JPH05345955A - Rail with surface damage resistance and high toughness - Google Patents

Abstract

PURPOSE:To prevent the occurrence of rolling fatigue damage at the surface of a railhead and to improve the fracture toughness of a rail. CONSTITUTION:The rail having surface damage resistance and high toughness can be formed by providing a two-layer structure where, in the cross section of a rail axis, the surface of the railhead is coated with a material constituting a layer between a railhead surface (a) and a position at a depth of 2-15mm from the railhead surface (a) and having a metallic structure composed of bainite structure with 370 Hv hardness, and further, the other part is formed of a high VN added material 2 having ferrite-pearlite structure and also having high toughness. As compared with the conventional rail, this rail has superior rolling fatigue damage resistance at the railhead surface and excellent electric conductivity, and further, the fracture toughness of the rail can remarkably be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rail which is laid on a railway and has a surface damage resistance and a high toughness.

[0002]

2. Description of the Related Art With the increase in efficiency and speed of railway transportation, the load capacity of trains and the operating speed of trains are being improved. With such changes in the railway transportation environment,
The environment in which rails are used has become severe, and in rails laid in sharp curves, the wear of the rail head surface increases sharply, and rail gauge corners (GC), which are the main contact points between rails and wheels. Fatigue damage that occurs from the inside of the part has become more frequent. As a countermeasure against this, conventionally, Cr, Mo
As-rolled alloy rails added with a large amount of alloying elements such as (Japanese Patent Laid-Open No. 50-140316), heat-treated rails manufactured by accelerating cooling of the rail head or the whole without adding alloys (Japanese Patent Publication No. 55-23885), a low alloy heat-treated rail (Japanese Patent Publication No. 59-19173) in which not only the wear resistance and the damage resistance but also the hardness decrease of the welded portion is improved by adding an alloy having a relatively low content. , Etc., and the characteristic of these rails is that they are single-strength high-strength rails exhibiting a pearlite structure made of high-carbon steel with improved wear resistance and internal fatigue damage resistance. ..

In recent years, in a straight section rail for high speed operation in which wear or internal fatigue damage does not become a big problem, rolling fatigue damage occurs on the rail head surface due to repeated contact between the wheel and the rail, and peeling damage or rail head surface occurs. There are some cases in which fatigue cracks branch off inside the rail head and cause lateral crack damage (dark spot damage). Furthermore, as the train speed has increased, the dynamic load that acts on the rails while the train is running has increased, and it has become necessary to improve not only the strength of the rails but also the fracture toughness at the same time. However, in such a section, the occurrence of damage on the surface of the rail head becomes apparent, and although the fracture toughness of the rail is required to be improved, rolling of a single structure having a conventional pearlite structure is performed. Rails are still used.

As a result of investigating the cause of damage to the rail head surface portion, the present inventors have found that the cause is that a fatigue damage layer caused by repeated contact between the wheel and the rail accumulates on the rail head surface portion. It was confirmed that it was because. As a countermeasure against this, there is a method of grinding the fatigue damage layer on the rail head surface with a grinder or the like, but there are problems of cost and time constraints such as high grinder work cost and working time limited to night. It was Another method is to improve the wear rate of the rail head surface and remove the fatigue layer by wear before fatigue damage is accumulated. However, with conventional materials that exhibit a pearlite structure, increasing the wear rate also causes a decrease in strength, and rail head surface damage such as cracks (creaking cracks) accompanying plastic deformation (metal flow) on the rail head surface. Was frequently generated.

Further, in order to improve the fracture toughness of the rail having the conventional pearlite structure, there is a method of improving the toughness by heat-treating the rail leg portion at a low temperature.
However, this low-temperature heat treatment has a problem that the rail manufacturing cost is high because the rail needs to be reheated, and the productivity in rail manufacturing is significantly reduced due to the reheating. As described above, in the conventional single-layer rail having the pearlite structure, it is difficult to prevent damage to the rail head surface portion and simultaneously improve the fracture toughness of the rail.

[0006]

In order to prevent the occurrence of damage to the rail head surface portion as described above, the fatigue damage layer on the rail head surface is removed by moderate wear, and plastic deformation (metal flow) is applied. It is also possible to manufacture rails using restrained high strength materials. However, materials satisfying such characteristics have to be added with more alloys such as Cr and Mo in order to improve the strength as compared with conventional pearlite steels, resulting in high rail component cost, and at the same time, The addition of this alloy causes a significant decrease in electrical conductivity, which causes a problem of signal interruption or failure. Further, in the above-mentioned materials, since the strength is improved as compared with the pearlite steel, it is possible to prevent the occurrence of damage on the rail head surface portion, but at the same time, improvement in fracture toughness could not be expected.

Therefore, in order to solve this problem, the inventors of the present invention used a rail head having a bainite structure having an Hv hardness of 370 or more and a material having a ferrite / pearlite structure at other portions. It was confirmed by experiments that this problem can be solved by using a layered structure. That is, an object of the present invention is to provide a high toughness rail having a double-layered structure in which the rail head surface portion and other portions are different and which is excellent in surface damage resistance.

[0008]

The present invention achieves the above object, and the gist thereof is that at least the rail head surface portion and the head side surface portion of the rail axial section are 2 mm from the rail outer surface surface portion. Hv hardness of 3 at a depth of ~ 15mm
It exhibits a bainite structure of 70 or more, and the other parts are wt%, C: 0.15 to 0.45%, Si: 0.15 to
1.20%, Mn: 0.50 to 2.00%, V: 0.0
35 to 0.100%, N: 0.010 to 0.025%, or if necessary Cu, Ni, Cr, M
A surface damage-resistant and high-toughness rail characterized in that it contains one or more of o and Nb and has a metal structure of a two-layer structure exhibiting a ferrite-pearlite structure.

The present invention will be described below with reference to the drawings. 1 and 2 show an embodiment of the present invention in a cross section of a rail shaft. In the figure, 1 has an Hv hardness of 37
It is a material exhibiting a bainite structure of 0 or more. As shown in FIG. 1, a material having a bainite structure having an Hv hardness of 370 or more is arranged on the rail head surface portion a, or
As shown in FIG. 2, various forms can be adopted, such as covering the entire surface of the rail head surface portion a and the rail head side portion b to the rail leg portion c with a material having a bainite structure having an Hv hardness of 370 or more. That is, the material 1 exhibiting a bainite structure having an Hv hardness of 370 or more exists in at least only the rail head surface portion a and the rail head side portion b where the wheel comes into contact with the rail in the rail shaft cross section, and the coating thickness thereof. Is 2 mm to 15 mm from the head surface and the head side of the rail. The material 1 having a Hv hardness of 370 or more and a bainite structure has a bainite metal structure,
In addition, the components are not particularly limited as long as the hardness is Hv370 or more after rolling or heat treatment.

2 is C: 0.15 to 0.45%, Si:
0.15 to 1.20%, Mn: 0.50 to 2.00%,
V: 0.035 to 0.100%, N: 0.010 to 0.
Contains 025%, and if necessary Cu, Ni, Cr, M
a material containing one or more of O and Nb and having a ferrite-pearlite structure, and covered with the material 1 having a pearlite structure from the rail head surface part a and the rail head side part b to the rail leg. It is arranged on a portion other than the surface of the portion c.

Further, among these chemical components, C is an essential element for ensuring a certain strength, and if it is less than 0.15%, it becomes difficult to secure the strength as rail steel, and 0.45 %, A large amount of pearlite structure with low fracture toughness is generated, so 0.15 to 0.45%
Limited to. Si is an element that improves strength by forming a solid solution in the ferrite structure, but it is also necessary to add 0.15% or more as a deoxidizing element, and 1.20%
When the value exceeds 0.1, problems of surface flaws at the time of rail manufacturing and inconvenience to the bondability at the time of rail welding are brought about.
Limited to 20%. Mn has the effect of enhancing the hardenability of the steel and making the ferrite grains fine, and at the same time improving the strength and toughness, but if it is less than 0.50%, the effect is small, and
If it exceeds 2.00%, a large amount of pearlite structure is generated and the toughness is lowered, so the content is limited to 0.50 to 2.00%.

By combining V with N to form a nitride of VN, the γ grains at the time of reheating by rolling are made finer, and further the ferrite grains at the time of ferrite transformation are made finer (the γ / α conversion ratio is increased. However, if the content is less than 0.035%, it is difficult to form a nitride, so the content is limited to 0.035% or more. Also, 0.1
When it exceeds 00%, the amount of N, which is a chemically equivalent amount for forming a VN nitride, exceeds 0.025%, and when molten steel is melted, internal defects such as blowholes are easily generated by N. It was limited to 100% or less. N combines with V to form a VN nitride, thereby making the γ grains during rolling reheating finer, and by further refining the ferrite grains during ferrite transformation, significantly improving toughness, and at the same time precipitation hardening However, if it is less than 0.010%, it becomes difficult to form a nitride, so the content is limited to 0.010% or more. Also, 0.025
%, The content is limited to 0.025% or less because internal defects such as blowholes due to N are likely to be generated during molten steel melting. Cu is an element that improves the strength without impairing the toughness of steel. The effect is the largest in the range of 0.05 to 0.50%, and the addition of more than 0.50% causes an excessive increase in the strength and a decrease in the toughness.
The range was limited to 0.50%. Ni also has the effect of improving the strength without impairing the toughness of the steel, but if it is less than 0.05%, the effect is remarkably small, and if it is added in excess of 1.00%, the effect is expected to be sufficiently improved. Since it cannot be performed, the range is limited to 0.05 to 1.00%. Cr, Mo
Is effective in improving the strength, but if less than 0.05%, the effect is small, and addition in excess of 0.50% causes a decrease in toughness, so it is limited to the range of 0.05 to 0.50%. did. Nb is an element that improves strength by precipitation hardening similar to V, and its effect is large at 0.01% or more,
The addition of more than 0.05% is limited to the range of 0.01 to 0.05% in order to reduce the effects of V and N described above.

In the present invention, the reason why the metal material exhibiting a bainite structure having an Hv hardness of 370 or more after rolling or heat treatment is selected is that a material exhibiting a conventional pearlite structure has a constant value in straight and gentle curve sections. Surface damage starting from the rail head surface (dark spot damage) occurs after the train passes tonnage. As a measure against this,
There is a method of increasing the hardness of the rail, but even if the hardness is increased, the effect of preventing the occurrence of this damage is not recognized.
On the contrary, in the case of the material exhibiting a pearlite structure of 70 or more, the tendency of causing this damage frequently was found. Considering the cause of this damage from the correlation between the amount of rail wear and the condition of this damage,
It is considered that by suppressing the wear of the rail head surface, fatigue damage due to rolling contact between the rail and the wheel accumulates on the rail head surface portion, causing a fatigue crack and causing the main damage. In addition, if the material exhibiting the pearlite structure is softened to promote the wear of the rail head surface part, the rail head surface becomes insufficient in strength against the wheel load of the train and the tangential force of the wheel with the rail, resulting in plastic deformation. (Metal flow) occurs, and a crack is generated from the plastic deformation layer on the surface of the rail head, causing peeling damage at the surface of the rail head. Therefore, a bainite steel having strength equal to or higher than that of a material exhibiting a pearlite structure and having a large amount of wear at the same hardness was selected. However, in bainite steel having an Hv hardness of 370 or less, the Hv hardness is limited to 370 or more because surface damage occurs due to plastic deformation of the rail head surface.

In the present invention, the Hv hardness is 370.
The reason why the coating thickness of the material 1 having the above bainite structure is limited to 2 mm to 15 mm from the rail head surface and the rail head side portion is that the maximum shear stress acting position inside the rail due to rolling contact between the rail and the wheel is the rail contour. From the head surface 1
The position is from mm to 2 mm. If the coating thickness is less than 2 mm, there may be a two-layer interface near the position where maximum shear stress is applied inside the rail, and rolling fatigue damage occurs at the position near the interface where the material strength distribution changes significantly. To do. Therefore, in order to prevent the occurrence of rolling damage near such an interface, the coating thickness is limited to 2 mm or more. Also, when the coating thickness is 15 mm or more, the area ratio of the surface layer portion in the rail axial cross section becomes large, the electric conduction characteristic of the rail is deteriorated, and it may cause signal interruption or failure in the signal section. .. The life of the rail is defined by the maximum amount of wear of the rail head, and its limit wear amount is about 15 mm. Therefore, even if a material with excellent surface damage resistance is coated for 15 mm or more, the The rail life was determined and a coating thickness of 15 mm or more was not necessary, limiting the coating thickness to 15 mm or less.

Further, H is provided on the rail head surface portion and the head side surface portion.
Material 1 having a v hardness of 370 or more and exhibiting a bainite structure is arranged, and C: 0.15 to 0.45%, S in other portions.
i: 0.15 to 1.20%, Mn: 0.50 to 2.00
%, V: 0.035 to 0.100%, N: 0.010
Contains 0.025% or, if necessary, C
The reason for disposing the material 2 containing one or more of u, Ni, Cr, Mo, and Nb and having a metal structure exhibiting a ferrite / pearlite structure is that H is formed on the rail head surface portion.
By arranging the material 1 having a bainite structure having a v hardness of 370 or more, rolling fatigue damage on the rail head surface due to repeated contact between the wheel and the rail, which has been a problem in a rail having a pearlite structure, is prevented, The rail life can be greatly improved as compared with the conventional single layer rail having the pearlite structure. In addition, by arranging a material exhibiting a ferrite-pearlite structure inside thereof, the fracture toughness of the rail can be greatly improved, and at the same time,
The cost of the rail component can be reduced by using a two-layer structure with a low-priced ferrite / pearlite material. Further, since the alloy component can be reduced as compared with the single-layer rail exhibiting the bainite structure, it is possible to secure the electric conductivity characteristic equivalent to that of the ordinary rail.

As described above, the present invention having a two-layer structure of dissimilar metals in the axial section of the rail can be applied to any method such as explosive-rolling, clad method, cast-in-fill casting method, laminated dispersion casting method, and multi-layer continuous casting method. After the bloom or slab is manufactured in, the rail is manufactured by the normal hot forming and rolling method. Further, if necessary, a heat treatment is performed to improve the material of the head and other parts of the rail. The rail manufactured in this manner has surface damage resistance and fracture toughness required for rails for railways, and also secures electrical conductivity characteristics equivalent to those of ordinary rails.

[0017]

EXAMPLES Next, examples of the present invention will be described. Using 6 types of double-layered rails of the present invention and 5 types of comparative rails, a rolling fatigue test on a rail head surface by an actual rail and an actual wheel and a drop weight test by an actual rail were performed. Reference symbol A: Two-layer structure rail of cross-sectional shape Fig. 1 Initial surface layer thickness of part 1 on rail top surface; 5.2 mm Rail head surface part; Hv hardness 370 exhibiting bainite structure 370
Materials other than the above; Material exhibiting ferrite-pearlite structure Code B; Cross-sectional shape dual-layer rail in Fig. 2 Initial surface layer thickness of part 1 on rail top surface; 5.4 mm Rail head surface part; Bainite structure Hv hardness of 370
Materials other than the above; Material exhibiting ferrite / pearlite structure Code C; Cross-sectional shape dual-layer rail of Fig. 1 Initial surface layer thickness of part 1 on rail top surface; 10.2 mm Rail head surface part; Bainite structure Hv hardness of 370
Materials other than the above; Materials exhibiting ferrite / pearlite structure Code D: Cross-sectional shape dual-layer rail in Fig. 2 Initial surface thickness of part 1 at rail top surface; 11.2 mm Rail head surface; Bainite structure Hv hardness of 370
Materials other than the above; Material exhibiting ferrite-pearlite structure Code E; Cross-sectional shape dual-layer rail of Fig. 1 Initial surface layer thickness of part 1 on rail top surface; 14.9 mm Rail head surface part; Bainite structure Hv hardness of 370
Materials other than the above; Material exhibiting ferrite-pearlite structure Code F; Two-layer structure rail of cross-sectional shape Fig. 2 Initial surface thickness of part 1 at rail top surface; 14.7 mm Rail head surface part; Bainite structure Hv hardness of 370
Materials other than the above; Material exhibiting ferrite-pearlite structure Code G; Cross-sectional shape dual-layer rail of Fig. 2 Initial surface layer thickness of part 1 on rail top surface; 1.8 mm Rail head surface part; Bainite structure Hv hardness of 370
Material other than the above: Material having ferrite / pearlite structure Code H: Single layer rail having Hv hardness of 370 or more exhibiting bainite structure Code I: Single layer rail having Hv hardness of 370 or less exhibiting bainite structure Code J: Perlite structure Ordinary high carbon steel single layer rail with K code; Head heat treated high carbon steel single layer rail with pearlite structure Rolling fatigue test conditions (common to all test pieces) ・ Testing machine; Rolling fatigue testing machine ・ Test load; Radial load 8.0t, thrust load 0t ・ Test speed; wheel traveling speed 210km / h ・ Number of repetitions (passage tonnage); until damage occurs or up to 200 million tons ・ Atmosphere; water atmosphere Drop weight test conditions (all test pieces・ The tester; Drop weight tester ・ Length of test piece: 1.3m ・ Distance between fulcrums; 1.0m ・ Support method: Head up Experience to (dropping the drop weight on the head) and weight of the falling weight; of 1000 kg, falling weight height; 10 m-test temperature; +. 20 to-100 ° C.

Table 1 shows the rolling fatigue test results. Table 1
As shown in Fig. 2, the conventional high carbon steel single layer rail J exhibiting the conventional pearlite structure and the head heat treated high carbon steel single layer rail K exhibiting the conventional pearlite structure have the same rail head table as in the case of the actual line. Surface damage (dark spot damage) occurred on the part. Further, in the single-layer rail I having a hardness of Hv370 or less exhibiting a bainite structure, surface damage occurs due to plastic deformation (metal flow) of the rail head surface, and in the single-layer rail H having a hardness of Hv370 or more exhibiting a bainite structure. Although no surface damage was observed on the rail head surface up to 200 million tons, the electric resistivity value of the rail was very high compared to the high carbon single layer rails J and K,
The electric conduction characteristics are significantly deteriorated. In addition, the rail G has a two-layer structure in which the initial surface layer thickness of the rail top portion 1 is less than 2 mm.
Peeling damage was caused by the initiation of fatigue cracks from the interface between the two layers. On the other hand, the two-layer rail A of the present invention,
Compared with conventional rails, B, C, D, E, and F greatly improve rail life by preventing rolling fatigue damage on the rail head surface, and at the same time ensure electrical resistivity equivalent to conventional rails. To do.

Table 2 shows the results of the drop weight test. For each test condition, the presence or absence of breakage of the four rails after the drop weight test is shown in the table. As shown in Table 2, the rails A, B, C, D, E, and F of the two-layer structure of the present invention and the comparative rail G are −9.
It was revealed that all four rails did not break up to 0 ° C.

From the results of the rolling fatigue test and the drop weight test,
The rails A, B, C, D, E and F of the two-layer structure of the present invention are
By preventing rolling fatigue damage on the rail head surface compared to conventional rails, rail life is greatly improved and fracture toughness is also greatly improved.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

[0025]

As described above, according to the present invention, the rail head surface portion has a bainite structure and has a two-layer structure with the other part having a pearlite / ferrite structure so that it has excellent wear resistance. Moreover, it is possible to manufacture a high toughness rail having high fracture toughness.

[Brief description of drawings]

FIG. 1 is a cross-sectional explanatory view showing an example of a rail of the present invention.

FIG. 2 is a sectional explanatory view showing another example of the rail of the present invention.

[Explanation of symbols]

 1 Material part showing pearlite structure 2 Material part showing ferrite / pearlite structure a Rail head surface part b Rail head side part c Rail leg part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuo Sugino 1-1 Hibahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works

Claims (2)

[Claims] 1. At least a head surface portion and a head-side surface portion of the rail in the rail shaft cross section are 2 more than the rail outer surface portion.
The metallographic structure of the layer up to the depth of 15 mm to 15 mm has an Hv hardness of 37.
It has a bainite structure of 0 or more, and the other parts are wt%.
, C: 0.15 to 0.45%, Si: 0.15 to 1.
20%, Mn: 0.50 to 2.00%, V: 0.035
˜0.100%, N: 0.010 to 0.025%, and the metal structure thereof has a two-layer structure exhibiting a ferrite / pearlite structure, which is a surface damage resistant / high toughness rail. 2. In the rail axial cross section, at least the head surface portion and the head side surface portion of the rail are more than 2 from the rail outer surface portion.
The metallographic structure of the layer up to the depth of 15 mm to 15 mm has an Hv hardness of 37.
It has a bainite structure of 0 or more, and the other parts are wt%.
, C: 0.15 to 0.45%, Si: 0.15 to 1.
20%, Mn: 0.50 to 2.00%, V: 0.035
.About.0.100%, N: 0.010 to 0.025%, Cu: 0.05 to 0.50%, Ni: 0.
05-1.00%, Cr: 0.05-0.50%, M
o: 0.05 to 0.50%, Nb: 0.01 to 0.05
%, And a metal structure having a two-layer structure exhibiting a ferrite / pearlite structure is contained, and a surface damage resistant and high toughness rail.