JP5176634B2 - 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 enclose arc welding (see, for example, Patent Document 1). Enclosed arc welding is a method in which the operator manually arcs the rails and is highly mobile, so it is often used as a welding method on the track. The beads formed by enclosing arc welding are polished away after welding.

  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 hammer peening, a grinder process, and a method using TIG dressing as disclosed in Patent Document 2, for example.

In the shot peening, fatigue strength can be improved by hitting a steel ball having a diameter of several millimeters to the material, plastically deforming the surface layer of the material and hardening it, and 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 uneven processing tends to occur. For example, Non-Patent Document 1 shows that wrinkle-shaped grooves produced by processing are affected 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-6-292968 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 enclosing arc welding of two rails, and at a toe end of a bead formed in a welded portion and a discontinuous portion of the bead, within 50 μm from the surface The structure has pearlite, and in the cross section perpendicular to the surface, 60% or more lamellar of the pearlite forms an angle of ± 45 ° or less with respect to the surface, and the toe portion of the bead And a long rail in which an ultrasonic peening process is performed on the discontinuous portion of the bead .

(2) In the cross section, 40% or more lamellar of pearlite located within 50 μm from the surface of the toe portion of the bead and the discontinuous portion forms an angle of ± 15 ° or less with respect to the surface. The long rail as described in (1) above, wherein
(3) In each of the toe portion and the discontinuous portion of the bead, 10% or more of pearlite within 50 μm from the surface has a lamellar spacing of 70 nm or less in the cross section. ) Long rail as described in.
(4) In (3) above, 5% or more of the pearlite located within 50 μm from the surface in each of the toe portion and the discontinuous portion of the bead has a lamellar spacing of 50 nm or less in the cross section. Long rail described.
(5) In the cross section in the longitudinal direction of the long rail, the radius of curvature of the toe portion of the bead is 1.5 mm or more, according to any one of the above (1) to (4), Long rail.
(6) The surface of each of the toe portion and the discontinuous portion of the bead is characterized in that the residual stress in the longitudinal direction of the long rail is neutral or compressed. Long rail described in one.
(7) The long rail according to any one of (1) to (6) above, wherein a fatigue limit at 2 million load cycles is 300 MPa or more.
(8) The long rail as described in any one of (1) to (7) above, wherein the beads formed on the rail column part and the rail foot part are not removed in the welded part.

  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. 1A is a side view of the long rail in the longitudinal direction, and FIG. 1B is a cross-sectional view taken along the line AA ′ of FIG. The long rail shown in the figure is manufactured by enclosing arc welding of 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. 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.

  2A and 3A are perspective views of the long rail, respectively. 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.

  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.

  2A and 3A, the foot 3 and the head 1 are cut in the longitudinal direction, and a cross section of the bead 10 is shown.

  2B is a view showing a longitudinal section of the foot portion 3 in FIG. 2A, and FIG. 3B is a view showing a longitudinal section of the foot portion 3 in FIG. is there. In the enclosed arc welding, on the surface of the bead 10, a toe portion 10a, that is, a boundary portion between the bead 10 and the rail body, and a discontinuous portion 10b, that is, a boundary portion in the bead 10 are formed. The stress generated by the passing of the wheel concentrates on the toe 10a and the discontinuous portion 10b of the bead 10. For this reason, in most cases, the fatigue failure of the long rail manufactured by the enclosed arc welding starts from the toe portion 10a or the discontinuous portion 10b of the bead 10 of the foot portion 3. In particular, the foot surface 3b of the welded portion 11 has a residual stress that is a tensile stress due to a difference in thermal history for each part. Therefore, the surface of the toe portion 10a of the bead 10 located on the foot surface 3b is , Easy to become a starting point of destruction.

  From the above, in order to improve the fatigue strength of the weld portion 11 of the long rail manufactured by the enclosed arc welding, the fatigue strength of the surface of the toe portion 10a of the bead 10 and the discontinuous portion 10b of the foot portion 3 is achieved. To prevent cracking, to relieve stress concentration on the toe 10a and the discontinuous part 10b of the foot 3, and the residual stress of the toe 10a and the discontinuous part 10b of the foot 3 The three points of making the neutral or compression direction 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 orientation 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 10a and the discontinuous portion 10b is set to an angle of ± 15 ° or less with respect to the surface, the fatigue strength is particularly improved.
The hardness also increases when the lamellar interval (center interval between two adjacent ferrite phases) of 10% or more of pearlite within 50 μm from the surfaces of the toe portion 10a and the discontinuous portion 10b is 70 nm or less (for example, Hv50 Therefore, the fatigue strength is particularly improved. In this case, when the lamellar spacing of 5% or more of pearlite located within 50 μm from the surfaces of the toe portion 10a and the discontinuous portion 10b is 50 nm or less, the fatigue strength is further improved.

  In order to relieve stress concentration on the toe portion 10a and the discontinuous portion 10b, the radius of curvature of the cross section of the toe portion 10a and the discontinuous portion 10b is set to 1.5 mm or more, and the toe portion 10a and the discontinuous portion 10b. It is effective to smooth the surface shape.

  The surface of the toe portion 10a and the discontinuous portion 10b is made to have the above-described structure, the radius of curvature of the cross section of the toe portion 10a and the discontinuous portion 10b is set to the above value, and the toe portion 10a and the discontinuous portion 10b remain. As a method of making the stress neutral or compressive, there is a method of performing ultrasonic peening treatment (UIT: Ultrasonic Impact Treatment) on the toe portion 10a and the discontinuous portion 10b. Thereby, the fatigue limit of the welded part at the load repetition number of 2 million times can be increased by 50 MPa or more and 300 MPa or more as compared with the non-treated material. In addition, the area | region which performs an ultrasonic peening process may be all of the toe part 10a and discontinuous part 10b located in the pillar part 2 and the leg part 3, but may be a part. In the latter case, it is necessary to include at least all of the toe portion 10a and the discontinuous portion 10b located at the foot portion 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.

  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 . Name your, until six times the number of passes of the ultrasonic peening treatment, but the fatigue strength is increased each time to increase the number of times, fatigue strength in the more than seven times hardly increases. 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.

  In addition, when performing ultrasonic peening treatment on the toe portion 10a and the discontinuous portion 10b of the bead 10, the conventional method of removing the bead 10 with a grinder or the like without performing the ultrasonic peening treatment without removing the bead 10. Compared with, the fatigue strength of the welding part 11 becomes high. The labor and skill level required to perform the ultrasonic peening process can be lower than the labor and skill level required to polish and remove the bead 10 with a grinder or the like. Moreover, it is not necessary to perform ultrasonic peening immediately after welding. Conventionally, the application of the enclosed arc welding has been limited in the line section where the working time cannot be secured, but this limitation can be relaxed by the present invention.

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 carry out 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.

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.

(Reference example)
First, a reference example will be described. A long rail was manufactured using the thermite welding method, and a plurality of samples were prepared by subjecting the toe end of the bead formed in the welded portion of the long rail to ultrasonic peening treatment under the above-described conditions. 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. 4A is a cross-sectional SEM photograph showing the structure of the bead toe portion in a sample with three passes of the ultrasonic peening process, and FIG. 4B is the bead toe of FIG. 4A. 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. The thickness of each region having an angle of 45 ° or less and ± 15 ° or less was measured (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 was 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. 5 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. 6 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 ultrasonic peening processing 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.

  FIGS. 7A, 7B and 7C show Vickers hardness Hv of 50 μm below the surface of the bead toe (FIG. 7A), residual stress in the longitudinal direction of the rail on the surface of the bead toe (FIG. 7A). 7 (B)) and the processing depth of the bead toe portion (FIG. 7C) 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. 8 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. The welding method is thermite welding. 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.

  From this, in the long rail manufactured using the thermite welding method, the structure within 50 μm from the surface of the bead toe is obtained by performing ultrasonic peening treatment of a predetermined amount or more on the bead toe of the weld. 60% or more lamellar of the pearlite forms an angle of ± 45 ° or less with respect to the surface, the rail longitudinal stress is neutral or compression direction, and the curvature of the bead toe It was shown that the radius was 1.5 mm or more. As a result, it was shown that the fatigue strength of the welded portion is 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. It was shown that the lap fatigue strength increased by 60 MPa or more.

Example 1
A long rail is manufactured using an enclosing arc welding method, and ultrasonic peening is performed on the toe and discontinuous portions of the bead formed on the welded portion of the long rail under the above-described conditions, thereby A sample was prepared. The number of ultrasonic peening treatments is different between a plurality of samples, but other conditions are the same. Further, as a comparative example, a long rail from which the beads were removed without performing the ultrasonic peening process was produced.

  FIG. 9 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. In addition, the fatigue strength of the welded portion in the comparative example was 280 MPa, whereas the number of passes of the ultrasonic peening treatment was 1, 3, 6, and 12 in each of the samples. The 2 million times fatigue strength was 290 MPa, 320 MPa, 360 MPa, and 375 MPa, respectively.

  From this, in the long rail manufactured using the enclosed arc welding method, by performing ultrasonic peening treatment of a predetermined amount or more on the bead toe portion and the discontinuous portion of the welded portion, the same effect as the reference example is obtained. It was shown that the fatigue strength of the welded part of the long rail is 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. It was shown that the lap fatigue strength increased by 80 MPa or more. Moreover, the effort which performs an ultrasonic peening process was small compared with the effort which removes a bead.

(Example 2)
A long rail is manufactured using the enclosed arc welding method, and ultrasonic peening treatment (UIT treatment) is performed on the toe and discontinuous portions of the bead formed in the welded portion of this long rail under the above-mentioned conditions. The bead was further polished and removed. In addition, as a comparative example, a long rail was manufactured by using an enclosed arc welding method, and a sample in which a bead formed on the welded portion of the long rail was left as it was and a sample in which the bead was polished and removed by a grinder were prepared. .

  FIG. 10 is a graph showing the fatigue test results of the welds in these samples. For comparison, the fatigue test result of the welded part in the sample of Example 1 with three passes is also shown. In the enclose arc welding method, the sample from which the bead is removed has a higher fatigue strength at the welded portion than the sample from which the bead is not removed. However, it has been shown that when ultrasonic peening is performed on the toe and discontinuous portions of the beads, the fatigue strength is increased without removing the beads as compared with the case where the beads are removed. In addition, when the bead was polished and removed after the ultrasonic peening treatment was performed on the toe portion and the discontinuous portion of the bead, the fatigue strength was slightly improved.

(Example 3)
Table 3 shows a case where the present invention is applied to a long rail manufactured using an enclosed arc welding method ( Reference Examples A1 to A21, Invention Examples A22 to A24) and a case where the present invention is not applied (Comparative Examples A1 to A10). 2 million times fatigue limit (MPa) is shown. All of Reference Examples A1 to A21 and Invention Examples A22 to A24 had a fatigue limit of 2 million times of 310 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.

Specifically, in Reference Examples A1 to A3, the structure having an angle between the surface and the pearlite lamellar of ± 45 ° or less is 60% or more, and thus the 2 million times fatigue limit was 310 MPa.
In Reference Examples A4 to A6, the structure where the angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the structure where the angle between the surface and the pearlite lamellar is ± 15 ° or less is 40% or more. The fatigue limit of 2 million times was 310 MPa.

In Reference Examples A7 to A9, the structure where the angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the pearlite whose lamellar interval is 70 nm or less is 10% or more. Was 315 MPa.
In Reference Examples A10 to A12, the structure where the angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the pearlite whose lamellar interval is 50 nm or less is 5% or more. Was 320 MPa.

In Reference Examples A13 to A15, the structure having an angle of ± 45 ° or less between the surface and the pearlite lamellar is 60% or more, and the radius of curvature of the bead toe is 1.5 mm or more. The limit was 315 MPa or more.

In Reference Examples A16 to A18, the structure where the angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the residual stress at the toe of the bead is in a neutral or compressive direction. The limit was 320 MPa or more.

Further, in Reference Examples A19 to A21, the structure whose surface and pearlite lamellar angle is ± 45 ° or less is 60% or more, the structure whose pearlite lamellar angle is ± 15 ° or less is 40% or more, and the lamellar spacing is Perlite with a thickness of 70 nm or less is 10% or more, Perlite with a lamellar spacing of 50 nm or less is 5% or more, the radius of curvature of the bead toe is 1.5 mm or more, and the residual stress at the bead toe is neutral. Or since it is a compression direction, the 2 million times fatigue limit became 350 Mpa or more.

In addition, the reference examples A1 to A21 described above have removed the rail pillar part and the bead of the foot part, but as shown in the invention examples A22 to A24, the structure in which the angle between the surface and the pearlite lamellar is ± 45 ° or less. Is 60% or more, the structure of the pearlite lamellar angle is ± 15 ° or less is 40% or more, the pearlite with a lamellar interval of 70 nm or less is 10% or more, and the pearlite with a lamellar interval of 50 nm or less is 5% or more. If the radius of curvature of the bead toe is 1.5 mm or more and the residual stress at the bead toe is neutral or in the compression direction, the rail pillar and the foot bead need not be removed. The fatigue limit of 2 million times was 330 MPa or more.

  On the other hand, in Comparative Example A1, since the structure of the beat toe portion is not pearlite but bainite, the fatigue limit of 2 million cycles was 230 MPa even though ultrasonic peening was performed. In Comparative Examples A2 to A8, the structure having an angle between the surface and the pearlite lamellar is ± 45 ° or less is 50% or less. Met. Further, in Comparative Example A9, since the structure where the angle between the surface and the pearlite lamellar is ± 45 ° or less is 30%, the ultrasonic peening treatment was performed and the rail pillar portion and the foot bead were removed. The fatigue limit of 2 million times was 280 MPa. In Comparative Example 10, the peening treatment frequency (frequency) was less than the ultrasonic range, and the tip diameter of the striking member was small, so the 2 million fatigue limit was 280 MPa.

  From this, in the long rail manufactured using the enclosed arc welding method, the structure within 50 μm from the surface of the bead toe includes pearlite, and 60% or more of the pearlite has a lamellar structure of ±± It has been shown that the fatigue strength of the welded portion is improved when an angle of 45 ° or less is formed. 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 the fatigue strength of the welded portion is further improved when the proportion of the structure having a lamellar spacing of 50 nm or less is 5% or more. Moreover, 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.

Example 4
Table 4 shows an ultrasonic peening process for the toe end of a long rail manufactured using the enclosed arc welding method, and the bead on the rail and the foot is removed by removing the bead on the rail head. It is a table | surface which shows the value of a 2 million times fatigue limit when not carrying out (invention example), and when removing the bead of a rail, without performing an ultrasonic peening process (comparative example). The value of the 2 million times fatigue limit is shown by the difference from the value of the 2 million times fatigue limit (280 MPa) of Comparative Example B1 that was not subjected to UIT treatment and bead removal.

  Inventive Examples B1 to B5 were subjected to ultrasonic peening treatment on the bead toes, and therefore the fatigue limit of 2 million cycles compared to the non-treated material despite the fact that the rail pillar and the foot bead were not removed. Improved by 40 MPa or more. In particular, 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 50 MPa or more compared to the non-treated material.

  On the other hand, since Comparative Example B1-B3 did not perform the ultrasonic peening process, when the bead of a rail head part, a pillar part, and a foot part is removed (Comparative Example B1), the bead of a rail pillar part and a foot part is removed. In each case of removing (Comparative Example B2) and removing the rail head bead (Comparative Example B3), the fatigue limit was not improved 2 million times compared to the non-treated material.

(Example 5)
Table 5 shows the results of fatigue tests and treatment times when various fatigue strength improvement measures are applied to the rail's enclosure arc weld. The value of the 2 million times fatigue limit is shown by the difference from the value of the 2 million times fatigue limit (280 MPa) of Comparative Example C4 where UIT was not treated and beads were removed.
Comparative Example C4 is an example in which the beads 10 are removed smoothly by grinding, and is the same as Comparative Example B1 of Example 4 described above. A small disc grinder was used as the polishing tool. Enclosed arc welding beads have a width of about 20 mm and a thickness of about 5 mm, and the processing operation requires 30 minutes or more. Rail welding on railroads is considered the standard method, but rail welding performed during the passage of trains has severe work time restrictions, and it is desirable to be able to reduce the time of this polishing work.
Comparative Example C6 is a joint that is in a welded state and has a weld bead left as it is. The fatigue strength is about 20 MPa lower than the grinding material of Comparative Example C4, which is a standard method.

  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 36 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 part is about 1.4 m from the surface of the rail base material in the deep part, and the fatigue strength is rather lower than that of the non-treated material.

Comparative Example C3 is an example in which hammer peening is performed on the front and back of the foot within a range of 10 mm from the toe of the bead 10 in the length direction. 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. The fatigue strength was equivalent to that of Comparative Example C4 of the grinding treatment as a comparative material.
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. The fatigue strength was equivalent to that of Comparative Example C4 of the grinding treatment as a comparative material. However, in order to prevent delayed cracking, it was necessary to preheat to 400 ° C., which required 20 minutes.

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 fatigue performance can be improved efficiently with a short processing time. It was.
As described above, it has been 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. (A) is a perspective view of a long rail, (B) is a figure which shows the cross section of the foot part 3 in the longitudinal direction. (A) is a perspective view of a long rail, (B) is a figure which shows the cross section of the foot part 3 in the longitudinal direction. It is a reference example, (A) is the cross-sectional SEM photograph which shows the structure | tissue of the bead toe part in the sample whose frequency | count of the ultrasonic peening process is 3 times, (B) is the surface part of the bead toe part of (A) An enlarged cross-sectional SEM photograph. This is a reference example, and shows the relationship between the ratio of the tissue having a lamellar orientation angle of ± 45 ° or less and the ratio of the tissue having an angle of ± 15 ° or less and the number of passes of the ultrasonic peening process. Graph. In the pearlite located in the range of 50 μm depth from the surface of the bead toe, 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 path of ultrasonic peening treatment A graph showing the relationship with the number of times. It is a reference example, Vickers hardness Hv ((A)) 50 μm below the surface of the bead toe, the residual stress in the rail longitudinal direction on the surface of the bead toe ((B)), and the processing depth of the bead toe (C) is a graph showing how each changes according to the number of passes of the ultrasonic peening process. The graph which is a reference example and shows the fatigue test result of the welding part of a long rail using the thermite welding method. 3 is a graph showing a fatigue test result of a welded portion in Example 1. 6 is a graph showing fatigue test results of welds in Example 2. 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 part, 2 ... Rail pillar part, 3 ... Rail foot part, 3a ... Rail foot sole, 3b ... Rail foot surface, 10 ... Bead, 10a ... Stop end part, 10b ... Discontinuous part, 11 …welded part

Claims (8)

It is a long rail manufactured by enclosing arc welding of two rails, and the structure within 50 μm from the surface is pearlite at the toe end of the bead formed in the weld and the discontinuous part of the bead. 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 , and the toe portion of the bead, and the A long rail characterized in that ultrasonic peening is applied to the discontinuous part of the bead .   In the cross section, 40% or more lamellar of pearlite located within 50 μm from the surface of the toe portion of the bead and the discontinuous portion forms an angle of ± 15 ° or less with respect to the surface. The long rail according to claim 1.   3. The long rail according to claim 1, wherein in each of the toe end portion and the discontinuous portion of the bead, 10% or more of pearlite within 50 μm from the surface has a lamellar spacing of 70 nm or less in the cross section. .   4. The long rail according to claim 3, wherein in each of the toe end portion and the discontinuous portion of the bead, 5% or more of pearlite located within 50 μm from the surface has a lamellar spacing of 50 nm or less in the cross section. .   5. The long rail according to claim 1, wherein a radius of curvature of a toe portion of the bead is 1.5 mm or more in a longitudinal section of the long rail.   The long rail according to claim 1, wherein the surface of each of the toe portion and the discontinuous portion of the bead is neutral or compressed in the longitudinal direction of the long rail. .   The long rail according to any one of claims 1 to 6, wherein a fatigue limit at a load repetition number of 2 million times is 300 MPa or more.   The long rail according to any one of claims 1 to 7, wherein a bead formed on the rail column portion and the rail foot portion is not removed in the welded portion.