeling Example 24 R1-4 P 1135 W1-6 P 621 1.83 no peeling no peeling Example 25 R1-8 P 896 W1-14 P 452 1.98 — 733500 Comparative Example 26 R1-13 P 857 W1-15 P 1028 0.83 1522800 — Comparative Example 27 R1-6 P 1135 W1-16 P 579 1.96 — 688500 Comparative Example 28 R1-22 P 822 W1-17 P 1166 0.70 1458000 — Comparative Example 29 R1-11 P 1006 W1-18 P 502 2.00 — 666000 Comparative Example 30 R1-23 P 931 W1-19 P 1179 0.79 1666800 — Comparative Example 31 R1-4 P 1135 W1-20 P 576 1.97 — 697500 Comparative Example 32 R1-13 P 857 W1-21 P 1221 0.70 1342800 — Comparative Example 33 R1-11 P 1006 W1-22 P 627 1.60 no peeling no peeling Example 34 R1-13 P 857 W1-23 P 580 1.48 no peeling no peeling Example 35 R1-24 P 838 W1-23 P 999 0.84 1386000 — Comparative Example 36 R1-25 P 1144 W1-23 P 583 1.96 — 742500 Comparative Example *P: pearlite, M: martensite.

Example 2

(57) Tests were conducted under the same conditions as in Example 1 except that rail steels having the compositions listed in Table 4 and wheel steels having the compositions in Table 5 were used. Table 6 lists the rail steel and wheel steel combinations used and the evaluation results. 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 YS.sub.R/YS.sub.W satisfy the conditions disclosed herein.

(58) TABLE-US-00004 TABLE 4 Steel Chemical composition of rail steel (mass %)* No. C Si Mn P S Cr Cu Ni Mo V Nb Al W B Ti Remarks R2-1 0.84 0.55 0.55 0.014 0.005 0.79 — — — 0.05 — — — — — Conforming Steel R2-2 0.84 0.51 0.61 0.008 0.004 0.74 — — — 0.30 — — — — — Conforming Steel R2-3 0.84 0.25 1.10 0.006 0.005 0.25 — — — — 0.04 — — — — Conforming Steel R2-4 0.84 0.35 1.05 0.003 0.004 0.29 — — 0.3 — — — — — — Conforming Steel R2-5 0.84 0.55 0.55 0.011 0.005 0.62 0.5 1.0 — — — — — — — Conforming Steel R2-6 0.84 0.25 1.20 0.004 0.005 0.29 — — — — — 0.07 0.20 — — Conforming Steel R2-7 0.84 0.88 0.55 0.005 0.005 0.45 — — — — — — — 0.003 0.05 Conforming Steel R2-8 0.84 0.95 0.56 0.011 0.005 0.79 — — — 0.05 — — — — — Conforming Steel *The balance consists of Fe and inevitable impurities.

(59) TABLE-US-00005 TABLE 5 Steel Chemical composition of wheel steel (mass %)* No. C Si Mn P S Cr Cu Ni Mo V Nb Al W B Ti Remarks W2-1 0.78 0.25 0.80 0.012 0.005 0.25 — — — 0.10 0.05 — — — Conforming Steel W2-2 0.79 0.21 0.75 0.015 0.008 0.20 0.5 1.0 — — — — — — — Conforming Steel W2-3 0.81 0.35 0.78 0.019 0.004 0.28 — — 0.2 — — — — — — Conforming Steel W2-4 0.84 0.33 0.80 0.007 0.009 0.25 — — — 0.20 — — — — — Conforming Steel W2-5 0.78 0.25 0.80 0.012 0.005 0.74 — — — — 0.05 — 0.20 — — Conforming Steel W2-6 0.81 0.27 0.70 0.019 0.007 0.22 — — — — — — — 0.003 0.05 Conforming Steel W2-7 0.84 0.99 0.84 0.008 0.007 0.35 — — — — 0.05 — — — — Conforming Steel W2-8 0.79 0.11 0.82 0.005 0.003 0.29 — 0.10 — 0.05 — — — — — Conforming Steel *The balance consists of Fe and inevitable impurities.

(60) TABLE-US-00006 TABLE 6 Rail Wheel Yield Yield Yield strength Number of revolutions Steel Steel strength Steel Steel strength Ratio when peeling occurred No. No. microstructure* YS.sub.R (MPa) No. microstructure* YS.sub.W (MPa) R/W Rail Wheel Remarks 1 R2-1 P 924 W2-3 P 776 1.19 no peeling no peeling Example 2 R2-2 P 918 W2-8 P 727 1.26 no peeling no peeling Example 3 R2-3 P 871 W2-1 P 716 1.22 no peeling no peeling Example 4 R2-4 P 881 W2-2 P 701 1.26 no peeling no peeling Example 5 R2-5 P 885 W2-7 P 952 0.93 no peeling no peeling Example 6 R2-6 P 896 W2-5 P 849 1.06 no peeling no peeling Example 7 R2-7 P 886 W2-6 P 737 1.20 no peeling no peeling Example 8 R2-8 P 981 W2-4 P 823 1.19 no peeling no peeling Example *P: pearlite, M: martensite.

Example 3

(61) Tests were conducted under the same conditions as in Example 1 except that rail steels having the chemical compositions listed in Table 7 and wheel steels having the compositions in Table 8 were used. In addition, the Vickers hardness H.sub.R of the finally obtained rail steel and the Vickers hardness H.sub.W of the finally obtained wheel steel were measured using a Vickers hardness testing machine with a load of 98 N, and the ratio H.sub.R/H.sub.W of the hardness H.sub.R of the rail steel to the hardness H.sub.W of the wheel steel was determined. Table 9 lists the rail steel and wheel steel combinations used and the evaluation results.

(62) 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 YS.sub.R/YS.sub.W 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 H.sub.R/H.sub.W of the hardness H.sub.R of the rail steel to the hardness H.sub.W 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 YS.sub.R/YS.sub.W 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.

(63) TABLE-US-00007 TABLE 7 Steel Chemical composition of rail steel (mass %)* No. C Si Mn P S Cr Others Remarks R3-1 0.84 0.55 0.55 0.014 0.005 0.79 — Conforming Steel R3-2 0.84 0.95 0.61 0.008 0.004 0.74 — Conforming Steel R3-3 0.80 0.15 1.10 0.006 0.005 0.25 — Conforming Steel R3-4 0.70 0.15 1.05 0.003 0.004 0.29 — Conforming Steel R3-5 0.80 0.55 0.55 0.011 0.005 0.55 — Conforming Steel R3-6 0.84 0.25 1.20 0.004 0.005 0.29 — Conforming Steel R3-7 0.84 0.88 0.55 0.005 0.005 0.51 — Conforming Steel R3-8 0.85 0.90 0.61 0.011 0.004 0.81 — Conforming Steel R3-9 0.85 1.50 0.22 0.015 0.006 1.22 — Conforming Steel R3-10 0.85 0.25 0.81 0.015 0.006 0.25 — Conforming Steel R3-11 0.73 0.50 0.65 0.015 0.012 0.45 — Conforming Steel *The balance consists of Fe and inevitable impurities.

(64) TABLE-US-00008 TABLE 8 Steel Chemical composition of wheel steel (mass %)* No. C Si Mn P S Cr Others Remarks W3-1 0.78 0.25 0.80 0.012 0.005 0.25 — Conforming Steel W3-2 0.79 0.21 0.75 0.015 0.008 0.20 — Conforming Steel W3-3 0.81 0.35 0.78 0.019 0.004 0.28 — Conforming Steel W3-4 0.79 0.99 0.84 0.008 0.007 0.35 — Conforming Steel W3-5 0.69 0.25 0.75 0.012 0.005 0.27 — Conforming Steel W3-6 0.68 0.27 0.70 0.019 0.007 0.22 — Conforming Steel W3-7 0.84 0.33 0.80 0.007 0.009 0.25 — Conforming Steel W3-8 0.79 0.11 0.82 0.005 0.003 0.29 — Conforming Steel W3-9 0.63 0.69 0.81 0.011 0.005 0.39 — Conforming Steel W3-10 0.85 0.39 0.91 0.015 0.006 0.72 — Conforming Steel W3-11 0.75 0.40 0.20 0.021 0.002 0.85 Ni: 0.10 Conforming Steel *The balance consists of Fe and inevitable impurities.

(65) TABLE-US-00009 TABLE 9 Rail Wheel Yield Yield Steel strength Steel Hardness H.sub.R Steel strength Steel Hardness H.sub.W No. No. YS.sub.R (MPa) microstructure* HV No. YS.sub.W (MPa) microstructure* HV 1 R3-1 924 P 412 W3-3 776 P 359 2 R3-2 978 P 429 W3-8 727 P 357 3 R3-3 823 P 371 W3-1 716 P 343 4 R3-4 772 P 346 W3-2 701 P 342 5 R3-5 831 P 386 W3-7 823 P 385 6 R3-6 896 P 403 W3-5 569 P 330 7 R3-7 899 P 406 W3-6 533 P 314 8 R3-8 1008  P 435 W3-4 874 P 400 9 R3-9 1143  P 455 W3-9 584 P 353 10 R3-10 838 P 400 W3-10 998 P 400 11 R3-11 910 P 420 W3-11 880 Tempering M 360 Yield Hardness Number of revolutions strength ratio ratio when peeling occurred No. YS.sub.R/YS.sub.W H.sub.R/H.sub.W Rail Wheel Remarks 1 1.19 1.15 no peeling no peeling Example 2 1.35 1.20 no peeling no peeling Example 3 1.15 1.08 1436400 — Comparative Example 4 1.10 1.01 1080000 — Comparative Example 5 1.01 1.00 no peeling no peeling Example 6 1.57 1.22 — 481500 Comparative Example 7 1.69 1.29 — 472500 Comparative Example 8 1.15 1.09 no peeling no peeling Example 9 1.96 1.29 — 481500 Comparative Example 10 0.84 1.00 1436400 — Comparative Example 11 1.03 1.17 — 1440000  Comparative Example *P: pearlite, M: martensite.

REFERENCE SIGNS LIST

(66) 1 wheel material 2 rail material