JPH05271871A - Rail excellent in surface scratching resistance - Google Patents

Abstract

PURPOSE:To provide a high strength rail having a bainitic structure effective for improving the rolling fatigue scratching resistance of the surface of a rail head required under the high speed running conditions of a passenger railroad or heavy load conditions in foreign countries. CONSTITUTION:The objective steel rail is constituted of steel contg., by weight, 0.15 to 0.45% C, 0.15 to 1.20% Si, 0.30 to 1.00% Mn, 2.00 to 4.00% Cr and 0.20 to 0.60% Mo or furthermore contg. one or >=two kinds among Nb, V and Ti and having a bainitic structure as well as >=370Hv hardness after rolling. The bainitic steel rail is one in which wear is increased more than that of the conventional rail for preventing the generation of dark spot scratching on the surface of a linear rail head over the interval of high speed running as well as the formation of flaking scratching over the interval of a slow curved line by imparting high strength thereto, and by laying it over the high speed interval of the Shinkansen(super-express railways) and conventional lines, the rail service life can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is effective in improving rolling fatigue damage resistance of a rail head surface required under high-speed operating conditions of passenger railways or under heavy load conditions such as overseas mining railways. The present invention relates to a high strength rail having a bainite structure.

[0002]

2. Description of the Related Art In recent years, as a means of improving the efficiency of rail transportation, the weight of cargo is increased or the speed of transportation is increased. Such efficient rail transportation means that the rail environment is severe, and further improvement in rail material has been required. Specifically, the wear of the rail laid in the sharp curve section increases rapidly, and fatigue damage frequently occurs from inside the rail gauge corner (GC) where the wheel flange is strongly pressed. The following methods have been adopted as countermeasures. An as-rolled alloy steel rail containing a large amount of alloy elements such as Cr and Mo (see Japanese Patent Laid-Open No. 50-140316). Heat-treated rails produced by accelerating cooling of the rail head or the whole without adding alloy (Japanese Patent Publication No.
(See JP 5-23885). A low alloy heat treatment rail (see Japanese Patent Publication No. 59-19173) in which not only wear resistance and damage resistance but also a decrease in hardness of a welded portion is improved by adding an alloy having a relatively low content. A characteristic of these rails is a high-strength rail having a fine pearlite structure made of high carbon content steel, and the purpose thereof is to improve wear resistance and internal fatigue damage resistance.

On the other hand, in rails in straight and gentle curve sections where wear or internal fatigue damage does not pose a problem, rolling fatigue damage accumulates on the rail head surface due to repeated wheel contact, resulting in peeling damage or fatigue cracks generated from the rail surface. However, it splits inside the rail head, causing lateral crack damage. This typical damage is called "crown shelling" or "dark spot damage" that occurs mainly in straight sections of high-speed railways such as the Shinkansen. Conventionally, as-rolled plain carbon steel rails have been used as straight or gently curved rails, but most of them are pearlite structure rails of high carbon content steel.

[0004]

Rolling fatigue damage originating from the rail head surface is generated on a rail in a straight or gentle curve section of a passenger railway mainly after a certain specific period (train passing tonnage) has passed. The cause is
It is considered that the fatigue damage caused by repeated contact of wheels occurs on the rail head surface.
As a countermeasure for this, a method of periodically grinding the rail head surface with a grinder etc. is considered, but there are problems that the grinding wheel is expensive and that the grinding time cannot be taken sufficiently due to the travel distance of the train. There is. Another possible solution is to improve the wear rate of the rail head surface and eliminate fatigue damage by wear.
The wear of conventional rail steel is dominated by the hardness of the pearlite structure rail material, and in order to promote wear,
It is only necessary to reduce the hardness of the rail steel. However, if the hardness is simply reduced, the rail head is plastically deformed, and cracks and delamination are generated due to the plastic deformation. Even if plastic deformation does not occur, a plastic flow occurs on the rail head surface due to the contact of the wheels, resulting in the formation of microcracks along the plastic deformation of the metal structure, and eventually continuous delamination damage on the rail head surface. Occur. That is,
Up to now, the pearlite structure that has been used as rail steel is a layered structure of a low hardness ferrite structure and a plate-like hard cementite structure, and a soft ferrite structure is squeezed out on the track surface through which the wheel passes. Only hard cementite is piled up, and work hardening is added to ensure wear resistance. However, on the track surface, a laminar structure flow occurs inward of the rail, and a crack is generated along the flow.

On the other hand, in a bainite structure steel having a carbon content lower than that of the pearlite structure and a bainite structure due to a large alloy content, a fine ferrite cementite is dispersed in a soft ferrite structure ground, When the wheel is running, cementite is easily picked up together with the ferrite material, and the flow of the structure that is the starting point of the crack is small, and the wear progresses shortly before the accumulation of fatigue damage that causes the crack. However, in a bainite structure steel with a small amount of alloy addition, since the ferrite base is large and the distribution of granular cementite is coarse, a flow of a structure opposite to the traveling direction of the wheel occurs just below the running raceway surface of the wheel, and microcracks are generated. It is easy to generate.

[0006] In view of such a situation, the present invention promotes wear without lowering rail hardness, picks up rolling fatigue damage by wear, and prevents metal plastic flow from being generated. It is intended to provide a high strength rail having resistance.

[0007]

In order to achieve the above object, the present invention provides C: 0.15 to 0.45%, Si: 0.1
5 to 1.20%, Mn: 0.30 to 1.00%, Cr:
2.00 to 4.00%, Mo: 0.20 to 0.60%, and optionally one or more of Nb, V, and Ti, consisting of bainite structure, It is a high-strength rail excellent in surface damage resistance, characterized by having a hardness of Hv 370 or more.

The present invention will be described in detail below. The present invention can be applied to a converter or an electric furnace with C: 0.15 to 0.4.
5%, Si: 0.15 to 1.20%, Mn: 0.30
1.00%, Cr: 2.00 to 4.00%, Mo: 0.
20 to 0.60%, or Nb: 0.
01 to 0.05%, V: 0.05 to 0.30%, Ti:
Steel containing 0.01 to 0.05% of one kind or two kinds or more, the balance of which is iron and unavoidable impurities is melted and hot-rolled into a rail shape.

Among these chemical steel components, C is an essential element for ensuring a certain hardness, and if it is 0.15% or less, it becomes difficult to secure the hardness as rail steel at low cost. If it is 0.45% or more, a pearlite structure harmful to rolling fatigue damage accumulation is generated, so 0.15% is used.
Limited to ~ 0.45%.

Si is an element that increases the strength by forming a solid solution in the base material in the bainite structure.
%, It is not possible to expect an increase in strength, and if it exceeds 1.20%, not only problems of surface defects during rail manufacturing, inconvenience to bondability during rail welding, but also material embrittlement occur. It was limited to 15 to 1.20%.

Similar to C, Mn is an element that inexpensively increases the rail hardness, but if it is less than 0.3%, its effect is small, and if it exceeds 1.00%, a pearlite structure harmful to rolling fatigue damage accumulation is formed. 0.3 because it is generated
It was limited to 0 to 1.00%.

[0012] Cr is an important element for finely dispersing cementite in the bainite structure to secure the strength, but if it is less than 2.00%, the dispersion of cementite in the bainite structure becomes coarse and the plastic flow of the metal structure is caused. Is no longer effective in preventing On the other hand, if it is 4.00% or more, not only coarsening of carbide occurs but also hardness increases, which makes it difficult to process the rail. Therefore, the additive component range of Cr is 2.00 to 4.
Limited to 00%.

Mo is an element indispensable for stabilizing the bainite structure, but if it is less than 0.20%, its effect is not sufficient, and if it is 0.60% or more, a hard and brittle martensite structure is present in the bainite structure. Will be generated. Therefore, the Mo additive component range is limited to 0.30 to 0.60%.

Further, according to the present invention, the toughness and ductility of the rail steel can be ensured by adding the austenite grain refiner elements such as Nb, V and Ti. At this time, the effective amount of Nb added is 0.01% or more, and if it exceeds 0.05%, an intermetallic compound of Nb is formed, which rather causes embrittlement. Therefore, the component range of Nb is limited to 0.01 to 0.05%.

V shows a tendency similar to that of Nb, but V
The bainite structure can be strengthened by the precipitation of (C, N), and this precipitation hardening does not contribute to wear and is therefore effective in generating surface damage due to the metal plastic flow. The lower limit of addition that can be expected for this effect is 0.05%.
When the content exceeds V, addition of V causes embrittlement due to coarsening of V (C, N). For this reason, the addition amount of V is set to 0.
It was limited to 05 to 0.30%.

By utilizing the fact that precipitated Ti (C, N) does not melt even at high temperatures, Ti contributes to the refinement of austenite crystal grains during rolling and heating of the rail. But 0.0
If it is less than 1%, its effect is small, and if it exceeds 0.05%, coarsening of TiN mainly occurs, which becomes a nucleus of fatigue damage inside the rail head and is harmful. Therefore, the Ti component range is 0.0
It was limited to 1 to 0.05%.

Next, the reason why the high strength bainite structure of Hv370 or higher is used as an important constituent element in the present invention will be described. First, in ordinary carbon steel rails with a conventional pearlite structure, "dark spot damage" starting from the top of the rail in the straight-line high-speed operation section is generated after a certain number of tons of train passage, and the rail hardness at this time Is Hv2
It was 60, and even if the hardness of the rail material was further increased, it was not effective in preventing damage from occurring, and it was observed that dark spot damage tended to occur more frequently by increasing the hardness to Hv 370 or higher. This is considered to be because rolling wear damage is accumulated and fatigue cracks are generated after a certain period of time by suppressing wear of the rail head surface. Also, when trying to promote wear by softening the ordinary carbon steel rail to Hv 260 or less, the rail head cannot bear the train load and plastic deformation occurs, and a crack is generated from this plastic deformation toe and continuous cracking occurs. It causes peeling damage to the rail head. Further, the reason why the bainite structure is selected is that the bainite has larger wear at the same hardness than the pearlite structure and is effective in promoting wear and preventing dark spot damage.
In the following soft materials, flaking damage due to plastic flow on the head surface occurs at the upper part of the rail gauge corner, causing peeling damage. Therefore, in the present invention, the wear rate can be improved by the bainite structure, the dark spot damage on the rail crown surface can be prevented, and the occurrence of metal plastic flow can be suppressed and the flaking damage can be prevented by increasing the strength to Hv370 or higher. it can.

[0018]

[Examples] Table 1 shows the chemical composition of the present invention steel and the comparative steel, and Table 2 shows the hardness of the present invention steel and the comparative steel and wear after repeated 500,000 times under dry conditions in the Nishihara type wear test. The results of quantity comparison tests are shown. In addition, Table 3 shows the comparison test results of the surface damage occurrence life (the number of times the wheel passes) after the water lubrication test using a disk test piece in which the shapes of the rail and the wheel were scaled down to 1/4. As is apparent from the test results, the rail of the present invention having the bainite structure is superior in the characteristics required for the rail to the rail for comparison having the bainite structure deviating from the pearlite structure or the steel composition of the present invention.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

INDUSTRIAL APPLICABILITY As described above, the rail of the present invention can promote wear without lowering the hardness of the rail by specifying the components and structure, and can prevent rolling fatigue damage to prevent dark spot damage from occurring, and By increasing the strength, it is possible to obtain a long-life rail that prevents flaking damage generation.

Claims (2)

[Claims] 1. By weight, C: 0.15 to 0.45%, Si: 0.15 to 1.20%, Mn: 0.30 to 1.00%, Cr: 2.00 to 4.00%. , Mo: 0.20 to 0.60%, the balance being steel consisting of iron and unavoidable impurities, having a bainite structure, and having a hardness of Hv370 after rolling.
A rail excellent in surface damage resistance, characterized by having the above. 2. By weight, C: 0.15 to 0.45%, Si: 0.15 to 1.20%, Mn: 0.30 to 1.00%, Cr: 2.00 to 4.00%. , Mo: 0.20 to 0.60%, Nb: 0.01 to 0.05%, V: 0.05 to 0.30%, Ti: 0.01 to 0.05% Alternatively, a rail having excellent surface damage resistance, which is characterized by containing two or more kinds, the balance being steel consisting of iron and inevitable impurities, having a bainite structure, and having a hardness after rolling of Hv 370 or more. ..