JP2002363701A - Pearlitic rail - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pearlitic rail which has excellent ductility, and has excellent wear resistance and rolling fatigue damage resistance. SOLUTION: In the pearlitic rail, at least a part of the head has a pearlitic structure essentially consisting of lamellas consisting of ferrite and cementite. The interval between the lamellas in the pearlitic structure is <=150 nm, and also, the grain size of pearlitic colonies is <=50 μm. The rail has chemical components containing, by mass, 0.75 to 0.84% C, <=1% Si, 0.4 to 2.5% Mn, <=0.035% P and <=0.035% S, and the balance substantially Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

2. Description of the Related Art In a railway mainly transporting heavy goods such as ores, the load applied to the wheel axle of a vehicle is much larger than that of a general railway passenger car. Conventionally, pearlite steel has been mainly used for rail steel used in railways from the viewpoint of emphasizing wear resistance. In particular, in recent years, as the loading capacity of freight cars has increased and the amount of wear on the rail head has increased, further improvement in wear resistance has been required, and the C content of pearlite steel has been increased to 0.85% or more. Attempts have been made to increase the cementite fraction in steel to improve wear resistance. JP-A-8-109439 and JP-A-8-144016 disclose that the C content is 0.85% or more.
1.20% high wear-resistant pearlitic rail
JP-A-246100 and JP-A-8-246101 disclose a technique for a pearlite-based rail having a C content of 0.85% to 1.20% and heat-treated on the rail head and having excellent wear resistance. ing.

[0002]

Generally, rails are caused by thermal expansion and contraction due to a temperature difference between summer (sometimes 80 ° C or more) and winter (sometimes -30 ° C or less). It is known that a thermal stress is applied and a break may occur. In particular, when the rail is poor in ductility, the rail is likely to break, which may cause an accident. Further, in recent years, long rails having a length of 200 to 1500 m obtained by welding a plurality of rails have been frequently used, and the amount of expansion and contraction at seams at rail ends has been further increased. For this reason, rails are required to be more ductile than ever before.

[0003] On the other hand, in a severe use environment such as a high axle heavy railway, the rails are severely worn, so that further improvement in wear resistance is desired. However, as described above, in order to improve wear resistance, when the C content of pearlite steel is increased,
Accordingly, the ductility is greatly reduced, so that it is difficult to improve the wear resistance and at the same time to improve the ductility. There is also a method of reducing the wear of the rail by applying a lubricant such as oil without demanding the wear resistance of the rail itself. Due to the contact pressure and the plastic flow of the rail surface caused by sliding, the occurrence of rolling fatigue damage called flaking has become a problem.

Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a pearlite-based rail which is excellent in ductility and excellent in wear resistance and rolling contact fatigue damage resistance.

[0005]

The features of the present invention for solving such a problem are as follows.

(1) A pearlite-based rail having a pearlite structure mainly composed of lamellar composed of ferrite and cementite at least a part of a rail head,
A pearlite-type rail, wherein a lamella spacing in a pearlite structure is 150 nm or less, and a particle size of a pearlite colony is 50 μm or less.

(2) The chemical composition of the rail is C:
0.75-0.84%, Si: 1% or less, Mn: 0.4-2.5%, P:
The pearlitic rail according to (1), wherein the pearlite rail contains 0.035% or less, S: 0.035% or less, and the balance substantially consists of Fe.

(3) Nb: 0.03-0.5% by mass%
And / or V: 0.03 to 0.5% of the pearlite-based rail according to (2),

(4) Further, Cr: 1.5% or less by mass%;
The pearlite rail according to (2) or (3), comprising one or more selected from Cu: 1% or less, Ni: 1% or less, and Mo: 1% or less.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. The rail of the present invention has improved abrasion resistance, rolling contact fatigue damage resistance, and ductility more than conventional hypoeutectoid, eutectoid, and hypereutectoid pearlite rails by refinement of the pearlite structure. FIG. 1 shows a schematic diagram of the structure of the pearlite structure of the rail of the present invention. As shown in FIG. 1, the pearlite structure is an aggregate of a lamella structure in which ferrite 1 and cementite 2 form a layered (lamellar) structure, and the lamella structure is one unit (pearlite colony 3).
In the present invention, in such a pearlite structure, the lamellar spacing a is made finer, and the wear resistance and the rolling contact fatigue damage resistance are improved. It is intended to improve the ductility and ductility. Here, perlite colony 3
Is defined as a circle diameter equivalent to the area of pearlite colonies, and the average particle diameter of all pearlite colonies is used. Hereinafter, the particle size of the pearlite colony 3 is referred to as pearlite colony size or colony size.

The rail of the present invention has a pearlite structure in which at least a part of the rail head is mainly composed of a lamellar structure composed of ferrite and cementite as described above. However, both the lamella spacing and the colony size need to be reduced in order to significantly improve the wear resistance and the rolling fatigue resistance. Conventionally used par light rail (0.68% C
In order to improve wear resistance by at least 20% and rolling fatigue damage resistance by at least 25% compared to heat-treated pearlite steel, the lamellar spacing of pearlite colonies should be 150 nm or less and the colony size should be 50 μm or less. I do. It is not clear why the wear resistance and rolling contact fatigue damage resistance are improved by the refinement of the lamella spacing and colony size, but the relaxation of stress concentration at the pearlite colony boundary, the generation of local slip due to the refinement of the lamella spacing, etc. A mechanism that suppresses the occurrence of cracks and peeling is conceivable. In addition, ductility is improved until the elongation (El) of 10% or more can be surely ensured by the fine structure.

The pearlite-based rail of the present invention has a lamellar interval of 150 nm or less and a pearlite colony size of 50 nm.
Although it has a pearlite structure mainly composed of lamellar which is not more than μm, at least a part of the rail head has a pearlite structure mainly composed of lamella having a lamella interval of 150 nm or less, and a pearlite colony size of 50 μm or less. It is effective if it has a pearlite structure mainly composed of a lamella having a lamellar interval of 150 nm or less, and a pearlite colony size of 50 μm or less, at least at the top of the rail where the wheels contact the rail head. Preferably, there is. It is more effective if the entire rail head has the above-mentioned structure, but it is also effective if only the surface of the rail head has the above-mentioned structure. Also, the entire rail is very effective if the lamellar interval is 150 nm or less, and the pearlite structure mainly composed of lamellar colonies with a pearlite colony size of 50 μm or less,
Manufacturing costs rise. However, safety is improved because the entire rail has a uniform structure. When only the surface layer of the entire rail has the above-described structure, the safety is appropriately improved, and the increase in manufacturing cost is suppressed.

The pearlite structure of the rail according to the present invention is mainly composed of a lamellar structure composed of ferrite and cementite as described above. When the volume fraction of the structure other than the lamellar structure is low, the effect is negligible. Therefore, it may contain another metal structure of 3% or less in total volume fraction, for example, ferrite or the like.

In order to obtain the pearlite structure of the present invention,
Rail is mass% C: 0.75-0.84%, Si: 1% or less, M
It is preferable that n: 0.4 to 2.5%, P: 0.035% or less, S: 0.035% or less, and the balance have a chemical component substantially consisting of Fe. The reasons for limiting each component are described below.

C: It is preferable to be 0.75 to 0.84%. C
Is an essential element for forming cementite in the pearlite structure and ensuring wear resistance, and the wear resistance improves with an increase in the amount of addition. If it is less than 0.75%, it is difficult to obtain excellent wear resistance as compared with conventional heat-treated pearlite steel rails. On the other hand, if the content exceeds 0.85%, proeutectoid cementite is formed at the γ grain boundary during transformation after hot rolling, and the ductility is significantly reduced. Therefore, the C content is preferably set to 0.75 to 0.84%.

Si: It is preferable that the content be 1% or less. Si
Is effective as a deoxidizing agent, but if it exceeds 1%, the weldability deteriorates due to the high bonding force of Si with oxygen. Therefore, the amount of Si is preferably set to 1% or less.

Mn: 0.4 to 2.5% is preferable. Mn
Is an element that contributes to increasing the strength and ductility of the rail by reducing the transformation temperature and reducing the lamella spacing of the pearlite structure. However, if the content is less than 0.4%, a sufficient effect cannot be obtained. If the content exceeds 2.5%, a martensite structure is liable to occur due to micro-segregation of the steel, and hardening or embrittlement occurs during heat treatment and welding, which is not preferable because the material is deteriorated.
Therefore, the amount of Mn is preferably set to 0.4 to 2.5%.

P: It is preferable to be 0.035% or less. 0.
Addition of P exceeding 035% deteriorates ductility. Therefore,
The P content is preferably set to 0.035% or less.

S: It is preferred to be 0.035% or less. S
Is mainly present in the form of inclusions in the steel, but if the content exceeds 0.035%, the amount of the inclusions increases significantly and causes embrittlement of the material. Therefore, it is preferable to set the content to 0.035% or less.

The other components are substantially Fe. Substantially Fe means that the substance containing other trace elements including unavoidable impurities can be included in the scope of the present invention, as long as the effects of the present invention are not lost.

In order to obtain the pearlite structure of the present invention, it is more effective to add the following elements in addition to the above chemical components.

Nb: 0.03-0.5% and / or V: 0.03-
Add 0.5%. Both Nb and V are precipitated as carbides during and after rolling in association with C in the steel, and effectively act on the refinement of the pearlite colony size,
It also precipitates in ferrite forming lamellars and contributes to making the apparent lamellar spacing finer. As a result, wear resistance,
It greatly improves rolling fatigue resistance and ductility, and greatly contributes to extending the life of rails. However, if the added amount of these elements is less than 0.03%, a sufficient effect cannot be obtained. Also 0.5
%, Wear resistance, rolling contact fatigue damage resistance,
The effect of improving ductility saturates, and an effect commensurate with the added amount cannot be obtained. Therefore, the addition amount of Nb and V is preferably set to 0.03 to 0.5%, and Nb and / or V is added as needed.

One or more selected from the group consisting of Cr: 1.5% or less, Cu: 1% or less, Ni: 1% or less, and Mo: 1% or less are added as necessary.

Cr: 1.5% or less is preferable. Cr
Is an element for achieving higher strength by solid solution strengthening. However, if it exceeds 1.5%, the hardenability increases,
Martensite is formed, and wear resistance and ductility decrease.
Therefore, when added, the Cr content is preferably set to 1.5% or less.

Cu: 1% or less is preferable. Cu is C
Like r, it is an element for achieving higher strength by solid solution strengthening. However, if it exceeds 1%, Cu cracks occur. Therefore, when added, the Cu content is preferably set to 1% or less.

Ni: 1% or less is preferable. Ni is an element for increasing the strength without deteriorating the ductility. In addition, in order to suppress Cu cracking by adding Cu in combination, it is desirable to add Ni when Cu is added. However, if added over 1%, the hardenability increases, martensite is generated, and the wear resistance and rolling contact fatigue damage decrease. Therefore, when adding, the amount of Ni is preferably set to 1% or less.

Mo: 1% or less is preferable. Mo is an element for achieving higher strength by solid solution strengthening. However, if it exceeds 1%, a bainite structure is likely to be generated, and the wear resistance is reduced. Therefore, when adding, it is preferable that the amount of Mo is 1% or less.

In order to obtain a rail having a pearlite structure according to the present invention, any manufacturing method can be used. However, steel having the above-mentioned components is heated to 1200 ° C. or less, and the rolling reduction at 1000 ° C. or less is reduced to 40%. %, The rolling finish temperature is 850 ° C or less, and it is rolled into a rail shape, and after rolling, the rail head is cooled at a cooling rate of 2 ° C / s or more to 650 ° C or less, and thereafter, it can be cooled and manufactured. preferable. The higher the cooling rate, the smaller the lamellar interval, which is preferable. In order to reduce the pearlite colony size, it is effective to lower the rolling finishing temperature.

[0029]

EXAMPLES In Examples 1 and 2 below, the pearlitic rails were evaluated for wear resistance, rolling contact fatigue damage resistance, and ductility. For the evaluation of wear resistance and rolling contact fatigue damage resistance, instead of evaluating the wear amount and damage occurrence behavior in actual laying, a comparative test simulating actual contact conditions was performed using a Nishihara type abrasion tester .

Regarding the abrasion, a Nishihara type abrasion test piece having a diameter of 30 mm was taken from the rail head, and the abrasion amount after 100,000 rotations was measured under dry conditions under a contact pressure of 1.4 GPa, a slip ratio of -10%. . The content of C currently used as a rail is 0.6
8% (0.68% C) heat-treated pearlite steel rail is used as a standard for wear and compared with this heat-treated pearlite steel rail.
When a value with a small amount of wear of 20% or more was obtained, it was determined that the wear resistance was sufficiently improved.

With respect to rolling fatigue damage, a 30 mm diameter Nishihara type abrasion test piece having a contact surface curved with a radius of curvature of 15 mm was taken from the rail head, and the contact pressure was 2.2 GPa and the slip ratio was −
The specimen surface was observed every 25,000 revolutions under oil lubrication conditions of 20%, and the point at which a crack of 0.5 mm or more occurred was defined as the rolling fatigue life, compared with a 0.68% C heat-treated pearlite steel rail. 25
When the rolling fatigue life was increased by more than%, it was judged that the rolling fatigue damage resistance was sufficiently improved.

Regarding ductility, AREMA (American Railway
(Engineering & Maintenance of Way Association: Association of North American railway companies) As a standard of ductility to suppress breakage due to expansion and contraction of end joints on long rails, a tensile test using an ASTM round bar test piece (parallel part: 9φ, gauge length: 36 mm) showed an elongation of 10% or more. Yes (AREMA standard chapter
er 4 Rail) was judged to be sufficiently ductile.

Example 1 A pearlite steel (steel types A and B) having the chemical components shown in Table 1 was rolled and cooled under the conditions shown in Table 2 to produce a rail. Cooling was performed only on the rail head, and after cooling was stopped, it was allowed to cool. As a result, it was possible to manufacture rails in which the lamella spacing of the pearlite structure on the rail head and the pearlite colony size were changed (Nos. 1-1 to 1-12).

[0034]

[Table 1]

[0035]

[Table 2]

Each of the rail heads was photographed using a scanning electron microscope at a magnification of 400 to 2000 times and about 10 fields of view. Using the obtained photographs, in each visual field, 50
After selecting individual pieces and measuring the area of each pearlite colony with an image processing apparatus, the average area of the pearlite colonies was converted into the diameter of a circle corresponding to the area to determine the particle size (colony size) of the pearlite colonies. The lamella interval was also measured at the same time, and the average value was determined from the value for each visual field. For these rails,
The above-mentioned wear test, rolling fatigue test and tensile test were performed. Lamella spacing of each rail head, colony size, wear amount,
Table 3 shows the measurement results of rolling fatigue life and ductility.

[0037]

[Table 3]

No. 1-1 is a conventionally used heat-treated pearlite steel rail as a reference for comparison, which has a wide lamellar interval and a coarse colony size.
No.1-1 has a wear amount of 0.75 g and a rolling fatigue life of 1.5 h. As described above, if the wear amount is 0.6 g or less and the rolling fatigue life is 1.88 h or more, sufficient wear resistance is obtained. It was determined that the rolling contact fatigue damage resistance was improved.

No. 1 coarse in both lamellar spacing and colony size
-2, 3, and 4 showed little improvement in wear resistance and rolling contact fatigue damage resistance. On the other hand, No.1-5, 6, 7, and 8 with only small lamellar spacing or colony size have slightly improved abrasion resistance and rolling fatigue resistance compared to the standard No.1-1. However, no improvement of 20% or more in wear resistance and 25% or more in rolling contact fatigue damage resistance was observed. Nos. 1-9, 10, 11, and 12, in which both the lamella spacing and the pearlite colony size were within the scope of the present invention, had improved wear resistance and rolling fatigue damage resistance, respectively. The ductility was sufficient for all rails.

FIGS. 2 and 3 summarize these results. FIG. 2 shows the effect of the lamella spacing on the wear / rolling fatigue characteristics, and FIG. 3 shows the effect of the colony size on the wear / rolling fatigue characteristics. Show the effect. The dotted lines in Fig. 2 and Fig. 3 show the wear amount reduced by 20% and the rolling fatigue life improved by 25% compared with the standard 0.68% C heat-treated pearlite steel rail. It is a criterion for improving the performance. According to FIG. 2, it can be seen that the wear resistance and the rolling contact fatigue damage resistance are both improved by reducing the lamellar interval, but it is necessary to sufficiently improve the wear resistance and the rolling contact fatigue damage resistance. It was found that it was necessary to use a rail having a colony size of 50 μm or less in a region with a lamellar interval of 150 nm or less and further indicated by a black circle (●). FIG. 3 shows that the wear resistance and the rolling contact fatigue damage resistance are improved with the miniaturization of the colony size. However, the colony is not sufficiently improved in the wear resistance and the rolling contact fatigue damage resistance. It was found that it is necessary to use a rail having a lamellar interval of 150 nm or less, which is indicated by a black circle (●), in a region having a size of 50 μm or less. By setting the lamellar interval to 150 nm or less and the colony size to 50 μm or less, the abrasion resistance is improved by 20% or more and the rolling contact fatigue damage resistance is improved by 25% or more as compared with the pearlite rail conventionally used as a standard. . Therefore, refining only the lamellar spacing or colony size does not provide a significant improvement in wear resistance and rolling contact fatigue damage resistance. It was found that the rolling fatigue resistance was significantly improved.

Example 2 Test steels (Nos. 2-1 to 15) having different chemical components shown in Table 4 were heated to 1100 ° C., rolled at 800 ° C., and rails were manufactured. 3 ° C / s head
After air-cooling to 350 ° C., the mixture was allowed to cool. No.2-1 is
It is a conventional steel rail which is a reference for comparison similarly to No. 1-1 of Example 1, and only this is heated at 1250 ° C, and after rolling at 1000 ° C, air up to 500 ° C at 2 ° C / s. It was cooled and allowed to cool to produce. The microstructure of these rails was observed, and the wear properties, rolling fatigue properties, and ductility were measured. Table 5 shows the results.

[0042]

[Table 4]

[0043]

[Table 5]

In No. 2-2 having a C content of more than 0.85%, proeutectoid cementite was formed and ductility was significantly reduced. Mn
In No. 2-4 having a content exceeding 2.5%, martensite is formed, and all of the wear characteristics, rolling fatigue characteristics, and ductility are deteriorated. No.2- with Nb or V content exceeding 0.5%
6, No. 2-7, Nb, V content is within the scope of the present invention No. 2-8, N
No improvement compared to o.2-9. In No.2-10 with Cr content exceeding 1.5%, martensite was generated,
All of the wear characteristics, rolling fatigue characteristics, and ductility are deteriorated. In No. 2-12 in which the Ni content exceeds 1%, martensite is formed and the wear characteristics are deteriorated. In No. 2-14 in which the Mo content exceeds 1%, bainite is formed and the wear characteristics are significantly reduced. On the other hand, each additive element is within the scope of the present invention No. 2-3, 2-5, 2-8, 2-9, 2-11, 2-13, 2-15 microstructure is pearlite, It is mainly composed of lamellar consisting of ferrite and cementite with a lamellar spacing of 150 nm or less and a pearlite colony size of 50 μm or less, and has a wear resistance of at least 20% compared to the standard No.2-1 and rolling resistance. 25% or more improvement in fatigue damage and ductility
It showed a value of 10% or more.

[0045]

As described above, the pearlitic rail of the present invention exhibits excellent wear resistance, rolling fatigue resistance, and ductility, which are suitable mainly for high-axle heavy railways. The service life and safety of the rail can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the structure of a pearlite structure.

FIG. 2 is a graph showing the effect of lamellar spacing on wear and rolling fatigue characteristics.

FIG. 3 is a graph showing the effect of colony size on wear and rolling fatigue characteristics.

[Explanation of symbols]

 1, ferrite 2, cementite 3, pearlite colony a, lamella spacing

Claims (4)

[Claims] 1. A pearlite-type rail having a pearlite structure mainly composed of a lamella composed of ferrite and cementite at least a part of a rail head, wherein the lamella interval in the pearlite structure is 150 nm or less and pearlite colonies are formed. A perlite rail having a diameter of 50 μm or less. 2. The chemical composition of the rail is C: 0.75% by mass.
0.84%, Si: 1% or less, Mn: 0.4 to 2.5%, P: 0.035
%, S: 0.035% or less, with the balance being substantially Fe
The perlite rail according to claim 1, wherein the rail comprises: 3. The pearlite rail according to claim 2, further comprising Nb: 0.03 to 0.5% and / or V: 0.03 to 0.5% by mass. 4. The composition according to claim 1, further comprising:
4. The pearlite-based rail according to claim 2, wherein one or two or more selected from u: 1% or less, Ni: 1% or less, and Mo: 1% or less are contained.