JP4987772B2 - 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 flash butt welding (see, for example, Patent Document 1) and gas pressure welding (see, for example, Patent Document 2). Although flash butt welding and gas pressure welding are highly efficient, they require a large-scale apparatus and are used in welding factories or welding bases. These two welding methods are pressure welding methods, and a bead is formed by expanding the rail cross section by pressurization in the rail longitudinal direction. Most of the bead is deleted by a hydraulic tool or the like at a high temperature after welding, and the remainder of the bead is polished and removed by using a grinder or the like.

  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, for example, there is a method using shot peening, hammer peening, grinder processing, or TIG dressing as described in Patent Document 3.

  In the shot peening treatment, a steel ball having a diameter of several millimeters is hit against the material to plastically deform the material surface layer, the residual stress is 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 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-56-136292 JP 11-270810 A 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 rail, it is necessary to further improve the fatigue strength of the welded portion of the rail. 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 circumstances, 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 flash pad welding or gas pressure welding of two rails,
One toe end of the bead remaining in the welded part immediately after punching out the bead during welding has an acute angle with respect to the surface of the rail compared to the other toe part,
The one toe portion has a pearlite structure within 50 μm from the surface, and 60% or more lamellar of the pearlite is ± 45 ° or less with respect to the surface in a cross section perpendicular to the surface. A long rail characterized by an angle.

(2) In the cross section, 40% or more lamellar of pearlite located within 50 μm from the surface of the one toe portion forms an angle of ± 15 ° or less with respect to the surface. The long rail described in (1).
(3) The long end as described in (1) or (2) above, wherein 10% or more of the pearlite within 50 μm from the surface in the cross section has a lamellar spacing of 70 nm or less. rail.
(4) The long rail according to (3) above, wherein a lamellar spacing of 50% or less of the pearlite located within 50 μm from the surface of the one toe is 50 nm or less in the cross section.
(5) In the longitudinal section of the long rail, the one toe portion has a radius of curvature of 1.5 mm or more, according to any one of the above (1) to (4), Long rail. (6) The long surface according to any one of the above (1) to (5), wherein the surface of the one toe portion is neutral or compressive in the longitudinal direction of the long rail. rail.
(7) The long rail as described in any one of (1) to (6) above, wherein a fatigue limit at 2 million load cycles is 330 MPa or more.
(8) The long rail as described in any one of (1) to (7) above, wherein an ultrasonic peening process is performed on the one toe.

  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 this figure is manufactured by flash butt welding or gas pressure 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. 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.

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

  FIG. 2B is a view showing a longitudinal section of the foot 3 in FIG. In flash butt welding and gas pressure welding, the bead 1 is formed by expanding the rail cross section by pressurization in the rail longitudinal direction. Most of the bead 10 is punched and removed by a hydraulic cutting tool having substantially the same cross-sectional shape as the rail at a high temperature after welding. For this reason, immediately after the bead punching during welding, the toe 10a located on the upstream side in the punching direction among the remaining parts of the bead 10 is smooth, but on the toe 10b located on the downstream side in the punching direction. The angle formed by the base of the toe portion 10b with respect to the surface of the foot portion 3 (for example, less than 90 °) is the angle formed by the base of the toe portion 10a with respect to the surface of the foot portion 3 (for example, It becomes an acute angle compared to (over 90 °). For this reason, the stress generated by the passing of the wheel concentrates on the toe 10b of the bead 10 of the foot 3. The fatigue failure of the long rail manufactured by flash butt welding or gas pressure welding is almost always the toe 10b of the bead 10 of the foot 3. In particular, the rail foot table 3b in the welded portion 11 has a tensile stress due to the residual stress due to the difference in the thermal history of each part, so that the toe 10b of the bead 10 located on the rail foot table 3b The surface is likely to be a starting point for destruction.

  From the above, in order to improve the fatigue strength of the welded part 11 of the long rail manufactured by flash butt welding and gas pressure welding, at least the bead 10 in the punching direction of the bead 10 located at the foot 3 is used. Increase the fatigue strength of the surface of the toe portion 10b located on the downstream side to make it difficult to crack, relieve stress concentration on at least the toe portion 10b located on the foot portion 3, and at least on the foot portion 3 The three points of making the residual stress of the toe portion 10b positioned to be neutral or compressive 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 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 toe portion 10b of the bead 10, 60% or more of the pearlite contained in the structure within 50 μm from the surface of these portions is made to the surface of the toe portion 10a. It is effective to make an angle of ± 45 ° or less with respect to the surface in a vertical cross section (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. In addition, although the structure within 50 μm from the surface of the toe portion 10a of the bead 10 is mostly pearlite, it may be proeutectoid ferrite or may remain.

In this case, in the above-described cross section, when the lamellar of 40% or more of pearlite located within 50 μm from the surface of the end portion 10b 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 interval (center interval between two adjacent ferrite phases) of 10% or more of pearlite within 50 μm from the surface of the toe portion 10b 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 portion 10b in the above-described cross section is 50 nm or less, the fatigue strength is further improved.

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

  As a method of making the surface of the toe portion 10b as described above, setting the radius of curvature of the cross section of the toe portion 10b to the above value, and setting the residual stress of the toe portion 10b to a neutral or compression direction, the toe portion There is a method of performing ultrasonic peening treatment (UIT) in 10b. Thereby, the fatigue limit of the welded part when the number of load repetitions is 2 million times can be increased by 30 MPa or more and 330 MPa or more compared to the non-treated material. In addition, although the area | region which performs ultrasonic peening process may be the whole region of the toe part 10b located in the pillar part 2 and the foot | leg part 3 among the beads 10, it is in the pillar part 2 and the foot | leg part 3 among the beads 10. It may be the entire area of the toe portions 10a and 10b that are positioned. Moreover, the whole area of the toe part 10b located in the foot part 3 may be sufficient, and the whole area of the toe part 10a, 10b located in the foot part 3 may be sufficient. In addition, when the ultrasonic peening process is performed on the toe portion 10b, the warp described above may not be an acute angle.

  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. 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.

  When ultrasonic peening is performed on the toe 10b of the bead 10, the remaining part of the bead 10 is removed by a grinder or the like without performing the ultrasonic peening process even if the remaining part of the bead 10 is not removed. As a result, the fatigue strength of the welded portion 11 increases. The labor and skill level necessary for performing the ultrasonic peening process can be lower than the labor and skill level necessary for polishing and removing the remainder of the bead 10 with a grinder or the like. For this reason, the welding productivity of a rail improves.

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. 3 (A) is a cross-sectional SEM photograph showing the structure of the bead toe portion in a sample with three passes of ultrasonic peening, and FIG. 3 (B) is the bead toe of FIG. 3 (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. 4 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 within a range of 50 μm in 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. 5 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 treatment 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.

  6A, 6B and 6C show the Vickers hardness Hv of 50 μm below the surface of the bead toe (FIG. 6A), the residual stress in the longitudinal direction of the rail on the surface of the bead toe (see FIG. 6). 6 (B)) and the processing depth of the bead toe portion (FIG. 6C) 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. 7 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
Long rails were manufactured using flash butt welding, and most of the formed beads were stamped and removed at high temperatures. And the several sample was produced by performing the ultrasonic peening process on the above-mentioned conditions in the whole area of the toe part of the remainder of a bead. 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 was prepared by polishing and removing the remainder of the bead with a grinder without performing ultrasonic peening.

  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. 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 part in the comparative example was 300 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 315 MPa, 350 MPa, 390 MPa, and 405 MPa, respectively.

  For this reason, in long rails manufactured using the flash butt welding method, by performing ultrasonic peening treatment at a predetermined amount or more on the bead toes of the welded portion, welding of the long rail is performed in the same manner as in the reference example. It was shown that the fatigue strength of the part is improved. Further, if the number of passes of ultrasonic peening treatment exceeds 3 times, the fatigue strength of 2 million times increases by 50 MPa or more compared to the comparative example, and if it exceeds 6 times, the fatigue strength of 2 million times fatigue strength of 90 MPa compared to the comparative example. It was shown that it increased more. Moreover, the effort which performs an ultrasonic peening process was small compared with the effort which removes a bead.

(Example 2)
A plurality of long rails were manufactured using the gas pressure welding method, and most of the beads formed in the welded portions of these long rails were removed by hot punching using a cutting tool. And the some sample was produced by performing ultrasonic peening process on the above-mentioned conditions to the toe part of the remaining bead. 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 was prepared by polishing and removing the remainder of the bead with a grinder or the like.

  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 part in the comparative example was 300 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 320 MPa, 350 MPa, 395 MPa, and 410 MPa, respectively.

  From this, in the long rail manufactured using the gas pressure welding method, the fatigue strength is improved by the same action as the reference example by performing ultrasonic peening treatment of a predetermined amount or more on the bead toe portion of the welded portion. It was shown that. Further, when the number of passes of ultrasonic peening treatment exceeds 3 times, the fatigue strength of 2 million times increases by 50 MPa or more compared to the comparative example, and when it exceeds 6 times, the fatigue strength of 2 million times fatigue strength becomes 95 MPa compared with the comparative example. It was shown that it increased more.

Example 3
Long rails were manufactured using the flash bad welding method, and most of the beads formed on the welds of these long rails were removed by hot punching with a tool. Then, the ultrasonic peening process (UIT process) was performed on the entire area of the toe end of the bead formed in the welded part of the long rail under the above-described conditions, and the remaining part of the bead was polished and removed. In addition, as a comparative example, a long rail was manufactured using a flash butt welding method, and a sample which was not subjected to ultrasonic peening treatment while leaving a bead formed on the welded portion of the long rail, and a welded portion of the long rail A long rail was prepared by punching out and removing most of the beads formed on the substrate and polishing and removing the remainder of the beads with a grinder or the like.

  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 flash butt welding method, the sample from which the remaining bead is removed has a higher fatigue strength at the weld than the sample from which no bead is removed. However, it was shown that when ultrasonic peening is performed on the toe portion of the remaining portion of the bead, the fatigue strength is increased even if the remaining portion of the bead is not removed, as compared with the case where the remaining portion of the bead is removed. In addition, when the remaining part of the bead was polished and removed after performing ultrasonic peening treatment on the toe part of the remaining part of the bead, the fatigue strength was slightly improved.

Example 4
Long rails were manufactured using the gas pressure welding method, and most of the beads formed in the welded portions of these long rails were removed by hot punching using a cutting tool. Then, the ultrasonic peening process (UIT process) was performed on the entire area of the toe end of the bead formed in the welded part of the long rail under the above-described conditions, and the remaining part of the bead was polished and removed. In addition, as a comparative example, a long rail was manufactured using a gas pressure welding method, and a sample which was not subjected to ultrasonic peening treatment while leaving a bead formed on the welded portion of the long rail was directly applied to a welded portion of the long rail. A long rail was produced in which most of the formed beads were punched and removed, and the remainder of the beads were removed by grinding with a grinder or the like.

  FIG. 11 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 2 with three passes is also shown. In the gas pressure welding method, the sample from which the remaining part of the bead is removed has higher fatigue strength at the welded portion than the sample from which the bead is not removed at all. However, it was shown that when ultrasonic peening is performed on the toe portion of the remaining portion of the bead, the fatigue strength is increased even if the remaining portion of the bead is not removed, as compared with the case where the remaining portion of the bead is removed. In addition, when the remaining part of the bead was polished and removed after performing ultrasonic peening treatment on the toe part of the remaining part of the bead, the fatigue strength was slightly improved.

(Example 5)
Table 3 shows 2 million times when the present invention is applied to the long rail manufactured using the flash pad welding method or gas pressure welding (Invention Examples A1 to A24) and when not applied (Comparative Examples A1 to A9). The fatigue limit (MPa) is shown. Inventive Examples A1 to A24 all had a fatigue limit of 2 million cycles of 325 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 Invention Examples A1 to A3, since the structure having an angle of ± 45 ° or less between the surface and the pearlite lamellar is 60% or more, the fatigue limit of 2 million cycles was 325 MPa.
In Invention Examples A4 to A6, the structure having an angle between the surface and the pearlite lamellar is ± 45 ° or less is 60% or more, and the structure having an angle between the surface and the pearlite lamellar is ± 15 ° or less is 40% or more. The 2 million fatigue limit was 330 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. Became 330 MPa or more.
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 335 MPa or more.

  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 340 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 340 MPa or more.

  In addition, as shown in Invention Examples A19 to A21, there was an example in which the fatigue limit of 2 million cycles was 345 MPa or more without performing ultrasonic peening treatment.

  In Invention Examples A22 to A24, 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 340 MPa. That's it.

  On the other hand, in Comparative Example A1, the structure of the beat toe is not pearlite but bainite, so that ultrasonic peening was performed and the active residual stress direction was neutral, but the fatigue limit of 2 million times Was 270 MPa. In Comparative Examples A2 to A8, since the structure having an angle between the surface and the pearlite lamellar is ± 45 ° or less is 50% or less, the fatigue limit of 2 million cycles is 300 MPa or less despite the ultrasonic peening treatment. Met. In Comparative Example 9, since the frequency (frequency) of the peening treatment was less than the ultrasonic region and the tip diameter of the striking member was small, the 2 million times fatigue limit was 300 MPa.

  From this, in the long rail manufactured using the flash bad welding method or gas pressure welding, the structure within 50 μm from the surface of the bead toe includes pearlite, and 60% or more lamellar of the pearlite is less than the surface. It has been shown that the fatigue strength of the welded portion is improved when the angle becomes ± 45 ° or less. 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 6)
Table 4 shows that in a long rail manufactured by using the flash bad welding method or gas pressure welding, the bead was punched and removed with a cutting tool, and an ultrasonic peening process was performed on the beat toe end on the downstream side in the punching direction. In the cases (Invention Examples B1 to B4) and the case where ultrasonic peening is applied to the beat toe without removing the beads (Comparative Examples B1, B3 to B5), the fatigue limit value of 2 million times is shown. It is a table. In addition, the value of the 2 million times fatigue limit is the value of the 2 million times fatigue limit (230 MPa) when the bead is not removed and the ultrasonic peening treatment is not performed on the beat toe (Comparative Example B2). The difference is shown.

  Inventive Examples B1 to B4 have a 2 million times fatigue limit improved by 40 MPa or more compared to Comparative Example B2. In particular, in Invention Examples B2 and B4, since the moving speed of the striking member is 5 mm / second or more and 20 mm / second or less, the fatigue limit of 2 million cycles is improved by 80 MPa or more compared to the non-treated material.

  On the other hand, in Comparative Examples B1, B3 to B5, since the ultrasonic peening treatment was not performed on the beat toe, the fatigue limit of 2 million times was improved only by 30 MPa as compared with the non-treated material. In particular, in Comparative Example B1, the fatigue limit of 2 million cycles was improved only by 10 MPa as compared with the non-treated material, even though the moving speed of the striking member was 5 mm / second or more and 20 mm / second or less.

(Example 7)
Table 5 shows the fatigue test results and processing time when various fatigue strength improvement measures are applied to the flash butt weld of the rail. The value of the 2 million times fatigue limit is shown by the difference from the value of the 2 million times fatigue limit (300 MPa) of Comparative Example C4 in which the beads were removed and further subjected to the grinding treatment.
Comparative Example C4 is an example in which the beads 10 are completely removed by grinding. A small disc grinder was used as the polishing tool. The flash butt welding bead has a width of about 20 mm and a thickness of about 2 mm, and the processing operation requires 20 minutes or more. In order to improve productivity, it is desirable that this polishing operation can be shortened.
Comparative Example C6 is a state where the weld bead is punched out with a hydraulic cutting tool at a high temperature. The fatigue strength is about 30 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 slightly improved as compared with the comparative material. However, it is necessary to introduce a large shot peening apparatus.

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 was about 1.6 mm from the surface of the rail base material in the deep part, and the fatigue strength slightly decreased 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. The processing time required about 30 minutes. 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 decreased rather than the comparative material. Further, it took 20 minutes to preheat to 400 ° C. in order to prevent cracking of the treated portion.

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, and fatigue performance can be improved efficiently in a short 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 longitudinal direction of the leg part 3 of (A). 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 fatigue test results of welds in Example 1. FIG. 6 is a graph showing a fatigue test result of a welded portion in Example 2. 10 is a graph showing a fatigue test result of a welded portion in Example 3. 10 is a graph showing a fatigue test result of a welded portion in Example 4. 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, 10b ... Stop end part, 11 ... Welded part

Claims (8)

  A long rail manufactured by flash-bad welding or gas pressure welding of two rails, and one toe of the bead remaining in the weld immediately after the bead is pushed out during welding is the other stop The angle formed with respect to the surface of the rail as compared with the end portion is an acute angle, and the one end portion has a pearlite structure within 50 μm from the surface, and is a cross section perpendicular to the surface. In the long rail, 60% or more of the pearlite has an angle of ± 45 ° or less with respect to the surface.   2. The lamellar of 40% or more of pearlite located within 50 μm from the surface of the one toe in the cross section forms an angle of ± 15 ° or less with respect to the surface. Long rail described.   3. The long rail according to claim 1, wherein the one toe portion has 10% or more of pearlite within 50 μm from the surface in the cross section and a lamellar spacing of 70 nm or less.   4. The long rail according to claim 3, wherein a lamellar interval of 50% or less of the pearlite located within 50 μm from the surface of the one toe portion is 50 nm or less in the cross section.   5. The long rail according to claim 1, wherein a radius of curvature of the one toe portion is 1.5 mm or more in a longitudinal section of the long rail.   6. The long rail according to claim 1, wherein the surface of the one toe portion has a neutral or compressive residual stress 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 2 million load cycles is 330 MPa or more.   The long rail according to any one of claims 1 to 7, wherein an ultrasonic peening process is performed on the one toe.