JP2013086163A - Crack repairing method - Google Patents

Abstract

A laser beam is applied to a crack generated in a steel material of an existing steel structure to repair the crack.
A laser beam 11 having a predetermined spot diameter is cracked from a first through hole 20 formed in one end portion 5a of a crack 5 generated in a steel material 3 of an existing steel structure 1 to the other end portion 5b of the crack 5. By irradiating along 5 and melting and erasing the crack 5, the crack 5 generated in the steel material 3 of the existing steel structure can be repaired easily and reliably.
[Selection] Figure 5

Description

  The present invention relates to a crack repairing method for repairing a crack generated in a steel material of an existing steel structure.

  Conventionally, when a crack has occurred in a steel material of an existing steel structure such as a bridge, a building, or a building due to aging or fatigue, the crack has been repaired by the following repair method. For example, as shown in FIG. 1 and FIG. 2, when repairing the crack 5 generated in the belly plate 4 of the I-shaped steel 3 used for the main girder 2 of the bridge 1, first, as shown in FIG. As shown, in order to alleviate the stress concentration of the crack 5 and prevent the crack 5 from proceeding, a stop hole 21 made of a through hole is formed in at least one of the end portions 5a and 5b of the crack 5 or in the vicinity thereof. Next, as shown in FIG. 17 (B), the abdominal plate 4 is provided with a pair of patch plates 100, 100 so as to cover the cracks from both sides of the front and back surfaces of the abdominal plate 4 for reinforcement. The pair of contact plates 100, 100 are attached by a plurality of fastening members 101 such as bolts and nuts. As described above, conventionally, a crack generated in a steel material of an existing steel structure is repaired (see Patent Document 1).

  However, such a crack repairing method is heavier than before because the caul plate 100 and the fastening member 101 are added. Further, in such a repair method for cracks, insertion holes are formed in the repair work site so as to correspond to a plurality of insertion holes through which the bolts previously formed in the contact plate 100 are inserted. It is necessary to do this, and it is cumbersome and workability is poor. Further, in such a crack repairing method, since the caul plate 100 is attached, the aesthetic appearance is impaired.

Japanese Patent No. 4613287

  An object of this invention is to provide the crack repair method which can repair the crack which generate | occur | produced in the steel materials of the existing steel structure easily and reliably.

  In the crack repair method according to the present invention, a circular first through hole having a diameter larger than the width of one end of the crack is penetrated at one end of the crack generated in the steel material of the existing steel structure. Laser light with a predetermined spot diameter is irradiated along the crack from the hole to the other end of the crack, and the crack is melted and erased. Of course, the first through hole may be previously drilled.

  Further, the steel material may be formed with a second through hole at the other end of the crack.

  Furthermore, the spot diameter of the laser light is preferably set larger than the width of the crack. Furthermore, the spot diameter of the laser beam may be smaller than the first through hole and the second through hole. Alternatively, the spot diameter of the laser beam may be larger than the first through hole and the second through hole. Specifically, the spot diameter of the laser light is preferably set to 3 to 7 mm.

  Further, the focal point of the laser beam may be located outside the steel material. Or you may make it the focus of a laser beam be located in the inside of steel materials.

  Furthermore, you may make it irradiate a laser beam from the one side of the thickness direction of steel materials. Moreover, you may make it irradiate a laser beam from the both sides of the thickness direction of steel materials. Further, when the laser beam is irradiated from both sides in the thickness direction of the steel material, it is preferable that the laser beam melts at least half of the thickness of the steel material by one irradiation.

  Furthermore, after repairing the crack, build-up welding may be performed on the reduced thickness portion, the first through hole, the second through hole, or the like. In this case, the build-up welding is preferably laser hot wire welding.

  In the present invention, a laser beam having a predetermined spot diameter is irradiated along the crack from the first through hole formed at one end of the crack generated in the steel material of the existing steel structure to the other end of the crack. By melting and erasing, it is possible to repair cracks generated in the steel material of the existing steel structure.

  Further, in the present invention, when irradiating the crack with the laser beam, the laser beam having a predetermined spot diameter is irradiated, so that the construction margin is improved and the laser beam can be easily and reliably irradiated with the laser beam. The crack can be easily and reliably erased.

  Furthermore, in the present invention, when the crack is melted and erased, the crack can be easily and surely erased by starting the irradiation of the laser beam from the first through hole formed at one end of the crack. .

It is the perspective view which showed the crack repaired with the repair method of the crack to which this invention is applied. The crack repaired with the repair method of the crack to which this invention is applied is shown, (A) is a side view, (B) is a front view. It is the schematic diagram which showed schematically the laser crack repairing apparatus. It is the front view which showed the state provided so that the focus of a laser beam might be located inside a belly board. It is process drawing of the repair method of the crack to which this invention is applied. FIG. 2 shows a crack repair method to which the present invention is applied, wherein (A) is a side view schematically showing a state of a crack before repair, and (B) is a schematic view of a crack state during repair. It is the side view shown in. It is the side view which showed the repair method of the crack in which the stop hole was formed in the one end part, and showed typically the state of the crack under repair. The repair method of the crack in which the stop hole was formed in the other end part is shown, (A) is the side view which showed the state of the crack before repair typically, (B) is the crack under repair It is the side view which showed typically the state. It is the side view which showed the repair method of the crack in which the stop hole was formed in both ends, and showed typically the state of the crack under repair. The crack repair method which repairs a crack by irradiating a laser beam from both sides of a stomach plate is shown, (A) is the front view which showed the state which irradiates a laser beam from the one main surface side of a stomach plate (B) is the front view which showed the state which irradiates a laser beam from the other main surface side of a belly board. The repair method of a crack in case there is a thinning part with a crack is shown, (A) is the front view showing the state which repairs a crack, and (B) shows the state which repairs the thinning part FIG. It is the figure which showed the state after the repair of the comparative example 1. FIG. It is the figure which showed the state after the repair of the comparative example 2. FIG. It is the figure which showed the state after the repair of Example 1. FIG. It is the figure which showed the state after the repair of Example 2. FIG. It is the figure which showed the state after the repair of Example 3. FIG. It is the side view which showed the repair method of the conventional crack, (A) shows the state which formed the stop hole, (B) has shown the state which attached the patch plate.

  Hereinafter, a crack repair method to which the present invention is applied will be described in the following order with reference to the drawings.

1. Overview 2. 2. Laser crack repair device How to repair cracks

[1. Overview]
The crack repairing method to which the present invention is applied is a crack repairing method for repairing a crack caused by aging deterioration, fatigue, or the like to a steel material of an existing steel structure such as a bridge, a building, and a building with a laser beam. Hereinafter, as shown in FIG. 1 and FIG. 2, an example of repairing a crack 5 generated in the belly plate 4 of the I-shaped steel 3 used in the main girder 2 of the bridge 1 will be described.

  In addition, the repair method of the crack to which this invention is applied is not only repairing the crack 5 which generate | occur | produced in the belly board 4 of the I-shaped steel 3 used for the main girder 2 of the bridge 1, but other repair of the main girder 2 The present invention can also be applied to repairing a part or, for example, a crack 5 generated in a steel deck deck 6 of a bridge 1 as shown in FIG. Furthermore, the crack repairing method to which the present invention is applied can be applied not only to the bridge 1 but also to repairing cracks 5 occurring in steel materials such as other buildings and buildings that are existing steel structures. I can do it.

[2. Laser crack repair device]
First, a laser crack repairing apparatus 10 that implements a crack repairing method to which the present invention is applied will be described. As shown in FIG. 3, a laser crack repairing apparatus 10 that performs a crack repairing method to which the present invention is applied has a laser generation mechanism 12 that irradiates a laser beam 11 to a crack 5. The laser generation mechanism 12 includes a laser oscillator 13 that oscillates a laser beam 11, a laser torch 15 that includes a condensing optical system 14, and an optical fiber that guides the laser beam 11 oscillated from the laser oscillator 13 to the laser torch 15. 16. Such a laser generation mechanism 12 condenses the laser beam 11 oscillated from the laser oscillator 13 and guided to the laser torch 15 through the optical fiber 16 at a predetermined position by the condensing optical system 14. Irradiate the crack 5 of the plate 4 with a predetermined spot diameter. Here, the focal point 17 of the laser beam 11 is provided so as to be located outside the abdominal plate 4. The laser crack repairing apparatus 10 is fixed by fixing means so that the distance between the laser torch 15 and the abdominal plate 4 is constant. Thereby, the laser crack repairing apparatus 10 can stably irradiate the laser beam having the energy for melting the belly plate 4 around the crack 5.

  Note that the focus position of the laser beam is not particularly limited as long as the energy for melting the crack 5 is constant and the laser beam 11 is irradiated. For example, as shown in FIG. 4, the laser generating mechanism 12 irradiates the crack 5 of the abdominal plate 4 with a predetermined spot diameter so that the focal point 17 of the laser beam 11 is located inside the abdominal plate 4. May be. At this time, the laser generating mechanism 12 may move the laser torch 15 itself toward the abdominal plate 4 so that the focal point 17 of the laser light 11 is positioned inside the abdominal plate 4. Is moved close to the abdominal plate 4 by a focus adjustment unit (not shown) that moves up and down so as to be close to or away from the abdominal plate 4, so that the focal point 17 of the laser light 11 is inside the abdominal plate 4. You may be made to be located in.

  Further, if the laser generating mechanism 12 irradiates the crack 5 of the abdominal plate 4 with a predetermined spot diameter, the laser generation mechanism 12 irradiates the crack 5 of the abdominal plate 4 in the defocused state. The laser beam 11 may be irradiated to the crack 5 of the abdominal plate 4 in a focused state without being limited thereto. Further, the laser generating mechanism 12 may irradiate the crack 5 of the abdominal plate 4 with a predetermined spot diameter in a parallel light state by an optical system different from the condensing optical system 14. Further, the laser generation mechanism 12 may irradiate the crack 5 of the abdominal plate 4 with a predetermined spot diameter in the state of parallel light directly from the laser oscillator 13.

  The laser crack repairing apparatus 10 having the above configuration melts and erases the crack 5 by irradiating the crack 5 generated in the belly plate 4 of the I-shaped steel 3 with the laser beam 11 having a predetermined spot diameter. Can be repaired.

  Further, the laser crack repairing apparatus 10 irradiates the crack 5 with the laser beam 11 to irradiate the laser beam with a predetermined spot diameter, so that the construction margin is improved and the laser crack repairing apparatus 10 is constructed at the site where the bridge 1 is constructed. Even in such a situation, the crack 5 can be easily and surely irradiated with the laser beam 11, and the crack 5 can be easily and reliably erased.

[3. Crack repair method]
Next, a crack repairing method for repairing the crack 5 using the laser crack repairing apparatus 10 will be described with reference to the process diagram of FIG.

  First, in step S1, as shown in FIG. 6 (A), one end portion 5a of the crack 5 generated in the belly plate 4 of the I-shaped steel 3 is compared with the width of the crack 5 as shown in FIG. 6 (B). The start hole 20 to be a large circular first through hole is formed. The start hole 20 becomes a starting point for irradiating the laser beam 11.

  Next, in step S2, as shown in FIG. 3, the laser beam 11 having a predetermined spot diameter is irradiated from one side (one surface) in the thickness direction of the abdominal plate 4 by the laser generation mechanism 12 of the laser crack repairing apparatus 10. To do. At this time, the laser beam is irradiated along the crack 5 from the start hole 20 serving as the first through hole to the other end 5b of the crack 5 as indicated by an arrow X in FIG. Thereby, the crack 5 is melted and erased. The spot diameter of the laser beam 11 here is set larger than the width of the crack 5 and smaller than the start hole 20. Specifically, the spot diameter is set to 3 to 7 mm. The crack 5 of the belly plate 4 of the I-shaped steel 3 is repaired as described above.

  In the crack repairing method to which the present invention is applied, a start hole 20 is formed at one end portion 5 a of the crack 5, and a laser beam 11 having a spot diameter larger than the width of the crack 5 and smaller than the start hole 20 from the start hole 20. When the irradiation is started, the laser beam 11 is inserted into the start hole 20, and the base end portion of the crack 5 connected to the start hole 20 by the laser beam 11 inserted into the start hole 20. However, the whole is heated substantially uniformly in the thickness direction of the abdominal plate 4. Here, for example, when the irradiation of the laser beam 11 is started without forming the start hole 20, and the main surface of the abdominal plate 4 is irradiated by the laser beam 11, the main portion of the abdominal plate 4. Since only the surface is heated, the temperature is different in the thickness direction of the abdominal plate 4, it is difficult to melt evenly in the thickness direction of the abdominal plate 4, and there is a possibility that cracks remain in an underheated state, There is a possibility that the periphery of the crack may sag due to an excessive and / or excessive heat input state. In the crack repairing method to which the present invention is applied, since the crack 5 is heated substantially uniformly in the thickness direction of the abdominal plate 4 as described above, only one main surface of the abdominal plate 4 is heated by the laser beam 11. The cracks 5 can be melted more uniformly and more easily in the thickness direction of the abdominal plate 4 than in the case of the above.

  The spot diameter of the laser beam 11 is not limited to being provided smaller than the start hole 20 but may be provided larger than the start hole 20.

  Here, conventionally, when a crack 5 occurs in a steel material such as the I-shaped steel 3, in order to alleviate the stress concentration of the crack 5 and prevent the crack 5 from proceeding, the ends 5a and 5b of the crack 5 are used. It is known to form a stop hole 21 made of a through hole in at least one of the above.

  In the crack repair method to which the present invention is applied, for example, as shown in FIG. 7, when the repair work of the crack 5 is performed after the stop hole 21 serving as the first through hole is formed in the one end portion 5 a of the crack 5, Using this stop hole 21 as the start hole 20, the laser beam 11 is irradiated along the crack 5 from the stop hole 21 at one end 5 a of the crack 5 to the other end 5 b of the crack 5 to melt the crack 5. May be erased.

  Further, in the crack repairing method to which the present invention is applied, for example, as shown in FIG. 8A, after the stop hole 21 serving as the second through hole is formed in the other end portion 5b of the crack 5, the crack 5 When the repair work is performed, as shown in FIG. 8B, a start hole 20 serving as a first through hole is formed in one end portion 5a of the crack 5, and the laser beam 11 is irradiated with the start hole in the one end portion 5a of the crack 5. Irradiation along the crack 5 from 20 to the stop hole 21 at the other end 5b of the crack 5 may be performed by melting the crack 5 and erasing it.

  Furthermore, in the crack repair method to which the present invention is applied, for example, as shown in FIG. 9, when repair work of the crack 5 is performed after the stop holes 21 and 21 are formed at both ends 5 a and 5 b of the crack 5, Using the stop hole 21 at one end 5a of the crack 5 serving as the first through hole as the start hole 20, the laser beam 11 is transmitted from the stop hole 21 at the one end 5a of the crack 5 to the crack 5 serving as the second through hole. Irradiation along the crack 5 to the stop hole 21 at the end 5b may be performed to melt and erase the crack 5.

  Furthermore, in the crack repair method to which the present invention is applied, the laser beam 11 is not limited to being irradiated from one side of the abdominal plate 4, and may be irradiated from both sides of the abdominal plate 4. Specifically, as shown in FIG. 10A, the laser beam 11 having a predetermined spot diameter is applied from the start surface 20 of the one end 5a of the crack 5 from the one main surface 4a side of the abdominal plate 4. Irradiation is performed along the crack 5 up to the other end 5b or the stop hole 21, and the crack 5 is melted by ½ or more of the thickness of the abdomen 4 to erase a part of the crack 5. Next, as shown in FIG. 10B, the laser beam 11 having a predetermined spot diameter is started from the other main surface 4b side opposite to the one main surface 4a of the abdomen 4 and the one end portion 5a of the crack 5 is started. Irradiation is performed along the crack 5 from the hole 20 to the other end 5b of the crack 5 or the stop hole 21, and the crack 5 is melted by more than ½ of the thickness of the abdomen 4 to erase the remaining crack 5. As described above, the crack 5 of the abdominal plate 4 may be repaired.

  Furthermore, as shown in FIG. 11A, the abdominal plate 4 may have a thinned portion 7 that is thinned by corrosion along with the crack 5. In such a case, in the crack repair method to which the present invention is applied, first, the ground treatment such as grinder polishing by removing the dirt and rust of the thinned portion 7 is performed, as shown in FIG. After repairing the crack 5 by the crack repairing method as described above, as shown in FIG. 11B, build-up welding is performed on the thinned portion 7, and the thinned portion 7 is filled with the built-up portion 8. Then, the thinned portion 7 may be repaired.

  For example, overlay welding for repairing the thinned portion 7 is performed by laser hot wire welding. When laser hot wire welding is performed, the laser crack repairing apparatus 10 melts and erases the crack 5 by irradiating the crack 5 of the abdominal plate 4 with the laser beam 11 in a defocused state, as shown in FIG. In addition to the laser generation mechanism 12, a filler wire supply mechanism 30 that supplies a filler wire 31 heated to a temperature close to the melting point is provided at an irradiation site of the laser light 11 irradiated from the laser generation mechanism 12.

  The filler wire supply mechanism 30 includes a wire drum 32, a wire straightening machine 33, and a wire holder 34. The filler wire 31 sent from the wire drum 32 is sent to the wire holder 34 with the curl corrected by the wire straightening machine 33. The wire holder 34 serves as a power supply unit for the filler wire 31, and a heating current is supplied to the filler wire 31 by the power supply unit 35 between the wire holder 34 and the abdomen 4 of the welding member. By supplying electric current to the filler wire 31, the filler wire 31 is heated by Joule heat to such an extent that the filler wire 31 melts when the tip contacts the molten pool. Then, the filler wire 31 delivered from the wire holder 34 is supplied to the irradiated portion of the laser light 11. As described above, build-up welding may be performed on the thinned portion 7 of the abdominal plate 4 by the laser crack repairing apparatus 10 to repair the thinned portion 7.

  The crack repairing method to which the present invention is applied is not limited to repairing the thinned portion 7 by performing overlay welding by laser hot wire welding, but by other conventionally known overlay welding. You may do it.

  Furthermore, in the crack repairing method to which the present invention is applied, after the crack 5 is repaired, overlay welding may be performed by the laser crack repairing apparatus 10 to fill the start hole 20 and the stop hole 21. Further, in addition to overlay welding, the start hole 20 and the stop hole 21 may be filled with putty or the like. The crack repairing method to which the present invention is applied may be variously modified without departing from the gist of the present invention.

  Next, the inventors confirmed the effect of the crack repair method to which the present invention was applied.

<Comparative Example 1>
In Comparative Example 1, the start hole 20 and / or the stop hole 21 are not formed in the crack 5 generated in the belly plate 4 of the I-shaped steel 3, and as shown in FIG. The crack 5 was repaired by irradiating a laser beam 11 having a predetermined spot diameter from the main surface 4a side.

  In Comparative Example 1, the belly plate 4 of the I-shaped steel 3 was made of carbon steel (SM400) and 6 mm thick. Further, the laser beam 11 was set so that the laser output was 10 kW, the torch moving speed was 0.30 m / min, and the spot diameter was φ5 mm.

  In Comparative Example 1 as described above, as shown in FIG. 12, an excessive heat input state occurred and the periphery of the crack was dropped.

<Comparative example 2>
In Comparative Example 2, the start hole 20 and / or the stop hole 21 is not formed in the crack 5 generated in the belly plate 4 of the I-shaped steel 3, and as shown in FIG. The crack 5 was repaired by irradiating a laser beam 11 having a predetermined spot diameter from the main surface 4a side.

  In Comparative Example 2, the belly plate 4 of the I-shaped steel 3 was made of carbon steel (SM400) and 6 mm thick. Further, the laser beam 11 was set so that the laser output was 10 kW, the torch moving speed was 0.50 m / min, and the spot diameter was φ5 mm.

  In Comparative Example 2 as described above, as shown in FIG. 13, cracks remained due to an excessive heat input state.

<Example 1>
In Example 1, as shown in FIGS. 6 (A) and 6 (B), a start hole 20 is formed at one end portion 5a of the crack 5 generated in the belly plate 4 of the I-shaped steel 3, and this start hole is formed. As shown in FIG. 3, the laser beam 11 having a predetermined spot diameter is irradiated from 20 to the other end 5 b of the crack 5 from the main surface 4 a side of the abdominal plate 4, Repaired.

  In Example 1, the belly plate 4 of the I-shaped steel 3 was made of carbon steel (SM400) and the plate thickness was 6 mm. Further, the laser beam 11 was set so that the laser output was 10 kW, the torch moving speed was 0.32 m / min, and the spot diameter was φ5 mm.

  In such Example 1, as shown in FIG. 14, the crack 5 is formed in an excessive heat input state without causing the periphery of the crack to sag, and in an under heat input state without the crack remaining. It was possible to erase by melting evenly in the thickness direction of the plate 4.

<Example 2>
In Example 2, as in Example 1, as shown in FIGS. 6 (A) and 6 (B), a start hole 20 is formed at one end 5a of the crack 5 generated in the belly plate 4 of the I-shaped steel 3. The laser beam 11 having a predetermined spot diameter is formed from one main surface 4a side of the abdominal plate 4 from the start hole 20 to the other end portion 5b of the crack 5 along the crack 5 as shown in FIG. Irradiation repaired crack 5.

  In Example 2, the belly plate 4 of the I-shaped steel 3 was made of carbon steel (SM400) with a plate thickness of 12 mm. Further, the laser beam 11 was set so that the laser output was 10 kW, the torch moving speed was 0.40 m / min, and the spot diameter was 3 mm.

  In the second embodiment, as shown in FIG. 15, the crack 5 is formed in an excessive heat input state without causing the periphery of the crack to sag, and in an under heat input state without the crack remaining. It was possible to erase by melting evenly in the thickness direction of the plate 4.

<Example 3>
In Example 3, as shown in FIGS. 6 (A) and 6 (B), a start hole 20 is formed at one end 5a of the crack 5 generated in the belly plate 4 of the I-shaped steel 3, and this start hole is formed. After irradiating the laser beam 11 with a predetermined spot diameter from one principal surface 4a side of the abdomen plate 4 from 20 to the other end 5b of the crack 5 along the crack 5, As shown in FIG. 10B, the crack 5 was repaired by irradiating a laser beam 11 having a predetermined spot diameter from the other main surface 4b side of the abdominal plate 4.

  In Example 3, the belly plate 4 of the I-shaped steel 3 was made of carbon steel (SM400) and the plate thickness was 12 mm. Further, the laser beam 11 was set so that the laser output was 10 kW, the torch moving speed was 1.00 m / min, and the spot diameter was 3 mm.

  In Example 3, as shown in FIG. 16, the crack 5 was uniformly melted in the thickness direction of the abdominal plate 4 and could be erased without remaining under crack due to an excessive heat input state.

  The crack repair method to which the present invention is applied is not limited to the above-described construction conditions. For example, the laser beam 11 has a laser output of 1 kW to 20 kW and a torch moving speed of 0.2 m / min. The spot diameter may be set within a range of .about.1.0 m / min and a spot diameter of .phi.0.2 mm to .phi.10 mm. Further, the steel materials to be repaired for the crack 5 are, in addition to those described in the comparative example and Examples 1 to 4, a plate thickness of 3.2 mm to 30 mm, a material of carbon steel (SS, SM), and a high strength carbon steel ( HT690, HT780, HT980), stainless steel (SUS) or the like may be used.

  1 Bridge, 2 Main Girder, 3 I-shape Steel, 4 Abdominal Plate, 4a One Main Surface, 4b Other Main Surface, 5 Crack, 5a One End, 5b Other End, 6 Steel Floor Deck, 7 Thinning Section, 8 Overlaying part, 10 Laser crack repair device, 11 Laser light, 12 Laser generation mechanism, 13 Laser oscillator, 14 Condensing optical system, 15 Laser torch, 16 Optical fiber, 17 Focus, 20 Start hole, 21 Stop hole, 30 Filler Wire supply mechanism, 31 filler wire, 32 wire drum, 33 wire straightening machine, 34 wire holder, 35 power supply unit, 100 backing plate, 101 fastening member

Claims (14)

  A laser beam with a predetermined spot diameter is irradiated along the crack from the first through hole formed at one end of the crack generated in the steel material of the existing steel structure to melt the crack. A method for repairing a crack, characterized in that it is erased.   The crack repairing method according to claim 1, wherein the first through hole is formed in a circular shape having a diameter larger than the width of the crack.   The method for repairing a crack according to claim 1, wherein a second through hole is formed in the other end of the crack in the steel material.   The method for repairing a crack according to claim 1, wherein a spot diameter of the laser light is larger than a width of the crack.   The spot diameter of the said laser beam is smaller than said 1st through-hole and said 2nd through-hole, The repair method of the crack in any one of Claims 1-4 characterized by the above-mentioned.   The method for repairing a crack according to claim 1, wherein a spot diameter of the laser beam is larger than that of the first through hole and the second through hole.   The method for repairing a crack according to claim 1, wherein a spot diameter of the laser beam is 3 to 7 mm.   The method for repairing a crack according to claim 1, wherein the focal point of the laser beam is located outside the steel material.   The method for repairing a crack according to any one of claims 1 to 7, wherein the focal point of the laser beam is located inside the steel material.   The method for repairing a crack according to claim 1, wherein the laser beam is irradiated from one side of the steel material in the thickness direction.   The method for repairing a crack according to claim 1, wherein the laser light is irradiated from both sides in the thickness direction of the steel material.   The method of repairing a crack according to claim 11, wherein the laser beam melts ½ or more of the thickness of the steel material by one irradiation.   The crack repairing method according to claim 1, wherein build-up welding is performed after repairing the crack.   The crack repairing method according to claim 13, wherein the build-up welding is laser hot wire welding.

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