JP4987773B2 - Long rail - Google Patents

Description

  The present invention relates to a long rail and a manufacturing method thereof. In particular, the present invention relates to a long rail having improved fatigue strength as compared with the prior art and a method for manufacturing the same.

  The portion of the rail that is most susceptible to damage and is costly to maintain is the rail joint. The seam is the main source of noise and vibration generated when passing through the train. Since speeding up of passenger railways and high loading of freight railways are being promoted in Japan and overseas, the technology of making long rails by welding rail joints having the above-mentioned problems has become common.

  One of the main rail welding methods is thermite welding (see, for example, Patent Document 1). The thermite welding method is a welding method in which molten steel produced by a chemical reaction between aluminum and iron oxide is poured into a welded portion to weld the rail. Specifically, the rails to be welded are placed facing each other with a gap of 20 to 30 mm between the end faces, the gap between the end faces of the rails is surrounded by a mold, and the molten steel generated by the thermite reaction is poured into the gap between the end faces of the rail for welding. To do. Thermite welding method is widely used as a welding method in the field because of its small size and high mobility.

  On the other hand, when the train passes, a bending load is applied to the rail, and a tensile stress is generated at the bottom of the rail. Since this stress is generated every time the wheel passes, the rail needs high fatigue strength. Since the welded portion of the rail changes in cross-sectional shape and material, the fatigue strength often decreases compared to other portions. As a technique for improving the fatigue strength of the welded portion of the rail, there are a method using shot peening, a method using hammer peening, a grinder process, and a TIG dressing as disclosed in Patent Document 2, for example.

In the shot peening, a steel ball having a diameter of several millimeters is hit against the material, the material surface layer is plastically deformed and work hardened, and the fatigue strength can be improved by compressing the residual stress. However, the treatment requires large-scale equipment for projecting, collecting, and preventing dust from the steel balls, and its application is limited to large welds. In addition, it is necessary to replenish the wear and damage of the projection material, and a running cost for that is required.
The hammer peening is said to improve fatigue strength by striking the tip of the tool against the material to give plastic deformation to the weld, introducing compressive stress, and reducing stress concentration by plastic deformation. . However, in addition to a large vibration at the time of impact and a heavy burden on the operator, fine control is difficult and processing unevenness is likely to occur. For example, Non-Patent Document 1 shows that a wrinkle-like groove portion generated by processing affects depending on processing conditions, and the effect of improving fatigue strength is small.

  In addition, the grinder treatment can be expected to improve the fatigue strength by reducing the stress concentration by smoothing the toe end of the bead. Since the reduction is caused, there is a disadvantage that the processing requires skill and the work takes a long time.

  Further, the TIG dressing improves the fatigue strength by reducing the stress concentration by remelting the toe portion of the weld bead with an arc generated from a tungsten electrode and resolidifying it into a smooth shape. . However, highly difficult skills and strict construction management are required for difficult-to-weld materials such as rails.

JP-A-48-95337 JP-A-3-249127 Miki, Ami, Tani, Sugimoto, "Improvement of fatigue strength by improving weld toe", Journal of the Japan Welding Society, Vol. 17, No. 1, P111-119 (1999)

  In order to improve the durability of the long rail, it is necessary to further improve the fatigue strength of the weld. Further, it is required to improve the fatigue strength more effectively than the conventional techniques such as shot peening, hammer peening, grinder processing, and TIG dressing which improve the fatigue strength of the welded portion described above.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide a long rail having improved fatigue strength as compared with the prior art and a method for manufacturing the same.

  When a fatigue test is performed on the welded portion of the rail, a fatigue crack is generated at the toe portion of the bead formed in the welded portion, and the fracture occurs starting from the fatigue crack. The present invention improves the fatigue strength of the welded portion of the rail by making it difficult for fatigue cracks to occur at the toe portion of the bead.

That is, the gist of the present invention is as follows.
(1) A long rail manufactured by thermite welding of two rails, wherein a cast burr formed on the toe end of the bead of the welded portion is 1 mm or less at the rail foot, and the rail foot Cast burrs are removed at the toe portion of the bead located in the front part, and the structure within 50 μm from the surface has pearlite in at least the region located in the rail foot surface part of the toe part of the bead. A long rail in which 60% or more lamellar of the pearlite forms an angle of ± 45 ° or less with respect to the surface in a cross section perpendicular to the surface.

(2) In the cross section of the region located at the rail foot surface portion of the toe portion of the bead, 40% or more lamellar of pearlite located within 50 μm from the surface is ± 15 ° or less with respect to the surface The long rail according to (1), wherein the long rail is formed.
(3) 10% or more of pearlite within 50 μm from the surface has a lamellar spacing of 70 nm or less in the region located at least on the rail foot surface portion of the toe portion of the bead (1) ) Or the long rail described in (2).
(4) In the cross section of the region located at the rail foot surface portion of the toe portion of the bead, 5% or more of pearlite within 50 μm from the surface has a lamellar spacing of 50 nm or less. The long rail described in (3).
(5) The above-mentioned (1) characterized in that the radius of curvature of the cross section in the longitudinal section of the long rail is 1.5 mm or more in the region of the toe portion of the bead located at the rail foot surface portion. The long rail according to any one of (4) to (4).
(6) The region located at the rail foot surface portion of the surface of the toe portion of the bead is characterized in that the residual stress in the longitudinal direction of the long rail is neutral or compressed. The long rail as described in any one of 5).
(7) The region located at the rail foot surface portion of the toe portion of the bead is subjected to ultrasonic peening treatment, as described in any one of (1) to (6) above Long rail.
(8) The long rail as described in any one of (1) to (7) above, wherein a fatigue limit at 2 million load repetitions is 280 MPa or more.

  According to the present invention, fatigue strength of the welded portion of the rail can be improved by making it difficult for fatigue cracks to occur at the toe portion of the bead.

  First, the shape of the long rail will be described with reference to FIG. FIG. 1 is a side view of the long rail in the longitudinal direction. Long rails are manufactured by welding at least two rails. For this reason, the welded portion 11 is included in the long rail. A bead 10 is formed on the welded portion 11.

  FIG. 2 is a view showing a cross section of the rail before welding together with a mold used in the thermite welding method, and corresponds to the AA ′ cross section of FIG. The rail has a head 1 that is an upper part of the rail that comes into contact with a wheel, a foot 3 that is a lower part of the rail that contacts the sleepers, and a pillar 2 that is a vertical part between the head 1 and the foot 3. Hereinafter, the back side of the foot portion 3 is referred to as a rail foot surface 3a, and the front side is referred to as a rail foot surface 3b. The range of the rail foot 3a includes a straight line portion on the bottom surface of the rail, and the rail foot surface 3b includes a straight line portion on the surface side of the foot portion 3 and a curved portion between the foot portion 3 and the column portion 2.

  When the train passes, a bending load acts on the rail every time the wheel passes, and tensile stress is generated on the rail. When the residual stress of the rail is a compressive stress, the stress caused by the bending load is offset by the residual stress, and the effective stress is reduced. Conversely, when the residual stress of the rail is a tensile stress, the effective stress increases due to the synergistic effect of the stress caused by the bending load and the residual stress.

  The mold for thermite welding shown in this figure is a two-part form composed of right and left two-part molds 4a and 4b. The two-part molds 4a and 4b are manufactured so that the space for fitting into the rail of the mold is somewhat larger (1 to 2 mm) than the standard cross section of the rail in order to cope with variations in the manufacturing dimensions of the rail and the mold. Therefore, when the mold is set on the rail, a gap is generated between the rail and the mold. When molten steel is inserted into this gap during welding, cast burrs are generated. This cast burr causes stress concentration and reduces the fatigue characteristics of the rail.

  3A is a cross-sectional view taken along the line AA ′ of FIG. 1, that is, a cross-sectional view of the rail welded portion 11, and FIG. 3B is a cross-sectional view taken along the line BB ′ of FIG. As described above, the bead 10 is formed in the weld portion 11 of the long rail, and the stress generated by the passage of the wheel concentrates on the toe portion 10a of the bead 10, that is, the boundary portion between the bead 10 and the rail body. For this reason, in most cases, the fatigue failure of the long rail starts from the toe 10a of the bead 10 of the foot 3. Further, the rail foot table 3b of the welded portion 11 is caused by a difference in thermal history for each part, and the residual stress is a tensile stress. Therefore, the toe portion 10a of the bead 10 located on the rail foot table 3b. The surface of is likely to be a starting point for destruction. In particular, when the rail is welded using the thermite welding method, as described above, a gap is generated between the rail and the mold, and therefore, molten steel is inserted into the gap during welding to generate a cast burr. This cast burr further causes stress concentration and lowers the fatigue characteristics of the rail. In order to reduce stress concentration due to casting burr, the thickness of casting burr should be 1mm or less as shown in FIG.

  From the above, in order to improve the fatigue strength of the weld portion 11 of the long rail manufactured using the thermite welding method, the fatigue strength of the surface of the toe portion 10a of the bead 10 of the foot portion 3 is increased. Making it difficult to crack, relieving stress concentration on the toe 10a of the foot 3, making the residual stress on the surface of the toe 10a of the foot 3 neutral or compressive, and the foot 3 Four points of eliminating the cast burr are effective.

  The lamellar structure of the pearlite structure is a layered structure of cementite tramler and ferrite lamellar. The lamellar spacing can be observed by mirror-polishing the sample and then revealing the lamellar structure with a 1 to 10% nitric acid alcohol solution (nital). In the present invention, the lamellar spacing is measured in a cross section including the longitudinal direction of the joint and perpendicular to the bead direction, as shown in FIG. The lamellar interval L is the center interval of the cementite tramler 50 or the ferrite lamellar 52 as shown in the enlarged view of the section A in FIG. The lamellar direction and the angle θ of the surface 40 are measured as the angle between the lamellar orientation at the measurement site and the line parallel to the surface closest to the measurement site, as shown in FIG.

  In order to increase the fatigue strength of the surface of the toe portion 10a of the bead 10, 60% or more of pearlite contained in the structure within 50 μm from the surface of these portions is applied to the surface of the toe portion 10a. It is effective to have an angle of ± 45 ° or less with respect to the surface in a cross section perpendicular to the surface (for example, a cross section in the longitudinal direction of the rail). This is because the orientation direction of general pearlite lamellar is random, but the orientation direction of more than half of pearlite lamellar is nearly perpendicular to the direction in which stress is applied, which is anisotropic to the strength of the tissue. It is because it can have sex. The structure within 50 μm from the surface of the toe portion 10a of the bead 10 is mostly pearlite. However, if it is pro-eutectoid ferrite or remains, or if the weld metal covers the toe portion 10a, the ferrite Or bainite may be included.

In this case, when the lamellar of 40% or more of pearlite located within 50 μm from the surface of the toe portion 10a in the cross section described above is set to an angle of ± 15 ° or less with respect to the surface, the fatigue strength is particularly improved.
Further, in the above-described cross section, the hardness increases (for example, Hv50 or more) when the lamellar spacing (center spacing between two adjacent ferrite phases) of 10% or more of pearlite within 50 μm from the surface of the toe portion 10a is 70 nm or less. Therefore, fatigue strength is particularly improved. In this case, when the lamellar spacing of 5% or more of pearlite located within 50 μm from the surface of the toe 10a is 50 nm or less, the fatigue strength is further improved.

  In order to alleviate the stress concentration on the toe portion 10a, it is effective to make the surface shape of the toe portion 10a smooth by setting the curvature radius of the cross section of the toe portion 10a to 1.5 mm or more.

  As a method of making the surface of the toe portion 10a as described above, setting the radius of curvature of the cross section of the toe portion 10a to the above value, and setting the residual stress of the toe portion 10a to a neutral or compression direction, the toe portion There is a method of performing ultrasonic peening treatment (UIT) in 10a. Thereby, the fatigue limit of the welded part when the load is repeated 2 million times can be increased by 30 MPa or more and 280 MPa or more compared to the non-treated material. In addition, when there exists the casting burr | flash 10b, it is preferable to remove the casting burr | flash 10b before performing an ultrasonic peening process to the toe part 10a.

  Further, as a method of eliminating the cast burr 10b of the toe portion 10a, for example, a cast burr 10b formed on the rail sole 3a by bringing the inner surface of a two-part mold used for thermite welding into close contact with the rail sole 3a is used. It is effective to remove the cast burr 10b formed at the toe portion 10a of the rail foot surface 3b to 1 mm or less. As a method of removing the cast burr 10b of the rail foot surface 3b, there is a method of performing an ultrasonic peening process on the toe portion 10a as shown in FIG. By performing the ultrasonic peening process, the cast burr 10 b is lifted from the rail and then separated from the bead 10. Further, in this case, the ultrasonic peening process can be performed on the toe portion 10a in the same process as the removal of the cast burr 10b.

  The reason why the mold is brought into close contact with the rail sole 3a is due to consideration for leakage of hot water. In other words, if the molten steel does not stay on the outer surface of the mold and leaks further out of the mold, welding failure will result. If the mold is set so that the gap between the mold and the rail is increased on the rail foot surface 3b, it is easy to detect even if the molten steel leaks, and the rail foot surface 3b is higher in position than the rail foot surface 3a. The water pressure is somewhat lower, the momentum at the time of hot water leak is small, and there is an advantage that it is easy to deal with leakage prevention.

  The region where the ultrasonic peening process is performed may be the entire area of the toe portion 10a located in the column portion 2 and the foot portion 3, or may be a part thereof. In the latter case, it is necessary to include at least the entire area of the toe portion 10 a located at the foot 3.

  The ultrasonic peening process is a process in which a striking member (for example, a pin shape) is vibrated with an amplitude of 10 μm to 100 μm and a frequency of 15 kHz or more (preferably 20 kHz), and the surface of the processing target is hit with the tip of the striking member. . In the ultrasonic peening process, the impact energy per time is smaller than hammer peening and larger than shot peening. Also, the ultrasonic peening process gives a very large number of hits to the surface. For this reason, the effect which is not in hammer peening and shot peening can be acquired by performing ultrasonic peening processing.

  When the frequency of the ultrasonic peening process is less than 20 kHz (particularly less than 15 kHz), the vibration frequency is in the audible sound range, which affects the operator and the environment. The higher the frequency of the ultrasonic peening process, the higher the machining energy, which is preferable. However, considering the manufacturing cost of the ultrasonic peening process tool, 60 kHz is the upper limit.

  The temperature at which ultrasonic peening is performed will be described below. When the steel material temperature is 500 ° C. or higher, the yield point of the steel material is extremely low, and an extremely deep dent is generated by the ultrasonic peening treatment, and the stress concentration in the welded portion increases. Further, since a recovery phenomenon occurs at a high temperature, the effect of compressing residual stress due to processing cannot be obtained. As the temperature decreases, the yield point of the steel material recovers, and at 300 ° C. or less, it becomes 80% to 90% of the room temperature state, and at 100 ° C., it becomes 90% or more of the room temperature state. Therefore, in order to avoid the formation of dents deeper than necessary and to obtain the effect of compressing the residual stress, it is desirable to perform the treatment at a material temperature of 300 ° C. or lower, more preferably 100 ° C. or lower.

  On the other hand, as the steel material temperature further decreases, the toughness and ductility of the steel material decrease. For this reason, when the ambient temperature falls below −20 ° C., there is a concern that the ultrasonic peening treatment portion may be cracked due to processing. Therefore, it is desirable to perform the treatment at a temperature of −20 ° C. or higher.

  Moreover, when the amplitude of the ultrasonic peening process is less than 10 μm, the processing energy is reduced and the processing time is increased, which is not preferable. In addition, when the amplitude exceeds 100 μm, the ultrasonic peening tool is increased in size and the processing efficiency is not greatly improved.

  In order to obtain the above-described effect by the ultrasonic peening treatment, it is preferable to move the striking member at least 3 passes along the toe portion 10a at a speed of 5 mm / second or more and 20 mm / second or less. When the number of passes is two or less, a portion that is insufficiently processed (for example, a region where the boundary line between the bead 10 and the rail body remains) remains, and there is a possibility that fatigue cracks may occur starting from this insufficient portion. In addition, when the number of passes of the ultrasonic peening process is up to 6, the fatigue strength increases with each increase, but the fatigue strength hardly increases even when the number of passes is 7 or more. For this reason, it is preferable that the number of passes of the ultrasonic peening process is 6 or less.

  The diameter of the striking member is preferably 2 mm or more and 5 mm or less. When the diameter of the striking member is less than 2 mm, the processing area in one pass becomes small, and it becomes difficult to make the radius of curvature of the cross section of the toe portion 10a 1.5 mm or more. On the other hand, when the diameter of the striking member exceeds 5 mm, the processing energy is dispersed and the processing efficiency is lowered, which is not preferable.

  Moreover, it is preferable that the curvature radius of the front-end | tip of a striking member is 1 mm or more and 4 mm or less. When the radius of curvature is less than 1 mm, it is difficult to make the radius of curvature of the cross section of the toe portion 10a 1.5 mm or more. Moreover, when the curvature radius of the front-end | tip of a striking member exceeds 4 mm, since the processing area required in order to smooth the surface of the toe part 10a will spread, and processing efficiency will fall, it is unpreferable.

The fatigue strength evaluation test was conducted by a three-point bending method. The rail welded portion cut to 1.5 m was placed in an upright posture at the center of the pedestal set at a distance of 1 m, and a load was applied to the center from the rail head with a pressing jig. The curvature radius of the part which touches the rail of a base and a pushing jig | tool was 100 mmR. The applied load was adjusted so that a strain gauge was bonded to the sole of the rail and the indicated value was set stress. The test stress was 3 kgf / mm ² as the minimum stress, and the maximum stress was set according to the stress range to be tested.
The load repetition rate was 5 Hz, and the test was terminated when the weld was broken. Moreover, when the load repetition number was not broken up to 2 million times, the test was terminated there.

Example 1
A plurality of long rails were manufactured using a thermite welding method using a two-part mold. At this time, the inner surface of the mold was brought into close contact with the rail sole, and the cast burr formed on the rail sole was reduced to 1 mm or less. And the casting burr | flash of the toe part of the bead located in a rail foot surface part was removed by the ultrasonic peening method, and also the ultrasonic peening process was performed to the toe part of a bead, and the some sample was produced. Although the number of ultrasonic peening treatments is different among a plurality of samples, other production conditions are the same. In addition, as a comparative example, a long rail was manufactured as-welded, that is, not subjected to ultrasonic peening.

  FIG. 6 (A) is a cross-sectional SEM photograph showing the structure of the bead toe portion in a sample with three passes of ultrasonic peening treatment, and FIG. 6 (B) is the bead toe of FIG. 6 (A). It is the cross-sectional SEM photograph which expanded the surface part of the part. This cross section is a cross section perpendicular to the surface of the bead toe, and is a cross section in the longitudinal direction of the rail. As is clear from this figure, by performing ultrasonic peening treatment at a predetermined amount or more, the structure of the surface portion of the bead toe becomes pearlite, and many lamellars of the pearlite have an angle of ± 45 ° or less with respect to the surface. Became.

  Further, in each of the sample 1 in which the number of passes of the ultrasonic peening process is 5 and the sample 2 in which the number of passes is 3 times, the lamellar orientation angle is within ± 50 μm from the surface of the bead toe. Measured using the thickness of each region having an angle of 45 ° or less and ± 15 ° or less (Table 1), and the thickness of each region having a lamellar spacing of 100 nm or less, 70 nm or less, and 50 nm or less. Measured (Table 2). A cross-sectional SEM photograph was used for these measurements. Each table also includes numerical values indicating how much the thickness of each region occupies within a range of 50 μm in depth.

  In sample 1, the thicknesses of the regions where the lamellar orientation angles are ± 45 ° or less and ± 15 ° or less are 40 μm and 30 μm, respectively, and in sample 2, the lamellar orientation angles are ± 45 ° or less and ± 15. The thickness of the region having an angle of less than or equal to ° was 33 μm.

  In Sample 1, the thicknesses of the regions where the lamellar spacing is 100 nm or less, 70 nm or less, and 50 nm or less are 9 μm, 8 μm, and 5 μm, respectively, and in Sample 2, the lamellar spacing is 100 nm or less, 70 nm or less, And the thickness of the area | region which is 50 nm or less was 25 micrometers, 20 micrometers, and 18 micrometers, respectively.

  The hardness, residual stress, working depth, and fatigue strength of Sample 1 are Hv410, 180-200 MPa, 100-200 μm, and 300 MPa, and the hardness, residual stress, working depth, and fatigue strength of Sample 2 are Hv520, 180-200 MPa, 100-200 μm, and 300 MPa.

  FIG. 7 shows the ratio of the structure in which the lamellar orientation angle is an angle of ± 45 ° or less and the angle of ± 15 ° or less in the pearlite located in the range of 50 μm depth from the surface of the bead toe. It is a graph which shows the relationship between the ratio of a structure | tissue, and the frequency | count of a pass of an ultrasonic peening process. The above-mentioned structure ratio was calculated by visually observing a cross-sectional SEM photograph.

  When the number of passes of the ultrasonic peening process is 2 times or less, the ratio of the structure having a lamellar orientation angle of ± 45 ° or less is less than 60%, and the lamellar orientation angle is ± 15 ° or less. The ratio of the organization is less than 40%. On the other hand, when the number of passes was 3 or more, the ratio of the structure having a lamellar orientation angle of ± 45 ° or less was 70% or less. In particular, the ratio of the structure having a lamellar orientation angle of ± 15 ° or less increased rapidly to around 65%.

  Therefore, by performing ultrasonic peening treatment at a certain level or more, in pearlite located within a range of 50 μm in depth from the surface of the bead toe, a lamella of 60% or more of pearlite is ± 45 ° or less with respect to the surface. It was shown that a lamellar of 40% or more can be made an angle of ± 15 ° or less with respect to the surface.

  FIG. 8 shows the ratio of the regions where the lamellar spacing is 100 nm or less, 70 nm or less, and 50 nm or less and the number of passes of the ultrasonic peening process in the pearlite located within the range of 50 μm depth from the surface of the bead toe. It is a graph which shows the relationship. The above-mentioned structure ratio was calculated by visually observing a cross-sectional SEM photograph.

  When the number of passes of the ultrasonic peening process is 2 or less, the ratios of the regions where the lamellar spacing is 100 nm or less, 70 nm or less, and 50 nm or less were less than 20%, less than 8%, and less than 5%, respectively. It was. On the other hand, when the number of passes is 3 or more, the ratio of the regions where the lamellar spacing is 100 nm or less, 70 nm or less, and 50 nm or less is more than 45%, more than 35%, and more than 25%, respectively. there were.

  From these facts, by performing ultrasonic peening treatment at a certain level or more, 10% or more of pearlite located within a range of 50 μm in depth from the surface of the bead toe is set to a lamellar spacing of 70 nm or less, and 5% or more. It was shown that the lamellar spacing can be 50 nm or less.

  9A, 9B and 9C show the Vickers hardness Hv of 50 μm below the surface of the bead toe (FIG. 9A), the residual stress in the rail longitudinal direction on the surface of the bead toe (see FIG. 9). 9 (B)) and the processing depth of the bead toe portion (FIG. 9 (C)) are graphs showing how each changes depending on the number of passes of the ultrasonic peening process. The pressing force when measuring the Vickers hardness Hv was 100 N, and the residual stress was measured by a cutting method using a strain gauge. The processing depth is the depth of the dent formed by the ultrasonic peening process. When the processing depth is deep, the radius of curvature of the bead toe becomes large and smooth.

  When the number of passes of the ultrasonic peening process was 6 or less, the Vickers hardness Hv increased as the number of passes increased, the residual stress changed from the tensile direction to the compression direction, and the processing depth increased. And the curvature radius of the bead toe part became 1.5 mm or more. However, even when the number of passes was 7 or more, there was almost no change in any of the Vickers hardness, the residual stress, and the processing depth as compared with the case of 6 passes.

  FIG. 10 is a graph showing the fatigue test results of the welded portion of the long rail, with the vertical axis indicating the stress amplitude and the horizontal axis indicating the number of breaks. When the number of passes of ultrasonic peening treatment was 6 or less, the number of breaks increased at any stress amplitude, but even when the number of passes exceeded 7, the number of breaks hardly increased. Specifically, the fatigue strength of the welded part in the comparative example, ie, the sample not subjected to the ultrasonic peening treatment, was 230 MPa, whereas the number of passes of the ultrasonic peening treatment was 1, 3, 6 The 2 million times fatigue strength of the welded part in each of the 12th and 12th samples was 240 MPa, 270 MPa, 300 MPa, and 305 MPa, respectively.

  To summarize the above, in the long rail manufactured using thermite welding method, the inner surface of the mold is brought into close contact with the rail sole, the cast burr formed on the rail sole is 1 mm or less, and the rail foot surface is formed. The cast burr at the toe portion of the positioned bead was removed by ultrasonic peening. Furthermore, by performing ultrasonic peening of a predetermined amount or more on the toe of the bead located on the rail foot surface, the tissue within 50 μm from the surface contains pearlite, and 60% or more lamellar of the pearlite is applied to the surface. The ratio of the structure with a lamellar spacing of 70 nm or less is 10% or more, and the radius of curvature of the cross section of the bead toe located on the rail foot surface is 1.5 mm or more. And the compressive stress of the rail longitudinal direction of the bead toe part located in a rail foot surface part was made into the neutral or compression direction.

  As a result, the fatigue strength of the welded portion of the long rail was improved. Further, when the number of passes of the ultrasonic peening treatment exceeds 3 times, the fatigue strength increases by 30 MPa or more compared to the comparative example, that is, the non-treated material, and when it exceeds 6 times, 2 million times compared with the non-treated material. The fatigue strength increased by 60 MPa or more.

  Table 3 shows the fatigue limit of 2 million times (MPa) when the present invention is applied to the long rail manufactured using the thermite welding method (Invention Examples A1 to A24) and when not applied (Comparative Examples A1 to A9). ). Inventive Examples A1 to A24 all had a fatigue limit of 2 million cycles of 240 MPa or more. Hereinafter, in detail, in this description, the structure, the lamella ratio, and the pearlite ratio are the results of viewing the structure within 50 μm from the surface of the bead toe. Moreover, all of invention example A1-A24 and comparative example A2-A9 are removing the cast burr | flash of the foot surface part of a bead toe part.

Specifically, in Invention Examples A1 to A3, the structure having an angle of ± 45 ° or less between the surface and the pearlite lamellar is 60% or more, and therefore the 2 million fatigue limit is 240 MPa.
In Invention Examples A4 to A6, the structure having a surface and pearlite lamellar angle of ± 45 ° or less is 60% or more, and the structure of the surface and pearlite lamellar angle is ± 15 ° or less is 40% or more. The 2 million fatigue limit was 240 MPa.

In Invention Examples A7 to A9, the structure where the angle between the surface and the pearlite lamellar angle is ± 45 ° or less is 60% or more, and the pearlite whose lamellar interval is 70 nm or less is 10% or more. Was 250 MPa.
In Invention Examples A10 to A12, the structure where the angle between the surface and the pearlite lamellar angle is ± 45 ° or less is 60% or more, and the pearlite whose lamellar interval is 50 nm or less is 5% or more. Was 260 MPa.

  In Invention Examples A13 to A15, the structure in which the angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the radius of curvature of the bead toe is 1.5 mm or more. The limit was 260 MPa or more.

  In Invention Examples A16 to A18, the structure in which the angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the residual stress at the bead toe is in the neutral or compressive direction. The limit was 270 MPa or more.

  In Invention Examples A19 to A21, the structure where the angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the bead toe is subjected to ultrasonic peening treatment, so the fatigue limit of 2 million times is 260 MPa. That's it.

  In addition, as shown in Invention Examples A22 to A24, there was an example in which the fatigue limit of 2 million times was 280 MPa or more.

  On the other hand, Comparative Example A1 does not remove the cast burr on the foot front portion of the beat toe portion and the cast burr on the sole portion is 1.5 mm, so the ultrasonic peening treatment was performed. The fatigue limit of 2 million times was 190 MPa. Further, in Comparative Examples A2 to A7, the structure having an angle between the surface and the pearlite lamellar is ± 45 ° or less is 50% or less, and thus the fatigue limit of 2 million cycles is 225 MPa or less despite the ultrasonic peening treatment. Met. In Comparative Examples 8 and 9, the frequency (frequency) of the peening treatment was less than the ultrasonic range, and the tip diameter of the striking member was small, so the 2 million fatigue limit was 220 MPa.

  For this reason, in long rails manufactured using thermite welding method, the cast burr formed on the rail foot is 1 mm or less, and the cast burr at the toe of the bead located on the rail foot is removed. And when the structure within 50 μm from the surface of the bead toe includes pearlite, and 60% or more lamellar of the pearlite forms an angle of ± 45 ° or less with respect to the surface, the fatigue strength of the welded portion is improved. Was shown to do. Furthermore, it was shown that when the ratio of the structure having a lamellar spacing of 70 nm or less is 10% or more, the fatigue strength of the welded portion is further improved. Furthermore, it was shown that when the ratio of the structure having a lamellar spacing of 50 nm or less is 5% or more, the fatigue strength of the welded portion is further improved. Furthermore, it was shown that when the stress in the rail longitudinal direction becomes neutral or in the compression direction, the fatigue strength of the welded portion is further improved. Further, it was shown that when the radius of curvature of the bead toe portion is 1.5 mm or more, the fatigue strength of the welded portion is further improved.

  Table 4 shows the movement speed of the striking member for performing ultrasonic peening and the fatigue limit of 2 million times when ultrasonic peening is performed on the bead toe of a long rail manufactured using thermite welding method. It is a table | surface which shows the relationship of a value. In addition, the value of the 2 million times fatigue limit is shown by the difference from the 2 million times fatigue limit value (230 MPa) of Comparative Example 1 in which cast burr was not removed and ultrasonic peening was not performed.

  In invention examples B1 and B3, the moving speed of the striking member is 3 mm / second and 25 mm / second, respectively, but the mold is brought into close contact with the bottom of the rail, and the ultrasonic peening is performed on the toe of the bead. Since the cast burr of the part was removed, the fatigue limit of 2 million cycles was improved by 25 MPa and 30 MPa compared to Comparative Example 1. Further, in Invention Examples B2 and B4, since the moving speed of the striking member was 5 mm / second or more and 20 mm / second or less, the fatigue limit of 2 million cycles was improved by 40 MPa or more compared to the non-treated material.

  On the other hand, in Comparative Examples B2 to B4, ultrasonic peening is not performed on the bead toe, the cast burr on the rail foot surface is not removed, and the moving speed of the striking member is less than 5 mm / second or 20 mm. Since it is more than / sec, the fatigue limit of 2 million cycles was improved only by 10 MPa at the maximum compared to the non-treated material.

(Example 2)
A plurality of long rails were manufactured using a thermite welding method using a two-part mold. At this time, the inner surface of the mold was brought into close contact with the rail foot portion, and the cast burr formed on the rail foot portion was set to 1 mm or less. Then, a sample 7 (denoted as “foot table UIT (non-root)” in FIG. 11A) in which the cast burr at the toe portion of the bead located at the rail foot surface portion is removed by an ultrasonic peening method, The cast burr at the toe portion of the bead located on the front portion was removed by an ultrasonic peening method, and an additional ultrasonic peening treatment was further performed on the bead toe portion in three passes (in FIG. Table UIT treatment ”). The bead toe of the sample 7 is subjected to some ultrasonic peening treatment when removing cast burrs.

  In addition, as a comparative example, a sample 9 (described as “as-welded” in FIG. 11A) was prepared as-welded, that is, not subjected to ultrasonic peening. Moreover, as a comparative example, the sample 10 (FIG. 10) was subjected to ultrasonic peening treatment on the toe ends of the bead on the rail foot surface and the sole part without removing the cast burr at the toe portion of the bead by the ultrasonic peening method. 11 (A) described as “double-sided UIT processing”).

  The graph of FIG. 11 (A) shows the fatigue test results of the welds of Samples 7-10. In this graph, the vertical axis represents the stress amplitude, and the horizontal axis represents the number of breaks.

  Furthermore, as a comparative example, in the thermite welding method using a two-part mold, the inner surface of the mold is brought into close contact with the rail foot surface portion, and the sample burr formed on the rail foot surface portion is 1 mm or less (in FIG. 11B) "Described as welded") was prepared. Furthermore, as a comparative example, the inner surface of the mold is brought into close contact with the rail foot surface portion, the cast burr formed on the rail foot surface portion is reduced to 1 mm or less, and ultrasonic peening is applied to the bead toe portion of the rail foot surface portion in 3 passes. A sample 12 (described as “foot table UIT treatment” in FIG. 11B) was prepared.

  The graph of FIG. 11 (B) shows the fatigue test results of the welds of Samples 11 and 12. In this graph, the vertical axis represents the stress amplitude, and the horizontal axis represents the number of breaks.

  As shown in the graphs of FIGS. 11A and 11B, the fatigue strengths of the samples 7 and 8 were higher than those of the samples 9, 11, and 12. Specifically, the 2 million times fatigue strength of the welds of Samples 7, 8, 9, 10, 11, and 12 were 300 MPa, 310 MPa, 230 MPa, and 290 MPa, 260 MPa, and 270 MPa, respectively. Note that the fatigue strength of the sample 10 was slightly smaller than those of the samples 7 and 8. However, since it is difficult to perform ultrasonic peening on the sole of the rail at the site where the rail is laid, it is not very practical.

(Example 5)
Table 5 shows the results of fatigue tests and processing times when various fatigue strength improvement measures are applied to thermite welds of rails.

  Comparative example C1 is an application example of shot peening. As the shot material, a steel ball having a diameter of 1 mmφ and a hardness of Hv500 was used. The treatment range was 100 mm on each side of the welded portion in the longitudinal direction and 200 mm in total width, and the shot material was projected on the rail foot surface portion 3 b and the column portion 2. The treatment time was 10 minutes for each rail side. The injection amount of the shot material was about 0.5 kg / second, and the collision speed of the shot material to the steel material was 5 m / second. As a result, the fatigue strength was improved by about 40 MPa compared to the non-treated material. However, it is difficult to carry in large-scale shot peening equipment for on-site welding of rails, and its industrialization seems difficult.

Comparative examples C2 and C3 are examples in which hammer peening is applied. Compressed air was used as the power for hammer peening, and the striking frequency was 40 times per second. The moving speed of the tool was 20 mm / second.
The comparative example C2 is an example in which the toe end of the bead 10 is intensively hammer peened using a tool having a tip curvature of 15 mmφ hitting the steel material surface. Processing was repeated 10 passes at the same position. The depth of the processed portion was about 1.5 mm from the surface of the rail base material in the deep portion, and the fatigue strength was rather lowered compared to the non-treated material.

  Comparative Example C3 is an example in which hammer peening was performed on the front and back of the foot within a range of 10 mm in the length direction from the toe portion of the bead 10. Using a tool with a radius of curvature of 4 mmφ at the tip of the tool, the air pressure was lowered (5 bar) so that the dent would not become large. Although the fatigue strength was improved by about 20 MPa, the processing time required about 40 minutes.

Comparative Example C4 is an example in which the toe portion of the bead 10 is smoothed by grinding. A small pencil grinder was used as the tool. Although the fatigue strength was improved, the treatment time required 20 minutes.
Comparative Example C5 is an example in which the toe portion of the bead 10 is remelted in a range of about 5 mm in width by a TIG welder and re-solidified into a smooth shape. Although the fatigue strength was improved as compared with the non-treated material, it took 20 minutes to preheat to 400 ° C. in order to prevent cracking of the treated part.

Invention Example C1 is an example in which ultrasonic peening is applied to the bead toe, and the processing conditions are the same as those of Invention Example A23, but the processing performance is shorter than that of the above Comparative Example. Was obtained.
From this, it was shown that ultrasonic peening is more efficient and effective in improving fatigue strength than other methods for improving fatigue strength.

The side view of the longitudinal direction of a long rail. The figure which showed the cross section of the rail before welding with the casting_mold | template used with thermite welding method. (A) is AA 'sectional drawing of FIG. 1, ie, sectional drawing of the welding part 11 of a rail, (B) is BB' sectional drawing of (A). The figure which shows the relationship between the thickness of the cast burr | flash of a rail foot table, and fatigue strength. The figure explaining the method of removing a casting burr | flash by ultrasonic peening processing. (A) is a cross-sectional SEM photograph showing the structure of the bead toe in a sample with three passes of ultrasonic peening treatment, and (B) is an enlarged cross-sectional SEM of the surface of the bead toe in (A). Photo. The graph which shows the relationship between the ratio of the structure | tissue where the lamellar orientation angle is an angle of less than +/- 45 degrees, and the ratio of the structure | tissue which is an angle of less than +/- 15 degree, and the frequency | count of the ultrasonic peening process. The relationship between the ratio of the area where the lamellar spacing is 100 nm or less, 70 nm or less, and 50 nm or less and the number of passes of the ultrasonic peening process in the pearlite located within a range of 50 μm in depth from the surface of the bead toe. Graph showing. Vickers hardness Hv ((A)) of 50 μm below the surface of the bead toe, residual stress in the rail longitudinal direction on the surface of the bead toe ((B)), and processing depth of the bead toe ((C)) ) A graph showing how each changes depending on the number of passes of the ultrasonic peening process. The graph which shows the fatigue test result of the welding part of the long rail in Example 1. FIG. (A), (B) is a graph which shows the fatigue test result of the sample in Example 2. FIG. The figure for demonstrating the measuring method of a lamellar space | interval and a lamellar angle. The enlarged view of the A section of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Rail head, 2 ... Rail pillar part, 3 ... Rail foot part, 3a ... Rail foot sole, 3b ... Rail foot surface, 10 ... Bead, 10a ... Stop end part, 10b ... Cast burr, 11 ... welded part

Claims (8)

  It is a long rail manufactured by thermite welding of two rails, and the cast burr formed on the toe end of the bead of the welded portion is 1 mm or less at the rail foot, and the rail foot surface is Cast burrs are removed at the toe portion of the bead positioned, and the structure within 50 μm from the surface of the bead toe portion has a pearlite at least in the region located on the rail foot surface portion. A long rail characterized in that 60% or more lamellar of the pearlite forms an angle of ± 45 ° or less with respect to the surface in a cross section perpendicular to the surface.   In the cross section of the region located at the rail foot surface portion of the toe portion of the bead, 40% or more lamellar of pearlite located within 50 μm from the surface forms an angle of ± 15 ° or less with respect to the surface. The long rail according to claim 1, wherein:   The lamellar interval is 70 nm or less in 10% or more of pearlite within 50 μm from the surface in the cross section of the region located on the rail foot surface portion of the toe portion of the bead. 2. A long rail as described in 2.   4. The lamellar spacing is 50 nm or less in 5% or more of pearlite within 50 μm from the surface in the cross section of the region located on the rail foot surface portion of the toe end portion of the bead. 5. Long rail described.   The region located in the rail foot surface portion of the toe portion of the bead has a radius of curvature of a cross section in a longitudinal cross section of the long rail of 1.5 mm or more. Long rail as described in any one.   The region located on the rail foot surface portion of the surface of the toe portion of the bead has a residual stress in the longitudinal direction of the long rail being neutral or compressed. Long rail described in the section.   The long rail according to any one of claims 1 to 6, wherein a region located in the rail foot surface portion of the toe end portion of the bead is subjected to an ultrasonic peening process.   The long rail according to any one of claims 1 to 7, wherein a fatigue limit at 2 million load repetitions is 280 MPa or more.