Patent Application
20200002780
The disclosure relates to a rail, particularly a high-strength pearlitic rail, and to a method for producing the same. 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 shipping, which is 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 straightening treatment before natural aging is effective to improve the 0.2% proof stress of a pearlitic rail. In particular, rails used in heavy haul railways in mines have a long transport period from the shipping to the laying on foreign and domestic mining sites. Thus, based on the findings that using the period for natural aging is advantageous, we completed the disclosure.
The disclosure is based on the findings described above and has the following primary features.
1. A rail accompanied by a steel material inspection certificate which describes at least a measurement result of a 0.2% proof stress of a head of the rail, 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,
wherein the rail exhibits, at least 90 days after a preparation date of the steel material inspection certificate, an improvement margin of a 0.2% proof stress of 40 MPa or more, relative to the 0.2% proof stress described in the steel material inspection certificate.
2. The 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.
3. 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 100 tf or more; and
preparing a steel material inspection certificate including at least a measurement result of a 0.2% proof stress of a head of the rail within 480 hours after the straightening.
4. The method according to 3, 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 obtains an excellent 0.2% proof stress by aging and thus can be suitably used in heavy haul railways.
In the accompanying drawings:
[Chemical Composition]
Our rail will be specifically explained below. It is important that our rail has the chemical composition described above. The reasons for limiting the chemical composition as described above are explained first. 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 by natural aging. 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 natural aging. 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 has an effect 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. More preferably, the P content is 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, the S content is 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.
[Improvement Margin of a 0.2% Proof Stress of 40 MPa or more]
In the disclosure, it is important that the rail has the aforementioned chemical composition, and additionally exhibits, at least 90 days after a preparation date of a steel material inspection certificate of the rail which describes at least a measurement result of a 0.2% proof stress of a head of the rail, an improvement margin of a 0.2% proof stress of 40 MPa or more, relative to the 0.2% proof stress described in the steel material inspection certificate.
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, straightening treatment and aging treatment under the aforementioned conditions are effective. The 0.2% proof stress at least after the passage of 90 days can be improved by straightening treatment under optimal loads and natural aging for an optimal period
When the improvement margin of a 0.2% proof stress is as small as less than 40 MPa, a plastic flow is easily caused on a surface of the rail, and fatigue layers easily accumulate on a surface of the rail, decreasing the improvement margin of rolling contact fatigue resistance. Therefore, the improvement margin of a 0.2% proof stress is set to 40 MPa or more. As used herein, the “improvement margin of a 0.2% proof stress” can be determined as a difference between a 0.2% proof stress of the rail at a preparation date of a “steel material inspection certificate” after production of the rail, specifically, almost at the time of shipping (hereinafter, referred to as before-aging rail) and a 0.2% proof stress obtained by performing a tensile test on test pieces collected from the rail at least 90 days after the preparation date (hereinafter, referred to as aged rail) (a 0.2% proof stress of the aged rail - a 0.2% proof stress of the before-aging rail).
The improvement margin of a 0.2% proof stress at least after the passage of 90 days is used to evaluate a 0.2% proof stress of the rail which has sufficiently exhibited strain aging, in which C is stuck to strains introduced into the rail in rail straightening, thus improving the 0.2% proof stress.
The steel material inspection certificate describes a result of testing mechanical properties on the rail obtained through a final process of producing the rail. The rail is shipped to a customer attached with the steel material inspection certificate. The rail of the disclosure is attached with the steel material inspection certificate, which describes at least a measurement result of a 0.2% proof stress of a head of the rail. The described measurement result of a 0.2% proof stress is a value obtained in a tensile test on samples collected from a head of the rail. Further, at least 90 days after a preparation date of the steel material inspection certificate, the 0.2% proof stress is improved by 40 MPa or more, relative to the 0.2% proof stress described in the steel material inspection certificate. Thus, considering that there are typically 90 days or more from the shipping of the rail to the rail laying, the rail obtains more improved rolling contact fatigue resistance than expected from the 0.2% proof stress described in the steel material inspection certificate. Even if the time from a preparation date of the steel material inspection certificate to the rail laying is less than 90 days, the rail typically has a service life substantially longer than 90 days. Thus, during use of the rail, 90 days pass from the preparation date of the steel material inspection certificate before rolling contact fatigue occurs in the rail, and thus the 0.2% proof stress is increased, improving rolling contact fatigue resistance.
The improvement margin of a 0.2% proof stress may be measured at least 90 days after a preparation date of the steel material inspection certificate and does not need to be measured in 90 days from a preparation date. In other words, the improvement margin of a 0.2% proof stress may be measured in 90 days from a preparation date, or 1 year or more after a preparation date of the steel material inspection certificate.
The improvement of a 0.2% proof stress of 40 MPa or more may be considered as a safety margin for a predicted value of the rail service life. Alternatively, 40 MPa is further added to a 0.2% proof stress described in the steel material inspection certificate and the obtained value may be used to predict the rail service life.
[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 100 tf or more.
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 100 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 100 tf or more, enough strains can be accumulated in the rail to produce a sufficient improvement margin of a 0.2% proof stress relative a 0.2% proof stress measured within 480 hours after the straightening.
Within 480 hours after the bend straightening of the rail is performed, a steel material inspection certificate is prepared which includes at least the measurement result of a 0.2% proof stress of a head of the rail. When material properties of the rail including a 0.2% proof stress are inspected after a long period of time which causes strain aging has passed after the bend straightening of the rail, the 0.2% proof stress is found to have been increased relative to that inspected immediately after the straightening. Thus, the rail can be shipped as a rail having a high 0.2% proof stress. However, storing the rail in factories for a long period of time after the straightening treatment is impossible because of limitation of the extent of a repository. Therefore, the steel material inspection certificate which describes at least the measurement result of a 0.2% proof stress of a head of the rail is prepared immediately after the bend straightening, that is, within 480 hours after the bend straightening.
A rail made from a steel raw material having the aforementioned chemical composition exhibits an improvement margin of a 0.2% proof stress of 40 MPa or more at least 90 days after a preparation date of the steel material inspection certificate by virtue of natural aging for at least 90 days after the straightening 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 cooling rate of 3° C./s. Subsequently, the cooled rails were subjected to straightening treatment under conditions listed in Table 2.
A tensile test was performed on the obtained rails to measure their 0.2% proof stress, tensile strength, and elongation. Further, a rolling contact fatigue resistance test was performed to measure rolling contact fatigue resistance of the rails. Table 2 also lists these results. The measurement method was as follows. The tensile test was performed between the straightening treatment and the preparation of a steel material inspection certificate. Additionally, the tensile test was also performed in the rails other than No. 1 after natural aging treatment.
[Tensile Test]
For heads of the obtained rails, tensile test pieces were collected from the portion illustrated in
The tensile test was performed within 480 hours after the straightening treatment on test pieces of heads of the rails collected from immediately after (within 480 hours after) the straightening treatment. As to the rails other than No. 1, the tensile test was also performed 90 days after the preparation of a steel material inspection certificate. Further, test pieces were collected from heads of the rails after the rails had gone through natural aging treatment for a natural aging treatment period as listed in Table 2. On the test pieces, a tensile test was performed after time (days) from the preparation of a steel material inspection certificate to the tensile test as listed in Table 2 had passed. Then, the improvement margin (MPa) of a 0.2% proof stress after natural aging treatment was measured, relative to the 0.2% proof stress measured in the tensile test immediately after the straightening treatment.
[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 after natural aging treatment as illustrated in
The wheel material illustrated in
0.69
18
A3
—
Standard
19
34
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 a 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. Table 4 lists conditions of straightening treatment and aging treatment, and measurement results.
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 a 0.2% proof stress and rolling contact fatigue resistance.
0.68
0.86
0.04
1.55
0.34
1.55
0.01
1.55
B11
32
B12
B13
39
B14
B15
30
B16
B17
36
—
Standard
32
39
31
38
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-054992 | Mar 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/011193 | 3/20/2018 | WO | 00 |
