Patent Application
20200277682
The disclosure relates to method for producing a rail, in particular a high-strength pearlitic rail. Specifically, because this kind of rail is used under severe high axle load conditions such as in mining railways which are weighted with heavy freight cars and often have steep curves, the disclosure provides a method for providing a high-strength pearlitic rail having excellent rolling contact fatigue resistance which is suitable for prolonging the rail service life.
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 such a rail used in heavy haul railways, specifically, in railways on which trains and freight cars run with high loading weight, steel having a pearlite structure is conventionally primarily used, from the viewpoint of the importance of rolling contact fatigue resistance. In recent years, however, to increase loading weight on freight cars and improve the efficiency of transportation, there has been demand for further improvement of rolling contact fatigue resistance of rails.
Consequently, there have been made various studies for further improvement of rolling contact fatigue resistance. For example, JP 5292875 B (PTL 1) proposes a rail having excellent wear resistance, rolling contact fatigue resistance, and delayed fracture resistance, the rail having defined ratios of the Mn content and the Cr content, and of the V content and the N content. JP 5493950 B (PTL 2) proposes a method for producing a pearlitic rail having excellent wear resistance and ductility, in which the pearlitic rail has defined contents of C and Cu and is subjected to post heat treatment at heating temperature of 450° C. to 550° C. for 0.5 h to 24 h. JP 2000-219939 A (PTL 3) proposes a pearlitic rail having excellent wear resistance and surface damage resistance, the pearlitic rail having a defined C content and structure and further having a 0.2% proof stress of 600 MPa to 1200 MPa. JP 5453624 B (PTL 4) proposes a pearlite steel rail having a 0.2% proof stress of more than 500 MPa and less than 800 MPa, the pearlite steel rail having defined contents of C, Si, Mn, P, S, and Cr, and a defined sum of contents of C, Si, Mn, and Cr.
PTL 1: JP 5292875 B
PTL 2: JP 5493950 B
PTL 3: JP 2000-219939 A
PTL 4: JP 5453624 B
A rail obtained through hot rolling and accelerated cooling is typically subjected to straightening treatment to eliminate a bend of the rail. In this straightening treatment, the 0.2% proof stress is significantly decreased by the Bauschinger effect. Specifically, to impart straightness to a rail, for example, the rail has to be straightened with a load of 30 tf to 70 tf. When straightening treatment is performed with such a high load, the 0.2% proof stress after the straightening treatment is significantly decreased as compared with before the treatment.
Then, alloying elements need to be added to sufficiently enhance the 0.2% proof stress before straightening treatment of a rail, but adding a large amount of alloying elements rather causes an abnormal structure other than a pearlite structure. Thus, adding more alloying elements than the present level is difficult. Therefore, a decrease in the 0.2% proof stress caused by the Bauschinger effect needs to be prevented by a method other than the addition of alloying elements.
All the techniques described in PTL 1 to PTL 4, however, merely improve the 0.2% proof stress in a stage before a rail is subjected to straightening treatment. Any of the techniques cannot avoid a decrease in the 0.2% proof stress after straightening treatment.
Specifically, the technique described in PTL 1 defines a ratio of the Mn content and the Cr content, and a ratio of the V content and the N content, but the rail loses the 0.2% proof stress in straightening treatment as described above. Thus, the 0.2% proof stress cannot be sufficiently maintained after straightening treatment only by defining the ratio of alloying elements.
PTL 2 proposes to define contents of C and Cu and to perform post heat treatment at heating temperature of 450° C. to 550° C. for 0.5 h to 24 h, but the heating temperature is high only to decrease the 0.2% proof stress because of recovery of dislocation. Thus, the 0.2% proof stress is more decreased after straightening treatment.
The technique described in PTL 3 sets the C content to more than 0.85% and increases the amount of cementite, thus ensuring a high 0.2% proof stress. On the other hand, a decrease in elongation tends to cause cracking, thus making it difficult to ensure rolling contact fatigue resistance.
The pearlite steel rail of PTL 4 has a 0.2% proof stress as low as less than 800 MPa, and actually has difficulties to ensure rolling contact fatigue resi stance.
The disclosure has been developed in light of the above circumstances. It could be helpful to provide a method for achieving a high 0.2% proof stress in a rail after straightening treatment, the high 0.2% proof stress being effective at improving rolling contact fatigue resistance of the rail.
We studied to address this issue, and found that optimizing the chemical composition of a rail, and additionally, properly performing heating treatment after straightening treatment is effective at improving the 0.2% proof stress of a pearlitic rail which has been subjected to straightening treatment. Based on the findings, we completed the disclosure.
The disclosure is based on the findings described above and has the following primary features.
1. A method for producing a rail comprising: hot rolling a steel raw material to obtain a rail, the steel raw material having a chemical composition containing (consisting of), in mass %,
C: 0.70% to 0.85%,
Si: 0.1% to 1.5%,
Mn: 0.4% to 1.5%,
P: 0.035% or less,
S: 0.010% or less, and
Cr: 0.05% to 1.50%
with the balance being Fe and inevitable impurities; straightening the rail with a load of 50 tf or more; and subsequently subjecting the rail to heat treatment in which the rail is held in a temperature range of 150° C. or more and 400° C. or less for 0.5 hours or more and 10 hours or less.
2. The method for producing a rail according to 1., wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
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,
Al: 0.07% or less,
W: 1.0% or less,
B: 0.005% or less, and
Ti: 0.05% or less.
According to the disclosure, it is possible to provide a high-strength pearlitic rail which exhibits an excellent 0.2% proof stress after straightening treatment and thus can be suitably used in heavy haul railways.
In the accompanying drawings:
Our method for producing a rail will be specifically explained below.
First, it is important that a steel raw material to produce a rail has the chemical composition described above. Reasons for limiting the chemical composition as described above are explained for each element. The unit of the content of each component is “mass %”, but it is abbreviated as “%”.
C: 0.70% to 0.85%
C is an element that forms cementite in a pearlite structure and has the effect of improving the 0.2% proof stress in heat treatment after straightening treatment. Therefore, the addition of C is necessary to ensure the 0.2% proof stress in a rail. As the C content increases, the 0.2% proof stress is improved. Specifically, when the C content is less than 0.70%, it is difficult to obtain an excellent 0.2% proof stress after the heat treatment. On the other hand, when the C content is beyond 0.85%, pro-eutectoid cementite is formed at prior austenite grain boundaries, ending up deteriorating rolling contact fatigue resistance of a rail. Therefore, the C content is set to 0.70% to 0.85%, and preferably, 0.75% to 0.85%.
Si: 0.1% to 1.5%
Si is an element that functions as a deoxidizer. Further, Si has an effect of improving the 0.2% proof stress of a rail by solid solution strengthening of ferrite in pearlite. Therefore, the Si content needs to be 0.1 or more. On the other hand, a Si content beyond 1.5% produces a large amount of oxide-based inclusions because Si has a high strength of bonding with oxygen, thus deteriorating rolling contact fatigue resistance. Therefore, the Si content is set to 0.1% to 1.5%, and preferably, 0.15% to 1.5%.
Mn: 0.4% to 1.5%
Mn is an element that improves the strength of a rail by decreasing the transformation temperature of steel to thereby shorten the lamellar spacing. A Mn content less than 0.4%, however, cannot achieve a sufficient effect. On the other hand, a Mn content beyond 1.5% tends to generate a martensite structure by microsegregation of steel, thus deteriorating rolling contact fatigue resistance. Therefore, the Mn content is set to 0.4% to 1.5%, and preferably, 0.4% to 1.4%.
P: 0.035% or less
A P content beyond 0.035% deteriorates ductility of a rail. Therefore, the P content is set to 0.035% or less. On the other hand, the lower limit of the P content is not limited, and may be 0%, although industrially more than 0%. Excessively decreasing the P content causes an increase in refining cost. Thus, from the perspective of economic efficiency, the P content is preferably set to 0.001% or more, and more preferably, 0.025% or less.
S: 0.010% or less
S exists in steel mainly in the form of an A type (sulfide-based) inclusion. A S content beyond 0.010% significantly increases the amount of the inclusions and generates coarse inclusions, thus deteriorating rolling contact fatigue resistance. Setting the S content to less than 0.0005% causes an increase in refining cost. Thus, from the perspective of economic efficiency, the S content is preferably set to 0.0005% or more, more preferably, 0.009% or less.
Cr: 0.05% to 1.50%
Cr is an element that has an effect of improving the 0.2% proof stress by solid solution strengthening of cementite in pearlite. To achieve this effect, the Cr content needs to be 0.05% or more. On the other hand, a Cr content beyond 1.50% generates a martensite structure by solid solution strengthening of Cr, ending up deteriorating rolling contact fatigue resistance. Therefore, the Cr content is set to 0.05% to 1.50%, and preferably 0.10% to 1.50%.
Our rail comprises the aforementioned composition as a steel raw material, with the balance being Fe and inevitable impurities. The balance may be Fe and inevitable impurities, and may further contain the following elements within a range which does not substantially affect the action and effect of the disclosure.
Specifically, the balance may further contain as necessary at least one selected from the group consisting of
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,
Al: 0.07% or less,
W: 1.0% or less,
B: 0.005% or less, and
Ti: 0.05% or less.
V: 0.30% or less
V is an element that has an effect of precipitating as a carbonitride during and after rolling and improving the 0.2% proof stress by precipitation strengthening. Therefore, 0.001% or more of V is preferably added. On the other hand, a V content beyond 0.30% causes the precipitation of a large amount of coarse carbonitrides, thus deteriorating rolling contact fatigue resistance. Therefore, in the case of adding V, the V content is preferably set to 0.30% or less.
Cu: 1.0% or less
As with Cr, Cu is an element that has an effect of improving the 0.2% proof stress by solid solution strengthening. Therefore, 0.001% or more of Cu is preferably added. On the other hand, a Cu content beyond 1.0% causes Cu cracking. Therefore, in the case of adding Cu, the Cu content is preferably set to 1.0% or less.
Ni: 1.0% or less
Ni has an effect of improving the 0.2% proof stress without deteriorating ductility. Therefore, 0.001% or more of Ni is preferably added. In addition, adding Ni along with Cu can prevent Cu cracking. Thus, in the case of adding Cu, Ni is preferably added. On the other hand, a Ni content beyond 1.0% increases quench hardenability to produce martensite, deteriorating rolling contact fatigue resistance. Therefore, in the case of adding Ni, the Ni content is preferably set to 1.0% or less.
Nb: 0.05% or less
Nb precipitates as a carbonitride during and after rolling and improves the 0.2% proof stress of a pearlitic rail. Therefore, 0.001% or more of Nb is preferably added. On the other hand, a Nb content beyond 0.05% causes the precipitation of a large amount of coarse carbonitrides, thus deteriorating ductility. Therefore, in the case of adding Nb, the Nb content is preferably set to 0.05% or less.
Mo: 0.5% or less
Mo precipitates as a carbonitride during and after rolling and improves the 0.2% proof stress by precipitation strengthening. Therefore, 0.001% or more of Mo is preferably added. On the other hand, a Mg content beyond 0.5% produces martensite, thus deteriorating rolling contact fatigue resistance. Therefore, in the case of adding Mo, the Mo content is preferably set to 0.5% or less.
Al: 0.07% or less
Al is an element that is added as a deoxidizer. Therefore, 0.001% or more of Al is preferably added. On the other hand, an Al content beyond 0.07% produces a large amount of oxide-based inclusions because Al has a high strength of bonding with oxygen, thus deteriorating rolling contact fatigue resistance. Therefore, the Al content is preferably set to 0.07% or less.
W: 1.0% or less
W precipitates as a carbonitride during and after rolling and improves the 0.2% proof stress by precipitation strengthening. Therefore, 0.001% or more of W is preferably added. On the other hand, a W content beyond 1.0% produces martensite, thus deteriorating rolling contact fatigue resistance. Therefore, in the case of adding W, the W content is preferably set to 1.0% or less.
B: 0.005% or less
B precipitates as a nitride during and after rolling, and improves the 0.2% proof stress by precipitation strengthening. Therefore, 0.0001% or more of B is preferably added. A B content beyond 0.005% produces martensite, thus deteriorating rolling contact fatigue resistance. Therefore, in the case of adding B, the B content is preferably set to 0.005% or less.
Ti: 0.05% or less
Ti precipitates as a carbide, a nitride, or a carbonitride during and after rolling, and improves the 0.2% proof stress by precipitation strengthening. Therefore, 0.001% or more of Ti is preferably added. On the other hand, a Ti content beyond 0.05% produces coarse carbides, nitrides, or carbonitrides, thus deteriorating rolling contact fatigue resistance. Therefore, in the case of adding Ti, the Ti content is preferably 0.05% or less.
[Producing Conditions]
Next, a method for producing our rail will be described.
Our rail can be produced by making a rail through hot rolling and cooling according to a usual method and subsequently subjecting the rail to straightening treatment with loads of 50 tf or more, and then to heat treatment under predetermined conditions.
The rail is produced by hot rolling, for example, in accordance with the following procedures.
First, steel is melted in a converter or an electric heating furnace and subjected as necessary to secondary refining such as degassing.
Subsequently, the chemical composition of the steel is adjusted within the aforementioned range. Next, the steel is subjected to continuous casting to make a steel raw material such as bloom. Subsequently, the steel raw material is heated in a heating furnace to 1200° C. to 1350° C. and hot rolled to obtain a rail. The hot rolling is preferably performed at rolling finish temperature: 850° C. to 1000° C. and the rail after the hot rolling is preferably cooled at cooling rate: 1° C./s to 10° C./s.
After the cooling following the hot rolling is finished, the rail is subjected to straightening treatment with loads of 50 tf or more to straighten a bend of the rail. The bend of the rail is straightened by passing the rail through straightening rollers disposed in zigzag along the feed direction of the rail and subjecting the rail to repeated bending/bend restoration deformation.
Strains accumulated in the rail by straightening treatment is changed depending on the straightening load and the cross-sectional area of the rail (size of the rail) to be subjected to the straightening treatment. Here, the rail to be used under high axle load conditions which is mainly targeted in the disclosure has a size of 115 lbs, 136 lbs, and 141 lbs in the North America AREMA Standard which has a relatively large cross-section, and a size of 50 kgN and 60 kgN in the JIS Standard. When the rail having such a size is applied with a straightening load of 50 tf or more, enough strains can be accumulated in the rail to sufficiently improve a 0.2% proof stress after heat treatment.
After the straightening treatment, it is important to perform heat treatment in which a rail is held in a temperature range of 150° C. or more and 400° C. or less for 0.5 hours or more and 10 hours or less. Specifically, when the holding temperature is less than 150° C. or more than 400° C., improvement margins of a 0.2% proof stress and rolling contact fatigue resistance are decreased. Further, when the holding time in the temperature range is less than 0.5 hours or more than 10 hours, improvement margins of a 0.2% proof stress and rolling contact fatigue resistance are decreased. For the heat treatment, a furnace or a high-frequency heat treatment device can be used.
By subjecting a rail made from a steel raw material having the aforementioned chemical composition to the aforementioned heat treatment after the straightening treatment, a 0.2% proof stress after the heat treatment is improved by 40 MPa or more relative to a 0.2% proof stress before the heat treatment.
Specifically, to improve rolling contact fatigue resistance of the rail, the 0.2% proof stress of the rail needs to be improved to limit a plastic deformation area as much as possible. The 0.2% proof stress can be improved by adding alloying elements, which, however, rather deteriorates rolling contact fatigue resistance of the rail by the generation of an abnormal structure such as martensite. To prevent the generation of an abnormal structure and improve the 0.2% proof stress, heat treatment under the aforementioned conditions is effective. The 0.2% proof stress can be improved by performing optimal heat treatment.
As used herein, the “improvement margin of a 0.2% proof stress” can be determined as a difference between 0.2% proof stresses obtained in tensile tests before and after aging and heat treatment (a 0.2% proof stress after aging and heat treatment—a 0.2% proof stress before aging and heat treatment).
Steel raw materials (bloom) having a chemical composition listed in Table 1 were hot rolled to obtain rails having a size listed in Table 2. At that time, the heating temperature before the hot rolling was 1250° C., and the delivery temperature was 900° C. The hot-rolled rails were cooled to 400° C. at an average rate of 3° C./s. Subsequently, the cooled rails were subjected to straightening treatment under conditions listed in Table 2, and then to heat treatment under conditions listed in Table 2. The rails of Comparative Examples of No. 1 and No. 2 were not subjected to heat treatment.
A tensile test was performed on each obtained rail to measure its 0.2% proof stress, tensile strength, and elongation. Further, a rolling contact fatigue resistance test was performed to measure rolling contact fatigue resistance of each rail. The measurement method was as follows.
[Tensile Test]
For heads of the obtained rails, tensile test pieces were collected from the portion illustrated in
The tensile test was performed on test pieces of heads of the rails collected from immediately after the straightening treatment. For rails of No. 1 and No. 2, the tensile test was also performed on test pieces of heads of the rails collected 10 hours after the straightening treatment without the heat treatment. For the other rails than those of No. 1 and No. 2, the tensile test was also performed on test pieces of heads of the rails collected after the heat treatment under heat treatment conditions listed in Table 2.
[Rolling Contact Fatigue Resistance]
Rolling contact fatigue resistance was evaluated using a Nishihara type wear test apparatus and simulating actual contact conditions between a rail and a wheel. Specifically, cylinder test pieces having a diameter of 30 mm (an outer diameter of 30 mm and an inner diameter of 16 mm) with a contact surface being a curved surface having a radius of curvature of 15 mm were collected from heads of the rails as illustrated in
The wheel material illustrated in
The rail of Comparative Example No. 1 in Example 1 was an actually-used pearlitic rail having the C content of 0.81%. As seen from the results listed in Table 2, rails of Examples according to the disclosure had a more excellent 0.2% proof stress than the rail of Comparative Example No. 1 by 40 MPa or more and exhibited an improvement margin of rolling contact fatigue resistance of 10% or more. On the other hand, the rails of Comparative Examples which did not satisfy the conditions of the disclosure were inferior in at least one of 0.2% proof stress, elongation, and rolling contact fatigue resistance.
Rails were made in the same procedures as in Example 1 other than using steel having a chemical composition listed in Table 3. A tensile test and measurement of rolling contact fatigue resistance were performed on the rails in the same way as in Example 1. Heat treatment conditions and the measurement results are presented in Table 4.
As seen from the results listed in Table 4, the rails of Examples satisfying the conditions of the disclosure had a more excellent 0.2% proof stress than the rail of Comparative Example No. 1 by 40 MPa or more and exhibited an improvement margin of rolling contact fatigue resistance of 10% or more. On the other hand, the rails of Comparative Examples which did not satisfy the conditions of the disclosure were inferior in at least one of 0.2% proof stress and rolling contact fatigue resistance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-054989 | Mar 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/011191 | 3/20/2018 | WO | 00 |
